A dual-function DNA probe-enabled split crRNA/Cas12a system for microRNA detection

Dual-function DNA probe (dfDNA) enables a split crRNA/Cas12a (SCas12a) assay. • The dfDNA acts as both a Cas12a activator and a fluorescent reporter. • The dfDNA activates both the cis - and trans -cleavage activities of Cas12a. • The dfDNA/SCas12a assay detects microRNA-21 (miR-21) without amplification. • Bead-assisted dfDNA format improves the sensitivity for miR-21 detection. MicroRNA-21 (miR-21) is a promising non-invasive biomarker for tumor diagnosis, but its short length and low abundance make amplification-free detection challenging. Here, we developed a dual-function DNA probe (dfDNA)-enabled split crRNA-Cas12a (SCas12a) system for miR-21 detection. This design separates crRNA into a constant scaffold fragment (sc-crRNA) and a variable spacer fragment (sp-crRNA), and miR-21 functionally replaces the spacer fragment to activate Cas12a in the presence of dfDNA. The dfDNA acts as both a target-recognition probe and a fluorescent reporter: it hybridizes with miR-21, activates Cas12a, and presents a single-stranded fluorescent overhang that can be cleaved after Cas12a activation. Because target recognition and signal reporting are integrated within a single probe, dfDNA positions the activated Cas12a complex close to the cleavable overhang, improving signal conversion compared with the conventional single-stranded DNA reporter and fully complementary double-stranded DNA probe format. Under optimized conditions, the limit of detection (LOD) with the homogeneous ssDNA reporter-based assay was 347.9 pM for miR-21, whereas the homogeneous dfDNA-based assay reduced the LOD to 19.8 pM. When transferred to a magnetic bead (MB)-assisted format, the dfDNA-based system achieved an LOD of 1.1 pM, supporting that the localized reaction accelerates the trans -cleavage kinetics of Cas12a. The MB-assisted dfDNA assay achieves comparable or even superior sensitivity for miR-21 detection compared with previously reported split crRNA-based fluorescence methods. Further optimization, including target enrichment or signal amplification strategies, is needed before this method can be reliably used for low-abundance miRNAs in clinical serum samples. Download: Download high-res image (88KB) Download: Download full-size image