Cas12a base editors induce efficient and specific editing with low DNA damage response (RNA sequencing)

The advent of base editors (BEs) holds a promising potential in correcting pathogenic-related point mutations to treat relevant diseases. Unexpectedly, Cas9 nickase (nCas9) derived BEs lead to DNA double-strand breaks, which can trigger unwanted cellular responses including a p53-mediated DNA damage response (DDR). Here, we showed that catalytically-dead-Cas12a (dCas12a) conjugated BEs induced no DNA break and minimally activated DDR proteins including H2AX, ATM, ATR and p53. We further developed a BEACON (Base Editing induced by human APOBEC3A and Cas12a without DNA break) system that fuses dCas12a to the engineered APOBEC3A with enhanced deamination efficiency and editing specificity. By using BEACON, efficient C-to-T editing was achieved at levels comparable to AncBE4max and only low levels of DDR and RNA off-target (OT) effects were triggered in mammalian cells. BEACON also induced in vivo base editing in mouse embryos and targeted C-to-T conversions were detected in F0 mice. Examination of RNA off-targets induced by the Cas12a base editors