MADISON - Using stem cells as a window to the earliest developmental processes in the human brain, scientists have found that a group of genes critical for brain development is selectively disrupted in Down syndrome.
Writing in the recent issue (Jan. 26, 2002) of the British medical journal The Lancet, a team of scientists from the University of Cambridge, University College London and the University of Wisconsin-Madison report findings from a genetic study based on stem cells derived from Down syndrome and normal fetal tissue.
The results illuminate some of the key cellular and molecular processes that give rise to Down syndrome, one of the most common causes of developmental disability in humans. The study is the first of its kind using human cells.
The central finding of the study, according to Clive N. Svendsen, a UW-Madison professor of anatomy and neurology and a co-author of the report, is that a faulty genetic circuit results in dramatic changes in the development of the cells that make up the early brain.
"These findings point to a serious deficit in specific genes known to be important for neuronal development," said Svendsen who is currently director of the stem cell research program at the UW-Madison Waisman Center, one of the world's leading centers for the study of human development, developmental disabilities, and neurodegenerative diseases.
The Lancet study, which Svendsen co-authored with lead author Sabine Bahn, University of Cambridge, and others has begun to shed light on the earliest genetic events in humans that give rise to a serious cognitive disability.
It has long been known that most instances of Down syndrome, which affects nearly 350,000 people in the United States alone, results from an extra chromosome, chromosome 21, in the cells of those who have the condition. However, the precise genetic events that lead to the abnormal brain development of people with Down syndrome have not been understood.
"Until now, we have only had mouse models of Down syndrome, which have not been so faithful in reproducing all aspects of Down syndrome," Svendsen said. "Now we have a complementary source of human stem cells with extra chromosome 21, and which can be grown indefinitely and used by a large number of scientists."
Although the results presented in the current Lancet study are preliminary, identifying the faulty behavior of key genes that give rise to Down syndrome could ultimately lead to better treatments, including new drug and, possibly, gene therapies.
"This provides a nice model for drug or other types of intervention to try and get (the developing brain) back to normal neuronal production from the stem cells," Svendsen explained.
The study was made possible, according to Svendsen, by new advances that permit scientists to grow stem cells in culture, monitor gene activity within the cells, and observe the cells as they progress down the developmental pathway to becoming the cells that make up the human brain.
In the Down syndrome cells, it was found that there is a significant reduction in the percentage of cells that go on to form neurons compared to non-Down syndrome cells. Moreover, nerve fibers from the Down syndrome cells were shorter and misshapen.
Using techniques that permit scientists to see which genes are active in both a Down syndrome sample and a non-Down syndrome sample, the group was able to home in on the genes that seemed to be responsible. Knowing which genes are involved is critical because it opens the door to developing gene and drug therapies that could prevent the onset or development of the syndrome.
"I think it is early in the game, but now we can work with a new model for Down syndrome which uses human cells rather than mouse cells," said Svendsen. "If we can understand the loss of neurons in Down syndrome, I think it may lead to some novel treatments in the future."
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