What Causes Autism? New Research Unlocks More Secrets

A number of disorders exist on the autism spectrum (ASD). These include autism, pervasive developmental disorder not otherwise specified, and Asperger syndrome. ASD holds its secrets closely, but researchers are working hard to understand its causes and find ways to improve the lives of those who have the disorder, and the families who love them.

People with ASD have a different way of learning, paying attention or reacting to things. The ability to learn, think and problem solve varies greatly in people with ASD, from gifted to severely challenged. They also show differences in the way they relate to people and the way they communicate or deal with emotion. The severity and combination of symptoms can vary vastly from person to person, but the symptoms are likely to include:

  • a resistance to change,
  • difficulty adapting to changes in routine,
  • repetitive actions,
  • repetitive play,
  • repetition of words or phrases,
  • little or no interest in other people or objects,
  • may show interest in people but not able to relate to them,
  • difficulty understanding other people’s feelings and expressing their own,
  • avoids or resists being cuddled or seem to ignore people when spoken to, but responsive to other sounds,
  • difficulty expressing what they want,
  • unusual reactions to the way things look, sound, smell, taste or feel,
  • obsessive interests,
  • prefers to ply alone,
  • difficult to comfort during distress,
  • reverses pronouns (‘you’ instead of ‘I’),
  • does not play pretend games.

What Causes Autism?

We don’t know exactly what causes ASD. Up to now, differences in brain development have been thought to be the cause. New research, published in the journal Cell, has found that there seems to be more to it than that.

A study in mice has found that some symptoms of ASD, such as touch perception, anxiety and social difficulties, are caused by problems with the nerve cells that send sensory information (such as information about touch) to the brain. They are the nerves that are found in the arms and legs, fingers and toes, and other parts of the body. (Researchers often use mice in their studies because of genetic and biological similarities between mice and humans.)

It is as though the volume of these nerve cells is turned up, so the sensation of touch is exaggerated and intense. This seems to lead to anxiety and the behavioural problems that are often associated with ASD.

“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.” David Ginty, Professor of Neurobiology at Harvard Medical School.

The Research. What they did.

Though the exact cause of ASD is unknown, there does seem to be a genetic basis. Exactly how this genetic vulnerability leads to the development of ASD is unclear, and this is where the work lies for researchers. Is there a specific combination of genes? Do the gene mutations interact with something in the environment? So many questions, but researchers are getting closer to uncovering more of the secrets of ASD.

As part of the study, researchers looked at a number of genes mutations that are known to be associated with ASD in humans.  They genetically engineered the mice to have these mutations only in the cells of their peripheral sensory nerve cells. These are the nerve cells in the extremities of the body – arms, legs, fingers toes.

They also looked at two other genes that have been associated with behaviours that are typical of ASD. These genes are crucial for nerve cells to function normally, and previous research has connected the mutations to problems with the way nerve cells communicate with each other.

(For the scientific ones out there, researchers were looking at mutations in the Mecp2, Gabrb3, Shank3, and Fmr1 genes.)

“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 … 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.” David Ginty.

Sensitivity to touch.

The researchers looked at how the mice reacted when they were touched gently. In the study, the touch was from a gentle puff of air on their backs. The study also explored whether the mice could tell the difference between objects that had different textures.

The mice that were bred to have the ASD gene mutation in only their sensory nerve cells showed:

  • a heightened sensitivity to touch;
  • an inability to tell the difference between textures;
  • an abnormality in the transmission of impulses between the nerves in the skin and spinal cord – these are the nerves that send touch signals to the brain.
Anxiety and Social Interactions

The researchers then turned their attention to anxiety and the way the mice interacted socially. They looked at how much the mice avoided being out in the open and how they interacted with unfamiliar mice.

The mice that were bred to have the ASD gene mutations showed heightened levels of anxiety. They also interacted less with the mice they hadn’t seen before.

‘A key aspect of this work is that we’ve shown that a tactile, somatosensory dysfunction contributes to behavioral deficits, something that hasn’t been seen before … In this case, that deficit is anxiety and problems with social interactions.’ David Ginty.

The research has revealed the ‘what’, but the ‘how’ is still vague. What we know is that the mutations in the sensory nerve cells cause problems for the way the body interprets touch. This seems to contribute to anxiety and social problems, but exactly how it contributes isn’t yet clear.

‘Based on our findings, we think mice with these ASD-associated gene mutations have a major defect in the ‘volume switch’ in their peripheral sensory neurons,’ Dr Lauren Orefice, researcher.

Because the volume of these nerve cells seems to be turned all the way up, the sensation of touch is strong and severe.

‘The sense of touch is important for mediating our interactions with the environment, and for how we navigate the world around us … 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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