Mitochondrial DNA replacement successful in Rhesus monkeys

New procedure may halt some serious inherited diseases, a study suggests

Scientists may have found a way to prevent the transfer of serious inherited mitochondrial diseases from mother to child. By shuttling DNA from an egg cell to a donor cell, the technique enabled the birth of four healthy Rhesus monkey males, researchers report online August 26 in Nature.

LIVING ON BORROWED DNA These twin Rhesus monkeys, born on April 24, 2009, developed from cells with donor mitochondrial DNA, a technique that may halt the transmission of certain kinds of inherited diseases. Mito (on the left) and Tracker (on the right) were named for a dye called mitotracker, which labels mitochondria. Oregon National Primate Research Center at OHSU
HEALTHY BABY Spindler, the third of four infants born thanks to this technique, arrived healthy on May 8, 2009. Oregon National Primate Research Center at OHSU

“We consider this a big achievement,” study coauthor Shoukhrat Mitalipov of the Oregon National Primate Research Center in Beaverton said in a news briefing August 25. “We believe that the technique can be applied very quickly to humans, and we believe it will work.”

Mitochondria, power-producing organelles in cells, carry their own DNA, distinct from the DNA held in cells’ nuclei. Healthy or otherwise, mitochondrial DNA is passed from mother to child. In recent years, researchers have identified more than 150 harmful mutations in mitochondrial DNA, some of which can cause serious and debilitating diseases (SN: 2/28/09, p. 20).

“This whole field of mitochondria medicine is very new,” says Douglas Wallace, a mitochondria expert at the University of California, Irvine. “It is a very important problem that’s been pretty much ignored. It affects lots of people, but we have very little to offer them.” Some estimates report that 1 in 6,000 people may have inherited a mitochondrial DNA disorder. Other estimates put the number higher, Wallace says.

A single cell can have thousands of copies of mitochondrial DNA. Usually, all of these copies are the same, healthy type, but occasionally a cell can have a mixture of normal and mutant mitochondrial DNA, a condition called heteroplasmy.

Heteroplasmy in an egg cell makes it nearly impossible to determine if a baby is going to inherit a severe mitochondrial disease, says Jo Poulton of the University of Oxford in England. “You can get quite a big range of how much mitochondrial DNA is transferred to children. There’s been a lot of debate on whether you can or can’t do genetic counseling” for these women, she says.

To get around the guesswork surrounding inherited mitochondrial diseases, the researchers took the mother’s mitochondrial DNA completely out of the picture. In the new work, researchers identified nuclear DNA in a mother’s egg cell by the DNA’s attachment to structures called spindles. Researchers removed the nuclear DNA (leaving the original mitochondrial DNA behind) and then put it into different egg cells lacking nuclear DNA but replete with healthy donor mitochondrial DNA. With the help of an inactive virus, the nuclear DNA fused into the donor cells.

Next, these modified egg cells were fertilized with donor sperm and implanted into Rhesus females to develop. Male twins, named Mito and Tracker, were born healthy, followed later by two more individual males, named Spindler and Spindy, from different mothers.

Researchers found no traces of the original egg cell’s mitochondrial DNA in the offspring, indicating that the process successfully prevented its transfer.

“I’m very pleased to see that, in nonhuman primates, this paper shows conceptually that this system might work,” Wallace comments.

The researchers used Rhesus monkey mothers with no mitochondrial DNA mutations because there are no established primate groups with such mutations, Mitalipov says. However, genetic signatures from the two cell groups—the mother and the egg donor—were distinct enough to tell apart easily, he adds.

Mitalipov and colleagues plan to monitor the monkeys as they age. “We’d like to see the growth and development of the offspring, to see if they have any abnormalities,” he says. Seeing how the mitochondrial DNA fares in subsequent generations will also be important, he adds. Doing that, though, will require female offspring, since mitochondria are passed on maternally. Mitalipov says they are now trying for a girl.

Many of the techniques used in the experiment are already in use at human fertility clinics, Mitalipov says, although the procedure as a whole will need to gain approval from the Food and Drug Administration before being used. The procedure, he says, “is offering real treatments to real patients.” 

Others remain cautious. Wallace says that the virus used to integrate the nuclear DNA into a donor cell would need to be scrutinized for safety. The virus is “something FDA would look at very carefully,” before approving the procedure, he says. What’s more, no one knows what will happen as the monkeys get older. “They look normal now, but who knows what they’ll look like as they age,” Wallace says.

Poulton points out that techniques like this performed in animals might not reveal subtle defects, such as the mild hearing loss associated with some mitochondrial DNA mutations. “Doing it in an animal is quite a long way off from doing it in humans,” she says.

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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