A new gene editing technique, based on retrons, could avoid the potential collateral damage caused by CRISPR genetic scissors.
The CRISPR technique revolutionized gene editing when it was created in 2012. These are ” genetic scissors »: From a protein, it is possible to edit parts of the DNA, thus modifying the genetic heritage. This opens the way for many useful medical uses, for example against cancer. But this tool has certain limitations, notably a lack of precision. In a research article to be published on May 4, 2021 in PNAS, a group of eight researchers, notably affiliated with the Wyss Institute at Harvard, describe a new technique of gene editing.
This new tool is based on retrons. How does it work ? Can it be more efficient than CRISPR?
Avoid the collateral damage of “genetic scissors”
To edit the genome, CRISPR relies on the Cas9 enzyme, accompanied by RNA as a guide. In this way, breaks are induced in the DNA double strand. These breaks make it possible to modify the targeted gene, or even to reduce or increase its expression. From this principle is taken the name of “genetic scissors”, because it is actually a question of cutting the DNA.
However, to modify DNA to incorporate desired mutations, ” the cell must be made to use a new piece of DNA to repair the break », Explains the group of researchers recently published in PNAS. « This priming process can be complicated to orchestrate and can even be toxic to the cells, because Cas9 [l’enzyme utilisée]often cuts unintentional, off-target sites. This collateral damage is a problem.
For this reason, these researchers have developed a technique they call Retron Library Recombineering (RLR), which relies on retrons, genetic sequences found in certain bacteria that produce a single strand of DNA and unique. The major difference with CRISPR: it consists in introducing a piece of DNA into a cell during the phase when it replicates its genome. This helps to avoid breaking DNA, by riding a natural genetic process.
- In the use of CRISPR, the double-stranded DNA is physically broken to induce the cell to incorporate the desired mutation (carried by the Cas9 enzyme) during the repair process;
- With the RLR technique presented in this study, a mutation is introduced in the form of a DNA strand when the cell replicates, so that the resulting sister cells then replicate this mutation naturally. Using retrons therefore makes it possible to ” producing single-stranded DNA inside the cells we wanted to modify, rather than trying to force them into the cell from the outside, and without damaging the native DNA, two very compelling qualities ».
A “simpler and more flexible” tool than CRISPR scissors
The technique based on retrons therefore has the major advantage of not damaging DNA, avoiding collateral damage from CRISPR genetic scissors. In addition, the genetic sequence of retrons can act as a “bar code” allowing them to be identified. This level of precision offers researchers the possibility of carrying out genetic editing on a group of several targets in which we want to insert mutations. For research, this is used to ” perform millions of experiments simultaneously, which allows us to observe the effects of mutations on the entire genome, as well as how these mutations may interact with each other ».
The authors conclude that the RLR is ” a simpler and more flexible genetic editing tool “, Which can be used for collaborative experiments, while eliminating” the toxicity often seen with CRISPR “And improving” the ability of researchers to explore genome-level mutations “. The research group believes that this technique could be combined with CRISPR, or simply serve as a replacement in situations where CRISPR genetic scissors are deemed to be risky.
This retron-based technique is not the first innovation to present itself as an alternative to CRISPR for gene editing. In October 2019, geneticists presented, in Nature, the prime editing. Here again, this is to avoid the deleterious effects of cutting: this technique aims to simply notch DNA to rewrite its code.
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