This study extends our previous findings on the methylation levels at 16 CpG dinucleotide sites in the OPRM1 promoter region in Caucasians (Nielsen et al. 2009) to Africans American and Hispanics. We found ethnic differences in the methylation levels at specific CpG dinucleotides. Although the methylation levels at each site differed depending upon ethnicity, we found that the methadone stabilized former heroin addicts had an overall hypermethylation of their OPRM1 CpG sites as compared to controls. In cell culture studies of undifferentiated and differentiated P19 carcinoma cells, it has been reported that OPRM1 gene expression increases in concert with the demethylation of the OPRM1 promoter during differentiation or by the action of the demethylating agent azacitidine (Hwang et al. 2007). In NS20Y neuroblastoma cells that normally do not express OPRM1, the expression of OPRM1 can be induced by the demethylating agent azacitidine (Hwang et al. 2008).
