Regulatable systems: applications in gene therapy and replicating viruses

Safety switches for adoptive cell therapy

Adoptive transfer of allogeneic and genetically modified T cells, such as CAR-T and TCR-T cells, can induce profound immune reactivity against cancer tissue. At the same time, these therapies are associated with severe off-target and on-target toxicities. For this reason, the development of genetic safety switches that can be used to control the activity of T cells in vivo has become an active field of research. With the spectrum of technologies developed, reversible control of cell products either by supply or removal of small molecules, by supply of protein-based regulators, or by physical stimuli such as light, ultrasound or heat, has become feasible. In this review, we describe the mechanistic classes of genetic safety switches, such as transcription-based or protein-based control of antigen receptors, split receptors, small molecule responsive antibodies, as well as universal remote controls, and discuss their advantages and limitations.

Therapeutic Applications of Aptamer-Based Riboswitches

Aptamers bind to their targets with high affinity and specificity through structure-based complementarity, instead of sequence complementarity that is used by most of the oligonucleotide-based therapeutics. This property has been exploited in using aptamers as multifunctional therapeutic units, by attaching them to therapeutic drugs, nanoparticles, or imaging agents, or as direct molecular decoys for inducing loss-of-function or gain-of-function of targets. One of the most interesting fields of aptamer application is their development as molecular sensors to regulate artificial riboswitches. Naturally, the riboswitches sense small-molecule metabolites and respond by regulating the expression of the corresponding metabolic genes. Riboswitches are cis-acting RNA structures that consist of the sensing (aptamer) and the regulating (expression platform) domains. In principle, diverse riboswitches can be engineered and applied to control different steps of gene expression in bacterial species as well as eukaryotes, by simply replacing aptamers against various endogenous and/or exogenous targets. Although these engineered aptamer-based riboswitches are recently gaining attention, it is clear that aptamer-based riboswitches have a potential for next-generation therapeutics against various diseases because of their controllability, specificity, and modularity in regulating gene expression through various cellular processes, including transcription, splicing, stability, RNA interference, and translation. In this review, we provide a summary of the recently developed and engineered aptamer-based riboswitches focusing on their therapeutic availability and further discuss their clinical potential.

Systematic transfer of prokaryotic sensors and circuits to mammalian cells

Prokaryotic regulatory proteins respond to diverse signals and represent a rich resource for building synthetic sensors and circuits. The TetR family contains >10(5) members that use a simple mechanism to respond to stimuli and bind distinct DNA operators. We present a platform that enables the transfer of these regulators to mammalian cells, which is demonstrated using human embryonic kidney (HEK293) and Chinese hamster ovary (CHO) cells. The repressors are modified to include nuclear localization signals (NLS) and responsive promoters are built by incorporating multiple operators. Activators are also constructed by modifying the protein to include a VP16 domain. Together, this approach yields 15 new regulators that demonstrate 19- to 551-fold induction and retain both the low levels of crosstalk in DNA binding specificity observed between the parent regulators in Escherichia coli, as well as their dynamic range of activity. By taking advantage of the DAPG small molecule sensing mediated by the PhlF repressor, we introduce a new inducible system with 50-fold induction and a threshold of 0.9 μM DAPG, which is comparable to the classic Dox-induced TetR system. A set of NOT gates is constructed from the new repressors and their response function quantified. Finally, the Dox- and DAPG- inducible systems and two new activators are used to build a synthetic enhancer (fuzzy AND gate), requiring the coordination of 5 transcription factors organized into two layers. This work introduces a generic approach for the development of mammalian genetic sensors and circuits to populate a toolbox that can be applied to diverse applications from biomanufacturing to living therapeutics.

Novel and emerging therapies safeguarding health of humans and their companion animals: a review

Modern medicine has helped to a great extent to eradicate and cure several diseases of mankind and animals. But the existence of incurable diseases like cancer, Acquired Immunodeficiency Syndrome (AIDS), diabetes or rheumatoid arthritis, side effects of allopathic medicine, increasing trend of antibiotic resistance and chemicals and biopesticides causing dietary risk have made the situation more critical than ever before. Thus, it has become a matter of concern for the scientists and researchers to develop novel therapies. Bacteriophage therapy to treat pathogenic bacterial infections, virophage therapy for conservation of global system and avian egg yolk antibody therapy for designing prophylactic strategies against Gastrointestinal (GI) diseases are interesting approaches. Others include the use of cytokines as adjunctive immunomodulators, gene therapy focusing on diseases caused by single gene defects, RNAi technology to suppress specific gene of interest and apoptins for cancer treatment. Stem cell therapy against several diseases and ailments has also been discussed. The use of nanoparticles for better drug delivery, even though costly, has been given equal importance. Nevertheless, immunomodulation, be it through physiological, chemical or microbial products, or through essential micronutrients, probiotics, herbs or cow therapy prove to be cost-effective, causing minimum adverse reactions when compared to allopathy. Development in the field of molecular biology has created an enormous impact on vaccine development. The present review deals with all these novel and emerging therapies essential to safeguard the health of humans and companion animals.

Cardiac gene therapy with SERCA2a: from bench to bedside

While progress in conventional treatments is making steady and incremental gains to reduce mortality associated with heart failure, there remains a need to explore potentially new therapeutic approaches. Heart failure induced by different etiologies such as coronary artery disease, hypertension, diabetes, infection, or inflammation results generally in calcium cycling dysregulation at the myocyte level. Recent advances in understanding of the molecular basis of these calcium cycling abnormalities, together with the evolution of increasingly efficient gene transfer technology, have placed heart failure within reach of gene-based therapy. Furthermore, the recent successful completion of a phase 2 trial targeting the sarcoplasmic reticulum calcium pump (SERCA2a) ushers in a new era for gene therapy for the treatment of heart failure. This article is part of a Special Section entitled "Special Section: Cardiovascular Gene Therapy".

Separating lentiviral vector injection and induction of gene expression in time, does not prevent an immune response to rtTA in rats

Background

Lentiviral gene transfer can provide long-term expression of therapeutic genes such as erythropoietin. Because overexpression of erythropoietin can be toxic, regulated expression is needed. Doxycycline inducible vectors can regulate expression of therapeutic transgenes efficiently. However, because they express an immunogenic transactivator (rtTA), their utility for gene therapy is limited. In addition to immunogenic proteins that are expressed from inducible vectors, injection of the vector itself is likely to elicit an immune response because viral capsid proteins will induce “danger signals” that trigger an innate response and recruit inflammatory cells.

Methodology and Principal Findings

We have developed an autoregulatory lentiviral vector in which basal expression of rtTA is very low. This enabled us to temporally separate the injection of virus and the expression of the therapeutic gene and rtTA. Wistar rats were injected with an autoregulatory rat erythropoietin expression vector. Two or six weeks after injection, erythropoietin expression was induced by doxycycline. This resulted in an increase of the hematocrit, irrespective of the timing of the induction. However, most rats only responded once to doxycycline administration. Antibodies against rtTA were detected in the early and late induction groups.

Conclusions

Our results suggest that, even when viral vector capsid proteins have disappeared, expression of foreign proteins in muscle will lead to an immune response.

Doxycycline regulated lentiviral vectors

Lentiviral vectors are a powerful tool to achieve regulated expression of transgenes in vivo and in vitro. The doxycycline-inducible system is well characterized and can be used to regulate expression mediated by lentiviral vectors. Because many different doxycycline-inducible lentiviral vectors have been described, choosing the best vector system can be difficult. This chapter can be used as a guide to select the optimal system for a particular application.

Activation of transgene expression in skeletal muscle by focused ultrasound

To correlate thermal dose from focused ultrasound (FUS) with gene expression and tissue injury, a temperature plateau strategy was employed. Plasmids encoding luciferase gene under the control of hsp70B promoter were transfected into the right gastrocnemius muscle in a rat via electroporation. One day after transfection, hind limbs were treated with 3.3-MHz focused ultrasound, using one of four different temperature plateaus with spatial-peak time-average focal temperatures (T(SPTA)) of 46 degrees C, 48 degrees C, 51 degrees C and 62 degrees C. The treatment duration at the plateau temperature was varied from 0 to 30s. Gene expression was analyzed in vivo one day following FUS treatment, and H&E staining was employed to assess tissue injury. Gene activation and tissue damage correlated closely with thermal dose. The highest level of gene activation was induced by FUS at T(SPTA)=51 degrees C for 20s, which was found to be statistically equivalent to that produced by water-bath hyperthermia.

Molecular engineering of viral gene delivery vehicles

Viruses can be engineered to efficiently deliver exogenous genes, but their natural gene delivery properties often fail to meet human therapeutic needs. Therefore, engineering viral vectors with new properties, including enhanced targeting abilities and resistance to immune responses, is a growing area of research. This review discusses protein engineering approaches to generate viral vectors with novel gene delivery capabilities. Rational design of viral vectors has yielded successful advances in vitro, and to an extent in vivo. However, there is often insufficient knowledge of viral structure-function relationships to reengineer existing functions or create new capabilities, such as virus-cell interactions, whose molecular basis is distributed throughout the primary sequence of the viral proteins. Therefore, high-throughput library and directed evolution methods offer alternative approaches to engineer viral vectors with desired properties. Parallel and integrated efforts in rational and library-based design promise to aid the translation of engineered viral vectors toward the clinic.

Development of an immortalised, post-pubertal gonadotrophin-releasing hormone neuronal cell line

Gonadotrophin-releasing hormone (GnRH) is important in reproduction, although some of the mechanisms for its synthesis and release remain elusive. Progress in understanding the GnRH neurone has been hampered by the limited number and diffuse distribution of the neurone in the mammalian brain. Several stable GnRH-expressing cell lines have been developed using in vivo expression of the simian virus 40 T Antigen (TAg), and they have been helpful for the study of gene expression and neuronal function. However, expression of an immortalising gene may interfere with normal cellular function. We developed a novel GnRH-secreting cell line transgenic mouse model suitable for targeted transformation in post-pubertal mice using a tetracycline-regulated TAg transgene. This clonal cell line, GRT, expresses neuronal markers and GnRH. GRT cells grown in medium containing tetracycline-free serum express increasing mRNA levels of GnRH associated with declining levels of TAg expression. The novelty and ultimately the usefulness of this cell line is that TAg expression, which could affect the GnRH neuronal phenotype, can be regulated by tetracycline.

Laser-based gene transfection and gene therapy

The plasma membrane of mammalian cells can be transiently permeablized by optical means and exogenous materials or genes can be introduced into the cytoplasm of living cells. Until now, few mechanisms were exploited for the manipulation: laser is directly and tightly focused on the cells for optoinjection, laser-induced stress waves, photochemical internalization, and irradiation of selective cell targeting with light-absorbing particles. During the past few years, extensive progress and numerous breakthroughs have been made in this area of research. This review covers four different laser-assisted transfection techniques and their advantages and disadvantages. Universality towards various cell lines is possibly the main advantage of laser-assisted optoporation in comparison with presently existing methods of cell transfection.

Synthetic tetracycline-inducible regulatory networks: computer-aided design of dynamic phenotypes

BACKGROUND

Tightly regulated gene networks, precisely controlling the expression of protein molecules, have received considerable interest by the biomedical community due to their promising applications. Among the most well studied inducible transcription systems are the tetracycline regulatory expression systems based on the tetracycline resistance operon of Escherichia coli, Tet-Off (tTA) and Tet-On (rtTA). Despite their initial success and improved designs, limitations still persist, such as low inducer sensitivity. Instead of looking at these networks statically, and simply changing or mutating the promoter and operator regions with trial and error, a systematic investigation of the dynamic behavior of the network can result in rational design of regulatory gene expression systems. Sophisticated algorithms can accurately capture the dynamical behavior of gene networks. With computer aided design, we aim to improve the synthesis of regulatory networks and propose new designs that enable tighter control of expression.

RESULTS

In this paper we engineer novel networks by recombining existing genes or part of genes. We synthesize four novel regulatory networks based on the Tet-Off and Tet-On systems. We model all the known individual biomolecular interactions involved in transcription, translation, regulation and induction. With multiple time-scale stochastic-discrete and stochastic-continuous models we accurately capture the transient and steady state dynamics of these networks. Important biomolecular interactions are identified and the strength of the interactions engineered to satisfy design criteria. A set of clear design rules is developed and appropriate mutants of regulatory proteins and operator sites are proposed.

CONCLUSION

The complexity of biomolecular interactions is accurately captured through computer simulations. Computer simulations allow us to look into the molecular level, portray the dynamic behavior of gene regulatory networks and rationally engineer novel ones with useful applications. We are able to propose, test and accept or reject design principles for each network. Guided by simulations, we develop a set of design principles for novel tetracycline-inducible networks.

New strategies for cardiovascular gene therapy: regulatable pre-emptive expression of pro-angiogenic and antioxidant genes

Cardiovascular diseases are among the major targets for gene therapy. Initially, clinical experiments of gene transfer of vascular endothelial growth factor (VEGF) improved vascularization and prevented the amputation in patients with critical leg ischemia. However, the majority of trials did not provide conclusive results and therefore further preclinical studies are required. Importantly, data indicate the necessity of regulated expression of angiogenic factors, particularly VEGF, to obtain the therapeutic effect. It is also suggested that the combined delivery of two or more genes may improve the formation of mature vasculature and therefore may be more effective in the amelioration of ischemia. Moreover, experimental approaches in animal models displayed the promise of gene transfer modulating the inflammatory processes and oxidant status of the cells. Particularly, the concept of preemptive gene therapy has been tested, and recent studies have demonstrated that overexpression of heme oxygenase-1 or extracellular superoxide dismutase can prevent heart injury by myocardial infarction induced several weeks after gene instillation. The combination of a preemptive strategy with regulated gene expression, using the vectors in which the therapeutic transgene is driven by exogenously or endogenously controllable promoter, offers another modality. However, we hypothesize that regulatable gene therapy, dependent on the activity of endogenous factors, might be prone to limitations owing to the potential disturbance in the expression of endogenous genes. Here, we demonstrated some indications of these drawbacks. Therefore, the final acceptance of these promising strategies for clinical trials requires careful validation in animal experiments.

Gene therapy: twenty-first century medicine

Broadly defined, the concept of gene therapy involves the transfer of genetic material into a cell, tissue, or whole organ, with the goal of curing a disease or at least improving the clinical status of a patient. A key factor in the success of gene therapy is the development of delivery systems that are capable of efficient gene transfer in a variety of tissues, without causing any associated pathogenic effects. Vectors based upon many different viral systems, including retroviruses, lentiviruses, adenoviruses, and adeno-associated viruses, currently offer the best choice for efficient gene delivery. Their performance and pathogenicity has been evaluated in animal models, and encouraging results form the basis for clinical trials to treat genetic disorders and acquired diseases. Despite some initial success in these trials, vector development remains a seminal concern for improved gene therapy technologies.

Liver-specific expression of interferon gamma following adenoviral gene transfer controls hepatitis B virus replication in mice

Interferons control viral replication and the growth of some malignant tumors. Since systemic application may cause severe adverse effects, tissue-specific expression is an attractive alternative. Liver-directed interferon gene therapy offers promising applications such as chronic viral hepatitis B or C or hepatocellular carcinoma and thus needs testing in vivo in suitable animal models. We therefore used the Tet-On system to regulate gene expression in adenoviral vectors, and studied the effect of liver-specific and regulated interferon gamma expression in a mouse model of chronic hepatitis B virus (HBV) infection. In a first generation adenoviral vector, genes encoding for firefly luciferase and interferons alpha, beta or gamma, respectively, were coexpressed under control of the bidirectional tetracycline-regulated promoter P(tet)bi. Liver-specific promoters driving expression of the reverse tetracycline controlled transactivator ensured local expression in the livers of HBV transgenic mice. Following gene transfer, we demonstrated low background, tight regulation and a 1000-fold induction of gene expression by doxycycline. Both genes within the bidirectional transcription unit were expressed simultaneously, and in a liver-specific fashion in cell culture and in living mice. Doxycycline-dependent interferon gamma expression effectively controlled HBV replication in mice, but did not eliminate HBV transcripts. This system will help to study the effects of local cytokine expression in mouse disease models in detail.

New complex Ad vectors incorporating both rtTA and tTS deliver tightly regulated transgene expression both in vitro and in vivo

Regulation of transgene expression is a major goal of gene therapy research. Previously, we have developed a complex adenovirus (Ad) vector with tetracycline-regulated expression of a Fas ligand (FasL)-green fluorescent protein (GFP) fusion protein. This vector delivered high levels of activity that was regulated by doxycycline. However, this regulation was limited by the low but significant background activity of the TRE promoter. Recently, the Tet-regulated transcriptional silencer, tTS, was reported to suppress efficiently basal TRE activity without affecting induced expression levels. Here, we report development of Ad vectors that incorporate tTS in combination with that of reverse transactivator (rtTA) coupled with TRE promoter driving transgene expression. Incorporation of tTS improved control of transgene expression in vitro, so that an induction range of over three orders of magnitude was achieved in some cell lines. Effective regulation of transgene expression was also seen in a mouse model in vivo, following systemic vector delivery. In the case of FasL-GFP expression, significant improvement in the control of apoptotic activity both in vitro and in a mouse hepatotoxicity model was demonstrated when using rtTA-tTS vectors. In conclusion, a highly effective transgene regulation system, deliverable by a single adenoviral vector, is now available.

Construction of a novel expression system on a human artificial chromosome

Efficient regulation of transgene would greatly facilitate the analysis of gene function in biological systems for basic research and clinical applications. The tetracycline-regulatable system (TRS) has proven to be a promising tool for such purposes. Despite their widespread application, a number of challenges are still associated with the use of TRS, including clonal variability in the regulation and copy number. We have recently constructed a novel human artificial chromosome (HAC) called 21DeltaqHAC. By housing a TRS-based DNA-PKcs expression cassette in this HAC, we were able to circumvent the problems associated with conventional TRS-based vectors. We achieved tight control of DNA-PKcs expression and rescued the radiosensitive phenotype of DNA-PKcs-deficient CHO cells. The combined use of HAC and the TRS serves as a model for controllable and fixed copy number expression vectors. Our study also demonstrates the suitability of the HAC to accommodate multi-subunit constructs such as that of the TRS.

Comparison of single regulated lentiviral vectors with rtTA expression driven by an autoregulatory loop or a constitutive promoter

Regulated expression of a therapeutic gene is crucial for safe and efficacious gene therapy. Many inducible regulatory systems use a constitutive promoter to express a regulatory protein, such as rtTA in the Tet-On system, which may restrict their use because of cytotoxicity and immunogenicity. Autoregulatory expression of rtTA provides extremely low levels of rtTA when transgene expression is off, with rapid transgene induction upon addition of doxycycline. Lentiviral vectors efficiently transfer genes to dividing and non-dividing cells with long-term gene expression both in vitro and in vivo. We compared regulatory function in a single lentiviral vector where rtTA was either expressed from a constitutive promoter or placed in an autoregulatory loop. Autoregulatory expression of rtTA was superior to constitutive promoter expression, resulting in higher viral titers, undetectable levels of both rtTA and transgene expression in the absence of doxycycline, improved induction kinetics and increased induction levels in all cells tested. We further expanded the utility of the autoregulatory vector by using an improved rtTA variant with an increased sensitivity to doxycycline. This lentiviral vector with doxycycline-regulated transgene expression may be useful for gene therapy applications and in experimental settings where strict temporal expression of a transgene is required.

Identification and characterization of novel human glioma-specific peptides to potentiate tumor-specific gene delivery

Glioblastomas account for approximately 20% of all primary brain tumors in adults. Glioblastoma multiforme (GBM) is a highly malignant tumor. In spite of advances in surgery, chemotherapy, and radiotherapy, the life expectancy of the patient with glioblastoma is approximately 11 months. To enhance glioma-specific gene delivery, we employed a 12-mer phage display peptide library to isolate phages that bind specifically to human glioma cell lines. Here, we report the isolation and functional characterization of novel glioma-specific peptides that target transgenes specifically to a wide array of human glioblastomas in vitro and in vivo. One of the isolated peptides, tentatively denoted as MG11, is demonstrated to be glioma specific and gives an in vitro-binding enrichment of more than 5-fold for glioma cells when compared with nonglioma cells. Intravenous injection of phages bearing the MG11 peptide-binding motif enables the phages to home specifically to glioma xenografts. Most significantly, when Lissamine rhodamine-labeled MG11 peptide is injected intratumorally, it targets specifically to glioma xenografts instead of non-glioma-derived xenografts. In summary, our results suggest that the MG11 peptide is able to target specifically to tumors of glial origin, which would allow the design of applications related to the diagnosis and treatment of human gliomas.

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.

HLA-A*0201-restricted cytolytic responses to the rtTA transactivator dominant and cryptic epitopes compromise transgene expression induced by the tetracycline on system

The tetracycline-controlled transcription system (Tet-on) is widely used to regulate gene expression in mammalian cells. In gene therapy applications, immune responses to Tet-on proteins such as the rtTA transcription factor have been reported, raising concerns about their occurrence in humans. To monitor the HLA class I cytolytic responses against Tet-on regulators, we characterized the immunogenic CD8+ epitopes within rtTA and tTS regulators using HLA-A*0201 class I transgenic mice. Epitope prediction programs, HLA-A*0201 binding assays, and peptide immunization were used to select a set of immunogenic peptides within rtTA and tTS sequences. To identify further the rejection epitopes, we expressed Tet-on protein components in vivo and found a single dominant rtTA186 CTL epitope in the rtTA tetracycline repressor domain. Target cells expressing rtTA were susceptible to CTL lysis, and rtTA expression compromised muscle transgene engraftment. To reduce the occurrence of immune responses to rtTA protein, we mutated the dominant rtTA186 epitope and found that this leads to the appearance of subdominant epitopes. As a result, we think that an epitope modification strategy is not applicable to blunt the immune response in this model. Moreover, the identification of HLA-A*0201 rtTA epitopes allowed us to demonstrate here that the delivery of the Tet-on system with weakly immunogenic rAAV vectors does not trigger primary CTL responses in mice, in contrast to DNA transfer. Altogether, the existence of HLA-A*0201 rtTA epitopes may lead to the occurrence of immune responses depending on vectors and local inflammation in gene therapy applications involving rtTA-based regulatory systems.

Dynamics of gene regulatory networks with cell division cycle

This paper focuses on modeling and analyzing the nonlinear dynamics of gene regulatory networks with the consideration of a cell division cycle with duplication process of DNA, in particular for switches and oscillators of synthetic networks. We derive two models that may correspond to the eukaryotic and prokaryotic cells, respectively. A biologically plausible three-gene model ( lac, tetR, and cI ) and a repressilator as switch and oscillator examples are used to illustrate our theoretical results. We show that the cell cycle may play a significant role in gene regulation due to the nonlinear dynamics of a gene regulatory network although gene expressions are usually tightly controlled by transcriptional factors.

A caspase-6 and anti-human epidermal growth factor receptor-2 (HER2) antibody chimeric molecule suppresses the growth of HER2-overexpressing tumors

Clinical studies have suggested that human epidermal growth factor receptor-2 (HER2) provide a useful target for antitumor therapy. We previously described the generation of a chimeric HER2-targeted immunocasp-3 protein. In this study, we extend the repertoire of chimeric proapoptotic proteins with immunocasp-6, a construct that comprises a HER2-specific single-chain Ab, a single-chain Pseudomonas exotoxin A, and an active caspase-6, which can directly cleave lamin A leading to nucleus damage and inducing programmed cell death. We demonstrate that the secreted immunocasp-6 molecule selectively recognizes and induces apoptosis in HER2-overexpressing tumor cells in vitro, but not in cells with undetectable HER2. The immunocasp-6 gene was next transferred into BALB/c athymic mice bearing human breast SK-BR-3 tumors by i.m. injection of liposome-encapsulated vectors, by intratumor injection of adenoviral vectors, or by i.v. injection of PBMC modified by retroviral infection. Regardless of the method used, expression of immunocasp-6 suppressed tumor growth and prolonged animal survival significantly. Our data show that the chimeric immunocasp-6 molecule can recognize HER2-positive tumor cells, promptly attack their nucleus, and induce their apoptotic death, suggesting the potential of this strategy for the treatment of human cancers that overexpress HER2.

Prolonged and inducible transgene expression in the liver using gutless adenovirus: a potential therapy for liver cancer

BACKGROUND AND AIMS

Gene therapy of liver diseases would benefit from systems allowing prolonged, regulable, and tissue-specific transgene expression. We attempted to produce a vector fulfilling these requirements.

METHODS

We generated gutless adenoviral vectors containing a mifepristone (RU486)-inducible system for controlled and liver-specific expression of human interleukin-12 (hIL-12) (GL-Ad/RUhIL-12) and mouse IL-12 (mIL-12) (GL-Ad/RUmIL-12). The properties of these vectors were tested both in vitro and in vivo.

RESULTS

Infection of cells with GL-Ad/RUhIL-12 resulted in high level of hIL-12 expression in the presence of RU486 only in hepatocytic cells. In animals injected with GL-Ad/RUhIL-12, the administration of RU486 induced a transient rise of serum hIL-12 that peaked at 10 hours and completely disappeared by 72 hours. The peak value of hIL-12 was dependent on the doses of the vector and the inducer. High and sustained serum levels of hIL-12 could be attained by continuing administration of RU486 every 12 or 24 hours. Repetitive induction of hIL-12 could be obtained over, at least, a period of 48 weeks after a single injection of GL-Ad/RUhIL-12. Although the vector was detected in many tissues after systemic injection, transcription of the transgene was only found in the liver. Treatment of liver metastases with 5 x 10(8) infectious units of GL-Ad/RUmIL-12 plus RU846 resulted in complete tumor regression in all animals.

CONCLUSION

Gutless adenoviral vectors allow liver-specific and regulable transgene expression for prolonged periods of time. These vectors are promising tools for gene therapy of liver cancer and could also be useful for other forms of hepatic disease.

Conditional suppression of cellular genes: lentivirus vector-mediated drug-inducible RNA interference

RNA interference has emerged as a powerful technique to downregulate the expression of specific genes in cells and in animals, thus opening new perspectives in fields ranging from developmental genetics to molecular therapeutics. Here, we describe a method that significantly expands the potential of RNA interference by permitting the conditional suppression of genes in mammalian cells. Within a lentivirus vector background, we subjected the polymerase III promoter-dependent production of small interfering RNAs to doxycycline-controllable transcriptional repression. The resulting system can achieve the highly efficient and completely drug-inducible knockdown of cellular genes. As lentivirus vectors can stably transduce a wide variety of targets both in vitro and in vivo and can be used to generate transgenic animals, the present system should have broad applications.

Conditional suppression of cellular genes: lentivirus vector-mediated drug-inducible RNA interference

RNA interference has emerged as a powerful technique to downregulate the expression of specific genes in cells and in animals, thus opening new perspectives in fields ranging from developmental genetics to molecular therapeutics. Here, we describe a method that significantly expands the potential of RNA interference by permitting the conditional suppression of genes in mammalian cells. Within a lentivirus vector background, we subjected the polymerase III promoter-dependent production of small interfering RNAs to doxycycline-controllable transcriptional repression. The resulting system can achieve the highly efficient and completely drug-inducible knockdown of cellular genes. As lentivirus vectors can stably transduce a wide variety of targets both in vitro and in vivo and can be used to generate transgenic animals, the present system should have broad applications.

Modeling genetic switches with positive feedback loops

In this paper, we develop a new methodology to design synthetic genetic switch networks with multiple genes and time delays, by using monotone dynamical systems. We show that the networks with only positive feedback loops have no stable oscillation but stable equilibria whose stability is independent of the time delays. In other words, such systems have ideal properties for switch networks and can be designed without consideration of time delays, because the systems can be reduced from functional spaces to Euclidian spaces. Therefore, we can ensure that the designed switches function correctly even with uncertain delays. We first prove the basic properties of the genetic networks composed of only positive feedback loops, and then propose a procedure to design the switches, which drastically simplifies analysis of the switches and makes theoretical analysis and design tractable even for large-scaled systems. Finally, to demonstrate our theoretical results, we show biologically plausible examples by designing a synthetic genetic switch with experimentally well investigated lacI, tetR, and cI genes for numerical simulation.

Regulated production of proteins from muscle using gene transfer: potential therapeutic applications

The ability to produce high-level transgene expression following the introduction of genetic material into a host cell has been well documented. Various vectors and methods for in vivo gene delivery have been shown to provide long-term expression from many different tissue types in rodents and large animals. However, many potential therapeutic targets for gene therapy involve the production of proteins that are toxic or lead to undesirable effects if overexpressed. Thus, the ability to achieve regulated gene expression following treatment will be required to ensure the safety of long-acting gene therapy products. Skeletal muscle, in particular, has been widely used as a target for gene therapy protocols, due to the ease of accessibility and ability to produce and secrete some proteins at very high levels. This review focuses on regulated gene therapy systems that are being evaluated for use in muscle, and discusses two classes of system: those dependent on exogenously administered drugs and those dependent on endogenously produced metabolites.

Tetracycline-regulated gene expression in replication-incompetent herpes simplex virus vectors

Although herpes simplex virus (HSV) vectors appear to have great potential as gene delivery vectors both in vitro and in vivo, the expression of foreign genes in such vectors cannot be easily regulated. Of the known eukaryotic regulatory systems, the tetracycline-inducible gene expression system is perhaps the most widely used because of its induction characteristics and because of the well-known pharmacological properties of tetracycline (Tet) and analogs such as doxycycline. Here, we describe the adaptation of the Tet-inducible system for use in replication-incompetent HSV vectors. HSV vectors were constructed that contained several types of Tet-inducible promoters for foreign gene expression. These promoters contained a tetracycline response element (TRE) linked to either a minimal cytomegalovirus (CMV) immediate-early promoter, a minimal HSV ICP0 promoter, or a truncated HSV ICP0 promoter containing one copy of the HSV TAATGARAT cis-acting immediate-early regulatory element (where R represents a prime base). All three promoter constructs were regulated appropriately by doxycycline, as shown by the expression of the marker gene lacZ in cell lines engineered to express Tet transactivators. The ICP0 promoter constructs expressed the highest and most sustained levels of lacZ, but the CMV promoter construct had the highest relative level of induction, suggesting their use in different applications. To extend the utility of Tet-regulated HSV vectors, vectors were constructed that coexpressed an inducible Tet transactivator in addition to the inducible lacZ marker gene. This modification resulted in tetracycline-inducible gene expression that was not restricted to specific cell lines, and this vector was capable of inducible expression in irreversibly differentiated NT2 cells (NT-neurons) for several days. Finally, HSV vectors were constructed that expressed modified Tet transactivators, resulting in improved induction properties and indicating the flexibility of the Tet-regulated system for regulation of foreign gene expression in HSV vector-infected cells.

Lack of an immune response against the tetracycline-dependent transactivator correlates with long-term doxycycline-regulated transgene expression in nonhuman primates after intramuscular injection of recombinant adeno-associated virus

We previously documented persistent regulation of erythropoietin (Epo) secretion in mice after a single intramuscular (i.m.) injection of a recombinant adeno-associated virus (rAAV) vector harboring both the tetracycline-dependent transactivator (rtTA) and the Epo cDNA (D. Bohl, A. Salvetti, P. Moullier, and J. M. Heard, Blood 92:1512-1517, 1998). Using the same vector harboring the cynomolgus macaque Epo cDNA instead, the present study evaluated the ability of the tetracycline-regulatable (tetR) system to establish long-term transgene regulation in nonhuman primates. The vector was administered i.m., after which 5-day induction pulses were performed monthly for up to 13 months by using doxycycline (DOX), a tetracycline analog. We show that initial inductions were successful in all individuals and that there was a tight regulation and a rapid deinduction pattern upon DOX withdrawal. For one macaque, regulation of Epo secretion was maintained during the entire experimental period; for the five remaining macaques, secreted Epo became indistinguishable from endogenous Epo upon repeated DOX inductions. We investigated the mechanism involved and showed that, except in the animal in which secretion persisted, delayed humoral and cellular immune responses were directed against the rtTA transactivator protein associated with the reduction of vector DNA in transduced muscles. This study provides some evidence that, when the immune system is not mobilized against the rtTA transactivator, the tetR-regulatable system is able to support long-term transgene regulation in the context of an rAAV in nonhuman primates. In addition, our results suggest potential improvements for vector design.

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

Control of gene expression for gene therapy application requires the design of a sophisticated system embodying multiple properties. The ideal system should present the following features: (1) low or undetectable gene expression in the absence of inducer; (2) strong expression upon induction; and (3) fast kinetics of induction in the presence of inducers and rapid reversal of induction after its withdrawal. To evaluate these parameters, the features of the latest generation tetracycline-sensitive reverse-transactivator (rtTA2(s)-M2) alone or in combination with Tet-repressor (tTS-Kid) were explored in the context of helper-dependent adenovirus vector. Various genetic elements were assembled in a series of vectors and the ability to control secreted alkaline phosphatase expression evaluated in vitro in HeLa cells and in vivo by intramuscular injection in both C57/B6 and Balb/C nude mice. The results allow us to draw some general conclusions about the combination of transcription regulators and their relative orientation to the transgene to achieve maximal induction, while minimizing leakiness of expression.

Long-term doxycycline-regulated secretion of erythropoietin by encapsulated myoblasts

We developed an ex vivo gene therapy approach for the regulated delivery of therapeutic proteins based on the implantation of encapsulated, genetically engineered C(2)C(12) myoblasts. We investigated doxycycline-based regulation of gene expression to modulate the secretion of erythropoietin (EPO) from encapsulated myoblasts in a mouse model. An autoregulatory tet-off system provided high induction levels with low basal expression in the noninduced state. Stable C(2)C(12) clones constitutively secreted between 25 and 50 IU mouse EPO/10(6)cells/24 hours in the on-state. The clone C15, selected for its in vivo survival characteristics, displayed a desirable secretion profile when encapsulated. Devices released 5 IU EPO per capsule in the on-state, with EPO levels being undetectable upon the addition of doxycycline (dox). Capsules subcutaneously implanted in DBA/2J mice demonstrated a tightly regulated secretion of EPO through up to four on-off cycles during a period lasting 40 weeks. Hematocrits could be modulated between basal levels (40-50%) and elevated levels (70-90%) through the presence or absence of dox in the drinking water. Hematocrit returned to normal levels, paralleling the kinetics observed following capsule explantation, 6 to 8 weeks following dox administration to polycythemic mice. The results of this study suggest that encapsulation and implantation of a tet-off regulated C(2)C(12) cell clone represents a safe method for the controlled long-term delivery of proteins in vivo.

Genetic approaches to craniofacial tissue repair

This review discusses in some detail the opportunities and challenges of applying gene therapy to the important clinical problem of wound repair and regeneration.

Endogenous regulation of a therapeutic transgene restores homeostasis in arthritic joints

The treatment of chronic inflammatory diseases is complicated by their unpredictable, relapsing clinical course. Here, we describe a new strategy in which an inflammation-regulated therapeutic transgene is introduced into the joints to prevent recurrence of arthritis. To this end, we designed a recombinant adenoviral vector containing a two-component, inflammation-inducible promoter controlling the expression of human IL-10 (hIL-10) cDNA. When tested in vitro, this system had a low-level basal activity and was activated four to five orders of magnitude by various inflammatory stimuli, including TNF-alpha, IL-1 beta, IL-6, and LPS. When introduced in joints of rats with recurrent streptococcal cell wall-induced arthritis, the IL-10 transgene was induced in parallel with disease recurrence and effectively prevented the influx of inflammatory cells and the associated swelling of the joints. Levels of inflammation-inducible hIL-10 protein within the joints correlated closely with the severity of recurrence. An endogenously regulated therapeutic transgene can thus establish negative feedback and restore homeostasis in vivo while minimizing host exposure to the recombinant drug.

Chemokines: agents for the immunotherapy of cancer?

Chemokines, a superfamily of small cytokine-like molecules, regulate leukocyte transport in the body. In recent years, we have witnessed the transition of immunotherapeutic strategies from the laboratory to the bedside. Here, we review the role of chemokines in tumour biology and the development of the host's anti-tumour defence. We summarize the current knowledge of chemokine-receptor expression by relevant cellular components of the immune system and the role of their ligands in the organization of the antitumour immune response. Finally, we discuss recent findings which indicate that chemokines have therapeutic potential as adjuvants or treatments in antitumour immunotherapy, as well as remaining questions and perspectives for translating experimental evidence into clinical practice.

Gene therapy strategy for long-term myocardial protection using adeno-associated virus-mediated delivery of heme oxygenase gene

BACKGROUND

Ischemia and oxidative stress are the leading mechanisms for tissue injury. An ideal strategy for preventive/protective therapy would be to develop an approach that could confer long-term transgene expression and, consequently, tissue protection from repeated ischemia/reperfusion injury with a single administration of a therapeutic gene. In the present study, we used recombinant adeno-associated virus (rAAV) as a vector for direct delivery of the cytoprotective gene heme oxygenase-1 (HO-1) into the rat myocardium, with the purpose of evaluating this strategy as a therapeutic approach for long-term protection from ischemia-induced myocardial injury.

METHODS AND RESULTS

Human HO-1 gene (hHO-1) was delivered to normal rat hearts by intramyocardial injection. AAV-mediated transfer of the hHO-1 gene 8 weeks before acute coronary artery ligation and release led to a dramatic reduction (>75%) in left ventricular myocardial infarction. The reduction in infarct size was accompanied by decreases in myocardial lipid peroxidation and in proapoptotic Bax and proinflammatory interleukin-1beta protein abundance, concomitant with an increase in antiapoptotic Bcl-2 protein level. This suggested that the transgene exerts its cardioprotective effects in part by reducing oxidative stress and associated inflammation and apoptotic cell death.

CONCLUSIONS

This study documents the beneficial therapeutic effect of rAAV-mediated transfer, before myocardial injury, of a cytoprotective gene that confers long-term myocardial protection from ischemia/reperfusion injury. Our data suggest that this novel "pre-event" gene transfer approach may provide sustained tissue protection from future repeated episodes of injury and may be beneficial as preventive therapy for patients with or at risk of developing coronary ischemic events.

A system for small-molecule control of conditionally replication-competent adenoviral vectors

Replication-competent adenoviral vectors are potentially far more efficient than replication-defective vectors. However, for reasons of safety, there is a need to restrict viral replication both spatially, by limiting replication to certain cell types, and temporally. To control replication temporally, we have developed a system, based on the small-molecule dimerizer rapamycin, for regulating the replication of adenoviral vectors. In this system, one adenoviral vector, AdC4, expresses transcription factors whose activity is regulated by the non-immunosuppressive rapamycin analog AP21967. A second vector, Ad(Z12-I-E1aE1b19k), contains E1 genes placed downstream of binding sites for the regulated transcription factor. Co-infection of several cell lines by the vector pair leads to dimerizer-dependent E1 expression and an increase in viral replication, as shown by Southern blots and replication assays. Furthermore, expression of a reporter gene from a replication-defective vector, Ad-GM-CSF, can be augmented by up to 18-fold by co-infection with the pair of conditionally replicating vectors in the presence of dimerizer. Similar results are obtained when the vectors are directly injected into subcutaneous HT1080 xenograft tumors in nude mice. We believe that vectors based on this principle will be a useful additional tool to enhance efficiency and safety of gene delivery for anti-cancer therapy.

Stringent control of gene expression in vivo by using novel doxycycline-dependent trans-activators

The tetracycline (Tet)-dependent regulatory system has been widely used for controlling gene expression. The Tet-on version of the system, in which the reverse Tet-responsive transcriptional activator (rtTA) is positively regulated by Tet or its analogs, such as doxycycline (Dox), is of potential utility for gene therapy applications in humans. However, rtTA may display a high basal activity, especially when delivered in vivo by using episomal vectors such as plasmids. Two novel Dox-inducible activators, called rtTA2(S)-S2 and rtTA2(S)-M2, which have a significantly lower basal activity than rtTA in stably transfected cell lines, have been described. In this study we tested the capability of these trans-activators to control expression of mouse erythropoietin (mEpo) and to modulate hematocrit (Hct) increase in vivo on delivery of plasmids into quadriceps muscles of adult mice by DNA electroinjection. Both rtTA2(S)-M2 and rtTA2(S)-S2 displayed a considerably lower background activity and higher window of induction than rtTA in vivo. Moreover, a stringent control of mEpo gene expression and Hct levels in the absence of any background activity was maintained over a 10-month period by injecting as little as 1 microg of a single plasmid containing the rtTA2(S)-S2 expression cassette and the Tet-responsive mEpo cDNA. This constitutes the first report of a stringent ligand-dependent control of gene expression in vivo obtained by delivering a single plasmid encoding both the trans-activator and the regulated gene. Notably, the rtTA2(S)-S2-based system was induced by oral doses of doxycycline comparable to those normally used in clinical practice in humans.

Gene therapy: promises and problems

Gene therapy can be broadly defined as the transfer of genetic material to cure a disease or at least to improve the clinical status of a patient. One of the basic concepts of gene therapy is to transform viruses into genetic shuttles, which will deliver the gene of interest into the target cells. Based on the nature of the viral genome, these gene therapy vectors can be divided into RNA and DNA viral vectors. The majority of RNA virus-based vectors have been derived from simple retroviruses like murine leukemia virus. A major shortcoming of these vectors is that they are not able to transduce nondividing cells. This problem may be overcome by the use of novel retroviral vectors derived from lentiviruses, such as human immunodeficiency virus (HIV). The most commonly used DNA virus vectors are based on adenoviruses and adeno-associated viruses. Although the available vector systems are able to deliver genes in vivo into cells, the ideal delivery vehicle has not been found. Thus, the present viral vectors should be used only with great caution in human beings and further progress in vector development is necessary.

Recombinant proteins for neurodegenerative diseases: the delivery issue

Tackling neurodegenerative diseases represents a formidable challenge for our ageing society. Recently, major achievements have been made in understanding the molecular mechanisms responsible for such diseases, and, simultaneously, numerous proteins such as neurotrophic factors, anti-apoptotic or anti-oxidant have been identified as potential therapeutic agents. Although many neurotrophic factors have been tested on individuals suffering from various neurodegenerative disorders, to date none has shown efficacy. Inadequate protein delivery is believed to be part of the problem. Recent improvements in pump technology, as well as in cell and gene therapy, are providing innovative ways to allow localized, regulatable delivery of proteins in brain parenchyma, opening new avenues for clinical trials in the not so distant future.