Binding and Ligand Activation Driven Enrichment-Directed Evolution of SaCas9 gRNAs Improves Gene Editing Efficiency.

Abstract

Clustered regularly interspaced short palindromic repeats-based editing is inefficient at over two-thirds of genetic targets. A primary cause is ribonucleic acid (RNA) misfolding that can occur between the spacer and scaffold regions of the gRNA, which hinders the formation of functional Cas9 ribonucleoprotein (RNP) complexes. Here, we uncover hundreds of highly efficient gRNA variant scaffolds for Staphylococcus aureus (Sa)Cas9 utilizing an innovative binding and ligand activation driven enrichment (BLADE) methodology, which leverages asymmetrical product dissociation over rounds of evolution. SaBLADE-derived gRNA scaffolds contain 7%-42% of nucleotide variation relative to wild type. gRNA variants are able to improve gene editing efficiency at all targets tested, and they achieve their highest levels of editing improvement (>400%) at the most challenging DNA target sites for the wild-type SaCas9 gRNA. This arsenal of SaBLADE-derived gRNA variants showcases the power and flexibility of combinatorial chemistry and directed evolution to enable efficient gene editing at challenging, or previously intractable, genomic sites.