Interestingly, the S147C (rs17174794) variant genotyped in EAs was previously found to have increased potency for morphine [31]; however, our association was not statistically significant (p=0.19). The function of rs17174801 (N152D) has also been assessed in mammalian cell lines and the mutant allele leads to reduced expression of the receptor [5]. However, N152D does not appear to increase risk for drug addiction, since no significant association was observed in our AA population (p=0.65). rs1799971 (N40D) leads to the loss of a glycosylation site in the extracellular N-terminal domain of the MOR. Functional studies have found differential expression, receptor binding, signaling efficiency, and altered intracellular effects to be associated with this variant [9, 14, 22, 29, 41]. While rs1799971 has been associated with heroin and cocaine addiction [13, 14, 23, 26, 33], negative findings have also been reported [2, 15]. In support of the latter findings, we did not find rs1799971 to be associated with the risk for drug addiction (p=0.57).
