We previously reported association of SNPs in this region to two different SD disorders: viz. AD and ND. These results led us to ask whether one of these phenotypes (i.e., AD), or another that is highly comorbid with it (i.e., drug dependence (DD)), is driving the association. Further, some or all of the associations within this cluster that we reported could be attributable to a common risk factor for multiple forms of SD. Since dopamine is a major constituent of reward pathways and is strongly believed to be involved in many forms of SD (Connor et al., 2007; Sasabe et al., 2007; Shahmoradgoli Najafabadi et al., 2005; Volkow et al., 2006; Xu et al., 2004), this seems a likely possibility. One way to answer this question is to subdivide samples characterized by AD, based on the presence or absence of DD comorbidity. We performed this study with the intention to determine whether some, or all, of the risk loci we identified in this gene cluster are specific to AD or comorbid AD+DD.
