UNEXPECTED HLA HAPLOTYPE SHARING IN DIZYGOTIC TWIN PAIRS DISCORDANT FOR RHEUMATOID-ARTHRITIS

Dizygotic twins are generally believed to be no more genetically simil ar than sibs born from separate pregnancies. In the present study, a p anel of 93 dizygotic twin pairs discordant for rheumatoid arthritis we re typed for HLA-A, -B, -Cw, and -DR antigens. HLA haplotype sharing i dentical by descent between the twins showed a trend towards increased sharing of both HLA haplotypes; this increased sharing was statistica lly significant when the female/female twin pairs were considered sepa rately. In contrast, the pattern of HLA haplotype sharing in sib pairs (n = 128) was consistent with a 1:2:1 ratio of 2, 1, or 0 haplotypes shared. An analysis of 16 normal dizygotic twin pairs was consistent w ith these results raising the possibility that dizygotic twins in gene ral are genetically more similar at the HLA complex than sibs born fro m separate pregnancies.