1% of Autism Due to Unstable Genes

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 causes the Prader-Willi and Angelman autism 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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By Daniel DeNoon
Reviewed by Louise Chang
©2005-2006 WebMD, Inc. All rights reserved

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