Montreal

A technique for detecting genetic abnormalities in embryos is itself coming under the microscope. Preimplantation genetic diagnosis (PGD) is used in fertility treatment to check the condition of an embryo before it is implanted in a mother's womb. But studies now suggest that there is still much to learn about the procedure.

The findings suggest that genetically ‘abnormal’ embryos could still give rise to normal embryonic stem-cell lines. And they lend urgency to a newly launched effort to track the safety of the procedure, which is becoming increasingly popular but is regulated differently around the world.

In PGD, a cell is taken from the embryo at the eight-cell stage and tested for genetic abnormalities. If the cell is given the all-clear, the embryo is implanted in the mother's uterus. Babies born after the procedure seem to be fine, but few data have been collected on the children later in life.

It is also difficult to know how well the technique works in different cases. PGD is currently targeted at older women trying to have a baby. But whether it actually cuts the rate of miscarriage among these prospective mothers is only now being examined, researchers told a joint meeting of the American Society for Reproductive Medicine and the Canadian Fertility and Andrology Society in Montreal this week.

And results presented at the meeting show that PGD picks up surprisingly high rates of genetic abnormalities in younger women who donate eggs to infertile couples. That led some specialists, such as Jeffrey Nelson of the Huntington Reproductive Center in Pasadena, California, to suggest that the technique should be used more widely. “You can make an argument that PGD should be done in younger women”, not just older women, he says.

But the most intriguing data presented at the conference suggest that embryos diagnosed as abnormal might be able to correct their genetic defects as they grow. A team including Santiago Munné of Reprogenetics, a company specializing in PGD based in West Orange, New Jersey, grew 55 embryos previously diagnosed as abnormal to the preimplantation stage, called the blastocyst. They found that many of the embryos' cells were genetically normal. One embryo contained 76% normal cells after 12 days in culture. On average, 48% of the embryos' cells were genetically normal by the blastocyst stage.

These results hint that such embryos might be an ‘ethical’ source of embryonic stem cells for use in research. Normally, producing these cells means destroying a viable embryo, leading many groups to brand the practice as unacceptable (see page 1076). But the genetic abnormalities of embryos rejected by PGD mean that they are unlikely to develop into a healthy baby. “These results suggest that we could get normal stem cells from abnormal embryos,” says Munné. “These embryos will never implant, so this is ethically the best way of making them.”

But the results also raise questions about whether cells that are not directly sampled by PGD are actually defective. If they aren't, the potential of these embryos remains unclear. “These are the really important data we have to get from PGD,” says Paulette Browne, a reproductive endocrinologist at the Shady Grove Fertility Center in Virginia. “It could be that some of the cells are abnormal and some are not, and we're starting to collect the rest of those data right now.”

Together, the latest studies highlight the importance of a new effort to accumulate data on PGD, described at this week's meeting. Scientists and policy analysts announced the creation of a US database to track the safety of the procedure. A similar database exists in Europe, where PGD is tightly regulated, but US clinics, which carry out thousands of procedures, are not required to report their data.

The US database, set up by the Johns Hopkins University's Genetics and Public Policy Center in Washington DC, will collect information on who is using the technique in the country, the reasons for its use, and whether it results in healthy children.