A novel electrochemical biosensor for sensitive detection of telomerase activity based on structure-switching DNA

Sensitive naked-eye detection of telomerase activity based on exponential amplification reaction and lateral flow assay

Telomerase is a promising diagnostic and prognostic biomarker for cancers. Sensitive, simple, and reliable telomerase activity detection is vital for cancer diagnosis. Herein, we developed an ultrasensitive visualized assay for telomerase activity that combined the exponential amplification reaction (EXPAR) and lateral flow assay for easy and quick signal readout, which we termed as a lateral flow readout-EXPAR (LFR-EXPAR) assay. In the LFR-EXPAR assay, telomerase elongation products initiate the exponential amplification reaction, the generated trigger hybridizes with the reporter to form the recognition site of the nicking enzyme, and the nicking enzyme cuts the reporter strand. The degradation of the reporter can be detected with a universal lateral flow dipstick and read out with the naked eye. After conducting a series of proof-of-concept investigations, the LFR-EXPAR assay was found to achieve a sensitivity comparable to that of a TRAP (telomere repeat amplification protocol) assay. The LFR-EXPAR assay can be used to realize ultrasensitive and point-of-care detection of telomerase without requiring specialized instruments, holding great promise for early cancer diagnosis.

Test Strip Platform Spin-Off for Telomerase Activity Detection: Development of an Electrochemical Biosensor

Telomerase overexpression has been associated directly with cancer, and the enzyme itself is recognized within the scientific community as a cancer biomarker. BIDEA's biosensing strip (BBS) is an innovative technology capable of detecting the presence of telomerase activity (TA) using electrochemical impedance spectroscopy (EIS). This BBS is an interdigital gold (GID) electrode array similar in size and handling to a portable glucose sensor. For the detection of the biomarker, BBS was modified by the immobilization of a telomere-like single strand DNA (ssDNA) on its surface. The sensor was exposed to telomerase-positive extract from commercially available cancer cells, and the EIS spectra were measured. Telomerase recognizes the sequence of this immobilized ssDNA probe on the BBS, and the reverse transcription process that occurs in cancer cells is replicated, resulting in the ssDNA probe elongation. This surface process caused by the presence of TA generates changes in the capacitive process on the electrode array microchip surface, which is followed by EIS as the sensing tool and correlated with the presence of cancer cells. The telomerases' total cell extraction protocol results demonstrate significant changes in the charge-transfer resistance (R ct) change rate after exposure to telomerase-positive extract with a detection limit of 2.94 × 104 cells/mL. Finally, a preliminary study with a small set of "blind" uterine biopsy samples suggests the feasibility of using the changes in the R ct magnitude change rate (Δ(ΔR ct/R cti)/Δt) to distinguish positive from negative endometrial adenocarcinoma samples by the presence or absence of TA.

Ultrasensitive fluorescent detection of telomerase activity based on tetrahedral DNA nanostructures as carriers for DNA-templated silver nanoclusters

Precise evaluation of telomerase activity is essential for the clinical diagnosis of early tumors. Herein, we have ingeniously designed a tetrahedral DNA nanostructure, with hairpin-shaped DNA probes rich in cytosine bases at four vertices for telomerase detection. The DNA-templated silver nanoclusters can be formed after the addition of Ag. Then the introduction of telomerase adds the single-strand TTAGGG extension, which can "turn on" the fluorescence of silver nanoclusters quickly by the proximity of the resulting guanine-rich sequences to silver nanoclusters and realize accurate detection of telomerase activity. In this study, integration of high stability tetrahedral DNA nanostructure and fluorescence signal amplification of four DNA-templated silver nanoclusters offers the advantage of high sensitivity, with a low detection limit of 1 cell. More than that, this method is low-cost, facile, and feasible for practical clinical applications.

Electrochemical Resistive DNA Biosensor for the Detection of HPV Type 16

In this work, a low-cost and rapid electrochemical resistive DNA biosensor based on the current relaxation method is described. A DNA probe, complementary to the specific human papillomavirus type 16 (HPV-16) sequence, was immobilized onto a screen-printed gold electrode. DNA hybridization was detected by applying a potential step of 30 mV to the system, composed of an external capacitor and the modified electrode DNA/gold, for 750 µs and then relaxed back to the OCP, at which point the voltage and current discharging curves are registered for 25 ms. From the discharging curves, the potential and current relaxation were evaluated, and by using Ohm's law, the charge transfer resistance through the DNA-modified electrode was calculated. The presence of a complementary sequence was detected by the change in resistance when the ssDNA is transformed in dsDNA due to the hybridization event. The target DNA concentration was detected in the range of 5 to 20 nM. The results showed a good fit to the regression equation ΔRtotal(Ω)=2.99 × [DNA]+81.55, and a detection limit of 2.39 nM was obtained. As the sensing approach uses a direct current, the electronic architecture of the biosensor is simple and allows for the separation of faradic and nonfaradaic contributions. The simple electrochemical resistive biosensor reported here is a good candidate for the point-of-care diagnosis of HPV at a low cost and in a short detection time.

Single plasmonic nanostructures for biomedical diagnosis

The field of single nanoparticle plasmonics has grown enormously. There is no doubt that a wide diversity of the nanoplasmonic techniques and nanostructures represents a tremendous opportunity for fundamental biomedical studies as well as sensing and imaging applications. Single nanoparticle plasmonic biosensors are efficient in label-free single-molecule detection, as well as in monitoring real-time binding events of even several biomolecules. In the present review, we have discussed the prominent advantages and advances in single particle characterization and synthesis as well as new insight into and information on biomedical diagnosis uniquely obtained using single particle approaches. The approaches include the fundamental studies of nanoplasmonic behavior, two typical methods based on refractive index change and characteristic light intensity change, exciting innovations of synthetic strategies for new plasmonic nanostructures, and practical applications using single particle sensing, imaging, and tracking. The basic sphere and rod nanostructures are the focus of extensive investigations in biomedicine, while they can be programmed into algorithmic assemblies for novel plasmonic diagnosis. Design of single nanoparticles for the detection of single biomolecules will have far-reaching consequences in biomedical diagnosis.

An all-in-one telomerase assay based on CRISPR-Cas12a trans-cleavage while telomere synthesis

As one of the crucial factors associated with human life span and cancer progression, telomerase is regarded as an emerging biomarker for cancer diagnosis. Therefore, a facile, rapid and sensitive approach for telomerase activity detection with point-of-care (POC) diagnosis potential is in great demands. Herein, an all-in-one telomerase activity detection assay was established based on the telomere synthesis activated CRISPR-Cas12a system. A telomerase extension reaction generated telomere repeats sequences (TTAGGG)_n, which was recognized by a customized CRISPR-guided RNA (crRNA) simultaneously, and finally activated a typical trans-cleavage based CRISPR-Cas12a detection assay. With the inherent sensitivity of CRISPR-Cas12a, this approach achieved a great linear regression ranging from 100 to 2000 HeLa cells and a limitation of detection down to 26 HeLa cells. Moreover, by using the proposed method, telomerase can be detected in one pot under isothermal condition (37 °C) by a simple and fast workflow (one step within 1 h). Due to its excellent performance, this all-in-one method shows great potential in POC detection of the telomerase activity.

A stepwise recognition strategy for the detection of telomerase activity via direct electrochemical analysis of metal-organic frameworks

The detection of telomerase is of great significance for monitoring cell canceration. The conventional methods depend on the extension of telomerase towards its primer to conduct signal transduction. Herein, a specific and reliable detection strategy based on stepwise recognition was developed for tandem detection of metal ions and enzymes. We first synthesized an electrically active metal-organic framework (MIL-101(Fe)), which can act directly as a signal reporter in phosphate buffered saline after being modified with capture DNA (cDNA). When the zinc ion is added as a coenzyme factor, the modified hairpin DNA on the electrode is cleaved by DNAzyme to yield the activated primer. After the addition of telomerase, the cleaved DNA strand would be extended, and the resulting sequence will be hybridized with the signal label of MIL-101(Fe)-cDNA. Therefore, a signal-on strategy for the detection of telomerase was achieved based on the direct electrochemical analysis of MIL-101(Fe). Moreover, this electrochemical biosensor can discriminate telomerase activity among different cell lines. The stepwise recognition ensured the advantages of an electrochemical biosensor such as high sensitivity and specificity during the detection process, providing a novel method for monitoring and diagnosis of diseases.

Sensitive detection of intracellular telomerase activity via double signal amplification and ratiometric fluorescence resonance energy transfer

As an important and universal tumor marker, the reliable and in situ detection of intracellular telomerase activity is crucial for cancer diagnosis. Herein, a ratiometric fluorescence resonance energy transfer (FRET) method was developed for detecting intracellular telomerase activity. It takes full advantage of manganese dioxide nanosheets (MnO2NS) that can carry DNA probes with different conformations into cells and then completely release the DNA probes via decomposition of MnO2NS by intracellular reduced glutathione (GSH). In the presence of telomerase, a telomere substrate (TS) could be extended to form long telomerase extension products (TEPs), which trigger the cycling strand displacement reaction (SDR) between two fluorophore-labeled hairpin DNA probes to form lots of DNA duplexes. The close contact of two fluorophores led to an effective ratiometric FRET for reliable detection of telomerase activity. Fluorescence confocal imaging demonstrated that the activity of telomerase in tumor cells was reliably detected. The inhibition of telomerase activity by an inhibitor resulted in a decrease in FRET signal. For extracellular detection, the FRET ratio (FA/FD) shows a good linear relationship with the number of HeLa cells in the range of 20-1000 cells. Therefore, it offers a more facile method for reliable and sensitive detection of intracellular telomerase activity.

Electrocatalysis of cerium metal-organic frameworks for ratiometric electrochemical detection of telomerase activity

A ratiometric electrochemical biosensor was constructed to detect telomerase activity based on electrocatalysis of cerium-based metal-organic frameworks (CeMOFs) and conformation switch of hairpin DNA. First, the CeMOFs were synthesized using Ce as nodes and 1,3,5-benzenetricarboxylic acid as linker in a green method, and then functionalized with gold nanoparticles. The resulted Au@CeMOF tags demonstrated an excellent electrocatalysis toward hydroquinone oxidation. Meanwhile, a methylene blue (MB) modified hairpin probe was designed with telomerase primer (TP) hybridized "stem" and immobilized on the electrode surface via Au-S chemistry. In the presence of the dNTPs and telomerase, the extended TP can open the hairpin DNA and keep the MB away from the electrode surface, resulting in a decrease of electrochemical signal. In the meantime, the TP-extended part could capture the Au@CeMOF-cDNA tags on the electrode surface via hybridization, leading to the increase electrochemical signal of hydroquinone oxidation catalyzed by Au@CeMOF-cDNA tags. Thus, a ratiometric signal output mode was developed for the electrochemical detection of telomerase activity. This biosensor showed wide dynamic correlation of telomerase activity from 2 × 10² to 2 × 10⁶ cells mL^-1 with the limit of detection of 27 cells mL^-1, and was applied to evaluate telomerase activity in single cell. The ratiometric electrochemical strategy based on the catalysis of MOFs provides a new avenue on signal transduction in telomerase detection.

A PCR-free voltammetric telomerase activity assay using a substrate primer on a gold electrode and DNA-triggered capture of gold nanoparticles

The paper describes a voltammetric method for the quantitation of the activity of telomerase extracted from cancer cells. A thiolated single-stranded telomerase substrate primer was firstly immobilized on a gold electrode. In the presence of a mixture of telomerase and deoxynucleotide triphosphates, the primer becomes elongated and contains repetitive nucleotide sequences (TTAGGG)_n. After hybridization with blocker DNA, gold nanoparticles are added and captured by the elongated single-stranded DNA. This reduces the charge transfer resistance of the gold electrode. The telomerase activity is then quantified via differential pulse voltammetry, typically at 0.12 V (vs. SCE). The method is PCR-free, rapid, and convenient. It was applied to the detection of HeLa cells via the telomerase activity of lysed cells. The detection range was from 500 to 50,000 cells/mL and the detection limit was as low as 500 cells/mL. Graphical abstract A telomerase substrate (TS) primer is immobilized on a gold electrode as the sensing interface to detect the activity of telomerase extracted from cancer cells. Unmodified gold nanoparticles (AuNPs) are utilized which change the electrochemical responses.

Ratiometric Fluorescent Biosensor for Visual Discrimination of Cancer Cells with Different Telomerase Expression Levels

Telomerase is inactive in normal somatic cells but highly activated in tumor cells to maintain their indefinite proliferation and immortal phenotype. As a specific marker for the generation and progress of almost all tumors, the detection of telomerase activity by classical PCR techniques has served in the biological research of tumors. However, the detection of in situ telomerase activity in cell extracts to evaluate the malignancy, progress, and metastasis of tumors remains a daunting challenge. Here, a precisely designed FRET-based ratiometric fluorescent oligonucleotide probe has achieved high-fidelity detection of telomerase activity for accurate discrimination of different cancer cells toward advanced diagnosis of tumors. Our method is superior to other methods in its capabilities to quantify telomerase activity in cell extracts and visualize various tumor cell extracts with different telomerase expression levels by the naked eye for clinical diagnosis. In particular, the ratiometric fluorescent probe used in the assay could exclude other experimental factors influence, and further avoid false positive signal generation. The method reported here could provide a reliable, accurate, and convenient way in medical diagnostics and therapeutic response assessment.

A facile graphene oxide-based fluorescent nanosensor for the in situ “turn-on” detection of telomerase activity

A graphene oxide-based fluorescent nanosensor has been proposed for the quantitative detection of telomerase and in situ imaging of intracellular telomerase.

A triple-amplification strategy for sensitive detection of telomerase at the single-cell level

We develop a triple-amplification strategy for sensitive detection of telomerase from cancer cells at the single-cell level.

A PCR-free colorimetric strategy for visualized assay of telomerase activity

A simple yet powerful polymerase chain reaction (PCR)-free strategy for visualized assay of human telomerase activity was reported in this work. Gold nanoparticles (AuNPs) based colorimetric strategy was applied with well-designed enzyme-aided cyclic amplification. Briefly, the detection relies on the elongated primers of telomerase substrate (TS) induced by telomerase, which open the hairpin DNA and hybridize with linker DNA, the trigger of AuNPs aggregation. Nicking endonuclease was added in the sensing system, which cleaved linker DNA after hybridization and released complimentary strand for cyclic hybridization with linker DNA, resulted in high sensitivity for the detection of telomerase. Down to 25 HeLa cells with high expression of telomerase could be recognized. The proposed strategy provides a good platform for the determination of telomerase activity, differentiation of cancer cell lines from normal cell line and screening of telomerase-targeted anticancer drugs.

An AIEgens and exonuclease III aided quadratic amplification assay for detecting and cellular imaging of telomerase activity

Monitoring telomerase activity with high sensitive and reliable is of great importance to cancer analysis. In this paper, we report a sensitive and facile method to detect telomerase activity using AIEgens modified probe (TPE-Py-DNA) as a fluorescence reporter and exonuclease III (Exo III) as a signal amplifier. With the aid of telomerase, repeat units (TTAGGG)n are extended from the end of template substrate oligonucleotides (TS primer) that form duplex DNAs with TPE-Py-DNA. Then, Exo III catalyzes the digestion of duplex DNAs, liberating elongation product and releasing hydrophobic TPE-Py. The released hydrophobic TPE-Py aggregate together and produce a telomerase-activity-related fluorescence signal. The liberated product hybridizes with another TPE-Py-DNA probe, starting the second cycle. Finally, we obtain the target-to-signal amplification ratio of 1:N². This strategy exhibits good performance for detecting clinical urine samples (distinguishing 15 cancer patients' samples from 8 healthy ones) and checking intracellular telomerase activity (differentiating cell lines including HeLa, MDA-MB-231, MCF-7, A375, HLF and MRC-5 from the cells pretreated with telomerase-related drug), which shows its potential in clinical diagnosis as well as therapeutic monitoring of cancer.

Catalytic assembly of DNA nanostructures on a nanoporous gold array as 3D architectures for label-free telomerase activity sensing

Telomerase, an enzyme known to catalyze telomere elongation by adding TTAGGG [thymine (T), adenine (A), and guanine (G)] repeats to the end of telomeres, is vital for cell proliferation. Overexpression of telomerase has been found in most tumor cells, resulting in telomere dysfunction and uncontrolled cellular proliferation. Thus, telomerase has been considered as a potential cancer biomarker, as well as a potential target in cancer therapy. In this study, telomerase-catalyzed growth of tandem G-quadruplex (G4) assembled on a nanoporous gold array (NPGA) resulted in the formation of three-dimensional hybrid nanoarchitectures. The generated nanostructure then captured malachite green (MG) (reporter molecule) without the need of a complicated labeling process. Upon laser irradiation, the captured MG molecules produced a surface-enhanced Raman scattering (SERS) signal that was generated by an abundant amount of plasmonic hot spots in the NPGA substrates. A limit of detection (LOD) of 10^-10 IU along with a linear range, which was 3 orders of magnitude, was achieved, which was equivalent to the telomerase amount extracted from 20 HeLa cells. The LOD is 2 orders of magnitude better than that of the commercial enzyme-linked immunosorbent assay (ELISA), and it approaches that of the most sensitive technique, telomeric repeat amplification protocols (TRAP), which require a laborious and equipment-intensive polymerase chain reaction (PCR). In addition, X-ray photoelectron spectroscopy (XPS) was used to chemically identify and quantify the telomerase activity on the sensitized NPGA surface. Furthermore, the sensor was applied to screen the effectiveness of anti-telomerase drugs such as zidovudine, thus demonstrating the potential use of the sensor in telomerase-based diagnosis and drug development. Moreover, the framework represents a novel paradigm of collaborative plasmonic intensification and catalytic multiplication (c-PI/CM) for label-free biosensing.

Advances in the detection of telomerase activity using isothermal amplification

Telomerase plays a significantly important role in keeping the telomere length of a chromosome. Telomerase overexpresses in nearly all tumor cells, suggesting that telomerase could be not only a promising biomarker but also a potential therapeutic target for cancers. Therefore, numerous efforts focusing on the detection of telomerase activity have been reported from polymerase chain reaction (PCR)-based telomeric repeat amplification protocol (TRAP) assays to PCR-free assays such as isothermal amplification in recent decade. In this review, we highlight the strategies for the detection of telomerase activity using isothermal amplification and discuss some of the challenges in designing future telomerase assays as well.

Design and development of PCR-free highly sensitive electrochemical assay for detection of telomerase activity using Nano-based (liposomal) signal amplification platform

Telomerase, which has been detected in almost all kinds of cancer tissues, is considered as an important tumor marker for early cancer diagnostics. In the present study, an electrochemical method based on liposomal signal amplification platform is proposed for simple, PCR-free, and highly sensitive detection of human telomerase activity, extracted from A549 cells. In this strategy, telomerase reaction products, which immobilized on streptavidin-coated microplate, hybridized with biotinylated capture probes. Then, dopamine-loaded biotinylated liposomes are attached through streptavidin to biotinylated capture probes. Finally, liposomes are ruptured by methanol and the released-dopamine is subsequently measured using differential pulse voltammetry technique by multi-walled carbon nanotubes modified glassy carbon electrode. Using this strategy, the telomerase activity extracted from 10 cultured cancer cells could be detected. Therefore, this approach affords high sensitivity for telomerase activity detection and it can be regarded as an alternative to telomeric repeat amplification protocol assay, having the advantages of simplicity and less assay time.

Platinum nanoparticles encapsulated metal–organic frameworks for the electrochemical detection of telomerase activity

A one-pot strategy was developed for the preparation of platinum nanoparticle encapsulated metal–organic frameworks as an electrocatalytic tracer for sensing telomerase activity.

A PCR-free fluorescence strategy for detecting telomerase activity via double amplification strategy

As a universal tumor biomarker, research on the activity and inhibition of telomerase is of great importance for cancer diagnosis and therapy. Although the telomeric repeat amplification protocol (TRAP) has served as a powerful assay for detecting telomerase activity, its application has been significantly limited by amplification related errors and time-consuming procedure. To address the limitations of PCR-based protocol, a dual amplification fluorescence assay was developed for PCR-free detecting telomerase activity. Briefly, we designed an arch-structure DNA probe to specifically control strand displacement reaction and subsequent enzyme-aided amplification. Telomerase substrate (TS) primer was extended by telomerase to form long elongation products which contain several TTAGGG repeat units. So, one elongation product can release more than one trigger DNA (t-DNA) via strand displacement reaction to realize first amplification. Subsequently, t-DNA specifically opened molecular beacon (MB) to restore the fluorescence of MB. Meanwhile, t-DNA was recycled by the aid of nicking endonuclease to continuously open more and more MBs, leading to a second amplification. Owing to the double amplification strategy, the proposed method allowed the measurement of telomerase activity in crude cell extracts equivalent to 5 HeLa cells and 10 CCRF-CEM cells without PCR amplification. Besides, the influence of telomere-binding ligands on the telomerase activity demonstrated that the proposed method holds the potential to evaluate the inhibition efficiency of telomerase inhibitors.

Cancer biomarker detection: recent achievements and challenges

We provide an overview covering the existing challenges and latest developments in achieving high selectivity and sensitivity cancer-biomarker detection.

Analysis of telomerase activity based on a spired DNA tetrahedron TS primer

The development of sensitive telomerase biosensors is hindered by the restricted accessibility of telomere strand (TS) primer and the limited enzyme reaction space, which is mainly confined by the vertical distance. In this work, we designed an electrochemical telomerase biosensor based on a spired DNA tetrahedron TS primer (STTS). By adding a rigid dsDNA spire onto the top of the DNA tetrahedron, we successfully regulated the distance between the TS primer and the surface, and thus greatly facilitated the telomerase elongation on surface. The signal-to-noise ratio was 2 times higher than TSP without the spire structure. The limit of detection was calculated to be lower than 10 HeLa cells, which is at least 2 magnitudes lower than other surface extension-based electrochemical telomerase sensors without amplification. The practicability of STTS sensor was also demonstrated by analysing various other cell lines including cancer cells, stem cells of high telomerase activity and somatic cells of low telomerase activity.

Real-Time Detection of Telomerase Activity in Cancer Cells using a Label-Free Electrochemical Impedimetric Biosensing Microchip

The enzyme telomerase is present in about 85% of human cancers which makes it not only a good target for cancer treatment but also an excellent marker for cancer detection. Using a single stranded DNA probe specific for telomerase binding and reverse transcription tethered to an interdigital gold electrode array surface, the chromosome protection provided by the telomerase was replicated and followed by Electrochemical Impedance Spectroscopy as an unlabeled biosensor. Using this system designed in-house, easy and affordable, impedance measurements were taken while incubating at 37 °C and promoting the probe elongation. This resulted in up to 14-fold increase in the charge transfer resistance when testing a telomerase-positive nuclear extract from Jurkat cells compared to the heat-inactivated telomerase-negative nuclear extract. The electron transfer process at the Au electrodes was studied before the elongation, at different times after the elongation, and after desorption of non-specific binding.