Improvement of HSV-1 based amplicon vectors for a safe and long-lasting gene therapy in non-replicating cells

Viral and cellular insulators promote sustained HSV vector-mediated transgene expression in brain

We have developed a gene therapy platform based on non-toxic, high-capacity replication-defective (rd) herpes simplex virus type 1 (HSV-1) vectors. We previously determined that transgene expression from rdHSV-1 vectors requires strategic placement of insulators-small DNA elements that overcome the host's epigenetic silencing of foreign DNA-to maintain transgenes in euchromatin regions. Transgene expression was rescued by replacing either the latency associated transcript (LAT) or the the infected cell protein 4 (ICP4) gene with the transgene cassette close to naturally occurring viral insulators. The ICP4 locus was more permissive for transgene expression than the LAT locus in neurons in vitro. Following in vivo brain delivery, transgene expression from both loci lasted for at least 4 months. However, the level of expression tended to decline over time. To enhance transgene expression, we designed a novel insulator environment by combining cellular insulators with the resident viral insulators. In combination, these elements provided significantly higher levels of transgene expression in the brain than the viral insulators alone, lasting for at least 11.7 months. This new cassette design extends transgene activity in neurons compared with previous designs and holds promise for gene therapy applications in treating brain disorders.

Next-generation replication-defective HSV vectors for delivery of large DNA payloads

The application of gene therapy to the treatment of human disease with complex etiology and pathology will often require the delivery of large payloads exceeding 10 kbp in size. This is generally not possible with the most popular vectors such as adeno-associated viruses (AAVs), lentiviruses (LVs), retroviruses (RVs), and many nonviral delivery systems. There is a high likelihood that the correction of many human gene defects such as those associated with neurodegenerative diseases and inflammatory processes will require single large genes or complex genetic payloads that will often necessitate precise regulatory control of the specificity, timing, and duration of corrective gene expression. The regulation of cellular gene products typically depends on genomic promoter systems and splicing-driven transcription variants, necessitating a delivery vector with substantial payload capacity. Replication-defective herpes simplex virus (rdHSV) mutants lack at least one essential viral gene product and are propagated in host cells that supply the missing gene product. This review explores next-generation rdHSV vectors, which do not express viral genes, offer high payload capacity, and can be engineered for safe, long-term transgene expression. These advanced vectors enable the correction of complex diseases affecting neurons and other tissues, paving the way for large or intricate gene replacement strategies.

State-of-the-art gene therapy in epilepsy

PURPOSE OF REVIEW

Gene therapy in epilepsy has undergone a rapid expansion in recent years. This has largely been driven by both advances in our understanding of epilepsy genetics and mechanisms, and also significant advances in gene therapy tools, in particular safe and effective viral vectors. Epilepsy remains an ideal target disease for gene therapy and this review highlights recent developments in this area.

RECENT FINDINGS

There have been continued advances in the development of antisense oligonucleotide therapies to knock down genes in the treatment of monogenic epilepsies with some now entering clinical trial. However, the greatest recent advances have been in vector gene therapy, which offers a more permanent solution by delivering therapeutic genes directly to the brain as a one-off therapy. In particular, there has been a growth in methods that target focal epilepsy. Such promising approaches close to or in clinical trial include expressing NPY and its Y2 receptor, knocking-down GluK5, a kainate receptor subunit, and the over-expression of Kv1.1, an endogenous potassium channel.In the future, it is likely that we will take advantage of approaches of regulating more precisely network excitability by using methods such as optogenetics, designer receptors exclusively activated by designer drugs (DREADDs), 'inhibitory' glutamate receptors activated by excessive glutamate spill-over, and activity-dependent promoters, which target gene expression to the 'hyperactive' neurons.

SUMMARY

Gene therapies offer a novel approach to the treatment of not just genetic epilepsies but any form of epilepsy and may in the future offer an alternative to drug and surgical therapies, allowing more precise, permanent and targeted treatment with fewer adverse effects.

Non-replicative herpes simplex virus genomic and amplicon vectors for gene therapy - an update

Two major types of defective vectors have been derived from herpes simplex virus type 1 (HSV-1), non-replicative genomic vectors (nrHSV-1), and amplicon vectors. This review recapitulates the main features of both vector types and summarizes recent improvements in our understanding of virus/vector biology, particularly with regard to the critical role played by the overpowering of antiviral cellular defenses and the epigenetic control of viral gene expression. Over the past years, significant breakthroughs in vector design, genetic engineering, and HSV-1 biology have accelerated the development of nrHSV-1 vectors. The low immunogenicity and enhanced safety profiles allowed the successful translation of these vectors into several clinical trials, with some being approved by the FDA. Regarding amplicons, despite their advantage in carrying very large or multiple transgenes, and their potential to avoid genome dilution in dividing cells, the absence of production procedures capable of generating large amounts of helper-free amplicons at reasonable cost with GMP compliance, still limits the translation of these outstanding vectors to clinical trials.

Molecular Engineering of Virus Tropism

Engineered viral vectors designed to deliver genetic material to specific targets offer significant potential for disease treatment, safer vaccine development, and the creation of novel biochemical research tools. Viral tropism, the specificity of a virus for infecting a particular host, is often modified in recombinant viruses to achieve precise delivery, minimize off-target effects, enhance transduction efficiency, and improve safety. Key factors influencing tropism include surface protein interactions between the virus and host-cell, the availability of host-cell machinery for viral replication, and the host immune response. This review explores current strategies for modifying the tropism of recombinant viruses by altering their surface proteins. We provide an overview of recent advancements in targeting non-enveloped viruses (adenovirus and adeno-associated virus) and enveloped viruses (retro/lentivirus, Rabies, Vesicular Stomatitis Virus, and Herpesvirus) to specific cell types. Additionally, we discuss approaches, such as rational design, directed evolution, and in silico and machine learning-based methods, for generating novel AAV variants with the desired tropism and the use of chimeric envelope proteins for pseudotyping enveloped viruses. Finally, we highlight the applications of these advancements and discuss the challenges and future directions in engineering viral tropism.

The importance of synthetic pharmacotherapy for recessive cerebellar ataxias

INTRODUCTION

The last decade has witnessed major breakthroughs in identifying novel genetic causes of hereditary ataxias, deepening our understanding of disease mechanisms, and developing therapies for these debilitating disorders.

AREAS COVERED

This article reviews the currently approved and most promising candidate pharmacotherapies in relation to the known disease mechanisms of the most prevalent autosomal recessive ataxias. Omaveloxolone is an Nrf2 activator that increases antioxidant defense and was recently approved for treatment of Friedreich ataxia. Its therapeutic effect is modest, and further research is needed to find synergistic treatments that would halt or reverse disease progression. Promising approaches include upregulation of frataxin expression by epigenetic mechanisms, direct protein replacement, and gene replacement therapy. For ataxia-telangiectasia, promising approaches include splice-switching antisense oligonucleotides and small molecules targeting oxidative stress, inflammation, and mitochondrial function. Rare recessive ataxias for which disease-modifying therapies exist are also reviewed, emphasizing recently approved therapies. Evidence supporting the use of riluzole and acetyl-leucine in recessive ataxias is discussed.

EXPERT OPINION

Advances in genetic therapies for other neurogenetic conditions have paved the way to implement feasible approaches with potential dramatic benefits. Particularly, as we develop effective treatments for these conditions, we may need to combine therapies, consider newborn testing for pre-symptomatic treatment, and optimize non-pharmacological approaches.

Safety of non-replicative and oncolytic replication-selective HSV vectors

Herpes simplex virus type 1 (HSV-1) is a DNA virus and human pathogen used to construct promising therapeutic vectors. HSV-1 vectors fall into two classes: replication-selective oncolytic vectors for cancer therapy and defective non-replicative vectors for gene therapy. Vectors from each class can accommodate ≥30 kb of inserts, have been approved clinically, and demonstrate a relatively benign safety profile. Despite oncolytic HSV (oHSV) replication in tumors and elicited immune responses, the virus is well tolerated in cancer patients. Current non-replicative vectors elicit only limited immune responses. Seropositivity and immune responses against HSV-1 do not eliminate either the vector or infected cells, and the vectors can therefore be re-administered. In this review we highlight vectors that have been translated to the clinic and host-virus immune interactions that impact on the safety and efficacy of HSVs.

RNA therapies for CNS diseases

Neurological disorders are a diverse group of conditions that pose an increasing health burden worldwide. There is a general lack of effective therapies due to multiple reasons, of which a key obstacle is the presence of the blood-brain barrier, which limits drug delivery to the central nervous system, and generally restricts the pool of candidate drugs to small, lipophilic molecules. However, in many cases, these are unable to target key pathways in the pathogenesis of neurological disorders. As a group, RNA therapies have shown tremendous promise in treating various conditions because they offer unique opportunities for specific targeting by leveraging Watson-Crick base pairing systems, opening up possibilities to modulate pathological mechanisms that previously could not be addressed by small molecules or antibody-protein interactions. This potential paradigm shift in disease management has been enabled by recent advances in synthesizing, purifying, and delivering RNA. This review explores the use of RNA-based therapies specifically for central nervous system disorders, where we highlight the inherent limitations of RNA therapy and present strategies to augment the effectiveness of RNA therapeutics, including physical, chemical, and biological methods. We then describe translational challenges to the widespread use of RNA therapies and close with a consideration of future prospects in this field.

Between hope and reality: treatment of genetic diseases through nucleic acid-based drugs

Rare diseases (RD) affect a small number of people compared to the general population and are mostly genetic in origin. The first clinical signs often appear at birth or in childhood, and patients endure high levels of pain and progressive loss of autonomy frequently associated with short life expectancy. Until recently, the low prevalence of RD and the gatekeeping delay in their diagnosis have long hampered research. The era of nucleic acid (NA)-based therapies has revolutionized the landscape of RD treatment and new hopes arise with the perspectives of disease-modifying drugs development as some NA-based therapies are now entering the clinical stage. Herein, we review NA-based drugs that were approved and are currently under investigation for the treatment of RD. We also discuss the recent structural improvements of NA-based therapeutics and delivery system, which overcome the main limitations in their market expansion and the current approaches that are developed to address the endosomal escape issue. We finally open the discussion on the ethical and societal issues that raise this new technology in terms of regulatory approval and sustainability of production.

Viral vectors for gene delivery to the central nervous system

Brain diseases with a known or suspected genetic basis represent an important frontier for advanced therapeutics. The central nervous system (CNS) is an intricate network in which diverse cell types with multiple functions communicate via complex signaling pathways, making therapeutic intervention in brain-related diseases challenging. Nevertheless, as more information on the molecular genetics of brain-related diseases becomes available, genetic intervention using gene therapeutic strategies should become more feasible. There remain, however, several significant hurdles to overcome that relate to (i) the development of appropriate gene vectors and (ii) methods to achieve local or broad vector delivery. Clearly, gene delivery tools must be engineered for distribution to the correct cell type in a specific brain region and to accomplish therapeutic transgene expression at an appropriate level and duration. They also must avoid all toxicity, including the induction of inflammatory responses. Over the last 40 years, various types of viral vectors have been developed as tools to introduce therapeutic genes into the brain, primarily targeting neurons. This review describes the most prominent vector systems currently approaching clinical application for CNS disorders and highlights both remaining challenges as well as improvements in vector designs that achieve greater safety, defined tropism, and therapeutic gene expression.

An HSV-1-H129 amplicon tracer system for rapid and efficient monosynaptic anterograde neural circuit tracing

Monosynaptic viral tracers are essential tools for dissecting neuronal connectomes and for targeted delivery of molecular sensors and effectors. Viral toxicity and complex multi-injection protocols are major limiting application barriers. To overcome these barriers, we developed an anterograde monosynaptic H129_Amp tracer system based on HSV-1 strain H129. The H129_Amp tracer system consists of two components: an H129-dTK-T2-pac^Flox helper which assists H129_Amp tracer's propagation and transneuronal monosynaptic transmission. The shared viral features of tracer/helper allow for simultaneous single-injection and subsequent high expression efficiency from multiple-copy of expression cassettes in H129_Amp tracer. These improvements of H129_Amp tracer system shorten experiment duration from 28-day to 5-day for fast-bright monosynaptic tracing. The lack of toxic viral genes in the H129_Amp tracer minimizes toxicity in postsynaptic neurons, thus offering the potential for functional anterograde mapping and long-term tracer delivery of genetic payloads. The H129_Amp tracer system is a powerful tracing tool for revealing neuronal connectomes.