Inhibition of DNA repair by a herpes simplex virus vector enhances the radiosensitivity of human glioblastoma cells

The Current Landscape of Oncolytic Herpes Simplex Viruses as Novel Therapies for Brain Malignancies

Despite advances in surgical resection and chemoradiation, high-grade brain tumors continue to be associated with significant morbidity/mortality. Novel therapeutic strategies and approaches are, therefore, desperately needed for patients and their families. Given the success experienced in treating multiple other forms of cancer, immunotherapy and, in particular, immunovirotherapy are at the forefront amongst novel therapeutic strategies that are currently under investigation for incurable brain tumors. Accordingly, herein, we provide a focused mini review of pertinent oncolytic herpes viruses (oHSV) that are being investigated in clinical trials.

The Oncolytic Virus in Cancer Diagnosis and Treatment

Cancer has always been an enormous threat to human health and survival. Surgery, radiotherapy, and chemotherapy could improve the survival of cancer patients, but most patients with advanced cancer usually have a poor survival or could not afford the high cost of chemotherapy. The emergence of oncolytic viruses provided a new strategy for us to alleviate or even cure malignant tumors. An oncolytic virus can be described as a genetically engineered or naturally existing virus that can selectively replicate in cancer cells and then kill them without damaging the healthy cells. There have been many kinds of oncolytic viruses, such as herpes simplex virus, adenovirus, and Coxsackievirus. Moreover, they have different clinical applications in cancer treatment. This review focused on the clinical application of oncolytic virus and predicted the prospect by analyzing the advantages and disadvantages of oncolytic virotherapy.

Oncolytic herpes simplex virus-based strategies: toward a breakthrough in glioblastoma therapy

Oncolytic viruses (OV) are a class of antitumor agents that selectively kill tumor cells while sparing normal cells. Oncolytic herpes simplex virus (oHSV) has been investigated in clinical trials for patients with the malignant brain tumor glioblastoma for more than a decade. These clinical studies have shown the safety of oHSV administration to the human brain, however, therapeutic efficacy of oHSV as a single treatment remains unsatisfactory. Factors that could hamper the anti-glioblastoma efficacy of oHSV include: attenuated potency of oHSV due to deletion or mutation of viral genes involved in virulence, restricting viral replication and spread within the tumor; suboptimal oHSV delivery associated with intratumoral injection; virus infection-induced inflammatory and cellular immune responses which could inhibit oHSV replication and promote its clearance; lack of effective incorporation of oHSV into standard-of-care, and poor knowledge about the ability of oHSV to target glioblastoma stem cells (GSCs). In an attempt to address these issues, recent research efforts have been directed at: (1) design of new engineered viruses to enhance potency, (2) better understanding of the role of the cellular immunity elicited by oHSV infection of tumors, (3) combinatorial strategies with different antitumor agents with a mechanistic rationale, (4) “armed” viruses expressing therapeutic transgenes, (5) use of GSC-derived models in oHSV evaluation, and (6) combinations of these. In this review, we will describe the current status of oHSV clinical trials for glioblastoma, and discuss recent research advances and future directions toward successful oHSV-based therapy of glioblastoma.

Pediatric glioma stem cells: biologic strategies for oncolytic HSV virotherapy

While glioblastoma multiforme (GBM) is the most common adult malignant brain tumor, GBMs in childhood represent less than 10% of pediatric malignant brain tumors and are phenotypically and molecularly distinct from adult GBMs. Similar to adult patients, outcomes for children with high-grade gliomas (HGGs) remain poor. Furthermore, the significant morbidity and mortality yielded by pediatric GBM is compounded by neurotoxicity for the developing brain caused by current therapies. Poor outcomes have been attributed to a subpopulation of chemotherapy and radiotherapy resistant cells, termed “glioma stem cells” (GSCs), “glioma progenitor cells,” or “glioma-initiating cells,” which have the ability to initiate and maintain the tumor and to repopulate the recurring tumor after conventional therapy. Future innovative therapies for pediatric HGG must be able to eradicate these therapy-resistant GSCs. Oncolytic herpes simplex viruses (oHSV), genetically engineered to be safe for normal cells and to express diverse foreign anti-tumor therapeutic genes, have been demonstrated in preclinical studies to infect and kill GSCs and tumor cells equally while sparing normal brain cells. In this review, we discuss the unique aspects of pediatric GSCs, including markers to identify them, the microenvironment they reside in, signaling pathways that regulate them, mechanisms of cellular resistance, and approaches to target GSCs, with a focus on the promising therapeutic, genetically engineered oHSV.

The roles of viruses in brain tumor initiation and oncomodulation

While some avian retroviruses have been shown to induce gliomas in animal models, human herpesviruses, specifically, the most extensively studied cytomegalovirus, and the much less studied roseolovirus HHV-6, and Herpes simplex viruses 1 and 2, currently attract more and more attention as possible contributing or initiating factors in the development of human brain tumors. The aim of this review is to summarize and highlight the most provoking findings indicating a potential causative link between brain tumors, specifically malignant gliomas, and viruses in the context of the concepts of viral oncomodulation and the tumor stem cell origin.

Oncolytic viruses in radiation oncology

Oncolytic viruses are investigational cancer treatments. They are currently being assessed as single agents or in combination with standard therapies such as external beam radiotherapy - a DNA damaging agent that is a standard of care for many tumour types. Preclinical data indicate that combinations of oncolytic viruses and radiation therapy are promising, showing additional or synergistic antitumour effects in in vitro and in vivo studies. This interaction has the potential to be multifaceted: viruses may act as radiosensitizing agents, but radiation may also enhance viral oncolysis by increasing viral uptake, replication, gene expression and cell death (apoptosis, autophagy or necrosis) in irradiated cells. Phase I and II clinical trials investigating combinations of viruses and radiation therapy have been completed, paving the way for ongoing phase III studies. The aim of this review is to focus on the therapeutic potential of these combinations and to highlight their mechanistic bases, with particular emphasis on the role of the DNA damage response.

Increased oncolytic efficacy for high-grade gliomas by optimal integration of ionizing radiation into the replicative cycle of HSV-1

Oncolytic viruses have been combined with standard cancer therapies to increase therapeutic efficacy. Given the sequential activation of herpes viral genes (herpes simplex virus-1, HSV-1) and the temporal cellular changes induced by ionizing radiation, we hypothesized an optimal temporal sequence existed in combining oncolytic HSV-1 with ionizing radiation. Murine U-87 glioma xenografts were injected with luciferase encoding HSV-1, and ionizing radiation (IR) was given at times before or after viral injection. HSV-1 replication and tumor-volume response were followed. Radiation given 6-9 h after HSV-1 injection resulted in maximal viral luciferase expression and infectious viral production in tumor xenografts. The greatest xenograft regression was also seen with radiation given 6 h after viral injection. We then tested if HSV-1 replication had a dose response to ionizing radiation. HSV-1 luciferase expression exhibited a dose response as xenografts were irradiated from 0 to 5 Gy. There was no difference in viral luciferase expression as IR dose increased from 5 Gy up to 20 Gy. These results suggest that the interaction of IR with the HSV-1 lytic cycle can be manipulated for therapeutic gain by delivering IR at a specific time within viral replicative cycle.

Engineered herpes simplex virus vectors for antitumor therapy and vaccine delivery

Genetically modified herpes simplex viruses (HSVs) have been exploited for both antitumor therapy and vaccine delivery. These mutant viruses retain their ability to replicate and lyse permissive cells, including many tumor types, and are referred to as oncolytic HSVs. In addition, deletion of nonessential genes permits the introduction of foreign genes to augment the antitumor effect by either immune stimulation, targeting for select tumors, or expression of tumor or vaccine antigens. This article reviews the development of oncolytic HSVs as an anticancer therapy, as well as the application of HSV-1 vectors for delivery of targeted antigens or as vaccine adjuvants. The impact of these novel vectors with respect to enhanced antitumor activity and development of antitumor vaccination strategies is discussed.

Genomes in conflict: maintaining genome integrity during virus infection

The cellular surveillance network for sensing and repairing damaged DNA prevents an array of human diseases, and when compromised it can lead to genomic instability and cancer. The carefully maintained cellular response to DNA damage is challenged during viral infection, when foreign DNA is introduced into the cell. The battle between virus and host generates a genomic conflict. The host attempts to limit viral infection and protect its genome, while the virus deploys tactics to eliminate, evade, or exploit aspects of the cellular defense. Studying this conflict has revealed that the cellular DNA damage response machinery comprises part of the intrinsic cellular defense against viral infection. In this review we examine recent advances in this emerging field. We identify common themes used by viruses in their attempts to commandeer or circumvent the host cell's DNA repair machinery, and highlight potential outcomes of the conflict for both virus and host.

A viral E3 ligase targets RNF8 and RNF168 to control histone ubiquitination and DNA damage responses

The ICP0 protein of herpes simplex virus type 1 is an E3 ubiquitin ligase and transactivator required for the efficient switch between latent and lytic infection. As DNA damaging treatments are known to reactivate latent virus, we wished to explore whether ICP0 modulates the cellular response to DNA damage. We report that ICP0 prevents accumulation of repair factors at cellular damage sites, acting between recruitment of the mediator proteins Mdc1 and 53BP1. We identify RNF8 and RNF168, cellular histone ubiquitin ligases responsible for anchoring repair factors at sites of damage, as new targets for ICP0-mediated degradation. By targeting these ligases, ICP0 expression results in loss of ubiquitinated forms of H2A, mobilization of DNA repair proteins and enhanced viral fitness. Our study raises the possibility that the ICP0-mediated control of histone ubiquitination may link DNA repair, relief of transcriptional repression, and activation of latent viral genomes.

Chromatin at the intersection of viral infection and DNA damage

During infection, viruses cause global disruption to nuclear architecture in their attempt to take over the cell. In turn, the host responds with various defenses, which include chromatin-mediated silencing of the viral genome and activation of DNA damage signaling pathways. Dynamic exchanges at chromatin, and specific post-translational modifications on histones have recently emerged as master controllers of DNA damage signaling and repair. Studying viral control of chromatin modifications is identifying histones as important players in the battle between host and virus for control of cell cycle and gene expression. These studies are revealing new complexities of the virus-host interaction, uncovering the potential of chromatin as an anti-viral defense mechanism, and also providing unique insights into the role of chromatin in DNA repair.

Viral manipulation of DNA repair and cell cycle checkpoints

Recognition and repair of DNA damage is critical for maintaining genomic integrity and suppressing tumorigenesis. In eukaryotic cells, the sensing and repair of DNA damage are coordinated with cell cycle progression and checkpoints, in order to prevent the propagation of damaged DNA. The carefully maintained cellular response to DNA damage is challenged by viruses, which produce a large amount of exogenous DNA during infection. Viruses also express proteins that perturb cellular DNA repair and cell cycle pathways, promoting tumorigenesis in their quest for cellular domination. This review presents an overview of strategies employed by viruses to manipulate DNA damage responses and cell cycle checkpoints as they commandeer the cell to maximize their own viral replication. Studies of viruses have identified key cellular regulators and revealed insights into molecular mechanisms governing DNA repair, cell cycle checkpoints, and transformation.

ICP0 inhibits the decrease of HSV amplicon-mediated transgene expression

The herpes simplex virus (HSV) amplicon vector produces an initial host response that limits transgene expression. In this study, we hypothesized that restoration of the HSV gene infected cell protein (ICP0) into the amplicon could circumvent this host response and thus overcome silencing of encoded transgenes. To test this, we constructed an amplicon vector that encodes the ICP0 under control of its native promoter (ICP0+ amplicon). Expression of ICP0 was transient and, at a multiplicity of infection (MOI) of 1, did not significantly alter interferon (IFN)-based responses against the vector or cell kinetics/apoptosis of infected cells. Chromatin immunoprecipitation (ChIP) PCR analysis revealed that conventional amplicon DNA became associated with histone deacetylase 1 (HDAC1) immediately after infection, whereas ICP0+ amplicon DNA remained relatively unbound by HDAC1 for at least 72 hours after infection. Mice administered systemic ICP0+ amplicon exhibited significantly greater and more sustained transgene expression in their livers than did those receiving conventional amplicon, likely due to increased transcriptional or post-transcriptional activity rather than increased copy numbers of vector DNA. These findings indicate that restoration of ICP0 expression may be employed within HSV amplicon constructs to decrease transgene silencing in vitro and in vivo.

Therapeutic efficacy of a herpes simplex virus with radiation or temozolomide for intracranial glioblastoma after convection-enhanced delivery

The herpes simplex virus-1 (HSV-1)-infected cell protein 0 (ICP0) is an E3 ubiquitin ligase implicated in cell cycle arrest and DNA repair inhibition. Convection-enhanced delivery (CED) of either the replication-defective, ICP0-producing HSV-1 mutant, d106, or the recombinant d109, devoid of all viral genome expression, was performed to determine the in vivo efficacy of ICP0 in combination with ionizing radiation (IR) or systemic temozolomide (TMZ) in the treatment of glioblastoma multiforme (GBM). Intracranial U87-MG xenografts were established in athymic nude mice. Animal survival was determined after mice underwent intracranial CED of either the replication-defective d106 or d109 viruses, or Hanks' balanced salt solution (HBSS), before a single session of whole-brain irradiation or TMZ treatment. Median survival for animals that underwent treatment with HBSS alone, d109 alone, d106 alone, HBSS + IR, HBSS + TMZ, d109 + IR, d106 + IR, and d106 + TMZ was 28, 35, 41, 39, 44, 39, 68 (P < 0.01), and 66 days (P < 0.01), respectively. Intracerebral d106 CED resulted in a significant increase in athymic nude mouse survival when combined with IR or TMZ. d106 CED allows for distribution of HSV-1 in human GBM xenografts and persistent viral infection.