Cytosine base editing systems with minimized off-target effect and molecular size

Abstract Structure-based rational engineering of the cytosine base editing system Target-AID was performed to minimize its off-target effect and molecular size. By intensive and careful truncation, DNA-binding domain of its deaminase PmCDA1 was eliminated and additional mutations were introduced to restore enzyme function. The resulting tCDA1EQ was effective in N-terminal fusion (AID-2S) or inlaid architecture (AID-3S) with Cas9, showing minimized gRNA-independent off-targets, as assessed in yeast and human cells. Combining with the smaller Cas9 ortholog system, the smallest cytosine base editing system was created that is within the size limit of AAV vector.