Cytosine base editors induce prevalent unwanted out-of-protospacer editing and target-strand editing
The Detect-seq provides an unbiased method for genome-wide identification of CBE induced off-targets inside of cells.     Purpose: Cytosine base editors (CBEs) have the potential to correct human pathogenic point mutations. However, its genome-wide specificity remains poorly understood, precluding its therapeutic applications. Unbiased tools are urgently needed to comprehensively evaluate CBE off-targets at the genome-wide level.     Methods: The genomic DNA from CBE transfected cells are processed with fragmentation, endogenous d5fC protection, damage repair, biotin-dUTP and 5fdCTP labeling, and pull-down steps. After the sequencing library preparation, the  FASTQ files are produced by the Illumina HiSeq XTen platform. To guarantee the reproducibility, each case of the experiment is present with two biological replicates.     Results: Through Detect-seq, we comprehensively profiled the genome-wide off-targets of CBE for several representative sgRNAs, and observed both weak, “random-like” off-targets as well as strong Cas9-dependent off-targets in different human cell lines. Cas9-dependent off-targets can be prevalent for several sgRNAs, and demonstrate a divergent degrees of sequence similarity to the sgRNA. These CBE-induced off-target sites differ greatly from those caused by Cas9 nuclease alone, suggesting inherently different properties among these genome editing tools. Targeted amplicon sequencing further corroborated the novel off-target sites by Detect-seq; quite a few sites showed editing ratio that is on the same order of magnitude or even comparable to the on-target sites. Detect-seq also revealed two unexpected types of Cas9-dependent off-targets, which are out-of-protospacer editing and target-strand editing. These novel off-targets were likely caused by an unstable structure at the PAM-distal side of the Cas9-sgRNA-DNA complex. Based upon such understanding to the off-target editing, we engineered several CBE variants bearing mutations in APOBEC1 and obtained improved CBEs with significantly reduced off-target editing activities.    Detect-seq utilized a dU-mapping strategy to profile on-target and off-target editing events by CBE at the whole-genome level. We applied Detect-seq to off-target evaluation in HEK293T and MCF7 cells for BE4max with several frequently used sgRNAs: “VEGFA site 2”, “HEK293 site 4”, “EMX1”, and “RNF2”. Overall design: 26 samples.