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New prenatal test for Down syndrome less risky
Blood-sample discoveries could make amniocentesis test obsolete

by Sue Dremann

A new blood-sample test could make risky prenatal tests such as amniocentesis a thing of the past.

The new method counts the number of DNA fragments from chromosomes known to cause genetic diseases, such as Down syndrome, that are found in a mother's blood with 100-percent accuracy and has no risk of miscarriage, according to researchers at Stanford University, Lucile Packard Children's Hospital and the Howard Hughes Medical Institute.

The study was published online this month in the Proceedings of the National Academy of Sciences.

The study looked at 18 women, 12 of whom had aneuploid pregnancies — an abnormality in the number of fetal chromosomes — while six had normal pregnancies.

The test scanned for extra chromosomes, including extra chromosome 21, which causes Down syndrome, and chromosomes 13 and 18, which cause Patau syndrome and Edwards syndrome, respectively.

One in 800 births result in Down syndrome. Affected individuals have mental retardation, characteristic facial features and, often, heart defects and other health problems, according to the Centers for Disease Control and Prevention.

Patau syndrome affects 1 in 5,000 to 1 in 2,000 live births and features severe craniofacial abnormalities, extra digits, seizures, deafness, kidney problems and mental retardation. Edwards syndrome — the most common abnormality after Down syndrome — affects 1 in 3,000 and 1 in 8,000 births and exhibits hand and foot abnormalities, low-set ears, cardiac defects, muscle spasticity, kidney problems and mental retardation.

"The technique is on the leading edge of a flood of different ways that rapid DNA sequencing will be used in medicine," said Stephen Quake, Ph.D., professor of bioengineering and the study's senior author.

Other researchers had struggled to tease the fetal genetic fragments apart from a mother's DNA. But Quake's team's method has no need to distinguish between maternal and fetal DNA, he said.

The researchers took a 16- to 20- milliliter blood sample — one-tenth the volume of a can of soda — from the women. They separated maternal blood into cells and plasma and looked at the liquid plasma's DNA fragments, which come from both mother and fetus. They counted the DNA fragments and used DNA sequencing to read each one. The researchers tallied how many gene fragments originated from each chromosome.

"Every [pregnant] woman carries 3 to 6 percent of their baby's DNA in her bloodstream," said Dr. Yair Blumenfeld, a maternal-fetal medicine fellow at Stanford and co-author of a paper describing the technique. But women with Down-syndrome fetuses had more chromosome-21 fragments in their blood than the women who had normal pregnancies.

Because fetal DNA shows up in maternal blood early in pregnancy, finding the extra genetic material in a mother's blood could provide much earlier diagnosis for fetal aneuploidy, he said.

The results are exciting because the test was 100 percent accurate, he added.

The premise behind the test differs from other approaches, according to Blumenfeld.

"The idea of the test is to diagnose, not screen," he said.

The method is also safer.

Invasive testing such as amniocentesis and chorionic villus sampling requires inserting a needle into the mother's uterus. The test has a risk of miscarriage of approximately half a percent. But in the small study, the blood sampling had no risk to the mother or fetus, Blumenfeld said.

Potentially, the test could detect other forms of chromosomal abnormalities, but more study needs to be done, he added.

The next step is to repeat the study in a larger number of women. If the study holds up in further research, the test is expected to be simple and inexpensive and could be available within two years, he said.

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Staff Writer Sue Dremann can be e-mailed at sdremann@paweekly.com.