Failure to genotype herpes simplex virus by real-time PCR assay and melting curve analysis due to sequence variation within probe binding sites

RT-PCR based SARS-CoV-2 variant screening assays require careful quality control

Background

Distinctive genotypes of SARS-CoV-2 have emerged that are or may be associated with increased transmission, pathogenicity, and/or antibody escape. In many countries, clinical and diagnostic laboratories are under mandate to identify and report these so-called variants of concern (VOC).

Objectives

We used an external quality assessment scheme to determine the scope, accuracy, and reliability of laboratories using various molecular diagnostic assays to identify current VOC (03 March 2021).

Study design

Participant laboratories were sent the same five patient-derived samples and were asked to provide their variant detection methods, variant detection results and interpretation of results.

Results

Twenty-five laboratories reported a range of RT-qPCR-based assays to identify specific variations in the SARS-CoV-2 spike protein that are characteristic of three VOC lineages. Laboratories that detected VOC-associated nucleotide mutations at four specific sites had the highest ratio of correct classification. Low template copy number and additional variation in target regions resulted in loss of confidence and accuracy in sample classification.

Conclusions

Melting-curve-based assays to identify genomic variants are less time-consuming and require less bioinformatic analysis compared to partial or whole genome sequencing. However, our results suggest that correct classification of a given genotype/lineage (e.g., a VOC) relies on the ability to detect more than one variant site, adequate template in the sample (i.e., relatively high viral load/copy number) and results may be unclear in certain samples with additional genetic variations. These initial results suggest that some diagnostic laboratories may require additional training to interpret and report complex genetic information about a dynamic emerging virus.

[RETRACTED: High-throughput sequencing in diagnostics and prevention of herpes simplex virus infection (Herpesviridae, Alphaherpesvirinae, Simplexvirus, Human alphaherpesvirus 1)]

RETRACTEDHerpes simplex viruses types 1 (HSV-1) and 2 (HSV-2) are among the most common viruses in the human population. The clinical manifestations of HSV infection vary widely, which necessitates reliable molecular methods for the timely diagnosis of herpes virus infection, as well as for detection of mutations in the genes responsible for drug resistance. PCR is often unable to detect HSV isolates with nucleotide substitutions at the primer binding site. Sanger sequencing of the whole genome reveals mutations mainly at the consensus level, which accumulate at advanced stages of viral infection. High-throughput sequencing (HTS, next generation sequencing) offers an obvious advantage both in early diagnosis of herpes virus infection and identification of HSV variants.

Development of an EvaGreen-based real-time PCR assay for detection of Aleutian mink disease virus

The objective of this study was to develop a rapid, sensitive and specific EvaGreen (EG)-based real-time PCR assay capable of detecting Aleutian mink disease virus (AMDV) and to evaluate the reliability of the assay for analysis of blood or tissue samples. For this assay, a pair of primers was designed based on a nonstructural protein (NS)-encoding gene of AMDV, and the identity of PCR products was identified based on a melting temperature of 82.8°C. The EG-based real-time PCR assay did not detect canine distemper virus or mink enteritis virus, and the assay could be used to detect Chinese and American AMDV strains, in contrast to a commercial TaqMan kit that could only detect American AMDV strains. The amplification efficiencies of the EG assay were 104.8% for the Chinese strain and 94.4% for the American strain, and the detection limit was 1 copy/μL of AMDV plasmid or 3 pg/μL of viral DNA (Chinese strain). The intra- and inter-assay variation coefficients of melting temperature were all lower than 0.15%, confirming the high reproducibility of the assay. Forty-five clinical blood samples were simultaneously analyzed using the EG real-time PCR, TaqMan kit and conventional PCR, and the detection rates were 91.1%, 0.0% and 86.7%, respectively. Serum samples were also collected from the corresponding blood samples and tested using the counterimmunoelectrophoresis (CIEP) assay, where positive samples accounted for 24.4% of the 45 samples. In conclusion, EG-based real-time PCR is a rapid, sensitive, universal assay that can be effectively utilized as a reliable and specific tool for detection and quantitation of AMDV.

Herpes Simplex Virus Disease Management and Diagnostics in the Era of High-Throughput Sequencing

Herpes simplex virus (HSV) serotypes 1 and 2 are among the most widespread human viruses. HSV disease has a complex phenotype, with symptoms that can range from mild lesions to encephalitis. In the clinical setting, this diversity of outcomes poses a major challenge, making timely disease diagnosis and treatment challenging. High-throughput sequencing (HTS) has been one of the breakthrough technologies in the modern era of molecular biology, and it is revolutionizing the study of pathogen biology and clinical diagnostics. Here, we review recent studies that have used HTS to answer questions related to the evolution of drug resistance, transmission and spread, virulence marker identification, and the design of better antiviral therapeutics for HSV. We also highlight practical considerations for handling computational analysis of HSV genomes and adoption of HTS as a routine diagnostic procedure in the clinical laboratories.

Problematic molecular diagnosis of HSV-1 infection due to a single nucleotide polymorphism in the US7 gene

BACKGROUND

HSV-1 infection is very common worldwide and can be associated with severe medical conditions such as encephalitis and neonatal herpes. Current diagnosis relies heavily on molecular assays targeting the viral genome. One of the pitfalls of these assays is genetic variability, as viral polymorphisms located in the target region can impair the detection of HSV-1.

OBJECTIVES

The aim of this study was to determine if genetic diversity of HSV-1 could account for equivocal assay results we obtained during routine diagnosis of HSV-1 in a genital specimen with the HSV-1 HSV-2 VZV R-gene® assay.

STUDY DESIGN

The presence of HSV-1 in the genital swab studied was assessed using the HSV-1 HSV-2 VZV R-gene® qPCR assay and viral culture. Genetic variability of the patient's HSV-1 isolate was determined using molecular cloning, sequencing and comparison of the sequence of the isolate with those of previously published strains.

RESULTS

We identified an HSV-1 isolate carrying a novel single-nucleotide polymorphism (SNP) located in the US7 gene from a genital swab. This SNP resulted in a missense substitution in the gI protein. It was responsible for the generation of an altered amplification curve when the genital specimen was checked for HSV-1 presence with the Argene HSV-1 HSV-2 VZV R-gene® assay, preventing unequivocal determination of the HSV-1 status of the sample analyzed.

CONCLUSIONS

This study raises awareness of the risk of misdiagnosis of HSV-1 infections when "single-target" molecular assays are employed.

Development of a rapid, sensitive TaqMan real-time RT-PCR assay for the detection of Rose rosette virus using multiple gene targets

Rose rosette virus (RRV), belonging to the genus Emaravirus, is a highly destructive pathogen that causes rose rosette disease. The disease is a major concern for the rose industry in the U.S. due to the lack of highly sensitive methods for early detection of RRV. This is critical, as early identification of the infected plants and eradication is necessary in minimizing the risks associated with the spread of the disease. A highly reliable, specific and sensitive detection assay is thus required to test and confirm the presence of RRV in suspected plant samples. In this study a TaqMan real-time reverse transcription-polymerase chain reaction (RT-PCR) assay was developed for the detection of RRV from infected roses, utilizing multiple gene targets. Four pairs of primers and probes; two of them (RRV_2-1 and RRV_2-2) based on the consensus sequences of the glycoprotein gene (RNA2) and the other two (RRV_3-2 and RRV_3-5) based on the nucleocapsid gene (RNA3) were designed. The specificity of the primers and probes was evaluated against other representative viruses infecting roses, belonging to the genera Alfamovirus, Cucumovirus, Ilarvirus, Nepovirus, Tobamovirus, and Tospovirus and one Emaravirus (Wheat mosaic virus). Dilution assays using the in vitro transcripts (spiked with total RNA from healthy plants, and non-spiked) showed that all the primers and probes are highly sensitive in consistently detecting RRV with a detection limit of 1 fg. Testing of the infected plants over a period of time (three times in monthly intervals) indicated high reproducibility, with the primer/probe RRV_3-5 showing 100% positive detection, while RRV_2-1, RRV_2-2 and RRV_3-2 showed 90% positive detection. The developed real-time RT-PCR assay is reliable, highly sensitive, and can be easily used in diagnostic laboratories for testing and confirmation of RRV.

Detection of torque teno sus virus 1 and 2 in porcine tissues by in situ hybridization using multi-strained pooled probes

Porcine torque teno sus virus (TTSuV) has been suggested as a co-factor for the development of porcine circovirus-associated diseases. However, the pathogenic role of TTSuV is still inconclusive, and the target cell and tissue tropism of this virus are also ambiguous. In the present study, a multi-strained pooled probe-based in situ hybridization was established to detect the nucleic acids of TTSuV1 and TTSuV2 in the tissue. The strategy of using polymerase chain reaction-derived digoxigenin-labeled multi-strained pooled probe, instead of single-strained probe or oligonucleotide, was to overcome the fact of high sequence diversity among TTSuV strains and simultaneous infection with distinct strains of TTSuV in the same animal. The cell tropism and tissue distribution were evaluated by grading system with tissues from major organs. Lymphoid tissues, including superficial inguinal, mesenteric, and hilar lymph nodes, tonsil, intestinal lamina propria of mucosa and Peyer's patches, and sometimes spleen, generally contained higher levels of positive signals and are considered as the target sites for TTSuV. Morphologically, the distribution of TTSuV-positive signals had a strong correlation with the T lymphocyte zone. T lymphocytes are, thus, speculated as the major target cells for TTSuV.

Nucleic acid amplification technology for the diagnosis of genital herpes infection

Nucleic acid amplification technologies are well-characterized methods for the rapid and sensitive diagnosis of a wide variety of infectious diseases. Herpes simplex virus amplification is no exception, demonstrating as much as a ninefold enhancement in sensitivity over viral culture in some settings. Currently, there are no US Food and Drug Administration-approved systems for herpes simplex virus amplification; hence, most laboratories utilize in-house-developed PCR-based systems. Unfortunately, the utilization of herpes simplex virus amplification has been confined to the investigations of suspected herpes simplex virus encephalitis, largely due to cost and the need for appropriately trained technical staff. Recent methodological advances will help to render herpes simplex virus nucleic acid amplification technologies more applicable to routine practice. However, the use of nucleic acid amplification technologies for the diagnosis of genital herpes infection remains highly controversial.

Characterization of a novel melt curve by use of the Roche LightCycler HSV 1/2 analyte-specific reagent real-time PCR assay: frequencies of this novel (low) melt curve and commonly encountered (intermediate) melt curves

We characterize a novel probe binding-site polymorphism detectable solely by melt curve analysis using the Roche LightCycler HSV 1/2 analyte-specific reagent real-time PCR assay. The frequencies of this novel (47°C) and previously described intermediate (60 to 62°C) melt curves were 0.016% and 4.9%, respectively.

EvaGreen real-time PCR protocol for specific 'Candidatus Phytoplasma mali' detection and quantification in insects

In this paper the validation and implementation of a Real-time PCR protocol based on ribosomal protein genes has been carried out for sensitive and specific quantification of 'Candidatus (Ca.) Phytoplasma mali' (apple proliferation phytoplasma, APP) in insects. The method combines the use of EvaGreen(®) dye as chemistry detection system and the specific primer pair rpAP15f-mod/rpAP15r3, which amplifies a fragment of 238 bp of the ribosomal protein rplV (rpl22) gene of APP. Primers specificity was demonstrated by running in the same Real-time PCR 'Ca. Phytoplasma mali' samples with phytoplasmas belonging to the same group (16SrX) as 'Ca. Phytoplasma pyri' and 'Ca. Phytoplasma prunorum', and also phytoplasmas from different groups, as 'Ca. Phytoplasma phoenicium' (16SrIX) and Flavescence dorée phytoplasma (16SrV). 'Ca. Phytoplasma mali' titre in insects was quantified using a specific approach, which relates the concentration of the phytoplasma to insect 18S rDNA. Absolute quantification of APP and insect 18S rDNA were calculated using standard curves prepared from serial dilutions of plasmids containing rplV-rpsC and a portion of 18S rDNA genes, respectively. APP titre in insects was expressed as genome units (GU) of phytoplasma per picogram (pg) of individual insect 18S rDNA. 'Ca. Phytoplasma mali' concentration in examined samples (Cacopsylla melanoneura overwintered adults) ranged from 5.94 × 10(2) to 2.51 × 10(4) GU/pg of insect 18S rDNA. Repeatability and reproducibility of the method were also evaluated by calculation of the coefficient of variation (CV%) of GU of phytoplasma and pg of 18S rDNA fragment for both assays. CV less than 14% and 9% (for reproducibility test) and less than 10 and 11% (for repeatability test) were obtained for phytoplasma and insect qPCR assays, respectively. Sensitivity of the method was also evaluated, in comparison with conventional 16S rDNA-based nested-PCR procedure. The method described has been demonstrated reliable, sensitive and specific for the quantification of 'Ca. Phytoplasma mali' in insects. The possibility to study the trend of phytoplasma titre in the vectors will allow a deepen investigation on the epidemiology of the disease.

Diagnostics for Herpes Simplex Virus

Herpes simplex virus (HSV) is one of the most common, yet frequently overlooked, sexually transmitted infections. Since the type of HSV infection affects prognosis and subsequent counseling, type-specific testing to distinguish HSV-1 from HSV-2 is recommended. Although PCR has been the diagnostic standard for HSV infections of the central nervous system, until now viral culture has been the test of choice for HSV genital infection. However, HSV PCR, with its consistently and substantially higher rate of HSV detection, will likely replace viral culture as the gold standard for the diagnosis of genital herpes in people with active mucocutaneous lesions, regardless of anatomic location or viral type. Alternatively, type-specific serologic tests based on glycoprotein G should be the test of choice to establish the diagnosis of HSV infection when no active lesion is present. Given the difficulty in making the clinical diagnosis of HSV, the growing worldwide prevalence of genital herpes and the availability of effective antiviral therapy, there is an increased demand for rapid, accurate laboratory diagnosis of patients with HSV.

Development of a multi-target TaqMan assay to detect eastern equine encephalitis virus variants in mosquitoes

Disease outbreaks caused by eastern equine encephalitis virus (EEEV; Togaviridae, Alphavirus) may be prevented by implementing effective surveillance and intervention strategies directed against the mosquito vector. Methods for EEEV detection in mosquitoes include a real-time reverse transcriptase PCR technique (TaqMan assay), but we report its failure to detect variants isolated in Connecticut in 2011, due to a single base-pair mismatch in the probe-binding site. To improve the molecular detection of EEEV, we developed a multi-target TaqMan assay by adding a second primer/probe set to provide redundant targets for EEEV detection. The multi-target TaqMan assay had similar performance characteristics to the conventional assay, but also detected newly-evolving strains of EEEV. The approach described here increases the reliability of the TaqMan assay by creating back-up targets for virus detection without sacrificing sensitivity or specificity.

Improved detection of genetic markers of antimicrobial resistance by hybridization probe-based melting curve analysis using primers to mask proximal mutations: examples include the influenza H275Y substitution

OBJECTIVES

Numerous real-time PCR assays have been described for detection of the influenza A H275Y alteration. However, the performance of these methods can be undermined by sequence variation in the regions flanking the codon of interest. This is a problem encountered more broadly in microbial diagnostics.

METHODS

In this study, we developed a modification of hybridization probe-based melting curve analysis, whereby primers are used to mask proximal mutations in the sequence targets of hybridization probes, so as to limit the potential for sequence variation to interfere with typing. The approach was applied to the H275Y alteration of the influenza A (H1N1) 2009 strain, as well as a Neisseria gonorrhoeae mutation associated with antimicrobial resistance. Assay performances were assessed using influenza A and N. gonorrhoeae strains characterized by DNA sequencing.

RESULTS

The modified hybridization probe-based approach proved successful in limiting the effects of proximal mutations, with the results of melting curve analyses being 100% consistent with the results of DNA sequencing for all influenza A and N. gonorrhoeae strains tested. Notably, these included influenza A and N. gonorrhoeae strains exhibiting additional mutations in hybridization probe targets. Of particular interest was that the H275Y assay correctly typed influenza A strains harbouring a T822C nucleotide substitution, previously shown to interfere with H275Y typing methods.

CONCLUSIONS

Overall our modified hybridization probe-based approach provides a simple means of circumventing problems caused by sequence variation, and offers improved detection of the influenza A H275Y alteration and potentially other resistance mechanisms.

SYTO dyes and EvaGreen outperform SYBR Green in real-time PCR

Background

Real-time PCR can be carried out using either probes or DNA dyes. SYBR Green has been used the most, but it suffers from several drawbacks. Numerous other DNA dyes are commercially available, but with limited structural information. Dye behavior in real time PCR is difficult to predict, so empirical data are needed. In the work described here, a panel of 23 different DNA dyes--including green, orange, and red SYTO dyes, EvaGreen, and SYBR Green--were evaluated with respect to their performance in real time PCR.

Findings

Data were analyzed for reaction inhibition, effects on amplicon melting temperature, fluorescent signal strength, and reaction efficiency. This is the first report of reaction efficiency using alternatives to SYBR Green. Results indicated substantial variation in performance even within the SYTO dye family. EvaGreen and the SYTO dyes 13, 16, 80, and 82 performed better than SYBR Green in general, and high reaction efficiencies can be achieved using these dyes.

Conclusions

Empirical data were generated for 23 DNA dyes. This paper confirms and extends previous findings that among commercially available DNA dyes, EvaGreen and certain SYTO dyes are the most desirable alternatives to the commonly used SYBR Green in real-time PCR.

Molecular detection of herpesviruses

In routine molecular diagnostics, detection of herpesviruses has made a major impact. Infection with herpesviruses is indicated by demonstrating the presence of the virus in selected specimens. Rapid and reliable detection of herpesvirus DNA helps to decrease the lethality as well as the sequelae of herpesvirus infection in patients at risk. This chapter discusses specimen types and both laboratory-developed and commercially available assays useful for molecular detection of herpesviruses. To meet the need for reliable laboratory results, it is advisable to employ maximum automated and standardized kits based on reagents and standards of reproducible high quality. In the routine diagnostic laboratory, introduction of IVD/CE and/or FDA-labeled tests is preferred.

Development of SYBR green-based real-time PCR and duplex nested PCR assays for quantitation and differential detection of species- or type-specific porcine Torque teno viruses

Porcine Torque teno virus (TTV), a single-stranded circular DNA virus, has been incriminated in swine diseases recently. Multiple infection with porcine TTV species 1 (PTTV1) and species 2 (PTTV2), each consisting of two types (PTTV1a and 1b) or subtypes (PTTV2b and 2c), in a single pig had been reported by our group previously. The present study described three novel assays for quantitation and differential detection of porcine TTV. First, we developed two SYBR green-based real-time PCR assays to quantify viral loads of two porcine TTV species, respectively. The PTTV1- and PTTV2-specific real-time PCR primer sequences were selected to target conserved regions identified by multiple alignments of ten available porcine TTV full-length genomes. Furthermore, by coupling the two singleplex PCR assays, a duplex real-time PCR assay followed by melting curve analysis was established for simultaneous detection and differentiation of PTTV1 and PTTV2. In addition, a type-specific duplex nested PCR was also developed to simultaneously detect and distinguish between the two types, PTTV1a and 1b, in PTTV1 species. These assays provide rapid and practical tools for molecular diagnosis of species- or type-specific porcine TTV.

Identification of two GH18 chitinase family genes and their use as targets for detection of the crayfish-plague oomycete Aphanomyces astaci

Background

The oomycete Aphanomyces astaci is regarded as the causative agent of crayfish plague and represents an evident hazard for European crayfish species. Native crayfish populations infected with this pathogen suffer up to 100% mortality. The existence of multiple transmission paths necessitates the development of a reliable, robust and efficient test to detect the pathogen. Currently, A. astaci is diagnosed by a PCR-based assay that suffers from cross-reactivity to other species. We developed an alternative closed-tube assay for A. astaci, which achieves robustness through simultaneous amplification of multiple functionally constrained genes.

Results

Two novel constitutively expressed members of the glycosyl hydrolase (GH18) gene family of chitinases were isolated from the A. astaci strain Gb04. The primary amino acid sequence of these chitinase genes, termed CHI2 and CHI3, is composed of an N-terminal signal peptide directing the post-translational transport of the protein into the extracellular space, the catalytic GH18 domain, a proline-, serine-, and threonine-rich domain and a C-terminal cysteine-rich putative chitin-binding site. The A. astaci mycelium grown in a pepton-glucose medium showed significant temporal changes in steady-state CHI2 and CHI3 mRNA amounts indicating functional constraint. Their different temporal occurrence with maxima at 48 and 24 hours of incubation for CHI2 and CHI3, respectively, is in accordance with the multifunctionality of GH18 family members. To identify A. astaci-specific primer target sites in these novel genes, we determined the partial sequence homologs in the related oomycetes A. frigidophilus, A. invadans, A. helicoides, A. laevis, A. repetans, Achlya racemosa, Leptolegnia caudata, and Saprolegnia parasitica, as well as in the relevant fungi Fusarium solani and Trichosporon cutaneum. An A. astaci-specific primer pair targeting the novel genes CHI2 and CHI3 as well as CHI1 - a third GH18 family member - was multiplexed with primers targeting the 5.8S rRNA used as an endogenous control. A species was typed unambiguously as A. astaci if two peaks were concomitantly detected by melting curve analysis (MCA). For sensitive detection of the pathogen, but also for quantification of agent levels in susceptible crayfish and carrier crayfish, a TaqMan-probe based real-time PCR (qPCR) assay was developed. It targets the same chitinase genes and allows quantification down to 25 target sequences.

Conclusion

The simultaneous qualitative detection of multiple sequences by qPCR/MCA represents a promising approach to detect species with elevated levels of genetic variation and/or limited available sequence information. The homogenous closed-tube format, reduced detection time, higher specificity, and the considerably reduced chance of false negative detection achieved by targeting multiple genes (CHI1, CHI2, CHI3, and the endogenous control) at least two of which are subject to high functional constraint, are the major advantages of this multiplex assay compared to other diagnostic methods.

Sensitive quantification achieved with TaqMan qPCR facilitates to monitor infection status and pathogen distribution in different tissues and can help prevent disease transmission.

False-negative results in nucleic acid amplification tests-do we need to routinely use two genetic targets in all assays to overcome problems caused by sequence variation?

Nucleic acid amplification tests (NAATs) have numerous advantages over traditional diagnostic techniques and so are now widely used by diagnostic laboratories for routine detection of infectious agents. However, there is some concern over the increasing numbers of reports of NAAT false-negative results caused by sequence variation. Highly conserved NAAT target sequences have been reported for many organisms, yet sequence-related problems continue to be observed in commercial and in-house assays targeting a broad range of microbial pathogens. In light of these ongoing problems, it may be time to consider the use of two genetic targets in NAAT methods to reduce the potential for sequence-related false-negative results.

Description and validation of a novel real-time RT-PCR enterovirus assay

BACKGROUND

Enteroviruses are a leading cause of aseptic meningitis in adult and pediatric populations. We describe the development of a real-time RT-PCR assay that amplifies a small target in the 5' nontranslated region upstream of the classical Rotbart enterovirus amplicon. The assay includes an RNA internal control and incorporates modified nucleotide chemistry.

METHODS

We evaluated the performance characteristics of this design and performed blinded parallel testing on clinical samples, comparing the results with a commercially available RT-PCR assay (Pan-Enterovirus OligoDetect kit) that uses an enzyme immunoassay-like plate end detection.

RESULTS

We tested 778 samples and found 14 discrepant samples between the 2 assays. Of these, the real-time assay detected 6 samples that were negative by the OligoDetect kit, 5 of which were confirmed as positive by sequence analysis using an alternative primer set. Eight discrepant samples were positive by the OligoDetect kit and real-time negative, with 6 confirmed by sequencing. Overall, detection rates of 97% and 96% were obtained for the OligoDetect kit and real-time assays, respectively. Sequence analysis revealed the presence of a number of single nucleotide polymorphisms in the targeted region. The comparative sensitivities of the 2 assays were equivalent, with the limit of detection for the real-time assay determined to be approximately 430 copies per milliliter in cerebrospinal fluid.

CONCLUSIONS

This novel real-time enterovirus assay is a sensitive and suitable assay for routine clinical testing. The presence of single nucleotide polymorphisms can affect real-time PCR assays.

Unlabeled probes for the detection and typing of herpes simplex virus

BACKGROUND

Unlabeled probe detection with a double-stranded DNA (dsDNA) binding dye is one method to detect and confirm target amplification after PCR. Unlabeled probes and amplicon melting have been used to detect small deletions and single-nucleotide polymorphisms in assays where template is in abundance. Unlabeled probes have not been applied to low-level target detection, however.

METHODS

Herpes simplex virus (HSV) was chosen as a model to compare the unlabeled probe method to an in-house reference assay using dual-labeled, minor groove binding probes. A saturating dsDNA dye (LCGreen Plus) was used for real-time PCR. HSV-1, HSV-2, and an internal control were differentiated by PCR amplicon and unlabeled probe melting analysis after PCR.

RESULTS

The unlabeled probe technique displayed 98% concordance with the reference assay for the detection of HSV from a variety of archived clinical samples (n = 182). HSV typing using unlabeled probes was 99% concordant (n = 104) to sequenced clinical samples and allowed for the detection of sequence polymorphisms in the amplicon and under the probe.

CONCLUSIONS

Unlabeled probes and amplicon melting can be used to detect and genotype as few as 10 copies of target per reaction, restricted only by stochastic limitations. The use of unlabeled probes provides an attractive alternative to conventional fluorescence-labeled, probe-based assays for genotyping and detection of HSV and might be useful for other low-copy targets where typing is informative.

Routine human papillomavirus genotyping by DNA sequencing in community hospital laboratories

Background

Human papillomavirus (HPV) genotyping is important for following up patients with persistent HPV infection and for evaluation of prevention strategy for the individual patients to be immunized with type-specific HPV vaccines. The aim of this study was to optimize a robust "low-temperature" (LoTemp™) PCR system to streamline the research protocols for HPV DNA nested PCR-amplification followed by genotyping with direct DNA sequencing. The protocol optimization facilitates transferring this molecular technology into clinical laboratory practice. In particular, lowering the temperature by 10°C at each step of thermocycling during in vitro DNA amplification yields more homogeneous PCR products. With this protocol, template purification before enzymatic cycle primer extensions is no longer necessary.

Results

The HPV genomic DNA extracted from liquid-based alcohol-preserved cervicovaginal cells was first amplified by the consensus MY09/MY11 primer pair followed by nested PCR with GP5+/GP6+ primers. The 150 bp nested PCR products were subjected to direct DNA sequencing. The hypervariable 34–50 bp DNA sequence downstream of the GP5+ primer site was compared to the known HPV DNA sequences stored in the GenBank using on-line BLAST for genotyping. The LoTemp™ ready-to-use PCR polymerase reagents proved to be stable at room temperature for at least 6 weeks. Nested PCR detected 107 isolates of HPV in 513 cervicovaginal clinical samples, all validated by DNA sequencing. HPV-16 was the most prevalent genotype constituting 29 of 107 positive cases (27.2%), followed by HPV-56 (8.5%). For comparison, Digene HC2 test detected 62.6% of the 107 HPV isolates and returned 11 (37.9%) of the 29 HPV-16 positive cases as "positive for high-risk HPV".

Conclusion

The LoTemp™ ready-to-use PCR polymerase system which allows thermocycling at 85°C for denaturing, 40°C for annealing and 65°C for primer extension can be adapted for target HPV DNA amplification by nested PCR and for preparation of clinical materials for genotyping by direct DNA sequencing. HPV genotyping is performed by on-line BLAST algorithm of a hypervariable L1 region. The DNA sequence is included in each report to the physician for comparison in following up patients with persistent HPV infection, a recognized tumor promoter in cancer induction.

Invader plus method detects herpes simplex virus in cerebrospinal fluid and simultaneously differentiates types 1 and 2

We report here on the development and validation of a prototype Invader Plus method for the qualitative detection of herpes simplex virus types 1 and 2 in cerebrospinal fluid (CSF). The method combines PCR and Invader techniques in a single, closed-tube, continuous-reaction format that gives an analytical sensitivity of approximately 10 copies per reaction. The clinical sensitivity and specificity were 100.0% and 98.6%, respectively, when the results of the method were validated against the results obtained with a PCR colorimetric microtiter plate system by use of clinical CSF specimens.

Quantification of the slug parasitic nematode Phasmarhabditis hermaphrodita from soil samples using real time qPCR

Phasmarhabditis hermaphrodita is a nematode parasite that infects and kills several species of slugs. The nematode is produced commercially as a biological control agent for slug pests of agriculture and horticulture. Given the difficulties of distinguishing this species from other nematode species in soil samples, very little is known about its natural ecology or its behaviour and persistence following application for biological control. Here we describe a method to quantify P. hermaphrodita in soil samples based on real time PCR. We designed primers and a dual labelled fluorescent probe that can be used to quantify numbers of P. hermaphrodita and which is capable of distinguishing this species from the morphologically identical Phasmarhabditis neopapillosa. We compared different methods whereby the entire nematode community is extracted prior to DNA extraction, and three methods to extract DNA directly from soil samples. Both nematode extraction and DNA extraction from large (10 g) samples of soil gave reliable estimates of nematode numbers, but methods which extracted DNA from small (1g or less) soil samples substantially underestimated numbers. However, direct extraction of DNA from soils may overestimate numbers of live nematodes as DNA from dead nematodes was found to persist in soil for at least 6 days. The technique could be modified for detection and quantification of all soil borne parasitic nematodes.

Evaluation of the Cepheid herpes simplex virus typing real-time PCR assay using dermal and genital specimens

The Cepheid herpes simplex virus (HSV) (Cepheid, Sunnyvale, CA) typing multiplex real-time polymerase chain reaction (PCR) assay was evaluated for its ability to detect HSV in dermal and genital specimens stored in M5 media. Swab specimens (n = 114) for HSV testing were placed in M5 media and split between our laboratory and a highly experienced reference laboratory. Aliquots for testing with the Cepheid assay were processed using a simple boil-and-go procedure and then run in a SmartCycler II (Cepheid). Aliquots tested at the reference laboratory were processed using a MagNA Pure LC DNA extractor (Roche Molecular Systems, Alameda, CA) and tested by the Roche HSV real-time PCR assay. Both laboratories detected 35 positives. Of the positive specimens, the Cepheid assay typed 16 as HSV 1 and 19 as HSV 2; the reference laboratory typed 15 as HSV 1, 19 as HSV 2, and 1 as HSV indeterminate. Our results demonstrate that the Cepheid real-time PCR assay, using specimens subjected to minimal specimen processing, performed as well as the Roche real-time PCR assay, using DNA extracts, for the detection of HSV DNA in genital and dermal specimens.

Real-time PCR in clinical microbiology: applications for routine laboratory testing

Real-time PCR has revolutionized the way clinical microbiology laboratories diagnose many human microbial infections. This testing method combines PCR chemistry with fluorescent probe detection of amplified product in the same reaction vessel. In general, both PCR and amplified product detection are completed in an hour or less, which is considerably faster than conventional PCR detection methods. Real-time PCR assays provide sensitivity and specificity equivalent to that of conventional PCR combined with Southern blot analysis, and since amplification and detection steps are performed in the same closed vessel, the risk of releasing amplified nucleic acids into the environment is negligible. The combination of excellent sensitivity and specificity, low contamination risk, and speed has made real-time PCR technology an appealing alternative to culture- or immunoassay-based testing methods for diagnosing many infectious diseases. This review focuses on the application of real-time PCR in the clinical microbiology laboratory.

Rapid and accurate detection of the CFTR gene mutation 1811+1.6kbA>G by real-time fluorescence resonance energy transfer PCR

The CFTR gene mutation 1811+1.6 kbA>G has been reported as associated with a severe phenotype of cystic fibrosis with pancreatic insufficiency. This mutation has been identified as a rather common one in the South West of France and in the Iberian Peninsula. Because of the precise geographical origin of the subjects and its frequency, the mutation has to be investigated with accuracy. We have developed an original real-time Fluorescence Resonance Energy Transfer (FRET) PCR assay for genotyping the mutation 1811+1.6 kbA>G. It is based on the amplification of a region spanning the mutation with simultaneous detection of the amplicon by hybridization with a bi-probe followed by a melting curve analysis. The results obtained are identical with those resulting from either restriction fragment length polymorphism analysis or sequencing. The distinction between the wild type and the mutation 1811+1.6 kbA>G is easy because the corresponding melting points shows a difference of 6 or 9.5 degrees C depending on the associated SNP A/T located 16 bp downstream. We demonstrated that a FRET assay showed enough sensitivity to discriminate between two nucleotide polymorphisms (SNPs) in the sequence of the sensor. In conclusion, this method is specific, fast, easy to perform, reproducible, inexpensive as it uses only one bi-probe and well adapted to daily practice.

Detection and typing of herpes simplex DNA in genital swabs by real-time polymerase chain reaction

The LightCycler polymerase chain reaction (PCR) is a sensitive assay for the detection of Herpes simplex virus (HSV) DNA in muco-cutaneous swabs. Software-based analysis of the probe melting temperature (Tm) can be used to discriminate between HSV types (HSV-1 and HSV-2). Among 76 HSV DNA positive genital swabs, atypical Tms were observed in 14 (18%). The 14 samples were all typed as HSV-2 by sequence alignment. In 4/14 samples, the atypical Tm was associated with sequence variation at the probe-binding site. Among 10 samples with conserved sequences, Tms were influenced by the specimen preparation method prior to PCR. These findings indicate that multiple factors including, but not limited to sequence variation complicate melting curve analysis following real-time PCR. Alternative typing methods are recommended for specimens with atypical melting curves.

Detection and typing of Herpes Simplex virus (HSV) in mucocutaneous samples by TaqMan PCR targeting a gB segment homologous for HSV types 1 and 2

Herpes simplex virus types 1 (HSV-1) and 2 (HSV-2) are major causes of mucocutaneous lesions and severe infections of the central nervous system. Here a new semiautomated method for detecting and typing of HSV was used to analyze 479 mucocutaneous swab samples. After DNA extraction using a Magnapure LC robot, a 118-bp segment of the gB region was amplified by real-time PCR utilizing type-specific TaqMan probes to identify HSV-1 or HSV-2. HSV detection in a single well using probes labeled with carboxyfluorescein (FAM) for HSV-1 and JOE (6-carboxy-4',5'-dichloro-2',7'-dimethoxyfluorescein) for HSV-2 had a sensitivity similar to that seen in separate reactions. All but one of 217 samples (99.5%) that had been positive by virus culture were positive by TaqMan PCR, with a correct identification of type in all cases. Out of 262 samples negative by virus culture, 48 (18.3%) were positive by TaqMan PCR, with higher Ct values compared with culture positive samples (P < 0.0001). Overall, the Ct values for HSV-1 were lower than for HSV-2 (mean, 25.5 versus 27.9), but to some extent this could be due to weaker fluorescence by JOE. Lower C(t) values for HSV-1 were seen also in the 202 genital samples (79 HSV-1, 122 HSV-2, 1 HSV-1 and HSV-2), indicating that HSV-1 replicates as well as HSV-2 in the genital area. HSV-1 constituted 40% of genital infections and was associated with lower mean age (29.2 versus 36.4 years), probably reflecting the fact that recurrent genital HSV-1 infections are rare.

Effect of sequence polymorphisms on performance of two real-time PCR assays for detection of herpes simplex virus

Herpes simplex virus (HSV) is the most common cause of acquired, sporadic encephalitis in the United States. PCR identification of HSV in spinal fluid has become the diagnostic gold standard due to its sensitivity and potential for speed, replacing other methods such as culture. We developed a real-time PCR assay to detect HSV, using a new type of hybridization probe, the Eclipse probe. In this study, we ran 323 samples (171 positives and 152 negatives) with the Eclipse real-time PCR assay and compared these results with another PCR assay using gel detection. The real-time assay agreed with our reference method for 319 out of the 323 samples tested (99%). Using two different real-time PCR platforms, we discovered that SNPs within the amplicon's probe binding region that are used to distinguish HSV-1 from HSV-2 can decrease assay sensitivity. This problem is potentially a general one for assays using fluorescent probes to detect target amplification in a real-time format. While real-time PCR can be a powerful tool in the field of infectious disease, careful sequence evaluation and clinical validation are essential in creating an effective assay.

Sequencing and resolution of amplified herpes simplex virus DNA with intermediate melting curves as genotype 1 or 2 by LightCycler PCR assay

DNA from 101 specimens containing herpes simplex virus (HSV) produced atypical intermediate melting curves compared with those expected for HSV type 1 or HSV type 2 subsequent to real-time PCR. Nucleic acid sequence analysis of amplified target DNA revealed 1- or 3-bp polymorphisms in the probe region which allowed designation of these viruses as HSV genotype 1 or HSV genotype 2. These two subpopulations of HSV were also identified as HSV genotype 1 or HSV genotype 2 using another commercially available PCR method. Amplified HSV target DNA producing intermediate melting curves could be designated as HSV genotype 1 or HSV genotype 2 without performing sequencing or another PCR method with 96/101 (95%) specimens by adding known intermediate HSV DNA characteristic for the two subpopulations as controls.

Use of two real-time polymerase chain reactions (PCRs) to detect herpes simplex type 1 and 2-DNA after automated extraction of nucleic acid

Herpes simplex virus infections may be diagnosed by several techniques, including conventional cell culture and the polymerase chain reaction (PCR). This prospective study compares the analytical performances and usefulness of an in-house real-time PCR method and the Light Cycler HSV (1/2) detection kit (Roche Diagnostics, Mannheim, Germany). The results of both PCRs were then compared to those obtained by conventional cell culture. A total of 313 samples were tested (70 dermal samples, 81 cerebrospinal fluids (CSF), 47 ocular, 42 anogenital, 34 throat swabs, and 33 oral samples, 3 whole blood, 2 biopsies, and 1 bronchoalveolar lavage). Samples for molecular assays were extracted twice with the MagNa Pure instrument (Roche Molecular Biochemicals, Mannheim, Germany) and tested blind in parallel by the two PCR methods. Most (226) samples were also examined by cell culture. Forty three samples were found positive by both PCRs, whereas 267 were negative. The HSV-1 and -2 typing of positive samples was identical. Three of the samples were positive in the in-house PCR and negative in the Light Cycler HSV (1/2) detection kit. There was no statistically significant difference between the two tests. Only one sample gave an invalid result due to negative PCR and negative internal control result. Seven samples were positive by both real-time PCRs and negative in conventional culture. The PCRs were significantly (P < 0.05) more sensitive. The results show good agreement between the two real-time PCR methods, with the molecular tests being more sensitive than cell culture.

SYBR green-based real-time quantitative PCR assay for detection of West Nile Virus circumvents false-negative results due to strain variability

Real-time quantitative PCR is used routinely for the high-throughput diagnosis of viral pathogens, such as West Nile virus (WNV). Rapidly evolving RNA viruses present a challenge for diagnosis because they accumulate mutations that may render them undetectable. To explore the effect of sequence variations on assay performance, we generated every possible single point mutation within the target region of the widely used TaqMan assay for WNV and found that the TaqMan assay failed to detect 47% of possible single nucleotide variations in the probe-binding site and was unable to detect any targets with more than two mutations. In response, we developed and validated a less expensive assay with the intercalating dye SYBR green. The SYBR green-based assay was as sensitive as the TaqMan assay for WNV. Importantly, it detected 100% of possible WNV target region variants. The assay developed here adds an additional layer of protection to guard against false-negative results that result from natural variations or drug-directed selection and provides a rapid means to identify such variants for subsequent detailed analysis.

Herpes simplex virus genotyping: multiple optional PCR-based RFLP systems and a non-isotopic single-strand conformation polymorphism method

Polymerase chain reaction (PCR)-based methods for herpes simplex virus (HSV) types I/II genotyping are described. These methods are based on the single-stranded conformation properties of DNA molecules obtained by PCR (PCR-SSCP) and restriction fragment analysis of PCR products (PCR-RFLP). With the aim to analyse these two genotyping techniques, genomic DNA from the standard viral strains KOS and G, and also 79 normal cervical samples were studied for HSV I/II. Sequence analysis showed 19 different RFLP possible systems for HSV typing. Five systems were used in this study and all led to the expected fragments obtained by sequence analysis. PCR-SSCP showed distinct patterns for both viral types and had 100% of concordance with PCR-RFLP results. The prevalence of herpes simplex virus in the normal cervical samples were 21.5% (17 out of 79), the most frequent viral type was HSV-2, with a prevalence of 71% (12 out of 17). Both techniques appeared suitable for HSV I/II genotyping and are easy to perform in most clinical laboratories.