Superior Fidelity and Distinct Editing Outcomes of SaCas9 Compared with SpCas9 in Genome Editing

A series of clustered regularly interspaced short palindromic repeats(CRISPR)-CRISPR associated protein 9(Cas9)systems have been engineered for genome editing.The most widely used Cas9 is SpCas9 from Streptococcus pyogenes and SaCas9 from Staphylococcus aureus.However,a comparison of their detailed gene editing outcomes is still lacking.By characterizing the editing outcomes of 11 sites in human induced pluripotent stem cells(iPSCs)and K562 cells,we found that SaCas9 could edit the genome with greater efficiencies than SpCas9.We also compared the effects of spacer lengths of single-guide RNAs(sgRNAs;18–21 nt for SpCas9 and 19–23 nt for SaCas9)and found that the optimal spacer lengths were 20 nt and 21 nt for SpCas9 and SaCas9,respectively.However,the optimal spacer length for a particular sgRNA was 18–21 nt for SpCas9 and 21–22 nt for SaCas9.Furthermore,SpCas9 exhibited a more substantial bias than SaCas9 for nonhomologous end-joining(NHEJ)+1 insertion at the fourth nucleotide upstream of the protospacer adjacent motif(PAM),indicating a characteristic of a staggered cut.Accordingly,editing with SaCas9 led to higher efficiencies of NHEJ-mediated double-stranded oligodeoxynucleotide(dsODN)insertion or homology-directed repair(HDR)-mediated adeno-associated virus serotype 6(AAV6)donor knock-in.Finally,GUIDE-seq analysis revealed that SaCas9 exhibited significantly reduced off-target effects compared with SpCas9.Our work indicates the superior performance of SaCas9 to SpCas9 in transgene integration-based therapeutic gene editing and the necessity to identify the optimal spacer length to achieve desired editing results.

GREPore-seq: A Robust Workflow to Detect Changes After Gene Editing Through Long-range PCR and Nanopore Sequencing

To achieve the enormous potential of gene-editing technology in clinical therapies,one needs to evaluate both the on-target efficiency and unintended editing consequences comprehensively.However,there is a lack of a pipelined,large-scale,and economical workflow for detecting genome editing outcomes,in particular insertion or deletion of a large fragment.Here,we describe an approach for efficient and accurate detection of multiple genetic changes after CRISPR/Cas9 editing by pooled nanopore sequencing of barcoded long-range PCR products.Recognizing the high error rates of Oxford nanopore sequencing,we developed a novel pipeline to capture the barcoded sequences by grepping reads of nanopore amplicon sequencing(GREPore-seq).GREPore-seq can assess nonhomologous end-joining(NHEJ)-mediated double-stranded oligodeoxynucleotide(dsODN)insertions with comparable accuracy to Illumina next-generation sequencing(NGS).GREPore-seq also reveals a full spectrum of homology-directed repair(HDR)-mediated large gene knock-in,correlating well with the fluorescence-activated cell sorting(FACS)analysis results.Of note,we discovered low-level fragmented and full-length plasmid backbone insertion at the CRISPR cutting site.Therefore,we have established a practical workflow to evaluate various genetic changes,including quantifying insertions of short dsODNs,knock-ins of long pieces,plasmid insertions,and large fragment deletions after CRISPR/Cas9-mediated editing.GREPore-seq is freely available at GitHub(https://github.com/lisiang/GREPore-seq)and the National Genomics Data Center(NGDC)BioCode(https://ngdc.cncb.ac.cn/biocode/tools/BT007293).