Genetic tool for mitochondria –

Editing the human genome may one day help treat serious inherited diseases – preliminary approaches have already been developed. But one component of the genome has been largely overlooked: mitochondrial DNA. Now, for the first time, a research team has developed a tool that can also convert basic DNA adenine into basic guanine in mitochondrial DNA. After all, such a correction could fix 39 of the 90 known disease-causing mutations in the mitochondrial genome – opening up new approaches to research and therapy.

Even a single “letter” change in the base code of our genome can, in extreme cases, lead to serious and sometimes even fatal diseases. So far, most of these diseases are incurable. Only with the development of new gene editing technologies, such as the CRISPR / Cas9 gene scissors, are the possibilities of gene therapy for such diseases within reach. However, these tools for exchanging single DNA bases or entire sections of genes only work with the DNA in the nucleus of our cells. They are not suitable for the genome of the mitochondria, the “powerhouse” of cells, because they usually cannot penetrate the mitochondria. As a result, treatments for diseases caused by mutations in mitochondrial DNA were lacking. After all, about one person in 5,000 is affected by this largely hereditary mitochondrial disease, and these diseases can also cause severe suffering and even be fatal.

Three molecular tools combined

So far, however, there are only a few ways to correct genetic defects in mitochondrial DNA. It was not until 2020 that American scientists for the first time developed a molecular tool that can convert basic DNA cytosine in mitochondrial DNA into basic DNA thymine. However, since this editing technique is only effective for some misplaced cytosine bases, the medical effect is limited: even if this editing tool is ready for clinical use, it will only be able to repair nine of the approximately 90 known mitochondrial mutations. “That’s why we looked for ways to overcome this limitation,” explains lead author Sung-Ik Cho of the Daejeon Institute for Fundamental Research in South Korea.

By combining three different molecular components, the Cho team succeeded in developing an editing tool that can now also convert DNA basal adenine into guanine in the mitochondrial genome. On the one hand, it consists of a variant of cytosine deaminase already developed by a team from the USA, which is combined with the so-called . The third ingredient is TadA8e, an adenine deaminase that promotes the conversion of adenine to guanine. By testing with human cell lines, the team was able to optimize the combination to the point where this gene tool replaces up to 49 percent of defective adenine bases in the mitochondria of the test cells with the correct guanine base. In testing, the editing tool was also found to not damage cells and did not destabilize mitochondrial DNA.

Possible correction of 39 out of 90 pathogenic mutations

“Our new TALED platform greatly expands the mitochondrial genome editing capabilities,” says Cho. Because with this exchange of bases, a further 39 out of 90 pathogenic mutations can now be corrected. “This could significantly contribute to the development of new disease models as well as the development of therapies,” says the researcher. Until now, research into treatments for mitochondrial disease has also been hampered by the fact that it has not previously been possible to replicate mutations occurring in patients in animal models – since genome editing is usually required. “In the long term, TALEDs could pave the way to correct disease-causing mutations in mitochondrial DNA in embryos, fetuses, newborns or adult patients, ushering in a new era of mitochondrial gene therapy,” writes Cho and his team.

However, before the new genetic tool can be used in animals or even humans, further research and optimization are still needed, as the researchers also acknowledge. Because the efficiency as well as the specifics of the replacement of bases still have to be significantly increased. Admittedly, when this method was used on human cells, no unwanted base conversions – so-called extra-target effects – occurred in the nuclear DNA of the cells. However, the team reported that there was a two- to four-fold increase in the rate of unwanted conversions in the mitochondrial DNA of the treated cells.

Source: Sung-Ik Cho (Institute of Fundamental Sciences, Daejeon) et al., Cell, doi: 10.1016 / j.cell.2022.03.039

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