MONDAY, March 17, 2003 (HealthDayNews) -- Researchers have made a discovery that may lead to simpler and less-invasive genetic tests for babies still in the womb.
Specifically, the scientists have been able to identify genetic material from the fetus that leaves the placenta and circulates in the mother's bloodstream, where it can be picked up in a routine blood test.
"It puts us a little bit closer," says Dr. John Williams III, director of reproductive genetics at Cedars-Sinai Medical Center in Los Angeles. "This is a big step in the direction in which they want to go. It's actually very exciting."
The findings appear in this week's issue of the Proceedings of the National Academy of Sciences.
Right now, the only reliable tests for genetic problems in unborn babies are amniocentesis and chorionic villus sampling (CVS). CVS involves taking a sample of the placenta either by way of the vagina and cervix or via a needle in the woman's stomach. With amniocentesis, a long needle is inserted directly into the womb.
"[Amniocentesis] causes a lot of psychological stress to the women concerned and, also, in the best of hands, there's a 1 percent risk of aborting the baby even though the baby is healthy," says study author Dr. Dennis Lo, a professor in the chemical pathology department at the Chinese University of Hong Kong.
Consequently, much research has been devoted to devising methods that are safer and easier. Measuring hormone levels in the mother's blood certainly fits these two criteria, but is not particularly accurate, Lo explains.
Scientists have also tried to isolate fetal cells in the mother's bloodstream, but this has turned out to be a needle-in-a-haystack proposition.
"You're talking one cell in a million," Lo says.
In 1997, researchers discovered a baby does release its DNA into the mother's blood plasma, but because DNA between different people is so similar, the only way to distinguish baby DNA from maternal DNA was if the baby was a boy and had a Y chromosome. This method effectively excludes 50 percent of fetuses from testing.
Lo and his colleagues hypothesized that the mother's placenta, which provides nourishment to the fetus, might also be a source of fetal genetic material. They tested maternal blood plasma for placental messenger RNA (mRNA), which is produced when a gene becomes active, and found it was indeed present in the mother's bloodstream.
"What's exciting about this is that by looking at the RNA, these markers can be done independent of the gender of fetus," Williams says.
Unlike DNA and fetal cells, this RNA seems to disappear from the blood soon after the baby is born. Some fetal cells can exist in the maternal bloodstream for months, so if a woman ended a pregnancy because her fetus had Down Syndrome, those cells might still be floating around during a second pregnancy.
"You wouldn't know if those cells were from the previous pregnancy," Williams says. "The fact that this disappears right away, if you find an abnormal condition in a subsequent pregnancy, it should be indicative of finding a disorder."
Now that there's a way of identifying the RNA in the mother's blood, Lo and his colleagues need to figure out a way to decipher the different genes.
"Now, suddenly, with RNA, we literally have hundreds, if not thousands, of markers to play with," Lo says. "We have solved many of the obstacles. We can see what genes are being expressed, and then you just go through the list of those genes."
Lo already has a paper in press on detecting genetic predisposition for preeclampsia, or pregnancy-related high blood pressure, and he expects that within one to two years they'll have examples of diseases in which the RNA test can be used.
More information
The March of Dimes has information on amniocentesis and on birth defects and genetics.
