Cognitive functioning in affected sibling pairs with ADHD: familial clustering and dopamine genes

A family-based study of genetic and epigenetic effects across multiple neurocognitive, motor, social-cognitive and social-behavioral functions

Many psychiatric and neurodevelopmental disorders are known to be heritable, but studies trying to elucidate the genetic architecture of such traits often lag behind studies of somatic traits and diseases. The reasons as to why relatively few genome-wide significant associations have been reported for such traits have to do with the sample sizes needed for the detection of small effects, the difficulty in defining and characterizing the phenotypes, partially due to overlaps in affected underlying domains (which is especially true for cognitive phenotypes), and the complex genetic architectures of the phenotypes, which are not wholly captured in traditional case-control GWAS designs. We aimed to tackle the last two issues by performing GWASs of eight quantitative neurocognitive, motor, social-cognitive and social-behavioral traits, which may be considered endophenotypes for a variety of psychiatric and neurodevelopmental conditions, and for which we employed models capturing both general genetic association and parent-of-origin effects, in a family-based sample comprising 402 children and their parents (mostly family trios). We identified 48 genome-wide significant associations across several traits, of which 3 also survived our strict study-wide quality criteria. We additionally performed a functional annotation of implicated genes, as most of the 48 associations were with variants within protein-coding genes. In total, our study highlighted associations with five genes (TGM3, CACNB4, ANKS1B, CSMD1 and SYNE1) associated with measures of working memory, processing speed and social behavior. Our results thus identify novel associations, including previously unreported parent-of-origin associations with relevant genes, and our top results illustrate new potential gene → endophenotype → disorder pathways.

Set Shifting and Inhibition Deficits as Potential Endophenotypes for Depression

The etiology of Major Depressive Disorder (MDD) is poorly understood, and identifying endophenotypes, or intermediate processes implicated in pathophysiology, for MDD may inform treatment and identification/prevention efforts. Impaired set-shifting and inhibition are commonly observed in MDD; however, few studies have examined they are endophenotypes for MDD. Thus, the present study tested whether set-shifting and/or inhibition satisfy several endophenotype criteria: specifically, whether they were (1) impaired in current MDD, (2) impaired in remitted MDD, and (3) familial (i.e., correlated within sibling pairs). Set-shifting and inhibition were assessed using subtests from the Delis-Kaplan Executive Function System. Psychopathology was assessed using the Structured Clinical Interview for DSM-5. Results indicated set-shifting deficits were familial and present in both current MDD and in remitted MDD individuals who had no current disorders, suggesting they may be state-independent. Inhibition was familial, but was generally not impaired in current nor remitted MDD (although the remitted MDD group with no current disorders exhibited impairments on one of the two inhibition tasks). These findings indicate that impaired set-shifting is a promising endophenotype candidate for MDD. Findings are limited to young adults, and further research is needed to test generalizability to other populations, evaluate longitudinal relationships, and examine other endophenotype criteria.

Dopamine Related Genes Differentially Affect Declarative Long-Term Memory in Healthy Humans

In humans, monetary reward can promote behavioral performance including response times, accuracy, and subsequent recognition memory. Recent studies have shown that the dopaminergic system plays an essential role here, but the link to interindividual differences remains unclear. To further investigate this issue, we focused on previously described polymorphisms of genes affecting dopaminergic neurotransmission: DAT1 40 base pair (bp), DAT1 30 bp, DRD4 48 bp, and cannabinoid receptor type 1 (CNR1). Specifically, 669 healthy humans participated in a delayed recognition memory paradigm on two consecutive days. On the first day, male vs. female faces served as cues predicting an immediate monetary reward upon correct button presses. Subsequently, participants performed a remember/know recognition memory task on the same day and 1 day later. As predicted, reward increased accuracy and accelerated response times, which were modulated by DAT 30 bp. However, reward did not promote subsequent recognition memory performance and there was no interaction with any genotype tested here. Importantly, there were differential effects of genotype on declarative long-term memory independent of reward: (a) DAT1 40 bp was linked to the quality of memory with a more pronounced difference between recollection and familiarity in the heterozygous and homozygous 10-R as compared to homozygous 9-R; (b) DAT1 30 bp was linked to memory decay, which was most pronounced in homozygous 4-R; (c) DRD4 48 bp was linked to overall recognition memory with higher performance in the short allele group; and (d) CNR1 was linked to overall memory with reduced performance in the homozygous short group. These findings give new insights into how polymorphisms, which are related to dopaminergic neuromodulation, differentially affect long-term recognition memory performance.

Fluid Reasoning Mediates the Association of Birth Weight With ADHD Symptoms in Youth From Multiplex Families With ADHD

OBJECTIVE

We tested mediation of birth weight and ADHD symptoms by multiple biologically plausible neurocognitive functions and evaluated familiality of observed indirect effects.

METHOD

647 youth from 284 multiplex families with ADHD completed the Arithmetic, Digit Span, Vocabulary, and Block Design subtests of the Wechsler Intelligence Scale for Children (WISC). Multiple mediation tested WISC subtests as mediators of birth weight and multi-informant ADHD symptoms. Familiality of indirect effects was estimated via moderated mediation comparing conditional indirect effects across siblings concordant and discordant for ADHD.

RESULTS

Controlling for IQ and demographic factors, Arithmetic uniquely mediated birth weight and ADHD symptoms. Conditional indirect effects through Arithmetic did not differ across ADHD concordant and discordant siblings.

CONCLUSION

These cross-sectional findings support previous prospective longitudinal research implicating Arithmetic (i.e., fluid reasoning) as a preliminary causal mediator of birth weight and ADHD symptoms, and suggest that this pathway is independent of genetic influences on ADHD.

Pathways from Birth Weight to ADHD Symptoms through Fluid Reasoning in Youth with or without Intellectual Disability

Although individual differences in fluid reasoning reliably mediate predictions of attention-deficit/hyperactivity disorder (ADHD) symptoms from birth weight in youth with typical cognitive development (TD), it is unknown if this indirect effect operates similarly in the development of ADHD symptoms secondary to intellectual disability (ID). Thus, we evaluated mediation by fluid reasoning in a longitudinal sample of 163 youth (45% female) with (n = 52) or without (n = 111) ID who were followed prospectively from age 5 to age 13. At age 9, youth completed the Arithmetic subtest of the Wechsler Intelligence Scale for Children, a measure of fluid reasoning. At ages 9 and 13, mothers and teachers separately rated youth ADHD symptoms and mothers completed a diagnostic interview. Mediation was tested via path analysis with bootstrapped confidence intervals, and moderated mediation estimated whether indirect effects differed between ID and TD youth or based on youth IQ. Controlling for demographic factors and age 9 ADHD symptoms, age 9 Arithmetic mediated birth weight and multi-method/informant age 13 ADHD symptoms, such that birth weight positively predicted Arithmetic, which negatively predicted ADHD symptoms. Neither ID status nor IQ moderated the observed indirect effect through Arithmetic, suggesting that it was similar for ID and TD youth as well as across the range of youth IQs. These findings support previous evidence that fluid reasoning, as measured by Arithmetic, may causally mediate birth weight and ADHD symptoms, and suggest that this pathway operates similarly with respect to the development of ADHD symptoms in youth with ID.

COMT and DAT1 polymorphisms moderate the indirect effect of parenting behavior on youth ADHD symptoms through neurocognitive functioning

Although gene × environment interactions contribute to youth attention-deficit/hyperactivity disorder (ADHD) symptoms, the pathways mediating these influences are unknown. We tested genetic moderation of indirect effects from parenting behavior to youth ADHD symptoms through multiple neurocognitive factors. Two hundred and twenty-nine youth with and without ADHD were assessed at baseline (Wave 1; ages 5-10) and at a 2-year follow-up (Wave 2; ages 7-13). At Wave 1, youth completed a neurocognitive battery including measures of response inhibition, visuospatial working memory, and fluid reasoning, and a standardized parent-child interaction task yielding observational measures of positive and negative parenting. At Wave 2, youth psychopathology was rated by parents and teachers using multiple methods (i.e., structured interview, rating scale). We employed moderated multiple mediation and compared conditional indirect effects across youth genotypes at two biologically plausible genetic loci. Controlling for parent ADHD symptoms as well as youth demographic factors and co-occurring externalizing symptoms, these genetic factors moderated the indirect effect from Wave 1 parenting to multi-method/informant Wave 2 ADHD symptoms through Wave 1 neurocognitive functioning. This preliminary study is the first to identify genetic moderation of mediated effects underlying ADHD symptoms and suggests that specific gene × parenting interactions may underlie neurocognitive functioning deficits and subsequent ADHD.

Neurocognitive Functioning Mediates the Prospective Association of Birth Weight With Youth ADHD Symptoms

Although birth weight is a potential causal risk factor for attention-deficit/hyperactivity disorder (ADHD) symptoms, both the specificity of this association and its mediating pathways are largely unknown. We carefully assessed youth with and without ADHD (i.e., Wave 1), and followed them prospectively for 2 years (i.e., Wave 2). We (a) tested the association of birth weight with Wave 2 ADHD symptoms, and (b) evaluated biologically plausible neurocognitive functions from Wave 1 as temporally ordered mediators of birth weight and Wave 2 ADHD symptoms in a multiple mediation framework. At Wave 1, 222 ethnically diverse youth (30% female; ages 5-10) completed the Digit Span, Vocabulary, Symbol Search, and Arithmetic subtests of the Wechsler Intelligence Scale for Children-IV. At both Wave 1 and Wave 2 (ages 7-13), multiple informants (i.e., parents, teachers) rated youth ADHD symptoms and co-occurring psychopathology using multiple methods (i.e., structured interview, rating scale). Controlling for demographic factors, gestational age, and co-occurring externalizing and internalizing psychopathology, birth weight inversely predicted Wave 2 ADHD symptoms across multiple methods and informants. Additionally, controlling for Wave 1 ADHD symptoms and relevant covariates, Wave 1 Arithmetic uniquely mediated the association of birth weight with multi-method/informant Wave 2 ADHD symptoms. These findings suggest that birth weight is a relatively specific risk factor for youth ADHD symptoms and they implicate individual differences in fluid reasoning as a preliminary causal mediator of this association. We discuss implications for future research evaluating causal mechanisms underlying risk factors for ADHD.

A functional variant in MIR137, a candidate gene for schizophrenia, affects Stroop test performance in young adults

MIR137, a brain expressed miRNA, has been identified as a top novel susceptibility gene for schizophrenia (SZ). 230 healthy participants completed the Stroop test and were genotyped for a functional Variable Number Tandem Repeat (VNTR) in MIR137 gene. MIR137 VNTR genotypes were associated with differences in Stroop facilitation and accuracies in congruent trials and for the total number of errors. This is the first study of the functional VNTR in MIR137 gene and Stroop test performance in healthy subjects. Our results could have important implications for the identification of genetic candidates for endophenotypes for SZ.

DRD2 and DRD4 genes related to cognitive deficits in HIV-infected adults who abuse alcohol

Background

HIV-infected individuals continue to experience neurocognitive deterioration despite virologically successful treatments. The causes of neurocognitive impairment are still unclear. However, several factors have been suggested including the role of genetics. There is evidence suggesting that neurocognitive impairment is heritable and individual differences in cognition are strongly driven by genetic variations. The contribution of genetic variants affecting the metabolism and activity of dopamine may influence these individual differences.

Methods

The present study explored the relationship between two candidate genes (DRD4 and DRD2) and neurocognitive performance in HIV-infected adults. A total of 267 HIV-infected adults were genotyped for polymorphisms, DRD4 48 bp-variable number tandem repeat (VNTR), DRD2 rs6277 and ANKK1 rs1800497. The Short Category (SCT), Color Trail (CTT) and Rey-Osterrieth Complex Figure Tests (ROCT) were used to measure executive function and memory.

Results

Results showed significant associations with the SNP rs6277 and impaired executive function (odds ratio = 3.3, 95 % CI 1.2–2.6; p = 0.004) and cognitive flexibility (odds ratio = 1.6, 95 % CI 2.0–5.7; p = 0.001). The results were further stratified by race and sex and significant results were seen in males (odds ratio = 3.5, 95 % CI 1.5–5.5; p = 0.008) and in African Americans (odds ratio = 3.1, 95 % CI 2.3–3.5; p = 0.01). Also, DRD4 VNTR 7-allele was significantly associated with executive dysfunction.

Conclusion

The study shows that genetically determined differences in the SNP rs6277 DRD2 gene and DRD4 48 bp VNTR may be risk factors for deficits in executive function and cognitive flexibility.

Dopamine receptor D4 (DRD4) gene modulates the influence of informational masking on speech recognition

Listeners vary substantially in their ability to recognize speech in noisy environments. Here we examined the role of genetic variation on individual differences in speech recognition in various noise backgrounds. Background noise typically varies in the levels of energetic masking (EM) and informational masking (IM) imposed on target speech. Relative to EM, release from IM is hypothesized to place greater demand on executive function to selectively attend to target speech while ignoring competing noises. Recent evidence suggests that the long allele variant in exon III of the DRD4 gene, primarily expressed in the prefrontal cortex, may be associated with enhanced selective attention to goal-relevant high-priority information even in the face of interference. We investigated the extent to which this polymorphism is associated with speech recognition in IM and EM conditions. In an unscreened adult sample (Experiment 1) and a larger screened replication sample (Experiment 2), we demonstrate that individuals with the DRD4 long variant show better recognition performance in noise conditions involving significant IM, but not in EM conditions. In Experiment 2, we also obtained neuropsychological measures to assess the underlying mechanisms. Mediation analysis revealed that this listening condition-specific advantage was mediated by enhanced executive attention/working memory capacity in individuals with the long allele variant. These findings suggest that DRD4 may contribute specifically to individual differences in speech recognition ability in noise conditions that place demands on executive function.

Biomarkers in the diagnosis of ADHD--promising directions

The etiology and pathogenesis of attention-deficit/hyperactivity disorder (ADHD) are unclear and a more valid diagnosis would certainly be welcomed. Starting from the literature, we built an hypothetical pyramid representing a putative set of biomarkers where, at the top, variants in DAT1 and DRD4 genes are the best candidates for their associations to neuropsychological tasks, activation in specific brain areas, methylphenidate response and gene expression levels. Interesting data come from the noradrenergic system (norepinephrine transporter, norepinephrine, 3-methoxy-4-hydroxyphenylglycol, monoamine oxidase, neuropeptide Y) for their altered peripheral levels, their association with neuropsychological tasks, symptomatology, drugs effect and brain function. Other minor putative genetic biomarkers could be dopamine beta hydroxylase and catechol-O-methyltransferase. In the bottom, we placed endophenotype biomarkers. A more deep integration of "omics" sciences along with more accurate clinical profiles and new high-throughput computational methods will allow us to identify a better list of biomarkers useful for diagnosis and therapies.

Response inhibition and interference control in obsessive-compulsive spectrum disorders

Over the past 20 years, motor response inhibition and interference control have received considerable scientific effort and attention, due to their important role in behavior and the development of neuropsychiatric disorders. Results of neuroimaging studies indicate that motor response inhibition and interference control are dependent on cortical–striatal–thalamic–cortical (CSTC) circuits. Structural and functional abnormalities within the CSTC circuits have been reported for many neuropsychiatric disorders, including obsessive–compulsive disorder (OCD) and related disorders, such as attention-deficit hyperactivity disorder, Tourette’s syndrome, and trichotillomania. These disorders also share impairments in motor response inhibition and interference control, which may underlie some of their behavioral and cognitive symptoms. Results of task-related neuroimaging studies on inhibitory functions in these disorders show that impaired task performance is related to altered recruitment of the CSTC circuits. Previous research has shown that inhibitory performance is dependent upon dopamine, noradrenaline, and serotonin signaling, neurotransmitters that have been implicated in the pathophysiology of these disorders. In this narrative review, we discuss the common and disorder-specific pathophysiological mechanisms of inhibition-related dysfunction in OCD and related disorders.

Electroencephalography correlates of spatial working memory deficits in attention-deficit/hyperactivity disorder: vigilance, encoding, and maintenance

In the current study we sought to dissociate the component processes of working memory (WM) (vigilance, encoding and maintenance) that may be differentially impaired in attention-deficit/ hyperactivity disorder (ADHD). We collected electroencephalographic (EEG) data from 52 children with ADHD and 47 typically developing (TD) children, ages 7-14 years, while they performed a spatial Sternberg working memory task. We used independent component analysis and time-frequency analysis to identify midoccipital alpha (8-12 Hz) to evaluate encoding processes and frontal midline theta (4-7 Hz) to evaluate maintenance processes. We tested for effects of task difficulty and cue processing to evaluate vigilance. Children with ADHD showed attenuated alpha band event-related desynchronization (ERD) during encoding. This effect was more pronounced when task difficulty was low (consistent with impaired vigilance) and was predictive of memory task performance and symptom severity. Correlated with alpha ERD during encoding were alpha power increases during the maintenance period (relative to baseline), suggesting a compensatory effort. Consistent with this interpretation, midfrontal theta power increases during maintenance were stronger in ADHD and in high-load memory conditions. Furthermore, children with ADHD exhibited a maturational lag in development of posterior alpha power whereas age-related changes in frontal theta power deviated from the TD pattern. Last, subjects with ADHD showed age-independent attenuation of evoked responses to warning cues, suggesting low vigilance. Combined, these three EEG measures predicted diagnosis with 70% accuracy. We conclude that the interplay of impaired vigilance and encoding in ADHD may compromise maintenance and lead to impaired WM performance in this group.

Olanzapine induced DNA methylation changes support the dopamine hypothesis of psychosis

Background

The dopamine (DA) hypothesis of schizophrenia proposes the mental illness is caused by excessive transmission of dopamine in selected brain regions. Multiple lines of evidence, including blockage of dopamine receptors by antipsychotic drugs that are used to treat schizophrenia, support the hypothesis. However, the dopamine D2 receptor (DRD2) blockade cannot explain some important aspects of the therapeutic effect of antipsychotic drugs. In this study, we hypothesized that antipsychotic drugs could affect the transcription of genes in the DA pathway by altering their epigenetic profile.

Methods

To test this hypothesis, we examined the effect of olanzapine, a commonly used atypical antipsychotic drug, on the DNA methylation status of genes from DA neurotransmission in the brain and liver of rats. Genomic DNA isolated from hippocampus, cerebellum, and liver of olanzapine treated (n = 2) and control (n = 2) rats were analyzed using rat specific methylation arrays.

Results

Our results show that olanzapine causes methylation changes in genes encoding for DA receptors (dopamine D1 receptor, dopamine D2 receptor and dopamine D5 receptor), a DA transporter (solute carrier family 18 member 2), a DA synthesis (differential display clone 8), and a DA metabolism (catechol-O-methyltransferase). We assessed a total of 40 genes in the DA pathway and found 19 to be differentially methylated between olanzapine treated and control rats. Most (17/19) genes showed an increase in methylation, in their promoter regions with in silico analysis strongly indicating a functional potential to suppress transcription in the brain.

Conclusion

Our results suggest that chronic olanzapine may reduce DA activity by altering gene methylation. It may also explain the delayed therapeutic effect of antipsychotics, which occurs despite rapid dopamine blockade. Furthermore, given the common nature of epigenetic variation, this lends insight into the differential therapeutic response of psychotic patients who display adequate blockage of dopamine receptors.

A potential electroencephalography and cognitive biosignature for the child behavior checklist-dysregulation profile

OBJECTIVE

The Child Behavior Checklist-Dysregulation Profile (CBCL/DP) identifies youth at increased risk for significant psychopathology. Although the genetic architecture and several biological correlates of the CBCL/DP have been described, little work has elucidated its underlying neurobiology. We examined the potential utility of electroencephalography (EEG), along with behavioral and cognitive assessments, in differentiating individuals based on the CBCL/DP.

METHOD

Participants aged 7 to 14 years of age were categorized into 3 age- and sex-matched groups based on clinical assessment and CBCL/DP: typically developing controls without attention-deficit/hyperactivity disorder (ADHD) (n = 38), individuals with ADHD without the CBCL/DP (ADHD/DP-) (n = 38), and individuals with the CBCL/DP (CBCL/DP+) (n = 38). Groups were compared with EEG and measures of clinical phenomenology and cognition.

RESULTS

ADHD/DP- and CBCL/DP+ groups had increased inattention, but the CBCL/DP+ group had increased hyperactive/impulsive symptoms, disruptive behavior, mood, and anxiety comorbidities compared with the group with ADHD alone. Cognitive profiles suggested that ADHD/DP-participants had fast impulsive responses, whereas CBCL/DP+ participants were slow and inattentive. On EEG, CBCL/DP+ had a distinct profile of attenuated δ-band and elevated α-band spectral power in the central and parietal regions compared to ADHD/DP- and controls. The low-δ/high-α profile was correlated with measures of emotion and behavior problems and not with inattentive symptomatology or cognitive measures. There were no EEG differences between the ADHD/DP- and control groups.

CONCLUSIONS

An EEG/cognitive profile suggests a distinct pattern of underlying neural dysfunction with the CBCL/DP that might ultimately serve as a biosignature. Further work is required to identify potential relationships with clinically defined psychiatric disorders, particularly those of dysregulated mood.

Do different ADHD-related etiological risks involve specific neuropsychological pathways? An analysis of mediation processes by inhibitory control and delay aversion

BACKGROUND

Inhibitory control (IC) has been regarded as a neuropsychological basic deficit and as an endophenotype of attention deficit/hyperactivity disorder (ADHD). Implicated here are mediation processes between etiological factors and ADHD symptoms. We thus analyze whether and to what extent executive IC and delay aversion (DA; i.e., reward-related IC) performance mediate the associations of familial, prenatal, and psychosocial risks with ADHD symptoms.

METHODS

The study sample consisted of 130 preschool children (3-6 years; 50% boys), including 20% (n = 26) with a positive family history of ADHD (familial risk). Prenatal risks were mainly taken from medical records. Psychosocial risks were assessed by a structured interview. ADHD symptoms were assessed by structured interviews and questionnaires completed by parents and teachers. A set of neuropsychological tasks on IC and DA was conducted with the children.

RESULTS

Familial, prenatal, and psychosocial risks were significantly associated with ADHD symptoms. IC and DA also correlated significantly with ADHD symptoms. While the familial risk significantly correlated with IC and DA, psychosocial and prenatal risks were only weakly associated with these measures. The link between the familial risk and ADHD symptoms was partially mediated by IC and DA.

CONCLUSIONS

The results indicate different neuropsychological pathways related to 'positive family history of ADHD' and prenatal risks. Given a cross-validation in future studies, the results underscore the endophenotypic character of IC and DA in preschool ages.

ADHD among adolescents with intellectual disabilities: pre-pathway influences

Children and adolescents with intellectual disabilities (ID) are at heightened risk for developing ADHD. However, the validity of ADHD as a diagnosis for youth with ID remains controversial. To advance research on validity, the present study examined the hypothesized precursors to ADHD in typically developing adolescents (TD) and adolescents with ID, specifically with regard to family history of ADHD, molecular genetics, and neuropsychological functioning. Results indicated that youth ADHD symptoms were related to parental ADHD symptoms regardless of the adolescent's cognitive functioning. Additionally, findings suggested that the DRD4 genetic variant and adolescent set-shifting abilities were related to adolescent ADHD symptoms independent of cognitive functioning. This study provides an initial investigation of the biological correlates of ADHD among youth with ID.

Is there a role for rare variants in DRD4 gene in the susceptibility for ADHD? Searching for an effect of allelic heterogeneity

Although several studies have demonstrated an association between the 7-repeat (7R) allele in the 48-bp variable number of tandem repeats (VNTRs) in the exon 3 at dopamine receptor D4 (DRD4) gene and attention-deficit/hyperactivity disorder (ADHD), others failed to replicate this finding. In this study, a total of 786 individuals with ADHD were genotyped for DRD4 exon 3 VNTR. All 7R homozygous subjects were selected for VNTR re-sequencing. Subjects homozygous for the 4R allele were selected paired by age, ancestry and disorder subtypes in order to have a sample as homogeneous as possible with 7R/7R individuals. Using these criteria, 103 individuals (66 with ADHD and 37 control individuals) were further investigated. An excess of rare variants were observed in the 7R alleles of ADHD patient when compared with controls (P=0.031). This difference was not observed in 4R allele. Furthermore, nucleotide changes that predict synonymous and non-synonymous substitutions were more common in the 7R sample (P=0.008 for total substitutions and P=0.043 for non-synonymous substitutions). In silico prediction of structural/functional alterations caused by these variants have also been observed. Our findings suggest that not only repeat length but also DNA sequence should be assessed to better understand the role of DRD4 exon 3 VNTR in ADHD genetic susceptibility.

The dopamine receptor D4 7-repeat allele influences neurocognitive functioning, but this effect is moderated by age and ADHD status: an exploratory study

OBJECTIVES

Evidence suggests the involvement of the dopamine D4 receptor gene (DRD4) in the pathogenesis of ADHD, but the exact mechanism is not well understood. Earlier reports on the effects of DRD4 polymorphisms on neurocognitive and neuroimaging measures are inconsistent. This study investigated the functional consequences of the 7-repeat allele of DRD4 on neurocognitive endophenotypes of ADHD in the Dutch subsample of the International Multicenter ADHD Genetics study.

METHODS

Participants were 350 children (5-11.5 years) and adolescents (11.6-19 years) with ADHD and their 195 non-affected siblings. An overall measure of neuropsychological functioning was derived by principal component analysis from five neurocognitive and five motor tasks. The effects of DRD4 and age were examined using Linear Mixed Model analyses.

RESULTS

The analyses were stratified for affected and non-affected participants after finding a significant three-way interaction between ADHD status, age and the 7-repeat allele. Apart from a main effect of age, a significant interaction effect of age and DRD4 was found in non-affected but not in affected participants, with non-affected adolescent carriers of the 7-repeat allele showing worse neuropsychological performance. In addition, carrying the 7-repeat allele of DRD4 was related to a significantly worse performance on verbal working memory in non-affected siblings, independent of age.

CONCLUSIONS

These results might indicate that the effect of the DRD4 7-repeat allele on neuropsychological functioning is dependent on age and ADHD status.

Neuropsychological endophenotypes in attention-deficit/hyperactivity disorder: a review of genetic association studies

As a relatively large body of research has been published up to now, it may be informative to explore whether the use of endophenotypes has produced consistent findings in attention-deficit hyperactivity disorder (ADHD). We reviewed the results of genetic studies investigating associations between putative susceptibility genes for ADHD and neuropsychological traits relevant for this disorder. A PubMed database search identified 47 studies. Most of them (n = 36) examined a single candidate gene, while seven studies examined two or three genes and only four studies examined 10 genes or more. The most investigated genes were DRD4, DAT1, COMT, MAOA, and DBH. Regarding DRD4, association of high reaction time variability with the 7-R allele absence appears to be the most consistent result. Speed of processing, set shifting, and cognitive impulsiveness were less frequently investigated, but seem to be altered in the 7-R allele carriers. Regarding DAT1, majority of studies reported negative results indicating that this gene may have a modulating effect rather than direct influence on cognitive functioning. The other genes were investigated in fewer studies, and the reported findings need to be replicated. The principal methodological issues that could represent confounding factors and may explain conflicting results are discussed.

Set shifting in a rodent model of attention-deficit/hyperactivity disorder

Two experiments compared spontaneously hypertensive rats (SHRs; a rodent model of attention-deficit/hyperactivity disorder) and Wistar rats (a normoactive control strain), on the acquisition of a set-shifting strategy. In Experiment 1, SHRs and Wistar rats were equivalent in trials to criterion to learn a brightness or a texture discrimination but SHRs were faster than Wistar rats in shifting to the opposite discrimination when there was 1 or 2 days between the initial discrimination and the shift. In Experiment 2, SHRs and Wistar rats were equivalent in shifting when the shift between discriminations occurred immediately after a criterion had been met in the first discrimination. The results are discussed in terms of a failure of SHRs to store or retrieve an initial discrimination and/or latent inhibition over a delay, leading to faster acquisition of a set-shift. This failure in storage or retrieval may be the result of a hypoactive dopamine system in the prefrontal cortex and nucleus accumbens shell as well as abnormalities in entorhinal cortex in SHRs.

Dopamine β-hydroxylase gene associates with stroop color-word task performance in Han Chinese children with attention deficit/hyperactivity disorder

The cognitive deficits observed in attention deficit/hyperactivity disorder (ADHD) are candidate endophenotypes for genetic association studies. Dopamine β-hydroxylase (DβH) converts dopamine to norepinephrine, and its activity is under strong genetic control. Prior studies suggest association between ADHD and DBH gene. The present study examined associations between a putative functional single nucleotide polymorphism (SNP) at DBH with performance on the Stroop task in patients with ADHD and in healthy control subjects. A total of 812 Han Chinese youths with DSM-IV ADHD and 233 unaffected controls were included in the study. Comprehensive phenotype data were collected, including performance on a series of Stroop interference tests examining inhibition of response to interfering stimuli. DBH SNP -1021C/T was genotyped using the 5'-exonuclease (TaqMan®) method. Compared to unaffected controls, children with ADHD performed significantly worse in all categories of the Stroop test. In ADHD cases, DBH genotype at -1021C/T significantly associates with reaction times of incongruent color word parts but not the interference times, with TT genotype performing significantly better in both reaction time and interference time than other two genotype groups. DBH genotype did not associate with cognitive performance in unaffected controls or in the combined group. DBH genotype at -1021C/T associates with differences in performance on the Stroop task in children with ADHD.

Psychopathological aspects of dopaminergic gene polymorphisms in adolescence and young adulthood

Dopamine hypotheses of several psychiatric disorders are based upon the clinical benefits of drugs affecting dopamine transporter or receptors, and have prompted intensive candidate gene research within the dopaminergic system during the last two decades. The aim of this review is to survey the most important findings concerning dopaminergic gene polymorphisms in attention deficit hyperactivity disorder (ADHD), Tourette syndrome (TS), obsessive compulsive disorder, and substance abuse. Also, genetic findings of related phenotypes, such as inattention, impulsivity, aggressive behavior, and novelty seeking personality trait are presented, because recent studies have applied quantitative trait measures using questionnaires, symptom scales, or other objective endophenotypes. Unfortunately, genetic variants with minor effects are problematic to detect in these complex inheritance disorders, often leading to contradictory results. The most consistent association findings relate to ADHD and the dopamine transporter and the dopamine D4 receptor genes. Meta-analyses also support the association between substance abuse and the D2 receptor gene. The dopamine catabolizing enzyme genes, such as monoamine oxidase (MAO) A and catechol-O-methyltransferase (COMT) genes, have been linked to aggressive behaviors.

ADHD familial loading and abnormal EEG alpha asymmetry in children with ADHD

OBJECTIVE

Abnormal brain laterality (ABL) is indicated in ADHD. ADHD and brain laterality are heritable. Genetic factors contributing to lateralization of brain function may contribute to ADHD. If so, increased ADHD family loading should be associated with greater ABL. Previous studies have shown increased rightward alpha asymmetry in ADHD. We tested whether this was more pronounced in ADHD children with increased ADHD family loading.

METHODS

We compared EEG alpha asymmetry at rest and during the Conner's Continuous Performance Test (CPT) in ADHD children with and without ADHD affected parents, and replicated our findings in a second larger sample. The replication study additionally stratified the parent-affected sample by parental persistent versus non-persistent ADHD status, increased spatial resolution of EEG measures, and assessed low versus high-alpha.

RESULTS

Study-1: the parent-affected group showed increased rightward asymmetry across frontal and central regions and reduced rightward parietal asymmetry during an eyes closed (EC) condition, as well as increasing rightward parietal asymmetry with advancing age during the CPT. Study-2 replicated these findings and further delineated influences of low versus high-alpha, recording site, and effects of parental persistent versus non-persistent ADHD status.

CONCLUSION

Increased ADHD familial loading was associated with increased rightward frontal asymmetry. In contrast, increased rightward parietal asymmetry was associated with reduced ADHD family loading. Frontal results are consistent with an ADHD endophenotype. Parietal results suggest an ADHD adaptive trait prevalent with less ADHD family loading. Age effects indicate a unique developmental course among ADHD children whose parents have non-persistent ADHD.

Insight into the relationship between impulsivity and substance abuse from studies using animal models

Drug use disorders are often accompanied by deficits in the capacity to efficiently process reward-related information and to monitor, suppress, or override reward-controlled behavior when goals are in conflict with aversive or immediate outcomes. This emerging deficit in behavioral flexibility and impulse control may be a central component of the progression to addiction, as behavior becomes increasingly driven by drugs and drug-associated cues at the expense of more advantageous activities. Understanding how neural mechanisms implicated in impulse control are affected by addictive drugs may therefore prove a useful strategy in the search for new treatment options. Animal models of impulsivity and addiction could make a significant contribution to this endeavor. Here, some of the more common behavioral paradigms used to measure different aspects of impulsivity across species are outlined, and the importance of the response to reward-paired cues in such paradigms is discussed. Naturally occurring differences in forms of impulsivity have been found to be predictive of future drug self-administration, but drug exposure can also increase impulsive responding. Such data are in keeping with the suggestion that impulsivity may contribute to multiple stages within the spiral of addiction. From a neurobiological perspective, converging evidence from rat, monkey, and human studies suggest that compromised functioning within the orbitofrontal cortex may critically contribute to the cognitive sequelae of drug abuse. Changes in gene transcription and protein expression within this region may provide insight into the mechanism underlying drug-induced cortical hypofunction, reflecting new molecular targets for the treatment of uncontrolled drug-seeking and drug-taking behavior.

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.

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.

A review and analysis of the relationship between neuropsychological measures and DAT1 in ADHD

Meta-analyses indicate that the gene coding for the dopamine transporter (DAT1 or SLC6A3) is associated with an increased risk for ADHD. The mechanisms of this gene for ADHD are unclear. We systematically reviewed studies linking the VNTR in the 3' UTR of the DAT1 to neurophysiological and neuropsychological measures. In addition, a broad set of executive/cognitive and motor tests was administered to 350 children (5-11 years) and adolescents (11-19 years) with ADHD and 195 non-affected siblings. Two VNTRs (in intron 8 and the 3' UTR) and four SNPs (two 5' and two 3') in DAT1 were genotyped. The effect of the polymorphisms on neuropsychological functioning was studied. The review indicated that the majority of studies did not find a relation between DAT1 and neurophysiological or neuropsychological measures. In our sample, several of the polymorphisms of DAT1 were associated with ADHD and ADHD was associated with impaired neuropsychological functioning. However, none of the DAT1 polymorphisms was convincingly associated with neuropsychological dysfunctioning. This suggests that the effect of DAT1 on ADHD was not mediated by neuropsychological performance. However, since DAT1 is mainly expressed in the striatum and not the prefrontal cortex, it may influence striatum-related functions (such as delay aversion) more heavily than prefrontal related functions (such as executive functions). Associations of DAT1 with ADHD were only found in adolescents, which may suggest that DAT1 mainly exerts its effect in adolescence, and/or that having a more persistent form of ADHD may mark a more severe or homogeneous genetic form of the disorder.