Two-sample Mendelian randomization analysis of causal relationship between eczema and autoimmune diseases.

Objectives: The causal relationship between eczema and autoimmune diseases has not been previously reported. This study aims to evaluate the causal relationship between eczema and autoimmune diseases.

Methods: The two-sample Mendelian randomization (MR) method was used to assess the causal effect of eczema on autoimmune diseases. Summary data from the Genome-Wide Association Study Catalog (GWAS) were obtained from the Integrative Epidemiology Unit (IEU) database. For eczema and autoimmune diseases, genetic instrument variants (GIVs) were identified according to the significant difference (P<5×10^-8). Causal effect estimates were generated using the inverse-variance weighted (IVW) method. MR Egger, maximum likelihood, MR-PRESSO, and MR-RAPS methods were used for alternative analyses. Sensitivity tests, including heterogeneity, horizontal pleiotropy, and leave-one-out analyses, were performed. Finally, reverse causality was assessed.

Results: Genetic susceptibility to eczema was associated with an increased risk of Crohn's disease (OR=1.444, 95% CI 1.199 to 1.738, P<0.001) and ulcerative colitis (OR=1.002, 95% CI 1.001 to 1.003, P=0.002). However, no causal relationship was found for the other 6 autoimmune diseases, including systemic lupus erythematosus (SLE) (OR=0.932, P=0.401), bullous pemphigoid (BP) (OR=1.191, P=0.642), vitiligo (OR=1.000, P=0.327), multiple sclerosis (MS) (OR=1.000, P=0.965), ankylosing spondylitis (AS) (OR=1.001, P=0.121), rheumatoid arthritis (RA) (OR=1.000, P=0.460). Additionally, no reverse causal relationship was found between autoimmune diseases and eczema.

Conclusions: Eczema is associated with an increased risk of Crohn's disease and ulcerative colitis. No causal relationship is found between eczema and SLE, MS, AS, RA, BP, or vitiligo.