Detection of genome-edited cells by oligoribonucleotide interference-PCR

Oligoribonucleotide-Mediated Blockade of DNA Extension by Taq DNA Polymerases Increases Specificity and Sensitivity for Detecting Single-Nucleotide Differences

Blocking PCR is a method that inhibits amplification of DNA possessing a nucleotide sequence complementary to that of a blocker; the method can be used to suppress amplification of target wild-type DNA while amplifying mutated DNA. Previously, we demonstrated that an oligoribonucleotide (ORN) functions as a cost-effective and sequence-specific blocker. This blocking PCR system, named ORN interference-PCR (ORNi-PCR), is compatible with DNA polymerases lacking 5'-3' exonuclease activity but not with those possessing the activity (e.g., Taq DNA polymerase), which can remove a hybridized ORN during DNA extension. Here, we demonstrate that under specific experimental conditions, an intact or phosphorothioated ORN strongly suppresses extension of target DNA by Taq DNA polymerases. This method was applied successfully to real-time ORNi-PCR and one-step real-time reverse transcription-ORNi-PCR using a dual-labeled fluorescent probe to detect a single-nucleotide mutation in DNA and RNA in a sequence-specific manner. The results reaffirm the utility of blocking PCR and provide technical hints for its improvement.

Oligoribonucleotide interference-PCR: principles and applications

Polymerase chain reaction (PCR) amplification of multiple templates using common primers is used widely for molecular biological research and clinical diagnosis. However, amplifying a specific DNA sequence harboring a mutation that is present in a small number of mutant cells within a large population of normal cells (e.g., as in cancer) in a tissue is difficult using the original PCR protocol. Thus, some measures are necessary to suppress amplification of background signals. To achieve this, we developed the oligoribonucleotide (ORN) interference-PCR (ORNi-PCR) technology in which an ORN (short RNA) hybridizes with a complementary DNA sequence to inhibit PCR amplification across the specific target sequence. ORNs can be prepared inexpensively, and ORNi-PCR can be carried out easily by adding ORNs to the PCR reaction mixture. Suppressing amplification of target sequences by ORNi-PCR is useful for detecting target sequence mutations. We showed that ORNi-PCR can discriminate single-nucleotide mutations in cancer cells and indel mutations introduced by genome editing. We also showed that ORNi-PCR can identify the CpG methylation status of a target sequence within bisulfite-treated DNA, and can enrich DNA sequences of interest from a DNA mixture by suppressing amplification of unwanted sequences. Thus, ORNi-PCR has many potential applications in various fields, including medical diagnosis and molecular biology. In this review, we outline the principles of the ORNi-PCR method and its use to detect nucleotide mutations in a variety of specimens.

Protein or ribonucleoprotein-mediated blocking of recombinase polymerase amplification enables the discrimination of nucleotide and epigenetic differences between cell populations

Isothermal DNA amplification, such as recombinase polymerase amplification (RPA), is well suited for point-of-care testing (POCT) as it does not require lengthy thermal cycling. By exploiting DNA amplification at low temperatures that do not denature heat-sensitive molecules such as proteins, we have developed a blocking RPA method to detect gene mutations and examine the epigenetic status of DNA. We found that both nucleic acid blockers and nuclease-dead clustered regularly interspaced short palindromic repeats (CRISPR) ribonucleoproteins suppress RPA reactions by blocking elongation by DNA polymerases in a sequence-specific manner. By examining these suppression events, we are able to discriminate single-nucleotide mutations in cancer cells and evaluate genome-editing events. Methyl-CpG binding proteins similarly inhibit elongation by DNA polymerases on CpG-methylated template DNA in our RPA reactions, allowing for the detection of methylated CpG islands. Thus, the use of heat-sensitive molecules such as proteins and ribonucleoprotein complexes as blockers in low-temperature isothermal DNA amplification reactions markedly expands the utility and application of these methods.

Fujita et al. investigate the use of oligoribonucleotides, proteins, and ribonucleoprotein complexes as sequence-specific blockers of DNA extension by DNA polymerases. They demonstrate the value of proteins and ribonucleoprotein complexes as blockers in low-temperature isothermal DNA amplification reactions for discrimination of nucleotide and epigenetic differences.

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.

Discrimination of CpG Methylation Status and Nucleotide Differences in Tissue Specimen DNA by Oligoribonucleotide Interference-PCR

Oligoribonucleotide (ORN) interference-PCR (ORNi-PCR) is a method in which PCR amplification of a target sequence is inhibited in a sequence-specific manner by the hybridization of an ORN with the target sequence. Previously, we reported that ORNi-PCR could detect nucleotide mutations in DNA purified from cultured cancer cell lines or genome-edited cells. In this study, we investigated whether ORNi-PCR can discriminate nucleotide differences and CpG methylation status in damaged DNA, such as tissue specimen DNA and bisulfite-treated DNA. First, we showed that ORNi-PCR could discriminate nucleotide differences in DNA extracted from acetone-fixed paraffin-embedded rat liver specimens or formalin-fixed paraffin-embedded human specimens. Rat whole blood specimens were compatible with ORNi-PCR for the same purpose. Next, we showed that ORNi-PCR could discriminate CpG methylation status in bisulfite-treated DNA. These results demonstrate that ORNi-PCR can discriminate nucleotide differences and CpG methylation status in multiple types of DNA samples. Thus, ORNi-PCR is potentially useful in a wide range of fields, including molecular biology and medical diagnosis.

Simultaneous Detection of the T790M and L858R Mutations in the EGFR Gene by Oligoribonucleotide Interference-PCR

A de novo single-nucleotide mutation in the EGFR gene can cause the development of lung cancer. EGFR tyrosine kinase inhibitors (EGFR-TKIs) are used for clinical treatment of such lung cancers, but acquired resistance often mitigates their efficacy. Accordingly, monitoring of de novo and acquired nucleotide mutations is essential for clinical treatment of lung cancers with EGFR-TKIs. Previously, we reported that oligoribonucleotide interference-PCR (ORNi-PCR) can accurately and cost-effectively detect single-nucleotide mutations. In this study, we applied ORNi-PCR to simultaneous detection of the de novo L858R and acquired T790M mutations in the EGFR gene in lung cancer cells. First, we established optimal experimental conditions for ORNi-PCR to simultaneously detect the two single-nucleotide mutations in genomic DNA from lung cancer cells. The conditions we established could also be used for ORNi-PCR using complementary DNA reverse-transcribed from extracted RNA. We found that ORNi-PCR could detect lung cancer cells possessing both single-nucleotide mutations among a large number of cells harboring wild-type sequences, even when the cancer cells constituted less than ~0.2% of all cells. Our findings demonstrate that ORNi-PCR can simultaneously detect multiple single-nucleotide mutations in a gene of interest and might therefore be useful for simultaneous detection of EGFR mutations in clinical examinations.

Target enrichment from a DNA mixture by oligoribonucleotide interference-PCR (ORNi-PCR)

Oligoribonucleotide (ORN) interference-PCR (ORNi-PCR) is a method that suppresses PCR amplification of target DNA in an ORN-specific manner. In this study, we examined whether ORNi-PCR can be used to enrich desirable DNA sequences from a DNA mixture by suppressing undesirable DNA amplification. ORNi-PCR enriched edited DNA sequences from a mixture of genomic DNA subjected to genome editing. ORNi-PCR enabled more efficient analysis of the types of insertion/deletion mutations introduced by genome editing. In addition, ORNi-PCR reduced the detection of 16S ribosomal RNA (16S rRNA) genes in 16S rRNA gene-based microbiome profiling, which might permit a more detailed assessment of populations of other 16S rRNA genes. Enrichment of desirable DNA sequences by ORNi-PCR may be useful in molecular biology, medical diagnosis, and other fields.

A refined two-step oligoribonucleotide interference-PCR method for precise discrimination of nucleotide differences

We previously developed oligoribonucleotide (ORN) interference-PCR (ORNi-PCR), in which an ORN hybridises with a complementary DNA sequence and inhibits PCR amplification across the sequence in a sequence-specific manner. Suppression of target amplification by ORNi-PCR can be used to detect nucleotide differences such as mutations in a target sequence. In the present study, we refined the ORNi-PCR method and established a detailed technical protocol to precisely discriminate single-nucleotide differences. We first revealed that a two-step (denaturing and annealing plus elongation) rather than a standard three-step (denaturing, annealing and elongation) method is more suitable for stably hybridising an ORN to its target DNA sequence for sequence-specific suppression of target amplification. We then optimised the ORNi-PCR method using two-step cycles and established a step-by-step technical protocol. The optimised Two-Step ORNi-PCR method could discriminate single-nucleotide differences in genomic DNA or cDNA introduced by genome editing or mutations in cancer cells. In addition, we showed that Two-Step ORNi-PCR can detect the cancer cells possessing a single nucleotide mutation in a target locus mixed with a large number of cells harboring wild-type sequences in the locus so that the number of the cancer cells is only 0.2% of the total cell number. Two-Step ORNi-PCR is useful for simple, precise, cost-effective and positive detection of nucleotide differences in a wide range of molecular biology and medical applications.