Sequential Proofreading Enhances Specificity and Potency of CRISPR-CasX-based Epigenetic Repressors

Epigenetic silencers are targeted therapeutics capable of persistently silencing genes without altering DNA sequence. However, many current silencers rely on direct fusion of a constitutively active DNA methyltransferase catalytic domain, that may cause off-target DNA methylation and cellular toxicity. In contrast, endogenous DNA methyltransferases contain a regulatory domain (ADD) that requires recognition of a permissive histone state before DNA methylation can occur. We reinstate this sequential proofreading logic in a compact CasX-based epigenetic repressor, generating Epigenetic Long-term CasX Repressors (ELXRs) that read local chromatin state before writing DNA methylation. In ELXRs, a repressor domain converts active chromatin into a state that unlocks the allosteric DNA methyltransferase. This creates a sequential, multi-gated process that preferentially restricts DNA methylation to the target site. Surprisingly, implementing this proofreading mechanism improved not only specificity, but also activity. ELXRs decreased off-target methylation up to 10-fold and rescued DNMT3A-dependent growth defects, with transcriptome profiling showing up to 150-fold fewer dysregulated genes, while also increasing on-target repression up to 4-fold across multiple loci. In mouse models, lipid nanoparticle delivery produced potent, durable PCSK9 silencing with precise promoter methylation. Together, these results demonstrate that coupling molecular recognition to effector activity through sequential proofreading can simultaneously improve specificity and potency.