The dopamine D2 receptor gene T aq I A polymorphism is not associated with Novelty Seeking, Harm Avoidance and Reward Dependence in healthy subjects

Relationship among methadone dose, polymorphisms of dopamine D2 receptor and tri-dimensional personality questionnaire in heroin-addicted patients

BACKGROUND

We investigated whether variation in the dopamine D2 receptor gene (DRD2) and tri-dimensional personality questionnaire (TPQ) scores could be used to aid adjustment of daily methadone requirements of heroin addicts. DRD2 TaqI B polymorphisms and TPQ scores were determined in 138 male Taiwanese heroin addicts who were receiving methadone treatment. Borderline index (harm avoidance + novelty seeking-reward dependence) was calculated for each subject, and three groups were defined: high (mean from all subjects plus 1 standard deviation, or greater), low (half of the calculated high score, or lower) and medium (all values between the high and low scores).

RESULTS

No significant differences in age (p = 0.60), mean methadone dose (p = 0.75) or borderline index group (p = 0.25) were observed between subjects bearing the B1/B1, B1/B2 and B2/B2 DRD2 TaqI genotypes. Among the individuals with low (≤10), medium (11-20) and high (≥21) borderline index scores, there was a significant difference in mean methadone dose (p = 0.04), but not age (p = 0.90). Further analysis showed that mean methadone dose was significantly higher in subjects with low borderline index scores than in those with high scores (62.5 vs. 47.0 mg/day, p = 0.03). The odds ratio for a daily methadone requirement ≥60 mg (median dose across the 138 subjects) was 2.64-fold greater in the low borderline index group than in the high group (p = 0.04).

CONCLUSIONS

Although the DRD2 TaqI B genotype was not associated with methadone use requirements, borderline index was revealed as a potential predictive marker for the adjustment of methadone dosage requirements in heroin addicts.

Biological mechanisms underlying evolutionary origins of psychotic and mood disorders

Psychotic and mood disorders are brain dysfunctions that are caused by gene environment interactions. Although these disorders are disadvantageous and involve behavioral phenotypes that decrease the reproductive success of afflicted individuals in the modern human society, the prevalence of these disorders have remained constant in the population. Here, we propose several biological mechanisms by which the genes associated with psychotic and mood disorders could be selected for in specific environmental conditions that provide evolutionary bases for explanations of when, why, and where these disorders emerged and have been maintained in humans. We discuss the evolutionary origins of psychotic and mood disorders with specific focuses on the roles of dopamine and serotonin in the conditions of social competitiveness/hierarchy and maternal care and other potential mechanisms, such as social network homophily and symbiosis.

Sexually dimorphic link between dopamine D2 receptor gene and neuroticism-anxiety

Prior theory-driven research probing the association between dopaminergic candidate genes and human personality has focused on the trait of novelty seeking. Here, we examined the association between the dopamine D2 receptor (DRD2) TaqI A polymorphism and two other personality traits, neuroticism-anxiety and agentic extraversion. We found no significant associations for agentic extraversion. However, for men, but not for women, we observed a strong and specific association between low neuroticism-anxiety and the A1+ allele of the DRD2 TaqI A polymorphism across two independent samples and across two alternative personality scales. We conclude that new theoretical models are needed to account for these and other recent reports of associations between neuroticism-anxiety and brain dopamine, which cannot be interpreted within the traditional framework.

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.

Advances in molecular genetics and the prevention and treatment of substance misuse: Implications of association studies of the A1 allele of the D2 dopamine receptor gene

Substance misuse is influenced by multiple genetic and environmental factors. Recent research has identified a number of potential genetic markers of risk and those associated with drug reward substrates show particular promise. The current study reexamines the extant published data of the association between the D2 dopamine receptor (DRD2) gene minor Taq 1A (A1) allele and substance misuse risk. A series of meta-analyses was performed on 64 studies examining DRD2 A1+ allelic status and substance misuse. In addition, personality was examined as a possible endophenotype. Significant association was found between the A1 allele and severe substance dependence in both Caucasian and non-Caucasian groups. The data did not support a significant association between the A1 allele and personality features. While the specific mechanism underlying these associations requires further elucidation, this genetic marker shows promise as a marker of brain reinforcement processes. Possible ways of utilising the A1 allele to inform prevention and treatment initiatives are discussed.

Genetic polymorphisms and personality in healthy adults: a systematic review and meta-analysis

A meta-analysis was conducted on studies reporting data on associations between candidate genes and human personality. Studies reporting data for psychiatric populations (including organic disease and substance abuse) were excluded. A total of 46 studies contributed to the analysis. Pooled data using a fixed-effects model suggested significant associations between the 5HTT LPR, DRD4 c>t, DRD4 length, DRD2 A1/A2, DRD3 A1/A2 polymorphisms and personality traits. A multivariate analysis using a mixed-effects model and including age, sex and predominant ethnicity as covariates was applied to the analyses of 5HTT LPR and DRD4 length polymorphism data. Only the association between the 5HTT LPR polymorphism and avoidance traits remained significant (P=0.038). However, sensitivity analyses excluding data from studies reporting allele frequencies not in Hardy-Weinberg equilibrium and unpublished data resulted in this association no longer being significant. Implications for the design of future association studies of human personality are discussed, including the likely sample sizes that will be required to achieve sufficient power and the potential role of moderating variables such as sex.

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.

The dopamine D2 receptor (DRD2) gene-a genetic risk factor in heavy smoking?

Indications of a genetic predisposition to nicotine dependence have been derived from numerous epidemiological data and from individual genetic studies suggesting the involvement of the dopaminergic D2 receptor. Previous association studies defined the TaqlA polymorphism as a risk factor for addiction, in particular for alcoholism and tobacco dependence. Results of investigations into this polymorphism in 110 severely addicted smokers and a control group of 60 population-matched German non-smokers did not support these findings. However, our results indicate an association between the DRD2-Fokl-1 allele and the onset and intensity of smoking.

Reappraisal of the association between the DRD2 gene, alcoholism and addiction

We analysed the impact of the TaqI A1 allele of the D2 dopamine receptor gene on the risk for alcoholism, trying to depict three explanations frequently proposed to explain discrepancies in association and linkage studies: that the A1 allele may act as a marker rather than as a vulnerability factor, that stratification biases and unevaluated controls may explain positive results, and that the A1 allele is modifying the phenotype rather than increasing the risk for alcoholism. We thus tested another (dinucleotide STRP) marker within the DRD2 gene, selected a new homogenous sample of 113 alcoholic patients and 49 unaffected controls strictly matched for ethnic origins, and systematically assessed both samples with a semi-structured interview to detect (in both samples) alcohol dependence, but also such related traits as specificities of complications. The frequency of the A1 allele was not significantly different between alcoholics and controls but when comparing different subgroups of alcoholics, the A1 allele was significantly more frequent in alcoholic patients with somatic complications (OR = 3.00, CI[1.37-6.62]), social and professional complications (OR = 2. 72, CI[1.25-5.90]), or with co-morbid dependence (OR = 2.88, 95% IC [1.16-7.15]). The association for co-morbid dependence and somatic complications was also positive when taking into consideration both STRP and TaqIA polymorphisms. The A1 allele does not increase the risk for alcoholism per se in our sample, but may be involved in a related trait which is partially dependent on the diagnosis of alcoholism, through a disequilibrium with another close mutation.