Elevated levels of DNA methylation at the OPRM1 promoter region in men with opioid use disorder

Background: The mu-opioid receptor, encoded by mu-opioid receptor gene (OPRM1), has an important role in the development of addiction to opioids. Its aberrant reduction on the cell membrane is responsible, at least in part, for tolerance and physical dependence. Objectives: The present study was designed to identify the relationship between opium consumption and epigenetic mechanisms involved in opium addiction. Methods: Genomic DNA was extracted from the peripheral blood of 66 men with opium use disorder and 57 healthy men as a control group. Genomic DNAs were treated with sodium bisulfite to convert the un-methylated cytosine to uracil, while methylated cytosine remained unaffected. Nested methylation-specific PCR (MSP) was used for analyses of region 1 (R1) and region 2 (R2) of the OPRM1 promoter DNA methylation. Results: All participants were 19-56 years old, and there was no significant difference in the mean age of both groups (P = 0.082). After Bonferroni correction, results showed that the DNA methylation status significantly increased the risk of opium addiction in the R2 region compared with un-methylation status (OR = 3.80, 95%CI = 1.77-8.17, P = 0.001). However, we found no significant difference in the R1 region DNA methylation between case and control groups (21.2% and 21.1%, respectively) (P = 1). Conclusion: Our findings demonstrated DNA hypermethylation of the R2 region of the OPRM1 promoter in leukocytes of opium use disorder. In peripheral tissues such as blood, changes of epigenetic endpoints with substance use can be considered as potentially clinically useful biomarkers in identifying individuals who may warrant further diagnostic assessment of a substance use disorder.