Transient expression analysis of allelic variants of a VNTR in the dopamine transporter gene (DAT1)

SLC6A3, HTR2C and HTR6 Gene Polymorphisms and the Risk of Haloperidol-Induced Parkinsonism

Antipsychotic-induced parkinsonism (AIP) is the most common type of extrapyramidal side effect (EPS), caused by the blockage of dopamine receptors. Since dopamine availability might influence the AIP risk, the dopamine transporter (DAT) and serotonin receptors (5-HTRs), which modulate the dopamine release, may be also involved in the AIP development. As some of the individual differences in the susceptibility to AIP might be due to the genetic background, this study aimed to examine the associations of SLC6A3, HTR2C and HTR6 gene polymorphisms with AIP in haloperidol-treated schizophrenia patients. The Extrapyramidal Symptom Rating Scale (ESRS) was used to evaluate AIP as a separate entity. Genotyping was performed using a PCR, following the extraction of blood DNA. The results revealed significant associations between HTR6 rs1805054 polymorphism and haloperidol-induced tremor and rigidity. Additionally, the findings indicated a combined effect of HTR6 T and SLC6A3 9R alleles on AIP, with their combination associated with significantly lower scores of ESRS subscale II for parkinsonism, ESRS-based tremor or hyperkinesia and ESRS subscales VI and VIII. These genetic predictors of AIP could be helpful in better understanding its pathophysiology, recognizing the individuals at risk of developing AIP and offering personalized therapeutic strategies for the patients suffering from this EPS.

Attention-deficit/hyperactive disorder updates

Background

Attention-deficit/hyperactive disorder (ADHD) is a neurodevelopmental disorder that commonly occurs in children with a prevalence ranging from 3.4 to 7.2%. It profoundly affects academic achievement, well-being, and social interactions. As a result, this disorder is of high cost to both individuals and society. Despite the availability of knowledge regarding the mechanisms of ADHD, the pathogenesis is not clear, hence, the existence of many challenges especially in making correct early diagnosis and provision of accurate management.

Objectives

We aimed to review the pathogenic pathways of ADHD in children. The major focus was to provide an update on the reported etiologies in humans, animal models, modulators, therapies, mechanisms, epigenetic changes, and the interaction between genetic and environmental factors.

Methods

References for this review were identified through a systematic search in PubMed by using special keywords for all years until January 2022.

Results

Several genes have been reported to associate with ADHD: DRD1, DRD2, DRD4, DAT1, TPH2, HTR1A, HTR1B, SLC6A4, HTR2A, DBH, NET1, ADRA2A, ADRA2C, CHRNA4, CHRNA7, GAD1, GRM1, GRM5, GRM7, GRM8, TARBP1, ADGRL3, FGF1, MAOA, BDNF, SNAP25, STX1A, ATXN7, and SORCS2. Some of these genes have evidence both from human beings and animal models, while others have evidence in either humans or animal models only. Notably, most of these animal models are knockout and do not generate the genetic alteration of the patients. Besides, some of the gene polymorphisms reported differ according to the ethnic groups. The majority of the available animal models are related to the dopaminergic pathway. Epigenetic changes including SUMOylation, methylation, and acetylation have been reported in genes related to the dopaminergic pathway.

Conclusion

The dopaminergic pathway remains to be crucial in the pathogenesis of ADHD. It can be affected by environmental factors and other pathways. Nevertheless, it is still unclear how environmental factors relate to all neurotransmitter pathways; thus, more studies are needed. Although several genes have been related to ADHD, there are few animal model studies on the majority of the genes, and they do not generate the genetic alteration of the patients. More animal models and epigenetic studies are required.

The dopamine transporter gene SLC6A3: multidisease risks

The human dopamine transporter gene SLC6A3 has been consistently implicated in several neuropsychiatric diseases but the disease mechanism remains elusive. In this risk synthesis, we have concluded that SLC6A3 represents an increasingly recognized risk with a growing number of familial mutants associated with neuropsychiatric and neurological disorders. At least five loci were related to common and severe diseases including alcohol use disorder (high activity variant), attention-deficit/hyperactivity disorder (low activity variant), autism (familial proteins with mutated networking) and movement disorders (both regulatory variants and familial mutations). Association signals depended on genetic markers used as well as ethnicity examined. Strong haplotype selection and gene-wide epistases support multimarker assessment of functional variations and phenotype associations. Inclusion of its promoter region's functional markers such as DNPi (rs67175440) and 5'VNTR (rs70957367) may help delineate condensate-based risk action, testing a locus-pathway-phenotype hypothesis for one gene-multidisease etiology.

Deficit in working memory and abnormal behavioral tactics in dopamine transporter knockout rats during training in the 8-arm maze

Understanding the role of the dopamine system in learning and memory processes is very important for uncovering central mechanisms underlying complex behavioral responses that can be impaired in patients with neuropsychiatric disorders caused by dopamine system dysfunction. One of the most useful animal models for dopaminergic dysregulation is the strain of dopamine transporter knockout (DAT-KO) rats that have no dopamine re-uptake and thus elevated extracellular dopamine levels. It is known that dopamine is involved in various cognitive processes such as learning, memory and attention. This investigation was focused on the ability of DAT-KO rats to learn and perform a behavioral task in the 8-arm radial maze test. It was found that DAT-KO rats are able to learn the behavioral task, but the level of task performance did not reach that of WT group. The behavioral tactics used by animals during training significantly differ in mutants. The behavioral tactics used by DAT-KO rats involved perseverations and resulted in worse task fulfillment in comparison to wild-type controls. The data obtained indicate that deficient dopamine reuptake results in an impairment of working memory and perseverative behavioral tactics in DAT-KO rats.

Association study and a systematic meta-analysis of the VNTR polymorphism in the 3'-UTR of dopamine transporter gene and attention-deficit hyperactivity disorder

Attention-deficit hyperactivity disorder (ADHD) has been postulated to associate with dopaminergic dysfunction, including the dopamine transporter (DAT1). Several meta-analyses showed small but significant association between the 10-repeat allele in the DAT1 gene in 3'-untranslated region variant number tandem repeat polymorphism and child and adolescent ADHD, whereas in adult ADHD the 9-repeat allele was suggested to confer as risk allele. Interestingly, recent evidence indicated that the long-allele variants (10 repeats and longer) might confer to lower expression of the transporter in comparison to the short-allele. Therefore, we assessed here the association in samples consisting of families with child and adolescent ADHD as well as a case-control sample, using either the 10- versus 9-repeat or the long- versus short-allele approach. Following, we conducted a systematic review and meta-analysis, including family and case-control studies, using the two aforementioned approaches as well as stratifying to age and ethnicity. The first approach (10-repeat) resulted in nominal significant association in child and adolescent ADHD (OR 1.1050 p = 0.0128), that became significant stratifying to European population (OR 1.1301 p = 0.0085). The second approach (long-allele) resulted in significant association with the whole ADHD population (OR 1.1046 p = 0.0048), followed by significant association for child and adolescent ADHD (OR 1.1602 p = 0.0006) and in Caucasian and in European child and adolescent ADHD (OR 1.1310 p = 0.0114; OR 1.1661 p = 0.0061; respectively). We were not able to confirm the association reported in adults using both approaches. In conclusion, we found further indication for a possible DAT1 gene involvement; however, further studies should be conducted with stringent phenotyping to reduce heterogeneity, a limitation observed in most included studies.

Variants of ACAN are associated with severity of lumbar disc herniation in patients with chronic low back pain

Introduction

Disc herniation is a complex spinal disorder associated with disability and high healthcare cost. Lumbar disc herniation is strongly associated with disc degeneration. Candidate genes of the aggrecan metabolic pathway may associate with the severity of lumbar disc herniation.

Objectives

This study evaluated the association of single nucleotide variants (SNVs) of the candidate genes of the aggrecan metabolic pathway with the severity of lumbar disc herniation in patients with chronic mechanical low back pain. In addition, we assessed the in-silico functional analysis of the significant SNVs and association of their haplotypes with the severity of lumbar disc herniation.

Methods

A descriptive cross sectional study was carried out on 106 patients. Severity of disc herniation and disc degeneration were assessed on T2-weighted mid sagittal lumbar MRI scan. Sixty two exonic SNVs of ten candidate genes of aggrecan metabolic pathway (ACAN, IL1A, IL1B, IL6, MMP3, ADAMTS4, ADAMTS5, TIMP1, TIMP2 and TIMP3) were genotyped on a Sequenom MassARRAY iPLEX platform. Multivariable linear regression analysis was carried out using PLINK 1.9 software adjusting for age, gender, body mass index and severity of disc degeneration. Four online bioinformatics tools (Provean, SIFT, PolyPhen and Mutation Taster) were used for in-silico functional analysis.

Results

Mean age was 52.42 ± 9.42 years and 69.8% were females. The mean severity of disc herniation was 2.81 ± 1.98. The rs2272023, rs35430524, rs2882676, rs2351491, rs938609, rs3825994, rs1042630, rs698621 and rs3817428 variants and their haplotypes of ACAN were associated with the severity of lumbar disc herniation. However, only the rs35430524, rs938609 and rs3817428 variants of ACAN were detected as pathogenic by in-silico functional analysis.

Conclusions

SNVs of ACAN and their haplotypes are associated with the severity of lumbar disc herniation. Functional genetic studies are necessary to identify the role of these significant SNVs in the pathogenesis of disc herniation.

Using iPSC-derived human DA neurons from opioid-dependent subjects to study dopamine dynamics

INTRODUCTION

The dopaminergic (DA) system plays important roles in addiction. However, human DA neurons from drug-dependent subjects were not available for study until recent development in inducible pluripotent stem cells (iPSCs) technology.

METHODS

In this study, we produced DA neurons differentiated using iPSCs derived from opioid-dependent and control subjects carrying different 3' VNTR (variable number tandem repeat) polymorphism in the human dopamine transporter (DAT or SLC6A3). In addition, the effects of valproic acid (VPA) exposures on iPSC-derived human DA neurons are also examined.

RESULTS

We present the first evidence suggesting that the 3' VNTR polymorphism in the hDAT gene affects DAT expression level in iPSC-derived human DA neurons. In human DA neurons, which provide an appropriate cellular milieu, VPA treatment alters the expression of several genes important for dopaminergic neuron function including DAT, Nurr1, and TH; this might partly explain its action in regulating addictive behaviors. VPA treatment also significantly increased DA D2 receptor (Drd2) expression, especially in the opioid-dependent iPSC cell lines.

CONCLUSIONS

Our data suggest that human iPSC-derived DA neurons may be useful in in vitro experimental model to examine the effects of genetic variation in gene regulation, to examine the underlying mechanisms in neurological disorders including drug addiction, and to serve as a platform for therapeutic development.

Genetic diversity of Thaumetopoea pityocampa in Greece: the role of Quaternary changes in Aegean Sea

Thaumetopoea pityocampa, the winter pine processionary moth, is one of the most important pests of pine trees in the Mediterranean region. To learn more about the refugial areas within the greater refugium of Greece, samples from 15 local populations were collected. Analysis of the Cytochrome Oxidase I region identified 15 haplotypes. One strongly supported clade was found, separating the Aegean island of Lesvos from the mainland populations. Mdiv analysis showed that this clade diverged from the general clade 155,000 y.a., suggesting the sea level changes during the Quaternary Period as a possible reason for its seclusion. Additionally, comparisons with similar studies in this region revealed a possible colonization of the eastern Aegean islands from the Turkish mainland rather than from Greece. However, no strong isolation-by-distance events were detected among the mainland populations, which could be attributed to the joint effect of regular gene flow and the lack of insurmountable geographic barriers. Finally, regarding population structure, Bayesian analysis as well as neutrality tests pointed towards an ongoing population expansion which verifies the potential invasiveness of this pest species, something that will have to be dealt with under the influence of climate change.

Functional effects of dopamine transporter gene genotypes on in vivo dopamine transporter functioning: a meta-analysis

Much psychiatric genetic research has focused on a 40-base pair variable number of tandem repeats (VNTR) polymorphism located in the 3'-untranslated region (3'UTR) of the dopamine active transporter (DAT) gene (SLC6A3). This variant produces two common alleles with 9- and 10-repeats (9R and 10R). Studies associating this variant with in vivo DAT activity in humans have had mixed results. We searched for studies using positron emission tomography (PET) or single-photon emission computed tomography (SPECT) to evaluate this association. Random effects meta-analyses assessed the association of the 3'UTR variant with DAT activity. We also evaluated heterogeneity among studies and evidence for publication bias. We found twelve studies comprising 511 subjects, 125 from PET studies and 386 from SPECT studies. The PET studies provided highly significant evidence that the 9R allele was associated with increased DAT activity in human adults. The SPECT studies were highly heterogeneous. As a group, they suggested no association between the 3'UTR polymorphism and DAT activity. When the analysis was limited to the most commonly used ligand, [123I]β-CIT, stratification by affection status dramatically reduced heterogeneity and revealed a significant association of the 9R allele with increased DAT activity for healthy subjects. In humans, the 9R allele of the 3'UTR polymorphism of SLC6A3 regulates dopamine activity in the striatal brain regions independent of the presence of neuropsychiatric illness. Differences in study methodology account for the heterogeneous results across individual studies.

Enhanced prepulse inhibition and low sensitivity to a dopamine agonist in HESR1 knockout mice

Transcription factor Hesr family genes are important in neuronal development. We demonstrated previously that HESR1 and HESR2 modified expression of the dopamine transporter (DAT) reporter gene. HESR-family genes have been investigated in development, but their functions, especially in relation to behaviors regulated by dopamine, in adult animals remain unclear. In the present study, we investigated the effects of Hesr1 and Hesr2 on behavior. A behavioral test battery to examine spontaneous activity, anxiety-like behavior, aggressive behavior, pain sensitivity, and sensorimotor gating was conducted in Hesr1 and Hesr2 knockout (KO) mice. Enhanced prepulse inhibition (PPI), which is a form of sensorimotor gating, was observed in only Hesr1 KO mice; other behavioral traits were mostly comparable to wild-type animals in both the Hesr1 and the Hesr2 KO lines. Next, we used a dopamine agonist, apomorphine, to confirm the involvement of the dopaminergic system. Injection of apomorphine reduced the enhanced PPI in Hesr1 KO mice. Additionally, dose-dependent sensitivity to the agonist was lower in the Hesr1 KO mice than in wild-type mice, suggesting that the enhanced PPI resulted from this alteration in dopamine sensitivity. Furthermore, DAT mRNA was downregulated in Hesr1 KO mice, whereas the dopamine D1 and D2 receptors were comparable. These findings suggest Hesr1 to be a novel factor that affects dopamine sensitivity and the sensorimotor gating system.

Good riddance to dopamine: roles for the dopamine transporter in synaptic function and dopamine-associated brain disorders

The neurotransmitter dopamine (DA) plays a critical role in CNS circuits that provide for attention, executive function, reward responses, motivation and movement. DA is inactivated by the cocaine- and amphetamine-sensitive DA transporter (DAT), a protein that also provides a pathway for non-vesicular DA release. After a brief review of DAT function and psychostimulant actions, we consider the importance DAT in relation to the distinct firing patterns of DA neurons that permit awareness of novelty and reward. Finally, we review recent efforts to gather direct support for DAT-linked disorders, with a specific focus on DAT mutations recently identified in subjects with ADHD.

The association study of polymorphisms in DAT, DRD2, and COMT genes and acute extrapyramidal adverse effects in male schizophrenic patients treated with haloperidol

Extrapyramidal symptoms (EPSs) are common adverse effects of antipsychotics. The development of acute EPSs could depend on the activity of dopaminergic system and its gene variants. The aim of this study was to determine the association between dopaminergic type 2 receptor (DRD2) dopamine transporter (SLC6A3) and catechol-O-methyltransferase (COMT) gene polymorphisms and acute EPSs in 240 male schizophrenic patients treated with haloperidol (15-mg/d) over a period of 2 weeks. Acute EPSs were assessed with Simpson-Angus Scale. Three dopaminergic gene polymorphisms, the DRD2 Taq1A, the SLC6A3 VNTR, and the COMT Val158Met, were determined. Extrapyramidal symptoms occurred in 116 (48.3%) of patients. Statistically significant associations were found for SLC6A3 VNTR and COMT Val158Met polymorphisms and EPS susceptibility. Patients with SLC6A3 9/10 genotype had almost twice the odds to develop EPSs compared with those with all other SLC6A3 genotypes (odds ratio, 1.9; 95% confidence interval, 1.13-3.30), and patients with COMT Val/Met genotype had 1.7 times greater odds to develop EPSs than those with all other COMT genotypes (odds ratio, 1.7; 95% confidence interval, 1.01-2.88). There was no statistically significant association between genotype and allele frequencies of DRD2, SLC6A3, or COMT polymorphisms and the development of particular EPSs.In conclusion, the results of the present study showed for the first time the association between acute haloperidol-induced EPSs and SLC6A3 VNTR and COMT Val158Met polymorphisms. Although the precise biological mechanisms underlying these findings are not yet understood, the results suggest that the dopaminergic gene variations could predict the vulnerability to the development of the acute EPSs in haloperidol-treated schizophrenic patients.

Functional genomics of attention-deficit/hyperactivity disorder (ADHD) risk alleles on dopamine transporter binding in ADHD and healthy control subjects

BACKGROUND

The main aim of this study was to examine the relationship between dopamine transporter (DAT) binding in the striatum in individuals with and without attention-deficit/hyperactivity disorder (ADHD), attending to the 3'-untranslated region of the gene (3'-UTR) and intron8 variable number of tandem repeats (VNTR) polymorphisms of the DAT (SLC6A3) gene.

METHODS

Subjects consisted of 68 psychotropic (including stimulant)-naïve and smoking-naïve volunteers between 18 and 55 years of age (ADHD n = 34; control subjects n = 34). Striatal DAT binding was measured with positron emission tomography with 11C altropane. Genotyping of the two DAT (SLC6A3) 3'-UTR and intron8 VNTRs used standard protocols.

RESULTS

The gene frequencies of each of the gene polymorphisms assessed did not differ between the ADHD and control groups. The ADHD status (t = 2.99; p<.004) and 3'-UTR of SLC6A3 9 repeat carrier status (t = 2.74; p<.008) were independently and additively associated with increased DAT binding in the caudate. The ADHD status was associated with increased striatal (caudate) DAT binding regardless of 3'-UTR genotype, and 3'-UTR genotype was associated with increased striatal (caudate) DAT binding regardless of ADHD status. In contrast, there were no significant associations between polymorphisms of DAT intron8 or the 3'-UTR-intron8 haplotype with DAT binding.

CONCLUSIONS

The 3'-UTR but not intron8 VNTR genotypes were associated with increased DAT binding in both ADHD patients and healthy control subjects. Both ADHD status and the 3'-UTR polymorphism status had an additive effect on DAT binding. Our findings suggest that an ADHD risk polymorphism (3'-UTR) of SLC6A3 has functional consequences on central nervous system DAT binding in humans.

Interaction of dopamine transporter (DAT1) genotype and maltreatment for ADHD: a latent class analysis

BACKGROUND

Although the association of the dopamine transporter (DAT1) gene and attention-deficit/hyperactivity disorder (ADHD) has been widely studied, far less is known about its potential interaction with environmental risk factors. Given that maltreatment is a replicated risk factor for ADHD, we explored the interaction between DAT1 and maltreatment with ADHD symptoms defined dimensionally and using latent class analysis (LCA).

METHOD

We tested the association of the 40 base-pair variable number of tandem repeats polymorphism in DAT1, maltreatment, and their interaction in 2,488 boys and girls from the National Longitudinal Study of Adolescent Health.

RESULTS

In boys, ADHD symptoms were optimally defined by four classes (Combined, Hyperactive/Impulsive, Inattentive, and Normal), whereas in girls, ADHD symptoms were defined by three classes (Combined, Combined-Mild, Normal). A significant DAT1 × maltreatment interaction revealed that maltreated girls homozygous for the 10-repeat allele had more symptoms of ADHD, and were also 2.5 times more likely to be classified in the Combined ADHD group than in the Normal Group.

CONCLUSIONS

The underlying structure of ADHD symptoms differed between boys and girls and DAT1 interacted with maltreatment to predict ADHD symptoms and ADHD status derived from LCA. Interactive exchanges between maltreatment and DAT1 for ADHD symptoms, and their implications for intervention, are discussed.

The androgen receptor facilitates inhibition of human dopamine transporter (DAT1) reporter gene expression by HESR1 and HESR2 via the variable number of tandem repeats

A functional genetic polymorphism in the 3'-untranslated region (UTR) within exon 15 of the human DAT gene (DAT1) has been described. This 3'-UTR contains a variable number of tandem repeats (VNTR) 40 bp in length; many association studies of psychiatric or developmental disorders with this VNTR have been conducted. We previously demonstrated that HESR1 (the Hairy/enhancer of split related transcriptional factor 1 with YRPW motif) and HESR2 reduced DAT reporter gene expression via this 3'-UTR. VNTR allele-dependent altered reporter gene expression was also observed. In the present study, we wanted to clarify the molecular characterization of HESR1 and HESR2, focusing on its cis-element and co-factor. Deletion of the VNTR domain increased reporter gene expression both with and without transfection of HESRs, suggesting that the VNTR inhibits DAT expression, and is responsive to HESRs. In the presence of transfected androgen receptor (AR), activity of the luciferase reporter with the nine-repeat allele (9r) decreased, while that with the ten-repeat allele (10r), the most frequent in the population, increased significantly. Furthermore, co-expression of HESR1 or HESR2 with AR increased the inhibitory effect of the HESRs. Our data indicate that a functional modification occurs when the HESRs are coupled with AR. This HESR-AR interaction could be the molecular basis of sexual dimorphisms in DAT expression, or other dopamine-related behavioral traits.

Neurogenetics of depression: a focus on reward processing and stress sensitivity

Major depressive disorder (MDD) is etiologically complex and has a heterogeneous presentation. This heterogeneity hinders the ability of molecular genetic research to reliably detect the small effects conferred by common genetic variation. As a result, significant research efforts have been directed at investigating more homogenous intermediate phenotypes believed to be more proximal to gene function and lie between genes and/or environmental effects and disease processes. In the current review we survey and integrate research on two promising intermediate phenotypes linked to depression: reward processing and stress sensitivity. A synthesis of this burgeoning literature indicates that a molecular genetic approach focused on intermediate phenotypes holds significant promise to fundamentally improve our understanding of the pathophysiology and etiology of depression, which will be required for improved diagnostic definitions and the development of novel and more efficacious treatment and prevention strategies. We conclude by highlighting challenges facing intermediate phenotype research and future development that will be required to propel this pivotal research into new directions.

Imaging genetics and the neurobiological basis of individual differences in vulnerability to addiction

BACKGROUND

Addictive disorders are heritable, but the search for candidate functional polymorphisms playing an etiological role in addiction is hindered by complexity of the phenotype and the variety of factors interacting to impact behavior. Advances in human genome sequencing and neuroimaging technology provide an unprecedented opportunity to explore the impact of functional genetic variants on variability in behaviorally relevant neural circuitry. Here, we present a model for merging these technologies to trace the links between genes, brain, and addictive behavior.

METHODS

We describe imaging genetics and discuss the utility of its application to addiction. We then review data pertaining to impulsivity and reward circuitry as an example of how genetic variation may lead to variation in behavioral phenotype. Finally, we present preliminary data relating the neural basis of reward processing to individual differences in nicotine dependence.

RESULTS

Complex human behaviors such as addiction can be traced to their basic genetic building blocks by identifying intermediate behavioral phenotypes, associated neural circuitry, and underlying molecular signaling pathways. Impulsivity has been linked with variation in reward-related activation in the ventral striatum (VS), altered dopamine signaling, and functional polymorphisms of DRD2 and DAT1 genes. In smokers, changes in reward-related VS activation induced by smoking abstinence may be associated with severity of nicotine dependence.

CONCLUSIONS

Variation in genes related to dopamine signaling may contribute to heterogeneity in VS sensitivity to reward and, ultimately, to addiction. These findings illustrate the utility of the imaging genetics approach for investigating the neurobiological basis for vulnerability to addiction.

Toward a mechanistic understanding of how variability in neurobiology shapes individual differences in behavior

Research has begun to identify how variability in brain function contributes to individual differences in complex behavioral traits. Examining variability in molecular signaling pathways with emerging and established methodologies such as pharmacologic fMRI, multimodal PET/fMRI, and hormonal assays are beginning to provide a mechanistic understanding of how individual differences in brain function arise. Against this background, functional genetic polymorphisms are being utilized to understand the origins of variability in signaling pathways as well as to efficiently model how such emergent variability impacts behaviorally relevant brain function and health outcomes. This chapter provides an overview of a research strategy that integrates these complimentary levels of analysis; existing empirical data is used to illustrate the effectiveness of this approach in illuminating the mechanistic neurobiology of individual differences in complex behavioral traits. This chapter also discusses how such efforts can contribute to the identification of predictive risk markers that interact with unique environmental factors to precipitate psychopathology.

A review on experimental and clinical genetic associations studies on fear conditioning, extinction and cognitive-behavioral treatment

Fear conditioning and extinction represent basic forms of associative learning with considerable clinical relevance and have been implicated in the pathogenesis of anxiety disorders. There is considerable inter-individual variation in the ability to acquire and extinguish conditioned fear reactions and the study of genetic variants has recently become a focus of research. In this review, we give an overview of the existing genetic association studies on human fear conditioning and extinction in healthy individuals and of related studies on cognitive-behavioral treatment (CBT) and exposure, as well as pathology development after trauma. Variation in the serotonin transporter (5HTT) and the catechol-o-methyltransferase (COMT) genes has consistently been associated with effects in pre-clinical and clinical studies. Interesting new findings, which however require further replication, have been reported for genetic variation in the dopamine transporter (DAT1) and the pituitary adenylate cyclase 1 receptor (ADCYAP1R1) genes, whereas the current picture is inconsistent for variation in the brain-derived neurotrophic factor (BDNF) gene. We end with a discussion of the findings and their limitations, as well as future directions that we hope will aid the field to develop further.

Dopamine transporter gene variant affecting expression in human brain is associated with bipolar disorder

The gene encoding the dopamine transporter (DAT) has been implicated in CNS disorders, but the responsible polymorphisms remain uncertain. To search for regulatory polymorphisms, we measured allelic DAT mRNA expression in substantia nigra of human autopsy brain tissues, using two marker SNPs (rs6347 in exon 9 and rs27072 in the 3'-UTR). Allelic mRNA expression imbalance (AEI), an indicator of cis-acting regulatory polymorphisms, was observed in all tissues heterozygous for either of the two marker SNPs. SNP scanning of the DAT locus with AEI ratios as the phenotype, followed by in vitro molecular genetics studies, demonstrated that rs27072 C>T affects mRNA expression and translation. Expression of the minor T allele was dynamically regulated in transfected cell cultures, possibly involving microRNA interactions. Both rs6347 and rs3836790 (intron8 5/6 VNTR) also seemed to affect DAT expression, but not the commonly tested 9/10 VNTR in the 3'UTR (rs28363170). All four polymorphisms (rs6347, intron8 5/6 VNTR, rs27072 and 3'UTR 9/10 VNTR) were genotyped in clinical cohorts, representing schizophrenia, bipolar disorder, depression, and controls. Only rs27072 was significantly associated with bipolar disorder (OR = 2.1, p = 0.03). This result was replicated in a second bipolar/control population (OR = 1.65, p = 0.01), supporting a critical role for DAT regulation in bipolar disorder.

Interactions between early parenting and a polymorphism of the child's dopamine transporter gene in predicting future child conduct disorder symptoms

Mounting evidence suggests that genetic risks for mental disorders often interact with the social environment, but most studies still ignore environmental moderation of genetic influences. The authors tested interactions between maternal parenting and the variable number tandem repeat (VNTR) polymorphism in the 3' untranslated region of the dopamine transporter gene in the child to increase understanding of gene-environment interactions involving early parenting. Participants were part of a 9-year longitudinal study of 4- to 6-year-old children who met criteria for attention-deficit/hyperactivity disorder (ADHD) and demographically matched controls. Maternal parenting was observed during standard mother-child interactions in Wave 1. The child's conduct disorder (CD) symptoms 5-8 years later were measured using separate structured diagnostic interviews of the mother and youth. Controlling for ADHD symptoms and child disruptive behavior during the mother-child interaction, there was a significant inverse relation between levels of both positive and negative parenting at 4-6 years and the number of later CD symptoms, but primarily among children with 2 copies of the 9-repeat allele of the VNTR. The significant interaction with negative parenting was replicated in parent and youth reports of CD symptoms separately.

Psychopathological aspects of dopaminergic gene polymorphisms in adolescence and young adulthood

Dopamine hypotheses of several psychiatric disorders are based upon the clinical benefits of drugs affecting dopamine transporter or receptors, and have prompted intensive candidate gene research within the dopaminergic system during the last two decades. The aim of this review is to survey the most important findings concerning dopaminergic gene polymorphisms in attention deficit hyperactivity disorder (ADHD), Tourette syndrome (TS), obsessive compulsive disorder, and substance abuse. Also, genetic findings of related phenotypes, such as inattention, impulsivity, aggressive behavior, and novelty seeking personality trait are presented, because recent studies have applied quantitative trait measures using questionnaires, symptom scales, or other objective endophenotypes. Unfortunately, genetic variants with minor effects are problematic to detect in these complex inheritance disorders, often leading to contradictory results. The most consistent association findings relate to ADHD and the dopamine transporter and the dopamine D4 receptor genes. Meta-analyses also support the association between substance abuse and the D2 receptor gene. The dopamine catabolizing enzyme genes, such as monoamine oxidase (MAO) A and catechol-O-methyltransferase (COMT) genes, have been linked to aggressive behaviors.

Allele and genotype frequencies of serotonin and dopamine transporter and receptor polymorphisms in a Norwegian population

Polymorphisms in genes coding for dopaminergic and serotonergic receptors and transporters have been associated with the clinical effects and adverse drug reactions of antipsychotic and antidepressant drugs. The objective of this study was to investigate the frequency and combinations of common polymorphisms in the dopamine transporter (DAT1), dopamine D(2) receptor (DRD2), dopamine D(3) receptor (DRD3), serotonin transporter (5HTT), and serotonin 2A receptor (5HTR2A) genes in a Norwegian population. To determine the background frequency in the population, 250 blood samples were consecutively collected from healthy Norwegian blood donors (125 men and 125 women; mean age: 48±11 years). Samples were tested for DAT1 VNTR, DRD2 Taq1A, DRD3 Ser9Gly, 5HTTLPR, and four polymorphisms (102 T>C, His452Tyr, 516 C>T, and Thr25Asn) in the 5HTR2A, using polymerase chain reaction and real-time polymerase chain reaction. We observed the frequency of the nine-repeat allele of DAT1 VNTR polymorphism as 20% (95% confidence interval [CI]: 0.18-0.23), the A1 allele of DRD2 Taq1A polymorphism as 21% (95% CI: 0.19-0.23), the A1 allele of DRD3 Ser9Gly polymorphism as 68% (95% CI: 0.66-0.70), the short allele of 5HTTLPR as 38% (95% CI: 0.36-0.40), and the T allele of 5HTR2A 102 T>C polymorphism as 41% (95% CI: 0.39-0.41), and the frequencies of 5HTR2A His452Tyr and 5HTR2A Thr25Asn were 93% and 95%, respectively. The tested polymorphisms showed differences compared with other European populations. Further studies are necessary to better understand the effect of these alleles and their combinations on personality, mental disorders, drug response, and adverse reactions of psychotropic drugs.

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.

Pharmacogenomic implications of variants of monoaminergic-related genes in geriatric psychiatry

Response to psychiatric medications in later life is highly heterogeneous and complex. Monoaminergic-related polymorphisms may influence medication response and susceptibility to side effects in elderly individuals. Individuals with the lower function short (S) allele of the serotonin transporter gene (SLC6A4) insertion/deletion (indel) promoter polymorphism (5-HTTLPR) have both increased the likelihood of adverse drug events and increased the need for higher antidepressant concentrations to obtain maximum antidepressant response. By contrast, carriers of the higher expression homozygous long allele (L/L) genotype may respond at lower concentrations. The differential role of these polymorphisms appears at early stages of treatment rather than in the final antidepressant outcome. Research findings suggest that the rs25531 SNP may influence functional expression of the L allele. Similarly, a variable number of tandem repeats in the second intron of the serotonin transporter gene may influence the expression of SLC6A4 and the implications of these variants may be influenced by aging. Two polymorphisms, rs2242466 (-182T/C) and rs5569 (1287G/A), in the norepinephrine transporter gene (SLC6A2 or NET) have been associated with antidepressant response. Studies in dopamine-related polymorphisms have focused on associations with neuroleptic-induced movement disorders. The rs1800497 variant (Taq1A) of the dopamine receptor D2 (DRD2) gene located in a noncoding 3´ region may regulate expression of D2 receptors. The rs6280 variant (Ser9Gly) of the dopamine receptor 3 (DRD3) gene may influence the binding affinity of D3 receptors as a result of serine to glycine substitution of the receptor protein. A multicenter collaborative research effort would be an effective strategy to increase sample sizes to further investigate how gene variants impact the pharmacodynamics and pharmacokinetics of psychotropic drugs in elderly persons.

Haplotypes of dopamine and serotonin transporter genes are associated with antisocial personality disorder in alcoholics

INTRODUCTION

A different genetic background is postulated for alcoholics with early onset and with antisocial personality disorder (type 2 alcoholics) compared with those with late onset and without antisocial personality disorder (type 1 alcoholics). The dopamine transporter (DAT) and the serotonin transporter (SERT) are involved in endophenotypes that are associated with these subtypes. Our study was aimed at investigating whether distinct haplotypes, defined by polymorphisms associated with the expressions of DAT and SERT, were associated with subgroups of alcohol dependence.

METHODS

Intron 8 variable number of tandem repeats (VNTR), exon 15 rs27072 and VNTR (DAT), promoter VNTR and rs25531, and intron 2 VNTR (SERT) were genotyped in a case-control sample comprising 360 alcoholics and 368 controls, and in a family-based sample of 65 trios, all of German origin.

RESULTS

DAT: The haplogenotypes 6-A-10/6-G-10 and 5-G-9/5-G-9 were more often present in type 2 alcoholics as compared with type 1 alcoholics [odds ratio (OR): 2.8], and controls (OR: 5.8), respectively. The daily ethanol consumption was associated with haplogenotypes. SERT: haplotypes SA-10 (OR: 2.3) and LG-12 (OR: 2.5) were more often present in type 2 alcoholics compared with controls. Haplotype LA-10 was less often present in type 2 alcoholics (OR: 0.5), and was more often transmitted, in families, to the affected offspring (transmission disequilibrium test: OR: 5.2; family-based association test: Z: 1.9). The haplotype LA-12 was significantly undertransmitted to affected offspring in the whole group (transmission disequilibrium test: OR: 0.216; family-based association test: Z: -2.2). A gene by environment interaction was observed with respect to the time course of the depression score after alcohol withdrawal and with respect to the positive family history of alcohol dependence.

CONCLUSION

Haplotype analysis, sub-grouping with respect to more homogeneous endophenotypes, and inclusion of quantifiable characteristics are sensible strategies to untangle the genetic background of such a complex disorder like alcohol dependence.

Influence of genotype on dopamine transporter availability in human striatum and sleep architecture

Genetic variants may modulate dopamine transporter (DAT) availability in the brain. A polymorphism within the intron 8 of the DAT1 gene was evaluated in 27 healthy men. No correlation between Int8 VNTR and either the inter-individual variability of the sleep architecture, or the DAT availability, as measured by single photon emission computed tomography (SPECT) and [(⁹⁹m)Tc]TRODAT-1 was observed.

Association between reward-related activation in the ventral striatum and trait reward sensitivity is moderated by dopamine transporter genotype

The impact of individual differences on human reward processing has been a focus of research in recent years, particularly, as they are associated with a variety of neuropsychiatric diseases including addiction and attention-deficit/hyperactivity disorder. Studies exploring the neural basis of individual differences in reward sensitivity have consistently implicated the ventral striatum (VS) as a core component of the human reward system. However, the mechanisms of dopaminergic neurotransmission underlying ventral striatal activation as well as trait reward sensitivity remain speculative. We addressed this issue by investigating the triadic interplay between VS reactivity during reward anticipation using functional magnetic resonance imaging, trait reward sensitivity, and dopamine (DA) transporter genotype (40-bp 3'VNTR of DAT, SLC6A3) affecting synaptic DA neurotransmission. Our results show that DAT variation moderates the association between VS-reactivity and trait reward sensitivity. Specifically, homozygote carriers of the DAT 10-repeat allele exhibit a strong positive correlation between reward sensitivity and reward-related VS activity whereas this relationship is absent in the DAT 9-repeat allele carriers. We discuss the possibility that this moderation of VS-trait relation might arise from DAT-dependent differences in DA availability affecting synaptic plasticity within the VS. Generally, studying the impact of dopaminergic gene variations on the relation between reward-related brain activity and trait reward sensitivity might facilitate the investigation of complex mechanisms underlying disorders linked to dysregulation of DA neurotransmission.

Imaging the effects of genetic polymorphisms on radioligand binding in the living human brain: A review on genetic neuroreceptor imaging of monoaminergic systems in psychiatry

Imaging genetics is a research field that describes the impact of genetic risk variants on brain structure and function. While magnetic resonance based imaging techniques are able to provide complex information on a system level, positron emission tomography (PET) and single photon emission computer tomography (SPECT) allow for determination of distribution and density of single receptor molecules in the human brain. Major psychiatric disorders are highly heritable, and have been associated with a dysregulation in brain dopamine and serotonin systems. Understanding the role of genetic polymorphisms within these neurotransmitter systems on brain phenotype is essential. This review tries to cover the literature on the impact of gene variants implicated in psychiatric disorders on serotonin, dopamine, and MAO-A radioligand binding in living humans. The majority of PET and SPECT studies investigated the role of polymorphisms within genes coding for the serotonin and dopamine transporters, the serotonin 1A receptor, and the dopamine D2 receptor on G protein coupled receptors or transporter proteins critically involved in serotonin or dopamine neurotransmission. Other studies investigated the impact of variants in genes for monoamine oxidase-A (MAO-A) or brain derived neurotrophic factor on monoamine transporters, receptors, or MAO-A activity. Two main findings in healthy subjects emerge from the current literature: one is an increased binding of the selective ligand [(11)C]DASB to serotonin transporters in subjects homozygous for the triallelic 5-HTTLPR LA allele. The other one is decreased binding of the radioligand [(11)C]raclopride to dopamine D2 receptors in D2 Taq1 A1 allele carriers. Other findings reported are highly interesting but require independent replication.

Association between dopaminergic polymorphisms and borderline personality traits among at-risk young adults and psychiatric inpatients

BACKGROUND

In the development of borderline personality disorder (BPD) both genetic and environmental factors have important roles. The characteristic affective disturbance and impulsive aggression are linked to imbalances in the central serotonin system, and most of the genetic association studies focused on serotonergic candidate genes. However, the efficacy of dopamine D2 receptor (DRD2) blocking antipsychotic drugs in BPD treatment also suggests involvement of the dopamine system in the neurobiology of BPD.

METHODS

In the present study we tested the dopamine dysfunction hypothesis of impulsive self- and other-damaging behaviors: borderline and antisocial traits were assessed by Structured Clinical Interview for Diagnosis (SCID) for DSM-IV in a community-based US sample of 99 young adults from low-to-moderate income families. For the BPD trait analyses a second, independent group was used consisting of 136 Hungarian patients with bipolar or major depressive disorder filling out self-report SCID-II Screen questionnaire. In the genetic association analyses the previously indicated polymorphisms of the catechol-O-methyl-transferase (COMT Val158Met) and dopamine transporter (DAT1 40 bp VNTR) were studied. In addition, candidate polymorphisms of the DRD2 and DRD4 dopamine receptor genes were selected from the impulsive behavior literature.

RESULTS

The DRD2 TaqI B1-allele and A1-allele were associated with borderline traits in the young adult sample (p = 0.001, and p = 0.005, respectively). Also, the DRD4 -616 CC genotype appeared as a risk factor (p = 0.02). With severity of abuse accounted for in the model, genetic effects of the DRD2 and DRD4 polymorphisms were still significant (DRD2 TaqIB: p = 0.001, DRD2 TaqIA: p = 0.008, DRD4 -616 C/G: p = 0.002). Only the DRD4 promoter finding was replicated in the independent sample of psychiatric inpatients (p = 0.007). No association was found with the COMT and DAT1 polymorphisms.

CONCLUSIONS

Our results of the two independent samples suggest a possible involvement of the DRD4 -616 C/G promoter variant in the development of BPD traits. In addition, an association of the DRD2 genetic polymorphisms with impulsive self-damaging behaviors was also demonstrated.

Functional analysis of intron 8 and 3' UTR variable number of tandem repeats of SLC6A3: differential activity of intron 8 variants

Association studies have found that variation in the dopamine transporter gene (SLC6A3) is important in the susceptibility to attention-deficit hyperactivity disorder (ADHD) and response to methylphenidate treatment. An understanding of the biological mechanisms underlying these associations is still inconclusive. We assessed the relative activity of variable number tandem repeat (VNTR) alleles of SLC6A3 under basal and stimulated cellular conditions, as well as in the presence of pharmacological blockade of the dopamine transporter using gene-reporter constructs. The intron 8 VNTR 5-repeat allele is more active than the 6-repeat allele. In the presence of forskolin, both alleles were significantly induced. Blockade of the dopamine transporter did not influence activity of either allelic construct. No difference in activity between 9- and 10-repeat alleles of the 3'-untranslated region VNTR was observed under any experimental condition. These data suggest that the intron 8 VNTR is a functional variant with an ADHD susceptibility allele having reduced activity. The lack of enhanced allele-specific activity in response to treatment regimes suggests that differential activity under basal conditions is the primary mode of action.

The neurobiology of individual differences in complex behavioral traits

Neuroimaging, especially BOLD fMRI, has begun to identify how variability in brain function contributes to individual differences in complex behavioral traits. In parallel, pharmacological fMRI and multimodal PET/fMRI are identifying how variability in molecular signaling pathways influences individual differences in brain function. Against this background, functional genetic polymorphisms are being utilized to understand the origins of variability in signaling pathways as well as to model efficiently how such emergent variability impacts behaviorally relevant brain function. This article provides an overview of a research strategy seeking to integrate these complementary technologies and utilizes existing empirical data to illustrate its effectiveness in illuminating the neurobiology of individual differences in complex behavioral traits. The article also discusses how such efforts can contribute to the identification of predictive markers that interact with environmental factors to precipitate disease and to develop more effective and individually tailored treatment regimes.

Dopamine transporter polymorphism modulates oculomotor function and DAT1 mRNA expression in schizophrenia

Smooth pursuit eye movement (SPEM) deficit is an established schizophrenia endophenotype with a similar neurocognitive construct to working memory. Frontal eye field (FEF) neurons controlling SPEM maintain firing when visual sensory information is removed, and their firing rates directly correlate with SPEM velocity. We previously demonstrated a paradoxical association between a functional polymorphism of dopamine signaling (COMT gene) and SPEM. Recent evidence implicates the dopamine transporter gene (DAT1) in modulating cortical dopamine and associated neurocognitive functions. We hypothesized that DAT1 10/10 genotype, which reduces dopamine transporter expression and increases extracellular dopamine, would affect SPEM. We examined the effects of DAT1 genotype on: Clinical diagnosis in the study sample (n = 418; 190 with schizophrenia), SPEM measures in a subgroup with completed oculomotor measures (n = 200; 87 schizophrenia), and DAT1 gene expression in FEF tissue obtained from postmortem brain samples (n = 32; 16 schizophrenia). DAT1 genotype was not associated with schizophrenia. DAT1 10/10 genotype was associated with better SPEM in healthy controls, intermediate SPEM in unaffected first-degree relatives of schizophrenia subjects, and worse SPEM in schizophrenia subjects. In the gene expression study, DAT1 10/10 genotype was associated with significantly reduced DAT1 mRNA transcript in FEF tissue from healthy control donors (P < 0.05), but higher expression in schizophrenia donors. Findings suggest regulatory effects of another gene(s) or etiological factor in schizophrenia, which modulate DAT1 gene function.

New genetic evidence for involvement of the dopamine system in migraine with aura

In order to systematically test the hypothesis that genetic variation in the dopamine system contributes to the susceptibility to migraine with aura (MA), we performed a comprehensive genetic association study of altogether ten genes from the dopaminergic system in a large German migraine with aura case-control sample. Based on the genotyping results of 53 variants across the ten genes in 270 MA cases and 272 controls, three genes-DBH, DRD2 and SLC6A3-were chosen to proceed to additional genotyping of 380 MA cases and 378 controls. Four of the 26 genotyped polymorphisms in these three genes displayed nominally significant allelic P-values in the sample of 650 MA patients and 650 controls. Three of these SNPs [rs2097629 in DBH (uncorrected allelic P value = 0.0012, OR = 0.77), rs7131056 in DRD2 (uncorrected allelic P value = 0.0018, OR = 1.28) and rs40184 in SLC6A3 (uncorrected allelic P value = 0.0082, OR = 0.81)] remained significant after gene-wide correction for multiple testing by permutation analysis. Further consideration of imputed genotype data from 2,937 British control individuals did not affirm the association with DRD2, but supported the associations with DBH and SLC6A3. Our data provide new evidence for an involvement of components of the dopaminergic system-in particular the dopamine-beta hydroxylase and dopamine transporter genes-to the pathogenesis of migraine with aura.

Genetic variation in components of dopamine neurotransmission impacts ventral striatal reactivity associated with impulsivity

Individual differences in traits such as impulsivity involve high reward sensitivity and are associated with risk for substance use disorders. The ventral striatum (VS) has been widely implicated in reward processing, and individual differences in its function are linked to these disorders. Dopamine (DA) plays a critical role in reward processing and is a potent neuromodulator of VS reactivity. Moreover, altered DA signaling has been associated with normal and pathological reward-related behaviors. Functional polymorphisms in DA-related genes represent an important source of variability in DA function that may subsequently impact VS reactivity and associated reward-related behaviors. Using an imaging genetics approach, we examined the modulatory effects of common, putatively functional DA-related polymorphisms on reward-related VS reactivity associated with self-reported impulsivity. Genetic variants associated with relatively increased striatal DA release (DRD2 -141C deletion) and availability (DAT1 9-repeat), as well as diminished inhibitory postsynaptic DA effects (DRD2 -141C deletion and DRD4 7-repeat), predicted 9-12% of the interindividual variability in reward-related VS reactivity. In contrast, genetic variation directly affecting DA signaling only in the prefrontal cortex (COMT Val158Met) was not associated with variability in VS reactivity. Our results highlight an important role for genetic polymorphisms affecting striatal DA neurotransmission in mediating interindividual differences in reward-related VS reactivity. They further suggest that altered VS reactivity may represent a key neurobiological pathway through which these polymorphisms contribute to variability in behavioral impulsivity and related risk for substance use disorders.

Striatal dopamine transporter availability associated with polymorphisms in the dopamine transporter gene SLC6A3

Polymorphisms in the dopamine transporter (DAT) gene SLC6A3 are associated with human striatal DAT expression, but the exact effects on DAT expression are not clear. A variable number of tandem repeats (VNTR) in the 3' untranslated region of the DAT gene was previously investigated in relation to striatal DAT availability, but the results were inconclusive. Other polymorphisms in the DAT gene were not extensively studied. Therefore, we investigated whether polymorphisms in both 3' and 5' ends of the DAT gene show association with in vivo striatal DAT expression.

METHODS

The subjects were an ethnically homogeneous group of 79 healthy young adults. Striatal DAT availability was measured with 123I-(2-beta-carbomethoxy-3-beta(4-iodophenyl)-tropane) (123I-beta-CIT) SPECT. The 40-base-pair VNTR in the 3' untranslated region of the DAT gene and the 2 single nucleotide polymorphisms (SNPs) rs2652511 and rs2937639 in the 5' end of the DAT gene were genotyped. Multiple-regression analysis was performed for each of the 3 polymorphisms. Analysis of the combination of the polymorphisms (haplotype analysis) was conducted for the triad rs2652511-rs2937639-VNTR.

RESULTS

For the VNTR, the 9-repeat (9R) allele was associated with significantly higher striatal DAT expression than was the 10-repeat (10R) allele (P=0.002). Subanalysis suggested a dominant effect for the 9R allele. Neither SNP rs2652511 nor SNP rs2937639 was associated with striatal DAT availability. The haplotype T-A-9R (rs2652511-rs2937639-VNTR) was significantly more associated with higher striatal DAT expression than were the other haplotypes (P=0.009).

CONCLUSION

The DAT VNTR 9R carriers have higher striatal DAT availability than do 10R homozygotes. This finding replicates former studies that included healthy subjects and also used 123I-beta-CIT SPECT. Our haplotype analysis identified a subgroup of 9R carriers, the T-A-9R, which appears to be mainly responsible for the association with higher striatal DAT availability. Thus, a combination of polymorphisms in both the 3' and the 5' ends of the DAT gene is associated with in vivo striatal DAT expression. This finding in healthy subjects may contribute to research on DAT availability and genotype in neuropsychiatric disorders.

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.

A neurogenetic approach to impulsivity

Impulsivity is a complex and multidimensional trait that is of interest to both personality psychologists and to clinicians. For investigators seeking the biological basis of personality traits, the use of neuroimaging techniques such as positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) revolutionized personality psychology in less than a decade. Now, another revolution is under way, and it originates from molecular biology. Specifically, new findings in molecular genetics, the detailed mapping and the study of the function of genes, have shown that individual differences in personality traits can be related to individual differences within specific genes. In this article, we will review the current state of the field with respect to the neural and genetic basis of trait impulsivity.

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.

Family based association study of pediatric bipolar disorder and the dopamine transporter gene (SLC6A3)

The dopamine transporter gene (SLC6A3) is a compelling candidate for pediatric bipolar disorder because (a) it has been associated with ADHD, (b) bipolar comorbidity with ADHD has been hypothesized to be an etiologically distinct familial subtype (c) blockade of the dopamine transporter with psychostimulants can induce mania in susceptible individuals and (d) previous studies have implicated the gene in bipolar disorder in adults. We conducted a family-based association study of SLC6A3 in 170 affected offspring trios defined by a child (12.9 +/- 5.3 years of age)with DSM-IV Bipolar-I disorder. Twenty-eight tag SNPs were chosen from the CEU (European) population of the International HapMap project (www.hapmap.org). Results indicated nominally positive association for 4 SNPs (rs40184, rs11133767, rs3776512, and rs464049), but only rs40184 survived correction for multiple statistical comparisons (P = 0.038). This is the first examination of the association with SLC6A3 and bipolar disorder in children and, like previous findings in adults with bipolar disorder, we found evidence of association with SNPs in the 3' region of the gene. These data provide suggestive evidence supporting a role for SLC6A3 in the etiology of pediatric bipolar disorder.

Common genetic variations in human brain-specific tryptophan hydroxylase-2 and response to antidepressant treatment

OBJECTIVE

Genetic variability within the serotoninergic system may predict the response to antidepressant drugs. Several polymorphisms in the gene coding for the brain-specific tryptophan hydroxylase (TPH2) have been associated with susceptibility to psychiatric diseases. In this study, we analyzed the correlation between TPH2 polymorphisms and response to antidepressant drugs.

METHODS

The study included 182 patients who received drug treatment for major depression. To assess the variability in the TPH2 gene, four single nucleotide polymorphisms (SNPs) tagging the common TPH2 haplotypes and six SNPs medically relevant according to data from other studies were analyzed in a multiplex single base primer extension reaction.

RESULTS

Two SNPs, rs10897346 and rs1487278, were significantly associated with response to therapy (P=0.003 and 0.007). The rs10897346 variant showed the highest predictive values with carriers of null C alleles showing a 2.6-fold increased risk (95% confidence interval 1.4-4.8) for nonresponse compared with the others. The effect was found in all major types of antidepressant medications administered in this study and was statistically significant in the subgroup on selective serotonin reuptake inhibitors. Multiple logistic regression analyses confirmed the rs10879346 polymorphism as an independent predictor of the antidepressant response (odds ratio: 3.86; 1.75-8.55, P=0.0008). The therapeutically relevant variant rs10897346 is completely linked with the functional Pro312Pro polymorphism, which is known to affect TPH2 expression and may influence serotonin synthesis in the brain.

CONCLUSION

The polymorphisms rs10897346 and Pro312Pro in the TPH2 gene might play an important role for TPH2 expression and antidepressant drug response.

Attention deficit hyperactivity disorder

Attention deficit hyperactivity disorder (ADHD) is a behavioral diagnosis based on the presence of developmentally inappropriate levels of impulsivity, overactivity, and inattentiveness. It is a familial condition with a complex pattern of inheritance. Variation of several genes involved in the regulation of dopamine, norepinephrine, and serotonin neurotransmission is associated with ADHD. We highlight the two most prominent findings with the dopamine D4 receptor (DRD4) gene and the dopamine transporter (DAT1) gene, and their implications for the understanding of the cellular and neurobiological basis for ADHD. Cognitive and functional studies using electrophysiology and brain imaging frequently indicate altered processing in ADHD during performance on cognitive tasks hypothesized to measure a "core" deficit, such as response inhibition. Yet, children with ADHD appear to suffer from a more general deficit, including impairment in attentional alerting, orienting, response preparation, and control. Reward processes are also altered and, further, a strong association emerges with intraindividual variability, with several causal hypotheses being proposed. Task performance correlates with underactivation of, especially, frontostriatal areas of the brain, but an extended network of brain regions is also implicated. Electroencephalography studies indicate abnormalities in ADHD in relation to slow-wave activity, linked to underarousal. These advances in the areas of genetics, cognitive function, neurophysiology, and neuroanatomy of ADHD give important leads for interdisciplinary research that aims to delineate the causal pathways. Such research is only at its beginning, but is illustrated by recent findings of an association between DAT1 and increased response variability in ADHD.