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Children with ADHD may benefit when their parents and preschool teachers learn strategies to control problem behaviours. ((George Widman/Associated Press))

Children with attention deficit hyperactivity disorder are more likely to have extra or missing DNA compared with normal children, British researchers have found.

The study compared the genomes of 366 white British children from five to 17 years old with attention deficit hyperactivity, or ADHD, to those of more than 1,000 similar children without the disorder.

ADHD affects about around one in 50 children, and makes them restless, impulsive, and distractible. Currently there is no cure for the condition, but symptoms can be treated with a combination of medications and behavioural interventions.

Among children without ADHD in the study, about seven per cent had deleted or doubled DNA segments. Among those with the disorder, about 14 per cent showed the duplications or deletions — the first direct evidence that that ADHD is a neurodevelopmental disorder, the researchers said.

There were also major overlaps between these segments, which are known as copy number variants or CNVs, and those linked with autism and schizophrenia.

ADHD isn't caused by a single genetic change but is likely caused by a number of genetic changes (including CNVS) interacting with environmental factors, said study author Dr. Kate Langley of the MRC Centre in Neuropsychiatric Genetics and Genomics and Department of  Psychological Medicine and Neurology, at Cardiff University, U.K.

Genes and brain development

It is too early to tell if the findings could affect diagnosis or treatment, said study author Anita Thapar, a professor at the centre.

The findings were made in people of European Caucasian descent and its not known if the results apply in other ethnicities.

The researchers said they hope the findings help to clear up misunderstandings about ADHD so those affected and their families face less stigma.

"This knowledge will eventually enter the clinic and might affect the way people think about and treat neurodevelopmental disorders by accounting for the biological consequence of the specific patient's genotype," Dr. Peter Burbach of the department of neuroscience and pharmacology at Rudolf Magnus Institute of Neuroscience at University Medical Center in Utrecht, Netherlands, wrote in a journal commentary accompanying Wednesday's study in The Lancet.

Environmental factors such as severe social deprivation at an early age should still be considered a cause of the disorder, said  Philip Asherson, a professor of molecular psychiatry at the Institute of Psychiatry at King's College London.

"The study doesn't tell us a lot about what's going on in the brains of people with ADHD," said Asherson. "If we can find out more about these genes and how they affect brain development, that may give us inroads, but it's hard to say when that will be."

The study was paid for by Action Research, Baily Thomas Charitable Trust, the Wellcome Trust, Britain's Medical Research Council and the European Union.

With files from The Associated Press