Concordance Apoptosis antagonist rates for autoimmune diseases in MZ twins are largely below 50% with few exceptions, but remain higher compared to DZ twins

or siblings [2]. In the case of SSc, similar concordance rates have been observed in MZ (4·2%) and DZ twins (5·6%) in a cross-sectional study [3], while a recent genome-wide association study (GWAS) has reported significant associations in subgroups of patients [4,5]. Accordingly, environmental factors remain crucial in SSc development and are thought to impact gene expression through epigenetic changes [6–8], particularly DNA methylation, which manifests a partial instability responsible for phenotypic differences across genetic identical organisms [9,10]. An additional clue to SSc pathogenesis comes from its female predominance with a sex ratio as high as 12:1 [11] and from the proposed theories related to X chromosome changes [12]. Peripheral lymphocytes from women with SSc manifest an enhanced rate of X chromosome loss (i.e. X monosomy) [13] and possibly a more frequently skewed X inactivation MG-132 chemical structure pattern [14,15], which may contribute to an haplo-insufficiency of X-linked genes predisposing to autoimmunity. Recent experimental evidence suggests that some genes variably escape X chromosome inactivation in women and thus epigenetic differences in X-linked genes could explain both the female preponderance

and low monozygotic twin concordance in autoimmune disorders such as SSc [16]. We herein report the first study of the X chromosome-wide DNA methylation profile in the unique model of MZ twins discordant and concordant for SSc. Using this approach, we identify many differentially methylated genes that will be useful in dissecting the epigenetic bases of the disease. Genomic DNA was extracted from peripheral blood mononuclear cells (PBMC) from eight pairs of MZ twins which in seven cases were discordant and in one case concordant for SSc (in the latter case one subject had diffuse and one had limited SSc). The age of discordant twins ranged between 41 and 59 years,

while the concordant set was 62 years old at the time of enrolment. Twin sets only included women and have already been described in a previous work, along with the DNA extraction methods [3]. The protocol was approved by the IRB of the University of California at Davis and all subjects provided written informed consent. DNA samples were sheared randomly by sonication to generate fragments between 300 and 500 base pairs (bp), which were immunoprecipitated with a monoclonal mouse antibody against 5-methylcytidine (Ab108005; Abcam, Cambridge, UK). The MeDIP efficiency was verified by polymerase chain reaction (PCR). After the enrichment of MeDIP DNA was validated, genomic MeDIP and control fragments were converted to PCR-amplifiable OmniPlex™ Library (Rubicon Genomics, Ann Arbor, MI, USA) molecules flanked by universal priming sites.