We also showed that large, rare CNVs identified in our ADHD cohort were significantly enriched for chromosomal loci previously implicated in autism and schizophrenia. ADHD is currently thought to be entirely separate from these disorders. However, there is some overlap between ADHD and autism in terms of clinical symptoms and cognitive deficits.33 Autistic traits and ADHD behaviours in the general population (not clinical disorder) also seem to be affected by shared heritability.34 Our results suggest that there could also be a shared biological basis to these two childhood-onset disorders. So far, possibly because of the dearth of relevant studies, there are no clinical or genetic data clearly pointing to overlap between ADHD and schizophrenia. In view of the strong evidence for association between duplications at 16p13.11 and schizophrenia,22 we note with particular interest that our ADHD cohort was significantly enriched for duplications at the same locus, a finding that was independently replicated in the Icelandic population. Moreover, further analysis of two duplications at 16p13.11 revealed that one was de novo, adding further support that this locus is functionally relevant to ADHD. Future studies analysing the segregation patterns of familial CNVs will be needed to estimate disease penetrance.
