Single HSV-amplicon vector mediates drug-induced gene expression via dimerizer system

Gene Therapy Tools for Brain Diseases

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

Evaluation of an AAV2-based rapamycin-regulated glial cell line-derived neurotrophic factor (GDNF) expression vector system

Effective regulation of transgene product in anatomically circumscribed brain tissue is dependent on the pharmacokinetics of the regulating agent, the kinetics of transcriptional activation and degradation of the transgene product. We evaluated rapamycin-regulated AAV2-GDNF expression in the rat brain (striatum). Regulated (a dual-component system: AAV2-FBZhGDNF + AAV2-TF1Nc) and constitutive (CMV-driven) expression vectors were compared. Constitutively active AAV2-GDNF directed stable GDNF expression in a dose-dependent manner and it increased for the first month, thereafter reaching a plateau that was maintained over a further 3 months. For the AAV2-regGDNF, rapamycin was administered in a 3-days on/4-days off cycle. Intraperitoneal, oral, and direct brain delivery (CED) of rapamycin were evaluated. Two cycles of rapamycin at an intraperitoneal dose of 10 mg/kg gave the highest GDNF level (2.75±0.01 ng/mg protein). Six cycles at 3 mg/kg resulted in lower GDNF values (1.36±0.3 ng/mg protein). Interestingly, CED of rapamycin into the brain at a very low dose (50 ng) induced GDNF levels comparable to a 6-week intraperitoneal rapamycin cycle. This study demonstrates the effectiveness of rapamycin regulation in the CNS. However, the kinetics of the transgene in brain tissue, the regulator dosing amount and schedule are critical parameters that influence the kinetics of accumulation and zenith of the encoded transgene product.

Prevention of diabetic neuropathy by regulatable expression of HSV-mediated erythropoietin

Previous studies have demonstrated that gene transfer of genes coding for neurotrophic factors to the dorsal root ganglion (DRG) using nonreplicating herpes simplex virus (HSV)-based vectors injected subcutaneously can prevent the progression of diabetic neuropathy. Because prolonged expression of neurotrophic factors could potentially have unwanted adverse effects, we constructed a nonreplicating HSV vector, vHrtEPO, to express erythropoietin (EPO) under the control of a tetracycline response element (TRE)-minimal cytomegalovirus (CMV) fusion promoter. Primary DRG neurons in culture infected with vHrtEPO express and release EPO in response to exposure to doxycycline (DOX). Animals infected with vHrtEPO by footpad inoculation demonstrated regulated expression of EPO in DRG under the control of DOX administered by gavage. Mice rendered diabetic by injection of streptozotocin (STZ), inoculated with vHrtEPO, and treated with DOX 4 days out of 7 each week for 4 weeks were protected against the development of diabetic neuropathy as assessed by electrophysiologic and behavioral measures. These studies indicate that intermittent expression of EPO in DRG achieved from a regulatable vector is sufficient to protect against the progression of neuropathy in diabetic animals, and provides proof-of-principle preclinical evidence for the development of such vectors for clinical trial.

Herpes Virus Amplicon Vectors

Since its emergence onto the gene therapy scene nearly 25 years ago, the replication-defective Herpes Simplex Virus Type-1 (HSV-1) amplicon has gained significance as a versatile gene transfer platform due to its extensive transgene capacity, widespread cellular tropism, minimal immunogenicity, and its amenability to genetic manipulation. Herein, we detail the recent advances made with respect to the design of the HSV amplicon, its numerous in vitro and in vivo applications, and the current impediments this virus-based gene transfer platform faces as it navigates a challenging path towards future clinical testing.

HSV-1-derived amplicon vectors: recent technological improvements and remaining difficulties--a review

Amplicons are defective and non-integrative vectors derived from herpes simplex virus type 1. As the vector genome carries no virus genes, amplicons are both non-toxic for the infected cells and non-pathogenic for the inoculated organisms. In addition, the large transgenic capacity of amplicons, which allow delivery of up to 150 Kbp of foreign DNA, makes these vectors one of the most powerful, interesting and versatile gene delivery platforms. We present here recent technological developments that have significantly improved and extended the use of amplicons, both in cultured cells and in living organisms. In addition, this review also discusses the many difficulties still pending to be solved, in order to achieve stable and physiologically regulated transgene expression.

Intraparenchymal spinal cord delivery of adeno-associated virus IGF-1 is protective in the SOD1G93A model of ALS

The potent neuroprotective activities of neurotrophic factors, including insulin-like growth factor 1 (IGF-1), make them promising candidates for treatment of amyotrophic lateral sclerosis (ALS). In an effort to maximize rate of motor neuron transduction, achieve high levels of spinal IGF-1 and thus enhance therapeutic benefit, we injected an adeno-associated virus 2 (AAV2)-based vector encoding human IGF-1 (CERE-130) into lumbar spinal cord parenchyma of SOD1(G93A) mice. We observed robust and long-term intraspinal IGF-1 expression and partial rescue of lumbar spinal cord motor neurons, as well as sex-specific delayed disease onset, weight loss, decline in hindlimb grip strength and increased animal survival.

HSV-1 amplicon vectors: a promising and versatile tool for gene delivery

Amplicons are defective and non-integrative vectors derived from herpes simplex virus type 1. They carry no virus genes in the vector genome and are, therefore, not toxic to the infected cells or pathogenic for the transduced organisms, making these vectors safe. In addition, the large transgenic capacity of amplicons, which allow delivery of < or = 150 Kbp of foreign DNA, make these vectors one of the most powerful, interesting and versatile gene delivery platforms. Here, the authors present recent technological developments that have significantly improved and extended the use of amplicons, both in cultured cells and in living organisms. In addition, this review illustrates the many possible applications that are presently being developed with amplicons and discuss the many difficulties still pending to be solved in order to achieve stable and physiologically regulated transgenic expression.

Viral vectors: a wide range of choices and high levels of service

Viruses are intracellular parasites with simple DNA or RNA genomes. Virus life revolves around three steps: infection of a host cell, replication of its genome within the host cell environment, and formation of new virions; this process is often but not always associated with pathogenic effects against the host organism. Since the mid-1980s, the main goal of viral vectorology has been to develop recombinant viral vectors for long-term gene delivery to mammalian cells, with minimal associated toxicity. Today, several viral vector systems are close to achieving this aim, providing stable transgenic expression in many different cell types and tissues. Here we review application characteristics of four vector systems, derived from adeno-associated viruses, adenoviruses, retroviruses and herpes simplex virus-1, for in vivo gene delivery. We discuss the transfer capacity of the expression vectors, the stability of their transgenic expression, the tropism of the recombinant viruses, the likelihood of induction of immunotoxicity, and the ease (or difficulty) of the virus production. In the end, we discuss applications of these vectors for delivery of three molecular systems for conditional mutagenesis, two for inducible transcriptional control of transgenic expression (the tet and the dimerizer systems), and the third one for inducible control of endogenous gene expression based on RNA interference.

Bioluminescence imaging after HSV amplicon vector delivery into brain

BACKGROUND

Firefly luciferase (Fluc) has routinely been used to quantitate and analyze gene expression in vitro by measuring the photons emitted after the addition of ATP and luciferin to a test sample. It is now possible to replace luminometer-based analysis of luciferase activity and measure luciferase activity delivered by viral vectors directly in live animals over time using digital imaging techniques.

METHODS

An HSV amplicon vector expressing Fluc cDNA from an inducible promoter was delivered to cells in culture and into the mouse brain. In culture, expression of Fluc was measured after induction in a dose-dependent manner by a biochemical assay, and then confirmed by Western blot analysis and digital imaging. The vectors were then stereotactically injected into the mouse brain and Fluc expression measured non-invasively using bioluminescence imaging.

RESULTS

Rapamycin-mediated induction of Fluc from an HSV amplicon vector in culture resulted in dose-dependent expression of Fluc when measured using a luminometer and by digital analysis. In mouse cortex, a single injection of an HSV amplicon vector (2 microl, 1x10(8) transducing units (t.u.)/ml) expressing Fluc from a viral promoter (CMV) was sufficient to detect robust luciferase activity for at least 1 week. Similarly, an HSV amplicon vector expressing Fluc under an inducible promoter was also detectable in the mouse cortex after a single dose (2 microl, 1x10(8) t.u./ml) for up to 5 days, with no detectable signal in the uninduced state.

CONCLUSIONS

This HSV amplicon vector-based system allows for fast, non-invasive, semi-quantitative analysis of gene expression in the brain.

Highly efficient regulation of gene expression by tetracycline in a replication-defective herpes simplex viral vector

Employing the tetracycline repressor tetR and the wild-type hCMV major immediate-early promoter, we have developed a highly sensitive tetracycline-inducible transcription switch in mammalian cells (T-REx; Invitrogen, Carlsbad, CA, USA). In view of the previous difficulty in achieving regulatable gene expression in recombinant HSV vector systems, we constructed a T-REx-encoding replication-defective HSV-1 recombinant, QR9TO-lacZ, that encodes two copies of the tetR gene controlled by the HSV-1 immediate-early ICP0 promoter and a reporter, the LacZ gene, under the control of the tetO-bearing hCMV major immediate-early promoter. Infection of cells, such as Vero, PC12, and NGF-differentiated PC12 cells, with QR9TO-lacZ led to 300- to 1000-fold tetracycline-regulated gene expression. Moreover, the expression of the LacZ gene by QR9TO-lacZ can be finely controlled by tetracycline in a dose-dependent fashion. Efficiently regulated gene expression can also be achieved in vivo following intracerebral and footpad inoculations in mice. The demonstrated capability of T-REx for achieving high levels of sensitively regulated gene expression in the context of the HSV-1 genome will significantly expand the utility of HSV-based vector systems for studying gene function in the nervous system and delivering regulated gene expression in therapeutic applications, particularly in the treatment of CNS diseases.

Dimerizer regulation of AADC expression and behavioral response in AAV-transduced 6-OHDA lesioned rats

Recombinant AAV vectors containing a dimerizer-inducible system of transcriptional activation provide a strategy for control of therapeutic gene expression in the CNS. Here we explored this system for regulated expression of human aromatic L-amino acid decarboxylase (hAADC) in a rodent model of Parkinson disease. Expression of hAADC, the enzyme that converts L-dopa to dopamine, was dependent on reconstitution of a functional transcription factor (TF) by the dimerizer rapamycin. Two vectors, AAV-CMV-TF and AAV-Z12-hAADC, were infused into striata of 6-OHDA-lesioned rats. Rapamycin-induced increases in expression of hAADC repeatedly produced robust rotational behavior in response to low doses of L-dopa. Seven weeks after vector infusion, AADC expression in brain was quantitated by both stereology and Western blot analysis following the final rapamycin treatment. While a low level of hAADC was observed in rats that were not induced with rapamycin, this basal expression was not significant enough to elicit a rotational response to L-dopa. This study demonstrated a robust behavioral response of parkinsonian rats to regulated hAADC expression. Recombinant AAV vectors controlled by rapamycin or its analogs show promise as candidates for CNS therapies in which regulation of the transgene is desired.

Long-term pharmacologically regulated expression of erythropoietin in primates following AAV-mediated gene transfer

Gene therapy is a potential route for the delivery of secreted therapeutic proteins, but pharmacologic control of expression will generally be required for optimal safety and efficacy. Previous attempts to achieve regulated expression in largeanimal models have been thwarted by transient expression or immune responses to regulatory proteins. We evaluated the ability of the dimerizer-regulated gene expression system to achieve controlled, long-term production of erythropoietin (Epo) following intramuscular administration of adeno-associated virus (AAV) vectors to 16 primates. All animals showed dose-responsive and completely reversible elevation of Epo and hematocrit in response to the dimerizer rapamycin, or analogs with reduced immunosuppressive activity, administered intravenously or orally. Animals that received optimized dual vectors showed persistent regulated expression for the duration of the study, with no apparent immune response to Epo or the regulatory proteins. Similar results were obtained with single vectors incorporating both the Epo and regulatory genes, including those packaged into serotype 1 AAV vectors to allow use of lower viral doses. For the longest-studied animal, regulated expression has persisted for more than 6 years and 26 induction cycles. These data indicate that one-time or infrequent gene transfer followed by dimerizer regulation is a promising approach for delivery of therapeutic proteins.

Approaches for trigger-inducible viral transgene regulation in gene-based tissue engineering

Recent advances in mammalian transgene expression dosing have resulted in a portfolio of mutually compatible systems that can adjust therapeutic transgene levels in response to antibiotics, hormone analogues, quorum-sensing messengers and secondary metabolites. The molecular merger of trigger-inducible expression technology with the latest generation of virus-derived transduction systems has enabled unmatched clinical interventions to shape desired therapeutic cell and tissue phenotypes for the treatment of complex human diseases.

Inducible release of TRAIL fusion proteins from a proapoptotic form for tumor therapy

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) can selectively kill neoplastic cells and control of its activity could enhance tumor therapy. We have developed means to control the secretion of a novel recombinant (r) TRAIL fusion protein using a viral protease. This system uses the endoplasmic reticulum (ER) as a storage depot for rTRAIL, because TRAIL acts by binding to its cognate receptors on the cell surface. We have engineered two TRAIL variants: (a) a secretable form that enhances apoptosis via a bystander effect; and (b) an ER-targeted TRAIL that is retained in the ER until selectively released by the viral protease. Gene delivery can be monitored in vivo by systemic administration of a near infrared fluorescent (NIRF) probe activated by the protease. This study serves as a template for design of recombinant proteins to enhance and control apoptosis of tumor cells via specific viral proteases and for use of viral proteases as in vivo reporters for cancer therapy.