Antiprimer quenching-based real-time PCR and its application to the analysis of clinical cancer samples

Designing Allele-Specific Competitive-Extension PCR-Based Assays for High-Throughput Genotyping and Gene Characterization

Polymerase chain reaction (PCR) is a simple and rapid method that can detect nucleotide polymorphisms and sequence variation in basic research applications, agriculture, and medicine. Variants of PCR, collectively known as allele-specific PCR (AS-PCR), use a competitive reaction in the presence of allele-specific primers to preferentially amplify only certain alleles. This method, originally named by its developers as Kompetitive Allele Specific PCR (KASP), is an AS-PCR variant adapted for fluorescence-based detection of amplification results. We developed a bioinformatic tool for designing probe sequences for PCR-based genotyping assays. Probe sequences are designed in both directions, and both single nucleotide polymorphisms (SNPs) and insertion-deletions (InDels) may be targeted. In addition, the tool allows discrimination of up to four-allelic variants at a single SNP site. To increase both the reaction specificity and the discriminative power of SNP genotyping, each allele-specific primer is designed such that the penultimate base before the primer’s 3′ end base is positioned at the SNP site. The tool allows design of custom FRET cassette reporter systems for fluorescence-based assays. FastPCR is a user-friendly and powerful Java-based software that is freely available (http://primerdigital.com/tools/). Using the FastPCR environment and the tool for designing AS-PCR provides unparalleled flexibility for developing genotyping assays and specific and sensitive diagnostic PCR-based tests, which translates into a greater likelihood of research success.

Multiplex real-time PCR using double-strand primers and probes for the detection of nucleic acids

Multiplex PCR encounters difficulties in primer designing with all the primer pairs working at the same annealing temperature. In this study, we have developed a double-strand primer-mediated multiple strand displacement reaction for the detection of SARS-COV-2 ORF, N and E genes (as examples). The double primer is composed of a 5'-modified fluorophore strand, which does not impact polymerase extension and a 3'-modified quencher strand, which cannot impact elongation. At the annealing temperature, the fluorophore strand combined with the template, extended and resulted in fluorescence signal release. Results showed that the double-strand primer relatively exhibits a wide annealing temperature range and good compatibility between three pairs of primers and probes. These merits allow the simple and multiplex real-time fluorescence quantification of nucleic acids. The detection limit was 400 copies/mL, and the detection time was approximately 2 h. In addition to its extreme specificity and simplicity, this method has a wide range of applications such as multiple PCR and SNP detection.

Denaturation-Enhanced Droplet Digital PCR for Liquid Biopsies

BACKGROUND

Although interest in droplet-digital PCR technology (ddPCR) for cell-free circulating DNA (cfDNA) analysis is burgeoning, the technology is compromised by subsampling errors and the few clinical targets that can be analyzed from limited input DNA. The paucity of starting material acts as a "glass ceiling" in liquid biopsies because, irrespective how analytically sensitive ddPCR techniques are, detection limits cannot be improved past DNA input limitations.

METHODS

We applied denaturation-enhanced ddPCR (dddPCR) using fragmented genomic DNA (gDNA) with defined mutations. We then tested dddPCR on cfDNA from volunteers and patients with cancer for commonly-used mutations. gDNA and cfDNA were tested with and without end repair before denaturation and digital PCR.

RESULTS

By applying complete denaturation of double-stranded DNA before ddPCR droplet formation the number of positive droplets increased. dddPCR using gDNA resulted in a 1.9-2.0-fold increase in data-positive droplets, whereas dddPCR applied on highly-fragmented cfDNA resulted in a 1.6-1.7-fold increase. End repair of cfDNA before denaturation enabled cfDNA to display a 1.9-2.0-fold increase in data-positive signals, similar to gDNA. Doubling of data-positive droplets doubled the number of potential ddPCR assays that could be conducted from a given DNA input and improved ddPCR precision for cfDNA mutation detection.

CONCLUSIONS

dddPCR is a simple and useful modification in ddPCR that enables extraction of more information from low-input clinical samples with minor change in protocols. It should be applicable to all ddPCR platforms for mutation detection and, potentially, for gene copy-number analysis in cancer and prenatal screening.

A novel RFLP-ARMS TaqMan PCR-based method for detecting the BRAF V600E mutation in melanoma

To enable the rapid and sensitive screening of the BRAF V600E mutation in clinical samples, a novel method combining restriction fragment length polymorphism (RFLP) analysis with the popular amplification refractory mutation system (ARMS) TaqMan quantitative (qPCR) genotyping method in a single reaction tube was developed. A total of 2 primer pairs were designed to enrich for and genotype the BRAF mutational hotspot (RFLP primers and ARMS primers) and a restriction enzyme was used to remove the wild-type alleles. The analysis revealed that this method detected mutant alleles in mixed samples containing >0.1% mutant sequences. In a survey of 53 melanoma samples, this method detected 21 mutation-positive samples. This novel RFLP-ARMS TaqMan qPCR protocol may prove useful for detecting mutations in clinical samples containing only a small proportion of mutant alleles.

Methylation-sensitive enrichment of minor DNA alleles using a double-strand DNA-specific nuclease

Aberrant methylation changes, often present in a minor allelic fraction in clinical samples such as plasma-circulating DNA (cfDNA), are potentially powerful prognostic and predictive biomarkers in human disease including cancer. We report on a novel, highly-multiplexed approach to facilitate analysis of clinically useful methylation changes in minor DNA populations. Methylation Specific Nuclease-assisted Minor-allele Enrichment (MS-NaME) employs a double-strand-specific DNA nuclease (DSN) to remove excess DNA with normal methylation patterns. The technique utilizes oligonucleotide-probes that direct DSN activity to multiple targets in bisulfite-treated DNA, simultaneously. Oligonucleotide probes targeting unmethylated sequences generate local double stranded regions resulting to digestion of unmethylated targets, and leaving methylated targets intact; and vice versa. Subsequent amplification of the targeted regions results in enrichment of the targeted methylated or unmethylated minority-epigenetic-alleles. We validate MS-NaME by demonstrating enrichment of RARb2, ATM, MGMT and GSTP1 promoters in multiplexed MS-NaME reactions (177-plex) using dilutions of methylated/unmethylated DNA and in DNA from clinical lung cancer samples and matched normal tissue. MS-NaME is a highly scalable single-step approach performed at the genomic DNA level in solution that combines with most downstream detection technologies including Sanger sequencing, methylation-sensitive-high-resolution melting (MS-HRM) and methylation-specific-Taqman-based-digital-PCR (digital Methylight) to boost detection of low-level aberrant methylation-changes.

Copy-number variation of housekeeping gene rpl13a in rat strains selected for nervous system excitability

We evaluated copy number variation (CNV) for four genes in rat strains differing in nervous system excitability. rpl13a copy number is significantly reduced in hippocampus and bone marrow in rats with a high excitability threshold and stress. The observed phenomenon may be associated with a role for rpl13a in lipid metabolism.

Elimination of unaltered DNA in mixed clinical samples via nuclease-assisted minor-allele enrichment

Presence of excess unaltered, wild-type (WT) DNA providing no information of biological or clinical value often masks rare alterations containing diagnostic or therapeutic clues in cancer, prenatal diagnosis, infectious diseases or organ transplantation. With the surge of high-throughput technologies there is a growing demand for removing unaltered DNA over large pools-of-sequences. Here we present nuclease-assisted minor-allele enrichment with probe-overlap (NaME-PrO), a single-step approach with broad genome coverage that can remove WT-DNA from numerous sequences simultaneously, prior to genomic analysis. NaME-PrO employs a double-strand-DNA-specific nuclease and overlapping oligonucleotide-probes interrogating WT-DNA targets and guiding nuclease digestion to these sites. Mutation-containing DNA creates probe-DNA mismatches that inhibit digestion, thus subsequent DNA-amplification magnifies DNA-alterations at all selected targets. We demonstrate several-hundred-fold mutation enrichment in diverse human samples on multiple clinically relevant targets including tumor samples and circulating DNA in 50-plex reactions. Enrichment enables routine mutation detection at 0.01% abundance while by adjusting conditions it is possible to sequence mutations down to 0.00003% abundance, or to scan tumor-suppressor genes for rare mutations. NaME-PrO introduces a simple and highly parallel process to remove un-informative DNA sequences and unmask clinically and biologically useful alterations.

Metastasis-associated MCL1 and P16 copy number alterations dictate resistance to vemurafenib in a BRAFV600E patient-derived papillary thyroid carcinoma preclinical model

BRAF^V600E mutation exerts an essential oncogenic function in many tumors, including papillary thyroid carcinoma (PTC). Although BRAF^V600E inhibitors are available, lack of response has been frequently observed. To study the mechanism underlying intrinsic resistance to the mutant BRAF^V600E selective inhibitor vemurafenib, we established short-term primary cell cultures of human metastatic/recurrent BRAF^V600E-PTC, intrathyroidal BRAF^V600E-PTC, and normal thyroid (NT). We also generated an early intervention model of human BRAF^V600E-PTC orthotopic mouse. We find that metastatic BRAF^V600E-PTC cells elicit paracrine-signaling which trigger migration of pericytes, blood endothelial cells and lymphatic endothelial cells as compared to BRAF^WT-PTC cells, and show a higher rate of invasion. We further show that vemurafenib therapy significantly suppresses these aberrant functions in non-metastatic BRAF^V600E-PTC cells but lesser in metastatic BRAF^V600E-PTC cells as compared to vehicle treatment. These results concur with similar folds of down-regulation of tumor microenvironment–associated pro-metastatic molecules, with no effects in BRAF^WT-PTC and NT cells. Our early intervention preclinical trial shows that vemurafenib delays tumor growth in the orthotopic BRAF^WT/V600E-PTC mice. Importantly, we identify high copy number gain of MCL1 (chromosome 1q) and loss of CDKN2A (P16, chromosome 9p) in metastatic BRAF^V600E-PTC cells which are associated with resistance to vemurafenib treatment. Critically, we demonstrate that combined vemurafenib therapy with BCL2/MCL1 inhibitor increases metastatic BRAF^V600E-PTC cell death and ameliorates response to vemurafenib treatment as compared to single agent treatment. In conclusion, short-term PTC and NT cultures offer a predictive model for evaluating therapeutic response in patients with PTC. Our PTC pre-clinical model suggests that combined targeted therapy might be an important therapeutic strategy for metastatic and refractory BRAF^V600E-positive PTC.

Enhanced ratio of signals enables digital mutation scanning for rare allele detection

The use of droplet digital PCR (ddPCR) for low-level DNA mutation detection in cancer, prenatal diagnosis, and infectious diseases is growing rapidly. However, although ddPCR has been implemented successfully for detection of rare mutations at pre-determined positions, no ddPCR adaptation for mutation scanning exists. Yet, frequently, clinically relevant mutations reside on multiple sequence positions in tumor suppressor genes or complex hotspot mutations in oncogenes. Here, we describe a combination of coamplification at lower denaturation temperature PCR (COLD-PCR) with ddPCR that enables digital mutation scanning within approximately 50-bp sections of a target amplicon. Two FAM/HEX-labeled hydrolysis probes matching the wild-type sequence are used during ddPCR. The ratio of FAM/HEX-positive droplets is constant when wild-type amplicons are amplified but deviates when mutations anywhere under the FAM or HEX probes are present. To enhance the change in FAM/HEX ratio, we employed COLD-PCR cycling conditions that enrich mutation-containing amplicons anywhere on the sequence. We validated COLD-ddPCR on multiple mutations in TP53 and in EGFR using serial mutation dilutions and cell-free circulating DNA samples, and demonstrate detection down to approximately 0.2% to 1.2% mutation abundance. COLD-ddPCR enables a simple, rapid, and robust two-fluorophore detection method for the identification of multiple mutations during ddPCR and potentially can identify unknown DNA variants present in the target sequence.

Current methods for fluorescence-based universal sequence-dependent detection of nucleic acids in homogenous assays and clinical applications

BACKGROUND

Specific and sensitive nucleic acid (NA) testing in research and clinical diagnostics is usually performed by use of labeled oligonucleotide probes. However, the use of target-specific fluorogenic probes increases the cost of analysis. Therefore, universal sequence-dependent (USD) NA detection methods have been developed to facilitate cost-effective target detection using standardized reagents.

CONTENT

We provide a comprehensive review of the current methods for fluorescence-based USD NA detection. Initially, we focus on the emergence of these methods as a means to overcome the shortcomings of common NA detection methods, such as hydrolysis probes and molecular beacons. Thereafter, we provide a critical evaluation of the individual detection methods. These methods include (a) target amplification with bipartite primers introducing a universal detection tag to the amplicon (UniPrimer PCR, universal fluorescence energy transfer probe PCR, attached universal duplex probe PCR, and universal strand displacement amplification) or combined with bipartite probes comprising a universal detection region (mediator probe PCR, universal strand displacement amplification, universal quenching probe PCR) and (b) amplification-independent assays employing either a universal variant of the invader assay or universal NA hybridization sensors. We discuss differences between the methods and review clinical applications.

SUMMARY

The current methods for USD NA testing are cost-effective and flexible and have concordant analytical performance in comparison with common probe-based techniques. They can detect any target sequence by the simple use of a label-free, low-cost primer or probe combined with a universal fluorogenic reporter. The methods differ in the number of target specificities, capability of multiplexing, and incubation requirements (isothermal/thermocycling). Extensive clinical applications comprise detection of single-nucleotide polymorphisms, study of gene expression, in situ PCR, and quantification of pathogen load.

BRCA1 gene mutations frequency estimation by allele-specific real-time PCR of pooled genomic DNA samples

The frequencies of four mutations in the BRCA1 gene (185delAG, T300G, 4153delA, and 5382insC) were estimated in the Russian population. The mutations were analyzed in 7920 DNA samples obtained from randomly selected Novosibirsk citizens and 570 samples from breast cancer patients at Siberian hospitals. The mutations were detected by allele-specific real-time PCR. The mutation analysis was performed with pooled DNA samples to reduce the cost of the study. The 5382insC mutation was found in 20 of 7920 (0.25%) population DNA samples and in 14 of 570 (2.46%) breast cancer samples; the T300G mutation was detected in 4 population samples (0.05%) and in 2 breast cancer samples (0.35%); the 185delAG or 4153delA mutations were not identified in any of the 7920 samples and were detected in 1 (0.18%) breast cancer case each.

Restriction endonuclease-mediated real-time digestion-PCR for somatic mutation detection

PCR is a powerful platform for clinical and diagnostic applications, but challenges remain in detecting somatic mutations, as mutant cells are often mixed with more numerous wild-type cells at the tissue-sample sites. Here, we describe a novel method that couples PCR with restriction endonuclease digestion (designated real-time digestion-PCR, or RTD-PCR) in a one-step reaction tube for detecting somatic mutations from a minority of cells. The PCR mixture contains a thermostable restriction enzyme that digests wild-type alleles during the PCR program, allowing selective amplification of the mutant alleles. To validate this method, we used real-time digestion-PCR for the specific detection of the EGFR (epidermal growth factor receptor) treatment resistance-inducing mutation, T790M, combining with three different platforms: Sanger sequencing, TaqMan probe PCR and Sequenom MassArray. From 78 clinical samples, seven T790M mutations were consistently detected on all three platforms, indicating that RTD-PCR may be a useful clinical tool for analyzing the T790M point mutation.

Fluorescence quenchers for hydrazone and oxime orthogonal bioconjugation

We describe the synthesis and properties of new fluorescence quenchers containing aldehyde, hydrazine, and aminooxy groups, allowing convenient bioconjugation as oximes or hydrazones. Conjugation to oligonucleotides proceeded in high yield with aniline as catalyst. Kinetics studies of conjugation show that, under optimal conditions, a hydrazine or aminooxy quencher can react with aldehyde-modified DNA to form a stable hydrazone or oxime adduct in as little as five minutes. The resulting quencher-containing DNAs were assessed for their ability to quench the emission of fluorescein in labeled complements and compared to the commercially available dabcyl and Black Hole Quencher 2 (BHQ2), which were conjugated as phosphoramidites. Results show that the new quenchers possess slightly different absorbance properties compared to dabcyl and are as efficient as the commercial quenchers in quenching fluorescein emission. Hydrazone-based quenchers were further successfully incorporated into molecular beacons and shown to give high signal to background ratios in single nucleotide polymorphism detection in vitro. Finally, aminooxy and hydrazine quenchers were applied to quenching of an aldehyde-containing fluorophore associated with living cells, demonstrating cellular quenching within one hour.

Multi-path quenchers: efficient quenching of common fluorophores

Fluorescence quenching groups are widely employed in biological detection, sensing, and imaging. To date, a relatively small number of such groups are in common use. Perhaps the most commonly used quencher, dabcyl, has limited efficiency with a broad range of fluorophores. Here, we describe a molecular approach to improve the efficiency of quenchers by increasing their electronic complexity. Multi-Path Quenchers (MPQ) are designed to have multiple donor or acceptor groups in their structure, allowing for a multiplicity of conjugation pathways of varied length. This has the effect of broadening the absorption spectrum, which in turn can increase quenching efficiency and versatility. Six such MPQ derivatives are synthesized and tested for quenching efficiency in a DNA hybridization context. Duplexes placing quenchers and fluorophores within contact distance or beyond this distance are used to measure quenching via contact or FRET mechanisms. Results show that several of the quenchers are considerably more efficient than dabcyl at quenching a wider range of common fluorophores, and two quench fluorescein and TAMRA as well as or better than a Black Hole Quencher.

Coamplification at lower denaturation temperature-PCR increases mutation-detection selectivity of TaqMan-based real-time PCR

BACKGROUND

DNA genotyping with mutation-specific TaqMan(R) probes (Applied Biosystems) is broadly used in detection of single-nucleotide polymorphisms but is less so for somatic mutations because of its limited selectivity for low-level mutations. We recently described coamplification at lower denaturation temperature-PCR (COLD-PCR), a method that amplifies minority alleles selectively from mixtures of wild-type and mutation-containing sequences during the PCR. We demonstrate that combining COLD-PCR with TaqMan technology provides TaqMan genotyping with the selectivity needed to detect low-level somatic mutations.

METHODS

Minor-groove binder-based or common TaqMan probes were designed to contain a nucleotide that matches the desired mutation approximately in the middle of the probe. The critical denaturation temperature (T(c)) of each amplicon was then experimentally determined. COLD-PCR/TaqMan genotyping was performed in 2 steps: denaturation at the T(c), followed by annealing and extension at a single temperature (fast COLD-PCR). The threshold cycle was used to identify mutations on the basis of serial dilutions of mutant DNA into wild-type DNA and to identify TP53 (tumor protein p53) and EGFR [epidermal growth factor receptor (erythroblastic leukemia viral (v-erb-b) oncogene homolog, avian)] mutations in tumors.

RESULTS

COLD-PCR/TaqMan genotyping identified G>A mutations within TP53 exon 8 (codon 273 mutation hot spot) and C>T mutations within the EGFR gene (drug-resistance mutation T790M) with a selectivity improvement of 15- to 30-fold over regular PCR/TaqMan genotyping. A second round of COLD-PCR/TaqMan genotyping improved the selectivity by another 15- to 30-fold and enabled detection of 1 mutant in 2000 wild-type alleles. Use of COLD-PCR/TaqMan genotyping allowed quantitative identification of low-level TP53 and T790 mutations in colon tumor samples and in non-small-cell lung cancer cell lines treated with kinase inhibitors.

CONCLUSIONS

The major improvement in selectivity provided by COLD-PCR enables the popular TaqMan genotyping method to become a powerful tool for detecting low-level mutations in clinical samples.

PCR-based methods for the enrichment of minority alleles and mutations

BACKGROUND

The ability to identify low-level somatic DNA mutations and minority alleles within an excess wild-type sample is becoming essential for characterizing early and posttreatment tumor status in cancer patients. Over the past 2 decades, much research has focused on improving the selectivity of PCR-based technologies for enhancing the detection of minority (mutant) alleles in clinical samples. Routine application in clinical and diagnostic settings requires that these techniques be accurate and cost-effective and require little effort to optimize, perform, and analyze.

CONTENT

Enrichment methods typically segregate by their ability to enrich for, and detect, either known or unknown mutations. Although there are several robust approaches for detecting known mutations within a high background of wild-type DNA, there are few techniques capable of enriching and detecting low-level unknown mutations. One promising development is COLD-PCR (coamplification at lower denaturation temperature), which enables enrichment of PCR amplicons containing unknown mutations at any position, such that they can be subsequently sequenced to identify the exact nucleotide change.

SUMMARY

This review summarizes technologies available for detecting minority DNA mutations, placing an emphasis on newer methods that facilitate the enrichment of unknown low-level DNA variants such that the mutation can subsequently be sequenced. The enrichment of minority alleles is imperative in clinical and diagnostic applications, especially in those related to cancer detection, and continued technology development is warranted.

Anchor-based fluorescent amplicon generation assays (FLAG) for real-time measurement of human cytomegalovirus, Epstein-Barr virus, and varicella-zoster virus viral loads

BACKGROUND

Monitoring the human cytomegalovirus (HCMV), Epstein-Barr virus (EBV), or varicella-zoster virus (VZV) viral load is an important factor in the management of immunosuppressed patients, such as recipients of solid-organ or bone marrow transplants. The advent of real-time PCR technologies has prompted the widespread development of quantitative PCR assays for the detection of viral loads and other diagnostic purposes.

METHODS

The fluorescent amplicon generation (FLAG) technology uses the PspGI restriction enzyme to monitor PCR product generation. We modified the FLAG technology by introducing an accessory oligonucleotide "anchor" that stabilizes the binding of the forward primer to the target sequence (a-FLAG). We developed assays for HCMV, EBV, and VZV that incorporated an internal amplification-control reaction to validate negative results and extensively analyzed the performance of the HCMV a-FLAG assay.

RESULTS

The 3 assays performed similarly with respect to reaction efficiency and linear range. Compared with a commercially available kit, the HCMV a-FLAG assay results showed good correlation with calculated concentrations (r = 0.9617), excellent diagnostic sensitivity and specificity (99% and 95%, respectively), and similar values for the linear range (1-10(7) copies/microL), analytical sensitivity (0.420 copies/microL), and intra- and interassay imprecision.

CONCLUSIONS

The a-FLAG assay is an alternative real-time PCR technology suitable for detecting and quantifying target-DNA sequences. For clinical applications such as the measurement of viral load, a-FLAG assays provide multiplex capability, internal amplification control, and high diagnostic sensitivity and specificity.

A novel universal real-time PCR system using the attached universal duplex probes for quantitative analysis of nucleic acids

Background

Real-time PCR techniques are being widely used for nucleic acids analysis, but one limitation of current frequently employed real-time PCR is the high cost of the labeled probe for each target molecule.

Results

We describe a real-time PCR technique employing attached universal duplex probes (AUDP), which has the advantage of generating fluorescence by probe hydrolysis and strand displacement over current real-time PCR methods. AUDP involves one set of universal duplex probes in which the 5' end of the fluorescent probe (FP) and a complementary quenching probe (QP) lie in close proximity so that fluorescence can be quenched. The PCR primer pair with attached universal template (UT) and the FP are identical to the UT sequence. We have shown that the AUDP technique can be used for detecting multiple target DNA sequences in both simplex and duplex real-time PCR assays for gene expression analysis, genotype identification, and genetically modified organism (GMO) quantification with comparable sensitivity, reproducibility, and repeatability with other real-time PCR methods.

Conclusion

The results from GMO quantification, gene expression analysis, genotype identification, and GMO quantification using AUDP real-time PCR assays indicate that the AUDP real-time PCR technique has been successfully applied in nucleic acids analysis, and the developed AUDP real-time PCR technique will offer an alternative way for nucleic acid analysis with high efficiency, reliability, and flexibility at low cost.

MS-FLAG, a Novel Real-Time Signal Generation Method for Methylation-Specific PCR

Background: Aberrant promoter methylation is a major mechanism for silencing tumor suppressor genes in cancer. Detection of hypermethylation is used as a molecular marker for early cancer diagnosis, as a prognostic index, or to define therapeutic targets for reversion of aberrant methylation. We report on a novel signal generation technology for real-time PCR to detect gene promoter methylation.

Methods: FLAG (fluorescent amplicon generation) is a homogeneous signal generation technology based on the exceptionally thermostable endonuclease PspGI. FLAG provides real-time signal generation during PCR by PspGI-mediated cleavage of quenched fluorophores at the 5′ end of double-stranded PCR products. Methylation-specific PCR (MSP) applied on bisulfite-treated DNA was adapted to a real-time format (methylation-specific FLAG; MS-FLAG) for quantifying methylation in the promoter of CDKN2A (p16), GATA5, and RASSF1. We validated MS-FLAG on plasmids and genomic DNA with known methylation status and applied it to detection of methylation in a limited number of clinical samples. We also conducted bisulfite sequencing on these samples.

Results: Real-time PCR results obtained via MS-FLAG agreed with results obtained via conventional, gel-based MSP. The new technology showed high specificity, sensitivity (2–3 plasmid copies), and selectivity (0.01% of methylated DNA) on control samples. It enabled correct prediction of the methylation status of all 3 gene promoters in 21 lung adenocarcinoma samples, as confirmed by bisulfite sequencing. We also developed a multiplex MS-FLAG assay for GATA5 and RASSF1 promoters.

Conclusion: MS-FLAG provides a new, quantitative, high-throughput method for detecting gene promoter methylation and is a convenient alternative to agarose gel-based MSP for screening methylation. In addition to methylation, FLAG-based real-time signal generation may have broad applications in DNA diagnostics.

FLAG assay as a novel method for real-time signal generation during PCR: application to detection and genotyping of KRAS codon 12 mutations

Real-time signal generation methods for detection and characterization of low-abundance mutations in genomic DNA are powerful tools for cancer diagnosis and prognosis. Mutations in codon 12 of the oncogene KRAS, for example, are frequently found in several types of human cancers. We have developed a novel real-time PCR technology, FLAG (FLuorescent Amplicon Generation) and adapted it for simultaneously (i) amplifying mutated codon 12 KRAS sequences, (ii) monitoring in real-time the amplification and (iii) genotyping the exact nucleotide alteration. FLAG utilizes the exceptionally thermostable endonuclease PspGI for real-time signal generation by cleavage of quenched fluorophores from the 5′-end of the PCR products and, concurrently, for selecting KRAS mutations over wild type. By including peptide-nucleic-acid probes in the reaction, simultaneous genotyping is achieved that circumvents the requirement for sequencing. FLAG enables high-throughput, closed-tube KRAS mutation detection down to ∼0.1% mutant-to-wild type. The assay was validated on model systems and compared with allele-specific PCR sequencing for screening 27 cancer specimens. Diverse applications of FLAG for real-time PCR or genotyping applications in cancer, virology or infectious diseases are envisioned.

DNA degradation test predicts success in whole-genome amplification from diverse clinical samples

The need to apply modern technologies to analyze DNA from diverse clinical samples often stumbles on suboptimal sample quality. We developed a simple approach to assess DNA fragmentation in minute clinical samples of widely different origin and the likelihood of success of degradation-tolerant whole genome amplification (restriction and circularization-aided rolling circle amplification, RCA-RCA) and subsequent polymerase chain reaction (PCR). A multiplex PCR amplification of four glyceraldehyde-3-phosphate dehydrogenase amplicons of varying sizes was performed using genomic DNA from clinical samples, followed by size discrimination on agarose gel or fluorescent denaturing high-performance liquid chromatography (dHPLC). RCA-RCA followed by real-time PCR was also performed, for correlation. Even minimal quantities of longer PCR fragments ( approximately 300 to 400 bp), visible via high-sensitivity fluorescent dHPLC or agarose gel, were essential for the success of RCA-RCA and subsequent PCR-based assays. dHPLC gave a more accurate correlation between DNA fragmentation and sample quality than agarose gel electrophoresis. Multiplex-PCR-dHPLC predicted correctly the likelihood of assay success in formalin-fixed, paraffin-embedded samples fixed under controlled conditions and of different ages, in laser capture microdissection samples, in tissue print micropeels, and plasma-circulating DNA. Estimates of the percent information retained relative to snap-frozen DNA are derived for real-time PCR analysis. The assay is rapid and convenient and can be used widely to characterize DNA from any clinical sample of unknown quality.

Anti-primer quenching-based real-time PCR for simplex or multiplex DNA quantification and single-nucleotide polymorphism genotyping

Nucleic acid amplification and detection plays an increasingly important role in genetic analysis of clinical samples, medical diagnostics and drug discovery. We present a new quantitative PCR method that allows versatile and flexible nucleic acid target quantification. One of the PCR primers is modified by an oligonucleotide "tail" fluorescently labeled at the 5' end. An oligonucleotide complementary to this tail, carrying a 3'-quencher ("anti-primer"), is included in the PCR along with the two primers. Following primer extension, the reaction temperature is lowered such that the anti-primer hybridizes to and quenches the fluorescence of only the free primer and not the double-stranded PCR product, allowing real-time fluorescent quantification of the latter. This anti-primer-based quantitative real-time PCR (aQRT-PCR) allows simplex or multiplex quantification or single-nucleotide polymorphism genotyping in clinical samples of widely differing quality (e.g., fresh samples, formalin-fixed paraffin-embedded samples and plasma-circulating DNA) and provides a practical alternative to existing, more expensive approaches. The process of aQRT-PCR takes 1.5-2 h.