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re:http://www.newscient...
http://www.newscientist.com/article/dn17041-gene-variant-found-in-65-of-autism-cases.html
Gene variant found in 65% of autism cases
18:00 28 April 2009 by Linda Geddes
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The quest to understand the genetic underpinnings of autism has taken a big stride forward with the discovery of a genetic variant carried by more than two thirds of people with the disorder.
Previous studies that have identified several genes that are implicated in autism, but Hakon Hakonarson of the Children's Hospital of Philadelphia, who led the new study, says they are extremely rare and account for a very small proportion of autism. "We have for the first time identified a common variant, present in over 65 per cent of autism cases."
To identify the gene, Hakonarson and his colleagues screened DNA from more than 2500 people with autism and more than 7000 healthy controls, searching for common gene variants that were associated with the condition.
The gene lies between two other genes called CAD 10 and CAD 9, which encode adhesion molecules that enable neurons to connect and communicate with one another – and CAD 10 is already known to be expressed in the very regions of the brain that appear to malfunction in people with autism.
Unknown factors
Because other, rarer genes associated with autism also encode cell adhesion molecules, researchers had previously speculated that a breakdown in neural connections during development might lie at the root of the condition. The current study provides the first genetic evidence that a similar problem could be implicated in a large number of cases, Hakonarson says.
However, although the variant is carried by around 65 per cent of people with autism, Hakonarson calculates that it only plays a central role in around 15 per cent of cases. In the remaining cases, other unknown factors may play a greater role, he says.
The variant is carried by around 60 per cent of people without autism. "There are probably numerous factors that cause autism, and these have to interact with environmental factors in order to express the [autistic] phenotype," Hakonarson says.
Simon Baron-Cohen of the Autism Research Centre at the University of Cambridge says that the new findings add to the very large number of genes – 133 – linked to autism spectrum conditions.
"The challenge for future research will be to establish which aspects of autism they can explain, how many of these genes are necessary and sufficient to cause autism, and how they may interact with environmental factors," he says.
Unknown role
Angelica Ronald, an autism researcher at Birkbeck College, London, agrees that the findings are exciting.
"Common variants are by definition common in the general population, and may explain a proportion of risk for familial autism spectrum disorders," she says. "Importantly, the finding is replicated across two independent samples within the study, which makes it seem pretty robust. The next steps are to figure out the role these neuronal cell adhesion molecules play in leading to the specific symptoms of ASD."
Understanding the genetic underpinnings of autism is important, as it might eventually be possible to screen babies and identify those at risk, and then use early behavioural interventions to encourage their brains to develop more normally. Ultimately, drugs might also be developed that could correct the genetic deficits.
Journal reference: Nature (DOI: 10.1038/nature07999)
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