Multiple loci are linked with anti-red blood cell antibody production in NZB mice -- comparison with other phenotypes implies complex modes of action

The New Zealand Black (NZB) mouse strain is a model of autoimmune haemolytic anaemia (AHA) and systemic lupus erythematosus (SLE), characterized by the production of anti-red blood cell (RBC) antibodies and anti-nuclear antibodies (ANA), respectively. A linkage analysis was carried out in an (NZB x BALB/c) F(2) cross in order to identify loci involved in the production of both anti-RBC IgM and IgG antibodies. These regions of linkage were compared with linkage data to ANA from the same cohort and other linkage analyses involving New Zealand mice. Four previously described NZB loci linked to anti-RBC antibodies were confirmed, and eight novel loci linked to this trait were also mapped: five of which were of NZB origin, and three derived from the non-autoimmune BALB/c background. A comparison between loci linked with anti-RBC antibodies and ANA demonstrated many that co-localize, suggesting the presence of genes that result in the general breaking of tolerance to self-antigen. Furthermore, the observation that some loci were associated only with the anti-RBC response suggests an antigen specific mechanism in addition to a general breaking of tolerance. A locus linked with anti-RBC antibodies and ANA on distal chromosome 7 in this cohort is orthologous to one on the q arm of human chromosome 11, a region linked to AHA and ANA in human SLE.

The candidate gene approach: have murine models informed the study of human SLE?

Genome wide linkage studies in human SLE have identified seven highly significant loci linked to SLE, and more than 20 other loci showing suggestive linkage to disease. However, pin-pointing the susceptibility alleles in candidate genes within these linkage regions is challenging, due the genetic heterogeneity, racial differences and environmental influences on disease aetiology. Utilization of murine models of spontaneous lupus nephritis provide a complementary approach, which may then identify candidate genes for analysis in human cases. This review highlights the utility of cross-species approach to identify and characterize the effect of given candidate genes in lupus. The examples described in this review demonstrate the importance of bringing together both genetic and functional information in human and mouse studies.

Evidence for an interferon-inducible gene, Ifi202, in the susceptibility to systemic lupus

The Nba2 locus is a major genetic contribution to disease susceptibility in the (NZB x NZW)F(1) mouse model of systemic lupus. We generated C57BL/6 mice congenic for this NZB locus, and these mice produced antinuclear autoantibodies characteristic of lupus. F(1) offspring of congenic and NZW mice developed high autoantibody levels and severe lupus nephritis similar to (NZB x NZW)F(1) mice. Expression profiling with oligonucleotide microarrays revealed only two differentially expressed genes, interferon-inducible genes Ifi202 and Ifi203, in congenic versus control mice, and both were within the Nba2 interval. Quantitative PCR localized increased Ifi202 expression to splenic B cells and non-T/non-B cells. These results, together with analyses of promoter region polymorphisms, strain distribution of expression, and effects on cell proliferation and apoptosis, implicate Ifi202 as a candidate gene for lupus.

Autoantigen glycoprotein 70 expression is regulated by a single locus, which acts as a checkpoint for pathogenic anti-glycoprotein 70 autoantibody production and hence for the corresponding development of severe nephritis, in lupus-prone PXSB mice

Retroviral envelope glycoprotein gp70 is present in the sera of immunologically normal and autoimmune-prone strains of mice. However, only lupus-prone mice spontaneously develop gp70-anti-gp70 immune complexes (gp70IC), and these have been implicated in the development of nephritis. We investigated the genetic factors that affect the production of both free serum gp70 and gp70IC in the lupus-prone BXSB mouse strain by analyzing (BXSB x (C57BL/10 x BXSB)F(1))- and (C57BL/10 x (C57BL/10 x BXSB)F(1))-backcrossed male mice. Production of gp70 mapped to a single major locus located on chromosome 13 (Bxs6) with a maximum log likelihood of the odds of 36.7 (p = 1.6 x 10(-38)). The level of gp70IC was highly dependent on Bxs6-related gp70 production, and high titer autoantibody production only occurred when serum gp70 levels were greater than a threshold value of approximately 4.0 microg/ml. The subdivision of the (BXSB x (C57BL/10 x BXSB)F(1))-backcrossed mice into those homozygous or heterozygous for Bxs6 enabled a remarkable association to be observed between high levels of gp70IC and severe nephritis in the Bxs6 homozygote population. A further mapping study in these two subgroups identified a previously unrecognized interval associated with the production of autoantibodies.

Genetic control of glycoprotein 70 autoantigen production and its influence on immune complex levels and nephritis in murine lupus

The F1 hybrids of New Zealand Black (NZB) and New Zealand White (NZW) mice spontaneously develop an autoimmune disease that serves as a model for human systemic lupus erythematosus. Autoimmunity in (NZB x NZW)F1 mice includes the production of autoantibodies to the endogenous retroviral envelope glycoprotein, gp70, and gp70-anti-gp70 immune complexes (gp70 IC) have been implicated in the development of lupus nephritis in these animals. We used backcross and intercross combinations of C57BL/6 (B6; low gp70 levels) and NZB mice (high gp70 levels) to examine the contribution of serum gp70 Ag levels to the development of gp70 IC and nephritis. Analysis of (B6.H2z x NZB)F1 x NZB backcross mice and (NZB x B6)F2 mice showed a much stronger association of gp70 IC with kidney disease compared with IgG anti-chromatin autoantibodies in both populations of mice. Serum levels of gp70 correlated with production of gp70 IC in mice producing autoantibodies, although the overall effect on nephritis appeared to be small. Genetic mapping revealed three NZB-derived regions on chromosomes 2, 4, and 13 that were strongly linked with increased gp70 levels, and together, accounted for over 80% of the variance for this trait. However, additional linkage analyses of these crosses showed that loci controlling autoantibody production rather than gp70 levels were most important in the development of nephritogenic immune complexes. Together, these studies characterize a set of lupus-susceptibility loci distinct from those that control autoantibody production and provide new insight into the components involved in the strong association of gp70 IC with murine lupus nephritis.

The genetics of systemic lupus erythematosus

BACKGROUND

There is a genetic predisposition to human systemic lupus erythematosus (SLE). The genes that contribute to susceptibility are, for the most part, unknown. The introduction of new gene mapping techniques has opened the way to explore lupus genetics on a genome-wide basis.

METHODS

Microsatellites are simple sequence repeats widely distributed throughout eukaryotic genomes. They exhibit length variation. This polymorphism can be exploited to provide a panoply of genome-wide markers. Thereby, loci linked with lupus have been mapped in lupus-prone mouse strains and in recently published studies in multi-case human families.

RESULTS

More than 20 non-MHC (major histocompatibility complex) loci have now been linked with murine lupus. Nine non-MHC loci have been corroborated in human SLE. Some of the mouse intervals are syntenic with human loci raising the tantalizing possibility of common susceptibility genes. Although we await the results of formal gene identification, functional studies in back-cross and congenic analyses indicate that, in the mouse at least, disease genes act at multiple levels in disease development.

CONCLUSIONS

A large number of genes are involved in the pathogenesis of SLE. The data also suggest that even the MHC contribution is multiple. Having mapped disease loci, geneticists now face the task of closing down on the actual aetiological alleles and demonstrating how they might operate. This undertaking will add significantly to our understanding of disease development.

Enhanced susceptibility to lupus contributed from the nonautoimmune C57BL/10, but not C57BL/6, genome

Genes from New Zealand Black and New Zealand White mice have been implicated in the development of a disease similar to human systemic lupus erythematosus. In an attempt to define the MHC class II genes involved in disease, we previously studied similarly designed backcrosses of New Zealand Black mice with C57BL/6 (B6) mice transgenic for Ez genes or with C57BL/10 (B10) mice transgenic for Az genes. Although the transgenes showed no effect on the development of autoantibody production or lupus nephritis in either backcross, surprisingly, there was greatly increased expression of these disease traits in the backcrosses involving B10 compared with B6 mice. These studies therefore implicated genetic contributions in B10 vs B6 backgrounds, despite their 98% identity. A genome-wide linkage analysis uncovered a B10 locus on mid-chromosome 13, which enhanced nephritis and was strongly linked with the production of pathogenic retroviral gp70-anti-gp70 immune complexes when contributed by B10, but not B6, mice. The subsequent identification of a single marker polymorphic between B10 and B6, along with the extreme genetic similarity between the two strains in this region, is likely to permit expedited identification of the lupus-susceptibility gene from this nonautoimmune strain.

Control of separate pathogenic autoantibody responses marks MHC gene contributions to murine lupus

Previous studies have suggested that MHC and non-MHC genes contribute to the development of autoimmune disease in F1 hybrids of New Zealand black (NZB) and white (NZW) mice. We conducted a genome-wide screen of 148 female (NZB x NZW)F1 x NZB backcross mice to map dominant NZW genetic loci linked with lupus disease traits. In this backcross analysis, inheritance of the NZW MHC (H2(d/z) vs. H2(d/d)) was strongly linked with the development of lupus nephritis (P approximately 1 x 10(-16)), increasing the risk of disease by over 30-fold. H2(d/z) was also linked with elevated serum levels of IgG autoantibodies to single-stranded DNA, double-stranded DNA, histones, and chromatin but not with anti-gp70 autoantibodies, measured as circulating gp70-anti-gp70 immune complexes. Non-MHC contributions from NZW seemed weak in comparison to MHC, although NZW loci on chromosomes 7 and 16 were noted to be suggestively linked with autoantibody production. Strikingly, H2(d/z) (compared with H2(d/d)) enhanced antinuclear antibodies in a coordinate fashion but did not affect anti-gp70 production in the current backcross. However, the opposite influence was noted for H2(d/z) (compared with H2(z/z)) when (NZB x NZW)F1 x NZW backcross mice were analyzed. These results suggest that H2(z) and H2(d) haplotypes differentially regulate two different sets of nephritogenic autoantibody responses. This study confirms a critical role for H2(z) compared with other dominant NZW loci in (NZB x NZW)F1 mice and provides an explanation as to why H2(d/z) heterozygosity is required for full expression of disease in this model.

Analysis of MHC class II genes in the susceptibility to lupus in New Zealand mice

Hybrids of New Zealand Black (NZB) and New Zealand White (NZW) mice spontaneously develop a disease similar to human systemic lupus erythematosus. MHC and non-MHC genes contribute to disease susceptibility in this murine model. Multiple studies have shown that the NZW H2z locus is strongly associated with the development of lupus-like disease in these mice. The susceptibility gene(s) within H2z is not known, but different lines of evidence have pointed to class II MHC genes, either H2-E or H2-A (Ez or Az in NZW). Recent studies from our laboratory showed that Ez does not supplant H2z in the contribution to lupus-like disease. In the present work we generated C57BL/10 (B10) mice transgenic for Aaz and Abz genes (designated B10.Az mice) and used a (B10.Az x NZB)F1 x NZB backcross to assess the contributions of Az genes to disease. A subset of backcross mice produced high levels of IgG autoantibodies and developed severe nephritis. However, no autoimmune phenotype was linked to the Az transgenes. Surprisingly, in the same backcross mice, inheritance of H2b from the nonautoimmune B10 strain was strongly linked with both autoantibody production and nephritis. Taken together with our previous Ez studies, the present work calls into question the importance of class II MHC genes for lupus susceptibility in this model and provides new insight into the role of MHC in lupus-like autoimmunity.

Thymic microenvironment and NZB mice: the abnormal thymic microenvironment of New Zealand mice correlates with immunopathology

There are distinct microenvironmental abnormalities of thymic architecture in several murine models of SLE defined using immunohistochemistry and a panel of mAb dissected at thymic epithelial markers. To address the issue of the relationship between the thymic microenvironment and autoimmunity, we studied backcross (NZB x NZW) F1 x NZW mice in which 50% of offspring develop nephritis associated with proteinuria and anti-DNA antibodies. We reasoned that if thymic abnormalities are associated with development of disease, the correlation of abnormalities with lupus-like disease in individual backcross mice will form the foundation for identification of the mechanisms involved. In parallel, we directed a genetic linkage analysis, using markers previously shown to be linked to nephritis and IgG autoantibody production, to determine if such loci were similarly associated with microenvironmental changes. Our data demonstrate that all (NZB x NZW) F1 x NZW backcross mice with disease have microenvironmental defects. Although the microenvironmental defects are not sufficient for development of autoimmune disease, the severity of thymic abnormalities correlates with titers of IgG autoantibodies to DNA and with proteinuria. Consistent with past studies of (NZB x NZW) F1 x NZW mice, genetic markers on proximal chromosome 17 (near MHC) and distal chromosome 4 showed trends for linkage with nephritis. Although the markers chosen only covered about 10-15% of the genome, the results demonstrated trends for linkage with thymic medullary abnormalities for loci on distal chromosome 4 and distal chromosome 1. We believe it will be important to define the biochemical nature of the molecules recognized by these mAbs to understand the relationships between thymic architecture and immunopathology.

Genetic susceptibility to systemic lupus erythematosus

Considerable evidence suggests that the development of systemic lupus erythematosus (SLE) has a strong genetic basis. Recent studies have emphasized that this disease, like other autoimmune diseases, is a complex genetic trait with contributions from major histocompatibility complex (MHC) genes and multiple non-MHC genes. Etiologic genes in these disorders determine susceptibility, and no particular gene is necessary or sufficient for disease expression. Studies of murine models of lupus have provided important insight into the immunopathogenesis of IgG autoantibody production and lupus nephritis, and genetic analyses of these mice overcome certain obstacles encountered when studying patients. Genome-wide linkage studies of different crosses have mapped the position of at least 12 non-MHC disease-susceptibility loci in the New Zealand hybrid model of lupus. Although the identity of the actual genes is currently unknown, recent studies have begun to characterize how these genetic contributions may function in the autoimmune process, especially in terms of their role in autoantibody production. Studies of MHC gene contributions in New Zealand mice have shown that heterozygosity for particular haplotypes greatly increases pathogenic autoantibody production and the incidence of severe nephritis. The mechanism for this effect appears to be genetically complex. Studies in human SLE have mostly focused on the association of disease with alleles of immunologically relevant genes, especially in the MHC. Associations with various complement component deficiencies and an allele of a particular Fc gamma receptor gene (FCGR2A) also have been described. In a diversion from previous association studies, a recent directed linkage analysis of sibpairs with SLE was based on mapping studies in murine lupus and may be an important step toward identifying a new disease-susceptibility gene in patients. Since the genes that predispose to autoimmunity are probably related to key events in pathogenesis, their identification in patients and murine models will almost certainly provide important insight into the breakdown of immunological self-tolerance and the cause of autoimmune disease.

Contributions of Ea(z) and Eb(z) MHC genes to lupus susceptibility in New Zealand mice

Unlike parental New Zealand Black (NZB) or New Zealand White (NZW) mice, (NZB x NZW)F1 mice exhibit a lupus-like disease characterized by IgG autoantibody production and severe immune complex-mediated nephritis. In studies of the genetic susceptibility to disease in this F1 model, the NZW MHC (H2z) has been strongly linked with the development of disease, and it was hypothesized that class II MHC genes, particularly Ez genes, may underlie this genetic contribution. In the present study, we bred transgenic B6 mice expressing I-Ez or congenic B6 mice carrying H2z with NZB mice and used a backcross analysis to test the hypothesis that Ea(z) and/or Eb(z) genes account for the effect of H2z on disease. The genetic analysis of different backcross combinations showed that unlike mice carrying H2z, mice inheriting Ez transgenes do not demonstrate increased IgG autoantibody production or increased incidence of nephritis. Surprisingly, in the same transgenic backcross mice, inheritance of the endogenous H2b from the B6 strain was strongly linked with the production of IgG autoantibodies, but not with disease. Additional experiments suggested that the level of IgG3 autoantibody production, which is controlled by H2, may be important in the pathogenesis of renal disease. Contributions to autoantibody production were also detected from an NZB locus on distal chromosome 1 (previously named Nba2). Together, these studies provide new insight into the role of MHC in lupus-like autoimmunity.

Control of multiple autoantibodies linked with a lupus nephritis susceptibility locus in New Zealand black mice

An NZB locus on distal chromosome 1 has been linked to murine lupus nephritis in backcross analyses of New Zealand mice. This locus, designated Nba2 for New Zealand Black autoimmunity 2, was found to colocalize in both (NZB x SM/J)F1 x NZW and (B6.H2z x NZB)F1 x NZB backcrosses, and was most likely situated between 92 and 97 cM from the centromere. This region of mouse chromosome 1 encodes several candidate genes, including the low affinity Fc gamma receptor genes. Both backcrosses were examined by interval mapping for quantitative trait loci linked with autoantibody and total Ig production. Nba2 was linked with elevated serum levels of multiple autoantibodies, including a variety of antinuclear Abs (anti-dsDNA, anti-chromatin and anti-histone) and autoantibodies to gp70, in both backcrosses. Nba2 was also linked (or showed a trend for linkage) with hypergammaglobulinemia and IgG1, IgG2a, and/or IgG3 levels in each backcross. In the (B6.H2z x NZB)F1 x NZB backcross, MHC was an additional genetic contribution that interacted with Nba2 in the production of autoantibodies and the development of nephritis. Together, these data provide new insight into the nature of one important genetic contribution to murine lupus and suggest that Nba2 may act as an immune response gene that influences Ag-driven B cell responses to self and possibly to exogenous Ags.

Control of multiple autoantibodies linked with a lupus nephritis susceptibility locus in New Zealand black mice

An NZB locus on distal chromosome 1 has been linked to murine lupus nephritis in backcross analyses of New Zealand mice. This locus, designated Nba2 for New Zealand Black autoimmunity 2, was found to colocalize in both (NZB x SM/J)F1 x NZW and (B6.H2z x NZB)F1 x NZB backcrosses, and was most likely situated between 92 and 97 cM from the centromere. This region of mouse chromosome 1 encodes several candidate genes, including the low affinity Fc gamma receptor genes. Both backcrosses were examined by interval mapping for quantitative trait loci linked with autoantibody and total Ig production. Nba2 was linked with elevated serum levels of multiple autoantibodies, including a variety of antinuclear Abs (anti-dsDNA, anti-chromatin and anti-histone) and autoantibodies to gp70, in both backcrosses. Nba2 was also linked (or showed a trend for linkage) with hypergammaglobulinemia and IgG1, IgG2a, and/or IgG3 levels in each backcross. In the (B6.H2z x NZB)F1 x NZB backcross, MHC was an additional genetic contribution that interacted with Nba2 in the production of autoantibodies and the development of nephritis. Together, these data provide new insight into the nature of one important genetic contribution to murine lupus and suggest that Nba2 may act as an immune response gene that influences Ag-driven B cell responses to self and possibly to exogenous Ags.

Effect of genetic background on the contribution of New Zealand black loci to autoimmune lupus nephritis

Autoimmune diseases such as systemic lupus erythematosus are complex genetic traits with contributions from major histocompatibility complex (MHC) genes and multiple unknown non-MHC genes. Studies of animal models of lupus have provided important insight into the immunopathogenesis of disease, and genetic analyses of these models overcome certain obstacles encountered when studying human patients. Genome-wide scans of different genetic crosses have been used to map several disease-linked loci in New Zealand hybrid mice. Although some consensus exists among studies mapping the New Zealand Black (NZB) and New Zealand White (NZW) loci that contribute to lupus-like disease, considerable variability is also apparent. A variable in these studies is the genetic background of the non-autoimmune strain, which could influence genetic contributions from the affected strain. A direct examination of this question was undertaken in the present study by mapping NZB nephritis-linked loci in backcrosses involving different non-autoimmune backgrounds. In a backcross with MHC-congenic C57BL/6J mice, H2z appeared to be the strongest genetic determinant of severe lupus nephritis, whereas in a backcross with congenic BALB/cJ mice, H2z showed no influence on disease expression. NZB loci on chromosomes 1, 4, 11, and 14 appeared to segregate with disease in the BALB/cJ cross, but only the influence of the chromosome 1 locus spanned both crosses and showed linkage with disease when all mice were considered. Thus, the results indicate that contributions from disease-susceptibility loci, including MHC, may vary markedly depending on the non-autoimmune strain used in a backcross analysis. These studies provide insight into variables that affect genetic heterogeneity and add an important dimension of complexity for linkage analyses of human autoimmune disease.

Genetic basis of systemic lupus erythematosus

Recent studies have emphasized that systemic lupus erythematosus is a complex genetic trait with contributions from the MHC and multiple non-MHC genes. Genome-wide linkage studies in murine models of lupus have mapped the positions of a number of non-MHC loci, but the contributing genes have not yet been identified. Recent studies in human systemic lupus erythematosus have found an association with a particular FCGR2A allele. Although susceptibility genes in lupus are unlikely to involve mutations with severe functional consequences, murine knockout models that develop lupus-like features may provide insight into the pathogenetic mechanisms and contributing genes in the human disease.

Genetic linkage of IgG autoantibody production in relation to lupus nephritis in New Zealand hybrid mice

F1 hybrids of New Zealand black (NZB) and New Zealand white (NZW) mice are a model of human systemic lupus erythematosus. These mice develop a severe immune com-plex-mediated nephritis, in which antinuclear autoantibodies are believed to play the major role. We used a genetic analysis of (NZB x NZW)F1 x NZW backcross mice to provide insight into whether different autoantibodies are subject to separate genetic influences and to determine which autoantibodies are most important in the development of lupus-like nephritis. The results showed one set of loci that coordinately regulated serum levels of IgG antibodies to double-stranded DNA, single-stranded DNA, total histones, and chromatin, which overlapped with loci that were linked to the production of autoantibodies to the viral glycoprotein, gp70. Loci linked with anti-gp70 compared with antinuclear antibodies demonstrated the strongest linkage with renal disease, suggesting that autoantibodies to gp70 are the major pathogenic antibodies in this model of lupus nephritis. Interestingly, a distal chromosome 4 locus, Nba1, was linked with nephritis but not with any of the autoantibodies measured, suggesting that it contributes to renal disease at a checkpoint distal to autoantibody production.

Backcross analysis of genes linked to autoantibody production in New Zealand White mice

F1 hybrids of New Zealand Black (NZB) and New Zealand White (NZW) mice are genetically predisposed to develop a lupus-like autoimmune disease characterized by IgG autoantibody production and an immune complex glomerulonephritis. Genes from both parental strains contribute to autoimmunity in the F1 animal. NZW mice produce mostly non-pathogenic autoantibodies to ssDNA and histones as their major autoimmune trait. We studied the genetics of this trait in order to gain insight into the NZW contribution to F1 disease. Genome-wide mapping of (NZW x BALB/c)F1 x NZW backcross mice showed that four NZW non-MHC loci on chromosomes 1, 11, 16, and 19 were linked with IgG autoantibody production. Another NZW locus on chromosome 14 appeared to be selectively linked with IgG anti-histone Abs. In this backcross, contributions from the nonautoimmune BALB/c strain were also apparent. Heterozygosity for the BALB/c MHC (H2d) was linked with IgG autoantibody production. This influence of H2d is therefore similar to that seen in (NZW x NZB)F1 mice, in which heterozygosity for H2d enhances autoantibody production and disease. Surprisingly, two non-MHC BALB/c loci were linked with IgM autoantibody levels, whereas no NZW loci had such an effect. Neither of these two loci have been previously linked with autoimmunity in lupus-prone mice. These data show that autoantibody production in NZW mice is a polygenic trait that is influenced by contributions from MHC and non-MHC genes. The results also support the hypothesis that NZW genes act to class-switch the autoantibody response, an effect that appears to contribute to disease in (NZB x NZW)F1 mice.

Backcross analysis of genes linked to autoantibody production in New Zealand White mice

F1 hybrids of New Zealand Black (NZB) and New Zealand White (NZW) mice are genetically predisposed to develop a lupus-like autoimmune disease characterized by IgG autoantibody production and an immune complex glomerulonephritis. Genes from both parental strains contribute to autoimmunity in the F1 animal. NZW mice produce mostly non-pathogenic autoantibodies to ssDNA and histones as their major autoimmune trait. We studied the genetics of this trait in order to gain insight into the NZW contribution to F1 disease. Genome-wide mapping of (NZW x BALB/c)F1 x NZW backcross mice showed that four NZW non-MHC loci on chromosomes 1, 11, 16, and 19 were linked with IgG autoantibody production. Another NZW locus on chromosome 14 appeared to be selectively linked with IgG anti-histone Abs. In this backcross, contributions from the nonautoimmune BALB/c strain were also apparent. Heterozygosity for the BALB/c MHC (H2d) was linked with IgG autoantibody production. This influence of H2d is therefore similar to that seen in (NZW x NZB)F1 mice, in which heterozygosity for H2d enhances autoantibody production and disease. Surprisingly, two non-MHC BALB/c loci were linked with IgM autoantibody levels, whereas no NZW loci had such an effect. Neither of these two loci have been previously linked with autoimmunity in lupus-prone mice. These data show that autoantibody production in NZW mice is a polygenic trait that is influenced by contributions from MHC and non-MHC genes. The results also support the hypothesis that NZW genes act to class-switch the autoantibody response, an effect that appears to contribute to disease in (NZB x NZW)F1 mice.

The molecular basis of hereditary complement factor I deficiency

The molecular basis of hereditary complement factor I deficiency is described in two pedigrees. In one pedigree, there were two factor I-deficient siblings, one of whom was asymptomatic and the other suffered from recurrent pyogenic infections. Their factor I mRNA was analyzed by reverse transcription of fibroblast RNA followed by amplification using the polymerase chain reaction. Both siblings were homozygous for the same transversion (adenine to thymine) at nucleotide 1282 in the cDNA. This mutation causes histidine-400 to be replaced by leucine. The altered histidine is a semi-conserved residue within the serine proteinase family, although no function has been ascribed to it. The proband of the second pedigree studied was found to be a compound heterozygote. One allele had the same mutation as the first family, the second allele had a donor splice site mutation that resulted in the deletion of the mRNA encoded in the fifth exon (a low-density lipoprotein receptor domain) from its transcript.