A hairpin reporter-driven feedback CRISPR/Cas signal amplification loop for terminal deoxynucleotidyl transferase activity detection.

The CRISPR/Cas12a system has become a powerful tool in biosensing because of its specific target recognition ability and highly efficient trans-cleavage activity. However, a problem faced by the CRISPR/Cas12a system when directly used for trace detection is the linear amplification efficiency of single-cycle digestion. Here, we present a novel hairpin reporter-driven CRISPR/Cas12a (HR-CRISPR) amplification system that establishes a positive feedback loop within the CRISPR/Cas12a platform to finish an exponential and sensitive signal amplification in a one-step reaction. As proof of concept, we applied this strategy to the terminal deoxynucleotidyl transferase (TdT) activity assay without pre-amplification procedure. The polyT strand extended by TdT hybridizes with crRNA, activating Cas12a, which then cleaves the FQ-hairpin reporter. The cleavage products are further elongated by reverse transcriptase using crRNA as a template, reactivating Cas12a and producing exponentially amplified fluorescence signals. This assay offers a simple yet highly sensitive approach for quantifying TdT activity, achieving a low detection limit of 4.55 × 10-6 U. Moreover, it is applicable for inhibitor screening and monitoring TdT activity in human serum samples.