Editing mitochondrial DNA
Scientists have made targeted changes to mitochondrial DNA for the first time
Denis Gordeev, Naked Science
Most cells in our body have two "genetic libraries" at once. One of them is located in the nucleus, the second is in the mitochondria, the "power stations" of the cell. Until now, scientists knew how to make directional changes only in nuclear DNA. An article published by American researchers in Nature (Mok et al., A bacterial cytidine deaminase toxin enables CRISPR-free mitochondrial base editing, describes a method for targeted editing of mitochondrial nucleic acids.
The Ddda toxin became the basis for the technology that allows making changes to the mitochondrial genome. It is synthesized by gram-negative bacteria Burkholderia cenocepacia, helping them compete for nutritional resources with other microorganisms. This toxin catalyzes changes in DNA, converting one of its nucleosides, cytosine, into uracil, a nitrogenous base characteristic of RNA.
This is not the first time that microorganisms have been found to have "tools" that help make changes to genes. In genetic engineering, deaminases are actively used — enzymes that also convert cytosine into uracil, replacing the amino group NH2 with an oxygen atom. However, this enzyme only works on single-stranded DNA, whereas in our mitochondria it is double-stranded.
Theoretically, this problem could be solved using the CRISPR-Cas9 gene editing system in combination with deaminases and additional enzymes for unwinding double-stranded DNA. However, the use of such an approach is complicated by the selective mitochondrial membrane: in particular, it does not allow the relatively bulky RNA matrices necessary in such a technique to identify certain DNA sequences.
The use of Ddda, which is able to work with double-stranded DNA, allowed us to circumvent these obstacles. The researchers combined the toxin with the so-called TALE protein, which is able to recognize certain domains in DNA and bind to them. The Ddda-TALE complex finds the desired sequence in mitochondrial genes and changes cytosine to uracil in it; the latter later turns into thymine, another nitrogenous base characteristic of DNA.
Laboratory tests have shown that Ddda-TALE gives scientists the necessary changes in the genetic code in about 50% of cases. At first glance, this does not seem to be such a good result. However, in the absence of other suitable candidates for the role of a tool for editing mitochondrial genes, and also given the absence of any signs of potentially catastrophic changes beyond the target sequences, this achievement is a serious scientific success.
Like mutations in nuclear DNA, genetic changes in mitochondrial genes can affect the state of the body, causing various pathologies that are transmitted to offspring. Further development of Ddda-TALE technology should help in the development and implementation of a reliable tool that will eliminate such mutations.
Earlier we wrote that the technology of CRISPR genetic editing directly in the patient's body was used for the first time to treat blindness in humans, and physiologists from the United States are considering the prospects of intervention in human epigenetics and genetics to protect DNA from cosmic radiation.
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