For the first time, David Liu's team has achieved precise gene editing of mitochondrial DNA.

But studying such diseases has been difficult because scientists lack a way to create animal models of mitochondrial genomes with the same mutationsTherefore, the development of a tool that can edit mtDNA is a long-standing goal of mitochondrial genetics scientistsSo why not use "Magic Cut" CRISPR-Cas9? Because CRISPR-Cas9 can edit genes in almost any organismThe RNA strands used by the tool can guide Cas9 enzymes to the dna regions that scientists want to editThe problem is that this works well for DNA in the nucleus, but RNA cannot penetrate the mitochondria that are surrounded by membranesIn the face of mitochondria, "magic scissors" seem a little helplessIn late 2018, David Liu of the Broad Institute learned that Joseph Mougous, a microbiologist at the University of Washington, led a team that had discovered a "strange" enzymeIt is a toxin (Ddda) produced by the bacteria Burkholdia cenocepacia, which is converted into uemesin (U) when it encounters DNA cytosine (C)Since uncommon U in DNA is similar to T, enzymes can be copied from cell DNACopy it to T, effectively converting C in the genome sequence to TDavid Liu has previously used similar enzymes in base editing, but these enzymes usually work only on single-stranded DNALiu had to rely on the Cas9 enzyme to destroy the double-stranded DNA and create an unwound single-stranded DNA region that made the enzyme workBecause it relies on the RNA strands that guide Cas9, the technology cannot reach the mitochondrial genomeHowever, the Mougous team found that DdA can act directly on double-stranded DNA without relying on The Cas9 enzyme to destroy itLiu and Mougous are ready to go - DddA can be paired with non-CRISPR DNA location systems for mitochondrial genome editingThe findings were published July 8 in the journal NatureOf course, Liu and Mougous have overcome many challenges in order to make DddA for mitochondrial genome editing First, DdA is toxic to mammalian cells To do this, the researchers divided the toxin region of DdDA into two (split-dddAtox halves) and turned into two inactive fragments The two fragments are then fused with the TALE protein and bound to a specific DNA sequence, which is activated only when they reach a specific site Finally, to deliver this gene-editing tool to the mitochondrial matrix, they must pass through the mitochondria's double-layer membrane As a result, the team used the amino acid sequence of mitochondrial target signals to mark the built gene editing tool For mitochondrial double membranes, this protein-based import mechanism is more advantageous than RNA-based import systems such as CRISPR-Cas9 Ultimately, this resulted in a DddA-derived mitochondrial base editor (DdCBE) derived from CRISPR that enables precise editing of the mitochondrial genome and brings new tools for research and treatment of mitochondrial-related diseases All genome editing tools need to consider off-target effects The team compared treated and untreated cells and found no deviation from the target in the nuclear genome MtDNA has low off-target activity Next, the team studied the therapeutic potential of DdCBE and found that it could repair 49 percent of the harmful mt DNA mutations known DdCBE reduces the proportion of mutated mtDNA carried without reducing the number of copies Therefore, when the mitochondrial mutation load is high, it may be the preferred or even the only option Liu stressed that the study is a long way from being used clinically Although preliminary studies have found little change in off-target DNA, more research on different cell types is needed The findings are important advances in the development of gene therapy for mtDNA disease, according to the journal Nature In addition, by using this tool to experimentally alter the mitochondrial genome, the correlation between mtDNA mutations and complex diseases, cancer, and age-related cellular dysfunction can be better understood Related: s1 Scientists make gene gene edits to mitochondrial DNA for first time Mitochondrial genome editing gets precise.