Concept: Taq polymerase
Most molecular diagnostic assays require upfront sample preparation steps to isolate the target’s nucleic acids, followed by its amplification and detection using various nucleic acid amplification techniques. Because molecular diagnostic methods are generally rather difficult to perform manually without highly trained users, automated and integrated systems are highly desirable but too costly for use at point-of-care or low-resource settings. Here, we showcase the development of a low-cost and rapid nucleic acid isolation and amplification platform by modifying entry-level 3D printers that cost between $400 and $750. Our modifications consisted of replacing the extruder with a tip-comb attachment that houses magnets to conduct magnetic particle-based nucleic acid extraction. We then programmed the 3D printer to conduct motions that can perform high-quality extraction protocols. Up to 12 samples can be processed simultaneously in under 13 minutes and the efficiency of nucleic acid isolation matches well against gold-standard spin-column-based extraction technology. Additionally, we used the 3D printer’s heated bed to supply heat to perform water bath-based polymerase chain reactions (PCRs). Using another attachment to hold PCR tubes, the 3D printer was programmed to automate the process of shuttling PCR tubes between water baths. By eliminating the temperature ramping needed in most commercial thermal cyclers, the run time of a 35-cycle PCR protocol was shortened by 33%. This article demonstrates that for applications in resource-limited settings, expensive nucleic acid extraction devices and thermal cyclers that are used in many central laboratories can be potentially replaced by a device modified from inexpensive entry-level 3D printers.
The ability to make rapid diagnosis of infectious diseases broadly available in a portable, low-cost format would mark a great step forward in global health. Many molecular diagnostic assays are developed based on using thermal cyclers to carry out polymerase chain reaction (PCR) and reverse-transcription PCR for DNA and RNA amplification and detection, respectively. Unfortunately, most commercial thermal cyclers are expensive and need continuous electrical power supply, so they are not suitable for uses in low-resource settings. We have previously reported a low-cost and simple approach to amplify DNA using vacuum insulated stainless steel thermoses food cans, which we have named it thermos thermal cycler or TTC. Here, we describe the use of an improved set up to enable the detection of viral RNA targets by reverse-transcription PCR (RT-PCR), thus expanding the TTC’s ability to identify highly infectious, RNA virus-based diseases in low resource settings. The TTC was successful in demonstrating high-speed and sensitive detection of DNA or RNA targets of sexually transmitted diseases, HIV/AIDS, Ebola hemorrhagic fever, and dengue fever. Our innovative TTC costs less than $200 to build and has a capacity of at least eight tubes. In terms of speed, the TTC’s performance exceeded that of commercial thermal cyclers tested. When coupled with low-cost endpoint detection technologies such as nucleic acid lateral-flow assay or a cell-phone-based fluorescence detector, the TTC will increase the availability of on-site molecular diagnostics in low-resource settings.
Next-generation sequencing technology has enabled the detection of rare genetic or somatic mutations and contributed to our understanding of disease progression and evolution. However, many next-generation sequencing technologies first rely on DNA amplification, via the Polymerase Chain Reaction (PCR), as part of sample preparation workflows. Mistakes made during PCR appear in sequencing data and contribute to false mutations that can ultimately confound genetic analysis. In this report, a single-molecule sequencing assay was used to comprehensively catalog the different types of errors introduced during PCR, including polymerase misincorporation, structure-induced template-switching, PCR-mediated recombination and DNA damage. In addition to well-characterized polymerase base substitution errors, other sources of error were found to be equally prevalent. PCR-mediated recombination by Taq polymerase was observed at the single-molecule level, and surprisingly found to occur as frequently as polymerase base substitution errors, suggesting it may be an underappreciated source of error for multiplex amplification reactions. Inverted repeat structural elements in lacZ caused polymerase template-switching between the top and bottom strands during replication and the frequency of these events were measured for different polymerases. For very accurate polymerases, DNA damage introduced during temperature cycling, and not polymerase base substitution errors, appeared to be the major contributor toward mutations occurring in amplification products. In total, we analyzed PCR products at the single-molecule level and present here a more complete picture of the types of mistakes that occur during DNA amplification.
Many modern molecular diagnostic assays targeting nucleic acids are typically confined to developed countries or to the national reference laboratories of developing-world countries. The ability to make technologies for the rapid diagnosis of infectious diseases broadly available in a portable, low-cost format would mark a revolutionary step forward in global health. Many molecular assays are also developed based on polymerase chain reactions (PCR), which require thermal cyclers that are relatively heavy (>20 pounds) and need continuous electrical power. The temperature ramping speed of most economical thermal cyclers are relatively slow (2 to 3°C/s) so a polymerase chain reaction can take 1 to 2 hours. Most of all, these thermal cyclers are still too expensive ($2k to $4k) for low-resource setting uses.
The primary method for analysis of low template DNA (LTDNA) is known as the low copy number (LCN) method involving an increased number of PCR cycles (typically 34). In common with other LTDNA methods, LCN profiles are characterized by allelic imbalance, drop in, and drop out that require complicated interpretation rules. They often require replicate PCR reactions to generate a “consensus” profile in a specialized facility. An ideal method for analysis of LTDNA should enhance profiling outcomes without elevated error rates and be performed using standard facilities, with minimum additional cost.
Chimeric sequences falsely increase the apparent diversity within samples. To examine chimera formation in PCR products from environmental DNA, clone libraries were prepared using different ramp speeds to reach the designated temperatures for each step of the PCR program. Slowing the thermocycler ramp speed to 1°C sec(-1) reduced chimera formation.
The polymerase chain reaction (PCR) is a sought-after nucleic acid amplification technique used in the detection of several diseases. However, one of the main limitations of this and other nucleic acid amplification assays is the complexity, size, maintenance, and cost of their operational instrumentation. This limits the use of PCR applications in settings that cannot afford the instruments-yet may have access to basic electrical, electronic, and optical components and the expertise to build them. To provide a more accessible platform, we developed a low-cost, palm-size and portable instrument to perform real-time PCR (qPCR). The thermocycler leverages a copper-sheathed power resistor and a computer fan, in tandem with basic electronic components controlled from a single-board computer. The instrument incorporates a 3D-printed chassis and a custom-made fluorescence optical setup based on a CMOS camera and a blue LED. Results are displayed in real-time on a tablet. We also fabricated simple acrylic microdevices consisting of 4 wells (2 μL in volume each) where PCR reactions take place. To test our instrument, we performed qPCR on a series of cDNA dilutions spanning 4 orders of magnitude, achieving similar limits of detection as a benchtop thermocycler. We envision our instrument being utilized to enable routine monitoring and diagnosis of certain diseases in low-resource areas.
We report on the development of a novel and flexible on-line digital PCR (dPCR) system. The system was composed of three parts: an inkjet for generating the droplets, a coiled fused-silica capillary for thermal cycling and a laser induced fluorescence detector (LIFD) for positive droplet counting. Upon inkjet printing, monodisperse droplets were continu-ously generated in the oil phase and then introduced into the capillary in the form of a stable dispersion. The droplets containing one or zero molecules of target DNA passed through the helical capillary that was attached to a cylindrical thermal cycler for PCR amplification, resulting in the generation of fluorescence for the DNA-positive droplet. After 36 PCR cycles, the fluorescence signal intensity was detected by laser-induced fluorescence located at the downstream of the capillary, followed by a positive/negative counting. The present system was successfully applied to the absolute quantification of the HPV sequence in Caski cells with dynamic ranges spanning four orders of magnitude.
Prosthetic Joint Infection (PJI) is a rare but devastating complications with high morbitity and mortality. The identification of the causal microorganism remains crucial and determines therapeutic strategies and success. Microbiology cultures remain the common method to diagnose PJI. Unfortunately, 14% of intra-articular punctures remain negative after culture. The microorganisms are best detected by inoculation of microbiology samples in blood culture bottles (Bactec), or after sonication of the implant and polymerase chain reaction (PCR). The identification of the causal microorganism remains crucial and determines therapeutic success.
- Biochemical and biophysical research communications
- Published almost 2 years ago
Although compartmentalized self-replication (CSR) and compartmentalized partnered replication (CPR) are powerful tools for directed evolution of proteins and gene circuits, limitations remain in the emulsion PCR process with the wild-type Taq DNA polymerase used so far, including long run times, low amounts of product, and false negative results due to inhibitors. In this study, we developed a high-efficiency mutant of DNA polymerase I from Thermus thermophilus HB27 (Tth pol) suited for CSR and CPR. We modified the wild-type Tth pol by (i) deletion of the N-terminal 5' to 3' exonuclease domain, (ii) fusion with the DNA-binding protein Sso7d, (iii) introduction of four known effective point mutations from other DNA polymerase mutants, and (iv) codon optimization to reduce the GC content. Consequently, we obtained a mutant that provides higher product yields than the conventional Taq pol without decreased fidelity. Next, we performed four rounds of CSR selection with a randomly mutated library of this modified Tth pol and obtained mutants that provide higher product yields in fewer cycles of emulsion PCR than the parent Tth pol as well as the conventional Taq pol.