Functional effects of the DAT1 polymorphism on EEG measures in ADHD

Association Between Single-Dose and Longer Term Clinical Response to Stimulants in Attention-Deficit/Hyperactivity Disorder: A Systematic Review of Randomized Controlled Trials

Objectives: Stimulants, such as methylphenidate (MPH) and amphetamines, represent the first-line pharmacological option for attention-deficit/hyperactivity disorder (ADHD). Randomized controlled trials (RCTs) have demonstrated beneficial effects at a group level but could not identify characteristics consistently associated with varying individual response. Thus, more individualized approaches are needed. Experimental studies have suggested that the neurobiological response to a single dose is indicative of longer term response. It is unclear whether this also applies to clinical measures. Methods: We carried out a systematic review of RCTs testing the association between the clinical response to a single dose of stimulants and longer term improvement. Potentially suitable single-dose RCTs were identified from the MED-ADHD data set, the European ADHD Guidelines Group RCT Data set (https://med-adhd.org/), as updated on February 1, 2024. Quality assessment was carried out using the Cochrane Risk of Bias (RoB) 2.0 tool. Results: A total of 63 single-dose RCTs (94% testing MPH, 85% in children) were identified. Among these, only a secondary analysis of an RCT tested the association between acute and longer term clinical response. This showed that the clinical improvement after a single dose of MPH was significantly associated with symptom improvement after a 4-week MPH treatment in 46 children (89% males) with ADHD. The risk of bias was rated as moderate. A further RCT used near-infrared spectroscopy, thus did not meet the inclusion criteria, and reported an association between brain changes under a single-dose and longer term clinical response in 22 children (82% males) with ADHD. The remaining RCTs only reported single-dose effects on neuropsychological, neuroimaging, or neurophysiological measures. Conclusion: This systematic review highlighted an important gap in the current knowledge. Investigating how acute and long-term response may be related can foster our understanding of stimulant mechanism of action and help develop stratification approaches for more tailored treatment strategies. Future studies need to investigate potential age- and sex-related differences.

Genetics in the ADHD Clinic: How Can Genetic Testing Support the Current Clinical Practice?

Attention-deficit/hyperactivity disorder (ADHD) is a neurodevelopmental disorder with a childhood prevalence of 5%. In about two-thirds of the cases, ADHD symptoms persist into adulthood and often cause significant functional impairment. Based on the results of family and twin studies, the estimated heritability of ADHD approximates 80%, suggests a significant genetic component in the etiological background of the disorder; however, the potential genetic effects on disease risk, symptom severity, and persistence are unclear. This article provides a brief review of the genome-wide and candidate gene association studies with a focus on the clinical aspects, summarizing findings of ADHD disease risk, ADHD core symptoms as dimensional traits, and other traits frequently associated with ADHD, which may contribute to the susceptibility to other comorbid psychiatric disorders. Furthermore, neuropsychological impairment and measures from neuroimaging and electrophysiological paradigms, emerging as potential biomarkers, also provide a prominent target for molecular genetic studies, since they lie in the pathway from genes to behavior; therefore, they can contribute to the understanding of the underlying neurobiological mechanisms and the interindividual heterogeneity of clinical symptoms. Beyond the aforementioned aspects, throughout the review, we also give a brief summary of the genetic results, including polygenic risk scores that can potentially predict individual response to different treatment options and may offer a possibility for personalized treatment for the therapy of ADHD in the future.

Characterizing Underlying Cognitive Components of ADHD Presentations and Co-morbid Diagnoses: A Diffusion Decision Model Analysis

OBJECTIVE

To Explore whether subtypes and comorbidities of attention-deficit hyperactivity disorder (ADHD) induce distinct biases in cognitive components involved in information processing.

METHOD

Performance on the Integrated Visual and Auditory Continuous Performance Test (IVA-CPT) was compared between 150 children (aged 7 to 10) with ADHD, grouped by DSM-5 presentation (ADHD-C, ADHD-I) or co-morbid diagnoses (anxiety, oppositional defiant disorder [ODD], both, neither), and 60 children without ADHD. Diffusion decision modeling decomposed performance into cognitive components.

RESULTS

Children with ADHD had poorer information integration than controls. Children with ADHD-C were more sensitive to changes in presentation modality (auditory/visual) than those with ADHD-I and controls. Above and beyond these results, children with ADHD+anxiety+ODD had larger increases in response biases when targets became frequent than children with ADHD-only or with ADHD and one comorbidity.

CONCLUSION

ADHD presentations and comorbidities have distinct cognitive characteristics quantifiable using DDM and IVA-CPT. We discuss implications for tailored cognitive-behavioral therapy.

The Interplay Between Prenatal Adversity, Offspring Dopaminergic Genes, and Early Parenting on Toddler Attentional Function

Background: Few studies have explored the complex gene-by-prenatal environment-by-early postnatal environment interactions that underlie the development of attentional competence. Here, we examined if variation in dopamine-related genes interacts with prenatal adversity to influence toddler attentional competence and whether this influence is buffered by early positive maternal behavior. Methods: From the Maternal Adversity, Vulnerability and Neurodevelopment cohort, 134 participants (197 when imputing missing data) had information on prenatal adversity (prenatal stressful life events, prenatal maternal depressive symptoms, and birth weight), five dopamine-related genes (DAT1, DRD4, DRD2, COMT, BDNF), observed maternal parenting behavior at 6 months and parent-rated toddler attentional competence at 18 and 24 months. The Latent Environmental and Genetic Interaction (LEGIT) approach was used to examine genes-by-prenatal environment-by-postnatal environment interactions while controlling for sociodemographic factors and postnatal depression. Results: Our hypothesis of a three-way interaction between prenatal adversity, dopamine-related genes, and early maternal parenting behavior was not confirmed. However, consistent two-way interactions emerged between prenatal adversity and dopamine-related genes; prenatal adversity and maternal parenting behavior, and dopamine-related genes and maternal parenting behavior in relation to toddler attentional competence. Significant interaction effects were driven by the DAT1, COMT, and BDNF genotypes; prenatal stressful life events; maternal sensitivity, tactile stimulation, vocalization, and infant-related activities. Conclusions: Multiple dopamine-related genes affected toddler attentional competence and they did so in interaction with prenatal adversity and the early rearing environment, separately. Effects were already visible in young children. Several aspects of early maternal parenting have been identified as potential targets for intervention.

Brain potentials reveal reduced attention and error-processing during a monetary Go/No-Go task in procrastination

Procrastination is a self-regulatory problem of voluntarily and destructively delaying intended and necessary or personally important tasks. Previous studies showed that procrastination is associated with executive dysfunctions that seem to be particularly strong in punishing contexts. In the present event-related potential (ERP) study a monetary version of the parametric Go/No-Go task was performed by high and low academic procrastinators to verify the influence of motivational context (reward vs. punishment expectation) and task difficulty (easy vs. hard) on procrastination-related executive dysfunctions. The results revealed increased post-error slowing along with reduced P300 and error-related negativity (ERN) amplitudes in high (vs. low) procrastination participants—effects that indicate impaired attention and error-related processing in this group. This pattern of results did not differ as a function of task difficulty and motivation condition. However, when the task got more difficult executive attention deficits became even more apparent at the behavioral level in high procrastinators, as indexed by increased reaction time variability. The findings substantiate prior preliminary evidence that procrastinators show difficulties in certain aspects of executive functioning (in attention and error processing) during execution of task-relevant behavior, which may be more apparent in highly demanding situations.

Convergent neural substrates of inattention in bipolar disorder patients and dopamine transporter-deficient mice using the 5-choice CPT

OBJECTIVES

Bipolar disorder (BD) is a debilitating psychiatric illness affecting 2%-5% of the population. Although mania is the cardinal feature of BD, inattention and related cognitive dysfunction are observed across all stages. Since cognitive dysfunction confers poor functional outcome in patients, understanding the relevant neural mechanisms remains key to developing novel-targeted therapeutics.

METHODS

The 5-choice continuous performance test (5C-CPT) is a mouse and fMRI-compatible human attentional task, requiring responding to target stimuli while inhibiting responding to nontarget stimuli, as in clinical CPTs. This task was used to delineate systems-level neural deficits in BD contributing to inattentive performance in human subjects with BD as well as mouse models with either parietal cortex (PC) lesions or reduced dopamine transporter (DAT) expression.

RESULTS

Mania BD participants exhibited severe 5C-CPT impairment. Euthymic BD patients exhibited modestly impaired 5C-CPT. High impulsivity BD subjects exhibited reduced PC activation during target and nontarget responding compared with healthy participants. In mice, bilateral PC lesions impaired both target and nontarget responding. In the DAT knockdown mouse model of BD mania, knockdown mice exhibited severely impaired 5C-CPT performance versus wildtype littermates.

CONCLUSIONS

These data support the role of the PC in inattention in BD-specifically regarding identifying the appropriate response to target vs nontarget stimuli. Moreover, the findings indicate that severely reduced DAT function/hyperdopaminergia recreates the attentional deficits observed in BD mania patients. Determining the contribution of DAT in the PC to attention may provide a future target for treatment development.

Cumulative Dopamine Genetic Score predicts behavioral and electrophysiological correlates of response inhibition via interactions with task demand

Functional genetic polymorphisms in the brain dopamine (DA) system have been suggested to underlie individual differences in response inhibition, namely the suppression of a prepotent or inappropriate action. However, findings on associations between single DA polymorphisms and inhibitory control often are mixed, partly due to their small effect sizes. In the present study, a cumulative genetic score (CGS) was used: alleles previously associated with both impulsive behavior and lower baseline DA level, precisely the DRD4 Exon III 7-repeat, DAT1 VNTR 10-repeat and the COMT 158val allele, each added a point to the DA-CGS. Participants (N = 128) completed a Go/No-Go task varying in difficulty and EEG recordings were made with focus on the NoGo-P3, an ERP that reflects inhibitory response processes. We found a higher DA-CGS (lower basal/tonic DA level) to be associated with better performance (lower %FA and more adaptive responding) in the very demanding/rapid than in the less demanding/rapid condition, whereas the reverse pattern was true for individuals with a lower DA-CGS. A similar interaction pattern of DA-CGS and task condition was found for NoGo-P3 amplitude. In line with assumptions of distinct optimum DA levels for different cognitive demands, a DA-CGS-dependent variation of tonic DA levels could have modulated the balance between cognitive stability and flexibility, thereby affecting the optimal DA level required for the specific task condition. Moreover, a task demand-dependent phasic DA release might have added to the DA-CGS-related basal/tonic DA levels, thereby additionally affecting the balance between flexibility and stability, in turn influencing performance and NoGo-P3.

Genetic associations between ADHD and dopaminergic genes (DAT1 and DRD4) VNTRs in Korean children

It is well known that dopaminergic genes affect the development of attention deficit hyperactivity disorder (ADHD) in various populations. Many studies have shown that variable number tandem repeats (VNTRs) located within the 3'-untranslated region of DAT1 and in exon 3 of DRD4 are associated with ADHD development; however, these results were inconsistent. Therefore, we investigated the genetic association between two VNTRs and ADHD in Korean children. We determined the VNTRs using PCR. We examined genotype and allele frequency differences between the experimental and control groups, along with the odds ratios, using Chi square and exact tests. We observed a significant association between the children with ADHD and the control group in the 10R/10R genotype of DAT1 VNTRs (p = 0.025). In addition, the 11R allele of DAT1 VNTRs showed a higher frequency in the control group than in the ADHD group (p = 0.023). Also, the short repeat (without 11R) and long repeat alleles (including 11R) were associated with ADHD (p < 0.05). The analysis of DRD4 VNTRs revealed that the 2R allele is associated with ADHD (p = 0.025). A significant result was also observed in long and short repeats (p < 0.05). Additionally, ADHD subtypes showed that the DRD4 VNTRs are associated with combined and hyperactive-impulsive subtype groups (p < 0.05). Therefore, our results suggest that DAT1 VNTRs and DRD4 VNTRs play a role in the genetic etiology of ADHD in Korean children.

The Effects of Methylphenidate on the Neural Signatures of Sustained Attention

BACKGROUND

Although it is well established that methylphenidate (MPH) enhances sustained attention, the neural mechanisms underpinning this improvement remain unclear. We examined how MPH influenced known electrophysiological precursors of lapsing attention over different time scales.

METHODS

We measured the impact of MPH, compared with placebo, on behavioral and electrocortical markers while healthy adults (n = 40) performed a continuous monitoring paradigm designed to elicit attentional lapses.

RESULTS

MPH led to increased rates of target detection, and electrophysiological analyses were conducted to identify the mechanisms underlying these improvements. Lapses of attention were reliably preceded by progressive increases in alpha activity that emerged over periods of several seconds. MPH led to an overall suppression of alpha activity across the entire task but also diminished the frequency of these maladaptive pretarget increases through a reduction of alpha variability. A drug-related linear increase in the amplitude of the frontal P3 event-related component was also observed in the pretarget timeframe (3 or 4 seconds). Furthermore, during immediate target processing, there was a significant increase in the parietal P3 amplitude with MPH, indicative of enhanced perceptual evidence accumulation underpinning target detection. MPH-related enhancements occurred without significant changes to early visual processing (visual P1 and 25-Hz steady-state visual evoked potential).

CONCLUSIONS

MPH serves to reduce maladaptive electrophysiological precursors of lapsing attention by acting selectively on top-down endogenous mechanisms that support sustained attention and target detection with no significant effect on bottom-up sensory excitability. These findings offer candidate markers to monitor the therapeutic efficacy of psychostimulants or to predict therapeutic responses.

Dopamine transporter gene polymorphism in children with ADHD: A pilot study in Indonesian samples

INTRODUCTION

Several studies showed that DAT1 polymorphism closed related with ADHD although the results were not consistently found. Studies in China, South Korea, Japan revealed that 10-repeat allele gave a risk for ADHD. Based on that understanding, this study tried to identify whether the similar polymorphism of DAT1 was also apparent in Indonesian children with ADHD.

METHOD

This was a case - control study. Case was 50 Indonesian origin children with ADHD and without any other mental disorders and metal retardation. Control is Indonesian origin children without ADHD, other mental disorders and mental retardation. ADHD diagnosis was taken after doing the psychiatric interview and observation based on the DSM-IV TR diagnostic criteria for ADHD at the Child and Adolescent Psychiatry Out-patient Clinic, Dr. Cipto Mangunkusumo National Referral Hospital - Faculty of Medicine Universitas Indonesia. DNA isolation, DNA purity and concentration were measured. PCR was done by using a primer based on Homo sapiens solute carrier family 6 (neurotransmitter transporter), member 3 (SLC6A3), RefSeq Gene on chromosome 5 with accession number NG_015885.1. To identify the serial of repeated allele, we used the sequencing technique.

RESULTS

There were 47 children with ADHD and 48 children without ADHD that involved in the final analysis. The mean of age amongst ADHD group was 9.18 (2.42) and 8.10 (2.46) years old in non-ADHD group. The 10-repeated allele of DAT1 was the highest proportion in both.

CONCLUSION

This finding was apparently similar with other studies on DAT1 polymorphism across Asian.

The Contribution of Maternal ADHD Symptomatology, Maternal DAT1, and Home Atmosphere to Child ADHD Symptomatology at 7 Years of Age

Children of mothers with attention-deficit/hyperactivity disorder (ADHD) have an increased genetic and environmental risk for ADHD. The unique and interactive contributions of a maternal dopamine receptor gene (DAT1), maternal ADHD symptoms (hyperactive- impulsive, inattentive), and home atmosphere to the prediction of ADHD symptoms (hyperactive- impulsive, inattentive) in 7- year-old boys (N = 96) were examined using data from a longitudinal study of familial risk for ADHD. During the first 6 months of the study, mothers and their spouses completed a questionnaire about the mother's ADHD symptoms. Home atmosphere questionnaire data were collected 4 years later. At the 7-year assessment, mothers reported on their child's ADHD symptoms. Negative home atmosphere was significantly associated with child hyperactive-impulsive and inattentive symptoms. Maternal inattentive symptoms were significantly correlated with both child symptom dimensions. Regression models, with child genotype and maternal education controlled, showed main effects for maternal inattentive symptoms, maternal DAT1 10/10 genotype, and home atmosphere in the prediction of child inattentive symptoms. Only home atmosphere predicted child hyperactive-impulsive symptoms. There was a significant home atmosphere x maternal hyperactive-impulsive symptoms interaction in the prediction of child hyperactive-impulsive symptoms. Boys with higher levels of symptoms came from homes characterized by higher levels of negative atmosphere and had mothers with higher levels of hyperactive-impulsive symptoms. There was also a trend (p = 0.075) for a maternal DAT1 x home atmosphere interaction. Boys with higher levels of inattentive symptoms came from homes with higher levels of negative atmosphere and had mothers with the homozygous 10/10 genotype. The maternal heterozygous 9/10 genotype did not predict child symptoms.

Brain imaging genetics in ADHD and beyond - Mapping pathways from gene to disorder at different levels of complexity

Attention-deficit/hyperactivity disorder (ADHD) is a common and often persistent neurodevelopmental disorder. Beyond gene-finding, neurobiological parameters, such as brain structure, connectivity, and function, have been used to link genetic variation to ADHD symptomatology. We performed a systematic review of brain imaging genetics studies involving 62 ADHD candidate genes in childhood and adult ADHD cohorts. Fifty-one eligible research articles described studies of 13 ADHD candidate genes. Almost exclusively, single genetic variants were studied, mostly focussing on dopamine-related genes. While promising results have been reported, imaging genetics studies are thus far hampered by methodological differences in study design and analysis methodology, as well as limited sample sizes. Beyond reviewing imaging genetics studies, we also discuss the need for complementary approaches at multiple levels of biological complexity and emphasize the importance of combining and integrating findings across levels for a better understanding of biological pathways from gene to disease. These may include multi-modal imaging genetics studies, bioinformatic analyses, and functional analyses of cell and animal models.

The Relationship between the SNAP-25 Polymorphism and Omission Errors in Korean Children with Attention Deficit Hyperactivity Disorder

Objective

This study aimed to investigate the association between the synaptosomal-associated protein 25 kDa (SNAP-25) genotype and performance on the continuous performance test (CPT) in Korean children with attention-deficit/hyperactivity disorder (ADHD).

Methods

Eighty-seven children with ADHD (mean age, 9.23±1.99 years) participated in this study. Omission errors, commission errors, reaction time, and reaction time variability on the CPT were analyzed. The single-nucleotide polymorphism (SNP) rs3746544 (1065 T>G) of SNAP-25 was genotyped to examine the association with CPT performance.

Results

We found significantly more omission errors on the CPT among children with the TT genotype of SNAP-25 (t=2.56, p=0.012) after correcting for multiple testing.

Conclusion

Our results suggest the possible involvement of the SNAP-25 1065 T>G polymorphism in the inattention phenotype in children with ADHD. Further studies with more refined neuropsychological measures and much larger sample sizes are needed to confirm our findings.

Intratask Variability As a Correlate for DRD4 and SLC6A3 Variants: A Pilot Study in ADHD

Behavioral variability may be an ADHD key feature. Currently used ex-Gaussian/Fast Fourier Transform analyses characterize general distribution and oscillatory/rhythmic components of performance but are unable to demonstrate slow cumulative changes over entire tasks.

OBJECTIVE

To explore how performance of ADHD children and unaffected sibs gradually evolves in relation to genetic variants linked to ADHD.

METHOD

A total of 40 kids (20 ADHD-discordant sib pairs) between 8 and 13 years resolved a visual Go/NoGo with 10% NoGo probability. Variable number tandem repeats (VNTRs) at DRD4 and SLC6A3 were identified following standard protocols. Performance changes were assessed by linear/logistic mixed-effect models.

RESULTS

Models exploring SLC6A3 effects demonstrated less accentuated increments of response time (RT) (p = .046) and cumulative increments in the correct responses to "NoGo" (p = .00027) in 10R/10R participants. Models for DRD4 showed faster decline of correct responses to "Go" (p = .0078) in 2R/7R carriers.

CONCLUSION

Dynamical analysis of attention/inhibition measures may unravel new correlates to DRD4 and SLC6A3 variants.

Mesocorticolimbic dopamine functioning in primary psychopathy: A source of within-group heterogeneity

Despite similar emotional deficiencies, primary psychopathic individuals can be situated on a continuum that spans from controlled to disinhibited. The constructs on which primary psychopaths are found to diverge, such as self-control, cognitive flexibility, and executive functioning, are crucially regulated by dopamine (DA). As such, the goal of this review is to examine which specific alterations in the meso-cortico-limbic DA system and corresponding genes (e.g., TH, DAT, COMT, DRD2, DRD4) might bias development towards a more controlled or disinhibited expression of primary psychopathy. Based on empirical data, it is argued that primary psychopathy is generally related to a higher tonic and population activity of striatal DA neurons and lower levels of D2-type DA receptors in meso-cortico-limbic projections, which may boost motivational drive towards incentive-laden goals, dampen punishment sensitivity, and increase future reward-expectancy. However, increasingly higher levels of DA activity in the striatum (moderate versus pathological elevations), lower levels of DA functionality in the prefrontal cortex, and higher D1-to-D2-type receptor ratios in meso-cortico-limbic projections may lead to increasingly disinhibited and impetuous phenotypes of primary psychopathy. Finally, in order to provide a more coherent view on etiological mechanisms, we discuss interactions between DA and serotonin that are relevant for primary psychopathy.

Altered white matter tract property related to impaired focused attention, sustained attention, cognitive impulsivity and vigilance in attention-deficit/ hyperactivity disorder

BACKGROUND

The neural substrate for clinical symptoms and neuropsychological performance in individuals with attention-deficit/hyperactivity disorder (ADHD) has rarely been studied and has yielded inconsistent results. We sought to compare the microstructural property of fibre tracts associated with the prefrontal cortex and its association with ADHD symptoms and a wide range of attention performance in youth with ADHD and healthy controls.

METHODS

We assessed youths with ADHD and age-, sex-, handedness-, coil- and intelligence-matched controls using the Conners' Continuous Performance Test (CCPT) for attention performance and MRI. The 10 target tracts, including the bilateral frontostriatal tracts (caudate to dorsolateral prefrontal cortex, ventrolateral prefrontal cortex and orbitofrontal cortex), superior longitudinal fasciculus (SLF) and cingulum bundle were reconstructed using diffusion spectrum imaging tractography. We computed generalized fractional anisotropy (GFA) values to indicate tract-specific microstructural property.

RESULTS

We included 50 youths with ADHD and 50 healthy controls in our study. Youths with ADHD had lower GFA in the left frontostriatal tracts, bilateral SLF and right cingulum bundle and performed worse in the CCPT than controls. Furthermore, alteration of the right SLF GFA was most significantly associated with the clinical symptom of inattention in youths with ADHD. Finally, youths with ADHD had differential association patterns of the 10 fibre tract GFA values with attention performance compared with controls.

LIMITATIONS

Ten of the youths with ADHD were treated with methylphenidate, which may have long-term effects on microstructural property.

CONCLUSION

Our study highlights the importance of the SLF, cingulum bundle and frontostriatal tracts for clinical symptoms and attention performance in youths with ADHD and demonstrates the involvement of different fibre tracts in attention performance in these individuals.

Association of norepinephrine transporter (NET, SLC6A2) genotype with ADHD-related phenotypes: findings of a longitudinal study from birth to adolescence

Variation in the gene encoding for the norepinephrine transporter (NET, SLC6A2) has repeatedly been linked with ADHD, although there is some inconsistency regarding the association with specific genes. The variants for which most consistent association has been found are the NET variants rs3785157 and rs28386840. Here, we tested for their association with ADHD diagnosis and ADHD-related phenotypes during development in a longitudinal German community sample. Children were followed from age 4 to age 15, using diagnostic interviews to assess ADHD. Between the ages of 8 and 15 years, the Child Behavior Checklist (CBCL) was administered to the primary caregivers. The continuous performance task (CPT) was performed at age 15. Controlling for possible confounders, we found that homozygous carriers of the major A allele of the functional promoter variant rs28386840 displayed a higher rate of ADHD lifetime diagnosis. Moreover, homozygous carriers of the minor T allele of rs3785157 were more likely to develop ADHD and showed higher scores on the CBCL externalizing behavior scales. Additionally, we found that individuals heterozygous for rs3785157 made fewer omission errors in the CPT than homozygotes. This is the first longitudinal study to report associations between specific NET variants and ADHD-related phenotypes during the course of development.

Indirect association of DAT1 genotype with executive function through white matter volume in orbitofrontal cortex

The dopamine transporter (DAT1) gene has been associated with impulsivity and executive functioning. Further, DAT1 has been associated with brain structural characteristics and resting state connectivity. This study tested an indirect effect model in which DAT1 genotype (9-repeat (9R) carriers vs 10-repeat (10R) homozygotes) is linked to phenotypes representing impulsivity and executive function (planning behavior) through effects on white matter (WM) volumes in prefrontal cortex (PFC), particularly orbitofrontal cortex (OFC). Adolescents (ages 14-18, n=38) were recruited from substance use treatment (n=22) and the community (n=16) to increase phenotype variation. Results indicated that DAT1 10/10 genotype was associated with lower WM volume in the PFC, specifically the left OFC. Further, lower WM volume in the left OFC predicted more difficulties in self-reported planning behavior, but not impulsivity. Indirect effect analysis indicated that lower WM volume in the left OFC mediated the association between DAT1 10/10 genotype and difficulties in planning behavior. Results suggest a brain structural mechanism, involving lower WM volume in the left OFC, as a link in the association between DAT1 genotype and a specific aspect of executive function. Genetic effects on regional WM volume that are linked to behavioral outcomes could ultimately inform the development of tailored interventions that address an individual׳s unique risk factors.

Reaction time variability and related brain activity in methamphetamine psychosis

BACKGROUND

This study investigated the dynamics of cognitive control instability in methamphetamine (MA) abuse, as well its relationship to substance-induced psychiatric symptoms and drug use patterns.

METHODS

We used an ex-Gaussian reaction time (RT) distribution to examine intraindividual variability (IIV) and excessively long RTs (tau) in an individual's RT on a Stroop task in 30 currently drug-abstinent (3 months to 2 years) MA abusers compared with 27 nonsubstance-abusing control subjects. All subjects underwent functional magnetic resonance imaging while performing the Stroop task, which allowed us to measure the relationship between IIV and tau to functional brain activity.

RESULTS

Elevated IIV in the MA compared with the control group did not reach significance; however, when the MA group was divided into those subjects who had experienced MA-induced psychosis (MAP+) (n = 19) and those who had not (n = 11), the MAP+ group had higher average IIV compared with the other groups (p < .03). In addition, although control subjects displayed a relationship between IIV and conflict-related brain activity in bilateral prefrontal cortex such that increased IIV was associated with increased activity, the MAP+ group displayed this relationship in right prefrontal cortex only, perhaps reflecting elevated vigilance in the MAP+ group. Greater IIV did not correlate with severity of use or months MA abstinent. No group differences emerged in tau values.

CONCLUSIONS

These results suggest increased cognitive instability in those MA-dependent subjects who had experienced MA-induced psychosis.

Methylphenidate effects on brain activity as a function of SLC6A3 genotype and striatal dopamine transporter availability

We pharmacologically challenged catecholamine reuptake, using methylphenidate, to investigate its effects on brain activity during a motor response inhibition task as a function of the 3'-UTR variable number of tandem repeats (VNTR) polymorphism of the dopamine transporter (DAT) gene (SLC6A3) and the availability of DATs in the striatum. We measured the cerebral hemodynamic response of 50 healthy males during a Go/No-Go task, a measure of cognitive control, under the influence of 40 mg methylphenidate and placebo using 3T functional magnetic resonance imaging. Subjects were grouped into 9-repeat (9R) carriers and 10/10 homozygotes on the basis of the SLC6A3 VNTR. During successful no-go trials compared with oddball trials, methylphenidate induced an increase of blood oxygen level-dependent (BOLD) signal for carriers of the SLC6A3 9R allele but a decrease in 10/10 homozygotes in a thalamocortical network. The same pattern was observed in caudate and inferior frontal gyrus when successful no-go trials were compared with successful go trials. We additionally investigated in a subset of 35 participants whether baseline striatal DAT availability, ascertained with (123)I-FP-CIT single photon emission computed tomography, predicted the amount of methylphenidate-induced change in hemodynamic response or behavior. Striatal DAT availability was nominally greater in 9R carriers compared with 10/10 homozygotes (d=0.40), in line with meta-analyses, but did not predict BOLD or behavioral changes following MPH administration. We conclude that the effects of acute MPH administration on brain activation are dependent on DAT genotype, with 9R carriers showing enhanced BOLD following administration of a prodopaminergic compound.

RT distributional analysis of cognitive-control-related brain activity in first-episode schizophrenia

Impairments in cognitive control are a defining feature of schizophrenia. Aspects of cognitive control include proactive control-the maintenance of task rules or goals to bias attention and maintain preparedness-and reactive control-the engagement of attention in reaction to changing cognitive demands. Proactive control is thought to be particularly impaired in schizophrenia. We sought to examine proactive and reactive control in schizophrenia, as measured by reaction time (RT) variability, and especially long RTs, which are thought to represent lapses in proactive control, during the Stroop paradigm. Furthermore, we sought to examine the neural underpinnings of lapses in proactive control and the subsequent engagement of reactive control in those with schizophrenia, as compared to healthy controls, using fMRI. We found that patients with schizophrenia displayed greater RT variability and more extremely long RTs than controls suggesting that proactive control was weaker in the schizophrenia than in the control group. All of the subjects engaged regions of the cognitive control network during long RTs, consistent with an engagement of reactive control following a failure in proactive control on these trials. The schizophrenia group, however, displayed significantly diminished activity in these regions relative to controls. Our results suggest increased failures in proactive control, but also impaired reactive control, in schizophrenia as compared to healthy subjects.

Dopamine DRD2/ANKK1 Taq1A and DAT1 VNTR polymorphisms are associated with a cognitive flexibility profile in pathological gamblers

Like drug addiction, pathological gambling (PG) has been associated with impairments in executive functions and alterations in dopaminergic functioning; however, the role of dopamine (DA) in the executive profile of PG remains unclear. The aim of this study was to identify whether the DRD2/ANKK1 Taq1A-rs1800497 and the DAT1-40 bp VNTR polymorphisms are associated with cognitive flexibility (measured by Wisconsin Card Sorting Test (WCST) and Trail Making Test (TMT)) and inhibition response (measured by Stroop Color and Word Test (SCWT)), in a clinical sample of 69 PG patients. Our results showed an association between DA functioning and cognitive flexibility performance. The Taq1A A1+ (A1A2/A1A1) genotype was associated with poorer TMT performance (p<0.05), while DAT1 9-repeat homozygotes displayed better WCST performance (p<0.05) than either 10-repeat homozygotes or heterozygotes. We did not find any association between the DRD2 or DAT1 polymorphisms and the inhibition response. These results suggested that pathological gamblers with genetic predispositions toward lower availability of DA and D2 receptor density are at a higher risk of cognitive flexibility difficulties. Future studies should aim to shed more light on the genetic mechanisms underlying the executive profile in PG.

Effects of methylphenidate on cognitive functions in children and adolescents with attention-deficit/hyperactivity disorder: evidence from a systematic review and a meta-analysis

BACKGROUND

Attention-deficit/hyperactivity disorder (ADHD) is associated with a broad range of neuropsychological impairments. The relationship between these neuropsychological deficits and the defining symptoms of ADHD seems more complex than originally thought. Methylphenidate (MPH) is an effective treatment for ADHD symptoms, but its impact on cognition is less clearly understood.

METHODS

With a common systematic search strategy and a rigorous coding and data extraction strategy across domains, we searched electronic databases to identify published placebo controlled trials that compared MPH and placebo on executive and nonexecutive memory, reaction time, reaction time variability and response inhibition in children and adolescents (5-18 years) with a formal diagnosis of ADHD.

RESULTS

Sixty studies were included in the review, of which 36 contained sufficient data for meta-analysis. Methylphenidate was superior to placebo in all five meta-analyses: executive memory, standardized mean difference (SMD) .26, 95% confidence interval (CI): -.39 to -.13; non-executive memory, SMD .60, 95% CI: -.79 to -.41; reaction time, SMD .24, 95% CI: -.33 to -.15; reaction time variability, SMD .62, 95% CI: -.90 to -.34; response inhibition, SMD .41, 95% CI: -.55 to -.27.

CONCLUSIONS

These data support the potentially important effects of MPH on various aspects of cognition known to be associated with ADHD. Consideration should be given to adding cognitive outcomes to the assessment of treatment outcome in ADHD, considering the complexity of the relationship between ADHD symptoms and cognition.

Influence of the COMT val(108/158)met polymorphism on continuous performance task indices

The Continuous Performance Task (CPT) is a widely-used measure of sustained attention and impulsivity. Deficits in CPT performance have been found in several psychiatric disorders, such as Attention-Deficit/Hyperactivity Disorder (ADHD) and schizophrenia. Molecular genetic studies of CPT performance are currently limited and have generally revealed inconsistent findings. The current study tested the associations of the COMT val(108/158)met polymorphism with AX-CPT indices (i.e., omission and commission errors, d׳, and lnβ), as well as the variability of these indices across blocks, in a sample of clinic-referred and non-referred children (N=380). We found significant associations between COMT and variability in the Signal Detection Theory (SDT) indices d׳ and lnβ across blocks, as well as a statistical trend for association between COMT and commission errors. Higher externalizing psychopathology was associated with general impairment on AX-CPT performance, and for some indices (i.e., d׳ variability and lnβ variability) the effect of COMT was stronger at higher levels of psychopathology. Our findings support the role of COMT in components of CPT performance and highlight the potential utility of using SDT indices, particularly in relation to variability in performance. Moreover, our results suggest that for some indices the effect of COMT is stronger at higher levels of externalizing psychopathology. Our study yields some preliminary insights regarding the neurobiology of CPT performance, which may elucidate the mechanisms by which specific genes confer risk for various cognitive deficits, as well as relevant disorders characterized by these deficits.

Genetic overlap between ADHD symptoms and EEG theta power

Biological markers that are grounded in neuroscience may facilitate understanding of the pathophysiology of complex psychiatric disorders. One of the most consistent and robust neural abnormalities in attention deficit hyperactivity disorder (ADHD) is increased EEG power in the theta band at rest (4-8Hz). The present study used a twin design to estimate the extent of genetic overlap between increased theta power and risk for ADHD in order to validate theta power as a marker of genetic risk for ADHD. At rest, EEG was measured in 30 monozygotic and dizygotic adolescent twin pairs concordant or discordant for high ADHD symptom scores and 37 monozygotic and dizygotic control twin pairs with low ADHD symptom scores. Structural equation modelling was used to estimate the heritability of theta power and partition the genetic and environmental contributions to the overlap between ADHD and theta power. A significant phenotypic correlation between ADHD symptoms and elevated theta power was found. Theta power demonstrated moderate to high heritability estimates (0.77) and moderate genetic correlations with ADHD (0.35) suggesting shared genetic influences. Increased theta power is a candidate biological marker of genetic risk for ADHD, which warrants further investigation of the neurobiological mechanisms that underlie the genetic relationship.

Potential contribution of dopaminergic gene variants in ADHD core traits and co-morbidity: a study on eastern Indian probands

Association of dopaminergic genes, mainly receptors and transporters, with Attention Deficit Hyperactivity Disorder (ADHD) has been investigated throughout the world due to the importance of dopamine (DA) in various physiological functions including attention, cognition and motor activity, traits. However, till date, etiology of ADHD remains unknown. We explored association of functional variants in the DA receptor 2 (rs1799732 and rs6278), receptor 4 (exon 3 VNTR and rs914655), and transporter (rs28363170 and rs3836790) with hyperactivity, cognitive deficit, and co-morbid disorders in eastern Indian probands. Diagnostic and Statistical Manual for Mental Disorders-IV was followed for recruitment of nuclear families with ADHD probands (N = 160) and ethnically matched controls (N = 160). Cognitive deficit and hyperactive traits were measured using Conner's parents/teachers rating scale. Peripheral blood was collected after obtaining informed written consent and used for genomic DNA isolation. Genetic polymorphisms were analyzed by PCR-based methods followed by population- as well as family-based statistical analyses. Association between genotypes and cognitive/hyperactivity traits and co-morbidities was analyzed by the Multifactor dimensionality reduction (MDR) software. Case-control analysis showed statistically significant difference for rs6278 and rs28363170 (P = 0.004 and 1.332e-007 respectively) while family-based analysis exhibited preferential paternal transmission of rs28363170 '9R' allele (P = 0.04). MDR analyses revealed independent effects of rs1799732, rs6278, rs914655, and rs3836790 in ADHD. Significant independent effects of different sites on cognitive/hyperactivity traits and co-morbid disorders were also noticed. It can be summarized from the present investigation that these gene variants may influence cognitive/hyperactive traits, thereby affecting the disease etiology and associated co-morbid features.

Evidence of an association between 10/10 genotype of DAT1 and endophenotypes of attention deficit/hyperactivity disorder

INTRODUCTION

Genetic variance of attention deficit-hyperactivity disorder (ADHD) is a strong determinant of this disorder. The 40 base pairs (bp) variable number tandem repeat (VNTR) located in the 3' untranslated region (UTR) of DAT1 gene increases the expression of the dopamine transporter. Therefore, DAT1 has been associated with susceptibility to ADHD.

OBJECTIVE

To determine the association between the VNTR of DAT1 and the phenotype of ADHD or its endophenotypes in a sample of children aged between 6 and 15 years from Bogotá.

SUBJECTS AND METHODS

We selected 73 patients with ADHD and 54 controls. WISC test was applied in all subjects and executive functions were assessed. The VNTR of DAT1 was polymerase chain reaction-amplified. Data regarding population genetics and statistical analysis were obtained. Correlation and association tests between genotype and neuropsychological testing were performed.

RESULTS

The DAT1 polymorphism was not associated with ADHD (P=.85). Nevertheless, the 10/10 genotype was found to be correlated with the processing speed index (P<.05). In the hyperactivity subtype, there was a genotypic correlation with some subtests of executive function (cognitive flexibility) (P≤.01). In the combined subtype, the 10/10 genotype was associated with verbal comprehension index of WISC (P<.05).

CONCLUSIONS

A correlation was found between DAT1 VNTR and the subtest "processing speed index" of WISC and the subtest "cognitive flexibility" of executive functions. To our knowledge, this is the first report to assess DAT1 gene in a Colombian population.

Methylphenidate HCl: therapy for attention deficit hyperactivity disorder

Methylphenidate is a first-line therapy for attention deficit hyperactivity disorder, the most prevalent neuropsychiatric disorder of childhood. The compound is a piperidine and the D-threo-isomer is considered the biologically active form. The compound is available in multiple short- and long-acting preparations, having different delivery systems leading to varying kinetics without clear superiority in efficacy or tolerability at the group level. Common adverse effects are insomnia, appetite disturbance, stomach ache, headache and dizziness. Its mechanism of action is linked to the monoamines dopamine and norepinephrine. The compound appears to predominantly increase the synaptic concentration of dopamine, presumably via inhibition of the dopamine transporter DAT1. There also appears to be effects on presynaptic vesicular trafficking and distribution of dopamine. Both immediate- and sustained-release preparations of methylphenidate have proven efficacy in children and adults with attention deficit hyperactivity disorder. Analysis of the National Institutes of Health-sponsored multimodal treatment study of attention deficit hyperactivity disorder supports a combined medication and behavioral therapy approach.

A phenotypic structure and neural correlates of compulsive behaviors in adolescents

BACKGROUND

A compulsivity spectrum has been hypothesized to exist across Obsessive-Compulsive disorder (OCD), Eating Disorders (ED), substance abuse (SA) and binge-drinking (BD). The objective was to examine the validity of this compulsivity spectrum, and differentiate it from an externalizing behaviors dimension, but also to look at hypothesized personality and neural correlates.

METHOD

A community-sample of adolescents (N=1938; mean age 14.5 years), and their parents were recruited via high-schools in 8 European study sites. Data on adolescents' psychiatric symptoms, DSM diagnoses (DAWBA) and substance use behaviors (AUDIT and ESPAD) were collected through adolescent- and parent-reported questionnaires and interviews. The phenotypic structure of compulsive behaviors was then tested using structural equation modeling. The model was validated using personality variables (NEO-FFI and TCI), and Voxel-Based Morphometry (VBM) analysis.

RESULTS

Compulsivity symptoms best fit a higher-order two factor model, with ED and OCD loading onto a compulsivity factor, and BD and SA loading onto an externalizing factor, composed also of ADHD and conduct disorder symptoms. The compulsivity construct correlated with neuroticism (r=0.638; p ≤ 0.001), conscientiousness (r=0.171; p ≤ 0.001), and brain gray matter volume in left and right orbitofrontal cortex, right ventral striatum and right dorsolateral prefrontal cortex. The externalizing factor correlated with extraversion (r=0.201; p ≤ 0.001), novelty-seeking (r=0.451; p ≤ 0.001), and negatively with gray matter volume in the left inferior and middle frontal gyri.

CONCLUSIONS

Results suggest that a compulsivity spectrum exists in an adolescent, preclinical sample and accounts for variance in both OCD and ED, but not substance-related behaviors, and can be differentiated from an externalizing spectrum.

MR imaging of the effects of methylphenidate on brain structure and function in attention-deficit/hyperactivity disorder

Methylphenidate is the first-choice pharmacological intervention for the treatment of Attention-Deficit/Hyperactivity Disorder (ADHD). The pharmacological and behavioral effects of methylphenidate are well described, but less is known about neurochemical brain changes induced by methylphenidate. This level of analysis may be informative on how the behavioral effects of methylphenidate are established. This paper reviews structural and functional MRI studies that have investigated effects of methylphenidate in children with ADHD. Structural MRI studies provide evidence that long-term stimulant treatment may normalize structural brain changes found in the white matter, the anterior cingulate cortex, the thalamus, and the cerebellum in ADHD. Moreover, preliminary evidence suggests that methylphenidate treatment may normalize the trajectory of cortical development in ADHD. Functional MRI has provided evidence that methylphenidate administration has acute effects on brain functioning, and even suggests that methylphenidate may normalize brain activation patterns as well as functional connectivity in children with ADHD during cognitive control, attention, and during rest. The effects of methylphenidate on the developing brain appear highly specific and dependent on numerous factors, including biological factors such as genetic predispositions, subject-related factors such as age and symptom severity, and task-related factors such as task difficulty. Future studies on structural and functional brain changes in ADHD may benefit from inclusion strategies guided by current medication status and medication history. Further studies on the effects of methylphenidate treatment on structural and functional MRI parameters are needed to address unresolved issues of the long-term effects of treatment, as well as the mechanism through which medication-induced brain changes bring about clinical improvement.

The dopamine transporter haplotype and reward-related striatal responses in adult ADHD

Attention deficit/hyperactivity disorder (ADHD) is a highly heritable disorder and several genes increasing disease risk have been identified. The dopamine transporter gene, SLC6A3/DAT1, has been studied most extensively in ADHD research. Interestingly, a different haplotype of this gene (formed by genetic variants in the 3' untranslated region and intron 8) is associated with childhood ADHD (haplotype 10-6) and adult ADHD (haplotype 9-6). The expression of DAT1 is highest in striatal regions in the brain. This part of the brain is of interest to ADHD because of its role in reward processing is altered in ADHD patients; ADHD patients display decreased striatal activation during reward processing. To better understand how the DAT1 gene exerts effects on ADHD, we studied the effect of this gene on reward-related brain functioning in the area of its highest expression in the brain, the striatum, using functional magnetic resonance imaging. In doing so, we tried to resolve inconsistencies observed in previous studies of healthy individuals and ADHD-affected children. In a sample of 87 adult ADHD patients and 77 healthy comparison subjects, we confirmed the association of the 9-6 haplotype with adult ADHD. Striatal hypoactivation during the reward anticipation phase of a monetary incentive delay task in ADHD patients was again shown, but no significant effects of DAT1 on striatal activity were found. Although the importance of the DAT1 haplotype as a risk factor for adult ADHD was again demonstrated in this study, the mechanism by which this gene increases disease risk remains largely unknown.

Meta-analysis of the association between dopamine transporter genotype and response to methylphenidate treatment in ADHD

Attention-deficit/hyperactivity disorder (ADHD) is a prevalent childhood-onset neuropsychiatric disorder. Treatment with methylphenidate, which blocks dopamine and noradrenaline transporters, is clinically efficacious in reducing the symptoms of ADHD. However, a considerable proportion of patients show no or only insufficient response to methylphenidate. Following a pharmacogenetic approach, a number of studies have suggested that heterogeneity in treatment response across subjects might to some extent be due to genetic factors. In particular, a variable number tandem repeat (VNTR) polymorphism in the 3' untranslated region of the SLC6A3 gene, which codes for the dopamine transporter, has been considered as a predictor of treatment success. However, the literature has so far been inconsistent. Here we present results of a meta-analysis of studies investigating the moderating effect of the SLC6A3 VNTR on response to methylphenidate treatment in subjects with ADHD. Outcome measures from 16 studies including data from 1572 subjects were entered into a random-effects model. There was no significant summary effect for the SLC6A3 VNTR on the response to methylphenidate treatment (P>0.5) and no effect on specific symptom dimensions of hyperactivity/impulsivity and inattention (all P>0.2). However, in a subanalysis of naturalistic trials, we observed a significant effect of d=-0.36 (P=0.03), indicating that 10R homozygotes show less improvement in symptoms following treatment than the non-10/10 carriers. This meta-analysis indicates that SLC6A3 VNTR is not a reliable predictor of methylphenidate treatment success in ADHD. Our study leaves unanswered the question of whether other genetic polymorphisms or nongenetic factors may contribute to the observed heterogeneity in treatment response across ADHD subjects.

Dopaminergic Gene Polymorphisms Affect Long-term Forgetting in Old Age: Further Support for the Magnification Hypothesis

Emerging evidence from animal studies suggests that suboptimal dopamine (DA) modulation may be associated with increased forgetting of episodic information. Extending these observations, we investigated the influence of DA-relevant genes on forgetting in samples of younger (n = 433, 20–31 years) and older (n = 690, 59–71 years) adults. The effects of single nucleotide polymorphisms of the DA D2 (DRD2) and D3 (DRD3) receptor genes as well as the DA transporter gene (DAT1; SLC6A3) were examined. Over the course of one week, older adults carrying two or three genotypes associated with higher DA signaling (i.e., higher availability of DA and DA receptors) forgot less pictorial information than older individuals carrying only one or no beneficial genotype. No such genetic effects were found in younger adults. The results are consistent with the view that genetic effects on cognition are magnified in old age. To the best of our knowledge, this is the first report to relate genotypes associated with suboptimal DA modulation to more long-term forgetting in humans. Independent replication studies in other populations are needed to confirm the observed association.

Emotive interference during cognitive processing in major depression: an investigation of lower alpha 1 activity

BACKGROUND

Individuals with Major Depressive Disorder (MDD) tend to be more susceptible to distraction by negative emotional material than their non-depressed counterparts. This extends to an enhanced vulnerability to interference from mood-congruent stimuli during cognitive processing. The current study investigated the electrophysiological correlates of competing cognitive and emotional processing demands in MDD.

METHODS

Event-related alpha activity within the lower alpha 1 band was examined during the online information retention phase of a non-emotive WM task with extraneous emotional stimuli (positive, negative and neutral) presented as background images. EEG activity over posterior parietal cortex was compared between 15 acutely depressed and 16 never depressed right-handed women.

RESULTS

A valence specific dissociation in lower alpha 1 activity was observed between the two groups, consistent with greater attentional resource allocation to positive distracters in control participants and to negative distracters in MDD participants. No group differences were seen when neutral distracters were displayed.

CONCLUSIONS

These results demonstrate that activity within the lower alpha 1 band is sensitive to competing emotional and cognitive processing demands and highlight the importance of posterior parietal regions in depression-related susceptibility to affective distractibility during cognitive processing.

Quantitative and molecular genetics of ADHD

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

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

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

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.