When women have children late in life, there is an increased risk that those children will have a condition that results in stunted growth, significant cognitive deficits, a flat face with a protruding tongue, and often short fingers and poor muscle tone. Down syndrome occurs in one in 35 babies born to women over 45, as compared to one in 400 babies born to women under 35; having a baby with Down syndrome is itself a risk factor, raising the chances of subsequent children also having the condition.
The most common form of Down syndrome is called trisomy 21 because it is caused by an extra 21st chromosome, three rather than the usual two. The other, vastly rarer, forms can also be traced to excess 21st-chromosome material, either present in some cells but not others (called mosaic Down syndrome) or attached to other chromosomes (called translocation Down syndrome). The translocation type accounts for about one in 25 cases of the condition and is the only type that can be inherited.
The extra chromosomal material, scientists now believe, appears to affect stem cell regulation. The primary culprit appears to be a gene on the chromosome called Usp16; an extra copy of the gene means more stem cells are used in development. While that one gene is almost certainly not solely responsible for all the symptoms of the condition, treatments that dial it back causes neural cells that typically grow slowly in Down patients to instead grow in the usual way.
This suggests a possible direction for research into genetic-based treatments for Down syndrome. The study that identified the role of Usp16 included tests in human cells, so there is some insight into how it works in people. Current treatments for Down syndrome are focused on early intervention and team care to address the symptoms and possible complications—specialists in developmental pediatrics, cardiology, speech therapy, neurology, and all the other areas the condition touches—but gene therapy may in the future prevent the condition from showing symptoms in the first place.