Molecular genetic studies of ADHD: 1991 to 2004.

Attention deficit hyperactivity disorder (ADHD)is highly heritable but is likely a complex disorder involving multiple genes of moderate effect (Smalley [1997: Am J Hum Genet 60:1276-12821]). Over 100 studies have examined the genetics of ADHD by linkage or association, though no article has presented a comprehensive overview of all published reports. We reviewed all ADHD studies, including 3 genome-wide linkage studies, and association studies of 94 polymorphisms in 33 candidate genes. To simplify comparisons across heterogeneous articles, demographics and comorbidity were ignored; analyses of subtype and haplotypes were excluded; and only the most positive finding for each polymorphism in a study was reported. Thirty-six percent of all findings were positive (P< 0.05), 17% were trends (0.05

0.15). Studies utilizing dimensional measures of ADHD tended to result in higher rates of positive findings than those using categorical diagnoses (X(2) = 5.6, P = 0.018), and case-control studies tended to result in higher rates of positive findings than family-based studies (X(2) = 18.8, P < 0.001). However, for either dichotomy, no significant difference remained when analyzing only studies using both methods within the same population and polymorphism. Evidence for association exists for four genes in ADHD: the dopamine D4 and D5 receptors, and the dopamine and serotonin transporters; others are promising but need further replication, including the dopamine D2 and serotonin 2A receptors. All candidate gene approaches continue to face the problem of relatively low power, given modest odds ratios for even the best replicated genes.