SCIENTISTS READY TO TEST RISK-FREE ANALYSIS FOR DOWN SYNDROME

Scientists are preparing to carry out clinical trials of a revolutionary technique that could result in all fetuses being screened for Down syndrome without any risk to either mother or baby.

The technique is designed to collect and study tissues from an unborn child without invading its mother's womb.It relies on technology, to be tested next year, which "filters out" and analyzes fetal cells that circulate in a mother's blood, and should pose no risk of triggering miscarriages, say researchers.

Current techniques, amniocentesis and chorionic villus sampling, use needles which are pushed inside the womb and are linked with small increases in rates of miscarriages.

"There is no invasion of the womb with fetal cell separation and no risk of miscarriage," said Dr. Bill Braun of Integrated Genetics, the Boston biotechnology firm which is pioneering fetal cell separation technology in the United States.

Scientists have known for more than a century that cells from an unborn child pass into its mother's bloodstream, however, concentrations are extremely low: Only about one in 10 million cells in maternal blood comes from the fetus.

Concentrating these cells in order to test for major chromosomal disorders, such as Down Syndrome, has therefore been a major stumbling block for doctors. However, in the past few years scientists have developed techniques that have allowed them to begin this cellular distillation in earnest.

Their principal approach has been the use of monoclonal antibodies - artificial versions of naturally occurring proteins that latch on to key molecules on the surfaces of cells. The crucial point is that these antibodies are very specific. They will bind to one, and only one, surface molecule found on only one type of cell.

To exploit this specificity, these antibodies can be attached to magnetic beads which are then dropped into a solution of cells, such as blood. Those cells that display a desired marker molecule stick to its corresponding antibody - and therefore to the beads, which can then be removed by a magnet. Alternatively, an antibody can be linked to a fluorescent dye which can reveal the identity of a cell.

The principle is simple and allows scientists to create concentrated solutions of desired cells. But scientists had to determine which type of antibodies should be used for fetal testing - in other words, which target cells, found only in fetal blood but not a mother's, should be their targets.