Familial versus sporadic ankylosing spondylitis. Two different diseases?

To define potential differences and the possible contribution of susceptibility or severity genes in familial versus sporadic ankylosing spondylitis (AS).

Three hundred twenty patients with AS were studied: 160 who had first-degree relatives with AS (familial) and 160 age- and sex-matched controls who had no first-degree relative with the disease (sporadic). Disease expression in the two groups was evaluated using an index of physical, psychological, and social functioning (the Arthritis Impact Measurement Scales [AIMS]) and an assessment of spinal mobility.

Familial disease was significantly milder than sporadic disease as assessed by all measures, e.g., spinal mobility score (mean 4.08 versus 4.65, P < 0.038), AIMS overall impact score (mean 2.63 versus 3.59, P = 0.002), AIMS physical activity score (4.19 versus 5.10 [P = 0.004]), AIMS social function score (4.02 versus 4.60, P = 0.023), and AIMS pain score (4.15 versus 5.33, P = 0.002).

The greater prevalence of AS in at-risk families may be explained by the occurrence of more AS "susceptibility" genes in those families, whereas the more severe disease, seen in patients with sporadic AS, is conferred by the presence of more "severity" genes than "susceptibility" genes.