Excess admixture proportion of extended major histocompatability complex haplotypes of Caucasian origin among rheumatoid arthritis associated haplotypes in African Americans and afro‐Caribbeans

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Abstract
Several extended major histocompatability complex (MHC) haplotypes are associated with susceptibility to autoimmune disease in Caucasian populations. It is known that African Americans and Afro‐Caribbeans are ethnic groups descended from west, central and southern black African populations which are admixed with Caucasians. To examine the possible association of some marker of Caucasian MHC genes and susceptibility to rheumatoid arthritis (RA) in African Americans, we studied extended MHC haplotypes (HLA‐B, complement and DR) in a sample of 18 African American and Afro‐Caribbean probands with RA, their first degree relatives and in 15 non‐RA families. We defined 36 disease‐associated RA haplotypes among the probands and 96 normal haplotypes in normal individuals. To obtain the most conservative estimate, we excluded recognized Caucasian, DR4‐bearing, extended MHC haplotypes from the analysis. Admixture proportions for non‐HLA‐DR4 extended MHC haplotypes of known Caucasian origin among RA‐associated and normal haplotypes were computed (0.40 versus 0.163 respectively). When we compared the difference in proportions between RA and normal haplotypes, the proportion of extended MHC haplotypes of known Caucasian origin was significantly increased among RA‐associated haplotypes (Z = 3.16, p (one sided) < 0.001, p (adjusted) < 0.008). Our results suggest that racial admixture with Caucasian MHC genes may augment RA susceptibility and thus may be one mechanism to explain the higher prevalence of RA in African Americans and Afro‐Caribbeans than in black African populations.