A panel of eGFP reporters for single base editing by APOBEC-Cas9 editosome complexes.

The prospect of introducing a single C-to-T change at a specific genomic location has become feasible with APOBEC-Cas9 editing technologies. We present a panel of eGFP reporters for quantification and optimization of single base editing by APOBEC-Cas9 editosomes. Reporter utility is demonstrated by comparing activities of seven human APOBEC3 enzymes and rat APOBEC1 (BE3). APOBEC3A and RNA binding-defective variants of APOBEC3B and APOBEC3H display the highest single base editing efficiencies. APOBEC3B catalytic domain complexes also elicit the lowest frequencies of adjacent off-target events. However, unbiased deep-sequencing of edited reporters shows that all editosomes have some degree of local off-target editing. Thus, further optimization is required to generate true single base editors and the eGFP reporters described here have the potential to facilitate this process.