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.