Ever since the discovery that a pregnant woman’s blood contains traces of her baby’s DNA, researchers have been looking for ways to screen that DNA for genetic abnormalities. A new test developed by Stephen Quake and his colleagues at Stanford University takes us one step closer to a noninvasive blood test to diagnose disorders like Down syndrome in a fetus.
The new test uses powerful DNA sequencing techniques to amplify short fragments of a baby’s DNA from its mother’s blood, and to map the chromosomes. The method reveals the extra copies of chromosomes–aneuploidy–characteristic of certain genetic disorders, including Down syndrome, in which there are three copies of chromosome 21 rather than two.
“Now we’re getting closer to the time when there will be not a screening test but a definitive noninvasive test,” says Joe Leigh Simpson of Florida International University. Simpson wasn’t involved in Quake’s work.
The test picked up all nine cases of Down syndrome among 18 women in a study reported today in the Proceedings of the National Academy of Sciences. It also detected two cases of Edward syndrome, in which there is an extra copy of chromosome 18, and one instance of Patau syndrome, characterized by three copies of chromosome 13.
Currently, the gold standard for prenatal diagnosis of Down syndrome is amniocentesis, “which requires a big needle to be stuck into the mom right next to the baby” to extract a sample of amniotic fluid, explains Quake, who is also an investigator for the Howard Hughes Medical Institute. There’s an attendant risk of miscarriage, infection, or damage to the fetus. A diagnosis may also be made by chorionic villus sampling, which uses tissue from the placenta but has a higher risk of miscarriage than amniocentesis. The samples of amniotic fluid or placenta are examined to see if the fetus has three copies of chromosome 21.
Quake’s technique takes advantage of the fact that small amounts of DNA from a fetus circulate in its mother’s blood. Some DNA is within intact fetal cells, but about a decade ago, researchers discovered that a pregnant woman’s blood also carries free-floating fetal DNA. Unlike intact cells, cell-free fetal DNA doesn’t linger from one pregnancy to another. It’s also more abundant than that from intact cells, but still rare enough to make it difficult to detect.