Viral vectors for in vivo gene transfer in Parkinson's disease: properties and clinical grade production

Intracerebral lentiviral ABCD1 gene therapy in an early disease onset ALD mouse model

X-linked adrenoleukodystrophy (ALD) is a genetic disorder of the ABCD1 gene. We aimed to treat ALD via direct intracerebral injection of lentiviral ABCD1 (LV.ABCD1). Lentiviral vectors (LVs) were injected into the brain of wild type mice to access toxicities and biodistribution. Confocal microscopy illustrated supraphysiological ABCD1 expression surrounding the injection sites, and LVs were also detected in the opposite site of the unilaterally injected brain. In multi-site bilateral injections (4, 6, 8, and 9 sites), LV.ABCD1 transduced most brain regions including the cerebellum. Investigation of neuronal loss, astrogliosis and microglia activation did not detect abnormality. For efficacy evaluation, a novel ALD knockout (KO) mouse model was established by deleting exons 3 to 9 of the ABCD1 gene based on CRISPR/Cas9 gene editing. The KO mice showed behavioral deficit in open-field test (OFT) and reduced locomotor activities in rotarod test at 6 and 7 months of age, respectively. We treated 3-month-old KO mice with bilateral LV.ABCD1 injections into the external capsule and thalamus. ABCD1 expression was detected 15 days later, and the impaired motor ability was gradually alleviated. Our studies established an early onset ALD model and illustrated neurological improvement after LV.ABCD1 intracerebral injection without immunopathological toxicity.

Adeno-Associated Virus Expression of α-Synuclein as a Tool to Model Parkinson's Disease: Current Understanding and Knowledge Gaps

Parkinson's disease (PD) is the second most common neurodegenerative disorder in the aging population and is characterized by a constellation of motor and non-motor symptoms. The abnormal aggregation and spread of alpha-synuclein (α-syn) is thought to underlie the loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNc), leading to the development of PD. It is in this context that the use of adeno-associated viruses (AAVs) to express a-syn in the rodent midbrain has become a popular tool to model SNc DA neuron loss during PD. In this review, we summarize results from two decades of experiments using AAV-mediated a-syn expression in rodents to model PD. Specifically, we outline aspects of AAV vectors that are particularly relevant to modeling a-syn dysfunction in rodent models of PD such as changes in striatal neurochemistry, a-syn biochemistry, and PD-related behaviors resulting from AAV-mediated a-syn expression in the midbrain. Finally, we discuss the emerging role of astrocytes in propagating a-syn pathology, and point to future directions for employing AAVs as a tool to better understand how astrocytes contribute to a-syn pathology during the development of PD. We envision that lessons learned from two decades of utilizing AAVs to express a-syn in the rodent brain will enable us to develop an optimized set of parameters for gaining a better understanding of how a-syn leads to the development of PD.

Advantages and Recent Developments of Autologous Cell Therapy for Parkinson's Disease Patients

Parkinson’s Disease (PD) is a progressive degenerative disease characterized by tremor, bradykinesia, rigidity and postural instability. There are approximately 7–10 million PD patients worldwide. Currently, there are no biomarkers available or pharmaceuticals that can halt the dopaminergic neuron degeneration. At the time of diagnosis about 60% of the midbrain dopamine (mDA) neurons have already degenerated, resulting in a depletion of roughly 70% of striatal dopamine (DA) levels and synapses. Symptomatic treatment (e.g., with L-dopa) can initially restore DA levels and motor function, but with time often lead to side-effects like dyskinesia. Deep-brain-stimulation can alleviate these side-effects and some of the motor symptoms but requires repeat procedures and adds limitations for the patients. Restoration of dopaminergic synapses using neuronal cell replacement therapy has shown benefit in clinical studies using cells from fetal ventral midbrain. This approach, if done correctly, increases DA levels and restores synapses, allowing biofeedback regulation between the grafted cells and the host brain. Drawbacks are that it is not scalable for a large patient population and the patients require immunosuppression. Stem cells differentiated in vitro to mDA neurons or progenitors have shown promise in animal studies and is a scalable approach that allows for cryopreservation of transplantable cells and rigorous quality control prior to transplantation. However, all allogeneic grafts require immunosuppression. HLA-donor-matching, reduces, but does not completely eliminate, the need for immunosuppression, and is currently investigated in a clinical trial for PD in Japan. Since immune compatibility is very important in all areas of transplantation, these approaches may ultimately be of less benefit to the patients than an autologous approach. By using the patient’s own somatic cells, reprogrammed to induced pluripotent stem cells (iPSCs) and differentiated to mDA neurons immunosuppression is not required, and may also present with several biological and functional advantages in the patients, as described in this article. The proof-of-principle of autologous iPSC mDA restoration of function has been shown in parkinsonian non-human primates (NHPs), and this can now be investigated in clinical trials in addition to the allogeneic and HLA-matched approaches. In this review, we focus on the autologous approach of cell therapy for PD.

Endothelial cell-targeted pVEGF165 polyplex plays a pivotal role in inhibiting intimal thickening after vascular injury

Upregulation of vascular endothelial growth factor (VEGF) expression can inhibit intimal thickening after vascular injury. However, the lack of efficient gene delivery systems leads to insufficient VEGF expression, which prevents its application in gene therapy. In the present study, to improve the delivery of the plasmid vector with the VEGF gene (pVEGF165) to the injured vessel wall, we explored the potentially important difference between endothelial cell-targeted and nontargeted polymeric carriers. The αvβ3 integrin is overexpressed on activated endothelial cells but not on normal quiescent vessels. In this study, CDG2-cRGD, synthesized by conjugating an αvβ3 integrin-binding cyclic arginylglycylaspartic acid (cRGD) peptide with the Generation 2 polycation polyamidoamine (PAMAMG2)-g-cyclodextrin (termed as CDG2), was developed as a targetable carrier. It was observed that the specific integrin-ligand interactions greatly enhanced cellular internalization of CDG2-cRGD in human umbilical vein endothelial cells (HUVECs), which are notoriously difficult to transfect. Consequently, HUVECs were found to show remarkably high levels of VEGF165 expression induced by the CDG2-cRGD polyplex. Interestingly, VEGF165 overexpression in vivo was more complex than that in vitro, and in vivo assays demonstrated that the stimulus response to balloon injury in arteries could obviously upregulate VEGF165 expression in the saline-treated group, although it was not enough to prevent intimal thickening. In gene-transfected groups, intravascular delivery of pVEGF165 with the CDG2-cRGD polyplex into rabbits after vascular injury resulted in a significant inhibition of intimal thickening at 4 weeks, whereas the low therapeutic efficacy in the nontargeted CDG2-treated group was only comparable to that in the saline-treated group. It is becoming clear that the conflicting results of VEGF165 gene therapy in two gene-transfected groups are reflective of the pivotal role of the cRGD-conjugated carriers in achieving the beneficial therapeutic effects of vascular gene therapy.

Neurotensin-polyplex-mediated brain-derived neurotrophic factor gene delivery into nigral dopamine neurons prevents nigrostriatal degeneration in a rat model of early Parkinson's disease

Background

The neurotrophin Brain-Derived Neurotrophic Factor (BDNF) influences nigral dopaminergic neurons via autocrine and paracrine mechanisms. The reduction of BDNF expression in Parkinson’s disease substantia nigra (SN) might contribute to the death of dopaminergic neurons because inhibiting BDNF expression in the SN causes parkinsonism in the rat. This study aimed to demonstrate that increasing BDNF expression in dopaminergic neurons of rats with one week of 6-hydroxydopamine lesion recovers from parkinsonism. The plasmids phDAT-BDNF-flag and phDAT-EGFP, coding for enhanced green fluorescent protein, were transfected using neurotensin (NTS)-polyplex, which enables delivery of genes into the dopaminergic neurons via neurotensin-receptor type 1 (NTSR1) internalization.

Results

Two weeks after transfections, RT-PCR and immunofluorescence techniques showed that the residual dopaminergic neurons retain NTSR1 expression and susceptibility to be transfected by the NTS-polyplex. phDAT-BDNF-flag transfection did not increase dopaminergic neurons, but caused 7-fold increase in dopamine fibers within the SN and 5-fold increase in innervation and dopamine levels in the striatum. These neurotrophic effects were accompanied by a significant improvement in motor behavior.

Conclusions

NTS-polyplex-mediated BDNF overexpression in dopaminergic neurons has proven to be effective to remit hemiparkinsonism in the rat. This BDNF gene therapy might be helpful in the early stage of Parkinson’s disease.

Gene therapy for Parkinson's disease: state-of-the-art treatments for neurodegenerative disease

Pharmacological and surgical treatments offer symptomatic benefits to patients with Parkinson's disease; however, as the condition progresses, patients experience gradual worsening in symptom control, with the development of a range of disabling complications. In addition, none of the currently available therapies have convincingly shown disease-modifying effects - either in slowing or reversing the disease. These problems have led to extensive research into the possible use of gene therapy as a treatment for Parkinson's disease. Several treatments have reached human clinical trial stages, providing important information on the risks and benefits of this novel therapeutic approach, and the tantalizing promise of improved control of this currently incurable neurodegenerative disorder.

Gene therapy for misfolding protein diseases of the central nervous system

Protein aggregation as a result of misfolding is a common theme underlying neurodegenerative diseases. Accordingly, most recent studies aim to prevent protein misfolding and/or aggregation as a strategy to treat these pathologies. For instance, state-of-the-art approaches, such as silencing protein overexpression by means of RNA interference, are being tested with positive outcomes in preclinical models of animals overexpressing the corresponding protein. Therapies designed to treat central nervous system diseases should provide accurate delivery of the therapeutic agent and long-term or chronic expression by means of a nontoxic delivery vehicle. After several years of technical advances and optimization, gene therapy emerges as a promising approach able to fulfill those requirements. In this review we will summarize the latest improvements achieved in gene therapy for central nervous system diseases associated with protein misfolding (e.g., amyotrophic lateral sclerosis, Alzheimer's, Parkinson's, Huntington's, and prion diseases), as well as the most recent approaches in this field to treat these pathologies.

Dysfunction of dopamine homeostasis: clues in the hunt for novel Parkinson's disease therapies

Parkinson's disease is the second most common neurodegenerative disorder and, at present, has no cure. Both environmental and genetic factors have been implicated in the etiology of the disease; however, the pathogenic pathways leading to neuronal degeneration are still unclear. Parkinson's disease is characterized by the preferential death of a subset of neurons in the mesencephalon that use dopamine as neurotransmitter for synaptic communication. Dopamine is a highly reactive molecule that can lead to cytotoxicity if not properly stored and metabolized. Targeting any of the pathways that tightly control this neurotransmitter holds great therapeutic expectations. In this article we present a comprehensive overview of the cellular pathways that control dopamine fate and discuss potential therapeutic approaches to counteract or slow Parkinson's disease onset and progression.

Optimizing NTS-polyplex as a tool for gene transfer to cultured dopamine neurons

The study of signal transduction in dopamine (DA)-containing neurons as well as the development of new therapeutic approaches for Parkinson's disease requires the selective expression of transgenes in such neurons. Here we describe optimization of the use of the NTS-polyplex, a gene carrier system taking advantage of neurotensin receptor internalization, to transfect mouse DA neurons in primary culture. The plasmids DsRed2 (4.7 kbp) and VGLUT2-Venus (11 kbp) were used to compare the ability of this carrier system to transfect plasmids of different sizes. We examined the impact of age of the neurons (1, 3, 5 and 8 days after seeding), of culture media used during the transfection (Neurobasal with B27 vs. conditioned medium) and of three molar ratios of plasmid DNA to carrier. While the NTS-polyplex successfully transfected both plasmids in a control N1E-115 cell line, only the pDsRed2 plasmid could be transfected in primary cultured DA neurons. We achieved 20% transfection efficiency of pDsRed2 in DA neurons, with 80% cell viability. The transfection was demonstrated pharmacologically to be dependent on activation of neurotensin receptors and to be selective for DA neurons. The presence of conditioned medium for transfection was found to be required to insure cell viability. Highest transfection efficiency was achieved in the most mature neurons. In contrast, transfection with the VGLUT2-Venus plasmid produced cell damage, most likely due to the high molar ratios required, as evidenced by a 15% cell viability of DA neurons at the three molar ratios tested (1:36, 1:39 and 1:42). We conclude that, when used at molar ratios lower than 1:33, the NTS-polyplex can selectively transfect mature cultured DA neurons with only low levels of toxicity. Our results provide evidence that the NTS-polyplex has good potential for targeted gene delivery in cultured DA neurons, an in vitro system of great use for the screening of new therapeutic approaches for Parkinson's disease.

Gene-based therapy of Parkinson's Disease: Translation from animal model to human clinical trial employing convection enhanced delivery

The existing treatment of Parkinson's disease (PD) is directed towards substituting dopamine loss with either dopamine replacement therapy or pharmacological therapies aimed at increasing dopamine at the synapse level. Emerging viable alternatives include the use of cell-based and gene-based therapeutics. In this review, we discuss efforts in developing in vitro and in vivo models and their translation to human clinical trials for gene-based therapy of this distressing and prevalent neurodegenerative disorder. Given the mismatch between expectations from preclinical data and results of human pivotal trials, drug delivery has been identified as the key emerging area for translational research due to limitation of limited efficacy. The chief highlights of the current topic include use of improved delivery methods of gene-based therapeutic agents. Convection-enhanced delivery (CED), an advanced infusion technique with demonstrated utility in ex vivo and in vivo animal models has recently been adopted for PD gene-based therapy trials. Several preclinical studies suggest that magnetic resonance imaging (MRI)-guided navigation for accurately targeting and real time monitoring viral vector delivery (rCED) in future clinical trials involving detection of gene expression and restoration of dopaminergic function loss using pro-drug approach will greatly enhance these PD treatments.

Gene therapy for Parkinson's disease

Current pharmacological and surgical treatments for Parkinson's disease offer symptomatic improvements to those suffering from this incurable degenerative neurological disorder, but none of these has convincingly shown effects on disease progression. Novel approaches based on gene therapy have several potential advantages over conventional treatment modalities. These could be used to provide more consistent dopamine supplementation, potentially providing superior symptomatic relief with fewer side effects. More radically, gene therapy could be used to correct the imbalances in basal ganglia circuitry associated with the symptoms of Parkinson's disease, or to preserve or restore dopaminergic neurons lost during the disease process itself. The latter neuroprotective approach is the most exciting, as it could theoretically be disease modifying rather than simply symptom alleviating. Gene therapy agents using these approaches are currently making the transition from the laboratory to the bedside. This paper summarises the theoretical approaches to gene therapy for Parkinson's disease and the findings of clinical trials in this rapidly changing field.

Production and purification of recombinant adeno-associated vectors

The use of recombinant adeno-associated virus (rAAV) vectors in gene therapy for preclinical studies in animal models and human clinical trials is increasing, as these vectors have been shown to be safe and to mediate persistent transgene expression in vivo. Constant improvement in rAAV manufacturing processes (upstream production and downstream purification) has paralleled this evolution to meet the needs for larger vector batches, higher vector titer, and improved vector quality and safety. This chapter provides an overview of existing production and purification systems used for adeno-associated virus (AAV) vectors, and the advantages and disadvantages of each system are outlined. Regulatory guidelines that apply to the use of these systems for clinical trials are also presented. The methods described are examples of protocols that have been utilized for establishing rAAV packaging cell lines, production of rAAV vectors using recombinant HSV infection, and for chromatographic purification of various AAV vector serotypes. A protocol for the production of clinical-grade rAAV type 2 vectors using transient transfection and centrifugation-based purification is also described.

Optimized quantities of GDNF overexpressed by engineered astrocytes are critical for protection of neuroblastoma cells against 6-OHDA toxicity

Optimized levels of glial cell line-derived neurotrophic factor (GDNF) are critical for protection of dopaminergic neurons against parkinsonian cell death. Recombinant lentiviruses harboring GDNF coding sequence were constructed and used to infect astrocytoma cell line 1321N1. The infected astrocytes overexpressed GDNF mRNA and secreted an average of 2.2 ng/mL recombinant protein as tested in both 2 and 16 weeks post-infection. Serial dilutions of GDNF-enriched conditioned medium from infected astrocytes added to growing neuroblastoma cell line SK-N-MC resulted in commensurate resistance against 6-OHDA toxicity. SK-N-MC cell survival rate rose from 51% in control group to 84% in the cells grown with astro-CM containing 453 pg secreted GDNF, an increase that was highly significant (P < 0.0001). However, larger volumes of the GDNF-enriched conditioned medium failed to improve cell survival and addition of volumes that contained 1,600 pg or more GDNF further reduced survival rate to below 70%. Changes in cell survival paralleled to changes in the percent of apoptotic cell morphologies. These data demonstrate the feasibility of using astrocytes as minipumps to stably oversecrete neurotrophic factors and further indicate that GDNF can be applied to neuroprotection studies in PD pending the optimization of its concentrations.

Cell transplantation and gene therapy in Parkinson's disease

Parkinson's disease is a progressive neurodegenerative disorder affecting, in part, dopaminergic motor neurons of the ventral midbrain and their terminal projections that course to the striatum. Symptomatic strategies focused on dopamine replacement have proven effective at remediating some motor symptoms during the course of disease but ultimately fail to deliver long-term disease modification and lose effectiveness due to the emergence of side effects. Several strategies have been experimentally tested as alternatives for Parkinson's disease, including direct cell replacement and gene transfer through viral vectors. Cellular transplantation of dopamine-secreting cells was hypothesized as a substitute for pharmacotherapy to directly provide dopamine, whereas gene therapy has primarily focused on restoration of dopamine synthesis or neuroprotection and restoration of spared host dopaminergic circuitry through trophic factors as a means to enhance sustained controlled dopamine transmission. This seems now to have been verified in numerous studies in rodents and nonhuman primates, which have shown that grafts of fetal dopamine neurons or gene transfer through viral vector delivery can lead to improvements in biochemical and behavioral indices of dopamine deficiency. However, in clinical studies, the improvements in parkinsonism have been rather modest and variable and have been plagued by graft-induced dyskinesias. New developments in stem-cell transplantation and induced patient-derived cells have opened the doors for the advancement of cell-based therapeutics. In addition, viral-vector-derived therapies have been developed preclinically with excellent safety and efficacy profiles, showing promise in clinical trials thus far. Further progress and optimization of these therapies will be necessary to ensure safety and efficacy before widespread clinical use is deemed appropriate.

COS-1 cells as packaging host for production of lentiviruses

We present a protocol for in vitro production of recombinant lentiviruses using COS-1 African green monkey kidney epithelial cells and HEK293T human embryonic kidney epithelial cells as packaging cells. COS-1 and HEK293T express SV40 large T antigen, amplifying transfected circular plasmids harboring SV40 replication origin. Support protocols for evaluation of transfection efficiency by in situ β-galactosidase enzyme activity assay and titer of infection-capable virions are also provided. Advantages of using COS-1 packaging cells over the standard HEK293T cells for contamination-sensitive applications or automated processing are discussed.

Adeno-associated virus serotype 2 mediated transduction and coexpression of the human apoAI and SR-BI gene in HepG2 cells

Cholesterol efflux is the first step in the reverse cholesterol transport (RCT) pathway, removing excess cholesterol from tissues, including the arterial wall, thus preventing the development of atherosclerosis. Adeno-associated virus (rAAV) has demonstrated significant promise as a DNA-delivery vector to treat serious human diseases. In this study, we constructed recombinant adeno-associated viruses coexpressing apoAI and SR-BI successfully, the double gene mRNA and protein were both strongly expressed in transduced HepG2 cells. A novel safe and efficient method of promoting the reverse cholesterol transport (RCT) may be established. These results may provide a new method for gene therapy of Arteriosclerosis.

Molecular chaperones in Parkinson's disease--present and future

Parkinson's disease, like many other neurodegenerative disorders, is characterized by the progressive accumulation of pathogenic protein species and the formation of intracellular inclusion bodies. The cascade by which the small synaptic protein α-synuclein misfolds to form distinctive protein aggregates, termed Lewy bodies and Lewy neurites, has been the subject of intensive research for more than a decade. Genetic and pathological studies in Parkinson's disease patients as well as experimental studies in disease models have clearly established altered protein metabolism as a key element in the pathogenesis of Parkinson's disease. Alterations in protein metabolism include misfolding and aggregation, post-translational modification and dysfunctional degradation of cytotoxic protein species. Protein folding and re-folding are both mediated by a highly conserved network of molecules, called molecular chaperones and co-chaperones. In addition to the regulatory role in protein folding, molecular chaperone function is intimately associated with pathways of protein degradation, such as the ubiquitin-proteasome system and the autophagy-lysosomal pathway, to effectively remove irreversibly misfolded proteins. Because of the central role of molecular chaperones in maintaining protein homeostasis, we herein review our current knowledge on the involvement of molecular chaperones and co-chaperones in Parkinson's disease. We further discuss the capacity of molecular chaperones to prevent or modulate neurodegeneration, an important concept for future neuroprotective strategies and summarize the current progress in preclinical studies in models of Parkinson's disease and other neurodegenerative disorders. Finally we include a discussion on the future potential of using molecular chaperones as a disease modifying therapy.

Gene therapy in Parkinson's disease: rationale and current status

Parkinson's disease is the second most common age-related neurodegenerative disorder, typified by the progressive loss of substantia nigra pars compacta dopamine neurons and the consequent decrease in the neurotransmitter dopamine. Patients exhibit a range of clinical symptoms, with the most common affecting motor function and including resting tremor, rigidity, akinesia, bradykinesia and postural instability. Current pharmacological interventions are palliative and largely aimed at increasing dopamine levels through increased production and/or inhibition of metabolism of this key neurotransmitter. The gold standard for treatment of both familial and sporadic Parkinson's disease is the peripheral administration of the dopamine precursor, levodopa. However, many patients gradually develop levodopa-induced dyskinesias and motor fluctuations. In addition, dopamine enhancement therapies are most useful when a portion of the nigrostriatal pathway is intact. Consequently, as the number of substantia nigra dopamine neurons and striatal projections decrease, these treatments become less efficacious. Current translational research is focused on the development of novel disease-modifying therapies, including those utilizing gene therapeutic approaches. Herein we present an overview of current gene therapy clinical trials for Parkinson's disease. Employing either recombinant adeno-associated virus type 2 (rAAV2) or lentivirus vectors, these clinical trials are focused on three overarching approaches: augmentation of dopamine levels via increased neurotransmitter production; modulation of the neuronal phenotype; and neuroprotection. The first two therapies discussed in this article focus on increasing dopamine production via direct delivery of genes involved in neurotransmitter synthesis (amino acid decarboxylase, tyrosine hydroxylase and GTP [guanosine triphosphate] cyclohydrolase 1). In an attempt to bypass the degenerating nigrostriatal pathway, a third clinical trial utilizes rAAV2 to deliver glutamic acid decarboxylase to the subthalamic nucleus, converting a subset of excitatory neurons to GABA-producing cells. In contrast, the final clinical trial is aimed at protecting the degenerating nigrostriatum by striatal delivery of rAAV2 harbouring the neuroprotective gene, neurturin. Based on preclinical studies, this gene therapeutic approach is posited to slow disease progression by enhancing neuronal survival. In addition, we discuss the outcome of each clinical trial and discuss the potential rationale for the marginal yet incremental clinical advancements that have thus far been realized for Parkinson's disease gene therapy.

Progress and prospects: Immunobiology of gene therapy for neurodegenerative disease: prospects and risks

Gene therapy for neurological, and in particular neurodegenerative, disease is now a reality. A number of early phase clinical trials have been completed and several are currently in progress. In view of this, it is critically important to evaluate the immunological risk associated with neurological gene therapy, which has clear implications for trial safety and efficacy. Moreover, it is imperative in particular to identify factors indicating potential high risk. In the light of recent advances in understanding immune regulation in the central nervous system (CNS) and with the continued development of new gene delivery vectors, this review critically assesses the current knowledge of immunobiology within the CNS in terms of likely immunological risk pertaining to viral vectors and gene therapy applications for neurodegenerative disease.

Large-scale adeno-associated viral vector production using a herpesvirus-based system enables manufacturing for clinical studies

The ability of recombinant adeno-associated viral (rAAV) vectors to exhibit minimal immunogenicity and little to no toxicity or inflammation while eliciting robust, multiyear gene expression in vivo are only a few of the salient features that make them ideally suited for many gene therapy applications. A major hurdle for the use of rAAV in sizeable research and clinical applications is the lack of efficient and versatile large-scale production systems. Continued progression toward flexible, scalable production techniques is a prerequisite to support human clinical evaluation of these novel biotherapeutics. This review examines the current state of large-scale production methods that employ the herpes simplex virus type 1 (HSV) platform to produce rAAV vectors for gene delivery. Improvements have substantially advanced the HSV/AAV hybrid method for large-scale rAAV manufacture, facilitating the generation of highly potent, clinical-grade purity rAAV vector stocks. At least one human clinical trial employing rAAV generated via rHSV helper-assisted replication is poised to commence, highlighting the advances and relevance of this production method.

Consideration of gene therapy for paediatric neurotransmitter diseases

The paediatric neurotransmitter diseases (PNDs) are a group of inborn errors of metabolism characterized by abnormalities of neurotransmitter synthesis or metabolism. Although some children may react favourably to neurotransmitter augmentation treatment, optimal response is not universal and other modes of treatment should be sought. The genes involved in many of the currently known monoamine PNDs have been utilized in pre-clinical and in phase I clinical trials in Parkinson disease (PD) and the basic principles could be applied to the therapy of PNDs with some modifications regarding the targeting and distribution of vectors. However, issues that go beyond neurotransmitter replacement are important considerations in PD and even more so in PNDs. Understanding the pathophysiology of PNDs including abnormal development resulting from the neurotransmitter deficiency will be critical for rational therapeutic approaches. Better animal models of PNDs are necessary to test gene therapy before clinical trials can be attempted.

Scientific rationale for the development of gene therapy strategies for Parkinson's disease

The ever-evolving understanding of the neuronal systems involved in Parkinson's disease together with the recent advances in recombinant viral vector technology has led to the development of several gene therapy applications that are now entering into clinical testing phase. To date, four fundamentally different approaches have been pursued utilizing recombinant adeno-associated virus and lentiviruses as vectors for delivery. These strategies aim either to restore the lost brain functions by substitution of enzymes critical for synthesis of neurotransmitters or neurotrophic factors as a means to boost the function of remaining neurons in the diseased brain. In this review we discuss the differences in mechanism of action and describe the scientific rationale behind the currently tested gene therapy approaches for Parkinson's disease in some detail and pinpoint their individual unique strengths and weaknesses.

Advantages of COS-1 monkey kidney epithelial cells as packaging host for small-volume production of high-quality recombinant lentiviruses

The HEK293T human embryonic kidney cells have been used widely as a packaging host for transfection-based production of recombinant lentiviruses. The present study describes advantages of using COS-1 African green monkey kidney cells versus HEK293T cells as a packaging host for small-volume production of high-quality recombinant lentiviruses. The particle performance index, defined as the ratio of infection-competent viral particles to the total number of particles, was three- to four-fold greater in transfection supernatants generated using COS-1 cells than that generated using HEK293T cells. Adhesion of HEK293T cells to the cell culture-treated plastic surface was weak, causing significant HEK293T cell contamination in the transfection supernatants produced by laboratory automation using the 96-well cell culture plates. In contrast, COS-1 cells adhered strongly to the plastic surface, and cell contamination was not detected in the transfection supernatants. These results suggest that COS-1 cells may be a useful alternative packaging host for use for automated generation of large numbers of high-quality lentivirus reagents, particularly because they eliminate the need for additional purification steps to remove viral particles from cell culture supernatant.

alpha-Synuclein: a therapeutic target for Parkinson's disease?

Parkinson's disease is a progressive age-related neurodegenerative disease with invariant loss of substantia nigra dopamine neurons and striatal projections. This disorder is well known for the associated motoric symptoms including resting tremor and the inability to initiate movement. However, it is now apparent that Parkinson's disease is a multisystem disorder with patients exhibiting symptoms derived from peripheral nervous system and extra-nigral dysfunctions in addition to the prototypical nigrostriatal damage. Although the etiology for sporadic Parkinson's disease is unknown, information gleaned from both familial forms of the disease and animal models places misfolded alpha-synuclein at the forefront. The disease is currently without a cure and most therapies target the motoric symptoms relying on increasing dopamine tone. In this review, the role of alpha-synuclein in disease pathogenesis and as a potential therapeutic target focusing on toxic conformers of this protein is considered. The addition of protofibrillar/oligomer-directed neurotherapeutics to the existing armamentarium may extend the symptom-free stage of Parkinson's disease as well as alleviate pathogenesis.

Preclinical development of gene therapy for Parkinson's disease

Multiple targets and pathways may be amenable to the development of gene therapy approaches for Parkinson's disease. This article discusses some of the cellular and brain circuit pathways relevant to Parkinson's disease that would be clinically amenable to gene therapy. Approaches could be classified according to two main categories, i.e. symptomatic vs. neuroprotective/neurorestorative strategies. Examples of the different possibilities currently in development are given and feature both dopaminergic and non-dopaminergic symptomatic treatments of parkinsonian symptoms and/or L-DOPA-induced side effects, anti-apoptotic neuroprotective strategies and growth-factor delivery for neuroprotection/neurorestoration. While gene therapy has been mostly used so far for enhancing the expression of the target gene, the use of dominant negative or siRNA opens new possibilities. This, combined with the key feature of gene delivery that offers access to intracellular signalling pathways, is likely to further expand the number of proposed targets to be studied.