Tuesday, January 09, 2007

Picking the Best Embryo from the Bunch

New embryo-screening technologies could weed out embryos with major genetic abnormalities and improve IVF success rates.

By Emily Singer

Scientists remove a single cell from an embryo derived from in vitro fertilization so that it can be used for genetic screening. Credit: Credit: Pascal Goetgheluck / Photo Researchers, Inc

More and more of the thousands of women undergoing in vitro fertilization (IVF) are asking their doctors for preimplantation genetic screening, a special type of embryo testing designed to weed out embryos with abnormal chromosomes. Such embryos are less likely to lead to successful pregnancies--they either fail to implant or miscarry, or if they do come to term, they can produce babies with disabilities such as Down's syndrome. Current screening techniques can only detect a subset of abnormal embryos, and doctors disagree about whether the screening substantially improves a patient's chance of pregnancy. But new screening technologies about to hit the market could change that, potentially bringing a big boost in IVF success rates.

About 50 percent of human embryos are chromosomally abnormal, meaning they carry either one or three copies of a chromosome, rather than the typical two, or they have a chromosome with an abnormal structure. This percentage increases with a woman's age--it's up to 80 percent in women over 40. "We think the majority of IVF procedures that fail do so because of chromosomal abnormalities," says David Adamson, incoming president of the American Society for Reproductive Medicine.

Chromosome screening is just one type of genetic test available for embryo screening; others include tests for genetic diseases such as cystic fibrosis. But chromosome screening is by far the most common, accounting for about two-thirds of preimplantation genetic testing. Despite its popularity, however, such tests are controversial. Studies comparing the successful-pregnancy rates of those who have had their embryos screened with those who have not have produced conflicting results.

Current screening methods use specially designed fluorescent probes that bind to the different chromosomes, revealing if there are either extra copies or not enough. But only 10 to 12 different probes can be used at a time, meaning that about half of the embryo's 23 chromosome pairs go unscreened. "So you're probably still transferring embryos that are abnormal," says David Grainger, president of the Society for Assisted Reproductive Technology.

New methods that use DNA microarrays--small chips coated with specific DNA sequences--could provide a more accurate screening method because researchers can simultaneously analyze many more spots on the chromosomes. But the major barrier to using microarrays to analyze embryonic DNA is generating enough genetic material for the test. The single cell used for genetic screening, which scientists carefully remove from the embryo, contains far too little DNA for the tests. And the most common duplication method, known as the polymerase chain reaction, is too error prone to be used in this case.