A promoter polymorphism (-839 C > T) at the dopamine transporter gene is associated with attention deficit/hyperactivity disorder in Brazilian children

Functional SLC6A3 polymorphisms differentially affect autism spectrum disorder severity: a study on Indian subjects

Imbalance in dopamine (DA) signaling is proposed to play a potential role in the etiology of Autism spectrum disorder (ASD) since, as a neuromodulator, DA regulates executive function, motor activity, social peering, attention as well as perception and subjects with ASD often exhibit deficit in these traits. Level of DA in the synaptic cleft is maintained by dopamine transporter (DAT) and hence, to identify the role of DAT in ASD, we have analyzed four functional genetic variants, rs28363170, rs3836790, rs2652511, rs27072, in nuclear families with ASD probands. Subjects were diagnosed based on Diagnostic and Statistical Manual for Mental Disorders and trait severity was assessed by Childhood Autism Rating Scale 2-Standard test. Informed written consent was obtained from the parents/care givers before recruitment followed by collection of peripheral blood for genomic DNA isolation. Target sites were investigated by PCR-based methods and data obtained was analyzed by population- as well as family-based statistical methods. Case-control analysis revealed significant higher frequencies of 9 repeat (9R) and 5 repeat (5R) alleles of rs28363170 and rs3836790 respectively in the ASD probands. Family-based analysis showed statistically significant higher paternal transmission of rs28363170 9R and rs2652511 T alleles. In the presence of rs28363170 9R, rs27072 C, rs3836790 6R6R, and rs2652511 CC variants, trait scores were higher. Studied variants showed independent as well as interactive effects, which varied based on gender of the probands. We infer that altered DA availability mediated through DAT may affect autistic traits warranting further in depth investigation in the field.

Presence of recombination hotspots throughout SLC6A3

The human dopamine transporter gene SLC6A3 is involved in substance use disorders (SUDs) among many other common neuropsychiatric illnesses but allelic association results including those with its classic genetic markers 3'VNTR or Int8VNTR remain mixed and unexplainable. To better understand the genetics for reproducible association signals, we report the presence of recombination hotspots based on sequencing of the entire 5' promoter regions in two small SUDs cohorts, 30 African Americans (AAs) and 30 European Americans (EAs). Recombination rate was the highest near the transcription start site (TSS) in both cohorts. In addition, each cohort carried 57 different promoter haplotypes out of 60 and no haplotypes were shared between the two ethnicities. A quarter of the haplotypes evolved in an ethnicity-specific manner. Finally, analysis of five hundred subjects of European ancestry, from the 1000 Genome Project, confirmed the promoter recombination hotspots and also revealed several additional ones in non-coding regions only. These findings provide an explanation for the mixed results as well as guidance for selection of effective markers to be used in next generation association validation (NGAV), facilitating the delineation of pathogenic variation in this critical neuropsychiatric gene.

The dopamine transporter role in psychiatric phenotypes

The dopamine transporter (DAT) is one of the most relevant and investigated neurotransmitter transporters. DAT is a plasma membrane protein which plays a homeostatic role, controlling both extracellular and intracellular concentrations of dopamine (DA). Since unbalanced DA levels are known to be involved in numerous mental disorders, a wealth of investigations has provided valuable insights concerning DAT role into normal brain functioning and pathological processes. Briefly, this extensive but non-systematic review discusses what is recently known about the role of SLC6A3 gene which encodes the dopamine transporter in psychiatric phenotypes. DAT protein, SLC6A3 gene, animal models, neuropsychology, and neuroimaging investigations are also concisely discussed. To conclude, current challenges are reviewed in order to provide perspectives for future studies.

Research Domain Criteria versus DSM V: How does this debate affect attempts to model corticostriatal dysfunction in animals?

For decades, the nosology of mental illness has been based largely upon the descriptions in the Diagnostic and Statistical Manual of the American Psychiatric Association (DSM). A recent challenge to the DSM approach to psychiatric nosology from the National Institute on Mental Health (USA) defines Research Domain Criteria (RDoC) as an alternative. For RDoC, psychiatric illnesses are not defined as discrete categories, but instead as specific behavioral dysfunctions irrespective of DSM diagnostic categories. This approach was driven by two primary weaknesses noted in the DSM: (1) the same symptoms occur in very different disease states; and (2) DSM criteria lack grounding in the underlying biological causes of mental illness. RDoC intends to ground psychiatric nosology in those underlying mechanisms. This review addresses the suitability of RDoC vs. DSM from the view of modeling mental illness in animals. A consideration of all types of psychiatric dysfunction is beyond the scope of this review, which will focus on models of conditions associated with frontostriatal dysfunction.

Separating the wheat from the chaff: systematic identification of functionally relevant noncoding variants in ADHD

Attention deficit hyperactivity disorder (ADHD) is a highly heritable psychiatric condition with negative lifetime outcomes. Uncovering its genetic architecture should yield important insights into the neurobiology of ADHD and assist development of novel treatment strategies. Twenty years of candidate gene investigations and more recently genome-wide association studies have identified an array of potential association signals. In this context, separating the likely true from false associations ('the wheat' from 'the chaff') will be crucial for uncovering the functional biology of ADHD. Here, we defined a set of 2070 DNA variants that showed evidence of association with ADHD (or were in linkage disequilibrium). More than 97% of these variants were noncoding, and were prioritised for further exploration using two tools-genome-wide annotation of variants (GWAVA) and Combined Annotation-Dependent Depletion (CADD)-that were recently developed to rank variants based upon their likely pathogenicity. Capitalising on recent efforts such as the Encyclopaedia of DNA Elements and US National Institutes of Health Roadmap Epigenomics Projects to improve understanding of the noncoding genome, we subsequently identified 65 variants to which we assigned functional annotations, based upon their likely impact on alternative splicing, transcription factor binding and translational regulation. We propose that these 65 variants, which possess not only a high likelihood of pathogenicity but also readily testable functional hypotheses, represent a tractable shortlist for future experimental validation in ADHD. Taken together, this study brings into sharp focus the likely relevance of noncoding variants for the genetic risk associated with ADHD, and more broadly suggests a bioinformatics approach that should be relevant to other psychiatric disorders.

Further evidence for the association between a polymorphism in the promoter region of SLC6A3/DAT1 and ADHD: findings from a sample of adults

The dopamine transporter (SLC6A3/DAT1) plays a key role in the regulation of dopaminergic neurotransmission and is the major site of action for methylphenidate, a first-line medication for attention deficit hyperactivity disorder (ADHD). Most genetic association studies with ADHD have investigated a 40-bp variable number of tandem repeats (VNTR) polymorphism in the 3'-untranslated region (UTR) of the DAT1, but these investigations have reported heterogeneous findings. The few studies focused on the 5' region have reported promising results. Despite rs2652511 not being included, nor having any proxy SNP available in GWAS, the few candidate gene studies that analyzed it suggested an association with ADHD and schizophrenia. Here, we analyzed the -839 C/T (rs2652511) promoter variant and the 3'-UTR and intron 8 (Int8) VNTR polymorphisms in 522 adults with ADHD and 628 blood donor controls. The diagnostic procedures followed the DSM-IV criteria. A significant association was detected (P = 0.002) between the rs2652511 C-allele with ADHD. In addition, the 6-repeat allele of Int8 VNTR was associated with higher inattention scores (P = 0.034). The haplotype analysis including DAT1 3'-UTR and Int8 VNTR polymorphisms did not reveal associations with ADHD susceptibility or severity dimensions. These findings extend to adult samples previous findings from children samples on the role of the rs2652511 polymorphism in the promoter region of DAT1 as a risk factor for ADHD susceptibility.

Polymorphisms in the dopamine transporter gene are associated with visual hallucinations and levodopa equivalent dose in Brazilians with Parkinson's disease

The requirement for dopaminergic drugs in Parkinson's disease (PD) is highly variable. Visual hallucinations are a frequent and serious complication of chronic levodopa therapy. Polymorphisms in the DAT1 gene might affect the reuptake of dopamine in the synaptic cleft, but the influence of this variability on adverse effects or levodopa equivalent dose on PD patients is still poorly investigated. Therefore, the aim of the present study was to investigate DAT1 gene polymorphisms on levodopa equivalent dose and visual hallucination occurrence in PD patients. Altogether, 196 PD patients in treatment with at least 200 mg levodopa equivalent dose for at least 1 yr were included. These patients were genotyped for the -839 C > T and 3' VNTR DAT1 polymorphisms by PCR-based methodologies. Visual hallucinations occurred in 25.5% of the sample. After controlling for confounders, the dopamine transporter (DAT) -839 C allele was associated with visual hallucinations (prevalence ratio 2.5, 95% confidence intervals 1.13-5.5, p = 0.02). Levodopa equivalent dose was lower in carriers of the nine repeat allele of the DAT 3'UTR VNTR (741.2 ± 355.0 vs. 843.4 ± 445.7), explaining 21% of dose variability (p = 0.01). Our results support an effect of DAT1 polymorphisms in adverse effects of anti-Parkinsonian drugs and in levodopa equivalent dose usage.

The Brazilian contribution to Attention-Deficit/Hyperactivity Disorder molecular genetics in children and adolescents

Attention-Deficit/Hyperactivity Disorder (ADHD) is a common psychiatric condition of children worldwide. This disorder is defined by a combination of symptoms of inattention and hyperactivity/impulsivity. Diagnosis is based on a sufficient number of symptoms causing impairment in these two domains determining several problems in personal and academic life. Although genetic and environmental factors are important in ADHD etiology, how these factors influence the brain and consequently behavior is still under debate. It seems to be consensus that a frontosubcortical dysfunction is responsible, at least in part, for the ADHD phenotype spectrum. The main results from association and pharmacogenetic studies performed in Brazil are discussed. The investigations performed so far on ADHD genetics in Brazil and elsewhere are far from conclusive. New plausible biological hypotheses linked to neurotransmission and neurodevelopment, as well as new analytic approaches are needed to fully disclose the genetic component of the disorder.

Quantitative and molecular genetics of ADHD

ADHD is a common and highly heritable disorder. Family, twin, and adoption studies confirm a strong genetic influence in risk for ADHD and there has been a great deal of interest in identifying the genetic factors involved. Quantitative genetic studies find that genetic risk for ADHD is continuously distributed throughout the population, that there are both shared and unique genetic influences on inattention and hyperactivity-impulsivity, and that ADHD shares genetic risk factors with commonly co-occurring clinical syndromes and traits. ADHD is found at all ages and the underlying genetic architecture is similar across the lifespan. In terms of specific genetic findings, there is consistent evidence of monoamine neurotransmitter involvement with the best evidence coming from genetic markers in or near the dopamine D4 and D5 receptor genes. Recent genome-wide association studies have identified new association findings, including genes involved in cell division, cell adhesion, neuronal migration, and neuronal plasticity. However, as yet, none of these pass genome-wide levels of significance. Finally, recent data confirm an important role for rare copy number variants, including those that are found in schizophrenia and autism. Future work should use genetic association data to determine the nature of the cognitive, neuronal and cellular processes that mediate genetic risks on behaviour, and identify environmental factors that interact with genetic risks for ADHD.

Polymorphisms in neuropsychiatric and neuroinflammatory disorders and the role of next generation sequencing in early diagnosis and treatment

A number of polymorphisms have been implicated in different neuropsychiatric and neurological disorders. Polymorphisms in neurological disorders with a central immune component are well described, mainly due to their role in increasing neurodegeneration. For example, the role of polymorphisms in Alzheimer's disease in accumulation of amyloid plaques is now well established. In contrast, polymorphisms resulting in or affecting psychiatric disorders are less well studied and frequently are not replicated by meta-analysis. Furthermore, even if a significant association has been confirmed, the role of the identified polymorphism in causing and/or augmenting the disorder is often difficult to rationalize. Here, we review polymorphisms found associated with different neuroinflammatory and neuropsychiatric disorders and discuss the role of next generation sequencing in early diagnosis and treatment and as a tool in studying their functional consequences.

The role of dopaminergic signalling during larval zebrafish brain development: a tool for investigating the developmental basis of neuropsychiatric disorders

Neurodevelopment depends on intrinsic and extrinsic factors that influence the overall pattern of neurogenesis and neural circuit formation, which has a direct impact on behaviour. Defects in dopamine signalling and brain morphology at a relatively early age, and mutations in neurodevelopmental genes are strongly correlated with several neuropsychiatric disorders. This evidence supports the hypothesis of a neurodevelopmental origin of at least some forms of mental illness. Zebrafish (Danio rerio) has emerged as an important vertebrate model system in biomedical research. The ease with which intrinsic and extrinsic factors can be altered during early development, the relatively conserved dopaminergic circuit organisation in the larval brain, and the emergence of simple sensorimotor behaviours very early in development are some of the appealing features that make this organism advantageous for developmental brain and behaviour research. Thus, examining the impact of altered dopamine signalling and disease related genetic aberrations during zebrafish development presents a unique opportunity to holistically analyse the in vivo biochemical, morphological and behavioural significance of altered dopamine signalling during a crucial period of development using a highly tractable vertebrate model organism. Ultimately, this information will shed new light on potential therapeutic targets for the treatment of schizophrenia and perhaps serve as a paradigm for investigating the neurodevelopmental origin of other psychiatric disorders.

Overview of animal models of attention deficit hyperactivity disorder (ADHD)

Attention-deficit/hyperactivity disorder (ADHD) is a heterogeneous, highly heritable, behavioral disorder that affects ∼5% to 10% of children worldwide. Although animal models cannot truly reflect human psychiatric disorders, they can provide insight into the disorder that cannot be obtained from human studies because of the limitations of available techniques. Genetic models include the spontaneously hypertensive rat (SHR), the Naples High Excitability (NHE) rat, poor performers in the 5-choice serial reaction time (5-CSRT) task, the dopamine transporter (DAT) knock-out mouse, the SNAP-25 deficient mutant coloboma mouse, mice expressing a human mutant thyroid hormone receptor, a nicotinic receptor knock-out mouse, and a tachykinin-1 (NK1) receptor knock-out mouse. Chemically induced models of ADHD include prenatal or early postnatal exposure to ethanol, nicotine, polychlorinated biphenyls, or 6-hydroxydopamine (6-OHDA). Environmentally induced models have also been suggested; these include neonatal anoxia and rat pups reared in social isolation. The major insight provided by animal models was the consistency of findings regarding the involvement of dopaminergic, noradrenergic, and sometimes also serotonergic systems, as well as more fundamental defects in neurotransmission.

Neither single-marker nor haplotype analyses support an association between the dopamine transporter gene and heroin dependence in Han Chinese

Much evidence suggests that dysfunction of dopamine transporter-mediated dopamine transmission may be involved in the pathophysiology of substance abuse and dependence. The aim of this study was to examine whether the dopamine transporter gene (DAT1; SLC6A3) is associated with the development of heroin dependence (HD) and whether DAT1 influences personality traits in patients with HD. Polymorphisms of DAT1 were analyzed in a case-control study of 1046 Han Chinese (615 patients and 431 controls). All participants were screened using a Chinese version of the modified Schedule of Affective Disorder and Schizophrenia-Lifetime and all patients met the criteria for HD. Furthermore, a Chinese version of the Tridimensional Personality Questionnaire (TPQ) was used to assess personality traits in the patient group and examine the association between their personality traits and DAT1 polymorphisms. Of the patient group, 271 completed the TPQ. No statistically significant differences in allele or genotype frequencies of all investigated variants between HD patients and controls were observed. In haplotype analyses, four haplotype blocks of DAT1 were not associated with the development of HD. These DAT1 polymorphisms did not influence novelty seeking and harm avoidance scores in HD patients. This study suggests that the DAT1 gene may not contribute to the risk of HD and specific personality traits in HD among the Han Chinese population.

Molecular genetics of attention-deficit/hyperactivity disorder: an overview

As heritability is high in attention-deficit/hyperactivity disorder (ADHD), genetic factors must play a significant role in the development and course of this disorder. In recent years a large number of studies on different candidate genes for ADHD have been published, most have focused on genes involved in the dopaminergic neurotransmission system, such as DRD4, DRD5, DAT1/SLC6A3, DBH, DDC. Genes associated with the noradrenergic (such as NET1/SLC6A2, ADRA2A, ADRA2C) and serotonergic systems (such as 5-HTT/SLC6A4, HTR1B, HTR2A, TPH2) have also received considerable interest. Additional candidate genes related to neurotransmission and neuronal plasticity that have been studied less intensively include SNAP25, CHRNA4, NMDA, BDNF, NGF, NTF3, NTF4/5, GDNF. This review article provides an overview of these candidate gene studies, and summarizes findings from recently published genome-wide association studies (GWAS). GWAS is a relatively new tool that enables the identification of new ADHD genes in a hypothesis-free manner. Although these latter studies could be improved and need to be replicated they are starting to implicate processes like neuronal migration and cell adhesion and cell division as potentially important in the aetiology of ADHD and have suggested several new directions for future ADHD genetics studies.

Association of promoter variants of human dopamine transporter gene with schizophrenia in Han Chinese

OBJECTIVE

Although dopamine was implicated in the etiology of schizophrenia, the human dopamine transporter gene (DAT1; SLC6A3) has not consistently been associated with schizophrenia. The purpose of this study was to examine whether six polymorphisms within the DAT1 gene are associated with schizophrenia.

METHODS

Six polymorphisms of the DAT1 gene (3 SNPs [rs6413429, rs2652511, and rs2975226] in the promoter region, one SNP [rs6347] in exon 9, and one SNP [rs27072]/one variable number tandem repeat [VNTR] in exon 15) were analyzed in 352 Chinese patients with schizophrenia and in 311 healthy controls. Pretreatment psychopathology was assessed using the Positive and Negative Syndrome Scale in a subset of 160 hospitalized schizophrenia patients who were drug-free or drug-naïve.

RESULTS

A statistically significant difference in two polymorphisms (rs2652511 and rs2975226) and a promoter region haplotype (rs2652511, rs2975226, and rs6413429) was found between patients and healthy controls. No association with schizophrenia was found for other polymorphisms and another haplotype (3' region). Symptoms severity (PANSS global, positive, negative and general symptoms scores) was similar regardless of DAT1 polymorphism.

CONCLUSION

The promoter region of the DAT1 gene may play a role in increasing susceptibility to schizophrenia, but does not affect the severity of psychotic symptoms in Han Chinese.

The genetics of attention-deficit/hyperactivity disorder

Attention-deficit/hyperactivity disorder (ADHD) is a complex neurodevelopmental disorder that almost certainly represents the common outcome of multiple causal pathways and it is now generally accepted that genetic factors make a significant contribution to these pathways. Behavioral studies suggest a heritability of approximately 0.76. While molecular genetic approaches have identified a range of potential candidate genes, it is now clear that the genetics of ADHD are characterized by a number of genes each of which makes a small but significant contribution to the overall risk. Several genome-wide linkage studies have been conducted and, although there are considerable differences in findings between studies, several regions have been supported across several studies (bin 16.4, 5p13, 11q22-25, 17p11). The contribution of several candidate genes has been supported by meta-analyses (DRD4, DRD5, DAT1, HTR1B and SNAP25). Genome-wide association scans are starting to appear but have not yet had sufficient power to produce conclusive results. Gene-environment interactions, which are as yet relatively understudied, are likely to be of importance in fully understanding the role of genes in ADHD and will be discussed.

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.

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.

A review of psychiatric genetics research in the Brazilian population

OBJECTIVE AND METHOD: A large increase in the number of Brazilian studies on psychiatric genetics has been observed in the 1970's since the first publications conducted by a group of researchers in Brazil. Here we reviewed the literature and evaluated the advantages and difficulties of psychiatric genetic studies in the Brazilian population. CONCLUSION: The Brazilian population is one of the most heterogeneous populations in the world, formed mainly by the admixture between European, African and Native American populations. Although the admixture process is not a particularity of the Brazilian population, much of the history and social development in Brazil underlies the ethnic melting pot we observe nowadays. Such ethnical heterogeneity of the Brazilian population obviously brings some problems when performing genetic studies. However, the Brazilian population offers a number of particular characteristics that are of major interest when genetic studies are carried out, such as the presence of isolated populations. Thus, differences in the genetic profile and in the exposure to environmental risks may result in different interactions and pathways to psychopathology.

The genetic overlap of attention deficit hyperactivity disorder and autistic spectrum disorder

Autistic spectrum disorders (ASD) and attention deficit hyperactivity disorder (ADHD) are classified as distinct disorders within the DSM-IV-TR (1994). The manual excludes simultaneous use of both diagnoses in case of overlap on a symptomatic level. However this does not always represent clinical observations and findings of previous studies. This review explores the genetic basis of the phenomenological overlap between ADHD and ASD. Based on an extensive review of twin-, linkage-, association studies, and reported structural genomic abnormalities associated with these disorders, we have identified seventeen regions on the human genome that can be related to both disorders. These regions of shared genetic association are: 2q35, 3p14, 4p16.1, 4p16.3, 5p15.31, 5p15.33, 7p12.3, 7p22, 7q21, 8q24.3, 14q12, 15q11–12, 16p13, 17q11, 18q21–23, 22q11.2, Xp22.3. The presented data are of interest for future genetic studies and appear to suggest the existence of a phenotype partition that may differ from the current classification of psychiatric disorders.

Activity-dependent regulation of the dopamine transporter is mediated by Ca(2+)/calmodulin-dependent protein kinase signaling

The mechanisms regulating expression of the dopamine transporter are poorly understood. We tested the hypothesis that neuronal activity is one of the non-genetic determinants of dopamine transporter abundance. Sustained changes in neuronal activity caused by tetrodotoxin and 4-aminopyridine altered the dopamine uptake and abundance of dopamine transporter and its mRNA in rat mesencephalic cultures. The altered neuronal activity caused by these two drugs is accompanied by changes in intracellular calcium concentrations and Ca(2+)/calmodulin-dependent protein (CaM) kinase II activity in dopamine neurons. Chronic treatment with an L-type calcium channel blocker (nifedipine) or CaM kinase inhibitor (KN93) decreased dopamine transporter-mediated uptake and occluded the effects of tetrodotoxin and 4-aminopyridine. These data suggest that neuronal activity can regulate dopamine transporter function and abundance via calcium/CaM kinase II signaling.

Association study of promoter polymorphisms at the dopamine transporter gene in Attention Deficit Hyperactivity Disorder

BACKGROUND

Attention deficit hyperactivity disorder (ADHD) is a complex neurobehavioral disorder. The dopamine transporter gene (DAT1/SLC6A3) has been considered a good candidate for ADHD. Most association studies with ADHD have investigated the 40-base-pair variable number of tandem repeat (VNTR) polymorphism in the 3'-untranslated region of DAT1. Only few studies have reported association between promoter polymorphisms of the gene and ADHD.

METHODS

To investigate the association between the polymorphisms -67A/T (rs2975226) and -839C/T (rs2652511) in promoter region of DAT1 in ADHD, two samples of ADHD patients from the UK (n = 197) and Taiwan (n = 212) were genotyped, and analysed using within-family transmission disequilibrium test (TDT).

RESULTS

A significant association was found between the T allele of promoter polymorphism -67A/T and ADHD in the Taiwanese population (P = 0.001). There was also evidence of preferential transmission of the T allele of -67A/T polymorphism in combined samples from the UK and Taiwan (P = 0.003). No association was detected between the -839C/T polymorphism and ADHD in either of the two populations.

CONCLUSION

The finding suggests that genetic variation in the promoter region of DAT1 may be a risk factor in the development of ADHD.

A common haplotype at the dopamine transporter gene 5' region is associated with attention-deficit/hyperactivity disorder

The dopamine transporter (DAT) is the major site of methylphenidate action, which is one of the main drugs used to treat attention-deficit/hyperactivity disorder (ADHD). Most association studies with ADHD focused in a VNTR at the 3'-untranslated region of the gene (3'UTR) presenting conflicting results. However, the most common explanation to inconsistent results is variable linkage disequilibrium with an adjacent functional variant, just a few number of DAT1 studies have reported LD structure across the gene. In this study, we screened 16 polymorphisms across the DAT1 gene to understand LD structure in a Brazilian sample of families with ADHD probands and to verify if there were evidence for a biased transmission of alleles and haplotypes from parents to their 243 children with ADHD. In the DSM-IV combined subtype, we observed a preferential transmission of the haplotype A/C/C/C/A derived from five SNPs (rs2550948, rs11564750, rs261759, rs2652511, rs2975223) in 5' region (P corrected = 0.018) and no association with any allele/haplotype at the 3' region of the gene, including the 3' VNTR and the VNTR of intron 8. These results suggest a role for the promoter region in ADHD susceptibility and that allele heterogeneity should be highly considered in DAT1 gene association studies highlighting the importance of this gene in the genetics of the disorder.

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.

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.