Precise and Predictable CRISPR Chromosomal Rearrangement Editing Reveals Principles of Cas9-mediated Nucleotide Insertion

DNA-fragment editing or chromosomal rearrangements including inversions, deletions, and duplications by Cas9 with paired sgRNAs are important to investigate structural genome variations and developmental gene regulation, but little is known about the underlying mechanism. Here we report that disrupting CtIP, which is thought to function in NHEJ, enhances precise DNA-fragment deletion. In addition, by analyzing the inserted nucleotides at the junctions of DNA-fragment editing and characterizing the cleaved products, we find that Cas9 endonucleolytically cleaves the noncomplementary strand with a flexible scissile profile upstream of -3 position of the PAM site in vivo and in vitro, generating overhanged DSB ends. Moreover, we find that engineered Cas9 nucleases have distinct cleavage profiles. Finally, Cas9-mediated nucleotide insertions are nonrandom and are equal to the combined sequences upstream of both PAM sites with predicted frequencies. Thus, precise and predictable DNA-fragment editing could be achieved by perturbing DNA repair genes and using appropriate PAM configurations. These findings have important implications regarding 3D chromatin folding and enhancer insulation during gene regulation.