Rheumatoid arthritis genetics: 2009 update.

Recent human genetic discoveries have increased our understanding of rheumatoid arthritis (RA) susceptibility. Genome-wide association studies have expanded the number of validated RA risk loci beyond HLA-DRB1 "shared epitope" alleles to include additional major histocompatibility complex (MHC) risk alleles and more than 10 regions outside the MHC. The newly discovered risk alleles are common in the general population, have a modest effect on RA risk, and together explain less than 5% of the variance in disease risk. Whereas the actual causal mutation and causal gene for most loci remain to be determined, these studies are beginning to reveal general themes: many risk loci are associated with other autoimmune diseases; many genes fall within discrete biological pathways (eg, the nuclear factor kappa-light-chain-enhancer of activated B cells signaling pathway); and human genetics can group diseases into clinically meaningful subset categories (eg, presence or absence of autoantibodies). This review discusses recent RA genetic discoveries in terms of their potential to improve patient care.