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Using base editing, mutations in mitochondrial DNA can be corrected, resulting in improved function.
Desearchers in Utrecht are using a new gene-editing technique to correct harmful mutations in mitochondrial DNA in human cells. Their ultimate aim is to treat mitochondrial diseases (PLOS Biology, 24 June).
Mitochondria, the energy suppliers of cells, contain their own DNA. This differs from nuclear DNA in that it is circular and transmitted exclusively through the egg cell. Mitochondrial mutations can lead to hereditary diseases and cancer. The now widely used CRISPR-Cas technology can correct errors in nuclear DNA, but does not work for mitochondrial mutations.
Tested
‘CRISPR-Cas is very precise and efficient, but has the drawback that it cannot penetrate the mitochondria to edit mitochondrial DNA there. A different type of editor, the base editor DdCBE, can do this. It makes use of programmable DNA-binding proteins – so-called TALENs – to recognise the correct DNA sequence,’ explains last author Martijn Koppens, a medical biotechnologist at UMC Utrecht, in an e-mail. The technique allows researchers to correct a single base without having to cut the DNA strand. This new publication stems from one of the first studies in which the method has been extensively tested on cells from patients with mitochondrial diseases.
‘CRISPR-Cas is very precise and efficient, but has the drawback that it cannot penetrate the mitochondria’
The Utrecht researchers applied DdCBE to liver cells with an artificially reduced ATP production, and to skin cells from a patient with Gitelman syndrome – a hereditary condition in which the kidneys lose too much salt, caused by a mitochondrial mutation. In both cases, they succeeded in at least partially restoring the affected mitochondrial functions.
Nanoparticles
‘We were surprised by the efficiency of the editor,’ says Koppens. ‘Whereas CRISPR-Cas only needs to edit two copies per cell, mitochondrial editors have to tackle hundreds to thousands of copies. Yet we observed a high percentage of editing in many cell types.’ The researchers use messenger RNA packaged in lipid nanoparticles to deliver the editor into the cells. This approach proves more efficient and less toxic than earlier techniques, such as the use of plasmids. Moreover, DdCBE remains highly specific. ‘Although it works well in human cells, it still needs to be tested in whole organisms, such as mice,’ says Koppens. ‘Mitochondrial base editors will need to be further developed, just as CRISPR-Cas has been, before clinical application becomes possible.
‘We now want to continue working on delivery strategies and on developing models to better understand the complex genetics of mitochondrial diseases.’