More genetic mutations may be needed to give rise to autism in girls than in boys. The finding supports the notion that the female brain is somehow protected against autism, and this may in turn explain why four times as many males have autism than females.
Although some cases of autism are associated with one mutation, most are thought to involve several genetic abnormalities. In the past few years, hundreds of mutations have been discovered that can make people more vulnerable to the condition.
To see if the mutations affect men and women differently, Sbastien Jacquemont at the University Hospital of Lausanne in Switzerland and colleagues measured the frequency of two different kinds of mutation in 762 families that had a child with autism.
Among the children with autism, one class of mutation known as a copy number variation deletions or duplications of a large chunk of genetic material was three times more common in girls than in boys. The team also found that substitutions of a single letter of DNA were about one-third more common in affected girls.
Jacquemont says this suggests it takes more mutations for autism to arise in girls than in boys. "Females function a lot better than males with similar mutations," he says.
The results reflect the "shielding" effect of being female, he says. "There's something that's protecting [their] brain development."
A larger, as yet unpublished, study of about 2400 people with autism, conducted as part of the Autism Genome Project - an attempt to sequence the whole genome of 10,000 individuals affected by the condition has produced similar results, says Joseph Buxbaum of Mount Sinai Hospital in New York.
The hope is that understanding the mechanism behind the "female protective effect" can be translated into a treatment for autism, says Buxbaum, who was not part of Jacquemont's study. "That's why we are doing this research."
Jacquemont says the finding is compatible with the controversial idea that autism represents an extreme version of the male brain, first put forward by Simon Baron-Cohen at the University of Cambridge. According to the theory, males have, on average, more tendencies that could be thought of as mildly autistic so it makes sense that it would take fewer mutations for autism to arise.
Journal reference: The American Journal of Human Genetics, DOI: 10.1016/j.ajhg.2014.02.001
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Fewer mutations needed for autism in boys than girls