The dyad symmetry element of Epstein-Barr virus is a dominant but dispensable replication origin

Episomes and Transposases-Utilities to Maintain Transgene Expression from Nonviral Vectors

The efficient delivery and stable transgene expression are critical for applications in gene therapy. While carefully selected and engineered viral vectors allowed for remarkable clinical successes, they still bear significant safety risks. Thus, nonviral vectors are a sound alternative and avoid genotoxicity and adverse immunological reactions. Nonviral vector systems have been extensively studied and refined during the last decades. Emerging knowledge of the epigenetic regulation of replication and spatial chromatin organisation, as well as new technologies, such as Crispr/Cas, were employed to enhance the performance of different nonviral vector systems. Thus, nonviral vectors are in focus and hold some promising perspectives for future applications in gene therapy. This review addresses three prominent nonviral vector systems: the Sleeping Beauty transposase, S/MAR-based episomes, and viral plasmid replicon-based EBV vectors. Exemplarily, we review different utilities, modifications, and new concepts that were pursued to overcome limitations regarding stable transgene expression and mitotic stability. New insights into the nuclear localisation of nonviral vector molecules and the potential consequences thereof are highlighted. Finally, we discuss the remaining limitations and provide an outlook on possible future developments in nonviral vector technology.

Cell-cycle-dependent EBNA1-DNA crosslinking promotes replication termination at oriP and viral episome maintenance

Epstein-Barr virus (EBV) is an oncogenic human herpesvirus that persists as a multicopy episome in proliferating host cells. Episome maintenance is strictly dependent on EBNA1, a sequence-specific DNA-binding protein with no known enzymatic activities. Here, we show that EBNA1 forms a cell cycle-dependent DNA crosslink with the EBV origin of plasmid replication oriP. EBNA1 tyrosine 518 (Y518) is essential for crosslinking to oriP and functionally required for episome maintenance and generation of EBV-transformed lymphoblastoid cell lines (LCLs). Mechanistically, Y518 is required for replication fork termination at oriP in vivo and for formation of SDS-resistant complexes in vitro. EBNA1-DNA crosslinking corresponds to single-strand endonuclease activity specific to DNA structures enriched at replication-termination sites, such as 4-way junctions. These findings reveal that EBNA1 forms tyrosine-dependent DNA-protein crosslinks and single-strand cleavage at oriP required for replication termination and viral episome maintenance.

Control of Viral Latency by Episome Maintenance Proteins

The human DNA tumor viruses Epstein-Barr virus (EBV), Kaposi's sarcoma-associated herpesvirus (KSHV), and human papillomavirus (HPV) share the common property of persisting as multicopy episomes in the nuclei of rapidly dividing host cells. These episomes form the molecular basis for viral latency and are etiologically linked to virus-associated cancers. Episome maintenance requires epigenetic programming to ensure the proper control of viral gene expression, DNA replication, and genome copy number. For these viruses, episome maintenance requires a dedicated virus-encoded episome maintenance protein (EMP), namely LANA (KSHV), EBNA1 (EBV), and E2 (HPV). Here, we review common features of these viral EMPs and discuss recent advances in understanding how they contribute to the epigenetic control of viral episome maintenance during latency.

Hitchhiking of Viral Genomes on Cellular Chromosomes

Persistent viral infections require a host cell reservoir that maintains functional copies of the viral genome. To this end, several DNA viruses maintain their genomes as extrachromosomal DNA minichromosomes in actively dividing cells. These viruses typically encode a viral protein that binds specifically to viral DNA genomes and tethers them to host mitotic chromosomes, thus enabling the viral genomes to hitchhike or piggyback into daughter cells. Viruses that use this tethering mechanism include papillomaviruses and the gammaherpesviruses Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus. This review describes the advantages and consequences of persistent extrachromosomal viral genome replication.

Bacterial artificial chromosomes establish replication timing and sub-nuclear compartment de novo as extra-chromosomal vectors

The role of DNA sequence in determining replication timing (RT) and chromatin higher order organization remains elusive. To address this question, we have developed an extra-chromosomal replication system (E-BACs) consisting of ∼200 kb human bacterial artificial chromosomes (BACs) modified with Epstein-Barr virus (EBV) stable segregation elements. E-BACs were stably maintained as autonomous mini-chromosomes in EBNA1-expressing HeLa or human induced pluripotent stem cells (hiPSCs) and established distinct RT patterns. An E-BAC harboring an early replicating chromosomal region replicated early during S phase, while E-BACs derived from RT transition regions (TTRs) and late replicating regions replicated in mid to late S phase. Analysis of E-BAC interactions with cellular chromatin (4C-seq) revealed that the early replicating E-BAC interacted broadly throughout the genome and preferentially with the early replicating compartment of the nucleus. In contrast, mid- to late-replicating E-BACs interacted with more specific late replicating chromosomal segments, some of which were shared between different E-BACs. Together, we describe a versatile system in which to study the structure and function of chromosomal segments that are stably maintained separately from the influence of cellular chromosome context.

Chromatin Structure of Epstein-Barr Virus Latent Episomes

EBV latent infection is characterized by a highly restricted pattern of viral gene expression. EBV can establish latent infections in multiple different tissue types with remarkable variation and plasticity in viral transcription and replication. During latency, the viral genome persists as a multi-copy episome, a non-integrated-closed circular DNA with nucleosome structure similar to cellular chromosomes. Chromatin assembly and histone modifications contribute to the regulation of viral gene expression, DNA replication, and episome persistence during latency. This review focuses on how EBV latency is regulated by chromatin and its associated processes.

Keeping it quiet: chromatin control of gammaherpesvirus latency

The human gammaherpesviruses Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV) establish long-term latent infections associated with diverse human cancers. Viral oncogenesis depends on the ability of the latent viral genome to persist in host nuclei as episomes that express a restricted yet dynamic pattern of viral genes. Multiple epigenetic events control viral episome generation and maintenance. This Review highlights some of the recent findings on the role of chromatin assembly, histone and DNA modifications, and higher-order chromosome structures that enable gammaherpesviruses to establish stable latent infections that mediate viral pathogenesis.

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.

Identification of a novel human papillomavirus by metagenomic analysis of samples from patients with febrile respiratory illness

As part of a virus discovery investigation using a metagenomic approach, a highly divergent novel Human papillomavirus type was identified in pooled convenience nasal/oropharyngeal swab samples collected from patients with febrile respiratory illness. Phylogenetic analysis of the whole genome and the L1 gene reveals that the new HPV identified in this study clusters with previously described gamma papillomaviruses, sharing only 61.1% (whole genome) and 63.1% (L1) sequence identity with its closest relative in the Papillomavirus episteme (PAVE) database. This new virus was named HPV_SD2 pending official classification. The complete genome of HPV-SD2 is 7,299 bp long (36.3% G/C) and contains 7 open reading frames (L2, L1, E6, E7, E1, E2 and E4) and a non-coding long control region (LCR) between L1 and E6. The metagenomic procedures, coupled with the bioinformatic methods described herein are well suited to detect small circular genomes such as those of human papillomaviruses.

Timeless-dependent DNA replication-coupled recombination promotes Kaposi's Sarcoma-associated herpesvirus episome maintenance and terminal repeat stability

Kaposi's Sarcoma-associated herpesvirus (KSHV) is maintained as a stable episome in latently infected pleural effusion lymphoma (PEL) cells. Episome maintenance is conferred by the binding of the KSHV-encoded LANA protein to the viral terminal repeats (TR). Here, we show that DNA replication in the KSHV TR is coupled with DNA recombination and mediated in part through the cellular replication fork protection factors Timeless (Tim) and Tipin. We show by two-dimensional (2D) agarose gel electrophoresis that replication forks naturally stall and form recombination-like structures at the TR during an unperturbed cell cycle. Chromatin immunoprecipitation (ChIP) assays revealed that Tim and Tipin are selectively enriched at the KSHV TR during S phase and in a LANA-dependent manner. Tim depletion inhibited LANA-dependent TR DNA replication and caused the loss of KSHV episomes from latently infected PEL cells. Tim depletion resulted in the aberrant accumulation of recombination structures and arrested MCM helicase at TR. Tim depletion did not induce the KSHV lytic cycle or apoptotic cell death. We propose that KSHV episome maintenance requires Tim-assisted replication fork protection at the viral terminal repeats and that Tim-dependent recombination-like structures form at TR to promote DNA repeat stability and viral genome maintenance.

EBNA1 and host factors in Epstein-Barr virus latent DNA replication

Epstein-Barr virus episomes (EBV) replicate once per cell cycle during latent infection from the latent origin, oriP. This replication requires the viral EBNA1 protein, which specifically recognizes sequences in oriP and recruits cellular proteins to this origin. Replication from oriP requires the cellular origin recognition and MCM helicase complexes and also involves telomeric factors (including TRF2) that associate with repeated nonameric sequences at the origin. Replication from oriP occurs late in S-phase and this timing appears to be important for efficient replication. Replication from oriP has proven to be a valuable system for elucidating cellular proteins and mechanisms of origin activation.

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.