Digital Isothermal Quantification of Nucleic Acids via Simultaneous Chemical Initiation of Recombinase Polymerase Amplification Reactions on SlipChip

Department of Chemistry, Institute for Biophysical Dynamics, The University of Chicago, Illinois 60637, United States.
Analytical Chemistry (Impact Factor: 5.64). 05/2011; 83(9):3533-40. DOI: 10.1021/ac200247e
Source: PubMed


In this paper, digital quantitative detection of nucleic acids was achieved at the single-molecule level by chemical initiation of over one thousand sequence-specific, nanoliter isothermal amplification reactions in parallel. Digital polymerase chain reaction (digital PCR), a method used for quantification of nucleic acids, counts the presence or absence of amplification of individual molecules. However, it still requires temperature cycling, which is undesirable under resource-limited conditions. This makes isothermal methods for nucleic acid amplification, such as recombinase polymerase amplification (RPA), more attractive. A microfluidic digital RPA SlipChip is described here for simultaneous initiation of over one thousand nL-scale RPA reactions by adding a chemical initiator to each reaction compartment with a simple slipping step after instrument-free pipet loading. Two designs of the SlipChip, two-step slipping and one-step slipping, were validated using digital RPA. By using the digital RPA SlipChip, false-positive results from preinitiation of the RPA amplification reaction before incubation were eliminated. End point fluorescence readout was used for "yes or no" digital quantification. The performance of digital RPA in a SlipChip was validated by amplifying and counting single molecules of the target nucleic acid, methicillin-resistant Staphylococcus aureus (MRSA) genomic DNA. The digital RPA on SlipChip was also tolerant to fluctuations of the incubation temperature (37-42 °C), and its performance was comparable to digital PCR on the same SlipChip design. The digital RPA SlipChip provides a simple method to quantify nucleic acids without requiring thermal cycling or kinetic measurements, with potential applications in diagnostics and environmental monitoring under resource-limited settings. The ability to initiate thousands of chemical reactions in parallel on the nanoliter scale using solvent-resistant glass devices is likely to be useful for a broader range of applications.

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Available from: Wenbin Du, Aug 11, 2015
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    • "The commonest isothermal methods are strand displacement amplification (SDA), nucleic acid sequence-based amplification (NASBA), helicase-dependent amplification (HDA), isothermal recombinase polymerase amplification (RPA), loop-mediated isothermal amplification (LAMP), and multiple displacement amplification (MDA) (Yan et al., 2014; Zanoli & Spoto, 2013). The performance of RPA (Piepenburg, Williams, Stemple, & Armes, 2006) and MDA (Dean et al., 2002) offers an interesting high-throughput analytical system , as demonstrated in a bright approach (digital RPA) proposed for the detection of a single pathogen on a chip (Shen et al., 2011). However, these methods have not been described for multiplex strategies. "
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    ABSTRACT: a b s t r a c t A valid solution for micro-analytical systems is the selection of a compatible amplification reaction with a simple, highly-integrated efficient design that allows the detection of multiple genomic targets. Two approaches under isothermal conditions are presented: recombinase polymerase amplification (RPA) and multiple displacement amplification (MDA). Both methods were applied to a duplex assay specific for Salmonella spp. and Cronobacter spp., with excellent amplification yields (0.2–8.6 Á 10 8 fold). The pro-posed approaches were successfully compared to conventional PCR and tested for the milk sample anal-ysis as a microarray format on a compact disc (support and driver). Satisfactory results were obtained in terms of resistance to inhibition, selectivity, sensitivity (10 1 –10 2 CFU/mL) and reproducibility (below 12.5%). The methods studied are efficient and cost-effective, with a high potential to automate microor-ganisms detection by integrated analytical systems working at a constant low temperature.
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    • "At temperatures just above room temperature an amplification of complex DNA targets can be achieved in less than 30 minutes. Recently, several applications of this method have been demonstrated for the detection of DNA and RNA targets [10-14] and the integration of the RPA in different instrumentations was shown [15-18]. "
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    ABSTRACT: Nucleic acid amplification is the most sensitive and specific method to detect Plasmodium falciparum. However the polymerase chain reaction remains laboratory-based and has to be conducted by trained personnel. Furthermore, the power dependency for the thermocycling process and the costly equipment necessary for the read-out are difficult to cover in resource-limited settings. This study aims to develop and evaluate a combination of isothermal nucleic acid amplification and simple lateral flow dipstick detection of the malaria parasite for point-of-care testing. A specific fragment of the 18S rRNA gene of P. falciparum was amplified in 10 min at a constant 38[degree sign]C using the isothermal recombinase polymerase amplification (RPA) method. With a unique probe system added to the reaction solution, the amplification product can be visualized on a simple lateral flow strip without further labelling. The combination of these methods was tested for sensitivity and specificity with various Plasmodium and other protozoa/bacterial strains, as well as with human DNA. Additional investigations were conducted to analyse the temperature optimum, reaction speed and robustness of this assay. The lateral flow RPA (LF-RPA) assay exhibited a high sensitivity and specificity. Experiments confirmed a detection limit as low as 100 fg of genomic P. falciparum DNA, corresponding to a sensitivity of approximately four parasites per reaction. All investigated P. falciparum strains (n = 77) were positively tested while all of the total 11 non-Plasmodium samples, showed a negative test result. The enzymatic reaction can be conducted under a broad range of conditions from 30-45[degree sign]C with high inhibitory concentration of known PCR inhibitors. A time to result of 15 min from start of the reaction to read-out was determined. Combining the isothermal RPA and the lateral flow detection is an approach to improve molecular diagnostic for P. falciparum in resource-limited settings. The system requires none or only little instrumentation for the nucleic acid amplification reaction and the read-out is possible with the naked eye. Showing the same sensitivity and specificity as comparable diagnostic methods but simultaneously increasing reaction speed and dramatically reducing assay requirements, the method has potential to become a true point-of-care test for the malaria parasite.
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    ABSTRACT: This paper presents a protocol using theoretical methods and free software to design and analyze multivolume digital PCR (MV digital PCR) devices; the theory and software are also applicable to design and analysis of dilution series in digital PCR. MV digital PCR minimizes the total number of wells required for "digital" (single molecule) measurements while maintaining high dynamic range and high resolution. In some examples, multivolume designs with fewer than 200 total wells are predicted to provide dynamic range with 5-fold resolution similar to that of single-volume designs requiring 12,000 wells. Mathematical techniques were utilized and expanded to maximize the information obtained from each experiment and to quantify performance of devices and were experimentally validated using the SlipChip platform. MV digital PCR was demonstrated to perform reliably, and results from wells of different volumes agreed with one another. No artifacts due to different surface-to-volume ratios were observed, and single molecule amplification in volumes ranging from 1 to 125 nL was self-consistent. The device presented here was designed to meet the testing requirements for measuring clinically relevant levels of HIV viral load at the point-of-care (in plasma, <500 molecules/mL to >1,000,000 molecules/mL), and the predicted resolution and dynamic range was experimentally validated using a control sequence of DNA. This approach simplifies digital PCR experiments, saves space, and thus enables multiplexing using separate areas for each sample on one chip, and facilitates the development of new high-performance diagnostic tools for resource-limited applications. The theory and software presented here are general and are applicable to designing and analyzing other digital analytical platforms including digital immunoassays and digital bacterial analysis. It is not limited to SlipChip and could also be useful for the design of systems on platforms including valve-based and droplet-based platforms. In a separate publication by Shen et al. (J. Am. Chem. Soc., 2011, DOI: 10.1021/ja2060116), this approach is used to design and test digital RT-PCR devices for quantifying RNA.
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