We present the first report of a genome-wide association analysis of comorbid depressive syndrome and alcohol dependence. No marker met genome-wide significance criteria, but 10 had p-values <10−5, and 938 had p-values <10−3. Indeed, given the genomic complexity and phenotypic heterogeneity of alcohol dependence and depressive syndrome, we might not expect that the effect size of any individual marker is large enough to reach genome-wide significance criteria in a study of this size; rather, many common variants of small effect likely influence these traits, with affected individuals each harboring an overlapping but unique set of risk-conferring alleles (Purcell et al., 2009; Wellcome Trust Case Control Consortium, 2007).
