The HLA system and the arthropathies associated with psoriasis.

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Abstract
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.