‘Splice-at-will’ Cas12a crRNA engineering enabled direct quantification of ultrashort RNAs

Abstract

We present a robust ‘splice-at-will’ CRISPR RNA (crRNA) engineering mechanism that overcomes the limitations of clustered regularly interspaced short palindromic repeats (CRISPR)/Cas system in directly detecting ultrashort RNAs. In this strategy, an intact Cas12a crRNA can be split from almost any site of the spacer region to obtain a truncated crRNA (tcrRNA) that cannot activate Cas12a even after binding an auxiliary DNA activator. While splicing tcrRNAs with a moiety of ultrashort RNA, the formed combination can work together to activate Cas12a efficiently, enabling ‘splice-at-will’ crRNA engineering. Importantly, the ‘splice-at-will’ crRNA exhibits almost the same trans-cleavage activation efficiency as that of a conventional intact crRNA. Therefore, by rationally designing a DNA auxiliary activator with a conserved tcrRNA-complementary sequence and an arbitrary short RNA-of-interest recognition domain, a general sensing system is established that directly utilizes traditional DNA-activated Cas12a to detect ultrashort RNAs. This ‘splice-at-will’ crRNA engineering strategy could faithfully detect ultrashort RNA sequences as short as 6–8 nt, which cannot be achieved by conventional Cas12a and Cas13a systems. Additionally, through flexible splicing site design, our method can precisely distinguish single-base differences in microRNA and other short RNA sequences. This work has significantly expanded the Cas12a-based diagnostic toolbox and opened new avenues for ultrashort RNA detection.