Cytosine base editors with minimized unguided DNA and RNA off-target events and high on-target activity

Cytosine base editors (CBEs) enable efficient, programmable reversion of T center dot A to C center dot G point mutations in the human genome. Recently, cytosine base editors with rAPOBEC1 were reported to induce unguided cytosine deamination in genomic DNA and cellular RNA. Here we report eight next-generation CBEs (BE4 with either RrA3F [wt, F130L], AmAPOBEC1, SsAPOBEC3B [wt, R54Q], or PpAPOBEC1 [wt, H122A, R33A]) that display comparable DNA on-target editing frequencies, whilst eliciting a 12- to 69-fold reduction in C-to-U edits in the transcriptome, and up to a 45-fold overall reduction in unguided off-target DNA deamination relative to BE4 containing rAPOBEC1. Further, no enrichment of genome-wide C center dot G to T center dot A edits are observed in mammalian cells following transfection of mRNA encoding five of these next-generation editors. Taken together, these next-generation CBEs represent a collection of base editing tools for applications in which minimized off-target and high on-target activity are required. Cytosine base editors have been reported to induce off-target mutations in DNA and RNA. Here the authors identify next-generation CBEs with reduced guide-independent off-target editing profiles and retain high on-target editing activity.