Attention-deficit hyperactivity disorder (ADHD) is a common childhood-onset behavioural disorder affecting about 5% of school-age children. It has been shown that ADHD is highly heritable, and polymorphisms in several genes appear to increase susceptibility to the disorder. Most of these genes encode proteins involved in the brains’ dopaminergic neurotransmitter system, regulating processes like attention, movement, and reward.
One gene that has been widely associated with ADHD is the dopamine transporter gene (DAT1). The dopamine transporter mediates the reuptake of dopamine into neurons in the brain, and is a major target for various pharmacologically active drugs including methylphenidate – the most common medication given to children with ADHD. A number of studies have found that the 10-repeat allele of a variable number of tandem repeats (VNTR) polymorphism located in the 3’ untranslated-region (3’UTR) of the gene is more common in children with ADHD than in unaffected individuals. However, this finding has not been ubiquitously replicated, and it is possible that another unidentified polymorphism in the DAT1 gene may be the real risk variant.
In research to be published in the American Journal of Medical Genetics Part B, researchers at the Toronto Western Research Institute have investigated a number of polymorphisms in DAT1 in a large sample of children with ADHD. While the 10-repeat allele of the VNTR was not found to be associated with ADHD in this sample, Feng et al. report that a nearby variant, just 480 base-pairs upstream, does significantly increase susceptibility to the disorder. Furthermore, haplotype analyses investigating several polymorphisms together provided further evidence that DAT1 is associated with ADHD.
The authors conclude that their findings support the notion that DAT1 is involved in the aetiology of ADHD, but suggest that polymorphisms other than the VNTR in the 3’UTR of the gene may be mediating susceptibility.
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