A second characteristic of our dataset that differs from previously published studies is the enrichment of our sample for substance dependence. Although we did not see a statistical interaction between comorbid diagnosis and the genetic association, our sample was primarily ascertained for substance dependence (alcohol and cocaine). Of interest, the relationship between this region and alcohol dependence has been noted in the literature(22, 33–34). This highlights the complex relationship between comorbid substance use disorders and genetic susceptibilities. Further, although this analysis shows a GWAS-significant association with FTCD-based nicotine dependence that was also seen in a large meta-analysis using CPD as the primary phenotype, it would be interesting to examine the association with this variant in other datasets that have measured FTCD.
