Uninvasive Prenatal Test for Down Syndrome Developed by Researchers

A team of researchers have developed a test for trisomy 21, the most common form of Down Syndrome, that is much less invasive than currently available diagnostic tests. Their findings were published online-before-print on Monday in the Proceedings of the National Academy of Sciences.

According to a Wikipedia article on Down Syndrome, two prenatal screening techniques -- maternal serum screening, which tests for levels of certain proteins and hormones in the mother's blood, and ultrasound screening, which can detect characteristic fluid accumulation behind a fetus's neck -- can provide an estimate of the chance that a fetus has Down Syndrome, although a number of circumstances can cause the screening to result in a false positive.

To actually diagnose Down Syndrome before birth requires invasive procedures (such as amniocentesis) to recover cells from the fetus for testing. These procedures carry a small chance of risk of miscarriage or fetal injury. To contrast, the test for trisomy 21 described in the new research paper carries almost no risk to the mother or fetus, as it only requires a maternal blood sample.

Trisomy 21 is caused by the presence of three copies of chromosome 21 in the cells of the fetus, as opposed to the normal two copies; this occurs due to nondisjunction, or failed separation, of chromosome 21 in cell division during the formation of either the sperm or egg. While tests for Trisomy 21 normally require the recovery of fetal cells for DNA comparison, researchers developed a new way to analyze very small amounts of cell-free fetal DNA that can be found in a mother's plasma.

The new paper describes how researches used a technique called digital PCR to count different types of DNA molecules isolated from maternal blood, and compare subtle differences in the number of molecules derived from chromosome 21. Elevated levels of DNA from chromosome 21 indicated trisomy 21 in the fetus. The team used this technique to design a testing procedure which they say can correctly diagnose trisomy 21 greater than 97% of the time, comperable to the 98-99% accurate diagnosis offered by currently available prenatal diagnostic tools (National Down Syndrome Society).

With further refinement of techniques, particularly those used to concentrate fetal DNA collected from maternal plasma, the researchers believe the uninvasive test could be made available in the future as lower-risk alternative to the tests that are currently available. It is also possible that similar techniques could be used to diagnose or identify other conditions which result from chromosomal abnormalities.

References:

Y. M. Dennis Lo et al., "Digital PCR for the molecular detection of fetal chromosomal aneuploidy" Abstract and link to pdf are available at: http://www.pnas.org/cgi/content/abstract/0705765104v1

Wikipedia, "Down Syndrome" http://en.wikipedia.org/wiki/Down_syndrome

National Down Syndrome Society, "Prenatal Testing and Diagnosis" http://www.ndss.org/index.php?option=com_content&task=category§ionid=23&id=60&Itemid=234