Transcriptome-wide off-target RNA editing induced by CRISPR-guided DNA base editors [P2A-EGFP control]. Transcriptome-wide off-target RNA editing induced by CRISPR-guided DNA base editors [P2A-EGFP control]

Base Editing has been touted the most intelligent and precise application of the CRISPR platform so far, merging the simplicity of RNA-guided nucleases with deaminases that allow for the programmable generation of single base substitutions - without introduction of double-strand breaks. Even though the two-component system has been expected to cause off-target substitutions, studies involving cytosine base editors (CBEs) showed that in most cases, relatively few single base off-targets could be detected on DNA. We introduce the concept of multi-dimensional off-targeting, presenting an extensive amount of RNA cytidines being edited by DNA base editors. Epitranscriptomic off-target effects affected different cell lines and were independent of the guide RNAs used, suggesting Cas9-independent activity of the cytidine deaminase rAPOBEC1 on single-stranded RNA. With the help of protein engineering, we developed CBE variants with massively reduced inadvertent mutation of RNA that preserve and enhance DNA base editing capabilities. Overall design: HEK293T or HepG2 cells were transfected with P2A-EGFP. Cells were sorted for top 5% GFP based on FITC signal. RNA-seq was performed to measure transcriptional changes associated with different constructs.