Fully Modified SpyCas9 Guide RNAs Enable Robust Genome Editing In Cells and In Vivo

Precision engineering of CRISPR/Cas9 components has advanced genome editing toward therapeutic applications. Completely chemically stabilized guide RNAs (gRNAs) have the potential to improve in vivo editing efficacy while enabling greater flexibility in delivery strategies. However, previous generations of fully modified guides have been associated with reduced Cas9 activity. Here, we employed an iterative, structure-guided optimization strategy to systematically introduce chemical modifications at each position of SpyCas9 gRNAs. Extending beyond commonly used nucleotide modifications, we incorporated 2’-amino-RNA, 4’-thio-RNA, and extended nucleic acid (exNA) to generate gRNA designs in which 90-100% of the nucleotides are sugar- or backbone-modified. Although certain modification patterns exhibit sequence-dependent variability, we have established a growing repertoire of guides that consistently maintain or enhance editing efficacy when applied both in vitro and in vivo . Collectively, our heavily and fully modified gRNAs hold potential for applications in nuclease editing, base editing, and other genome editing tools.