HSV-1-based vectors for gene therapy of neurological diseases and brain tumors: part II. Vector systems and applications

Novel Gene Therapy Approaches for Targeting Neurodegenerative Disorders: Focusing on Delivering Neurotrophic Genes

Neurodegenerative illnesses (NDDs) like Alzheimer's, Parkinson's, amyotrophic lateral sclerosis, spinal muscular atrophy, and Huntington's disease have demonstrated considerable potential for gene therapy as a viable therapeutic intervention. NDDs are marked by the decline of neurons, resulting in changes in both behavior and pathology within the body. Strikingly, only symptomatic management is available without a cure for the NDDs. There is an unmet need for a permanent therapeutic approach. Many studies have been going on to target the newer therapeutic molecular targets for NDDs including gene-based therapy. Gene therapy has the potential to provide therapeutic benefits to a large number of patients with NDDs by offering mechanisms including neuroprotection, neuro-restoration, and rectification of pathogenic pathways. Gene therapy is a medical approach that aims to modify the biological characteristics of living cells by controlling the expression of specific genes in certain neurological disorders. Despite being the most complex and well-protected organ in the human body, there is clinical evidence to show that it is possible to specifically target the central nervous system (CNS). This provides hope for the prospective application of gene therapy in treating NDDs in the future. There are several advanced techniques available for using viral or non-viral vectors to deliver the therapeutic gene to the afflicted region. Neurotrophic factors (NTF) in the brain are crucial for the development, differentiation, and survival of neurons in the CNS, making them important in the context of various neurological illnesses. Gene delivery of NTF has the potential to be used as a therapeutic approach for the treatment of neurological problems in the brain. This review primarily focuses on the methodologies employed for delivering the genes of different NTFs to treat neurological disorders. These techniques are currently being explored as a viable therapeutic approach for neurodegenerative diseases. The article exclusively addresses gene delivery approaches and does not cover additional therapy strategies for NDDs. Gene therapy offers a promising alternative treatment for NDDs by stimulating neuronal growth instead of solely relying on symptom relief from drugs and their associated adverse effects. It can serve as a long-lasting and advantageous treatment choice for the management of NDDs. The likelihood of developing NDDs increases with age as a result of neuronal degradation in the brain. Gene therapy is an optimal approach for promoting neuronal growth through the introduction of nerve growth factor genes.

Cationic cycloamylose based nucleic acid nanocarriers

Nucleic acid delivery by viral and non-viral methods has been a cornerstone for the contemporary gene therapy aimed at correcting the defective genes, replacing of the missing genes, or downregulating the expression of anomalous genes is highly desirable for the management of various diseases. Ostensibly, it becomes paramount for the delivery vectors to intersect the biological barriers for accessing their destined site within the cellular environment. However, the lipophilic nature of biological membranes and their potential to limit the entry of large sized, charged, hydrophilic molecules thus presenting a sizeable challenge for the cellular integration of negatively charged nucleic acids. Furthermore, the susceptibility of nucleic acids towards the degrading enzymes (nucleases) in the lysosomes present in cytoplasm is another matter of concern for their cellular and nuclear delivery. Hence, there is a pressing need for the identification and development of cationic delivery systems which encapsulate the cargo nucleic acids where the charge facilitates their cellular entry by evading the membrane barriers, and the encapsulation shields them from the enzymatic attack in cytoplasm. Cycloamylose bearing a closed loop conformation presents a robust candidature in this regard owing to its remarkable encapsulating tendency towards nucleic acids including siRNA, CpG DNA, and siRNA. The presence of numerous hydroxyl groups on the cycloamylose periphery provides sites for its chemical modification for the introduction of cationic groups, including spermine, (3-Chloro-2 hydroxypropyl) trimethylammonium chloride (Q188), and diethyl aminoethane (DEAE). The resulting cationic cycloamylose possesses a remarkable transfection efficiency and provides stability to cargo oligonucleotides against endonucleases, in addition to modulating the undesirable side effects such as unwanted immune stimulation. Cycloamylose is known to interact with the cell membranes where they release certain membrane components such as phospholipids and cholesterol thereby resulting in membrane destabilization and permeabilization. Furthermore, cycloamylose derivatives also serve as formulation excipients for improving the efficiency of other gene delivery systems. This review delves into the various vector and non-vector-based gene delivery systems, their advantages, and limitations, eventually leading to the identification of cycloamylose as an ideal candidate for nucleic acid delivery. The synthesis of cationic cycloamylose is briefly discussed in each section followed by its application for specific delivery/transfection of a particular nucleic acid.

Induced Pluripotent Stem Cells (iPSCs) and Gene Therapy: A New Era for the Treatment of Neurological Diseases

To date, gene therapy has employed viral vectors to deliver therapeutic genes. However, recent progress in molecular and cell biology has revolutionized the field of stem cells and gene therapy. A few years ago, clinical trials started using stem cell replacement therapy, and the induced pluripotent stem cells (iPSCs) technology combined with CRISPR-Cas9 gene editing has launched a new era in gene therapy for the treatment of neurological disorders. Here, we summarize the latest findings in this research field and discuss their clinical applications, emphasizing the relevance of recent studies in the development of innovative stem cell and gene editing therapeutic approaches. Even though tumorigenicity and immunogenicity are existing hurdles, we report how recent progress has tackled them, making engineered stem cell transplantation therapy a realistic option.

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.

Viral tools for neuroscience

Recombinant viruses are the workhorse of modern neuroscience. Whether one would like to understand a neuron's morphology, natural activity patterns, molecular composition, connectivity or behavioural and physiologic function, most studies begin with the injection of an engineered virus, often an adeno-associated virus or herpes simplex virus, among many other types. Recombinant viruses currently enable some combination of cell type-specific, circuit-selective, activity-dependent and spatiotemporally resolved transgene expression. Viruses are now used routinely to study the molecular and cellular functions of a gene within an identified cell type in the brain, and enable the application of optogenetics, chemogenetics, calcium imaging and related approaches. These advantageous properties of engineered viruses thus enable characterization of neuronal function at unprecedented resolution. However, each virus has specific advantages and disadvantages, which makes viral tool selection paramount for properly designing and executing experiments within the central nervous system. In the current Review, we discuss the key principles and uses of engineered viruses and highlight innovations that are needed moving forward.

Anti-Epidermal Growth Factor Receptor Gene Therapy for Glioblastoma

Glioblastoma multiforme (GBM) is the most common and aggressive primary intracranial brain tumor in adults with a mean survival of 14 to 15 months. Aberrant activation of the epidermal growth factor receptor (EGFR) plays a significant role in GBM progression, with amplification or overexpression of EGFR in 60% of GBM tumors. To target EGFR expressed by GBM, we have developed a strategy to deliver the coding sequence for cetuximab, an anti-EGFR antibody, directly to the CNS using an adeno-associated virus serotype rh.10 gene transfer vector. The data demonstrates that single, local delivery of an anti-EGFR antibody by an AAVrh.10 vector coding for cetuximab (AAVrh.10Cetmab) reduces GBM tumor growth and increases survival in xenograft mouse models of a human GBM EGFR-expressing cell line and patient-derived GBM. AAVrh10.CetMab-treated mice displayed a reduction in cachexia, a significant decrease in tumor volume and a prolonged survival following therapy. Adeno-associated-directed delivery of a gene encoding a therapeutic anti-EGFR monoclonal antibody may be an effective strategy to treat GBM.

Glioblastoma multiforme: State of the art and future therapeutics

BACKGROUND

Glioblastoma multiforme (GBM) is the most common and lethal primary malignancy of the central nervous system (CNS). Despite the proven benefit of surgical resection and aggressive treatment with chemo- and radiotherapy, the prognosis remains very poor. Recent advances of our understanding of the biology and pathophysiology of GBM have allowed the development of a wide array of novel therapeutic approaches, which have been developed. These novel approaches include molecularly targeted therapies, immunotherapies, and gene therapy.

METHODS

We offer a brief review of the current standard of care, and a survey of novel therapeutic approaches for treatment of GBM.

RESULTS

Despite promising results in preclinical trials, many of these therapies have demonstrated limited therapeutic efficacy in human clinical trials. Thus, although survival of patients with GBM continues to slowly improve, treatment of GBM remains extremely challenging.

CONCLUSION

Continued research and development of targeted therapies, based on a detailed understanding of molecular pathogenesis can reasonably be expected to yield improved outcomes for patients with GBM.

Evaluation and optimization of the administration of a selectively replicating herpes simplex viral vector to the brain by convection-enhanced delivery

The direct intraparenchymal administration of oncolytic viral vectors by convection-enhanced delivery (CED) represents a promising new treatment strategy for malignant gliomas. However, there is no evidence to suggest that oncolytic viruses as large as herpes simplex virus-1 (HSV-1) can be administered by CED, as this has not been systematically examined in an animal model. In this study, the administration of a herpes simplex viral vector, HSV1, has been evaluated in detail in the gray and white matter of both rat and pig models, using high flow-rate infusions, co-infusing heparin or preinfusing the tissue with an isotonic albumin solution. Rat HSV-1 infusions at both slow (0.5 μl min(-1)) and high infusion rates (2.5 μl min(-1)) led to extensive tissue damage and negligible cell transduction. Co-infusion with heparin led to extensive hemorrhage. Preinfusion of tissue with an isotonic albumin solution facilitated widespread vector distribution and cell transduction in white matter only. Using this approach in pig brain led to widespread vector distribution with extensive transduction of astrocytes and activated microglia. In rat brain, enhanced green fluorescent protein expression peaked 48 h after vector administration and was associated with a vigorous immune response. These findings indicate that direct infusions of HSV-1-based viral vectors into the brain lead to minimal vector distribution, negligible cell transduction and extensive damage. Tissue preinfusion with an isotonic solution prior to vector administration represents an effective technique for achieving widespread HSV-1 distribution.

Vector therapies for malignant glioma: shifting the clinical paradigm

BACKGROUND

Malignant glioma represents one of the most aggressive and devastating forms of human cancer. At present, there exists no successful treatment for this disease. Gene therapy, or vector therapy, has emerged as a viable experimental treatment method for intracranial malignancies.

OBJECTIVE

Vector therapy paradigms that have entered the clinical arena have shown adequate safety; however, the majority of the studies failed to observe significant clinical benefits. As such, researchers have refocused their efforts on developing novel vectors as well as new delivery methods to enhance the therapeutic effect of a particular vector. In this review, we discuss common vector therapy approaches used in clinical trials, their drawbacks and potential ways of overcoming these challenges.

METHODS

We focus on the experimental evaluation of cell-based vector therapies and adenoviral and herpes simplex virus type 1 vectors in the treatment of malignant glioma.

CONCLUSION

Vector therapy remains a promising treatment strategy for malignant glioma. Although significant questions remain to be answered, early clinical data suggest safety of this approach and future studies will likely address the efficacy of the proposed therapy.

Molecular imaging-guided gene therapy of gliomas

Gene therapy of patients with glioblastoma using viral and non-viral vectors, which are applied by direct injection or convection-enhanced delivery (CED), appear to be satisfactorily safe. Up to date, only single patients show a significant therapeutic benefit as deduced from single long-term survivors. Non-invasive imaging by PET for the identification of viable target tissue and for assessment of transduction efficiency shall help to identify patients which might benefit from gene therapy, while non-invasive follow-up on treatment responses allows early and dynamic adaptations of treatment options. Therefore, molecular imaging has a critical impact on the development of standardised gene therapy protocols and on efficient and safe vector applications in humans.

Gene therapy approaches for stem cell protection

Cytotoxic exposure of bone marrow and other non-hematopoietic organs containing self-renewing stem cell populations is associated with damage to the supportive microenvironment. Recent evidence indicates that radical oxygen species resulting from the initial oxidative stress persist for months after ionizing irradiation exposure of tissues including oral cavity, esophagus, lung and bone marrow. Antioxidant gene therapy using manganese superoxide dismutase plasmid liposomes has provided organ-specific radiation protection associated with delay or prevention of acute and late toxicity. Recent evidence has suggested that manganese superoxide dismutase transgene expression in cells of the organ microenvironment contributes significantly to the mechanism of protection. Incorporating this knowledge into designs of novel approaches for stem cell protection is addressed in the present review.

Imaging-Guided Gene Therapy of Experimental Gliomas

To further develop gene therapy for patients with glioblastomas, an experimental gene therapy protocol was established comprising a series of imaging parameters for (i) noninvasive assessment of viable target tissue followed by (ii) targeted application of herpes simplex virus type 1 (HSV-1) amplicon vectors and (iii) quantification of treatment effects by imaging. We show that viable target tissue amenable for application of gene therapy vectors can be identified by multitracer positron emission tomography (PET) using 2-(18)F-fluoro-2-deoxy-D-glucose, methyl-(11)C-L-methionine, or 3'-deoxy-3'-(18)F-fluoro-L-thymidine ([(18)F]FLT). Targeted application of HSV-1 amplicon vectors containing two therapeutic genes with synergistic antitumor activity (Escherichia coli cytosine deaminase, cd, and mutated HSV-1 thymidine kinase, tk39, fused to green fluorescent protein gene, gfp) leads to an overall response rate of 68%, with 18% complete responses and 50% partial responses. Most importantly, we show that the "tissue dose" of HSV-1 amplicon vector-mediated gene expression can be noninvasively assessed by 9-[4-(18)F-fluoro-3-(hydroxymethyl)butyl]guanine ([(18)F]FHBG) PET. Therapeutic effects could be monitored by PET with significant differences in [(18)F]FLT accumulation in all positive control tumors and 72% in vivo transduced tumors (P = 0.01) as early as 4 days after prodrug therapy. For all stably and in vivo transduced tumors, cdIREStk39gfp gene expression as measured by [(18)F]FHBG-PET correlated with therapeutic efficiency as measured by [(18)F]FLT-PET. These data indicate that imaging-guided vector application with determination of tissue dose of vector-mediated gene expression and correlation to induced therapeutic effect using multimodal imaging is feasible. This strategy will help in the development of safe and efficient gene therapy protocols for clinical application.

Human gene therapy and imaging in neurological diseases

Molecular imaging aims to assess non-invasively disease-specific biological and molecular processes in animal models and humans in vivo. Apart from precise anatomical localisation and quantification, the most intriguing advantage of such imaging is the opportunity it provides to investigate the time course (dynamics) of disease-specific molecular events in the intact organism. Further, molecular imaging can be used to address basic scientific questions, e.g. transcriptional regulation, signal transduction or protein/protein interaction, and will be essential in developing treatment strategies based on gene therapy. Most importantly, molecular imaging is a key technology in translational research, helping to develop experimental protocols which may later be applied to human patients. Over the past 20 years, imaging based on positron emission tomography (PET) and magnetic resonance imaging (MRI) has been employed for the assessment and "phenotyping" of various neurological diseases, including cerebral ischaemia, neurodegeneration and brain gliomas. While in the past neuro-anatomical studies had to be performed post mortem, molecular imaging has ushered in the era of in vivo functional neuro-anatomy by allowing neuroscience to image structure, function, metabolism and molecular processes of the central nervous system in vivo in both health and disease. Recently, PET and MRI have been successfully utilised together in the non-invasive assessment of gene transfer and gene therapy in humans. To assess the efficiency of gene transfer, the same markers are being used in animals and humans, and have been applied for phenotyping human disease. Here, we review the imaging hallmarks of focal and disseminated neurological diseases, such as cerebral ischaemia, neurodegeneration and glioblastoma multiforme, as well as the attempts to translate gene therapy's experimental knowledge into clinical applications and the way in which this process is being promoted through the use of novel imaging approaches.

Variability in infectivity of primary cell cultures of human brain tumors with HSV-1 amplicon vectors

We investigated the variability in infectivity of cells in primary brain tumor samples from different patients using an HSV-1 amplicon vector. We studied the infectivity of HSV-1 amplicon vectors in tumor samples derived from neurosurgical resections of 20 patients. Cells were infected with a definite amount of HSV-1 amplicon vector HSV-GFP. Transduction efficiency in primary tumor cell cultures was compared to an established human glioma line. Moreover, duration of transgene expression was monitored in different tumor cell types. All primary cell cultures were infectable with HSV-GFP with variable transduction efficiencies ranging between 3.0 and 42.4% from reference human Gli36 Delta EGFR glioma cells. Transduction efficiency was significantly greater in anaplastic gliomas and meningiomas (26.7+/-17.4%) compared to more malignant tumor types (glioblastomas, metastases; 11.2+/-8.5%; P=0.05). To further investigate the possible underlying mechanism of this variability, nectin-1/HevC expression was analyzed and was found to contribute, at least in part, to this variability in infectability. The tumor cells expressed the exogenous gene for 7 to 61 days with significant shorter expression in glioblastomas (18+/-13 d) compared to anaplastic gliomas (42+/-24 d; P<0.05). Interindividual variability of infectivity by HSV-1 virions might explain, at least in part, why some patients enrolled in gene therapy for glioblastoma in the past exhibited a sustained response to HSV-1-based gene- and virus therapy. Infectivity of primary tumor samples from respective patients should be tested to enable the development of efficient and safe herpes vector-based gene and virus therapy for clinical application.

Potential of equine herpesvirus 1 as a vector for immunization

Key problems using viral vectors for vaccination and gene therapy are antivector immunity, low transduction efficiencies, acute toxicity, and limited capacity to package foreign genetic information. It could be demonstrated that animal and human cells were efficiently transduced with equine herpesvirus 1 (EHV-1) reconstituted from viral DNA maintained and manipulated in Escherichia coli. Between 13 and 23% of primary human CD3+, CD4+, CD8+, CD11b+, and CD19+ cells and more than 70% of CD4+ MT4 cells or various human tumor cell lines (MeWo, Huh7, HeLa, 293T, or H1299) could be transduced with one infectious unit of EHV-1 per cell. After intranasal instillation of EHV-1 into mice, efficient transgene expression in lungs was detectable. Successful immunization using EHV-1 was shown after delivery of the human immunodeficiency virus type 1 Pr55gag precursor by the induction of a Gag-specific CD8+ immune response in mice. Because EHV-1 was not neutralized by human sera containing high titers of antibodies directed against human herpesviruses 1 to 5, it is concluded that this animal herpesvirus has enormous potential as a vaccine vector, because it is able to efficiently transduce a variety of animal and human cells, has high DNA packaging capacity, and can conveniently be maintained and manipulated in prokaryotic cells.

Human cytomegalovirus plasmid-based amplicon vector system for gene therapy

We have constructed and evaluated the utility of a helper-dependent virus vector system that is derived from Human Cytomegalovirus (HCMV). This vector is based on the herpes simplex virus (HSV) amplicon system and contains the HCMV orthologs of the two cis-acting functions required for replication and packaging of HSV genomes, the complex HCMV viral DNA replication origin (oriLyt), and the cleavage packaging signal (the a sequence). The HCMV amplicon vector replicated independently and was packaged into infectious virions in the presence of helper virus. This vector is capable of delivering and expressing foreign genes in infected cells including progenitor cells such as human CD34+ cells. Packaged defective viral genomes were passaged serially in fibroblasts and could be detected at passage 3; however, the copy number appeared to diminish upon serial passage. The HCMV amplicon offers an alternative vector strategy useful for gene(s) delivery to cells of the hematopoietic lineage.

Molecular imaging of gene therapy for cancer

Gene therapy of cancer has been one of the most exciting and elusive areas of scientific and clinical research in the past decade. One of the most critical issues for ensuring success of this therapy is the development of technology for noninvasive monitoring of the location, magnitude and duration of vector-mediated gene expression, as well as the distribution and targeting of vector particles in vivo. In recent years many advances have been made in high-resolution, in vivo imaging methods, including: radionuclide imaging, such as positron emission tomography (PET) and single photon emission tomography (SPECT), magnetic resonance (MR) imaging and spectroscopy, bioluminescence imaging and various fluorescence imaging techniques, including fluorescence-mediated tomography (FMT) and near-infrared fluorescence (NIRF) reflectance imaging. A variety of factors determine the choice of specific imaging system, some of them are the imaging requirements (single or repeated), intended use (animal or human) and spatial requirements (organs versus cellular resolution and depth). This review provides descriptions of modalities applicable to imaging different parameters of vector-mediated gene expression in tumors and stem cell tracking in vivo.

Imaging in gene therapy of patients with glioma

Over 10 years ago, the first successful gene therapy paradigms for experimental brain tumors models have been conducted, and they were thought to revolutionize the treatment of patients with gliomas. Application of gene therapy has been quickly forced into clinical trials, the first patients being enrolled in 1994, with overall results being disappointing. However, single patients seemed to benefit from gene therapy showing long-term treatment response, and most of these patients bearing small glioblastomas. Whereas the gene therapy itself has been performed with high sophistication, limited attention has been paid on technologies, which (i) allow an identification of viable target tissue in heterogenous glioma tissue and which (ii) enable an assessment of successful vector administration and vector-mediated gene expression in vivo. However, these measures are a prerequisite for the development of successful gene therapy in the clinical application. As biological treatment strategies such as gene and cell-based therapies hold promise to selectively correct disease pathogenesis, successful clinical implementation of these treatment strategies rely on the establishment of molecular imaging technology allowing the non-invasive assessment of endogenous and exogenous gene expression in vivo. Imaging endogenous gene expression will allow the characterization and identification of target tissue for gene therapy. Imaging exogenously introduced cells and genes will allow the determination of the 'tissue dose' of transduced cell function and vector-mediated gene expression, which in turn can be correlated to the induced therapeutic effect. Only these combined strategies of non-invasive imaging of gene expression in vivo will enable the establishment of safe and efficient vector administration and gene therapy protocols for clinical application. Here, we review some aspects of imaging in gene therapy trials for glioblastoma, and we present a 'proof-of-principle' 2nd-generation gene therapy protocol integrating molecular imaging technology for the establishment of efficient gene therapy in clinical application.

Cell-type-specific gene delivery into neuronal cells in vitro and in vivo

The avian retroviruses reticuloendotheliosis virus strain A (REV-A) and spleen necrosis virus (SNV) are not naturally infectious in human cells. However, REV-A-derived viral vectors efficiently infect human cells when they are pseudotyped with envelope proteins displaying targeting ligands specific for human cell-surface receptors. Here we report that vectors containing the gag region of REV-A and pol of SNV can be pseudotyped with the envelope protein of vesicular stomatitis virus (VSV) and the glycoproteins of different rabies virus (RV) strains. Vectors pseudotyped with the envelope protein of the highly neurotropic RV strain CVS-N2c facilitated cell type-specific gene delivery into mouse and human neurons, but did not infect other human cell types. Moreover, when such vector particles were injected into the brain of newborn mice, only neuronal cells were infected in vivo. Cell-type-specific gene delivery into neurons may present quite specific gene therapy approaches for many degenerative diseases of the brain.

Leukemia vaccines

Evidence that immunological effector mechanisms contribute to the elimination of leukemic blasts in allogeneic bone marrow transplantation supports the concept that the immune system plays a prominent role in the control of leukemic disease. For patients with high-risk acute leukemia, relapse prevention in the setting of minimal residual disease is paramount. This review discusses vaccine strategies aimed to stimulate a leukemia-specific immune response in vivo.

Prospects of molecular imaging in neurology

Molecular imaging aims towards the non-invasive kinetic and quantitative assessment and localization of biological processes of normal and diseased cells in vivo in animal models and humans. Due to technological advances during the past years, imaging of molecular processes is a rapidly growing field, which has the potential of broad applications in the study of cell biology, biochemistry, gene/protein function and regulation, signal transduction, characterization of transgenic animals, development of new treatment strategies (gene or cell-based) and their successful implementation into clinical application. Most importantly, the possibility to study these parameters in the same subject repeatedly over time makes molecular imaging an attractive technology to obtain reliable data and to safe recourse; for example, molecular imaging enables the assessment of an exogenously introduced therapeutic gene and the related alterations of endogenously regulated gene functions directly in the same subject. Therefore, molecular imaging will have great implications especially when molecular diagnostic and treatment modalities have to be translated from experimental into clinical application. Here, we review the three main imaging technologies, which have been developed for studying molecular processes in vivo, the disease models, which have been studied so far, and the potential future applications.

Gene therapy: a primer for neurosurgeons

Gene therapy involves the transfer of genes into cells with therapeutic intent. Although several methods can accomplish this, vectors based on viruses still provide the most efficient approach. For neurosurgical purposes, preclinical and clinical applications in the areas of glioma therapy, spinal neurosurgery, and neuroprotection for treatment of Parkinson's disease and cerebral ischemia are reviewed. In general, therapies applied in the neurosurgical realm have proven relatively safe, despite occasional, well-publicized cases of morbidity and death in non-neurosurgical trials. However, continued clinical and preclinical research in this area is critical, to fully elucidate potential toxicities and to generate truly effective treatments that can be applied in neurological diseases.

PET-based molecular imaging in neuroscience

Positron emission tomography (PET) allows non-invasive assessment of physiological, metabolic and molecular processes in humans and animals in vivo. Advances in detector technology have led to a considerable improvement in the spatial resolution of PET (1-2 mm), enabling for the first time investigations in small experimental animals such as mice. With the developments in radiochemistry and tracer technology, a variety of endogenously expressed and exogenously introduced genes can be analysed by PET. This opens up the exciting and rapidly evolving field of molecular imaging, aiming at the non-invasive localisation of a biological process of interest in normal and diseased cells in animal models and humans in vivo. The main and most intriguing advantage of molecular imaging is the kinetic analysis of a given molecular event in the same experimental subject over time. This will allow non-invasive characterisation and "phenotyping" of animal models of human disease at various disease stages, under certain pathophysiological stimuli and after therapeutic intervention. The potential broad applications of imaging molecular events in vivo lie in the study of cell biology, biochemistry, gene/protein function and regulation, signal transduction, transcriptional regulation and characterisation of transgenic animals. Most importantly, molecular imaging will have great implications for the identification of potential molecular therapeutic targets, in the development of new treatment strategies, and in their successful implementation into clinical application. Here, the potential impact of molecular imaging by PET in applications in neuroscience research with a special focus on neurodegeneration and neuro-oncology is reviewed.

Construction of a cytomegalovirus-based amplicon: a vector with a unique transfer capacity

Cytomegalovirus (CMV) has a number of interesting properties that qualifies it as a vector for gene transfer. Especially appealing is the ability of the CMV genome to persist in hematopoietic progenitor cells and the packaging capacity of the viral capsid that accommodates a DNA genome of 230 kbp. In order to exploit the packaging capacity of the CMV capsid we investigated whether the principles of an amplicon vector can be applied to CMV. Amplicons are herpesviral vectors, which contain only the cis-active sequences required for replication and packaging of the vector genome. For construction of a CMV amplicon the sequences comprising the lytic origin of replication (orilyt) and the cleavage packaging recognition sites (pac) of human CMV were cloned onto a plasmid. A gene encoding the green fluorescent protein was used as a model transgene. The amplicon plasmid replicated in the presence of a CMV helper virus and was packaged into CMV particles, with replication and packaging being dependent on the presence of the orilyt and pac sequences. The packaged amplicon could be transferred to recipient cells and reisolated from the transduced cells. Analysis of the DNA isolated from CMV capsids revealed that the CMV amplicon was packaged as a concatemer with a size of approximately 210 kbp. The CMV amplicon vector has the potential to transfer therapeutic genes with a size of more than 200 kbp and thus provides a unique transfer capacity among viral vectors.

Viral therapy for glioblastoma

Malignant gliomas remain amongst the most difficult cancer to treat. Viral-based gene therapies have been employed for the last decade in preclinical and clinical modes as a novel treatment modality. In this review, such therapies are summarized. The overwhelming majority of clinical studies point one to conclude that methodologies that will increase tumor infection/transduction will lead to enhanced therapeutic results.

Improved herpes simplex virus type 1 amplicon vectors for proportional coexpression of positron emission tomography marker and therapeutic genes

For the development of efficient and safe gene therapy protocols for clinical application it is desirable to determine the tissue dose of vector-mediated therapeutic gene expression noninvasively in vivo. The herpes simplex virus type 1 thymidine kinase gene (HSV-1-tk) has been shown to function as a marker gene for the direct noninvasive in vivo localization of thymidine kinase (TK) expression by positron emission tomography (PET). Using bicistronic or multicistronic gene-expressing cassettes with tk as the PET marker gene, the quantitative analysis of tk gene expression may indirectly indicate the distribution and the level of expression of linked and proportionally coexpressed genes. Here, we describe the construction and functional evaluation of HSV-1 amplicon vectors mediating proportional coexpression of HSV-1-tk as PET marker gene and the enhanced green fluorescent protein gene (gfp) as proof of principle and cell culture marker gene and the Escherichia coli cytosine deaminase (cd) as therapeutic gene. Several double-/triple-gene constructs expressing HSV-1-tk, gfp, and E. coli cd were engineered based on gene fusion or the use of an internal ribosome entry site (IRES). Functional analysis in cell culture (green fluorescent protein [GFP] fluorescence and sensitivity to the prodrugs ganciclovir [GCV] and 5-fluorocytosine [5-FC]) and Western blots were carried out after infection of proliferating rat 9L gliosarcoma and human Gli36 glioma cells with helper virus-free packaged HSV-1 amplicon vectors. To study the ability of PET to differentiate various levels of tk expression noninvasively in vivo, retrovirally transduced and selected populations of rat F98 and human Gli36dEGFR glioma cells with defined levels of proportionally coexpressed tk and gfp genes were grown as subcutaneous tumors in nude rats and nude mice, and tk imaging by PET was performed. To study HSV-1 amplicon vector-mediated gene coexpression in vivo, HSV-1 amplicon vectors bearing coexpression constructs were injected (4 x 10(7) to 1 x 10(8) transducing units) into subcutaneously growing Gli36dEGFR gliomas in nude animals, and tk imaging was performed 24 hr later. All vector constructs mediated GFP expression and sensitized 9L and Gli36 cells toward GCV- and 5-FC-mediated cell killing in a drug dose-dependent manner, respectively. The levels of gene expression varied depending on the location of the genes within the constructs indicating the influence of the IRES on the level of expression of the second gene. Moreover, functional proportional coexpression of the PET marker gene HSV-1-tk and the linked therapeutic E. coli cd gene was observed. In selected tumor cell populations, subtle IRES-dependent differences of tk gene expression could be noninvasively distinguished by PET with good correlation between quantitative assays for IRES-dependent attenuated GFP and TK expression in culture and in vivo. After infection of subcutaneously growing gliomas with HSV-1 amplicon vectors, various levels of TK expression were found ranging from 0.011-0.062 percentage injected dose per gram (%ID/g). These values were 4.0- to 5.7-fold lower than positive control tumor cells. TK expression could be imaged by PET in vivo even with the tk gene located at the weak position downstream from the IRES. In conclusion, these HSV-1 amplicon vectors carrying HSV-1-tk as PET marker gene and any linked therapeutic gene will serve an indirect noninvasive assessment of the distribution of therapeutic gene expression by PET. Monitoring the correlation between primary transduction and therapeutic efficiency of a given vector is highly desirable for the development of safe and efficient gene therapy and vector application protocols in clinical applications.

Modified herpes simplex virus delivery of enhanced GFP into the central nervous system

Controlled expression of proteins is a key experimental approach to a deeper understanding of the molecular basis of neuronal function. Here we evaluate the HSV-1 (herpes simplex virus) amplicon vector for gene delivery into the brains of living rats. We demonstrate that HSV-1 amplicon vectors expressing enhanced green fluorescent protein (EGFP) can reliably infect neurons after it is injected into cortex, striatum and thalamus in rats, producing sufficient numbers of infected neurons for electrophysiological experiments in acute brain slices. Expression of EGFP delivered by the HSV-1 amplicon was detected for up to 5 weeks post-infection. We detected no changes in the morphology or the electrophysiological properties of thalamic, striatal or cortical neurons within a period of at least 2 weeks after HSV-1 amplicon injection. We conclude that the HSV-1 amplicon is a valuable tool for gene delivery in the rat central nervous system.

Dynamics of transgene expression in human glioblastoma cells mediated by herpes simplex virus/adeno-associated virus amplicon vectors

One of the challenges in gene therapy is to ensure stable transgene expression at the site of disease with a high degree of accuracy and safety. In this paper, we examine both viral and cellular elements that may affect the level of transgene expression mediated by herpes simplex virus type 1 (HSV-1) adeno-associated virus (AAV) amplicon vectors. These elements include the AAV inverted terminal repeats (ITRs), the AAV Rep proteins, and the allelic status of 19q in human glioma cell lines. The latter is of particular interest because the AAV integration site (AAVS1) is located on the long arm of chromosome 19 and 30-40% of human glioblastoma tumors are reported to have loss of heterozygosity in this region of chromosome 19q. Fluorescence-activated cell-sorting analysis results indicate that inclusion of minimal or full-length AAV ITRs in HSV-1 amplicon vectors markedly increases the efficiency of transgene expression. On the other hand, insertion of the AAV rep gene decreases the level of transgene expression, apparently because of the cytotoxic effects of Rep proteins. Further, the levels of transgene expression appear to be independent of 19q allelic status or the number of endogenous AAVS1 sequences in the various glioma cell lines studied. Taken together, these data support employing AAV ITRs, in the context of HSV-1 amplicon vectors, to enhance short-term levels of transgene expression.

Potential new methods for antiepileptic drug delivery

Use of novel drug delivery methods could enhance the efficacy and reduce the toxicity of antiepileptic drugs (AEDs). Slow-release oral forms of medication or depot drugs such as skin patches might improve compliance and therefore seizure control. In emergency situations, administration via rectal, nasal or buccal mucosa can deliver the drug more quickly than can oral administration. Slow-release oral forms and rectal forms of AEDs are already approved for use, nasal and buccal administration is currently off-label and skin patches for AEDs are an attractive but currently hypothetical option. Therapies under development may result in the delivery of AEDs directly to the regions of the brain involved in seizures. Experimental protocols are underway to allow continuous infusion of potent excitatory amino acid antagonists into the CSF. In experiments with animal models of epilepsy, AEDs have been delivered successfully to seizure foci in the brain by programmed infusion pumps, acting in response to computerised EEG seizure detection. Inactive prodrugs can be given systemically and activated at the site of the seizure focus by locally released compounds. One such drug under development is DP-VPA (or DP16), which is cleaved to valproic acid (sodium valproate) by phospholipases at the seizure focus. Liposomes and nanoparticles are engineered micro-reservoirs of a drug, with attached antibodies or receptor-specific binding agents designed to target the particles to a specific region of the body. Liposomes in theory could deliver a high concentration of an AED to a seizure focus. Penetration of the blood-brain barrier can be accomplished by linking large particles to iron transferrin or biological toxins that can cross the barrier. In the near future, it is likely that cell transplants that generate neurotransmitters and neuromodulators will accomplish renewable endogenous drug delivery. However, the survival and viability of transplanted cells have yet to be demonstrated in the clinical setting. Gene therapy also may play a role in local drug delivery with the use of adenovirus, adeno-associated virus, herpesvirus or other delivery vectors to induce brain cells to produce local modulatory substances. New delivery systems should significantly improve the therapeutic/toxic ratio of AEDs.

Molecular Imaging of Gliomas

Gliomas are the most common types of brain tumors. Although sophisticated regimens of conventional therapies are being carried out to treat patients with gliomas, the disease invariably leads to death over months or years. Before new and potentially more effective treatment strategies, such as gene- and cell-based therapies, can be effectively implemented in the clinical application, certain prerequisites have to be established. First of all, the exact localization, extent, and metabolic activity of the glioma must be determined to identify the biologically active target tissue for a biological treatment regimen; this is usually performed by imaging the expression of up-regulated endogenous genes coding for glucose or amino acid transporters and cellular hexokinase and thymidine kinase genes, respectively. Second, neuronal function and functional changes within the surrounding brain tissue have to be assessed in order to save this tissue from therapy-induced damage. Third, pathognomonic genetic changes leading to disease have to be explored on the molecular level to serve as specific targets for patient-tailored therapies. Last, a concerted noninvasive analysis of both endogenous and exogenous gene expression in animal models as well as the clinical setting is desirable to effectively translate new treatment strategies from experimental into clinical application. All of these issues can be addressed by multimodal radionuclide and magnetic resonance imaging techniques and fall into the exciting and fast growing field of molecular and functional imaging. Noninvasive imaging of endogenous gene expression by means of positron emission tomography (PET) may reveal insight into the molecular basis of pathogenesis and metabolic activity of the glioma and the extent of treatment response. When exogenous genes are introduced to serve for a therapeutic function, PET imaging may reveal the assessment of the “location,” “magnitude,” and “duration” of therapeutic gene expression and its relation to the therapeutic effect. Detailed reviews on molecular imaging have been published from the perspective of radionuclide imaging (Gambhir et al., 2000; Blasberg and Tjuvajev, 2002) as well as magnetic resonance and optical imaging (Weissleder, 2002). The present review focuses on molecular imaging of gliomas with special reference on the status and perspectives of imaging of endogenous and exogenously introduced gene expression in order to develop improved diagnostics and more effective treatment strategies of gliomas and, in that, to eventually improve the grim prognosis of this devastating disease.

[Herpes simplex virus infection: host-virus interaction]

Herpes simplex virus infections still cause significant morbidity in immunocompetent and immunodeficient patients, in spite of the availability of specific anti-viral therapy. Latency and neurovirulence are central to the pathogenesis of infection, and the mechanisms underlying these properties are not yet completely known. Major advances of the recent years should shed light on the mechanisms governing host-virus interaction, and are illustrated in this review. They include the discovery of molecules acting as receptors for the virus, models for studying virus-neuron interaction during latency and reactivation, and animal models of neurovirulence.

Molecular and functional imaging technology for the development of efficient treatment strategies for gliomas

Gliomas are the most common types of brain tumors, which invariably lead to death over months or years. Before new and potentially more effective treatment strategies, such as gene therapy, can be effectively introduced into clinical application the following goals must be reached: (1) the determination of localization, extent and metabolic activity of the glioma; (2) the assessment of functional changes within the surrounding brain tissue; (3) the identification of genetic changes on the molecular level leading to disease; and in addition (4) a detailed non-invasive analysis of both endogenous and exogenous gene expression in animal models and in the clinical setting. Non-invasive imaging of endogenous gene expression by means of positron emission tomography (PET) may reveal insight into the molecular basis of pathogenesis and metabolic activity of the glioma and the extent of treatment response. When exogenous genes are introduced to serve for a therapeutic function, PET imaging techniques may reveal the assessment of the location, magnitude and duration of therapeutic gene expression and its relation to the therapeutic effect. Here, we review the main principles of PET imaging and its key roles in neurooncology research.

Towards non-invasive imaging of HSV-1 vector-mediated gene expression by positron emission tomography

The overall goals of the broad and growing field of molecular medicine is to identify fundamental errors of disease and to develop corrections of them on the molecular level. At the same time, real-time imaging of gene expression in vivo aims towards a detailed analysis of both endogenous and exogenous gene expression in animal models of disease and in the clinical setting. Non-invasive imaging of endogenous gene expression may reveal insight into the molecular basis of disease pathogenesis and the extent of treatment response. When exogenous genes are introduced, e.g. by herpes simplex virus type 1 (HSV-1)-based vectors, to ameliorate a genetic defect or to add an additional gene function to cells, imaging techniques may reveal the assessment of the location, magnitude and duration of therapeutic gene expression and its correlation to the therapeutic effect. Here, we review the main approaches of non-invasive imaging techniques of gene expression in vivo with special reference to HSV-1 vector-mediated gene expression.

Gene therapy of pain: emerging strategies and future directions

Gene therapy to alleviate pain could appear surprising and perhaps not appropriate when opioids and other active molecules are available. However, the possibility of introducing a therapeutic protein into some targeted structures, where it would be continuously synthesised and exert its biological effect in the near vicinity of, or inside the cells, might avoid some drawbacks of "classical" drugs. Moreover, the gene-transfer techniques might improve present therapies or lead to novel ones. The recent significant and constant advances in vector systems design suggest that these techniques will be available in the near future for safe application in humans. The first experimental protocols attempting the transfer of opioid precursors genes, leading to their overexpression at the spinal level, demonstrated the feasibility and the potential interest of these approaches. Indeed, overproduction of opioid peptides in primary sensory neurones or spinal cord induced antihyperalgesic effects in various animal models of persistent pain. However, numerous other molecules involved in pain processing or associated with chronic pain have been identified and the gene-based techniques might be particularly adapted for the evaluation of the possible therapeutic interest of these new potential targets.

Ionizing radiation does not alter the antitumor activity of herpes simplex virus vector G207 in subcutaneous tumor models of human and murine prostate cancer

Viral gene therapy against malignant tumors holds great promise for tumors that are susceptible to the oncolytic activity of viruses. One advantage of oncolytic viral therapy is that it can potentially be combined with other therapies, such as radiotherapy, to obtain an enhanced tumor response. In the case of prostate cancer, herpes simplex virus-mediated therapies have been shown to be highly effective in animal models; however, studies of the efficacy of combined viral and radiation therapy have not yet been reported. In this study, we have combined G207, a multimutated HSV type 1 vector, with external beam radiation therapy of prostate tumors grown subcutaneously in mice. We examined both the human LNCaP tumor in athymic mice and the mouse transgenic TRAMP tumor in either athymic mice or its syngeneic host, C57BL/6 mice. Virus was delivered either intravenously, in the case of LNCaP, or intratumorally, in the case of TRAMP. We found that individually, either G207 or radiation was effective in delaying tumor growth in these models. However, delivering the treatments simultaneously did not produce an enhanced effect.

Genetic engineering within the adult brain: implications for molecular approaches to behavioral neuroscience

Currently, the most popular technology used to modify the molecular makeup of the nervous system is through germline modifications of early embryos. This allows to construct gene 'knock-ins' (gene overexpression) or 'knock-outs' (gene deletions). This technology leads to gene additions or deletions from the earliest developmental stages. This can potentially lead to compensatory genetic changes. The technology to achieve inducible and cell-type-specific changes in gene expression in transgenic animals has been established. However, it is not yet possible, to reliably turn a particular gene 'on' or 'off' exclusively in adult animals. Alternatively, the use of gene transfer technology in fully mature animals could overcome many of these shortcomings. Gene therapy is the use of nucleic acids as drugs, and uses gene transfer technology to genetically engineer adult animals. Viral and nonviral vectors have been modified to serve as vectors for nucleic acid sequences of interest. Thus, over the last two decades, methods have been developed to deliver particular nucleic acids directly to target tissues. Further technological advances allow delivery of transgenes or antisense mRNAs directly to predetermined cell types, as well as their delivery under the control of inducible promoter elements. Combined transgenic (i.e., germline modifications) and viral vector technology will also be very powerful in allowing the genetic modification of selected neuronal populations in adult animals. In this review, we discuss the potential of gene delivery to the brain to analyze the effect of genetic engineering of particular neuronal groups on behavior, as well as recent developments and applications of newly engineered vector systems to allow transgenesis within nervous structures of adult animals.

Therapeutic efficacy of G207 in a novel peripheral nerve sheath tumor model

Nerve involvement poses a significant obstacle for the management of peripheral nervous system tumors, and nerve injury provides a frequent source of postoperative morbidity. The lack of suitable animal models for peripheral nerve tumors has impeded the development of alternative nerve-sparing therapies. To evaluate the effect of a multimutated replication-competent herpes simplex virus (G207) on the growth of peripheral nerve tumors and on nerve function, we developed a novel peripheral nerve sheath tumor model. Human neuroblastoma-derived cells injected into murine sciatic nerve consistently caused tumor development within the nerve sheath after 2 weeks followed by increasingly severe impairment of nerve function. Tumor treatment by a single intratumoral injection of G207 resulted in significant reduction of functional impairment, inhibition of tumor growth and prolonged survival. Direct injection of G207 viral particles into the healthy nerve sheath caused no obvious neurologic sequelae, whereas injections of wild-type virus resulted in uniform lethality. The results indicate that viral therapy might be considered as a safe alternative to surgical removal of tumors with peripheral nerve involvement.

Role of tumor-host interactions in interstitial diffusion of macromolecules: cranial vs. subcutaneous tumors

The large size of many novel therapeutics impairs their transport through the tumor extracellular matrix and thus limits their therapeutic effectiveness. We propose that extracellular matrix composition, structure, and distribution determine the transport properties in tumors. Furthermore, because the characteristics of the extracellular matrix largely depend on the tumor-host interactions, we postulate that diffusion of macromolecules will vary with tumor type as well as anatomical location. Diffusion coefficients of macromolecules and liposomes in tumors growing in cranial windows (CWs) and dorsal chambers (DCs) were measured by fluorescence recovery after photobleaching. For the same tumor types, diffusion of large molecules was significantly faster in CW than in DC tumors. The greater diffusional hindrance in DC tumors was correlated with higher levels of collagen type I and its organization into fibrils. For molecules with diameters comparable to the interfibrillar space the diffusion was 5- to 10-fold slower in DC than in CW tumors. The slower diffusion in DC tumors was associated with a higher density of host stromal cells that synthesize and organize collagen type I. Our results point to the necessity of developing site-specific drug carriers to improve the delivery of molecular medicine to solid tumors.

Combination suicide/cytokine gene therapy as adjuvants to a defective herpes simplex virus-based cancer vaccine

We have used syngeneic, established bilateral subcutaneous tumor models to examine the antitumor activity of herpes simplex virus (HSV) vectors, including the induction of an immune response against non-inoculated distant tumors. In such a model with CT26 murine colon adenocarcinoma, unilateral intratumoral inoculation of replication-deficient HSV-1 tsK inhibited the growth of both the inoculated and noninoculated established tumors. To enhance this limited antitumor immune response, we generated a defective HSV vector, dvIL12-tk encoding both interleukin-12 (IL-12) and HSV thymidine kinase (TK), with tsK as the helper virus. In a 'suicide gene' strategy, ganciclovir (GCV) treatment after intratumoral inoculation of dvlacZ-tk/tsK, encoding E. coli lacZ instead of IL-12, resulted in enhanced antitumor activity. Antitumor activity was also enhanced by local expression of IL-12 from dvIL12-tk/tsK. The combination of IL-12 cytokine therapy with GCV treatment was the most efficacious approach, with significantly greater inhibition of tumor growth than IL-12 or TK + GCV alone. These results illustrate the power of combining different cancer therapy approaches; 'suicide gene' therapy, cytokine therapy, and HSV vector infection. HSV vectors are particularly well suited to this because they can accommodate the insertion of large and multiple gene sequences.

Viral shedding and biodistribution of G207, a multimutated, conditionally replicating herpes simplex virus type 1, after intracerebral inoculation in aotus

G207 is a multimutated, conditionally replicating herpes simplex virus type 1 (HSV-1) that is currently in clinical trial for patients with malignant glioma. G207 exhibits an efficient oncolytic activity in tumor cells, yet minimal toxicity in normal tissue when injected into the brains of HSV-susceptible mice or nonhuman primates. In this study, we evaluated the shedding and biodistribution of clinical-grade G207 after intracerebral inoculation (3 x 10(7) pfu) in four New World owl monkeys (Aotus nancymae). Using PCR analyses and viral cultures, neither infectious virus nor viral DNA was detected from tear, saliva, or vaginal secretion samples at any time point up to 1 month postinoculation. Analyses of tissues obtained at necropsy at 1 month from two of the four monkeys, plus one monkey inoculated with laboratory-grade G207 (10(9) pfu) 2 years earlier, showed the distribution of G207 DNA restricted to the brain, although infectious virus was not isolated. Histopathology revealed normal brain tissues including the sites of inoculation. A measurable increase of serum anti-HSV antibody titer was observed in all monkeys, as early as 21 days postinoculation. The results ascertain the safety of G207 in the brain and indicate that strict biohazard management may not be required for G207-treated patients.

Hybrid vector designs to control the delivery, fate and expression of transgenes

One of the greatest challenges to gene therapy is the targetting of gene delivery selectively to the sites of disease and regulation of transgene expression without adverse effects. Ultimately, the successful realization of these goals is dependent upon improvements in vector design. Over the years, viral vector design has progressed from various types of replication-defective viral mutants to replication-conditioned viruses and, more recently, to 'gutted' and hybrid vectors, which have, respectively, eliminated expression of non-relevant or toxic viral genes and incorporated desired elements of different viruses so as to increase the efficacy of gene delivery in vivo. This review will focus on the different viral and cellular elements which have been incorporated into virus vectors to: improve transduction efficiencies; alter the entry specificity of virions; control the fate of transgenes in the host cells; and regulate transgene expression.

HSV-1 infected cell proteins influence tetracycline-regulated transgene expression

BACKGROUND

This study investigates elements of herpes simplex virus type 1 (HSV-1) which influence transgene expression in tetracycline-regulated expression systems.

METHODS

Different HSV-1 mutants were used to infect Vero cells that had been transfected with plasmids containing the luciferase gene under the control of tet-off or tet-on tetracycline-regulation systems.

RESULTS

The baseline level of luciferase expression was elevated after infection with HSV-1 mutants lacking one or more immediate early genes encoding transactivating factors: ICP27, ICP4 and ICP0. With the tet-off system, not only was baseline expression elevated, but there was a complete loss of induction upon removal of tet when this regulatory system was brought into the cell by infection with helper virus-free amplicon vectors. Elevation of luciferase expression was also observed upon infection with the same HSV-1 mutants following transfection with a plasmid containing only a CMV minimal promoter driving luciferase (pUHC13-3). Only one HSV mutant (14Hdelta3), which bears a disruption in the transactivation domain of VP16 and is deleted for both ICP4 genes, did not increase baseline luciferase expression after transfection of pUHC13-3. The disregulating effects were dependent on virus dose and were not influenced by treatment with interferon (IFN)-alpha, which suppresses viral gene expression. Additional assays involving cotransfection of pUHC13-3 with a plasmid encoding of the HSV-1 transactivating factor ICP4 revealed that ICP4 was the most potent inducer of gene expression from the tetO/CMV minimal promoter.

CONCLUSION

These results indicate that proteins encoded in the HSV-1 genome, especially the transactivating immediate early gene products (ICP4, ICP27 and ICP0) and the VP16 tegument protein can activate the tetO/ minimal CMV promoter and thereby interfere with the integrity of tetracycline-regulated transgene expression.