The brain isn’t only source of autism symptoms, study finds

For years, autism and neurodevelopmental disorders like it have been treated as though they were caused exclusively by issues with brain development, but new research published in the latest edition of the journal Cell suggests that there may be other factors at play.

According to senior author David Ginty, a professor of neurobiology at Harvard Medical School and a Howard Hughes Medical Institute investigator, and his colleagues, experiments conducted in mice revealed that some aspects of the disorder, including anxiety, touch perception, and some of the social abnormalities, are associated with issues elsewhere in the nervous system.

In fact, his team’s research has demonstrated that peripheral nerves found throughout the limbs, digits, and other parts of the body, which communicate sensory information to the brain, are also significant contributors to the symptoms associated with autism spectrum disorders (ASDs).

“An underlying assumption has been that ASD is solely a disease of the brain, but we’ve found that may not always be the case,” Ginty explained in a statement. “Advances in mouse genetics have made it possible for us to study genes linked to ASD by altering them only in certain types of nerve cells and studying the effects.”

Elevated sense of touch may explain some ASD-related behaviors

As part of their work, the study authors examined the effects of genetic mutations known to be linked to ASD in humans, including Mecp2, which a disorder known as Rett syndrome which is often linked to ASD, and Gabrb3. These genes are believed to be vital for nerve cells to function normally, and mutations in them have been linked to issues with synaptic function.

“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,” explained Ginty. “By engineering mice that have these mutations only in their peripheral sensory neurons, which detect light touch stimuli acting on the skin, we’ve shown that mutations there are both necessary and sufficient for creating mice with an abnormal hypersensitivity to touch.”

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He and his colleagues measured how mice reacted to touch stimuli, as well as their ability to tell the difference between the textures of different objects. Mice that had ASD gene mutations only in the sensory neurons were more sensitive to touch and could not discriminate between textures. They also had abnormalities in the transmission of neural impulses from touch-sensitive neurons in the skin and in the spinal cord neurons that rely touch-related signals to the brain.

Having established that mice with ASD-associated gene mutations had tactile perception issues, the scientists went on to evaluate their anxiety and social interaction levels by testing the mice to see how vigorously they would avoid being out in the open and how well (or poorly) they would interact with other mice. They found that the creatures that had autism-related mutations only in their peripheral sensory neurons had higher anxiety levels and interacted less with other mice.

“Mice with these ASD-associated gene mutations have a major defect in the ‘volume switch’ in their peripheral sensory neurons,” causing them to feel touch at a highly elevated level, explained first author Lauren Orefice. “An abnormal sense of touch is only one aspect of ASD, and while we don’t claim this explains all the pathologies seen in people, defects in touch processing may help to explain some of the behaviors observed in patients with ASD.”

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Image credit: Thinkstock

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