Fluorogenic RNA Aptamer-Based Amplification and Transcription Strategy for Label-free Sensing of Methyltransferase Activity in Complex Matrixes

DNA methyltransferase is significant in cellular activities and gene expression, and its aberrant expression is closely linked to various cancers during initiation and progression. Currently, there is a great demand for reliable and label-free techniques for DNA methyltransferase evaluation in tumor diagnosis and cancer therapy. Herein, a low-background fluorescent RNA aptamer-based sensing approach for label-free quantification of cytosine-guanine (CpG) dinucleotides methyltransferase (M.SssI) is reported. The fluorogenic light-up RNA aptamers-based strategy exhibits high selectivity via restriction endonuclease, padlock-based recognition, and RNA transcription. By combining rolling circle amplification (RCA), and RNA transcription with fluorescence response of RNA aptamers of Spinach-dye compound, the proposed platform exhibited efficiently ultrahigh sensitivity toward M.SssI. Eventually, the detection can be achieved in a linear range of 0.02-100 U mL-1 with a detection limit of 1.6 × 10-3 U mL-1. Owing to these superior features, the method is further applied in serum samples spiked M.SssI, which delivers a recovery ranging from 92.0 to 107.0% and a relative standard deviation <7.0%, providing a promising and practical tool for determining M.SssI in complex biological matrices.

Ultrasensitive fluorescence detection of multiple DNA methyltransferases based on DNA walkers and hyperbranched rolling circle amplification

The accurate and ultrasensitive detection of multiple methyltransferases was in great request for clinical diagnosis and epigenetic therapy. Here, a novel fluorescence assay was proposed for ultrasensitive CpG methyltransferase (M.SssI) and DNA adenine methyltransferase (Dam) activity detection based on hyperbranched rolling circle amplification (HRCA) and DNA walkers. The biosensor showed an extremely high sensitivity due to the dual-amplification strategy of HRCA and DNA walker. The LOD of the biosensor for M.SssI and Dam methyltransferase was estimated at 0.0004 U/mL and 0.001 U/mL, respectively. Without the presence of M.SssI methyltransferase, the corresponding recognition site of hairpin H_M was cleaved by HpaII endonuclease, generating a DNA fragment (T-DNA) and inducing the DNA walker-HRCA reaction. Since the HRCA products contained numerous double-strand DNA (dsDNA), SYBR Green I could be embedded in the dsDNA, leading to a high fluorescent signal. In the presence of M.SssI methyltransferase, the corresponding recognition site of hairpin H_M was methylated and the HpaII endonuclease-catalyzed stem of hairpin H_M dissociation was hindered, leading to no DNA fragment (T-DNA) present. Hence, the DNA walker-HRCA reaction was not initiated and the fluorescent signal of SYBR Green I remained at a low level. Similarly, DNA adenine methyltransferase (Dam) and its inhibitors could also be detected by redesigning hairpin H_D with the Dam recognition sequences. Furthermore, the sensing system was applied to analyze the endogenic Dam methyltransferase in the real samples such as E. coli cell lysate.

Simple Detection of DNA Methyltransferase with an Integrated Padlock Probe

DNA methyltransferases (MTases) can be regarded as biomarkers, as demonstrated by many studies on genetic diseases. Many researchers have developed biosensors to detect the activity of DNA MTases, and nucleic acid amplification, which need other probe assistance, is often used to improve the sensitivity of DNA MTases. However, there is no integrated probe that incorporates substrates and template and primer for detecting DNA MTases activity. Herein, we first designed a padlock probe (PP) to detect DNA MTases, which combines target detection with rolling circle amplification (RCA) without purification or other probe assistance. As the substrate of MTase, the PP was methylated and defended against HpaII, lambda exonuclease, and ExoI cleavage, as well as digestion, by adding MTase and the undestroyed PP started RCA. Thus, the fluorescent signal was capable of being rapidly detected after adding SYBR^TM Gold to the RCA products. This method has a detection limit of approximately 0.0404 U/mL, and the linear range was 0.5–110 U/mL for M.SssI. Moreover, complex biological environment assays present prospects for possible application in intricacy environments. In addition, the designed detection system can also screen drugs or inhibitors for MTases.

Construction of an APE1-Mediated Cascade Signal Amplification Platform for Homogeneously Sensitive and Rapid Measurement of DNA Methyltransferase in Escherichia coli Cells

DNA methylation is an essential genomic epigenetic behavior in both eukaryotes and prokaryotes. Deregulation of DNA methyltransferase (Dam MTase) can change the DNA methylation level and cause various diseases. Herein, we develop an apurinic/apyrimidinic endonuclease 1 (APE1)-mediated cascade signal amplification platform for homogeneously sensitive and rapid measurement of Dam MTase in Escherichia coli cells. This assay involves a partial double-stranded DNA (dsDNA) substrate and two hairpin signal probes (HP1 and HP2) that are modified with Cy5 and BHQ2 at two ends, respectively. When Dam MTase is present, it methylates the dsDNA substrate, and subsequently, endonuclease DpnI cleaves the methylated substrate, yielding trigger probe 1. Hybridization of trigger probe 1 with HP1 forms a partial dsDNA containing an apurinic/apyrimidinic (AP) site, which is cleaved by APE1 to induce the cyclic cleavage of HP1 and the production of abundant trigger probe 2. Subsequent hybridization of trigger probe 2 with HP2 forms a partial dsDNA with an AP site, inducing the cyclic cleavage of HP2 by APE1. Consequently, cyclic cleavage of HP1 and HP2 induces the generation of abundant Cy5 molecules, which are easily measured by single-molecule imaging. This assay can be performed homogeneously and rapidly within 2 h, which is the shortest among the reported amplification-based assays. Moreover, it exhibits good selectivity and high sensitivity, and it can discriminate Dam MTase from other enzymes and screen inhibitors. Importantly, it can accurately measure the Dam MTase activity in serum and E. coli cells, with promising applications in clinical diagnosis and drug discovery.

An Isothermal Autocatalytic Hybridization Reaction Circuit for Sensitive Detection of DNA Methyltransferase and Inhibitors Assay

Abnormal DNA methylation contributes to the annoying tumorigenesis and the elevated expression of methylation-related methyltransferase (MTase) is associated with many diseases. Hence DNA MTase could serve as a promising biomarker for cancer-specific diagnosis as well as a potential therapeutic target. Herein, we developed an isothermal autocatalytic hybridization reaction (AHR) circuit for the sensitive detection of MTase and its inhibitors by integrating the catalytic hairpin assembly (CHA) converter with the hybridization chain reaction (HCR) amplifier. The initiator-mediated HCR amplifier could generate amplified fluorescent readout, as well as numerous newly activated triggers for motivating the CHA converter. The CHA converter is designed to expose the identical sequence of HCR initiators that reversely powered the HCR amplifier. Thus, the trace amount of target could produce exponentially amplified fluorescent readout by the autocatalytic feedback cycle between HCR and CHA systems. Then an auxiliary hairpin was introduced to mediate the assay of Dam MTase via the well-established AHR circuit. The Dam MTase-catalyzed methylation of auxiliary hairpin leads to its subsequent efficient cleavage by DpnI endonuclease, thus resulting in the release of HCR initiators to initiate the AHR circuit. The programmable nature of the auxiliary hairpin allows its easy adaption into other MTase assay by simply changing the recognition site. This proposed AHR circuit permits a sensitive, robust, and versatile analysis of MTase with the limit of detection (LOD) of 0.011 U/mL. Lastly, the AHR circuit could be utilized for MTase analysis in real complex samples and for evaluating the cell-cycle-dependent expression of MTase. This developed MTase-sensing strategy holds promising potential for biomedical analysis and clinical diagnosis.

No-nonspecific recognition-based amplification strategy for endonuclease activity screening with dual-color DNA nano-clew

The inevitable nonspecific recognition severely restricted widely used nucleic acid amplification strategies, which has become an urgent problem in current scientific research. Herein, we developed a novel no-nonspecific recognition-based amplification strategy to construct dual-color dye loaded nano-clew as ultrabright illuminant for screening endonuclease activity with Escherichia coliRY13 I (EcoR I) as a model, which overcame some major drawbacks such as nonspecific recognition and photobleaching. Typically, the target endonuclease induces cleavage of the customized dumbbell-shape substrate (DSS) to generate two same triggers that can initiate the rolling circle amplification (RCA) to prepare long single-strand DNA (lssDNA), which could self-assemble into irregular DNA nano-clew based on the electrostatic interactions with Mg²⁺ to furtherly capture the donor and accepter fluorophore proximately, constructing the dye loaded nano-clew with dual-color fluorescence (FL) emission to resist photobleaching. Importantly, in absence of EcoR I, even if the DSS could combine with circular template a little, the reaction system performed hardly RCA reaction due to no cohesive terminus, resulting an extremely low background fluorescence signal because of the prevention of nonspecific RCA reaction. As expected, the proposed sensing platform with a low limit of detection (LOD) of 3.4 × 10^-7 U/μL was demonstrated to work well for endonuclease inhibitors screening also. Furthermore, the proposed no-nonspecific recognition strategy could be readily extended to various DNA or RNA enzymes such as DNA methyltransferase, DNA repair-related enzymes and polynucleotide kinase just by simply changing the recognition sequence in the DNA substrate, performing great potential of endonucleases-related clinical diagnosis and drugs discovery.

Nonamplification Multiplexed Assay of Endonucleases and DNA Methyltransferases by Colocalized Particle Counting

Restriction endonucleases (ENases) and DNA methyltransferases (MTases) are important enzymes in biological processes, and detection of ENases/MTases activity is significant for biological and pharmaceutical studies. However, available nonamplification methods with a versatile design, desirable sensitivity, and signal production mode of unbiased quantification toward multiple nucleases are rare. By combining deliberately designed hairpin DNA probes with the colocalized particle counting technique, we present a nonamplification, separation-free method for multiplexed detection of ENases and MTases. In the presence of target ENases, the hairpin DNA is cleaved and the resulting DNA sequence forms a sandwich structure to tie two different-colored fluorescent microbeads together to generate a colocalization signal that can be easily detected using a standard fluorescence microscope. The multiplexed assay is realized via different color combinations. For the assay of methyltransferase, methylation by MTases prevents cleavage of the hairpin by the corresponding ENase, leading to decreased colocalization events. Three ENases can be simultaneously detected with high selectivity, minimal cross-talk, and detection limits of (4.1-6.4) × 10^-4 U/mL, and the corresponding MTase activity can be measured without a change of the probe design. The potential for practical application is evaluated with human serum samples and different ENase and MTase inhibitors with satisfactory results. The proposed method is separation-free, unbiased toward multiple targets, and easy to implement, and the strategy has the potential to be extended to other targets.