Molly Malone

Identification of DNA variants in the SNAP-25 gene and linkage study of these polymorphisms and attention-deficit hyperactivity disorder

The gene for the synaptic vesicle docking fusion protein, synaptosomal-associated protein of 25 kDa (SNAP-25), has been implicated in the etiology of attention-deficit hyperactivity disorder (ADHD) based on the mouse mutant strain coloboma. This neutron-irradiation induced mouse strain is hemizygous for the deletion of the SNAP-25 gene and displays spontaneous hyperactivity that is responsive to dextroamphetamine. Because of these characteristics, this strain has been suggested to be a mouse model for ADHD. We identified using single stranded conformational polymorphism analysis (SSCP) four DNA sequence variants in the 3′ untranslated region of the human SNAP-25 gene. We searched for polymorphisms in the 3′ untranslated region because the intron/exon structure of this gene has not yet been determined. We tested for linkage of this gene and ADHD using two of the identified polymorphisms that change a restriction enzyme recognition site. We examined the transmission of the alleles of each of these polymorphisms and the haplotypes of both polymorphisms using the transmission disequilibrium test in a sample of 97 small nuclear families consisting of a proband with ADHD, their parents, and affected siblings. We observed biased transmission of the haplotypes of the alleles of these two polymorphisms. Our findings are suggestive of a role of this gene in ADHD.

The serotonin 5-HT1B receptor gene and attention deficit hyperactivity disorder.

Recent research has suggested that serotonin, in addition to dopamine, may be involved in the development of attention deficit hyperactivity disorder (ADHD). Serotonin regulates dopaminergic neurotransmission in some areas of the brain via several 5-HT receptors including 5-HT1B. Animal studies have suggested the involvement of the 5-HT1B receptors in locomotor behaviour. For these reasons, we hypothesized that the 5-HT1B receptor gene may be a good candidate for genetic studies of ADHD. We tested for linkage disequilibrium between the 5-HT1B G861C polymorphism and ADHD in 115 families using the transmission disequilibrium test (TDT). We found evidence for a trend towards excess transmission of the 861G allele (χ2=2.91, P=0.09) that when further analysed for parental allele transmissions exhibited significantly greater paternal transmission of the G allele (χ2=4.80, P=0.03) to the affected child. Although preliminary, results from this study provide additional evidence that serotonin genes may be important risk factors for the development of ADHD.

Evidence for the serotonin HTR2A receptor gene as a susceptibility factor in attention deficit hyperactivity disorder (ADHD)

A recent study demonstrated that treatment of hyperactive mice with psychostimulants and serotonergic agents produced a calming effect that was dependent on serotonergic neurotransmission and was not associated with any changes in extracellular dopamine levels.1 The complex interaction between the serotonergic and dopaminergic neurotransmitter systems suggests that a balance between the two systems may be necessary for mediating hyperactive behaviour. Defects in serotonin system genes, therefore, may disrupt normal brain serotonin function causing an imbalance between these neurotransmitter systems leading to the development of attention deficit hyperactivity disorder (ADHD). Using the transmission disequilibrium test (TDT), the current study assesses for linkage disequilibrium between polymorphisms in the serotonin HTR2A receptor gene and ADHD. One hundred and fifteen families with a total of 143 children diagnosed with ADHD (DSM-IV) were genotyped for the His452Tyr and the T102C polymorphisms in the serotonin HTR2A receptor gene. TDT analysis revealed a preferential transmission of the 452Tyr allele to the affected offspring (P = 0.03), suggesting linkage disequilibrium of this polymorphism with ADHD. This may open a new door in ADHD molecular genetics research, expanding the existing view of a catecholaminergic hypothesis to include a serotonergic hypothesis and should help elucidate the complex interplay among the neurotransmitter systems in the etiology of ADHD.

Further evidence from haplotype analysis for linkage of the dopamine D4 receptor gene and attention-deficit hyperactivity disorder

Several studies have suggested a possible association of a polymorphism at the dopamine D4 receptor gene and attention-deficit hyperactivity disorder [LaHoste et al., 1996; Rowe et al., 1998; Smalley et al., 1998; Sunohara et al., submitted; Swanson et al., 1998]. The allele reported to be associated with attention-deficit hyperactivity disorder (ADHD) is the allele with seven copies of the 48 bp repeat in the third exon. We extend our study of the dopamine D4 gene and ADHD by testing for linkage using two additional polymorphisms in the dopamine D4 receptor gene and a polymorphism in the closely linked gene, tyrosine hydroxylase. We also searched for two previously reported deletions, a 13 bp and a 21 bp deletion in the first exon. We examined the haplotypes of three polymorphisms of the D4 receptor gene and observed biased transmission of two of these haplotypes. Our findings further support the role of the dopamine D4 gene in ADHD.

Effect of Methylphenidate on Attention in Children with Attention Deficit Hyperactivity Disorder (ADHD): ERP Evidence

Methylphenidate is the most common treatment for attention deficit hyperactivity disorder (ADHD) and has been shown to improve attention and behaviour. However, the precise nature of methylphenidate on specific aspects of attention at different dose levels remains unclear. We studied methylphenidate effects in ADHD from a neurophysiological perspective, recording event-related potentials (ERPs) during attention task performance in normal controls and children with ADHD under different dose conditions. Twenty children with ADHD and 20 age matched controls were assessed with a continuous performance task requiring subjects to identify repeating alphabetic characters. ERPs and behavioural measures were recorded and analyzed for trials where a correct response was made. The ADHD group was assessed off drug (baseline) and on placebo, low (0.28 mg/kg) and high (0.56 mg/kg) dose levels of methylphenidate. The results showed that the ADHD group at baseline was more impulsive and inattentive than controls and had shorter P2 and N2 latencies and longer P3 latencies. Low dose methylphenidate was associated with reduced impulsivity (fewer false alarms) and decreased P3 latencies, whereas the higher dose level was associated with reduced impulsivity and less inattention (more hits), as well as increased P2 and N2 latencies and decreased P3 latencies. Amplitudes were unaffected and there were no adverse effects of the higher dose for any of the children. These results suggest differential dosage effects and a dissociation between dose levels and aspects of processing.

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.

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.

Hemispheric Processing and Methylphenidate Effects in Attention-Deficit Hyperactivity Disorder

To advance our understanding of attention-deficit hyperactivity disorder and medication effects we draw upon the evidence for (1) a neurotransmitter imbalance between norepinephrine and dopamine in attention-deficit hyperactivity disorder and (2) an asymmetric neural control system that links the dopaminergic pathways to left hemispheric processing and links the noradrenergic pathways to right hemispheric processing. It appears that attention-deficit hyperactivity disorder may involve a bihemispheric dysfunction characterized by reduced dopaminergic and excessive noradrenergic functioning. In turn, favorable medication effects may be mediated by a restoration in neurotransmitter balance and by increased control over the allocation of attentional resources between hemispheres. (J Child Neurol 1994;9:181-189).

The SNAP25 gene as a susceptibility gene contributing to attention-deficit hyperactivity disorder.

The synaptosomal-associated protein of 25 kDa gene (SNAP25) has been suggested as a genetic susceptibility factor in attention-deficit hyperactivity disorder (ADHD) based on the mouse strain coloboma. This strain is hemizygous for the SNAP25 gene and displays hyperactivity that responds to dextroamphetamine, but not to methylphenidate. Previously, we reported association of SNAP25 and ADHD using two polymorphisms. To further investigate this gene, we screened the exons for DNA variation and genotyped ten additional polymorphisms in an expanded sample of families from Toronto and a second sample of families collected in Irvine, CA. Significant results were observed in the Toronto sample for four markers, although not in the Irvine sample. The paper discusses the possible influence of the selection criteria on these differential results. The Irvine sample selected subjects that met the DSM-IV combined subtype diagnosis, whereas the Toronto sample included all subtypes. Analysis of the DSM-IV subtypes in the Toronto sample indicated that the differential results were not attributable to ADHD subtype. Differences in ethnicity, differential medication response, and other clinical characteristics of the samples cannot be ruled out at this time. Quantitative analysis of the dimensions of hyperactivity/impulsivity and inattention in the Toronto sample found that both behavioral traits were associated with SNAP25. Our findings continue to support SNAP25 in the susceptibility to ADHD.

Attention-deficit hyperactivity disorder and the gene for the dopamine D5 receptor

A recent study has suggested a possible association of a polymorphism near the dopamine D5 receptor gene (DRD5) and attention-deficit hyperactivity disorder.1 The polymorphism studied was a (CA)n repeat located in the cosmid containing the D5 receptor gene2 and the allele that was reported to be associated with attention-deficit hyperactivity disorder (ADHD) was the 148-bp allele. In this study we sought to replicate this finding by testing for biased transmission of the alleles at this same polymorphism in a sample of 92 families with an ADHD proband. We did not observe significant evidence for biased transmission of the 148-bp allele, however we did observe biased transmission of two other alleles, the 136-bp allele and the 146-bp allele. For these two alleles the bias was for these two alleles not to be transmitted to the ADHD children. The number of informative transmissions for these two alleles was small, therefore it would be premature to make any conclusions from our study concerning the role of DRD5 in ADHD.