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BALTIMORE — Mice share many biological features with humans and experience many of the same diseases. For this reason, they are especially helpful to scientists developing new medical treatments. Unfortunately, many human diseases do not naturally occur in mice. One of those conditions is Down syndrome. In a new study however, scientists say they can now circumvent this problem by genetically modifying mice so they develop similar symptoms as humans.

Normally, babies are born with 47 chromosomes (threadlike structures of tightly coiled DNA). Down syndrome occurs when a baby is born with either a partial or entire extra copy of the 21st chromosome. This extra chromosomal material causes the characteristic symptoms of Down syndrome, including a distinct facial appearance, developmental delays, and intellectual disability.

A complex and difficult disorder to study

Researchers from Johns Hopkins Medicine say Down syndrome is difficult to study due to its complexity. While some genetic diseases are the result of a relatively small number of genetic changes, Down syndrome involves an entire chromosome which expresses hundreds of genes.

“There are more than 500 genes on chromosome 21 that can be overexpressed,” says lead author Roger Reeves in a media release. “So, in comparison to many other genetic conditions, Down syndrome is vastly more complex.”

While genetic mouse models for Down syndrome do exist, they don’t closely replicate the human version of the disease. To address this problem, Reeves and his team set out to create a more precise mouse model of the disorder.

To create the new model, researchers insert a human version of chromosome 21 into mice which allows the mouse cells to copy the extra chromosome into new cells when they divide. It also allows mice to pass the extra chromosome on to their offspring.

The mouse model, which the researchers dub TcMAC21, closely mimics the symptoms of Down syndrome in humans. The study reveals TcMAC21 mice have distinct facial features like humans with Down syndrome. They also display learning difficulties, greater likelihood of congenital heart defects, and an unusually small cerebellum.

The first step towards new Down syndrome research?

The researchers point out that, although mouse models can’t perfectly replicate human diseases, it is a critical step in the right direction.

“Our goal is to improve the health of people with Down syndrome to give them the best chance at achieving their full potential,” adds Reeves, a professor of physiology at Johns Hopkins University School of Medicine.

The study is published in the journal eLife.

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