Researchers have found evidence that suggests autism spectrum disorder (ASD) – conditions characterised by impaired social ability, repetitive behaviours, and abnormal reactions to sensory stimuli – are not merely the result of defects in brain development - a possibility that scientists have been mulling over for years.
Instead, at least some effects of ASD go beyond the brain, and could impact the peripheral nerves that send sensory information from limbs to the brain, the researchers suggest. This means that other parts of the nervous system - besides the brain - could partly be responsible for the way people with ASD experience and react to stimuli.
The new study, led by David Ginty from the Harvard Medical School, was performed on mice, and looked at several genetic mutations that are commonly found in people with ASD.
Two of these variants affect the genes Mecp2 and Gabrb3. Both of these genes are known to impact synaptic function in the brain, meaning they're essential for neurons to communicate. The variants are typically found in people with ASD.
"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," Ginty said in a statement. "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."
When the team bred mice with these genetic mutations, they noticed that they could no longer use their sense of touch to navigate different objects, and also responded differently to puffs of air being blown on their necks. Based on this reaction, the team suggests that the altered mice experience touch in a completely different way from regular mice.
Next, they studied how these genes affected social interactions – one of the hallmark symptoms of ASD – by introducing genetically altered mice to other mice and examining how often they ventured into an open space inside their habitats.
They found that the altered mice showed signs of heightened anxiety compared to the unaltered mice. The team says this anxiety could stem from their altered sense of touch.
How does this work? The experience of touch and other sensory stimuli might be overloading the mice with information, causing them to get overly nervous for what seems like no reason, though the team is still investigating this hypothesis.
"A key aspect of this work is that we've shown that a tactile, somatosensory dysfunction contributes to behavioural deficits, something that hasn't been seen before," Ginty said. "In this case, that deficit is anxiety and problems with social interactions."
The team’s findings suggest that ASD might not just be brain related. Instead, genetic mutations could make those with ASD actually feel and sense the world differently – on a physical level – than those without it.
The best way to think about these findings is to imagine that your sense of touch has a volume knob. For most of us, this knob would be turned to, say, five. For someone with an ASD, it might be as high as 10, making everyday tasks feel heightened to the point of chaos. The team is basically saying that this 'volume knob' is controlled by genes, not a brain condition.
"The sense of touch is important for mediating our interactions with the environment, and for how we navigate the world around us," said one of the team, Luren 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 behaviours observed in patients with ASD."
It’s important to point out that the study was only done with mice. While these results are promising, they need to be replicated in humans for them to have relevance for future treatments.
But it's an intriguing insight into what could be going on in a condition that we still don't really understand, and which affects roughly 3.5 million Americans.
The study has been published in the journal Cell.