Objective: To examine the joint effects of a dopamine transporter (DAT) polymorphism and maternal prenatal smoking on childhood hyperactivity-impulsivity and inattentiveness.
Study design: A cohort of 161 children was followed prospectively from age 6 months to 60 months. Primary outcomes were the DSM-IV hyperactive-impulsive and inattentive scales of the Conners' Parent Rating Scale Revised-Long Version (CPRS R:L). A secondary outcome was the oppositional scale. Predictors included DAT genotype and maternal report of prenatal smoking. Children homozygous for the 480-bp DAT allele (DAT +/+) were compared with all other children (DAT +/- or -/-).
Results: In multivariate analyses, children with both prenatal smoke exposure and the DAT +/+ genotype had significantly elevated hyperactive-impulsive scores (beta, 7.5; SE, 2.9; P<.01) compared with children with no smoke exposure and DAT +/- or -/-. Inattentive scores were not significantly elevated in this group, but oppositional scores were a full standard deviation higher. Neither prenatal smoke exposure alone nor DAT +/+ genotype alone was significantly associated with increased scores.
Conclusions: Child hyperactivity-impulsivity and oppositional behaviors were associated with a DAT polymorphism but only when the child also had exposure to maternal prenatal smoking. This study emphasizes the importance of incorporating environmental cofactors in genetic studies of attention deficit hyperactivity disorder.