Bifunctional chimeric SuperCD suicide gene -YCD: YUPRT fusion is highly effective in a rat hepatoma model

In Vitro Sensitivity of Neuroendocrine Neoplasms to an Armed Oncolytic Measles Vaccine Virus

Neuroendocrine neoplasms represent a heterogenous group of rare tumors whose current therapeutic options show only limited efficacy. Oncolytic viruses exert their mode of action through (onco-)lysis of infected tumor cells and the induction of a systemic antitumoral immune response in a virus-induced inflammatory micromilieu. Here, we investigated the potential of our well-established second-generation suicide-gene armed oncolytic measles vaccine virus (MeV-SCD) in five human NEN cell lines. First, (i) expression of the MeV receptor CD46 and (ii) its correlation with primary infection rates were analyzed. Next, (iii) promising combination partners for MeV-SCD were tested by employing either the prodrug 5-fluorocytosine, which is converted into the chemotherapeutic compound 5-fluorouracil, or the mTOR-inhibitor everolimus. As a result, MeV-SCD was found to kill all NEN tumor cell lines efficiently in a dose-dependent manner. This oncolytic effect was further enhanced by exploiting the prodrug-converting system, which was found to be highly instrumental in overcoming the partial resistance found in a single NEN cell line. Furthermore, viral replication was unaffected by everolimus, which is a basic requirement for combined use in NEN patients. These data suggest that MeV-SCD has profound potential for patients with NEN, thus paving the way for early clinical trials.

Potent bystander effect and tumor tropism in suicide gene therapy using stem cells from human exfoliated deciduous teeth

Herpes simplex virus thymidine kinase (HSVTK)/ganciclovir (GCV) suicide gene therapy has a long history of treating malignant gliomas. Recently, stem cells from human exfoliated deciduous teeth (SHED), which are collected from deciduous teeth and have excellent harvestability, ethical aspects, and self-renewal, have been attracting attention mainly in the field of gene therapy. In the present study, we assessed SHED as a novel cellular vehicle for suicide gene therapy in malignant gliomas, as we have previously demonstrated with various cell types. SHED was transduced with the HSVTK gene (SHEDTK). In vitro experiments showed a significant bystander effect between SHEDTK and glioma cell lines in coculture. Furthermore, apoptotic changes caused by caspase 3/7 activation were simultaneously observed in SHEDTK and glioma cells. Mice implanted with a mixture of U87 and SHEDTK and treated with intraperitoneal GCV survived for longer than 100 days. Additionally, tumors in treatment model mice were significantly reduced in size during the treatment period. SHEDTK implanted at the contralateral hemisphere migrated toward the tumor crossing the corpus callosum. These results suggested that SHEDTK-based suicide gene therapy has potent tumor tropism and a bystander-killing effect, potentially offering a new promising therapeutic modality for malignant gliomas.

A Three-Dimensional Organoid Model of Primary Breast Cancer to Investigate the Effects of Oncolytic Virotherapy

Background: Although several oncolytic viruses have already been tested in early-stage clinical studies of breast cancer, there is still an urgent need to develop patient-derived experimental systems that mimic the response of breast cancer to oncolytic agents in preparation of testing different oncolytic viruses in clinical trials. We addressed this need by developing a protocol to study the effects of oncolytic viruses in stable organoid cell cultures derived from breast cancer tissue.

Methods: We used an established three-dimensional organoid model derived from tissue of 10 patients with primary breast cancer. We developed an experimental protocol for infecting organoid cultures with oncolytic viruses and compared the oncolytic effects of a measles vaccine virus (MeV) and a vaccinia virus (GLV) genetically engineered to express either green fluorescent protein (MeV-GFP) and red fluorescent protein (GLV-0b347), respectively, or a suicide gene encoding a fusion of cytosine deaminase with uracil phosphoribosyltransferase (MeV-SCD and GLV-1h94, respectively), thereby enabling enzymatic conversion of the prodrug 5-fluorocytosine (5-FC) into cytotoxic compounds 5-fluorouracil (5-FU) and 5-fluorouridine monophosphate (5-FUMP).

Results: The method demonstrated that all oncolytic viruses significantly inhibited cell viability in organoid cultures derived from breast cancer tissue. The oncolytic effects of the oncolytic viruses expressing suicide genes (MeV-SCD and GLV-1h94) were further enhanced by virus-triggered conversion of the prodrug 5-FC to toxic 5-FU and toxic 5-FUMP.

Conclusions: We were able to develop a protocol to assess the effects of two different types of oncolytic viruses in stable organoid cell cultures derived from breast cancer tissue. The greatest oncolytic effects were observed when the oncolytic viruses were engineered to express a suicide gene (MeV-SCD and GLV-1h94) in the presence of the prodrug 5-FC. The model therefore provides a promising in vitro method to help further testing and engineering of new generations of virotherapeutic vectors for in vivo use.

Dental Pulp Stem Cells Derived From Adult Human Third Molar Tooth: A Brief Review

The fields of regenerative medicine and stem cell-based tissue engineering have the potential of treating numerous tissue and organ defects. The use of adult stem cells is of particular interest when it comes to dynamic applications in translational medicine. Recently, dental pulp stem cells (DPSCs) have been traced in third molars of adult humans. DPSCs have been isolated and characterized by several groups. DPSCs have promising characteristics including self-renewal capacity, rapid proliferation, colony formation, multi-lineage differentiation, and pluripotent gene expression profile. Nevertheless, genotypic, and phenotypic heterogeneities have been reported for DPSCs subpopulations which may influence their therapeutic potentials. The underlying causes of DPSCs’ heterogeneity remain poorly understood; however, their heterogeneity emerges as a consequence of an interplay between intrinsic and extrinsic cellular factors. The main objective of the manuscript is to review the current literature related to the human DPSCs derived from the third molar, with a focus on their physiological properties, isolation procedures, culture conditions, self-renewal, proliferation, lineage differentiation capacities and their prospective advances use in pre-clinical and clinical applications.

Suicide gene‑armed measles vaccine virus for the treatment of AML

Virotherapy comprises a novel therapeutic approach to selectively eliminate cancer cells. Preclinical, as well as clinical data have demonstrated the efficacy of tumor‑selective (oncolytic) viruses in hematological malignancies. In this study, we infected AML cell lines and primary AML cells from patients with measles vaccine virus either expressing GFP or armed with super cytosine deaminase, which converts the prodrug, 5‑fluorocytosine, into the chemotherapeutic compound, 5‑fluorouracil. Target cell density of the measles entry receptor, CD46, infection rates of targeted leukemic cells, tumor cell viability, and apoptotic rates were determined. We found that measles vaccine virus infected the leukemic blasts and profoundly diminished the number and viability of leukemic cells via the induction of apoptosis. The conversion of 5‑fluorocytosine to 5‑fluorouracil exerted a potent additive tumoricidal effect. This was also observed in cases when leukemic cells displayed only moderate susceptibility to the oncolytic virus and hence direct oncolysis. Taken together, in this study, we provide a first characterization of the combinatorial use of measles vaccine virus and 5‑fluorouracil for treatment of AML. Our approach to site‑specifically produce the active drug and combine this agent with the direct lytic effect of virotherapy may overcome present limitations and constitutes a feasible method with which to introduce 5‑fluorouracil in the treatment of AML.

The Antitumor Efficacy of a Complex Based on Two-Vector System for Coexpression of the Suicide Gene Fcu1 and Cre Recombinase

In this study, we evaluated the antitumor activity of a gene therapy complex in which the tumor-specific control of the expression of the effector suicide gene FCU1 was performed using a two-vector system based on the site-specific Cre-LoxP recombinase system. The complex of interest showed a high therapeutic potential in a mouse colon adenocarcinoma model.

Advances in the mechanisms of action of cancer-targeting oncolytic viruses

Cancer virotherapy mediated by oncolytic viruses (OV), has emerged as a novel and effective strategy in cancer therapeutics. Preclinical models have demonstrated anticancer activity against numerous types of cancer. Currently, a number of recombinant viruses are in late phase clinical trials, many of which have demonstrated promising results regarding the safety and reliability of the treatments, particularly when combined with standard antineoplastic therapies. In addition to molecular-targeted therapeutics, genetic engineering of the viruses allows functional complementation to chemotherapy or radiotherapy agents. Co-administration of chemotherapy or radiotherapy is imperative for an effective treatment regime. Additionally, these approaches may be used in combination with current treatments to assist in cancer management. The near future may reveal whether this renewed interest in oncological virotherapy will result in meaningful therapeutic effects in patients. The aim of the present review was to highlight how the knowledge of oncolytic viral specificity and cytotoxicity has advanced in recent years, with a view to discuss OV in clinical application and the future directions of this field.

Combination of the oral histone deacetylase inhibitor resminostat with oncolytic measles vaccine virus as a new option for epi-virotherapeutic treatment of hepatocellular carcinoma

Epigenetic therapies such as histone deacetylase inhibitors (HDACi) not only have the capability to decrease tumor cell proliferation and to induce tumor cell death but also to silence antiviral response genes. Here, we investigated whether the combination of an oncolytic measles vaccine virus (MeV) with the novel oral HDACi resminostat (Res), being in clinical testing in patients with hepatocellular carcinoma (HCC), results in an enhanced efficacy of this epi-virotherapeutic approach compared to any of the two corresponding monotherapies. When testing a panel of human hepatoma cell lines, we found (i) a significantly improved rate of primary infections when using oncolytic MeV under concurrent treatment with resminostat, (ii) a boosted cytotoxic effect of the epi-virotherapeutic combination (Res + MeV) with enhanced induction of apoptosis, and, quite importantly, (iii) an absence of any resminostat-induced impairment of MeV replication and spread. Beyond that, we could also show that (iv) resminostat, after hepatoma cell stimulation with exogenous human interferon (IFN)-β, is able to prevent the induction of IFN-stimulated genes, such as IFIT-1. This finding outlines the possible impact of resminostat on cellular innate immunity, being instrumental in overcoming resistances to MeV-mediated viral oncolysis. Thus, our results support the onset of epi-virotherapeutic clinical trials in patients exhibiting advanced stages of HCC.

Pulsed versus continuous application of the prodrug 5-fluorocytosine to enhance the oncolytic effectiveness of a measles vaccine virus armed with a suicide gene

Abstract Oncolytic virotherapy with measles vaccine virus (MeV) already has been demonstrated to be safe. However, early clinical results pointed out the necessity for an enhancement of oncolytic effectiveness of MeV-based virotherapeutics. In our work, we are developing an armed measles vaccine virus (MeV-SCD) encoding a suicide fusion gene of yeast cytosine deaminase/uracil phosphoribosyltransferase, converting the nontoxic prodrug 5-fluorocytosine (5-FC) to the chemotherapeutic drug 5-fluorouracil (5-FU). To preclinically investigate what an optimal prodrug-assisted therapeutic regimen might look like, we added 5-FC at various time points after infection with MeV-SCD and either let the prodrug remain in the tumor cell culture medium continuously for various time periods ("continuous" 5-FC application) or applied it only temporarily for defined shorter periods of time ("pulsed" 5-FC application); we also varied the time point at which 5-FC was added after infection with MeV-SCD. As a result, addition of the prodrug at early times postinfection (e.g., at 3 hr postinfection) was found to be inferior concerning the overall oncolytic effectiveness when compared with addition of 5-FC at later time points (e.g., at 24 hr postinfection). Next, oncolytic effectiveness was found to correlate positively with the overall duration of incubation of MeV-infected tumor cells with 5-FC. Of note, this was true despite our finding that addition of the prodrug could also exert an inhibitory effect on the generation of infectious progeny viral particles, that is, on virus replication. These findings should be helpful for the rational design of further trials (preclinical, clinical) using suicide gene armed virotherapeutics, such as MeV-SCD.

A novel armed oncolytic measles vaccine virus for the treatment of cholangiocarcinoma

Cholangiocarcinoma (CC) is curable only in early stages by complete surgical resection. Thus, in advanced disease stages in which a complete removal of the tumor mass is no longer possible and palliative chemotherapy achieves only modest success, therapeutics employing new methods of action are desperately needed. Oncolytic viruses employed in clinical studies have been shown to spread preferentially in cancer cells. Beyond that, virotherapeutic cell killing can be enhanced by virus-based expression of suicide genes. We engineered a measles vaccine virus (MeV) vector expressing super cytosine deaminase (SCD), a fusion protein of yeast cytosine deaminase and uracil phosphoribosyltransferase, which converts the prodrug 5-fluorocytosine (5-FC) to 5-fluorouracil (5-FU) and subsequently to 5-fluorouridine-monophosphate. This novel vector was evaluated using three different human-derived CC cell lines. In vitro, all CC cell lines were found to be permissive to MeV infection. Partial blocking of MeV-mediated oncolysis could be overcome by employment of the SCD transgene together with administration of 5-FC. In vivo, intratumoral application of SCD-armed MeV together with a systemic 5-FC treatment showed a significant reduction in tumor size in a TFK-1 xenograft mouse model when compared with virus-only treatment. In a second animal experiment employing a HuCCT1 xenograft tumor model, an enhanced SCD-armed MeV vector, in which the SCD transgene was expressed from a different genomic position, led not only to reduced tumor volumes, but also to a significant survival benefit. On the basis of these encouraging preclinical data on employment of SCD-armed MeV for the virotherapeutic treatment of chemotherapy-resistant CC, a clinical virotherapy trial is set up currently.

An armed oncolytic measles vaccine virus eliminates human hepatoma cells independently of apoptosis

Due to late diagnosis and a pronounced chemoresistance, most patients with hepatocellular carcinoma (HCC) have an overall poor prognosis. Measles vaccine viruses (MeV) have been shown to possess anti-tumor properties and their efficacy has been enhanced by arming with suicide genes. To test armed MeV for the treatment of HCC, we equipped it with the suicide gene Super-cytosine deaminase (SCD) and tested the efficacy in cell culture and in a mouse xenograft model of human HCC. Prodrug conversion was investigated in cell culture and quantified by high-performance liquid chromatography. We observed a strong oncolytic activity of MeV-SCD against human HCC in vitro and in vivo. The prodrug was efficiently converted in infected cells leading to a significant enhancement of the cytotoxic effect. Treatment of HCC xenografts with MeV caused long-term virus replication in tumor tissue. We show that the suicide gene therapy induces an apoptosis-like cell death but is not dependent on intact apoptosis pathways. These results demonstrate that MeV-based suicide gene therapy is a promising novel therapy regimen for HCC overcoming resistance towards conventional therapy. The independence from apoptosis raises hopes for the treatment of patients whose tumor cells exert defects in this cell death mechanism.

Innate immune defense defines susceptibility of sarcoma cells to measles vaccine virus-based oncolysis

The oncolytic potential of measles vaccine virus (MeV) has been demonstrated in several tumor entities. Here, we investigated the susceptibility of eight sarcoma cell lines to MeV-mediated oncolysis and found five to be susceptible, whereas three proved to be resistant. In the MeV-resistant cell lines, we often observed an inhibition of viral replication along with a strong upregulation of the intracellular virus-sensing molecule RIG-I and of the interferon (IFN)-stimulated gene IFIT1. Not only expression of IFIT1 but also phosphorylation of IFN-stimulated Stat1 took place rapidly and were found to be persistent over time. In contrast, susceptible cell lines showed a much weaker, delayed, or completely missing expression of IFIT1 as well as a delayed or only transient phosphorylation of Stat1, whereas exogenic stimulation with beta interferon (IFN-β) resulted in a comparable profound activation of Stat1 and expression of IFIT1 in all cell lines. Pretreatment with IFN-β rendered three of the susceptible cell lines more resistant to MeV-mediated oncolysis. These data suggest that differences in the innate immune defense often account for different degrees of susceptibility of sarcoma cell lines to MeV-mediated oncolysis. From a therapeutic perspective, we were able to overcome resistance to MeV by increasing the multiplicity of infection (MOI) and by addition of the prodrug 5-fluorocytosine (FC), thereby exploiting the suicide gene function of virotherapeutic vector MeV-SCD armed with the SCD fusion protein, which consists of yeast cytosine deaminase and yeast uracil phosphoribosyltransferase.

Single agent- and combination treatment with two targeted suicide gene therapy systems is effective in chemoresistant small cell lung cancer cells

BACKGROUND

Transcriptional targeted suicide gene (SG) therapy driven by the insulinoma-associated 1 (INSM1) promoter makes it possible to target suicide toxin production and cytotoxicity exclusively to small cell lung cancer (SCLC) cells and tumors. It remains to be determined whether acquired chemoresistance, as observed in the majority of SCLC patients, desensitizes SCLC cells to INSM1 promoter-driven SG therapy.

METHODS

A panel of SCLC cell lines resistant to clinically relevant chemotherapeutics was characterized regarding the expression of proteins involved in response to chemotherapy and regarding INSM1 promoter activity. Sensitivity towards INSM1 promoter-driven SG therapy was tested using different systems: Yeast cytosine deaminase-uracil phosphoribosyl transferase (YCD-YUPRT) in combination with the prodrug 5-fluorocytosine (5-FC) or Escherichia coli nitroreductase (NTR) together with the bromomustard prodrug SN27686.

RESULTS

The chemoresistant cell lines displayed heterogeneous expression profiles of molecules involved in multidrug resistance, apoptosis and survival pathways. Despite this, the INSM1 promoter activity was found to be unchanged or increased in SCLC chemoresistant cells and xenografts compared to chemosensitive variants. INSM1 promoter-driven SG therapy with YCD-YUPRT/5-FC or NTR/SN27686, was found to induce high levels of cytotoxicity in both chemosensitive and chemoresistant SCLC cells. Moreover, the combination of INSM1 promoter-driven YCD-YUPRT/5-FC therapy and chemotherapy, as well as the combination of INSM1 promoter-driven YCD-YUPRT/5-FC and NTR/SN27686 therapy, was observed to be superior to single agent therapy in chemoresistant SCLC cells.

CONCLUSIONS

Collectively, the present study demonstrates that targeted SG therapy is a potent therapeutic approach for chemoresistant SCLC patients, with the highest efficacy achieved when applied as combination SG therapy or in combination with standard chemotherapy.

Suicidal gene therapy in the effective control of primary human hepatocellular carcinoma as monitored by noninvasive bioimaging

Hepatocellular carcinoma (HCC) is usually refractory to the available treatments. For cancer gene therapy purposes, real-time imaging of therapeutic gene expression is of great importance because there are multiple factors that modulate the therapeutic gene expression in a complex tumor microenvironment. As a consequence, multiple doses of therapeutic viral vectors may be required for improved efficacy. In the present study, the luciferase reporter gene and the yeast cytosine deaminase (yCD) genes were bicistronically expressed using the foot-and-mouth disease virus 2A peptide under the regulation of the cytomegalovirus (CMV) promoter. The effectiveness of the yCD/5-FC (5-fluorocytosine) killing efficacy mediated by the herpes simplex virus type 1 (HSV-1) amplicon viral vector was shown using HCC and non-HCC cell lines in vitro. In addition, in vivo experiment also showed tumor regression of a primary HCC 26-1004 tumor xenograft in tumor expressing high levels of the yCD gene (as determined by noninvasive imaging) after intratumoral injection of 1.5 × 10(6) TU HGCX-L2C HSV-1 amplicon viral vector and 5-FC administration. The HSV-1 amplicon viral vector coupled with the yCD/5-FC prodrug activated suicide gene could potentially be of use in clinical gene therapy for HCC.

Promoters with cancer cell-specific activity for melanoma gene therapy

Melanoma is one of the most aggressive tumors. It develops from pigment-forming cells (melanocytes) and results in a high number of lethal outcomes. The use of genetic constructs with the ability to specifically kill melanoma cells, but not normal cells, might increase the lifespan of patients, as well as improve their quality of life. One of the methods to achieve a selective impact for therapeutic genes on cancer cells is to utilize a transcriptional control mechanism using promoters that are specifically activated only in cancerous cells. In this review, promoters of the genes that are preferentially expressed in melanoma cells are described. These promoters, and other highly melanoma-specific regulatory elements, reduce the unspecific expression of therapeutic genes in normal tissues. Moreover, cancer-specific promoters and their elements are advantageous for the development of universal anticancer drugs. Examples of the use of double promoters that have a high potential as instruments in cancer gene therapy are also given in this review.

Comparative analysis of enzyme and pathway engineering strategies for 5FC-mediated suicide gene therapy applications

Bacterial- and yeast- encoded cytosine deaminases (bCD and yCD, respectively) are widely investigated suicide enzymes used in combination with the prodrug 5-fluorocytosine (5FC) to achieve localized cytotoxicity. Yet characteristics such as poor turnover rates of 5FC (bCD) and enzyme thermolability (yCD) preclude their full therapeutic potential. We previously applied regio-specific random mutagenesis and computational design to create novel bCD and yCD variants with altered substrate preference (bCD(1525)) or increased thermostability (yCD(double), yCD(triple)) to aid in overcoming these limitations. Others have utilized pathway engineering in which the microbial enzyme uracil phosphoribosyltransferase (UPRT) is fused with its respective CD, creating bCD/bUPRT or yCD/yUPRT. In this study, we evaluated whether the overlay of CD mutants onto their respective CD/UPRT fusion construct would further enhance 5FC activation, cancer cell prodrug sensitivity and bystander activity in vitro and in vivo. We show that all mutant fusion enzymes allowed for significant reductions in IC(50) values relative to their mutant CD counterparts. However, in vivo the CD mutants displayed enhanced tumor growth inhibition capacity relative to the mutant fusions, with bCD(1525) displaying the greatest tumor growth inhibition and bystander activity. In summary, mutant bCD(1525) appears to be the most effective of all bacterial or yeast CD or CD/UPRT enzymes examined and as such is likely to be the best choice to significantly improve the clinical outcome of CD/5FC suicide gene therapy applications.

Novel strong tissue specific promoter for gene expression in human germ cells

BACKGROUND

Tissue specific promoters may be utilized for a variety of applications, including programmed gene expression in cell types, tissues and organs of interest, for developing different cell culture models or for use in gene therapy. We report a novel, tissue-specific promoter that was identified and engineered from the native upstream regulatory region of the human gene NDUFV1 containing an endogenous retroviral sequence.

RESULTS

Among seven established human cell lines and five primary cultures, this modified NDUFV1 upstream sequence (mNUS) was active only in human undifferentiated germ-derived cells (lines Tera-1 and EP2102), where it demonstrated high promoter activity (approximately twice greater than that of the SV40 early promoter, and comparable to the routinely used cytomegaloviral promoter). To investigate the potential applicability of the mNUS promoter for biotechnological needs, a construct carrying a recombinant cytosine deaminase (RCD) suicide gene under the control of mNUS was tested in cell lines of different tissue origin. High cytotoxic effect of RCD with a cell-death rate approximately 60% was observed only in germ-derived cells (Tera-1), whereas no effect was seen in a somatic, kidney-derived control cell line (HEK293). In further experiments, we tested mNUS-driven expression of a hyperactive Sleeping Beauty transposase (SB100X). The mNUS-SB100X construct mediated stable transgene insertions exclusively in germ-derived cells, thereby providing further evidence of tissue-specificity of the mNUS promoter.

CONCLUSIONS

We conclude that mNUS may be used as an efficient promoter for tissue-specific gene expression in human germ-derived cells in many applications. Our data also suggest that the 91 bp-long sequence located exactly upstream NDUFV1 transcriptional start site plays a crucial role in the activity of this gene promoter in vitro in the majority of tested cell types (10/12), and an important role--in the rest two cell lines.

Targeted cytosine deaminase-uracil phosphoribosyl transferase suicide gene therapy induces small cell lung cancer-specific cytotoxicity and tumor growth delay

PURPOSE

Small cell lung cancer (SCLC) is a highly malignant cancer for which there is no curable treatment. Novel therapies are therefore in great demand. In the present study we investigated the therapeutic effect of transcriptionally targeted suicide gene therapy for SCLC based on the yeast cytosine deaminase (YCD) gene alone or fused with the yeast uracil phosphoribosyl transferase (YUPRT) gene followed by administration of 5-fluorocytosine (5-FC) prodrug.

EXPERIMENTAL DESIGN

The YCD gene or the YCD-YUPRT gene was placed under regulation of the SCLC-specific promoter insulinoma-associated 1 (INSM1). Therapeutic effect was evaluated in vitro in SCLC cell lines and in vivo in SCLC xenografted nude mice using the nonviral nanoparticle DOTAP/cholesterol for transgene delivery.

RESULTS

INSM1-YCD/5-FC and INSM1-YCD-YUPRT/5-FC therapy induced high cytotoxicity in a range of SCLC cell lines. The highest therapeutic effect was obtained from the YCD-YUPRT fusion gene strategy. No cytotoxicity was induced after treatment of cell lines of other origin than SCLC. In addition the INSM1-YCD-YUPRT/5-FC therapy was superior to an established suicide gene system consisting of the herpes simplex virus thymidine kinase (HSVTK) gene and the prodrug ganciclovir. The superior effect was in part due to massive bystander cytotoxicity of YCD-YUPRT-produced toxins. Finally, INSM1-YCD-YUPRT/5-FC therapy induced significant tumor growth delay in SCLC xenografts compared with control-treated xenografts.

CONCLUSIONS

The current study is the first to test cytosine deaminase-based suicide gene therapy for SCLC and the first to show an antitumor effect from the delivery of suicide gene therapeutics for SCLC in vivo.

Unique functions of repetitive transcriptomes

Repetitive sequences occupy a huge fraction of essentially every eukaryotic genome. Repetitive sequences cover more than 50% of mammalian genomic DNAs, whereas gene exons and protein-coding sequences occupy only ~3% and 1%, respectively. Numerous genomic repeats include genes themselves. They generally encode "selfish" proteins necessary for the proliferation of transposable elements (TEs) in the host genome. The major part of evolutionary "older" TEs accumulated mutations over time and fails to encode functional proteins. However, repeats have important functions also on the RNA level. Repetitive transcripts may serve as multifunctional RNAs by participating in the antisense regulation of gene activity and by competing with the host-encoded transcripts for cellular factors. In addition, genomic repeats include regulatory sequences like promoters, enhancers, splice sites, polyadenylation signals, and insulators, which actively reshape cellular transcriptomes. TE expression is tightly controlled by the host cells, and some mechanisms of this regulation were recently decoded. Finally, capacity of TEs to proliferate in the host genome led to the development of multiple biotechnological applications.

Bystander or no bystander for gene directed enzyme prodrug therapy

Gene directed enzyme prodrug therapy (GDEPT) of cancer aims to improve the selectivity of chemotherapy by gene transfer, thus enabling target cells to convert nontoxic prodrugs to cytotoxic drugs. A zone of cell kill around gene-modified cells due to transfer of toxic metabolites, known as the bystander effect, leads to tumour regression. Here we discuss the implications of either striving for a strong bystander effect to overcome poor gene transfer, or avoiding the bystander effect to reduce potential systemic effects, with the aid of three successful GDEPT systems. This review concentrates on bystander effects and drug development with regard to these enzyme prodrug combinations, namely herpes simplex virus thymidine kinase (HSV-TK) with ganciclovir (GCV), cytosine deaminase (CD) from bacteria or yeast with 5-fluorocytodine (5-FC), and bacterial nitroreductase (NfsB) with 5-(azaridin-1-yl)-2,4-dinitrobenzamide (CB1954), and their respective derivatives.

Adenovirus-mediated cytosine deaminase/5-fluorocytosine suicide gene therapy of human hepatoblastoma in vitro

BACKGROUND

Multidrug resistance is a key factor for the sobering outcome of relapsed and metastatic human hepatoblastoma (HB). Gene directed treatment approaches were recently identified as possible treatment options against advanced HB, in which standard chemotherapy regimens are partially insufficient. The aim of this study was to systematically analyze the effects of suicide gene therapy in three HB cell lines using a yeast-derived cytosine deaminase (YCD)-combined yeast uracil phosphoribosyltransferase (YUPRT)-based adenovirus-mediated gene transfer.

PROCEDURE

YCD and YUPRT were fused to form the bifunctional suicide gene SuperCD. Adeonoviral vectors were used for transduction. Tumor cells transduced at MOI 50 were incubated with 5-fluorocytosine (5-FC) in ascending concentrations.

RESULTS

Transduction rates were 87.8% (+6.7) in the mixed HB cell line HUH6, 98.6% (+1.4) in the epithelial HB cell line HepT1 and 93.6% (+0.6) in the multifocal HB embryonal cell line HepT3, respectively. In HepT3 and HepT1 cells suicide gene therapy with SuperCD/5-FC was highly effective leading to HB cell damage far above those of application of the prodrug 5-FC only. In HUH6 cells the approach had no effect due to a lack in activity of the CMV promoter being employed for transcription of the SuperCD transgene.

CONCLUSION

Assuming employment of fully active promoters, the SuperCD/5-FC approach may serve as a potentially useful anti-tumor strategy against advanced HB.

Abrogation of microsatellite-instable tumors using a highly selective suicide gene/prodrug combination

A substantial fraction of sporadic and inherited colorectal and endometrial cancers in humans is deficient in DNA mismatch repair (MMR). These cancers are characterized by length alterations in ubiquitous simple sequence repeats, a phenotype called microsatellite instability. Here we have exploited this phenotype by developing a novel approach for the highly selective gene therapy of MMR-deficient tumors. To achieve this selectivity, we mutated the VP22FCU1 suicide gene by inserting an out-of-frame microsatellite within its coding region. We show that in a significant fraction of microsatellite-instable (MSI) cells carrying the mutated suicide gene, full-length protein becomes expressed within a few cell doublings, presumably resulting from a reverting frameshift within the inserted microsatellite. Treatment of these cells with the innocuous prodrug 5-fluorocytosine (5-FC) induces strong cytotoxicity and we demonstrate that this owes to multiple bystander effects conferred by the suicide gene/prodrug combination. In a mouse model, MMR-deficient tumors that contained the out-of-frame VP22FCU1 gene displayed strong remission after treatment with 5-FC, without any obvious adverse systemic effects to the mouse. By virtue of its high selectivity and potency, this conditional enzyme/prodrug combination may hold promise for the treatment or prevention of MMR-deficient cancer in humans.

In vitro gene targeting in human hepatoblastoma

Poor treatment results in advanced hepatoblastoma (HB) made alternative treatment approaches desirable. Gene-directed tumor therapy is increasingly investigated in different malignancies. The aim of this study was to analyze possible alternatives of gene transfer into HB cells and to study therapeutic applications based on different strategies. Liposomal transfection of HB cells was assessed using liver-specific promoters, and adenovirus and Sendai virus transductions were performed in vitro. Transfer efficiencies were measured via flow cytometry determining expression of vector-encoded marker gene green fluorescent protein. Gene silencing of the anti-apoptotic bcl-2 gene in HUH6 cells was performed using lipofection of small interfering RNA (siRNA). Additionally, suicide gene therapy was carried out through a yeast-derived cytosine deaminase (YCD)-combined yeast uracil phosphoribosyltransferase (YUPRT)-based adenovirus-mediated gene transfer, leading to a potent intracellular prodrug transformation of 5-fluorocytosine into 5-fluorouracil. Treatment efficiencies were monitored via MTT viability assay. Highest gene transfer rates (86%) were observed using adenovirus transduction. We furthermore observed a significant therapeutic effect of adenovirus-mediated YCD::YUPRT suicide gene transfer. Liposomal-mediated anti-bcl-2 siRNA transfer led to a significant improvement of cisplatin treatment in HUH6 cells. Liver-specific promoters were found to be strongly active in HUH6 cells (mixed HB-derived), but less active in HepT1 cells (embryonal HB-derived). Liposomal transfection and viral transduction are effective approaches to genetically manipulate HB cells in vitro. For the first time, we demonstrate a positive effect of siRNA gene silencing in this malignancy. Additionally, we successfully investigated a model of adenovirus-based suicide gene therapy in HB cell cultures. Our data strongly encourage further studies assessing these alternative treatment approaches.