Comordibidty as a means of identifying genetic hetrogeneity in ADHD

Attention Deficit Hyperactivity Disorder (ADHD) is a common and highly heritable condition with most estimates of h2 lying between 0.75 and 0.9. The very prevalence of ADHD makes it unlikely that it is genetically homogeneous and we have previously shown the DSM-IV subtypes are genetically distinct with only 6% of MZ twins pairs differing in subtype, compared with 12% of DZ twins. Up to 80% of ADHD children meet the criteria for another behavioral problem. Here we demonstrate the role of such comorbidity in identifying genetically distinct forms of ADHD. Data comes from the Australian Twin ADHD Project (ATAP) which now has information about 18,000 family members. 46% of children with the Inattentive or Combined types of ADHD have speech and/or reading problems. 25% of MZ twins have the same pattern of ADHD with or without speech problems, compared with only 13% of DZ twins, indicating genetic heterogeneity within the ADHD subtypes. A similar pattern was found for another common comorbidity, separation anxiety. The potential of this approach for genetically subdividing ADHD indicates the importance of identifying comorbid conditions and of population- based studies, since some comorbidities such as reading problems and conduct disorder may be barriers to recruitment.