potential interventions against bmpr2 related pulmonary ... · potential interventions against...

Potential Interventions Against BMPR2-Related Pulmonary Hypertension

James West, PhDVanderbilt University

Medical CenterDepartment of MedicineNashville, Tennessee

James E. Loyd, MDVanderbilt University

Medical CenterDepartment of MedicineNashville, Tennessee

Rizwan Hamid, MD, PhDVanderbilt University

Medical CenterDepartments of Genetics

and PediatricsNashville, Tennessee

For more than 60 years, researchers have sought to understand the molecularbasis of idiopathic pulmonary arterial hypertension (PAH). Recognition of theheritable form of the disease led to the creation of patient registries in the1980s and 1990s, and discovery of BMPR2 as the cause of roughly 80% ofheritable PAH in 2000. With discovery of the disease gene came opportunity forintervention, with focus on 2 alternative approaches. First, it may be possibleto correct the effects of BMPR2 mutation directly through interventions tar-geted at correction of trafficking defects, increasing expression of the unmu-tated allele, and correction of splicing defects. Second, therapeutic interven-tions are being targeted at the signaling consequences of BMPR2 mutation. Inparticular, therapies targeting cytoskeletal and metabolic defects caused byBMPR2 mutation are currently in trials, or will be ready for human trials in thenear future. Translation of these findings into therapies is the culmination ofdecades of research, and holds great promise for treatment of the underlyingmolecular bases of disease.

Prior to development of cardiac catheter-ization in the late 1940s, primary pulmo-nary hypertension (PPH) was rarely sus-pected or confirmed prior to autopsy. PPHfirst became a clinical entity when inves-tigators were able to measure pulmonaryartery pressure and thus make the diagno-sis of pulmonary hypertension for the firsttime in living patients.1,2 Cardiac cathe-terization also brought new understandingabout the various causes of pulmonaryhypertension and how these could be dis-tinguished from one another. Physicianshave long been fascinated by the manyunique features of PPH, including itsprevalence in healthy young women, itsrarity, and its focal pathology within thepulmonary vascular bed, sparing the sys-temic circulation. Speculation by authori-ties about the origins of PPH favored ab-normal vasoreactivity or microthrombosis,or both, until the past decade. Widespreadinterest for PPH among the general publicand medical community was generated inthe 1970s by the first large anorexigen epi-demic related to aminorex/Menocil use inEurope,3 and the subsequent pivotal WHOmeeting in 1973, which developed, in part,as a consequence.4

Dr David Dresdale described the firstPPH family in 1951, including a mother,her son, and her sister.5 During the next 3decades, several families (13) were re-ported in the US. In 1980, we met a younglady with PPH at Vanderbilt, and she de-scribed several young female familymembers with premature cardiorespira-tory death, as young as age 23. That year,Dr John Newman joined the Vanderbiltpulmonary faculty and stimulated our in-vestigation. We contacted the authors ofthe earlier reports, and they generouslycontacted the former families to join ourdeveloping study. We described 9 newcases, which occurred in 8 families duringthe interval after the original reports.6 Thetransmission pattern was readily apparenteven then, as vertical transmission (highlyindicative of a single dominant gene) andfather to son transmission (excluding X orY linkage) were evident in multiple fam-ilies. Incomplete penetrance, with multi-ple skip generations, was apparent; thisphenomenon is still not understood andconfounds any attempt at disease predic-tion or genetic counseling.

In the mid-1980s, Dr Newman servedas Vanderbilt principal investigator in the

National Institutes of Health (NIH) natu-ral history study of PPH and enrolled 11patients from Vanderbilt.7 At semiannualNIH meetings for the study, the other in-vestigators around the US learned of ourgrowing interest in PPH families and en-couraged PPH families at their centers toparticipate in our fledgling familial PPHregistry. The NIH natural history studywas the benchmark that defined its clini-cal features and identified a positive fam-ily history in 6% of the 187 patients en-rolled.7

By the late 1980s, before any genesearch, a major concern arose about theheterogeneity of the cohort. The patho-logic literature in that era suggested thatPPH was actually many different diseases,including plexogenic PPH, thomboem-bolic PPH, pulmonary veno-occlusivedisease, isolated medial hypertrophy, andpulmonary capillary hemangiomatosis.8

That set of pathologic observations andthe related notion that PPH was not asingle disease entity imposed serious lim-itations on a gene search: it would beillogical to search for one gene as a causeof different diseases. This conundrum wasresolved by a study of the breadth of thepathologic findings in autopsies within thesame family.9 One family suffered 3 pa-tients with very different autopsy mani-festations, and thus we concluded that the

Key Words—heritable pulmonary arterial hypertension, history, treatment, vasculatureCorrespondence: [email protected]

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various pathologic findings are not differ-ent diseases, but have a single basis, be-cause they occurred in a family with ver-tical transmission indicating a single gene.With that information we concluded thatall the PPH families in our cohort could bejoined into one group to conduct a genesearch, which would be complete withinthe decade.

The familial PPH registry continued togrow, and collaboration by geneticist JohnPhillips after he moved to Vanderbilt in1985 provided direction to envision agene search. He urged us to bank patientand family specimens in the hope that agene search would eventually becomefeasible. Gene discovery methods im-proved in the 1990s, and Bill Nichols atMichigan conducted a PPH microsatellitemarker search. Using the patient and fam-ily samples from our familial PPH regis-try, he identified linkage on chromosome2q32 in 1997,10 and subsequently the un-derlying mutation responsible in mostfamilies was identified in bone morpho-genic protein receptor 2 (BMPR2) in2000.11 Similar studies of linkage of PPHand discovery that BMPR2 is the gene ofinterest were also accomplished indepen-dently in a nearly identical time frame atColumbia-Presbyterian by a team led byDrs Robyn Barst, Jane Morse, and JimKnowles.12 Mutation in BMPR2 is nowknown to be the basis for the vast majorityof families with PAH, and more than 120BMPR2 mutation families are known inthe USA, with estimates of 500 familiesworldwide. Other genes related to TGF-�are less frequently responsible for the dis-ease we now call hereditary pulmonaryarterial hypertension (HPAH): ACVRL1(ALK1) and endoglin, which more fre-quently cause hereditary hemorrhagic tel-angiectasia.13 More recently, an associa-tion between HPAH and SMAD 9 wasreported.14 In summary, germline BMPR2gene mutations cause HPAH in 80%-85%of families with a family history of PAH,while 5%-25% of patients diagnosed ashaving idiopathic pulmonary arterial hy-pertension (IPAH) actually have a detect-able germline mutation in BMPR2, aswell.11,15-18 BMPR2 mutations thus con-stitute the largest known risk for develop-ing PAH.

THERAPEUTICINTERVENTIONS AGAINSTBMPR2 EXPRESSION,SPLICING, AND TRAFFICKINGCurrently there is no therapy known toprevent, delay, or reverse the pulmonaryvasculopathy of pulmonary arterial hyper-tension (PAH). Perhaps the greatest bar-riers to developing effective treatmentsare 2 closely related knowledge gaps:what is the exact role of BMPR2 in thepathologic signaling of PAH and whatmechanisms cause the pulmonary vascu-lar disease itself. Certain unique featuresof BMPR2-related PAH provide tantaliz-ing clues into disease pathogenesis. Forexample, one of the most striking featuresof HPAH is its reduced penetrance; a mu-tation carrier has only a 20% chance ofdeveloping PAH. Thus nearly 80% ofBMPR2 mutation carriers have no clinicalsymptoms but can produce offspring thatare affected. The first clue toward under-standing reduced penetrance came fromthe recognition that BMPR2 mutationscan either be haploinsufficient (HI) ordominant negative. This distinction is im-portant to understand, as it providesunique insights into disease pathogenesisand opens avenues toward better HPAHdiagnosis and treatment. RNA studieshave shown that some BMPR2 mutationsproduce stable transcripts, while otherscontain premature termination codons(PTC) and are rapidly degraded throughthe nonsense mediated decay (NMD)pathway.19 NMD is an mRNA surveil-lance system that degrades transcriptscontaining PTCs to prevent translation ofunnecessary or harmful transcripts.20,21

HPAH patients with BMPR2 mutationsthat do not cause PTC and are thereforenot subject to NMD (NMD-) have diseasedue to dominant negative effects of themutated protein, while patients with mu-tations subject to NMD (NMD�) havedisease due to functional HI of BMPR2(NMD degradation of the mutated allele’smRNA).19,22 Thus, it appears that HI is aheterozygous state in which the normalallele of BMPR2 has insufficient expres-sion to maintain normal cellular func-tion and prevent disease (this “thresholdeffect” is illustrated in Figure 1). Muta-tions that cause HI of BMPR2 are

slightly more common (�55%-60% ofHPAH).

Thus, HI mutations teach us that totalcellular BMPR2 mRNA or protein levelsare important for disease penetrance. Thisobservation in families fits with thebroader observation that decreasedBMPR2 expression is present in other hu-man and experimental forms of pulmo-nary hypertension, though it remains un-clear whether this is a precipitating eventor a side effect.23 For example, decreasedBMPR2 expression is present in pulmo-nary vascular tissue of patients withIPAH23,24 and in multiple experimentalanimal models of PAH, including thoseinduced by monocrotaline, chronic hyp-oxia, and chronic systemic-to-pulmonaryshunting.25-27 These studies thus clearlydemonstrate that BMPR2 expression isimportant in many forms of PAH.

In studying HI NMD� BMPR2 muta-tion carriers, we noticed that mutation car-riers had variation in their total cellularBMPR2 levels. Analysis of 4 HPAH kin-dreds showed that, indeed, the expressionof wild-type BMPR2 transcript was lowerin afflicted patients compared to unaf-fected mutation carriers (P � .0.005).28

This association of transcript levels withpenetrance was not limited to a single typeof NMD� mutation since all 4 of thekindreds analyzed had different NMD�mutations. These data strongly suggestthat the level of expression of the wild-type (WT) (normal) BMPR2 allele pre-dicts the clinical development of HPAHin individuals who carry HI BMPR2 mu-tations, and thus the expression of the WTBMPR2 allele may be a primary modifierof HPAH penetrance.28 These data alsosuggest that there is likely a cellularthreshold for BMPR2 expression; ie, oncethe cell loses one BMPR2 allele secondaryto an HI mutation, the cellular BMPR2levels are determined by the expression ofthe remaining (normal, wild-type) allele(Figure 1). Thus decreased expression ofthe WT allele in an HI background(caused by the heterozygous NMD� mu-tation) lowers total BMPR2 expressionbelow a critical threshold needed forproper cellular function, thus causing dis-ease. In contrast, higher expression of theWT BMPR2 allele (higher than the pre-

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sumed threshold) with the same heterozy-gous NMD� mutation may prevent clin-ical disease in the mutation carrier.28

Such modulation of disease penetranceby WT transcripts has until now beenthought to be a rare phenomenon, beingpreviously reported in only 3 geneticdisorders: dominantly inherited erythro-poietic protoporphyria, hereditary ellipto-cytosis, and autosomal dominant retinitispigmentosa.29-32 One explanation for howlevels of WT transcripts might affect

HPAH penetrance comes from the findingthat BMPR2 forms a heterotrimeric com-plex with BMPR1A and BMPR1B. Thedegree of deficiency of normal BMPR2could affect the receptor complex stoichi-ometry, leading to decreased signalingand disease.28

The molecular mechanisms behind thisvariability in BMPR2 expression are notknown. It is unlikely to be related toBMPR2 promoter mutations (unpublisheddata) and more likely due to cis (function

of proximal regulatory regions) or a trans(function of distal genes) effect. Our datashowing that normal individuals havebaseline variability in BMPR2 expression(unpublished data) certainly support thishypothesis.

BMPR2 expression may also help ex-plain another interesting aspect of this dis-ease: that females are 1.9- to 4.1-foldmore likely to develop disease thanmales.33 It was recently shown that theBMPR2 promoter contains an evolution-arily conserved estrogen receptor bindingsite that responds to estrogen by suppress-ing BMPR2 expression.34 This observa-tion provides one clue as to why femalesare more likely to develop PAH thanmales and further highlights the impor-tance of BMPR2 expression levels in PAHpathogenesis.

Recent data, however, suggest that theeventual role of BMPR2 expression inHPAH is likely to be even more complexthen levels of total WT BMPR2 expres-sion. These data show that BMPR2 alter-native splicing may play a role in HPAHpathogenesis––it may not be a simplequestion of total BMPR2 expression, butin fact the relative levels of alternativelyspliced BMPR2 transcripts. Alternativesplicing is a mechanism by which a singlegene can generate multiple transcriptswith likely different functions through in-ternal deletion (“skipping”) of exons invarious combinations. This mRNA pro-cessing can have clinical consequencesand has been shown to play a role in manyhuman diseases.35,36 In lung disease, al-ternative splicing has been shown to beimportant in chronic obstructive pulmo-nary disease (COPD), bronchopulmonarydysplasia (BPD), chronic interstitial lungdisease, familial and sporadic interstitiallung disease,35-39 and cystic fibrosis.40,41

BMPR2 has 13 exons and is alternativelyspliced to produce 2 primary transcripts:isoform A, which is the full length geneproduct containing all 13 exons of thegene and isoform B, a much rarer tran-script missing exon 12.42-44 Several stud-ies have hinted at the importance of iso-form B in proper functioning of BMPR2and in the development of PAH. Deletionof exon 12 is a common BMPR2 mutationfound in HPAH patients, and previous

Figure 1: Cellular levels of BMPR2 mRNA are determined only by the WTallele if one inherits a mutated allele that degrades the product transcribedfrom the mutated allele. In this case if the expression from the nonmutatedallele is enough to reach a presumed threshold for BMPR2 mRNA expression,that patient will not get PAH (hypothesis from Hamid et al. Hum Mutat.2009).

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studies have shown that it can disruptBMPR2 function in a dominant negativefashion.45-47 Furthermore, studies in micehave shown that overexpression of aBMPR2 transcript with an exon 12 dele-tion results in PAH.48 Interestingly, ourdata (unpublished, in review) suggest thatcells from patients are more likely to havehigher levels of isoform B relative to lev-els of isoform A (B/A ratio) compared tocarriers. Thus the relationship betweenBMPR2 expression and PAH pathogene-sis is complex and involves not only theexpression of WT allele but also alterna-tive splicing. The relative contributions ofeither of these mechanisms toward HPAHpathogenesis are currently not known;however, it is quite likely that there issome overlap at a molecular level.

These data then suggest several poten-tially novel approaches toward disease di-agnosis and treatment. For example, couldwe use this information to design betterdiagnostic tools for HPAH patients? Thefact that we cannot predict disease devel-opment in BMPR2 carriers with extensivefamily history of PAH results in signifi-cant physical, emotional, and economicburden. Mutation carriers do not knowwhether or when they will develop clini-cal disease. Moreover, at the time of di-agnosis, 75% of subjects with HPAH al-ready have symptoms in New York HeartAssociation functional class III or IV, andthis more advanced symptom complexpredicts poor survival despite availabletreatment. Thus, any approach that willpredict which mutation carriers are likelyto develop disease would be tremendouslyhelpful both for those likely to developdisease and those likely to remain diseasefree. If ongoing studies prove that expres-sion of the normal BMPR2 allele predictsdisease development, this finding couldpossibly be used as a diagnostic tool toreassure some patients and provide opti-mal surveillance for those at higher risk.

These data also raise intriguing treat-ment possibilities—what if cellularBMPR2 expression levels could be in-creased over this critical threshold? Sev-eral approaches can now be used to iden-tify drugs that will alter BMPR2 cellularexpression. For example, the ConnectivityMap database49 offers a novel way to

identify and test drugs that may modulateBMPR2 expression. Furthermore, sincedrugs within the Map are already FDA-approved, the time frame from bench tobedside is significantly shortened, withobvious benefits to patients.50 Drugs thatcould upregulate BMPR2 expressioncould be potential PAH treatments, whileit might be wise to avoid those that down-regulate BMPR2 expression, which couldincrease an individual’s risk of develop-ing disease.

Modification of BMPR2 alternative splic-ing may also offer an approach to changedisease course. Several studies, includingour own, have shown that splicing is a dy-namic process and can be altered by theenvironment, including exposure todrugs.51,52 This raises the intriguing pos-sibility that we inadvertently make thedisease worse by our selection of partic-ular pharmacological agents. It is likelythat BMPR2 alternative splicing is dy-namic both in its response to medication(patient’s pharmacological milieu) andother environmental signals (such as hyp-oxia). A better understanding of the envi-ronmental determinants for alternativesplicing could be highly relevant for cli-nicians if, in fact, the drugs we use favor-ably (or unfavorably) influence BMPR2expression and thus cellular function inHPAH patients.

THERAPEUTICINTERVENTIONS AGAINSTSIGNALING CONSEQUENCESOF BMPR2 MUTATIONAn alternative to targeting the expressionand splicing of the BMPR2 receptor itselfis to target therapies at the downstreamsignaling consequences of BMPR2 muta-tion. This will probably be necessary formost classes of NMD- (dominant nega-tive) BMPR2 mutation. Moreover, themolecular pathways affected in mostIPAH patients are nearly identical to thosein HPAH patients.53 Since most IPAHpatients lack identifiable defects inBMPR2, targeting downstream signalingwill be required. In the 12 years sinceBMPR2 was identified as the primary her-itable PAH gene, substantial progress hasbeen made in understanding the signalingconsequences of BMPR2 mutation in the

pulmonary vasculature; some of theseconsequences are approaching readinessfor therapeutic intervention in patients.

BMPR2 is a 1038 amino acid singlepass transmembrane protein. Dimers ofBMPR2 in combination with dimers oftype 1 BMP receptors, usually BMPR1Aor BMPR1B, bind to BMP ligand54 andsignal directly through several differentmechanisms (Figure 2). There may be al-terations in signaling specificity based onwhether the receptor complex is pre-formed or assembles in the presence ofligand,55 and the specific type 1 receptorspresent in the complex may be important,but these details are currently poorly un-derstood. The receptor itself consists of 4domains: an extracellular ligand bindingdomain, a short transmembrane domain, akinase domain, and a long cytoplasmictail. BMPR2 is highly homologous toother type 2 TGF-� superfamily recep-tors, with the exception of its cytoplasmictail domain, which is both unique andhighly conserved across species.

The most well studied method bywhich BMPR2 signals is through phos-phorylation and activation of a type 1receptor. The cytoplasmic tail domain isdispensable for this function; both tail do-main mutations in BMPR2 and naturallyoccurring BMPR2 alternative splice iso-form B, are capable of phosphorylatingthe type 1 receptor. On the other hand, theBMPR2 cytoplasmic tail appears to beindispensable for binding and activationof at least 3 targets related to regulation ofintracellular trafficking and the cytoskel-eton, probably also requiring a functionalBMPR2 kinase domain. These targets areSRC, dynein light chain tctex-1(DYNLT1), and LIM domain kinase 1(LIMK1, Figure 2).

BMPR2 SIGNALING THROUGHTHE BMPR1A OR BMPR1BWhen phosphorylated by BMPR2, thetype 1 receptor signals through 2 distinctpathways: it phosphorylates and activatesSMAD transcription factors 1, 5, or 8, andunder some circumstances also regulatesthe TGF-� activated kinase 1 and its bind-ing protein (TAK1/TAB1) complexbridged by X-linked inactivator of apo-ptosis (XIAP).56

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When phosphorylated, SMAD1, 5,and/or 8 bind to the co-SMAD, SMAD4,and enter the nucleus to drive transcrip-tion. The most well studied SMAD tran-scription targets regulate terminal dif-ferentiation of cells, a role for whichBMP has been extensively studied in thedevelopmental literature. The BMPpathway also suppresses inflammatorymarkers including interleukin-6 andSTAT3 through SMAD-mediated mecha-nisms.57,58 SMAD proteins also regulatemicro RNA (miRNA) splicing through anontranscriptional mechanism, not requir-ing binding to SMAD4, which promotesmiRNA processing by DROSHA.59 Mi-cro RNAs are increasingly recognized asimportant signals in health and disease,including pulmonary hypertension. Thus,SMAD signaling regulates smooth muscledifferentiation state60,61 in 2 ways: by di-rect transcriptional regulation of genes in-volved in maintaining a fully differenti-ated, contractile state (in particular, KLF

genes62,63) and by regulating maturationof miRNAs, which further regulate differ-entiation state. In animal models, BMPR2mutations affecting SMAD signalingdrive smooth muscle from a contractile toa synthetic state,60 likely resulting in sig-nificant changes in cellular proliferation,extracellular matrix deposition, vascularinflammation, and mechanical stiffness.

In addition, decreased BMPR2 signal-ing through the SMAD transcription fac-tors may cause increased TGF-� signal-ing. Both pathways compete for use of thesame co-SMAD, SMAD4, and so reduceduse of SMAD4 by BMP may result inincreased availability for TGF-� signal,68

although there are hints that reciprocalregulation between the BMP and TGF-�signaling pathways is more complex thanthis. TGF-� -signaling in the lung is gen-erally regarded as profibrotic and proin-flammatory, and increased TGF-� signal-ing contributes to remodeling in someanimal models.69 Thus, decreased

BMPR2 mediated signaling may contrib-ute to remodeling by allowing an increasein TGF-�-signaling.

There are potential interventions thatcould be targeted against defects causedby reduced BMPR2 signaling throughBMPR1 receptors; however, such inter-ventions are all currently at a very earlystage of development. It would also besensible to target the increased interleukin6 and STAT3 signaling or to suppressincreased TGF-� signaling. InhaledmiRNA targeting these or other BMPR2pathways would also be logical. However,these approaches would first need to bevetted for safety and efficacy in robustanimal models, and such highly targetedtherapies are likely more than a decadeaway.

BMPR2 SIGNALING THROUGHSRC, DYNLT1, AND LIMK1In 2003, Ora Bernard’s group in Australiafound that the BMPR2 cytoplasmic tailphysically interacted with and regulatedLIMK1,70 and that when bound toBMPR2, LIMK1 function was reduced.BMPR2 mutation led to decreased LIMK1binding and increased activity. The pri-mary phosphorylation target for LIMK1 isthe actin binding and reorganization pro-tein cofilin (CFL1); we have confirmedthat BMPR2 mutation leads to increasedCFL1 phosphorylation in lungs fromBMPR2R899X mice, and this would havefunctional consequences to promote thepulmonary hypertension phenotype.

Also in 2003, Richard Trembath andNick Morrell’s group found that BMPR2colocalizes with, binds, and phosphory-lates DYNLT1.71 These functions weredisrupted by PAH-causing mutationswithin the cytoplasmic tail of BMPR2.DYNLT1 also physically interacts withboth the mitochondrial membrane perme-ability protein VDAC172 and the Rho/RAC guanine nucleotide exchange factorARHGEF2.73 We have found profounddefects in energy metabolism in BMPR2mutant mice, cells, and patients,74 and anincrease in GTP-bound RAC1 in bothwhole mouse lung, vascular smooth mus-cle, and pulmonary microvascular endo-thelium with BMPR2 mutations.75 Espe-cially because altered energy metabolism

Figure 2: BMPR2 signals through multiple mechanisms. In addition toactivation of SMAD transcription factors (green rectangles), BMPR2 signalingthrough the type 1 receptor regulates the TAB1/TAK1 complex, bridged byXIAP (green hexagons), and thence NF-kB and MAPK. In addition, the uniqueBMPR2 cytoplasmic tail binds and regulates SRC, LIMK1, and DYNLT1 (blueovals), resulting in altered regulation of actin organization, caveoli, focaladhesions, and mitochondrial metabolism.

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and Rho-kinase signaling are increasinglyrecognized as important to PAH patho-genesis in humans, our observationsstrongly implicate DYNLT1 as a func-tionally important downstream target ofBMPR2 mutations.

In summary, it seems that the BMPR2cytoplasmic tail regulates multiple criticalcytoskeletal functions. Expression arrayexperiments in mice and cells culturedfrom mice with BMPR2 mutation specificto the cytoplasmic tail show changes inmetabolic, cytoskeletal, adhesion, andmicrotubule-related genes.67,75,78 BMPR2regulation of the cytoskeleton may ex-plain the defects in endothelial barrierfunction,79 mitochondrial fission and fu-sion,80 and motility81 found in PAH.

Therapies targeted at the cytoskeletal orthe metabolic defects are either currentlyin trials, or will be ready for human trialsin the near future. Dichloroacetate is adirect inhibitor of pyruvate dehydroge-nase kinase, a key enzyme regulating gly-colysis, thus enhancing glucose oxidation,which has entered a Phase I trial for PAHat University of Alberta and Imperial Col-lege London.82 Because this addressesonly one of multiple metabolic defectsfound secondary to BMPR2 mutation, itmay not be sufficient in itself to restorenormal metabolic function in PAH, but itis a solid first step. BMPR2-related PAH,both in patients and in animal models, isrefractory to treatment: our group hastried multiple therapies on BMPR2 mutantmice with limited success. The only classof treatment that reverses BMPR2-related PAH, done independently by 2groups, is intervention targeted at re-storing endothelial cell-cell junctions.Novartis has tested the Schering-Ploughdrug SCH 527123 against PAH inendothelial-specific BMPR2 knockoutmice, and found dramatic efficacy in im-proving cell-cell junctions, reducing leu-kocyte recruitment, and reversing PAH.83

Our group used ACE2, which reversed theSRC and RAC1 defects, as well as manyof the downstream metabolic conse-quences, and achieved reversal of ele-vated right ventricular systolic pressure inmice expressing the cytoplasmic tail do-main mutant BMPR2R899X.75 Both ofthese drugs are currently in Phase I or II

trials for non-PAH conditions,84 and soare readily translatable to clinical trials forhuman PAH.

CONCLUSIONThe search for the molecular etiology ofPAH has stretched over decades, kickinginto high gear in 2000 with the discoverythat BMPR2 was the most common heri-table PAH gene. Now, therapies targetedat correcting the BMPR2 mutation itselfor at correcting the downstream conse-quences of BMPR2 mutation are rapidlyapproaching human translation, with thepromise of new, more effective treatmentstargeted specifically at the molecular de-fects that give rise to disease.

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