This is the first report of a genome-wide fine mapping of DNA methylation in MZ twins discordant and concordant for SSc. Interestingly, we found that consistent differences between the studied twins affect only genes located on the X chromosome, thus possibly contributing to the aetiology of SSc female predominance. The study of individual susceptibility to autoimmune diseases is hampered by numerous issues which apply well to SSc, including the rare prevalence, AUY-922 molecular weight the long latency between the exposure to specific environmental factors and disease onset and the limited applicability of GWAS data gathered in recent studies [18–25]. These limitations

are well represented by the variable concordance rates in MZ twins for specific autoimmune diseases and suggest that epigenetic changes may constitute the missing link between individual susceptibility and environmental factors. Data on the epigenetics of SSc are limited to the observation that DNA from CD4+ T cell of patients with SSc is hypomethylated significantly compared to healthy controls, along with a reduced expression of enzymes crucial to DNA methylation such as DNMT1, MBD3 and MBD4

[26]. More specifically, the FL1 promoter is down-regulated by CpG methylation in SSc fibroblasts, thus influencing the expression of collagen alpha 1 and other matrix proteins [27]. Conversely, a growing amount of data is being produced by epigenome-wide studies PARP inhibition of peripheral blood cells from MZ twins discordant for autoimmune diseases such as type 1 diabetes [28], multiple sclerosis [29], systemic lupus erythematosus [30] and psoriasis [31]. These studies are performed mainly on effector cell subpopulations (monocytes, T cells) to identify differentially expressed genes possibly preceding disease onset [28]. Investigating

DMRs in peripheral blood mononuclear cells (PBMC) from MZ twins discordant and concordant for SSc to search for aetiological factors and biomarkers for SSc is expected to be a powerful tool in spite of the limited number of samples examined. First, the limited number of samples should not be considered as a limit of this study based on the low prevalence of the disease in the general population, ranging from 71 to 433 cases per million [32], the low rate of MZ twinning (approximately three to four per 1000 pregnancies) [33] ADAMTS5 and the low concordance for SSc in such twins [3]; these factors suggest that our series of twins is representative of a general population of 10 million individuals. Secondly, several studies investigated the expression signature in PBMC from patients affected by complex diseases [34–36], including SSc, with reported correlations with defined subsets of SSc and different organ involvements [37]. Among such identified markers, interferon (IFN)-induced protein 44 seems to be one of the most highly differentially expressed gene in SSc monocytes and CD4+ T cells as well as IL-1α and IL-16 [38].