Attention deficit hyperactivity disorder (ADHD) is a common neurodevelopmental disorder, starting in early childhood and persisting into adulthood in the majority of cases. Family and twin studies have demonstrated the importance of genetic factors and candidate gene association studies have identified several loci that exert small but significant effects on ADHD. To provide further clarification of reported associations and identify novel associated genes, we examined 1038 single-nucleotide polymorphisms (SNPs) spanning 51 candidate genes involved in the regulation of neurotransmitter pathways, particularly dopamine, norepinephrine and serotonin pathways, in addition to circadian rhythm genes. Analysis used within family tests of association in a sample of 776 DSM-IV ADHD combined type cases ascertained for the International Multi-centre ADHD Gene project. We found nominal significance with one or more SNPs in 18 genes, including the two most replicated findings in the literature: DRD4 and DAT1. Gene-wide tests, adjusted for the number of SNPs analysed in each gene, identified associations with TPH2, ARRB2, SYP, DAT1, ADRB2, HES1, MAOA and PNMT. Further studies will be needed to confirm or refute the observed associations and their generalisability to other samples. Molecular Psychiatry (2006) 11, 934-953.

Several studies have implicated the dopamine transporter gene (DAT1) as conferring susceptibility to attention deficit hyperactivity disorder (ADHD), in particular, a VNTR situated at the 3' end of the gene. In addition, the 10-repeat VNTR allele associated with ADHD has been reported to be associated with an over-active transporter protein (DAT). Thus children possessing this variant might be particularly responsive to methylphenidate, a drug known to act by blocking DAT. We have examined this hypothesis and now report an association between the 10-repeat VNTR DAT1 polymorphism and retrospectively rated methylphenidate response in a sample of 119 Irish children with ADHD (chi(2) = 7.918, df = 1, P = 0.005). Our findings suggest a role for the 10-repeat DAT1 risk allele in medication response and may help to predict positive clinical outcome in ADHD.

Reduced central serotonergic activity has been implicated in poor impulse regulation and aggressive behaviour in animals, adults and also young children.1,2 Two recently published studies have implicated variation at a polymorphism in the promoter of the serotonin transporter (5HTT; hSERT) in influencing susceptibility to ADHD.3,4 Consistent with these results we have also found a trend for the long allele of the promoter polymorphism to influence susceptibility to ADHD in a sample of 113 ADHD parent proband trios (65 transmissions vs 49 non-transmissions, 2 = 2.25, P = 0.13). A pooled analysis of our, and these published results demonstrated a significant over representation of the long allele of the promoter in ADHD probands compared to controls ( 2 = 7.14, P = 0.008). We have also examined two other 5HTT polymorphisms ( Hyperkinetic disorder or attention deficit hyperactivity disorder (ADHD) affects 3-9% of school aged children with more boys diagnosed than girls.5 ADHD is characterised by marked and pervasive inattention, overactivity and impulsiveness 6 and causes significant social, educational and psychological problems. Quantitative genetic research over the last decade, from family, twin and adoption studies has firmly established that ADHD has a significant genetic contribution. 7,8 As a result, considerable effort has been directed towards identifying susceptibility genes for ADHD. The most effective treatments for ADHD are stimulant medications which act to inhibit the dopamine transporter eg methylphenidate (Ritalin). As a result, a hyperdopaminergic hypothesis has been postulated and is supported by an animal model: a knockout mouse for the dopamine transporter gene (DAT1) exhibits extreme hyperactivity. 9 Of particular interest is the finding that the calming effect of psychostimulants in this mouse appears to be influenced by serotonergic neurotransmission.10

Attention-deficit/hyperactivity disorder (ADHD) is a highly heritable, heterogeneous disorder of early onset, consisting of a triad of symptoms: inattention, hyperactivity, and impulsivity. The disorder has a significant genetic component, and theories of etiology include abnormalities in the dopaminergic system, with DRD4, DAT1, SNAP25, and DRD5 being implicated as major susceptibility genes. An initial report of association between ADHD and the common 148-bp allele of a microsatellite marker located 18.5 kb from the DRD5 gene has been followed by several studies showing nonsignificant trends toward association with the same allele. To establish the postulated association of the (CA)(n) repeat with ADHD, we collected genotypic information from 14 independent samples of probands and their parents, analyzed them individually and, in the absence of heterogeneity, analyzed them as a joint sample. The joint analysis showed association with the DRD5 locus (P=.00005; odds ratio 1.24; 95% confidence interval 1.12-1.38). This association appears to be confined to the predominantly inattentive and combined clinical subtypes.

Attention deficit hyperactivity disorder (ADHD) is a common, highly heritable, neurodevelopmental disorder with onset in early childhood. Genes involved in neuronal development and growth are, thus, important etiological candidates and brain-derived neurotrophic factor (BDNF), has been hypothesized to play a role in the pathogenesis of ADHD. BDNF is a member of the neurotrophin family and is involved in the survival and differentiation of dopaminergic neurons in the developing brain (of relevance because drugs that block the dopamine transporter can be effective therapeutically). The common Val66Met functional polymorphism in the human BDNF gene (rs 6265) was genotyped in a collaborative family-based sample of 341 white UK or Irish ADHD probands and their parents. We found evidence for preferential transmission of the valine (G) allele of BDNF (odds ratio, OR ¼ 1.6, P ¼ 0.02) with a strong paternal effect (paternal transmissions: OR ¼ 3.2, P ¼ 0.0005; maternal transmissions: OR ¼ 1.00; P ¼ 1.00). Our findings support the hypothesis that BDNF is involved in the pathogenesis of ADHD. The transmission difference between parents raises the possibility that an epigenetic process may be involved. Keywords: attention deficit hyperactivity disorder; association study; neurotrophic factor; polymorphism Attention deficit hyperactivity disorder (ADHD) is a neurodevelopmental disorder that affects 2-5% of school-aged children with more boys diagnosed than girls. 1 It is characterized by marked and pervasive inattention, overactivity and impulsiveness and causes significant social, educational and psychological problems. Quantitative genetic research over the last decade, from family, twin and adoption studies has firmly established that ADHD has a significant genetic contribution. 2 The medications most often used for ADHD are psychostimulants including methylphenidate and dexamphetamine. Precise therapeutic mechanisms of these drugs in ADHD remain unclear, although methylphenidate is known to inhibit the dopamine transporter. As a result, most candidate gene studies to date have focused on the dopamine system. 3 Positive findings have been successfully replicated for variants in the D4 dopamine receptor (DRD4), 4 D5 dopamine receptor (DRD5) 5 and the dopamine transporter (DAT1). 6 These family-based meta-analyses each report small effect sizes with odds ratios of less than 1.5, indicating that if they are true susceptibility variants for ADHD their contribution to the overall phenotype is small. Given that ADHD is a neurodevelopmental disorder, genes involved in neuronal development and growth represent an important set of candidates for involvement in the pathogenesis. One such candidate that has been postulated to play a role in ADHD is BDNF (MIM 113505). 7 The gene encoding BDNF is located at 11p13 and codes for a precursor peptide (proBDNF), which is proteolytically cleaved to form the mature protein. Only one nonsynonymous polymorphism in the human BDNF gene (rs 6265) has been identified, a single nucleotide polymorph...

Genetic and pharmacological studies have emphasised the role of serotonin 5-hydroxytryptamine (5-HT) as a possible etiologic factor in the development of attention-deficit hyperactivity disorder (ADHD). Tryptophan hydroxylase (TPH) is a rate-limiting enzyme in the biosynthesis of serotonin from tryptophan. Originally, the TPH gene was thought to be widely expressed, but a second form of TPH, TPH2, was recently identified and the TPH2 gene was found to be solely expressed in the brain. We examined eight single nucleotide polymorphisms (SNP) in the TPH2 gene for association with ADHD in 179 Irish nuclear families. Transmission disequilibrium test analysis revealed significant association between the T allele of marker rs1843809 with the disorder (v 2 ¼ 12.2, P ¼ 0.0006, OR ¼ 2.36). Stratifying data by the sex of the transmitting parent showed that this association was enhanced when paternal transmission was considered (OR ¼ 3.7). In addition, several haplotypes (all including the associated marker) were associated with ADHD. These preliminary findings suggest that TPH2 is a susceptibility locus for ADHD. Further confirmation, preferably from different ethnic groups, is required to firmly implicate TPH2 in the pathophysiology of ADHD. Molecular Psychiatry ( Keywords: attention-deficit hyperactivity disorder (ADHD); tryptophan hydroxylase (TPH2); transmission disequilibrium test (TDT); serotonergic system, genetic association Attention-deficit hyperactivity disorder (ADHD) is a common neurological disorder affecting 3-6% of children worldwide. 1 It is one of the most common causes of referrals in family practice, paediatric neurology and child psychiatry. 2 Children can be diagnosed with ADHD as early as 5 years of age with males being more affected than females (ratio ranging from 3 : 1-9 : 1). Major symptoms of ADHD include inability to sustain attention, hyperactivity, impulsivity and aggressive behaviour. The consequences are devastating for family and peer relations as well as academic life. Behaviour at home and at school is disrupted and ADHD children have poor school performance compared to healthy and age-matched children. 1 Symptoms persist into adulthood in at least 30% of cases. The disorder is clinically heterogeneous and its aetiology remains unknown. However, several lines of evidence have implicated genetic and environmental factors as predisposing to the disorder. Heritability of ADHD is estimated to be between 70 and 90%. 3,4 Environmental factors such as low birth weight, delivery complications, alcohol consumption and smoking during pregnancy, environmental toxins and diet are all implicated. 5 Serotonin 5-hydroxytryptamine (5-HT) is a neurotransmitter involved in a variety of functions such as attention, sleep, memory and learning, locomotion, control of appetite, anxiety and drug abuse. 6 Reduction in serotonergic function has been linked to ADHD symptoms such as aggression and impulsivity. 6,7 In addition, recent pharmacological evidence supports the potential involvement of serotonergic syste...

Associations between attention deficit hyperactivity disorder (ADHD) and genetic variants within the dopamine D4 receptor gene have been much reported. Variants investigated include the 7-repeat allele of a VNTR within the third exon, and two SNPs (-521 and -616) located with the promoter region. We investigated the relationship between the VNTR, -521, and -616 SNPs and sustained attention performance in 54 ADHD probands, relative to a non-genotyped control group. Participants performed the Sustained Attention to Response Task (SART) in which the response to an unpredictably occurring target digit must be inhibited. This task, therefore, challenged sustained attention and included a response inhibition component. Consistent with previous reports, ADHD participants possessing the 7-repeat allele of the VNTR outperformed those children not possessing this allele, both in terms of errors and response variability. In family based analyses, better performance on the SART tended to predict biased transmission of the 7-repeat allele from heterozygous parents. For the -521 SNP, A allele homozygotes showed greater impairment than heterozygotes or those not possessing this allele, both in terms of total errors and response variability. Family based analysis showed that higher total errors on the SART predicted transmission of the A allele from heterozygous parents. There were no effects of the -616 SNP. Our results suggest dissociable effects of the "associated alleles" of DRD4 gene variants on sustained attention: while the 7-repeat allele of the VNTR is associated with relatively better performance, the A allele of the -521 SNP is associated with poorer performance.

Abnormalities in dopaminergic neurotransmission are now accepted as factors in predisposing to ADHD. Evidence of associations between dopamine transporter gene polymorphism and ADHD was first reported by Cook et al. We confirmed the DAT1 association and also identified two additional susceptibility loci at the DRD5 and DBH. Notably, none of the associated variants at these three genes are known to be expressed. Other variants within or closely mapped to the associated alleles are likely to be relevant. In this investigation, we analyse additional markers creating a high-density map across and flanking these genes, and measure intermarker linkage disequilibrium (LD). None of the newly examined markers were more strongly associated with ADHD. At DAT1, the pattern of intermarker LD and haplotype association with the phenotype between exon 9 and the 3 0 of the gene suggests that the functional variant at DAT1 may be located to this region. For DRD5, three markers, covering a region of approximately 68 kb including the single DRD5 exon are all associated with disease, and thus do not provide localizing information. However, the data for DBH point to a region close to the centre of the gene. Correlation between D 0 and physical distance was observed between markers at DAT1 and DRD5 for distances less than 50 kb. This was not the case for DBH, where LD breakdown was observed between the intron 5 and intron 9 polymorphisms although they are only 9 kb apart. Further genetic analysis is unlikely to refine the location of susceptibility variants and functional assessment of variants within associated regions is required.