Widespread spinal cord transduction by intrathecal injection of rAAV delivers efficacious RNAi therapy for amyotrophic lateral sclerosis

Silencing strategies for therapy of SOD1-mediated ALS

Amyotrophic lateral sclerosis (ALS) is an adult-onset, lethal, paralytic disorder caused by the degeneration of motor neurons. Our understanding of this disease has been greatly facilitated by studies of familial ALS caused by mutations in the gene encoding superoxide dismutase 1 (SOD1). Evidence indicates that misfolded wild-type SOD1 may also be pathogenic in sporadic ALS. Mutant SOD1 is neurotoxic through multiple mechanisms. Because the pathogenicity of mutant SOD1 is proportional to the dose of the toxic protein, a rational approach to treating SOD1-related ALS is to reduce levels of the toxic SOD1 species. An advantage of this strategy is that it potentially obviates intervening in multiple, downstream pathological cascades. In recent years, several strategies to silence gene expression have been developed. The most clinically promising are predicated on approaches that enhance degradation of RNA, such as anti-sense oligonucleotides (ASO) and RNA interference (RNAi); the latter include small inhibitory RNA (siRNA), short hairpin RNA (shRNA) and microRNA (miR). Agents such as shRNA and either native or synthetic miR are capable of permeating the central nervous system (CNS) and efficiently silencing genes in the brain and spinal cord. Here we review recent progress in silencing SOD1, focusing on studies using artificial shRNA or miRNA in combination with potent viral vector delivery systems to mediate SOD1 silencing within the CNS in transgenic SOD1^G93A mice and non-human primates.

Adeno Associated Viral Vector Delivered RNAi for Gene Therapy of SOD1 Amyotrophic Lateral Sclerosis

Amyotrophic Lateral Sclerosis (ALS) is a fatal neurodegenerative disease caused by progressive loss of upper and lower motor neurons. Mutations in superoxide dismutase 1 (SOD1) are a leading cause of ALS, responsible for up to 20% of familial cases. Although the exact mechanism by which mutant SOD1 causes disease remains unknown, multiple studies have shown that reduction of the mutant species leads to delayed disease onset and extension of lifespan of animal models. This makes SOD1 an ideal target for gene therapy coupling adeno associated virus vector (AAV) gene delivery with RNAi molecules. In this review we summarize the studies done thus far attempting to decrease SOD1 gene expression, using AAV vectors as delivery tools, and RNAi as therapeutic molecules. Current hurdles to be overcome, such as the need for widespread gene delivery through the entire central nervous system (CNS), are discussed. Continued efforts to improve current AAV delivery methods and capsids will accelerate the application of these therapeutics to the clinic.

scAAV9-VEGF prolongs the survival of transgenic ALS mice by promoting activation of M2 microglia and the PI3K/Akt pathway

Amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disease that leads to paralysis and death three to five years after diagnosis in most patients. The disease is incurable, and the mechanism of motoneuron degeneration remains unknown, although research has demonstrated that activated microglia are involved in motor neuron death. Here, we used a simple method to deliver AAV9 virus by direct intrathecal injection and found that scAAV9-VEGF-165 improved the motor performance and prolonged the life span of SOD1-G93A mice. Furthermore, scAAV9-VEGF-165 activated the PI3K/Akt survival pathway and increased the level of Bcl-2, which contributed to the protection of motor neurons. Additionally, scAAV9-VEGF-165 attenuated the expression of classically activated (M1) microglial markers and enhanced the expression of alternatively activated (M2) microglial markers. Taken together, the results of our study suggest that simple, direct intrathecal injection of scAAV9-VEGF-165 may have a curative effect for ALS.

Adeno-Associated Virus-Based Gene Therapy for CNS Diseases

Gene therapy is at the cusp of a revolution for treating a large spectrum of CNS disorders by providing a durable therapeutic protein via a single administration. Adeno-associated virus (AAV)-mediated gene transfer is of particular interest as a therapeutic tool because of its safety profile and efficiency in transducing a wide range of cell types. The purpose of this review is to describe the most notable advancements in preclinical and clinical research on AAV-based CNS gene therapy and to discuss prospects for future development based on a new generation of vectors and delivery.

Adeno-associated virus-delivered artificial microRNA extends survival and delays paralysis in an amyotrophic lateral sclerosis mouse model

OBJECTIVE

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by loss of motor neurons, resulting in progressive muscle weakness, paralysis, and death within 5 years of diagnosis. About 10% of cases are inherited, of which 20% are due to mutations in the superoxide dismutase 1 (SOD1) gene. Riluzole, the only US Food and Drug Administration-approved ALS drug, prolongs survival by only a few months. Experiments in transgenic ALS mouse models have shown decreasing levels of mutant SOD1 protein as a potential therapeutic approach. We sought to develop an efficient adeno-associated virus (AAV)-mediated RNAi gene therapy for ALS.

METHODS

A single-stranded AAV9 vector encoding an artificial microRNA against human SOD1 was injected into the cerebral lateral ventricles of neonatal SOD1(G93A) mice, and impact on disease progression and survival was assessed.

RESULTS

This therapy extended median survival by 50% and delayed hindlimb paralysis, with animals remaining ambulatory until the humane endpoint, which was due to rapid body weight loss. AAV9-treated SOD1(G93A) mice showed reduction of mutant human SOD1 mRNA levels in upper and lower motor neurons and significant improvements in multiple parameters including the numbers of spinal motor neurons, diameter of ventral root axons, and extent of neuroinflammation in the SOD1(G93A) spinal cord. Mice also showed previously unexplored changes in pulmonary function, with AAV9-treated SOD1(G93A) mice displaying a phenotype reminiscent of patient pathophysiology.

INTERPRETATION

These studies clearly demonstrate that an AAV9-delivered SOD1-specific artificial microRNA is an effective and translatable therapeutic approach for ALS.

Cre-dependent selection yields AAV variants for widespread gene transfer to the adult brain

Recombinant adeno-associated viruses (rAAVs) are commonly used vehicles for in vivo gene transfer. However, the tropism repertoire of naturally occurring AAVs is limited, prompting a search for novel AAV capsids with desired characteristics. Here we describe a capsid selection method, called Cre recombination-based AAV targeted evolution (CREATE), that enables the development of AAV capsids that more efficiently transduce defined Cre-expressing cell populations in vivo. We use CREATE to generate AAV variants that efficiently and widely transduce the adult mouse central nervous system (CNS) after intravenous injection. One variant, AAV-PHP.B, transfers genes throughout the CNS with an efficiency that is at least 40-fold greater than that of the current standard, AAV9 (refs. 14,15,16,17), and transduces the majority of astrocytes and neurons across multiple CNS regions. In vitro, it transduces human neurons and astrocytes more efficiently than does AAV9, demonstrating the potential of CREATE to produce customized AAV vectors for biomedical applications.

Gene Therapy for the Treatment of Neurological Disorders: Amyotrophic Lateral Sclerosis

Gene therapy is a powerful tool for treating diseases, including neurological disorder such at amyotrophic lateral sclerosis. When delivered to the CNS, gene therapy vectors can provide prosurvival signals to neurons, knock down the expression of toxic proteins, or restore lost function. How to best deliver this type of therapeutic depends on the nature of the disease and the expected function of the transgene. Here we describe a method for parenchymal injection into rodent models, allowing for localized delivery of gene therapy vectors and other therapeutic molecules. This technique has been a robust mechanism for proof-of-principle experiments.

Clinical Improvement of Alpha-mannosidosis Cat Following a Single Cisterna Magna Infusion of AAV1

Lysosomal storage diseases (LSDs) are debilitating neurometabolic disorders for most of which long-term effective therapies have not been developed. Gene therapy is a potential treatment but a critical barrier to treating the brain is the need for global correction. We tested the efficacy of cisterna magna infusion of adeno-associated virus type 1 (AAV1) expressing feline alpha-mannosidase gene in the postsymptomatic alpha-mannosidosis (AMD) cat, a homologue of the human disease. Lysosomal alpha-mannosidase (MANB) activity in the cerebrospinal fluid (CSF) and serum were increased above the control values in untreated AMD cats. Clinical neurological signs were delayed in onset and reduced in severity. The lifespan of the treated cats was significantly extended. Postmortem histopathology showed resolution of lysosomal storage lesions throughout the brain. MANB activity in brain tissue was significantly above the levels of untreated tissues. The results demonstrate that a single cisterna magna injection of AAV1 into the CSF can mediate widespread neuronal transduction of the brain and meaningful clinical improvement. Thus, cisterna magna gene delivery by AAV1 appears to be a viable strategy for treatment of the whole brain in AMD and should be applicable to many of the neurotropic LSDs as well as other neurogenetic disorders.

In vivo kinetic approach reveals slow SOD1 turnover in the CNS

Therapeutic strategies that target disease-associated transcripts are being developed for a variety of neurodegenerative syndromes. Protein levels change as a function of their half-life, a property that critically influences the timing and application of therapeutics. In addition, both protein kinetics and concentration may play important roles in neurodegeneration; therefore, it is essential to understand in vivo protein kinetics, including half-life. Here, we applied a stable isotope-labeling technique in combination with mass spectrometric detection and determined the in vivo kinetics of superoxide dismutase 1 (SOD1), mutation of which causes amyotrophic lateral sclerosis. Application of this method to human SOD1-expressing rats demonstrated that SOD1 is a long-lived protein, with a similar half-life in both the cerebral spinal fluid (CSF) and the CNS. Additionally, in these animals, the half-life of SOD1 was longest in the CNS when compared with other tissues. Evaluation of this method in human subjects demonstrated successful incorporation of the isotope label in the CSF and confirmed that SOD1 is a long-lived protein in the CSF of healthy individuals. Together, the results of this study provide important insight into SOD1 kinetics and support application of this technique to the design and implementation of clinical trials that target long-lived CNS proteins.

A Single Injection of Recombinant Adeno-Associated Virus into the Lumbar Cistern Delivers Transgene Expression Throughout the Whole Spinal Cord

The lack of methods to deliver transgene expression in spinal cord has hampered investigation of gene function and therapeutic targets for spinal cord diseases. Here, we report that a single intrathecal injection of recombinant adeno-associated virus rhesus-10 (rAAVrh10) into the lumbar cistern led to transgene expression in 60 to 90 % of the cells in the spinal cord. The transgene was expressed in all cell types, including neurons, glia, ependymal cells, and endothelial cells. Additionally, the transgene was expressed in some brain areas up to the frontal cortex and the olfactory bulb. The rAAV was distributed predominantly in the spinal cord, where its genome copy was over ten times that of the peripheral organs. Compared with intravenous injection, another method for rAAV delivery to the broad central nervous system (CNS), the intrathecal injection reduced the dosage of rAAV required to achieve similar or higher levels of transgene expression in the CNS by ~100-fold. Finally, the transduced areas were co-localized with the perivascular spaces of Virchow-Robin, from which the rAAV spreads further into the CNS parenchyma, thus suggesting that rAAV penetrated the CNS parenchyma through this pathway. Taken together, we have defined a fast and efficient method to deliver widespread transgene expression in mature spinal cord in mice. This method can be applied to stably overexpress or silence gene expression in the spinal cord to investigate gene functions in mammalian CNS. Additionally, this method can be applied to validate therapeutic targets for spinal cord diseases.

RNA-targeted Therapeutics for ALS

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease leading to cell death of predominantly motor neurons. Despite extensive research in this disease, finding a way to slow the progress of the disease has been challenging. RNA-targeted therapeutic approaches, including small interfering RNA and antisense oligonucleotides are being developed for genetic forms of ALS. ALS provides an unique opportunity for the use of RNA inhibition strategies given a well-defined animal model, extensive available information regarding the causative genes, and recent experience in phase 1 clinical trial.

Initial gene vector dosing for studying symptomatology of amyotrophic lateral sclerosis in non-human primates

BACKGROUND

Most amyotrophic lateral sclerosis (ALS) research has focused on mice, but there are distinct differences in the functional neuroanatomy of the corticospinal pathway in primates vs. rodents. A non-human primate model may be more sensitive and more predictive for therapeutic efficacy.

METHODS

Rhesus macaques received recombinant adeno-associated virus (AAV9) encoding either the ALS-related pathological protein TDP-43 or a green fluorescent protein (GFP) control by intravenous administration. Motor function and electromyography were assessed over a nine-month expression interval followed by post-mortem analyses.

RESULTS

Recombinant TDP-43 or GFP was stably expressed long term. Although the TDP-43 subjects did not manifest severe paralysis and atrophy, there were trends of a partial disease state in the TDP-43 subjects relative to the control.

CONCLUSIONS

These data indicate that a higher gene vector dose will likely be necessary for more robust effects, yet augur that a relevant primate model is feasible.

SOD1 silencing in motoneurons or glia rescues neuromuscular function in ALS mice

Objective

Amyotrophic lateral sclerosis is an incurable disorder mainly characterized by motoneuron degeneration. Mutations in the superoxide dismutase 1 (SOD1) gene account for 20% of familial forms of the disease. Mutant SOD1 exerts multiple pathogenic effects through the gain of toxic properties in both neurons and glial cells. Here, we compare AAV-based gene therapy suppressing expression of mutant SOD1 in either motoneurons or astrocytes.

Methods

AAV vectors encoding microRNA against human SOD1 were administered to ^G93ASOD1 mice either by intracerebroventricular injections in pups or by lumbar intrathecal injections in adults. Vector systems were designed to suppress SOD1 expression predominantly in either spinal motoneurons or astrocytes. Electrophysiological and behavioral tests were performed on treated animals to evaluate disease progression.

Results

Following vector injection in ^G93ASOD1 pups, efficient silencing of SOD1 expression was achieved in motoneurons and/or astrocytes. Most complete protection of motor units was obtained when targeting human SOD1 predominantly in motoneurons. Suppressing SOD1 mainly in astrocytes led to preserved muscle innervation despite only partial protection of spinal motoneurons. In both cases, injection in pups led to full recovery of neuromuscular function and significantly prolonged survival. Vector injections in adult mice also achieved significant protection of neuromuscular function, which was highest when motoneurons were targeted.

Interpretation

These results suggest that AAV-mediated SOD1 silencing is an effective approach to prevent motoneuron degeneration caused by SOD1 mutation. AAV vectors suppressing SOD1 in motoneurons delay disease onset and show effective neuroprotection. On the other hand, AAV-based SOD1 silencing in astrocytes rescues neuromuscular function following initial denervation.

Efficient central nervous system AAVrh10-mediated intrathecal gene transfer in adult and neonate rats

Intracerebral administration of recombinant adeno-associated vector (AAV) has been performed in several clinical trials. However, delivery into the brain requires multiple injections and is not efficient to target the spinal cord, thus limiting its applications. To assess widespread and less invasive strategies, we tested intravenous (IV) or intrathecal (that is, in the cerebrospinal fluid (CSF)) delivery of a rAAVrh10-egfp vector in adult and neonate rats and studied the effect of the age at injection on neurotropism. IV delivery is more efficient in neonates and targets predominantly Purkinje cells of the cerebellum and sensory neurons of the spinal cord and dorsal root ganglia. A single intra-CSF administration of AAVrh10, single strand or oversized self-complementary, is efficient for the targeting of neurons in the cerebral hemispheres, cerebellum, brainstem and spinal cord. Green fluorescent protein (GFP) expression is more widespread in neonates when compared with adults. More than 50% of motor neurons express GFP in the three segments of the spinal cord in neonates and in the cervical and thoracic regions in adults. Neurons are almost exclusively transduced in neonates, whereas neurons, astrocytes and rare oligodendrocytes are targeted in adults. These results expand the possible routes of delivery of AAVrh10, a serotype that has shown efficacy and safety in clinical trials concerning neurodegenerative diseases.

AAVrh.10 immunogenicity in mice and humans. Relevance of antibody cross-reactivity in human gene therapy

Simian adeno-associated virus (AAV) serotype rh.10 is a promising gene therapy tool, achieving safe, sustained transgene expression in the nervous system, lung, liver and heart in animal models. To date, preexisting immunity in humans has not been confirmed, though exposure is unexpected. We compared the humoral immune response with serotypes AAVrh.10 and AAV9 in mice, and AAVrh.10, AAV9 and AAV2 in 100 healthy humans. Mice, injected-intravenously, raised significantly more anti-AAV9 than anti-AAVrh.10 IgG (immunoglobulins), and sera demonstrated greater neutralizing capacity, correspondingly. Antibody cross-binding studies in mice showed negligible cross-recognition between AAVrh.10, AAV9 and AAV2. In humans, IgG prevalence against the most common human serotype, AAV2, was 72%; AAV9, 47% and AAVrh.10, a surprising, 59%. Yet, neutralizing-antibody seroprevalences were 71% for AAV2, 18% for AAV9 and 21% for AAVrh.10. Thus, most anti-AAV9 and anti-AAVrh.10 IgG were nonneutralizing. Indeed, sera generally neutralized AAV2 more strongly than AAVrh.10. Further, all samples neutralizing AAVrh.10 or AAV9 also neutralized AAV2, suggesting antibody cross-recognition. This contrasts with the results in mice, and highlights the complexity of tailoring gene therapy to minimize the immune response in humans, when multiple-mixed infections during a lifetime evoke a broad repertoire of preexisting antibodies capable of cross reacting with non-human serotypes.

Gene therapy for the nervous system: challenges and new strategies

Current clinical treatments for central nervous system (CNS) diseases, such as Parkinson's disease and glioblastoma do not halt disease progression and have significant treatment morbidities. Gene therapy has the potential to "permanently" correct disease by bringing in a normal gene to correct a mutant gene deficiency, knocking down mRNA of mutant alleles, and inducing cell-death in cancer cells using transgenes encoding apoptosis-inducing proteins. Promising results in clinical trials of eye disease (Leber's congenital aumorosis) and Parkinson's disease have shown that gene-based neurotherapeutics have great potential. The recent development of genome editing technology, such as zinc finger nucleases, TALENS, and CRISPR, has made the ultimate goal of gene correction a step closer. This review summarizes the challenges faced by gene-based neurotherapeutics and the current and recent strategies designed to overcome these barriers. We have chosen the following challenges to focus on in this review: (1) delivery vehicles (both virus and nonviral), (2) use of promoters for vector-mediated gene expression in CNS, and (3) delivery across the blood-brain barrier. The final section (4) focuses on promising pre-clinical/clinical studies of neurotherapeutics.

State-of-the-art human gene therapy: part II. Gene therapy strategies and clinical applications

In Part I of this Review (Wang and Gao, 2014), we introduced recent advances in gene delivery technologies and explained how they have powered some of the current human gene therapy applications. In Part II, we expand the discussion on gene therapy applications, focusing on some of the most exciting clinical uses. To help readers to grasp the essence and to better organize the diverse applications, we categorize them under four gene therapy strategies: (1) gene replacement therapy for monogenic diseases, (2) gene addition for complex disorders and infectious diseases, (3) gene expression alteration targeting RNA, and (4) gene editing to introduce targeted changes in host genome. Human gene therapy started with the simple idea that replacing a faulty gene with a functional copy can cure a disease. It has been a long and bumpy road to finally translate this seemingly straightforward concept into reality. As many disease mechanisms unraveled, gene therapists have employed a gene addition strategy backed by a deep knowledge of what goes wrong in diseases and how to harness host cellular machinery to battle against diseases. Breakthroughs in other biotechnologies, such as RNA interference and genome editing by chimeric nucleases, have the potential to be integrated into gene therapy. Although clinical trials utilizing these new technologies are currently sparse, these innovations are expected to greatly broaden the scope of gene therapy in the near future.

Systemic RNAi delivery to the muscles of ROSA26 mice reduces lacZ expression

RNAi has potential for therapeutically downregulating the expression of dominantly inherited genes in a variety of human genetic disorders. Here we used the ROSA26 mouse, which constitutively expresses the bacterial lacZ gene in tissues body wide, as a model to test the ability to downregulate gene expression in striated muscles. Recombinant adeno-associated viral vectors (rAAVs) were generated that express short hairpin RNAs (shRNAs) able to target the lacZ mRNA. Systemic delivery of these rAAV6 vectors led to a decrease of β-galactosidase expression of 30-50-fold in the striated muscles of ROSA26 mice. However, high doses of vectors expressing 21 nucleotide shRNA sequences were associated with significant toxicity in both liver and cardiac muscle. This toxicity was reduced in cardiac muscle using lower vector doses. Furthermore, improved knockdown in the absence of toxicity was obtained by using a shorter (19 nucleotide) shRNA guide sequence. These results support the possibility of using rAAV vectors to deliver RNAi sequences systemically to treat dominantly inherited disorders of striated muscle.

Developments in treatments for amyotrophic lateral sclerosis via intracerebroventricular or intrathecal delivery

INTRODUCTION

Amyotrophic lateral scleroses (ALS) are neurodegenerative disorders primarily affecting the motor system. These incurable disorders are relentlessly progressive and typically limit survival to 2 - 5 years after disease onset. An improved knowledge about disease-causing genes, disease proteins and pathways has revealed considerable heterogeneity in ALS. Novel targeted therapies are being developed, but getting these beyond the BBB remains a challenge.

AREAS COVERED

The authors review the intracerebroventricular and intrathecal delivery of drugs for the treatment of ALS in preclinical and clinical studies.

EXPERT OPINION

Lack of BBB permeability should not hold back the development of promising treatments for ALS, as the available evidence suggest that direct intrathecal or intracerebroventricular administration of drug is a feasible delivery route in patients with ALS.

Methods for gene transfer to the central nervous system

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