Genetics of Childhood Disorders: XX. ADHD
Is ADHD Genetically Heterogeneous?
Reviews of the literature leave no doubt that genes influence the etiology of attention-deficit/hyperactivity disorder (ADHD) (Faraone et al., 1998). Notably, twin studies show the heritability of ADHD to be about 0.80, indicating that the effect of genes is substantial.
These genetic epidemiological studies have motivated molecular genetic studies of ADHD that have produced intriguing but conflicting results (Faraone and Biederman, 1998).
Researchers have focused on genes in dopamine pathways because animal models, theoretical considerations, and the effectiveness of stimulant treatment implicate dopaminergic dysfunction in the pathophysiology of the disorder.
Two genes that have been intensively studied are the dopamine transporter gene (DAT) and the dopamine D4 receptor gene (DRD4). Some studies of these genes strongly suggest that they influence susceptibility to ADHD. There are, however, several negative studies for each gene.
The inconsistent results from molecular genetic studies could mean that rather than being a unitary disorder, ADHD comprises several disorders having different genetic and nongenetic etiologies.
Author/s: Stephen V. Faraone
Issue: Nov, 2000