Attention-Deficit Hyperactivity Disorder in the post-genomic era

Unraveling the Role of miR-200b-3p in Attention-Deficit/Hyperactivity Disorder (ADHD) and Its Therapeutic Potential in Spontaneously Hypertensive Rats (SHR)

Attention-deficit/hyperactivity disorder (ADHD) is a prevalent neurodevelopmental disorder in children with unknown etiology. Impaired learning ability was commonly reported in ADHD patients and has been associated with dopamine uptake in the striatum of an animal model. Another evidence also indicated that micro-RNA (miR)-200b-3p is associated with learning ability in various animal models. However, the association between miR-200b-3p and ADHD-related symptoms remains unclear. Therefore, the current study investigated the role of miR-200b-3p in ADHD-related symptoms such as inattention and striatal inflammatory cytokines. To verify the influence of miR-200b-3p in ADHD-related symptoms, striatal stereotaxic injection of miR-200b-3p antagomir (AT) was performed on spontaneously hypertensive rats (SHR). The antioxidant activity and expressions of miR-200b-3p, slit guidance ligand 2 (Slit2), and inflammatory cytokines in the striatum of SHR were measured using quantitative real-time polymerase chain reaction (RT-qPCR), immunohistochemistry (IHC), immunoblotting, and enzyme-linked immunosorbent assay (ELISA). The spontaneous alternation of SHR was tested using a three-arm Y-shaped maze. The administration of miR-200b-3p AT or taurine significantly decreased striatal tumor necrosis factor (TNF)-α, interleukin (IL)-1β, and IL-6 in SHR, along with increased super-oxide dismutase (SOD) and glutathione peroxidase (GSH-Px) activities and significantly higher spontaneous alternation. In this paper, we show that miR-200b-3p AT and taurine alleviates ADHD-related symptoms in SHR. These findings provide insights into ADHD's molecular basis and suggest miR-200b-3p as a potential therapeutic target. Concurrently, this study also suggests broad implications for treating neurodevelopmental disorders affecting learning activity such as ADHD.

Disordered eating in early childhood: DRD4 and DAT1 gene polymorphisms and quality of mother-child interaction

PURPOSE

Eating disturbances are complex heritable conditions that can be influenced by both genetic and environmental factors but are poorly studied in early development. The aim of this research was to investigate the association of genetic polymorphisms within dopaminergic pathways with early feeding problems.

METHODS

We analyzed the presence of VNTR polymorphisms of DRD4 (rs1805186) and DAT1 (rs28363170) in overeating (N = 45), undereating (N = 48) and control (N = 44) young children. We also assessed presence of externalizing, internalizing and dysregulation symptoms by the Child Behavior Checklist and quality of mother-child interactions during feeding by the Italian adaptation of the Scale for the Assessment of Feeding Interaction, respectively.

RESULTS

Both polymorphisms were associated with children's eating behavior, psychological symptoms and quality of interaction with their mothers, suggesting that: (a) the DRD4 4-repeat allele behaves as a protective factor, the 2-repeats and 7-repeats alleles as risk factors, for undereating behavior, the general quality of mother-child interaction and internalizing, externalizing and dysregulated symptoms; and (b) the DAT1 9-repeats allele behaves as a protective factor, the 10-repeats allele as a risk factor, for overeating behavior, the general quality of mother-child interaction, internalizing, externalizing and dysregulated symptoms. Finally, a gene x gene interaction is suggested between the DAT1 9-repeat or 10-repeat allele and the DRD4 4-repeat allele.

CONCLUSIONS

Our results suggest a role for DRD4 and DAT1 in an early susceptibility to eating disturbances.

LEVEL OF EVIDENCE III

Evidence obtained from well-designed case-control analytic study.

Prenatal Tobacco Exposure Modulated the Association of Genetic variants with Diagnosed ADHD and its symptom domain in children: A Community Based Case-Control Study

The purpose of our study was to test the hypothesis that prenatal tobacco smoking exposure (PSE) could modulate the association of genetic variants with ADHD. A community based case-control study was conducted among Chinese children and 168 ADHD patients and 233 controls were recruited by using combination diagnosis of DSM-IV, SNAP-IV and semi-structured clinical interview. Logistic regression analysis was performed to estimate the effect of prenatal tobacco smoking exposure and genotype frequencies on ADHD susceptibility individually by adjustment for potential confounders. Multiplicative and additive interaction analysis were performed to evaluate the interactions between risk genes and PSE with regard to ADHD. Prenatal tobacco smoke exposure was a significant risk factor of ADHD even after adjusted for other potential confounders. ADRA2A rs553668, DRD2 rs1124491 and SLC6A4 rs6354 were identified to be associated with ADHD. A significant multiplicative and additive gene-environment interactions were observed between the PSE and the ADRA2A rs553668 in relation to ADHD and ADHD-ODD. The risk of the genetic variants in ADHD was increased significantly if the child had prenatal tobacco exposure. The genetic risk for ADHD could be influenced by the presence of environmental risks. The environmental and the genetic risks are not distinct to each other. More gene-environment interaction studies were needed to reveal the etiology of ADHD.

Risk factors for the existence of attention deficit hyperactivity disorder symptoms in children with autism spectrum disorders

Over the years, several authors have reported symptoms of attention deficit hyperactivity disorder (ADHD) in patients with autism spectrum disorders (ASD); however, studies on the risk factors of ADHD symptoms in children with ASD are lacking. The aim of this cross-sectional study was to identify the risk factors for the development of ADHD symptoms in children with ASD. The sample consisted of 67 children with ASD who were assessed with Conner's Parent Rating Scale-Revised (CPRS-R), and with a semi-structured detailed interview administered to parents, to collect a series of clinical data such as coexisting somatic and neuropsychiatric problems and familial and pre/peri/postpartum risk factors. We found that 55% of ASD children exceeded the cut-off of CPRS-R Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV), total scale. The univariate analyses showed that children's age (P=0.048), motor delay (P=0.039), enuresis (P=0.014), allergies (P<0.01), comorbid oppositional defiant disorder (P=0.026) and intellectual disabilities comorbidities (P=0.034) were associated to the CPRS-R DSM-IV total score. Some familial predictors such as neuropsychiatric family history of intellectual disabilities (P=0.003) and psychosis (P=0.039) were related to the CPRS-R DSM-IV total score. In particular, a model including allergies (P=0.000) and family history of psychosis (P=0.03) explained 25% (corrected R²=0.25) of the variance of the DSM-IV ADHD score. In conclusion, we identified some risk factors associated with the development of ADHD symptoms in ASD children that need to be studied further.

Analysis of personal and family factors in the persistence of attention deficit hyperactivity disorder: results of a prospective follow-up study in childhood

OBJECTIVES

To study the course of ADHD during childhood and analyze possible personal and family predictor variables of the results.

METHOD

Sixty-one children with ADHD who were between 6 and 12 years old at the baseline assessment were evaluated 30 months later (mean age at baseline: 8.70 ± 1.97; mean age at follow-up: 10.98 ± 2.19). Status of ADHD in follow-up was identified as persistent (met DSM-IV-TR criteria according to parents' and teachers' ratings), contextually persistent (met ADHD criteria according to one informant, and there was functional impairment) and remitted ADHD (with subthreshold clinical symptomatology). Associated psychological disorders of the three groups were analyzed in the follow-up with the Conners' Rating Scales. The groups were compared on ADHD characteristics (symptoms of ADHD and impairment), child psychopathology, executive functioning (EF; inhibition, working memory) and parenting characteristics (parental stress and discipline styles) at baseline.

RESULTS

At the follow-up, 55.7% of the children continued to meet the DSM-IV-TR criteria for ADHD, 29.5% showed contextual persistence, and 14.8% presented remission of the disorder. The persistent and contextually persistent ADHD groups showed more associated psychological disorders. Inattention, oppositional problems, cognitive problems and impairment at baseline distinguished the remitted ADHD children from the persistent and contextually persistent ADHD children. Moreover, the persistent groups had significantly more emotional liability and higher parental stress than the group in remission, while no differences in EF where found among the groups.

CONCLUSIONS

ADHD children continue to present symptoms, as well as comorbid psychological problems, during adolescence and early adulthood. These findings confirm that persistence of ADHD is associated with child psychopathology, parental stress and impairment in childhood.

Association between societal costs and treatment response in children and adolescents with ADHD and their parents. A cross-sectional study in the Netherlands

Attention Deficit Hyperactivity Disorder (ADHD) is associated with considerable burden of illness at a patient, family and societal level. Although pharmacological treatment is recommended by authoritative guidelines, evidence on its influence on the broader burden of illness is limited. As treatment induces costs, proper healthcare decision making requires evidence on the associated societal costs or benefits and particularly the difference that response to treatment can make. Data on ADHD related resource use of patients 8-18 years and parents were collected by means of a cross-sectional, online survey amongst members of the Dutch parent association. Children were stratified to responders and non-responders to treatment according to pre-defined expert definitions. Analyses were performed on 618 questionnaires (428 responders; 190 non-responders to treatment). Children were 11.8 years on average and mainly boys (82 %). Total monthly costs for children were €578 and €839 for responders and non-responders, respectively (p = 0.021), with a breakdown to direct medical costs (€322 vs. €512; p = 0.068), direct non-medical costs (€222 vs. €296; p = 0.090), and indirect non-medical costs (€34 vs. €57; p < 0.001). For parents, total costs were €246 vs. €399 for the responding and non-responding children, respectively (p = 0.006), with a breakdown to direct medical costs (€130 vs. €211; p = 0.010) and indirect non-medical costs (€116 vs. €181; p = 0.092). Total monthly costs of children and their parents together were €824 and €1228 for responders and non-responders to treatment, respectively (p = 0.002). These results stress the importance of a focus on response to treatment, not only beneficial for patients and their family, but also resulting in considerable societal benefits.

A meta-analysis of the prevalence of attention deficit hyperactivity disorder in incarcerated populations

BACKGROUND

Studies report the variable prevalence of attention deficit hyperactivity disorder (ADHD) in incarcerated populations. The aim of this meta-analysis was to determine the prevalence of ADHD in these populations.

METHOD

Primary research studies reporting the prevalence (lifetime/current) of ADHD in incarcerated populations were identified. The meta-analysis used a mixed log-binomial model, including fixed effects for each covariate and a random study effect, to estimate the significance of various risk factors.

RESULTS

Forty-two studies were included in the analysis. ADHD prevalence was higher with screening diagnoses versus diagnostic interview (and with retrospective youth diagnoses versus current diagnoses). Using diagnostic interview data, the estimated prevalence was 25.5% and there were no significant differences for gender and age. Significant country differences were noted.

CONCLUSIONS

Compared with published general population prevalence, there is a fivefold increase in prevalence of ADHD in youth prison populations (30.1%) and a 10-fold increase in adult prison populations (26.2%).

Moderator effects of working memory on the stability of ADHD symptoms by dopamine receptor gene polymorphisms during development

We tested the hypothesis that dopamine D1 and D2 receptor gene (DRD1 and DRD2, respectively) polymorphisms and the development of working memory skills can interact to influence symptom change over 10 years in children with attention-deficit/hyperactivity disorder (ADHD). Specifically, we examined whether improvements in working memory maintenance and manipulation from childhood to early adulthood predicted the reduction of ADHD symptoms as a function of allelic variation in DRD1 and DRD2. Participants were 76 7-11-year-old children with ADHD who were genotyped and prospectively followed for almost 10 years. ADHD symptoms were rated using the Attention Problems scale on the Child Behavior Checklist, and verbal working memory maintenance and manipulation, measured by Digit Span forward and backward, respectively, were assessed at baseline and follow-up. After correction for multiple testing, improvements in working memory manipulation, not maintenance, predicted reduction of symptomatology over development and was moderated by major allele homozygosity in two DRD1 polymorphisms (rs4532 and rs265978) previously linked with variation in D1 receptor expression. Depending on genetic background, developmental factors including age-dependent variation in DRD1 penetrance may facilitate the link between improvements in higher-order working memory and the remission of symptoms in individuals with childhood-diagnosed ADHD. Furthermore, the current findings suggest that DRD1 might contribute minimally to the emergence of symptoms and cognitive difficulties associated with ADHD in childhood, but may act as a modifier gene of these clinical features and outcome during later development for those with ADHD.

The role of sleep problems and circadian clock genes in childhood psychiatric disorders

CLOCK gene research and the analysis of circadian rhythmicity on the behavioural, cellular and molecular level are increasingly contributing to accumulate clinically relevant knowledge in the fields of neuroscience, psychopharmacology and adult psychiatry. However, the role of circadian phenomena, including sleep alterations in mental disorders during childhood and adolescence remains largely enigmatic. Fortunately, recent publications have addressed this problem and there is now some evidence available highlighting the relevance of CLOCK genes in conditions, such as ADHD, mood disorders, schizophrenia and anxiety disorders.

Independent oscillatory patterns determine performance fluctuations in children with attention deficit/hyperactivity disorder

The maintenance of stable goal-directed behaviour is a hallmark of conscious executive control in humans. Notably, both correct and error human actions may have a subconscious activation-based determination. One possible source of subconscious interference may be the default mode network that, in contrast to attentional network, manifests intrinsic oscillations at very low (<0.1 Hz) frequencies. In the present study, we analyse the time dynamics of performance accuracy to search for multisecond periodic fluctuations of error occurrence. Attentional lapses in attention deficit/hyperactivity disorder are proposed to originate from interferences from intrinsically oscillating networks. Identifying periodic error fluctuations with a frequency<0.1 Hz in patients with attention deficit/hyperactivity disorder would provide a behavioural evidence for such interferences. Performance was monitored during a visual flanker task in 92 children (7- to 16-year olds), 47 with attention deficit/hyperactivity disorder, combined type and 45 healthy controls. Using an original approach, the time distribution of error occurrence was analysed in the frequency and time-frequency domains in order to detect rhythmic periodicity. Major results demonstrate that in both patients and controls, error behaviour was characterized by multisecond rhythmic fluctuations with a period of ∼12 s, appearing with a delay after transition to task. Only in attention deficit/hyperactivity disorder, was there an additional 'pathological' oscillation of error generation, which determined periodic drops of performance accuracy each 20-30 s. Thus, in patients, periodic error fluctuations were modulated by two independent oscillatory patterns. The findings demonstrate that: (i) attentive behaviour of children is determined by multisecond regularities; and (ii) a unique additional periodicity guides performance fluctuations in patients. These observations may re-conceptualize the understanding of attentive behaviour beyond the executive top-down control and may reveal new origins of psychopathological behaviours in attention deficit/hyperactivity disorder.

The impact of study design and diagnostic approach in a large multi-centre ADHD study: Part 2: Dimensional measures of psychopathology and intelligence

BACKGROUND

The International Multi-centre ADHD Genetics (IMAGE) project with 11 participating centres from 7 European countries and Israel has collected a large behavioural and genetic database for present and future research. Behavioural data were collected from 1068 probands with ADHD and 1446 unselected siblings. The aim was to describe and analyse questionnaire data and IQ measures from all probands and siblings. In particular, to investigate the influence of age, gender, family status (proband vs. sibling), informant, and centres on sample homogeneity in psychopathological measures.

METHODS

Conners' Questionnaires, Strengths and Difficulties Questionnaires, and Wechsler Intelligence Scores were used to describe the phenotype of the sample. Data were analysed by use of robust statistical multi-way procedures.

RESULTS

Besides main effects of age, gender, informant, and centre, there were considerable interaction effects on questionnaire data. The larger differences between probands and siblings at home than at school may reflect contrast effects in the parents. Furthermore, there were marked gender by status effects on the ADHD symptom ratings with girls scoring one standard deviation higher than boys in the proband sample but lower than boys in the siblings sample. The multi-centre design is another important source of heterogeneity, particularly in the interaction with the family status. To a large extent the centres differed from each other with regard to differences between proband and sibling scores.

CONCLUSIONS

When ADHD probands are diagnosed by use of fixed symptom counts, the severity of the disorder in the proband sample may markedly differ between boys and girls and across age, particularly in samples with a large age range. A multi-centre design carries the risk of considerable phenotypic differences between centres and, consequently, of additional heterogeneity of the sample even if standardized diagnostic procedures are used. These possible sources of variance should be counteracted in genetic analyses either by using age and gender adjusted diagnostic procedures and regional normative data or by adjusting for design artefacts by use of covariate statistics, by eliminating outliers, or by other methods suitable for reducing heterogeneity.

The impact of study design and diagnostic approach in a large multi-centre ADHD study. Part 1: ADHD symptom patterns

BACKGROUND

The International Multi-centre ADHD Genetics (IMAGE) project with 11 participating centres from 7 European countries and Israel has collected a large behavioural and genetic database for present and future research. Behavioural data were collected from 1068 probands with the combined type of attention deficit/hyperactivity disorder (ADHD-CT) and 1446 'unselected' siblings. The aim was to analyse the IMAGE sample with respect to demographic features (gender, age, family status, and recruiting centres) and psychopathological characteristics (diagnostic subtype, symptom frequencies, age at symptom detection, and comorbidities). A particular focus was on the effects of the study design and the diagnostic procedure on the homogeneity of the sample in terms of symptom-based behavioural data, and potential consequences for further analyses based on these data.

METHODS

Diagnosis was based on the Parental Account of Childhood Symptoms (PACS) interview and the DSM-IV items of the Conners' teacher questionnaire. Demographics of the full sample and the homogeneity of a subsample (all probands) were analysed by using robust statistical procedures which were adjusted for unequal sample sizes and skewed distributions. These procedures included multi-way analyses based on trimmed means and winsorised variances as well as bootstrapping.

RESULTS

Age and proband/sibling ratios differed between participating centres. There was no significant difference in the distribution of gender between centres. There was a significant interaction between age and centre for number of inattentive, but not number of hyperactive symptoms. Higher ADHD symptom frequencies were reported by parents than teachers. The diagnostic symptoms differed from each other in their frequencies. The face-to-face interview was more sensitive than the questionnaire. The differentiation between ADHD-CT probands and unaffected siblings was mainly due to differences in hyperactive/impulsive symptoms.

CONCLUSIONS

Despite a symptom-based standardized inclusion procedure according to DSM-IV criteria with defined symptom thresholds, centres may differ markedly in probands' ADHD symptom frequencies. Both the diagnostic procedure and the multi-centre design influence the behavioural characteristics of a sample and, thus, may bias statistical analyses, particularly in genetic or neurobehavioral studies.

Polymorphisms of the steroid sulfatase (STS) gene are associated with attention deficit hyperactivity disorder and influence brain tissue mRNA expression

Previous studies in animals and humans have implicated the X-chromosome STS gene in the etiology of attentional difficulties and attention deficit hyperactivity disorder (ADHD). This family based association study has fine mapped a region of the STS gene across intron 1 and 2 previously associated with ADHD, in an extended sample of 450 ADHD probands and their parents. Significant association across this region is demonstrated individually with 7 of the 12 genotyped SNPs, as well as an allele specific haplotype of the 12 SNPs. The over transmitted risk allele of rs12861247 was also associated with reduced STS mRNA expression in normal human post-mortem frontal cortex brain tissue compared to the non-risk allele (P = 0.01). These results are consistent with the hypothesis arising from previous literature demonstrating that boys with deletions of the STS gene, and hence no STS protein are at a significantly increased risk of developing ADHD. Furthermore, this study has established the brain tissue transcript of STS, which except from adipose tissue, differs from that seen in all other tissues investigated. © 2010 Wiley-Liss, Inc.

Separation of cognitive impairments in attention-deficit/hyperactivity disorder into 2 familial factors

CONTEXT

Attention-deficit/hyperactivity disorder (ADHD) is associated with widespread cognitive impairments, but it is not known whether the apparent multiple impairments share etiological roots or separate etiological pathways exist. A better understanding of the etiological pathways is important for the development of targeted interventions and for identification of suitable intermediate phenotypes for molecular genetic investigations.

OBJECTIVES

To determine, by using a multivariate familial factor analysis approach, whether 1 or more familial factors underlie the slow and variable reaction times, impaired response inhibition, and choice impulsivity associated with ADHD.

DESIGN

An ADHD and control sibling-pair design.

SETTING

Belgium, Germany, Ireland, Israel, Spain, Switzerland, and the United Kingdom.

PARTICIPANTS

A total of 1265 participants, aged 6 to 18 years: 464 probands with ADHD and 456 of their siblings (524 with combined-subtype ADHD), and 345 control participants.

MAIN OUTCOME MEASURES

Performance on a 4-choice reaction time task, a go/no-go inhibition task, and a choice-delay task.

RESULTS

The final model consisted of 2 familial factors. The larger factor, reflecting 85% of the familial variance of ADHD, captured 98% to 100% of the familial influences on mean reaction time and reaction time variability. The second, smaller factor, reflecting 13% of the familial variance of ADHD, captured 62% to 82% of the familial influences on commission and omission errors on the go/no-go task. Choice impulsivity was excluded in the final model because of poor fit.

CONCLUSIONS

The findings suggest the existence of 2 familial pathways to cognitive impairments in ADHD and indicate promising cognitive targets for future molecular genetic investigations. The familial distinction between the 2 cognitive impairments is consistent with recent theoretical models--a developmental model and an arousal-attention model--of 2 separable underlying processes in ADHD. Future research that tests the familial model within a developmental framework may inform developmentally sensitive interventions.

Twin studies and their implications for molecular genetic studies: endophenotypes integrate quantitative and molecular genetics in ADHD research

OBJECTIVE

To describe the utility of twin studies for attention-deficit/hyperactivity disorder (ADHD) research and demonstrate their potential for the identification of alternative phenotypes suitable for genomewide association, developmental risk assessment, treatment response, and intervention targets.

METHOD

Brief descriptions of the classic twin study and genetic association study methods are provided, with illustrative findings from ADHD research. Biometrical genetics refers to the statistical modeling of data gathered from one or more group of known biological relation; it was apparently coined by Francis Galton in the 1860s and led to the "Biometrical School" at the University of London. Twin studies use genetic correlations between pairs of relatives, derived using this theoretical framework, to parse the individual differences in a trait into latent (unmeasured) genetic and environmental influences. This method enables the estimation of heritability, i.e., the percentage of variance due to genetic influences. It is usually implemented with a method called structural equation modeling, which is a statistical technique for fitting models to data, typically using maximum likelihood estimation. Genetic association studies aim to identify those genetic variants that account for the heritability estimated in twin studies. Measurements other than those used for the clinical diagnosis of the disorder are popular phenotype choices in current ADHD research. It is argued that twin studies have great potential to refine phenotypes relevant to ADHD.

RESULTS

Prior studies have consistently found that the majority of the variance in ADHD symptoms is due to genetic factors. To date, genomewide association studies of ADHD have not identified replicable associations that account for the heritable variation. Possibly, the application of genomewide association studies to these alternative phenotypic measurements will assist in identifying the pathways from genetic variants to ADHD.

CONCLUSION

Power to detect associations should be improved by the study of highly heritable endophenotypes for ADHD and by reducing the number of phenotypes to be considered. Therefore, twin studies are an important research tool in the development of endophenotypes, defined as alternative, more highly heritable traits that act at earlier stages of the pathway from genes to behavior. Although genetic variation in liability to ADHD is likely polygenic, the proposed approach should help to identify improved alternative measurements for genetic association studies.

Association study between a polymorphism at the 3'-untranslated region of CLOCK gene and attention deficit hyperactivity disorder

Background

The circadian locomotor output cycles kaput (CLOCK) gene encodes protein regulation circadian rhythm and also plays some roles in neural transmitter systems including the dopamine system. Several lines of evidence implicate a relationship between attention-deficit hyperactivity disorder (ADHD), circadian rythmicity and sleeping disturbances. A recent study has reported that a polymorphism (rs1801260) at the 3'-untranslated region of the CLOCK gene is associated with adult ADHD.

Methods

To investigate the association between the polymorphism (rs1801260) in ADHD, two samples of ADHD probands from the United Kingdom (n = 180) and Taiwan (n = 212) were genotyped and analysed using within-family transmission disequilibrium test (TDT). Bonferroni correction procedures were used to just for multiple comparisons.

Results

We found evidence of increased transmission of the T allele of the rs1801260 polymorphism in Taiwanese samples (P = 0.010). There was also evidence of preferential transmission of the T allele of the rs1801260 polymorphism in combined samples from the Taiwan and UK (P = 0.008).

Conclusion

This study provides evidence for the possible involvement of CLOCK in susceptibility to ADHD.

DRD4 and DAT1 in ADHD: Functional neurobiology to pharmacogenetics

Attention deficit/hyperactivity disorder (ADHD) is a common and potentially very impairing neuropsychiatric disorder of childhood. Statistical genetic studies of twins have shown ADHD to be highly heritable, with the combination of genes and gene by environment interactions accounting for around 80% of phenotypic variance. The initial molecular genetic studies where candidates were selected because of the efficacy of dopaminergic compounds in the treatment of ADHD were remarkably successful and provided strong evidence for the role of DRD4 and DAT1 variants in the pathogenesis of ADHD. However, the recent application of non-candidate gene strategies (eg, genome-wide association scans) has failed to identify additional genes with substantial genetic main effects, and the effects for DRD4 and DAT1 have not been replicated. This is the usual pattern observed for most other physical and mental disorders evaluated with current state-of-the-art methods. In this paper we discuss future strategies for genetic studies in ADHD, highlighting both the pitfalls and possible solutions relating to candidate gene studies, genome-wide studies, defining the phenotype, and statistical approaches.

Decreased Serum Cu/Zn SOD Associated with High Copper in Children with Attention Deficit Hyperactivity Disorder (ADHD)

Aim

To assess serum Cu/Zn SOD (Superoxide Dismutase) concentration in children with ADHD and evaluate its possible relationship to Cu and Zn levels.

Subjects and methods

Serum from 22 children with ADHD and 20 healthy control children without ADHD and 19 autistic children without ADHD were tested for Cu/Zn SOD using ELISAs and levels of serum Cu and Zn using inductively-coupled plasma-mass spectrometry.

Results

Serum Cu/Zn SOD levels of ADHD children were significantly lower than age and gender matched healthy non-ADHD controls (P < 0.001). Serum Cu/Zn SOD of ADHD children was significantly lower in individuals with high serum copper (P = 0.024). There was no significant correlation between Cu/Zn SOD levels and Zinc or Cu/Zn in ADHD individuals.

Discussion

These results suggest an association between Cu/Zn SOD serum levels and ADHD, particularly ADHD children with high serum copper.

Negative priming within a stroop task in children and adolescents with attention-deficit hyperactivity disorder, their siblings, and independent controls

OBJECTIVE

Negative priming (NP) is the slowed response to a stimulus that was previously ignored. Response times in NP task conditions were compared with the interference provided by congruent/incongruent stimuli in a Stroop condition in the same task in children diagnosed with attention-deficit hyperactivity disorder (ADHD), their unaffected siblings, and independent controls.

METHOD

Speed, accuracy, and variability of responses were compared using a computerized NP Stroop test for 35 children with ADHD, 24 siblings without diagnosis, and 37 independent healthy controls aged 6 to 17 years.

RESULTS

NP was evident at test onset for congruent trials in children without a diagnosis and was reduced initially in those with ADHD occurring in the absence of a significant Stroop interference effect and independently of age or symptom severity. Incongruency masked NP effects. Cases showed more intraindividual response-time variability.

CONCLUSIONS

Both NP in normal children and its reduction in ADHD cases attenuated across trials reflecting the increased facilitation from previous stimulation.

Shared heritability of attention-deficit/hyperactivity disorder and autism spectrum disorder

Attention-deficit/hyperactivity disorder (ADHD) and autism spectrum disorder (ASD) are both highly heritable neurodevelopmental disorders. Evidence indicates both disorders co-occur with a high frequency, in 20-50% of children with ADHD meeting criteria for ASD and in 30-80% of ASD children meeting criteria for ADHD. This review will provide an overview on all available studies [family based, twin, candidate gene, linkage, and genome wide association (GWA) studies] shedding light on the role of shared genetic underpinnings of ADHD and ASD. It is concluded that family and twin studies do provide support for the hypothesis that ADHD and ASD originate from partly similar familial/genetic factors. Only a few candidate gene studies, linkage studies and GWA studies have specifically addressed this co-occurrence, pinpointing to some promising pleiotropic genes, loci and single nucleotide polymorphisms (SNPs), but the research field is in urgent need for better designed and powered studies to tackle this complex issue. We propose that future studies examining shared familial etiological factors for ADHD and ASD use a family-based design in which the same phenotypic (ADHD and ASD), candidate endophenotypic, and environmental measurements are obtained from all family members. Multivariate multi-level models are probably best suited for the statistical analysis.

Performance variability, impulsivity errors and the impact of incentives as gender-independent endophenotypes for ADHD

BACKGROUND

Attention-deficit hyperactivity disorder (ADHD) is one of the most common and highly heritable child psychiatric disorders. There is strong evidence that children with ADHD show slower and more variable responses in tasks such as Go/Nogo tapping aspects of executive functions like sustained attention and response control which may be modulated by motivational factors and/or state-regulation processes. The aim of this study was (1) to determine if these executive functions may constitute an endophenotype for ADHD; (2) to investigate for the first time whether known modulators of these executive functions may also be familial; and (3) to explore whether gender has an impact on these measures.

METHODS

Two hundred and five children with ADHD combined type, 173 nonaffected biological siblings and 53 controls with no known family history of ADHD were examined using a Go/Nogo task in the framework of a multi-centre study. Performance-measures and modulating effects of event-rate and incentives were examined. Shared familial effects on these measures were assessed, and the influence of gender was tested.

RESULTS

Children with ADHD responded more slowly and variably than nonaffected siblings or controls. Nonaffected siblings showed intermediate scores for reaction-time variability, false alarms and omission errors under fast and slow event-rates. A slower event-rate did not lead to reduced performance specific to ADHD. In the incentive condition, mean reaction-times speeded up and became less variable only in children with ADHD and their nonaffected siblings, while accuracy was improved in all groups. Males responded faster, but also committed more false alarms. There were no interactions of group by gender.

CONCLUSIONS

Reaction-time variability and accuracy parameters could be useful neuropsychological endophenotypes for ADHD. Performance-modulating effects of incentives suggested a familially driven motivational dysfunction which may play an important role on etiologic pathways and treatment approaches for ADHD. The effects of gender were independent of familial effects or ADHD-status, which in turn suggests that the proposed endophenotypes are independent of gender.

NK1 (TACR1) receptor gene 'knockout' mouse phenotype predicts genetic association with ADHD

Mice with functional genetic ablation of the Tacr1 (substance P-preferring receptor) gene (NK1R-/-) are hyperactive. Here, we investigated whether this is mimicked by NK1R antagonism and whether dopaminergic transmission is disrupted in brain regions that govern motor performance. The locomotor activity of NK1R-/- and wild-type mice was compared after treatment with an NK1R antagonist and/or psychostimulant (d-amphetamine or methylphenidate). The inactivation of NK1R (by gene mutation or receptor antagonism) induced hyperactivity in mice, which was prevented by both psychostimulants. Using in vivo microdialysis, we then compared the regulation of extracellular dopamine in the prefrontal cortex (PFC) and striatum in the two genotypes. A lack of functional NK1R reduced (>50%) spontaneous dopamine efflux in the prefrontal cortex and abolished the striatal dopamine response to d-amphetamine. These behavioural and neurochemical abnormalities in NK1R-/- mice, together with their atypical response to psychostimulants, echo attention deficit hyperactivity disorder (ADHD) in humans. These findings prompted genetic studies on the TACR1 gene (the human equivalent of NK1R) in ADHD patients in a case-control study of 450 ADHD patients and 600 screened supernormal controls. Four single-nucleotide polymorphisms (rs3771829, rs3771833, rs3771856, and rs1701137) at the TACR1 gene, previously known to be associated with bipolar disorder or alcoholism, were strongly associated with ADHD. In conclusion, our proposal that NK1R-/- mice offer a mouse model of ADHD was borne out by our human studies, which suggest that DNA sequence changes in and around the TACR1 gene increase susceptibility to this disorder.

Duration discrimination in the range of milliseconds and seconds in children with ADHD and their unaffected siblings

BACKGROUND

Detecting genetic factors involved in attention deficit hyperactivity disorder (ADHD) is complicated because of their small effect sizes and complex interactions. The endophenotype approach eases this by coming closer to the relevant genes. Different aspects of temporal information processing are known to be affected in ADHD. Thus, some of these aspects could represent candidate endophenotypes for ADHD.

METHOD

Fifty-four sib-pairs with at least one child with ADHD and 40 control children aged 6-18 years were recruited and asked to perform two duration discrimination tasks, one with a base duration of 50 ms on automatic timing and one with a base duration of 1000 ms on cognitively controlled timing.

RESULTS

Whereas children with ADHD, but not their unaffected siblings, were impaired in discrimination of longer intervals, both groups were impaired in discriminating brief intervals. Furthermore, a significant within-family correlation was found for discrimination of brief intervals. Task performances of subjects of the control group correlated with individual levels of hyperactivity/impulsivity for discrimination of brief intervals, but not of longer intervals.

CONCLUSIONS

Cognitively controlled and also automatic processes of temporal information processing are impaired in children with ADHD. Discrimination of longer intervals appears as a typical 'disease marker' whereas discrimination of brief intervals shows up as a 'vulnerability marker'. Discrimination of brief intervals was found to be familial and linked to levels of hyperactivity/impulsivity. Taken together, discrimination of brief intervals represents a candidate endophenotype of ADHD.

Investigation of the serotonin 2C receptor gene in attention deficit hyperactivity disorder in UK samples

Background

Attention deficit hyperactivity disorder (ADHD) is a common, childhood-onset neurodevelopmental disorder that is more frequent in males than females. Several genes on the X chromosome have been studied as candidate risk factors for ADHD including the 5-HT_2C receptor (HTR2C) gene. Association between polymorphisms in HTR2C and ADHD were reported in a recent study.

Findings

In this study we investigated the association between ADHD and two polymorphisms C-759T (rs3813929) and G-697C (rs518147) in the promoter region of the HTR2C gene using a sample of 180 UK ADHD probands and their parents. We have shown that the -697G allele was significantly over-transmitted to affected ADHD probands (P = 0.017). No association was detected between the C-759T polymorphism and ADHD. Haplotype analysis of the two markers revealed no significantly increased transmission of any haplotype to ADHD.

Conclusion

The findings provide evidence that the G-allele of the G-697C HTR2C polymorphism may be involved in the development of ADHD. The results replicate one of the findings published recently.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

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.

DSM-IV combined type ADHD shows familial association with sibling trait scores: a sampling strategy for QTL linkage

Attention deficit hyperactivity disorder (ADHD) is a discrete clinical syndrome characterized by the triad of inattention, hyperactivity, and impulsivity in the context of marked impairments. Molecular genetic studies have been successful in identifying genetic variants associated with ADHD, particularly with DSM-IV inattentive and combined subtypes. Quantitative trait locus (QTL) approaches to linkage and association mapping have yet to be widely used in ADHD research, although twin studies investigating individual differences suggest that genetic liability for ADHD is continuously distributed throughout the population, underscoring the applicability of quantitative dimensional approaches. To investigate the appropriateness of QTL approaches, we tested the familial association between 894 probands with a research diagnosis of DSM-IV ADHD combined type and continuous trait measures among 1,135 of their siblings unselected for phenotype. The sibling recurrence rate for ADHD combined subtype was 12.7%, yielding a sibling recurrence risk ratio (lambda(sib)) of 9.0. Estimated sibling correlations around 0.2-0.3 are similar to those estimated from the analysis of fraternal twins in population twin samples. We further show that there are no threshold effects on the sibling risk for ADHD among the ADHD probands; and that both affected and unaffected siblings contributed to the association with ADHD trait scores. In conclusion, these data confirm the main requirement for QTL mapping of ADHD by demonstrating that narrowly defined DSM-IV combined type probands show familial association with dimensional ADHD symptom scores amongst their siblings.

Genetic heterogeneity in ADHD: DAT1 gene only affects probands without CD

Previous studies have found heterogeneous association between DAT1-3'-UTR-VNTR and attention deficit hyperactivity disorder (ADHD). Various proportions of conduct disorder (CD) comorbidity in their ADHD samples may partially explain the observational discrepancies. Evidence for this comes from family and twin studies which found ADHD probands with CD (ADHD + CD) are genetically different from those without CD (ADHD - CD). Genotypes of 20 DAT1 markers were analyzed in 576 trios, consisting of 141 ADHD + CD and 435 ADHD - CD. In addition to the classical TDT test, a specific genetic heterogeneity test was performed to identify variants that have different transmission patterns in the two phenotypic subgroups. After multiple-test correction, rs40184 and rs2652511 were significant in TDT tests. Further heterogeneity test found the two SNPs had a significant transmission pattern difference between ADHD + CD and ADHD - CD children, indicating that DAT1 has a significantly greater genetic influence on ADHD without CD. Although the result needs further replications, it does highlight the importance of selecting genetically homogeneous samples for molecular genetic analyses of ADHD.

Parent of origin effects in attention/deficit hyperactivity disorder (ADHD): analysis of data from the international multicenter ADHD genetics (IMAGE) program

There are conflicting reports suggesting that the parental origin of transmitted risk alleles may play a role in the etiology of attention deficit/hyperactivity disorder (ADHD). A recent report by Hawi and colleagues observed a generalized paternal over-transmission of alleles associated with ADHD. This was not replicated in more recent studies. Using data from a large multicenter study we examined the overall and gene-specific parent of origin effect in 554 independent SNPs across 47 genes. Transmission disequilibrium and explicit parent of origin test were performed using PLINK. Overall parent of origin effect was tested by Chi-square. There was no overall parent of origin effect in the IMAGE sample (chi(1)(2) = 1.82, P = 0.117). Five markers in three genes, DDC, TPH2, and SLC6A2 showed nominal association (P < 0.01) with ADHD combined subtype when restricted to maternal or paternal transmission only. Following the initial report by Hawi and co-workers three studies, including this one, found no evidence to support an overall parent of origin effect for markers associated with ADHD. We cannot however, exclude gene-specific parent of origin effect in the etiology ADHD.

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

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

Linkage to chromosome 1p36 for attention-deficit/hyperactivity disorder traits in school and home settings

BACKGROUND

Limited success has been achieved through previous attention-deficit/hyperactivity disorder (ADHD) linkage scans, which were all designed to map genes underlying the dichotomous phenotype. The International Multi-centre ADHD Genetics (IMAGE) project performed a whole genome linkage scan specifically designed to map ADHD quantitative trait loci (QTL).

METHODS

A set of 1094 single selected Caucasian ADHD nuclear families was genotyped on a highly accurate and informative single nucleotide polymorphism (SNP) panel. Two quantitative traits measuring the children's symptoms in home and school settings were collected and standardized according to a population sample of 8000 children to reflect the developmental nature and gender prevalence difference of ADHD. Univariate linkage test was performed on both traits and their mean score.

RESULTS

A significant common linkage locus was found at chromosome 1p36 with a locus-specific heritability of 5.1% and a genomewide empirical p < .04. Setting-specific suggestive linkage signals were also found: logarithm of odds (LOD) = 2.2 at 9p23 for home trait and LOD = 2.6 at 11q21 for school trait.

CONCLUSIONS

These results indicate that given large samples with proper phenotypic measures, searching for ADHD genes with a QTL strategy is an important alternative to using the clinical diagnosis. The fact that our linkage region 1p36 overlaps with the dyslexia QTL DYX8 further suggests it is potentially a pleiotropic locus for ADHD and dyslexia.

Action monitoring in boys with attention-deficit/hyperactivity disorder, their nonaffected siblings, and normal control subjects: evidence for an endophenotype

BACKGROUND

Attention-deficit/hyperactivity disorder (ADHD) is a very common and highly heritable child psychiatric disorder associated with dysfunctions in fronto-striatal networks that control attention and response organization. The aim of this study was to investigate whether features of action monitoring related to dopaminergic functions represent endophenotypes that are brain functions on the pathway from genes and environmental risk factors to behavior.

METHODS

Action monitoring and error processing as indicated by behavioral and electrophysiological parameters during a flanker task were examined in boys with ADHD combined type according to DSM-IV (n = 68), their nonaffected siblings (n = 18), and healthy control subjects with no known family history of ADHD (n = 22).

RESULTS

Boys with ADHD displayed slower and more variable reaction-times. Error negativity (Ne) was smaller in boys with ADHD compared with healthy control subjects, whereas nonaffected siblings displayed intermediate amplitudes following a linear model predicted by genetic concordance. The three groups did not differ on error positivity (Pe). The N2 amplitude enhancement due to conflict (incongruent flankers) was reduced in the ADHD group. Nonaffected siblings also displayed intermediate N2 enhancement.

CONCLUSIONS

Converging evidence from behavioral and event-related potential findings suggests that action monitoring and initial error processing, both related to dopaminergically modulated functions of anterior cingulate cortex, might be an endophenotype related to ADHD.

High heritability for a composite index of children's activity level measures

Despite the high heritability of children's activity level, which forms part of the core symptom domain of hyperactivity-impulsivity within attention deficit hyperactivity disorder (ADHD), there has only been a limited success with identifying candidate genes involved in its etiology. This may reflect a lack of understanding about the different measures used to define activity level across studies. We aimed to study the genetic and environmental etiology across three measures of activity level: parent and teacher ratings of hyperactivity-impulsivity and actigraph measurements, within a population-based sample of 463 7-9 year old twin pairs. We further examined ways in which the three measures could be combined for future molecular studies. Phenotypic correlations across measures were modest, but a common underlying phenotypic factor was highly heritable (92%); as was a simple aggregation of all three measurements (77%). This suggests that distilling what is common to all three measures may be a good method for generating a quantitative trait suitable for molecular studies of activity level in children. The high heritabilities found are encouraging in this respect.

Intelligence in DSM-IV combined type attention-deficit/hyperactivity disorder is not predicted by either dopamine receptor/transporter genes or other previously identified risk alleles for attention-deficit/hyperactivity disorder

A major goal of genetic studies of attention deficit hyperactivity disorder (ADHD) is to identify individual characteristics that might help segregate the disorder's inherent heterogeneity. [Mill et al. (2006); Arch Ger Psychiatry 63:462-469] recently reported a potentially important association between two dopamine-related risk polymorphisms (DRD4 variable number tandem repeat (VNTR) in exon 3 and DAT1 VNTR in the 3' UTR) and lowered IQ in ADHD. The objective of the current study was to replicate the [Mill et al. (2006); Arch Ger Psychiatry 63:462-469] findings in a clinical sample and to extend the analysis to a large range of alternative SNP markers of putative ADHD risk alleles identified in a recent study [Brookes et al. (2006); Mol Genet 11:934-953]. Participants were 1081 children and adolescents with a research-confirmed combined type ADHD diagnosis and 1300 unaffected siblings who took part in the International Multi-centre ADHD Genetics (IMAGE) project. They were recruited from multiple settings from across Europe: Belgium, Britain, Germany, Ireland, Israel, Netherlands, Spain and Switzerland. The results were that ADHD was associated with reduced IQ. However, there was no association between the two dopamine-related risk polymorphisms and IQ in either the probands or their siblings. Furthermore, other selected genetic markers previously demonstrated to be associated with ADHD in this sample were not associated with IQ. This large scale study with a clinically ascertained and regorously diagnosed sample failed to replicate the association between genetic polymorphisms in the dopamine system and IQ in ADHD. We also observed no association of other SNPs with IQ in ADHD.

Co-transmission of conduct problems with attention-deficit/hyperactivity disorder: familial evidence for a distinct disorder

Common disorders of childhood and adolescence are attention-deficit/hyperactivity disorder (ADHD), oppositional defiant disorder (ODD) and conduct disorder (CD). For one to two cases in three diagnosed with ADHD the disorders may be comorbid. However, whether comorbid conduct problems (CP) represents a separate disorder or a severe form of ADHD remains controversial. We investigated familial recurrence patterns of the pure or comorbid condition in families with at least two children and one definite case of DSM-IV ADHDct (combined-type) as part of the International Multicentre ADHD Genetics Study (IMAGE). Using case diagnoses (PACS, parental account) and symptom ratings (Parent/Teacher Strengths and Difficulties [SDQ], and Conners Questionnaires [CPTRS]) we studied 1009 cases (241 with ADHDonly and 768 with ADHD + CP), and their 1591 siblings. CP was defined as > or =4 on the SDQ conduct-subscale, and T > or = 65, on Conners' oppositional-score. Multinomial logistic regression was used to ascertain recurrence risks of the pure and comorbid conditions in the siblings as predicted by the status of the cases. There was a higher relative risk to develop ADHD + CP for siblings of cases with ADHD + CP (RRR = 4.9; 95%CI: 2.59-9.41); p < 0.001) than with ADHDonly. Rates of ADHDonly in siblings of cases with ADHD + CP were lower but significant (RRR = 2.9; 95%CI: 1.6-5.3, p < 0.001). Children with ADHD + CP scored higher on the Conners ADHDct symptom-scales than those with ADHDonly. Our finding that ADHD + CP can represent a familial distinct subtype possibly with a distinct genetic etiology is consistent with a high risk for cosegregation. Further, ADHD + CP can be a more severe disorder than ADHDonly with symptoms stable from childhood through adolescence. The findings provide partial support for the ICD-10 distinction between hyperkinetic disorder (F90.0) and hyperkinetic conduct disorder (F90.1).

Attention deficit hyperactivity disorder

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

For others: Epistasis and the evolutionary survival of an extreme tail of the quantitative distribution of autistic assets

The ongoing paradigm shift from the traditional qualitative dichotomy concept to the quantitative framework increases the necessity of an evolutionary implication and interpretation of the presence of a hypo-reproductive behavioral extreme (autism) with strong genetic contribution. As a theoretical challenge to explain the survival of the dimensional distribution of autistic traits, an epistasis-associated oscillation of fitness outcomes is proposed. In this hypothesis, an allele could contribute to the existence of both phenotypic extreme tails and the hypothesized genetic machinery (quantitative trait loci) for autism would necessarily be common in the entire human population. The postulated autism genes would allow autistics to enjoy autistic traits and assets and all of the residual non-autistic individuals could owe their social skills and reproductive advantages to the same autism genes. Importantly, the reported modest correlations between core autistic dimensions can be illustrated using unsynchronized epistatic pleiotropy.

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

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

Current issues and trends in the diagnosis and treatment of adults with ADHD

Attention-deficit/hyperactivity disorder (ADHD) has been commonly thought of as a childhood disorder that diminished over time. It is one of the most common developmental disorders and it is estimated that ADHD affects 5-10% of children. Two-thirds of children with ADHD will continue to have symptoms of ADHD that persist throughout adolescence. Longitudinal studies have demonstrated that symptoms of ADHD can also remain in adulthood, affecting 4.4% of the adult population. However, diagnosing adults with ADHD can prove difficult because they often find that their symptoms are egosyntonic. In addition, the development of comorbid conditions, such as anxiety, depression, personality disorders or substance abuse, can often overshadow underlying ADHD symptoms. Nonetheless, treatments such as stimulant and nonstimulant medication (e.g., atomoxetine), and cognitive-behavior therapy have been effective in treating adults with ADHD. This article reviews the prevalence of adults with ADHD, followed by a discussion of the neurobiological and genetic underpinnings of the disorder. Issues regarding the diagnosis and treatment of ADHD are also addressed.

Functional MRI in ADHD: a systematic literature review

Functional MRI (fMRI) research in attention-deficit/hyperactivity disorder (ADHD) is a fast developing and very complex field. Every study appears to show differences in patterns of brain activation between cases and controls, but the interpretation of such differences is not as straightforward as it may seem. We present here a systematic review of the fMRI literature in ADHD; areas covered include executive functions, reward processing, the effects of methylphenidate, comorbidity and spontaneous brain activity in the resting state. To facilitate the interpretation of research in this area, we discuss important conceptual issues, such as the need to take group differences in performance into account or to consider the role of errors. We present common themes that emerge from these studies and we discuss possible reasons for the many discrepancies that were observed. Finally, based on existing literature and current advancements in fMRI research, we discuss the role that fMRI could play in the future as a diagnostic tool or in treatment outcome predictions, and we make predictions for the future directions of research in this field.

Association study between the monoamine oxidase A gene and attention deficit hyperactivity disorder in Taiwanese samples

BACKGROUND

Attention deficit hyperactivity disorder (ADHD) is a common and highly heritable disorder of childhood characterized by inattention, hyperactivity and impulsivity. Molecular genetic and pharmacological studies suggest the involvement of the dopaminergic, serotonergic and noradrenergic neurotransmitter systems in the pathogenesis of ADHD. Monoamine oxidase A (MAO-A) encodes an enzyme that degrades biogenic amines, including neurotransmitters such as norepinephrine, dopamine and serotonin. In this study we examined a 30 bp promoter variable number tandem repeat (VNTR) and a functional G/T single nucleotide polymorphism (SNP) at position 941 in exon 8 (941G/T) of MAO-A for association with ADHD in a Taiwanese sample of 212 ADHD probands.

METHODS

Within-family transmission disequilibrium test (TDT) was used to analyse association of MAO-A polymorphisms with ADHD in a Taiwanese population.

RESULTS

A nominally significant association was found between the G-allele of 941G/T in MAO-A and ADHD (TDT: P = 0.034. OR = 1.57). Haplotype analysis identified increased transmission of a haplotype consisting of the 3-repeat allele of the promoter VNTR and the G-allele of the 941G/T SNP (P = 0.045) to ADHD cases which the strong association with the G-allele drove.

CONCLUSION

These findings suggest the importance of the 941G/T MAO-A polymorphism in the development of ADHD in the Taiwanese population. These results replicate previously published findings in a Caucasian sample.

Family-based association study between brain-derived neurotrophic factor gene polymorphisms and attention deficit hyperactivity disorder in UK and Taiwanese samples

Brain-derived neurotrophic factor (BDNF) plays an important role in normal neuronal development. Several lines of evidence implicate the involvement of BDNF in attention-deficit hyperactivity disorder (ADHD). This study investigated the role of two common BDNF variants (Val66Met, C270T) in two samples of ADHD probands from the United Kingdom (n = 180) and Taiwan (n = 212). We found evidence of increased transmission of the C allele of the C270T in Taiwanese samples (TDT: chi(2) = 6.78, P = 0.009) and the two samples pooled together (TDT: chi(2) = 7.24, P = 0.007). No association was found between the Val66Met polymorphism and ADHD in either of the two populations. Analysis of haplotypes demonstrated a significant decreased transmission of haplotypes containing the Val66 allele and the 270T allele in the Taiwanese samples (TDT: chi(2) = 4.57, P = 0.032) and the pooled sample set (TDT: chi(2) = 5.82, P = 0.016). This study provides evidence for the possible involvement of BDNF in susceptibility to ADHD.

Adult attention-deficit hyperactivity disorder: recognition and treatment in general adult psychiatry

Attention-deficit hyperactivity disorder (ADHD) is a common disorder affecting children and adults. Many young people treated with stimulants, as well as those in whom ADHD went unrecognised in childhood, need treatment as adults. Stimulants and atomoxetine effectively reduce ADHD symptoms at all ages and should be a standard treatment in general adult psychiatric practice.

Endophenotype approach to developmental psychopathology: implications for autism research

This paper discusses the utility of the endophenotype approach in the study of developmental psychopathology. It is argued that endophenotype research holds considerable promise for the study of gene-brain/cognition-behaviour pathways for developmental disorders. This paper outlines the criteria for determining useful endophenotypes. Possible endophenotypes for autism are discussed as an example of an area where endophenotype research on developmental disorders may be fruitful. It is concluded that although the endophenotype approach holds promise for the study of gene-brain/cognition-behaviour pathways, much work remains to be done in order to validate endophenotype measures. It is also noted that the changing nature of any developmental psychopathology poses a particular challenge to this type of research.

The IMAGE project: methodological issues for the molecular genetic analysis of ADHD

The genetic mechanisms involved in attention deficit hyperactivity disorder (ADHD) are being studied with considerable success by several centres worldwide. These studies confirm prior hypotheses about the role of genetic variation within genes involved in the regulation of dopamine, norepinephrine and serotonin neurotransmission in susceptibility to ADHD. Despite the importance of these findings, uncertainties remain due to the very small effects sizes that are observed. We discuss possible reasons for why the true strength of the associations may have been underestimated in research to date, considering the effects of linkage disequilibrium, allelic heterogeneity, population differences and gene by environment interactions.

With the identification of genes associated with ADHD, the goal of ADHD genetics is now shifting from gene discovery towards gene functionality – the study of intermediate phenotypes ('endophenotypes'). We discuss methodological issues relating to quantitative genetic data from twin and family studies on candidate endophenotypes and how such data can inform attempts to link molecular genetic data to cognitive, affective and motivational processes in ADHD.

The International Multi-centre ADHD Gene (IMAGE) project exemplifies current collaborative research efforts on the genetics of ADHD. This European multi-site project is well placed to take advantage of the resources that are emerging following the sequencing of the human genome and the development of international resources for whole genome association analysis. As a result of IMAGE and other molecular genetic investigations of ADHD, we envisage a rapid increase in the number of identified genetic variants and the promise of identifying novel gene systems that we are not currently investigating, opening further doors in the study of gene functionality.

A QTL on rat chromosome 7 modulates prepulse inhibition, a neuro-behavioral trait of ADHD, in a Lewis x SHR intercross

Background

Attention deficit hyperactivity disorder (ADHD) is a complex neuropsychiatric disorder with a substantial genetic component. The Spontaneously Hypertensive Rats (SHR), considered as a good animal model of ADHD, also show less anxiety-like behaviors than Lewis (LEW) rats. The use of these inbred rat strains led us to the mapping of two quantitative trait loci (QTL), named Ofil1 (on chromosome 4) and Ofil2 (on chromosome 7), related to locomotion in the central and aversive area of an open field. Herein, we examined whether LEW and SHR rats differ in the acoustic startle reflex, a test used to study the neurobiology of anxiety, and in the prepulse inhibition of the startle response, which is known to be impaired in ADHD patients. The effect of the two aforementioned loci on these behavioral responses was also studied.

Methods

For this latter purpose, rats deriving from an F2 intercross between the LEW and SHR strains were selected according to their genotype at markers flanking the QTLs and bred to obtain lines of rats homozygous LEW/LEW or SHR/SHR for each of the two loci, thus generating 4 genotypic combinations.

Results

The SHR rats displayed decreased startle and prepulse inhibition levels when compared to LEW rats. Ofil2 affected prepulse inhibition in female rats only.

Conclusion

The results suggest that the LEW and SHR strains are appropriate for studying mechanisms of sensorimotor gating and indicate that the locus Ofil2 on rat chromosome 7 contain genes controlling prepulse inhibition, a neuro-behavioral trait of ADHD.

Clinical assessment and treatment of attention deficit hyperactivity disorder in adults

Attention deficit hyperactivity disorder (ADHD) is a common childhood disorder that frequently persists into adulthood, with significant levels of inattentive, hyperactive and impulsive behavior. Impairments associated with adult ADHD include distress from the symptoms, impaired ability to function in work and academic settings, and problems sustaining stable relationships. The disorder is commonly associated with volatile moods, antisocial behavior, and drug and alcohol misuse. There is an increased risk of developing comorbid anxiety, depression, personality disorders, and drug and alcohol dependence. Despite the proven effectiveness of drugs such as methylphenidate, dexamphetamine and atomoxetine, few cases of ADHD are recognized and treated in the UK. The reasons for this are unclear, since most psychiatrists working with children and adolescents are aware that ADHD commonly persists into adult life and they also see the disorder affecting parents of children with ADHD. Issues of transition from the care of child to adult psychiatry and the need to refer adult relatives of children with ADHD to suitable psychiatric services are a major concern. Furthermore, many cases of adult ADHD go unrecognized or are seen by mental health teams that are not familiar with the subtleties of the adult presentation. As a result, misdiagnosis and treatment for conditions such as atypical depression, mixed affective disorder, cyclothymia, and borderline and unstable emotional personality disorders is not uncommon. There is therefore a requirement for further training in this area. This review will describe the common clinical presentation and provide guidelines for the diagnosis and treatment of ADHD in adults. Any psychiatrically trained physician using standard psychiatric assessment procedures can perform clinical evaluations for adult ADHD. As with other psychiatric disorders in adulthood, ADHD has its own characteristic onset, course and psychopathology. Symptoms of ADHD are trait-like, being stable characteristics from early childhood, and commonly co-occur with affective instability. Stimulants are the mainstay of treatment and are effective in around 70% of cases. Psychotherapeutic interventions also have an important role. These guidelines will assist psychiatrists and other adult mental health workers in identifying and treating individuals with adult ADHD.