Correction of the lack of commutability between plasmid DNA and genomic DNA for quantification of genetically modified organisms using pBSTopas as a model

Collaborative Ring Trial of the Applicability of a Reference Plasmid DNA Calibrant in the Quantitative Analysis of GM Maize Event MON810

Certified reference materials (CRMs) is one of the critical requirements in a quantitative analytical method, such as in the quantification of genetically modified (GM) contents in food/feed products. Plasmid-DNA-based CRMs are becoming essential in GM content quantification. Herein, we report the construction of one plasmid DNA calibrant, pMON810, for the quantification of the GM maize event MON810 which is commercially planted and used for food/feeds worldwide, and the collaborative ring trial was used to validate its applicability. pMON10 was proven to have high specificity for the MON810 event. The limit of detection (LOD) and limit of quantification (LOQ) of real-time PCR assays of MON810 event and maize endogenous gene using pMON810 as calibrant was 2 copies/μL and 5 copies/μL, respectively. A total of eight laboratories participated in the ring trial and returned valid test results. Each sample was performed with three repeats and three parallels in each repeat. Statistical analysis of the ring trial results showed that pMON810 as a calibrant had high PCR efficiency (ranging from 0.885 to 1.008) and good linearity (ranging from 0.9933 to 0.9997) in MON810 and endogenous gene real-time PCR assays. The bias between the test values and true values ranged from 4.60 to 20.00% in the quantification of five blind samples. These results indicate that pMON810 is suitable for use as a calibrant for the quantification of MON810 events in routine lab analysis or to evaluate detection methods for MON810, as well as being used as a substitute for the matrix-based CRM of MON810.

A New Reference Plasmid “pGMT27” Provides an Efficient Transgenic Detection Method for Flue-Cured Tobacco

Owing to the economic value of its foliage, tobacco (Nicotiana tabacum) is cultivated all across the world. For the detection of genetically modified (GM) tobacco, there is a lack of universal standard material which ultimately limits the detection methods because the accuracy and comparability of the results cannot be ensured. Here, we prepared a reference plasmid “pGMT27” for the detection of GM tobacco, which was 18,296 bp in length harboring two of the tobacco endogenous and seven exogenous genes. By using qualitative PCR test for the nine genes, 10 copies were used for plasmid sensitivity. In the quantitative real-time PCR (qPCR) assays with pGMT27 as a calibrator, the reaction efficiencies for P-35S and NR were 101.427% and 98.036%, respectively, whereas the limit of detection (LOD) and limit of quantification (LOQ) were 5 copies and 10 copies per reaction. For standard deviation (SD) and relative standard deviation (RSD) of the Ct values, the repeatability values were from 0.04 to 0.42 and from 0.18% to 1.29%, respectively; and the reproducibility values were from 0.04 to 0.39 and from 0.18% to 1.14%, respectively. For the unknown sample test, the average conversion factor (Cf) was 0.39, and the accuracy bias was from −15.55% to 1.93%; for precision, the SD values ranged from 0.02 to 0.62, while RSD values were from 1.34% to 10.6%. We concluded that using the pGMT27 plasmid as a calibrator provided a highly efficient transgenic detection method for flue-cured tobacco.

Development of a new genetic reference material system based on Saccharomyces cerevisiae cells

As an important quality control link of molecular diagnosis, genetic reference materials (RMs) are widely used in various gene detection platforms such as mutation detection, gene quantification, and second generation sequencing. However, contamination, construction, and storage of existing genetic RMs still remain challenges. Here, we established a new genetic RM system based on Saccharomyces cerevisiae. We chose the non-small cell lung cancer (NSCLC) mutation hotspots in Kirsten rat sarcoma viral oncogene (KRAS) and epidermal growth factor receptor (EGFR), using clustered regularly interspaced short palindromic repeats and CRISPR-associated protein (CRISPR-Cas9) system-mediated gene editing technology, combined with the high homologous recombination efficiency of Saccharomyces cerevisiae. A single copy of the target gene was inserted into the yeast genome, and the inserted target gene was stably inherited with the passage of yeast cells. The copy number calculation for the target gene can replays by cell counting. The RM system was evaluated by sequence, copy number, stability, and homogeneity. In summary, the recombinant yeast cell line has ease of construction and screening, stable genetic characteristics, accurate copy number calculation, and convenient culture and preservation. Our findings may provide new ideas and directions for the research and industrialization of genetic RMs.

Sensitive and Facile Electrochemiluminescent Immunoassay for Detecting Genetically Modified Rapeseed Based on Novel Carbon Nanoparticles

A highly sensitive electrochemiluminescent (ECL) immunoassay targeting PAT/ bar protein was facilely developed for genetically modified (GM) rapeseed detection using carbon nanoparticles (CNPs) originally prepared from printer toner. In this work, CNPs linked with antibody for PAT/ bar protein were used to modify a working electrode. After an immunoreaction between the PAT/ bar protein and its antibody, the immunocomplex formed on the electrode receptor region resulted in an inhibition of electron transfer between the electrode surface and the ECL substance, thus led to a decrease of ECL response. Under the optimal conditions, the ECL responses linearly decreased as the increase of the PAT/ bar protein concentration and the GM rapeseed RF3 content in the ranges of 0.10-10 ng/mL and 0.050-1.0%, with the limits of detection of 0.050 ng/mL and 0.020% (S/N = 3). These results open a facile, sensitive, and rapid approach for the safety control of agricultural GM rape.

One Novel Multiple-Target Plasmid Reference Molecule Targeting Eight Genetically Modified Canola Events for Genetically Modified Canola Detection

Multiple-target plasmid DNA reference materials have been generated and utilized as good substitutes of matrix-based reference materials in the analysis of genetically modified organisms (GMOs). Herein, we report the construction of one multiple-target plasmid reference molecule, pCAN, which harbors eight GM canola event-specific sequences (RF1, RF2, MS1, MS8, Topas 19/2, Oxy235, RT73, and T45) and a partial sequence of the canola endogenous reference gene PEP. The applicability of this plasmid reference material in qualitative and quantitative PCR assays of the eight GM canola events was evaluated, including the analysis of specificity, limit of detection (LOD), limit of quantification (LOQ), and performance of pCAN in the analysis of various canola samples, etc. The LODs are 15 copies for RF2, MS1, and RT73 assays using pCAN as the calibrator and 10 genome copies for the other events. The LOQ in each event-specific real-time PCR assay is 20 copies. In quantitative real-time PCR analysis, the PCR efficiencies of all event-specific and PEP assays are between 91% and 97%, and the squared regression coefficients (R²) are all higher than 0.99. The quantification bias values varied from 0.47% to 20.68% with relative standard deviation (RSD) from 1.06% to 24.61% in the quantification of simulated samples. Furthermore, 10 practical canola samples sampled from imported shipments in the port of Shanghai, China, were analyzed employing pCAN as the calibrator, and the results were comparable with those assays using commercial certified materials as the calibrator. Concluding from these results, we believe that this newly developed pCAN plasmid is one good candidate for being a plasmid DNA reference material in the detection and quantification of the eight GM canola events in routine analysis.

Development and application of a general plasmid reference material for GMO screening

The use of analytical controls is essential when performing GMO detection through screening tests. Additionally, the presence of taxon-specific sequences is analyzed mostly for quality control during GMO detection. In this study, 11 commonly used genetic elements involving three promoters (P-35S, P-FMV35S and P-NOS), four marker genes (Bar, NPTII, HPT and Pmi), and four terminators (T-NOS, T-35S, T-g7 and T-e9), together with the reference gene fragments from six major crops of maize, soybean, rapeseed, rice, cotton and wheat, were co-integrated into the same single plasmid to construct a general reference plasmid pBI121-Screening. The suitability test of pBI121-Screening plasmid as reference material indicated that the non-target sequence on the pBI121-Screening plasmid did not affect the PCR amplification efficiencies of screening methods and taxon-specific methods. The sensitivity of screening and taxon-specific assays ranged from 5 to 10 copies of pBI121-Screening plasmid, meeting the sensitivity requirement of GMO detection. The construction of pBI121-Screening solves the lack of a general positive control for screening tests, thereby reducing the workload and cost of preparing a plurality of the positive control.

Developing a matrix reference material for screening of transgenic rice

Certified reference materials (CRMs) that are compatible with detection methods are needed to detect genetically modified organisms (GMOs). Screening is the first detection step in determining the possible presence of GMO ingredients in food or feed; however, screening has been hindered by the lack of GMO CRMs. In this study, transgenic rice materials were developed via the transformation of a construct harboring 11 commonly used screening elements. Digital PCR was utilized to identify a homozygous single-copy line termed SDrice. The qualitative detections of 11 elements in 21 transgenic materials demonstrated that the genomic DNA of the SDrice was suitable for use as a positive control in the screening of GMO ingredients. The suitability of SDrice as reference material was further checked by testing the sensitivity of 11 known conventional PCR assays, ranging from 10 to 50 copies of the SDrice genome. The standard curves that were created using SDrice DNA series as calibrators all exhibited good linearities in the relationships of the Ct values with the template copy numbers in these 11 real-time PCR assays. The LODs of the real-time PCR assays were estimated to be two to five copies of the SDrice genome. Comparisons of the SDrice with other GM rice revealed that significant differences existed in both the intercepts of the standard curves and the ΔCt values of the exogenous and reference genes for the P-35S, T-nos, HPT, T-35S, and Bar assays; the SDrice was not fit for quantification of other GM rice events. This study provided a matrix reference material (RM) that was suitable for screening GM rice, determination of sensitivity and a LOD of PCR assays, and overcame some of the drawbacks of plasmid DNA as reference material.