Mitochondrial defects affect an estimated 1 in 4,000 children, and can cause rare and often fatal diseases such as carnitine deficiency, which prevents the body from using fats for energy. They are also implicated in a wide range of more common diseases affecting children and adults, such as multiple sclerosis and Parkinson’s disease. Mitochondria have their own DNA and are inherited only from the mother, so replacing defective mitochondria in eggs from mothers who have a high risk of passing on such diseases could spare the children. Three years ago, a team led by Shoukhrat Mitalipov, a reproductive biologist at Oregon Health and Science University in Beaverton, created1 eggs with donor mitochondria that developed into healthy rhesus monkeys (Macaca mulatta). Today, the same team reports2 the creation of human embryos in which all of the mitochondria come from a donor. The method needs to be tweaked to increase efficiency and gain regulatory clearance, but it is ready for the clinic, says Mitalipov. “You can expect the first healthy child to be born [using this method] within three years.”
Just as they did with the monkeys, Mitalipov and his colleagues removed the nucleus from an unfertilized egg, leaving behind all of that cell’s mitochondria, and injected it into another unfertilized egg that had had its nucleus removed. They then fertilized the egg in vitro. In the previous experiment, the team proved in convincing fashion that the fertilized monkey eggs were good — by implanting them in uteri, where they produced four healthy offspring. To evaluate the results with human cells, the researchers had to settle for developing the embryos to the blastocyst stage — a ball of about 100 cells. They used cells from the blastocysts to produce embryonic cell lines, and then carrying out various tests on them. The cells looked like those from normal embryos, but with mitochondria exclusively from the donor.