It is a question that has long stumped researchers.
But now light has been shed on why boys are more at risk of autism.
University of Iowa scientists believe they have collected the first ever evidence of a ‘protective effect’ in females.
Trials on mice showed males who had a known genetic cause of autism showed signs of being on the spectrum. However, females weren’t affected.
This genetic deletion, or a missing stretch of DNA, plays a role in one in every 200 cases of autism spectrum disorder (ASD), experts claim.
Figures suggest four boys are diagnosed with autism – which often causes sufferers to struggle with social interaction – to every one girl.
University of Iowa scientists believe they have collected the first ever evidence of a ‘protective effect’ in females which explains why they are less likely to be autistic
The new research suggests that the medical community are ‘on the right track’ in finding a cure for ASD.
The first evidence to date
Dr Nicola Grissom, lead author, said: ‘This is some of the first evidence in a mouse model of autism of a “female protective effect”.
‘These findings shed valuable new light on the science of neurodevelopmental disorders, many of which are more common in boys.
‘However, they also address the broader question of how sex and gender influence the neurobiology of how we learn and behave.’
Dr Ted Abel, senior author of the study published in Molecular Psychiatry, said: ‘We think we are on the right track.’
He said the findings, based on a signaling pathway in the brain, may explain why ‘neurodevelopmental disorders predominantly affect boys’.
WHAT DID THE RESEARCHERS FIND?
One of the genes contained in the missing section of DNA is an important signaling protein called ERK1.
Activity of this protein affects the function of the striatum – the part of the brain that’s involved in reward learning and motivation.
The researchers found male mice carrying this genetic deletion have increased activation of ERK1 in the striatum.
On top of this, they also had decreased amounts of another protein that reduces ERK1 activity.
In contrast, the female mice carrying the genetic deletion do not have over-activated ERK1.
In addition, despite the genetic deletion, the female deletion mice have higher levels of ERK1 than the male deletion mice.
All of these molecular differences mean that ERK1 signaling is particularly sensitive to disruption in male mice.
Charities estimate there is around 700,000 people who are on the autism spectrum in the UK. In the US, it is as high as 3.5 million.
Missing chunk of DNA
Nearly one in every 200 cases of autism is caused by the deletion of a section of DNA on a particular chromosome.
The mice involved in the study were missing this same stretch of DNA.
Mice were tested for any abnormalities in their reward-learning behaviour, which is disrupted in people with autism.
This type of learning is mediated by a part of the brain called the striatum.
Only the rodents with the autism-associated genetic deletion were found to display signs of this abnormal behaviour.
However, the findings, published in Molecular Psychiatry, were only true for males – as females appeared to be unaffected.
Signaling pathways affected
Further tests revealed some behavioural differences between male and female mice, linked to signaling pathways in the striatum.
The researchers believe that the disrupted reward-learning behaviour explains why those with autism struggle to interact socially.
Dr Abel, director of the university’s Neuroscience Institute, added: ‘They don’t find it rewarding in the same way.
‘It could explain why people with autism have restricted interests, because they find only very selective things rewarding.’