Advanced Genetic Tests Spot More Defects Before Birth
Birth defects can be spotted more precisely with advanced genetic tests that are performed on a fetus than with current prenatal tests, and may help pinpoint abnormalities that cause stillbirths, researchers said.
Two studies published yesterday in the New England Journal of Medicine suggest the tests called chromosomal microarrays could become standard for women with high-risk pregnancies. They may eventually be offered to all pregnant women, said Ronald Wapner, vice chairman of research at Columbia University Medical Center’s department of obstetrics and gynecology in New York.
The microarray tests are widely used to diagnose problems in newborns with developmental delays, autism disorders or congenital anomalies, Lorraine Dugoff, a physician and associate professor of obstetrics and gynecology at the University of Pennsylvania, wrote in an editorial accompanying the studies. The question is when the sensitive tests should be given because doctors don’t know the significance of everything they find.
“These reports highlight the power and complexity, and some of the pitfalls, of using new genomic technology in clinical practice,” Dugoff wrote. “Chromosomal microarrays can detect almost all the chromosomal imbalances” found with older tests, in addition to much smaller changes, she said.
Finding variants with uncertain significance could cause stress and anxiety for parents, who may be considering terminating the pregnancy, she wrote. They are also more expensive, costing two or three times the $500 to $600 price of conventional tests, Wapner said.
Current tests called karyotypes are performed using samples taken from the amniotic fluid or placenta. Doctors do a visual exam of chromosomes in the sample of cells, which Wapner compared to looking at a group of aircraft carriers while flying overhead. Major structural changes can be seen, though requires a substantial alteration or a large piece of missing DNA.
The newer tests are much more precise, like picking out the individual airplanes on the carriers’ decks, he said in a telephone interview. They can also look at specific locations to identify much smaller changes, he said. The microarrays also can spot pieces that are missing or overrepresented, he said.
“The biggest advantage of microarray is it can give us a lot more information,” Wapner said. “We are expanding the information a person can get out of prenatal testing, identifying additional and more serious problems.”
Affymetrix Inc. (AFFX) and Agilent Technologies Inc. (A), both based in Santa Clara, California, donated the microarray kits and reagents used in the study. Laboratory Corp. of America Holdings’s Integrated Genetics unit conducted the conventional testing.
Wapner’s study involved 4,406 women who were scheduled for prenatal testing because they were older mothers, had an abnormal result on a screening test for Down’s syndrome or had an ultrasound that showed a birth defect or something similar.
The microarrays, which compare fetal DNA with a normal sample, were as accurate as conventional tests for changes like extra or missing chromosomes that cause conditions such as Down’s syndrome.
Microarrays also found genetic changes in 6 percent of women carrying a fetus with a structural abnormality who had normal karyotype results. Another 1.6 percent of older women or those with abnormal screenings told they had normal karyotype tests had abnormalities detected with the microarray.
“Prior to the microarray, we would have said there was no genetic cause,” Wapner said. “In 6 percent of them, there was a genetic cause, it was just too small to be identified by karyotype. This allows us to better counsel a patient.”
For example, a heart defect that has no genetic cause may be limited to the heart, he said. That could be corrected by a surgeon. If the defect is genetic, however, the infant may have other health problems, he said.
The second study used microarrays to analyze 532 stillbirths. The cause of a stillbirth can be particularly difficult to decipher, since karyotyping and other tests require dividing cells. No reason is found for 25 percent to 60 percent of stillbirths, previous studies have shown.
Microarrays were able to determine the cause of the stillbirth in 87 percent of cases, compared with 71 percent using conventional testing, the study found. Genetic variations with unknown significance were found in 2.6 percent of the cases.
“Microarray was significantly more successful at returning clinically relevant information because, unlike karyotyping, it does not require cultured cells,” said Brynn Levy, co-director of personalized genomic medicine at Columbia. “Viability does not come into play at all. DNA can be extracted from tissue that is no longer living.”
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