Researchers have developed and validated a candidate Reference Measurement Procedure designed to bring metrological traceability and reproducibility to MET gene copy number quantification using droplet digital polymerase chain reaction. Reporting in Biological Procedures Online, a team led by Jessica Petiti and Carla Divieto established a duplex droplet digital PCR assay that simultaneously amplifies a target sequence within the MET oncogene and the diploid reference gene RPPH1. The methodology addresses an enduring challenge in precision oncology, where discrepancies among clinical diagnostic platforms have hindered the harmonization of genomic measurements. By demonstrating high quantitative linearity, tight repeatability, and a fully modeled measurement uncertainty budget, the authors provide an analytical framework intended to serve as a benchmark for molecular diagnostic laboratories, support the production of certified reference materials, and standardize copy number evaluations across translational cancer research.
Copy number variation involving the MET proto-oncogene, situated on chromosome 7q31, represents an established driver of tumorigenesis, disease progression, and acquired therapy resistance in malignancies such as non-small cell lung cancer and colorectal cancer. In clinical settings, elevated MET copy number carries prognostic significance and helps identify patients likely to respond to targeted MET inhibitors, including crizotinib and capmatinib. Despite the clear utility of tracking this genomic biomarker, diagnostic platforms commonly used in routine pathology—including fluorescence in situ hybridization, quantitative PCR, and next-generation sequencing—exhibit notable operational limitations. These conventional approaches frequently suffer from limited sensitivity, reliance on standard curves, procedural complexity, and variable reproducibility. Crucially, they lack metrological traceability, resulting in disparate reporting units, divergent reference intervals, and poor comparability among testing laboratories.
Digital polymerase chain reaction has emerged as a robust technology capable of resolving the analytical limitations associated with conventional copy number platforms. Unlike quantitative PCR, digital PCR does not rely on standard curves, provides absolute quantification, and is more tolerant to variations in reaction efficiency. This partitioned architecture provides high analytical sensitivity, strong tolerance to variations in amplification efficiency, and resilience against sample quality defects, including DNA fragmentation and low template abundance. Because the system operates through direct digital quantification rather than relative standard curves, digital PCR provides an ideal technical architecture for establishing Reference Measurement Procedures that tie clinical gene measurements directly to International System of Units standards, fulfilling a critical requirement for diagnostic standardization initiatives.
To construct their candidate measurement procedure, the researchers designed a duplex droplet digital PCR assay configured to target exon 14 of the MET gene alongside the reference gene RPPH1, located on chromosome 14q11.2. Exon 14 was chosen because it represents a clinically meaningful mutational hotspot frequently involved in oncogenic alterations, whereas RPPH1 was selected as the calibrator due to its stable diploid copy number. The team utilized Primer3Plus software to generate primer pairs and sequence-specific hydrolysis probes tailored specifically to digital PCR cycling kinetics, followed by in silico validation via NCBI Primer-BLAST to guarantee target specificity throughout the human genome. The primer and probe designs avoided known genomic polymorphisms and restriction enzyme cleavage sites within target amplicons, incorporating a previously validated RPPH1 configuration derived from the CCQM-K176 international metrological comparison study.
Validation of the newly designed assay required an array of synthetic constructs, well-characterized cell line models, and commercial reference materials to evaluate analytical performance across controlled genomic backgrounds. The research group engineered two custom plasmid templates using the pEX-A128 vector to carry sequence-verified insert fragments matching targeted regions of MET exon 14 and RPPH1. Plasmids were quantified with the Qubit double-stranded DNA High Sensitivity assay and preserved in tris-EDTA buffer at minus twenty degrees Celsius. Genomic DNA was isolated using the QIAamp DNA Mini Kit from four human cell lines characterized by normal diploid MET status—P-EP/SVTERT28-2, P-MSC/TERT308, MG-63, and THP-1—alongside two authenticated MET-amplified positive control lines, EBC1 and GTL16, obtained from the Candiolo Cancer Institute. Concentrations and purity were quantified using a NanoQuant plate, providing baseline matrices for dilution experiments and spiking models.
Preliminary assay optimization commenced with real-time quantitative PCR to assess amplification efficiency and verify reaction specificity before transitioning to digital droplet partitioning. Using SYBR Green master mix on a CFX96 system, the MET primer set demonstrated an amplification efficiency of 103.6 percent with a determination coefficient of 0.981, while the RPPH1 assay achieved 110.6 percent efficiency with an R-squared value of 0.997, both within acceptable performance limits. Melting curve analyses revealed sharp, isolated thermal peaks for both amplicons without non-specific products or primer-dimer artifacts. The investigators subsequently conducted thermal gradient testing on the QX200 Droplet Digital PCR System across a temperature span from 55 to 65 degrees Celsius. Although acceptable performance emerged between 61 and 65 degrees Celsius, an annealing temperature of 63 degrees Celsius provided the clearest droplet separation with minimal rain, establishing the optimal cycling condition.
The team next verified that combining the two target assays into a single reaction mixture did not compromise analytical sensitivity or introduce competitive interference. The duplex format coupled a FAM-labeled hydrolysis probe for MET exon 14 with a HEX-labeled probe for RPPH1, tested against approximately 1.3e-3 picograms of synthetic plasmid template. Direct comparisons between singleplex and duplex configurations demonstrated no detectable degradation in droplet cluster resolution, signal amplitude, or computed target concentrations. Further experiments systematically titrated total genomic DNA input across eight concentrations ranging from 0.1 nanograms to 100 nanograms per twenty-microliter reaction. Consistent quantification and robust droplet separation were maintained between 5 nanograms and 20 nanograms of input DNA, whereas lower concentrations increased statistical variability and excessive inputs diminished droplet formation efficiency. Additionally, treating genomic DNA with EcoRI restriction enzyme digestion showed negligible differences compared to undigested templates, confirming that pre-digestion was unnecessary.
Quantitative linearity and baseline accuracy under normal physiological conditions were evaluated by generating a seven-point standard curve in which MET-bearing plasmid constructs were spiked into a background matrix of equimolar genomic DNA pooled from the four diploid cell lines. Across an expected dynamic range spanning approximately two to seven copies, the duplex assay exhibited robust linearity with a coefficient of determination of 0.988. Regression analysis demonstrated a proportional relationship characterized by a slope of 1.04, with a 95 percent confidence interval of 0.99 to 1.09, an intercept not statistically divergent from zero, and a regression slope significantly different from zero at p less than 0.0001. Residual analysis confirmed that deviations remained within plus or minus 0.25 copies without non-linear trends. Concurrently, baseline measurements in unspiked genomic DNA from the diploid cell lines and a wild-type reference standard yielded a mean MET copy number of 2.25 plus or minus 0.30, validating diploid accuracy under physiological circumstances.
Precision profiling was conducted using four certified reference standards from the Seraseq ctDNA Mutation Mix version 4 series, covering variant allele frequencies of 0.1 percent, 0.5 percent, and 5 percent alongside a wild-type control across copy numbers up to 15. The materials were partitioned into independent aliquots and assayed in triplicate across four non-consecutive days by a single operator, yielding twelve measurements per material to evaluate repeatability and reproducibility. The pooled within-run repeatability showed an average coefficient of variation of 3.8 percent, comprising individual values of 4.2 percent for the 0.1 percent material, 3.7 percent for 0.5 percent, 3.3 percent for 5 percent, and 4.2 percent for wild-type controls. Inter-run reproducibility across experimental days demonstrated a corresponding mean coefficient of variation of 3.9 percent, with individual values of 4.2 percent, 3.7 percent, 3.3 percent, and 4.3 percent respectively, confirming high operational consistency across separate analytical runs.
In alignment with formal metrological guidelines, the authors established a comprehensive measurement uncertainty budget conforming to the Guide to the Expression of Uncertainty in Measurement and ISO 20395:2019 standards, adhering to methodologies endorsed by the CCQM Nucleic Acid Working Group. The uncertainty model integrated three primary uncertainty components: the volumetric dilution factor, the statistical partitioning ratio of negative to total droplets, and droplet volume variation. The resulting overall relative expanded measurement uncertainty, calculated using a coverage factor of k equals 2, averaged 23.8 percent across the reference panel, ranging from 14.1 percent for the 0.1 percent standard, 38.0 percent for 0.5 percent, 23.0 percent for 5 percent, to 20.0 percent for wild-type controls. Furthermore, measured copy number quantities displayed high concordance with manufacturer declared values, achieving a Pearson correlation coefficient of 0.999 with statistical significance at p equals 0.014.
Despite the strong analytical performance demonstrated by this candidate reference procedure, the authors noted several technical constraints that must be addressed prior to clinical implementation. The validation framework relied predominantly on synthetic plasmids, well-characterized cell line cultures, and commercially prepared reference materials, which do not fully replicate the biochemical complexity encountered in routine clinical biospecimens. Diagnostic samples such as formalin-fixed paraffin-embedded tissue blocks frequently suffer from DNA fragmentation and chemical modifications that can alter amplification efficiency and copy number estimates. Similarly, tumors with highly unstable genomes may present widespread structural alterations that complicate the interpretation of copy number variation results. The researchers emphasize that the stability of RPPH1 must be verified across specific tumor types, recommending that future multi-center trials incorporate their standardized operating procedure to harmonize molecular testing pipelines globally.
Subject of Research: Candidate reference measurement procedure for MET copy number variation quantification using droplet digital PCR
Article Title: New Candidate Reference Measurement Procedures for MET CNV Detection and Quantification Using Digital PCR
Article References: Petiti, J., Caria, S., Revel, L., Fava, M., Carrà , G., Albano, R., Gilardi, S., Venesio, T., & Divieto, C. (2026). New Candidate Reference Measurement Procedures for MET CNV Detection and Quantification Using Digital PCR. Biological Procedures Online, 28(1), Article 10. https://doi.org/10.1186/s12575-026-00325-5
Image Credits: AI Generated
DOI: 10.1186/s12575-026-00325-5
Keywords: droplet digital PCR, MET copy number variation, reference measurement procedure, metrological traceability, RPPH1 reference gene, precision oncology, molecular diagnostics, measurement uncertainty, non-small cell lung cancer, colorectal cancer, GUM principles, ISO 20395, analytical validation, repeatability, reproducibility, duplex ddPCR, crizotinib
News Source: Nathaniel Bowman. (November 13, 2026). Candidate Duplex Droplet Digital PCR Procedure Validated for MET Copy Number Quantification. Scienmag.
