A functional polymorphism within the mu-opioid receptor gene and risk for abuse of alcohol and other substances.

Genetic association studies investigating the role of the +118A allele of the human mu-opioid receptor gene in risk for alcohol dependency have produced inconsistent findings, possibly because of the failure to recognize sampling methodology difficulties inherent in association studies of polygenic disorders. We examined the frequency of the AA genotype and A allele in several groups of substance-dependent cases, unrestricted controls, and super controls screened for the use of alcohol and cigarettes. Our findings and analyses suggest that the OPRM1 +118 polymorphism is a general risk gene for substance dependence, but is not specific to a particular substance. The nature of the conferred risk is likely to be in use of multiple substances, but it is not yet determined if the risk could be expressed in severity of use of any particular substance. The contribution of the gene to risk for substance dependence is small, and is detected most easily in studies that use control samples that are screened for all forms of substance dependence.

Association of a functional mu-opioid receptor allele (+118A) with alcohol dependency.

Genetic association studies have implicated the TaqI A1 allele of the human dopamine D2 receptor gene (DRD2) as a risk-determining factor for alcohol dependency. However, as alcoholism is a disease of polygenic inheritance, the percentage of overall disease variance explained by the TaqI A1 allele is small. In searching for other genetic loci that may, either alone or in combination with DRD2, enhance prediction of alcoholism, we have found a novel association between a functional coding variant (+118A) within the human mu-opioid receptor gene and alcohol dependency. However, no association was detected between the DRD2 TaqI A1 allele and alcoholism in our sample nor did we find synergy between +118A and TaqI A1 alleles on prediction of risk for the disease. These results suggest that, at the molecular level, the endogenous mu-opioid receptor system is a contributing factor to the etiology of alcoholism.