RNA sends message from fetus

NewScientist.com news service

Tests on blood taken from a pregnant woman can reveal which genes are switched on in the placenta. The finding paves the way for non-invasive, safer, pre-natal tests for a range of disorders, including Down's syndrome and pre-eclampsia.

In blood samples from pregnant women, researchers isolated molecules produced only by genes in the placenta. The molecules, messenger RNA, give genetic information about the fetus, not the mother, because genetically the placenta is part of the baby.

"While DNA is the genetic blueprint, mRNA will tell you which genes are actually being expressed," says team leader Dennis Lo at the Chinese University of Hong Kong. "And many diseases, genetic or otherwise, might be associated with abnormalities in RNA expression."

Almost all current tests for fetal chromosome, genetic and biochemical disorders depend on invasive procedures to provide fetal cells for culture or DNA analysis, points out Malcolm Ferguson-Smith of the University of Cambridge, UK.

These tests can be risky. For every two fetuses identified as suffering from Down's syndrome using a cell-sampling test, five healthy babies miscarry as a direct result of the procedure. The new findings have "considerable potential for non-invasive prenatal screening and diagnosis," Ferguson-Smith writes in a commentary on the research.

Fluorescent probes

Lo's team used fluorescent probes to measure levels of mRNA encoding two hormones, human placenta lactogen and human chorionic gonadotropin (hCG). Elevated levels of hCG are associated with Down's syndrome, so tests for hCG mRNA might form a safer method of diagnosis than amniocentesis, says Lo.

Previous work has shown that a high level of fetal DNA in the mother's bloodstream is also associated with Down's syndrome. But tests based on analysing this DNA have two major drawbacks.

First, maternal and fetal DNA is so similar that the two can only be easily distinguished if the child is male. Second, elevated foetal DNA levels could indicate a wide range of disorders. But these disorders are also likely to produce increased levels of fetal RNA in the mother's blood, says Lo. "And we would expect different diseases to have specific patterns of RNA expression," he says.

The next step is to isolate other types of mRNA produced in the placenta, but not by the mother. "We should pick 10 or 20 and then document their levels in relation to different diseases," says Lo.

Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0637450100)