amir rattner, hui sun, and jeremy nathans 1,4...

P1: FKZ/FHS/FOB P2: FKZ/fgp QC: FhN/anil T1: FhN

October 16, 1999 16:58 Annual Reviews AR095-04

?Annu. Rev. Genet. 1999. 33:89–131

Copyright c© 1999 by Annual Reviews. All rights reserved

MOLECULAR GENETICS OF HUMAN RETINAL

DISEASE

Amir Rattner,1,4 Hui Sun,1,4 and Jeremy Nathans1–41Department of Molecular Biology and Genetics,2Department of Neuroscience,3Department of Ophthalmology,4Howard Hughes Medical Institute, Johns HopkinsUniversity School of Medicine, Baltimore, Maryland 21205; e-mail: [email protected]

Key Words macular degeneration, retinitis pigmentosa, photoreceptor, ophthalmicgenetics

■ Abstract The past decade has witnessed extraordinary progress in retinal dis-ease gene identification, the analysis of animal and tissue culture models of diseaseprocesses, and the integration of this information with clinical observations and withretinal biochemistry and physiology. During this period over twenty retinal diseasegenes were identified and for many of these genes there are now significant insightsinto their role in disease. This review presents an overview of the basic and clini-cal biology of the retina, summarizes recent progress in understanding the molecularmechanisms of inherited retinal diseases, and offers an assessment of the role thatgenetics will play in the next phase of research in this area.

CONTENTS

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90The Human Retina. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90Summary of Some Common Clinical Phenotypes. . . . . . . . . . . . . . . . . . . . . . . . . 93Mechanisms of Cell Death in Retinitis Pigmentosa. . . . . . . . . . . . . . . . . . . . . . . 95Diagnosis of Retinal Disease. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

Specific Gene Defects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97Phototransduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97Structure and Biosynthesis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101The Visual Cycle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106Retinal Pigment Epithelium. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108Development. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109Metabolism. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111Miscellaneous. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

Future Directions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114Genetic Analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114Prospects for Therapy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

0066-4197/99/1215-0089$08.00 89



?90 RATTNER ■ SUN ■ NATHANS

INTRODUCTION

This review summarizes recent work on the molecular mechanisms of inheritedretinal disease. It emphasizes those retinal diseases for which some insight existsinto pathogenic mechanisms and gives only cursory coverage to systemic diseasesthat have retinal manifestations and to retinal diseases for which the responsiblegenes have not yet been identified. Had this review been written ten years ago, itcould have provided comprehensive coverage of this topic in only a few pages.However, the past decade has witnessed extraordinary progress in retinal diseasegene identification, the analysis of animal and tissue culture models of diseaseprocesses, and the integration of this information with clinical observations andwith retinal biochemistry and physiology. As a result, this review can present onlya summary of current information.1

For additional information, the reader may wish to consult the following sour-ces. Retinal physiology with an emphasis on clinical applications is presented inReference 89 and with an emphasis on fundamental processes in Reference 199.A comprehensive treatment of the retinal pigment epithelium can be found inReference 146, and descriptions of the natural history and management of retinaldiseases can be found in References 92 and 177. An invaluable on-line resource,RetNet (http://wwwsph.uth.tmc.edu/Retnet/), has an up-to-date compendium ofreferences on inherited retinal disease and is maintained by Dr. Stephen Daigerat the University of Texas. Recent reviews on inherited retinal disease includeReferences 17, 20, 59, 64, and 84.

The Human Retina

Structure and Cell Biology The retina is an outpouching of the central nervoussystem (CNS) that covers the back wall of the eye. Three layers of cells comprise

1Abbreviations: Amino acids are referred to by the single letter code; amino acid sub-stitutions are referred to by the identity of the original amino acid, the codon number,and the identity of the new amino acid, e.g. K296E refers to the substitution of gluta-mate for lysine at codon 296; ABCR, retina-specific ABC transporter; ADRP, autosomaldominant RP; AMD, age-related macular degeneration; ARRP, autosomal recessive RP;CACNA1F, calcium channelα-subunit; CNCG, cyclic nucleotide gated channel; CNS,central nervous system; CRALBP, cellular retinaldehyde binding protein; CRX, cone-rodhomeobox; CSNB, congenital stationary (i.e. nonprogressive) night blindness; ECM, ex-tracellular matrix; EOG, electrooculogram; ERG, electroretinogram; GA, gyrate atrophy;GCAP, guanylate cyclase activating protein; LHON, Leber hereditary optic neuropathy;MMP, matrix metalloproteinase; ND, Norrie Disease; NRL, neural retina leucine zip-per; OAT, ornithine amino transferase; PDE, phosphodiesterase; rds, retinal degenerationslow; rd, retinal degeneration; REP-1, rab escort protein-1; RetGC, retinal guanylate cy-clase; RGS, regulator of G-protein signaling; ROM-1, rod outer segment membrane protein-1; ROS, rod outer segment; RP, retinitis pigmentosa; RPE, retinal pigment epithelium;RPGR, retinitis pigmentosa GTPase regulator; SFD, Sorsby fundus dystrophy; TIMP, tis-sue inhibitor of metalloproteinases; XLRP, X-linked RP; XLRS, X-linked retinoschisis.



?HUMAN RETINAL DISEASE 91

Figure 1 Schematic of the vertebrate retina and the adjacent choroid. The vitreous isat the bottom and the sclera is at the top. Most of the major cell types and structuresare shown: a choroidal blood vessel, Bruch’s membrane separating the choroid fromthe RPE, an RPE cell, rods and cones, bipolar cells, a ganglion cell, and a Mullerglial cell. Horizontal and amacrine cells that process information in the outer and innerplexiform layers, respectively, are not shown.

the neural retina (Figure 1). The outer layer contains the photoreceptor cells, therods and cones, which mediate dim light (scotopic) and bright light (photopic)vision, respectively. In humans only 5% of photoreceptor cells are cones and 95%are rods. Cones are found throughout the retina but are most concentrated withina small central region, the fovea. A somewhat larger zone, called the macula,is centered on the fovea and also includes the immediately surrounding rod-richretina. Cones mediate color vision, and in humans come in three types that differdepending upon which one of three light sensing pigments (visual pigments) ispresent. The phototransduction proteins are located within a modified and greatlyenlarged cilium, referred to as an outer segment, that protrudes from the apicalface of each photoreceptor. The outer segment is filled with flattened membranesacs called discs and is constantly renewed by new synthesis and assembly at itsbase, and by shedding of older material from its tip. The second (inner) layer ofcells contains bipolar cells, the second-order neurons onto which photoreceptorssynapse, as well as horizontal and amacrine cells, interneurons that process visualinformation in the outer and inner retina, respectively. The third and innermost




Figure 2 The visual cycle (adapted from Reference 191).

cell layer contains ganglion cells, the output units of the retina, as well as someamacrine cells. Ganglion cell axons track along the inner surface of the retina,coming together at the optic disc to form the optic nerve.

Adjacent to the retina is the retinal pigment epithelium (RPE), a CNS derivativethat separates the retina from the choroidal circulation, the blood supply for theouter retina. The RPE lies in close apposition to the outer segments and eachRPE cell contacts approximately 45 outer segments. As one of its functions, theRPE engulfs and digests the distal 10% of each outer segment daily, a total biomassequivalent to several average-size cells. This digestion, which continues throughoutlife, makes the RPE cell the most active phagocytic cell in the human body. TheRPE also participates in a unique exchange with the photoreceptors known as thevisual cycle, which plays a critical role in visual pigment regeneration (Figure 2).All vertebrate visual pigments consist of an apoprotein, opsin, linked covalently to achromophore 11-cisretinal (an aldehyde derivative of vitamin A). Photoactivationisomerizes retinal from 11-cis to all-trans, after which all-transretinal dissociatesfrom opsin, to be replaced by a new molecule of 11-cisretinal. In rods, the releasedall-trans retinal chromophore is reduced to all-trans retinol (the correspondingalcohol derivative of vitamin A) and transported to the RPE. Within the RPE it isesterified to a lipid, chemically isomerized to the 11-cisconfiguration, hydrolyzedfrom the ester linkage, and finally oxidized to the aldehyde and returned to thephotoreceptor. As discussed below, the isomerization cycle for cones may occurwithin the retina rather than the RPE. The flux through the visual cycle is extremelyhigh: gazing at the blue sky on a sunny day produces 20,000 photoisomerizations




per rod per second (199). Under these viewing conditions, the visual cycle in eachRPE cell therefore processes approximately 1,000,000 chromophores per second.

Phototransduction The excitation phase of phototransduction begins with pho-toactivation of a visual pigment, which induces a conformational change thatcatalyzes the activation of a photoreceptor G protein, transducin (Figure 3A). Eachactivated alpha subunit of transducin, now bound to GTP, displaces the inhibitorysubunit of a cGMP phosphodiesterase (PDE), which then catalyzes the hydrolysisof cGMP to GMP. A cGMP-gated cation channel located in the outer segmentplasma membrane closes in response to lower intracellular cGMP, leading to adecrease in sodium and calcium influx. The resulting membrane hyperpolariza-tion leads to a graded attenuation in neurotransmitter release at the photoreceptorsynapse. Within the outer segment, the drop in intracellular calcium triggers anegative feedback loop that mediates light adaptation in the case of a sustainedstimulus, or restores the cell to its preactivated state in the case of a transient stim-ulus. This feedback loop is described in greater detail under “Guanylate Cyclaseand Guanylate Cyclase Activating Protein (GCAP).”

The recovery phase of phototransduction involves active turnoff and recyclingof the transduction components (Figure 3A). Rhodopsin is inactivated by the com-bined action of rhodopsin kinase, which specifically phosphorylates photoactivatedrhodopsin, and arrestin, which binds phosphorylated rhodopsin (see below under“Arrestin and Rhodopsin Kinase”). Transducin hydrolyzes GTP to GDP and Pi, areaction that is accelerated by a member of the regulator of G-protein signaling(RGS) family, RGS-9, thereby allowing PDE to return to its inactive state. Theaction of guanylate cyclase leads to an increase in the cytosolic cGMP concen-tration, which in turn leads to an increase in the number of open cGMP-gatedchannels. The locations within the outer segment of the main structural and pho-totransduction proteins are shown in Figure 3B. More extensive descriptions ofphototransduction can be found in References 115, 188, and 262.

Summary of Some Common Clinical Phenotypes

In the industrialized world, the most common diseases involving the retina are dia-betic retinopathy, glaucoma, and age-related macular degeneration (AMD), whichtogether affect several percent of the population. Each of these diseases has bothgenetic and nongenetic components. By contrast, the simple Mendelian retinal dis-eases, which are the focus of this review, affect in aggregate approximately one per-son in 2000. Many of the Mendelian diseases have an earlier onset and some havea more severe clinical course than typically observed for the three more commondisorders listed above and, for the most part, they are untreatable. These charac-teristics, together with the possibility of using genetic approaches to understanddisease mechanisms, have focused attention on the Mendelian disorders.

Hereditary retinal diseases are characterized by age of onset, severity and topo-graphic pattern of visual loss, rod vs cone involvement, ophthalmoscopic findings,



?Figure 3 (A) The phototransduction cascade.Upper, the excitation phase;lower, therecovery phase. Activated species are shown initalics with an asterisk (∗) appended.Catalytic reactions that amplify the signal are shown asbold arrows. Arr, arrestin;RK, rhodopsin kinase; R, dark rhodopsin, i.e. with 11-cis retinal attached; R∗, pho-toactivated rhodopsin, i.e. with all-trans retinal attached; T, transducin; PDE, cGMPphosphodiesterase; GCAP, guanylate cyclase activating proteins; G cyclase, guanylatecyclase; CNCG, cGMP-gated channel; RGS, regulator of G-protein signaling. (B) Sub-cellular localization of rod outer segment proteins. Each of the four schematic diagramsshows a rod outer segment with a stack of (internal) disc membranes. The indicatedproteins are localized to the following regions (left to right): the edge of the discs, theplasma membrane, the disc and plasma membranes, the cytosol. Several proteins indi-cated as cytosolic are tethered to the cytosolic leaflet of the membrane by a covalentlylinked lipid.




and family history. Retinitis pigmentosa (RP), a clinically and genetically hetero-geneous group of disorders, is classically characterized by impaired rod function,a progressive degeneration of the retina beginning in the midperiphery, and acharacteristic retinal deposit, the appearance of which has given it the name “bonespicule” pigmentary deposit (91, 176). RP usually spares the central retina, whichmediates high-acuity vision, until late in the disease. Eventually, most RP patientslose both rod and cone function. In a minority of patients with RP or RP-likediseases, cone dysfunction occurs early in the disease; this is referred to as cone-rod dystrophy. RP can have X-linked (XLRP), autosomal recessive (ARRP), orautosomal dominant (ADRP) modes of inheritance, and as described below, eachof these forms shows both locus and allelic heterogeneity. Macular degenerationsshow the converse anatomic pattern, preferentially affecting the central retina andcausing a loss of acuity often with minimal impairment in peripheral vision. Lebercongenital amaurosis is the name given to nonsyndromic retinal dystrophies thatare diagnosed shortly after birth or in infancy. Additional descriptions of clinicalphenotypes are presented in the text below.

Mechanisms of Cell Death in Retinitis Pigmentosa

In humans with RP (162) and in mouse models of RP (30, 140, 187), photorecep-tor cell death occurs by apoptosis as determined by analysis of DNA fragmen-tation and by the absence of an inflammatory response. Interestingly, in humanRP retinas there is patchy loss of both rod and cone photoreceptors (34), and inchimeric or mosaic mouse models of RP, diseased or dying photoreceptor cellsinduce cell death in adjacent genetically normal photoreceptor cells (102, 117).The deleterious effect of proximity to defective and/or dying cells is presumablyresponsible for the eventual loss of cones in those RP patients who carry rod-specific gene defects. Within this group of patients, the progressive loss of cone-mediated vision has a far greater impact on quality of life than does the absenceof rod function. One reasonable therapeutic goal for this group of patients mightbe to diminish cone loss by preserving rod viability even in the absence of rodfunction.

Diagnosis of Retinal Disease

Genetic analysis depends on accurate disease classification. Retinal diseases rep-resent a favorable group in this respect because, in contrast to most tissues, theappearance and function of the retina can be monitored with high precision usingmethods that are minimally invasive. The following paragraphs summarize theprincipal methods used for clinical testing of retinal disease (16, 25, 255).

Fundus Imaging The back of the eye, the fundus, can be observed throughthe ophthalmoscope. The clinician can therefore monitor the appearance of theretina and its vasculature, and can detect gross abnormalities such as defects in theRPE, retinal and subretinal deposits, neovascularization, and retinal detachments.




Fundus photography can be combined with intravenous fluorescein injection tomonitor intraocular blood flow, a technique referred to as fluorescein angiography.Fluorescein angiography can detect the neovascularization that often accompa-nies macular degeneration and diabetic retinopathy. Fluorescein angiography isalso useful for identifying defects in the RPE, since fluorescent signals from thechoroidal circulation are normally attenuated by melanin within the RPE.

Several novel imaging methods show great promise for high-resolution nonin-vasive monitoring of retinal disease processes. Optical coherence tomography useslaser interferometry to measure optical reflectivity and generates cross-sectionalimages of the retina with a spatial resolution of∼10 µm (95, 101). A relatedtechnique, autofluorescent imaging of the fundus, measures the distribution andlevels of autofluorescent pigments deposited within the RPE, a characteristic of theaging human eye that appears to be a risk factor for macular degeneration (245).Most recently, adaptive optics, a method developed to enhance the quality of astro-nomical images, has been used to view the human retina at single-cell resolution(201).

Psychophysics By definition, psychophysical testing relies on a behavioral re-sponse to a stimulus. Human visual psychophysics enjoys the advantages of anenormous range of stimuli and a highly developed nervous system. Visual stim-uli can vary in intensity, wavelength, duration, and spatial extent, and they canbe superimposed on adapting stimuli or convey a complex pattern. In practice,the clinical assessment is usually confined to tests of visual acuity, color vision,and the visual field using either static or kinetic stimuli to map out the regionswithin the retina that show decreased or absent sensitivity (scotomas). Less com-monly, dark adaptation is assessed. For this purpose, the subject is placed in adark room following exposure to a bright light, and at frequent intervals over theensuing 30–60 minutes the threshold for detection of a brief test flash of variableintensity is determined. Subjects with retinitis pigmentosa, congenital stationarynight blindness (CSNB), or any of a number of diseases that affect scotopic visioncan show altered kinetics of dark adaptation.

Electrophysiology The retina generates a complex electrical response to illu-mination that can be recorded by measuring the fluctuating potential between acorneal and a reference electrode. This signal, referred to as the electroretinogram(ERG), consists of sequential waves of activity that originate in the photoreceptors(the a-wave), the inner retina (the b-wave), and the RPE (the c-wave). Ganglioncell and optic nerve activity do not contribute to the ERG response to flashes oflight. The ERG can selectively measure rod or cone responses: Rod responses areobtained under scotopic conditions, and cone responses are obtained under pho-topic conditions or by using a high-frequency stimulus train that the more sluggishrods cannot track. A patient with photoreceptor loss over a fraction of the retinawill show a proportional decrease in ERG amplitude under full-field (ganzfeld)illumination. Because of its high information content and because it is an objective




test, the ERG has become an important clinical tool in diagnosing retinal disease.A second test, the electrooculogram (EOG), measures a standing potential acrossthe eye that originates in the RPE. Its greatest utility is in assessing diffuse diseaseof the RPE, especially Best vitelliform macular dystrophy (described below).

SPECIFIC GENE DEFECTS

In this section we briefly describe each of the inherited retinal diseases for which theresponsible gene has been identified. The normal functions of the protein productsof some of these genes are not well understood, and for most of the genes ourunderstanding of the pathophysiological consequences of mutation is incomplete.In particular, we cannot easily connect what is known of the function or abundanceof the encoded protein with those aspects of disease that are most important to thepatient—the rapidity and severity of visual loss. Despite these limitations in ourcurrent knowledge base, we have taken the liberty in this review of grouping thegenes described below by the known or likely mechanism of action of their proteinproducts or the cell types in which the disease genes are likely to act. Genes listedin the miscellaneous group are ones for which there are insufficient data to assigna mechanistic or cell-type category.

Phototransduction

As described in the introductory paragraphs, rods and cones utilize a G-protein–coupled enzyme cascade to amplify the signal derived from a photoactivated visualpigment. For rod photoreceptors, the major protein components of this cascadehave been identified and the corresponding cDNAs encoding them have beenisolated. Although the possibility exists that additional regulatory proteins remainto be discovered, the broad outlines of the phototransduction cascade are nowwell established. In general, phototransduction in cones appears to utilize proteinsthat are homologous to, but distinct from, those in rods. Because cones constituteonly a small minority of photoreceptors in most mammalian retinas, including thehuman retina, cone phototransduction components have been identified principallyby using the cloned rod cDNAs as hybridization probes.

One of the most successful approaches to identifying retinal disease genes hasbeen systematic mutation screening of the approximately 15 rod phototransductiongenes using a panel of several hundred unrelated patients who are affected by adiverse set of retinal diseases (53). In its first application—the identification ofrhodopsin gene mutations in ADRP (60)—this effort was focused by informationobtained from linkage analysis (157), but for the most part it has been appliedwithout linkage information. Systematic mutation screening is well suited to thestudy of retinal diseases because of the extreme genetic heterogeneity within thispopulation and the high frequency of recessive disease that together precludeeffective linkage analysis for the majority of patients.




Rhodopsin As described below under “Structure and Biosynthesis,” most rho-dopsin mutations that produce a phenotype in humans appear to affect folding,stability, or intracellular trafficking and cause ADRP. Exceptions to this pat-tern are seen in two rhodopsin mutations (G90D and A292E) in patients withCSNB (54, 222). In vitro analysis of these proteins indicates that they are active inG-protein signaling in the absence of the retinal chromophore (54, 189). In bothcases, the newly acquired carboxylate appears to weaken the salt bridge betweenlysine296, the site of covalent attachment of the chromophore in the seventh trans-membrane domain, and its counterion, glutamate113, in the third transmembranedomain. A large body of work shows that this salt bridge is essential for maintain-ing opsin in an inert state (190). In heterozygotes these mutant proteins greatlyreduce the amplitude of the rod ERG, presumably because the small fraction ofmutant protein that is unliganded produces a signal that mimics a desensitizinglight stimulus. The lack of rod degeneration in carriers of the G90D and A292Emutations most likely reflects the ability of these proteins to bind 11-cis retinal,which stabilizes them and holds them in an inactive state.

Two mutant rhodopsins that are unable to bind 11-cis retinal due to substitu-tion at lysine296 (K296E and K296M) are found in patients with ADRP. In vitroexperiments indicate that the lysine296mutants can activate transducin (196, 197),reflecting a lack of the inhibitory salt bridge between residues 113 and 296. Trans-genic mice expressing low levels of K296E opsin show minimal rod ERG abnor-malities early in life, but their rods degenerate over the ensuing several monthsregardless of whether they are or are not exposed to light (133). Rod outer segments(ROS) from the K296E transgenic mice contain significant quantities of K296Eopsin that is constitutively phosphorylated and bound by arrestin. Removal of ar-restin (by washing the ROS membranes in urea) and dephosphorylation of opsinand rhodopsin reveals the expected light-independent transducin activation by theK296E opsin. Taken together, the in vivo and in vitro experiments suggest that theK296E opsin exhibits constitutive activity in vivo and is shut off by phosphorylationand arrestin binding. The mechanism by which K296E opsin causes photoreceptorcell death may be related to the considerably lower stability of opsin compared torhodopsin (circa 15–20◦C; 104).

Transducin The single known transducin alpha mutation (G38D) is of specialinterest because it is responsible for autosomal dominant CSNB in one of thelargest and most famous pedigrees in human genetics, the Nougaret family, whichtraces its ancestry to Jean Nougaret (1637–1719), who along with many of hisdescendents experienced severely reduced night vision (58). Residue 38 formsa critical part of the active site for GTP hydrolysis as judged by its positionwithin a loop that hydrogen bonds with the beta and gamma phosphates of GTP(124). Mutations in the corresponding position (codon 12) in the small monomericG-protein p2lrasdiminish GTPase activity and are oncogenic as a result of excessivesignaling (129). The G38D mutation in the alpha chain of transducin presumablyleads to excessive signaling in response to light, thereby elevating the background




noise within rod photoreceptors. A defect at the level of the outer segment wouldeliminate the a-wave in the electroretinogram, a finding that has been observed insome CSNB subjects (27, 28). Other CSNB subtypes are associated with a normala-wave but abnormal or absent b-waves, indicative of defects beyond the outersegment, for example, in neurotransmission (see below under “Calcium ChannelCACNA1F”).

Arrestin and Rhodopsin Kinase These two proteins act together to shut offrhodopsin activity (Figure 3A). Within one or a few hundred milliseconds ofphotoactivation, rhodopsin kinase phosphorylates photoexcited rhodopsin, whichis then capped by arrestin binding. Rhodopsin regeneration following photoacti-vation, which involves all-trans retinal release, 11-cis retinal binding, rhodopsindephosphorylation, and arrestin release, occurs on a time scale of minutes. Reces-sive loss-of-function mutations in either arrestin (72) or rhodopsin kinase (35, 259)produce Oguchi disease, a variant of CSNB. Oguchi disease due to rhodopsin ki-nase mutation causes slowing of both rod and cone recovery after light exposure,which implicates rhodopsin kinase in cone pigment phosphorylation (35). A diag-nostic fundus abnormality in Oguchi disease is the Mizuo-Nakamura phenomenon,a metallic golden-yellow color change that is induced by light. The origin of theMizuo-Nakamura phenomenon is still uncertain, but it is also seen in patients withX-linked retinoschisis (46) and X-linked recessive cone dystrophy (94). It has beensuggested to arise from an abnormal concentration of extracellular potassium (46).Interestingly, the same arrestin gene mutation responsible for most cases of Oguchidisease in Japan (a frame shift at codon 309) is responsible for RP in several per-cent of Japanese patients with ARRP (171). In this study, one member of a pairof siblings who were homozygous for a codon 309 frameshift mutation exhibitedclassic RP without the Mizuo-Nakamura phenomenon whereas the other siblinghad Oguchi disease.

cGMP PDE The alpha and beta chains of PDE are each approximately 90 kDaand together form the catalytically active complex. The gamma subunit associateswith alpha and beta to form an enzymatically inactive alpha1beta1gamma2 het-erotetramer. When liganded to GTP, the alpha subunit of transducin induces thedissociation of the gamma subunits, thereby activating the alpha-beta heterodimer.Recessive mutations within the alpha (103) and beta (45, 155, 156, 242) subunitsof PDE account for several percent of patients with ARRP. This disease associa-tion was predicted by earlier work on theretinal degeneration (rd)mouse, whichcarries a defective beta PDE gene and exhibits a marked elevation in cGMP and aprogressive loss of rod photoreceptors (63). In therd mouse and in this subset ofARRP patients, increased cGMP presumably leads to an increase in the numberof cGMP gated channels that are open, an increase in sodium and calcium influxacross the ROS plasma membrane, and a resulting increase in the requirementfor high-energy phosphate to extrude intracellular sodium. No disease-causingmutations have yet been identified in the gamma subunit of PDE.




In one family, a beta-PDE missense mutation (H258D) has been found to causeautosomal dominant CSNB (75). This mutation maps near the PDE gamma bindingsite in the amino-terminal half of the protein, whereas most of the ARRP mutationsaffect the carboxy-terminal catalytic domain. The CSNB mutation may act bydecreasing the effectiveness of PDE inhibition by the gamma subunit, leading toconstitutive hydrolysis of cGMP and rod desensitization.

Cyclic Nucleotide Gated Channel (CNCG)Rod and cone CNCGs consist ofheterotetramers of two homologous subunits (264). Mutations in the alpha subunitof the rod CNCG are responsible for 1–2% of ARRP (55). The mutations identifiedthus far either produce early truncations or destabilize the channel as determinedby a decrease in the number of channels in the plasma membrane of transientlytransfected cells. Missense mutations within the alpha subunit of the cone CNCGare found in autosomal recessive rod monochromacy, a nonprogressive disordercharacterized by a complete absence of cone function (121). The functional prop-erties of the mutant cone CNCGs have not yet been characterized, but severalmutations are located within the cGMP-binding domain and therefore may impairgating by cGMP.

Guanylate Cyclase and Guanylate Cyclase Activating Protein (GCAP)In thephotoreceptor outer segment, cGMP hydrolysis in response to light is balanced bycGMP synthesis (from GTP) by guanylate cyclase (Figure 3A). The photoreceptor-specific guanylate cyclases, RetGC-1 and RetGC-2, are members of a large fam-ily of guanylate cyclases that have an extracellular domain, a single membranespan, and a large intracellular domain within which the catalytic domain occu-pies the extreme carboxy-terminus (51, 141, 260, 261). Guanylate cyclase activityis tightly regulated by calcium—it increases when the calcium concentration dropsduring light exposure and it decreases when calcium levels rise in the dark. Thisregulation is mediated by two homologous calcium-binding proteins, guanylatecyclase activating protein-1 and -2 (GCAP-1 and -2), which each have four EF-hands (186). Intracellular calcium declines following light exposure becauseclosure of the plasma membrane cGMP gated channel blocks calcium entry whileintracellular calcium continues to be extruded via the plasma membrane sodium/calcium/potassium exchanger. Thus, the calcium-dependent modulation of guany-late cyclase activity serves to restore the outer-segment cGMP level to baseline.This feedback loop is the principal mechanism by which photoreceptors adapt tolight and damp out fluctuations in membrane potential (262).

Recessive missense and frameshift mutations inRETGC-1are responsible fora subset of cases of Leber congenital amaurosis (184). A naturally occurring nullmutation in the chicken orthologue ofRETGC-1produces a recessive photore-ceptor degeneration that begins after hatching, thus providing an animal modelfor this form of Leber congenital amaurosis (217). Two missense mutations inadjacent codons within a putative intracellular dimerization domain in RetGC-1(127), E837D and R838C, are responsible for one form of autosomal dominant




cone-rod dystrophy (119). The adjacent locations of these two mutations suggeststhat they impart a specific gain of function to the mutant protein and that cone-roddystrophy in these families does not arise from haploinsufficiency, a conclusionthat is reinforced by the lack of a phenotype in carriers of the recessive null mu-tations inRETGC-1responsible for Leber congenital amaurosis. An unansweredquestion at present is why the second photoreceptor guanylate cyclase, RetGC-2,cannot compensate for defects in RetGC1.

Affected individuals in a single family with dominant cone dystrophy havebeen found to carry a missense mutation (Y99C) in the third EF hand of guany-late cyclase activating protein-1 (182). In vitro, GCAP-1 (Y99C) resembles wild-type GCAP-1 in stimulating the guanylate cyclase activity of recombinant RetGC-1and of crude bovine rod outer segments in low (50 nM) calcium (50, 225). However,in high (2µM) calcium, a concentration at which wild-type GCAP-1 shows nostimulatory activity, GCAP1 (Y99C) exhibits 30–50% of the stimulatory activityseen in low calcium. Thus, GCAP1 (Y99C) would be predicted to inappropriatelystimulate cGMP synthesis in dark-adapted photoreceptors and lead to a derange-ment similar to that observed with loss-of-function mutations in cGMP PDE. Thepreferential degeneration of cones in patients carrying the GCAP-1 (Y99C) muta-tion may reflect the high level of expression of GCAP-1 in cone compared to rodphotoreceptors (82, 100).

Structure and Biosynthesis

Rhodopsin Approximately 30% of cases of ADRP are caused by rhodopsin genemutations, most of which produce single amino acid substitutions (56, 232). Asdescribed below, nearly all of the approximately 35 ADRP mutant rhodopsins thathave been studied in transfected cells and/or in transgenic mice have been foundto differ from wild-type rhodopsin in one or more biochemical or cell biologicalproperties.

One group of rhodopsin mutations, referred to as class I, cluster at the extremecarboxy-terminus (Figure 4). In transfected 293 or COS cells, class I rhodopsin mu-tants are indistinguishable from wild type in all aspects examined thus far: yield,efficiency of localization to the plasma membrane, 11-cis retinal binding, trans-ducin activation, and phosphorylation by rhodopsin kinase (116, 163, 233–235). Intransgenic mice, one class I mutant (Q344ter) shows inefficient outer segment lo-calization of the mutant opsin but not the endogenous wild-type opsin (234), and asecond mutant (P347S) induces massive accumulation of vesicles at the base of theouter segment (134a). Experiments in which synthetic peptides from rhodopsin’scarboxy-terminus were added to cell-free homogenates of retina reveal a partial in-hibition of cell-free vesicular transport of opsin by wild type but not by three class Imutant carboxy-terminal peptides (49). The role of rhodopsin’s carboxy-terminusin protein transport and sorting to the outer segment has recently been clarifiedwith the discovery that the wild-type carboxy-terminus binds to the Tctex-1 pro-tein, a widely expressed dynein light chain, whereas various single amino acid




Figure 4 (Right) Mutations responsible for ADRP that are near the carboxy-terminusof rhodopsin (44) and (Left) amino acid substitutions near the carboxy-terminus of ver-tebrate cone and rod pigments. The compendium of vertebrate rod and cone pigmentsis from human, mouse, goldfish, zebrafish, chicken, gecko, and frog.

substitutions of rhodopsin’s carboxy-terminus responsible for ADRP fail to bind(237a). Tctex-1 is abundant in photoreceptor inner segments and is likely to guidethe transport of rhodopsin-laden post-Golgi vesicles to their destination.

As seen in Figure 4, a compendium of ADRP mutations near rhodopsin’scarboxy-terminus reveals numerous amino acid substitutions at positions 345 and347, as well as frameshift and stop codon mutations that alter or remove the lastfew amino acids. Figure 4 also shows that among vertebrate visual pigments,positions 345 and 347 are distinguished from other positions near the carboxy-terminus by their high degree of evolutionary conservation. Taken together, thesedata indicate that valine345, proline347, and possibly the free carboxylate followingthe terminal amino acid at position 348 comprise a protein sorting signal that isnot required for ER–Golgi–plasma membrane movement in cultured cells but thatplays a specialized role in outer segment protein localization.

The second group of rhodopsin mutants, referred to as class II, show defects instability and/or protein folding when expressed in cultured cells (76, 116, 137, 163,233–235). The mutant proteins accumulate to reduced levels relative to the wildtype, are localized predominantly to the endoplasmic reticulum, and bind 11-cisretinal variably or not at all. Most class II mutants are single amino acid substi-tutions and are distributed throughout rhodopsin’s transmembrane and intradiscaldomains (the latter is topologically equivalent to an extracellular domain), a patternthat was anticipated by earlier mutagenesis studies aimed at identifying domainsthat contribute to the stability and folding of bovine rhodopsin (52). The asym-metric location of amino acids involved in stability and/or folding may reflecta requirement for flexibility in the cytosolic domain of rhodopsin, which must




undergo a conformational transition to interact with transducin, rhodopsin kinase,and arrestin.

In the analysis of class II mutants, it is noteworthy that different investigatorsstudying the same amino acid substitutions have observed different degrees offunctional impairment, as assessed by protein yield, degree of 11-cis retinal bind-ing, and subcellular localization. In general, a greater degree of impairment hasbeen reported with human rhodopsin mutants expressed in 293 cells and recon-stituted with 11-cis retinal after membrane purification (233, 235) relative to thecorresponding mutants in bovine rhodopsin expressed in COS cells and reconsti-tuted in intact cells (116, 137, 163). In ADRP, the photoreceptor cell demise thatresults from a mixture of normal and unstable or misfolded opsin may reflect thehigh steady-state level of rhodopsin (5× 107 rhodopsins per cell) and the highrate of rhodopsin synthesis (5× 106 rhodopsins per cell per day). In keeping withthis hypothesis, inDrosophilathe most frequent class of dominant photoreceptorcell degeneration mutants obtained after chemical mutagenesis carry point muta-tions in the rhodopsin gene that resemble class II rhodopsin mutations in ADRP(36, 123, 249).

One class II rhodopsin mutant, T17M, exhibits a significantly increased yieldand a conversion from ER to plasma membrane localization if the transfectedcells are grown in the dark in the continuous presence of 11-cis retinal (134). Intransgenic mice that express T17M rhodopsin, disease progression is slowed bya diet high in vitamin A, as determined by the amplitude of the ERG and by thethickness of the photoreceptor cell layer. A similar slowing of disease progressionis not observed in transgenic mice expressing P347S rhodopsin, a class I mutant.These observations probably reflect the 15–20◦C increment in stability that 11-cisretinal binding imparts to opsin (104), and they suggest that rhodopsin mutantsthat are marginally stable in the absence of 11-cisretinal may achieve a significantdegree of stability upon 11-cis retinal binding. If 11-cis retinal binds to newlysynthesized opsin in the rod inner segment, as current data suggest (227), thatbinding may permit the transport of unstable mutants to the outer segment, apattern that is observed in transgenic mice expressing the class II mutant proteinsP23H and T17M (134, 180).

Premature termination mutations at codons 64 and 249 in the rhodopsin genehave been characterized in two families and found to cause dominant and re-cessive RP, respectively (142, 202). Presumably, the codon 64 mutation producesa toxic protein fragment, whereas the codon 249 mutation either produces lessprotein as a result of mRNA instability [often seen in mRNAs carrying prematuretermination codons (154)] or a protein fragment that is less toxic. Mice carryingrhodopsin gene knockouts show rapid retinal degeneration in the homozygous stateand a very slow degeneration in the heterozygous state (105, 128). The heterozy-gous knockout phenotype is in contrast to the rapid degeneration seen in miceexpressing various ADRP rhodopsin transgenes, which suggests that this subtypeof ADRP is caused by a dominant gain-of-function (i.e. toxic) protein rather thanhaploinsufficiency.




Cone Pigments Approximately 8–10% of Caucasian males and about 5% ofAsian and African males carry rearrangements within the red and green visualpigment gene array on the X-chromosome that produce benign anomalies of colorvision (175). Rarely, alterations within this locus either eliminate a locus controlregion upstream of the array or introduce deleterious missense or stop codonmutations within an array that carries only a single gene (173, 174, 193). Theserare alterations produce blue cone monochromacy or one of its variants, disordersassociated with nearly complete color blindness and a loss of visual acuity. Asubset of individuals with blue cone monochromacy or its variants experience aprogressive degeneration of the central retina (68, 173, 174, 193), presumably viamechanisms analogous to those responsible for the progressive degeneration of theperipheral retina in RP patients with rhodopsin gene mutations. A similar RP-likemechanism probably accounts for the loss of blue cone function seen in autosomaldominant blue-blindness (tritanopia), a disorder caused by missense mutations inthe transmembrane domains of the blue pigment that are predicted to disrupt itsfolding and/or stability (253, 254).

Peripherin and Rod Outer Segment Membrane Protein-1 (ROM-1)Periphe-rin/rds and ROM-1 are homologous integral membrane proteins located at the discrim in rod and cone photoreceptors. Peripherin/rds was identified independently inbovine rod outer segments (38, 167) and by virtue of its mutation in theretinal de-generation slow (rds)mouse (240). ROM-1 was identified by differential hybridiza-tion as the product of a retina-specific cDNA (9). Peripherin/rds and ROM-1 formheterotetrameric complexes that are linked by a disulfide bond in the intradiscal do-main of the proteins to form larger oligomeric complexes (80, 81). The abundance,disc rim location, and oligomeric properties of these proteins strongly suggest thatthey play an important role in stabilizing the high membrane curvature at the discrim and possibly in anchoring the discs to the adjacent plasma membrane (166).

Over 40 mutations have been identified in theperipherin/rdsgene in patientswith a wide variety of retinal diseases with dominant modes of inheritance: ADRP,macular dystrophies of various types, and disorders associated with an accumu-lation of yellow/white deposits in the retina and/or RPE (118). This degree ofdiversity in disease phenotypes is unusual among retinal disease genes. Whenproduced in transfected cells, many of the mutant peripherin/rds proteins foundin human retinopathies fail to correctly fold and multimerize with coexpressedROM-1 (78). Although the spontaneous murineperipherin/rdsmutation, whichinvolves a large rearrangement and appears to be a null, is generally considered arecessive allele (206), heterozygous animals exhibit disorganization and shorten-ing of outer segments (90).

Differentperipherin/rdsalleles do not correlate in a simple way with the variousdisease phenotypes defined ophthalmoscopically (118). For example, one familycarrying a deletion in codons 153–154 has affected members with RP, patterndystrophy, and fundus flavimaculatus (256). The effect of genetic background onperipherin/rdsphenotypes has been dramatically demonstrated in several families




in which individuals who are heterozygous for both a Ll85P allele ofperiph-erin/rds and a null allele ofROM-1have RP. By contrast, family members whoare single heterozygotes are not affected, the first clear example in human geneticsof a digenic disorder (57, 114). In transfected cells, L185P peripherin/rds doesnot form homotetramers as does wild-type peripherin/rds, but it can form het-erotetramers with ROM-1 (79). Presumably, in double heterozygotes a reductionin ROM-1 concentration unmasks the homomultimerization defect in the 50% ofthe peripherin/rds protein that is mutant. Despite macular involvement in manyperipherin/rds-based retinopathies, no sequence alterations in this gene have yetbeen associated with AMD.

Rab Escort Protein-1 (REP-1) Choroideremia is an X-linked disorder that af-fects approximately in 1 in 50,000 males and shows clinical symptoms much likeRP (40). It is distinguished from RP by a dramatic and early loss of the RPE ac-companied by a progressive atrophy of the choroidal vasculature (93, 153). Theresponsible gene,REP-1, was independently discovered by positional cloning(42) and by purification of geranylgeranyl transferase, a multiprotein complex thatadds the prenyl group geranylgeranyl to the carboxy-termini of rab (and possi-bly other) proteins (213, 214). In the biochemistry literature, the REP-1 proteinis also referred to as component A of geranylgeranyl transferase. The identityof REP-1 as the choroideremia gene product has been demonstrated not onlyby mutation analysis of patient DNA (243) but also by the finding that ger-anylgeranyl transferase activity is deficient in lymphoblastoid cell lines fromchoroideremia patients (212). The rab proteins are involved in a variety of intra-cellular trafficking processes and require prenylation for membrane localizationand function. REP-1 is distinct from the catalytic component of the geranylger-anyl transferase complex, and appears to act by delivering newly synthesized rabproteins to the catalytic component and then to their target membranes (1, 6).

In Saccharomyces cerevisiae, theREP-1orthologue is essential for viability,as are the catalytic components of geranylgeranyl transferase. If geranylgerany-lation is also essential for viability in mammals, then the restricted phenotype ofREP-1–deficient patients might be explained by the existence of REP-2, a REP-1homologue that has partially overlapping but nonidentical substrate specificity(39, 215). As bothREP-1andREP-2are widely expressed, they might partiallycompensate for each other in many tissues.

The spectrum ofREP-1mutations in choroideremia patients is distinctive in thatit consists overwhelmingly of apparent null mutations (243). This pattern of muta-tions suggests either that low levels of REP-1 activity might be sufficient for normalocular development and function, or conversely that partially active alleles interferewith geranylgeranylation and produce a more severe and/or extraocular phenotype.

Myosin VIIA and USH2A Usher syndrome is a recessive, genetically hetero-geneous group of disorders characterized by the combination of congenital sen-sorineural hearing loss and retinitis pigmentosa (22, 67, 91, 176). Usher syndrome




affects approximately 1 in 25,000 persons. Three subtypes of Usher syndrome aredistinguished by the severity of auditory and vestibular dysfunction: Type I pa-tients are profoundly deaf and lack vestibular function, type II patients have mildhearing loss and normal vestibular function, and type III patients have progressivehearing loss with variable vestibular function. At least five loci are responsible forUsher syndrome type I (USH1), and one of these,USH1B, encodes myosin VIIA,an unconventional myosin with a predictedMr of ∼250 kDa (130, 139, 251). Adistinct group of recessive mutations in this gene are responsible for isolated (i.e.nonsyndromic) deafness (252), and recessive mutations in the orthologous murinegene cause the deafness and vestibular dysfunction seen in theshakermouse (77).In one study, myosin VIIA was localized by immunoelectron microscopy of bothprimate and rodent retinas to the ciliary base of rod and cone outer segments andto the apical microvilli of the RPE, which interdigitate between the outer segments(138). However, a second research group observed myosin VIIA immunoreactiv-ity in primate but not rodent photoreceptors, which led to the suggestion that aspecies difference in protein distribution accounts for the lack of retinal pathologyin theshakermouse (61). Notwithstanding the partial inconsistency in immunolo-calization, the data suggest a role for myosin VIIA in intracellular transport, andspecifically in outer segment biogenesis in the human retina.

Recently, a second Usher syndrome gene,USH2A, has been shown to be re-sponsible for Usher syndrome IIA (62).USH2Aencodes a putative extracellularmatrix protein of 1551 amino acids that is expressed in the retina and inner ear,and at lower levels in other tissues.

The Visual Cycle

Cellular Retinaldehyde Binding Protein (CRALBP) CRALBP is one of severalintra- and extracellular retinoid binding proteins in the retina and RPE that orches-trate the visual cycle (Figure 2). CRALBP preferentially binds 11-cis retinol andis present in the RPE and in Muller glia in the retina. In vitro CRALBP promotesoxidation of 11-cis retinol to 11-cis retinal and inhibits the esterification of 11-cisretinol (205). The oxidation reaction is the final chemical step in the recyclingpathway, whereas esterification produces a stored form of retinol.

A variety of mutations in CRALBP have been found in recessively inheritedprogressive retinal degenerations that are variably characterized by night blindness,maculopathy, and yellow/white deposits in the fundus (26, 152, 168). RecombinantCRALBP carrying one of the disease-causing mutations, R150Q, is unable to bind11-cis retinal, suggesting that this mutation impairs the oxidation of 11-cis retinolto 11-cis retinal in vivo and, by inference, reduces the 11-cis retinal available forregeneration of visual pigment (152).

11-cis Retinol Dehydrogenase As noted above, conversion of 11-cis retinolto 11-cis retinal occurs within the RPE. This reaction is catalyzed by 11-cisretinol dehydrogenase, encoded by the RDH5 gene (223a). Two recessive missense




mutations within this gene have been identified in patients with delayed dark adap-tation and yellow/white deposits within the fundus (259a). When produced intransfected mammalian cells, the mutant enzymes had a tenfold lower yield anda greatly reduced specific activity relative to the wild-type enzyme. In one of thepatients, the slower time course of both rod and cone adaptation implies a delayedregeneration of both rod and cone pigments and therefore suggests that the productof the RDH5 gene processes 11-cis retinal for both photoreceptor types.

RPE65 RPE65 is an abundant microsomal protein that is specifically expressedin the RPE (10, 88). Several forms of autosomal recessive early onset retinal de-generation are caused by mutations inRPE65(85, 145, 169).Rpe65 (−/−) micelack rhodopsin and rod-mediated light responses, but they retain cone-mediatedlight responses. In these mice, excess all-trans retinyl esters accumulate in theRPE but 11-cis retinyl esters are missing, indicative of a block at the all-trans to11-cis isomerization step. These observations, together with earlier data indicat-ing a physical association between RPE65 and 11-cis retinol dehydrogenase, linkRPE65 to the retinoid isomerization reaction, although its exact role is yet to bedetermined. The preservation of cone function inRpe65 (−/−) mice representsthe best evidence to date that, in the mammalian retina, the visual cycle that sup-plies cones is distinct from that which supplies rods. Earlier work suggested thatthe cone visual cycle occurs within Muller glia and/or cones (97, 113).

ABCR ABCR is a rod outer segment–specific member of the ATP-bindingcassette family of transporters. It resembles the cystic fibrosis transmembraneconductance regulator (CFTR), another family member, in possessing two ATPbinding cassettes linked to homologous sets of six putative transmembrane seg-ments. ABCR was independently identified as a relatively abundant∼250 kDaouter segment membrane protein (7, 108, 181, 239) and as the protein product ofa retina-specific gene that is mutated in chromosome 1–linked autosomal reces-sive Stargardt disease (2). Several dominant Stargardt-like diseases have also beendefined but the responsible genes have not yet been identified (265). Stargardt dis-ease, which has also been called fundus flavimaculatus, is an autosomal recessiveearly onset macular dystrophy that is associated with impaired dark adaptationand an accumulation of yellow deposits in the RPE and retina (21). ABCR is notdetectable in cones despite the fact that Stargardt disease is considered a maculardystrophy. This paradox may be more apparent than real, as the area over whichyellow pigment accumulates and dark adaptation is defective extends far beyondthe cone-rich fovea. Like many “macular” diseases, including those associated withperipherin/RDSmutations, at a molecular level Stargardt disease is a pan-retinaldisease in which the macular region shows a greater susceptibility to degeneration.

Within rod outer segments, ABCR localizes to the disc rather than the plasmamembrane, and within the disc membrane it is confined to the rim (108, 231). Thislocalization strongly suggests that ABCR catalyzes the intracellular rather thanintercellular transport of a molecule or ion. ABCR has recently been purified




to homogeneity from bovine rod outer segments and functionally reconstitutedinto lipid vesicles (230). All-trans and 11-cis retinal both stimulate the ATPaseactivity of the purified protein. A kinetic analysis of ATPase stimulation showsthat all-trans retinal interacts with ABCR in a saturable manner at a single classof binding sites and at a rate-limiting step in the ATP hydrolysis pathway toproduce uncompetitive activation, the behavior expected for a transport substrate.The recently described phenotype ofABCRknockout mice strongly supports arole for ABCR in intraphotoreceptor retinoid transport: these mice show more all-transretinal and less all-transretinol in the retina following acute light exposure,and over time they accumulate A2-E, a diretinal-ethanolamine adduct, in the RPE,due presumably to the build-up of all-trans retinal within the photoreceptor discmembranes (257a). These data suggest that ABCR normally transports or extractsall-transretinal from the disc membrane (following its release from photoactivatedrhodopsin) so that it can serve as a substrate for all-transretinal dehydrogenase, theenzyme within the outer segment that converts all-transretinal to all-trans-retinolprior to its export and subsequent reisomerization in the RPE.

The mutations inABCRresponsible for Stargardt disease are found throughoutthe protein coding region (2, 132, 151, 172, 203), and thus far no Stargardt diseasepatient has been identified with two null mutations. However, homozygous pre-sumptive null (frameshift and splice site) mutations have been identified in patientswith ARRP (41, 149), which suggests that Stargardt disease represents a partialloss-of-function phenotype and that RP is the null phenotype. As approximately 2%of humans carry anABCRallele that confers partial or complete loss of function,an intriguing question is whether these carriers are at higher risk for mild or lateonset retinal disease, in particular AMD. Two research teams have addressed thisquestion and have come to opposite conclusions (3, 228). In a recent study, either oftwo ABCR mutations, G1961E and D2177N, were found in the heterozygous statein 4% of AMD patients (n= 1300) and 0.9 % of control subjects (n= 1500) ascer-tained in the United States and Europe, a statistically significant association (3a).Ongoing investigations with larger sample sizes should further clarify this issue.

Retinal Pigment Epithelium

Bestrophin Best disease (also called Best vitelliform dystrophy) is an autosomaldominant disorder that in its classic form is characterized by bilateral cystic mac-ular lesions, the appearance of which has given rise to the name “egg yolk cyst.”Histologic analyses show accumulation of lipofuscin-like material in the RPE andin the subretinal space (the space between the retina and RPE). Affected indi-viduals who show minimal retinal changes ophthalmoscopically can be identifiedby a characteristic combination of an abnormal EOG and a normal ERG (131).Best disease is caused by mutations in a gene that is expressed predominantlywithin the RPE and codes for a 585-amino acid protein (147, 185). Currently, theonly clues to the function of this protein, named bestrophin, are its hydropathyprofile, which predicts multiple transmembrane domains, and its homology to afamily of C. elegansproteins (the RFP family) of unknown function. A variety of




uncharacterized human, mouse, andDrosophilasequences in the EST databaseare also members of this family.

Tissue Inhibitor of Metalloproteinases-3 (TIMP-3) Sorsby fundus dystrophy(SFD) is an autosomal dominant macular degeneration characterized by abnormallipid-rich deposits in Bruch’s membrane (the acellular fibrous layer between theRPE and choroid) and subretinal neovascularization. SFD is caused by mutationswithin the gene encoding the tissue inhibitor of metalloproteinases-3 (TIMP-3),one of four homologous proteins that form 1:1 complexes with various membersof the matrix metalloproteinase (MMP) family (250). The MMPs comprise a largefamily of extracellular proteases implicated in tissue remodeling in a wide range ofdevelopmental, homeostatic, and pathologic settings. The four TIMPs are built on acommon plan: the amino-terminal domain interacts with the target MMP, whereasthe carboxy-terminal domain appears to have other bioactivities, for example,antiangiogenic activity in the case of TIMP-2 (170). TIMP-3 is distinct fromother TIMPs in being associated with the extracellular matrix (ECM), and thisassociation is mediated by the carboxy-terminal domain (126). TIMP-3 is presentin a variety of tissues, and in both normal eyes and eyes from patients with SFD itis concentrated in Bruch’s membrane (48, 65, 66).

Interestingly, all of the TIMP-3 mutations identified thus far in SFD patientsresult in single amino acid substitutions that add a cysteine to the carboxy-terminaldomain, which in the wild type has six cysteines arranged in three intramolecu-lar disulfide bonds. When one of the SFD mutant proteins (S181C) was studiedin transfected COS cells, it was found to retain the ability to inhibit MMP andassociate with the ECM (126). The restricted nature and localization of the SFDmutations and the essentially normal inhibitory activity of the S181C mutant sug-gest that the SFD mutations may represent dominant gain-of-function mutations,perhaps leading to an increase in ECM association with a resulting increase in theconcentration of TIMP-3 and a decrease in MMP activity in Bruch’s membrane.

In SFD and other retinal dystrophies associated with a thickened Bruch’s mem-brane or with deposits beneath the RPE, one attractive hypothesis for the patho-genesis of disease is that impaired diffusion of oxygen and/or nutrients from thechoroidal circulation leads to retinal dysfunction and degeneration (148). In onetest of this hypothesis, Jacobson and coworkers showed that high-dose oral vita-min A reversibly corrects the reduced rod sensitivity that is a characteristic of SFD(111). The impaired diffusion hypothesis could also account for aberrant bloodvessel growth through Bruch’s membrane (choroidal neovascularization) that isseen in SFD and other macular dystrophies, including AMD. This growth couldbe induced by angiogenic signals released from the RPE and/or retina in responseto impaired oxygen or nutrient delivery.

Development

Transcription Factors: Cone Rod Homeobox (CRX) and Neural Retina LeucineZipper (NRL) Numerous transcriptional regulatory proteins are essential for




ocular development (70), but few are expressed specifically in the retina and thusfar only two—CRX and NRL—have been found to cause nonsyndromic retinaldisease in humans. CRX is an otd/Otx-like homeobox protein that is expressedprincipally in rod and cone photoreceptors (31, 69, 74). In vitro CRX binds to a con-sensus sequence present in or near the promoters of many photoreceptor-specificgenes, and in cotransfection experiments it transactivates those promoters (31, 74).Introduction of a dominant negative form of CRX into developing photoreceptorcells via retroviral transduction inhibits photoreceptor morphogenesis (74). In hu-mans, several clinical phenotypes are associated with CRX mutations, includingautosomal dominant cone-rod dystrophy, Leber congenital amaurosis, and domi-nant retinitis pigmentosa (69, 71, 110, 224, 236). In some cases of Leber congenitalamaurosis, affected subjects appear to carry only a single mutant allele. About halfof the mutations identified thus far are amino acid substitutions, including muta-tions within the homeodomain that impair binding (31), and half are frameshiftmutations that truncate the protein distal to the homeodomain. Whether differ-ent human phenotypes arise from haploinsufficiency or from a dominant negativeeffect remains to be determined.

NRLwas identified by differential hybridization as a retina-specific sequenceand was subsequently shown to be expressed throughout the retina (237). In co-transfection experiments, NRL transactivates photoreceptor promoters both alone(122, 192) and in synergy with CRX (31). Recently, anNRL missense mutation(S50T) has been identified in a family with ADRP (19). In mammalian cells, themutant NRL synergizes more efficiently than wild-type NRL with coexpressedCRX to transactivate a rhodopsin promoter. The increase in transcriptional activ-ity associated with the mutant NRL may relate to the observation that overexpres-sion of wild-type rhodopsin in transgenic mice causes photoreceptor degeneration(180). The phenotypes associated with defects in CRX andNRL suggest that inthe photoreceptor cell the high rate of outer segment renewal constrains the levelof production of outer segment proteins to a narrow window and that even modestover- or underproduction may be deleterious.

X-Linked Retinoschisis (XLRS) X-linked juvenile retinoschisis (XLRS) is themost common cause of juvenile macular degeneration in males. XLRS is character-ized by cystic lesions within the nerve fiber layer at the inner aspect of the retina thatare believed to arise from a defect in Muller glia, and that ultimately lead to large re-gions over which the inner retina is split (a schisis cavity). The XLRS gene encodesa putative secreted protein of approximately 20 kDa that shares homology withdiscoidin and other proteins implicated in cell-cell interactions (207). In mice, theXLRS orthologue is predominantly expressed in the retina, and within the retina,it is found exclusively in photoreceptor cells, which suggests that the XLRS pro-tein mediates communication between these cells and the Muller glia (194). In acohort of 234 XLRS patients, 82 different mutations were observed (238). Themost common mutation was found in 15% of subjects, and most mutations werefound only once.




Norrie Disease (ND) ND is an X-linked developmental disorder characterizedby congenital blindness associated with retinal dysplasia, aberrant cell proliferationwithin the eye, and shrinkage of the globe. At least one third of cases have otherCNS manifestations, including progressive sensorineural deafness and mentalretardation. TheND gene encodes a secreted protein of 133 amino acids witha cysteine-rich C terminus that is homologous to a cysteine-rich extracellularmotif found in mucins, von Willibrand factor, and theDrosophila slit protein(12, 13, 15, 33, 158). The pattern of cysteines suggests that the C terminus con-forms to the cysteine knot tertiary structure seen in members of the transforminggrowth factor beta family (160). TheNDgene is expressed in both the fetal and adultCNS, but its mechanism of action is currently unknown. A mouse knockout of theNDgene shows vitreous opacities, disorganization of the retinal ganglion cell layer,and occasional degeneration of photoreceptors (14). In ND, most mutations ap-pear to be null or severe loss-of-function mutations. Mutations in theND gene thatwould be predicted to confer partial loss of function have been identified in patientswith X-linked familial exudative vitreoretinopathy (FEVR), a disorder that looks inmany respects like a mild version of ND (32, 218). Interestingly,NDmutations havealso been identified in a subset of patients diagnosed with retinopathy of prematu-rity, a retinal vascular disease for which low birth weight, prematurity, and exposureto high oxygen tension during neonatal treatment constitute risk factors (219).

Metabolism

Ornithine Amino Transferase (OAT) Gyrate atrophy (GA) of the choroid andretina is a rare recessive disorder in which visual loss resembles that seen in severeRP (241). However, GA can be distinguished from RP ophthalmoscopically by thepresence of sharply demarcated regions of complete chorioretinal degeneration thatfirst appear in the periphery and then slowly enlarge and coalesce until they ulti-mately encompass the entire retina (257). GA is caused by complete or nearly com-plete deficiency in OAT, a mitochondrial enzyme present throughout the body thatlinks the metabolism of ornithine, proline, and glutamate. OAT deficiency pro-duces a 10–20-fold increase in ornithine within virtually all bodily fluids. Over 50different OAT mutations have been defined, and many of the corresponding mutantproteins have been studied biochemically. Targeted disruption of the mouse OATgene produces a slowly progressive photoreceptor degeneration associated withearly morphologic defects within the RPE (248).

Mitochondria Leber hereditary optic neuropathy (LHON), first described in1871, is a variably penetrant adult onset degeneration of retinal ganglion cellsand the optic nerve (99, 112, 246). The onset of visual loss is typically sudden andirreversible in an otherwise healthy individual, although in some patients there areextraophthalmic manifestations of disease, such as peripheral neuropathy and heartconduction defects. The ERG and EOG are normal in the vast majority of patients,indicative of normal photoreceptor and inner nuclear layer function (220). The




distinguishing genetic feature of LHON is maternal inheritance, reflecting its originin sequence changes within the mitochondrial genome (247). In one study of 159LHON families, 96% were found to carry a single nucleotide substitution at posi-tion 3460, 11778, or 14484 in the mitochondrial genome. These mutations producesingle amino acid substitutions in the ND1, ND4, and ND6 subunits, respectively,of oxidative phosphorylation complex I (NADH:ubiquinone oxidoreductase; 143).The role of rare mitochondrial sequence variants as a cause of LHON and the pos-sible modifying role of more common DNA sequence variants or polymorphismsother than the three mutations noted above are controversial (99, 112, 246).

Partial deficiencies in respiratory chain function have been observed in cul-tured cells from LHON patients and in cybrids in which LHON mitochondria areused to populate the cytosol of recipient cells that lack mitochondrial DNA (244).These experiments are consistent with the generally accepted view that mitochon-drial diseases affect most severely those tissues that have the greatest requirementfor oxidative phosphorylation. However, applying this model to the retina raisesan apparent paradox. From studies of isolated, perfused mammalian retinas, it iswell established that photoreceptors consume far more oxygen than inner retinalneurons, which are powered largely by glycolysis (5, 136, 221). Thus the suscep-tibility of ganglion cells and the resistance of photoreceptors to LHON mutationssuggest that the degenerative process in LHON may reflect a defect other thansimply a decreased capacity for ATP generation. An interesting discussion of thisquestion and of the possible relationship between ganglion cell loss in LHON andin glaucoma may be found in Reference 99.

Miscellaneous

Calcium Channel Cacna1F Two subtypes of CSNB have been mapped to dif-ferent loci on the X-chromosome. X-linked complete CSNB (CSNB1) maps toXp11.4 and is associated with absent rod and normal cone sensitivity. X-linkedincomplete CSNB (CSNB2) maps to Xp11.23 and is associated with a reduction inboth rod and cone sensitivity. In both types of X-linked CSNB, the ERG shows es-sentially normal rod a-waves and greatly diminished b-waves, indicative of a defectin signal transmission to second-order neurons (27, 164, 165, 183, 204). The generesponsible for CSNB2 has been identified and shown to encode a retina-specificL-type voltage-gated calcium channel alpha subunit, CACNA1F (11, 229). Mostof the CSNB2 mutations are predicted to be null. In situ hybridization localizesCACNA1F transcripts to all three cell layers within the retina, with strongest hy-bridization in the outer and inner nuclear layers. The ERG findings could be mostsimply explained if CACNA1F were found to mediate voltage-dependent gluta-mate release from photoreceptor synaptic terminals, a process known to involvean L-type calcium channel (209, 258).

Alpha-Tocopherol Carrier Protein Vitamin E (alpha-tocopherol) is the majorantioxidant in photoreceptor outer segments, where it is presumed to block photo-oxidation damage, especially among polyunsaturated fatty acids that are present




at high concentration (43, 106). Dietary vitamin E deficiency in animals causes aprogressive retinal degeneration (198), and retinal changes have been observed inhumans with vitamin E malabsorption (98). Low serum vitamin E levels have beenidentified in two patients with retinitis pigmentosa and one patient with Friedreich’sataxia, all of whom carry homozygous H101Q mutations in the alpha-tocopheroltransfer protein, an intracellular vitamin E carrier protein (83, 263). In these pa-tients, high-dose oral vitamin E supplementation raises serum vitamin E to normallevels and appears to be beneficial.

Epidermal Growth Factor Repeat (EGF)-Containing Fibrillin-Like 1 Extracel-lular Matrix Protein-1 (EFEMP1) Two separately defined autosomal dominantdisorders, Malattia Leventinese and Doyne honeycomb retinal dystrophy, are asso-ciated with yellow/white deposits beneath the RPE and a progressive loss of centralvision. Ophthalmoscopically similar deposits, referred to as drusen, are associatedwith AMD. Linkage mapping and positional cloning have identified a R345W sub-stitution in the EFEMP1 gene as the likely cause of both disorders in all affectedpatients tested thus far (228a). This mutation was not found in over 400 unaffectedethnically matched control subjects, nor in nearly 500 subjects with age-relatedmacular degeneration. EFEMP1 is a widely expressed protein with homology tothe fibulin family of extracellular matrix glycoproteins. The R345W mutation fallswithin the last epidermal growth factor repeat in the EFEMP-1 protein.

TULP1 TULP1 is one of three members of the TULP family of related proteins,the founding member of which, TUB, is mutated in thetubbymouse and leads toobesity, deafness, and retinal degeneration (120, 178).TUB is expressed in manytissues, including the brain and retina;TULP1 is expressed only in the retina; andTULP2 is expressed in the retina and testis (179). Mutations inTULP1have beenidentified in approximately 1% of patients with recessive RP (8, 87). Currently, theonly clue to the function of the TULP proteins is that this family is very ancient,and is found in both plants and animals.

RP2 and RP3 (Retinitis Pigmentosa GTPase Regulator; Rpgr)Among themultiple subtypes of RP, X-linked RP (XLRP) displays the most severe clinicalcourse, with partial or complete blindnesss by the third or fourth decade of life.The genes responsible for two forms of XLRP,RP2andRP3, have been position-ally cloned and found to encode widely expressed proteins of unknown function.RPGR, the product of theRP3gene, is a 90-kDa protein that has homology inits N-terminal half to RCC-1, a guanine nucleotide exchange factor for the smallGTP binding protein Ran (159, 200). RCC-1 has been characterized as a nuclearprotein that regulates chromosome condensation, and Ran is an abundant, highlyconserved protein that has been implicated in protein transport between nucleusand cytoplasm. One report indicates an interaction between RPGR and a recentlydescribed 17-kDa protein that is a member of a conserved family of proteins that areproposed to regulate the trafficking or localization of prenylated proteins (135).Thus far, mutation screening has detected mutations inRPGRin only 10–20%




of X-linked RP patients (24, 73, 159, 200), less than the 60–70% expected basedon linkage mapping and clinical criteria, which suggests the existence of unusualRPGRmutations or additionalRPgenes in the vicinity ofRPGR.

The product of theRP2gene is a soluble intracellular protein with weak ho-mology to cofactor C, a protein involved in the folding of beta-tubulin (211).

FUTURE DIRECTIONS

Genetic Analysis

During the past decade, the genes responsible for Mendelian retinal diseases havebeen identified at an accelerating rate. These genes were identified by positionalcloning without prior biological data (e.g.REP-1, XLRP-2, andXLRP-3), by highthroughput mutation screening of candidate genes in the absence of linkage data(e.g. CNCG andPDE), and by a combination of the two (the positional candi-date method; e.g. TIMP-3 and CRX). However, many monogenic retinal disordersremain to be identified, including the vast majority of subtypes of Leber con-genital amaurosis and ARRP, at least eight ADRP subtypes (109), and variousmaculopathies (44, 265).

The completion within the next several years of both the human genomesequence and a high-density map of polymorphic markers, together with the devel-opment of high-throughput automated array technologies for screening polymor-phisms, promise to transform positional cloning (29, 37). Candidate gene analysiswill also be transformed by the construction of a complete human transcript mapin which all exons are identified and the tissue distributions of all transcripts aredetermined. It seems reasonable to assume that, with these resources in hand, allknown monogenic disorders will be rapidly linked to well-defined chromosomalloci and high-throughput mutation screening will be used in an increasingly effi-cient manner to identify the causal mutations. At that point, the identification offamilies with inherited disorders will become the rate-limiting step in the effort tocatalogue all disease-causing genes.

The great challenge for the future will be to identify the causes of the morecommon disorders, in particular AMD and glaucoma, which have both genetic andnongenetic components (68a, 68b, 96, 107, 161, 216, 223). One approach to identi-fying genetic risk factors in multifactorial disorders is to look for sequence changesin genes that are responsible for phenotypically similar monogenic disorders. Forexample, this approach has revealed sequence alterations in themyocillin/TIGRgene, which was initially identified as a gene responsible for juvenile open-angleglaucoma, in several percent of patients with late onset glaucoma (4). With the ex-ception of one controversial report of sequence variants within theABCRgene in asubset of patients with AMD (see above), this approach has thus far failed to providea direct connection between AMD and the rare Mendelian retinal disease genes.

A second approach to identifying genetic risk factors in multifactorial disordersis to look throughout the genome for linkage between disease susceptibility lociand polymorphic markers. Multiple experimental designs are possible with this




approach (125, 195, 208). Linkage with susceptibility loci can be established bycomparing polymorphic markers within extended pedigrees or among concordantand discordant sib or other relative pairs. Susceptibility loci can be further localizedby identifying linkage disequilibrium between polymorphic markers and an an-cestral mutation by analyzing transmission disequilibrium within pedigrees or bycomparing distantly related patients. For multifactorial disorders, especially thosewith a late onset such as AMD, large pedigrees with multiple affected membersare unusual. For these disorders, genetically isolated populations with a strongfounder effect, as seen for example in the Finnish population, offer a favorablesituation for linkage disequilibrium analysis (47). A potentially powerful resourcethat is currently being assembled is a database of the entire Icelandic populationthat will include complete medical and genealogical data to be used in conjunctionwith large-scale genotyping (86). This database will allow efficient genome-widelinkage analysis with multiple distantly related pedigrees.

A third approach, which can identify both genetic and nongenetic risk fac-tors, is to define biochemical pathways that are relevant to disease, and then touse this knowledge to identify other components within each pathway. Three ex-amples will illustrate this point. First, the retinal disease phenotype caused bymutations in genes that encode known or likely components of the visual cycle,such as CRALBP, ABCR, and RPE65, suggests that defects in other componentsof the visual cycle will cause retinal disease. Second, the identification of a defec-tive signal for protein sorting in class I rhodopsin mutants responsible for ADRPsuggests the existence of retinal diseases caused by defects in the proteins that com-prise the sorting machinery in the photoreceptor inner segment and at the base ofthe outer segment. Third, the retinal disease phenotype produced by mutations intheCRXandNRLgenes suggests that additional transcription factor defects will befound in retinal disease. For each of these examples, the identification of a subsetof disease genes will accelerate the identification of additional candidate genes,because the known genes or proteins can be used to develop assays in vitro or intransfected cells to identify novel interacting or functionally related proteins.

The identification of new genetic and nongenetic risk factors will also be ac-celerated by the use of known disease genes to create knockout, knock-in, andtransgenic animal models. Genetically engineered animals can be used to identifyand ultimately isolate modifier genes by positional cloning, and they can also beused to identify environmental effects by studies of diet, lighting, and drug or toxicchemical treatment. Finally, new insights into disease mechanisms and risk factorsshould emerge from studying the transcriptional response to disease processes ingenetically engineered animals using differential cDNA hybridization, an approachthat will become increasingly powerful with the use of DNA array technology.

Prospects for Therapy

For many retinal diseases, the goal of efficacious treatment has proven elusive, andfor those that are treatable, improved efficacy would be highly desirable. Currently,there is no accepted treatment for the vast majority of monogenic retinopathies




including RP. [One group has reported a modest slowing of disease progression inRP patients with high-dose oral vitamin A (18), but the interpretation of these datahas been controversial (150).] The only currently accepted treatment for AMDis laser photocoagulation of choroidal neovascular membranes in the exudativesubtype of the disorder, but this treatment is applicable to only a small minor-ity of patients (144). Current surgical and drug therapy for glaucoma is directedexclusively at reducing intraocular pressure, although the existence of glaucomapatients with normal intraocular pressure points to additional risk factors that maybe intrinsic to retinal ganglion cells or their microenvironment (210).

How can genetics further the goal of improved treatment or prevention of reti-nal disease? First, diagnostic and prognostic information will come increasinglyfrom genetic testing, and genetic counseling and prenatal diagnosis will becomeincreasingly common as the catalogue of disease-causing mutations becomes morecomplete and the cost of genetic testing declines. Second, as noted in the previoussection, genetic methods can be used to create animal models of disease. Third,as also noted in the previous section, genetic approaches have the potential toidentify biochemical pathways that may be relevant to the pathogenesis of bothgenetic and nongenetic diseases, as seen, for example, in the discovery of low-density lipoprotein receptor defects in familial hypercholesterolemia (23). In thisregard, we note that the frequency of a given gene defect in the disease populationis not necessarily a good measure of the likelihood that it will lead to significantinsights into disease mechanisms or therapy. Although insulin plays a central role inmetabolic homeostasis and is the mainstay of therapy in diabetes, mutations in theinsulin gene are extremely rare in the diabetic population (226). Fourth, large-scale genotyping can be used to divide patient populations into groups that sharea common disease mechanism. This division is important in designing and inter-preting clinical trials or epidemiologic studies of a heterogeneous disease suchas RP, because it may reveal treatment or environmental effects in one subgroupand not another. Fifth, genetic therapies—including the introduction of gene orribozyme constructs into the retina, or the localized expression of growth, survival,or antiangiogenic factors—are being studied with a variety of delivery systems,although this approach faces special challenges posed by the fragility and relativeinaccessibility of the retina. Finally, the ultimate identification of all genes ex-pressed in the retina and the cataloguing of their roles in normal and disease stateswill provide the raw material for screening of lead compounds for drug develop-ment, a general strategy that is likely to dominate pharmaceutical research in thetwenty-first century.

ACKNOWLEDGMENTS

The authors thank Ms Teri Carter for excellent secretarial assistance, Drs. SamuelJacobson, Patrick Tong, and Donald Zack for helpful comments on the manuscript,and the Howard Hughes Medical Institute, The Foundation Fighting Blindness,and the National Eye Institute for financial support.




Visit the Annual Reviews home page at http://www.AnnualReviews.org

LITERATURE CITED

1. Alexandrov K, Horiuchi H, Steele-Morti-moer O, Seabra MC, Zerial M. 1994. Rabescort protein-1 is a multifunctional pro-tein that accompanies newly prenylated rabproteins to their target membranes.EMBOJ. 13:5262–73

2. Allikmets R, Singh N, Sun H, ShroyerNF, Hutchinson A, et al. 1997. A photo-receptor cell-specific ATP-binding trans-porter gene (ABCR) is mutated in recessiveStargardt’s macular dystrophy.Nat. Genet.15:236–46

3. Allikmets R, Shroyer NF, Singh N, Sed-don JM, Lewis RA, et al. 1997. Mutationof the Stargardt disease gene (ABCR) inage-related macular degeneration.Science277:1805–7

3a. Allikmets and the International ABCRScreening Consortium. 1999. Associationof G1961E and D2177N variants in theABCR gene with age-related macular de-generation.Invest. Opthalmol. Vis. Sci.40:S775

4. Alward WLM, Fingert JH, Coote MA,Johnson AT, Lerner SF, et al. 1998. Clinicalfeatures associated with mutations in thechromosome 1 open-angle glaucoma gene(GLC1A). N. Engl. J. Med.338:1022–27

5. Ames A, Li Y-Y, Heher E, Kimble CR.1992. Energy metabolism of rabbit retinaas related to function: high cost of sodiumtransport.J. Neurosci.12:840–53

6. Andres DA, Seabra MC, Brown MS, Arm-strong SA, Smeland TE, et al. 1993. cDNAcloning of component A of Rab geranyl-geranyl transferase and demonstration ofits role as a rab escort protein.Cell 73:1091–99

7. Azarian SM, Travis GH. 1997. The pho-toreceptor rim protein is an ABC trans-porter encoded by the gene for recessiveStargardt’s disease (ABCR).FEBS Lett.409:247–52

8. Banerjee P, Kleyn PW, Knowles JA, LewisCA, Ross BM, et al. 1998. TULP1 muta-tion in two extended Dominican kindredswith autosomal recessive retinitis pigmen-tosa.Nat. Genet.18:177–79

9. Bascom RA, Manara S, Collins L, Mol-day RS, Kalnins VI, McInnes RR. 1992.Molecular cloning of the cDNA for a novelphotoreceptor-specific membrane protein(rom-1) identifies a disk rim proteinfamily implicated in human degenerativeretinopathies.Neuron8:1171–84

10. Bavik CO, Levy F, Hellma U, Wern-stedt C, Eriksson U. 1993. The retinalpigment epithelial membrane receptor forplasma retinol binding protein: isolationand cDNA cloning of a 63 kDa protein.J.Biol. Chem.268:20540–46

11. Bech-Hansen NT, Naylor M, MaybaumTA, Pearce WG, Koop B, et al. 1998.Loss-of-function mutations in a calcium-channel alpha-1 subunit gene in Xp11.23cause incomplete X-linked congenital sta-tionary night blindness.Nat. Genet.19:264–67

12. Berger W. 1998. Molecular dissection ofNorrie disease.Acta Anat.162:95–100

13. Berger W, Meindl A, van de Pol TJR, Cre-mers FMP, Ropers HH, et al. 1992. Isola-tion of a candidate gene for Norrie diseaseby positional cloning.Nat. Genet.1:199–203

14. Berger W, van de Pol D, Bachner D, Oerle-mans F, Winkens H, et al. 1996. An animalmodel for Norrie disease (ND): gene tar-geting of the mouse ND gene.Hum. Mol.Genet.5:51–59

15. Berger W, van de Pol D, Warburg M, Gal A,Bleeker-Wagemekers L, et al. 1992. Muta-tions in the candidate gene for Norrie dis-ease.Hum. Mol. Genet.1:461–65

16. Berson EL. 1992. Electrical phenomena inthe retina. See Ref. 89, pp. 641–707




17. Berson EL. 1993. Retinitis pigmentosa.In-vest. Ophthalmol. Vis. Sci.34:1659–76

18. Berson EL, Rosner B, Sandberg MA,Hayes KC, Nicholson BW, et al. 1993.A randomized trial of vitamin A and vi-tamin E supplementation for retinitis pig-mentosa.Arch. Ophthalmol.111:761–72

19. Bessant DAR, Payne AM, Mitton KP,Wang Q-L, Swain PK, et al. 1999. A muta-tion in NRL is associated with autosomaldominant retinitis pigmentosa.Nat. Genet.21:355–56

20. Bird AC. 1995. Retinal photoreceptor dys-trophies. Am. J. Ophthalmol.119:543–62

21. Blacharski PA. 1988. Fundus flavimacula-tus. See Ref. 177, pp. 135–59

22. Boughman JA, Vernon M, Shaver KA.1983. Usher syndrome: definition and esti-mate of prevalence from two high-risk pop-ulations.J. Chronic Dis.36:595–603

23. Brown MS, Goldstein JL. 1986. Areceptor-mediated pathway for cholesterolhomeostasis.Science232:34–47

24. Buraczynska M, Wu W, Fujita R, Bu-raczynska K, Phelps E, et al. 1997. Spec-trum of mutations in the RPGR gene thatare identified in 20% of families with X-linked retinitis pigmentosa.Am. J. Hum.Genet.61:1287–92

25. Bursell S-E, Mainster MA. 1988. Methodsof vitreoretinal evaluation. See Ref. 177,pp. 5–20

26. Burstedt MSI, Sandgren O, HolmgrenG, Forsman-Semb K. 1999. Bothnia dys-trophy caused by mutations in the cel-lular retinaldehyde-binding protein gene(RLBP1) on chromosome 15q26.Invest.Ophthalmol. Vis. Sci.40:995–1000

27. Carr RE. 1974. Congenital stationary nightblindness.Trans. Am. Ophthalmol. Soc.72:448–87

28. Carr RE, Ripps H, Siegel IM, Weale RA.1966. Rhodopsin and the electrical activityof the retina in congenital night blindness.Invest. Ophthalmol. Vis. Sci.5:497–507

29. Chakravarti A. 1999. Population gene-

tics—making sense out of sequence.Nat.Genet.21(Suppl.):56–60

30. Chang GQ, Hao Y, Wong F. 1993. Apopto-sis: final common pathway of photorecep-tor death in rd, rds, and rhodopsin mutantmice.Neuron11:595–605

31. Chen S, Wang Q-L, Nie Z, Sun H, LennonF, et al. 1997. Crx, a novel otx-like paired-homeodomain protein, binds to and trans-activates photoreceptor cell-specific genes.Neuron19:1017–30

32. Chen Z-Y, Battinelli EM, Fiedler A,Bundey S, Sims K, et al. 1993. A muta-tion in the Norrie disease gene (NDP) as-sociated with X-linked familial exudativevitreoretinopathy.Nat. Genet.5:180–82

33. Chen Z-Y, Hendriks RW, Jobling MA,Powell JF, Breakefield XO, et al. 1992.Isolation and characterization of a candi-date gene for Norrie disease.Nat. Genet.1:204–8

34. Cideciyan AV, Hoo DC, Huang Y, Banin E,Li Z-Y, et al. 1998. Disease sequence frommutant rhodopsin allele to rod and conephotoreceptor degeneration in man.Proc.Natl. Acad. Sci. USA95:7103–8

35. Cideciyan AV, Zhao X, Nielson L, KhanSC, Jacobson SG, Palczewski K. 1998.Null mutation in the rhodopsin kinasegene slows recovery kinetics of rod andcone phototransduction in man.Proc. Natl.Acad. Sci. USA95:328–33

36. Colley NJ, Casill JA, Baker EK, ZukerCS. 1995. Defective intracellular trans-port is the molecular basis of rhodopsin-dependent retinal degeneration.Proc. Natl.Acad. Sci. USA92:3070–74

37. Collins FS, Patrinos A, Jordan E,Chakravarti A, Gesteland R, et al. 1998.New goals for the U.S. Human GenomeProject: 1998–2003.Science282:682–89

38. Connell G, Molday RS. 1990. Molecularcloning, primary structure, and orientationof the vertebrate photoreceptor cell proteinperipherin in the rod outer segment discmembrane.Biochemistry29:4691–98

39. Cremers FPM, Armstrong SA, Seabra MC,




Brown MS, Goldstein JL. 1994. REP2, a Rab escort protein encoded by thechoroideremia-like gene.J. Biol. Chem.269:2111–17

40. Cremers FPM, Ropers H-H. 1995. Choroi-deremia. See Ref. 211a, pp. 4311–24

41. Cremers FPM, van de Pol DJR, van DrielM, den Hollander AI, van Haren FJJ, et al.1998. Autosomal recessive retinitis pig-mentosa and cone-rod dystrophy causedby splice site mutations in the Stargardt’sdisease gene ABCR.Hum. Mol. Genet.7:355–62

42. Cremers FPM, van de Pol DJR, vanKerkhoff LPM, Wieringa B, Ropers H-H. 1990. Cloning of a gene that is rear-ranged in patients with choroideremia.Na-ture347:674–77

43. Daemen FJM. 1973. Vertebrate rod outersegment membranes.Biochem. Biophys.Acta300:255–88

44. Daiger SP, Sullivan LS, Rossiter BJF. 1999.Retnet Retinal Information Network Website. www.sph.uth.tmc.edu/Retnet/home.htm

45. Danciger M, Heilbron V, Gao YQ, ZhaoDY, Jacobson SG, Farber DB. 1996. A ho-mozygous PDE6B mutation in a familywith autosomal recessive retinitis pigmen-tosa.Mol. Vis.2:10

46. de Jong RTVM, Zrenner E, van Meel GJ,Kennen JEE, van Norren D. 1991. Mizuophenomenon in X-linked retinoschisis.Arch. Ophthalmol.104:1104–8

47. de la Chapelle A, Wright FA. 1998. Link-age disequilibrium mapping in isolatedpopulations: the example of Finland revis-ited.Proc. Natl. Acad. Sci. USA95:12416–23

48. Delia NG, Campochiaro P, Zack DJ. 1996.Localization of TIMP-3 mRNA expressionto the retinal pigment epithelium.Invest.Ophthalmol. Vis. Sci.37:1921–24

49. Deretic D, Schmerl S, Hargrave PA, ArendtA, McDowell JH. 1998. Regulation of sort-ing and post-Golgi trafficking of rhodopsinby its C-terminal sequence QVS(A)PA.

Proc. Natl. Acad. Sci. USA95:10620–2550. Dizhoor AM, Boikov SG, Olshevskaya EV.

1998. Constitutive activation of photore-ceptor guanylate cyclase by Y99C mutantof GCAP-1.J. Biol. Chem.273:17311–14

51. Dizhoor AM, Lowe DG, Olshevskaya EV,Laura R, Hurley JB. 1994. The human pho-toreceptor membrane guanylate cyclase,ret GC, is present in outer segments andis regulated by calcium and soluble activa-tor. Neuron12:1345–52

52. Doi T, Molday RS, Khorana HG. 1990.Role of the intradiscal domain in rhodopsinassembly and function.Proc. Natl. Acad.Sci. USA87:4991–95

53. Dryja TR, 1997. Gene-based approach tohuman gene-phenotype correlations.Proc.Natl. Acad. Sci. USA94:12117–21

54. Dryja TP, Berson EL, Rao VR, Oprian DD.1993. Heterozygous missense mutation inthe rhodopsin gene as a cause of congeni-tal stationary night blindness.Nat. Genet.4:280–83

55. Dryja TP, Finn JT, Peng Y-W, McGee TL,Berson EL, Yau K-W. 1995. Mutations inthe gene encoding the alpha subunit ofthe rod cGMP-gated channel in autosomalrecessive retinitis pigmentosa.Proc. Natl.Acad. Sci. USA92:10177–81

56. Dryja TP, Hahn LB, Cowley GS, McGeeTL, Berson EL. 1991. Mutation spectrumof the rhodopsin gene among patients withautosomal dominant retinitis pigmentosa.Proc. Natl. Acad. Sci. USA88:9370–74

57. Dryja TP, Hahn LB, Kajiwara K, BersonEL. 1997. Dominant and digenic mutationsin the peripherin/RDS and ROM1 genes inretinitis pigmentosa.Invest. Ophthalmol.Vis. Sci.38:1972–82

58. Dryja TP, Hahn LB, Reboul T, Arnaud B.1996. Missense mutation in the gene en-coding the alpha subunit of rod transducinin the Nougaret form of congenital station-ary night blindness.Nat. Genet.13:358–60

59. Dryja TP, Li T. 1995. Molecular geneticsof retinitis pigmentosa.Hum. Mol. Genet.4:1739–43




60. Dryja TP, McGee TL, Reichel E, HahnLB, Cowley GS, et al. 1990. A point mu-tation of the rhodopsin gene in one formof retinitis pigmentosa.Nature343:364–66

61. El-Amraoui A, Sahly I, Picaud S, Sahel J,Abitbol M, Petit C. 1996. Human Usher1B/mouseshaker-1: the retinal phenotypediscrepancy explained by the presence/absence of myosin VIIA in the photore-ceptor cells.Hum. Mol. Genet.5:1171–78

62. Eudy JID, Weston MID, Yao SF, HooverDM, Rehm HL, et al. 1998. Mutation of agene encoding a protein with extracellu-lar matrix motifs in Usher syndrome typeIIa. Science280:1753–57

63. Farber DB. 1995. From mice to men: thecyclic GMP phosphodiesterase gene in vi-sion and disease.Invest. Ophthalmol. Vis.Sci.36:263–75

64. Farber DB, Danciger M. 1997. Identi-fication of genes causing photoreceptordegenerations leading to blindness.Curr.Opin. Neurobiol.7:666–73

65. Fariss RN, Apte SS, Luthert PJ, Bird AC,Milam AH. 1998. Accumulation of tissueinhibitor of metalloproteinases-3 in hu-man eyes with Sorsby’s fundus dystrophyor retinitis pigmentosa.Br. J. Ophthalmol.82:1329–34

66. Fariss RN, Apte SS, Olsen BR, IwataK, Milarn AH. 1997. Tissue inhibitor ofmetalloproteinases-3 is a component ofBruch’s membrane of the eye.Am. J.Pathol.150:323–28

67. Fishman GA, Kumar A, Joseph ME,Torok N, Anderson RJ. 1983. Usher’ssyndrome.Arch. Ophthalmol.101:1367–74

68. Fleischman JA, O’Donnell FE. 1981.Congenital X-linked incomplete achro-matopsia: evidence for slow progression,carrier fundus findings, and possible ge-netic linkage with glucose-6-phosphatedehydrogenase locus.Arch. Ophthalmol.99:468–72

68a. Francois J. 1966. Genetics and primary

open-angle glaucoma.Am. J. Ophthal-mol.61:652–65

68b. Friedman JS, Walter MA. 1999. Glau-coma genetics, present and future.Clin.Genet.55:71–79

69. Freund CL, Gregory-Evans CY, Furu-kawa T, Papaioannou M, Looser J, et al.1997. Cone-rod dystrophy due to mu-tations in a novel photoreceptor-specifichomeobox gene (CRX) essential formaintenance of the photoreceptor.Cell91:543–53

70. Freund C, Horsford DJ, McInnes RR.1996. Transcription factor genes and thedeveloping eye: a genetic perspective.Hum. Mol. Genet.5:1471–88

71. Freund CL, Wang Q-L, Chen S, MuskatBL, Wiles CD, et al. 1998. De novo mu-tations in the CRX homeobox gene asso-ciated with Leber congenital amaurosis.Nat. Genet.18:311–12

72. Fuchs S, Nakazawa M, Maw M, Tamai M,Oguchi Y, Gal A. 1995. A homozygous1-base pair deletion in the arrestin geneis a frequent cause of Oguchi disease inJapanese.Nat. Genet.10:360–62

73. Fujita R, Buraczynska M, Gieser L, WuW, Forsythe P, et al. 1997. Analysis ofthe RPGR gene in 11 pedigrees withthe retinitis pigmentosa type 3 genotype:paucity of mutations in the coding regionbut splice defects in two families.Am. J.Hum. Genet.61:571–80

74. Furukawa T, Morrow EM, Cepko CL.1997. Crx, a novel otx-like homeoboxgene, shows photoreceptor-specific ex-pression and regulates photoreceptor dif-ferentiation.Cell 9l:531–41

75. Gal A, Orth U, Baehr W, Schwinger E,Rosenberg T. 1994. Heterozygous mis-sense mutation in the rod cGMP phospho-diesterase beta-subunit gene in autosomaldominant stationary nightblindness.Nat.Genet.13:358–60

76. Garriga P, Liu X, Khorana HG. 1996.Structure and function in rhodopsin:correct folding and misfolding in point




mutants at and in proximity to the site ofthe retinitis pigmentosa mutation leu125-arg in the transmembrane helix C.Proc.Natl. Acad. Sci. USA93:4560–64

77. Gibson F, Walsh J, Mburu P, Varela A,Brown KA, et al. 1995. A type VII myosinencoded by the mouse deafness geneshaker-1. Nature374:62–64

78. Goldberg AFX, Loewen CJ, Molday RS.1998. Cysteine residues of photoreceptorperipherin/rds: role in subunit assemblyand autosomal dominant retinitis pigmen-tosa.Biochemistry37:680–85

79. Goldberg AFX, Molday RS. 1996. Defec-tive subunit assembly underlies a digenicform of retinitis pigmentosa linked to mu-tations in peripherin/rds and rom-1.Proc.Natl. Acad. Sci. USA93:13726–30

80. Goldberg AFX, Molday RS. 1996. Sub-unit composition of the peripherin/rds-rom-1 disk rim complex from rod pho-toreceptors: hydrodynamic evidence for atetrameric quaternary structure.Biochem-istry 35:6144–49

81. Goldberg AFX, Moritz OL, Molday RS.1995. Heterologous expression of photore-ceptor peripherin/rds and rom-1 in COS-1cells: assembly, interactions and localiza-tion of multisubunit complexes.Biochem-istry 34:14213–19

82. Gorczyna WA, Polans AS, SurguchevaIG, Subbaraya I, Baehr W, PalczewskiK. 1995. Guanylyl cyclase activating pro-tein: a calcium-sensitive regulator of pho-totransduction.J. Biol. Chem.270:22029–36

83. Gotoda T, Arita M, Arai H, Inoue K,Yokota, et al. 1995. Adult-onset spinocere-bellar dysfunction caused by a mutation inthe gene for the alpha-tocopherol-transferprotein.N. Engl. J. Med.333:1313–18

84. Gregory-Evans K, Bhattacharya SS. 1998.Genetic blindness: current concepts in thepathogenesis of human outer retinal dys-trophies.Trends Genet.14:103–8

85. Gu S-M, Thompson DA, Srikumari CRS,Lorenz B, Finckh U, et al. 1997. Mutations

in RPE65 cause autosomal recessive child-hood severe onset retinal dystrophy.Nat.Genet.17:194–97

86. Gulcher J, Stefansson K. 1998. Populationgenomics: laying the groundwork for ge-netic disease modeling and targeting.Clin.Chem. Lab. Med.36:523–27

87. Hagstrom SA, North MA, Nishina PM,Berson EL, Dryja TR. 1998. Recessive mu-tations in the gene encoding the tubby-likeprotein TULP1 in patients with retinitispigmentosa.Nat. Genet.18:174–76

88. Hamel CR, Tsilou E, Pfeffier BA, HooksJJ, Detrick B, Redmond TM. 1993.Molecular cloning and expression ofRPE65, a novel retinal pigment epithelium-specific microsomal protein that is post-translationally regulated in vitro.J. Biol.Chem.268:15751–57

89. Hart WM. 1992.Adler’s Physiology of theEye. St. Louis: Mosby. 888 pp.

90. Hawkins RK, Jansen HG, Sanyal S. 1985.Development and degeneration of retina inrds mutant mice: photoreceptor abnormal-ities in heterozygotes.Exp. Eye Res.41:7011–20

91. Heckenlively JR. 1988. RP syndromes. SeeRef. 92, pp. 221–52

92. Heckenlively JR. 1988.Retinitis Pigmen-tosa. Philadelphia: Lippincott. 269 pp.

93. Heckenlively JR, Bird AC. 1988. Choroi-deremia. See Ref. 92, pp. 176–87

94. Heckenlively JR, Weleber RG. 1986.X-linked recessive cone dystrophy withtapetal-like sheen: a newly recognized en-tity with Mizuo-Nakamura phenomenon.Arch. Ophthalmol.104:1322–28

95. Hee MR, Izatt JA, Swanson EA, HuangD, Schuman JS, et al. 1995. Optical co-herence tomography of the human retina.Arch. Ophthalmol.113:325–32

96. Heiba IM, Elston RC, Klein BEK, Klein R.1994. Sibling correlations and segregationanalysis of age-related maculopathy: theBeaver Dam eye study.Genet. Epidemiol.11:51–67

97. Hood DC, Hock PA. 1973. Recovery of




cone receptor activity in the frog’s iso-lated retina.Vis. Res.13:1943–51

98. Howard L, Ovesen L, Satya-Murti S, ChuR. 1982. Reversible neurological symp-toms caused by vitamin E deficiency in apatient with short bowel syndrome.Am.J. Clin. Nutr.36:1243–49

99. Howell N. 1996. Leber hereditary opticneuropathy: mitochondrial mutations anddegeneration of the optic nerve.Vis. Res.37:3495–507

100. Howes K, Bronson JD, Dang LY, Li N,Zhang K, et al. 1998. Gene array and ex-pression of mouse retina guanylate cy-clase activating proteins 1 and 2.Invest.Ophthalmol. Vis. Sci.39:867–75

101. Huang D, Swanson EA, Lin CP, SchumanJS, Stinson WG, et al. 1991. Optical co-herence tomography.Science254:1178–81

102. Huang PC, Gaitan AE, Hao Y, PettersRM, Wong F. 1993. Cellular interactionsimplicated in the mechanism of photore-ceptor degeneration in transgenic miceexpressing a mutant rhodopsin.Proc.Natl. Acad. Sci. USA90:8484–89

103. Huang SH, Pittler SJ, Huang X, OlivieraL, Berson EL, Dryja TR. 1995. Autoso-mal recessive retinitis pigmentosa causedby mutations in the alpha subunit of rodcGMP phosphodiesterase.Nat. Genet.11:468–71

104. Hubbard R. 1958. The thermal stabilityof rhodopsin and opsin.J. Gen. Physiol.42:259–80

105. Humphries MM, Rancourt D, Farrar GJ,Kenna P, Hazel M, et al. 1997. Retinopa-thy induced in mice by targeted disrup-tion of the rhodopsin gene.Nat. Genet.15:216–19

106. Hunt DF, Organisciak DT, Wang HM, WuRL. 1984. Alpha-tocopherol in the devel-oping rat retina: a high pressure liquidchromatographic analysis.Curr. Eye Res.3:1281–88

107. Hyman LG, Lilenfeld AM, Ferris FL,Fine SL. 1983. Senile macular degener-

ation: a case-control study.Am. J. Epi-demiol.118:213–27

108. Illing M, Molday LL, Molday RS. 1997.The 220 kDa rim protein of retinal rodouter segments is a member of the ABCtransporter superfamily.J. Biol. Chem.272:10303–10

109. Inglehearn CF, Tarttelin EE, Plant C, Pea-cock RE, Al-Maghtheh M, et al. 1998.A linkage survey of 20 dominant retini-tis pigmentosa families: frequency of thenine known loci and evidence for furtherheterogeneity.J. Med. Genet.35:1–5

110. Jacobson SG, Cideciyan AV, Huang Y,Hanna DB, Freund CL, et al. 1998.Retinal degenerations with truncationmutations in the cone-rod homeobox(CRX) gene.Invest. Ophthalmol. Vis. Sci.39:2417–26

111. Jacobson SG, Cideciyan AV, RegunathG, Rodriguez FJ, Vandenburgh K, et al.1995. Nightblindness in Sorsby’s fundusdystrophy is reversed by vitamin A.Nat.Genet.11:27–32

112. Johns DR. 1995. Mitochondrial DNAand disease.N. Engl. J. Med.333:638–44

113. Jones GJ, Crouch RK, Wiggert B, Corn-wall MC, Chader GJ. 1989. Retinoid re-quirements for recovery of sensitivity af-ter visual pigment bleaching in isolatedphotoreceptors.Proc. Natl. Acad. Sci.USA86:9606–10

114. Kajiwara K, Berson EL, Dryja TR. 1994.Digenic retinitis pigmentosa due to muta-tions at the unlinked peripherin/RDS andROM1 loci.Science264:1604–8

115. Kaupp UB, Koch KW. 1992. Role ofcGMP and calcium in vertebrate photore-ceptor excitation and adaptation.Annu.Rev. Physiol.54:153–75

116. Kaushel S, Khorana HG. 1994. Struc-ture and function in rhodopsin: point mu-tations associated with autosomal dom-inant retinitis pigmentosa.Biochemistry33:6121–28

117. Kedzierski W, Bok D, Travis GH. 1998.




Non-cell-autonomous photoreceptor de-generation in rds mutant mice mosaic forexpression of a rescue transgene.J. Neu-rosci.18:4076–82

118. Keen TJ, Inglehearn CF. 1996. Muta-tions and polymorphisms in the humanperipherin-RDS gene and their involve-ment in inherited retinal degeneration.Hum. Mutat.8:297–303

119. Kelsell RE, Gregory-Evans K, Payne AM,Perrault I, Kaplan J, et al. 1998. Mutationsin the retinal guanylate cyclase (RETGC-1) gene in dominant cone-rod dystrophy.Hum. Mol. Genet.7:1179–84

120. Kleyn PW, Fan W, Kovats SG, Lee JJ,Pulido JC, et al. 1996. Identification andcharacterization of the mouse obesitygene tubby: a member of a novel genefamily. Cell 85:281–90

121. Kohl S, Marx T, Giddings I, Jagle H, Ja-cobson SG, et al. 1998. Total colourblind-ness is caused by mutations in the geneencoding the alpha-subunit of the conephotoreceptor cGMP-gated cation chan-nel.Nat. Genet.19:257–59

122. Kumar R, Chen S, Scheurer D, Wang QL,Duh E, et al. 1996. The bZIP transcriptionfactor Nrl stimulates rhodopsin promotoractivity in primary retinal cell cultures.J.Biol. Chem.271:29612–18

123. Kurada P, O’Tousa JE. 1995. Retinal de-generation caused by dominant rhodopsinmutants in Drosophila.Neuron14:571–79

124. Lambright DG, Noel J, Hamm HE, SiglerPB. 1994. Structural determinants for theactivation of the alpha subunit of a hetero-trimeric G-protein.Nature369:621–28

125. Lander E, Schork N. 1994. Genetic dis-section of complex traits.Science265:2037–48

126. Langton KP, Barker MID, McKie N.1998. Localization of the functionaldomains of human tissue inhibitor ofmetalloproteinase-3 and the effects of aSorsby’s fundus dystrophy mutation.J.Biol. Chem.273:16778–81

127. Laura RP, Dizhoor AM, Hurley JB. 1996.The membrane guanylyl cyclase, retinalguanylyl cyclase-1, is activated throughits intracellular domain.J. Biol. Chem.271:11646–51

128. Lem J, Krasnoperova NV, Calvert PD,Kosaras B, Cameron DA, et al. 1999. Mor-phological, physiological, and biochemi-cal changes in rhodopsin knockout mice.Proc. Natl. Acad. Sci. USA96:736–41

129. Levinson AD. 1986. Normal and acti-vated ras oncogenes and their encodedproducts.Trends Genet.2:81–85

130. Levy G, Leci-Acobas F, Blanchard S,Gerber S, Larget-Piet D. 1997. MyosinVIIA gene: heterogeneity of the mu-tations responsible for Usher syndrometype IB.Hum. Mol. Genet.6:111–16

131. Lewis RA. 1988. Juvenile hereditary mac-ular dystrophies. See Ref. 177, pp. 126–30

132. Lewis RA, Shroyer NF, Singh N, Al-likmets R, Hutchinson A, et al. 1999.Genotype/phenotype analysis of a photo-receptor-specific ATP-binding cassettetransporter gene, ABCR, in Stargardt dis-ease.Am. J. Hum. Genet.64:422–34

133. Li T, Franson WK, Gordon JW, BersonEL, Dryja TR. 1995. Constitutive activa-tion of phototransduction by K296E opsinis not a cause of photoreceptor degenera-tion.Proc. Natl. Acad. Sci. USA92:3551–55

134. Li T, Sandberg MA, Pawlyk BS, RosnerB, Hayes KC, et al. 1998. Effect of vi-tamin A supplementation on rhodopsinmutants threonine-17-methionine andproline-347-serine in transgenic mice andin cell cultures.Proc. Natl. Acad. Sci. USA95:11933–38

134a. Li T, Snyder WK, Olsson JE, Dryja TR.1996. Transgenic mice carrying the dom-inant rhodopsin mutation P347S: evi-dence for defective vectorial transport ofrhodopsin to the outer segments.Proc.Natl. Acad. Sci. USA93:14176–81

135. Linari M, Ueffing M, Manson F, Wright




A, Meitinger T, Becker J. 1999. Theretinitis pigmentosa GTPase regulator,RPGR, interacts with the delta subunit ofrod cyclic GMP phosphodiesterase.Proc.Natl. Acad. Sci. USA96:1315–20

136. Linsenmeier RA. 1986. Effects of lightand darkness on oxygen distribution andconsumption in the cat retina.J. Gen.Physiol.88:521–42

137. Liu X, Garriga P, Khorana HG. 1996.Structure and function in rhodopsin: cor-rect folding and misfolding in two pointmutants in the intradiscal domain ofrhodopsin identified in retinitis pigmen-tosa.Proc. Natl. Acad. Sci. USA93:4554–59

138. Liu X, Vansant G, Udovichenko IP, Wol-frum U, Williams IDS. 1997. MyosinVIIa, the product of the Usher 1B syn-drome gene, is concentrated in the con-necting cilia of photoreceptor cells.CellMotil. Cytoskel.37:240–52

139. Liu X-Z, Hope C, Walsh J, Newton V,Ke XM, et al. 1998. Mutations in themyosin VIIAa gene cause a wide pheno-typic spectrum, including atypical Ushersyndrome.Am. J. Hum. Genet.63:909–12

140. Lolley RN, Rong H, Craft CM. 1994.Linkage of photoreceptor degeneration byapoptosis with inherited defect in photo-transduction.Invest. Ophthalmol. Vis. Sci.35:358–62

141. Lowe DG, Dizhoor AM, Liu K, Gu Q,Spencer M, et al. 1995. Cloning andexpression of a second photoreceptor-specific membrane guanylate cyclase(RetGC), RetGC-2.Proc. Natl. Acad. Sci.USA92:5535–39

142. Macke JR, Davenport CM, JacobsonSG, Hennessey JC, Gonzalez-FernandezF, et al. 1993. Identification of novelrhodopsin mutations responsible for auto-somal dominant retinitis pigmentosa: im-plications for the structure and function ofrhodopsin.Am. J. Hum. Genet.53:80–89

143. Mackey D, Oostra R-J, Rosenberg T,Nikoskelainen F, Bronte-Stewart J, et al.

1996. Primary pathogenic mtDNA muta-tions in multigeneration pedigrees withLeber hereditary optic neuropathy.Am. J.Hum. Genet.59:481–85

144. Macular Photocoagulation Study Group.1991. Laser photocoagulation for sub-foveal neovascular lesions of age relatedmacular degeneration: results of a ran-domized clinical trial.Arch. Ophthalmol.109:1219–31

145. Marlhens F, Bareil C, Griffoin J-M, Zren-ner E, Amalric P, et al. 1997. Mutations inRPE65 cause Leber’s congenital amauro-sis.Nat. Genet.17:139–41

146. Marmor MF, Wolfensberger TJ. 1998.The Retinal Pigment Epithelium. Oxford:Oxford Univ. Press. 745 pp.

147. Marquardt A, Stohr H, Passmore LA,Kramer F, Rivera A, Weber BH. 1998.Mutations in a novel gene, VMD2, encod-ing a protein of unknown properties causejuvenile-onset vitelliform macular dys-trophy (Best’s disease).Hum. Mol. Genet.7:1517–25

148. Marshall J, Hussain A, Starita C, MooreIDJ, Patmore AL. 1998. Aging andBruch’s membrane. See Ref. 146, pp.669–92

149. Martinez-Mir A, Paloma E, Allikmets R,Ayoso C, del Rio T, et al. 1998. Retini-tis pigmentosa caused by a homozygousmutation in the Stargardt disease geneABCR. Nat. Genet.18:11–12

150. Massof RW, Finkelstein D. 1993. Edito-rial: supplemental vitamin A retards lossof ERG amplitude in retintis pigmentosa.Arch. Ophthalmol.111:751–54

151. Maugeri A, van Driel MA, van de PolIDJR, Klevering BJ, van Haren FJJ, et al.1999. The 2588G to C mutation in theABCR gene is a mild frequent foundermutation in the western European pop-ulation and allows the classification ofABCR mutations in patients with Star-gardt disease.Am. J. Hum. Genet.64:1024–35

152. Maw MA, Kennedy BA, Knight A,




Bridges R, Roth KE, et al. 1997. Mu-tations in the gene encoding cellularretinaldehyde-binding protein in autoso-mal recesssive retinitis pigmentosa.Nat.Genet.17:198–200

153. McCulloch C. 1988. Choroideremia andother choroidal atrophies. See Ref. 177,pp. 285–95

154. McIntosh I, Hamosh A, Dietz HC.1993. Nonsense mutations and dimin-ished mRNA levels.Nat. Genet.4:219

155. McLaughlin ME, Ehrhart TL, Berson EL,Dryja TR. 1995. Mutation spectrum ofthe gene encoding the beta subunit of rodphosphodiesterase among patients withautosomal recessive retinitis pigmentosa.Proc. Natl. Acad. Sci. USA92:3249–53

156. McLaughlin ME, Sandberg MA, BersonEL, Dryja TR. 1993. Recessive mutationsin the gene encoding the beta-subunit ofrod phosphodiesterase in patients withretinitis pigmentosa.Nat. Genet.4:130–34

157. McWilliam P, Farrar GJ, Kenna P, BradleyDG, Humphries MM, et al. 1989. Au-tosomal dominant retinitis pigmentosa(ADRP): localization of an ADRP gene tothe long arm of chromosome 3.Genomics5:619–22

158. Meindl A, Berger W, Meitinger T, Van dePol D, Achatz H, et al. 1992. Norrie dis-ease is caused by mutations in an extracel-lular protein resembling the C-terminalglobular domain of mucins.Nat. Genet.2:139–43

159. Meindl A, Dry K, Herrmann K, Man-son F, Ciccodicola A, et al. 1996. Agene (RPGR) with homology to the RCC1guanine nucleotide exchange factor ismutated in X-linked retinitis pigmentosa(RP3).Nat. Genet.13:35–42

160. Meitinger T, Meindl A, Bork P, Rost B,Sander C, et al. 1993. Molecular mod-elling of the Norrie disease protein pre-dicts a cystine knot growth factor tertiarystructure.Nat. Genet.5:376–80

161. Meyers SM, Greene T, Gutman FA. 1995.A twin study of age-related macular de-generation.Am. J. Ophthalmol.120:757–66

162. Milam AH, Li ZY, Fariss RN. 1998.Histopathology of the human retina in re-tinitis pigmentosa.Prog. Retinal Eye Res.17:175–205

163. Min KC, Zvyaga TA, Cypess AM, Sak-mar TP. 1993. Characterization of mutantrhodopsins responsible for autosomaldominant retinitis pigmentosa: mutationson the cytoplasmic surface affect trans-ducin activation.J. Biol. Chem.268:9400–4

164. Miyake Y, Horiuchu M, Ota I, Shi-royama N. 1987. Characteristic ERGflicker anomaly in incomplete congenitalstationary night blindness.Invest. Oph-thalmol. Vis. Sci.28:1816–23

165. Miyake Y, Yagasaki K, Horiguchi M,Kawase Y, Kanda T. 1986. Congeni-tal stationary night blindness with neg-ative electroretinogram.Arch. Ophthal-mol.104:1013–20

166. Molday RS. 1998. Photoreceptor mem-brane proteins, phototransduction, andretinal degenerative diseases.Invest. Oph-thalmol. Vis. Sci.39:2493–513

167. Molday RS, Hicks D, Molday LL. 1987.Peripherin: a rim-specific membrane pro-tein of rod outer segment disks.Invest.Ophthalmol. Vis. Sci.28:50–61

168. Morimura H, Berson EL, Dryja TP. 1999.Recessive mutations in the RLBP1 geneencoding cellular retinaldehyde bindingprotein in a form of retinitis punctataalbescens.Invest. Ophthalmol. Vis. Sci.40:1000–4

169. Morimura H, Fishman GA, Grover SA,Fulton AB, Berson EL, Dryja TP. 1998.Mutations in the RPE65 gene in patientswith autosomal retinitis pigmentosa orLeber congenital amaurosis.Proc. Natl.Acad. Sci. USA95:3088–93

170. Murphy AN, Unsworth EJ, Stetler-Stevenson WG. 1993. Tissue inhibitor




of metalloproteinases-2 inhibits bFGF-induced human microvascular endothe-lial cell proliferation.J. Cell Physiol.157:351–58

171. Nakazawa M, Wada Y, Tamai M. 1998.Arrestin gene mutations in autosomal re-cessive retinitis pigmentosa.Arch. Oph-thalmol.116:498–501

172. Nasonkin I, Illing M, Koehler MR,Schmid M, Molday RS, Weber BHF.1998. Mapping of the rod photorecep-tor ABC transporter ABCR to 1p21-p22.1and identification of novel mutations inStargardt’s disease.Hum. Genet.102:21–26

173. Nathans J, Davenport CM, Maumenee IH,Lewis RA, Hejtmancik JF, et al. 1989.Molecular genetics of human blue conemonochromacy.Science245:831–38

174. Nathans J, Maumenee IH, Zrenner E,Sadowski B, Sharpe LT, et al. 1993.Genetic heterogeneity among blue conemonochromats.Am. J. Hum. Genet.53:987–1000

175. Nathans J, Piantanida TP, Eddy RL,Shows TB, Hogness DS. 1986. Moleculargenetics of inherited variation in humancolor vision.Science232:203–10

176. Newsome DA. 1988. Retinitis pigmen-tosa, Usher’s syndrome, and other pig-mentary retinopathies. See Ref. 177, pp.161–94

177. Newsome DA. 1988.Retinal Dystrophiesand Degenerations. New York: Raven.382 pp.

178. Noben-Trauth K, Naggert JK, North MA,Nishina PM. 1996. A candidate genefor the mouse mutation tubby.Nature380:534–38

179. North MA, Naggert JK, Yan Y, Noben-Trauth K, Nishina PM. 1997. Molecu-lar characterization of TUB, TULP1, andTULP2, members of the novel tubby genefamily and their possible relation to ocu-lar disease.Proc. Natl. Acad. Sci. USA94:3128–33

180. Olsson JE, Gordon JW, Pawlyk BS, Roof

D, Hayes A, et al. 1992. Transgenic micewith a rhodopsin mutation (Pro23His):a mouse model of retinitis pigmentosa.Neuron9:815–30

181. Papermaster DS, Schneider BG, ZornMA, Kraehenbuhl JR. 1978. Immunocy-tochemical localization of a large intrin-sic membrane protein to the incisure andmargins of frog rod outer segment disks.J. Cell Biol.78:415–25

182. Payne AM, Downes SM, Bessant DAR,Taylor R, Holder GE, et al. 1998. A mu-tation in guanylate cyclase activator 1A(GUCA1A) in an autosomal dominantcone dystrophy pedigree mapping to anew locus on chromosome 6p21.1.Hum.Mol. Genet.7:273–77

183. Pearce WG, Reedyk M, Coupland SG.1990. Variable expressivity in X-linkedcongenital stationary night blindness.Can. J. Ophthalmol.25:3–10

184. Perrault I, Rozet JM, Calvas P, Ger-ber S, Camuzat A, et al. 1996. Retinal-specific guanylate cyclase gene muta-tions in Leber’s congenital amaurosis.Nat. Genet.14:461–64

185. Petrukhin K, Koisti MJ, Bakall B, Li W,Xie G, et al. 1998. Identification of thegene responsible for Best macular dys-trophy.Nat. Genet.19:241–47

186. Polans A, Baehr W, Palczewski K. 1996.Turned on by calcium: the physiology andpathology of calcium-binding proteins inthe retina.Trends Neurosci.19: 547–54

187. Portera-Cailliau C, Sung C-H, Nathans J,Adler R. 1994. Apoptotic photoreceptorcell death in mouse models of retinitispigmentosa.Proc. Natl. Acad. Sci. USA91:974–78

188. Pugh EN, Lamb TD. 1990. Cyclic GMPand calcium: the internal messengers ofexcitation and adaptation in vertebratephotoreceptors.Vis. Res.30:1923–48

189. Rao VR, Cohen GB, Oprian DD. 1994.Rhodopsin mutation G90D and a molec-ular mechanism for congenital stationarynight blindness.Nature367:639–42




190. Rao VR, Oprian DD. 1996. Activat-ing mutations of rhodopsin and otherG protein-coupled receptors.Annu. Rev.Biophys. Biomol. Struct.25:287–314

191. Rando RR. 1990. The chemistry of vita-min A and vision.Angew. Chem. Int. Ed.Engl.29:461–80

192. Rehemtulla A, Warwar R, Kumar R, JiX, Zack DJ, Swaroop A. 1996. The ba-sic motif leucine zipper transcription fac-tor Nrl can positively regulate rhodopsingene expression.Proc. Natl. Acad. Sci.USA93:191–95

193. Reichel E, Bruce AM, Sandberg MA,Berson EL. 1989. An electroretino-graphic and molecular genetic study ofX-linked cone degeneration.Am. J. Oph-thalmol.108:540–47

194. Reid SNM, Akhmedov NB, Piriev NI,Kozak CA, Danciger M, Farber DB. 1999.The mouse X-linked juvenile retinoschi-sis cDNA: expression in photoreceptors.Gene227:257–66

195. Risch N, Merikangas K. 1996. The futureof genetic studies of complex human dis-orders.Science273:1516–17

196. Rim J, Oprian DD. 1995. Constitutive ac-tivation of opsin: interaction of mutantswith rhodopsin kinase and arrestin.Bio-chemistry34:11938–45

197. Robinson PR, Cohen GB, ZhukovskyEA, Oprian DD. 1992. Constitutively ac-tive mutants of rhodopsin.Neuron9:719–25

198. Robinson WG, Kuwabara T, Bier JG.1982. The role of vitamin E and unsat-urated fatty acids in the visual process.Retina4:263–81

199. Rodieck RW. 1998.The First Steps in See-ing. Sunderland, MA: Sinauer. 562 pp.

200. Roepman R, van Duijnhoven G,Rosenberg T, Pinckers AJLG, Bleeker-Wagemakers LM, et al. 1996. Positionalcloning of the gene for X-linked re-tinitis pigmentosa 3: homology withthe guanine-nucleotide-exchange factorRCC1.Hum. Mol. Genet.5:1035–41

201. Roorda A, Williams DR. 1999. The ar-rangement of the three classes of conesin the living human eye.Nature397:520–22

202. Rosenfeld PJ, Cowley GS, McGee TIL,Sandberg MA, Berson EL, Dryja TP.1992. A null mutation in the rhodopsingene causes rod photoreceptor dysfunc-tion and autosomal recessive retinitis pig-mentosa.Nat. Genet.1:209–13

203. Rozet JM, Gerber S, Souied E, PerraultI, Chatelin S, et al. 1998. Spectrum ofABCR gene mutations in autosomal re-cessive macular dystrophies.Eur. J. Hum.Genet.6:291–95

204. Ruether K, Apfelstedt-Sylla E, ZrennerE. 1993. Clinical findings in patients withcongenital stationary night blindness ofthe Schubert-Bornschein type.Ger. J.Ophthalmol.2:429–35

205. Saari JC, Bredberg DL, Noy N. 1994.Control of substrate flow at a branch in thevisual cycle.Biochemistry33:3106–12

206. Sanyal S, Jansen HG. 1981. Absence ofreceptor outer segments in the retina of rdsmutant mice.Neurosci. Lett.21:23–26

207. Sauer CG, Gehrig A, Warneke-WittstockR, Marquardt A, Ewing CC, et al. 1997.Positional cloning of the gene associatedwith X-linked juvenile retinoschisis.Nat.Genet.17:164–70

208. Schaid DJ. 1998. Statistical Genetics ’98:transmission disequilibrium, family con-trols, and great expectations.Am. J. Hum.Genet.63:935–41

209. Schmitz Y, Witkovsky P. 1997. Depen-dence of photoreceptor glutamate releaseon a dihydropyridine-sensitive calciumchannel.Neuroscience78:1209–16

210. Schumer RA, Podos SM. 1994. The nerveof glaucoma!Arch. Ophthalmol.112:37–44

211. Schwahn U, Lenzner S, Dong J, FeilS, Hinzmann B, et al. 1998. Positionalcloning of the gene for X-linked retinitispigmentosa 2.Nat. Genet.19:327–32

211a. Scriver CR, Beaudet AL, Sly WS, Valle




D, eds. 1995.The Metabolic and Molecu-lar Basis of Inherited Disease. New York:McGraw Hill

212. Seabra MC, Brown MS, Goldstein JL.1993. Retinal degeneration in choroi-deremia: deficiency of rab geranylgeranyltransferase.Science259:377–81

213. Seabra MC, Brown MS, Slaughter CA,Sudhof TC, Goldstein JL. 1992. Pu-rification of component A of Rab ger-anylgeranyl transferase: possible iden-tity with the choroideremia gene product.Cell 70:1049–57

214. Seabra MC, Goldstein JL, Sudhof TC,Brown MS. 1992. Rab geranylgeranyltransferase: a multisubunit enzyme thatprenylates GTP-binding proteins termi-nating in cys-X-cys or cys-cys.J. Biol.Chem.267:14497–503

215. Seabra MC, Ho YK, Anant JS. 1995.Deficient geranylgeranylation of Ram/Rab27 in chroideremia.J. Biol. Chem.270:24420–27

216. Seddon JM, Ajani UA, Mitchell BD.1997. Familial aggregation of age-related maculopathy.Am. J. Ophthalmol.123:199–206

217. Semple-Rowland SL, Lee NR, VanHooser JP, Palczewski K, Baehr W. 1998.A null mutation in the photoreceptorguanylate cyclase gene causes the retinaldegeneration chicken phenotype.Proc.Natl. Acad. Sci. USA95:1271–76

218. Shastry BS, Hejtmancik JF, Trese MT.1997. Identification of novel missensemutations in the Norrie disease gene as-sociated with one X-linked and four spo-radic cases of familial exudative vitreo-retinopathy.Hum. Mutat.9:396–401

219. Shastry BS, Pendergast SD, Hartzer MK,Liu X, Trese MT. 1997. Identification ofmissense mutations in the Norrie diseasegene associated with advanced retinopa-thy of prematurity.Arch. Ophthalmol.115:651–55

220. Sherman J, Kleiner L. 1994. Visual sys-tem dysfunction in Leber’s hereditary op-

tic neuropathy.Clin. Neurosci.2:121–29221. Sickel W. 1972. Retinal metabolism in

dark and light. InPhysiology of Pho-toreceptor Organs, ed. MGF Fuortes, pp.667–727. Berlin: Springer-Verlag

222. Sieving PA, Richards JE, NaarendorpF, Bingham EL, Scott K, Alpern M.1995. Dark-light: model for nightblind-ness from the human rhodopsin gly-90-asp mutation.Proc. Natl. Acad. Sci. USA92:880–84

223. Silvestri G, Johnson PB, Hughes AE.1994. Is genetic predisposition an impor-tant risk factor in age-related macular de-generation?Eye8:564–68

223a. Simon A, Hellma U, Wernstedt C,Eriksson U. 1995. The retinal-pigmentepithelial-specific 11-cisretinol dehydro-genase belongs to the family of shortchain alcohol dehydrogenases.J. Biol.Chem.270:1107–12

224. Sohocki MM, Sullivan LS, Mintz-HittnerHA, Birch D, Heckenlively JR, et al.1998. A range of clinical phenotypes as-sociated with mutations in CRX, a pho-toreceptor transcription factor gene.Am.J. Hum. Genet.63:1307–15

225. Sokal I, Li N, Surgucheva I, Warren MJ,Payne AM, Bhattacharya SS, et al. 1998.GCAP1 (Y99C) mutant is constitutivelyactive in autosomal dominant cone dys-trophy.Mol. Cell 2:129–33

226. Steiner DF, Tager HS, Nanjo K, ChanSJ, Rubenstein AH. 1995. Familial syn-dromes of hyperproinsulinemia and hy-perinsulinemia with mild diabetes. SeeRef. 211a, pp. 897–904

227. St. Jules RS, Wallingford JC, Smith SB,O’Brien PJ. 1989. Addition of the chro-mophore to rat rhodopsin is an early post-translational step.Exp. Eye Res.48:653–65

228. Stone EM, Webster AR, Vandenburgh K,Streb LM, Hockey RR, et al. 1998. Allelicvariation in ABCR associated with Star-gardt disease but not age-related maculardegeneration.Nat. Genet.20:328–29




228a. Stone EM, Lotery AJ, Munier FL, HeonE, Piguet B, et al. 1999. A singleEFEMP1 mutation associated with bothMalattia Leventinese and Doyne hon-eycomb retinal dystrophy.Nat. Genet.22:199–202

229. Strom TM, Nyakatura G, Apfelstedt-Sylla E, Hellebrand H, Lorenz B, et al.1997. An L-type calcium-channel genemutated in incomplete X-linked con-genital stationary night blindness.Nat.Genet.19:260–63

230. Sun H, Molday RS, Nathans J. 1999.Retinal stimulates ATP hydrolysis bypurified and reconstituted ABCR, thephotoreceptor-specific ABC transporterresponsible for Stargardt disease.J. Biol.Chem.274:8269–81

231. Sun H, Nathans J. 1997. Stargardt’sABCR is localized to the disc mem-brane of retinal rod outer segments.Nat.Genet.17:15–16

232. Sung C-H, Davenport CM, HennesseyJC, Maumenee IH, Jacobson SG, et al.1991. Rhodopsin mutations in auto-somal dominant retinitis pigmentosa.Proc. Natl. Acad. Sci. USA88:6481–85

233. Sung C-H, Davenport C, Nathans J.1993. Rhodopsin mutations responsi-ble for autosomal dominant retinitispigmentosa: clustering of functionalclasses along the polypeptide chain.J.Biol. Chem.268: 26645–49

234. Sung C-H, Makino C, Baylor D, NathansJ. 1994. A rhodopsin gene mutation re-sponsible for autosomal dominant retini-tis pigmentosa results in a protein that isdefective in localization to the photore-ceptor outer segment.J. Neurosci.14:5818–33

235. Sung C-H, Schneider BG, AgarwalN, Papermaster D, Nathans J. 1991.Functional heterogeneity of mutantrhodopsins responsible for autosomaldominant retinitis pigmentosa.Proc.Natl. Acad. Sci. USA88:8840–44

236. Swain PK, Chen S, Wang QL, Affati-gato LM, Coats CL, et al. 1998. Mu-tations in the cone-rod homeobox geneare associated with the cone-rod dystro-phy photoreceptor degeneration.Neuron19:1329–36

237. Swaroop A, Xu J, Pawar H, Jackson A,Skolnick C, Agarwal N. 1992. A con-served retina-specific gene encodes a ba-sic motif-leucine zipper domain.Proc.Natl. Acad. Sci. USA89:266–270

237a. Tai AW, Chuang J-Z, Bode C, Wol-frum U, Sung C-H. 1999. Rhodopsin’scarboxy-terminal cytoplasmic tail actsas a membrane receptor for cytoplasmicdynein by binding to the dynein lightchain Tctex-1.Cell 97:877–87

238. The Retinoschisis Consortium. 1998.Functional implications of the spectrumof mutations found in 234 cases withX-linked juvenile retinoschisis.Hum.Mol. Genet.7:1185–92

239. Thomson JL, Brzeski H, Dunbar B, For-rester JV, Fothergill JE, Converse CA.1997. Photoreceptor rim protein: partialsequences of cDNA show a high degreeof similarity to ABC transporters.Curr.Eye Res.16:741–45

240. Travis GH, Brennan MB, Danielson PE,Kozak CA, Sutcliffe JG. 1989. Identifi-cation of a photoreceptor specific mRNAencoded by the gene responsible forretinal degeneration slow (rds).Nature338:70–73

241. Valle D, Simell O. 1995. The hyperor-nithinemias. See Ref. 211a, pp. 1147–85

242. Valverde D, Baiget M, Seminago R, delRio E, Garcia-Sandoval B, et al. 1996.Identification of a novel R5520 muta-tion in exon 13 of the beta subunit ofrod phosphodiesterase gene in a Spanishfamily with autosomal recessive retinitispigmentosa.Hum. Mutat.8:393–94

243. van den Hurk JAJM, Schwartz M, vanBokhoven H, van de Pol TJR, Bogerd L,et al. 1997. Molecular basis of choroi-deremia (CHM): mutations involving




the rab escort protein-1 (REP-1) gene.Hum. Mutat.9:110–17

244. Vergani L, Martinuzzi A, Carelli V,Cortelli P, Montagna P, et al. 1995.MtDNA mutations associated withLeber’s herditary optic neuropathy:studies on cytoplasmic hybrid (cybrid)cells. Biochem. Biophys. Res. Commun.210:880–88

245. von Ruckmann A, Fitzke FW, Bird AC.1998. Autofluorescence imaging of thehuman fundus. See Ref. 146, pp. 224–34

246. Wallace DC. 1995. Mitochondrial DNAvariation in human evolution, degener-ative disease, and aging.Am. J. Hum.Genet.57:201–23

247. Wallace DC, Singh G, Lott MT, HodgeJA, Schurr TG, et al. 1988. Mitochon-drial DNA mutation associated withLeber’s hereditary optic neuropathy.Sci-ence242:1427–30

248. Wang T, Milam AH, Steel G, Valle D.1996. A mouse model of gyrate atrophy ofthe choroid and retina: early pigment ep-ithelium damage and progressive retinaldegeneration.J. Clin. Invest.97:2753–62

249. Washburn T, O’Tousa JE. 1989. Molec-ular defects in Drosophila rhodopsinmutants. J. Biol. Chem. 264:15464–66

250. Weber BHF, Bogt G, Pruett RC, Stohr H,Felbor U. 1994. Mutations in the tissueinhibitor of metalloproteinase-3 (TIMP3)in patients with Sorby’s fundus dystrophy.Nat. Genet.8:352–55

251. Weil D, Blanchard S, Kaplan J, Gull-ford P, Gibson F, et al. 1995. Defec-tive myosin VIIA gene responsible forUsher syndrome type 1B.Nature374:60–61

252. Weil D, Kussel P, Blanchard S, Levy G,Levi-Acobas F, et al. 1997. The autoso-mal recessive isolated deafness, DFNB2,and the Usher 1B syndrome are allelicdefects of the myosin-VIIA gene.Nat.Genet.16:191–93

253. Weitz CJ, Miyake Y, Shinzato K, Mon-tag E, Zrenner E, et al. 1992. Human tri-tanopia associated with two amino acidsubstitutions in the blue-sensitive opsin.Am. J. Hum. Genet.50:498–507

254. Weitz CJ, Went LN, Nathans J. 1992.Human tritanopia associated with a thirdamino acid substitution in the blue sen-sitive visual pigment gene.Am. J. Hum.Genet.51:444–46

255. Weleber RG, Eisner A. 1988. Retinalfunction and physiologic studies. See Ref.177, pp. 21–69

256. Weleber RG, Carr RE, Murphey WH,Sheffield VC, Stone EM. 1993. Pheno-typic variation including retinitis pig-mentosa, pattern dystrophy and fundusflavimaculatus in a single family witha deletion of codon 153 or 154 of theperipherin/rds gene.Arch. Ophthalmol.111:1531–42

257. Weleber RG, Kennaway NG. 1988. Gy-rate atrophy of the choroid and retina. SeeRef. 92, pp. 198–220

257a. Weng J, Mata NL, Azarian SM, TzekovRD, Birch DG, Travis GH. 1999. In-sights into the function of Rim proteinin photoreceptors and etiology of Star-gardt’s disease from the phenotype inabcrknockout mice.Cell 98:13–23

258. Witkovsky P, Schmitz Y, Akopian A,Krizaj D, Tranchina D. 1997. Gain of rodto horizontal cell synaptic transfer: rela-tion to glutamate release and a dihydropy-ridine-sensitive calcium current.J. Neu-rosci.17:7297–306

259. Yamamoto S, Sippel KC, Berson EL,Dryja TR. 1997. Defects in the rhodopsinkinase gene in the Oguchi form of station-ary night blindness.Nat. Genet.15:175–78

259a. Yamamoto H, Simon A, Eriksson U,Harris E, Berson EL, Dryja TP. 1999.Mutations in the gene encoding 11-cisretinol dehydrogenase cause delayed darkadaptation and fundus albipunctatus.Nat.Genet.22:188–91




260. Yang R-B, Foster DC, Garbers DL, FulleH-J. 1995. Two membrane forms ofguanylyl cyclase found in the eye.Proc.Natl. Acad. Sci. USA92:602–6

261. Yang R-B, Garbers DL. 1997. Two eyeguanylyl cyclases are expressed in thesame photoreceptor cells and form homo-mers in preference to heteromers.J. Biol.Chem.272:13738–42

262. Yau K-W. 1994. Phototransduction mech-anism in retinal rods and cones.Invest.Ophthalmol. Vis. Sci.35:9–32

263. Yokota T, Shiojiri T, Gotoda T, Arai H.1996. Retinitis pigmentosa and ataxiacaused by a mutation in the gene forthe alpha-tocopherol-transfer protein.N.Engl. J. Med.335:1770–71

264. Zagotta WN, Siegelbaum SA. 1996.Structure and function of cyclic nucleo-tide-gated channels.Annu. Rev. Neurosci.19:235–63

265. Zhang K, Yeon H, Han M, Donoso LA.1996. Molecular genetics of macular dys-trophies.Br. J. Ophthalmol.80:1018–22

amir rattner, hui sun, and jeremy nathans 1,4...

Documents