Dopamine D 4 receptor gene polymorphism and extraversion revisited: results from the Munich gene bank project for alcoholism

Associations Between the Dopamine D4 Receptor and DAT1 Dopamine Transporter Genes Polymorphisms and Personality Traits in Addicted Patients

Many factors are involved in addiction. The dopaminergic system is thought to be the key element in this process. The mesolimbic dopamine system is a crucial element in the reward system. Changes in this system are thought to be leading to substance use disorders and dependence. Therefore, for our study we chose an analysis of two polymorphisms in genes (Variable Number of Tandem Repeats in DRD4 and DAT1) responsible for dopaminergic transmission, which might be implicated in the scores of personality traits measured by the NEO-FFI test. The study group consisted of 600 male volunteers—299 addicted subjects and 301 controls. Both groups were recruited by psychiatrists; in the case group addiction was diagnosed; in the controls a mental illness was excluded. In both groups the same psychometric test and genotyping by the PCR VNTR method were performed. The results were investigated by a multivariate analysis of the main effects ANOVA. In the presented study no DRD4 main effects were found for any of the analyzed traits but the DRD4 main effects approximated to the statistical significance for the extraversion scale. However, no DAT1 main effects were found for any of the analyzed traits but the DAT1 main effects approximated to the statistical significance for the agreeability scale.These associations open new possibilities for addiction research.

Personality as an intermediate phenotype for genetic dissection of alcohol use disorder

Genetic and environmental interactive influences on predisposition to develop alcohol use disorder (AUD) account for the high heterogeneity among AUD patients and make research on the risk and resiliency factors complicated. Several attempts have been made to identify the genetic basis of AUD; however, only few genetic polymorphisms have consistently been associated with AUD. Intermediate phenotypes are expected to be in-between proxies of basic neuronal biological processes and nosological symptoms of AUD. Personality is likely to be a top candidate intermediate phenotype for the dissection of the genetic underpinnings of different subtypes of AUD. To date, 38 studies have investigated personality traits, commonly assessed by the Cloninger's Tridimensional Personality Questionnaire (TPQ) or Temperament and Character Inventory (TCI), in relation to polymorphisms of candidate genes of neurotransmitter systems in alcohol-dependent patients. Particular attention has been given to the functional polymorphism of the serotonin transporter gene (5-HTTLPR), however, leading to contradictory results, whereas results with polymorphisms in other candidate monoaminergic genes (e.g., tryptophan hydroxylase, serotonin receptors, monoamine oxidases, dopamine receptors and transporter) are sparse. Only one genome-wide association study has been performed so far and identified the ABLIM1 gene of relevance for novelty seeking, harm avoidance and reward dependence in alcohol-dependent patients. Studies investigating genetic factors together with personality could help to define more homogenous subgroups of AUD patients and facilitate treatment strategies. This review also urges the scientific community to combine genetic data with psychobiological and environmental data to further dissect the link between personality and AUD.

Val158Met COMT polymorphism and risk of aggression in alcohol dependence

Aggression, violence and antisocial behavior are common in alcoholism, but their biological basis is poorly understood. Several studies and recent meta-analyses indicate that in schizophrenia the catecholamine-O-methyltransferase (COMT) Val158Met genotype may be associated with aggression, most often in methionine allele carriers. We tested this hypothesis in a sample of treatment-seeking alcohol-dependent in-patients (293 German patients and 499 controls, and additional 190 Polish patients as replication sample). As expected, patients with a history of violent or non-violent crime were more often male, had an earlier onset of alcoholism and more withdrawal seizures and delirium tremens, and were more likely to have a history of suicide attempts. COMT genotype was not associated with a history of violent or non-violent crime. More studies are needed on the neurobiological basis of aggression and violence in alcoholism.

Lack of association between SNPs in the NEUROD2 gene and alcohol dependence in a German patient sample

Results of a human post mortem study performed by our own group have suggested that the transcription factor NEUROD2, which plays a role in neuronal development, as well as in the development of anxiety and risk behavior in mice, might be a susceptibility factor for addictive disorders. Therefore the aim of the present study was to analyze a possible relation between genetic variants in the NEUROD2 gene and alcohol dependence in a sample of the Munich Gene Bank of Alcoholism (MGBA). We performed single SNP (single nucleotide polymorphism) and haplotype studies in 430 alcohol-dependent patients and 365 healthy controls with four SNPs covering the gene region of NEUROD2. Neither single SNP nor haplotype analysis could detect significant associations with alcohol dependence. Additionally we could not detect any relation of the analyzed genetic variants to Cloninger's Type 1/2 or Babor's Type A/B classification, to the age of onset or to the amount of alcohol intake. Our results do not provide evidence for an involvement of NEUROD2 polymorphisms in the pathophysiology of alcohol dependence. Further association studies are needed to confirm our findings.

GABA-A2 receptor subunit gene (GABRA2) polymorphisms and risk for alcohol dependence

Gamma-aminobutyric acid (GABA) A receptors are believed to mediate some of the physiological and behavioral actions of ethanol. Recent studies have suggested that genetic variants of the GABA-A receptor alpha2 subunit gene (GABRA2) are associated with alcohol dependence. The aim of this study is to confirm and extend the role of GABRA2 haplotypes in the liability to alcohol dependence. 291 (231 male) treatment-seeking alcohol-dependent individuals and 295 (153 male) control subjects were enrolled into the study. Characteristics of alcohol dependence were obtained using the SSAGA (semi-structured assessment of the genetics of alcoholism, German Version). Genotyping of 10 SNPs across the GABRA2 gene was performed following previous reports and using PCR. One genetic variant was detected to significantly differ between alcohol-dependent subjects and controls. Two common 8 SNP haplotypes and their complementary alleles were identified containing this SNP and were present in 89.9% of controls and 93.4% of the alcohol-dependent individuals. One of the haplotypes (T-C-A-C-A-T-T-C) was significantly associated with alcohol dependence and characteristics of alcohol withdrawal and severity of alcohol dependence (delirium tremens, withdrawal seizures). These findings support and extend the three previous studies implicating a GABA-A receptor subunit as contributing to the genetic risk for alcohol dependence. Possible implications of these findings are discussed.

Approach to the genetics of alcoholism: A review based on pathophysiology

Alcohol dependence is a common disorder with a heterogenous etiology. The results of family, twin and adoption studies on alcoholism are reviewed. These studies have revealed a heritability of alcoholism of over 50%. After evaluating the results, it was epidemiologically stated that alcoholism is heterogenous complex disorder with a multiple genetic background. Modern molecular genetic techniques allow examining specific genes involved in the pathophysiology of complex diseases such as alcoholism. Strategies for gene identification are introduced to the reader, including family-based and association studies. The susceptibility genes that are in the focus of this article have been chosen because they are known to encode for underlying mechanisms that are linked to the pathophysiology of alcoholism or that are important for the pharmacotherapeutic approaches in the treatment of alcohol dependence. Postulated candidate genes of the metabolism of alcohol and of the involved neurotransmitter systems are introduced. Genetic studies on alcoholism examining the metabolism of alcohol and the dopaminergic, GABAergic, glutamatergic, opioid, cholinergic and serotonergic neurotransmitter systems as well as the neuropeptide Y are presented. The results are critically discussed followed by a discussion of possible consequences.

The association of DRD4 and novelty seeking is found in a nonhuman primate model

OBJECTIVE

The association of novelty seeking with a repeat polymorphism in the coding region of the dopamine D4 receptor gene (DRD4) has been demonstrated in several human populations, but not in others. The objective of this study was to test the generality of the association in a captive nonhuman primate population of known history, using objective methods for assessing novelty seeking and a pedigree-based association design.

METHODS

Four hundred and fifty two socially-living vervet monkeys (Cercopithecus aethiops) from a large multigenerational pedigree at the UCLA-VA Vervet Research Colony were studied. Two variants in the 48 base pair repeat in exon III of the DRD4 gene have been found in this population, a six-repeat (92%) and a less common five-repeat (8%). Novelty seeking was measured by the latency to approach a large and potentially threatening novel object placed in the home enclosure. Heritability of novelty seeking and the association of novelty seeking with the DRD4 polymorphism were assessed using variance component modeling as implemented in Sequential Oligogenic Linkage Analysis Routines.

RESULTS

The variance component analysis indicated that the DRD4 variant explained a significant portion of the total variance in novelty seeking. The final model included a significant effect of the DRD4 polymorphism (P=0.03), which explained 13% of the phenotypic variance, and a significant remaining genetic effect (h=0. 467+/-0.095, P<0.0001).

CONCLUSIONS

The association of DRD4 with novelty seeking has now been replicated in a nonhuman primate species, the vervet monkey.

The C(-1019)G 5-HT1A promoter polymorphism and personality traits: no evidence for significant association in alcoholic patients

The 5HT1A receptor is one of at least 14 different receptors for serotonin which has a role in moderating several brain functions and may be involved in the aetiology of several psychiatric disorders. The C(-1019)G 5-HT1A promoter polymorphism was reported to be associated with major depression, depression-related personality traits and suicidal behavior in various samples. The G(-1019) allele carriers are prone to depressive personality traits and suicidal behavior, because serotonergic neurotransmission is reduced. The aim of this study is to replicate previous findings in a sample of 185 Alcohol-dependent individuals. Personality traits were evaluated using the NEO FFI and TCI. History of suicidal behavior was assessed by a standardized semistructured interview (SSAGA). No significant differences across C(-1019)G 5-HT1A genotype groups were found for TCI temperament and character traits and for NEO FFI personality scales. No association was detected between this genetic variant and history of suicide attempts. These results neither support a role of C(-1019)G 5-HT1A promoter polymorphism in the disposition of personality traits like harm avoidance or neuroticism, nor confirm previous research reporting an involvement of the G allele in suicidal behavior in alcoholics. Significant associations, however, were detected between Babor's Type B with number of suicide attempts in history, high neuroticism and harm avoidance scores in alcoholics.

Ionotropic glutamate receptor gene GRIK3 SER310ALA functional polymorphism is related to delirium tremens in alcoholics

Upregulation of glutamatergic neurotransmission resulting from chronic ethanol intoxication may cause a hyperexcitable state during alcohol withdrawal, which may lead to seizures and delirium tremens. The aim of our study was to evaluate the association between a history of alcohol withdrawal-induced seizures and delirium tremens, and a functional polymorphism (Ser310Ala) of the GRIK3 gene coding for the glutamatergic kainate receptor subunit GlurR7 in a sample of well-characterized alcoholics compared to controls. In total, 233 patients meeting DSM-IV alcohol dependence criteria and 309 controls, all of German descent, were investigated. GRIK3 functional polymorphism was determined using PCR (polymerase chain reaction) of lymphocyte DNA. History of alcohol withdrawal-induced delirium tremens and seizures were obtained using the SSAGA (Semi-Structured Assessment for the Genetics of Alcoholism). Data were cross-checked with in-patients' clinical files. While a significant relationship between history of delirium tremens and the Ser310 allele was detected, no significant results were obtained for alcohol withdrawal-related seizures. Although this result is suggestive for a significant role of this polymorphism in the pathogenesis of delirium tremens in alcohol-dependent individuals, further investigation and confirmation are warranted.

Impact of different personality dimensions (NEO Five-Factor Inventory) on the outcome of alcohol-dependent patients 6 and 12 months after treatment

There is growing evidence that outpatient treatment might be equally effective as inpatient treatment for alcohol-dependent patients. Nevertheless, it is still unclear which patient group benefits most from outpatient treatment. Some personality traits such as emotional instability and high neuroticism are known risk factors for relapse after treatment. The purpose of the following study was to investigate the possible relationship between personality traits, measured by the NEO Five-Factor Inventory (NEO-FFI), and outcome after an intensive outpatient treatment program. Seventy-four alcohol-dependent patients who had completed the outpatient treatment program were included. Patients were examined at the beginning and end of the treatment, and 6 and 12 months after completion of the treatment. At the 6- and 12-month follow-up, 97% of the patients were successfully located and personally interviewed. Analyses revealed that 54 patients (73%) had been abstinent for the entire 12-month follow-up period. Twenty patients (27.4%) were found to be non-abstinent. Of these patients, 10 (13.5%) were found to be improved and 10 (13.5%) to have relapsed. Results indicate a remarkable stability of NEO-FFI scores over time, giving evidence for the usefulness of this instrument for the assessment of personality dimensions in alcohol-dependent patients. Patients who relapsed within 12 months after treatment had significantly higher scores on the personality domains of "neuroticism" and lower scores on the personality domains of "conscientiousness" than abstainers. These results are in line with prior research on this topic.

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.

Polymorphisms in the NMDA subunit 2B are not associated with alcohol dependence and alcohol withdrawal-induced seizures and delirium tremens

OBJECTIVE

Ethanol-inhibited glutamatergic neurotransmission has been shown to mediate pathophysiological mechanisms in the development of alcoholism, including withdrawal symptoms. NMDA-receptor 2B (NR2B) is a subunit that confers a high sensitivity to ethanol-induced inhibition. Previously we had reported a lack of association between the single nucleotide polymorphism (SNP) rs1806201 in the NR2B gene (GRIN2B) and alcoholism. Shortly thereafter, an association between the polymorphism and early-onset alcoholism has been reported. One aim of the present study was to test whether the association between the GRIN2B polymorphism rs1806201 and early-onset alcoholism can be replicated in a larger sample. Moreover, we hypothesized that another genetic variation within GRIN2B (rs1806191) may have an effect in the etiology of alcoholism or withdrawal-related traits.

METHODS

We extended our original study sample to a size of 377 patients and 464 healthy volunteers and performed a replication study, including the second GRIN2B SNP. Associations between allele, genotype and haplotype frequencies of the two polymorphisms and alcoholism as well as with patients' phenotypes were investigated.

RESULTS

No associations were found between any of the two polymorphisms, tested individually or as haplotypes, and alcoholism, respectively withdrawal-related traits.

CONCLUSION

Neither the analyzed SNPs nor any of their haplotypes likely modify susceptibility to alcohol dependence or withdrawal-related phenotypes.

Neurobiological dimensional models of personality: a review of the models of Cloninger, Depue, and Siever

Neurobiological dimensional models of personality aim to account for the structure of personality traits and disorders through links to neurotransmitter systems. Three such models are reviewed: those of Cloninger, Depue, and Siever. While these proposals have heuristic value, none of them has obtained strong empirical support. Our current understanding of neurobiology is insufficient to develop a model of personality on this basis. At this point, dimensions of personality should be derived from factor analysis rather than from neurobiological theories.

GRIN1 locus may modify the susceptibility to seizures during alcohol withdrawal

N-Methyl-D-aspartate (NMDA) receptors, members of the glutamate receptor channel superfamily, are generally inhibited by alcohol. The expression and alternative splicing of the obligatory NR1 subunit is altered by alcohol exposure, emphasizing the involvement of the NR1 subunit, which is coded by the GRIN1 gene, in alcohol-mediated effects. We performed an association study in patients with alcohol dependence with the GRIN1 locus. Two independent case control samples consisting of a total of 442 alcohol-dependent patients and 442 unrelated controls were included. There was no overall difference in allele or genotype frequency between patients and controls. However, the 2108A allele and A-containing genotypes were over-represented in the patients with a history of withdrawal-induced seizures when compared to healthy volunteers (allele: chi(2) = 5.412, df = 1, P = 0.020) or an independent sample of patients without a history of seizures (allele: chi(2) = 4.185, df = 1, P = 0.041). Age at onset, years of alcohol dependence, and a history of delirium tremens did not differ between genotype or allele groups. These findings support the hypothesis that the GRIN1 locus may modify the susceptibility to seizures during alcohol withdrawal. This novel finding warrants replication.

Genetic variants implicated in personality: a review of the more promising candidates

Alleles of the serotonin transporter gene (SERT) and the dopamine 4 receptor gene (DRD4) were first associated with anxiety-related and novelty-seeking personality traits, respectively, in 1996. These early successes precipitated a flood of research into the genetic basis of personality; a quest that has yet to yield decisive answers. Here, both the theoretical and the empirical evidence implicating specific loci-in particular SERT and DRD4-in the development of personality is evaluated. Despite a paucity of statistically significant results following post-hoc analysis, and an excess of positive results derived from studies with small sample sizes, the existence of a genuine effect is argued for: a gene-personality relationship rendered periodically latent through genetic epistasis, gene-environment interactions, variation in genetic background, and the presence of other confounding variables.

No association of CRH1 receptor polymorphism haplotypes, harm avoidance and other personality dimensions in alcohol dependence: results from the Munich gene bank project for alcoholism

Because corticotrophin-releasing hormone (CRH) plays a central role in stress regulation, the possible role of CRH1 polymorphism for anxiety-related personality variables such as harm avoidance possibly associated with alcoholism was studied. The research instruments used to phenotype patients were adopted partly from the US collaborative study of the genetics of alcoholism and include a number of personality inventories such as the temperament and character inventory (TCI). Based on the examination of 170 alcoholic subjects no association was found between CRH1 receptor haplotypes of four single nuclotid polymorphisms (SNPs) and low and high temperament traits of harm avoidance, novelty seeking and reward dependence. The possible implications of these findings are discussed.

A dyslexia susceptibility locus (DYX7) linked to dopamine D4 receptor (DRD4) region on chromosome 11p15.5

Dyslexia is a disability in acquiring reading and spelling skills that is independent of general intelligence and educational opportunity, and is highly heritable. It is known that dyslexia often co-occurs with attention deficit hyperactivity disorder (ADHD), and the 7-repeat allele of the 48-bp tandem repeat in exon 3 of the dopamine D4 receptor (DRD4) has been implicated in ADHD. We, therefore, investigated DRD4 as a candidate gene for dyslexia by testing for linkage and association with 14 markers at and around the DRD4 locus on chromosome 11p15.5. Using 100 families having at least two siblings affected with dyslexia, model-free linkage analysis revealed evidence for linkage to the DRD4-exon 3 repeat (two-point MFLOD = 2.27, P = 0.001) and to HRAS located just proximal to DRD4 (two-point MFLOD = 2.68, P = 0.0004). Evidence for linkage was maximal between DRD4 and HRAS (three-point MFLOD = 3.57, P = 0.00005). However, linkage disequilibrium analysis showed no significant evidence for association between dyslexia and DRD4 or HRAS. In particular, dyslexic subjects showed no significant increase of the DRD4 7-repeat allele associated with ADHD. It is possible that other DRD4 variants, not in strong linkage disequilibrium with the exon 3 repeat polymorphism, or alternatively, another gene very closely linked to DRD4, may influence susceptibility to dyslexia.

Effects of dopamine receptor D4 variation on alcohol and tobacco use and on novelty seeking: multivariate linkage and association analysis

The dopamine D4 receptor gene contains a polymorphic sequence consisting of a variable number of 48-base-pair (bp) repeats, and there have been a number of reports that this polymorphism is associated with variation in novelty seeking or in substance abuse and addictive behaviors. In this study we have assessed the linkage and association of DRD4 genotype with novelty seeking, alcohol use, and smoking in a sample of 377 dizygotic twin pairs and 15 single twins recruited from the Australian Twin Registry (ATR). We found no evidence of linkage or association of the DRD4 locus with any of the phenotypes. We made use of repeated measures for some phenotypes to increase power by multivariate genetic analysis, but allelic effects were still non-significant. Specifically, it has been suggested that the DRD4 7-repeat allele is associated with increased novelty seeking in males but we found no evidence for this, despite considerable power to do so. We conclude that DRD4 variation does not have an effect on use of alcohol and the problems that arise from it, on smoking, or on novelty seeking behavior.

Genetics of personality: are we making progress?

For centuries, scientists are intrigued by the differences in personality between individuals. As early as in the ancient Greek civilization, people tried to formulate theories to systematize this diversity. With the increased interest in behavior genetics, personality was also considered a challenging phenotype. From the early start, studies suggested a heritable component in personality. After the successes of molecular genetic studies in unraveling the genetic basis of (mostly) monogenic diseases, the focus shifted towards complex traits, including psychiatric disorders. It was observed in several studies that personality measures differed between patients with psychiatric disorders and healthy controls. Therefore, normal personality was considered a viable endophenotype in the search for genes involved in psychiatric disorders such as affective disorders, ADHD and substance dependence. Genes that were to be found in studies on personality could be candidate genes for particular psychiatric disorders. In the course of time, however the study of genes for personality turned out to be at least as hard as the search for genes involved in other complex disorders. In this review, past studies, present problems and future directions concerning the study of personality genetics are discussed.