Other dopaminergic gene variants have been associated with substance addiction with varying success (Doehring et al., 2009, Gelernter et al., 1994, Guindalini et al., 2006, Hou & Li, 2009). These include a variable nucleotide tandem repeat (VNTR) polymorphism in the 3’UTR (rs28363170) of the dopamine transporter gene (DAT1) and the Taq1A allele now known to reside upstream of DRD2 in ANKK1 . A recent study found evidence of an epistatic interaction between the splice polymorphism in DRD2 and a VNTR in intron 8 of DAT1 (rs3836790) and cocaine use that contributes to risk for lethal cocaine intoxication (Sullivan et al., 2013). Therefore, future studies of the role of dopaminergic gene variants and drug dependence would benefit from studying the epistatic effects of polymorphism on the risk for addiction.
