Scaffold-Proximal DNA Extensions Enhance Cas12a Trans-cleavage for Direct and Broad-Scope Nucleic Acid Detection

The CRISPR/Cas12a system has revolutionized nucleic acid diagnostics, but its direct application for RNA detection remains constrained by the requisite reverse transcription step, insufficient sensitivity, and poor compatibility across diverse RNA targets such as microRNAs (miRNAs), long mRNAs, and single nucleotide polymorphisms (SNPs). Here, we introduce a rationally engineered DNA activator architecture that markedly enhances Cas12a trans-cleavage activity and expands its analytical utility. A key mechanistic finding is the strong positional dependence of activator extensions: appending an overhang specifically at the scaffold-proximal end of the DNA activator (termed Proximal-Extended Activator, PEA) potently boosts Cas12a activation through ribonucleoprotein (RNP) stabilization, whereas distal extensions are inhibitory due to steric hindrance of essential interdomain motions. This optimized PEA system facilitates direct, amplification-free RNA detection, achieving exceptional sensitivity with detection limits of 1.3 fM for miRNA and 93 fM for mRNA, all without reverse transcription. Furthermore, a Split-PEA format confers exceptional discriminatory power for SNPs, enabling robust identification of the EGFR T790 M mutation at a 0.1% allelic frequency. This work establishes a facile and versatile platform where simple sequence modification enables highly sensitive and specific analysis of a broad range of nucleic acid targets, effectively overcoming a significant hurdle in CRISPR-based diagnostics.