CRISPR/Cas12a-mediated DNAzyme/split-aptamer cascade for label-free detection of site-specific DNA methylation

DNA methylation serves as a critical epigenetic biomarker for early disease screening and clinical diagnostics, exemplified by the FDA-approved detection of methylated Septin9 gene in peripheral blood for colorectal cancer (CRC) screening. Conventional CRISPR/Cas12a-based detection systems rely on fluorophore-quencher-labeled ssDNA probes to transduce trans-cleavage activity into fluorescence signal, which yet suffers from limitations including complex probe synthesis, background noise induced by incomplete quenching and high cost. In this study, we developed a cascaded and label-free CRISPR/Cas12a system for Septin9 methylation detection using DNAzyme and split DAP-10. The method employed methylation-sensitive AciI digestion to selectively hydrolyze non-methylated DNA, leaving methylated Septin9 DNA fragment intact to activate CRISPR/Cas12a system. Activated Cas12a triggered trans-cleavage of DNAzyme, a bifunctional molecule that acted as both trans-cleavage substrate of Cas12a and hydrolysis regulator of linker DNA that facilitated split DAP-10 reassembly to light-up Auramine O (AO) fluorescence proportionally to methylation levels. The assay achieved a linear range from 2 to 200 nM with a detection limit of 1.74 nM and robust performance in accuracy, precision, and selectivity. The proposed method was successfully applied in methylated Septin9 detection in cell lines, clinical tissues and blood samples, which indicated application potential for CRC screening.