The methylation differences found between cases and controls could be due to several factors (Nielsen et al. 2009). Methylation levels at specific CpG sites in the OPRM1 promoter region may be a predisposing factor for vulnerability to develop heroin addiction. Although methylation may be inherited through genomic imprinting, examination of our published data (Nielsen et al. 2009), in which we determined methylation on individual strands of cloned DNA, showed that the number of methylated CpG sites in each individual had a unimodal distribution. Imprinted genes show a bimodal methylated CpG distribution (e.g. Durcova-Hills et al. 2006; El-Maarri et al. 2007); therefore, it is unlikely that genomic imprinting occurs in the OPRM1 promoter region. The alterations in DNA methylation may be due to major life events occurring prior to heroin addiction, to chronic heroin use, or to long-term methadone maintenance pharmacotherapy.
