A faster prenatal test for Down syndrome

Scientists in California have advanced toward rapid testing for pre-natal detection of Down syndrome and other birth defects involving an abnormal number of chromosomes.

In a study to be published by the journal Analytical Chemistry, Stanford University's Prof Stephen Quake and his graduate student Christina Fan point out that most existing pre-natal testing depends on a technique termed karyotyping, requiring a two-week wait for results.

Laboratory tests with their new method produced accurate results within two hours.

The test is a variation of the polymerase chain reaction (PCR) — the basis of the genetic engineering revolution — which produces thousands of identical copies of tiny samples of DNA.

Using a technique known as the digital PCR, 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 were able to quickly determine which samples had the extra chromosome indicating 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, while those without DNA did not glow. The technique enabled researchers to confirm the presence of abnormal chromosomes typical of Down syndrome with great accuracy.

Rapid testing alternatives already exist, but according to Quake they are either too labour-intensive or not applicable to the whole population.

"The technique we present in this paper can overcome these limitations. It is rapid and simple. We estimate that the entire procedure from sample collection to result readout would take only a few hours, substantially reducing the anxiety of the expectant parents," Quake said.

The test is also potentially cheaper than other available methods and semi-automated, reducing the workload of lab personnel. It is also readily available since the digital PCR technique is based on commercially available lab equipment.

"We are confident that it will work on clinical samples of amniotic fluid or chorionic villus," Fan said.

The next step is to begin clinical trials to evaluate the sensitivity and specificity of the new test. The authors believe the new test could be available in as little as one year.