Gene delivery to rat and human Schwann cells and nerve segments: a comparison of AAV 1–9 and lentiviral vectors

Novel AAV variants with improved tropism for human Schwann cells

Gene therapies and associated technologies are transforming biomedical research and enabling novel therapeutic options for patients living with debilitating and incurable genetic disorders. The vector system based on recombinant adeno-associated viral vectors (AAVs) has shown great promise in recent clinical trials for genetic diseases of multiple organs, such as the liver and the nervous system. Despite recent successes toward the development of novel bioengineered AAV variants for improved transduction of primary human tissues and cells, vectors that can efficiently transduce human Schwann cells (hSCs) have yet to be identified. Here, we report the application of the functional transduction-RNA selection method in primary hSCs for the development of AAV variants for specific and efficient transgene delivery to hSCs. The two identified capsid variants, Pep2hSC1 and Pep2hSC2, show conserved potency for delivery across various in vitro, in vivo, and ex vivo models of hSCs. These novel AAV capsids will serve as valuable research tools, forming the basis for therapeutic solutions for both SC-related disorders or peripheral nervous system injury.

AAV-mediated editing of PMP22 rescues Charcot-Marie-Tooth disease type 1A features in patient-derived iPS Schwann cells

BACKGROUND

Charcot-Marie-Tooth disease type 1A (CMT1A) is one of the most common hereditary peripheral neuropathies caused by duplication of 1.5 Mb genome region including PMP22 gene. We aimed to correct the duplication in human CMT1A patient-derived iPS cells (CMT1A-iPSCs) by genome editing and intended to analyze the effect on Schwann cells differentiated from CMT1A-iPSCs.

METHODS

We designed multiple gRNAs targeting a unique sequence present at two sites that sandwich only a single copy of duplicated peripheral myelin protein 22 (PMP22) genes, and selected one of them (gRNA3) from screening their efficiencies by T7E1 mismatch detection assay. AAV2-hSaCas9-gRNAedit was generated by subcloning gRNA3 into pX601-AAV-CMV plasmid, and the genome editing AAV vector was infected to CMT1A-iPSCs or CMT1A-iPSC-derived Schwann cell precursors. The effect of the genome editing AAV vector on myelination was evaluated by co-immunostaining of myelin basic protein (MBP), a marker of mature myelin, and microtubule-associated protein  2(MAP2), a marker of neurites or by electron microscopy.

RESULTS

Here we show that infection of CMT1A-iPS cells (iPSCs) with AAV2-hSaCas9-gRNAedit expressing both hSaCas9 and gRNA targeting the tandem repeat sequence decreased PMP22 gene duplication by 20-40%. Infection of CMT1A-iPSC-derived Schwann cell precursors with AAV2-hSaCas9-gRNAedit normalized PMP22 mRNA and PMP22 protein expression levels, and also ameliorated increased apoptosis and impaired myelination in CMT1A-iPSC-derived Schwann cells.

CONCLUSIONS

In vivo transfer of AAV2-hSaCas9-gRNAedit to peripheral nerves could be a potential therapeutic modality for CMT1A patient after careful examinations of toxicity including off-target mutations.

Human Schwann Cells in vitro III. Analytical Methods and a Practical Approach for Quality Control

This paper introduces simple analytical methods and bioassays to promptly assess the identity and function of in vitro cultured human Schwann cells (hSCs). A systematic approach is proposed to unequivocally discriminate hSCs from other glial cells, non-glial cells, and non-human SCs (authentication), identify hSCs at different stages of differentiation, and determine whether individual hSCs are proliferative or senescent. Examples of how to use distinct cell-based approaches for quality control and routine troubleshooting are provided to confirm the constitution (identity, purity, and heterogeneity) and potency (bioactivity) of hSC cultures from multiple sources. The bioassays are valuable for rapidly gauging the responses of hSCs to mitogenic and differentiating factors and ascertaining the cells' basic properties before performing co-culture or cell grafting studies. The assays are image based and use adherent hSCs established in monoculture to simplify the experimental setup and interpretation of results. Finally, all sections contain thorough background information, notes, and references to facilitate decision making, data interpretation, and ad hoc method development for diverse applications.

Human Schwann Cells in vitro II. Passaging, Purification, Banking, and Labeling of Established Cultures

This manuscript describes step-by-step procedures to establish and manage fresh and cryopreserved cultures of nerve-derived human Schwann cells (hSCs) at the desired scale. Adaptable protocols are provided to propagate hSC cultures through serial passaging and perform routine manipulations such as enzymatic dissociation, purification, cryogenic preservation, live-cell labeling, and gene delivery. Expanded hSCs cultures are metabolically active, proliferative, and phenotypically stable for at least three consecutive passages. Cell yields are expected to be variable as determined by the rate of growth of individual batches and the rounds of subculture. The purity, however, can be maintained high at >95% hSC regardless of passage. The cells obtained in this manner are suitable for various applications, including small drug screens, in vitro modeling of neurodevelopmental processes, and cell transplantation. One caveat of this protocol is that continued expansion of same-batch hSC populations is eventually restricted due to senescence-linked growth arrest.

AAV2/9-mediated silencing of PMP22 prevents the development of pathological features in a rat model of Charcot-Marie-Tooth disease 1 A

Charcot-Marie-Tooth disease 1 A (CMT1A) results from a duplication of the PMP22 gene in Schwann cells and a deficit of myelination in peripheral nerves. Patients with CMT1A have reduced nerve conduction velocity, muscle wasting, hand and foot deformations and foot drop walking. Here, we evaluate the safety and efficacy of recombinant adeno-associated viral vector serotype 9 (AAV2/9) expressing GFP and shRNAs targeting Pmp22 mRNA in animal models of Charcot-Marie-Tooth disease 1 A. Intra-nerve delivery of AAV2/9 in the sciatic nerve allowed widespread transgene expression in resident myelinating Schwann cells in mice, rats and non-human primates. A bilateral treatment restore expression levels of PMP22 comparable to wild-type conditions, resulting in increased myelination and prevention of motor and sensory impairments over a twelve-months period in a rat model of CMT1A. We observed limited off-target transduction and immune response using the intra-nerve delivery route. A combination of previously characterized human skin biomarkers is able to discriminate between treated and untreated animals, indicating their potential use as part of outcome measures.

AAV Targeting of Glial Cell Types in the Central and Peripheral Nervous System and Relevance to Human Gene Therapy

Different glial cell types are found throughout the central (CNS) and peripheral nervous system (PNS), where they have important functions. These cell types are also involved in nervous system pathology, playing roles in neurodegenerative disease and following trauma in the brain and spinal cord (astrocytes, microglia, oligodendrocytes), nerve degeneration and development of pain in peripheral nerves (Schwann cells, satellite cells), retinal diseases (Müller glia) and gut dysbiosis (enteric glia). These cell type have all been proposed as potential targets for treating these conditions. One approach to target these cell types is the use of gene therapy to modify gene expression. Adeno-associated virus (AAV) vectors have been shown to be safe and effective in targeting cells in the nervous system and have been used in a number of clinical trials. To date, a number of studies have tested the use of different AAV serotypes and cell-specific promoters to increase glial cell tropism and expression. However, true glial-cell specific targeting for a particular glial cell type remains elusive. This review provides an overview of research into developing glial specific gene therapy and discusses some of the issues that still need to be addressed to make glial cell gene therapy a clinical reality.

Comparative intra-articular gene transfer of seven adeno-associated virus serotypes reveals that AAV2 mediates the most efficient transduction to mouse arthritic chondrocytes

Osteoarthritis (OA) is the most common arthropathy, characterized by progressive degeneration of the articular cartilage. Currently, there are no disease-modifying approaches for OA treatment. Adeno-associated virus (AAV)-mediated gene therapy has recently become a potential treatment for OA due to its exceptional characteristics; however, the tropism and transduction efficiency of different AAV serotypes to articular joints and the safety profile of AAV applications are still unknown. The present study aims to screen an ideal AAV serotype to efficiently transfer genes to arthritic cartilage. AAV vectors of different serotypes expressing eGFP protein were injected into the knee joint cavities of mice, with all joint tissues collected 30 days after AAV injection. The transduction efficiency of AAVs was quantified by assessing the fluorescent intensities of eGFP in the cartilage of knee joints. Structural and morphological changes were analyzed by toluidine blue staining. Changes to ECM metabolism and pyroptosis of chondrocytes were determined by immunohistochemical staining. Fluorescence analysis of eGFP showed that eGFP was expressed in the cartilage of knee joints injected with each AAV vector. Quantification of eGFP intensity indicated that AAV2, 7 and 8 had the highest transduction efficiencies. Both toluidine blue staining and Mankin score showed that AAV6 aggravated cartilage degeneration. The analysis of key molecules in ECM metabolism suggested that AAV5 and 7 significantly reduced collagen type II, while AAV9 increased ADAMTS-4 but decreased MMP-19. In addition, transduction with AAV2, 5, 7 and 8 had no obvious effect on pyroptosis of chondrocytes. Comprehensive score analysis also showed that AAV2 had the highest score in intra-articular gene transfer. Collectively, our findings point to AAV2 as the best AAV serotype candidate for gene transfer on arthritic cartilage, resulting in minimal impact to ECM metabolism and pyroptosis of chondrocytes.

GDNF Gene Therapy to Repair the Injured Peripheral Nerve

A spinal root avulsion is the most severe proximal peripheral nerve lesion possible. Avulsion of ventral root filaments disconnects spinal motoneurons from their target muscles, resulting in complete paralysis. In patients that undergo brachial plexus nerve repair, axonal regeneration is a slow process. It takes months or even years to bridge the distance from the lesion site to the distal targets located in the forearm. Following ventral root avulsion, without additional pharmacological or surgical treatments, progressive death of motoneurons occurs within 2 weeks (Koliatsos et al., 1994). Reimplantation of the avulsed ventral root or peripheral nerve graft can act as a conduit for regenerating axons and increases motoneuron survival (Chai et al., 2000). However, this beneficial effect is transient. Combined with protracted and poor long-distance axonal regeneration, this results in permanent function loss. To overcome motoneuron death and improve functional recovery, several promising intervention strategies are being developed. Here, we focus on GDNF gene-therapy. We first introduce the experimental ventral root avulsion model and discuss its value as a proxy to study clinical neurotmetic nerve lesions. Second, we discuss our recent studies showing that GDNF gene-therapy is a powerful strategy to promote long-term motoneuron survival and improve function when target muscle reinnervation occurs within a critical post-lesion period. Based upon these observations, we discuss the influence of timing of the intervention, and of the duration, concentration and location of GDNF delivery on functional outcome. Finally, we provide a perspective on future research directions to realize functional recovery using gene therapy.

Magnetic separation of peripheral nerve-resident cells underscores key molecular features of human Schwann cells and fibroblasts: an immunochemical and transcriptomics approach

Nerve-derived human Schwann cell (SC) cultures are irreplaceable models for basic and translational research but their use can be limited due to the risk of fibroblast overgrowth. Fibroblasts are an ill-defined population consisting of highly proliferative cells that, contrary to human SCs, do not undergo senescence in culture. We initiated this study by performing an exhaustive immunological and functional characterization of adult nerve-derived human SCs and fibroblasts to reveal their properties and optimize a protocol of magnetic-activated cell sorting (MACS) to separate them effectively both as viable and biologically competent cells. We next used immunofluorescence microscopy imaging, flow cytometry analysis and next generation RNA sequencing (RNA-seq) to unambiguously characterize the post-MACS cell products. High resolution transcriptome profiling revealed the identity of key lineage-specific transcripts and the clearly distinct neural crest and mesenchymal origin of human SCs and fibroblasts, respectively. Our analysis underscored a progenitor- or stem cell-like molecular phenotype in SCs and fibroblasts and the heterogeneity of the fibroblast populations. In addition, pathway analysis of RNA-seq data highlighted putative bidirectional networks of fibroblast-to-SC signaling that predict a complementary, yet seemingly independent contribution of SCs and fibroblasts to nerve regeneration. In sum, combining MACS with immunochemical and transcriptomics approaches provides an ideal workflow to exhaustively assess the identity, the stage of differentiation and functional features of highly purified cells from human peripheral nerve tissues.

NF1, Neurofibromin and Gene Therapy: Prospects of Next-Generation Therapy

Neurofibromatosis type 1 [NF1] is an autosomal dominant genetic disorder affecting multiple organs. NF1 is well known for its various clinical manifestations, including café-au-late macules, Lisch nodules, bone deformity and neurofibromas. However, there is no effective therapy for NF1. Current therapies are aimed at alleviating NF1 clinical symptoms but not curing the disease. By altering pathogenic genes, gene therapy regulates cell activities at the nucleotide level. In this review, we described the structure and functions of neurofibromin domains, including GAP-related domain [GRD], cysteine-serine rich domain [CSRD], leucine-rich domain [LRD] and C-terminal domain [CTD], which respectively alter downstream pathways. By transfecting isolated sequences of these domains, researchers can partially restore normal cell functions in neurofibroma cell lines. Furthermore, recombinant transgene sequences may be designed to encode truncated proteins, which is functional and easy to be packaged into viral vectors. In addition, the treatment effect of gene therapy is also determined by various factors such as the vectors selection, transgene packaging strategies and drug administration. We summarized multiple NF1 gene therapy strategies and discussed their feasibility from multiple angles. Different protein domains alter the function and downstream pathways of neurofibromin.

Gene Therapy Options as New Treatment for Inherited Peripheral Neuropathy

Inherited peripheral neuropathy (IPN) is caused by heterogeneous genetic mutations in more than 100 genes. So far, several treatment options for IPN have been developed and clinically evaluated using small molecules. However, gene therapy-based therapeutic strategies have not been aggressively investigated, likely due to the complexities of inheritance in IPN. Indeed, because the majority of the causative mutations of IPN lead to gain-of-function rather than loss-of-function, developing a therapeutic strategy is more difficult, especially considering gene therapy for genetic diseases began with the simple idea of replacing a defective gene with a functional copy. Recent advances in gene manipulation technology have brought novel approaches to gene therapy and its clinical application for IPN treatment. For example, in addition to the classically used gene replacement for mutant genes in recessively inherited IPN, other techniques including gene addition to modify the disease phenotype, modulations of target gene expression, and techniques to edit mutant genes have been developed and evaluated as potent therapeutic strategies for dominantly inherited IPN. In this review, the current status of gene therapy for IPN and future perspectives will be discussed.

The properties of human Schwann cells: Lessons from in vitro culture and transplantation studies

Human Schwann cells (hSCs) can be isolated directly from peripheral nerve and cultured using methods similar to those used for SCs from other species. Yet, important interspecies differences are revealed when the primary or expanded hSCs are compared to their nonhuman counterparts. This review addresses the special properties of nerve-derived hSCs that have resulted to date from both in vitro studies and in vivo research on cell transplantation in animal models and human subjects. A consensus has yet to emerge about the essential attributes of cultured normal hSCs. Thus, an emphasis is placed on the importance of validating hSC cultures by means of purity, identity, and biological activity to reliably use them as in vitro models of the SC phenotype and cell therapy products for injury repair. Combining traditional immunological methods, high-resolution omics approaches, and assorted cell-based assays is so far the best approach to unequivocally identify hSC populations obtained by direct isolation or derivation from stem cells. Special considerations are required to understand and manage the variability and heterogeneity proper of donor batches, as well as to evaluate risk factors. This is particularly important if the intended use of the hSCs is implantation in the human body, diagnosis of disease, or drug testing aimed at targeting any aspect of SC function in human patients. To conclude, in view of their unique properties, new concepts and methods are needed to better understand the biology of hSCs and exploit their full potential in basic science and regenerative medicine.

Gene therapy approaches targeting Schwann cells for demyelinating neuropathies

Charcot-Marie-Tooth disease (CMT) encompasses numerous genetically heterogeneous inherited neuropathies, which together are one of the commonest neurogenetic disorders. Axonal CMT types result from mutations in neuronally expressed genes, whereas demyelinating CMT forms mostly result from mutations in genes expressed by myelinating Schwann cells. The demyelinating forms are the most common, and may be caused by dominant mutations and gene dosage effects (as in CMT1), as well as by recessive mutations and loss of function mechanisms (as in CMT4). The discovery of causative genes and increasing insights into molecular mechanisms through the study of experimental disease models has provided the basis for the development of gene therapy approaches. For demyelinating CMT, gene silencing or gene replacement strategies need to be targeted to Schwann cells. Progress in gene replacement for two different CMT forms, including CMT1X caused by GJB1 gene mutations, and CMT4C, caused by SH3TC2 gene mutations, has been made through the use of a myelin-specific promoter to restrict expression in Schwann cells, and by lumbar intrathecal delivery of lentiviral viral vectors to achieve more widespread biodistribution in the peripheral nervous system. This review summarizes the molecular-genetic mechanisms of selected demyelinating CMT neuropathies and the progress made so far, as well as the remaining challenges in the path towards a gene therapy to treat these disorders through the use of optimal gene therapy tools including clinically translatable delivery methods and adeno-associated viral (AAV) vectors.

Gene replacement therapy after neuropathy onset provides therapeutic benefit in a model of CMT1X

X-linked Charcot-Marie-Tooth disease (CMT1X), one of the commonest forms of inherited demyelinating neuropathy, results from GJB1 gene mutations causing loss of function of the gap junction protein connexin32 (Cx32). The aim of this study was to examine whether delayed gene replacement therapy after the onset of peripheral neuropathy can provide a therapeutic benefit in the Gjb1-null/Cx32 knockout model of CMT1X. After delivery of the LV-Mpz.GJB1 lentiviral vector by a single lumbar intrathecal injection into 6-month-old Gjb1-null mice, we confirmed expression of Cx32 in lumbar roots and sciatic nerves correctly localized at the paranodal myelin areas. Gjb1-null mice treated with LV-Mpz.GJB1 compared with LV-Mpz.Egfp (mock) vector at the age of 6 months showed improved motor performance at 8 and 10 months. Furthermore, treated mice showed increased sciatic nerve conduction velocities, improvement of myelination and reduced inflammation in lumbar roots and peripheral nerves at 10 months of age, along with enhanced quadriceps muscle innervation. Plasma neurofilament light (NEFL) levels, a clinically relevant biomarker, were also ameliorated in fully treated mice. Intrathecal gene delivery after the onset of peripheral neuropathy offers a significant therapeutic benefit in this disease model, providing a proof of principle for treating patients with CMT1X at different ages.

Feasibility of using NF1-GRD and AAV for gene replacement therapy in NF1-associated tumors

Neurofibromatosis type 1, including the highly aggressive malignant peripheral nerve sheath tumors (MPNSTs), is featured by the loss of functional neurofibromin 1 (NF1) protein resulting from genetic alterations. A major function of NF1 is suppressing Ras activities, which is conveyed by an intrinsic GTPase-activating protein-related domain (GRD). In this study, we explored the feasibility of restoring Ras GTPase via exogenous expression of various GRD constructs, via gene delivery using a panel of adeno-associated virus (AAV) vectors in MPNST and human Schwann cells (HSCs). We demonstrated that several AAV serotypes achieved favorable transduction efficacies in those cells and a membrane-targeting GRD fused with an H-Ras C-terminal motif (C10) dramatically inhibited the Ras pathway and MPNST cells in a NF1-specific manner. Our results opened up a venue of gene replacement therapy in NF1-related tumors.

Differentially Branched Ester Amine Quadpolymers with Amphiphilic and pH-Sensitive Properties for Efficient Plasmid DNA Delivery

Development of highly effective nonviral gene delivery vectors for transfection of diverse cell populations remains a challenge despite utilization of both rational and combinatorial driven approaches to nanoparticle engineering. In this work, multifunctional polyesters are synthesized with well-defined branching structures via A2 + B2/B3 + C1 Michael addition reactions from small molecule acrylate and amine monomers and then end-capped with amine-containing small molecules to assess the influence of polymer branching structure on transfection. These Branched poly(Ester Amine) Quadpolymers (BEAQs) are highly effective for delivery of plasmid DNA to retinal pigment epithelial cells and demonstrate multiple improvements over previously reported leading linear poly(beta-amino ester)s, particularly for volume-limited applications where improved efficiency is required. BEAQs with moderate degrees of branching are demonstrated to be optimal for delivery under high serum conditions and low nanoparticle doses further relevant for therapeutic gene delivery applications. Defined structural properties of each polymer in the series, including tertiary amine content, correlated with cellular transfection efficacy and viability. Trends that can be applied to the rational design of future generations of biodegradable polymers are elucidated.

The therapeutic potential of RNA regulation in neurological disorders

INTRODUCTION

Gene regulation is the term used to describe the mechanisms by which a cell increases or decreases the amount of a gene product (RNA or protein). In complex organs such as the brain, gene regulation is of the utmost importance; aberrations in the regulation of specific genes can lead to neurological disorders. Understanding these mechanisms can create new strategies for targeting these disorders and progress is being made. Two drugs that function at the RNA level (nusinersen and eteplirsen) have now been approved by the FDA for the treatment of Spinomuscular atrophy and Duchenne muscular dystrophy, respectively; several other compounds for neurological disease are currently being investigated in preclinical studies and clinical trials. Areas covered: We highlight how gene regulation at the level of RNA molecules can be used as a therapeutic strategy to treat neurological disorders. We provide examples of how such an approach is being studied or used and discuss the current hurdles. Expert opinion: Targeting gene expression at the RNA level is a promising strategy to treat genetic neurological disorders. Safe administration, long-term efficacy, and potential side effects, however, still need careful evaluation before RNA therapeutics can be applied on a larger scale.

In vitro efficacy of a gene-activated nerve guidance conduit incorporating non-viral PEI-pDNA nanoparticles carrying genes encoding for NGF, GDNF and c-Jun

Despite the success of tissue engineered nerve guidance conduits (NGCs) for the treatment of small peripheral nerve injuries, autografts remain the clinical gold standard for larger injuries. The delivery of neurotrophic factors from conduits might enhance repair for more effective treatment of larger injuries but the efficacy of such systems is dependent on a safe, effective platform for controlled and localised therapeutic delivery. Gene therapy might offer an innovative approach to control the timing, release and level of neurotrophic factor production by directing cells to transiently sustain therapeutic protein production in situ. In this study, a gene-activated NGC was developed by incorporating non-viral polyethyleneimine-plasmid DNA (PEI-pDNA) nanoparticles (N/P 7 ratio, 2 μg dose) with the pDNA encoding for nerve growth factor (NGF), glial derived neurotrophic factor (GDNF) or the transcription factor c-Jun. The physicochemical properties of PEI-pDNA nanoparticles, morphology, size and charge, were shown to be suitable for gene delivery and demonstrated high Schwann cell transfection efficiency (60 ± 13%) in vitro. While all three genes showed therapeutic potential in terms of enhancing neurotrophic cytokine production while promoting neurite outgrowth, delivery of the gene encoding for c-Jun showed the greatest capacity to enhance regenerative cellular processes in vitro. Ultimately, this gene-activated NGC construct was shown to be capable of transfecting both Schwann cells (S42 cells) and neuronal cells (PC12 and dorsal root ganglia) in vitro, demonstrating potential for future therapeutic applications in vivo.

STATEMENT OF SIGNIFICANCE

The basic requirements of biomaterial-based nerve guidance conduits have now been well established and include being able to bridge a nerve injury to support macroscopic guidance between nerve stumps, while being strong enough to withstand longitudinal tension and circumferential compression, in addition to being mechanically sound to facilitate surgical handling and implantation. While meeting these criteria, conduits are still limited to the treatment of small defects clinically and might benefit from additional biochemical stimuli to enhance repair for the effective treatment of larger injuries. In this study, a gene activated conduit was successfully developed by incorporating non-viral nanoparticles capable of efficient Schwann cell and neuronal cell transfection with therapeutic genes in vitro, which showed potential to enhance repair in future applications particularly when taking advantage of the transcription factor c-Jun. This innovative approach may provide an alternative to conduits used as platforms for the delivery neurotrophic factors or genetically modified cells (viral gene therapy), and a potential solution for the unmet clinical need to repair large peripheral nerve injury effectively.

Combining Gene and Stem Cell Therapy for Peripheral Nerve Tissue Engineering

Despite a substantially increased understanding of neuropathophysiology, insufficient functional recovery after peripheral nerve injury remains a significant clinical challenge. Nerve regeneration following injury is dependent on Schwann cells, the supporting cells in the peripheral nervous system. Following nerve injury, Schwann cells adopt a proregenerative phenotype, which supports and guides regenerating nerves. However, this phenotype may not persist long enough to ensure functional recovery. Tissue-engineered nerve repair devices containing therapeutic cells that maintain the appropriate phenotype may help enhance nerve regeneration. The combination of gene and cell therapy is an emerging experimental strategy that seeks to provide the optimal environment for axonal regeneration and reestablishment of functional circuits. This review aims to summarize current preclinical evidence with potential for future translation from bench to bedside.

Gene therapy and peripheral nerve repair: a perspective

Clinical phase I/II studies have demonstrated the safety of gene therapy for a variety of central nervous system disorders, including Canavan's, Parkinson's (PD) and Alzheimer's disease (AD), retinal diseases and pain. The majority of gene therapy studies in the CNS have used adeno-associated viral vectors (AAV) and the first AAV-based therapeutic, a vector encoding lipoprotein lipase, is now marketed in Europe under the name Glybera. These remarkable advances may become relevant to translational research on gene therapy to promote peripheral nervous system (PNS) repair. This short review first summarizes the results of gene therapy in animal models for peripheral nerve repair. Secondly, we identify key areas of future research in the domain of PNS-gene therapy. Finally, a perspective is provided on the path to clinical translation of PNS-gene therapy for traumatic nerve injuries. In the latter section we discuss the route and mode of delivery of the vector to human patients, the efficacy and safety of the vector, and the choice of the patient population for a first possible proof-of-concept clinical study.