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

Cytosine base editing enables the installation of specific point mutations without double-strand breaks in DNA and is advantageous for various applications such as gene therapy, but further reduction of off-target risk and development of efficient delivery methods are desired. Here we show structure-based rational engineering of the cytosine base editing system Target-AID to minimize its off-target effect and molecular size. By intensive and careful truncation, DNA-binding domain of its deaminase PmCDA1 is eliminated and additional mutations are introduced to restore enzyme function. The resulting tCDA1EQ is effective in N-terminal fusion (AID-2S) or inlaid architecture (AID-3S) with Cas9, showing minimized RNA-mediated editing and gRNA-dependent/independent DNA off-targets, as assessed in human cells. Combining with the smaller Cas9 ortholog system (SaCas9), a cytosine base editing system is created that is within the size limit of AAV vector.