The serotonin 5-HT1B receptor gene and attention deficit hyperactivity disorder

Serotonin dysfunction in ADHD

It is well accepted that attention deficit hyperactivity disorder (ADHD) is in part driven by dysfunction in the monoaminergic neurotransmitter system, but both the extent of dysfunction and possible therapeutic avenues presented by serotonergic neurotransmission is frequently overlooked. As such, we present key evidence for dysfunction in serotonergic transmission, as seen from biochemical, genetic and pharmacological perspectives. An overall deficit in serotonin availability is a common theme throughout the literature, thus this review aims to explore possible dysfunctions in the serotonin synthesis pathway which result in this reduced bioavailability, and investigate whether such dysfunctions could be loci of change in ADHD. We have identified several steps in transmission, namely the conversion of tryptophan to 5-hydroxytryptophan and its use of cofactor tetrahydrobiopterin, which could present promising avenues for development of novel clinical interventions for ADHD.

Laying the groundwork: Exploring pesticide exposure and genetic factors in south-eastern Brazilian farmers

Brazil is the world leader in pesticide consumption, and its indiscriminate use puts farmers' health at risk. The CYP2C9 gene encodes the CYP2C9 enzyme, which metabolizes several endogenous substrates and specific xenobiotics, especially pesticides. Our goal is to study the risk of pesticide use, especially the herbicide glyphosate, in the development of diseases and the association with two CYP2C9 polymorphisms, in farmers living in the southern region of Espírito Santo state, Brazil. The allelic frequency of CYP2C9*1, CYP2C9*2 and CYP2C9*3 was determined in blood samples from individuals exposed or not to pesticides using real-time PCR. 304 blood samples were analyzed, dividing CYP2C9 genotypes into three metabolization classes: normal, intermediate, and slow. Our results indicate that normal metabolizers may be more susceptible to conditions such as high blood pressure, cardiovascular disease, and kidney problems. Intermediate metabolizers show an association with attention deficit disorder and miscarriages, suggesting that farmers' symptoms correlated with their CYP2C9 genotype. Insufficient data prevented conclusions about slow metabolizers (*2 and/or *3). These findings suggest that the CYP2C9 genotype may influence the way farmers exposed to pesticides respond, but more research is needed to clarify causality and investigate other possible health effects. As an introductory effort, this study provides insights into the complex relationship between genetic variations and pesticide exposure, laying the groundwork for future research. This pioneering work on associations between specific genetic variations and health risks with pesticide exposure, emphasizes the importance of personalized medicine and stricter regulation of pesticide use for public health and occupational safety.

New Advances in Attention-Deficit/Hyperactivity Disorder-like Dogs

Similar to humans, dogs could suffer an Attention-Deficit/Hyperactivity Disorder-like syndrome (ADHD-like). In fact, several studies highlight the use of dogs as a model for studying ADHD. This condition entails behavioral problems expressed through impulsivity, attention issues, hyperactivity, and/or aggression, compromising the quality of life for both the caregiver and the dog. The pathophysiology of ADHD-like is complex and is associated with dysregulation of various neurotransmitters such as serotonin and dopamine. The expression of ADHD-like behavior in dogs would appear to depend on a classical gene-environment interaction as is the case with many neurological disorders in humans. In addition to the described symptomatology, ADHD-like dogs can exhibit strong comorbidities with compulsive behaviors, aggressiveness, inappropriate elimination and fearfulness, in addition to epilepsy, foreign body ingestion, and pruritus. In spite of the fact that there is no veterinary consensus about the diagnosis of ADHD-like, some validated questionnaires could be helpful, but these cannot be used as a unique diagnostic tool. The use of drugs, such as fluoxetine, in addition to an adequate environmental enrichment, relaxation protocols, and behavior modification can achieve an adequate quality of life for both the dog and caregivers.

Viloxazine extended-release capsules as an emerging treatment for attention-deficit/hyperactivity disorder in children and adolescents

INTRODUCTION

Attention-deficit/hyperactivity disorder (ADHD) is a neurodevelopmental disorder characterized by inattention and/or hyperactivity and impulsivity. Viloxazine extended-release (ER) capsules (Qelbree®) is a US Food and Drug Administration-approved nonstimulant treatment option for children, adolescents, and adults with ADHD.

AREAS COVERED

This review manuscript summarizes the neurobiology of ADHD and currently available treatment options before discussing viloxazine pharmacology, efficacy, safety, and tolerability data from phase II and III trials in children and adolescents (6-17 years old). Viloxazine clinical efficacy has also been further demonstrated by post hoc analyses of pediatric clinical trial results.

EXPERT OPINION

Current stimulant and nonstimulant treatments for ADHD may be suboptimal given low response rates and that tolerability issues are frequently experienced. Preclinical and clinical evidence has implicated both the role of catecholamine and serotonin signaling in the pathophysiology of ADHD and the pharmacologic effect of viloxazine on these critical neurotransmitter systems. With a relatively rapid onset of action, sustained symptom improvement, and clinical benefit in ADHD-associated impairments (functional and social), viloxazine ER represents a novel and emerging ADHD treatment option.

BH4 as a Therapeutic Target for ADHD: Relevance to Neurotransmitters and Stress-Driven Symptoms

Tetrahydrobiopterin (BH4) is a critical cofactor in a variety of metabolic pathways that have been linked to ADHD. There have been no previous studies utilizing BH4 as a supplement for ADHD. BH4 has been approved as a treatment for phenylketonuria (PKU). Individuals with PKU and ADHD appear to have low DA levels in common, suggesting that the hypodopaminergic state seen in both illnesses could be a relationship between the two. Clinical research involving supplementation of BH4 has shown low occurrence of adverse. In experiments, BH4 has also been found to have good blood-brain barrier permeability. BH4 also has the ability in scavenging ROS activity, which is an implication of stress and is seen in ADHD. BH4's significance in ADHD is reviewed in this paper because of its involvement in numerous neurodevelopmental metabolic pathways, and we anticipate that exogenous BH4 can be used to treat ADHD.

Serotonin and Dopamine Blood Levels in ADHD-Like Dogs

As with humans, dogs can suffer from attention deficit hyperactivity disorder-like (ADHD-like) behaviors naturally and exhibit high levels of hyperactivity/impulsivity and attention deficit problems, making the domestic dog a potential animal model for ADHD. ADHD has a very complex pathophysiology in which many neurotransmitters are involved, such as serotonin and dopamine. The aim of the study was to evaluate serum serotonin and dopamine levels in dogs with ADHD-like symptomatology. Fifty-eight dogs were studied, of which, thirty-six were classified as ADHD-like after physical and behavioral assessments. Additionally, the dogs’ owners performed a series of scientifically validated questionnaires which included C-BARQ, the Dog Impulsivity Assessment Scale, and the Dog-ADHD rating scale. Serum from every animal was collected after the behavioral assessments and analyzed with commercial ELISA tests for serotonin and dopamine determination. Kruskal–Wallis tests and Lasso regressions were applied to assess the relationships between both neurotransmitters and the ADHD-like behaviors (as assessed by clinical evaluation and through the different questionnaires). The dogs clinically classified as ADHD-like showed lower serotonin and dopamine concentrations. Further, serotonin and dopamine levels were also linked to aggression, hyperactivity, and impulsivity. Decreased serotonin concentrations were also related to fear, attachment, and touch sensitivity. Finally, it must be noted that our data suggested a strong relationship between serotonin and dopamine and ADHD-like behaviors.

Modelling ADHD-Like Phenotypes in Zebrafish

The use of multiple species to model complex human psychiatric disorders, such as ADHD, can give important insights into conserved evolutionary patterns underlying multidomain behaviors (e.g., locomotion, attention, and impulsivity). Here we discuss the advantages and challenges in modelling ADHD-like phenotypes in zebrafish (Danio rerio), a vertebrate species that has been widely used in neuroscience and behavior research. Moreover, multiple behavioral tasks can be used to model the core symptoms of ADHD and its comorbidities. We present a critical review of current ADHD studies in zebrafish, and how this species might be used to accelerate the discovery of new drug treatments for this disorder.

[The combined effect of genetic factors and attention deficit hyperactivity disorder on the development of dependence on synthetic cannabinoids]

AIM

To develop a model of assessment of individual risk of dependence on synthetic cannabinoids based on genetic factors and diagnosis of attention deficit hyperactivity disorder (ADHD).

MATERIAL AND METHODS

The study included 146 male adolescents using synthetic cannabinoids and 136 healthy people. The genetic study considered the combination of dependence on synthetic cannabinoids and ADHD. Six polymorphisms in the genes of dopaminergic and serotonergic systems were genotyped.

RESULTS AND CONCLUSION

In general, the results of this work confirm the important role of the dopaminergic and serotonergic systems in the pathogenesis of substance use disorders, and the significance of changes in the nucleotide sequences of DRD2, SLC6A3, HTR2A genes in the development of dependence on synthetic cannabinoids with ADHD.

Evidence of sexual dimorphism of HTR1B gene on major adult ADHD comorbidities

Attention-deficit/hyperactivity disorder (ADHD) is a very common psychiatric disorder across the life cycle and frequently presents comorbidities. Since ADHD is highly heritable, several studies have focused in the underlying genetic factors involved in its etiology. One of the major challenges in this search is the phenotypic heterogeneity, which could be partly attributable to the sexual dimorphism frequently seen in psychiatric disorders. Taking into account the well-known sexual dimorphic effect observed in serotonergic system characteristics, we differentially tested the influence of HTR1B SNPs (rs11568817, rs130058, rs6296 and rs13212041) on ADHD susceptibility and on its major comorbidities according to sex. The sample comprised 564 adults with ADHD diagnosed according to DSM-IV criteria and 635 controls. There was no association of any HTR1B SNPs tested in relation to ADHD susceptibility. As for the comorbidities evaluated, after correction for multiple tests, significant associations were observed for both rs11568817 and rs130058 with substance use disorders (P_corr = 0.009 and P_corr = 0.018, respectively) and for rs11568817 with nicotine dependence (P_corr = 0.025) in men with ADHD. In women with ADHD, the same rs11568817 was associated with generalized anxiety disorder (P_corr = 0.031). The observed effects of rs11568817 G allele presence conferring risk to either substance use disorders or generalized anxiety disorder according to sex, suggest an overall scenario where a higher transcriptional activity of HTR1B, resulting from the presence of this allele, is related to externalizing behaviors in men and internalizing behaviors in women. These results are consistent with and expand previous evidence of sexual dimorphism of the serotoninergic system.

Expanding the toolbox of ADHD genetics. How can we make sense of parent of origin effects in ADHD and related behavioral phenotypes?

Genome-wide association (GWA) studies have shown that many different genetic variants cumulatively contribute to the risk of psychiatric disorders. It has also been demonstrated that various parent-of-origin effects (POE) may differentially influence the risk of these disorders. Together, these observations have provided important new possibilities to uncover the genetic underpinnings of such complex phenotypes. As POE so far have received little attention in neuropsychiatric disorders, there is still much progress to be made. Here, we mainly focus on the new and emerging role of POE in attention-deficit hyperactivity disorder (ADHD). We review the current evidence that POE play an imperative role in vulnerability to ADHD and related disorders. We also discuss how POE can be assessed using statistical genetics tools, expanding the resources of modern psychiatric genetics. We propose that better comprehension and inspection of POE may offer new insight into the molecular basis of ADHD and related phenotypes, as well as the potential for preventive and therapeutic interventions.

Lack of association of polymorphisms in six candidate genes in colombian adhd patients

BACKGROUND

Attention Deficit and Hyperactivity Disorder (ADHD) is a common childhood neuropsychiatric condition. The disorder has a multifactorial background, with heritability estimates of around 76%, suggesting an important role of genetic factors. Candidate genes include those related to dopaminergic (e.g. DRD4, DRD5, SLC6A3 and DBH)and serotoninergic (e.g.HTR1B and SLC6A4) pathways.

PURPOSE

To explore the association of common polymorphisms in six genes (DRD4, DRD5, SLC6A3, DBH, HTR1B and SLC6A4) and the susceptibility to ADHD in a Colombian sample population.

METHODS

trios and 152 healthy controls were recruited. Genotyping of the six polymorphisms was performed using described PCR-based protocols. A TDT analysis was used to test if there was preferential allelic transmission for any of the six polymorphisms. Additionally, a case-control analysis was performed to test for association of the serotoninergic (HTR1B and SLC6A4) polymorphisms with ADHD.

RESULTS

Through the TDT analysis there was no preferential allelic transmission for any of the studied variants. Case-control analysis did not show association.

CONCLUSION

This is the first study in Latin America to describe six polymorphisms in a group of patients with ADHD. There was no evidence of association for any of the studied polymorphic variants in this Colombian ADHD sample. Further research, with larger sample sizes and study of endophenotypes, is needed in this population to confirm and extend the results.

Studying the effects of dietary body weight-adjusted acute tryptophan depletion on punishment-related behavioral inhibition

Background

Alterations in serotonergic (5-HT) neurotransmission are thought to play a decisive role in affective disorders and impulse control.

Objective

This study aims to reproduce and extend previous findings on the effects of acute tryptophan depletion (ATD) and subsequently diminished central 5-HT synthesis in a reinforced categorization task using a refined body weight–adjusted depletion protocol.

Design

Twenty-four young healthy adults (12 females, mean age [SD]=25.3 [2.1] years) were subjected to a double-blind within-subject crossover design. Each subject was administered both an ATD challenge and a balanced amino acid load (BAL) in two separate sessions in randomized order. Punishment-related behavioral inhibition was assessed using a forced choice go/no-go task that incorporated a variable payoff schedule.

Results

Administration of ATD resulted in significant reductions in TRP measured in peripheral blood samples, indicating reductions of TRP influx across the blood–brain barrier and related brain 5-HT synthesis. Overall accuracy and response time performance were improved after ATD administration. The ability to adjust behavioral responses to aversive outcome magnitudes and behavioral adjustments following error contingent punishment remained intact after decreased brain 5-HT synthesis. A previously observed dissociation effect of ATD on punishment-induced inhibition was not observed.

Conclusions

Our results suggest that neurodietary challenges with ATD Moja–De have no detrimental effects on task performance and punishment-related inhibition in healthy adults.

Serotonin mediated immunoregulation and neural functions: Complicity in the aetiology of autism spectrum disorders

Serotonergic system has long been implicated in the aetiology of autism spectrum disorders (ASD), since platelet hyperserotonemia is consistently observed in a subset of autistic patients, who respond well to selective serotonin reuptake inhibitors. Apart from being a neurotransmitter, serotonin functions as a neurotrophic factor directing brain development and as an immunoregulator modulating immune responses. Serotonin transporter (SERT) regulates serotonin level in lymphoid tissues to ensure its proper functioning in innate and adaptive responses. Immunological molecules such as cytokines in turn regulate the transcription and activity of SERT. Dysregulation of serotonergic system could trigger signalling cascades that affect normal neural-immune interactions culminating in neurodevelopmental and neural connectivity defects precipitating behavioural abnormalities, or the disease phenotypes. Therefore, we suggest that a better understanding of the cross talk between serotonergic genes, immune systems and serotonergic neurotransmission will open wider avenues to develop pharmacological leads for addressing the core ASD behavioural deficits.

Parent-of-origin effects in ADHD: distinct influences of paternal and maternal ADHD on neuropsychological functioning in offspring

OBJECTIVE

The authors examined parent-of-origin effects in transmission of ADHD and neuropsychological functioning. Proof of these effects can identify more etiologically homogeneous ADHD subgroups and facilitate genetic studies.

METHOD

The authors included 238 ADHD and 147 control families. ADHD in children was assessed using parent and teacher ratings, while parents completed self-reports. Children were assessed with neuropsychological paradigms measuring IQ, motor, timing, and executive functions.

RESULTS

Paternal and maternal ADHD were equally positively related to ADHD in offspring. Paternal ADHD was related to poorer time reproduction in offspring and to lower verbal and total IQ in daughters. Maternal ADHD was related to poorer inhibition and motor control in offspring. No mediating effects of neuropsychological functions were found between parent and offspring ADHD symptoms.

CONCLUSION

Neuropsychological functions may be more sensitive to parent-of-origin effects than ADHD symptoms and possibly useful in detecting the transmission of different gene-brain network pathways depending on parental sex.

Loss of neuronal 3D chromatin organization causes transcriptional and behavioural deficits related to serotonergic dysfunction

The interior of the neuronal cell nucleus is a highly organized three-dimensional (3D) structure where regions of the genome that are linearly millions of bases apart establish sub-structures with specialized functions. To investigate neuronal chromatin organization and dynamics in vivo, we generated bitransgenic mice expressing GFP-tagged histone H2B in principal neurons of the forebrain. Surprisingly, the expression of this chimeric histone in mature neurons caused chromocenter declustering and disrupted the association of heterochromatin with the nuclear lamina. The loss of these structures did not affect neuronal viability but was associated with specific transcriptional and behavioural deficits related to serotonergic dysfunction. Overall, our results demonstrate that the 3D organization of chromatin within neuronal cells provides an additional level of epigenetic regulation of gene expression that critically impacts neuronal function. This in turn suggests that some loci associated with neuropsychiatric disorders may be particularly sensitive to changes in chromatin architecture.

Evaluation of single nucleotide polymorphisms in the miR-183-96-182 cluster in adulthood attention-deficit and hyperactivity disorder (ADHD) and substance use disorders (SUDs)

Attention deficit-hyperactivity disorder (ADHD) is a neuropsychiatric disorder characterized by inappropriate and impaired levels of hyperactivity, impulsivity and inattention. Around 75% of adults with ADHD show comorbidity with other psychiatric disorders such as disruptive behavior disorders or substance use disorders (SUDs). Recently, there has been growing interest in studying the role of microRNAs (miRNAs) in the susceptibility to complex disorders. Interestingly, converging evidence suggests that single nucleotide polymorphisms (SNPs) within miRNAs or miRNA target sites may modulate the miRNA-mediated regulation of gene expression through the alteration of the miRNA maturation, structure or expression pattern as well as the silencing mechanisms of target genes. Genetic studies and animal models support the involvement of the serotonin receptor (HTR1B) in ADHD. We evaluated the contribution of one SNP in the miR-96 target site at HTR1B and eight tagSNPs within the genomic region containing this miRNA in 695 adults with ADHD (266 and 396 subjects with and without comorbid SUD, respectively), 403 subjects with SUD without life-time diagnosis of ADHD and 485 sex-matched controls from Spain. Single and multiple marker analyses revealed association between two SNPs located at the 3' region of miR-96 (rs2402959 and rs6965643) and ADHD without SUD. Our results provide preliminary evidence for the contribution of two sequence variants at the miR-183-96-182 cluster to ADHD without comorbid SUD, and emphasize the need to take comorbidities into account in genetic studies to minimize the effect of heterogeneity and to clarify these complex phenotypes.

Attention deficit/hyperactivity disorder symptoms and depression symptoms as mediators in the intergenerational transmission of smoking

BACKGROUND

Attention deficit/hyperactivity disorder and depression have been found to be comorbid with smoking behaviors, and all three behavioral syndromes have been shown to be familially transmitted. The present paper reports on the results of analyses testing whether child attention deficit/hyperactivity disorder and depression symptoms were mediators in the intergenerational transmission of cigarette smoking.

METHOD

Path analyses using bootstrapped mediation procedures were conducted on data from a community sample of 764 families (one or both parents and one adolescent offspring) from the Indiana University Smoking Survey. Parents reported on their smoking behaviors, ADHD, and depression and their child's ADHD, while offspring reported on their smoking behaviors and depression.

RESULTS

Although fathers' and mothers' smoking status, depression, and ADHD were not significantly correlated with boys' smoking initiation, there was a significant mediated (indirect) pathway from mothers' depression to boys' smoking initiation through boys' depression. Several parental variables were significantly correlated with smoking initiation in girls, and the pathways from mothers' smoking status, mothers' ADHD, and fathers' smoking status to girls' smoking initiation were significantly mediated by girls' ADHD.

CONCLUSIONS

For adolescent girls, the intergenerational transmission of ADHD appears to be important in understanding the intergenerational transmission of cigarette smoking. Sex differences in the intergenerational transmission of psychopathology as it leads to smoking initiation were also discussed.

Normative database of the serotonergic system in healthy subjects using multi-tracer PET

The highly diverse serotonergic system with at least 16 different receptor subtypes is implicated in the pathophysiology of most neuropsychiatric disorders including affective and anxiety disorders, obsessive compulsive disorder, post-traumatic stress disorder, eating disorders, sleep disturbance, attention deficit/hyperactivity disorder, drug addiction, suicidal behavior, schizophrenia, Alzheimer, etc. Alterations of the interplay between various pre- and postsynaptic receptor subtypes might be involved in the pathogenesis of these disorders. However, there is a lack of comprehensive in vivo values using standardized procedures. In the current PET study we quantified 3 receptor subtypes, including the major inhibitory (5-HT(1A) and 5-HT(1B)) and excitatory (5-HT(2A)) receptors, and the transporter (5-HTT) in the brain of healthy human subjects to provide a database of standard values. PET scans were performed on 95 healthy subjects (age=28.0 ± 6.9 years; 59% males) using the selective radioligands [carbonyl-(11)C]WAY-100635, [(11)C]P943, [(18)F]altanserin and [(11)C]DASB, respectively. A standard template in MNI stereotactic space served for region of interest delineation. This template follows two anatomical parcellation schemes: 1) Brodmann areas including 41 regions and 2) AAL (automated anatomical labeling) including 52 regions. Standard values (mean, SD, and range) for each receptor and region are presented. Mean cortical and subcortical binding potential (BP) values were in good agreement with previously published human in vivo and post-mortem data. By means of linear equations, PET binding potentials were translated to post-mortem binding (provided in pmol/g), yielding 5.89 pmol/g (5-HT(1A)), 23.5 pmol/g (5-HT(1B)), 31.44 pmol/g (5-HT(2A)), and 11.33 pmol/g (5-HTT) being equivalent to the BP of 1, respectively. Furthermore, we computed individual voxel-wise maps with BP values and generated average tracer-specific whole-brain binding maps. This knowledge might improve our interpretation of the alterations taking place in the serotonergic system during neuropsychiatric disorders.

Biochemical and genetic analyses of childhood attention deficit/hyperactivity disorder

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

Epigenetics: genetics versus life experiences

Epigenetics is the field of research that examines alterations in gene expression caused by mechanisms other than changes in DNA sequence. ADHD is highly heritable; however, epigenetics are considered relevant in potentially explaining the variance not accounted for by genetic influence. In this chapter, some of the well-known processes of epigenetics, such as chromosome organization, DNA methylation, and effects of transcriptional factors are reviewed along with studies examining the role of these processes in the pathophysiology of ADHD. Potential epigenetic factors conferring risk for ADHD at various developmental stages, such as alcohol, tobacco, toxins, medications, and psychosocial stressor are discussed. Animal studies investigating ADHD medications and changes in CNS Gene/Protein Expression are also explored since they provide insight into the neuronal pathways involved in ADHD pathophysiology. The current limited data suggest that identification of the epigenetic processes involved in ADHD is extremely important and may lead to potential interventions that may be applied to modify the expression of deleterious, as well as protective, genes.

Quantitative and molecular genetics of ADHD

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

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

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

Serotonin and molecular neuroimaging in humans using PET

The serotonergic system is one of the most important modulatory neurotransmitter systems in the human brain. It plays a central role in major physiological processes and is implicated in a number of psychiatric disorders. Along with the dopaminergic system, it is also one of the phylogenetically oldest human neurotransmitter systems and one of the most diverse, with 14 different receptors identified up to this day, many of whose function remains to be understood. The system's functioning is even more diverse than the number of its receptors, since each is implicated in a number of different processes. This review aims at illustrating the distribution and summarizing the main functions of the serotonin (5-hydroxytryptamin, 5-HT) receptors as well as the serotonin transporter (SERT, 5-HTT), the vesicular monoamine transporter 2, monoamine oxidase type A and 5-HT synthesis in the human brain. Recent advances in in vivo quantification of these different receptors and enzymes that are part of the serotonergic system using positron emission tomography are described.

A randomized targeted amino acid therapy with behaviourally at-risk adopted children

BACKGROUND

Increasing numbers of children are at-risk for behavioural and emotional disorders, a phenomenon contributing to increased use of pharmacological interventions for paediatric clients. Adverse side effects and other risks associated with pharmacological approaches have helped fuel interest in nutritional interventions for behaviourally at-risk children.

METHODS

The current randomized clinical trial evaluates the efficacy of a neurochemical intervention involving the glutamine and glutamate analogue L-theanine and 5-hydroxytryptophan, the precursor for serotonin, with children adopted from traumatic backgrounds.

RESULTS

Results include significant increases in urinary levels of the biomarkers for serotonin and gamma-aminobutyric acid, coupled with significant decreases in parent reports of the children's behaviour problems.

CONCLUSIONS

While further research is needed, these initial findings are encouraging and are consistent with a growing number of studies indicating the efficacy of nutritional approaches to help behaviourally at-risk children.

Pharmacokinetic evaluation of eltoprazine

INTRODUCTION

Approvals and availability of drugs and delivery systems to treat attention-deficit/hyperactivity disorder (ADHD) have increased steadily in the last decade. The development of new pharmacological agents to treat ADHD symptoms in adults allows for both single and combination therapy for optimal efficacy. Eltoprazine hydrochloride, a serotonergic agent currently under development, is the focus of the current paper.

AREAS COVERED

This paper provides an overview of the pharmacokinetics (PK) of the non-stimulant medication eltoprazine. The PK profile of eltoprazine, based on standard PK sampling accompanied by safety monitoring, is described. A literature search and review of the studies published on eltoprazine were carried out using the PubMed database up to November 2010.

EXPERT OPINION

Although clinical studies of eltoprazine did not conclusively demonstrate its efficacy in treating pathological aggression in humans, eltoprazine has shown to be a well-tolerated drug in both healthy volunteers and patients across several indications studied thus far. More recently, the role of eltoprazine in reducing symptoms of ADHD in adults has been explored in a clinical trial. Future research may provide endophenotypical characteristics to help identify specific subgroups of patients with ADHD, who can benefit from the development of eltoprazine, to maximize efficacy while minimizing adverse reactions.

Test of variables of attention (TOVA) as a predictor of early attention complaints, an antecedent to dementia

The goal of this study was to determine if impairments detected by the test of variables of attention (TOVA) may be used to predict early attention complaints and memory impairments accurately in a clinical setting. We performed a statistical analysis of outcomes in a patient population screened for attention deficit hyperactivity disorder or attention complaints, processing errors as measured by TOVA and the Wechsler Memory Scale (WMS-III) results. Attention deficit disorder (ADD) checklists, constructed using the Diagnostic and Statistical Manual of Mental Disorders 4th Edition criteria, which were completed by patients at PATH Medical, revealed that 72.8% of the patients had more than one attention complaint out of a total of 16 complaints, and 41.5% had more than five complaints. For the 128 males with a significant number of ADD complaints, individuals whose scores were significantly deviant or borderline (SDB) on TOVA, had a significantly greater number of attention complaints compared with normals for omissions (P < 0.02), response time (P < 0.015), and variability (P < 0.005), but not commissions (P > 0.50). For males, the mean scores for auditory, visual, immediate, and working memory scores as measured by the WMS-III were significantly greater for normals versus SDBs on the TOVA subtest, ie, omission (P < 0.01) and response time (P < 0.05), but not variability or commissions. The means for auditory, visual, and immediate memory scores were significantly greater for normals versus SDBs for variability (P < 0.045) only. In females, the mean scores for visual and working memory scores were significantly greater for normals versus SDBs for omissions (P < 0.025). The number of SDB TOVA quarters was a significant predictor for "impaired" or "normal" group membership for visual memory (P < 0.015), but not for the other three WMS-III components. For males, the partial correlation between the number of attention complaints and the number of SDB TOVA quarters was also significant (r = 0.251, P < 0.005). For the 152 females with a significant number of attention complaints, no significant differences between SDBs and normals were observed (P > 0.15). This is the first report, to our knowledge, which provides evidence that TOVA is an accurate predictor of early attention complaints and memory impairments in a clinical setting. This finding is more robust for males than for females between the ages of 40 and 90 years.

A case-control association study of serotonin 1A receptor gene and tryptophan hydroxylase 2 gene in attention deficit hyperactivity disorder

Serotonergic system-related genes are likely to be involved in mechanisms underlying attention deficit hyperactivity disorder (ADHD). We investigated the association of serotonin the 1A receptor C-1019G single nucleotide polymorphism (HTR1A C-1019G SNP) and tryptophan hydroxylase 2 gene -703G/T (TPH2 -703G/T) SNP with ADHD. All of the ADHD subjects completed a comprehensive and standardized diagnostic and psychological evaluation battery including the parents' Korean version of the ADHD Rating Scale-IV (ARS). The genotype and allele frequencies of 78 ADHD patients and 107 normal controls were analyzed for 5-HTR1A C-1019G and TPH2 -703G/T. There were statistically significant differences in the genotype distributions and allele frequencies of HTR1A C-1019G between the ADHD group and the control group. The homozygous allele C frequency was significantly higher in ADHD patients than in controls. However, no differences in either genotype distribution or in allele frequencies of TPH2 -703G/T were observed between the ADHD patients and the controls. In the ADHD patients, ANCOVA revealed that there were no significant differences in the subscales and total score between the ADHD probands with the CC genotype and those with the CG and GG genotypes in ARS and the Continuous Performance Test (CPT) when adjusting for age and gender. The odds ratio comparing the CC genotype group with the CG genotype group and the C allele with G was 2.12 and 1.79 respectively. Therefore, genotype CC was associated with higher risk of ADHD. Our results suggest that the HTR1A C-1019G SNP may affect susceptibility to ADHD. Further investigation with a larger number of subjects is needed in order to confirm this finding.

ATRX partners with cohesin and MeCP2 and contributes to developmental silencing of imprinted genes in the brain

Human developmental disorders caused by chromatin dysfunction often display overlapping clinical manifestations, such as cognitive deficits, but the underlying molecular links are poorly defined. Here, we show that ATRX, MeCP2, and cohesin, chromatin regulators implicated in ATR-X, RTT, and CdLS syndromes, respectively, interact in the brain and colocalize at the H19 imprinting control region (ICR) with preferential binding on the maternal allele. Importantly, we show that ATRX loss of function alters enrichment of cohesin, CTCF, and histone modifications at the H19 ICR, without affecting DNA methylation on the paternal allele. ATRX also affects cohesin, CTCF, and MeCP2 occupancy within the Gtl2/Dlk1 imprinted domain. Finally, we show that loss of ATRX interferes with the postnatal silencing of the maternal H19 gene along with a larger network of imprinted genes. We propose that ATRX, cohesin, and MeCP2 cooperate to silence a subset of imprinted genes in the postnatal mouse brain.

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

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

Molecular genetics of attention deficit hyperactivity disorder

Although twin studies demonstrate that ADHD is a highly heritable condition, molecular genetic studies suggest that the genetic architecture of ADHD is complex. The handful of genome-wide linkage and association scans that have been conducted thus far show divergent findings and are, therefore, not conclusive. Similarly, many of the candidate genes reviewed here (ie, DBH, MAOA, SLC6A2, TPH-2, SLC6A4, CHRNA4, GRIN2A) are theoretically compelling from neurobiological systems perspective but available data are sparse and inconsistent. However, candidate gene studies of ADHD have produced substantial evidence implicating several genes in the etiology of the disorder, with meta-analyses supportive of a role of the genes coding for DRD4, DRD5, SLC6A3, SNAP-25, and HTR1B in the etiology of ADHD.

Functional polymorphisms in the serotonin 1B receptor gene (HTR1B) predict self-reported anger and hostility among young men

We examined associations between haplotypes of the serotonin 1B receptor gene and individual differences in anger and hostility. Data were analyzed from a study of 361 university students (47% male). Participants were genotyped at five polymorphisms in the HTR1B gene (rs11568817, rs130058, rs6296, rs6297, rs13212041), including promoter and 3'UTR polymorphisms with opposite functional effects on gene expression. Participants reported their emotional states across 30 consecutive days for up to 4 years. Haplotype pairs were constructed statistically and assigned to a level of HTR1B expression based on the presence of the functional polymorphisms. Six haplotypes accounted for >97% of chromosomes. Three low expression haplotypes contained the 3'UTR variant (rs13212041 A-allele) that enables a microRNA-mediated reduction in expression. One intermediate expression haplotype contained the 3'UTR A-allele paired with the high-activity promoter. Two high expression haplotypes contained the 3'UTR variant (rs13212041 G-allele) that attenuates microRNA-mediated reduction in expression. Men with low expression haplotypes reported greater anger and hostility than men with one or two high expression haplotypes. Diplotype classification accounted for 8.4% of the variance in men's anger and hostility, primarily due to the 3'UTR polymorphism (rs13212041), but with some contribution of the functional promoter combination (rs11568817, rs130058). Associations with anger and hostility were not found in women. These findings extend our understanding of the genetic basis of anger and hostility by showing that newly characterized HTR1B haplotypes, particularly those with rs13212041, which modulates microRNA-mediated regulation of HTR1B expression, may have important implications for aggression-related phenotypes among young men.

The genetics of attention-deficit/hyperactivity disorder

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

High-resolution imaging of brain 5-HT 1B receptors in the rhesus monkey using [11C]P943

The serotonin 5-HT(1B) receptors regulate the release of serotonin and are involved in various disease states, including depression and schizophrenia. The goal of the study was to evaluate a high affinity and high selectivity antagonist, [(11)C]P943, as a positron emission tomography (PET) tracer for imaging the 5-HT(1B) receptor. [(11)C]P943 was synthesized via N-methylation of the precursor with [(11)C]methyl iodide or [(11)C]methyl triflate using automated modules. The average radiochemical yield was approx. 10% with radiochemical purity of >99% and specific activity of 8.8+/-3.6 mCi/nmol at the end-of-synthesis (n=37). PET imaging was performed in non-human primates with a high-resolution research tomograph scanner with a bolus/infusion paradigm. Binding potential (BP(ND)) was calculated using the equilibrium ratios of regions to cerebellum. The tracer uptake was highest in the globus pallidus and occipital cortex, moderate in basal ganglia and thalamus, and lowest in the cerebellum, which is consistent with the known brain distribution of 5-HT(1B) receptors. Infusion of tracer at different specific activities (by adding various amount of unlabeled P943) reduced BP(ND) values in a dose-dependent manner, demonstrating the saturability of the tracer binding. Blocking studies with GR127935 (2 mg/kg iv), a selective 5-HT(1B)/5-HT(1D) antagonist, resulted in reduction of BP(ND) values by 42-95% across regions; for an example, in occipital region from 0.71 to 0.03, indicating a complete blockade. These results demonstrate the saturability and specificity of [(11)C]P943 for 5-HT(1B) receptors, suggesting its suitability as a PET radiotracer for in vivo evaluations of the 5-HT(1B) receptor system in humans.

Genetics of attention-deficit hyperactivity disorder (ADHD)

Attention-deficit hyperactivity disorder (ADHD) is a clinically and genetically heterogeneous syndrome which is comorbid with childhood conduct disorder, alcoholism, substance abuse, dis-social personality disorder, and affective disorders. A small but consistent overlap with autistic symptoms has also been established. Twin and family studies of ADHD show a substantial genetic heritability with little or no family environmental effect. Linkage and association studies have conclusively implicated the dopamine transporter gene (DAT1). DAT1 has also been confirmed as being associated with bipolar disorder. Remarkably, and for the first time in psychiatry, genetic markers at the DAT1 locus appear to be able to predict clinical heterogeneity because the non-conduct disordered subgroup of ADHD is associated with DAT1 whereas other subgroups do not appear to be associated. The second most well replicated susceptibility gene encodes the DRD4 dopamine receptor and many other dopamine related genes appear to be implicated. It is becoming increasingly clear that genes causing bipolar mania overlap with genes for a subtype of ADHD. The key to understanding the genetics of ADHD is to accept very considerable heterogeneity with different genes having effects in different families and in different individuals. It is too early to interpret the new wave of genome-wide association and copy number variant studies but preliminary data support the overlap with affective disorder genes and also with CNS connectivity genes likely to be involved in autism and affective disorders.

Suggestive linkage of ADHD to chromosome 18q22 in a young genetically isolated Dutch population

Attention deficit/hyperactivity disorder (ADHD) is a common, highly heritable, neuropsychiatric disorder among children. Linkage studies in isolated populations have proved powerful to detect variants for complex diseases, such as ADHD. We performed a genome-wide linkage scan for ADHD in nine patients from a genetically isolated population in the Netherlands, who were linked to each other within 10 generations through multiple lines of descent. The genome-wide scan was performed with a set of 400 microsatellite markers with an average spacing of +/-10-12 cM. We performed multipoint parametric linkage analyses using both recessive and dominant models. Our genome scan pointed to several chromosomal regions that may harbour ADHD susceptibility genes. None exceeded the empirical genome-wide significance threshold, but the Log of odds (LOD) scores were >1.5 for regions 6p22 (Heterogenetic log of odds (HLOD)=1.67) and 18q21-22 (HLOD=2.13) under a recessive model. We followed up these two regions in a larger sample of ADHD patients (n=21, 9 initial and 12 extra patients). The LOD scores did not increase after increasing the sample size (6p22 (HLOD=1.51), 18q21-22 (HLOD=1.83)). However, the LOD score on 6p22 increased to 2 when a separate analysis was performed for the inattentive type ADHD children. The linkage region on chromosome 18q overlaps with the findings of association of rs2311120 (P=10(-5)) and rs4149601 (P=10(-4)) in the genome-wide association analysis for ADHD performed by the Genetic Association Information Network consortium. Furthermore, there was an excess of regions harbouring serotonin receptors (HTR1B, HTR1E, HTR4, HTR1D, and HTR6) that showed a LOD score >1 in our genome-wide scan.

A common polymorphism in serotonin receptor 1B mRNA moderates regulation by miR-96 and associates with aggressive human behaviors

Non-coding regulatory elements can transduce the human genome's response to environmental stimuli. Thus, there is a possibility that variation in non-coding regulatory elements may underlie some of the diversity in human behavior. However, this idea has remained largely untested due to the difficulty in accurately identifying regulatory elements in the 98% of the human genome that does not encode protein. The recent recognition that small trans-acting RNAs anneal to mRNA and regulate gene expression provides a means to identify and test such variants. Here, we show that microRNA-directed silencing of mRNA can be attenuated by a common human polymorphism. We have identified an element (A-element) within serotonin receptor 1B (HTR1B) mRNA that confers repression by miR-96. The repressive activity of this element is attenuated by a common human variant (G-element) that disrupts a nucleotide critical for its interaction with miR-96. Because deletion of the HTR1B gene leads to an aggressive phenotype in mice, we hypothesized an association between the A/G polymorphism and aggressive phenotypes in a sample of 359 college students. As predicted, individuals homozygous for the ancestral A-element reported more conduct-disorder behaviors than individuals with the G-element. Our studies suggest that such functional variants may be common and may help to refine the search for genes involved in complex behavioral disorders.

Imprinted genes and neuroendocrine function

Imprinted genes are monoallelically expressed in a parent-of-origin dependent manner. Whilst the full functional repertoire of these genes remains obscure, they are generally highly expressed in the brain and are often involved in fundamental neural processes. Besides influencing brain neurochemistry, imprinted genes are important in the development and function of the hypothalamus and pituitary gland, key sites of neuroendocrine regulation. Moreover, imprinted genes may directly modulate hormone-dependent signalling cascades, both in the brain and elsewhere. Much of our knowledge about imprinted gene function has come from studying knockout mice and human disorders of imprinting. One such disorder is Prader-Willi syndrome, a neuroendocrine disorder characterised by hypothalamic abnormalities and aberrant feeding behaviour. Through examining the role of imprinted genes in neuroendocrine function, it may be possible to shed light on the neurobiological basis of feeding and aspects of social behaviour and underlying cognition, and to provide insights into disorders where these functions go awry.

Genetics of attention deficit hyperactivity disorder

Results of behavioral genetic and molecular genetic studies have converged to suggest that both genetic and nongenetic factors contribute to the development of attention deficit hyperactivity disorder (ADHD). Family, twin, and adoption studies provide compelling evidence that genes play a strong role in mediating susceptibility to ADHD. In contrast to a handful of genome-wide scans conducted thus far, many candidate gene studies of ADHD have produced substantial evidence implicating several genes in the etiology of the disorder. Yet, even these associations are small and consistent with the idea that the genetic vulnerability to ADHD is mediated by many genes of small effects. These small effects emphasize the need for future candidate gene studies to implement strategies that will provide enough statistical power to detect such small effects.

Molecular biology of 5-HT receptors

Serotonin (5-hydroxytryptamine; 5-HT) is a monoamine neurotransmitter whose effects are mediated by at least 13 distinct G protein-coupled receptors (GPCRs) of the type A family which includes the monoamine receptors and a combination of ligand-gated ion channels (5-HT3) of the Cys loop family which constitutes heteropentamers. 5-HT receptors are currently divided into seven classes (5-HT1 to 5-HT7), based on structural, transductional and operational features. While this degree of physical diversity clearly underscores the physiological importance of serotonin, evidence for an even greater degree of operational diversity is supported by the existence of a great number of splice and editing variants for several 5-HT receptors, their possible modulation by accessory proteins and chaperones, as well as their potential to form homo or heteromers both at the GPCR and at the ligand-gated channel level.

Serotonergic functioning and trait-impulsivity in attention-deficit/hyperactivity-disordered boys (ADHD): influence of rapid tryptophan depletion

OBJECTIVE

The present study investigated the effects of rapid tryptophan depletion (RTD) and the ensuing reduction of central nervous levels of serotonin (5-HT) on reactive aggression with respect to personality factors comprising aspects of trait-impulsivity and -aggression in boys with attention-deficit/hyperactivity-disorder (ADHD).

METHODS

Twenty-two male adolescent patients with ADHD received the RTD test on one day, and, on another day a tryptophan balanced placebo in a double-blind within-subject crossover design. Impulsive personality factors and trait-impulsivity were assessed in advance of the study. Aggression was provoked using a competitive reaction time game 270 min after RTD/placebo intake.

RESULTS

RTD had a significant effect on increased aggressive behaviour with which low-grade impulsive patients responded. High-grade impulsive patients were not affected by RTD or even responded with increased aggressive behaviour while receiving placebo treatment.

CONCLUSIONS

The present study supports the hypothesis that 5-HT functioning in ADHD patients influences reactive aggression depending on aspects of trait-impulsivity. Future studies are necessary in order to detect the specific influence of ADHD as regards the relevance to 5-HT-induced changed aggressive responding.

The effect of genetic variation of the serotonin 1B receptor gene on impulsive aggressive behavior and suicide

Impulsive-aggressive behaviors (IABs) are regarded as possible suicide intermediate phenotypes, mediating the relationship between genes and suicide outcome. In this study, we aimed to investigate the putative relationship between genetic variation at the 5-HT1B receptor gene, which in animal models is involved in impulse-aggression control, IABs, and suicide risk. We investigated the relationship of variation at five 5-HT1B loci and IAB measures in a sample of 696 subjects, including 338 individuals who died by suicide and 358 normal epidemiological controls. We found that variation at the 5-HT1B promoter A-161T locus had a significant effect on levels of IABs, as measured by the Buss-Durkee Hostility Inventory (BDHI). Suicides also differed from controls in distribution of variants at this locus. The A-161T locus, which seems to impact 5-HT1B transcription, could play a role in suicide predisposition by means of mediating impulsive-aggressive behaviors.

Role of the serotonin system in ADHD: treatment implications

A limited number of studies have considered whether the activity of serotonin (5-hydroxytryptamine [5-HT]) contributes to the problems experienced by youngsters with attention-deficit/hyperactivity disorder (ADHD). The aim of this article is to review this work and propose interpretations. Peripheral measures of 5-HT and its metabolite do not point to a widespread association with the diagnosis. However, separate consideration of the major domains of dysfunction (motor activity, inattention and impulsivity) support a more differentiated assessment. The marked innervation of motor regions of the brain by 5-HT projections and the clear involvement of 5-HT systems in the control of locomotion in animals suggests a likely node for dysfunction in ADHD. The few relevant studies do not show evidence of this, but more attention should be accorded to the issue. The situation is different for attention-related processes; here, there are deficiencies in perceptual sensitivity and the appropriate designation of saliency to stimulation. These are attributable, in part, to altered 5-HT activity. Marked and opposite changes of 5-HT responsivity are associated with behavioral and cognitive impulsivity. There is also a growing series of studies demonstrating preferential transmission of various genetic markers for 5-HT receptors that are expressed in ADHD. Currently, the heterogeneity of methods in this young discipline restricts the possibilities of definition of these markers and the types of ADHD in which they are expressed.

A review of association and linkage studies for genetical analyses of learning disorders

Learning disorders (LD) commonly comprise of a heterogeneous group of disorders manifested by unexpected problems in some children's experiences in the academic performance arena. These problems especially comprise of a variety of disorders which may be subclassified to attention-deficit hyperactivity disorder (ADHD), reading disability (RD), specific language impairment (SLI), speech-sound disorder (SSD), and dyspraxia. The aim of this review is to summarize the current molecular studies and some of the most exciting recent developments in molecular genetic research on LD. The findings for the association and linkage of LD with candidate genes will help to set the research agendas for future studies to follow.

Investigation of parent-of-origin effects in ADHD candidate genes

Attention deficit hyperactivity disorder (ADHD) is a common early-onset childhood disorder with a strong genetic component. Results from previous studies have suggested that there may be a parent-of-origin effect for ADHD candidate genes. In particular, a recent investigation identified a pattern of paternal over-transmission of risk alleles for nine ADHD candidate genes. We examined this phenomenon in a sample of 291 trios for five genes previously associated with ADHD (HTR1B, SNAP-25, DRD5, DAT1, and BDNF). Using a dense map of markers and two analytic methods in this relatively large family-based sample, we do not find any evidence for significant paternal over-transmission of risk alleles in these candidate loci. Thus, we conclude that a substantial parent-of-origin effect is unlikely for these leading ADHD candidate genes.