Ankylosing spondylitis in Danish and Norwegian twins: Occurrence and the relative importance of genetic vs. environmental effectors in disease causation

Objective: To estimate the influence of genetic effects in the aetiology and pathogenesis of ankylosing spondylitis (AS). Methods: The study comprised one Norwegian and two Danish nationwide twin surveys. In 1994 and 2002, respectively, 37 388 and 46 331 Danish twin individuals were asked by questionnaire if they had AS. Similarly, in 1998, 12 718 Norwegian twins were asked if they had AS using a questionnaire phrased according to the Danish survey. Twins reporting AS were categorized according to the modified New York criteria. Results: A total of 113 twin individuals reported AS, of whom 81 (72.3%) participated in validation of the diagnosis. After validation, 39 probands were diagnosed with AS. Subsequent invitation of co-twins resulted in 27 complete pairs. The point prevalence and the annual incidence of AS was 0.1% and 3/100 000 person-years (pyr) among the Danish twins. The positive predictive value of self-reported AS was 49.3%. Probandwise concordance rates on AS were (2/5) 40% in monozygotic (MZ) and (1/23) 4% in dizygotic (DZ) twins [difference 35% (95% CI 2.9-72.8), p = 0.26]. Heredity analysis including previously published and the present HLA-B27-positive twin pairs indicated that additive genetic effects account for 94% (95% CI 0.56-0.99) of the variance in the causation of AS. Conclusion: Self-reported AS needs careful validation. The occurrence of AS in a Danish twin population was 0.1% and accords well with previous studies on singletons in hospital settings. The present study adds to previous evidence of a major genetic effect in the pathogenesis of AS.