An Epstein-Barr Virus Anti-Apoptotic Protein Constitutively Expressed in Transformed Cells and Implicated in Burkitt Lymphomagenesis: The Wp/BHRF1 Link

Two factors contribute to Burkitt lymphoma (BL) pathogenesis, a chromosomal translocation leading to c-myc oncogene deregulation and infection with Epstein-Barr virus (EBV). Although the virus has B cell growth–transforming ability, this may not relate to its role in BL since many of the transforming proteins are not expressed in the tumor. Mounting evidence supports an alternative role, whereby EBV counteracts the high apoptotic sensitivity inherent to the c-myc–driven growth program. In that regard, a subset of BLs carry virus mutants in a novel form of latent infection that provides unusually strong resistance to apoptosis. Uniquely, these virus mutants use Wp (a viral promoter normally activated early in B cell transformation) and express a broader-than-usual range of latent antigens. Here, using an inducible system to express the candidate antigens, we show that this marked apoptosis resistance is mediated not by one of the extended range of EBNAs seen in Wp-restricted latency but by Wp-driven expression of the viral bcl2 homologue, BHRF1, a protein usually associated with the virus lytic cycle. Interestingly, this Wp/BHRF1 connection is not confined to Wp-restricted BLs but appears integral to normal B cell transformation by EBV. We find that the BHRF1 gene expression recently reported in newly infected B cells is temporally linked to Wp activation and the presence of W/BHRF1-spliced transcripts. Furthermore, just as Wp activity is never completely eclipsed in in vitro–transformed lines, low-level BHRF1 transcripts remain detectable in these cells long-term. Most importantly, recognition by BHRF1-specific T cells confirms that such lines continue to express the protein independently of any lytic cycle entry. This work therefore provides the first evidence that BHRF1, the EBV bcl2 homologue, is constitutively expressed as a latent protein in growth-transformed cells in vitro and, in the context of Wp-restricted BL, may contribute to virus-associated lymphomagenesis in vivo

Protein-protein interaction of HSV-1 tegument proteins

Herpes simplex virus type 1 virions contain a proteinaceous layer between the nucleocapsid and the virus envelope termed the tegument. The mechanism underlying tegumentation remains largely undefined for all herpesviruses, as does the role of many tegument proteins in virus replication. The networks of protein interactions involved in virus assembly have been largely explored and although large-scale studies have been carried out using yeast two hybrid analyses of herpesvirus protein interactions, few of the identified networks have been validated in infected cells. Here, the molecular interactions that occur between the major tegument proteins VP22, VP16 and VP13/14 and a range of glycoproteins and tegument proteins were defined in detail. Two alternative studies were performed from infected cells, however one based on the purification of GFP-tagged proteins and their protein partners proved more successful. These studies validated previous findings and also identified VP13/14, UL21, UL16 and vhs as novel binding partners of VP22, and VP22, UL21, UL16 and vhs as novel binding partners of VP13/14. Thus, these results have led to the identification of two discrete tegument protein complexes in the infected cell: VP22-VP16-VP13/14-vhs and VP22-VP13/14-UL21-UL16. To investigate the nature of the VP22-VP16-VP13/14-vhs complex in more detail, a number of techniques were used and showed that VP22 and VP13/14 both bind directly to the C-terminus of VP16, but were unable to interact with each other. As anticipated from other studies on transfected cell extracts, vhs was shown to be incorporated into this complex by virtue of its direct binding to VP16 during infection, and did not have the capacity to interact directly with VP22. This work has established a defined network of protein-protein interactions encompassing over one third of tegument proteins, and will improve our understanding of the wider protein interaction networks that lead to the assembly of the herpesvirus tegument.Open Acces

A network of protein interactions around the herpes simplex virus tegument protein VP22

Assembly of the herpesvirus tegument is poorly understood but is believed to involve interactions between outer tegument proteins and the cytoplasmic domains of envelope glycoproteins. Here, we present the detailed characterization of a multicomponent glycoprotein-tegument complex found in herpes simplex virus 1 (HSV-1)-infected cells. We demonstrate that the tegument protein VP22 bridges a complex between glycoprotein E (gE) and glycoprotein M (gM). Glycoprotein I (gI), the known binding partner of gE, is also recruited into this gE-VP22-gM complex but is not required for its formation. Exclusion of the glycoproteins gB and gD and VP22's major binding partner VP16 demonstrates that recruitment of virion components into this complex is highly selective. The immediate-early protein ICP0, which requires VP22 for packaging into the virion, is also assembled into this gE-VP22-gM-gI complex in a VP22-dependent fashion. Although subcomplexes containing VP22 and ICP0 can be formed when either gE or gM are absent, optimal complex formation requires both glycoproteins. Furthermore, and in line with complex formation, neither of these glycoproteins is individually required for VP22 or ICP0 packaging into the virion, but deletion of gE and gM greatly reduces assembly of both VP22 and ICP0. Double deletion of gE and gM also results in small plaque size, reduced virus yield, and defective secondary envelopment, similar to the phenotype previously shown for pseudorabies virus. Hence, we suggest that optimal gE-VP22-gM-gI-ICP0 complex formation correlates with efficient virus morphogenesis and spread. These data give novel insights into the poorly understood process of tegument acquisition

Virion Incorporation of the Herpes Simplex Virus Type 1 Tegument Protein VP22 Occurs via Glycoprotein E-Specific Recruitment to the Late Secretory Pathway▿

The mechanism by which herpesviruses acquire their tegument is not yet clear. One model is that outer tegument proteins are recruited by the cytoplasmic tails of viral glycoproteins. In the case of herpes simplex virus tegument protein VP22, interactions with the glycoproteins gE and gD have been shown. We have previously shown that the C-terminal half of VP22 contains the necessary signal for assembly into the virus. Here, we show that during infection VP22 interacts with gE and gM, as well as its tegument partner VP16. However, by using a range of techniques we were unable to demonstrate VP22 binding to gD. By using pulldown assays, we show that while the cytoplasmic tails of both gE and gM interact with VP22, only gE interacts efficiently with the C-terminal packaging domain of VP22. Furthermore, gE but not gM can recruit VP22 to the Golgi/trans-Golgi network region of the cell in the absence of other virus proteins. To examine the role of the gE-VP22 interaction in infection, we constructed a recombinant virus expressing a mutant VP22 protein with a 14-residue deletion that is unable to bind gE (ΔgEbind). Coimmunoprecipitation assays confirmed that this variant of VP22 was unable to complex with gE. Moreover, VP22 was no longer recruited to its characteristic cytoplasmic trafficking complexes but exhibited a diffuse localization. Importantly, packaging of this variant into virions was abrogated. The mutant virus exhibited poor growth in epithelial cells, similar to the defect we have observed for a VP22 knockout virus. These results suggest that deletion of just 14 residues from the VP22 protein is sufficient to inhibit binding to gE and hence recruitment to the viral envelope and assembly into the virus, resulting in a growth phenotype equivalent to that produced by deleting the entire reading frame

Multiple post-transcriptional strategies to regulate the herpes simplex virus type 1 vhs endoribonuclease

The HSV1 virion host shutoff (vhs) protein is an endoribonuclease that binds to the cellular translation initiation machinery and degrades associated mRNAs, resulting in shut-off of host protein synthesis. Hence its unrestrained activity is considered to be lethal, and it has been proposed that vhs is regulated by two other virus proteins, VP22 and VP16. We have found that during infection, translation of vhs requires VP22 but not the VP22-VP16 complex. Moreover, in the absence of VP22, vhs is not overactive against cellular or viral transcripts. In transfected cells, vhs was also poorly translated, correlating with aberrant localization of its mRNA. Counterintuitively, vhs mRNA was predominantly nuclear in cells where vhs protein was detected. Likewise, transcripts from co-transfected plasmids were also retained in the same nuclei where vhs mRNA was located, while polyA binding protein (PABP) was relocalised to the nucleus in a vhs-dependent manner, implying a general block to mRNA export. Co-expression of VP16 and VP22 rescued cytoplasmic localization of vhs mRNA but failed to rescue vhs translation. We identified a 230-nucleotide sequence in the 5’ region of vhs that blocked its translation and, when transferred to a heterologous GFP transcript, reduced translation without altering mRNA levels or localization. We propose that expression of vhs is tightly regulated by a combination of inherent untranslatability and auto-induced nuclear retention of its mRNA that results in a negative feedback loop, with nuclear retention but not translation of vhs mRNA being the target of rescue by the vhs-VP16-VP22 complex

Methylation Status of theEpstein-Barr Virus (EBV) BamHI W Latent Cycle Promoter and Promoter Activity: Analysis with Novel EBV-Positive Burkitt and Lymphoblastoid Cell Lines

The Epstein-Barr virus (EBV) latent cycle promoter Wp, present in each tandemly arrayed copy of the BamHI W region in the EBV genome, drives expression of the EB viral nuclear antigens (EBNAs) at the initiation of virus-induced B-cell transformation. Thereafter, an alternative EBNA promoter, Cp, becomes dominant, Wp activity declines dramatically, and bisulfite sequencing of EBV-transformed lymphoblastoid cell lines (LCLs) shows extensive Wp methylation. Despite this, Wp is never completely silenced in LCLs. Here, using a combination of bisulfite sequencing and methylation-specific PCR, we show that in standard LCLs transformed with wild-type EBV isolates, some Wp copies always remain unmethylated, and in LCLs transformed with a recombinant EBV carrying just two BamHI W copies, Wp is completely unmethylated. Furthermore, we have analyzed rare LCLs, recently established using wild-type EBV isolates, and rare Burkitt lymphoma (BL) cell clones, recently established from tumors carrying EBNA2-deleted EBV genomes, which express EBNAs exclusively from Wp-initiated transcripts. Here, in sharp contrast to standard LCL and BL lines, all resident copies of Wp appear to be predominantly hypomethylated. Thus, studies of B cells with atypical patterns of Wp usage emphasize the strong correlation between the presence of unmethylated Wp sequences and promoter activity

Integrated systems-genetic analyses reveal a network target for delaying glioma progression

OBJECTIVETo identify a convergent, multitarget proliferation characteristic for astrocytoma transformation that could be targeted for therapy discovery.METHODS Using an integrated functional genomics approach, we prioritized networks associated with astrocytoma progression using the following criteria: differential co-expression between grade II and grade III IDH1-mutated and 1p/19q euploid astrocytomas, preferential enrichment for genetic risk to cancer, association with patient survival and sample-level genomic features. Drugs targeting the identified multitarget network characteristic for astrocytoma transformation were computationally predicted using drug transcriptional perturbation data and validated using primary human astrocytoma cells.RESULTS A single network, M2, consisting of 177 genes, was associated with glioma progression on the basis of the above criteria. Functionally, M2 encoded physically interacting proteins regulating cell cycle processes and analysis of genome-wide gene-regulatory interactions using mutual information and DNA-protein interactions revealed the known regulators of cell cycle processes FoxM1, B-Myb, and E2F2 as key regulators of M2. These results suggest functional disruption of M2 via gene mutation or altered expression as a convergent pathway regulating astrocytoma transformation. By considering M2 as a multitarget drug target regulating astrocytoma transformation, we identified several drugs that are predicted to restore M2 expression in anaplastic astrocytoma toward its low-grade profile and of these, we validated the known antiproliferative drug resveratrol as down-regulating multiple nodes of M2 including at nanomolar concentrations achievable in human cerebrospinal fluid by oral dosing.INTERPRETATION Our results identify M2 as a multitarget network characteristic for astrocytoma progression and encourage M2-based drug screening to identify new compounds for preventing glioma transformation.</p

Integrated systems-genetic analyses reveal a network target for delaying glioma progression

OBJECTIVETo identify a convergent, multitarget proliferation characteristic for astrocytoma transformation that could be targeted for therapy discovery.METHODS Using an integrated functional genomics approach, we prioritized networks associated with astrocytoma progression using the following criteria: differential co-expression between grade II and grade III IDH1-mutated and 1p/19q euploid astrocytomas, preferential enrichment for genetic risk to cancer, association with patient survival and sample-level genomic features. Drugs targeting the identified multitarget network characteristic for astrocytoma transformation were computationally predicted using drug transcriptional perturbation data and validated using primary human astrocytoma cells.RESULTS A single network, M2, consisting of 177 genes, was associated with glioma progression on the basis of the above criteria. Functionally, M2 encoded physically interacting proteins regulating cell cycle processes and analysis of genome-wide gene-regulatory interactions using mutual information and DNA-protein interactions revealed the known regulators of cell cycle processes FoxM1, B-Myb, and E2F2 as key regulators of M2. These results suggest functional disruption of M2 via gene mutation or altered expression as a convergent pathway regulating astrocytoma transformation. By considering M2 as a multitarget drug target regulating astrocytoma transformation, we identified several drugs that are predicted to restore M2 expression in anaplastic astrocytoma toward its low-grade profile and of these, we validated the known antiproliferative drug resveratrol as down-regulating multiple nodes of M2 including at nanomolar concentrations achievable in human cerebrospinal fluid by oral dosing.INTERPRETATION Our results identify M2 as a multitarget network characteristic for astrocytoma progression and encourage M2-based drug screening to identify new compounds for preventing glioma transformation.</p