As well as possible future treatments for humans, the hope is this approach may one day provide an alternative for mitochondrial replacement therapy used in IVF to produce babies free of mitochondrial disease.
Published last week in Nature Medicine, two back-to-back papers describe how genome editing technologies were harnessed in live mice to help correct a mitochondrial disorder.
'These are remarkable findings that make it possible to even consider doing this in humans,' Dr Martin Picard at the Columbia University Irving Medical Centre in New York City, who was not involved in the work, told Science.
There have been long-standing hopes of using gene therapy to treat diseases of these key organelles – often called the 'powerhouses' of the cell as mitochondria are responsible for energy production, and so when they do not work fully there can be serious health consequences.
Most of the genes that control mitochondrial behaviour are in the mitochondria themselves, separate from the DNA in the nucleus of a cell. This difference makes mitochondrial disease gene therapy very challenging, as tools developed for regular genes will not always translate. There are, however, usually many mitochondria per cell with slightly different DNA, which presents an opportunity.
'One idea for treating these devastating diseases is to reduce the amount of mutated mitochondrial DNA by selectively destroying the mutated DNA, and allowing healthy DNA to take its place,' said Dr Michal Minczuk of the UK's Medical Research Council Mitochondrial Biology Unit in Cambridge, and a senior author of one of the studies.
Both groups made use of the same mouse model of mitochondrial disease, which has the same mutation found in some human patients. Each group then used a different technique to target that specific mutation for elimination.
These techniques – called transcription activator-like effector nucleases (TALENs) and zinc-finger nucleases (ZFNs) – use specific protein sequences to target and chop up target DNA. TALENs and ZFNs were designed to eliminate the mutant mitochondrial DNA in the mice.
The teams used these approaches to genome editing, rather than the more ubiquitous CRISPR/Cas9, because CRISPR uses RNA guide molecules to target the DNA site for cutting. It is more difficult to get RNA into mitochondria than TALEN and ZFN guide proteins, although the latter are more laborious and costly to produce.
The teams inserted the DNA encoding the genome editing tools into a modified virus and infected the mice. One group injected it directly into a leg muscle, while the other group injected it into the mice's circulation, from which it could make it to the muscle cells of the heart. Both studies showed that the gene therapy made it into the muscle cells and successfully targeted the mutant mitochondria, tipping the balance so that healthy mitochondria could take over.
The study that performed vein injection further showed that the metabolism of the cardiac muscle cells seemed to improve following this treatment, indicating the process seemed to have the desired effect.
'The approach appears safe in mice, and we would like to move it into humans,' said Dr Carlos Moraes at the University of Miami Miller School of Medicine in Florida, senior author on the study harnessing TALENs. 'Of course, the delivery of genes to several tissues is still a challenge.'