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

The Use of Drosophila to Understand Psychostimulant Responses

The addictive properties of psychostimulants such as cocaine, amphetamine, methamphetamine, and methylphenidate are based on their ability to increase dopaminergic neurotransmission in the reward system. While cocaine and methamphetamine are predominately used recreationally, amphetamine and methylphenidate also work as effective therapeutics to treat symptoms of disorders including attention deficit and hyperactivity disorder (ADHD) and autism spectrum disorder (ASD). Although both the addictive properties of psychostimulant drugs and their therapeutic efficacy are influenced by genetic variation, very few genes that regulate these processes in humans have been identified. This is largely due to population heterogeneity which entails a requirement for large samples. Drosophila melanogaster exhibits similar psychostimulant responses to humans, a high degree of gene conservation, and allow performance of behavioral assays in a large population. Additionally, amphetamine and methylphenidate reduce impairments in fly models of ADHD-like behavior. Therefore, Drosophila represents an ideal translational model organism to tackle the genetic components underlying the effects of psychostimulants. Here, we break down the many assays that reliably quantify the effects of cocaine, amphetamine, methamphetamine, and methylphenidate in Drosophila. We also discuss how Drosophila is an efficient and cost-effective model organism for identifying novel candidate genes and molecular mechanisms involved in the behavioral responses to psychostimulant drugs.

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.

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

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

Overweight in Boys With ADHD Is Related to Candidate Genes and Not to Deficits in Cognitive Functions

OBJECTIVE

The aim of the study was to assess the relationship of overweight, the polymorphisms of selected candidate genes, and deficits in the executive functions among children with ADHD.

METHOD

We examined 109 boys with ADHD aged between 7 and 17 years. The study indicated variants of 14 polymorphisms in eight candidate genes. We applied seven neuropsychological tests to evaluate the executive functions. Overweight was diagnosed on the basis of the guidelines of the International Obesity Task Force.

RESULTS

Analyses revealed significant association between DRD4 rs1800955, SNAP25 rs363039 and rs363043, 5HTR2A rs17288723, and overweight in boys with ADHD. There were no significant differences in the level of neuropsychological test results between patients with overweight and without overweight.

CONCLUSION

Overweight in boys with ADHD is associated with polymorphisms in three candidate genes: DRD4, SNAP25, and 5HTR2A, but not through conditioning deficits in cognitive functions.

Analysis of shared homozygosity regions in Saudi siblings with attention deficit hyperactivity disorder

AIM

Genetic and clinical complexities are common features of most psychiatric illnesses that pose a major obstacle in risk-gene identification. Attention deficit hyperactivity disorder (ADHD) is the most prevalent child-onset psychiatric illness, with high heritability. Over the past decade, numerous genetic studies utilizing various approaches, such as genome-wide association, candidate-gene association, and linkage analysis, have identified a multitude of candidate loci/genes. However, such studies have yielded diverse findings that are rarely reproduced, indicating that other genetic determinants have not been discovered yet. In this study, we carried out sib-pair analysis on seven multiplex families with ADHD from Saudi Arabia. We aimed to identify the candidate chromosomal regions and genes linked to the disease.

PATIENTS AND METHODS

A total of 41 individuals from multiplex families were analyzed for shared regions of homozygosity. Genes within these regions were prioritized according to their potential relevance to ADHD.

RESULTS

We identified multiple genomic regions spanning different chromosomes to be shared among affected members of each family; these included chromosomes 3, 5, 6, 7, 8, 9, 10, 13, 17, and 18. We also found specific regions on chromosomes 8 and 17 to be shared between affected individuals from more than one family. Among the genes present in the regions reported here were involved in neurotransmission (GRM3, SIGMAR1, CHAT, and SLC18A3) and members of the HLA gene family (HLA-A, HLA-DPA1, and MICC).

CONCLUSION

The candidate regions identified in this study highlight the genetic diversity of ADHD. Upon further investigation, these loci may reveal candidate genes that enclose variants associated with ADHD. Although most ADHD studies were conducted in other populations, our study provides insight from an understudied, ethnically interesting population.

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.

Comorbidity of Internet use disorder and attention deficit hyperactivity disorder: Two adult case-control studies

Objectives There is good scientific evidence that attention deficit hyperactivity disorder (ADHD) is both a predictor and a comorbidity of addictive disorders in adulthood. These associations not only focus on substance-related addictions but also on behavioral addictions like gambling disorder and Internet use disorder (IUD). For IUD, systematic reviews have identified ADHD as one of the most prevalent comorbidities besides depressive and anxiety disorders. Yet, there is a need to further understand the connections between both disorders to derive implications for specific treatment and prevention. This is especially the case in adult clinical populations where little is known about these relations so far. This study was meant to further investigate this issue in more detail based on the general hypothesis that there is a decisive intersection of psychopathology and etiology between IUD and ADHD. Methods Two case-control samples were examined at a university hospital. Adult ADHD and IUD patients ran through a comprehensive clinical and psychometrical workup. Results We found support for the hypothesis that ADHD and IUD share psychopathological features. Among patients of each group, we found substantial prevalence rates of a comorbid ADHD in IUD and vice versa. Furthermore, ADHD symptoms were positively associated with media use times and symptoms of Internet addiction in both samples. Discussion Clinical practitioners should be aware of the close relationships between the two disorders both diagnostically and therapeutically. When it comes to regain control over one's Internet use throughout treatment and rehabilitation, a potential shift of addiction must be kept in mind on side of practitioners and patients.

Dopamine transporter (DAT1/SLC6A3) polymorphism and the association between being born small for gestational age and symptoms of ADHD

Being small for gestational age (SGA) has been established as a risk factor for Attention Deficit Hyperactivity Disorder (ADHD). Likewise, several molecular genetic studies have found a link between DAT1 and ADHD. This study investigated whether SGA moderates the effect of dopamine transporter gene variants on the risk of ADHD. A total of 546 children of European descent were genotyped at age 11 for seven DAT1 SNPs (rs6347, rs11564774, rs40184, rs1042098, rs2702, rs8179029 and rs3863145). The Strengths and Difficulties Questionnaire was used to measure symptoms of ADHD at ages 3.5, 7 and 11. We found significant gene-environment interactions between birth weight and DAT1 SNPs (rs6347, rs40184, rs1042098, rs3863145) on ADHD symptoms at 3.5 years only. Results suggest that genotypic variation of DAT1 may confer a relative protective effect against ADHD in SGA individuals. This study supports the idea that being born SGA moderates the effect of the DAT1 gene on ADHD symptoms in the preschool years and may help to explain some of the heterogeneity in ADHD outcomes.

Genetic associations with reflexive visual attention in infancy and childhood

This study elucidates genetic influences on reflexive (as opposed to sustained) attention in children (aged 9-16 years; N = 332) who previously participated as infants in visual attention studies using orienting to a moving bar (Dannemiller, 2004). We investigated genetic associations with reflexive attention measures in infancy and childhood in the same group of children. The genetic markers (single nucleotide polymorphisms and variable number tandem repeats on the genes APOE, BDNF, CHRNA4, COMT, DRD4, HTR4, IGF2, MAOA, SLC5A7, SLC6A3, and SNAP25) are related to brain development and/or to the availability of neurotransmitters such as acetylcholine, dopamine, or serotonin. This study shows that typically developing children have differences in reflexive attention associated with their genes, as we found in adults (Lundwall, Guo & Dannemiller, 2012). This effort to extend our previous findings to outcomes in infancy and childhood was necessary because genetic influence may differ over the course of development. Although two of the genes that were tested in our adult study (Lundwall et al., 2012) were significant in either our infant study (SLC6A3) or child study (DRD4), the specific markers tested differed. Performance on the infant task was associated with SLC6A3. In addition, several genetic associations with an analogous child task occurred with markers on CHRNA4, COMT, and DRD4. Interestingly, the child version of the task involved an interaction such that which genotype group performed poorer on the child task depended on whether we were examining the higher or lower infant scoring group. These findings are discussed in terms of genetic influences on reflexive attention in infancy and childhood.

Genetic Influence on Slope Variability in a Childhood Reflexive Attention Task

Individuals are not perfectly consistent, and interindividual variability is a common feature in all varieties of human behavior. Some individuals respond more variably than others, however, and this difference may be important to understanding how the brain works. In this paper, we explore genetic contributions to response time (RT) slope variability on a reflexive attention task. We are interested in such variability because we believe it is an important part of the overall picture of attention that, if understood, has the potential to improve intervention for those with attentional deficits. Genetic association studies are valuable in discovering biological pathways of variability and several studies have found such associations with a sustained attention task. Here, we expand our knowledge to include a reflexive attention task. We ask whether specific candidate genes are associated with interindividual variability on a childhood reflexive attention task in 9–16 year olds. The genetic makers considered are on 11 genes: APOE, BDNF, CHRNA4, COMT, DRD4, HTR4, IGF2, MAOA, SLC5A7, SLC6A3, and SNAP25. We find significant associations with variability with markers on nine and we discuss the results in terms of neurotransmitters associated with each gene and the characteristics of the associated measures from the reflexive attention task.

Dopamine transporter gene may be associated with bipolar disorder and its personality traits

Dopamine transporter and its genetic factors have been suggested to play a critical role in the development of bipolar disorder (BPD). However, the importance of the dopamine transporter gene (DAT1) in the pathogenesis of BPD remains unclear. The aims of this study were to assess 18 polymorphisms of the DAT1 gene to determine whether this gene is associated with BPD and whether it influences personality traits of patients with BPD. DAT1 polymorphisms were analyzed in 492 BPD (374 BPDI and 118 BPDII) patients and 436 controls. All participants were screened using the same assessment tool, and all met the criteria for BPD. The Tridimensional Personality Questionnaire was used to assess personality traits in both patients and controls. Several polymorphisms had a weak association with BPD, including rs2550948, rs2652511, and rs2975226 in allele distribution analysis (P < 0.05). Furthermore, the promoter G-A-C-G haplotype (rs6350-rs2975226-rs2652511-rs6413429) was over-represented in the BPD patients compared to the controls (P = 0.007). In personality assessment, the BPDII patients had the highest harm avoidance score, followed by the BPDI patients and controls (P = 3.7 × 10(-32)). In addition, a significant association between rs40184 and harm avoidance was found in the patients with BPD. The DAT1 promoter may be associated with vulnerabilities in BPD. The BPD patients had a higher rate of harm avoidance personality traits than the controls, and DAT1 variants may influence personality traits in patients with BPD.

Biomarkers in the diagnosis of ADHD--promising directions

The etiology and pathogenesis of attention-deficit/hyperactivity disorder (ADHD) are unclear and a more valid diagnosis would certainly be welcomed. Starting from the literature, we built an hypothetical pyramid representing a putative set of biomarkers where, at the top, variants in DAT1 and DRD4 genes are the best candidates for their associations to neuropsychological tasks, activation in specific brain areas, methylphenidate response and gene expression levels. Interesting data come from the noradrenergic system (norepinephrine transporter, norepinephrine, 3-methoxy-4-hydroxyphenylglycol, monoamine oxidase, neuropeptide Y) for their altered peripheral levels, their association with neuropsychological tasks, symptomatology, drugs effect and brain function. Other minor putative genetic biomarkers could be dopamine beta hydroxylase and catechol-O-methyltransferase. In the bottom, we placed endophenotype biomarkers. A more deep integration of "omics" sciences along with more accurate clinical profiles and new high-throughput computational methods will allow us to identify a better list of biomarkers useful for diagnosis and therapies.

The neurobiological link between OCD and ADHD

Obsessive compulsive disorder (OCD) and attention deficit hyperactivity disorder (ADHD) are two of the most common neuropsychiatric diseases in paediatric populations. The high comorbidity of ADHD and OCD with each other, especially of ADHD in paediatric OCD, is well described. OCD and ADHD often follow a chronic course with persistent rates of at least 40–50 %. Family studies showed high heritability in ADHD and OCD, and some genetic findings showed similar variants for both disorders of the same pathogenetic mechanisms, whereas other genetic findings may differentiate between ADHD and OCD. Neuropsychological and neuroimaging studies suggest that partly similar executive functions are affected in both disorders. The deficits in the corresponding brain networks may be responsible for the perseverative, compulsive symptoms in OCD but also for the disinhibited and impulsive symptoms characterizing ADHD. This article reviews the current literature of neuroimaging, neurochemical circuitry, neuropsychological and genetic findings considering similarities as well as differences between OCD and ADHD.

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

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

Psychoendocrine and psychoneuroimmunological mechanisms in the comorbidity of atopic eczema and attention deficit/hyperactivity disorder

Epidemiological data indicate that atopic eczema (AE) in infancy significantly increases the risk for attention deficit/hyperactivity disorder (ADHD) in later life. The underlying pathophysiological mechanisms of this comorbidity are unknown. We propose that the release of inflammatory cytokines caused by the allergic inflammation and/or elevated levels of psychological stress as a result of the chronic disease interfere with the maturation of prefrontal cortex regions and neurotransmitter systems involved ADHD pathology. Alternatively, increased stress levels in ADHD patients may trigger AE via neuroimmunological mechanisms. In a third model, AE and ADHD may be viewed as two separate disorders with one or more shared risk factors (e.g., genetics, prenatal stress) that increase the susceptibility for both disorders leading to the co-occurrence of AE and ADHD. Future investigation of these three models may lead to a better understanding of the mechanisms underlying the observed comorbidity between AE and ADHD and further, to targeted interdisciplinary primary prevention and treatment strategies.

Drug therapy of attention deficit hyperactivity disorder: current trends

Attention deficit hyperactivity disorder is a developmental disorder with an age onset prior to 7 years. Children with ADHD have significantly lower ability to focus and sustain attention and also score higher on impulsivity and hyperactivity. Stimulants, such as methylphenidate, have remained the mainstay of ADHD treatment for decades with evidence supporting their use. However, recent years have seen emergence of newer drugs and drug delivery systems, like osmotic release oral systems and transdermal patches, to mention a few. The use of nonstimulant drugs like atomoxetine and various other drugs, such as α-agonists, and a few antidepressants, being used in an off-label manner, have added to the pharmacotherapy of ADHD. This review discusses current trends in drug therapy of ADHD and highlights the promise pharmacogenomics may hold in the future.

Biochemical and genetic analyses of childhood attention deficit/hyperactivity disorder

Attention deficit/hyperactivity disorder (ADHD) in children is a neurobehavioral disorder characterized by inattention, hyperactivity, and/or impulsivity. The biochemical abnormalities and genetic factors play significant roles in the etiology of ADHD. These symptoms affect the behavior performance and social relationships of children in school and at home. Recently, many studies about biochemical abnormalities in ADHD have been published. Several research groups have also suggested the genetic contribution to ADHD, and attempted to identify susceptibility and candidate genes for this disorder through the genetic linkage and association studies. To date, these studies have reported substantial evidence implicating several genes (dopaminergic: DRD4, DAT1, DRD5, COMT; noradrenergic: DBH, ADRA2A; serotonergic: 5-HTT, HTR1B, HTR2A; cholinergic: CHRNA4, and central nervous system development pathway: SNAP25, BDNF) in the etiology of ADHD. Understanding the biochemistry and genetics of ADHD will allow us to provide a useful addition with other treatment procedures for ADHD.

Dopamine Genes and Sensory Sensitivity as a Temperamental Trait

The regulative theory of temperament defines sensory sensitivity as the capacity to react to sensory stimuli of low stimulating value. Some evidence already exists indicating that dopamine has the potential to modulate different aspects of sensory sensitivity. The present study sought to identify the relationships between several polymorphisms in dopamine genes (DRD2, DRD3, DRD4, DAT1, ANKK1, SNAP-25, and COMT) and sensory sensitivity as a temperamental trait. The method used in this study was family-based. The study was run on 149 biological families with one or two children aged 3–12 (M = 6.83; SD = 1.9). Phenotypic data were obtained using the Temperament Inventory for Children. We found a significant association with rs463379, the single nucleotide polymorphism in intron 4 of dopamine transporter gene (DAT1). Furthermore, we found a significant association with haplotypes in DAT1 and SNAP-25 (the synaptosomal associated protein of 25 kDa) genes. The data collected suggest that variability in dopamine genes may have an impact on the development of sensory sensitivity.

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.

Genetic factors modulating the response to stimulant drugs in humans

Individuals vary in their responses to stimulant drugs, and several lines of evidence suggest that the basis for this variation is at least partially genetic in origin. Association studies have examined the effects of polymorphisms in specific genes on acute and chronic responses to stimulant drugs. Several of these genetic polymorphisms are also associated with other psychiatric dimensions and disorders.This chapter examines the evidence for genetic associations between the genes that have been most carefully examined for their influence on the response to stimulant drugs.

Genome-wide copy number variation analysis in attention-deficit/hyperactivity disorder: association with neuropeptide Y gene dosage in an extended pedigree

Attention-deficit/hyperactivity disorder (ADHD) is a common, highly heritable neurodevelopmental syndrome characterized by hyperactivity, inattention and increased impulsivity. To detect micro-deletions and micro-duplications that may have a role in the pathogenesis of ADHD, we carried out a genome-wide screen for copy number variations (CNVs) in a cohort of 99 children and adolescents with severe ADHD. Using high-resolution array comparative genomic hybridization (aCGH), a total of 17 potentially syndrome-associated CNVs were identified. The aberrations comprise 4 deletions and 13 duplications with approximate sizes ranging from 110 kb to 3 Mb. Two CNVs occurred de novo and nine were inherited from a parent with ADHD, whereas five are transmitted by an unaffected parent. Candidates include genes expressing acetylcholine-metabolizing butyrylcholinesterase (BCHE), contained in a de novo chromosome 3q26.1 deletion, and a brain-specific pleckstrin homology domain-containing protein (PLEKHB1), with an established function in primary sensory neurons, in two siblings carrying a 11q13.4 duplication inherited from their affected mother. Other genes potentially influencing ADHD-related psychopathology and involved in aberrations inherited from affected parents are the genes for the mitochondrial NADH dehydrogenase 1 α subcomplex assembly factor 2 (NDUFAF2), the brain-specific phosphodiesterase 4D isoform 6 (PDE4D6) and the neuronal glucose transporter 3 (SLC2A3). The gene encoding neuropeptide Y (NPY) was included in a ∼3 Mb duplication on chromosome 7p15.2-15.3, and investigation of additional family members showed a nominally significant association of this 7p15 duplication with increased NPY plasma concentrations (empirical family-based association test, P=0.023). Lower activation of the left ventral striatum and left posterior insula during anticipation of large rewards or losses elicited by functional magnetic resonance imaging links gene dose-dependent increases in NPY to reward and emotion processing in duplication carriers. These findings implicate CNVs of behaviour-related genes in the pathogenesis of ADHD and are consistent with the notion that both frequent and rare variants influence the development of this common multifactorial syndrome.

Association between the dopamine transporter gene and the inattentive subtype of attention deficit hyperactivity disorder in Taiwan

Attention deficit hyperactivity disorder (ADHD) is a common heritable childhood psychiatric disorder. Since methylphenidate, one of the main drugs used to treat ADHD, targets the dopamine transporter, this study examined the linkage disequilibrium (LD) structure of the dopamine transporter gene (DAT1) and investigated whether the DAT1 gene was associated with ADHD. This Chinese family-based association sample consisted of 273 DSM-IV diagnosed ADHD probands and their family members (n=906). We screened 15 polymorphisms across the DAT1 gene, including 14 single nucleotide polymorphism (SNP) markers and the variable number of tandem repeat (VNTR) polymorphism in 3'-untranslated region (3'UTR). Calculations of pairwise LD revealed three main haplotype blocks (HBs): HB1 (intron 2 through intron 6), HB2 (intron 8 through intron 11), and HB3 (3'UTR). Family-Based Association Tests showed that no allele was significantly more transmitted than expected to the ADHD children for these 15 markers. Haplotype-Based Association Tests showed that a haplotype rs27048 (C)/rs429699 (T) was significantly associated with the inattentive subtype (P=0.008). In quantitative analyses, this haplotype also demonstrated significant association with the inattention severity (P=0.012). Our finding of the haplotype rs27048 (C)/rs429699 (T) as a novel genetic marker in the inattentive ADHD subtype suggests that variation in the DAT1 gene may primarily affect the inattentive subtype of ADHD.

Monoamine transporters: vulnerable and vital doorkeepers

Transporters of dopamine, serotonin, and norepinephrine have been empirically used as medication targets for several mental illnesses in the last decades. These protein-targeted medications are effective only for subpopulations of patients with transporter-related brain disorders. Since the cDNA clonings in early 1990s, molecular studies of these transporters have revealed a wealth of information about the transporters' structure-activity relationship (SAR), neuropharmacology, cell biology, biochemistry, pharmacogenetics, and the diseases related to the human genes encoding these transporters among related regulators. Such new information creates a unique opportunity to develop transporter-specific medications based on SAR, mRNA, DNA, and perhaps transporter trafficking regulation for a number of highly relevant diseases including substance abuse, depression, schizophrenia, and Parkinson's disease.

Toward a better understanding of ADHD: LPHN3 gene variants and the susceptibility to develop ADHD

During the past 15 years, an impressive amount of genetic information has become available in the research field of psychiatry, particularly as it relates to attention-deficit/hyperactivity disorder (ADHD). However, the classical clinical approach to ADHD has minimally affected and not significantly been improved by this genetic revolution. It is difficult to predict how long it will take for genetic findings to alter the way clinicians treat patients with ADHD. New medications or treatment protocols may take years to become routine clinical practice. However, when taken together, recent successes in genomics, pharmacogenomics, and genetic epidemiology have the potential (1) to prevent comorbid consequences of ADHD, (2) to individualize therapies for patients with ADHD, and (3) to define new epidemiological policies to aid with the impact of ADHD on society. Here, we present an overview of how genetic research may affect and improve the quality of life of patients with ADHD: as an example, we use the discovery of LPHN3, a new gene in which variants have recently been shown to be associated with ADHD.

The role of DAT1 gene on the rapid detection of task novelty

In an environment with a myriad of different stimuli, the fast detection of novel and behaviorally relevant signals becomes crucial for an adaptive behavior. The detection of task-novelty has been related to striatum-prefrontal cortex (PFC) pathways involving dopaminergic (DA) neurotransmission. Here we thus tested the hypothesis that DA regulates the detection of task novelty through the modulation of the auditory N1 potential, an auditory potential peaking at 100 ms and previously shown to be modulated by the detection of sensory novelty. Thirty-five healthy volunteers were divided in two groups according to the presence or absence of the 9-repetition allele (9R) of the SLC6A3/DAT1 gene for the dopamine transporter. Participants performed a cued task-switching paradigm that dissociated the effects of exogenous sensory novelty from those of endogenous task novelty. Individuals with the 9R allele showed an amplitude enhancement of the auditory N1 elicited to sensory changes requiring a task-set reconfiguration as compared to sensory changes with no task novelty. In contrast, individuals without the 9R allele did not have their N1 waveform modulated by task novelty. The present results suggest that individuals homozygous for the 10-repeat allele fail to detect the behavioral relevance of new stimuli at early stages.

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.

Does the difference between physically active and couch potato lie in the dopamine system?

Obesity and other inactivity related diseases are increasing at an alarming rate especially in Western societies. Because of this, it is important to understand the regulating mechanisms involved in physical activity behavior. Much research has been done in regard to the psychological determinants of physical activity behavior; however, little is known about the underlying genetic and biological factors that may contribute to regulation of this complex trait. It is true that a significant portion of any trait is regulated by genetic and biological factors. In the case of voluntary physical activity behavior, these regulating mechanisms appear to be concentrated in the central nervous system. In particular, the dopamine system has been shown to regulate motor movement, as well as motivation and reward behavior. The pattern of regulation of voluntary physical activity by the dopamine system is yet to be fully elucidated. This review will summarize what is known about the dopamine system and regulation of physical activity, and will present a hypothesis of how this signaling pathway is mechanistically involved in regulating voluntary physical activity behavior. Future research in this area will aid in developing personalized strategies to prevent inactivity related diseases.

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.

Gender differences in genetic and environmental influences on gambling: results from a sample of twins from the National Longitudinal Study of Adolescent Health

AIMS

To examine the extent to which genetic factors and shared and non-shared environmental factors are implicated in the development of gambling behaviors and to examine whether there are gender differences in the genetic and environmental contributors to gambling behaviors.

DESIGN

A genetically informative analysis was performed by using DeFries-Fulker (DF) analysis.

SETTING

Analysis of secondary data drawn from the National Longitudinal Study of Adolescent Health (Add Health).

PARTICIPANTS

A total of 324 monozygotic (MZ) twins and 278 same-sex dizygotic (DZ) twins were included in the analysis. Of these twins, there were 150 male MZ twins, 144 male DZ twins, 174 female MZ twins and 134 female DZ twins.

MEASUREMENTS

Gambling behavior was measured through eight self-reported questions that tapped a range of items designed to measure problems related to gambling. Self-reported measures of self-control and delinquent involvement were also included to examine the degree to which these factors covaried with gambling behavior.

FINDINGS

The results of the DF analysis indicated that when male and female twin pairs were analyzed simultaneously, genetic factors explained approximately 70% of the variance in gambling and non-shared environmental factors explained the remaining variance. When gender-specific models were calculated, substantial gender differences emerged. For males, genetic factors explained approximately 85% of the variance in gambling, with the non-shared environment accounting for the remaining variance. For females, genetic factors explained none of the variance in gambling behaviors, while the shared environment explained 45% of the variance and the non-shared environment explained 55% of the variance.

CONCLUSIONS

Analysis of twins from the Add Health data suggests that there are significant gender differences in the genetic and environmental underpinnings to gambling behaviors.

Progress and promise of attention-deficit hyperactivity disorder pharmacogenetics

One strategy for understanding variability in attention-deficit hyperactivity disorder (ADHD) medication response, and therefore redressing the current trial-and-error approach to ADHD medication management, is to identify genetic moderators of treatment. This article summarizes ADHD pharmacogenetic investigative efforts to date, which have primarily focused on short-term response to methylphenidate and largely been limited by modest sample sizes. The most well studied genes include the dopamine transporter and dopamine D(4) receptor, with additional genes that have been significantly associated with stimulant medication response including the adrenergic alpha(2A)-receptor, catechol-O-methyltransferase, D(5) receptor, noradrenaline (norepinephrine) transporter protein 1 and synaptosomal-associated protein 25 kDa. Unfortunately, results of current ADHD pharmacogenetic studies have not been entirely consistent, possibly due to differences in study design, medication dosing regimens and outcome measures. Future directions for ADHD pharmacogenetics investigations may include examination of drug-metabolizing enzymes and a wider range of stimulant and non-stimulant medications. In addition, researchers are increasingly interested in going beyond the individual candidate gene approach to investigate gene-gene interactions or pathways, effect modification by additional environmental exposures and whole genome approaches. Advancements in ADHD pharmacogenetics will be facilitated by multi-site collaborations to obtain larger sample sizes using standardized protocols. Although ADHD pharmacogenetic efforts are still in a relatively early stage, their potential clinical applications may include the development of treatment efficacy and adverse effect prediction algorithms that incorporate the interplay of genetic and environmental factors, as well as the development of novel ADHD treatments.

Polymorphisms in dopamine transporter (SLC6A3) are associated with stimulant effects of D-amphetamine: an exploratory pharmacogenetic study using healthy volunteers

Individuals vary in their subjective responses to stimulant drugs, and these differences are believed to be partially genetic in origin. We evaluated associations between mood, cognitive and cardiovascular responses to d-amphetamine and four polymorphisms in the dopamine transporter (SLC6A3): rs460000, rs3756450, rs37022 and rs6869645. Healthy Caucasian male and female volunteers (N = 152) participated in a double-blind, crossover design study in which they received placebo, 10 and 20 mg of d-amphetamine. We measured self-reported rating of mood, performance on the Digit Symbol Substitution Task, blood pressure and heart rate. Individuals with the C/C genotype at rs460000 (N = 83) reported approximately twofold higher ratings of stimulation and euphoria relative to the A/A+A/C (N = 69) genotype group, at both the 10 and 20 mg doses. No other responses or SNPs showed significant effects. rs460000 is in perfect LD with rs463379 (CEU: D' = 1; r (2) = 1), which was not studied here, but has been associated with etiology of Attention Deficit Hyperactivity Disorder (ADHD). These findings suggest a pleiotropic effect of this polymorphic locus on both ADHD and sensitivity to the subjective effects of amphetamine.

Familiality and molecular genetics of attention networks in ADHD

Indices from a more elementary neuropsychological level might be useful in the search for genes for complex psychiatric disorders, such as ADHD. In this study we investigated systematically whether attentional performance as measured with the Attention Network Test (ANT) is suited for the identification of endophenotypes of ADHD. Attentional performance in affected sib pairs with ADHD (n = 181) was compared to unaffected control siblings (n = 121). Intrafamilial correlation patterns were calculated. In addition, linkage and association analyses were conducted between quantitative scores of attentional functions and dopamine receptor D4 (DRD4) and dopamine transporter (DAT1 or SLC6A3) gene variants. Only the executive attention network was significantly impaired in subjects with ADHD compared to controls (P < 0.05) and showed evidence for familiality in both affected and unaffected families. Linkage analyses revealed the highest LOD score for a severity score based on DSM-IV inattentive symptoms in the DAT1 chromosomal region (LOD score 2.6 at 15 cM). However, a SNP (rs6350) at the DAT1 locus showed a tendency for association with both alerting performance (P = 0.02) and executive attention (P = 0.01) although it did not survive alpha adjustment for multiple testing. No evidence was found for association of any of the investigated phenotypes with the VNTR in the DRD4. Thus, our data suggest that the quantitative behavioral ratings of inattentive symptoms might be more useful when searching for new genes associated with ADHD, however, among the ANT measures the executive attention network seems to be best suited for further endophenotype analyses.

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.

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

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

Exploring the genetic link between RLS and ADHD

Attention deficit/hyperactivity disorder (ADHD) is a highly heritable neurodevelopmental disorder of childhood onset. Clinical and biological evidence points to shared common central nervous system (CNS) pathology of ADHD and restless legs syndrome (RLS). It was hypothesized that variants previously found to be associated with RLS in two large genome-wide association studies (GWA), will also be associated with ADHD. SNPs located in MEIS1 (rs2300478), BTBD9 (rs9296249, rs3923809, rs6923737), and MAP2K5 (rs12593813, rs4489954) as well as three SNPs tagging the identified haplotype in MEIS1 (rs6710341, rs12469063, rs4544423) were genotyped in a well characterized German sample of 224 families comprising one or more affected sibs (386 children) and both parents. We found no evidence for preferential transmission of the hypothesized variants to ADHD. Subsequent analyses elicited nominal significant association with haplotypes consisting of the three SNPs in BTBD9 (chi2 = 14.8, df = 7, nominal p = 0.039). According to exploratory post hoc analyses, the major contribution to this finding came from the A-A-A-haplotype with a haplotype-wise nominal p-value of 0.009. However, this result did not withstand correction for multiple testing. In view of our results, RLS risk alleles may have a lower effect on ADHD than on RLS or may not be involved in ADHD. The negative findings may additionally result from genetic heterogeneity of ADHD, i.e. risk alleles for RLS may only be relevant for certain subtypes of ADHD. Genes relevant to RLS remain interesting candidates for ADHD; particularly BTBD9 needs further study, as it has been related to iron storage, a potential pathophysiological link between RLS and certain subtypes of ADHD.

Candidate gene analysis in an on-going genome-wide association study of attention-deficit hyperactivity disorder: suggestive association signals in ADRA1A

OBJECTIVES

Attention-deficit hyperactivity disorder (ADHD) is a highly heritable, common developmental disorder. Although a few confirmed associations have emerged from candidate gene studies, these have shown the same limitations that have become evident in the study of other complex diseases, often with inconsistent and nonreplicated results across different studies.

METHODS

In this report, 27 ADHD candidate genes were explored in greater depth using high-density tag single nucleotide polymorphism (SNP) genotyping. Association with 557 SNPs was tested using the transmission disequilibrium test in 270 nuclear pedigrees selected from an ongoing ADHD genetic study that includes all disease subtypes.

RESULTS

SNPs in seven genes including SLC1A3, SLC6A3, HTR4, ADRA1A, HTR2A, SNAP25, and COMT showed a nominal level of association with ADHD (P values <0.05), but none remained significant after a stringent correction for the total number of tests performed.

CONCLUSION

The strongest signal emerged from SNPs in the promoter region (rs3808585) and in an intron (rs17426222, rs4732682, rs573514) of ADRA1A, all located within the same haplotype block. Some of the SNPs in HTR2A and COMT have already been reported by others, whereas other SNPs will need confirmation in independent samples.

The effect of methylphenidate on Internet video game play in children with attention-deficit/hyperactivity disorder

OBJECTIVE

A number of studies about attention-deficit/hyperactivity disorder (ADHD) and Internet video game play have examined the prefrontal cortex and dopaminergic system. Stimulants such as methylphenidate (MPH), given to treat ADHD, and video game play have been found to increase synaptic dopamine. We hypothesized that MPH treatment would reduce Internet use in subjects with co-occurring ADHD and Internet video game addictions.

METHODS

Sixty-two children (52 males and 10 females), drug-naive, diagnosed with ADHD, and Internet video game players, participated in this study. At the beginning of the study and after 8 weeks of treatment with Concerta (OROS methylphenidate HCl, Seoul, Korea), participants were assessed with Young's Internet Addiction Scale, Korean version (YIAS-K), Korean DuPaul's ADHD Rating Scale, and the Visual Continuous Performance Test. Their Internet usage time was also recorded.

RESULTS

After 8 weeks of treatment, the YIAS-K scores and Internet usage times were significantly reduced. The changes in the YIAS-K scores between the baseline and 8-week assessments were positively correlated with the changes in total and inattention scores from the Korean DuPaul's ADHD Rating Scale, as well as omission errors from the Visual Continuous Performance Test. There was also a significant difference in the number of omission errors among non-Internet-addicted, mildly Internet addicted, and severely Internet addicted participants.

DISCUSSION

We suggest that Internet video game playing might be a means of self-medication for children with ADHD. In addition, we cautiously suggest that MPH might be evaluated as a potential treatment of Internet addiction.

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.

Chromosome 5 and Gilles de la Tourette syndrome: Linkage in a large pedigree and association study of six candidates in the region

Gilles de la Tourette Syndrome (TS) is a neuropsychiatric disorder characterized by both motor and vocal tics. In our previous genome scan for TS we identified evidence for linkage to the centromeric region of chromosome 5 in a single large family of 32 individuals with 10 family members with TS or chronic multiple tics (CMT). In this paper we report further analyses of the 5p-centromeric region in this pedigree. An additional 11 family members were identified and screened for TS. Using a set of 14 microsatellite markers we refined the linked region to a approximately 28 Mb interval between the markers D5S1506 and D5S76. A set of six candidate genes located in this region were selected to be tested for genetic association with TS. These genes were GDNF, ITGA1, ISL1, FGF10, HCN1 and SLC1A3. The TDT statistic was used for the association tests in a sample of 171 independent nuclear families with 241 affected children with TS. We found no evidence for an association between TS and markers in these genes in this sample of families. This study represents the first efforts to narrow the linkage region in the extended pedigree and the first tests of candidate genes in the chromosome 5 region linked to TS.

Atypical antipsychotics clozapine and quetiapine attenuate prepulse inhibition deficits in dopamine transporter knockout mice

Sensorimotor gating disruptions are seen in various psychiatric illnesses with putatively different pathologies, including schizophrenia and bipolar disorder. Interestingly, mice lacking the dopamine (DA) transporter (DAT) gene display markedly increased levels of DA, deficits in sensorimotor gating, and hyperactivity relative to wild-type mice. Atypical antipsychotics are effective treatments of schizophrenia and manic symptoms, presumably in part by antagonizing DA receptors. Here we report that treatment with clozapine (3 mg/kg) or quetiapine (2.5 mg/kg) attenuated prepulse inhibition deficits in male DAT knockout mice. Thus male DAT knockout mice may provide a useful animal model for predicting the efficacy of novel drugs in treating psychiatric illnesses characterized by a dysregulated DA system.

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.

Meta-analysis of genome-wide linkage scans of attention deficit hyperactivity disorder

Genetic contribution to the development of attention deficit hyperactivity disorder (ADHD) is well established. Seven independent genome-wide linkage scans have been performed to map loci that increase the risk for ADHD. Although significant linkage signals were identified in some of the studies, there has been limited replications between the various independent datasets. The current study gathered the results from all seven of the ADHD linkage scans and performed a Genome Scan Meta Analysis (GSMA) to identify the genomic region with most consistent linkage evidence across the studies. Genome-wide significant linkage (P(SR) = 0.00034, P(OR) = 0.04) was identified on chromosome 16 between 64 and 83 Mb. In addition there are nine other genomic regions from the GSMA showing nominal or suggestive evidence of linkage. All these linkage results may be informative and focus the search for novel ADHD susceptibility genes.