This is the first study to examine whether the 118A>G polymorphism in OPRM1 is associated with an increased risk for opioid dependence in ethnic Bulgarian and Roma active heroin users. Previous association studies in several populations worldwide have produced mixed results in that regard. Our results indicate that 118A>G is not directly associated with heroin dependence in our sample, consistent with other studies in African-American, Caucasian, Han Chinese and European American populations (Crowley et al., 2003; Franke et al., 2001; Zhang et al., 2007; Zhang et al., 2006a). It is unlikely that other polymorphisms in exon 1 of OPRM1 played a role. In other work, we sequenced this exon in a separate sample of 101 heroin dependent cases and 101 controls, all ethnic Bulgarians (ABI310, Applied Biosystems). The frequency of the 118G allele was very close to what we ultimately observed in the main study (14% compared to 13.8%). Other known SNPs, including rs1799972, rs1799973, rs1042753, rs9282817 and rs9282819 were virtually monomorphic. The rare ins/del GGC at codon 63 (rs9282818) was observed in only one subject.
