A valveless rotary microfluidic device for multiplex point mutation identification based on ligation-rolling circle amplification

A Novel Rolling Circle Amplification-Based Detection of SARS-CoV-2 with Multi-Region Padlock Hybridization

SARS-CoV-2 has remained a global health burden, primarily due to the continuous evolution of different mutant strains. These mutations present challenges to the detection of the virus, as the target genes of qPCR, the standard diagnostic method, may possess sequence alterations. In this study, we develop an isothermal one-step detection method using rolling circle amplification (RCA) for SARS-CoV-2. This novel strategy utilizes a multi-padlock (MP-RCA) approach to detect viral-RNA via a simplified procedure with the reliable detection of mutated strains over other procedures. We designed 40 padlock-based probes to target different sequences across the SARS-CoV-2 genome. We established an optimal one-step isothermal reaction protocol utilizing a fluorescent output detected via a plate reader to test a variety of padlock combinations. This method was tested on RNA samples collected from nasal swabs and validated via PCR. S-gene target failure (SGTF)-mutated strains of SARS-CoV-2 were included. We demonstrated that the sensitivity of our assay was linearly proportional to the number of padlock probes used. With the 40-padlock combination the MP-RCA assay was able to correctly detect 45 out 55 positive samples (81.8% efficiency). This included 10 samples with SGTF mutations which we were able to detect as positive with 100% efficiency. We found that the MP-RCA approach improves the sensitivity of the MP-RCA assay, and critically, allows for the detection of SARS-CoV-2 variants with SGTF. Our method offers the simplicity of the reaction and requires basic equipment compared to standard qPCR. This method provides an alternative approach to overcome the challenges of detecting SARS-CoV-2 and other rapidly mutating viruses.

Rolling Circle Amplification as an Efficient Analytical Tool for Rapid Detection of Contaminants in Aqueous Environments

Environmental contaminants are a global concern, and an effective strategy for remediation is to develop a rapid, on-site, and affordable monitoring method. However, this remains challenging, especially with regard to the detection of various contaminants in complex water environments. The application of molecular methods has recently attracted increasing attention; for example, rolling circle amplification (RCA) is an isothermal enzymatic process in which a short nucleic acid primer is amplified to form a long single-stranded nucleic acid using a circular template and special nucleic acid polymerases. Furthermore, this approach can be further engineered into a device for point-of-need monitoring of environmental pollutants. In this paper, we describe the fundamental principles of RCA and the advantages and disadvantages of RCA assays. Then, we discuss the recently developed RCA-based tools for environmental analysis to determine various targets, including heavy metals, organic small molecules, nucleic acids, peptides, proteins, and even microorganisms in aqueous environments. Finally, we summarize the challenges and outline strategies for the advancement of this technique for application in contaminant monitoring.

Homogeneous circle-to-circle amplification for real-time optomagnetic detection of SARS-CoV-2 RdRp coding sequence

Circle-to-circle amplification (C2CA) is a specific and precise cascade nucleic acid amplification method consisting of more than one round of padlock probe ligation and rolling circle amplification (RCA). Although C2CA provides a high amplification efficiency with a negligible increase of false-positive risk, it contains several step-by-step operation processes. We herein demonstrate a homogeneous and isothermal nucleic acid quantification strategy based on C2CA and optomagnetic analysis of magnetic nanoparticle (MNP) assembly. The proposed homogeneous circle-to-circle amplification eliminates the need for additional monomerization and ligation steps after the first round of RCA, and combines two amplification rounds in a one-pot reaction. The second round of RCA produces amplicon coils that anneal to detection probes grafted onto MNPs, resulting in MNP assembly that can be detected in real-time using an optomagnetic sensor. The proposed methodology was applied for the detection of a synthetic complementary DNA of SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2, also known as 2019-nCoV) RdRp (RNA-dependent RNA polymerase) coding sequence, achieving a detection limit of 0.4 fM with a dynamic detection range of 3 orders of magnitude and a total assay time of ca. 100 min. A mathematical model was set up and validated to predict the assay performance. Moreover, the proposed method was specific to distinguish SARS-CoV and SARS-CoV-2 sequences with high similarity.

Effects of Microchannel Shape and Ultrasonic Mixing on Microfluidic Padlock Probe Rolling Circle Amplification (RCA) Reactions

The fluorescence in situ hybridization (FISH)-based padlock probe and rolling circle amplification (RCA) method allows for the detection of point mutations. However, it requires multiple reaction steps and solution exchanges, making it costly, labor-intensive, and time-consuming. In this study, we aimed to improve the efficiency of padlock/RCA by determining the effects of microchannel shape and ultrasonic solution mixing. Using a circular-shaped microchamber and ultrasonic mixing, the efficiency of microfluidic padlock/RCA was improved, and the consumption of the expensive probe solution was reduced from 10 µL to approximately 3.5 µL. Moreover, the fluorescent probe hybridization time was reduced to 5 min, which is four times faster than that of the standard protocol. We used this method to successfully detect mitochondrial DNA and transcripts of β-actin and K-ras proto-oncogene codon 12 in cells. Our method offers improvements over current padlock/RCA methods and will be helpful in optimizing other microfluidics-based FISH-related analyses.

Screening oligonucleotide sequences for silver staining and d-galactose visual detection using RCA silver staining in a tube

Oligonucleotides were screened for strongly silver-stained repetitive sequences. An 'AG'-clustered purine sequence showed strong staining, and the staining density can be compromised by disrupting the continuity of the 'AG'-clustered sequence. The staining-favored sequence was then employed in rolling circle amplification (RCA) for its product detection. A tube-staining method was developed for convenient and visual RCA assay. Moreover, by introducing GalR into RCA, d-galactose was detected by RCA tube-staining with naked eyes without any equipment. About 10 mM d-galactose can be easily identified, and the detection of d-galactose was specific in comparison with that of several other monosaccharides.

A simple and ultrasensitive fluorescence assay for single-nucleotide polymorphism

In this report, a simple, label-free and highly efficient nucleic acid amplification technique is developed for ultrasensitive detection of single-nucleotide polymorphism (SNP). Briefly, a designed padlock probe is first circularized by a DNA ligase when it perfectly complements to a mutant gene. Then, the mutant gene functions as a primer to initiate branched rolling circle amplification reaction (BRCA), generating a large number of branched DNA strands and a lot of pyrophosphate molecules which is equivalent to the number of nucleotides consumed. With the addition of a terpyridine-Zn(II) complex, pyrophosphate molecules can be sensitively detected owing to the formation of a fluorescent terpyridine-Zn(II)-pyrophosphate complex. The fluorescence intensity is directly associated with the content of the mutant gene in a sample solution. On the other hand, the circulation of the padlock probe is prohibited when it hybridizes with the wild-type gene. In this assay, the accumulative nature of the BRCA process produces a detection limit of 0.1 pM and an excellent selectivity factor of 1000 toward SNP. As little as 0.1% mutant in the wild-type gene can be successfully detected. The simple procedure, high sensitivity, and high selectivity of this assay offer a potentially viable alternative for routine SNP analysis. Graphical abstract A simple and label-free fluorescence assay for SNP detection by coupling BRCA with selective fluorescence detection of pyrophosphate using the terpyridine-Zn(II) complex.

Multiplexed isothermal nucleic acid amplification

Multiplexed isothermal amplification and detection of nucleic acid sequences and biomarkers is of increasing importance in diverse areas including advanced diagnostics, food quality control and environmental monitoring. Whilst there are several very elegant isothermal amplification approaches, multiplexed amplification remains a challenge, requiring careful experimental design and optimisation, from judicious primer design in order to avoid the formation of primer dimers and non-specific amplification, applied temperature as well as the ratio and concentration of primers. In this review, we describe the various approaches that have been reported to date for multiplexed isothermal amplification, for both "one-pot" multiplexing as well as parallelised multiplexing using loop-mediated isothermal amplification, strand-displacement amplification, helicase-dependent amplification, rolling circle amplification, nucleic acid sequence-based amplification, with a particular focus on recombinase polymerase amplification.

Rolling circle amplification as isothermal gene amplification in molecular diagnostics

Rolling circle amplification (RCA) developed in the mid-1990s has been widely used as an efficient isothermal DNA amplification process for molecular diagnosis. This enzymatic process amplifies target DNA sequences with high fidelity and specificity by using the strand displacing DNA polymerases. The product of RCA is long single-stranded DNA that contains tandem repeat of target sequence. Isothermal reaction amplification condition of RCA has an advantage over conventional polymerase chain reaction, because no temperature cycling devices are needed for RCA. Thus, RCA is suitable tool for point-of-care detection of target nucleic acids as well as facile detection of target genes. Combined with various detection methods, RCA could amplify and detect femtomolar scale of target nucleic acids with a specificity of one or two base discrimination. Herein, RCA technology is reviewed with an emphasis on molecular diagnosis of microRNAs, infectious pathogens, and point mutations.

Production of dumbbell probe through hairpin cleavage-ligation and increasing RCA sensitivity and specificity by circle to circle amplification

Dumbbell probe (DP) attracts increasing interests in rolling circle amplification (RCA). A universal DP production method through cleavage-ligation of hairpin was proposed and optimized. The production is characterized by restriction endonuclease (RE)-induced cleavage ends ligation. It has the advantage of phosphorylation-free, splint-free and purification-free. To optimize designing, we found that the position of RE cleavage sequence in the stem and the primer position in the loop affected the formation and amplification of DP obviously. Both sticky and blunt ends cleaved by RE produce DP efficiently. Moreover, we introduced this DP into circle to circle (C2C) RCA based on the same cleavage-ligation principle, and acquired high sensitivity. By combining a two-ligation design and the C2C strategy, specificity for detecting let-7 family members was increased extremely. Furthermore, coreaction of different steps facilitated convenient formation and amplification process of DP.