Colorimetric Detection of PCR Product with DNAzymes Induced by 5′-Nuclease Activity of DNA Polymerases

Development of a Novel Multiplex PCR Method for the Rapid Detection of SARS-CoV-2, Influenza A Virus, and Influenza B Virus

Objective

A sensitive and specific multiplex fluorescence rapid detection method was established for simultaneous detection of SARS-CoV-2, influenza A virus, and influenza B virus in a self-made device within 30 min, with a minimum detection limit of 200 copies/mL.

Methods

Based on the genome sequences of SARS-CoV-2, influenza A virus (FluA), and influenza B virus (FluB) with reference to the Chinese Center for Disease Control and Prevention and related literature, specific primers were designed, and a multiplex fluorescent PCR system was established. The simultaneous and rapid detection of SARS-CoV-2, FluA, and FluB was achieved by optimizing the concentrations of Taq DNA polymerase as well as primers, probes, and Mg2+. The minimum detection limits of the nucleic acid rapid detection system for SARS-CoV-2, FluA, and FluB were evaluated.

Results

By optimizing the amplification system, the N enzyme with the best amplification performance was selected, and the optimal concentration of Mg2+ in the multiamplification system was 3 mmol/L; the final concentrations of SARS-CoV-2 NP probe and primer were 0.15 μmol/L and 0.2 μmol/L, respectively; the final concentrations of SARS-CoV-2 ORF probe and primer were both 0.15 μmol/L; the final concentrations of FluA probe and primer were 0.2 μmol/L and 0.3 μmol/L, respectively; the final concentrations of FluB probe and primer were 0.15 μmol/L and 0.25 μmol/L, respectively.

Conclusion

A multiplex real-time quantitative fluorescence RT-PCR system for three respiratory viruses of SARS-CoV-2, FluA, and FluB was established with a high amplification efficiency and sensitivity reaching 200 copies/mL for all samples. Combined with the automated microfluidic nucleic acid detection system, the system can achieve rapid detection in 30 minutes.

Solid-Phase PCR-Amplified DNAzyme Activity for Real-Time FO-SPR Detection of the MCR-2 Gene

The polymerase chain reaction (PCR) has been the gold standard molecular analysis technique for decades and has seen quite some evolution in terms of reaction components, methodology, and readout mechanisms. Nucleic acid enzymes (NAzymes) have been used to further exploit the applications of PCR, but so far the work was limited to the colorimetric G-quadruplex or fluorescent substrate cleaving NAzymes. In this study, a solid-phase, fiber optic surface plasmon resonance (FO-SPR) technique is presented as an alternative readout for PCR utilizing NAzymes. First, the surface cleavage activity of DNAzyme-extended amplicons (DNAzyme-amps) is established, followed by optimization of the PCR conditions, which are required for compatibility with the FO-SPR system. Next, by integrating the complement of a 10-23 DNAzyme into the primer pair, PCR-amplified DNAzyme-amps were generated, tested, and validated on qPCR for the detection of the antimicrobial resistance gene MCR-2. Once validated, this primer concept was developed as a one-step assay, driven by PCR-amplified DNAzymes, for FO-SPR-based sensitive and specific detection. Using gold nanoparticle labeled RNA-DNA hybrid strands as substrate for the DNAzyme, PCR-amplified DNAzyme-amps generated in the presence of MCR-2 gene were monitored in real-time, which resulted in an experimental limit of detection of 4 × 10⁵ copy numbers or 6.6 fM. In addition, the DNAzyme-based FO-PCR assay was able to discriminate between the MCR-1 and MCR-2 genes, to further prove the specificity of this assay. Henceforth, this DNAzyme-based fiber optic PCR assay provides a universally applicable, real-time system for the detection of virtually any target NA, in a specific and sensitive manner.

Bioanalytical Application of Peroxidase-Mimicking DNAzymes: Status and Challenges

DNAzymes with peroxidase-mimicking activity are a new class of catalytically active DNA molecules. This system is formed as a complex of hemin and a G-quadruplex structure created by oligonucleotides rich in guanine. Considering catalytic activity, this DNAzyme mimics horseradish peroxidase, the enzyme most commonly used for signal generation in bioassays. Because DNAzymes exhibit many advantages over protein enzymes (thermal stability, easy and cheap synthesis and purification) they can successfully replace HRP in bioanalytical applications. HRP-like DNAzymes have been applied in the detection of several DNA sequences. Many amplification techniques have been conjugated with DNAzyme systems, resulting in ultrasensitive bioassays. On the other hand, the combination of aptamers and DNAzymes has led to the development of aptazymes for specific targets. An up-to-date summary of the most interesting DNAzyme-based assays is presented here. The elaborated systems can be used in medical diagnosis or chemical and biological studies.

A Novel Colorimetric PCR-Based Biosensor for Detection and Quantification of Hepatitis B Virus

Hepatitis B virus (HBV) can cause viral infection that attacks the liver and it is a major global health problem that puts people at a high risk of death from cirrhosis of the liver and liver cancer. HBV has infected one-third of the worldwide population, and 350 million people suffer from chronic HBV infection. For these reasons, development of an accurate, sensitive, and expedient detection method for diagnosing, monitoring, and assessing therapeutic response of HBV is very necessary and urgent for public health and disease control. Here we report a new strategy for detection of viral load quantitation of HBV based on colorimetric polymerase chain reaction (PCR) with DNAzyme-containing probe. The special DNAzyme adopting a G-quadruplex structure exhibited peroxidase-like activity in the presence of hemin to report colorimetric signal. This method has shown a broad range of linearity and high sensitivity. This study builds an important foundation to achieve the specific and accurate detection level of HBV DNA with a low-cost and effective method in helping diagnosing, preventing and protecting human health form HBV all over the world, and especially in developing countries.

Integrating Deoxyribozymes into Colorimetric Sensing Platforms

Biosensors are analytical devices that have found a variety of applications in medical diagnostics, food quality control, environmental monitoring and biodefense. In recent years, functional nucleic acids, such as aptamers and nucleic acid enzymes, have shown great potential in biosensor development due to their excellent ability in target recognition and catalysis. Deoxyribozymes (or DNAzymes) are single-stranded DNA molecules with catalytic activity and can be isolated to recognize a wide range of analytes through the process of in vitro selection. By using various signal transduction mechanisms, DNAzymes can be engineered into fluorescent, colorimetric, electrochemical and chemiluminescent biosensors. Among them, colorimetric sensors represent an attractive option as the signal can be easily detected by the naked eye. This reduces reliance on complex and expensive equipment. In this review, we will discuss the recent progress in the development of colorimetric biosensors that make use of DNAzymes and the prospect of employing these sensors in a range of chemical and biological applications.

DNA polymerase-catalyzed incorporation of nucleotides modified with a G-quadruplex-derived DNAzyme

We show that nucleotides which are modified with a G-quadruplex-derived DNAzyme are substrates for DNA polymerases. Based on this finding we developed a naked-eye detection system that allows the detection of single nucleotide variations in DNA.

Sequence selective naked-eye detection of DNA harnessing extension of oligonucleotide-modified nucleotides

DNA polymerases can efficiently and sequence selectively incorporate oligonucleotide (ODN)-modified nucleotides and the incorporated oligonucleotide strand can be employed as primer in rolling circle amplification (RCA). The effective amplification of the DNA primer by Φ29 DNA polymerase allows the sequence-selective hybridisation of the amplified strand with a G-quadruplex DNA sequence that has horse radish peroxidase-like activity. Based on these findings we develop a system that allows DNA detection with single-base resolution by naked eye.

G-quadruplex DNAzymes-induced highly selective and sensitive colorimetric sensing of free heme in rat brain

Direct selective determination of free heme in the cerebral system is of great significance due to the crucial roles of free heme in physiological and pathological processes. In this work, a G-quadruplex DNAzymes-induced highly sensitive and selective colorimetric sensing of free heme in rat brain is established. Initially, the conformation of an 18-base G-rich DNA sequence, PS2.M (5'-GTGGGTAGGGCGGGTTGG-3'), in the presence of K(+), changes from a random coil to a "parallel" G-quadruplex structure, which can bind free heme in the cerebral system with high affinity through π-π stacking. The resulted heme/G-quadruplex complex exhibits high peroxidase-like activity, which can be used to catalyze the oxidation of colorless ABTS(2-) to green ABTS˙(-) by H2O2. The concentration of heme can be evaluated by the naked eye and determined by UV-vis spectroscopy. The signal output showed a linear relationship for heme within the concentration range from 1 to 120 nM with a detection limit of 0.637 nM. The assay demonstrated here was highly selective and free from the interference of physiologically important species such as dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), ascorbate acid (AA), cysteine, uric acid (UA), glucose and lactate in the cerebral system. The basal dialysate level of free heme in the microdialysate from the striatum of adult male Sprague-Dawley rats was determined to be 32.8 ± 19.5 nM (n = 3). The analytic protocol possesses many advantages, including theoretical simplicity, low-cost technical and instrumental demands, and responsible detection of heme in rat brain microdialysate.

Fluorescence reporting of G-quadruplex structures and modulating their DNAzyme activity using polyethylenimine-pyrene conjugate

Four-stranded G-quadruplex structure is one of the most important non-canonical secondary structures of DNA formed by guanine (G)-rich sequences. G-rich DNA sequences are known to occur in the human genome, especially in the telomere 3' end and in oncogene promoters such as c-MYC and c-KIT. In this context, we designed pyrene-conjugated polyethylenimine (PEI-Py) as a fluorescence reporter for the recognition and detection of G-quadruplex structures of G-rich deoxyoligonucleotides and human telomere and gene promoter sequences, under ambient conditions. PEI-Py exhibited prominent pyrene excimer emission in the presence of G-quadruplex structures of G-rich deoxyoligonucleotides and biologically relevant DNA sequences. PEI-Py further displayed the modulation of DNAzyme activity of various G-quadruplex structures in the presence of hemin and hydrogen peroxide.

A novel colorimetric PCR-based biosensor for detection and quantification of hepatitis B virus

Hepatitis B virus (HBV) can cause viral infection that attacks the liver and it is a major global health problem that put people at a high risk of death from cirrhosis of the liver and liver cancer. HBV has infected one third of the worldwide population, and 350 million people suffer from chronic HBV infection. For these reasons, development of an accurate, sensitive and expedient detection method for diagnosing, monitoring and assessing therapeutic response of HBV is very necessary and urgent for public health and disease control. Here we report a new strategy for detection of viral load quantitation of HBV based on colorimetric polymerase chain reaction (PCR) with DNAzyme-containing probe. The special DNAzyme adopting a G-quadruplex structure exhibited peroxidase-like activity in the presence of hemin to report colorimetric signal. This method has shown a broad range of linearity and high sensitivity. This study builds important foundation to achieve the specific and accurate detection level of HBV DNA with a low-cost and effective method in helping diagnosing, preventing and protecting human health form HBV generally all over the world and especially in developing countries.

Nucleic acid detection using G-quadruplex amplification methodologies

In the last decade, there has been an explosion in the use of G-quadruplex labels to detect various analytes, including DNA/RNA, proteins, metals and other metabolites. In this review, we focus on strategies for the detection of nucleic acids, using G-quadruplexes as detection labels or as enzyme labels that amplify detection signals. Methods to detect other analytes are briefly mentioned. We highlight various strategies, including split G-quadruplex, hemin-G-quadruplex conjugates, molecular beacon G-quadruplex or inhibited G-quadruplex probes. The tandem use of G-quadruplex labels with various DNA-modifying enzymes, such as polymerases (used for rolling circle amplification), exonucleases and endonucleases, is also discussed. Some of the detection modalities that are discussed in this review include fluorescence, colorimetric, chemiluminescence, and electrochemical methods.

RNA pathogen detection with one-step reverse transcription PCR and strand-displacement based signal amplification

A novel detection method for RNA pathogens based on one-step reverse transcription PCR is introduced here. This method utilized the reverse transcriptase activity and the 5'-nuclease activity of TaqM1 DNA polymerase to transform target RNA into cDNA. The following PCR process released a fragment from the 5' end as a specific probe. Afterwards this fragment triggered a strand-displacement based signal amplification to release large amounts of G-quadruplex DNAzymes. All the probes applied in our method were unmodified DNA oligonucleotides. The detection results could be reported without sophisticated instruments either in the colorimetric way through oxidizing ABTS or in the fluorometric way by using tyramine as substrate. This approach could successfully detect HIV-1 in a blood sample and it has a linear concentration range of 6 fM to 60 pM.

DNA based identification of medicinal materials in Chinese patent medicines

Chinese patent medicines (CPM) are highly processed and easy to use Traditional Chinese Medicine (TCM). The market for CPM in China alone is tens of billions US dollars annually and some of the CPM are also used as dietary supplements for health augmentation in the western countries. But concerns continue to be raised about the legality, safety and efficacy of many popular CPM. Here we report a pioneer work of applying molecular biotechnology to the identification of CPM, particularly well refined oral liquids and injections. What's more, this PCR based method can also be developed to an easy to use and cost-effective visual chip by taking advantage of G-quadruplex based Hybridization Chain Reaction. This study demonstrates that DNA identification of specific Medicinal materials is an efficient and cost-effective way to audit highly processed CPM and will assist in monitoring their quality and legality.