Researchers in Toronto have found a genetic clue that helps explain why autism affects four times more boys than girls.
They discovered a genetic glitch on the X-chromosome that appears to play a role in autism spectrum disorder, or ASD, a range of conditions which affect communication and social interaction. It often causes intellectual deficits, and in some cases, is so severe children can't speak. ASD affects an estimated one in every 280 girls, but one in 70 boys.
"The male gender bias in autism has intrigued us for years and now we have an indicator that starts to explain why this may be," say Stephen Scherer, senior scientist and director of the centre for applied genomics at the Hospital for Sick Children in Toronto.
He and his colleague John Vincent at the Centre for Addiction and Mental Health in Toronto found that about 1 per cent of boys with ASD in their large study had a mutation on a gene called PTCHD1 on the X-chromosome. A similar mutation was not found in thousands of boys tested who didn't have autism. Sisters carrying the same mutation were also unaffected.
The reason that boys are at a high risk of developing ASD from the mutation, the scientists explained, is because they inherit an X-chromosome from their mother and one Y-chromosome from their father. If a boy's X-chromosome is missing the PTCH1 gene or other nearby DNA sequences, they don't have a fall-back X-chromosome. Girls with the mutation, on the other hand, always carry a second X-chromosome which protects from getting ASD. Autism, however, could appear in subsequent generations of boys in their families.
Scientists believe ASD is triggered in the majority of individuals through a series of mutations and environmental factors, but a minority, explained Dr. John Vincent, who led the project, may get it from a single mutation. This new discovery, he said, "is one step, but a step in the right direction nonetheless, another piece in the genetic puzzle."
One of the issues with ASD is that it is often diagnosed later in childhood, when interventions and treatment may not work as well. By next January, the research team hopes to have a diagnostic test that will allow boys with the disorder to learn if they have the mutation – if so, any younger brothers could also be tested at a very young age.
"That would direct clinicians to early interventions and early therapies, which would be far more effective if carried out sooner rather than later."
With a report from Anne McIlroy
Erin Anderssen