Mutation detection in KRAS Exon 1 by constant denaturant capillary electrophoresis in 96 parallel capillaries

Detection and Quantification of ctDNA for Longitudinal Monitoring of Treatment in Non-Small Cell Lung Cancer Patients Using a Universal Mutant Detection Assay by Denaturing Capillary Electrophoresis

Background: Observation of anticancer therapy effect by monitoring of minimal residual disease (MRD) is becoming an important tool in management of non-small cell lung cancer (NSCLC). The approach is based on periodic detection and quantification of tumor-specific somatic DNA mutation in circulating tumor DNA (ctDNA) extracted from patient plasma. For such repetitive testing, complex liquid-biopsy techniques relying on ultra-deep NGS sequencing are impractical. There are other, cost-effective, methods for ctDNA analysis, typically based on quantitative PCR or digital PCR, which are applicable for detecting specific individual mutations in hotspots. While such methods are routinely used in NSCLC therapy prediction, however, extension to cover broader spectrum of mutations (e.g., in tumor suppressor genes) is required for universal longitudinal MRD monitoring.

Methods: For a set of tissue samples from 81 NSCLC patients we have applied a denaturing capillary electrophoresis (DCE) for initial detection of somatic mutations within 8 predesigned PCR amplicons covering oncogenes and tumor suppressor genes. Mutation-negative samples were then subjected to a large panel NGS sequencing. For each patient mutation found in tissue was then traced over time in ctDNA by DCE.

Results: In total we have detected a somatic mutation in tissue of 63 patients. For those we have then prospectively analyzed ctDNA from collected plasma samples over a period of up to 2 years. The dynamics of ctDNA during the initial chemotherapy therapy cycles as well as in the long-term follow-up matched the clinically observed response.

Conclusion: Detection and quantification of tumor-specific mutations in ctDNA represents a viable complement to MRD monitoring during therapy of NSCLC patients. The presented approach relying on initial tissue mutation detection by DCE combined with NGS and a subsequent ctDNA mutation testing by DCE only represents a cost-effective approach for its routine implementation.

Scanning the mitochondrial genome for mutations by cycling temperature capillary electrophoresis

To bypass possible nuclear contamination and to exclusively amplify DNA from the mitochondrion, a set of 23 primers was selected. On the mitochondrial DNA selection fragments, a second set of fragments was used to amplify and identify mutant fractions with a detection limit of 1% . This mutation scanning method analyzed 76% of the mitochondrial genome and was used to examine 94 tumours from different tissues of origin. In all, 87 tumours had one or more mutations, leaving seven samples without observed mutations. Sanger sequencing verified samples carrying mutations with a mutant fraction exceeding 30%. The generated data validate that several regions of the mitochondrial DNA have more mutations than others.

Automated amplicon design suitable for analysis of DNA variants by melting techniques

Background

The technological development of DNA analysis has had tremendous development in recent years, and the present deep sequencing techniques present unprecedented opportunities for detailed and high-throughput DNA variant detection. Although DNA sequencing has had an exponential decrease in cost per base pair analyzed, focused and target-specific methods are however still much in use for analysis of DNA variants. With increasing capacity in the analytical procedures, an equal demand in automated amplicon and primer design has emerged.

Results

We have constructed a web-based tool that is able to batch design DNA variant assay suitable for analysis by denaturing gel/capillary electrophoresis and high resolution melting. The tool is developed as a computational workflow that implements one of the most widely used primer design tools, followed by validation of primer specificity, as well as calculation and visualization of the melting properties of the resulting amplicon, with or without an artificial high melting domain attached. The tool will be useful for scientists applying DNA melting techniques in analysis of DNA variations. The tool is freely available at http://meltprimer.ous-research.no/.

Conclusion

Herein, we demonstrate a novel tool with respect to covering the whole amplicon design workflow necessary for groups that use melting equilibrium techniques to separate DNA variants.

Genome wide single cell analysis of chemotherapy resistant metastatic cells in a case of gastroesophageal adenocarcinoma

Background

Metastatic progression due to development or enrichment of therapy-resistant tumor cells is eventually lethal. Molecular characterization of such chemotherapy resistant tumor cell clones may identify markers responsible for malignant progression and potential targets for new treatment. Here, in a case of stage IV adenocarcinoma of the gastroesophageal junction, we report the successful genome wide analysis using array comparative genomic hybridization (CGH) of DNA from only fourteen tumor cells using a bead-based single cell selection method from a bone metastasis progressing during chemotherapy.

Case presentation

In a case of metastatic adenocarcinoma of the gastroesophageal junction, the progression of bone metastasis was observed during a chemotherapy regimen of epirubicin, oxaliplatin and capecitabine, whereas lung-, liver and lymph node metastases as well as the primary tumor were regressing. A bone marrow aspirate sampled at the site of progressing metastasis in the right iliac bone was performed, and single cell molecular analysis using array-CGH of Epithelial Specific Antigen (ESA)-positive metastatic cells, and revealed two distinct regions of amplification, 12p12.1 and 17q12-q21.2 amplicons, containing the KRAS (12p) and ERBB2 (HER2/NEU) (17q) oncogenes. Further intrapatient tumor heterogeneity of these highlighted gene copy number changes was analyzed by fluorescence in situ hybridization (FISH) in all available primary and metastatic tumor biopsies, and ErbB2 protein expression was investigated by immunohistochemistry.

ERBB2 was heterogeneously amplified by FISH analysis in the primary tumor, as well as liver and bone metastasis, but homogenously amplified in biopsy specimens from a progressing bone metastasis after three initial cycles of chemotherapy, indicating a possible enrichment of erbB2 positive tumor cells in the progressing bone marrow metastasis during chemotherapy. A similar amplification profile was detected for wild-type KRAS, although more heterogeneously expressed in the bone metastasis progressing on chemotherapy. Correspondingly, the erbB2 protein was found heterogeneously expressed by immunohistochemical staining of the primary tumor of the gastroesophageal junction, while negative in liver and bone metastases, but after three initial cycles of palliative chemotherapy with epirubicin, oxaliplatin and capecetabine, the representative bone metastasis stained strongly positive for erbB2.

Conclusion

Global analysis of genetic aberrations, as illustrated by performing array-CGH analysis on genomic DNA from only a few selected tumor cells of interest sampled from a progressing bone metastasis, can identify relevant therapeutic targets and genetic aberrations involved in malignant progression, thus emphasizing the importance and feasibility of this powerful tool on the road to more personalized cancer therapies in the future.

Clinical relevance of KRAS in human cancers

The KRAS gene (Ki-ras2 Kirsten rat sarcoma viral oncogene homolog) is an oncogene that encodes a small GTPase transductor protein called KRAS. KRAS is involved in the regulation of cell division as a result of its ability to relay external signals to the cell nucleus. Activating mutations in the KRAS gene impair the ability of the KRAS protein to switch between active and inactive states, leading to cell transformation and increased resistance to chemotherapy and biological therapies targeting epidermal growth factor receptors. This review highlights some of the features of the KRAS gene and the KRAS protein and summarizes current knowledge of the mechanism of KRAS gene regulation. It also underlines the importance of activating mutations in the KRAS gene in relation to carcinogenesis and their importance as diagnostic biomarkers, providing clues regarding human cancer patients' prognosis and indicating potential therapeutic approaches.

A tumor sorting protocol that enables enrichment of pancreatic adenocarcinoma cells and facilitation of genetic analyses

Molecular profiling of human cancer is complicated by both stromal contamination and cellular heterogeneity within samples from tumor biopsies. In this study, we developed a tissue-processing protocol using mechanical dissociation and flow cytometric sorting that resulted in the respective enrichment of stromal and tumor fractions from frozen pancreatic adenocarcinoma samples. Molecular profiling of DNA from the sorted populations using high-density single nucleotide polymorphism arrays revealed widespread chromosomal loss of heterozygosity in tumor fractions but not in either the stromal fraction or unsorted tissue specimens from the same sample. Similarly, a combination of KRAS mutations and chromosomal copy number changes at key pancreatic cancer loci, such as CDK2NA and TP53, was detected in a substantial proportion of the tumor fractions but not in matched stromal fractions from the same sample. This approach to tissue processing could greatly expand the amount of archived tissue that is available for molecular profiling of human cancer and enable a more accurate diagnosis of genetic alterations in patient samples.

Single nucleotide polymorphism detection in the hMSH2 gene using conformation-sensitive CE

CE allows for highly reproducible analysis of DNA fragments which can be used to detect DNA mutations including SNPs. We have utilized a simple and direct CE analysis method for SNP analysis called conformation-sensitive CE (CSCE), based on the principle of single nucleotide different to produce conformational changes in the mildly denaturing solvent system. This method was applied to analysis of a mutation in the promoter region of the hMSH2 gene. This gene belongs to the human DNA mismatch repair system, which is responsible for recognizing and repairing mispaired nucleotides, and mutations in the hMSH2 gene are known to cause hereditary nonpolyposis colorectal cancer (HNPCC). PCR fragments generated from the promoter region of the hMSH2 gene, displaying either a C/C homozygote, C/T heterozygote, or T/T homozygote genotype, did not require further pretreatment before electrokinetic injection. The CE separation, using a 1xTris-borate-EDTA (TBE) buffer containing 3% w/v hydroxylethyl cellulose (HEC) and 6 M urea, was performed under reverse polarity with a separation temperature of 15 degrees C. The genotypes of 204 healthy volunteers and 13 colorectal cancer patients were determined using CSCE, and the results confirmed by DNA sequencing. While the CSCE separations were shown to be highly reproducible and sensitive for screening large populations, no correlation was observed between cancer patients and this hMSH2 gene polymorphism.

Scanning for DNA variants by denaturant capillary electrophoresis

Analysis and detection of DNA variation is important in any field of biology. Hence, numerous methods have been developed to analyze DNA. A simple yet effective way of analyzing DNA is by denaturant capillary electrophoresis (DCE). The method is in theory applicable to 95% of the human genome. The method involves three steps; fragment design, PCR amplification and allele separation. The allele separation can in principle be performed with any DNA capillary sequencing instrument.

Analysis of mutational spectra by denaturing capillary electrophoresis

The point mutational spectrum over nearly any 75- to 250-bp DNA sequence isolated from cells, tissues or large populations may be discovered using denaturing capillary electrophoresis (DCE). A modification of the standard DCE method that uses cycling temperature (e.g., +/-5 degrees C), CyDCE, permits optimal resolution of mutant sequences using computer-defined target sequences without preliminary optimization experiments. The protocol consists of three steps: computer design of target sequence including polymerase chain reaction (PCR) primers, high-fidelity DNA amplification by PCR and mutant sequence separation by CyDCE and takes about 6 h. DCE and CyDCE have been used to define quantitative point mutational spectra relating to errors of DNA polymerases, human cells in development and carcinogenesis, common gene-disease associations and microbial populations. Detection limits are about 5 x 10(-3) (mutants copies/total copies) but can be as low as 10(-6) (mutants copies/total copies) when DCE is used in combination with fraction collection for mutant enrichment. No other technological approach for unknown mutant detection and enumeration offers the sensitivity, generality and efficiency of the approach described herein.

High resolution melting analysis for the rapid and sensitive detection of mutations in clinical samples: KRAS codon 12 and 13 mutations in non-small cell lung cancer

BACKGROUND

The development of targeted therapies has created a pressing clinical need for the rapid and robust molecular characterisation of cancers. We describe here the application of high-resolution melting analysis (HRM) to screen for KRAS mutations in clinical cancer samples. In non-small cell lung cancer, KRAS mutations have been shown to identify a group of patients that do not respond to EGFR targeted therapies and the identification of these mutations is thus clinically important.

METHODS

We developed a high-resolution melting (HRM) assay to detect somatic mutations in exon 2, notably codons 12 and 13 of the KRAS gene using the intercalating dye SYTO 9. We tested 3 different cell lines with known KRAS mutations and then examined the sensitivity of mutation detection with the cell lines using 189 bp and 92 bp amplicons spanning codons 12 and 13. We then screened for KRAS mutations in 30 non-small cell lung cancer biopsies that had been previously sequenced for mutations in EGFR exons 18-21.

RESULTS

Known KRAS mutations in cell lines (A549, HCT116 and RPMI8226) were readily detectable using HRM. The shorter 92 bp amplicon was more sensitive in detecting mutations than the 189 bp amplicon and was able to reliably detect as little as 5-6% of each cell line DNA diluted in normal DNA. Nine of the 30 non-small cell lung cancer biopsies had KRAS mutations detected by HRM analysis. The results were confirmed by standard sequencing. Mutations in KRAS and EGFR were mutually exclusive.

CONCLUSION

HRM is a sensitive in-tube methodology to screen for mutations in clinical samples. HRM will enable high-throughput screening of gene mutations to allow appropriate therapeutic choices for patients and accelerate research aimed at identifying novel mutations in human cancer.

Evaluation of sieving matrices used to separate alleles by cycling temperature capillary electrophoresis

Denaturing CE (DCE) is a powerful tool for analysis of DNA variation. The development of commercial multi-CE instruments allows large-scale studies of DNA variation (many samples and many fragments). However, the cost of consumables like capillary arrays and sieving matrix might limit the use of DCE in such studies. Thus, we have tested 72 different in-house formulated sieving matrices' ability to suppress EOF and separate PCR-amplified alleles with the DCE variant, cycling temperature CE (CTCE). The data herein demonstrate that alleles can be baseline-separated by use of PVP and poly(N,N-dimethyl acrylamide) polymers at various percentages and pH. Allele separation by CTCE is matrix-independent and consequently applicable to any capillary instrument used for DNA separation. Formulation of sieving matrix for CTCE was done by dissolving appropriate amount of polymer powder into the running buffers. Allele separation was observed at different pH (7.5-8.5), concentrations and molecular size of the polymer, without compromising the separation and reproducibility. Finally, the cost reduction of homemade matrices is more than 1000-fold as compared to commercial sieving matrices.

High-throughput gender determination using automated denaturant gel capillary electrophoresis

Sex determination of anonymous samples is a requirement before analysis of DNA variation on X or Y chromosomes. Based on this, we designed a method for screening samples on different DNA capillary sequencing instruments with a sensitivity that is able to quantify sex chromosome abnormalities. The two different amelogenin alleles sited on the X and Y chromosomes were polymerase chain reaction amplified with the same set of primers and separated by denaturant capillary electrophoresis (DCE). Sex chromosome ratios could be reproducibly determined with a relative standard deviation of 8.7%, which is sufficient to distinguish a normal XY karyotype from an XYY karyotype associated with Klinefelter syndrome. Reconstruction experiments demonstrated sensitivity down to a simulated Y:X allelic ratio of 1:127 in all three instruments, enabling the prediction of sex chromosomal aneuploidies. When tested on anonymous pooled and single samples, DCE gave a good prediction of the male to female ratio in pools of 1000 blood donors. In conclusion, DCE is a simple and robust method for sex determination that can be readily performed on commercially available CE systems.

Design of an automated multicapillary instrument with fraction collection for DNA mutation discovery by constant denaturant capillary electrophoresis (CDCE)

A fundamental goal ingenomics is the discovery of genetic variation that contributes to disease states or to differential drug responses. Single nucleotide polymorphism (SNP) detection has been the focus of much attention in the study of genetic variation over the last decade. These SNPs typically occur at a frequency greater than 1% in the human genome. Recently, low-frequency alleles are also being increasingly recognized as critical to obtain an improved understanding of the correlation between genetic variation and disease. Although many methods have been reported for the discovery and scoringof SNPs, sensitive, automated, and cost-effective methods and platforms for the discovery of low-frequency alleles are not yet readily available. We describe here an automated multicapillary instrument for high-throughput detection of low-frequency alleles from pooled samples using constant denaturant capillary electrophoresis. The instrument features high optical sensitivity (1 x 10(-12) M fluorescein detection limit), precise and stable temperature control (+/- 0.01degrees C), and automation for sample delivery, injection, matrix replacement, and fraction collection. The capillary array is divided into six groups of four capillaries, each of which can be independently set at any temperature ranging from room temperature to 90 degrees C. The key performance characteristics of the instrument are reported.

Review of denaturant capillary electrophoresis in DNA variation analysis

Analyses of germline and somatic single-nucleotide DNA variations are important in both population genetics research and clinical practice. Reliable and inexpensive methods that are flexible and designed for automation are required for these analyses. Present day DNA sequencing technology is too expensive for testing all 22-25 000 human genes in populations genetics studies or in scanning large numbers of tumors for novel mutations. Denaturant capillary electrophoresis (DCE) has the potential to meet the need for large-scale analysis of DNA variants. Several different analyses can be performed by DCE, including mutation analysis, single-nucleotide polymorphism (SNP) discovery in individual and pooled samples, detection of allelic imbalance, and determination of microhaplotypes. Here we review the theoretical background of the method, its sensitivity, specificity, detection limit, throughput, and repeatability in the light of current literature in the field.

Detection and frequency estimation of rare variants in pools of genomic DNA from large populations using mutational spectrometry

DNA variants underlying the inheritance of risk for common diseases are expected to have a wide range of population allele frequencies. The detection and scoring of the rare alleles (at frequencies of <0.01) presents significant practical problems, including the requirement for large sample sizes and the limitations inherent in current methodologies for allele discrimination. In the present report, we have applied mutational spectrometry based on constant denaturing capillary electrophoresis (CDCE) to DNA pools from large populations in order to improve the prospects of testing the role of rare variants in common diseases on a large scale. We conducted a pilot study of the cytotoxic T lymphocyte-associated antigen-4 gene (CTLA4) in type 1 diabetes (T1D). A total of 1228 bp, comprising 98% of the CTLA4 coding sequence, all adjacent intronic mRNA splice sites, and a 3' UTR sequence were scanned for unknown point mutations in pools of genomic DNA from a control population of 10,464 young American adults and two T1D populations, one American (1799 individuals) and one from the United Kingdom (2102 individuals). The data suggest that it is unlikely that rare variants in the scanned regions of CTLA4 represent a significant proportion of T1D risk and illustrate that CDCE-based mutational spectrometry of DNA pools offers a feasible and cost-effective means of testing the role of rare variants in susceptibility to common diseases.

DNA variants in the ATM gene are not associated with sporadic rectal cancer in a Norwegian population-based study

BACKGROUND AND AIMS

A large number of DNA single-nucleotide polymorphisms (SNPs) have been discovered following the Human Genome Project. Several projects have been launched to find associations between SNPs and various disease cohorts. This study examined the possible association between the reported SNPs and sporadic rectal cancer. It has been proposed that SNPs in the ataxi-telangiectasia mutated (ATM) gene modulate the penetrance of some cancers. The investigated target sequence harbors three polymorphisms (IVS38-8 T/C in intron 38, 5557 G/A and 5558 A/T in exon 39), resulting in eight possible microhaplotypes at the DNA level. Furthermore, the two exonic SNPs are sited next to each other, allowing four possible amino acids in the same codon.

METHODS

We report on a new method analyzing SNPs and microhaplotypes based on theoretical thermodynamics and migration of variant fragments by cycling temperature capillary electrophoresis. Fluorophore-labeled PCR products were analyzed without any post-PCR steps on a standard 96 capillary-sequencing instrument under denaturing conditions.

RESULTS

More than 7000 alleles were microhaplotyped based on peak migration patterns of individual samples and sequencing results. The ATM polymorphisms and microhaplotypes examined did not significantly differ between sporadic rectal cancer and normal population.

CONCLUSION

No associations were found between the IVS38-8 T/C, 5557 G/A and 5558 A/T polymorphisms and microhaplotypes in the ATM gene with respect to sporadic rectal cancer.