Molecular biology of varicella zoster virus

Comparison of the complete DNA sequences of the Oka varicella vaccine and its parental virus

The DNA sequences of the Oka varicella vaccine virus (V-Oka) and its parental virus (P-Oka) were completed. Comparison of the sequences revealed 42 base substitutions, which led to 20 amino acid conversions and length differences in tandem repeat regions (R1, R3, and R4) and in an origin of DNA replication. Amino acid substitutions existed in open reading frames (ORFs) 6, 9A, 10, 21, 31, 39, 50, 52, 55, 59, 62, and 64. Of these, 15 base substitutions, leading to eight amino acid substitutions, were in the gene 62 region alone. Further DNA sequence analysis showed that these substitutions were specific for V-Oka and were not present in nine clinical isolates. The immediate-early gene 62 product (IE62) of P-Oka had stronger transactivational activity than the mutant IE62 contained in V-Oka in 293 and CV-1 cells. An infectious center assay of a plaque-purified clone (S7-01) from the V-Oka with 8 amino acid substitutions in ORF 62 showed smaller plaque formation and less-efficient virus-spreading activity than did P-Oka in human embryonic lung cells. Another clone (S-13) with only five substitutions in ORF 62 spread slightly faster than S7-01 but not as effectively as P-Oka. Moreover, transient luciferase assay in 293 cells showed that transactivational activities of IE62s of S7-01 and S7-13 were lower than that of P-Oka. Based on these results, it appears that amino acid substitutions in ORF 62 are responsible for virus growth and spreading from infected to uninfected cells. Furthermore, the Oka vaccine virus was completely distinguishable from P-Oka and 54 clinical isolates by seven restriction-enzyme fragment length polymorphisms that detected differences in the DNA sequence.

Varicella-zoster virus latency in human ganglia

Varicella-zoster virus (VZV) is a human herpesvirus which causes varicella (chickenpox) as a primary infection, and, following a variable period during which it remains in latent form in trigeminal and dorsal root ganglia, reactivates in later life to cause herpes zoster (shingles). VZV is a significant cause of neurological disease including post-herpetic neuralgia which may be persistent and highly resistant to treatment, and small and large vessel encephalitis. VZV infections are more frequent with advancing age and in immunocompromised individuals. An understanding of the mechanisms of latency is crucial in developing effective therapies for VZV infections of the nervous system. Such studies have been hampered by the difficulties in working with the virus and also the lack of a good animal model of VZV latency. It is known that the ganglionic VZV burden during latency is low. Two of the key questions that have been addressed are the cellular site of latent VZV and the identity of the viral genes which are transcribed during latency. There is now a consensus that latent VZV resides predominantly in ganglionic neurons with less frequent infection of non-neuronal satellite cells. There is considerable evidence to show that at least five viral genes are transcribed during latency. Unlike herpes simplex virus-1 latency, viral protein expression has been demonstrated during VZV latency. A precise knowledge of which viral genes are expressed is crucial in devising novel antiviral therapy using expressed genes as therapeutic targets. Whether gene expression at both the transcriptional and translational levels is more extensive than currently reported will require much more work and probably new molecular technology.

Varicella-Zoster virus gene expression in latently infected rat dorsal root ganglia

Latent infection of human ganglia with Varicella-Zoster virus (VZV) is characterized by a highly restricted pattern of viral gene expression. To enhance understanding of this process we used in situ hybridization (ISH) in a rat model of VZV latency to examine expression of RNA corresponding to eight different VZV genes in rat dorsal root ganglia (DRG) at various times after footpad inoculation with wild-type VZV. PCR in situ amplification was also used to determine the cell specificity of latent VZV DNA. It was found that the pattern of viral gene expression at 1 week after infection was different from that observed at the later times of 1 and 18 months after infection. Whereas multiple genes were expressed at 1 week after infection, gene expression was restricted at the later time points when latency had been established. At the later time points after infection the RNA transcripts expressed most frequently were those for VZV genes 21, 62, and 63. Gene 63 was expressed more than any other gene studied. While VZV DNA was detected almost exclusively in 5-10% of neurons, VZV RNA was detected in both neurons and nonneuronal cells at an approximate ratio of 3:1. A newly described monoclonal antibody to VZV gene 63-encoded protein was used to detect this protein in neuronal nuclei and cytoplasm in almost half of the DRG studied. These results demonstrate that (1) this rat model of latency has close similarities in terms of viral gene expression to human VZV latency which makes it a useful tool for studying this process and its experimental modulation and (2) expression of VZV gene 63 appears to be the single most consistent feature of VZV latency.

Varicella-zoster virus gene expression in latently infected and explanted human ganglia

A consistent feature of varicella-zoster virus (VZV) latency is the restricted pattern of viral gene expression in human ganglionic tissues. To understand further the significance of this gene restriction, we used in situ hybridization (ISH) to detect the frequency of RNA expression for nine VZV genes in trigeminal ganglia (TG) from 35 human subjects, including 18 who were human immunodeficiency virus (HIV) positive. RNA for VZV gene 21 was detected in 7 of 11 normal and 6 of 10 HIV-positive subjects, RNA for gene 29 was detected in 5 of 14 normal and 11 of 11 HIV-positive subjects, RNA for gene 62 was detected in 4 of 10 normal and 6 of 9 HIV-positive subjects, and RNA for gene 63 was detected in 8 of 17 normal and 12 of 15 HIV-positive subjects. RNA for VZV gene 4 was detected in 2 of 13 normal and 4 of 9 HIV-positive subjects, and RNA for gene 18 was detected in 4 of 15 normal and 5 of 15 HIV-positive subjects. By contrast, RNAs for VZV genes 28, 40, and 61 were rarely or never detected. In addition, immunocytochemical analysis detected the presence of VZV gene 63-encoded protein in five normal and four HIV-positive subjects. VZV RNA was also analyzed in explanted fresh human TG and dorsal root ganglia from five normal human subjects over a period of up to 11 days in culture. We found a very different pattern of gene expression in these explants, with transcripts for VZV genes 18, 28, 29, 40, and 63 all frequently detected, presumably as a result of viral reactivation. Taken together, these data provide further support for the notion of significant and restricted viral gene expression in VZV latency.

Variable R1 region in varicella zoster virus in fulminant type of acute retinal necrosis syndrome

BACKGROUND/AIMS

Varicella zoster virus (VZV) is a causative agent in acute retinal necrosis (ARN) syndrome. However, in spite of aggressive antiviral therapy, clinical characteristics among patients have varied. Different viral strains were examined to determine their respective role in producing clinical characteristics. The viral strains were also compared with those of previously reported ones.

METHODS

To differentiate VZV strains R1 and R5, variable regions of VZV were amplified by nested polymerase chain reaction (PCR) in 11 eyes of 10 patients. Sequence analysis was also performed.

RESULTS

Four cases had strains diverted only at the tip of the 3' end of the R1 variable region, similar to that of the H-N3 strain, which was previously reported. Conversely, other cases were diverted to other regions. Interestingly, some of the latter cases showed multiple PCR products in the R1 region that were generated by the truncation of either the 5' or 3' R1 region. Final visual acuities of these patients were less than 0.2. The former cases showed final visual acuities more than 0.4. Only two variants were from the R5 region. No patient had the same viral strain as the European Dumas type.

CONCLUSION

These results showed that variable VZV strains participated in ARN. Using PCR of the R1 variable region, it was estimated that patients with a more fulminant type of ARN may have diverse viruses with extensive replication in the affected eyes.

Experimental investigation of herpes simplex virus latency

The clinical manifestations of herpes simplex virus infection generally involve a mild and localized primary infection followed by asymptomatic (latent) infection interrupted sporadically by periods of recrudescence (reactivation) where virus replication and associated cytopathologic findings are manifest at the site of initial infection. During the latent phase of infection, viral genomes, but not infectious virus itself, can be detected in sensory and autonomic neurons. The process of latent infection and reactivation has been subject to continuing investigation in animal models and, more recently, in cultured cells. The initiation and maintenance of latent infection in neurons are apparently passive phenomena in that no virus gene products need be expressed or are required. Despite this, a single latency-associated transcript (LAT) encoded by DNA encompassing about 6% of the viral genome is expressed during latent infection in a minority of neurons containing viral DNA. This transcript is spliced, and the intron derived from this splicing is stably maintained in the nucleus of neurons expressing it. Reactivation, which can be induced by stress and assayed in several animal models, is facilitated by the expression of LAT. Although the mechanism of action of LAT-mediated facilitation of reactivation is not clear, all available evidence argues against its involving the expression of a protein. Rather, the most consistent models of action involve LAT expression playing a cis-acting role in a very early stage of the reactivation process.

Lessons to be learned from varicella-zoster virus

Varicella-zoster virus (VZV) is an alphaherpesvirus responsible for two human diseases: chicken pox and shingles. The virus has a respiratory port of entry. After two successive viremias, it reaches the skin where it causes typical lesions. There, it penetrates the peripheral nervous system and it remains latent in dorsal root ganglia. It is still debatable whether VZV persists in neurons or in satellite cells. During latency, VZV expresses a limited set of transcripts of its immediate early (IE) and early (E) genes but no protein has been detected. Mechanisms of reactivation from ganglia have not been identified. However, dysfunction of the cellular immune system appears to be involved in this process. The cell-associated nature of VZV has made it difficult to identify a temporal order of gene expression, but there appears to be a cascade mechanism as for HSV-1. The lack of high titre cell-free virions or recombination mutants has hindered so far the understanding of VZV gene functions. Five genes, ORFs 4, 10, 61, 62, and 63 that encode regulatory proteins could be involved in VZV latency. ORF4p activates gene promoters with basal activities. ORF10p seems to activate the ORF 62 promoter. ORF61p has trans-activating and trans-repressing activities. The major IE protein ORF62p, a virion component, has DNA-binding and regulatory functions, transactivates many VZV promoters and even regulates its own expression. ORF63p is a nuclear IE protein of yet unclear regulatory functions, abundantly expressed very early in infection. We have established an animal model of VZV latency in the rat nervous system, enabling us to study the expression of viral mRNA and protein expression during latency, and yielding results similar to those found in humans. This model is beginning to shed light on the molecular events in VZV persistent infection and on the regulatory mechanisms that maintain the virus in a latent stage in nerve cells.

Varicella-zoster virus gene 63 encodes an immediate-early protein that is abundantly expressed during latency

Varicella-zoster virus (VZV) gene 63 encodes a protein with a predicted molecular mass of 30.5 kDa which has amino acid similarities with the immediate-early (IE) protein 22 (ICP-22) of herpes simplex virus type 1. In order to study the expression of this protein during lytic and latent infection, gene 63 was cloned in frame and downstream from the glutathione-S-transferase gene, expressed as a fusion protein, and purified. In VZV-infected Vero cells, antibodies directed against this protein detect two polypeptides of 45 and 38 kDa which are localized both in the cytoplasm and in the nucleus. Using a sequential combination of transcription and protein synthesis inhibitors (actinomycin D and cycloheximide, respectively), we demonstrated the immediate-early nature of this protein, which can thus be named IE63. Using a rat model of VZV latency, we showed that IE63 is heavily expressed, essentially in neurons, during latency. IE63 can also be detected in the skin of patients showing early herpes zoster symptoms.

Mutational analysis of varicella-zoster virus major immediate-early protein IE62

The varicella-zoster virus (VZV) open reading frame 62 encodes an immediate-early protein (IE62) that transactivates expression of various VZV promoters and autoregulates its own expression in transient expression assays. In Vero cells, IE62 was shown to transactivate the expression of all putative immediate-early (IE) and early (E) genes of VZV with an up-regulating effect at low intracellular concentrations. To define the functional domains involved in the regulatory properties of IE62, a large number of in-frame insertions and deletions were introduced into a plasmid-borne copy of the gene encoding IE62. Studies of the regulatory activities of the resultant mutant polypeptides in transient expression assays allowed to delineate protein regions important for repression of its own promoter and for transactivation of a VZV putative immediate-early gene (ORF61) promoter and an early gene (ORF29) promoter. This mutational analysis resulted in the identification of a new functional domain situated at the border between regions 4 and 5 which plays a crucial role in the IE62 regulatory functions. This domain turned out to be very well conserved amongst homologous alphaherpesvirus regulatory proteins and appeared to be rich in bulky hydrophobic and proline residues, similar to the proline-rich region of the CAAT box binding protein CTF-1. By immunofluorescence, a nuclear localization signal has been mapped in region 3.

Identification of varicella-zoster virus strains by PCR analysis of three repeat elements and a PstI-site-less region

We established a method of identifying varicella-zoster virus (VZV) strains, especially those of the Oka vaccine, in patients with clinical VZV infections. The DNAs of 30 clinically isolated strains and 4 laboratory strains including the Oka vaccine strain and its parent VZV strain, were analyzed by PCR with four sets of primers for the four variable regions, R2, R4, R5, and a region without a PstI site (PS). R4 was unstable in four laboratory VZV strains and was excluded from the study. The other regions were stable in several passages in cell culture. The number of copies in R2 and R5 were distributed from 2 to 13 and from 1 to 3, respectively, in the strains analyzed. The vaccine strain had seven copies in R2 and two copies in R5, and it was PS negative. Among 30 clinical isolates, 3, 23, and 11 strains had the same characteristics as the vaccine strain in R2, R5, and PS, respectively. Therefore, by this method, 97.2% of the isolates were distinguished from the Oka vaccine strain. This strategy will be useful in diagnosing VZV infections induced by the vaccine strain.

Overview: the biology of varicella-zoster virus infection

Varicella-zoster virus infection is manifested initially as chickenpox. The virus persists for life in sensory nerve ganglia, from which it reactivates in many people to cause zoster. Among the many recognized complications of these infections, post-zoster neuralgia is the most frequently debilitating. The molecular events of virus replication, latency, and reactivation, and the pathogenesis of post-zoster neuralgia, are incompletely understood and inadequately addressed by current therapeutic strategies.

Characterization and immunogenicity of a candidate subunit vaccine against varicella-zoster virus

This study describes the properties of an inactivated subunit antigen preparation from varicella-zoster virus (VZV)-infected MRC-5 cells by treatment with detergent and formaldehyde, ultracentrifugation over sucrose and acetone precipitation. The method preserved the antigenicity of VZV proteins and several VZV-specific glycoproteins, while virus DNA was less than 20 pg/250 micrograms protein--a putative vaccine dose. The vaccine was immunogenic in rabbits and stimulated antibodies to the major capsid protein as well as to glycoproteins; an immunoprecipitin was shared with a known immune human serum. The preparation contained no infectious VZV with no evidence of side effects in a rabbit or in five human vaccinees during a follow-up period of 6-10 years.

Characterization of the regulatory functions of varicella-zoster virus open reading frame 4 gene product

Varicella-zoster virus (VZV) open reading frame 4 (ORF4) encodes a protein with a predicted molecular weight of 51,540 presenting amino acid sequence homology with the immediate-early regulatory protein ICP27 of herpes simplex virus type 1. To investigate the regulatory properties of the ORF4 gene product, we performed a series of transient expression assays in Vero cells, using a plasmid expressing ORF4 as effector and several VZV genes and heterologous genes as targets. The VZV target plasmids contained promoter/regulatory regions from genes belonging to the three putative VZV kinetic classes fused to the chloramphenicol acetyltransferase (CAT) gene. The heterologous target plasmids consisted of promoter/regulatory regions of human cytomegalovirus, Rous sarcoma virus, and human immunodeficiency virus type 1 fused to the reporter gene. These experiments demonstrated that the ORF4 gene product activated expression of ORF62 in a dose-dependent fashion but had no effect on the expression of the three other putative immediate-early genes (ORF4, ORF61, and ORF63). When various amounts of ORF4 were transfected in the presence of early gene promoters, dose-dependent transactivation was evidenced with the thymidine kinase gene (ORF36) and the major DNA-binding protein gene (ORF29) promoters; interestingly, little activity was detected with the promoter of the DNA polymerase gene (ORF28). No activation of late gene expression, represented by the glycoprotein I and glycoprotein II genes, was seen even over a wide range of concentrations of input ORF4 plasmid. Expression of pCMVCAT, pRSVCAT, and pHIVCAT was also stimulated by the ORF4 gene product. CAT mRNA analysis showed that activation of VZV target promoters occurs at the transcriptional and/or posttranscriptional level.

Absence of varicella zoster DNA in varicella embryopathy tissue utilizing the polymerase chain reaction

We have previously examined a case of varicella embryopathy (VE) occurring in a 41-week gestational age infant, showing some of the most severe lesions reported to be associated with the syndrome to date. A previous survey of the postmortem tissue with two anti-varicella zoster antibodies was negative. It was hypothesized that if varicella-zoster virus (VZV) was associated with VE, a latent viral infection might be expected, similar to that occurring in adults infected with VZV. Subsequently, an attempt was made to identify DNA encoding three separate and specific VZV genes using the highly sensitive polymerase chain reaction from formalin-fixed tissues of the central nervous system including cerebral cortex, cerebellum, and brainstem, in addition to frozen tissue of liver, adrenal, and thymus. Although amplifications utilizing primers for a normal human DNA sequence (ME491) demonstrated the ability of the samples to serve as a target for the PCR, none of the samples contained amplifiable VZV DNA. Substitution of biotin 11-dUTP in the PCR and subsequent detection of the biotinylated amplimers resulted in a large increase of the sensitivity of the PCR amplimer detection, but still failed to detect the presence of VZV DNA in the samples. The most likely explanation for these findings suggests a complete destruction of fetal tissue tropic for VZV with a subsequent inability for a latent VZV infection to be established.

Regulation of varicella-zoster virus gene expression in human T lymphocytes

Varicella-zoster virus (VZV), a neurotropic alphaherpesvirus, is the etiologic agent of chicken pox and shingles (zoster) in humans. Using an in vitro transient expression assay, we have evaluated the ability of the putative immediate early VZV genes, ORF4, ORF61, and ORF62 (the analogs of the herpes simplex virus alpha 27, alpha 0, and alpha 4 genes, respectively), to modulate the expression of VZV genes of different putative kinetic classes in a human T lymphocyte cell line. These cells are of the type in which VZV can be readily detected in the viremic phase of human infection. We present evidence to indicate that, in this system, the gene product of ORF62 (IE62) is a major regulatory protein in VZV and is capable of activating VZV genes of all putative kinetic classes. In addition, we demonstrate that the gene product of ORF4 and, unlike the apparent situation in Vero cells, the gene product of ORF61 may play an accessory regulatory role in synergizing the activation of VZV genes induced by the gene product of ORF62 in human T lymphocytes.

Characterization of regulatory functions of the varicella-zoster virus gene 63-encoded protein

Varicella-zoster virus (VZV) gene 63 encodes a protein (IE63) with a predicted molecular mass of 30.5 kDa which has amino acid similarities to the immediate-early (IE) protein 22 (ICP22) of herpes simplex virus type 1. ICP22 is a polypeptide synthesized in herpes simplex virus type 1-infected cells, and as is the case for its VZV counterpart, its regulatory functions are unknown. On the basis of the VZV DNA sequence, it has been shown that IE63 exhibits hydrophilic and acidic properties, suggesting that this protein could play a regulatory role during the infectious cycle. We report in this article cotransfection experiments which demonstrate that the VZV gene 63 protein strongly represses, in a dose-dependent manner, the expression of VZV gene 62. On the other hand, transient expression of the VZV gene 63 protein can promote activation of the thymidine kinase gene but cannot affect the expression of the genes encoding glycoproteins I and II. The results of transient expression experiments strongly suggest that the VZV gene 63 protein could play a pivotal role in the repression of IE gene expression as well as in the activation of early gene expression.

Biology and molecular aspects of herpes simplex and varicella-zoster virus infections

The herpes simplex and varicella-zoster viruses are members of the subfamily alpha herpesviruses with specific properties of the virion and with the capacity to establish latent infections in humans. The genome of each of these viruses has been determined with an estimate of the number of genes and proteins encoded. The biology and molecular events of the herpes simplex virus productive and latent infection have been detailed with the use of both in vitro and in vivo model systems. The neuron is the site of latency in the ganglia with a limited transcription of genes expressed during the latent period. The specific molecular regulation of latency and reactivation are not well established. There are co-cultivation, electron microscopy, and biochemical studies that support the concept of corneal latency, although this has not been proven conclusively. Details about the varicella-zoster virus biology and molecular events are not as well advanced since animal models have been lacking. The biology of the productive infection (varicella) is different from herpes simplex virus infection since the portal of entry is the respiratory system. Data support the concept of the maintenance of latency within satellite cells in the ganglia rather than within neurons. There are multiple genes expressed during this latency. These features may explain the different clinical presentations and course of reactivation (zoster) compared with herpes simplex virus reactivation.

The glycoproteins of the human herpesviruses

The herpesvirus family contains several important human pathogens. Human herpesviruses include herpes simplex virus type 1 and 2, varicella-zoster virus, human cytomegalovirus, Epstein-Barr virus and human T-cell lymphotropic virus. The general property of herpesviruses is their ability to establish latency and to be periodically reactivated. All human herpesviruses contain a subset of genes encoding viral glycoproteins that are clearly homologous, and their similarity is significantly greater among members of the same subfamily. Membrane glycoproteins specified by human herpesviruses are important determinants of viral pathogenicity. They are exposed on the viral envelope and on the surface of infected cells. They mediate entry of the virus into cells and cell-to-cell spread of infection and also influence tissue tropism and host range. Viral membrane glycoproteins are also the most important elicitors of protective immune response and are therefore the best candidates for subunit vaccines.