Effects of covariates and interactions on a genome-wide association analysis of rheumatoid arthritis.

While genetic and environmental factors and their interactions influence susceptibility to rheumatoid arthritis (RA), causative genetic variants have not been identified. The purpose of the present study was to assess the effects of covariates and genotype x sex interactions on the genome-wide association analysis (GWAA) of RA using Genetic Analysis Workshop 16 Problem 1 data and a logistic regression approach as implemented in PLINK. After accounting for the effects of population stratification, effects of covariates and genotype x sex interactions on the GWAA of RA were assessed by conducting association and interaction analyses. We found significant allelic associations, covariate, and genotype x sex interaction effects on RA. Several top single-nucleotide polymorphisms (SNPs) (~22 SNPs) showed significant associations with strong p-values (p < 1 x 10-4 - p < 1 x 10-24). Only three SNPs on chromosomes 4, 13, and 20 were significant after Bonferroni correction, and none of these three SNPs showed significant genotype x sex interactions. Of the 30 top SNPs with significant (p < 1 x 10-4 - p < 1 x 10-6) interactions, ~23 SNPs showed additive interactions and ~5 SNPs showed only dominance interactions. Those SNPs showing significant associations in the regular logistic regression failed to show significant interactions. In contrast, the SNPs that showed significant interactions failed to show significant associations in models that did not incorporate interactions. It is important to consider interactions of genotype x sex in addition to associations in a GWAA of RA. Furthermore, the association between SNPs and RA susceptibility varies significantly between men and women.