Family studies and HLA typing in ankylosing spondylitis and sacroiliitis.

The families of 21 ankylosing spondylitis (AS) and 16 sacroiliitis (SI) patients were investigated and typed for HLA markers. The association of HLA B27 with AS was confirmed, but no strong evidence for the same or other HLA markers being associated with SI was found. Inheritance patterns in families were analyzed according to the multifactorial and monofactorial models. It is proposed that a major gene associated or interacting with the B27 product controls the susceptibility to AS, and that this gene behaves as a dominant with incomplete penetrance. The problem as to whether linkage disequilibrium maintained by selective pressure, or functional epistasis between the "disease gene" and the B27 antigen may be the acting mechanism of association, remains to be elucidated.