genetics ofb27-associated diseases 1 · genetics ofb27-associated diseases 1 j. c. woodrow fromthe...

Ann. rheum. Dis. (1979), 38, Supplement p. 135

Genetics of B27-associated diseases 1

J. C. WOODROW

From the Department of Medicine, University ofLiverpool, Liverpool

The genetic analysis of those conditions that showfamilial aggregation, in the absence of recognisablepatterns of Mendelian segregation, has proved to bevery difficult. A generalised model would allow forthe possibility of more than one disease susceptibilitygene and for the influence of environmental factors.The pedigree illustrated in Fig. 1 includes patientswith Reiter's syndrome (RS), ankylosing spondylitis(AS) and uveitis, ulcerative colitis, and psoriasis. Itcan be seen that the associations with the HLA-Blocus genes enable us to achieve an understanding ofsome genetic structure in pedigrees of this kind. Withregard to the arthropathies being considered at thismeeting, clearly the HLA-linked 'ankylosingspondylitis gene' (which may or may not be B27itself) and genes that play a role in susceptibility topsoriasis and inflammatory bowel disease have to beconsidered.

If we look first at AS itself I think we must look attwo phenomena. Firstly, there are the genetic andenvironmental factors that determine w-hetheran individual gets or does not get the disease.Secondly, there is the question of the severity.Experience in recent years increasingly suggests thatwe are dealing here with a very graded phenotypefrom mild asymptomatic radiologically detectablesacroiliitis to the full picture of severe spinal disease.

This evidence comes from the study of members ofthe families of patients with spondylitis and fromstudies of populations, particularly those made up ofB27-positive individuals.

It seems to me that clinically there is a muchgreater difference between a B27-positive individualwith asymptomatic radiological sacroiliitis and onewith very severe sacroiliitis than between the formerand a normal B27-negative individual. I think it isgoing to be just as important to ask why someB27-positive individuals get very severe disease whilein others the only evidence of disease is a slightradiological change in the sacroiliac joints. Thesource of variation in relation to both these questionsmust be either genetic or environmental or both.

If we accept for the moment that the 'spondylitisgene' is B27 itself then clearly B27 has an essentialrole in determining whether any degree of sacroiliitisor spondylitis occurs. We can produce a generalmodel (Fig. 2) in which the much increased relativerisk associated with B27-something of the order of100 to 150-is evident but which allows for furthervariation in liability within B27-negative andB27-positive individuals. This underlying variabilitycould be environmental or genetic. Evidenceof important environmental factors comes froma study of identical twins in which it can be

8 27 17

j[Reiter's syndrome spAnkyliisn A Psoriosis M Diabetes Ulcerative colitis

Fig. 1 Pedigree offamily showing association ofHLA-B alleles with Reiter's syndrome, ankylosing spondylitis andother conditions, ulcerative colitis, andpsoriasis.

by copyright. on F

ebruary 25, 2021 by guest. Protected

http://ard.bmj.com

/A

nn Rheum

Dis: first published as 10.1136/ard.38.S

uppl_1.135 on 1 January 1979. Dow

nloaded from

http://ard.bmj.com/

Suppl. p. 136 Annals of the Rheumatic Diseases

Lic'hility

Threshold

Fig. 2 Multifactorial model for liability to ankylosingspondylitis, with much increased liability in HLA-B27positive individuals.

shown that a B27-positive identical twin of a severespondylitic may be, as far as we can tell, completelyfree of disease.95 Fig. 3a, b shows the sacroiliac jointsin a pair of identical B27-positive twins, one withgrade 4 sacroiliitis and the other clinically andradiologically normal.

There are several ways of looking at the geneticsof AS in relation to B27. We can look to see if therisk for homozygous B27-positive individuals is thesame as that for heterozygotes, and I think itprobably is. We can look at the association of B27and spondylitis in different populations. Previously Ihave suggested that the relative risk in B27-positiveindividuals is apparently constant in differentpopulations* in spite of great variation in theincidence of B27.327 We can look at the incidence of*Other contributors have data at variance with thisstatement-Editor.

spondylitis, firstly, in B27-positive individuals in thegeneral population and, secondly, in the families ofaffected probands. By relating these we can attemptto dissect out the genetics of the disease a littlefurther.

Table 1 presents data from four studies of popula-tions of apparently healthy B27-positive blooddonors. 54 64, 236, 315 Although there is some overallagreement about the incidence of sacroiliitis orspondylitis-that is, of the order of 20 %-there areimportant differences. No doubt there is appreciableselection bias here, but among our own subjects wefound only one male who had suggestive symptomsand definite radiological changes. The remaining fivemales had very mild radiological changes and norelevant history. We found nothing in the females.This contrasts with the study from Calfornia,54

Table 1 Incidence of sacroiliitis or spondylitis infour studies ofB27-positive individualsAuthors B27- positive Spondylitis

subjects

Definite Lesserchanges

No. M. F. M. F. M. F.

Calin and Fries54 78 30 48 6 (20%) 7 (14%) 2 4Cohen et al.64 24 24 - 3 (12-5%)- 3 -

Truog et al.315 44 8 (18 %) - -Nichol andWoodrow236 48 33 15 1 (3 Y.) 0 5 0

(3a)

by copyright. on F


http://ard.bmj.com

/A

nn Rheum



nloaded from

http://ard.bmj.com/

IV. Aetiopathogenetic features Suppl. p. 137

(3b)Fig. 3 Appearance of sacroiliac joints in a pair of identical twins aged 52 years. one showing grade 4 sacroiliitis(a) and the other normal appearance (b).

which puzzles me a little because of (a) the highincidence of what seems to have been severe disease,and (b) the relatively high incidence in women.

There are obvious problems here because it isextremely difficult to get a completely randomsample of B27-positive individuals. I believe we are

all selecting a possibly biased population when we dothese studies, and perhaps we have been selecting indifferent ways.

If we accept, for the sake of further argument, that20% of B27-positive males get some degree ofdisease there are three simple independentpossibilities that can be put forward. (1) That B27 isitself the 'spondylitis gene' and that 20% ofindividuals are exposed to the appropriate environ-mental stimulus. (2) That B27 is the 'spondylitisgene' but that 20% of individuals inherit theadditional gene or genes necessary for the fullgenotype. (3) That the 'spondylitis gene' is not B27but is in linkage disequilibruim with B27 to theextent that it occurs on 20% of chromosomes havingB27.

Evidence for the first possibility comes from thestudy of identical twins previously referred to. Somesuggestive evidence for the second possibility-that

is, that there are other interacting genes-comesfrom families of the type shown in Fig. 4. The father,who has a brother with severe spondylitis, is notB27-positive but may have supplied a gene or geneswhich interacted with B27 from the mother toproduce the full genotype in the affected son. I haveseen other families like this, and in theory anextensive family study could be designed to provewhether this hypothesis is true.

Evidence for the third possibility has been putforward by those who describe families in which thereis disassociation of spondylitis from B27. It would,

'(Behets disease)

" 0 D09,21 9,2710,16 3, 5

A9,279,21

* Ankylosing spondylitis

Fig. 4 Pedigree offamily of B27-positive proband withankylosing spondylitis. The B27-negative father has abrother with severe spondylitis.

by copyright. on F


http://ard.bmj.com

/A

nn Rheum



nloaded from

http://ard.bmj.com/


however, be premature to talk in terms of recom-bination here without further evidence. In our ownfamily studies, with one interesting exception, over30 affected relatives of B27-positive (affected)probands were themselves B27-positive. If thehypothesis were true we should now have entirelyB27-negative families in which there are two or morespondylitics. Some suggestive evidence for theassociation between B27 and the Bf polymorphismhas been briefly reported from Iceland,16 but thisneeds confirmation.

If we now turn to family studies we can ask whatare the implications of these simple hypotheses forthe expected incidence of disease in B27-positiverelatives of probands (Fig. 5). We will look at theparents of spondylitic patients and presume aB27-positive father, because this largely avoids theproblem of the variable age of onset and diseaseexpression according to sex. For each of the followingmodels we can suppose an expected frequency ofdisease.Assuming, on the first model, that there is no

particular environment relevant to AS shared by themembers of the family over a considerable period oftime, we would expect something like 20% of fathersto have some evidence of disease-that is, nodifferent from the risk for B27-positive individuals inthe general population.On the second model the incidence in fathers

would depend on the genetic behaviour of theadditional genes. These are unlikely to be recessive.Therefore on a simple basis one might suppose anindependently segregating gene of dominant effectwhich would be present in the B27-positive father in5000 of cases, and therefore we would expect that500% or thereabouts of the B27-positive fathers tohave the disease.On the third model we would expect all the fathers,

if positive for B27, to have the disease, assuming alow rate of recombination.We have examined 21 B27-positive fathers.

Five have definite ankylosing spondylitis and fivehave only radiological sacroiliitis (Fig. 5). Thus 50%of B27-positive fathers had evidence of sacroiliitis orspondylitis. This is higher than expected on the first

(? 201/.) (? 50./o) (? 100°/o)27 27 27D--

27 27X 27 AS27+ environmental 27+ gene(s)X ASgene linkfactor (? in 201.) (? in 201/.) to 27 (? in 20°Liverpool study AS Radiological sacroil

21 B27- positive fathers 5 5(22 mothers 1 0

model, and suggests that being B27-positive andexposed to an environmental factor is not the solebasis for development of disease.The fact that the observed incidence fits the second

model raises two difficulties. Firstly, that there is noroom for an environmental factor, and, secondly,that identical twins (with the full presumed twogene-genotype) do not always get detectable disease.Out of 15 reported pairs nine have been concordantfor AS and six discordant. (However, the problemsassociated with the ascertainment and selectivereporting of disease in twins must lead toconsiderable caution in drawing general conclusionsfrom the concordance rate.)

If, nevertheless, we accept these data as they stand-that is, that only 600% of those with the rightgenotype get disease-then on the second model wewould expect 300% of B27-positive fathers to showdisease in some degree. If we turn to the findings inrandom B27-positive individuals the implicationwould be that an independently segregating gene ofdominant effect would have to have a gene frequencyof 0-2 to give a 20%0 incidence of disease in thisgroup.The expected incidence on the third model-that

is, a 'spondylitis gene' in linkage disequilibruim withB27-is clearly too high to fit in with the observedincidence.Combining the first and third models, and again

assuming that an environmental factor occurs in60% of individuals, the data do not fit too badly. Theimplication when we come to the incidence ofdisease in B27-positive individuals in the populationis that the second 'spondylitis gene' would have to bepresent on 35 %-40% of chromosomes that have theB27 gene.

In summary, none of these simple models will bythemselves explain all the available evidence. Thedata in regard to concordance in identical twins andto the incidence of disease in B27-positive relatives ofprobands and in B27 random individuals are allsubject to the dangers of biased ascertainment. Ihope that with the collection of further data in thesefields a more definitive genetic analysis may bepossible.

Fig. 5 Expected incidence of sacroiliitis orspondylitis in B27-positive fathers ofprobands with

,/d) spondylitis based on simple genetic models. Theactual Liverpool study findings (Nichol and

liitis Woodrow, unpublished) are shown.

by copyright. on F


http://ard.bmj.com

/A

nn Rheum



nloaded from

http://ard.bmj.com/


You will notice that in the mothers in the Liverpoolstudy (Fig. 5) there was a much lower incidence. Ithink this supports the general experience that thedisease is much less common in women and does notsupport the thesis of Calin and Fries54 in this respect.The pedigree shown in Fig. 6 gives food for thought.One of two B27-positive identical twins has severespondylitis and the other has asymptomatic radio-logical sacroiliitis. A further brother, also B27-positive, is clinically and radiologically normal. Yetall three have produced severely affected children.None of us can be certain at present of the genetic

structure of AS, but the evidence I have put forwardand the fact of the enormous variability in expressionof the disease itself should make us think of otherpossible genetic factors. Of course, variation inexposure to the relevant environmental agents couldalso account for variability of expression.

It is interesting to consider the relationshipbetween sacroiliitis or spondylitis and inflammatorybowel disease (IBD) and psoriasis, each of which hasits own genetic predisposition. The facts that 40% ofpatients with spondylitis and IBD are B27-negative,that two groups of workers79 161 have shown thatthere is a considerable incidence of asymptomaticsacroiliitis in B27-negative individuals with IBD, andthat it can be estimated that about 50% of B27-positive patients with IBD get spondylitis (which ismore than expected from controls) point to genesunderlying IBD giving rise to susceptibility tosacroiliitis and spondylitis. Two pedigrees areexceptional in our studies. The first (Fig. 7) showsour only case of disassociation between B27 andspondylitis within a family but ulcerative colitis ispresent in the B27-negative spondylitic. The second(Fig. 8) shows two brothers with spondylitis, bothB27-negative but one with ulcerative colitis.There is little doubt that genes underlyingI t o1 t2

III 42,27 2,w15 2 73.7 9,w16 40

2.27 I'2, 27 2279,17 2,w15 2,8

J Ankylosing spondylitismf Radiological sacroiliitisFl Uveitis

Fig. 6 Pedigree offamily including three B27-positivebrothers of whom two are identical twins. They showgreat variability of expression ofspondylitis but all haveaffected children.

ASUc

27+ 27- 27-

Fig. 7 Pedigree showingfather with B27-positivespondylitis and three siblings-two B27-negative, onewith spondylitis and ulcerative colitis, and a third withulcerative colitis alone.

I

II23,5 3,527 9sw35

Ankylosing spondylitisj Ulcerative colitis

Fig. 8 Pedigree showing two B27-negative brotherswith spondylitis, one also having ulcerative colitis.

susceptibility to psoriasis play a role in the geneticsof some cases of sacroiliitis and spondylitis. About30% of patients with spondylitis and psoriasis areB27-negative. Our analysis94 suggests (a) that genesfor psoriasis provide susceptibility to peripheralpsoriatic arthritis and to sacroiliitis or spondylitis,and (b) that B27 also provides susceptibility to thesetwo patterns of joint involvement. Some of the casesof spondylitis with psoriasis are, I think, the purelycoincidental occurrence of the two diseases in thesame patient. The pedigree shown in Fig. 9 illustratesthe probable interaction of B27 and genes forpsoriasis in predisposing to peripheral psoriaticarthritis.

_2 27 (b....I.IIAC,.9, 222 273,14/ Uveitis

O PsoriasisPsoriaticc'rthritis

Fig. 9 Pedigree showing separate inheritance ofB27and genes for psoriasis, the proband having peripheralpsoriatic arthritis.

by copyright. on F


http://ard.bmj.com

/A

nn Rheum



nloaded from

http://ard.bmj.com/


RS has important genetic relationships with bothspondylitis and psoriasis. We would expect RS andspondylitis to occur in the same individuals and inrelated individuals to a degree greater than chanceexpectation. This is in fact the case. The pedigree(Fig. 10) shows an example in a woman whodeveloped chlamydia-positive RS on her honeymoonand who shares the same haplotype, including B27,with an uncle and cousin who have spondylitis.Lawrence's studies confirm the supposition.199

This must lead to a modification of the way we

look at sacroiliitis or spondylitis and RS. Findingthe former does not mean that it must be a complica-tion of the latter. Six of my patients already hadevidence of established spondylitis when they hadtheir first attack of RS. Indeed, we would expect atleast 15 %of patients with RS to develop sacroiliitis orspondylitis at some stage independently of the RS.

In addition to spondylitis, B27-positive individualsmay get any one or more of the following more

episodic diseases: lower limb arthropathy of a

characteristic type; RS; arthritis reactive tosalmonella, shigella, yersinia, and possiblybrucellosis154; and anterior uveitis. These may occur

at any time before or after the onset of the spondylitisand in varying combinations in the same patient. Ihave seen one patient who has had both shigellaarthritis and sexually acquired RS, the genetic basisbeing the same with the environmental agent beingdifferent.The incidence of spondarthropathies in B27-

positive individuals exposed to various environ-mental stimuli is interesting (Table 2). In thoseconditions that appear to follow a clear-cut infectivestimulus a near 20% incidence of arthritis is ratherstriking and inevitably suggests a common basis.

II~~~~~~~~~~~~~~I )1 2b

II 2,w15]t1w32 w40 w32,w40 2,w15

IV 2) 3

-,18 2,w15 w30/31,27w30/31,27 w32N,w40 28,w35

[ Reiter's syndrome 3 Ankylosingspondylitis

Fig. 10 Pedigree showing a B27-positive female withReiter's syndrome whose uncle and cousin haveankylosing spondylitis and share the same haplotypeincluding B27.

Table 2 Incidence of various arthropathies in B27-positive individuals calculatedfrom published data

Clinlical syndrome Authors % B27-positiveindividuals affected

Ankylosing spondylitis Calin and Fries54 20-25Cohen et al.64Truog et al.315Nichol and Woodrow236

Salmonella arthritis Friis and Svejgaard12' 20Hakansson et al.144Aho et al.3

Shigella arthritis Calin and Fries54 20NSU arthritis Keat et al.175 20

Within each group all individuals have had the sameenvironmental stimulus. The occurrence of diseasein only 20% of B27-positive individuals must beattributed to other genes being necessary for sus-ceptibility or, as mentioned before, to the 'sus-ceptibility gene' being not B27 but close to this Blocus and present on 20% of chromosomes havingthe B27 allele.We now can relate this to the incidence of

spondylitis in B27-positive individuals. I think wemust be careful about concluding that there is acommon mechanisms here, because this wouldimmediately imply that all B27-positive individualsare exposed to the relevant agent. The evidence fromidentical twins suggests the contrary. Moreover, theimplication would be that all who develop thesepostinfective arthropathies must get sacroiliitis orspondylitis in some degree. This seems unlikely onthe evidence. In other words, the genetic mechanismfor these postinfective arthropathies may differ fromthat for spondylitis.We have, I think, made progress in understanding

the genetic structure underlying these disorders.Much more needs to be done. In particular, furtherdefinitive family studies should help to clarify someof the present uncertainties.

General discussion

DR. E. ALBERT: It is difficult not to be impressed bythe many families that Dr. Woodrow has shown.Indeed, one of the best ways of identifying previouslyrecognised or new clinical entities is to study families.We have seen beautiful data from Professor Bitter'scomputer analysis rounding-up a neat clinical picture.I would like to see such a disease segregate in one ortwo families with that particular expression. Onlythen would I say that we have a new disease entity.Another issue, the genetic background of B27-

associated diseases, raises the major questionwhether B27-associated gene(s) are different whencoding for such different diseases as AS, or RS, orreactive arthritis, and so on. Let us assume that they

by copyright. on F


http://ard.bmj.com

/A

nn Rheum



nloaded from

http://ard.bmj.com/


are different (for this hypothesis is easier to test). Adifferent gene for RS, for AS, and for reactivearthritis would call for allelic forms of a diseasesusceptibility gene which is associated with B27.This seems unlikely to me. Indeed, ifwe had differentgenes, different allelic forms, different susceptibilitycoding associated with B27, each causing a differentform of rheumatic disease, then within a given familythe disease expression and set of rheumatologicalsymptoms should be constant.

Alternatively, if we have one gene that broadlydetermines reactivity to all sorts of environmentalrisk factors that cause the disease these will deter-mine, to a certain extent, how much variability weare going to see expressed within a population andwithin a family. But there again the degree ofvariability of disease expression is going to be bestillustrated in monozygotic twins. If environmentalinfluences play a major role we expect discordance.However, we would expect a high degree of con-cordance if the environmental impact was rathernegligible and the gene played the major role.Dizygotic twins should be very different if bothenvironmental conditions and genetic coding areoperative.

Dr. Woodrow has shown us discordant pairs ofmonozygotic twins. That at once indicates that wehave a strong environmental influence in theexpression of the disease. That in a way-I amsorry, Dr. Woodrow-invalidates right away someof the models that you have produced. You don'tactually have to assume the action of a secondgene if a father of a B27-positive ankylosingspondylitic child either lacks the B27 or does nothave AS. You don't have to assume that there is adifferent gene inherited by the child, you don't haveto assume that there is a second locus involved. Tome, the picture is somewhat more simple. I thinkthat we are dealing with one gene that determines thebroad susceptibility and that we are dealing with agreat variety of environmental influences (chlamydia,mycoplasma, etc.) that may or may not influencemore or less strongly the expression of a disease.For that reason I presume RS and AS are based on

the same genetic mechanism. Indeed, we haveexplored the various possibilities of another geneaction (Fig. 11) but, by and large, we do not have anyevidence for such a hypothesis-either for an allelicform of B27-associated gene or even for cross-

Centromere PGM3 GLO HLA-D BFHLA-B HLA-C HLA-AI

C2, C4, RG,CH

Fig. 11 Mapping of loci on human chromosome C6.

reacting antigens such as B7, B22, B13, and B40. Onthe other hand, we have looked at the distributionof B27-positive haplotypes associated withankylosing spondylitis-that is, a certain degree oflinkage disequilibrium with an antigen of the A locus(as had been reported by Terasaki)-but we did notfind any evidence. We have not ourselves searchedfor a gene or an allele in the D locus, but the UCLAgroup (with Dr. Bluestone) has not found any suchassociation.

Finally, Dr. Woodrow has mentioned the studyof the Bf locus in AS.16 This is not all that puzzlingto me. Why should there not be a linkage disequili-brium between the AS-associated B27 gene and a Bftype found in that Icelandic population? However,we have no direct information for a second gene.One other definitely genetic factor I haven't

mentioned is the sex, which directly or indirectlyinfluences the expression ofAS and perhaps RS. I donot believe that-as Cepellini used to say-womencomplain less about their back and therefore we seefewer clinical cases of AS in women. I would ratherthink that the sex has a major influence on theexpression of the disease on the one hand, and, onthe other, despite dominant inheritance,183' 312 wemight actually be dealing with a very low degree ofpenetrance and hence a major environmentalimpact. Thus the B27-positive son of a B27-positivediseased father does not have to have the disease.From the dominant mode of inheritance of the B27-associated susceptibility genes we conclude that thepathogenesis of, for example, AS is based on a 'toomuch' rather than a 'too little', while in recessivediseases we usually have a lacking gene product andone of the two genes required for the product aremissing. A dominant disease, on the other hand, isusually caused by a gene that codes for a pathologicaldevelopment such as we see, for example, inankylosing spondylitis, but we could also assume thatthe dominant gene codes for too little co-ordination,for example, or defective immune reactivity tobacteria.

by copyright. on F


http://ard.bmj.com

/A

nn Rheum



nloaded from

http://ard.bmj.com/

genetics ofb27-associated diseases 1 · genetics ofb27-associated diseases 1 j. c. woodrow fromthe...

Documents