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Article |
The Laboratory of the Biology of Addictive Diseases, The Rockefeller University, New York, New York
Abstract
Abstract I. Introduction II. Addictions A. Opiates and Other Opioids 1. Heroin. 2. Codeine. 3. Noncodeine Prescription Opioids. B. Cocaine III. Molecular Genetics of Opioid and Cocaine Addictions A. Epidemiology B. Molecular Genetic Studies 1. Family and Linkage Studies. 2. Case Control-Association Studies. 3. Haplotype Analysis. C. Selected Identified Genes 1. Opioid-Related Genes. a. µ Opioid Receptor Gene (OPRM1). b. {kappa} Opioid Receptor Gene (OPRK1). c. {delta} Opioid Receptor Gene (OPRD1). d. Preproenkephalin Gene (PENK). e. Preprodynorphin Gene (PDYN). 2. Monoaminergic-Related Genes. a. Dopamine {beta}-Hydroxylase Gene (D{beta}H). b. Dopamine Receptor D2 Gene (DRD2). c. Dopamine Transporter Gene (SLC6A3). d. Serotonin Transporter Gene (SLC6A4). e. Norepinephrine Transporter Gene (SLC6A2). IV. Treatment of Addictions A. Pharmacotherapies 1. Opiate and Opioid Addictions. a. Methadone. b. Levo-{alpha}-acetylmethadol. c. Buprenorphine. d. Naltrexone. e. Clonidine. 2. Cocaine Addiction. a. Antidepressants. b. Disulfiram. c. Dopaminergic Agents. d. GABAA-GABAB Directed Drugs V. Pharmacogenetics Related to the Treatment of Addictions A. Metabolism/Biotransformation of Opiates and Other Opioids 1. Morphine and Heroin. 2. Codeine. 3. Methadone, Levo-{alpha}-acetylmethadol, and Buprenorphine. B. Metabolism/Biotransformation of Cocaine VI. Summary
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 marshaled 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 µ 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 
-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.
Opiate and cocaine addictions are chronic relapsing diseases with complex etiologies, significant comorbidities (e.g., human immunodeficiency virus, hepatitis B and C infections, depressive and anxiety disorders, and other psychiatric illnesses), and major negative socioeconomic consequences. Opiate addicts frequently suffer from cocaine addiction, and cocaine addicts frequently have comorbid alcohol addiction. There is also significant comorbid nicotine addiction in both groups. In addition, opiate and cocaine addicts often abuse marijuana and benzodiazepines. As with other complex chronic diseases such as hypertension, coronary artery disease, type 2 diabetes mellitus, and unipolar depression, the addictions develop through the interaction of various social-behavioral, physiological, and genetic factors. Because the development of the addictive diseases is predicated on the exposure to a drug of abuse, pharmacological and pharmacogenetic factors must be considered and, for opiate and cocaine addictions, will be the focus of this review.
The addictive diseases are clinically defined through a combination of physiological and behavioral criteria. Currently, the most widely used diagnostic criteria for the addictive diseases are those of the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV1) (Diagnostic and Statistical Manual of Mental Disorders, 1994). The DSM-IV was written not as a research tool but as a diagnostic tool for identifying patients in need of treatment. Unlike earlier versions (e.g., DSM-III-R), DSM-IV criteria for substance dependence include but do not require physiological phenomena such as tolerance and dependence. Other nondiagnostic tools such as the Addiction Severity Index are available to assess duration and mode of drug use but do little to ascertain the extent of drug exposure (McLellan et al., 1980
, 1992). We have recently developed and validated against DSM-IV definitions of opiate, cocaine, and alcohol dependencies a brief instrument, the Kreek-McHugh-Schluger-Kellogg scale, that assesses the amount of drug use at its peak level of intake (Kellogg et al., 2003). The Kreek-McHugh-Schluger-Kellogg scale allows for quantitative analysis of drug use characteristics and may be useful in future genetic and pharmacological studies of the addictive diseases.
According to DSM-IV, a diagnosis of substance dependence is met if three or more of the following occur in the same 12-month period: 1) tolerance, defined by the need for increased amount of substance to achieve the desired effect or diminished effect with continued use of the same amount of the substance; 2) development of a characteristic withdrawal syndrome when the substance is stopped or the use of the substance to prevent the onset of withdrawal; 3) increased or prolonged use; 4) a desire or unsuccessful attempts to cut down or control use; 5) significant time spent in activities related to drug procurement, use, and recovery; 6) important social, occupational, or recreational activities are sacrificed because of substance use; and 7) ongoing use despite knowledge of ongoing physical or psychological harm related to substance use. This is in contrast to the initial, and current, federal criteria for entrance into methadone maintenance treatment for opiate addiction (Rettig and Yarmolinsky, 1995; Kreek and Vocci, 2002). The federal criteria, which require greater than 1 year of multiple daily opiate self-administrations, far exceed the DSM-IV criteria for opioid dependence in that tolerance and withdrawal may develop within several weeks following regular opiate use.
By contrast, the DSM-IV criteria for substance abuse do not include the physiological effects of repeated drug use. Rather, a diagnosis of substance abuse is met if one or more of the following result in clinically significant impairment or distress during a 12-month period: 1) substance use results in failure to fulfill major work, school, or home obligations; 2) recurrent substance use in hazardous situations; 3) recurrent substance-related legal problems; and 4) continued use despite persistent or recurrent social or interpersonal problems related to substance use.
Although the DSM-IV refers to "substance dependence", this causes confusion in nomenclature since frequently encountered, medically expected and accepted dependencies occur with chronically administered therapeutic agents such as all opioid analgesics, adrenocorticosteroids, and several antihypertensives. Throughout this review we will use the term "addiction" rather than "dependence" since it better describes the maladaptive and harmful aspects of persistent compulsive drug use.
Whereas opiates and cocaine are discussed in this review, all drugs of abuse share a common characteristic which underlies their abuse potential: initial use in the mode and pattern of abuse leads to rapid increase or decrease of receptor and/or transporter function, neurotransmitter/neuropeptide activity, and secondary messenger signaling. Changes in the gene expression of target proteins follow frequent, repeated exposure. Cessation of drug use leads to similarly profound changes. Thus, recurrent "on-off" use of short-acting drugs produces long-term, perhaps permanent, alterations in these affected neuronal systems and underlies the development of tolerance, dependence, withdrawal, and relapse characteristic of the addictive diseases.
Opiates are the drugs derived from opium and include morphine, codeine, their congeners (e.g., heroin and oxycodone), and other semisynthetic derivatives of the-baine. Opioids, however, consist of all agonist drugs with morphine-like activity whether they are naturally occurring or synthetic. For the purposes of this review, opiates and opioids are discussed together. The initial effects of opiates are mediated through the endogenous opioid system. Although there are three classes of opioid receptors (µ, 
, and 
), abused opiates primarily interact with µ opioid receptors (MOR). This seven transmembrane G protein-coupled receptor modulates diverse physiological systems including response to pain and reward, stress responsivity, gastrointestinal motility, and immune function. The endogenous ligands for MOR are the 31-amino acid protein -endorphin and the smaller enkephalin molecules. By inhibiting 
-aminobutyric acid (GABAergic) neurons, stimulation of MOR also results in disinhibition of mesolimbic-mesocortical dopamine pathways central to the reinforcing properties of opiates and other drugs of abuse. Repeated administration of and withdrawal from opiates disrupts these pathways and results in the physiological and behavioral effects of opiate addiction.
1. Heroin.
The 2002 National Survey on Drug Use and Health estimated that in 2002, 19.5 million Americans age 12 or older were illicit drug users (Substance Abuse and Mental Health Services Administration, 2003b). Over 3.5 million Americans have used heroin and there are over 1 million heroin addicts in the United States. From 1995 to 2002, the prevalence of lifetime heroin use has increased among youths aged 12 to 25 with greater than 100,000 heroin initiates annually during this period.
2. Codeine.
Codeine is an orally administered prescription opiate rarely administered as a sole agent. Codeine is frequently prepared in combination with decongestants and expectorants as a cough remedy and nonopiate analgesics such as acetaminophen for the treatment of pain. Although codeine itself is modestly potent, it is, in part, metabolized into the potent and reinforcing opiate morphine. Recent trends indicate that there has been a decline in the illicit use of codeine (Substance Abuse and Mental Health Services Administration, 2003b). Rather than a sign of decreased demand for prescription opiates, this decrease seems to be offset by increased illicit use of more potent prescription opioids.
3. Noncodeine Prescription Opioids.
The prevalence of illicitly used codeine- and noncodeine-containing prescription opioids has been difficult to determine. An estimated 1.9 million Americans used oxycodone illicitly in 2002 (Substance Abuse and Mental Health Services Administration, 2003b). According to the Drug Abuse Warning Network, from 1995 to 2002 emergency room mentions of illicitly used prescription opiates increased 116% (2775 mentions in 2002) for morphine-containing analgesics, 560% (22,397 mentions in 2002) for oxycodone-containing analgesics, to over 6000% (1506 mentions in 2002) for fentanyl-containing analgesics (Substance Abuse and Mental Health Services Administration, 2003a).
Cocaine primarily acts through inhibition of presynaptic dopamine transporters as well as the serotonin and norepinephrine transporters. Increased levels of synaptic dopamine and, thereby, dopamine receptor binding following cocaine administration is a key mechanism through which cocaine is reinforcing. Cocaine also modulates the endogenous opioid system, especially MOR, opioid receptors (KOR), and preprodynorphin. Whereas stimulation of dopaminergic pathways may be sufficient to cause the reinforcing effects of cocaine, dopamine transporter gene deletion studies have shown that this pathway is not essential to the development of cocaine self-administration (see below). Selective gene disruption of the MOR will, however, prevent the development of cocaine self-administration (see below).
Close to 34 million Americans have used cocaine and there are over 1.5 million cocaine addicts in the United States (Substance Abuse and Mental Health Services Administration, 2003b). Discussion of other stimulant drugs such as methamphetamine and 3,4-methylenedioxy-methamphetamine (or ecstasy) as well as illicitly used prescription stimulants is beyond the scope of this review.
III. Molecular Genetics of Opioid and Cocaine Addictions
The human genome contains approximately 25–40,000 genes encoded in 3.2 billion nucleotides of DNA (Lander et al., 2001; Venter et al., 2001). It has been predicted that any two genomes, when compared, are nearly 99.9% identical (Kruglyak and Nickerson, 2001). A substantial portion of the 0.1% genetic variability between individuals is due to the 11 million single nucleotide polymorphisms (SNPs) estimated to occur in the human genome with allelic frequencies greater than 1% (Kruglyak and Nickerson, 2001). Variability is also introduced by such processes as alternative splicing of mRNA transcripts and imprinting.
Polymorphisms (or variants) in genes, which code for proteins that are in the pathways where heroin or cocaine act, especially when their expression results in altered protein amounts or when they code for aberrant forms of proteins, may be responsible for some of the observed differences between individuals in their physiological, biochemical, and behavioral responses to those drugs. The acquisition and persistence of, relapse to, and treatment of the addictions may also be influenced by genetic variations.
Although some diseases, such as sickle cell anemia and cystic fibrosis, are single gene disorders, vulnerability to addiction undoubtedly has a more complex genetic basis. Complex diseases may be polygenic (being caused by many genes), but are generally considered to be oligogenic (when only a few genes play a significant role). Classical pharmacogenomics has concentrated on the genetics of an individual and how it relates to their response to therapeutic agents. This has focused on genetic variation related to the absorption, toxicity, and biotransformation of therapeutic agents. However, it has become apparent that physiogenetics, the genetic variability in physiological processes (e.g., endocrine regulation, intercellular signal transduction pathways, and neurochemistry), are of importance. Hence, pharmacogenetics will expand its scope to include physiogenetics.
Many genes have been significantly associated or have displayed evidence of linkage with opiate or cocaine addiction. However, only a small subset of these genes has a polymorphism for which an alteration in function has been verified. Polymorphisms in genes may modify transcriptional regulation or rate, mRNA splicing or stability, protein translation, function (such as enzymatic activity or binding), or stability. There are many factors that may confound results obtained in association or linkage studies, such as population admixture or environmental variables. These factors may obscure the influence of various genetic polymorphisms in the specific cohorts studied, not allowing the studies to reach significance.
In this review, we will discuss several genes for which convincing evidence has been published to indicate that they may be involved in the predisposition to opiate or cocaine addiction. These genes have been chosen due to either a functional alteration in a variant allele affecting expression of this gene, because genetic linkage and/or association have been replicated in a number of different studies and populations, or due to their known role in the manifestation of drug effects. Hence, the genes chosen here for review are hypothesized to be involved in the vulnerability to develop opioid or cocaine addiction based on laboratory studies and, moreover, have been shown to be involved in vulnerability in one or more studies (see Table 1).
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Three main factors contribute to the development of addiction. These are: 1) environmental factors, 2) drug-induced physiological effects (such as those effects on neurochemistry, neural networks, mRNA, peptide and protein levels), and 3) genetic factors. Genetic factors may be quite complex in nature. Genetic variants of one gene may interact with other genes, or the effects of one gene may mask the effect of another gene (epistasis). Genes have been shown to interact with environmental factors. For example, Caspi and colleagues have found that in males with a variant of the monoamine oxidase A gene encoding a monoamine oxidase A enzyme with low activity (a possible risk factor for violence) and also who had experienced high levels of maltreatment as children had the highest risk of developing antisocial behavior (Caspi et al., 2002). Drug addictions are complex disorders that are likely to involve multiple genes having multiple polymorphisms that work in various combinations with each other in different people and interact with the environment. In addition, many of these alleles may contribute to either protection from, or vulnerability to, the development of a drug addiction and will have frequencies which vary by ethnic/cultural group. The genetic, developmental, and environmental context in which particular variants exist may affect their influence on the ultimate phenotype.
Early studies to determine whether addiction is a heritable disease were family studies on alcoholism (e.g., Kaij, 1960; Partanen et al., 1966; Cloninger et al., 1981). More recently, other addictions have been studied and both similarities and differences between the addictions have been observed. Some of the physiology, neurobiology, and treatments have been found to overlap in various aspects between the addictions, but often they are quite distinct. In addition, genetic variants may be both nonspecific (shared) and specific for various drugs of abuse. However, rigorous studies by leading geneticists into the specificity of genetic factors have found divergent results. For example, the genetic influences determining drug abuse vulnerability were studied by Tsuang and colleagues (Tsuang et al., 1998). Using a study population of 3372 predominately Caucasian (90% non-Hispanic white) male twin pairs, it was found that stimulant abuse/addiction (e.g., cocaine and amphetamines) had a total genetic variance of 0.3, whereas heroin use/addiction had a total genetic variance of 0.5. In this study, stimulants had a low specific genetic variance of 0.09, whereas heroin had a high specific genetic variance of 0.4. A similar study, but with very different findings, was performed by Kendler and colleagues using 1196 U.S. Caucasian male twin pairs (Kendler et al., 2003). Common additive genetic factors for use of cocaine were estimated to have a variance of 0.5, whereas for opiates the variance was estimated at 0.4. However, in contrast with the study of Tsuang and colleagues, the substance-specific additive genetic variance was only 0.07 for cocaine and 0.00 for opiates.
Family studies have also documented familiar transmission of cocaine abuse. Siblings of cocaine-dependent probands had a relative risk of 1.7 to develop cocaine dependence themselves (Bierut et al., 1998). A comparison of other addictions occurring in the probands suggested that the addictions had independent causative factors. Another family study found that the adjusted odds ratio for having the same drug disorder in adult first-degree relatives was over 7 for cocaine and over 10 for opioids, again indicating an involvement of genetic factors (Merikangas et al., 1998).
1. Family and Linkage Studies. There are two main types of studies, linkage and association studies, conducted to establish whether genes and their variants may be involved in vulnerability to drug addiction. Linkage studies use families to provide evidence of how close a genetic marker is to an allele causing the phenotype in question, whereas association studies may be performed with unrelated individuals or with parent-offspring trios.
Linkage studies have been used to study subjects with addictive diseases and comorbid conditions. One early and continuing study is the Collaborative Study on the Genetics of Alcoholism (COGA), a multicenter effort to identify genes involved in alcoholism (description available at <www.niaaa.nih.gov/extramural/projcoga.htm>) (Edenberg, 2002; also, see Begleiter et al., 1999). This group, as well as studies by others, have identified many chromosomal regions (including regions on chromosomes 1, 2, 3, 4, 7, and 11) using linkage scans that appear to be involved in the vulnerability to develop alcoholism as well as other addictions (Long et al., 1998; Reich et al., 1998; Foroud et al., 2000; Bergen et al., 2003; Ma et al., 2003; Stallings et al., 2003; Wyszynski et al., 2003). These studies have identified regions that can be more finely mapped, and the genes within these regions may be studied further.
2. Case Control-Association Studies. Another method to identify variants involved in addictions is to select genes that are likely to be involved in the physiological effect of the specific drug under consideration in a neurotransmitter system related to drug taking behaviors or genes for which there is previous evidence to be of interest. Genetic variants are identified in these candidate genes. Cases and control groups are genotyped for the variant(s), and statistical approaches are then employed to measure the probability that a given variant allele is associated with the drug addiction.
In association studies, the cultural identity and the ethnicity of the subjects must be carefully evaluated because some genes' allelic frequencies vary widely among ethnic/cultural groups. Several studies have found that although approximately 90% of the total genetic variation is between individuals, approximately 10% occurs between ethnic/cultural groups (reviewed in Brown and Armelagos, 2001). Hence, it appears that this 10% difference may be enough to introduce population stratification into association analyses when ethnicity is not matched between subjects and controls. Failure to do so may cause both false-positive and false-negative errors. Such population admixture, or stratification, repeatedly has been shown to confound results of association studies (reviewed in Thomas and Witte, 2002).
The techniques for conducting association studies have been recently expanded to include thousands of variants using gene microarray technology. To locate and identify genes and chromosomal regions that are associated with specific addictions, genome-wide scans can be performed on affected and control subjects. To map genes involved in vulnerability to drug abuse, Uhl and colleagues have used microarray technology (using an Affymetrix microarray that can genotype 1494 SNPs in a single hybridization) (Uhl et al., 2001). Multiple pools, each containing DNA from 20 individuals who were either polysubstance abusers or controls and who were of the same ethnic/cultural group, were genotyped. Using this association genome scanning technique, they identified 42 chromosomal regions that potentially are involved in vulnerability to drug abuse in both European Americans and African Americans. Since these studies were conducted using subjects with alcohol, nicotine, or polysubstance addictions, the chromosomal regions identified may contain loci for vulnerability to addiction to multiple substances. When the variants associated with polysubstance abuse in the microarray study were compared with those regions identified in linkage studies of alcohol (Long et al., 1998; Reich et al., 1998; Foroud et al., 2000) and nicotine addiction (Straub et al., 1999), 15 candidate regions had positive results in at least two of the studies (Uhl et al., 2002b). Again, since these studies were based on polysubstance abuse, alcohol, or nicotine addiction, the regions identified as candidates are most likely associated with vulnerability to drug abuse in general.
3. Haplotype Analysis. Polymorphic variants are known to associate in clusters. Analyses of variants in pedigrees have shown that some alleles that are in close proximity with each other are inherited as a block, whereas others have lower linkage disequilibrium. Linkage disequilibrium patterns of variants have shown that there are characteristic patterns of linkage disequilibrium across the human genome. A map of these patterns is a haplotype map and is a description of variants in a region and the linkage disequilibrium pattern involved. The haplotype map is intended to describe the common variation patterns and to provide a minimal set of variants that capture the full set of diversity in the population.
A haplotype is a defined region of one chromosome from each pair of chromosomes. An individual has two sets of haplotypes, one derived from each parent. Hence, haplotype blocks in a population represent chromosomal regions that were inherited with little past recombination. Haplotypes may be determined directly through the use of various techniques or may be inferred using one of several algorithms. These algorithms predict haplotypes that are most likely to occur. However, haplotypes of equal likelihood are not predicted by these methods, although they may be the real haplotypes.
Haplotypes vary among populations. It has been demonstrated that in European and Asian populations the mean haplotype block is about 22,000 nucleotides with about four common haplotypes per block (Gabriel et al., 2002). In African American and Yoruban (Nigerian) populations, the mean haplotype block is only 11,000 nucleotides with five common haplotypes per block. This is similar to the finding of Reich and colleagues who found that, in populations of European descent, linkage disequilibrium extends to approximately 60,000 nucleotides from common alleles, although only extending less than 5000 nucleotides in a Nigerian population (Reich et al., 2001).
When a new variant arises in a population, it occurs on a specific haplotype. In subsequent generations, the variant and its ancestral haplotype are altered only by recombination or the acquisition of new variants. Hence, it should be possible to identify a rare variant associated with a disease by identifying the ancestral region on which it was first introduced.
Two examples relevant to this review which will be detailed below are the µ opioid receptor (OPRM1) and dopamine -hydroxylase (DH) genes. A haplotype analysis of OPRM1 was conducted with European American and African American subjects (Luo et al., 2003). Eight variants were genotyped and analyzed using an expectation-maximization (E-M) algorithm (Long et al., 1995). Six haplotypes were predicted in the European Americans and seven in the African Americans. In the European Americans, a significant difference in haplotype frequency was found between the groups who were both alcohol- and opioid-dependent combined versus controls.
In the DH gene, Cubells and colleagues genotyped two variants, one 3000 nucleotides upstream of the transcription start site (DBH*5'-ins/del) and the other at nucleotide 444 (DBH*444g/a) (Cubells et al., 2000). These variants, separated by approximately 8400 nucleotides, were in linkage disequilibrium. One of the haplotypes, "Del-a", was associated with low plasma DH and with cocaine-induced paranoia in European Americans.
1. Opioid-Related Genes.
For millennia, compounds derived from the opium poppy have been used for their medicinal properties as analgesics, antitussives, soporifics, and antidiarrheals. Also, opium and derivatives such as morphine and heroin have long been recognized as drugs of addiction. Endogenous receptors for opiate drugs were first postulated to exist in 1954 (Beckett and Casey, 1954). During the ensuing two decades, efforts to identify these receptors were performed using stereo-specific ligand-binding assays (Ingoglia and Dole, 1970; Goldstein et al., 1971), and in 1973, opioid receptors were discovered independently by three groups (Pert and Snyder, 1973; Simon et al., 1973; Terenius, 1973). The availability of increasingly selective opioid ligands subsequently allowed the identification of three receptor types, which were named the µ, , and opioid receptors (abbreviated in this review as MOR, KOR, and DOR, respectively). The first endogenous ligands for opioid receptors were identified in 1975 by Hughes and colleagues, who isolated Leu- and Met-enkephalins, which are five amino acid peptides with "opiate-like" or "opioid" activity (Hughes et al., 1975). Subsequently, two other endogenous opioid peptides were discovered, -endorphin and dynorphin A (Bradbury et al., 1976; Cox et al., 1976; Li and Chung, 1976; Goldstein et al., 1979).
The endogenous opioid peptides are derived through proteolytic processing of three propeptide precursors. Met- and Leu-enkephalin, as well as Met-enkephalin-Arg-Phe and Met-enkephalin-Arg-Gly-Leu are derived from proenkephalin. -Endorphin is derived from proopiomelanocortin (which also yields adrenocorticotropic hormone and melanocyte stimulating hormone in equimolar amounts), and processing of prodynorphin produces dynorphins A and B and 
-neoendorphin. Each of these classic endogenous opioid peptides contains the N-terminal sequence Tyr-Gly-Gly-Phe, which confers their opiate-like properties.
The cDNA and gene sequences of the endogenous opioid peptide precursors were determined a few years after their discovery (enkephalin: Comb et al., 1982; Noda et al., 1982a,b; dynorphin: Kakidani et al., 1982; Horikawa et al., 1983; pro-opiomelanocortin: Nakanishi et al., 1979, 1981; Takahashi et al., 1981, 1983). Opioid receptors, on the other hand, were first cloned and sequenced almost 20 years following their discovery. The groups of Evans and Kieffer, working independently, isolated, defined, and sequenced cDNA clones of the mouse opioid receptor in 1992 (Evans et al., 1992; Kieffer et al., 1992). Following publication of the sequence of the DOR, many groups used homology screening to identify and sequence clones of the MOR and KOR from several species (µ: Chen et al., 1993; Fukuda et al., 1993; Thompson et al., 1993; Wang et al., 1993; : Li et al., 1993; Meng et al., 1993; Minami et al., 1993; Yasuda et al., 1993; Mansson et al., 1994; Simonin et al., 1995, Zhu et al., 1995; : Knapp et al., 1994; Simonin et al., 1994).
Binding assays with selective ligands allowed classification of the opioid receptors into three general classes: the MOR, which has as endogenous ligands the enkephalins and -endorphin, the DOR, which selectively binds enkephalins, and the KOR, for which dynorphins are the endogenous ligands. Binding assays using cells expressing cloned receptors indicate that there exists considerable cross-selectivity for some of the peptide ligands.
The endogenous opioid system is centrally important in the responses to addictive opiate drugs such as morphine, codeine, and heroin, as well as to synthetic opioid narcotics such as fentanyl. The receptors mediate both the analgesic and rewarding properties of opioid compounds and opioid effects on the hypothalamic-pituitary-adrenal (HPA) stress-responsive axis, respiratory and pulmonary function, gastrointestinal motility, immune responses, and other functions. Additionally, this system is important in modulating the responses to cocaine and other psychostimulants, as well as to alcohol and other drugs (e.g., see Kreek et al., 2002). Components of the endogenous opioid system and the genes encoding them have therefore been the focus of research into specific addictions since their discovery.
a. µ Opioid Receptor Gene (OPRM1).
OPRM1 has been selected as a candidate for human genetic studies of the addictions for many reasons. The MOR is the molecular target of the active biotransformation products of heroin (6-monoacetylmorphine and morphine), as well as most opiate and opioid analgesic medications such as oxycodone, hydromorphone, and fentanyl, each of which has significant potential for addiction. Abuse of and addiction to these MOR-directed agents is increasingly recognized to constitute a major addiction problem. From our early work that led to the development of methadone maintenance treatment for heroin addiction in the 1960s, we know that µ-selective agonists with long-acting pharmacokinetics such as methadone and levo--acetylmethadol (LAAM) or partial agonists (buprenorphine) are the most effective treatments for this disorder (e.g., Dole et al., 1966; Kreek et al., 2002). Clinical studies also point to a relative "endorphin deficiency" in active and former heroin addicts and also in active or recently abstinent cocaine addicts, although whether this is drug-induced or pre-exists and contributes to the development of addiction is still an unanswered question (e.g., Kreek et al., 1984; Schluger et al., 2001).
Studies of quantitative trait loci in mice identified a chromosomal region containing the µ opioid receptor gene as contributing to a substantial amount of the variance in analgesic and reward responses to morphine (Belknap and Crabbe, 1992; Berrettini et al., 1994; Kozak et al., 1994; Belknap et al., 1995; Crabbe et al., 1999). Also, studies of mice with targeted deletion of the µ opioid receptor gene definitively established this receptor as essential for morphine analgesia, physical dependence, and reward as measured by antinociception, withdrawal, conditioned place preference, and self-administration studies (Matthes et al., 1996; Sora et al., 1997; Kitanaka et al., 1998; Loh et al., 1998; Becker et al., 2000). Chronic morphine administration or heroin self-administration alters naloxone efficacy measured using 5'-O-(3-[35S]thio)triphosphate ([35S]GTPS) binding in several brain regions by increasing MOR binding and decreasing MOR-activated G proteins (Sim et al., 1996; Sim-Selley et al., 2000; Kruzich et al., 2003).
Although the initial molecular target of cocaine is monoamine transporters, expression and function of the MOR is also affected by cocaine, particularly in a chronic experimental administration paradigm and also in longterm human cocaine abusers. For example, in rats administered cocaine for 14 days in a "binge" paradigm, MOR binding density increased in several regions containing terminals of the mesocorticolimbic dopaminergic system, including areas of the cingulate cortex, the nucleus accumbens, caudate-putamen, and basolateral amygdala (Unterwald et al., 1992, 1994). A direct molecular effect of acute or 3-day cocaine administration on MOR mRNA levels has also been observed with increases in dopaminergically innervated brain regions reported (Azaryan et al., 1996; Yuferov et al., 1999). A positron emission tomography (PET) study in cocaine-dependent men also showed increases in MOR binding that were associated with cocaine craving (Zubieta et al., 1996). Additionally, MOR knockout mice show reduced reward responses to both cocaine and alcohol (Roberts et al., 2000; Hall et al., 2001; Becker et al., 2002).
Many variants, particularly SNPs, in the coding, untranslated, flanking, and intronic regions of this gene have been identified and tested for association with opiate, cocaine, and other addictions. A few nonsynonymous variants in the coding region have been studied using in vitro assays to assess possible functional alterations in the encoded proteins.
The most common coding region SNP, the A118G variant, which alters amino acid sequence, was one of the first discovered in the OPRM1 gene (Bergen et al., 1997; Bond et al., 1998) and has been evaluated in a number of genetic studies of opiate, alcohol, and mixed drug addiction. This variant has also been shown to be of physiogenetic (genetically based difference in physiology) and pharmacogenetic (genetically based difference in response to a pharmacotherapeutic agent) importance, discussed below. The frequency of this polymorphism varies widely across populations, occurring at less than 2% in some populations up to almost 50% in others (Table 2). This SNP is in the first exon of the OPRM1 gene, and the 118G allele encodes a variant receptor with an aspartic acid at amino acid position 40 instead of asparagine, which is at this position in the prototype receptor. This substitution leads to a charge difference and also to the loss of a putative N-linked glycosylation site in the N-terminal domain of the receptor (Bergen et al., 1997; Bond et al., 1998). Glycosylation of G protein-coupled receptors has been reported to be important in mediating appropriate protein conformation that allows receptor trafficking to the cell membrane (George et al., 1986; Hughes et al., 1997; Petäjä-Repo et al., 2000).
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In an extensive in vitro study of function of the variant receptors encoded by the 118A and 118G alleles, we used stably transfected cell lines expressing either the variant or prototype receptors and performed binding assays using several ligands, including the five amino acid residue endogenous peptide ligands Met- and Leu-enkephalin, the four amino acid residue peptides endomorphin-1 and -2, the synthetic µ-selective peptide D-Ala2,N-Me-Phe4,Gly-ol5-enkephalin, the -selective endogenous peptide dynorphin A(1–17), the µ-preferring opioid agonists morphine, methadone, fentanyl, and the opioid antagonist naloxone. Each of these compounds had similar binding affinities to the prototype and 118G variant receptors (Bond et al., 1998).
We also tested the µ- and -selective 31-residue endogenous peptide -endorphin, which is the longest of the endogenous opioid peptides, has the longest half-life, and is found both centrally and peripherally. In our in vitro studies, -endorphin bound the 118G variant receptors with approximately three times greater affinity than those containing the prototype sequence (Bond et al., 1998).
We also studied the 118A and 118G variant receptors with respect to an important cellular activity of the MOR, that of agonist-mediated regulation of G protein-activated inwardly rectifying K+ channels. The prototype or 118G receptors were coexpressed in Xenopus oocytes with G protein-activated inwardly rectifying K+ channels; several peptide agonists, including -endorphin and endomorphin-1 and -2, were tested for their ability to activate the K+ channels. The short peptide agonists tested in this assay all showed similar EC50 values for the prototype and variant receptor; however, -endorphin caused an approximately 3-fold greater potency in activation of K+ current in the 118G variant receptor compared with the prototype (Bond et al., 1998).
The differences in binding of -endorphin and activation of the 118G variant receptors following -endorphin binding in these in vitro cellular studies led us to predict that persons carrying the gene expressing the variant receptor might show altered function of physiological systems under the control of the MOR, including, for example, pain perception, reproductive function, and responses to stress mediated by the HPA stress-responsive axis, which is under tonic inhibitory control of this receptor (Bond et al., 1998; Kreek 2000; LaForge et al., 2000b). Two studies have identified such a difference in the HPA stress-responsive axis of persons with the 118A/118G or 118G/118G genotype (Wand et al., 2002; Hernandez-Avila et al., 2003). In these studies, healthy control individuals were administered naloxone; subjects who carried one or more 118G alleles showed a greater activation of the axis, as measured by plasma cortisol, demonstrating a physiogenetic role for the A118G variant. Additional studies also suggest that this polymorphism may result in altered pharmacogenetic responses. Differences in physiological and analgesic responses to morphine, its active metabolite morphine-6-glucuronide (M6G), and to alfentanil have been reported in individuals carrying a 118G allele (Lötsch et al., 2002; Skarke et al., 2003a). Also, 118A/118G or 118G/118G genotypes have been reported to be associated with an improved response to treatment of alcoholism with naltrexone (Oslin et al., 2003) as well as to treatment of smoking with transdermal nicotine replacement therapy (Lerman et al., 2004).
The A118G variant has been studied in a number of genetic studies, primarily case-control studies of opiate and other addictions. In some studies, evidence for an association of specific alleles in specific populations has been found; other studies have not obtained such evidence. In the first report of an association of this variant with opiate addiction, we observed a higher proportion of the 118G allele in Hispanic control subjects, but not in American Caucasian or African American subjects (Bond et al., 1998). Tan and colleagues found a higher proportion of the 118A allele in opioid-dependent cases of Indian ancestry who were recruited and studied in Singapore; this finding, however was not replicated in other ethnic groups in that study, including Chinese and Malay subjects (Tan et al., 2003). In both of these studies, the numbers of individuals in each of the groups with a positive association was small (58 cases and 9 controls in the study of Bond and colleagues, 20 cases and 117 controls in the study of Tan and colleagues); also, the likelihood of population admixture in these groups would be expected to be high, which may have contributed to the findings of a positive association with this allele.
Conversely, in two other studies conducted in selected populations, the 118G allele was found to be strongly associated with opiate addiction. These studies had a more robust sample size and, to reduce the possibility of admixture confounding results, were performed with subjects who had parents of the same ethnic/cultural group as the probands. Szeto and colleagues studied 200 cases and 97 controls. Subjects were male Han Chinese individuals with all first-degree relatives from that ethnic group. An association of both the 118G allele and genotypes containing that allele with opiate-dependent subjects was found (Szeto et al., 2001).
In a recent study of heroin addiction in subjects from a geographic region known to have minimal admixture (central Sweden) we studied primarily Swedish individuals (both male and female) from the Stockholm area (139 cases and 170 controls). Like the study of Szeto and colleagues, we observed an association of 118G alleles and genotypes containing at least one 118G allele with opiate addiction (Bart et al., 2004a). Moreover, in that study, we found a substantial attributable risk for heroin addiction contributed by the 118G allele. In this study, the findings were significant if all subjects were included (attributable risk 18%), as well as if analyses were limited to Swedish subjects with both parents Swedish (attributable risk 21%) (Bart et al., 2004a).
Other genetic studies in which the A118G SNP has been evaluated have not found evidence that either allele of the A118G variant is associated with the development of opiate addiction. These include two studies of Han Chinese populations in Sichuan and Nanjing Provinces, respectively, one of German Caucasians and one in a sample of American Caucasians and African Americans (Li et al., 2000; Franke et al., 2001; Shi et al., 2002; Crowley et al., 2003). Li and colleagues studied 282 heroin-abusing subjects (DSM-IV criteria, determined by clinical interview) and 258 control subjects without neurological or psychiatric disorders (determined by questionnaire) recruited from college staff, medical students, and acute medical inpatients from a general hospital. All subjects were Han Chinese recruited in the Sichuan Province. No differences in genotype or allele frequencies between cases and controls were observed (Li et al., 2000). Franke and colleagues used case control and allele transmission analyses to test for an association of the A118G SNP with opiate addiction in German Caucasians from the Bonn area. In the case control analysis, 287 individuals meeting DSM-III-R criteria for opiate dependence and 365 control subjects (with 133 screened using a semistructured instrument and 232 blood donors who were briefly screened with respect to drug abuse history) were genotyped for the A118G variant. No differences in genotype or allele distributions were found between cases and controls (Franke et al., 2001). Additionally, in the family-controlled allele transmission test with 111 opiate-dependent probands, no preferential transmission of alleles of the A118G SNP were observed (Franke et al., 2001). Shi and colleagues studied 148 former heroin addicts in methadone maintenance treatment at the Nanjing Drug Abuse Control Bureau and 48 control individuals (with no drug or alcohol abuse as confirmed by questionnaire). All subjects were Han Chinese. The authors did not find a difference in 118A/118A versus 118A/118G + 118G/118G genotype frequencies in cases compared with controls. However, among former heroin-abusing individuals, those carrying the 118G allele along with a specific allele of an intron 2 SNP of the OPRM1 gene (the IVS2 + 31A allele) reported higher heroin intake dosages than other addicts (Shi et al., 2002). Finally, Crowley and colleagues studied 225 opioid-dependent (determined by review of medical records) and 200 screened controls matched for ethnicity (European American and African American) recruited from the Philadelphia area. No differences between cases and controls in genotype or allele frequencies for the A118G SNP were found within either ethnic group (Crowley et al., 2003).
The second most common coding region variant of the µ opioid receptor gene is the C17T SNP, which encodes a variant receptor with a valine at amino acid position 6 instead of alanine in the N-terminal domain of the receptor (Bergen et al., 1997; Berrettini et al., 1997; Bond et al., 1998). Allele frequencies of this substitution also vary widely across population groups (Table 3). The potential functional significance of this alteration in protein structure was studied with no differences between prototype and variant receptors in agonist binding to several peptide and nonpeptidic ligands, in agonist-induced 5'-O-(3-[35S]thio)triphosphate ([35S]GTPS)-binding assays (a measure of receptor activation), or in agonist-mediated receptor down-regulation (Befort et al., 2001). It should be noted, however, that in this study no differences in -endorphin binding between the 118A and 118G receptors were also found and that an undefined biological reason (such as systematically decreased presentation of functional receptors on the plasma membrane) or, alternatively, technical matters may underlie the difference in findings with our earlier study in which binding differences were observed (Bond et al., 1998).
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Several studies have also evaluated this variant with respect to specific addictions in human genetic studies. Berrettini and colleagues reported that the 17T allele was associated with opiate- and/or cocaine-dependent subjects at a borderline level of significance (p = 0.05, ethnic groups combined), a finding similar to our study of opioid-dependent individuals in which individuals were stratified by ethnic/cultural group (Berrettini et al., 1997; Bond et al., 1998). Two studies in other populations did not find an association of this variant with alcohol or mixed (opiate and/or cocaine) dependence (Gelernter et al., 1999). Because the 17T allele is at low frequency or absent in a number of the populations in which this variant has been characterized, its usefulness as a genetic marker may be limited to those populations in which it has a higher prevalence (see Table 3, and Bond et al., 1998; Gelernter et al., 1999; Tan et al., 2003; Bart et al., 2004a,b).
Several other variants, primarily SNPs, of the µ opioid receptor gene have been evaluated in the context of opiate, cocaine, and other addictions. For example, an allelic association was reported for the IVS2 C1031G variant in male Han Chinese with both parents also from that ethnic group (Szeto et al., 2001); however, this finding was not observed in a separate study in several populations including Han Chinese, Indian, Thai, and Malay individuals recruited in Singapore (Tan et al., 2003).
Three groups have also reported haplotype approaches to test for associations of this gene with addiction to opiates with or without addiction to other substances. One study used a clustering approach of 43 variants in coding, flanking, and intronic regions in a study group of European Americans and African Americans with opiate and/or cocaine dependencies compared with ethnically matched controls; they reported that specific haplotypes were associated with addiction to these substances (Hoehe et al., 2000). This type of analysis, however, raises issues concerning corrections for multiple testing, an issue that has come under increasing scrutiny in the field of statistical genetics (Levenstien et al., 2003). A second study of six biallelic variants (C-2044A, T-1793A, -1699i nsT, T-1469C, A-1320G, and C-111T) in the 5'-flanking region along with the A118G and C17T SNPs, reported specific differences in haplotype frequencies comparing controls to alcohol- and opioid-dependent patients combined or to alcohol-, opioid-, or cocaine-dependent patients combined (DSM-III-R criteria), although this finding was limited to European American subjects and was not found in African Americans (Luo et al., 2003). An additional study conducted in European American and African American subjects evaluated three 5'-flanking region variants (T-1793A, -1699insT, and A-1320G) along with the two common exon 1 SNPs and did not find evidence for an association of haplotypes with severe opiate dependence (cases had a family history of substance dependence and the age of onset of opiate dependence was 20 or below) (Crowley et al., 2003). In each of the haplotype studies, significant differences in haplotype frequencies were found between the two ethnic/cultural groups. Also, a high degree of linkage disequilibrium was reported across the 5'-flanking and exon 1 variants (Crowley et al., 2003; Luo et al., 2003).
b. Opioid Receptor Gene (OPRK1).
The opioid receptor is also involved in the responses to addictive drugs, particularly cocaine, but also to opiates. agonists, including dynorphin, the primary endogenous peptide ligand of this receptor, lower levels of dopamine in the nucleus accumbens and act in a counter-modulatory manner to attenuate increases in synaptic dopamine levels caused by cocaine's blockade of dopamine reuptake into presynaptic terminals (see Section III.C.1.e. and III.C.2.). Increases in synaptic dopamine in the nucleus accumbens is reinforcing and is thought to be an important, although not necessarily, essential component of addiction to cocaine, opiates, alcohol, and other substances.
Chronic binge administration of cocaine lowers basal dopamine levels in both the caudate-putamen and nucleus accumbens (Maisonneuve and Kreek, 1994; Maisonneuve et al., 1995). Chronic binge cocaine administration also leads to increases in opioid receptor-binding density in several brain regions that play a counter-modulatory role in reward or locomotor effects of psychostimulants, including the rostral cingulate cortex, caudal olfactory tubercle, rostral caudate-putamen, and the ventral tegmental area (Unterwald et al., 1994). These two phenomena may be related since cocaine induces dynorphin release from the striatum, and either acute or chronic cocaine administration by a number of paradigms persistently increases preprodynorphin mRNA transcription in the caudate-putamen (e.g., Sivam, 1989; Hurd et al., 1992; Hurd and Herkenham, 1992; Daunais et al., 1993, 1995; Spangler et al., 1993a,b, 1996).
Dynorphin, presumably acting through the opioid receptor, also modulates µ opioid receptor responses that may be relevant for opiate addiction. For example, dynorphin A(1–13), a dynorphin peptide that has been approved for study in humans and is a shortened form of the naturally occurring peptide dynorphin A(1–17), attenuates opiate withdrawal symptoms and augments µ opioid receptor-mediated analgesia in chronic pain patients (Specker et al., 1998; Portenoy et al., 1999). Also of interest is the finding that in methadone-maintained patients administered dynorphin A(1–13) the attenuated increase of serum prolactin (which is regulated by the tuberoinfundibular dopaminergic system) is lower than in control subjects, suggesting that alterations in dopaminergic tone may exist in long-term opioid-dependent individuals (Bart et al., 2003).
The initial cloning of the human opioid receptor gene identified three exons that contained the complete coding sequence (Simonin et al., 1995; Zhu et al., 1995). However, the rat and mouse genes have been shown to contain an exon upstream of the translation start site that encodes most of the 5'-untranslated region of the mRNA (Liu et al., 1995; Yakovlev et al., 1995).
Recently, Yuferov and colleagues defined an additional 5' exon in the human opioid receptor gene and identified several transcription initiation sites, as well as determining the poly(A) addition site of the mRNA (Yuferov et al., 2004). This has allowed a renumbering of the exons of the gene, and the previously numbered exons 1, 2, and 3 will be referred to as 2, 3, and 4 in this review.
Seven SNPs in the human opioid receptor gene have been reported (Höllt, 2000; LaForge et al., 2000a; Mayer and Höllt, 2001). These include the substitutions G36T in exon 2, C459T in exon 3, and A843G, C846T, C852T, C948T, and C1008T in exon 4. The first to be reported were identified in a population of German Caucasians; these were G36T (14%), C459T (2%), C843C (8%), and C846T (2%) (Höllt, 2000) (with allele frequencies in parentheses reported in Mayer and Höllt, 2001). These variants were also detected in our further studies conducted in New York with a study sample of primarily Caucasian, African American, and Hispanic subjects with the identification of additional SNPs that have overall allele frequencies ranging from <1% to 3% (Yuferov et al., 2004). Additionally, preliminary evidence from that study suggests that the G36T SNP may be associated with opiate addiction (Yuferov et al., 2004).
c. Opioid Receptor Gene (OPRD1).
The DOR functions in nociceptive responses, but has also been shown to be involved in modulating the effects of MOR-directed compounds. For example, mice with targeted deletion of the OPRD1 gene do not develop tolerance to the analgesic effects of morphine, although still becoming physically dependent on the drug (Zhu et al., 1999; Nitsche et al., 2002). The DOR also may play a role in responses to cocaine. Chronic binge cocaine administration attenuated the ability of the selective agonist [D-penicillamine2, D-penicillamine5] enkephalin to inhibit adenylyl cyclase in the caudate-putamen and nucleus accumbens (Unterwald et al., 1993).
Three studies have evaluated variants of the OPRD1 gene for possible association with opiate addiction. Cloning of the OPRD1 identified three exons containing the complete coding region; however, given that the rat and mouse genes have four exons, the possibility of yet-undiscovered exons remain (Simonin et al., 1994). Two coding region SNPs in the OPRD1 gene have been defined and studied. These are both in the first exon: a T80G substitution, which results in the replacement of the amino acid phenylalanine with cysteine at position 27, and the synonymous substitution T921C (Mayer et al., 1997; Gelernter and Kranzler, 2000).
Mayer and colleagues studied 103 heroin addicts from the Rhine-Ruhr, Berlin, and Southern Bavaria areas of Germany and 115 unaffected individuals from the Rhine-Ruhr and Southern Bavaria regions, with the T921C variant ascertained in cases and controls (Mayer et al., 1997). This SNP is very common in this population, with 39% frequency of the 921C allele in control subjects. This study identified an association of the 921C allele with opiate addiction. No differences in genotype or allele frequencies were observed between samples collected from different regions within Germany within the case or control groups, respectively.
In a second study of 233 German heroin addicts meeting DSM-III-R criteria for heroin dependence and 173 ethnically and geographically matched controls (all subjects were from the Bonn area), no differences were found in genotype or allele frequencies for the T921C SNP between cases and controls (Franke et al., 1999). Also, an allele transmission test was performed on 90 additional heroin addicts and their parents. No difference in allele transmission to affected offspring was detected (Franke et al., 1999). A third study analyzed the OPRD1 T921C SNP in 405 patients hospitalized for heroin detoxification (all meeting DSM-IV criteria for opiate abuse or dependence) from two cities in Southwestern China and population-based controls from the same geographic region (Xu et al., 2002). In this study, no differences were observed in genotype or allele frequencies between cases and controls. Genotyping of two unrelated (genomic control) variants (ADH2 47 His>Arg and ALDH2 487 Glu>Lys) suggested that cases and controls were appropriately matched in this sample. These studies bring into question the findings of Mayer and colleagues, and additional studies in diverse populations will be necessary to determine the importance of variants of the OPRD1 gene in the development of opiate addiction.
d. Preproenkephalin Gene (PENK).
Enkephalin peptides, which are derived from the processing of proenkephalin, function in the modulation of pain perception and are also suggested to play a role in reward and the addictions. For example, "knockout" mice lacking the preproenkephalin gene, develop physical dependence following morphine administration but do not develop tolerance to the antinociceptive effects of the drug (Nitsche et al., 2002). Additionally, mice lacking the enkephalin gene show reduced responding to food reinforcers, which suggests a general reduction in hedonic response (Hayward et al., 2002).
One study of 31 opioid-dependent non-Hispanic Caucasians and several matched control/contrast groups found an association of genotypes and alleles of a (CA)n repeat polymorphism in the 3'-flanking sequence and opiate dependence, as measured by DSM-III-R criteria (Comings et al., 1999). Preliminary findings of studies conducted by our group also suggest a difference in genotype frequencies between opioid-dependent cases and controls in Caucasian subjects, but not in other ethnic/cultural groups studied (K. S. LaForge, unpublished findings).
e. Preprodynorphin Gene (PDYN).
As discussed above, naturally occurring peptides derived from prodynorphin (including dynorphin A, dynorphin B, and -neoendorphin) are the primary endogenous ligands of the opioid receptor. Microdialysis studies in rats demonstrated that dynorphin A(1–17) lowers basal dopamine tone in the nucleus accumbens, and studies in mice demonstrate that this peptide attenuates the increases in dopamine release in the nucleus accumbens caused by cocaine administration (Claye et al., 1997; Zhang et al., 2004). It is therefore hypothesized that dynorphins acting through the opioid receptor may counter-modulate the responses of the dopaminergic system to psychostimulants and, possibly, other drugs of abuse that also directly or indirectly increase synaptic dopamine release in brain reward circuitry. The well documented increases in striatal dynorphin peptide release and increases in mRNA levels caused by psychostimulant administration provide additional evidence that dynorphin is relevant for addictions (e.g., Sivam, 1989; Hurd and Herkenham, 1992; Hurd et al., 1992; Daunais et al., 1993; Spangler et al., 1993a,b).
Variants of the preprodynorphin gene have been studied in addiction to opiates, cocaine, and alcohol, as well as in in vitro functional studies. The most interesting variant is a 68-base repeat polymorphism located in the promoter region. This repeat was identified in the initial sequencing report of the gene (Horikawa et al., 1983) and is located approximately 1200 nucleotides upstream from the primary transcription initiation site identified by Geijer and colleagues (Geijer et al., 1995), thus placing it in the putative promoter region of the gene. Zimprich and colleagues studied this region in heroin addicts and controls and showed that the repeat is polymorphic, with alleles of one, two, three, and four copies identified (Zimprich et al., 2000). In that study, the alleles had overall frequencies of 2.7%, 32.0%, 63.5%, and 1.8%, for the one, two, three, and four repeat alleles, respectively, in control subjects of German Caucasian origin. Interestingly, the repeat contains a near-canonical activator protein 1 (AP-1) binding site that specifically binds the AP-1 protein complex. Using reporter gene constructs containing one to four copies of the repeat, Zimprich and colleagues also found that phorbol ester-induced transcription was greater in cells transfected with plasmids containing three or four copies of the repeat compared with one or two copies, and therefore this 68-base repeat polymorphism represents a potentially functional gene variant (Zimprich et al., 2000).
In their study of this variant and opiate addiction, Zimprich and colleagues found no differences in allele frequencies between heroin-addicted and control subjects (Zimprich et al., 2000). However, given the interactions between cocaine, dopamine, and dynorphin described above, we suggested this variant as an interesting candidate for cocaine addiction (LaForge et al., 2000b; Chen et al., 2002). In our study of cocaine abusing and dependent subjects and controls recruited in New York, we found that alleles containing three or four copies of the repeat were more common in control subjects compared with cases. These findings suggest that longer alleles (i.e., those with three or four copies of the repeat), which may result in greater transcriptional activation of the dynorphin gene and, therefore, greater levels of dynorphin peptides, may be protective against cocaine addiction. Given these provocative findings and the concordance of evidence from human genetic, animal, and in vitro molecular biological studies, further genetic studies into the role of allelic variants of the dynorphin gene in cocaine and other addictions are warranted.
2. Monoaminergic-Related Genes.
The monoaminergic neurotransmitter systems include the catecholaminergic and the serotonergic systems. Dopamine, a catecholamine, functions as a neurotransmitter in the central nervous system, whereas norepinephrine, another catecholamine, is also a neurotransmitter in the brain and is the main postganglionic, sympathetic neurotransmitter (Molinoff and Axelrod, 1971). Dopamine is synthesized from the amino acid tyrosine by the subsequent actions of the tyrosine hydroxylase and 3-hydroxytyrosine decarboxylase enzymes. Dopamine can then be biotransformed to norepinephrine by the action of the enzyme D
