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Biomedicine

The Fingerprints of Embryos

Using DNA fingerprinting, researchers strive to improve in vitro fertilization.

When multiple embryos are transferred into a woman’s uterus during in vitro fertilization (IVF) and a single baby is born, there has previously been no way to know which embryo successfully implanted and developed. Now researchers have harnessed DNA fingerprinting, the same technique used to settle paternity suits and implicate criminals, to match an embryo to the baby it ultimately becomes. This technique may help researchers develop tests to more reliably discriminate between viable embryos and their nonviable siblings.

From blastocyst to baby: At five days after fertilization, this embryo–known as a blastocyst–has differentiated into two kinds of cells. One clump, called the inner cell mass (visible here in the upper right quadrant), will become the fetus. The other clump, called the trophectoderm (lining the blastocyst’s inner wall), will become the placenta. Australian researchers developed a technique to extract trophectoderm cells and obtain their DNA fingerprints. When multiple embryos are used for in vitro fertilization, these fingerprints can indicate which blastocyst became which baby. The researchers used a laser to zap a tiny hole in the blastocyst’s outer coating, directly across from the inner cell mass, and then sucked out 8 to 20 trophectoderm cells for analysis.

Currently, doctors select embryos for transfer based on a crude visual inspection under the microscope. But this tactic fails some 50 percent of the time. Consequently, multiple embryos are often transferred at one time, to increase the chances of a successful pregnancy. Multiple births, which put both mother and babies at risk, often result.

“The next big advance in IVF is going to come from our ability to select embryos better,” says David Adamson, president of the American Society for Reproductive Medicine, who was not involved in the project. “It will allow us to increase the pregnancy rate and decrease the multiple-pregnancy rate.”

In a study published last week in the journal Human Reproduction, researchers at Monash University, in Australia, led by IVF and stem-cell pioneer Alan Trounson, worked with a group of 48 women undergoing IVF. Five days after fertilization, the researchers used a laser to nick the outer coating of each embryo, known at that stage as a blastocyst. The nick allowed the researchers to extract a small number of cells from the layer of the blastocyst that eventually forms the placenta. The following day, each blastocyst was transferred into the woman’s uterus.

Such embryo biopsies are commonly used in IVF to obtain cells for prenatal genetic screening and diagnosis. But they are usually performed three days after fertilization, when the embryo has just six to eight cells. Because the window between fertilization and successful transfer is narrow, a day-five biopsy is usually too late to allow enough time for the cells to be tested, an informed decision to be made, and the embryo to be transferred. In this study, the biopsied cells were not used to make reproductive decisions, so the embryo could be transferred before test results came in. An earlier biopsy wouldn’t yield enough cells for meaningful analysis.

The researchers zeroed in on a group of 18 women in whom some but not all of the transferred blastocysts implanted and developed into babies. By obtaining each embryo’s DNA fingerprint from the extracted cells and comparing it with each baby’s DNA fingerprint, they could definitively tell which embryos had given rise to healthy babies.

Infertility specialists say that the fingerprinting approach may enable a rigorously controlled study of potential techniques for distinguishing between viable and nonviable embryos, a goal that has thus far proved elusive. Many groups are exploring such techniques; some are hunting for genetic markers specific to viable embryos, while others are pursuing less invasive analyses of the substances that a viable embryo secretes.

“In terms of doing sound science and figuring out what parameters are important to look at, no matter what technique you’re using, you have to have fingerprinting,” says Boston IVF medical director Michael Alper. (Alper was not involved in the fingerprinting study.)

In addition to piloting the fingerprinting technique, the Monash University study investigated one possible approach to sorting embryos by viability. Using extracted cells, the researchers compared the expression levels of some 45,000 genes between viable and nonviable embryos. In this case, they didn’t use the fingerprinting technique; because they could only extract a small number of cells, they could not perform both comprehensive gene-expression analysis and DNA fingerprinting on a single blastocyst. Instead, they pooled cells from viable embryos from women in whom all transferred blastocysts developed into babies, and from nonviable embryos from women who did not become pregnant but had no known uterine deficiencies.

“We felt that embryos that were capable of implantation and growing through to term would have a unique gene-expression profile,” says Gayle Jones, a coauthor of the study. More than 7,000 genes differed between viable and nonviable embryos, although the findings are still very preliminary. Ultimately, the researchers hope to narrow down this pool to a handful of markers that could identify those embryos most likely to succeed.

Biopsied blastocyst cells could be tested for these markers in a fast, straightforward procedure. “Then you could decide which embryos you would transfer–which would be the most viable,” says David Cram, also a coauthor of the research. And because this smaller-scale analysis would require less genetic starting material, it could be paired with DNA fingerprinting to validate its predictive power.

The study’s authors did not observe any negative effects of the blastocyst biopsy, but Marcelle Cedars, director of reproductive endocrinology and infertility at UCSF Medical Center, cautions that much is still unknown about the procedure and that further studies are needed to ensure its safety. Even if the biopsy itself proves harmless, growing the embryo in a dish for six days may be detrimental to development; traditionally, embryos are implanted sooner.

Cedars emphasizes the need to develop less invasive techniques in which embryo biopsy is unnecessary. For now, the DNA fingerprinting approach may prove useful in refining those techniques.

Regardless of their differing approaches, all these research efforts share a common ambition. “We’re hoping that this technique will ultimately eliminate multiple pregnancies” associated with IVF, says Jones.

“The goal is no longer just getting a woman pregnant,” says Cedars, “but really trying to have a healthy singleton birth.”

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