easier to spot. . Down syndrome affects one in 1000 in the general population
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A technology guru may have solved a problem that has long vexed the Obstetricians: how to test for Down syndrome, without poking a needle into the uterus. By sequencing the floating fetal DNA in maternal blood, bioengineer Stephen Quake's team at Stanford University in Palo Alto, California, has detected new cases of the disease with 100% accuracy. If this small study is in larger trials, the test could become routine for pregnant women.
Down syndrome is caused by an extra copy of chromosome 21. This results in mental retardation and other health problems, and some women choose to abort fetuses with Down syndrome. Although there is a noninvasive screening test that indicates the likelihood of a fetus has Down syndrome, pregnant women who want a definite answer now have two choices: amniocentesis, which involves inserting a needle into the uterus to remove the fluid, or another procedure that extracts a piece of placental tissue. These tests, performed at about 10 to 16 weeks, are 99% accurate, but cost about $ 1000 and, in rare cases, can cause miscarriage. Therefore, usually only women over 35 who are at higher risk for carrying a baby down to get tested.
In the last decade, researchers have developed tests for prenatal sex and Rh blood group based on tests for mutations in fragments of fetal DNA circulating in the blood of pregnant women ( science , September 2, 05, p. 1476). But the detection of disorders such as Down syndrome using maternal blood was more difficult because the test is to identify an extra chromosome, and DNA from the mother swamps on the DNA of the fetus. In the most advanced approach, the Sequenom, based in San Diego, California, uses the messenger RNA produced by the placenta to detect Down. However, the test is only a screen because it relies on DNA markers on chromosome 21, which vary according to ancestry, and missing about 7% of the population in the United States.
Quake team tried a brute force approach to work around this problem: They put samples of maternal blood through a DNA sequencing machine. Although the researchers sequenced about 2% of the genome of the mother and the fetus, it was enough data to distinguish the levels of chromosome 21 in mothers carrying a baby syndrome down to those with normal fetuses from 14 weeks. In all nine cases of Down syndrome tested but not six normal cases, this ratio was greater than one - which indicates that the condition was present. The technique also detected three cases of other chromosomal disorders that lead to abnormal development, the group says Quake online today in the Proceedings of the National Academy of Sciences .
The test costs $ 700 per sample, but the price should drop with the latest sequencing technologies, says Quake, who hopes the approach commercialized. "I think it's going better and better," agrees Department of Medical Genetics of Energy Edward Rubin, who said the newspaper Quake was the speech of a meeting of the genome recently in Boston
experts fetal testing are also impressed. "the results are amazing," says molecular biologist Sinuhe Hahn Basel university hospital in Switzerland. Hahn and others say it is too early to tell, however, if the test will prove greater than of Sequenom. It must be studied on a much larger scale to be sure he can pick up all the 1000 births of Down syndrome in the general population, says Diana Bianchi of Tufts University in Boston. Although she says the study is "a significant step forward."
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