Sensitive detection of uracil-DNA glycosylase (UDG) activity based on terminal deoxynucleotidyl transferase-assisted formation of fluorescent copper nanoclusters (CuNCs)

Abstract

As an important base excision repair (BER) enzyme, uracil-DNA glycosylase (UDG) can repair the uracil-induced DNA lesion and maintain the genomic integrity. Herein, we have proposed a sensitive UDG assay based on the terminal deoxynucleotidyl transferase (TdT)-assisted formation of fluorescent copper nanoclusters (CuNCs). In this study, a uracil-containing stem-loop DNA substrate is rationally designed with its 3′-end blocked with 2′, 3′-dideoxycytosine (ddC). UDG can remove the uracil in the DNA substrate to generate an apurinic/apyrimidinic (AP) site, which can then be specifically cleaved by endonuclease IV (Endo IV) to expose a 3′-OH terminus. TdT will initiate the template-free DNA extension along the exposed 3′-OH terminus to produce a quite long poly(T) tail, which will perfectly template the production of fluorescent CuNCs. By recording the fluorescence of the CuNCs, the UDG activity can be faithfully detected. In contrast, if UDG is absent, the 3′-ddC terminus of the DNA substrate cannot be recognized by TdT and thus no TdT-based extension and formation of CuNCs will occur. The use of ddC as a 3′-end blocker can greatly decrease the nonspecific DNA extension and improve the signal-to-noise ratio. Furthermore, TdT is a template-free and sequence-independent DNA polymerase, which can effectively catalyze the tailing process up to a maximum of thousands of thymines, and each tail can form many fluorescent CuNCs. Therefore, an ultrahigh sensitivity is achieved and as low as 0.00005 U/mL of UDG can be clearly detected. Moreover, with an additional AP site-contained poly(A) oligonucleotide during TdT-mediated extension, a branched amplification mechanism is also preliminarily devised, which can further push the detection limit of UDG to an extremely low level of 0.000002 U/mL.