Treating the Untreatable: Can we edit mtDNA at the base level to cure diseases?

Scientists are now able to edit mtDNA in the hearts of live mice. Could this pave the path for the development of novel therapies to cure mitochondrial diseases?

Author: Ismat Ghuman
Artist: Zach Ng
Editor: Sara Maria Majernikova

From a young age, we have been repeatedly reminded of the utmost significance a mitochondria houses within a cell, even going so far as to call it the cell’s “powerhouse”. Each mitochondria is encoded by a minute quantity of mitochondrial DNA (mtDNA) which comprises only 0.1% of the entire human genome and is exclusively transmitted from mother to child. Being the site of vital metabolic processes, necessary for providing energy to the cell, one can only imagine how catastrophic the effects of mutations in mtDNA would be on an individual. mtDNA mutations can impair mitochondrial function, resulting in mitochondrial diseases, affecting approximately 1 in 5,000 individuals and are often fatal. These mutations manifest themselves in the form of organ-specific implications at some point in an individual’s life. Despite their detrimental effects, these diseases are considered largely incurable since they lack sufficient research and the available treatments are mostly preventive in nature rather than disease-specific. This was the case until recently, when Silva-Pinheiro and colleagues made an advancement in this field by suggesting a capability to edit mtDNA base pairs in a live animal. They propose using adeno-associated virus (AAV) vectors to deliver a DddA-derived cytosine base editor (DdCBE, which makes a cytosine to thymine base change) into the heart of a mouse. This research lends an insight as to how we might be able to fix mtDNA sequences in humans to treat new symptoms of mitochondrial diseases.

The original issue with manipulating mtDNA arose from difficulties encountered while trying to import nucleic acids into the mitochondria. To counter this, programmable nucleases have been used in the past, but only to a limited extent, since they are incapable of introducing novel mtDNA variants. DdCBE, a cytosine base editor, has been shown to help with C:G to T:A conversions by catalysing cytidine deamination Through the use of adult and neonatal mice’s hearts, it was shown that AAV vectors are capable of delivering DdCBE to install mutations in mtDNA that are deemed desirable. This sheds light on the possible use of DdCBE for tissue–specific mtDNA mutagenesis in vivo, in turn opening up a new door to future treatments involving the correction of somatic mitochondrial genes to treat mitochondrial diseases. By “reverse engineering” the genome of mitochondria, this kind of technology could be used to fix mtDNA point mutations and treat the symptoms of primary mitochondrial disease (PMD).

Despite these advantages, the technique could face some shortcomings in-vivo. The process involves the deamination of cytosine to produce uracil (C->U), but in order to do so,  mitochondrial base excision repair (BER) would need to be inhibited. Since BER’s efficacy in-vivo is not fully understood, further research would be required to establish whether it would have an effect on editing mtDNA with DdCBE. Along with this issue, it is uncertain if the level of mammalian mtDNA replication is high enough to be successfully edited by DdCBEs, although this was the case in neonatal and adult mice, with the latter taking a longer duration. The earlier DdCBE was introduced to neonatal mice, the more efficient the editing. This may be the case because the AAV vector-to-cell ratio was higher when it was given early, which would make it easier for the virus to get into their hearts. 

When looking at how DdCBE-mediated editing of mtDNA could be used in future treatments, these issues may make people wonder how effective these therapies would be on adults instead of infants, whose mtDNA replicates at a much higher rate. The scope of this research is also worth taking into consideration since DdCBE is only capable of editing C:G to T:A, and further studies could try incorporating other base editors to manipulate mtDNA in-vivo. Nevertheless, this discovery opens the door  to mtDNA editing in a live organism, which could be a potential therapy for treating mitochondrial diseases in humans. DdCBE has been shown to be a tool that can be used to manipulate mtDNA in post-mitotic tissue. It may be capable of changing pathogenic variants back to normal sequences and could potentially help patients recover from mitochondrial diseases completely.

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