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1% of Autism Due to Unstable Genes

Specific Genetic Instability Causes 1 in 100 Autism Cases
By
WebMD Health News
Reviewed by Louise Chang, MD

Jan. 9, 2008 -- Duplication or deletion of genes in a "hot spot of genetic instability" accounts for 1% of early autism cases, researchers find.

The genes are located on an arm of the human chromosome that's still young in terms of evolution. This means it carries highly similar stretches of genetic code that cross over in the earliest stages of embryo development. This crossover means that genes in the segment can be cut out (deleted) or copied too many times (duplicated).

Deletion or duplication of genes in this region "carries substantial susceptibility to autism and appear to account for approximately 1% of cases," report Massachusetts General Hospital researchers Lauren A. Weiss, PhD, Mark J. Daly, PhD, and colleagues.

This is at least as large a subset of autism cases as any other single factor is known to cause, the researchers note. A duplicated gene in another chromosome is associated with the Prader-Willi and Angelman syndromes, which account for about 1% of autism cases.

So far, researchers believe about 10% of autism cases have genetic causes. Many of these genetic abnormalities cause other profound disabilities in addition to autism.

The newly discovered abnormality, at a chromosomal site called 16p11.2, appears to cause early-onset autism. Developmental regression -- in which apparently normal, usually 14- to 24-month old children seem to backslide into autism -- was not seen in children with the 16p11.2 abnormality.

Weiss and colleagues found the 16p11.2 abnormality using sophisticated genetic techniques to search the genomes of 751 families with two or more cases of autism. The abnormality was seen only in people with autism.

Because it is caused by a misreading of the genetic code, the genetic abnormality is not directly inherited from parents. Many cases of autism may result from various rare genetic deletions or duplications. This could be why it's been so hard to identify specific "autism genes."

On the other hand, researchers now see autism as a collection of many different syndromes -- and even within these syndromes there may be very different autistic symptoms. As we learn more and more about autism, it's like peeling back the layers of an onion, suggest Evan E. Eichler, PhD, and Andrew W. Zimmerman, MD, in an editorial accompanying the Weiss report.

They suggest that larger studies, using the new techniques pioneered by Weiss and colleagues, "may be needed to peel away the remaining layers of the onion."

The Weiss study, and the Eichler/Zimmerman editorial, appear ahead of print in the Jan. 9 Online First issue of The New England Journal of Medicine.

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