Women are able to carry higher levels of genetic defects without getting brain development disorders such as autism, supporting the possibility of a "female protective effect," finds a new study.
The study gives clues as to why 50 percent more males typically have an intellectual disability than females, and why boys are four times more likely to have autism than girls.
Professor Evan Eichler of the University of Washington, USA, led a Swiss-U.S. collaboration which published their results in The American Journal of Human Genetics today.
The team looked for genetic defects in 15,585 people who had been diagnosed with a range of disorders all thought to be due to faulty brain development -- known as neurodevelopmental disorders. Some of these people had autism spectrum disorders but they were not separated out from the rest.
The defects they were looking for were large "copy number variants" (CNVs) -- sections of chromosomes carrying perhaps a dozen genes which are either missing, or present as multiple copies.
Surprisingly, the females in the sample had more CNVs than the males. Although both sexes in the study had neurodevelopmental disorders, the females were carrying a bigger "burden" of genetic damage.
Eichler says this fits with females somehow being better protected from the effects of the CNVs. "It takes a lot more mutational hits to make a woman cross the threshold for a disorder," he said.
"The next question was... do you see this in autism?" said Eichler.
The team then focused on autism alone using a separate group of 762 families with autism spectrum disorders. The females in this group carried an even greater burden of CNVs.
These women were also more likely to carry tiny harmful mutations, affecting just a couple of base pairs in the DNA, than the men in the group.
Why are males more vulnerable?
Eichler now wants to do more research on thousands rather than hundreds of people with autism spectrum disorder, to validate and extend the results. He'd like to pinpoint the genes that most put people at risk of the disease.
As to why females are protected - he speculates that hormonal influences may be involved and adds "there is some data to suggest that we are hard-wired differently, but this is not my area of expertise."
And females are "genetically more robust because they have two X chromosomes while males are stuck with a single X and all the mutations...that are on it."
Commenting on the paper, Professor Cheryl Dissanayake, director of the Olga Tennison Autism Research Centre at La Trobe University, says she is not a geneticist but believes the paper sheds light on the question of why males are more affected than females.
"We've often speculated about a 'female protective factor' but never really understood why that might be."
"This is basically saying females need more genetic damage to have autism or a neurodevelopmental disorder than do the males -- it provides a clue."
The question that now needs to be answered, she agreed, is "what makes males more vulnerable to these genetic defects than females?"