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 s timulant medications act primarily on the dopaminergic system, dopamine sys tem 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 thir d exon of the dopamine D4 receptor. Method: The authors sought to replicate these previous findings by collecting an independent sample of families fr om Toronto, Ontario, Canada, and confirming this finding in an expanded sam ple of ADHD families collected from Irvine, California. Using the transmiss ion disequilibrium test (TDT), the authors tested for biased transmission o f the 7-repeat allele at the exon III polymorphism of the dopamine D4 recep tor locus in these samples of ADHD subjects. Results: Biased transmission o f the 7-repeat allele from parents to ADHD probands and their affected sibl ings was observed in the 2 new samples of families collected in Toronto and irvine (TDT chi (2) = 2.711, 1 df, one-sided p Value = .050) and for these samples combined with the 52 families previously reported from Irvine (TDT chi (2) = 6.428, 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.