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Last week marked the end of a two-year waiting period for several scientists at Harvard University who are planning to start human cloning experiments, a crucial first step in generating stem cell lines matched to specific patients. While the announcement came amid much fanfare, the researchers will now start a new waiting game. They need fresh human eggs to begin their experiments – and they have no idea how many women will step forward to undergo the lengthy and potentially risky donation procedure.

Much of the attention on human cloning has centered around ethical issues associated with destroying embryos. In fact, getting access to an adequate number of eggs may end up being the biggest hurdle in the thorny field of human therapeutic cloning, also known as somatic cell nuclear transfer. Scientists have no idea how many eggs it will take to clone a human embryo, and therefore don’t know how big a pool of egg donors they’ll need for success.

To clone an embryo, scientists first strip an egg of its genetic material. Then they insert DNA from an adult cell, such as a skin cell from a diabetic patient, into the egg. Through an unknown process, the egg turns back the clock on the adult DNA and begins to develop as a normally fertilized egg would. Researchers can then collect from the embryo a specialized ball of cells, which can be coaxed into stem cells. So far, though, no one has successfully performed this feat.

Since these stem cells are genetically matched to the DNA donor, they could be used to generate new, more accurate models of complex genetic diseases, such as diabetes. Eventually, scientists also hope to use the cells as the basis for cell transplant therapies (see “Stem Cells Reborn”).

Multiple U.S. institutions have started or will start human cloning research. Last week Harvard announced it had approved research proposals from three scientists to do human nuclear transfer, after lengthy ethical and scientific reviews. Under the new Harvard project, Kevin Eggan and Doug Melton, of the Harvard Stem Cell Institute, aim to create models for diabetes and neurological diseases. George Daley of Children’s Hospital Boston plans to develop cell transplants for children with blood diseases.

Last month, with much less pomp, scientists at the University of California, San Francisco started human nuclear transfer experiments. Like Harvard, they aim to create patient-specific stem cells, as well as to study the reprogramming process that takes place when an egg is fertilized. In addition to the two universities, Advanced Cell Technologies (ACT), an embryonic stem cell company with offices in Worcester, MA, plans to restart its human cloning program, which was shut down in 2004 due to a lack of funds and an inadequate supply of eggs.

But all these groups face an potentially insurmountable problem. Unlike other embryonic stem cell research, these experiments require unfertilized human eggs. The egg donation procedure is uncomfortable, potentially painful, and carries some medical risk. Women must undergo hormone treatments to stimulate ovulation, counseling sessions to understand the risks involved, and a medical procedure in which a needle is inserted into the vagina to remove eggs from the ovary. A small percentage of donors develop ovarian hyperstimulation syndrome, which in rare cases can cause kidney damage.

Harvard scientists say they have taken steps to ensure the safety of egg donors. “We are limiting the number of oocytes we will retrieve from one women to a small number in order to decrease or eliminate any risk of ovarian hyperstimulation,” says Eggan. He estimates that they will be able to harvest 8 to 10 eggs from each woman who undergoes the procedure. Women will be monitored throughout the process to make sure they are not having an adverse hormonal reaction.

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Tagged: Biomedicine

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