Prenatal Diagnosis Using Mother's Blood.

Researchers have found a way to decipher the chromosomal makeup of a 10-week-old fetus by peering into a drop of its mother's blood. The study was a small one, involving just two pregnant women -- one who feared that her baby might be born with sickle-cell anemia and another who worried that her baby might inherit a hemoglobin deficiency called thalassemia. Both fetuses were normal.

Despite the study's limitations, the implications for prenatal diagnosis of genetic disease are profound, asserts Yuet Wai Kan of the Howard Hughes Medical Institute at the University of California, San Francisco. "The method could be applied to any genetic disease in which the mutation is known," he says in the November Nature Genetics.

Just 30 years ago, prenatal diagnosis did not exist. Now, amniocentesis and chorionic villus sampling can tell expectant parents whether their child will be born with sickle-cell anemia, thalassemia, or certain other genetic diseases. Parents can then decide whether to continue the pregnancy.

These diagnostic procedures carry slight risks because they require doctors to invade the uterus with a slender needle to obtain fetal cells. Amniocentesis provokes miscarriage in a fraction of 1 percent of cases. Chorionic villus sampling causes miscarriage about 1 percent of the time. Using maternal blood would eliminate those risks.

The Kan study was inspired, in part, by research showing that fetal cells circulate in a mother's blood. Indeed, scientists showed recently that lymphocytes descended from a fetus' white blood cells may flourish in the mother's blood for decades (Science News. 2/10/96, p.85).

Although the presence of fetal cells in a woman's bloodstream makes the new technique possible, the ability of the cells to replicate long after a baby's birth also means that doctors might recover cells stemming from an earlier pregnancy. To eliminate this possibility, the researchers zeroed in on a red blood cell, called an erythroblast, that has a brief life span. First, they sorted out maternal and fetal red cells from all others and then they mixed these cells with antibodies to fetal hemoglobin. These antibodies, which had been tagged with an intense red dye, latched onto the fetal cells and stained them a brilliant scarlet.

Then came the painstaking task of picking out each dyed fetal cell from the thousands of other cells under a microscope. Once this task was complete, the researchers analyzed the fetal cells' DNA to determine whether the threatening mutations were present.

-- Science News, 11/2/96