IMAGINE if, a mere three days into your existence, a cell is plucked from your eight-cell being and couriered off to a lab in New Jersey or Illinois. There, the chromosomes and genes of the confiscated cell are assessed, and the remainder of you will live or die by the verdict. Twenty-four hours later you are given the all-clear, and are placed in a womb, where you implant and develop into a healthy baby.
Around 5000 people now belong to this PGD generation 鈥� individuals who grew from embryos specially selected using 鈥減reimplantation genetic diagnosis鈥�. So far, there is no evidence that removing a single cell early in development is harmful. But at a meeting of the American Society for Reproductive Medicine last week in Montreal, Canada, reproductive scientists debated just how safe the technique is, whether it is worth the risk, and who should be allowed to use it.
鈥淲hen you take a cell from an eight-cell embryo, you can鈥檛 say it has no consequence,鈥� admits Jacques Cohen, research director of Reprogenetics, a firm based in West Orange, New Jersey, which specialises in PGD. 鈥淵ou have just taken one-eighth of the biomass.鈥� Indeed, research using frozen embryos indicates that taking two cells, as is routinely done in some European labs, compromises the viability of the embryo and lessens its chance of resulting in a live birth, he says.
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Questions are also being raised over which cell to remove. Earlier this year, researchers showed that the eight cells in a 3-day-old embryo might not be equal, having possibly differentiated more than was first assumed (快猫短视频, 12 March, p 15). And Nicole Noyes, a reproductive endocrinologist at New York University School of Medicine, points out that we also do not know whether breaching the zona pellucida, the protective covering around the embryo, has any impact on an embryo鈥檚 development.
Until now, PGD has been mostly limited to testing embryos from women who are older or who suffer chronic miscarriage, or in failed IVF cases. But that may be about to change. Evidence presented at the meeting shows that almost half of all embryos donated by even young women for use in IVF are abnormal (快猫短视频 online, www.newscientist.com/dn8174). 鈥淚t may be beneficial to routinely use PGD [for IVF],鈥� says research team member Jeffrey Nelson, who is at the Huntington Reproductive Center in Pasadena, California.
A study conducted by Noyes showed that, at least in the high-risk groups, PGD dramatically improved the rate at which successful embryos are selected. Fertility specialists usually select embryos for implantation primarily on the basis of their morphology 鈥� how intact and how advanced they look. 鈥淚t鈥檚 a beauty contest,鈥� says Noyes. But when she reviewed 787 embryos in her clinic, she found 75 per cent of those that appeared to be top grade morphologically actually had chromosomal abnormalities. And only 18 per cent of embryos confirmed as healthy by PGD had been decreed usable on the morphological criteria, she told the meeting. In some cases, she found that relying on morphology alone would have led to only abnormal embryos being selected for implantation. 鈥淚t was very disturbing,鈥� she says.
鈥淭hree-quarters of embryos that appeared to be top grade morphologically actually had chromosomal abnormalities鈥�
鈥淭he best way to select an embryo, normal from abnormal, is PGD,鈥� says Yury Verlinsky, director of the Reproductive Genetics Institute in Chicago, which provides fertility treatments including PGD. There is evidence that PGD cuts miscarriage rates and means that fewer embryos need be transferred to create a viable pregnancy, reducing the incidence of multiple births.
But while Noyes agrees that PGD does have such benefits, she says the evidence is not yet strong enough to mean the technique should be used widely, especially in cases where there is no medical indication, such as sex selection. 鈥淚 don鈥檛 think the facts are clear enough,鈥� she says.
