Eliminating base-editor-induced genome-wide and transcriptome-wide off-target mutations

The fusion of CRISPR-Cas9 with cytidine deaminases leads to base editors (BEs) capable of programmable C-to-T editing, which has potential in clinical applications but suffers from off-target (OT) mutations. Here, we used a cleavable deoxycytidine deaminase inhibitor (dCDI) domain to construct a transformer BE (tBE) system that induces efficient editing with only background levels of genome-wide and transcriptome-wide OT mutations. After being produced, the tBE remains inactive at OT sites with the fusion of a cleavable dCDI, therefore eliminating unintended mutations. When binding at on-target sites, the tBE is transformed to cleave off the dCDI domain and catalyses targeted deamination for precise base editing. After delivery into mice through a dual-adeno-associated virus (AAV) system, the tBE system created a premature stop codon in Pcsk9 and significantly reduced serum PCSK9, resulting in a similar to 30-40% decrease in total cholesterol. The development of tBE establishes a highly specific base editing system and its in vivo efficacy has potential for therapeutic applications.

Automated summary

The study introduces a novel base editor system, tBE, which efficiently performs C-to-T editing by fusing a cleavable dCDI domain, reducing off-target mutations. The tBE system remains inactive at off-target sites, preventing unintended mutations, and activates for precise base editing at on-target sites.