October 2002

Fetal Fortunes

Continued from page 3

By Jon Cohen

Fishing for DNA

One of the most promising prenatal diagnostic tests in the works might one day eliminate altogether the need for amniocentesis or chorionic villus sampling. Its strategy: testing the mother's blood for fetal DNA. For years, Diana Bianchi, a neonatologist and medical geneticist at the Tufts-New England Medical Center in Boston, and others had hoped that fetal cells found in maternal blood might provide a noninvasive means for assessing a baby's genetic makeup. But, Bianchi explains, the cells proved too few in number and thus too difficult to find. In 1997 a team led by Dennis Lo of the Chinese University of Hong Kong first reported the discovery of fetal DNA floating freely in maternal serum and plasma. Since then, preliminary evidence from Bianchi's group and other researchers has shown that this fetal DNA can be assessed for chromosomal or gene defects.

Bianchi cautions that the approach has several critical limitations right now-not the least of which is that researchers have determined how to fish out only male fetal DNA, which is easier to see because of its distinctive Y chromosome. But she predicts that given the speed with which this technology has moved forward, in five years it may have developed far enough to actually turn up in the clinic. "Fetal DNA opens a whole area of new research that is going to be huge," predicts Bianchi.

As ever more information pours out about the relationship between genes and diseases, and as screening tools become more sophisticated, prospective parents will inevitably face harder decisions about the types of babies they want to bring into the world. New tools such as DNA microarrays could soon enter the arena of preimplantation and prenatal genetic diagnosis, allowing physicians to screen DNA for a host of genetic diseases simultaneously.

Prenatal screens and diagnostics cannot eliminate all of the uncertainties about the health of a newborn. They can dramatically reduce them, though, and that is the aim of the technologies. But the same diagnostics also raise vexing ethical questions. As the technology continues to advance, parents will gain the ability to determine whether their offspring have genes that "predispose" them to diseases such as Alzheimer's or breast cancer. In the end, these future tests could themselves introduce ethical ambiguities that no amount of science and technology can overcome.

Genetic Diagnostic Tools

Technique	Applications	Limitations
Comparative genomic hybridization	Chromosomal or genetic diagnosis before implantation	Limited to in vitro fertilization
Amniocentesis	Genetic diagnosis in the second trimester	Invasive, high risk of miscarriage
Chorionic villus sampling	Genetic diagnosis in the first trimester	High risk of miscarriage
Free-cell fetal DNA	Noninvasive genetic diagnosis	Under development