The Epstein-Barr virus nuclear antigen-1 promotes genomic instability via induction of reactive oxygen species

EBNA1 binding and epigenetic regulation of gastrokine tumor suppressor genes in gastric carcinoma cells

BACKGROUND

Epstein-Barr Virus (EBV) latently infects ~10% of gastric carcinomas (GC). Epstein-Barr Nuclear Antigen 1 (EBNA1) is expressed in EBV-associated GC, and can bind host DNA, where it may impact cellular gene regulation. Here, we show that EBNA1 binds directly to DNA upstream of the divergently transcribed GC-specific tumor suppressor genes gastrokine 1 (GKN1) and gastrokine 2 (GKN2).

METHODS

We use ChIP-Seq, ChIP-qPCR, and EMSA to demonstrate that EBNA1 binds directly to the GKN1 and GKN2 promoter locus. We generate AGS-EBV, and AGS-EBNA1 cell lines to study the effects of EBNA1 on GKN1 and GKN2 mRNA expression with or without 5' azacytidine treatment.

RESULTS

We show that gastrokine genes are transcriptionally silenced by DNA methylation. We also show that latent EBV infection further reduces GKN1 and GKN2 expression in AGS gastric carcinoma cells, and that siRNA depletion of EBNA1 partially alleviates this repression. However, ectopic expression of EBNA1 slightly increased GKN1 and GKN2 basal mRNA levels, but reduced their responsiveness to demethylating agent.

CONCLUSIONS

These findings demonstrate that EBNA1 binds to the divergent promoter of the GKN1 and GKN2 genes in GC cells, and suggest that EBNA1 contributes to the complex transcriptional and epigenetic deregulation of the GKN1 and GKN2 tumor suppressor genes in EBV positive GC.

Upregulation of TFAM and mitochondria copy number in human lymphoblastoid cells

Mitochondria are central to several physiological and pathological conditions in humans. In the present study, we performed copy number analysis of nuclear encoded mitochondrial genes, in peripheral blood mononuclear cells (PBMCs) and its representative lymphoblastoid cells (LCLs). We have observed hyper diploid copies of mitochondrial transcription factor A (TFAM) gene in the LCLs along with increased mtDNA copy number, mitochondrial mass, intracellular ROS and mitochondrial membrane potential, suggesting elevated mitochondrial biogenesis in LCLs. Gene expression analysis confirmed TFAM over-expression in LCLs when compared to PBMC. Based on our observation, we suggest that increased copy number of TFAM gene upregulates its expression, increases mtDNA copy numbers and protects it from oxidative stress induced damage in the transformed LCLs.

Necrosis is increased in lymphoblastoid cell lines from children with autism compared with their non-autistic siblings under conditions of oxidative and nitrosative stress

Autism spectrum disorders are a heterogeneous group of neurodevelopmental conditions characterised by impairments in reciprocal social interaction, communication and stereotyped behaviours. As increased DNA damage events have been observed in a range of other neurological disorders, it was hypothesised that they would be elevated in lymphoblastoid cell lines (LCLs) obtained from children with autism compared with their non-autistic siblings. Six case-sibling pairs of LCLs from children with autistic disorder and their non-autistic siblings were obtained from the Autism Genetic Resource Exchange (AGRE) and cultured in standard RPMI-1640 tissue culture medium. Cells were exposed to medium containing either 0, 25, 50, 100 and 200 µM hydrogen peroxide (an oxidative stressor) or 0, 5, 10, 20 and 40 µM s-nitroprusside (a nitric oxide producer) for 1h. Following exposure, the cells were microscopically scored for DNA damage, cytostasis and cytotoxicity biomarkers as measured using the cytokinesis-block micronucleus cytome assay. Necrosis was significantly increased in cases relative to controls when exposed to oxidative and nitrosative stress (P = 0.001 and 0.01, respectively). Nuclear division index was significantly lower in LCLs from children with autistic disorder than their non-autistic siblings when exposed to hydrogen peroxide (P = 0.016), but there was no difference in apoptosis, micronucleus frequency, nucleoplasmic bridges or nuclear buds. Exposure to s-nitroprusside significantly increased the number of micronuclei in non-autistic siblings compared with cases (P = 0.003); however, other DNA damage biomarkers, apoptosis and nuclear division did not differ significantly between groups. The findings of this study show (i) that LCLs from children with autism are more sensitive to necrosis under conditions of oxidative and nitrosative stress than their non-autistic siblings and (ii) refutes the hypothesis that children with autistic disorder are abnormally susceptible to DNA damage.

Reactive oxygen species mediate Epstein-Barr virus reactivation by N-methyl-N'-nitro-N-nitrosoguanidine

N-nitroso compounds (NOCs) and Epstein-Barr virus (EBV) reactivation have been suggested to play a role in the development of nasopharyngeal carcinoma (NPC). Although chemicals have been shown to be a risk factor contributing to the carcinogenesis of NPC, the underlying mechanism is not fully understood. We demonstrated recently that N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) enhances the genomic instability and tumorigenicity of NPC cells via induction of EBV reactivation. However, the mechanisms that trigger EBV reactivation from latency remain unclear. Here, we address the role of ROS in induction of EBV reactivation under MNNG treatment. EBV reactivation was induced in over 70% of EBV-positive NA cells and the promoter of Rta (Rp) was activated after MNNG treatment. Inhibitor experiments revealed ATM, p38 MAPK and JNK were activated by ROS and involved in MNNG-induced EBV reactivation. Significantly, ROS scavengers N-acetyl-L-cysteine (NAC), catalase and reduced glutathione inhibited EBV reactivation under MNNG and H₂O₂ treatment, suggesting ROS mediate EBV reactivation. The p53 was essential for EBV reactivation and the Rp activation by MNNG. Moreover, the p53 was phosphorylated, translocated into nucleus, and bound to Rp following ROS stimulation. The results suggest ROS play an important role in initiation of EBV reactivation by MNNG through a p53-dependent mechanism. Our findings demonstrate novel signaling mechanisms used by NOCs to induce EBV reactivation and provide a novel insight into NOCs link the EBV reactivation in the contribution to the development of NPC. Notably, this study indicates that antioxidants might be effective for inhibiting N-nitroso compound-induced EBV reactivation and therefore could be promising preventive and therapeutic agents for EBV reactivation-associated malignancies.

Telomere dysfunction and activation of alternative lengthening of telomeres in B-lymphocytes infected by Epstein-Barr virus

Malignant cells achieve replicative immortality by two alternative mechanisms, a common one dependent on de novo synthesis of telomeric DNA by telomerase, and a rare one based on telomere recombination known as alternative lengthening of telomeres (ALT). Epstein-Barr virus (EBV) transforms human B-lymphocytes into lymphoblastoid cell lines with unlimited growth potential in vitro and in vivo. Here we show that newly EBV-infected cells exhibit multiple signs of telomere dysfunction, including the occurrence of extra-chromosomal telomeres, telomere fusion and telomere length heterogeneity, and undergo progressive increase in telomere length without a parallel increase in telomerase activity. This phenotype is accompanied by the accumulation of telomere-associated promyelocytic leukemia nuclear bodies and telomeric-sister chromatid exchange, suggesting that EBV infection promotes the activation of ALT. Newly infected cells also display a significant reduction of telomere-associated TRF2 and express low levels of TRF1, TRF2, POT1 and ATRX, pointing to telomere de-protection as an important correlate of ALT activation. Collectively, these findings highlight the involvement of recombination-dependent mechanisms for maintenance of telomere homeostasis in EBV-induced B-cell immortalization.

Regulation of ROS in transmissible gastroenteritis virus-activated apoptotic signaling

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TGEV infection induced ROS accumulation.

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ROS accumulation is involved in TGEV-induced mitochondrial integrity impairment.

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ROS is associated with p53 activation and apoptosis occurrence in TGEV-infected cells.

Transmissible gastroenteritis virus (TGEV), an enteropathogenic coronavirus, causes severe lethal watery diarrhea and dehydration in piglets. Previous studies indicate that TGEV infection induces cell apoptosis in host cells. In this study, we investigated the roles and regulation of reactive oxygen species (ROS) in TGEV-activated apoptotic signaling. The results showed that TGEV infection induced ROS accumulation, whereas UV-irradiated TGEV did not promote ROS accumulation. In addition, TGEV infection lowered mitochondrial transmembrane potential in PK-15 cell line, which could be inhibited by ROS scavengers, pyrrolidinedithiocarbamic (PDTC) and N-acetyl-l-cysteine (NAC). Furthermore, the two scavengers significantly inhibited the activation of p38 MAPK and p53 and further blocked apoptosis occurrence through suppressing the TGEV-induced Bcl-2 reduction, Bax redistribution, cytochrome c release and caspase-3 activation. These results suggest that oxidative stress pathway might be a key element in TGEV-induced apoptosis and TGEV pathogenesis.

Interplay between DNA tumor viruses and the host DNA damage response

Viruses encounter many challenges within host cells in order to replicate their nucleic acid. In the case of DNA viruses, one challenge that must be overcome is recognition of viral DNA structures by the host DNA damage response (DDR) machinery. This is accomplished in elegant and unique ways by different viruses as each has specific needs and sensitivities dependent on its life cycle. In this review, we focus on three DNA tumor viruses and their interactions with the DDR. The viruses Epstein-Barr virus (EBV), Kaposi's sarcoma-associated herpesvirus (KSHV), and human papillomavirus (HPV) account for nearly all of the virus-associated human cancers worldwide. These viruses have also been excellent models for the study of oncogenic virus-mediated cell transformation. In this review, we will discuss how each of these viruses engage and subvert aspects of the host DDR. The first level of DDR engagement is a result of the genetic linkage between the oncogenic potential of these viruses and their ability to replicate. Namely, the promotion of cells from quiescence into the cell cycle to facilitate virus replication can be sensed through aberrant cellular DNA replication structures which activate the DDR and hinder cell transformation. DNA tumor viruses subvert this growth-suppressive DDR through changes in viral oncoprotein expression which ultimately facilitate virus replication. An additional level of DDR engagement is through direct detection of replicating viral DNA. These interactions parallel those observed in other DNA virus systems in that the need to subvert these intrinsic sensors of aberrant DNA structure in order to replicate must be in place. DNA tumor viruses are no exception. This review will cover the molecular features of DNA tumor virus interactions with the host DDR and the consequences for virus replication.

IL1- and TGFβ-Nox4 signaling, oxidative stress and DNA damage response are shared features of replicative, oncogene-induced, and drug-induced paracrine 'bystander senescence'

Many cancers arise at sites of infection and inflammation. Cellular senescence, a permanent state of cell cycle arrest that provides a barrier against tumorigenesis, is accompanied by elevated proinflammatory cytokines such as IL1, IL6, IL8 and TNFα. Here we demonstrate that media conditioned by cells undergoing any of the three main forms of senescence, i.e. replicative, oncogene- and drug-induced, contain high levels of IL1, IL6, and TGFb capable of inducing reactive oxygen species (ROS)-mediated DNA damage response (DDR). Persistent cytokine signaling and activated DDR evoke senescence in normal bystander cells, accompanied by activation of the JAK/STAT, TGFβ/SMAD and IL1/NFκB signaling pathways. Whereas inhibition of IL6/STAT signaling had no effect on DDR induction in bystander cells, inhibition of either TGFβ/SMAD or IL1/NFκB pathway resulted in decreased ROS production and reduced DDR in bystander cells. Simultaneous inhibition of both TGFβ/SMAD and IL1/NFκB pathways completely suppressed DDR indicating that IL1 and TGFβ cooperate to induce and/or maintain bystander senescence. Furthermore, the observed IL1- and TGFβ-induced expression of NAPDH oxidase Nox4 indicates a mechanistic link between the senescence-associated secretory phenotype (SASP) and DNA damage signaling as a feature shared by development of all major forms of paracrine bystander senescence.

Characterization of Epstein-Barr virus reactivation in a modeled spaceflight system

Epstein-Barr virus (EBV) is the causative agent of mononucleosis and is also associated with several malignancies, including Burkitt's lymphoma, Hodgkin's lymphoma, and nasopharyngeal carcinoma, among others. EBV reactivates during spaceflight, with EBV shedding in saliva increasing to levels ten times those observed pre-and post-flight. Although stress has been shown to increase reactivation of EBV, other factors such as radiation and microgravity have been hypothesized to contribute to reactivation in space. We used a modeled spaceflight environment to evaluate the influence of radiation and microgravity on EBV reactivation. BJAB (EBV-negative) and Raji (EBV-positive) cell lines were assessed for viability/apoptosis, viral antigen and reactive oxygen species expression, and DNA damage and repair. EBV-infected cells did not experience decreased viability and increased apoptosis due to modeled spaceflight, whereas an EBV-negative cell line did, suggesting that EBV infection provided protection against apoptosis and cell death. Radiation was the major contributor to EBV ZEBRA upregulation. Combining modeled microgravity and radiation increased DNA damage and reactive oxygen species while modeled microgravity alone decreased DNA repair in Raji cells. Additionally, EBV-infected cells had increased DNA damage compared to EBV-negative cells. Since EBV-infected cells do not undergo apoptosis as readily as uninfected cells, it is possible that virus-infected cells in EBV seropositive individuals may have an increased risk to accumulate DNA damage during spaceflight. More studies are warranted to investigate this possibility.

Enhanced and prolonged baculovirus-mediated expression by incorporating recombinase system and in cis elements: a comparative study

Baculovirus (BV) is a promising gene vector but mediates transient expression. To prolong the expression, we developed a binary system whereby the transgene in the substrate BV was excised by the recombinase (ΦC31o, Cre or FLPo) expressed by a second BV and recombined into smaller minicircle. The recombination efficiency was lower by ΦC31o (≈40–75%), but approached ≈90–95% by Cre and FLPo in various cell lines and stem cells [e.g. human adipose-derived stem cells (hASCs)]. Compared with FLPo, Cre exerted higher expression level and lower negative effects; thus, we incorporated additional cis-acting element [oriP/Epstein–Barr virus nuclear antigen 1 (EBNA1), scaffold/matrix attached region or human origin of replication (ori)] into the Cre-based BV system. In proliferating cells, only oriP/EBNA1 prolonged the transgene expression and maintained the episomal minicircles for 30 days without inadvertent integration, whereas BV genome was degraded in 10 days. When delivering bmp2 or vegf genes, the efficient recombination/minicircle formation prolonged and enhanced the growth factor expression in hASCs. The prolonged bone morphogenetic protein 2 expression ameliorated the osteogenesis of hASCs, a stem cell with poor osteogenesis potential. Altogether, this BV vector exploiting Cre-mediated recombination and oriP/EBNA1 conferred remarkably high recombination efficiency, which prolonged and enhanced the transgene expression in dividing and non-dividing cells, thereby broadening the applications of BV.

Epigenetic regulation of EBV and KSHV latency

The gammaherpesviruses are unique for their capacity to establish a variety of gene expression programs during latent and lytic infection. This capacity enables the virus to control host-cell proliferation, prevent programmed cell death, elude immune cell detection, and ultimately adapt to a wide range of environmental and developmental changes in the host cell. This remarkable plasticity of gene expression results from the combined functionalities of viral and host factors that biochemically remodel and epigenetically modify the viral chromosome. These epigenetic modifications range from primary DNA methylations, to chromatin protein post-translational modifications, to higher-order chromosome conformations. In addition, gammaherpesviruses have acquired specialized tools to modulate the epigenetic processes that promote viral genome propagation and host-cell survival.

NADPH Oxidases NOXs and DUOXs as putative targets for cancer therapy

Reactive oxygen species (ROS) form a class of molecules with both positive and negative impacts on cellular health. Negatively, ROS may react with cellular constituents including proteins, lipids, and DNA to generate an array of oxidative lesions. These lesions may compromise genome stability which is critical for long-term cellular homeostasis and healthy progeny. Paradoxically, ROS also function as strong signalling molecules that mediate various growth-related responses, so their presence is also essential for cellular metabolism. While ROS are generated in an unregulated manner by physical stresses such as exposure to ionizing radiation and biochemical malfunctions such as mitochondrial leakage, cells also contain the NADPH oxidases NOXs and DUOXs, which specifically generate ROS in a wide variety of tissues. While the NOXs/DUOXs may be involved in maintaining optimal cellular redox levels, there is also accumulating evidence that NADPH oxidases-derived ROS may elevate the risk for genomic instability and cancer. Cancer cells may produce high levels of ROS, and in some cases, the source of these ROS has been linked to NOX/DUOX deregulation as reported for prostate cancer (NOX1 and NOX5), melanoma and glioblastoma (NOX4) among others. In addition, recent studies reveal that targeting NADPH oxidases with NOXs inhibitors may impair tumor growth in vivo; indicating that these proteins may be useful targets in future clinical strategies to fight cancer. This review provides an overview of the current knowledge concerning these enzymes, their roles in cancer, and their potential as targets in future cancer therapies.

An update on viral association of human cancers

Up to now, seven viruses that infect humans have been identified as oncogenic and are closely associated with different human cancers. Most of them encode oncogenes whose products play important roles in the development of cancers in the context of environmental and genetic factors; others may act via indirect mechanisms. The transforming activities of the human oncogenic viruses have much in common with the well-studied tumorigenic processes elicited by the acutely transforming murine retroviruses. Many of these mechanisms have been elucidated for or are represented in the successive steps leading to the efficient in vitro immortalization by the lymphotropic herpesvirus Epstein-Barr virus, although the establishment of malignancy in vivo takes longer. The development of cancer is a complicated process involving multiple factors, from the host and the environment. Although any one of these etiologic factors may exert an effect on the carcinogenic process, vaccination against the viral pathogen in several cases has shown efficacy in preventing the spread of the virus and, in turn, the development of the associated cancers. Modern laboratory techniques can be expected to facilitate the identification of new emerging viruses whose association with malignancies is suggested by epidemiologic and clinical data.

The Epstein-Barr virus nuclear antigen-1 reprograms transcription by mimicry of high mobility group A proteins

Viral proteins reprogram their host cells by hijacking regulatory components of protein networks. Here we describe a novel property of the Epstein-Barr virus (EBV) nuclear antigen-1 (EBNA1) that may underlie the capacity of the virus to promote a global remodeling of chromatin architecture and cellular transcription. We found that the expression of EBNA1 in transfected human and mouse cells is associated with decreased prevalence of heterochromatin foci, enhanced accessibility of cellular DNA to micrococcal nuclease digestion and decreased average length of nucleosome repeats, suggesting de-protection of the nucleosome linker regions. This is a direct effect of EBNA1 because targeting the viral protein to heterochromatin promotes large-scale chromatin decondensation with slow kinetics and independent of the recruitment of adenosine triphosphate-dependent chromatin remodelers. The remodeling function is mediated by a bipartite Gly-Arg rich domain of EBNA1 that resembles the AT-hook of High Mobility Group A (HMGA) architectural transcription factors. Similar to HMGAs, EBNA1 is highly mobile in interphase nuclei and promotes the mobility of linker histone H1, which counteracts chromatin condensation and alters the transcription of numerous cellular genes. Thus, by regulating chromatin compaction, EBNA1 may reset cellular transcription during infection and prime the infected cells for malignant transformation.

Potential cellular functions of Epstein-Barr Nuclear Antigen 1 (EBNA1) of Epstein-Barr Virus

Epstein-Barr Nuclear Antigen 1 (EBNA1) is a multifunctional protein encoded by EBV. EBNA1’s role in maintaining EBV in latently proliferating cells, by mediating EBV genome synthesis and nonrandom partitioning to daughter cells, as well as regulating viral gene transcription, is well characterized. Less understood are the roles of EBNA1 in affecting the host cell to provide selective advantages to those cells that harbor EBV. In this review we will focus on the interactions between EBNA1 and the host cell that may provide EBV-infected cells selective advantages beyond the maintenance of EBV.

Telomeres and viruses: common themes of genome maintenance

Genome maintenance mechanisms actively suppress genetic instability associated with cancer and aging. Some viruses provoke genetic instability by subverting the host's control of genome maintenance. Viruses have their own specialized strategies for genome maintenance, which can mimic and modify host cell processes. Here, we review some of the common features of genome maintenance utilized by viruses and host chromosomes, with a particular focus on terminal repeat (TR) elements. The TRs of cellular chromosomes, better known as telomeres, have well-established roles in cellular chromosome stability. Cellular telomeres are themselves maintained by viral-like mechanisms, including self-propagation by reverse transcription, recombination, and retrotransposition. Viral TR elements, like cellular telomeres, are essential for viral genome stability and propagation. We review the structure and function of viral repeat elements and discuss how they may share telomere-like structures and genome protection functions. We consider how viral infections modulate telomere regulatory factors for viral repurposing and can alter normal host telomere structure and chromosome stability. Understanding the common strategies of viral and cellular genome maintenance may provide new insights into viral-host interactions and the mechanisms driving genetic instability in cancer.

Variations of Epstein-Barr virus nuclear antigen 1 in Epstein-Barr virus-associated gastric carcinomas from Guangzhou, southern China

Epstein-Barr virus (EBV) nuclear antigen 1 (EBNA1) is the only viral protein consistently expressed in all EBV-associated malignancies, and play a critical role in the onset, progression, and/or maintenance of these tumors. Based on the signature changes at amino acid residue 487, EBNA1 is classified into five distinct subtypes: P-ala, P-thr, V-leu, V-val and V-pro. In the present study, the sequence variations of EBNA1 in EBV-associated gastric carcinoma (EBVaGC) and throat washing (TW) samples of healthy EBV carriers in Guangzhou, southern China, where nasopharyngeal carcinoma (NPC) is endemic, were analyzed by PCR and DNA sequencing. V-val subtype was the most predominant (53.6%, 15/28) in EBVaGC, followed by P-ala (42.9%, 12/28) and V-leu (32.1%, 9/28) subtypes. In TWs of healthy EBV carriers, V-val subtype was also predominant (85.7%, 18/21). The sequence variations of EBNA1 in EBVaGC were similar to those in TW of healthy EBV carriers (p>0.05), suggesting that the EBV strains in EBVaGC might originate from the viral strains prevalent within the background population. The predominance of V-val subtype in EBVaGC in Guangzhou was similar to that in EBVaGC in northern China and Japan, but was different from that in EBVaGC in America, suggesting that the variations of EBNA1 in EBVaGC represent geographic-associated polymorphisms rather than tumor-specific mutations. In addition, the EBNA1 variations in EBVaGC in gastric remnant carcinoma were also determined. V-leu subtype was detected in all 4 (100%) cases, although 2 cases occurred as mixed infection with P-ala subtype. This is different from the predominant V-val subtype in EBVaGC in conventional gastric carcinoma, suggesting that V-leu might be a subtype that adapts particularly well to the microenvironment within the gastric stump and enters the remnant gastric mucosa epithelia easily. This, to our best knowledge, is the first investigation of EBNA1 polymorphisms in EBVaGC from endemic area of NPC.

Reactive oxygen species are induced by Kaposi's sarcoma-associated herpesvirus early during primary infection of endothelial cells to promote virus entry

The entry of Kaposi's sarcoma-associated herpesvirus (KSHV) into human dermal microvascular endothelial cells (HMVEC-d), natural in vivo target cells, via macropinocytosis is initiated through a multistep process involving the binding of KSHV envelope glycoproteins with cell surface α3β1, αVβ3, and αVβ5 integrin molecules and tyrosine kinase ephrin-A2 receptor, followed by the activation of preexisting integrin-associated signaling molecules such as focal adhesion kinase (FAK), Src, c-Cbl, phosphoinositide 3-kinase (PI-3K), and Rho-GTPases. Many viruses, including KSHV, utilize cellular reactive oxygen species (ROS) for viral genomic replication and survival within host cells; however, the role of ROS in early events of viral entry and the induction of signaling has not been elucidated. Here we show that KSHV induced ROS production very early during the infection of HMVEC-d cells and that ROS production was sustained over the observation period (24 h postinfection). ROS induction was dependent on the binding of KSHV to the target cells, since pretreatment of the virus with heparin abolished ROS induction. Pretreatment of HMVEC-d cells with the antioxidant N-acetylcysteine (NAC) significantly inhibited KSHV entry, and consequently gene expression, without affecting virus binding. In contrast, H(2)O(2) treatment increased the levels of KSHV entry and infection. In addition, NAC inhibited KSHV infection-induced translocation of αVβ3 integrin into lipid rafts, actin-dependent membrane perturbations, such as blebs, observed during macropinocytosis, and activation of the signal molecules ephrin-A2 receptor, FAK, Src, and Rac1. In contrast, H(2)O(2) treatment increased the activation of ephrin-A2, FAK, Src, and Rac1. These studies demonstrate that KSHV infection induces ROS very early during infection to amplify the signaling pathways necessary for its efficient entry into HMVEC-d cells via macropinocytosis.

Similarities between the Epstein-Barr Virus (EBV) Nuclear Protein EBNA1 and the Pioneer Transcription Factor FoxA: Is EBNA1 a "Bookmarking" Oncoprotein that Alters the Host Cell Epigenotype?

EBNA1, a nuclear protein expressed in all EBV-associated neoplasms is indispensable for the maintenance of the viral episomes in latently infected cells. EBNA1 may induce genetic alterations by upregulating cellular recombinases, production of reactive oxygen species (ROS) and affecting p53 levels and function. All these changes may contribute to tumorigenesis. In this overview we focus, however, on the epigenetic alterations elicited by EBNA1 by drawing a parallel between EBNA1 and the FoxA family of pioneer transcription factors. Both EBNA1 and FoxA induce local DNA demethylation, nucleosome destabilization and bind to mitotic chromosomes. Local DNA demethylation and nucleosome rearrangement mark active promoters and enhancers. In addition, EBNA1 and FoxA, when associated with mitotic chromatin may “bookmark” active genes and ensure their reactivation in postmitotic cells (epigenetic memory). We speculate that DNA looping induced by EBNA1-EBNA1 interactions may reorganize the cellular genome. Such chromatin loops, sustained in mitotic chromatin similarly to the long-distance interactions mediated by the insulator protein CTCF, may also mediate the epigenetic inheritance of gene expression patterns. We suggest that EBNA1 has the potential to induce patho-epigenetic alterations contributing to tumorigenesis.

Contributions of Epstein-Barr nuclear antigen 1 (EBNA1) to cell immortalization and survival

Epstein–Barr virus (EBV) immortalizes host cells as part of its latent mode of infection. As a result of this ability to promote cell proliferation and survival, EBV infection contributes to the development of several kinds of B-cell lymphomas and epithelial tumours. The EBV Epstein–Barr nuclear antigen 1 (EBNA1) protein is the only EBV protein expressed in all EBV-associated tumours and plays multiple important roles in EBV latency. In addition to its well-studied roles in viral DNA replication, segregation and transcriptional activation, several studies have identified roles of EBNA1 in manipulating cellular processes that result in reduced apoptosis and increased cell survival. This review discusses these cellular effects of EBNA1 and mechanisms by which they occur.

Epstein-Barr nuclear antigen 1 induces expression of the cellular microRNA hsa-miR-127 and impairing B-cell differentiation in EBV-infected memory B cells. New insights into the pathogenesis of Burkitt lymphoma

Epstein-Barr Virus (EBV) is a γ-herpesvirus that infects >90% of the human population. Although EBV persists in its latent form in healthy carriers, the virus is also associated with several human cancers. EBV is strongly associated with Burkitt lymphoma (BL), even though there is still no satisfactory explanation of how EBV participates in BL pathogenesis. However, new insights into the interplay between viruses and microRNAs (miRNAs) have recently been proposed. In particular, it has been shown that B-cell differentiation in EBV-positive BL is impaired at the post-transcriptional level by altered expression of hsa-miR-127. Here, we show that the overexpression of hsa-miR-127 is due to the presence of the EBV-encoded nuclear antigen 1 (EBNA1) and give evidence of a novel mechanism of direct regulation of the human miRNA by this viral product. Finally, we show that the combinatorial expression of EBNA1 and hsa-miR-127 affects the expression of master B-cell regulators in human memory B cells, confirming the scenario previously observed in EBV-positive BL primary tumors and cell lines. A good understanding of these mechanisms will help to clarify the complex regulatory networks between host and pathogen, and favor the design of more specific treatments for EBV-associated malignancies.

Extensive natural variation for cellular hydrogen peroxide release is genetically controlled

Natural variation in DNA sequence contributes to individual differences in quantitative traits. While multiple studies have shown genetic control over gene expression variation, few additional cellular traits have been investigated. Here, we investigated the natural variation of NADPH oxidase-dependent hydrogen peroxide (H₂O₂ release), which is the joint effect of reactive oxygen species (ROS) production, superoxide metabolism and degradation, and is related to a number of human disorders. We assessed the normal variation of H₂O₂ release in lymphoblastoid cell lines (LCL) in a family-based 3-generation cohort (CEPH-HapMap), and in 3 population-based cohorts (KORA, GenCord, HapMap). Substantial individual variation was observed, 45% of which were associated with heritability in the CEPH-HapMap cohort. We identified 2 genome-wide significant loci of Hsa12 and Hsa15 in genome-wide linkage analysis. Next, we performed genome-wide association study (GWAS) for the combined KORA-GenCord cohorts (n = 279) using enhanced marker resolution by imputation (>1.4 million SNPs). We found 5 significant associations (p<5.00×10−8) and 54 suggestive associations (p<1.00×10−5), one of which confirmed the linked region on Hsa15. To replicate our findings, we performed GWAS using 58 HapMap individuals and ∼2.1 million SNPs. We identified 40 genome-wide significant and 302 suggestive SNPs, and confirmed genome signals on Hsa1, Hsa12, and Hsa15. Genetic loci within 900 kb from the known candidate gene p67phox on Hsa1 were identified in GWAS in both cohorts. We did not find replication of SNPs across all cohorts, but replication within the same genomic region. Finally, a highly significant decrease in H₂O₂ release was observed in Down Syndrome (DS) individuals (p<2.88×10−12). Taken together, our results show strong evidence of genetic control of H₂O₂ in LCL of healthy and DS cohorts and suggest that cellular phenotypes, which themselves are also complex, may be used as proxies for dissection of complex disorders.

Epstein-Barr virus immortalization of human B-cells leads to stabilization of hypoxia-induced factor 1 alpha, congruent with the Warburg effect

BACKGROUND

Epstein-Barr virus (EBV) encodes six nuclear transformation-associated proteins that induce extensive changes in cellular gene expression and signaling and induce B-cell transformation. The role of HIF1A in EBV-induced B-cell immortalization has not been previously studied.

METHODS AND FINDINGS

Using Western blotting and Q-PCR, we found that HIF1A protein is stabilized in EBV-transformed lymphoblastoid cells. Western blotting, GST pulldown assays, and immunoprecipitation showed that EBV-encoded nuclear antigens EBNA-5 and EBNA-3 bind to prolylhydroxylases 1 and 2, respectively, thus inhibiting HIF1A hydroxylation and degradation. Immunostaining and Q-PCR showed that the stabilized HIF1A translocates to the nucleus, forms a heterodimer with ARNT, and transactivates several genes involved in aerobic glycolysis. Using biochemical assays and Q-PCR, we also found that lymphoblastoid cells produce high levels of lactate, lactate dehydrogenase and pyruvate.

CONCLUSIONS

Our data suggest that activation of the aerobic glycolytic pathway, corresponding to the Warburg effect, occurs in EBV-transformed lymphoblastoid cells, in contrast to mitogen-activated B-cells.

The biology of EBV infection in human epithelial cells

EBV-associated human malignancies may originate from B cells and epithelial cells. EBV readily infects B cells in vitro and transforms them into proliferative lymphoblastoid cell lines. In contrast, infection of human epithelial cells in vitro with EBV has been difficult to achieve. The lack of experimental human epithelial cell systems for EBV infection has hampered the understanding of biology of EBV infection in epithelial cells. The recent success to infect human epithelial cells with EBV in vitro has allowed systematic investigations into routes of EBV entry, regulation of latent and lytic EBV infection, and persistence of EBV infection in infected epithelial cells. Understanding the biology of EBV infection in human epithelial cells will provide important insights to the role of EBV infection in the pathogenesis of EBV-associated epithelial malignancies including nasopharyngeal carcinoma and gastric carcinoma.

Role of microglia in oxidative toxicity associated with encephalomycarditis virus infection in the central nervous system

The single-stranded RNA encephalomyocarditis virus (EMCV) can replicate in the central nervous system (CNS) and lead to prominent brain lesions in the stratum pyramidale hippocampus and the stratum granulosum cerebelli. Activated microglia cells infected by EMCV produce a massive burst of reactive oxygen species (ROS) via NADPH oxidase 2 (NOX2) activation, leading to neuronal death. Balancing this effect is mechanisms by which ROS are eliminated from the CNS. Cellular prion protein (PrP^C) plays an important antioxidant role and contributes to cellular defense against EMCV infection. This review introduces recent knowledge on brain injury induced by EMCV infection via ROS generation as well as the involvement of various mediators and regulators in the pathogenesis.

Familial and large-scale case-control studies identify genes associated with nasopharyngeal carcinoma

Nasopharyngeal carcinoma (NPC) is an epithelial malignancy and has a remarkable geographic distribution, which is highly prevalent in southern China, Southeast Asia, and North Africa. Although most of the NPC are sporadic cases, the familial clustering of NPC has been demonstrated worldwide. Accumulating studies have proposed that the etiology of NPC is multi-stage and multi-factorial, involving genetic lesions, Epstein-Barr virus infection, and environmental exposure. Genetic variations result in differences in gene function, which in turn lead to different susceptibility to disease. Many studies have been carried out to dissect the genetic variants that contribute to NPC susceptibility. This article reviews the current progress of genetic studies to identify genes associated with NPC, focusing on the familial linkage and large-scale case-control study designs.

Differential role of PKC-induced c-Jun in HTLV-1 LTR activation by 12-O-tetradecanoylphorbol-13-acetate in different human T-cell lines

We have previously shown that TPA activates HTLV-1 LTR in Jurkat T-cells by inducing the binding of Sp1-p53 complex to the Sp1 site residing within the Ets responsive region 1 (ERR-1) of the LTR and that this activation is inhibited by PKCalpha and PKCepsilon. However, in H9 T-cells TPA has been noted to activate the LTR in two consecutive stages. The first stage is activation is mediated by PKCetta and requires the three 21 bp TRE repeats. The second activation mode resembles that of Jurkat cells, except that it is inhibited by PKCdelta. The present study revealed that the first LTR activation in H9 cells resulted from PKCetta-induced elevation of non-phosphorylated c-Jun which bound to the AP-1 site residing within each TRE. In contrast, this TRE-dependent activation did not occur in Jurkat cells, since there was no elevation of non-phosphorylated c-Jun in these cells. However, we found that PKCalpha and PKCepsilon, in Jurkat cells, and PKCetta and PKCdelta, in H9 cells, increased the level of phosphorylated c-Jun that interacted with the Sp1-p53 complex. This interaction prevented the Sp1-p53 binding to ERR-1 and blocked, thereby, the ERR-1-mediated LTR activation. Therefore, this PKC-inhibited LTR activation started in both cell types after depletion of the relevant PKCs by their downregulation. In view of these variable activating mechanisms we assume that there might be additional undiscovered yet modes of HTLV-1 LTR activation which vary in different cell types. Moreover, in line with this presumption we speculate that in HTLV-1 carriers the LTR of the latent provirus may also be reactivated by different mechanisms that vary between its different host T-lymphocyte subclones. Since this reactivation may initiate the ATL process, understanding of these mechanisms is essential for establishing strategies to block the possibility of reactivating the latent virus as preventive means for ATL development in carriers.

The DNA damage response in viral-induced cellular transformation

The DNA damage response (DDR) has emerged as a critical tumour suppressor pathway responding to cellular DNA replicative stress downstream of aberrant oncogene over-expression. Recent studies have now implicated the DDR as a sensor of oncogenic virus infection. In this review, we discuss the mechanisms by which tumour viruses activate and also suppress the host DDR. The mechanism of tumour virus induction of the DDR is intrinsically linked to the need for these viruses to promote an S-phase environment to replicate their nucleic acid during infection. However, inappropriate expression of viral oncoproteins can also activate the DDR through various mechanisms including replicative stress, direct interaction with DDR components and induction of reactive oxygen species. Given the growth-suppressive consequences of activating the DDR, tumour viruses have also evolved mechanisms to attenuate these pathways. Aberrant expression of viral oncoproteins may therefore promote tumourigenesis through increased somatic mutation and aneuploidy due to DDR inactivation. This review will focus on the interplay between oncogenic viruses and the DDR with respect to cellular checkpoint control and transformation.

The Epstein-Barr Virus EBNA1 Protein

Epstein-Barr virus (EBV) is a widespread human herpes virus that immortalizes cells as part of its latent infection and is a causative agent in the development of several types of lymphomas and carcinomas. Replication and stable persistence of the EBV genomes in latent infection require the viral EBNA1 protein, which binds specific DNA sequences in the viral DNA. While the roles of EBNA1 were initially thought to be limited to effects on the viral genomes, more recently EBNA1 has been found to have multiple effects on cellular proteins and pathways that may also be important for viral persistence. In addition, a role for EBNA1 in lytic infection has been recently identified. The multiple roles of EBNA1 in EBV infection are the subject of this paper.

Role of EBNA1 in NPC tumourigenesis

EBNA1 is expressed in all NPC tumours and is the only Epstein-Barr virus protein needed for the stable persistence of EBV episomes. EBNA1 binds to specific sequences in the EBV genome to facilitate the initiation of DNA synthesis, ensure the even distribution of the viral episomes to daughter cells during mitosis and to activate the transcription of other viral latency genes important for cell immortalization. In addition, EBNA1 has been found to alter cellular pathways in multiple ways that likely contribute to cell immortalization and malignant transformation. This chapter discusses the known functions and cellular effects of EBNA1, especially as pertains to NPC.

Contributions of the Epstein-Barr virus EBNA1 protein to gastric carcinoma

Approximately 10% of gastric carcinomas (GC) are comprised of cells latently infected with Epstein-Barr virus (EBV); however, the mechanism by which EBV contributes to the development of this malignancy is unclear. We have investigated the cellular effects of the only EBV nuclear protein expressed in GC, EBNA1, focusing on promyelocytic leukemia (PML) nuclear bodies (NBs), which play important roles in apoptosis, p53 activation, and tumor suppression. AGS GC cells infected with EBV were found to contain fewer PML NBs and less PML protein than the parental EBV-negative AGS cells, and these levels were restored by silencing EBNA1. Conversely, EBNA1 expression was sufficient to induce the loss of PML NBs and proteins in AGS cells. Consistent with PML functions, EBNA1 expression decreased p53 activation and apoptosis in response to DNA damage and resulted in increased cell survival. In addition, EBNA1 mutants unable to bind CK2 kinase or ubiquitin-specific protease 7 had decreased ability to induce PML loss and to interfere with p53 activation. PML levels in EBV-positive and EBV-negative GC biopsy specimens were then compared by immunohistochemistry. Consistent with the results in the AGS cells, EBV-positive tumors had significantly lower PML levels than EBV-negative tumors. The results indicate that EBV infection of GC cells leads to loss of PML NBs through the action of EBNA1, resulting in impaired responses to DNA damage and promotion of cell survival. Therefore, PML disruption by EBNA1 is one mechanism by which EBV may contribute to the development of gastric cancer.

Changes in the nasopharyngeal carcinoma nuclear proteome induced by the EBNA1 protein of Epstein-Barr virus reveal potential roles for EBNA1 in metastasis and oxidative stress responses

Epstein-Barr virus (EBV) infection is causatively associated with a variety of human cancers, including nasopharyngeal carcinoma (NPC). The only viral nuclear protein expressed in NPC is EBNA1, which can alter cellular properties in ways that may promote oncogenesis. Here, we used 2-dimensional difference gel electrophoresis (2-D DiGE) to profile changes in the nuclear proteome that occur after stable expression of EBNA1 in the EBV-negative NPC cell line CNE2. We found that EBNA1 consistently altered the levels of a small percentage of the nuclear proteins. The identification of 19 of these proteins by mass spectrometry revealed that EBNA1 upregulated three proteins affecting metastatic potential (stathmin 1, maspin, and Nm23-H1) and several proteins in the oxidative stress response pathway, including the antioxidants superoxide dismutase 1 (SOD1) and peroxiredoxin 1 (Prx1). Western blot analysis verified that EBNA1 expression upregulated and EBNA1 silencing downregulated these proteins. In addition, transcripts for stathmin 1 were induced by EBNA1, whereas EBNA1 only affected Prx1 and SOD1 at the protein level. Further investigation of the EBNA1 effects on the redox pathway showed that long-term EBNA1 expression in NPC resulted in increased reactive oxygen species (ROS) and increased levels of the NADPH oxidases NOX1 and NOX2, known to generate ROS. In addition, EBNA1 depletion in EBV-positive cells decreased NOX2 and ROS. The results show multiple roles for EBNA1 in the oxidative stress response pathway and suggest mechanisms by which EBNA1 may promote NPC metastases.

Hydrogen peroxide: a Jekyll and Hyde signalling molecule

Reactive oxygen species (ROS) are a group of molecules produced in the cell through metabolism of oxygen. Endogenous ROS such as hydrogen peroxide (H₂O₂) have long been recognised as destructive molecules. The well-established roles they have in the phagosome and genomic instability has led to the characterisation of these molecules as non-specific agents of destruction. Interestingly, there is a growing body of literature suggesting a less sinister role for this Jekyll and Hyde molecule. It is now evident that at lower physiological levels, H₂O₂ can act as a classical intracellular signalling molecule regulating kinase-driven pathways. The newly discovered biological functions attributed to ROS include proliferation, migration, anoikis, survival and autophagy. Furthermore, recent advances in detection and quantification of ROS-family members have revealed that the diverse functions of ROS can be determined by the subcellular source, location and duration of these molecules within the cell. In light of this confounding paradox, we will examine the factors and circumstances that determine whether H₂O₂ acts in a pro-survival or deleterious manner.

ROS-generating NADPH oxidase NOX4 is a critical mediator in oncogenic H-Ras-induced DNA damage and subsequent senescence

Activated Ras oncogene induces DNA-damage response by triggering reactive oxygen species (ROS) production and this is critical for oncogene-induced senescence. Until now, little connections between oncogene expression, ROS-generating NADPH oxidases and DNA-damage response have emerged from different studies. Here we report that H-RasV12 positively regulates the NADPH oxidase system NOX4-p22(phox) that produces H(2)O(2). Knocking down the NADPH oxidase with small interference RNA decreases H-RasV12-induced DNA-damage response detected by γ-H2A.X foci analysis. Using HyPer, a specific probe for H(2)O(2), we detected an increase in H(2)O(2) in the nucleus correlated with NOX4-p22(phox) perinuclear localization. DNA damage response can be caused not only by H-RasV12-driven accumulation of ROS but also by a replicative stress due to a sustained oncogenic signal. Interestingly, NOX4 downregulation by siRNA abrogated H-RasV12 regulation of CDC6 expression, an essential regulator of DNA replication. Moreover, senescence markers, such as senescence-associated heterochromatin foci, PML bodies, HP1β foci and p21 expression, induced under H-RasV12 activation were decreased with NOX4 inactivation. Taken together, our data indicate that NADPH oxidase NOX4 is a critical mediator in oncogenic H-RasV12-induced DNA-damage response and subsequent senescence.

Functional modulation of the metastatic suppressor Nm23-H1 by oncogenic viruses

Evidence over the last two decades from a number of disciplines has solidified some fundamental concepts in metastasis, a major contributor to cancer associated deaths. However, significant advances have been made in controlling this critical cellular process by focusing on targeted therapy. A key set of factors associated with this invasive phenotype is the nm23 family of over twenty metastasis-associated genes. Among the eight known isoforms, Nm23-H1 is the most studied potential anti-metastatic factor associated with human cancers. Importantly, a growing body of work has clearly suggested a critical role for Nm23-H1 in limiting tumor cell motility and progression induced by several tumor viruses, including Epstein-Barr virus (EBV), Kaposi's sarcoma associated herpes virus (KSHV) and human papilloma virus (HPV). A more in depth understanding of the interactions between tumor viruses encoded antigens and Nm23-H1 will facilitate the elucidation of underlying mechanism(s) which contribute to virus-associated cancers. Here, we review recent studies to explore the molecular links between human oncogenic viruses and progression of metastasis, in particular the deregulation of Nm23-H1 mediated suppression.

At a crossroads: human DNA tumor viruses and the host DNA damage response

Human DNA tumor viruses induce host cell proliferation in order to establish the necessary cellular milieu to replicate viral DNA. The consequence of such viral-programmed induction of proliferation coupled with the introduction of foreign replicating DNA structures makes these viruses particularly sensitive to the host DNA damage response machinery. In fact, sensors of DNA damage are often activated and modulated by DNA tumor viruses in both latent and lytic infection. This article focuses on the role of the DNA damage response during the life cycle of human DNA tumor viruses, with a particular emphasis on recent advances in our understanding of the role of the DNA damage response in EBV, Kaposi's sarcoma-associated herpesvirus and human papillomavirus infection.

Ultrafine anaphase bridges, broken DNA and illegitimate recombination induced by a replication fork barrier

Most DNA double-strand breaks (DSBs) in S- and G2-phase cells are repaired accurately by Rad51-dependent sister chromatid recombination. However, a minority give rise to gross chromosome rearrangements (GCRs), which can result in disease/death. What determines whether a DSB is repaired accurately or inaccurately is currently unclear. We provide evidence that suggests that perturbing replication by a non-programmed protein-DNA replication fork barrier results in the persistence of replication intermediates (most likely regions of unreplicated DNA) into mitosis, which results in anaphase bridge formation and ultimately to DNA breakage. However, unlike previously characterised replication-associated DSBs, these breaks are repaired mainly by Rad51-independent processes such as single-strand annealing, and are therefore prone to generate GCRs. These data highlight how a replication-associated DSB can be predisposed to give rise to genome rearrangements in eukaryotes.

An ATM/Chk2-mediated DNA damage-responsive signaling pathway suppresses Epstein-Barr virus transformation of primary human B cells

Epstein-Barr virus (EBV), an oncogenic herpesvirus that causes human malignancies, infects and immortalizes primary human B cells in vitro into indefinitely proliferating lymphoblastoid cell lines, which represent a model for EBV-induced tumorigenesis. The immortalization efficiency is very low suggesting that an innate tumor suppressor mechanism is operative. We identify the DNA damage response (DDR) as a major component of the underlying tumor suppressor mechanism. EBV-induced DDR activation was not due to lytic viral replication nor did the DDR marks co-localize with latent episomes. Rather, a transient period of EBV-induced hyper-proliferation correlated with DDR activation. Inhibition of the DDR kinases ATM and Chk2 markedly increased transformation efficiency of primary B cells. Further, the viral latent oncoproteins EBNA3C was required to attenuate the EBV-induced DNA damage response We propose that heightened oncogenic activity in early cell divisions activates a growth-suppressive DDR which is attenuated by viral latency products to induce cell immortalization.

High avidity binding to DNA protects ubiquitylated substrates from proteasomal degradation

Protein domains that act as degradation and stabilization signals regulate the rate of turnover of proteasomal substrates. Here we report that the bipartite Gly-Arg repeat of the Epstein-Barr virus (EBV) nuclear antigen (EBNA)-1 acts as a stabilization signal that inhibits proteasomal degradation in the nucleus by promoting binding to cellular DNA. Protection can be transferred by grafting the domain to unrelated proteasomal substrates and does not involve changes of ubiquitylation. Protection is also afforded by other protein domains that, similar to the Gly-Arg repeat, mediate high avidity binding to DNA, as exemplified by resistance to detergent extraction. Our findings identify high avidity binding to DNA as a portable inhibitory signal that counteracts proteasomal degradation.

The Epstein-Barr virus nuclear antigen-1 promotes telomere dysfunction via induction of oxidative stress

The Epstein–Barr virus (EBV) nuclear antigen (EBNA)-1 promotes the accumulation of chromosomal aberrations in malignant B cells by inducing oxidative stress. Here we report that this phenotype is associated with telomere dysfunction. Stable or conditional expression of EBNA1 induced telomere abnormalities including loss or gain of telomere signals, telomere fusion and heterogeneous length of telomeres. This was accompanied by the accumulation of extrachromosomal telomeres, telomere dysfunction-induced foci (TIFs) containing phosphorylated histone H2AX and the DNA damage response protein 53BP1, telomere-associated promyelocytic leukemia nuclear bodies (APBs), telomeric-sister chromatid exchanges and displacement of the shelterin protein TRF2. The induction of TIFs and APBs was inhibited by treatment with scavengers of reactive oxygen species (ROS) that also promoted the relocalization of TRF2 at telomeres. These findings highlight a novel mechanism by which EBNA1 may promote malignant transformation and tumor progression.

Comprehensive pathway-based association study of DNA repair gene variants and the risk of nasopharyngeal carcinoma

DNA repair plays a central role in protecting against environmental carcinogenesis, and genetic variants of DNA repair genes have been reported to be associated with several human malignancies. To assess whether DNA gene variants were associated with nasopharyngeal carcinoma (NPC) risk, a candidate gene association study was conducted among the Cantonese population within the Guangdong Province, China, the ethnic group with the highest risk for NPC. A 2-stage study design was utilized. In the discovery stage, 676 tagging SNPs covering 88 DNA repair genes were genotyped in a matched case-control study (cases/controls = 755/755). Eleven SNPs with P(trend) < 0.01 were identified. Seven of these SNPs were located within 3 genes, RAD51L1, BRCA2, and TP53BP1. In the validation stage, these 11 SNPs were genotyped in a separate Cantonese population (cases/controls = 1,568/1,297). Two of the SNPs (rs927220 and rs11158728), both in RAD51L1, remained strongly associated with NPC. The SNP rs927220 had a significant P(combined) of 5.55 × 10(-5), with OR = 1.20 (95% CI = 1.10-1.30), Bonferroni corrected P = 0.0381. The other SNP (rs11158728), which is in strong linkage disequilibrium with rs927220 (r(2) = 0.7), had a significant P(combined) of 2.0 × 10(-4), Bonferroni corrected P = 0.1372. Gene-environment interaction analysis suggested that the exposures of salted fish consumption and cigarette smoking had potential interactions with DNA repair gene variations, but need to be further investigated. Our findings support the notion that DNA repair genes, in particular RAD51L1, play a role in NPC etiology and development.

Hybrid herpes simplex virus/Epstein-Barr virus amplicon viral vectors confer enhanced transgene expression in primary human tumors and human bone marrow-derived mesenchymal stem cells

BACKGROUND

Herpes simplex virus type-1 (HSV-1) amplicon vectors are attractive tools for gene transfer because of their large DNA insert capacity, their broad host range of vector transduction and a minimal immune response as a result of the absence of helper viruses during viral packaging. However, the transient gene expression remains a challenge for the translation of HSV-1 amplicon based therapeutic strategies to a clinical setting. Although oriP/EBV nuclear antigen (EBNA)-1 elements of Epstein-Barr virus (EBV) have been successfully employed to achieve prolonged transgene expression, little is known about the stability of the EBNA-1 elements in the context of HSV-1 amplicon viral vectors.

METHODS

We have generated HSV/EBV hybrid vectors expressing the mutant EBNA-1 gene with the luciferase reporter gene bicistronically to enable monitoring of EBNA-1 expression in real-time, both in vitro and in vivo.

RESULTS

The results obtained showed that the HSV/EBV hybrid vectors could mediate high levels of transgene expression (ranging from approximately two-fold to nine-fold) in primary human tumor cells and human bone marrow-derived mesenchymal stem cells compared to the control vector. Prolonged transgene expression could also be observed in primary patient-derived human hepatocellular carcinoma xenografts and in the mouse brain parenchyma up to a period of 17 and 365 days, respectively.

CONCLUSIONS

Taken together, we have demonstrated that these hybrid vectors could be promising tools as carriers of therapeutic genes in mesenchymal stem cells or even provide an alternative non-integrating platform for the generation of induced pluripotent stem cells.

Developing extrachromosomal gene expression vector technologies: an overview

Extrachromosomal, or episomal, vectors offer a number of advantages for therapeutic and scientific applications compared to integrating vectors. Extrachromosomal vectors persist in the nucleus without the requirement to integrate into the host genome, hence avoiding the recent concerns surrounding the genotoxic effects of vector integration. By avoiding integration, episomal vectors avoid vector rearrangement, which can occur at integration, and also avoid any effect of surrounding DNA activity on transgene expression ("position effect"). Extrachromosomal vectors offer a very high transgene capacity, allowing either the incorporation of large promoter and regulatory elements into an expression cassette, or the use of complete genomic loci of up to 100 kb or larger as transgenes. Whole genomic loci transgenes offer an elegant means to express genes under physiological and developmental-stage regulation, to express multiple transcript variants from a single locus, and to express multiple genes from a single tract of genomic DNA. The combined advantages of episomal vectors of prolonged transgene persistence in the absence of vector integration, avoiding silencing by flanking heterochromatin, and high capacity, facilitating delivery and expression of genomic DNA transgenes, will be reviewed here and potential therapeutic and scientific uses outlined.