Fu-Chao Liu, Chang-Fu Kuo, Lai-Chu See, Hsin-I Tsai, Huang-Ping Yu

Myasthenia gravis (MG) is clinically heterogeneous and can be life-threatening if bulbar or respiratory muscles are involved. However, relative contributions of genetic, shared, and nonshared environmental factors to MG susceptibility remain unclear. The aim of this study was to examine the familial aggregation and heritability of MG and the relative risks (RRs) of other autoimmune diseases in the relatives of patients with MG. A population-based family study using the Taiwan National Health Insurance (NHI) Database was conducted. Participants included all individuals (N=23,422,955) who were actively registered in the NHI Database in 2013, 15,066 of whom had at least one first-degree relative with MG. We identified 8,638 parent-child relationships, 3,279 with an affected offspring, 3,134 with affected siblings, and 26 with affected twins. Prevalence and RRs of MG and other autoimmune diseases in the relatives of patients as well as the relative contributions of heritability, shared, and nonshared environmental factors to MG susceptibility were measured. RRs (95% confidence intervals [CIs]) for MG were 17.85 (8.71-36.56) for patients' siblings, 5.33 (2.79-10.18) for parents, 5.82 (3.03-11.16) for offspring, and 1.42 (0.20-10.10) for spouses without genetic similarities. RRs (95% CIs) in individuals with a first-degree relative with MG were 2.18 (1.53-3.12) for systemic lupus erythematosus, 1.73 (1.09-2.74) for primary Sjögren's syndrome, 1.90 (1.66-2.18) for autoimmune thyroid disease, and 1.68 (1.22-2.30) for rheumatoid arthritis. Accountability for the phenotypic variance of MG was 82.1% for familial transmission and 17.9% for nonshared environmental factors. Individual risks of MG and other autoimmune diseases are increased in the relatives of patients with MG. Familial transmission of MG was estimated to be 82.1%.