Further evidence from haplotype analysis for linkage of the dopamine D4 receptor gene and attention-deficit hyperactivity disorder

ADHD co-morbidities: A review of implication of gene × environment effects with dopamine-related genes

ADHD is a major burden in adulthood, where co-morbid conditions such as depression, substance use disorder and obesity often dominate the clinical picture. ADHD has substantial shared heritability with other mental disorders, contributing to comorbidity. However, environmental risk factors exist but their interaction with genetic makeup, especially in relation to comorbid disorders, remains elusive. This review for the first time summarizes present knowledge on gene x environment (GxE) interactions regarding the dopamine system. Hitherto, mainly candidate (GxE) studies were performed, focusing on the genes DRD4, DAT1 and MAOA. Some evidence suggest that the variable number tandem repeats in DRD4 and MAOA may mediate GxE interactions in ADHD generally, and comorbid conditions specifically. Nevertheless, even for these genes, common variants are bound to suggest risk only in the context of gender and specific environments. For other polymorphisms, evidence is contradictory and less convincing. Particularly lacking are longitudinal studies testing the interaction of well-defined environmental with polygenic risk scores reflecting the dopamine system in its entirety.

A neurocomputational account of reward and novelty processing and effects of psychostimulants in attention deficit hyperactivity disorder

Computational models of reinforcement learning have helped dissect discrete components of reward-related function and characterize neurocognitive deficits in psychiatric illnesses. Stimulus novelty biases decision-making, even when unrelated to choice outcome, acting as if possessing intrinsic reward value to guide decisions toward uncertain options. Heightened novelty seeking is characteristic of attention deficit hyperactivity disorder, yet how this influences reward-related decision-making is computationally encoded, or is altered by stimulant medication, is currently uncertain. Here we used an established reinforcement-learning task to model effects of novelty on reward-related behaviour during functional MRI in 30 adults with attention deficit hyperactivity disorder and 30 age-, sex- and IQ-matched control subjects. Each participant was tested on two separate occasions, once ON and once OFF stimulant medication. OFF medication, patients with attention deficit hyperactivity disorder showed significantly impaired task performance (P = 0.027), and greater selection of novel options (P = 0.004). Moreover, persistence in selecting novel options predicted impaired task performance (P = 0.025). These behavioural deficits were accompanied by a significantly lower learning rate (P = 0.011) and heightened novelty signalling within the substantia nigra/ventral tegmental area (family-wise error corrected P < 0.05). Compared to effects in controls, stimulant medication improved attention deficit hyperactivity disorder participants' overall task performance (P = 0.011), increased reward-learning rates (P = 0.046) and enhanced their ability to differentiate optimal from non-optimal novel choices (P = 0.032). It also reduced substantia nigra/ventral tegmental area responses to novelty. Preliminary cross-sectional evidence additionally suggested an association between long-term stimulant treatment and a reduction in the rewarding value of novelty. These data suggest that aberrant substantia nigra/ventral tegmental area novelty processing plays an important role in the suboptimal reward-related decision-making characteristic of attention deficit hyperactivity disorder. Compared to effects in controls, abnormalities in novelty processing and reward-related learning were improved by stimulant medication, suggesting that they may be disorder-specific targets for the pharmacological management of attention deficit hyperactivity disorder symptoms.

Amphetamine improves mouse and human attention in the 5-choice continuous performance test

Non-medical use of prescription stimulants amongst college students is common, with claims of cognitive and academic benefits. The mechanism, magnitude, and pervasiveness of the cognitive enhancing effects of stimulants in healthy adults remain poorly understood however. The present study determined the effects of dextroamphetamine (D-amp) on the 5-choice continuous performance test (5C-CPT) of attention in healthy young adult humans and mice. A mixed gender sample received placebo (n = 29), 10 (n = 17) or 20 mg D-amp (n = 25) in a double-blind fashion before 5C-CPT testing. In addition, male C57BL/6J mice were trained on a touchscreen adaptation of the 5C-CPT and tested after receiving saline or D-amp (0.1, 0.3, 1.0 mg/kg; n = 8/dose). In humans, D-amp significantly improved 5C-CPT performance. Both doses improved signal detection driven by increased hit rate (reduced omissions). Both doses also improved response accuracy and reduced hit reaction time (HRT) variability. In mice, similar effects (improved signal detection, hit rate, and response accuracy) were observed at the moderate dose (0.3 mg/kg). In contrast to human participants however, no effect on HRT variability was detected in mice, with no effect on HRT in either species. Human 5C-CPT performance was consistent with prior studies and consistent with alternative CPT paradigms. The performance of C57BL/6J mice on the touchscreen 5C-CPT mirrored performance of this strain on 5-hole operant chambers. Importantly, comparable facilitation of attention with D-amp was observed in both species. The 5C-CPT provides a cross-species paradigm by which the cognitive enhancing properties of stimulants and the neural underpinnings of attention can be assessed.

The 5 choice continuous performance test (5C-CPT): A novel tool to assess cognitive control across species

BACKGROUND

Neurodevelopmental disorders including Tourette's syndrome (TS) and attention deficit hyperactivity disorder (ADHD) are characterized by significant impairment in attention and cognitive control. These cognitive deficits persist throughout development, contribute significantly to socio-occupational impairment, and are relatively impervious to available treatment. A critical challenge in pro-cognitive drug discovery is translatability of findings across species, underscoring the need for developing valid and reliable cross-species cognitive tasks.

NEW METHOD

Here we describe a cross-species 5 choice continuous performance task that was developed to measure cognitive control processes of attention, vigilance, and response inhibition, enabling the translation of findings for pro-cognitive drug discovery across species and delineate neural mechanisms underlying cognitive control construct.

RESULTS

Construct validity of 5C-CPT has been verified by multiple cross-species studies. Several lines of evidence report consistent findings across species including, deficits resulting from 36-h sleep deprivation studies, engagement of parietal cortex in human brain imaging and rodent lesion studies, and vigilance decrements over time.

COMPARISON WITH EXISTING METHOD

Unlike the widely used rodent 5 choice serial reaction time task (5CSRTT) and the sustained attention task (SAT), the rodent 5C-CPT includes both target and non-target stimuli that allow measuring of cognitive control elements including response inhibition, an ability to inhibit pre-potent response during non-target trials, detect vigilance decrement and calculate signal detection parameters in rodents analogous to human CPT.

CONCLUSION

The cross-species 5C-CPT is a robust translational tool to characterize the neurobiological substrates underlying cognitive control deficits in clinical population including, ADHD and TS and develop targeted pro-cognitive therapeutics.

Interactions Between Early Trauma and Catechol-O-Methyltransferase Genes on Inhibitory Deficits in Children With ADHD

OBJECTIVE

To investigate the interaction between childhood trauma exposure with the catechol-O-methyltransferase (COMT) and dopamine receptor D4 (DRD4) polymorphisms in relation to neuropsychological measures in children with ADHD.

METHOD

A cross-sectional examination of early traumatic experiences and the continuous performance test (CPT) were performed in 55 children with ADHD. Participants were also genotyped for the DRD4 exon III 48-bp variable number of tandem repeats (VNTR) polymorphism and the COMT Val¹⁵⁸-Met (rs4680) polymorphism.

RESULTS

There was significant interaction between the effects of the COMT genotype and trauma in commission errors. In participants with ADHD carrying the COMT Val/Val genotype, the group exposed to trauma showed significantly higher commission errors than the non-traumatized group. However, for the participants with other genotypes, no significant differences were found.

CONCLUSION

This study suggests that there exists a genetic influence on the association between childhood trauma and the severity of inhibitory deficits in children with ADHD.

Dopamine D4 Receptor Gene Polymorphism in a Sample of Egyptian Children With Attention-Deficit Hyperactivity Disorder (ADHD)

This study aimed to detect DRD4 receptor gene polymorphisms in attention-deficit hyperactivity disorder (ADHD) children and to correlate their phenotype-genotype. Fifty children with ADHD were diagnosed by Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, criteria and were subjected to Conners Parent Rating Scale. All cases and controls were subjected to history taking, physical examination, IQ assessment, and dopamine receptor D4 (DRD4) exon 3 genotyping. The 7-repeat allele was present only in controls, whereas 2-repeat allele was present in the ADHD children (heterozygous 2-repeat allele in 16% and homozygous in 26% of cases). Eight percent of cases had homozygous 4-repeat allele vs 28% of controls, whereas 10% of cases had heterozygous 4-repeat allele vs 6% of controls, with its predominance in controls. The 2-repeat and 4-repeat alleles have been associated with more inattention, hyperactivity, and impulsivity phenotypes. In conclusion, children with ADHD had a significant presence of the 2-repeat allele and absence of the 7-repeat allele.

Dopamine, cognitive function, and gamma oscillations: role of D4 receptors

Cognitive deficits in individuals with schizophrenia (SCZ) are considered core symptoms of this disorder, and can manifest at the prodromal stage. Antipsychotics ameliorate positive symptoms but only modestly improve cognitive symptoms. The lack of treatments that improve cognitive abilities currently represents a major obstacle in developing more effective therapeutic strategies for this debilitating disorder. While D4 receptor (D4R)-specific antagonists are ineffective in the treatment of positive symptoms, animal studies suggest that D4R drugs can improve cognitive deficits. Moreover, recent work from our group suggests that D4Rs synergize with the neuregulin/ErbB4 signaling pathway, genetically identified as risk factors for SCZ, in parvalbumin (PV)-expressing interneurons to modulate gamma oscillations. These high-frequency network oscillations correlate with attention and increase during cognitive tasks in healthy subjects, and this correlation is attenuated in affected individuals. This finding, along with other observations indicating impaired GABAergic function, has led to the idea that abnormal neural activity in the prefrontal cortex (PFC) in individuals with SCZ reflects a perturbation in the balance of excitation and inhibition. Here we review the current state of knowledge of D4R functions in the PFC and hippocampus, two major brain areas implicated in SCZ. Special emphasis is given to studies focusing on the potential role of D4Rs in modulating GABAergic transmission and to an emerging concept of a close synergistic relationship between dopamine/D4R and neuregulin/ErbB4 signaling pathways that tunes the activity of PV interneurons to regulate gamma frequency network oscillations and potentially cognitive processes.

DRD4 and TH gene polymorphisms are associated with activity, impulsivity and inattention in Siberian Husky dogs

Both dopamine receptor D4 (DRD4) exon 3 and tyrosine hydroxylase (TH) intron 4 repeat polymorphisms have been linked to activity and impulsivity in German Shepherd dogs (GSDs). However, the results in GSDs may not be generalisable to other breeds, as allelic frequencies vary markedly among breeds. We selected the Siberian Husky for further study, because it is highly divergent from most dog breeds, including the GSD. The study sample consisted of 145 racing Siberian Huskies from Europe and North America. We found that this breed possesses seven DRD4 length variants, two to five more variants than found in other breeds. Among them was the longest known allele, previously described only in wolves. Short alleles of the DRD4 and TH repeat polymorphisms were associated with higher levels of activity, impulsivity and inattention. Siberian Huskies possessing at least one short allele of the DRD4 polymorphism displayed greater activity in a behavioural test battery than did those with two long alleles. However, the behavioural test was brief and may not have registered variation in behaviour across time and situations. Owners also completed the Dog-Attention Deficit Hyperactivity Disorder Rating Scale (Dog-ADHD RS), a more general measure of activity and attention. Siberian Huskies from Europe with two short alleles of the TH polymorphism received higher ratings of inattention on the Dog-ADHD RS than did those with the long allele. Investigation of the joint effect of DRD4 and TH showed that dogs possessing long alleles at both sites were scored as less active-impulsive than were others. Our results are aligned with previous studies showing that DRD4 and TH polymorphisms are associated with activity-impulsivity related traits in dogs. However, the prevalence of variants of these genes differs across breeds, and the functional role of specific variants is unclear.

No Association between the Response to Methylphenidate and DRD4 Gene Polymorphism in Korean Attention Deficit Hyperactivity Disorder: A Case Control Study

Objective

Recently the relationship between alleles frequency distribution, drug response and the attention deficit hyperactivity disorder (ADHD), has been actively researched. We investigated the association between the genetic type, alleles and drug response for the dopamine receptor D4 (DRD4) gene in ADHD patients in Korea.

Methods

One hundred fourteen patients diagnosed with ADHD according to the the Diagnostic and Statistical Manual of Mental Disorders version IV (DSM-IV) diagnostic criteria were selected for the study. The clinical features of patients were confirmed by Korean version of Conners' parent rating scale, Attention deficit Diagnostic System, Korean version of Spielberger state-trait anxiety scale. Blood samples were taken from the 198 subjects. DNA was extracted from blood lymphocytes, PCR was performed for DRD4 Polymorphism. Alleles, genotype frequencies, the Clinical Global Impression (CGI) improvement score were compared using the chi-square test. Korean ADHD Rating Scale (K-ARS) and CGI severity scores were compared using the t-test.

Results

In comparing the ADHD with 4/4 repeats group and without the ADHD with 4/4 repeats group, no significant difference was seen between the DRD4 genetic type, alleles distribution, and CGI drug response.

Conclusion

As a result, it is viewed that there is no relationship between ADHD and DRD4, but final decision is indefinite. Follow up studies with larger patient or pure subgroups are expected.

Children’s Perceptions of Their ADHD Symptoms

This study investigated the perceptions of children with Attention-Deficit/Hyperactivity Disorder (ADHD) of their ADHD symptoms in terms of the positive illusory bias (PIB), their attributions for their problem behaviors, and their beliefs about whether their problem behaviors and disorder are stigmatizing. Participants were 152 9- to 14-year-old children (86 ADHD). Children with ADHD demonstrated a PIB for their ADHD symptoms; the discrepancy between their self-reports and parent reports was greater than for children without ADHD. They also viewed their self-identified most problematic behavior as more uncontrollable and stigmatizing (i.e., bothersome to others, leading to differential treatment, and embarrassing) than comparison children. Their perception that their problem behaviors and their disorder were stigmatizing was negatively associated with behavioral self-concept and self-esteem. These results suggest that school psychologists should interpret the self-reports of problem behaviors of children with ADHD with caution while being aware of the vulnerability of these children to feelings of stigma.

Role of dopamine receptors in ADHD: a systematic meta-analysis

The dopaminergic system plays a pivotal role in the central nervous system via its five diverse receptors (D1-D5). Dysfunction of dopaminergic system is implicated in many neuropsychological diseases, including attention deficit hyperactivity disorder (ADHD), a common mental disorder that prevalent in childhood. Understanding the relationship of five different dopamine (DA) receptors with ADHD will help us to elucidate different roles of these receptors and to develop therapeutic approaches of ADHD. This review summarized the ongoing research of DA receptor genes in ADHD pathogenesis and gathered the past published data with meta-analysis and revealed the high risk of DRD5, DRD2, and DRD4 polymorphisms in ADHD.

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

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

The role of dopamine in bipolar disorder

OBJECTIVE

Despite effective pharmacological treatments for bipolar disorder, we still lack a comprehensive pathophysiological model of the illness. Recent neurobiological research has implicated a number of key brain regions and neuronal components in the behavioural and cognitive manifestations of bipolar disorder. Dopamine has previously been investigated in some depth in bipolar disorder, but of late has not been a primary focus of attention. This article examines the role of dopamine in bipolar disorder, incorporating recent advances into established models where possible.

METHODS

A critical evaluation of the literature was undertaken, including a review of behavioural, neurochemical, receptor, and imaging studies, as well as genetic studies focusing on dopamine receptors and related metabolic pathways. In addition, pharmacologic manipulation of the central dopaminergic pathways and comparisons with other disease states such as schizophrenia were considered, principally as a means of exploring the hypothesised models.

RESULTS

Multiple lines of evidence, including data from pharmacological interventions and structural and functional magnetic resonance imaging studies, suggest that the dopaminergic system may play a central role in bipolar disorder.

CONCLUSION

Future research into the pathophysiological mechanisms of bipolar disorder and the development of new treatments for bipolar disorder should focus on the dopaminergic system.

Phthalates exposure and attention-deficit/hyperactivity disorder in school-age children

BACKGROUND

Very few studies have examined the association between attention-deficit/hyperactivity disorder (ADHD) and phthalate exposure in humans. The aim of this study was to investigate the impact of phthalates on symptoms of ADHD in school-age children.

METHODS

A cross-sectional examination of urine phthalate concentrations was performed, and scores on measures of ADHD symptoms and neuropsychological dysfunction with regard to attention and impulsivity were obtained from 261 Korean children, age 8-11 years.

RESULTS

Mono-2-ethylheyl phthalate (MEHP) and mono-2-ethyl-5-oxohexylphthalate (MEOP) for metabolites of Di-2-ethylhexylphthalate (DEHP) and mono-n-butyl phthalate (MNBP) for metabolites of dibutyl phthalate (DBP) were measured in urine samples. The mean concentrations of MEHP, MEOP, and MNBP were 34.0 microg/dL (SD = 36.3; range: 2.1-386.7), 23.4 microg/dL (SD = 23.0; range: .75-244.8), and 46.7 microg/L (SD = 21.4; range: 13.2-159.3), respectively. After adjustment for covariates, teacher-rated ADHD scores were significantly associated with DEHP metabolites but not with DBP metabolites. We also found significant relationships between the urine concentrations of metabolites for DBP and the number of omission and commission errors in continuous performance tests (CPT) after adjustment for covariates.

CONCLUSION

The present study showed a strong positive association between phthalate metabolites in urine and symptoms of ADHD among school-age children.

D2 dopamine receptor subtype-mediated hyperactivity and amphetamine responses in a model of ADHD

Low doses of psychostimulants produce beneficial behavioral effects in ADHD patients but the mechanisms underlying the response are not understood. Here we use the hyperactive mouse mutant coloboma to identify D2-like dopamine receptor subtypes that mediate the hyperactivity and response to amphetamine; we have previously demonstrated that D1-like dopamine receptors are not involved. Targeted deletion of the D2, but not the D3 or the D4, dopamine receptor in coloboma mice eliminated the hyperactivity; depleting D2 dopamine receptors also restored the excess dopamine overflow that may drive the hyperactivity to normal concentrations. Similar to its effects on ADHD patients, amphetamine reduced the hyperactivity of coloboma mice. The D2 dopamine receptor-selective antagonist L-741,626, but not D3 or D4 dopamine receptor-selective antagonists, blocked the amphetamine-induced reduction in locomotor activity. Thus, the D2 dopamine receptor subtype mediates both the hyperactivity and response to amphetamine, suggesting a specific target for novel therapeutics in ADHD.

Association between Dopamine D4 Receptor Gene Polymorphism and Scores on a Continuous Performance Test in Korean Children with Attention Deficit Hyperactivity Disorder

OBJECTIVE

The aim of this study was to evaluate the association between a variable number of tandem repeats polymorphism at the dopamine D4 receptor gene (DRD4) and the performance of children with attention deficit hyperactivity disorder (ADHD) in a continuous performance test (CPT).

METHODS

This study included 72 ADHD children (mean age=9.39+/-2.05 years) who were recruited from one child psychiatric clinic. The omission errors, commission errors, reaction time and reaction standardization in the CPT were computed. The number of 48-base pairs tandem repeats in the exon III of DRD4 was analyzed in a blind manner.

RESULTS

The homozygosity of the 4-repeat allele at DRD4 was significantly associated with fewer commission errors (t=2.364, df=28.685, p=0.025) and standard deviation of reaction time (t=2.351, df=24.648, p=0.027) even after adjusting for age. The results of analyses of CPT measured values among three groups showed that the group with higher frequency of the 4-repeat allele showed a lower mean score of commission errors (F=4.268, df=2, p=0.018).

CONCLUSION

These results suggest a protective role of 4-repeat allele of the DRD4 polymorphisms on commission errors in the CPT in children with ADHD.

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.

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.

Electrophysiological evidence of atypical motivation and reward processing in children with attention-deficit hyperactivity disorder

Behavioral and neurophysiological evidence suggest that attention-deficit hyperactivity disorder (ADHD) is characterized by the impact of abnormal reward prediction error signals carried by the midbrain dopamine system on frontal brain areas that implement cognitive control. To investigate this issue, we recorded the event-related brain potential (ERP) from typical children and children with ADHD as they navigated a "virtual maze" to find monetary rewards, and physically gave them their accumulated rewards halfway through the task and at the end of the experiment. We found that the amplitude of a reward-related ERP component decreased somewhat for typical children after they received their first payment, but increased for children with ADHD following the payment. This result indicates that children with ADHD are unusually sensitive to the salience of reward and suggests that such sensitivity may be mediated in part by the midbrain dopamine system.

Association of 4-repeat allele of the dopamine D4 receptor gene exon III polymorphism and response to methylphenidate treatment in Korean ADHD children

In the present study, we investigated the association between the 4-repeat allele at the dopamine receptor D4 (DRD4) gene and the response to treatment with methylphenidate (MPH) in Korean children with attention deficit hyperactivity disorder (ADHD). The study subjects were 83 children with ADHD (8.40+/-1.73 years) who were recruited from two child psychiatric clinics in South Korea. All of the drug-naive ADHD children were treated with MPH for about 8 weeks. An improvement of more than or equal to [corrected] 50% in the ADHD Rating Scale-IV (ARS) scores after 8 weeks of treatment compared with the baseline ARS scores before the treatment was considered as a 'good response', whereas an improvement of less than [corrected] 50% was considered as a 'poor response'. After the genotyping for DRD4 was performed, we investigated the association between the genotype at DRD4 and the response to MPH treatment. We performed a comparison of the response to MPH treatment between the two largest groups, viz. the subjects with and without the 4/4 genotype at DRD4. According to the ARS scores of the subjects as assessed by their parents and by their teachers, we found that while 71.1 and 80.0% (32/45 and 24/30), respectively, of those with a good response to MPH treatment showed the 4/4 genotype at DRD4, only 31.6 and 37.7% (12/38 and 20/53), respectively, of those with a poor response to MPH treatment showed the 4/4 genotype at DRD4 (Pearson chi2-values=12.926 and 13.737, respectively, both df=1, and both p<0.01). Our findings support the existence of an association between the 4-repeat allele at DRD4 and good response to MPH in Korean ADHD children.

Dopamine Genes and Pathological Gambling in Discordant Sib-Pairs

Pathological gambling (PG) is an impulse control disorder that has been considered as a behavioral addiction. Recent studies have suggested the involvement of the dopaminergic system in addictions and impulse control disorders and associations of dopamine receptor genes (DRD1, DRD2, and DRD4) and PG have been reported. In the present study, 140 sib-pairs discordant for the diagnosis of PG (70 males and 70 females on each group) were recruited through the Gambling Outpatient Unit at the Institute of Psychiatry, University of Sao Paulo and were assessed by trained psychiatrists. A family-based association design was chosen to prevent population stratification. All subjects were genotyped for dopamine receptor genes (DRD1 -800 T/C, DRD2 TaqIA RFLP, DRD3 Ser9Gly, DRD4 48bp exon III VNTR, DRD5 (CA) repeat) and the dopamine transporter gene (SCL6A3 40 bp VNTR). Our results suggest the association of PG with DRD1 -800 T/C allele T (P = .03).

The analysis of 51 genes in DSM-IV combined type attention deficit hyperactivity disorder: association signals in DRD4, DAT1 and 16 other genes

Attention deficit hyperactivity disorder (ADHD) is a common neurodevelopmental disorder, starting in early childhood and persisting into adulthood in the majority of cases. Family and twin studies have demonstrated the importance of genetic factors and candidate gene association studies have identified several loci that exert small but significant effects on ADHD. To provide further clarification of reported associations and identify novel associated genes, we examined 1,038 single-nucleotide polymorphisms (SNPs) spanning 51 candidate genes involved in the regulation of neurotransmitter pathways, particularly dopamine, norepinephrine and serotonin pathways, in addition to circadian rhythm genes. Analysis used within family tests of association in a sample of 776 DSM-IV ADHD combined type cases ascertained for the International Multi-centre ADHD Gene project. We found nominal significance with one or more SNPs in 18 genes, including the two most replicated findings in the literature: DRD4 and DAT1. Gene-wide tests, adjusted for the number of SNPs analysed in each gene, identified associations with TPH2, ARRB2, SYP, DAT1, ADRB2, HES1, MAOA and PNMT. Further studies will be needed to confirm or refute the observed associations and their generalisability to other samples.

An examination of the behavioral and neuropsychological correlates of three ADHD candidate gene polymorphisms (DRD4 7+, DBH TaqI A2, and DAT1 40 bp VNTR) in hyperactive and normal children followed to adulthood

Several candidate gene polymorphisms have been implicated in attention deficit hyperactivity disorder (ADHD), including DAT1 40bp VNTR, DRD4 7+, and DBH TaqI A2 alleles. We used the Milwaukee longitudinal study of hyperactive (n = 122) and normal (n = 67) children to compare participants with and without these respective polymorphisms on ADHD-related behavioral ratings at childhood, 8 years later in adolescence, and 13+ years later into young adulthood. Neuropsychological tests were given at the adolescent and young adulthood follow-up. No differences were found between the DRD4-7+ and 7- repeat polymorphism. The DBH TaqI A2 allele, when homozygous, was associated with being more hyperactive in childhood, having more pervasive behavior problems at adolescence, and earning less money on a card playing task in adulthood. At adolescence, poorer test scores were also found only in the hyperactive group with homozygous for this allele. The DAT1 40bp VNTR heterozygous 9/10 repeat, however, differed from the 10/10 repeat pair in many respects, having greater ADHD and externalizing symptoms at all three follow-ups, more cross-situational behavioral problems at both childhood and adolescence, poorer mother-teen relations at adolescence, and lower class rankings in high school. Participants with the 9/10 pair in the control group also had lower work performance, a lower grade point average in high school, greater teacher rated externalizing symptoms at adolescence, and greater omission errors on a continuous performance test in adulthood. The DAT1 40bp VNTR 9/10 polymorphism pairing appears to be reliably associated with greater symptoms of ADHD and externalizing behavior from childhood to adulthood, and with family, educational, and occupational impairments. We also present a contrary view on the appropriate endophenotypes for use in behavioral genetic research on ADHD.

Role of electroencephalography in attention-deficit hyperactivity disorder

Attention-deficit hyperactivity disorder (ADHD) is characterized by symptoms of inattention, hyperactivity and impulsivity, and affects 3-5% of school-aged children. Recommendations regarding the relative importance of electroencephalography (EEG) in ADHD are ambiguous. Most guidelines for ADHD diagnostics only recommend an EEG in cases with clinical suggestion of seizure disorders or degenerative conditions and not for routine use. Although in most cases of ADHD, an EEG is indeed unnecessary, without a routine EEG, some children with absences or rolandic spikes will not be identified and, therefore, will not be treated correctly. The EEGs of children with ADHD demonstrated increased theta activity and fewer alpha waves compared with controls. Research on event-related potentials is helpful in identifying underlying attentional deficits in ADHD. Future studies that combine EEG analysis with functional magnetic resonance imaging data, positron emission tomography studies or genetic research will help to improve our knowledge about the pathophysiology of ADHD and perhaps lead to a better, more individual treatment in well defined subgroups.

Attention-deficit disorder (attention-deficit/ hyperactivity disorder without hyperactivity): a neurobiologically and behaviorally distinct disorder from attention-deficit/hyperactivity disorder (with hyperactivity)

Most studies of attention-deficit/hyperactivity disorder (ADHD) have focused on the combined type and emphasized a core problem in response inhibition. It is proposed here that the core problem in the truly inattentive type of ADHD (not simply the subthreshold combined type) is in working memory. It is further proposed that laboratory measures, such as complex-span and dual-task dichotic listening tasks, can detect this. Children with the truly inattentive type of ADHD, rather than being distractible, may instead be easily bored, their problem being more in motivation (underarousal) than in inhibitory control. Much converging evidence points to a primary disturbance in the striatum (a frontal-striatal loop) in the combined type of ADHD. It is proposed here that the primary disturbance in truly inattentive-type ADHD (ADD) is in the cortex (a frontal-parietal loop). Finally, it is posited that these are not two different types of ADHD, but two different disorders with different cognitive and behavioral profiles, different patterns of comorbidities, different responses to medication, and different underlying neurobiologies.

The genetics of attention deficit hyperactivity disorder

Over the past 15 years, considerable progress has been made in understanding the etiology of childhood Attention Deficit Hyperactivity Disorder (ADHD), largely due to the publication of numerous twin studies which are consistent in suggesting substantial genetic influences (i.e., heritabilities ranging from 60% to 90%), non-shared environmental influences that are small-to-moderate in magnitude (i.e., ranging from 10% to 40%), and little-to-no shared environmental influences. Following from these quantitative genetic findings, numerous molecular genetic studies of association and linkage between ADHD and a variety of candidate genes have been conducted during the past 10 years. The majority of the candidate genes studied underlie various facets of the dopamine, norepinephrine, and serotonin neurotransmitter systems, although the etiological role of candidate genes outside of neurotransmitter systems (e.g., involved in various aspects of brain and nervous system development) have also been examined. In this paper, we review recent findings from candidate gene studies of childhood ADHD and highlight those candidate genes for which associations are most replicable and which thus appear most promising. We conclude with a consideration of some of the emerging themes that will be important in future studies of the genetics of ADHD.

Association of dopamine D4 receptor (DRD4) polymorphisms with attention deficit hyperactivity disorder in Indian population

Attention deficit hyperactivity disorder (ADHD) is a childhood onset neurobehavioral disorder. Several studies worldwide have implicated a possible association between ADHD and transmission of different polymorphisms of the dopamine D4 receptor gene (DRD4) in different ethnic groups. However, this is the first report on the transmission of different polymorphisms of DRD4 in Indian subjects. Association of 5' flanking 120-bp duplication, exon 1 12-bp duplication, and exon 3 48-bp variable numbers of tandem repeats (VNTR) were analyzed in 50 ADHD cases. Haplotype-based haplotype relative risk (HHRR) analysis and transmission disequilibrium test (TDT) were carried out to ascertain the association of these polymorphisms with the disorder. Linkage disequilibria (LD) between the polymorphisms were calculated using EH+ and 2LD programs. Our preliminary data showed lack of association between ADHD and transmission of the 5' flanking 120-bp duplication and exon 1 12-bp duplication. But, the transmissions of 6 and 7 repeat alleles of exon 3 48-bp VNTR showed significant association with ADHD. We have also examined the haplotype frequencies and biased transmission of one haplotype was observed in ADHD probands. LD analysis showed very strong disequilibrium between exon 1 12-bp duplication and exon 3 48-bp VNTR. Strong LD was also observed between the 5' flanking 120-bp duplication and exon 1 12-bp duplication. The observed association between higher repeat alleles of exon 3 48-bp VNTR and Indian ADHD children is consistent with some of the earlier reports.

Sequence variation in the 3'-untranslated region of the dopamine transporter gene and attention-deficit hyperactivity disorder (ADHD)

The dopamine transporter gene (DAT1) has been reported to be associated with attention-deficit hyperactivity disorder (ADHD) in a number of studies [Cook et al. (1995): Am J Human Genet 56(4):9993-998; Gill et al. (1997): Mol Psychiatry 2(4):311-313; Waldman et al. (1998): Am J Human Genet 63(6):1767-1776; Barr et al. (2001): Biol Psychiatry 49(4):333-339; Curran et al. (2001): Mol Psychiatry 6(4):425-428; Chen et al. (2003): Mol Psychiatry 8(4):393-396]. Specifically, the 10-repeat allele of the 40-bp variable number of tandem repeats (VNTR) polymorphism located in the 3' untranslated region (UTR) of the gene has been found to be associated with ADHD. There is evidence from in vitro studies indicating that variability in the repeat number, and sequence variation in the 3'-UTR of the DAT1 gene may influence the level of the dopamine transporter protein [Fuke et al. (2001): Pharmacogenomics J 1(2):152-156; Miller and Madras (2002): Mol Psychiatry 7(1):44-55]. In this study, we investigated whether DNA variation in the DAT1 3'UTR contributed to ADHD by genotyping DNA variants around the VNTR region in a sample of 178 ADHD families. These included a MspI polymorphism (rs27072), a DraI DNA change (T/C) reported to influence DAT1 expression levels, and a BstUI polymorphism (rs3863145) in addition to the VNTR. We also screened the VNTR region by direct resequencing to determine if there was sequence variation within the repeat units that could account for the association. Our results indicate that DAT1 is associated with ADHD in our sample but not with alleles of the VNTR polymorphism. We did not find any variation in the sequence for either the 10- or 9-repeat alleles in the probands screened nor did we observe the reported DraI (T/C) variation. Our results therefore refute the possibility of the reported DraI variation or alleles of the VNTR as the functional variants contributing to the disorder.

Attention-deficit disorder (attention-deficit/ hyperactivity disorder without hyperactivity): a neurobiologically and behaviorally distinct disorder from attention-deficit/hyperactivity disorder (with hyperactivity)

Most studies of attention-deficit/hyperactivity disorder (ADHD) have focused on the combined type and emphasized a core problem in response inhibition. It is proposed here that the core problem in the truly inattentive type of ADHD (not simply the subthreshold combined type) is in working memory. It is further proposed that laboratory measures, such as complex-span and dual-task dichotic listening tasks, can detect this. Children with the truly inattentive type of ADHD, rather than being distractible, may instead be easily bored, their problem being more in motivation (underarousal) than in inhibitory control. Much converging evidence points to a primary disturbance in the striatum (a frontal-striatal loop) in the combined type of ADHD. It is proposed here that the primary disturbance in truly inattentive-type ADHD (ADD) is in the cortex (a frontal-parietal loop). Finally, it is posited that these are not two different types of ADHD, but two different disorders with different cognitive and behavioral profiles, different patterns of comorbidities, different responses to medication, and different underlying neurobiologies.

Méta-analyse des gènes candidats dans le trouble déficit attentionnel avec hyperactivité (TDAH)

Attention-deficit hyperactivity disorder (ADHD) is a common behavioral disorder observed during childhood, detected in 3% to 5% of school-age children. The disorder is characterised by marked inattention, hyperactivity, and impulsiveness. In most cases, symptoms can be treated by catecholamine-releasing drugs, such as methylphenidate. Children with ADHD are at higher risk for substance abuse and oppositional, conduct and mood disorders. Familial and adoption studies shed light on the genetic vulnerability of ADHD. Twin studies estimated the broad heritability to range between 40% and 90%. The mode of transmission is yet unknown, but is likely polygenic. Molecular genetic studies in ADHD should contribute to a greater understanding of the pathophysiology of the disorder (genetics of the vulnerability), and could help to select a more rational type of treatment (pharmacogenetic). Family-based association studies already performed are reviewed in this manuscript. Association studies, using haplotype relative risk (HRR) or transmission disequilibrium test (TDT) have focused on candidate genes which code for proteins potentially involved in the etiopathogenesis of the disorder. Genes involved in dopamine, serotonin, and noradrenalin systems have thus been assessed for their role in core features of ADHD, such as motor overactivity, inattention, and impulsiveness. According to a meta-analysis, the DAT1 gene, an obvious candidate gene in ADHD vulnerability, does not appear to be involved (OR = 1.13, p = 0.21). On the other hand, DRD4 (OR = 1.26, p = 0.01) and DRD5 (OR = 1.4, p = 0.01) are significantly associated to ADHD according to the present meta-analysis, confirming previous ones. Recent studies showed a trend for an association between one allele of the 5-HTT (considering case-control studies) and DBH (OR = 1.27, p = 0.06) genes and ADHD, but these positive findings have to be replicated. ADHD is a complex disorder with potentially many different risk factors. Genetic and phenotypic heterogeneity could explain why some association studies are positive, whereas others are negative. For instance, different developmental pathways are likely to lead to similar clinical outcomes. More clear-cut phenotypes, such as ADHD with conduct disorder, or ADHD with bipolar disorder, could be more homogenous, the genes involved being therefore more easy to detect. These phenotypes are beginning to be specifically studied in molecular genetics. In addition, the development of pharmacogenetics could help to identify predictors of clinical response for a specific type of treatment, which would be clearly helpful in clinical practice.

Molecular genetics of attention-deficit hyperactivity disorder (ADHD): an update

Attention-deficit hyperactivity disorder (ADHD) is a heritable and behavioral condition of childhood, affecting 5-10% of school-age children worldwide. Affected patients exhibit various behavioral problems such as carelessness, restlessness, disobedience and failure to stay quiet in class. The etiology of ADHD is not known. However, family, twin and adoption studies have provided strong evidence for a genetic etiology of the disorder. A genome-wide scan has identified six chromosomal loci with LOD scores suggestive of linkage. Animal studies suggest the involvement of the brain dopamine pathway and its alteration in ADHD but there is no direct evidence to support this hypothesis. In addition, there are at least 20 candidate genes of small effect that have been studied but none of them appear to be the major gene causing ADHD. Medical intervention along with psychosocial therapy proved to be beneficial for controlling ADHD, although some undesirable side effects have been encountered during medical treatment. In the future, identification of environmental factors, study of additive gene effects and the interaction of genes and environmental factors may provide better insight into the pathophysiology of ADHD. This may lead to an effective new treatment strategy.

Transmission disequilibrium testing of dopamine-related candidate gene polymorphisms in ADHD: confirmation of association of ADHD with DRD4 and DRD5

Attention-deficit hyperactivity disorder (ADHD) is one of the most common childhood behavioral disorders. Genetic factors contribute to the underlying liability to develop ADHD. Reports implicate variants of genes important for the synthesis, uptake, transport and receptor binding of dopamine in the etiology of ADHD, including DRD4, DAT1, DRD2, and DRD5. In the present study, we genotyped a large multiplex sample of ADHD affected children and their parents for polymorphisms in genes previously reported to be associated with ADHD. Associations were tested by the transmission disequilibrium test (TDT). The sample is sufficient to detect genotype relative risks (GRRs) for putative risk alleles. The DRD4 gene 120-bp insertion/deletion promoter polymorphism displayed a significant bias in transmission of the insertion (chi(2)=7.58, P=0.006) as suggested by an analysis of a subset of these families. The seven repeat allele of the DRD4 48-bp repeat polymorphism (DRD4.7) was not significantly associated with ADHD in the large sample in contrast to our earlier findings in a smaller subset. We replicate an association of a dinucleotide repeat polymorphism near the DRD5 gene with ADHD by showing biased nontransmission of the 146-bp allele (P=0.02) and a trend toward excess transmission of the 148-bp allele (P=0.053). No evidence for an association was found for polymorphisms in DRD2 or DAT1 in this sample. The DRD5 146-bp (DRD5.146) allele and the DRD4 240-bp (DRD4.240) allele of the promoter polymorphism emerge as the two DNA variants showing a significant association in this large sample of predominantly multiplex families with ADHD, with estimated GRRs of 1.7 and 1.37, respectively.

Neurobiologie der Aufmerksamkeitsdefizit-/ Hyperaktivitätsstörung (ADHS)

Zusammenfassung. Bei der Aufmerksamkeitsdefizit-Hyperaktivitätsstörung (ADHS) handelt es sich um eine in der Kindheit beginnende, klinisch und wahrscheinlich auch ätiologisch heterogene Störung, die durch die Symptom-Trias Unaufmerksamkeit, Hyperaktivität und Impulsivität gekennzeichnet ist. Obwohl die genauen neurobiologischen Ursachen der ADHS weiterhin unbekannt sind, konnten in den Forschungsbereichen Genetik, psychosoziale Umweltfaktoren, Neuropsychologie, Psychophysiologie, funktionelle und strukturelle Bildgebung und Biochemie bedeutende Fortschritte erzielt und verschiedene, hypothetische Modelle zur Pathophysiologie entwickelt werden. Die vorliegende Übersichtsarbeit fasst die aktuellen Erkenntnisse der unterschiedlichen Gebiete zusammen und gibt so eine Zusammenfassung über den aktuellen Wissensstand zur Pathophysiologie der ADHS.

Breed differences in genotype and allele frequency of catechol O-methyltransferase gene polymorphic regions in dogs

Catechol O-methyltransferase (COMT) inactivates catecholamines and catechol-containing drugs such as L-DOPA. The common genetic polymorphism Val158Met in the human COMT gene is suspected to be associated with "persistence" or risk for schizophrenia. In this study, we attempted to identify the canine COMT gene fragment and to find a similar polymorphism and to reveal its genetic distribution among five representative canine breeds. We found that the amplified gene consisted of 663 bp nucleotides and was 84% homologous with the human COMT gene. The single nucleotide polymorphisms, guanine adenine substitution, were observed at the 39th, 216th and 482nd nucleotides. From the genotyping of the 216th polymorphism among 266 dogs by the polymerase chain reaction-restriction fragment length polymorphism method with restriction enzyme EagI, and that of the 482nd polymorphism with restriction enzyme SfcI, we found inter-breed variations of genotypes as well as of allelic frequencies for both of these polymorphic regions. These results suggest that the identified polymorphisms will be useful tools in elucidating the genetic background of canine behavioral traits.

Response disinhibition may be explained as an extinction deficit in an animal model of attention-deficit/hyperactivity disorder (ADHD)

Attention-deficit/hyperactivity disorder (ADHD) is a disorder affecting between 2 and 12% of grade-school children disturbing social, academic, and occupational functioning. Problems related to social adjustment and functioning and/or psychiatric problems will exist in 50-70% of adolescents and young adults diagnosed with ADHD as children. It has been suggested that altered reinforcement and extinction processes may cause the symptoms of ADHD. The present study investigated extinction processes in spontaneously hypertensive rats (SHR), possibly the best-validated animal model of ADHD. Extinction was tested after either a variable interval (VI) or a fixed interval (FI) schedule of reinforcement with and without the presence of a conditioned reinforcer (light in the water cubicle). The results indicate a slower extinction process in the SHR compared to the normal controls, especially during the initial transition from scheduled reinforcement to extinction. Also, more responses were retained in the SHR during the later part of extinction. The extinction deficit in the SHR may be linked to reinforcer unpredictability and the presence of conditioned reinforcers, and may explain response disinhibition seen in children with ADHD.

Childhood inattention and dysphoria and adult obesity associated with the dopamine D4 receptor gene in overeating women with seasonal affective disorder

There is significant evidence that altered dopamine activity plays a role in seasonal affective disorder (SAD). The current study examined three separate genetic hypotheses for SAD related to the 7-repeat allele (7R) of the dopamine-4 receptor gene (DRD4), a variant associated with decreased affinity for dopamine. We examined the possible contribution of 7R to the overall expression of SAD, attention deficit disorder (ADD) comorbidity, and body weight regulation. As part of an ongoing genetic study of increased eating behavior and mood in female subjects, 108 women with winter SAD and carbohydrate craving/weight gain were administered the Wender-Utah Rating Scale to measure childhood ADD symptomatology, and a questionnaire to assess maximal lifetime body mass index (BMI). To test for an association between 7R and the categorical diagnosis of SAD, the transmission disequilibrium test (TDT) was used in a subsample of probands providing familial DNA. Standard parametric tests were used to compare childhood ADD symptoms and maximal lifetime BMI across the two genotypic groups defined by the presence or absence of 7R. The TDT found no initial evidence for an association between 7R and the categorical diagnosis of SAD. However, 7R carriers reported significantly greater inattention and dysphoria in childhood (p=0.01 and 0.001, respectively) and a higher maximal lifetime BMI (p=0.007) than did probands without this allele. Furthermore, excluding probands with extreme obesity (maximal BMI >40), a strong correlation was found linking childhood inattentive symptoms and maximal lifetime BMI (r=0.35, p=0.001). In overeating women with SAD, the 7R allele of DRD4 may be associated with a unique developmental trajectory characterized by attentional deficits and dysphoria in childhood and mild to moderate obesity in adulthood. This developmental course may reflect different manifestations of the same underlying vulnerability related to central dopamine dysfunction. Given the possibility of population stratification when studying genotype/phenotype relationships, future use of genomic controls and replication of our findings in other overeating and/or ADD populations are needed to confirm these initial results.

Dopamine phenotype and behaviour in animal models: in relation to attention deficit hyperactivity disorder

The phenotypic expression of behaviour is the outcome of interacting neuronal networks and is modulated by different subcortical systems. In the present paper the role of a major subcortical neurochemical system, dopamine (DA), is reviewed. In particular, knockout (KO) technology has given an overwhelming insight into the effects of specific component of the dopaminergic system. Therefore, the behavioural profile of dopamine transporter (DAT), tyrosine hydroxylase (TH), DA and cAMP-regulated phosphoprotein (DARPP 32), and D1, D2, D3, D4 and D5 dopamine receptors knockouts (and their combination) is reviewed.TH, D1, D2, D4 KO mice exhibit decreased locomotor activity, perhaps due to decreased motivational level. D3 KO and DAT KO mice show an increase in basal and novelty-induced activity respectively. It is possible that the increased dopamine levels in DAT KO mice enhance motivation. These observations support the hyperDA hypothesis in hyperactive phenotypes. Moreover, they suggest that the inhibitory effect of psychostimulant drugs, such as methylphenidate and amphetamines, in Attention Deficit Hyperactivity Disorder may be the outcome of an altered balance between auto- and hetero-receptors. However, since KO technology is hampered by blockade of the target at early stages of development, some alternatives have been proposed, such as inducible mutagenesis and inhibitory small RNAs conveyed to target by viral vectors in adulthood.

Association of the dopamine beta hydroxylase gene with attention deficit hyperactivity disorder: genetic analysis of the Milwaukee longitudinal study

Attention deficit hyperactivity disorder (ADHD) is a highly heritable and common disorder that partly reflects disturbed dopaminergic function in the brain. Recent genetic studies have shown that candidate genes involved in dopamine signaling and metabolism contribute to ADHD susceptibility. We have initiated genetic studies in a unique cohort of 158 ADHD and 81 control adult subjects who have been followed longitudinally since childhood in the Milwaukee study of ADHD. From this cohort, genetic analysis was performed in 105 Caucasian subjects with ADHD and 68 age and ethnicity-matched controls for the DRD4 exon 3 VNTR, the SLC6A3 (DAT1) 3' UTR VNTR, dopamine beta hydroxylase (DBH) TaqI A polymorphism, and the DBH GT microsatellite repeat polymorphism that has been quantitatively associated with serum levels of DBH activity, but not previously studied in ADHD. Results indicate a significant association between the DBH TaqI A1 allele and ADHD (P = 0.018) with a relative risk of 1.33. The DBH GT repeat 4 allele, which is associated with high serum levels of DBH, occurred more frequently in the ADHD group than controls, but the difference did not reach statistical significance. Associations were not found with the SLC6A3 10 repeat or DRD4 7 repeat alleles. These results indicate that the DBH TaqI A allele, or another polymorphism in linkage disequilibrium with this allele, may confer increased susceptibility towards ADHD.

The short DRD4 repeats confer risk to attention deficit hyperactivity disorder in a family-based design and impair performance on a continuous performance test (TOVA)

One particular candidate gene, the dopamine D4 receptor (DRD4), has been the focus of intense study regarding ADHD since the original investigation by La Hoste et al, an observation confirmed by a recent metaanalysis. However, two previous studies from Israel failed to observe this association. We have now recruited an additional sample and, overall, in the combined sample of 178 triads we observe using the transmission disequilibrium test, preferential transmission of the short allele. Additionally, we now report the effect of the DRD4 repeat region on the Test Of Variables of Attention (TOVA), a widely used computerized continuous performance test. Probands with the short exon III repeat performed significantly worse on the TOVA measured both by errors of commission and response time variable. Intriguingly, a 'dose effect' was observed. Increasing repeat size is accompanied by a reduced number of errors of commission and a significant difference is observed between the 2 vs 7 repeats. On the whole, our results lend credence to the notion that the relationship between the DRD4 receptor and ADHD is complex and may be reflecting linkage disequilibrium between the 7 or long DRD4 exon III repeats and a 'true' risk allele in this gene or a neighboring locus.

Dopamine system genes and attention deficit hyperactivity disorder: a meta-analysis

The dopamine system may play a major role in the development of attention deficit hyperactivity disorder (ADHD). We applied a random-effects model meta-analysis to family-based studies of association between ADHD and the dopamine system genes DRD4, DRD5 and DAT1. A statistical test of heterogeneity was conducted for each group of studies. The meta-analysis of DRD4 included data from 13 studies, with a total of 571 informative meioses. The pooled odds ratio estimate was 1.41 (95% CI 1.20-1.64, =1.57 x 10 ), demonstrating positive association. For DRD5, data from five studies, with a total of 340 informative meioses, were combined yielding a pooled odds ratio of 1.57 (95% CI 1.25-1.96, =8.28 x 10 ). Eleven studies examining DAT1, with a total of 824 informative meioses, yielded a non-significant pooled odds ratio estimate of 1.27 (95% CI 0.99-1.63, 0.06). There was no support of heterogeneity between the studies. Overall, the meta-analyses support the involvement of the dopamine system genes in ADHD liability variation and suggest the need for studies examining interactions between these genes.

Genes de suscetibilidade no transtorno de déficit de atenção e hiperatividade

O transtorno de déficit de atenção e hiperatividade (TDAH) é um dos transtornos mais comuns da infância e adolescência, afetando entre 3% a 6% das crianças em idade escolar. Essa patologia caracteriza-se por sintomas de desatenção, hiperatividade e impulsividade, apresentando ainda uma alta heterogeneidade clínica. Embora as causas precisas do TDAH não estejam esclarecidas, a influência de fatores genéticos é fortemente sugerida pelos estudos epidemiológicos, cujas evidências impulsionaram um grande número de investigações com genes candidatos. Atualmente, apesar da ênfase dada a este tópico, nenhum gene pode ser considerado necessário ou suficiente ao desenvolvimento do TDAH, e a busca de genes que influenciam este processo ainda é o foco de muitas pesquisas. O objetivo desse artigo é, portanto, sumarizar e discutir os principais resultados das pesquisas com genes candidatos no TDAH.

The norepinephrine transporter gene and attention-deficit hyperactivity disorder

The adrenergic system plays a known role in attentional systems and a suspected causal role in attention-deficit hyperactivity disorder (ADHD), based on evidence from pharmacological interventions and animal models. The efficacy of the highly selective noradrenergic reuptake inhibitor, tomoxetine, in treating ADHD symptoms supports the system's role in ADHD and points to the norephinephrine transporter as a candidate gene. This study tested the gene for the norepinephrine transporter (NET1) as a susceptibility factor in ADHD using three polymorphisms located in exon 9, intron 9, and intron 13. We examined the inheritance of these polymorphisms in a sample of 122 families with a total of 155 children with ADHD identified through an ADHD proband. Use of the transmission disequilibrium test failed to show significant evidence for biased transmission of any of the alleles or the haplotypes of these polymorphisms. We further investigated this gene by screening the probands for five known amino acid variants to determine if they contributed to the ADHD phenotype but observed only one (Thr99Ile) in our sample. Since the frequency of this variant (1.8%) was similar to that previously reported in a control sample (2.2%), it is unlikely that this variant is related to the ADHD phenotype. Our results do not support the NET1 gene as a major genetic susceptibility factor in ADHD.

Psychiatric pharmacogenetics: personalizing psychostimulant therapy in attention-deficit/hyperactivity disorder

There is a substantial amount of variation in response and adverse drug reactions to psychostimulant therapy in attention-deficit/hyperactivity disorder (ADHD). Psychiatric pharmacogenetics is a rapidly developing field, which can be applied to identify genetic predictors of this variability in outcome to psychostimulant medications. This article will briefly review ADHD and its pharmacotherapy. This will be followed by an overview of the pharmacokinetics and pharmacodynamics of methylphenidate, the most commonly used psychostimulant in the US. Then the field of psychiatric pharmacogenetics will be introduced and its methodology will be described. This will be followed by a discussion about how pharmacogenetics can be applied to children afflicted with ADHD. The future of psychiatric pharmacogenetics will then be presented with an emphasis being placed on developing prospects that will ensure the continued advancement of this field.

Association of DRD4 in children with ADHD and comorbid conduct problems

Recent family and twin study findings suggest that ADHD when comorbid with conduct problems may represent a particularly familial and heritable form of ADHD. Although several independent groups have shown association between the DRD4 7 repeat allele and ADHD, others have failed to replicate this finding. Previous TDT analyses of UK and Eire samples had also been negative. We set out to further examine the role of DRD4 but selecting a subgroup of children with ADHD and comorbid conduct problems. Families were recruited from Manchester, Ireland, Birmingham and London clinics. From these, 67 children who fulfilled diagnostic criteria for ADHD and who displayed conduct disorder symptoms were selected. TDT analysis, which had previously yielded negative results for the total sample, showed evidence of association between DRD4 and "ADHD with conduct problems" (7 repeat allele-24 transmissions, 13 non-transmissions; one-tailed P=0.05). These results provide further support for the role of DRD4 in ADHD. Furthermore, these results when considered together with family and twin study findings, suggest that those children with ADHD and comorbid conduct problems may be particularly informative for molecular genetic studies of ADHD. Further work is needed to examine these phenotype issues.

Estado atual e perspectivas da genética e epidemiologia do alcoolismo

O alcoolismo é um problema de saúde pública de escala mundial. O abuso e dependência combinados afetam aproximadamente 8% da população brasileira, gerando um grande custo social. O reconhecimento da existência de uma herdabilidade significativa contribuiu para o entendimento do problema como uma doença específica com origem biológica. Os avanços no conhecimento da neurobiologia da dependência permitiram delimitar uma série de genes candidatos para a predisposição. Atualmente, iniciam-se os estudos sobre o papel de polimorfismos genéticos na resposta ao tratamento. A integração de abordagens clínicas, epidemiológicas e de genética molecular pode identificar grupos clínicos mais responsivos a abordagens terapêuticas específicas.

Susceptibility genes for a trait measure of attention deficit hyperactivity disorder: a pilot study in a non-clinical sample of twins

Attention deficit hyperactivity disorder (ADHD) is a highly heritable disorder, and molecular genetic studies are underway, with most researchers focusing on identifying susceptibility genes in clinical samples with ADHD. An alternative approach is to search for quantitative trait loci underlying the trait measure of ADHD in non-clinical samples. Positive findings of association of the dopamine transporter DAT1 480 bp allele (allele 10) and the DRD4 7 repeat allele with clinical ADHD have been previously reported. In this pilot study, we examined these polymorphisms in a selected population-based sample of twins (50 high scoring pairs, 42 low scoring pairs). There was a trend for an increase in the frequency of the dopamine receptor DRD4 7 repeat allele in the high-scoring concordant monozygotic twins (odds ratio=1.4). Although this result was not statistically significant, the frequency of the 7 repeat allele was similar to that reported for our clinic sample of ADHD patients drawn from the same geographical area. There was a non-significant trend for an increased frequency of the DAT1 allele 10 (odds ratio=1.3). These results suggest that a molecular genetic study based on a questionnaire-derived measure of ADHD in a non-clinical sample is feasible and the results appear to be comparable with those from studies of clinical cases. However, sample size and power are key issues to consider when using this approach.

Is attention-deficit/hyperactivity disorder an energy deficiency syndrome?

Attention-deficit/hyperactivity disorder (ADHD) is a highly heritable yet clinically heterogeneous syndrome associated with hypocatecholamine function in subcortical and prefrontal cortical regions and clinical response to medications that enhance catecholamine function. The goal of this article is to present a hypothesis about the etiology of ADHD by synthesizing these findings with recent experiments indicating that activity-dependent neuronal energy consumption is regulated by cortical astrocytes. The scientific literature was searched from 1966 to the present using MEDLINE and relevant key words. Inattention and impulsivity may be related to hypofunctionality of catecholamine projection pathways to prefrontal cortical areas, resulting in decreased neuronal energy availability. This may be mediated by astrocyte catecholamine receptors that normally regulate energy availability during neuronal activation. At least some forms of ADHD may be viewed as cortical, energy-deficit syndromes secondary to catecholamine-mediated hypofunctionality of astrocyte glucose and glycogen metabolism, which provides activity-dependent energy to cortical neurons. Several tests of this hypothesis are proposed.

Attention-deficit hyperactivity disorder: a study of association with both the dopamine transporter gene and the dopamine D4 receptor gene

Attention-deficit hyperactivity disorder (ADHD) is one of the most common psychiatric disorders of childhood. The role of genetic factors in its etiology is strongly supported by family, adoption, and twin studies. Several investigations have reported associations between ADHD and both the 7-repeat allele of the 48 bp VNTR at the DRD4 gene and the 10-repeat allele of the 40 bp VNTR at the DAT1 gene, but the results have been inconsistent. A sample of 81 Brazilian ADHD children and adolescents and their parents were screened for these DRD4 and DAT1 VNTRs. An excess of the DRD4 7-repeat allele was observed when both ADHD probands and their parents were compared with an ethnically matched control sample (chi-square = 11.55, P = 0.03; chi-square = 12.17, P = 0.03, respectively). However, haplotype relative risk (HRR) analysis showed no preferential transmission of the DRD4 7-repeat allele. No evidence of association with the DAT1 polymorphism was detected by both approaches. Nevertheless, an interaction effect of both genes on ADHD hyperactive/impulsive dimension was observed (F = 4.68; P = 0.03). These results add to the group of studies that together suggest a small effect of these genes in the susceptibility to ADHD.

Examining for association between candidate gene polymorphisms in the dopamine pathway and attention-deficit hyperactivity disorder: a family-based study

Attention-deficit hyperactivity disorder (ADHD) is a highly heritable childhood-onset psychiatric condition characterized by developmentally inappropriate inattention, hyperactivity, and impulsiveness. The pathophysiology of ADHD is currently unknown. However, the therapeutic effects of stimulant medication together with findings from animal and neuroimaging studies as well as from several molecular genetic studies of the dopamine receptor D4 gene and dopamine transporter gene have implicated involvement of the dopaminergic system. To test the dopaminergic hypothesis further, we have looked for association between ADHD and alleles of seven dopamine-related candidate genes using a family-based association approach in a sample of 150 children diagnosed with ADHD. We tested polymorphisms in genes encoding three dopamine receptors (DRD3, DRD4, and DRD5) and four dopamine-relevant enzymes: tyrosine hydroxylase [tyrosine hydroxylase (TH)], dopamine beta hydroxylase (DbetaH), catechol-O-methyltransferase (COMT), and monoamine oxidase A (MAOA). We were unable to detect a significant association with any of the polymorphisms genotyped, although there was a trend for preferential transmission of the DRD5 148 bp marker allele and the MAOA 122 bp marker allele. We conclude that none of the alleles we have tested makes a major contribution to ADHD, although much larger samples are required to exclude small effects.