The HLA system and the arthropathies associated with psoriasis.

Histocompatibility typing was carried out in 74 patients with psoriasis and an inflammatory arthropathy. In 40 patients with peripheral arthropathy characterized by distal interphalangeal joint involvement, 13 (32-5%) were HLA-B27 positive, significantly higher than the control frequency (P = 5-8 X 10 (-6). 26 of the 40 patients did not have ankylosing spondylitis or radiological sacroiliitis and 7 were HLA-B27 positive, also significantly higher than in controls (P = 0-0049). All 7 patients with psoriasis and ankylosing spondylitis without peripheral arthropathy were HLA-B27 positive. The 10 patients with ankylosing spindylitis or radiological sacroliitis who were HLA-B27 negative all had peripheral arthropathy. It is suggested that being HLA-B27 positive increases the risk of a psoriatic patient developing both peripheral arthropathy and ankylosing spondylitis. In addition, some of the genes involved in susceptibility to psoriasis also have a role in the pathogenesis of both types of arthropathy. A hypothesis is put forward that some of the genes for psoriasis may be aetiologically important in some HLA-B27 negative patients with ankylosing spondylitis.