Genotypic selection methods for the direct analysis of point mutations

Development of CAPS Markers for Evaluation of Genetic Diversity and Population Structure in the Germplasm of Button Mushroom (Agaricus bisporus)

Agaricus bisporus is a globally cultivated mushroom with high economic value. Despite its widespread cultivation, commercial button mushroom strains have little genetic diversity and discrimination of strains for identification and breeding purposes is challenging. Molecular markers suitable for diversity analyses of germplasms with similar genotypes and discrimination between accessions are needed to support the development of new varieties. To develop cleaved amplified polymorphic sequences (CAPs) markers, single nucleotide polymorphism (SNP) mining was performed based on the A. bisporus genome and resequencing data. A total of 70 sets of CAPs markers were developed and applied to 41 A. bisporus accessions for diversity, multivariate, and population structure analyses. Of the 70 SNPs, 62.85% (44/70) were transitions (G/A or C/T) and 37.15% (26/70) were transversions (A/C, A/T, C/G, or G/T). The number of alleles per locus was 1 or 2 (average = 1.9), and expected heterozygosity and gene diversity were 0.0-0.499 (mean = 0.265) and 0.0-0.9367 (mean = 0.3599), respectively. Multivariate and cluster analyses of accessions produced similar groups, with F-statistic values of 0.134 and 0.153 for distance-based and model-based groups, respectively. A minimum set of 10 markers optimized for accession identification were selected based on high index of genetic diversity (GD, range 0.299-0.499) and major allele frequency (MAF, range 0.524-0.817). The CAPS markers can be used to evaluate genetic diversity and population structure and will facilitate the management of emerging genetic resources.

Phenotypic characterization of auxotrophic mutant of nontyphoidal Salmonella and determination of its cytotoxicity, tumor inhibiting cytokine gene expression in cell line models

An auxotrophic mutant of nontyphoidal Salmonella (NTS) strain (Salmonella Oslo) was phenotypically characterized in this study. The characterization was based on phenotype, morphology, motility, biofilm forming ability, growth kinetics, etc. The phenotypic results from the above experiments determined that the mutant showed variation in phenotypic characters from that of wild-type strain. Subsequently, mutant and wild-type NTS were subjected to epithelial cell invasion and intracellular replication assays. The real-time PCR analysis was also performed to analyse expression of tumor inhibiting cytokine genes and virulence genes post-bacterial infection in cell lines. The mutant showed highest invasion potential than wild-type NTS whereas the replication of mutant was slower in both the cell lines. Similar to the wild-type strain, the mutant also retained the cytotoxic potential when analysed in vitro. Furthermore, the expression of proinflammatory cytokine genes such as TNF-α and IL-1β was upsurged with the downregulation of anti-inflammatory cytokine genes like TGF-β, IL-6 and IL-10 post-infection of the mutant strain in cell lines. In addition, virulence genes of Salmonella pathogenicity island one and two of mutant were downregulated in vitro except invA in HeLa cell line. Therefore, the auxotrophic mutant showed positive attributes of a potential antitumor agent in terms of expressing tumor inhibiting cytokine genes when assessed in vitro. Though the study did not check the tumor inhibitory effect of NTS strain directly, findings of the study emphasizes on the development of a novel strain of NTS with less virulence and more immunogenic traits to inhibit tumor cells.

Next-Generation Genotoxicology: Using Modern Sequencing Technologies to Assess Somatic Mutagenesis and Cancer Risk

Mutations have a profound effect on human health, particularly through an increased risk of carcinogenesis and genetic disease. The strong correlation between mutagenesis and carcinogenesis has been a driving force behind genotoxicity research for more than 50 years. The stochastic and infrequent nature of mutagenesis makes it challenging to observe and to study. Indeed, decades have been spent developing increasingly sophisticated assays and methods to study these low-frequency genetic errors, in hopes of better predicting which chemicals may be carcinogens, understanding their mode of action, and informing guidelines to prevent undue human exposure. While effective, widely used genetic selection-based technologies have a number of limitations that have hampered major advancements in the field of genotoxicity. Emerging new tools, in the form of enhanced next-generation sequencing platforms and methods, are changing this paradigm. In this review, we discuss rapidly evolving sequencing tools and technologies, such as error-corrected sequencing and single cell analysis, which we anticipate will fundamentally reshape the field. In addition, we consider a variety emerging applications for these new technologies, including the detection of DNA adducts, inference of mutational processes based on genomic site and local sequence contexts, and evaluation of genome engineering fidelity, as well as other cutting-edge challenges for the next 50 years of environmental and molecular mutagenesis research. Environ. Mol. Mutagen. 61:135-151, 2020. © 2019 The Authors. Environmental and Molecular Mutagenesis published by Wiley Periodicals, Inc. on behalf of Environmental Mutagen Society.

Detection of Pig-a Mutant Erythrocytes in the Peripheral Blood of Rats and Mice

The endogenous X-linked phosphatidyl inositol glycan class A gene (Pig-a) can be used as a reporter of in vivo somatic cell mutation in rats and mice. Pig-a mutant cells are deficient in specific protein surface markers and can be identified and quantified by immunofluorescent staining followed by high-throughput flow cytometry. Pig-a mutation detection is commonly performed with red blood cells (RBCs) because: (1) the low volumes of blood required for determining mutant frequencies in RBCs allow multiple samplings on small laboratory animals over extended periods of time; (2) the execution of the RBC assay is easy and the interpretation of the results is straightforward; and (3) RBC Pig-a mutant frequencies are known within hours of sample collection. Two endpoints are determined in the assay: the frequency of mutant total RBCs and the frequency of mutant reticulocytes. When Pig-a mutation is used to assess the in vivo mutagenic potential of suspect hazards, the frequency of mutant reticulocytes is an early indicator of mutagenic potential, while the mutant frequency in total RBCs can be measured more rapidly and with greater precision.

Topological Transformation-Based Nanobarcoding for Detection and Enumeration of MicroRNAs and Single Nucleotide Polymorphism

RNA biomarkers have been recently reported to be associated tightly with the diagnosis and prognosis of many diseases. Particularly, cancers considered to be a serious threat to primates are known to be vastly dominated by genetic networks where RNA plays a key role. RNAs are thus recognized as a major target group that can be used for numerous cancer treatments and it is still required to identify and enumerate them in an effective manner. Here, a new topological transformation-based nanobarcoding technique (TNT) is first reported using fluorescence-DNA barcodes engaged with graphene oxide (GO_x ) for effectively discriminating short RNAs such as miRNAs and their single nucleotide polyporphisms in tissue and plasma. Through topological transformation into 3D DNA-RNA polygonal structures, various kinds of microRNAs have been read at the same time and analyzed quantitatively. Also, it positively discerned epidermal growth factor receptor (EGFR) mutations known as single base variations of typical lung cancer specific RNAs. A single variant of 0.785% in target EGFR mutations is explicitly detected. It is speculated that the TNT may be a versatile method for polymerase chain reaction (PCR)-free practical diagnosis of several clinical genetic deviations such as significant biotic RNA and genic fragments and would be a promising alternative to conventional PCR terrains.

MTHFR C677T polymorphism analysis: A simple, effective restriction enzyme-based method improving previous protocols

Background

5,10‐Methylentetrahydrofolate reductase (MTHFR) C677T polymorphism is one of the most studied genetic variations in the human genome. Polymerase chain reaction‐restriction fragment length polymorphism (PCR‐RFLP) is one of the most used techniques to characterize the point mutations in genomic sequences because of its suitability and low cost. The most widely used method for the MTHFR C677T polymorphism characterization was developed by Frosst et al. (1995) but appears to have some technical limitations. The aim of this study was to propose a novel PCR‐RFLP method for the detection of this polymorphism.

Methods

In order to retrieve all published articles possibly describing any PCR‐RFLP methods useful to analyze MTHFR C677T polymorphism, we performed systematic queries on PubMed, using a combination of Boolean operators (AND/OR) and MeSH terms. Amplify software was used in order to design a new primer pair following the optimal standard criteria. Primer‐BLAST software was used to check primer pair's biological specificity.

Results

The analysis of previous literature showed that PCR‐RFLP method remains the most used technique. None of the 108 primer pairs described was ideal with regard to main accepted primer pair biochemical technical parameters. The new primer pair amplifies a DNA‐fragment of 513 base pair (bp) that, in the presence of the polymorphism, is cut by Hinf I enzyme in two pieces of 146 bp and 367 bp and clearly visible on 2% agarose gel. The level of expertise and the materials required are minimal and the protocol takes one day to carry out.

Conclusion

Our original PCR‐RFLP strategy, specifically designed to make the analysis optimal with respect to PCR primers and gel analysis, fits the ideal criteria compared to the widely used strategy by Frosst et al (1995) as well as any other PCR‐RFLP strategies proposed for MTHFR C677T polymorphism genotyping to date.

Enhancing the accuracy of next-generation sequencing for detecting rare and subclonal mutations

Mutations, the fuel of evolution, are first manifested as rare DNA changes within a population of cells. Although next-generation sequencing (NGS) technologies have revolutionized the study of genomic variation between species and individual organisms, most have limited ability to accurately detect and quantify rare variants among the different genome copies in heterogeneous mixtures of cells or molecules. We describe the technical challenges in characterizing subclonal variants using conventional NGS protocols and the recent development of error correction strategies, both computational and experimental, including consensus sequencing of single DNA molecules. We also highlight major applications for low-frequency mutation detection in science and medicine, describe emerging methodologies and provide our vision for the future of DNA sequencing.

Wild-type Blocking PCR Combined with Direct Sequencing as a Highly Sensitive Method for Detection of Low-Frequency Somatic Mutations

Accurate detection and identification of low frequency mutations can be problematic when assessing residual disease after therapy, screening for emerging resistance mutations during therapy, or when patients have few circulating tumor cells. Wild-type blocking PCR followed by sequencing analysis offers high sensitivity, flexibility, and simplicity as a methodology for detecting these low frequency mutations. By adding a custom designed locked nucleic acid oligonucleotide to a new or previously established conventional PCR based sequencing assay, sensitivities of approximately 1 mutant allele in a background of 1,000 WT alleles can be achieved (1:1,000). Sequencing artifacts associated with deamination events commonly found in formalin fixed paraffin embedded tissues can be partially remedied by the use of uracil DNA glycosylase during extraction steps. The optimized protocol here is specific for detecting MYD88 mutation, but can serve as a template to design any WTB-PCR assay. Advantages of the WTB-PCR assay over other commonly utilized assays for the detection of low frequency mutations including allele specific PCR and real-time quantitative PCR include fewer occurrences of false positives, greater flexibility and ease of implementation, and the ability to detect both known and unknown mutations.

RAS testing in metastatic colorectal cancer: advances in Europe

Personalized medicine shows promise for maximizing efficacy and minimizing toxicity of anti-cancer treatment. KRAS exon 2 mutations are predictive of resistance to epidermal growth factor receptor-directed monoclonal antibodies in patients with metastatic colorectal cancer. Recent studies have shown that broader RAS testing (KRAS and NRAS) is needed to select patients for treatment. While Sanger sequencing is still used, approaches based on various methodologies are available. Few CE-approved kits, however, detect the full spectrum of RAS mutations. More recently, "next-generation" sequencing has been developed for research use, including parallel semiconductor sequencing and reversible termination. These techniques have high technical sensitivities for detecting mutations, although the ideal threshold is currently unknown. Finally, liquid biopsy has the potential to become an additional tool to assess tumor-derived DNA. For accurate and timely RAS testing, appropriate sampling and prompt delivery of material is critical. Processes to ensure efficient turnaround from sample request to RAS evaluation must be implemented so that patients receive the most appropriate treatment. Given the variety of methodologies, external quality assurance programs are important to ensure a high standard of RAS testing. Here, we review technical and practical aspects of RAS testing for pathologists working with metastatic colorectal cancer tumor samples. The extension of markers from KRAS to RAS testing is the new paradigm for biomarker testing in colorectal cancer.

Liquid biopsy for detection of actionable oncogenic mutations in human cancers and electric field induced release and measurement liquid biopsy (eLB)

Oncogenic activations by mutations in key cancer genes such as EGFR and KRAS are frequently associated with human cancers. Molecular targeting of specific oncogenic mutations in human cancer is a major therapeutic inroad for anti-cancer drug therapy. In addition, progressive developments of oncogene mutations lead to drug resistance. Therefore, the ability to detect and continuously monitor key actionable oncogenic mutations is important to guide the use of targeted molecular therapies to improve long-term clinical outcomes in cancer patients. Current oncogenic mutation detection is based on direct sampling of cancer tissue by surgical resection or biopsy. Oncogenic mutations were recently shown to be detectable in circulating bodily fluids of cancer patients. This field of investigation, termed liquid biopsy, permits a less invasive means of assessing the oncogenic mutation profile of a patient. This paper will review the analytical strategies used to assess oncogenic mutations from biofluid samples. Clinical applications will also be discussed.

Microspore Induced Doubled Haploids Production from Ethyl Methanesulfonate (EMS) Soaked Flower Buds Is an Efficient Strategy for Mutagenesis in Chinese Cabbage

Chinese cabbage buds were soaked with Ethyl methanesulfonate (EMS) to induce mutagenesis. The influence of different EMS concentrations and treatment durations on microspore development, embryo production rate and seedling rate were evaluated in five Chinese cabbage genotypes. Mutations in four color-related genes were identified using high resolution melting (HRM) curves of their PCR products. The greatest embryo production and seedling rates were observed when buds were treated with 0.03 to 0.1% EMS for 5 to 10 min, while EMS concentrations greater than 0.1% were lethal to the microspores. In total, 142 mutants with distinct variations in leaf shape, leaf color, corolla size, flower color, bolting time and downy mildew resistance were identified from 475 microspore culture derived Doubled Haploids. Our results demonstrate that microspore derived Doubled Haploids from EMS soaked buds represents an efficient approach to rapidly generate homozygous Chinese cabbage mutants.

Molecular analysis of ciprofloxacin resistance mechanisms in Malaysian ESBL-producing Klebsiella pneumoniae isolates and development of mismatch amplification mutation assays (MAMA) for rapid detection of gyrA and parC mutations

Ninety-three Malaysian extended-spectrum β-lactamase (ESBL)-producing Klebsiella pneumoniae isolates were investigated for ciprofloxacin resistance. Two mismatch amplification mutation (MAMA) assays were developed and used to facilitate rapid detection of gyrA and parC mutations. The isolates were also screened for plasmid-mediated quinolone resistance (PMQR) genes including aac(6′)-Ib-cr, qepA, and qnr. Ciprofloxacin resistance (MICs 4– ≥ 32 μg/mL) was noted in 34 (37%) isolates, of which 33 isolates had multiple mutations either in gyrA alone (n = 1) or in both gyrA and parC regions (n = 32). aac(6′)-Ib-cr was the most common PMQR gene detected in this study (n = 61), followed by qnrB and qnrS (n = 55 and 1, resp.). Low-level ciprofloxacin resistance (MICs 1-2 μg/mL) was noted in 40 (43%) isolates carrying qnrB accompanied by either aac(6′)-Ib-cr (n = 34) or a single gyrA 83 mutation (n = 6). Ciprofloxacin resistance was significantly associated with the presence of multiple mutations in gyrA and parC regions. While the isolates harbouring gyrA and/or parC alteration were distributed into 11 PFGE clusters, no specific clusters were associated with isolates carrying PMQR genes. The high prevalence of ciprofloxacin resistance amongst the Malaysian ESBL-producing K. pneumoniae isolates suggests the need for more effective infection control measures to limit the spread of these resistant organisms in the hospital.

Detection of Pig-a mutant erythrocytes in the peripheral blood of rats and mice

The endogenous X-linked phosphatidyl inositol glycan class A gene (Pig-a) can be used as a reporter of in vivo somatic cell mutation in rats and mice. Pig-a mutant cells are deficient in specific protein surface markers and can be identified and quantified by immunofluorescent staining followed by high-throughput flow cytometry. Pig-a mutation detection is commonly performed with red blood cells (RBCs) because (1) the low volumes of blood required for determining mutant frequencies in RBCs allow multiple samplings on small laboratory animals over extended periods of time; (2) the execution of the RBC assay is easy and the interpretation of the results is straightforward; and (3) RBC Pig-a mutant frequencies are known within hours of sample collection. Two endpoints are determined in the assay: the frequency of mutant total RBCs and the frequency of mutant reticulocytes. When Pig-a mutation is used to assess the in vivo mutagenic potential of suspect hazards, the frequency of mutant reticulocytes is an early indicator of mutagenic potential, while the mutant frequency in total RBCs can be measured more rapidly and with greater precision.

RANTES gene G-403A polymorphism and coronary artery disease: a meta analysis of observational studies

Objective

The G-403A polymorphism in RANTES gene may be involved in the development of coronary artery disease (CAD) through increasing RANTES-mediated leukocyte trafficking and activation. However, studies investigating the relationship between G-403A polymorphism and CAD yielded contradictory and inconclusive results. In order to shed some light on these inconsistent findings, a meta analysis was performed to clarify the role of G-403A polymorphism of RANTES gene in the susceptibility of CAD.

Methods

A systemic literature search of PubMed and EMBASE was conducted from their inception to March 23, 2012, to retrieve related studies. In addition, Conference Proceedings Citation Index-Science was searched, authors of relevant studies were contacted, and reference lists of the included studies and their related citations in PubMed were reviewed for additional pertinent studies.

Results

A total of 8 eligible studies were identified, with a total of 4252 CAD cases and 2150 controls. There was no evidence of significant association between G-403A polymorphism and CAD risk in any genetic model or pairwise comparisons (additive model: OR = 1.046, 95% CI = 0.883–1.239, I² = 65.9%; recessive model: OR = 1.140, 95% CI = 0.774–1.678, I² = 53.1%; dominant model: OR = 1.000, 95% CI = 0.820–1.21), I² = 62.6%; AA vs GG: OR = 1.141, 95% CI = 0.734–1.773, I² = 61.2%; GA vs GG: OR = 0.993, 95% CI = 0.800–1.232, I² = 64.6%). Subgroup analysis and meta regression indicated that ethnicity and genotyping method accounted for the significant heterogeneity among studies. In the stratified analysis by ethnic group, G-403A polymorphism was found to be associated with increased CAD risk in Caucasian population whereas its protective role was observed in Asian population in some but not all comparisons.

Conclusion

Data from the current meta-analysis do not support the existence of a relationship between G-403A polymorphism and the development of CAD, and large sample size study employing unified genotyping method is needed to further evaluate the influence of G-403A polymorphism on susceptibility of CAD.

EGFR tyrosine kinase mutation testing in the treatment of non-small-cell lung cancer

BACKGROUND

Non-small-cell lung cancer (nsclc) tumours with activating mutations of the epidermal growth factor receptor (efgr) tyrosine kinase are highly sensitized to the effects of oral tyrosine kinase inhibitors such as gefitinib and erlotinib, suggesting the possibility of targeted treatment of nsclc based on EFGR mutation status. However, no standardized method exists for assessing the EGFR mutation status of tumours. Also, it is not known if available methods are feasible for routine screening. To address that question, we conducted a validation study of methods used for detecting EGFR mutations in exons 19 and 21 at molecular laboratories located in five specialized Canadian cancer centres.

METHODS

The screening methods were first optimized using cell lines harbouring the mutations in question. A validation phase using anonymized patient samples followed.

RESULTS

The methods used at the sites were highly specific and sensitive in detecting both mutations in cell-line dna (specificity of 100% and sensitivity of at least 1% across all centres). In the validation phase, we observed excellent concordance between the laboratories for detecting mutations in the patient samples. Concordant results were obtained in 26 of 30 samples (approximately 87%). In general, the samples for which results were discordant were also less optimal, containing small amounts of tumour.

CONCLUSIONS

Our results suggest that currently available methods are capable of reliably detecting exon 19 and exon 21 mutations of EFGR in tumour samples (provided that sufficient tumour material is available) and that routine screening for those mutations is feasible in clinical practice.

High Resolution Melt (HRM) analysis is an efficient tool to genotype EMS mutants in complex crop genomes

BACKGROUND

Targeted Induced Loci Lesions IN Genomes (TILLING) is increasingly being used to generate and identify mutations in target genes of crop genomes. TILLING populations of several thousand lines have been generated in a number of crop species including Brassica rapa. Genetic analysis of mutants identified by TILLING requires an efficient, high-throughput and cost effective genotyping method to track the mutations through numerous generations. High resolution melt (HRM) analysis has been used in a number of systems to identify single nucleotide polymorphisms (SNPs) and insertion/deletions (IN/DELs) enabling the genotyping of different types of samples. HRM is ideally suited to high-throughput genotyping of multiple TILLING mutants in complex crop genomes. To date it has been used to identify mutants and genotype single mutations. The aim of this study was to determine if HRM can facilitate downstream analysis of multiple mutant lines identified by TILLING in order to characterise allelic series of EMS induced mutations in target genes across a number of generations in complex crop genomes.

RESULTS

We demonstrate that HRM can be used to genotype allelic series of mutations in two genes, BraA.CAX1a and BraA.MET1.a in Brassica rapa. We analysed 12 mutations in BraA.CAX1.a and five in BraA.MET1.a over two generations including a back-cross to the wild-type. Using a commercially available HRM kit and the Lightscanner™ system we were able to detect mutations in heterozygous and homozygous states for both genes.

CONCLUSIONS

Using HRM genotyping on TILLING derived mutants, it is possible to generate an allelic series of mutations within multiple target genes rapidly. Lines suitable for phenotypic analysis can be isolated approximately 8-9 months (3 generations) from receiving M3 seed of Brassica rapa from the RevGenUK TILLING service.

PCR enrichment techniques to identify the diet of predators

The increasing sensitivity of PCR has meant that in the last two decades PCR has emerged as a major tool in diet studies, enabling us to refine our understanding of trophic links and to elucidate the diets of predators whose prey is as yet uncharacterized. The achievements and methods of PCR-based diet studies have been reviewed several times, but here we review an important development in the field: the use of PCR enrichment techniques to promote the amplification of prey DNA over that of the predator. We first discuss the success of using group-specific primers either in parallel single reactions or in multiplex reactions. We then concentrate on the more recent use of PCR enrichment techniques such as restriction enzyme digests, peptide nucleic acid clamping, DNA blocking and laser capture microdissection. We also survey the vast literature on enrichment techniques in clinical biology, to ascertain the pitfalls of enrichment techniques and what refinements have yielded some highly sensitive methods. We find that while there are several new approaches to enrichment, peptide nucleic acid clamping and DNA blocking are generally sufficient techniques for the characterization of diets of predators and highlight the most important considerations of the approach.

Competitive amplification of differentially melting amplicons (CADMA) enables sensitive and direct detection of all mutation types by high-resolution melting analysis

Sensitive and specific mutation detection is of particular importance in cancer diagnostics, prognostics, and individualized patient treatment. However, the majority of molecular methodologies that have been developed with the aim of increasing the sensitivity of mutation testing have drawbacks in terms of specificity, convenience, or costs. Here, we have established a new method, Competitive Amplification of Differentially Melting Amplicons (CADMA), which allows very sensitive and specific detection of all mutation types. The principle of the method is to amplify wild-type and mutated sequences simultaneously using a three-primer system. A mutation-specific primer is designed to introduce melting temperature decreasing mutations in the resulting mutated amplicon, while a second overlapping primer is designed to amplify both wild-type and mutated sequences. When combined with a third common primer very sensitive mutation detection becomes possible, when using high-resolution melting (HRM) as detection platform. The introduction of melting temperature decreasing mutations in the mutated amplicon also allows for further mutation enrichment by fast coamplification at lower denaturation temperature PCR (COLD-PCR). For proof-of-concept, we have designed CADMA assays for clinically relevant BRAF, EGFR, KRAS, and PIK3CA mutations, which are sensitive to, between 0.025% and 0.25%, mutated alleles in a wild-type background. In conclusion, CADMA enables highly sensitive and specific mutation detection by HRM analysis.

Ice-COLD-PCR enables rapid amplification and robust enrichment for low-abundance unknown DNA mutations

Identifying low-abundance mutations within wild-type DNA is important in several fields of medicine, including cancer, prenatal diagnosis and infectious diseases. However, utilizing the clinical and diagnostic potential of rare mutations is limited by sensitivity of the molecular techniques employed, especially when the type and position of mutations are unknown. We have developed a novel platform that incorporates a synthetic reference sequence within a polymerase chain reaction (PCR) reaction, designed to enhance amplification of unknown mutant sequences during COLD-PCR (CO-amplification at Lower Denaturation temperature). This new platform enables an Improved and Complete Enrichment (ice-COLD-PCR) for all mutation types and eliminates shortcomings of previous formats of COLD-PCR. We evaluated ice-COLD-PCR enrichment in regions of TP53 in serially diluted mutant and wild-type DNA mixtures. Conventional-PCR, COLD-PCR and ice-COLD-PCR amplicons were run in parallel and sequenced to determine final mutation abundance for a range of mutations representing all possible single base changes. Amplification by ice-COLD-PCR enriched all mutation types and allowed identification of mutation abundances down to 1%, and 0.1% by Sanger sequencing or pyrosequencing, respectively, surpassing the capabilities of other forms of PCR. Ice-COLD-PCR will help elucidate the clinical significance of low-abundance mutations and our understanding of cancer origin, evolution, recurrence-risk and treatment diagnostics.

Oncomutations as biomarkers of cancer risk

Cancer risk assessment impacts a range of societal needs, from the regulation of chemicals to achieving the best possible human health outcomes. Because oncogene and tumor suppressor gene mutations are necessary for the development of cancer, such mutations are ideal biomarkers to use in cancer risk assessment. Consequently, DNA-based methods to quantify particular tumor-associated hotspot point mutations (i.e., oncomutations) have been developed, including allele-specific competitive blocker-PCR (ACB-PCR). Several studies using ACB-PCR and model mutagens have demonstrated that significant induction of tumor-associated oncomutations are measureable at earlier time points than are used to score tumors in a bioassay. In the particular case of benzo[a]pyrene induction of K-Ras codon 12 TGT mutation in the A/J mouse lung, measurement of tumor-associated oncomutation was shown to be an earlier and more sensitive endpoint than tumor response. The measurement of oncomutation by ACB-PCR led to two unexpected findings. First, oncomutations are present in various tissues of control rodents and "normal" human colonic mucosa samples at relatively high frequencies. Approximately 60% of such samples (88/146) have mutant fractions (MFs) >10(-5), and some have MFs as high as 10(-3) or 10(-4). Second, preliminary data indicate that oncomutations are present frequently as subpopulations in tumors. These findings are integrated into a hypothesis that the predominant preexisting mutations in particular tissues may be useful as generic reporters of carcinogenesis. Future research opportunities using oncomutation as an endpoint are described, including rodent to human extrapolation, dose-response assessment, and personalized medicine.

Generation, function, and prognostic utility of somatic mitochondrial DNA mutations in cancer

Exciting new studies are increasingly strengthening the link between mitochondrial mutagenesis and tumor progression. Here we provide a comprehensive review and meta-analysis of studies reporting on mitochondrial DNA mutations in common human cancers. We discuss possible mechanisms by which mitochondrial DNA mutations may influence carcinogenesis, outline important caveats for interpreting the detected mutations--particularly differentiating causality from association--and suggest how new mutational assays may help resolve fundamental controversies in the field and delineate the origin and expansion of neoplastic cell lineages. Finally, we discuss the potential clinical utility of mtDNA mutations for improving the sensitivity of early cancer diagnosis, rapidly detecting cancer recurrence, and predicting the disease outcome.

Detection of low-level KRAS mutations using PNA-mediated asymmetric PCR clamping and melting curve analysis with unlabeled probes

Detection of somatic mutations in clinical cancer specimens is often hampered by excess wild-type DNA. The aim of this study was to develop a simple and economical protocol without using fluorescent probes to detect low-level mutations. In this study, we combined peptide nucleic acid (PNA)-clamping PCR with asymmetric primers and a melting curve analysis using an unlabeled detection probe. PNA-clamping PCR, which suppressed amplification of the wild-type allele, was more sensitive for KRAS codon 12 mutation detection than nonclamping PCR in 5 different mutant cell lines. Three detection probes were tested (a perfectly matched antisense, a mismatched antisense, and a mismatched sense), and the mismatched sense detection probe showed the highest sensitivity (0.1% mutant detection) under clamping conditions. With this probe, we were able to detect not only the perfectly matched KRAS mutation, but also 4 other mismatched mutations of KRAS. We then applied this protocol to 10 human colon cancer tissues with KRAS codon 12 mutations, successfully detecting the mutations in all of them. Our data indicate that the combination of perfectly matched antisense PNA and a mismatched sense detection probe can detect KRAS mutations with a high sensitivity in both cell lines and human tissues. Moreover, this study might prove an easily applicable protocol for the detection of low-level mutations in other cancer genes.

A consensus linkage map identifies genomic regions controlling fruit maturity and beta-carotene-associated flesh color in melon (Cucumis melo L.)

The nutritional value and yield potential of US Western Shipping melon (USWS; Cucumis melo L.) could be improved through the introgression of genes for early fruit maturity (FM) and the enhancement of the quantity of beta-carotene (QbetaC) in fruit mesocarp (i.e., flesh color). Therefore, a set of 116 F(3) families derived from the monoecious, early FM Chinese line 'Q 3-2-2' (no beta-carotene, white mesocarp) and the andromonoecious, late FM USWS line 'Top Mark' (possessing beta-carotene, orange mesocarp) were examined during 2 years in Wisconsin, USA to identify quantitative trait loci (QTL) associated with FM and QbetaC. A 171-point F(2-3) based map was constructed and used for QTL analysis. Three QTL associated with QbetaC were detected, which explained a significant portion of the observed phenotypic variation (flesh color; R (2) = 4.0-50.0%). The map position of one QTL (beta-carM.E.9.1) was uniformly aligned with one carotenoid-related gene (Orange gene), suggesting its likely role in QbetaC in this melon population and putative relationship with the melon white flesh (wf) gene. Two major (FM.6.1 and FM.11.1; R (2) >or= 20%) and one minor QTL (FM.2.1; R (2) = 8%) were found to be associated with FM. This map was then merged with a previous recombinant inbred line (RIL)-based map used to identify seven QTL associated with QbetaC in melon fruit. This consensus map [300 molecular markers (187 co-dominant melon and 14 interspecific; 10 LG)] provides a framework for the further dissection and cloning of published QTL, which will consequently lead to more effective trait introgression in melon.

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.

Quantification of random mutations in the mitochondrial genome

Mitochondrial DNA (mtDNA) mutations contribute to the pathology of a number of age-related disorders, including Parkinson disease [A. Bender et al., Nat. Genet. 38 (2006) 515,Y. Kraytsberg et al., Nat. Genet. 38 (2006) 518], muscle-wasting [J. Wanagat, Z. Cao, P. Pathare, J.M. Aiken, FASEB J. 15 (2001) 322], and the metastatic potential of cancers [K. Ishikawa et al., Science 320 (2008) 661]. The impact of mitochondrial DNA mutations on a wide variety of human diseases has made it increasingly important to understand the mechanisms that drive mitochondrial mutagenesis. In order to provide new insight into the etiology and natural history of mtDNA mutations, we have developed an assay that can detect mitochondrial mutations in a variety of tissues and experimental settings [M. Vermulst et al., Nat. Genet. 40 (2008) 4, M. Vermulst et al., Nat. Genet. 39 (2007) 540]. This methodology, termed the Random Mutation Capture assay, relies on single-molecule amplification to detect rare mutations among millions of wild-type bases [J.H. Bielas, L.A. Loeb, Nat. Methods 2 (2005) 285], and can be used to analyze mitochondrial mutagenesis to a single base pair level in mammals.

Effect of chronic azathioprine treatment on germ-line transmission of Hprt mutation in mice

Azathioprine (Aza), a prodrug of 6-mercaptopurine, is used in human medicine to prevent transplant rejection and for the treatment of autoimmune diseases. Extremely high HPRT lymphocyte mutant frequencies (MFs) are found in humans and mice chronically treated with Aza, and these elevated MFs appear to be caused by selection and amplification of pre-existing HPRT mutant lymphocytes. In the present study, we investigated if in vivo selection by Aza also promotes the germ-line transmission of Hprt mutants. Fifty-five male C57BL/6 mice were treated with 10 mg/kg Aza three times/week for 24 weeks; 10 control mice were treated with the vehicle. Each of these males then was bred to unexposed females for a total of 8 weeks. Analysis of the Aza-treated males after the breeding period indicated that 12 had highly elevated Hprt lymphocyte MFs (1 x 10(-4)-2.5 x 10(-1) vs. normal MFs of <1 x 10(-5)), indicating that the Aza treatment successfully selected somatic cell mutants. The female offspring from the breeding were sacrificed at 28 days of age and Hprt MFs were measured in spleen lymphocytes. Most of the 364 female offspring (332 from Aza-treated fathers) had Hprt MFs of 0-6 x 10(-6), but seven of the offspring had moderately elevated MFs of 16 x 10(-6)-55 x 10(-6). Since one of these mice was fathered by a control male, these relatively high MFs appear to be part of the normal variation in lymphocyte Hprt MF. The present results provide no evidence that long-term Aza treatment promotes high levels of germ-line Hprt mutation transmission in mice.

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.

Detection of ultrarare somatic mutation in the human TP53 gene by bidirectional pyrophosphorolysis-activated polymerization allele-specific amplification

The detection of ultra-rare mutation in the presence of excess amounts of normal genomic DNA is highly advantageous in a number of circumstances, including: 1) identification of minimal residual disease for improved cancer chemotherapy; 2) measurement of mutation load to assess environmental mutagen exposure or endogenous DNA repair; and 3) prenatal diagnosis of paternally-derived mutations within fetal cells in the maternal circulation. Bidirectional pyrophosphorolysis activated polymerization allele-specific amplification (Bi-PAP-A) utilizes two opposing 3'-terminal blocked oligonucleotides (P(*)s) with 1 nucleotide overlap at their 3' termini. The selectivity of Bi-PAP-A derives from the serial coupling of pyrophosphorolysis and DNA polymerization. A total of 13 Bi-PAP-A assays were developed and validated for the human p53 gene (TP53). The sensitivity and specificity of each assay were determined with mutated and wild-type DNA templates, respectively. Bi-PAP-A has a sensitivity of one molecule for most assays and a selectivity (sensitivity:specificity) greater than 1:10(7)-1:10(9) for four of all six mutation types. Four assays with high selectivity were used to detect rare somatic mutations in blood white cells. The silent g.13147C>G (p.R156) mutation was present at an estimated frequency of 1.1 x 10(-7). The g.14523A>T (p.E285V), g.14487G>C (p.R273P), and g.14060G>C (p.G245R) mutations were undetectable with frequencies less than 2.0 x 10(-8). We conclude that Bi-PAP-A is a general and rapid method for detecting ultra-rare mutations.

Use of genotypic selection to detect P53 codon 273 CGT>CTT transversion: application to an occupationally exposed population

CGT>CTT transversion in codon 273 of the P53 tumor-suppressor gene is one of the major mutations detected in human tumors. Within an epidemiological framework, we investigated the use of a genotypic selection method to measure this point mutation. The allele-specific polymerase chain reaction (AS-PCR) that was developed was able to detect 10 mutant copies of the gene among a total of 5 x 10(5) wild-type copies. We used this assay to detect CGT>CTT transversions in buccal cell DNA of production workers (n=76) from a viscose factory exposed to carbon disulfide (amongst other pollutants) and in the DNA of non-exposed office workers (n=67). The mutation appeared more frequently in the exposed than in the non-exposed worker who were smokers. The results of the study indicate that occupational exposure results in a significant increase in P53 CGT>CTT transversions and more especially identified occupational exposure in combination with smoking as a significant risk factor for the mutation. We conclude that AS-PCR of the P53 273rd codon transversions is a suitable technique for studying the effects of occupational exposure.

Direct transduction of allele-specific primer extension into electrical signal using genetic field effect transistor

We have developed a genetic field effect transistor (FET) for single nucleotide polymorphism (SNP) genotyping, which is based on potentiometric detection of molecular recognition on the gate insulator. Here, we report direct transduction of allele-specific primer extension on the gate surface into electrical signal using the genetic FETs. This method is based on detection of intrinsic negative charges of polynucleotide synthesized by DNA polymerase. The charge density change at the gate surface could be monitored during primer extension reaction. Moreover, three different genotypes could be successfully distinguished without any labeling for target DNA by the use of the genetic FET in combination with allele-specific primer extension. The platform based on the genetic FETs is suitable for a simple, accurate and inexpensive system for SNP genotyping in clinical diagnostics.

Distributions of five common point mutants in the human tracheal-bronchial epithelium

The mutations C742T, G746T, G747T in the TP53 gene and G35T in the KRAS gene have been repeatedly found in sectors of human tumors by direct DNA sequencing. The mutation G508A in the HPRT1 gene has been repeatedly found among peripheral T lymphocytes by clonal expansion under selective conditions. To discover if these mutations also occur frequently in normal tissues from which tumors arise, we have developed and validated allele-specific mismatch amplification mutation assays (MAMA) for each mutation. Reconstruction experiments demonstrated linearity in the range of 9-3000 mutant alleles among 3 x 10(6) wild-type alleles. The cumulative distributions of all negative controls established robust detection limits (P<0.05) of 34-125 mutants per 10(6) copies assayed depending on the mutation. One hundred and seventy-seven micro-anatomical samples of approximately (0.5-6)x10(6) tracheal-bronchial epithelial cells from nine non-smokers were assayed representing en toto the equivalent of approximately 1.6 human bronchial trees to the fifth bifurcation. Statistically significant mutant copy numbers were found in 257 of 463 assays. Clusters of mutant copies ranged from 10 to 1000 in 239/257 positive samples. As all five point mutations were detected at mutant fractions of >10(-5) in two or more lungs, we infer that they are mutational hotspots generated in lung epithelial stem cells. As the cancer-associated mutations did not differ in cluster size distribution from the HPRT1 mutation, we infer that none of the mutations conferred a growth advantage to somatic heterozygous clusters or maintenance turnover units. Specific mutants appeared in very large copy numbers, 1000-35,000, in 18/257 positive assays. Various hypotheses to account for the observed cluster size distributions are offered.

Wild-type blocking polymerase chain reaction for detection of single nucleotide minority mutations from clinical specimens

Detection and sequencing of mutations from clinical specimens is often complicated by the presence of an excess of nonmutated cells. To facilitate the detection and sequencing of minority mutations from clinical specimens, we developed wild-type blocking polymerase chain reaction (WTB-PCR). This technique allows sensitive detection of minority mutations in a tissue sample containing excess wild-type DNA. In WTB-PCR, a nonextendable locked nucleic acid (LNA) oligonucleotide binds tightly to a region of wild-type DNA known to develop point mutations. This LNA sequence blocks amplification of wild-type DNA during PCR while permitting amplification of mutant exon 15. Our results show that the LNA blocking oligonucleotide inhibits amplification of wild-type DNA in a dose-dependent manner. WTB-PCR was able to detect mutant DNA in clinical samples of melanoma tissue containing an excess of nonmelanoma cells. This method was also able to detect small amounts of point mutated or tandem mutated DNA diluted with a much larger concentration of wild-type DNA. This rapid and simple assay overcomes the limitations of current methods to detect minority mutations. The potential applications of WTB-PCR include early diagnosis and prognosis of various cancers.

Levels of H-ras codon 61 CAA to AAA mutation: response to 4-ABP-treatment and Pms2-deficiency

DNA mismatch repair (MMR) deficiencies result in increased frequencies of spontaneous mutation and tumor formation. In the present study, we tested the hypothesis that a chemically-induced mutational response would be greater in a mouse with an MMR-deficiency than in the MMR-proficient mouse models commonly used to assay for chemical carcinogenicity. To accomplish this, the induction of H-ras codon 61 CAA-->AAA mutation was examined in Pms2 knockout mice (Pms2-/-, C57BL/6 background) and sibling wild-type mice (Pms2+/+). Groups of five or six neonatal male mice were treated with 0.3 micromol 4-aminobiphenyl (4-ABP) or the vehicle control, dimethylsulfoxide. Eight months after treatment, liver DNAs were isolated and analysed for levels of H-ras codon 61 CAA-->AAA mutation using allele-specific competitive blocker-PCR. In Pms2-proficient and Pms2-deficient mice, 4-ABP treatment caused an increase in mutant fraction (MF) from 1.65x10(-5) to 2.91x10(-5) and from 3.40x10(-5) to 4.70x10(-5), respectively. Pooling data from 4-ABP-treated and control mice, the approximately 2-fold increase in MF observed in Pms2-deficient as compared with Pms2-proficient mice was statistically significant (P=0.0207) and consistent with what has been reported previously in terms of induction of G:C-->T:A mutation in a Pms2-deficient background. Pooling data from both genotypes, the increase in H-ras MF in 4-ABP-treated mice, as compared with control mice, did not reach the 95% confidence level of statistical significance (P=0.0606). The 4-ABP treatment caused a 1.76-fold and 1.38-fold increase in average H-ras MF in Pms2-proficient and Pms2-deficient mice, respectively. Furthermore, the levels of induced mutation in Pms2-proficient and Pms2-deficient mice were nearly identical (1.26x10(-5) and 1.30x10(-5), respectively). We conclude that Pms2-deficiency does not result in an amplification of the H-ras codon 61 CAA-->AAA mutational response induced by 4-ABP.

Detailed review of transgenic rodent mutation assays

Induced chromosomal and gene mutations play a role in carcinogenesis and may be involved in the production of birth defects and other disease conditions. While it is widely accepted that in vivo mutation assays are more relevant to the human condition than are in vitro assays, our ability to evaluate mutagenesis in vivo in a broad range of tissues has historically been quite limited. The development of transgenic rodent (TGR) mutation models has given us the ability to detect, quantify, and sequence mutations in a range of somatic and germ cells. This document provides a comprehensive review of the TGR mutation assay literature and assesses the potential use of these assays in a regulatory context. The information is arranged as follows. (1) TGR mutagenicity models and their use for the analysis of gene and chromosomal mutation are fully described. (2) The principles underlying current OECD tests for the assessment of genotoxicity in vitro and in vivo, and also nontransgenic assays available for assessment of gene mutation, are described. (3) All available information pertaining to the conduct of TGR assays and important parameters of assay performance have been tabulated and analyzed. (4) The performance of TGR assays, both in isolation and as part of a battery of in vitro and in vivo short-term genotoxicity tests, in predicting carcinogenicity is described. (5) Recommendations are made regarding the experimental parameters for TGR assays, and the use of TGR assays in a regulatory context.

Quantifying levels of p53 mutation in mouse skin tumors

Allele-specific competitive blocker PCR (ACB-PCR) amplification and quantification was developed for mouse p53 codon 270 CGT-->TGT base substitution and codon 244/245 AAC/CGC-->AAT/TGC tandem mutation. PCR products corresponding to p53 mutant and wild-type DNA sequences were generated. These DNAs were mixed in known proportions to construct samples with defined mutant fractions and the allele-specific detection of each mutation was systematically optimized. Each assay was used to analyze eight simulated solar light (SSL)-induced tumors. By analyzing mutant fraction (MF) standards in parallel with PCR products generated from tumor samples, p53 mutants could be quantified as subpopulations within the tumors. All eight tumors contained detectable levels of p53 codon 270 CGT-->TGT mutation. Three tumors had p53 MFs between 10(-4) and 10(-3). Five tumors had p53 MFs between 10(-3) and 10(-2). None of the eight mouse skin tumors had measurable levels of p53 codon 244/245 tandem mutation. Frequent detection of p53 codon 270 CGT-->TGT mutation provides additional evidence that a pyrimidine dinucleotide overlapping a methylated CpG site (Pyr(me)CG) is a susceptible target for SSL-induced mutagenesis. The absence of p53 codon 244/245 mutation in tumors may be explained by its mutant p53 phenotype and/or indicate that this site is not methylated. These initial results indicate that p53 codon 270 CGT-->TGT mutation may be a sensitive biomarker for SSL- or UV-induced mutagenesis. This mutational endpoint may be useful for evaluating the co-carcinogenicity of compounds administered in combination with UV or SSL.

Levels of 4-aminobiphenyl-induced somatic H-ras mutation in mouse liver DNA correlate with potential for liver tumor development

The utility of liver H-ras codon 61 CAA to AAA mutant fraction as a biomarker of liver tumor development was investigated using neonatal male mice treated with 4-aminobiphenyl (4-ABP). Treatment with 0.1, 0.3, or 1.0 mumol 4-ABP produced dose-dependent increases in liver DNA adducts in B6C3F(1) and C57BL/6N mice. Eight months after treatment with 0.3 mumol 4-ABP or the DMSO vehicle, H-ras codon 61 CAA to AAA mutant fraction was measured in liver DNA samples (n = 12) by allele-specific competitive blocker-polymerase chain reaction (ACB-PCR). A significant increase in average mutant fraction was found in DNA of 4-ABP-treated mice, with an increase from 1.3 x 10(-5) (control) to 44.9 x 10(-5) (treated) in B6C3F(1) mice and from 1.4 x 10(-5) to 7.0 x 10(-5) in C57BL/6N mice. Compared with C57BL/6N mutant fractions, B6C3F(1) mutant fractions were more variable and included some particularly high mutant fractions, consistent with the more rapid development of liver foci expected in B6C3F(1) mouse liver. Twelve months after treatment, liver tumors developed in 79.2% of 4-ABP-treated and 22.2% of control B6C3F(1) mice; thus measurement of H-ras mutant fraction correlated with subsequent tumor development. This study demonstrates that ACB-PCR can directly measure background levels of somatic oncogene mutation and detect a carcinogen-induced increase in such mutation.

Quantification of random genomic mutations

Cancer cells contain numerous clonal mutations. It has been theorized that malignant cells sustain an elevated mutation rate and, as a consequence, harbor yet larger numbers of random point mutations. Testing this hypothesis has been precluded by lack of an assay to measure random mutations-that is, mutations that occur in only one or a few cells of a population. We have established a method that has permitted us to detect and identify rare random mutations in human cells, at a frequency of 1 per 10(8) base pairs. The assay is based on gene capture, by hybridization with a uracil-containing probe, followed by magnetic separation. Mutations that render the mutational target sequence non-cleavable by a restriction enzyme are quantified by dilution to single molecules and real-time quantitative PCR amplification. The assay can be extended to quantify mutation in any DNA-based organism, at different sites in the genome, in introns and exons, in unselected and selected genes, and in proliferating and quiescent cells.

PAP: detection of ultra rare mutations depends on P* oligonucleotides: "sleeping beauties" awakened by the kiss of pyrophosphorolysis

Pyrophosphorolysis-activated polymerization (PAP) was initially developed to enhance the specificity of allele-specific PCR for detection of known mutations in the presence of a great excess of wild-type allele. The high specificity of PAP derives from the serial coupling of activation of a 3' blocked pyrophosphorolysis-activable oligonucleotide (P(*)) with extension of the unblocked, activated P(*). In theory, PAP can detect a copy of a single base mutation present in 3x10(11) copies of the wild-type allele. In practice, the selectivity of detection is limited by polymerase extension errors, a bypass reaction, from the unblocked oligonucleotide annealed to the opposing strand. Bi-directional PAP allele-specific amplification (Bi-PAP-A) is a derivative of PAP that uses two opposing pyrophosphorolysis activable oligonucleotides (P(*)) with one nucleotide overlap at their 3' termini. This eliminates the problematic bypass reaction. The selectivity of Bi-PAP-A was examined using lambda phage DNA as a model system. Bi-PAP-A selectively detected two copies of a rare mutated allele in the presence of at least 2x10(9) copies of the wild-type lambda phage DNA. We then applied Bi-PAP-A to direct detection of spontaneous somatic mutations in the lacI transgene in BigBlue transgenic mice at a frequency as low as 3x10(-9). A 370-fold variation in the frequency of a specific somatic mutation among different mouse samples was found, implying hyper-Poisson variance and clonal expansion of mutation occurring during early development. Bi-PAP-A is a simple, rapid, and general method capable of automation and particularly suited to detection of ultra rare mutations. We also show that P(*) oligonucleotides have the novel and unexpected property of high specificity to mismatches with the template throughout lengths of the P(*). Thus, PAP also can form the basis of microarray-based scanning or resequencing methods to detect virtually all mutations.

PCR-based detection of minority point mutations

The need for detection of minority mutations (i.e., a few mutants within a high excess of wild-type alleles) arises frequently in the field of cancer and molecular genetics. Current mutation detection technologies are limited by several technical factors when it comes to the detection of minority point mutations, including generation of misincorporations by the DNA polymerase during PCR amplification. Primer ligation-mediated PCR methodologies for detection of mutations in an excess wild-type sequences are described, that can be applied for detection of both known and unknown minority point mutations. Furthermore, a new methodology is described, hairpin-PCR, which has the potential to completely eliminate PCR errors from amplified sequences, prior to minority mutation detection. Combination of these technologies can effectively tackle the problem of minority mutation detection, in order to pursue demanding applications such as identification of cancer cells at an early stage, detection of mutations in single cells, identification of minimal residual disease, or investigation of mechanisms of spontaneous mutagenesis.