APOBEC3s induce mutations during the repair of CRISPR-Cas9-generated DNA breaks.

APOBEC-AID family of cytidine deaminase prefers single-stranded nucleic acids for cytidine to uracil deamination. Single-stranded nucleic acids are commonly involved in the DNA repair system for breaks generated by CRISPR-Cas9. Here, we show in human cells that APOBEC3s can trigger the cytidine deamination of single-stranded oligodeoxynucleotides, which ultimately results in base substitution mutations in genomic DNA through the homology-directed repair (HDR) of Cas9-generated double-strand breaks . In addition, the APOBEC3-catalyzed deamination in genomic single-stranded DNA formed during the repair of Cas9 nickase-generated single-strand breaks can be further processed to yield mutations mainly involving insertions or deletions (indels). Mechanistically, both APOBEC3-mediated deamination and DNA repair proteins play important roles in the generation of these indels. Correspondingly, optimizing conditions for the repair of CRISPR-Cas9-generated DNA breaks, such as using double-stranded donors in HDR or temporarily suppressing endogenous APOBEC3s, can substantially repress these unwanted mutations in genomic DNA. Overall design: Determination of base substituitons and insertions/deletions induced by APOBECs during the repair of CRISPR-Cas9-generated DNA breaks