No association between low- and high-activity catecholamine-methyl-transferase (COMT) and attention deficit hyperactivity disorder (ADHD) in a sample of Turkish children

Genetic Variations in Attention Deficit Hyperactivity Disorder Subtypes and Treatment Resistant Cases

OBJECTIVE

ObjectiveaaWe evaluated the distribution of alpha-2A adrenergic receptor (ADRA2A) and catechol-o-methyltransferase (COMT) single nucleotide polymorphisms (SNPs) among ADHD subtypes and other homogeneous patient populations including treatment-resistant cases and patients with high symptom severity.

METHODS

Methodsaa121 ADHD patients aged 6-18 years were included in the study. Diagnosis and subtypes designation were confirmed using the Kiddie Schedule for Affective Disorders and Schizophrenia (K-SADS) and symptoms were evaluated using the Conners' Parent (CPRS) and Teacher Rating Scales (CTRS). The response to methylphenidate was assessed objectively using the Clinical Global Impression-Severity Scale (CGI-S) and Global Assessment of Functioning Scale (GAS) as well as the Continuous Performance (CPT) and Trail Making tests (TMT-A, B). Patients were genotyped for ADRA2A (rs1800544) and COMT (rs4680) SNPs by PCR/RFLP and compared to a gender-matched control group.

RESULTS

Although there was no association of COMT (rs4680) SNP with symptoms or diagnosis, the ADRA2A polymorphism, low socioeconomic status (SES), and comorbid psychiatric diagnosis were all associated with poor response to methylphenidate in logistic regression analysis.

CONCLUSION

Clinicians may consider adjuvant strategies when these negative factors are present to increase the success of tailored ADHD treatments in the future.

The Effect of Single Dose Methylphenidate on Neurometabolites according to COMT Gene Val158Met Polymorphism in the Patient with Attention Deficit Hyperactivity Disorder: A Study Using Magnetic Resonance Spectroscopy

OBJECTIVE

Attention deficit/hyperactivity disorder (ADHD) is a common neurodevelopmental disorder. Thus, the present study aimed to determine the effects of a single dose of methylphenidate (Mph) on neurometabolite levels according to polymorphisms of the catechol-O-methyltransferase (COMT) gene.

METHODS

This study evaluated the neurometabolite levels including N-acetylaspartate (NAA), creatine (Cr), and choline (Cho) of ADHD patients, before and after treatment with Mph (10 mg) according to the presence of COMT polymorphisms. The spectra were obtained from the dorsolateral prefrontal cortex (DLPFC), anterior cingulate cortex (ACC), cerebellum, and striatum.

RESULTS

The NAA levels of the val/val and val genotype carriers (val/val and val/met genotypes) increased in the DLPFC and ACC, respectively, following Mph treatment. The NAA/Cr ratio was lower in the DLPFC of val carriers than in the met/met genotype carriers prior to Mph administration. The Cho levels of the val/met genotype and val carriers increased in the striatum following Mph treatment. Following Mph treatment, the Cr levels of the met/met genotype carriers were higher than those of the val/met genotype and val carriers. Additionally, after Mph treatment, there was a significant increase in Cr levels in the DLPFC of the met/met genotype carriers but a significant decrease in such levels in the striatum of val/val genotype carriers.

CONCLUSION

These findings suggest that polymorphisms of the COMT gene can account for individual differences in neurochemical responses to Mph among ADHD patients. Therefore, further studies are needed to fully characterize the effects of the Val158met polymorphism of the COMT gene on treatment outcomes in patients with ADHD.

No association between catechol-O-methyltransferase (COMT) genotype and attention deficit hyperactivity disorder (ADHD) in Japanese children

OBJECTIVE

This study ascertained the association between attention deficit/hyperactivity disorder (ADHD) in Japanese children and a polymorphism of catechol-O-methyltransferase (COMT), a dopamine-control gene. The secondary aim of the study was the evaluation of a putative association between methylphenidate (MPH) effect/adverse effects and the COMT genotype.

METHODS

To ascertain the distribution of the Val158Met variant of COMT, 50 children meeting ADHD inclusion criteria were compared with 32 healthy children. Clinical improvement and the occurrence of adverse effects were measured before and 3 months after MPH administration in children with ADHD, and analyzed for genotype association. Wechsler Intelligence Scale for Children-Third Edition (WISC-III), age, MPH dose were included as co-variables.

RESULTS

The occurrence of the COMT Val/Val genotype was significantly higher in children with ADHD (χ(2)(1)=7.13, p<0.01). However, there was no significant difference in the Val/Val genotype according to disorder, and WISC and ADHD rating scale scores, after correcting for the interaction between disorder and COMT genotype. Furthermore, no significant difference in MPH effect/adverse effects was observed in association with the COMT genotype in the ADHD group.

CONCLUSIONS

These results showed a lack of association between the COMT Val/Val genotype and ADHD in Japan.

Attention-deficit/hyperactivity disorder phenotype is influenced by a functional catechol-O-methyltransferase variant

The catechol-O-methyltransferase gene (COMT) plays a crucial role in the metabolism of catecholamines in the frontal cortex. A single nucleotide polymorphism (Val(158)Met SNP, rs4680) leads to either methionine (Met) or valine (Val) at codon 158, resulting in a three- to fourfold reduction in COMT activity. The aim of the present study was to assess the COMT Val(158)Met SNP as a risk factor for attention-deficit/hyperactivity disorder (ADHD), ADHD symptom severity and co-morbid conduct disorder (CD) in 166 children with ADHD. The main finding of the present study is that the Met allele of the COMT Val(158)Met SNP was associated with ADHD and increased ADHD symptom severity. No association with co-morbid CD was observed. In addition, ADHD symptom severity and early adverse familial environment were positive predictors of lifetime CD. These findings support previous results implicating COMT in ADHD symptom severity and early adverse familial environment as risk factors for co-morbid CD, emphasizing the need for early intervention to prevent aggressive and maladaptive behavior progressing into CD, reducing the overall severity of the disease burden in children with ADHD.

Association between catechol-O-methyltransferase gene polymorphism and attention-deficit hyperactivity disorder in Korean population

Recently, the relationship between allele frequency distribution and attention-deficit hyperactivity disorder (ADHD) has been actively studied. In Korea, the relationship between the genetic type and alleles for catechol-O-methyltransferase (COMT) gene has been studied in ADHD patients. ADHD was diagnosed in 60 patients according to the Diagnostic and Statistical Manual of Mental Disorders Version IV (DSM-IV) diagnostic criteria and Schedule for Affective Disorders and Schizophrenia for School-Age Children--Present and Lifetime Version (K-SADS-PL), and they were selected for the study. For the control group, normal volunteers were chosen. Blood samples were taken from the 160 subjects. DNA was extracted from blood lymphocytes, and PCR was performed for COMT NlaIII VNTR polymorphism. For the case-control analyses, allele and genotype frequencies were compared using the chi(2) method. When the ADHD group and the normal control group were compared, significant difference was seen on the COMT genetic type, but was not seen on the allele distribution. As a result, it is viewed that there is no relationship between ADHD and the COMT gene, but final decision is indefinite.

Candidate gene analysis in an on-going genome-wide association study of attention-deficit hyperactivity disorder: suggestive association signals in ADRA1A

OBJECTIVES

Attention-deficit hyperactivity disorder (ADHD) is a highly heritable, common developmental disorder. Although a few confirmed associations have emerged from candidate gene studies, these have shown the same limitations that have become evident in the study of other complex diseases, often with inconsistent and nonreplicated results across different studies.

METHODS

In this report, 27 ADHD candidate genes were explored in greater depth using high-density tag single nucleotide polymorphism (SNP) genotyping. Association with 557 SNPs was tested using the transmission disequilibrium test in 270 nuclear pedigrees selected from an ongoing ADHD genetic study that includes all disease subtypes.

RESULTS

SNPs in seven genes including SLC1A3, SLC6A3, HTR4, ADRA1A, HTR2A, SNAP25, and COMT showed a nominal level of association with ADHD (P values <0.05), but none remained significant after a stringent correction for the total number of tests performed.

CONCLUSION

The strongest signal emerged from SNPs in the promoter region (rs3808585) and in an intron (rs17426222, rs4732682, rs573514) of ADRA1A, all located within the same haplotype block. Some of the SNPs in HTR2A and COMT have already been reported by others, whereas other SNPs will need confirmation in independent samples.

Catechol-O-methyltransferase Val158Met polymorphism is associated with methylphenidate response in ADHD children

Methylphenidate is the most frequently prescribed drug in the treatment of attention deficit hyperactivity disorder (ADHD) but it is not effective in every case. Therefore, identifying genetic and/or biological markers predicting drug-response is increasingly important. Here we present a case-control study and pharmacogenetic association analyses in ADHD investigating three dopaminergic polymorphisms. Previous studies suggested variable number of tandem repeats (VNTR) in the dopamine D4 receptor (DRD4) and the dopamine transporter (DAT1) genes as genetic risk factors for ADHD and as possible markers of methylphenidate response. Our results did not indicate substantial involvement of these two VNTRs in ADHD, however, both the case-control and the pharmacogenetic analyses showed significant role of the high activity Val-allele of cathecol-O-methyltransferase (COMT) Val158Met polymorphism in our ADHD population. The Val-allele was more frequent in the ADHD group (n = 173) compared to the healthy population (P = 0.016). The categorical analysis of 90 responders versus 32 non-responders showed an association between the Val-allele or Val/Val genotype and good methylphenidate response (P = 0.009 and P = 0.034, respectively). Analyzing symptom severity as a continuous trait, significant interaction of COMT genotype and methylphenidate was found on the Hyperactivity-Impulsivity scale (P = 0.044). Symptom severity scores of all three genotype groups decreased following methylphenidate administration (P < 0.001), however Val/Val homozygote children had significantly less severe symptoms than those with Met/Met genotype after treatment (P = 0.015). This interaction might reflect the regulatory effect of COMT dominated prefrontal dopamine transmission on subcortical dopamine systems, which are the actual site of methylphenidate action.

COMT Val108/158Met polymorphism and the modulation of task-oriented behavior in children with ADHD

It has been suggested that the symptoms of attention-deficit/hyperactivity disorder (ADHD), including inattention and/or hyperactivity/impulsivity, translate into deficits in task-oriented behavior or problem-focused activity. The frontosubcortical dopamine pathway has been implicated in ADHD. One of the key modulators of extracellular dopamine levels in the prefrontal cortex is catechol-O-methyltransferase (COMT). The objective of this study was to examine the association of the COMT Val(108/158)Met polymorphism with (1) task-oriented behavior in children with ADHD, and (2) response of this behavior given methylphenidate (MPH) treatment. Children of Caucasian ethnicity, having ADHD (n=188), were assessed using the Restricted Academic Situation Scale (RASS). The RASS uses a simulated academic environment within the research clinic, to assess the child's ability for independent, sustained orientation to an assignment of math problems. Each child was administered placebo and MPH (0.5 mg/kg in a divided b.i.d. dose), each for a 1-week period, in a randomized, double-blind, crossover trial. On day 3 of the respective treatment week, the child was administered placebo/MPH in the clinic, and the acute change in behavior (before and 1 h after treatment) was evaluated on the RASS. Analysis was carried out using mixed model analysis of variance. Significant main effects of COMT genotype (F(2,184)=5.12, p=0.007) and treatment (F(1,184)=44.26, p<0.001) on task-oriented behavior were observed. However, no genotype by treatment interaction was observed. These results suggest that the COMT Val(108/158)Met polymorphism modulates task-oriented behavior, but it does not modulate the response of this behavior with MPH treatment.

Evaluation of potential gene-gene interactions for attention deficit hyperactivity disorder in the Han Chinese population

Several lines of evidence suggest that attention-deficit/hyperactivity disorder (ADHD) is a polygenic disorder produced by the interaction of several genes with minor effects. To explore potential gene-gene interactions among candidate genes for ADHD, we studied the dopamine D2 receptor (DRD2), dopamine D4 receptor (DRD4), dopamine transporter (DAT1), and catechol-O-methyltransferase (COMT) genes in the Han Chinese population. A sample of 340 children with ADHD was diagnosed according to the DSM-IV criteria. We also recruited 226 unrelated controls. Identified polymorphisms included a 48-base-pair-repeat in Exon 3 of DRD4, a 40-base-pair-repeat in the 3' untranslated region of DAT1, a restriction-fragment-length polymorphism at codon 158 of COMT, and a -241A > G transition in the promoter of DRD2. Associations of polymorphisms with ADHD and its subtypes were examined by comparing allele frequencies between probands and controls. Binary logistic regression analysis was used to examine the potential gene-gene interactions. Binary logistic regression analysis with the sample of refined phenotypes showed that male gender and long-repeat genotypes of DRD4 and DAT1 were independent risk factors for ADHD. We found no evidence for gene-gene interactions among the candidate genes studied. The present study suggests that dopamine candidate genes are associated with increased vulnerability to ADHD in the Han Chinese population.

Meta-analysis of association between a catechol-O-methyltransferase gene polymorphism and attention deficit hyperactivity disorder

There have been conflicting reports on the association between the Val158/108Met polymorphism of the catechol-O-methyltransferase (COMT) gene and attention deficit hyperactivity disorder (ADHD). Therefore we would like to perform a meta-analysis of previous studies to assess the overall magnitude and significance of the association. Family-based and case-control studies of the association between the COMT gene polymorphism and ADHD were searched systematically and comprehensively. Odds ratios (OR) of association were pooled by the fixed effects model if no significant heterogeneity was present among different studies. Subgroup analysis by gender and ADHD subtypes were also performed. Eleven family-based and two case-control studies were identified. After pooling the results, no significant association between the COMT Vall58/108Met polymorphism and ADHD was found (OR 0.99 (95% CI: 0.88-1.12), P = 0.87). There was also no significant association when the results were stratified by gender or ADHD subtype. There was no significant statistical heterogeneity (chi2 = 12.27, P = 0.2) although clinical heterogeneity was present in the studies, especially the ethnicity of subjects. Sensitivity analysis demonstrated absence of undue influence of any single study. Standard regression analysis showed no significant publication bias. We concluded that no significant association was present between the most common COMT gene polymorphism and ADHD. Further studies should employ larger sample size in more homogeneous subjects. Further investigations in moderator variables and gene-gene and gene-environment interactions are also warranted.

The genetics of attention deficit hyperactivity disorder

Over the past 15 years, considerable progress has been made in understanding the etiology of childhood Attention Deficit Hyperactivity Disorder (ADHD), largely due to the publication of numerous twin studies which are consistent in suggesting substantial genetic influences (i.e., heritabilities ranging from 60% to 90%), non-shared environmental influences that are small-to-moderate in magnitude (i.e., ranging from 10% to 40%), and little-to-no shared environmental influences. Following from these quantitative genetic findings, numerous molecular genetic studies of association and linkage between ADHD and a variety of candidate genes have been conducted during the past 10 years. The majority of the candidate genes studied underlie various facets of the dopamine, norepinephrine, and serotonin neurotransmitter systems, although the etiological role of candidate genes outside of neurotransmitter systems (e.g., involved in various aspects of brain and nervous system development) have also been examined. In this paper, we review recent findings from candidate gene studies of childhood ADHD and highlight those candidate genes for which associations are most replicable and which thus appear most promising. We conclude with a consideration of some of the emerging themes that will be important in future studies of the genetics of ADHD.

Méta-analyse des gènes candidats dans le trouble déficit attentionnel avec hyperactivité (TDAH)

Attention-deficit hyperactivity disorder (ADHD) is a common behavioral disorder observed during childhood, detected in 3% to 5% of school-age children. The disorder is characterised by marked inattention, hyperactivity, and impulsiveness. In most cases, symptoms can be treated by catecholamine-releasing drugs, such as methylphenidate. Children with ADHD are at higher risk for substance abuse and oppositional, conduct and mood disorders. Familial and adoption studies shed light on the genetic vulnerability of ADHD. Twin studies estimated the broad heritability to range between 40% and 90%. The mode of transmission is yet unknown, but is likely polygenic. Molecular genetic studies in ADHD should contribute to a greater understanding of the pathophysiology of the disorder (genetics of the vulnerability), and could help to select a more rational type of treatment (pharmacogenetic). Family-based association studies already performed are reviewed in this manuscript. Association studies, using haplotype relative risk (HRR) or transmission disequilibrium test (TDT) have focused on candidate genes which code for proteins potentially involved in the etiopathogenesis of the disorder. Genes involved in dopamine, serotonin, and noradrenalin systems have thus been assessed for their role in core features of ADHD, such as motor overactivity, inattention, and impulsiveness. According to a meta-analysis, the DAT1 gene, an obvious candidate gene in ADHD vulnerability, does not appear to be involved (OR = 1.13, p = 0.21). On the other hand, DRD4 (OR = 1.26, p = 0.01) and DRD5 (OR = 1.4, p = 0.01) are significantly associated to ADHD according to the present meta-analysis, confirming previous ones. Recent studies showed a trend for an association between one allele of the 5-HTT (considering case-control studies) and DBH (OR = 1.27, p = 0.06) genes and ADHD, but these positive findings have to be replicated. ADHD is a complex disorder with potentially many different risk factors. Genetic and phenotypic heterogeneity could explain why some association studies are positive, whereas others are negative. For instance, different developmental pathways are likely to lead to similar clinical outcomes. More clear-cut phenotypes, such as ADHD with conduct disorder, or ADHD with bipolar disorder, could be more homogenous, the genes involved being therefore more easy to detect. These phenotypes are beginning to be specifically studied in molecular genetics. In addition, the development of pharmacogenetics could help to identify predictors of clinical response for a specific type of treatment, which would be clearly helpful in clinical practice.

Association of the dopamine transporter (DAT1) 10/10-repeat genotype with ADHD symptoms and response inhibition in a general population sample

Association between attention-deficit hyperactivity disorder (ADHD) and the 10-repeat allele of the dopamine transporter gene (DAT1) has been reported in independent clinical samples using a categorical clinical definition of ADHD. The present study adopts a quantitative trait loci (QTL) approach to examine the association between DAT1 and a continuous measure of ADHD behaviours in a general-population sample, as well as to explore whether there is an independent association between DAT1 and performance on neuropsychological tests of attention, response inhibition, and working memory. From an epidemiological sample of 872 boys aged 6-11 years, we recruited 58 boys scoring above the 90th percentile for teacher reported ADHD symptoms (SWAN ADHD scale) and 68 boys scoring below 10th percentile for genotyping and neuropsychological testing. A significant association was found between the DAT1 homozygous 10/10-repeat genotype and high-scoring boys (chi(2)square=4.6, P<0.03; odds ratio=2.4, 95% CI 1.1-5.0). Using hierarchical linear regression, a significant independent association was found between the DAT1 10/10-repeat genotype and measures of selective attention and response inhibition after adjusting for age, IQ, and ADHD symptoms. There was no association between DAT1 and any component of working memory. Furthermore, performance on tasks of selective attention although associated with DAT1 was not associated with SWAN ADHD high scores after controlling for age and IQ. In contrast, impairment on tasks that tapped sustained attention and the central executive component of working memory were found in high-scoring boys after adjusting for age and IQ. The results suggest that DAT1 is a QTL for continuously distributed ADHD behaviours in the general population and the cognitive endophenotype of response inhibition.

Molecular 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). We review this literature, with a particular emphasis on molecular genetic studies. Family, twin, and adoption studies provide compelling evidence that genes play a strong role in mediating susceptibility to ADHD. This fact is most clearly seen in the 20 extant twin studies, which estimate the heritability of ADHD to be .76. Molecular genetic studies suggest that the genetic architecture of ADHD is complex. The few genome-wide scans conducted thus far are not conclusive. In contrast, the many candidate gene studies of ADHD have produced substantial evidence implicating several genes in the etiology of the disorder. For the eight genes for which the same variant has been studied in three or more case-control or family-based studies, seven show statistically significant evidence of association with ADHD on the basis of the pooled odds ratio across studies: DRD4, DRD5, DAT, DBH, 5-HTT, HTR1B, and SNAP-25.

A family based study of catechol-O-methyltransferase (COMT) and attention deficit hyperactivity disorder (ADHD)

Neurobiological studies have suggested that altered dopaminergic function may contribute to the etiology of attention deficit hyperactivity disorder (ADHD). The gene encoding catechol-O-methyltransferase (COMT) is an attractive candidate for ADHD susceptibility as it plays a major role in the degradation of dopamine. Moreover, a functional Val158Met polymorphism in COMT that alters the activity of the encoded protein has been strongly implicated in frontal lobe function, with the high activity Valine allele being associated with poorer performance, and ADHD is thought to involve fronto-striatal pathways. We have examined this functional variant for association with ADHD in a family based association sample comprising 279 probands and their parents. We have also examined two other markers in the COMT gene (rs737865, rs165599) which, together with the Val/Met variant, have recently been shown to be associated with altered COMT expression rather than enzyme activity. No evidence for association was observed with any single marker or haplotype in a sample of 279 affected children and their parents.

The methionine allele of the COMT polymorphism impairs prefrontal cognition in children and adolescents with ADHD

ADHD is a highly heritable psychiatric disorder of childhood. A functional polymorphism (Val158Met) of the catechol-O-methyltransferase (COMT) gene has attracted interest as a candidate gene for ADHD. The high-activity valine variant of this polymorphism degrades prefrontal dopamine three to four times more quickly than the low-activity methionine variant and could therefore contribute to the proposed hypodopaminergic state in ADHD. Here we tested for association of this polymorphism with ADHD and examined its influence on prefrontal cognition in ADHD. We have previously reported no association of the Val158Met COMT gene polymorphism in 94 Irish ADHD families (Hawi et al. (2000) Am J Med Genet 96:282-284). Here we re-examined this finding with an extended sample of 179 ADHD cases using a family control design. We also examined the performance of children and adolescents with ADHD (n = 61) on a standardised test of sustained attention. Analysis confirmed the absence of an association between the Val158Met COMT gene polymorphism and the clinical phenotype of ADHD. COMT genotype, however, affected prefrontal cognition in ADHD: ADHD children who were homozygous for the valine variant had significantly better sustained attention than those ADHD children possessing at least one copy of the methionine variant. Children possessing the methionine variant performed significantly below age-related norms on tests of sustained attention. Contrary to expectations, the methionine variant of the Val158Met COMT gene polymorphism impaired prefrontally-mediated cognition in ADHD. This effect may be understood by positing a hyper-functioning of prefrontal dopaminergic systems. Against this background, the slower clearance of dopamine associated with the methionine variant of the COMT gene polymorphism may be disadvantageous to cognition in ADHD.

The catechol-O-methyltransferase polymorphism: relations to the tonic-phasic dopamine hypothesis and neuropsychiatric phenotypes

Diverse phenotypic associations with the catechol-O-methyltransferase (COMT) Val158Met polymorphism have been reported. We suggest that some of the complex effects of this polymorphism be understood from the perspective of the tonic-phasic dopamine (DA) hypothesis. We hypothesize that the COMT Met allele (associated with low enzyme activity) results in increased levels of tonic DA and reciprocal reductions in phasic DA in subcortical regions and increased D1 transmission cortically. This pattern of effects is hypothesized to yield increased stability but decreased flexibility of neural network activation states that underlie important aspects of working memory and executive functions; these effects may be beneficial or detrimental depending on the phenotype, a range of endogenous factors, and environmental exigencies. The literature on phenotypic associations of the COMT Val158Met polymorphism is reviewed, highlighting areas where this hypothesis may have explanatory value, and pointing to possible directions for refinement of relevant phenotypes and experimental evaluation of this hypothesis.

Attention-deficit hyperactivity disorder (ADHD)

Approaches to the diagnosis and treatment of attention-deficit hyperactivity disorder (ADHD) are undergoing a major change as a result of information from studies on the genetics of ADHD and the use of new neuroimaging technologies. Moreover, pharmacogenomics, although still in its infancy, will provide a basis for much more sophisticated treatment strategies for ADHD, particularly once more information is available about the genetics of ADHD. Even at this point in time, there is some pertinent information available that, although not ready for application in clinical settings, nonetheless provides a broader perspective for the clinician. In terms of etiology, ADHD is a neuropsychiatric disorder. There is a genetic basis in about 80% of the cases, involving a number of different genes, and in about 20% of the cases, ADHD is the result of an acquired insult to the brain. Some individuals likely have both genetic and acquired forms. Although medication works well in many cases of ADHD, optimal treatment of ADHD requires integrated medical and behavioral treatment. The family plays a crucial role in the management of children with ADHD. Because there is often a very high degree of comorbidity between ADHD and learning disabilities, teachers also have a great deal to contribute in the day-to-day management of these children. Early recognition and treatment prevent the development of more serious psychopathology in adolescence and adulthood.

Molecular genetics of attention-deficit hyperactivity disorder (ADHD): an update

Attention-deficit hyperactivity disorder (ADHD) is a heritable and behavioral condition of childhood, affecting 5-10% of school-age children worldwide. Affected patients exhibit various behavioral problems such as carelessness, restlessness, disobedience and failure to stay quiet in class. The etiology of ADHD is not known. However, family, twin and adoption studies have provided strong evidence for a genetic etiology of the disorder. A genome-wide scan has identified six chromosomal loci with LOD scores suggestive of linkage. Animal studies suggest the involvement of the brain dopamine pathway and its alteration in ADHD but there is no direct evidence to support this hypothesis. In addition, there are at least 20 candidate genes of small effect that have been studied but none of them appear to be the major gene causing ADHD. Medical intervention along with psychosocial therapy proved to be beneficial for controlling ADHD, although some undesirable side effects have been encountered during medical treatment. In the future, identification of environmental factors, study of additive gene effects and the interaction of genes and environmental factors may provide better insight into the pathophysiology of ADHD. This may lead to an effective new treatment strategy.

Attention deficit/hyperactivity disorder (ADHD): complex phenotype, simple genotype?

Complex genetic traits refer to those phenotypes not fitting patterns of Mendelian segregation and/or assortment but exhibiting a preferential familial clustering that cannot be explained by cultural or environmental causes. Attention-deficit/hyperactivity disorder (ADHD) is the most common neurodevelopmental disorder of childhood and probably the most controversial. ADHD has been considered a complex genetic trait based upon the absence of a clear-cut boundary between affected and unaffected status. Furthermore, its high comorbidity with other disorders strongly suggests complex epistatic or pleiotropic effects acting in common with the environmental influences. This implies that the same gene or genes is or are associated with different and concurrently occurring phenotypes. In this study, we will review clinical and epidemiological aspects related to the ADHD phenotype, which are considered either as categorical or continuous traits. We also will discuss genetic models underlying the complexity of this behavioral phenotype and the probable role of epistatic interactions between major genes contributing to the ADHD phenotype.

No evidence of association between Catechol-O-Methyltransferase (COMT) Val158Met genotype and performance on neuropsychological tasks in children with ADHD: a case-control study

BACKGROUND

Several studies have suggested an association between the functional Val158Met polymorphism in the Catechol-O-Methyltransferase (COMT) gene and neurocognitive performance. Two studies showed that subjects with the low activity Met allele performed better on the Wisconsin Card Sorting Test (WCST) and another study found an effect on processing speed and attention.

METHODS

We set out to examine the association between the Val158Met polymorphism and performance on neurocognitive tasks including those tapping working memory, attention and speed, impulsiveness and response inhibition in a sample of 124 children with ADHD. Task performance for each genotypic group was compared using analysis of variance.

RESULTS

There was no evidence of association with performance on any of the neurocognitive tasks.

CONCLUSIONS

We conclude that Val158Met COMT genotype is not associated with neurocognitive performance in our sample.

Genetics of adult attention-deficit/hyperactivity disorder

Results of behavioral genetic investigations using family twin and adoption studies converge with those of molecular genetic studies in showing that genes influence susceptibility to'attention-deficit/hyperactivity disorder (ADHD). These finding suggest that genetic mechanisms that predispose individuals to ADHD are complex. It seems likely that the disorder is caused by the combined actions of several genes. It is equally clear that aberrant genes create a vulnerability to the disorder that is not expressed in all environments. The literature about the genetics of adult ADHD is relatively small, but it suggests not only that the persistent form of ADHD is familial, but that it is more familial than the nonpersistent form. More work from twin and molecular genetic studies is needed to determine if the increased familiality of persistent ADHD reflects the actions of genes or of familial environmental causes.

Family-based and case-control association studies of catechol-O-methyltransferase in attention deficit hyperactivity disorder suggest genetic sexual dimorphism

Attention deficit hyperactivity disorder (ADHD) is the most common childhood-onset behavioral disorder. Boys are more often affected than girls. Family, twin, and adoption studies have supported a strong genetic basis. Some studies show that a catechol-O-methyltransferase (COMT) polymorphism affecting enzyme activity was associated with personality characteristics and diseases, such as novelty-seeking personality, substance abuse, and heroin addiction, whose features are similar to ADHD or are associated with ADHD. These findings suggest that the COMT gene may be a candidate gene for ADHD. TDT, HHRR, and case-control association studies were conducted within a sample of 202 nuclear ADHD families, 340 ADHD cases, and 226 controls in the Han Chinese population. Diagnoses and ADHD subtypes were ascertained according to DSM-IV criteria using American Clinical Diagnostic Interviewing Scales. The HHRR analysis suggested that the low enzyme-activity COMT Met allele was preferentially transmitted to ADHD boys (160 trios, chi(2) = 3.858, P = 0.05, df = 1) but not girls. This association is particularly pronounced among male ADHD probands without any comorbidity (50 trios, HHRR: chi(2) = 5.128, P = 0.024, df = 1; TDT: chi(2) = 4.558, P = 0.033, df = 1), especially the ADHD-I subtype (32 trios, HHRR: chi(2) = 5.792, P = 0.016, df = 1; TDT: chi(2) = 5.333, P = 0.021, df = 1). The case-control study revealed that the Val allele was more frequent in females meeting ICD-10 or DSM-IV criteria for ADHD than in female controls (86 and 79.5%, respectively, chi(2) = 4.059, P = 0.044, df = 1). Although these results suggest the COMT gene exerts some influence on the risk for ADHD in the Han Chinese population, given the potential for Type I error, these findings require replication before drawing definitive conclusions.

Genes de suscetibilidade no transtorno de déficit de atenção e hiperatividade

O transtorno de déficit de atenção e hiperatividade (TDAH) é um dos transtornos mais comuns da infância e adolescência, afetando entre 3% a 6% das crianças em idade escolar. Essa patologia caracteriza-se por sintomas de desatenção, hiperatividade e impulsividade, apresentando ainda uma alta heterogeneidade clínica. Embora as causas precisas do TDAH não estejam esclarecidas, a influência de fatores genéticos é fortemente sugerida pelos estudos epidemiológicos, cujas evidências impulsionaram um grande número de investigações com genes candidatos. Atualmente, apesar da ênfase dada a este tópico, nenhum gene pode ser considerado necessário ou suficiente ao desenvolvimento do TDAH, e a busca de genes que influenciam este processo ainda é o foco de muitas pesquisas. O objetivo desse artigo é, portanto, sumarizar e discutir os principais resultados das pesquisas com genes candidatos no TDAH.

Significance of serotonin transporter gene 5-HTTLPR and variable number of tandem repeat polymorphism in attention deficit hyperactivity disorder

The purpose of this study was to evaluate the relationship between attention deficit hyperactivity disorder (ADHD) and polymorphism of the two regions of the 5-HTT gene [variable number of tandem repeats (VNTR) and 5-HTTLRR] in a sample of Turkish children. Using the PCR technique, these polymorphisms were assessed in 71 patients with ADHD and 128 healthy controls. The 5-HTTLPR S/S genotype was significantly lower in the patients than in the controls (p = 0.018). Homozygous and heterozygous L variant predominated in the ADHD group. But the VNTR STin2.12/12 genotype was significantly less found in the patients than in the controls (p = 0.001). There was no significant difference between the frequency of the short (S), long, 10, and 12 alleles of both groups. The lack of an S/S variant of 5-HTTLPR polymorphism of the STin2.12/12 variant of VNTR polymorphism appears to be associated with an increased risk of ADHD.

Examining for association between candidate gene polymorphisms in the dopamine pathway and attention-deficit hyperactivity disorder: a family-based study

Attention-deficit hyperactivity disorder (ADHD) is a highly heritable childhood-onset psychiatric condition characterized by developmentally inappropriate inattention, hyperactivity, and impulsiveness. The pathophysiology of ADHD is currently unknown. However, the therapeutic effects of stimulant medication together with findings from animal and neuroimaging studies as well as from several molecular genetic studies of the dopamine receptor D4 gene and dopamine transporter gene have implicated involvement of the dopaminergic system. To test the dopaminergic hypothesis further, we have looked for association between ADHD and alleles of seven dopamine-related candidate genes using a family-based association approach in a sample of 150 children diagnosed with ADHD. We tested polymorphisms in genes encoding three dopamine receptors (DRD3, DRD4, and DRD5) and four dopamine-relevant enzymes: tyrosine hydroxylase [tyrosine hydroxylase (TH)], dopamine beta hydroxylase (DbetaH), catechol-O-methyltransferase (COMT), and monoamine oxidase A (MAOA). We were unable to detect a significant association with any of the polymorphisms genotyped, although there was a trend for preferential transmission of the DRD5 148 bp marker allele and the MAOA 122 bp marker allele. We conclude that none of the alleles we have tested makes a major contribution to ADHD, although much larger samples are required to exclude small effects.