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Genetic factors in systemic lupus erythematosus
Genetic Factors
in Systemic
Lupus Erythematosus
By A. S. Russell
C
AN systemic lupus erythematosus (SLE) be regarded as a homogeneous disease? As dermatologists, nephrologists, hematologists, and rheumatologists, our approach to SLE in the past has been reminiscent of the blind man to the elephant. In recent years this descriptive dilemma has been resolved, and we have slowly framed a coherent clinical and laboratory picture of the disease. Despite this undoubted progress, it is not at all certain that the attempt to fit the disease into a uniform phenotype is a valid one. One of the many advantages of genetic analyses is that they can help provide answers to such questions. Answers based on clinical features alone can be misleading, both in suggesting differences where similarities are more important and, perhaps more commonly, suggesting uniformity where crucial differences exist. Marfan syndrome provides an example of the first type of error. It is a heritable disorder of connective tissue in which the mode of inheritance has been established in family pedigrees as an autosomal dominant. Expression of that disorder is variable, and even within one family it may be present as minor arachnodactyly in one member and the complete syndrome in another.’ It has a moderate penetrance in that no manifestations whatsoever may be seen in a member of one generation, but many or all of the features may be evident in the offspring. This presumably reflects the influence of other genes, and the example illustrates that a major variation in disease expression may still reflect one underlying defect. A similar finding may be seen in the well characterized single amino acid defect of sickle cell disease. If a subject who is homozygous for this condition also has a trait causing persistence of synthesis of fetal hemoglobin, the ensuing sickle cell disease is much less severe in its expression.* Thus, genetically homogeneous diseases may have different appearances, that is, a single disease genotype may have varying phenotypes. The obverse of the coin may be even more relevant to SLE-phenotypically similar diseases may be related to different genotypes. For example, the San Felippo syndromes A and B3 Seminars
in Arthritis and Rheumatism, Vol. 10. No. 4 (May), 198 1
differ in the enzyme defect that is inherited, and yet clinically they are identical. Rapid gastric emptying or increased blood levels of pepsinogen I, both genetically controlled abnormalities, may present simply as one “disease’‘-peptic ulceration. This type of genetic diversity is so common, McKusick put forward the dictum that a genetic disease that appears at first to be homogeneous, will be found to comprise two or more separate and distinct entities on further study. It seems very likely that this is the case with SLE. SLE can already be divided into clinical subgroups:4 (1) drug-induced disease, (2) disease associated with complement deficiencies, (3) mixed connective tissue disease, and (4) other SLE. It is quite possible that the further division of the SLE syndrome on the basis of clinical and serologic criteria will continue. While there may be genetic or environmental factors in common between all the groups, the improved analysis that results from subgrouping may help delineate a role for separable genetic factors, as has occurred, for example, in diabetes mellitus.5 One of the problems to be faced in assessing the role of genetic factors in SLE or, indeed, in many other diseases, is in deciding which individual can be designated as having the disease. This is particularly a problem where (1) the disease is probably heterogeneous, (2) there are no clear and specific criteria that can be used to define affected subjects under varying epidemiologic conditions, (3) the variability of age of clinical expression makes it difficult to exclude the possibility that the disease may develop in a given individual at a later age, and (4) there is almost complete ignorance of the basic underlying disorder(s). Some of these factors are particularly important in trying to determine the mode of inheriFrom the Rheumatic Disease Unit. Department of Medicine, University of Alberta, Edmonton. Alberta, Canada. Address reprint requests to Dr. A. S. Russell. Rheumatic Disease Unit. Department of Medicine, University of Alberta, Edmonton, Alberta, T6G. 2G3. Canada. 0 I981 by Grune & Stratton, Inc. 0049-0172/81/1004-0002$01.00/0
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A. S. RUSSELL
tance once a role for genetic factors has been established. If we knew what basic defects or disease processes more closely reflect the genetic component, then we could perhaps ascertain them directly. The inheritance pattern of the development of frank peptic ulceration remains unclear if this aspect is studied alone. In those families with peptic ulcer and hyperpepsinogenemia I, a study of the increased blood levels of pepsinogen I discloses a clear pattern of dominant inheritance.6 Perhaps the most important single factor in evaluating evidence provided by the types of approach detailed below is the stringency of the selection and assessment of the control populations. Even with reliable data, problems with interpretation may still persist. Genetic factors will normally be reflected in a familial aggregation of disease, but the same phenomenon can be seen with social and environmental factors. In this context, it should be remembered that because of a familial aggregation of the conditions leading to nicotinic acid deficiency, pellagra was, at one time, thought to be an inherited disease. Chronic alcoholism is familial and the controversy between genetic and environmental influences still rages. The Bittner agent-now known to be a virus-is responsible for inducing breast cancer in mice. It was the discovery that infection is transmitted by breast milk that enabled it to be distinguished from a genetic disease. It is unwise to consider a role for genetic factors and to evaluate the significance of genetic models on statistical grounds alone, without the relevant clinical context. To illustrate the problems that can arise, Lillienfeld used genetic models based on family studies and was able to show that attendance at medical school was consistent with the inheritance of an autosomal recessive gene.’ Evidence relating to the inheritability of SLE will be considered in the following paragraphs. POPULATION STUDIES SLE Prevalence
and Racial Variation
SLE appears to be an uncommon disorder. With no simple, reliable technique for mass screening to allow testing of whole populations, less reliable estimates have been used. The data of Siegel and Lee’ were based on the assumption
that all patients would eventually require hospital or clinic care because of the severity of their illness and that when there, they would be correctly diagnosed. They found an increased prevalence of spontaneous-but not drug induced-SLE among the nonwhite population of Manhattan. The prevalence in whites was 8.7 x 10e5 and in nonwhite 38.8 x IO-‘. On the basis of additional stratifications, they could not account for this by environmental differences. A recent study in Hawaii,’ which was based on medical record discharge diagnoses, found 107 patients with definite SLE while the ageadjusted prevalence rates for whites were 5.8 x 10m5, for Chinese 24 x lo-‘, and for Japanese 18 x 10 -‘. In a study of 75 North American Indian tribes using inpatient records that were checked and diagnosed according to the American Rheumatism Association (ARA) criteria, the prevalence of systemic lupus was roughly in accord with that of other studies in the US.” However, for three tribes, the prevalence was higher, and for full-breed Sioux, it was 31 x 10--5, but for half-breed Sioux it was only 21 x 10m5. The highest prevalence study reported to date is from the Kaiser Permanente Foundation in California, which recorded a prevalence in adult black females of 400 x 10m5.” Associations
of SLE With Known
Genetic Factors
Gender While female gender is a genetically determined trait associated with SLE, the evidence suggests that it is hormonal factors that are responsible for the female predisposition to this disease: (1) The sex ratio of patients with onset of disease prior to puberty or late in life shows a lower prominence of females.‘* (2) Eight patients have been described with both SLE and XXY Klinefelter syndrome,‘-’ that is, a decreased androgenic stimulus and probable increased estrogen formation. (3) The data from Lahita et al. at the Rockefeller Institutei demonstrate an increase in 16-hydroxyestrone in females and males with SLE. (4) The increased susceptibility of castrated male BW mice to SLE and the amelioration of the disease in females by testosterone implants.15 In this context, one has to bear in mind that the major histocompatibility complex in mice is associated with gene loci that may influence sex
GENETICS OF S.L.E.
257
hormone levels, as well as the end-organ sensitivity to some of these agents.16 One of the few reported pairs of monozygotic twins who were discordant for SLE may illustrate this hormonal factor.” The “resistant” twin had a total oophorectomy 16 yr prior to the onset of SLE in her twin. Despite a chronic biologic false-positive STS, no other evidence of SLE was present 7 yr after it developed in the twin. Acetylator Phenotype The ability to rapidly acetylate and remove drugs characterized by the presence of an aromatic amino group is inherited. Rapid acetylation is dominant,” and there is an increased proportion of patients with a slow acetylator phenotype in hydralazineand procainamideinduced SLE.19 Several studies have found a similar result in patients with spontaneous SLE.*’ This remains in question,*’ and there is a tendency to report positive results, but it seems probable that this observation, while not uniform, may be a further indication of an underlying disease heterogeneity. Immunoglobulin
Allotypes
Some genes controlling immune responses in mice are also associated with allotypic markers on IgG. These loci are not associated with H-2 or HLA. After the initial studies of Gm markers in rheumatoid arthritis drew a blank, interest waned. It is now reviving and there has been an association demonstrated between neuroblastoma and a specific Gm group,** and a similar finding has been described in patients with myasthenia gravis.23 Furthermore, there is early evidence that Ig allotypes Km1 are associated with increased antibody response after immunization with meningococcal polysaccharide.24 The area of immunoglobulin allotypes has not yet been explored in SLE. Complement Deficiency There are now many reports of SLE or a “lupus-like disease” occurring in association with several types of complement deficiencies.*’ The proportion of patients with homozygous C2 deficiency that have SLE is impressive; nevertheless, initial suggestions that this might be an artifact of selection are not entirely disproven, as some of the patients were ascertained because of their SLE and referred
because of their low complement level. On the other hand, the numbers of such homozygous deficient patients present within a population of patients with SLE, although small, seem above that of a random population sample.*’ There are, in addition, a number of reports to suggest that the spectrum of lupus seen does vary somewhat from the classical mould.26 The C2 loci are linked to the major histocompatibility complex and, while it seems likely that the association with SLE results from the effects of the complement deficiency, it is possible that the C2 null gene is acting as a marker for a disease susceptibility gene. One report suggested that the heterozygous C2 deficiency is also associated with SLE.*‘j This would argue against the requirement for a major functional defect in the complement pathway, but the statistical calculations to support this important observation are based on relatively small numbers and require substantiation. The relationship to other deficiencies of complement, many of which are not HLA related, is less clear-cut but still highly suggestive. Several associations have been reported between SLE and the Cl esterase inhibitor deficiency-hereditary angioedema.25 Patients with this latter disorder have decreases in C2 and C4 varying in both degree and duration, depending on the severity of the disease. This defect was found in a twin with SLE. The other monozygotic twin had a much less severe reduction in C2 and did not have SLE” (additional details in ref. 25). This single twin study suggests that the role of the complement deficiency is not a genetic one. It is possible that decreased complement levels may allow undefended entry of some environmental factor responsible for the disease. Lennon made the suggestion that as regulatory antiidiotype antibodies require complement in order to eliminate abnormal lymphocytes bearing that idiotype, the absence of complement might instead allow stimulation rather than elimination of such cells. Thus, complement deficiency could, in this way, markedly affect immune regulation.28 HLA The demonstration of an association between various postulated human disease susceptibility genes and specific HLA antigens does not explain the mechanism underlying this associa-
258
tion, although there is no shortage of hypotheses. Nevertheless, this type of demonstration is important. The presence of such a marker for a disease susceptibility gene not only facilitates the detection and study of other disease susceptibility factors, it also allows the dissection and better definition of clinical syndromes. Thus, the clinical validity of the subgrouping of patients with diabetes mellitus, chronic active hepatitis, and juvenile chronic arthritis can now be supported by the results of HLA typing. SLE was one of the first diseases to be investigated by HLA typing. The original study showed an increased prevalence of both HLA-B8 and HLA-Bwl 5.29 Other reports have, in general, supported the increased prevalence of B8 at least in whites4 Recent work has suggested that DR3-in linkage disequilibrium with B8-may be more closely associated with lupus than B8 itse1f.30 This has, of course, proved to be the pattern in the other B8-associated diseases, including Sjogren syndrome. Rather than using HLADR-specific antisera, another approach has been to use less well characterized antisera with a broader anti-B-cell specificity and to determine whether any show a particular relationship to the antigens on cells from patients with SLE when compared with contro1s.3’.32 One such serum marked 70% of SLE lymphocytes compared to 40% of those of a control popu1ation.3’ Absorption experiments suggested that at least two populations of antibodies were involved, probably related to determinants DR2 and DR3.3’ An advantage of this type of broad spectrum antiserum is that it may disclose other allelic groups or supergroups, and it is possible that the new HLA antigens on the MB or MT series may be involved. A final recognition that both DR2 and DR3 are involved would suggest a genetic basis for disease heterogeneity that could be more closely examined at a clinical or laboratory level. Hawkins33 and others have made the interesting observation that in their patients with SLE, the haplotype Al-B8 appears to be significantly associated with severe lupus, whereas the antigens A2-B7 (not as a haplotype) are found in increased frequency in mild lupus. This could provide an interesting corollary to the above findings with DR2 and DR3. In contrast to the above results with spontaneous SLE, hydralazine-induced lupus has been shown to be closely
A. S. RUSSELL
associated with HLA-DR4. Seventy-three percent of patients with this disease carried this antigen, compared to approximately 25% of patients with spontaneous SLE.34 This clearly demonstrates a basis for genetic heterogeneity between these two major subgroups of SLE. If we postulate one or more lupus disease susceptibility gene(s) situated near the major histocompatibility complex, we must not let the hypothesis that it reflects an immune response gene blind us to the possibility that it could exist well outside the designated HLA region. Thus, if multiple sclerosis can be regarded as a homogeneous disease with a disease susceptibility gene associated with the MHC and linked to Dw2, this gene is probably situated some distance from the D 10~~s~~and much closer to PGM,, that is at a distance of 20 recombination units from HLA. Studies including an assessment of linkage with GLO and PGM, allotypes are therefore important. FAMILY STUDIES
Familial Aggregation
of Disease
Despite the reservations discussed earlier, family studies generally provide the initial evidence of the importance of genetic factors. The clinical observation that SLE seemed to occur in some families with an unexpected frequency was put forward by Davis,36 and Adams3’ in the early 1950s. Subsequent findings have been reported by Arnett and Shu1man.38 The prevalence of SLE in family members of an affected proband will depend on the severity of the criteria used to affirm the diagnosis in these relatives. Thus, in Marfan syndrome, minor manifestations of the disease are recognized only because of the relationship with an affected proband. In the absence of this demonstrated relationship, the relative might have been considered a normal variant. Similarly, the presence of a close relative with SLE will cast a different light on a subject with otherwise unexplained synovitis and a positive antinuclear antibody (ANA). In pedigree analysis (intrafamilial genetic studies), it is probably reasonable and important to designate such a subject as “affected.” In studies setting out to demonstrate familial aggregation, it is important not only that the same standard criteria be used for the relatives
GENETICS OF S.L.E.
of an affected
proband as for the control population, but that ideally, the control population should be selected in the same way as the lupus families because of a primary relationship with a nonaffected proband. This approach has not been adopted in any studies to date. Estimates of the observed prevalence of SLE, in the firstdegree relatives of a lupus patient have been quoted as approximately 1%,39 and while they seem convincingly higher than would be found in a control population, the hard data necessary to demonstrate this with any degree of conviction are not available. Pedigree Analysis
Mode of Inheritance of Disease or Defects With an uncommon disease, intrafamily studies have an advantage in that they should largely eliminate the possibility of disease heterogeneity. To date, there have not been enough family studies to allow answers to specific questions, to avoid ascertainment bias, and certainly not to infer any mode of inheritance. Many of the early studies were of families with affected siblings and normal parents,40 which would tend to maximize the proportion of patients with an autosoma1 recessive mode of inheritance. Recent studies include more families with multiple generations, which would maximize a dominant mode. However, from the data currently available, it is clear that the disease can exist in siblings who have normal parents, and in all four types of single parent-child combinations.38 To date, there is no evidence to suggest increased consanguinity in the parents of patients with SLE although this is an area that has not yet been formally examined. It is possible that several modes of inheritance may be found, once again reflecting disease heterogeneity. More data are required in relation to the patterns of inheritance of the disease, but particularly in relation to the patterns of inheritance of possible basic defects that may lead to disease expression. Basic defects. In an exciting attempt to identify a genetic defect of basic importance, Miller and Schwartz looked at the integrity of an immune suppressor cell system in both patients and relatives of patients with SLE.4’ They incubated peripheral lymphocytes with either pokeweed mitogen (PWM) alone or with PWM plus
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Con-A and measured the reduction in response, that is of IgG synthesis, consequent on the introduction of the Con-A. After assessment of the normal suppressor response, they showed that 11 of 15 patients had a decreased response, more than 3 SD below the normal mean. A depressed response was found in 13 of the 50 first-degree relatives; 12 of these 13 were female, compared to 12 of 37 of the remainder. No correlation with lymphocytotoxic antibodies was found nor, seemingly, with ANA, although the number of positives was small. Serologic abnormalities: ANA. Early studies found a mild but significantly increased prevalence of ANA and of hypergammaglobulinemia in first-degree relatives of lupus patients. The prevalence of ANA generally ranged between 4% and 14% (reviewed in ref. 42), although one report was presented that described it in 33%. Some additional studies did not include even a rudimentary “control” population. A recent study of several large families found an ANA prevalence of 55% in consanguineous relatives.43 In one of the largest families, 20 of 40 relatives had a positive ANA and while most of the positives were over 20 yr of age, 14 of the 20 negatives were under 20 yr. It was also seen in 50% of nonconsanguinious relatives of probands with SLE, suggesting a nongenetic influence and illustrating the dangers of using spouses as “controls.“43 The high prevalence of ANA found in this study was unusual and may result from specific selection of these families because of the presence of multiple cases. More recent studies on smaller families by Miller and Schwartz4’ and Raum et a1.44found a prevalence similar to that seen in the earlier studies. Lymphocytotoxic and anti-RNA antibodies. In someJ5 but not a1144studies, these antibodies have been found, not only in relatives of patients with SLE, but in household contacts, providing further evidence for the importance of nongenetic factors. Association with Genetic Markers HLA. Recent pedigree studies43 suggested an association between susceptibility to SLE and the inheritance of a particular HLA haplotype, although the specific HLA markers differed for each family. In the largest family, several individuals were seen with the designated HLA
A. S. RUSSELL
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haplotype but without SLE, and one was noted to have SLE even in the absence of the HLA marker. This latter could be easily explained by postulating a recombination event between a lupus disease susceptibility gene and the HLA markers. Some evidence exists to suggest an increased prevalence of recombinations in patients with connective tissue diseases.46 An alternative explanation is that this may simply reflect the difficulty of trying to assess linkage between a genetic marker and a multifactorial trait. The interesting possibility also exists that in some families, or with some “types” of lupus, the disease may be HLA linked, but that statistics on groups of families might be diluted by those families or types where it is not so linked.
TWIN STUDIES
Studies of the relative disease concordance in monozygotic and dizygotic twins provide the most cogent evidence of disease heritability. Recently, seven additional pairs of definite monozygotic twins were described, four of which were concordant for SLE,42 all using ARA criteria for the diagnosis of the disease. Of the three seemingly discordant pairs, in one pair, the “normal” twin had been followed for 1 yr since the onset of disease in her twin and had marked serologic abnormalities. The second pair had been followed for 2 yr and the third for 6 yr, and had persistent leukopenia (approximately 2000). It seems possible that at least the first of these “normal” twins might have developed a full expression of SLE if observed over a longer period. There are two other reports of definite monozygotic twins discordant for SLE where follow-up was 7 yr in each case (see ref. 42 for a comprehensive review). In 3 other sets of identical twins the proband does not fit the ARA criteria. In one case simply because of a deforming arthritis,“’ but as in the twins reported by Arnett and Shulman3* the diagnosis nevertheless appears unquestioned. Of those sets of twins demonstrated to be monozygotic and where the ARA criteria are fulfilled, (or enough information is available to convince me that at least one of the twins has SLE) 13 of 21 are concordant for the disease. There is a remarkable similarity in the date of onset of SLE in a concordant twin pair (Table l), and it is much closer than in
Table 1. Systemic Lupus Erythematosus in Definite* Monozygotic Twins Total PWS
Concordant
All systemic lupus erythematosus
21
13
Using ARA criteria
17
11
7
0
Time interval between onsets of diseaset Dizvgotic
0.9 yr
*Data excluded if monozygotity in doubt. tWhere data are available.
other sibling pairs. The concordance of ciinical features is also closer than seen with nontwin sibling pairs.3s This factor is difficult to interpret, however, as parent-child concordance was statistically even closer than between the members of the twin pair.38 There is much less information available on dizygotic twins. Eight pairs have been reported, most as an addendum to the above report from New York,42 and all but one are discordant for SLE. The mean follow-up on the four discordant pairs with available data is 4.5 yr from diagnosis. A demonstration of disease concordance in monozygotic twins does not prove a genetic etiology. It remains possible that a vertically transmitted agent or some event early in life may be responsible. The difference in the disease concordance in mono- compared to dizygotic twins is more important evidence and indicates the major importance of genetic factors. Nevertheless, the same findings could be seen were an infection responsible, and genetic factors would then be of importance only in the expression of the disease. In this context, it is important to remember the elegant family studies involving twins performed in relation to tuberculosis.48 It was stated that, “the chance of developing manifest tuberculosis increases in strict proportion to the degree of blood relationship to a tuberculous index case.“48 The authors noted that the morbidity rate in monozygotic twins was approximately 3 times that for dizygotic twins, and in turn, the morbidity rate for dizygotic twins was approximately the same as for any other sibling. The discrepancy was even more marked if one studied the similarities in the extent, course, and eventual outcome of the disease in the two groups. As a counterpoise to this, it is helpful to mention the set of twins described by Koroleva, Ermakova, et al. and reviewed in detail by the
GENETICS OF S.L.E.
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New York group.42 These twins, after going to an orphanage at 11 days of age, were widely separated at 16 mo of age and yet developed SLE at age 14 with only 1 mo between the dates of onset in the two children. The reported twin studies could include some ascertainment bias, as nonconcordant dizygotic twins are less likely to be brought to attention and dizygotic twins themselves may not be assessed quite so carefully or continually. Nevertheless, these observations provide crucial evidence of an inherited factor in the pathogenesis of SLE. Equally, of course, the fact that only 60% of the monozygotic identical twins are concordant indicates that nongenetic factors are also important. A strange paradox exists between the results of twin studies in rheumatoid arthritis and SLE. Concordance for rheumatoid arthritis in monozygotic twins is low,49 of the order of 10% and not significantly above the concordance found in dizygotic twins. This factor appeared to confirm the result of many family studies, suggesting that rheumatoid arthritis was not a familial disease.” The association with a clearly identical genetic marker (HLA-DR4) now seems to suggest otherwise. It remains possible that this marker relates to a factor increasing disease severityl* rather than predisposing to disease induction, and we must be aware of this possibility underlying disease heterogeneity in SLE.
genes or clusters of genes are responsible for this renal disorder and that two of them come from the “normal” NZW and only one from the NZB.54 More recent developments have produced MRL and BXSB strains, mice from quite different genetic backgrounds, but which develop a similar lupus-like disease.” There are many differences; for example, BXSB disease is more rapid and progressive in males, MRL do not have thymocytotoxic antibodies. Information is not yet available regarding a genetic basis for the defect in feedback control of certain Ly-123 T cells noted by NZB mice,s6 nor whether this factor plays a role in the development of autoimmunity in the other strains. The lesson to be learned for human disease is that murine lupus is genetically heterogeneous. The different manifestations, i.e., tendency to DNA antibodies, antilymphocyte antibodies, glomerulonephritis, etc., may have quite separate genetic factors, perhaps distinguishable from and subservient to a basic genetic “predisposition” to autoimmune responses. It is also impressive how much variation in disease phenotype is induced by genes from normal mice. Finally, even in these inbred strains, it seems important to remember the dramatic effect of environmental manipulation. Thus, significant protein calorie malnutrition will eventually cure NZB/W disease.” CONCLUSIONS
ANIMAL
MODELS
The prototype animal model for SLE has been the spontaneous autoimmune disease of New Zealand mice, especially the first generation cross between NZB and NZW mice. This disease in its expression is similar to SLE, but extrapolation from the mice to the human disease must be guarded. NZB mice develop antibodies to thymic lymphocytes and to DNA. Genetic studies involving backcrosses have shown that the propensity to develop these antibodies is inherited via single but separate nonlinked loci.s2 Other studies have shown that it is the genes contributed by the relatively normal NZW parent that enable the B/W mice to respond and develop antibodies to native DNA.53 Analysis of the end result, that is, the glomerulonephritis, suggested again that three separate
We are deficient in family studies of systemic lupus erythematosus and of course large families may be especially useful. It is clear that to minimize the noise factor of heterogeneity, data should be examined from each family separately. The attempts to ascertain and study the inheritance of basic defects should prove of interest and may even help prove or disprove the postulated importance of these factors. The murine studies suggest that while the types of autoimmune responses seen may be under separate genetic control, there is also control at a more basic level relating to the propensity to express autoimmune phenomena. The defect in Ly-123 cells may reflect this, as may the demonstration of decreased suppressor cell function in patients and relatives of patients with SLE. A further aspect of immune regulation that
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deserves to be studied on a family basis in patients with SLE is the demonstrated defect in the autologous mixed lymphocyte reaction in lupus patients.58 This appears to relate to a defect in B-cell recognition and is present in lupus patients even when the disease is relatively
inactive. Therefore, this could well prove to be a useful genetic marker of disease susceptibility. ACKNOWLEDGMENT
I am particularly reviewing
grateful the manuscript.
to Dr. J.S. Percy for his help in
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19. Perry HM, Tan EM, Carmody S, et al: Relationship of acetyl transferase activity to antinuclear antibodies and toxic symptoms in hypertensive patients treated with hydralazine. J Lab Clin Med 1970; 76:114-125. 20. Reidenberg MM, Levy M, Drayer DE, Zylber-Katz E, Robbins WC: Acetylator phenotype in idiopathic systemic lupus erythematosus. Arthritis Rheum 1980; 23:569-573. 21. Foad B, Litwin A, Zimmer H, et al: Acetylator phenotype in systemic lupus erythematosus. Arthritis Rheum 1977; 20:8 15-8 18. 22. Morel1 A, Kaser H, Scherz R, et al: Uncommon Gm phenotypes in sera from neuroblastoma patients. J lmmunol 1977; 118:1083-1085. 23. Nakad Y, Matsumoto H, Miyazaki T, et al: Gm allotypes in myasthenia gravis. Lancet 1980; 1:677-680. 24. Pandy JP, Fudenberg HH, Virella G, et al: Association between immunoglobulin allotypes and immune responses to H. influenzae and meningococcus polysaccharide. Lancet 1979; 1:19&194. 25. Agnello V: Association of systemic lupus erythematosus and SLE-like syndromes with hereditary and acquired complement deficiency states. Arthritis Rheum 1978; 2l:Sl46S160. 26. Glass DN, Raum D, Gibson D, et al: Inherited deficiency of the second component of complement. Rheumatic disease associations. J Clin Invest 1976; 58:853. 27. Rosenfeld GB, Partridge REH, Bartholemew W. et al: Hereditary angioneurotic adema and systemic lupus erythematosus in one of identical twin girls. J Allergy Clin lmmunol (abstr) 1974; 53:69-70. 28. Lennon V: Genetic Control of Autoimmune Disease. Rose NR, Bigazzi PE, Warner NL, eds. New York: Elsevier N Holland, 1978; 339 29. Grumet FC, Couckell A, Bodner JG, et al: Histocompatibility antigens associated with systemic lupus erythematosus. N Engl J Med 1971; 285:193-196. 30. Celada A, Barras C, Benzonana G, et al: Increased frequency of HLA-DRW3 in systemic lupus erythematosus. Tissue Antigens 1980; 15:283-288. 31. Gibofsky A, Winchester RJ, Patarroyo M, et al: Disease association of the Ia-like human alloantigens. J Exp Med 1978; 1725-1732. 32. Reinertsen JL, Klippel JH, Johnson AH, et al: BP lymphocyte alloantigens associated with systemic lupus erythematosus. N Engl .I Med 1978; 299:5 15-5 18. 33. Hawkins BR, Dawkins RL, Richmond J, et al: Immunogenetic factors in systemic lupus erythematosus. Arthritis Rheum 1919; 22194. 34. Batchelor JR, Welsh KI, Tinoco RM, et al: Hydralazine-induced systemic lupus erythematosus: influence of HLA-DR and sex on susceptibility. Lancet 1980; 1:l 1071109.
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35. Tuvari JL, Hodge SE, Terasaki PI, et al: HLA + the inheritance of multiple sclerosis: linkage analysis of 72 pedigrees. Am J Hum Genet 1980; 32:103-l 11. 36. Davis MW, Gutridge GH: Disseminated lupus erythematosus in identical twin sisters associated with diabetes mellitus. J Missouri Med Assoc 1951; 48:446450. 37. Adams JL: The familial occurrence of lupus erythematosus. N Z Med J 1954, 53:504-507. 38. Arnett FC, Shulman LE: Studies in familial systemic lupus erythematosus. Medicine 1976; 55:313-322. 39. Christian CL: Clues from genetic and epidemiologic studies. Arthritis Rheum 1978; 2l:Sl30-S133. 40. Joseph RR, Zarafonetis CJD: Fatal systemic lupus in identical twins. Case report and review of the literature. Am J Med Sci 1965; 249:19&199. 41. Miller KB, Schwartz RS: Familial abnormalities of
suppressor cell function in systemic lupus erythematosus. N Engl J Med 1979; 301:803-809. 42. Block SR, Winfield JB, Lockshin MD, et al: Studies of twins with systemic lupus erythematosus. Am J Med 1975; 59:533-552. 43. Cleland LG, Bell DA, Willans M, et al: Familial lupus: Family studies of HLA and serologic findings. Arthritis Rheum 1978; 21:183-191. 44. Raum D, Glass D, Soter NA, et al: Lymphocytotoxic antibodies, HLA antigen associations, disease associations and family studies. Arthritis Rheum 1977; 20:933-936. 45. DeHoratius RJ, Messner RP: Lymphocytotoxic antibodies in family members of patients with systemic lupus erythematosus. J Clin Invest 1975; 55: 1254-l 258. 46. Raum D, Glass D, Carpenter CB, et al: The chromosomal order of genes controlling the major histocompatibility complex, properdin factor B, and deficiency of the second component of complement. J Clin Invest 1976; 58:124& 1248. 47. Larsson 0, Leonhardt T: Hereditary hypergammaglobulinemia and systemic lupus erythematosus. Acta Med Stand 1959; 165:371-393. 48. Kallmann FJ, Reisner D: Twin studies on the signilicance of genetic factors in tuberculosis. Am Rev Tuberculosis 1943; 47:549-574.
49. Jacox RF, Meyerowitz twins discordant for rheumatoid and psychological study of eight PHN, eds. Population Studies Amsterdam: Excerpta Med Ser,
S, Hess EV: Monozygotic arthritis: a genetic clinical sets. In: Bennett PH, Wood of the Rheumatic Diseases. 148, 1968:128-35.
50. Veenot-Garmann AM, Steiner FJF, WestendorpBoerma F, et al: The prevalence of rheumatoid arthritis and rheumatoid factor in relatives and spouses of seropositive and seronegative patients suffering from definite or classical rheumatoid arthritis. In: Bennett PH, Wood PHN, eds. Population Studies in the Rheumatic Diseases. Amsterdam: Excerpta Med Ser, 148, 1968: 123-27. 51. Roitt IM, Corbett M, Festenstein H, et al: HLADRW4 and prognosis in rheumatoid arthritis. Lancet 1978; 1:990. 52. Raveche ES, Steinberg AD, Klassen LW, et al: Genetic studies in NZB mice. I. Spontaneous autoantibody production. J Exp Med 1978; 147:1487-1502. 53. Paporan R, Talal N: Ability of NZW but not NZB antigen presenting cells to support Txell proliferative responses to DNA-M albumin. J Immunol 1980; 124:515519. 54. Knight JG, Adams DD: Three genes for lupus nephritis in NZB and NZW mice. J Exp Med 1978; 147:16531660. 55. Houston DP, Steinberg AD: Animal models of human systemic lupus erythematosus. Yale J Biol Med 1979; 52:289-305. 56. Cantor H, McVay-Boudreau L, Hugenberger J, et al: Immunoregulatory circuits among T-cell sets: physiologic role of feedback inhibition in vivo: absence in NZB mice. J Exp Med 1978; 147:1116-I 125. 57. Gardner MB, Ihle JN, Pillarisetty RJ, et al: Type C virus expression and host response in diet cured NZBW mice. Nature 1977; 268:341-344. 58. Kuntz MM. Innes JB. Webster ME: The cellular basis of the impaired autologous mixed lymphocyte reaction in patients with system lupus erythematosus. J Clin Invest 1979; 63:151-l 53.
in Systemic
Lupus Erythematosus
By A. S. Russell
C
AN systemic lupus erythematosus (SLE) be regarded as a homogeneous disease? As dermatologists, nephrologists, hematologists, and rheumatologists, our approach to SLE in the past has been reminiscent of the blind man to the elephant. In recent years this descriptive dilemma has been resolved, and we have slowly framed a coherent clinical and laboratory picture of the disease. Despite this undoubted progress, it is not at all certain that the attempt to fit the disease into a uniform phenotype is a valid one. One of the many advantages of genetic analyses is that they can help provide answers to such questions. Answers based on clinical features alone can be misleading, both in suggesting differences where similarities are more important and, perhaps more commonly, suggesting uniformity where crucial differences exist. Marfan syndrome provides an example of the first type of error. It is a heritable disorder of connective tissue in which the mode of inheritance has been established in family pedigrees as an autosomal dominant. Expression of that disorder is variable, and even within one family it may be present as minor arachnodactyly in one member and the complete syndrome in another.’ It has a moderate penetrance in that no manifestations whatsoever may be seen in a member of one generation, but many or all of the features may be evident in the offspring. This presumably reflects the influence of other genes, and the example illustrates that a major variation in disease expression may still reflect one underlying defect. A similar finding may be seen in the well characterized single amino acid defect of sickle cell disease. If a subject who is homozygous for this condition also has a trait causing persistence of synthesis of fetal hemoglobin, the ensuing sickle cell disease is much less severe in its expression.* Thus, genetically homogeneous diseases may have different appearances, that is, a single disease genotype may have varying phenotypes. The obverse of the coin may be even more relevant to SLE-phenotypically similar diseases may be related to different genotypes. For example, the San Felippo syndromes A and B3 Seminars
in Arthritis and Rheumatism, Vol. 10. No. 4 (May), 198 1
differ in the enzyme defect that is inherited, and yet clinically they are identical. Rapid gastric emptying or increased blood levels of pepsinogen I, both genetically controlled abnormalities, may present simply as one “disease’‘-peptic ulceration. This type of genetic diversity is so common, McKusick put forward the dictum that a genetic disease that appears at first to be homogeneous, will be found to comprise two or more separate and distinct entities on further study. It seems very likely that this is the case with SLE. SLE can already be divided into clinical subgroups:4 (1) drug-induced disease, (2) disease associated with complement deficiencies, (3) mixed connective tissue disease, and (4) other SLE. It is quite possible that the further division of the SLE syndrome on the basis of clinical and serologic criteria will continue. While there may be genetic or environmental factors in common between all the groups, the improved analysis that results from subgrouping may help delineate a role for separable genetic factors, as has occurred, for example, in diabetes mellitus.5 One of the problems to be faced in assessing the role of genetic factors in SLE or, indeed, in many other diseases, is in deciding which individual can be designated as having the disease. This is particularly a problem where (1) the disease is probably heterogeneous, (2) there are no clear and specific criteria that can be used to define affected subjects under varying epidemiologic conditions, (3) the variability of age of clinical expression makes it difficult to exclude the possibility that the disease may develop in a given individual at a later age, and (4) there is almost complete ignorance of the basic underlying disorder(s). Some of these factors are particularly important in trying to determine the mode of inheriFrom the Rheumatic Disease Unit. Department of Medicine, University of Alberta, Edmonton. Alberta, Canada. Address reprint requests to Dr. A. S. Russell. Rheumatic Disease Unit. Department of Medicine, University of Alberta, Edmonton, Alberta, T6G. 2G3. Canada. 0 I981 by Grune & Stratton, Inc. 0049-0172/81/1004-0002$01.00/0
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tance once a role for genetic factors has been established. If we knew what basic defects or disease processes more closely reflect the genetic component, then we could perhaps ascertain them directly. The inheritance pattern of the development of frank peptic ulceration remains unclear if this aspect is studied alone. In those families with peptic ulcer and hyperpepsinogenemia I, a study of the increased blood levels of pepsinogen I discloses a clear pattern of dominant inheritance.6 Perhaps the most important single factor in evaluating evidence provided by the types of approach detailed below is the stringency of the selection and assessment of the control populations. Even with reliable data, problems with interpretation may still persist. Genetic factors will normally be reflected in a familial aggregation of disease, but the same phenomenon can be seen with social and environmental factors. In this context, it should be remembered that because of a familial aggregation of the conditions leading to nicotinic acid deficiency, pellagra was, at one time, thought to be an inherited disease. Chronic alcoholism is familial and the controversy between genetic and environmental influences still rages. The Bittner agent-now known to be a virus-is responsible for inducing breast cancer in mice. It was the discovery that infection is transmitted by breast milk that enabled it to be distinguished from a genetic disease. It is unwise to consider a role for genetic factors and to evaluate the significance of genetic models on statistical grounds alone, without the relevant clinical context. To illustrate the problems that can arise, Lillienfeld used genetic models based on family studies and was able to show that attendance at medical school was consistent with the inheritance of an autosomal recessive gene.’ Evidence relating to the inheritability of SLE will be considered in the following paragraphs. POPULATION STUDIES SLE Prevalence
and Racial Variation
SLE appears to be an uncommon disorder. With no simple, reliable technique for mass screening to allow testing of whole populations, less reliable estimates have been used. The data of Siegel and Lee’ were based on the assumption
that all patients would eventually require hospital or clinic care because of the severity of their illness and that when there, they would be correctly diagnosed. They found an increased prevalence of spontaneous-but not drug induced-SLE among the nonwhite population of Manhattan. The prevalence in whites was 8.7 x 10e5 and in nonwhite 38.8 x IO-‘. On the basis of additional stratifications, they could not account for this by environmental differences. A recent study in Hawaii,’ which was based on medical record discharge diagnoses, found 107 patients with definite SLE while the ageadjusted prevalence rates for whites were 5.8 x 10m5, for Chinese 24 x lo-‘, and for Japanese 18 x 10 -‘. In a study of 75 North American Indian tribes using inpatient records that were checked and diagnosed according to the American Rheumatism Association (ARA) criteria, the prevalence of systemic lupus was roughly in accord with that of other studies in the US.” However, for three tribes, the prevalence was higher, and for full-breed Sioux, it was 31 x 10--5, but for half-breed Sioux it was only 21 x 10m5. The highest prevalence study reported to date is from the Kaiser Permanente Foundation in California, which recorded a prevalence in adult black females of 400 x 10m5.” Associations
of SLE With Known
Genetic Factors
Gender While female gender is a genetically determined trait associated with SLE, the evidence suggests that it is hormonal factors that are responsible for the female predisposition to this disease: (1) The sex ratio of patients with onset of disease prior to puberty or late in life shows a lower prominence of females.‘* (2) Eight patients have been described with both SLE and XXY Klinefelter syndrome,‘-’ that is, a decreased androgenic stimulus and probable increased estrogen formation. (3) The data from Lahita et al. at the Rockefeller Institutei demonstrate an increase in 16-hydroxyestrone in females and males with SLE. (4) The increased susceptibility of castrated male BW mice to SLE and the amelioration of the disease in females by testosterone implants.15 In this context, one has to bear in mind that the major histocompatibility complex in mice is associated with gene loci that may influence sex
GENETICS OF S.L.E.
257
hormone levels, as well as the end-organ sensitivity to some of these agents.16 One of the few reported pairs of monozygotic twins who were discordant for SLE may illustrate this hormonal factor.” The “resistant” twin had a total oophorectomy 16 yr prior to the onset of SLE in her twin. Despite a chronic biologic false-positive STS, no other evidence of SLE was present 7 yr after it developed in the twin. Acetylator Phenotype The ability to rapidly acetylate and remove drugs characterized by the presence of an aromatic amino group is inherited. Rapid acetylation is dominant,” and there is an increased proportion of patients with a slow acetylator phenotype in hydralazineand procainamideinduced SLE.19 Several studies have found a similar result in patients with spontaneous SLE.*’ This remains in question,*’ and there is a tendency to report positive results, but it seems probable that this observation, while not uniform, may be a further indication of an underlying disease heterogeneity. Immunoglobulin
Allotypes
Some genes controlling immune responses in mice are also associated with allotypic markers on IgG. These loci are not associated with H-2 or HLA. After the initial studies of Gm markers in rheumatoid arthritis drew a blank, interest waned. It is now reviving and there has been an association demonstrated between neuroblastoma and a specific Gm group,** and a similar finding has been described in patients with myasthenia gravis.23 Furthermore, there is early evidence that Ig allotypes Km1 are associated with increased antibody response after immunization with meningococcal polysaccharide.24 The area of immunoglobulin allotypes has not yet been explored in SLE. Complement Deficiency There are now many reports of SLE or a “lupus-like disease” occurring in association with several types of complement deficiencies.*’ The proportion of patients with homozygous C2 deficiency that have SLE is impressive; nevertheless, initial suggestions that this might be an artifact of selection are not entirely disproven, as some of the patients were ascertained because of their SLE and referred
because of their low complement level. On the other hand, the numbers of such homozygous deficient patients present within a population of patients with SLE, although small, seem above that of a random population sample.*’ There are, in addition, a number of reports to suggest that the spectrum of lupus seen does vary somewhat from the classical mould.26 The C2 loci are linked to the major histocompatibility complex and, while it seems likely that the association with SLE results from the effects of the complement deficiency, it is possible that the C2 null gene is acting as a marker for a disease susceptibility gene. One report suggested that the heterozygous C2 deficiency is also associated with SLE.*‘j This would argue against the requirement for a major functional defect in the complement pathway, but the statistical calculations to support this important observation are based on relatively small numbers and require substantiation. The relationship to other deficiencies of complement, many of which are not HLA related, is less clear-cut but still highly suggestive. Several associations have been reported between SLE and the Cl esterase inhibitor deficiency-hereditary angioedema.25 Patients with this latter disorder have decreases in C2 and C4 varying in both degree and duration, depending on the severity of the disease. This defect was found in a twin with SLE. The other monozygotic twin had a much less severe reduction in C2 and did not have SLE” (additional details in ref. 25). This single twin study suggests that the role of the complement deficiency is not a genetic one. It is possible that decreased complement levels may allow undefended entry of some environmental factor responsible for the disease. Lennon made the suggestion that as regulatory antiidiotype antibodies require complement in order to eliminate abnormal lymphocytes bearing that idiotype, the absence of complement might instead allow stimulation rather than elimination of such cells. Thus, complement deficiency could, in this way, markedly affect immune regulation.28 HLA The demonstration of an association between various postulated human disease susceptibility genes and specific HLA antigens does not explain the mechanism underlying this associa-
258
tion, although there is no shortage of hypotheses. Nevertheless, this type of demonstration is important. The presence of such a marker for a disease susceptibility gene not only facilitates the detection and study of other disease susceptibility factors, it also allows the dissection and better definition of clinical syndromes. Thus, the clinical validity of the subgrouping of patients with diabetes mellitus, chronic active hepatitis, and juvenile chronic arthritis can now be supported by the results of HLA typing. SLE was one of the first diseases to be investigated by HLA typing. The original study showed an increased prevalence of both HLA-B8 and HLA-Bwl 5.29 Other reports have, in general, supported the increased prevalence of B8 at least in whites4 Recent work has suggested that DR3-in linkage disequilibrium with B8-may be more closely associated with lupus than B8 itse1f.30 This has, of course, proved to be the pattern in the other B8-associated diseases, including Sjogren syndrome. Rather than using HLADR-specific antisera, another approach has been to use less well characterized antisera with a broader anti-B-cell specificity and to determine whether any show a particular relationship to the antigens on cells from patients with SLE when compared with contro1s.3’.32 One such serum marked 70% of SLE lymphocytes compared to 40% of those of a control popu1ation.3’ Absorption experiments suggested that at least two populations of antibodies were involved, probably related to determinants DR2 and DR3.3’ An advantage of this type of broad spectrum antiserum is that it may disclose other allelic groups or supergroups, and it is possible that the new HLA antigens on the MB or MT series may be involved. A final recognition that both DR2 and DR3 are involved would suggest a genetic basis for disease heterogeneity that could be more closely examined at a clinical or laboratory level. Hawkins33 and others have made the interesting observation that in their patients with SLE, the haplotype Al-B8 appears to be significantly associated with severe lupus, whereas the antigens A2-B7 (not as a haplotype) are found in increased frequency in mild lupus. This could provide an interesting corollary to the above findings with DR2 and DR3. In contrast to the above results with spontaneous SLE, hydralazine-induced lupus has been shown to be closely
A. S. RUSSELL
associated with HLA-DR4. Seventy-three percent of patients with this disease carried this antigen, compared to approximately 25% of patients with spontaneous SLE.34 This clearly demonstrates a basis for genetic heterogeneity between these two major subgroups of SLE. If we postulate one or more lupus disease susceptibility gene(s) situated near the major histocompatibility complex, we must not let the hypothesis that it reflects an immune response gene blind us to the possibility that it could exist well outside the designated HLA region. Thus, if multiple sclerosis can be regarded as a homogeneous disease with a disease susceptibility gene associated with the MHC and linked to Dw2, this gene is probably situated some distance from the D 10~~s~~and much closer to PGM,, that is at a distance of 20 recombination units from HLA. Studies including an assessment of linkage with GLO and PGM, allotypes are therefore important. FAMILY STUDIES
Familial Aggregation
of Disease
Despite the reservations discussed earlier, family studies generally provide the initial evidence of the importance of genetic factors. The clinical observation that SLE seemed to occur in some families with an unexpected frequency was put forward by Davis,36 and Adams3’ in the early 1950s. Subsequent findings have been reported by Arnett and Shu1man.38 The prevalence of SLE in family members of an affected proband will depend on the severity of the criteria used to affirm the diagnosis in these relatives. Thus, in Marfan syndrome, minor manifestations of the disease are recognized only because of the relationship with an affected proband. In the absence of this demonstrated relationship, the relative might have been considered a normal variant. Similarly, the presence of a close relative with SLE will cast a different light on a subject with otherwise unexplained synovitis and a positive antinuclear antibody (ANA). In pedigree analysis (intrafamilial genetic studies), it is probably reasonable and important to designate such a subject as “affected.” In studies setting out to demonstrate familial aggregation, it is important not only that the same standard criteria be used for the relatives
GENETICS OF S.L.E.
of an affected
proband as for the control population, but that ideally, the control population should be selected in the same way as the lupus families because of a primary relationship with a nonaffected proband. This approach has not been adopted in any studies to date. Estimates of the observed prevalence of SLE, in the firstdegree relatives of a lupus patient have been quoted as approximately 1%,39 and while they seem convincingly higher than would be found in a control population, the hard data necessary to demonstrate this with any degree of conviction are not available. Pedigree Analysis
Mode of Inheritance of Disease or Defects With an uncommon disease, intrafamily studies have an advantage in that they should largely eliminate the possibility of disease heterogeneity. To date, there have not been enough family studies to allow answers to specific questions, to avoid ascertainment bias, and certainly not to infer any mode of inheritance. Many of the early studies were of families with affected siblings and normal parents,40 which would tend to maximize the proportion of patients with an autosoma1 recessive mode of inheritance. Recent studies include more families with multiple generations, which would maximize a dominant mode. However, from the data currently available, it is clear that the disease can exist in siblings who have normal parents, and in all four types of single parent-child combinations.38 To date, there is no evidence to suggest increased consanguinity in the parents of patients with SLE although this is an area that has not yet been formally examined. It is possible that several modes of inheritance may be found, once again reflecting disease heterogeneity. More data are required in relation to the patterns of inheritance of the disease, but particularly in relation to the patterns of inheritance of possible basic defects that may lead to disease expression. Basic defects. In an exciting attempt to identify a genetic defect of basic importance, Miller and Schwartz looked at the integrity of an immune suppressor cell system in both patients and relatives of patients with SLE.4’ They incubated peripheral lymphocytes with either pokeweed mitogen (PWM) alone or with PWM plus
259
Con-A and measured the reduction in response, that is of IgG synthesis, consequent on the introduction of the Con-A. After assessment of the normal suppressor response, they showed that 11 of 15 patients had a decreased response, more than 3 SD below the normal mean. A depressed response was found in 13 of the 50 first-degree relatives; 12 of these 13 were female, compared to 12 of 37 of the remainder. No correlation with lymphocytotoxic antibodies was found nor, seemingly, with ANA, although the number of positives was small. Serologic abnormalities: ANA. Early studies found a mild but significantly increased prevalence of ANA and of hypergammaglobulinemia in first-degree relatives of lupus patients. The prevalence of ANA generally ranged between 4% and 14% (reviewed in ref. 42), although one report was presented that described it in 33%. Some additional studies did not include even a rudimentary “control” population. A recent study of several large families found an ANA prevalence of 55% in consanguineous relatives.43 In one of the largest families, 20 of 40 relatives had a positive ANA and while most of the positives were over 20 yr of age, 14 of the 20 negatives were under 20 yr. It was also seen in 50% of nonconsanguinious relatives of probands with SLE, suggesting a nongenetic influence and illustrating the dangers of using spouses as “controls.“43 The high prevalence of ANA found in this study was unusual and may result from specific selection of these families because of the presence of multiple cases. More recent studies on smaller families by Miller and Schwartz4’ and Raum et a1.44found a prevalence similar to that seen in the earlier studies. Lymphocytotoxic and anti-RNA antibodies. In someJ5 but not a1144studies, these antibodies have been found, not only in relatives of patients with SLE, but in household contacts, providing further evidence for the importance of nongenetic factors. Association with Genetic Markers HLA. Recent pedigree studies43 suggested an association between susceptibility to SLE and the inheritance of a particular HLA haplotype, although the specific HLA markers differed for each family. In the largest family, several individuals were seen with the designated HLA
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haplotype but without SLE, and one was noted to have SLE even in the absence of the HLA marker. This latter could be easily explained by postulating a recombination event between a lupus disease susceptibility gene and the HLA markers. Some evidence exists to suggest an increased prevalence of recombinations in patients with connective tissue diseases.46 An alternative explanation is that this may simply reflect the difficulty of trying to assess linkage between a genetic marker and a multifactorial trait. The interesting possibility also exists that in some families, or with some “types” of lupus, the disease may be HLA linked, but that statistics on groups of families might be diluted by those families or types where it is not so linked.
TWIN STUDIES
Studies of the relative disease concordance in monozygotic and dizygotic twins provide the most cogent evidence of disease heritability. Recently, seven additional pairs of definite monozygotic twins were described, four of which were concordant for SLE,42 all using ARA criteria for the diagnosis of the disease. Of the three seemingly discordant pairs, in one pair, the “normal” twin had been followed for 1 yr since the onset of disease in her twin and had marked serologic abnormalities. The second pair had been followed for 2 yr and the third for 6 yr, and had persistent leukopenia (approximately 2000). It seems possible that at least the first of these “normal” twins might have developed a full expression of SLE if observed over a longer period. There are two other reports of definite monozygotic twins discordant for SLE where follow-up was 7 yr in each case (see ref. 42 for a comprehensive review). In 3 other sets of identical twins the proband does not fit the ARA criteria. In one case simply because of a deforming arthritis,“’ but as in the twins reported by Arnett and Shulman3* the diagnosis nevertheless appears unquestioned. Of those sets of twins demonstrated to be monozygotic and where the ARA criteria are fulfilled, (or enough information is available to convince me that at least one of the twins has SLE) 13 of 21 are concordant for the disease. There is a remarkable similarity in the date of onset of SLE in a concordant twin pair (Table l), and it is much closer than in
Table 1. Systemic Lupus Erythematosus in Definite* Monozygotic Twins Total PWS
Concordant
All systemic lupus erythematosus
21
13
Using ARA criteria
17
11
7
0
Time interval between onsets of diseaset Dizvgotic
0.9 yr
*Data excluded if monozygotity in doubt. tWhere data are available.
other sibling pairs. The concordance of ciinical features is also closer than seen with nontwin sibling pairs.3s This factor is difficult to interpret, however, as parent-child concordance was statistically even closer than between the members of the twin pair.38 There is much less information available on dizygotic twins. Eight pairs have been reported, most as an addendum to the above report from New York,42 and all but one are discordant for SLE. The mean follow-up on the four discordant pairs with available data is 4.5 yr from diagnosis. A demonstration of disease concordance in monozygotic twins does not prove a genetic etiology. It remains possible that a vertically transmitted agent or some event early in life may be responsible. The difference in the disease concordance in mono- compared to dizygotic twins is more important evidence and indicates the major importance of genetic factors. Nevertheless, the same findings could be seen were an infection responsible, and genetic factors would then be of importance only in the expression of the disease. In this context, it is important to remember the elegant family studies involving twins performed in relation to tuberculosis.48 It was stated that, “the chance of developing manifest tuberculosis increases in strict proportion to the degree of blood relationship to a tuberculous index case.“48 The authors noted that the morbidity rate in monozygotic twins was approximately 3 times that for dizygotic twins, and in turn, the morbidity rate for dizygotic twins was approximately the same as for any other sibling. The discrepancy was even more marked if one studied the similarities in the extent, course, and eventual outcome of the disease in the two groups. As a counterpoise to this, it is helpful to mention the set of twins described by Koroleva, Ermakova, et al. and reviewed in detail by the
GENETICS OF S.L.E.
261
New York group.42 These twins, after going to an orphanage at 11 days of age, were widely separated at 16 mo of age and yet developed SLE at age 14 with only 1 mo between the dates of onset in the two children. The reported twin studies could include some ascertainment bias, as nonconcordant dizygotic twins are less likely to be brought to attention and dizygotic twins themselves may not be assessed quite so carefully or continually. Nevertheless, these observations provide crucial evidence of an inherited factor in the pathogenesis of SLE. Equally, of course, the fact that only 60% of the monozygotic identical twins are concordant indicates that nongenetic factors are also important. A strange paradox exists between the results of twin studies in rheumatoid arthritis and SLE. Concordance for rheumatoid arthritis in monozygotic twins is low,49 of the order of 10% and not significantly above the concordance found in dizygotic twins. This factor appeared to confirm the result of many family studies, suggesting that rheumatoid arthritis was not a familial disease.” The association with a clearly identical genetic marker (HLA-DR4) now seems to suggest otherwise. It remains possible that this marker relates to a factor increasing disease severityl* rather than predisposing to disease induction, and we must be aware of this possibility underlying disease heterogeneity in SLE.
genes or clusters of genes are responsible for this renal disorder and that two of them come from the “normal” NZW and only one from the NZB.54 More recent developments have produced MRL and BXSB strains, mice from quite different genetic backgrounds, but which develop a similar lupus-like disease.” There are many differences; for example, BXSB disease is more rapid and progressive in males, MRL do not have thymocytotoxic antibodies. Information is not yet available regarding a genetic basis for the defect in feedback control of certain Ly-123 T cells noted by NZB mice,s6 nor whether this factor plays a role in the development of autoimmunity in the other strains. The lesson to be learned for human disease is that murine lupus is genetically heterogeneous. The different manifestations, i.e., tendency to DNA antibodies, antilymphocyte antibodies, glomerulonephritis, etc., may have quite separate genetic factors, perhaps distinguishable from and subservient to a basic genetic “predisposition” to autoimmune responses. It is also impressive how much variation in disease phenotype is induced by genes from normal mice. Finally, even in these inbred strains, it seems important to remember the dramatic effect of environmental manipulation. Thus, significant protein calorie malnutrition will eventually cure NZB/W disease.” CONCLUSIONS
ANIMAL
MODELS
The prototype animal model for SLE has been the spontaneous autoimmune disease of New Zealand mice, especially the first generation cross between NZB and NZW mice. This disease in its expression is similar to SLE, but extrapolation from the mice to the human disease must be guarded. NZB mice develop antibodies to thymic lymphocytes and to DNA. Genetic studies involving backcrosses have shown that the propensity to develop these antibodies is inherited via single but separate nonlinked loci.s2 Other studies have shown that it is the genes contributed by the relatively normal NZW parent that enable the B/W mice to respond and develop antibodies to native DNA.53 Analysis of the end result, that is, the glomerulonephritis, suggested again that three separate
We are deficient in family studies of systemic lupus erythematosus and of course large families may be especially useful. It is clear that to minimize the noise factor of heterogeneity, data should be examined from each family separately. The attempts to ascertain and study the inheritance of basic defects should prove of interest and may even help prove or disprove the postulated importance of these factors. The murine studies suggest that while the types of autoimmune responses seen may be under separate genetic control, there is also control at a more basic level relating to the propensity to express autoimmune phenomena. The defect in Ly-123 cells may reflect this, as may the demonstration of decreased suppressor cell function in patients and relatives of patients with SLE. A further aspect of immune regulation that
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A. S. RUSSELL
deserves to be studied on a family basis in patients with SLE is the demonstrated defect in the autologous mixed lymphocyte reaction in lupus patients.58 This appears to relate to a defect in B-cell recognition and is present in lupus patients even when the disease is relatively
inactive. Therefore, this could well prove to be a useful genetic marker of disease susceptibility. ACKNOWLEDGMENT
I am particularly reviewing
grateful the manuscript.
to Dr. J.S. Percy for his help in
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