Use of nonintegrating lentiviral vectors for gene therapy

Avasimibe can cooperate with a DC-targeting and integration-deficient lentivector to induce stronger HBV specific T cytotoxic response by regulating cholesterol metabolism

We have reported a lentivector which could effectively induce HBV-specific cytotoxic T lymphocytes (CTLs). Avasimibe is an inhibitor of acetyl-CoA acetyltransferase-1 (ACAT1), and has been shown to enhance T lymphocyte cytotoxicity on tumor cells. However, the role of avasimibe in lentivector-induced HBV-specific T cytotoxic response remains unknown. Based on previous study, we constructed an integration-deficient lentivector LVDC-ID-HBV (harboring HBcAg expression), and the in vitro experiments showed that the combination of avasimibe exhibited better efficacy in inducing HBV-specific CTL responses including cell proliferation, production of cytokines, as well as CTL killing activities. Mechanism experiments showed that increasing cell membrane cholesterol levels by MβCD-coated cholesterol or ACAT1 inhibition efficiently promoted TCR clustering, signaling transduction and immunological synapse formation, thereby mediating augmented CTL responses. Nevertheless, the depletion of plasma membrane cholesterol with MβCD led to obviously decreased CTL responses. The avasimibe-mediated strengthened immune effects were also determined in animal experiments and the results were in agreement with those from the in vitro research. In particular, the in vivo CTL killing activities were identified by the CFSE or BV-labeled splenocyte lysis assay. Moreover, the experiments in HBV transgenic mice showed that the LVDC-ID-HBV plus avasimibe group demonstrated the lowest serum HBsAg and HBV DNA levels, as well as the lowest expression of HBsAg and HBcAg in liver tissues. We concluded that the HBV-specific CTL immune responses could be potentiated by avasimibe through regulating plasma membrane cholesterol levels. Avasimibe may be a potential adjuvant for lentivector vaccine against HBV infection.

Advances in RNA Viral Vector Technology to Reprogram Somatic Cells: The Paramyxovirus Wave

Ethical issues are a significant barrier to the use of embryonic stem cells in patients due to their origin: human embryos. To further the development of stem cells in a patient application, alternative sources of cells were sought. A process referred to as reprogramming was established to create induced pluripotent stem cells from somatic cells, resolving the ethical issues, and vectors were developed to deliver the reprogramming factors to generate induced pluripotent stem cells. Early viral vectors used integrating retroviruses and lentiviruses as delivery vehicles for the transcription factors required to initiate reprogramming. However, because of the inherent risk associated with vectors that integrate into the host genome, non-integrating approaches were explored. The development of non-integrating viral vectors offers a safer alternative, and these modern vectors are reliable, efficient, and easy to use to achieve induced pluripotent stem cells suitable for direct patient application in the growing field of individualized medicine. This review summarizes all the RNA viral vectors in the field of reprogramming with a special focus on the emerging delivery vectors based on non-integrating  Paramyxoviruses, Sendai and measles viruses. We discuss their design and evolution towards being safe and efficient reprogramming vectors in generating induced pluripotent stem cells from somatic cells.

Tax Induces the Recruitment of NF-κB to Unintegrated HIV-1 DNA To Rescue Viral Gene Expression and Replication

We previously reported that the normally essential step of integration of the HIV-1 proviral DNA intermediate into the host cell genome becomes dispensable in T cells that express the human T cell leukemia virus 1 (HTLV-1) Tax protein, a known activator of cellular NF-κB. The rescue of integrase (IN)-deficient HIV-1 replication by Tax results from the strong activation of transcription from the long terminal repeat (LTR) promoter on episomal HIV-1 DNA, an effect that is closely correlated with the recruitment of activating epigenetic marks, such as H3Ac, and depletion of repressive epigenetic marks, such as H3K9me3, from chromatinized unintegrated proviruses. In addition, activation of transcription from unintegrated HIV-1 DNA coincides with the recruitment of NF-κB to the two NF-κB binding sites found in the HIV-1 LTR enhancer. Here, we report that the recruitment of NF-κB to unintegrated viral DNA precedes, and is a prerequisite for, Tax-induced changes in epigenetic marks, so that an IN^- HIV-1 mutant lacking both LTR NF-κB sites is entirely nonresponsive to Tax and fails to undergo the epigenetic changes listed above. Interestingly, we found that induction of Tax expression at 24 h postinfection, when unintegrated HIV-1 DNA is already fully repressed by inhibitory chromatin modifications, is able to effectively reverse the epigenetic silencing of that DNA and rescue viral gene expression. Finally, we report that heterologous promoters introduced into IN-deficient HIV-1-based vectors are transcriptionally active even in the absence of Tax and do not increase their activity when the HIV-1 promoter and enhancer, located in the LTR U3 region, are deleted, as has been recently proposed. IMPORTANCE Integrase-deficient expression vectors based on HIV-1 are becoming increasingly popular as tools for gene therapy in vivo due to their inability to cause insertional mutagenesis. However, many IN^- lentiviral vectors are able to achieve only low levels of gene expression, and methods to increase this low level have not been extensively explored. Here, we analyzed how the HTLV-1 Tax protein is able to rescue the replication of IN^- HIV-1 in T cells, and we describe IN^- lentiviral vectors, lacking any inserted origin of replication, that are able to express a heterologous gene effectively.

Large-Scale Production of Lentiviral Vectors: Current Perspectives and Challenges

Lentiviral vectors (LVs) have gained value over recent years as gene carriers in gene therapy. These viral vectors are safer than what was previously being used for gene transfer and are capable of infecting both dividing and nondividing cells with a long-term expression. This characteristic makes LVs ideal for clinical research, as has been demonstrated with the approval of lentivirus-based gene therapies from the Food and Drug Administration and the European Agency for Medicine. A large number of functional lentiviral particles are required for clinical trials, and large-scale production has been challenging. Therefore, efforts are focused on solving the drawbacks associated with the production and purification of LVsunder current good manufacturing practice. In recent years, we have witnessed the development and optimization of new protocols, packaging cell lines, and culture devices that are very close to reaching the target production level. Here, we review the most recent, efficient, and promising methods for the clinical-scale production ofLVs.

Placenta-directed gene therapy for fetal growth restriction

Fetal growth restriction (FGR) is a serious pregnancy complication affecting ∼8% of all pregnancies. There is no treatment to increase fetal growth in the uterus. Gene therapy presents a promising treatment strategy for FGR, with the use of adenoviral vectors encoding for proteins such as vascular endothelial growth factor (VEGF) and insulin-like growth factor demonstrating improvements in fetal growth, placental function, and neonatal outcome in preclinical studies. Safety assessments suggest no adverse risk to the mother or fetus for VEGF maternal gene therapy; a clinical trial is in development. This review assesses research into placenta-directed gene therapy for FGR, investigating the use of transgenes and vectors, their route of administration in obstetrics, and the steps that will be needed to take this treatment modality into the clinic.

* CRISPR-Based Epigenome Editing of Cytokine Receptors for the Promotion of Cell Survival and Tissue Deposition in Inflammatory Environments

Musculoskeletal diseases have been associated with inflammatory cytokine action, particularly action by TNF-α and IL-1β. These inflammatory cytokines promote apoptosis and senescence of cells in diseased tissue and extracellular matrix breakdown. Stem cell-based therapies are being considered for the treatment of musculoskeletal diseases, but the presence of these inflammatory cytokines will have similar deleterious action on therapeutic cells delivered to these environments. Methods that prevent inflammatory-induced apoptosis and proinflammatory signaling, in cell and pathway-specific manners are needed. In this study we demonstrate the use of clustered regularly interspaced short palindromic repeats (CRISPR)-based epigenome editing to alter cell response to inflammatory environments by repressing inflammatory cytokine cell receptors, specifically TNFR1 and IL1R1. We targeted CRISPR/Cas9-based repressors to TNFR1 and IL1R1 gene regulatory elements in human adipose-derived stem cells (hADSCs) and investigated the functional outcomes of repression of these genes. Efficient signaling regulation was demonstrated in engineered hADSCs, as activity of the downstream transcription factor NF-κB was significantly reduced or maintained at baseline levels in the presence of TNF-α or IL-1β. Pellet culture of undifferentiated hADSCs demonstrated improved survival in engineered hADSCs treated with TNF-α or IL-1β, while having little effect on their immunomodulatory properties. Furthermore, engineered hADSCs demonstrated improved chondrogenic differentiation capacity in the presence of TNF-α or IL-1β, as shown by superior production of glycosaminglycans in this inflammatory environment. Overall this work demonstrates a novel method for modulating cell response to inflammatory signaling that has applications in engineering cells delivered to inflammatory environments, and as a direct gene therapy to protect endogenous cells exposed to chronic inflammation, as observed in a broad spectrum of degenerative musculoskeletal pathology.

Histones Are Rapidly Loaded onto Unintegrated Retroviral DNAs Soon after Nuclear Entry

Chromosomal structure of nuclear DNA is usually maintained by insertion of nucleosomes into preexisting chromatin, both on newly synthesized DNA at replication forks and at sites of DNA damage. But during retrovirus infection, a histone-free DNA copy of the viral genome is synthesized that must be loaded with nucleosomes de novo. Here, we show that core histones are rapidly loaded onto unintegrated Moloney murine leukemia virus DNAs. Loading of nucleosomes requires nuclear entry, but does not require viral DNA integration. The histones associated with unintegrated DNAs become marked by covalent modifications, with a delay relative to the time of core histone loading. Expression from unintegrated DNA can be enhanced by modulation of the histone-modifying machinery. The data show that histone loading onto unintegrated DNAs occurs very rapidly after nuclear entry and does not require prior establishment of an integrated provirus.

In vivo genome editing as a potential treatment strategy for inherited retinal dystrophies

In vivo genome editing represents an emerging field in the treatment of monogenic disorders, as it may constitute a solution to the current hurdles in classic gene addition therapy, which are the low levels and limited duration of transgene expression. Following the introduction of a double strand break (DSB) at the mutational site by highly specific endonucleases, such as TALENs (transcription activator like effector nucleases) or RNA based nucleases (clustered regulatory interspaced short palindromic repeats - CRISPR-Cas), the cell's own DNA repair machinery restores integrity to the DNA strand and corrects the mutant sequence, thus allowing the cell to produce protein levels as needed. The DNA repair happens either through the error prone non-homologous end-joining (NHEJ) pathway or with high fidelity through homology directed repair (HDR) in the presence of a DNA donor template. A third pathway called microhomology mediated endjoining (MMEJ) has been recently discovered. In this review, the authors focus on the different DNA repair mechanisms, the current state of the art tools for genome editing and the particularities of the retina and photoreceptors with regard to in vivo therapeutic approaches. Finally, current attempts in the field of retinal in vivo genome editing are discussed and future directions of research identified.

Engineered Viruses as Genome Editing Devices

Genome editing based on sequence-specific designer nucleases, also known as programmable nucleases, seeks to modify in a targeted and precise manner the genetic information content of living cells. Delivering into cells designer nucleases alone or together with donor DNA templates, which serve as surrogate homologous recombination (HR) substrates, can result in gene knockouts or gene knock-ins, respectively. As engineered replication-defective viruses, viral vectors are having an increasingly important role as delivery vehicles for donor DNA templates and designer nucleases, namely, zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs) and clustered, regularly interspaced, short palindromic repeats (CRISPR)-associated Cas9 (CRISPR−Cas9) nucleases, also known as RNA-guided nucleases (RGNs). We review this dual role played by engineered viral particles on genome editing while focusing on their main scaffolds, consisting of lentiviruses, adeno-associated viruses, and adenoviruses. In addition, the coverage of the growing body of research on the repurposing of viral vectors as delivery systems for genome editing tools is complemented with information regarding their main characteristics, pros, and cons. Finally, this information is framed by a concise description of the chief principles, tools, and applications of the genome editing field as a whole.

Genome editing at the crossroads of delivery, specificity, and fidelity

Genome editing (GE) entails the modification of specific genomic sequences in living cells for the purpose of determining, changing, or expanding their function(s). Typically, GE occurs after delivering sequence-specific designer nucleases (e.g., ZFNs, TALENs, and CRISPR/Cas9) and donor DNA constructs into target cells. These designer nucleases can generate gene knockouts or gene knock-ins when applied alone or in combination with donor DNA templates, respectively. We review progress in this field, with an emphasis on designer nuclease and donor template delivery into mammalian target cell populations. We also discuss the impact that incremental improvements to these tools are having on the specificity and fidelity attainable with state-of-the-art DNA-editing procedures. Finally, we identify areas that warrant further investigation.

Gene delivery in tissue engineering and regenerative medicine

As a promising strategy to aid or replace tissue/organ transplantation, gene delivery has been used for regenerative medicine applications to create or restore normal function at the cell and tissue levels. Gene delivery has been successfully performed ex vivo and in vivo in these applications. Excellent proliferation capabilities and differentiation potentials render certain cells as excellent candidates for ex vivo gene delivery for regenerative medicine applications, which is why multipotent and pluripotent cells have been intensely studied in this vein. In this review, gene delivery is discussed in detail, along with its applications to tissue engineering and regenerative medicine. A definition of a stem cell is compared to a definition of a stem property, and both provide the foundation for an in-depth look at gene delivery investigations from a germ lineage angle.

Development and Challenges of Nanovectors in Gene Therapy

In recent years, hundreds of genes have been linked to a variety of human diseases, and the field of gene therapy has emerged as a way to treat this wide range of diseases. The main goal of gene therapy is to find a gene delivery vehicle that can successfully target diseased cells and deliver therapeutic genes directly to their cellular compartment. The two main types of gene delivery vectors currently being investigated in clinical trials are recombinant viral vectors and synthetic nonviral vectors. Recombinant viral vectors take advantage of the evolutionarily optimized viral mechanisms to deliver genes, but they can be hard to specifically target in vivo and are also associated with serious side effects. Synthetic nonviral vectors are made out of highly biocompatible lipids or polymers, but they are much less efficient at delivering their genetic payload due to the lack of any active delivery mechanism. This mini review will introduce the current state of gene delivery in clinical trials, and discuss the specific challenges associated with each of these vectors. It will also highlight some specific gaps in knowledge that are limiting the advancement of this field and touch on the current areas of research being explored to overcome them.

S/MAR sequence confers long-term mitotic stability on non-integrating lentiviral vector episomes without selection

Insertional oncogene activation and aberrant splicing have proved to be major setbacks for retroviral stem cell gene therapy. Integrase-deficient human immunodeficiency virus-1-derived vectors provide a potentially safer approach, but their circular genomes are rapidly lost during cell division. Here we describe a novel lentiviral vector (LV) that incorporates human ß-interferon scaffold/matrix-associated region sequences to provide an origin of replication for long-term mitotic maintenance of the episomal LTR circles. The resulting 'anchoring' non-integrating lentiviral vector (aniLV) achieved initial transduction rates comparable with integrating vector followed by progressive establishment of long-term episomal expression in a subset of cells. Analysis of aniLV-transduced single cell-derived clones maintained without selective pressure for >100 rounds of cell division showed sustained transgene expression from episomes and provided molecular evidence for long-term episome maintenance. To evaluate aniLV performance in primary cells, we transduced lineage-depleted murine hematopoietic progenitor cells, observing GFP expression in clonogenic progenitor colonies and peripheral blood leukocyte chimerism following transplantation into conditioned hosts. In aggregate, our studies suggest that scaffold/matrix-associated region elements can serve as molecular anchors for non-integrating lentivector episomes, providing sustained gene expression through successive rounds of cell division and progenitor differentiation in vitro and in vivo.

Lentiviral vectors encoding zinc-finger nucleases specific for the model target locus HPRT1

Zinc-finger nucleases (ZFNs) are artificial proteins designed to induce double-stranded DNA breaks (DSBs) at predefined chromosomal positions. These site-specific genomic lesions facilitate the study of translocations and cellular DNA repair processes and serve as powerful stimuli for the editing of complex genomes. The delivery of ZFNs into a wide range of cell types is of utmost importance for the broad evaluation and deployment of the technology. Lentiviral vectors (LVs) are versatile gene delivery vehicles that transduce alike transformed and primary cells regardless of their division rate. In this chapter, we describe the generation of conventional and integrase-defective LVs encoding ZFNs targeting the human hypoxanthine phosphoribosyltransferase 1 (HPRT1) locus. Furthermore, we introduce a general LV titration method based on a cost-effective quantitative PCR protocol and implement a rapid and simple restriction enzyme site polymorphism assay to detected DSB formation at the HPRT1 target sequence. Owing in part to the small molecule-based clone selection schemes conferred by HPRT1 allelic knockouts, this X-linked gene has become a "classical" target model locus in mammalian cells. The reagents and techniques detailed herein yield LV preparations that induce HPRT1-specific DSBs. As a result, they should constitute a valuable resource to increase the robustness and decrease the timelines of the various protocols based on HPRT1 gene disruption and targeting.

Lentiviral vector-mediated RNA silencing in the central nervous system

RNA silencing is an established method for investigating gene function and has attracted particular interest because of the potential for generating RNA-based therapeutics. Using lentiviral vectors as an efficient delivery system that offers stable, long-term expression in postmitotic cells further enhances the applicability of an RNA-based gene therapy for the CNS. In this review we provide an overview of both lentiviral vectors and RNA silencing along with design considerations for generating lentiviral vectors capable of RNA silencing. We go on to describe the current preclinical data regarding lentiviral vector-mediated RNA silencing for CNS disorders and discuss the concerns of side effects associated with lentiviral vectors and small interfering RNAs and how these might be mitigated.

Histone deacetylase inhibition rescues gene knockout levels achieved with integrase-defective lentiviral vectors encoding zinc-finger nucleases

Zinc-finger nucleases (ZFNs) work as dimers to induce double-stranded DNA breaks (DSBs) at predefined chromosomal positions. In doing so, they constitute powerful triggers to edit and to interrogate the function of genomic sequences in higher eukaryotes. A preferred route to introduce ZFNs into somatic cells relies on their cotransduction with two integrase-defective lentiviral vectors (IDLVs) each encoding a monomer of a functional heterodimeric pair. The episomal nature of IDLVs diminishes the risk of genotoxicity and ensures the strict transient expression profile necessary to minimize deleterious effects associated with long-term ZFN activity. However, by deploying IDLVs and conventional lentiviral vectors encoding HPRT1- or eGFP-specific ZFNs, we report that DSB formation at target alleles is limited after IDLV-mediated ZFN transfer. This IDLV-specific underperformance stems, to a great extent, from the activity of chromatin-remodeling histone deacetylases (HDACs). Importantly, the prototypic and U.S. Food and Drug Administration-approved inhibitors of metal-dependent HDACs, trichostatin A and vorinostat, respectively, did not hinder illegitimate recombination-mediated repair of targeted chromosomal DSBs. This allowed rescuing IDLV-mediated site-directed mutagenesis to levels approaching those achieved by using their isogenic chromosomally integrating counterparts. Hence, HDAC inhibition constitutes an efficacious expedient to incorporate in genome-editing strategies based on transient IDLV-mediated ZFN expression. Finally, we compared two of the most commonly used readout systems to measure targeted gene knockout activities based on restriction and mismatch-sensitive endonucleases. These experiments indicate that these enzymatic assays display a similar performance.

An HIV-1 replication pathway utilizing reverse transcription products that fail to integrate

Integration is a central event in the replication of retroviruses, yet ≥ 90% of HIV-1 reverse transcripts fail to integrate, resulting in accumulation of unintegrated viral DNA in cells. However, understanding what role, if any, unintegrated viral DNA plays in the natural history of HIV-1 has remained elusive. Unintegrated HIV-1 DNA is reported to possess a limited capacity for gene expression restricted to early gene products and is considered a replicative dead end. Although the majority of peripheral blood CD4(+) T cells are refractory to infection, nonactivated CD4 T cells present in lymphoid and mucosal tissues are major targets for infection. Treatment with cytokine interleukin-2 (IL-2), IL-4, IL-7, or IL-15 renders CD4(+) T cells permissive to HIV-1 infection in the absence of cell activation and proliferation and provides a useful model for infection of resting CD4(+) T cells. We found that infection of cytokine-treated resting CD4(+) T cells in the presence of raltegravir or with integrase active-site mutant HIV-1 yielded de novo virus production following subsequent T cell activation. Infection with integration-competent HIV-1 naturally generated a population of cells generating virus from unintegrated DNA. Latent infection persisted for several weeks and could be activated to virus production by a combination of a histone deacetylase inhibitor and a protein kinase C activator or by T cell activation. HIV-1 Vpr was essential for unintegrated HIV-1 gene expression and de novo virus production in this system. Bypassing integration by this mechanism may allow the preservation of genetic information that otherwise would be lost.

Differential integrity of TALE nuclease genes following adenoviral and lentiviral vector gene transfer into human cells

The array of genome editing strategies based on targeted double-stranded DNA break formation have recently been enriched through the introduction of transcription activator-like type III effector (TALE) nucleases (TALENs). To advance the testing of TALE-based approaches, it will be crucial to deliver these custom-designed proteins not only into transformed cell types but also into more relevant, chromosomally stable, primary cells. Viral vectors are among the most effective gene transfer vehicles. Here, we investigated the capacity of human immunodeficiency virus type 1- and adenovirus-based vectors to package and deliver functional TALEN genes into various human cell types. To this end, we attempted to assemble particles of these two vector classes, each encoding a monomer of a TALEN pair targeted to a bipartite sequence within the AAVS1 ‘safe harbor’ locus. Vector DNA analyses revealed that adenoviral vectors transferred intact TALEN genes, whereas lentiviral vectors failed to do so, as shown by their heterogeneously sized proviruses in target cells. Importantly, adenoviral vector-mediated TALEN gene delivery resulted in site-specific double-stranded DNA break formation at the intended AAVS1 target site at similarly high levels in both transformed and non-transformed cells. In conclusion, we demonstrate that adenoviral, but not lentiviral, vectors constitute a valuable TALEN gene delivery platform.

Histone deacetylase inhibition activates transgene expression from integration-defective lentiviral vectors in dividing and non-dividing cells

Integration-defective lentiviral vectors (IDLVs) are being increasingly deployed in both basic and preclinical gene transfer settings. Often, however, the IDLV transgene expression profile is muted when compared to that of their integration-proficient counterparts. We hypothesized that the episomal nature of IDLVs turns them into preferential targets for epigenetic silencing involving chromatin-remodeling histone deacetylation. Therefore, vectors carrying an array of cis-acting elements and transcriptional unit components were assembled with the aid of packaging constructs encoding either the wild-type or the class I mutant D116N integrase moieties. The transduction levels and transgene-product yields provided by each vector class were assessed in the presence and absence of the histone deacetylase (HDAC) inhibitors sodium butyrate and trichostatin A. To investigate the role of the target cell replication status, we performed experiments in growth-arrested human mesenchymal stem cells and in post-mitotic syncytial myotubes. We found that IDLVs are acutely affected by HDACs regardless of their genetic makeup or target cell replication rate. Interestingly, the magnitude of IDLV transgene expression rescue due to HDAC inhibition varied in a vector backbone- and cell type-dependent manner. Finally, investigation of histone modifications by chromatin immunoprecipitation followed by quantitative PCR (ChIP-qPCR) revealed a paucity of euchromatin marks distributed along IDLV genomes when compared to those measured on isogenic integration-competent vector templates. These findings support the view that IDLVs constitute preferential targets for epigenetic silencing involving histone deacetylation, which contributes to dampening their full transcriptional potential. Our data provide leads on how to most optimally titrate and deploy these promising episomal gene delivery vehicles.

Development of safer gene delivery systems to minimize the risk of insertional mutagenesis-related malignancies: a critical issue for the field of gene therapy

Integrating gene delivery systems allow for a more stable transgene expression in mammalian cells than the episomal ones. However, the integration of the shuttle vector within the cellular chromosomal DNA is associated with the risk of insertional mutagenesis, which, in turn, may cause malignant cell transformation. The use of a retroviral-derived vector system was responsible for the development of leukemia in five children, who participated in various clinical trials for the treatment of severe combined immunodeficiency (SCID-X1) in France and in the United Kingdom. Unfortunately, the hematological malignancy claimed the life of one patient in 2004, who was enrolled in the French clinical trial. In addition, adeno-associated-viral-(AAV-) mediated gene transfer induced tumors in animal models, whereas the Sleeping Beauty (SB) DNA transposon system was associated with insertional mutagenesis events in cell culture systems. On these grounds, it is necessary to develop safer gene delivery systems for the genetic manipulation of mammalian cells. This paper discusses the latest achievements that have been reported in the field of vector design.

Lentiviral vectors encoding short hairpin RNAs efficiently transduce and knockdown LINGO-1 but induce an interferon response and cytotoxicity in central nervous system neurones

BACKGROUND

Knocking down neuronal LINGO-1 using short hairpin RNAs (shRNAs) might enhance axon regeneration in the central nervous system (CNS). Integration-deficient lentiviral vectors have great potential as a therapeutic delivery system for CNS injuries. However, recent studies have revealed that shRNAs can induce an interferon response resulting in off-target effects and cytotoxicity.

METHODS

CNS neurones were transduced with integration-deficient lentiviral vectors in vitro. The transcriptional effect of shRNA expression was analysed using quantitative real time-polymerase chain reaction and northern blots were used to assess shRNA production.

RESULTS

Integration-deficient lentiviral vectors efficiently transduced CNS neurones and knocked down LINGO-1 mRNA in vitro. However, an increase in cell death was observed when lentiviral vectors encoding an shRNA were applied or when high vector concentrations were used. We demonstrate that high doses of vector or the use of vectors encoding shRNAs can induce an up-regulation of interferon-stimulated genes (2',5'-oligoadenylate synthase 1 and protein kinase R although not myxovirus resistance 1) and a down-regulation of off-target genes (including p75(NTR) and Nogo receptor 1). Furthermore, the northern blot demonstrated that these negative consequences occur even when lentiviral vectors express low levels of shRNAs. Taken together, these results may explain why neurite outgrowth was not enhanced on an inhibitory substrate following transduction with lentiviral vectors encoding an shRNA targeting LINGO-1.

CONCLUSIONS

These findings highlight the importance of including appropriate controls to verify silencing specificity and the requirement to check for an interferon response when conducting RNA interference experiments. However, the potential benefits that RNA interference and viral vectors offer to gene-based therapies to CNS injuries cannot be overlooked and demand further investigation.

Viral vectors for gene delivery to the central nervous system

The potential benefits of gene therapy for neurological diseases such as Parkinson's, Amyotrophic Lateral Sclerosis (ALS), Epilepsy, and Alzheimer's are enormous. Even a delay in the onset of severe symptoms would be invaluable to patients suffering from these and other diseases. Significant effort has been placed in developing vectors capable of delivering therapeutic genes to the CNS in order to treat neurological disorders. At the forefront of potential vectors, viral systems have evolved to efficiently deliver their genetic material to a cell. The biology of different viruses offers unique solutions to the challenges of gene therapy, such as cell targeting, transgene expression and vector production. It is important to consider the natural biology of a vector when deciding whether it will be the most effective for a specific therapeutic function. In this review, we outline desired features of the ideal vector for gene delivery to the CNS and discuss how well available viral vectors compare to this model. Adeno-associated virus, retrovirus, adenovirus and herpesvirus vectors are covered. Focus is placed on features of the natural biology that have made these viruses effective tools for gene delivery with emphasis on their application in the CNS. Our goal is to provide insight into features of the optimal vector and which viral vectors can provide these features.

Genetic modification of lymphocytes by retrovirus-based vectors

The genetic modification of lymphocytes is an important topic in the emerging field of gene therapy. Many clinical trials targeting immunodeficiency syndromes or cancer have shown therapeutic benefit; further applications address inflammatory and infectious disorders. Retroviral vector development requires a detailed understanding of the interactions with the host. Most researchers have used simple gammaretroviral vectors to modify lymphocytes, either directly or via hematopoietic stem and progenitor cells. Lentiviral, spumaviral (foamyviral) and alpharetroviral vectors were designed to reduce the necessity for cell stimulation and to utilize potentially safer integration properties. Novel surface modifications (pseudotyping) and transgenes, built using synthetic components, expand the retroviral toolbox, altogether promising increased specificity and potency. Product consistency will be an important criterion for routine clinical use.

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.

Non-integrating lentiviral vectors for specific killing of Epstein-Barr virus nuclear antigen 1-positive B cell lymphoma cells

BACKGROUND

Epstein-Barr virus (EBV) causes a range of life-threatening B-lymphocyte malignancies but, despite the use of various strategies, treatment remains problematic.

METHODS

In the present study, we developed a non-integrating lentiviral vector (NILV) that mediates specific killing of EBV nuclear antigen 1 (EBNA1)-expressing cells with minimal toxicity to EBNA1-negative cells. The EBV family of repeats (FR) was cloned intok the NILV genome upstream of various transgenes.

RESULTS

The presence of the FR in the NILV genome induced transcriptional up-regulation and prolonged the expression of a transgene specifically in EBNA1-positive B cells. Transgene expression from an FR-containing NILV was also prolonged in EBV-transformed cells compared to an FR-negative NILV. We found that the delivery of an FR-containing NILV encoding herpes simplex virus 1 thymidine kinase (TK) lead to the killing of more than 99% of EBNA1-positive B cells with minimal toxicity to EBNA1-negative cells in the presence of gancyclovir. EBNA1-positive cells were not killed by an FR-negative vector containing the TK gene. An FR-TK-containing NILV also specifically killed EBNA1-containing cells in a mixed population of EBNA1-positive and EBNA1-negative cells, thus confirming that NILV-FR-TK-mediated killing is specific for EBNA1-expressing cells.

CONCLUSIONS

Transgene expression from our NILVs is both EBNA1-specific and dependent upon the presence of the FR. The results obtained in the present study indicate that NILVs have potential use in the treatment of EBV-associated B cell malignancies.

Ocular gene delivery using lentiviral vectors

Substantial advances in our understanding of lentivirus lifecycles and their various constituent proteins have permitted the bioengineering of lentiviral vectors now considered safe enough for clinical trials for both lethal and non-lethal diseases. They possess distinct properties that make them particularly suitable for gene delivery in ophthalmic diseases, including high expression, consistent targeting of various post-mitotic ocular cells in vivo and a paucity of associated intraocular inflammation, all contributing to their ability to mediate efficient and stable intraocular gene transfer. In this review, the intraocular tropisms and therapeutic applications of both primate and non-primate lentiviral vectors, and how the unique features of the eye influence these, are discussed. The feasibility of therapeutic targeting using these vectors in animal models of both anterior and posterior ophthalmic disorders has been established, and has, in combination with substantial progress in enhancing lentiviral vector bio-safety over the past two decades, paved the way for the first human ophthalmic clinical trials using lentivirus-based gene transfer vectors.

Methods of cell purification: a critical juncture for laboratory research and translational science

Research in cell biology and the development of translational technologies are driven by competition, public expectations, and regulatory oversight, putting these fields at a critical juncture. Success in these fields is quickly becoming dependent on the ability of researchers to identify and isolate specific cell populations from heterogeneous mixtures accurately and efficiently. Many methods for cell purification have been developed, and each has advantages and disadvantages that must be considered in light of the intended application. Current cell separation strategies make use of surface proteins, genetic expression, and physics to isolate specific cells by phenotypic traits. Cell purification is also dependent on the cellular reagents available for use and the intended application, as these factors may preclude certain mechanisms used in the processes of labeling and sorting cells.

Neuron-specific gene transfer through retrograde transport of lentiviral vector pseudotyped with a novel type of fusion envelope glycoprotein

The lentiviral vector system is used extensively in gene therapy trials for various neurological and neurodegenerative disorders. The vector system permits efficient and sustained gene expression in many cell types through integration of the transgene into the host cell genome. However, there is a significant issue concerning the therapeutic use of lentiviral vectors, that transgene insertion may lead to tumorigenesis by altering the expression of proto-oncogenes adjacent to the integration sites. One useful approach for improving safety is to restrict vector transduction to neuronal cells. We have reported the use of human immunodeficiency virus type 1 (HIV-1)-based vectors for efficient retrograde transport by pseudotyping with rabies virus glycoprotein (RV-G) or fusion glycoprotein B type, in which the cytoplasmic domain of RV-G was substituted with the counterpart of vesicular stomatitis virus glycoprotein (VSV-G). Here we developed a novel vector system for neuron-specific retrograde gene transfer (termed NeuRet) by pseudotyping the HIV-1 vector with fusion glycoprotein C type (FuG-C), in which a short C-terminal segment of the extracellular domain and the transmembrane/cytoplasmic domains of RV-G were replaced with the corresponding regions of VSV-G. FuG-C pseudotyping caused efficient gene transfer, mainly through retrograde transport, into neuronal cells in diverse brain regions, whereas the pseudotyping resulted in less efficiency for the transduction of glial and neural stem/progenitor cells. Our NeuRet vector system achieves efficient retrograde gene delivery for therapeutic trials and improves their safety by greatly reducing the risk of gene transduction of dividing cells in the brain.

Immunization delivered by lentiviral vectors for cancer and infectious diseases

The increasing level of understanding of the lentivirus biology has been instrumental in shaping the design strategy of creating therapeutic lentiviral delivery vectors. As a result, lentiviral vectors have become one of the most powerful gene transfer vehicles. They are widely used for therapeutic purposes as well as in studies of basic biology, due to their unique characteristics. Lentiviral vectors have been successfully employed to mediate durable and efficient antigen expression and presentation in dendritic cells both in vitro and in vivo, leading to the activation of cellular immunity and humoral responses. This capability makes the lentiviral vector an ideal choice for immunizations that target a wide range of cancers and infectious diseases. Further advances into optimizing the vector system and understanding the relationship between the immune system and diseases pathogenesis will only augment the potential benefits and utility of lentiviral vaccines for human health.

Gammaretroviral vectors: biology, technology and application

Retroviruses are evolutionary optimized gene carriers that have naturally adapted to their hosts to efficiently deliver their nucleic acids into the target cell chromatin, thereby overcoming natural cellular barriers. Here we will review—starting with a deeper look into retroviral biology—how Murine Leukemia Virus (MLV), a simple gammaretrovirus, can be converted into an efficient vehicle of genetic therapeutics. Furthermore, we will describe how more rational vector backbones can be designed and how these so-called self-inactivating vectors can be pseudotyped and produced. Finally, we will provide an overview on existing clinical trials and how biosafety can be improved.

Organ targeted prenatal gene therapy--how far are we?

Prenatal gene therapy aims to deliver genes to cells and tissues early in prenatal life, allowing correction of a genetic defect, before long-term tissue damage has occurred. In contrast to postnatal gene therapy, prenatal application can target genes to a large population of dividing stem cells, and the smaller fetal size allows a higher vector-to-target cell ratio to be achieved. Early-gestation delivery may allow the development of immune tolerance to the transgenic protein which would facilitate postnatal repeat vector administration if needed. Targeting particular organs will depend on manipulating the vector to achieve selective tropism and on choosing the most appropriate gestational age and injection method for fetal delivery. Intra-amniotic injection reaches the skin, and other organs that are bathed in the fluid however since gene transfer to the lung and gut is usually poor more direct injection methods will be needed. Delivery to the liver and blood can be achieved by systemic delivery via the umbilical vein or peritoneal cavity. Gene transfer to the central nervous system in the fetus is difficult but newer vectors are available that transduce neuronal tissue even after systemic delivery.

Avoiding cytotoxicity of transposases by dose-controlled mRNA delivery

The Sleeping Beauty (SB) transposase and its newly developed hyperactive variant, SB100X, are of increasing interest for genome modification in experimental models and gene therapy. The potential cytotoxicity of transposases requires careful assessment, considering that residual integration events of transposase expression vectors delivered by physicochemical transfection or episomal retroviral vectors may lead to permanent transposase expression and resulting uncontrollable transposition. Comparing retrovirus-based approaches for delivery of mRNA, episomal DNA or integrating DNA, we found that conventional SB transposase, SB100X and a newly developed codon-optimized SB100Xo may trigger premitotic arrest and apoptosis. Cell stress induced by continued SB overexpression was self-limiting due to the induction of cell death, which occurred even in the absence of a co-transfected transposable element. The cytotoxic effects of SB transposase were strictly dose dependent and heralded by induction of p53 and c-Jun. Inactivating mutations in SB's catalytic domain could not abrogate cytotoxicity, suggesting a mechanism independent of DNA cleavage activity. An improved approach of retrovirus particle-mediated mRNA transfer allowed transient and dose-controlled expression of SB100X, supported efficient transposition and prevented cytotoxicity. Transposase-mediated gene transfer can thus be tuned to maintain high efficiency in the absence of overt cell damage.

Construction of a lentiviral vector carrying an MCFP shRNA and generation of a HepG2 cell line stably transfected with this vector

AIM: To construct a lentiviral vector carrying a short hairpin RNA (shRNA) targeting the mitochondrial carrier functional protein SLC25A40 (MCFP) gene and to detect the silencing effect of the vector in HepG2 cell line.

METHODS: A double-stranded shRNA targeting the MCFP gene was designed, synthesized and cloned into the pSiCoR vector. The resulting lentiviral vector containing the MCFP shRNA was named pSiCoR-MCFP. HepG2 cells were transfected with the pSiCoR-MCFP lentivirus to obtain a cell line stably expressing the MCFP shRNA. After transfection, the mRNA and protein expression of MCFP in HepG2 cells was detected by RT-PCR and Western blot, respectively.

RESULTS: A lentiviral vector carrying an shRNA targeting the MCFP gene was successfully constructed and a HepG2 cell line stably transfected with the vector was established. The recombinant lentivirus and control lentivirus harvested from 293 cells had a titer of 1.78 × 10¹⁰ pfu/L and 1.45 × 10¹⁰ pfu/L, respectively. RT-PCR and Western blot analyses confirmed that the expression of MCFP was down-regulated in HepG2 cell line stably transfected with the recombinant vector (both P < 0.001).

CONCLUSION: A lentiviral vector carrying an shRNA targeting the MCFP gene was successfully constructed and a HepG2 cell line stably transfected with the vector was established.

Advancements in reprogramming strategies for the generation of induced pluripotent stem cells

Direct reprogramming of somatic cells into induced pluripotent stem (iPS) cells has emerged as an invaluable method for generating patient-specific stem cells of any lineage without the use of embryonic materials. Following the first reported generation of iPS cells from murine fibroblasts using retroviral transduction of a defined set of transcription factors, various new strategies have been developed to improve and refine the reprogramming technology. Recent developments provide optimism that the generation of safe iPS cells without any genomic modification could be derived in the near future for the use in clinical settings. This review summarizes current and evolving strategies in the generation of iPS cells, including types of somatic cells for reprogramming, variations of reprogramming genes, reprogramming methods, and how the advancement iPS cells technology can lead to the future success of reproductive medicine.

Influence of insulators on transgene expression from integrating and non-integrating lentiviral vectors

BACKGROUND

The efficacy and biosafety of lentiviral gene transfer is influenced by the design of the vector. To this end, properties of lentiviral vectors can be modified by using cis-acting elements such as the modification of the U3 region of the LTR, the incorporation of the central flap (cPPT-CTS) element, or post-transcriptional regulatory elements such as the woodchuck post-transcriptional regulatory element (WPRE). Recently, several studies evaluated the influence of the incorporation of insulators into the integrating lentiviral vector genome on transgene expression level and position effects.

METHODS

In the present study, the influence of the matrix attachment region (MAR) of the mouse immunoglobulin-κ (Ig-κ) or the chicken lysozyme (ChL) gene was studied on three types of HIV-1-derived lentiviral vectors: self-inactivating (SIN) lentiviral vectors (LV), double-copy lentiviral vectors (DC) and non-integrating lentiviral vectors (NILVs) in different cell types: HeLa, HEK293T, NIH-3T3, Raji, and T Jurkat cell lines and primary neural progenitors.

RESULTS AND DISCUSSION

Our results demonstrate that the Ig-κ MAR in the context of LV slightly increases transduction efficiency only in Hela, NIH-3T3 and Jurkat cells. In the context of double-copy lentiviral vectors, the Ig-κ MAR has no effect or even negatively influences transduction efficiency. In the same way, in the context of non-integrating lentiviral vectors, the Ig-κ MAR has no effect or even negatively influences transduction efficiency, except in differentiated primary neural progenitor cells.The ChL MAR in the context of integrating and non-integrating lentiviral vectors shows no effect or a decrease of transgene expression in all tested conditions.

CONCLUSIONS

This study demonstrates that MAR sequences not necessarily increase transgene expression and that the effect of these sequences is probably context dependent and/or vector dependent. Thus, this study highlights the importance to consider a MAR sequence in a given context. Moreover, other recent reports pointed out the potential effects of random integration of insulators on the expression level of endogenous genes. Taken together, these results show that the use of an insulator in a vector for gene therapy must be well assessed in the particular therapeutic context that it will be used for, and must be balanced with its potential genotoxic effects.

LDLR-Gene therapy for familial hypercholesterolaemia: problems, progress, and perspectives

Coronary artery diseases (CAD) inflict a heavy economical and social burden on most populations and contribute significantly to their morbidity and mortality rates. Low-density lipoprotein receptor (LDLR) associated familial hypercholesterolemia (FH) is the most frequent Mendelian disorder and is a major risk factor for the development of CAD. To date there is no cure for FH. The primary goal of clinical management is to control hypercholesterolaemia in order to decrease the risk of atherosclerosis and to prevent CAD. Permanent phenotypic correction with single administration of a gene therapeutic vector is a goal still needing to be achieved. The first ex vivo clinical trial of gene therapy in FH was conducted nearly 18 years ago. Patients who had inherited LDLR gene mutations were subjected to an aggressive surgical intervention involving partial hepatectomy to obtain the patient's own hepatocytes for ex vivo gene transfer with a replication deficient LDLR-retroviral vector. After successful re-infusion of transduced cells through a catheter placed in the inferior mesenteric vein at the time of liver resection, only low-level expression of the transferred LDLR gene was observed in the five patients enrolled in the trial. In contrast, full reversal of hypercholesterolaemia was later demonstrated in in vivo preclinical studies using LDLR-adenovirus mediated gene transfer. However, the high efficiency of cell division independent gene transfer by adenovirus vectors is limited by their short-term persistence due to episomal maintenance and the cytotoxicity of these highly immunogenic viruses. Novel long-term persisting vectors derived from adeno-associated viruses and lentiviruses, are now available and investigations are underway to determine their safety and efficiency in preparation for clinical application for a variety of diseases. Several novel non-viral based therapies have also been developed recently to lower LDL-C serum levels in FH patients. This article reviews the progress made in the 18 years since the first clinical trial for gene therapy of FH, with emphasis on the development, design, performance and limitations of viral based gene transfer vectors used in studies to ameliorate the effects of LDLR deficiency.

Vaccines delivered by integration-deficient lentiviral vectors targeting dendritic cells induces strong antigen-specific immunity

We report a study of an integration-deficient lentiviral vector (IDLV) enveloped with a Sindbis virus glycoprotein mutant (SVGmu) capable of selectively binding to dendritic cells (DCs) for its potential as a vaccine carrier. The in vitro assays showed that the D64V point mutation in the catalytic domain of HIV-1 integrase efficiently inhibited the integration of the transgene upon vector transduction, while the targeting specificity of the vector to preferentially transduce and mediate durable expression in DCs was maintained. Substantial immune responses in C57BL/6 mice and complete protection against a challenge with the C57BL/6 thymoma EG.7 tumor expressing a delivered ovalbumin (OVA) antigen in mice have been achieved through the direct injection of the DC-directed IDLV encoding OVA. Thus, this DC-directed IDLV system represents a promising and efficient vector platform with remarkably improved safety for the future development of DC-based immunotherapy.

Nonintegrating foamy virus vectors

Foamy viruses (FVs), or spumaviruses, are integrating retroviruses that have been developed as vectors. Here we generated nonintegrating foamy virus (NIFV) vectors by introducing point mutations into the highly conserved DD35E catalytic core motif of the foamy virus integrase sequence. NIFV vectors produced high-titer stocks, transduced dividing cells, and did not integrate. Cells infected with NIFV vectors contained episomal vector genomes that consisted of linear, 1-long-terminal-repeat (1-LTR), and 2-LTR circular DNAs. These episomes expressed transgenes, were stable, and became progressively diluted in the dividing cell population. 1-LTR circles but not 2-LTR circles were found in all vector stocks prior to infection. Residual integration of NIFV vectors occurred at a frequency 4 logs lower than that of integrase-proficient FV vectors. Cre recombinase expressed from a NIFV vector mediated excision of both an integrated, floxed FV vector and a gene-targeted neo expression cassette, demonstrating the utility of these episomal vectors. The broad host range and large packaging capacity of NIFV vectors should make them useful for a variety of applications requiring transient gene expression.

Nonintegrating lentiviral vectors can effectively deliver ovalbumin antigen for induction of antitumor immunity

It has been demonstrated that nonintegrating lentiviral vectors (NILVs) are efficient in maintaining transgene expression in vitro and in vivo. Gene delivery by NILVs can significantly reduce nonspecific vector integration, which has been shown to cause malignant transformation in patients receiving gene therapy for X-linked severe combined immunodeficiency. Strong and sustained immune responses were observed after a single immunization with NILVs carrying viral antigens. However, there is no report to date that evaluates the efficacy of NILVs in inducing antigen-specific antitumor immunity. Using a well-characterized tumor model, we tested in vivo immunization with a self-inactivating lentiviral vector harboring a defective integrase. A high frequency of ovalbumin peptide (OVAp1)-specific CD8(+) T cells and a substantial antibody response were detected in naive mice immunized with an NILV encoding an OVA transgene. Furthermore, this immunization method completely protected the mice against the growth of E.G7 tumor cells expressing the OVA antigen. Thus, this study provides evidence that immunization using NILVs can be a safe and promising approach for exploring cancer immunotherapy.

Recent advances in lentiviral vector development and applications

Lentiviral vectors (LVs) have emerged as potent and versatile vectors for ex vivo or in vivo gene transfer into dividing and nondividing cells. Robust phenotypic correction of diseases in mouse models has been achieved paving the way toward the first clinical trials. LVs can deliver genes ex vivo into bona fide stem cells, particularly hematopoietic stem cells, allowing for stable transgene expression upon hematopoietic reconstitution. They are also useful to generate induced pluripotent stem cells. LVs can be pseudotyped with distinct viral envelopes that influence vector tropism and transduction efficiency. Targetable LVs can be generated by incorporating specific ligands or antibodies into the vector envelope. Immune responses toward the transgene products and transduced cells can be repressed using microRNA-regulated vectors. Though there are safety concerns regarding insertional mutagenesis, their integration profile seems more favorable than that of gamma-retroviral vectors (gamma-RVs). Moreover, it is possible to minimize this risk by modifying the vector design or by employing integration-deficient LVs. In conjunction with zinc-finger nuclease technology, LVs allow for site-specific gene correction or addition in predefined chromosomal loci. These recent advances underscore the improved safety and efficacy of LVs with important implications for clinical trials.

Viral vectors for gene transfer: current status of gene therapeutics

Gene therapy for the correction of inherited or acquired disease has gained increasing importance in recent years. Successful treatment of children suffering from severe combined immunodeficiency (SCID) was achieved using retrovirus vectors for gene transfer. Encouraging improvements of vision were reported in a genetic eye disorder (LCA) leading to early childhood blindness. Adeno-associated virus (AAV) vectors were used for gene transfer in these trials. This chapter gives an overview of the design and delivery of viral vectors for the transport of a therapeutic gene into a target cell or tissue. The construction and production of retrovirus, lentivirus, and AAV vectors are covered. The focus is on production methods suitable for biopharmaceutical upscaling and for downstream processing. Quality control measures and biological safety considerations for the use of vectors in clinical trials are discussed.

Integrase defective, nonintegrating lentiviral vectors

Lentiviral vectors are a powerful tool for gene transfer into target cells in vitro and in vivo. However, there are concerns about safety with regard to their use in gene transfer protocols because of insertional mutagenesis following viral infection. Once in the target cells, and in addition to the integrated proviral DNA, lentiviral vectors produce episomal forms of DNA (E-DNA), which are transcriptionally active. Therefore, one strategy to improve safety would envision the block integration of the lentiviral vector while allowing production of E-DNA. Such nonintegrating lentiviral vectors can be produced by introducing mutations in the Integrase (IN) protein of the parental packaging vector. These vectors are fundamentally different from the parental IN competent counterpart, thus opening new avenues for this class of lentiviral vectors as a new gene delivery system for gene therapy strategies, vaccination protocols and as a tool for anti-Integrase drug discovery.

Viral vectors for neurotrophic factor delivery: a gene therapy approach for neurodegenerative diseases of the CNS

The clinical manifestation of most diseases of the central nervous system results from neuronal dysfunction or loss. Diseases such as stroke, epilepsy and neurodegeneration (e.g. Alzheimer's disease and Parkinson's disease) share common cellular and molecular mechanisms (e.g. oxidative stress, endoplasmic reticulum stress, mitochondrial dysfunction) that contribute to the loss of neuronal function. Neurotrophic factors (NTFs) are secreted proteins that regulate multiple aspects of neuronal development including neuronal maintenance, survival, axonal growth and synaptic plasticity. These properties of NTFs make them likely candidates for preventing neurodegeneration and promoting neuroregeneration. One approach to delivering NTFs to diseased cells is through viral vector-mediated gene delivery. Viral vectors are now routinely used as tools for studying gene function as well as developing gene-based therapies for a variety of diseases. Currently, many clinical trials using viral vectors in the nervous system are underway or completed, and seven of these trials involve NTFs for neurodegeneration. In this review, we discuss viral vector-mediated gene transfer of NTFs to treat neurodegenerative diseases of the central nervous system.

Lentiviral delivery of short hairpin RNAs

In less than a decade after discovery, RNA interference-mediated gene silencing is already being tested as potential therapy in clinical trials for a number of diseases. Lentiviral vectors provide a means to express short hairpin RNA (shRNA) to induce stable and long-term gene silencing in both dividing and non-dividing cells and thus, are being intensively investigated for this purpose. However, induction of long-term shRNA expression can also cause toxicities by inducing off-target effects and interference with the endogenous micro-RNA (miRNA) pathway that regulates cellular gene expression. Recently, several advances have been made in the shRNA vector design to mimic cellular miRNA processing and to express multiplex siRNAs in a tightly regulated and reversible manner to overcome toxicities. In this review we describe some of these advances, focusing on the progress made in the development of lentiviral shRNA delivery strategies to combat viral infections.

Development and use of SIV-based Integrase defective lentiviral vector for immunization

Integrase (IN) defective lentiviral vectors have a high safety profile and might prove useful as immunizing agents especially against HIV-1. However, IN defective SIV-based vectors must be developed in order to test their potential in the non-human primate models (NHP) of AIDS. To this aim we tested a novel SIV-based IN defective lentiviral vector for its ability to induce sustained immune responses in mice. BALB/c mice were immunized once intramuscularly with a SIV-based IN defective lentiviral vector expressing the model antigen enhanced green fluorescence protein (eGFP). Immune responses were evaluated 90 days after the injection and compared with those elicited with the IN competent counterpart. The IN defective vector was able to efficiently elicit specific and long-lasting polyfunctional immune responses as evaluated by enzyme-linked immunospot (ELISPOT) assays for interferon-gamma (IFN-gamma) in spleens, bone marrow (BM) and draining lymph nodes, and by intracellular staining (ICS) for IFN-gamma, Interleukin-2 (IL-2) and tumor necrosis factor (TNF-alpha) in both splenocytes and BM cells without integration of the vector into the host genome. This is the first demonstration that an IN defective SIV-based lentiviral vector provides effective immunization, thus paving the way for the construction of IN defective vectors expressing SIV antigen(s) and test their efficacy against a SIV virus challenge in the NHP model of AIDS.

Integration-deficient lentiviral vectors: a slow coming of age

Lentiviral vectors are very efficient at transducing dividing and quiescent cells, which makes them highly useful tools for genetic analysis and gene therapy. Traditionally this efficiency was considered dependent on provirus integration in the host cell genome; however, recent results have challenged this view. So called integration-deficient lentiviral vectors (IDLVs) can be produced through the use of integrase mutations that specifically prevent proviral integration, resulting in the generation of increased levels of circular vector episomes in transduced cells. These lentiviral episomes lack replication signals and are gradually lost by dilution in dividing cells, but are stable in quiescent cells. Compared to integrating lentivectors, IDLVs have a greatly reduced risk of causing insertional mutagenesis and a lower risk of generating replication-competent recombinants (RCRs). IDLVs can mediate transient gene expression in proliferating cells, stable expression in nondividing cells in vitro and in vivo, specific immune responses, RNA interference, homologous recombination (gene repair, knock-in, and knock-out), site-specific recombination, and transposition. IDLVs can be converted into replicating episomes, suggesting that if a clinically applicable system can be developed they would also become highly appropriate for stable transduction of proliferating tissues in therapeutic applications.