The influence of MU Opioid receptor polymorphism on ligand mediated signalling

Abstract

The human OPRMl gene and a number of its polymorphic variants have been confirmed to be associated with addictions, particularly opiate addiction. The C17T and Al18G opioid receptor variants are the two OPRMl receptor variants with the greatest relevance in relation to opiate addiction and within a clinical context open the floor to the suggestion that carriers of these specific variants could possibly benefit from individualised pharmacogenetic dosing and treatment with respect to opiate replacement therapy. There is little in vitro molecular signalling data involving opioid ligands clinically relevant to opiate withdrawal treatment which exists to support this premise. This study therefore, aimed to investigate the effects of ligand mediated signalling in the clinically significant C17T and A118G OPRMl receptor variants and the wild type OPRMl receptor, in order to lay the foundation for further studies with potential bearing on the pharmacogenetic treatment of opiate withdrawal therapy. A mammalian expression vector containing the wild type OPRMl receptor cDNA was generated and used to generate the C17T and A118G OPRMl SNPs. The wild type and mutant constructs were expressed in the COS-7 mammalian cell line. The expressed wild type and mutant receptors were treated with five clinically relevant opioid ligands in the presence of IBMX and forskolin and cAMP assays were subsequently carried out on both the wild type and mutant receptor transfectants. Two-tailed t-test analysis of the signalling data showed a significant decrease in cAMP inhibition (P=O.0203) for COS-7 cells expressing the C17T receptor variant treated with buprenorphine when compared to signalling by the expressed buprenorphine-treated wild type receptor. A similar but not significant trend was observed for the same C17T variant in methadone-treated cells. Signalling could not be compared to the Al18G receptor variant, as this was excluded from the study. The results observed suggest that buprenorphine-treated OPRMl C17T patients may require higher doses than their wild type counterparts, in order to exhibit the same clinical efficacy. In view of the emerging role of buprenorphine as a preferred drug for opiateaddiction therapy, this may of particular importance in C17T patients.