ANKYLOSING-SPONDYLITIS IN WEST AFRICANS - EVIDENCE FOR A NON-HLA-B27 PROTECTIVE EFFECT

Objective-To determine the prevalence of ankylosing spondylitis in the Fula ethnic group in The Gambia, and relate the disease prevalence to the B27 frequency and subtype distribution of that population. Method s-215 first degree relatives of 48 B27 positive Fula twin pairs, and 9 00 adult Fula males were screened for ankylosing spondylitis by clinic al and, where appropriate, radiographic means. The B27 prevalence was determined by PCR/sequence specific oligonucleotides on finger prick s amples from 100 unrelated Fula, and B27 subtype distribution by SSCP o n unrelated B27 positive individuals. This data were then compared wit h the prevalence of ankylosing spondylitis among B27 positive Caucasia ns. Results-No case of ankylosing losing spondylitis was seen. Six per cent of Fula are B27 positive, of which 32% are B2703 and 68% B*2705 . Assuming the penetrance of ankylosing spondylitis in B27 positive Fu la is the same as in B27 positive Caucasians, the probability of not o bserving any cases of ankylosing spondylitis among the Fula examined i s remote (P = 6.7 x 10(-6)). Similarly, the chance of not seeing any c ases among those expected to be either B2705 or B*2703 was small (P = 3.2 x 10(-4) for B2705, and P = 0.02 for B*2703). Conclusions-The ri sk of developing ankylosing spondylitis in B27 positive Fula is lower than in B27 positive Caucasians. This is not explained by the B27 subt ype distribution among Fula, and suggests the presence of some non-B27 protective factor reducing the prevalence of ankylosing spondylitis i n this population.