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Turning off the extra chromosome in Down syndrome

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Scientists have discovered a way to silence the genes on the third copy of chromosome 21 that causes Down syndrome. The results of the study were published on July 17, 2013 in the journal Nature.

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In Connecticut, an estimated 40 to 50 babies are born with Down syndrome each year (about 6,000 babies in the U.S. each year). That’s about 1 in every 700 babies born. According to the Connecticut Down Syndrome Congress, there are an estimated 3,000 people in Connecticut with Down syndrome (personal correspondence).

Normally, human cells contain 23 pairs of chromosomes. One chromosome in each pair comes from the father, the other from the mother. People with Down syndrome are born with an extra copy of chromosome 21. This extra genetic material changes way the body and brain develops and is responsible for the characteristic features and developmental problems of Down syndrome.

Researchers at the University of Massachusetts Medical School have silenced the extra copy of chromosome 21 in a model of Down syndrome. They used an enzyme to introduce an RNA gene (called XIST) into cultured stem cells from Down syndrome patients. The XIST RNA coated the extra third copy of chromosome 21, silencing the genes on it. This helped correct the unusual patterns of cell growth and differentiation seen in Down syndrome cells.

The study was inspired by a naturally occurring event – during development, XIST switches off one of the two X chromosomes in female embryos, ensuring that daughters do not express a double dose of X chromosome genes.

This study was performed in the laboratory on a cell culture model of Down syndrome. However, the results provide a step towards ‘whole chromosome therapy’ for disorders such as Down syndrome and will help scientists understand the basic biology underlying this disorder. This research also provides a model to study normal chromosome silencing.

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