A protective effect in females may help explain one of the biggest mysteries of autism: Why boys are five times more likely to develop the developmental brain disorder than girls.
A new, preliminary study suggests that developing females are much better able than males to fight off genetic pressure to develop symptoms of autism.
The findings aren't definitive and don't point to a treatment or cure. Still, "first steps like this are important" and could lead to greater understanding of autism, said study lead author Elise Robinson, an instructor in the department of medicine at Harvard Medical School in Boston.
An estimated 1 in 88 children in the United States has an "autism spectrum
disorder." The condition, which can range from mild to severe, is characterised
by impaired communication and social interaction. Prevalence of autism is
increasing in the United States, although it's not clear if that's largely
because there's more awareness of the disorder.
Boys more prone than girls
According to the US Centers for Disease Control and Prevention, boys are almost five times more likely than girls to have a form of the disorder - 1 in 54 boys compared to 1 in 252 girls.
There are many theories about why this autism "gender gap" exists. One idea is that males are threatened in the womb by exposure to testosterone, the male hormone. Another theory holds that females might be somehow inherently better protected against the threat of the condition.
This may fit in with the idea that males are weaker in the womb than females. Boys are generally more prone to develop neurological disorders than girls, noted Dr Andrew Zimmerman, director of clinical trials at the Lurie Center for Autism at Massachusetts General Hospital.
"We poor males are vulnerable," he said. "Girls have two X chromosomes, so if there's a problem on one, they have a spare." On the other hand, males have one X chromosome and "a Y chromosome that has very few genes," added Zimmerman, who was not involved in the study.
How the study was done
In the new study, published online, researchers aimed to figure out how females fared who were born into families that appeared to have a higher genetic risk of autism symptoms.
The study authors looked at more than 3 800 pairs of non-identical twins from Great Britain and more than 6 000 pairs of non-identical twins from Sweden. They then tried to figure out how a family risk of autism symptoms (not diagnosed autism itself) affected the twins.
They found evidence that it takes greater family risk - meaning a higher genetic load - for a girl to develop autism symptoms. In other words, girls appeared to be more resilient against the threat of autism symptoms compared to boys.
"There's more pressure on them to get it, and once they get it, it's more obvious than in their relatives," said Zimmerman.
Genetics researchers should spend more time studying families with autistic girls in order to better understand the disorder, the research team concluded. This gender-specific research might also help identify ways to prevent autism, they said.
For more about autism see the US National Library of Medicine.