Haplotypes at the DRD2 locus and severe alcoholism

DRD2/ANKK1 TaqI A1 polymorphism associates with overconsumption of unhealthy foods and biochemical abnormalities in a Mexican population

PURPOSE

The dopamine receptor 2/ankyrin repeat domain and content kinase 1 (DRD2/ANKK1) TaqIA polymorphism (rs1800497) has been associated with rewarding behaviors. This study aimed to investigate the association of DRD2/ANKK1 TaqIA polymorphism with the dietary intake, the intake frequency of food groups and biochemical profile in Mexican mestizo subjects.

METHODS

A cross-sectional/analytical study with 276 Mexican subjects was performed. Dietary intake was assessed with a 24-h recall and a food frequency questionnaire (FFQ). An allelic discrimination assay evaluated DRD2/ANKK1 TaqIA genotypes. Anthropometric and biochemical data were evaluated.

RESULTS

Genotype frequencies were A1A1 (18.48%), A1A2 (45.29%) and A2A2 (36.23%). TaqI A1 allele carriers had a higher intake of carbohydrates (p = 0.038), meats (p = 0.005), fried dishes (p = 0.039), and sugars (p = 0.009). Male TaqI A1 carriers consumed more carbohydrates (p = 0.009) and meats (p = 0.018) while females consumed fewer legumes (p = 0.005). TaqI A1 carriers had glucose (p = 0.037) and triglycerides (p = 0.011) abnormalities. TaqI A1 was associated with higher risk of consumption of unhealthy foods such as fried dishes (OR 3.79, 95% CI 1.53-9.35, p = 0.002) and meats (OR 2.31, 95% CI 1.32-4.05, p = 0.003), and lower healthy foods (OR 1.89, 95% CI 1.04-3.29, p = 0.038). TaqI A1 allele was associated with risk of abnormal glucose, triglycerides, and VLDL levels (OR 2.148, 95% CI 1.068-4.322, p = 0.036; OR 1.999, 95% CI 1.194-3.348, p = 0.011; OR 2.021, 95% CI 1.203-3.392, p = 0.007), respectively.

CONCLUSIONS

The presence of the TaqI A1 allele in Mexicans is a genetic risk factor for detrimental dietary quality that may predispose to metabolic disturbances.

LEVEL OF EVIDENCE

Level III, case-control analytic study.

Shaping vulnerability to addiction - the contribution of behavior, neural circuits and molecular mechanisms

Substance use disorders continue to impose increasing medical, financial and emotional burdens on society in the form of morbidity and overdose, family disintegration, loss of employment and crime, while advances in prevention and treatment options remain limited. Importantly, not all individuals exposed to abused substances effectively develop the disease. Genetic factors play a significant role in determining addiction vulnerability and interactions between innate predisposition, environmental factors and personal experiences are also critical. Thus, understanding individual differences that contribute to the initiation of substance use as well as on long-term maladaptations driving compulsive drug use and relapse propensity is of critical importance to reduce this devastating disorder. In this paper, we discuss current topics in the field of addiction regarding individual vulnerability related to behavioral endophenotypes, neural circuits, as well as genetics and epigenetic mechanisms. Expanded knowledge of these factors is of importance to improve and personalize prevention and treatment interventions in the future.

Altered regional brain volumes in elderly carriers of a risk variant for drug abuse in the dopamine D2 receptor gene (DRD2)

Dopamine D2 receptors mediate the rewarding effects of many drugs of abuse. In humans, several polymorphisms in DRD2, the gene encoding these receptors, increase our genetic risk for developing addictive disorders. Here, we examined one of the most frequently studied candidate variant for addiction in DRD2 for association with brain structure. We tested whether this variant showed associations with regional brain volumes across two independent elderly cohorts, totaling 1,032 subjects. We first examined a large sample of 738 elderly participants with neuroimaging and genetic data from the Alzheimer's Disease Neuroimaging Initiative (ADNI1). We hypothesized that this addiction-related polymorphism would be associated with structural brain differences in regions previously implicated in familial vulnerability for drug dependence. Then, we assessed the generalizability of our findings by testing this polymorphism in a non-overlapping replication sample of 294 elderly subjects from a continuation of the first ADNI project (ADNI2) to minimize the risk of reporting false positive results. In both cohorts, the minor allele-previously linked with increased risk for addiction-was associated with larger volumes in various brain regions implicated in reward processing. These findings suggest that neuroanatomical phenotypes associated with familial vulnerability for drug dependence may be partially mediated by DRD2 genotype.

Updated findings of the association and functional studies of DRD2/ANKK1 variants with addictions

Both nicotine and alcohol addictions are severe public health hazards worldwide. Various twin and family studies have demonstrated that genetic factors contribute to vulnerability to these addictions; however, the susceptibility genes and the variants underlying them remain largely unknown. Of susceptibility genes investigated for addictions, DRD2 has received much attention. Considering new evidence supporting the association of DRD2 and its adjacent gene ankyrin repeat and kinase domain containing 1 (ANKK1) with various addictions, in this paper, we provide an updated view of the involvement of variants in DRD2 and ANKK1 in the etiology of nicotine dependence (ND) and alcohol dependence (AD) based on linkage, association, and molecular studies. This evidence shows that both genes are significantly associated with addictions; however the association with ANKK1 appears to be stronger. Thus, both more replication studies in independent samples and functional studies of some of these variants are warranted.

Variation of types of alcoholism: review and subtypes identified in Han Chinese

Alcoholism, as it has been hypothesized, is caused by a highly heterogeneous genetic load. Since 1960, many reports have used the bio-psycho-social approach to subtype alcoholism; however, no subtypes have been genetically validated. We reviewed and compared the major single-gene, multiple-gene, and gene-to-gene interaction studies on alcoholism published during the past quarter-century, including many recent studies that have made contributions to the subtyping of alcoholism. Four subtypes of alcoholism have been reported: [1] pure alcoholism, [2] anxiety/depression alcoholism, [3] antisocial alcoholism, and [4] mixed alcoholism. Most of the important studies focused on three genes: DRD2, MAOA, and ALDH2. Therefore, our review focuses on these three genes.

Polymorphisms of genes in neurotransmitter systems were associated with alcohol use disorders in a Tibetan population

Studies of linkage and association in various ethnic populations have revealed many predisposing genes of multiple neurotransmitter systems for alcohol use disorders (AUD). However, evidence often is contradictory regarding the contribution of most candidate genes to the susceptibility of AUD. We, therefore, performed a case-control study to investigate the possible associations of genes selected from multiple neurotransmitter systems with AUD in a homogeneous Tibetan community population in China. AUD cases (N = 281) with an alcohol use disorder identification test (AUDIT) score ≥10, as well as healthy controls (N = 277) with an AUDIT score ≤5, were recruited. All participants were genotyped for 366 single nucleotide polymorphisms (SNPs) of 34 genes selected from those involved in neurotransmitter systems. Association analyses were performed using PLINK version 1.07 software. Allelic analyses before adjustment for multiple tests showed that 15 polymorphisms within seven genes were associated with AUD (p<0.05). After adjustment for the number of SNPs genotyped within each gene, only the association of a single marker (rs10044881) in HTR4 remained statistically significant. Haplotype analysis for two SNPs in HTR4 (rs17777298 and rs10044881) showed that the haplotype AG was significantly associated with the protective effect for AUD. In conclusion, the present study discovered that the HTR4 gene may play a marked role in the pathogenesis of AUD. In addition, this Tibetan population sample marginally replicated previous evidence regarding the associations of six genes in AUD.

Dopamine D2 receptors and striatopallidal transmission in addiction and obesity

Drug addiction and obesity share the core feature that those afflicted by the disorders express a desire to limit drug or food consumption yet persist despite negative consequences. Emerging evidence suggests that the compulsivity that defines these disorders may arise, to some degree at least, from common underlying neurobiological mechanisms. In particular, both disorders are associated with diminished striatal dopamine D2 receptor (D2R) availability, likely reflecting their decreased maturation and surface expression. In striatum, D2Rs are expressed by approximately half of the principal medium spiny projection neurons (MSNs), the striatopallidal neurons of the so-called 'indirect' pathway. D2Rs are also expressed presynaptically on dopamine terminals and on cholinergic interneurons. This heterogeneity of D2R expression has hindered attempts, largely using traditional pharmacological approaches, to understand their contribution to compulsive drug or food intake. The emergence of genetic technologies to target discrete populations of neurons, coupled to optogenetic and chemicogenetic tools to manipulate their activity, have provided a means to dissect striatopallidal and cholinergic contributions to compulsivity. Here, we review recent evidence supporting an important role for striatal D2R signaling in compulsive drug use and food intake. We pay particular attention to striatopallidal projection neurons and their role in compulsive responding for food and drugs. Finally, we identify opportunities for future obesity research using known mechanisms of addiction as a heuristic, and leveraging new tools to manipulate activity of specific populations of striatal neurons to understand their contributions to addiction and obesity.

The association between DRD2/ANKK1 and genetically informed measures of alcohol use and problems

In 1990, Blum and colleagues first reported an association between DRD2 and alcoholism. While there have been subsequent replications of this genetic association, there have also been numerous studies that failed to detect an association between DRD2 and alcohol dependence. We propose that one aspect contributing to this inconsistency is the variation in alcohol phenotype used across studies. Within the population-based Finnish twin sample, FinnTwin16, we previously performed multivariate twin analyses to extract latent genetic factors, which account for the variation across seven measures of alcohol consumption (frequency of drinking, frequency × quantity, frequency of heavy drinking, frequency of intoxication and maximum drinks in a 24-hour period) and problems (the Rutgers Alcohol Problem Index-RAPI and the Mälmö-modified Michigan Alcohol Screen Test-MmMAST) in 3065 twins. In the present study, we examined the association between 31 DRD2/ANKK1 single-nucleotide polymorphisms (SNPs) and the genetic factor scores generated by twin analyses in a subset of FinnTwin16 (n = 602). We focus on two of the genetic factors: a general alcohol consumption and problems factor score, which represents shared genetic variance across alcohol measures, and a alcohol problems genetic factor score, which loads onto the two indices of problematic drinking (MAST and RAPI). After correction for multiple testing across SNPs and phenotypes, of the 31 SNPs genotyped across DRD2/ANKK1, one SNP (rs10891549) showed significant association with the general alcohol consumption and problems factor score (P = 0.004), and four SNPs (rs10891549, rs1554929, rs6275, rs6279), representing two independent signals after accounting for linkage disequilibrium, showed significant association with the alcohol problems genetic factor score (P = 0.005, P = 0.005, P = 0.003, P = 0.003). In this study, we provide additional positive evidence for the association between DRD2/ANKK1 and alcohol outcomes, including frequency of drinking and drinking problems. Additionally, post hoc analyses indicate stronger association signals using genetic factor scores than individual measures, which suggest that accounting for the genetic architecture of the alcohol measures reduces genetic heterogeneity in alcohol dependence outcomes in this sample and enhances the ability to detect association.

Association of DRD2 and ANKK1 polymorphisms with prolactin increase in olanzapine-treated women

Dopamine D2 receptors, encoded by DRD2, play a role in regulating serum prolactin concentration. Single nucleotide polymorphisms (SNPs), rs2734842(C), rs6275(T), and rs6279(C) located within DRD2, have been shown to be associated with prolactin increase in olanzapine/fluoxetine combination (OFC)-treated women. The present analyses seek to replicate these results and test other SNPs in DRD2 and neighboring gene ANKK1 for associations with prolactin increase in women, using data from 3 pooled studies of olanzapine, and 2 previously examined studies OFC. An ANCOVA was used to test whether change from baseline in the natural log of prolactin concentration (ln[prolactin]) was associated with SNPs in the pooled olanzapine studies. A meta-analysis was also performed using the inverse chi-square method, pooling p-values from the 2 previously examined studies and the 3 olanzapine studies. Negative strand alleles rs2734842(C), rs6275(T), and rs6279(C) were significantly associated with increased prolactin in olanzapine-treated women, replicating previous results. These SNPs also showed moderate association with increased prolactin in olanzapine-treated and OFC-treated women in the meta-analysis, as did rs4938016, rs2734848, rs2734841, rs1124493, and rs1076562. Five of these SNPs fall in or are adjacent to an LD block spanning DRD2 intron 7, exon 7, 5' untranslated region and ANKK1.

CLINICAL TRIAL REGISTRATION

www.clinicaltrial.gov.

Reward mechanisms in obesity: new insights and future directions

Food is consumed in order to maintain energy balance at homeostatic levels. In addition, palatable food is also consumed for its hedonic properties independent of energy status. Such reward-related consumption can result in caloric intake exceeding requirements and is considered a major culprit in the rapidly increasing rates of obesity in developed countries. Compared with homeostatic mechanisms of feeding, much less is known about how hedonic systems in brain influence food intake. Intriguingly, excessive consumption of palatable food can trigger neuroadaptive responses in brain reward circuitries similar to drugs of abuse. Furthermore, similar genetic vulnerabilities in brain reward systems can increase predisposition to drug addiction and obesity. Here, recent advances in our understanding of the brain circuitries that regulate hedonic aspects of feeding behavior will be reviewed. Also, emerging evidence suggesting that obesity and drug addiction may share common hedonic mechanisms will also be considered.

Genetic diagnostics of functional variants of the human dopamine D2 receptor gene

INTRODUCTION

The importance of dopamine D2 receptors (DRD2) for central nervous dopaminergic signalling makes variants in the DRD2 gene potential modulators of the risk or course of various behavioural, psychiatric or neurologic diseases (e.g. addiction, schizophrenia, Parkinson's disease). We developed Pyrosequencing genetic screening assays for single nucleotide polymorphisms spanning the whole range of the DRD2 gene locus up to the functionally related ankyrin repeat and kinase domain containing 1 gene (ANKK1) located at approximately 10 kb downstream of DRD2.

METHODS

Assays for 11 genetic variants with reported functional association were developed in DNA samples from 300 unrelated healthy Caucasians and validated by independent conventional sequencing.

RESULTS

In all DNA samples the DRD2/ANKK1 genetic variants were identified correctly as verified by the control samples. The observed frequencies of homozygous, heterozygous and noncarriers of the minor alleles were in agreement with the Hardy-Weinberg equilibrium. Observed minor allele frequencies were DRD2 rs12364283T>C: 6.5%, rs1799978A>G: 4.8%, rs1799732C del: 14.2%, rs4648317C>T: 12.8%, rs1079597G>A: 13.8%, rs1076560G>T: 14.5%, rs1800496C>T: 0.2%, rs1801028C>G: 3.0%, rs6275C>T: 32.7%, rs6277C>T: 53.0% and ANKK1 rs1800497C>T: 17.5%.

CONCLUSION

The presently developed Pyrosequencing assays are provided to facilitate further research toward personalized approaches to pathophysiological conditions involving behavioural, psychiatric and neurologic disorders including addiction, schizophrenia and Parkinson's disease.

The role of microRNAs in drug addiction: a big lesson from tiny molecules

Alcoholism is a multifactorial disease of unclear molecular underpinnings. Currently, we are witnessing a major shift in our understanding of the functional elements of the genome, which could help us to discover novel insights into the nature of alcoholism. In humans, the vast majority of the genome encodes non-protein-coding DNA with unclear function. Recent research has started to unveil this mystery by describing the functional relevance of microRNAs, and examining which genes are regulated by non-protein-coding DNA. Here, I describe alcohol regulation of microRNAs and provide examples of microRNAs that control the expression of alcohol-relevant genes. Emphasis is put on the potential of microRNAs in explaining the polygenic nature of alcoholism and prospects of microRNA research and future directions of this burgeoning field.

Dopamine D2 receptor genotype is associated with increased mortality at a 10-year follow-up of alcohol-dependent individuals

AIMS

Because the TAQ1 A1 allele may be associated with alcohol-related medical illnesses, and medical illnesses in alcohol-dependent individuals are associated with increased mortality, we test the hypothesis that the TAQ1 A1 allele of the DRD2 gene is associated with increased mortality in alcohol-dependent individuals.

METHODS

Following an index treatment episode, a 10-year follow-up study in 366 alcohol-dependent individuals was performed. The TAQ1 A1/A2 DRD2 genotype and allele frequencies were compared between those deceased and those still living at the 10-year point. In addition, the genotype and allele frequencies of these alcohol-dependent individuals were compared to that in 578 control subjects.

RESULTS

The prevalence of the A1 allele differed between the deceased and living patients and the controls: 47% of the deceased were A1+, compared to 37% of the living patients and 32% of the controls. The frequency of the TAQ1 A1/A2 genotype also differed between the groups. Thus, 43% had the A1/A2 genotype in comparison with 32% in the living patients and 29% in the controls. The TAQ 1 A1 allele frequency differed between the groups. The frequency of A1 allele was 25% in the deceased patients compared to 21% in the living patients and 17% in the controls.

CONCLUSION

The TAQ I A1 allele of the DRD2 gene (or DRD2 gene region) was associated with increased mortality over a 10-year period in alcohol-dependent individuals.

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.

Candidate genes for cannabis use disorders: findings, challenges and directions

AIM

Twin studies have shown that cannabis use disorders (abuse/dependence) are highly heritable. This review aims to: (i) review existing linkage studies of cannabis use disorders and (ii) review gene association studies, to identify potential candidate genes, including those that have been tested for composite substance use disorders and (iii) to highlight challenges in the genomic study of cannabis use disorders.

METHODS

Peer-reviewed linkage and candidate gene association studies are reviewed.

RESULTS

Four linkage studies are reviewed: results from these have homed in on regions on chromosomes 1, 3, 4, 9, 14, 17 and 18, which harbor candidates of predicted biological relevance, such as monoglyceride lipase (MGLL) on chromosome 3, but also novel genes, including ELTD1[epidermal growth factor (EGF), latrophilin and seven transmembrane domain containing 1] on chromosome 1. Gene association studies are presented for (a) genes posited to have specific influences on cannabis use disorders: CNR1, CB2, FAAH, MGLL, TRPV1 and GPR55 and (b) genes from various neurotransmitter systems that are likely to exert a non-specific influence on risk of cannabis use disorders, e.g. GABRA2, DRD2 and OPRM1.

CONCLUSIONS

There are challenges associated with (i) understanding biological complexity underlying cannabis use disorders (including the need to study gene-gene and gene-environment interactions), (ii) using diagnostic versus quantitative phenotypes, (iii) delineating which stage of cannabis involvement (e.g. use versus misuse) genes influence and (iv) problems of sample ascertainment.

The A1 allele of the D2 dopamine receptor gene region, alcohol expectancies and drinking refusal self-efficacy are associated with alcohol dependence severity

Psychological risk and genetic risk for alcohol dependence are rarely examined in concert. The current study used path analysis (via structural equation modelling) to explore the relationship between the A(1) allele of the D2 dopamine receptor DRD2 gene region, age of problem drinking onset, alcohol expectancy and drinking refusal self-efficacy towards alcohol consumption and dependence severity. One hundred and forty-three (93 male, 50 female) alcohol dependent inpatients provided an extensive clinical history, including age of onset of problem drinking and quantity and frequency of alcohol consumption. The Drinking Expectancy Profile and the Alcohol Dependence Scale were completed, and 10 milliliters of blood were obtained for genetic analysis. The results showed that the posited model fitted the data set well and support the pattern of direct (allele status to drinking) and indirect (allele status influenced by psychosocial variables) relationships hypothesised for the model. A formal test of mediation showed some support for a psychosocial mediational model. The results are discussed in terms of a possible developmental trajectory that involves both genetic risk that influences brain dopamine activity and reinforcement expectancies that both operate via diminished drinking refusal self-efficacy. The prevention and treatment possibilities that arise from understanding this trajectory are examined.

Genetic association of DRD2 polymorphisms with anxiety scores among alcohol-dependent patients

The dopaminergic neurotransmission system is one of the major factors in development of alcoholism and also contributes to anxiety and depression. In this study, the associations of the dopamine receptor type 2 (DRD2) polymorphisms with the symptoms of anxiety were analyzed. A total of 573 alcoholics and 273 controls were enrolled in the study from the Korean population. Five DRD2 SNPs, including -32869 A>G, -32768 insdel C, +11890 C>G, +11915 C>T, and +32806 C>T, were genotyped using a TaqMan assay and analyzed with various alcoholic phenotypes. Although no DRD2 polymorphisms were associated with the risk of alcoholism, +32806C>T and Block2-ht1 showed associations (in dominant models) with both the state anxiety level scale (STAI-S) and the trait anxiety level scale (STAI-T) (P=0.004 and P=0.003, and P=0.01 and P=0.005, respectively). In addition, the -32768 insdel C also showed positive association with both anxiety level scales in recessive models (P=0.01 and P=0.02, respectively).

Family-Based Association Analyses of Alcohol Dependence Phenotypes Across DRD2 and Neighboring Gene ANKK1

BACKGROUND

There is an extensive and inconsistent literature on the association of the dopamine D2 receptor gene (DRD2) with alcohol dependence. Conflicting results have been attributed to differences in the severity of the alcohol dependence phenotype across studies, failure to exclude related disorders from comparison groups, and artifacts of population-stratification. Recently the genetic polymorphism most widely analyzed in DRD2, Taq1A, has been discovered to reside in a neighboring gene, ankyrin repeat and kinase domain containing 1 (ANKK1), located 10 kb downstream from DRD2.

METHODS

To more carefully characterize evidence for association across this region, we genotyped 26 single nucleotide polymorphisms (SNPs) spanning DRD2 and ANKK1 in a sample of 219 Caucasian families (n = 1,923) from the Collaborative Study on the Genetics of Alcoholism (COGA), making this the most extensive analysis to date of association between this region and alcohol dependence. We used family-based analyses robust to population-stratification, and we made use of rich phenotypic data to analyze alcohol dependence and subtypes hypothesized in the literature to be more directly influenced by DRD2.

RESULTS

We found that the evidence for association is strongest in the 5' linkage disequilibrium block of ANKK1 (that does not contain Taq1A), with weak evidence of association with a small number of SNPs in DRD2. The association in ANKK1 is strongest among the subsets of alcoholics with medical complications and with antisocial personality disorder.

CONCLUSIONS

More extensive genotyping across DRD2 and ANKK1 suggests that the association with alcohol dependence observed in this region may be due to genetic variants in the ANKK1 gene. ANKK1 is involved in signal transduction pathways and is a plausible biological candidate for involvement in addictive disorders.

Neurobiology of addiction. An integrative review

Evidence that psychoactive substance use disorders, bulimia nervosa, pathological gambling, and sexual addiction share an underlying biopsychological process is summarized. Definitions are offered for addiction and addictive process, the latter being the proposed designation for the underlying biopsychological process that addictive disorders are hypothesized to share. The addictive process is introduced as an interaction of impairments in three functional systems: motivation-reward, affect regulation, and behavioral inhibition. An integrative review of the literature that addresses the neurobiology of addiction is then presented, organized according to the three functional systems that constitute the addictive process. The review is directed toward identifying candidate neurochemical substrates for the impairments in motivation-reward, affect regulation, and behavioral inhibition that could contribute to an addictive process.

Between- and within-family association test of the dopamine receptor D2 TaqIA polymorphism and alcohol abuse and dependence in a general population sample of adults

OBJECTIVE

Dopaminergic dysfunction has been hypothesized to play an important role in the etiology of alcohol-use disorders. A restriction fragment length polymorphism (RFLP) in the 3' untranslated region (3'UTR) of the DRD2 gene affects gene expression and has been implicated as a risk factor for alcohol dependence. This polymorphism (TaqIA) has been reported as positively associated with alcohol-use disorders in case-control samples, but these results have not been replicated in family-based association studies. The mixed results of association between the DRD2 TaqIA polymorphism and alcohol-use disorders may be the result of differences in sample size, phenotype definition, heterogeneity of the samples, and genetic admixture.

METHOD

We conducted tests of association in a sample of 838 adults participating in the National Youth Survey Family Study (NYSFS). We examined whether the DRD2 TaqIA polymorphism was associated with a symptom-count measure of alcohol abuse and dependence derived from the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, and the Craving Withdrawal Model.

RESULTS

Tests of association were nonsignificant across each classification system examined. Power calculations suggested that these results were despite the ability to detect an effect size of 1%.

CONCLUSIONS

This study supports other family-based association tests that have reported no association between the DRD2 TaqIA polymorphism and alcohol abuse and dependence.

THE TAQI DRD2 A1 ALLELE IS ASSOCIATED WITH ALCOHOL-DEPENDENCE ALTHOUGH ITS EFFECT SIZE IS SMALL

BACKGROUND

Numerous studies of the relationship between the TaqIA DRD2 A1 allele and alcohol-dependence have been performed and many of these have shown an association whereas others have not (Noble, 2003). This has consequently generated some controversy as to whether such an association actually exists (Noble, 2003). In the two recent meta-analyses by Noble (2003) and Young et al. (2004) some very important methodological issues have been discussed, which need to be addressed in forthcoming studies. Thus, the sample size is of great importance. In case-control studies it has been estimated that to detect the role of genes with small effect size of approximately 2, which is in the range of the DRD2 A1 allele-alcoholism relationship, case-control sets of 300-400 subjects are necessary (Noble, 2003).

METHODS

In the present study, we have consequently recruited a large number of subjects, 375 alcohol-dependent individuals, who were treated as inpatients for alcohol withdrawal symptoms and out of these 357 could be evaluated. As controls, 578 individuals screened and 254 individuals unscreened for alcohol consumption were used. Thus, the total number of subjects was 1217.

RESULTS

In the present study, in which the TaqI A1/A2 DRD2 polymorphism was in Hardy-Weinberg equilibrium in the patient group and the two control groups, we found that the TaqI DRD2 A1/A2 genotype frequency differed significantly between the alcohol-dependent group and both the total and screened control groups. Furthermore, the TaqI DRD2 A1 allele frequency was significantly overrepresented in the alcohol-dependent subjects as compared with both the total and screened control groups. The odds ratio for alcohol-dependency being associated with the A1 allele was 1.34.

CONCLUSIONS

Consequently, the findings in this study lend further support to the notion of an association between the DRD2 A1 allele and alcohol-dependence, although the effect size of the DRD2 A1 allele is small.

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.

Evolution of the DRD2 gene haplotype and its association with alcoholism in Mexican Americans

The human D2 dopamine receptor gene (DRD2) plays a central role in the neuromodulation of appetitive behaviors and is implicated in having a possible role in susceptibility to alcoholism. We genotyped an SNP in DRD2 Exon 8 in 251 nonalcoholic, unrelated, healthy controls and 200 alcoholic Mexican Americans. The DRD2 haplotypes were analyzed using the Exon 8 genotype in combination with five other SNP genotypes, which were obtained from our previous study. The ancestral origins of the DRD2 polymorphisms have been determined by sequencing the homologous region in other higher primates. Twenty DRD2 haplotypes, defined as H1 to H20 based on their frequency from high to low, were obtained in this major minority population. The ancestral haplotype "I-B2-G-C-G-A1" and two one-step mutation haplotypes were absent in our study population. The haplotype H1, "I-B1-T-C-A-A1", with the highest frequency in the population, is a three-step mutation from the ancestral form. The first five or eight major haplotypes make up 87% or 95% of the entire population, respectively. The prevalence of the haplotype H1+ (H1/H1 and H1/Hn genotypes) is significantly higher in alcoholics and alcoholic subgroups, including early onset drinkers and benders, than in their respective control groups. The Promoter -141C allele is in linkage disequilibrium (LD) with five other loci in the nonalcoholic group, but not in the alcoholic group. All of the other five loci are in LD in both the alcoholic and control groups. The DRD2 TaqI B allele is in complete LD with the allele located in intron 6. Five SNPs, Promoter -141C, TaqI B (or Intron 6), Exon 7, Exon 8, and TaqI A, are sufficient to define the DRD2 haplotypes in Mexican Americans. Our data indicate that the DRD2 haplotypes are associated with alcoholism in Mexican Americans.

Nicotine addiction through a neurogenomic prism: ethics, public health, and smoking

Studies are under way to examine the neurogenetic factors contributing to smoking behaviors. The combined approaches of genomics, molecular biology, neuroscience, and pharmacology are expected to fuel developments in pharmacogenetics, to create new genetic tests, and ultimately to provide the basis for innovative strategies for smoking cessation and prevention. The emergence of a neurogenomic understanding of nicotine addiction is likely to induce fundamental changes in popular, clinical, and public health views of smoking, which could significantly shape existing practices and policies to reduce tobacco use. Still a nascent area of research, nicotine addiction provides an excellent case study through which to anticipate key ethical and policy issues in both behavioral genetics and the neurogenomics of addictive behaviors.

ADH1B*1, ADH1C*2, DRD2 (-141C Ins), and 5-HTTLPR are associated with alcoholism in Mexican American men living in Los Angeles

BACKGROUND

The aim of the present study was to use a candidate gene approach to identify the genetic risk factors for alcoholism in Mexican Americans residing in the Los Angeles area. The genes selected include alcohol metabolizing genes and neurotransmitter genes, which have been shown in the literature to be associated with alcoholism in other ethnic groups.

METHODS

Thirteen allelic variants from seven genes were evaluated for their role in alcoholism using alcoholic (n = 200) and nonalcoholic (n = 251) Mexican Americans. Those polymorphic sites include alcohol dehydrogenase (ADH1B, ADH1C), aldehyde dehydrogenase (ALDH2), cytochrome P-450 2E1 (CYP2E1) TaqI, DraI, RsaI, dopamine D2 receptor (DRD2) TaqI A, B, intron 6, exon 7, -141C Ins/Del, serotonin transporter (5-HTTLPR), and GABAA receptor beta3 subunit (GABRbeta3).

RESULTS

The results demonstrate that Mexican Americans have extremely low allele frequency for both ALDH2*2 and ADH1B*2 and a relatively high frequency of ADH1C*2 and CYP2E1 c2 alleles. ADH1B*1, ADH1C*2, DRD2 (-141C Ins), and 5-HTTLPR were associated with alcoholism in Mexican Americans (p < 0.05). DRD2 Ins was associated with alcoholism in those alcoholics who carried the ADH1B*2 or ADH1C*1 protective alleles (p = 0.032 in genotype level and p = 0.015 in allele level). DRD2 TaqI A and B alleles were associated with early age of onset for drinking (p = 0.016 for TaqI A1 and p = 0.049 for TaqI B1 allele).

CONCLUSIONS

Together, the data reveal unique genetic patterns in Mexican Americans that may be in part responsible for the heightened risk for alcoholism and alcohol-associated health problems in this population.

Possible interaction of alcohol dehydrogenase and aldehyde dehydrogenase genes with the dopamine D2 receptor gene in anxiety-depressive alcohol dependence

BACKGROUND

The role of the dopamine D2 receptor (DRD2) gene in the development of alcohol abuse or dependence is controversial. The controversy is due in part to the disparate definitions pertaining to the control groups used and to the definitions of subtypes in alcohol dependence. In the Han Chinese population, the alcohol dehydrogenase 1B*2/*2 (ADH1B*2/*2) genotype and the aldehyde dehydrogenase 2*2 (ALDH2*2) allele have been considered as protective factors against alcohol abuse or dependence. Moreover, the ADH1B and ALDH2 genes might be involved in dopamine metabolism. We hypothesized that the ADH1B and ALDH2 genes might interact with the DRD2 gene and that the association between the DRD2 gene and alcohol dependence might be affected by different ADH1B and ALDH2 genotypes. This study examined whether the DRD2 gene is associated with specific subtypes of alcohol dependence and evaluated the relationship between the DRD2 gene and alcohol-metabolizing genes in a specific subtype of alcohol dependence.

METHODS

Of the 465 Han Chinese subjects who were recruited for the study, 71 were classified with pure alcohol dependence, 113 with both alcohol dependence and anxiety-depression (ANX/DEP ALC), and 129 with anxiety-depression but without alcohol dependence (ANX/DEP). The remaining 152 subjects were supernormal controls. All subjects were interviewed with the Chinese version of the modified Schedule of Affective Disorders and Schizophrenia-Lifetime; all alcohol dependence, anxiety, and major depressive diagnoses were made according to DSM-IV criteria.

RESULTS

The DRD2 gene was not found to be associated with pure alcohol dependence or ANX/DEP, but was found to be associated with ANX/DEP ALC. Furthermore, the association between the DRD2 gene and ANX/DEP ALC was shown to be under the control of the ALDH2*1/*1 and ADH1B*1/*2 genotypes.

CONCLUSIONS

ANX/DEP ALC is a specific subtype of alcohol dependence. Because ANX/DEP ALC was associated with the DRD2 gene only under the stratification of ADH1B*1/*2 or ALDH2*1/*1, the DRD2 gene might interact with the ADH1B gene and the ALDH2 gene, respectively, in the development of ANX/DEP ALC in the Taiwan Han Chinese population.

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.

Association of seven polymorphisms of the D2 dopamine receptor gene with brain receptor-binding characteristics

Association of alleles at the Taq1 A, Taq1 B, intron 6, Taq1 D, exon 7, exon 8, and promoter-141C sites of the D2 dopamine receptor gene with D2 dopamine receptor binding characteristics in the caudate nucleus of Caucasian alcoholic and nonalcoholic subjects was determined. For the Taq1 D, exon 7, exon 8, and promoter- 141C sites there were no significant allelic differences in Bmax (number of binding sites) or Kd (binding affinity) of the D2 dopamine receptors. However, subjects having the minor alleles at the Taq1 A, Taq1 B, and intron 6 sites had significantly lower Bmax than subjects not having them. None of these three polymorphisms had any significant effect on Kd. Highly significant linkage disequilibria were observed among the Taq1 A, Taq1 B, and intron 6 polymorphic sites, but linkage disequilibria between these three sites and each of the Taq1 D, exon 7, exon 8, and promoter-141C sites were of lesser or of no significance. Taken together, these findings suggest that the Taq1 A, Taq1 B, and intron 6 polymorphisms, but not the Taq1 D, exon 7, exon 8, and promoter-141C polymorphisms, are in linkage disequilibrium with a functional allelic variant that affects D2 dopamine receptor expression.

Neurodevelopmental liabilities in alcohol dependence: central serotonin and dopamine dysfunction

Alcoholism is a complex disorder with symptoms ranging from abuse to dependence, often comorbid with depression, antisocial personality, or anxiety. Neurodevelopmental causes of the disorder are unknown but inferences are possible from current knowledge. Neurobiological studies implicate multiple brain changes, which may be characterized as premorbid or morbid. These studies have also examined specific aspects of the alcohol dependence syndrome, including alcohol reinforcement and craving. Here, we review the evidence for vulnerability factors in alcohol dependence, with an emphasis on central serotonin (5-HT) and dopamine (DA). Serotonin dysfunction likely contributes to the development of alcoholism since studies of alcohol-preferring rodents show decreased 5-HT function on many measures. We have shown that serotonin-enhancing drugs reduce consumption and craving in mild to moderate alcoholics, yet similar studies in severely dependent individuals remain inconclusive. Studies indicate that serotonin dysfunction may contribute to the development of dependence via impaired impulse control and/or mood regulation. The mesocorticolimbic dopamine pathway represents another important pathophysiological target in alcoholism. Differences in D(2) receptor density, dopamine sensitivity, and gene expression have been linked to consumption, reinforcement, craving, and relapse. However, while DA agonists reduce self-administration in animals, we found no effect in humans with long-acting bromocriptine, a D(2) agonist. Dopamine may contribute differentially to the development of dependence via its effects on alcohol wanting, reinforcement, and reward memory. Although animal experiments show consistent roles for serotonin and dopamine in alcohol dependence, human studies are not always concordant. Such discrepancies highlight the complexity of dependence-related behaviors in humans and of identifying vulnerabilities to alcoholism.

Association of a CB1 cannabinoid receptor gene (CNR1) polymorphism with severe alcohol dependence

Due to the involvement of the endogenous cannabinoid system in brain reward mechanisms a silent polymorphism (1359G/A; Thr453Thr) in the single coding exon of the CB1 human cannabinoid receptor gene (CNR1) was analysed in 121 severely affected Caucasian alcoholics and 136 most likely non-alcoholic controls. The observed frequency of the A allele was 31.2% for controls and 42.1% for alcoholics with severe withdrawal syndromes (P=0.010). Post-hoc exploration indicated that this allelic association resulted from an excess of the homozygous A/A genotype in patients with a history of alcohol delirium (P=0.031, DF 2), suggesting s an increased risk of delirium (OR=2.45, 95% CI 1.14--5.25). This finding suggests that the homozygous genotype CNR1 1359A/A confers vulnerability to alcohol withdrawal delirium.

Estado atual e perspectivas da genética e epidemiologia do alcoolismo

O alcoolismo é um problema de saúde pública de escala mundial. O abuso e dependência combinados afetam aproximadamente 8% da população brasileira, gerando um grande custo social. O reconhecimento da existência de uma herdabilidade significativa contribuiu para o entendimento do problema como uma doença específica com origem biológica. Os avanços no conhecimento da neurobiologia da dependência permitiram delimitar uma série de genes candidatos para a predisposição. Atualmente, iniciam-se os estudos sobre o papel de polimorfismos genéticos na resposta ao tratamento. A integração de abordagens clínicas, epidemiológicas e de genética molecular pode identificar grupos clínicos mais responsivos a abordagens terapêuticas específicas.

Genetic polymorphisms of the promoter region of dopamine D2 receptor and dopamine transporter genes and alcoholism among four aboriginal groups and Han Chinese in Taiwan

This study aims to examine the relationship between the functional polymorphism at the promoter region of the dopamine D2 receptor (DRD2) gene (i.e. -141C Ins/Del) and variable number of tandem repeat polymorphism at the 3' untranslated region of the dopamine transporter (DAT) gene (SLC6A3) with alcoholism in a case-control study. The cases (n = 203) were alcohol dependents with withdrawal symptoms, and the controls (n = 213) were sex- and ethnicity-matched individuals who were screened to exclude those with alcohol problems among four aboriginal groups (Atayal, Ami, Bunun, and Paiwan) and Han Chinese in Taiwan. To control for potential confounding factors, we excluded tobacco abusers from control subjects in part of the analysis and compared the distribution of the genetic polymorphisms in alcoholics with severe medical complications versus those with less severe medical complications. There were no differences in allele and genotype frequencies of these two distinct genetic markers between alcoholics and control subjects in these five different ethnic groups. There was no significant linkage disequilibrium between the -141C polymorphism and two other DRD2 polymorphisms (TaqI A and NcoI). The results remained unchanged when cases were limited to alcoholics with more severe medical complications or when tobacco abusers were excluded from control subjects. The results suggest that both the DRD2 promoter region and the DAT gene do not play a significant role in conferring vulnerability to alcoholism.

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.