Abstract
DNA base editors have enabled genome editing without generating DNA double strand breaks. The applications of this technology have been reported in a variety of animal and plant systems, however, their editing specificity in human stem cells has not been studied by unbiased genome-wide analysis. Here we investigate the fidelity of cytidine deaminase-mediated base editing in human induced pluripotent stem cells (iPSCs) by whole genome sequencing after sustained or transient base editor expression. While base-edited iPSC clones without significant off-target modifications are identified, this study also reveals the potential of APOBEC-based base editors in inducing unintended point mutations outside of likely in silico-predicted CRISPR-Cas9 off-targets. The majority of the off-target mutations are C:G->T:A transitions or C:G->G:C transversions enriched for the APOBEC mutagenesis signature. These results demonstrate that cytosine base editor-mediated editing may result in unintended genetic modifications with distinct patterns from that of the conventional CRISPR-Cas nucleases.