Identification of a DNA-binding protein of human herpesvirus 6, a putative DNA polymerase stimulatory factor

Latent, Early or Late Human Herpes Virus-6B Expression in Adult Mesial Temporal Lobe Epilepsy: Association of Virus Life Cycle with Inflammatory Cytokines in Brain Tissue and Cerebral Spinal Fluid

BACKGROUND

Human herpes virus-6B (HHV-6B) was suggested as an important etiologic factor of mesial temporal lobe epilepsy, while the mechanism is still unknown. Here, we aimed to analyze antigens representing latent, early and late HHV-6B infection and the association with inflammatory cytokines in brain tissue and cerebral spinal fluid (CSF) from MTLE patients with HHV-6B-positivity.

METHODS

Nested polymerase chain reaction (nPCR), real-time PCR, immunohistochemistry (ICH) and suspension bead array for cytokines were performed.

RESULTS

Nested polymerase chain reaction (nPCR) in brain tissue revealed HHV-6B DNA in 19 of 49 MTLE patients (39%) and 1 of 19 controls (5%) (P < 0.001), but not in CSF. ICH showed HHV-6B early antigen (P41) positivity in 3 patients (6%), late antigen (gp116/54/64) positivity in 5 patients (10%), latent antigen (U94) positivity in 8 patients (16%), and multiple antigen (early and late or/and latent) positivity in 9 patients (18%). None of these HHV-6B related proteins were found positive in control brain tissue. PCR revealed significant up-regulation of IL-1a, IL-2 and IL-7 mRNA levels in the brain tissue from MTLE patients expressing early antigens compared to those expressing late, latent, multiple antigens, negative antigens and the controls. Suspension bead array of the CSF confirmed significant up-regulation of IL-1a and IL-7 protein expression from MTLE patients expressing early antigens compared to the other groups.

CONCLUSIONS

Our finding suggests HHV-6B is a common etiologic agent of MTLE. Different virus life cycle may play an important modifying role in inflammatory biology that warrants further investigation. Though virus DNA is difficult detected in CSF, up-regulation of IL-1a and IL-7 in CSF indicates the two cytokines may be taken as indirect biomarker of HHV-6B infection.

Human herpesvirus 6A U27 plays an essential role for the virus propagation

Human herpesvirus 6A (HHV-6A) is a member of the genus Roseolovirus and the subfamily Betaherpesvirinae. It is similar to and human cytomegalovirus (HCMV). HHV-6A encodes a 41 kDa nuclear phosphoprotein, U27, which acts as a processivity factor in the replication of the viral DNA. HHV-6A U27 has 43% amino acid sequence homology with HCMV UL44, which is important for DNA replication. A previous study on HHV-6A U27 revealed that it greatly increases the in vitro DNA synthesis activity of HHV-6A DNA polymerase. However, the role of U27 during the HHV-6A virus replication process remains unclear. In this study, we constructed a U27-deficient HHV-6A mutant (HHV-6ABACU27mut) with a frameshift insertion at the U27 gene using an HHV-6A bacterial artificial chromosome (BAC) system. Viral reconstitution from the mutant BAC DNA was not detected, in contrast to the wild type and the revertant from the U27 mutant. This suggests that U27 plays a critical role in the life cycle of HHV-6A.

The Presence of Human Herpesvirus 6 in the Brain in Health and Disease

The human herpesvirus 6 (HHV-6) -A and -B are two dsDNA beta-herpesviruses infectingalmost the entire worldwide population. These viruses have been implicated in multipleneurological conditions in individuals of various ages and immunological status, includingencephalitis, epilepsy, and febrile seizures. HHV-6s have also been suggested as playing a role inthe etiology of neurodegenerative diseases such as multiple sclerosis and Alzheimer's disease. Theapparent robustness of these suggested associations is contingent on the accuracy of HHV-6detection in the nervous system. The effort of more than three decades of researching HHV-6 in thebrain has yielded numerous observations, albeit using variable technical approaches in terms oftissue preservation, detection techniques, sample sizes, brain regions, and comorbidities. In thisreview, we aimed to summarize current knowledge about the entry routes and direct presence ofHHV-6 in the brain parenchyma at the level of DNA, RNA, proteins, and specific cell types, inhealthy subjects and in those with neurological conditions. We also discuss recent findings relatedto the presence of HHV-6 in the brains of patients with Alzheimer's disease in light of availableevidence.

A Conserved Tripeptide Sequence at the C Terminus of the Poxvirus DNA Processivity Factor D4 Is Essential for Protein Integrity and Function

Vaccinia virus (VACV) is a poxvirus, and the VACV D4 protein serves both as a uracil-DNA glycosylase and as an essential component required for processive DNA synthesis. The VACV A20 protein has no known catalytic function itself but associates with D4 to form the D4-A20 heterodimer that functions as the poxvirus DNA processivity factor. The heterodimer enables the DNA polymerase to efficiently synthesize extended strands of DNA. Upon characterizing the interaction between D4 and A20, we observed that the C terminus of D4 is susceptible to perturbation. Further analysis demonstrated that a conserved hexapeptide stretch at the extreme C terminus of D4 is essential for maintaining protein integrity, as assessed by its requirement for the production of soluble recombinant protein that is functional in processive DNA synthesis. From the known crystal structures of D4, the C-terminal hexapeptide is shown to make intramolecular contact with residues spanning the inner core of the protein. Our mutational analysis revealed that a tripeptide motif (²¹⁵GFI²¹⁷) within the hexapeptide comprises apparent residues necessary for the contact. Prediction of protein disorder identified the hexapeptide and several regions upstream of Gly²¹⁵ that comprise residues of the interface surfaces of the D4-A20 heterodimer. Our study suggests that ²¹⁵GFI²¹⁷ anchors these potentially dynamic upstream regions of the protein to maintain protein integrity. Unlike uracil-DNA glycosylases from diverse sources, where the C termini are disordered and do not form comparable intramolecular contacts, this feature may be unique to orthopoxviruses.

GP96 interacts with HHV-6 during viral entry and directs it for cellular degradation

CD46 and CD134 mediate attachment of Human Herpesvirus 6A (HHV-6A) and HHV-6B to host cell, respectively. But many cell types interfere with viral infection through rapid degradation of viral DNA. Hence, not all cells expressing these receptors are permissive to HHV-6 DNA replication and production of infective virions suggesting the involvement of additional factors that influence HHV-6 propagation. Here, we used a proteomics approach to identify other host cell proteins necessary for HHV-6 binding and entry. We found host cell chaperone protein GP96 to interact with HHV-6A and HHV-6B and to interfere with virus propagation within the host cell. In human peripheral blood mononuclear cells (PBMCs), GP96 is transported to the cell surface upon infection with HHV-6 and interacts with HHV-6A and -6B through its C-terminal end. Suppression of GP96 expression decreased initial viral binding but increased viral DNA replication. Transient expression of human GP96 allowed HHV-6 entry into CHO-K1 cells even in the absence of CD46. Thus, our results suggest an important role for GP96 during HHV-6 infection, which possibly supports the cellular degradation of the virus.

Nuclear transport of Epstein-Barr virus DNA polymerase is dependent on the BMRF1 polymerase processivity factor and molecular chaperone Hsp90

Epstein-Barr virus (EBV) replication proteins are transported into the nucleus to synthesize viral genomes. We here report molecular mechanisms for nuclear transport of EBV DNA polymerase. The EBV DNA polymerase catalytic subunit BALF5 was found to accumulate in the cytoplasm when expressed alone, while the EBV DNA polymerase processivity factor BMRF1 moved into the nucleus by itself. Coexpression of both proteins, however, resulted in efficient nuclear transport of BALF5. Deletion of the nuclear localization signal of BMRF1 diminished the proteins' nuclear transport, although both proteins can still interact. These results suggest that BALF5 interacts with BMRF1 to effect transport into the nucleus. Interestingly, we found that Hsp90 inhibitors or knockdown of Hsp90β with short hairpin RNA prevented the BALF5 nuclear transport, even in the presence of BMRF1, both in transfection assays and in the context of lytic replication. Immunoprecipitation analyses suggested that the molecular chaperone Hsp90 interacts with BALF5. Treatment with Hsp90 inhibitors blocked viral DNA replication almost completely during lytic infection, and knockdown of Hsp90β reduced viral genome synthesis. Collectively, we speculate that Hsp90 interacts with BALF5 in the cytoplasm to assist complex formation with BMRF1, leading to nuclear transport. Hsp90 inhibitors may be useful for therapy for EBV-associated diseases in the future.

Identification of inhibitors that block vaccinia virus infection by targeting the DNA synthesis processivity factor D4

Smallpox was globally eradicated 30 years ago by vaccination. The recent threat of bioterrorism demands the development of improved vaccines and novel therapeutics to effectively preclude a reemergence of smallpox. One new therapeutic target is the vaccinia poxvirus processivity complex, comprising D4 and A20 proteins that enable the viral E9 DNA polymerase to synthesize extended strands. Five compounds identified from an AlphaScreen assay designed to disrupt A20:D4 binding were shown to be effective in: (i) blocking vaccinia processive DNA synthesis in vitro, (ii) preventing cellular infection with minimal cytotoxicity, and (iii) binding to D4, as evidenced by ThermoFluor. The EC(50) values for inhibition of viral infectivity ranged from 9.6 to 23 μM with corresponding selectivity indices (cytotoxicity CC(50)/viral infectivity EC(50)) of 3.9 to 17.8. The five compounds are thus potential therapeutics capable of halting smallpox DNA synthesis and infectivity through disruptive action against a component of the vaccinia processivity complex.

Human herpesvirus 6B induces phenotypic maturation without IL-10 and IL-12p70 production in dendritic cells

Human herpesvirus 6B (HHV-6B) is the causative agent of the common childhood febrile illness, exanthema subitum. The virus is predominantly regarded as a T-cell tropic virus, although in reality it has the ability to infect a wide variety of cell types including monocytes, macrophages and dendritic cells (DC). Although DC are important immune regulators, the modulating effects of HHV-6B on DC are controversial. Here, we examine the phenotypic and functional consequences of HHV-6B infection of DC. The addition of HHV-6B to immature DC led to expression of the nuclear viral p41 protein and cell surface expression of the viral glycoprotein gp60/110 consistent with HHV-6B infection. Nevertheless, HHV-6B did not induce noticeable cytopathogenic effects or cell death in infected DC. Importantly, HHV-6B infection induced a partial phenotypic maturation of immature DC as demonstrated by a substantial increase in the expression of HLA-DR, CD86 and CD40, whereas only a minor increase in CD80 and CD83 was observed. This phenotypic maturation was, however, not followed by functional maturation, because HHV-6B infection did not induce IL-10 and IL-12p70 production in immature DC. However, infected DC were still able to react to bacteria-derived stimuli such as lipopolysaccaharide by an even more pronounced production of IL-10 and IL-12p70 when compared to that of uninfected DC.

The flexible loop of the human cytomegalovirus DNA polymerase processivity factor ppUL44 is required for efficient DNA binding and replication in cells

Phosphoprotein ppUL44 of the human cytomegalovirus (HCMV) DNA polymerase plays an essential role in viral replication, conferring processivity to the DNA polymerase catalytic subunit pUL54 by tethering it to the DNA. Here, for the first time, we examine in living cells the function of the highly flexible loop of ppUL44 (UL44-FL; residues 162 to 174 [PHTRVKRNVKKAP(174)]), which has been proposed to be directly involved in ppUL44's interaction with DNA. In particular, we use a variety of approaches in transfected cells to characterize in detail the behavior of ppUL44Deltaloop, a mutant derivative in which three of the five basic residues within UL44-FL are replaced by nonbasic amino acids. Our results indicate that ppUL44Deltaloop is functional in dimerization and binding to pUL54 but strongly impaired in binding nuclear structures within the nucleus, as shown by its inability to form nuclear speckles, reduced nuclear accumulation, and increased intranuclear mobility compared to wild-type ppUL44. Moreover, analysis of cellular fractions after detergent and DNase treatment indicates that ppUL44Deltaloop is strongly reduced in DNA-binding ability, in similar fashion to ppUL44-L86A/L87A, a point mutant derivative impaired in dimerization. Finally, ppUL44Deltaloop fails to transcomplement HCMV oriLyt-dependent DNA replication in cells and also inhibits replication in the presence of wild-type ppUL44, possibly via formation of heterodimers defective for double-stranded DNA binding. UL44-FL thus emerges for the first time as an important determinant for HCMV replication in cells, with potential implications for the development of novel antiviral approaches by targeting HCMV replication.

Role of homodimerization of human cytomegalovirus DNA polymerase accessory protein UL44 in origin-dependent DNA replication in cells

The presumed processivity subunit of human cytomegalovirus (HCMV) DNA polymerase, UL44, forms homodimers. The dimerization of UL44 is important for binding to DNA in vitro; however, whether it is also important for DNA replication in a cellular context is unknown. Here we show that UL44 point mutants that are impaired for dimerization, but not for nuclear localization or interaction with the C terminus of the polymerase catalytic subunit, are not capable of supporting HCMV oriLyt-dependent DNA replication in cells. These data suggest that the disruption of UL44 homodimers could represent a novel anti-HCMV strategy.

Processivity factor of KSHV contains a nuclear localization signal and binding domains for transporting viral DNA polymerase into the nucleus

Kaposi's sarcoma-associated human herpesvirus (KSHV) encodes a processivity factor (PF-8, ORF59) that forms homodimers and binds to viral DNA polymerase (Pol-8, ORF9). PF-8 is essential for stabilizing Pol-8 on template DNA so that Pol-8 can incorporate nucleotides continuously. Here, the intracellular interaction of these two viral proteins was examined by confocal immunofluorescence microscopy. When individually expressed, PF-8 was observed exclusively in the nucleus, whereas Pol-8 was found only in the cytoplasm. However, when co-expressed, Pol-8 was co-translocated with PF-8 into the nucleus. Mutational analysis revealed that PF-8 contains a nuclear localization signal (NLS) as well as domains located at the N-terminus and the C-proximal regions that are required for Pol-8 binding. This study suggests that the mechanism that enables PF-8 to transport Pol-8 into the nucleus is the first critical step required for Pol-8 and PF-8 to function processively in KSHV DNA synthesis.

Human herpesvirus 6B induces cell cycle arrest concomitant with p53 phosphorylation and accumulation in T cells

We studied the interactions between human herpesvirus 6B (HHV-6B) and its host cell. Productive infections of T-cell lines led to G1/S- and G2/M-phase arrest in the cell cycle concomitant with an increased level and enhanced DNA-binding activity of p53. More than 70% of HHV-6B-infected cells did not bind annexin V, indicating that the majority of cells were not undergoing apoptosis. HHV-6B infection induced Ser20 and Ser15 phosphorylation on p53, and the latter was inhibited by caffeine, an ataxia telangiectasia mutated kinase inhibitor. Thus, a productive HHV-6B infection suppresses T-cell proliferation concomitant with the phosphorylation and accumulation of p53.

Human Kaposi's sarcoma herpesvirus processivity factor-8 functions as a dimer in DNA synthesis

In the absence of other proteins, the DNA polymerase (Pol-8) of Kaposi's sarcoma herpesvirus incorporates only several nucleotides from a primer template. However, association with the Kaposi's sarcoma herpesvirus processivity factor (PF-8) enables Pol-8 to incorporate thousands of nucleotides. Unlike the well described sliding clamp processivity factors, eukaryotic proliferating cell nuclear antigen and Escherichia coli beta-subunit, PF-8 and other herpesvirus processivity factors do not require a clamp loader or ATP to bind to template DNA. To begin to understand the mechanism used by PF-8 to achieve processivity, we have now purified PF-8 and demonstrated that it is a dimer both in solution and on the DNA. Mutational analysis of the PF-8 protein (396R) indicates that residues between 277 and 304 as well as the N-terminal 21 amino acids are required for dimerization. The results further correlate PF-8 dimerization with binding to Pol-8 and stabilizing Pol-8 on primer template. Notably, although removal of only 26 residues from the C terminus of PF-8 does not affect its ability to form dimers on DNA or to bind Pol-8, only short DNA chains (<100 nucleotides) are synthesized. This indicates that full-length PF-8 is necessary to enable Pol-8 to incorporate thousands of nucleotides. Interestingly, cross-linking of the processivity factor UL44 of cytomegalovirus reveals that it is a dimer in solution also.

Recognition of a novel stage of betaherpesvirus latency in human herpesvirus 6

Latency-associated transcripts of human herpesvirus 6 (H6LTs) (K. Kondo et al. J. Virol. 76:4145-4151, 2002) were maximally expressed at a fairly stable intermediate stage between latency and reactivation both in vivo and in vitro. H6LTs functioned as sources of immediate-early protein 1 at this stage, which up-regulated the viral reactivation.

Phosphorylation of the Epstein-Barr virus (EBV) DNA polymerase processivity factor EA-D by the EBV-encoded protein kinase and effects of the L-riboside benzimidazole 1263W94

A member of the family of L-riboside benzimidazole compounds, 1263W94, was shown recently to inhibit replication of Epstein-Barr virus (EBV) (V. L. Zacny, E. Gershburg, M. G. Davis, K. K. Biron, and J. S. Pagano, J. Virol. 73:7271-7277, 1999). In the present report the effect of 1263W94 on the phosphorylation pattern of the EBV DNA polymerase processivity factor, EA-D, during viral reactivation in latently EBV-infected Akata cells is analyzed. This pattern specifically changes with progression of cytolytic infection. In the presence of 1263W94 the appearance of the hyperphosphorylated form of EA-D is mainly affected. Next, coexpression of the cloned EBV-encoded protein kinase (EBV PK), BGLF4, with EA-D demonstrated the ability of EBV PK to phosphorylate EA-D to its hyperphosphorylated form in transient assays. However, the phosphorylation of EA-D was not directly inhibited by 1263W94 in these coexpression assays. The results indicate that the EBV PK appears to be responsible for the hyperphosphorylation of EA-D, imply that the phosphorylation status of EA-D is important for viral replication, and suggest that 1263W94 acts at a level other than direct inhibition of EA-D phosphorylation by EBV PK.

Human herpesvirus 6 limbic encephalitis after stem cell transplantation

Central nervous system complications are common in stem cell transplant recipients, but selective involvement of the medial temporal area is unusual. The 5 patients reported here presented after stem cell transplantation with increased hippocampal T2 signal on magnetic resonance imaging and increased hippocampal glucose uptake on [F-18]fluorodeoxyglucose-positron emission tomography (FDG-PET) associated with short-term memory loss, insomnia, and temporal lobe electrographic seizure activity. The initial scalp electroencephalograms (EEGs) failed to detect seizure activity in these patients, although the memory dysfunction along with the magnetic resonance imaging and FDG-PET findings suggested subcortical seizure activity. However, extended EEG monitoring revealed repetitive temporal lobe electrographic seizure activity. Follow-up MRIs in 2 patients and postmortem findings on 1 patient suggested that hippocampal sclerosis had developed following the clinical syndrome. Cerebrospinal fluid studies revealed the presence of human herpesvirus 6, variant B, DNA in all of 3 patients who had lumbar punctures. Immunohistochemical staining for the P41 and P101 human herpesvirus 6 protein antigens showed numerous immunoreactive astrocytes and neurons in the hippocampus of 1 of the patients who died from other causes. Because of its subtle clinical presentation, this syndrome may be underrecognized, but can be diagnosed with appropriate magnetic resonance imaging techniques, EEG monitoring, and cerebrospinal fluid viral studies.

Definition of a divergent epitope that allows differential detection of early protein p41 from human herpesvirus 6 variants A and B

The human herpesvirus 6 (HHV-6) early protein, p41, encoded by the U27 gene has been detected in oligodendrocytes of multiple sclerosis (MS) patients by using a monoclonal antibody (MAb to p41/38). We here report the antigenic epitope of HHV-6 p41 recognized by this MAb. First, we established that the MAb to p41/38 recognizes a nuclear antigen in HHV-6A strain GS-infected cells but not in HHV-6B strain Z29-infected cells. Secondly, we compared the reactivity of the MAb to p41/38 to that of another p41-specific MAb (MAb to p41) on immunoblots with purified p41-glutathione S-transferase fusion protein from strains GS and Z29 and GS- and Z29-infected-cell lysates. The two MAbs were tested in an enzyme-linked immunosorbent assay against a panel of synthetic peptides covering the amino acid substitutions between the GS- and Z29-derived p41 proteins, as determined by DNA sequencing of our cloned isolates of the U27 gene. The MAb to p41/38 reacted specifically with a peptide comprising p41 residues 321 to 340 from strain GS. The critical residue in this peptide was serine 328, as the substitution S328N in the Z29 strain rendered the corresponding peptide nonreactive. The p41 S328 marker was present in three of three HHV-6A strains, while four of four sequenced p41 genes from HHV-6B strains had N328. Our findings are of value for the interpretation of previous findings of p41 expression in brains of MS patients and may allow a more detailed analysis of the role of HHV-6 variants in other disorders.

The RGD sequence in the cytomegalovirus DNA polymerase accessory protein can mediate cell adhesion

The murine cytomegalovirus (MCMV) polymerase processivity factor ppM44 (also referred to as pp50) is an abundant phosphoprotein found in MCMV-infected cells. Sequence analysis of the MCMV M44 open reading frame revealed an "RGD" motif that is also present in the human cytomegalovirus (HCMV) UL44 open reading frame. In this report, histidine-tagged M44 protein produced in Escherichia coli or the vaccinia/T7 expression system was purified to near homogeneity by metal chelation affinity chromatography using His*Bind resins. We demonstrated that recombinant M44 protein could mediate cell adhesion via its conserved "RGD" motif, because a single amino acid change (RGD to RGE) abolished cell attachment. In addition, cell adhesion was abolished in the presence of EDTA. We next showed that recombinant HCMV UL44, but not human herpesvirus type 6 p41, which lacks the RGD motif, could mediate cell adhesion in a similar manner. We also provided evidence that ppM44 was present in the culture medium during virus infection. Thus these results suggested that in addition to its primary role as the polymerase processivity factor, MCMV ppM44 may serve as a substrate for integrin-binding via its conserved RGD motif, with the potential for a novel role in the MCMV replication cycle.

Characterization of human herpesvirus 7 U27 gene product and identification of its nuclear localization signal

A monoclonal antibody, 5H4, that recognizes human herpesvirus 7 (HHV-7) was used in Western analysis to probe HHV-7-infected SupT1 cells. This antibody recognizes a 40-kDa virus-specific polypeptide that is expressed in the absence of viral DNA synthesis. By screening a lambdagt11 HHV-7 cDNA library, the gene encoding the protein was identified as the U27 open reading frame previously reported [J. Virol. (1996) 70, 5975-5989]. Immunofluorescent studies showed a punctate nuclear localization of the protein in both HHV-7-infected cells and transfected cells. A computer program predicted two classic nuclear localization signals (NLSs) in the middle and C-terminal regions of the protein. A C-terminal deletion mutant of the protein could not enter the nucleus, whereas green fluorescent protein or maltose binding protein fused to the C-terminal region of the protein was transported into the nucleus. These findings demonstrate that the predicted C-terminal, but not middle, NLS of the protein actually function as NLS. In addition, nuclear transport of a maltose binding protein-fusion protein containing the C-terminal NLS of the U27 protein was inhibited by both wheat germ agglutinin and a Q69L Ran-GTP mutant, indicating that the U27 protein is transported into the nucleus from the cytoplasm by means of classic nuclear transport machinery. Interestingly, this NLS motif is highly conserved at the C-termini of all herpesvirus DNA polymerase processivity factors that have been examined.

The 41-kDa protein of human herpesvirus 6 specifically binds to viral DNA polymerase and greatly increases DNA synthesis

We previously isolated a 41-kDa early antigen of human herpesvirus 6 (HHV-6), which exhibited nuclear localization and DNA-binding activity (Agulnick et al., 1993). In this study, we observed that a 110-kDa protein was coimmunoprecipitated with p41 from HHV-6-infected cells by an anti-p41 antibody. This 110-kDa protein was identified as the HHV-6 DNA polymerase (Pol-6) by an antibody raised against the N terminus of Pol-6. Reciprocal immunoprecipitation and Western blot analyses confirmed that p41 complexes with Pol-6 in HHV-6-infected cells. In addition, both p41 and Pol-6 were expressed in vitro and shown to form a specific complex. An in vitro DNA synthesis assay using primed M13 single-stranded DNA template demonstrated that p41 not only increased the DNA synthesis activity of Pol-6 but also allowed Pol-6 to synthesize DNA products corresponding to full-length M13 template (7249 nucleotides). By contrast, Pol-6 alone could only synthesize DNA of <100 nucleotides. The functional interaction between Pol-6 and p41 appears to be specific because they could not be physically or functionally substituted in vitro by their herpes simplex virus 1 homologues. Moreover, as revealed by mutational analysis, both the N and C termini of Pol-6 contribute to its binding to p41. In the case of p41, the N terminus is required for increasing DNA synthesis but not binding to Pol-6, whereas the C terminus is totally dispensable.

Expression of human herpesvirus-6 antigens in benign and malignant lymphoproliferative diseases

Immunohistochemistry was used to look for the expression of human herpesvirus-6 (HHV-6) antigens in a well characterized series of benign, atypical, and malignant lymphoid lesions, which tested positive for the presence of HHV-6 DNA. A panel of specific antibodies against HHV-6 antigens, characteristic either of the early (p41) or late (p101K, gp106, and gp116) phases of the viral cycle, was applied to the lymphoid tissues from 15 non-Hodgkin's lymphomas, 14 Hodgkin's disease cases, 5 angioimmunoblastic lymphadenopathies with dysproteinemia, 14 reactive lymphadenopathies, and 2 cases of sinus histiocytosis with massive lymphadenopathy (Rosai-Dorfman disease). In lymphomatous tissues, the expression of late antigens was documented only in reactive cells, and mainly in plasma cells. Of interest, the expression of the early p41 antigen was detected in the so-called "mummified" Reed-Sternberg cells, in two Hodgkin's disease cases. In reactive lymphadenopathies, the HHV-6 late antigen-expressing cells were plasma cells, histiocytes, and rare granulocytes distributed in interfollicular areas. In both cases of Rosai-Dorfman disease, the p101K showed an intense staining in follicular dendritic cells of germinal centers, whereas the gp106 exhibited an intense cytoplasmic reaction in the abnormal histiocytes, which represent the histological hallmark of the disease. The expression of HHV-6 antigens is tightly controlled in lymphoid tissues. The lack of HHV-6 antigen expression in neoplastic cells and the limited expression in degenerating Reed-Sternberg cells argue against a major pathogenetic role of the virus in human lymphomagenesis. The detection of a rather unique pattern of viral late antigen expression in Rosai-Dorfman disease suggests a possible pathogenetic involvement of HHV-6 in some cases of this rare lymphoproliferative disorder.

Cloning and functional analysis of Kaposi's sarcoma-associated herpesvirus DNA polymerase and its processivity factor

Kaposi's sarcoma-associated herpesvirus (KSHV), or human herpesvirus 8, is a newly identified virus with tumorigenic potential. Here, we cloned and expressed the DNA polymerase (Pol-8) of KSHV and its processivity factor (PF-8). Pol-8 bound specifically to PF-8 in vitro. Moreover, the DNA synthesis activity of Pol-8 was shown in vitro to be strongly dependent on PF-8. Addition of PF-8 to Pol-8 allowed efficient synthesis of fully extended DNA products corresponding to the full-length M13 template (7,249 nucleotides), whereas Pol-8 alone could incorporate only several nucleotides. The specificity of PF-8 and Pol-8 for each other was demonstrated by their inability to be functionally replaced by the DNA polymerases and processivity factors of herpes simplex virus 1 and human herpesvirus 6.

Physical and genetic maps of the human herpesvirus 7 strain SB genome

Human herpesvirus 7 (HHV-7) is a close relative of human herpesvirus 6A (HHV-6A) and human herpesvirus 6B (HHV-6B) based on limited biologic and genetic data. In this work we describe physical and genetic maps for HHV-7 strain SB [HHV-7(SB)], which was obtained from the saliva of a healthy adult. The HHV-7(SB) genome length is approximately 144 kb by clamped homogeneous electric field gel electrophoresis and approximately 135 kb by summation of restriction endonuclease fragments. We constructed plasmid clones and PCR amplimers that span the HHV-7 genome, except for the genomic termini, and determined the maps of the restriction endonuclease cleavage sites for BamHI, PstI, and SacI. The HHV-7(SB) genome is composed of a single unique region of approximately 122 kb bounded at each end by a 6 kb direct repeat. Homologs to thirty-five herpesvirus genes were identified. The highest similarity was with the HHV-6 genes, with an average amino acid identity of 50%, followed by the human cytomegalovirus counterpart. The genomic and genetic maps indicated that the HHV-7 and HHV-6 genomes are colinear. There was no sequence variation in a segment of the gene encoding the DNA polymerase-associated factor homolog among six HHV-7 isolates, while the corresponding segment of the HHV-6A and HHV-6B counterparts differed by 4.6%. These data support previous observations that the closest genetic relatives of HHV-7 are betaherpesviruses.

Transcriptional patterns of the pCD41 (U27) locus of human herpesvirus 6

Human herpesvirus 6 (HHV-6) is a lymphotropic herpesvirus, and in vitro, it can productively infect many of the same cell types that human immunodeficiency virus (HIV) infects. Simultaneous infection of T cells by HIV and HHV-6 can lead to both activation of the HIV promoter and acceleration of the cytopathic effects. Several HHV-6 genes have been demonstrated to activate HIV promoter expression. Among them is a cDNA clone, pCD41 (U27), which codes for the HHV-6 DNA polymerase accessory protein. We have now further characterized the transcription pattern in the pCD41 locus and identified at least six RNA species, ranging in size from 1.2 to 4.5 kb. Northern (RNA) blot analyses showed no significant difference in RNA patterns between the HHV-6 variant A (GS) and variant B (Z29) viruses. All the RNA species detected by pCD41 are polyadenylated and polyribosome associated, suggesting that they may be actively engaged in protein synthesis. Cycloheximide and phosphonoacetic acid inhibition assay results indicate that all the pCD41 RNA species belong to the herpesviral early-late family. Using primer extension and S1 mapping techniques, the 5' and 3' ends of each transcript were mapped to different positions, and no splicing was observed.

HHV-6 antigen and viral DNA detected in cervical cells from archived tissue using histochemical staining and hybridization

BACKGROUND

Human herpesvirus type 6 (HHV-6), an ubiquitous virus, is the causative agent for exanthem subitum. The virus is frequently associated with lymphoproliferative disorders and other diseases. Recently, we have reported the frequent presence of HHV-6 in oral carcinoma and the present study extends the observation to cervical carcinoma.

OBJECTIVE

To examine the presence of HHV-6 in cervical carcinoma.

STUDY DESIGN

Formalin-fixed, paraffin-embedded cervical carcinoma tissues were examined for the presence of HHV-6 by immunohistochemistry using two monoclonal antibodies that react to HHV-6-encoded p41/38 and gp116/64/54. In situ hybridization with variant-specific probes were used to type the HHV-6 DNA sequences present.

RESULTS

A total of 14/26 (53.9%) carcinoma tissue specimens and 5/8 (62.5%) normal tissue specimens were positive for viral antigens. In situ hybridization studies revealed the presence of HHV-6 DNA sequences in 10/26 (38.5%) carcinoma tissue specimens and 1/8 (12.5%) normal tissue specimens. In the normal tissue, the HHV-6 was present in the endocervical ciliated columnar-epithelial cells and some cells in the subepithelial mucosa but in the carcinoma, the transformed cells were positive for the virus.

CONCLUSIONS

HHV-6 viral proteins and DNA were found in more than one third of the cervical tissue examined suggesting possible viral expression in these tumours. The significance of the distribution and role of the HHV-6 in cervical tissue remains unclear. Since HHV-6 has an oncogenic potential, the virus may cooperate with other transforming agents for the progression of the disease.

Mutations that specifically impair the DNA binding activity of the herpes simplex virus protein UL42

The herpes simplex virus DNA polymerase is a heterodimer consisting of a catalytic subunit and the protein UL42, which functions as a processivity factor. It has been hypothesized that UL42 tethers the catalytic subunit to the DNA template by virtue of DNA binding activity (J. Gottlieb, A. I. Marcy, D. M. Coen, and M. D. Challberg, J. Virol. 64:5976-5987, 1990). Relevant to this hypothesis, we identified two linker insertion mutants of UL42 that were unable to bind to a double-stranded-DNA-cellulose column but retained their ability to bind the catalytic subunit. These mutants were severely impaired in the stimulation of long-chain-DNA synthesis by the catalytic subunit in vitro. In transfected cells, the expressed mutant proteins localized to the nucleus but were nonetheless deficient in complementing the growth of a UL42 null virus. Thus, unlike many other processivity factors, UL42 appears to require an intrinsic DNA binding activity for its function both in vitro and in infected cells. Possible mechanisms for the activity of UL42 and its potential as a drug target are discussed.

Plaque-associated expression of human herpesvirus 6 in multiple sclerosis

Representational difference analysis was used to search for pathogens in multiple sclerosis brains. We detected a 341-nucleotide fragment that was 99.4% identical to the major DNA binding protein gene of human herpesvirus 6 (HHV-6). Examination of 86 brain specimens by PCR demonstrated that HHV-6 was present in > 70% of MS cases and controls and is thus a commensal virus of the human brain. By DNA sequencing, 36/37 viruses from MS cases and controls were typed as HHV-6 variant B group 2. Other herpesviruses, retroviruses, and measles virus were detected infrequently or not at all. HHV-6 expression was examined by immunocytochemistry with monoclonal antibodies against HHV-6 virion protein 101K and DNA binding protein p41. Nuclear staining of oligodendrocytes was observed in MS cases but not in controls, and in MS cases it was observed around plaques more frequently than in uninvolved white matter. MS cases showed prominent cytoplasmic staining of neurons in gray matter adjacent to plaques, although neurons expressing HHV-6 were also found in certain controls. Since destruction of oligodendrocytes is a hallmark of MS, these studies suggest an association of HHV-6 with the etiology or pathogenesis of MS.

Cloning, sequencing, and functional characterization of the two subunits of the pseudorabies virus DNA polymerase holoenzyme: evidence for specificity of interaction

The pseudorabies virus (PRV) genes encoding the two subunits of the DNA polymerase were located on the genome by hybridization to their herpes simplex virus type 1 (HSV-1) homologs, pol and UL42, and subsequently were sequenced. Like the HSV-1 homologs, in vitro translation products of the PRV gene encoding the catalytic subunit (pol) possessed activity in the absence of the Pol accessory protein (PAP). However, the PRV PAP stimulated the activity of Pol fourfold in the presence of 150 mM KCl, using an activated calf thymus DNA template. The stimulation of Pol activity by PAP under high-salt conditions and the inhibition of Pol activity by PAP when assayed in low salt (0 mM KCl) together were used to determine the specificity with which PAP interacted with Pol. Despite functional similarity, HSV-1 UL42 and PRV PAP could neither stimulate the noncognate Pols at high salt nor inhibit them at low salt. Furthermore, a PRV Pol mutant lacking the 30 C-terminal amino acids retained basal Pol activity but could be neither stimulated nor inhibited by the PRV PAP. Sequence comparisons of the Pol proteins of the alphaherpesviruses reveal a conserved domain in the C terminus which terminates immediately before the last 41 residues of both PRV and HSV-1 proteins. These results indicate that the ability and specificity for interaction of the PRV Pol with PAP most likely resides predominantly in the extreme Pol C terminus.

Specific inhibition of herpes simplex virus DNA polymerase by helical peptides corresponding to the subunit interface

The herpes simplex virus DNA polymerase consists of two subunits--a catalytic subunit and an accessory subunit, UL42, that increases processivity. Mutations affecting the extreme C terminus of the catalytic subunit specifically disrupt subunit interactions and ablate virus replication, suggesting that new antiviral drugs could be rationally designed to interfere with polymerase heterodimerization. To aid design, we performed circular dichroism (CD) spectroscopy and analytical ultracentrifugation studies, which revealed that a 36-residue peptide corresponding to the C terminus of the catalytic subunit folds into a monomeric structure with partial alpha-helical character. CD studies of shorter peptides were consistent with a model where two separate regions of alpha-helix interact to form a hairpin-like structure. The 36-residue peptide and a shorter peptide corresponding to the C-terminal 18 residues blocked UL42-dependent long-chain DNA synthesis at concentrations that had no effect on synthesis by the catalytic subunit alone or by calf thymus DNA polymerase delta and its processivity factor. These peptides, therefore, represent a class of specific inhibitors of herpes simplex virus DNA polymerase that act by blocking accessory-subunit-dependent synthesis. These peptides or their structures may form the basis for the synthesis of clinically effective drugs.

A novel cis element essential for stimulated transcription of the p41 promoter of human herpesvirus 6

The p41 DNA-binding protein of human herpesvirus 6 is an apparent processivity factor important for viral DNA replication. The p41 promoter was characterized to understand how this processivity factor is regulated. A single transcription start site and a functional TATA box are located 48 and 74 bp, respectively, upstream of the start codon. A reporter construct containing 1,027 bp of the sequence upstream of the p41 start codon was inactive in uninfected T cells but functioned as a strong promoter in human herpesvirus 6-infected cells. Mutational analysis identified a 21-bp element (the EA site) which is located at -73 to -52 bp relative to the transcription start site and is essential for promoter activity. The ability of the EA site to stimulate transcription optimally appears to be strictly dependent upon its distance from the p41 basal promoter. The EA site contains three overlapping sequences, a CAAT-enhancer-binding protein (C/EBP) transcription factor recognition site and two repeat elements. Mobility shift assays using the EA site identified four binding activities (C1 to C4). C1 and C2 are present in both uninfected and infected cells and do not contain C/EBP factors. In infected cells, point mutation of the EA site abrogates C1 and C2 binding activities and destroys transcriptional activity of the p41 promoter. C3 and C4 are present in uninfected cells only and were found to contain C/EBP factors. These findings indicate that in infected cells, transcriptional stimulation of the p41 promoter by the EA site requires C1 and C2 binding activities. These results further suggest that transcriptional activity may also depend upon the elimination of C3 and C4 binding activities.