A WOMAN has become pregnant through a procedure that combines a controversial IVF method with one of the techniques used for cloning. But the fetuses that resulted, although they did not survive to term, were certainly not clones. In fact, they had three genetic parents.
The procedure involves transferring a fertilised nucleus from the mother鈥檚 egg to an egg from another women (see Graphic). Any child born this way would inherit the vast majority of its genes from its mother and father, like a normal baby, but the handful of genes found outside the nucleus, in the cell structures called mitochondria, would come from another woman.
The US team that developed the method say it could be used as a form of genetic therapy, to allow women who have genetic defects in their mitochondria to have normal children. It could also help the small proportion of women who are infertile because of faults in their eggs. 鈥淚t鈥檚 nothing to do with human cloning,鈥� says team leader Jamie Grifo of the New York University School of Medicine. 鈥淚t is in no way an attempt to create a genetic copy of an adult.鈥�
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Babies with three parents have already being born in the US (麻豆传媒, 12 May 2001, p 7). They were created by injecting cytoplasm (along with the mitochondria) from one egg into another. The use of this method now requires FDA approval.
The latest work was done by John Zhang鈥檚 team at the Sun Yat-sen University of Medical Science in Guangzhou, China, with the help of Grifo鈥檚 team. They took eggs from a 30-year-old woman for whom IVF had failed, because her embryos stopped developing after two days. Other eggs were obtained from donors. The eggs were fertilised by intracytoplasmic sperm injection (ICSI).
The donor eggs were then emptied of their nuclei, and a fertilised nucleus from the woman鈥檚 eggs transferred into each one. Five of the resulting embryos were implanted, resulting in a triplet pregnancy. Doctors reduced this to twins, but one fetus died at 24 weeks, and the other at 29 weeks. The details were presented at a meeting of the American Society for Reproductive Medicine (ASRM) in San Antonio, Texas, this week.
Azim Surani at Cambridge University, who has used a similar technique in mice, agrees the method is not comparable with cloning. The key problem in cloning, where a nucleus from an adult cell is transferred to an empty egg, is that the adult nucleus has to be genetically 鈥渞eprogrammed鈥� if it is to develop into an embryo. In contrast, a fertilised nucleus from another egg already has the necessary programming.
鈥淗owever, that doesn鈥檛 mean that the procedure they used won鈥檛 lead to serious abnormalities when applied to humans,鈥� says cloning expert Bob Lanza of biotech company Advanced Cell Technology of Massachusetts. 鈥淲e know that many of the manipulations they used in this study are associated with a significant increase in genetic defects and other abnormalities.鈥� But Grifo thinks the miscarriage was unrelated to the technique used.
鈥淭he charge that this is going to lead to human cloning could really be levelled at any infertility treatment,鈥� adds Sean Tipton of the ASRM. And Surani points out that ICSI, which has a higher risk of defects, could be considered as a form of nuclear transfer.
The future of the method is unclear. China is introducing regulations that will ban it, but it is legal in some countries.