What causes ASD?
We have learnt a great deal about the causes of autism spectrum disdorder in recent decades, but there is much more work that needs to be done. For example, we confidently know that it is not due to parentinig style, or vaccination.
Both genes and environmental influences are important, and there is also evidence that the ASD brain is wired differently.
Genetics
Autism spectrum disorder can sometimes run in families, indicating that genetics plays a key role in the disorder. Early research into the genetics of ASD began with studies of twins, both identical and non-identical. Scientists studied the concordance of autism spectrum disorder in these sets of twins – the percentage chance that if one twin had ASD, the other would too. They found a significantly higher concordance in identical twins, who have largely identical DNA, suggesting that ASD has a genetic basis. Under current estimates, there is a 50 to 80 per cent concordance for identical twins; this compares to an overall ~1.5 per cent chance that any two people will have ASD. If parents have one child with ASD, there is a 5 to 20 per cent chance that their second child will also have it.
We now know that as with numerous other traits, including height and intelligence, autism spectrum disorder involves many different genetic variations acting together. No single gene is enough to produce ASD, but the more mutations you have in specific parts of your DNA, the greater your chances of a diagnosis. To identify the exact regions of DNA associated with the disorder, scientists study large numbers of people affected by ASD. This approach is called a genome-wide association study (GWAS).
Environmental factors
As with genetics, no single environmental factor definitively causes ASD. Instead, a range of environmental influences seems to exist. For example, an epidemiological study led by QBI Professor John McGrath found that women deficient in Vitamin D during pregnancy had children with an increased risk of ASD.
A lack of Vitamin D is just one risk factor, but it can also interact with other environmental variables. Professor Thomas Burne, for example, is looking at how Vitamin D deficiency can combine with low levels of alcohol during pregnancy to raise the risk of ASD-like behaviours in mice.
Another environmental risk is viral or bacterial infection of the pregnant mother during the first trimester, which has become a common way to study ASD in model animals like mice. The laboratory of Professor Darryl Eyles has shown that the presence of Vitamin D protects against ASD-like behaviours that are triggered by infections of the mother. The work of Professors McGrath, Eyles and Burne converges in the long-term hope that by supplementing pregnant mothers with Vitamin D, ASD rates might be reduced.
Finally, more research by Professor McGrath has found that older fathers – particularly those over 50 at the time of conception – have a greater chance of having a child with ASD, potentially as a result of new genetic mutations that affect brain development.
Brain wiring
In the developing brain, connections between and within brain regions are still being formed. Sometimes, though, these connections don’t form in the normal way, which can produce lasting changes in behaviour.
One example relevant to ASD is a congenital condition called agenesis of the corpus callosum (AgCC). The corpus callosum is a thick bundle of nerve fibres that connects one brain hemisphere to the other. In people with AgCC, the corpus callosum is partly or even completely missing. Strikingly, one third of patients with AgCC meet the diagnostic criteria for ASD. What’s more, the genes involved in AgCC and ASD are similar. Other studies, however, have found the opposite profile in some people with ASD – an enlarged corpus callosum.
