Viral vector-mediated gene transfer for CNS disease

Importance of the field: Gene therapy is a promising strategy for the treatment of many neurological disorders that currently lack effective treatment. Recent improvements in vectorology and vector engineering have improved overall safety and delivery of viral vectors.

Areas covered in this review: This review discusses the current state of viral vector development and clinical use, as well as routes of delivery, and clinical trials for neurological disorders.

What the reader will gain: Viral vectors may be delivered directly or remotely to the CNS, largely depending on the nature of the disease and the tropism of the vector. Nonetheless, delivery remains one of the major limitations of successful gene transfer to the CNS.

Take home message: Although the majority of clinical trials have centered on gene replacement and neuroprotection approaches, the field is advancing in the direction of neuromodulation, gene silencing and other newer strategies.     

Recombinant AAV-directed gene therapy for type I glycogen storage diseases

Introduction: Glycogen storage disease (GSD) type Ia and Ib are disorders of impaired glucose homeostasis affecting the liver and kidney. GSD-Ib also affects neutrophils. Current dietary therapies cannot prevent long-term complications. In animal studies, recombinant adeno-associated virus (rAAV) vector-mediated gene therapy can correct or minimize multiple aspects of the disorders, offering hope for human gene therapy.

Areas covered: A summary of recent progress in rAAV-mediated gene therapy for GSD-I; strategies to improve rAAV-mediated gene delivery, transduction efficiency and immune avoidance; and vector refinements that improve expression.

Expert opinion: rAAV-mediated gene delivery to the liver can restore glucose homeostasis in preclinical models of GSD-I, but some long-term complications of the liver and kidney remain. Gene therapy for GSD-Ib is less advanced than for GSD-Ia and only transient correction of myeloid dysfunction has been achieved. A question remains as to whether a single rAAV vector can meet the expression efficiency and tropism required to treat all aspects of GSD-I, or if a multi-pronged approach is needed. An understanding of the strengths and weaknesses of rAAV vectors in the context of strategies to achieve efficient transduction of the liver, kidney and hematopoietic stem cells is required for treating GSD-I.     

Gene therapy for muscular dystrophy: current progress and future prospects

Muscular dystrophies refer to a group of inherited disorders characterized by progressive muscle weakness, wasting and degeneration. So far, there is no effective treatment but new gene-based therapies are currently being developed with particular noted advances in using conventional gene replacement strategies, RNA-based approaches, or cell-based gene therapy with a main focus on Duchenne muscular dystrophy (DMD). DMD is the most common and severe form of muscular dystrophy and current treatments are far from adequate. However, genetic and cell-based therapies, in particular exon skipping induced by antisense strategies, and corrective gene therapy via functionally engineered dystrophin genes hold great promise, with several clinical trials ongoing. Proof-of-concept of exon skipping has been obtained in animal models, and most recently in clinical trials; this approach represents a promising therapy for a subset of patients. In addition, gene-delivery-based strategies exist both for antisense-induced reading frame restoration, and for highly efficient delivery of functional dystrophin mini- and micro-genes to muscle fibres in vivo and muscle stem cells ex-vivo. In particular, AAV-based vectors show efficient systemic gene delivery to skeletal muscle directly in vivo, and lentivirus-based vectors show promise of combining ex vivo gene modification strategies with cell-mediated therapies.     

Update on gene and stem cell therapy approaches for spinal muscular atrophy

Introduction: Spinal muscular atrophy (SMA) is the leading genetic cause of pediatric death to which at present there is no effective therapeutic. The genetic defect is well characterized as a mutation in exon 7 of the survival of motor neuron (SMN) gene. The current gene therapy approach focuses on two main methodologies, the replacement of SMN1 or augmentation of SMN2 readthrough. The most promising of the current work focuses on the delivery of SMN via AAV9 vectors via intravenous delivery.

Areas covered: In the review the authors examine the current research in the field of stem cell and gene therapy approaches for SMA. Also focusing on delivery methods, timing of administration and general caveats that must be considered with translational work for SMA.

Expert opinion: Gene therapy currently offers the most promising avenue of research for a successful therapeutic for SMA. There are many important practical and ethical considerations which must be carefully considered when dealing with clinical trial in infants such as the invasiveness of the surgery, the correct patient cohort and the potential risks.     

The progress of AAV-mediated gene therapy in neuromuscular disorders

Introduction: The well-defined genetic causes and monogenetic nature of many neuromuscular disorders, including Duchenne muscular dystrophy (DMD) and spinal muscular atrophy (SMA), present gene therapy as a prominent therapeutic approach. The novel variants of adeno-associated virus (AAV) can achieve satisfactory transduction efficiency of exogenous genes through the central nervous system and body-wide in skeletal muscle.

Areas covered: In this review, we summarize the strategies of AAV gene therapy that are currently under preclinical and clinical evaluation for the treatment of degenerative neuromuscular disorders, with a focus on diseases such as DMD and SMA. In addition to gene replacement strategy, we provide an overview of other approaches such as AAV-mediated RNA therapy and gene editing in the treatment of muscular dystrophies.

Expert opinion: AAV gene therapy has achieved striking therapeutic efficacy in clinical trials in infants with SMA. Promising results have also come from the preclinical studies in small and large animal models of DMD and several clinical trials are now on the way. This strategy shows great potential as a therapy for various neuromuscular disorders. Further studies are still required to confirm its long-term safety and improve the efficacy.     

The advent of AAV9 expands applications for brain and spinal cord gene delivery

Introduction: Straightforward studies compared adeno-associated virus (AAV) serotypes to determine the most appropriate one for robust expression in the CNS. AAV9 was efficient when directly injected into the brain, but more surprisingly, AAV9 produced global expression in the brain and spinal cord after a peripheral, systemic route of administration to neonatal mice.

Areas covered: Topics include AAV9 gene delivery from intraparenchymal, intravenous, intrathecal and intrauterine routes of administration, and related preclinical studies and disease models. Systemic AAV9 gene transfer yields remarkably consistent neuronal expression, though only in early development. AAV9 is versatile to study neuropathological proteins: microtubule-associated protein tau and transactive response DNA-binding protein 43 kDa (TDP-43).

Expert opinion: AAV9 will be more widely used based on current data, although other natural serotypes and recombineered vectors may also support or improve upon wide-scale expression. A peripheral-to-central gene delivery that can affect the entire CNS without having to inject the CNS is promising for basic functional experiments, and potentially for gene therapy. Systemic or intra-cerebrospinal fluid routes of AAV9 administration should be considered for spinal muscular atrophy, lysosomal storage diseases and amyotrophic lateral sclerosis, if more neuronal expression can be achieved in adults, or if glial expression can be exploited.     

Evolving targeted therapies for right ventricular failure

Introduction: Although right and left ventricular embryological origins, morphology and cardiodynamics differ, the notion of selectively targeted right ventricular therapies remains controversial.

Areas covered: This review focuses on both the currently evolving pharmacologic agents targeting right ventricular failure (metabolic modulators, phosphodiesterase type V inhibitors) and future therapeutic approaches including epigenetic modulation by miRNAs, chromatin binding complexes, long non-coding RNAs, genomic editing, adoptive gene transfer and gene therapy, cell regeneration via cell transplantation and cell reprogramming and cardiac tissue engineering.

Expert opinion: Strategies for adult right ventricular regeneration will require a more holistic approach than strategies for adult left ventricular failure. Instances of right ventricular failure requiring global reconstitution of right ventricular myocardium, attractive approaches include: i) myocardial patches seeded with cardiac fibroblasts reprogrammed into cardiomyocytes in vivo by small molecules, miRNAs or other epigenetic modifiers; and ii) administration of miRNAs, lncRNAs or small molecules by non-viral vector delivery systems targeted to fibroblasts (e.g., episomes) to stimulate in vivo reprogramming of fibroblasts into cardiomyocytes. For selected heritable genetic myocardial diseases, genomic editing affords exciting opportunities for allele-specific silencing by site-specific directed silencing, mutagenesis or gene excision. Genomic editing by adoptive gene transfer affords similarly exciting opportunities for restoration of myocardial gene expression.     

Current advances in cell therapy strategies for muscular dystrophies

Introduction: Muscular dystrophies are a heterogeneous group of genetic diseases characterized by muscle weakness, wasting and degeneration. Cell therapy consists of delivering myogenic precursor cells to damaged tissue for the complementation of missing proteins and/or the regeneration of new muscle fibres.

Areas covered: We focus on human candidate cells described so far (myoblasts, mesoangioblasts, pericytes, myoendothelial cells, CD133⁺ cells, aldehyde-dehydrogenase-positive cells, mesenchymal stem cells, embryonic stem cells, induced pluripotent stem cells), gene-based strategies developed to modify cells prior to injection, animal models (dystrophic and/or immunodeficient) used for pre-clinical studies, and clinical trials that have been performed using cell therapy strategies. The approaches are reviewed in terms of feasibility, hurdles, potential solutions and/or research areas from where the solution may come and potential application in terms of types of dystrophies and targets.

Expert opinion: Cell therapy for muscular dystrophies should be put in the context of which dystrophy or muscle group is targeted, what tools are available at hand, but even more importantly what can cell therapy bring as compared with and/or in combination with other therapeutic strategies. The solution will probably be the right dosage of these combinations adapted to each dystrophy, or even to each type of mutation within a dystrophy.     

An update on RNA interference-mediated gene silencing in cancer therapy

Introduction: Based on our previous review, this article presents the new progress in RNA interference (RNAi)-mediated gene silencing in cancer therapy, and reviews the hurdles and how they might be overcome.

Areas covered: RNAi-mediated gene silencing approaches have been demonstrated in humans, and ongoing clinical trials hold promise for treating cancer or providing alternatives to traditional chemotherapies. Here we describe the broad range of approaches to achieve targeted gene silencing for cancer therapy, discuss the progress made in developing RNAi as therapeutics for cancer and highlight challenges and emerging solutions associated with its clinical development.

Expert opinion: Although the field of RNAi-based cancer therapy is still an emerging one, we have yet to get solutions for overcoming all obstacles associated with its clinical development. The current rapid advances in development of new targeted delivery strategies and noninvasive imaging methods will be big steps to explore RNAi as a new and potent clinical modality in humans.     

Gene therapy as future treatment of erectile dysfunction

Importance of the field: Erectile dysfunction (ED) is a major men's health problem. Although the high success rate of treating ED by phosphodiesterase 5 (PDE5) inhibitors has been reported, there are a significant number of ED patients who do not respond to currently available treatment modalities.

Areas covered in this review: To elucidate the current status of gene therapy applications for ED, gene therapy approaches for ED treatment are reviewed.

What the reader will gain: Gene therapy strategies that can enhance nitric oxide (NO) production or NO-mediated signaling pathways, growth factor-mediated nerve regeneration or K⁺ channel activity in the smooth muscle could be promising approaches for the treatment of ED. Although the majority of gene therapy studies are still in the preclinical phase, the first clinical trial using non-viral gene transfer of Ca²⁺-activated, large-conductance K⁺ channels into the corpus cavernosum of ED patients showed positive results.

Take home message: Gene therapy represents an exciting future treatment option for ED, especially for people with severe ED unresponsive to current first-line therapies such as PDE5 inhibitors although the long-term safety of both viral and non-viral gene therapies should be established.     

siRNA therapy for cancer and non-lethal diseases such as arthritis and osteoporosis

Importance of the field: Gene silencing mediated by siRNA has been widely investigated as a potential therapeutic approach. The success of these therapies depends on effective systems capable of selectively and efficiently conveying siRNA to targeted cells/organs with minimal toxicity.

Areas covered in this review: This review discusses current experimental approaches to siRNA delivery strategies available for arthritis treatment and the management of other musculoskeletal disorders. The review covers literature on the subject from 2000 to 2010.

What the reader will gain: In the last decade, extensive improvements have been made to optimize siRNA-based gene therapy and have been tested on several arthritis and orthopedic conditions. However, except for Phase I – II DNA-based gene therapy trials on arthritis, no clinical studies have reported siRNA application in these domains.

Take home message: Most musculoskeletal disorders, such as rheumatoid arthritis, osteoarthritis, fracture, aseptic loosening, cartilage and intervertebral disc degeneration are non-fatal and age-related chronic inflammatory conditions, but represent significant morbidity and a socio-economic burden. siRNA-based gene therapy offers treatment opportunities that are less invasive, more effective and less expensive than existing modalities. Future directions for siRNA therapy include the development of safe and more efficient delivery systems and the selection of optimal gene targets for disease control.     

TNFα gene delivery therapy for solid tumors

Importance of the field: Multimodality therapy, including adjuvant and neoadjuvant chemotherapy and radiotherapy, is now the mainstay of treatment for the majority of non-hematologic cancers. Host toxicity can, however, be significant, which may contribute to local and/or systemic failures. Novel adjunctive treatments that can limit systemic exposure while synergizing with standard therapy hold promise in the fight against an increasing number of cancers.

Areas covered in this review: We discuss a TNFα gene delivery system used to generate high levels of intratumoral TNFα, while limiting systemic exposure. The delivery system utilizes a replication-deficient adenoviral vector. When injected intratumorally and activated by external beam radiation, infected cells synthesize and locally secrete large amounts of TNFα.

What the reader will gain: This review will provide the reader with a thorough understanding of the gene-based TNFα delivery system with special emphasis on product characteristics, mechanisms of action, clinical efficacy, safety and tolerability.

Take home message: The TNFα gene delivery system holds promise as an adjunctive agent for improved local control and increasing resectability rates for many solid tumors. The completion of several ongoing randomized trials will help to better define the role for TNFα gene delivery therapy in the treatment of solid tumors.     

AAV gene delivery to the spinal cord: serotypes,methods, candidate diseases,and clinical trials

Introduction: Adeno-associated viral (AAV) vector-mediated gene delivery to the spinal cord has finally entered the pathway towards regulatory approval. Phase 1 clinical trials using AAV gene therapy for pediatric disorders – spinal muscular atrophy (SMA) and giant axonal neuropathy (GAN) – are now underway.

Areas covered: This review addresses the latest progress in the field of AAV gene delivery to the spinal cord, particularly focusing on the most prominent AAV serotypes and delivery methodologies to the spinal cord. Candidate diseases and scaling up experiments in large animals are also discussed.

Expert opinion: Intravenous (IV) and intrathecal (IT) deliveries seem to undoubtedly be the preferred routes of administration for diffuse spinal cord delivery of therapeutic AAV vectors that can cross the blood-brain barrier (BBB) and correct inherited genetic disorders. Conversely, intraparenchymal delivery is still an undervalued but very viable approach for segmental therapy in afflictions such as ALS or Pompe Disease as a means to prevent respiratory dysfunction.     

Transduction Efficiency and Immune Response Associated With the Administration of AAV8 Vector Into Dog Skeletal Muscle

Recombinant adeno-associated virus (rAAV)-mediated gene transfer is an attractive approach to the treatment of Duchenne muscular dystrophy (DMD). We investigated the muscle transduction profiles and immune responses associated with the administration of rAAV2 and rAAV8 in normal and canine X-linked muscular dystrophy in Japan (CXMD_J) dogs. rAAV2 or rAAV8 encoding the lacZ gene was injected into the skeletal muscles of normal dogs. Two weeks after the injection, we detected a larger number of β-galactosidase-positive fibers in rAAV8-transduced canine skeletal muscle than in rAAV2-transduced muscle. Although immunohistochemical analysis using anti-CD4 and anti-CD8 antibodies revealed less T-cell response to rAAV8 than to rAAV2, β-galactosidase expression in rAAV8-injected muscle lasted for <4 weeks with intramuscular transduction. Canine bone marrow-derived dendritic cells (DCs) were activated by both rAAV2 and rAAV8, implying that innate immunity might be involved in both cases. Intravenous administration of rAAV8-lacZ into the hind limb in normal dogs and rAAV8-microdystrophin into the hind limb in CXMD_J dogs resulted in improved transgene expression in the skeletal muscles lasting over a period of 8 weeks, but with a declining trend. The limb perfusion transduction protocol with adequate immune modulation would further enhance the rAAV8-mediated transduction strategy and lead to therapeutic benefits in DMD gene therapy.     

Gene therapy for pulmonary hypertension: prospects and challenges

Introduction: Recent evidence shows that pulmonary arterial hypertension (PAH) remains a fatal disease despite the introduction of new pharmacological treatments. New options are therefore needed and gene therapy approaches are a rational consideration based on emerging understanding of the genetic basis of PAH.

Areas covered: This review briefly discusses the recent developments in clinical management of PAH and the investigation of gene delivery techniques for pulmonary vascular disease from 1997 to 2010, relating this to improved understanding of disease pathogenesis during this period. There is a focus on bone morphogenetic protein receptor type 2, as mutations in this gene are clearly linked to disease pathogenesis and outcomes. The reader will gain insight into the gene vector strategies being used, the target cells and the specific genes being delivered as candidate therapeutic approaches for PAH.

Expert opinion: Various genes and strategies for delivery have achieved improvements in PAH in animal models, which is encouraging for the development of this technology for human application. The main limiting factor for clinical progress relates to gene delivery vector technology.     

Embryonic stem cells as a treatment for macular degeneration

Introduction: Retinal degenerations are typically characterized by loss of highly differentiated cell types within the neurosensory retina, such as photoreceptors, or retinal pigment epithelium (RPE). RPE loss is the final common pathway in a number of degenerations including the leading cause of new blindness in the developed world: age-related macular degeneration (AMD).

Areas covered: This paper presents the pathophysiologic case for RPE transplantation with stem cell (SC)-derived tissue, a review of the preclinical data substantiating the hypothesis and the initial clinical trials safety data from early human trials.

Expert opinion: Targeting the RPE for transplantation with SC-derived tissue presents a reasonable therapeutic opportunity in a variety of important, otherwise untreatable, blinding conditions. Success of cellular replacement strategies is contingent on finding a viable source of replacement cells, establishing a safe technique for delivery and survival of transplanted cells within the host, restoration of normal retinal architecture and stabilization or improvement of vision.     

AAV-mediated Overexpression of Human α7 Integrin Leads to Histological and Functional Improvement in Dystrophic Mice

Duchenne muscular dystrophy (DMD) is a severe muscle disease caused by mutations in the DMD gene, with loss of its gene product, dystrophin. Dystrophin helps link integral membrane proteins to the actin cytoskeleton and stabilizes the sarcolemma during muscle activity. We investigated an alternative therapeutic approach to dystrophin replacement by overexpressing human α7 integrin (ITGA7) using adeno-associated virus (AAV) delivery. ITGA7 is a laminin receptor in skeletal and cardiac muscle that links the extracellular matrix (ECM) to the actin skeleton. It is modestly upregulated in DMD muscle and has been proposed to be an important modifier of dystrophic symptoms. We delivered rAAV8.MCK.ITGA7 to the lower limb of mdx mice through isolated limb perfusion (ILP) of the femoral artery. We demonstrated ~50% of fibers in the tibialis anterior (TA) and extensor digitorum longus (EDL) overexpressing α7 integrin at the sarcolemma following AAV gene transfer. The increase in ITGA7 in skeletal muscle significantly protected against loss of force following eccentric contraction-induced injury compared with untreated (contralateral) muscles while specific force following tetanic contraction was unchanged. Reversal of additional dystrophic features included reduced Evans blue dye (EBD) uptake and increased muscle fiber diameter. Taken together, this data shows that rAAV8.MCK.ITGA7 gene transfer stabilizes the sarcolemma potentially preserving mdx muscle from further damage. This therapeutic approach demonstrates promise as a viable treatment for DMD with further implications for other forms of muscular dystrophy.     

Thymosin β4 attenuates microcirculatory and hemodynamic destabilization in sepsis

Objective: The actin polymerization regulator Thymosin β4 (Tβ4) has been shown to be involved in angiogenesis, wound healing, cell survival and anti-inflammatory responses. We have previously shown that Tβ4 is capable of recruiting pericytes, thus stabilizing the endothelial barrier function. Here, we analyzed whether treatment with Tβ4 is able to reduce the pericytes loss in lipopolysaccharides (LPS)-induced sepsis and to improve the hemodynamic function and survival in C57BL/6 mice.

Methods: Fourteen days before LPS injection, the mice were injected with an adeno-associated virus carrying the Tβ4 (rAAV.Tβ4) or LacZ gene (rAAV.LacZ). A sepsis-severity score was assessed, and non-invasive hemodynamic and permeability measurements were performed. Heart and muscle samples were analyzed for PECAM-1⁺ capillaries and NG2⁺pericytes.

Results: At 36 h, there was a decrease of sepsis severity score in rAAV.Tβ4-treated animals as compared to rAAV.LacZ-treated control. rAAV.Tβ4-treated animals displayed lower perivascular leakage and higher blood pressure compared to control. Of note, the rAAV.Tβ4 group showed a higher pericyte count in heart and peripheral muscle samples. Finally, Tβ4-treatment reduced mortality compared to control.

Conclusion: The data indicate a preventive role of Tβ4 in septic hypercirculation and highlight Tβ4 as a potential therapeutic target in severe sepsis.