There is extensive genetic sharing between rheumatoid arthritis (RA) and other autoimmune and associated diseases. Such sharing is evidenced by high familial risks of discordant autoimmune diseases in parents and offspring, singleton siblings, twins, and spouses.
Hemminki and associates used data from 47,361 patients with RA and their family members to estimate associations of RA with any of 33 autoimmune diseases and related conditions. Standardized incidence ratios (SIRs) were calculated, and familial risks were determined for male and female patients with relatives who had concordant or discordant autoimmune diseases.
The SIR for RA in offspring of affected parents was 3.02. The significant SIRs for RA in offspring according to parental proband were ankylosing spondylitis, 2.96; localized scleroderma, 2.40; Sjgren syndrome, 2.25; systemic lupus erythematosus, 2.13; systemic sclerosis, 1.65; Hashimoto thyroiditis/hypothyroidism, 1.54; pernicious anemia, 1.53; sarcoidosis, 1.40; psoriasis, 1.36; Wegener granulomatosis, 1.34; and asthma or polymyalgia rheumatica, 1.32. Other SIRs for RA were 4.64 in siblings, 9.31 in multiplex families, 6.48 in twins, and 1.17 in spouses.
The authors noted that this study was the first to compare the familial risks of RA in relation to a large number of autoimmune diseases and related conditions using data from a single population.