Autism impacts kids and adults socially and behaviorally — but it also impacts their senses. For instance autistic individuals may dislike being touched, or be especially averse to things like scratchy wool or buzzing refrigerators.
New research, for the first time, gives a hint as to why. A study in mice finds that autism has effects not just on the brain but also on the nerves in the skin that respond to touch. And this heightened sensitivity affects not just their sensory experience of the world but also their social interactions.
David Ginty and colleagues at Harvard Medical School studied mice with genetic mutations that are associated with autism and also with a developmental syndrome called Rett syndrome that often causes autism. Mice with these mutations share some similarities to patients with autism and Rett syndrome — they’re anxious, avoid social contact, and have a hypersensitivity to touch.
The team engineered mice with the mutation in only some of their cells, in either the brain or in touch receptor cells throughout their skin that go into the spinal cord.
When they tested the mice’s reaction to puffs of air on their backs, they found that the Rett syndrome mutation in the nerves caused them to be especially startled, which wasn’t true if the mutation was just in the brain. Looking at the cellular level, scientists saw that the touch nerves were overactive. “Everything’s coming in with the volume turned all the way up,” Ginty said.
And over time these mice don’t acclimate to touch the way that normal mice do — akin to feeling your clothes on your skin all the time, Ginty said. “We would argue that’s pretty disruptive.”
Perhaps most intriguingly, the mutation just in touch cells had dramatic effects on the mice’s social lives.
When the researchers placed the mice in cages with a strange mouse, the mice with touch sensitivity were as antisocial as mice with the gene mutation in their whole bodies.
That is one of the most fascinating questions the study raises, Ginty said: “What is it about having normal sensation that shapes the brain to give rise to normal cognitive behavior?”
The findings were published Thursday in Cell.
“I’m very excited about this study,” said Caroline Robertson, a postdoctoral autism researcher at Harvard University who was not involved in the study “Importantly, it demonstrates a direct link between very low level symptoms,” like touch sensation, “and higher order differences in behavior,” like antisocial behavior.
However, Robertson points out that autism in humans is more complicated than a single mouse model. “The tricky thing about autism is that everyone has their own hypersensitivities, and sometimes even hyposensitivities,” or things they have reduced sensitivity toward.
Helen Bateup, a professor of neurobiology at the University of California, Berkeley, agrees. “There are a lot of autism genes that are only expressed in the brain,” and therefore wouldn’t affect the nerves in the skin. “I don’t see how this could be a general mechanism for all of autism. [But] this might be an interesting mechanism for some kinds of causes of autism.”
Its meaning for treatment is less clear at this point. Ginty is looking into it: “Can we add back the gene in sensory neurons and reverse the tactile dysfunction — and maybe even the anxiety-like behavior?” A drug therapy could also theoretically tamp down overactive touch nerves, and maybe reduce touch sensitivity, though whether that would affect social interactions would need to be studied.