TH is the rate-limiting enzyme in DA synthesis. Dahmen and colleagues (2005) showed an increased frequency of the Val allele (Val81Met polymorphism) of TH (11p15) in patients with early-onset AD. Association studies of ADHD have been negative (Faraone and Khan, 2006), and no rigorous reports have examined any of the other disinhibitory traits. The final enzyme in the synthesis pathway, DDC, converts L-DOPA to DA. No published studies have reported an association with DDC (7p12.2) and AD or any disinhibitory phenotypes that we studied, including ASPD, ADHD, CD, DD, or NS.
