Genome-wide copy number variation study associates metabotropic glutamate receptor gene networks with attention deficit hyperactivity disorder

Glutamate/glutamine and neuronal integrity in adults with ADHD: a proton MRS study

There is increasing evidence that abnormalities in glutamate signalling may contribute to the pathophysiology of attention-deficit hyperactivity disorder (ADHD). Proton magnetic resonance spectroscopy ([1H]MRS) can be used to measure glutamate, and also its metabolite glutamine, in vivo. However, few studies have investigated glutamate in the brain of adults with ADHD naive to stimulant medication. Therefore, we used [1H]MRS to measure the combined signal of glutamate and glutamine (Glu+Gln; abbreviated as Glx) along with other neurometabolites such as creatine (Cr), N-acetylaspartate (NAA) and choline. Data were acquired from three brain regions, including two implicated in ADHD-the basal ganglia (caudate/striatum) and the dorsolateral prefrontal cortex (DLPFC)-and one 'control' region-the medial parietal cortex. We compared 40 adults with ADHD, of whom 24 were naive for ADHD medication, whereas 16 were currently on stimulants, against 20 age, sex and IQ-matched healthy controls. We found that compared with controls, adult ADHD participants had a significantly lower concentration of Glx, Cr and NAA in the basal ganglia and Cr in the DLPFC, after correction for multiple comparisons. There were no differences between stimulant-treated and treatment-naive ADHD participants. In people with untreated ADHD, lower basal ganglia Glx was significantly associated with more severe symptoms of inattention. There were no significant differences in the parietal 'control' region. We suggest that subcortical glutamate and glutamine have a modulatory role in ADHD adults; and that differences in glutamate-glutamine levels are not explained by use of stimulant medication.

Evidence-based guidelines for the pharmacological management of attention deficit hyperactivity disorder: update on recommendations from the British Association for Psychopharmacology

Attention deficit hyperactivity disorder (ADHD) is a common condition with a high societal burden. The present guidelines summarise current literature, generating expert consensus recommendations for the treatment of ADHD in children and adults. These guidelines also provide a review of recent research in the fields of neuroimaging, neuropsychology and genetics of ADHD. Novel discoveries in these areas have informed physiological models for the disease. Since the publication of the previous British Association for Psychopharmacology guidelines in 2008, new drugs have been licensed and further compounds are being investigated. The publication of randomised controlled trials of psychological interventions has contributed to the range of treatment options for ADHD. As the disorder has been diagnosed more frequently there has been greater focus on comorbid conditions and how they impact treatment. Services have continued to develop for the treatment of ADHD in adults and care agreements have been introduced to facilitate access to treatment.

Glutamate receptor mutations in psychiatric and neurodevelopmental disorders

Alterations in glutamatergic neurotransmission have long been associated with psychiatric and neurodevelopmental disorders (PNDD), but only recent advances in high-throughput DNA sequencing have allowed interrogation of the prevalence of mutations in glutamate receptors (GluR) among afflicted individuals. In this review we discuss recent work describing GluR mutations in the context of PNDDs. Although there are no strict relationships between receptor subunit or type and disease, some interesting preliminary conclusions have arisen. Mutations in genes coding for ionotropic glutamate receptor subunits, which are central to synaptic transmission and plasticity, are mostly associated with intellectual disability and autism spectrum disorders. In contrast, mutations of metabotropic GluRs, having a role on modulating neural transmission, are preferentially associated with psychiatric disorders. Also, the prevalence of mutations among GluRs is highly heterogeneous, suggesting a critical role of certain subunits in PNDD pathophysiology. The emerging bias between GluR subtypes and specific PNDDs may have clinical implications.

A case of 17q21.31 microduplication and 7q31.33 microdeletion, associated with developmental delay, microcephaly, and mild dysmorphic features

Concurrent cryptic microdeletion and microduplication syndromes have recently started to reveal themselves with the advent of microarray technology. Analysis has shown that low-copy repeats (LCRs) have allowed chromosome regions throughout the genome to become hotspots for nonallelic homologous recombination to take place. Here, we report a case of a 7.5-year-old girl who manifests microcephaly, developmental delay, and mild dysmorphic features. Microarray analysis identified a microduplication in chromosome 17q21.31, which encompasses the CRHR1, MAPT, and KANSL1 genes, as well as a microdeletion in chromosome 7q31.33 that is localised within the GRM8 gene. To our knowledge this is one of only a few cases of 17q21.31 microduplication. The clinical phenotype of patients with this microduplication is milder than of those carrying the reciprocal microdeletions, and suggests that the lower incidence of the former compared to the latter may be due to underascertainment.

Genome-wide copy number variation analysis in adult attention-deficit and hyperactivity disorder

Attention-deficit and hyperactivity disorder (ADHD) is a common psychiatric disorder with a worldwide prevalence of 5-6% in children and 4.4% in adults. Recently, copy number variations (CNVs) have been implicated in different neurodevelopmental disorders such as ADHD. Based on these previous reports that focused on pediatric cohorts, we hypothesize that structural variants may also contribute to adult ADHD and that such genomic variation may be enriched for CNVs previously identified in children with ADHD. To address this issue, we performed for the first time a whole-genome CNV study on 400 adults with ADHD and 526 screened controls. In agreement with recent reports in children with ADHD or in other psychiatric disorders, we identified a significant excess of insertions in ADHD patients compared to controls. The overall rate of CNVs >100 kb was 1.33 times higher in ADHD subjects than in controls (p = 2.4e-03), an observation mainly driven by a higher proportion of small events (from 100 kb to 500 kb; 1.35-fold; p = 1.3e-03). These differences remained significant when we considered CNVs that overlap genes or when structural variants spanning candidate genes for psychiatric disorders were evaluated, with duplications showing the greatest difference (1.41-fold, p = 0.024 and 2.85-fold, p = 8.5e-03, respectively). However, no significant enrichment was detected in our ADHD cohort for childhood ADHD-associated CNVs, CNVs previously identified in at least one ADHD patient or CNVs previously implicated in autism or schizophrenia. In conclusion, our study provides tentative evidence for a higher rate of CNVs in adults with ADHD compared to controls and contributes to the growing list of structural variants potentially involved in the etiology of the disease.

Genome-wide analysis of rare copy number variations reveals PARK2 as a candidate gene for attention-deficit/hyperactivity disorder

Attention-deficit/hyperactivity disorder (ADHD) is a common, highly heritable neurodevelopmental disorder. Genetic loci have not yet been identified by genome-wide association studies. Rare copy number variations (CNVs), such as chromosomal deletions or duplications, have been implicated in ADHD and other neurodevelopmental disorders. To identify rare (frequency ≤1%) CNVs that increase the risk of ADHD, we performed a whole-genome CNV analysis based on 489 young ADHD patients and 1285 adult population-based controls and identified one significantly associated CNV region. In tests for a global burden of large (>500 kb) rare CNVs, we observed a nonsignificant (P=0.271) 1.126-fold enriched rate of subjects carrying at least one such CNV in the group of ADHD cases. Locus-specific tests of association were used to assess if there were more rare CNVs in cases compared with controls. Detected CNVs, which were significantly enriched in the ADHD group, were validated by quantitative (q)PCR. Findings were replicated in an independent sample of 386 young patients with ADHD and 781 young population-based healthy controls. We identified rare CNVs within the parkinson protein 2 gene (PARK2) with a significantly higher prevalence in ADHD patients than in controls (P=2.8 × 10(-4) after empirical correction for genome-wide testing). In total, the PARK2 locus (chr 6: 162 659 756-162 767 019) harboured three deletions and nine duplications in the ADHD patients and two deletions and two duplications in the controls. By qPCR analysis, we validated 11 of the 12 CNVs in ADHD patients (P=1.2 × 10(-3) after empirical correction for genome-wide testing). In the replication sample, CNVs at the PARK2 locus were found in four additional ADHD patients and one additional control (P=4.3 × 10(-2)). Our results suggest that copy number variants at the PARK2 locus contribute to the genetic susceptibility of ADHD. Mutations and CNVs in PARK2 are known to be associated with Parkinson disease.

Copy number variation at 22q11.2: from rare variants to common mechanisms of developmental neuropsychiatric disorders

Recently discovered genome-wide rare copy number variants (CNVs) have unprecedented levels of statistical association with many developmental neuropsychiatric disorders, including schizophrenia, autism spectrum disorders, intellectual disability and attention deficit hyperactivity disorder. However, as CNVs often include multiple genes, causal genes responsible for CNV-associated diagnoses and traits are still poorly understood. Mouse models of CNVs are in use to delve into the precise mechanisms through which CNVs contribute to disorders and associated traits. Based on human and mouse model studies on rare CNVs within human chromosome 22q11.2, we propose that alterations of a distinct set of multiple, noncontiguous genes encoded in this chromosomal region, in concert with modulatory impacts of genetic background and environmental factors, variably shift the probabilities of phenotypes along a predetermined developmental trajectory. This model can be further extended to the study of other CNVs and may serve as a guide to help characterize the impact of genes in developmental neuropsychiatric disorders.

Candidate genetic pathways for attention-deficit/hyperactivity disorder (ADHD) show association to hyperactive/impulsive symptoms in children with ADHD

OBJECTIVE

Because multiple genes with small effect sizes are assumed to play a role in attention-deficit/hyperactivity disorder (ADHD) etiology, considering multiple variants within the same analysis likely increases the total explained phenotypic variance, thereby boosting the power of genetic studies. This study investigated whether pathway-based analysis could bring scientists closer to unraveling the biology of ADHD.

METHOD

The pathway was described as a predefined gene selection based on a well-established database or literature data. Common genetic variants in pathways involved in dopamine/norepinephrine and serotonin neurotransmission and genes involved in neuritic outgrowth were investigated in cases from the International Multicentre ADHD Genetics (IMAGE) study. Multivariable analysis was performed to combine the effects of single genetic variants within the pathway genes. Phenotypes were DSM-IV symptom counts for inattention and hyperactivity/impulsivity (n = 871) and symptom severity measured with the Conners Parent (n = 930) and Teacher (n = 916) Rating Scales.

RESULTS

Summing genetic effects of common genetic variants within the pathways showed a significant association with hyperactive/impulsive symptoms ((p)empirical = .007) but not with inattentive symptoms ((p)empirical = .73). Analysis of parent-rated Conners hyperactive/impulsive symptom scores validated this result ((p)empirical = .0018). Teacher-rated Conners scores were not associated. Post hoc analyses showed a significant contribution of all pathways to the hyperactive/impulsive symptom domain (dopamine/norepinephrine, (p)empirical = .0004; serotonin, (p)empirical = .0149; neuritic outgrowth, (p)empirical = .0452).

CONCLUSION

The present analysis shows an association between common variants in 3 genetic pathways and the hyperactive/impulsive component of ADHD. This study demonstrates that pathway-based association analyses, using quantitative measurements of ADHD symptom domains, can increase the power of genetic analyses to identify biological risk factors involved in this disorder.

RBFOX1 and RBFOX3 mutations in rolandic epilepsy

Partial deletions of the gene encoding the neuronal splicing regulator RBFOX1 have been reported in a range of neurodevelopmental diseases, including idiopathic generalized epilepsy. The RBFOX1 protein and its homologues (RBFOX2 and RBFOX3) regulate alternative splicing of many neuronal transcripts involved in the homeostatic control of neuronal excitability. In this study, we explored if structural microdeletions and exonic sequence variations in RBFOX1, RBFOX2, RBFOX3 confer susceptibility to rolandic epilepsy (RE), a common idiopathic focal childhood epilepsy. By high-density SNP array screening of 289 unrelated RE patients, we identified two hemizygous deletions, a 365 kb deletion affecting two untranslated 5′-terminal exons of RBFOX1 and a 43 kb deletion spanning exon 3 of RBFOX3. Exome sequencing of 242 RE patients revealed two novel probably deleterious variants in RBFOX1, a frameshift mutation (p.A233Vfs*74) and a hexanucleotide deletion (p.A299_A300del), and a novel nonsense mutation in RBFOX3 (p.Y287*). Although the three variants were inherited from unaffected parents, they were present in all family members exhibiting the RE trait clinically or electroencephalographically with only one exception. In contrast, no deleterious mutations of RBFOX1 and RBFOX3 were found in the exomes of 6503 non-RE subjects deposited in the Exome Variant Server database. The observed RBFOX3 exon 3 deletion and nonsense mutation suggest that RBFOX3 represents a novel risk factor for RE, indicating that exon deletions and truncating mutations of RBFOX1 and RBFOX3 contribute to the genetic variance of partial and generalized idiopathic epilepsy syndromes.

The role of rs2237781 within GRM8 in eating behavior

INTRODUCTION

The glutamate receptor, metabotropic 8 gene (GRM8) encodes a G-protein-coupled glutamate receptor and has been associated with smoking behavior and liability to alcoholism implying a role in addiction vulnerability. Data from animal studies suggest that GRM8 may be involved in the regulation of the neuropeptide Y and melanocortin pathways and might influence food intake and metabolism. This study aimed to investigate the effects of the genetic variant rs2237781 within GRM8 on human eating behavior.

METHODS

The initial analysis included 548 Sorbs from Germany who have been extensively phenotyped for metabolic traits and who completed the German version of the three-factor eating questionnaire. In addition, we analyzed two independent sample sets comprising 293 subjects from another German cohort and 430 Old Order Amish individuals. Genetic associations with restraint, disinhibition, and hunger were assessed in an additive linear regression model.

RESULTS

Among the Sorbs the major G allele of rs2237781 was significantly associated with increased restraint scores in eating behavior (P = 1.9 × 10(-4); β = +1.936). The German cohort and the Old Order Amish population revealed a trend in the same direction for restraint (P = 0.242; β = +0.874; P = 0.908; β = +0.096; respectively). A meta-analysis resulted in a combined P = 3.1 × 10(-3) (Z-score 2.948).

CONCLUSION

Our data suggest that rs2237781 within GRM8 may influence human eating behavior factors probably via pathways involved in addictive behavior.

Genetic causes of developmental disorders

PURPOSE OF REVIEW

Developmental disorders, including intellectual disability, autism and attention deficit hyperactivity disorder (ADHD), are neuropsychiatric disorders that manifest in early childhood as deviations from the normal development. At present, in the majority of cases a cause cannot be found. However, in the past 5 years major advances have been made in the identification of specific genetic causes of these disorders. Here, we review these findings and discuss possible implications for our current understanding of the cause of developmental disorders.

RECENT FINDINGS

In addition to the disorders with known genetic cause that are associated with intellectual disability, autism and ADHD, an increasing number of novel recurrent structural variants are identified in association with these developmental disorders. These variants, as well as the genetic variants identified through sequencing approaches indicate the involvement of a large number of genes.

SUMMARY

Similar to what is the case for intellectual disability, recent genetic studies indicate a large degree of genetic heterogeneity for autism and ADHD. Many of the disease risk variants display incomplete penetrance, indicating that additional genetic, and possibly nongenetic, factors are relevant. Despite the high number of causative or contributing genes, functional studies of these genes indicate a large degree of convergence into a smaller number of neurobiological pathways. Elucidating these shared biological mechanisms is a crucial step towards the rational development of novel therapeutic interventions.

Functional properties of rare missense variants of human CDH13 found in adult attention deficit/hyperactivity disorder (ADHD) patients

The CDH13 gene codes for T-cadherin, a GPI-anchored protein with cell adhesion properties that is highly expressed in the brain and cardiovascular system. Previous studies have suggested that CDH13 may be a promising candidate gene for Attention Deficit/Hyperactivity Disorder (ADHD). The aims of this study were to identify, functionally characterize, and estimate the frequency of coding CDH13 variants in adult ADHD patients and controls. We performed sequencing of the CDH13 gene in 169 Norwegian adult ADHD patients and 63 controls and genotyping of the identified variants in 641 patients and 668 controls. Native and green fluorescent protein tagged wild type and variant CDH13 proteins were expressed and studied in CHO and HEK293 cells, respectively. Sequencing identified seven rare missense CDH13 variants, one of which was novel. By genotyping, we found a cumulative frequency of these rare variants of 2.9% in controls and 3.2% in ADHD patients, implying that much larger samples are needed to obtain adequate power to study the genetic association between ADHD and rare CDH13 variants. Protein expression and localization studies in CHO cells and HEK293 cells showed that the wild type and mutant proteins were processed according to the canonical processing of GPI-anchored proteins. Although some of the mutations were predicted to severely affect protein secondary structure and stability, no significant differences were observed between the expression levels and distribution of the wild type and mutant proteins in either HEK293 or CHO cells. This is the first study where the frequency of coding CDH13 variants in patients and controls is reported and also where the functional properties of these variants are examined. Further investigations are needed to conclude whether CDH13 is involved in the pathogenesis of ADHD or other conditions.

Copy number variants in the kallikrein gene cluster

The kallikrein gene family (KLK1-KLK15) is the largest contiguous group of protease genes within the human genome and is associated with both risk and outcome of cancer and other diseases. We searched for copy number variants in all KLK genes using quantitative PCR analysis and analysis of inheritance patterns of single nucleotide polymorphisms. Two deletions were identified: one 2235-bp deletion in KLK9 present in 1.2% of alleles, and one 3394-bp deletion in KLK15 present in 4.0% of alleles. Each deletion eliminated one complete exon and created out-of-frame coding that eliminated the catalytic triad of the resulting truncated gene product, which therefore likely is a non-functional protein. Deletion breakpoints identified by DNA sequencing located the KLK9 deletion breakpoint to a long interspersed element (LINE) repeated sequence, while the deletion in KLK15 is located in a single copy sequence. To search for an association between each deletion and risk of prostate cancer (PC), we analyzed a cohort of 667 biopsied men (266 PC cases and 401 men with no evidence of PC at biopsy) using short deletion-specific PCR assays. There was no association between evidence of PC in this cohort and the presence of either gene deletion. Haplotyping revealed a single origin of each deletion, with most recent common ancestor estimates of 3000-8000 and 6000-14 000 years for the deletions in KLK9 and KLK15, respectively. The presence of the deletions on the same haplotypes in 1000 Genomes data of both European and African populations indicate an early origin of both deletions. The old age in combination with homozygous presence of loss-of-function variants suggests that some kallikrein-related peptidases have non-essential functions.

Background, clinical features and treatment of attention deficit hyperactivity disorder in children

INTRODUCTION

Attention deficit hyperactivity disorder (ADHD) is an early onset, clinically heterogeneous, complex neurobiological disorder, defined by symptoms of inattention and hyperactivity/impulsivity and has been associated with a broad range of impairments for those affected. Additionally, ADHD in children and adolescents is frequently associated with psychiatric comorbidities. This review provides an overview of the epidemiology, neurobiology, genetics, diagnosis and most recent pharmacological approaches for treatment with a focus on safety and efficacy and describes the use of medications used to treat ADHD in special populations.

AREAS COVERED

PubMed, Cochrane database, Essential Evidence and Uptodate were searched for relevant articles about stimulant and non-stimulant pharmacological approaches in ADHD.

EXPERT OPINION

Data supporting the safety and efficacy of both stimulant and non-stimulant formulations have significantly grown over the past decade and more efforts are being made to tailor medications to the needs of the patients and their families. Pharmacogenomics research is evolving, but predictors of treatment response and side effects remain largely unknown. Other unmet clinical needs include long-term follow-up studies of the safety and efficacy of medications for those with ADHD alone, or with comorbidities and in special populations including preschoolers.

GStream: improving SNP and CNV coverage on genome-wide association studies

We present GStream, a method that combines genome-wide SNP and CNV genotyping in the Illumina microarray platform with unprecedented accuracy. This new method outperforms previous well-established SNP genotyping software. More importantly, the CNV calling algorithm of GStream dramatically improves the results obtained by previous state-of-the-art methods and yields an accuracy that is close to that obtained by purely CNV-oriented technologies like Comparative Genomic Hybridization (CGH). We demonstrate the superior performance of GStream using microarray data generated from HapMap samples. Using the reference CNV calls generated by the 1000 Genomes Project (1KGP) and well-known studies on whole genome CNV characterization based either on CGH or genotyping microarray technologies, we show that GStream can increase the number of reliably detected variants up to 25% compared to previously developed methods. Furthermore, the increased genome coverage provided by GStream allows the discovery of CNVs in close linkage disequilibrium with SNPs, previously associated with disease risk in published Genome-Wide Association Studies (GWAS). These results could provide important insights into the biological mechanism underlying the detected disease risk association. With GStream, large-scale GWAS will not only benefit from the combined genotyping of SNPs and CNVs at an unprecedented accuracy, but will also take advantage of the computational efficiency of the method.

Polygenic transmission and complex neuro developmental network for attention deficit hyperactivity disorder: genome-wide association study of both common and rare variants

Attention-deficit hyperactivity disorder (ADHD) is a complex polygenic disorder. This study aimed to discover common and rare DNA variants associated with ADHD in a large homogeneous Han Chinese ADHD case-control sample. The sample comprised 1,040 cases and 963 controls. All cases met DSM-IV ADHD diagnostic criteria. We used the Affymetrix6.0 array to assay both single nucleotide polymorphisms (SNPs) and copy number variants (CNVs). Genome-wide association analyses were performed using PLINK. SNP-heritability and SNP-genetic correlations with ADHD in Caucasians were estimated with genome-wide complex trait analysis (GCTA). Pathway analyses were performed using the Interval enRICHment Test (INRICH), the Disease Association Protein-Protein Link Evaluator (DAPPLE), and the Genomic Regions Enrichment of Annotations Tool (GREAT). We did not find genome-wide significance for single SNPs but did find an increased burden of large, rare CNVs in the ADHD sample (P = 0.038). SNP-heritability was estimated to be 0.42 (standard error, 0.13, P = 0.0017) and the SNP-genetic correlation with European Ancestry ADHD samples was 0.39 (SE 0.15, P = 0.0072). The INRICH, DAPPLE, and GREAT analyses implicated several gene ontology cellular components, including neuron projections and synaptic components, which are consistent with a neurodevelopmental pathophysiology for ADHD. This study suggested the genetic architecture of ADHD comprises both common and rare variants. Some common causal variants are likely to be shared between Han Chinese and Caucasians. Complex neurodevelopmental networks may underlie ADHD's etiology.

Common obesity risk alleles in childhood attention-deficit/hyperactivity disorder

Children with attention-deficit/hyperactivity disorder (ADHD) have a higher rate of obesity than children without ADHD. Obesity risk alleles may overlap with those relevant for ADHD. We examined whether risk alleles for an increased body mass index (BMI) are associated with ADHD and related quantitative traits (inattention and hyperactivity/impulsivity). We screened 32 obesity risk alleles of single nucleotide polymorphisms (SNPs) in a genome-wide association study (GWAS) for ADHD based on 495 patients and 1,300 population-based controls and performed in silico analyses of the SNPs in an ADHD meta-analysis comprising 2,064 trios, 896 independent cases, and 2,455 controls. In the German sample rs206936 in the NUDT3 gene (nudix; nucleoside diphosphate linked moiety X-type motif 3) was associated with ADHD risk (OR: 1.39; P = 3.4 × 10(-4) ; Pcorr  = 0.01). In the meta-analysis data we found rs6497416 in the intronic region of the GPRC5B gene (G protein-coupled receptor, family C, group 5, member B; P = 7.2 × 10(-4) ; Pcorr  = 0.02) as a risk allele for ADHD. GPRC5B belongs to the metabotropic glutamate receptor family, which has been implicated in the etiology of ADHD. In the German sample rs206936 (NUDT3) and rs10938397 in the glucosamine-6-phosphate deaminase 2 gene (GNPDA2) were associated with inattention, whereas markers in the mitogen-activated protein kinase 5 gene (MAP2K5) and in the cell adhesion molecule 2 gene (CADM2) were associated with hyperactivity. In the meta-analysis data, MAP2K5 was associated with inattention, GPRC5B with hyperactivity/impulsivity and inattention and CADM2 with hyperactivity/impulsivity. Our results justify further research on the elucidation of the common genetic background of ADHD and obesity.

Dances with black widow spiders: dysregulation of glutamate signalling enters centre stage in ADHD

Attention-deficit/hyperactivity disorder (ADHD) is a common neurodevelopmental disorder with impairments across the lifespan. The persistence of ADHD is associated with considerable liability to neuropsychiatric co-morbidity such as depression, anxiety and substance use disorder. The substantial heritability of ADHD is well documented and recent genome-wide analyses for risk genes revealed synaptic adhesion molecules (e.g. latrophilin-3, LPHN3; fibronectin leucine-rich repeat transmembrane protein-3, FLRT3), glutamate receptors (e.g. metabotropic glutamate receptor-5, GRM5) and mediators of intracellular signalling pathways (e.g. nitric oxide synthase-1, NOS1). These genes encode principal components of the molecular machinery that connects pre- and postsynaptic neurons, facilitates glutamatergic transmission, controls synaptic plasticity and empowers intersecting neural circuits to process and refine information. Thus, identification of genetic variation affecting molecules essential for the formation, specification and function of excitatory synapses is refocusing research efforts on ADHD pathogenesis to include the long-neglected glutamate system.

Impact of the ADHD-susceptibility gene CDH13 on development and function of brain networks

Attention-deficit/hyperactivity disorder (ADHD) is a common, early onset and enduring neuropsychiatric disorder characterized by developmentally inappropriate inattention, hyperactivity, increased impulsivity and motivational/emotional dysregulation with similar prevalence rates throughout different cultural settings. Persistence of ADHD into adulthood is associated with considerable risk for co-morbidities such as depression and substance use disorder. Although the substantial heritability of ADHD is well documented the etiology is characterized by a complex coherence of genetic and environmental factors rendering identification of risk genes difficult. Genome-wide linkage as well as single nucleotide polymorphism (SNP) and copy-number variant (CNV) association scans recently allow to reliably define aetiopathogenesis-related genes. A considerable number of novel ADHD risk genes implicate biological processes involved in neurite outgrowth and axon guidance. Here, we focus on the gene encoding Cadherin-13 (CDH13), a cell adhesion molecule which was replicably associated with liability to ADHD and related neuropsychiatric conditions. Based on its unique expression pattern in the brain, we discuss the molecular structure and neuronal mechanisms of Cadherin-13 in relation to other cadherins and the cardiovascular system. An appraisal of various Cadherin-13-modulated signaling pathways impacting proliferation, migration and connectivity of specific neurons is also provided. Finally, we develop an integrative hypothesis of the mechanisms in which Cadherin-13 plays a central role in the regulation of brain network development, plasticity and function. The review concludes with emerging concepts about alterations in Cadherin-13 signaling contributing to the pathophysiology of neurodevelopmental disorders.

Is there a role for endogenous retroviruses to mediate long-term adaptive phenotypic response upon environmental inputs?

Endogenous retroviruses (ERVs) are long terminal repeat-containing virus-like elements that have colonized approximately 10 per cent of the present day mammalian genomes. The intracisternal A particles (IAPs) are a class of ERVs that is currently highly active in the rodents. IAP elements can influence the transcription profile of nearby genes by providing functional promoter elements and modulating local epigenetic landscape through changes in DNA methylation and histone (H3K9) modifications. Despite the potential role for IAPs in gene regulation, the precise genomic locations where these elements are integrated are not well understood. To address this issue, we have identified more than 400 novel IAP insertion sites within/near annotated genes by searching the murine genome, which suggests that the impact of IAP elements on local and/or global gene regulation could be more profound than was previously expected. On the basis of our independent analyses and already published reports, here we argue that IAPs and ERV elements in general could have an evolutionary role for modulating phenotypic plasticity upon environmental inputs, and that this could be mediated through specific stages of embryonic development such as placentation during which the epigenetic constraints on IAP elements are partially relaxed.

Rare genomic structural variants in complex disease: lessons from the replication of associations with obesity

The limited ability of common variants to account for the genetic contribution to complex disease has prompted searches for rare variants of large effect, to partly explain the 'missing heritability'. Analyses of genome-wide genotyping data have identified genomic structural variants (GSVs) as a source of such rare causal variants. Recent studies have reported multiple GSV loci associated with risk of obesity. We attempted to replicate these associations by similar analysis of two familial-obesity case-control cohorts and a population cohort, and detected GSVs at 11 out of 18 loci, at frequencies similar to those previously reported. Based on their reported frequencies and effect sizes (OR≥25), we had sufficient statistical power to detect the large majority (80%) of genuine associations at these loci. However, only one obesity association was replicated. Deletion of a 220 kb region on chromosome 16p11.2 has a carrier population frequency of 2×10(-4) (95% confidence interval [9.6×10(-5)-3.1×10(-4)]); accounts overall for 0.5% [0.19%-0.82%] of severe childhood obesity cases (P = 3.8×10(-10); odds ratio = 25.0 [9.9-60.6]); and results in a mean body mass index (BMI) increase of 5.8 kg.m(-2) [1.8-10.3] in adults from the general population. We also attempted replication using BMI as a quantitative trait in our population cohort; associations with BMI at or near nominal significance were detected at two further loci near KIF2B and within FOXP2, but these did not survive correction for multiple testing. These findings emphasise several issues of importance when conducting rare GSV association, including the need for careful cohort selection and replication strategy, accurate GSV identification, and appropriate correction for multiple testing and/or control of false discovery rate. Moreover, they highlight the potential difficulty in replicating rare CNV associations across different populations. Nevertheless, we show that such studies are potentially valuable for the identification of variants making an appreciable contribution to complex disease.

Phenotypic impact of genomic structural variation: insights from and for human disease

Genomic structural variants have long been implicated in phenotypic diversity and human disease, but dissecting the mechanisms by which they exert their functional impact has proven elusive. Recently however, developments in high-throughput DNA sequencing and chromosomal engineering technology have facilitated the analysis of structural variants in human populations and model systems in unprecedented detail. In this Review, we describe how structural variants can affect molecular and cellular processes, leading to complex organismal phenotypes, including human disease. We further present advances in delineating disease-causing elements that are affected by structural variants, and we discuss future directions for research on the functional consequences of structural variants.

Ask the Experts: Pharmacogenomics and genome-wide association studies

Hakon Hakonarson is an associate professor of pediatrics at The University of Pennsylvania School of Medicine (PA, USA). He is a physician–scientist and director of The Children’s Hospital of Philadelphia’s Center for Applied Genomics, a high-throughput highly automated genotyping facility founded to identify the genetic causes of complex medical disorders in children, such as autism and cancer, with the objective of developing new therapies. The Center represents a US$40 million commitment from The Children’s Hospital of Philadelphia to genotype approximately 100,000 children; a research undertaking that has gained nationwide attention, including news features in prestigious newspapers and scientific journals. Hakonarson has an extensive track record in human genetics and has developed an international reputation among his peers. He has served previously in several senior posts in the biopharmaceutical industry and has been the principal and coprincipal investigator on several NIH-sponsored grants. He has published numerous high-impact papers on genomic discoveries and their translations in some of the most prestigious scientific medical journals. Time Magazine listed Hakonarson’s autism gene discovery, reported in Nature in 2009, among the top ten medical breakthroughs of that year. With over 15 years of experience in pioneering genomics and pharmacogenomics research, as well as genome-wide mapping and association studies, Hakonarson has intimate knowledge of the complexities of large-scale genomics projects and has put together the necessary infrastructure and workflow processes to unravel these complexities.

No association between general cognitive ability and rare copy number variation

There is increasing evidence for the role of rare copy-number variation (CNV) in the development of neuropsychiatric disorders. It is likely that such variants also have an effect on the variation of cognition in what is considered the "normal" phenotypic range. The role of rare CNV (>20 KB in length; frequency <5 %) on general cognitive ability is investigated in a sample of 800 individuals (mean age = 16.5, SD = 1.2) using copy-number variants called from the Illumina 610K SNP genotyping array with the software QuantiSNP. We assessed three measures of CNV burden--total CNV length, number of CNV and average CNV length--for both deletions and duplications in combination and separately. No correlation was found between any of the measures of CNV burden and IQ, or when comparing the top and bottom 10 % of the sample for IQ, both on a genome-wide scale and at individual positions across the genome.

Rare exonic deletions of the RBFOX1 gene increase risk of idiopathic generalized epilepsy

PURPOSE

Structural variations disrupting the gene encoding the neuron-specific splicing regulator RBFOX1 have been reported in three patients exhibiting epilepsy in comorbidity with other neuropsychiatric disorders. Consistently, the Rbfox1 knockout mouse model showed an increased susceptibility of seizures. The present candidate gene study tested whether exon-disrupting deletions of RBFOX1 increase the risk of idiopathic generalized epilepsies (IGEs), representing the largest group of genetically determined epilepsies.

METHODS

Screening of microdeletions (size: >40 kb, coverage >20 markers) affecting the genomic sequence of the RBFOX1 gene was carried out by high-resolution single-nucleotide polymorphism (SNP) arrays in 1,408 European patients with idiopathic generalized epilepsy (IGE) and 2,256 population controls. Validation of RBFOX1 deletions and familial segregation analysis were performed by quantitative polymerase chain reaction (qPCR).

KEY FINDINGS

We detected five exon-disrupting RBFOX1 deletions in the IGE patients, whereas none was observed in the controls (p = 0.008, Fisher's exact test). The size of the exonic deletions ranged from 68 to 896 kb and affected the untranslated 5'-terminal RBFOX1 exons. Segregation analysis in four families indicated that the deletions were inherited, display incomplete penetrance, and heterogeneous cosegregation patterns with IGE.

SIGNIFICANCE

Rare deletions affecting the untranslated 5'-terminal RBFOX1 exons increase risk of common IGE syndromes. Variable expressivity, incomplete penetrance, and heterogeneous cosegregation patterns suggest that RBFOX1 deletions act as susceptibility factor in a genetically complex etiology, where heterogeneous combinations of genetic factors determine the disease phenotype.

TNiK is required for postsynaptic and nuclear signaling pathways and cognitive function

Traf2 and NcK interacting kinase (TNiK) contains serine-threonine kinase and scaffold domains and has been implicated in cell proliferation and glutamate receptor regulation in vitro. Here we report its role in vivo using mice carrying a knock-out mutation. TNiK binds protein complexes in the synapse linking it to the NMDA receptor (NMDAR) via AKAP9. NMDAR and metabotropic receptors bidirectionally regulate TNiK phosphorylation and TNiK is required for AMPA expression and synaptic function. TNiK also organizes nuclear complexes and in the absence of TNiK, there was a marked elevation in GSK3β and phosphorylation levels of its cognate phosphorylation sites on NeuroD1 with alterations in Wnt pathway signaling. We observed impairments in dentate gyrus neurogenesis in TNiK knock-out mice and cognitive testing using the touchscreen apparatus revealed impairments in pattern separation on a test of spatial discrimination. Object-location paired associate learning, which is dependent on glutamatergic signaling, was also impaired. Additionally, TNiK knock-out mice displayed hyperlocomotor behavior that could be rapidly reversed by GSK3β inhibitors, indicating the potential for pharmacological rescue of a behavioral phenotype. These data establish TNiK as a critical regulator of cognitive functions and suggest it may play a regulatory role in diseases impacting on its interacting proteins and complexes.

Child development and structural variation in the human genome

Structural variation of the human genome sequence is the insertion, deletion, or rearrangement of stretches of DNA sequence sized from around 1,000 to millions of base pairs. Over the past few years, structural variation has been shown to be far more common in human genomes than previously thought. Very little is currently known about the effects of structural variation on normal child development, but such effects could be of considerable significance. This review provides an overview of the phenomenon of structural variation in the human genome sequence, describing the novel genomics technologies that are revolutionizing the way structural variation is studied and giving examples of genomic structural variations that affect child development.

Altered gene expression in the prefrontal cortex of young rats induced by the ADHD drug atomoxetine

Atomoxetine (ATX), a selective norepinephrine reuptake inhibitor, is a non-stimulant approved for the treatment of attention deficit/hyperactivity disorder (ADHD). Little is known about the molecular basis for its therapeutic effect. The objective of this animal study was to determine alterations in gene expression patterns in the prefrontal cortex after long-term administration of atomoxetine. Rats were treated for 21 days during childhood and early adolescent stages of development with a once-daily oral application of 0.05 g/kg atomoxetine, which resulted in plasma levels similar to those described in children. A whole genome RNA-microarray of rat prefrontal cortical gene expression after administration of atomoxetine versus sterile water revealed an mRNA increase in 114 genes (≥2-fold) while 11 genes were down-regulated (≤0.5-fold). By applying quantitative real-time PCR (qRT-PCR) and Western Blot we confirmed a significant increase in the expression of GABA A receptor subunits as well as ubiquinol-cytochrome c reductase complex core protein 2 (Uqcrc2). SNAP-25 (synaptosomal-associated protein of 25 kDa), which is an ADHD candidate gene and an important vesicle protein involved in axonal growth, synaptic plasticity and regulation of neurotransmitter release was also significantly upregulated on RNA- and protein level after atomoxetine treatment. In summary, we could show that long-term treatment with the ADHD drug atomoxetine induces the regulation of several genes in the prefrontal cortex of young rats. Especially the increased expression of SNAP-25 and GABA-A receptor subunits may indicate additional active therapeutic mechanisms for atomoxetine.

Association between the GRM7 rs3792452 polymorphism and attention deficit hyperacitiveity disorder in a Korean sample

Background

The purpose of this study was to investigate the association between the ionotropic and glutamate receptors, N-methyl D-asparate 2A (GRIN2A) and 2B (GRIN2B), and the metabotropic glutamate receptor mGluR7 (GRM7) gene polymorphisms and attention-deficit hyperactivity disorder (ADHD) in Korean population.

Methods

We conducted a case–control analysis of 202 ADHD subjects and 159 controls, performed a transmission disequilibrium test (TDT) on 149 trios, and compared scores from the continuous performance test (CPT), the Children’s Depression Inventory (CDI), and the State-Trait Anxiety Inventory for Children (STAIC) according to the genotype of the glutamate receptor genes.

Results

There were no significant differences in the genotype or allele frequencies of the GRIN2A rs8049651, GRIN2B rs2284411, or GRM7 rs37952452 polymorphisms between the ADHD and control groups. For 148 ADHD trios, the TDT analysis also showed no preferential transmission of the GRIN2A rs8049651 or GRIN2B rs2284411 polymorphisms. However, the TDT analysis of the GRM7 rs3792452 polymorphism showed biased transmission of the G allele (χ2 = 4.67, p = 0.031). In the ADHD probands, the subjects with GG genotype in the GRM7 rs37952452 polymorphism had higher mean T-scores for omission errors on the CPT than did those with the GA or AA genotype (t = 3.38, p = 0.001). In addition, the ADHD subjects who were homozygous for the G allele in the GRM7 rs37952452 polymorphism had higher STAIC-T (t = 5.52, p < 0.001) and STAIC-S (t = 2.74, p = 0.007) scores than did those with the GA or AA genotype.

Conclusions

These results provide preliminary evidence of an association between the GRM7 rs37952452 polymorphism and selective attention deficit and anxiety found within the Korean ADHD population.

ParseCNV integrative copy number variation association software with quality tracking

A number of copy number variation (CNV) calling algorithms exist; however, comprehensive software tools for CNV association studies are lacking. We describe ParseCNV, unique software that takes CNV calls and creates probe-based statistics for CNV occurrence in both case–control design and in family based studies addressing both de novo and inheritance events, which are then summarized based on CNV regions (CNVRs). CNVRs are defined in a dynamic manner to allow for a complex CNV overlap while maintaining precise association region. Using this approach, we avoid failure to converge and non-monotonic curve fitting weaknesses of programs, such as CNVtools and CNVassoc, and although Plink is easy to use, it only provides combined CNV state probe-based statistics, not state-specific CNVRs. Existing CNV association methods do not provide any quality tracking information to filter confident associations, a key issue which is fully addressed by ParseCNV. In addition, uncertainty in CNV calls underlying CNV associations is evaluated to verify significant results, including CNV overlap profiles, genomic context, number of probes supporting the CNV and single-probe intensities. When optimal quality control parameters are followed using ParseCNV, 90% of CNVs validate by polymerase chain reaction, an often problematic stage because of inadequate significant association review. ParseCNV is freely available at http://parsecnv.sourceforge.net.

Copy-number disorders are a common cause of congenital kidney malformations

We examined the burden of large, rare, copy-number variants (CNVs) in 192 individuals with renal hypodysplasia (RHD) and replicated findings in 330 RHD cases from two independent cohorts. CNV distribution was significantly skewed toward larger gene-disrupting events in RHD cases compared to 4,733 ethnicity-matched controls (p = 4.8 × 10(-11)). This excess was attributable to known and novel (i.e., not present in any database or in the literature) genomic disorders. All together, 55/522 (10.5%) RHD cases harbored 34 distinct known genomic disorders, which were detected in only 0.2% of 13,839 population controls (p = 1.2 × 10(-58)). Another 32 (6.1%) RHD cases harbored large gene-disrupting CNVs that were absent from or extremely rare in the 13,839 population controls, identifying 38 potential novel or rare genomic disorders for this trait. Deletions at the HNF1B locus and the DiGeorge/velocardiofacial locus were most frequent. However, the majority of disorders were detected in a single individual. Genomic disorders were detected in 22.5% of individuals with multiple malformations and 14.5% of individuals with isolated urinary-tract defects; 14 individuals harbored two or more diagnostic or rare CNVs. Strikingly, the majority of the known CNV disorders detected in the RHD cohort have previous associations with developmental delay or neuropsychiatric diseases. Up to 16.6% of individuals with kidney malformations had a molecular diagnosis attributable to a copy-number disorder, suggesting kidney malformations as a sentinel manifestation of pathogenic genomic imbalances. A search for pathogenic CNVs should be considered in this population for the diagnosis of their specific genomic disorders and for the evaluation of the potential for developmental delay.

Folate metabolism gene 5,10-methylenetetrahydrofolate reductase (MTHFR) is associated with ADHD in myelomeningocele patients

The objective of this study was to examine the relation between the 5, 10-methylenetetrahydrofolate reductase (MTHFR) gene and behaviors related to attention- deficit/hyperactivity disorder (ADHD) in individuals with myelomeningocele. The rationale for the study was twofold: folate metabolizing genes, (e.g. MTHFR), are important not only in the etiology of neural tube defects but are also critical to cognitive function; and individuals with myelomeningocele have an elevated incidence of ADHD. Here, we tested 478 individuals with myelomeningocele for attention-deficit hyperactivity disorder behavior using the Swanson Nolan Achenbach Pelham-IV ADHD rating scale. Myelomeningocele participants in this group for whom DNAs were available were genotyped for seven single nucleotide polymorphisms (SNPs) in the MTHFR gene. The SNPs were evaluated for an association with manifestation of the ADHD phenotype in children with myelomeningocele. The data show that 28.7% of myelomeningocele participants exhibit rating scale elevations consistent with ADHD; of these 70.1% had scores consistent with the predominantly inattentive subtype. In addition, we also show a positive association between the SNP rs4846049 in the 3'-untranslated region of the MTHFR gene and the attention-deficit hyperactivity disorder phenotype in myelomeningocele participants. These results lend further support to the finding that behavior related to ADHD is more prevalent in patients with myelomeningocele than in the general population. These data also indicate the potential importance of the MTHFR gene in the etiology of the ADHD phenotype.

Confluence of genes, environment, development, and behavior in a post Genome-Wide Association Study world

This article serves to outline a research paradigm to investigate main effects and interactions of genes, environment, and development on behavior and psychiatric illness. We provide a historical context for candidate gene studies and genome-wide association studies, including benefits, limitations, and expected payoffs. Using substance use and abuse as our driving example, we then turn to the importance of etiological psychological theory in guiding genetic, environmental, and developmental research, as well as the utility of refined phenotypic measures, such as endophenotypes, in the pursuit of etiological understanding and focused tests of genetic and environmental associations. Phenotypic measurement has received considerable attention in the history of psychology and is informed by psychometrics, whereas the environment remains relatively poorly measured and is often confounded with genetic effects (i.e., gene-environment correlation). Genetically informed designs, which are no longer limited to twin and adoption studies thanks to ever-cheaper genotyping, are required to understand environmental influences. Finally, we outline the vast amount of individual difference in structural genomic variation, most of which remains to be leveraged in genetic association tests. Although the genetic data can be massive and burdensome (tens of millions of variants per person), we argue that improved understanding of genomic structure and function will provide investigators with new tools to test specific a priori hypotheses derived from etiological psychological theory, much like current candidate gene research but with less confusion and more payoff than candidate gene research has to date.

Developmental psychopathology: The role of structural variation in the genome

A wide range of developmental disorders present with characteristic psychopathologies and behaviors, with diagnoses including, inter alia, cognitive disorders and learning disabilities, epilepsies, autism, and schizophrenia. Each, to varying extent, has a genetic component to etiology and is associated with cytogenetic abnormalities. Technological developments, particularly array-based comparative genome hybridization and single nucleotide polymorphism chips, has revealed a wide range of rare recurrent and de novo copy number variants (CNVs) to be associated with disorder and psychopathology. It is surprising that many apparently similar CNVs are identified across two or more disorders hitherto considered unrelated. This article describes the characteristics of CNVs and current technological restrictions that make accurately identifying small events difficult. It summarizes the latest discoveries for individual diagnostic categories and considers the implications for a shared neurobiology. It examines likely developments in the knowledge base as well as addressing the clinical implications going forward.

Attention-deficit/hyperactivity disorder genomics: update for clinicians

Attention deficit, hyperactivity disorder (ADHD) is familial and highly heritable. Several candidate genes involved in neurotransmission have been identified, however these confer minimal risk, suggesting that for the most part, ADHD is not caused by single common genetic variants. Advances in genotyping enabling investigation at the level of the genome have led to the discovery of rare structural variants suggesting that ADHD is a genomic disorder, with potentially thousands of variants, and common neuronal pathways disrupted by numerous rare variants resulting in similar ADHD phenotypes. Heritability studies in humans also indicate the importance of epigenetic factors, and animal studies are deciphering some of the processes that confer risk during gestation and throughout the post-natal period. These and future discoveries will lead to improved diagnosis, individualized treatment, cures, and prevention. These advances also highlight ethical and legal issues requiring management and interpretation of genetic data and ensuring privacy and protection from misuse.

De novo mutations in neurological and psychiatric disorders: effects, diagnosis and prevention

Neurological and psychiatric disorders account for a considerable proportion of the global disease burden. Although there is a high heritability and a significant genetic component in these disorders, the genetic cause of most cases has yet to be identified. Advances in DNA sequencing allowing the analysis of the whole human genome in a single experiment have led to an acceleration of the discovery of the genetic factors associated with human disease. Recent studies using these platforms have highlighted the important role of de novo mutations in neurological and psychiatric disorders. These findings have opened new avenues into the understanding of genetic disease mechanisms. These discoveries, combined with the increasing ease with which we can sequence the human genome, have important implications for diagnosis, prevention and treatment. Here, we present an overview of the recent discovery of de novo mutations using key examples of neurological and psychiatric disorders. We also discuss the impact of technological developments on diagnosis and prevention.

A role of genomic copy number variation in the complex behavioral phenotype of alcohol dependence: a commentary

In their paper "Copy number variations in 6q14.1 and 5q13.2 are associated with alcohol dependence" Lin and colleagues report on the association between alcohol dependence and 2 duplication CNVs in the genome sequence, one containing 8 genes within its boundaries and another that contains no genes. In this commentary, I point out some of the opportunities and challenges that arise from such a finding.

Phasing of many thousands of genotyped samples

Haplotypes are an important resource for a large number of applications in human genetics, but computationally inferred haplotypes are subject to switch errors that decrease their utility. The accuracy of computationally inferred haplotypes increases with sample size, and although ever larger genotypic data sets are being generated, the fact that existing methods require substantial computational resources limits their applicability to data sets containing tens or hundreds of thousands of samples. Here, we present HAPI-UR (haplotype inference for unrelated samples), an algorithm that is designed to handle unrelated and/or trio and duo family data, that has accuracy comparable to or greater than existing methods, and that is computationally efficient and can be applied to 100,000 samples or more. We use HAPI-UR to phase a data set with 58,207 samples and show that it achieves practical runtime and that switch errors decrease with sample size even with the use of samples from multiple ethnicities. Using a data set with 16,353 samples, we compare HAPI-UR to Beagle, MaCH, IMPUTE2, and SHAPEIT and show that HAPI-UR runs 18× faster than all methods and has a lower switch-error rate than do other methods except for Beagle; with the use of consensus phasing, running HAPI-UR three times gives a slightly lower switch-error rate than Beagle does and is more than six times faster. We demonstrate results similar to those from Beagle on another data set with a higher marker density. Lastly, we show that HAPI-UR has better runtime scaling properties than does Beagle so that for larger data sets, HAPI-UR will be practical and will have an even larger runtime advantage. HAPI-UR is available online (see Web Resources).

ADHD and autism: differential diagnosis or overlapping traits? A selective review

According to DSM-IV TR and ICD-10, a diagnosis of autism or Asperger Syndrome precludes a diagnosis of attention-deficit/hyperactivity disorder (ADHD). However, despite the different conceptualization, population-based twin studies reported symptom overlap, and a recent epidemiologically based study reported a high rate of ADHD in autism and autism spectrum disorders (ASD). In the planned revision of the DSM-IV TR, dsm5 (www.dsm5.org), the diagnoses of autistic disorder and ADHD will not be mutually exclusive any longer. This provides the basis of more differentiated studies on overlap and distinction between both disorders. This review presents data on comorbidity rates and symptom overlap and discusses common and disorder-specific risk factors, including recent proteomic studies. Neuropsychological findings in the areas of attention, reward processing, and social cognition are then compared between both disorders, as these cognitive abilities show overlapping as well as specific impairment for one of both disorders. In addition, selective brain imaging findings are reported. Therapeutic options are summarized, and new approaches are discussed. The review concludes with a prospectus on open questions for research and clinical practice.

Transient mGlu5R inhibition enhances the survival of granule cell precursors in the neonatal cerebellum

The generation of the most abundant neurons of the cerebellum, the granule cells, relies on a balance between clonal expansion and apoptosis during the first 10 days after birth in the external germinal layer (EGL). The amino acid glutamate controls such critical phases of cell development in other systems through specific receptors such as metabotropic glutamate receptor 5 (mGlu(5)R). However, the function of mGlu(5)Rs on the proliferation and survival of granule cell precursors (GCPs) remains elusive. We found mGlu(5)R mRNA transcripts in EGL using RT-PCR and observed mGlu(5)R-mediated Ca(2+) responses in GCPs in acute slices as early as postnatal day (P) 2-3. Using in vivo injections of the selective non-competitive mGlu(5)R antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) in P7-P9 mice, we found a 20% increase in the number of proliferative GCPs labeled at P7 with the S-phase marker bromodeoxyuridine (BrdU), but no increase in cell proliferation examined 2h following a BrdU injection. Furthermore, similar treatments led to a significant 70% decrease in the number of apoptotic GCPs in the EGL as determined by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining. In contrast, in vivo treatment with the mGlu(5)R agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG) resulted in a ∼60% increase in the number of TUNEL-labeled GCPs compared to control. These findings identify a unique role for glutamate acting at mGlu(5)Rs as a functional switch regulating GCP survival in the EGL, thus controlling the total number of cerebellar granule cells produced.

Small rare recurrent deletions and reciprocal duplications in 2q21.1, including brain-specific ARHGEF4 and GPR148

We have identified a rare small (~450 kb unique sequence) recurrent deletion in a previously linked attention-deficit hyperactivity disorder (ADHD) locus at 2q21.1 in five unrelated families with developmental delay (DD)/intellectual disability (ID), ADHD, epilepsy and other neurobehavioral abnormalities from 17 035 samples referred for clinical chromosomal microarray analysis. Additionally, a DECIPHER (http://decipher.sanger.ac.uk) patient 2311 was found to have the same deletion and presented with aggressive behavior. The deletion was not found in either six control groups consisting of 13 999 healthy individuals or in the DGV database. We have also identified reciprocal duplications in five unrelated families with autism, developmental delay (DD), seizures and ADHD. This genomic region is flanked by large, complex low-copy repeats (LCRs) with directly oriented subunits of ~109 kb in size that have 97.7% DNA sequence identity. We sequenced the deletion breakpoints within the directly oriented paralogous subunits of the flanking LCR clusters, demonstrating non-allelic homologous recombination as a mechanism of formation. The rearranged segment harbors five genes: GPR148, FAM123C, ARHGEF4, FAM168B and PLEKHB2. Expression of ARHGEF4 (Rho guanine nucleotide exchange factor 4) is restricted to the brain and may regulate the actin cytoskeletal network, cell morphology and migration, and neuronal function. GPR148 encodes a G-protein-coupled receptor protein expressed in the brain and testes. We suggest that small rare recurrent deletion of 2q21.1 is pathogenic for DD/ID, ADHD, epilepsy and other neurobehavioral abnormalities and, because of its small size, low frequency and more severe phenotype might have been missed in other previous genome-wide screening studies using single-nucleotide polymorphism analyses.

Genetics and hypertension: is it time to change my practice?

Recent advances in genotyping technology and in particular a number of large-scale genome-wide association studies have helped to unravel the genetic basis of hypertension. Although our knowledge is still far from being complete it is important to ask how genetic findings could be translated to clinical practice. In a first step we summarize the strategies to dissect the genetics of hypertension from candidate gene studies to genome-wide association studies and recent sequencing experiments. The greatest hope in this context is the development of new drugs that are based on newly discovered pathophysiological principles. We describe examples where significant therapeutic effects are achieved with agents targeting pathways that contribute only small amounts to the genetic variability of a phenotype. There are good reasons to believe that new drugs will be developed based on genetic data in hypertension. We also highlight the potential for pharmacogenetics and risk stratification. The former is not currently supported by a larger body of evidence, but well designed studies are under way. The latter needs to follow the same principles for evaluation of other novel biomarkers of cardiovascular risk and is unlikely to influence clinical practice in the next few years.

CNVs: harbingers of a rare variant revolution in psychiatric genetics

The genetic bases of neuropsychiatric disorders are beginning to yield to scientific inquiry. Genome-wide studies of copy number variation (CNV) have given rise to a new understanding of disease etiology, bringing rare variants to the forefront. A proportion of risk for schizophrenia, bipolar disorder, and autism can be explained by rare mutations. Such alleles arise by de novo mutation in the individual or in recent ancestry. Alleles can have specific effects on behavioral and neuroanatomical traits; however, expressivity is variable, particularly for neuropsychiatric phenotypes. Knowledge from CNV studies reflects the nature of rare alleles in general and will serve as a guide as we move forward into a new era of whole-genome sequencing.