Mar 16 2007
Two recent studies bolster our knowledge of the genetics of autism. One, published this week in Nature Genetics, is the product of the Autism Genome Project – a consortium of scientists putting the Human Genome Project to good use. They have identified numerous genes associated with autism risk. The second looks at the effects of spontaneous mutations and autism risk.
Autism, or autism spectrum disorder (ASD) is a complex neurological disorder that typically presents in toddlers and is characterized by poor social ability and may include abnormal movements and cognitive deficits. The spectrum ranges from barely detectible to severely disabling. ASD is at the center of swirling controversies concerning its cause(s) and treatment, with the mainstream scientific community often at odds with various ideological groups.
The Nature Genetics paper, written by Ami Klin and Dr. Fred Volkmar from my own institution of Yale, found that there are 5-6 major genes and about 30 other genes that have alleles that are strongly associated with the risk of developing autism. The next step is figuring out exactly what proteins each of the genes code for, what these proteins do, and how the variants associated with autism result in brain dysfunction.
The second study is also the product of a huge collaboration, between lead authors Drs. Jonathan Sebat and Michael Wigler with 30 other colleagues. They found that the rate of mutations in certain genes is much higher is sporadic cases of autism (about 10 times the rate in healthy controls) compared to autism where there is a prior family history (about twice the rate of healthy controls). This suggests that these new mutations are causally related to autism in these sporadic cases.
Of course, these two studies are just the most recent bits of evidence in a very large and growing body of scientific evidence clearly demonstrating that ASD has genetic roots. However, the exact relationship still needs to be elucidated. First it needs to be understood that ASD is likely not a single disease but rather a groups of diseases possibly with various underlying causes. Neurological disorders, like ASD, are determined by which part of the nervous system is not working – not by the mechanism of damage. So there may be multiple different causes resulting in the same populations of nerve cells or structure(s) in the brain malfunctioning, and therefore the signs and symptoms will be the same or similar.
So, it is possible that a subset of ASD is caused by genetic variants or mutations, but that another subset has a distinct cause – inflammatory, environmental, developmental, etc. It is also possible that certain genetic types are predisposed to environmental triggers. However, non-genetic causes or contributors to ASD remain highly speculative and controversial. Although the press tends to focus on these issues, including a lengthy article in the current issue of Discover magazine, the fact is that evidence for any particular environmental cause remains scant.
Perhaps the most controversial aspect of this debate has been the claim by some that ASD is caused by vaccines. First it was blamed on the MMR vaccine itself, and than later on thimerosal, a mercury based preservative in some childhood vaccines (although removed several years ago). So far the evidence is strongly against any association between vaccines and autism, but this notion has its dedicated adherents, including, of course, the anti-vaccine crowd.
At the center of much of this controversy is also the question of what is the incidence of ASD and is it increasing. It is true that the rates of ASD diagnosis have increased significantly in the last 20 years. Promoters of a vaccine or other environment cause of ASD claim that the rise in diagnosis represents a real epidemic – and this would strongly support an environmental over a genetic cause.
However, the mainstream scientific view is that the increase is primarily the result of increased surveillance and a broadening of the definition (although a small real increase cannot be ruled out). The data seems to support this conclusion, but as more detailed information is gathered over time we will be able to make more accurate statements about true incidence.
Autism is a serious neurological disorder that deserves resources to research and treat. The research is very promising and hopefully will lead to real advances soon. However, it is distressing that so many ideologues have targeted autism for campaigns of misinformation, conspiracy mongering, and pseudoscience. They have largely placed themselves at odds with the scientific and medical community, and if anything, despite their intentions, are hampering progress.
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