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June 13, 2012

Researchers predict genome of child without invasive screening

What if you could read much of your child's medical future while it was still in the womb? Taking a major step toward that goal, one fraught with therapeutic potential and ethical questions, scientists have now accurately predicted almost the whole genome of an unborn child by sequencing DNA from the mother's blood and DNA from the father's saliva.

At the moment, prenatal diagnosis for a small number of genetic conditions is usually done from fetal cells that doctors capture from fluid in the womb (amniocentesis) or a snippet of placental tissue (chorionic villus sampling). But these methods, which require the insertion of a needle or tube into the womb or placenta, can cause miscarriages in about 1% of all cases. In 1997, chemical pathologist Dennis Lo, now at the Chinese University of Hong Kong, discovered that roughly 10% of the cell-free DNA floating in a pregnant woman's blood stream stems from her fetus. In 2010 in a paper published in Science Translational Medicine, Lo's group showed that enough such fragments of fetal DNA are there to reconstruct the fetus's whole genome, and that it should be possible to use this DNA to test the unborn child for genetic diseases without exposing it to the risk of an invasive procedure. "The biggest advantage is that you are saving all those babies that would be lost," says Lo.

But it is tricky to distinguish fetal DNA in the blood from the mother's DNA. One strategy makes use of subtle genetic variations that exist between a mother's pairs of chromosomes. In most cases, for a particular genetic sequence on a specific chromosome, the variants from each pair should be represented equally in the woman's blood. But in an expectant woman, whose child has received only one variant as part of its genetic inheritance, her blood will contain a little more of that variant because of the free-floating fetal DNA. If the mother's patterns of genetic variants, or haplotypes, are known, statistics allow researchers to conclude what variants she passed on to her offspring. In 2010, Lo showed that with both parents' haplotypes known, it would be possible to predict the child's genome from the DNA in an expectant mom's blood.

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