Efficient and high-fidelity base editor with expanded PAM compatibility for cytidine dinucleotide

Cytidine base editor(CBE),which is composed of a cytidine deaminase fused to Cas9 nickase,has been widely used to induce C-to-T conversions in a wide range of organisms.However,the targeting scope of current CBEs is largely restricted to protospacer adjacent motif(PAM)sequences containing G,T,or A bases.In this study,we developed a new base editor termed“nNme2-CBE”with excellent PAM compatibility for cytidine dinucleotide,significantly expanding the genome-targeting scope of CBEs.Using nNme2-CBE,targeted editing efficiencies of 29.0%-55.0%and 17.3%—52.5%were generated in human cells and rabbit embryos,respectively.In contrast to conventional nSp-CBE,the nNme2-CBE is a natural high-fidelity base editing platform with minimal DNA off-targeting detected in vivo.Significantly increased efficiency in GC context and precision were determined by combining nNme2Cas9 with rationally engineered cytidine deaminases.In addition,the Founder rabbits with accurate single-base substitutions at Fgf5 gene loci were successfully generated by using the nNme2-CBE system.These novel nNme2-CBEs with expanded PAM compatibility and high fidelity will expand the base editing toolset for efficient gene modification and therapeutic applications.