Familial risk of Behcet's disease among first-degree relatives: a population-based aggregation study in Korea

Objective. Previous studies have indicated that Behcet's disease (BD) has a genetic component, however population-level familial risk estimates are unavailable. We quantified the familial incidence and risk of BD in first-degree relatives (FDR) according to age, sex and type of family relationship. Methods. Using the Korean National Health Insurance database, which has full population coverage and confirmed FDR information, we constructed a cohort of 21 940 795 individuals comprising 12 million families, which were followed for a familial occurrence of BD from 2002 to 2017. Age- and sex-adjusted incidence risk ratios for BD were calculated in individuals with affected FDR compared with those without affected FDR. Results. Among the total study population, 53 687 individuals had affected FDR, of whom 284 familial cases developed BD with an incidence of 3.57/10(4) person-years. The familial risk (incidence) for BD was increased to 13.1-fold (2.71/10(4) person-years) in individuals with an affected father, 13.9-fold (3.11/10(4) person-years) with affected mother, 15.2-fold (4.9/10(4) person-years) with an affected sibling and the highest risk was 165-fold (46/10(4) person-years) with an affected twin. Familial risks showed age dependence, being higher in younger age groups. The sex-specific familial risk was similar in males and females. Conclusion. This study provides quantified estimates of familial incidence and risk in FDR of BD patients in an entire population. Familial risks were higher within generation (sibling-sibling) vs between generations (parent-offspring). This implicates complex interactions between genetic factors and shared childhood environmental exposures in the pathogenesis of BD.