ABSTRACT: On the basis of the converging evidence showing regulation of drinking behavior by 5-HT3AB receptors and the serotonin transporter, we hypothesized that the interactive effects of genetic variations in the genes HTR3A, HTR3B, and SLC6A4 confer greater susceptibility to alcohol dependence (AD) than do their effects individually. We examined the associations of AD with 22 SNPs across HTR3A, HTR3B, and two functional variants in SLC6A4 in 500 AD and 280 healthy control individuals of European descent. We found that the alleles of the low-frequency SNPs rs33940208:T in HTR3A and rs2276305:A in HTR3B were inversely and nominally significantly associated with AD with odds ratio (OR) and 95 % confidence interval of 0.212 and 0.073, 0.616 (P = 0.004) and 0.261 and 0.088, 0.777 (P = 0.016), respectively. Further, our gene-by-gene interaction analysis revealed that two four-variant models that differed by only one SNP carried a risk for AD (empirical P < 1 × 10(-6) for prediction accuracy of the two models based on 10(6) permutations). Subsequent analysis of these two interaction models revealed an OR of 2.71 and 2.80, respectively, for AD (P < 0.001) in carriers of genotype combinations 5'-HTTLPR:LL/LS(SLC6A4)-rs1042173:TT/TG(SLC6A4)-rs1176744:AC(HTR3B)-rs3782025:AG(HTR3B) and 5'-HTTLPR:LL/LS(SLC6A4)-rs10160548:GT/TT(HTR3A)-rs1176744:AC(HTR3B)-rs3782025:AG(HTR3B). Combining all five genotypes resulted in an OR of 3.095 (P = 2.0 × 10(-4)) for AD. Inspired by these findings, we conducted the analysis in an independent sample, OZ-ALC-GWAS (N = 6699), obtained from the NIH dbGAP database, which confirmed the findings, not only for all three risk genotype combinations (Z = 4.384, P = 1.0 × 10(-5); Z = 3.155, P = 1.6 × 10(-3); and Z = 3.389, P = 7.0 × 10(-4), respectively), but also protective effects for rs33940208:T (? (2) = 3.316, P = 0.0686) and rs2276305:A (? (2) = 7.224, P = 0.007). These findings reveal significant interactive effects among variants in SLC6A4-HTR3A-HTR3B affecting AD. Further studies are needed to confirm these findings and characterize the molecular mechanisms underlying these effects.
