The dopamine D4 receptor and the hyperactivity phenotype: a developmental-epidemiological study

Overweight in Boys With ADHD Is Related to Candidate Genes and Not to Deficits in Cognitive Functions

OBJECTIVE

The aim of the study was to assess the relationship of overweight, the polymorphisms of selected candidate genes, and deficits in the executive functions among children with ADHD.

METHOD

We examined 109 boys with ADHD aged between 7 and 17 years. The study indicated variants of 14 polymorphisms in eight candidate genes. We applied seven neuropsychological tests to evaluate the executive functions. Overweight was diagnosed on the basis of the guidelines of the International Obesity Task Force.

RESULTS

Analyses revealed significant association between DRD4 rs1800955, SNAP25 rs363039 and rs363043, 5HTR2A rs17288723, and overweight in boys with ADHD. There were no significant differences in the level of neuropsychological test results between patients with overweight and without overweight.

CONCLUSION

Overweight in boys with ADHD is associated with polymorphisms in three candidate genes: DRD4, SNAP25, and 5HTR2A, but not through conditioning deficits in cognitive functions.

Is Adult ADHD a Childhood-Onset Neurodevelopmental Disorder? Evidence From a Four-Decade Longitudinal Cohort Study

OBJECTIVE

Despite a prevailing assumption that adult ADHD is a childhood-onset neurodevelopmental disorder, no prospective longitudinal study has described the childhoods of the adult ADHD population. The authors report follow-back analyses of ADHD cases diagnosed in adulthood, alongside follow-forward analyses of ADHD cases diagnosed in childhood, in one cohort.

METHOD

Participants belonged to a representative birth cohort of 1,037 individuals born in Dunedin, New Zealand, in 1972 and 1973 and followed to age 38, with 95% retention. Symptoms of ADHD, associated clinical features, comorbid disorders, neuropsychological deficits, genome-wide association study-derived polygenic risk, and life impairment indicators were assessed. Data sources were participants, parents, teachers, informants, neuropsychological test results, and administrative records. Adult ADHD diagnoses used DSM-5 criteria, apart from onset age and cross-setting corroboration, which were study outcome measures.

RESULTS

As expected, childhood ADHD had a prevalence of 6% (predominantly male) and was associated with childhood comorbid disorders, neurocognitive deficits, polygenic risk, and residual adult life impairment. Also as expected, adult ADHD had a prevalence of 3% (gender balanced) and was associated with adult substance dependence, adult life impairment, and treatment contact. Unexpectedly, the childhood ADHD and adult ADHD groups comprised virtually nonoverlapping sets; 90% of adult ADHD cases lacked a history of childhood ADHD. Also unexpectedly, the adult ADHD group did not show tested neuropsychological deficits in childhood or adulthood, nor did they show polygenic risk for childhood ADHD.

CONCLUSIONS

The findings raise the possibility that adults presenting with the ADHD symptom picture may not have a childhood-onset neurodevelopmental disorder. If this finding is replicated, then the disorder's place in the classification system must be reconsidered, and research must investigate the etiology of adult ADHD.

A randomized, double-blind, crossover comparison of MK-0929 and placebo in the treatment of adults with ADHD

OBJECTIVE

Preclinical models, receptor localization, and genetic linkage data support the role of D4 receptors in the etiology of ADHD. This proof-of-concept study was designed to evaluate MK-0929, a selective D4 receptor antagonist as treatment for adult ADHD.

METHOD

A randomized, double-blind, placebo-controlled, crossover study was conducted in adults with primary ADHD. The primary end point was changed from baseline in total score on the Adult ADHD Investigator Symptom Rating Scale following a 4-week treatment regimen. Additional measures included Clinical Global Impression-Severity Scale, Hospital Anxiety and Depression Scale, and Brown Attention Deficit Disorder Scale and D4 genotype analysis.

RESULTS

No statistically significant treatment differences were found between MK-0929 and placebo in any of the primary or secondary assessments.

CONCLUSION

Results from this study suggest that blockade of the D4 receptor alone is not efficacious in the treatment of adult ADHD.

Psychopathological aspects of dopaminergic gene polymorphisms in adolescence and young adulthood

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

Family-based association analysis of a statistically derived quantitative traits for ADHD reveal an association in DRD4 with inattentive symptoms in ADHD individuals

The objective of this study was to determine whether single nucleotide polymorphisms (SNPs) within candidate genes for ADHD are associated with quantitative phenotypes generated from inattentive and hyperactive-impulsive symptoms. One hundred forty-three SNPs were genotyped in and around five ADHD candidate genes. A highly heritable quantitative phenotype was generated at each SNP by weighting inattentive and hyperactive-impulsive symptoms. Once these phenotypes were generated, a screening procedure was used to select and test the five SNP/phenotype combinations with the greatest power to detect an association for each candidate gene. Adjacent SNPs in the promoter region of DRD4, hCV26775267 and hCV26775266, were associated with the quantitative phenotypes generated from the ADHD symptoms (corrected P-values = 0.012 for both SNPs). The correlations between the ADHD symptoms and quantitative phenotype revealed that inattentive symptoms had a strong influence on the generated phenotype. Subsequent family-based association test-principal components (FBAT-PC) analyses using inattentive symptoms only also had significant associations. SNPs in the promoter region of DRD4 are associated with the phenotypes generated from ADHD symptoms. The strong correlation of the inattentive symptoms with these quantitative phenotypes and the subsequent FBAT-PC analyses suggest this region is primarily associated with inattentive symptoms. This analysis adds to previous findings by suggesting that variants at these loci may be specifically associated with inattentive symptoms.

Genetic variation in the dopamine D4 receptor (DRD4) gene and smoking cessation: follow-up of a randomised clinical trial of transdermal nicotine patch

Smokers of European ancestry (n=720) who participated in a double-blind, randomised, placebo-controlled trial of transdermal nicotine replacement therapy, were genotyped for two functional polymorphisms (variable number of tandem repeats (VNTR) and a C to T transition at position -521 (C-521T)) in the dopamine D4 receptor gene (DRD4) gene. Logistic regression models of abstinence at 12- and 26-week follow-ups were carried out separately for each polymorphism. For the DRD4 VNTR models, the main effect of treatment was significant at both 12-week (P=0.001) and 26-week (P=0.006) follow-ups, indicating an increased likelihood of successful cessation on active nicotine replacement therapy transdermal patch relative to placebo. The main effect of DRD4 VNTR genotype was associated with abstinence at 12-week follow-up (P=0.034), with possession of one or more copies of the long allele associated with reduced likelihood of cessation (17 vs 23%), but this effect was not observed at 26-week follow-up. For the DRD4 C-521T models, no main effect or interaction terms involving genotype were retained in the models at either 12- or 26-week follow-up. These data are consistent with observations from studies of the DRD2 gene that genetic variants related to relatively decreased dopaminergic tone in the mesocorticolimbic system are associated with increased risk for relapse to smoking following a cessation attempt.

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.

Association of dopamine D4 receptor (DRD4) polymorphisms with attention deficit hyperactivity disorder in Indian population

Attention deficit hyperactivity disorder (ADHD) is a childhood onset neurobehavioral disorder. Several studies worldwide have implicated a possible association between ADHD and transmission of different polymorphisms of the dopamine D4 receptor gene (DRD4) in different ethnic groups. However, this is the first report on the transmission of different polymorphisms of DRD4 in Indian subjects. Association of 5' flanking 120-bp duplication, exon 1 12-bp duplication, and exon 3 48-bp variable numbers of tandem repeats (VNTR) were analyzed in 50 ADHD cases. Haplotype-based haplotype relative risk (HHRR) analysis and transmission disequilibrium test (TDT) were carried out to ascertain the association of these polymorphisms with the disorder. Linkage disequilibria (LD) between the polymorphisms were calculated using EH+ and 2LD programs. Our preliminary data showed lack of association between ADHD and transmission of the 5' flanking 120-bp duplication and exon 1 12-bp duplication. But, the transmissions of 6 and 7 repeat alleles of exon 3 48-bp VNTR showed significant association with ADHD. We have also examined the haplotype frequencies and biased transmission of one haplotype was observed in ADHD probands. LD analysis showed very strong disequilibrium between exon 1 12-bp duplication and exon 3 48-bp VNTR. Strong LD was also observed between the 5' flanking 120-bp duplication and exon 1 12-bp duplication. The observed association between higher repeat alleles of exon 3 48-bp VNTR and Indian ADHD children is consistent with some of the earlier reports.

The Italian preadolescent mental health project (PrISMA): rationale and methods

The Italian preadolescent mental health project (PrISMA--Progetto Italiano Salute Mentale Adolescenti) is the first Italian study designed to estimate the prevalence of mental disorders in preadolescents (10-14 years old) living in urban areas, and to analyse the demographic and biological correlates of emotional and behavioural problems. This paper describes the rationale, methods and the analysis plan of the project. The design of the study used a two-stage sampling procedure, one screening stage of emotional and behavioural problems in a large sample of subjects attending public and private schools and a second stage of diagnostic assessment in a sample including all high scorers and a proportion of low scorers. In the screening stage, parents of preadolescents were asked to fill in the Child Behavior Checklist (CBCL), whereas in the second stage preadolescents and their parents were administered the Development and Well Being Assessment for the assessment of mental disorders together with the Strengths and Difficulties Questionnaire and two scales (C-GAS and HoNOSCA) designed to evaluate the functioning of the preadolescent in different areas. Genetic samples were collected during the screening stage, after parents gave their informed written consent. The findings of this study are expected to allow an adequate planning of interventions for the prevention and the treatment of mental disorders in preadolescence as well as efficient health services.

No evidence for the association of DRD4 with ADHD in a Taiwanese population within-family study

Background

Attention Deficit Hyperactivity Disorder (ADHD) is a prevalent and highly heritable childhood disorder. The dopamine D4 receptor (DRD4) gene has shown a genetic association with ADHD in Caucasian populations with meta-analysis indicating a small but significant effect across datasets. It remains uncertain whether this association can be generalised to non-Caucasian ethnic groups. Here we investigate two markers within the DRD4 gene in a Taiwanese population, the exon 3 variable number tandem repeat (VNTR) and a 5' 120 base-pair duplication.

Methods

Within-family transmission disequilibrium tests of association of the 5' 120 base-pair duplication, and exon 3 VNTR in a Taiwanese population.

Results

No evidence of association of ADHD with either polymorphism in this population was observed.

Conclusion

The DRD4 gene markers investigated were not found to be associated with ADHD in this Taiwanese sample. Further work in Taiwanese and other Asian populations will therefore be required to establish whether the reports of association of DRD4 genetic variants in Caucasian samples can be generalised to Asian populations.

The polymorphic nature of the human dopamine D4 receptor gene: a comparative analysis of known variants and a novel 27 bp deletion in the promoter region

BACKGROUND

The human dopamine D4 receptor (DRD4) is a candidate gene of great interest in molecular studies of human personality and psychiatric disorders. This gene is unique in having an exceptionally high amount of polymorphic sites both in the coding and in the promoter region.

RESULTS

We report the identification of a new 27 bp deletion starting 524 bp upstream of the initiation codon (27 bp del) of the dopamine D4 receptor (DRD4) gene, in the close vicinity of the -521C>T SNP. The presence of the 27 bp deletion leads to the misgenotyping of the -616C>G SNP by the Sau96 I RFLP method, thus the genotype determination of the mutation is of additional importance. The frequency of this novel sequence variation is considerably low (allele frequency is = 0.16%), as no homozygotes, and only 3 heterozygote carriers were found in a healthy, unrelated Caucasian sample (N = 955).

CONCLUSION

Remarkably, the deleted region contains consensus sequences of binding sites for several known transcription factors, suggesting that the different alleles may affect the transcriptional regulation of the gene. A comparison of methods and results for the allelic variations of the DRD4 gene in various ethnic groups is also discussed, which has a high impact in psychiatric genetic studies.

Interaction between the tryptophan hydroxylase gene and the serotonin transporter gene in schizophrenia but not in bipolar or unipolar affective disorders

Increasing focus is being given to identify possible combinations of genes related to specific clinical phenotypes. In our sample of 814 patients comprising 114 with schizophrenia, 416 with bipolar affective disorder and 284 with unipolar affective disorder, we studied interactions between the tryptophan hydroxylase (TPH), the serotonin transporter (5-HTTLPR), and the dopamine receptor (DRD4) genes in relation to five major psychiatric symptomatology scores. There was significant interaction between the TPH and the 5-HTTLPR genes. With an increasing number of short (s) alleles of 5-HTTLPR, the scores for delusions, disorganization and negative symptoms were significantly decreasing among subjects having the TPH genotype AA but increasing among subjects having the TPH genotype AC, yielding the highest scores for the combinations AA x ll and AC x ss. Since high scores on just delusions, disorganization and negative symptoms but low scores on excitement and depression were found among subjects with schizophrenia, we conducted comparisons among the three diagnostic categories and controls as regards the combined TPH x 5-HTTLPR genotype distribution. Schizophrenia subjects had a significantly different distribution of the genotype combination for TPH x 5-HTTLPR as compared to 241 controls or to unipolar or bipolar subjects, and had significantly higher frequencies of AA x ll and of AC x ss. Thus, an interaction between TPH and 5-HTTLPR genes constitutes susceptibility to schizophrenia, thereby yielding apparent relationships between the major psychiatric symptomatology scores and genotype combinations in samples that are obtained by pooling schizophrenia with other diagnostic categories.

Quantitative trait locus analysis of candidate gene alleles associated with attention deficit hyperactivity disorder (ADHD) in five genes: DRD4, DAT1, DRD5, SNAP-25, and 5HT1B

It has been widely postulated that the categorical diagnosis of attention deficit hyperactivity disorder (ADHD) should be seen as the extreme end of a set of traits quantitatively distributed in the general population. A consequence of this is that the genes associated with DSM-IV ADHD should also influence these underlying traits in non-affected individuals. The aim of this study was to examine if specific candidate loci previously shown to be associated with DSM-IV ADHD, also act as quantitative trait loci (QTLs) for ADHD-symptoms in the general population. We have genotyped five candidate markers in a population-based sample of male dizygous twin-pairs (n = 329 pairs). We found little evidence to support a role for the previously-nominated alleles of a DRD4 VNTR, a 5HT1B SNP, or a microsatellite marker near to DRD5, in the distribution of ADHD-symptoms scores; however, we found some evidence to suggest that the DAT1 3'UTR VNTR and weak evidence that a microsatellite in SNAP-25 may have a role in continuous measures of ADHD-symptoms hyperactivity above and beyond their role in clinical ADHD.

Attention-Deficit Hyperactivity Disorder in the post-genomic era

BACKGROUND

ADHD is a common and complex genetic disorder. Genetic risk factors are expected to be multiple, have small effect sizes when considered individually and to interact with each other and with environmental factors.

OBJECTIVE

To describe the difficulties involved in the genetic investigation of such a complex disorder and give a prospective for the future.

METHODS

Review based on empirical literature and project description.

RESULTS

Considerable progress has been achieved through the association analysis of candidate gene loci. Linkage scans using affected sibling pairs have identified a number of potential loci that may lead to the identification of novel genes of moderate effect size. Quantitative trait locus (QTL) approaches provide powerful complementary strategies that have the potential to link the categorical disorder to continuously distributed traits associated more closely with underlying genetic liability in the general population. Success in identifying some associated genes has been complemented by functional studies that seek to understand the mode of action of such genes.

CONCLUSION

Progress in understanding the mechanisms involved has not been straightforward and many inconsistencies have arisen. In order to take advantage of the potential for progress that stems from the genetic findings it will be important to draw upon a variety of approaches and experimental paradigms. A functional genomic approach to ADHD means that investigation of gene function is carried out at various levels of analysis, not only at the level of molecular and cellular function but also at the level of psychological processes, neuronal networks, environmental interactions and behavioural outcomes.

Phenotype studies of the DRD4 gene polymorphisms in ADHD: association with oppositional defiant disorder and positive family history

The objective of this study was to investigate association of DRD4 polymorphisms with ADHD subtypes for which there is evidence for increased heritability. The genetic variants tested were the 120 bp insertion/deletion, the -616, -521, -376, and the 48 bp DRD4 VNTR. The primary analyses were of association with oppositional defiant disorder (ODD), conduct disorder (CD), and diagnostic subtypes. Secondary analyses of clinical subtype were exploratory in nature and included analysis of association of DRD4 polymorphisms with family history of ADHD. We observed significant association between DRD4 7-repeat allele transmission and ADHD children with comorbid ODD (chi(2) = 6.74, df = 1, P = 0.01, OR = 2.45) The DRD4 7-repeat allele was also significantly associated with family history positive ADHD (chi(2) = 10.12, df = 1, P = 0.0021, OR = 3.57). We observed no significant distortion in the transmission of any of the tested DRD4 variants with inattentive or hyperactive-impulsive subtypes or symptom dimensions. In conclusion, our findings of increased DRD4 7-repeat allele transmission in ODD extend those reported by Holmes et al. To our knowledge, this is the first study to report association of the DRD4 7-repeat allele with ADHD children who have a positive family history of ADHD. Overall, the results from this study support the investigation of clinical subtypes in molecular genetic studies of ADHD.

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.

Recent advances in the genetics of attention deficit hyperactivity disorder

In the past few years, interest in the molecular genetics of attention deficit hyperactivity disorder (ADHD) has grown enormously, with many groups searching for susceptibility genes, often through large collaborative efforts facilitated by the International ADHD Genetics Consortium. Association findings for several candidate genes within the dopaminergic system, the DRD4 and DRD5 receptor genes and the dopamine transporter gene, DAT1, have been well replicated, and the first of several ongoing genome linkage scan study results have been published. Current challenges in this field are to identify the actual functional variant(s) in these genes conferring susceptibility and other genetic and environmental risk factors for ADHD.

Polymorphisms in the dopamine D4 receptor gene and attention-deficit hyperactivity disorder

There is considerable evidence to support a role of dopamine-related genes in the molecular aetiology of attention-deficit hyperactivity disorder (ADHD). A 48 bp repeat in exon three of the dopamine D4 receptor gene has been widely studied in clinical ADHD samples, and a meta-analysis of published studies suggests it is associated with ADHD. A number of other polymorphisms across this gene have been characterised but not so thoroughly investigated in relation to ADHD. In this study we have genotyped five polymorphisms (a 120 bp promoter-region duplication, the -616 C/G substitution, the -521 C/T substitution, a poly-G repeat in intron 1, and the 48 bp exon 3 repeat) across the gene in a large clinical sample (n = 188) and their families. We found that none of the markers is individually associated with ADHD, although there is evidence to suggest that a haplotype of markers in the 5' promoter region of the gene (allele 2 of the 120 bp duplication, the C allele of the -616 substitution, and the C allele of the -521 substitution) may confer susceptibility.

Gene-environment interaction in psychiatric disorders as indicated by season of birth variations in tryptophan hydroxylase (TPH), serotonin transporter (5-HTTLPR) and dopamine receptor (DRD4) gene polymorphisms

Genetic and environmental factors, as well as their interactions, are likely to be involved in psychiatric disorders. Considerable progress has been made in association and linkage studies with various candidate genes, at times with conflicting or ambiguous results. An environmental factor that has persistently shown associations with several psychiatric and neurological disorders is the season of birth. If it is the interaction of a specific gene allele with a specific season of birth that constitutes an increased (or decreased) risk for a disorder, then the individuals with this disorder are likely to have a season of birth variation in this gene allele. We investigated the variations in TPH, 5-HTTLPR and DRD4 gene polymorphisms according to seasonality of birth in 954 patients with unipolar affective disorder, bipolar affective disorder, and schizophrenia, respectively, and in 395 controls. We first analyzed season of birth variations in the gene alleles with one cycle or two cycles per year, and then compared specified birth seasons with each other. We found season of birth variations in these gene alleles that were different for different psychiatric disorders. Significant differences between cases and controls could be obtained when restricting the analysis within certain birth seasons but not within others. Our results thus suggest an interaction between the seasons of birth and the expression of the candidate genes, and that season of birth is a confounding variable when investigating the role of the candidate genes in susceptibility to psychiatric disorders.