Toward a faster prenatal test for Dow...( Scientists in California are reporti...),Health

Scientists in California are reporting an advance toward rapid testing for pre-natal detection of Down syndrome and other birth defects that involve an abnormal number of chromosomes.

In a study scheduled for the Oct. 1, 2007 issue of ACS journal, Analytical Chemistry, Stanford University bioengineering professor and Howard Hughes Medical Institute researcher Stephen R. Quake and his graduate student H. Christina Fan point out that most existing pre-natal tests depend on a technique termed karyotyping. It requires a two-week wait for anxious parents, while cells taken with amniocentesis or chorionic villus sampling are grown in laboratory culture and analyzed.

Laboratory studies with the new method produced accurate results within two hours. The test is a variation of the famed polymerase chain reaction (PCR) the basis of the genetic engineering revolution which produces thousands of identical copies of minute samples of DNA.

Using a technique known as the digital polymerase chain reaction, Quake and Fan replicated DNA from two cultures of cells growing in the laboratory. One consisted of a normal human cell line and the other had human cells with the Down variant. The digital PCR process allowed the researchers to count DNA molecules from the samples, substituting for the two-week cell culture process traditionally needed to produce enough DNA for karyotyping. With the precision derived from counting individual DNA molecules, researchers then were able to move ahead without delay and determine which samples had the extra chromosome that indicates Down syndrome.

The digital PCR was performed in a commercially available microfluidic chip. The samples were loaded onto the chip, and then partitioned into thousands of chambers by microscopic mechanical valves. While PCR was performed, fluorescent material in the compartments containing individual DNA molecules lit up like an array of LEDs, while those without DNA did not gl
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