Improved helper virus-free packaging system for HSV amplicon vectors using an ICP27-deleted, oversized HSV-1 DNA in a bacterial artificial chromosome

Viral vectors engineered for gene therapy

Gene therapy has seen major progress in recent years. Viral vectors have made a significant contribution through efficient engineering for improved delivery and safety. A large variety of indications such as cancer, cardiovascular, metabolic, hematological, neurological, muscular, ophthalmological, infectious diseases, and immunodeficiency have been targeted. Viral vectors based on adenoviruses, adeno-associated viruses, herpes simplex viruses, retroviruses including lentiviruses, alphaviruses, flaviviruses, measles viruses, rhabdoviruses, Newcastle disease virus, poxviruses, picornaviruses, reoviruses, and polyomaviruses have been used. Proof-of-concept has been demonstrated for different indications in animal models. Therapeutic efficacy has also been achieved in clinical trials. Several viral vector-based drugs have been approved for the treatment of cancer, and hematological, metabolic, and neurological diseases. Moreover, viral vector-based vaccines have been approved against COVID-19 and Ebola virus disease.

Viral Vectors in Gene Therapy: Where Do We Stand in 2023?

Viral vectors have been used for a broad spectrum of gene therapy for both acute and chronic diseases. In the context of cancer gene therapy, viral vectors expressing anti-tumor, toxic, suicide and immunostimulatory genes, such as cytokines and chemokines, have been applied. Oncolytic viruses, which specifically replicate in and kill tumor cells, have provided tumor eradication, and even cure of cancers in animal models. In a broader meaning, vaccine development against infectious diseases and various cancers has been considered as a type of gene therapy. Especially in the case of COVID-19 vaccines, adenovirus-based vaccines such as ChAdOx1 nCoV-19 and Ad26.COV2.S have demonstrated excellent safety and vaccine efficacy in clinical trials, leading to Emergency Use Authorization in many countries. Viral vectors have shown great promise in the treatment of chronic diseases such as severe combined immunodeficiency (SCID), muscular dystrophy, hemophilia, β-thalassemia, and sickle cell disease (SCD). Proof-of-concept has been established in preclinical studies in various animal models. Clinical gene therapy trials have confirmed good safety, tolerability, and therapeutic efficacy. Viral-based drugs have been approved for cancer, hematological, metabolic, neurological, and ophthalmological diseases as well as for vaccines. For example, the adenovirus-based drug Gendicine® for non-small-cell lung cancer, the reovirus-based drug Reolysin® for ovarian cancer, the oncolytic HSV T-VEC for melanoma, lentivirus-based treatment of ADA-SCID disease, and the rhabdovirus-based vaccine Ervebo against Ebola virus disease have been approved for human use.

The HSV-1 Transcription Factor ICP4 Confers Liquid-Like Properties to Viral Replication Compartments

Herpes Simplex Virus Type-1 (HSV-1) forms progeny in the nucleus within distinct membrane-less inclusions, the viral replication compartments (VRCs), where viral gene expression, DNA replication, and packaging occur. The way in which the VRCs maintain spatial integrity remains unresolved. Here, we demonstrate that the essential viral transcription factor ICP4 is an intrinsically disordered protein (IDP) capable of driving protein condensation and liquid–liquid phase separation (LLPS) in transfected cells. Particularly, ICP4 forms nuclear liquid-like condensates in a dose- and time-dependent manner. Fluorescence recovery after photobleaching (FRAP) assays revealed rapid exchange rates of EYFP-ICP4 between phase-separated condensates and the surroundings, akin to other viral IDPs that drive LLPS. Likewise, HSV-1 VRCs revealed by EYFP-tagged ICP4 retained their liquid-like nature, suggesting that they are phase-separated condensates. Individual VRCs homotypically fused when reaching close proximity and grew over the course of infection. Together, the results of this study demonstrate that the HSV-1 transcription factor ICP4 has characteristics of a viral IDP, forms condensates in the cell nucleus by LLPS, and can be used as a proxy for HSV-1 VRCs with characteristics of liquid–liquid phase-separated condensates.

Multigene human artificial chromosome vector delivery with herpes simplex virus 1 amplicons

Gene expression studies and gene therapy require efficient gene delivery into cells. Different technologies by viral and non-viral mechanisms have been used for gene delivery into cells. Small gene vectors transfer across the cell membrane with a relatively high efficiency, but not large genes or entire loci spanning several kilobases, which do not remain intact following introduction. Previously, we developed an efficient delivery system based on herpes virus simplex type 1 (HSV-1) amplicons to transfer large fragments of DNA incorporated in human artificial chromosome (HAC) vectors into the nucleus of human cells. The HSV-1 amplicon lacks the signals for cleavage and replication of its own genome, yet each amplicon has the capacity to incorporate up to 150 kb of exogenous DNA. In this study, we investigated whether the capacity of gene delivery could be increased by simultaneously introducing multiple HSV-1 modified amplicons carrying a gene expressing HAC vector into cells with the aim of generating a single artificial chromosome containing the desired genes. Following co-transduction of two HSV-1 HAC amplicons, artificial chromosomes were successfully generated containing the introduced genes, which were appropriately expressed in different human cell types.

Enhanced Production of Herpes Simplex Virus 1 Amplicon Vectors by Gene Modification and Optimization of Packaging Cell Growth Medium

Herpes simplex virus 1 (HSV-1)-derived amplicon vectors are unique in their ability to accommodate large DNA molecules allowing whole genomic loci to be included with all of their regulatory elements. Additional advantages of these amplicons include their minimal toxicity and ability to persist as episomes, with negligible risk of insertional mutagenesis, being particularly well-suited for gene therapy of neurological disorders due to their outstanding ability to deliver genes into neurons and other neural cells. However, extensive gene therapy application has been hindered by difficulties in vector production. This work improved HSV-1 amplicons production by genetic modification of the packaging cell line and optimization of the culture medium. A stably-transfected Vero 2-2 cell line overexpressing the anti-apoptotic Bcl-2 protein was generated, exhibiting an increased resistance to apoptosis, prolonged culture duration, and a significant improvement in viral vector production. Additionally, supplementation of the growth medium with antioxidants, polyamines, amino acids, and reduced glutathione further increased the yield of packaged amplicon vectors. With these modifications, HSV-1 amplicons could be isolated from culture supernatants instead of cell lysates, leading to vector preparations with higher titer and purity and paving the way for generation of stable cell lines that are capable of continuous herpesviral vector production.

Preparation of Herpes Simplex Virus Type 1 (HSV-1)-Based Amplicon Vectors

Amplicon vectors, or amplicons, are defective, helper-dependent, herpes simplex virus type 1 (HSV-1)-based vectors. The main interest of amplicons as gene transfer tools stems from the fact that the genomes of these vectors do not carry protein-encoding viral sequences. Consequently, they are completely safe for the host and nontoxic for the infected cells. Moreover, the complete absence of virus genes provides a genomic space authorizing a very large payload, enough to accommodate foreign DNA sequences up to almost 150-kbp, the size of the HSV-1 genome. This transgene capacity can be used to deliver complete gene loci, including introns and exons, as well as long regulatory sequences conferring tissue-specific expression or stable maintenance of the transgene in proliferating cells. For many years the development of these vectors and their application in gene transfer experiments was hindered by the presence of contaminating toxic helper virus particles in the vector stocks. In recent years, however, two different methodologies have been developed that allow generating amplicon stocks either completely free of helper particles or only faintly contaminated with fully defective helper particles. This chapter describes these two methodologies.

HSV-1 Amplicon Vectors as Genetic Vaccines

HSV-1 amplicon vectors have been used as platforms for the generation of genetic vaccines against both DNA and RNA viruses. Mice vaccinated with such vectors encoding structural proteins from both foot-and-mouth disease virus and rotavirus were partially protected from challenge with wild-type virus (D'Antuono et al., Vaccine 28:7363-7372, 2010; Laimbacher et al., Mol Ther 20:1810-1820, 2012; Meier et al., Int J Mol Sci 18:431, 2017), indicating that HSV-1 amplicon vectors are attractive tools for the development of complex and safe genetic vaccines.This chapter describes the preparation and testing of HSV-1 amplicon vectors that encode individual or multiple viral structural proteins from a polycistronic transgene cassette. We further put particular emphasis on generating virus-like particles (VLPs) in vector-infected cells. Expression of viral genes is confirmed by Western blot and immune fluorescence analysis and generation of VLPs in vector-infected cells is demonstrated by electron microscopy. Furthermore, examples on how to analyze the immune response in a mouse model and possible challenge experiments are described.

U94 of human herpesvirus 6 down-modulates Src, promotes a partial mesenchymal-to-epithelial transition and inhibits tumor cell growth, invasion and metastasis

U94, the latency gene of human herpesvirus 6, was found to inhibit migration, invasion and proliferation of vascular endothelial cells (ECs). Because of its potent anti-migratory activity on ECs, we tested the capability of U94 to interfere with the individual steps of the metastatic cascade. We examined the U94 biological activity on the human breast cancer cell line MDA-MB 231, as a model of highly aggressive cancer cell. Here we show that the expression of U94 delivered by an HSV-1-based amplicon promoted down-modulation of Src and downstream molecules linked to cell motility and proliferation. Indeed, U94 expression strongly inhibited cell migration, invasiveness and clonogenicity. We investigated the effects of U94 in a three-dimensional rotary cell-culture system and observed the ability of U94 to modify tumor cell morphology by inducing a partial mesenchymal-to-epithelial transition. In fact, despite U94 did not induce any expression of the epithelial marker E-cadherin, it down-modulated different mesenchymal markers as β-catenin, Vimentin, TWIST, Snail1, and MMP2. In vivo data on the tumorigenicity of MDA-MB 231 displayed the capability of U94 to control tumor growth, invasiveness and metastasis, as well as tumor-driven angiogenesis. The antitumor U94 activity was also confirmed on the human cervical cancer cell line HeLa. The ability of U94 to inhibit cell growth, invasion and metastasis opens the way to a promising field of research aimed to develop new therapeutic approaches for treating tumor and cancer metastasis.

Evaluation of the immunogenicity of a recombinant HSV-1 vector expressing human group C rotavirus VP6 protein

Group C Rotavirus (RVC) has been associated globally with sporadic outbreaks of gastroenteritis in children and adults. RVC also infects animals, and interspecies transmission has been reported as well as its zoonotic potential. Considering its genetic diversity and the absence of effective vaccines, it is important and necessary to develop new generation vaccines against RVC for both humans and animals. The aim of the present study was to develop and characterize an HSV-1-based amplicon vector expressing a human RVC-VP6 protein and evaluate the humoral immune response induced after immunizing BALB/c mice. Local fecal samples positive for RVC were used for isolation and sequencing of the vp6 gene, which phylogenetically belongs to the I2 genotype. We show here that cells infected with the HSV[VP6C] amplicon vector efficiently express the VP6 protein, and induced specific anti-RVC antibodies in mice immunized with HSV[VP6C], in a prime-boost schedule. This work highlights that amplicon vectors are an attractive platform for the generation of safe genetic immunogens against RVC, without the addition of external adjuvants.

Sustained FXN expression in dorsal root ganglia from a nonreplicative genomic HSV-1 vector

BACKGROUND

Friedreich's ataxia (FA) is an autosomal recessive neurodegenerative disease caused by mutations in the frataxin gene (FXN), which lead to reduced levels of the essential mitochondrial protein frataxin. Currently, there is no effective cure.

METHODS

With the aim of developing a gene therapy for FA neuropathology, we describe the construction and preliminary characterization of a high-capacity nonreplicative genomic herpes simplex virus type 1 vector (H24B-FXNlac vector) carrying a reduced version of the human FXN genomic locus, comprising the 5-kb promoter and the FXN cDNA with the inclusion of intron 1.

RESULTS

We show that the transgene cassette contains the elements necessary to preserve physiological neuronal regulation of human FXN expression. Transduction of cultured fetal rat dorsal root ganglia neurons with the H24B-FXNlac vector results in sustained expression of human FXN transcripts and frataxin protein. Rat footpad inoculation with the H24B-FXNlac vector results in human FXN transgene delivery to the dorsal root ganglia, with expression persisting for at least 1 month.

CONCLUSIONS

The results of the present study support the feasibility of using this vector for sustained neuronal expression of human frataxin for FA gene therapy.

The Role of The RNA Demethylase FTO (Fat Mass and Obesity-Associated) and mRNA Methylation in Hippocampal Memory Formation

The formation of long-lasting memories requires coordinated changes in gene expression and protein synthesis. Although many studies implicate DNA modifications (DNA methylation, histone modifications) in memory formation, the contributions of RNA modifications remain largely unexplored. Here we investigated the role of mRNA methylation in hippocampal-dependent memory formation in mice. RNA modifications are highly dynamic and readily reversible. Methyltransferases add a methyl group to mRNA while demethylases remove methyl groups. Here we focused on examining the role of the best characterized RNA demethylase, FTO (fat mass and obesity-associated) in memory. We observed that FTO is expressed in the nuclei, dendrites and near dendritic spines of mouse dorsal hippocampal CA1 neurons. Next, we found that contextual fear conditioning transiently (0.5 h) decreased Fto levels in these neurons, with the largest decrease in FTO observed near synapses. The decrease in FTO observed shortly after contextual fear conditioning suggests that FTO normally constrains memory formation. To directly test this, we artificially decreased FTO levels in dorsal hippocampus of otherwise normal (wild-type) mice by microinjecting before training a single herpes simplex virus (HSV) vector expressing either CRISPR/Cas9 or shRNA targeted against Fto. Decreasing FTO using either method specifically enhanced contextual fear memory. Together, these results show the importance of FTO during memory formation and, furthermore, implicate mRNA modification and epi-transcriptomics as novel regulators of memory formation.

Synergistic effects of deleting multiple nonessential elements in nonreplicative HSV-1 BAC genomic vectors play a critical role in their viability

Nonreplicative Herpes simplex virus type-1 (HSV-1) genomic vectors have already entered into clinical trials for neurological gene therapy thanks to their scalable growth in permissive cells. However, the small transgene capacity of this type of HSV-1 vectors currently used in the clinic represents an important limiting factor as a gene delivery system. To develop high-capacity nonreplicative genomic HSV-1 vectors, in this study we have characterized a series of multiply deleted mutants which we have constructed in bacterial artificial chromosomes (BACs), removing up to 24 kb of unstable or dispensable genomic sequences to allow insertion of transgenes up to this size. We show that synergistic effects of deletions of: the HSV-1 replication origins ori_S and ori_L, the HSV-1 internal repeat region, the remaining ICP4 gene copy and the genes encoding for ICP27, U_L56, U_L55, can severely reduce the growth of these HSV-1 vectors. Given that several of these elements have been characterized as 'non-essential' for viral growth in cell culture by single-deletion experiments of wild-type HSV-1, our study highlights the need to re-evaluate their functional contribution in the context of multiply deleted nonreplicative HSV-1 genomic vectors. Our BAC mutants described here can serve as useful starting platforms to accelerate HSV-1 vector development.

Transfer of Anti-Rotavirus Antibodies during Pregnancy and in Milk Following Maternal Vaccination with a Herpes Simplex Virus Type-1 Amplicon Vector

Rotaviruses (RVs) are important enteric pathogens of newborn humans and animals, causing diarrhea and in rare cases death, especially in very young individuals. Rotavirus vaccines presently used are modified live vaccines that lack complete biological safety. Previous work from our laboratory suggested that vaccines based on in situ produced, non-infectious rotavirus-like particles (RVLPs) are efficient while being entirely safe. However, using either vaccine, active mucosal immunization cannot induce protective immunity in newborns due to their immature immune system. We therefore hypothesized that offspring from vaccinated dams are passively immunized either by transfer of maternal antibodies during pregnancy or by taking up antibodies from milk. Using a codon optimized polycistronic gene expression cassette packaged into herpesvirus particles, the simultaneous expression of the RV capsid genes led to the intracellular formation of RVLPs in various cell lines. Vaccinated dams developed a strong RV specific IgG antibody response determined in sera and milk of both mother and pups. Moreover, sera of naïve pups nursed by vaccinated dams also had RV specific antibodies suggesting a lactogenic transfer of antibodies. Although full protection of pups was not achieved in this mouse model, our observations are important for the development of improved vaccines against RV in humans as well as in various animal species.

Novel Mutant AAV2 Rep Proteins Support AAV2 Replication without Blocking HSV-1 Helpervirus Replication

As their names imply, parvoviruses of the genus Dependovirus rely for their efficient replication on the concurrent presence of a helpervirus, such as herpesvirus, adenovirus, or papilloma virus. Adeno-associated virus 2 (AAV2) is such an example, which in turn can efficiently inhibit the replication of each helpervirus by distinct mechanisms. In a previous study we have shown that expression of the AAV2 rep gene is not compatible with efficient replication of herpes simplex virus 1 (HSV-1). In particular, the combined DNA-binding and ATPase/helicase activities of the Rep68/78 proteins have been shown to exert opposite effects on the replication of AAV2 and HSV-1. While essential for AAV2 DNA replication these protein activities account for the Rep-mediated inhibition of HSV-1 replication. Here, we describe a novel Rep mutant (Rep-D371Y), which displayed an unexpected phenotype. Rep-D371Y did not block HSV-1 replication, but still supported efficient AAV2 replication, at least when a double-stranded AAV2 genome template was used. We also found that the capacity of Rep-D371Y to induce apoptosis and a Rep-specific DNA damage response was significantly reduced compared to wild-type Rep. These findings suggest that AAV2 Rep-helicase subdomains exert diverging activities, which contribute to distinct steps of the AAV2 life cycle. More important, the novel AAV2 mutant Rep-D371Y may allow deciphering yet unsolved activities of the AAV2 Rep proteins such as DNA second-strand synthesis, genomic integration or packaging, which all involve the Rep-helicase activity.

Polycistronic Herpesvirus Amplicon Vectors for Veterinary Vaccine Development

Heterologous virus-vectored vaccines, particularly those based on canarypox virus vectors, have established a firm place in preventive veterinary medicine. However, herpesvirus-based vaccines have paved the way for DIVA vaccines (discrimination of infected against vaccinated animals), which are particularly desirable for highly contagious livestock diseases that are otherwise combatted by culling of infected animals.In this chapter, we describe the design, the preparation, and the testing of a polycistronic herpesvirus amplicon vaccine against rotaviruses with a particular emphasis on generating heterologous virus-like particles for immunization. After the design, the procedure consists of three steps, first, transient expression of the construct in cell cultures, second, expression and antibody response in a mouse model, and third, application of the system to the desired host species. As a whole, the present information will facilitate the design of novel vaccines of veterinary interest from the designing process until pre-licensing.

The infectious BAC genomic DNA expression library: a high capacity vector system for functional genomics

Gene dosage plays a critical role in a range of cellular phenotypes, yet most cellular expression systems use heterologous cDNA-based vectors which express proteins well above physiological levels. In contrast, genomic DNA expression vectors generate physiologically-relevant levels of gene expression by carrying the whole genomic DNA locus of a gene including its regulatory elements. Here we describe the first genomic DNA expression library generated using the high-capacity herpes simplex virus-1 amplicon technology to deliver bacterial artificial chromosomes (BACs) into cells by viral transduction. The infectious BAC (iBAC) library contains 184,320 clones with an average insert size of 134.5 kb. We show in a Chinese hamster ovary (CHO) disease model cell line and mouse embryonic stem (ES) cells that this library can be used for genetic rescue studies in a range of contexts including the physiological restoration of Ldlr deficiency, and viral receptor expression. The iBAC library represents an important new genetic analysis tool openly available to the research community.

Gene Transfer of Brain-derived Neurotrophic Factor (BDNF) Prevents Neurodegeneration Triggered by FXN Deficiency

Friedreich's ataxia is a predominantly neurodegenerative disease caused by recessive mutations that produce a deficiency of frataxin (FXN). Here, we have used a herpesviral amplicon vector carrying a gene encoding for brain-derived neurotrophic factor (BDNF) to drive its overexpression in neuronal cells and test for its effect on FXN-deficient neurons both in culture and in the mouse cerebellum in vivo. Gene transfer of BDNF to primary cultures of mouse neurons prevents the apoptosis which is triggered by the knockdown of FXN gene expression. This neuroprotective effect of BDNF is also observed in vivo in a viral vector-based knockdown mouse cerebellar model. The injection of a lentiviral vector carrying a minigene encoding for a FXN-specific short hairpin ribonucleic acid (shRNA) into the mouse cerebellar cortex triggers a FXN deficit which is accompanied by significant apoptosis of granule neurons as well as loss of calbindin in Purkinje cells. These pathological changes are accompanied by a loss of motor coordination of mice as assayed by the rota-rod test. Coinjection of a herpesviral vector encoding for BDNF efficiently prevents both the development of cerebellar neuropathology and the ataxic phenotype. These data demonstrate the potential therapeutic usefulness of neurotrophins like BDNF to protect FXN-deficient neurons from degeneration.

Histone deacetylase 6 inhibition enhances oncolytic viral replication in glioma

Oncolytic viral (OV) therapy, which uses genetically engineered tumor-targeting viruses, is being increasingly used in cancer clinical trials due to the direct cytolytic effects of this treatment that appear to provoke a robust immune response against the tumor. As OVs enter tumor cells, intrinsic host defenses have the potential to hinder viral replication and spread within the tumor mass. In this report, we show that histone deacetylase 6 (HDAC6) in tumor cells appears to alter the trafficking of post-entry OVs from the nucleus toward lysosomes. In glioma cell lines and glioma-stem-like cells, HDAC6 inhibition (HDAC6i) by either pharmacologic or genetic means substantially increased replication of oncolytic herpes simplex virus type 1 (oHSV). Moreover, HDAC6i increased shuttling of post-entry oHSV to the nucleus. In addition, electron microscopic analysis revealed that post-entry oHSVs are preferentially taken up into glioma cells through the endosomal pathway rather than via fusion at the cell surface. Together, these findings illustrate a mechanism of glioma cell defense against an incoming infection by oHSV and identify possible approaches to enhance oHSV replication and subsequent lysis of tumor cells.

Use of Adeno-Associated and Herpes Simplex Viral Vectors for In Vivo Neuronal Expression in Mice

Adeno-associated viruses and the herpes simplex virus are the two most widely used vectors for the in vivo expression of exogenous genes. Advances in the development of these vectors have enabled remarkable temporal and spatial control of gene expression. This unit provides methods for storing, delivering, and verifying expression of adeno-associated and herpes simplex viruses in the adult mouse brain. It also describes important considerations for experiments using in vivo expression of these viral vectors, including serotype and promoter selection, as well as timing of expression. Additional protocols are provided that describe methods for preliminary experiments to determine the appropriate conditions for in vivo delivery.

Delivery of the 135 kb human frataxin genomic DNA locus gives rise to different frataxin isoforms

Friedreich's ataxia (FRDA) is the most common form of hereditary ataxia caused by recessive mutations in the FXN gene. Recent results have indicated the presence of different frataxin isoforms due to alternative gene expression mechanisms. Our previous studies demonstrated the advantages of using high-capacity herpes simplex virus type 1 (HSV-1) amplicon vectors containing the entire FXN genomic locus (iBAC-FXN) as a gene-delivery vehicle capable of ensuring physiologically-regulated and long-term persistence. Here we describe how expression from the 135 kb human FXN genomic locus produces the three frataxin isoforms both in cultured neuronal cells and also in vivo. Moreover, we also observed the correct expression of these frataxin isoforms in patient-derived cells after delivery of the iBAC-FXN. These results lend further support to the potential use of HSV-1 vectors containing entire genomic loci whose expression is mediated by complex transcriptional and posttranscriptional mechanisms for gene therapy applications.

Herpes simplex virus type 1 (HSV-1)-derived amplicon vectors for gene transfer and gene therapy

Amplicons are defective, helper -dependent, herpes simplex virus type 1 (HSV-1 )-derived vectors. The main interest of these vectors as gene transfer tools stems from the fact that the amplicon vector genomes do not carry protein-encoding viral sequences. Consequently, they are completely safe for the host and non-toxic for the infected cells. Moreover, the complete absence of virus genes provides space to accommodate very large foreign DNA sequences, up to almost 150-kbp, the size of the virus genome . This large transgene capacity can be used to deliver complete gene loci, including introns and exons, as well as long regulatory sequences, conferring tissue-specific expression, or stable maintenance of the transgene in proliferating cells. During many years the development of these vectors and their application in gene transfer experiments was hindered by the presence of contaminating toxic helper virus particles in the vector stocks. In recent years however, two different methodologies have been developed that allow generating amplicon stocks either completely free of helper particles or only faintly contaminated with fully defective helper particles. This chapter summarizes these two methodologies.

Murine Bone Marrow-Derived Dendritic Cells Transduced by Light-Helper-Dependent Herpes Simplex Virus-1 Amplicon Vector Acquire a Mature Dendritic Cell Phenotype

Dendritic cells (DCs) turn into the most potent antigen-presenting cells following a complex transforming process, which leads to their maturation. Herpes simplex virus-1 (HSV-1) amplicon vectors represent highly versatile viral vector platforms with the ability to transduce immature DCs at exceedingly high efficiencies, while the efficiency of infection of mature DCs is significantly low. However, the bacterial artificial chromosome (BAC)-dependent (BD) amplicon vectors tested so far do not result in the maturation of mouse bone marrow-derived DCs (BMDCs) in vitro. In this study we investigated the effects of light-helper-dependent (LHD) amplicon vectors produced with the replication-defective HSV-1 LaLΔJ helper virus system. First, we observed that transgene expression in BMDC cultures was equally potent between the LHD and the BD amplicon vectors. We determined that the percentage of transduced cells and the duration of transgene expression were negatively influenced by the presence of increasing levels of helper virus. Second, infection by the LHD amplicon vector as well as the helper HSV-1 LaLΔJ virus alone resulted in the phenotypic maturation of BMDCs and the expression of both interferon-stimulated genes and proinflammatory cytokines. Further comparisons of the gene expression of infected DCs showed that while interferon-stimulated genes such as Ifit1, Ifit3, Mx2, Isg15, and Cxcl10 were induced by both BD and LHD amplicon vectors, early proinflammatory cytokine gene expression (Tnfa, Il1a, Il1b, Il6, Il10, Il12b, Cxcl1, and Cxcl16) and DC maturation were mediated only by the LHD amplicons.

Herpes simplex virus type 1 (HSV-1)-derived amplicon vectors

Amplicons are defective, helper-dependent, herpes simplex virus type 1 (HSV-1)-derived vectors. The main interest of these vectors as gene transfer tools stems from the fact that the amplicon vector genomes do not carry protein-encoding viral sequences. Consequently, they are completely safe for the host and nontoxic for the infected cells. Moreover, the complete absence of virus genes provides space to accommodate very large foreign DNA sequences, up to almost 150-kb, the size of the virus genome. This large transgene capacity can be used to deliver complete gene loci, including introns and exons, as well as long regulatory sequences, conferring tissue-specific expression or stable maintenance of the transgene in proliferating cells. During many years the development of these vectors and their application in gene transfer experiments was hindered by the presence of contaminating toxic helper virus particles in the vector stocks. In recent years, however, two different methodologies have been developed that allow generating amplicon stocks either completely free of helper particles or only faintly contaminated with fully defective helper particles. This chapter describes these two methodologies.

Doxorubicin synergizes with 34.5ENVE to enhance antitumor efficacy against metastatic ovarian cancer

PURPOSE

Novel therapeutic regimens are needed to improve dismal outcomes associated with late-stage ovarian cancer. Oncolytic viruses are currently being tested in patients with ovarian cancer. Here, we tested the therapeutic efficacy of combining doxorubicin with 34.5ENVE, an oncolytic herpes simplex virus transcriptionally driven by a modified stem cell-specific nestin promoter, and encoding for antiangiogenic Vasculostatin-120 (VStat120) for use against progressive ovarian cancer.

EXPERIMENTAL DESIGN

Antitumor efficacy of 34.5ENVE was assessed in ovarian cancer cell lines, mouse ascites-derived tumor cells, and primary patient ascites-derived tumor cells by standard MTT assay. The ability of conditioned medium derived from 34.5ENVE-infected ovarian cancer cells to inhibit endothelial cell migration was measured by a Transwell chamber assay. Scope of cytotoxic interactions between 34.5ENVE and doxorubicin were evaluated using Chou-Talalay synergy analysis. Viral replication, herpes simplex virus receptor expression, and apoptosis were evaluated. Efficacy of oncolytic viral therapy in combination with doxorubicin was evaluated in vivo in the murine xenograft model of human ovarian cancer.

RESULTS

Treatment with 34.5ENVE reduced cell viability of ovarian cancer cell lines, and mouse ascites-derived and patient ascites-derived ovarian tumor cells. Conditioned media from tumor cells infected with 34.5ENVE reduced endothelial cell migration. When combined with doxorubicin, 34.5ENVE killed synergistically with a significant increase in caspase-3/7 activation, and an increase in sub-G1 population of cells. The combination of doxorubicin and 34.5ENVE significantly prolonged survival in nude mice bearing intraperitoneal ovarian cancer tumors.

CONCLUSIONS

This study indicates significant antitumor efficacy of 34.5ENVE alone, and in combination with doxorubicin against disseminated peritoneal ovarian cancer.

HSV-1 amplicon vectors as genetic vaccines

HSV-1 amplicon vectors have been used as platforms for the generation of genetic vaccines against both DNA and RNA viruses. Mice vaccinated with such vectors encoding structural proteins from both foot-and-mouth disease virus and rotavirus were partially protected from challenge with wild-type virus (D'Antuono et al. Vaccine 28: 7363-7372, 2010; Laimbacher et al. Mol Ther 20: 1810-1820, 2012), indicating that HSV-1 amplicon vectors are attractive tools for the development of complex and safe genetic vaccines. This chapter describes the use of HSV-1 amplicon vectors that encode individual or multiple viral structural proteins from a polycistronic transgene cassette in mammalian cells. More precisely, amplicon vectors that encode multiple structural viral proteins support the in situ production of viruslike particles (VLPs) in vector-infected cells. The expression of the viral genes is confirmed by Western blot and immune fluorescence analysis, and the generation of VLPs in vector-infected cells is demonstrated by electron microscopy.

Modification of HSV-1 to an oncolytic virus

Cancer-permissive viruses or oncolytic viruses consist of either genetically engineered or naturally occurring strains that possess relatively selective replicative and/or infection abilities for cancer vs. normal cells (Chiocca, Nat Rev Cancer 2: 938-950, 2002). They can also be armed with additional anticancer cDNAs (e.g., cytokines, prodrug-activating, anti-angiogenesis genes, and others) to extend therapeutic effects (Kaur et al., Curr Gene Ther 9: 341-355, 2009). Herpes simplex virus type 1 (HSV-1) possesses several advantages as an oncolytic virus such as a rapid lytic cycle and a large capacity for insertion of heterologous DNA sequences (Wade-Martins et al., Nat Biotechnol, 19: 1067-1070, 2001). However, the technical nuances of genetic manipulation of the HSV-1 genome may still be relatively challenging. Here, we describe a system that has been durable and consistent in providing the ability to generate multiple recombinant HSV-1. The HsvQuik technology utilizes an HSV-1 genome cloned in a bacterial artificial chromosome to recombine heterologous cDNAs in a relatively rapid and reliable manner (Terada et al., Gene Ther 13: 705-714, 2006).

A GAA repeat expansion reporter model of Friedreich's ataxia recapitulates the genomic context and allows rapid screening of therapeutic compounds

Friedreich's ataxia (FRDA) is caused by large GAA expansions in intron 1 of the frataxin gene (FXN), which lead to reduced FXN expression through a mechanism not fully understood. Understanding such mechanism is essential for the identification of novel therapies for FRDA and this can be accelerated by the development of cell models which recapitulate the genomic context of the FXN locus and allow direct comparison of normal and expanded FXN loci with rapid detection of frataxin levels. Here we describe the development of the first GAA-expanded FXN genomic DNA reporter model of FRDA. We modified BAC vectors carrying the whole FXN genomic DNA locus by inserting the luciferase gene in exon 5a of the FXN gene (pBAC-FXN-Luc) and replacing the six GAA repeats present in the vector with an ∼310 GAA repeat expansion (pBAC-FXN-GAA-Luc). We generated human clonal cell lines carrying the two vectors using site-specific integration to allow direct comparison of normal and expanded FXN loci. We demonstrate that the presence of expanded GAA repeats recapitulates the epigenetic modifications and repression of gene expression seen in FRDA. We applied the GAA-expanded reporter model to the screening of a library of novel small molecules and identified one molecule which up-regulates FXN expression in FRDA patient primary cells and restores normal histone acetylation around the GAA repeats. These results suggest the potential use of genomic reporter cell models for the study of FRDA and the identification of novel therapies, combining physiologically relevant expression with the advantages of quantitative reporter gene expression.

Oncolytic viral therapy with a combination of HF10, a herpes simplex virus type 1 variant and granulocyte-macrophage colony-stimulating factor for murine ovarian cancer

Ovarian cancer is the most frequent cause of gynecological cancer-related mortality as a majority of patients are diagnosed at an advanced stage with intraperitoneal dissemination because of the absence of initial symptoms. Granulocyte-macrophage colony-stimulating factor (GM-CSF) plays an important role in the maturation of specialized antigen-presenting cells. In this study, we utilized a herpes simplex virus (HSV) amplicon expressing murine GM-CSF combined with HF10 (mGM-CSF amplicon), a highly attenuated HSV type 1 strain functioning as a helper virus to strengthen anti-tumor immune response, for the treatment of ovarian cancer with intraperitoneal dissemination. A mouse ovarian cancer cell line, OV2944-HM-1 (HM-1), was intraperitoneally injected, following which HF10 only or the mGM-CSF amplicon was injected intraperitoneally three times. HF10 injection prolonged survival and decreased intraperitoneal dissemination, but to a lesser extent than the mGM-CSF amplicon. Although HF10 replication was not observed in HM-1 cells, expression of VP5, a late gene coding the major capsid protein of HSV, was detected. Moreover, mGM-CSF production was detected in transfected HM-1 cells. Immunohistochemical staining revealed the infiltration of CD4- and CD8-positive cells into the peritoneal tumor(s). A significantly increased CD4+ T cell concentration was observed in the spleen. Murine splenic cells after each treatment were stimulated with HM-1 cells, and the strongest immune response was observed in the mice that received mGM-CSF amplicon injections. These results suggested that the mGM-CSF amplicon is a promising agent for the treatment of advanced ovarian cancer with intraperitoneal dissemination.

Switching a replication-defective adenoviral vector into a replication-competent, oncolytic adenovirus

The adenovirus immediate early gene E1A initiates the program of viral gene transcription and reprograms multiple aspects of cell function and behavior. For adenoviral (Ad) vector-mediated gene transfer and therapy approaches, where replication-defective (RD) gene transfer is required, E1A has thus been the primary target for deletions. For oncolytic gene therapy for cancer, where replication-competent (RC) Ad viral gene expression is needed, E1A has been either mutated or placed under tumor-specific transcriptional control. A novel Ad vector that initially infected target tumor cells in an RD manner for transgene expression but that could be "switched" into an RC, oncolytic state when needed might represent an advance in vector technology. Here, we report that we designed such an Ad vector (proAdΔ24.GFP), where initial Ad replication is silenced by a green fluorescent protein (GFP) transgene that blocks cytomegalovirus (CMV)-mediated transcription of E1A. This vector functions as a bona fide E1A-deleted RD vector in infected tumor cells. However, because the silencing GFP transgene is flanked by FLP recombination target (FRT) sites, we show that it can be efficiently excised by Flp recombinase site-specific recombination, either when Flp is expressed constitutively in cells or when it is provided in trans by coinfection with a second RD herpes simplex virus (HSV) amplicon vector. This switches the RD Ad, proAdΔ24.GFP, into a fully RC, oncolytic Ad (rAdΔ24) that lyses tumor cells in culture and generates oncolytic progeny virions. In vivo, coinfection of established flank tumors with the RD proAdΔ24.GFP and the RD Flp-bearing HSV1 amplicon leads to generation of RC, oncolytic rAdΔ24. In an orthotopic human glioma xenograft tumor model, coinjection of the RD proAdΔ24.GFP and the RD Flp-bearing HSV1 amplicon also led to a significant increase in animal survival, compared to controls. Therefore, Flp-FRT site-specific recombination can be applied to switch RD Ad into fully oncolytic RC Ad for tumor therapy and is potentially applicable to a variety of gene therapy approaches.

Familial Alzheimer's disease coding mutations reduce Presenilin-1 expression in a novel genomic locus reporter model

We have generated a physiologically relevant bacterial artificial chromosome (BAC)-based genomic DNA expression model to study PS1 gene expression and function. The PS1-WT-BAC construct restored γ-secretase function, whereas the mutant PS1 BACs demonstrated partial to complete loss of enzymatic activity when stably expressed in a PS double knock-out clonal cell line. We then engineered WT and mutant human PS1-BAC-Luciferase whole genomic locus reporter transgenes, which we transiently transduced in mouse and human non-neuronal and neuronal-like cells, respectively. PS1 ΔE9 and C410Y FAD were found to lower PS1 gene expression in both cell lines, whereas PS1-M146V showed a neuron-specific effect. The nonclinical γ-secretase inactive PS1-D257A mutation did not alter gene expression in either cell line. This is the first time that pathogenic coding mutations in the PS1 gene have been shown to lower PS1 gene expression. These findings may represent a pathologic mechanism for PS1 FAD mutations independent of their effects on γ-secretase activity and demonstrate how dominant PS1 mutations may exert their pathogenic effects by a loss-of-function mechanism.

HSV-1 as a Model for Emerging Gene Delivery Vehicles

The majority of viral vectors currently used possess modest cargo capability (up to 40 kb) being based on retroviruses, lentiviruses, adenoviruses, and adenoassociated viruses. These vectors have made the most rapid transition from laboratory to clinic because their small genomes have simplified their characterization and modification. However, there is now an increasing need both in research and therapy to complement this repertoire with larger capacity vectors able to deliver multiple transgenes or to encode complex regulatory regions, constructs which can easily span more than 100 kb. Herpes Simplex Virus Type I (HSV-1) is a well-characterized human virus which is able to package about 150 kb of DNA, and several vector systems are currently in development for gene transfer applications, particularly in neurons where other systems have low efficiency. However, to reach the same level of versatility and ease of use as that of smaller genome viral vectors, simple systems for high-titer production must be developed. This paper reviews the major HSV-1 vector systems and analyses the common elements which may be most important to manipulate to achieve this goal.

Gene delivery using helper virus-free HSV-1 amplicon vectors

Herpes simplex virus type 1 (HSV-1)-based amplicon vectors contain only a very small percentage of the 152-kbp viral genome. Consequently, replication and packaging of amplicons depend on helper functions that are provided either by replication-defective mutants of HSV-1 or by replication-competent, but packaging-defective, HSV-1 genomes. Sets of cosmids that overlap and represent the entire HSV-1 genome can form, via homologous recombination, circular replication-competent viral genomes, which give rise to infectious virus progeny. However, if the DNA cleavage/packaging signals are deleted, reconstituted virus genomes are not packageable, but still provide all the helper functions required for the packaging of cotransfected amplicon DNA. The resulting stocks of packaged amplicon vectors are essentially free of contaminating helper virus. This unit describes the cotransfection of amplicon and cosmid or bacterial artificial chromosome (BAC) DNA into 2-2 cells by cationic liposome-mediated transfection and the harvesting of packaged vector particles. Support protocols provide methods for preparing cosmid and BAC DNA and determining the titers of amplicon stocks.

Rotavirus viroplasm fusion and perinuclear localization are dynamic processes requiring stabilized microtubules

Rotavirus viroplasms are cytosolic, electron-dense inclusions corresponding to the viral machinery of replication responsible for viral template transcription, dsRNA genome segments replication and assembly of new viral cores. We have previously observed that, over time, those viroplasms increase in size and decrease in number. Therefore, we hypothesized that this process was dependent on the cellular microtubular network and its associated dynamic components. Here, we present evidence demonstrating that viroplasms are dynamic structures, which, in the course of an ongoing infection, move towards the perinuclear region of the cell, where they fuse among each other, thereby gaining considerably in size and, simultaneouly, explaining the decrease in numbers. On the viral side, this process seems to depend on VP2 for movement and on NSP2 for fusion. On the cellular side, both the temporal transition and the maintenance of the viroplasms are dependent on the microtubular network, its stabilization by acetylation, and, surprisingly, on a kinesin motor of the kinesin-5 family, Eg5. Thus, we provide for the first time deeper insights into the dynamics of rotavirus replication, which can explain the behavior of viroplasms in the infected cell.

Anti-angiogenic gene therapy in the treatment of malignant gliomas

More than four decades ago, Dr. Judah Folkman hypothesized that angiogenesis was a critical process in tumor growth. Since that time, there have been significant advances in understanding tumor biology and groundbreaking research in cancer therapy that have validated his hypothesis. However, in spite of extensive research, glioblastoma multiforme (GBM), the most common and malignant primary brain tumor, has gained little in the way of improved median survival. There have been several angiogenesis targets that have resulted in drugs that are in clinical trials or FDA approved for clinical use in several cancers. GBM is a highly angiogenic tumor and several drugs are showing promise in clinical trials with one (bevacizumab), clinically approved for use. We will review several possible angiogenic targets in GBM as well as the vector methodologies used for delivery. In addition, GBMs present several therapeutic challenges related to structure, tumor immune microenvironment and resistance to angiogenesis. To overcome these challenges will require novel approaches to improve therapeutic gene expression and vector biodistribution in the glioma.

HSV-1 amplicon vectors launch the production of heterologous rotavirus-like particles and induce rotavirus-specific immune responses in mice

Virus-like particles (VLPs) are promising vaccine candidates because they represent viral antigens in the authentic conformation of the virion and are therefore readily recognized by the immune system. As VLPs do not contain genetic material they are safer than attenuated virus vaccines. In this study, herpes simplex virus type 1 (HSV-1) amplicon vectors were constructed to coexpress the rotavirus (RV) structural genes VP2, VP6, and VP7 and were used as platforms to launch the production of RV-like particles (RVLPs) in vector-infected mammalian cells. Despite the observed splicing of VP6 RNA, full-length VP6 protein and RVLPs were efficiently produced. Intramuscular injection of mice with the amplicon vectors as a two-dose regimen without adjuvants resulted in RV-specific humoral immune responses and, most importantly, immunized mice were partially protected at the mucosal level from challenge with live wild-type (wt) RV. This work provides proof of principle for the application of HSV-1 amplicon vectors that mediate the efficient production of heterologous VLPs as genetic vaccines.

Construction and packaging of herpes simplex virus/adeno-associated virus (HSV/AAV) Hybrid amplicon vectors

Herpes simplex virus type 1 (HSV-1)-based amplicon vectors conserve most properties of the parental virus: broad host range, the ability to transduce dividing and nondiving cells, and a large transgene capacity. This permits incorporation of genomic sequences as well as cDNA, large transcriptional regulatory sequences for cell-specific expression, multiple transgene cassettes, or genetic elements from other viruses. Hybrid vectors use elements from HSV-1 that allow replication and packaging of large-vector DNA into highly infectious particles, and elements from other viruses that confer genetic stability to vector DNA in the transduced cell. For example, adeno-associated virus (AAV) has the unique ability to integrate its genome into a specific site on human chromosome 19. The viral rep gene and the inverted terminal repeats (ITRs) that flank the AAV genome are sufficient for this process. However, AAV-based vectors have a very small transgene capacity and do not conventionally contain the rep gene to support site-specific genomic integration. HSV/AAV hybrid vectors contain both HSV-1 replication and packaging functions and the AAV rep gene and a transgene cassette flanked by the AAV ITRs. This combines the large transgene capacity of HSV-1 with the capability of site-specific genomic transgene integration and long-term transgene expression of AAV. This protocol describes the preparation of HSV/AAV hybrid vectors using a replication-competent/conditional, packaging-defective HSV-1 genome cloned as a bacterial artificial chromosome (BAC) to provide helper functions for vector replication and packaging. The advantages and limitations of such vectors compared to standard HSV-1 amplicon vectors are also discussed.

Virally mediated gene manipulation in the adult CNS

Understanding how the CNS functions poses one of the greatest challenges in modern life science and medicine. Studying the brain is especially challenging because of its complexity, the heterogeneity of its cellular composition, and the substantial changes it undergoes throughout its life-span. The complexity of adult brain neural networks results also from the diversity of properties and functions of neuronal cells, governed, inter alia, by temporally and spatially differential expression of proteins in mammalian brain cell populations. Hence, research into the biology of CNS activity and its implications to human and animal behavior must use novel scientific tools. One source of such tools is the field of molecular genetics—recently utilized more and more frequently in neuroscience research. Transgenic approaches in general, and gene targeting in rodents have become fundamental tools for elucidating gene function in the CNS. Although spectacular progress has been achieved over recent decades by using these approaches, it is important to note that they face a number of restrictions. One of the main challenges is presented by the temporal and spatial regulation of introduced genetic manipulations. Viral vectors provide an alternative approach to temporally regulated, localized delivery of genetic modifications into neurons. In this review we describe available technologies for gene transfer into the adult mammalian CNS that use both viral and non-viral tools. We discuss viral vectors frequently used in neuroscience, with emphasis on lentiviral vector (LV) systems. We consider adverse effects of LVs, and the use of LVs for temporally and spatially controllable manipulations. Especially, we highlight the significance of viral vector-mediated genetic manipulations in studying learning and memory processes, and how they may be effectively used to separate out the various phases of learning: acquisition, consolidation, retrieval, and maintenance.

Herpes simplex virus type 1-based amplicon vectors for fundamental research in neurosciences and gene therapy of neurological diseases

Somatic manipulation of the nervous system without the involvement of the germinal line appears as a powerful counterpart of the transgenic strategy. The use of viral vectors to produce specific, transient and localized knockout, knockdown, ectopic expression or overexpression of a gene, leads to the possibility of analyzing both in vitro and in vivo molecular basis of neural function. In this approach, viral particles engineered to carry transgenic sequences are delivered into discrete brain regions, to transduce cells that will express the transgenic products. Amplicons are replication-incompetent helper-dependent vectors derived from herpes simplex virus type 1 (HSV-1), with several advantages that potentiate their use in neurosciences: (1) minimal toxicity: amplicons do not encode any virus proteins, are neither toxic for the infected cells nor pathogenic for the inoculated animals and elicit low levels of adaptive immune responses; (2) extensive transgene capacity to carry up to 150-kb of foreign DNA; i.e., entire genes with regulatory sequences could be delivered; (3) widespread cellular tropism: amplicons can experimentally infect several cell types including glial cells, though naturally the virus infects mainly neurons and epithelial cells; (4) since the viral genome does not integrate into cellular chromosomes there is low probability to induce insertional mutagenesis. Recent investigations on gene transfer into the brain using these vectors, have focused on gene therapy of inherited genetic diseases affecting the nervous system, such as ataxias, or on neurodegenerative disorders using experimental models of Parkinson's or Alzheimer's disease. Another group of studies used amplicons to investigate complex neural functions such as neuroplasticity, anxiety, learning and memory. In this short review, we summarize recent data supporting the potential of HSV-1 based amplicon vector model for gene delivery and modulation of gene expression in primary cultures of neuronal cells and into the brain of living animals.

Infectious delivery and long-term persistence of transgene expression in the brain by a 135-kb iBAC-FXN genomic DNA expression vector

Novel gene-based therapies for disease will depend in many cases on long-term persistent transgene expression. To develop gene therapy strategies for Friedreich's ataxia (FRDA), we have examined the persistence of transgene expression in the brain in vivo provided by the entire 135 kb FXN genomic DNA locus delivered as an infectious bacterial artificial chromosome (iBAC) herpes simplex virus type 1 (HSV-1)-based vector injected in the adult mouse cerebellum. We constructed genomic DNA-reporter fusion vectors carrying a complete 135 kb FXN genomic locus with an insertion of the Escherichia coli lacZ gene at the ATG start codon (iBAC-FXN-lacZ). SHSY5Y human neuroblastoma cells transduced by iBAC-FXN-lacZ showed high efficiency of vector delivery and LacZ expression. Direct intracranial injection of iBAC-FXN-lacZ into the adult mouse cerebellum resulted in a large number of easily detectable transduced cells, with LacZ expression driven by the FXN genomic locus, which persisted for at least 75 days. Green fluorescent protein expression driven from the same vector but by the strong HSV-1 IE4/5 promoter was transient. Our data demonstrate for the first time sustained transgene expression in vivo by infectious delivery of a genomic DNA locus >100 kb in size. Such an approach may be suitable for gene rescue strategies in neurological disease, such as FRDA.

Herpes simplex virus type 1-derived recombinant and amplicon vectors

Herpes simplex virus type 1 (HSV-1) is a human pathogen whose lifestyle is based on a long-term dual interaction with the infected host, being able to establish both lytic and latent infections. The virus genome is a 153 kbp double-stranded DNA molecule encoding more than 80 genes. The interest of HSV-1 as gene transfer vector stems from its ability to infect many different cell types, both quiescent and proliferating cells, the very high packaging capacity of the virus capsid, the outstanding neurotropic adaptations that this virus has evolved, and the fact that it never integrates into the cellular chromosomes, thus avoiding the risk of insertional mutagenesis. Two types of vectors can be derived from HSV-1, recombinant vectors and amplicon vectors, and different methodologies have been developed to prepare large stocks of each type of vector. This chapter summarizes (1) the two approaches most commonly used to prepare recombinant vectors through homologous recombination, either in eukaryotic cells or in bacteria, and (2) the two methodologies currently used to generate helper-free amplicon vectors, either using a bacterial artificial chromosome (BAC)-based approach or a Cre/loxP site-specific recombination strategy.

Interactions between AAV-2 and HSV-1: implications for hybrid vector design

Herpes simplex virus type 1 (HSV-1)-based amplicon vectors have a transgene capacity of up to 150 kbp and can efficiently transduce many different cell types in culture and in vivo without causing cytopathic effects. However, these vectors do not support long-term transgene expression. Adeno-associated virus type 2 (AAV-2) has the capacity to integrate its genome into a specific site on human chromosome 19, but AAV-2-derived gene therapy vectors have a transgene capacity of only 4.5 kb. To combine the large transgene capacity of HSV-1 with the potential for site-specific genomic integration and long-term transgene expression of AAV-2, HSV/AAV hybrid vectors have been developed. This review describes the design, applications and limitations of these hybrid vectors. However, as HSV-1 is a full helper virus for AAV-2 replication, the main focus is the analysis of the molecular mechanisms of interaction between the two viruses. The knowledge of these interactions will have direct implications on the design of novel HSV/AAV hybrid vectors.

Developing extrachromosomal gene expression vector technologies: an overview

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

Episomal transgene expression in pluripotent stem cells

Herpes simplex type 1 (HSV-1) amplicon vectors possess a number of features that make them excellent vectors for the delivery of transgenes into stem cells. HSV-1 amplicon vectors are capable of efficiently transducing both dividing and nondividing cells and since the virus is quite large, 152 kb, it is of sufficient size to allow for incorporation of entire genomic DNA loci with native promoters. HSV-1 amplicon vectors can also be used to incorporate and deliver to cells a variety of sequences that allow extrachromosomal retention. These elements offer advantages over integrating vectors as they avoid transgene silencing and insertional mutagenesis. The construction of amplicon vectors carrying extrachromosomal retention elements, their packaging into HSV-1 viral particles, and the use of HSV-1 amplicons for stem cell transduction will be described.

Prospects for the use of artificial chromosomes and minichromosome-like episomes in gene therapy

Artificial chromosomes and minichromosome-like episomes are large DNA molecules capable of containing whole genomic loci, and be maintained as nonintegrating, replicating molecules in proliferating human somatic cells. Authentic human artificial chromosomes are very difficult to engineer because of the difficulties associated with centromere structure, so they are not widely used for gene-therapy applications. However, OriP/EBNA1-based episomes, which they lack true centromeres, can be maintained stably in dividing cells as they bind to mitotic chromosomes and segregate into daughter cells. These episomes are more easily engineered than true human artificial chromosomes and can carry entire genes along with all their regulatory sequences. Thus, these constructs may facilitate the long-term persistence and physiological regulation of the expression of therapeutic genes, which is crucial for some gene therapy applications. In particular, they are promising vectors for gene therapy in inherited diseases that are caused by recessive mutations, for example haemophilia A and Friedreich's ataxia. Interestingly, the episome carrying the frataxin gene (deficient in Friedreich's ataxia) has been demonstrated to rescue the susceptibility to oxidative stress which is typical of fibroblasts from Friedreich's ataxia patients. This provides evidence of their potential to treat genetic diseases linked to recessive mutations through gene therapy.

Constitutive and Inducible Innate Responses in Cells Infected by HSV-1-Derived Amplicon Vectors

Amplicons are helper-dependent herpes simplex virus type 1 (HSV-1)-based vectors that can deliver very large foreign DNA sequences and, as such, are good candidates both for gene delivery and vaccine development. However, many studies have shown that innate constitutive or induced cellular responses, elicited or activated by the entry of HSV-1 particles, can play a significant role in the control of transgenic expression and in the induction of inflammatory responses. Moreover, transgene expression from helper-free amplicon stocks is often weak and transient, depending on the particular type of infected cells, suggesting that cellular responses could be also responsible for the silencing of amplicon-mediated transgene expression. This review summarizes the current experimental evidence underlying these latter concepts, focusing on the impact on transgene expression of very-early interactions between amplicon particles and the infected cells, and speculates on possible ways to counteract the cellular protective mechanisms, thus allowing stable transgene expression without enhancement of vector toxicity.

Glycoprotein D of bovine herpesvirus 5 (BoHV-5) confers an extended host range to BoHV-1 but does not contribute to invasion of the brain

Bovine herpesvirus 1 (BoHV-1) and BoHV-5 are closely related pathogens of cattle, but only BoHV-5 is considered a neuropathogen. We engineered intertypic gD exchange mutants with BoHV-1 and BoHV-5 backbones in order to address their in vitro and in vivo host ranges, with particular interest in invasion of the brain. The new viruses replicated in cell culture with similar dynamics and to titers comparable to those of their wild-type parents. However, gD of BoHV-5 (gD5) was able to interact with a surprisingly broad range of nectins. In vivo, gD5 provided a virulent phenotype to BoHV-1 in AR129 mice, featuring a high incidence of neurological symptoms and early onset of disease. However, only virus with the BoHV-5 backbone, independent of the gD type, was detected in the brain by immunohistology. Thus, gD of BoHV-5 confers an extended cellular host range to BoHV-1 and may be considered a virulence factor but does not contribute to the invasion of the brain.

Inhibition of herpes simplex virus type 1 replication by adeno-associated virus rep proteins depends on their combined DNA-binding and ATPase/helicase activities

Adeno-associated virus (AAV) has previously been shown to inhibit the replication of its helper virus herpes simplex virus type 1 (HSV-1), and the inhibitory activity has been attributed to the expression of the AAV Rep proteins. In the present study, we assessed the Rep activities required for inhibition of HSV-1 replication using a panel of wild-type and mutant Rep proteins lacking defined domains and activities. We found that the inhibition of HSV-1 replication required Rep DNA-binding and ATPase/helicase activities but not endonuclease activity. The Rep activities required for inhibition of HSV-1 replication precisely coincided with the activities that were responsible for induction of cellular DNA damage and apoptosis, suggesting that these three processes are closely linked. Notably, the presence of Rep induced the hyperphosphorylation of a DNA damage marker, replication protein A (RPA), which has been reported not to be normally hyperphosphorylated during HSV-1 infection and to be sequestered away from HSV-1 replication compartments during infection. Finally, we demonstrate that the execution of apoptosis is not required for inhibition of HSV-1 replication and that the hyperphosphorylation of RPA per se is not inhibitory for HSV-1 replication, suggesting that these two processes are not directly responsible for the inhibition of HSV-1 replication by Rep.