Research demonstrates how genome editing can eliminate mitochondrial mutations

LONDON – Researchers have used genome editing to selectively eliminate mitochondria mutations in the germline of mice, a technique they suggest could be applied to the treatment of mitochondrial diseases in humans.

Editing mice genomes in oocytes and single-cell embryos to eliminate mutant mitochondrial DNA, while retaining normal wild-type mitochondrial genes, prevented transmission of mitochondrial diseases to offspring.

That would remove one of the main ethical concerns raised by the alternative approach of replacing the mitochondrial DNA from a woman carrier with mitochondrial DNA from a donor oocyte, which has prompted talk of "three-parent children."

In addition, the scientists succeeded in reducing the levels of human mutated mitochondrial DNA responsible for two different inherited mitochondrial diseases, in cells from human patients that had been introduced into a mouse oocyte.

Previous studies have shown that elimination of mutated mitochondrial DNA in somatic cells restores the normal energy-generating function of mitochondria in vitro, and the researchers said that demonstrates the potential of genome editing in treating mitochondrial disease.

The research by Pradeep Reddy and colleagues at the Salk Institute for Biological Studies in La Jolla, Calif., and a team of researchers based in labs in China, Japan, Spain and elsewhere in the U.S., was published in the April 23, 2015, issue of Cell.

The scientists confirmed that in addition to normal mitochondrial function, there were no off-target effects on the nuclear genome caused by manipulating maternal mitochondrial DNA in that way.

Furthermore, the mice offspring were fertile and analysis showed barely detectable levels of mutant mitochondrial DNA in the F2 generation.

The mitochondrial genome contains around 16,500 base pairs, against more than 3 billion base pairs in the nuclear genome. Those base pairs form 37 genes, compared to about 23,000 genes in the nuclear genome. There are more than 200 different mutations associated with mitochondrial diseases, and the researchers said they have developed genome-editing constructs that could be directed against virtually any of those mutations.

The paper has attracted attention from researchers in the UK, which earlier this year became the first country in the world to approve the use of mitochondria donation to prevent inherited mitochondrial diseases.

A law empowering the regulator, the Human Fertilization and Embryology Authority, to license the procedure will be passed before the end of 2015. (See BioWorld Today, Feb. 4, 2015.)

UK researchers applauded the study as potentially opening up a new avenue to treating mitochondrial diseases, but said much more work is required before the technique is ready for the clinic and that genome editing would not be relevant in all cases.

Marita Pohlschmidt, director of research at the charity Muscular Dystrophy UK, said although early stage, the results in mice are promising. "It does not require egg donation and could eventually be an important alternative to mitochondrial donation," she said.

The research elegantly illustrates how relatively simple methods can be used to deplete levels of disease-causing mitochondrial mutations, said Robert Lightowlers, director of the Institute for Cell and Molecular Biosciences at Newcastle University. "At least for women who have eggs that harbor both normal and disease-causing populations of mitochondrial DNA, it may be possible to use these methods to lessen the load of defective mitochondrial DNA and prevent transmission of mitochondrial disease," Lightowlers said.

Mitochondrial diseases are sparked when the percentage of mutant DNA molecules compromises the function of mitochondria. The threshold is estimated to range from 60 percent to 95 percent, depending on the severity of the mutations.

It is expected that the first UK license to carry out mitochondrial replacement will be granted to researchers in Newcastle who have pioneered the technique.

SAFETY CONCERNS

Doug Turnbull, professor of neurology at Newcastle University Hospital, said while it is important to try and widen the number of options for treating mitochondrial disease, mitochondrial donation will still be necessary for women whose oocytes have either large amounts of mutated mitochondrial DNA, or all mutated mitochondrial DNA. "It is these women who are most likely to benefit from mitochondrial donation," Turnbull said.

Other researchers noted that although genome editing is technically simpler, there is far more safety data for mitochondrial replacement.

"Although this clever alternative approach for correcting genetic errors in mitochondria is a technical masterpiece, it is unlikely to make the clinic in the near future," said Dusko Ilic, reader in stem cell science at King's College London. "Replacing faulty genes in human preimplantation embryos, germ cells or gametes poses serious risks and, with all ethical and especially safety implication, the therapeutic benefits are questionable," he said.

Reddy et al's study of editing the mitochondrial genome followed close on the heels of reports from Chinese scientists describing attempts to edit the nuclear genome of human embryos and recent calls for a moratorium on gene editing and gene modification on moral and ethical grounds.

Although not affecting the nuclear genome, U.S. researchers expressed similar concerns about mitochondrial genome editing.

It is true that in comparison to mitochondrial donation, the alternative approach of mitochondrial genome editing may solve a significant ethical issue for some, said Debra Matthews, assistant professor of pediatrics at Johns Hopkins University. However, "the question of safety and germline modification remain," she said.

Jennifer Kuzma, co-director of the Genetic Engineering and Society program at North Carolina State University, agreed. "I think the precautionary approach that the authors of the calls for moratoria make . . . should apply in this case."

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