Advances in genetic studies of attention-deficit/hyperactivity disorder

Developmental pyrethroid exposure disrupts molecular pathways for MAP kinase and circadian rhythms in mouse brain

Neurodevelopmental disorders (NDDs) are a category of pervasive disorders of the developing nervous system with few or no recognized biomarkers. A significant portion of the risk for NDDs, including attention deficit hyperactivity disorder (ADHD), is contributed by the environment, and exposure to pyrethroid pesticides during pregnancy has been identified as a potential risk factor for NDD in the unborn child. We recently showed that low-dose developmental exposure to the pyrethroid pesticide deltamethrin in mice causes male-biased changes to ADHD- and NDD-relevant behaviors as well as the striatal dopamine system. Here, we used an integrated multiomics approach to determine the broadest possible set of biological changes in the mouse brain caused by developmental pyrethroid exposure (DPE). Using a litter-based, split-sample design, we exposed mouse dams during pregnancy and lactation to deltamethrin (3 mg/kg or vehicle every 3 days) at a concentration well below the EPA-determined benchmark dose used for regulatory guidance. We raised male offspring to adulthood, euthanized them, and pulverized and divided whole brain samples for split-sample transcriptomics, kinomics and multiomics integration. Transcriptome analysis revealed alterations to multiple canonical clock genes, and kinome analysis revealed changes in the activity of multiple kinases involved in synaptic plasticity, including the mitogen-activated protein (MAP) kinase ERK. Multiomics integration revealed a dysregulated protein-protein interaction network containing primary clusters for MAP kinase cascades, regulation of apoptosis, and synaptic function. These results demonstrate that DPE causes a multi-modal biophenotype in the brain relevant to ADHD and identifies new potential mechanisms of action.

NEW & NOTEWORTHY

Here, we provide the first evidence that low-dose developmental exposure to the pyrethroid pesticide, deltamethrin, results in molecular disruptions in the adult mouse brain in pathways regulating circadian rhythms and neuronal growth (MAP kinase). This same exposure causes a neurodevelopmental disorder (NDD) relevant behavioral changes in adult mice, making these findings relevant to the prevention of NDDs.

Identification of factors associated with the efficacy of atomoxetine in adult attention-deficit/hyperactivity disorder

Aim

Atomoxetine (ATX) is a non‐central stimulant and a standard treatment for adult attention‐deficit/hyperactivity disorder (ADHD). The long‐term efficacy of Atomoxetine is about 40% at 6 months. The variability in efficacy between individuals is thought to be related to patient‐specific factors, but no detailed research has been conducted. In this retrospective cohort study, we aimed to identify the factors associated with Atomoxetine efficacy.

Methods

A total of 147 patients with attention‐deficit/hyperactivity disorder aged ≥18 years who were using Atomoxetine for the first time were included in this study. The outcome was treatment success (treatment maintained for at least 6 months and improvement in symptoms). Symptom assessment was based on the overall improvement in symptoms judged by an expert physician.

Results

Of the patient sample, 103 (70.1%) achieved the outcome. Logistic regression analysis identified “the maximum dose of ATX” and “gambling habit” as factors associated with efficacy ( P  < 0.05). In the process of Atomoxetine titration, the larger the maximum dose, the higher the efficacy was shown to be. Gambling habits may be indicative of impulsivity, which is among the core symptoms of attention‐deficit/hyperactivity disorder. Thus, a gambling habit may be considered a surrogate marker for impulsivity.

Conclusions

Knowledge of these factors will help healthcare professionals to predict the likely efficacy of Atomoxetine in a given patient before subscribing it, facilitating individualized pharmacotherapy for adult attention‐deficit/hyperactivity disorder.

We retrospectively examined factors associated with the efficacy of atomoxetine for adult ADHD. The maximum dose of atomoxetine and gambling habit were identified as significant factors related to the efficacy.

Inflammation, Anxiety, and Stress in Attention-Deficit/Hyperactivity Disorder

Attention-deficit/hyperactivity disorder (ADHD) is a prevalent and serious neurodevelopmental disorder characterized by symptoms of inattention and/or hyperactivity/impulsivity. Chronic and childhood stress is involved in ADHD development, and ADHD is highly comorbid with anxiety. Similarly, inflammatory diseases and a pro-inflammatory state have been associated with ADHD. However, while several works have studied the relationship between peripheral inflammation and stress in affective disorders such as depression or bipolar disorder, fewer have explored this association in ADHD. In this narrative review we synthetize evidence showing an interplay between stress, anxiety, and immune dysregulation in ADHD, and we discuss the implications of a potential disrupted neuroendocrine stress response in ADHD. Moreover, we highlight confounding factors and limitations of existing studies on this topic and critically debate multidirectional hypotheses that either suggest inflammation, stress, or anxiety as a cause in ADHD pathophysiology or inflammation as a consequence of this disease. Untangling these relationships will have diagnostic, therapeutic and prognostic implications for ADHD patients.

Dopamine transporter genotype modulates brain activity during a working memory task in children with ADHD

Dopamine active transporter gene (DAT1) is a candidate gene associated with attention-deficit/hyperactivity disorder (ADHD). The DAT1 variable number tandem repeat (VNTR)-3' polymorphism is functional and 9R carriers have been shown to produce more DAT than 10R homozygotes. We used functional magnetic resonance imaging (fMRI) to investigate the effects of this polymorphism on the neural substrates of working memory (WM) in a small but selected population of children with ADHD, naïve of any psychotropic treatment and without comorbidity. MRI and genotype data were obtained for 36 children (mean age: 10,36 +/- 1,49 years) with combined-type ADHD (9R n = 15) and 25 typically developing children (TDC) (mean age: 9,55 +/- 1,25 years) (9R n = 12). WM performance was similar between conditions. We found a cross-over interaction effect between gene (9R vs. 10R) and diagnosis (TDC vs. ADHD) in the orbito-frontal gyrus, cerebellum and inferior temporal lobe. In these areas, WM-related activity was higher for 9R carriers in ADHD subjects and lower in TDC. In ADHD children only, 10R homozygotes exhibited higher WM-related activity than 9R carriers in a network encompassing the parietal and the temporal lobes, the ventral visual cortex, the orbito-frontal gyrus and the head of the caudate nucleus. There was no significant results in TDC group. Our preliminary findings suggest that DAT1 VNTR polymorphism can modulate WM-related brain activity ADHD children.

Epigenetic Programming by Maternal Behavior in the Human Infant

: media-1vid110.1542/5804912859001PEDS-VA_2017-1890Video Abstract OBJECTIVES: We sought to determine if variations in maternal care alter DNA methylation in term, healthy, 5-month-old infants. This work was based on landmark studies in animal models demonstrating that nurturing care by dams would alter their newborns' stress responses through epigenetic mechanisms. We used breastfeeding as a proxy for animal maternal behavior. We hypothesized alterations in DNA methylation of the glucocorticoid receptor gene and less hypothalamic stress response in infants of mothers who breastfed their infants versus infants of mothers who did not breastfeed.

METHODS

A cohort study of term, healthy infants and their mothers who did (n = 21) or did not (n = 21) breastfeed for the first 5 months was used in this analysis. Cortisol stress reactivity was measured in infant saliva by using a mother-infant interaction procedure and DNA methylation of an important regulatory region of the glucocorticoid receptor gene. Changes in DNA methylation of this gene in humans were compared to homologous regions of the rat gene. DNA samples were prepared from cheek swabs and subjected to quantitative analysis of the extent of methylation by using sensitive sequencing techniques.

RESULTS

Breastfeeding was associated with decreased DNA methylation of the glucocorticoid receptor promoter and decreased cortisol reactivity in 5-month-old infants. Decreased DNA methylation occurred in the promoter region involved in regulation of the hypothalamic-pituitary-adrenal and immune system responses.

CONCLUSIONS

Maternal care in humans may impact the hypothalamic-pituitary-adrenal stress response through behavioral programming and manifest as offspring epigenetic change. These results explain, in part, some of the positive effects observed in children who are breastfed.

Prenatal exposure to bisphenol A and hyperactivity in children: a systematic review and meta-analysis

BACKGROUND

Attention-deficit hyperactivity disorder (ADHD) has increased in prevalence in the past decade. Studies attempting to identify a specific genetic component have not been able to account for much of the heritability of ADHD, indicating there may be gene-environment interactions underlying the disorder, including early exposure to environmental chemicals. Based on several relevant studies, we chose to examine bisphenol A (BPA) as a possible contributor to ADHD in humans. BPA is a widespread environmental chemical that has been shown to disrupt neurodevelopment in rodents and humans.

OBJECTIVES

Using the Office of Health Assessment and Translation (OHAT) framework, a systematic review and meta-analysis was designed to determine the relationship between early life exposure to BPA and hyperactivity, a key diagnostic criterion of ADHD.

DATA SOURCES

Searches of PubMed, Web of Science, and Toxline were completed for all literature to January 1, 2017.

STUDY ELIGIBILITY CRITERIA

For inclusion, the studies had to publish original data, be in the English language, include a measure of BPA exposure, and assess if BPA exposure affected hyperactive behaviors in mice, rats or humans. Exposure to BPA had to occur at <3 months of age for humans, up to postnatal day 35 for rats and up to postnatal day 40 for mice. Exposure could occur either gestationally (via maternal exposure) or directly to the offspring.

STUDY APPRAISAL AND SYNTHESIS METHODS

Studies were evaluated using the OHAT risk of bias tool. The effects in humans were assessed qualitatively. For rodents exposed to 20 μg/kg/day BPA, we evaluated the study findings in a random effects meta-analytical model.

RESULTS

A review of the literature identified 29 rodent and 3 human studies. A random effects meta-analysis showed significantly increased hyperactivity in male rodents. In humans, early BPA exposure was associated with hyperactivity in boys and girls.

LIMITATIONS, CONCLUSIONS, AND IMPLICATIONS OF KEY FINDINGS

We concluded that early life BPA exposure is a presumed human hazard for the development of hyperactivity. Possible limitations of this systematic review include deficiencies in author reporting, exclusion of some literature based on language, and insufficient similarity between human studies. SRs that result in hazard-based conclusions are the first step in assessing and mitigating risks. Given the widespread exposure of BPA and increasing diagnoses of ADHD, we recommend immediate actions to complete such risk analyses and take next steps for the protection of human health. In the meantime, precautionary measures should be taken to reduce exposure in pregnant women, infants and children. The present analysis also discusses potential mechanisms by which BPA affects hyperactivity, and the most effective avenues for future research.

SYSTEMATIC REVIEW REGISTRATION NUMBER

Not available.

DNA methylation: a mechanism linking environmental chemical exposures to risk of autism spectrum disorders?

There is now compelling evidence that gene by environment interactions are important in the etiology of autism spectrum disorders (ASDs). However, the mechanisms by which environmental factors interact with genetic susceptibilities to confer individual risk for ASD remain a significant knowledge gap in the field. The epigenome, and in particular DNA methylation, is a critical gene expression regulatory mechanism in normal and pathogenic brain development. DNA methylation can be influenced by environmental factors such as diet, hormones, stress, drugs, or exposure to environmental chemicals, suggesting that environmental factors may contribute to adverse neurodevelopmental outcomes of relevance to ASD via effects on DNA methylation in the developing brain. In this review, we describe epidemiological and experimental evidence implicating altered DNA methylation as a potential mechanism by which environmental chemicals confer risk for ASD, using polychlorinated biphenyls (PCBs), lead, and bisphenol A (BPA) as examples. Understanding how environmental chemical exposures influence DNA methylation and how these epigenetic changes modulate the risk and/or severity of ASD will not only provide mechanistic insight regarding gene-environment interactions of relevance to ASD but may also suggest potential intervention strategies for these and potentially other neurodevelopmental disorders.

Neurofeedback as a nonpharmacological treatment for adults with attention-deficit/hyperactivity disorder (ADHD): study protocol for a randomized controlled trial

BACKGROUND

Neurofeedback has been applied effectively in various areas, especially in the treatment of children with attention-deficit/hyperactivity disorder (ADHD). This study protocol is designed to investigate the effect of slow cortical potential (SCP) feedback and a new form of neurofeedback using near-infrared spectroscopy (NIRS) on symptomatology and neurophysiological parameters in an adult ADHD population. A comparison of SCP and NIRS feedback therapy methods has not been previously conducted and may yield valuable findings about alternative treatments for adult ADHD.

METHODS/DESIGN

The outcome of both neurofeedback techniques will be assessed over 30 treatment sessions and after a 6-month follow-up period, and then will be compared to a nonspecific biofeedback treatment. Furthermore, to investigate if treatment effects in this proof-of-principle study can be predicted by specific neurophysiological baseline parameters, regression models will be applied. Finally, a comparison with healthy controls will be conducted to evaluate deviant pretraining neurophysiological parameters, stability of assessment measures, and treatment outcome.

DISCUSSION

To date, an investigation and comparison of SCP and NIRS feedback training to an active control has not been conducted; therefore, we hope to gain valuable insights in effects and differences of these types of treatment for ADHD in adults.

TRIAL REGISTRATION

This study is registered with the German Registry of Clinical Trials: DRKS00006767 , date of registration: 8 October 2014.

Multifactorial Origin of Neurodevelopmental Disorders: Approaches to Understanding Complex Etiologies

A significant body of evidence supports the multifactorial etiology of neurodevelopmental disorders (NDDs) affecting children. The present review focuses on early exposure to environmental chemicals as a risk factor for neurodevelopment, and presents the major lines of evidence derived from epidemiological studies, underlying key uncertainties and research needs in this field. We introduce the exposome concept that, encompassing the totality of human environmental exposures to multiple risk factors, aims at explaining individual vulnerability and resilience to early chemical exposure. In this framework, we synthetically review the role of variable gene backgrounds, the involvement of epigenetic mechanisms as well as the function played by potential effect modifiers such as socioeconomic status. We describe laboratory rodent studies where the neurodevelopmental effects of environmental chemicals are assessed in the presence of either a “vulnerable” gene background or adverse pregnancy conditions (i.e., maternal stress). Finally, we discuss the need for more descriptive and “lifelike” experimental models of NDDs, to identify candidate biomarkers and pinpoint susceptible groups or life stages to be translated to large prospective studies within the exposome framework.

Early-life risk factors for chronic nonrespiratory diseases

We have witnessed a change in disease patterns contributing to the global burden of disease, with a shift from early childhood deaths due to the classic infectious communicable diseases to years lived with disability from chronic noncommunicable diseases. In both developing and developed countries, the years lived with disability attributable to chronic disease have increased: cardiovascular diseases by 17.7%; chronic respiratory disease by 8.5%; neurological conditions by 12.2%; diabetes by 30.0%; and mental and behavioural disorders by 5.0% over the past 20 years. Recognition of the contribution made by adverse environmental exposures in early life to noncommunicable diseases in later life is increasing. These early-life exposures appear to contribute to both chronic respiratory and chronic nonrespiratory diseases. In this State of the Art article, we aim to examine early-life environmental exposures that have an epidemiological association with chronic nonrespiratory diseases, such as obesity and type II diabetes, cardiovascular disease, and neurocognitive and behavioural problems. We will highlight the potential overlap in environmental risks with respiratory diseases, and point out knowledge gaps and research opportunities.

The Roots of Autism and ADHD Twin Study in Sweden (RATSS)

Neurodevelopmental disorders affect a substantial minority of the general population. Their origins are still largely unknown, but a complex interplay of genetic and environmental factors causing disturbances of the central nervous system's maturation and a variety of higher cognitive skills is presumed. Only limited research of rather small sample size and narrow scope has been conducted in neurodevelopmental disorders using a twin-differences design. The Roots of Autism and ADHD Twin Study in Sweden (RATSS) is an ongoing project targeting monozygotic twins discordant for categorical or dimensional autistic and inattentive/hyperactive-impulsive phenotypes as well as other neurodevelopmental disorders, and typically developing twin controls. Included pairs are 9 years of age or older, and comprehensively assessed for psychopathology, medical history, neuropsychology, and dysmorphology, as well as structural, functional, and molecular brain imaging. Specimens are collected for induced pluripotent (iPS) and neuroepithelial stem cells, genetic, gut bacteria, protein-/monoamine, and electron microscopy analyses. RATSS's objective is to generate a launch pad for novel surveys to understand the complexity of genotype-environment-phenotype interactions in autism spectrum disorder and attention-deficit hyperactivity disorder (ADHD). By October 2013, RATSS had collected data from 55 twin pairs, among them 10 monozygotic pairs discordant for autism spectrum disorder, seven for ADHD, and four for other neurodevelopmental disorders. This article describes the design, recruitment, data collection, measures, collected pairs' characteristics, as well as ongoing and planned analyses in RATSS. Potential gains of the study comprise the identification of environmentally mediated biomarkers, the emergence of candidates for drug development, translational modeling, and new leads for prevention of incapacitating outcomes.

Attention deficit hyperactivity disorder (ADHD): an affect-processing and thought disorder?

In the literature on child and adolescent psychoanalysis attention deficit hyperactivity disorder (ADHD) is described as complex syndrome with wide-ranging psychodynamic features. Broadly speaking, the disorder is divided into three categories: 1. a disorder in early object relations leading to the development of a maniform defence organization in which object-loss anxieties and depressed affects are not worked through via symbolization but are organized in a body-near manner; 2. a triangulation disorder in which the cathexis of the paternal position is not stable; structures providing little support alternate with excessive arousal, affect regulation is restricted; 3. current emotional stress or a traumatic experience. I suggest taking a fresh look at ADHD from a psychoanalytic vantage point. With respect to the phenomenology of the disorder, the conflict-dynamic approach should be supplemented by a perspective regarding deficits in α-function as constitutive for ADHD. These deficits cause affect-processing and thought disorders compensated for (though not fully) by the symptomatology. At a secondary level, a vicious circle develops through the mutual reinforcement of defective processing of sense data and affects into potential thought content, on the one hand, and secondary, largely narcissistic defence processes on the other. These considerations have major relevance for the improved understanding of ADHD and for psychoanalytic technique.

Lead, mercury, and cadmium exposure and attention deficit hyperactivity disorder in children

BACKGROUND

There is limited research examining the relationship between lead (Pb) exposure and medically diagnosed attention deficit hyperactivity disorder (ADHD) in children. The role of mercury (Hg) and cadmium (Cd) exposures in ADHD development is even less clear.

OBJECTIVES

To examine the relationship between Pb, Hg, and Cd and ADHD in children living inside and outside a Lead Investigation Area (LIA) of a former lead refinery in Omaha, NE.

METHODS

We carried out a case-control study with 71 currently medically diagnosed ADHD cases and 58 controls from a psychiatric clinic and a pediatric clinic inside and outside of the LIA. The participants were matched on age group (5-8, 9-12 years), sex, race (African American or Caucasians and others), and location (inside or outside LIA). We measured whole blood Pb, total Hg, and Cd using inductively coupled plasma mass spectrometry.

RESULTS

Inside the LIA, the 27 cases had blood Pb geometric mean (GM) 1.89 µg/dL and the 41 controls had 1.51 µg/dL. Outside the LIA, the 44 cases had blood Pb GM 1.02 µg/dL while the 17 controls had 0.97 µg/dL. After adjustment for matching variables and maternal smoking, socioeconomic status, and environmental tobacco exposure, each natural log unit blood Pb had an odds ratio of 2.52 with 95% confidence interval of 1.07-5.92. Stratification by the LIA indicated similar point estimate but wider CIs. No associations were observed for Hg or Cd.

CONCLUSIONS

Postnatal Pb exposure may be associated with higher risk of clinical ADHD, but not the postnatal exposure to Hg or Cd.

Review of current evidence on the impact of pesticides, polychlorinated biphenyls and selected metals on attention deficit / hyperactivity disorder in children

The aim of this review was to investigate the association between attention deficit / hyperactivity disorder (ADHD) or ADHD-related symptoms and industrial chemicals, such as organophosphates and organochlorine pesticides, polychlorinated biphenyls (PCBs), lead, mercury and manganese. Medline, PubMed and EBSCO searches were performed to identify the studies that analyzed the association of prenatal and postnatal child exposure to such toxicants and ADHD or ADHD-related symptoms. The review is restricted to human studies published in English in peer-reviewed journals since 2000. Most of the presented studies focused on pesticides, PCB and lead. The impact of mercury and manganese was investigated less frequently. The findings indicate that children's exposure to organophosphate pesticides may cause symptoms consistent with pervasive developmental disorder, ADHD or attention problems. Exposures to organochlorine pesticides and PCBs were associated with ADHD-like behaviors such as alertness, quality of alert response, and cost of attention. The studies provided evidence that blood lead level below 10 μg/dl was associated with ADHD or ADHD-related symptoms. Information on the association between exposure to mercury and neurotoxicity is limited, and requires further confirmation in future research. Two studies indicated that exposure to manganese is related to ADHD; such exposure and its impact on children neurodevelopment need to be further investigated. Future studies should use a prospective design with multiple biological samples collected over time for better assessment of exposure and its critical windows. Additionally, inclusion of potential confounding factors and co-exposures is crucial.

Assessment of physician practices in adult attention-deficit/hyperactivity disorder

OBJECTIVE

Adult attention-deficit/hyperactivity disorder (ADHD) is a common neuropsychiatric disorder, yet only 1 in 10 affected adults receives treatment. The study objective was to assess gaps in knowledge and describe current practice patterns of primary care physicians and psychiatrists in the United States in the management of adult patients with ADHD.

METHOD

Primary care physicians and psychiatrists completed an Internet survey as a needs assessment of customary care related to management of adults with ADHD. Adult clinical case vignettes were followed by practice, confidence, and barrier questions. Survey data were collected from April 15, 2010, to August 22, 2010, and were deidentified and analyzed in aggregate to maintain confidentiality. χ(2) and t tests were used to compare responses of primary care physicians with those of psychiatrists.

RESULTS

The survey was completed by 1,924 physicians: 1,216 primary care physicians and 708 psychiatrists. Fewer primary care physicians than psychiatrists were "extremely confident" in diagnosis (8% vs 28%, respectively, P < .001) and treatment (8% vs 27%, respectively, P < .001). Limited experience with ADHD diagnosis was more of a barrier in primary care than in psychiatry (44% vs 19%, respectively, P < .001). Mean scores on 12 evidence-based questions were lower for primary care physicians than for psychiatrists (6.1 vs 6.8 correct, respectively, P < .001). Awareness of adult ADHD prevalence was lower among primary care physicians than among psychiatrists (32% vs 47% correct, respectively, P < .001). Fewer primary care physicians than psychiatrists recognized comorbid substance use disorder (76% vs 82%, respectively, P = .002), but more primary care physicians than psychiatrists recognized eating disorders (35% vs 21%, respectively, P < .001).

CONCLUSIONS

The self-assessment survey results indicate physician practices in primary care and psychiatry differ and show areas in which further education will be useful to improve care for adults with ADHD.

A genetic association study of DNA methylation levels in the DRD4 gene region finds associations with nearby SNPs

BACKGROUND

Dopamine receptor D4(DRD4) polymorphisms have been associated with a number of psychiatric disorders, but little is known about the mechanism of these associations. DNA methylation is linked to the regulation of gene expression and plays a vital role in normal cellular function, with abnormal DNA methylation patterns implicated in a range of disorders. Recent evidence suggests DNA methylation can be influenced by cis-acting DNA sequence variation, that is, DNA sequence variation located nearby on the same chromosome.

METHODS

To investigate the potential influence of cis-acting genetic elements within DRD4, we analysed DRD4 promoter DNA methylation levels in the transformed lymphoblastoid cell-line DNA of 89 individuals (from 30 family-trios). Five SNPs located +/- 10kb of the promoter region were interrogated for associations with DNA methylation levels.

RESULTS

Four significant SNP associations were found with DNA methylation (rs3758653, rs752306, rs11246228 and rs936465). The associations of rs3758653 and rs936465 with DNA methylation were tested and nominally replicated (p-value < 0.05) in post-mortem brain tissue from an independent sample (N = 18). Interestingly, the DNA methylation patterns observed in post-mortem brain tissue were similar to those observed in transformed lymphoblastoid cell line DNA.

CONCLUSIONS

The link reported between DNA sequence and DNA methylation offers a possible functional role to seemingly non-functional SNP associations. DRD4 has been implicated in several psychiatric disease phenotypes and our results shed light upon the possible mode of action of SNP associations in this region.

Attention deficit/hyperactivity disorder-derived coding variation in the dopamine transporter disrupts microdomain targeting and trafficking regulation

Attention deficit/hyperactivity disorder (ADHD) is the most commonly diagnosed disorder of school-age children. Although genetic and brain-imaging studies suggest a contribution of altered dopamine (DA) signaling in ADHD, evidence of signaling perturbations contributing to risk is largely circumstantial. The presynaptic, cocaine- and amphetamine (AMPH)-sensitive DA transporter (DAT) constrains DA availability at presynaptic and postsynaptic receptors following vesicular release and is targeted by the most commonly prescribed ADHD therapeutics. Using polymorphism discovery approaches with an ADHD cohort, we identified a hDAT (human DAT) coding variant, R615C, located in the distal C terminus of the transporter, a region previously implicated in constitutive and regulated transporter trafficking. Here, we demonstrate that, whereas wild-type DAT proteins traffic in a highly regulated manner, DAT 615C proteins recycle constitutively and demonstrate insensitivity to the endocytic effects of AMPH and PKC (protein kinase C) activation. The disrupted regulation of DAT 615C parallels a redistribution of the transporter variant away from GM1 ganglioside- and flotillin1-enriched membranes, and is accompanied by altered CaMKII (calcium/calmodulin-dependent protein kinase II) and flotillin-1 interactions. Using C-terminal peptides derived from wild-type DAT and the R615C variant, we establish that the DAT 615C C terminus can act dominantly to preclude AMPH regulation of wild-type DAT. Mutagenesis of DAT C-terminal sequences suggests that phosphorylation of T613 may be important in sorting DAT between constitutive and regulated pathways. Together, our studies support a coupling of DAT microdomain localization with transporter regulation and provide evidence of perturbed DAT activity and DA signaling as a risk determinant for ADHD.

Attention deficit/hyperactivity disorder: a focused overview for children's environmental health researchers

OBJECTIVES

Attention deficit/hyperactivity disorder (ADHD) is the most frequently diagnosed childhood neurobehavioral disorder. Much research has been done to identify genetic, environmental, and social risk factors for ADHD; however, we are still far from fully understanding its etiology. In this review we provide an overview of diagnostic criteria for ADHD and what is known about its biological basis. We also review the neuropsychological functions that are affected in ADHD. The goal is to familiarize the reader with the behavioral deficits that are hallmarks of ADHD and to facilitate comparisons with neurobehavioral deficits associated with environmental chemical exposures.

DATA SOURCES

Relevant literature on ADHD is reviewed, focusing in particular on meta-analyses conducted between 2004 and the present that evaluated associations between measures of neuropsychological function and ADHD in children. Meta-analyses were obtained through searches of the PubMed electronic database using the terms "ADHD," "meta-analysis," "attention," "executive," and "neuropsychological functions." Although meta-analyses are emphasized, nonquantitative reviews are included for particular neuropsychological functions where no meta-analyses were available.

DATA SYNTHESIS

The meta-analyses indicate that vigilance (sustained attention), response inhibition, and working memory are impaired in children diagnosed with ADHD. Similar but somewhat less consistent meta-analytic findings have been reported for impairments in alertness, cognitive flexibility, and planning. Additionally, the literature suggests deficits in temporal information processing and altered responses to reinforcement in children diagnosed with ADHD. Findings from brain imagining and neurochemistry studies support the behavioral findings.

CONCLUSIONS

Behavioral, neuroanatomical, and neurochemical data indicate substantial differences in attention and executive functions between children diagnosed with ADHD and non-ADHD controls. Comparisons of the neurobehavioral deficits associated with ADHD and those associated with exposures to environmental chemicals may help to identify possible environmental risk factors for ADHD and/or reveal common underlying biological mechanisms.

Lead and PCBs as risk factors for attention deficit/hyperactivity disorder

OBJECTIVES

Attention deficit/hyperactivity disorder (ADHD) is the most frequently diagnosed neurobehavioral disorder of childhood, yet its etiology is not well understood. In this review we present evidence that environmental chemicals, particularly polychlorinated biphenyls (PCBs) and lead, are associated with deficits in many neurobehavioral functions that are also impaired in ADHD.

DATA SOURCES

Human and animal studies of developmental PCB or lead exposures that assessed specific functional domains shown to be impaired in ADHD children were identified via searches of PubMed using "lead" or "PCB exposure" in combination with key words, including "attention," "working memory," "response inhibition," "executive function," "cognitive function," "behavior," and "ADHD."

DATA SYNTHESIS

Children and laboratory animals exposed to lead or PCBs show deficits in many aspects of attention and executive function that have been shown to be impaired in children diagnosed with ADHD, including tests of working memory, response inhibition, vigilance, and alertness. Studies conducted to date suggest that lead may reduce both attention and response inhibition, whereas PCBs may impair response inhibition to a greater degree than attention. Low-level lead exposure has been associated with a clinical diagnosis of ADHD in several recent studies. Similar studies of PCBs have not been conducted.

CONCLUSIONS

We speculate that exposures to environmental contaminants, including lead and PCBs, may increase the prevalence of ADHD.

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

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

Behavioral genetics in antisocial spectrum disorders and psychopathy: a review of the recent literature

Behavioral geneticists are increasingly using the tools of molecular genetics to extend upon discoveries from twin, family, and adoption studies concerning the heritability of antisocial spectrum disorders and psychopathy. While there is a substantial body of research concerning antisocial spectrum disorders in the behavioral genetics literature, only a few studies could be located using the phenotype of psychopathy. In this report we summarize some of the recent molecular genetics work concerning antisocial spectrum disorders and psychopathy, with a focus on genes involved in the serotonergic and dopaminergic pathways, while also mentioning some of the novel genetic factors being considered. Monoamine oxidase (MAOA) and the serotonin transporter (5HTT) are reviewed at length, as these genes have received significant scientific attention in recent years and are sites of high biological plausibility in antisocial spectrum disorders and psychopathy.