Familial risk of systemic sclerosis and co-aggregation of autoimmune diseases in affected families

Background: Systemic sclerosis (SSc) is a rare and devastating disease affecting skin and internal organs. Familial aggregation of SSc and co-aggregation with other autoimmune diseases is rarely reported. Methods: We identified 23,658,577 beneficiaries registered with the National Health Insurance database in 2010, 1891 of whom had SSc. We identified 21,009,551 parent-child relationships and 17,168,340 full sibling pairs. The familial risks of SSc and other autoimmune diseases and familial transmission were estimated. Results: The prevalence of SSc in the general population was 0.008 %. There are 3801 individuals had at least one first-degree relative with SSc, among them 3 people had SSc which was equivalent to a prevalence of 0.08 %. The adjusted relative risk (RR) (95 % CI) for SSc was 81.21 (11.40-579.72) for siblings of SSc patients. The familial transmission (genetic plus shared environmental contribution to total phenotypic variance of SSc) was 0.72. However, 84.1 % of patients were expected to be sporadic cases. The RR (95 % CI) in first-degree relatives of SSc patients was 2.64 (1.46-4.75) for rheumatoid arthritis, 6.51 (4.05-10.46) for systemic lupus erythematosus, 2.77 (1.04-7.35) for Sjogren's syndrome, 8.05 (2.03-31.92) for idiopathic inflammatory myositis, and 1.52 (1.15-2.01) for psoriasis. Conclusions: The risks of SSc and other autoimmune diseases are increased in relatives of people with SSc, and family factors explain over two-thirds of the phenotypic variance of the disease. These findings may be useful in counselling families of patients with SSc and for further genetic studies.