Ethanol consumption and DRD2 gene TaqI a polymorphism among socially drinking males

Understanding the Global Problem of Drug Addiction is a Challenge for IDARS Scientists

IDARS is an acronym for the International Drug Abuse Research Society. Apart from our scientific and educational purposes, we communicate information to the general and scientific community about substance abuse and addiction science and treatment potential. Members of IDARS are research scientists and clinicians from around the world, with scheduled meetings across the globe. IDARS is developing a vibrant and exciting international mechanism not only for scientific interactions in the domain of addiction between countries but also ultimately as a resource for informing public policy across nations. Nonetheless, a lot more research needs to be done to better understand the neurobiological basis of drug addiction - A challenge for IDARS scientists.

Population-based case-control study of DRD2 gene polymorphisms and alcoholism

Several independent lines of evidence for genetic contributions to vulnerability to alcoholism exist. Dopamine is thought to play a major role in the mechanism of reward and reinforcement in response to alcohol. D2 dopamine receptor (DRD2) gene has been among the stronger candidate genes implicated in alcoholism. In this study, alcohol use was assessed in 196 randomly selected Kota individuals of Nilgiri Hills, South India. Six DRD2 SNPs were assessed in 81 individuals with alcoholism and 151 controls to evaluate the association between single nucleotide polymorphisms (SNPs) and alcoholism. Of the three models (dominant, recessive, and additive) tested for association between alcoholism and DRD2 SNPs, only the additive model shows association for three loci (rs1116313, TaqID, and rs2734835). Of six studied polymorphisms, five are in strong linkage disequilibrium forming onesingle haplotype block. Though the global haplotype analysis with these five SNPs was not significant, haplotype analysis using all six SNPs yielded a global P value of .033, even after adjusting for age. These findings support the importance of dopamine receptor gene polymorphisms in alcoholism. Further studies to replicate these findings in different populations are needed to confirm these results.

Association of the dopamine D2 receptor gene with alcohol dependence: haplotypes and subgroups of alcoholics as key factors for understanding receptor function

OBJECTIVES

The dopamine D2 receptor (DRD2) plays an important role in the reinforcing and motivating effects of ethanol. Several polymorphisms have been reported to affect receptor expression. The amount of DRD2, expressed in a given individual, is the result of the expression of both alleles, each representing a distinct haplotype. We examined the hypothesis that haplotypes composed of polymorphisms, associated with reduced receptor expression, are more frequent in alcoholics compared with healthy individuals.

METHODS

The polymorphisms -141ins/del, C957T, A1385G, and TaqlA were genotyped in a case-control sample comprising 360 alcoholics and 368 controls, and in a family-based sample of 65 trios. To investigate more homogenous groups, we constructed two subgroups with respect to age at onset and antisocial personality disorder. In addition, a subgroup with positive family history of alcoholism was investigated.

RESULTS

The haplotypes I-C-G-A2 and I-C-A-A1 occurred with a higher frequency in alcoholics [P=0.026, odds ratio (OR): 1.340; P=0.010, OR: 1.521, respectively]. The rare haplotype I-C-A-A2 occurred less often in alcoholics (P=0.010, OR: 0.507), and was also less often transmitted from parents to their affected offspring (1 vs.7). Among the subgroups, I-C-G-A2 and I-C-A-A1 had a higher frequency in Cloninger 1 alcoholics (P=0.083 and 0.001, OR: 1.917, respectively) and in alcoholics with a positive family history (P=0.031, OR: 1.478; P=0.073, respectively). Cloninger 2 alcoholics had a higher frequency of the rare haplotype D-T-A-A2 (P<0.001, OR: 4.614) always compared with controls. In patients with positive family history haplotype I-C-A-A2 (P=0.004, OR: 0.209), and in Cloninger 1 alcoholics haplotype I-T-A-A1 (P=0.045 OR: 0.460) were less often present.

CONCLUSION

We confirmed the hypothesis that haplotypes, which are supposed to induce a low DRD2 expression, are associated with alcohol dependence. Furthermore, supposedly high-expressing haplotypes weakened or neutralized the action of low-expressing haplotypes.

Cortical NMDA receptor expression in human chronic alcoholism: influence of the TaqIA allele of ANKK1

Real-time RT-PCR normalized to GAPDH was used to assay N-methyl-D-aspartate (NMDA) receptor NR1, NR2A and NR2B subunit mRNA in human autopsy cortex tissue from chronic alcoholics with and without comorbid cirrhosis of the liver and matched controls. Subunit expression was influenced by the subject's genotype. The TaqIA polymorphism selectively modulated NMDA receptor mean transcript expression in cirrhotic-alcoholic superior frontal cortex, in diametrically opposite ways in male and female subjects. Genetic make-up may differentially influence vulnerability to brain damage by altering the excitation: inhibition balance, particularly in alcoholics with comorbid cirrhosis of the liver. The TaqIA polymorphism occurs within the poorly characterised ankyrin-repeat containing kinase 1 (ANKK1) gene. Using PCR, ANKK1 mRNA transcript was detected in inferior temporal, occipital, superior frontal and primary motor cortex of control human brain. ANKK1 expression may mediate the influence of the TaqIA polymorphism on phenotype.

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.

Alcohol consumption is associated with an interaction between DRD2 exon 8 A/A genotype and self-directedness in males

BACKGROUND

Both reduced postsynaptic dopamine D(2) receptor function and the character variable self-directedness (SDD) are related to the level of alcohol consumption. We examined for interactions between DRD2 exon 8(rs6276), a polymorphism which has been associated with various alcohol-related phenotypes, SDD and alcohol consumption.

METHODS

A total of 144 male and 186 female probands with alcohol dependence or abuse diagnoses and without were included in the study. All subjects were assessed with the alcohol section of the Semi-Structured Assessment for the Genetics of Alcoholism and the Temperament and Character Inventory.

RESULTS

Male probands with A/A genotype reported significantly higher alcohol consumption in a typical week (ANOVA; p = 0.024); those with A/A genotype and low SDD showed particularly high consumption levels (interaction DRD2 x SDD: p = 0.019). Alcohol dependence/abuse (DSM-IV) but not nicotine dependence was also relevant for higher alcohol consumption (trend: p = 0.052). In the female group, only alcohol disorders predicted alcohol consumption.

CONCLUSIONS

Our findings support a role for a gene-personality interaction of DRD2 exon 8 x SDD in alcohol consumption in males.

Polymorphisms in human dopamine D2 receptor gene affect gene expression, splicing, and neuronal activity during working memory

Subcortical dopamine D2 receptor (DRD2) signaling is implicated in cognitive processes and brain disorders, but the effect of DRD2 variants remains ambiguous. We measured allelic mRNA expression in postmortem human striatum and prefrontal cortex and then performed single nucleotide polymorphism (SNP) scans of the DRD2 locus. A previously uncharacterized promoter SNP (rs12364283) located in a conserved suppressor region was associated with enhanced DRD2 expression, whereas previously studied DRD2 variants failed to affect expression. Moreover, two frequent intronic SNPs (rs2283265 and rs1076560) decreased expression of DRD2 short splice variant (expressed mainly presynaptically) relative to DRD2 long (postsynaptic), a finding reproduced in vitro by using minigene constructs. Being in strong linkage disequilibrium with each other, both intronic SNPs (but not rs12364283) were also associated with greater activity of striatum and prefrontal cortex measured with fMRI during working memory and with reduced performance in working memory and attentional control tasks in healthy humans. Our results identify regulatory DRD2 polymorphisms that modify mRNA expression and splicing and working memory pathways.

Test of association between TaqIA A1 allele and alcohol use disorder phenotypes in a sample of adolescent patients with serious substance and behavioral problems

Several studies have demonstrated a significant association between the A1 allele of the TaqIA polymorphism and various phenotypes of alcoholism, others have not, and two studies have shown the reversed association, where the A2 allele was associated with higher levels of alcohol consumption. We sought to test for an association between early onset (in childhood or adolescence) alcohol use disorders and the DRD2 TaqIA polymorphism and to resolve some of the hypothesized explanations for previous negative results, utilizing a larger sample than many previous studies.

METHODS

We selected individuals with a lifetime alcohol abuse or dependence (n=239) diagnosis from a clinically ascertained sample of youth (ages 13-19) with serious conduct and substance problems (about 90% also met criteria for conduct disorder and a cannabis use disorder) and compared them with individuals without a lifetime alcohol use disorder diagnosis ascertained from (1) community adolescent controls (n=151), (2) siblings of patients (n=87) and (3) other adolescent patients (n=92). Cases were compared with each control group, separately, by genotype using the chi(2)-test. Using 78 adolescent patients with an alcohol use disorder where genotypic information was available for both parents, we conducted the transmission disequilibrium test (TDT).

RESULTS

Case-control results were non-significant using the entire community control sample (chi(2)(2)=1.92; p=0.38) and when restricting the sample to Caucasians (chi(2)(2)=3.81; p=0.15) or Hispanics (chi(2)(2)=1.70; p=0.43). Case-control results using the other comparison groups and TDT results were also non-significant.

DISCUSSION

We did not find support for an association between the TaqIA polymorphism and early onset alcohol use disorders.

Imaging gene–substance interactions: The effect of the DRD2 TaqIA polymorphism and the dopamine agonist bromocriptine on the brain activation during the anticipation of reward

Dopamine is known as the main neurotransmitter modulating the activation of the reward system of the brain. The DRD2 TaqIA polymorphism is associated with dopamine D2 receptor density which plays an important role in the context of reward. Persons carrying an A1 allele have a lower D2 receptor density and a higher risk to show substance abuse. The present study was designed to investigate the influence of the DRD2 TaqIA polymorphism and the selective D2 receptor agonist bromociptine on the activation of the reward system by means of functional magnetic resonance imaging (fMRI). In a double-blind crossover study with 24 participants we found an increase of reward system activation from placebo to bromocriptine only in subjects carrying the A1 allele. Furthermore, only A1 carrier showed an increase of performance under bromocriptine. The results are interpreted as reflecting a specific sensitivity for dopamine agonists in persons carrying an A1 allele and may complement actual data and theories of the development of addiction disorders postulating a higher genetic risk for substance abuse in carrier of the A1 allele.

Association between the DRD2 gene Taq1A (C32806T) polymorphism and alcohol consumption in social drinkers

An association between the DRD2 Taq1A (C32806T) polymorphism and social alcohol consumption in the opposite direction to that reported for alcoholism has recently been reported in a male Finnish sample. We attempted to replicate these findings in two independent samples, and extend on previous work by including female participants. The DRD2 A1 allele was significantly associated with reduced alcohol consumption in sample one (P=0.004) and sample two (P=0.015). In sample two there was a significant genotype x sex interaction (P=0.016), with the association of the A1 allele and reduced alcohol consumption significant in men only. This interaction was marginally significant (P=0.042) in a meta-analysis of combined data from both samples, and the main effect of genotype highly significant (P<0.001). Age at time of data collection and cigarette consumption were entered as covariates in all analyses. These results replicate recent previous findings and suggest a possibility that this association may exist in men only, or be stronger in men.

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.