Temporal and rheostatic control of genome editing with a chemically-inducible Cas9.

Nuclease-mediated DNA cleavage and subsequent repair lie at the heart of genome editing, and the RNA-guided endonuclease Cas9 has emerged as the most widely-used tool for facilitating this process. Extensive biochemical and biophysical efforts have revealed much regarding the structure, mechanism, and cellular properties of Cas9. This has enabled engineering of Cas9 variants with enhanced activity, specificity, and other features. However, we lack a detailed understanding of the kinetics of Cas9-mediated DNA cleavage and repair in vivo. To study in vivo Cas9 cleavage kinetics and activity dose-dependence, we have engineered a chemically-inducible, single-component Cas9, ciCas9. ciCas9 allows for temporal and rheostatic control of Cas9 activity using a small molecule activator, A115. We have also developed a droplet-digital PCR-based assay (DSB-ddPCR) to directly quantify Cas9-mediated double-stranded breaks (DSBs). The methods in this chapter describe the application of ciCas9 and DSB-ddPCR to study the kinetics and dose-dependence of Cas9 editing in vivo.