People with ADHD have an unusually large number of "copy number variants" or CNVs -- chunks of DNA that are either missing or duplicated, says study researcher Anita Thapar, MD, professor of child and adolescent psychiatry at Cardiff University in Wales.
"These missing or duplicated chunks of DNA are in the areas of the chromosome that overlap with those implicated in autism and schizophrenia, [which are] established brain disorders," Thapar said at a news conference. "And we found that the most significant excess of these copy number variants was in a specific region ... that includes genes for brain development."
Thapar and colleagues analyzed genetic data from 366 children with ADHD ranging in age from 5 to 17 and from 1,047 matched children in the general population. They found that the kids with ADHD carried twice as many large CNVs.
This difference was even more pronounced in ADHD kids with intellectual disability. These children, with IQs of less than 70, had nearly six times more large CNVs than normal children. But regardless of intellectual disability, ADHD kids had significantly more large chunks of DNA that were missing and duplicated.
Because the findings don't identify a specific "ADHD gene," the kind of genetic analysis used in the study cannot be used as a test for ADHD.
"ADHD is a very complex disorder, which will have a number of different genetic factors involved, and also non-genetic, environmental factors. It is this combination that is the likely cause of ADHD," study researcher Kate Langley, PhD, a psychologist at Cardiff University, said at the news conference.
However, Thapar said the findings should be a great relief to parents and to people with ADHD who have been stigmatized by the disorder.
"There is a lot of misunderstanding about ADHD. Some say it is not a real disorder or that it is just the result of bad parenting," she said. "Our results show it should be considered a neurodevelopmental disorder like autism."
Indeed, kids with ADHD and kids with autism have some symptoms in common. This leads the researchers to suggest that there might be a biological link between the two disorders.
Thapar said that the study findings will spur more intensive research likely to uncover more specific information on the genetic factors that predispose a person to ADHD.
The study is reported in the Sept. 30 online issue of The Lancet.