Protein Linked to Development Problems - New York Times

The French geneticist Jérôme Lejeune discovered more than 50 years ago that Down syndrome is caused by the presence of an extra copy of chromosome 21. But to this day it has remained a mystery why that results in impaired physical and cognitive development. Now researchers at the Sanford-Burnham Medical Research Institute think they have found a clue.

The scientists, who were investigating Alzheimer’s disease, found that mice that lacked a protein known as SNX27 had many of the same learning and memory defects as mice with Down syndrome. Looking at the brains of people with the syndrome, the researchers discovered that they, too, lacked SNX27.

While chromosome 21 is not directly involved in SNX27 production, it does encode a regulator — miR-155 — that inhibits production. According to the study, published in the journal Nature Medicine, levels of miR-155 in the brains of people with Down syndrome correlate almost exactly with the decrease in SNX27.

“In the brain, SNX27 keeps certain receptors on the cell surface — receptors that are necessary for neurons to fire properly,” said the study’s senior author, Huaxi Xu, in a statement released by the institute. “So in Down syndrome, we believe lack of SNX27 is at least partly to blame for developmental and cognitive defects.”

To test their findings, Dr. Xu’s team introduced more SNX27 to mice with Down syndrome. As they expected, the mice showed immediate improvements in cognitive function and behavior. Now the researchers are investigating molecules that might increase production of SNX27 in the human brain. Douglas Quenqua

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