Children with ADHD are missing certain sections of their genetic code, and other sections are repeated, which may contribute to the development of the condition, according to a new study.
The study is one of the first to identify genetic variations linked to attention deficit hyperactivity disorder.
Children with ADHD have trouble paying attention and are excessively impulsive and restless.
Scientists have suspected ADHD is, at least in part, due to genetics, because it runs in families. However, attempts to identify specific ADHD genes have been inconclusive. So current studies are now searching not for specific gene sequences that are different in people with ADHD, but instead for chunks of DNA that may be duplicated or deleted within the genome.
The new findings add weight to the idea that ADHD is not solely due to environmental factors.
"Too often, people dismiss ADHD as being down to bad parenting or poor diet. As a clinician, it was clear to me that this was unlikely to be the case," study author Anita Thapar, of Cardiff University in Wales, said in a statement. "Now we can say with confidence that ADHD is a genetic disease and that the brains of children with this condition develop differently to those of other children."
The study also found similarities between the genetic variants associated with ADHD and variants tied to other psychiatric disorders, including schizophrenia and autism , which suggests these conditions may have common roots.
The study was published today (Sept. 29) in the journal The Lancet.
Searching the genome
Thapar and colleagues examined the genomes of 366 children in the United Kingdom who had been diagnosed with ADHD, and compared them to the genomes of 1,047 people without ADHD.
They looked for so-called copy number variants, or CNVs, which are segments of DNA that are duplicated or missing. They limited their analysis to rare CNVs, those that are present in only a small portion of the population.
The results showed these missing and duplicated DNA segments were twice as common in children with ADHD than in those without ADHD. And these rare CNVs were five times as common in children who had both ADHD and mental disabilities (with IQs lower than 70).
Some of these CNVs overlapped with sections of DNA previously suspected of playing a role in schizophrenia and autism. While these conditions are thought to be entirely separate, ADHD does share some symptoms with autism, the researchers said.
"Our results suggest that there could also be a shared biological basis to these two childhood-onset disorders," they wrote.
Not all researchers were convinced by the findings. Dr. Josephine Elia, a psychiatrist at the Children's Hospital of Philadelphia, noted that the average IQ of the children with ADHD in the study was 86. The problem, Elia said, is that the control group in the study was a general group of men and women, born in the United Kingdom in 1958, who would have an average IQ of about 100. Because that is higher than the children's average IQ, and because rare CNVs are known to be common in people with lower IQs, rare CNVs would naturally be harder to find among the control group.
"It's difficult to believe these conclusions, given that the IQ levels of the control were not matched to this sample," said Elia, who has studied the genetic basis of ADHD. "And they themselves are showing that you get more of these CNVs with lower IQs, so attributing it to ADHD doesn't make sense."
However, Elia said studying CNVs remains an important research avenue. It may turn out to be not the number of CNVs, but rather which genes are missing or duplicated that plays a more important role in the development of ADHD, she said.
The new study was funded by the Wellcome Trust.