mRNA-Engineered CRISPR-Cas Epigenetic Editors Enable Durable and Efficient Gene Silencing in Vivo [RNA-seq]

Programmable epigenetic editors that achieve long-term gene expression modulation hold immense therapeutic potential. However, the large size of current CRISPR-based epigenome editors has limited their widespread application. This study describes the engineering of compact, mRNA-delivered epigenetic repressors, termed CRISPR OFF-EE, using SpyCas9, intein-based split-SpyCas9 or the smaller SF01 Cas variant, to achieve highly efficient and durable silencing of endogenous genes. We developed "superfolder" V2 mRNA constructs with optimized codon usage, UTRs, and nuclear localization signals, which enhanced mRNA stability, translational output, and LNP packaging. These CRISPR OFF-EE V2 editors demonstrated significantly improved and durable gene silencing across multiple targets in variable cell lines. Remarkably, a single intravenous administration of lipid nanoparticles (LNPs) encapsulating the optimized OFF-EE-V2 mRNA and selected sgRNAs targeting PCSK9 in mice led to an approximately 90% reduction in circulating PCSK9 protein and a 55% reduction in LDL-C levels, with effects persisting for at least 180 days. Whole-genome bisulfite sequencing confirmed the high specificity of these editors, with SF01-based editors exhibiting fewer off-target methylation events than SpCas9-based editors. The platform also demonstrated multiplexing capability and a favorable safety profile with liver-restricted activity. These findings establish a robust foundation for advancing in vivo therapeutics based on precise and durable epigenetic silencing using transiently delivered mRNA editors.