DNA Swap May Prevent Rare Genetic Diseases: Cloning-Like Method Would Correct Mitochondrial Flaws

Article excerpt

A technique that puts one woman's nuclear DNA into another woman's donor egg cell maybe feasible for correcting inherited diseases caused by faulty cellular power sources. The technique has already produced healthy baby rhesus monkeys, and now it raises the possibility of preventing mitochondrial diseases in human newborns.

Mitochondria, energy-producing organelles inside cells, carry loops of DNA important for the power plants' function. Mutations of the mitochondrial DNA, which is passed to offspring directly by their mothers, can cause diseases that often affect energy-greedy organs such as the brain, heart, muscles, pancreas and kidneys with varying severity. An estimated 1,000 to 4,000 U.S. babies are born each year with mitochondrial diseases.

Swapping the nucleus, the cellular compartment where chromosomes are housed, from an egg with mutant mitochondria into an egg containing functional power plants could stop those diseases from happening in the first place. Offspring would inherit healthy mitochondria from the egg donor, while the rest of their genetic makeup would come from the mother and father.

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Researchers led by Shoukhrat Mitalipov, a reproductive and developmental biologist at Oregon Health & Science University in Beaverton, previously demonstrated that the technique works with rhesus monkeys. Now the team has succeeded in transferring the nuclei of unfertilized human eggs into donor eggs. Zygotes created by fertilizing those eggs produce embryonic stem cells, the team reports online October 24 in Nature. Short of actually transplanting the embryos into women to grow into fetuses, stem cell production is the clearest sign that the embryos are normal.

Performing the transfer procedure in the United States for women who carry faulty mitochondria, and implanting resulting embryos in the womb, will require approval of the federal Food and Drug Administration, which oversees clinical trials involving gene therapy.

Although the experimental therapy requires transfer of a nucleus into an egg, as human cloning would, it does not raise the same ethical concerns as human cloning, says Josephine Johnston, a research scholar at the Hastings Center, an organization in Garrison, N.Y., that examines the ethics of biological research. "To me it's not human cloning," Johnston says. "It's not the creation of an individual who is genetically identical to an existing person."

Not everyone is comfortable with the technique because children would carry DNA from three parents- a set of chromosomes each from the mother and father, and mitochondrial DNA from an egg donor. "Any time reproduction is proposed to take place in a novel way, some people get nervous," Johnston says. But if the technique is safe, she says, she sees no major ethical concerns barring it from reaching the clinic.

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Rhesus monkeys produced using the technique are now healthy 3-year-olds, Mitalipov and his colleagues report. "They are as normal as they can get," he says. …