Development of a peptide nucleic acid polymerase chain reaction clamping assay for semiquantitative evaluation of genetically modified organism content in food

Unravel the hidden protistan diversity: application of blocking primers to suppress PCR amplification of metazoan DNA

Planktonic protists, including both autotroph and heterotroph, have been recognized as a major contributor to primary production and consumers of bacteria, archaea and picophytoplankton. However, the understanding of protistan diversity is typically impeded by the large amount of metazoans when employing universal primers to environmental samples. In this study, we developed 3 blocking primers to inhibit the amplification of metazoan DNA in PCR. First, we optimized the design and concentration of blocking primers by using 2 metazoans and 3 protists in quantitative PCR. No significant difference (q > 0.05) was found in protistan community structure at phylum, family or OTU levels and genetic diversity between samples amplified with and without adding blocking primers, indicating that the blocking primer does not alter the composition of protistan community. An application to samples with a high abundance of metazoans showed that the blocking primer can reduce 42.1-72.4% of metazoan sequences, resulting the retrieval of a higher protistan richness and diversity. The use of the blocking primer helped to identify protistan community composition from a large size fraction, which is usually dominated by metazoans. Protistan community from the small (0.2-10 μm) and large (> 10 μm) size fractions exhibited a low similarity of 36.6% and shared 14.7% OTUs. About 63.8% of the OTUs were unique to the large fraction. Species from groups, such as Lepidodinium, Warnowia, Kareniaceae, Torodiniales, Phaeocystis, Chrysochromulina and Chrysophyceae, were enriched in the large fraction, indicating that they could be largely underestimated in studies that exclude the large-sized cells. Blocking primers are a promising tool to increase the efficiency for the characterization of protistan diversity in aquatic planktonic ecosystems.

Application of nonsense-mediated primer exclusion (NOPE) for preparation of unique molecular barcoded libraries

Background

Recently we proposed efficient method to exclude undesirable primers at any stage of amplification reaction, here termed NOPE (NOnsense-mediated Primer Exclusion). According to this method, added oligonucleotide overlapping with the 3′-end of unwanted amplification primer (NOPE oligo) simultaneously provides a template for its elongation. This elongation disrupts specificity of unwanted primer, preventing its further participation in PCR. The suggested approach allows to rationally manage the course of PCR reactions in order to facilitate analysis of complex DNA mixtures as well as to perform multistage PCR bypassing intermediate purification steps.

Results

Here we apply NOPE method to DNA library preparation for the high-throughput sequencing (HTS) with the PCR-based introduction of unique molecular identifiers (UMI). We show that NOPE oligo efficiently neutralizes UMI-containing oligonucleotides after introduction of UMI into sample DNA molecules, thus allowing to proceed with further amplification steps without purification and associated loss of starting material. At the same time, NOPE oligo does not affect the efficiency of target PCR amplification.

Conclusion

We describe a simple, robust and cheap modification of UMI-labeled HTS libraries preparation procedure, that allows to bypass purification step and thus to preserve starting material which may be limited, e.g. circulating tumor DNA, circulating fetal DNA, or small amounts of isolated cells of interest. Furthermore, demonstrated simplicity and robustness of NOPE method should make it popular in various PCR protocols.

Who is eating what: diet assessment using next generation sequencing

The analysis of food webs and their dynamics facilitates understanding of the mechanistic processes behind community ecology and ecosystem functions. Having accurate techniques for determining dietary ranges and components is critical for this endeavour. While visual analyses and early molecular approaches are highly labour intensive and often lack resolution, recent DNA-based approaches potentially provide more accurate methods for dietary studies. A suite of approaches have been used based on the identification of consumed species by characterization of DNA present in gut or faecal samples. In one approach, a standardized DNA region (DNA barcode) is PCR amplified, amplicons are sequenced and then compared to a reference database for identification. Initially, this involved sequencing clones from PCR products, and studies were limited in scale because of the costs and effort required. The recent development of next generation sequencing (NGS) has made this approach much more powerful, by allowing the direct characterization of dozens of samples with several thousand sequences per PCR product, and has the potential to reveal many consumed species simultaneously (DNA metabarcoding). Continual improvement of NGS technologies, on-going decreases in costs and current massive expansion of reference databases make this approach promising. Here we review the power and pitfalls of NGS diet methods. We present the critical factors to take into account when choosing or designing a suitable barcode. Then, we consider both technical and analytical aspects of NGS diet studies. Finally, we discuss the validation of data accuracy including the viability of producing quantitative data.

Application of blocking oligonucleotides to improve signal-to-noise ratio in a PCR

"Universal" or group-specific PCR primers have a tendency to predominately hybridise with the common sequences in samples with mixed templates. The result is that the rarer sequences are seldom retrieved by cloning or sequencing. The use of a blocking oligonucleotide (oligo) designed to specifically prevent amplification of dominant or unwanted DNA templates is an easy way to improve the amplification of rarer sequences. Here, we describe the different types of blocking principles and the different types of blocking oligos and give guidelines and examples of their application.

Food analysis and food authentication by peptide nucleic acid (PNA)-based technologies

This tutorial review will address the issue of DNA determination in food by using Peptide Nucleic Acid (PNA) probes with different technological platforms, with a particular emphasis on the applications devoted to food authentication. After an introduction aimed at describing PNAs structure, binding properties and their use as genetic probes, the review will then focus specifically on the use of PNAs in the field of food analysis. In particular, the following issues will be considered: detection of genetically modified organisms (GMOs), of hidden allergens, of microbial pathogens and determination of ingredient authenticity. Finally, the future perspectives for the use of PNAs in food analysis will be briefly discussed according to the most recent developments.

Testing for genetically modified organisms (GMOs): Past, present and future perspectives

This paper presents an overview of GMO testing methodologies and how these have evolved and may evolve in the next decade. Challenges and limitations for the application of the test methods as well as to the interpretation of results produced with the methods are highlighted and discussed, bearing in mind the various interests and competences of the involved stakeholders. To better understand the suitability and limitations of detection methodologies the evolution of transformation processes for creation of GMOs is briefly reviewed.

Simultaneous confirmatory analysis of different transgenic maize (zea mays) lines using multiplex polymerase chain reaction-restriction analysis and capillary gel electrophoresis with laser induced fluorescence detection

A novel analytical procedure based on the combination of multiplex PCR, restriction analysis, and CGE-LIF to unambiguosly and simultaneously confirm the presence of multiple lines of genetically modified corn is proposed. This methodology is based on the amplification of event-specific DNA regions by multiplex PCR using 6-FAM-labeled primers. Subsequently, PCR products are digested by a mixture containing specific restriction endonucleases. Thus, restriction endonucleases selectively recognize DNA target sequences contained in the PCR products and cleave the double-stranded DNA at a given cleavage site. Next, the restriction digest is analyzed by CGE-LIF corroborating the length of the expected restriction fragments, confirming (or not) the existence of GMOs. For accurate size determination of the DNA fragments by CGE-LIF a special standard DNA mixture was produced in this laboratory for calibration. The suitability of this mixture for size determination of labeled DNA fragments is also demonstrated. The usefulness of the proposed methodology is demonstrated through the simultaneous detection and confirmatory analysis of samples containing 0.5% of GA21 and MON863 maize plus an endogenous gene of maize as control.

Blocking primers to enhance PCR amplification of rare sequences in mixed samples - a case study on prey DNA in Antarctic krill stomachs

BACKGROUND

Identification of DNA sequence diversity is a powerful means for assessing the species present in environmental samples. The most common molecular strategies for estimating taxonomic composition depend upon PCR with universal primers that amplify an orthologous DNA region from a range of species. The diversity of sequences within a sample that can be detected by universal primers is often compromised by high concentrations of some DNA templates. If the DNA within the sample contains a small number of sequences in relatively high concentrations, then less concentrated sequences are often not amplified because the PCR favours the dominant DNA types. This is a particular problem in molecular diet studies, where predator DNA is often present in great excess of food-derived DNA.

RESULTS

We have developed a strategy where a universal PCR simultaneously amplifies DNA from food items present in DNA purified from stomach samples, while the predator's own DNA is blocked from amplification by the addition of a modified predator-specific blocking primer. Three different types of modified primers were tested out; one annealing inhibiting primer overlapping with the 3' end of one of the universal primers, another annealing inhibiting primer also having an internal modification of five dI molecules making it a dual priming oligo, and a third elongation arrest primer located between the two universal primers. All blocking primers were modified with a C3 spacer. In artificial PCR mixtures, annealing inhibiting primers proved to be the most efficient ones and this method reduced predator amplicons to undetectable levels even when predator template was present in 1000 fold excess of the prey template. The prey template then showed strong PCR amplification where none was detectable without the addition of blocking primer. Our method was applied to identifying the winter food of one of the most abundant animals in the world, the Antarctic krill, Euphausia superba. Dietary item DNA was PCR amplified from a range of species in krill stomachs for which we had no prior sequence knowledge.

CONCLUSION

We present a simple, robust and cheap method that is easily adaptable to many situations where a rare DNA template is to be PCR amplified in the presence of a higher concentration template with identical PCR primer binding sites.