Ziarih Hawi

Confirmation of association between attention deficit hyperactivity disorder and a dopamine transporter polymorphism

Attention deficit hyperactivity disorder (ADHD) is a common condition of childhood the symptoms of which include inattention, excessive motor activity, inpulsivity and distractibility. It is strongly familial1 and twin2 and adoption studies3,4 suggest that the familiality is due, at least in part, to shared genes. Gillis et al2 found concordance rates in ADHD for MZ and DZ twins of 81% and 29% respectively. Stimulant drugs (eg, methylphenidate) are effective in the treatment of ADHD5 and inhibit the dopamine transporter. This has led to the development of a hypodopaminergic hypothesis for the disease6. Cook et al7 examined a 3′ variable number of tandem repeat (VNTR) polymorphism at the dopamine transporter gene (DAT1) in a sample of 49 ADHD patients and their parents, using the haplotype relative risk (HRR) method. They found a significant association (χ2 = 7.29, 1 d.f., P = 0.007) between ADHD and the 480-bp DAT1 VNTR allele. The authors stressed the importance of independent replication and we have achieved this in a study of 40 probands and their parents, using the same robust HRR method. As in the study of Cook et al7 we found that the 480-bp allele was preferentially transmitted to ADHD probands (χ2 = 6.07, 1 d.f., P = 0.014).

Mapping susceptibility loci in attention deficit hyperactivity disorder: preferential transmission of parental alleles at DAT1, DBH and DRD5 to affected children

Attention deficit hyperactivity disorder (ADHD) is a common disorder of childhood characterized by inattention, excessive motor activity, impulsivity, and distractibility. It is associated with serious disability in children, adolescents and adults.1 The etiology of the disorder is unknown, but it has a strong genetic component. Pharmacological and biochemical studies have suggested that dopaminergic and noradrenergic systems are involved.2 Using a sample of affected children and their parents we have found preferential transmission of alleles at polymorphisms at the dopamine transporter (DAT1), RR = 1.2 (1.05–1.37), P = 0.006, re-confirming and extending our previous findings for DAT13 (new sample one-tailed P = 0.039); dopamine-β-hydroxylase (DBH), RR = 1.31 (1.09–1.56), P = 0.0027; and the dopamine D5 receptor (DRD5), RR = 1.67 (1.29–2.15), P = 0.00005. Transmission of the ‘associated’ alleles at DAT1 and DBH is stronger in familial cases, RRDAT1 = 1.29 (1.04–1.59), RRDBH = 1.49 (1.10–2.00), but for DRD5, transmission is stronger in non-familial cases, RR = 1.59 (1.05–2.42). TDT analysis of complete trios supports the HHRR analysis, with P < 0.05, for dat1 P < 0.005 and dbh and P < 0.01 for drd5. attributable fractions for dat1, dbh and drd5 are calculated at 0.08, 0.12 and 0.20 respectively.

Dopaminergic system genes in ADHD: toward a biological hypothesis

Converging evidence has implicated abnormalities of dopamine neurotransmission to the pathology of attention deficit hyperactivity disorder (ADHD). Several genetic association studies have been published, but so far, no DNA variants have been unequivocally demonstrated as contributing to ADHD susceptibility. Four dopamine related gene loci have been implicated, however: DAT1, DRD4, DBH, and DRD5. Each of these may influence the liability of ADHD to a small degree. Notably, all are involved in signal transduction at the neuronal synapse. In this article, we investigate as candidate genes for ADHD, DNA polymorphisms at dopamine receptors, the dopamine transporter, and genes known to be involved in dopamine synthesis and metabolism. In a recent article, we confirmed the previously reported association of DAT1 (480bp allele) with ADHD and identified polymorphisms at two additional loci showing preferential transmission to ADHD children of alleles at DRD5 (148bp allele) and at DBH (allele 2, Taq I polymorphism). Increased transmission of the 4bp deletion in the untranslated exon 1 of the DOPA decarboxylase gene was also observed but was of marginal significance. Nonsignificant trends of association were found for TH (allele 2) and DRD2 (Ser-311). No preferential transmission of alleles to ADHD children was observed for polymorphisms at DRD1, DRD2 (TaqI), DRD3, DRD4, and COMT. Analyzing the data by sex of transmitting parent showed significant preferential paternal transmission of alleles at TH (allele 2) and a nonsignificant trend for paternal transmission for DRD2 (Ser-311). We attempt to put these findings together with what is known of the function of the particular proteins, and suggest working hypotheses.

Joint Analysis of the DRD5 Marker Concludes Association with Attention-Deficit/Hyperactivity Disorder Confined to the Predominantly Inattentive and Combined Subtypes

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.

Genome-wide analysis of copy number variants in attention deficit hyperactivity disorder: the role of rare variants and duplications at 15q13.3

Objective: Attention deficit hyperactivity disorder (ADHD) is a common, highly heritable psychiatric disorder. Because of its multifactorial etiology, however, identifying the genes involved has been difficult. The authors followed up on recent findings suggesting that rare copy number variants (CNVs) may be important for ADHD etiology. Method: The authors performed a genome-wide analysis of large, rare CNVs (<1% population frequency) in children with ADHD (N=896) and comparison subjects (N=2,455) from the IMAGE II Consortium. Results: The authors observed 1,562 individually rare CNVs >100 kb in size, which segregated into 912 independent loci. Overall, the rate of rare CNVs >100 kb was 1.15 times higher in ADHD case subjects relative to comparison subjects, with duplications spanning known genes showing a 1.2-fold enrichment. In accordance with a previous study, rare CNVs >500 kb showed the greatest enrichment (1.28-fold). CNVs identified in ADHD case subjects were significantly enriched for loci implicated in autism and in schizophrenia. Duplications spanning the CHRNA7 gene at chromosome 15q13.3 were associated with ADHD in single-locus analysis. This finding was consistently replicated in an additional 2,242 ADHD case subjects and 8,552 comparison subjects from four independent cohorts from the United Kingdom, the United States, and Canada. Presence of the duplication at 15q13.3 appeared to be associated with comorbid conduct disorder. Conclusions: These findings support the enrichment of large, rare CNVs in ADHD and implicate duplications at 15q13.3 as a novel risk factor for ADHD. With a frequency of 0.6% in the populations investigated and a relatively large effect size (odds ratio=2.22, 95% confidence interval=1.5–3.6), this locus could be an important contributor to ADHD etiology.

Serotonergic system and attention deficit hyperactivity disorder (ADHD): a potential susceptibility locus at the 5-HT 1B receptor gene in 273 nuclear families from a multi-centre sample

Attention deficit hyperactivity disorder (ADHD) is a highly heritable and heterogeneous disorder, which usually becomes apparent during the first few years of childhood. Imbalance in dopamine neurotransmission has been suggested as a factor predisposing to ADHD. However, evidence has suggested an interaction between dopamine and serotonin systems in the pathophysiology of the disorder. Studies using selective agonists of the different 5-HT receptors microinjected into selected brain structures have shown a positive modulating effect on the functional activities of the mesotelencephalic dopaminergic system. This suggests that some of the genetic predisposition to ADHD might be due to DNA variation at serotonin system genes. In this study, we investigated polymorphisms in HTR1B and HTR2A (which encode the serotonin receptors 5-HT1B and 5-HT2A respectively) in a European ADHD sample. Using haplotype based haplotype relative risk (HHRR) and transmission disequilibrium test (TDT) analyses, we observed significant preferential transmission of the allele 861G of the HTR1B in the total sample (for HHRR; χ2 = 7.4, P = 0.0065 and TDT; (χ2 = 6.4, P = 0.014). Analysis of HTR2A failed to reveal evidence of association or linkage between the His452Tyr polymorphism and ADHD in the total sample. However, a significantly increased transmission of the allele 452His was observed in the Irish sample alone (χ2 = 4.9, P = 0.026). These preliminary data suggest an important role for the serotonin system in the development of ADHD. Further studies, preferentially including different ethnic groups are required to substantiate these findings.

Association of the 480 bp DAT1 allele with methylphenidate response in a sample of Irish children with ADHD.

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 (χ2 = 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.

Synaptosomal-associated protein 25 (SNAP-25) and attention deficit hyperactivity disorder (ADHD): evidence of linkage and association in the Irish population

Several lines of evidence have suggested that ADHD is a polygenic disorder produced by the interaction of several genes each of a minor effect. Synaptosomal-associated protein 25 (SNAP-25) is a presynaptic plasma membrane protein which is expressed highly and specifically in the nerve cells. The gene encodes a protein essential for synaptic vesicle fusion and neurotransmitter release. Animal model studies showed that the coloboma mouse mutant has a hyperactive phenotype similar to that of ADHD. The hyperactive phenotype of this model has been shown to be the result of a deletion of the SNAP-25 gene. DNA variations within or closely mapped to the SNAP-25 gene may alter the level of expression and hence may have an effect on the function of synaptic vesicle fusion and neurotransmitter release. Using HHRR and TDT we analysed 93 ADHD nuclear families from Ireland and found increased preferential transmission of SNAP-25/DdeI allelel to ADHD cases; HHRR (χ2 = 6.55, P = 0.01) and linkage (TDT) (χ2 = 6.5, P = 0.015). In contrast to our findings, Barr et al1 reported an increased transmission of allele 2 of the DdeI polymorphism though this was not statistically significant. However, they also reported a significantly increased transmission of a haplotype (made of allele 1 of MnlI and allele 2 of the DdeI) in their Canadian ADHD sample. It is not clear what the role of SNAP-25 in ADHD is until these findings are either confirmed or refuted in other ADHD samples.

Evidence that variation at the serotonin transporter gene influences susceptibility to attention deficit hyperactivity disorder (ADHD): analysis and pooled analysis.

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, χ2 = 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 (χ2 = 7.14, P = 0.008). We have also examined two other 5HTT polymorphisms (the VNTR in intron 2 and the 3′ UTR SNP). TDT analysis demonstrated preferential transmission of the T allele of the 3′ UTR SNP (χ2 = 4.06, P = 0.04). In addition, ETDT analysis of haplotypes demonstrated significant preferential transmission of haplotypes containing the T allele of the 3′ UTR SNP with the long allele of the promoter polymorphism (χ2 = 13.18, 3 df, P = 0.004) and the 10 repeat of the VNTR (χ2 = 8.77, 3 df, P = 0.03). This study provides further evidence for the possible involvement of the serotonin transporter in susceptibility to ADHD.

Case-control genome-wide association study of attention-deficit/ hyperactivity disorder

OBJECTIVE Although twin and family studies have shown attention-deficit/hyperactivity disorder (ADHD) to be highly heritable, genetic variants influencing the trait at a genome-wide significant level have yet to be identified. Thus additional genomewide association studies (GWAS) are needed. METHOD We used case-control analyses of 896 cases with DSM-IV ADHD genotyped using the Affymetrix 5.0 array and 2,455 repository controls screened for psychotic and bipolar symptoms genotyped using Affymetrix 6.0 arrays. A consensus SNP set was imputed using BEAGLE 3.0, resulting in an analysis dataset of 1,033,244 SNPs. Data were analyzed using a generalized linear model. RESULTS No genome-wide significant associations were found. The most significant results implicated the following genes: PRKG1, FLNC, TCERG1L, PPM1H, NXPH1, PPM1H, CDH13, HK1, and HKDC1. CONCLUSIONS The current analyses are a useful addition to the present literature and will make a valuable contribution to future meta-analyses. The candidate gene findings are consistent with a prior meta-analysis in suggesting that the effects of ADHD risk variants must, individually, be very small and/or include multiple rare alleles.