Discordance between HLA-B27 and ankylosing spondylitis: a family investigation.

Forty-one members in 4 generations of a family were evaluated clinically for ankylosing spondylitis (AS). Thirty-five individuals were HLA typed. The propositus and 4 male siblings demonstrated AS. A sister had sacroiliitis. Three of the affected sibs shared the B27 containing haplotype, but the remaining 3 individuals lacked the B27 but shared the other maternal haplotype. Two other first degree relatives with B27 did not show evidence of either sacroiliitis or AS. Four members of the family had psoriasis, but did not all share a common haplotype. The lack of association of known HLA antigens with disease expression in this family suggests a role for a putative disease susceptibility gene for psoriasis in the development of B27 negative spondylitis and illustrates the difficulty with the diagnosis of B27 negative AS, since when the family was first studied, it presented as a family of "pure" AS and only after several years of followup did the first evidence of psoriasis appear.