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

Genetic determinants of addiction to opioids and cocaine

OBJECTIVE

The completion of the human genome sequence has spurred investigation of the genetic contribution to substance dependence. In this article some of the recent scientific evidence for genetic determinants of opioid and cocaine dependence is reviewed.

METHOD

An electronic search of the medical literature was conducted to locate published studies relevant to the genetics of opioid and cocaine dependence. The collected information judged to be most pertinent is described and discussed.

RESULTS

Genetic epidemiologic studies support a high degree of heritable vulnerability for both opioid and cocaine dependence. Polymorphisms in the genes coding for dopamine receptors and transporter, opioid receptors, endogenous opioid peptides, cannabinoid receptors, and serotonin receptors and transporter all appear to be associated with the phenotypic expression of this vulnerability once opioids or cocaine are consumed.

CONCLUSIONS

Despite this initial progress, identification of specific genes and quantification of associated risk for the expression of each gene remain to be elucidated. While alteration of an individual's genome to change the phenotype seems remote, future interventions for treatment of opioid and cocaine dependence may include precise medications targeted to block the effects of proteins that have been identified through genetic research.

The A118G single nucleotide polymorphism of the mu-opioid receptor gene (OPRM1) is associated with pressure pain sensitivity in humans

Responses to painful stimuli are characterized by tremendous interindividual variability, and genetic factors likely account for some proportion of this variability. However, few studies have identified genetic contributions to experimental pain perception in humans. This experiment investigated whether the A118G single nucleotide polymorphism of the mu-opioid receptor gene ( OPRM1 ) was associated with responses to three different experimental pain modalities in a sample of 167 healthy volunteers (96 female, 71 male). Responses to thermal, mechanical, and ischemic pain were assessed in all subjects, and genotyping of OPRM1 was performed, which revealed that the rare A118G allele occurred in 24 females (25%) and 12 males (17%). Statistical analyses indicated that subjects with a rare allele had significantly higher pressure pain thresholds than those homozygous for the common allele. Also, a sex by genotype interaction emerged for heat pain ratings at 49 degrees C, such that the rare allele was associated with lower pain ratings among men but higher pain ratings among women. These data indicate an association of a common single nucleotide polymorphism of OPRM1 with mechanical pain responses and that this genotype may be associated with heat pain perception in a sex-dependent manner. This study examines the association of the A118G SNP of OPRM1 to experimental pain sensitivity. The results indicate that the rare allele is associated with higher pressure pain thresholds. These results support previous contentions that OPRM1 may be a pain-relevant gene; however, replication of these findings is needed.

Primate trace amine receptor 1 modulation by the dopamine transporter

Recently identified trace amine receptors are potential direct targets for drugs of abuse, including amphetamine and 3,4-methylenedioxymethamphetamine (MDMA). We cloned full-length rhesus monkey trace amine receptor 1 (rhTA(1)) that was 96% homologous to human TA(1). The trace amines tyramine and beta-phenylethylamine (PEA) and the monoamine transporter substrates (+/-)-amphetamine and (+/-)-MDMA stimulated cAMP accumulation in rhTA(1)-expressing cell lines, as measured by a cAMP response element-luciferase assay. Cocaine did not stimulate cAMP accumulation in rhTA(1) cells, but it blocked [(3)H]PEA transport mediated by the dopamine transporter. Cotransfection with the human dopamine transporter enhanced PEA-, amphetamine-, and MDMA-mediated rhTA(1) receptor activation, but it diminished tyramine activation of rhTA(1). Because TA(1) (EGFP-rhTA(1) chimera) was largely intracellular, conceivably the dopamine transporter can facilitate access of specific agonists to intracellular TA(1). rhTA(1) mRNA expression was detected in rhesus monkey substantia nigra, implying that TA(1) may be colocalized with the dopamine transporter in dopamine neurons. In summary, primate TA(1) receptors are direct targets of trace amines, amphetamine, and MDMA. These receptors could also be indirect targets of amphetamine, MDMA, and cocaine through modification of monoamine transporter function. Conceivably, rhTA(1) receptors may be located on pre- or postsynaptic membranes. Interference with the carrier function of monoamine transporters with a consequent rise of extracellular levels of trace amines could activate these receptors. The cloning of a highly homologous TA(1) from rhesus monkey and demonstration that rhTA(1) receptors are activated by drugs of abuse, indicate that nonhuman primates may serve to model physiological and pharmacological TA(1)-mediated responses in humans.

How individual sensitivity to opiates can be predicted by gene analyses

Opiate analgesics are widely used and abused drugs. Individual differences in opiate sensitivity can hamper effective pain treatments and increase risks of drug abuse. Although genetic factors might affect individual differences in opiate sensitivity, scientific evidence for specific genetic mechanisms that underlie these differences has been sparse. Recent studies using inbred and knockout mice have revealed that the mu opioid peptide (MOP) receptor encoded by the Oprm1 gene has a mandatory role in the analgesic and addictive properties of opiate drugs. Increasing evidence suggests that differences in Oprm1 gene sequences affect the amount of Oprm1 mRNA and sensitivity to opiates, and >100 polymorphisms have been identified in the human OPRM1 gene, some of which are related to vulnerability to drug dependence in some populations. Rapid advances in this research field are leading to improved understanding of the relationships between gene polymorphisms and opiate sensitivities that will enable more-accurate prediction of the opiate sensitivity and opiate requirements in individual patients.

Deficit in beta-endorphin peptide and tendency to alcohol abuse

Human and animal studies suggest that there is a correlation between endogenous opioid peptides, especially beta-endorphin, and alcohol abuse. It has been proven that the consumption of alcohol activates the endogenous opioid system. Consumption of alcohol results in an increase in beta-endorphin level in those regions of the human brain, which are associated with a reward system. However, it has also been observed that habitual alcohol consumption leads to a beta-endorphin deficiency. It is a well-documented phenomenon that people with a genetic deficit of beta-endorphin peptide are particularly susceptible to alcoholism. The plasma level of beta-endorphin in subjects genetically at high risk of excessive alcohol consumption shows lower basal activity of this peptide. Its release increases significantly after alcohol consumption. Clinical and laboratory studies confirm that certain genetically determined factors might increase the individual's vulnerability to alcohol abuse.

Association study of personality factors and the Asn40Asp polymorphism at the mu-opioid receptor gene (OPRM1)

INTRODUCTION

This study examined whether a functional polymorphism (A118G) in the gene encoding the mu-opioid receptor protein, which causes an Asn40Asp substitution in the receptor's extracellular domain, is associated with personality factors, as defined by the Five-Factor Model of Personality.

METHODS

Healthy subjects (n=446) and substance-dependent subjects (n=335) underwent a diagnostic interview, completed the NEO Five-Factor Inventory and provided a blood sample for genotyping. Controlling for demographic features and substance dependence diagnosis, analyses examined the allelic association of Asn40Asp with the five personality dimensions.

RESULTS

There was no evidence for allelic association with any of the five personality dimensions.

CONCLUSION

These findings fail to support the hypothesis that Asn40Asp alleles moderate the development of personality dimensions, as measured by the Five-Factor Model.

Pharmacogenetics and human molecular genetics of opiate and cocaine addictions and their treatments

Opiate and cocaine addictions are major social and medical problems that impose a significant burden on society. Despite the size and scope of these problems, there are few effective treatments for these addictions. Methadone maintenance is an effective and most widely used treatment for opiate addiction, allowing normalization of many physiological abnormalities caused by chronic use of short-acting opiates. There are no pharmacological treatments for cocaine addiction. Epidemiological, linkage, and association studies have demonstrated a significant contribution of genetic factors to the addictive diseases. This article reviews the molecular genetics and pharmacogenetics of opiate and cocaine addictions, focusing primarily on genes of the opioid and monoaminergic systems that have been associated with or have evidence for linkage to opiate or cocaine addiction. This evidence has been marshalled either through identification of variant alleles that lead to functional alterations of gene products, altered gene expression, or findings of linkage or association studies. Studies of polymorphisms in the mu opioid receptor gene, which encodes the receptor target of some endogenous opioids, heroin, morphine, and synthetic opioids, have contributed substantially to knowledge of genetic influences on opiate and cocaine addiction. Other genes of the endogenous opioid and monoaminergic systems, particularly genes encoding dopamine beta-hydroxylase, and the dopamine, serotonin, and norepinephrine transporters have also been implicated. Variants in genes encoding proteins involved in metabolism or biotransformation of drugs of abuse and also of treatment agents are reviewed.

Microarray studies of psychostimulant-induced changes in gene expression

Alterations in the expression of multiple genes in many brain regions are likely to contribute to psychostimulant-induced behaviours. Microarray technology provides a powerful tool for the simultaneous interrogation of gene expression levels of a large number of genes. Several recent experimental studies, reviewed here, demonstrate the power, limitations and progress of microarray technology in the field of psychostimulant addiction. These studies vary in the paradigms of cocaine or amphetamine administration, drug doses, route and also mode of administration, duration of treatment, animal species, brain regions studied and time of tissue collection after final drug administration. The studies also utilize different microarray platforms and statistical techniques for analysis of differentially expressed genes. These variables influence substantially the results of these studies. It is clear that current microarray techniques cannot detect small changes reliably in gene expression of genes with low expression levels, including functionally significant changes in components of major neurotransmission systems such as glutamate, dopamine, opioid and GABA receptors, especially those that may occur after chronic drug administration or drug withdrawal. However, the microarray studies reviewed here showed cocaine- or amphetamine-induced alterations in the expression of numerous genes involved in the modulation of neuronal growth, cytoskeletal structures, synaptogenesis, signal transduction, apoptosis and cell metabolism. Application of laser capture microdissection and single-cell cDNA amplification may greatly enhance microarray studies of gene expression profiling. The combination of rapidly evolving microarray technology with established methods of neuroscience, molecular biology and genetics, as well as appropriate behavioural models of drug reinforcement, may provide a productive approach for delineating the neurobiological underpinnings of drug responses that lead to addiction.

Gene polymorphisms of the mu opioid receptor in methamphetamine abusers

In drug addiction, the opioid system is thought to mediate motivational effects through dopamine-independent mechanisms. We have investigated associations of the mu-opioid receptor gene (OPRM) variations with methamphetamine (MAP) dependence/psychosis. The allelic frequency of A118G (Asn40Asp) in exon 1 of ORPM was 45.3% in our control subjects, but only 7.5-25.8% in the Caucasian or African-American population of previous studies. We have identified several novel polymorphisms in intron 1 and the 5' untranslated region (5'UTR) of OPRM. Polymorphisms in the functionally relevant 5' regulatory region of OPRM were different in our Japanese population from Caucasian or African-American populations. No significant differences between controls and MAP abusers were found in either genotype or allele frequency at any single nucleotide polymorphism (SNP) or (AC)n dinucleotide repeat in intron 1. A subdivision of our MAP group revealed that A118G of OPRM shows a significant association with MAP psychosis having latency less than three years. Further analysis should be capable of identifying associations between the OPRM variations and MAP dependence/psychosis.

Novel exonic mu-opioid receptor gene (OPRM1) polymorphisms not associated with opioid dependence

The mu-opioid receptor (MOR) mediates reward and dependence associated with opioids and other commonly abused substances. Variability in the MOR gene, OPRM1, may influence risk for opioid dependence. In this study, associations between two single nucleotide polymorphisms (SNPs), dbSNP rs540825 and dbSNP rs562859, and opioid dependence were investigated. The two SNPs are located in the protein coding region of the novel exon X of an alternative splice variant of OPRM1, and can be detected using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) methods. Genotyping at the two SNPs was performed for 170 severe opioid dependent individuals and 128 carefully screened controls. Although no differences were found between cases and controls, there were significant prevalence differences between African-American (AA) subjects and European-American (EA) subjects for SNP 540825 allele and genotype frequencies. The 540825 and 562859 polymorphisms were found to be in complete linkage disequilibrium (LD) for both ethnic groups, and LD existed between the 562859 SNP and the A(-1320)G SNP in the promoter region of OPRM1 in AAs, based on genotyping data previously carried out on the same subjects. LD between these two markers, separated by 55 kb, links the entire distance studied in this project. The results indicate that polymorphisms in the novel splice variant are not associated with opioid dependence, but are in LD with other polymorphisms in OPRM1.

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

The mu-opioid receptor (MOR), through its effects on reward and stress-responsivity, modulates alcohol intake in both animal and human laboratory studies. We have previously demonstrated that the frequently occurring A118G single-nucleotide polymorphism (SNP) in exon 1 of the MORgene (OPRM1), which encodes an amino-acid substitution, is functional and receptors encoded by the variant 118G allele bind the endogenous opioid peptide beta-endorphin with three-fold greater affinity than prototype receptors. Other groups subsequently reported that this variant alters stress-responsivity in normal volunteers and also increases the therapeutic response to naltrexone (a mu-preferring opioid antagonist) in the treatment of alcohol dependence. We compared frequencies of genotypes containing an 118G allele in 389 alcohol-dependent individuals and 170 population-based controls without drug or alcohol abuse or dependence. The A118G SNP was present in the Hardy-Weinberg equilibrium with an overall frequency of the 118G allele of 10.9%. There was a significant overall association between genotypes with an 118G allele and alcohol dependence (p=0.0074). The attributable risk for alcohol dependence in subjects with an 118G allele was 11.1%. There was no difference in A118G genotype between type 1 and type 2 alcoholics. In central Sweden, the functional variant 118G allele in exon 1 of OPRM1 is associated with an increased attributable risk for alcohol dependence.

Are μ-opioid receptor polymorphisms important for clinical opioid therapy?

Mutations in the mu-opioid receptor--the primary site of action of opioid analgesics--are candidates for the variability of clinical opioid effects. This has been substantiated by recent advances in genetic research. A common mu-opioid receptor polymorphism was associated with higher demands for alfentanil or morphine for pain relief. It also decreased the potency of morphine for pupil constriction and experimental analgesia, but its molecular mechanisms are unclear. Another opioid receptor mutation greatly impaired receptor signalling in vitro, but is very rare. The accumulated evidence provides a solid basis for continuing research that should address the underlying molecular mechanisms and the role and benefits of OPRM1 genotyping for clinical pain therapy.

Treating Tobacco Dependence: State of the Science and New Directions

Despite almost two decades of intensive tobacco control efforts, nearly one quarter of Americans continue to smoke. The two United States Food and Drug Administration–approved medications used to treat tobacco dependence, bupropion and nicotine replacement therapy, are effective for only a fraction of smokers. Investigations of medications approved for affective disorders and other forms of substance abuse, such as fluoxetine and naltrexone, have yielded mixed results as tobacco dependence treatments. A particular challenge in tobacco dependence treatment is the development of effective approaches for smokers with unique needs, such as cancer patients and pregnant women. Despite new developments in these areas, significant gaps in knowledge and practice remain. Basic research in the neurobiologic and genetic basis of nicotine dependence offers promise for the development of novel and more effective treatment approaches. For example, emerging research in pharmacogenetics explores how genetic variation in drug-metabolizing enzymes and drug targets modifies response to pharmacotherapy. These discoveries could someday help practitioners to individualize the type, dosage, and duration of tobacco dependence treatment based on genotype, and maximize the efficacy.

Morphine-6-glucuronide: Actions and mechanisms

Morphine-6-glucuronide (M6G) appears to show equivalent analgesia to morphine but to have a superior side-effect profile in terms of reduced liability to induce nausea and vomiting and respiratory depression. The purpose of this review is to examine the evidence behind this statement and to identify the possible reasons that may contribute to the profile of M6G. The vast majority of available data supports the notion that both M6G and morphine mediate their effects by activating the micro-opioid receptor. The differences for which there is a reasonable consensus in the literature can be summarized as: (1) Morphine has a slightly higher affinity for the micro-opioid receptor than M6G, (2) M6G shows a slightly higher efficacy at the micro-opioid receptor, (3) M6G has a lower affinity for the kappa-opioid receptor than morphine, and (4) M6G has a very different absorption, distribution, metabolism, and excretion (ADME) profile from morphine. However, none of these are adequate alone to explain the clinical differences between M6G and morphine. The ADME differences are perhaps most likely to explain some of the differences but seem unlikely to be the whole story. Further work is required to examine further the profile of M6G, notably whether M6G penetrates differentially to areas of the brain involved in pain and those involved in nausea, vomiting, and respiratory control or whether micro-opioid receptors in these brain areas differ in either their regulation or pharmacology.

Evolving perspectives on neurobiological research on the addictions: celebration of the 30th anniversary of NIDA

The roots of the Laboratory of the Biology of the Addictive Diseases are in the development of methadone maintenance for the treatment of opiate addiction. Methadone maintenance therapy continues to be one of the major effective forms of addiction pharmacotherapy and underscores the importance of biological factors in the physiology and treatment of the addictive diseases. Recent work in the Laboratory has focused on the neurobiological, neurochemical, neuroendocrine and behavioral aspects of addictive diseases (principally cocaine and the opiate addictions), using an interdisciplinary approach. The models we have focused on range from in vitro molecular biology and neuroscience, to in vivo animal models, to experiments in normal human populations and patients with specific addictive diseases, and most recently to the human molecular genetics of different addictive diseases. Two long-term corollary hypotheses have guided the Laboratory's work: (1) That the endogenous opioid peptide/receptor systems play a central role in the addictive states and therefore in their treatment. (2) That atypical responsivity to stressors (e.g., in the hypothalamic-pituitary-adrenal axis) plays a role in vulnerability and relapse to specific addictive diseases. This atypical responsivity may be drug-induced, environmentally acquired, and/or due to genetic variation.

Alcoholism: genes and mechanisms

Alcoholism is a chronic relapsing/remitting disease that is frequently unrecognized and untreated, in part because of the partial efficacy of treatment. Only approximately one-third of patients remain abstinent and one-third have fully relapsed 1 year after withdrawal from alcohol, with treated patients doing substantially better than untreated [1]. The partial effectiveness of strategies for prevention and treatment, and variation in clinical course and side effects, represent a challenge and an opportunity to better understand the neurobiology of addiction. The strong heritability of alcoholism suggests the existence of inherited functional variants of genes that alter the metabolism of alcohol and variants of other genes that alter the neurobiologies of reward, executive cognitive function, anxiety/dysphoria, and neuronal plasticity. Each of these neurobiologies has been identified as a critical domain in the addictions. Functional alleles that alter alcoholism-related intermediate phenotypes include common alcohol dehydrogenase 1B and aldehyde dehydrogenase 2 variants that cause the aversive flushing reaction; catechol-O-methyltransferase (COMT) Val158Met leading to differences in three aspects of neurobiology: executive cognitive function, stress/anxiety response, and opioid function; opioid receptor μ1 (OPRM1) Asn40Asp, which may serve as a gatekeeper molecule in the action of naltrexone, a drug used in alcoholism treatment; and HTTLPR, which alters serotonin transporter function and appears to affect stress response and anxiety/dysphoria, which are factors relevant to initial vulnerability, the process of addiction, and relapse.

Mutant G-protein-coupled receptors as a cause of human diseases

G-protein-coupled receptors (GPCR) are involved in directly and indirectly controlling an extraordinary variety of physiological functions. Their key roles in cellular communication have made them the target for more than 60% of all currently prescribed drugs. Mutations in GPCR can cause acquired and inherited diseases such as retinitis pigmentosa (RP), hypo- and hyperthyroidism, nephrogenic diabetes insipidus, several fertility disorders, and even carcinomas. To date, over 600 inactivating and almost 100 activating mutations in GPCR have been identified which are responsible for more than 30 different human diseases. The number of human disorders is expected to increase given the fact that over 160 GPCR have been targeted in mice. Herein, we summarize the current knowledge relevant to understanding the molecular basis of GPCR function, with primary emphasis on the mechanisms underlying GPCR malfunction responsible for different human diseases.

A Possible Genetic Mechanism Underlying Individual and Interstrain Differences in Opioid Actions: Focus on the Mu Opioid Receptor Gene

Individual differences in responses to opioids limit effective pain treatment with these drugs. Identifying the mechanism could help to improve the analgesic effects of them. Since the molecular cloning of the mu opioid receptor (muOR) gene, substantial advances in opioid research have been made, including the discoveries that muOR plays a mandatory role in the analgesic effects of opioids and that the sequence of the muOR gene varies from one individual to another. It is conceivable that the differences in the muOR gene cause individual differences in opioid actions. The present review summarizes the recent advances made in research on human and mouse muOR genes and proposes that the variances in the 3' untranslated region (39-UTR) of the muOR gene might participate in the variability of the opioid response.

Association of polymorphisms in nicotinic acetylcholine receptor alpha 4 subunit gene (CHRNA4), mu-opioid receptor gene (OPRM1), and ethanol-metabolizing enzyme genes with alcoholism in Korean patients

Findings obtained from several studies indicate that ethanol enhances the activity of alpha4beta2 neuronal nicotinic acetylcholine receptor and support the possibility that a polymorphism of the nicotinic acetylcholine receptor alpha4 subunit gene (CHRNA4) modulates enhancement of nicotinic receptor function by ethanol. To identify the association between the CfoI polymorphism of the CHRNA4 and alcoholism, we examined distribution of genotypes and allele frequencies in Korean patients diagnosed with alcoholism (n = 127) and Korean control subjects without alcoholism (n = 185) with polymerase chain reaction-restriction fragment length polymorphism methods. We were able to detect the association between the CfoI polymorphism of the CHRNA4 and alcoholism in Korean patients (genotype P = .023; allele frequency P = .047). The genotypes and allele frequencies of known polymorphisms in other alcoholism candidate genes, such as alcohol metabolism-related genes [alcohol dehydrogenase 2 (ADH2), aldehyde dehydrogenase 2 (ALDH2), alcohol dehydrogenase 3 (ADH3), and cytochrome P450 2E1 (CYP2E1)] and mu-opioid receptor gene (OPRM1), were studied. The polymorphisms of ADH2, ALDH2, and CYP2E1 were significantly different in Korean patients with alcoholism and Korean control subjects without alcoholism, but ADH3 and OPRM1 did not differ between the two groups.

Association of Functional Opioid Receptor Genotypes With Alcohol Dependence in Koreans

BACKGROUND

The functional polymorphism (A118G) of the mu-opioid receptor gene (OPRM1) is thought to have clinical significance in the treatment of alcohol dependence. This study compared Koreans with one or two copies of the A118G polymorphism seeking treatment for alcohol dependence with a group of non-alcohol-dependent controls.

METHODS

Patients hospitalized for alcohol dependence (n = 112) and a group of non-alcohol-dependent controls (n = 140) were interviewed on aspects of drinking history and psychiatric history. Patients and controls were excluded if they met criteria for any other major psychiatric disorder. Participants were genotyped at the OPRM1 locus.

RESULTS

The allele frequency of the Asp40 allele was 0.397 in the alcohol-dependent group, which is consistent with other literature demonstrating this polymorphism to be common in Asian populations. Within the alcohol-dependent subjects, being homozygous for the Asp40 allele was associated with more days drinking than those heterozygous or homozygous for the Asn40 allele. Differences in the allele frequencies between alcohol-dependent and non-alcohol-dependent controls were not significant.

CONCLUSIONS

These results suggest that having one or two copies of the A118G allele is common among Koreans and may be an important genetic factor in the etiology of alcohol dependence and the frequency of alcohol consumption.

Genes associated with addiction: alcoholism, opiate, and cocaine addiction

Drug addiction is a complex disorder that has a large spectrum of causes. Vulnerability to addiction has been shown in twin studies to have a robust genetic component. This genetic basis for addiction has general and specific components for each drug abused. Although many genes have been implicated in drug addiction, only a handful have either been replicated to have an association or to have an identified functional mechanism related to specific effects of abused drugs. A few selected genetic variants that currently look promising for the study of alcohol, opiate, and cocaine addiction are discussed in this article.

Effect of the A118G polymorphism on binding affinity, potency and agonist-mediated endocytosis, desensitization, and resensitization of the human mu-opioid receptor

The most prevalent single-nucleotide polymorphism (SNP) A118G in the human mu-opioid receptor gene predicts an amino acid change from an asparagine residue to an aspartatic residue in amino acid position 40. This N40D mutation, which has been implicated in the development of opioid addiction, was previously reported to result in an increased beta-endorphin binding affinity and a decreased potency of morphine-6-glucuronide. Therefore, in the present study we have investigated whether this mutation might affect the binding affinity, potency, and/or the agonist-induced desensitization, internalization and resensitization of the human mu-opioid receptor stably expressed in human embryonic kidney 293 cells. With the exception of a reduced expression level of N40D compared to human mu-opioid receptor (hMOR) in HEK293 cells, our analyses revealed no marked functional differences between N40D and wild-type receptor. Morphine, morphine-6-glucuronide and beta-endorphin revealed similar binding affinities and potencies for both receptors. Both the N40D-variant receptor and hMOR exhibited robust receptor internalization in the presence of the opioid peptide [d-Ala(2),N-MePhe(4),Glyol(5)]enkephalin (DAMGO) and beta-endorphin but not in response to morphine or morphine-6-glucuronide. After prolonged treatment with morphine, morphine-6-glucuronide or beta-endorphin both receptors showed similiar desensitization time courses. In addition, the receptor resensitization rates were nearly identical for both receptor types.

Mu opioid receptor gene as a candidate for the study of obsessive compulsive disorder with and without tics

Obsessive compulsive disorder (OCD) is a complex psychiatric disease characterized by recurring obsessions or compulsions that cause significant distress to the patient. The etiology of this disorder remains largely unknown, although a genetic component has been suggested. Many candidates genes have been evaluated based on a possible serotoninergic and dopaminergic brain dysfunction. We postulate the micro opioid receptor (MOR) gene as a candidate because some observations support a role of the opioid system in OCD. The opioid antagonist, naloxone, rapidly exacerbates OCD symptoms and the opioid agonist, tramadol, was reported to be effective in the treatment of some patients. We studied two single nucleotide polymorphisms (C17T and A118G) in 51 trios with OCD. Genotyping was analyzed with transmission desequilibrium test (TDT). The allelic variant +17T of the C17T polymorphism had a low frequency (1%) in our population that did not allow for statistic analysis. However, for the allelic variant +G of the A118G polymorphism we were able to performed statistical comparisons. Our results showed a trend toward significance (chi(2) McNemar = 3.6, P = 0.065) for TDT in patients with comorbid tics. It is an interesting finding that should be tested in a larger sample of OCD patients with tics.

Genetic variation and pharmacogenomics: concepts, facts, and challenges

The analysis of genetic variation in candidate genes is an issue of central importance in pharmacogenomics. The specific approaches taken will have a critical impact on the successful identification of disease genes, the molecular correlates of drug response, and the establishment of meaningful relationships between genetic variants and phenotypes of biomedical and pharmaceutical importance in general. Against a historical background, this article distinguishes different approaches to candidate gene analysis, reflecting different stages in human genome research. Only recently has it become feasible to analyze genetic variation systematically at the ultimate level of resolution, ie, the DNA sequence. In this context, the importance of haplotype-based approaches to candidate gene analysis has at last been recognized; the determination of the specific combinations of variants for each of the two sequences of a gene defined as a haplotype is essential. An up-to-date summary of such maximum resolution data on the amount, nature, and structure of genetic variation in candidate genes will be given. These data demonstrate abundant gene sequence and haplotype diversity. Numerous individually different forms of a gene may exist. This presents major challenges to the analysis of relationships between genetic variation, gene function, and phenotype. First solutions seem within reach. The implications of naturally occurring variation for pharmacogenomics and “personalized” medicine are now evident. Future approaches to the identification, evaluation, and prioritization of drug targets, the optimization of clinical trials, and the development of efficient therapies must be based on in-depth knowledge of candidate gene variation as an essential prerequisite.

Opioids and alcoholism

Although far from conclusive, evidence implicating the endogenous opioid system in the development and maintenance of alcoholism is growing. Currently available data suggest that ethanol increases opioid neurotransmission and that this activation is part of the mechanism responsible for its reinforcing effects. Findings from preclinical research indicate that ethanol consumption and ethanol-induced dopamine (DA) release are both reduced by opioid antagonists. Individual differences in endogenous opioid activity have been linked to inherited risks for alcoholism in studies comparing ethanol-preferring and nonpreferring rats, as well as in studies using targeted gene mutation (knockout) strategies. To a large extent, findings from human studies have paralleled those from the preclinical work. Persons who differ in family history of alcoholism have been shown to also differ in basal beta-endorphin activity, beta-endorphin response to alcohol, and subjective and HPA axis hormonal response to opioid antagonists. Findings from clinical trials indicate that opioid antagonists may reduce ethanol consumption in alcoholics, particularly in persons who have resumed drinking. Nevertheless, many questions remain unanswered about the use of opioid antagonists in alcoholism treatment and about the exact role of the opioid system in ethanol preference and reward. The progression of knowledge in this field suggests that many of these questions are imminently answerable, as our ability to characterize relationships between opioid activity and human behavior continues to develop. This paper summarizes both the progress that has been made and the gaps that remain in our understanding of the interactions between the endogenous opioid system and risk for alcoholism.

The functional mu opioid receptor (OPRM1) Asn40Asp variant predicts short-term response to nicotine replacement therapy in a clinical trial

To determine whether the functional mu-opioid receptor (OPRM1) Asn40Asp variant predicts the comparative efficacy of different forms of NRT, we conducted a clinical trial of transdermal nicotine (TN) vs nicotine nasal spray (NS) in 320 smokers of European ancestry. Smokers carrying the OPRM1 Asp40 variant (n=82) were significantly more likely than those homozygous for the Asn40 variant (n=238) to be abstinent at the end of treatment, and reported less mood disturbance and weight gain. The genotype effect on treatment outcome was most pronounced among smokers receiving TN, particularly during the 21 mg dose phase. Smokers who carry the OPRM1 Asp40 variant are likely to have a favorable response to TN and may benefit from extended therapy with the 21 mg dose.

Watson and Crick 50 years on. From double helix to pharmacogenomics

The second half of the 20th century has seen quantum leaps in our understanding of molecular biology. The technological advances, which facilitated the recent successful completion of the Human Genome Project, have provided the tools for deciphering the complexity of the human condition. At present, the function of only 50% of genes is known. However, as understanding of the human genome improves, a plethora of gene targets for treating disease will be uncovered - leading to therapies which will be considered revolutionary. Genome related science has begun to impact almost every facet of medicine including anaesthesia and intensive care. Better understanding of interindividual differences will enable better prediction of illness susceptibility as well as response to treatment. These insights will permit therapies to be tailored to individuals or racial groups. At present, there is only rudimentary knowledge of factors controlling gene regulation, but in the future, better understanding of gene-environment interactions and gene expression will enable pharmaceutical companies to develop new therapies and permit clinicians to optimise their effects, without recourse to current laborious testing regimens. As genomic science progresses, new ethical, legal, social and philosophical dilemmas will also continue to emerge.

Functional analysis of MOR-1 splice variants of the mouse mu opioid receptor gene Oprm

A series of mu opioid receptor gene Oprm splice variants have been reported that differ only at their C-terminus. These variants all contain exons 1, 2, and 3 of the gene, the exons responsible for coding all seven transmembrane domains. Whereas MOR-1 also has exon 4 that encodes for an additional 12 amino acids at the tip of the C-terminus, the other MOR-1 variants have unique amino acid sequences distinct from those in MOR-1 due to alternative splicing. All these variants are mu-selective in binding assays. The current study explored the ability of these variants to stimulate [35S]GTPgammaS binding to assess them functionally. Only mu opioids stimulated [35S]GTPgammaS binding. Among the mu opioids we noted marked differences in their maximal stimulation among the clones. This was most prominent with beta-endorphin, which stimulated [35S]GTPgammaS binding in the MOR-1E expressing cells to a greater degree than [D-Ala2,MePhe4,Gly(ol)5]enkephalin (DAMGO; 130%) and was far less effective than DAMGO in MOR-1C cells (44%). The rank order of maximal stimulation of the drugs varied among the clones as well. Dynorphin A, beta-endorphin and morphine were most effective in stimulating [35S]GTPgammaS binding in MOR-1E, while M6G and fentanyl were most effective in MOR-1 expressing cells. The potency (EC50) of some of the drugs also varied extensively among the clones, with a poor correlation between the potency of the drugs to stimulate [35S]GTPgammaS binding and their binding affinity. Together, these findings reveal marked functional differences among the variants that only can be explained by their structural differences at the tip of their C-terminus.

A mu-opioid receptor single nucleotide polymorphism in rhesus monkey: association with stress response and aggression

Variations in the human mu-opioid receptor gene have driven exploration of their biochemical, physiological and pathological relevance. We investigated the existence of variations in the nonhuman primate mu-opioid receptor gene to determine whether nonhuman primates can model genotype/phenotype associations of relevance to humans. Similar to the A118G single nucleotide polymorphism (SNP) in the human mu-opioid receptor gene, a SNP discovered in the rhesus monkey mu-opioid receptor gene (C77G) alters an amino acid in the N-terminal arm of the receptor (arginine for proline at position 26). Two mu-opioid receptor coding regions isolated from a single heterozygous (C77/G77) rhesus monkey brain were expressed in HEK-293 cells and characterized in radioreceptor assays. Paralleling the findings of increased affinity of beta-endorphin by the A118G allele in the human, the rhesus monkey mu-opioid receptor protein derived from the G77-containing clone demonstrated a 3.5-fold greater affinity for beta-endorphin than the receptor derived from the C77-containing clone. An assay developed to assess the incidence of the C77G SNP in a behaviorally and physiologically characterized cohort of rhesus monkeys (n=32) indicated that 44% were homozygous for C77-containing alleles, 50% were heterozygous and 6% were homozygous for G77-containing alleles. The presence of G77-containing alleles was associated with significantly lower basal and ACTH-stimulated plasma cortisol levels (P<0.03-0.05 and P<0.02, respectively) and a significantly higher aggressive threat score (P<0.05) in vivo. In a cohort of 20 monkeys, a trend towards an inverse correlation between aggressive threat and plasma cortisol levels was observed. The findings suggest that mu-opioid receptor haplotypes in monkeys can contribute to individual variability in stress response and related aggression. The data support the use of nonhuman primates to investigate mu-opioid receptor genotype/phenotype relations of relevance to humans.

Endogenous Opioid Receptor Genes and Alcohol Dependence Among Taiwanese Han

BACKGROUND

Nonselective opioid antagonists reduce alcohol consumption under various experimental situations, and several association studies have examined possible roles of opioid receptor mu (OPRM), delta (OPRD), and kappa (OPRK) genes in the development of alcohol dependence.

METHODS

We examined 20 single nucleotide polymorphisms (SNPs) across the OPRM, OPRD, and OPRK genes in 158 alcohol-dependent subjects and 149 controls. Differences in allele frequency and genotype distribution between case subjects and controls, as well as the deviation from Hardy-Weinberg equilibrium, were examined using Fisher's exact tests.

RESULTS

No significant difference in either allele or genotype frequency was found between case subjects and controls for each of the SNPs.

CONCLUSIONS

Our findings do not support a possible role of the opioid receptor genes for the proclivity to alcohol dependence in the Taiwanese Han.

Genetic Predictors of the Clinical Response to Opioid Analgesics

This review uses a candidate gene approach to identify possible pharmacogenetic modulators of opioid therapy, and discusses these modulators together with demonstrated genetic causes for the variability in clinical effects of opioids. Genetically caused inactivity of cytochrome P450 (CYP) 2D6 renders codeine ineffective (lack of morphine formation), slightly decreases the efficacy of tramadol (lack of formation of the active O-desmethyl-tramadol) and slightly decreases the clearance of methadone. MDR1 mutations often demonstrate pharmacogenetic consequences, and since opioids are among the P-glycoprotein substrates, opioid pharmacology may be affected by MDR1 mutations. The single nucleotide polymorphism A118G of the mu opioid receptor gene has been associated with decreased potency of morphine and morphine-6-glucuronide, and with decreased analgesic effects and higher alfentanil dose demands in carriers of the mutated G118 allele. Genetic causes may also trigger or modify drug interactions, which in turn can alter the clinical response to opioid therapy. For example, by inhibiting CYP2D6, paroxetine increases the steady-state plasma concentrations of (R)-methadone in extensive but not in poor metabolisers of debrisoquine/sparteine. So far, the clinical consequences of the pharmacogenetics of opioids are limited to codeine, which should not be administered to poor metabolisers of debrisoquine/sparteine. Genetically precipitated drug interactions might render a standard opioid dose toxic and should, therefore, be taken into consideration. Mutations affecting opioid receptors and pain perception/processing are of interest for the study of opioid actions, but with modern practice of on-demand administration of opioids their utility may be limited to explaining why some patients need higher opioid doses; however, the adverse effects profile may be modified by these mutations. Nonetheless, at a limited level, pharmacogenetics can be expected to facilitate individualised opioid therapy.

Drug dependence and the endogenous opioid system

The discovery of endogenous opioids has markedly influenced the research on the biology of drug dependence. Evidence has been presented that these brain substances are self-administered by laboratory animals. This finding, among others, has led to the hypothesis that endogenous opioids are involved in reinforcing habits, including dependence on drugs of abuse. The course of drug dependence is presented as a continuum from no drug use via controlled use to an actual dependence on the drug. Specific brain opioid systems belonging to four conceptualized brain circuits are described to be involved during the different phases of the drug dependence continuum. More recent research to delineate the role of endogenous opioid systems in drug dependence has focussed on genetic research in humans and animals. Among others, the findings obtained from studies of opioid receptor and opioid peptide precursor knockout mice provided further support for a role of endogenous opioid systems in drug dependence, in agreement with previous pharmacological studies.

Association between human mu-opioid receptor gene polymorphism, pain tolerance, and opioid addiction

Central to both pain responses and opioid addiction is activity at the micro -opioid receptor. To explore the role of the micro -opioid receptor gene (OPRM) in human pain tolerance and opioid addiction, we examined the relationships among OPRM genotype and experimental pain tolerance in opioid addicts in methadone treatment (n = 50) and healthy normal controls (n = 59). Pain phenotype (pain tolerant vs. pain intolerant) was operationalized as tolerance to a standardized noxious stimulus (either thermal or mechanical), and dichotomized based on distribution. One microsatellite and two single nucleotide polymorphisms, A118G and C17T, in exon 1 were typed to study the OPRM gene. Although the established relationship between the phenotypes of opioid addiction and pain intolerance was validated (P = 0.02), genotype differed neither between addict-affected vs. control, nor pain tolerant vs. intolerant subjects. The variant A118G was absent in all individuals and the C17T polymorphism appeared in only three African-American individuals (two addicts and one control). The absence of this polymorphism, the small sample size and the heterogeneous ethnic backgrounds of participants in the pilot study allow only tentative conclusions based on the results, thus the role of the opioid receptor in pain and opioid reward response remains uncertain.

A functional polymorphism of the mu-opioid receptor gene is associated with naltrexone response in alcohol-dependent patients

This study examined the association between two specific polymorphisms of the gene encoding the mu-opioid receptor and treatment outcomes in alcohol-dependent patients who were prescribed naltrexone or placebo. A total of 82 patients (71 of European descent) who were randomized to naltrexone and 59 who were randomized to placebo (all of European descent) in one of three randomized, placebo-controlled clinical trials of naltrexone were genotyped at the A(+118)G (Asn40Asp) and C(+17)T (Ala6Val) SNPs in the gene encoding the mu-opioid receptor (OPRM1). The association between genotype and drinking outcomes was measured over 12 weeks of treatment. In subjects of European descent, individuals with one or two copies of the Asp40 allele treated with naltrexone had significantly lower rates of relapse (p=0.044) and a longer time to return to heavy drinking (p=0.040) than those homozygous for the Asn40 allele. There were no differences in overall abstinence rates (p=0.611), nor were there differences in relapse rates or abstinence rates between the two genotype groups among those assigned to placebo. These preliminary results are consistent with prior literature demonstrating that the opioid system is involved in the reinforcing properties of alcohol and that allelic variation at OPRM1 is associated with differential response to a mu-receptor antagonist. If replicated, these results would help to identify alcohol-dependent individuals who may be most likely to respond to treatment with naltrexone.

Haplotypes at the OPRM1 locus are associated with susceptibility to substance dependence in European-Americans

Our objective was to investigate the relationship between the gene encoding the mu-opioid receptor (OPRM1) and susceptibility to substance dependence in European-American (EA) and African-American (AA) subjects. Eight single nucleotide polymorphisms (SNPs) at the OPRM1 locus, i.e., -2044C/A, -1793T/A, -1699insT, -1469T/C, -1320A/G, -111C/T, +17C/T (Ala6Val), and +118A/G (Asn40Asp) were genotyped in 676 subjects: 318 EA subjects and 124 AA subjects with substance dependence, and 179 EA normal controls, and 55 AA normal controls. Affection status was defined by each unique combination of alcohol, cocaine, and opioid dependence and analysis of association examined in relation to the possible combinations. We used a newly implemented permutation method to evaluate statistical significance. In EAs, a significant difference in haplotype frequency distributions was found between controls and "alcohol + opioid" dependent patients (P = 0.0036). This finding is also supported by logistic regression analysis and a simulation method. The frequencies of allele -2044A and haplotypes including -2044A are higher in these patients than in controls. In AAs, no allele, haplotype, or genotype frequencies were significantly different between cases and controls. There were highly significant differences in the allele, haplotype, and genotype frequencies between EA and AA controls. Four of the variants [-1793T/A, -1699insT, -1320A/G, and -111C/T] are in virtually complete linkage disequilibrium (LD) to compose a sequence pattern, which does not associate with any of the seven categories of substance dependence. In EAs, allele -2044A and haplotypes that include -2044A are the susceptibility allele and haplotypes for substance dependence. These findings suggest that OPRM1 may play a role in the pathophysiology of substance dependence and this role is population- and diagnosis-specific.

Interindividual variability of the clinical pharmacokinetics of methadone: implications for the treatment of opioid dependence

Methadone is widely used for the treatment of opioid dependence. Although in most countries the drug is administered as a racemic mixture of (R)- and (S)- methadone, (R)-methadone accounts for most, if not all, of the opioid effects. Methadone can be detected in the blood 15-45 minutes after oral administration, with peak plasma concentration at 2.5-4 hours. Methadone has a mean bioavailability of around 75% (range 36-100%). Methadone is highly bound to plasma proteins, in particular to alpha(1)-acid glycoprotein. Its mean free fraction is around 13%, with a 4-fold interindividual variation. Its volume of distribution is about 4 L/kg (range 2-13 L/kg). The elimination of methadone is mediated by biotransformation, followed by renal and faecal excretion. Total body clearance is about 0.095 L/min, with wide interindividual variation (range 0.02-2 L/min). Plasma concentrations of methadone decrease in a biexponential manner, with a mean value of around 22 hours (range 5-130 hours) for elimination half-life. For the active (R)-enantiomer, mean values of around 40 hours have been determined. Cytochrome P450 (CYP) 3A4 and to a lesser extent 2D6 are probably the main isoforms involved in methadone metabolism. Rifampicin (rifampin), phenobarbital, phenytoin, carbamazepine, nevirapine, and efavirenz decrease methadone blood concentrations, probably by induction of CYP3A4 activity, which can result in severe withdrawal symptoms. Inhibitors of CYP3A4, such as fluconazole, and of CYP2D6, such as paroxetine, increase methadone blood concentrations. There is an up to 17-fold interindividual variation of methadone blood concentration for a given dosage, and interindividual variability of CYP enzymes accounts for a large part of this variation. Since methadone probably also displays large interindividual variability in its pharmacodynamics, methadone treatment must be individually adapted to each patient. Because of the high morbidity and mortality associated with opioid dependence, it is of major importance that methadone is used at an effective dosage in maintenance treatment: at least 60 mg/day, but typically 80-100 mg/day. Recent studies also show that a subset of patients might benefit from methadone dosages larger than 100 mg/day, many of them because of high clearance. In clinical management, medical evaluation of objective signs and subjective symptoms is sufficient for dosage titration in most patients. However, therapeutic drug monitoring can be useful in particular situations. In the case of non-response trough plasma concentrations of 400 microg/L for (R,S)-methadone or 250 microg/L for (R)-methadone might be used as target values.

Corticotropin-releasing factor testing reveals a dose-dependent difference in methadone maintained vs control subjects

Opiate addiction is associated with abnormal function of the stress-responsive hypothalamic-pituitary-adrenal (HPA) axis. In general, addiction and withdrawal are associated with abnormal HPA responsivity as demonstrated by baseline, dexamethasone, and metyrapone testing. Following stabilization in methadone maintenance treatment, normalization of HPA axis responsivity is observed. To further investigate HPA axis function associated with heroin addiction and its treatment, saline placebo and human corticotropin-releasing factor (hCRF) were administered intravenously in two doses, one dose lower (0.5 microg/kg) and one dose higher (2.0 microg/kg) than the dose used in standard clinical diagnostics (100 microg), to 16 normal male volunteer controls (NV) and eight male stable-dose methadone-maintained former heroin addicts without ongoing drug or alcohol abuse or dependence (MM). Plasma adrenocorticotrophic hormone (ACTH) and cortisol levels were determined at serial time points. There was no difference in hormonal measurements between the two groups following placebo. NV as well as MM displayed a dose-response effect in plasma ACTH and cortisol levels. MM displayed a significantly greater increase in plasma ACTH levels than the NV following high-dose but not low-dose hCRF (p <0.05). There was no significant difference in plasma cortisol levels between the two groups following high-dose hCRF. Thus, despite earlier documented normalization of behavioral function and of several measures of neuroendocrine function during long-term methadone maintenance, some abnormalities in HPA axis responsivity that may be a consequence of heroin exposure, or that may have existed prior to the addiction, can persist during stable methadone treatment.

Sequence variability and candidate gene analysis in two cancer patients with complex clinical outcomes during morphine therapy

In this case report, we present genetic differences in two morphine-related gene sequences, UDP-glucuronosyltransferase 2B7 (UGT2B7) and mu opioid receptors (MOR1), in two cancer patients whose clinical responses to morphine were very different [i.e., sensitive (patient 1) and low responder (patient 2)]. In addition, allelic variants in the UGT2B7 gene were analyzed in 46 Japanese individuals. Amplified DNA fragments for the two genes of interest were screened using single strand conformation polymorphism and then sequenced. In the UGT2B7 gene, 12 single nucleotide polymorphisms (SNPs) were newly identified with an allelic frequency ranging from 0.022 to 0.978. Six SNPs in the promoter region (A-1302G, T-1295C, T-1111C, G-899A, A-327G, and T-125C) and two coding SNPs (UGT2B7*2 in exon 2 and C1059G in exon 4) appeared to be consistently linked. Remarkable differences in the nucleotide sequence of UGT2B7 were observed between the two patients; in contrast to patient 1 who had "reference" alleles at almost SNP positions, but a rare ATTGAT*2(AT)C haplotype as homozygosity, patient 2 was a homozygous carrier for the predominant GCCAGC*1(TC)G sequence. Serum morphine and two glucuronide concentrations in patient 2 suggest that the predominant GCCAGC*1G sequence was not associated with a "poor metabolizer" phenotype. In the MOR1 gene, patient 1 had no SNPs, whereas patient 2 was a heterozygous carrier for both the G-1784A and A118G alleles. The present study describes substantial differences in genotype patterns of two genes of interest between the two patients. The results necessitate larger trials to confirm these observations in larger case control studies.

Association between the cortisol response to opioid blockade and the Asn40Asp polymorphism at the mu-opioid receptor locus (OPRM1)

This study examined whether a reportedly functional polymorphism in the gene encoding the mu-opioid receptor protein (A118G, which causes an Asn40Asp substitution in the receptor's extracellular domain), modifies the cortisol response to the opioid antagonist naloxone. The polymorphism occurs commonly in European Americans and some other population groups, underscoring its potential phenotypic significance.

METHODS

Using a balanced, within-subject design involving two test sessions over a period of 3-7 days, we examined ACTH and cortisol responses to intravenous naloxone (125 microg/kg) or placebo in 30 healthy subjects (21 males, mean age = 24.4 years). Plasma ACTH and cortisol concentrations were measured over 120 min post infusion. DNA isolated from whole blood was PCR amplified and genotyped via restriction enzyme digestion, with genotypes assigned based on agarose gel size fractionation.

RESULTS

Subjects with one or more Asp40 alleles (n = 6; 5 heterozygotes and 1 homozygote) had significantly higher cortisol concentrations at baseline and at 15, 60, and 90 min after naloxone infusion than subjects homozygous for the Asn40 allele (n = 24). Subjects with the Asp40 allele also had a greater peak cortisol response and a greater area under the cortisol time curve than those homozygous for the Asn40 allele. There were no effects of the Asn40Asp polymorphism on plasma ACTH concentration or on self-reported anxiety or distress.

CONCLUSIONS

These findings are consistent with recent reports showing an enhanced cortisol response to naloxone and a reduced agonist effect of morphine-6-glucuronide among subjects with the Asp40 variant. Given evidence of its pharmacological significance, the clinical relevance of this polymorphism warrants further investigation.

Mu opioid receptor gene polymorphisms and heroin dependence in Asian populations

The distribution of three polymorphisms of the mu opioid receptor gene (OPRM1) was investigated in four different Asian populations, and in heroin-dependent subjects deriving from three of these populations. For the A118G polymorphism, we found significant differences in allele and genotype frequencies between different ethnic groups and highly significant association with heroin dependence in Indians for both genotype distribution (p = 0.024) and allele frequency (p = 0.009). For the C17 T polymorphism, the minor allele was documented in Chinese and Malays for the first time. Molecular haplotyping revealed complete linkage dis-equilibrium between the A118G and C17 T polymorphisms. Linkage disequilibrium between the A118G and C1031G polymorphisms was found to be almost complete in all four ethnic groups.

Introducing a new recruitment approach to sample collection for genetic association studies in opioid dependence

OBJECTIVE

In a modified case-control association study we tested the assumption that two polymorphisms (A(118)G in exon 1 and IVS2+31 in intron 2) of the human mu-opioid receptor gene (OPRM1) confer susceptibility to opioid dependence.

METHODS

In contrast to classical case-control studies both groups, opioid dependent cases and non-opioid dependent controls were recruited from individuals who have had access to drugs including opioids and who had been sentenced for violation of the "Dangerous Drugs Act" in Germany.

RESULTS

For the two allelic variants of OPRM1 under study we did not find evidence for association with opioid dependence. CONCLUSIONS ;Despite absence of association we think that this recruitment approach introduced here, is useful since it putatively offers a more adequate matching for case-control association studies of opioid dependent individuals.