Transmission disequilibrium testing of dopamine-related candidate gene polymorphisms in ADHD: confirmation of association of ADHD with DRD4 and DRD5

Attention-deficit hyperactivity disorder (ADHD) is one of the most common childhood behavioral disorders. Genetic factors contribute to the underlying liability to develop ADHD. Reports implicate variants of genes important for the synthesis, uptake, transport and receptor binding of dopamine in the etiology of ADHD, including DRD4, DAT1, DRD2, and DRD5. In the present study, we genotyped a large multiplex sample of ADHD affected children and their parents for polymorphisms in genes previously reported to be associated with ADHD. Associations were tested by the transmission disequilibrium test (TDT). The sample is sufficient to detect genotype relative risks (GRRs) for putative risk alleles. The DRD4 gene 120-bp insertion/deletion promoter polymorphism displayed a significant bias in transmission of the insertion (chi(2)=7.58, P=0.006) as suggested by an analysis of a subset of these families. The seven repeat allele of the DRD4 48-bp repeat polymorphism (DRD4.7) was not significantly associated with ADHD in the large sample in contrast to our earlier findings in a smaller subset. We replicate an association of a dinucleotide repeat polymorphism near the DRD5 gene with ADHD by showing biased nontransmission of the 146-bp allele (P=0.02) and a trend toward excess transmission of the 148-bp allele (P=0.053). No evidence for an association was found for polymorphisms in DRD2 or DAT1 in this sample. The DRD5 146-bp (DRD5.146) allele and the DRD4 240-bp (DRD4.240) allele of the promoter polymorphism emerge as the two DNA variants showing a significant association in this large sample of predominantly multiplex families with ADHD, with estimated GRRs of 1.7 and 1.37, respectively.     

Haplotype study of three polymorphisms at the dopamine transporter locus confirm linkage to attention-deficit/hyperactivity disorder.

BACKGROUND: Attention-deficit/hyperactivity disorder (ADHD) is often treated using methylphenidate, a psychostimulant that inhibits the dopamine transporter. This led E.H. Cook and colleagues to consider the dopamine transporter locus (DAT1) as a primary candidate gene for ADHD. That group reported a significant association between ADHD and the 480-base pair (bp) allele of the variable number of tandem repeats (VNTR) polymorphism located in the 3' untranslated region of the DAT1 gene. This association was later replicated in additional studies. METHODS: The DAT1 gene has additional common polymorphisms in intron 9 and exon 9. We investigated the possibility of linkage of DAT1 and ADHD using the VNTR polymorphism and two additional common polymorphisms in 102 nuclear families with an ADHD proband. Using the transmission disequilibrium test, we examined the transmission of the alleles of each of these polymorphisms, as well as the haplotypes of the polymorphisms. RESULTS: We did not observe significant evidence for the biased transmission of the alleles of either the VNTR or the additional two polymorphisms when examined individually, although there was a trend for the biased transmission of the 480-bp allele of the VNTR. When we examined the haplotypes of the three polymorphisms we found significant evidence for biased transmission of one of the haplotypes containing the 480-bp VNTR allele. We also genotyped six additional DNA sequence variants of the DAT1 gene. However, these variants were not sufficiently polymorphic in our sample to be informative. Two of the DNA variants that result in an amino acid change, Ala559Val and Glu602Gly, were not observed in our sample. CONCLUSIONS: Our results support previous findings of an association between the DAT1 gene and ADHD.     

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

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

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

Attention-deficit hyperactivity disorder (ADHD) affects 2-6% of school-age children and is a precursor of behavioural problems in adolescence and adulthood. Underlying the categorical definition of ADHD are the quantitative traits of activity, impulsivity, and inattention which vary continuously in the population. Both ADHD and quantitative measures of hyperactivity are heritable, and influenced by multiple genes of small effect. Several studies have reported an association between clinically defined ADHD and the seven-repeat allele of a 48-bp tandem repeat polymorphism in the third exon of the dopamine D4 receptor gene (DRD4). We tested this association in a large, unselected birth cohort (n = 1037) using multiple measures of the hyperactivity phenotype taken at multiple assessment ages across 20 years. This longitudinal approach allowed us to ascertain whether or not DRD4 has a general effect on the diagnosed (n = 49) or continuously distributed hyperactivity phenotype, and related personality traits. We found no evidence to support this association.     

Attention-deficit hyperactivity disorder and the adrenergic receptors alpha 1C and alpha 2C

The adrenergic system has been hypothesized to be involved in the etiology of attention-deficit hyperactivity disorder (ADHD) based on pharmacological interventions and animal models. Noradrenergic neurons are implicated in the modulation of vigilance, improvement of visual attention, initiation of adaptive response, learning and memory. In this study we tested the genes for two adrenergic receptors, alpha 1C (ADRA1C) located on chromosome 8p11.2, and alpha 2C (ADRA2C) located on chromosome 4p16, as genetic susceptibility factors in ADHD. For the adrenergic receptor alpha 1C we used a C to T polymorphism that results in a change of Cys to Arg at codon 492 for the linkage study. For the adrenergic receptor alpha 2C gene we examined a dinucleotide repeat polymorphism located approximately 6 kb from the gene. We examined these polymorphisms in a sample of 103 families ascertained through an ADHD proband. Using the transmission disequilibrium test, we did not observe biased transmission of any of the alleles of these polymorphisms. We conclude that the alleles at the polymorphisms tested in these two genes are not linked to the ADHD phenotype in this sample of families.     

High prevalence of rare dopamine receptor D4 alleles in children diagnosed with attention-deficit hyperactivity disorder

Associations have been reported of the 7-repeat (7R) allele of the human dopamine receptor D4 (DRD4) gene with both the personality trait of novelty seeking and attention-deficit/hyperactivity disorder (ADHD). The increased prevalence of the 7R allele in ADHD probands is consistent with the common variant-common disorder hypothesis, which proposes that the high frequency of many complex genetic disorders is related to common DNA variants. Recently, based on the unusual DNA sequence organization and strong linkage disequilibrium surrounding the DRD4 7R allele, we proposed that this allele originated as a rare mutational event, which nevertheless increased to high prevalence in human populations by positive selection. We have now determined, by DNA resequencing of 250 DRD4 alleles obtained from 132 ADHD probands, that most ADHD 7R alleles are of the conserved haplotype found in our previous 600 allele worldwide DNA sample. Interestingly, however, half of the 24 haplotypes uncovered in ADHD probands were novel (not one of the 56 haplotypes found in our prior population studies). Over 10 percent of the ADHD probands had these novel haplotypes, most of which were 7R allele derived. The probability that this high incidence of novel alleles occurred by chance in our ADHD sample is much less than 0.0001. These results suggest that allelic heterogeneity at the DRD4 locus may also contribute to the observed association with ADHD.     

Linkage of the dopamine D4 receptor gene and attention-deficit/hyperactivity disorder

OBJECTIVE: There is considerable evidence supporting a genetic component in the etiology of attention-deficit/hyperactivity disorder (ADHD). Because stimulant medications act primarily on the dopaminergic system, dopamine system genes are prime candidates for genetic susceptibility factors for ADHD. Previous studies by several groups have observed a significant association of ADHD and an allele with 7 copies of the 48 base pair repeat in the third exon of the dopamine D4 receptor. METHOD: The authors sought to replicate these previous findings by collecting an independent sample of families from Toronto, Ontario, Canada, and confirming this finding in an expanded sample of ADHD families collected from Irvine, California. Using the transmission disequilibrium test (TDT), the authors tested for biased transmission of the 7-repeat allele at the exon III polymorphism of the dopamine D4 receptor locus in these samples of ADHD subjects. RESULTS: Biased transmission of the 7-repeat allele from parents to ADHD probands and their affected siblings was observed in the 2 new samples of families collected in Toronto and Irvine (TDT chi2 = 2.711, 1 df, one-sided p value = .050) and for these samples combined with the 52 families previously reported from Irvine (TDT chi2 = 6.426, 1 df, one-sided p value = .006). CONCLUSIONS: The results of this study further support the possibility of a role of the dopamine D4 receptor locus in ADHD.     

Association of dopamine D4 receptor (DRD4) gene with attention-deficit/hyperactivity disorder (ADHD) in a high-risk community sample: a longitudinal study from birth to 11 years of age

Summary. Background: In recent years, a growing number of studies has focused on the dopamine D4 receptor gene (DRD4) as mediating the susceptibility to attention-deficit/hyperactivity disorder (ADHD). While their results are contradictory, the reason for this inconsistency remains as yet unclear. Method: The present study sought to examine the association between ADHD and the DRD4 exon III polymorphism during child development using longitudinal data from a high-risk community sample (n=265, 129 females, 126 males) who have been followed from birth to 11 years of age. Results: Higher rates of ADHD were observed in boys with the 7 repeat allele of exon III than in boys with other alleles at the ages of 4 1/2 (Fishers exact test, p=.061), 8 (p=.026), and 11 years (p=.005). Boys with this allele also exhibited higher rates of persistent disorder (p=.024). In girls, a trend towards an association (p=.055) with the 7 repeat allele emerged only at preschool age. Conclusions: These findings provide additional evidence for the role of the dopamine D4 receptor in ADHD during the course of child development.     

Association between DRD4 gene and performance of children with ADHD in a test of sustained attention.

BACKGROUND: The adoption of neuropsychological tests as endophenotypic measures can provide an increased sensitivity to specific dimensions of attention-deficit/hyperactivity disorder (ADHD). METHODS: The association between a variable number of tandem repeats polymorphism at the dopamine D4 receptor gene (DRD4) and the performance of children and adolescents with ADHD in a continuous performance test (CPT) was evaluated. The sample comprised 90 clinically referred children and adolescents with ADHD. Errors of omission and commission in the CPT were computed and the number of 48-base pairs tandem repeats in the exon III of DRD4 was assessed. RESULTS: The presence of a 7-repeat allele was associated with more errors of commission and the homozygosity of the 4-repeat allele was related to fewer errors of commission and omission even after adjusting for age. CONCLUSIONS: These findings bring further evidence on the role of DRD4 polymorphisms on the performance in sustained attention tasks among children and adolescents with ADHD diagnosis.     

Molecular genetic contribution to the developmental course of attention-deficit hyperactivity disorder

Objective  The developmental trajectory of attention-deficit hyperactivity disorder (ADHD) is variable. Utilizing a longitudinally assessed sample, we investigated the contribution of susceptibility gene variants, previously implicated through pooled or meta-analyses, to the developmental course of Attention-Deficit Hyperactivity Disorder over time. Methods  151 children (aged 6–12) who met diagnostic criteria for ADHD were assessed using research diagnostic interviews during childhood and 5 years later in adolescence. Severity was defined as total number of ADHD symptoms at baseline and reassessment. Association with variants at DRD4, DRD5, and the dopamine transporter gene, DAT was analyzed using linear regression. Results  As expected, affected individuals showed a decline in ADHD severity over time. The DRD4 48 bp VNTR 7-repeat and DRD5 CA(n) microsatellite marker 148 bp risk alleles were associated with persistent ADHD. Those possessing the DRD4 7 repeat risk allele showed less of a decline in severity at reassessment than those without the risk allele. Conclusions  Those carrying the DRD4 7 risk allele showed greater symptom severity at follow-up and less ADHD reduction over time. These findings support the hypothesis that some susceptibility genes for ADHD also influence its developmental course.     

Effects of source of DNA on genotyping success rates and allele percentages in the Preschoolers with Attention-Deficit/Hyperactivity Disorder Treatment Study (PATS)

OBJECTIVE: In children diagnosed with attention-deficit/hyperactivity disorder (ADHD) and their parents, who were participants of the Preschool ADHD Treatment Study (PATS), we assessed the effect of source of DNA (from buccal or blood cells) on the genotyping success rate and allele percentages for the five polymorphisms in three candidate genes (DAT1, DRD4, and SNAP 25) investigated in the PATS pharmacogenetic study of response to stimulant medication. METHOD: At baseline assessment, 241 individuals (113 probands and 128 parents) consented to participate; 144 individuals (52 probands and 92 parents) provided blood samples from venipuncture, and 97 individuals (61 probands and 36 parents) provided buccal samples from cheek swab as specimens for isolation of DNA. Three types of polymorphisms-variable number of tandem repeat (VNTR) polymorphism, tandem duplication polymorphism (TDP), and single nucleotide polymorphism (SNP)-were evaluated, including the DRD4 gene 48-bp VNTR in exon III, the DAT1 gene 40-bp VNTR in 3'-untranslated region, the DRD4 gene TDP 120-bp duplication in the promoter region, the SNAP-25 gene TC-1069 SNP, and the SNAP-25 gene TG-1065 SNP. Standard procedures were used to genotype individuals for each of these five polymorphisms. RESULTS: Using the methods available in 2004, the genotyping success rate was on the average much greater for DNA from blood cells than buccal cells (e.g., 91% vs. 54% in probands). For some polymorphisms (DRD4-VNTR, DRD4-TDP, and SNAP25-TC SNP), allele proportion also varied by blood versus buccal source of DNA (e.g., 26.5% vs. 18.6% for the 7-repeat allele of the DRD4 gene). CONCLUSIONS: The much lower success rate for genotyping based on DNA from buccal than blood cells is likely due to the quality of DNA derived from these two sources. The observed source differences in allele proportion may be due to self-selection related to choice of how specimens were collected (from cheek swab or venipuncture), or to a selective detection of some alleles based on differences in DNA quality.     

The dopamine transporter gene is associated with attention deficit hyperactivity disorder in a Taiwanese sample

Genetic variation of the dopamine transporter gene (DAT1) is of particular interest in the study of attention-deficit hyperactivity disorder (ADHD), since stimulant drugs interact directly with the transporter protein. Association between ADHD and the 10-repeat allele of a 40-bp VNTR polymorphism that lies within the 3'-UTR of DAT1 was first reported in 1995, a finding that has been replicated in at least six independent samples from Caucasian populations. We analysed the DAT1 polymorphism in a sample of 110 Taiwanese probands with a DSM-IV diagnosis of ADHD and found evidence of increased transmission of the 10-repeat allele using TRANSMIT (chi(2)=10.8, 1 d.f., p=0.001, OR=2.9, 95% CI 1.4-6.3). These data give rise to a similar odds ratio to that observed in Caucasian poplulations despite a far higher frequency of the risk allele in this Taiwanese population; 82.3% in the un-transmitted parental alleles and 94.5% in the ADHD probands. These data support the role of DAT1 in ADHD susceptibility among Asian populations.     

A family-based and case-control association study of the dopamine D4 receptor gene and dopamine transporter gene in attention deficit hyperactivity disorder

Attention deficit hyperactivity disorder (ADHD) is a highly heritable psychiatric condition of early childhood onset characterised by marked inattention, hyperactivity and impulsiveness. Molecular genetic investigations of ADHD have found positive associations with the 480-bp allele of a VNTR situated in the 3' untranslated region of DAT1 and allele 7 of a VNTR in exon 3 of DRD4. A number of independent studies have attempted to replicate these findings but the results have been inconsistent. We used both family-based and case control approaches to examine these polymorphisms in a sample of 137 children diagnosed with ICD-10, DSM-IV or DSM-III-R ADHD. We found no evidence of association with the DAT1 polymorphism, despite a sample size that has up to 80% power to detect a previously reported effect size. We observed a significant increase in the DRD4 7 repeat allele amongst ADHD probands (21.7%) and their parents (18.9% in mothers, 22.3% in fathers), compared to ethnically matched controls (12.8%). However TDT analysis showed no preferential transmission of allele 7 to ADHD probands.     

Do SNPs of DRD4 gene predict adult persistence of ADHD in a Chinese sample?

The dopamine D4 receptor (DRD4) gene has been frequently studied in relation to attention deficit hyperactivity disorder (ADHD) but little is known about the contribution of single nucleotide polymorphisms (SNPs) of the DRD4 gene to the development and persistence of ADHD. In the present study, we examined the association between two SNPs in DRD4 (rs1800955, rs916455) and adult ADHD persistence in a Chinese sample. Subjects (n=193) were diagnosed with ADHD in childhood and reassessed in young adulthood at an affiliated clinic of Peking University Sixth Hospital. Kaplan-Meier survival analyses and Cox proportional hazard models were used to test the association between ADHD remission and alleles of the two SNPs. DRD4 rs916455 C allele carriers were more likely to have persistent ADHD symptoms in adulthood. No significant association was found between rs1800955 allele and the course of ADHD. These newly detected associations between DRD4 polymorphisms and ADHD prognosis in adulthood may help to predict the persistence of childhood ADHD into adulthood.     

Replication test for association of the IL-1 receptor antagonist gene, IL1RN, with attention-deficit/hyperactivity disorder

Attention-deficit/hyperactivity disorder (ADHD) has a strong genetic basis, and aberrant brain dopaminergic and noradrenergic activity is implicated in its etiology. Interleukin-1 (IL-1), its antagonist, IL-1Ra, and IL-1 receptors are all present in the brain, and IL-1 has been shown to influence both dopaminergic and noradrenergic function. Recently, Segman et al. [1] tested the IL-1Ra gene, IL1RN, as a candidate for involvement in ADHD.Using the transmission/disequilibrium test (TDT) to examine 77 nuclear ADHD families for the inheritance of alleles of an intronic 86-bp VNTR polymorphism, they found significant evidence for biased transmission of the 4-repeat allele (p=0.04) and non-transmission of the 2-repeat allele (p=0.03). Here, we sought to replicate this in an independent sample of families. In contrast to the previous findings, our analysis of 178 ADHD families showed no evidence for biased transmission of these alleles (p=0.81 and p=1.00, respectively). Our lack of evidence for association of this IL1RN polymorphism with ADHD, based on a much larger sample of families, suggests that the original finding may have been a spurious (i.e. false-positive) result. These findings highlight the need for further investigations of this marker, in additional independent ADHD samples, in the future.     

Association of the calcyon gene (DRD1IP) with attention deficit/hyperactivity disorder

Attention deficit/hyperactivity disorder (ADHD) is a childhood-onset disorder characterized by marked inattention, hyperactivity and impulsivity. The dopaminergic system has been hypothesized to be involved in the development of ADHD. Positive associations have been found for the dopamine receptors D1 and D5 genes, suggesting that other genes involved in D1/D5 signalling may also contribute to ADHD. In this study, we tested the calcyon gene (DRD1IP), which encodes a brain-specific D1-interacting protein involved in D1/D5 receptors calcium signalling, for association with ADHD. The inheritance of nine polymorphisms in the calcyon gene was examined in a sample of 215 nuclear families, with 260 affected children, using the transmission/disequilibrium test. The most common haplotype, designated C1, demonstrated significant evidence for excess transmission. Quantitative trait analyses of this haplotype showed significant relationships with both the inattentive (parent's rating, P=0.006; teacher's rating, P=0.003) and hyperactive/impulsive (parent's rating, P=0.004) dimensions of the disorder. Two of the nine marker alleles included in haplotype C1, rs4838721A located approximately 10 kb 5' of the gene and rs2275723C located 10 bp upstream of the exon 5 acceptor splice site, also showed significant evidence for association when analysed individually. As these two variants are not predicted to alter calcyon function, we screened the gene exons by sequencing. No variation in the coding region was identified, suggesting that a causal variant allele resides elsewhere in a regulatory sequence of the gene. These findings support the proposed involvement of the calcyon gene in ADHD and implicate haplotype C1 as containing a risk allele.     

Dopamine D4 gene 7-repeat allele and attention deficit hyperactivity disorder.

OBJECTIVE: Family, twin, and adoption studies show attention deficit hyperactivity disorder (ADHD) to have a substantial genetic component, and some studies have reported an association between ADHD and the dopamine D4 (DRD4) gene. METHOD: The authors recruited 27 triads that comprised an ADHD adult, his or her spouse, and their ADHD child. These triads were assessed for ADHD, and their DNA was genotyped for DRD4 alleles. RESULTS: A multiallelic transmission disequilibrium test suggested an association between ADHD and the DRD4 7-repeat allele. Among family members, the number of 7-repeat alleles predicted the diagnosis of ADHD. CONCLUSIONS: Prior reports of an association between ADHD and DRD4 generalize to families recruited through clinically referred ADHD adults. However, because there are some conflicting studies, further work is needed to clarify the role of DRD4 in the etiology of the disorder.     

Linkage of the dopamine receptor D1 gene to attention-deficit/hyperactivity disorder

Attention-deficit/hyperactivity disorder (ADHD) has a strong genetic basis, and evidence from human and animal studies suggests the dopamine receptor D1 gene, DRD1, to be a good candidate for involvement. Here, we tested for linkage of DRD1 to ADHD by examining the inheritance of four biallelic DRD1 polymorphisms [D1P.5 (-1251HaeIII), D1P.6 (-800HaeIII), D1.1 (-48DdeI) and D1.7 (+1403Bsp1286I)] in a sample of 156 ADHD families. Owing to linkage disequilibrium between alleles at the four markers, only three haplotypes are common in our sample. Using the transmission/disequilibrium test (TDT), we observed a strong bias for transmission of Haplotype 3 (1.1.1.2) from heterozygous parents to their affected children (P=0.008). Furthermore, using quantitative trait TDT analyses, we found significant and positive relationships between Haplotype 3 transmission and the inattentive symptoms, but not the hyperactive/impulsive symptoms, of ADHD. These findings support the proposed involvement of DRD1 in ADHD, and implicate Haplotype 3, in particular, as containing a potential risk factor for the inattentive symptom dimension of the disorder. Since none of the four marker alleles comprising Haplotype 3 is predicted to alter DRD1 function, we hypothesize that a functional DRD1 variant, conferring susceptibility to ADHD, is on this haplotype. To search for such a variant we screened the DRD1 coding region, by sequencing, focusing on the children who showed preferential transmission of Haplotype 3. DNA from 41 children was analysed, and no sequence variations were identified, indicating that the putative DRD1 risk variant for ADHD resides outside of the coding region of the gene.