Inferring candidate genes for attention deficit hyperactivity disorder (ADHD) assessed by the World Health Organization Adult ADHD Self-Report Scale (ASRS)

Regional brain activation changes and abnormal functional connectivity of the ventrolateral prefrontal cortex during working memory processing in adults with attention-deficit/hyperactivity disorder

Previous studies on working memory (WM) function in adults with attention-deficit/hyperactivity disorder (ADHD) suggested aberrant activation of the prefrontal cortex and the cerebellum. Although it has been hypothesized that activation differences in these regions most likely reflect aberrant frontocerebellar circuits, the functional coupling of these brain networks during cognitive performance has not been investigated so far. In this study, functional magnetic resonance imaging (fMRI) and both univariate and multivariate analytic techniques were used to investigate regional activation changes and functional connectivity differences during cognitive processing in healthy controls (n = 12) and ADHD adults (n = 12). Behavioral performance during a parametric verbal WM paradigm did not significantly differ between adults with ADHD and healthy controls. During the delay period of the activation task, however, ADHD patients showed significantly less activation in the left ventrolateral prefrontal cortex (VLPFC), as well as in cerebellar and occipital regions compared with healthy control subjects. In both groups, independent component analyses revealed a functional network comprising bilateral lateral prefrontal, striatal, and cingulate regions. ADHD adults had significantly lower connectivity in the bilateral VLPFC, the anterior cingulate cortex, the superior parietal lobule, and the cerebellum compared with healthy controls. Increased connectivity in ADHD adults was found in right prefrontal regions, the left dorsal cingulate cortex and the left cuneus. These findings suggest both regional brain activation deficits and functional connectivity changes of the VLPFC and the cerebellum as well as functional connectivity abnormalities of the anterior cingulate and the parietal cortex in ADHD adults during WM processing.

Association between catechol O-methyltransferase (COMT) haplotypes and severity of hyperactivity symptoms in adults

It has been suggested that symptoms of attention-deficit/hyperactivity disorder (ADHD) is related to low dopamine levels in the prefrontal cortex. The enzyme catechol O-methyltransferase (COMT), which degrades dopamine and other catecholamines, is important for monoamine signaling in this brain-region, but genetic studies of the functional Val158Met (rs4680) polymorphism in ADHD have been inconsistent. However, recently it was shown that also common synonymous COMT variants modulate total COMT enzymatic activity by affecting the expression of the gene [Nackley et al. (2006); Science 314(5807):1930-1933]. We therefore hypothesized that analysis of haplotypes could reveal more about the association between COMT and ADHD symptoms than the Val158Met polymorphism alone. SNPs rs6269, rs4633, rs4818, and rs4680, tagging the common putative functional COMT haplotypes, were genotyped in 435 adult subjects with a clinical diagnosis of ADHD and 383 controls and analyzed for association with ADHD and the hyperactivity/impulsivity and inattention dimensions from the Adult ADHD Self-Report Scale (ASRS). All markers showed a trend for association with the hyperactivity/impulsivity scale, peaking at marker rs6269 (P = 0.007). Haplotype analysis revealed that the rs6269 risk allele tags the suggested high COMT-activity haplotype, which is associated with the highest hyperactivity/impulsivity score in our sample (P = 0.01). Our results also suggest that there is a stepwise decreased hyperactivity/impulsivity score associated with the proposed mid and low activity haplotypes described previously. In conclusion, we suggest that COMT haplotype variation is associated primarily with the hyperactivity/impulsivity dimension of ADHD and point to the importance of testing this hypothesis in future studies.

Neural hyporesponsiveness and hyperresponsiveness during immediate and delayed reward processing in adult attention-deficit/hyperactivity disorder

BACKGROUND

Dysfunctional reward processing, accompanied by a limited ability to tolerate reward delays, has been proposed as an important feature in attention-deficit/hyperactivity disorder (ADHD).

METHODS

Using functional magnetic resonance imaging (fMRI), brain activation in adult patients with ADHD (n=14) and healthy control subjects (n=12) was examined during a series of choices between two monetary reward options that varied by delay to delivery.

RESULTS

Compared with healthy control subjects, hyporesponsiveness of the ventral-striatal reward system was replicated in patients with ADHD and was evident for both immediate and delayed rewards. In contrast, delayed rewards evoked hyperactivation in dorsal caudate nucleus and amygdala of ADHD patients. In both structures, neural activity toward delayed rewards was significantly correlated with self-rated ADHD symptom severity.

CONCLUSIONS

The finding of ventral-striatal hyporesponsiveness during immediate and delayed reward processing in patients with ADHD further strengthens the concept of a diminished neural processing of rewards in ADHD. Hyperactivation during delayed reward processing, gradually increasing along the ventral-to-dorsal extension of the caudate nucleus, and especially the concomitant hyperactivation of the amygdala are in accordance with predictions of the delay aversion hypothesis.

The influence of serotonin- and other genes on impulsive behavioral aggression and cognitive impulsivity in children with attention-deficit/hyperactivity disorder (ADHD): Findings from a family-based association test (FBAT) analysis

BACKGROUND

Low serotonergic (5-HT) activity correlates with increased impulsive-aggressive behavior, while the opposite association may apply to cognitive impulsiveness. Both types of impulsivity are associated with attention-deficit/hyperactivity disorder (ADHD), and genes of functional significance for the 5-HT system are implicated in this disorder. Here we demonstrate the separation of aggressive and cognitive components of impulsivity from symptom ratings and test their association with 5-HT and functionally related genes using a family-based association test (FBAT-PC).

METHODS

Our sample consisted of 1180 offspring from 607 families from the International Multicenter ADHD Genetics (IMAGE) study. Impulsive symptoms were assessed using the long forms of the Conners and the Strengths and Difficulties parent and teacher questionnaires. Factor analysis showed that the symptoms aggregated into parent- and teacher-rated behavioral and cognitive impulsivity. We then selected 582 single nucleotide polymorphisms (SNPs) from 14 genes directly or indirectly related to 5-HT function. Associations between these SNPs and the behavioral/cognitive groupings of impulsive symptoms were evaluated using the FBAT-PC approach.

RESULTS

In the FBAT-PC analysis for cognitive impulsivity 2 SNPs from the gene encoding phenylethanolamine N-methyltransferase (PNMT, the rate-limiting enzyme for adrenalin synthesis) attained corrected gene-wide significance. Nominal significance was shown for 12 SNPs from BDNF, DRD1, HTR1E, HTR2A, HTR3B, DAT1/SLC6A3, and TPH2 genes replicating reported associations with ADHD. For overt aggressive impulsivity nominal significance was shown for 6 SNPs from BDNF, DRD4, HTR1E, PNMT, and TPH2 genes that have also been reported to be associated with ADHD. Associations for cognitive impulsivity with a SERT/SLC6A4 variant (STin2: 12 repeats) and aggressive behavioral impulsivity with a DRD4 variant (exon 3: 3 repeats) are also described.

DISCUSSION

A genetic influence on monoaminergic involvement in impulsivity shown by children with ADHD was found. There were trends for separate and overlapping influences on impulsive-aggressive behavior and cognitive impulsivity, where an association with PNMT (and arousal mechanisms affected by its activity) was more clearly involved in the latter. Serotonergic and dopaminergic mechanisms were implicated in both forms of impulsivity with a wider range of serotonergic mechanisms (each with a small effect) potentially influencing cognitive impulsivity. These preliminary results should be followed up with an examination of environmental influences and associations with performance on tests of impulsivity in the laboratory.

Genetics of attention deficit hyperactivity disorder

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

Role of the serotonin system in ADHD: treatment implications

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

Norepinephrine transporter and catecholamine-O-methyltransferase gene variants and attention-deficit/hyperactivity disorder symptoms in adults

Attention deficit/hyperactivity disorder (ADHD) is a complex, highly heritable psychiatric condition. Neuropsychological and pharmacological studies suggest a dysregulation of central noradrenergic neurotransmission in addition to dopaminergic and serotonergic mechanisms. Only a few studies have focused on the association of noradrenergic susceptibility genes with ADHD. In this study, we investigated the association of several ADHD symptom scores (German short form of the Wender Utah Rating Scale, WURS-k; ADHD self report, ADHD-SB, and the German validated version of the WRAADDS, WRI) with haplotypes of the catechol-O-methyltransferase (COMT) and the norepinephrine transporter (SLC6A2) genes. Subjects were genotyped for three SLC6A2 (rs5569, rs998424, rs2242447) and two COMT single nucleotide polymorphisms (rs4680, rs4818). In addition, psychosocial adversity in childhood was assessed in order to evaluate putative gene-environment interactions. We did not find main effects of the COMT and SLC6A2 NET1 gene haplotypes on any ADHD symptom severity score. Childhood psychosocial adversity was strongly associated with number of ADHD symptoms. No gene-environment interaction was found. A specific combination of two COMT and SLC6A2 gene haplotypes, containing the low functioning COMT variant was nominally associated with low ADHD scores in all scales. Results do not support the hypothesis that common variants in the SLC6A2 and COMT genes in particular are associated with ADHD, but might give some evidence for interactive effects between these gene variants on ADHD severity.

Schizotypy, attention deficit hyperactivity disorder, and dopamine genes

Previous research has suggested that there may be overlap between schizophrenia and attention-deficit hyperactivity disorder (ADHD). The relationship between schizotypal personality traits, ADHD features and polymorphisms was evaluated in dopamine-related genes. Thirty-one healthy, Caucasian men completed the Rust Inventory of Schizotypal Cognitions (RISC) and the ADHD Self-Report Scale (ASRS). Catechol-O-methyltransferase (COMT) Val158Met, dopamine receptors of the D3 type (DRD3) Ser9Gly, DRD4 variable number of tandem repeats (VNTR), and SLC6A3 VNTR polymorphisms were analyzed. RISC score was correlated with ASRS score (r = 0.54, P = 0.003). COMT Met homozygotes had higher ASRS scores than Val homozygotes (P = 0.005). These findings are consistent with evidence of overlap between schizophrenia and ADHD and support an involvement of COMT genotype in ADHD features.