- Email: [email protected]
Genetic dissection of systemic lupus erythematosus
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Genetic dissection of systemic lupus erythematosus Edward K Wakeland*, Amy E Wandstrat†, Kui Liu‡ and Laurence Morel§ Recent progress towards elucidating the genetic basis for susceptibility to systemic lupus erythematosus (SLE) has provided insights into the manner in which individual susceptibility genes contribute to disease pathogenesis. Studies in animal models of systemic autoimmunity suggest that genes in three separate pathways contribute to the initiation and progression of systemic autoimmunity. Linkage studies in humans suggest that at least some susceptibility genes mediating disease in lupus-prone mice may also contribute to susceptibility in humans. Addresses *†‡ Center for Immunology, The University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75235-9093, USA *email: [email protected] §Center for Mammalian Genetics and Department of Medicine, University of Florida, Gainesville, FL 32610-0275, USA Current Opinion in Immunology 1999, 11:701–707 0952-7915/99/$ — see front matter © 1999 Elsevier Science Ltd. All rights reserved.
genetic pathways that are critical to the development of severe lupus nephritis and have identified allele-specific, suppressive modifiers capable of dramatically influencing disease progression. The second important approach has been the ‘synthesis’ of mouse models of systemic autoimmunity via the production of targeted gene disruptions. These genetically synthesized models have identified specific genes or combinations of genes capable of causing disease pathogenesis when mutated. Here we will briefly summarize recent key discoveries in human and mouse genetics that have advanced our understanding of lupus susceptibility. In addition, we will present a hypothetical framework with which to view the role of specific genetic elements in disease progression. A comparison of the phenotypic features and linkage data in mice and humans reveals important similarities in the genetic mechanisms mediating systemic autoimmunity in the two species.
Linkage analysis of lupus nephritis in the mouse
Systemic lupus erythematosus (SLE) is a chronic autoimmune disease characterized by the production of autoantibodies to a spectrum of nuclear antigens. The clinical consequences of SLE are extremely heterogeneous, potentially leading to inflammatory damage in a variety of different organ systems. Although the factors responsible for the initiation of SLE are poorly understood, genetic predisposition is firmly established as a key element in susceptibility. Recently, several groups have completed initial linkage analyses in human families with SLE [1,2•–5•]. These studies have provided interesting insights into the complexity of the genetic interactions involved in SLE but have not yet resulted in the identification of specific genes or genetic pathways mediating SLE susceptibility. This reflects the difficulties inherent in the analysis of complex genetic diseases, coupled with the absence of sample repositories of sufficient size to allow definitive genetic analysis in humans.
The chromosomal locations of genes mediating susceptibility to lupus nephritis or systemic autoimmunity in the NZB/W, MRL and BXSB mouse models have been determined via genome scans [6,7••,8–13]. These studies have demonstrated that susceptibility to lupus in the mouse is inherited in a complex fashion that is reminiscent of human SLE, involving both genetic interactions and additive effects of individual genes. A compilation of the positions in the murine genome of the named SLE susceptibility loci from all these studies is provided in Figure 1. A total of 31 different gene designations have been defined thus far, distributed among 21 nonoverlapping 20 cM genomic intervals. This summary clearly illustrates the complexity of the genetic basis for susceptibility to autoimmunity. In addition to the susceptibility genes compiled in Figure 1, other investigators have determined the locations of loci affecting a variety of component phenotypes associated with systemic autoimmunity [14]. Taken together, these results indicate that the inheritance of systemic autoimmunity is extremely polygenic. However, as illustrated by the data in Figure 1, genomic segments on murine chromosomes 1, 4 and 7 are associated with disease susceptibility in multiple strain combinations — suggesting that these intervals may contain genes or clusters of genes that strongly influence autoimmunity.
Genetic analyses in the mouse have provided some important insights into the pathogenic processes mediating disease in experimental models of SLE. Two distinct approaches have been utilized to investigate the genetics of murine systemic autoimmunity. First, linkage analysis and congenic dissection have provided insights into the genetic basis for susceptibility in the classic lupus-prone mouse strains. These studies have delineated specific
The importance of epistatic interactions to the development of severe autoimmunity has recently been demonstrated via the analysis of congenic strains. Sle1 and Sle3 are NZWderived susceptibility genes for murine autoimmune lupus nephritis for which we produced B6-congenic strains [15]. Although neither of these genes mediates a severe disease when isolated on the B6 genome in interval-specific congenic strains, we recently demonstrated that Sle1 and Sle3 in
Abbreviations ANA antinuclear autoantibody Sap serum amyloid P component SLE systemic lupus erythematosus Sles SLE suppressor
Introduction
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Figure 1
D1MIT5
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D4MIT9 D4MIT31 D4Nds2 elp1
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Chromosomal locations of susceptibility loci (vertical white bars) associated with SLE. Linkage analysis was performed on the 19 murine autosomes (chromosome [c] 1–19) in murine test crosses [6,7••,8–13] involving NZB, NZW, NZM2410, BXSB and MRL/lpr mice. Each susceptibility locus is centered on the SSR (simple sequence repeat) marker, indicated by the horizontal line on the left,
that showed maximum linkage. Black vertical bars indicate the locations of suppressive modifiers responsible for the suppression of fatal disease in the NZW genome [7••]. The locus names are on the right of each bar. The length of each confidence interval (apart from Lrdm/Sle5) has been set arbitrarily at 20 cM, which typically corresponds to the support interval obtained for each locus.
combination as a bi-congenic strain (B6.NMZc1|c7) mediate the development of fatal lupus nephritis with a penetrance of >55% by 12 months of age [16•]. This result was surprising in that, although both of these genes are present in NZW mice, this strain does not develop severe humoral autoimmunity or glomerulonephritis — suggesting that the NZW genome must contain epistatic modifiers that suppress the development of severe autoimmunity. We
subsequently performed a genome scan and identified the positions of four epistatic modifiers that suppress autoimmunity in the NZW genome [7••]. These genes — which we designated SLE suppressor (Sles)1–Sles4 — are recessive, suppressive modifiers in the NZW genome whose cumulative effect accounted for the benign autoimmune phenotype of NZW. The positions of these suppressive modifiers are included with the susceptibility loci compiled
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in Figure 1. In this study we also found that Sles1 could specifically suppress the ability of Sle1 to cause a loss of immune tolerance to chromatin and that this effect was sufficient to inhibit fatal lupus nephritis [7••]. The detection of suppressive modifiers of systemic autoimmunity, which is consistent with observations in other complex traits [17], has significant implications for the genetics of systemic autoimmunity. Given the complexity of the immune system, it is reasonable to predict that epistasis will also impact SLE susceptibility in humans. If so, such interactions will create additional complications in the process of unraveling the genetics of human lupus. The presence of suppressive modifiers within specific families or ethnic groups would conceal the phenotypes of potent susceptibility genes in both linkage and allele-association studies. This could significantly weaken the power of such analyses. On the other hand, the presence of such potent suppressive modifiers in this disease suggests that a thorough understanding of the genetic pathways responsible for disease pathogenesis should reveal strategies for therapeutic intervention. The profound impact of epistasis on the expression of fatal lupus in the mouse supports the feasibility of identifying appropriate genetic targets for the development of effective therapies.
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Figure 2 Pathway 1: loss of tolerance to nuclear antigens
Susceptible individuals Sle1 Sap C1q Antinuclear autoantibodies 2–4% of population ANA positive Familial aggregation
Pathway 2: dysregulation of the immune system
Sle2 Sle3 Fas Lyn SHP-1 Pathogenic autoimmunity Familial aggregation Sle6 FcγRIII
Pathway 3: end-organ targeting Nephritis
Neurologic disorders
Arthritis
Vasculitis
Current Opinion in Immunology
Delineating genetic pathways in systemic autoimmunity Genetic analyses of interval-specific congenic strains have provided important insights into the component phenotypes contributed by individual genes to disease pathogenesis. Our analysis of congenic strains carrying genomic intervals with Sle1, Sle2 and Sle3 has provided a detailed characterization of the component autoimmune phenotype produced by each of these susceptibility genes. These results have allowed the development of a working model of the manner in which specific genes interact to mediate systemic autoimmunity. As diagrammed in Figure 2, the development of severe lupus nephritis in the mouse is controlled by interactions between genes in three separate biological pathways. The first pathway contains genes such as Sle1 that trigger the loss of immune tolerance to nuclear autoantigens and mediate the initiation of autoimmunity [18,19]. Genes in this pathway are capable of causing the initiation of a humoral autoimmune response to nuclear antigens; however, this response is minimally pathogenic in the absence of susceptibility genes in the other pathways. In this regard, many first-degree relatives of SLE probands exhibit a similar seropositive phenotype without severe disease pathogenesis [20]. The second biological pathway contributing to lupus susceptibility contains genes causing generalized immune hyper-responsiveness or dysregulation. Genes such as Sle2 [21], Sle3 [22], lpr [23], gld [23] and Yaa [24] would all be included in this pathway. These genes often do not generate autoimmune phenotypes in
A diagram of the hypothetical stages of pathogenesis in human SLE and murine lupus nephritis. The biological pathways responsible for the transition of individuals between different stages, together with a brief description of the key characteristics of each, are listed on the left. The susceptibility genes and genes/molecules that are involved within each biological pathway are listed to the right of each arrow. The arrows delineate the transitions of affected individuals between individual stages in disease pathogenesis and the newly acquired disease parameters are briefly described below the key component phenotype associated with each stage. Several separate pathways are potentially involved in the transition to the final stage because a variety of end organs may become involved in human SLE.
lupus-resistant genomes but strongly enhance the expansion of the autoimmune response when combined with genes that mediate the loss of tolerance to nuclear autoantigens. The final class of lupus susceptibility genes, represented by Sle6 in our model, potentiates end-organ damage. Theoretically, end-organ damage could be enhanced by a variety of molecular mechanisms — including genes that modify immune effector functions (such as Fc receptors; see below) and those that modify the end organ itself. Support for this model has recently been produced in our analysis of B6.NZMc1|c7 mice, which develop fatal lupus due to the epistatic interactions of genes in biological pathways 1 (Sle1) and 2 (Sle3) [16•]. Several groups have obtained, via the analysis of targeted gene disruptions, data implicating specific genes in the pathogenesis of systemic autoimmunity. Botto et al. [25••] recently demonstrated that mice carrying a targeted disruption of the gene encoding the C1q complement
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component spontaneously developed systemic autoimmunity with many of the phenotypes commonly associated with SLE. Interestingly, the severity of the autoimmune phenotype was strongly impacted by epistatic modifiers segregating between B6 and 129 (these are mouse strains) — suggesting that C1q deficiency may interact with other genes (possibly those in pathway 2 of Figure 2) to mediate severe systemic autoimmunity. Bickerstaff et al. [26••], in their analysis of the autoimmune phenotypes elicited by targeted disruptions of the gene encoding serum amyloid P component (Sap), obtained very similar results. These investigators postulate that Sap- and C1qdeficiencies potentiate the production of antinuclear autoantibodies (ANAs) due to the disruption of their functions in the pathway mediating degradation and removal of nuclear chromatin. The ability of these deficiencies to elicit ANA production is reminiscent of the phenotype produced by Sle1, suggesting that Sap and C1q would be elements of biological pathway 1 (see Figure 2). Cornall et al. [27••] recently characterized the impact of quantitative decreases in key components of the B cell antigen-receptor (BCR)-mediated signal transduction pathway on systemic autoimmunity. These investigators demonstrated that quantitative decreases in the signal transduction components encoded by Lyn, Cd22 and Shp-1 lower the activation thresholds of B cells — resulting in the development of B cell hyper-reactivity. The investigators postulate that they have synthesized a model system illustrating the potential cumulative impact of minor quantitative changes in the expression of signal transduction molecules on the immune system. They hypothesize that such changes would be an example of a polygenic autoimmune phenotype capable of mediating a dysregulation of the immune system. Such an effect is consistent with the phenotypes of genes such as Sle2, Sle3, lpr, gld and Yaa — suggesting that they are examples of genetic processes in biological pathway 2 (see Figure 2). Finally, Clynes et al. [28••] recently reported that a disruption of the Fc receptor γ chain gene, leading to a deficiency in Fc receptor expression, severely inhibits the development of fatal glomerulonephritis in the NZB/NZW mouse model of SLE. Since the production of autoantibodies was not impaired in these mice, the authors concluded that Fc receptor expression is essential for the triggering of the inflammatory pathway normally elicited by immune complex deposition. This phenotype is consistent with a function of Fc receptors in the elicitation of end-organ damage, placing this gene in biological pathway 3 (Figure 2). In summary, analyses of congenic strains and ‘synthetic’ mouse models of systemic autoimmunity delineate three biological pathways that appear to play important roles in disease pathogenesis. Although the working model of SLE pathogenesis depicted in Figure 2 is consistent with all these results, much more work will be necessary to firmly establish the precise genetic processes mediating each
component phenotype. However, the inhibition of disease mediated by suppression of these pathways [7••,28••] suggests that genes in these pathways will represent key targets for the development of therapeutic agents for the treatment of systemic autoimmunity.
Genetics of SLE susceptibility in humans Several techniques, including association studies with specific alleles and genome scans, have been used to analyze the genetic basis for SLE susceptibility in humans. Recently, three large linkage studies have been performed for SLE using sib-pairs and extended family pedigrees [3•–5•]. Table 1 presents the parameters and test populations for each study and lists nine genomic intervals that were detected in at least two of the three independent studies. All three scans detected linkage on chromosome 1q41–q44 and to the MHC region on 6p11–p21, although the MHC was detected quite weakly in two of the three studies. In addition, two of the three studies detected linkage with 1p36, 1q23–q24, 2q21–q32, 6p11–p21, 14q21–q23, 16q13, 20p12–p13 and 20q11–q13 (Table 1). The strong and consistent linkage of SLE susceptibility with the 1q23 and 1q41–q44 regions is especially interesting for several reasons. The Fc receptors, located at 1q21–q23, have been previously linked to lupus susceptibility by association and case/control studies [29,30]. The Fc receptors for IgG — FcγRI, FcγRII and FcγRIII — are all encoded in this region, are expressed in a variety of cell types and bind IgG-containing immune complexes with distinct affinities [31]. Two common alleles, encoding His131 and Arg131 of FcγRIIA, show distinct association patterns with SLE and lupus nephritis. His131 causes an increased affinity for IgG2 compared with that of Arg131 and correlates with a higher association with lupus, especially lupus nephritis in African-American and Korean populations [32]. Findings that the distribution of FcγRIIIA alleles differs in SLE patients when compared with that in the normal population have also been reported. When valine is encoded at amino acid 176 instead of phenylalanine, this natural killer cell receptor has a higher binding affinity for IgG1 and IgG3 [30]. Tsao et al. [1] originally linked the 1q41–q44 segment of chromosome 1 with susceptibility to SLE when they performed a genome scan limited to human genomic regions that were syntenic with the positions of lupus-susceptibility genes in the mouse. The 1q23–q42 segment of human chromosome 1 is syntenic with the telomeric region of murine chromosome 1, a segment of the murine genome containing several different susceptibility loci that were detected in different crosses (see Figure 1). These results support the hypothesis that the same genes may play a crucial role in dictating susceptibility to lupus nephritis in man and mouse. Figure 3 shows a comparative map of this syntenic region of chromosome 1 in the human and mouse genomes. Intriguing similarities in the genetics of disease susceptibility in humans and mice are revealed by this
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Table 1 Summary of human genome-scan results from three independent studies. Study parameters Number of families Type of study Number of affected individuals Number of unaffected individuals Number of ethnic groups Caucasian families Mexican-American families African-American families Hispanic families European-American families Asian families Mixed heritage families Number of loci analyzed Basis for linkage Statistical method Linkage regions found in common between at least two studies
Study 1 [4•]
Study 2 [3•]
Study 3 [5•]
105 Sib-pairs 220 155 5 84 0 6 8 0 3 4
94 Extended pedigrees 220 313 2 0 0 31 0 55 0 0
80 Extended pedigrees 188 246 2 37 43 0 0 0 0 0
341 LOD ≥1.0 Nonparametric
312 LOD ≥1.5 Model-based, then nonparametric
350 Zlp>1.5 Nonparametric
1p36 – 1q42 6p11–p21* 2q21–33 14q21–23 16q13 20p12 –
– 1q23* 1q41 6p11–p21 2q32 – – – 20q13
1p36 1q24 1q44* 6p22 – 14q23 16q13 20p13 20q11
*Indicates strongest linkage.
comparison. Both murine and human linkage studies have detected multiple linkages in this segment, suggesting that a cluster of genes affecting systemic autoimmunity may be located in this region (Figures 1 and 3). In this
regard, our fine-mapping analysis of Sle1 in this region in mice has revealed the presence of at least four closely linked genes (Sle1a–Sle1d) affecting autoimmunity (Figure 3). This region is also extremely potent in both species
Figure 3
Comparison of linkages with SLE in humans and mice. The diagram shows the synteny between the 1q23–q42 region on human chromosome 1 and the telomeric portion of murine chromosome 1. The positions of loci that form the basis for the similarity of genes found in these segments are listed above the human 1q chromosomal diagram and below the mouse c1 diagram. The approximate position of 1q23 and 1q42 are presented above the human chromosome. Three studies [3•,5•,30] have reported associations of SLE susceptibility with the 1q23 region of human chromosome 1q (for loci under 1q22–23 there is some variation in the mapping data); four studies [1,3•–5•] have reported association with the 1q42 region. The diagonal lines creating a break in the human and murine chromosomes between the CRP and PARP genes represent the nonlinear relationship between human and mouse chromosomes in this region. The distance between CRP and PARP is much greater in human chromosome 1 than between CRP and Adprp (the murine equivalent of PARP) on mouse
Position
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chromosome 1. The positions of three SLE susceptibility genes in the Sle1 interval of murine chromosome 1 are placed between the human and mouse chromosomes in positions
identifying their approximate locations; the shaded area delineates the critical interval defining the location of each murine SLE susceptibility gene on the mouse
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and appears to be associated with susceptibility in multiple ethnic groups in humans and in multiple mouse strains. Although more work will be needed, the 1q21–q44 region of human chromosome 1 appears to contain an important cluster of autoimmune-susceptibility genes that also plays a major role in susceptibility in the spontaneous lupusprone mouse strains. This region of chromosome 1 contains several interesting candidate genes in addition to the Fc receptor genes described above. The APT1LG1 gene is the homologue of the Fas-ligand gene (also known as gld), which when mutated expresses a phenotype quite similar to that of the lpr/lpr Fas mutation in the MRL/lpr mouse model of lupus [33]. Mutations in the Fas-ligand gene can cause systemic autoimmunity in mice, thus supporting a potential role for this gene in SLE pathogenesis. Unfortunately, human studies have revealed no abnormality in the expression or function of the Fas apoptosis pathway. In a screen of 75 SLE patients, only one patient was found with a defect in the Fas ligand and none with a defect in Fas [34]. It is therefore believed that these genes are associated with SLE in only a small portion of patients. Recently, Tsao et al. [35] followed up on their original detection of linkage to SLE with 1q31–q41 and narrowed the interval to an approximately 5 cM region in 1q41–q42 that they optimistically estimate to contain only 150 genes. Three candidate genes were screened in this study: ADPRT, TGFβ2, and HLX1. Only ADPRT, a gene that is involved in cellular proliferation and apoptosis, showed lower than normal levels of activity and allelic distortion in patients with SLE. An 85 base pair polymorphism in the promoter region was preferentially transmitted to affected offspring, primarily in Caucasian populations. Notably, however, lupus-like phenotypes have not been reported in Adprt-knockout mice. In summary, significant progress has been made in characterizing the genetic basis for susceptibility to SLE in human families. Although the sample sizes of these initial studies are relatively small, several intervals were detected with fairly robust statistical correlation in multiple independent studies. This supports the possibility that at least some of the SLE genes will be potent and shared by a reasonable portion of susceptible individuals. In addition, the 1q23–q44 segment appears to be an outstanding example of syntenic conservation in susceptibility intervals between humans and mice. Clearly, identifying the causative genes within this segment in man and mouse will provide a unique opportunity to assess the relationship of genetic susceptibility in experimental models of systemic autoimmunity and human SLE patients.
Conclusions and future directions The complex inheritance of susceptibility to SLE indicates that it will be difficult to delineate the genetic mechanisms mediating disease pathogenesis. Multiple loci contribute to disease in both humans and experimental
mouse models and disease pathogenesis is expressed as a complex genetic interaction that is undoubtedly influenced by environmental factors. Recent advances in mouse models of lupus suggest that genes in three 'biological pathways' contribute to the development of disease pathogenesis. However, the molecular mechanisms mediating disease susceptibility in these pathways currently are poorly defined and are likely to be genetically heterogeneous. Consequently, a more detailed analysis of the precise pathways responsible for these in vivo phenotypes would provide important insights into disease pathogenesis. Nonetheless, the working model in Figure 2 provides a framework with which to interpret the manner in which individual loci with disparate properties interact to cause in vivo disease progression. The observed genetic synteny between human and murine susceptibility regions on chromosome 1 is intriguing and strongly suggests that at least some genetic elements of disease susceptibility will be shared between the two species. Although the identification of individual disease genes is extremely difficult for complex traits in humans, such analyses can be performed in experimental mouse models. Consequently, it is reasonable to predict that the murine lupus susceptibility genes in the chromosome 1 interval will be identified in the near future. Their identification should provide key new insights into the genetic mechanisms that are disrupted in susceptible individuals and may provide important entry points for the analysis of human disease. Finally, the detection of locus-specific genetic modifiers capable of suppressing disease progression supports the feasibility of therapeutic intervention in disease progression. Current data indicate that the suppression of either pathway 1 or 3 can inhibit disease development. However, the development of immunotherapeutic strategies based on these initial findings will require a detailed understanding of the molecular mechanisms that are dysregulated by susceptibility genes in each of these pathways.
References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest •• of outstanding interest 1.
Tsao BP, Cantor RM, Kalunian C, Chen C-J, Badsha H, Singh R, Wallace DJ, Kitridou RC, Chen S, Shen N et al.: Evidence for linkage of a candidate chromosome 1 region to human systemic lupus erythematosus. J Clin Invest 1997, 99:725-731.
2. •
Harley JB, Moser KL, Gaffney PM, Behrens TW: The genetics of human systemic lupus erythematosus. Curr Opin Immunol 1998, 10:690-696. This paper provides a well-referenced review of genetic studies of SLE susceptibility in humans. 3. •
Moser KL, Neas BR, Salmon JE, Yu H, Gray-McGuire C, Asundi N, Bruner GR, Fox J, Kelly J, Henshall S et al.: Genome scan of human systemic lupus erythematosus: evidence for linkage on chromosome 1q in african-american pedigrees. Proc Natl Acad Sci USA 1998, 95:14869-14874. See annotation [5•].
4. •
Gaffney PM, Kearns GM, Shark KB, Ortmann WA, Selby SA, Malmgren ML, Rohlf KE, Ockenden TC, Messner RP, Rich S, Behrens TW: A genome-wide search for susceptibility genes in human systemic lupus erythematosus sib-pair families. Proc Natl
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Acad Sci USA 1998, 95:14875-14879. See annotation [5•]. 5. •
Shai R, Quismorio F Jr, Li L, Kwon O-J, Morrison J, Wallace D, Neuwelt C, Brautbar C, Gauderman W, Jacob CO: Genome-wide screen for systemic lupus erythematosus susceptibility genes in multiplex families. Hum Mol Genet 1999, 8:639-644. These three papers [3•–5•] present whole-genome scans of separate collections of SLE-affected sib-pair families. 6.
Morel L, Rudofsky UH, Longmate JA, Schiffenbauer J, Wakeland EK: Polygenic control of susceptibility to murine systemic lupus erythematosus. Immunity 1994, 1:219-229.
7. ••
Morel L, Tian X-H, Croker BP, Wakeland EK: Epistatic modifiers of autoimmunity in a murine model of lupus nephritis. Immunity 1999, 11:131-139. Data presented in this report demonstrate that susceptibility to lupus nephritis is impacted by allele-specific genetic interactions between genes enhancing and suppressing disease. 8.
Morel L, Mohan C, Schiffenbauer J, Rudofsky UH, Tian J, Longmate J, Wakeland EK: Multiplex inheritance of component phenotypes in a murine model of lupus. Mamm Genome 1999, 10:176-181.
9.
Drake CG, Babcok SK, Palmer E, Kotzin BK: Genetic analysis of the NZB contribution to lupus-like autoimmune disease. Proc Natl Acad Sci USA 1994, 91:4062-4065.
10. Kono DH, Burlingame RW, Owens DG, Kuramochi A, Balderas RS, Balomenos D, Theofilopoulos AN: Lupus susceptibility loci in New Zealand mice. Proc Natl Acad Sci USA 1994, 91:10168-10172. 11. Merino R, Shibata T, De Kossodo S, Izui S: Differential effect of the autoimmune Yaa and lpr genes on the acceleration of lupus-like syndrome in MRL/Mpj mice. Eur J Immunol 1989, 19:2131-2137. 12. Hogarth MB, Slingsby JH, Allen PJ, Thompson EM, Chandler P, Davies KA, Simpson E, Morley BJ, Walport MJ: Multiple lupus susceptibility loci map to chromosome 1 in BXSB mice. J Immunol 1998, 161:2753-2761. 13. Santiago ML, Mary C, Parzy D, Jacquet C, Montagutelli X, Parkhouse RM, Lemoine R, Izui S, Reininger L: Linkage of a major quantitative trait locus to Yaa gene-induced lupus-like nephritis in (NZW x C57BL/6)F1 mice. Eur J Immunol 1998, 28:4257-4267. 14. Ida A, Hirose S, Hamano Y, Kodera S, Jiang Y, Abe M, Zhang D, Nishimura H, Shirai T: Multigenic control of lupus-associated antiphospholipid syndrome in a model of (NZW x BXSB) F1 mice. Eur J Immunol 1998, 28:2694-2703. 15. Morel L, Yu Y, Blenman KR, Caldwell RA, Wakeland EK: Production of congenic mouse strains carrying SLE-susceptibility genes derived from the SLE-prone NZM/Aeg2410 strain. Mamm Genome 1996, 7:335-339. 16. Mohan C, Morel L, Yang P, Watanabe H, Croker BP, Gilkeson GS, • Wakeland EK: Genetic dissection of lupus pathogenesis: a recipe for nephrophilic autoantibodies. J Clin Invest 1999, 103:1685-1695. This study presents data demonstrating that fatal lupus nephritis can be produced in normal B6 mice by the introduction of Sle1 and Sle3 from the NZM2410 lupus-prone mouse strain. 17.
Dietrich WF, Lander ES, Smith JS, Moser AR, Gould KA, Luongo C, Borenstein N, Dove W: Genetic identification of Mom-1, a major modifier locus affecting Min-induced intestinal neoplasia in the mouse. Cell 1993, 75:631-639.
18. Morel L, Mohan C, Croker BP, Tian X-H, Wakeland EK: Functional dissection of systemic lupus erythematosus using congenic mouse strains. J Immunol 1997, 158:6019-6028. 19. Mohan C, Alas E, Morel L, Yang P, Wakeland EK: Genetic dissection of SLE pathogenesis: Sle1 on murine chromosome 1 leads to a selective loss of tolerance to H2A/H2B/DNA subnucelosomes. J Clin Invest 1998, 101:1362-1372. 20. Winchester R: Genetic susceptibility to systemic lupus erythematosus. In Systemic Lupus Erythematosus. Edited by Lahita RG. New York: Churchill Livingstone; 1992:65-85.
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21. Mohan C, Morel L, Yang P, Wakeland EK: Genetic dissection of SLE pathogenesis: Sle2 on murine chromosome 4 leads to B-cell hyperactivity. J Immunol 1997, 159:454-465. 22. Mohan C, Yu Y, Morel L, Yang P, Wakeland EK: Genetic dissection of SLE pathogenicity: Sle3 on murine chromosome 7 impacts T cell activation, differentiation, and cell death. J Immunol 1999, 162:6492-6502. 23. Cohen PL, Eisenberg RA: Lpr and gld: single gene models of systemic autoimmunity and lymphoproliferative disease. Annu Rev Immunol 1991, 9:243-269. 24. Izui S, Merino R, Fossati L, Iwamoto M: The role of the Yaa gene in lupus syndrome. Int Rev Immunol 1994, 11:211-230. 25. Botto M, Dell’Agnola C, Bygrave AE, Thompson EM, Cook HT, Petry F, •• Loos M, Pandolfi PP, Walport MJ: Homozygous C1q deficiency causes glomerulonephritis associated with multiple apoptotic bodies. Nat Genet 1998, 19:56-59. This study presents results demonstrating that C1q deficiencies mediate a loss in tolerance to chromatin antigens and lead to an increase in apoptotic cell bodies in diseased kidneys. 26. Bickerstaff MC, Botto M, Hutchinson WL, Herbert J, Tennent GA, •• Bybee A, Mitchell DA, Cook HT, Butler PJ, Walport MJ, Pepys MB: Serum amyloid P component controls chromatin degradation and prevents antinuclear autoimmunity. Nat Med 1999, 5:694-697. This study demonstrates the effect of deficiencies in Sap on immune tolerance to chromatin and demonstrates that Sap deficiency impacts the clearance of chromatin in vivo. 27. ••
Cornall RJ, Cyster JG, Hibbs ML, Dunn AR, Otibopy KL, Clark EA, Goodnow CC: Polygenic autoimmune traits: Lyn, CD22, and SHP-1 are limiting elements of a biochemical pathway regulating BCR signaling and selection. Immunity 1998, 8:497-508. This study demonstrates the cumulative effect of multiple quantitative decreases in B cell antigen-receptor (BCR) signal transduction molecules on the immune system and postulates that such effects may be responsible for polygenic models of immune dysregulation. 28. Clynes R, Dumitru C, Ravetch JV: Uncoupling of immune complex •• formation and kidney damage in autoimmune glomerulonephritis. Science 1998, 279:1052-1054. This study characterizes the role of Fc receptors in mediating the development of inflammatory damage as a consequence of immune complex deposition in the kidney. 29. Salmon JE, Millard S, Schachter LA, Arnett FC, Ginzler EM, Gourley MF, Ramsey-Goldman R, Peterson MGE, Kimberly RP: FcgammaRIIA alleles are heritable risk factors for lupus nephritis in african americans. J Clin Invest 1996, 97:1348-1354. 30. Wu J, Edberg JC, Redecha PB, Bansal V, Guyre PM, Coleman K, Salmon JE, Kimberly RP: A novel polymorphism of FcgammaRIIIa (CD16) alters receptor function and predisposes to autoimmune disease. J Clin Invest 1997, 100:1059-1070. 31. Gibson A, Wu J, Edberg JC, Kimberly RP: Fcgamma receptor polymorphisms: insights into pathogenesis. In Lupus: Molecular and Cellular Pathogenesis. Edited by Kammer G, Tsokos GC. Totowa, NJ, USA: Human Press, Inc.; 1999:557-573. 32. Parren P, Warmerdam P, Boeije J, Arts L, Westerdaal N, Vlug A, Capel P, Aarden L, van de Winkel J: On the interaction of IgG subclasses with the low affinity FcGammaIIA. J Clin Invest 1992, 90:1537-1546. 33. Nagata S, Suda T: Fas and Fas ligand: lpr and gld mutations. Immunol Today 1995, 16:39-43. 34. Wu J, Wilson J, Xiang L, Schur PH, Mountz JD: Fas ligand mutation in a patient with systemic lupus erythematosus and lymphoproliferative disease. J Clin Invest 1996, 98:1107-1113. 35. Tsao BP, Cantor RM, Grossman J, Shen N, Teophilov N, Wallace DC, Arnett FC, Hartung K, Goldstein R, Kalunian K et al.: PARP alleles within the linked chromosomal region are associated with systemic lupus erythematosus. J Clin Invest 1999, 103:1135-1140.
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Genetic dissection of systemic lupus erythematosus Edward K Wakeland*, Amy E Wandstrat†, Kui Liu‡ and Laurence Morel§ Recent progress towards elucidating the genetic basis for susceptibility to systemic lupus erythematosus (SLE) has provided insights into the manner in which individual susceptibility genes contribute to disease pathogenesis. Studies in animal models of systemic autoimmunity suggest that genes in three separate pathways contribute to the initiation and progression of systemic autoimmunity. Linkage studies in humans suggest that at least some susceptibility genes mediating disease in lupus-prone mice may also contribute to susceptibility in humans. Addresses *†‡ Center for Immunology, The University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75235-9093, USA *email: [email protected] §Center for Mammalian Genetics and Department of Medicine, University of Florida, Gainesville, FL 32610-0275, USA Current Opinion in Immunology 1999, 11:701–707 0952-7915/99/$ — see front matter © 1999 Elsevier Science Ltd. All rights reserved.
genetic pathways that are critical to the development of severe lupus nephritis and have identified allele-specific, suppressive modifiers capable of dramatically influencing disease progression. The second important approach has been the ‘synthesis’ of mouse models of systemic autoimmunity via the production of targeted gene disruptions. These genetically synthesized models have identified specific genes or combinations of genes capable of causing disease pathogenesis when mutated. Here we will briefly summarize recent key discoveries in human and mouse genetics that have advanced our understanding of lupus susceptibility. In addition, we will present a hypothetical framework with which to view the role of specific genetic elements in disease progression. A comparison of the phenotypic features and linkage data in mice and humans reveals important similarities in the genetic mechanisms mediating systemic autoimmunity in the two species.
Linkage analysis of lupus nephritis in the mouse
Systemic lupus erythematosus (SLE) is a chronic autoimmune disease characterized by the production of autoantibodies to a spectrum of nuclear antigens. The clinical consequences of SLE are extremely heterogeneous, potentially leading to inflammatory damage in a variety of different organ systems. Although the factors responsible for the initiation of SLE are poorly understood, genetic predisposition is firmly established as a key element in susceptibility. Recently, several groups have completed initial linkage analyses in human families with SLE [1,2•–5•]. These studies have provided interesting insights into the complexity of the genetic interactions involved in SLE but have not yet resulted in the identification of specific genes or genetic pathways mediating SLE susceptibility. This reflects the difficulties inherent in the analysis of complex genetic diseases, coupled with the absence of sample repositories of sufficient size to allow definitive genetic analysis in humans.
The chromosomal locations of genes mediating susceptibility to lupus nephritis or systemic autoimmunity in the NZB/W, MRL and BXSB mouse models have been determined via genome scans [6,7••,8–13]. These studies have demonstrated that susceptibility to lupus in the mouse is inherited in a complex fashion that is reminiscent of human SLE, involving both genetic interactions and additive effects of individual genes. A compilation of the positions in the murine genome of the named SLE susceptibility loci from all these studies is provided in Figure 1. A total of 31 different gene designations have been defined thus far, distributed among 21 nonoverlapping 20 cM genomic intervals. This summary clearly illustrates the complexity of the genetic basis for susceptibility to autoimmunity. In addition to the susceptibility genes compiled in Figure 1, other investigators have determined the locations of loci affecting a variety of component phenotypes associated with systemic autoimmunity [14]. Taken together, these results indicate that the inheritance of systemic autoimmunity is extremely polygenic. However, as illustrated by the data in Figure 1, genomic segments on murine chromosomes 1, 4 and 7 are associated with disease susceptibility in multiple strain combinations — suggesting that these intervals may contain genes or clusters of genes that strongly influence autoimmunity.
Genetic analyses in the mouse have provided some important insights into the pathogenic processes mediating disease in experimental models of SLE. Two distinct approaches have been utilized to investigate the genetics of murine systemic autoimmunity. First, linkage analysis and congenic dissection have provided insights into the genetic basis for susceptibility in the classic lupus-prone mouse strains. These studies have delineated specific
The importance of epistatic interactions to the development of severe autoimmunity has recently been demonstrated via the analysis of congenic strains. Sle1 and Sle3 are NZWderived susceptibility genes for murine autoimmune lupus nephritis for which we produced B6-congenic strains [15]. Although neither of these genes mediates a severe disease when isolated on the B6 genome in interval-specific congenic strains, we recently demonstrated that Sle1 and Sle3 in
Abbreviations ANA antinuclear autoantibody Sap serum amyloid P component SLE systemic lupus erythematosus Sles SLE suppressor
Introduction
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Figure 1
D1MIT5
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D4MIT9 D4MIT31 D4Nds2 elp1
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Chromosomal locations of susceptibility loci (vertical white bars) associated with SLE. Linkage analysis was performed on the 19 murine autosomes (chromosome [c] 1–19) in murine test crosses [6,7••,8–13] involving NZB, NZW, NZM2410, BXSB and MRL/lpr mice. Each susceptibility locus is centered on the SSR (simple sequence repeat) marker, indicated by the horizontal line on the left,
that showed maximum linkage. Black vertical bars indicate the locations of suppressive modifiers responsible for the suppression of fatal disease in the NZW genome [7••]. The locus names are on the right of each bar. The length of each confidence interval (apart from Lrdm/Sle5) has been set arbitrarily at 20 cM, which typically corresponds to the support interval obtained for each locus.
combination as a bi-congenic strain (B6.NMZc1|c7) mediate the development of fatal lupus nephritis with a penetrance of >55% by 12 months of age [16•]. This result was surprising in that, although both of these genes are present in NZW mice, this strain does not develop severe humoral autoimmunity or glomerulonephritis — suggesting that the NZW genome must contain epistatic modifiers that suppress the development of severe autoimmunity. We
subsequently performed a genome scan and identified the positions of four epistatic modifiers that suppress autoimmunity in the NZW genome [7••]. These genes — which we designated SLE suppressor (Sles)1–Sles4 — are recessive, suppressive modifiers in the NZW genome whose cumulative effect accounted for the benign autoimmune phenotype of NZW. The positions of these suppressive modifiers are included with the susceptibility loci compiled
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in Figure 1. In this study we also found that Sles1 could specifically suppress the ability of Sle1 to cause a loss of immune tolerance to chromatin and that this effect was sufficient to inhibit fatal lupus nephritis [7••]. The detection of suppressive modifiers of systemic autoimmunity, which is consistent with observations in other complex traits [17], has significant implications for the genetics of systemic autoimmunity. Given the complexity of the immune system, it is reasonable to predict that epistasis will also impact SLE susceptibility in humans. If so, such interactions will create additional complications in the process of unraveling the genetics of human lupus. The presence of suppressive modifiers within specific families or ethnic groups would conceal the phenotypes of potent susceptibility genes in both linkage and allele-association studies. This could significantly weaken the power of such analyses. On the other hand, the presence of such potent suppressive modifiers in this disease suggests that a thorough understanding of the genetic pathways responsible for disease pathogenesis should reveal strategies for therapeutic intervention. The profound impact of epistasis on the expression of fatal lupus in the mouse supports the feasibility of identifying appropriate genetic targets for the development of effective therapies.
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Figure 2 Pathway 1: loss of tolerance to nuclear antigens
Susceptible individuals Sle1 Sap C1q Antinuclear autoantibodies 2–4% of population ANA positive Familial aggregation
Pathway 2: dysregulation of the immune system
Sle2 Sle3 Fas Lyn SHP-1 Pathogenic autoimmunity Familial aggregation Sle6 FcγRIII
Pathway 3: end-organ targeting Nephritis
Neurologic disorders
Arthritis
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Delineating genetic pathways in systemic autoimmunity Genetic analyses of interval-specific congenic strains have provided important insights into the component phenotypes contributed by individual genes to disease pathogenesis. Our analysis of congenic strains carrying genomic intervals with Sle1, Sle2 and Sle3 has provided a detailed characterization of the component autoimmune phenotype produced by each of these susceptibility genes. These results have allowed the development of a working model of the manner in which specific genes interact to mediate systemic autoimmunity. As diagrammed in Figure 2, the development of severe lupus nephritis in the mouse is controlled by interactions between genes in three separate biological pathways. The first pathway contains genes such as Sle1 that trigger the loss of immune tolerance to nuclear autoantigens and mediate the initiation of autoimmunity [18,19]. Genes in this pathway are capable of causing the initiation of a humoral autoimmune response to nuclear antigens; however, this response is minimally pathogenic in the absence of susceptibility genes in the other pathways. In this regard, many first-degree relatives of SLE probands exhibit a similar seropositive phenotype without severe disease pathogenesis [20]. The second biological pathway contributing to lupus susceptibility contains genes causing generalized immune hyper-responsiveness or dysregulation. Genes such as Sle2 [21], Sle3 [22], lpr [23], gld [23] and Yaa [24] would all be included in this pathway. These genes often do not generate autoimmune phenotypes in
A diagram of the hypothetical stages of pathogenesis in human SLE and murine lupus nephritis. The biological pathways responsible for the transition of individuals between different stages, together with a brief description of the key characteristics of each, are listed on the left. The susceptibility genes and genes/molecules that are involved within each biological pathway are listed to the right of each arrow. The arrows delineate the transitions of affected individuals between individual stages in disease pathogenesis and the newly acquired disease parameters are briefly described below the key component phenotype associated with each stage. Several separate pathways are potentially involved in the transition to the final stage because a variety of end organs may become involved in human SLE.
lupus-resistant genomes but strongly enhance the expansion of the autoimmune response when combined with genes that mediate the loss of tolerance to nuclear autoantigens. The final class of lupus susceptibility genes, represented by Sle6 in our model, potentiates end-organ damage. Theoretically, end-organ damage could be enhanced by a variety of molecular mechanisms — including genes that modify immune effector functions (such as Fc receptors; see below) and those that modify the end organ itself. Support for this model has recently been produced in our analysis of B6.NZMc1|c7 mice, which develop fatal lupus due to the epistatic interactions of genes in biological pathways 1 (Sle1) and 2 (Sle3) [16•]. Several groups have obtained, via the analysis of targeted gene disruptions, data implicating specific genes in the pathogenesis of systemic autoimmunity. Botto et al. [25••] recently demonstrated that mice carrying a targeted disruption of the gene encoding the C1q complement
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component spontaneously developed systemic autoimmunity with many of the phenotypes commonly associated with SLE. Interestingly, the severity of the autoimmune phenotype was strongly impacted by epistatic modifiers segregating between B6 and 129 (these are mouse strains) — suggesting that C1q deficiency may interact with other genes (possibly those in pathway 2 of Figure 2) to mediate severe systemic autoimmunity. Bickerstaff et al. [26••], in their analysis of the autoimmune phenotypes elicited by targeted disruptions of the gene encoding serum amyloid P component (Sap), obtained very similar results. These investigators postulate that Sap- and C1qdeficiencies potentiate the production of antinuclear autoantibodies (ANAs) due to the disruption of their functions in the pathway mediating degradation and removal of nuclear chromatin. The ability of these deficiencies to elicit ANA production is reminiscent of the phenotype produced by Sle1, suggesting that Sap and C1q would be elements of biological pathway 1 (see Figure 2). Cornall et al. [27••] recently characterized the impact of quantitative decreases in key components of the B cell antigen-receptor (BCR)-mediated signal transduction pathway on systemic autoimmunity. These investigators demonstrated that quantitative decreases in the signal transduction components encoded by Lyn, Cd22 and Shp-1 lower the activation thresholds of B cells — resulting in the development of B cell hyper-reactivity. The investigators postulate that they have synthesized a model system illustrating the potential cumulative impact of minor quantitative changes in the expression of signal transduction molecules on the immune system. They hypothesize that such changes would be an example of a polygenic autoimmune phenotype capable of mediating a dysregulation of the immune system. Such an effect is consistent with the phenotypes of genes such as Sle2, Sle3, lpr, gld and Yaa — suggesting that they are examples of genetic processes in biological pathway 2 (see Figure 2). Finally, Clynes et al. [28••] recently reported that a disruption of the Fc receptor γ chain gene, leading to a deficiency in Fc receptor expression, severely inhibits the development of fatal glomerulonephritis in the NZB/NZW mouse model of SLE. Since the production of autoantibodies was not impaired in these mice, the authors concluded that Fc receptor expression is essential for the triggering of the inflammatory pathway normally elicited by immune complex deposition. This phenotype is consistent with a function of Fc receptors in the elicitation of end-organ damage, placing this gene in biological pathway 3 (Figure 2). In summary, analyses of congenic strains and ‘synthetic’ mouse models of systemic autoimmunity delineate three biological pathways that appear to play important roles in disease pathogenesis. Although the working model of SLE pathogenesis depicted in Figure 2 is consistent with all these results, much more work will be necessary to firmly establish the precise genetic processes mediating each
component phenotype. However, the inhibition of disease mediated by suppression of these pathways [7••,28••] suggests that genes in these pathways will represent key targets for the development of therapeutic agents for the treatment of systemic autoimmunity.
Genetics of SLE susceptibility in humans Several techniques, including association studies with specific alleles and genome scans, have been used to analyze the genetic basis for SLE susceptibility in humans. Recently, three large linkage studies have been performed for SLE using sib-pairs and extended family pedigrees [3•–5•]. Table 1 presents the parameters and test populations for each study and lists nine genomic intervals that were detected in at least two of the three independent studies. All three scans detected linkage on chromosome 1q41–q44 and to the MHC region on 6p11–p21, although the MHC was detected quite weakly in two of the three studies. In addition, two of the three studies detected linkage with 1p36, 1q23–q24, 2q21–q32, 6p11–p21, 14q21–q23, 16q13, 20p12–p13 and 20q11–q13 (Table 1). The strong and consistent linkage of SLE susceptibility with the 1q23 and 1q41–q44 regions is especially interesting for several reasons. The Fc receptors, located at 1q21–q23, have been previously linked to lupus susceptibility by association and case/control studies [29,30]. The Fc receptors for IgG — FcγRI, FcγRII and FcγRIII — are all encoded in this region, are expressed in a variety of cell types and bind IgG-containing immune complexes with distinct affinities [31]. Two common alleles, encoding His131 and Arg131 of FcγRIIA, show distinct association patterns with SLE and lupus nephritis. His131 causes an increased affinity for IgG2 compared with that of Arg131 and correlates with a higher association with lupus, especially lupus nephritis in African-American and Korean populations [32]. Findings that the distribution of FcγRIIIA alleles differs in SLE patients when compared with that in the normal population have also been reported. When valine is encoded at amino acid 176 instead of phenylalanine, this natural killer cell receptor has a higher binding affinity for IgG1 and IgG3 [30]. Tsao et al. [1] originally linked the 1q41–q44 segment of chromosome 1 with susceptibility to SLE when they performed a genome scan limited to human genomic regions that were syntenic with the positions of lupus-susceptibility genes in the mouse. The 1q23–q42 segment of human chromosome 1 is syntenic with the telomeric region of murine chromosome 1, a segment of the murine genome containing several different susceptibility loci that were detected in different crosses (see Figure 1). These results support the hypothesis that the same genes may play a crucial role in dictating susceptibility to lupus nephritis in man and mouse. Figure 3 shows a comparative map of this syntenic region of chromosome 1 in the human and mouse genomes. Intriguing similarities in the genetics of disease susceptibility in humans and mice are revealed by this
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Table 1 Summary of human genome-scan results from three independent studies. Study parameters Number of families Type of study Number of affected individuals Number of unaffected individuals Number of ethnic groups Caucasian families Mexican-American families African-American families Hispanic families European-American families Asian families Mixed heritage families Number of loci analyzed Basis for linkage Statistical method Linkage regions found in common between at least two studies
Study 1 [4•]
Study 2 [3•]
Study 3 [5•]
105 Sib-pairs 220 155 5 84 0 6 8 0 3 4
94 Extended pedigrees 220 313 2 0 0 31 0 55 0 0
80 Extended pedigrees 188 246 2 37 43 0 0 0 0 0
341 LOD ≥1.0 Nonparametric
312 LOD ≥1.5 Model-based, then nonparametric
350 Zlp>1.5 Nonparametric
1p36 – 1q42 6p11–p21* 2q21–33 14q21–23 16q13 20p12 –
– 1q23* 1q41 6p11–p21 2q32 – – – 20q13
1p36 1q24 1q44* 6p22 – 14q23 16q13 20p13 20q11
*Indicates strongest linkage.
comparison. Both murine and human linkage studies have detected multiple linkages in this segment, suggesting that a cluster of genes affecting systemic autoimmunity may be located in this region (Figures 1 and 3). In this
regard, our fine-mapping analysis of Sle1 in this region in mice has revealed the presence of at least four closely linked genes (Sle1a–Sle1d) affecting autoimmunity (Figure 3). This region is also extremely potent in both species
Figure 3
Comparison of linkages with SLE in humans and mice. The diagram shows the synteny between the 1q23–q42 region on human chromosome 1 and the telomeric portion of murine chromosome 1. The positions of loci that form the basis for the similarity of genes found in these segments are listed above the human 1q chromosomal diagram and below the mouse c1 diagram. The approximate position of 1q23 and 1q42 are presented above the human chromosome. Three studies [3•,5•,30] have reported associations of SLE susceptibility with the 1q23 region of human chromosome 1q (for loci under 1q22–23 there is some variation in the mapping data); four studies [1,3•–5•] have reported association with the 1q42 region. The diagonal lines creating a break in the human and murine chromosomes between the CRP and PARP genes represent the nonlinear relationship between human and mouse chromosomes in this region. The distance between CRP and PARP is much greater in human chromosome 1 than between CRP and Adprp (the murine equivalent of PARP) on mouse
Position
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chromosome 1. The positions of three SLE susceptibility genes in the Sle1 interval of murine chromosome 1 are placed between the human and mouse chromosomes in positions
identifying their approximate locations; the shaded area delineates the critical interval defining the location of each murine SLE susceptibility gene on the mouse
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and appears to be associated with susceptibility in multiple ethnic groups in humans and in multiple mouse strains. Although more work will be needed, the 1q21–q44 region of human chromosome 1 appears to contain an important cluster of autoimmune-susceptibility genes that also plays a major role in susceptibility in the spontaneous lupusprone mouse strains. This region of chromosome 1 contains several interesting candidate genes in addition to the Fc receptor genes described above. The APT1LG1 gene is the homologue of the Fas-ligand gene (also known as gld), which when mutated expresses a phenotype quite similar to that of the lpr/lpr Fas mutation in the MRL/lpr mouse model of lupus [33]. Mutations in the Fas-ligand gene can cause systemic autoimmunity in mice, thus supporting a potential role for this gene in SLE pathogenesis. Unfortunately, human studies have revealed no abnormality in the expression or function of the Fas apoptosis pathway. In a screen of 75 SLE patients, only one patient was found with a defect in the Fas ligand and none with a defect in Fas [34]. It is therefore believed that these genes are associated with SLE in only a small portion of patients. Recently, Tsao et al. [35] followed up on their original detection of linkage to SLE with 1q31–q41 and narrowed the interval to an approximately 5 cM region in 1q41–q42 that they optimistically estimate to contain only 150 genes. Three candidate genes were screened in this study: ADPRT, TGFβ2, and HLX1. Only ADPRT, a gene that is involved in cellular proliferation and apoptosis, showed lower than normal levels of activity and allelic distortion in patients with SLE. An 85 base pair polymorphism in the promoter region was preferentially transmitted to affected offspring, primarily in Caucasian populations. Notably, however, lupus-like phenotypes have not been reported in Adprt-knockout mice. In summary, significant progress has been made in characterizing the genetic basis for susceptibility to SLE in human families. Although the sample sizes of these initial studies are relatively small, several intervals were detected with fairly robust statistical correlation in multiple independent studies. This supports the possibility that at least some of the SLE genes will be potent and shared by a reasonable portion of susceptible individuals. In addition, the 1q23–q44 segment appears to be an outstanding example of syntenic conservation in susceptibility intervals between humans and mice. Clearly, identifying the causative genes within this segment in man and mouse will provide a unique opportunity to assess the relationship of genetic susceptibility in experimental models of systemic autoimmunity and human SLE patients.
Conclusions and future directions The complex inheritance of susceptibility to SLE indicates that it will be difficult to delineate the genetic mechanisms mediating disease pathogenesis. Multiple loci contribute to disease in both humans and experimental
mouse models and disease pathogenesis is expressed as a complex genetic interaction that is undoubtedly influenced by environmental factors. Recent advances in mouse models of lupus suggest that genes in three 'biological pathways' contribute to the development of disease pathogenesis. However, the molecular mechanisms mediating disease susceptibility in these pathways currently are poorly defined and are likely to be genetically heterogeneous. Consequently, a more detailed analysis of the precise pathways responsible for these in vivo phenotypes would provide important insights into disease pathogenesis. Nonetheless, the working model in Figure 2 provides a framework with which to interpret the manner in which individual loci with disparate properties interact to cause in vivo disease progression. The observed genetic synteny between human and murine susceptibility regions on chromosome 1 is intriguing and strongly suggests that at least some genetic elements of disease susceptibility will be shared between the two species. Although the identification of individual disease genes is extremely difficult for complex traits in humans, such analyses can be performed in experimental mouse models. Consequently, it is reasonable to predict that the murine lupus susceptibility genes in the chromosome 1 interval will be identified in the near future. Their identification should provide key new insights into the genetic mechanisms that are disrupted in susceptible individuals and may provide important entry points for the analysis of human disease. Finally, the detection of locus-specific genetic modifiers capable of suppressing disease progression supports the feasibility of therapeutic intervention in disease progression. Current data indicate that the suppression of either pathway 1 or 3 can inhibit disease development. However, the development of immunotherapeutic strategies based on these initial findings will require a detailed understanding of the molecular mechanisms that are dysregulated by susceptibility genes in each of these pathways.
References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest •• of outstanding interest 1.
Tsao BP, Cantor RM, Kalunian C, Chen C-J, Badsha H, Singh R, Wallace DJ, Kitridou RC, Chen S, Shen N et al.: Evidence for linkage of a candidate chromosome 1 region to human systemic lupus erythematosus. J Clin Invest 1997, 99:725-731.
2. •
Harley JB, Moser KL, Gaffney PM, Behrens TW: The genetics of human systemic lupus erythematosus. Curr Opin Immunol 1998, 10:690-696. This paper provides a well-referenced review of genetic studies of SLE susceptibility in humans. 3. •
Moser KL, Neas BR, Salmon JE, Yu H, Gray-McGuire C, Asundi N, Bruner GR, Fox J, Kelly J, Henshall S et al.: Genome scan of human systemic lupus erythematosus: evidence for linkage on chromosome 1q in african-american pedigrees. Proc Natl Acad Sci USA 1998, 95:14869-14874. See annotation [5•].
4. •
Gaffney PM, Kearns GM, Shark KB, Ortmann WA, Selby SA, Malmgren ML, Rohlf KE, Ockenden TC, Messner RP, Rich S, Behrens TW: A genome-wide search for susceptibility genes in human systemic lupus erythematosus sib-pair families. Proc Natl
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Acad Sci USA 1998, 95:14875-14879. See annotation [5•]. 5. •
Shai R, Quismorio F Jr, Li L, Kwon O-J, Morrison J, Wallace D, Neuwelt C, Brautbar C, Gauderman W, Jacob CO: Genome-wide screen for systemic lupus erythematosus susceptibility genes in multiplex families. Hum Mol Genet 1999, 8:639-644. These three papers [3•–5•] present whole-genome scans of separate collections of SLE-affected sib-pair families. 6.
Morel L, Rudofsky UH, Longmate JA, Schiffenbauer J, Wakeland EK: Polygenic control of susceptibility to murine systemic lupus erythematosus. Immunity 1994, 1:219-229.
7. ••
Morel L, Tian X-H, Croker BP, Wakeland EK: Epistatic modifiers of autoimmunity in a murine model of lupus nephritis. Immunity 1999, 11:131-139. Data presented in this report demonstrate that susceptibility to lupus nephritis is impacted by allele-specific genetic interactions between genes enhancing and suppressing disease. 8.
Morel L, Mohan C, Schiffenbauer J, Rudofsky UH, Tian J, Longmate J, Wakeland EK: Multiplex inheritance of component phenotypes in a murine model of lupus. Mamm Genome 1999, 10:176-181.
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