Replication-competent Retroviral Vector Research Articles

A Replication-Competent Retroviral Vector Expressing the HERV-W Envelope Glycoprotein is a Potential Tool for Cancer Gene Therapy.

The fusogenic membrane glycoprotein (FMG) derived from the human endogenous retrovirus-W (HERV-W) exhibits fusogenic properties, making it a promising candidate for cancer gene therapy. When cells are transfected with HERV-W FMG, they can fuse with neighboring cells expressing the receptor, resulting in the formation of syncytia. These syncytia eventually undergo cell death within a few days. In addition, it has been observed that an HERV-W env mutant, which is truncated after amino acid 483, displays increased fusogenicity compared to the wild-type HERV-W env. In this study, we observed syncytium formation upon transfection of HeLa and TE671 human cancer cells with plasmids containing the HERV-W 483 gene. To explore the potential of a semi-replication-competent retroviral (s-RCR) vector encoding HERV-W 483 for FMG-mediated cancer gene therapy, we developed two replication-defective retroviral vectors: a gag-pol vector encoding HERV-W 483 (MoMLV-HERV-W 483) and an env vector encoding VSV-G (pCLXSN-VSV-G-EGFP). When MoMLV-HERV-W 483 and pCLXSN-VSV-G-EGFP were co-transfected into HEK293T cells to produce the s-RCR vector, gradual syncytium formation was observed. However, the titers of the s-RCR virus remained consistently low. To enhance gene transfer efficiency, we constructed an RCR vector encoding HERV-W 483 (MoMLV-10A1-HERV-W 483), which demonstrated replication ability in HEK293T cells. Infection of A549 and HT1080 human cancer cell lines with this RCR vector induced syncytium formation and subsequent cell death. Consequently, both the s-RCR vector and RCR encoding HERV-W 483 hold promise as valuable tools for cancer gene therapy.

Production of a replicating retroviral vector expressing Reovirus fast protein for cancer gene therapy

Reovirus fusion-associated small transmembrane (FAST) proteins induce syncytium formation. Recently, several studies have shown that the use of recombinant vectors engineered to express fusion proteins is becoming attractive for the development of enhanced oncolytic viruses. In this study, we investigated the cytotoxic effect of four different FAST proteins (p10 FAST of Avian reovirus [ARV], p10 FAST of Pulau virus [PuV], p13 FAST of Broome virus [BroV], and p14 FAST of reptilian reovirus [RRV]). Plasmids encoding FASTs were transfected into Vero cells. All FAST proteins induced syncytium formation at varying intensities. To achieve high levels of FAST expression, four different FAST genes were inserted into the murine leukemia virus (MLV)-based replication-competent retroviral (RCR) vector. Two days after transfection in 293 T cells, only the MoMLV-10A1-p10(PuV) RCR vector showed syncytia formation. Based on these results, p10(Puv) was selected from the four FASTs. Next, we investigated the cytotoxicity of p10(PuV) on HeLa cervical carcinoma cells, HT1080 human fibrosarcoma cells, and U87 human glioma cells. Although three human cancer cell lines induced syncytium formation, U87 cells were highly susceptible to syncytia formation by transfection with p10(PuV). In addition, the viral supernatants from MoMLV-10A-p10(PuV) RCR vector-transfected 293 T cells also induced syncytium formation in HT1080, TE671, and U87 cells. This RCR vector encoding p10(PuV) is a promising candidate for cancer gene therapy.

Construction of a replication-competent retroviral vector for expression of the VSV-G envelope glycoprotein for cancer gene therapy.

Gibbon ape leukemia virus (GALV) can infect a wide variety of cells but fails to infect most cells derived from laboratory mice. Transduction of human hematopoietic stem cells with GALV retroviral vectors is more efficient than with amphotropic vectors. In this study, a Moloney murine leukemia virus-gibbon ape leukemia virus (MoMLV-GALV) vector was constructed by replacing the natural env gene of the full-length Moloney MLV genome with the GALV env gene. To monitor viral transmission by green fluorescent protein (GFP) expression, internal ribosomal entry site-enhanced GFP (IRES-EGFP) was positioned between the GALV env gene and the 3' untranslated region (3' UTR) to obtain pMoMLV-GALV-EGFP. The MoMLV-GALV-EGFP vector was able to replicate with high titer in TE671 human rhabdomyosarcoma cells and U-87 human glioma cells. To evaluate the potential of the MoMLV-GALV vector as a therapeutic agent, the gene for the fusogenic envelope G glycoprotein of vesicular stomatitis virus (VSV-G) was incorporated into the vector. Infection with the resulting MoMLV-GALV-VSV-G vector resulted in lysis of the U-87 cells due to syncytium formation. Syncytium formation was also observed in the transfected human prostate cancer cell line LNCaP after extended cultivation of cells. In addition, we deleted the GALV env gene from the MoMLV-GALV-VSV-G vector to improve viral genome stability. This MoMLV-VSV-G vector is also replication competent and induces syncytium formation in 293T, HT1080, TE671 and U-87 cells. These results suggest that replication of the MoMLV-GALV-VSV-G vector or MoMLV-VSV-G vector may directly lead to cytotoxicity. Therefore, the vectors developed in this study are potentially useful tools for cancer gene therapy.

Infectious Knockdown of CREB and HIF-1 for the Treatment of Metastatic Uveal Melanoma.

Uveal melanoma (UM) is the most prevalent primary intraocular cancer in adults. Up to half the patients develop metastases that are currently incurable, and most patients die within two years following the diagnosis of metastases. Therefore, novel therapeutic approaches are required. It has been established that tumor cells are more resistant to the hypoxia cue than non-malignant cells and can remain viable in hypoxia. Oxygen absence in hypoxic tumor areas means the absence of chemotherapeutics and the absence of the effector for radiotherapy (free oxygen radicals). To overcome this treatment resistance, we constructed MuLV-based replication-competent retroviral (RCR) vectors expressing shRNA targeting the hypoxia-response regulating genes CREB and HIF-1. These RCRs express shRNAs either against a single exon or against an exon and the poly-A signal to minimize the point-mutation resistance. These RCRs that only infect replicating cells will preferentially infect tumor cells. Pre-infected Mel270 UM subcutaneous xenografts in SCID mice were monitored weekly in vivo via bioluminescence. Here, we demonstrate that the knockdown of CREB or HIF-1 in UM cells dramatically decreases UM tumor progression. The reduction of the expression of Glut-1, which is a major glucose transporter in cancer cells, within tumors that are infected with the armed viruses may indicate UM’s dependence on glycolysis for tumor progression.

PRDM14 and BLIMP1 control the development of chicken primordial germ cells

The differentiation of primordial germ cells (PGCs) is a fundamental step in development. PR domain-containing protein 14 (PRDM14) and B lymphocyte-induced maturation protein 1 (BLIMP1) play pivotal roles in mouse PGC specification. In the present study, we assessed the roles of chicken orthologs of PRDM14 and BLIMP1 in PGC development. PRDM14 and BLIMP1 were expressed in blastodermal cells and PGCs. The in vivo knockdown of PRDM14 or BLIMP1 by introducing a replication-competent retroviral vector expressing shRNAs to the blastodermal stage of embryos reduced the number of SSEA-1 or chicken vasa homologue-positive PGCs on day 5.5–6.5. Since the inhibition of Activin receptor-like kinase 4/5/7 in cultured PGCs reduced the expression of PRDM14, BLIMP1, and NANOG, and that of MEK inhibited PRDM14 expression, the expression of these genes seems to be controlled by Activin A and FGF2 signaling. Overall, PRDM14, BLIMP1, and NANOG seem to be involved in the self-renewal of PGCs in cultured PGCs and embryos.

Construction of replication-competent oncolytic retroviral vectors expressing R peptide-truncated 10A1 envelope glycoprotein

Replication-deficient retroviral (RDR) vectors have been generally used for gene therapy, but clinically beneficial transduction efficiency is difficult to achieve with these vectors. In recent times, attention has been focused on the use of murine leukemia virus (MLV)-based replication-competent retroviral (RCR) vectors. RCR vectors have been shown to achieve efficient tumor reduction in a wide variety of cancer models. Most RCR vectors have been developed from amphotropic 4070 A MLV env, which is broadly applied in basic research. In this study, we generated RCR vectors based on Moloney MLV by replacing the native env gene in a full-length viral genome with the 10A1 env gene. 10A1 MLV can infect a wide variety of cells. Unlike amphotropic MLV, the 10A1 MLV can use amphotropic MLV receptor Pit2 or gibbon ape leukemia virus (GaLV) receptor Pit1. The resulting construct MoMLV-10A1-EGFP was able to replicate in 293 T, NIH3T3, and Mus dunni cells. To evaluate the potential of MoMLV-10A1 vector as a therapeutic agent, we incorporated the yeast cytosine deaminase (CD) suicide gene into vectors. The resulting vector MoMLV-10A1-CD could inhibit the growth of human 293T cells upon 5-fluorocytosine (5-FC) administration. In addition, to lyse tumor cells by syncytium, MoMLV-10A1-R(-)-EGFP was generated by replacing wild-type 10A1 env with the 16-amino acid R peptide-truncated 10A1 env gene. Syncytium formation was observed in the TE671 human tumor cells, 293 T and PG13 cells upon transfection of the MoMLV-10A1-R(-)-EGFP vector. This result suggests that replication of this vector could be oncolytic in itself. We also found that syncytium could contribute to enhance cell-to-cell transmission of the retroviral vectors. Our results thus show that the MoMLV-10A1 vectors can be potentially useful for cancer gene therapy.

Chicken IFN Kappa: A Novel Cytokine with Antiviral Activities

Interferons (IFNs) are essential components of the host innate immune system and define first-line of defence against pathogens. In mammals, several type I IFNs are identified, however, only limited data is available on the repertoire of IFNs in avian species. Here we report the characterization of chicken IFN-κ (chIFN-κ) near the type I IFN locus on the sex-determining Z chromosome. Genetic, evolutionary and syntenic analyses indicate that chIFN-κ is a type I IFN with conserved genetic features and promoter binding sites. chIFN-κ regulated the IFN-stimulated response element signalling pathways and activated a panel of IFN-regulated genes, antiviral mediators and transcriptional regulators. Priming of chicken primary fibroblasts and tracheal organ cultures with chIFN-κ imparted cellular protections against viral infections both in vitro and ex vivo. To determine whether chIFN-κ defines the antiviral state in developing chicken embryos, we used replication-competent retroviral RCAS vector system to generate transgenic chicken embryos that constitutively and stably expressed chIFN-κ. We could demonstrate that chIFN-κ markedly inhibited the replication of avian RNA viruses in ovo. Collectively, these results shed the light on the repertoire of IFNs in avian species and provide functional data on the interaction of the chIFN-κ with RNA viruses of poultry and public health importance.

Antiviral Activity of Lambda Interferon in Chickens

Interferons (IFNs) are essential components of the antiviral defense system of vertebrates. In mammals, functional receptors for type III IFN (lambda interferon [IFN-λ]) are found mainly on epithelial cells, and IFN-λ was demonstrated to play a crucial role in limiting viral infections of mucosal surfaces. To determine whether IFN-λ plays a similar role in birds, we produced recombinant chicken IFN-λ (chIFN-λ) and we used the replication-competent retroviral RCAS vector system to generate mosaic-transgenic chicken embryos that constitutively express chIFN-λ. We could demonstrate that chIFN-λ markedly inhibited replication of various virus strains, including highly pathogenic influenza A viruses, in ovo and in vivo, as well as in epithelium-rich tissue and cell culture systems. In contrast, chicken fibroblasts responded poorly to chIFN-λ. When applied in vivo to 3-week-old chickens, recombinant chIFN-λ strongly induced the IFN-responsive Mx gene in epithelium-rich organs, such as lungs, tracheas, and intestinal tracts. Correspondingly, these organs were found to express high transcript levels of the putative chIFN-λ receptor alpha chain (chIL28RA) gene. Transfection of chicken fibroblasts with a chIL28RA expression construct rendered these cells responsive to chIFN-λ treatment, indicating that receptor expression determines cell type specificity of IFN-λ action in chickens. Surprisingly, mosaic-transgenic chickens perished soon after hatching, demonstrating a detrimental effect of constitutive chIFN-λ expression. Our data highlight fundamental similarities between the IFN-λ systems of mammals and birds and suggest that type III IFN might play a role in defending mucosal surfaces against viral intruders in most if not all vertebrates.

Downregulation of GluA2 AMPA Receptor Subunits Reduces the Dendritic Arborization of Developing Spinal Motoneurons

AMPA receptors lacking the GluA2 subunit allow a significant influx of Ca2+ ions. Although Ca2+-permeable AMPA receptors are a familiar feature at early stages of development, the functional significance of these receptors during the maturation of the nervous system remains to be established. Chicken lumbar motoneurons express Ca2+-permeable AMPA receptors at E6 but the Ca2+ permeability of AMPA receptors decreases ∼3-fold by E11. Considering that activity-dependent changes in intracellular Ca2+ regulates dendritic outgrowth, in this study we investigated whether downregulation of GluA2 expression during a critical period of development alters the dendritic arborization of spinal motoneurons in ovo. We use an avian replication-competent retroviral vector RCASBP (B) carrying the marker red fluorescent protein (RFP) and a GluA2 RNAi construct to downregulate GluA2 expression. Chicken embryos were infected at E2 with one of the following constructs: RCASBP(B)-RFP, RCASBP(B)-RFP-scrambled RNAi, or RCASBP(B)-RFP-GluA2 RNAi. Infection of chicken embryos at E2 resulted in widespread expression of RFP throughout the spinal cord with ≥60% of Islet1/2-positive motoneurons infected, resulting in a significant reduction in GluA2 protein expression. Downregulation of GluA2 expression had no effect on the dendritic arborization of E6 motoneurons. However, downregulation of GluA2 expression caused a significant reduction in the dendritic arborization of E11 motoneurons. Neither motoneuron survival nor maturation of network activity was affected by changes in GluA2 expression. These findings demonstrate that increased GluA2 expression and changes in the Ca2+ permeability of AMPA receptors regulate the dendritic arborization of spinal cord motoneurons during a critical period of development.

RNAi-mediated gene silencing in tumour tissue using replication-competent retroviral vectors

RNAi represents a powerful technology to specifically downregulate the expression of target genes. For cancer research and therapy, an efficient in vivo delivery system is supposed to distribute RNAi to all tumour cells upon systemic administration. We present replication-competent murine leukaemia virus (MLV) vectors, which deliver RNAi to tumour tissue upon tail vein injection. In HT1080 cells stably expressing GFP or luciferase, GFP expression was suppressed by more than 80% and luciferase (luc) activity by more than 90%, even when only 0.1% of the cells were initially infected with reporter gene specific vectors. To demonstrate its potential, PLK1- and MMP14-specific small hairpin RNA expression cassettes were applied in the system. Upon infection, PLK1 and MMP14 levels were reduced on mRNA and protein level. MLV-shPLK1-infected cells were arrested in the G2-phase and underwent apoptosis. MLV-shMMP14-infected cells showed reduced MMP2 activity, as well as substantially reduced invasion and tumour growth. In vivo, MLV-shLuc silenced luc expression in HT1080-luc tumour tissue by more than 80% and MLV-shPLK1 reduced tumour growth substantially, demonstrating the therapeutic relevance of this system. This RNAi vector system allows long-term downregulation of target gene expression as well as efficient delivery to and distribution throughout tumour tissue in vivo.

Adenovirus–Retrovirus Hybrid Vectors Achieve Highly Enhanced Tumor Transduction and Antitumor Efficacy In Vivo

Murine leukemia virus (MLV)-based replication-competent retrovirus (RCR) vectors have been shown to mediate efficient, selective, and persistent tumor transduction, thereby achieving significant therapeutic benefit in a wide variety of cancer models. To further augment the efficiency of this strategy, we have developed a delivery method employing a gutted adenovirus encoding an RCR vector (AdRCR); thus, tumor cells transduced with the adenoviral vector transiently become RCR vector producer cells in situ. As expected, high-titer AdRCR achieved significantly higher initial transduction levels in human cancer cells both in vitro and in vivo, as compared to the original RCR vector itself. Notably, even at equivalent initial transduction levels, more secondary RCR progeny were produced from AdRCR-transduced cells as compared to RCR-transduced cells, resulting in further acceleration of subsequent RCR replication kinetics. In pre-established tumor models in vivo, prodrug activator gene therapy with high-titer AdRCR could achieve enhanced efficacy compared to RCR alone, in a dose-dependent manner. Thus, AdRCR hybrid vectors offer the advantages of high production titers characteristic of adenovirus and secondary production of RCR in situ, which not only accelerates subsequent vector spread and progressive tumor transduction, but can also significantly enhance the therapeutic efficacy of RCR-mediated prodrug activator gene therapy.

Abstract B33: Intratumoral conversion of 5-fluorocytosine to 5-fluorouracil using a novel prodrug activator gene transfer technology in an immune-competent model of glioblastoma

Abstract Background: Patients with the most common and aggressive form of brain cancer, glioblastoma multiforme (GBM), have poor prognosis and current treatments remain only palliative. Previous gene therapy approaches using replication-deficient viral vectors have demonstrated limited efficacy likely due to poor gene delivery to the tumor. Methods: We used a novel replication-competent retroviral vector (Toca 511) designed and engineered to efficiently deliver a modified cytosine deaminase (CD) prodrug activating gene to glioma cells. Previous studies showed that such a replicating vector can preferentially infect intracranial human xenografts in nude mice (Tai et al. Mol Ther. 2005;12:842) and lead to increased survival in treatment groups compared to controls. In preparation for clinical trials we have improved the CD gene activity and further stabilized the vector structure to create Toca 511. We investigated the functionality of Toca 511 using stereotactic, intratumoral delivery in a syngeneic, orthotopic glioma model in immune-competent B6C3F1 mice. Freshly excised tumors were analyzed by quantitative PCR, Western, and HPLC. Results: Brain tumors isolated from mice treated with Toca 511 and fluorocytosine (5FC) had observable expression of CD protein and vector copy numbers per genome that demonstrate efficient tumor infection. Extensive tumor specific viral spread with almost no spread of vector from the site of injection was observed in the first 24 days. Highly efficient intratumoral conversion of the 5FC prodrug into the anticancer drug 5-fluorouracil (5FU) was demonstrated after intraperitoneal or oral dosing with 5FC. Studies with Toca 511 at high and mid dose levels in combination with 5FC resulted in prolonged survival compared to controls (> 5 months, p<0.0036 and <0.0354 respectively) in this syngeneic glioma model. The highest Toca 511 dose level tested in combination with 5-FC yielded no safety related findings in the mice. Conclusions: Toca 511 efficiently delivers a functional CD gene for expression in glioma cells in a syngeneic, orthotopic model in immune-competent mice. Toca 511 increases survival through the delivery of CD gene and subsequent efficient conversion of 5FC into 5FU within the brain tumor with no safety related findings. A phase 1 ascending dose trial investigating the safety and tolerability of Toca 511 in combination with 5FC in patients with recurrent GBM is about to initiate. Citation Information: Clin Cancer Res 2010;16(14 Suppl):B33.

Comparative evaluation of preclinical in vivo models for the assessment of replicating retroviral vectors for the treatment of glioblastoma

Despite impressive improvements in neurosurgical techniques, radiation and chemotherapy during the past few years, little progress has been made in the treatment of malignant gliomas. Recently, the efficacy of suicide gene therapy based on replication-competent retroviral (RCR) vectors as delivery vehicles for the therapeutic gene has been described in the treatment of experimental cancer, including gliomas. In this study, we have thus critically evaluated a panel of human and rodent glioma/glioblastoma cell lines (U-87MG, U-118MG, LN-18, LN-229, 8-MG-BA, 42-MG-BA, A-172, T-98G, UVW, C6, 9L, G-26, GL-261, Tu-2449, Tu-9648) with respect to RCR virus vector spread, sensitivity towards the cytosine deaminase (CD)/5-flurocytosine (5-FC)/5-flurouracil (5-FU) suicide system, and orthotopic growth characteristics in mice to identify suitable preclinical animal models for the development of a glioblastoma gene therapy. Rapid virus spread was observed in eight out of nine human cell lines tested in vitro. As expected, only CD-expressing cells became sensitive to 5-FC, due to their ability to convert the prodrug in its toxic form, 5-FU. All LD(50) values were within the range of concentrations obtained in human body fluids after conventional antifungal 5-FC administration. In addition, a significant bystander effect was observed in all human glioma cell lines tested. Injection of the RCR vector into pre-established orthotopic mouse tumor xenografts revealed substantial infection and virus spread of tumor tissue from most cell types.

Intratumoral conversion of flucytosine to 5-fluorouracil using a novel MLV-based replication-competent retroviral vector carrying the cytosine deaminase gene (Toca 511) in an immune-competent model of glioblastoma.

e13541 Background: Patients with the most common and aggressive form of brain cancer, glioblastoma multiforme (GBM) have poor prognosis and few palliative options. Gene therapy approaches using replication-deficient viral vectors have been tried previously for GBM with limited efficacy likely due to poor gene delivery to the tumor. Methods: We used a novel replication-competent retroviral vector (Toca 511) engineered to efficiently deliver a modified cytosine deaminase (CD) prodrug activating gene to glioma cells. We investigated the functionality of Toca 511 using intratumoral delivery in a syngeneic, orthotopic glioma model in immune-competent B6C3F1 mice. Freshly excised tumors were analyzed by quantitative PCR, western, and HPLC. Results: Brain tumors isolated from mice treated with Toca 511 and flucystosine (5FC) had observable expression of CD protein and vector copy numbers per genome that demonstrate efficient tumor infection. Extensive tumor specific viral spread with almost no spread of vector from the site of injection was observed in the first 24 days. Highly efficient intratumoral conversion of the 5FC prodrug into the anticancer drug 5-fluorouracil (5FU) was demonstrated after intraperitoneal or oral dosing with 5FC. Studies with Toca 511 at high and mid dose levels in combination with 5FC resulted in prolonged survival compared to controls (> 5 months, p<0.0036 and <0.0354 respectively) in this syngeneic glioma model. The highest Toca 511 dose level tested yielded no safety related findings in the mice. Conclusions: Toca 511 efficiently delivers a functional CD gene for expression in glioma cells in a syngeneic, orthotopic model in immune-competent mice. Toca 511 increases survival through the delivery of CD gene and subsequent efficient conversion of 5FC into 5FU within the brain tumor with no safety related findings. A phase I ascending dose trial of the safety and tolerability of Toca 511 in combination with 5FC in patients with recurrent GBM is planned for first quarter 2010. Author Disclosure Employment or Leadership Position Consultant or Advisory Role Stock Ownership Honoraria Research Funding Expert Testimony Other Remuneration Tocagen, Inc. Tocagen, Inc. Tocagen, Inc.

Enhanced efficiency of prodrug activation therapy by tumor-selective replicating retrovirus vectors armed with the Escherichia coli purine nucleoside phosphorylase gene

Gene transfer of the Escherichia coli purine nucleoside phosphorylase (PNP) results in potent cytotoxicity after administration of the prodrug fludarabine phosphate (F-araAMP). Here, we have tested whether application of this strategy in the context of replication-competent retrovirus (RCR) vectors, which can achieve highly efficient tumor-restricted transduction as well as persistent expression of transgenes, would result in effective tumor inhibition, or, alternatively, would adversely affect viral replication. We found that RCR vectors could achieve high levels of PNP expression concomitant with the efficiency of their replicative spread, with significant cell killing activity in vitro and potent therapeutic effects in vivo. In U-87 xenograft models, replicative spread of the vector resulted in progressive transmission of the PNP transgene, as evidenced by increasing PNP enzyme activity with time after vector inoculation. On F-araAMP administration, high efficiency gene transfer of PNP by the RCR vector resulted in significant suppression of tumor growth and extended survival time. As the RCR mediates stable integration of the PNP gene and continuous expression, an additional round of F-araAMP administration resulted in further survival benefit. RCR-mediated PNP suicide gene therapy thus represents a highly efficient form of intracellular chemotherapy, and may achieve effective antitumor activity with less systemic toxicity.

334. Tumor-Selectivity of Replication-Competent Retrovirus (RCR) Vectors Regulated by Prostate-Specific Promoters and Cytotoxic Effect of Suicide Gene Therapy Mediated by RCR Vectors In Vitro

334. Tumor-Selectivity of Replication-Competent Retrovirus (RCR) Vectors Regulated by Prostate-Specific Promoters and Cytotoxic Effect of Suicide Gene Therapy Mediated by RCR Vectors In Vitro

831. Preclinical Studies Support the Efficacy of Prodrug Activator Gene Transfer in Experimental Glioma Models by Enhanced Replication-Competent Retrovirus Vector

831. Preclinical Studies Support the Efficacy of Prodrug Activator Gene Transfer in Experimental Glioma Models by Enhanced Replication-Competent Retrovirus Vector

492. Optimizing the Genomic Stability, Transgene Expression, and Therapeutic Efficacy of Replication-Competent Retrovirus Vectors for Prodrug Activator Gene Transfer

492. Optimizing the Genomic Stability, Transgene Expression, and Therapeutic Efficacy of Replication-Competent Retrovirus Vectors for Prodrug Activator Gene Transfer

675. RNAi-Mediated Gene Silencing in Tumour Tissue upon Systemic Application of Replication-Competent Retroviral Vectors

675. RNAi-Mediated Gene Silencing in Tumour Tissue upon Systemic Application of Replication-Competent Retroviral Vectors

The RCAS retroviral expression system in the study of skeletal development

RCAS is a replication-competent retroviral vector system that allows sustained misexpression of a gene of interest in avian cells. This tool has been used to gain fundamental insights into skeletal development in the chick embryo, and consequently into broader principles of morphogenesis. In this review, we discuss a range of RCAS-based strategies that have been employed to examine gene function during early patterning of the limb bud, later skeletal differentiation, and craniofacial morphogenesis. The potential for the RCAS system as a tool for loss-of-function studies is also discussed.