Li, X. et al. Nat. Biotechnol. 36, 324–327 (2018).

The use of the Cas9 nickase fused to cytidine deaminases such as APOBEC1 has achieved great success in converting C to T nucleotides. But Cas9 relies on an NGG protospacer-adjacent motif (PAM) sequence to bind its target, which limits its target range. Li et al. expanded the genome-wide reach of base editors (BEs) by fusing APOBEC1 to catalytically inactive Cpf1, which recognizes TTTA/C/G sequences. The optimized dCpf1-BE from Lachnospiraceae bacterium, which also included a nuclear localization signal and a uracil DNA glycosylase inhibitor, achieved editing rates of 44% in a plasmid-based reporter system and up to 37% in selected targets in human cells. The researchers assessed 40 predicted off-target sites for eight guide RNAs and found that dCpf1-BE showed activity at only three sites for one guide RNA. Further improvements to Cpf1 to narrow the editing window and expand recognition to different PAMs are in the works.