Increased attributable risk related to a functional mu-opioid receptor gene polymorphism in association with alcohol dependence in central Sweden

The importance of glucocorticoids in alcohol dependence and neurotoxicity

Alterations in hypothalamo-pituitary adrenal (HPA) function have been described in alcoholics and in rodents after chronic alcohol consumption but the role of glucocorticoids in alcohol consumption, and the mechanisms involved, has received little attention until recently. Both alcohol consumption and withdrawal from chronic alcohol intake raise circulating glucocorticoid levels, and prolonged high concentrations of glucocorticoids are known to have detrimental effects on neuronal function and cognition. This minireview covers the ways in which glucocorticoids may be involved in drinking behavior, from social drinking to dependence, and the negative consequences of alcohol consumption seen during withdrawal which may have a detrimental effect on treatment outcome. Research shows prolonged increases in brain glucocorticoid concentrations and decreased brain glucocorticoid receptor availability (consistent with increased levels of endogenous ligand) after withdrawal from chronic alcohol treatment. Evidence suggests that increased glucocorticoid levels in the brain after chronic alcohol treatment are associated with the cognitive deficits seen during abstinence which impact on treatment efficacy and quality of life. Studies on organotypic cultures also demonstrate the importance of glucocorticoids in the neuropathological consequences of alcohol dependence.

Psychobiological traits in the risk profile for overeating and weight gain

Our dramatically changed food environment--since periods in our history when food sources were highly constrained--has presented new challenges for obesity research. For example, these alterations have strongly emphasized the physiological differences between the homeostatic and the hedonic regulation of food intake--the latter being largely responsible for the pronounced increase in obesity in the past few decades. There is also increasing agreement that compulsive overeating shares many parallels with addiction disorders such as drug abuse. These factors have also fostered a renewed interest in identifying individual differences in personality and motivational systems that increase the risk for overeating and weight gain in our population. Reward sensitivity has been the focus of a recent body of compelling research, with evidence favoring two seemingly opposite points of view. On the one hand, studies have found support for a link between low reward sensitivity and obesity, whereas other evidence suggests that a strong appetitive motivation leads to overeating and weight gain. Arguments are provided to reconcile these apparently disparate theories. Finally, the role of impulsivity and its links with symptoms of attention deficit/hyperactivity disorder are discussed, as well as their respective roles in the risk profile for obesity.

A pharmacogenetic model of naltrexone-induced attenuation of alcohol consumption in rhesus monkeys

BACKGROUND

Variation at the human mu-opioid receptor has been associated with alcohol abuse. The A118G (N40D) polymorphism in humans is functionally mimicked by the C77G (P26R) polymorphism in rhesus monkeys; both show similar in vitro influences on ligand binding and in vivo correlations with physiological measures as well as behavioral measures including predilection towards alcohol consumption. Naltrexone, an antagonist at the receptor, has been used to treat alcoholism in humans and has been reported to show differences in effectiveness depending on genotype.

METHODS

Here we describe a study in which we a priori selected rhesus monkeys based on genotype at the OPRM1 C77G single nucleotide polymorphism, trained them to self-administer alcohol, and assessed naltrexone responsiveness.

RESULTS

Alcohol intake in rhesus monkeys varied with genotype across a range of alcohol concentrations (0.5-4%, w/v) such that animals with the G/G genotype drank consistently more alcohol than those animals with the C/C genotype. Additionally, naltrexone attenuated alcohol drinking in a dose- and genotype-dependent manner. Animals harboring the G/G genotype were more sensitive to the effects of naltrexone and showed greater reductions in alcohol consumption at lower naltrexone doses compared to animals with a C/G or C/C genotype.

CONCLUSIONS

This preliminary study demonstrates a pharmacogenomic response to naltrexone in rhesus monkeys that parallels that seen in humans. This finding provides a basis for developing a pharmacogenetic animal model for naltrexone effect that can expand further our understanding of the causes and treatments of alcohol use disorders.

OPRM1 SNP (A118G): involvement in disease development, treatment response, and animal models

Endogenous opioids acting at mu-opioid receptors mediate many biological functions. Pharmacological intervention at these receptors has greatly aided in the treatment of acute and chronic pain, in addition to other uses. However, the development of tolerance and dependence has made it difficult to adequately prescribe these therapeutics. A common single nucleotide polymorphism (SNP), A118G, in the mu-opioid receptor gene can affect opioid function and, consequently, has been suggested to contribute to individual variability in pain management and drug addiction. Investigation into the role of A118G in human disease and treatment response has generated a large number of association studies across various disease states as well as physiological responses. However, characterizing the functional consequences of this SNP and establishing if it causes or contributes to disease phenotypes have been significant challenges. In this manuscript, we will review a number of association studies as well as investigations of the functional impact of this gene variant. In addition, we will describe a novel mouse model that was generated to recapitulate this SNP in mice. Evaluation of models that incorporate known human genetic variants into a tractable system, like the mouse, will facilitate the understanding of discrete contributions of SNPs to human disease.

Pharmacotherapy in the treatment of addiction: methadone

Methadone maintenance treatment (MMT) is the most widely available pharmacotherapy for opioid addiction and has been shown to be an effective and safe treatment over a period of 40 years. Although women comprise approximately 40% of clients currently being treated in MMT programs, comparatively little research geared specifically toward this group has been published. This article begins with an overview of neurobiological studies on opioid addiction, including a discussion of gender differences, followed by a review of the pharmacology of methadone. The authors then examine the particular needs and differences of women being treated in MMTs, including co-dependence with other substances, women's health issues, and psychosocial needs unique to this population. Research shows that women have different substance abuse treatment needs in comparison to their male counterparts. One New York City MMT program that has attempted to address these differences is highlighted.

Drug-induced and genetic alterations in stress-responsive systems: Implications for specific addictive diseases

From the earliest work in our laboratory, we hypothesized, and with studies conducted in both clinical research and animal models, we have shown that drugs of abuse, administered or self-administered, on a chronic basis, profoundly alter stress-responsive systems. Alterations of expression of specific genes involved in stress responsivity, with increases or decreases in mRNA levels, receptor, and neuropeptide levels, and resultant changes in hormone levels, have been documented to occur after chronic intermittent exposure to heroin, morphine, other opiates, cocaine, other stimulants, and alcohol in animal models and in human molecular genetics. The best studied of the stress-responsive systems in humans and mammalian species in general is undoubtedly the HPA axis. In addition, there are stress-responsive systems in other parts in the brain itself, and some of these include components of the HPA axis, such as CRF and CRF receptors, along with POMC gene and gene products. Several other stress-responsive systems are known to influence the HPA axis, such as the vasopressin-vasopressin receptor system. Orexin-hypocretin, acting at its receptors, may effect changes which suggest that it should be properly categorized as a stress-responsive system. However, less is known about the interactions and connectivity of some of these different neuropeptide and receptor systems, and in particular, about the possible connectivity of fast-acting (e.g., glutamate and GABA) and slow-acting (including dopamine, serotonin, and norepinephrine) neurotransmitters with each of these stress-responsive components and the resultant impact, especially in the setting of chronic exposure to drugs of abuse. Several of these stress-responsive systems and components, primarily based on our laboratory-based and human molecular genetics research of addictive diseases, will be briefly discussed in this review.

Single-nucleotide polymorphism (A118G) in exon 1 of OPRM1 gene causes alteration in downstream signaling by mu-opioid receptor and may contribute to the genetic risk for addiction

The opioid receptor mu1 (OPRM1) mediates the action of morphine. Although genetic background plays an important role in the susceptibility toward abuse of drugs as evident from familial, adoption and twin studies, association of specific single-nucleotide polymorphisms of OPRM1 gene with narcotic addiction is to be established. Here, we demonstrate the involvement of A118G polymorphism of exon1 of human OPRM1 gene (hOPRM1), with heroin and alcohol addiction, in a population in eastern India. Statistical analysis exhibited a significant association of G allele with both heroin and alcohol addiction with a risk factor of P(trend) < 0.05. The functional significance of G allele in A118G single-nucleotide polymorphisms was evaluated by studying the regulation of protein kinase A (PKA), pCREB, and pERK1/2 by morphine in Neuro 2A cells, stably transfected with either wild type or A118G mutant hOPRM1. Unlike acute morphine treatment, both chronic morphine exposure and withdrawal precipitated by naloxone were differentially regulated by A118 and G118 receptor isoforms when both PKA and pERK1/2 activities were compared. Results suggest that the association of A118G polymorphism to heroin and alcohol addiction may be because of the altered regulation of PKA and pERK1/2 during opioid and alcohol exposures.

Initial evidence of an association between OPRM1 and adolescent alcohol misuse

BACKGROUND

Considerable research efforts have attempted to identify genes associated with alcoholism among adults, yet few studies have examined adolescents. Identifying genes associated with alcohol misuse in youth is important given that the relative contribution of genetic and environmental influences on alcoholism varies across development. The purpose of this study was to examine the association between a polymorphism of the mu-opioid receptor gene (OPRM1) and alcohol misuse in a sample of youth and to test whether heightened sensitivity to the reinforcing effects of alcohol mediated this relationship.

METHODS

Adolescents (n = 187; mean age = 15.4 years; 47.6% female) were genotyped for A118G (rs1799971), a single-nucleotide polymorphism (SNP) of the OPRM1 gene, and assessed for alcohol use disorder (AUD) diagnoses and other psychopathology. Alcohol misuse was also measured continuously to maximize detection of drinking problems in youth. Drinking motives were used to capture the extent to which youth consumed alcohol to enhance positive affect.

RESULTS

AUD groups differed significantly in terms of allelic distributions of the A118G SNP, such that 51.9% of youth with an AUD carried at least one copy of the G allele compared to 16.3% of non-AUD controls. Those who carried the G allele endorsed drinking to enhance positive affect more strongly than those who were homozygous for the A allele and drinking to enhance positive affect mediated the association between OPRM1 and alcohol-related problems.

CONCLUSIONS

These data build on findings from adult studies and provide the first evidence that a polymorphism of the OPRM1 receptor gene is associated with the development of early-onset alcohol-related problems during adolescence, in part, by heightening sensitivity to the reinforcing effects of alcohol.

The interaction of reward genes with environmental factors in contribution to alcoholism in mexican americans

BACKGROUND

Alcoholism is a polygenic disorder resulting from reward deficiency; polymorphisms in reward genes including serotonin transporter (5-HTT)-linked polymorphic region (5-HTTLPR), A118G in opioid receptor mu1 (OPRM1), and -141C Insertion/Deletion (Ins/Del) in dopamine receptor D2 (DRD2) as well as environmental factors (education and marital status) might affect the risk of alcoholism. Objective of the current study was to examine the main and interacting effect of these 3 polymorphisms and 2 environmental factors in contribution to alcoholism in Mexican Americans.

METHODS

Genotyping of 5-HTTLPR, OPRM1 A118G, and DRD2-141C Ins/Del was performed in 365 alcoholics and 338 nonalcoholic controls of Mexican Americans who were gender- and age-matched. Alcoholics were stratified according to tertiles of MAXDRINKS, which denotes the largest number of drinks consumed in one 24-hour period. Data analysis was done in the entire data set and in each alcoholic stratum. Multinomial logistic regression was conducted to explore the main effect of 3 polymorphisms and 2 environmental factors (education and marital status); classification tree, generalized multifactor dimensionality reduction (GMDR) analysis, and polymorphism interaction analysis version 2.0 (PIA 2) program were used to study factor interaction.

RESULTS

Main effect of education, OPRM1, and DRD2 was detected in alcoholic stratum of moderate and/or largest MAXDRINKS with education < or =12 years, OPRM1 118 A/A, and DRD2 -141C Ins/Ins being risk factors. Classification tree analysis, GMDR analysis, and PIA 2 program all supported education*OPRM1 interaction in alcoholics of largest MAXDRINKS with education < or =12 years coupled with OPRM1 A/A being a high risk factor; dendrogram showed synergistic interaction between these 2 factors; dosage-effect response was also observed for education*OPRM1 interaction. No definite effect of marital status and 5-HTTLPR in pathogenesis of alcoholism was observed.

CONCLUSIONS

Our results suggest main effect of education background, OPRM1 A118G, and DRD2 -141C Ins/Del as well as education*OPRM1 interaction in contribution to moderate and/or severe alcoholism in Mexican Americans. Functional relevance of these findings still needs to be explored.

The G allele of SNP E1/A118G at the µ-opioid receptor gene locus shows genomic evidence of recent positive selection

Opioid drug response and pain perception differs greatly amongst different individuals. The µ-opioid receptor (MOR) is the main receptor target for important opioid analgesics. As SNPs may contribute to interindividual differences in drug response, in silico signatures of recent positive selection (RPS) were utilized to seek out potentially functional SNPs in the MOR gene in order to facilitate the prioritization of SNPs for evaluation in genetic association studies. Out of over 1000 SNPs at the MOR locus, 184 high-frequency SNPs were interrogated for signatures of RPS. A total of five SNPs (four noncoding and one nonsynonymous coding) demonstrated in silico evidence of RPS. Significantly, the nonsynonymous E1/A118G SNP, which was previously reported to be functionally important, showed in silico evidence of RPS. This reaffirms the feasibility of utilizing in silico signatures of RPS to identify potentially functionally significant SNPs for association studies. Interestingly, the positively selected G allele of this RPS SNP was also predicted to create a novel exon splice enhancer as well as p53 binding sites.

Functional characterization of human variants of the mu-opioid receptor gene

Opioids and their receptors have an important role in analgesia and alcohol and substance use disorders (ASUD). We have identified several naturally occurring amino acid changing variants of the human mu-opioid receptor (MOR), and assessed the functional consequences of these previously undescribed variants in stably expressing cell lines. Several of these variants had altered trafficking and signaling properties. We found that an L85I variant showed significant internalization in response to morphine, in contrast to the WT MOR, which did not internalize in response to morphine. Also, when L85I and WT receptor were coexpressed, WT MOR internalized with the L85I MOR, suggesting that, in the heterozygous condition, the L85I phenotype would be dominant. This finding is potentially important, because receptor internalization has been associated with development of tolerance to opiate analgesics. In contrast, an R181C variant abolished both signaling and internalization in response to saturating doses of the hydrolysis-resistant enkephalin [D-Ala2,N-MePhe4,Gly5-ol]enkephalin (DAMGO). Coexpression of the R181C and WT receptor led to independent trafficking of the 2 receptors. S42T and C192F variants showed a rightward shift in potency of both morphine and DAMGO, whereas the S147C variant displayed a subtle leftward shift in morphine potency. These data suggest that these and other such variants may have clinical relevance to opioid responsiveness to both endogenous ligands and exogenous drugs, and could influence a broad range of phenotypes, including ASUD, pain responses, and the development of tolerance to morphine.

Dopamine for "wanting" and opioids for "liking": a comparison of obese adults with and without binge eating

Obesity research suffers from an overinclusion paradigm whereby all participants with a BMI beyond a certain cutoff value (e.g., 30) are typically combined in a single group and compared to those of normal weight. There has been little attempt to identify meaningful subgroups defined by their salient biobehavioral differences. In order to address this limitation, we examined genetic and psychological indicators of hedonic eating in obese adults with (n=66) and without (n=70) binge eating disorder (BED). Our analyses focused on dopamine (DA) and opioid genetic markers because of their conjoint association with the functioning of brain reward mechanisms. We targeted three functional polymorphisms related to the D2 receptor (DRD2) gene, as well as the functional A118G polymorphism of the mu-opioid receptor (OPRM1) gene. We found that significantly more obese controls had the "loss-of-function" A1 allele of Taq1A compared to their BED counterparts, whereas the "gain-of-function" G allele of A118G occurred with greater frequency in the BED group. A significant gene-gene combination chi2 analysis also indicated that of those participants with the gain-gain genotype (G+ and A1), 80% were in the BED group whereas only 35% with the loss-loss genotype (G- and A1+) were in this group. Finally, BED subjects had significantly higher scores on a self-report measure of hedonic eating. Our findings suggest that BED is a biologically based subtype of obesity and that the proneness to binge eating may be influenced by a hyper-reactivity to the hedonic properties of food--a predisposition that is easily exploited in our current environment with its highly visible and easily accessible surfeit of sweet and fatty foods.

The opioid peptides enkephalin and beta-endorphin in alcohol dependence

BACKGROUND

Experimental evidence indicates that the endogenous opioid system influences stress responses as well as reinforces effects of addictive drugs. Because stress is an important factor contributing to drug dependence and relapse, we have now studied ethanol preference in enkephalin- and beta-endorphin-deficient mice under baseline conditions and after stress exposure.

METHODS

In the present study we used a two-bottle choice paradigm to study ethanol consumption and stress-induced ethanol preference. To examine alcohol withdrawal symptoms the forced drinking procedure was employed. We performed an association analysis in two case-control samples of alcohol addicts to determine whether these opioid peptides also contribute to ethanol dependence in humans.

RESULTS

Ethanol consumption was significantly reduced in the absence of beta-endorphins, particularly in female knockout animals. Stress exposure results in an increased ethanol consumption in wild-type mice but did not influence ethanol-drinking in beta-endorphin knockouts. Enkephalin-deficient mice showed no difference from wild-type mice in baseline ethanol preference but also showed no stress-induced elevation of ethanol consumption. Interestingly, we found a two-marker haplotype in the POMC gene that was associated with alcohol dependence in females in both cohorts.

CONCLUSIONS

Together these results indicate a contribution of beta-endorphin to ethanol consumption and dependence.

Review. Genetics of addictions: strategies for addressing heterogeneity and polygenicity of substance use disorders

Addictions are common psychiatric disorders that exert high cost to the individual and to society. Addictions are a result of the interplay of multiple genetic and environmental factors. They are characterized by phenotypic and genetic heterogeneity as well as polygenicity, implying a contribution of different neurobiological mechanisms to the clinical diagnosis. Therefore, treatments for most substance use disorders are often only partially effective, with a substantial proportion of patients failing to respond. To address heterogeneity and polygenicity, strategies have been developed to identify more homogeneous subgroups of patients and to characterize genes contributing to their phenotype. These include genetic linkage and association studies as well as functional genetic analysis using endophenotypes and animal behavioural experimentation. Applying these strategies in a translational context aims at improving therapeutic response by the identification of subgroups of addiction patients for individualized, targeted treatment strategies. This article aims to discuss strategies addressing heterogeneity and polygenicity of substance use disorders by presenting results of recent research on genetic and environmental components of addiction. It will also introduce the European IMAGEN study that aims to integrate methodical approaches discussed in order to identify the genetic and neurobiological basis of behavioural traits relevant to the development of addictions.

Analgesic requirements after major abdominal surgery are associated with OPRM1 gene polymorphism genotype and haplotype

Aims: The association between SNPs of the human OPRM1 gene encoding the µ-opioid receptor and postoperative analgesic requirements in surgical patients remains controversial. Here, we evaluate whether any of the five tag SNPs (A118G, IVS2+G691C, IVS3+G5953A, IVS3+A8449G and TAA+A2109G) representing the four linkage disequilibrium blocks of the OPRM1 gene influences postoperative analgesic requirements. Materials & methods: We studied 138 adult Japanese patients who underwent major open abdominal surgery under combined general and epidural anesthesia and received continuous postoperative epidural analgesia with opioids. Results: The 118G homozygous (GG) patients required 24-h postoperative analgesics more than 118A homozygous (AA) and heterozygous (AG) patients. Tag SNP haplotypes also were associated with 24-h postoperative analgesic requirements. Conclusions: These results suggest that OPRM1 gene tag SNP genotypes and haplotypes can primarily contribute to prediction of postoperative analgesic requirements in individual patients undergoing major open abdominal surgery.

Pharmacogenetic treatments for drug addiction: alcohol and opiates

AIMS

Psychiatric pharmacogenetics involves the use of genetic tests that can predict the effectiveness of treatments for individual patients with mental illness such as drug dependence. This review aims to cover these developments in the pharmacotherapy of alcohol and opiates, two addictive drugs for which we have the majority of our FDA approved pharmacotherapies.

METHODS

We conducted a literature review using Medline searching terms related to these two drugs and their pharmacotherapies crossed with related genetic studies.

RESULTS

Alcohol's physiological and subjective effects are associated with enhanced beta-endorphin release. Naltrexone increases baseline beta-endorphin release blocking further release by alcohol. Naltrexone's action as an alcohol pharmacotherapy is facilitated by a putative functional single nucleotide polymorphism (SNP) in the opioid mu receptor gene (Al18G) which alters receptor function. Patients with this SNP have significantly lower relapse rates to alcoholism when treated with naltrexone. Caucasians with various forms of the CYP2D6 enzyme results in a 'poor metabolizer' phenotype and appear to be protected from developing opioid dependence. Others with a "ultra-rapid metabolizer" phenotype do poorly on methadone maintenance and have frequent withdrawal symptoms. These patients can do well using buprenorphine because it is not significantly metabolized by CYP2D6.

CONCLUSIONS

Pharmacogenetics has great potential for improving treatment outcome as we identify gene variants that affect pharmacodynamic and pharmacokinetic factors. These mutations guide pharmacotherapeutic agent choice for optimum treatment of alcohol and opiate abuse and subsequent relapse.

Bidirectional translational research: Progress in understanding addictive diseases

The focus of this review is primarily on recent developments in bidirectional translational research on the addictions, within the Laboratory of the Biology of Addictive Diseases at The Rockefeller University. This review is subdivided into major interacting aspects, including (a) Investigation of neurobiological and molecular adaptations (e.g., in genes for the opioid receptors or endogenous neuropeptides) in response to cocaine or opiates, administered under laboratory conditions modeling chronic patterns of human self-exposure (e.g., chronic escalating "binge"). (b) The impact of such drug exposure on the hypothalamic-pituitary-adrenal (HPA) axis and interacting neuropeptidergic systems (e.g., opioid, orexin and vasopressin). (c) Molecular genetic association studies using candidate gene and whole genome approaches, to define particular systems involved in vulnerability to develop specific addictions, and response to pharmacotherapy. (d) Neuroendocrine challenge studies in normal volunteers and current addictive disease patients along with former addicts in treatment, to investigate differential pharmacodynamics and responsiveness of molecular targets, in particular those also investigated in the experimental and molecular genetic approaches as described above.

The OPRD1 and OPRK1 loci in alcohol or drug dependence: OPRD1 variation modulates substance dependence risk

Eleven single-nucleotide polymorphisms (SNPs) spanning OPRD1 were examined in 1063 European Americans (EAs) (620 cases with substance dependence (SD), including 557 with alcohol dependence (AD), 225 with cocaine dependence (CD) and 111 with opioid dependence (OD), and 443 controls). Nominally significant associations (P<0.05) of five SNPs with SD were observed; only the association of the non-synonymous variant G80T with OD remained significant after correction for multiple testing using SNPSpD. Haplotype analyses with six tag SNPs indicated that a specific haplotype GCAACT, which harbors G80T G-allele and C921T C-allele, was significantly associated with AD (chi(2)=14.82, degrees of freedom (d.f.)=1, P<0.001), CD (chi(2)=9.19, d.f.=1, P=0.002) and OD (chi(2)=20.68, d.f.=1, P<0.001). Logistic regression analyses, with sex and age being considered, demonstrated that this haplotype had a risk effect on AD (P=0.03, beta=1.86, odds ratio (OR)=6.43) and especially on OD (P<0.001, beta=3.92, OR=50.57). Moreover, seven SNPs covering OPRK1 were examined in the majority of the above subjects (390 cases, including 327 AD, 177 CD and 97 OD subjects, and 358 controls). Although no significant differences in allele, genotype or haplotype frequency distributions were seen between cases and controls, a specific OPRK1 haplotype, GGCTTCT, was significantly associated with AD (chi(2)=8.12, d.f.=1, P=0.004). Logistic regression analyses also revealed its risk effect on AD (P=0.009, beta=1.06, OR=2.90). Population stratification artifact was not observed in the sample. Taken together, our findings supported a positive association between OPRD1 variants and SD, and a positive haplotypic association between OPRK1 and AD in EAs.

Limited effects of beta-endorphin compared to loperamide or fentanyl in a neuroendocrine biomarker assay in non-human primates

The in vivo pharmacodynamics of the opioid neuropeptide beta-endorphin (a major endogenous agonist at the mu-opioid receptor) is difficult to determine in non-human primate models with translational value, or in humans. The present studies therefore employed a neuroendocrine biomarker assay, prolactin release, to systematically compare the in vivo profile of i.v. beta-endorphin (0.01-0.32 mg/kg; i.v.) in gonadally intact male rhesus monkeys (n=4) to that of the peripherally selective mu-agonist loperamide (0.01-0.32 mg/kg; i.v.) and the centrally penetrating mu-agonist fentanyl (0.0056-0.018 mg/kg; i.v.). Studies utilized a standardized time course design (measuring prolactin levels 5-120 min after agonist administration). Beta-endorphin displayed only limited effectiveness in causing prolactin release when tested over this 30-fold dose range, compared to loperamide or fentanyl. Furthermore, two of the four subjects were only minimally responsive to beta-endorphin. This differential responsiveness was not due to the presence of a previously described single nucleotide polymorphism at the OPRM1 gene (C77G), known to affect beta-endorphin pharmacodynamics in vitro. In vivo biotransformation studies with MALDI-mass spectrometry determined that full-length beta-endorphin was detectable in all subjects up to at least 5 min after i.v. administration. Thus, the relative ineffectiveness of i.v. beta-endorphin in this assay does not appear to be principally due to rapid generation of non-opioid fragments of this neuropeptide.

Role of a functional human gene polymorphism in stress responsivity and addictions

Over the past decade, enormous progress has been made in both the technical approaches and the scientific information available for studying the human genome. Therefore, increasingly, scientists have begun to address not just single-gene disorders but complex disorders. The limiting factor in most of such studies remains appropriate, well-focused detailed phenotyping of the complex disorders under study, with careful ascertainment of subjects with the specific disorder, as well as healthy control subjects.

The single nucleotide polymorphism A118G alters functional properties of the human mu opioid receptor

The most common single nucleotide polymorphism in the coding region of the human mu opioid receptor gene is the A118G variant, an adenine to guanine transition at nucleotide position 118 of the coding sequence of the gene. This polymorphism codes for an asparagine to aspartic acid substitution at amino acid 40 in the amino-terminus, thereby removing a potential extracellular glycosylation site. Using in vitro cellular expression assays, this variant has been reported to change binding of the endogenous agonist beta-endorphin and signaling of the receptor following binding of beta-endorphin. Three clinical studies report that A118G genotype affects opioid antagonist-mediated increases in cortisol levels. These studies demonstrate a functional role of this variant in responses to endogenous and exogenous opioids. To further characterize function, we expressed the prototype and variant receptors in two types of cells (human 293 embryonic kidney cells and Syrian hamster adenovirus-12-induced tumor cells). Stable expression of variant and prototype receptors was characterized by differences in levels of cell surface binding capacity (B(max)), forskolin-induced cAMP accumulation, as well as agonist-induced accumulation of cAMP (EC(50)) for several agonists, but not for beta-endorphin. In contrast, transiently expressed variant receptors showed only a minor difference in cell surface binding capacity compared to the prototype, and no differences in cAMP EC(50) values.

Evaluation of OPRM1 variants in heroin dependence by family-based association testing and meta-analysis

OPRM1, which codes for the mu-opioid receptor, is the most frequently studied candidate gene for opioid dependence. Despite numerous allelic association studies, no definitive conclusion has been reached regarding the role of OPRM1 polymorphisms in determining risk for opioid dependence. We attempted to resolve this by conducting a family-based association study and meta-analysis which may be more robust and powerful, respectively, than traditional case-control analyses. First, we genotyped three single nucleotide polymorphisms (SNPs) of OPRM1 in 1208 individuals from 473 Han Chinese families ascertained on the basis of having two or more siblings with DSM-IV-defined opioid dependence. The Val6Ala and Arg111His SNPs were detected, but with low minor allele frequencies (0.002 and 0.001, respectively). The Asn40Asp SNP was more informative (minor allele frequency: 0.419), but no significant evidence was observed for either a dominant (p=0.810) or additive (p=0.406) effect of this polymorphism on risk for opioid dependence. In addition, a meta-analysis of case-control studies of opioid dependence was performed, and found a similar lack of evidence for an association with the Asn40Asp SNP (p=0.859). Although a role of OPRM1 polymorphisms in determining risk for opioid dependence cannot be entirely discounted, a major contribution of the Asn40Asp polymorphism seems unlikely. Further analysis is warranted in samples from specific ancestral groups. In addition, it is critical that other OPRM1 variants, including all haplotype-tagging and amino-acid-coding SNPs, be tested for an influence on risk for opioid dependence, since the Asn40Asp polymorphism is only one of several hundred known mutations in the gene.

The opioid system in alcohol and drug dependence: family-based association study

Opioid receptors and their endogenous peptide ligands play important roles in neurotransmission and neuromodulation in response to addictive drugs such as heroin, cocaine, and alcohol. In an earlier study, we reported that variation in the genes encoding the kappa-opioid receptor (OPRK1) and its peptide ligand (PDYN) were associated with the risk for alcoholism. We continued our investigation of the role of the opioid system in alcohol dependence by analyzing the genes encoding the micro- and delta-opioid receptors and their peptide ligands. We analyzed 18 OPRM1 SNPs, 18 OPRD1 SNPs, 7 PENK SNPs, and 7 POMC SNPs in a sample of 1923 European Americans from 219 multiplex alcohol dependent families. Employing a family-based test of association, we found no evidence that these four genes were significantly associated with alcohol dependence. We also did not find association between these genes and illicit drug dependence. Secondary analyses employing the narrower phenotype of opioid dependence (83 affected individuals) demonstrated association with SNPs in PENK and POMC, but not in OPRM1 or OPRD1. Haplotype analyses provided further support for the association of PENK and POMC with opioid dependence. Therefore, our data provide no support for the idea that variations in OPRM1, OPRD1, PENK and POMC are associated with alcohol dependence or general illicit drug dependence, but variations in PENK and POMC appear to be associated with the narrower phenotype of opioid dependence in these families.

Polymorphisms in the µ-opioid receptor gene (OPRM1) and the implications for alcohol dependence in humans

Twin and adoption studies have shown that alcohol dependence contains a substantial genetic component. In attempts to identify the genetic factors involved, association studies have linked the opioid system to alcohol dependence, with a main focus on the OPRM1 gene encoding the μ-opioid receptor. Our aim was to conduct a systematic review of the literature on the associations between polymorphisms in OPRM1 and alcohol dependence. We addressed findings of 12 studies that met our inclusion criteria. All studies employed a case–control design and included alcohol dependence as a dependent outcome measure. Our review showed that clinical studies do not unequivocally support an association between polymorphisms in OPRM1 and alcohol dependence. Factors that complicate genetic research on alcohol dependence, such as gene–environment interaction, and genetic and clinical heterogeneity, are discussed.

A functional polymorphism of the mu-opioid receptor gene (OPRM1) influences cue-induced craving for alcohol in male heavy drinkers

BACKGROUND

The mu-opioid receptor gene (OPRM1) codes for the mu-opioid receptor, which binds beta-endorphin. The A118G polymorphism in this gene affects beta-endorphin binding such that the Asp40 variant (G allele) binds beta-endorphin 3 times more tightly than the more common Asn40 variant (A allele). This study investigated the influence of the A118G polymorphism on cue reactivity after exposure to an alcoholic beverage in male heavy drinkers.

METHODS

Participants were either homozygous for the A allele (n=84) or carrying at least 1 copy of the G allele (n=24). All participants took part in a cue-reactivity paradigm where they were exposed to water and beer in 3-minute trials. The dependent variables of main interest were subjective craving for alcohol, subjective arousal, and saliva production.

RESULTS

G allele carriers reported significantly more craving for alcohol than the A allele participants (as indicated by the within-subject difference in craving after beer vs after water exposure). No differences were found for subjective arousal and saliva. Both groups did not differ in family history of alcoholism. Participants with the G allele reported a significantly higher lifetime prevalence of drug use than participants homozygous for the A allele.

CONCLUSIONS

A stronger urge to drink alcohol after exposure to an alcoholic beverage might contribute to a heightened risk for developing alcohol-related problems in individuals with a copy of the G allele. The G allele might also predispose to drug use in general.

Endogenous opioid blockade and impulsive responding in alcoholics and healthy controls

The opioid receptor antagonist naltrexone (NTX) is one of few approved treatments for alcoholism, yet the mechanism by which it reduces drinking remains unclear. In rats, NTX reduces morphine-induced impulsive choice bias; however, nothing is known about the drug's effect on discrete aspects of impulsive behavior in humans, such as decision-making and inhibitory control. Here, we used a modified delay discounting procedure to investigate whether NTX improves decision-making or inhibitory control in humans. We measured the effect of acute NTX (50 mg) on choice between smaller sooner (SS) and larger later monetary rewards and on response errors (motor mismatch) in a high conflict condition in a group of abstinent alcoholics (AA) and healthy control subjects (CS). We previously reported that AA selected the SS option significantly more often than did CS in this paradigm. If the choice bias of AA is due to enhanced endogenous opioid signaling in response to potential reward, NTX should reduce such bias in the AA group. We found that NTX did not reliably reduce impulsive choice in the AA group; however, NTX's effect on choice bias across individuals was robustly predictable. NTX's effect on choice bias was significantly correlated with scores on Rotter's Locus of Control (LOC) scale; increasingly internal LOC scores predicted increasing likelihood of impulsive choices on NTX. In addition, we found that NTX significantly enhanced control of motor responses, particularly within the CS group. These results suggest that endogenous opioids may impair response selection during decision-making under conflict, and that NTX's effects on explicit decision-making are personality-dependent. Determining the biological basis of this dependence could have important implications for effective alcoholism treatment.

Trait aggression and hostility in recovered alcoholics

There is a long-recognized association between alcohol consumption and aggressive behavior. This study was designed to examine aggression in a group of socially well-adapted recovered alcoholics (RA). The question addressed was whether the treatment, together with long-term abstinence from alcohol, could reduce aggression and hostility in RA. A group of male RA (n = 64), who did not meet the DSM-IV criteria for any psychiatric or personality disorder, were recruited to the study from aftercare groups. According to data from their group therapists, they were reliably abstinent for at least 3 years and socially well adapted. The study participants representing the control group (n = 69), diagnosed as being 'reliable nonalcoholics' (NA) by the Munich Alcoholism Test, were recruited from general practice. Data were derived from an in-house questionnaire on general characteristics of both groups, and aggressive and hostility traits were assessed using the Buss-Durkee Hostility Inventory (BDHI). The univariate and multivariate between-groups design was used for data analysis. Taking into account the BDHI dimensions of aggression and hostility, the difference between RA and NA groups was statistically significant [Wilks' lambda (8, 125) = 0.769; p = 0.00004]. There were statistically significant differences in the BDHI scales for indirect aggression, irritability, negativism, suspicion, resentment, and guilt. Both RA and Na groups did not differ significantly in variables that assessed physical and verbal aggression. After a 3-year abstinence, subjects from the RA group displayed signs of hostility and covert aggression.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).

Altered levels of basal cortisol in healthy subjects with a 118G allele in exon 1 of the Mu opioid receptor gene

The mu opioid receptor is centrally involved in the development of the addictive diseases. It also modulates the stress responsive hypothalamic-pituitary-adrenal axis. Receptors encoded by the variant 118G polymorphism in exon 1 of the mu opioid receptor gene have a threefold increase in beta-endorphin binding and beta-endorphin is three times more potent in receptor-mediated activation of G protein-coupled inwardly rectifying potassium channels. Humans with this variant have increased stress response following opioid antagonism. Here, we study basal levels of adrenocorticotropic hormone and cortisol in subjects with this variant. In all, 59 healthy adults were genotyped and had morning levels of adrenocorticotropic hormone and cortisol measured following intravenous administration of saline placebo. Subjects with a 118G allele had significantly greater levels of cortisol than subjects with the prototype gene. Groups did not differ in levels of adrenocorticotropic hormone. A planned comparison revealed significantly greater cortisol in females with at least one copy of the 118G allele compared to females with the prototype gene. There was no significant effect of gender alone, nor was there a significant interaction between gender and genotype, on ACTH or cortisol. Subjects with at least one copy of the 118G allele have increased basal levels of cortisol, which may influence the susceptibility to and treatment of the stress responsive dyscrasia.

A Genetic Association Study of the Functional A118G Polymorphism of the Human ??-Opioid Receptor Gene in Patients with Acute and Chronic Pain

In this prospective, observational study we explored whether A118G single nucleotide polymorphism in the human mu-opioid receptor (MOR) gene could explain the inter-individual differences in opioid analgesic requirements in patients with acute postoperative pain and chronic pain. The frequency of the wild-type A118 MOR (major) and variant G118 MOR (minor) alleles in the subjects with chronic, noncancer pain (n = 121) and opioid-naïve subjects with acute postoperative pain (n = 101), serving as the control group, were examined. The relationships among the A118G MOR genotype, opioid requirements, and the numerical pain score were analyzed in both groups. The frequency of the minor allele was significantly lower in the subjects with chronic pain when compared with the group with acute postoperative pain (0.079 versus 0.158; P = 0.009 by chi2 test). No statistically significant association was observed between the presence of A118G MOR polymorphism and the average postoperative pain score or the doses of morphine used in the immediate postoperative period. In the high-quartile, opioid utilization, chronic pain patients, the homozygotic carriers of the major allele required significantly higher opioid dose than did the carriers of the minor allele. The results indicate that although the presence of the minor allele does not appear to affect opioid analgesic use in acute postoperative pain, the minor allele is less common in chronic pain patients, especially in those requiring higher doses of opioid analgesics.

Genetic moderators of naltrexone's effects on alcohol cue reactivity

BACKGROUND

Naltrexone (NTX) reduces drinking and craving in alcoholic individuals in treatment and also in heavy drinkers. Polymorphisms in the D4 dopamine receptor (DRD4) gene and mu-opiate receptor gene (OPRM1) may moderate NTX's effects on craving. This study examined these candidate genes as moderators of the effects of NTX on cue-elicited urge to drink in non-treatment-seeking heavy drinkers.

METHOD

Data from the subset of 93 participants who consented for genetic testing in a larger study of medication effects were used to examine pharmacogenetic hypotheses. The non-treatment-seeking male and female heavy drinkers (62% alcohol dependent) were genotyped for the variable number of tandem repeats polymorphism in the DRD4 gene [L=7 or more (n=34), S=less than 7 (n=56)] and Asn40Asp single-nucleotide polymorphism in the OPRM1 gene [29 aspartate (Asp) carriers and 59 asparagine (Asn) homozygotes]. Ten days after randomization to NTX (50 mg) or placebo, participants completed an alcohol cue reactivity assessment.

RESULTS

Any medication effects were all accounted for by interaction with genotype. Naltrexone increased urge for alcohol in Asp carriers across alcohol and neutral beverage cue trials and had no effect on homozygous Asn carriers. Asp40 carriers on either medication had greater decreases (from resting baseline) in mean arterial blood pressure across all beverage cue trials compared with Asn carriers. For DRD4, no differential medication effects by DRD4 polymorphism were found. Alcohol dependence diagnosis did not moderate the effects of gene and medication on cue-elicited measures.

DISCUSSION

The differential responses to NTX due to variation in the OPRM1 gene may help explain conflicting results in clinical trials and suggest directions for patient-treatment matching.

Association of mu-opioid receptor gene polymorphism A118G with alcohol dependence in a Japanese population

Ethanol is considered to activate the brain reward system by increasing the release of an endogenous opioid receptor ligand, beta-endorphin. The polymorphism A118G in the mu-opioid receptor gene (OPRM1) causes the amino acid change Asn40Asp and has been reported to affect the affinity of the ligand for the receptor. The association of this polymorphism with the vulnerability to alcohol dependence has been studied in many populations, but not yet in Japanese people. In the present study, we compared the frequencies of the polymorphism OPRM1 A118G between patients with alcohol dependence and healthy control subjects living in a Japanese provincial prefecture. We also genotyped a polymorphism, G1510A, in the acetaldehyde dehydrogenase 2 gene (ALDH2), in which the A allele causes poor metabolism of acetaldehyde, a major metabolite of alcohol. Both OPRM1 118G and ALDH2 1510G were significantly associated with alcohol dependence. These results suggest that OPRM1 118G in addition to ALDH2 1510G might be one of the risk factors for alcohol dependence in Japanese people.

Targeting treatments for alcohol dependence: the pharmacogenetics of naltrexone

Alcohol dependence is one of the leading causes of morbidity worldwide, yet only a minority of those afflicted engages in treatment. While increasing access to treatment is an important public health approach, increasing the success of treatment is also likely to lead to greater engagement. However, alcohol dependence is a complex disorder likely to consist of several biological subtypes. Recent evidence from a number of different studies has suggested that genetic variation in the mu-opioid receptor has a significant influence on clinical presentation of alcohol problems and response to treatment with an opioid antagonist. Most notably, the A118G polymorphism of the mu-receptor gene has been demonstrated to predict clinical response to naltrexone in alcohol-dependent individuals. This article reviews the biological correlates and outlines a scientific agenda for better understanding the role of opioid neurotransmission in the etiology, maintenance and treatment of alcohol dependence.

Non-human primate models of inheritance vulnerability to alcohol use disorders

Many animal species have been used to model certain aspects of alcohol use and addiction. However, there are complex behavioral and social features of alcohol use disorders that are not easily modeled in animal species. This review considers both the limitations and advantages of using a non-human primate to model alcohol use disorders and discusses how non-human primates can be particularly useful for studying how genetic variants interact with social factors, temperament and alcohol response as motivating factors for alcohol consumption and abstinence. Genetic variants in rhesus macaques (Macaca mulatta) that are functionally equivalent to those increasing addiction vulnerability in humans influence temperament, stress reactivity and alcohol response in addition to voluntary alcohol consumption. Non-human primate models may also have translational value for understanding of how variants within addiction and abuse vulnerability genes influence alcohol-induced neuroadaptation, neuropathology and treatment response.

Association of an Asn40Asp (A118G) polymorphism in the mu-opioid receptor gene with substance dependence: a meta-analysis

INTRODUCTION

The mu-opioid receptor has been implicated in the pathogenesis of dependence on opioids, alcohol, nicotine, and cocaine. Studies examining the association of the mu-opioid receptor gene (genetic locus OPRM1) with substance dependence (SD) have focused on the Asn40Asp (A118G) single nucleotide polymorphism (SNP).

METHOD

We used meta-analysis to examine the literature on the association of Asn40Asp with SD. Twenty-two articles describing 28 distinct samples and over 8000 subjects were included. A variety of factors (i.e., ethnicity, type of SD, rigor with which controls were screened, severity of SD among cases) were examined as potential moderators of the association.

RESULTS

Four studies showed a significantly higher frequency of the Asp40 allele among SD cases, while three studies showed a significantly higher frequency of the Asp40 allele among controls. There was no significant association between Asn40Asp and SD (OR=1.01, 95%CI=0.86-1.19), nor was there substantial evidence of a moderator effect.

CONCLUSION

The Asn40Asp SNP in OPRM1 does not appear to affect risk for SD. Additional research is needed to determine whether these findings reflect no role for OPRM1 in determining risk for SD or whether another polymorphism in the gene influences receptor function and risk for SD.

Mu opioid receptor A118G polymorphism in association with striatal opioid neuropeptide gene expression in heroin abusers

Mu opioid receptors are critical for heroin dependence, and A118G SNP of the mu opioid receptor gene (OPRM1) has been linked with heroin abuse. In our population of European Caucasians (n = 118), approximately 90% of 118G allelic carriers were heroin users. Postmortem brain analyses showed the OPRM1 genotype associated with transcription, translation, and processing of the human striatal opioid neuropeptide system. Whereas down-regulation of preproenkephalin and preprodynorphin genes was evident in all heroin users, the effects were exaggerated in 118G subjects and were most prominent for preproenkephalin in the nucleus accumbens shell. Reduced opioid neuropeptide transcription was accompanied by increased dynorphin and enkephalin peptide concentrations exclusively in 118G heroin subjects, suggesting that the peptide processing is associated with the OPRM1 genotype. Abnormal gene expression related to peptide convertase and ubiquitin/proteosome regulation was also evident in heroin users. Taken together, alterations in opioid neuropeptide systems might underlie enhanced opiate abuse vulnerability apparent in 118G individuals.

New findings on the genetic influences on alcohol use and dependence

PURPOSE OF REVIEW

Alcohol dependence is a complex disorder with a well documented highly hereditary nature. This article reviews the recent advances in our understanding of the direct and indirect genetic influences on alcohol use and dependence.

RECENT FINDINGS

Recent findings can be summarized as follows: (a) twin studies have defined and estimated the risks of general and specific alcohol-related vulnerabilities. (b) Linkage studies have provided largely inconsistent findings, though several chromosomal regions have been implicated. (c) Quantitative trait loci analyses in animals have identified that the Mpdz gene predisposes to alcohol dependence and withdrawal. (d) Examination of family-based samples has identified several genes including GABRA2 and CHRM2 thought to be associated with alcohol dependence.

SUMMARY

Despite great advances in understanding of genetic vulnerability in alcohol use disorders, only two gene complexes, ADH and ALDH2, have been identified as having defined effects on alcohol use and liability to dependence in humans. New genes associated with increased risks for the disorder will certainly be added to this list in the near future. Neurobiological analyses of the effects of these genes will surely contribute to further understanding of the cause of alcohol dependence and the interindividual differences in risks.

An overview of the genetics of substance use disorders

Twin, family, and adoption studies show that genetic factors play a significant role in vulnerability to becoming addicted to drugs. Through a combination of genetic approaches, the variant alleles responsible for interindividual differences in susceptibility to addiction are being uncovered.

The genetics of alcohol dependence

Alcohol dependence is a common, complex disorder, which affects millions of people worldwide and causes considerable burden in terms of interpersonal and societal costs. Family, twin, and adoption studies have convincingly demonstrated that genes play an important role in the development of alcohol dependence, with heritability estimates in the range of 50% to 60% for both men and women. A number of studies are under way to identify specific genes involved in the predisposition toward alcohol dependence, and there is reason to be enthusiastic about recent progress. Several associated susceptibility genes are reviewed here, including genes involved in alcohol metabolism, as well as genes involved in GABAergic, endogenous opioid, dopaminergic, cholinergic, and serotonergic transmission. The next challenge will be to further characterize the risk associated with these susceptibility genes, examining how they may be related to comorbid disorders, developmental trajectories of risk, and potential moderation by environmental factors.

Association between two mu-opioid receptor gene (OPRM1) haplotype blocks and drug or alcohol dependence

We examined 13 single nucleotide polymorphisms (SNPs) spanning the coding region of the mu-opioid receptor gene (OPRM1), among 382 European Americans (EAs) affected with substance dependence [alcohol dependence (AD) and/or drug dependence (DD)] and 338 EA healthy controls. These SNPs delineated two haplotype blocks. Genotype distributions for all SNPs were in Hardy-Weinberg equilibrium (HWE) in controls, but in cases, four SNPs in Block I and three SNPs in Block II showed deviation from HWE. Significant differences were found between cases and controls in allele and/or genotype frequencies for six SNPs in Block I and two SNPs in Block II. Association of SNP4 in Block I with DD (allele: P=0.004), SNP5 in Block I with AD and DD (allele: P< or =0.005 for both) and two SNPs in Block II with AD (SNP11 genotype: P=0.002; SNP12 genotype: P=0.001) were significant after correction for multiple testing. Frequency distributions of haplotypes (constructed by five tag SNPs) differed significantly for cases and controls (P<0.001 for both AD and DD). Logistic regression analyses confirmed the association between OPRM1 variants and substance dependence, when sex and age of subjects and alleles, genotypes, haplotypes or diplotypes of five tag SNPs were considered. Population structure analyses excluded population stratification artifact. Additional supporting evidence for association between OPRM1 and AD was obtained in a smaller Russian sample (247 cases and 100 controls). These findings suggest that OPRM1 intronic variants play a role in susceptibility to AD and DD in populations of European ancestry.

Genetic influences on impulsivity, risk taking, stress responsivity and vulnerability to drug abuse and addiction

Genetic variation may partially underlie complex personality and physiological traits--such as impulsivity, risk taking and stress responsivity--as well as a substantial proportion of vulnerability to addictive diseases. Furthermore, personality and physiological traits themselves may differentially affect the various stages of addiction, defined chronologically as initiation of drug use, regular drug use, addiction/dependence and potentially relapse. Here we focus on recent approaches to the study of genetic variation in these personality and physiological traits, and their influence on and interaction with addictive diseases.

Pharmacogenetics of opioid receptors and addiction

It is generally assumed that combinations of polymorphic alleles of different genes contribute to polygenetic disorders. Variants of the opioid receptors are the obvious candidates underlying addiction. Most research has focused on the coding variation A118G of the mu opioid receptor (MOPr), which replaces asparagine at position 40 by aspartate (Asn40Asp). However, to date, no conclusive evidence exists regarding which physiological effects this mutation may cause. Other rare polymorphisms in the MOPr cause marked effects (e.g. impairment of G-protein coupling) but, due to their low frequency, their role in addiction is questionable. A large number of polymorphic sites have been found in the putative promoter region of the mu opioid receptor. These variations may alter the expression level of the receptors in neurones, but the functional relevance of promoter polymorphisms is hard to predict. In the delta opioid receptor, surprisingly few polymorphisms have been detected within the coding region, and a clear association with addiction has not been shown to date. The kappa opioid receptor contains mainly silent polymorphisms. Nevertheless, there are studies reporting positive associations of silent mutations in the three opioid receptors with drug addiction. The underlying mechanism remains unclear, but examples exist indicating that silent mutations affect mRNA stability. Taken together, the coding polymorphisms, which are rather frequent, reveal no convincing association. The vast number of non-coding, intronic or promoter polymorphisms in the opioid receptors may influence addictive behaviour, but these polymorphisms are far less studied, and their physiological significance remains to be demonstrated.