Highly precise base editing with CC context-specificity using engineered human APOBEC3G-nCas9 fusions

Cytidine base editors, composed of a cytidine deaminase fused to Cas9 nickase, enable efficient C-to-T conversion in various organisms. However, current base editors can induce unwanted bystander C-to-T conversions when more than one C is present in the activity window of cytidine deaminase, which negatively affects the precision. Here, we develop a new base editor with CC context-specificity using rationally engineered human APOBEC3G, thus significantly reduce unwanted bystander activities. In addition, efficient C-to-T conversion that can further recognize relaxed NG PAMs is achieved by combining an engineered SpCas9-NG variant. These novel base editors with improved precision and targeting scope will expand the toolset for precise gene modification in organisms.