Identification of single target taxon-specific reference assays for the most commonly genetically transformed crops using digital droplet PCR

cor1 Gene: A Suitable Marker for Identification of Opium Poppy (Papaver somniferum L.)

This paper discusses the development of rapid, reliable, and accurate polymerase chain reaction (PCR) assays for detecting opium poppy (Papaver somniferum L.) in food. Endpoint, quantitative, and digital PCRs were compared based on the amplification of a newly developed DNA marker targeting the NADPH-dependent codeinone reductase (COR) gene. Designed assays were shown to be highly specific and sensitive in discriminating opium poppy from other plant species, even in heat-treated and food samples. Digital PCR was the most sensitive, with a detection limit of up to 5 copies, i.e., approximately 14 pg of target DNA per reaction. Quantitative and digital PCR further allowed the quantification of opium poppy in up to 1.5 ng and 42 pg (15 copies) of target DNA in a sample, respectively. In addition, two duplex PCRs have been developed for the simultaneous detection of opium poppy DNA and representatives of (i) the Papaveraceae family or (ii) the Plantae kingdom. Finally, all designed assays were successfully applied for analysis of 15 commercial foodstuffs; two were suspected of being adulterated. The study results have an important impact on addressing food fraud and ensuring the safety and authenticity of food products. Beyond food adulteration, the study may also have significant implications for forensics and law enforcement.

Digital PCR for the characterization of reference materials

Well-characterized reference materials support harmonization and accuracy when conducting nucleic acid-based tests (such as qPCR); digital PCR (dPCR) can measure the absolute concentration of a specific nucleic acid sequence in a background of non-target sequences, making it ideal for the characterization of nucleic acid-based reference materials. National Metrology Institutes are increasingly using dPCR to characterize and certify their reference materials, as it offers several advantages over indirect methods, such as UV-spectroscopy. While dPCR is gaining widespread adoption, it requires optimization and has certain limitations and considerations that users should be aware of when characterizing reference materials. This review highlights the technical considerations of dPCR, as well as its role when developing and characterizing nucleic acid-based reference materials.

Expression of GM content in mass fraction from digital PCR data

Nowadays the quantification of the content of genetically modified (GM) constituents in food or feed products is performed by using either quantitative real-time PCR (qPCR) or digital PCR (dPCR). The latter is increasingly used. Therefore, experimental protocols for the quantification of 52 GM events authorised in the EU have been converted into a digital format and minimum performance characteristics for dPCR methods are detailed. Because of the need to harmonise the transformation of PCR results between two different measurement scales, 50 conversion factors for Certified Reference Materials (CF_CRM) have been experimentally determined by three and sometimes four independent expert laboratories. The uncertainty of each CF_CRM has been estimated to express dPCR results in mass fraction with a consistent uncertainty contribution. In 38 out of 58 cases, the validated qPCR methods (for 52 event-specific and 6 taxon-specific measurements) could easily be transferred into dPCR methods by using the same oligo sequences, final oligo concentration or annealing temperatures for the dPCR procedure. Laboratories have nevertheless used different strategies to improve the resolution or to reduce the so-called rain in their dPCR outcome. Those modifications were needed for PCR procedures that could not be converted without changes into a digital format. Therefore, exclusion/quality criteria such as the maximum rate of partitions with intermediate fluorescence “rain”, the minimum resolution and repeatability are suggested for dPCR methods. The CF_CRM determined in this study were generally in agreement with the declared zygosity of the GM parental donor for hemizygous maize events. In a limited number of GM events the CF_CRM values were significantly different when measured with different maize-specific (ZmAdh1 or hmgA) genes.

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Digital PCR protocols for the quantification of 52 GM events

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Minimum performance criteria for digital PCR results

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Conversion factors to express GM content determined by digital PCR in mass fraction

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Harmonised measurement system for official GMO controls in the EU

Integrated approach for the molecular characterization of edited plants obtained via Agrobacterium tumefaciens-mediated gene transfer

Agrobacterium tumefaciens-mediated gene transfer—actually the most used method to engineer plants—may lead to integration of multiple copies of T-DNA in the plant genome, as well as to chimeric tissues composed of modified cells and wild type cells. A molecular characterization of the transformed lines is thus a good practice to select the best ones for further investigation. Nowadays, several quantitative and semi-quantitative techniques are available to estimate the copy number (CN) of the T-DNA in genetically modified plants. In this study, we compared three methods based on (1) real-time polymerase chain reaction (qPCR), (2) droplet digital PCR (ddPCR), and (3) next generation sequencing (NGS), to carry out a molecular characterization of grapevine edited lines. These lines contain a knock-out mutation, obtained via CRISPR/Cas9 technology, in genes involved in plant susceptibility to two important mildew diseases of grapevine. According to our results, qPCR and ddPCR outputs are largely in agreement in terms of accuracy, especially for low CN values, while ddPCR resulted more precise than qPCR. With regard to the NGS analysis, the CNs detected with this method were often not consistent with those calculated by qPCR and ddPCR, and NGS was not able to discriminate the integration points in three out of ten lines. Nevertheless, the NGS method can positively identify T-DNA truncations or the presence of tandem/inverted repeats, providing distinct and relevant information about the transgene integration asset. Moreover, the expression analysis of Cas9 and single guide RNA (sgRNA), and the sequencing of the target site added new information to be related to CN data. This work, by reporting a practical case-study on grapevine edited lines, explores pros and cons of the most advanced diagnostic techniques available for the precocious selection of the proper transgenic material. The results may be of interest both to scientists developing new transgenic lines, and to laboratories in charge of GMO control.

Simple, precise, and less-biased GMO quantification by multiplexed genetic element-specific digital PCR

BACKGROUND

To provide the consumer with choices of GMO or non-GMO, official food labeling systems were established in many countries. Because the threshold GMO content values were set to distinguish between "non-GMO" and "GMO" designations, GMO content quantification method are required for ensuring the appropriateness of labeling.

OBJECTIVE

As the number of GMOs is continuously increasing around the world, we set out to develop a low-cost, simple and less-biased analytical strategy to cover all necessary detection targets.

METHODS

Digital PCR methods are advantageous compared to the conventional quantitative real-time PCR methods. We developed a digital PCR-based GMO quantification method to evaluate the GMO content in maize grains. To minimize the analytical workload, we adopted multiplex digital PCR targeting 35S promoter and NOS terminator, which are genetic elements commonly introduced in many GMOs.

RESULTS

Our method is significantly simpler and more precise than the conventional real-time PCR-based methods. Additionally, we found that this method enables to quantify the copy number of GM DNA without double counting multiple elements (P35S and TNOS) tandemly placed in a recombinant DNA construct.

CONCLUSION

This is the first report on the development of a GM maize quantification method using the multiplexed genetic element-specific digital PCR method. The tandem effect we report here is quite useful for reducing the bias in the analytical results.

HIGHLIGHTS

Multiplexed genetic element-specific digital PCR can simplify weight-based GMO quantification and thus should prove useful in light of the continuous increase in the numbers of GM events.

An Editing-Site-Specific PCR Method for Detection and Quantification of CAO1-Edited Rice

Genome-edited plants created by genome editing technology have been approved for commercialization. Due to molecular characteristics that differ from classic genetically modified organisms (GMOs), establishing regulation-compliant analytical methods for identification and quantification of genome-edited plants has always been regarded as a challenging task. An editing-site-specific PCR method was developed based on the unique edited sequence in CAO1-edited rice plants. Test results of seven primer/probe sets indicated that this method can identify specific CAO1-edited rice from other CAO1-edited rice and wild types of rice with high specificity and sensitivity. The use of LNA (locked nucleic acid) in a probe can efficiently increase the specificity of the editing-site-specific PCR method at increased annealing temperature which can eliminate non-specific amplification of the non-target. The genome-edited ingredient content in blinded samples at the level of 0.1% to 5.0% was accurately quantified by this method on the ddPCR platform with RSD of <15% and bias in the range of ±17%, meeting the performance requirements for GMO detection method. The developed editing-site-specific PCR method presents a promising detection and quantification technique for genome-edited plants with known edited sequence.

Ten years of proficiency testing reveals an improvement in the analytical performance of EU National Reference Laboratories for genetically modified food and feed

National Reference Laboratories (NRLs) in the Member States of the European Union (EU) monitor the implementation of the EU legislation on the presence of genetically modified organisms (GMOs) in food and feed. The EU Reference Laboratory for GM Food and Feed (EURL GMFF) supports the harmonisation of measurement procedures and the improvement of the analytical performance of these laboratories, among others through the organisation of a proficiency testing (PT) scheme. The PT results reported over 10 years have been analysed using common criteria applied to the reported data. The outcome revealed a gradual decrease of the relative standard deviation within the sets of the reported data with time. The extent of the deviation of the results from the assigned value also diminished between 2010 and 2019. The average deviation from the assigned value was independent of the GM content in the later PT rounds but it was affected by the complexity of the test item matrix. Performance scores were calculated for all results reported by the 62 NRLs. The number of unsatisfactory performance scores obtained decreased with time. The trends observed indicate an improvement in the analytical performance and an increased harmonisation of GMO testing within the EU enforcement laboratories.

Mass ratio quantitative detection for kidney bean in lotus seed paste using duplex droplet digital PCR and chip digital PCR

Two "mass ratio-DNA copy concentration ratio" formulas were established respectively on droplet digital PCR (ddPCR) and chip-based digital PCR (cdPCR) to determine the mass ratio of kidney bean, a common alternative plant-derived ingredient in lotus seed paste. The limit of detection for DNA copy concentration of kidney bean and lotus seed was 6 copies/μL. Quantitative detection range was set from 5 to 80%, and the limit of quantification for mass ratio of kidney bean in lotus seed paste was defined as 5%. Results of 6 simulated samples and 16 prepackaged pastes in this work offer compelling evidence that an innovative scheme for quantitative detection of kidney bean in lotus seed paste was available, and provide technical support for the identification of suspicious ingredients from fraudulent substitution or adventitious contamination. Graphical abstract Two "mass ratio-DNA copy concentration ratio" formulas were established respectively on droplet digital PCR (ddPCR) and chip digital PCR (cdPCR) to determine the mass ratio of kidney bean in adulterated lotus seed paste. It was the first time to quantify adulterate food by directly converting DNA copy concentration ratio obtained from digital PCR to mass ratio, which could provide strong technical support for quantitative detection of adulterated food.

Log transformation of proficiency testing data on the content of genetically modified organisms in food and feed samples: is it justified?

The outcome of proficiency tests (PTs) is influenced, among others, by the evaluation procedure chosen by the PT provider. In particular for PTs on GMO testing a log-data transformation is often applied to fit skewed data distributions into a normal distribution. The study presented here has challenged this commonly applied approach. The 56 data populations from proficiency testing rounds organised since 2010 by the European Union Reference Laboratory for Genetically Modified Food and Feed (EURL GMFF) were used to investigate the assumption of a normal distribution of reported results within a PT. Statistical evaluation of the data distributions, composed of 3178 reported results, revealed that 41 of the 56 datasets showed indeed a normal distribution. For 10 datasets, the deviation from normality was not statistically significant at the raw or log scale, indicating that the normality assumption cannot be rejected. The normality of the five remaining datasets was statistically significant after log-data transformation. These datasets, however, appeared to be multimodal as a result of technical/experimental issues with the applied methods. On the basis of the real datasets analysed herein, it is concluded that the log transformation of reported data in proficiency testing rounds is often not necessary and should be cautiously applied. It is further shown that the log-data transformation, when applied to PT results, favours the positive performance scoring for overestimated results and strongly penalises underestimated results. The evaluation of the participants' performance without prior transformation of their results may highlight rather than hide relevant underlying analytical problems and is recommended as an outcome of this study. Graphical abstract.

Performance assessment of digital PCR for the quantification of GM-maize and GM-soya events

Accurate quantitative methods are needed to determine the amount of transgenic material in ingredients and comply with labelling GMO thresholds. Quantitative real-time PCR methods are usually applied for GMO quantification, but since a few years, digital PCR (dPCR) has been described as a potential alternative by quantifying DNA molecules directly without any standard curves. In this study, the performance of dPCR to quantify 9 GM-soya events and 15 GM-maize events was assessed. Following GMO validation guidelines, the trueness and precision were determined on high, medium and low levels of transgenic content. Results showed biases below ± 25% and satisfactory precision data. Limits of quantification were determined for each GM-event and were between 12 and 31 target copies. The reliability of GMO quantification by dPCR was further confirmed by analysing several proficiency test samples. Overall, dPCR showed accurate and precise GMO quantification on all the tested GM-events, from high to low transgenic amount. With its ease-of-use, dPCR was found to be an appealing alternative technology for routine GMO testing laboratories. Graphical abstract.