SARS-CoV-2 subgenomic RNA: formation process and rapid molecular diagnostic methods

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) which caused coronavirus disease-2019 (COVID-19) is spreading worldwide and posing enormous losses to human health and socio-economic. Due to the limitations of medical and health conditions, it is still a huge challenge to develop appropriate discharge standards for patients with COVID-19 and to use medical resources in a timely and effective manner. Similar to other coronaviruses, SARS-CoV-2 has a very complex discontinuous transcription process to generate subgenomic RNA (sgRNA). Some studies support that sgRNA of SARS-CoV-2 can only exist when the virus is active and is an indicator of virus replication. The results of sgRNA detection in patients can be used to evaluate the condition of hospitalized patients, which is expected to save medical resources, especially personal protective equipment. There have been numerous investigations using different methods, especially molecular methods to detect sgRNA. Here, we introduce the process of SARS-CoV-2 sgRNA formation and the commonly used molecular diagnostic methods to bring a new idea for clinical detection in the future.

A joint Bayesian hierarchical model for estimating SARS-CoV-2 genomic and subgenomic RNA viral dynamics and seroconversion

Understanding the viral dynamics of and natural immunity to the severe acute respiratory syndrome coronavirus 2 is crucial for devising better therapeutic and prevention strategies for coronavirus disease 2019 (COVID-19). Here, we present a Bayesian hierarchical model that jointly estimates the genomic RNA viral load, the subgenomic RNA (sgRNA) viral load (correlated to active viral replication), and the rate and timing of seroconversion (correlated to presence of antibodies). Our proposed method accounts for the dynamical relationship and correlation structure between the two types of viral load, allows for borrowing of information between viral load and antibody data, and identifies potential correlates of viral load characteristics and propensity for seroconversion. We demonstrate the features of the joint model through application to the COVID-19 post-exposure prophylaxis study and conduct a cross-validation exercise to illustrate the model's ability to impute the sgRNA viral trajectories for people who only had genomic RNA viral load data.

Identification of Hammerhead-variant ribozyme sequences in SARS-CoV-2

The SARS-CoV-2 RNA virus and variants, responsible for the COVID-19 pandemic has become endemic, raised a need for further understanding of the viral genome and biology. Despite vast research on SARS-CoV-2, no ribozymes have been found in the virus genome. Here we report the identification of 39 Hammerhead-variant ribozyme sequences (CoV-HHRz) in SARS-CoV-2. These sequences are highly conserved within SARS-CoV-2 variants but show large diversity among other coronaviruses. In vitro CoV-HHRz sequences possess the characteristics of typical ribozymes; cleavage is pH and ion dependent, although their activity is relatively low and Mn2+ is required for cleavage. The cleavage sites of four CoV-HHRz coincide with the breakpoint of expressed subgenomic RNA (sgRNAs) in SARS-CoV-2 transcriptome data suggesting in vivo activity. The CoV-HHRz are involved in processing sgRNAs for ORF7b, ORF 10 and ORF1ab nsp13 which are essential for viral packaging and life cycle.

Experimental and analytical pipeline for sub-genomic RNA landscape of coronavirus by Nanopore sequencer

Coronaviruses (CoVs), including severe acute respiratory syndrome coronavirus 2, can infect a variety of mammalian and avian hosts with significant medical and economic consequences. During the life cycle of CoV, a coordinated series of subgenomic RNAs, including canonical subgenomic messenger RNA and non-canonical defective viral genomes (DVGs), are generated with different biological implications. Studies that adopted the Nanopore sequencer (ONT) to investigate the landscape and dynamics of viral RNA subgenomic transcriptomes applied arbitrary bioinformatics parameters without justification or experimental validation. The current study used bovine coronavirus (BCoV), which can be performed under biosafety level 2 for library construction and experimental validation using traditional colony polymerase chain reaction and Sanger sequencing. Four different ONT protocols, including RNA direct and cDNA direct sequencing with or without exonuclease treatment, were used to generate RNA transcriptomic libraries from BCoV-infected cell lysates. Through rigorously examining the k-mer, gap size, segment size, and bin size, the optimal cutoffs for the bioinformatic pipeline were determined to remove the sequence noise while keeping the informative DVG reads. The sensitivity and specificity of identifying DVG reads using the proposed pipeline can reach 82.6% and 99.6% under the k-mer size cutoff of 15. Exonuclease treatment reduced the abundance of RNA transcripts; however, it was not necessary for future library preparation. Additional recovery of clipped BCoV nucleotide sequences with experimental validation expands the landscape of the CoV discontinuous RNA transcriptome, whose biological function requires future investigation. The results of this study provide the benchmarks for library construction and bioinformatic parameters for studying the discontinuous CoV RNA transcriptome.IMPORTANCEFunctional defective viral genomic RNA, containing all the cis-acting elements required for translation or replication, may play different roles in triggering cell innate immune signaling, interfering with the canonical subgenomic messenger RNA transcription/translation or assisting in establishing persistence infection. This study does not only provide benchmarks for library construction and bioinformatic parameters for studying the discontinuous coronavirus RNA transcriptome but also reveals the complexity of the bovine coronavirus transcriptome, whose functional assays will be critical in future studies.

Quantitative and qualitative subgenomic RNA profiles of SARS-CoV-2 in respiratory samples: A comparison between Omicron BA.2 and non-VOC-D614G

The SARS-CoV-2 Omicron variants are notorious for their transmissibility, but little is known about their subgenomic RNA (sgRNA) expression. This study applied RNA-seq to delineate the quantitative and qualitative profiles of canonical sgRNA of 118 respiratory samples collected from patients infected with Omicron BA.2 and compared with 338 patients infected with non-variant of concern (non-VOC)-D614G. A unique characteristic profile depicted by the relative abundance of 9 canonical sgRNAs was reproduced by both BA.2 and non-VOC-D614G regardless of host gender, age and presence of pneumonia. Remarkably, such profile was lost in samples with low viral load, suggesting a potential application of sgRNA pattern to indicate viral activity of individual patient at a specific time point. A characteristic qualitative profile of canonical sgRNAs was also reproduced by both BA.2 and non-VOC-D614G. The presence of a full set of canonical sgRNAs carried a coherent correlation with crude viral load (AUC ​= ​0.91, 95% CI 0.88-0.94), and sgRNA ORF7b was identified to be the best surrogate marker allowing feasible routine application in characterizing the infection status of individual patient. Further potentials in using sgRNA as a target for vaccine and antiviral development are worth pursuing.

Dissemination of the Flavivirus Subgenomic Replicon Genome and Viral Proteins by Extracellular Vesicles

Extracellular vesicles (EVs) such as exosomes have been shown to play physiological roles in cell-to-cell communication by delivering various proteins and nucleic acids. In addition, several studies revealed that the EVs derived from the cells that are infected with certain viruses could transfer the full-length viral genomes, resulting in EVs-mediated virus propagation. However, the possibility cannot be excluded that the prepared EVs were contaminated with infectious viral particles. In this study, the cells that harbor subgenomic replicon derived from the Japanese encephalitis virus and dengue virus without producing any replication-competent viruses were employed as the EV donor. It was demonstrated that the EVs in the culture supernatants of those cells were able to transfer the replicon genome to other cells of various types. It was also shown that the EVs were incorporated by the recipient cells primarily through macropinocytosis after interaction with CD33 and Tim-1/Tim-4 on HeLa and K562 cells, respectively. Since the methods used in this study are free from contamination with infectious viral particles, it is unequivocally indicated that the flavivirus genome can be transferred by EVs from cell to cell, suggesting that this pathway, in addition to the classical receptor-mediated infection, may play some roles in the viral propagation and pathogenesis.

The subgenomic flaviviral RNA suppresses RNA interference through competing with siRNAs for binding RISC components

During the life cycle of mosquito-borne flaviviruses, substantial subgenomic flaviviral RNA (sfRNA) is produced via incomplete degradation of viral genomic RNA by host XRN1. Zika virus (ZIKV) sfRNA has been detected in mosquito and mammalian somatic cells. Human neural progenitor cells (hNPCs) in the developing brain are the major target cells of ZIKV, and antiviral RNA interference (RNAi) plays a critical role in hNPCs. However, whether ZIKV sfRNA was produced in ZIKV-infected hNPCs as well as its function remains not known. In this study, we demonstrate that abundant sfRNA was produced in ZIKV-infected hNPCs. RNA pulldown and mass spectrum assays showed ZIKV sfRNA interacted with host proteins RHA and PACT, both of which are RNA-induced silencing complex (RISC) components. Functionally, ZIKV sfRNA can antagonize RNAi by outcompeting small interfering RNAs (siRNAs) in binding to RHA and PACT. Furthermore, the 3' stem loop (3'SL) of sfRNA was responsible for RISC components binding and RNAi inhibition, and 3'SL can enhance the replication of a viral suppressor of RNAi (VSR)-deficient virus in a RHA- and PACT-dependent manner. More importantly, the ability of binding to RISC components is conversed among multiple flaviviral 3'SLs. Together, our results identified flavivirus 3'SL as a potent VSR in RNA format, highlighting the complexity in virus-host interaction during flavivirus infection.IMPORTANCEZika virus (ZIKV) infection mainly targets human neural progenitor cells (hNPCs) and induces cell death and dysregulated cell-cycle progression, leading to microcephaly and other central nervous system abnormalities. RNA interference (RNAi) plays critical roles during ZIKV infections in hNPCs, and ZIKV has evolved to encode specific viral proteins to antagonize RNAi. Herein, we first show that abundant sfRNA was produced in ZIKV-infected hNPCs in a similar pattern to that in other cells. Importantly, ZIKV sfRNA acts as a potent viral suppressor of RNAi (VSR) by competing with siRNAs for binding RISC components, RHA and PACT. The 3'SL of sfRNA is responsible for binding RISC components, which is a conserved feature among mosquito-borne flaviviruses. As most known VSRs are viral proteins, our findings highlight the importance of viral non-coding RNAs during the antagonism of host RNAi-based antiviral innate immunity.

Visualization of Early RNA Replication Kinetics of SARS-CoV-2 by Using Single Molecule RNA-FISH Combined with Immunofluorescence

SARS-CoV-2 infection remains a global burden. Despite intensive research, the mechanism and dynamics of early viral replication are not completely understood, such as the kinetics of the formation of genomic RNA (gRNA), sub-genomic RNA (sgRNA), and replication centers/organelles (ROs). We employed single-molecule RNA-fluorescence in situ hybridization (smRNA-FISH) to simultaneously detect viral gRNA and sgRNA and immunofluorescence to detect nsp3 protein, a marker for the formation of RO, and carried out a time-course analysis. We found that single molecules of gRNA are visible within the cytoplasm at 30 min post infection (p.i.). Starting from 2 h p.i., most of the viral RNA existed in clusters/speckles, some of which were surrounded by single molecules of sgRNA. These speckles associated with nsp3 protein starting at 3 h p.i., indicating that these were precursors to ROs. Furthermore, RNA replication was asynchronous, as cells with RNA at all stages of replication were found at any given time point. Our probes detected the SARS-CoV-2 variants of concern, and also suggested that the BA.1 strain exhibited a slower rate of replication kinetics than the WA1 strain. Our results provide insights into the kinetics of SARS-CoV-2 early post-entry events, which will facilitate identification of new therapeutic targets for early-stage replication to combat COVID-19.

Defining Distinct RNA-Protein Interactomes of SARS-CoV-2 Genomic and Subgenomic RNAs

Host RNA binding proteins recognize viral RNA and play key roles in virus replication and antiviral mechanisms. SARS-CoV-2 generates a series of tiered subgenomic RNAs (sgRNAs), each encoding distinct viral protein(s) that regulate different aspects of viral replication. Here, for the first time, we demonstrate the successful isolation of SARS-CoV-2 genomic RNA and three distinct sgRNAs (N, S, and ORF8) from a single population of infected cells and characterize their protein interactomes. Over 500 protein interactors (including 260 previously unknown) were identified as associated with one or more target RNA. These included protein interactors unique to a single RNA pool and others present in multiple pools, highlighting our ability to discriminate between distinct viral RNA interactomes despite high sequence similarity. Individual interactomes indicated viral associations with cell response pathways, including regulation of cytoplasmic ribonucleoprotein granules and posttranscriptional gene silencing. We tested the significance of three protein interactors in these pathways (APOBEC3F, PPP1CC, and MSI2) using siRNA knockdowns, with several knockdowns affecting viral gene expression, most consistently PPP1CC. This study describes a new technology for high-resolution studies of SARS-CoV-2 RNA regulation and reveals a wealth of new viral RNA-associated host factors of potential functional significance to infection.

Rescue of a Cilevirus from infectious cDNA clones

Reverse genetics systems represent an important tool for studying the molecular and functional processes of viral infection. Citrus leprosis virus C (CiLV-C) (genus Cilevirus, family Kitaviridae) is the main pathogen responsible for the citrus leprosis (CL) disease in Latin America, one of the most economically important diseases of the citrus industry. Molecular studies of this pathosystem are limited due to the lack of infectious clones. Here, we report the construction and validation of a CiLV-C infectious cDNA clone based on an agroinfection system. The two viral RNA segments (RNA1 and RNA2) were assembled into two binary vectors (pJL89 and pLXAS). Agroinfiltrated Nicotiana benthamiana plants showed a response similar to that observed in the natural infection process with the formation of localized lesions restricted to the inoculated leaves. The virus recovered from the plant tissue infected with the infectious clones can be mechanically transmitted between N. benthamiana plants. Detection of CiLV-C subgenomic RNAs (sgRNAs) from agroinfiltrated and mechanically inoculated leaves further confirmed the infectivity of the clones. Finally, partial particle-purification preparations or sections of CiLV-C-infected tissue followed by transmission electron microscopy (TEM) analysis showed the formation of CiLV-C virions rescued by the infectious clone. The CiLV-C reverse genetic system now provides a powerful molecular tool to unravel the peculiarities of the CL pathosystem.

Identification of subgenomic mRNAs derived from the coronavirus 1a/1b protein gene: Implications for coronavirus transcription

Synthesis of coronavirus subgenomic mRNA (sgmRNA) is guided by the transcription regulatory sequence (TRS). sgmRNA derived from the body TRS (TRS-B) located at the 1a/1b protein gene is designated 1ab/sgmRNA. In the current study, we comprehensively identified the 1ab/sgmRNAs synthesized from TRS-Bs located at the 1a/1b protein genes of different coronavirus genera both in vitro and in vivo by RT‒PCR and sequencing. The results suggested that the degree of sequence homology between the leader TRS (TRS-L) and TRS-B may not be a decisive factor for 1ab/sgmRNA synthesis. This observation led us to revisit the coronavirus transcription mechanism and to propose that the disassociation of coronavirus polymerase from the viral genome may be a prerequisite for sgmRNA synthesis. Once the polymerase can disassociate at TRS-B, the sequence homology between TRS-L and TRS-B is important for sgmRNA synthesis. The study therefore extends our understanding of transcription mechanisms.

Comparative analysis of bioinformatics tools to characterize SARS-CoV-2 subgenomic RNAs

During the replication of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), positive-sense genomic RNA and subgenomic RNAs (sgRNAs) are synthesized by a discontinuous process of transcription characterized by a template switch, regulated by transcription-regulating sequences (TRS). Although poorly known about makeup and dynamics of sgRNAs population and function of its constituents, next-generation sequencing approaches with the help of bioinformatics tools have made a significant contribution to expand the knowledge of sgRNAs in SARS-CoV-2. For this scope to date, Periscope, LeTRS, sgDI-tector, and CORONATATOR have been developed. However, limited number of studies are available to compare the performance of such tools. To this purpose, we compared Periscope, LeTRS, and sgDI-tector in the identification of canonical (c-) and noncanonical (nc-) sgRNA species in the data obtained with the Illumina ARTIC sequencing protocol applied to SARS-CoV-2-infected Caco-2 cells, sampled at different time points. The three software showed a high concordance rate in the identification and in the quantification of c-sgRNA, whereas more differences were observed in nc-sgRNA. Overall, LeTRS and sgDI-tector result to be adequate alternatives to Periscope to analyze Fastq data from sequencing platforms other than Nanopore.

Subgenomic Flaviviral RNAs of Dengue Viruses

Subgenomic flaviviral RNAs (sfRNAs) are produced during flavivirus infections in both arthropod and vertebrate cells. They are undegraded products originating from the viral 3' untranslated region (3' UTR), a result of the action of the host 5'-3' exoribonuclease, Xrn1, when it encounters specific RNA structures known as Xrn1-resistant RNAs (xrRNAs) within the viral 3' UTR. Dengue viruses generate three to four distinct species of sfRNAs through the presence of two xrRNAs and two dumbbell structures (DBs). The tertiary structures of xrRNAs have been characterized to form a ringlike structure around the 5' end of the viral RNA, effectively inhibiting the activity of Xrn1. The most important role of DENV sfRNAs is to inhibit host antiviral responses by interacting with viral and host proteins, thereby influencing viral pathogenicity, replicative fitness, epidemiological fitness, and transmission. In this review, we aimed to summarize the biogenesis, structures, and functions of DENV sfRNAs, exploring their implications for viral interference.

Dimerization of an umbravirus RNA genome activates subgenomic mRNA transcription

Many eukaryotic RNA viruses transcribe subgenomic (sg) mRNAs during infections to control expression of a subset of viral genes. Such transcriptional events are commonly regulated by local or long-range intragenomic interactions that form higher-order RNA structures within these viral genomes. In contrast, here we report that an umbravirus activates sg mRNA transcription via base pair-mediated dimerization of its plus-strand RNA genome. Compelling in vivo and in vitro evidence demonstrate that this viral genome dimerizes via a kissing-loop interaction involving an RNA stem-loop structure located just upstream from its transcriptional initiation site. Both specific and non-specific features of the palindromic kissing-loop complex were found to contribute to transcriptional activation. Structural and mechanistic aspects of the process in umbraviruses are discussed and compared with genome dimerization events in other RNA viruses. Notably, probable dimer-promoting RNA stem-loop structures were also identified in a diverse group of umbra-like viruses, suggesting broader utilization of this unconventional transcriptional strategy.

Construction and characterization of a new hepatitis C virus genotype 6a subgenomic replicon that is prone to render the sofosbuvir resistance

Hepatitis C virus (HCV) infection remains a challenge to human public health despite the development of highly effective direct-acting antivirals (DAAs). Sofosbuvir (SOF), a key component in most DAA-based anti-HCV cocktail regimens, is a potent viral RNA polymerase (NS5B) inhibitor with a high barrier to drug resistance. The serine-to-threonine mutation at NS5B 282 (S282T) confers the SOF resistance, but severely impairs viral replication in most HCV genotypes (GTs) and cannot be stably maintained after the termination of the SOF-based therapies. In this study, we first developed a new HCV GT-6a subgenomic replicon PR58D6. Next, we selected SOF-resistant PR58D6 variants by culturing the replicon cells in the presence of SOF. Interestingly, unlike many other HCV replicons which require additional mutations to compensate for the S282T-inducing fitness loss, S282T alone in PR58D6 is genetically stable and confers the SOF resistance without significantly impairing viral replication. Furthermore, we showed that amino acid residue at NS5B 74 (R74) and 556 (D556) which are conserved in GT 6a HCV contribute to efficient replication of PR58D6 containing S282T. Finally, we showed that the G556D mutation in NS5B could rescue the replication deficiency of the S282T in JFH1, a GT-2a replicon. In conclusion, we showed that a novel GT-6a HCV replicon may easily render SOF resistance, which may call for attention to potential drug resistance during DAA therapies of HCV GT-6a patients.

Quantitative SARS-CoV-2 subgenomic RNA as a surrogate marker for viral infectivity: Comparison between culture isolation and direct sgRNA quantification

Detection of subgenomic (sg) SARS-CoV-2 RNAs are frequently used as a correlate of viral infectiousness, but few data about correlation between sg load and viable virus are available. Here, we defined concordance between culture isolation and E and N sgRNA quantification by ddPCR assays in 51 nasopharyngeal swabs collected from SARS-CoV-2 positive hospitalized patients. Among the 51 samples, 14 were SARS-CoV-2 culture-positive and 37 were negative. According to culture results, the sensitivity and specificity of E and N sgRNA assays were 100% and 100%, and 84% and 86%, respectively. ROC analysis showed that the best E and N cut-offs to predict positive culture isolation were 32 and 161 copies/mL respectively, with an AUC (95% CI) of 0.96 (0.91-1.00) and 0.96 (0.92-1.00), and a diagnostic accuracy of 88% and 92%, respectively. Even if no significant correlations were observed between sgRNA amount and clinical presentation, a higher number of moderate/severe cases and lower number of days from symptoms onset characterized patients with sgRNA equal to or higher than sgRNA cut-offs. Overall, this study suggests that SARS-CoV-2 sgRNA quantification could be helpful to estimate the replicative activity of SARS-CoV-2 and can represent a valid surrogate marker to efficiently recognize patients with active infection. The inclusion of this assay in available SARS-CoV-2 diagnostics procedure might help in optimizing fragile patients monitoring and management.

A culture-free method for rapidly and accurately quantifying active SARS-CoV-2

Determining the quantity of active virus is the most important basis to judge the risk of virus infection, which usually relies on the virus median tissue culture infectious dose (TCID50) assay performed in a biosafety level 3 laboratory within 5-7 days. We have developed a culture-free method for rapid and accurate quantification of active severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) by targeting subgenomic RNA (sgRNA) based on reverse transcription digital PCR (RT-dPCR). The dynamic range of quantitative assays for sgRNA-N and sgRNA-E by RT-dPCR was investigated, and the result showed that the limits of detection (LoD) and quantification (LoQ) were 2 copies/reaction and 10 copies/reaction, respectively. The delta strain (NMDC60042793) of SARS-CoV-2 was cultured at an average titer of 106.13 TCID50/mL and used to evaluate the developed quantification method. Copy number concentrations of the cultured SARS-CoV-2 sgRNA and genomic RNA (gRNA) gave excellent linearity (R2 = 0.9999) with SARS-CoV-2 titers in the range from 500 to 105 TCID50/mL. Validation of 63 positive clinical samples further proves that the quantification of sgRNA-N by RT-dPCR is more sensitive for active virus quantitative detection. It is notable that we can infer the active virus titer through quantification of SARS-CoV-2 sgRNA based on the linear relationship in a biosafety level 2 laboratory within 3 h. It can be used to timely and effectively identify infectious patients and reduce unnecessary isolation especially when a large number of COVID-19 infected people impose a burden on medical resources.

Total and Subgenomic RNA Viral Load in Patients Infected With SARS-CoV-2 Alpha, Delta, and Omicron Variants

BACKGROUND

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) genomic and subgenomic RNA levels are frequently used as a correlate of infectiousness. The impact of host factors and SARS-CoV-2 lineage on RNA viral load is unclear.

METHODS

Total nucleocapsid (N) and subgenomic N (sgN) RNA levels were measured by quantitative reverse transcription polymerase chain reaction (RT-qPCR) in specimens from 3204 individuals hospitalized with coronavirus disease 2019 (COVID-19) at 21 hospitals. RT-qPCR cycle threshold (Ct) values were used to estimate RNA viral load. The impact of time of sampling, SARS-CoV-2 variant, age, comorbidities, vaccination, and immune status on N and sgN Ct values were evaluated using multiple linear regression.

RESULTS

Mean Ct values at presentation for N were 24.14 (SD 4.53) for non-variants of concern, 25.15 (SD 4.33) for Alpha, 25.31 (SD 4.50) for Delta, and 26.26 (SD 4.42) for Omicron. N and sgN RNA levels varied with time since symptom onset and infecting variant but not with age, comorbidity, immune status, or vaccination. When normalized to total N RNA, sgN levels were similar across all variants.

CONCLUSIONS

RNA viral loads were similar among hospitalized adults, irrespective of infecting variant and known risk factors for severe COVID-19. Total N and subgenomic RNA N viral loads were highly correlated, suggesting that subgenomic RNA measurements add little information for the purposes of estimating infectivity.

Viral target and metabolism-based rationale for combined use of recently authorized small molecule COVID-19 medicines: Molnupiravir, nirmatrelvir, and remdesivir

The COVID-19 pandemic remains a major health concern worldwide, and SARS-CoV-2 is continuously evolving. There is an urgent need to identify new antiviral drugs and develop novel therapeutic strategies. Combined use of newly authorized COVID-19 medicines including molnupiravir, nirmatrelvir, and remdesivir has been actively pursued. Mechanistically, nirmatrelvir inhibits SARS-CoV-2 replication by targeting the viral main protease (Mpro ), a critical enzyme in the processing of the immediately translated coronavirus polyproteins for viral replication. Molnupiravir and remdesivir, on the other hand, inhibit SARS-CoV-2 replication by targeting RNA-dependent RNA-polymerase (RdRp), which is directly responsible for genome replication and production of subgenomic RNAs. Molnupiravir targets RdRp and induces severe viral RNA mutations (genome), commonly referred to as error catastrophe. Remdesivir, in contrast, targets RdRp and causes chain termination and arrests RNA synthesis of the viral genome. In addition, all three medicines undergo extensive metabolism with strong therapeutic significance. Molnupiravir is hydrolytically activated by carboxylesterase-2 (CES2), nirmatrelvir is inactivated by cytochrome P450-based oxidation (e.g., CYP3A4), and remdesivir is hydrolytically activated by CES1 but covalently inhibits CES2. Additionally, remdesivir and nirmatrelvir are oxidized by the same CYP enzymes. The distinct mechanisms of action provide strong rationale for their combined use. On the other hand, these drugs undergo extensive metabolism that determines their therapeutic potential. This review discusses how metabolism pathways and enzymes involved should be carefully considered during their combined use for therapeutic synergy.

Comparative subgenomic mRNA profiles of SARS-CoV-2 Alpha, Delta and Omicron BA.1, BA.2 and BA.5 sub-lineages using Danish COVID-19 genomic surveillance data

BACKGROUND

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has rapidly spread worldwide in the population since it was first detected in late 2019. The transcription and replication of coronaviruses, although not fully understood, is characterised by the production of genomic length RNA and shorter subgenomic RNAs to make viral proteins and ultimately progeny virions. Observed levels of subgenomic RNAs differ between sub-lineages and open reading frames but their biological significance is presently unclear.

METHODS

Using a large and diverse panel of virus sequencing data produced as part of the Danish COVID-19 routine surveillance together with information in electronic health registries, we assessed the association of subgenomic RNA levels with demographic and clinical variables of the infected individuals.

FINDINGS

Our findings suggest no significant statistical relationship between levels of subgenomic RNAs and host-related factors.

INTERPRETATION

Differences between lineages and subgenomic ORFs may be related to differences in target cell tropism, early virus replication/transcription kinetics or sequence features.

FUNDING

The author(s) received no specific funding for this work.

Analytical validation of quantitative SARS-CoV-2 subgenomic and viral load laboratory developed tests conducted on the Panther Fusion® (Hologic) with preliminary application to clinical samples

OBJECTIVE

Validate the performance characteristics of two analyte specific, laboratory developed tests (LDTs) for the quantification of SARS-CoV-2 subgenomic RNA (sgRNA) and viral load on the Hologic Panther Fusion® using the Open Access functionality.

METHODS

Custom-designed primers/probe sets targeting the SARS-CoV-2 Envelope gene (E) and subgenomic E were optimized. A 20-day performance validation following laboratory developed test requirements was conducted to assess assay precision, accuracy, analytical sensitivity/specificity, lower limit of detection and reportable range.

RESULTS

Quantitative SARS-CoV-2 sgRNA (LDT-Quant sgRNA) assay, which measures intermediates of replication, and viral load (LDT-Quant VLCoV) assay demonstrated acceptable performance. Both assays were linear with an R2 and slope equal to 0.99 and 1.00, respectively. Assay precision was evaluated between 4-6 Log10 with a maximum CV of 2.6% and 2.5% for LDT-Quant sgRNA and LDT-Quant VLCoV respectively. Using negative or positive SARS-CoV-2 human nasopharyngeal swab samples, both assays were accurate (kappa coefficient of 1.00 and 0.92). Common respiratory flora and other viral pathogens were not detected and did not interfere with the detection or quantification by either assay. Based on 95% detection, the assay LLODs were 729 and 1206 Copies/mL for the sgRNA and VL load LDTs, respectively.

CONCLUSION

The LDT-Quant sgRNA and LDT-Quant VLCoV demonstrated good analytical performance. These assays could be further investigated as alternative monitoring assays for viral replication; and thus, medical management in clinical settings which could inform isolation/quarantine requirements.

Visualising SARS-CoV-2 infection of the lung in deceased COVID-19 patients

BACKGROUND

SARS-CoV-2 is a single-stranded positive-sense RNA virus. Several negative-sense SARS-CoV-2 RNA species, both full-length genomic and subgenomic, are produced transiently during viral replication. Methodologies for rigorously characterising cell tropism and visualising ongoing viral replication at single-cell resolution in histological sections are needed to assess the virological and pathological phenotypes of future SARS-CoV-2 variants. We aimed to provide a robust methodology for examining the human lung, the major target organ of this RNA virus.

METHODS

A prospective cohort study took place at the University Hospitals Leuven in Leuven, Belgium. Lung samples were procured postmortem from 22 patients who died from or with COVID-19. Tissue sections were fluorescently stained with the ultrasensitive single-molecule RNA in situ hybridisation platform of RNAscope combined with immunohistochemistry followed by confocal imaging.

FINDINGS

We visualised perinuclear RNAscope signal for negative-sense SARS-CoV-2 RNA species in ciliated cells of the bronchiolar epithelium of a patient who died with COVID-19 in the hyperacute phase of the infection, and in ciliated cells of a primary culture of human airway epithelium that had been infected experimentally with SARS-CoV-2. In patients who died between 5 and 13 days after diagnosis of the infection, we detected RNAscope signal for positive-sense but not for negative-sense SARS-CoV-2 RNA species in pneumocytes, macrophages, and among debris in the alveoli. SARS-CoV-2 RNA levels decreased after a disease course of 2-3 weeks, concomitant with a histopathological change from exudative to fibroproliferative diffuse alveolar damage. Taken together, our confocal images illustrate the complexities stemming from traditional approaches in the literature to characterise cell tropism and visualise ongoing viral replication solely by the surrogate parameters of nucleocapsid-immunoreactive signal or in situ hybridisation for positive-sense SARS-CoV-2 RNA species.

INTERPRETATION

Confocal imaging of human lung sections stained fluorescently with commercially available RNAscope probes for negative-sense SARS-CoV-2 RNA species enables the visualisation of viral replication at single-cell resolution during the acute phase of the infection in COVID-19. This methodology will be valuable for research on future SARS-CoV-2 variants and other respiratory viruses.

FUNDING

Max Planck Society, Coronafonds UZ/KU Leuven, European Society for Organ Transplantation.

Current outcomes of SARS-CoV-2 Omicron variant infection in high-risk haematological patients treated early with antivirals

OBJECTIVES

We aimed to describe the clinical outcomes and duration of viral shedding in high-risk patients with haematological malignancies hospitalized with COVID-19 during Omicron variant predominance who received early treatment with antivirals.

METHODS

We conducted a prospective observational study on high-risk haematological patients admitted in our hospital between December 2021 and March 2022. We performed detection techniques on viral subgenomic mRNAs until negative results were obtained to document active, prolonged viral replication.

RESULTS

This analysis included 60 consecutive adults with high-risk haematological malignancies and COVID-19. All of these patients underwent early treatment with remdesivir. Thirty-two (53%) patients received combined antiviral strategies, with sotrovimab or hyperimmune plasma being added to remdesivir. The median length of viral replication-as measured by real-time RT-PCR and/or subgenomic RNA detection-was 20 (IQR 14-28) days. Prolonged viral replication (6 weeks after diagnosis) was documented in six (10%) patients. Only two patients had prolonged infection for more than 2 months. Overall mortality was 5%, whereas COVID-19-related mortality was 0%.

CONCLUSIONS

Current outcomes of high-risk patients with haematological malignancies hospitalized with COVID-19 during Omicron variant predminance are good with the use of early antiviral strategies. Persistent viral shedding is uncommon.

TUT4/7-mediated uridylation of a coronavirus subgenomic RNAs delays viral replication

Coronaviruses are positive-strand RNA viruses with 3' polyadenylated genomes and subgenomic transcripts. The lengths of the viral poly(A) tails change during infection by mechanisms that remain poorly understood. Here, we use a splint-ligation method to measure the poly(A) tail length and poly(A) terminal uridylation and guanylation of the mouse hepatitis virus (MHV) RNAs. Upon infection of 17-CL1 cells with MHV, a member of the Betacoronavirus genus, we observe two populations of terminally uridylated viral transcripts, one with poly(A) tails ~44 nucleotides long and the other with poly(A) tails shorter than ~22 nucleotides. The mammalian terminal uridylyl-transferase 4 (TUT4) and terminal uridylyl-transferase 7 (TUT7), referred to as TUT4/7, add non-templated uracils to the 3'-end of endogenous transcripts with poly(A) tails shorter than ~30 nucleotides to trigger transcript decay. Here we find that depletion of the host TUT4/7 results in an increased replication capacity of the MHV virus. At late stages of infection, the population of uridylated subgenomic RNAs with tails shorter than ~22 nucleotides is reduced in the absence of TUT4/7 while the viral RNA load increases. Our findings indicate that TUT4/7 uridylation marks the MHV subgenomic RNAs for decay and delays viral replication.

Cell-penetrating porcine single-chain antibodies (transbodies) against nonstructural protein 1β (NSP1β) of porcine reproductive and respiratory syndrome virus inhibit virus replication

Porcine reproductive and respiratory syndrome virus (PRRSV) causes porcine reproductive and respiratory syndrome (PRRS) worldwide, especially in domestic pigs, with an enormous economic impact, estimated at $664 million in losses every year to the pig industry. Current vaccines confer limited protection, and no direct-acting anti-PRRS treatment is available. Non-structural protein (NSP) 1β, a cysteine-like protease (CLPro) of PRRSV plays an essential role in viral polyprotein processing, subgenomic RNA synthesis, and evasion of host innate immunity. Therefore, agents that interfere with the bioactivity of NSP1β would be expected to inhibit virus replication. In this study, a porcine single-chain antibody (scFv)-phage display library was constructed and used as a tool for production of NSP1β-specific porcine scFvs (pscFvs). The pscFvs to NSP1β were linked to a cell-penetrating peptide to form cell-penetrating pscFvs (transbodies), which could be internalized and inhibit PRRSV replication in infected cells. A computer simulation indicated that the effective pscFvs used several residues in multiple complementarity determining regions (CDRs) to interact with multiple residues in the CLPro and C-terminal motifs, which might explain the mechanism of pscFv-mediated inhibition of virus replication. Although experiments are needed to determine the antiviral mechanism of the transbodies, the current data indicate that transbodies can potentially be applied for treatment and prevention of PRRSV infection.

Modulation of Capsid Dynamics in Bromoviruses by the Host and Heterologous Viral Replicase

The three genomic and a single subgenomic RNA of Cowpea chlorotic mottle virus (CCMV), which is pathogenic to plants, is packaged into three morphologically indistinguishable icosahedral virions with T=3 symmetry. The two virion types, C1V and C2V, package genomic RNAs 1 (C1) and 2 (C2), respectively. The third virion type, C3+4V, copackages genomic RNA3 and its subgenomic RNA (RNA4). In this study, we sought to evaluate how the alteration of native capsid dynamics by the host and viral replicase modulate the general biology of the virus. The application of a series of biochemical, molecular, and biological assays revealed the following. (i) Proteolytic analysis of the three virion types of CCMV assembled individually in planta revealed that, while retaining the structural integrity, C1V and C2V virions released peptide regions encompassing the N-terminal arginine-rich RNA binding motif. In contrast, a minor population of the C3+4V virion type was sensitive to trypsin-releasing peptides encompassing the entire capsid protein region. (ii) The wild-type CCMV virions purified from cowpea are highly susceptible to trypsin digestion, while those from Nicotiana benthamiana remained resistant, and (iii) finally, the matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) analysis evaluated the relative dynamics of C3+4V and B3+4V virions assembled under the control of the homologous versus heterologous replicase. The role of viral replicase in modulating the capsid dynamics was evident by the differential sensitivity to protease exhibited by B3+4V and C3+4V virions assembled under the homologous versus heterologous replicase. Our results collectively conclude that constant modulation of capsid dynamics by the host and viral replicase is obligatory for successful infection. IMPORTANCE Infectious virus particles or virions are considered static structures and undergo various conformational transitions to replicate and infect many eukaryotic cells. In viruses, conformational changes are essential for establishing infection and evolution. Although viral capsid fluctuations, referred to as dynamics or breathing, have been well studied in RNA viruses pathogenic to animals, such information is limited among plant viruses. The primary focus of this study is to address how capsid dynamics of plant-pathogenic RNA viruses, namely, Cowpea chlorotic mottle (CCMV) and Brome mosaic virus (BMV), are modulated by the host and viral replicase. The results presented have improved and transformed our understanding of the functional relationship between capsid dynamics and the general biology of the virus. They are likely to provide stimulus to extend similar studies to viruses pathogenic to eukaryotic organisms.

The anti-immune dengue subgenomic flaviviral RNA is present in vesicles in mosquito saliva and is associated with increased infectivity

Mosquito transmission of dengue viruses to humans starts with infection of skin resident cells at the biting site. There is great interest in identifying transmission-enhancing factors in mosquito saliva in order to counteract them. Here we report the discovery of high levels of the anti-immune subgenomic flaviviral RNA (sfRNA) in dengue virus 2-infected mosquito saliva. We established that sfRNA is present in saliva using three different methods: northern blot, RT-qPCR and RNA sequencing. We next show that salivary sfRNA is protected in detergent-sensitive compartments, likely extracellular vesicles. In support of this hypothesis, we visualized viral RNAs in vesicles in mosquito saliva and noted a marked enrichment of signal from 3'UTR sequences, which is consistent with the presence of sfRNA. Furthermore, we show that incubation with mosquito saliva containing higher sfRNA levels results in higher virus infectivity in a human hepatoma cell line and human primary dermal fibroblasts. Transfection of 3'UTR RNA prior to DENV2 infection inhibited type I and III interferon induction and signaling, and enhanced viral replication. Therefore, we posit that sfRNA present in salivary extracellular vesicles is delivered to cells at the biting site to inhibit innate immunity and enhance dengue virus transmission.

Mucociliary transport deficiency and disease progression in Syrian hamsters with SARS-CoV-2 infection

Substantial clinical evidence supports the notion that ciliary function in the airways is important in COVID-19 pathogenesis. Although ciliary damage has been observed in both in vitro and in vivo models, the extent or nature of impairment of mucociliary transport (MCT) in in vivo models remains unknown. We hypothesize that SARS-CoV-2 infection results in MCT deficiency in the airways of golden Syrian hamsters that precedes pathological injury in lung parenchyma. Micro-optical coherence tomography was used to quantitate functional changes in the MCT apparatus. Both genomic and subgenomic viral RNA pathological and physiological changes were monitored in parallel. We show that SARS-CoV-2 infection caused a 67% decrease in MCT rate as early as 2 days postinfection (dpi) in hamsters, principally due to 79% diminished airway coverage of motile cilia. Correlating quantitation of physiological, virological, and pathological changes reveals steadily descending infection from the upper airways to lower airways to lung parenchyma within 7 dpi. Our results indicate that functional deficits of the MCT apparatus are a key aspect of COVID-19 pathogenesis, may extend viral retention, and could pose a risk factor for secondary infection. Clinically, monitoring abnormal ciliated cell function may indicate disease progression. Therapies directed toward the MCT apparatus deserve further investigation.

3D Structural Analysis of Long Noncoding RNA by Small Angle X-ray Scattering and Computational Modeling

Small angle X-ray scattering (SAXS) has been widely applied as an enabling integrative technique for comprehensive analysis of the structure of biomacromolecules by multiple, complementary techniques in solution. SAXS in combination with computational modeling can be a powerful strategy bridging the secondary and 3D structural analysis of large RNAs, including the long noncoding RNAs (lncRNA). Here, we outline the major procedures and techniques in the combined use of SAXS and computational modeling for 3D structural characterization of a lncRNA, the subgenomic flaviviral RNA from Zika virus. lncRNA production and purification, RNA buffer and sample preparation for SAXS experiments, SAXS data collection and analysis, SAXS-aided RNA 3D structure prediction, and computational modeling are described.

Sequence characterization of extracellular HBV RNA in patient plasma

Antiviral nucleos(t)ide analogue therapies inhibit HBV replication and suppress the HBV DNA levels in patients with chronic HBV infection. Since HBV RNAs are expressed from cccDNA or HBV integrated sequences, independently of viral genome replication, levels of HBV RNAs in plasma may remain high following treatment with nucleos(t)ide analogue. Thus, HBV RNAs have been proposed to be used as a viral biomarker for treatment outcome and disease progression. Recent investigations of plasma HBV RNAs described the presence of full length as well as subgenomic forms of RNA. To support the usage of plasma HBV RNAs as a viral biomarker, further understanding of HBV RNA composition in clinical samples is needed. Here, sequence of extracellular HBV RNAs was characterized in plasma samples of patients with chronic HBV infection using two independent RNA amplification methods that do not use HBV-specific primers for amplification: total RNA (NuGEN RNAseq) and mRNA (TruSeq RNAseq). Sequencing coverage was obtained across the full length of HBV genome for both methods, confirming the presence of full-length HBV RNA in plasma. The sequence of HBV RNA was nearly identical to plasma HBV DNA sequence in each sample with only 0-14 (median 4) mismatches over 3 kb. Thus, sequence of HBV RNA plasma reflects the intrahepatic viral reservoir and can be used for monitoring of sequence variants such as resistance in clinical trials. Additionally, RNA splice forms, different polyA tails start positions and presence of HBV-human chimeric transcript were identified.

Molecular Characteristics of Subgenomic RNAs and the Cap-Dependent Translational Advantage Relative to Corresponding Genomic RNAs of Tomato spotted wilt virus

Tomato spotted wilt virus (TSWV) causes severe viral diseases on many economically important plants of Solanaceae. During the infection process of TSWV, a series of 3'-truncated subgenomic RNAs (sgRNAs) relative to corresponding genomic RNAs were synthesized, which were responsible for the expression of some viral proteins. However, corresponding genomic RNAs (gRNAs) seem to possess the basic elements for expression of these viral proteins. In this study, molecular characteristics of sgRNAs superior to genomic RNAs in viral protein expression were identified. The 3' ends of sgRNAs do not cover the entire intergenic region (IGR) of TSWV genomic RNAs and contain the remarkable A-rich characteristics. In addition, the 3' terminal nucleotides of sgRNAs are conserved among different TSWV isolates. Based on the eIF4E recruitment assay and subsequent northern blot, it is suggested that the TSWV sgRNA, but not gRNA, is capped in vivo; this is why sgRNA is competent for protein expression relative to gRNA. In addition, the 5' and 3' untranslated region (UTR) of sgRNA-Ns can synergistically enhance cap-dependent translation. This study further enriched the understanding of sgRNAs of ambisense RNA viruses.