Fetal Genome Sequencing Could Spot Disorders

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At present, the only test routinely done that uses maternal blood plasma.
Corbis

THE GIST

- Researchers have mapped the genome of a fetus using DNA samples from parents.

- A test based on the new technique could detect the roughly 3,000 conditions known as Mendelian disorders.

- The hope is that researchers can develop a clinical test that detects many diseases noninvasively.

In a development scientists are calling a "tour de force," researchers have reconstructed the genome of a fetus using DNA samples from the parents.

Because their technique did not require an invasive test to take samples from the fetus itself, it's an important step toward what could become a low-risk way to identify genetic disorders early in development, experts say.

Currently, "when genetic testing is done, it's done for just a few diseases," said lead author Dr. Jay Shendure, an associate professor of genome sciences at the University of Washington.

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A test based on the new technique could detect the roughly 3,000 conditions known as Mendelian disorders, each of which are the result of a single mutated gene, Shendure said. Huntington's disease, hemophilia and sickle-cell anemia fall into this category.

While each of these disorders is relatively rare, together they affect about 1 percent of births, Shendure said.

"This is amazing," said Dr. Ada Hamosh, director of the Institute of Genetic Medicine at the Johns Hopkins University School of Medicine, of the findings. "On the other hand, in no way is this ready for prime time," said Hamosh, who was not involved with the research.

Shendure and colleagues put together the fetal genome using a saliva sample from the father, and a sample of blood plasma from the mother. About 13 percent of the DNA found outside of cells in a pregnant woman's body belongs to her fetus.

They sequenced the regions of DNA they were aiming for with 98.2 percent accuracy, as confirmed after the baby's birth with a test on blood taken from the umbilical cord blood.

Shendure estimated that reconstructing the fetal genome cost roughly between $20,000 and $50,000.

But with the cost of genome sequencing expected to come down, and as the technique is further refined, Shendure said, the hope is that researchers can develop a clinical test that detects many diseases noninvasively.

It's also hoped that testing could be done earlier in pregnancy than current tests, he said. In the study, the researchers sequenced the fetal genome twice, once in a fetus at 8 weeks into the pregnancy, and another time with a different fetus at 18 weeks. The highly accurate information only came at the later time.

The accuracy is affected variations between pregnancies, Shendure noted. "It's possible it could be done in the first trimester, it just may require more resources to do it."

At present, the only test routinely done that uses maternal blood plasma, as in the study, is for Down syndrome.

But to confirm the results of that blood test, an invasive test called amniocentesis, which requires fetal tissue samples, is used, said Dr. Laird Jackson, a professor of obstetrics and gynecology at the Drexel University School of Medicine.

Hamosh said that while a noninvasive test would be an improvement, at present, a full analysis of a fetus's genome would be overkill, because parents can be tested for specific genes of concern, and a fetus could be tested if the parents' tests reveal a potential problem.

"You don't need whole genome sequence information on a fetus," she said. If a family is known to carry a risky version of a gene, then a test for that gene can be done.

But as the techniques used in the study are improved, she said, they could be helpful in catching cases where new mutations arising in the fetus are the source of genetic diseases, rather than mutations inherited from the parents.

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"I think this paper is a tour de force," Hamosh said. "This was unthinkable six months ago. Is it ready for any kind of clinical application? Not at all yet."

While a noninvasive test may be at least several years from the clinic, Jackson said, ultimately they could be a boon for parents, providing more of them with more information about their future children.

With current invasive tests, "if you go to the right person, if you go to a center that has excellent [health professionals], the risk of losing that pregnancy is so small," Jackson said.

But many mothers can't get to those top-notch facilities, he said.

A noninvasive diagnostic test could increase access, Jackson said. "Now mommy only has to go some place where someone can draw her blood."

Pass it on: Scientists have successfully sequenced the genome of a developing fetus without needing an invasive test.

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