27 April 2015
Two recurring themes in the news so far this year - mitochondrial transfer techniques and gene editing – have converged, with the first published attempts at gene editing of mitochondrial DNA – albeit in this instance using mouse, rather than human, embryos.
Some forms of mitochondrial disease are caused by mutations in the mtDNA contained in cellular mitochondria, and separate from the nuclear DNA. Such diseases are maternally inherited, because each individual’s mitochondria are derived from those in their mother’s egg cell before fertilisation.
One of the complexities of mitochondrial diseases is that a cell can have a mixed mitochondrial population – some mitochondria containing mutated mtDNA and others not, a phenomenon known as mitochondrial heteroplasmy. This in turn can have a significant impact on disease severity (more significant where levels of diseased mitochondria are higher) and is the reason why preimplantation genetic diagnosis (PGD) to identify affected embryos can reduce but not eliminate the risk of disease in subsequent babies.
The purpose of mitochondrial transfer techniques is to avoid the transmission of maternal mtDNA mutations by using healthy donor eggs with normal mitochondria, combined with nuclear DNA from the mother.
Mitochondrial gene editing
Now researchers from the Salk Institute for Biological Studies in La Jolla, California, have attempted an alternative approach using targeted gene editing to correct mtDNA mutations. The technique is slightly different from the CRISPR/Cas9, as it uses a transcription activator-like effector nucleases (TALENs)-based system.
Writing in the journal Cell, they report that targeted gene editing allowed the selective removal of mutated mtDNA sequences, leaving corresponding healthy mtDNA sequences in place, in mouse eggs or embryos that subsequently gave rise to healthy adult mice. The offspring of these mice also showed reduced levels of mutated mtDNA
They also used mitochondria-targeted transcription activator-like effector nucleases (dubbed mito-TALENs) directed against two specific human mtDNA mutations in special human-mouse fusion egg cells. The mito-TALENS successfully reduced levels of the Leber’s hereditary optic neuropathy (LHOND) and neurogenic muscle weakness, ataxia, and retinitis pigmentosa (NARP) linked mutations. Even a reduction in the levels of mutated mtDNA (as opposed to complete elimination) can prevent disease transmission to offspring, since low levels may not compromise mitochondrial function enough to cause disease.
Barriers to application in humans: safety and semantics
The authors of the research conclude that their approaches ‘may be applied and developed to prevent the transgenerational transmission of human mitochondrial diseases’. However, as expert clinical geneticist Prof Frances Flinter told the BBC: "The biggest question to address will be the possibility that DNA cutting enzymes may disrupt adjacent genes that are important, leading to unintended adverse consequences".
Use in humans might also be said to constitute germline genetic modification, which in itself poses significant ethical barriers to application, although the UK government has defined germline genetic modification as the ‘germ-line modification of nuclear DNA’ so that mitochondrial gene editing, like mitochondrial transfer, would constitute germline modification but not genetic modification. However, the argument is rather weaker for a technique that literally involves editing the mtDNA, as opposed to replacing whole mitochondria, which has been likened to blood donation.
The techniques published in Cell differ significantly from mitochondrial transfer in another respect, too: no donor egg is required. This could be seen as an ethical benefit, and certainly as a practical one, since supply of donor eggs for fertility treatments and research is typically limited. Salk researcher Alejandro Ocampo observed: “The clinical application of our technique does not require donor eggs. We are just performing a single injection into the patient’s egg or one-cell embryo, which is technically easier than mitochondrial replacement”.
Insofar as they go, these latest research findings are interesting and open new possibilities for future approaches to preventing the transmission of mitochondrial diseases. However, their use in humans could also fall within the controversial arena of human germline gene editing, where technical capacity is already outstripping societal debate and ethical decision-making.