Long-term neuroprotection achieved with latency-associated promoter-driven herpes simplex virus gene transfer to the peripheral nervous system

Improving Control of Gene Therapy-Based Neurotrophin Delivery for Inner Ear Applications

Background: Survival and integrity of the spiral ganglion is vital for hearing in background noise and for optimal functioning of cochlear implants. Numerous studies have demonstrated that supplementation of supraphysiologic levels of the neurotrophins BDNF and NT-3 by pumps or gene therapy strategies supports spiral ganglion survival. The endogenous physiological levels of growth factors within the inner ear, although difficult to determine, are likely extremely low within the normal inner ear. Thus, novel approaches for the long-term low-level delivery of neurotrophins may be advantageous.

Objectives: This study aimed to evaluate the long-term effects of gene therapy-based low-level neurotrophin supplementation on spiral ganglion survival. Using an adenovirus serotype 28-derived adenovector delivery system, the herpes latency promoter, a weak, long expressing promoter system, has been used to deliver the BDNF or NTF3 genes to the inner ear after neomycin-induced ototoxic injury in mice.

Results: Treatment of the adult mouse inner ear with neomycin resulted in acute and chronic changes in endogenous neurotrophic factor gene expression and led to a degeneration of spiral ganglion cells. Increased survival of spiral ganglion cells after adenoviral delivery of BDNF or NTF3 to the inner ear was observed. Expression of BDNF and NT-3 could be demonstrated in the damaged organ of Corti after gene delivery. Hearing loss due to overexpression of neurotrophins in the normal hearing ear was avoided when using this novel vector–promoter combination.

Conclusion: Combining supporting cell-specific gene delivery via the adenovirus serotype 28 vector with a low-strength long expressing promoter potentially can provide long-term neurotrophin delivery to the damaged inner ear.

Deletion of the Virion Host Shut-off Gene Enhances Neuronal-Selective Transgene Expression from an HSV Vector Lacking Functional IE Genes

The ability of herpes simplex virus (HSV) to establish lifelong latency in neurons suggests that HSV-derived vectors hold promise for gene delivery to the nervous system. However, vector toxicity and transgene silencing have created significant barriers to vector applications to the brain. Recently, we described a vector defective for all immediate-early gene expression and deleted for the joint region between the two unique genome segments that proved capable of extended transgene expression in non-neuronal cells. Sustained expression required the proximity of boundary elements from the latency locus. As confirmed here, we have also found that a transgene cassette introduced into the ICP4 locus is highly active in neurons but silent in primary fibroblasts. Remarkably, we observed that removal of the virion host shutoff (vhs) gene further improved transgene expression in neurons without inducing expression of viral genes. In rat hippocampus, the vhs-deleted vector showed robust transgene expression exclusively in neurons for at least 1 month without evidence of toxicity or inflammation. This HSV vector design holds promise for gene delivery to the brain, including durable expression of large or complex transgene cassettes.

Regulatable Transgene Expression for Prevention of Chemotherapy-Induced Peripheral Neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is a debilitating complication associated with drug treatment of cancer for which there are no effective strategies of prevention or treatment. In this study, we examined the effect of intermittent expression of neurotophin-3 (NT-3) or interleukin-10 (IL-10) from replication-defective herpes simplex virus (HSV)-based regulatable vectors delivered by subcutaneous inoculation to the dorsal root ganglion (DRG) on the development of paclitaxel-induced peripheral neuropathy. We constructed two different tetracycline (tet)-on-based regulatable HSV vectors, one expressing NT-3 and the other expressing IL-10, in which the transactivator expression in the tet-on system was under the control of HSV latency-associated promoter 2 (LAP-2), and expression of the transgene was controlled by doxycycline (DOX). We examined the therapeutic effect of intermittent expression of the transgene in animals with paclitaxel-induced peripheral neuropathy modeled by intraperitoneal injection of paclitaxel (16 mg/kg) once a week for 5 weeks. Intermittent expression of either NT-3 or IL-10 3 days before and 1 day after paclitaxel administration protected animals against paclitaxel-induced peripheral neuropathy over the course of 5 weeks. These results suggest the potential of regulatable vectors for prevention of chemotherapy-induced peripheral neuropathy.

Gene transfer to the outflow tract

Elevated intraocular pressure is the primary cause of open angle glaucoma. Outflow resistance exists within the trabecular meshwork but also at the level of Schlemm's canal and further downstream within the outflow system. Viral vectors allow to take advantage of naturally evolved, highly efficient mechanisms of gene transfer, a process that is termed transduction. They can be produced at biosafety level 2 in the lab using protocols that have evolved considerably over the last 15-20 years. Applied by an intracameral bolus, vectors follow conventional as well as uveoscleral outflow pathways. They may affect other structures in the anterior chamber depending on their transduction kinetics which can vary among species when using the same vector. Not all vectors can express long-term, a desirable feature to address the chronicity of glaucoma. Vectors that integrate into the genome of the target cell can achieve transgene function for the life of the transduced cell but are mutagenic by definition. The most prominent long-term expressing vector systems are based on lentiviruses that are derived from HIV, FIV, or EIAV. Safety considerations make non-primate lentiviral vector systems easier to work with as they are not derived from human pathogens. Non-integrating vectors are subject to degradation and attritional dilution during cell division. Lentiviral vectors have to integrate in order to express while adeno-associated viral vectors (AAV) often persist as intracellular concatemers but may also integrate. Adeno- and herpes viral vectors do not integrate and earlier generation systems might be relatively immunogenic. Nonviral methods of gene transfer are termed transfection with few restrictions of transgene size and type but often a much less efficient gene transfer that is also short-lived. Traditional gene transfer delivers exons while some vectors (lentiviral, herpes and adenoviral) allow transfer of entire genes that include introns. Recent insights have highlighted the role of non-coding RNA, most prominently, siRNA, miRNA and lncRNA. SiRNA is highly specific, miRNA is less specific, while lncRNA uses highly complex mechanisms that involve secondary structures and intergenic, intronic, overlapping, antisense, and bidirectional location. Several promising preclinical studies have targeted the RhoA or the prostaglandin pathway or modified the extracellular matrix. TGF-β and glaucoma myocilin mutants have been transduced to elevate the intraocular pressure in glaucoma models. Cell based therapies have started to show first promise. Past approaches have focused on the trabecular meshwork and the inner wall of Schlemm's canal while new strategies are concerned with modification of outflow tract elements that are downstream of the trabecular meshwork.

Future Directions in Pain Management: Integrating Anatomically Selective Delivery Techniques With Novel Molecularly Selective Agents

Treatment for chronic, locoregional pain ranks among the most prevalent unmet medical needs. The failure of systemic analgesic drugs, such as opioids, is often due to their off-target toxicity, development of tolerance, and abuse potential. Interventional pain procedures provide target specificity but lack pharmacologically selective agents with long-term efficacy. Gene therapy vectors are a new tool for the development of molecularly selective pain therapies, which have already been proved to provide durable analgesia in preclinical models. Taken together, advances in image-guided delivery and gene therapy may lead to a new class of dual selective analgesic treatments integrating the molecular selectivity of analgesic genes with the anatomic selectivity of interventional delivery techniques.

A small linear peptide encompassing the NGF N-terminus partly mimics the biological activities of the entire neurotrophin in PC12 cells

Ever since the discovery of its neurite growth promoting activity in sympathetic and sensory ganglia, nerve growth factor (NGF) became the prototype of the large family of neurotrophins. The use of primary cultures and clonal cell lines has revealed several distinct actions of NGF and other neurotrophins. Among several models of NGF activity, the clonal cell line PC12 is the most widely employed. Thus, in the presence of NGF, through the activation of the transmembrane protein TrkA, these cells undergo a progressive mitotic arrest and start to grow electrically excitable neuritis. A vast number of studies opened intriguing aspects of NGF mechanisms of action, its biological properties, and potential use as therapeutic agents. In this context, identifying and utilizing small portions of NGF is of great interest and involves several human diseases including Alzheimer's disease. Here we report the specific action of the peptide encompassing the 1-14 sequence of the human NGF (NGF(1-14)), identified on the basis of scattered indications present in literature. The biological activity of NGF(1-14) was tested on PC12 cells, and its binding with TrkA was predicted by means of a computational approach. NGF(1-14) does not elicit the neurite outgrowth promoting activity, typical of the whole protein, and it only has a moderate action on PC12 proliferation. However, this peptide exerts, in a dose and time dependent fashion, an effective and specific NGF-like action on some highly conserved and biologically crucial intermediates of its intracellular targets such as Akt and CREB. These findings indicate that not all TrkA pathways must be at all times operative, and open the possibility of testing each of them in relation with specific NGF needs, biological actions, and potential therapeutic use.

Changing channels in pain and epilepsy: Exploiting ion channel gene therapy for disorders of neuronal hyperexcitability

Chronic pain and epilepsy together affect hundreds of millions of people worldwide. While traditional pharmacotherapy provides essential relief to the majority of patients, a large proportion remains resistant, and surgical intervention is only possible for a select few. As both disorders are characterised by neuronal hyperexcitability, manipulating the expression of the most direct modulators of excitability - ion channels - represents an attractive common treatment strategy. A number of viral gene therapy approaches have been explored to achieve this. These range from the up- or down-regulation of channels that control excitability endogenously, to the delivery of exogenous channels that permit manipulation of excitability via optical or chemical means. In this review we highlight the key experimental successes of each approach and discuss the challenges facing their clinical translation.

Herpes simplex virus-based nerve targeting gene therapy in pain management

Chronic pain represents a major medical burden not only in terms of suffering but also in terms of economic costs. Traditional medical approaches have so far proven insufficient in treating chronic pain and new approaches are necessary. Gene therapy with herpes simplex virus (HSV)-based vectors offers the ability to directly target specific regions of the neuraxis involved in pain transmission including the primary afferent nociceptor. This opens up new targets to interact with that are either not available to traditional systemic drugs or cannot be adequately acted upon without substantial adverse off-target effects. Having access to the entire neuron, which HSV-based vector gene therapy enables, expands treatment options beyond merely treating symptoms and allows for altering the basic biology of the nerve. In this paper, we discuss several HSV-based gene therapy vectors that our group and others have used to target specific neuronal functions involved in the processing of nociception in order to develop new therapies for the treatment of chronic pain.

Methods for gene transfer to the central nervous system

Gene transfer is an increasingly utilized approach for research and clinical applications involving the central nervous system (CNS). Vectors for gene transfer can be as simple as an unmodified plasmid, but more commonly involve complex modifications to viruses to make them suitable gene delivery vehicles. This chapter will explain how tools for CNS gene transfer have been derived from naturally occurring viruses. The current capabilities of plasmid, retroviral, adeno-associated virus, adenovirus, and herpes simplex virus vectors for CNS gene delivery will be described. These include both focal and global CNS gene transfer strategies, with short- or long-term gene expression. As is described in this chapter, an important aspect of any vector is the cis-acting regulatory elements incorporated into the vector genome that control when, where, and how the transgene is expressed.

Small-molecule modulation of neurotrophin receptors: a strategy for the treatment of neurological disease

Neurotrophins and their receptors modulate multiple signalling pathways to regulate neuronal survival and to maintain axonal and dendritic networks and synaptic plasticity. Neurotrophins have potential for the treatment of neurological diseases. However, their therapeutic application has been limited owing to their poor plasma stability, restricted nervous system penetration and, importantly, the pleiotropic actions that derive from their concomitant binding to multiple receptors. One strategy to overcome these limitations is to target individual neurotrophin receptors — such as tropomyosin receptor kinase A (TRKA), TRKB, TRKC, the p75 neurotrophin receptor or sortilin — with small-molecule ligands. Such small molecules might also modulate various aspects of these signalling pathways in ways that are distinct from the programmes triggered by native neurotrophins. By departing from conventional neurotrophin signalling, these ligands might provide novel therapeutic options for a broad range of neurological indications.

Gene therapy for the treatment of chronic peripheral nervous system pain

Chronic pain is a major health concern affecting 80 million Americans at some time in their lives with significant associated morbidity and effects on individual quality of life. Chronic pain can result from a variety of inflammatory and nerve damaging events that include cancer, infectious diseases, autoimmune-related syndromes and surgery. Current pharmacotherapies have not provided an effective long-term solution as they are limited by drug tolerance and potential abuse. These concerns have led to the development and testing of gene therapy approaches to treat chronic pain. The potential efficacy of gene therapy for pain has been reported in numerous pre-clinical studies that demonstrate pain control at the level of the spinal cord. This promise has been recently supported by a Phase-I human trial in which a replication-defective herpes simplex virus (HSV) vector was used to deliver the human pre-proenkephalin (hPPE) gene, encoding the natural opioid peptides met- and leu-enkephalin (ENK), to cancer patients with intractable pain resulting from bone metastases (Fink et al., 2011). The study showed that the therapy was well tolerated and that patients receiving the higher doses of therapeutic vector experienced a substantial reduction in their overall pain scores for up to a month post vector injection. These exciting early clinical results await further patient testing to demonstrate treatment efficacy and will likely pave the way for other gene therapies to treat chronic pain.

Herpes simplex virus vector-mediated expression of interleukin-10 reduces below-level central neuropathic pain after spinal cord injury

BACKGROUND

Neuroimmune activation in the spinal dorsal horn plays an important role in the pathogenesis of chronic pain after peripheral nerve injury.

OBJECTIVE

The aim of this study was to examine the role of neuroimmune activation in below-level neuropathic pain after traumatic spinal cord injury (SCI).

METHODS

Right hemilateral SCI was created in male Sprague-Dawley rats by controlled blunt impact through a T12 laminectomy. Pain-related behaviors were assessed using both evoked reflex responses and an operant conflict-avoidance test. Neuroimmune activation was blocked by the anti-inflammatory cytokine interleukin-10 (IL-10) delivered by a nonreplicating herpes simplex virus (HSV)-based gene transfer vector (vIL10). Markers of neuroimmune activation were assessed using immunohistochemistry and Western blot.

RESULTS

One week after SCI, injured animals demonstrated mechanical allodynia, thermal hyperalgesia, and mechanical hyperalgesia in the hind limbs below the level of injury. Animals inoculated with vIL10 had a statistically significant reduction in all of these measures compared to injured rats or injured rats inoculated with control vector. Conflict-avoidance behavior of injured rats inoculated with vIL10 was consistent with significantly reduced pain compared with injured rats injected with control vector. These behavioral results correlated with a significant decrease in spinal tumor necrosis factor α (mTNFα) expression assessed by Western blot and astrocyte activation assessed by glial fibrillary acidic protein immunohistochemistry.

CONCLUSION

Below-level pain after SCI is characterized by neuroimmune activation (increase mTNFα and astrocyte activation). Blunting of the neuroimmune response by HSV-mediated delivery of IL-10 reduced pain-related behaviors, and may represent a potential novel therapeutic agent.

Targeted drug delivery to the peripheral nervous system using gene therapy

Gene transfer to target delivery of neurotrophic factors to the primary sensory afferent for treatment of polyneuropathy, or of inhibitory neurotransmitters for relief of chronic pain, offers the possibility of a highly selective targeted release of bioactive molecules within the nervous system. Preclinical studies with non-replicating herpes simplex virus (HSV)-based vectors injected into the skin to transduce neurons in the dorsal root ganglion have demonstrated efficacy in reducing-pain related behaviors in animal models of inflammatory pain, neuropathic pain, and pain caused by cancer, and in preventing progression of sensory neuropathy caused by toxins, chemotherapeutic drugs or resulting from diabetes. Successful completion of the first phase 1 clinical trial of HSV-mediated gene transfer in patients with intractable pain from cancer has set the stage for further clinical trials of this approach.

Prolonged regulatable expression of EPO from an HSV vector using the LAP2 promoter element

We previously reported regulated expression of erythropoietin (EPO) over 4 weeks in the peripheral nerve in vivo, using a herpes simplex virus (HSV)-based vector containing a Tet-on regulatable gene expression cassette. To create a vector that would be appropriate for the treatment of chronic neuropathy, we constructed a HSV vector with expression of EPO under the control of the Tet-on system in which the HSV latency-associated promoter 2 element was used to drive the expression of the Tet-on transactivator. EPO expression from the vector was tightly controlled by administration of doxycycline (DOX) in vitro. One month after inoculation of the vector to transduce dorsal root ganglion (DRG) in vivo, administration of DOX-containing chow-induced expression of EPO. Mice with streptozotocin-induced diabetes, inoculated with the vector, were protected against the development of neuropathy by continuous administration of DOX-containing chow over the course of 3 months. Identical results were achieved when DOX was administered every other week over 3 months of diabetes, but administration of DOX, 1 week out of 3, provided only partial protection against the development of neuropathy. Taken together, these results suggest such a vector is well suited for clinical trial for the treatment of chronic or subacutely developing neuropathy.

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.

A human trial of HSV-mediated gene transfer for the treatment of chronic pain

Gene transfer to the dorsal root ganglion using replication defective herpes simplex virus (HSV)-based vectors reduces pain-related behaviors in rodent models having inflammatory pain, neuropathic pain and pain caused by cancer in bone. HSV vectors engineered to produce inhibitory neurotransmitters, including the delta opioid agonist peptide enkephalin, the mu opioid agonist peptide endomorphin-2 and glutamic acid decarboxylase (GAD), to effect the release of gamma amino butyric acid (GABA) act to inhibit nociceptive neurotransmission at the first synapse between primary nociceptive and second-order neuron in the dorsal horn of the spinal cord. HSV vectors engineered to release anti-inflammatory peptides, including interleukin (IL)-4, IL-10 and the p55 soluble tumor necrosis factor alpha (TNFalpha) receptor reduce neuroimmune activation in the spinal dorsal horn. The path leading from preclinical animal studies to the ongoing phase 1 human trial of the enkephalin-producing vector in patients with pain from cancer, and plans for an efficacy trial with an opioid-producing vector in inflammatory pain and an efficacy trial with a GAD-producing vector in diabetic neuropathic pain are outlined.

In vitro and in vivo gene therapy with CMV vector-mediated presumed dog beta-nerve growth factor in pyridoxine-induced neuropathy dogs

Due to the therapeutic potential of gene therapy for neuronal injury, many studies of neurotrophic factors, vectors, and animal models have been performed. The presumed dog β-nerve growth factor (pdβ-NGF) was generated and cloned and its expression was confirmed in CHO cells. The recombinant pdβ-NGF protein reacted with a human β-NGF antibody and showed bioactivity in PC12 cells. The pdβ-NGF was shown to have similar bioactivity to the dog β-NGF. The recombinant pdβ-NGF plasmid was administrated into the intrathecal space in the gene therapy group. Twenty-four hours after the vector inoculation, the gene therapy group and the positive control group were intoxicated with excess pyridoxine for seven days. Each morning throughout the test period, the dogs' body weight was taken and postural reaction assessments were made. Electrophysiological recordings were performed twice, once before the experiment and once after the test period. After the experimental period, histological analysis was performed. Dogs in the gene therapy group had no weight change and were normal in postural reaction assessments. Electrophysiological recordings were also normal for the gene therapy group. Histological analysis showed that neither the axons nor the myelin of the dorsal funiculus of L₄ were severely damaged in the gene therapy group. In addition, the dorsal root ganglia of L₄ and the peripheral nerves (sciatic nerve) did not experience severe degenerative changes in the gene therapy group. This study is the first to show the protective effect of NGF gene therapy in a dog model.

Gene therapy for the treatment of diabetic neuropathy

Neuropathy is a common, untreatable complication of type 1 and type 2 diabetes. In animal models peptide neurotrophic factors can be used to protect against the development of neuropathy, but the combination of short half-life and off-target effects of these potent pleiotropic peptides has limited translation to human therapy. Gene transfer is a promising strategy that may circumvent these limitations. In this article, we review the basic methods of gene transfer and the -preclinical data in rodent models that support the use of this approach in the treatment of diabetic neuropathy. The path to clinical applications and potential pitfalls in developing gene therapy for the treatment of diabetic neuropathy are considered.

Herpes vector-mediated gene transfer in the treatment of chronic pain

Chronic pain is a major health concern with up to 50% of patients finding little if any relief following traditional pharmacotherapy. This review describes the treatment of chronic pain using herpes simplex virus type 1 (HSV)-based vectors. HSV can be effectively used to deliver pain-modulating transgenes to sensory neurons in vivo following intradermal inoculation. The vector genome persists in peripheral nerve bodies in an episomal state and serves as a platform for expression of natural pain-relieving molecules that access endogenous antinociceptive circuitry. The vectors are mutated to prevent reactivation from latency or spread to the central nervous system. Dermatome selection for administration of HSV vectors provides targeted delivery of pain gene therapy to primary afferent neurons. This novel approach alleviates pain without systemic side effects or the induction of tolerance and can be used in combination with standard pain treatments.

Dysfunction of the recurrent laryngeal nerve and the potential of gene therapy

Injury to the recurrent laryngeal nerve causes vocal fold paresis or paralysis resulting in poor voice quality, and possibly swallowing dysfunction and/or airway compromise. Injury can occur as part of a neurodegenerative disease process or can be due to direct nerve trauma or tumor invasion. Management depends upon symptoms, the cause and severity of injury, and the prognosis for recovery of nerve function. Surgical treatment techniques can improve symptoms, but do not restore physiologic motion. Gene therapy may be a useful adjunct to enhance nerve regeneration in the setting of neurodegenerative disease or trauma. Remote injection of viral vectors into the recurrent laryngeal nerve is the least invasive way to deliver neurotrophic factors to the nerve's cell bodies within the nucleus ambiguus, and in turn to promote nerve regeneration and enhance both nuclear and nerve survival. The purpose of this review is to discuss the potential role for gene therapy in treatment of the unsolved problem of vocal fold paralysis.

Prolonged preservation of nerve function in diabetic neuropathy in mice by herpes simplex virus-mediated gene transfer

AIMS/HYPOTHESIS

The aim of this study was to determine whether prolonged expression of neurotrophin-3 (NT-3) in mice, achieved by herpes simplex virus (HSV)-mediated gene transfer with gene expression under the control of an HSV latency promoter, can provide protection against the progression of diabetic neuropathy over a 6 month period.

MATERIALS AND METHODS

Mice with diabetes induced by streptozotocin were inoculated s.c. into both hind feet with a non-replicating HSV vector containing the coding sequence for NT-3 under the control of the HSV latency-associated promoter 2 (LAP2) elements or with a control vector. Nerve function was evaluated by electrophysiological and behavioural measures over the course of 6 months after the onset of diabetes.

RESULTS

Animals inoculated with the NT-3-expressing vector, but not animals inoculated with control vector, showed preservation of sensory and motor nerve amplitude and conduction velocity measured electrophysiologically, small fibre sensory function assessed by withdrawal from heat, autonomic function measured by pilocarpine-induced sweating, skin innervation assessed by protein gene product 9.5 staining of axons, and density of calcitonin gene-related peptide terminals in the spinal cord measured by immunohistochemistry 5.5 months after vector inoculation.

CONCLUSIONS/INTERPRETATION

These results indicate that the continuous production of NT-3 by LAP2-driven expression of the transgene from an HSV vector over a 6 month period protects against progression of diabetic neuropathy in mice, and provide a proof-of-principle demonstration for the development of a novel therapy for preventing the progression of diabetic neuropathy.

Gene therapy applications for the treatment of neuropathic pain

Neuropathic pain is notoriously difficult to treat; currently available pharmaceutical drugs result in moderate analgesia in approximately a third of patients. As our understanding of the biological processes involved in the establishment and maintenance of neuropathic pain increases, so does the development of novel treatment options. Significant advancements have been made in the past few years in gene transfer, a very powerful potential therapy that can be used to directly target affected areas of the neuraxis or body tissues involved in neuropathic pain. Candidate gene products include directly analgesic proteins as well as proteins that interfere with pain-associated biochemical changes in nerve or other tissues underlying the disease process.

The therapeutic potential of gene transfer for the treatment of peripheral neuropathies

Peripheral neuropathy is a common medical problem with numerous aetiologies. Unfortunately, for the majority of cases there is no available medical solution for the underlying cause, and the only option is to try to treat the resulting symptoms. Treatment options exist when neuropathy results in positive symptoms such as pain, but there is a significant lack of treatments for negative symptoms such as numbness and weakness. Systemic application of growth factor peptides has shown promise in protecting nerves from neuropathic insults in preclinical animal studies, but translation into human trials has been problematic and disappointing. Significant advancements have been made in the past few years in utilising gene therapy approaches to treat peripheral neuropathy by expressing neuroprotective gene products either systemically or in specific nervous tissues. For example, plasmids expressing vascular endothelial growth factor injected into muscle, or herpes-simplex-virus-based vectors expressing neurotrophin gene products delivered to dorsal root ganglion neurons, have been used to protect peripheral nerve function in animal models of diabetes-associated peripheral neuropathy. Many published studies support the feasibility of this approach, although several questions still need to be addressed as gene therapy to treat peripheral neuropathy moves out of the laboratory and into the clinic.

HSV-mediated delivery of erythropoietin restores dopaminergic function in MPTP-treated mice

To investigate the neuroprotective effects of erythropoietin (EPO) in a rodent model of Parkinson disease, we inoculated a nonreplicating herpes simplex virus-based vector expressing EPO (vector DHEPO) into the striatum of mice 1 week prior to, or 2 weeks after, the start of continual administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) (4 mg/kg intraperitoneally, 5 of 7 days) for 6 weeks. Inoculation with DHEPO prior to MPTP intoxication preserved behavioral function measured by pellet retrieval and the histological markers of tyrosine hydroxylase-immunoreactive (TH-IR) neuronal cell bodies in the substantia nigra (SN) and TH-IR and dopamine transporter-immunoreactive (DAT-IR) terminals in striatum. Inoculation of DHEPO 2 weeks into a 6-week course of MPTP resulted in improvement of behavioral function and restoration of TH-IR cells in SN and TH- and DAT-IR in the striatum. The effects of vector-produced EPO were similar in magnitude to the effects of vector-mediated expression of glial-derived neurotrophic factor in the same model. These results demonstrate that vector-mediated EPO production may be used to reverse dopaminergic neurodegeneration in the face of continued toxic insult.

Gene therapy for the treatment of sensory neuropathy

Sensory polyneuropathy can be a serious problem, but for the majority of clinically important neuropathies there are no available therapies. Neurotrophic and neuroprotective peptide factors have been identified that prevent or reverse neuropathy in rodent models of disease, but delivery of these highly pleiotropic peptides has posed an obstacle for translation into effective human therapies. Gene transfer into muscle using viral or non-viral vectors, or into neurons of the dorsal root ganglion using herpes simplex virus-based vectors, provides an alternative means to achieve this end. Studies in animal models have been promising, and the first human trial, using a plasmid to transfer the gene coding for vascular endothelial growth factor into muscle for the treatment of diabetic neuropathy, is now underway. Evidence supporting the trial and the challenges facing this therapy are reviewed.