Association analysis of the GABA(A) receptor subunit genes cluster on 5q33-34 and alcohol dependence in a Japanese population

Association Between GABRG2 and Self-Rating of the Effects of Alcohol in a French Young Adult Sample

Purpose

Alcohol use is a leading risk factor for preventable death, injury, and disease globally. Low sensitivity to the effects of alcohol is influenced by genes and predicts risk for harmful alcohol use and alcohol use disorder (AUD). Alcohol induces effects partly by modulation of gamma-aminobutyric acid receptors type A (GABAARs). This study investigates the relationship between genetic variation in GABAAR subunit genes and individual alcohol sensitivity among French university students.

Patients and Methods

The study involved 1,409 French university students (34.5% women; mean age 20.3 years). Alcohol sensitivity was measured by the Self-Rating of the Effects of Alcohol Scale (SRE). SRE-scores from initial drinking, regular drinking, and heavy drinking were investigated for correlations with alcohol consumption and for associations with single nucleotide polymorphisms (SNPs) in GABAAR subunit genes (GABRA2, GABRG2, GABRA6).

Results

We replicated correlations between low alcohol sensitivity and high alcohol consumption. We further found an association between the minor allele in rs211014 (GABRG2) and higher SRE-scores, linked to dizziness and motor incoordination. Genetic variation in GABRG2 has previously been associated with processes involving motor coordination (alcohol withdrawal, febrile- and epileptic seizures).

Conclusion

The results from our study suggest that genetic variation in GABRG2 may influence alcohol sensitivity, which could inform strategies for assessing risk for harmful alcohol use and AUD.

GABRB2, a key player in neuropsychiatric disorders and beyond

The GABA receptors represent the main inhibitory system in the central nervous system that ensure synaptogenesis, neurogenesis, and the regulation of neuronal plasticity and learning. GABAA receptors are pentameric in structure and belong to the Cys-loop superfamily. The GABRB2 gene, located on chromosome 5q34, encodes the β2 subunit that combines with the α and γ subunits to form the major subtype of GABAA receptors, which account for 43% of all GABAA receptors in the mammalian brain. Each subunit probably consists of an extracellular N-terminal domain, four membrane-spanning segments, a large intracellular loop between TM3 and TM4, and an extracellular C-terminal domain. Alternative splicing of the RNA transcript of the GABRB2 gene gives rise at least to four long and short isoforms with dissimilar electrophysiological properties. Furthermore, GABRB2 is imprinted and subjected to epigenetic regulation and positive selection. It has been associated with schizophrenia first in Han Chinese, and subsequently validated in other populations. Gabrb2 knockout mice also exhibited schizophrenia-like behavior and neuroinflammation that were ameliorated by the antipsychotic drug risperidone. GABRB2 was also associated with other neuropsychiatric disorders including bipolar disorder, epilepsy, autism spectrum disorder, Alzheimer's disease, frontotemporal dementia, substance dependence, depression, internet gaming disorder, and premenstrual dysphoric disorder. Recently, it has been postulated that GABRB2 might be a potential marker for different cancer types. As GABRB2 has a pivotal role in the central nervous system and is increasingly recognized to contribute to human diseases, further understanding of its structure and function may expedite the generation of new therapeutic approaches.

Several behavioral traits relevant for alcoholism are controlled by ɣ2 subunit containing GABAA receptors on dopamine neurons in mice

The risk factors for developing alcohol addiction include impulsivity, high sensitivity to the rewarding action of ethanol, and low sensitivity to its sedative and intoxicating effects. Genetic variation in GABA_A receptor subunits, including the ɣ2 subunit (Gabrg2), affects the risk for developing alcoholism. Alcohol directly potentiates GABA_A receptors and activates the mesolimbic dopamine system. Here, we deleted Gabrg2 selectively in dopamine cells of adult mice. The deletion resulted in elevated firing of dopamine neurons and made them less sensitive to drugs acting at GABA_A receptors. At the behavioral level, the deletion increased exploratory behavior and augmented both correct and incorrect responding in the go/no-go task, a test often used to assay the response inhibition component of impulsivity. In addition, conditioned place preference to alcohol, but not to cocaine or morphine, was increased. Ethanol-induced locomotor activation was enhanced in the mice lacking Gabrg2 on dopaminergic cells, whereas the sedative effect of alcohol was reduced. Finally, the alcohol drinking, but not the alcohol preference, at a high concentration was increased in the mutant mice. In summary, deletion of Gabrg2 on dopamine cells induced several behavioral traits associated with high risk of developing alcoholism. The findings suggest that mice lacking Gabrg2 on dopaminergic cells could be used as models for individuals at high risk for developing alcoholism and that GABA_A receptors on dopamine cells are protective against the development of excessive alcohol drinking.

Methamphetamine-induced psychosis is associated with DNA hypomethylation and increased expression of AKT1 and key dopaminergic genes

Methamphetamine, one of the most frequently used illicit drugs worldwide, can induce psychosis in a large fraction of abusers and it is becoming a major problem for the health care institutions. There is some evidence that genetic and epigenetic factors may play roles in methamphetamine psychosis. In this study, we examined methamphetamine-induced epigenetic and expression changes of several key genes involved in psychosis. RNA and DNA extracted from the saliva samples of patients with methamphetamine dependency with and without psychosis as well as control subjects (each group 25) were analyzed for expression and promoter DNA methylation status of DRD1, DRD2, DRD3, DRD4, MB-COMT, GAD1, and AKT1 using qRT-PCR and q-MSP, respectively. We found statistically significant DNA hypomethylation of the promoter regions of DRD3 (P = 0.032), DRD4 (P = 0.05), MB-COMT (P = 0.009), and AKT1 (P = 0.0008) associated with increased expression of the corresponding genes in patients with methamphetamine psychosis (P = 0.022, P = 0.034, P = 0.035, P = 0.038, respectively), and to a lesser degree in some of the candidate genes in non-psychotic patients versus the control subjects. In general, methamphetamine dependency is associated with reduced DNA methylation and corresponding increase in expression of several key genes involved in the pathogenesis of psychotic disorders. While these epigenetic changes can be useful diagnostic biomarkers for psychosis in methamphetamine abusers, it is also consistent with the use of methyl rich diet for prevention or suppression of psychosis in these patients. However, this needs to be confirmed in future studies. © 2016 Wiley Periodicals, Inc.

Common single nucleotide variants underlying drug addiction: more than a decade of research

Drug-related phenotypes are common complex and highly heritable traits. In the last few years, candidate gene (CGAS) and genome-wide association studies (GWAS) have identified a huge number of single nucleotide polymorphisms (SNPs) associated with drug use, abuse or dependence, mainly related to alcohol or nicotine. Nevertheless, few of these associations have been replicated in independent studies. The aim of this study was to provide a review of the SNPs that have been most significantly associated with alcohol-, nicotine-, cannabis- and cocaine-related phenotypes in humans between the years of 2000 and 2012. To this end, we selected CGAS, GWAS, family-based association and case-only studies published in peer-reviewed international scientific journals (using the PubMed/MEDLINE and Addiction GWAS Resource databases) in which a significant association was reported. A total of 371 studies fit the search criteria. We then filtered SNPs with at least one replication study and performed meta-analysis of the significance of the associations. SNPs in the alcohol metabolizing genes, in the cholinergic gene cluster CHRNA5-CHRNA3-CHRNB4, and in the DRD2 and ANNK1 genes, are, to date, the most replicated and significant gene variants associated with alcohol- and nicotine-related phenotypes. In the case of cannabis and cocaine, a far fewer number of studies and replications have been reported, indicating either a need for further investigation or that the genetics of cannabis/cocaine addiction are more elusive. This review brings a global state-of-the-art vision of the behavioral genetics of addiction and collaborates on formulation of new hypothesis to guide future work.

Pathway analysis of genome-wide association datasets of personality traits

Although several genome-wide association (GWA) studies of human personality have been recently published, genetic variants that are highly associated with certain personality traits remain unknown, due to difficulty reproducing results. To further investigate these genetic variants, we assessed biological pathways using GWA datasets. Pathway analysis using GWA data was performed on 1089 Korean women whose personality traits were measured with the Revised NEO Personality Inventory for the 5-factor model of personality. A total of 1042 pathways containing 8297 genes were included in our study. Of these, 14 pathways were highly enriched with association signals that were validated in 1490 independent samples. These pathways include association of: Neuroticism with axon guidance [L1 cell adhesion molecule (L1CAM) interactions]; Extraversion with neuronal system and voltage-gated potassium channels; Agreeableness with L1CAM interaction, neurotransmitter receptor binding and downstream transmission in postsynaptic cells; and Conscientiousness with the interferon-gamma and platelet-derived growth factor receptor beta polypeptide pathways. Several genes that contribute to top-ranked pathways in this study were previously identified in GWA studies or by pathway analysis in schizophrenia or other neuropsychiatric disorders. Here we report the first pathway analysis of all five personality traits. Importantly, our analysis identified novel pathways that contribute to understanding the etiology of personality traits.

Association of gamma-aminobutyric acid A receptor α2 gene (GABRA2) with alcohol use disorder

Gamma-aminobutyric acid (GABA) is a major inhibitory neurotransmitter in mammalian brain. GABA receptor are involved in a number of complex disorders, including substance abuse. No variants of the commonly studied GABA receptor genes that have been associated with substance dependence have been determined to be functional or pathogenic. To reconcile the conflicting associations with substance dependence traits, we performed a meta-analysis of variants in the GABAA receptor genes (GABRB2, GABRA6, GABRA1, and GABRG2 on chromosome 5q and GABRA2 on chromosome 4p12) using genotype data from 4739 cases of alcohol, opioid, or methamphetamine dependence and 4924 controls. Then, we combined the data from candidate gene association studies in the literature with two alcohol dependence (AD) samples, including 1691 cases and 1712 controls from the Study of Addiction: Genetics and Environment (SAGE), and 2644 cases and 494 controls from our own study. Using a Bonferroni-corrected threshold of 0.007, we found strong associations between GABRA2 and AD (P=9 × 10(-6) and odds ratio (OR) 95% confidence interval (CI)=1.27 (1.15, 1.4) for rs567926, P=4 × 10(-5) and OR=1.21 (1.1, 1.32) for rs279858), and between GABRG2 and both dependence on alcohol and dependence on heroin (P=0.0005 and OR=1.22 (1.09, 1.37) for rs211014). Significant association was also observed between GABRA6 rs3219151 and AD. The GABRA2 rs279858 association was observed in the SAGE data sets with a combined P of 9 × 10(-6) (OR=1.17 (1.09, 1.26)). When all of these data sets, including our samples, were meta-analyzed, associations of both GABRA2 single-nucleotide polymorphisms remained (for rs567926, P=7 × 10(-5) (OR=1.18 (1.09, 1.29)) in all the studies, and P=8 × 10(-6) (OR=1.25 (1.13, 1.38)) in subjects of European ancestry and for rs279858, P=5 × 10(-6) (OR=1.18 (1.1, 1.26)) in subjects of European ancestry. Findings from this extensive meta-analysis of five GABAA receptor genes and substance abuse support their involvement (with the best evidence for GABRA2) in the pathogenesis of AD. Further replications with larger samples are warranted.

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.

Association between GABA(A) receptor subunit gene cluster and zolpidem-induced complex sleep behaviors in Han Chinese

STUDY OBJECTIVES

To investigate and elucidate the role of GABA(A) receptor subunits, specifically the 2 genetic markers at the GABA(A) α1 and GABA(A) α6 receptors, in zolpidem-induced complex sleep behaviors (CSBs).

DESIGN

Genetic association study.

SETTING

Kaohsiung Medical University-affiliated hospitals, Kaohsiung, Taiwan.

PATIENTS

30 zolpidem-induced CSB subjects and 37 controls.

INTERVENTIONS

N/A.

MEASUREMENTS AND RESULTS

The χ(2) test demonstrated an association between the A15G variant at the GABA(A) α1 receptor subunit gene and zolpidem-induced CSBs (P = 0.007). The adjusted odds ratio of the GABA(A) α1 receptor subunit genotype for the risk of zolpidem-induced CSBs was approximately 10 (OR = 9.99, 95% CI = 1.82, 74.87; P = 0.013).

CONCLUSIONS

The finding reveals that the A15G variant at the GABA(A) α1 receptor subunit gene confers a high risk of zolpidem-induced CSBs and may be considered in clinical services.

Potential role of GABAA receptor subunit; GABRA6, GABRB2 and GABRR2 gene polymorphisms in epilepsy susceptibility and pharmacotherapy in North Indian population

BACKGROUND

GABA(A) receptors influence the susceptibility to seizures, and variations in the receptor genes can contribute to antiepileptic drug resistance also.

METHODS

We investigated the possible associations of single nucleotide polymorphisms (SNPs) present in GABRA6 c. 1512 T>C, GABRB2 c. 1412 C>T, and GABRR2 c. IVS2C>G genes of GABA(A) receptors in epilepsy susceptibility and drug resistance in northern Indian patients with epilepsy. After screening a total of 202 healthy controls and 401 epilepsy patients were enrolled in study. The genotyping was done by PCR-RFLP methods.

RESULTS

The GABRA6 c. 1512 T>C, polymorphism was conferring risk for epilepsy susceptibility for TC (P=0.018), CC (P=0.0001) genotype and for C allele (P=0.0002). Another polymorphism GABRB2 c. 1412 C>T was also conferring high risk for epilepsy susceptibility CT (P=0.012), TT (P=0.778) genotype and for variant T allele (P=0.034) but was not associated with drug resistance. No association was found with epilepsy susceptibility or with drug resistance in case of GABRR2 c. IVS2C>G gene polymorphism.

CONCLUSION

Overall, our findings suggest significant involvement of alpha (GABRA6) and beta (GABRB2) subunits of GABA(A) receptor in epilepsy susceptibility in north Indian population.

Genetics of alcohol dependence

Twin, family, and adoption studies have consistently shown that genetic factors play an important role in the pathogenesis of alcohol dependence. Numerous studies have aimed to identify genes that contribute to susceptibility to alcohol dependence. Whole-genome linkage studies have identified several chromosomal regions that are linked with alcohol dependence. Association studies have also identified genes associated with alcohol dependence. Alcohol-metabolizing enzymes, such as alcohol dehydrogenase-1B and aldehyde dehydrogenase-2, are the most well-established genes that have polymorphisms associated with the risk for alcohol dependence. Polymorphisms in gamma-aminobutyric acid receptor genes are also reported to be associated with alcohol dependence. The polymorphism of opioid receptor mu 1 gene is of interest because it alters the treatment effects of naltrexone. Several genes related to neural transmission have been reported to be associated with alcohol dependence, but results are inconsistent among studies. One reason for these inconsistent results is the great heterogeneity of alcohol dependence. Classifying alcohol dependence into homogeneous phenotypes is a good strategy to solve this problem. Recently, several genome-wide association studies have been reported. Genome-wide association studies enable hypothesis-free genome mapping of vulnerability-contributing genes and are expected to add data to identify genes associated with the susceptibility to alcohol dependence. Knowledge of the genetic basis of alcohol dependence is growing and leads to a better understanding of the biological mechanisms of addiction, which can help with strategies to prevent and treat this disease.

Genetic association studies of methamphetamine use disorders: A systematic review and synthesis

Efforts to understand the biological processes that increase susceptibility to methamphetamine (METH) use disorders (i.e., abuse, dependence, and psychosis) have uncovered several putative genotypic variants. However, to date a synthesis of this information has not been conducted. Thus, systematic searches of the current literature were undertaken for genetic-association studies of METH use disorders. Each gene's chromosomal location, function, and examined polymorphic markers were extracted. Frequencies, odds ratios and 95% confidence intervals for risk alleles, as well as sample size and power, were calculated. We uncovered 38 studies examining 39 genes, of which 18 were found to have a significant genotypic, allelic, and/or haplotypic association with METH use disorders. Three genes (COMT, DRD4, and GABRA1) were associated with METH abuse, nine (ARRB2, BDNF, CYP2D6, GLYT1, GSTM1, GSTP1, PDYN, PICK1, and SLC22A3) with METH dependence, two (AKT1 and GABRG2) with METH abuse/dependence, and four (DTNBP1, OPRM1, SNCA, and SOD2) with METH psychosis. Limitations related to phenotypic classification, statistical power, and potential publication bias in the current literature were noted. Similar to other behavioral, psychiatric, and substance use disorders, the genetic epidemiology of METH use disorders is complex and likely polygenic. National and international collaborative efforts are needed to increase the availability of large population-based samples and improve upon the power to detect genetic associations of small magnitude. Further, replication of the findings reviewed here along with further development of more rigorous methodologies and reporting protocols will aid in delineating the complex genetic epidemiology of METH use disorders.

The role of GABA(A) receptors in the acute and chronic effects of ethanol: a decade of progress

The past decade has brought many advances in our understanding of GABA(A) receptor-mediated ethanol action in the central nervous system. We now know that specific GABA(A) receptor subtypes are sensitive to ethanol at doses attained during social drinking while other subtypes respond to ethanol at doses attained by severe intoxication. Furthermore, ethanol increases GABAergic neurotransmission through indirect effects, including the elevation of endogenous GABAergic neuroactive steroids, presynaptic release of GABA, and dephosphorylation of GABA(A) receptors promoting increases in GABA sensitivity. Ethanol's effects on intracellular signaling also influence GABAergic transmission in multiple ways that vary across brain regions and cell types. The effects of chronic ethanol administration are influenced by adaptations in GABA(A) receptor function, expression, trafficking, and subcellular localization that contribute to ethanol tolerance, dependence, and withdrawal hyperexcitability. Adolescents exhibit altered sensitivity to ethanol actions, the tendency for higher drinking and longer lasting GABAergic adaptations to chronic ethanol administration. The elucidation of the mechanisms that underlie adaptations to ethanol exposure are leading to a better understanding of the regulation of inhibitory transmission and new targets for therapies to support recovery from ethanol withdrawal and alcoholism.

Craving for alcohol and food during treatment for alcohol dependence: modulation by T allele of 1519T>C GABAAalpha6

BACKGROUND

Craving for alcohol and food has been studied in association with alcohol dependence and eating disorders, respectively. One subclass of the gamma-aminobutyric acid (GABA) receptor, 1519T>C GABA(A)alpha6 has been reported to be associated with both alcohol dependence and weight gain. In this study, we hypothesized that patients being treated for alcohol dependence would report decreased craving for alcohol, but an increased craving for food during a 4-week treatment period. We further hypothesized that the T allele of the 1519T>C GABA(A)alpha6 gene would modulate the extent of changes in craving for alcohol and food.

METHODS

This study included 98 male inpatients being treated for alcohol dependence. A 7-point visual analog scale was applied to evaluate relative levels of alcohol and food craving at baseline and again 4 weeks later. Body weight was also checked at the same periods. Genotyping of the 1519T>C SNP in GABA(A)alpha6 was carried out by restriction fragment length polymorphism.

RESULTS

There were significant changes in craving for alcohol and food in all patients with alcohol dependence. During the treatment period, body weight increased in all patients with alcohol dependence. Changes in alcohol and food craving in T-allele carriers (CT + TT) of 1519T>C GABA(A)alpha6 were greater than those observed in CC homozygotes. In T-allele carriers, body weight significantly increased and the changes in weight showed a negative correlation with the change in the craving for alcohol and a positive correlation with the changes in craving for food.

DISCUSSION

The current results suggest that in T-allele carriers the change in craving for alcohol during treatment for alcohol dependence is negatively associated with changes in craving for food. The T allele of the 1519T>C GABA(A)alpha6 gene may be one of the modulating factors associated with changes in craving for alcohol and food during treatment of patients with alcohol dependence.

The role of GABA(A) receptors in the development of alcoholism

Alcoholism is a common, heritable, chronic relapsing disorder. GABA(A) receptors undergo allosteric modulation by ethanol, anesthetics, benzodiazepines and neurosteroids and have been implicated in the acute as well as the chronic effects of ethanol including tolerance, dependence and withdrawal. Medications targeting GABA(A) receptors ameliorate the symptoms of acute withdrawal. Ethanol induces plasticity in GABA(A) receptors: tolerance is associated with generally decreased GABA(A) receptor activation and differentially altered subunit expression. The dopamine (DA) mesolimbic reward pathway originating in the ventral tegmental area (VTA), and interacting stress circuitry play an important role in the development of addiction. VTA GABAergic interneurons are the primary inhibitory regulators of DA neurons and a subset of VTA GABA(A) receptors may be implicated in the switch from heavy drinking to dependence. GABA(A) receptors modulate anxiety and response to stress; important elements of sustained drinking and relapse. The GABA(A) receptor subunit genes clustered on chromosome 4 are highly expressed in the reward pathway. Several recent studies have provided strong evidence that one of these genes, GABRA2, is implicated in alcoholism in humans. The influence of the interaction between ethanol and GABA(A) receptors in the reward pathway on the development of alcoholism together with genetic and epigenetic vulnerabilities will be explored in this review.

Markers in the 5'-region of GABRG1 associate to alcohol dependence and are in linkage disequilibrium with markers in the adjacent GABRA2 gene

Following an initial report, there have been multiple replications of an association of alcohol dependence (AD) to markers within a haplotype block that includes the 3'-half of the gene encoding the GABA(A) alpha-2 subunit (GABRA2), on chromosome 4p. We examined the intergenic extent of this haplotype block and the association to AD of markers in the adjacent 5' haplotype block in GABRG1, which encodes the GABA(A) receptor gamma-1 subunit. We genotyped 15 single nucleotide polymorphisms in the GABRG1-GABRA2 interval as well as at 34 ancestry informative markers in three samples: 435 AD and 635 screened control subjects from Connecticut and 812 participants from a multicenter AD treatment trial. We observed two large haplotype blocks in the GABRG1-GABRA2 intergenic interval with a region of increased recombination midway between the two genes. Markers in the two haplotype blocks were in moderate linkage disequilibrium. Compared with markers in the GABRA2 haplotype block, markers in the 5' GABRG1 haplotype showed greater allelic, genotypic and haplotypic association with AD in European Americans from both AD samples. Logistic regression analysis indicated that genetic elements in the GABRG1 haplotype block likely contribute to AD risk in an additive manner, whereas those in the GABRA2 haplotype block may act in a dominant manner in relation to risk of AD.

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.

Association analysis of GABA receptor subunit genes on 5q33 with heroin dependence in a Chinese male population

GABAA receptor subunit genes clustered on 5q33 play a role in the development of alcoholism and methamphetamine use disorder without psychosis. The present study explored the possible contribution of the same subunit genes to the development of heroin dependence. Single nucleotide polymorphisms (SNPs) of the GABAA receptor subunits GABRB2, GABRA6, GABRA1, and GABRG2 were examined in 178 male Han Chinese heroin-dependent and 170 male control subjects. A significant difference in allele frequency for the SNP rs211014 in the GABAAgamma2 receptor subunit gene between cases and controls was identified (P = 0.015). A possible mechanism for the involvement of the GABA receptor subunit genes on 5q33 in the development of heroin dependence is discussed.

Identifying the genetic determinants of emotionality in humans; insights from rodents

The identification of human quantitative trait genes underlying variation in emotionality has proved to be both difficult and expensive. Whilst several QTL have been consistently identified and independently replicated across both phenotypes and populations, little progression has been made towards the identification of underlying genes or even variants. In this review we discuss the potential for using cross-species QTL concordance as a tool for QTL dissection in behaviour genetics, using an affect-related locus mapped to human chromosome 1 as an example.

The polymorphism GABABR1 T1974C[rs29230] of the GABAB receptor gene is not associated with the diagnosis of alcoholism or alcohol withdrawal seizures

As the inhibitory neurotransmitter gamma aminobutyric acid (GABA) modulates ethanol consumption, alcohol withdrawal symptoms and seizure generation by interacting with the GABAB receptor, the genes encoding for the GABAB receptor can be considered as candidate genes for alcoholism and alcohol withdrawal seizures (AWS). As the polymorphism GABABR1 T1974C[rs29230] of the GABAB receptor gene had been associated with alcoholism and EEG abnormalities in prior studies, the present examination investigated if the polymorphism is associated with the diagnosis of alcoholism or AWS. After genotyping the allele and genotype frequencies of a group of alcoholics with a history of AWS (n = 69) were compared with the results of a group of alcoholics with only mild withdrawal symptoms (n = 97). Additionally a group of healthy controls (n = 101) was compared with individuals with the diagnosis of alcoholism (n = 220). As no significant differences were found between the compared groups, this study gave no further evidence for GABABR1 T1974C[rs29230] as a candidate for alcoholism or AWS.

New findings on the genetic influences on alcohol use and dependence

PURPOSE OF REVIEW

Alcohol dependence is a complex disorder with a well documented highly hereditary nature. This article reviews the recent advances in our understanding of the direct and indirect genetic influences on alcohol use and dependence.

RECENT FINDINGS

Recent findings can be summarized as follows: (a) twin studies have defined and estimated the risks of general and specific alcohol-related vulnerabilities. (b) Linkage studies have provided largely inconsistent findings, though several chromosomal regions have been implicated. (c) Quantitative trait loci analyses in animals have identified that the Mpdz gene predisposes to alcohol dependence and withdrawal. (d) Examination of family-based samples has identified several genes including GABRA2 and CHRM2 thought to be associated with alcohol dependence.

SUMMARY

Despite great advances in understanding of genetic vulnerability in alcohol use disorders, only two gene complexes, ADH and ALDH2, have been identified as having defined effects on alcohol use and liability to dependence in humans. New genes associated with increased risks for the disorder will certainly be added to this list in the near future. Neurobiological analyses of the effects of these genes will surely contribute to further understanding of the cause of alcohol dependence and the interindividual differences in risks.

Haplotype association between GABAA receptor gamma2 subunit gene (GABRG2) and methamphetamine use disorder

Psychostimulant use disorder and schizophrenia have a substantial genetic basis. Evidence from human and animal studies on the involvement of the gamma-aminobutyric acid (GABA) system in methamphetamine (METH) use disorder and schizophrenia is mounting. As we tested for the association of the human GABA(A) receptor gamma 2 subunit gene (GABRG2) with each diagnostic group, we used a case-control design with a set of 178 subjects with METH use disorder, 288 schizophrenics and 288 controls. First, we screened 96 controls and identified six SNPs in GABRG2, three of whom we newly reported. Next, we selected two SNPs, 315C>T and 1128+99C>A, as representatives of the linkage disequilibrium blocks for further case-control association analysis. Although no associations were found in either allelic or genotypic frequencies, we detected a haplotypic association in GABRG2 with METH use disorder, but not with schizophrenia. This finding partly replicates a recent case-control study of GABRG2 in METH use disorder, and thus indicates that GABRG2 may be one of the susceptibility genes of METH use disorder.

Association studies of neurotransmitter gene polymorphisms in alcoholic Caucasians

Ethanol enhances mesolimbic/cortical dopamine activity in reward and reinforcement circuits. We investigated the hypothesis that risk for alcoholism may be mediated by genes for neurotransmitters associated with the dopamine reward system as well as genes for enzymes involved in ethanol metabolism. DNA was extracted from brain tissue collected at autopsy from pathologically characterized alcoholics and controls. PCR-based assays showed that alcoholism was associated with polymorphisms of the dopamine D2 receptor (DRD2) TaqI B (P = .029) and the GABAA-beta2 subunit C1412T (P = .012) genes, but not with the glutamate receptor subunit gene NMDAR2B (366C/G), the serotonin transporter gene (5HTTL-PR), the dopamine transporter gene DAT1(SLC6A3), the dopamine D2 receptor gene DRD2 TaqI A, or the GABAA alpha1(A15G), alpha6(T1519C), and gamma2(G3145A) subunit genes. The glial glutamate transporter gene EAAT2 polymorphism G603A was associated with alcoholic cirrhosis (P = .048). The genotype for the most active alcohol dehydrogenase enzyme ADH1C was associated with a lower risk of alcoholism (P = .026) and was less prevalent in alcoholics with DRD2TaqIA2/A2 (P = .047), GABAA-beta2 1412C/C (P = .01), or EAAT2 603G/A (P = .022) genotypes. Combined DRD2TaqI A or B with GABAA-beta2 or EAAT2 G603A genotypes may have a concerted influence in the predisposition to alcoholism.

No association of the GABAA receptor genes on chromosome 5 with alcoholism in the collaborative study on the genetics of alcoholism sample

A substantial body of literature suggests that gamma-aminobutyric acid (GABA) may be involved in the neurochemical pathways contributing to alcohol use and related disorders. Chromosome 5 contains a cluster of GABA(A) receptor genes, GABRA1, GABRA6, GABRB2, and GABRG2, which have been among the most extensively studied in relation to alcohol use. These studies have yielded mixed results. Using data from large, multiplex alcoholic families collected as part of the Collaborative Study on the Genetics of Alcoholism (COGA), we sought to provide more conclusive evidence regarding the role of the GABA(A) receptor genes on chromosome 5. Multiple single nucleotide polymorphisms (SNPs) were tested in each of the four chromosome 5q GABA(A) receptor genes, and we conducted both classic trio-based association analyzes and extended pedigree analyzes. We found no consistent evidence of association with alcohol dependence or alcohol dependence comorbid with antisocial personality disorder (ASPD) for any of the regions tested in the chromosome 5 GABA(A) receptor genes. These analyses suggest that the GABA(A) receptor genes on chromosome 5 do not play a strong role in alcohol dependence. Future studies are planned to test whether these genes are more important in influencing behavioral endophenotypes related to the risk of alcohol dependence.

An Expanded Evaluation of the Relationship of Four Alleles to the Level of Response to Alcohol and the Alcoholism Risk

BACKGROUND

Alcoholism is a complex, genetically influenced disorder the cause of which may be better understood through the study of genetically influenced phenotypes that mediate the risk. One such intermediate phenotype is the low level of response (LR) to alcohol. This project used a case-control approach to search for genes that may contribute to LR.

METHODS

Data were available from alcohol challenges at approximately age 20 and regarding the development of alcohol use disorders over the subsequent 20 years for 85 men, including 40 reported in a previous genetic analysis. LR was evaluated using oral consumption of 0.75 ml/kg of alcohol, after which changes in subjective feelings of intoxication and body sway were measured. Alcohol abuse and dependence were diagnosed by DSM-III-R criteria through structured interviews administered to both the participant and an informant (usually the spouse) 10, 15, and 20 years after initial testing. Four polymorphisms were evaluated, including the serotonin transporter HTTLPR promoter ins/del, GABAAalpha6 Pro385Ser, NPY Leu7Pro, and catalase 262C>T. Two of these, HTTLPR and GABAAalpha6 Pro385Ser, had been previously associated with LR and alcoholism in a preliminary study.

RESULTS

The HTTLPR L allele was significantly related to both the LR and alcoholism in an allele-dosage (stepwise) manner. Furthermore, the association remained when L alleles were subdivided into recently reported functional subtypes: the lowest LR was associated with genotypes correlated with the highest serotonin transporter expression. The GABAAalpha6 Ser385 allele showed a nonsignificant trend for association to a low LR, as had been previously observed, although the Ser385 allele is uncommon, and only 18 heterozygotes were in the current group. However, the six men with both LL and Pro385/Ser385 genotypes had the lowest LR, and each had developed alcoholism during follow-up. Neither NPY nor catalase was associated with either LR or alcoholic outcomes, although the sample did not have sufficient power for definitive conclusions.

CONCLUSIONS

This report strengthens the support for a relationship between the HTTLPR L and GABAAalpha6 Ser385 alleles to low alcohol LR and to alcoholism in a prospectively studied cohort evaluated for LR in young adulthood and before the onset of alcohol dependence.

GABRA6 gene polymorphism and an attenuated stress response

The glucocorticoid component of the stress response has been the subject of intense scientific scrutiny because of the wide ranging pathological consequences resulting from excess glucocorticoid exposure, including mood and anxiety disorders, and cognitive impairment. Exposure to stress activates the hypothalamic-pituitary-adrenal (HPA) axis and the sympathetic adrenomedullary system, which are regulated by neuronal pathways, including the inhibitory GABAergic (gamma-aminobutyric acid) system. Approximately 60% of the variance in glucocorticod levels may be attributable to genetic individual differences. In the present study, 56 healthy subjects underwent genotyping to determine the influence of the T1521C single nucleotide polymorphism (SNP) in the GABA(A)alpha6 receptor subunit gene (GABRA6) on the hormonal and autonomic responses to psychological stress induced by the Trier Social Stress Test (TSST). Adrenocorticotropin (ACTH), cortisol, diastolic blood pressure, and mean blood pressure responses to the TSST were significantly greater in subjects homozygous for the T allele or heterozygous compared to subjects homozygous for the C allele. Behavioral data was collected employing the Revised NEO Personality Inventory (NEO PI-R); subjects homozygous for the C allele scored significantly lower on the Extraversion factor compared to subjects homozygous for the T allele or heterozygous. These results suggest that the T1521C polymorphism in the GABRA6 gene is associated with specific personality characteristics as well as a marked attenuation in hormonal and blood pressure responses to psychological stress.

The Search for Genes Contributing to the Low Level of Response to Alcohol: Patterns of Findings Across Studies

BACKGROUND

Alcoholism is a complex genetically influenced disorder in which multiple phenotypes [e.g., disinhibition, alcohol-metabolizing patterns, and the low level of response (LR) to alcohol] contribute to the risk. A low LR to alcohol is one of the more thoroughly studied risk phenotypes; data indicate that LR relates to the risk status, predicts future alcoholism, and has a heritability as high as 60%. This article reviews data from animal and human studies regarding the LR to alcohol, searching for a convergence of results that might lead to the identification of relevant genes.

METHODS

A literature search was performed regarding animal and human genetic studies focusing on genes that might affect the LR to alcohol as a risk factor for alcoholism. The goal was to synthesize these results and highlight potential patterns.

RESULTS

Focusing on both genetic linkage and association studies, a number of chromosomal regions and genes potentially relevant to findings across two or more sources were identified. The genes of potential interest fell into several categories, including second-messenger systems (e.g., G proteins, adenylyl cyclase, and protein kinases); neurotransmitters or drug-related receptors (e.g., gamma-aminobutyric acid-A, glutamate, serotonin, and cannabinoid and opioid receptors); genes that affect alcohol metabolism; and genes that might relate to an overlap in the risk for alcoholism and some psychiatric conditions (e.g., catechol-O-methyltransferase regarding schizophrenia and bipolar disorder).

CONCLUSIONS

The review identifies several genes that may contribute to a low LR to alcohol and, thus, to an increased risk for alcohol use disorders. The chromosomal regions and genes highlighted here may form the basis for more focused genetic studies of alcohol use disorders, with the goals of developing more specific and effective prevention and treatment approaches.

Gender-specific contribution of the GABA(A) subunit genes on 5q33 in methamphetamine use disorder

Family and twins studies have suggested that genetic factors are involved in the development of substance use disorders. Several unrelated case/control association studies have reported associations of the GABA(A) subunit genes on 5q33 with the development of alcohol dependence. We hypothesized that these particular GABA(A) subunit genes also contribute to the development of methamphetamine use disorder. To test our hypothesis, we recruited cases using a series of questionnaires. Among the polymorphic SNPs, significant differences between cases and controls were identified in the female sample in the rs2279020 of the GABA(A)alpha1 subunit gene, and the novel SNP rs4480617 in the GABA(A)gamma2 subunit gene. No associations were found in the male sample. Further haplotype analysis identified several marker blocks significantly associated with methamphetamine use disorder in females; each block consists of the rs4480617. Our study provides preliminary evidence that the GABA(A) subunit genes on 5q33 may preferentially contribute to methamphetamine use disorder in females.

Pharmacogenomics of alcohol response and addiction

Alcoholism is a complex psychiatric disorder that has high heritability (50-60%) and is relatively common; in the US the lifetime prevalence of alcohol dependence is 20% in men and 8% in women. Current psychosocial and pharmacological therapies have relatively modest effects. Treatment is complicated by the fact that alcoholism is often co-morbid with other disorders, including anxiety, depression, and antisocial personality disorder. Approximately 80% of alcoholics smoke cigarettes and there is considerable genetic overlap between nicotine and alcohol addiction. Convergent evidence supports the classification of alcoholics into two broad categories: type 1 - later onset with feelings of anxiety, guilt, and high harm avoidance; and type 2 - early age of onset, usually men, impulsive, antisocial, and with low levels of brain serotonin. The pharmacogenomics of alcohol response is well established; genetic variants for the principal enzymes of alcohol metabolism influence drinking behavior and protect against alcoholism. Vulnerability to alcoholism is likely to be due to multiple interacting genetic loci of small to modest effects. First-line therapeutic targets for alcoholism are neurotransmitter pathway genes implicated in alcohol use. Of particular interest are the 'reward pathway' (serotonin, dopamine, GABA, glutamate, and beta endorphin) and the behavioral stress response system (corticotrophin-releasing factor and neuropeptide Y). Common functional polymorphisms in these genes are likely to be predictive (although each with small effect) of individualized pharmacological responses. Genetic studies, including case-control association studies and genome wide linkage studies, have identified associations between alcoholism and common functional polymorphisms in several candidate genes. Meanwhile, the current pharmacological therapies for alcoholism are effective in some alcoholics but not all. Some progress has been made in elucidating the pharmacogenomic responses to these drugs, particularly in the context of the type 1/type 2 classification system for alcoholics.

Molecular characterization of new polymorphisms at the beta2, alpha1, gamma2 GABA(A) receptor subunit genes associated to a rat nonpreferring ethanol phenotype

Recent preclinical and clinical studies have indicated a possible involvement of the genes encoding for the GABA(A) receptor subunits alpha6, beta2, alpha1 and gamma2 in the genetic susceptibility to alcohol abuse. We have recently found an (R) to (Q) mutation in codon 100 of the alpha6 GABA(A) subunit, that segregated in a rat line selectively bred for its voluntary ethanol aversion, Sardinian alcohol nonpreferring (sNP), but not in their Sardinian alcohol preferring (sP) counterpart, selected for its ethanol preference. In the present study the molecular composition of other GABA(A) subunits (beta2, alpha1 and gamma2) were analyzed in order to further investigate the involvement of the GABA(A) receptors in the genetic predisposition to voluntary alcohol intake. Automated sequencing analysis indicated the presence of six new silent substitutions (289 T-->C in the beta2 gene; 115 G-->A in the alpha1 gene; 157 G-->A, 174 C-->T, 347 A-->G and 385 A-->T in the gamma2 gene), in sNP but not in sP rats. These polymorphisms were linked to the alpha6 R100Q mutation previously described in sNP rats. The strict association between the alpha6 point mutation and the new polymorphisms found in the beta2, alpha1 and gamma2 genes, demonstrate that such genes belong to the same cluster and are inherited together in the rat. These results sustain the synteny for these clusters between the rodent and human genomes, and suggest that mutated GABA(A) beta2, alpha6, alpha1 and gamma2 subunit genes might contribute to the expression of an ethanol nonpreferring phenotype in a rat line that voluntarily avoids alcoholic solutions.

Toward a molecular architecture of personality

Epidemiological studies provided a large body of evidence that personality dimensions are influenced by genetic factors and that the genetic component is highly complex, polygenic, and epistatic. However, consistent findings on the genetic basis of personality have yet remained sparse. In recent years, molecular genetics has begun to identify specific genes coding in particular for components of the serotonergic and dopaminergic neurotransmitter systems representing quantitative trait loci (QTLs) for behavioral traits. The QTL concept suggests that complex traits are not attributable to single genes. According to this polygenic model, the genetic basis of personality and behavior and its pathological variations thus results from additive or nonadditive interactions of various genes. As the number of suitable candidate genes constantly increases, the QTL model provides a reasonable explanation for the genetic basis of personality and its disorders. In this review, the current knowledge on the impact of a large number of candidate gene polymorphisms (e.g. variations in serotonin and dopamine receptor and serotonin transporter genes) on personality and temperament is summarized. Additionally, investigations of gene-gene and gene-environment interactions in humans and animals, which currently intensify the identification of genes that underlie behavioral variations, are examined. The findings converge on the notion that a probabilistic rather than deterministic impact of genes on the expression of behavior will contribute to the demystification of behavioral disorders.

Association of GABA(A) receptors and alcohol dependence and the effects of genetic imprinting

GABA receptor genes have been postulated as candidates affecting the risk for alcoholism. The potential association between genes encoding five subunits of the GABA(A) receptors and alcoholism (alcohol dependence) was analyzed in the multiplex alcoholic pedigrees collected by the Collaborative Study on the Genetics of Alcoholism (COGA) using family-based association tests. We found consistent, although weak, linkage disequilibrium between GABRB1 (located on chromosome 4) and alcoholism (P < 0.03). Genes encoding GABRA1 and GABRA6, on chromosome 5, did not provide evidence for association with alcoholism. GABRA5 and GABRB3, on chromosome 15, were reported to be expressed uniparentally from the paternal chromosome. Analyses of paternal transmission of alleles of GABRA5 provided evidence for association with alcoholism, particularly in the Caucasian population and with the stricter ICD-10 definition of alcoholism (P < 0.004). Evidence of association was also observed during paternal transmission with GABRB3 in the Caucasian population (P < 0.007). Maternal transmissions provided no evidence for association. These data are consistent with an association between the expressed alleles in the GABA(A)-gene cluster on chromosome 15 and alcoholism that is modulated by genetic imprinting.

Genetic contributions to addiction

Even the most extreme environmentalists along the nature-nurture continuum in psychology now acknowledge that genes often contribute to individual differences in behavior. Behavioral traits are complex, reflecting the aggregate effects of many genes. These genetic effects are interactive, inter se and with the environments in which they are expressed. Human studies of addictive behaviors have clearly implicated both environmental and genetic influences. This review selects drug dependence as a paradigmatic addiction, and further, concentrates on the extensive literature with genetic animal models. Both traditional studies with inbred strains and selected lines and studies exploiting the new molecularly based technologies of the genomics era are discussed. Future directions for further contribution of animal models studies to our understanding of the brain dysregulations characteristic of addictions are identified.

The genetics of alcoholism and alcohol abuse

Twin studies have established that there are substantial genetic influences on alcoholism (0.5-0.6) in both men and women. Our knowledge of behaviors predisposing to alcoholism, including anxiety and impulsivity, is advancing rapidly through animal and human studies. Although alcoholism is often comorbid with other substance abuse and psychiatric disorders, recent studies have shown that, with the exception of nicotine, the heritability of alcoholism is largely substance-specific. Increasing understanding of the neurobiology of addiction has identified neural pathways in which genetic variation at candidate genes could influence vulnerability. Some functional variants of these genes have been identified. Recent linkage analyses in humans and rodents have pointed to genomic regions harboring genes that influence alcoholism. Refinement of clinical phenotypes and use of intermediate phenotypes will improve chances of gene identification. All these advances in the understanding of the genetics of alcoholism should facilitate the development of more accurately targeted therapies using molecular diagnostic approaches.

Role of the GABA(A)beta2, GABA(A)alpha6, GABA(A)alpha1 and GABA(A)gamma2 receptor subunit genes cluster in drug responses and the development of alcohol dependence

gamma-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter of the central nervous system and it acts at the GABA(A) and GABA(B) receptors. A possible role for the GABA(A) receptors in alcohol action has been derived from in vitro cell models, animal studies and human research. GABA(A) subunit mRNA expression in cell models has suggested that the long form of the gamma2 subunit is essential for ethanol enhanced potentiation of GABA(A) receptors, by phosphorylation of a serine contained within the extra eight amino acids. Several animal studies have demonstrated that alterations in drug and alcohol responses may be caused by amino-acid differences at the GABA(A)alpha6 and GABA(A)gamma2 subunits. An Arg(100)/Glu(100) change at the GABA(A)alpha6 subunit conferring altered binding efficacy of the benzodiazepine inverse agonist Ro 15-4513, was found between the AT (alcohol tolerance) and ANT (alcohol non-tolerance) rats. Several loci related to alcohol withdrawal on mouse chromosome 11 which corresponds to the region containing four GABA(A) subunit (beta2, alpha6, alpha1 and gamma2) genes on human chromosome 5q33-34, were also identified. Gene knockout studies of the role of GABA(A)alpha6 and GABA(A)gamma2 subunit genes in mice have demonstrated an essential role in the modulation of other GABA(A) subunit expression and the efficacy of benzodiazepine binding. Absence of the GABA(A)gamma2 subunit gene has more severe effects with many of the mice dying shortly after birth. Disappointingly few studies have examined the effects of response to alcohol in these gene knockout mice. Human genetic association studies have suggested that the GABA(A)beta2, alpha6, alpha1 and gamma2 subunit genes have a role in the development of alcohol dependence, although their contributions may vary between ethnic group and phenotype. In summary, in vitro cell, animal and human genetic association studies have suggested that the GABA(A)beta2, alpha6, alpha1 and gamma2 subunit genes have an important role in alcohol related phenotypes (300 words).