Autism researchers are a little closer to solving the mystery of why autism is four times more common among males than females.

An international team of researchers led by Stephen Scherer at the Hospital for Sick Children in Toronto describes the new genetic clue in Thursday's online issue of the American Journal of Human Genetics.    


Bradley, centre, who turns three in a couple of weeks, was diagnosed with mild autism. (Tony Gutierrez/Associated Press)

"I think the key to actually developing a treatment for autism is to understand the difference between males and females, why female prevalence is much lower," Scherer said in an interview. "They must have some protective factors in their biology. So we think that this finding may lead us to discovering what those protective factors are."

When investigators analyzed genetic data from 1,158 Canadians and 456 Europeans with autism spectrum disorder, they found a rare family with four generations in which males carrying the glitch were affected but females were not.

The males had high-functioning autism spectrum disorder, showing high IQ and good verbal ability with poor social communication.

When male fetuses are missing one copy of the gene, it throws off their developmental process enough to lead to autism but female biology differs enough that it doesn't matter.

The discovery is important because now that doctors know that the SHANK 1 gene is involved and it can be tested for, they'll know to follow affected males very closely and offer treatments early on.

"These results will also have immediate relevance for clinical genetic testing in autism spectrum disorder," the study's authors concluded.

If 100 people diagnosed with autism spectrum disorder are given a full battery of genetic tests, Scherer estimated they could now account for the genetic cause in 20 per cent of those cases.

"We do have a handle on a significant proportion now," Scherer said.

On its own, SHANK1 would account for 0.1 per cent of the affected population, he said. It's a reflection of how scientists are chipping away at understanding what contributes to autism even though each individual is a "genetic snowflake" with a unique form.

Researchers have identified three SHANK genes that are involved in how neurons are formed and interact with each other in the brain.

The study was funded by the University of Toronto McLaughlin Centre, NeuroDevNet, Genome Canada/Ontario Genomics Institute, the Canadian Institutes of Health Research (CIHR), the Canadian  Institute for Advanced Research (CIFAR), the Canada Foundation for Innovation, the Ontario Ministry of  Economic Development and Innovation, Autism Speaks and SickKids Foundation.

With files from CBC's Amina Zafar