Mutation and association analysis of the 5' region of the dopamine D3 receptor gene in schizophrenia patients: identification of the Ala38Thr polymorphism and suggested association between DRD3 haplotypes and schizophrenia

Although the association between the Ser9Gly polymorphism of the dopamine D3 receptor gene (DRD3) and schizophrenia has been investigated by many research groups, it is not known whether the Ser9Gly polymorphism alone or a variation in linkage disequilibrium may effect susceptibility to schizophrenia. We searched the 5' region of the DRD3 gene and found three novel polymorphisms: -712G/C, -205A/G, and Ala38Thr. The Ala38Thr polymorphism is located in the first transmembrane region and is conserved in the monkey, mouse, and rat. Case-control comparisons in 153 Japanese schizophrenia patients and 122 Japanese controls did not suggest an association between Ala38Thr and schizophrenia. However, there was a marginally significant association between the Ser9 allele of the Ser9Gly polymorphisms and schizophrenia (P = 0.02). Furthermore, there was a highly significant association between haplotypes of the -712G/C, -205A/G, and Ser9Gly polymorphisms and schizophrenia (P = 0.0007, corrected P = 0.007). These positive findings were replicated in an additional 99 Japanese schizophrenia patients and 132 controls (P = 0.04 and 0.0004, respectively). The most allelic differences of the Ser9Gly polymorphism between patient and control groups arose from the chromosome carrying specific alleles of the other three polymorphisms. This study indicates unknown variant(s) in linkage disequilibrium with the DRD3 haplotypes associated with schizophrenia.     

No association between the DRD3 Ser9Gly polymorphism and schizophrenia

OBJECTIVE: To investigate the association between a Ser9Gly polymorphism of the dopamine D3 receptor gene (DRD3) and schizophrenia. METHODS: 408 schizophrenic patients and 172 control subjects were compared with regard to their DRD3 Ser9Gly genotypic and allelic frequencies. In addition, we carried out a family-based association study including 183 pedigrees (472 subjects) using the transmission disequilibrium test (TDT). RESULTS: No significant differences of genotype or homozygosity distribution were identified between patients and controls. When patients were stratified according to gender, response to treatment, age at onset, no significant differences were observed. Neither allele A (Ser), or G (Gly) were preferentially transmitted from parents to affected offspring. CONCLUSION: The hypothesis that the DRD3 Ser9Gly polymorphism plays a predisposing role in schizophrenia is not supported by this study.     

多巴胺D3受体基因多态性与精神分裂症亚型及利培酮疗效相关性分析

目的探讨多巴胺D3受体(dopamine D3 receptor,DRD3)基因第一外显子丝氨酸9甘氨酸(Ser9Gly)多态性与精神分裂症临床亚型、药物疗效的关联.方法 241 例汉族首发精神分裂症患者,采用限制性片段长度多态性(restriction fragment length polymorphism,RFLP)技术测定基因型.分析判断基因多态性与精神分裂症的临床亚型、药物疗效的关联. 结果精神分裂症各亚型Ser9Gly等位基因分布存在显著性差异(p <0.05).利培酮疗效不同的患者间Ser9Gly等位基因多态性均无显著性差异. 结论 DRD3受体基因第一外显子Ser9Gly多态性可能与精神分裂症亚型相关,而与患者对药物的反应不相关.     

Polymorphisms of DRD4 and DRD3 and risk of avoidant and obsessive personality traits and disorders

We investigated whether polymorphisms of the dopamine D4 receptor (DRD4) and polymorphisms of the dopamine D3 receptor (DRD3) were associated with personality disorder symptomatology rather than with personality traits such as novelty seeking. DNA was obtained from 145 depressed patients in a clinical trial. These patients were assessed for the presence of personality disorder symptoms and disorders. The 2-repeat allele of the DRD4 exon III polymorphism was associated with increased rates of avoidant and obsessive personality disorder symptomatology. The T,T genotype of the DRD4 -521 C>T polymorphism was also associated with increased rates of avoidant and obsessive personality disorder symptomatology. The Gly9,Gly9 genotype of the DRD3 Ser9Gly polymorphism was associated with increased rates of obsessive personality disorder symptomatology. None of these three polymorphisms were associated with novelty seeking or other temperament traits on the Temperament and Character Inventory. Our results suggest that genetic polymorphisms of DRD4 and DRD3 may well be associated with personality traits, and that conflicting findings to date may arise from the problem of phenotype definition.     

Novel, replicated associations between dopamine D3 receptor gene polymorphisms and schizophrenia in two independent samples.

BACKGROUND: Meta-analyses have suggested an association between schizophrenia (SZ) and a coding polymorphism (rs6280/Ser9Gly) at the dopamine D3 receptor gene (DRD3), but results have been inconsistent. Because most studies have evaluated only rs6280, the inconsistencies might reflect associations with other variants. METHODS: We analyzed polymorphisms spanning 109kb in two independent samples (United States: 13 single nucleotide polymorphisms (SNPs), 331 cases, 151 trios, 274 control subjects; India: 11 SNPs, 141 trios). RESULTS: In the U.S. samples, significant associations were detected with eight SNPs, including rs6280 (p = .001, odds ratio: 1.5). Consistent associations in the case-control and family-based analyses were detected with a common haplotype spanning intron 1 to the 3' region of the gene (rs324029-rs7625282-rs324030-rs2134655-rs10934254; case-control, p = .002; transmission disequilibrium test [TDT], p = .0009; global p-values = .002 and .007, respectively). In the Indian sample, one SNP was associated (rs10934254, p = .03). Moreover, over-transmission of the same common haplotype as the U.S. sample was observed in this cohort (TDT, p = .005; global test, p = .009). Ser9Gly (rs6280) was associated with SZ against this haplotype background but not other haplotypes. CONCLUSIONS: These data suggest previous inconsistencies might have resulted from associations with other DRD3 variants. A liability locus might be in linkage disequilibrium (LD) with or carried against, an associated haplotype 3' to rs6280. Comprehensive SNP evaluation in larger samples is needed.     

多巴胺D3受体基因多态性与精神分裂症

目的:探讨沈阳地区汉族人群中多巴胺D3受体基因(DRD3)第一外显子第9密码子A→G单核苷酸置换多态性(Ser9Gly多态)与精神分裂症的关联。方法:采用聚合酶链反应-限制性内切酶片断长度多态性(PCR-RFLP)技术对70例精神分裂症患者、94名健康对照者进行基因分型鉴定。比较患者组与对照组DRD3多态性分布频率、精神分裂症早发组与非早发组基因分布频率差异,并与其他国家人群进行比较。结果:患者组与对照组之间等位基因分布无明显差异,早发组与非早发组亦未发现明显差异,而该位点等位基因分布频率与巴西、英国人群有明显差异。结论:研究人群中未发现DRD3基因Ser9Gly多态与精神分裂症存在明显关联。     

多巴胺D_3受体基因多态性与精神分裂症临床表型

目的：探讨多巴胺D3受体（DRD3）基因多态性与精神分裂症临床表型的关系。方法：对73个精神分裂症核心家系83例精神分裂症患者（患者组）及其146名父母（父母组）应用聚合酶链反应、限制性内切酶消化方法、琼脂糖凝胶电泳结合紫外凝胶成像系统检测DRD3基因中的3个位点（Ser9Gly、Ala38Thr和-205A/G）的多态性;采用阳性与阴性症状量表（PANSS）评定精神分裂症临床表型。结果：两组DRD3基因中3个位点的基因型分布及等位基因的频率分布差异无显著性;但在患者组中,对Ser9Gly位点3种基因型（Ser9Ser、Ser9Gly和Gly9Gly）进行分组比较时,Gly9Gly组的PANSS抑郁因子评分明显高于Ser9Ser组和Ser9Gly组,3组间比较,差异有显著性（P=0.042）。对Ala38Thr位点3种基因型（Ala38Ala、Ala38Thr和Thr38Thr）进行分组比较,PANSS总分及各因子分在Ala38Ala、Ala38Thr和Thr38Thr组间差异均无显著性。对-205A/G位点3种基因型（A/A、A/G和G/G）进行分组比较,G/G组的PANSS反应缺乏因子评分明显高于A/A组和A/G组,3组间比较,差异有显著性（P=0.048）。结论：DRD3基因变异可能与精神分裂症的病理症状有关。     

DRD3 and DAT1 genes in schizophrenia: an association study

OBJECTIVE: To investigate the role of the dopamine receptor 3 (DRD3) and transporter 1 (DAT1) genes in schizophrenia or in modulating its phenotype. METHODS: a Ser9Gly polymorphism in codon 9 of the DRD3 and a VNTR polymorphism in the DAT1genes were examined in two groups of schizophrenic patients, one of excellent neuroleptic responders (N=42) and one of nonresponders (N=64). A group of healthy volunteers screened for major psychiatric disorders was also included (N=89). In addition, age at onset of psychotic symptoms, attention performance and family loading for schizophrenia spectrum disorders were compared between patients with different genotypes in the DRD3 and DAT1 genes. RESULTS: No significant differences in the allelic distribution of the DRD3 and DAT1 polymorphisms were detected between schizophrenic patients and controls. A trend toward an excess of DRD3 genotype Gly/Gly was observed in neuroleptic nonresponder schizophrenic patients compared to controls (chi(2)=3. 30, df=1, p=0.07). No significant differences in age at onset of psychotic symptoms, attention task performance or family loading for schizophrenia spectrum disorders were observed between groups with different DRD3 and DAT1 genotypes. CONCLUSION: These results do not support the role of either of these genes in increasing susceptibility to schizophrenia or in modulating its phenotype in the studied population.     

Characterisation,mutation detection,and association analysis of alternative promoters and 5' UTRs of the human dopamine D3 receptor gene in schizophrenia

The dopamine D(3) receptor gene (DRD3) is a candidate for a number of psychiatric conditions including schizophrenia, bipolar disorder and alcohol and drug abuse. Previous studies have reported associations between polymorphisms in DRD3 and these disorders, but these findings may have reflected linkage disequilibrium with pathogenic variants that are further upstream. We have isolated and sequenced approximately 9 kb of genomic sequence upstream of the human DRD3 translational start site. Using 5' RACE, we have identified within this region three additional exons and two putative promoter regions which show promoter activity in three different cell lines. A 5' UTR identified only in lymphoblasts is spread over three exons and is 353 bp long. A second 5' UTR, found in adult and fetal brain, lymphocytes, kidney and placenta is spread over two exons and is 516 bp long. A 260-bp sequence within this 9 kb corresponds to a previously reported EST, but corresponding mRNA could not be found in the tissues above. The EST, 5' UTRs and putative promoter regions have been analysed for polymorphisms, revealing 10 single nucleotide polymorphisms, seven of which were tested for association in a large sample of unrelated patients with schizophrenia and matched controls. No associations were observed with schizophrenia. In addition we failed to replicate previous findings of association with homozygosity of the Ser9Gly variant. The results from this study imply that neither the coding nor the regulatory region of DRD3 plays a major role in predisposition to schizophrenia.     

多巴胺D3受体基因Ser9Gly多态性与阿立哌唑及利培酮治疗效应的关联研究

目的比较阿立哌唑与利培酮治疗女性首发精神分裂症患者的疗效和血浆催乳素水平变化及其与多巴胺D3受体(DRD3)基因Ser9Gly(rs6280)多态性的关联。方法选择完成8周阿立哌唑或利培酮治疗的女性首发精神分裂症患者各60例,于治疗前和治疗8周后分别评测阳性与阴性症状量表(positive and negativesymptom scale,PANSS)。采用放射免疫法检测血浆催乳素水平,DNA测序技术检测DRD3基因Ser9Gly多态性,分析DRD3基因Ser9Gly多态性与两药疗效及血浆催乳素变化的关联。结果治疗8周后,两组PANSS减分率的差异无统计学意义[(59.79±23.48)vs.(63.30±22.66),P>0.05],但利培酮组血浆催乳素的变化值高于阿立哌唑组[(26.92±9.48)vs.(-25.25±8.07),P<0.05]。利培酮组中C等位基因携带者的血浆催乳素的增加明显高于未携带者[(52.48±27.01)ng/mL vs(36.07±17.46),P<0.05];而阿立哌唑组中未见此差异[(-23.27±8.36)vs.TT(-26.05±8.11),P>0.05]。两组8周后PANSS减分率(%)与DRD3基因Ser9Gly的差异均无统计学意义:阿立哌唑组[CC+CT(57.83±19.94)vs.TT(56.84±18.46),P>0.05];利培酮组[CC+CT(53.94±21.08)vs.TT(60.38±19.37),P>0.05]。结论阿立哌唑治疗女性首发精神分裂症疗效与利培酮相当,但引起血浆催乳素水平变化的幅度较小;利培酮引起血浆催乳素水平增加可能与DRD3基因Ser9Gly多态性有关联。     

The COMT and DRD3 genes interacted in bipolar I but not bipolar II disorder

Abstract

Objectives. Clarifying the association between bipolar I and bipolar II disorders at the genetic level is essential for improving our understanding of them. In this study, we evaluated the hypothesis that the dopaminergic polymorphisms are risk factors for bipolar disorders. We examined the association between the catechol-O-methyltransferase (COMT) Val158Met and dopamine D3 receptor (DRD3) Ser9Gly polymorphisms and bipolar I and II disorders, as well as possible interactions between these genes. Methods. Seven hundred and eleven participants were recruited: 205 with bipolar I, 270 with bipolar II, and 236 healthy controls. The genotypes of the COMT Val158Met and DRD3 Ser9Gly polymorphisms were determined using polymerase chain reactions plus restriction fragment length polymorphism analysis. Results. Logistic regression analyses showed a statistically significant main effect for the Met/Met genotype of the COMT Val158Met polymorphism (P=0.032) and a significant interaction effect for the Met/Met genotype of the COMT Val158Met and Ser/Ser genotypes of the DRD3 Ser9Gly polymorphism (P=0.001) predicted bipolar I patients. However, there was no association between the COMT Val158Met or DRD3 Ser9Gly and bipolar II. Conclusions. We provide initial evidence that the COMT Val158Met and DRD3 Ser9Gly genotypes interact in bipolar I and bipolar II disorders and that bipolar I and bipolar II are genetically distinct.     

The Ser9Gly polymorphism of the dopamine D3 receptor gene and risk of schizophrenia: an association study and a large meta-analysis

Dopamine D3 receptor (DRD3) binds antipsychotic drugs and is abundant in the limbic system of the brain. It has been shown to play important roles in schizophrenia. A number of studies investigated the Ser9Gly polymorphism of the DRD3 gene to test its possible association with schizophrenia; however, the results were inconsistent. Our study aims to further evaluate the possible association between the Ser9Gly polymorphism and schizophrenia using a case-control association study within the Han Chinese population as well as a meta-analysis covering all previous studies. Our study, based on 329 schizophrenic patients and 288 controls, found no significant difference in the genotype or allele distributions of Ser9Gly polymorphism, the meta-analysis showed that the Ser9Gly polymorphism was not associated with Schizophrenia. Our study does not support the contention that the Ser9Gly polymorphism of the DRD3 gene plays a major role in schizophrenia in the Chinese population.     

Effects of the DRD3 Ser9Gly polymorphism on aripiprazole efficacy in schizophrenic patients as modified by clinical factors

Aripiprazole, a novel antipsychotic agent, has a unique pharmacological action (partial agonist) on the dopamine neurotransmission system. Aripiprazole has high affinity for dopamine D2 and D3 receptors (DRD2 and DRD3). We investigated whether the efficacy of aripiprazole can be predicted by a functional DRD3 gene polymorphism Ser9Gly (rs6280) as modified by clinical factors in Han Chinese hospitalized patients with acutely exacerbated schizophrenia. After hospitalization, the patients (n = 128) were given aripiprazole for up to four weeks. Patients were genotyped for DRD3 Ser9Gly polymorphism by Restriction Fragment Length Polymorphism (RFLP) method. Clinical factors such as gender, age, duration of illness, education level, diagnostic subtype and medication dosage were recorded. Psychopathology was measured biweekly with the Positive and Negative Syndrome Scale (PANSS). The effects of genetic and clinical factors on PANSS performance after aripiprazole treatment were analyzed by a mixed model regression approach (SAS Proc MIXED). We found that, although the Ser carriers have numerically larger score reductions when compared with non-carriers in almost all PANSS dimensions, the difference of their effects are statically not significant. However, the clinical factors, including dosage of aripiprazole, age, duration of illness, and diagnostic subtype could influence PANSS performance after aripiprazole treatment. This study suggests that DRD3 Ser9Gly polymorphism may not contribute significantly to inter-individual differences in therapeutic efficacy of aripiprazole, but some clinical factors may predict treatment efficacy.     

Schizotypy, attention deficit hyperactivity disorder, and dopamine genes

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

The association of genotypic combination of the DRD3 and BDNF polymorphisms on the adhesio interthalamica and medial temporal lobe structures

Abnormal neurodevelopment in midline structures such as the adhesio interthalamica (AI), as well as in the medial temporal lobe structures has been implicated in schizophrenia, while its genetic mechanism is unknown. This magnetic resonance imaging study investigated the effect of the genotypic combination of the dopamine D3 receptor (DRD3) Ser9Gly and brain-derived neurotrophic factor (BDNF) Val66Met polymorphisms on the AI length and volumetric measures of the medial temporal lobe structures (amygdala, hippocampus, and parahippocampal gyrus) in 33 schizophrenia patients and 29 healthy controls. The subjects with a combination of the Ser/Ser genotype of DRD3 and Met-containing genotypes of BDNF (high-risk combination) had a shorter AI than those without it in the healthy controls, but not in the schizophrenia patients. The subjects carrying the high-risk combination had a smaller posterior hippocampus than those without it for both diagnostic groups. These genotypic combination effects on brain morphology were not explained by the independent effect of each polymorphism. These findings suggest the effect of gene-gene interaction between the DRD3 and BDNF variations on brain morphology in midline and medial temporal lobe structures, but do not support its specific role in the pathogenesis of schizophrenia.     

Linkage disequilibrium between dopamine D1 receptor gene (DRD1) and bipolar disorder.

BACKGROUND: Based on the dopamine hypothesis, the dopamine D1 receptor gene (DRD1) is considered to be a good candidate gene for bipolar disorder (BP). METHODS: In our study, three polymorphisms of the DRD1 gene, -800T/C, -48A/G, and 1403T/C, were analyzed in 286 BP trios. Both the transmission disequilibrium test (TDT) and haplotype TDT were performed on the genotype data to test for the presence of linkage disequilibrium between DRD1 and bipolar disorder. With the extended transmission disequilibrium test (ETDT), we also calculated the maternal transmission and paternal transmission for each allele. RESULTS: Although no association was found for each individual polymorphism, there is a significant association between DRD1 and BP for haplotype TDT analysis (chi(2) = 16.068, df = 3, p =.0011). CONCLUSIONS: These results indicate that DRD1 may play a role in the etiology of bipolar disorder.     

Family-based association testing strongly implicates DRD2 as a risk gene for schizophrenia in Han Chinese from Taiwan

The gene that codes for dopamine receptor D2 (DRD2 on chromosome 11q23) has long been a prime functional and positional candidate risk gene for schizophrenia. Collectively, prior case-control studies found a reliable effect of the Ser311Cys DRD2 polymorphism (rs1801028) on risk for schizophrenia, but few other polymorphisms in the gene had ever been evaluated and no adequately powered family-based association study has been performed to date. Our objective was to test 21 haplotype-tagging and all three known nonsynonymous single-nucleotide polymorphisms (SNPs) in DRD2 for association with schizophrenia in a family-based study of 2408 Han Chinese, including 1214 affected individuals from 616 families. We did not find a significant effect of rs1801028, but we did find significant evidence for association of schizophrenia with two multi-marker haplotypes spanning blocks of strong linkage disequilibrium (LD) and nine individual SNPs (Ps<0.05). Importantly, two SNPs (rs1079727 and rs2283265) and both multi-marker haplotypes spanning entire LD blocks (including one that contained rs1801028) remained significant after correcting for multiple testing. These results further add to the body of data implicating DRD2 as a schizophrenia risk gene; however, a causal variant(s) in DRD2 remains to be elucidated by further fine mapping of the gene, with particular attention given to the area surrounding the third through fifth exons.     

Allelic variation in the human prodynorphin gene promoter and schizophrenia

Experimental and clinical studies suggest an involvement of the opioid neuropeptide system in schizophrenia. In particular, the prodynorphin (PDYN), the precursor of the dynorphin opioid peptides, has been shown to play an important role in several aspects of human mental diseases. Recently, a functional polymorphism in the promoter of PDYN gene has been described. We studied the possible relationship between this polymorphism and schizophrenia and we found no significant difference in allelic and genotype distributions between schizophrenic patients and control subjects. However, we observed a significant interactive effect with the receptor 3 of dopamine gene (DRD3); in particular, the frequency of subjects carrying PDYN allele 3 being also homozygotes for DRD3 Gly allele (of Ser9Gly polymorphism) was significantly greater in patients than controls. We conclude that PDYN gene polymorphism alone does not alter the risk for schizophrenia but, by an epistatic interaction with the Gly allele of DRD3 gene, may contribute to the susceptibility to this disorder.     

The dopamine D2 receptor gene is a susceptibility locus for Parkinson's disease.

The dopamine D2 receptor (DRD2) gene has been proposed as a candidate gene underlying several psychiatric and neurologic disorders. The aim of the present study was to examine if selected polymorphisms in the DRD2 gene are associated with Parkinson's disease (PD). We determined the allelic frequencies for four polymorphisms located in the DRD2 gene in a sample of 135 patients with PD and 202 normal control subjects. No significant difference was observed in the allelic frequencies between patients with PD and control subjects with regard to the -141C Ins/Del and the Ser311/Cys311 variants. On the contrary, the A1 allele of the TaqIA polymorphism and the B1 allele of the TaqIB polymorphism were more frequent in patients with PD than in control subjects (control subjects: TaqIA A1 = 14.6%, TaqIB B1 = 10.6%; patients with PD: TaqIA A1 = 20.7%, TaqIB B1 = 17.4%). Patients carrying the A1 allele or the B allele had an increased risk of developing PD (TaqIA, odds ratio: 1.71, 95% confidence intervals: 1.08-2.73; TaqIB, odds ratio: 1.83, 95% confidence intervals: 1.12-3.02). The TaqIA and TaqIB polymorphisms were in strong linkage disequilibrium, suggesting that these two polymorphisms convey the same information about the risk of presenting with PD. Genetic variation in the DRD2 gene may influence the risk of developing PD, thus confirming that the DRD2 gene is a susceptibility locus for PD.     

Pedigree disequilibrium test (PDT) replicates association and linkage between DRD4 and ADHD in multigenerational and extended pedigrees from a genetic isolate

Association/linkage between dopamine D4 receptor (DRD4) polymorphisms and attention-deficit/hyperactivity disorder (ADHD) has been suggested by case-control- and nuclear-family-based studies. Here, we present a candidate gene analysis for DRD4 using 14 extended and multigenerational families segregating ADHD derived from the 'Paisa' community of Antioquia, Colombia, a genetic isolate. Two DRD4 polymorphisms (a 120 bp tandem duplication at the promoter and a 48 bp-VNTR at exon 3), reported associated to ADHD, were genotyped. Parametric and non-parametric linkage analyses, and a family-based association test (FBAT), the pedigree disequilibrium test (PDT), were applied to search for evidence of association/linkage. Two-point LOD scores were significantly negative, with values ranging from -3.21 (P=0.011158) to -7.66 (P=0.000091 at theta=0). Non-parametrical analysis resulted in nonsignificant evidence for linkage. The PDT showed a moderate trend toward significance of association/linkage between the 7-repeat (7R) allele at the 48 bp VNTR and ADHD (P=0.0578). Furthermore, the haplotype analysis shows a significant association/linkage of the 7R-240 bp haplotype (P=0.0467) with ADHD. Results suggest that either a moderate DRD4 genetic effect, or linkage disequilibrium of DRD4 with an ADHD disease locus in the vicinity or the linkage to a phenotypic component of the ADHD spectrum could be underlying this association/linkage. These results provide further evidence for the association of ADHD to genetic variation in or near to DRD4 and replicate the previously reported association between ADHD and the 7R allele.