A Ratiometric Nonfluorescent CRISPR Assay Utilizing Cas12a-Induced Plasmid Supercoil Relaxation

Abstract

Most CRISPR-based biosensors rely on labeled reporter molecules and expensive equipment for signal readout. A recent alternative approach quantifies analyte concentration by sizing native λ DNA reporters, providing a simple and label-free solution for ultrasensitive detection. However, this method faces challenges in accurately quantifying size reduction of long DNA reporters via gel electrophoresis due to the sensitivity of DNA band shift to other interferences such as gel distortion. To overcome these limitations, here we developed a simple and robust ratiometric signaling strategy using CRISPR-Cas12a-induced supercoil relaxation of dsDNA plasmid reporters. In the presence of target, we observed that the fraction of supercoiled plasmid DNA decreased, and the amount of relaxed conformation (circular) increased over time. The relative percentage of supercoiled DNA to the relaxed circular DNA was analyzed by gel electrophoresis to generate an intensity-based ratiometric signal for more accurate target concentration quantification. This simple and inexpensive method is ∼100 times more sensitive when compared with the typical fluorescent reporter system. This self-referenced strategy solves the potential application limitations of previously demonstrated DNA sizing-based CRISPR-Dx without compromising the sensitivity. Finally, we demonstrated the applicability of ratiometric sensing strategy using model DNA targets such as AAV and HPV 16, highlighting its feasibility for point-of-use CRISPR-Dx applications.Table of Contents (TOC):