exon 3 genotype and ADHD: Randomised pharmacodynamic investigation of treatment response to methylphenidate.

Dopamine plays an important role in modulating attention and motor behaviours, dimensions altered in attention deficit/hyperactivity disorder (ADHD). Numerous association studies have linked dopamine receptor 4 () to increased risk of ADHD. This study investigated the effect of exon 3 polymorphism on child behaviours in response to treatment with methylphenidate. A total of 374 children diagnosed with ADHD (ages 6-12 years) were evaluated under three experimental conditions: baseline, placebo and MPH (0.5 mg/kg/day). This was a 2-week prospective within-subject, placebo-controlled, crossover trial. The Conners' Global Index for parents and for teachers was used to evaluate the behaviours of the children. One-way repeated measures analysis of variance was used to test the effect of the interaction between genotype and experimental conditions. A significant interaction between genotype and treatment was detected when the child's behaviour was evaluated by the parents ( = 0.035, effect size of 0.014), driven by a better treatment response in children homozygous for long 7-repeat allele. According to the parent assessment, children homozygous for the long 7-repeat allele were more responsive to experimental condition. This is the largest pharmacogenetic investigation of the effect of exon 3 polymorphism in childhood ADHD. clinicaltrials.gov, identifier NCT00483106.