EVENING. Health.

Brave New Genetics

May 19, 1993|By Linda Roach Monroe, Knight-Ridder/Tribune.

Today's baby girl could face a very different world when she is grown and ready to have a baby of her own.

Before they even say "I do," the bride and groom could run their blood samples through a device that looks like a one-hour photo machine to scan for hundreds of genetic diseases they might harbor.

Armed with that information, the bride could have her eggs fertilized in the laboratory, then tested and genetically modified before she is impregnated with one of them.

The result: a world free of major genetic disorders. No more Down's syndrome. No more sickle-cell anemia. No more hereditary forms of Alzheimer's disease.

The technical and economic hurdles to such a scenario remain so formidable that researchers in the field won't speculate about how close it might be.

But medical geneticists do look forward to a day - probably in the next five years - when they will have perfected an unintrusive blood test that will open up a whole new world of risk-free, prenatal screening for all American women. They will be able to test a fetus' genetic makeup by finding the 1-in-10-million fetal cells circulating in the mother's bloodstream.

"Once you're assured you've got true fetal cells, you can do an endless number of things," said Dr. Sherman Elias, leader of a University of Tennessee-Memphis team developing the test.

Currently, only pregnant women over 35 receive routine prenatal testing for genetic abnormalities because available procedures carry a risk of miscarriage. In younger women, the risk is not considered worth taking because the incidence of chromosomal abnormalities is so low.

With the blood test, thousands more of America's 4 million expectant mothers every year would face a choice they didn't have the information to make before - deciding whether to bear a child with a birth defect.

If they are like those who have come before them, a large majority will answer "no." Currently, 70 percent to 90 percent of women whose fetus is found to have birth defects choose abortion, two Miami genetic experts estimate. And, when you consider that 80 percent of infants with Down's syndrome are born to women under 35 - the group not routinely tested now - the ramifications of such a test become apparent: There could be fewer Down's babies. Still, we're a long way from a world of "perfect" babies if, indeed, society would ever even want such a thing. For example, of 120,000 mentally retarded children born every year, only about 20,000 cases are because of genetic problems, according to the March of Dimes. The rest have a variety of non-genetic causes - including maternal infection, oxygen deprivation during birth, toxemia, prematurity, drug or alcohol abuse during pregnancy, or chronic diseases, such as diabetes.

So, even if every genetic case is detected prenatally and all those mothers choose abortion, this wouldn't eliminate mental retardation in children.

Even single-gene hereditary diseases will be hard to eliminate, scientists say.

"The number of human genes is estimated at anywhere from 10,000 to many more than that, and you can't be looking at every single gene on every single child," said Dr. Magdalena Plewinska, an instructor in clinical pediatrics and genetics at the University of Miami. "Science can do any one thing, but it can't do 10,000 at the same time. There'll always be that one more thing that you don't know and you can't control or predict."

Genes are the basis of all human heredity, from why a person's eyes are blue to why cancer or mental illness runs in certain families. They are contained on the body's chromosomes, long ribbons of the chemical DNA along which genes are strung like pearls on a necklace.

When chromosome sections are deleted, repeated or moved to the wrong place - as in Down's syndrome - a baby is born mentally retarded or with life-threatening physical problems. Easy to see under a microscope, these chromosome defects were the first of genetic disorders to be targeted by prenatal testing as it developed in the late 1970s and early '80s.

In the past few years, though, scientists have also learned to detect the genes that lead to hereditary diseases like sickle-cell anemia, Tay-Sachs disease, cystic fibrosis and even a rare familial form of Alzheimer's disease.

The effectiveness of the new blood test will be tested in 3,000 women over the next four years in a federal experiment. It will be an option added to two state-of-the-art prenatal tests currently used - amniocentesis and chorionic villus sampling (CVS) - which require inserting a needle into the uterus to draw fluid or placental tissue out. This raises the risk of miscarriage by between 0.5 to 0.8 percent.

But the maternal blood test being developed would remove the miscarriage danger and open the possibility of screening all women. Amniocentesis to confirm that blood test would follow at about 16 weeks of pregnancy.

"People always say this is going to replace amniocentesis, but I don't think that it is," said Dr. Diana Bianchi, a geneticist researching the blood test at Children's Hospital in Boston.

"I think what we're trying to offer is a way for low-risk women to know more about their pregnancy. ... What we're trying to do is get information to those women," she said.