Two separate groups of researchers have developed a non-invasive test for Down syndrome, using only a blood sample from the pregnant woman to examine the fetus' DNA. While the genetic tests are still in clinical trials, experts are hailing the achievement as significant because current prenatal tests like amniocentesis require inserting a needle in the uterus, and carry a risk of miscarriage. A biotechnology company called Sequenom says it will begin selling its test next June, while researchers from Stanford, who just published their results in the Proceedings of the National Academy of Sciences, are still planning a large-scale clinical trial. Some experts say that the results are somewhat preliminary, and should be viewed cautiously:
The Stanford test has been tried on only 18 blood samples. Sequenom has tried its test on only about 400 samples and has not yet published its results in peer-reviewed journals. Still, both tests have perfect records so far: no false negatives or false positives. “This is quite simply a major step forward, if it works at all like we expect it might,” [says Down Syndrome expert] Jacob A. Canick [The New York Times].
Currently, the most common prenatal tests for Down syndrome are amniocentesis--in which doctors take a sample of the amniotic fluid for genetic testing--and chorionic villus sampling (CVS), which requires a sample of placenta tissue. The new blood test method
exploits the ability of new and improved DNA technologies to reveal more detailed genetic information. "I am extraordinarily excited,'' said [Stanford researcher] Stephen Quake.... "I got into this because I have a family, with two kids. With the first kid, we used amnio. With the second, we used CVS. Both were nerve-wracking experiences" [San Jose Mercury News].
Down syndrome is a genetic disorder caused when a child inherits an extra copy of the 21st chromosome from either the mother or father. The new
test relies on finding tiny amounts of fetal DNA that circulate through a mother's blood.... Fetal DNA accounts for only a small proportion of the 'naked' DNA — that not contained within a cell — in the mother's blood. Most lab techniques are not sensitive enough to detect changes in circulating fetal DNA because the signal is drowned out by the mother's DNA [Nature News].
But researchers developed a method of looking for genes from the 21st chromosome that are active in the fetus, but not the mother. They then check to see if the fetus has a higher than normal number of those genes, which would indicate the extra chromosome that causes Down syndrome.
Image: flickr/Torsten Mangner