Single-nucleotide polymorphism in the human mu opioid receptor gene alters β-endorphin binding and activity: Possible implications for opiate addiction

Abstract

Opioid drugs play important roles in the clinical management of pain, as well as in the development and treatment of drug abuse. The mu opioid receptor is the primary site of action for the most commonly used opioids, including morphine, heroin, fentanyl, and methadone. By sequencing DNA from 113 former heroin addicts in methadone maintenance and 39 individuals with no history of drug or alcohol abuse or dependence, we have identified five different single-nucleotide polymorphisms (SNPs) in the coding region of the mu opioid receptor gene. The most prevalent SNP is a nucleotide substitution at position 118 (A118G), predicting an amino acid change at a putative N-glycosylation site. This SNP displays an allelic frequency of approximately 10% in our study population. Significant differences in allele distribution were observed among ethnic groups studied. The variant receptor resulting from the A118G SNP did not show altered binding affinities for most opioid peptides and alkaloids tested. However, the A118G variant receptor binds ␤-endorphin, an endogenous opioid that activates the mu opioid receptor, approximately three times more tightly than the most common allelic form of the receptor. Furthermore, ␤-endorphin is approximately three times more potent at the A118G variant receptor than at the most common allelic form in agonist-induced activation of G protein-coupled potassium channels. These results show that SNPs in the mu opioid receptor gene can alter binding and signal transduction in the resulting receptor and may have implications for normal physiology, therapeutics, and vulnerability to develop or protection from diverse diseases including the addictive diseases.The mu opioid receptor is the primary site of action of several of the endogenous opioid peptides including ␤-endorphin, Met-enkephalin-Arg-Phe, and the recently identified endomorphins (1). This receptor is also the major target for clinically important opioid analgesic agents including morphine, methadone, fentanyl, and related drugs (2, 3). Activation of this receptor has diverse physiological effects (4, 5). Furthermore, it is the major molecular site of action for heroin (6, 7). Rapid activation of the mu opioid receptor, such as that which occurs in the setting of drug abuse, results in a euphoric effect, thus conferring the reinforcing or rewarding effects of the drug, contributing to the development of addiction. Clinical observations have suggested that individuals have varied sensitivity to opioids, suggesting potential variability in the receptor protein and gene. Naturally occurring polymorphisms are well known to exist in human genes; some have been shown to produce profound effects on the function of the corresponding proteins. Molecular cloning of the mu opioid receptor (8-11) has made it possible to determine potential sequence polymorphism, as shown by two recent studies (12, 13). The mu opioid receptor is a member of the G protein-coupled receptor family (8,14). There are a number of well documented cases where natural...

A Functional Polymorphism of the μ-Opioid Receptor Gene is Associated with Naltrexone Response in Alcohol-Dependent Patients

“…In particular, two polymorphisms in exon 1 of the gene alter amino acid sequence, A +118 G (Asn40Asp) and C +17 T (Ala6Val), and these have received the most research attention. However, case-control studies have failed to demonstrate a consistent association between OPRM1 sequence variation and the presence of alcohol and/or drug dependence (Bergen et al, 1997;Berrettini et al, 1997;Bond et al, 1998;Kranzler et al, 1998;Sander et al, 1998;Gelernter et al, 1999;Town et al, 1999;Hoehe et al, 2000;Franke et al, 2001;Rommelspacher et al, 2001;Szeto et al, 2001;Schinka et al, 2002;Crowley et al, 2003).…”

“…In this respect, the A +118 G polymorphism is of particular interest, since functional effects of the polymorphism have been demonstrated both in vitro and in vivo. Bond et al (1998) showed that, in cell culture, m-opioid receptors encoded by the Asp40 variant bind b-endorphin and activate G-protein-coupled protein potassium ion channels with three times greater potency than receptors encoded by the Asn40 variant. Both Wand et al (2002) and Hernandez-Avila et al (2003) found that individuals with one or two copies of the Asp40 allele had altered HPA-axis activation induced by the opioid receptor antagonist naloxone, while Smolka et al (1999) showed that individuals with the Asp40 variant display greater dopaminergic sensitivity during acute alcohol withdrawal.…”

“…Consistent with evidence of greater effects of naloxone among subjects with the Asp40 variant, we hypothesized that this allele would predict greater clinical response among patients who underwent naltrexone treatment for a minimum of 5 weeks. At least one copy of the Asp40 variant is expected to be present in 24.3-36% of the general population of adults of European descent (Bergen et al, 1997;Bond et al, 1998;Gelernter et al, 1999;Crowley et al, 2003). Consequently, this polymorphism is also of potential importance on an epidemiological level, since the allele is sufficiently common to be clinically relevant if associated with treatment response.…”

Altered Levels of Basal Cortisol in Healthy Subjects with a 118G Allele in Exon 1 of the Mu Opioid Receptor Gene

“…We have shown that a functional single nucleotide polymorphism in the first exon of OPRM1 accounts for up to 21% of the attributable risk for developing heroin addiction and 11% of the risk for developing alcoholism in cohorts from central Sweden (Bart et al, 2004(Bart et al, , 2005. This A118G polymorphism (rs1799971) results in an asparagine to aspartic acid substitution at amino-acid position 40 and leads to increased receptor-binding affinity for the endogenous opioid beta-endorphin and increased potency of ion channel activation following beta-endorphin binding (Bond et al, 1998). Two other studies, one in transiently-expressing COS cells (Befort et al, 2001), the other in stable HEK293 cell lines (Beyer et al, 2004), did not replicate these findings.…”

“…Two other studies, one in transiently-expressing COS cells (Befort et al, 2001), the other in stable HEK293 cell lines (Beyer et al, 2004), did not replicate these findings. In our prior study (Bond et al, 1998), the variant and prototype receptors were stablytransfected AV-12 cells, which confer N-glycosylation. These cells were chosen because, in the mu opioid receptor, amino-acid position 40 is a putative site of glycosylation.…”