A stable but reversible integrated surrogate reporter for assaying CRISPR/Cas9-stimulated homology-directed repair

Yahong Wen, Grace Liao, Thomas Pritchard, Ting-Ting Zhao, Jon P Connelly, Shondra M Pruett-Miller, Valerie Blanc, Nicholas O Davidson, Blair B Madison

The discovery and application of CRISPR/Cas9 technology for genome editing has greatly accelerated targeted mutagenesis in a variety of organisms. CRISPR/Cas9-mediated site-specific cleavage is typically exploited for the generation of insertions or deletions (indels) following aberrant dsDNA repair via the endogenous non-homology end-joining (NHEJ) pathway, or alternatively, for enhancing homology directed repair (HDR) to facilitate the generation of a specific mutation (or knock-in). However, there is a need for efficient cellular assays that can measure Cas9/guide RNA (gRNA) activity. Reliable methods for enriching and identifying desired mutants are also lacking. Here we describe a method using the Piggybac transposon for stable genomic integration of an H2B-GFP reporter or a hygromycin resistance gene for assaying Cas9 target cleavage and homology-directed repair (HDR). The H2B-GFP fusion protein provides increased stability and an obvious pattern of nuclear localization. This method, called SRIRACCHA (i.e., a stable, but reversible, integrated reporter for assaying CRISPR/Cas-stimulated HDR activity), enables the enrichment of mutants via selection of GFP-positive or hygromycin-resistant mammalian cells (immortalized or non-immortalized) as a surrogate for the modification of the endogenous target site. Currently available hyperactive Piggybac transposase mutants allow both delivery and removal of the surrogate reporters, with minimal risk of generating undesirable mutations. This assay permits rapid screening for efficient gRNAs, and the accelerated identification of mutant clones, and is applicable to many cell types. We foresee the utility of this approach in contexts in which the maintenance of genomic integrity is essential, for example, when engineering cells for therapeutic purposes.