Haplotype study of three polymorphisms at the dopamine transporter locus confirm linkage to attention-deficit/hyperactivity disorder

Dopaminergic system and neurons: Role in multiple neurological diseases

The dopaminergic system is a complex and powerful neurotransmitter system in the brain. It plays an important regulatory role in motivation, reward, cognition, and motor control. In recent decades, research in the field of the dopaminergic system and neurons has increased exponentially and is gradually becoming a point of intervention in the study and understanding of a wide range of neurological diseases related to human health. Studies have shown that the dopaminergic system and neurons are involved in the development of many neurological diseases (including, but not limited to Parkinson's disease, schizophrenia, depression, attention deficit hyperactivity disorder, etc.) and that dopaminergic neurons either have too much stress or too weak function in the dopaminergic system can lead to disease. Therefore, targeting dopaminergic neurons is considered key to treating these diseases. This article provides a comprehensive review of the dopaminergic system and neurons in terms of brain region distribution, physiological function and subtypes of dopaminergic neurons, as well as the role of the dopaminergic system and neurons in a variety of diseases.

A person-centered data analytic approach to dopaminergic polygenic moderation of child maltreatment exposure

The present study illustrates the utility of latent class analysis, a person-centered data analytic approach, as an innovative method for identifying naturally occurring patterns of polygenic risk, specifically within the dopaminergic system. Moreover, this study tests whether latent classes of polygenic variation moderate the effect of child maltreatment exposure on internalizing symptoms among African ancestry youth. African ancestry youth were selected for this study because youth of color are overrepresented in the child welfare system and because African ancestry individuals are significantly underrepresented in genomics research. Results identified three latent classes of dopaminergic gene variation. Class 1 was marked predominately by homozygous minor alleles, Class 2 was characterized by homozygous major and heterozygous presentations, and Class 3 was marked by heterozygous alleles on the DAT-1 single-nucleotide polymorphisms (SNPs) and a combination of homozygous major and minor alleles on the other SNPs. Results indicated that a greater number of maltreatment subtypes experienced were associated with higher internalizing symptoms only for children with the latent polygenic Class 2 pattern. This latent class was distinctly characterized by more homozygous major or heterozygous allelic presentations along all three DAT-1 SNPs. This significant latent polygenic class by environment interaction was replicated in an independent replication sample. Together, findings suggest that African ancestry children with a pattern of dopaminergic variation characterized by this specific combination of polygenic variation are more vulnerable to developing internalizing symptoms following maltreatment exposure, relative to their peers with other dopamine-related polygenic patterns.

The Effects of COMT Polymorphism on Cortical Thickness and Surface Area Abnormalities in Children with ADHD

The catechol-O-methyltransferase (COMT) gene is associated with frontal cortex development and the pathophysiology of attention-deficit/hyperactivity disorder (ADHD). However, how the COMT gene impacts brain structure and behavior in ADHD remains unknown. In the present study, we identify the effect of COMT on cortical thickness and surface area in children with ADHD and children with typically developing (TD) using a machine learning approach. In a sample of 39 children with ADHD and 34 age- and IQ-matched TD children, we found that cortical thickness and surface area differences were predominantly observed in the frontal cortex. Furthermore, a path analysis revealed that a COMT genotype affected abnormal development of the frontal cortex in terms of both cortical thickness and surface area and was associated with working memory changes in children with ADHD. Our study confirms that the role of COMT in ADHD is not restricted to the development of behavior but may also affect the cortical thickness and surface area. Thus, our findings may help to improve the understanding of the neuroanatomic basis for the relationship between the COMT genotype and ADHD pathogenesis.

Catechol-O-methyltransferase Val158Met Genotype and Early-Life Family Adversity Interactively Affect Attention-Deficit Hyperactivity Symptoms Across Childhood

Attention-deficit hyperactivity disorder (ADHD) is among the most commonly diagnosed psychiatric disorders of childhood. The dopaminergic system has been shown to have substantial effects on its etiology, with both functional Catechol-O-methyltransferase (COMT) Val158Met genotype and early-life environmental adversity involved in the risk of inattention and hyperactivity/impulsivity symptoms. In this prospective longitudinal study, we examined for the first time the impact of proximal and distal early-life family adversity and COMT Val158Met polymorphism gene - both the direct and the interactive effects, on children's ADHD symptoms across childhood. Data came from the Family Life Project, a prospective longitudinal study of 1,292 children and families in high poverty from birth to 11 years. In infancy, data regarding socioeconomic (SES)-risk-factors, observed-caregiving behaviors, and DNA genotyping were collected. In early and middle childhood teachers rated the occurrence and severity of the child's ADHD symptoms. Multilevel growth curve models revealed independent effects of COMT, early-life SES-risk and negative caregiving on ADHD symptoms in early and middle childhood. Significant gene-environment interactions were found, indicating that overall, carriers of at least one COMT158Met allele were more sensitive to early-life adversity, showing higher inattention and hyperactivity/impulsivity symptoms severity in childhood when exposed to high SES-risk factors in infancy, compared to Val-Val carriers. Findings provide new insights into the complex etiology of ADHD and underline the need for further investigation of the neuronal mechanisms underlying gene-environment interactions. Findings might have implications for prevention and intervention strategies with a focus on early-life family relationships in genetically at-risk children.

Emotional insecurity as a mediator of the moderating role of dopamine genes in the association between interparental conflict and youth externalizing problems

This study tested whether the association between interparental conflict and adolescent externalizing symptoms was moderated by a polygenic composite indexing low dopamine activity (i.e., 7-repeat allele of DRD4; Val alleles of COMT; 10-repeat variants of DAT1) in a sample of seventh-grade adolescents (Mean age = 13.0 years) and their parents. Using a longitudinal, autoregressive design, observational assessments of interparental conflict at Wave 1 predicted increases in a multi-informant measurement of youth externalizing symptoms 2 years later at Wave 3 only for children who were high on the hypodopaminergic composite. Moderation was expressed in a “for better” or “for worse” form hypothesized by differential susceptibility theory. Thus, children high on the dopaminergic composite experienced more externalizing problems than their peers when faced with more destructive conflicts but also fewer externalizing problems when exposed to more constructive interparental conflicts. Mediated moderation findings indicated that adolescent reports of their emotional insecurity in the interparental relationship partially explained the greater genetic susceptibility experienced by these children. More specifically, the dopamine composite moderated the association between Wave 1 interparental conflict and emotional insecurity 1 year later at Wave 2 in the same “for better” or “for worse” pattern as externalizing symptoms. Adolescent insecurity at Wave 2, in turn, predicted their greater externalizing symptoms 1 year later at Wave 3. Post hoc analyses further revealed that the 7-repeat allele of the dopamine receptor D4 (DRD4) gene was the primary source of plasticity in the polygenic composite. Results are discussed as to how they advance process-oriented Gene x Environment models of emotion regulation.

No Influence of Dopamine System Gene Variations on Acute Effects of MDMA

3,4-Methylenedioxymethamphetamine (MDMA, ecstasy) is a recreational substance also investigated as medication for posttraumatic stress disorder. Dopamine (DA) system stimulation likely contributes to the acute mood effects of amphetamines, including MDMA. Genetic variants, such as single-nucleotide polymorphisms (SNPs), and polymorphic regions of the DA system genes may in part explain interindividual differences in the acute responses to MDMA in humans. We characterized the effects of common genetic variants within genes coding for key players in the DA system including the dopamine D2 receptor (DRD2/ANKK1 rs1800497, DRD2 rs6277, and rs107959), the dopamine transporter (DAT1 rs28363170, rs3836790, rs6347, rs11133767, rs11564774, rs460000, and rs463379), and dopamine D4 receptor [DRD4, variable-number tandem repeat (VNTR)] on the subjective and autonomic response to MDMA (125 mg) in pooled data from randomized, placebo-controlled, crossover studies in a total of 149 healthy subjects. Plasma concentrations of MDMA were used as covariate in the analysis to control for individual pharmacokinetic (metabolic and weight) differences. None of the tested genetic polymorphisms within the DA system altered effects of MDMA when adjusting for multiple comparisons. Genetic variations in genes coding for players of the DA system are unlikely to explain interindividual variations in the acute effects of MDMA in humans.

Application of Research Domain Criteria to childhood and adolescent impulsive and addictive disorders: Implications for treatment

The Research Domain Criteria (RDoC) initiative provides a large-scale, dimensional framework for the integration of research findings across traditional diagnoses, with the long-term aim of improving existing psychiatric treatments. A neurodevelopmental perspective is essential to this endeavor. However, few papers synthesizing research findings across childhood and adolescent disorders exist. Here, we discuss how the RDoC framework may be applied to the study of childhood and adolescent impulsive and addictive disorders in order to improve neurodevelopmental understanding and to enhance treatment development. Given the large scope of RDoC, we focus on a single construct highly relevant to addictive and impulsive disorders - initial responsiveness to reward attainment. Findings from genetic, molecular, neuroimaging and other translational research methodologies are highlighted.

Gene polymorphisms associated with temperament

When individuals are exposed to stressful environmental challenges, the response varies widely in one or more of three components: psychology, behavior and physiology. This variability among individuals can be defined as temperament. In recent years, an increasing large body of evidence suggests that the dimensions of temperament, as well as personality, psychological disorders and behavioral traits, are influenced by genetic factors, and much of the variation appears to involve variation in genes or gene polymorphisms in the hypothalamic-pituitary-adrenocortical (HPA) axis and the behavior-controlling neurotransmitter networks. Here, we review our current understanding of the probabilistic impact of a number of candidate gene polymorphisms that control temperament, psychological disorders and behavioral traits in animals and human, including the gene polymorphisms related to corticotrophin-releasing hormone (CRH) production and adrenal cortisol production involved in the HPA axis, and a large number of gene polymorphisms in the dopaminergic and serotonergic neurotransmitter networks. It will very likely to assist in diagnosis and treatment of human relevant disorders, and provide useful contributions to our understanding of evolution, welfare and conservation, for animals in the wild and in production systems. Additionally, investigations of gene-gene and gene-environment complex interactions in humans and animals need further clear illustration.

Child maltreatment, impulsivity, and antisocial behavior in African American children: Moderation effects from a cumulative dopaminergic gene index

A model examining the effects of an increasing number of maltreatment subtypes experienced on antisocial behavior, as mediated by impulsivity and moderated by a polygenic index of dopaminergic genotypes, was investigated. An African American sample of children (N = 1,012, M age = 10.07) with and without maltreatment histories participated. Indicators of aggression, delinquency, and disruptive peer behavior were obtained from peer- and counselor-rated measures to form a latent variable of antisocial behavior; impulsivity was assessed by counselor report. Five genotypes in four dopaminergic genes (dopamine receptors D4, D2, known as DRD4, DRD2; dopamine active transporter 1, known as DAT1; and catechol-O-methyltransferase, known as COMT) conferring heightened environmental sensitivity were combined into one polygenic index. Using structural equation modeling, a first-stage, moderated-mediation model was evaluated. Age and sex were entered as covariates, both as main effects and in interaction with maltreatment and the gene index. The model had excellent fit: χ2 (32, N = 1,012) = 86.51, p < .001; comparative fit index = 0.982, Tucker-Lewis index = 0.977, root mean square error of approximation = 0.041, and standardized root mean square residual = 0.022. The effect of maltreatment subtypes on antisocial behavior was partially mediated by impulsivity (β = 0.173, p < .001), and these relations were moderated by the number of differentiating dopaminergic genotypes. Specifically, a significant Gene × Environment interaction (β = 0.016, p = .013) indicated that the relation between maltreatment and impulsivity was stronger as children evinced more differentiating genotypes, thereby strengthening the mediational effect of impulsivity on antisocial behavior. These findings elucidate the manner by which maltreated children develop early signs of antisocial behavior, and the genetic mechanisms involved in greater vulnerability for maladaptation in impulse control within the context of child maltreatment.

Developmental stress elicits preference for methamphetamine in the spontaneously hypertensive rat model of attention-deficit/hyperactivity disorder

BACKGROUND

Developmental stress has been hypothesised to interact with genetic predisposition to increase the risk of developing substance use disorders. Here we have investigated the effects of maternal separation-induced developmental stress using a behavioural proxy of methamphetamine preference in an animal model of attention-deficit/hyperactivity disorder, the spontaneously hypertensive rat, versus Wistar Kyoto and Sprague-Dawley comparator strains.

RESULTS

Analysis of results obtained using a conditioned place preference paradigm revealed a significant strain × stress interaction with maternal separation inducing preference for the methamphetamine-associated compartment in spontaneously hypertensive rats. Maternal separation increased behavioural sensitization to the locomotor-stimulatory effects of methamphetamine in both spontaneously hypertensive and Sprague-Dawley strains but not in Wistar Kyoto rats.

CONCLUSIONS

Our findings indicate that developmental stress in a genetic rat model of attention-deficit/hyperactivity disorder may foster a vulnerability to the development of substance use disorders.

Neuropsychological performance measures as intermediate phenotypes for attention-deficit/hyperactivity disorder: A multiple mediation analysis

Genetic influences on dopaminergic neurotransmission have been implicated in attention-deficit hyperactivity disorder (ADHD) and are theorized to impact cognitive functioning via alterations in frontal-striatal circuitry. Neuropsychological functioning has been proposed to account for the potential associations between dopamine candidate genes and ADHD. However, to date, this mediation hypothesis has not been directly tested. Participants were 498 youth ages 6-17 years (mean M = 10.8 years, SD = 2.4 years, 55.0% male). All youth completed a multistage, multiple-informant assessment procedure to identify ADHD and non-ADHD cases, as well as a comprehensive neuropsychological battery. Youth provided a saliva sample for DNA analyses; the 480 base pair variable number of tandem repeat polymorphism of the dopamine active transporter 1 gene (DAT1) and the 120 base pair promoter polymorphism of the dopamine receptor D4 gene (DRD4) were genotyped. Multiple mediation analysis revealed significant indirect associations between DAT1 genotype and inattention, hyperactivity-impulsivity, and oppositionality, with specific indirect effects through response inhibition. The results highlight the role of neurocognitive task performance, particularly response inhibition, as a potential intermediate phenotype for ADHD, further elucidating the relationship between genetic polymorphisms and externalizing psychopathology.

Serotonin/dopamine interactions in a hyperactive mouse: reduced serotonin receptor 1B activity reverses effects of dopamine transporter knockout

Knockout (KO) mice that lack the dopamine transporter (SL6A3; DAT) display increased locomotion that can be attenuated, under some circumstances, by administration of drugs that normally produce psychostimulant-like effects, such as amphetamine and methylphenidate. These results have led to suggestions that DAT KO mice may model features of attention deficit hyperactivity disorder (ADHD) and that these drugs may act upon serotonin (5-HT) systems to produce these unusual locomotor decreasing effects. Evidence from patterns of brain expression and initial pharmacologic studies led us to use genetic and pharmacologic approaches to examine the influence of altered 5-HT_1B receptor activity on hyperactivity in DAT KO mice. Heterozygous 5-HT_1B KO and pharmacologic 5-HT_1B antagonism both attenuated locomotor hyperactivity in DAT KO mice. Furthermore, DAT KO mice with reduced, but not eliminated, 5-HT_1B receptor expression regained cocaine-stimulated locomotion, which was absent in DAT KO mice with normal levels of 5-HT_1B receptor expression. Further experiments demonstrated that the degree of habituation to the testing apparatus determined whether cocaine had no effect on locomotion in DAT KO or reduced locomotion, helping to resolve differences among prior reports. These findings of complementation of the locomotor effects of DAT KO by reducing 5-HT_1B receptor activity underscore roles for interactions between specific 5-HT receptors and dopamine (DA) systems in basal and cocaine-stimulated locomotion and support evaluation of 5-HT_1B antagonists as potential, non-stimulant ADHD therapeutics.

Association of gene polymorphisms encoding dopaminergic system components and platelet MAO-B activity with alcohol dependence and alcohol dependence-related phenotypes

The present study aimed to evaluate the association of alcohol dependence and alcohol dependence-related phenotypes with platelet monoamine oxidase type B (MAO-B) activity, Val108/158Met of catechol-o-methyltransferase (COMT), variable number of tandem repeats (VNTR) in the third exon of dopamine receptor D4 (DRD4) gene, VNTR in the 3'-untranslated region of dopamine transporter (DAT) gene, -1021C/T of dopamine beta-hydroxylase (DBH) and MAO-B intron 13 polymorphisms. The study included 1270 Caucasian men and women of Croatian origin: 690 patients with alcohol dependence and 580 healthy controls. Patients with alcohol dependence were subdivided according to the presence or absence of withdrawal symptoms, aggressive behavior, severity of alcohol dependence, delirium tremens, comorbid depression, suicidal behavior, lifetime suicide attempt and early/late onset of alcohol abuse. The results, corrected for multiple testing, revealed increased platelet MAO-B activity in patients with alcohol dependence, subdivided into those with or without alcohol-related liver diseases, compared to control subjects (P<0.001). In addition, we found an increased frequency of the COMT Met/Met genotype among suicidal (P=0.002) and patients who attempted suicide (P<0.001) and an increased frequency of COMT Val/Val genotype in patients with an early onset of alcohol dependence (P=0.004). This study provides data from a sample of ethnically homogeneous unrelated Caucasian subjects for future meta-analyses and suggests that the increased platelet MAO-B activity might be used as independent peripheral indicator of alcohol dependence, while COMT Val108/158Met polymorphism is associated with increased suicidality and early onset of alcohol dependence.

Stuttering candidate genes DRD2 but not SLC6A3 is associated with developmental dyslexia in Chinese population

Background

Dyslexia is a polygenic developmental disorder characterized by difficulties in reading and spelling despite normal intelligence, educational backgrounds and perception. Increasing evidences indicated that dyslexia may share similar genetic mechanisms with other speech and language disorders. We proposed that stuttering candidate genes, DRD2 and SLC6A3, might be associated with dyslexia.

Methods and results

The study was conducted in an unrelated Chinese cohort with 502 dyslexic cases and 522 healthy controls. In total, 23 Tag SNPs covering the two genes were selected for genotyping through Tagger program. Association analysis was performed on each SNP alone and in haplotypes. One SNP markers in DRD2 showed significant association with developmental dyslexia.

Conclusion

These findings indicate that polymorphism of DRD2 gene may be a risk factor of developmental dyslexia in the Chinese population.

Low dopamine function in attention deficit/hyperactivity disorder: should genotyping signify early diagnosis in children?

Attention deficit/hyperactivity disorder (ADHD) is present in 8% to 12% of children, and 4% of adults worldwide. Children with ADHD can have learning impairments, poor selfesteem, social dysfunction, and an increased risk of substance abuse, including cigarette smoking. Overall, the rate of treatment with medication for patients with ADHD has been increasing since 2008, with ≥ 2 million children now being treated with stimulants. The rise of adolescent prescription ADHD medication abuse has occurred along with a concomitant increase of stimulant medication availability. Of adults presenting with a substance use disorder (SUD), 20% to 30% have concurrent ADHD, and 20% to 40% of adults with ADHD have a history of SUD. Following a brief review of the etiology of ADHD, its diagnosis and treatment, we focus on the benefits of early and appropriate testing for a predisposition to ADHD. We suggest that by genotyping patients for a number of known, associated dopaminergic polymorphisms, especially at an early age, misdiagnoses and/or over-diagnosis can be reduced. Ethical and legal issues of early genotyping are considered. As many as 30% of individuals with ADHD are estimated to either have secondary side-effects or are not responsive to stimulant medication. We also consider the benefits of non-stimulant medication and alternative treatment modalities, which include diet, herbal medications, iron supplementation, and neurofeedback. With the goals of improving treatment of patients with ADHD and SUD prevention, we encourage further work in both genetic diagnosis and novel treatment approaches.

Attention deficit hyperactivity disorder

Over the last two decades, there have been numerous technical and methodological advances available to clinicians and researchers to better understand attention deficit hyperactivity disorder (ADHD) and its etiology. Despite the growing body of literature investigating the disorder's pathophysiology, ADHD remains a complex psychiatric disorder to characterize. This chapter will briefly review the literature on ADHD, with a focus on its history, the current genetic insights, neurophysiologic theories, and the use of neuroimaging to further understand the etiology. We address some of the major concerns that remain unclear about ADHD, including subtype instability, heterogeneity, and the underlying neural correlates that define the disorder. We highlight that the field of ADHD is rapidly evolving; the descriptions provided here will hopefully provide a sturdy foundation for which to build and improve our understanding of the disorder.

Association between the DAT1 gene and spatial working memory in attention deficit hyperactivity disorder

An association between attention deficit hyperactivity disorder (ADHD) and the dopamine transporter gene (DAT1) was reported in clinical samples. This study aimed to explore whether there was an association between DAT1 and spatial working memory (SWM), a promising endophenotype for ADHD. This family-based association sample consisted of 382 probands with DSM-IV ADHD and their family members (n = 1298) in Taiwan. The SWM task of the Cambridge Neuropsychological Test Automated Battery (CANTAB) was used to measure SWM of all participants. We screened 15 polymorphisms across the DAT1 gene, including 14 single nucleotide polymorphisms (SNPs) and the variable number of tandem repeat polymorphism in the 3'-untranslated region. We used the Family-Based Association Test (FBAT) to test the associations of genetic polymorphisms with the SWM measures. In single locus association analyses, two SNPs (rs2617605 and rs37020) were significantly associated with the double errors (adjusted p = 0.03 and 0.03, respectively) after adjustment for multiple testing. In haplotype analyses, a haplotype rs403636 (G)/rs463379 (C)/rs393795 (C)/rs37020 (G) was significantly associated with total within-search errors (minimal p = 0.001), within-search errors in eight boxes (minimal p = 0.002), total double errors (minimal p = 0.001) and double errors in eight boxes (minimal p = 0.004). Our finding of the haplotype rs403636 (G)/rs463379 (C)/rs393795 (C)/rs37020 (G) as a novel genetic marker for spatial working memory suggests that variation in DAT1 may provide insight into the pathways leading from genotype to phenotype of ADHD.

Relating dopaminergic and cholinergic polymorphisms to spatial attention in infancy

Early selective attention skills are a crucial building block for cognitive development, as attention orienting serves as a primary means by which infants interact with and learn from the environment. Although several studies have examined infants' attention orienting using the spatial cueing task, relatively few studies have examined neurodevelopmental factors associated with attention orienting during infancy. The present study examined the relationship between normative genetic polymorphisms affecting dopamine and acetylcholine signaling and attention orienting in 7-month-old infants during a spatial cueing task. We focused on 3 genes, including the CHRNA4 C¹⁵⁴⁵T SNP (rs10344946), DAT1 3'UTR VNTR, and COMT Val¹⁵⁸Met SNP (rs4680), as previous adult research has linked spatial attention skills to these polymorphisms. Behavioral measures included both facilitation of orienting at the cued location as well as inhibition of return (IOR), in which attention orienting is suppressed at the cued location. Results indicated that COMT Val carriers showed robust IOR relative to infants with the Met/Met genotype. However, COMT was unrelated to infants' facilitation responses, and there were no effects of CHRNA4 or DAT1 on either facilitation or IOR. Overall, this study suggests that variations in dopamine signaling, likely in prefrontal cortex, contribute to individual differences in orienting during early development.

Association of dopamine transporter gene variants with childhood ADHD features in bipolar disorder

Bipolar disorder (BD) and attention deficit hyperactivity disorder (ADHD) exhibit remarkably high rates of comorbidity, as well as patterns of familial co-segregation. Epidemiological data suggests that these disorders either share a common genetic architecture or that ADHD features in BD may represent an etiologically distinct subtype. We previously used the Wender Utah Rating Scale (WURS) to assess ADHD features in BD families and identified three heritable factors relating to impulsivity, mood instability, and inattention. Linkage analysis revealed a LOD score of 1.33 for the inattention factor on 5p15.3 near the dopamine transporter gene (DAT1), which has been associated with both BD and ADHD. Pharmacological evidence also suggests a role for DAT in both disorders. We have now evaluated the association of ten DAT1 variants for the WURS total score and factors in an overlapping sample of 87 BD families. Significant associations for three SNPs were observed across the WURS measures, notably for a SNP in intron 8 with the WURS total score (P = 0.007) and for variants in introns 9 and 13 with mood instability (P = 0.009 and 0.004, respectively). Analysis of an independent sample of 52 BD cases and 46 healthy controls further supported association of the intron 8 variant with mood instability (P = 0.005), and a combined analysis confirmed the associations of this SNP with WURS total score. Impulsivity and mood instability (P = 0.002, 0.007, and 8 × 10(-4), respectively). These data suggest that variants within DAT1 may predispose to a subtype of BD characterized by early prodromal features that include attentional deficits.

The effect of reduced dopamine D4 receptor expression in the 5-choice continuous performance task: Separating response inhibition from premature responding

Impairments in attention/vigilance and response disinhibition are commonly observed in several neuropsychiatric disorders. Validating animal models could help in developing therapeutics for cognitive deficits and improving functional outcomes in such disorders. The 5-choice continuous performance test (5C-CPT) in mice offers the opportunity to assess vigilance and two forms of impulsivity. Since reduced dopamine D4 receptor (DRD4) function is implicated in several disorders, DRD4 is a potential therapeutic target for cognition enhancement. We trained wildtype (WT), heterozygous (HT), and knockout (KO) mice of the murine Drd4 to perform the 5C-CPT under baseline and variable stimulus duration conditions. To dissect motor impulsivity (premature responding) from behavioral disinhibition (false alarms), we administered the 5-HT(2C) antagonist SB242084 during an extended inter-trial-interval session. We also examined the preattentive and exploratory profile of these mice in prepulse inhibition (PPI) and the Behavioral Pattern Monitor (BPM). Reduced Drd4 expression in HT mice, as confirmed by quantitative RT-PCR, resulted in response disinhibition and impaired 5C-CPT performance, while premature responding was unaffected. Conversely, SB242084 increased premature responding without affecting response inhibition or attentional measures. No genotypic differences were observed in PPI or BPM behavior. Thus, reduced Drd4 expression impairs attentional performance, but not other behaviors associated with neuropsychiatric disorders. Moreover, the use of signal and non-signal stimuli in the 5C-CPT enabled the differentiation of response disinhibition from motor impulsivity in a vigilance task.

Relationship between SLC6A3 genotype and striatal dopamine transporter availability: a meta-analysis of human single photon emission computed tomography studies

The human dopamine transporter (DAT) gene (SLC6A3) contains a 40-bp variable number of tandem repeats (VNTR) polymorphism. A number of studies have investigated the association of this VNTR with striatal DAT availability in humans using single photon emission computed tomography (SPECT). However, the results are not consistent. Therefore, we carried out a meta-analysis of the association between the SLC6A3 VNTR and striatal DAT binding measured in human SPECT studies. The meta-analysis of five samples of healthy individuals failed to find a significant difference in DAT availability between SLC6A3 9-repeat carriers and 10-repeat homozygotes (P = 0.22) although the 9R carriers had nominally higher striatal DAT levels (g = 0.66). The results remained nonsignificant after the inclusion of patient samples, namely schizophrenia, attention deficit hyperactivity disorder, and Parkinson's disease (four samples; all P > 0.18). To conclude, this meta-analysis provides no evidence to support the hypothesis that the SLC6A3 VNTR is significantly associated with interindividual differences in DAT availability in the human striatum. Further work is needed to clarify the molecular mechanisms by which this polymorphism may affect cognition and psychiatric disorders, if not through altered expression as measured by molecular imaging.

Adverse childhood experiences (ACEs), genetic polymorphisms and neurochemical correlates in experimentation with psychotropic drugs among adolescents

Epidemiological and clinical data show frequent associations between adverse childhood experiences (ACEs) and substance abuse susceptibility particularly in adolescents. A large body of evidences suggests that the possible dysregulation of neuroendocrine responses as well as neurotransmitters function induced by childhood traumatic experiences and emotional neglect could constitute one of the essential biological changes implementing substance abuse vulnerability. Moreover, genotype variables and its environment interactions have been associated with an increased risk for early onset substance abuse. In this paper we present several data that support the hypothesis of the involvement of hypothalamus-pituitary-adrenal (HPA) axis in mediating the combined effect of early adverse experiences and gene variants affecting neurotransmission. The presented data also confirm the relationship between basal plasma levels of cortisol and ACTH, on the one hand, and retrospective measures of neglect during childhood on the other hand: the higher the mother and father neglect (CECA-Q) scores are, the higher the plasma levels of the two HPA hormones are. Furthermore, such positive relationship has been proved to be particularly effective and important when associated with the "S" promoter polymorphism of the gene encoding the 5-HTT transporter, both in homozygote and heterozygote individuals.

The dopamine transporter gene, a spectrum of most common risky behaviors, and the legal status of the behaviors

This study tests the specific hypothesis that the 9R/9R genotype in the VNTR of the dopamine transporter gene (DAT1) exerts a general protective effect against a spectrum of risky behaviors in comparison to the 10R/9R and 10R/10R genotypes, drawing on three-time repeated measures of risky behaviors in adolescence and young adulthood on about 822 non-Hispanic white males from the Add Health study. Our data have established two empirical findings. The first is a protective main effect in the DAT1 gene against risky behaviors. The second finding is that the protective effect varies over age, with the effect prominent at ages when a behavior is illegal and the effect largely vanished at ages when the behavior becomes legal or more socially tolerated. Both the protective main effect and the gene-lifecourse interaction effect are replicated across a spectrum of most common risky behaviors: delinquency, variety of sexual partners, binge drinking, drinking quantity, smoking quantity, smoking frequency, marijuana use, cocaine use, other illegal drug use, and seatbelt non-wearing. We also compared individuals with the protective genotype and individuals without it in terms of age, physical maturity, verbal IQ, GPA, received popularity, sent popularity, church attendance, two biological parents, and parental education. These comparisons indicate that the protective effect of DAT1*9R/9R cannot be explained away by these background characteristics. Our work demonstrates how legal/social contexts can enhance or reduce a genetic effect on risky behaviors.

Genetics of attention-deficit hyperactivity disorder (ADHD)

Attention-deficit hyperactivity disorder (ADHD) is a clinically and genetically heterogeneous syndrome which is comorbid with childhood conduct disorder, alcoholism, substance abuse, dis-social personality disorder, and affective disorders. A small but consistent overlap with autistic symptoms has also been established. Twin and family studies of ADHD show a substantial genetic heritability with little or no family environmental effect. Linkage and association studies have conclusively implicated the dopamine transporter gene (DAT1). DAT1 has also been confirmed as being associated with bipolar disorder. Remarkably, and for the first time in psychiatry, genetic markers at the DAT1 locus appear to be able to predict clinical heterogeneity because the non-conduct disordered subgroup of ADHD is associated with DAT1 whereas other subgroups do not appear to be associated. The second most well replicated susceptibility gene encodes the DRD4 dopamine receptor and many other dopamine related genes appear to be implicated. It is becoming increasingly clear that genes causing bipolar mania overlap with genes for a subtype of ADHD. The key to understanding the genetics of ADHD is to accept very considerable heterogeneity with different genes having effects in different families and in different individuals. It is too early to interpret the new wave of genome-wide association and copy number variant studies but preliminary data support the overlap with affective disorder genes and also with CNS connectivity genes likely to be involved in autism and affective disorders.

Understanding genes, environment and their interaction in attention-deficit hyperactivity disorder: is there a role for neuroimaging?

Attention-deficit hyperactivity disorder (ADHD) has an established heritable component, but identifying the genes involved has proven difficult. To date, the two most investigated risk genes in ADHD are the DRD4 and DAT1-genes. However, individual risk genes have only explained up to 1% of the variance in the phenotype, suggesting that they represent only relatively small risk factors for ADHD. As such, the role of environmental factors, gene-gene and gene-environment interactions are being investigated. However, studies have not always been able to address the neurobiological mechanisms by which environmental factors and interactions with genes exert their effect on the ADHD-phenotype. Neuroimaging is being used as a tool to investigate the neurobiological effects of individual risk genes. We suggest it could also be applied to investigate the mechanisms involved in environmental effects and interactions between genetic and environmental factors.

Replication of an association of a promoter polymorphism of the dopamine transporter gene and Attention Deficit Hyperactivity Disorder

Genetic associations for Attention Deficit Hyperactivity Disorder (ADHD), a common highly heritable childhood behavioural disorder, require replication in order to establish whether they are true positive findings. The current study aims to replicate recent association findings from the International Multi-centre ADHD Genetics (IMAGE) project in one of the most studied genes related to ADHD, the dopamine transporter (DAT1) gene. In a family-based sample of 450 ADHD probands, three Single Nucleotide Polymorphism (SNP) markers have been genotyped using TaqMan assays. Transmission Disequilibrium Test analysis demonstrates that one of three SNP markers (rs11564750) in the 5' promoter region of the gene is significantly associated with ADHD (P=0.02). This provides further evidence that in addition to the well-known and investigated 3'UTR polymorphism associated with ADHD, there is potentially a further association signal emanating from the 5' promoter region of the gene. Further replication and functional studies are now required to fully understand the consequence of polymorphisms present at both the 5' and 3' ends of the DAT1 gene and their role in ADHD pathophysiology.

SNP discovery and haplotype analysis in the segmentally duplicated DRD5 coding region

The dopamine receptor 5 gene (DRD5) holds much promise as a candidate locus for contributing to neuropsychiatric disorders and other diseases influenced by the dopaminergic system, as well as having potential to affect normal behavioral variation. However, detailed analyses of this gene have been complicated by its location within a segmentally duplicated chromosomal region. Microsatellites and SNPs upstream from the coding region have been used for association studies, but we find, using bioinformatics resources, that these markers all lie within a previously unrecognized second segmental duplication (SD). In order to accurately analyze the DRD5 locus for polymorphisms in the absence of contaminating pseudogene sequences, we developed a fast and reliable method for sequence analysis and genotyping within the DRD5 coding region. We employed restriction enzyme digestion of genomic DNA to eliminate the pseudogenes prior to PCR amplification of the functional gene. This approach allowed us to determine the DRD5 haplotype structure using 31 trios and to reveal additional rare variants in 171 unrelated individuals. We clarify the inconsistencies and errors of the recorded SNPs in dbSNP and HapMap and illustrate the importance of using caution when choosing SNPs in regions of suspected duplications. The simple and relatively inexpensive method presented herein allows for convenient analysis of sequence variation in DRD5 and can be easily adapted to other duplicated genomic regions in order to obtain good quality sequence data.

Pharmacogenomic strategy for individualizing antidepressant therapy

Despite remarkable progress, pharmacotherapy in general, including that for the treatment of depressive conditions, has often ignored the magnitude and clinical significance of the huge interindividual variations in pharmacokinetics and pharmacodynamics, resulting in poor compliance, suboptimal therapeutic effects, and treatment resistance. Advances in pharmacogenomics and computer modeling technologies hold promise for achieving the goals of “individualized” (“personalized”) medicine. However, the challenges for realizing such goals remain substantial. These include the packaging and interpretation of genotyping results, changes in medical practice (innovation diffusion), and infrasiructural, financing, ethical, and organizational issues related to the use of new information.

A review and analysis of the relationship between neuropsychological measures and DAT1 in ADHD

Meta-analyses indicate that the gene coding for the dopamine transporter (DAT1 or SLC6A3) is associated with an increased risk for ADHD. The mechanisms of this gene for ADHD are unclear. We systematically reviewed studies linking the VNTR in the 3' UTR of the DAT1 to neurophysiological and neuropsychological measures. In addition, a broad set of executive/cognitive and motor tests was administered to 350 children (5-11 years) and adolescents (11-19 years) with ADHD and 195 non-affected siblings. Two VNTRs (in intron 8 and the 3' UTR) and four SNPs (two 5' and two 3') in DAT1 were genotyped. The effect of the polymorphisms on neuropsychological functioning was studied. The review indicated that the majority of studies did not find a relation between DAT1 and neurophysiological or neuropsychological measures. In our sample, several of the polymorphisms of DAT1 were associated with ADHD and ADHD was associated with impaired neuropsychological functioning. However, none of the DAT1 polymorphisms was convincingly associated with neuropsychological dysfunctioning. This suggests that the effect of DAT1 on ADHD was not mediated by neuropsychological performance. However, since DAT1 is mainly expressed in the striatum and not the prefrontal cortex, it may influence striatum-related functions (such as delay aversion) more heavily than prefrontal related functions (such as executive functions). Associations of DAT1 with ADHD were only found in adolescents, which may suggest that DAT1 mainly exerts its effect in adolescence, and/or that having a more persistent form of ADHD may mark a more severe or homogeneous genetic form of the disorder.

Association of the dopamine transporter gene and ADHD symptoms in a Canadian population-based sample of same-age twins

Attention deficit hyperactivity disorder (ADHD) is the most prevalent psychiatric disorder emerging during childhood. Psychostimulant medications (e.g., methylphenidate) noticeably reduce ADHD symptoms in most children. Since methylphenidate inhibits dopamine transporter activity, the dopamine transporter gene (DAT1) was considered to be the prime candidate risk gene in ADHD. Several studies found evidence for an association between the 10-repeat allele of the variable number of tandem repeat (VNTR) located in the 3' untranslated region and ADHD and/or ADHD symptoms in clinical and population-based samples. However, this finding was not replicated in all samples. In this study, we investigated the association between the DAT1 gene and ADHD symptoms in a population-based twin sample from Québec (Canada). We used two polymorphisms, the VNTR and rs27072, the last providing the most significant results in a clinical sample from Toronto (Ontario, Canada). No association was noted between the VNTR and ADHD symptoms in children at 6 and 7 years of age, as reported by teachers. However, a significant association was found for the rs27072 polymorphism and symptoms of inattention and hyperactivity/impulsivity. These findings indicate that the DAT1 gene contributes to ADHD symptoms in this sample and further suggest that the VNTR may not be the optimal polymorphism for study in all populations.

Association of ADHD with genetic variants in the 5'-region of the dopamine transporter gene: evidence for allelic heterogeneity

Multiple studies have reported an association between attention deficit hyperactivity disorder (ADHD) and the 10-repeat allele of a variable number tandem repeat (VNTR) polymorphism in the 3'-untranslated region (3'UTR) of the dopamine transporter gene (DAT1). Yet, recent meta-analyses of available data find little or no evidence for this association; although there is strong evidence for heterogeneity between datasets. This pattern of findings could arise for several reasons including the presence of relatively rare risk alleles on common haplotype backgrounds or the functional interaction of two or more loci within the gene. We previously described the importance of a specific haplotype at the 3' end of DAT1, as well as the identification of associated single nucleotide polymorphisms (SNPs) within or close to 5' regulatory sequences. In this study we replicate the association of SNPs at the 5' end of the gene and identify a specific risk haplotype spanning the 5' and 3' markers. These findings indicate the presence of at least two loci associated with ADHD within the DAT1 gene and suggest that either additive or interaction effects of these two loci on the risk for ADHD. Overall these data provide further evidence that genetic variants of the dopamine transporter gene confer an increased risk for ADHD.

Dopamine system genes and ADHD: a review of the evidence

The search for genes influencing the development of attention-deficit/hyperactivity disorder (ADHD) has identified a number of associated genes within, or influencing, the dopamine neurotransmitter system. The focus on this system as the site of genetic susceptibility was prompted by information from animal models, particularly transgenics, as well as the mechanism of action of the psychostimulants, the primary pharmacological treatment for ADHD. Thus far, genes in the dopamine system reported as associated with ADHD, by at least one study, include the dopamine transporter, the dopamine receptors D1, D4 and D5, as well as genes encoding proteins that control the synthesis, degradation and release of dopamine. For some of these genes, replication across studies provides evidence supporting the relationship; however, for others, the data is far from conclusive and further work is needed. The quick progress in the genetic findings was initially surprising given the complexity of the phenotype and the relatively small sample sizes used in the initial studies. However, the high heritability of ADHD, as indicated by twin studies, may have contributed to the success. The genes studied so far are estimated to contribute only weakly or moderately to the risk for the development of ADHD. This may be because these genes, in fact, make only a small contribution. However, few studies have comprehensively examined the genetic information across the gene. This will lead to underestimates of risk if the polymorphism(s) tested is/are not the functional change(s) actually contributing to the genetic susceptibility and if linkage disequilibrium between tested marker(s) and causal variant(s) is weak, or if there is substantial allelic heterogeneity. While the studies thus far are very promising, virtually nothing is known on precisely how genetic variation in these genes actually contributes to risk; thus, functional studies are now required.

Genotype and neuropsychological response inhibition as resilience promoters for attention-deficit/hyperactivity disorder, oppositional defiant disorder, and conduct disorder under conditions of psychosocial adversity

Whereas child personality, IQ, and family factors have been identified as enabling a resilient response to psychosocial adversity, more direct biological resilience factors have been less well delineated. This is particularly so for child attention-deficit/hyperactivity disorder (ADHD), which has received less attention from a resilience perspective than have associated externalizing disorders. Children from two independent samples were classified as resilient if they avoided developing ADHD, oppositional defiant disorder (ODD), or conduct disorder (CD) in the face of family adversity. Two protective factors were examined for their potential relevance to prefrontal brain development: neuropsychological response inhibition, as assessed by the Stop task, and a composite catecholamine genotype risk score. Resilient children were characterized in both samples by more effective response inhibition, although the effect in the second sample was very small. Genotype was measured in Sample 1, and a composite high risk genotype index was developed by summing presence of risk across markers on three genes expressed in prefrontal cortex: dopamine transporter, dopamine D4 receptor, and noradrenergic alpha-2 receptor. Genotype was a reliable resilience indicator against development of ADHD and CD, but not ODD, in the face of psychosocial adversity. Results illustrate potential neurobiological protective factors related to development of prefrontal cortex that may enable children to avoid developing ADHD and CD in the presence of psychosocial adversity.

No association between the DAT1 10-repeat allele and ADHD in the Iranian population

Association studies between attention-deficit hyperactivity disorder (ADHD) and the 10-repeat allele of a polymorphism (a 40 bp variable number of tandem repeats) in the dopamine transporter gene (DAT1) have resulted in mixed findings in different populations. We performed a case/control study to clarify the contribution of this allele with ADHD in the Iranian population. No association was observed between the 10-allele and disease (chi(2) = 0.081, P < 0.9). Furthermore, no significant difference was observed in the homozygosity of this allele between the case and control groups (chi(2) = 0.022, P < 0.9). Implication of the dopamine transporter gene in the pathophysiology of ADHD warrants investigation of other functional polymorphisms within this gene in the Iranian ADHD patients.

Relationship between VNTR polymorphisms of the human dopamine transporter gene and expression in post-mortem midbrain tissue

Attention deficit hyperactivity disorder (ADHD) is currently one of the most prevalent childhood behavioral disorders. The disorder is found to be highly heritable, suggesting a large genetic component. Association studies have repeatedly implicated the dopamine transporter (DAT1) gene, and in particular the 10-repeat allele of a variable number tandem repeat (VNTR) polymorphism located in the 3'UTR of the gene. Inconclusive data has been generated from several earlier studies on the functional effects of this polymorphism. Therefore, there is call for further investigation and thus the focus on data described here from TaqMan RT-PCR assays. The expression levels of the DAT1 gene from post-mortem midbrain tissue was measured in relation to the polymorphism present at the 3'UTR VNTR, together with a further VNTR marker located within intron 8 of the gene (Int8 VNTR). The findings suggest that the presence of the 10-repeat allele of the 3'UTR VNTR, the 3-repeat of the intron 8 VNTR and both VNTR markers are correlated with increased levels of the DAT1 transcript in midbrain post-mortem tissue. Further work using linear regression (LR) shows agreement with the correlation analysis, and either nominal significance or a trend in that direction. Given the small sample size, bootstrapping-derived confidence intervals were calculated for the LR. These empirical analyses suggest that the 3'UTR VNTR to show a significant main effect on relative DAT1 expression. Furthermore, a significant effect was found for the combined model (3'UTR and Int8 VNTR markers) on expression. These data provide further evidence on the plausible molecular mechanism underlying the aetiology of the disorder.

Interaction between BDNF Val66Met and dopamine transporter gene variation influences anxiety-related traits

The involvement in neural plasticity and the mediation of effects of repeated stress exposure and long-term antidepressant treatment on hippocampal neurogenesis supports a critical role of brain-derived neurotrophic factor (BDNF) in the pathophysiology of affective and other stress-related disorders. A previously reported valine to methionine substitution at amino-acid position 66 (BDNF Val66Met) seems to account for memory disturbance and hippocampal dysfunction. In the present study, we evaluated the impact of the BDNF Val66Met polymorphism on individual differences in personality traits in a sample of healthy volunteers in relation to other common gene variants thought to be involved in the pathophysiology of affective disorders, such as the serotonin transporter promoter polymorphism (5-HTTLPR) and a variable number of tandem repeat polymorphism of the dopamine transporter gene (DAT VNTR). Personality traits were assessed using the NEO personality inventory (NEO-PI-R) and Tridimensional Personality Questionnaire (TPQ). There was a significant DAT VNTR-dependent association between NEO-PI-R Neuroticism and the BDNF Val66Met polymorphism. Among individuals with at least one copy of the DAT 9-repeat allele, carriers of the BDNF Met allele exhibited significantly lower Neuroticism scores than noncarriers. This interaction was also observed for TPQ Harm Avoidance, a personality dimension related to Neuroticism. Our results support the notion that allelic variation at the BDNF locus--in interaction with other gene variants--influences anxiety- and depression-related personality traits.

Molecular genetics of monoamine transporters: relevance to brain disorders

We have demonstrated in both the human serotonin transporter gene (5HTT) and the dopamine transporter gene (DAT1) that specific polymorphic variants termed Variable Number Tandem Repeats (VNTRs), which correlate with predisposition to a number of neurological and psychiatric disorders, act as transcriptional regulatory domains. We have demonstrated that these domains can act as both tissue-specific and stimulus-inducible regulators of gene expression. As such they can act to be mechanistically associated with the progression or initiation of a behavioural disorder by altering the level of transporter mRNA, which in turn regulates the concentration of transporter in specific cells or in response to a challenge; chemical, environmental or physiological. The synergistic actions of such transcriptional domains will modulate gene expression. Our hypothesis is that these VNTR variants are one mechanism by which nurture can modify concentrations of neurotransmitters in a differential manner.

Association and linkage of allelic variants of the dopamine transporter gene in ADHD

Previously, we had reported a genome-wide scan for attention-deficit/hyperactivity disorder (ADHD) in 102 families with affected sibs of German ancestry; the highest multipoint LOD score of 4.75 was obtained on chromosome 5p13 (parametric HLOD analysis under a dominant model) near the dopamine transporter gene (DAT1). We genotyped 30 single nucleotide polymorphisms (SNPs) in this candidate gene and its 5' region in 329 families (including the 102 initial families) with 523 affected offspring. We found that (1) SNP rs463379 was significantly associated with ADHD upon correction for multiple testing (P=0.0046); (2) the global P-value for association of haplotypes was significant for block two upon correction for all (n=3) tested blocks (P=0.0048); (3) within block two we detected a nominal P=0.000034 for one specific marker combination. This CGC haplotype showed relative risks of 1.95 and 2.43 for heterozygous and homozygous carriers, respectively; and (4) finally, our linkage data and the genotype-IBD sharing test (GIST) suggest that genetic variation at the DAT1 locus explains our linkage peak and that rs463379 (P<0.05) is the only SNP of the above haplotype that contributed to the linkage signal. In sum, we have accumulated evidence that genetic variation at the DAT1 locus underlies our ADHD linkage peak on chromosome 5; additionally solid association for a single SNP and a haplotype were shown. Future studies are required to assess if variation at this locus also explains other positive linkage results obtained for chromosome 5p.