Transient in vivo base editing with no detectable RNA- and DNA off-target mutations
RNA-programmable deaminases, known as base editors (BEs), enable precise single base conversions on genomic DNA and hold great promise for therapeutic application in patients. Recent studies, however, have raised serious concern with regard to off-target effects, questioning translatability of BEs to the clinic. Here we analyze transcriptome- and genome-wide off-target effects following AAV-mediated delivery of cytosine base editors (CBEs) in vivo in an unbiased manner. We show that low expression of CBEs allows sufficient on-target editing to cure a disease phenotype with no increase in off-target effects compared to untreated controls. To further improve safety of in vivo base editing, we developed a lipid nanoparticle (LNP)-mediated delivery system to transiently express BEs. We reach up to 21% on-target editing with no detectable transcriptome- or genome-wide off-target effects, and are able to reverse the disease phenotype of a phenylketonuria mouse model. These results have important implications, underlining the feasibility of transient in vivo base editing for therapeutic use in patients.