Dimerizer regulation of AADC expression and behavioral response in AAV-transduced 6-OHDA lesioned rats

Applications of adeno-associated virus vector-mediated gene delivery for neurodegenerative diseases and psychiatric diseases: Progress, advances, and challenges

Neurodegeneration is the most common disease in the elderly population due to its slowly progressive nature of neuronal deterioration, eventually leading to executive dysfunction. The pathological markers of neurological disorders are relatively well-established, however, detailed molecular mechanisms of progression and therapeutic targets are needed to develop novel treatments in human patients. Treating known therapeutic targets of neurological diseases has been aided by recent advancements in adeno-associated virus (AAV) technology. AAVs are known for their low-immunogenicity, blood-brain barrier (BBB) penetrating ability, selective neuronal tropism, stable transgene expression, and pleiotropy. In addition, the usage of AAVs has enormous potential to be optimized. Therefore, AAV can be a powerful tool used to uncover the underlying pathophysiology of neurological disorders and to increase the success in human gene therapy. This review summarizes different optimization approaches of AAV vectors with their current applications in disease modeling, neural tracing and gene therapy, hence exploring progressive mechanisms of neurodegenerative diseases as well as effective therapy. Lastly, this review discusses the limitations and future perspectives of the AAV-mediated transgene delivery system.

Gene Therapy Tools for Brain Diseases

Neurological disorders affecting the central nervous system (CNS) are still incompletely understood. Many of these disorders lack a cure and are seeking more specific and effective treatments. In fact, in spite of advancements in knowledge of the CNS function, the treatment of neurological disorders with modern medical and surgical approaches remains difficult for many reasons, such as the complexity of the CNS, the limited regenerative capacity of the tissue, and the difficulty in conveying conventional drugs to the organ due to the blood-brain barrier. Gene therapy, allowing the delivery of genetic materials that encodes potential therapeutic molecules, represents an attractive option. Gene therapy can result in a stable or inducible expression of transgene(s), and can allow a nearly specific expression in target cells. In this review, we will discuss the most commonly used tools for the delivery of genetic material in the CNS, including viral and non-viral vectors; their main applications; their advantages and disadvantages. We will discuss mechanisms of genetic regulation through cell-specific and inducible promoters, which allow to express gene products only in specific cells and to control their transcriptional activation. In addition, we will describe the applications to CNS diseases of post-transcriptional regulation systems (RNA interference); of systems allowing spatial or temporal control of expression [optogenetics and Designer Receptors Exclusively Activated by Designer Drugs (DREADDs)]; and of gene editing technologies (CRISPR/Cas9, Zinc finger proteins). Particular attention will be reserved to viral vectors derived from herpes simplex type 1, a potential tool for the delivery and expression of multiple transgene cassettes simultaneously.

Gene Therapy for Parkinson's Disease, An Update

The current mainstay treatment of Parkinson's disease (PD) consists of dopamine replacement therapy which, in addition to causing several side effects, does not delay disease progression. The field of gene therapy offers a potential means to improve current therapy. The present review gives an update of the present status of gene therapy for PD. Both non-disease and disease modifying transgenes have been tested for PD gene therapy in animal and human studies. Non-disease modifying treatments targeting dopamine or GABA synthesis have been successful and promising at improving PD symptomatology in randomized clinical studies, but substantial testing remains before these can be implemented in the standard clinical treatment repertoire. As for disease modifying targets that theoretically offer the possibility of slowing the progression of disease, several neurotrophic factors show encouraging results in preclinical models (e.g., neurturin, GDNF, BDNF, CDNF, VEGF-A). However, so far, clinical trials have only tested neurturin, and, unfortunately, no trial has been able to meet its primary endpoint. Future clinical trials with neurotrophic factors clearly deserve to be conducted, considering the still enticing goal of actually slowing the disease process of PD. As alternative types of gene therapy, opto- and chemogenetics might also find future use in PD treatment and novel genome-editing technology could also potentially be applied as individualized gene therapy for genetic types of PD.

Riboswitch-mediated Attenuation of Transgene Cytotoxicity Increases Adeno-associated Virus Vector Yields in HEK-293 Cells

Cytotoxicity of transgenes carried by adeno-associated virus (AAV) vectors might be desired, for instance, in oncolytic virotherapy or occur unexpectedly in exploratory research when studying sparsely characterized genes. To date, most AAV-based studies use constitutively active promoters (e.g., the CMV promoter) to drive transgene expression, which often hampers efficient AAV production due to cytotoxic, antiproliferative, or unknown transgene effects interfering with producer cell performance. Therefore, we explored artificial riboswitches as novel tools to control transgene expression during AAV production in mammalian cells. Our results demonstrate that the guanine-responsive GuaM8HDV aptazyme efficiently attenuates transgene expression and associated detrimental effects, thereby boosting AAV vector yields up to 23-fold after a single addition of guanine. Importantly, riboswitch-harboring vectors preserved their ability to express functional transgene at high levels in the absence of ligand, as demonstrated in a mouse model of AAV-TGFβ1-induced pulmonary fibrosis. Thus, our study provides the first application-ready biotechnological system-based on aptazymes, which should enable high viral vector yields largely independent of the transgene used. Moreover, the RNA-intrinsic, small-molecule regulatable mode of action of riboswitches provides key advantages over conventional transcription factor-based regulatory systems. Therefore, such riboswitch vectors might be ultimately applied to temporally control therapeutic transgene expression in vivo.

A chemical switch for controlling viral infectivity

Chemically triggered molecular switches for controlling the fate and function of biological systems are fundamental to the emergence of synthetic biology and the development of biomedical applications. We here present the first chemically triggered switch for controlling the infectivity of adeno-associated viral (AAV) vectors.

A next step in adeno-associated virus-mediated gene therapy for neurological diseases: regulation and targeting

Recombinant adeno-associated virus (rAAV) vectors mediating long term transgene expression are excellent gene therapy tools for chronic neurological diseases. While rAAV2 was the first serotype tested in the clinics, more efficient vectors derived from the rh10 serotype are currently being evaluated and other serotypes are likely to be tested in the near future. In addition, aside from the currently used stereotaxy-guided intraparenchymal delivery, new techniques for global brain transduction (by intravenous or intra-cerebrospinal injections) are very promising. Various strategies for therapeutic gene delivery to the central nervous system have been explored in human clinical trials in the past decade. Canavan disease, a genetic disease caused by an enzymatic deficiency, was the first to be approved. Three gene transfer paradigms for Parkinson's disease have been explored: converting L-dopa into dopamine through AADC gene delivery in the putamen; synthesizing GABA through GAD gene delivery in the overactive subthalamic nucleus and providing neurotrophic support through neurturin gene delivery in the nigro-striatal pathway. These pioneer clinical trials demonstrated the safety and tolerability of rAAV delivery in the human brain at moderate doses. Therapeutic effects however, were modest, emphasizing the need for higher doses of the therapeutic transgene product which could be achieved using more efficient vectors or expression cassettes. This will require re-addressing pharmacological aspects, with attention to which cases require either localized and cell-type specific expression or efficient brain-wide transgene expression, and when it is necessary to modulate or terminate the administration of transgene product. The ongoing development of targeted and regulated rAAV vectors is described.

Enhancing the utility of adeno-associated virus gene transfer through inducible tissue-specific expression

The ability to regulate both the timing and specificity of gene expression mediated by viral vectors will be important in maximizing its utility. We describe the development of an adeno-associated virus (AAV)-based vector with tissue-specific gene regulation, using the ARGENT dimerizer-inducible system. This two-vector system based on AAV serotype 9 consists of one vector encoding a combination of reporter genes from which expression is directed by a ubiquitous, inducible promoter and a second vector encoding transcription factor domains under the control of either a heart- or liver-specific promoter, which are activated with a small molecule. Administration of the vectors via either systemic or intrapericardial injection demonstrated that the vector system is capable of mediating gene expression that is tissue specific, regulatable, and reproducible over induction cycles. Somatic gene transfer in vivo is being considered in therapeutic applications, although its most substantial value will be in basic applications such as target validation and development of animal models.

Vector and helper genome rearrangements occur during production of helper-dependent adenoviral vectors

Helper-dependent adenoviral vectors (HD Ad) hold extreme promise for gene therapy of human diseases. All viral genes are deleted in HD Ad vectors, and therefore, the presence of a helper virus is required for their production. Current methods to minimize helper contamination in large-scale preparations rely on the use of the Cre/loxP system. The inclusion of loxP sites flanking the packaging signal results in its excision in the presence of Cre recombinase, preventing helper genome encapsidation. It is well established that the level of Cre recombinase activity is important in determining the degree of helper contamination. However, there is little information on other mechanisms that could also play an important role. We have generated several HD Ad vectors containing a rapalog-inducible system to regulate transgene expression, or LacZ under the control of the elongation factor 1 α promoter. Large-scale production of these vectors resulted in abundant helper contamination. Viral DNA analysis revealed the presence of rearrangements between vector and helper genomes. The rearrangements involved a helper DNA molecule with a fragment of the left arm of the HD Ad vector, including its ITR, packaging signal, and some stuffer sequence. Overall, our data suggest that helper DNA molecules that accumulate after Cre recombinase activity are prone to rearrangements, resulting in helper genomes that have incorporated a packaging signal from the vector. Helper particles with rearranged genomes have a growth advantage. This study identifies a novel mechanism leading to helper contamination during helper-dependent adenoviral vector production.

Gene therapy for the treatment of Parkinson's disease: the nature of the biologics expands the future indications

The pharmaceutical industry's development of therapeutic medications for the treatment of Parkinson's disease (PD) endures, as a result of the continuing need for better agents, and the increased clinical demand due to the aging population. Each new drug offers advantages and disadvantages to patients when compared to other medical offerings or surgical options. Deep brain stimulation (DBS) has become a standard surgical remedy for the effective treatment of select patients with PD, for whom most drug regimens have failed or become refractory. Similar to DBS as a surgical option, gene therapy for the treatment of PD is evolving as a future option. In the four different PD gene therapy approaches that have reached clinical trials investigators have documented an excellent safety profile associated with the stereotactic delivery, viral vectors and doses utilized, and transgenes expressed. In this article, we review the clinically relevant gene therapy strategies for the treatment of PD, concentrating on the published preclinical and clinical results, and the likely mechanisms involved. Based on these presentations, we advance an analysis of how the nature of the gene therapy used may eventually expand the scope and utility for the management of PD.

Guided delivery of adeno-associated viral vectors into the primate brain

In this review, we discuss recent developments in the delivery of adeno-associated virus-based vectors (AAV), particularly with respect to the role of axonal transport in vector distribution in the brain. The use of MRI-guidance and new stereotactic aiming devices have now established a strong foundation for neurological gene therapy to become an accepted procedure in interventional neurology.

Gene regulation systems for gene therapy applications in the central nervous system

Substantial progress has been made in the development of novel gene therapy strategies for central nervous system (CNS) disorders in recent years. However, unregulated transgene expression is a significant issue limiting human applications due to the potential side effects from excessive levels of transgenic protein that indiscriminately affect both diseased and nondiseased cells. Gene regulation systems are a tool by which tight tissue-specific and temporal regulation of transgene expression may be achieved. This review covers the features of ideal regulatory systems and summarises the mechanics of current exogenous and endogenous gene regulation systems and their utility in the CNS.

Evaluation of an AAV2-based rapamycin-regulated glial cell line-derived neurotrophic factor (GDNF) expression vector system

Effective regulation of transgene product in anatomically circumscribed brain tissue is dependent on the pharmacokinetics of the regulating agent, the kinetics of transcriptional activation and degradation of the transgene product. We evaluated rapamycin-regulated AAV2-GDNF expression in the rat brain (striatum). Regulated (a dual-component system: AAV2-FBZhGDNF + AAV2-TF1Nc) and constitutive (CMV-driven) expression vectors were compared. Constitutively active AAV2-GDNF directed stable GDNF expression in a dose-dependent manner and it increased for the first month, thereafter reaching a plateau that was maintained over a further 3 months. For the AAV2-regGDNF, rapamycin was administered in a 3-days on/4-days off cycle. Intraperitoneal, oral, and direct brain delivery (CED) of rapamycin were evaluated. Two cycles of rapamycin at an intraperitoneal dose of 10 mg/kg gave the highest GDNF level (2.75±0.01 ng/mg protein). Six cycles at 3 mg/kg resulted in lower GDNF values (1.36±0.3 ng/mg protein). Interestingly, CED of rapamycin into the brain at a very low dose (50 ng) induced GDNF levels comparable to a 6-week intraperitoneal rapamycin cycle. This study demonstrates the effectiveness of rapamycin regulation in the CNS. However, the kinetics of the transgene in brain tissue, the regulator dosing amount and schedule are critical parameters that influence the kinetics of accumulation and zenith of the encoded transgene product.

Genetic therapy for the nervous system

Genetic therapy is undergoing a renaissance with expansion of viral and synthetic vectors, use of oligonucleotides (RNA and DNA) and sequence-targeted regulatory molecules, as well as genetically modified cells, including induced pluripotent stem cells from the patients themselves. Several clinical trials for neurologic syndromes appear quite promising. This review covers genetic strategies to ameliorate neurologic syndromes of different etiologies, including lysosomal storage diseases, Alzheimer's disease and other amyloidopathies, Parkinson's disease, spinal muscular atrophy, amyotrophic lateral sclerosis and brain tumors. This field has been propelled by genetic technologies, including identifying disease genes and disruptive mutations, design of genomic interacting elements to regulate transcription and splicing of specific precursor mRNAs and use of novel non-coding regulatory RNAs. These versatile new tools for manipulation of genetic elements provide the ability to tailor the mode of genetic intervention to specific aspects of a disease state.

Lentiviral vectors mediate nonimmunosuppressive rapamycin analog-induced production of secreted therapeutic factors in the brain: regulation at the level of transcription and exocytosis

Gene transfer may become a powerful clinical tool for the delivery of secreted therapeutic polypeptides, provided that the in situ production of these peptides can be tightly regulated by the administration of a small inducer molecule. Particularly efficient control may be achieved by simultaneously using two regulation systems that interfere with the biosynthesis of the therapeutic factor at two different levels. Therefore, we have developed a set of two lentiviral vectors containing two regulation systems. These systems are induced by nonimmunosuppressive derivatives of rapamycin ("rapalogs") and allow simultaneous control of expression and of exocytosis of secreted therapeutic polypeptides. The set of vectors was used to produce green fluorescent protein (GFP) and glial cell line-derived neurotrophic factor (GDNF); GFP served as a model factor to demonstrate expression and entry into the exocytotic pathway in transduced cells. The constructs allowed robust in vitro expression and secretion of the polypeptides in the presence of rapalog AP21967. Withdrawal of the inducer resulted in efficient downregulation. In vivo, tightly regulated production of GFP and GDNF was observed after injection of the constructs into the striata of mice. The vectors thus fulfill key requirements for application in gene therapy.

Biodistribution of adeno-associated virus type-2 in nonhuman primates after convection-enhanced delivery to brain

A combination treatment of AAV2-hAADC with oral levodopa is a novel therapeutic approach that is being developed for late-stage Parkinson's disease. Biodistribution of AAV2-hAADC was assessed over a wide range of vector dose in 12 monkeys with parkinsonian syndrome, 6 months after intraputamenal infusion. Quantitative PCR (Q-PCR) from all the major neuroanatomical regions of the brain indicated a dose-dependent increase in vector DNA, with 99% being detected in the target site and other basal ganglia tissues. Within these tissues, the distribution varied widely between the putamen (PT) and the globus pallidus, and this was attributed to differences in vector transport. Q-PCR and immunocytochemistry were consistent with results reported earlier for various measures of transgene expression including aromatic L-amino acid decarboxylase (AADC) activity assays, behavioral response, and in vivo imaging with positron emission tomography (PET). Outside of the brain, trace amounts of vector DNA were detected in the spleens of animals in the two highest dose groups, but not in any other peripheral tissue, blood, or cerebrospinal fluid. Some increase in neutralizing antibody titers to adeno-associated virus type-2 (AAV2) capsid protein was observed in monkeys that received high doses of AAV2-hAADC or control AAV2-GFP. This study further validates convection-enhanced delivery (CED) as the preferred method of viral vector delivery to the brain, and supports a Phase I clinical testing of AAV2-hAADC in humans with Parkinson's disease.

Antidepressive-like effects of rapamycin in animal models: Implications for mTOR inhibition as a new target for treatment of affective disorders

Lithium, the prototypic mood stabilizer, was recently demonstrated to enhance autophagy in cells. Recent hypotheses regarding the source of therapeutic effects of lithium as well as other mood stabilizers and antidepressants suggest that they may stem from increased neuroprotection, cellular plasticity and resilience. Hence it is clearly a possibility that enhanced autophagy may be involved in the therapeutic action by contributing to increased cellular resilience. A well-documented mechanism to induce autophagy is by inhibition of mTOR, a negative modulator of autophagy and rapamycin (sirolimus) is a commonly used inhibitor of mTOR. Accordingly, the present study was designed to evaluate the effects of rapamycin in animal models of antidepressant activity. A dose-response experiment in the mice forced swim test was performed and followed by additional testing of mice and rats in an open field, the forced swim test and the tail suspension test. Results show that sub-chronic, but not acute, administration of rapamycin doses of 10mg/kg and above, have an antidepressant-like effect in both mice and rats and in both the forced swim and the tail suspension tests with no effects on the amount or distribution of activity in the open field. Whereas it is tempting to conclude that the antidepressant-like effects are related to mTOR inhibition, they may also be the consequences of interactions with other intracellular pathways. Additional studies are now planned to further explore the behavioral range of rapamycin's effects as well as the biological mechanisms underlying these effects.

Gene therapy for Parkinson's disease

Advances in molecular biology and virology in recent years have enabled the technology of gene transfer to proceed forward. Parkinson's disease (PD) is a particularly appropriate target for gene therapy since the brain pathology is fully characterized and relatively well circumscribed largely within the nigrostriatal dopaminergic neurons. In addition, the search for genetic mutations responsible for familial forms of PD has accelerated in recent years with several genes or loci already identified. Mutations in the parkin gene are linked to the autosomal recessive form known as autosomal recessive juvenile parkinsonism, park2. Therefore, parkin gene therapy can be effective in PD caused by parkin gene mutations, which are inherited as an autosomal recessive trait. Intriguingly, several studies, including our reports, have suggested the possibility that parkin gene therapy could treat a subset of patients with PD who have mutations in the alpha-synuclein gene. Furthermore, if indeed parkin overexpression broadly corrects anatomical degeneration in the substantia nigra and striatum, this might be a potential therapy for alpha-synucleinopathy.

Rapamycin-regulated control of antiangiogenic tumor therapy following rAAV-mediated gene transfer

Regulated gene expression may be required for the clinical development of certain gene therapies. Several approaches have been developed that allow pharmacologic control of transgene expression, including the dimerizer-regulated transcriptional system in which rapamycin or its analogs function as transcriptional inducers. These compounds can also act as direct antitumor agents via inhibition of mammalian target of rapamycin (mTOR). We describe the development of an optimized recombinant adeno-associated virus (AAV) expression cassette that allows dimerizer-regulated gene expression from a single vector in vitro and in vivo. After demonstrating multiple cycles of rapamycin-dependent transgene induction following a single administration of an AAV vector in vivo, application of this regulated AAV gene expression system to the pharmacologic control of antiangiogenic therapy was evaluated in preclinical tumor models. Dimerizer-regulated vectors were constructed encoding a soluble inhibitor of the vascular endothelial growth factor (VEGF) pathway. In two subcutaneous models of glioblastoma, regulated expression of the VEGF inhibitor via recombinant AAV-mediated gene transfer, in combination with rapamycin, was shown to decrease tumor growth rate significantly. The dual properties of rapamycin--as a transcriptional inducer and mTOR inhibitor--are exploited in combination with an AAV-encoded antiangiogenic agent to provide a novel approach for the treatment of malignant diseases.

Gene Therapy Progress and Prospects – Vectorology: design and production of expression cassettes in AAV vectors

Adeno-associated virus (AAV) derived vectors are considered highly eligible vehicles for human gene therapy. Not only do they possess many great potential for clinical applications due to their wide range of tissue targets but also their excellent preclinical safety profile makes them particularly suitable candidates for treating serious diseases. Initial clinical trials have yielded encouraging results and prompted further improvements in their design and methods of production. Many studies have been performed to modify the tropism of recombinant (r)AAV by capsid modification. However, the precise control of spatial and temporal gene expression, which may be important in determining the safety and efficacy of gene transfer, lies in a rational choice and a subtle combination of various regulatory genetic elements to be inserted into the expression cassette. Moreover, new strategies based on such genetic sequences open new perspectives for enhancing vector genome persistence, disrupting or reducing pathogenic gene expression and even targeting genes.