A way of mapping the genome of an unborn child using DNA from the mother’s blood shows potential for broad genetic testing without risk to the fetus. While the technique is too expensive to be put into practice now, the research is important because it shows that the fetus’s entire genome is present in the mother’s blood.
“I think this paper is a major landmark to eventually offering noninvasive testing to all pregnant women,” says Arthur Beaudet, chairman of the department of molecular and human genetics at Baylor College of Medicine in Texas. Beaudet, who was not involved in the study, is pursuing similar research, to develop a method of isolating fetal cells from maternal blood.
The two most accurate prenatal tests for genetic abnormalities, such as Down syndrome, in use today are amniocentesis and chorionic villus sampling. Because these procedures are invasive and therefore carry a small risk of miscarriage, they are typically recommended only for women with known risk factors.
The discovery more than a decade ago of small amounts of free-floating fetal DNA circulating in maternal blood opened the possibility for noninvasive testing. A limited number of practical applications of this discovery are already in use. When a fetus is at risk for sex-linked diseases, some clinical labs analyze the free-floating DNA to determine the baby’s sex. It is also used to screen for Rh incompatibility syndrome, in which a blood-type mismatch between mother and fetus can trigger a dangerous immune reaction against the baby.
These two tests require analysis of only a single genetic factor. The broader analysis of the fetal DNA in maternal blood presents several challenges. For one thing, the DNA fragments are quite small—about 150 letters of DNA—making them difficult to piece together into a genome. For another, the fetal DNA accounts for only about 10 percent of the DNA fragments in blood, the remainder being the mother’s. That means scientists have to sequence the DNA about 60 times to sequence the fetal DNA six times. (It’s necessary to analyze the same stretch of DNA repeatedly in order to generate an accurate sequence.) The fact that the fetus shares half its DNA with its mother makes it especially difficult to determine which fragments are fetal and which are maternal.
“It’s like trying to assemble a jigsaw puzzle with millions of pieces, where 90 percent of the pieces are from another puzzle,” says Dennis Lo, a professor of chemical pathology at the Chinese University of Hong Kong, who first discovered fetal DNA in blood and is the lead author of the current study.