The development of genome editing tools has revolutionized the way we think and deal with genetics. The use of Cas9 or its variants allows modifications of specific sites in the human genome by inducing deletions and insertions in a more or less controlled way. In recent years, a new class of tools for genome editing has emerged: the base editors (BE), which result from the fusion of a modified Cas9, which serves to direct the BE to the target, and an active deaminase acting on the DNA, which mediates the C> T or A> G editing. These molecules allow the modification of single bases in a specific DNA region without inducing double strand breaks, and therefore could be safer than using Cas9 alone. As for C> T editing, the most commonly used deaminase is rat APOBEC1, a powerful DNA mutator that acts physiologically on RNA.
Despite their potential, several studies have shown that Base Editor expression in human cells causes off-target mutations in both RNA and DNA. To limit these mutations, we have used bacterial screening and assays in human cells to develop mutants of the APOBEC1 deaminase with substantially lower levels of off-target mutations both on RNA and on DNA. Their incorporation within base editors presents a considerably reduced number of off-target mutations both on RNA and DNA. These new base editors we have developed can be used to induce a safer genomic editing.
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