Gene-environment interaction in hyperkinetic conduct disorder (HD + CD) as indicated by season of birth variations in dopamine receptor (DRD4) gene polymorphism.

Recently, an interaction between season of birth and the expression of candidate genes has been suggested. Season of birth variations in tryptophan hydroxylase (TPH), the serotonin transporter (5-HTTLPR) and the dopamine D4 receptor (DRD4) gene polymorphisms are different for affective disorders and schizophrenia. The DRD4 gene has been postulated as a candidate gene for attention-deficit-hyperactivity disorder (ADHD), equivalent to hyperkinetic disorder (HD). The seven-repeat long variant of this gene (DRD4*7) in comparison to the short repeat variants of the DRD4 gene polymorphism, has been found to be associated with ADHD. A seasonal pattern of birth has also been proposed for different subtypes of ADHD. Therefore, in a subgroup of children with HD and conduct disorder (CD) and in healthy controls, we investigated a possible association between the DRD4*7 allele and HD + CD in association with the season of birth. Supporting this hypothesis, we found an interaction between the seasons of birth and the expression of the DRD4 candidate gene in children with HD + CD as well as in controls, which differ significantly from each other. Depending on the season of birth, children carrying the DRD4*7R allele showed different relative risks for developing HD + CD.