Miniaturized isothermal nucleic acid amplification, a review

Loop-mediated isothermal amplification (LAMP): recent progress in research and development

Loop-mediated isothermal amplification (LAMP) is an established technology that continues to attract the attention of researchers in many fields. Research and development efforts on LAMP technology in recent years have focused on two major areas; first, the study of its clinical application as an approved in vitro diagnostics tool in Japan and certain other countries; and second, research aimed at further simplifying the LAMP test process. This review provides an overview of the status of LAMP on these two topics by summarizing research work conducted, in the main, after our previous review article.

Label-free DNA quantification via a 'pipette, aggregate and blot' (PAB) approach with magnetic silica particles on filter paper

Reliable measurement of DNA concentration is essential for a broad range of applications in biology and molecular biology, and for many of these, quantifying the nucleic acid content is inextricably linked to obtaining optimal results. In its most simplistic form, quantitative analysis of nucleic acids can be accomplished by UV-Vis absorbance and, in more sophisticated format, by fluorimetry. A recently reported new concept, the 'pinwheel assay', involves a label-free approach for quantifying DNA through aggregation of paramagnetic beads in a rotating magnetic field. Here, we describe a simplified version of that assay adapted for execution using only a pipet and filter paper. The 'pipette, aggregate, and blot' (PAB) approach allows DNA to induce bead aggregation in a pipette tip through exposure to a magnetic field, followed by dispensing (blotting) onto filter paper. The filter paper immortalises the extent of aggregation, and digital images of the immortalized bead conformation, acquired with either a document scanner or a cell phone camera, allows for DNA quantification using a noncomplex algorithm. Human genomic DNA samples extracted from blood are quantified with the PAB approach and the results utilized to define the volume of sample used in a PCR reaction that is sensitive to input mass of template DNA. Integrating the PAB assay with paper-based DNA extraction and detection modalities has the potential to yield 'DNA quant-on-paper' devices that may be useful for point-of-care testing.

Isothermal amplification methods for the detection of nucleic acids in microfluidic devices

Diagnostic tools for biomolecular detection need to fulfill specific requirements in terms of sensitivity, selectivity and high-throughput in order to widen their applicability and to minimize the cost of the assay. The nucleic acid amplification is a key step in DNA detection assays. It contributes to improving the assay sensitivity by enabling the detection of a limited number of target molecules. The use of microfluidic devices to miniaturize amplification protocols reduces the required sample volume and the analysis times and offers new possibilities for the process automation and integration in one single device. The vast majority of miniaturized systems for nucleic acid analysis exploit the polymerase chain reaction (PCR) amplification method, which requires repeated cycles of three or two temperature-dependent steps during the amplification of the nucleic acid target sequence. In contrast, low temperature isothermal amplification methods have no need for thermal cycling thus requiring simplified microfluidic device features. Here, the use of miniaturized analysis systems using isothermal amplification reactions for the nucleic acid amplification will be discussed.

IsoPCR: An Analytically Sensitive, Nested, Multiplex Nucleic Acid Amplification Method

BACKGROUND

Nucleic acid tests that can simultaneously detect multiple targets with high sensitivity, specificity, and speed are highly desirable. To meet this need, we developed a new approach we call the isoPCR method.

METHODS

The isoPCR method is a 2-stage nested-like nucleic acid amplification method that combines a single multiplex preamplification PCR with subsequent distinct detection of specific targets by use of isothermal amplification. We compared isoPCR to nested quantitative PCR (qPCR), loop-mediated isothermal amplification (LAMP), and nested LAMP (PCR followed by LAMP), for detection of DNA from Candida glabrata. We evaluated the method's multiplex capability for detecting low copy numbers of pathogens commonly involved in sepsis.

RESULTS

IsoPCR provided detection of 1 copy of Candida glabrata, an LOD that was 5-fold lower than a nested qPCR assay (5 copies), while the amplification time was simultaneously halved. Similarly, the LOD for isoPCR was lower than that for a LAMP assay (1000 copies) and a nested LAMP assay (5 copies). IsoPCR required recognition of 6 regions for detection, thereby providing a theoretically higher specificity compared to nested qPCR (4 regions). The isoPCR multiplexing capability was demonstrated by simultaneous detection of 4 pathogens with individual LODs of 10 copies or fewer. Furthermore, the specificity of isoPCR was demonstrated by successful pathogen detection from samples with more than 1 pathogen present.

CONCLUSIONS

IsoPCR provides a molecular diagnostic tool for multiplex nucleic acid detection, with an LOD down to 1 copy, high theoretical specificity, and halving of the amplification time compared to a nested qPCR assay.

Development of a panel of recombinase polymerase amplification assays for detection of biothreat agents

Syndromic panels for infectious disease have been suggested to be of value in point-of-care diagnostics for developing countries and for biodefense. To test the performance of isothermal recombinase polymerase amplification (RPA) assays, we developed a panel of 10 RPAs for biothreat agents. The panel included RPAs for Francisella tularensis, Yersinia pestis, Bacillus anthracis, variola virus, and reverse transcriptase RPA (RT-RPA) assays for Rift Valley fever virus, Ebola virus, Sudan virus, and Marburg virus. Their analytical sensitivities ranged from 16 to 21 molecules detected (probit analysis) for the majority of RPA and RT-RPA assays. A magnetic bead-based total nucleic acid extraction method was combined with the RPAs and tested using inactivated whole organisms spiked into plasma. The RPA showed comparable sensitivities to real-time RCR assays in these extracts. The run times of the assays at 42°C ranged from 6 to 10 min, and they showed no cross-detection of any of the target genomes of the panel nor of the human genome. The RPAs therefore seem suitable for the implementation of syndromic panels onto microfluidic platforms.

An integrated allele-specific polymerase chain reaction-microarray chip for multiplex single nucleotide polymorphism typing

An integrated allele-specific polymerase chain reaction (AS PCR) and microarray chip has been developed for multiplex single nucleotide polymorphism (SNP) typing on a portable genetic analyzer instrumentation. We applied the integrated PCR-microarray system for on-site Hanwoo (Korean indigenous beef cattle) identification. Eleven sets of primers were designed, among which ten sets of primers targeted ten SNP loci to discriminate Hanwoo from the imported beef cattle and one primer set was used as a positive PCR control. The AS PCR for multiplex SNP typing was conducted on a glass-based microchip consisting of four layers: a microchannel plate for microfluidic control, a Pt-electrode plate for a resistance temperature detector (RTD), a poly(dimethylsiloxane) (PDMS) membrane and a manifold glass for micropump and microvalve function. The resultant AS PCR products were mixed with a hybridization buffer in a micromixer channel through the micropumping operation, and then the microarray assay was performed in the downstream process. Eleven duplicate probes were spotted in a glass slide, which was connected at the end of the micromixer channel unit. When the mixed solution was injected into the disposable microarray chip, pneumatically actuated micropumping was executed to speed up the hybridization process by inducing the convective flow. The fluorescence signals on each spot were monitored by a miniaturized fluorescence scanner, and the Hanwoo was verified by detecting the number of fluorescent spots with three or fewer among eleven. An integrated portable PCR-microarray genetic analysis microsystem was first demonstrated for rapid, accurate, and on-site multiplex SNP typing to differentiate animal species.

A low-cost, label-free DNA detection method in lab-on-chip format based on electrohydrodynamic instabilities, with application to long-range PCR

In order to evolve from a "chip in the lab" to a "lab on a chip" paradigm, there is still a strong demand for low-cost, portable detection technologies, notably for analytes at low concentrations. Here we report a new label-free DNA detection method with direct electronic read, and apply it to long-range PCR. This method uses a nonlinear electrohydrodynamic phenomenon: when subjected to high electric fields (typically above 100 V cm(-1)), suspensions of large polyelectrolytes, such as long DNA molecules, create "giant" dynamic concentration fluctuations. These fluctuations are associated with large conductivity inhomogeneities, and we use here a contact-mode local conductivity detector to detect these fluctuations. In order to decouple the detection electronics from the high voltage excitation one, an original "doubly symmetric" floating mode battery-operated detection scheme was developed. A wavelet analysis was then applied, to unravel from the chaotic character of the electohydrodynamic instabilities a scalar signal robustly reflecting the amplification of DNA. As a first proof of concept, we measured the products of the off-chip amplification of 10 kbp DNA from lambda phage DNA, achieving a sensitivity better than 100 fg DNA in the original 50 μl sample. This corresponds to the amplification products of less than 100 initial copies of target DNA. The companion enabling technologies developed to implement this new concept, i.e. the doubly symmetric contact conductivity detection and wavelet analysis, may also find various other applications in lab-on-chips.

Signal amplification by glyco-qPCR for ultrasensitive detection of carbohydrates: applications in glycobiology

Tiny amounts of carbohydrates (ca. 1 zmol) can be detected quantitatively by a real-time method based on the conjugation of carbohydrates with DNA markers (see picture). The proposed method (glyco-qPCR) provides uniform, ultrasensitive detection of carbohydrates, which can be applied to glycobiology, as well as carbohydrate-based drug discovery.

Adapting enzyme-free DNA circuits to the detection of loop-mediated isothermal amplification reactions

Loop-mediated isothermal amplification of DNA (LAMP) is a powerful isothermal nucleic acid amplification technique that can accumulate ~10(9) copies from less than 10 copies of input template within an hour or two. Unfortunately, while the amplification reactions are extremely powerful, the quantitative detection of LAMP products is still analytically difficult. In this article, to both improve the specificity of LAMP detection and to make direct readout of LAMP amplification simpler and much more reliable, we have developed a nonenzymatic nucleic acid circuit (catalyzed hairpin assembly, CHA) that can both amplify and integrate the specific sequence signals present in LAMP amplicons. Through a hairpin acceptor, one of the four loop products amplified from the LAMP is transduced to an active catalyst ssDNA which can in turn trigger a CHA reaction. After CHA detection, even less than 10 molecules/μL model templates (M13mp18) can produce significant signal, and both nonspecific template and parasitic amplicons cannot bring interference at all. More importantly, to further enhance the specificity, we have designed a dual-CHA circuit that only gave positive responses in presence of two LAMP loops. The AND-GATE detector will act as a simultaneous, specific readout of the LAMP product, rather than of competing and parasitic amplicons.

Multifunctional sample preparation kit and on-chip quantitative nucleic acid sequence-based amplification tests for microbial detection

This study reports a quantitative nucleic acid sequence-based amplification (Q-NASBA) microfluidic platform composed of a membrane-based sampling module, a sample preparation cassette, and a 24-channel Q-NASBA chip for environmental investigations on aquatic microorganisms. This low-cost and highly efficient sampling module, having seamless connection with the subsequent steps of sample preparation and quantitative detection, is designed for the collection of microbial communities from aquatic environments. Eight kinds of commercial membrane filters are relevantly analyzed using Saccharomyces cerevisiae, Escherichia coli, and Staphylococcus aureus as model microorganisms. After the microorganisms are concentrated on the membrane filters, the retentate can be easily conserved in a transport medium (TM) buffer and sent to a remote laboratory. A Q-NASBA-oriented sample preparation cassette is originally designed to extract DNA/RNA molecules directly from the captured cells on the membranes. Sequentially, the extract is analyzed within Q-NASBA chips that are compatible with common microplate readers in laboratories. Particularly, a novel analytical algorithmic method is developed for simple but robust on-chip Q-NASBA assays. The reported multifunctional microfluidic system could detect a few microorganisms quantitatively and simultaneously. Further research should be conducted to simplify and standardize ecological investigations on aquatic environments.

Detection of food-borne pathogens with DNA arrays on disk

A DNA oligonucleotide array for duplex pathogen detection on a DVD platform is developed. The assay involves hybridization of PCR products and optical detection using compact disc technology. Different DNA array constructions for attachment of synthetic oligonucleotides on to DVD surface are evaluated, finding that streptavidin-biotin coupling method yielded the highest sensitivity in combination with enzymatic signal amplification. Issues of importance for the DNA array construction such immobilized probes design, PCR product labeling strategy and composition of the hybridization buffer were addressed. The methodology was proved scoring single nucleotide polymorphisms with high selectivity. The assay capability was also demonstrated by the identification of two pathogenic microorganisms in powder milk samples. In fifty minutes, the DVD-array system identifies Salmonella spp. and Cronobacter spp. (previously named Enterobacter sakazakii) precise and simultaneously with a sensitivity of 10(0) and 10(2) cfu/mL, respectively, in infant milk. Results were in good agreement with those obtained by quantitative real-time PCR.

A microfluidic platform for real-time and in situ monitoring of virus infection process

Microfluidic chip is a promising platform for studying virus behaviors at the cell level. However, only a few chip-based studies on virus infection have been reported. Here, a three-layer microfluidic chip with low shear stress was designed to monitor the infection process of a recombinant Pseudorabies virus (GFP-PrV) in real time and in situ, which could express green fluorescent protein during the genome replication. The infection and proliferation characteristics of GFP-PrV were measured by monitoring the fluorescence intensity of GFP and determining the one-step growth curve. It was found that the infection behaviors of GFP-PrV in the host cells could hardly be influenced by the microenvironment in the microfluidic chip. Furthermore, the results of drug inhibition assays on the microfluidic chip with a tree-like concentration gradient generator showed that one of the infection pathways of GFP-PrV in the host cells was microtubule-dependent. This work established a promising microfluidic platform for the research on virus infection.

A lateral flow assay for quantitative detection of amplified HIV-1 RNA

Although the accessibility of HIV treatment in developing nations has increased dramatically over the past decade, viral load testing to monitor the response of patients receiving therapy is often unavailable. Existing viral load technologies are often too expensive or resource-intensive for poor settings, and there is no appropriate HIV viral load test currently available at the point-of-care in low resource settings. Here, we present a lateral flow assay that employs gold nanoparticle probes and gold enhancement solution to detect amplified HIV RNA quantitatively. Preliminary results show that, when coupled with nucleic acid sequence based amplification (NASBA), this assay can detect concentrations of HIV RNA that match the clinically relevant range of viral loads found in HIV patients. The lateral flow test is inexpensive, simple and rapid to perform, and requires few resources. Our results suggest that the lateral flow assay may be integrated with amplification and sample preparation technologies to serve as an HIV viral load test for low-resource settings.

Microfluidic designs and techniques using lab-on-a-chip devices for pathogen detection for point-of-care diagnostics

Effective pathogen detection is an essential prerequisite for the prevention and treatment of infectious diseases. Despite recent advances in biosensors, infectious diseases remain a major cause of illnesses and mortality throughout the world. For instance in developing countries, infectious diseases account for over half of the mortality rate. Pathogen detection platforms provide a fundamental tool in different fields including clinical diagnostics, pathology, drug discovery, clinical research, disease outbreaks, and food safety. Microfluidic lab-on-a-chip (LOC) devices offer many advantages for pathogen detection such as miniaturization, small sample volume, portability, rapid detection time and point-of-care diagnosis. This review paper outlines recent microfluidic based devices and LOC design strategies for pathogen detection with the main focus on the integration of different techniques that led to the development of sample-to-result devices. Several examples of recently developed devices are presented along with respective advantages and limitations of each design. Progresses made in biomarkers, sample preparation, amplification and fluid handling techniques using microfluidic platforms are also covered and strategies for multiplexing and high-throughput analysis, as well as point-of-care diagnosis, are discussed.

A paper and plastic device for performing recombinase polymerase amplification of HIV DNA

Despite the importance of early diagnosis and treatment of HIV, only a small fraction of HIV-exposed infants in low- and middle-income countries are tested for the disease. The gold standard for early infant diagnosis, DNA PCR, requires resources that are unavailable in poor settings, and no point-of-care HIV DNA test is currently available. We have developed a device constructed of layers of paper, glass fiber, and plastic that is capable of performing isothermal, enzymatic amplification of HIV DNA. The device is inexpensive, small, light-weight, and easy to assemble. The device stores lyophilized enzymes, facilitates mixing of reaction components, and supports recombinase polymerase amplification in five steps of operation. Using commercially available lateral flow strips as a detection method, we demonstrate the ability of our device to amplify 10 copies of HIV DNA to detectable levels in 15 min. Our results suggest that our device, which is designed to be used after DNA extraction from dried-blood spots, may serve in conjunction with lateral flow strips as part of a point-of-care HIV DNA test to be used in low resource settings.

Isothermal nucleic acid amplification technologies for point-of-care diagnostics: a critical review

Nucleic Acid Testing (NAT) promises rapid, sensitive and specific diagnosis of infectious, inherited and genetic disease. The next generation of diagnostic devices will interrogate the genetic determinants of such conditions at the point-of-care, affording clinicians prompt reliable diagnosis from which to guide more effective treatment. The complex biochemical nature of clinical samples, the low abundance of nucleic acid targets in the majority of clinical samples and existing biosensor technology indicate that some form of nucleic acid amplification will be required to obtain clinically relevant sensitivities from the small samples used in point-of-care testing (POCT). This publication provides an overview and thorough review of existing technologies for nucleic acid amplification. The different methods are compared and their suitability for POCT adaptation are discussed. Current commercial products employing isothermal amplification strategies are also investigated. In conclusion we identify the factors impeding the integration of the methods discussed in fully automated, sample-to-answer POCT devices.

A point-of-care instrument for rapid multiplexed pathogen genotyping

We are leveraging recent advances in rapid nucleic acid amplification chemistries, self-powered microfluidics, and low-cost optoelectronics to develop instrumentation for pathogen genotyping in the developing world. A growing number of correlations are emerging between genetic mutations in pathogens and their infectivity, origin, and drug resistance. Particularly for diseases like tuberculosis, where multi-drug resistance is a growing concern, a rapid diagnostic which could inform prescription decisions for newly diagnosed patients would not only save lives and reduce prolonged sickness but would help slow the emergence of more virulent strains. Additionally, for pathogens such as HIV, there is a need for new assay formats which can inexpensively and quantitativly monitor pathogen load. We have developed a portable instrument which uses disposable microfluidic assay cartridges pre-loaded with lyophilized reagents for genetic amplification of multiple markers. The cartridges can be adapted for a variety of sample types (blood, sputum, saliva). The instrument controls assay temperature and quantitatively monitors real-time fluorescence signals from 96 individual reaction chambers. The platform can be tailored for different economic situations--from a quantitative electronic readout to a simple binary readout with the naked eye.

Recombinase polymerase amplification assay for rapid detection of Francisella tularensis

Several real-time PCR approaches to develop field detection for Francisella tularensis, the infectious agent causing tularemia, have been explored. We report the development of a novel qualitative real-time isothermal recombinase polymerase amplification (RPA) assay for use on a small ESEQuant Tube Scanner device. The analytical sensitivity and specificity were tested using a plasmid standard and DNA extracts from infected rabbit tissues. The assay showed a performance comparable to real-time PCR but reduced the assay time to 10 min. The rapid RPA method has great application potential for field use or point-of-care diagnostics.

Mixing process of ternary solvents prepared through microchannels in a microchip under laminar flow conditions

The mixing process of ternary solvents (water-hydrophilic/hydrophobic organic mixture) prepared in microchannels in a microchip was examined by fluorescence observation of the dyes dissolved in the solvents under laminar flow conditions. A microchip incorporating microchannels was used. In it, three narrow channels were combined to form one wide channel. Water-acetonitrile (hydrophilic) mixture containing relatively hydrophilic Eosin Y (green) was fed into the narrow center channel and an acetonitrile-ethyl acetate (hydrophobic) mixture containing hydrophobic perylene (blue) was fed into the two narrow side channels in the microchip. The mixtures in the narrow channels combined in the wide channel to prepare the ternary solvents of water-acetonitrile-ethyl acetate, causing the tube radial distribution of the solvents. We observed the mixing process of the ternary solvents in the wide channel through fluorescence of the green and blue dyes, including an aqueous-organic interface. For example, the green dye that was fed into the center channel was distributed near the inner side walls and the blue dye that was fed into the two side channels was distributed around the center area in the wide channel. Such specific mixing behavior was not observed for two-component solvents in the wide channel, such as water-acetonitrile mixture and water-ethyl acetate mixture.

Integrated allele-specific polymerase chain reaction-capillary electrophoresis microdevice for single nucleotide polymorphism genotyping

An integrated allele-specific (AS) polymerase chain reaction (PCR) and capillary electrophoresis (CE) microdevice has been developed for multiplex single nucleotide polymorphism (SNP) genotyping on a portable instrumentation, which was applied for on-site identification of HANWOO (Korean indigenous beef cattle). Twelve sets of primers were designed for targeting beef cattle's eleven SNP loci for HANWOO verification and one primer set for a positive PCR control, and the success rate for identification of HANWOO was demonstrated statistically. The AS PCR and CE separation for multiplex SNP typing was carried out on a glass-based microchip consisting of four layers: a microchannel plate for microfluidic control, a Pt-electrode plate for a resistance temperature detector (RTD), a poly(dimethylsiloxane) (PDMS) membrane and a manifold glass for microvalve function. The operation of the sample loading, AS PCR, microvalve, and CE on a chip was automated with a portable genetic analyzer, and the laser-induced fluorescence detection was performed on a miniaturized fluorescence detector. The blind samples were correctly identified as a HANWOO by showing one or two amplicon peaks in the electropherogram, while the imported beef cattle revealed more than five peaks. Our genetic analysis platform provides rapid, accurate, and on-site multiplex SNP typing.

Enrichment of viable bacteria in a micro-volume by free-flow electrophoresis

Macro- to micro-volume concentration of viable bacteria is performed in a microfluidic chip. The enrichment principle is based on free flow electrophoresis and is demonstrated for Gram positive bacteria. Bacteria from a suspension flow are trapped on a gel interface that separates the trapping location from integrated actuation electrodes in order to enable non-destructive trapping. The microfluidic chip contains integrated electrolytic gas expulsion structures and phaseguides for gel and liquid handling. Trapping efficiency is systematically optimized to reach 25 times the initial concentration from a theoretical maximum of 30. Finally, enrichment from analytically relevant concentrations down to 3 × 10(2) colony forming units per millilitre is demonstrated with a trapping efficiency of 80% which represents the most important parameter in enrichment.

Microfluidics and the life sciences

The field of microfluidics, often also referred to as "Lab-on-a-Chip" has made significant progress in the last 15 years and is an essential tool in the development of new products and protocols in the life sciences. This article provides a broad overview on the developments on the academic as well as the commercial side. Fabrication technologies for polymer-based devices are presented and a strategy for the development of complex integrated devices is discussed, together with an example on the use of these devices in pathogen detection.

Rethinking approaches to improve the utilization of nucleic acid amplification tests for detection and characterization of influenza A in diagnostic and reference laboratories

Influenza A virus (IFVA) is a significant cause of respiratory infections worldwide and was also responsible for a recent pandemic in 2009. Laboratory identification of IFVA can guide antiviral therapy, assist in cohorting of patients and prevent antibiotic use. Characterization of the virus can track the emergence of novel strains, identify resistance and determine how circulating strains match with vaccine components. The gold standard for detection and characterization of IFVA is nucleic acid amplification technology (e.g., reverse transcriptase PCR [RT-PCR]), which must contend with a constantly evolving viral genome. Although molecular technology has been available for over two decades, there is still an operational gap between assay design and utilization of these tests for the diagnosis and characterization of IFVA. This review will discuss issues surrounding the implementation and use of RT-PCR for the identification and characterization of IFVA, and speculate on why RT-PCR has not been used more widely in clinical laboratories or moved closer to the patient. Newer, less widely used technologies that may change our laboratory practices will be identified and the authors will close with an attempt to identify some future applications of RT-PCR-based technologies for the detection and characterization of IFVA.

A self-heating cartridge for molecular diagnostics

A disposable, water-activated, self-heating, easy-to-use, polymeric cartridge for isothermal nucleic acid amplification and visual fluorescent detection of the amplification products is described. The device is self-contained and does not require any special instruments to operate. The cartridge integrates chemical, water-triggered, exothermic heating with temperature regulation facilitated with a phase-change material (PCM) and isothermal nucleic acid amplification. The water flows into the exothermic reactor by wicking through a porous paper. The porous paper's characteristics control the rate of water supply, which in turn controls the rate of exothermic reaction. The PCM material enables the cartridge to maintain a desired temperature independent of ambient temperatures in the range between 20 °C and 40 °C. The utility of the cartridge is demonstrated by amplifying and detecting Escherichia coli DNA with loop mediated isothermal amplification (LAMP). The device can detect consistently as few as 10 target molecules in the sample. With proper modifications, the cartridge also can work with other isothermal nucleic acid amplification technologies for detecting nucleic acids associated with various pathogens borne in blood, saliva, urine, and other body fluids as well as in water and food. The device is suitable for use at home, in the field, and in poor-resource settings, where access to sophisticated laboratories is impractical, unaffordable, or nonexistent.