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Mouse Study Says Down Syndrome Is Genetically Complex

By Cheryl Simon Silver

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For three decades many researchers who study Down syndrome have assumed that a region of chromosome 21 is critical in the development of the syndrome’s characteristic features. Down syndrome is the most common genetic cause of mental retardation and congenital heart disease, and there is no treatment.

The region, known as “Down syndrome critical region,” has long been thought to cause many of the distinctive features of the syndrome such as cranial and facial shape.

This notion is apparently refuted by researchers at Johns Hopkins University School of Medicine in Baltimore and their colleagues who manipulated mice bred to carry the chromosome abnormalities like those associated with Down syndrome in humans.

The researchers created mice to carry one, two, or three copies of the critical region. When they measured the facial bones of the mice, they found that mice with three copies of the critical region did not have the unambiguous facial features of Down syndrome. In fact, the mice had skeletal changes that appeared to be the opposite of what was expected.

Roger H. Reeves of Johns Hopkins and his colleagues reported the findings in Science.

Based on their findings, the researchers suggest that the genetics of Down syndrome are more complex and less well understood than some may have assumed.

The researchers propose that while three copies of a single gene may not be sufficient to cause the characteristic features of Down syndrome, this gene in triplicate may contribute to syndrome through interactions with other genes.

Reeves R.H. et al. Chromosome 21 critical region does not cause specific down syndrome phenotypes. Science306 687-690. (October 22, 2004).

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