Aging has been shown to increase DNA methylation globally and at specific genes (Fraga et al. 2005; Ronn et al. 2008; Sarter et al. 2005). We found several CpG sites with point-wise significant increases in DNA methylation with age. These were at the −10 CpG site in African-American controls, the −60 CpG site in Hispanic controls, and the −18 CpG site in Hispanic cases. When these results were adjusted for multiple testing, no significant experiment-wise correlation was found for DNA methylation levels at any of the CpG sites in the OPRM1 promoter region with subject age, and therefore, do not confound our analyses of ethnic or drug related differences in methylation.
