Revisting The Genetics Of Autism

This past month, a new twin study of autism spectrum disorder (ASD) was published in the Archives of General Psychiatry. And it may have a profound effect on future research on ASD. etiology

The key contribution of this study was that it suggested that environmental factors are a major source of influence on ASD, even after controlling for genetic factors. More specifically, the authors found evidence for substantial shared environmental effects. What this means is that a large portion of twin similarity for ASD was attributable to sharing a common environment rather than common genes. The implication is that exposure to some type of environmental influence has a strong effect as a risk factor for ASD.

Having experience in analyzing and publishing data from twin studies, I’d like to explain why this paper is so influential. First, it is based on a large sample, which gives us confidence that the results are solid and not due to chance findings. Second, the sample is community-based, meaning that it is representative (there isn’t some type of sampling bias that would skew the results). Third — and this is a big one — it uses modern diagnostic criteria and hence provides the most relevant picture on the genetic epidemiology of autism (particularly as prior twin studies used older criteria). And lastly, the conclusion is radically different than that drawn from prior twin studies, which suggested that ASD was primarily due to genes.

The prior twin studies always rested on a surprising finding: that dizygotic (or fraternal) twins were essentially uncorrelated for ASD. This was strange, as sibling studies always showed evidence of increased rates of ASD (and keep in mind, of course, that dizygotic twins are — genetically speaking — full biological siblings). What the new twin study found was a much higher concordance rate for the dizygotic twins as compared to previous studies. So even though the concordance rate for monozygotic (or identical) twins was greater than that found for dizygotic twins (which is consistent with a genetic etiology), the difference in concordance rates was less than would be suggested based on genetics, and hence the conclusion that non-genetic (or environmental) factors play a critical role in ASD.

Given all this, the reason this paper is so important is that it suggests that focusing exclusively on the genetics of autism would miss a major source of influence on this disorder. The twin study certainly confirmed a role for genetics and hence supports the intensive molecular genetic studies of ASD. That said, this research recalibrated how important genes may be and pointed to non-genetic effects as a key etiological factor. This is the kind of ground breaking study that can reorient a field and suggest that environmental influences become an intensive focus of research (do keep in mind that this twin study did not examine specific sources of this environmental effect). This message is very important because biomedical research in general has become “genome-centric.” The double helix is a critical piece of the ASD puzzle, but it’s not the only piece — and in fact in might not be, in the end, the largest piece either.

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