Circular DNAzyme-Switched CRISPR/Cas12a Assay for Electrochemiluminescent Response of Demethylase Activity

DNA nanostructure provides powerful tools for DNA demethylase activity detection, but its stability has been significantly challenged. By virtue of circular DNA with resistance to exonuclease degradation, herein, the circular DNAzyme duplex with artificial methylated modification was constructed to identify the target and output the DNA activators to drive the CRISPR/Cas12a, constructing an "on–off–on" electrochemiluminescence (ECL) biosensor for monitoring the activity of the O6-methylguanine-DNA methyltransferase (MGMT). Specifically, the circular DNAzyme duplex consisted of the chimeric RNA–DNA substrate ring with double activator sequences and two single-stranded DNAzymes, whose catalytic domains were premodified with the methyl groups. When the MGMT was present, the methylated DNAzymes were repaired and restored the catalytic activity to cleave the chimeric RNA–DNA substrates, followed by the output of DNA activators to initiate the CRISPR/Cas12a. Subsequently, the ECL signals of silver nanoparticle-modified SnO2 nanospheres (Ag@SnO2) were recovered by releasing the ferrocene-labeled quenching probes (Fc-DNA) from the electrode surface because of the trans-cleavage activity of CRISPR/Cas12a, thus achieving the specific and sensitive ECL detection of MGMT from 2.5 × 10–4 to 2.5 × 102 ng/mL with a low limit (9.69 × 10–5 ng/mL). This strategy affords novel ideas and insights into research on how to project stable nucleic acid probes to detect DNA demethylases beyond traditional methods.