No association between D2 dopamine receptor (DRD2) alleles or haplotypes and cocaine dependence or severity of cocaine dependence in European- and African-Americans

The Streetlight Effect: Reappraising the Study of Addiction in Light of the Findings of Genome-wide Association Studies

Drug dependence has long been thought to have a genetic component. Research seeking to identify the genetic basis of addiction has gone through important transitions over its history, in part based upon the emergence of new technologies, but also as the result of changing perspectives. Early research approaches were largely dictated by available technology, with technological advancements having highly transformative effects on genetic research, but the limitations of technology also affected modes of thinking about the genetic causes of disease. This review explores these transitions in thinking about the genetic causes of addiction in terms of the "streetlight effect," which is a type of observational bias whereby people search for something only where it is easiest to search. In this way, the genes that were initially studied in the field of addiction genetics were chosen because they were the most "obvious," and formed current understanding of the biological mechanisms underlying the actions of drugs of abuse and drug dependence. The problem with this emphasis is that prior to the genomic era the vast majority of genes and proteins had yet to be identified, much less studied. This review considers how these initial choices, as well as subsequent choices that were also driven by technological limitations, shaped the study of the genetic basis of drug dependence. While genome-wide approaches overcame the initial biases regarding which genes to choose to study inherent in candidate gene studies and other approaches, genome-wide approaches necessitated other assumptions. These included additive genetic causation and limited allelic heterogeneity, which both appear to be incorrect. Thus, the next stage of advancement in this field must overcome these shortcomings through approaches that allow the examination of complex interactive effects, both gene × gene and gene × environment interactions. Techniques for these sorts of studies have recently been developed and represent the next step in our understanding of the genetic basis of drug dependence.

Two Threats to Precision Medicine Equity

In January 2015, President Barack Obama unveiled the "Precision Medicine Initiative," a nationwide research effort to help bring an effective, preventive, and therapeutic approach to medicine. The purpose of the initiative is to bring a precise understanding of the genetic and environmental determinants of disease into clinical settings across the United States.¹ The announcement was coupled with $216 million provided in the President's proposed budget for a million-person national research cohort including public and private partnerships with academic medical centers, researchers, foundations, privacy experts, medical ethicists, and medical product innovators. The Initiative promises to expand the use of precision medicine in cancer research and modernize regulatory approval processes for genome sequencing technologies. In response, Congress passed the 21st Century Cures Act in December 2016, authorizing a total of $1.5 billion over 10 years for the program.² Although the Precision Medicine Initiative heralds great promise for the future of disease treatment and eradication, its implementation and development must be carefully guided to ensure that the millions of federal dollars expended will be spent equitably. This commentary discusses two key threats to the Precision Medicine Initiative's ability to proceed in a manner consistent with the United States Constitutional requirement that the federal government shall not "deny to any person . . . the equal protection of the laws."³ In short, this commentary sounds two cautionary notes, in order to advance precision medicine equity. First, achieving precision medicine equity will require scientists and clinicians to fulfill their intellectual, moral, and indeed legal duty to work against abusive uses of precision medicine science to advance distorted views of racial group variation. Precision medicine scientists must decisively denounce and distinguish this Initiative from the pseudo-science of eugenics - the immoral and deadly pseudo-science that gave racist and nationalist ideologies what Troy Duster called a "halo of legitimacy" during the first half of the 20th century.⁴ Second, to combat the social threat to precision medicine, scientists must incorporate a comprehensive, ecological understanding of the fundamental social and environmental determinants of health outcomes in all research. Only then will the Precision Medicine Initiative live up to its potential to improve and indeed transform health care delivery for all patients, regardless of race, color, or national origin.

Associations between the dopamine D4 receptor gene polymorphisms and personality traits in elite athletes

Personality traits and temperament may affect sports performance. Previous studies suggest that dopamine may play an important role in behavior regulation and physical exercise performance. The aim of this study is to determine associations between dopamine D4 receptor gene (DRD4 Ex3) polymorphisms and personality traits (such as neuroticism, extraversion, openness, agreeability and conscientiousness) in elite combat athletes. A total of 302 physically active, unrelated, self-reported Caucasian participants were recruited for this study. The participants consisted of 200 elite male combat athletes and 102 healthy male participants (control group). For personality trait measurements, the NEO Five-Factor Personality Inventory (NEO-FFI) and the State-Trait Anxiety Inventory questionnaires were used. For the genetic assays, blood was collected and all samples were genotyped using the real-time PCR method. A 2 x 3 factorial ANOVA revealed statistically significant differences on the Openness NEO Five Factor Inventory scale for both examined factors, i.e. sport status and genetics DTD4 Ex3. Combat athletes achieved higher scores on the Conscientiousness NEO-FFI scale when compared to controls (7.18 vs 5.98). On the other hand, combat athletes scored lower on the Openness scale in comparison with control group (4.42 vs. 4.63). Subjects with the DRD4 Ex3 s/s genotype had lower results on the openness scale in comparison with participants with the DRD4 Ex3 s/1 genotype (4.01 vs. 4.57) and higher DRD4 Ex3 1/1 genotype (4,01 vs. 3,50). In conclusion, we found an association between the dopamine D4 receptor gene in variable number tandem repeat (VNTR) polymorphisms and athletic status for two NEO-FFI factors: Openness and Conscientiousness. The DRD4 exon 3 polymorphism may be associated with the selected personality traits in combat athletes, thereby modulating athletes’ predisposition to participate in high risk sports.

HIV-related cognitive impairment shows bi-directional association with dopamine receptor DRD1 and DRD2 polymorphisms in substance-dependent and substance-independent populations

It has been postulated that drugs of abuse act synergistically with HIV, leading to increased neurotoxicity and neurocognitive impairment. The CNS impacts of HIV and drug use converge on the mesocorticolimbic dopamine (DA) system, which contains two main receptor subtypes: dopamine receptors 1 (DRD1) and 2 (DRD2). DRD1 and DRD2 have been linked to substance dependence; whether they predict HIV-associated neurocognitive disorder (HAND) is unclear. Using an advanced-stage HIV+ population, we sought to determine if drug dependence impacts the contribution of DA receptor polymorphisms on neurocognition. We observed that both DRD1 and DRD2 polymorphisms were associated with opiate and cocaine dependence (P < 0.05) in Caucasian subjects, but not African-American individuals. Using linear regression analysis, we examined the polymorphisms for associations with neuropsychological performance in global and cognitive domain T-scores (Motor, Processing Speed, Verbal Fluency, Learning, Memory, Executive Functioning, Working Memory) while controlling for opiate and cocaine dependency. In the Motor domain, we observed an association for two DRD2 polymorphisms (P < 0.05) in Caucasian subjects. The effects differed for substance dependence groups as the direction of the correlations with DRD2 were opposite to what was seen in subjects without these dependencies. In African-American subjects, associations were observed in nearly every domain, and again, the direction of the correlation differed between substance-dependent and substance-independent groups. We conclude that studies to examine genetic risk for HAND must carefully account for substance dependence patterns when assaying dopaminergic systems, as the neurobiological substrates of cognition in HIV populations may vary with tonic alterations secondary to chronic substance exposures.

Implications of genome wide association studies for addiction: are our a priori assumptions all wrong?

Substantial genetic contributions to addiction vulnerability are supported by data from twin studies, linkage studies, candidate gene association studies and, more recently, Genome Wide Association Studies (GWAS). Parallel to this work, animal studies have attempted to identify the genes that may contribute to responses to addictive drugs and addiction liability, initially focusing upon genes for the targets of the major drugs of abuse. These studies identified genes/proteins that affect responses to drugs of abuse; however, this does not necessarily mean that variation in these genes contributes to the genetic component of addiction liability. One of the major problems with initial linkage and candidate gene studies was an a priori focus on the genes thought to be involved in addiction based upon the known contributions of those proteins to drug actions, making the identification of novel genes unlikely. The GWAS approach is systematic and agnostic to such a priori assumptions. From the numerous GWAS now completed several conclusions may be drawn: (1) addiction is highly polygenic; each allelic variant contributing in a small, additive fashion to addiction vulnerability; (2) unexpected, compared to our a priori assumptions, classes of genes are most important in explaining addiction vulnerability; (3) although substantial genetic heterogeneity exists, there is substantial convergence of GWAS signals on particular genes. This review traces the history of this research; from initial transgenic mouse models based upon candidate gene and linkage studies, through the progression of GWAS for addiction and nicotine cessation, to the current human and transgenic mouse studies post-GWAS.

Imaging the effects of genetic polymorphisms on radioligand binding in the living human brain: A review on genetic neuroreceptor imaging of monoaminergic systems in psychiatry

Imaging genetics is a research field that describes the impact of genetic risk variants on brain structure and function. While magnetic resonance based imaging techniques are able to provide complex information on a system level, positron emission tomography (PET) and single photon emission computer tomography (SPECT) allow for determination of distribution and density of single receptor molecules in the human brain. Major psychiatric disorders are highly heritable, and have been associated with a dysregulation in brain dopamine and serotonin systems. Understanding the role of genetic polymorphisms within these neurotransmitter systems on brain phenotype is essential. This review tries to cover the literature on the impact of gene variants implicated in psychiatric disorders on serotonin, dopamine, and MAO-A radioligand binding in living humans. The majority of PET and SPECT studies investigated the role of polymorphisms within genes coding for the serotonin and dopamine transporters, the serotonin 1A receptor, and the dopamine D2 receptor on G protein coupled receptors or transporter proteins critically involved in serotonin or dopamine neurotransmission. Other studies investigated the impact of variants in genes for monoamine oxidase-A (MAO-A) or brain derived neurotrophic factor on monoamine transporters, receptors, or MAO-A activity. Two main findings in healthy subjects emerge from the current literature: one is an increased binding of the selective ligand [(11)C]DASB to serotonin transporters in subjects homozygous for the triallelic 5-HTTLPR LA allele. The other one is decreased binding of the radioligand [(11)C]raclopride to dopamine D2 receptors in D2 Taq1 A1 allele carriers. Other findings reported are highly interesting but require independent replication.

Gender effect on association between DRD2 polymorphism and substance dependence in a Spanish sample

Our aim was to examine a possible association between substance dependence and the TaqIA polymorphism of the D2 dopamine receptor (DRD2), a single nucleotide polymorphism (SNP) located at the 3' UTR region of the DRD2 gene. A case-control design stratified by gender was used to analyze the genotypes of this SNP in a sample of 125 substance-dependent patients according to DSM-IV and 203 blood donors recruited as controls in two general city hospitals in Madrid, Spain. Genomic DNA from peripheral blood samples was amplified through PCR to identify the variants of the SNP in the DRD2 gene. Analyses performed with Chi(2) tests revealed that the A1 allele (A1/A1 and A1/A2 genotypes) of the Taq 1A SNP of the DRD2 gene was significantly associated with substance dependence in males, but not in the whole sample. Male patients had significantly higher rates of the A1-containing genotypes than male controls. The finding of an association between substance dependence and the DRD2 gene TaqIA SNP only in males suggests the existence of gender-specific differences in the genetic underpinnings of substance dependence.

Association of variants in MANEA with cocaine-related behaviors

CONTEXT

Cocaine dependence (CD) and related behaviors are highly heritable, but no genetic association has been consistently demonstrated. A recent genome-wide study of drug dependence identified an association between cocaine-induced paranoia (CIP) and a single-nucleotide polymorphism (SNP) in the alpha-endomannosidase (MANEA) locus in a family-based sample of European Americans and African Americans.

OBJECTIVE

To conduct a comprehensive genetic association study of the MANEA locus with CD and CIP.

DESIGN

Genome-wide association study.

SETTING

Four university hospitals.

PARTICIPANTS

A total of 3992 individuals from 2 family-based and 2 case-control samples.

INTERVENTION

Participants were classified as having CD or CIP or as a control using the Semi-Structured Assessment for Drug Dependence and Alcoholism. They were genotyped for 11 SNPs spanning MANEA and its surrounding region.

MAIN OUTCOME MEASURE

Association of CD and CIP with individual SNPs and haplotypes.

RESULTS

Cocaine-induced paranoia was associated with 6 SNPs in the European American families and 9 SNPs in the African American families. The strongest evidence in the total sample of families was observed in 3 markers located in the promoter and 3' untranslated regions (P < .001). The association of MANEA SNPs with CD in both family samples was much weaker. In the African American case-control sample, multiple markers were significantly associated with CIP and CD; CIP and CD were also significantly associated with a 2-SNP haplotype in the European American case-control sample. The A allele of the 3' untranslated region SNP rs9387522 was associated with increased risk of CIP in all 4 data sets.

CONCLUSIONS

Our findings suggest that CD and associated behaviors may involve biological pathways not typically thought to be associated with brain metabolism.

Association analysis between polymorphisms in the conserved dopamine neurotrophic factor (CDNF) gene and cocaine dependence

Cocaine-induced neuroplasticity changes in the mesocorticolimbic dopamine systems are thought to be involved in the pathophysiology of cocaine dependence. Since neurotrophic factors have been observed to prevent/reverse and mimic cocaine-induced neurobiological changes in the brain, related genes are plausible candidates for susceptibility to cocaine dependence. The novel conserved dopamine neurotrophic factor protein (CDNF) promotes the survival, growth, and function of dopamine-specific neurons and is expressed in brain regions that undergo cocaine-induced neuroplasticity. In this study, we hypothesize that polymorphisms in the CDNF gene (CDNF/ARMETL1) contribute to increased risk for cocaine dependence. Cocaine dependent individuals (n=351) and unaffected controls (n=257) of African descent were genotyped for four single nucleotide polymorphisms (SNPs) in the CDNF gene (rs11259365, rs7094179, rs7900873, rs2278871). We observed no significant differences in allele, genotype, or haplotype frequencies between cases and controls for any of the tested SNPs. Our study suggests that there is no association between variants in the CDNF gene and cocaine dependence. However, additional studies using larger sample sizes, comprehensive SNP coverage, and clinically homogenous populations are necessary before confidently excluding CDNF as a significant genetic risk factor for cocaine dependence.

Genetic variants in the cocaine- and amphetamine-regulated transcript gene (CARTPT) and cocaine dependence

Dopaminergic brain systems have been implicated to play a major role in drug reward, thus making genes involved in these circuits plausible candidates for susceptibility to substance use disorders. The cocaine- and amphetamine-regulated transcript peptide (CARTPT) is involved in reward and feeding behavior and has functional characteristics of an endogenous psychostimulant. In this study we tested the hypothesis that variation in the CARTPT gene increases susceptibility to cocaine dependence in individuals of African descent. Genotypes of three HapMap tagging SNPs (rs6894758; rs11575893; rs17358300) across the CARTPT gene region were obtained in cocaine dependent individuals (n=348) and normal controls (n=256). All subjects were of African descent. There were no significant differences in allele, genotype or haplotype frequencies between cases and controls for any of the tested SNPs. Our results do not support an association of the CARTPT gene with cocaine dependence; however, additional studies using larger samples, comprehensive SNP coverage, and different populations are necessary to conclusively rule out CARTPT as a contributing factor in the etiology of cocaine dependence.

Meta-analysis of the association of the Taq1A polymorphism with the risk of alcohol dependency: a HuGE gene-disease association review

The human dopamine 2 receptor Taq1A allele has been implicated as a vulnerability factor for alcohol dependence in a number of studies and reviews. To determine whether this allele is associated with alcoholism, the authors conducted a Human Genome Epidemiology review and meta-analysis. Forty-four studies with 9,382 participants were included. An odds ratio of 1.38 (95% confidence interval: 1.20, 1.58; heterogeneity, 50.5%) was found for the A1A1 + A1A2 versus the A2A2 genotype. Sensitivity analyses suggested lack of ethnic matching as a possible source of heterogeneity; a small, significant association was detected in studies with ethnic-matched controls (odds ratio = 1.26, 95% confidence interval: 1.02, 1.56; heterogeneity, 37%). Significant associations were also found in analyses restricted to studies reporting use of blinding and those with adequate screening of controls for alcohol dependency. For the A1A1 versus the A1A2 + A2A2 genotype, the odds ratio was 1.22 (95% confidence interval: 1.05, 1.43; heterogeneity, 0%). Sensitivity analyses on groups of studies reporting use of ethnic-matched controls and those that screened controls for alcohol dependency still showed significant associations. The relatively small effect for the association of the A1 allele, or another genetic variant linked to it, with alcohol dependence indicates a multigene causality for this complex disorder.

Polymorphisms TaqI A of the DRD2, BalI of the DRD3, exon III repeat of the DRD4, and 3' UTR VNTR of the DAT: association with childhood ADHD in male African-Caribbean cocaine dependents?

The conflicting results reported by genetic studies with the variants of the genes coding for the dopaminergic system in cocaine addicts could be partially explained by the difficulties to constitute homogenous sample of patients. Childhood attention-deficit/hyperactivity disorder (ADHD), and/or impulsivity are frequently associated with cocaine addiction and could participate in the heterogeneity of the samples in cocaine addicts. Accordingly, it is hypothesized that cocaine addiction would be associated with the variants of the genes coding for the dopamine system in an homogenized sample of cocaine addicts, especially in individuals with childhood ADHD comorbidity, or with a high impulsivity score. The potential association of the variants TaqI A of the DRD2, BalI of the DRD3, exon III repeat of the DRD4, and 3' UTR VNTR of the DAT was examined in African-Caribbean males, smoked-cocaine dependents. All the subjects were assessed with the Diagnostic Interview of Genetic Studies, the Barratt's impulsivity scale, and the Wender Utah rating scale for childhood ADHD. A positive association was found with the DRD2 and DRD4 polymorphisms in the subgroups of patients with childhood ADHD, or with a high impulsivity score, which represented, respectively, 53.3 and 73.0% of the patients. Conversely, no positive association was found for any of the polymorphisms studied when the group of patients was examined as a whole. Therefore, our results suggest that the clinical dimensions of childhood ADHD and of impulsivity could be taken into account to homogenize the samples of patients in cocaine association studies.

Gene-environment contributions to young adult sexual partnering

To date, there has been relatively little work on gene-environment contributions to human sexuality, especially molecular analyses examining the potential contributions of specific polymorphisms in conjunction with life experiences. Using Wave III data from 717 heterozygous young adult sibling pairs included in the National Longitudinal Study of Adolescent Health, this article examined the combined contributions of attendance at religious services and three genetic polymorphisms (in the dopamine D4 receptor [DRD4]), dopamine D2 receptor [DRD2]), and the serotonin transporter promoter [5HTT]) to sensation seeking, a personality construct related to sexual behavior, and the number of vaginal sex partners participants had in the year before interview. Data analyses used an Allison mixed model approach to account for population stratification and correlated observations. DRD4 was unrelated to sensation seeking and to the number of sex partners in tests of both main effects and in interaction with religious attendance. Contrary to hypothesis, presence of the A1 DRD2 allele was associated with having had fewer sex partners in the past year. Associations between the 5HTT allele and sex partners varied by religious attendance, but again the patterns of associations were contrary to hypothesized relationships and were small in magnitude. These findings underscore the necessity of using more comprehensive multiple gene-multiple life experience approaches to investigations of complex behaviors such as sexual patterns.

Haplotype spanning TTC12 and ANKK1, flanked by the DRD2 and NCAM1 loci, is strongly associated to nicotine dependence in two distinct American populations

Nicotine dependence (ND) is a moderately heritable trait. We ascertained a set of 1615 subjects in 632 families [319 African-American (AA) and 313 European-American (EA)] based on affected sibling pairs with cocaine or opioid dependence. Subjects were interviewed with the Semi-Structured Assessment for Drug Dependence and Alcoholism (SSADDA). Previously, we identified a modest linkage peak (LOD score =1.97) for ND in the EA part of the sample on chromosome 11q23, a region that includes the NCAM1-TTC12-ANKK1-DRD2 gene cluster. DRD2 and NCAM1 are functional candidate genes for substance dependence; the TTC12 and ANKK1 loci are not well characterized. We genotyped a set of 43 single nucleotide polymorphisms (SNPs) spanning this region, and performed family-based association and haplotype analysis. There was relatively weak evidence for association of the flanking DRD2 and NCAM1 markers to ND, but very strong evidence of association of multiple SNPs at TTC12 and ANKK1 in both populations (minimal P=0.0007 in AAs and minimal P=0.00009 in EAs), and in the pooled sample, as well as strong evidence for highly significant association of a single haplotype spanning TTC12 and ANKK1 to ND in the pooled sample (P=0.0000001). We conclude that a risk locus for ND, important both in AAs and EAs, maps to a region that spans TTC12 and ANKK1. Functional studies of these loci are warranted. These results provide additional information useful in evaluating the many earlier discrepant findings regarding association of DRD2 with substance dependence.

Perspectives on current directions in the neurobiology of addiction disorders relevant to genetic risk factors

There is a significant heritability of drug addiction disorders, but potential genes that may underlie such vulnerability have not been clearly identified. Common neurobiological candidates for drug abuse include genes related to dopamine, opioid neuropeptide, and glutamate transmission that play important roles in drug reward and inhibitory control. This article provides an overview of genetic polymorphisms linked to these neurobiological systems, particularly in relation to psychostimulant- and opioid-addiction vulnerability.

Symbiotic relationship of pharmacogenetics and drugs of abuse

Pharmacogenetics/pharmacogenomics is the study of how genetic variation affects pharmacology, the use of drugs to treat disease. When drug responses are predicted in advance, it is easier to tailor medications to different diseases and individuals. Pharmacogenetics provides the tools required to identify genetic predictors of probable drug response, drug efficacy, and drug-induced adverse events-identifications that would ideally precede treatment decisions. Drug abuse and addiction genetic data have advanced the field of pharmacogenetics in general. Although major findings have emerged, pharmacotherapy remains hindered by issues such as adverse events, time lag to drug efficacy, and heterogeneity of the disorders being treated. The sequencing of the human genome and high-throughput technologies are enabling pharmacogenetics to have greater influence on treatment approaches. This review highlights key studies and identifies important genes in drug abuse pharmacogenetics that provide a basis for better diagnosis and treatment of drug abuse disorders.

Association of a polymorphism in the Homer1 gene with cocaine dependence in an African American population

OBJECTIVE

While twin and adoption studies have demonstrated that up to 70% of the risk for becoming addicted to cocaine is due to genetic factors, identifying specific genes involved in the development or progression of cocaine dependence has been difficult. The purpose of this study is to determine whether single-nucleotide polymorphisms in the Homer1 and Homer2 genes associate with the cocaine-dependent phenotype in an African American population.

METHODS

This study utilized a case-control design in which the genotype and allele frequencies for four single-nucleotide polymorphisms in the Homer1 gene and three single-nucleotide polymorphisms in the Homer2 gene were compared between African American individuals with a diagnosis of cocaine dependence (n=170) and African American individuals with no history of substance abuse (n=90).

RESULTS

The data indicate that one single-nucleotide polymorphism, rs6871510, located in intron 1 of the Homer1 gene significantly (P=0.029) associates with cocaine dependence at the genotype level, and trends toward a significant association at the allele frequency level (chi=2.62, df=1, P=0.106, OR=1.71). None of the single-nucleotide polymorphisms analyzed in the Homer2 gene associates with cocaine dependence.

CONCLUSIONS

The results of this study suggest that a polymorphism in the Homer1 gene, rs6871510, is a potential risk factor for the development of cocaine dependence in an African American population, whereas polymorphisms in the Homer2 gene are not.

Association study of dopamine D2 and D3 receptor gene polymorphisms with cocaine dependence

Genetic factors play a role in the vulnerability to cocaine dependence. The reinforcing properties of cocaine are related to the dopaminergic system, and, in particular, the dopamine receptors have been linked to the reward mechanisms. The present study examines the role of the variants TaqI A of the dopamine D2 receptor gene and BalI of the dopamine D3 receptor gene in a Brazilian sample consisting of 730 cocaine dependents and 782 healthy controls. The studied polymorphisms did not show any difference in allelic frequencies or genotypic distribution between the groups. Our data do not support a role for the dopamine D2 receptor gene TaqI A and dopamine D3 receptor gene BalI gene polymorphisms in the susceptibility to cocaine dependence in a Brazilian sample.

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.

Genetic epidemiology of anxiety disorders

This chapter reviews the genetic epidemiology of the major subtypes of anxiety disorders including panic disorder, phobic disorders, generalized anxiety disorder, and obsessive-compulsive disorder. Controlled family studies reveal that all of these anxiety subtypes are familial, and twin studies suggest that the familial aggregation is attributable in part to genetic factors. Panic disorder and, its spectrum have the strongest magnitude of familial clustering and genetic underpinnings. Studies of offspring of parents with anxiety disorders an increased risk of mood and anxiety disorders, but there is far less specificity of the manifestations of anxiety in children and young adolescents. Although there has been a plethora of studies designed to identify genes underlying these conditions, to date, no specific genetic loci have been identified and replicated in independent samples.

Advances in molecular genetics and the prevention and treatment of substance misuse: Implications of association studies of the A1 allele of the D2 dopamine receptor gene

Substance misuse is influenced by multiple genetic and environmental factors. Recent research has identified a number of potential genetic markers of risk and those associated with drug reward substrates show particular promise. The current study reexamines the extant published data of the association between the D2 dopamine receptor (DRD2) gene minor Taq 1A (A1) allele and substance misuse risk. A series of meta-analyses was performed on 64 studies examining DRD2 A1+ allelic status and substance misuse. In addition, personality was examined as a possible endophenotype. Significant association was found between the A1 allele and severe substance dependence in both Caucasian and non-Caucasian groups. The data did not support a significant association between the A1 allele and personality features. While the specific mechanism underlying these associations requires further elucidation, this genetic marker shows promise as a marker of brain reinforcement processes. Possible ways of utilising the A1 allele to inform prevention and treatment initiatives are discussed.

Genetics and drug use as a complex phenotype

Drug use is a complex behavior influenced by multiple biological, family, and sociocultural factors. The concurrent use/misuse of multiple drugs is often seen and drug use also co-occurs with other psychiatric conditions. Behavior and molecular genetic studies support an important posited role of genes in drug use. This posited genetic risk does not appear to be conferred by one or two major genes manifesting large effects, but rather by a number of genes manifesting smaller effects. Genetic factors explain, on average, only about half of the total variability in drug use, with the remaining variability influenced by environmental factors. Also, genetic risk may be differentially expressed in the presence vs. absence of particular environmental conditions. Thus, investigation of environmental factors and their interaction with genetic risk is a necessary component of genetic research. While the full potential of genetic investigations for the prevention of drug misuse has yet to be realized, an example of the impact of risk factor modification under various conditions of gene-environment interaction is provided, and the implications for use of genetic information in drug-misuse prevention are discussed. The multifactorial nature of drug use necessitates coordinated investigation from multiple disciplines and timely dissemination of scientific findings. In addition, this work demands adherence to the highest standards of confidentiality and ethical use of genetic information to best inform future prevention efforts.

D2 dopamine receptor gene in psychiatric and neurologic disorders and its phenotypes

The D2 dopamine receptor (DRD2) has been one of the most extensively investigated gene in neuropsychiatric disorders. After the first association of the TaqI A DRD2 minor (A1) allele with severe alcoholism in 1990, a large number of international studies have followed. A meta-analysis of these studies of Caucasians showed a significantly higher DRD2 A1 allelic frequency and prevalence in alcoholics when compared to controls. Variants of the DRD2 gene have also been associated with other addictive disorders including cocaine, nicotine and opioid dependence and obesity. It is hypothesized that the DRD2 is a reinforcement or reward gene. The DRD2 gene has also been implicated in schizophrenia, posttraumatic stress disorder, movement disorders and migraine. Phenotypic differences have been associated with DRD2 variants. These include reduced D2 dopamine receptor numbers and diminished glucose metabolism in brains of subjects who carry the DRD2 A1 allele. In addition, pleiotropic effects of DRD2 variants have been observed in neurophysiologic, neuropsychologic, stress response, personality and treatment outcome characteristics. The involvement of the DRD2 gene in certain neuropsychiatric disorders opens up the potential of a targeted pharmacogenomic approach to the treatment of these disorders.

No association for D2 and D4 dopamine receptor polymorphisms and methamphetamine abuse in Chinese males

The D2 and D4 dopamine receptors (DRD2 and DRD4) play major roles in the central effects of psychostimulants and in the reward system. Previous studies, although not all, have demonstrated associations between the DRD2 TaqI and the DRD4 exon III variable number tandem repeat (VNTR) polymorphisms and substance dependence. For this study, we have investigated the associations between these two polymorphisms and methamphetamine (MAP) dependence, as manifested in a Chinese-male sample population. No significant difference was demonstrated for genotype or allele frequency when comparing MAP-dependent and control cases for the DRD2 TaqI and the DRD4 gene exon III VNTR polymorphisms, suggesting that these two polymorphisms do not play major roles in MAP dependence for our sample of Chinese males.

Serotonin transporter (5-HTT) gene polymorphisms and susceptibility to cocaine dependence among African-American individuals

Studies indicate that the serotonin system, particularly the serotonin transporter (5-HTT), may modulate the central effects of cocaine. We investigated whether a polymorphism in the 5' promotor region (5-HTTLPR) of the 5-HTT gene confers susceptibility to cocaine dependence. One hundred and ninety-seven cocaine-dependent African-American subjects and 101 controls were studied. Polymerase chain reaction based genotyping of a biallelic repeat polymorphism in the 5' promotor region yielded 2 alleles containing 484 (S) and 528 bp (L) repeats, respectively. There were no significant differences between controls of European background (n = 40) and African-American controls (n = 61) in distribution of genotypes (European: LL = 32.5%, LS = 40.0%, SS = 27.5%; African-American: LL = 27.9%, LS = 57.4%, SS = 14.7%) (chi(2) = 3.60, df = 2, p = 0.16) or allele frequencies (European: L = 52.5%, S = 47.5%; African-American: L = 56.6%, S = 43.4%) (chi(2) = 2.21, df = 1, p = 0.13). When cocaine patients were compared to an ethnically diverse control group (n = 101), frequencies of the L variant (65.0%) were significantly higher while the S variant (35.0%) was less frequent among cocaine patients compared to controls (L = 53.9%, S = 46.1%) (chi(2) = 6.83, df = 1, p < 0.01). Similarly, there were more cocaine patients with the LL genotype (41.1%) and less with the SS genotype (11.2%) compared to controls (LL = 29.7%, SS = 21.8%) (chi(2) = 7.43, df = 2, p < 0.05). However, after restricting controls to African-American individuals only (n = 61), cocaine subjects and controls did not differ significantly with respect to genotype distribution (chi(2) = 4.24, df = 2, p = 0.12) or allele frequencies (chi(2) = 2.83, df = 1, p = 0.10). In conclusion, although comparisons with a heterogeneous control group indicated a possible association between allelic variants of 5-HTTLPR and cocaine dependence among African-American cocaine subjects, this relationship was not observed when the control group was limited to African-American people only. Our findings need to be confirmed on larger samples of ethnically matched individuals.

Imaging neurochemical endophenotypes: promises and pitfalls

A large number of polymorphisms in genes coding for neurotransmitter receptors and transporters have been associated with neuropsychiatric conditions, although few of these associations have been consistently replicated. These proteins are critical targets of psychoactive drugs and the clarification of the functional significance of these polymorphisms might offer important leads for drug development and therapeutic applications. Brain imaging techniques such as single photon emission computed tomography (SPECT) and positron emission tomography (PET) provide the means to monitor the expression and function of many of these proteins in the living human brain. This paper reviews brain imaging studies designed to evaluate the significance of polymorphisms in genes coding for important drug targets (e.g., the serotonin transporter [SERT], the dopamine transporter [DAT] and the dopamine D(2) receptor) in terms of expression or function. These studies illustrate the unique opportunities, as well as the pitfalls, generated by combining genetic analysis with brain imaging studies.

Attention deficit/hyperactivity disorder and substance abuse. Diagnostic and therapeutic considerations

Patterns of association between attention deficit hyperactivity disorder (ADHD) and substance-use disorders are considered. Recent investigations have found that up to 50% of individuals with continuing ADHD symptoms have a substance-use disorder. ADHD appears to represent an independent risk factor for substance abuse. We review clinical challenges posed by the diagnosis of ADHD in substance-abusing populations. Nicotine dependence is also substantially more common among adults with ADHD (40%) than in the general population (26%). While several classes of substances of abuse may ameliorate various symptoms of ADHD, individuals with ADHD may also be vulnerable to substance use because of poor judgment or impulsive behavior in social settings. Evidence is reviewed from genetic studies examining the role of the dopamine D2 (DRD2) gene in the etiology of ADHD. The presence of ADHD may affect the course of adolescent substance abuse in several ways: predicting earlier age of onset, longer duration of substance-use disorder, and progression of alcohol abuse to another drug-use disorder. Individuals with ADHD have been noted to have a shorter interval between the onsets of drug abuse and drug dependence. Such individuals are also at greater risk for treatment failure, as their disruptive behaviors interfere with treatment access and response. Lastly, we review advances in pharmacotherapeutic agents used for treating ADHD and consider the impact of these interventions on comorbid substance-use disorders. We suggest promising areas of focus for clinical research trials targeting the subpopulation of substance abusers with concurrent ADHD.

Polymorphisms in dopamine receptors: what do they tell us?

Many genetic studies have focussed on dopamine receptors and their relationship to neuropsychiatric disease. Schizophrenia, bipolar disorder, and substance abuse have been the most studied, but no conclusive linkage or association has been found. The possible influence of dopamine receptor variants on drug response has not received as much attention. While there is some evidence that polymorphisms and mutations in dopamine receptors can alter functional activity and pharmacological profiles, no conclusive data link these gene variants to drug response or disease. The lack of unequivocal findings may be related, in part, to the subtle changes in receptor pharmacology that these polymorphisms and mutations mediate. These subtle effects may be obscured by the influence of genes controlling drug metabolism and kinetics. Further insight into the pharmacogenetics of dopamine receptors may require not just more studies, but novel approaches to the study of complex genetic traits and diseases.

Family-based study of DRD2 alleles in alcohol and drug dependence

Numerous case-control studies have addressed the hypothesis that variant alleles of the dopamine D2 receptor gene (DRD2) increase the liability for alcohol and/or drug dependence, and both positive and negative results have been reported. Because population frequencies of these alleles vary considerably, the conflicting results could be due to population stratification bias. Using the transmission disequilibrium test, the present study examined linkage disequilibrium of alcohol and drug (opioid and/or cocaine) dependence with three DRD2 polymorphic systems: (a) TaqI A, (b) TaqI D, and (c) the functional -141CIns/Del promoter systems. DNA samples were collected from small nuclear families (SNFs), where one or more offspring met DSM-III-R or DSM-IV criteria for alcohol and/or drug dependence. Because positive association between DRD2 alleles and alcohol and/or drug dependence has been reported only in populations of European ancestry, we limited the present study to European Americans (EAs). No evidence for linkage disequilibrium was found for any of the polymorphic systems when examined in relation to any substance dependence, alcohol dependence (with or without drug dependence), or drug dependence (with or without alcohol dependence). These results are consistent with those from a recent family-based study of alcohol dependence. Together, these studies suggest that the conflicting findings from case-control studies of the association between alleles of DRD2 and substance dependence may be attributable to population stratification in some samples.

Functional variants at CYP2A6: new genotyping methods, population genetics, and relevance to studies of tobacco dependence

Cytochrome P450CYP2A6 (CYP2A6) is the predominant enzyme responsible for the metabolism of nicotine to cotinine. Two variants have been identified that encode products presumed to have little or no activity. A previous study suggested that carriers of at least one copy of either null variant may be protected against tobacco dependence, while tobacco-dependent carriers smoke fewer cigarettes. However, different laboratories have reported widely disparate CYP2A6 allele frequencies across European populations. These differences prompted us to reexamine the genotyping methods for CYP2A6. We developed an improved genotyping strategy using CYP2A6-specific nested PCR, and differential restriction enzyme digestion to identify variant nucleotides in exon 3. We used sequencing to verify genotype results and to assess the sequence of exon 4, which previous work predicted should correspond to "wild-type" CYP2A6 sequence. In addition, we developed a new nomenclature in which CYP2A6*1 is designated CYP2A6*A1-*B1, CYP2A6*2 is CYP2A6*A2, and CYP2A6*3 is CYP2A6*B2. The frequencies of CYP2A6*A2 and CYP2A6*B2 were then estimated in samples from six populations. Sequencing confirmed CYP2A6*A2 genotypes in all cases. Unexpectedly, sequencing demonstrated exon 4 sequence corresponding to CYP2A7 in samples genotyped as CYP2A6*B2. In the population study, we found consistently low allele frequencies (</=5%) for CYP2A6*A2 and CYP2A6*B2 in all samples examined. The molecular data from this study suggest that further physical mapping may be necessary to clarify the structure of CYP2A6*B2. The population results suggest that in many populations the frequencies of the variants examined are quite low, and might therefore limit the power of future genetic association studies of tobacco dependence based on these variants.

Genetic vulnerability to drug abuse

Addiction to various substances, including drugs and alcohol, probably arises from a combination of environmental and genetic factors. The genetic vulnerability to drug addiction is supported by several familial, adoption and twin studies. However, as in other mental disorders, the genetic vulnerability to drug addiction appears complex: these disorders do not follow the rules of Mendelian inheritance. Instead, they are probably influenced by multiple susceptibility genes, each of which contributes to the disorder. The more genes necessary for a disorder, the harder it is to detect any of them. This difficulty is magnified by the role of environmental factors. Association studies using the candidate gene approach can identify susceptibility genes for drug abuse supported by the pathophysiological hypothesis of the illness. This review will focus on the clinical and molecular genetic studies in drug abuse.

A haplotype at the DBH locus, associated with low plasma dopamine beta-hydroxylase activity, also associates with cocaine-induced paranoia

Low levels of dopamine beta-hydroxylase (DbetaH) protein in the plasma or cerebrospinal fluid (CSF) are associated with greater vulnerability to positive psychotic symptoms in several psychiatric disorders. DbetaH level is a stable, genetically controlled trait. DBH, the locus encoding DbetaH protein, is the major quantitative trait locus controlling plasma and CSF DbetaH levels. We therefore hypothesized that DBH variants or haplotypes, associated with low levels of DbetaH in the plasma, would also associate with greater vulnerability to cocaine-induced paranoia. To test this hypothesis, we first showed that a di-allelic variant, DBH*5'-ins/del, located approximately 3 kb 5' to the DBH transcriptional start site, significantly associates with plasma DbetaH activity in European-Americans (n = 66). Linkage disequilibrium analysis of that polymorphism and DBH*444g/a, another di-allelic variant associated with DbetaH levels, demonstrated that alleles of similar association to DbetaH levels are in positive disequilibrium. We then estimated DBH haplotype frequencies in cocaine-dependent European Americans rated for cocaine-induced paranoia (n = 45). As predicted, the low-DbetaH-associated haplotype, Del-a, was significantly more frequent (P = 0.0003) in subjects endorsing cocaine-induced paranoia (n = 29) than in those denying it (n = 16). Comparison to control haplotype frequencies (n = 145 healthy European-Americans) showed that the association predominantly reflected under-representation of Del-a haplotypes in those denying cocaine-induced paranoia. We conclude that: (a) the two DBH polymorphisms we studied are associated with plasma DBH levels; (b) those two polymorphisms are in significant linkage disequilibrium in European Americans, with alleles of similar association to DbetaH levels in positive disequilibrium; and (c) the haplotype associated with low DBH activity is also associated with cocaine-induced paranoia. Molecular Psychiatry (2000) 5, 56-63.

No association between D2 dopamine receptor (DRD2) "A" system alleles, or DRD2 haplotypes, and posttraumatic stress disorder

BACKGROUND

Association studies between marker alleles at the D2 dopamine receptor gene (DRD2) and various psychiatric illnesses have produced conflicting results. Reports of allelic associations were originally made with alcoholism, but were then extended to other psychiatric disorders, including posttraumatic stress disorder (PTSD).

METHODS

We studied allele frequency of the DRD2 TaqI "A," "B," and "D" system markers in 52 European-American subjects with diagnoses of PTSD (based on structured interviews).

RESULTS

Frequency of the A1 allele in this sample was .15, not significantly different from the .19 allele frequency seen in 87 control subjects. We were thus unable to replicate the previous reports of allelic association between the DRD2 TaqI "A1" allele and PTSD. There were also no significant differences in allele frequency for the "B" or "D" systems. We then computed three marker (TaqI "A," "B," and "D" system) haplotypes for the sample; DRD2 haplotype frequencies also did not differ between control subjects and subjects with PTSD.

CONCLUSIONS

We conclude that DRD2 alleles are not associated with PTSD in this sample, and that genetic variation at the DRD2 locus is not likely to be an important contributor to risk for this disorder.