Examination of the Cell Cycle Dependence of Cytosine and Adenine Base Editors

We report here the first systematic study of the cell cycle dependence of base editing using cell synchronization experiments. We find that nickase derived BEs (which introduce DNA backbone nicks opposite the uracil or inosine base) function independently of the cell cycle, while non-nicking BEs are highly dependent on S-phase (DNA synthesis phase). We found that synchronization in G1 (growth phase) during the process of cytosine base editing causes significant increases in C:G to A:T byproduct introduction rates, which can be leveraged to discover new strategies for precise C:G to A:T base editing. We observe that endogenous expression levels of DNA damage repair pathways are sufficient to process base editing intermediates into desired editing outcomes, and the process of base editing does not significantly perturb transcription levels.