Objective: To examine recurrence of manifestations belonging to the spectrum of spondylarthropathy (SpA) in first-degree relatives of SpA patients, and to estimate recurrence risk ratio.
Methods: Parents and siblings of consecutive SpA probands have been thoroughly investigated, including clinical data collection, pelvic x-ray and HLA-B27 status determination. The diagnosis of SpA was made according to European Spondylarthropathy Study Group and/or Amor's criteria. The recurrence risk ratio λ1, which gives an estimate of the weight of genetic factors was calculated as the ratio of the recurrence risk of SpA in first-degree relatives, compared with the population prevalence of SpA. The λnon-HLA was obtained by similar calculations restricted to HLA-B27+ individuals.
Results: Most manifestations of SpA were more frequent among the 157 HLA-B27+ relatives of 83 probands, than among their 111 HLA-B27- relatives. A diagnosis of SpA was made in 50 relatives of 31 (37%) probands. Recurrence was very similar between parents and siblings, without gender difference, resulting in overall recurrence risk of 12% in first-degree relatives, and of 22.7% in HLA-B27+ relatives. The λ1 value was 40, and the λnon-HLA value was 6.5, very close to the λHLA value of 6.25 estimated from linkage study in SpA.
Conclusions: Similar recurrence risk of SpA was observed between parents and siblings, consistent with a model of inheritance with no dominance variance, and without sex influence. The weight of non-HLA genetic component was equivalent to that estimated for the HLA locus, and fitted a model of multiplicative interaction between HLA, and non-HLA genetic components.