An isatin-beta-thiosemicarbazone-resistant vaccinia virus containing a mutation in the second largest subunit of the viral RNA polymerase is defective in transcription elongation

Current Status of Epidemiology, Diagnosis, Therapeutics, and Vaccines for the Re-Emerging Human Monkeypox Virus

Monkeypox (Mpox) virus, a member of the Poxviridae family, causes a severe illness similar to smallpox, which is characterized by symptoms such as high fever, rash, and pustules. Human-to-human transmission cases have been reported but remained low since the first recorded case of human infection occurred in the Congo in 1970. Recently, Mpox has re-emerged, leading to an alarming surge in infections worldwide since 2022, originating in the United Kingdom. Consequently, the World Health Organization (WHO) officially declared the '2022-23 Mpox outbreak'. Currently, no specific therapy or vaccine is available for Mpox. Therefore, patients infected with Mpox are treated using conventional therapies developed for smallpox. However, the vaccines developed for smallpox have demonstrated only partial efficacy against Mpox, allowing viral transmission among humans. In this review, we discuss the current epidemiology of the ongoing Mpox outbreak and provide an update on the progress made in diagnosis, treatment, and development of vaccines for Mpox.

Anticancer Thiosemicarbazones: Chemical Properties, Interaction with Iron Metabolism, and Resistance Development

SIGNIFICANCE

During the past decades, thiosemicarbazones were clinically developed for a variety of diseases, including tuberculosis, viral infections, malaria, and cancer. With regard to malignant diseases, the class of α-N-heterocyclic thiosemicarbazones, and here especially 3-aminopyridine-2-carboxaldehyde thiosemicarbazone (Triapine), was intensively developed in multiple clinical phase I/II trials. Recent Advances: Very recently, two new derivatives, namely COTI-2 and di-2-pyridylketone 4-cyclohexyl-4-methyl-3-thiosemicarbazone (DpC) have entered phase I evaluation. Based on the strong metal-chelating/metal-interacting properties of thiosemicarbazones, interference with the cellular iron (and copper) homeostasis is assumed to play an important role in their biological activity.

CRITICAL ISSUES

In this review, we summarize and analyze the data on the interaction of (α-N-heterocyclic) thiosemicarbazones with iron, with the special aim of bridging the current knowledge on their mode of action from chemistry to (cell) biology. In addition, we highlight the difference to classical iron(III) chelators such as desferrioxamine (DFO), which are used for the treatment of iron overload.

FUTURE DIRECTIONS

We want to emphasize that thiosemicarbazones are not solely removing iron from the cells/organism. In contrast, they should be considered as iron-interacting drugs influencing diverse biological pathways in a complex and multi-faceted mode of action. Consequently, in addition to the discussion of physicochemical properties (e.g., complex stability, redox activity), this review contains an overview on the diversity of cellular thiosemicarbazone targets and drug resistance mechanisms.

RNA Polymerase Mutations Selected during Experimental Evolution Enhance Replication of a Hybrid Vaccinia Virus with an Intermediate Transcription Factor Subunit Replaced by the Myxoma Virus Ortholog

High-throughput DNA sequencing enables the study of experimental evolution in near real time. Until now, mutants with deletions of nonessential host range genes were used in experimental evolution of vaccinia virus (VACV). Here, we guided the selection of adaptive mutations that enhanced the fitness of a hybrid virus in which an essential gene had been replaced with an ortholog from another poxvirus genus. Poxviruses encode a complete system for transcription, including RNA polymerase and stage-specific transcription factors. The abilities of orthologous intermediate transcription factors from other poxviruses to substitute for those of VACV, as determined by transfection assays, corresponded with the degree of amino acid identity. VACV in which the A8 or A23 intermediate transcription factor subunit gene was replaced by the myxoma (MYX) virus ortholog exhibited decreased replication. During three parallel serial passages of the hybrid virus with the MYXA8 gene, plaque sizes and virus yields increased. DNA sequencing of virus populations at passage 10 revealed high frequencies of five different single nucleotide mutations in the two largest RNA polymerase subunits, RPO147 and RPO132, and two different Kozak consensus sequence mutations predicted to increase translation of the MYXA8 mRNA. Surprisingly, there were no mutations within either intermediate transcription factor subunit. Based on homology with Saccharomyces cerevisiae RNA polymerase, the VACV mutations were predicted to be buried within the internal structure of the enzyme. By directly introducing single nucleotide substitutions into the genome of the original hybrid virus, we demonstrated that both RNA polymerase and translation-enhancing mutations increased virus replication independently.IMPORTANCE Previous studies demonstrated the experimental evolution of vaccinia virus (VACV) following deletion of a host range gene important for evasion of host immune defenses. We have extended experimental evolution to essential genes that cannot be deleted but could be replaced by a divergent orthologous gene from another poxvirus. Replacement of a VACV transcription factor gene with one from a distantly related poxvirus led to decreased fitness as evidenced by diminished replication. Serially passaging the hybrid virus at a low multiplicity of infection provided conditions for selection of adaptive mutations that improved replication. Notably, these included five independent mutations of the largest and second largest RNA polymerase subunits. This approach should be generally applicable for investigating adaptation to swapping of orthologous genes encoding additional essential proteins of poxviruses as well as other viruses.

Combined Proteomics/Genomics Approach Reveals Proteomic Changes of Mature Virions as a Novel Poxvirus Adaptation Mechanism

DNA viruses, like poxviruses, possess a highly stable genome, suggesting that adaptation of virus particles to specific cell types is not restricted to genomic changes. Cowpox viruses are zoonotic poxviruses with an extraordinarily broad host range, demonstrating their adaptive potential in vivo. To elucidate adaptation mechanisms of poxviruses, we isolated cowpox virus particles from a rat and passaged them five times in a human and a rat cell line. Subsequently, we analyzed the proteome and genome of the non-passaged virions and each passage. While the overall viral genome sequence was stable during passaging, proteomics revealed multiple changes in the virion composition. Interestingly, an increased viral fitness in human cells was observed in the presence of increased immunomodulatory protein amounts. As the only minor variant with increasing frequency during passaging was located in a viral RNA polymerase subunit and, moreover, most minor variants were found in transcription-associated genes, protein amounts were presumably regulated at transcription level. This study is the first comparative proteome analysis of virus particles before and after cell culture propagation, revealing proteomic changes as a novel poxvirus adaptation mechanism.

Ectromelia virus accumulates less double-stranded RNA compared to vaccinia virus in BS-C-1 cells

Most orthopoxviruses, including vaccinia virus (VACV), contain genes in the E3L and K3L families. The protein products of these genes have been shown to combat PKR, a host defense pathway. Interestingly, ectromelia virus (ECTV) contains an E3L ortholog but does not possess an intact K3L gene. Here, we gained insight into how ECTV can still efficiently evade PKR despite lacking K3L. Relative to VACV, we found that ECTV-infected BS-C-1 cells accumulated considerably less double-stranded (ds) RNA, which was due to lower mRNA levels and less transcriptional read-through of some genes by ECTV. The abundance of dsRNA in VACV-infected cells, detected using a monoclonal antibody, was able to activate the RNase L pathway at late time points post-infection. Historically, the study of transcription by orthopoxviruses has largely focused on VACV as a model. Our data suggest that there could be more to learn by studying other members of this genus.

Emergence of a Viral RNA Polymerase Variant during Gene Copy Number Amplification Promotes Rapid Evolution of Vaccinia Virus

Viruses are under relentless selective pressure from host immune defenses. To study how poxviruses adapt to innate immune detection pathways, we performed serial vaccinia virus infections in primary human cells. Independent courses of experimental evolution with a recombinant strain lacking E3L revealed several high-frequency point mutations in conserved poxvirus genes, suggesting important roles for essential poxvirus proteins in innate immune subversion. Two distinct mutations were identified in the viral RNA polymerase gene A24R, which seem to act through different mechanisms to increase virus replication. Specifically, a Leu18Phe substitution encoded within A24R conferred fitness trade-offs, including increased activation of the antiviral factor protein kinase R (PKR). Intriguingly, this A24R variant underwent a drastic selective sweep during passaging, despite enhanced PKR activity. We showed that the sweep of this variant could be accelerated by the presence of copy number variation (CNV) at the K3L locus, which in multiple copies strongly reduced PKR activation. Therefore, adaptive cases of CNV can facilitate the accumulation of point mutations separate from the expanded locus. This study reveals how rapid bouts of gene copy number amplification during accrual of distant point mutations can potently facilitate poxvirus adaptation to host defenses.

IMPORTANCE

Viruses can evolve quickly to defeat host immune functions. For poxviruses, little is known about how multiple adaptive mutations emerge in populations at the same time. In this study, we uncovered a means of vaccinia virus adaptation involving the accumulation of distinct genetic variants within a single population. We identified adaptive point mutations in the viral RNA polymerase gene A24R and, surprisingly, found that one of these mutations activates the nucleic acid sensing factor PKR. We also found that gene copy number variation (CNV) can provide dual benefits to evolving virus populations, including evidence that CNV facilitates the accumulation of a point mutation distant from the expanded locus. Our data suggest that transient CNV can accelerate the fixation of mutations conferring modest benefits, or even fitness trade-offs, and highlight how structural variation might aid poxvirus adaptation through both direct and indirect actions.

Monkeypox virus induces the synthesis of less dsRNA than vaccinia virus, and is more resistant to the anti-poxvirus drug, IBT, than vaccinia virus

Monkeypox virus (MPXV) infection fails to activate the host anti-viral protein, PKR, despite lacking a full-length homologue of the vaccinia virus (VACV) PKR inhibitor, E3. Since PKR can be activated by dsRNA produced during a viral infection, we have analyzed the accumulation of dsRNA in MPXV-infected cells. MPXV infection led to less accumulation of dsRNA than VACV infection. Because in VACV infections accumulation of abnormally low amounts of dsRNA is associated with mutations that lead to resistance to the anti-poxvirus drug isatin beta-thiosemicarbazone (IBT), we investigated the effects of treatment of MPXV-infected cells with IBT. MPXV infection was eight-fold more resistant to IBT than wild-type vaccinia virus (wtVACV). These results demonstrate that MPXV infection leads to the accumulation of less dsRNA than wtVACV, which in turn likely leads to a decreased capacity for activation of the dsRNA-dependent host enzyme, PKR.

Synthesis and antioxidant activities of transition metal complexes based 3-hydroxysalicylaldehyde-S-methylthiosemicarbazone

The nickel(II), iron(III), oxovanadium(IV) complexes of the 3-hydroxysalicylidene-S-methyl-thiosemicarbazone (L) were obtained from the 3-hydroxysalicyldehyde-S-methylthiosemicarbazone with the R1-substituted-salicylaldehyde (R1: H, 3-OH) in the presence of Ni(II), Fe(III), VO(IV) as template ion. The ligand and its complexes were characterized by elemental analysis, electronic, UV/Vis., (1)HNMR, EPR and IR studies. The free ligand and its metal complexes have been tested for in vitro antioxidant capacity by reduction of copper(II) neocuproine (Cu(II)-Nc) using the CUPRAC method. The ligand exhibited more potent in vitro antioxidant capacity than its complexes. The obtained trolox equivalent antioxidant capacity (TEAC) value of the iron(III) complex (TEACCUPRAC=3.27) was higher than those of other complexes. Furthermore, the antioxidant activity of the free ligand and its complexes were determined by in vitro methods measuring the scavenging activity of reactive oxygen species (ROS) including hydroxyl radical (OH), superoxide anion radical (O2(-)), and hydrogen peroxide (H2O2), showing that especially the V(IV) and Fe(III) complexes had significant scavenging activity for ROS.

Activation of stress response pathways promotes formation of antiviral granules and restricts virus replication

The formation of protein-RNA granules is a part of both natural cellular function (P-bodies and nuclear HNRNPs) and the response to cellular stress (stress granules and ND10 bodies). To better understand the role of stress-induced granules in viral infection, we have studied the ability of cells to restrict poxvirus replication through the formation of antiviral granules (AVGs). Of cells infected with a wild-type poxvirus, a small number spontaneously formed AVGs. In these AVG-positive cells, viral gene expression was inhibited. The addition of compounds that altered RNA helicase activity, induced oxidative stress, or stimulated translation initiation factor phosphorylation significantly increased the number of AVG-positive cells. When AVGs formed, both viral translation and titers were decreased even when host translation persisted. Treatment with the antiviral compound isatin β-thiosemicarbazone (IBT), a compound that was used to treat smallpox infections, induced AVGs, suggesting a role for these structures in the pharmacological inhibition of poxvirus replication. These findings provide evidence that AVGs are an innate host response that can be exogenously stimulated to combat virus infection. Since small molecules are able to stimulate AVG formation, it is a potential target for new antiviral development.

Identification of a pyridopyrimidinone inhibitor of orthopoxviruses from a diversity-oriented synthesis library

Orthopoxviruses include the prototypical vaccinia virus, the emerging infectious agent monkeypox virus, and the potential biothreat variola virus (the causative agent of smallpox). There is currently no FDA-approved drug for humans infected with orthopoxviruses. We screened a diversity-oriented synthesis library for new scaffolds with activity against vaccinia virus. This screen identified a nonnucleoside analog that blocked postreplicative intermediate and late gene expression. Viral genome replication was unaffected, and inhibition could be elicited late in infection and persisted upon drug removal. Sequencing of drug-resistant viruses revealed mutations predicted to be on the periphery of the highly conserved viral RNA polymerase large subunit. Consistent with this, the compound had broad-spectrum activity against orthopoxviruses in vitro. These findings indicate that novel chemical synthesis approaches are a potential source for new infectious disease therapeutics and identify a potentially promising candidate for development to treat orthopoxvirus-infected individuals.

Determinants of vaccinia virus early gene transcription termination

Vaccinia virus early gene transcription requires the vaccinia termination factor, VTF, nucleoside triphosphate phosphohydrolase I, NPH I, ATP, the virion RNA polymerase, and the motif, UUUUUNU, in the nascent RNA, found within 30 to 50 bases from the poly A addition site, in vivo. In this study, the relationships among the vaccinia early gene transcription termination efficiency, termination motif specificity, and the elongation rate were investigated. A low transcription elongation rate maximizes termination efficiency and minimizes specificity for the UUUUUNU motif. Positioning the termination motif over a 63 base area upstream from the RNA polymerase allowed efficient transcript release, demonstrating a remarkable plasticity in the transcription termination complex. Efficient transcript release was observed during ongoing transcription, independent of VTF or UUUUUNU, but requiring both NPH I and either ATP or dATP. This argues for a two step model: the specifying step, requiring both VTF and UUUUUNU, and the energy-dependent step employing NPH I and ATP. Evaluation of NPH I mutants for the ability to stimulate transcription elongation demonstrated that ATPase activity and a stable interaction between NPH I and the Rap94 subunit of the viral RNA polymerase are required. These observations demonstrate that NPH I is a component of the elongating RNA polymerase, which is catalytically active during transcription elongation.

A targeted approach to identification of vaccinia virus postreplicative transcription elongation factors: genetic evidence for a role of the H5R gene in vaccinia transcription

Treatment of wild-type vaccinia virus infected cells with the anti-poxviral drug isatin-beta-thiosemicarbazone (IBT) induces the viral postreplicative transcription apparatus to synthesize longer-than-normal mRNAs through an unknown mechanism. Prior studies have shown that virus mutants resistant to or dependent on IBT affect proteins involved in control of viral postreplicative transcription elongation, including G2, J3, and the viral RNA polymerase. Prior studies also suggest that there exist additional unidentified vaccinia genes that influence transcription elongation. The present study was undertaken to target candidate transcription elongation factor genes in an error-prone mutagenesis protocol to determine whether IBT-resistant or -dependent alleles could be isolated in those candidate genes. Mutagenesis of genes in which IBT resistance alleles have previously been isolated, namely A24R (encoding the second largest RNA polymerase subunit, rpo132) and G2R (encoding a positive transcription elongation factor), resulted in isolation of novel IBT resistance and dependence alleles therefore providing proof of principle of the targeted mutagenesis technique. The vaccinia H5 protein has been implicated previously in transcription elongation by virtue of its association with the positive elongation factor G2. Mutagenesis of the vaccinia H5R gene resulted in a novel H5R IBT resistance allele, strongly suggesting that H5 is a positive transcription elongation factor.

Mapping and phenotypic analysis of spontaneous isatin-beta-thiosemicarbazone resistant mutants of vaccinia virus

Treatment of wild type vaccinia virus infected cells with the anti-poxviral drug isatin-beta-thiosemicarbazone (IBT) induces the viral postreplicative transcription apparatus to synthesize longer-than-normal mRNAs through an unknown mechanism. Previous studies have shown that virus mutants resistant to or dependent on IBT affect genes involved in control of viral postreplicative transcription elongation. This study was initiated in order to identify additional viral genes involved in control of vaccinia postreplicative transcription elongation. Eight independent, spontaneous IBT resistant mutants of vaccinia virus were isolated. Marker rescue experiments mapped two mutants to gene G2R, which encodes a previously characterized postreplicative gene positive transcription elongation factor. Three mutants mapped to the largest subunit of the viral RNA polymerase, rpo147, the product of gene J6R. One mutant contained missense mutations in both G2R and A24R (rpo132, the second largest subunit of the RNA polymerase). Two mutants could not be mapped, however sequence analysis demonstrated that neither of these mutants contained mutations in previously identified IBT resistance or dependence genes. Phenotypic and biochemical analysis of the mutants suggests that they possess defects in transcription elongation that compensate for the elongation enhancing effects of IBT. The results implicate the largest subunit of the RNA polymerase (rpo147) in the control of elongation, and suggest that there exist additional gene products which mediate intermediate and late transcription elongation in vaccinia virus.

In vitro and in vivo evaluation of isatin-beta-thiosemicarbazone and marboran against vaccinia and cowpox virus infections

It has been reported previously that some thiosemicarbazone compounds have prophylactic activity against smallpox disease and therapeutic activity against vaccinia virus (VV) infections. In these studies, isatin-beta-thiosemicarbazone (IBT) and marboran were administered once daily by intraperitoneal (ip) injection to mice using 30, 10 or 3 mg/kg for 5 days beginning 24, 48 or 72 h after inoculation with VV or cowpox virus (CV). Both compounds were highly effective (p < 0.01) at preventing mortality due to VV even when treatment was delayed up to 72 h postinfection. In CV-infected mice, neither IBT nor Marboran were effective in preventing mortality at any dosage tested when administered at 24 h postinoculation. Viral replication in liver, spleen and kidney was delayed or reduced by 100-to 10,000-fold by 10 mg/kg of marboran, but not IBT, in VV infections. Neither compound was effective against CV infection. Neither IBT nor marboran treatment of mice cutaneously infected with VV or CV reduced viral replication or clinical disease. These results suggest that this class of compound has little therapeutic potential for orthopoxvirus infections since the in vivo activity against CV, a surrogate virus for variola, is lacking.