To determine whether comorbid alcohol dependence may have influenced our results, we compared DNA methylation levels at each of the sixteen CpG sites and at all CpG sites combined between the former severe heroin addicts without a history of alcohol dependence and former severe heroin addicts with a history of alcohol dependence. No point-wise significant association was found in the level of DNA methylation of all 16 CpG sites combined with history of alcoholism in either the African-American or the Hispanic former severe heroin addicts. No significant point-wise association was found for DNA methylation level at any of specific CpG sites with history of alcoholism in the African-American former severe heroin addicts. In the Hispanic former severe heroin addicts, there was a significant point-wise association of history of alcoholism with DNA methylation level at the −93 and the −60 CpG sites (11.8% without alcoholism, 8.6% with alcoholism, P = 0.020; 10.5% without alcoholism, 8.0% with alcoholism, P = 0.028, respectively). Neither of these sites had a significant association of DNA methylation levels with history of alcoholism after adjusting for multiple testing.
