RNA from fetuses can slip through the placenta and into their mothers' blood, a new study has found. Surprisingly, the RNA manages to escape destruction by enzymes. The finding could lead to no-risk tests for preeclampsia, chromosomal abnormalities, and other diseases.
In 1996, researchers discovered that fetal DNA could seep into maternal blood, offering the opportunity to screen for diseases without jabbing a needle into the amniotic fluid or other invasive techniques, which bear a slight risk of miscarriage (ScienceNow, 31 October 1996). Certain diseases can be diagnosed because they send more fetal DNA in the mother's bloodstream. But the amount could only be measured when fetal DNA differed from the mother's, such as when the fetus was male or inherited a known genetic difference from its father. Looking for RNA expressed only in the fetus or placenta could avoid that problem, but enzymes in blood that digest RNA were thought to make it unsuitable for screening.
But molecular biologist Dennis Lo and colleagues at the Chinese University of Hong Kong report otherwise in the 17 March online edition of the Proceedings of the National Academies of Science. Using real-time polymerase chain reaction, a technique based on fluorescent molecules that glow more brightly the more target RNA is present, they tested mothers' blood for two types of messenger RNA expressed in all normal placentas. They found fetal RNA in all of the 63 women tested.
RNA will most likely be useful for identifying preeclampsia, which affects about 6% of pregnancies, plus Down's syndrome and other disorders in which placental abnormalities might create telltale changes in the amount of RNA. The researchers are now testing larger numbers of women to establish baseline levels of fetal RNA, in hopes of finding markers for these and other diseases.
Geneticist Diana Bianchi of the Tufts School of Medicine in Boston says it's surprising that RNA is so stable in maternal blood. "This paper adds further evidence for the trafficking of cells and nucleic acids between the mother and the fetus," she says, and that opens many new possibilities for prenatal screening.