Background: Rheumatoid arthritis (RA) is a common complex autoimmune disorder which is influenced by both genetic and environmental factors. While multiple factors that influence susceptibility to and outcome of disease have been identified there is still a large proportion of missing heritability and limited understanding of disease pathogenesis. In recent years, biologic drug therapies have advanced treatment of RA; however good disease control is achieved in just 30% of patients, making identification of predictors of treatment response important. One area of research which is yet to be explored in relation to treatment response, and requires further evaluation in RA susceptibility, is epigenetics. Epigenetics is the study of modifications of the DNA which can influence gene expression but do not alter genetic sequence, and the most commonly studied epigenetic phenomenon, to date, is DNA methylation.Objectives: To identify DNA methylation signatures predictive of treatment response to anti-TNF biologics, to explore the role of DNA methylation in RA susceptibility using disease discordant monozygotic (MZ) twins, and to assess the effect of cryopreservation of cells on DNA methylation.Methods: Genome-wide DNA methylation levels were measured using the HumanMethylation450 BeadChip in pre-treatment whole blood DNA samples from individuals who had extremely good or extremely poor response to the anti-TNF therapies, etanercept and adalimumab, and in MZ twins discordant for RA (n=79 pairs). I also compared genome-wide methylation in cells which had been cryopreserved with fresh cells, to investigate if this technique is suitable for epigenetic investigations.Results: I identified four methylation sites which were significantly related to response to etanercept at a false discovery rate of 5%, the most significantly differentially methylated of which maps to the LRPAP1 gene (p=1.46E-8). Indeed, four other sites at the same locus also showed evidence for differential methylation indicating that this represents a differentially methylated region. No sites were significantly associated with response to adalimumab after correction for multiple testing. I identified subtle differences in DNA methylation between RA discordant twins. Although these were not statistically significant following adjustment for cell composition, one of the most differentially methylated positions mapped to the ZNF74 gene (p=4.97E-6), and replicated a methylation difference identified in the largest previous epigenome-wide association study of RA cases and unrelated healthy controls. I found that cryopreservation of cells does not significantly alter the methylome, an important observation that will impact upon design of future studies.Conclusions: In the largest studies of DNA methylation in RA treatment response and RA discordant MZ twins to date, I identified significant differential methylation in etanercept response, but not adalimumab response, and found small differences in methylation in RA discordant MZ twins. I also concluded that cryopreservation does not significantly alter the methylome.