Tight control of gene expression by a helper-dependent adenovirus vector carrying the rtTA2(s)-M2 tetracycline transactivator and repressor system

Development of an RNA Virus-Based Episomal Vector Capable of Switching Transgene Expression

Viral vectors are efficient gene delivery systems, although most of these vectors still present limitations to their practical use, such as achieving only transient transgene expression and a risk of insertional mutations. We have recently developed an RNA virus-based episomal vector (REVec), based on nuclear-replicating Borna disease virus (BoDV). REVec can transduce transgenes into various types of cells and stably express transgenes; however, an obstacle to the practical use of REVec is the lack of a mechanism to turn off transgene expression once REVec is transduced. Here, we developed a novel REVec system, REVec-L2b9, in which transgene expression can be switched on and off by using a theophylline-dependent self-cleaving riboswitch. Transgene expression from REVec-L2b9 was suppressed in the absence of theophylline and induced by theophylline administration. Conversely, transgene expression from REVec-L2b9 was switched off by removing theophylline. To our knowledge, REVec-L2b9 is the first nuclear-replicating RNA virus vector capable of switching transgene expression on and off as needed, which will expand the potential for gene therapies by increasing safety and usability.

Nuclear estrogen receptor activation by insulin-like growth factor-1 in Neuro-2A neuroblastoma cells requires endogenous estrogen synthesis and is mediated by mutually repressive MAPK and PI3K cascades

Non-canonical mechanisms of estrogen receptor activation may continue to support women's cognitive health long after cessation of ovarian function. These mechanisms of estrogen receptor activation may include ligand-dependent actions via locally synthesized neuroestrogens and ligand-independent actions via growth factor-dependent activation of intracellular kinase cascades. We tested the hypothesis that ligand-dependent and ligand-independent mechanisms interact to activate nuclear estrogen receptors in the Neuro-2A neuroblastoma cell line in the absence of exogenous estrogens. Transcriptional output of estrogen receptors was measured following treatment with insulin-like growth factor-1 (IGF-1) in the presence of specific inhibitors for mitogen-activated protein kinase (MAPK), phosphoinositde-3 kinase (PI3K), and neuroestrogen synthesis. Results indicate that IGF-1-dependent activation of nuclear estrogen receptors is mediated by MAPK, is opposed PI3K, and requires concomitant endogenous neuroestrogen synthesis. We conclude that both cellular signaling context and endogenous ligand availability are important modulators of ligand-independent nuclear estrogen receptor activation.

A Synthetic Mammalian Therapeutic Gene Circuit for Sensing and Suppressing Inflammation

Inflammation, which is a highly regulated host response against danger signals, may be harmful if it is excessive and deregulated. Ideally, anti-inflammatory therapy should autonomously commence as soon as possible after the onset of inflammation, should be controllable by a physician, and should not systemically block beneficial immune response in the long term. We describe a genetically encoded anti-inflammatory mammalian cell device based on a modular engineered genetic circuit comprising a sensor, an amplifier, a "thresholder" to restrict activation of a positive-feedback loop, a combination of advanced clinically used biopharmaceutical proteins, and orthogonal regulatory elements that linked modules into the functional device. This genetic circuit was autonomously activated by inflammatory signals, including endogenous cecal ligation and puncture (CLP)-induced inflammation in mice and serum from a systemic juvenile idiopathic arthritis (sIJA) patient, and could be reset externally by a chemical signal. The microencapsulated anti-inflammatory device significantly reduced the pathology in dextran sodium sulfate (DSS)-induced acute murine colitis, demonstrating a synthetic immunological approach for autonomous anti-inflammatory therapy.

A novel adenoviral vector carrying an all-in-one Tet-On system with an autoregulatory loop for tight, inducible transgene expression

Background

One of the most commonly used vectors for gene therapy is the adenoviral vector; its ability to tightly regulate transgene expression is critical for optimizing therapeutic outcomes. The tetracycline-regulated system (especially the Tet-On system) for gene expression is one of the most valuable tools for controlling gene expression. The major problem of an adenoviral vector carrying a Tet-On system is suboptimal regulation of transgene expression.

Results

We constructed a single adenoviral vector carrying in its E1 region a novel “all-in-one” Tet-On system with an autoregulatory loop. This system had improved Dox-inducible gene expression in terms of low basal expression, high induced expression and high responsiveness to Dox. To our knowledge, this is the first reported adenovirus-based, all-in-one Tet-On system with an autoregulatory loop inserted into a single region of adenoviral genome. This system was further tested by inducible expression of soluble tumor necrosis factor-related apoptosis-inducing ligand (sTRAIL). The adenovirus that expressed soluble TRAIL under the control of this novel Tet-On system showed tumor-derived cells inhibitory activity in SW480 cells only under induced conditions.

Conclusions

Our novel, single adenoviral vector carrying in its E1 region an all-in-one Tet-On system with an autoregulatory loop displayed tight regulation of transgene expression in vitro. This system has great potential for a variety of applications, including gene therapy and the study of gene function.

Promoter control over foreign antigen expression in a murine cytomegalovirus vaccine vector

Previous studies have reported on the development of a recombinant murine cytomegalovirus (rMCMV) containing the mouse zona pellucida 3 (mZP3) gene for use as a virally vectored immunocontraceptive (VVIC). This study aimed to alter promoter control over foreign antigen expression and cellular localisation of the antigen expressed in order to overcome virus attenuation previously encountered. Early studies reported on the mZP3 gene expressed by a strong constitutive human cytomegalovirus immediate-early 1 promoter (pHCMV IE1). This virus was able to induce >90% infertility in BALB/c mice despite being heavily attenuated in vivo. In this study the mZP3 was placed under the control of the MCMV early 1 (pMCMV E1) promoter and the inducible tetracycline promoter (Tet-On). In both instances the recombinant virus was able to induce infertility in directly infected mice. However, the viruses remained attenuated. This study demonstrated the capacity to manipulate the nature of the immune response by altering promoter control over foreign antigen expression and cellular localisation of the expressed antigen. We were able to demonstrate that by using the MCMV E1 promoter it was still possible to sterilize female BALB/c mice with an MCMV vector expressing mZP3. The use of the MCMV E1 promoter provides an added level of safety to any MCMV based VVIC approach as it only allows for transgene expression in MCMV permissive cells.

Characterization of a molecular switch system that regulates gene expression in mammalian cells through a small molecule

BACKGROUND

Molecular switch systems that activate gene expression by a small molecule are effective technologies that are widely used in applied biological research. Nuclear receptors are valuable candidates for these regulation systems due to their functional role as ligand activated transcription factors. Previously, our group engineered a variant of the retinoid x receptor to be responsive to the synthetic compound, LG335, but not responsive to its natural ligand, 9-cis-retinoic acid.

RESULTS

This work focuses on characterizing a molecular switch system that quantitatively controls transgene expression. This system is composed of an orthogonal ligand/nuclear receptor pair, LG335 and GRQCIMFI, along with an artificial promoter controlling expression of a target transgene. GRQCIMFI is composed of the fusion of the DNA binding domain of the yeast transcription factor, Gal4, and a retinoid x receptor variant. The variant consists of the following mutations: Q275C, I310M, and F313I in the ligand binding domain. When introduced into mammalian cell culture, the switch shows luciferase activity at concentrations as low as 100 nM of LG335 with a 6.3 +/- 1.7-fold induction ratio. The developed one-component system activates transgene expression when introduced transiently or virally.

CONCLUSIONS

We have successfully shown that this system can induce tightly controlled transgene expression and can be used for transient transfections or retroviral transductions in mammalian cell culture. Further characterization is needed for gene therapy applications.

Construction and application of an inducible system for homogenous expression levels in bulk cell lines

Stringently controlled conditional expressing systems are crucial for the functional characterization of genes. Currently, screening of multiple clones to identify the tightly controlled ones is necessary but time-consuming. Here, we describe a system fusing Tet (tetracycline)-inducible elements, BAC (bacterial artificial chromosome) and Gateway technology together to allow tight control of gene expression in BAC-transfected eukaryotic bulk cell cultures. Recombinase cloning into the shuttle vector and the BAC facilitates vector construction. An EGFP (enhanced green fluorescent protein) allows FACS (fluorescence activated cell sorting) and the BAC technology ensures tight control of gene expression that is independent of the integrating site. In the current first application, our gene of interest encodes a beta-catenin-ERalpha fusion protein. Tested by luciferase assay and western blotting, in HTB56 lung cancer cells the final BAC E11-IGR-beta-catenin-ERalpha vector demonstrated sensitive inducibility by Tet or Dox (doxycycline) in a dose-dependent manner with low background, and the EGFP was an effective selection marker by FACS in bulk culture HTB56 and myeloblastic 32D cells. This is a highly efficient tool for the rapid generation of stringently controlled Tet-inducible systems in cell lines.

In vivo gene regulation using tetracycline-regulatable systems

Numerous preclinical studies have demonstrated the efficacy of viral gene delivery vectors, and recent clinical trials have shown promising results. However, the tight control of transgene expression is likely to be required for therapeutic applications and in some instances, for safety reasons. For this purpose, several ligand-dependent transcription regulatory systems have been developed. Among these, the tetracycline-regulatable system is by far the most frequently used and the most advanced towards gene therapy trials. This review will focus on this system and will describe the most recent progress in the regulation of transgene expression in various organs, including the muscle, the retina and the brain. Since the development of an immune response to the transactivator was observed following gene transfer in the muscle of nonhuman primate, focus will be therefore, given on the immune response to transgene products of the tetracycline inducible promoter.

Regulation of episomal gene expression by KRAB/KAP1-mediated histone modifications

KAP1 is an essential cofactor of KRAB zinc finger proteins, a family of vertebrate-specific epigenetic repressors of largely unknown functions encoded in the hundreds by the mouse and human genomes. So far, KRAB/KAP1-mediated gene regulation has been studied within the environment of chromosomal DNA. Here we demonstrate that KRAB/KAP1 regulation is fully functional within the context of episomal DNA, such as adeno-associated viral and nonintegrated lentiviral vectors, and is correlated with histone modifications typically associated with this epigenetic regulator.

Intronically encoded siRNAs improve dynamic range of mammalian gene regulation systems and toggle switch

Applications of conditional gene expression, whether for therapeutic or basic research purposes, are increasingly requiring mammalian gene control systems that exhibit far tighter control properties. While numerous approaches have been used to improve the widely used Tet-regulatory system, many applications, particularly with respect to the engineering of synthetic gene networks, will require a broader range of tightly performing gene control systems. Here, a generically applicable approach is described that utilizes intronically encoded siRNA on the relevant transregulator construct, and siRNA sequence-specific tags on the reporter construct, to minimize basal gene activity in the off-state of a range of common gene control systems. To demonstrate tight control of residual expression the approach was successfully used to conditionally express the toxic proteins RipDD and Linamarase. The intronic siRNA concept was also extended to create a new generation of compact, single-vector, autoinducible siRNA vectors. Finally, using improved regulation systems a mammalian epigenetic toggle switch was engineered that exhibited superior in vitro and in vivo induction characteristics in mice compared to the equivalent non-intronic system.

Turning the gene tap off; implications of regulating gene expression for cancer therapeutics

Cancer poses a tremendous therapeutic challenge worldwide, highlighting the critical need for developing novel therapeutics. A promising cancer treatment modality is gene therapy, which is a form of molecular medicine designed to introduce into target cells genetic material with therapeutic intent. Anticancer gene therapy strategies currently used in preclinical models, and in some cases in the clinic, include proapoptotic genes, oncolytic/replicative vectors, conditional cytotoxic approaches, inhibition of angiogenesis, inhibition of growth factor signaling, inactivation of oncogenes, inhibition of tumor invasion and stimulation of the immune system. The translation of these novel therapeutic modalities from the preclinical setting to the clinic has been driven by encouraging preclinical efficacy data and advances in gene delivery technologies. One area of intense research involves the ability to accurately regulate the levels of therapeutic gene expression to achieve enhanced efficacy and provide the capability to switch gene expression off completely if adverse side effects should arise. This feature could also be implemented to switch gene expression off when a successful therapeutic outcome ensues. Here, we will review recent developments related to the engineering of transcriptional switches within gene delivery systems, which could be implemented in clinical gene therapy applications directed at the treatment of cancer.

Tightly regulated gene expression in human hematopoietic stem cells after transduction with helper-dependent Ad5/35 vectors

OBJECTIVE

Inducible and transient expression of transcription factors, growth factors, or mitogenic factors can be used to influence proliferation or differentiation of hematopoietic progenitor/stem cells (HSC). Furthermore, transient expression of proteins with site-specific endonuclease activity, potentially, can support targeted integration of exogenous transgenes into specific sites in the genome, a task that is currently a focus in development of gene therapy vectors.

MATERIALS AND METHODS

We constructed a set of helper-dependent adenovirus (Ad) vectors with serotype 35 fiber knob domains (HD-Ad5/35), which can efficiently transduce human CD34(+) cells, particularly subsets with potential stem cell capacity. These vectors contain Tet-inducible expression systems that were shielded by insulators and transcription stop signals to minimize unspecific interference by transcriptional elements present in viral and stuffer DNA. We compared two vectors, containing a fusion between the Krüppel-associated box (KRAB) domain and the tetracycline repressor (HD-Ad5/35.Tet-1) or an autoregulated rtTA (HD-Ad5/35.Tet-2) for regulated transgene expression in Mo7e cells, a model for HSC, and primary human CD34(+) cells.

RESULTS

HD-Ad5/35.Tet-1 conferred lower background expression than HD-Ad5/35.Tet-2, although levels of induced gene expression were higher for HD-Ad5/35.Tet-2. In CD34(+) cells, while HD-Ad5/35.Tet-1 allowed for activated gene expression in all transduced cells, induced gene expression from HD-Ad5/35.Tet-2 was restricted to a small subset of CD34(+) cells. Importantly, clonogeneic assays and repopulation studies in nonobese diabetic/severe combined immunodeficient mice showed that both HD-Ad5/35.Tet-1 and -2 vectors mediated induced gene expression in primitive hematopoietic cells. These studies also confirmed that transduction of CD34(+) cells with HD-Ad5/35 vectors is not associated with cytotoxicity, a problem observed with first-generation Ad5/35 vectors.

CONCLUSIONS

Both HD-Ad5/35 vector expression systems confer tightly regulated, transient transgene expression in human HSC.

Protection against aminoglycoside-induced ototoxicity by regulated AAV vector-mediated GDNF gene transfer into the cochlea

Since standard aminoglycoside treatment progressively causes hearing disturbance with hair cell degeneration, systemic use of the drugs is limited. Adeno-associated virus (AAV)-based vectors have been of great interest because they mediate stable transgene expression in a variety of postmitotic cells with minimal toxicity. In this study, we investigated the effects of regulated AAV1-mediated glial cell line-derived neurotrophic factor (GDNF) expression in the cochlea on aminoglycoside-induced damage. AAV1-based vectors encoding GDNF or vectors encoding GDNF with an rtTA2s-S2 Tet-on regulation system were directly microinjected into the rat cochleae through the round window at 5 x 10(10) genome copies/body. Seven days after the virus injection, a dose of 333 mg/kg of kanamycin was subcutaneously given twice daily for 12 consecutive days. GDNF expression in the cochlea was confirmed and successfully modulated by the Tet-on system. Monitoring of the auditory brain stem response revealed an improvement of cochlear function after GDNF transduction over the frequencies tested. Damaged spiral ganglion cells and hair cells were significantly reduced by GDNF expression. Our results suggest that AAV1-mediated expression of GDNF using a regulated expression system in the cochlea is a promising strategy to protect the cochlea from aminoglycoside-induced damage.

Helper-dependent adenovirus for the gene therapy of proliferative retinopathies: stable gene transfer, regulated gene expression and therapeutic efficacy

BACKGROUND

Ocular neovascular disorders, such as diabetic retinopathy and age-related macular degeneration, are the principal causes of blindness in developed countries. Current treatments are of limited efficacy, whereas a therapy based on intraocular gene transfer of angiostatic factors represents a promising alternative. For the first time we have explored the potential of helper-dependent adenovirus (HD-Ad), the last generation of Ad vectors, in the therapy of retinal neovascularization.

METHODS

We first analyzed efficiency and stability of intraretinal gene transfer following intravitreous injection in mice. A HD-Ad vector expressing green fluorescent protein (GFP) under the control of the cytomegalovirus (CMV) promoter (HD-Ad/GFP) was compared with a first-generation (E1/E3-deleted) Ad vector carrying an identical GFP expression cassette (FG-Ad/GFP). We also constructed HD-Ad vectors expressing a soluble form of the VEGF receptor (sFlt-1) in a constitutive (HD-Ad/sFlt-1) or doxycycline (dox)-inducible (HD-Ad/S-M2/sFlt-1) manner and tested their therapeutic efficacy upon intravitreous delivery in a rat model of oxygen-induced retinopathy (OIR).

RESULTS

HD-Ad/GFP promoted long-lasting (up to 1 year) transgene expression in retinal Müller cells, in marked contrast with the short-term expression observed with FG-Ad/GFP. Intravitreous injection of HD-Ad vectors expressing sFlt-1 resulted in detectable levels of sFlt-1 and inhibited retinal neovascularization by more than 60% in a rat model of OIR. Notably, the therapeutic efficacy of the inducible vector HD-Ad/S-M2/sFlt-1 was strictly dox-dependent.

CONCLUSIONS

HD-Ad vectors enable stable gene transfer and regulated expression of angiostatic factors following intravitreous injection and thus are attractive vehicles for the gene therapy of neovascular diseases of the retina.

Gene therapy with an improved doxycycline-regulated plasmid encoding a tumour necrosis factor-alpha inhibitor in experimental arthritis

Inhibition of tumour necrosis factor (TNF)-alpha with biological molecules has proven an effective treatment for rheumatoid arthritis, achieving a 20% improvement in American College of Rheumatology score in up to 65% of patients. The main drawback to these and many other biological treatments has been their expense, which has precluded their widespread application. Biological molecules could alternatively be delivered by gene therapy as the encoding DNA. We have developed novel plasmid vectors termed pGTLMIK and pGTTMIK, from which luciferase and a dimeric TNF receptor II (dTNFR) are respectively expressed in a doxycycline (Dox)-regulated manner. Regulated expression of luciferase from the self-contained plasmid pGTLMIK was examined in vitro in a variety of cell lines and in vivo following intramuscular delivery with electroporation in DBA/1 mice. Dox-regulated expression of luciferase from pGTLMIK of approximately 1,000-fold was demonstrated in vitro, and efficient regulation was observed in vivo. The vector pGTTMIK encoding dTNFR was delivered by the same route with and without administration of Dox to mice with collagen-induced arthritis. When pGTTMIK was delivered after the onset of arthritis, progression of the disease in terms of both paw thickness and clinical score was inhibited when Dox was also administered. Vectors with similar regulation characteristics may be suitable for clinical application.

Inducible expression of phospholipid transfer protein (PLTP) in transgenic mice: acute effects of PLTP on lipoprotein metabolism

One main determinant in high-density lipoprotein (HDL) metabolism is phospholipid transfer protein (PLTP), a plasma protein that is associated with HDL. In transgenic mice overexpressing human PLTP we found that elevated plasma PLTP levels dose-dependently increased the susceptibility to diet-induced atherosclerosis. This could be mainly due to the fact that most functions of PLTP are potentially atherogenic, such as decreasing plasma HDL levels. To further elucidate the role of PLTP in lipoprotein metabolism and atherosclerosis we generated a novel transgenic mouse model that allows conditional expression of human PLTP. In this mouse model a human PLTP encoding sequence is controlled by a Tet-On system. Upon induction of PLTP expression, our mouse model showed a strongly increased PLTP activity (from 3.0 +/- 0.6 to 11.4 +/- 2.8 AU, p < 0.001). The increase in PLTP activity resulted in an acute decrease in plasma cholesterol of 33% and a comparable decrease in phospholipids. The decrease in total plasma cholesterol and phospholipids was caused by a 35% decrease in HDL-cholesterol level and a 41% decrease in HDL-phospholipid level. These results demonstrate the feasibility of our mouse model to induce an acute elevation of PLTP activity, which is easily reversible. As a direct consequence of an increase in PLTP activity, HDL-cholesterol and HDL-phospholipid levels strongly decrease. Using this mouse model, it will be possible to study the effects of acute elevation of PLTP activity on lipoprotein metabolism and pre-existing atherosclerosis.

Tetracycline-inducible expression systems: new strategies and practices in the transgenic mouse modeling

To accurately analyze the function of transgene(s) of interest in transgenic mice, and to generate credible transgenic animal models for multifarious human diseases to precisely mimic human disease states, it is critical to tightly regulate gene expression in the animals in a conditional manner. The ability to turn gene expression on or off in the restricted cells or tissues at specific time permits unprecedented flexibility in dissecting gene functions in health and disease. Pioneering studies in conditional transgene expression have brought about the development of a wide variety of controlled gene expression systems, which meet this criterion. Among them, the tetracycline-controlled expression systems (e.g. Tet-off system and Tet-on system) have been used extensively in vitro and in vivo. In recent years, some strategies derived from tetracycline-inducible system alone, as well as the combined use of Tet-based systems and Cre/lox P switching gene expression system, have been newly developed to allow more flexibility for exploring gene functions in health and disease, and produce credible transgenic animal models for various human diseases. In this review these newly developed strategies are discussed.

Immune responses against tetracycline-dependent transactivators affect long-term expression of mouse erythropoietin delivered by a helper-dependent adenoviral vector

BACKGROUND

Helper-dependent adenoviral (HD-Ad) vectors give rise to sustained gene expression after delivery in a variety of organisms. In particular, we previously documented persistent expression of erythropoietin (EPO) in mice after a single intramuscular (i.m.) injection of a HD-Ad vector harboring the mouse EPO cDNA.

METHODS

We use the same vector harboring the tetracycline (tet)-dependent transactivator (rtTA2S-M2) and silencer (tTS) and mouse EPO cDNA to analyze the capacity of the dual tet-dependent transactivator system to control long-term EPO gene expression and to study the effect of an eventual immune response against these artificial proteins after i.m. delivery in immuno-competent mice.

RESULTS

In the present study we demonstrate that i.m. injection of this vector in immuno-competent mice generates a cellular immune response to the rtTA2S-M2 transcription factor. This response curtails the duration of mouse EPO expression in mice, presumably by destroying the cells that co-express transcription factors and the therapeutic gene. Nonetheless, regulation of mouse EPO secretion was maintained during the entire experimental period, both when the vector dosage was reduced and when the tet-dependent transcription factors were put under the control of a muscle-specific promoter.

CONCLUSIONS

Delivery of the tet transactivators using as vehicle a HD-Ad vector induced an immune response directed against the transactivators themselves, causing short-term therapeutic transgene expression. Regulated, long-term therapeutic transgene expression was, however, obtained by reducing the vector dose or expressing the transactivators under the control of a muscle-specific promoter.

[Gene therapy for osteoarticular disorders]

Osteoarticular disorders are the major cause of disability in Europe and North America. It is estimated that rheumatoid arthritis affects 1 % of the population and that more than two third of people over age 55 develop osteoarthritis. Because there are no satisfactory treatments, gene therapy offers a new therapeutic approach. The delivery of cDNA encoding anti-arthritic proteins to articular cells has shown therapeutic efficacy in numerous animal models in vivo. Through the development and the experimental progresses that have been made for both rheumatoid arthritis and osteoarthritis, this review discusses the different gene therapy strategies available today and the safety issues with which they may be associated. Among the different vectors available today, adeno-associated virus seems the best candidate for a direct in vivo gene delivery approach for the treatment of joint disorders.

Viral vectors: a wide range of choices and high levels of service

Viruses are intracellular parasites with simple DNA or RNA genomes. Virus life revolves around three steps: infection of a host cell, replication of its genome within the host cell environment, and formation of new virions; this process is often but not always associated with pathogenic effects against the host organism. Since the mid-1980s, the main goal of viral vectorology has been to develop recombinant viral vectors for long-term gene delivery to mammalian cells, with minimal associated toxicity. Today, several viral vector systems are close to achieving this aim, providing stable transgenic expression in many different cell types and tissues. Here we review application characteristics of four vector systems, derived from adeno-associated viruses, adenoviruses, retroviruses and herpes simplex virus-1, for in vivo gene delivery. We discuss the transfer capacity of the expression vectors, the stability of their transgenic expression, the tropism of the recombinant viruses, the likelihood of induction of immunotoxicity, and the ease (or difficulty) of the virus production. In the end, we discuss applications of these vectors for delivery of three molecular systems for conditional mutagenesis, two for inducible transcriptional control of transgenic expression (the tet and the dimerizer systems), and the third one for inducible control of endogenous gene expression based on RNA interference.

Regulatable gutless adenovirus vectors sustain inducible transgene expression in the brain in the presence of an immune response against adenoviruses

In view of recent serious adverse events and advances in gene therapy technologies, the use of regulatable expression systems is becoming recognized as indispensable adjuncts to successful clinical gene therapy. In the present work we optimized high-capacity adenoviral (HC-Ad) vectors encoding the novel tetracycline-dependent (TetOn)-regulatory elements for efficient and regulatable gene expression in the rat brain in vivo. We constructed two HC-Ad vectors encoding beta-galactosidase (beta-gal) driven by a TetOn system containing the rtTAS(s)M2 transactivator and the tTS(Kid) repressor under the control of the murine cytomegalovirus (mCMV) (HC-Ad-mTetON-beta-Gal) or the human CMV (hCMV) promoter (HC-Ad-hTetON-beta-Gal). Expression was tightly regulatable by doxycycline (Dox), reaching maximum expression in vivo at 6 days and returning to basal levels at 10 days following the addition or removal of Dox, respectively. Both vectors achieved higher transgene expression levels compared to the expression from vectors encoding the constitutive mCMV or hCMV promoter. HC-Ad-mTetON-beta-Gal yielded the highest transgene expression levels and expressed in both neurons and astrocytes. Antivector immune responses continue to limit the clinical use of vectors. We thus tested the inducibility and longevity of HC-Ad-mediated transgene expression in the brain of rats immunized against adenovirus by prior intradermal injections of RAds. Regulated transgene expression from HC-Ad-mTetON-beta-Gal remained active even in the presence of a significant systemic immune response. Therefore, these vectors display two coveted characteristics of clinically useful vectors, namely their regulation and effectiveness even in the presence of prior immunization against adenovirus.

A bidirectional Tet-dependent promotor construct regulating the expression of E1A for tight control of oncolytic adenovirus replication

Tight regulation of oncolytic adenoviruses (oAdV) represents an important requirement for their safe application. Here we describe a new doxycycline (Dox)-dependent oAdV with a bidirectional expression cassette, which drives the expression of the reverse tetracycline-controlled transactivator (rtTA(s)-M2) from a lung tumor-specific promoter and, in the opposite direction, the expression of the adenoviral E1A gene from a second generation TetO(7) sequence linked to an isolated TATA box. In H441 lung cancer cells, this oAdV showed a strictly Dox-dependent E1A expression, adenoviral replication, cell killing activity and a 450-fold induction of progeny virus production. The virus could be shut off again by withdrawal of Dox and, in contrast to a control oAdV expressing E1A directly from the SP-B promoter, did not replicate in non-target cells. However, the absolute values of virus production and the cell killing activity in the presence of the inducer were still reduced as compared to the control oAdV. The results demonstrate, for the first time, Dox-dependent oAdV replication from a single adenoviral vector genome. Future improvement of the Dox-dependent E1A regulation cassette should lead to the generation of an oAdV well suited to meet the demands for a highly regulated and efficient oncolytic virus for in vivo applications.

Generation and characterization of tTS-H4: a novel transcriptional repressor that is compatible with the reverse tetracycline-controlled TET-ON system

BACKGROUND

Conditional gene regulatory systems ensuring tight and adjustable expression of therapeutic genes are central for developing future gene therapy strategies. Among various regulatory systems, tetracycline-controlled gene expression has emerged as a safe and reliable option. Moreover, the tightness of tetracycline-regulated gene switches can be substantially improved by complementing transcriptional activators with antagonizing repressors.

METHODS

To develop novel tetracycline-responsive transcriptional repressors, we fused various transcriptional silencing domains to the TetR (B/E) DNA-binding and dimerization domain of the Tn10-encoded tetracycline resistance operon (TetR (B/E)). The resulting fusion proteins were individually tested for their ability to repress transcription of the constitutively active hypoxanthine phosphoribosyltransferase (HPRT) promoter. In addition, compatibility with the commonly used reverse tetracycline-controlled transactivator system (rtTA-system) and responsiveness to the pharmacological effector doxycycline (DOX) were evaluated. Finally, inducibility, effector-dependent promoter activity and the modification of histone H3 and H4 of the active versus the repressed target promoter were determined.

RESULTS

Fusion of the human deacetylase 4 (HDAC4) carboxy-terminal silencing domain to TetR (B/E) resulted in a functional transcriptional repressor. This novel repressor, termed tTS-H4, efficiently reduced the activity of the murine HPRT promoter and a constitutively active human cytomegalovirus (hCMV) minimal promoter. Furthermore, combining tTS-H4 with the rtTA transcriptional activator allowed for grading, turning off and resuming target gene expression over several orders of magnitude without background.

CONCLUSIONS

The tTS-H4 repressor is compatible with the commonly used rtTA transcriptional activation system and is a versatile new tool for tightly and adjustably regulating conditional gene expression.

Virus-based gene therapy strategies for bone regeneration

Gene therapy has emerged as a promising strategy for the repair and regeneration of damaged musculoskeletal tissues. Application of this paradigm to bone healing has shown enhanced efficacy in preclinical animal studies compared to conventional bone grafting approaches. This review discusses current and emerging virus-based genetic engineering strategies for the delivery of therapeutic molecules which promote skeletal regeneration. Viral gene delivery vectors are discussed in the context of bone repair in order to illustrate the challenges and applications of these methods with tissue-specific examples. Moreover the concepts discussed can be broadly applied to promote healing in a wide range of tissues. We also present important considerations involved in the application of these gene therapy techniques to a variety of osteogenic (e.g. bone marrow-derived cells) and non-osteogenic (e.g. fibroblasts and skeletal myoblasts) cell types. Criteria for the selection of regenerative molecules with soluble versus intracellular modes of action and emerging combinatorial approaches are also discussed. Overall, gene transfer technologies have the potential to overcome limitations associated with existing bone grafting approaches and may enable investigators to design therapies which more closely mimic the complex spatial and temporal cascade of proteins involved in endogenous bone development and repair.

Regulatable gene expression systems for gene therapy

It is feasible to restrict transgene expression to a tissue or region in need of therapy by using promoters that respond to focusable physical stimuli. The most extensively investigated promoters of this type are radiation-inducible promoters and heat shock protein gene promoters that can be activated by directed, transient heat. Temporal regulation of transgenes can be achieved by various two- or three-component gene switches that are triggered by an appropriate small molecule inducer. The most commonly considered gene switches that are reviewed herein are based on small molecule-responsive transactivators derived from bacterial tetracycline repressor, insect or mammalian steroid receptors, or mammalian FKBP12/FRAP. A new generation of gene switches combines a heat shock protein gene promoter and a small molecule-responsive gene switch and can provide for both spatial and temporal regulation of transgene activity.

A retroviral expression system based on tetracycline-regulated tricistronic transactivator/repressor vectors for functional analyses of antiproliferative and toxic genes

Establishment of stably transfected mammalian cells with conditional expression of antiproliferative or proapoptotic proteins is often hampered by varying expression within bulk-selected cells and high background in the absence of the inducing drug. To overcome such limitations, we designed a gene expression system that transcribes the tetracycline-dependent rtTA2-M2-activator, TRSID-silencer, and selection marker as a tricistronic mRNA from a single retroviral vector. More than 92% of bulk-selected cells expressed enhanced green fluorescent protein or luciferase over more than three orders of magnitude in an almost linear, dose-dependent manner. To functionally test this system, we studied how dose-dependent expression of p27(Kip1) affects proliferation and viability of SH-EP neuroblastoma cells. Low to moderate p27(Kip1) expression caused transient G(0)-G(1) accumulation without reduced viability, whereas high p27(Kip1) levels induced significant apoptosis after 72 hours. This proves that this expression system allows concentration-dependent analysis of gene function and implicates p27(Kip1) as a critical regulator of both proliferation and apoptosis in SH-EP neuroblastoma cells.

A gene regulation system with four distinct expression levels

BACKGROUND

The amount of a particular protein, and not just its presence or absence, frequently determines the outcome of a developmental process or disease progression. These dosage effects can be studied by conditionally expressing such proteins at different levels. With typical gene regulation systems like the Tet-On system, intermediate expression levels can be obtained by varying the effector concentration. However, this strategy is limited to situations in which these concentrations can be precisely controlled and, thus, not suited for animal models or gene therapy approaches. Here, we present a Tet transregulator setup that allows establishment of four levels of promoter activity largely independent of effector concentration.

METHODS

A newly introduced transsilencer is combined with a reverse transactivator. As the regulators respond differentially to tetracycline derivatives, four expression levels are obtained by adding different effectors. To facilitate integration of the components, we generated versatile all-in-one vectors. Apart from a cassette expressing the transregulators and a selection marker, these vectors encode a bidirectional, regulated promoter driving expression of GFP and the gene of interest. The features of this stepwise regulation system were analyzed by transient and stable transfections of human cell lines.

RESULTS

We demonstrate in a variety of experimental settings that coexpression of these transregulators leads to robust stepwise regulation. Depending on the respective effectors, four expression levels are achieved with different responsive promoters, cell lines and target genes.

CONCLUSIONS

This system shows that a promoter can be adjusted to different activities and provides an excellent strategy to investigate protein dosage effects.

Inducible regulation of Runx2-stimulated osteogenesis

Ex vivo gene therapy is a promising approach to orthopedic regenerative medicine. These strategies typically focus on the constitutive overexpression of osteogenic factors to induce osteoblastic differentiation and matrix mineralization. However, the unregulated production of osteoinductive molecules has also resulted in abnormal bone formation and tumorigenesis. To address these limitations, this work describes a retroviral system to deliver the Runx2 osteoblastic transcription factor under control of the tetracycline-inducible (tet-off) promoter in primary skeletal myoblasts. Runx2 expression was tightly regulated by anhydrotetracyline (aTc) concentration in cell culture media. Osteoblastic gene expression, alkaline phosphatase activity, and matrix mineralization were also controlled by aTc in a dose-dependent manner. Additionally, osteoblastic differentiation was temporally regulated by adding and removing aTc from the culture media. Engineered cells were seeded onto collagen scaffolds and implanted intramuscularly in the hind limbs of syngeneic mice. In vivo mineralization by these constructs was regulated by supplementing the drinking water with aTc, as demonstrated by micro-computed tomography and histological analyses. Collectively, these results present a novel system for regulating osteoblastic differentiation of a clinically relevant autologous cell source. This system is significant to developing controlled and effective orthopedic gene therapy strategies and studying the regulation of osteoblastic differentiation.

Improved transgene expression fine-tuning in mammalian cells using a novel transcription–translation network

Following the discovery of RNA interference (RNAi) and related phenomena, novel regulatory processes, attributable to small non-protein-coding RNAs, continue to emerge. Capitalizing on the ability of artificial short interfering RNAs (siRNAs) to trigger degradation of specific target transcripts, and thereby silence desired gene expression, we designed and characterized a generic transcription-translation network in which it is possible to fine-tune heterologous protein production by coordinated transcription and translation interventions using macrolide and tetracycline antibiotics. Integration of siRNA-specific target sequences (TAGs) into the 5' or 3' untranslated regions (5'UTR, 3'UTR) of a desired constitutive transcription unit rendered transgene-encoded protein (erythropoietin, EPO; human placental alkaline phosphatase, SEAP; human vascular endothelial growth factor 121, VEGF(121)) production in mammalian cells responsive to siRNA levels that can be fine-tuned by macrolide-adjustable RNA polymerase II- or III-dependent promoters. Coupling of such macrolide-responsive siRNA-triggered translation control with tetracycline-responsive transcription of tagged transgene mRNAs created an antibiotic-adjustable two-input transcription-translation network characterized by elimination of detectable leaky expression with no reduction in maximum protein production levels. This transcription-translation network revealed transgene mRNA depletion to be dependent on siRNA and mRNA levels and that translation control was able to eliminate basal expression inherent to current transcription control modalities. Coupled transcription-translation circuitries have the potential to lead the way towards composite artificial regulatory networks, to enable complex therapeutic interventions in future biopharmaceutical manufacturing, gene therapy and tissue engineering initiatives.

Lentiviral Vectors for Use in the Central Nervous System

Lentiviral vectors have been used extensively as gene transfer tools for the central nervous system throughout the past decade since they transduce most cell types in the brain, resulting in high-level and long-term transgene expression. This review discusses some of the recent progress in this field, including preclinical gene therapy experiments in disease models, development of regulated vectors, and the application of siRNA's using lentiviral vectors. We also describe some of the features that make lentiviral vectors a likely candidate for human gene therapy in the brain.

Gutless adenovirus: last-generation adenovirus for gene therapy

Last-generation adenovirus vectors, also called helper-dependent or gutless adenovirus, are very attractive for gene therapy because the associated in vivo immune response is highly reduced compared to first- and second-generation adenovirus vectors, while maintaining high transduction efficiency and tropism. Nowadays, gutless adenovirus is administered in different organs, such as the liver, muscle or the central nervous system achieving high-level and long-term transgene expression in rodents and primates. However, as devoid of all viral coding regions, gutless vectors require viral proteins supplied in trans by a helper virus. To remove contamination by a helper virus from the final preparation, different systems based on the excision of the helper-packaging signal have been generated. Among them, Cre-loxP system is mostly used, although contamination levels still are 0.1-1% too high to be used in clinical trials. Recently developed strategies to avoid/reduce helper contamination were reviewed.

Endogenous GD3 ganglioside induces apoptosis in U-1242 MG glioma cells

GD3 ganglioside induces apoptosis in several cell types, but the molecular events through which this occurs are largely unknown. We investigated the apoptotic effects of GD3 expression using U-1242 MG glioblastoma cells, as these cells synthesize almost exclusively GM3 and GM2 but not GD3. To express GD3 under the control of the TetOn system with minimum leakage, we modified the system by constructing a single tri-cistronic retrovirus vector containing three genes separated by two internal ribosome entry sites: (a) transcriptional silencer, tTS; (b) mutant of reverse transcriptional activator, rtTA2(S)-M2 (provided by H. Bujard, Heidelberg, Germany); and (c) enhanced green fluorescent protein (EGFP), as an indicator of the tri-cistronic gene expression. Using flow cytometry, we selected glioma cells (U1242MG-GD3 clone) that express high levels of GD3 in response to doxycycline. Expression of GD3 was associated with apoptosis as verified by annexin-V binding, TdT-mediated dUTPnick end-labelling assay (TUNEL), and EGFP degradation. GD3-induced apoptosis occurred via caspase-8 activation, as GD3 caused cleavage of caspase-8 and inhibition of caspase-8 activation by zlETD-fmk minimized GD3-induced apoptosis.

Pharmacologic transgene control systems for gene therapy

Pharmacologic transgene-expression dosing is considered essential for future gene therapy scenarios. Genetic interventions require precise transcription or translation fine-tuning of therapeutic transgenes to enable their titration into the therapeutic window, to adapt them to daily changing dosing regimes of the patient, to integrate them seamlessly into the patient's transcriptome orchestra, and to terminate their expression after successful therapy. In recent years, decisive progress has been achieved in designing high-precision trigger-inducible mammalian transgene control modalities responsive to clinically licensed and inert heterologous molecules or to endogenous physiologic signals. Availability of a portfolio of compatible transcription control systems has enabled assembly of higher-order control circuitries providing simultaneous or independent control of several transgenes and the design of (semi-)synthetic gene networks, which emulate digital expression switches, regulatory transcription cascades, epigenetic expression imprinting, and cellular transcription memories. This review provides an overview of cutting-edge developments in transgene control systems, of the design of synthetic gene networks, and of the delivery of such systems for the prototype treatment of prominent human disease phenotypes.

An improved Tet-On regulatable FasL-adenovirus vector system for lung cancer therapy

Gene therapy is a new therapeutic approach for the treatment of human cancers. Gene expression systems that can be regulated by drugs have been developed to improve the safety and efficacy of therapeutic transgene delivery. One of the most promising systems is the tetracycline (Tet)-responsive system in the Tet-On configuration. A major problem of the Tet-On system if used in viral vectors is the high basal activity of the Tet response element (TRE) promoter leading to leaky expression of transgenes under uninduced conditions. We therefore evaluated novel TRE promoters for controlling gene expression in an adenovirus vector (AdV) Tet-On system and further investigated them for expression of the pro-apoptotic CD95/Fas ligand (FasL) in human epithelial carcinoma cell line (HeLa) and lung cancer cells. Plasmid-based reporter gene assays showed that modifications within the tetO (7) and minimal immediate early cytomegalovirus promoter (CMV)(min) sequence of the TRE promoter reduced its leakiness and led to a markedly improved regulatability by doxycycline. Among several TRE promoters tested, a new construct (TRE-Tight1) containing modifications of both the tetO (7) sequence and the CMV(min) showed 11-fold reduced leakiness and 1.5-fold increased absolute transgene expression levels after induction, as compared to the original TRE. Under induced conditions, a TRE-Tight1 promoter-dependent AdV expressing the pro-apoptotic CD95L/FasL induced apoptosis and cell lysis in HeLa cells as efficiently as an AdV containing the original TRE promoter. In contrast to the latter, however, the vector with the modified TRE promoter left cells totally unaffected in the absence of the inducer. Stringently regulated induction of apoptosis and cell death by TRE-Tight1-AdV was also demonstrated in three human lung cancer cell lines. These data show that the novel TRE-Tight1 promoter has a high potential for closely controlled and efficient expression of cytotoxic genes in AdV-based anti-cancer approaches.

Regulatable gene expression systems for gene therapy applications: progress and future challenges

Gene therapy aims to revert diseased phenotypes by the use of both viral and nonviral gene delivery systems. Substantial progress has been made in making gene transfer vehicles more efficient, less toxic, and nonimmunogenic and in allowing long-term transgene expression. One of the key issues in successfully implementing gene therapies in the clinical setting is to be able to regulate gene expression very tightly and consistently as and when it is needed. The regulation ought to be achievable using a compound that should be nontoxic, be able to penetrate into the desired target tissue or organ, and have a half-life of a few hours (as opposed to minutes or days) so that when withdrawn or added (depending on the regulatable system used) gene expression can be turned "on" or "off" quickly and effectively. Also, the genetic switches employed should ideally be nonimmunogenic in the host. The ability to switch transgenes on and off would be of paramount importance not only when the therapy is no longer needed, but also in the case of the development of adverse side effects to the therapy. Many regulatable systems are currently under development and some, i.e., the tetracycline-dependent transcriptional switch, have been used successfully for in vivo preclinical applications. Despite this, there are no examples of switches that have been employed in a human clinical trial. In this review, we aim to highlight the main regulatable systems currently under development, the gene transfer systems employed for their expression, and also the preclinical models in which they have been used successfully. We also discuss the substantial challenges that still remain before these regulatable switches can be employed in the clinical setting.

Efficient control of gene expression in the hematopoietic system using a single Tet-on inducible lentiviral vector

This work addresses the problem of efficient control of gene expression in the context of viral vectors, which still represents a difficult challenge. A number of lentiviral vectors incorporating the different elements of regulatable transcriptional systems have been described, but they fail to perform satisfactorily either because of a poor dynamic range of transcription levels or because they display high background activities in the uninduced state and mediocre inducer response. We report here on the systematic comparison of vector designs containing the elements of the doxycycline-inducible Tet-on system in their most advanced versions (rtTA2S-M2 transactivator and tTS(Kid) repressor). We show that a simple "all-in-one" vector can be obtained and used for efficient control of transgene expression in long-term tissue culture and in the hematopoietic system of mice following bone marrow transplantation. Using this vector, the uninduced state can be kept at background levels and induction factors of 100-fold are repeatedly obtained over months both in tissue culture and in vivo. Interestingly, the low background activity of the all-in-one vector renders the use of the tTS repressor dispensable, avoiding the problem of progressive loss of inducibility over time associated with irreversible modifications of the chromatin surrounding proviral sequences.

Construction and characterization of multiple human colon cancer cell lines for inducibly regulated gene expression

Validation of targets for cancer drug discovery requires robust experimental models. Systems based on inducible gene expression are well suited to this purpose but are difficult to establish in several epithelial cell types. Using the recently discovered transcriptional transactivator (rtTA2S-M2), we developed a strategy for fast and efficient generation of Tet On cells. Multiple clones of HCT116, SW480, and HT29 human colon cancer cells for doxycycline-regulated gene expression were constructed that constitutively express green fluorescent protein (GFP) for selection/maintenance purposes. The cell lines displayed good fold inducibility (49-124xHCT116; 178-621xSW480; 261-787xHT29) and minimal leakiness after transient transfection with a luciferase reporter or with vectors driving inducible expression of red fluorescent protein (dsRed2), constitutively active c-Src or dominant negative K-Ras4B. The clones preserved their transformed phenotype as demonstrated by comparing their properties to respective wild type cells, in terms of growth in vitro and in vivo (as tumor xenografts), cell cycle traverse, and sensitivity to drugs used in chemotherapy. These engineered cell lines enabled tightly controlled inducible gene expression both in vitro and in vivo, and proved well suited for construction of double-stable cell lines inducibly expressing a protein of interest. As such they represent a useful research tool for example, to dissect oncogene function(s) in colon cancer. Supplementary material for this article be found at http://www.mrw.interscience.wiley.com/suppmat/0730-2312/suppmat/94/suppmat_welman.doc.

Reproducible doxycycline-inducible transgene expression at specific loci generated by Cre-recombinase mediated cassette exchange

Comparative analysis of mutants using transfection is complicated by clones exhibiting variable levels of gene expression due to copy number differences and genomic position effects. Recombinase-mediated cassette exchange (RMCE) can overcome these problems by introducing the target gene into pre-determined chromosomal loci, but recombination between the available recombinase targeting sites can reduce the efficiency of targeted integration. We developed a new LoxP site (designated L3), which when used with the original LoxP site (designated L2), allows highly efficient and directional replacement of chromosomal DNA with incoming DNA. A total of six independent LoxP integration sites introduced either by homologous recombination or retroviral delivery were analyzed; 70-80% of the clones analyzed in hamster and human cells were correct recombinants. We combined the RMCE strategy with a new, tightly regulated tetracycline induction system to produce a robust, highly reliable system for inducible transgene expression. We observed stable inducible expression for over 1 month, with uniform expression in the cell population and between clones derived from the same integration site. This system described should find significant applications for studies requiring high level and regulated transgene expression and for determining the effects of various stresses or oncogenic conditions in vivo and in vitro.

Stringent doxycycline-dependent control of gene activities using an episomal one-vector system

Conditional expression systems are of pivotal importance for the dissection of complex biological phenomena. Here, we describe a novel EBV-derived episomally replicating plasmid (pRTS-1) that carries all the elements for conditional expression of a gene of interest via Tet regulation. The vector is characterized by (i) low background activity, (ii) high inducibility in the presence of doxycycline (Dox) and (iii) graded response to increasing concentrations of the inducer. The chicken beta actin promoter and an element of the murine immunoglobin heavy chain intron enhancer drive constitutive expression of a bicistronic expression cassette that encodes the highly Dox-sensitive reverse tetracycline controlled transactivator rtTA2(S)-M2 and a Tet repressor-KRAB fusion protein (tTS(KRAB)) (silencer) placed downstream of an internal ribosomal entry site. The gene of interest is expressed from the bidirectional promoter P(tet)bi-1 that allows simultaneous expression of two genes, of which one may be used as surrogate marker for the expression of the gene of interest. Tight down regulation is achieved through binding of the silencer tTS(KRAB) to P(tet)bi-1 in the absence of Dox. Addition of Dox releases repression and via binding of rtTA2(S)-M2 activates P(tet)bi-1.

Regulated and liver-specific tamarin alpha interferon gene delivery by a helper-dependent adenoviral vector

Gene therapy approaches based on liver-restricted and regulated alpha interferon (IFN-alpha) expression, recently shown to be effective in different murine hepatitis models, appear promising alternatives to inhibit hepatitis C virus (HCV) replication in patients and minimize side effects. Tamarins (Saguinus species) infected by GB virus B (GBV-B) are considered a valid surrogate model for hepatitis C to study the biology of HCV infection and the development of new antiviral drugs. To test the efficacy of local delivery and expression of IFN-alpha in this model, we have developed HD-TET-tIFN, a helper-dependent adenovirus vector expressing tamarin IFN-alpha (tIFN) under the control of the tetracycline-inducible transactivator rtTA2s-S2. Expression of tIFN was successfully induced both in vitro and in vivo in rodents by doxycycline administration with consequent activation of IFN-responsive genes. More importantly, tIFN efficiently inhibited GBV-B replicon in a Huh-7 hepatoma cell line at low HD-TET-tIFN doses. A certain degree of transcriptional control of tIFN was achieved in tamarins injected with HD-TET-tIFN, but under the conditions used in this study, infection and replication of GBV-B were only delayed and not totally abrogated upon virus challenge. Hepatic delivery and regulated expression of IFN-alpha appear to be a possible approach for the cure of hepatitis, but this approach requires more studies to increase its efficacy. To our knowledge, this is the first report showing a regulated gene expression in a nonhuman primate hepatitis model.

Regulated delivery of glial cell line-derived neurotrophic factor into rat striatum, using a tetracycline-dependent lentiviral vector

In this study, a tetracycline-regulated lentiviral vector system, based on the tetracycline-dependent transactivator rtTA2(S)-M2, was developed for controlled expression of glial cell line-derived neurotrophic factor (GDNF) in the rat brain. Expression of the marker gene green fluorescent protein (GFP) and GDNF was tightly regulated in a dose-dependent manner in neural cell lines in vitro. Injection of high-titer lentiviral vectors into the rat striatum resulted in a 7-fold induction of GDNF tissue levels (1060 pg/mg tissue), when doxycycline (a tetracycline analog) was added to the drinking water. However, low levels of GDNF (150 pg/mg tissue) were also detected in animals that did not receive doxycycline, indicating a significant background leakage from the vector system in vivo. The level of basal expression was markedly reduced when a 10-fold lower dose of the tetracycline-regulated GDNF vector was injected into the striatum (3-11 pg/mg tissue), and doxycycline-induced GDNF tissue levels obtained in these animals were about 190 pg/mg tissue. Doxycycline-induced expression of GDNF resulted in a significant downregulation of the tyrosine hydroxylase (TH) protein in the intact striatum. Removal of doxycycline from the drinking water rapidly (within 3 days) turned off transgenic GDNF mRNA expression and GDNF protein levels in the tissue were completely reduced by 2 weeks, demonstrating the dynamics of the system in vivo. Accordingly, TH protein expression returned to normal by 2-8 weeks after removal of doxycycline, indicating that GDNF-induced downregulation of TH is a reversible event.

Tight control of transgene expression by lentivirus vectors containing second-generation tetracycline-responsive promoters

BACKGROUND

The goal of this study was to design improved regulatable lentivirus vector systems. The aim was to design tetracycline (tet)-regulatable lentivirus vectors based on the Tet-on system displaying low background expression in the absence of the doxycycline (DOX) inducer and high transgene expression levels in the presence of DOX.

METHODS

We constructed a binary lentivirus vector system that is composed of a self-inactivating (SIN) lentivirus vector bearing inducible first- or second-generation tet-responsive promoter elements (TREs) driving expression of a transgene and a second lentivirus vector encoding a reverse tetracycline-controlled transactivator (rtTA) that activates transgene expression from the TRE in the presence of DOX.

RESULTS

We evaluated a number of different rtTAs and found rtTA2S-M2 to induce the highest levels of transgene expression. Regulated transgene expression was stable in human breast carcinoma cells implanted into nude mice for up to 11 weeks. In an attempt to minimize background expression levels, the chicken beta-globin cHS4 insulator element was cloned into the 3' long terminal repeat (LTR) of the transgene transfer vector. The cHS4 insulator element reduced background expression but expression levels following DOX addition were lower than those observed with vectors lacking an insulator sequence. In a second strategy, vectors bearing second-generation TREs harboring repositioned tetracycline operator elements were used. Such vectors displayed greatly reduced leakiness in the absence of DOX and induced transgene expression levels were up to 522-fold above those seen in the absence of DOX.

CONCLUSIONS

Inducible lentivirus vectors bearing insulators or second-generation TREs will likely prove useful for applications demanding the lowest levels of background expression.

Conditional expression of a myocardium-specific transgene in zebrafish transgenic lines

To develop the first heart-specific tetracycline (Tet)-On system in zebrafish, we constructed plasmids in which the cardiac myosin light chain 2 promoter of zebrafish was used to drive the reverse Tet-controlled transactivator (rtTA) and the green fluorescent protein (GFP) reporter gene was preceded by an rtTA-responsive element. In the zebrafish fibroblast cell-line, rtTA-M2, one of rtTA's derivatives, demonstrated the highest increase in luciferase activity upon doxycycline (Dox) induction. We then generated two germ lines of transgenic zebrafish: line T03 was derived from microinjection of a plasmid containing rtTA-M2 and a plasmid containing a responsive reporter gene, whereas line T21 was derived from microinjection of a single dual plasmid. Results showed that line T21 was superior to line T03 in terms of greater GFP intensity after induction and with of minimal leakiness before induction. The photographic images of induced GFP in the heart of F2 larvae showed that the fluorescent level of GFP was dose-responsive. The level of GFP expressed in the F3 3 days postfertilization larvae that were treated with Dox for 1 hr decreased gradually after the withdrawal of the inducer; and the fluorescent signal disappeared after 5 days. The GFP induction and reduction were also tightly controlled by Dox in the F3 adult fish from line T21. This Tet-On system developed in zebrafish shows much promise for the study of the gene function in a specific tissue at the later developmental stage.

Optimal design of a single recombinant adeno-associated virus derived from serotypes 1 and 2 to achieve more tightly regulated transgene expression from nonhuman primate muscle

Recombinant adeno-associated virus (rAAV) vector supports long-term transgene expression from skeletal muscle in most mammals, including human. In some instances, the requirement for tight control of the transgene expression is expected. The original tetracycline-dependent system using the rtTA (Dox-on) transactivator displayed a baseline activity in the off state but improved versions are now available and need to be evaluated in a single-rAAV-vector strategy. In the present study we cloned, in three different orientations, the two expression cassettes responsible for doxycycline-mediated transgene regulation and further evaluated the basal and inducible activity of the recently described rtTA2S-S2, rtTA2S-M2, and rtTA2S-M2nls transactivators. Evaluations were conducted in vivo in mice and nonhuman primates using the respective homologous erythropoietin cDNA as a reporter gene because of its sensitive detection by ELISA. The woodchuck hepatitis virus posttranscriptional regulatory element sequence was also introduced to enhance further the stringency with respect to basal activity in the absence of inducer.

Approaches for trigger-inducible viral transgene regulation in gene-based tissue engineering

Recent advances in mammalian transgene expression dosing have resulted in a portfolio of mutually compatible systems that can adjust therapeutic transgene levels in response to antibiotics, hormone analogues, quorum-sensing messengers and secondary metabolites. The molecular merger of trigger-inducible expression technology with the latest generation of virus-derived transduction systems has enabled unmatched clinical interventions to shape desired therapeutic cell and tissue phenotypes for the treatment of complex human diseases.

Gene therapy for autoimmune diseases: quo vadis?

Biological therapies using antibodies and cytokines are becoming widespread for the treatment of chronic inflammatory autoimmune diseases. However, these treatments have several limitations - such as expense, the need for repeated injections and unwanted side-effects - that can be overcome by genetic delivery. This review summarizes the ingenuity, sophistication and variety of gene-therapy approaches that have been taken in the design of therapeutic molecules and vectors, the engineering of cells and the regulation of gene expression for the targeting of disease outcome. We focus our attention on multiple sclerosis, type 1 diabetes and rheumatoid arthritis.

Rat tyrosine hydroxylase promoter directs tetracycline-inducible foreign gene expression in dopaminergic cell types

A prerequisite for creating animal models in which gene expression is spatially and temporally controlled is the development of promoters to target genetic switches to specific populations of cells. Here we used the dopaminergic biosynthetic enzyme, tyrosine hydroxylase (TH) to test various combinations of tetracycline (Tet) system elements to determine the optimal configuration for inducible, tissue-specific expression. The present study shows that the degree of expression and level of leakiness associated with the Tet transactivators rtTA, rtTA2S-M2, tTS/rtTA or tTS/rtTA2S-M2 was dependent upon both the promoter and cell type utilized. Specifically, CMV-driven tTS/rtTA2S-M2 exhibited the highest level of inducibility in HEK cells (approximately 1000-fold) versus the dopaminergic cell line, MN9D (approximately 70-fold). In contrast, TH-driven rtTA2S-M2 yielded the highest level of expression with the least background in dopaminergic cell types versus HEK cells. Moreover, the TH promoter could be combined with the bi-directional Tet response system, BiTetO, allowing for the co-expression and regulation of two genes in the same cell. To further test the feasibility of this system we replaced the reporter gene with human Bcl-2. Consistent with previous studies, induction of Bcl-2 expression in dopaminergic cell types attenuated cell death due to the neurotoxin, MPP+. Taken together, these data suggest that targeted, inducible gene expression can be achieved in dopaminergic cell types.

Optimization of the Tet-on system to regulate interleukin 12 expression in the liver for the treatment of hepatic tumors

Interleukin 12 (IL-12) is a potent antitumoral cytokine, but it can be toxic at high doses. Therapy of liver tumors might benefit from the use of vectors enabling tight control of IL-12 expression in hepatic tissue for long periods of time. To this aim, we have improved the Tet-on system by modifying the minimal region of the inducible promoter and adjusting the level of the trans-activator using liver-specific promoters with graded activities. The resulting vectors allowed hepato-specific gene regulation with lower basal activity and higher inducibility compared with the original system in the absence of repressor molecules. The basal and final protein levels depend on the strength of the promoter that directs the transcripcional activator as well as the relative orientation of the two genes in the same plasmid. We have selected the construct combining minimal leakage with higher level of induced gene expression to regulate IL-12 after DNA transfer to mouse liver. Administration of doxycycline (Dox) enhanced IL-12 expression in a dose-dependent manner, whereas it was undetectable in serum in the noninduced state. Gene activation could be repeated several times, and sustained levels of IL-12 were achieved by daily administration of Dox. The antitumor effect of IL-12 was evaluated in a mouse model of metastatic colon cancer to the liver. Complete eradication of liver metastasis and prolonged survival was observed in all mice receiving Dox for 10 days. These data demonstrate the potential of a naked DNA gene therapy strategy to achieve tight control of IL-12 within the liver for the treatment of cancer.