A nuclear matrix attachment region organizes the Epstein-Barr viral plasmid in Raji cells into a single DNA domain

A vector system encoding histone H3 mutants facilitates manipulations of the neuronal epigenome

The differentiation of developmental cell lineages is associated with genome-wide modifications in histone H3 methylation. However, the causal role of histone H3 methylation in transcriptional regulation and cell differentiation has been difficult to test in mammals. The experimental overexpression of histone H3 mutants carrying lysine-to-methionine (K-to-M) substitutions has emerged as an alternative tool for inhibiting the endogenous levels of histone H3 methylation at specific lysine residues. Here, we leverage the use of histone K-to-M mutants by creating Enhanced Episomal Vectors that enable the simultaneous depletion of multiple levels of histone H3 lysine 4 (H3K4) or lysine 9 (H3K9) methylation in projection neurons of the mouse cerebral cortex. Our approach also facilitates the simultaneous depletion of H3K9 and H3K27 trimethylation (H3K9me3 and H3K27me3, respectively) in cortical neurons. In addition, we report a tamoxifen-inducible Cre-FLEX system that allows the activation of mutant histones at specific developmental time points or in the adult cortex, leading to the depletion of specific histone marks. The tools presented here can be implemented in other experimental systems, such as human in vitro models, to test the combinatorial role of histone methylations in developmental fate decisions and the maintenance of cell identity.

Pheophorbide a: State of the Art

Chlorophyll breakdown products are usually studied for their antioxidant and anti-inflammatory activities. The chlorophyll derivative Pheophorbide a (PPBa) is a photosensitizer that can induce significant anti-proliferative effects in several human cancer cell lines. Cancer is a leading cause of death worldwide, accounting for about 9.6 million deaths, in 2018 alone. Hence, it is crucial to monitor emergent compounds that show significant anticancer activity and advance them into clinical trials. In this review, we analyze the anticancer activity of PPBa with or without photodynamic therapy and also conjugated with or without other chemotherapic drugs, highlighting the capacity of PPBa to overcome multidrug resistance. We also report other activities of PPBa and different pathways that it can activate, showing its possible applications for the treatment of human pathologies.

Epigenetic lifestyle of Epstein-Barr virus

Epstein-Barr virus (EBV) is a model of herpesvirus latency and epigenetic changes. The virus preferentially infects human B-lymphocytes (and also other cell types) but does not turn them straight into virus factories. Instead, it establishes a strictly latent infection in them and concomitantly induces the activation and proliferation of infected B cells. How the virus establishes latency in its target cells is only partially understood, but its latent state has been studied intensively by many. During latency, several copies of the viral genome are maintained as minichromosomes in the nucleus. In latently infected cells, most viral genes are epigenetically repressed by cellular chromatin constituents and DNA methylation, but certain EBV genes are spared and remain expressed to support the latent state of the virus in its host cell. Latency is not a dead end, but the virus can escape from this state and reactivate. Reactivation is a coordinated process that requires the removal of repressive chromatin components and a gain in accessibility for viral and cellular factors and machines to support the entire transcriptional program of EBV's ensuing lytic phase. We have a detailed picture of the initiating events of EBV's lytic phase, which are orchestrated by a single viral protein - BZLF1. Its induced expression can lead to the expression of all lytic viral proteins, but initially it fosters the non-licensed amplification of viral DNA that is incorporated into preformed capsids. In the virions, the viral DNA is free of histones and lacks methylated cytosine residues which are lost during lytic DNA amplification. This review provides an overview of EBV's dynamic epigenetic changes, which are an integral part of its ingenious lifestyle in human host cells.

Epigenetic specifications of host chromosome docking sites for latent Epstein-Barr virus

Epstein-Barr virus (EBV) genomes persist in latently infected cells as extrachromosomal episomes that attach to host chromosomes through the tethering functions of EBNA1, a viral encoded sequence-specific DNA binding protein. Here we employ circular chromosome conformation capture (4C) analysis to identify genome-wide associations between EBV episomes and host chromosomes. We find that EBV episomes in Burkitt's lymphoma cells preferentially associate with cellular genomic sites containing EBNA1 binding sites enriched with B-cell factors EBF1 and RBP-jK, the repressive histone mark H3K9me3, and AT-rich flanking sequence. These attachment sites correspond to transcriptionally silenced genes with GO enrichment for neuronal function and protein kinase A pathways. Depletion of EBNA1 leads to a transcriptional de-repression of silenced genes and reduction in H3K9me3. EBV attachment sites in lymphoblastoid cells with different latency type show different correlations, suggesting that host chromosome attachment sites are functionally linked to latency type gene expression programs.

A chimeric protein composed of NuMA fused to the DNA binding domain of LANA is sufficient for the ori-P-dependent DNA replication

The Kaposi's sarcoma-associated herpesvirus (KSHV) genome is stably maintained in KSHV-infected PEL cell lines during cell division. We previously showed that accumulation of LANA in the nuclear matrix fraction could be important for the latent DNA replication, and that the functional significance of LANA should be its recruitment of ori-P to the nuclear matrix. Here, we investigated whether the forced localization of the LANA-DNA binding domain (DBD) to the nuclear matrix facilitated ori-P-containing plasmid replication. We demonstrated that chimeric proteins constructed by fusion of LANA DBD with the nuclear mitotic apparatus protein (NuMA), which is one of the components of the nuclear matrix, could bind with ori-P and enhance replication of an ori-P-containing plasmid, compared with that in the presence of DBD alone. These results further suggested that the ori-P recruitment to the nuclear matrix through the binding with DBD is important for latent viral DNA replication.

Role of epigenetics in EBV regulation and pathogenesis

Epigenetic modifications of the viral and host cell genomes regularly occur in EBV-associated lymphomas and carcinomas. The cell type-dependent usage of latent EBV promoters is determined by the cellular epigenetic machinery. Viral oncoproteins interact with the very same epigenetic regulators and alter the cellular epigenotype and gene-expression pattern: there are common gene sets hypermethylated in both EBV-positive and EBV-negative neoplasms of different histological types. A group of hypermethylated promoters may represent, however, a unique EBV-associated epigenetic signature in EBV-positive gastric carcinomas. By contrast, EBV-immortalized B-lymphoblastoid cell lines are characterized by genome-wide demethylation and loss and rearrangement of heterochromatic histone marks. Early steps of EBV infection may also contribute to reprogramming of the cellular epigenome.

Applying horizontal gene transfer phenomena to enhance non-viral gene therapy

Horizontal gene transfer (HGT) is widespread amongst prokaryotes, but eukaryotes tend to be far less promiscuous with their genetic information. However, several examples of HGT from pathogens into eukaryotic cells have been discovered and mimicked to improve non-viral gene delivery techniques. For example, several viral proteins and DNA sequences have been used to significantly increase cytoplasmic and nuclear gene delivery. Plant genetic engineering is routinely performed with the pathogenic bacterium Agrobacterium tumefaciens and similar pathogens (e.g. Bartonella henselae) may also be able to transform human cells. Intracellular parasites like Trypanosoma cruzi may also provide new insights into overcoming cellular barriers to gene delivery. Finally, intercellular nucleic acid transfer between host cells will also be briefly discussed. This article will review the unique characteristics of several different viruses and microbes and discuss how their traits have been successfully applied to improve non-viral gene delivery techniques. Consequently, pathogenic traits that originally caused diseases may eventually be used to treat many genetic diseases.

The origin recognition complex in human diseases

ORC (origin recognition complex) serves as the initiator for the assembly of the pre-RC (pre-replication complex) and the subsequent DNA replication. Together with many of its non-replication functions, ORC is a pivotal regulator of various cellular processes. Notably, a number of reports connect ORC to numerous human diseases, including MGS (Meier-Gorlin syndrome), EBV (Epstein-Barr virus)-infected diseases, American trypanosomiasis and African trypanosomiasis. However, much of the underlying molecular mechanism remains unclear. In those genetic diseases, mutations in ORC alter its function and lead to the dysregulated phenotypes; whereas in some pathogen-induced symptoms, host ORC and archaeal-like ORC are exploited by these organisms to maintain their own genomes. In this review, I provide detailed examples of ORC-related human diseases, and summarize the current findings on how ORC is involved and/or dysregulated. I further discuss how these discoveries can be generalized as model systems, which can then be applied to elucidating other related diseases and revealing potential targets for developing effective therapies.

Interpreting the Epstein-Barr Virus (EBV) epigenome using high-throughput data

The Epstein-Barr virus (EBV) double-stranded DNA genome is subject to extensive epigenetic regulation. Large consortiums and individual labs have generated a vast number of genome-wide data sets on human lymphoblastoid and other cell lines latently infected with EBV. Analysis of these data sets reveals important new information on the properties of the host and viral chromosome structure organization and epigenetic modifications. We discuss the mapping of these data sets and the subsequent insights into the chromatin structure and transcription factor binding patterns on latent EBV genomes. Colocalization of multiple histone modifications and transcription factors at regulatory loci are considered in the context of the biology and regulation of EBV.

DNA of a circular minichromosome linearized by restriction enzymes or other reagents is resistant to further cleavage: an influence of chromatin topology on the accessibility of DNA

The accessibility of DNA in chromatin is an essential factor in regulating its activities. We studied the accessibility of the DNA in a ∼170 kb circular minichromosome to DNA-cleaving reagents using pulsed-field gel electrophoresis and fibre-fluorescence in situ hybridization on combed DNA molecules. Only one of several potential sites in the minichromosome DNA was accessible to restriction enzymes in permeabilized cells, and in growing cells only a single site at an essentially random position was cut by poisoned topoisomerase II, neocarzinostatin and γ-radiation, which have multiple potential cleavage sites; further sites were then inaccessible in the linearized minichromosomes. Sequential exposure to combinations of these reagents also resulted in cleavage at only a single site. Minichromosome DNA containing single-strand breaks created by a nicking endonuclease to relax any unconstrained superhelicity was also cut at only a single position by a restriction enzyme. Further sites became accessible after ≥95% of histones H2A, H2B and H1, and most non-histone proteins were extracted. These observations suggest that a global rearrangement of the three-dimensional packing and interactions of nucleosomes occurs when a circular minichromosome is linearized and results in its DNA becoming inaccessible to probes.

Epigenetic dysregulation of epstein-barr virus latency and development of autoimmune disease

Epstein-Barr virus (EBV) is ahumanherpesvirus thatpersists in the memory B-cells of the majority of the world population in a latent form. Primary EBV infection is asymptomatic or causes a self-limiting disease, infectious mononucleosis. Virus latency is associated with a wide variety of neoplasms whereof some occur in immune suppressed individuals. Virus production does not occur in strict latency. The expression of latent viral oncoproteins and nontranslated RNAs is under epigenetic control via DNA methylation and histone modifications that results either in a complete silencing of the EBV genome in memory B cells, or in a cell-type dependent usage of a couple of latency promoters in tumor cells, germinal center B cells and lymphoblastoid cells (LCL, transformed by EBV in vitro). Both, latent and lytic EBV proteins elicit a strong immune response. In immune suppressed and infectious mononucleosis patients, an increased viral load can be detected in the blood. Enhanced lytic replication may result in new infection- and transformation-events and thus is a risk factor both for malignant transformation and the development of autoimmune diseases. An increased viral load or a changed presentation of a subset of lytic or latent EBV proteins that cross-react with cellular antigens may trigger pathogenic processes through molecular mimicry that result in multiple sclerosis (MS), systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA).

Viral hit and run-oncogenesis: genetic and epigenetic scenarios

It is well documented that viral genomes either inserted into the cellular DNA or co-replicating with it in episomal form can be lost from neoplastic cells. Therefore, "hit and run"-mechanisms have been a topic of longstanding interest in tumor virology. The basic idea is that the transient acquisition of a complete or incomplete viral genome may be sufficient to induce malignant conversion of host cells in vivo, resulting in neoplastic development. After eliciting a heritable change in the gene expression pattern of the host cell (initiation), the genomes of tumor viruses may be completely lost, i.e. in a hit and run-scenario they are not necessary for the maintenance of the malignant state. The expression of viral oncoproteins and RNAs may interfere not only with regulators of cell proliferation, but also with DNA repair mechanisms. DNA recombinogenic activities induced by tumor viruses or activated by other mechanisms may contribute to the secondary loss of viral genomes from neoplastic cells. Viral oncoproteins can also cause epigenetic dysregulation, thereby reprogramming cellular gene expression in a heritable manner. Thus, we expect that epigenetic scenarios of viral hit and run-tumorigenesis may facilitate new, innovative experiments and clinical studies in spite of the fact that the regular presence of a suspected human tumor virus in an early phase of neoplastic development and its subsequent regular loss have not been demonstrated yet. We propose that virus-specific "epigenetic signatures", i.e. alterations of the host cell epigenome, especially altered DNA methylation patterns, may help to identify viral hit and run-oncogenic events, even after the complete loss of tumor viruses from neoplastic cells.

Identifying a property of origins of DNA synthesis required to support plasmids stably in human cells

The plasmid origin of replication, oriP, of Epstein-Barr Virus (EBV) was identified in an assay to detect autonomously replicating sequences (ARSs) in human cells. Raji ori, a second origin in EBV, functions in vivo but fails in long-term ARS assays. We examined the initiating element, DS, within oriP and Raji ori to resolve this paradox. DS, but not Raji ori, binds EBNA1; whereas both act as ARSs in short-term assays, with DS being more efficient, only DS can act as an ARS in long-term assays. Surprisingly, we found that DS supported the establishment of a plasmid with Raji ori in cis and that after deletion of DS, Raji ori could now act as an ARS in the long term. This finding explains the frequent failure of ARS assays in mammalian cells. More origins can initially act as ARSs than can be established. We identified one requirement for ARSs to be established: They must function efficiently enough initially to generate a wide distribution of numbers of plasmids per cell. Only the cells that have more than a threshold number of plasmids can survive selections imposed on the cells to retain these replicons.

Analysis of cis-elements that facilitate extrachromosomal persistence of human papillomavirus genomes

Human papillomaviruses (HPVs) are maintained latently in dividing epithelial cells as nuclear plasmids. Two virally encoded proteins, E1, a helicase, and E2, a transcription factor, are important players in replication and stable plasmid maintenance in host cells. Recent experiments in yeast have demonstrated that viral genomes retain replication and maintenance function independently of E1 and E2 [Angeletti, P.C., Kim, K., Fernandes, F.J., and Lambert, P.F. (2002). Stable replication of papillomavirus genomes in Saccharomyces cerevisiae. J. Virol. 76(7), 3350-8; Kim, K., Angeletti, P.C., Hassebroek, E.C., and Lambert, P.F. (2005). Identification of cis-acting elements that mediate the replication and maintenance of human papillomavirus type 16 genomes in Saccharomyces cerevisiae. J. Virol. 79(10), 5933-42]. Flow cytometry studies of EGFP-reporter vectors containing subgenomic HPV fragments with or without a human ARS (hARS), revealed that six fragments located in E6-E7, E1-E2, L1, and L2 regions showed a capacity for plasmid stabilization in the absence of E1 and E2 proteins. Interestingly, four fragments within E7, the 3' end of L2, and the 5' end of L1 exhibited stability in plasmids that lacked an hARS, indicating that they possess both replication and maintenance functions. Two fragments lying in E1-E2 and the 3' region of L1 were stable only in the presence of hARS, that they contained only maintenance function. Mutational analyses of HPV16-GFP reporter constructs provided evidence that genomes lacking E1 and E2 could replicate to an extent similar to wild type HPV16. Together these results support the concept that cellular factors influence HPV replication and maintenance, independently, and perhaps in conjunction with E1 and E2, suggesting a role in the persistent phase of the viral lifecycle.

An autonomous replicating element within the KSHV genome

Members of the herpesviridae family including Kaposi's sarcoma-associated herpesvirus (KSHV) persist latently in their hosts and harbor their genomes as closed circular episomes. Propagation of the KSHV genome into new daughter cells requires replication of the episome once every cell division and is considered critically dependent on expression of the virus encoded latency-associated nuclear antigen (LANA). This study demonstrates a LANA-independent mechanism of KSHV latent DNA replication. A cis-acting DNA element within a discreet KSHV genomic region termed the long unique region (LUR) can initiate and support replication of plasmids lacking LANA-binding sequences or a eukaryotic replication origin. The human cellular replication machinery proteins ORC2 and MCM3 associated with the LUR element and depletion of cellular ORC2 abolished replication of the plasmids indicating that recruitment of the host cellular replication machinery is important for LUR-dependent replication. Thus, KSHV can initiate replication of its genome independent of any trans-acting viral factors.

Epigenotypes of latent herpesvirus genomes

Epigenotypes are modified cellular or viral genotypes which differ in transcriptional activity in spite of having an identical (or nearly identical) DNA sequence. Restricted expression of latent, episomal herpesvirus genomes is also due to epigenetic modifications. There is no virus production (lytic viral replication, associated with the expression of all viral genes) in tight latency. In vitro experiments demonstrated that DNA methylation could influence the activity of latent (and/or crucial lytic) promoters of prototype strains belonging to the three herpesvirus subfamilies (alpha-, beta-, and gamma-herpesviruses). In vivo, however, DNA methylation is not a major regulator of herpes simplex virus type 1 (HSV-1, a human alpha-herpesvirus) latent gene expression in neurons of infected mice. In these cells, the promoter/enhancer region of latency-associated transcripts (LATs) is enriched with acetyl histone H3, suggesting that histone modifications may control HSV-1 latency in terminally differentiated, quiescent neurons. Epstein-Barr virus (EBV, a human gamma-herpesvirus) is associated with a series of neoplasms. Latent, episomal EBV genomes are subject to host cell-dependent epigenetic modifications (DNA methylation, binding of proteins and protein complexes, histone modifications). The distinct viral epigenotypes are associated with distinct EBV latency types, i.e., cell type-specific usage of latent EBV promoters controlling the expression of latent, growth transformation-associated EBV genes. The contribution of major epigenetic mechanisms to the regulation of latent EBV promoters is variable. DNA methylation contributes to silencing of Wp and Cp (alternative promoters for transcripts coding for the nuclear antigens EBNA 1-6) and LMP1p, LMP2Ap, and LMP2Bp (promoters for transcripts encoding transmembrane proteins). DNA methylation does not control, however, Qp (a promoter for EBNA1 transcripts only) in lymphoblastoid cell lines (LCLs), although in vitro methylated Qp-reporter gene constructs are silenced. The invariably unmethylated Qp is probably switched off by binding of a repressor protein in LCLs.

Epi-CHO, an episomal expression system for recombinant protein production in CHO cells

This study describes the development of a transient expression system for CHO cells based on autonomous replication and retention of transfected plasmid DNA. A transient expression system that allows extrachromosomal amplification of plasmids permits more plasmid copies to persist in the transfected cell throughout the production phase leading to a significant increase in transgene expression. The expression system, named Epi-CHO comprises (1) a CHO-K1 cell line stably transfected with the Polyomavirus (Py) large T (LT) antigen gene (PyLT) and (2) a DNA expression vector, pPyEBV encoding the Py origin (PyOri) for autonomous plasmid amplification and encoding Epstein-Barr Virus (EBV) nuclear antigen-1 (EBNA-1) and OriP for plasmid retention. The CHO-K1 cell line expressing PyLT, named CHO-T was adapted to suspension growth in serum-free media to facilitate large-scale transient transfection and recombinant gene expression. Enhanced green fluorescent protein (EGFP) and human growth hormone (hGH) were used as reporter proteins to demonstrate transgene expression and productivity. Transfection of suspension-growing CHO-T cells with the vector pPyEBV encoding hGH resulted in a final concentration of 75 mg L(-1) of hGH in culture supernatants 11 days following transfection.

Site-directed mutagenesis of large DNA palindromes: construction and in vitro characterization of herpes simplex virus type 1 mutants containing point mutations that eliminate the oriL or oriS initiation function

Technical challenges associated with mutagenesis of the large oriL palindrome have hindered comparisons of the functional roles of the herpes simplex virus type 1 (HSV-1) origins of DNA replication, oriL and oriS, in viral replication and pathogenesis. To address this problem, we have developed a novel PCR-based strategy to introduce site-specific mutations into oriL and other large palindromes. Using this strategy, we generated three plasmids containing mutant forms of oriL, i.e., pDoriL-I(L), pDoriL-I(R), and pDoriL-I(LR), containing point mutations in the left, right, and both copies, respectively, of the origin binding protein (OBP) binding site (site I) which eliminate OBP binding. In in vitro DNA replication assays, plasmids with mutations in only one arm of the palindrome supported origin-dependent DNA replication, whereas plasmids with symmetrical mutations in both arms of the palindrome were replication incompetent. An analysis of the cloned mutant plasmids used in replication assays revealed that a fraction of each plasmid mutated in only one arm of the palindrome had lost the site I mutation. In contrast, plasmids containing symmetrical mutations in both copies of site I retained both mutations. These observations demonstrate that the single site I mutations in pDoriL-I(L) and pDoriL-I(R) are unstable upon propagation in bacteria and suggest that functional forms of both the left and right copies of site I are required to initiate DNA replication at oriL. To examine the role of oriL and oriS site I in virus replication, we introduced the two site I mutations in pDoriL-I(LR) into HSV-1 DNA to yield the mutant virus DoriL-I(LR) and the same point mutations into the single site I sequence present in both copies of oriS to yield the mutant virus DoriS-I. In Vero cells and primary rat embryonic cortical neurons (PRN) infected with either mutant virus, viral DNA synthesis and viral replication were efficient, confirming that the two origins can substitute functionally for one another in vitro. Measurement of the levels of oriL and oriS flanking gene transcripts revealed a modest alteration in the kinetics of ICP8 transcript accumulation in DoriL-I(LR)-infected PRN, but not in Vero cells, implicating a cell-type-specific role for oriL in regulating ICP8 transcription.

Epigenetic regulation of lymphoid specific gene sets

Coregulation of lymphoid-specific gene sets is achieved by a series of epigenetic mechanisms. Association with higher-order chromosomal structures (nuclear subcompartments repressing or favouring gene expression) and locus control regions affects recombination and transcription of clonotypic antigen receptors and expression of a series of other lymphoid-specific genes. Locus control regions can regulate DNA methylation patterns in their vicinity. They may induce tissue- and site-specific DNA demethylation and affect, thereby, accessibility to recombination-activating proteins, transcription factors, and enzymes involved in histone modifications. Both DNA methylation and the Polycomb group of proteins (PcG) function as alternative systems of epigenetic memory in lymphoid cells. Complexes of PcG proteins mark their target genes by covalent histone tail modifications and influence lymphoid development and rearrangement of IgH genes. Ectopic expression of protein noncoding microRNAs may affect the generation of B-lineage cells, too, by guiding effector complexes to sites of heterochromatin assembly. Coregulation of lymphoid and viral promoters is also possible. EBNA 2, a nuclear protein encoded by episomal Epstein-Barr virus genomes, binds to the cellular protein CBF1 (C promoter binding factor 1) and operates, thereby, a regulatory network to activate latent viral promoters and cellular promoters associated with CBF1 binding sites.Key words : lymphoid cells, coregulation of gene batteries, epigenetic regulation, nuclear subcompartment switch, locus control region, DNA methylation, Polycomb group of proteins, histone modifications, microRNA, Epstein-Barr virus, EBNA 2, regulatory network.

Improved gene expression pattern using Epstein-Barr virus (EBV)-based plasmid and cationic emulsion

To improve transgene expression of a non-viral gene delivery system, an Epstein-Barr virus (EBV)-based plasmid and cationic emulsion complex was prepared and evaluated. Cationic emulsion was formulated with castor oil, 3-N-(N',N'-dimethylaminoethane)-carbamoyl cholesterol (DC-Chol) and other co-emulsifiers. An EBV-based plasmid containing the two EBV components, origin of replication (oriP) and EBV nuclear antigen 1 (EBNA-1), was constructed. The physical characteristics of the emulsion and the emulsion/DNA complex were determined. After cells were transfected with cationic emulsion/EBV-based plasmid complex, transfection efficiency and expression pattern were evaluated using green fluorescent protein (GFP) as a reporter. The average particle size and zeta potential of the emulsion itself were 96 nm and + 17 mV, respectively. The emulsion showed stable size distribution up to at least one month. With an increase of emulsion to DNA ratio, zeta-potential increased from negative to positive and the particle size decreased to 200-300 nm. The complex was stable against DNase I digestion and showed comparable transfection efficiency with Lipofectin for several tested cell lines. An enhanced and prolonged gene expression was achieved using EBV-based plasmid and cationic emulsion complex. Combining physically stable emulsion with self-replicating EBV-based plasmid may confer more effective gene expression.

Gene transfer into hepatocytes mediated by herpes simplex virus–Epstein-Barr virus hybrid amplicons

Gene transfer into hepatocytes is highly desirable for the long-term goal of replacing deficient proteins and correcting metabolic disorders. Vectors based on herpes simplex virus type-1 (HSV-1) have been demonstrated to mediate efficient gene transfer into hepatocytes both in vitro and in vivo. Large transgene capacity and extrachromosomal persistence make HSV-1/EBV hybrid amplicon vectors an attractive candidate for hepatic gene replacement therapy. To assess liver-directed gene transfer, we constructed (i) a conventional HSV-1 amplicon vector encoding a secreted reporter protein (secreted alkaline phosphatase, SEAP) under the control of the HSV-1 immediate-early 4/5 promoter; (ii) a HSV-1 amplicon encoding SEAP under the control of the artificial CAG promoter (the chicken beta-actin promoter and cytomegalovirus (CMV) immediate-early enhancer); and (iii) a HSV-1/EBV hybrid amplicon, also encoding SEAP under the control of the CAG promoter. While all three vector constructs yielded high SEAP concentrations in vitro and in vivo, use of HSV-1/EBV hybrid amplicon vectors significantly prolonged the duration of gene expression. Using conventional amplicon vectors in cultured hepatocytes, SEAP was detected for two weeks, whereas SEAP was detected for at least six weeks when HSV-1/EBV amplicons were used. Intraparenchymal injection into the liver of SICD mice yielded high (up to 77 ng of SEAP per milliliter serum) and sustained (greater than three weeks) expression of SEAP. Serum transaminases (ALT/AST) were measured at different time points to monitor for hepatocellular damage. While initially elevated four times above baseline, the transaminase levels returned to normal after three to seven days. These results demonstrate the usefulness of HSV-1-based amplicons and SEAP for the evaluation of gene replacement strategies in the liver.

A 30 kb region of the Epstein-Barr virus genome is colinear with the rearranged human immunoglobulin gene loci: implications for a "ping-pong evolution" model for persisting viruses and their hosts. A review

The left part of the Epstein-Barr virus (EBV) genome exhibits a strong colinearity of structural and functional elements with the immunoglobulin (Ig) gene loci which is only partially reflected in nucleotide sequence homologies. We propose that this colinearity may be the result of an inter-dependent co-evolution of the immunoglobulin loci together with EBV. Our observation could help elucidating the mechanisms of somatic hypermutation, explaining the ability of EBV to accidentally cause tumors, and shedding more light on the general mechanisms of viral and organismal evolution. We suggest that persisting viruses served as a complement for the organismal germline like in a ping-pong game and outline The Ping-Pong Evolution Hypothesis.

The Function of Nuclear Architecture: A Genetic Approach

Eukaryotic genomes are distributed on linear chromosomes that are grouped together in the nucleus, an organelle separated from the cytoplasm by a characteristic double membrane studded with large proteinaceous pores. The chromatin within chromosomes has an as yet poorly characterized higher-order structure, but in addition to this, chromosomes and specific subchromosomal domains are nonrandomly positioned in nuclei. This review examines functional implications of the long-range organization of the genome in interphase nuclei. A rigorous test of the physiological importance of nuclear architecture is achieved by introducing mutations that compromise both structure and function. Focussing on such genetic approaches, we address general concepts of interphase nuclear order, the role of the nuclear envelope (NE) and lamins, and finally the importance of spatial organization for DNA replication and heritable gene expression.

The LCR of EBV makes Burkitt's lymphoma endemic

The spectacular ability of Epstein-Barr virus (EBV) to immortalize and morphologically transform human B cells in vitro to lymphoblastoid cell lines (LCLs) is central to most molecular models of viral oncogenesis. However, binding of transcription factor and oncoprotein c-Myc to the major locus control region (LCR) of the viral genome directs us to an alternative model for the origin of Burkitt's lymphoma (BL). In this model, improved nuclear maintenance of the viral genome and the continuous expression of anti-apoptotic functions in B cells exhibiting class I EBV latency contribute to the generation of BL, without any detour through EBV nuclear antigen (EBNA) 2-driven B-cell immortalization (also called class III latency).

In vivo dynamics of EBNA1-oriP interaction during latent and lytic replication of Epstein-Barr virus

The Epstein-Barr virus (EBV) nuclear antigen 1 (EBNA1) is required for maintenance of the viral genome DNA during the latent phase of EBV replication but continues to be synthesized after the induction of viral productive replication. An EBV genome-wide chromatin immunoprecipitation assay revealed that EBNA1 constantly binds to oriP of the EBV genome during not only latent but also lytic infection. Although the total levels of EBNA1 proved constant throughout the latter, the levels of the oriP-bound form were increased as lytic infection proceeded. EBV productive DNA replication occurs at discrete sites in nuclei, called replication compartments, where viral replication proteins are clustered. Confocal laser microscopic analyses revealed that whereas EBNA1 was distributed broadly in nuclei as fine punctate dots during the latent phase of infection, the protein became redistributed to the viral replication compartments and localized as distinct spots within and/or nearby the compartments after the induction of lytic replication. Taking these findings into consideration, oriP regions of the EBV genome might be organized by EBNA1 into replication domains that may set up scaffolding for lytic replication and transcription.

EBV-associated neoplasms: alternative pathogenetic pathways

We propose that there are two main classes of Epstein-Barr virus (EBV) associated lymphomas: primarily malignant Burkitt's Lymphoma (BL) and Hodgkin's Disease (HD), on one hand, and primarily benign lymphoproliferations, e.g., post-transplant lymphoproliferative disease (PTLD) on the other hand. PTLD may start as a benign lymphoproliferation which becomes malignant if out of T cell control for too long. Our discovery of a binding site for the oncoprotein c-Myc at a central position of the EBV genome favours a distinction of pathogenetic pathways or scenarios for the proposed lymphoma classes. In the first scenario nuclear maintenance of the EBV genome and activation of viral anti-apoptotic functions with the help of c-Myc are indispensable for the origin of malignant tumours (BL, HD) from the germinal centre B-cell. In the second scenario expression of the main viral transforming protein EBNA2 is essential for immortalisation and non-malignant morphological transformation of any (germinal centre derived or non-germinal centre) B-cell in the absence of T cell control. Although EBNA2 expression is permissible, under specific circumstances, in malignant B-cells, it is not required for oncogenesis.

Host cell-dependent expression of latent Epstein-Barr virus genomes: regulation by DNA methylation

Epstein-Barr virus (EBV) is a ubiquitous human gammaherpesvirus associated with a wide spectrum of malignant neoplasms. Expression of latent (growth transformation-associated) EBV genes is host cell specific. Transcripts for EBV-encoded nuclear antigens (EBNAs) are initiated at one of the alternative promoters: Wp, Cp (for EBNA1-6), or Qp (for EBNA1 only). Wp is active shortly after EBV infection of human B cells in vitro but is progressively methylated and silenced in established lymphoblastoid cell lines (LCLs). In parallel Cp, an unmethylated, lymphoid-specific promoter is switched on. In contrast, Cp is methylated and silent in Burkitt's lymphoma (BL) cell lines, which keep the phenotype of BL biopsy cells (group I BL lines). These cells use Qp for the initiation of EBNA1 messages. Qp is unmethylated both in group I BLs (Qp on) and in LCLs (Qp off). Thus, DNA methylation does not play a role in silencing Qp. In LCLs and nasopharyngeal carcinoma (NPC) cells, transcripts for latent membrane protein 1 (LMP1) are initiated from LMP1p, a promoter regulated by CpG methylation. LMPlp is silent in group I BL lines but can be activated by demethylating agents. Promoter silencing by CpG methylation involves both direct interference with transcription factor binding (Wp, Cp) and indirect mechanisms involving the recruitment of histone deacetylases (LMPlp). A dyad symmetry sequence(DS) within oriP (the latent origin of EBV replication) and intragenic RNA polymerase III control regions of EBER 1 and 2 transcription units are invariably unmethylated in EBV-carrying cells.

Telomere repeat binding factors TRF1, TRF2, and hRAP1 modulate replication of Epstein-Barr virus OriP

Epstein-Barr virus OriP confers cell cycle-dependent DNA replication and stable maintenance on plasmids in EBNA1-positive cells. The dyad symmetry region of OriP contains four EBNA1 binding sites that are punctuated by 9-bp repeats referred to as nonamers. Previous work has shown that the nonamers bind to cellular factors associated with human telomeres and contribute to episomal maintenance of OriP. In this work, we show that substitution mutation of all three nonamer sites reduces both DNA replication and plasmid maintenance of OriP-containing plasmids by 2.5- to 5-fold. The nonamers were required for high-affinity binding of TRF1, TRF2, and hRap1 to the dyad symmetry element but were not essential for the binding of EBNA1 as determined by DNA affinity purification from nuclear extracts. Chromatin immunoprecipitation assays indicated that TRF1, TRF2, and hRap1 bound OriP in vivo. Cell cycle studies indicate that TRF2 binding to OriP peaks in G(1)/S while TRF1 binding peaks in G(2)/M. OriP replication was inhibited by transfection of full-length TRF1 but not by deletion mutants lacking the myb DNA binding domain. In contrast, OriP replication was not affected by transfection of full-length TRF2 or hRap1 but was potently inhibited by dominant-negative TRF2 or hRap1 amino-terminal truncation mutants. Knockdown experiments with short interfering RNAs (siRNAs) directed against TRF2 and hRap1 severely reduced OriP replication, while TRF1 siRNA had a modest stimulatory effect on OriP replication. These results indicate that TRF2 and hRap1 promote, while TRF1 antagonizes, OriP-dependent DNA replication and suggest that these telomeric factors contribute to the establishment of replication competence at OriP.

Intravascular naked DNA vaccine encoding glycoprotein B induces protective humoral and cellular immunity against herpes simplex virus type 1 infection in mice

Naked plasmid DNA (pDNA) vaccine expressing herpes simplex virus type 1 (HSV-1) glycoprotein B (gB) was tested for protective activity against acute HSV-1 infection in mice. The pDNA was intravenously injected into Balb/c mice via their tail vein under high pressure, and the vaccination was performed two times at an interval of 7 days. The gB gene vaccination significantly protected the mice from subsequent intraperitoneal challenge with a lethal dose of HSV-1, which killed all the animals given control plasmid or saline. The protective activity was correlated with the dose of the plasmid inoculated, the survival rate reaching 83% in mice vaccinated with 5 microg of pDNA. The vaccinated mice were also protected from latent HSV infection. The immunized mice showed significant elevation in neutralizing antibody against HSV-1 as well as serum levels of interleukin-12 (IL-12) and interferon-gamma (IFN-gamma). When mice were immunized with 5 microg of an Epstein-Barr virus (EBV)-based plasmid vector harboring the gB, the cytotoxic T lymphocytes (CTLs) activity and proliferative response for HSV-1 were also induced. The results strongly suggest that intravenous immunization of naked pDNA may induce humoral and cellular immune responses against the virus, leading to a significant prophylactic outcome against HSV-1 infection in mice.

Electrochemo-gene therapy of cancer: intratumoral delivery of interleukin-12 gene and bleomycin synergistically induced therapeutic immunity and suppressed subcutaneous and metastatic melanomas in mice

To treat established melanoma in mice, intratumoral transfer of bleomycin and/or an interleukin (IL)-12 expression vector was performed by means of electroporation. Although either bleomycin alone or the IL12 gene alone significantly suppressed the subcutaneous tumors, the combination therapy drastically improved the therapeutic outcome. Three of eight mice (37.5%) that received both bleomycin and the IL12 gene showed complete remission of the preestablished tumors and rejected subsequent rechallenge with the tumor cells. We also examined whether electrochemo-gene therapy for subcutaneous tumor mass induced suppression of pulmonary metastasis that had been established by intravenous inoculation of the melanoma cells. Although metastatic foci were significantly reduced in number in groups that were given IL12 gene alone or bleomycin plus IL12 gene, it was only the combination therapy that significantly prolonged the mean survival period of the tumor-bearing animals. Natural killer (NK) and cytotoxic T lymphocyte cytolytic activities were markedly enhanced in the mice that received the chemo-gene therapy, while IL12 gene therapy alone partially elevated the NK cytotoxicity. The present study suggests that the electroporation-mediated delivery of the IL12 gene and bleomycin synergistically elicits innate and adaptive anti-melanoma immune responses, resulting in marked suppression of the treated tumors as well as bystander metastatic lesions.

Interaction of EBV latent origin of replication with the nuclear matrix: identification of S/MAR sequences and protein components

During latency, Epstein Barr virus (EBV) genome, as an episome, is attached to the nuclear matrix (NM) via the latent origin of replication ori P. Within this element, we have found that a region, 580 bp long, encompassing the replicator DS element, shows the strongest affinity for the NM. In addition, by cross-linking with cis-diamminedichloroplatinum, we have identified two NM proteins with an apparent molecular weight of 85 and 60 kDa that, with high affinity and specificity, bind ori P. These proteins are not induced by EBV infection, but their interaction with ori P is lost upon induction of EBV lytic cycle. These data strongly suggest that the binding of ori P to specific components of the NM is required for EBV latent replication.

Successful genetic transduction in vivo into synovium by means of electroporation

This present study aims at establishing a novel in vivo gene delivery system for intra-articular tissues. Plasmid DNA (pDNA) carrying the firefly luciferase or enhanced green fluorescent protein (EGFP) genes as markers was injected into a joint space and electric stimuli were given percutaneously with a pair of electrodes. Injection with naked pDNA alone did not induce any detectable level of luciferase activity, whereas electroporation at 25-500 V/0.7 cm resulted in a significant expression of the marker gene in the synovium. The expression level depended on the voltage, the optimum transfection being achieved at 150 V/0.7 cm. When the Epstein-Barr virus (EBV)-based plasmid vectors harboring the EBV nuclear antigen 1 (EBNA1) gene and oriP sequence were substituted for conventional pDNA, the transfection efficiency was increased approximately 5-10 times. Histological examination of the EGFP gene-transfected joints revealed that the marker gene was expressed in the synovial membrane while other intra-articular tissues such as articular cartilage were negative for the transgene product. Transgene-specific mRNA was demonstrated in synovium but not in other organs as estimated by RT-PCR analysis. The present results strongly suggest that in vivo electroporation is a quite simple, safe, and effective gene delivery method that could be applicable to gene therapy against articular diseases.

Replication licensing of the EBV oriP minichromosome

The latent EBV genome may persist in the integrated form as well as the circular episomal form. However, most of the latent viral DNA molecules are known to exist in the circular episomal form, which binds to host chromosomes during mitosis. The DS element of oriP in the circular episomal DNA functions as a replication origin. As it replicates once in a single S phase, it is possible that oriP is regulated by the cellular replication licensing mechanism including the MCM family of replication licensing factors. Transient replication analysis using the oriP plasmid and HeLa/EB1 cells revealed that the DS element requires early G1 phase for the next round of replication, the same cell-cycle window in which the replication licensing of cellular chromatin occurs. After this phase, the sedimentation velocity of the oriP minichromosome increases. MCM2 associates with the oriP minichromosome at late G1 but not at G2/M, and this association requires the DS element in the plasmid. The interaction of EBNA1 and the MCM proteins on the DS element was also suggested. These results suggested that the cellular licensing mechanism controls the replication from oriP. This also suggested a similarity in the replication machinery of the cellular chromatin and the latent EBV genome. In addition to DS-dependent replication, the EBV genome replicates in a manner independent of the DS element in several cultured cell lines. The DS-dependent replication is likely to be suppressed in these cell lines by the expression of other viral proteins. In contrast, EBV-positive Burkitt's lymphoma and circulating EBV-infected B cells express only EBNA1 or both EBNA1 and LMP2. DS-dependent replication may play a major role in these EBNA1-only cells, and the licensing regulation of oriP is important for maintenance of the EBV genome during this latent period of the viral life cycle. EBNA1 is required for efficient nuclear retention and partitioning of oriP-carrying plasmid by its binding to the FR element, thus providing stable persistence of the latent EBV genome during cell division. The copy number of latent EBV DNA molecules in B-cell lines remains fairly constant during multiple passage in culture. However, very little is known about the mechanism by which the viral DNA molecules are equally segregated into daughter cells. To understand the mechanisms responsible for stable nuclear retention and partitioning of the latent viral genome, it is essential to analyze the episomal and integrated viral DNAs at a single-cell level by FISH and other techniques.

Telomeric proteins regulate episomal maintenance of Epstein-Barr virus origin of plasmid replication

Episomal maintenance and DNA replication of EBV origin of plasmid replication (OriP) plasmid maintenance is mediated by the viral encoded origin binding protein, EBNA1, and unknown cellular factors. We found that telomeric repeat binding factor 2 (TRF2), TRF2-interacting protein hRap1, and the telomere-associated poly(ADP-ribose) polymerase (Tankyrase) bound to the dyad symmetry (DS) element of OriP in an EBNA1-dependent manner. TRF2 bound cooperatively with EBNA1 to the three nonamer sites (TTAGGGTTA), which resemble telomeric repeats. Mutagenesis of the nonamers reduced plasmid maintenance function and increased plasmid sensitivity to genotoxic stress. DS affinity-purified proteins possessed poly(ADP-ribose) polymerase (PARP) activity, and EBNA1 was subject to NAD-dependent posttranslational modification in vitro. OriP plasmid maintenance was sensitive to changes in cellular PARP/Tankyrase activity. These findings imply that telomere-associated proteins regulate OriP plasmid maintenance by PAR-dependent modifications.

Polyethylenimine-mediated suicide gene transfer induces a therapeutic effect for hepatocellular carcinoma in vivo by using an Epstein-Barr virus-based plasmid vector

The present study aimed to establish a novel efficient nonviral strategy for suicide gene transfer in hepatocellular carcinoma (HCC) in vivo. We employed branched polyethylenimine (PEI) and combined it with Epstein-Barr virus (EBV)-based plasmid vectors. The HCC cells transfected with an EBV-based plasmid carrying the herpes simplex virus-1 thymidine kinase (HSV-1 Tk) gene (pSES.Tk) showed up to 30-fold higher susceptibilities to ganciclovir (GCV) than those transfected with a conventional plasmid vector carrying the HSV-1 Tk gene (pS.Tk). The therapeutic effect in vivo was tested by intratumoral injection of the plasmids into HuH-7 hepatomas transplanted into C.B-17 scid/scid mutant (SCID) mice and subsequent GCV administrations. Treatment with pSES.Tk, but not pS.Tk, markedly suppressed growth of hepatomas in vivo, resulting in a significantly prolonged survival period of the mice. These findings suggest that PEI-mediated gene transfer system can confer efficient expression of the suicide gene in HCC cells in vivo by using EBV-based plasmid vectors.

Sequences adjacent to oriP improve the persistence of Epstein-Barr virus-based episomes in B cells

Epstein-Barr virus (EBV) oriP and the EBV nuclear antigen 1 (EBNA-1) protein allow persistence of EBV-based episomes. A nuclear matrix attachment region (MAR) spans oriP and the adjacent region of the EBV genome containing the EBV-expressed RNAs. Here, we show that episomes with the MAR are retained significantly more efficiently in EBV-positive B cells than episomes containing oriP alone.

Highly efficient gene transfer into murine liver achieved by intravenous administration of naked Epstein-Barr virus (EBV)-based plasmid vectors

Naked plasmid DNA (pDNA) injection could become an alternative procedure to viral and nonviral gene delivery systems. We have previously shown that Epstein-Barr virus (EBV)-based plasmid vectors containing the EBV nuclear antigen 1 (EBNA1) gene and the oriP sequence enable quite high and long-lasting expression in various in vitro and in vivo transfection systems. The EBV-based plasmids were intravenously injected into mice via their tail vein under high pressure. A large amount of the marker gene product was expressed in the liver; as much as 320 microg of luciferase was demonstrated per gram of liver at 8 to 24 h after a single injection with 10 microg of DNA. More than 70% of liver cells stained with X-gal when beta-gal gene was transferred. The expression level was significantly higher than that obtained by conventional pDNA lacking the EBNA1 gene and oriP. On day 35 after the transfection, the expression from the EBV-based plasmid was approximately 100-fold stronger than the conventional pDNA gene expression. Both the EBNA1 gene and oriP are a prerequisite for the augmentation of the transfection efficiency. These results suggest that the intravascular transfection with naked EBV-based plasmid may provide a quite efficient, simple and convenient means to transduce therapeutic genes in vivo into the liver.

In vivo electroporation-mediated transfer of interleukin-12 and interleukin-18 genes induces significant antitumor effects against melanoma in mice

Direct intratumoral transfection of cytokine genes was performed by means of the in vivo electroporation as a novel therapeutic strategy for cancer. Plasmid vectors carrying the firefly luciferase, interleukin (IL)-12 and IL-18 genes were injected into established subcutaneous B16-derived melanomas followed by electric pulsation. When plasmid vectors with Epstein--Barr virus (EBV) nuclear antigen 1 (EBNA1) gene were employed, the expression levels of the transgenes were significantly higher in comparison with those obtained with conventional plasmid vectors. In consequence of the transfection with IL-12 and IL-18 genes, serum concentrations of the cytokines were significantly elevated, while interferon (IFN)-gamma also increased in the sera of the animals. The IL-12 gene transfection resulted in significant suppression of tumor growth, while the therapeutic effect was further improved by co-transfection with IL-12 and IL-18 genes. Repetitive co-transfection with IL-12 and IL-18 genes resulted in significant prolongation of survival of the animals. Natural killer (NK) and cytotoxic T lymphocyte (CTL) activities were markedly enhanced in the mice transfected with the cytokine genes. The present data suggest that the cytokine gene transfer can be successfully achieved by in vivo electroporation, leading to both specific and nonspecific antitumoral immune responses and significant therapeutic outcome.

Variant chromatin structure of the oriP region of Epstein-Barr virus and regulation of EBER1 expression by upstream sequences and oriP

Most of the Epstein-Barr virus genome in latently infected cells is in a standard nucleosomal structure, but the region encompassing oriP and the Epstein-Barr virus-encoded small RNA (EBER) genes shows a distinctive pattern when digested with micrococcal nuclease. This pattern corresponds to a previously mapped nuclear matrix attachment region. Although the EBER genes are adjacent to oriP, there is only a two- to fourfold effect of oriP on EBER expression. However, sequences containing a consensus ATF site upstream of EBER1 are important for EBER1 expression.

Activation of TRAF5 and TRAF6 signal cascades negatively regulates the latent replication origin of Epstein-Barr virus through p38 mitogen-activated protein kinase

Latent Epstein-Barr virus (EBV) is maintained by the virus replication origin oriP that initiates DNA replication with the viral oriP-binding factor EBNA1. However, it is not known whether oriP's replicator activity is regulated by virus proteins or extracellular signals. By using a transient replication assay, we found that a low level of expression of viral signal transduction activator latent membrane protein 1 (LMP1) suppressed oriP activity. The binding site of the tumor necrosis factor receptor-associated factor (TRAF) of LMP1 was essential for this suppressive effect. Activation of the TRAF signal cascade by overexpression of TRAF5 and/or TRAF6 also suppressed oriP activity. Conversely, blocking of TRAF signaling with dominant negative mutants of TRAF5 and TRAF6, as well as inhibition of a downstream signal mediator p38 MAPK, released the LMP1-induced oriP suppression. Furthermore, activation of TRAF6 signal cascade by lipopolysaccharides (LPS) resulted in loss of EBV from Burkitt's lymphoma cell line Akata, and inhibition of p38 MAPK abolished the suppressive effect of LPS. These results suggested that the level of oriP activity is regulated by LMP1 and extracellular signals through TRAF5- and TRAF6-mediated signal cascades.

Expression and replication timing patterns of wildtype and translocated BCL2 genes

Translocation of the BCL2 gene from chromosome 18 to chromosome 14 results in constitutive expression of the gene. We have recently demonstrated that the major breakpoint region (mbr) of BCL2, which is implicated in 70% of t(14;18) translocations present in human follicular lymphoma, is a matrix attachment region. Since these regions are implicated in control of both transcription and replication, we wished to determine whether BCL2 translocation was also accompanied by changes in replication timing of the translocated allele. Using both fluorescence in situ hybridization and allele-specific PCR, we have demonstrated that the translocated allele replicates at the G1/S boundary, while the wildtype allele continues to replicate as usual in mid-S phase. These differences are accompanied by allele-specific changes in BCL2 expression. Since the net structural effect of t(14;18) translocations within the mbr is to disrupt the BCL2 MAR and replace it with the IGH MARs located just downstream of each breakpoint, we conclude that MAR exchange is a significant, selectable outcome of these translocations. We propose that subsequent changes of replication and transcriptional patterns for the translocated BCL2 allele result from this exchange and represent important early steps in lymphomagenesis.

Prolonged transgene expression in glomeruli using an EBV replicon vector system combined with HVJ liposomes

BACKGROUND

Various gene transfer vectors as well as delivery systems have been developed; however, many problems remain to be solved. We already achieved a technique to introduce genes into glomerular mesangial cells by hemagglutinating virus of Japan (HVJ) liposome-mediated gene transfer via renal artery. The main limitation of this method is the transient transgene expression.

METHOD

For long-term gene expression in glomeruli, Epstein-Barr virus (EBV) replicon-based plasmid was employed, containing the latent viral DNA replication origin (oriP) and EBV nuclear antigen-1 (EBNA-1), which are the minimum EBV component of transgene-nuclear retention. To examine the effect of EBV replicon apparatus on the duration of transgene expression in glomeruli in vivo, the EBV replicon vector pEBActLuc, and the control plasmid vector pActLuc were adopted. These plasmid vectors were transferred into the kidney via renal artery by using artificial viral envelope (AVE)-type HVJ liposome method, and glomerular luciferase activities were analyzed at various time points after transfection.

RESULTS

On day 4, pEBActLuc and pActLuc transfer resulted in equal glomerular luciferase activity, and the luciferase gene expression was sustained for at least 56 days in glomeruli transfected with pEBActLuc, whereas it was reduced on seven days in glomeruli transfected with pActLuc.

CONCLUSION

The combination of EBV replicon apparatus and HVJ liposomes appears to be a powerful tool for long-term gene expression in vivo, and furthermore, it may be a promising new therapeutic method for the progression of renal disease.

Non-replicating Epstein-Barr virus-based plasmids extend gene expression and can improve gene therapy in vivo

To date, no gene transfer vector has produced prolonged gene expression following a single intravenous injection and then efficiently re-expressed the delivered gene following repeated systemic injection into immunocompetent hosts. To overcome these limitations, a gene therapy regimen using non-replicating Epstein-Barr virus (EBV)-based expression plasmids was developed. One plasmid contains the FR (EBV family of repeats) sequence and the expressed gene. The other encodes Epstein-Barr nuclear antigen 1 (EBNA-1), but lacks FR. Although unable to replicate in mice, intravenous co-injection of EBV-based plasmids in cationic liposome-DNA complexes (CLDCs) substantially prolonged luciferase gene expression. The use of a two-vector system limited host exposure to the EBNA-1 gene product. Furthermore, this EBV-based vector system could be intravenously re-injected multiple times into immunocompetent mice without loss of transfection efficiency. Use of this vector system significantly improved the therapeutic efficacy of the biologically important human granulocyte colony-stimulating factor gene. Delivery of the human granulocyte colony-stimulating factor gene in EBV-based plasmids increased circulating white blood counts for at least 2 months following a single CLDC-based intravenous co-injection. Conversely, white blood counts were never elevated following injection of CLDCs lacking EBV-derived elements. Thus, this EBV-based plasmid vector system both markedly prolongs gene expression at therapeutic levels and efficiently and repeatedly re-transfects immunocompetent hosts. These properties of EBV-based plasmid vectors appear to be due, at least in part, to the documented abilities of the EBNA-1 protein both to retain FR-containing DNA intracellularly and within the nucleus and to block anti-EBNA-1 cytotoxic T cell responses.

Episomal vectors for gene expression in mammalian cells

An important reason for preferring mammalian cells for heterologous gene expression is their ability to make authentic proteins containing post-translational modifications similar to those of the native protein. The development of expression systems for mammalian cells has been ongoing for several years, resulting in a wide variety of effective expression vectors. The aim of this review is to highlight episomal expression vectors. Such episomal plasmids are usually based on sequences from DNA viruses, such as BK virus, bovine papilloma virus 1 and Epstein-Barr virus. In this review we will mainly focus on the improvements made towards the usefulness of these systems for gene expression studies and gene therapy.

Long-term replication of Epstein-Barr virus-derived episomal vectors in the rodent cells

Plasmids containing the origin of replication, oriP, of the Epstein-Barr virus (EBV) and EBV nuclear antigen-1 genes replicate extrachromosomally in primate cells. However, these plasmids have been believed not to replicate in rodent cells. We demonstrate here that these plasmids can replicate in some types of rodent cells over a long period. This result should offer not only the new insight into the mechanisms of species-specific replication of EBV, but also the possibility that an EBV-based vector can be used for gene transfer experiments in non-primate cells and an animal experiment regarding human gene therapy.

De novo DNA methylation at nonrandom founder sites 5' from an unmethylated minimal origin of DNA replication in latent Epstein-Barr virus genomes

Latent episomal genomes of Epstein-Barr virus, a human gammaherpesvirus, represent a suitable model system for studying replication and methylation of chromosomal DNA in mammals. We analyzed the methylation patterns of CpG dinucleotides in the latent origin of DNA replication of Epstein-Barr virus using automated fluorescent genomic sequencing of bisulfite-modified DNA samples. We observed that the minimal origin of DNA replication was unmethylated in 8 well-characterized human cell lines or clones carrying latent Epstein-Barr virus genomes as well as in a prototype virus producer marmoset cell line. This observation suggests that unmethylated DNA domains can function as initiation sites or zones of DNA replication in human cells. Furthermore, 5' from this unmethylated region we observed focal points of de novo DNA methylation in nonrandom positions in the majority of Burkitt's lymphoma cell lines and clones studied while the corresponding CpG dinucleotides in viral genomes carried by lymphoblastoid cell lines and marmoset cells were completely unmethylated. Clustering of highly methylated CpG dinucleotides suggests that de novo methylation of unmethylated double-stranded episomal viral genomes starts at discrete founder sites in vivo. This is the first comparative high-resolution methylation analysis of a latent viral origin of DNA replication in human cells.