Cellular Antisilencing Elements Support Transgene Expression from Herpes Simplex Virus Vectors in the Absence of Immediate Early Gene Expression

Safety of non-replicative and oncolytic replication-selective HSV vectors

Herpes simplex virus type 1 (HSV-1) is a DNA virus and human pathogen used to construct promising therapeutic vectors. HSV-1 vectors fall into two classes: replication-selective oncolytic vectors for cancer therapy and defective non-replicative vectors for gene therapy. Vectors from each class can accommodate ≥30 kb of inserts, have been approved clinically, and demonstrate a relatively benign safety profile. Despite oncolytic HSV (oHSV) replication in tumors and elicited immune responses, the virus is well tolerated in cancer patients. Current non-replicative vectors elicit only limited immune responses. Seropositivity and immune responses against HSV-1 do not eliminate either the vector or infected cells, and the vectors can therefore be re-administered. In this review we highlight vectors that have been translated to the clinic and host-virus immune interactions that impact on the safety and efficacy of HSVs.

Evaluation of parameters for efficient purification and long-term storage of herpes simplex virus-based vectors

Replication competent oncolytic herpes simplex virus (HSV) vectors have been used extensively to treat solid tumors with promising results. However, highly defective HSV vectors will be needed for applications that require sustained therapeutic gene expression in the absence of vector-related toxicity or inflammation. These vectors require complementing cell lines for their manufacture, creating significant challenges to achieve high yields of infectious virus particles. We recently described an improved upstream process for the production of a non-cytotoxic HSV vector for gene therapy applications. Here, we sought to optimize the downstream conditions for purification and long-term storage of the same vector, JΔNI5. We compared different methods to remove cellular impurities and concentrate the vector by monitoring both physical and biological titers, resulting in the establishment of optimal conditions for vector production. To optimize the long-term storage parameters for non-cytotoxic HSV vectors, we evaluated vector stability at low temperature and sensitivity to freeze-thaw cycles. We report that suboptimal purification and storage methods resulted in loss of vector viability. Our results describe effective and reproducible protocols for purification and storage of HSV vectors for pre-clinical studies.

Construction and optimization of herpes simplex virus vectors for central nervous system gene delivery based on CRISPR/Cas9-mediated genome editing

AIMS

We aim to define parameters affecting the safety and long-term transgene expression of attenuated HSV-1 vectors and optimize the expression cassettes to achieve robust and sustained expression in CNS.

BACKGROUND

Engineered, attenuated Herpes simplex virus (HSV) vectors are promising vehicles for gene delivery to the peripheral and central nervous systems. The virus latent promoter (LAP) is commonly used to drive exogenous gene expression; however, parameters affecting the safety and long-term transgene expression of attenuated HSV-1 vectors have not been fully understood.

OBJECTIVE

This study aimed to construct attenuated HSV-1 vectors using the CRISPR-Cas9 system and examine the influence of transgene cassette construction and insertion site on transgene expression and vector safety.

METHOD

In this study, we used a CRISPR-Cas9 system to accurately and efficiently edit attenuated HSV-1 strain 1716, and constructed two series of recombinant virus LMR and LMRx with different sets of gene cassettes insertion in Exon1(LAP2) and 2.0 kb intron downstream of LAP, respectively. The transgene expression and viral gene transcriptional kinetics were compared in in-vitro cell lines. The reporter gene expression and safety profiles of each vector were further evaluated in the mouse hippocampus gene transduction model.

RESULT

The in-vitro cell line analysis indicated that the insertion of a gene expression cassette would disrupt virus gene transcription. Mouse hippocampus transducing analysis suggested that complete expression cassette insertion at 2.0 kb intron could achieve robust and longtime gene expression than the other constructs. Recombinants with gene expression cassettes lacked Poly (A), which induced significant neuronal inflammation due to persistent viral antigen expression and microglia activation.

CONCLUSION

Our results indicated that the integrity of LAT transcripts was not necessary for the establishment of long-term latent expression. Exogenous strong promoters (like cBh promoter) could remain active during latency when placed in Exon1 or 2.0 Kb Intron of LAT locus, although their transcriptional activity declined with time. Consistent with previous research, the foreign gene expression would last much longer when the gene cassette was located downstream of Exon1, which suggested a role of LAP2 in maintaining promoter activity during latency. Besides, over-transcription of the downstream part of LAT may induce continuous activation of the attenuated vectors, suggesting an important role of LAT in maintaining viral reactivation potential.

Chromatin-mediated epigenetic regulation of HSV-1 transcription as a potential target in antiviral therapy

The ability to establish, and reactivate from, latent infections is central to the biology and pathogenesis of HSV-1. It also poses a strong challenge to antiviral therapy, as latent HSV-1 genomes do not replicate or express any protein to be targeted. Although the processes regulating the establishment and maintenance of, and reactivation from, latency are not fully elucidated, the current general consensus is that epigenetics play a major role. A unifying model postulates that whereas HSV-1 avoids or counteracts chromatin silencing in lytic infections, it becomes silenced during latency, silencing which is somewhat disrupted during reactivation. Many years of work by different groups using a variety of approaches have also shown that the lytic HSV-1 chromatin is distinct and has unique biophysical properties not shared with most cellular chromatin. Nonetheless, the lytic and latent viral chromatins are typically enriched in post translational modifications or histone variants characteristic of active or repressed transcription, respectively. Moreover, a variety of small molecule epigenetic modulators inhibit viral replication and reactivation from latency. Despite these successes in culture and animal models, it is not obvious how epigenetic modulation would be used in antiviral therapy if the same epigenetic mechanisms governed viral and cellular gene expression. Recent work has highlighted several important differences between the viral and cellular chromatins, which appear to be of consequence to their respective epigenetic regulations. In this review, we will discuss the distinctiveness of the viral chromatin, and explore whether it is regulated by mechanisms unique enough to be exploited in antiviral therapy.

Improvement of HSV-1 based amplicon vectors for a safe and long-lasting gene therapy in non-replicating cells

A key factor for developing gene therapy strategies for neurological disorders is the availability of suitable vectors. Currently, the most advanced are adeno-associated vectors that, while being safe and ensuring long-lasting transgene expression, have a very limited cargo capacity. In contrast, herpes simplex virus-based amplicon vectors can host huge amounts of foreign DNA, but concerns exist about their safety and ability to express transgenes long-term. We aimed at modulating and prolonging amplicon-induced transgene expression kinetics in vivo using different promoters and preventing transgene silencing. To pursue the latter, we deleted bacterial DNA sequences derived from vector construction and shielded the transgene cassette using AT-rich and insulator-like sequences (SAm technology). We employed luciferase and GFP as reporter genes. To determine transgene expression kinetics, we injected vectors in the hippocampus of mice that were longitudinally scanned for bioluminescence for 6 months. To evaluate safety, we analyzed multiple markers of damage and performed patch clamp electrophysiology experiments. All vectors proved safe, and we managed to modulate the duration of transgene expression, up to obtaining a stable, long-lasting expression using the SAm technology. Therefore, these amplicon vectors represent a flexible, efficient, and safe tool for gene delivery in the brain.

Protocol Optimization for the Production of the Non-Cytotoxic JΔNI5 HSV Vector Deficient in Expression of Immediately Early Genes

Non-toxic herpes simplex virus (HSV) vectors can be generated by functional deletion of all immediate-early (IE) genes, providing a benign vehicle with potential for gene therapy. However, deletion of multiple IE genes raises manufacturing concerns and thus limits clinical application of these vectors. To address this issue, we previously developed a novel production cell line, called U2OS-ICP4/27, by lentiviral transduction of human osteosarcoma U2OS cells with two essential HSV IE genes, ICP4 and ICP27. To optimize the process of vector manufacturing on this platform, we evaluated several cell culture parameters of U2OS-ICP4/27 for high-titer and -quality production of non-toxic HSV vectors, revealing that the yields and functionality of these vectors can be significantly influenced by culturing conditions. We also found that several chemical compounds can enhance the replication of non-toxic HSV vectors and their release from producer cells into the supernatants. Notably, the vector produced by our optimized protocol displayed a greatly improved vector yield and quality and showed elevated transgene expression in cultures of primary dorsal root ganglion neurons. Taken together, our optimized production approach emerges as a relevant protocol for high-yield and high-quality preparation of non-toxic HSV-based gene therapy vectors.

Herpes Simplex Virus: A Versatile Tool for Insights Into Evolution, Gene Delivery, and Tumor Immunotherapy

Herpesviruses are prevalent throughout the animal kingdom, and they have coexisted and coevolved along with their host species for millions of years. Herpesviruses carry a large (120-230 kb) double-stranded DNA genome surrounded by a protein capsid, a tegument layer consisting of viral and host proteins, and a lipid bilayer envelope with surface glycoproteins. A key characteristic of these viruses is their ability to enter a latent state following primary infection, allowing them to evade the host's immune system and persist permanently. Herpesviruses can reactivate from their dormant state, usually during times of stress or when the host's immune responses are impaired. While herpesviruses can cause complications with severe disease in immune-compromised people, most of the population experiences few ill effects from herpesvirus infections. Indeed, herpes simplex virus 1 (HSV-1) in particular has several features that make it an attractive tool for therapeutic gene delivery. Herpes simplex virus 1 targets and infects specific cell types, such as epithelial cells and neurons. The HSV-1 genome can also accommodate large insertions of up to 14 kb. The HSV-1-based vectors have already achieved success for the oncolytic treatment of melanoma. In addition to serving as a vehicle for therapeutic gene delivery and targeted cell lysis, comparative genomics of herpesviruses HSV-1 and 2 has revealed valuable information about the evolutionary history of both viruses and their hosts. This review focuses on the adaptability of HSV-1 as an instrument for gene delivery and an evolutionary marker. Overall, HSV-1 shows great promise as a tool for treating human disease and studying human migration patterns, disease outbreaks, and evolution.

Chromatin dynamics and the transcriptional competence of HSV-1 genomes during lytic infections

During latent infections with herpes simplex virus 1 (HSV-1), viral transcription is restricted and the genomes are mostly maintained in silenced chromatin, whereas in lytically infected cells all viral genes are transcribed and the genomes are dynamically chromatinized. Histones in the viral chromatin bear markers of silenced chromatin at early times in lytic infection or of active transcription at later times. The virion protein VP16 activates transcription of the immediate-early (IE) genes by recruiting transcription activators and chromatin remodelers to their promoters. Two IE proteins, ICP0 and ICP4 which modulate chromatin epigenetics, then activate transcription of early and late genes. Although chromatin is involved in the mechanism of activation of HSV- transcription, its precise role is not entirely understood. In the cellular genome, chromatin dynamics often modulate transcription competence whereas promoter-specific transcription factors determine transcription activity. Here, biophysical fractionation of serially digested HSV-1 chromatin followed by short-read deep sequencing indicates that nuclear HSV-1 DNA has different biophysical properties than protein-free or encapsidated HSV-1 DNA. The entire HSV-1 genomes in infected cells were equally accessible. The accessibility of transcribed or non-transcribed genes under any given condition did not differ, and each gene was entirely sampled in both the most and least accessible chromatin. However, HSV-1 genomes fractionated differently under conditions of generalized or restricted transcription. Approximately 1/3 of the HSV-1 DNA including fully sampled genes resolved to the most accessible chromatin when HSV-1 transcription was active, but such enrichment was reduced to only 3% under conditions of restricted HSV-1 transcription. Short sequences of restricted accessibility separated genes with different transcription levels. Chromatin dynamics thus provide a first level of regulation on HSV-1 transcription, dictating the transcriptional competency of the genomes during lytic infections, whereas the transcription of individual genes is then most likely activated by specific transcription factors. Moreover, genes transcribed to different levels are separated by short sequences with limited accessibility.

Although chromatin epigenetics modulate transcription of the nuclear replicating DNA viruses, and play major roles in the process of establishment of, and reactivation from, latency, the specific mechanisms of this modulation are not totally clear. Chromatin often regulates the transcriptional competency of cellular genes, rather than the actual level of transcription of individual genes. Here, we show that chromatin dynamics regulate the transcription competency of entire herpes simplex virus 1 (HSV-1) genomes, rather than the actual transcription level of individual genes. Moreover, CTCF/ insulator containing sequences flanking the immediate-early gene loci are more inaccessible when these genes are highly transcribed in a context of little transcription from the rest of the genome than when no gene was highly transcribed or all genes were. We postulate that chromatin dynamics modulate the transcriptional competency of the HSV-1 genome. Genes in genomes rendered transcriptionally inactive by chromatin dynamics cannot be transcribed, whereas transcription of individual genes, or of group of genes, is regulated separately in the transcriptionally competent genomes.

Gene Therapy Tools for Brain Diseases

Neurological disorders affecting the central nervous system (CNS) are still incompletely understood. Many of these disorders lack a cure and are seeking more specific and effective treatments. In fact, in spite of advancements in knowledge of the CNS function, the treatment of neurological disorders with modern medical and surgical approaches remains difficult for many reasons, such as the complexity of the CNS, the limited regenerative capacity of the tissue, and the difficulty in conveying conventional drugs to the organ due to the blood-brain barrier. Gene therapy, allowing the delivery of genetic materials that encodes potential therapeutic molecules, represents an attractive option. Gene therapy can result in a stable or inducible expression of transgene(s), and can allow a nearly specific expression in target cells. In this review, we will discuss the most commonly used tools for the delivery of genetic material in the CNS, including viral and non-viral vectors; their main applications; their advantages and disadvantages. We will discuss mechanisms of genetic regulation through cell-specific and inducible promoters, which allow to express gene products only in specific cells and to control their transcriptional activation. In addition, we will describe the applications to CNS diseases of post-transcriptional regulation systems (RNA interference); of systems allowing spatial or temporal control of expression [optogenetics and Designer Receptors Exclusively Activated by Designer Drugs (DREADDs)]; and of gene editing technologies (CRISPR/Cas9, Zinc finger proteins). Particular attention will be reserved to viral vectors derived from herpes simplex type 1, a potential tool for the delivery and expression of multiple transgene cassettes simultaneously.

Herpes Simplex Virus Vectors for Gene Transfer to the Central Nervous System

Neurodegenerative diseases (NDs) have a profound impact on human health worldwide and their incidence is predicted to increase as the population ages. ND severely limits the quality of life and leads to early death. Aside from treatments that may reduce symptoms, NDs are almost completely without means of therapeutic intervention. The genetic and biochemical basis of many NDs is beginning to emerge although most have complex etiologies for which common themes remain poorly resolved. Largely relying on progress in vector design, gene therapy is gaining increasing support as a strategy for genetic treatment of diseases. Here we describe recent developments in the engineering of highly defective herpes simplex virus (HSV) vectors suitable for transfer and long-term expression of large and/or multiple therapeutic genes in brain neurons in the complete absence of viral gene expression. These advanced vector platforms are safe, non-inflammatory, and persist in the nerve cell nucleus for life. In the near term, it is likely that HSV can be used to treat certain NDs that have a well-defined genetic cause. As further information on disease etiology becomes available, these vectors may take on an expanded role in ND therapies, including gene editing and repair.