Original article 169 A genetic association study of the mu opioid receptor and severe opioid dependence James J. Crowley a , David W. Oslin b , Ashwin A. Patkar c , Edward Gottheil c , Peter A. DeMaria Jr. c , Charles P. O’Brien b,d , Wade H. Berrettini a,b,d and Dorothy E. Grice b Objectives Twin, family and adoption studies have sug- gested that vulnerability to opioid dependence may be a partially inherited trait (Cadoret et al., 1986; Merikangas et al., 1998; Tsuang et al., 1998, 2001). Studies using animal models also support a role for genetic factors in opioid dependence, and point to a locus of major effect on mouse chromosome 10 (Berrettini et al., 1994; Alexander et al., 1996), which harbors the mu opioid receptor gene (Mor1) (Kozak et al., 1994). The gene encoding the human mu opioid receptor (OPRM1) is thus an obvious candidate gene for contributing to opioid dependence. A recent report (Hoehe et al., 2000) found a significant association between a specific combination of OPRM1 single nucleotide polymorphisms (SNPs) and substance dependence. Methods In the current study, we genotyped 213 subjects with severe opioid dependence (89 African-Americans, 124 European-Americans) and 196 carefully screened ‘super- control’ subjects (96 African-Americans, 100 European- Americans) at five SNPs residing in the OPRM1 gene. The polymorphisms include three in the promoter region (T –1793 A, –1699 T insertion and A –1320 G) and two in exon 1 (C + 17 T [Ala6Val] and A + 118 G [Asp40Asn]). Results Statistical analysis of the allele frequency differ- ences between opioid-dependent and control subjects for each of the polymorphisms studied yielded P values in the range of 0.444–1.000. Haplotype analysis failed to identify any specific combination of SNPs associated with the phenotype. Conclusions Despite reasonable statistical power we found no evidence of association between the five mu opioid receptor polymorphisms studied and severe opioid dependence in our sample. There were, however, signifi- cant allele frequency differences between African- Americans and European-Americans for all five poly- morphisms, irrespective of drug-dependent status. Linkage disequilibrium analysis of the African-American genotypes indicated linkage disequilibrium (P < 0.0001) across the five-polymorphism, 1911 base pair region. In addition, only four haplotypes of these five polymorphisms are predicted to exist in African-Americans. Psychiatr Genet 13:169–173  c 2003 Lippincott Williams & Wilkins. Psychiatric Genetics 2003, 13:169–173 Keywords: opioid dependence, substance dependence, genetics, mu opioid receptor gene, OPRM1, single nucleotide polymorphism, association study, heroin a Department of Pharmacology, b Department of Psychiatry, University of Pennsylvania, Philadelphia, Pennsylvania, c Department of Psychiatry and Human Behavior, Jefferson Medical College, Philadelphia, Pennsylvania and d MIRECC, Veteran Affairs Medical Center, Philadelphia, Pennsylvania, USA. Sponsorship: Supported in part by NIH grants K12 DA 00357 (D.E.G.), R01 DA 11835 (W.H.B.), K08-DA-00340 (A.A.P.), P60 DA 05186-16 (C.P.O.,W.H.B.) and T32 GM 008076-17 (J.J.C.). Correspondence to Dorothy E. Grice, Center for Neurobiology and Behavior, Room 135A, CRB, University of Pennsylvania, 415 Curie Boulevard, Philadelphia, PA 19104, USA. Tel: +1 215 573 4582; fax: +1 215 573 2041; e-mail: dgrice@mail.med.upenn.edu Received 25 July 2002 Accepted 6 January 2003 Introduction The mu opioid receptor is the molecular target for endogenous opioid peptides and several exogenous opioid drugs, including morphine. The mu opioid receptor mediates several properties of morphine, including the rewarding effects and the development of tolerance and dependence (Matthes et al., 1996). It also seems to be involved in the reinforcing effects of non-opioid drugs, including alcohol and cocaine (Kreek, 1996). Sequence variability in the gene encoding the human mu opioid receptor (OPRM1) may create a receptor with altered expression, structure or function, and as a consequence may increase or decrease an individual’s susceptibility to substance dependence (Lichtermann et al., 2000). Several case–control studies have investigated associations be- tween OPRM1 sequence variability and substance dependence, with inconsistent results. Most of these studies have focused on two polymorphisms found in OPRM1 exon 1 that alter amino acid sequence, A + 118 G (Asn40Asp) and C + 17 T (Ala6Val). The A + 118 G polymorphism is of particular interest since functional effects of the A + 118 G polymorphism have been demonstrated both in vitro and in vivo. Bond et al. 0955-8829  c 2003 Lippincott Williams & Wilkins DOI: 10.1097/01.ypg.0000071762.90004.45 Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.