Obesity-Related Traits and the Development of Rheumatoid Arthritis: Evidence From Genetic Data

Objective To investigate the association between obesity-related traits and risk of rheumatoid arthritis (RA). Methods We conducted genetic correlation analysis and a 2-sample Mendelian randomization (MR) study, using genome-wide genetic data based on >850,000 individuals of European ancestry. Summary statistics were collected from the largest genome-wide association study conducted to date for body mass index (BMI; n = 806,810), waist-to-hip ratio (WHR; n = 697,734), WHR adjusted for BMI (WHRadjBMI; n = 694,649), and RA (n(case) = 14,361, n(control) = 43,923). We conducted cross-trait linkage disequilibrium score regression and rho-HESS analyses to quantify genetic correlation between pairs of traits (causal overlap). For each obesity-related exposure, we utilized independent, genome-wide significant single-nucleotide polymorphisms (P < 5 x 10(-9)) as instruments to perform MR analysis (causal relationship). We interrogated the causal relationship both in the general population and in a sex-specific manner and calculated odds ratios (ORs) and 95% confidence intervals (95% CIs). Sensitivity analyses were performed to validate MR model assumptions. Results Despite a negligible overall genetic correlation between the 3 obesity-related traits and RA, we found significant local genetic correlations at several regions on chromosome 6 (positions 28-29M, 30-35M, and 50-52M), highlighting a shared genetic basis. We further observed an increased risk of RA per SD increment (4.8 kg/m(2)) in genetically predicted BMI (OR 1.22 [95% CI 1.09-1.37]). The effect was consistent across sensitivity analyses and comparable between sexes (OR 1.22 [95% CI 1.04-1.44] in male subjects and 1.19 [95% CI 1.04-1.36] in female subjects). However, we did not find evidence supporting a causal role of either WHR (OR 0.98 [95% CI 0.84-1.14]) or WHRadjBMI (OR 0.90 [95% CI 0.79-1.04]) in RA. Conclusion Genetically predicted BMI significantly increases RA risk. Future studies are needed to understand the biologic mechanisms underlying this link.