Palindromic molecular beacon-based intramolecular strand-displacement amplification strategy for ultrasensitive detection of K-ras gene

Nonenzymatic Autonomous Assembly of Cross-Linked Network Structures from Only Two Palindromic DNA Components for Intracellular Fluorescence Imaging of miRNAs

The abnormal expression of miRNA-21 is often found in tumor specimens and cell lines, and thus, its specific detection is an urgent need for the diagnosis and effective therapy of cancers. In this contribution, we demonstrate a palindrome-based hybridization chain reaction (PHCR) upon the stimuli of a short oligonucleotide trigger to perform the autonomous assembly of cross-linked network structures (CNSs) for the amplification detection of miRNA-21 and sensitive fluorescence imaging of cancerous cells. The building blocks are only two palindromic hairpin-type DNA strands that are separately modified with different fluorophores (Cy3 and Cy5), which is easily combined with the catalytic hairpin assembly (CHA) technique that can further amplify the signal output. Utilizing the CHA-PHCR assay system, a small amount of miRNA-21 can activate many triggers via CHA and in turn induce the PHCR-based CNS assembly from more DNA building blocks, bringing Cy3 and Cy5 into close proximity to each other and generating ultrasensitive fluorescence resonance energy transfer signals. As a result, target miRNA can be quantitatively detected down to as low as 10 pM with high assay specificity. The coexisting nontarget miRNAs and other biomacromolecules do not interfere with signal transduction. The developed assay system is suitable for screening different expression levels of miRNA-21 in living cells by fluorescence imaging. The palindrome-based cross-linking assembly can enhance the intracellular stability of assembled nanostructures by at least fivefold and exhibit the good universality for the detection of other miRNAs. Moreover, cancerous cells can be distinguished from healthy cells, and the CHA-PHCR assay is in good accordance with the gold standard PCR method, indicating a promising platform for the diagnosis of human cancers and other genetic diseases.

Palindromic-assisted self-annealing transcription amplification for reliable genotyping of epidermal growth factor receptor exon mutations

Reliable discrimination of specific epidermal growth factor receptor (EGFR) gene mutations plays a critical role in guiding lung cancer therapeutics. Until now, convenient and accurate recognition of the specific deletion of EGFR exons has remained particularly challenging. Herein, we propose a palindromic-assisted self-annealing transcription amplification (PASTA) strategy for the reliable detection of circulating EGFR exon mutations. We designed a palindromic DNA hairpin nanorobot consisting of a palindromic tail, a T7 promoter, a target recognition region, and a transcription template. The nanorobot enabled prompt self-assembly into a target-hairpin/hairpin-target dimer in the presence of single-stranded DNA target and further triggered in vitro transcription. In a proof-of-concept experiment for detecting circulating 15n-del EGFR mutation, a detection limit of 0.8 fM and a linear detection range of 1 fM to 100 pM was achieved, and an accuracy of 100% was reached in clinical validation by analyzing 20 samples from clinical lung cancer patients. Empowered by the intrinsic sensitivity and selectivity, the proposed PASTA approach will lead to the development of a universal platform for reliable molecular subtyping.

A highly sensitive electrochemical aptasensor for vascular endothelial growth factor detection based on toehold-mediated strand displacement reaction

An electrochemical aptasensor with high sensitivity, specificity, and good intra-day reproducibility is reported to meet the detection needs of vascular endothelial growth factor (VEGF). The toehold-mediated strand displacement recycling amplification and VEGF aptamer are integrated in the biosensor. The probe A is hybridized with the VEGF aptamer to form the probe A-aptamer complex. When VEGF is introduced, the aptamer specifically binds with VEGF, and probe A can be liberated. Then, the free probe A captures the toehold region of the Hp1, leading the exposure of the toehold region on the other end of Hp1. Similarly, Hp2 and Hp3 are also immobilized on the surface of the electrode; thus, the methylene blue labelled on Hp2 and Hp3 causes the current response. With the signal transduction mechanism, the expression level of VEGF can be detected quantitatively. With a series of optimizations of sensor parameters, high sensitivity and specificity of the VEGF detection sensor can be achieved with a detection limit as low as 10 pg mL^-1. This significant performance has good intra-day reproducibility, and it can be applied to human biological samples such as serum, urine, and saliva to detect the VEGF content.

Dual Catalytic Hairpin Assembly-Based Automatic Molecule Machine for Amplified Detection of Auxin Response Factor-Targeted MicroRNA-160

MicroRNA160 plays a crucial role in plant development by negatively regulating the auxin response factors (ARFs). In this manuscript, we design an automatic molecule machine (AMM) based on the dual catalytic hairpin assembly (D-CHA) strategy for the signal amplification detection of miRNA160. The detection system contains four hairpin-shaped DNA probes (HP1, HP2, HP3, and HP4). For HP1, the loop is designed to be complementary to miRNA160. A fragment of DNA with the same sequences as miRNA160 is separated into two pieces that are connected at the 3′ end of HP2 and 5′ end of HP3, respectively. In the presence of the target, four HPs are successively dissolved by the first catalytic hairpin assembly (CHA1), forming a four-way DNA junction (F-DJ) that enables the rearrangement of separated DNA fragments at the end of HP2 and HP3 and serving as an integrated target analogue for initiating the second CHA reaction, generating an enhanced fluorescence signal. Assay experiments demonstrate that D-CHA has a better performance compared with traditional CHA, achieving the detection limit as low as 10 pM for miRNA160 as deduced from its corresponding DNA surrogates. Moreover, non-target miRNAs, as well as single-base mutation targets, can be detected. Overall, the D-CHA strategy provides a competitive method for plant miRNAs detection.

Ion-exchanged glass microrods as hybrid SERS/fluorescence substrates for molecular beacon-based DNA detection

Ion-exchange in molten nitrate salts containing metal ions (i.e. silver, copper, etc.) represents a well-established technique able to modify the chemical-physical properties of glass materials. It is widely used not only in the field of integrated optics (IO) but also, more recently, in plasmonics due to the possibility to induce the formation of metal nanoparticles in the glass matrix by an ad hoc thermal post-process. In this work, the application of this technology for the realisation of low-cost and stable surface-enhanced Raman scattering (SERS) active substrates, based on soda-lime glass microrods, is reported. The microrods, with a radius of a few tens of microns, were obtained by cutting the end of an ion-exchanged soda-lime fibre for a length less than 1 cm. As ion source, silver nitrate was selected due to the outstanding SERS properties of silver. The ion-exchange and thermal annealing post-process parameters were tuned to expose the embedded silver nanoparticles on the surface of the glass microrods, avoiding the use of any further chemical etching step. In order to test the combined SERS/fluorescence response of these substrates, labelled molecular beacons (MBs) were immobilised on their surface for deoxyribonucleic acid (DNA) detection. Our experiments confirm that target DNA is attached on the silver nanoparticles and its presence is revealed by both SERS and fluorescence measurements. These results pave the way towards the development of low-cost and stable hybrid fibres, in which SERS and fluorescence interrogation techniques are combined in the same optical device.

Hairpin DNA-Mediated isothermal amplification (HDMIA) techniques for nucleic acid testing

Nucleic acid detection is of great importance in a variety of areas, from life science and clinical diagnosis to environmental monitoring and food safety. Unfortunately, nucleic acid targets are always found in trace amounts and their response signals are difficult to be detected. Amplification mechanisms are then practically needed to either duplicate nucleic acid targets or enhance the detection signals. Polymerase chain reaction (PCR) is one of the most popular and powerful techniques for nucleic acid analysis. But the requirement of costly devices for precise thermo-cycling procedures in PCR has severely hampered the wide applications of PCR. Fortunately, isothermal molecular reactions have emerged as promising alternatives. The past decade has witnessed significant progress in the research of isothermal molecular reactions utilizing hairpin DNA probes (HDPs). Based on the nucleic acid strand interaction mechanisms, the hairpin DNA-mediated isothermal amplification (HDMIA) techniques can be mainly divided into three categories: strand assembly reactions, strand decomposition reactions, and strand creation reactions. In this review, we introduce the basics of HDMIA methods, including the sensing principles, the basic and advanced designs, and their wide applications, especially those benefiting from the utilization of G-quadruplexes and nanomaterials during the past decade. We also discuss the current challenges encountered, highlight the potential solutions, and point out the possible future directions in this prosperous research area.

Periodically programmed building and collapse of DNA networks enables an ultrahigh signal amplification effect for ultrasensitive nucleic acids analysis

ANALYSIS

of molecular species is needed for applications in diagnosis of infections and genetic diseases. Herein, we demonstrate a target DNA-responsive ultrahigh fluorescence signal-on DNA amplification system via periodically programmed building and collapse of DNA networks. In this system, a pair of oligonucleotides of padlock probe (PP) and palindromic hairpin probe (PHP) are utilized. The presence of target DNA firstly hybridizes with PP, allowing the occurrence of rolling circle amplification (RCA) to produce RCA products with tandem repeats in abundance to bind and unfold numbers of PHPs. The conformational change of PHPs enables the building of DNA networks via the intermolecular palindrome pairing, but then makes the DNA networks collapsed via the palindrome-induced strand displacement polymerization. The displaced RCA products are dynamically reused to undergo periodically programmed multiple rounds of DNA network building and collapse. Depend on the bidirectional DNA assembly and disassembly, a strikingly amplified fluorescence can be collected to ultrasensitive and specific detection of target DNA. The practicability has been demonstrated by evaluating target-spiked human serum, saliva, and urine samples with acceptable recoveries and reproducibility. Therefore, this newly explored method opens a promising avenue for the detection of nucleic acids with low abundance in biochemical analysis and diseases diagnosis.

Self-assembly of a polythymine embedded activatable molecular beacon for one-step quantification of terminal deoxynucleotidyl transferase activity

We describe an isothermal, single-reaction, and one-step method for signal-on quantification of terminal deoxynucleotidyl transferase (TdT) activity based on the periodic elongation and assembly of polythymine embedded activatable molecular beacon (PTA-MB) into DNA nanostructures. PTA-MB is easily designed according to the rule of the conventional molecular beacon (MB) but engineered with a polyT composed loop. Upon exposure to the specific target TdT, the MB is first elongated with an adenine-rich (A-rich) long chain so that it can then act as the anchoring substrate to capture many original PTA-MBs along its strand. Their unfolding contributes to preliminary fluorescence emission. Significantly, the assembled PTA-MBs can also be elongated and hybridized with residual free PTA-MBs for the second round of signal amplification. Accordingly, multiple rounds of elongation, assembly, and activation of initial PTA-MBs can lead to the formation of DNA nanostructures and induce a dramatically enhanced fluorescence signal for qualitative and quantitative evaluation of TdT activity. The final assay indicated a limit of detection (LOD) of 0.042 U mL^-1 TdT and showed excellent selectivity for TdT versus other common enzymes. Moreover, the practical applicability was validated by direct/absolute quantification of TdT in real biological specimens and accurate monitoring of the activity of TdT pretreated by low/high temperature and heavy metal ions. These findings demonstrated that this functional PTA-MB and its unique assembly behavior is most likely to promote the study of oligonucleotide probe-based DNA assembly, providing a reliable, convenient, and universal platform for precise and point-of-care monitoring of various biomolecules.

Three-way junction-induced isothermal amplification for nucleic acid detection

We, herein, describe a three-way junction (3WJ)-induced isothermal amplification (ThIsAmp) reaction for target nucleic acid detection. In this strategy, target nucleic acid induces the formation of 3WJ structure by associating a specially designed ThIsAmp template and ThIsAmp primer. Upon the formation of 3WJ structure, ThIsAmp primer is subjected to continuously repeated extension and nicking reaction by the combined activities of DNA polymerase and nicking endonuclease, consequently producing a large number of trigger strands. The trigger strands then initiate two separate but interconnected pathways by binding to either 3' overhang of ThIsAmp template within the 3WJ structure or free ThIsAmp template. As a consequence, a large number of final double-stranded DNA products are produced under an isothermal condition, which can be monitored in real-time by detecting the fluorescence intensity resulting from SYBR Green I staining. Based on this principle, we successfully detected target DNA down to 78.1 aM with excellent specificity. The sophisticated design principle employed in this work would provide great insight for the development of self-operative isothermal amplifying system enabling target nucleic acid detection.