shown that in heroin addicts, there was elevated methylation at several CpG sites in the promoter of a μ-opioid receptor gene, OPRM1, in lymphocytes, which might result in reduced expression of that gene (Nielsen et al., 2009). Differential DNA methylation at the promoter of the pro-opiomelanocortin gene (POMC) was found to be associated with alcoholism in a human study (Muschler et al., 2010). DNA hypomethylation was associated with activation of endogenous retroviruses in alcoholic brain (Ponomarev et al., 2012).
