Here we present an approach that combines a clustered regularly interspaced short palindromic repeats (CRISPR) system that simultaneously targets hundreds of epigenetically diverse endogenous genomic sites with high-throughput sequencing to measure Cas9 dynamics and cellular responses at scale. This massive multiplexing of CRISPR is enabled by means of multi-target guide RNAs (mgRNAs), degenerate guide RNAs that direct Cas9 to a pre-determined number of well-mapped sites. mgRNAs uncovered generalizable insights into Cas9 binding and cleavage, revealing rapid post-cleavage Cas9 departure and repair factor loading at protospacer adjacent motif-proximal genomic DNA. Moreover, by bypassing confounding effects from guide RNA sequence, mgRNAs unveiled that Cas9 binding is enhanced at chromatin-accessible regions, and cleavage by bound Cas9 is more efficient near transcribed regions. Combined with light-mediated activation and deactivation of Cas9 activity, mgRNAs further enabled high-throughput study of the cellular response to double-strand breaks with high temporal resolution, revealing the presence, extent (under 2 kb) and kinetics (~1 h) of reversible DNA damage-induced chromatin decompaction. Altogether, this work establishes mgRNAs as a generalizable platform for multiplexing CRISPR and advances our understanding of intracellular Cas9 activity and the DNA damage response at endogenous loci.
CRISPR-Cas9 nucleases, and particularly Streptococcus pyogenes Cas9, are widespread tools for genome editing. However, many aspects of intracellular Cas9 activity and the ensuing DNA damage response remain incompletely characterized. In order to address these issues, we developed a multiplexed CRISPR approach, where a single, degenerate multi-target gRNA (mgRNA) directs the Cas9 enzyme to target hundred endogenous sites at once. When combined with next-generation sequencing readouts, this system enables interrogation of Cas9 activity and DNA double-strand break (DSB) repair response in high-throughput. Here, we present a step-by-step protocol to deliver a Cas9:mgRNA ribonucleoprotein complex into cultured cells and measure key processes related to Cas9 activity and DSB repair.
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