Research Also Sheds Light on Why Boys Are More Affected by Autism Than Girls
By Kathleen Doheny
WebMD Health News
Reviewed by Laura J. Martin, MD
June 8, 2011 -- Genetic mutations not inherited from parents appear to explain some cases of autism, new research suggests. And the mutations may number in the hundreds.
While the new research is a step forward, it is a small puzzle piece. "It could explain up to 2% of all autism cases," says researcher Stephan J. Sanders, MD, a postdoctoral research associate at Yale University's Child Study Center.
Even so, he says the new research -- reported as a trio of studies in the journal Neuron -- provides a solid foundation to a better understanding of the biology of the disorder, eventually leading to better treatments.
In the new research, scientists also found new clues about why boys seem to be more vulnerable to the disorder than girls.
''In combination with some other research studies, this new research shows pretty clearly there is indeed a strong genetic component to autism, and that the individual genes can be identified," say Alan Packer, PhD, associate director for research at the Simons Foundation. It funds autism research and provided the sample populations studied in the new research.
About one in 110 U.S. children has autism or autism spectrum disorder, the neurodevelopmental disorders marked by impaired communication, social interaction problems, repetitive behaviors, and other problems.
Genetic Causes of Autism: Trio of Studies
In two of the new studies, researchers analyzed more than 1,000 families who have one autistic child and unaffected siblings. They evaluated their DNA from blood samples. The researchers used a highly sophisticated technique that can detect duplications or deletions of one or more sections of DNA.
These duplications or deletions are called copy number variants or CNVs. If they occur at random, or sporadically, and aren't inherited, they are known as de novo CNVs.
Some CNVs ''are normal parts of being human," Sanders tells WebMD. "It's very difficult to find the ones that matter. We looked for ones that were new in the child.''
They found more new CNVs in autistic children than in unaffected children, which they expected.
They zeroed in on many regions linked with these rare sporadic mutations, Sanders says, confirming previous research on which areas matter. "Basically five regions really stand out now," he says.
These include areas of chromosome 7, 15, 16, 17 and Neurexin 1.
The team estimates ''there are 130-234 CNV regions that could be linked with autism," he says.
The researchers also found that the long arm of chromosome 7, a region associated with Williams syndrome, a genetic disorder in which people are highly social and overly friendly with strangers, may also be associated with autism.
"For a long time it has been known if you have a deletion there, it causes Williams syndrome," Sanders says.
They found the children with autism were more likely to have duplications in this region.
So it appears having a duplication may make you less social -- one of the characteristics of autism.
Autism Genes: Girls vs. Boys
In a second study, researchers from Cold Spring Harbor Laboratory and other institutions analyzed the same families. They used a similar approach.
They, too, confirmed a contribution from the sporadic mutations. They estimated the number of regions involved even higher, at up to 300.
They also found that girls appear to have a greater resistance to autism from genetic causes than do boys.
''When we looked at affected females, we don't see small mutations, but very large mutations," says Michael Ronemus, PhD, a researcher at Cold Spring Harbor Laboratory. "Girls are more resistant," he says. It appears to take a larger mutation to affect girls with autism compared with boys.
It's long been known that boys are much more likely to get autism than are girls.
In a third report, Ronemus and his colleagues developed a method for analysis of the genetic associations. They used a new method to help identify a network of genes affected by the rare mutations in autism.
"The genes we are finding are typically involved in early brain development, forming connections in the brain," Ronemus says. These genes are related to the development of synapses, the point of connection between two nerve cells, among other tasks, the researchers say.
Genetic Causes of Autism: Implications
While the new research provides more clues to the genetic underpinnings of autism, "the message is not 'go out and get tested,'" says Sanders."We really are not at that stage yet."
The new research provides new information ''but also confirms a lot of things we have already known," says Andy Shih, PhD, vice president of scientific affairs for Autism Speaks, an advocacy and research organization.
"It confirms that rare copy number variants are the main risk factors for many families," he says. However, he says, it also confirms that it's still impossible to explain the majority of cases of autism.
The sporadic mutations appear to play more of a role in families with just one child affected, Shih says. Eventually, the genetic findings could be useful information during genetic counseling in families who have one affected child, he says.
The findings are called a critical first step in the eventual goal of developing targeted treatments.
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