Single-Point Mutations in the N Gene of SARS-CoV-2 Adversely Impact Detection by a Commercial Dual Target Diagnostic Assay

Neglected SARS-CoV-2 variants and potential concerns for molecular diagnostics: a framework for nucleic acid amplification test target site quality assurance

IMPORTANCE

Molecular tests like polymerase chain reaction were widely used during the COVID-19 pandemic but as the pandemic evolved, so did SARS-CoV-2. This virus acquired mutations, prompting concerns that mutations could compromise molecular test results and be falsely negative. While some manufacturers may have in-house programs for monitoring mutations that could impact their assay performance, it is important to promptly report mutations in circulating viral strains that could adversely impact a diagnostic test result. However, commercial test target sites are proprietary, making independent monitoring difficult. In this study, SARS-CoV-2 test target sites were sequenced to monitor and assess mutations impact, and 29 novel mutations impacting SARS-CoV-2 detection were identified. This framework for molecular test target site quality assurance could be adapted to any molecular test, ensuring accurate diagnostic test results and disease diagnoses.

SARS-CoV-2 Nucleocapsid Protein Mutations Found in Switzerland Disrupt N-Gene Amplification in Commonly Used Multiplex RT-PCR Assay

At the end of 2021, we observed an increase in N-gene target failures (NGTF) with the TaqPathTM COVID-19 CE-IVD RT-PCR Kit from Thermo Fisher Scientific (TaqPath). We subsequently used whole-genome sequencing (Oxford Nanopore Technology) to identify potential issues with N-gene PCR efficacy. Among 168,101 positive samples with a cycle threshold (CT) value <30 from August 2021 to May 2022, 194 specimens without N-gene amplification by PCR were identified (0.12%). Most NGTF samples originated from a wave of infection attributable to the Delta variant (B.1.617.2) and its sublineages. Sequencing revealed the nucleotide substitution G28922T (A217S) in 151 samples (88.8%). The substitution G215C, a hallmark mutation for Delta lineages, was concurrently present in all of these samples. Ten samples (5.9%) carried the deletion 28,913-28,918 (del214/215), eight samples (4.7%) the deletion 28,913-28,915 (del214) and one sample (0.6%) the deletion 28,892-28,930 (del207-219). Samples showing intact N-gene amplification by PCR lacked these specific mutations, but delayed-type amplification (i.e., partial or pNGTF) was attributable to the exclusive presence of A217S. As the N gene is a common target in many RT-PCR methods for SARS-CoV-2, an in-depth analysis of single-target failures using a combination with viral whole genome sequencing may allow for the identification of diagnostic flaws and eventual new variants.

Genomic characterization of SARS-CoV-2 from Uganda using MinION nanopore sequencing

SARS-CoV-2 undergoes frequent mutations, affecting COVID-19 diagnostics, transmission and vaccine efficacy. Here, we describe the genetic diversity of 49 SARS-CoV-2 samples from Uganda, collected during the COVID-19 waves of 2020/2021. Overall, the samples were similar to previously reported SARS-CoV-2 from Uganda and the Democratic Republic of Congo (DRC). The main lineages were AY.46 and A.23, which are considered to be Delta SARS-CoV-2 variants. Further, a total of 268 unique single nucleotide variants and 1456 mutations were found, with more than seventy percent mutations in the ORF1ab and S genes. The most common mutations were 2042C>G (83.4%), 14143C>T (79.5%), 245T>C (65%), and 1129G>T (51%), which occurred in the S, ORF1ab, ORF7a and N genes, respectively. As well, 28 structural variants-21 insertions and 7 deletions, occurred in 16 samples. Our findings point to the possibility that most SARS-CoV-2 infections in Uganda at the time arose from local spread and were not newly imported. Moreover, the relatedness of variants from Uganda and the DRC reflects high human mobility and interaction between the two countries, which is peculiar to this region of the world.

A mutation responsible for impaired detection by the Xpert SARS-CoV-2 assay independently emerged in different lineages during the SARS-CoV-2 pandemic

BACKGROUND

COVID-19 diagnosis lies on the detection of SARS-CoV-2 on nasopharyngeal specimens by RT-PCR. The Xpert-Xpress SARS-CoV-2 assay provides results in less than one hour from specimen reception, which makes it suitable for clinical/epidemiological circumstances that require faster responses. The analysis of a COVID-19 outbreak suspected in the neonatology ward from our institution showed that the Ct values obtained for the targeted genes in the Xpert assay were markedly different within each specimen (N Ct value > 20 cycles above the E Ct value).

RESULTS

We identified the mutation C29200T in the N gene as responsible for an impairment in the N gene amplification by performing whole genome sequencing of the specimens involved in the outbreak (Omicron variant). Subsequently, a retrospective analysis of all specimens sequenced in our institution allowed us to identify the same SNP as responsible for similar impairments in another 12 cases (42% of the total cases reported in the literature). Finally, we found that the same SNP emerged in five different lineages independently, throughout almost all the COVID-19 pandemic.

CONCLUSIONS

We demonstrated for the first time the impact of this SNP on the Xpert assay, when harbored by new Omicron variants. We extend our observation period throughout almost all the COVID-19 pandemic, offering the most updated observations of this phenomenon, including sequences from the seventh pandemic wave, until now absent in the reports related to this issue. Continuous monitoring of emerging SNPs that could affect the performance of the most commonly used diagnostic tests, is required to redesign the tests to restore their correct performance.

The Mutational Landscape of SARS-CoV-2

Mutation research is crucial for detecting and treating SARS-CoV-2 and developing vaccines. Using over 5,300,000 sequences from SARS-CoV-2 genomes and custom Python programs, we analyzed the mutational landscape of SARS-CoV-2. Although almost every nucleotide in the SARS-CoV-2 genome has mutated at some time, the substantial differences in the frequency and regularity of mutations warrant further examination. C>U mutations are the most common. They are found in the largest number of variants, pangolin lineages, and countries, which indicates that they are a driving force behind the evolution of SARS-CoV-2. Not all SARS-CoV-2 genes have mutated in the same way. Fewer non-synonymous single nucleotide variations are found in genes that encode proteins with a critical role in virus replication than in genes with ancillary roles. Some genes, such as spike (S) and nucleocapsid (N), show more non-synonymous mutations than others. Although the prevalence of mutations in the target regions of COVID-19 diagnostic RT-qPCR tests is generally low, in some cases, such as for some primers that bind to the N gene, it is significant. Therefore, ongoing monitoring of SARS-CoV-2 mutations is crucial. The SARS-CoV-2 Mutation Portal provides access to a database of SARS-CoV-2 mutations.

Cycle Threshold (Ct) Values of SARS-CoV-2 Detected with the GeneXpert® System and a Mutation Associated with Different Target Gene Failure

SARS-CoV-2 nucleic acid detection tests enable rapid virus detection; however, it is challenging to identify genotypes to comprehend the local epidemiology and infection routes in real-time qRT-PCR. At the end of June 2022, our hospital experienced an in-hospital cluster of COVID-19. When examined using the GeneXpert^® System, the cycle threshold (Ct) value of the N2 region of the nucleocapsid gene of SARS-CoV-2 was approximately 10 cycles higher than that of the envelope gene. Sanger sequencing revealed a G29179T mutation in the primer and probe binding sites. A review of past test results revealed differences in Ct values in 21 of 345 SARS-CoV-2-positive patients, of which 17 cases were cluster-related and 4 were not. Including these 21 cases, 36 cases in total were selected for whole-genome sequencing (WGS). The viral genomes in the cluster-related cases were identified as BA.2.10, and those in the non-cluster cases were closely related and classified as being downstream of BA.2.10 and other lineages. Although WGS can provide comprehensive information, its use is limited in various laboratory settings. A measurement platform reporting and comparing Ct values of different target genes can improve test accuracy, enhance our understanding of infection spread, and be applied to the quality control of reagents.

Impact of G29179T mutation on two commercial PCR assays for SARS-CoV-2 detection

Nucleic acid amplification test (NAAT) is the gold standard for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) detection. However, genetic mutations in the virus can affect the result. Cycle threshold (Ct) values of N genes and their association with mutations using SARS-CoV-2 positive specimens diagnosed by the Xpert Xpress SARS-CoV-2 were examined in this study. In total, 196 nasopharyngeal swab specimens were tested for SARS-CoV-2 infection using the Xpert Xpress SARS-CoV-2, and 34 were positive. WGS was performed for four outlier samples with increased ΔCt identified by Scatterplot analysis as well as seven control samples without increased ΔCt in the Xpert Xpress SARS-CoV-2. The presence of the G29179T mutation was identified as a cause of increased ΔCt. PCR using the Allplex™ SARS-CoV-2 Assay did not show a similar increase in ΔCt. Previous reports focusing on N-gene mutations and their effects on SARS-CoV-2 testing including the Xpert Xpress SARS-CoV-2 were also summarized. While a single mutation that impacts one target of a multiplex NAAT is not a true detection failure, mutation compromising NAAT target region can cause confusion of the results and render the assay susceptible to diagnostic failure.

Using Real-Time PCR Fluorescence Reaction Values to Improve SARS-CoV-2 Virus Detection and Benefit Clinical Decision-Making

This study aimed to compare the SARS-CoV-2 nucleic acid detection results of the BD MAX™ System and other platforms to formulate an optimized laboratory verification process. The re-examination of 400 samples determined as positive by BD MAX™ indicated that the inconsistency rate between BD MAX™ and the other platforms was 65.8%; the inconsistency rate of single-gene-positive results was as high as 99.2%. A receiver operating characteristic curve was drawn for the relative light unit (RLU) values of samples positive for a single gene, and RLU 800 was used as the cutoff. After setting the retest standard as single-gene positive and RLU ≥ 800, the number of the 260 BD MAX™ single-gene positives that needed to be confirmed again was 36 (13.8%) and the number that could be directly reported as negative was 224 (86.2%). This verification process can shorten the reporting period and speed up the epidemic adjustment time and turnover rate of special wards, thereby improving SARS-CoV-2 detection efficiency and clinical decision-making.

Global landscape of SARS-CoV-2 mutations and conserved regions

BACKGROUND

At the end of December 2019, a novel strain of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) disease (COVID-19) has been identified in Wuhan, a central city in China, and then spread to every corner of the globe. As of October 8, 2022, the total number of COVID-19 cases had reached over 621 million worldwide, with more than 6.56 million confirmed deaths. Since SARS-CoV-2 genome sequences change due to mutation and recombination, it is pivotal to surveil emerging variants and monitor changes for improving pandemic management.

METHODS

10,287,271 SARS-CoV-2 genome sequence samples were downloaded in FASTA format from the GISAID databases from February 24, 2020, to April 2022. Python programming language (version 3.8.0) software was utilized to process FASTA files to identify variants and sequence conservation. The NCBI RefSeq SARS-CoV-2 genome (accession no. NC_045512.2) was considered as the reference sequence.

RESULTS

Six mutations had more than 50% frequency in global SARS-CoV-2. These mutations include the P323L (99.3%) in NSP12, D614G (97.6) in S, the T492I (70.4) in NSP4, R203M (62.8%) in N, T60A (61.4%) in Orf9b, and P1228L (50.0%) in NSP3. In the SARS-CoV-2 genome, no mutation was observed in more than 90% of nsp11, nsp7, nsp10, nsp9, nsp8, and nsp16 regions. On the other hand, N, nsp3, S, nsp4, nsp12, and M had the maximum rate of mutations. In the S protein, the highest mutation frequency was observed in aa 508-635(0.77%) and aa 381-508 (0.43%). The highest frequency of mutation was observed in aa 66-88 (2.19%), aa 7-14, and aa 164-246 (2.92%) in M, E, and N proteins, respectively.

CONCLUSION

Therefore, monitoring SARS-CoV-2 proteomic changes and detecting hot spots mutations and conserved regions could be applied to improve the SARS-CoV-2 diagnostic efficiency and design safe and effective vaccines against emerging variants.

Analytical performance of rapid nucleic acid detection assays and routine RT-qPCR assays for detection of SARS-CoV-2 in Shanghai, China in 2022

Diagnostic accuracy of COVID-19 varies among different assays. In this study, the analytical performance of 1 rapid nucleic acid detection assay (Coyote assay) and 2 routine RT-qPCR assays (BioGerm assay and DaAn assay) was evaluated, using 1196 clinical samples. Disagreement in the results of 2 paired targets occurred in all 3 assays. The Coyote assay failed to detect 15 samples, and the DaAn assay failed to detect 5 samples. The Cohen's kappa coefficient was 0.970 between the BioGerm and DaAn assays, 0.907 between the Coyote and BioGerm assays, and 0.936 between the Coyote and DaAn assays. The positive percent agreement, and negative percent agreement of the Coyote assay were 84.04%, and 100%, respectively. Our study revealed that the results of the Coyote, BioGerm, and DaAn assays were highly consistent, which provided reference for the application of these assays for diagnosis of COVID-19.

Development and Validation of a Novel COVID-19 nsp8 One-Tube RT-LAMP-CRISPR Assay for SARS-CoV-2 Diagnosis

Accurate and simple diagnostic tests for coronavirus disease 2019 (COVID-19) are essential components of the pandemic response. In this study, we evaluated a one-tube reverse transcription-loop-mediated isothermal amplification (RT-LAMP) assay coupled with clustered regularly interspaced short palindromic repeat (CRISPR)-associated protein-mediated endpoint detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) RNA in clinical samples. RT-LAMP-CRISPR is fast and affordable, does not require bulky thermocyclers, and minimizes carryover contamination risk. Results can be read either visually or with a fluorometer. RT-LAMP-CRISPR assays using primers targeting a highly expressed nsp8 gene and previously described nucleocapsid (N) gene primers were designed. The analytical characteristics and diagnostic performance of RT-LAMP-CRISPR assays were compared to those of a commercial real-time RT-PCR E gene assay. The limits of detection (LODs) of the nsp8 and N RT-LAMP-CRISPR assays were 750 and 2,000 copies/mL, which were higher than that of the commercial real-time RT-PCR assay (31.3 copies/mL). Despite the higher LOD, RT-LAMP-CRISPR assays showed diagnostic sensitivity and specificity of 98.6% and 100%, respectively, equivalent to those of the real-time RT-PCR assay (P = 0.5). The median fluorescence reading from the nsp8 assay (378.3 raw fluorescence unit [RFU] [range, 215.6 to 592.6]) was significantly higher than that of the N gene assay (342.0 RFU [range, 143.0 to 576.6]) (P < 0.0001). In conclusion, we demonstrate that RT-LAMP-CRISPR assays using primers rationally designed from highly expressed gene targets are highly sensitive, specific, and easy to perform. Such assays are a valuable asset in resource-limited settings. IMPORTANCE Accurate tests for the diagnosis of SARS-CoV-2, the virus causing coronavirus disease 2019 (COVID-19), are important for timely treatment and infection control decisions. Conventional tests such as real-time reverse transcription-PCR (RT-PCR) require specialized equipment and are expensive. On the other hand, rapid antigen tests suffer from a lack of sensitivity. In this study, we describe a novel assay format for the diagnosis of COVID-19 that is based on principles of loop-mediated isothermal amplification (LAMP) and clustered regularly interspaced short palindromic repeat (CRISPR)-Cas chemistry. A major advantage of this assay format is that it does not require expensive equipment to perform, and results can be read visually. This method proved to be fast, easy to perform, and inexpensive. The test compared well against an RT-PCR assay in terms of the ability to detect SARS-CoV-2 RNA in clinical samples. No false-positive test results were observed. The new assay format is ideal for SARS-CoV-2 diagnosis in resource-limited settings.

Detection of SARS-CoV-2 VOC-Omicron using commercial sample-to-answer real-time RT-PCR platforms and melting curve-based SNP assays

Objectives

The World Health Organization (WHO) had designated the SARS-CoV-2 lineage B.1.1.529 as the new Variant of Concern Omicron (VOC-Omicron) on 26^th November 2021¹. Real-time reverse transcription polymerase chain reaction (RT-PCR), single nucleotide polymorphisms (SNP) and whole genome sequencing (WGS) tests were widely employed to detect SARS-CoV-2 and its variant. Yet, the SARS-CoV-2 Omicron detection performance of commercial real-time RT-PCR platforms and SARS-CoV-2 spike SNP assays remain to be elucidated.

Methods

In the first part of this study, we evaluated the VOC-Omicron detection performance of three commercial RT-PCR sample-to-answer platforms i.e. Roche cobas® 6800/8800, Roche cobas® Liat®, and Cepheid GeneXpert® systems. The detection performances were compared to one commercial conventional real-time RT-PCR assay (TIB MOLBIOL LightMix Modular SARS and Wuhan CoV E-gene) and one in-house real-time RT-PCR assay targeting RNA-dependent RNA polymerase (RdRP) gene of SARS-CoV-2 in the WHO COVID-19 Reference Laboratory at Public Health Laboratory Services Branch, Centre for Health Protection, Department of Health, The Government of the Hong Kong Special Administrative Region. In the second part of this study, we evaluated the SNP detection performance of four TIB MOLBIOL melting curve-based assays (1. Spike S371L/S373P, 2. Spike E484A, 3. Spike E484K and 4. Spike N501Y) in clinical samples obtained from hospitalized COVID-19 patients in Hong Kong. The SNP results were compared to whole genome sequences generated by Illumina platform.

Results

The VOC-Omicron detection limits of three commercial sample-to-answer assays were tested to be ≤ 2.35 Log₁₀ dC/ml. The detection performances of the sample-to-answer platforms were comparable to the two tested conventional real-time RT-PCR assays. The test sensitivities of TIB MOLBIOL VirSNiP SARS-CoV-2 Spike S371L/S373P assay and the Spike E484A assays were 100% and 96.6% respectively and the test specificities of both assays were 100%. An aberrant melting peak at Tm 42-44°C was observed when the specimens with Omicron variant were tested with the TIB MOLBIOL VirSNiP SARS-CoV-2 Spike E484K assay. Notably, the TIB MOLBIOL VirSNiP SARS-CoV-2 Spike N501Y assay failed to detect the spike N501Y mutation of Omicron variant in the tested specimens.

Conclusions

The SARS-CoV-2 detection sensitivity of three commercial platforms, Roche cobas® 6800/8800, Roche cobas® Liat®, and Cepheid GeneXpert® systems were shown not to be impacted by the large number of mutations of VOC-Omicron. Also, the signature mutations i.e. Spike S371L/Spike S373P and Spike E484A in VOC-Omicron were correctly identified by the TIB MOLBIOL VirSNiP SARS-CoV-2 Spike S371L/S373P and VirSNiP SARS-CoV-2 Spike E484A assays. Unexpected findings including a shifted melting peak or absence of amplification curve/melting peak were observed when specimens with Omicron variant were tested with the TIB MOLBIOL VirSNiP SARS-CoV-2 Spike E484K assay and Spike N501Y assay, suggesting a potential alert for Omicron variant, prior confirmation by whole genome sequencing.

Continent-wide evolutionary trends of emerging SARS-CoV-2 variants: dynamic profiles from Alpha to Omicron

The ongoing SARS-CoV-2 evolution process has generated several variants due to its continuous mutations, making pandemics more critical. The present study illustrates SARS-CoV-2 evolution and its emerging mutations in five directions. First, the significant mutations in the genome and S-glycoprotein were analyzed in different variants. Three linear models were developed with the regression line to depict the mutational load for S-glycoprotein, total genome excluding S-glycoprotein, and whole genome. Second, the continent-wide evolution of SARS-CoV-2 and its variants with their clades and divergence were evaluated. It showed the region-wise evolution of the SARS-CoV-2 variants and their clustering event. The major clades for each variant were identified. One example is clade 21K, a major clade of the Omicron variant. Third, lineage dynamics and comparison between SARS-CoV-2 lineages across different countries are also illustrated, demonstrating dominant variants in various countries over time. Fourth, gene-wise mutation patterns and genetic variability of SARS-CoV-2 variants across various countries are illustrated. High mutation patterns were found in the ORF10, ORF6, S, and low mutation pattern E genes. Finally, emerging AA point mutations (T478K, L452R, N501Y, S477N, E484A, Q498R, and Y505H), their frequencies, and country-wise occurrence were identified, and the highest event of two mutations (T478K and L452R) was observed.

Dealing with inconclusive SARS-CoV-2 PCR samples-Our experience

Purpose

Early confirmation of SARS-CoV-2 is a key point in the timely management of infected patients and contact persons. Routine diagnostics of COVID-19 cases relies on RT-PCR detection of two or three unique sequences of the virus. A serious problem for the laboratories is how to interpret inconclusive samples which are positive for only one of the SARS-CoV-2 specific genes.

Materials and methods

A total of 16364 naso-oropharyngeal swabs were collected and tested with SARS-CoV-2 Real-TM kit (Sacace Biotechnologies, Italy) between May and September 2020. We retrospectively analyzed their amplification plots to determine the number of inconclusive samples. We also reviewed the medical records to summarize the patient’s COVID-19 testing history and basic demographic characteristics.

Results

We obtained 136 (0.8%) inconclusive samples with amplification signal only for the N-gene. Thirty-nine of the samples were excluded from further analysis as no additional data were available for them. Of the rest of the samples, the majority– 48% (95% CI 38–59%) had a previous history of SARS-CoV-2 positivity, 14% (95% CI 8–23%)–a subsequent history of positivity and 37% (95% CI 28–48%) were considered as false positive.

Conclusion

A substantial proportion of the inconclusive results should be considered as positive samples at the beginning or the end of the infection. However, the number of false-positive results is also significant and each patient’s result should be analyzed separately following the clinical symptoms and epidemiological data.

Presumptive positive with the Cepheid Xpert Xpress SARS-CoV-2 Assay due to N mutations in the Delta variant

The Cepheid Xpert® Xpress SARS-CoV-2 assay is 1 of the several real-time reverse transcription polymerase chain reaction (RT-PCR) assays that received Emergency Use Authorization from the United States Food and Drug Administration (FDA) for detection of SARS-CoV-2. Here we report 4 SARS-CoV-2 samples that were reported as presumptive positives on the Cepheid platform while reported as positives on alternative RT-PCR platforms. Whole genome sequencing indicated that the samples were Delta variants and had point mutations in the N gene which potentially interfered with SARS-CoV-2 detection. Two types of point mutations were found in these samples in the US CDC 2019-nCoV Real time PCR N2 Probe region: C29203T and C29200T. C29203T is a novel point mutation, and C29200T has not been previously reported in the Delta variants. This underlines the fact that mutations in the real-time RT-PCR assay target region could hinder accurate detection of SARS-CoV-2.

Comparative Diagnostic Performance of a Novel Reverse Transcription Loop-Mediated Isothermal Amplification (RT-LAMP) Kit for the Rapid Detection of SARS-CoV-2

Although the reverse transcription-polymerase chain reaction (RT-PCR) is considered a standard-of-care assay for the laboratory diagnosis of SARS-CoV-2, several limitations of this method have been described. Reverse transcription loop-mediated isothermal amplification (RT-LAMP) is an alternative molecular assay and is potentially able to overcome some intrinsic shortcomings of RT-PCR. In this study, we evaluated the diagnostic performance of the novel HG COVID-19 RT-LAMP assay. In this retrospective analysis, a total of 400 routinely collected leftover nasopharyngeal samples with a known RT-PCR result were tested by means of the HG COVID-19 RT-LAMP assay. The overall sensitivity and specificity values of HG COVID-19 RT-LAMP versus RT-PCR were 97.0% (95% CI: 93.6-98.9%) and 98.5% (95% CI: 95.7-99.7%), respectively. Inter-assay agreement was almost perfect (κ = 0.96). Concordance was perfect in samples with high viral loads (cycle threshold < 30). The average time to a positive result on RT-LAMP was 17 min. HG COVID-19 RT-LAMP is a reliable molecular diagnostic kit for detecting SARS-CoV-2, and its performance is comparable to that of RT-PCR. Shorter turnaround times and the possibility of performing molecular diagnostics in the point-of-care setting make it a valuable option for facilities without sophisticated laboratory equipment.