French investigators have identified in 70 families a second allele predisposing to a group of inflammatory rhematic diseases—the spondyloarthropathies (SpA). MHC HLA-B27 is associated with all familial SpA—ankylosing spondylitis; reactive arthritis; a subspecies of psoriatic arthritis; arthritis with inflammatory bowel disease; and undifferentiated spondyloarthropathy—but now HLA-DR4 has also been shown to be significantly associated, independently of linkage disequilibrium with HLA-B27.

The investigators found that the gene frequencies of HLA-A, B, C in the HLA-B27 haplotypes in these families were similar to those in a random sample of the French population; the same was true for HLA-DR except for DR 13, which was overrepresented. Transmission disequilibrium tests for HLA-DR alleles performed on all haplotypes showed excess transmission of HLA-DR4 to family members affected with SpA (52 positive v 20 negative; 169 tested) and reduced transmission in unaffected siblings with HLA-B27 (10 positive v 18 negative; 71 tested). Repeating the test with HLA-B27 negative haplotypes showed excess transmission of HLA-DR4 to family members with SpA (20 positive v 6 negative; 156 tested) but the reverse for unaffected siblings with HLA-B27 (2 positive v 8 negative; 70 tested), confirming HLA-DR4 was not in linkage disequilibrium with HLA-B27.

The study population included 188 patients and first degree relatives from 70 families with at least two affected members and 198 HLA-B27 negative haplotypes randomly selected from the French population.

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