Familial aggregation of rheumatoid arthritis and co-aggregation of autoimmune diseases in affected families: a nationwide population-based study

The aim was to estimate familial relative risk (RR) for RA and other autoimmune diseases and the genetic contribution to RA phenotypic variance (heritability). This study used the Taiwan National Health Insurance Research Database to identify all National Health Insurance registered beneficiaries (n = 23 658 577) in 2010; among them, 37 482 individuals had RA. We estimated familial RRs and 95% CIs of RA and other autoimmune diseases using marginal Cox proportional models and heritability of RA using a threshold liability model. The RR (95% CI) for RA was 328.27 (135.95, 795.63) for twins of RA patients; 11.97 (8.68, 16.52) for siblings; 4.86 (4.16, 5.67) for parents; 4.65 (3.92, 5.50) for offspring; and 2.32 (1.83, 2.95) for spouses. Using a threshold liability model, we estimated that familial transmission was 59.4% (95% CI: 50.3, 69.5%) and that heritability was 43.5% (33.9, 54.1%). The RR (95% CI) in individuals with a first-degree relative with RA was 2.91 (2.49, 3.42) for SLE; 2.92 (1.62, 5.25) for SSc; 3.13 (2.50, 3.93) for primary SS; 0.95 (0.36, 2.51) for idiopathic inflammatory myositis; 1.96 (1.54, 2.48) for type 1 diabetes mellitus; 3.32 (1.82, 5.95) for multiple sclerosis; 1.31 (1.31, 2.43) for IBD; 2.76 (2.46, 3.10) for AS; and 1.65 (1.54, 1.77) for psoriasis. The risks of RA and other autoimmune diseases increased in individuals with an RA family history. Approximately two-thirds of RA phenotypic variation is explained by familial factors.