Epigenetic changes: the missing link

The association of rheumatoid arthritis (RA) with a number of genetic risk loci is well established; however, only part of the risk to develop the disease is based on genetics. Environmental factors significantly contribute to the pathogenesis. A gene-environment interaction for smoking and certain major histocompatibility complex (MHC) class II alleles has been shown to promote anti-citrullinated protein antibody (ACPA)-positive RA; however, the molecular mechanisms of interaction remain unclear. In contrast to the genetic background, epigenetic factors are responsive to external stimuli and can modulate gene expression. Therefore, epigenetic mechanisms may function as intermediaries between genetic risk alleles and environmental factors. In this review, epigenetic mechanisms are explained and the evidence for epigenetic changes relevant for the pathogenesis of RA and potential therapeutic applications are discussed.