Mitochondrial replacement therapy, where a nucleus from a mother's egg cell is transferred into a donor egg containing healthy mitochondria, shows promise for preventing the inheritance of mitochondrial DNA diseases. However, small amounts of mitochondrial DNA can sometimes hitch a ride with the transferred nucleus, and a study publishing May 19 in Cell Stem Cell shows that this DNA can override the mitochondria in the donor cell. The findings may call into question the beneficial effect of nuclear transfer for mitochondrial replacement therapy.

"We identified a challenge to making mitochondrial replacement therapy safe and effective," says senior study author Dieter Egli of the New York Stem Cell Foundation. "We anticipate that the findings will inform decisions regarding when and how mitochondrial replacement in humans will be done clinically."

Mitochondria generate most of a cell's energy, and mutations in mitochondrial DNA can result in serious health problems including developmental delays, seizures, dementia, heart failure, liver dysfunction, vision loss, deafness, and premature death. Although mitochondrial replacement therapy to prevent these conditions has not been approved by the US Food and Drug Administration, it has been approved in the United Kingdom, and the National Academies of Sciences, Engineering, and Medicine recently concluded it is ethically permissible to conduct clinical investigations in the United States.

These are cardiomyocytes derived after mitochondrial replacement (red: troponinT, green aSMA). Credit: Courtesy of Greg Lallos and Bruce Sun.

Mitochondrial DNA is exclusively transmitted through the mother's egg cells--sperm don't contribute mitochondria to a fertilized embryo. Therefore, one technique for preventing the transmission of mutated mitochondrial DNA is nuclear transfer, in which the nucleus of an egg cell containing mutated mitochondrial DNA is transferred to a healthy donated egg cell whose nucleus has been removed. The goal is to generate an egg cell that contains the intended mother's nuclear DNA and the female donor's healthy mitochondrial DNA.

In the new study, Egli and his collaborators examined the reproducibility and consistency of the nuclear transfer procedure. Using healthy human egg cells, the researchers transferred the nuclear genome of an "original" egg cell into an unfertilized "donated" egg cell. Half of the resulting cell lines contained a low percentage of mitochondrial DNA from the original egg cell, not just the donated egg cell.

Over a period of six months, the researchers continued to measure the amount of transferred mitochondrial DNA in the cell lines as the mitochondrial DNA continuously turned over and underwent genetic drift. The transferred mitochondrial DNA vanished over time in most cases, showing that complete mitochondrial DNA replacement is possible. But a few of the cell colonies (containing mitochondria of widely different genotypes) underwent complete reversion, with up to 100% of the mitochondrial DNA matching that of the transferred DNA.

In future research, Egli and his team will look for ways to avoid mitochondrial DNA carryover during the nuclear transfer procedure. Possible strategies include reducing the transfer of cytoplasm during nuclear transfer, or only selecting embryos without detectable levels of "extra" mitochondrial DNA.

"We will also examine whether matching of mitochondrial genotypes will be able to avoid mitochondrial genotype instability, and how precise this match needs to be, whether it has to be of the same ethnic group, or even of the same maternal lineage," Egli says. "Progress on either of these fronts should provide a path to therapeutic translation."