Cytosine base editor-DNA binding domain fusions for editing window modulation in the RNP format

Base editing technologies allow for the precise and efficient installation of defined nucleotide substitutions into a target genome without the introduction of double strand breaks or DNA templates. Here we describe two recombinant, protein format cytosine base editors (CBEs) that efficiently catalyze the installation of cytosine-to-thymine edits, termed "Flexible" and "Precision." Flexible exhibits a wide editing window, while Precision uses a fused single-stranded DNA binding protein to narrow the editing window, lowering the risk of editing multiple cytosine residues at the target site. We show that co-transfection with uracil glycosylase inhibitor protein increases the proportion of substitutions that are C-to-T and the ratio of C-to-T editing to indel formation, thus reducing undesired editing outcomes. We use in vitro editing assays to characterize our editors and show a preference for cytosine residues preceded by thymine (TpC dinucleotides) and unmethylated cytosine residues.