Candidate genes for alcohol dependence: a review of genetic evidence from human studies

Association analysis of the dopamine receptor D2 (DRD2) SNP rs1076560 in alcoholic patients

The dopamine system plays a well-established role in alcoholism. In this study, we examined the association between the single-nucleotide polymorphism (SNP) rs1076560 of the dopamine receptor D2 (DRD2) gene and susceptibility to alcoholism. SNP rs1076560 (C/A) is located in intron 6 of DRD2, where it is 1.4 kb downstream from alternative exon 6 and 83 bp upstream from exon 7. A total of 248 alcoholic patients and 322 healthy controls, all Japanese males, were genotyped for rs1076560 polymorphism by direct sequencing and allele-specific PCR. Data were analyzed using standard chi(2) statistics and a backwards logistic regression approach to adjust for the contribution of aldehyde dehydrogenase-2 (ALDH2) genotype status. The DRD2 risk allele A was more prevalent in the alcoholic patients (40.1%) than in the healthy controls (34.0%) (P=0.034, odds ratio=1.300, 95% confidence interval=1.020-1.657). These data identify SNP rs1076560 as a potentially important variable in the development of alcoholism.

A rational approach to the pharmacotherapy of alcohol dependence

The use of alcohol is widespread in the world, and although there are many regular users, some individuals drink excessively. Understanding the time course of the development and maintenance of alcohol dependence is important in assessing the potential risk/benefit of an intervention and in accurately treating the problem. Most, although not all, literature suggests that alcoholism is a chronic, relapsing disorder and that there is a general progression from less to more severe problems. Understanding the neurobiology that underlies alcohol dependence as it relates to different clinical stages may help in the development of effective targeted pharmacological treatments. Important clinical stages that may be amenable to pharmacological intervention include the transition from alcohol use to heavy drinking particularly in vulnerable individuals, the cessation of heavy drinking in individuals who want to quit, and the prevention of relapse in individuals who have initiated abstinence but may struggle with craving or the desire to resume alcohol use. Neurotransmitter systems implicated in these stages include glutamate, gamma-aminobutyric acid, opioid, and serotonin systems that may act directly or via the indirect modulation of dopamine function. The treatment implications will also be discussed.

Genomics and complex liver disease: Challenges and opportunities

The concept of genetic susceptibility in the contribution to human disease is not new. What is new is the emerging ability of the field of genomics to detect, assess, and interpret genetic variation in the study of susceptibility to development of disease. Deciphering the human genome sequence and the publication of the human haplotype map are key elements of this effort. However, we are only beginning to understand the contribution of genetic predisposition to complex liver disease through its interaction with environmental risk factors. In the coming decade, we anticipate the development of human studies to better dissect the genotype/phenotype relationship of complex liver diseases. This endeavor will require large, well-phenotyped patient populations of each disease of interest and proper study designs aimed at answering important questions of hepatic disease prognosis, pathogenesis, and treatment. Teamwork between patients, physicians, and genomics scientists can ensure that this opportunity leads to important biological discoveries and improved treatment of complex disease.

Autosomal linkage analysis for the level of response to alcohol

BACKGROUND

The level of response (LR) to alcohol is a genetically-influenced phenotype related to the alcoholism risk. Usually measured by evaluating psychological and physiological changes that follow the administration of alcohol, the heritability of LR is estimated to be between 0.4 and 0.6, and efforts are being made to find genes related to this phenotype. This paper presents data from a family-based genome with linkage analysis focusing on alcohol challenge determinants of LR.

METHODS

The subjects were 18-to-29-year-old sibling pairs with at least one parent who was alcohol-dependent and who had experience with alcohol but were not yet alcohol-dependent themselves. Both members of the sibling pairs were given oral alcohol challenges (0.75-0.90 ml/kg of ethanol for females and males, respectively), with LR established using the Subjective High Assessment Scale (SHAS) and changes in body sway (BS) repeatedly over a 3.5-hr. period. Blood samples from siblings and at least one parent were genotyped using 811 microsatellite markers, with results evaluated using several related variance component approaches as implemented in SOLAR for continuous traits. In addition, association was tested using single nucleotide polymorphisms (SNPs) within the KCNMA1, HTR7 and SLC18A2 genes that may relate to a finding on chromosome 10.

RESULTS

Data were generated from 238 sib-pairs representing 365 individuals (41.6% were males) from 165 families. The most consistent results across methods and samples were observed for SHAS on chromosome 10 between 120 and 140 cM (with a maximum LOD score of 2.6 at 122 cM), and a second region of possible interest at 173 cM (LOD = 1.2). Statistical analysis with the KCNMA1, HTR7 and SLC18A2 genes, which lie in the support region of interest revealed no evidence for association after correction for multiple comparisons.

CONCLUSIONS

These evaluations from the largest known alcohol challenge-based genetic study to date highlight the potential importance of genes on chromosome 10 as possible contributors to the low LR to alcohol as a risk factor for alcoholism.

Rapid ethanol tolerance mediated by adaptations in acute tolerance in inbred mouse strains

It has been postulated that decreased acute sensitivity to ethanol is an important genetically-mediated risk factor for the development of alcoholism. Previous work in mice and rats has indicated that ethanol sensitivity can be reduced in a genotype-dependent manner by a single dose of ethanol 24 h prior to testing, so-called 'rapid' tolerance. The current studies were undertaken to determine if the observed rapid tolerance was mediated by alterations in initial sensitivity or acute functional tolerance (AFT), the two primary components of acute sensitivity. Separate groups of C57BL/6, DBA/2, ILS, and ISS inbred mouse strains were administered a single pretreatment dose of saline or ethanol (5 g/kg). The original and modified versions of the loss of righting reflex test, ethanol-induced hypothermia, and ataxia on a stationary dowel rod were tested 24 h later. Dependent on the test and strain, varying degrees of rapid tolerance were observed; a pronounced sensitization was detected in one case. There was a concomitant increase in the rate and/or magnitude of AFT with little change in initial sensitivity suggesting that rapid tolerance was mediated primarily by alterations in AFT. This conclusion may have implications for the contribution of acute sensitivity to human alcoholism.

Genetics and alcoholism: how close are we to potential clinical applications?

Rapid advancement of genetic knowledge has provided a wealth of data demonstrating a significant contribution of genes to the development of alcoholism but has suggested little in the way of clinical applicability. Twin and adoption studies suggest that 50% to 60% of the development of alcoholism is due to heritable factors, and linkage and association studies have identified chromosomal regions and individual genes that likely contribute to the development of this condition. Most of these genes are related to neurotransmitter systems and to alcohol metabolizing enzymes. We briefly review the evidence for this before discussing intermediate phenotypes of alcoholism under genetic control, pharmacogenetic aspects of alcoholism treatment, and the possibility of future clinical applications based on these areas.

Association between alcoholism and the genetic polymorphisms of the GABAA receptor genes on chromosome 5q33-34 in Korean population

Family, twin, and adoption studies have demonstrated that genes play an important role in the development of alcoholism. We investigated the association between alcoholism and the genetic polymorphisms of the GABAA receptor genes on chromosome 5q33-34 in Korean population. The genotype of the GABAA receptor gene polymorphisms were determined by performing polymerase chain reaction genotyping for 172 normal controls and 162 male alcoholics who are hospitalized in alcoholism treatment institute. We found a significant association between the genetic polymorphisms of the GABAA alpha1 and GABAA alpha6 receptor gene and alcoholism. The GG genotype of the GABAA alpha1 receptor gene was associated with the onset age of alcoholism and alcohol withdrawal symptoms, and a high score on the Korean version of the ADS. However, there was no association between the genetic polymorphisms of the GABAA beta2 and gamma2 receptor gene and alcoholisms. Our finding suggest that genetic polymorphisms of the GABAA alpha1 and GABAA alpha6 receptor gene may be associated with the development of alcoholism and that the GG genotype of the GABAA alpha1 receptor gene play an important role in the development of the early onset and the severe type of alcoholism.

Challenges in genetic studies of the etiology of substance use and substance use disorders: introduction to the special issue

Misuse of psychoactive substances is associated with substantial costs to users and to society. A growing literature suggests individual differences in vulnerability to develop substance related problems are influenced to a large degree by genetic factors. We review the evidence from genetic epidemiologic and molecular genetic studies of problematic use of alcohol, tobacco, and other drugs, then discuss the challenges for the next generation of studies of genetic influences on substance use. These challenges are addressed in the remaining papers of this special issue. The papers cover a variety of approaches, substances, and non-human as well as human studies, but are united by their focus on going beyond heritability estimates to address the mechanisms and processes underlying the development of substance use and substance related problems, including measurement, precursors of substance abuse, stages of substance involvement, and specificity of genetic influences.

Clinical and genetic characteristics of Korean male alcoholics with and without attention deficit hyperactivity disorder

AIMS

To examine the clinical and genetic characteristics of Korean male alcoholics with and without attention deficit hyperactivity disorder (ADHD).

METHODS

The present study included 85 male alcoholics who were diagnosed as having DSM-IV alcohol dependence. A total of 28 (32.9%) alcoholics were diagnosed as having DSM-IV ADHD with ongoing symptoms in adulthood. For the evaluation of their psychiatric conditions, the alcohol dependence scale (ADS), Beck depression inventory (BDI), Beck anxiety inventory (BAI), Barratt impulsiveness scale (BIS), brief anger-aggression questionnaire (BAQ), overt aggression scale (OAS), codependence test, and obsessive compulsive drinking scale (OCDS) were administered. The genotype frequencies of the dopamine type 2 receptor gene (DRD2), aldehyde dehydrogenase type 2 gene (ALDH2), functional polymorphism in the regulatory region of the serotonin transporter gene (5-HTTLPR), and catechol-O-methyltransferase gene (COMT) polymorphisms were examined.

RESULTS

Compared with alcoholics without ADHD, the mean ages for the onset of pathological drinking and alcohol withdrawal hallucinations were significantly earlier in alcoholics with ADHD. There was also a significant difference in the history of antisocial behaviour between the two groups. Compared with alcoholics without ADHD, the mean scores of the ADS, BDI, BAI, OAS, and OCDS were significantly higher in alcoholics with ADHD. With regard to the codependence test results, the mean scores of the interpersonal problem, low self-esteem and anxiety/fear subscales, and the mean total score of the codependence test were significantly higher in alcoholics with ADHD when compared with those without ADHD. There were no significant differences in the genotype frequencies of the DRD2, ALDH2, 5-HTTLPR, and COMT polymorphisms between alcoholics with and without ADHD.

CONCLUSIONS

The results of this study suggest that the comorbidity of alcohol dependence and ADHD in this Korean sample forms a distinct clinical phenotype that shows an increased severity of alcohol-related symptoms and behavioural/emotional problems and that ADHD is associated with an increased risk for the early onset of alcohol dependence in Korean male alcoholics.

Integrative strategies to identify candidate genes in rodent models of human alcoholism

The search for genes underlying alcohol-related behaviours in rodent models of human alcoholism has been ongoing for many years with only limited success. Recently, new strategies that integrate several of the traditional approaches have provided new insights into the molecular mechanisms underlying ethanol's actions in the brain. We have used alcohol-preferring C57BL/6J (B6) and alcohol-avoiding DBA/2J (D2) genetic strains of mice in an integrative strategy combining high-throughput gene expression screening, genetic segregation analysis, and mapping to previously published quantitative trait loci to uncover candidate genes for the ethanol-preference phenotype. In our study, 2 genes, retinaldehyde binding protein 1 (Rlbp1) and syntaxin 12 (Stx12), were found to be strong candidates for ethanol preference. Such experimental approaches have the power and the potential to greatly speed up the laborious process of identifying candidate genes for the animal models of human alcoholism.

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.

Major depression and immunity in alcohol-dependent persons

Altered immunity has been associated with both alcoholism and major depression (MD). We investigated the contribution of MD, as well as alcoholism, to in vitro measures of immunity in inner-city alcohol-dependent (SCID-DSM-III-R) persons and community nonabusers, all otherwise in good health.

METHODS

Alcohol-dependent persons at an ambulatory alcohol treatment center who did not abuse other substances were studied along with the comparison sample (total n=122). Enumerative and functional immune measures included leukocyte and lymphocyte subsets, mitogen response, natural killer cell activity (NKCA), and granulocytic phagocytosis.

RESULTS

Controlling for alcohol dependence, age, gender, racial background, and medical status, MD was associated with decreased phytohemagglutinin (PHA) responses (p<.03), possibly decreased NKCA (p<.08), and increased circulating monocytes (p<.04). Controlling for MD, age, gender, racial background, and medical status, alcohol dependence was associated with decreased circulating B lymphocytes (p<.02), possibly decreased CD56+ (NK) cells (p<.06), and increased monocytes (p<.04). Responses to concanavalin A and pokeweed mitogen, granulocyte functions, and the composition of other leukocyte and lymphocyte subsets showed no evidence of being associated with MD or with alcoholism (p>.1). Secondary analyses exploring factors such as recent alcohol use, cigarette use, and nutrition suggested that these factors accounted for the altered lymphocyte subsets associated with alcoholism and the possibly decreased NKCA with MD. They did not account for the association of MD with increased monocytes and decreased PHA.

DISCUSSION

MD-associated immune changes in alcoholics are modest and consistent with those seen in MD without alcoholism. Some MD- and many alcoholism-associated immune effects appear related to factors such as cigarette use and recent alcohol exposure.

Application of the regression of offspring on mid-parent method to detect associations between single-nucleotide polymorphisms and the beta 2 electroencephalogram phenotype in the COGA data

The beta 2 electroencephalogram (EEG) phenotype is used as a quantitative measure related to alcoholism, and evidence of linkage and association has previously been reported in the Collaborative Study on the Genetics of Alcoholism data. In this study, associations between the beta 2 EEG phenotype and single nucleotide polymorphisms from whole-genome Illumina and Affymetrix panels were investigated with the regression of offspring on mid-parent method to identify significant genetic effects and to estimate their heritability. Separate regressions on father and mother were performed to identify parent-specific effects. Estimates of the heritability of the beta 2 EEG phenotype were 0.68 ± 0.12 and 0.52 ± 0.07 based on father-offspring and mother-offspring pairs, respectively. Significant associations at the 0.0005 level, some of which were parent-specific, were found on chromosomes 1, 2, 5, 6, 7, 8, 11, 12, 15, 16, 17, 18, and 19 with heritability attributable to each SNP ranging from 0.01 to 8%.

Comparison of genotype- and haplotype-based approaches for fine-mapping of alcohol dependence using COGA data

It is generally assumed that the detection of disease susceptibility genes via fine-mapping association study is facilitated by consideration of marker haplotypes. In this study, we compared the performance of genotype-based and haplotype-based association studies using the Collaborative Study of Genetics of Alcoholism dataset, on several chromosomal regions showing evidence for linkage with ALDX1. After correction for multiple testing, the most significant results were observed with the genotype-based analyses on two regions of chromosomes 2 and 7. Interestingly, the analyses results from this dataset showed that there was no advantage of the haplotype-based analyses over genotype-based (single-locus) analyses. However, caution should be taken when generalizing these results to other chromosomal regions or to other populations.

Analysis of alcohol dependence phenotype in the COGA families using covariates to detect linkage

Linkage analysis methods that incorporate etiological heterogeneity of complex diseases are likely to demonstrate greater power than traditional linkage analysis methods. Several such methods use covariates to discriminate between linked and unlinked pedigrees with respect to a certain disease locus. Here we apply several such methods including two mixture models, ordered subset analysis, and a conditional logistic model to genome scan data on the DSM-IV alcohol dependence phenotype on the Collaborative Studies on Genetics of Alcoholism families, and compare the results to traditional nonparametric linkage analysis. In general, there was little agreement among the various covariate-based linkage statistics. Linkage signals with empirical p-values less than 0.001 were detected on chromosomes 3, 4, 7, 10, and 12, with the highest peak occurring at the GABRB1 gene using the ecb21 covariate.

A genome-wide tree- and forest-based association analysis of comorbidity of alcoholism and smoking

Genetic mechanisms underlying alcoholism are complex. Understanding the etiology of alcohol dependence and its comorbid conditions such as smoking is important because of the significant health concerns. In this report, we describe a method based on classification trees and deterministic forests for association studies to perform a genome-wide joint association analysis of alcoholism and smoking. This approach is used to analyze the single-nucleotide polymorphism data from the Collaborative Study on the Genetics of Alcoholism in the Genetic Analysis Workshop 14. Our analysis reaffirmed the importance of sex difference in alcoholism. Our analysis also identified genes that were reported in other studies of alcoholism and identified new genes or single-nucleotide polymorphisms that can be useful candidates for future studies.

Norepinephrine Transporter: A Candidate Gene for Initial Ethanol Sensitivity in Inbred Long-Sleep and Short-Sleep Mice

BACKGROUND

Altered noradrenergic neurotransmission is associated with depression and may contribute to drug abuse and alcoholism. Differential initial sensitivity to ethanol is an important predictor of risk for future alcoholism, making the inbred long-sleep (ILS) and inbred short-sleep (ISS) mice a useful model for identifying genes that may contribute to alcoholism.

METHODS

In this study, molecular biological, neurochemical, and behavioral approaches were used to test the hypothesis that the norepinephrine transporter (NET) contributes to the differences in ethanol-induced loss of righting reflex (LORR) in ILS and ISS mice.

RESULTS

We used these mice to investigate the NET as a candidate gene contributing to this phenotype. The ILS and ISS mice carry different DNA haplotypes for NET, showing eight silent differences between allelic coding regions. Only the ILS haplotype is found in other mouse strains thus far sequenced. Brain regional analyses revealed that ILS mice have 30 to 50% lower [3H]NE uptake, NET binding, and NET mRNA levels than ISS mice. Maximal [3H]NE uptake and NET number were reduced, with no change in affinity, in the ILS mice. These neurobiological changes were associated with significant influences on the behavioral phenotype of these mice, as demonstrated by (1) a differential response in the duration of ethanol-induced LORR in ILS and ISS mice pretreated with a NET inhibitor and (2) increased ethanol-induced LORR in LXS recombinant inbred (RI) strains, homozygous for ILS in the NET chromosomal region (44-47 cM), compared with ISS homozygous strains.

CONCLUSIONS

This is the first report to suggest that the NET gene is one of many possible genetic factors influencing ethanol sensitivity in ILS, ISS, and LXS RI mouse strains.

The impact of emotional stress early in life on adult voluntary ethanol intake-results of maternal separation in rats

The combination of genetic and environmental factors determines the individual vulnerability for excessive ethanol intake, possibly leading to dependence. The environmental influences early in life represent examples of determinant factors for adult behaviour and can be protective as well as risk factors. Maternal separation is one model to examine the long-term consequences of early environmental experiences on neurochemistry and behaviour, including drug-taking behaviour in experimental animals. In the present review, findings from studies using repeated short and prolonged periods of maternal separation, with emphasis on effects on voluntary ethanol intake in rats with or without a genetic predisposition for high voluntary ethanol intake, are summarized. Despite some contradictory results, the general picture emerging shows that short periods of maternal separation during the postnatal period result in a lower adult voluntary ethanol intake in male rats. Prolonged periods of maternal separation were found to induce a high voluntary ethanol intake in male rats, including rats with a genetic predisposition for high ethanol intake. Results from the literature also show that changes were not just related to time of separation but were also related to the degree of handling. Interestingly, in terms of voluntary ethanol intake, female rats were generally not affected by postnatal maternal separation. The reasons for these sex differences need further investigation. In terms of neurobiological consequences of maternal separation, conclusive data are sparse and one of the future challenges will, therefore, be to identify and characterize underlying neurobiological mechanisms, especially in the individual animal.

Association between alcoholism and gamma-amino butyric acid alpha2 receptor subtype in a Russian population

BACKGROUND

Two recent large genetic studies in the US population have reported association between genetic variation in gamma-amino butyric acid alpha2 receptor subtype (GABRA2) and risk for alcohol dependence. The goal of this study was to test whether GABRA2 is associated with alcohol dependence in a sample of Russian alcohol-dependent men.

METHODS

A total of 113 Russian alcohol-dependent men and 100 male population control subjects were recruited in St. Petersburg and genotyped for seven GABRA2 single-nucleotide polymorphisms (SNPs) using real-time PCR (TaqMan). Six SNPs were located in a GABRA2 haplotype block previously associated with alcohol dependence (AD) in the US population. SNPs and haplotypes were tested for an association to AD using chi analysis and a likelihood ratio-based statistic implemented in the software COCAPHASE.

RESULTS

Significant associations between two SNPs and AD were observed (p < 0.05). In addition, a trend-level association was observed between AD and three adjacent SNPs (p < 0.1). Associated alleles were carried in a haplotype that was present at frequencies of 0.37 and 0.48 in the control and alcohol-dependent populations, respectively (p < 0.06). Tight linkage disequilibrium spanning from the central portion of the gene to the 3' end was observed in this population. Comparison of the findings to the previously published studies in the US population revealed a highly similar linkage disequilibrium pattern in this population.

CONCLUSIONS

These findings suggest that genetic variants of GABRA2 increase risk for AD in the Russian population and provide additional support to the hypothesis that polymorphic variation at the GABRA2 locus plays an important role in predisposing to AD at least in European-ancestry populations.

Genetic determinants of addiction to opioids and cocaine

OBJECTIVE

The completion of the human genome sequence has spurred investigation of the genetic contribution to substance dependence. In this article some of the recent scientific evidence for genetic determinants of opioid and cocaine dependence is reviewed.

METHOD

An electronic search of the medical literature was conducted to locate published studies relevant to the genetics of opioid and cocaine dependence. The collected information judged to be most pertinent is described and discussed.

RESULTS

Genetic epidemiologic studies support a high degree of heritable vulnerability for both opioid and cocaine dependence. Polymorphisms in the genes coding for dopamine receptors and transporter, opioid receptors, endogenous opioid peptides, cannabinoid receptors, and serotonin receptors and transporter all appear to be associated with the phenotypic expression of this vulnerability once opioids or cocaine are consumed.

CONCLUSIONS

Despite this initial progress, identification of specific genes and quantification of associated risk for the expression of each gene remain to be elucidated. While alteration of an individual's genome to change the phenotype seems remote, future interventions for treatment of opioid and cocaine dependence may include precise medications targeted to block the effects of proteins that have been identified through genetic research.

Novelty seeking: interaction between parental alcohol use and dopamine D4 receptor gene exon III polymorphism over 17 years

Recent meta-analyses have questioned the association between the dopamine receptor D4 (DRD4) gene polymorphism and the temperament trait of novelty seeking, and proposed an interaction between the polymorphism and other factors. We wanted to test whether parental alcohol use during childhood moderated the effect of an offspring dopamine receptor gene (DRD4) polymorphism on the temperament trait of novelty seeking in adulthood. A population-based sample of children and adolescents (n=2149) and their parents was examined in 1980 and 1983 on parental alcohol use and rearing practices. In 1997, study participants completed the Temperament and Character Inventory for the novelty-seeking temperament trait, and a subsample (n=150) was genotyped for the DRD4 exon III polymorphism. For the participants with the father, but not the mother, reporting more frequent alcohol consumption or drunkenness in examinations 17 and/or 14 years before the novelty-seeking assessment, an association between the short (two- or five-repeat) alleles of the DRD4 gene and extremely high novelty-seeking scores was observed. When the father reported less frequent alcohol consumption or drunkenness, the genotype was not associated with novelty seeking. The association remained after controlling for sex, age, and maternal child-rearing. These results provide preliminary information on gene-environment interaction on the temperament trait of novelty seeking and may partly explain the heterogeneity of findings concerning the association between DRD4 polymorphisms and novelty seeking.

Development of medications for alcohol use disorders: recent advances and ongoing challenges

During the past decade, efforts to develop medications for alcoholism have burgeoned. Three agents, disulfiram, naltrexone and acamprosate, are now approved in a large number of countries. Although many patients have benefited from existing medications, their effects are moderate, and some alcoholics fail to respond to them. A host of new agents are currently under active investigation. Critical barriers must be overcome to ensure that future efforts in the development of medications for alcohol use disorders reach full fruition. These challenges include: establishing key targets for drug discovery; validating animal and human screening models; and developing biomarkers to help predict treatment success. In addition, it is important to formulate methodological and statistical strategies to efficiently conduct alcohol pharmacotherapy trials; to specify genetic and phenotypic patient characteristics associated with efficacy and safety for lead compounds; to forge productive alliances among governmental agencies, the pharmaceutical industry and academic researchers to further drug development; and, ultimately and perhaps most difficult, to engage the practitioner community to incorporate medications into the alcohol treatment process.

Genetic influences on quantity of alcohol consumed by adolescents and young adults

OBJECTIVE

To examine genetic and environmental influences on drinking in a nationally representative study of genetically informative adolescents followed into young adulthood.

METHOD

The average quantity of alcohol used per drinking episode during the past year was analyzed in 4432 youth assessed during adolescence (mean age of 16) and then 1 and 6 years later. The variance of quantity of alcohol consumed was decomposed into three components: additive genetic (a2), shared environmental (c2), non-shared environmental (e2). Four candidate genes were tested for association.

RESULTS

Wave 1 a2-0.52e2-0.48, Wave 2 a2-0.28e2-0.72, Wave 3 a2-0.30e2-0.70. Genetic correlations between Waves 1 and 2 were 0.85, Waves 1 and 3 were 0.34. The DAT1 440 allele was associated at Wave 1 (p=0.007). DRD2 TaqI A1/A2 was associated at Wave 3 (p=0.007). DRD4 and 5HTT were not associated. The DAT1 and DRD2 polymorphisms accounted for 3.1% and 2.0% of the variation, respectively.

CONCLUSION

Genetic influence on drinking behavior was common in adolescents longitudinally assessed 1 year apart, but was less correlated between these adolescents and their assessment as young adults at a subsequent time point. Polymorphisms in genes of the dopaminergic system appear to influence variation in drinking behavior.

Alcohol and the liver

PURPOSE OF REVIEW

To highlight salient recent discoveries and results of clinical trials in alcoholic liver disease (ALD). The burden of care for ALD patients is hefty and the prevalence of alcohol abuse may be increasing in both the developed and the underdeveloped world.

RECENT FINDINGS

Molecular mechanisms of alcoholism are being identified but not of the predisposition to alcoholic liver injury, except perhaps for polymorphism of a cytotoxic T-cell antigen. The Mayo End-stage Liver Disease (MELD) score performs well in assessing the prognosis of ALD; serological biomarkers for predicting ALD outcome are of uncertain value. Concomitant liver disease (e.g., obesity, hepatitis C, and iron overload) aggravates the severity of ALD; conversely, alcohol abuse may be a cryptic co-factor in some cases of non-alcoholic fatty liver. For alcoholic hepatitis, nutritional support is the mainstay of treatment; steroids are considered by some (but not all) as safe and effective therapy, whereas manipulations of tumor necrosis factor-alpha activity have been disappointing, or of unproven benefit at best. In liver transplantation for ALD, methods are being devised to monitor recidivism and to ameliorate its risk and that of co-morbid psychiatric conditions.

SUMMARY

Much of the pathogenesis of ALD has been identified and headway has been made in predicting its prognosis. However, much remains to be done to elucidate the molecular genetics of the risk of developing ALD and in formulating safe, effective therapies for alcoholic hepatitis.

Functionality of allelic variations in human alcohol dehydrogenase gene family: assessment of a functional window for protection against alcoholism

Alcohol dehydrogenase (ADH) catalyses the rate-determining reaction in ethanol metabolism. Genetic association studies of diverse ethnic groups have firmly demonstrated that the allelic variant ADH1B*2 significantly protects against alcoholism but that ADH1C*1, which is in linkage with ADH1B*2, produces a negligible protection. The influence of other potential candidate genes/alleles within the human ADH family, ADH1B*3 and ADH2, remains unclear or controversial. To address this question, functionalities of ADH1B3 and ADH2 were assessed at a physiological level of coenzyme and substrate range. Ethanol-oxidizing activities of recombinant ADH1B1, ADH1B2, ADH1B3, ADH1C1, ADH1C2 and ADH2 were determined at pH 7.5 in the presence of 0.5 mm NAD with 2-50 mm ethanol. The activity differences between ADH1B2 and ADH1B1 were taken as a threshold for effective protection against alcoholism and those between ADH1C1 and ADH1C2 as a threshold for null protection. Over 2-50 mm ethanol, the activities of ADH1B3 were found 2.9-23-fold lower than those of ADH1B2, largely attributed to the Km effect (ADH1B2, 1.8 mm; ADH1B3, 61 mm). Strikingly, the ADH1B3 activity was only 84% that of ADH1B1 at a low ethanol concentration, 2 mm, but increased 10-fold at 50 mm. Corrected for relative expression levels of the enzyme in liver, the hepatic ADH2 activities were estimated to be 18-97% those of ADH1B1 over 2-50 mm ethanol and were 28-140% of the activity differences between ADH1C1 and ADH1C2. The assessment based on the proposed functional window for the human ADH gene family indicates that ADH1B*3 may show some degree of protection against alcoholism and that the ADH2 functional variants appear to be negligible for this protection.

Short and Prolonged Periods of Maternal Separation and Voluntary Ethanol Intake in Male and Female Ethanol-Preferring AA and Ethanol-Avoiding ANA Rats

BACKGROUND

Genetic as well as environmental factors can affect the propensity for psychopathology and/or drug dependence. Maternal separation represents an animal experimental model that is useful in studies of effects of early life experiences. The authors have established a protocol for short and prolonged periods of maternal separation to study adult neurochemistry, behavior, and ethanol intake and have previously reported alterations in ethanol intake in Wistar rats and ethanol-preferring rats. The aim of the current study was to more thoroughly study how early life experiences affect an inherited propensity for high and low ethanol intake, respectively, in male and female ethanol-preferring AA (Alko alcohol) and ethanol-avoiding ANA (Alko, Non-Alcohol) rats.

METHODS

AA and ANA pups were assigned to one of three different rearing conditions: 15 min (MS15) or 360 min (MS360) of daily maternal separation in litters or normal animal facility rearing (AFR) during postnatal days 1 to 21. In adulthood, voluntary ethanol intake was investigated using the two-bottle free choice paradigm.

RESULTS

In male ethanol-preferring AA rats, MS15 resulted in a lower intake and fewer high-preferring animals at 8% and 10% ethanol compared with MS360 rats. The male MS360 rats had a higher ethanol intake at 8% and 10% ethanol in comparison with AFR rats. In contrast, the female AA MS15 and MS360 rats had a lower ethanol intake and a lower preference for the 10% ethanol solution compared with the female AA AFR rats. In male and female ANA rats, no major separation-induced effects were found.

CONCLUSIONS

The current results show that genetic inheritance can be affected by environmental manipulations in AA rats with an inherent high ethanol intake. The findings in female ethanol-preferring AA rats give further evidence of a differential outcome of maternal separation in male and female rats, as previously shown.

Ebrii gignunt ebrios--One drunkard begets another: the genetics of alcohol dependence

Family, twin, and adoption studies provide convincing evidence of a genetic contribution to the predisposition to alcohol dependence. Two main molecular genetic approaches, namely linkage and association, have been adopted to identify the genes that underpin that genetic vulnerability. Robust findings have implicated genes involved in alcohol metabolism, particularly when studied in Asian populations. These include the genes encoding aldehyde dehydrogenase 2 and several alcohol dehydrogenases. Other findings have proven to be less resilient, but have often implicated genes of the dopaminergic, GABA-ergic and serotonergic systems. Improvements in sample collection and characterization, technological and statistical advances, combined with a developmental approach, should enable molecular genetics to deliver on its initial promise of identifying the genes involved. These findings will have important implications for the identification and targeting of treatment. However it is anticipated that this information will have little impact on risk alteration and consequently population screening will be highly unlikely. Genetic factors always should be considered in the context of their developmental interplay with those of the environment.

Bitter receptor gene (TAS2R38), 6-n-propylthiouracil (PROP) bitterness and alcohol intake

BACKGROUND

Phenylthiocarbamide (PTC) and 6-n-propylthiouracil (PROP), chemically related compounds, are probes for genetic variation in bitter taste, although PROP is safer with less sulfurous odor. Threshold for PROP distinguishes nontasters (increased threshold) from tasters (lower threshold); perceived intensity subdivides tasters into medium tasters (PROP is bitter) and supertasters (PROP is very bitter). Compared with supertasters, nontasters have fewer taste papillae on the anterior tongue (fungiform papillae) and experience less negative (e.g., bitterness) and more positive (eg, sweetness) sensations from alcohol. We determined whether the TAS2R38 gene at 7q36 predicted PROP bitterness, alcohol sensation and use.

METHODS

Healthy adults (53 women, 31 men; mean age 36 years)--primarily light and moderate drinkers--reported the bitterness of five PROP concentrations (0.032-3.2 mM) and intensity of 50% ethanol on the general Labeled Magnitude Scale. PROP threshold and density of fungiform papillae were also measured. Subjects had common TAS2R38 gene haplotypes [alanine-valine-isoleucine (AVI) and proline-alanine-valine (PAV)].

RESULTS

PROP bitterness varied significantly across genotypes with repeated measures ANOVA: 26 AVI/AVI homozygotes tasted less bitterness than either 37 PAV/AVI heterozygotes or 21 PAV/PAV homozygotes. The PAV/PAV group exceeded the PAV/AVI group for bitterness only for the top PROP concentrations. The elevated bitterness was musch less than if we defined the groups using psychophysical criteria. With multiple regression analyses, greater bitterness from 3.2 mM PROP was a significant predictor of greater ethanol intensity and less alcohol intake--effects separate from age and sex. Genotype was a significant predictor of alcohol intake, but not ethanol intensity. With ANOVA, AVI/AVI homozygotes reported higher alcohol use than either PAV/AVI heterozygotes or PAV/PAV homozygotes. When age effects were minimized, PROP bitterness explained more variance in alcohol intake than did the TAS2R38 genotype.

CONCLUSIONS

These results support taste genetic effects on alcohol intake. PROP bitterness serves as a marker of these effects.

Race and ethnicity in the genome era: The complexity of the constructs

The vast amount of biological information that is now available through the completion of the Human Genome Project presents opportunities and challenges. The genomic era has the potential to advance an understanding of human genetic variation and its role in human health and disease. A challenge for genomics research is to understand the relationships between genomics, race, and ethnicity and the implications of uncovering these relationships. Robust and scholarly discourse on the concept of race and ethnicity in genomic research should be expanded to include social and behavioral scientists. Interdisciplinary research teams are needed in which psychologists, as well as other social and behavioral scientists, work collaboratively with geneticists and other natural scientists.

Alcoholism: genes and mechanisms

Alcoholism is a chronic relapsing/remitting disease that is frequently unrecognized and untreated, in part because of the partial efficacy of treatment. Only approximately one-third of patients remain abstinent and one-third have fully relapsed 1 year after withdrawal from alcohol, with treated patients doing substantially better than untreated [1]. The partial effectiveness of strategies for prevention and treatment, and variation in clinical course and side effects, represent a challenge and an opportunity to better understand the neurobiology of addiction. The strong heritability of alcoholism suggests the existence of inherited functional variants of genes that alter the metabolism of alcohol and variants of other genes that alter the neurobiologies of reward, executive cognitive function, anxiety/dysphoria, and neuronal plasticity. Each of these neurobiologies has been identified as a critical domain in the addictions. Functional alleles that alter alcoholism-related intermediate phenotypes include common alcohol dehydrogenase 1B and aldehyde dehydrogenase 2 variants that cause the aversive flushing reaction; catechol-O-methyltransferase (COMT) Val158Met leading to differences in three aspects of neurobiology: executive cognitive function, stress/anxiety response, and opioid function; opioid receptor μ1 (OPRM1) Asn40Asp, which may serve as a gatekeeper molecule in the action of naltrexone, a drug used in alcoholism treatment; and HTTLPR, which alters serotonin transporter function and appears to affect stress response and anxiety/dysphoria, which are factors relevant to initial vulnerability, the process of addiction, and relapse.

Alcohol and gene expression in the central nervous system

AIMS

To describe recent research focusing on the analysis of gene and protein expression relevant to understanding ethanol consumption, dependence and effects, in order to identify common themes.

METHODS

A selective literature search was used to collate the relevant data.

RESULTS

Over 160 genes have been individually assessed before or after ethanol administration, as well as in genetically selected lines. Techniques for studying gene expression include northern blots, differential display, real time reverse transcriptase-polymerase chain reaction (RT-PCR) and in situ hybridization. More recently, high throughput functional genomic technology, such as DNA microarrays, has been used to examine gene expression. Recent gene expression analyses have dramatically increased the number of candidate genes (nine array papers have illuminated 600 novel gene transcripts that may contribute to alcohol abuse and alcoholism).

CONCLUSIONS

Although functional genomic experiments (transcriptome analysis) have failed to identify a single alcoholism gene, they have illuminated important pathways and gene products that may contribute to the risk of alcohol abuse and alcoholism.

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.

Genetic polymorphism in ethanol metabolism: acetaldehyde contribution to alcohol abuse and alcoholism

Acetaldehyde, the first product of ethanol metabolism, has been speculated to be involved in many pharmacological and behavioral effects of ethanol. In particular, acetaldehyde has been suggested to contribute to alcohol abuse and alcoholism. In the present paper, we review current data on the role of acetaldehyde and ethanol metabolism in alcohol consumption and abuse. Ethanol metabolism involves several enzymes. Whereas alcohol dehydrogenase metabolizes the bulk of ethanol within the liver, other enzymes, such as cytochrome P4502E1 and catalase, also contributes to the production of acetaldehyde from ethanol oxidation. In turn, acetaldehyde is metabolized by the enzyme aldehyde dehydrogenase. In animal studies, acetaldehyde is mainly reinforcing particularly when injected directly into the brain. In humans, genetic polymorphisms of the enzymes alcohol dehydrogenase and aldehyde dehydrogenase are also associated with alcohol drinking habits and the incidence of alcohol abuse. From these human genetic studies, it has been concluded that blood acetaldehyde accumulation induces unpleasant effects that prevent further alcohol drinking. It is therefore speculated that acetaldehyde exerts opposite hedonic effects depending on the localization of its accumulation. In the periphery, acetaldehyde is primarily aversive, whereas brain acetaldehyde is mainly reinforcing. However, the peripheral effects of acetaldehyde might also be dependent upon its peak blood concentrations and its rate of accumulation, with a narrow range of blood acetaldehyde concentrations being reinforcing.

Mouse striatal transcriptome analysis: effects of oral self-administration of alcohol

Results of recent studies support the notion that substance self-administration is partially a genetically controlled component of addiction tied to habit formation and cellular modification of the striatum. Aiming to define pathways among genomic, neural, and behavioral determinants of addiction, we investigated global striatal gene expression in a paradigm of oral self-administration of alcohol by using genomically very similar alcohol-nonpreferring B6.Cb(5)i(7)-alpha 3/Vad (C5A3) and alcohol-preferring B6.Ib(5)i(7)-beta 25A/Vad (I5B25A) quasi-congenic mouse strains and their progenitors, C57BL/6By (B6By) and BALB/cJ. Expression of 12,488 genes and expressed sequence tags (ESTs) was studied by using 24 high-density oligonucleotide microarrays. Transcript signal intensity differences were analyzed with z test after iterative median normalization across groups and Hochberg step-down Bonferroni procedure. As expected, striatal transcriptome differences were far more extensive between the independently derived progenitor strains than between the quasi-congenic strains and their background partner, B6By. However, the genes, which were differentially expressed between the quasi-congenic strains and their background partner, were not subsets of the progenitorial differences and were not located on the chromosome segments introgressed into the quasi-congenic strains from the donor BALB/cJ strain that have been so far defined. Although 25 transcripts showed significantly different expression between the progenitor strains, only two transcripts, phosphatidylserine decarboxylase and a hypothetical 21.2-kDa protein, and one transcript, molybdenum co-factor synthesis 2, showed significantly different expression between C5A3 and I5B25A, and between B6By and I5B25A, respectively. The latter three transcripts are not located on previously identified chromosome segments introgressed from the donor BALB/cJ strain, supporting the suggestion of trans-acting regulatory variations among strains. Exposure to alcohol did not induce statistically significant striatal gene expression changes in any of the mouse strains. In conclusion, the results support the hypothesis that in functional genomic studies the chance of detecting function-relevant genes can be increased by the comparative analysis of quasi-congenic and background strains because the number of functionally irrelevant, differentially expressed genes between genomically similar strains is reduced. Lack of statistically significant alcohol-induced changes in transcript abundance indicated that oral self-administration had subtle effects on striatal gene expression and directed attention to important implications for the experimental design of future microarray gene expression studies on complex behaviors.

Acute effects of alcohol on larval zebrafish: a genetic system for large-scale screening

Larval zebrafish are used extensively for developmental genetic studies due to their salient features, such as small size, external development, optical transparency, and accessibility in large numbers. However, their use for the study of drug and alcohol abuse has not been explored. Here we investigated the response of larval zebrafish to acute treatment of alcohol. Our analyses showed that like adults, the larval zebrafish exhibited a dose-dependent locomotor response to ethanol: intermediate doses led to hyperactivity, whereas high doses have a neurodepressive effect resulting in hypoactivity and sedation. Alcohol also induced morphological changes of melanocytes, providing a visible cellular measure of the biological effects of alcohol in vivo. In addition, alcohol induced thigmotaxis behavior (preference for the edge of a compartment). In the behaviors we analyzed, genetic background influenced the locomotor responses to alcohol. The present study demonstrates that larval zebrafish exert a response to the acute treatment of alcohol, which is genetically modifiable. Therefore, the larval zebrafish represent a tractable vertebrate model system for a large-scale genetic analysis of the biological effects of alcohol.

Association of GABRG3 With Alcohol Dependence

BACKGROUND

Evidence from human, animal, and in vitro cell models suggests that gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter in the human central nervous system, is involved in many of the neurochemical pathways that affect alcohol use, abuse, and dependence. Both linkage and association to the region on chromosome 15q that contains a cluster of GABAA receptor genes have previously been reported, but the role of these genes in alcoholism remains inconclusive.

METHODS

We conducted family-based association analyses by using a large sample of multiplex alcoholic families collected as part of the Collaborative Study on the Genetics of Alcoholism, to test for an association between alcohol dependence and the GABAA receptor genes clustered on chromosome 15q. Multiple single-nucleotide polymorphisms were tested in each of the three chromosome 15q GABAA receptor genes: GABRA5, GABRB3, and GABRG3.

RESULTS

Using both classic trio-based analyses and extended-family analyses, we found consistent evidence of association between alcohol dependence and GABRG3. Nearly all single-nucleotide polymorphisms across the gene yielded evidence of association, and haplotype analyses were highly significant. No consistent evidence of association was observed with either GABRA5 or GABRB3, nor was there evidence for parent-of-origin effects with any of the genes.

CONCLUSIONS

These analyses suggest that GABRG3 may be involved in the risk for alcohol dependence. These findings support the theory that the predisposition to alcoholism may be inherited as a general state of central nervous system disinhibition/hyperexcitability that results from an altered responsiveness to GABA.

Genetics and drug use as a complex phenotype

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