Tejaswee Pathare

Sequence variation in the 3'-untranslated region of the dopamine transporter gene and attention-deficit hyperactivity disorder (ADHD).

The dopamine transporter gene (DAT1) has been reported to be associated with attention‐deficit hyperactivity disorder (ADHD) in a number of studies [Cook et al. (1995): Am J Human Genet 56(4):9993–998; Gill et al. (1997): Mol Psychiatry 2(4):311–313; Waldman et al. (1998): Am J Human Genet 63(6):1767–1776; Barr et al. (2001): Biol Psychiatry 49(4):333–339; Curran et al. (2001): Mol Psychiatry 6(4):425–428; Chen et al. (2003): Mol Psychiatry 8(4):393–396]. Specifically, the 10‐repeat allele of the 40‐bp variable number of tandem repeats (VNTR) polymorphism located in the 3′ untranslated region (UTR) of the gene has been found to be associated with ADHD. There is evidence from in vitro studies indicating that variability in the repeat number, and sequence variation in the 3′‐UTR of the DAT1 gene may influence the level of the dopamine transporter protein [Fuke et al. (2001): Pharmacogenomics J 1(2):152–156; Miller and Madras (2002): Mol Psychiatry 7(1):44–55]. In this study, we investigated whether DNA variation in the DAT1 3′UTR contributed to ADHD by genotyping DNA variants around the VNTR region in a sample of 178 ADHD families. These included a MspI polymorphism (rs27072), a DraI DNA change (T/C) reported to influence DAT1 expression levels, and a BstUI polymorphism (rs3863145) in addition to the VNTR. We also screened the VNTR region by direct resequencing to determine if there was sequence variation within the repeat units that could account for the association. Our results indicate that DAT1 is associated with ADHD in our sample but not with alleles of the VNTR polymorphism. We did not find any variation in the sequence for either the 10‐ or 9‐repeat alleles in the probands screened nor did we observe the reported DraI (T/C) variation. Our results therefore refute the possibility of the reported DraI variation or alleles of the VNTR as the functional variants contributing to the disorder.

Gene for the serotonin transporter and ADHD: no association with two functional polymorphisms.

Evidence from both human and animal studies implicates the serotonergic system in the development of attention‐deficit hyperactivity disorder (ADHD) including positive association studies for several key serotonergic genes. The serotonin transporter (HTT) regulates the availability of serotonin by reuptake of the neurotransmitter from the synaptic cleft. Several studies have reported an association of this gene to ADHD, specifically the long variant of a common insertion/deletion polymorphism located in the promoter of this gene that results in increased transcription and higher HTT expression. An additional study found no evidence for an association with this polymorphism. Recently, an A/G single nucleotide polymorphism (SNP) was found within the promoter polymorphism with functional studies indicating that the long variant containing the G allele at this site behaves like the short variant. This previously unidentified functional change may have confounded earlier association studies. We investigated the relationship of several variants to ADHD: the promoter polymorphisms, SNP in the 3′ untranslated region (3′UTR) with a reported association to ADHD and a rare, non‐synonymous coding SNP. These polymorphisms were genotyped in 209 ADHD families identified through an affected proband. We did not find evidence for an association of these polymorphisms, or haplotypes of these polymorphisms, to ADHD in this sample.

Association of Attention-Deficit/Hyperactivity Disorder with a Candidate Region for Reading Disabilities on Chromosome 6p

BACKGROUND Reading disabilities (RD) and attention-deficit hyperactivity/disorder (ADHD) are two common childhood disorders that co-occur by chance more often than expected. Twin studies and overlapping genetic linkage findings indicate that shared genetic factors partially contribute to this comorbidity. Linkage of ADHD to 6p, an identified RD candidate locus, has previously been reported, suggesting the possibility of a pleiotropic gene at this locus. RD has been previously associated with five genes in the region, particularly DCDC2 and KIAA0319. METHODS To test whether these genes also contribute to ADHD, we investigated markers previously associated with RD for association with ADHD and ADHD symptoms in a sample of families with ADHD (n = 264). Markers were located in two subregions, VMP/DCDC2 and KIAA0319/TTRAP. RESULTS Across all analyses conducted, strong evidence for association was observed in the VMP/DCDC2 region. Association was equally strong with symptoms of both inattention and hyperactivity/impulsivity, suggesting that this locus contributes to both symptom dimensions. Markers were also tested for association with measures of reading skills (word identification, decoding); however, there was virtually no overlap in the markers associated with ADHD and those associated with reading skills in this sample. CONCLUSIONS Overall this study supports a previous linkage study of ADHD indicating a risk gene for ADHD on 6p and points to VMP or DCDC2 as the most likely candidates.

Investigation of the relationship of attention deficit hyperactivity disorder to the EKN1 gene on chromosome 15q21

Recently a gene, termed EKN1, has been identified because of a chromosomal breakpoint that occurred in this gene. This chromosomal breakpoint was found in 4 family members that had specific reading disabilities (RDs), indicating that disruption of this gene may be contributing to the risk of developing RDs. This gene was further supported as contributing to RD by association studies. Because of the evidence from twin studies for shared genetic factors contributing to RD and attention deficit hyperactivity disorder (ADHD), particularly inattention symptoms, we investigated the relationship of DNA markers in this gene to ADHD and ADHD symptoms in a sample of 186 nuclear families (probands, their parents, and affected siblings) collected through a proband with ADHD. We used 6 polymorphic DNA markers located across the gene, including the 2 markers previously reported to be associated with RD in a Finnish sample and a marker associated with RD in a sample of families collected in Toronto. We found a trend for association for several markers to the ADHD phenotype analyzed as a categorical trait using the transmission disequilibrium test and significant evidence for biased transmission of the haplotypes containing these markers, χ2(3) = 9.312, p =. 025. Using quantitative analysis, we observed evidence for association of one of the haplotypes to the inattention and hyperactive/impulsive symptom dimensions as reported by parents and to the inattention symptoms as reported by teachers, as well as a trend for association with the reading phenotypes of word identification and decoding. The results provide preliminary support for the role of the EKN1 chromosomal region in ADHD, suggesting that this region may contribute to ADHD symptoms in addition to RD.

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 study for genes at chromosome 5p13-q11 in attention deficit hyperactivity disorder†

Linkage of attention deficit hyperactivity disorder (ADHD) to the short arm‐centromeric region of chromosome 5 has been reported in multiple studies. The overlapping region (5p13‐q11) contains a number of strong candidate genes for ADHD, based on their role in brain function or neurodevelopment. The aim of this study was to investigate some of the top candidates among these genes in relation to ADHD in a sample of 245 nuclear families from the Toronto area. We investigated the genes for the glial cell‐derived neurotropic factor (GDNF), the fibroblast growth factor 10 (FGF10), islet‐1 (ISL1), the hyperpolarized potassium channel (HCN1) and the integrin alpha 1 (ITGA1). In addition to these genes, we assessed the 3′region of the SLC1A3 gene, a glutamate transporter implicated in ADHD by a previous association study. A total of 36 polymorphisms were selected across the six genes. We performed family‐based association and haplotype analyses. ADHD is a dimensional disorder, with symptoms of inattention and hyperactivity‐impulsivity therefore, we also conducted quantitative analysis in relation to symptom scores for both dimensions. Single marker and haplotype analyses yielded little evidence of association for any of the genes tested in this study. Moreover, we were unable to replicate the positive association findings reported for SLC1A3. Our results suggest that these six genes are unlikely to be susceptibility genes in the chromosome 5p13‐q11 region and other genes should now be considered for priority study.

The gene for synapsin III and attention-deficit hyperactivity disorder.

Objective Recent studies have implicated the involvement of proteins regulating neurotransmitter release in the etiology of attention deficit hyperactivity disorder. On the basis of the role of synapsin III in the modulation of neurotransmitter release, we tested this gene as a candidate contributing to the genetic susceptibility of attention deficit hyperactivity disorder. Method In this study, we genotyped five markers across the gene on 177 small, nuclear families consisting of an attention deficit hyperactivity disorder proband, their parents, and 43 affected siblings. We examined the transmission of the alleles at each one of these sites and the haplotypes of the polymorphisms using the transmission disequilibrium test. Result Our observations did not yield any evidence of biased transmission of the alleles at any polymorphism or haplotype. On the basis of the evidence for synapsins in learning and memory from animal models, we also investigated the relationship of this gene to verbal short-term and working memory as measured by digit span forward and backwards. No evidence was found for an association of this gene to these traits. Conclusion Our findings with this particular sample do not support the synapsin III locus as a major susceptibility locus contributing to attention deficit hyperactivity disorder.

No evidence for genetic association between DARPP-32 (PP1R1B) polymorphisms and attention deficit hyperactivity disorder.

Attention deficit hyperactivity disorder (ADHD) has a strong genetic basis, and evidence from human and animal studies suggests that a dopamine system dysfunction plays a role in the disorder pathophysiology. Several genes involved in dopamine neurotransmission have shown replicated genetic association with ADHD. These include the dopamine receptors D4 (DRD4), D5 (DRD5), and the dopamine transporter (DAT1) genes. Recently, evidence has also accumulated in favor of the dopamine receptor D1 gene (DRD1). The dopamine‐ and cAMP‐regulated phosphoprotein of relative molecular mass of 32 kDa (DARPP‐32) is a key component of dopamine signaling, acting as a converging point for several neurotransmitter systems influencing dopaminergic neurons and regulating a wide variety of downstream effectors. Here, we tested the DARPP‐32 gene, PPP1R1B, for association with ADHD using four polymorphic markers selected across the gene in a sample of 255 ADHD families. We did not detect evidence of association of individual marker alleles and haplotype analysis did not reveal significant association in this sample of families. Moreover, we found no relationship between the same alleles or haplotypes and symptom scores of inattention or hyperactivity/impulsivity in these families using a quantitative approach. In conclusion, albeit a key regulatory role in dopamine signaling, our data do not support a major contribution of the DARPP‐32 gene in ADHD.