The association of HLA-DR4 with rheumatoid arthritis strongly implicates genes of the major histocompatibility complex (MHC) as contributing to disease susceptibility. Molecular analysis of MHC genes expressed on haplotypes in association with HLA-DR4 reveals that at least six different alleles of the DRβ1 locus and at least three different alleles of the DQβ locus occur on different DR4+ haplotypes. Some of these allelic differences are quite substantial, and others are rather subtle, involving as few as two amino acids. The analysis of individual DR and DQ alleles in rheumatoid arthritis identifies some DR4+ genes strongly associated with disease susceptibility and some DR4+ genes which are not. The Dw4(DR4) and Dw14(DR4) DRβ1 genes appear to represent specific alleles which confer disease risk in RA; other DRβ1 genes, such as Dw10(DR4), may represent DRβ genes altered during evolution which have lost their contribution to RA susceptibility. DQβ 3.1(DQw3) and DQβ 3.2(DQw3) DQβ genes, which are present on DR4+ haplotypes, represent discrete variable alleles not directly implicated in RA, but which account for HLA-DR4 associations with other diseases, such as the association of DQβ 3.2(DQw3) with Type I diabetes. © 1987 Plenum Publishing Corporation.
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