Integrated Extreme Real-Time PCR and High-Speed Melting Analysis in 52 to 87 Seconds.

BACKGROUND: Extreme PCR in <30 s and high-speed melting of PCR products in <5 s are recent advances in the turnaround time of DNA analysis. Previously, these steps had been performed on different specialized instruments. Integration of both extreme PCR and high-speed melting with real-time fluorescence monitoring for detection and genotyping is presented here. METHODS: A microfluidic platform was enhanced for speed using cycle times as fast as 1.05 s between 66.4 °C and 93.7 °C, with end point melting rates of 8 °C/s. Primer and polymerase concentrations were increased to allow short cycle times. Synthetic sequences were used to amplify fragments of hepatitis B virus (70 bp) and Clostridium difficile (83 bp) by real-time PCR and high-speed melting on the same instrument. A blinded genotyping study of 30 human genomic samples at F2 c.*97, F5 c.1601, MTHFR c.665, and MTHFR c.1286 was also performed. RESULTS: Standard rapid-cycle PCR chemistry did not produce any product when total cycling times were reduced to <1 min. However, efficient amplification was possible with increased primer (5 µmol/L) and polymerase (0.45 U/µL) concentrations. Infectious targets were amplified and identified in 52 to 71 s. Real-time PCR and genotyping of single-nucleotide variants from human DNA was achieved in 75 to 87 s and was 100% concordant to known genotypes. CONCLUSIONS: Extreme PCR with high-speed melting can be performed in about 1 min. The integration of extreme PCR and high-speed melting shows that future molecular assays at the point of care for identification, quantification, and variant typing are feasible.

High-Speed Melting Analysis: The Effect of Melting Rate on Small Amplicon Microfluidic Genotyping.

BACKGROUND: High-resolution DNA melting analysis of small amplicons is a simple and inexpensive technique for genotyping. Microfluidics allows precise and rapid control of temperature during melting. METHODS: Using a microfluidic platform for serial PCR and melting analysis, 4 targets containing single nucleotide variants were amplified and then melted at different rates over a 250-fold range from 0.13 to 32 °C/s. Genotypes (n = 1728) were determined manually by visual inspection after background removal, normalization, and conversion to negative derivative plots. Differences between genotypes were quantified by a genotype discrimination ratio on the basis of inter- and intragenotype differences using the absolute value of the maximum vertical difference between curves as a metric. RESULTS: Different homozygous curves were genotyped by melting temperature and heterozygous curves were identified by shape. Technical artifacts preventing analysis (0.3%), incorrect (0.06%), and indeterminate (0.4%) results were minimal, occurring mostly at slow melting rates (0.13-0.5 °C/s). Genotype discrimination was maximal at around 8 °C/s (2-8 °C/s for homozygotes and 8-16 °C/s for heterozygotes), and no genotyping errors were made at rates >0.5 °C/s. PCR was completed in 10-12.2 min, followed by melting curve acquisition in 4 min down to <1 s. CONCLUSIONS: Microfluidics enables genotyping by melting analysis at rates up to 32 °C/s, requiring <1 s to acquire an entire melting curve. High-speed melting reduces the time for melting analysis, decreases errors, and improves genotype discrimination of small amplicons. Combined with extreme PCR, high-speed melting promises nucleic acid amplification and genotyping in < 1 min.

Microfluidic genotyping by rapid serial PCR and high-speed melting analysis.

BACKGROUND: Clinical molecular testing typically batches samples to minimize costs or uses multiplex lab-on-a-chip disposables to analyze a few targets. In genetics, multiple variants need to be analyzed, and different work flows that rapidly analyze multiple loci in a few targets are attractive. METHODS: We used a microfluidic platform tailored to rapid serial PCR and high-speed melting (HSM) to genotype 4 single nucleotide variants. A contiguous stream of master mix with sample DNA was pulsed with each primer pair for serial PCR and melting. Two study sites each analyzed 100 samples for F2 (c.*97G>A), F5 (c.1601G>A), and MTHFR (c.665C>T and c.1286A>C) after blinding for genotype and genotype proportions. Internal temperature controls improved melting curve precision. The platform's liquid-handling system automated PCR and HSM. RESULTS: PCR and HSM were completed in a total of 12.5 min. Melting was performed at 0.5 °C/s. As expected, homozygous variants were separated by melting temperature, and heterozygotes were identified by curve shape. All samples were correctly genotyped by the instrument. Follow-up testing was required on 1.38% of the assays for a definitive genotype. CONCLUSIONS: We demonstrate genotyping accuracy on a novel microfluidic platform with rapid serial PCR and HSM. The platform targets short turnaround times for multiple genetic variants in up to 8 samples. It is also designed to allow automatic and immediate reflexive or repeat testing depending on results from the streaming DNA. Rapid serial PCR provides a flexible genetic work flow and is nicely matched to HSM analysis.

Quasi-digital PCR: Enrichment and quantification of rare DNA variants.

Rare variant enrichment and quantification was achieved by allele-specific, competitive blocker, digital PCR for aiming to provide a noninvasive method for detecting rare DNA variants from circulating cells. The allele-specific blocking chemistry improves sensitivity and lowers assay cost over previously described digital PCR methods while the instrumentation allowed for rapid thermal cycling for faster turnaround time. Because the digital counting of the amplified variants occurs in the presence of many wild-type templates in each well, the method is called "quasi-digital PCR". A spinning disk was used to separate samples into 1000 wells, followed by rapid-cycle, allele-specific amplification in the presence of a molecular beacon that serves as both a blocker and digital indicator. Monte Carlo simulations gave similar results to Poisson distribution statistics for mean number of template molecules and provided an upper and lower bound at a specified confidence level and accounted for input DNA concentration variation. A 111 bp genomic DNA fragment including the BRAF p.V600E mutation (c.T1799A) was amplified with quasi-digital PCR using cycle times of 23 s. Dilution series confirmed that wild-type amplification was suppressed and that the sensitivity for the mutant allele was <0.01 % (43 mutant alleles amongst 500,000 wild-type alleles). The Monte Carlo method presented here is publically available on the internet and can calculate target concentration given digital data or predict digital data given target concentration.

Spinning disk platform for microfluidic digital polymerase chain reaction.

An inexpensive plastic disk disposable was designed for digital polymerase chain reaction (PCR) applications with a microfluidic architecture that passively compartmentalizes a sample into 1000 nanoliter-sized wells by centrifugation. Well volumes of 33 nL were attained with a 16% volume coefficient of variation (CV). A rapid air thermocycler with aggregate real-time fluorescence detection was used, achieving PCR cycle times of 33 s and 94% PCR efficiency, with a melting curve to validate product specificity. A CCD camera acquired a fluorescent image of the disk following PCR, and the well intensity frequency distribution and Poisson distribution statistics were used to count the positive wells on the disk to determine the number of template molecules amplified. A 300 bp plasmid DNA product was amplified within the disk and analyzed in 50 min with 58-1000 wells containing plasmid template. Target concentrations measured by the spinning disk platform were 3 times less than that predicted by absorbance measurements. The spinning disk platform reduces disposable cost, instrument complexity, and thermocycling time compared to other current digital PCR platforms.

Solution-phase DNA mutation scanning and SNP genotyping by nanoliter melting analysis.

Solution-phase, DNA melting analysis for heterozygote scanning and single nucleotide polymorphism (SNP) genotyping was performed in 10 nl volumes on a custom microchip. Human genomic DNA was PCR amplified in the presence of the saturating fluorescent dye, LCGreen Plus, and placed within microfluidic channels that were created between two glass slides. The microchip was heated at 0.1 degrees C/s with a Peltier device and viewed with an inverted fluorescence microscope modified for photomulitiplier tube detection. The melting data was normalized and the negative first derivative plotted against temperature. Mutation scanning for heterozygotes was easily performed by comparing the shape of the melting curve to homozygous standards. Genotyping of homozygotes by melting temperature (T(m)) required absolute temperature comparisons. Mutation scanning of ATM exon 17 and CFTR exon 10 identified single base change heterozygotes in 84 and 201 base-pair (bp) products, respectively. All genotypes at HFE C282Y were distinguished by simple melting analysis of a 40-bp fragment. Sequential analysis of the same sample on the gold-standard, commercial high-resolution melting instrument HR-1, followed by melting in a 10 nl reaction chamber, produced similar results. DNA melting analysis requires only minutes after PCR and is a simple method for genotyping and scanning that can be reduced to nanoliter volumes. Microscale systems for performing DNA melting reduce the reagents/DNA template required with a promise for high throughput analysis in a closed chamber without risk of contamination.