Autism is not just a disorder of the brain, according to a new study which suggests that at least some aspects of the disorder – including how touch is perceived, anxiety and social abnormalities – are linked to defects in another area of the nervous system.
Autism spectrum disorders (ASDs) are characterised by impaired social interactions and repetitive behaviours, often accompanied by abnormal reactions to sensory stimuli. ASD is generally thought to be caused by deficits in brain development, researchers said.
The study in mice now suggests that at least some aspects of the disorder – including how touch is perceived, anxiety, and social abnormalities – are linked to defects in another area of the nervous system, the peripheral nerves found throughout the limbs, digits, and other parts of the body that communicate sensory information to the brain, they said. “An underlying assumption has been that ASD is solely a disease of the brain, but we have found that may not always be the case,” said David Ginty from Harvard University in the US.
For the study, researchers examined the effects of gene mutations known to be associated with ASD in humans. In particular, they focused on Mecp2, which causes Rett syndrome, a disorder that is often associated with ASD, and Gabrb3, which also is implicated in ASD. They looked at two other genes connected to ASD-like behaviours as well. These genes are believed to be essential for the normal function of nerve cells, and previous studies have linked these mutations to problems with synaptic function – how neurons communicate with each other, researchers said. “Although we know about several genes associated with ASD, a challenge and a major goal has been to find where in the nervous system the problems occur,” said Ginty.
“By engineering mice that have these mutations only in their peripheral sensory neurons, which detect light touch stimuli acting on the skin, we have shown that mutations there are both necessary and sufficient for creating mice with an abnormal hypersensitivity to touch,” he said.
Researchers measured how the mice reacted to touch stimuli, such as a light puff of air on their backs, and tested whether they could discriminate between objects with different textures.
Mice with ASD gene mutations in only their sensory neurons exhibited heightened sensitivity to touch stimuli and were unable to discriminate between textures, researchers said. The transmission of neural impulses between the touch-sensitive neurons in the skin and the spinal cord neurons that relay touch signals to the brain was also abnormal, they said.
Researchers also examined anxiety and social interactions in the mice using established tests looking at how much mice avoided being out in the open and how much they interacted with mice they had never seen before. The findings were published in the journal Cell.