There’s never been a great way to safely and accurately test what’s going on in the womb. The mother’s bloodstream contains some fetal cells, but not many of them, so a maternal blood sample rarely yields enough for a useful analysis. Now Christina Fan has come up with an approach to measuring the chromosomes and genes in the fetus without having to isolate the fetal cells, enabling her to develop tests for Down syndrome and a range of inherited and other conditions.
While still a graduate student in bioengineering at Stanford, Fan developed a DNA sequencing technique as well as an algorithm for estimating how many of certain chromosomes—such as chromosome 21, the one implicated in Down syndrome—should be present in a sample of the mother’s blood if the fetus is contributing the expected number. Any excess in the sample means the fetus has more than the normal number of the chromosome, indicating that the child is likely to have Down syndrome. There are other blood tests for this condition, which affects cognitive and physical development, but these tests are much less accurate. There are also more accurate tests performed on fluid drawn from the amniotic sac, but collecting this fluid carries a small chance of triggering a miscarriage.
Fan realized that to expand her work to other inherited conditions, she had to go beyond simply counting chromosomes and look at the genes associated with those conditions. She was able to adapt her chromosome technique for these other conditions by calculating, from an analysis of both the mother’s and father’s cells, how much of a certain type of gene ought to turn up in a sample of the mother’s blood. If the sample contains higher levels than expected, the excess is coming from the fetus. “We used this method to build the entire inherited fetal genome from maternal blood,” she explains.
Some of the conditions that are detectable this way can be prevented from causing problems if they’re treated promptly at birth. The metabolic disorder phenylketonuria, for example, can be managed through diet if that begins when the patient is a newborn.