Inducible control of gene expression: prospects for gene therapy

A simple, rapid fluorescent reporter-based method for detection of ectopic cre recombinase expression in presumed retinal cell type-targeted mouse lines

Although cell type-specific Cre recombinase-expressing mouse lines are commonly used to generate conditional knockout of genes of interest, germline recombination and ectopic "leakiness" in Cre recombinase expression in non-specific cell types has been observed in several neuronal and glial-specific Cre lines. This often leads to inadvertent loss of conditional mouse lines, requiring rederivation. It is therefore imperative to be able to monitor and validate cell type-specific Cre recombinase-mediated gene editing. Herein, we describe a simple, inexpensive, rapid ZsGreen fluor-reporter-based strategy for genotype-free identification of ectopic leakiness using a custom-designed, 3-D blue LED light box. We assessed cell type-specific expression in several allegedly specific Cre recombinase mouse lines commonly used in vision research: retinal pigment epithelium (RPE)-specific (VMD2 (Best1) Cre, RPE65 Cre); astrocyte-specific (GFAP Cre); as well as photoreceptor-bipolar progenitor cell-specific (CRX Cre). Our standardized workflow allows facile, rapid identification of ectopic and non-specific Cre recombinase expression in any presume specific Cre mouse line, without the need for genotyping and without causing animal distress.

Gene therapy strategies to improve strength and quality of flexor tendon healing

INTRODUCTION

Rupture of the repair and adhesion around a tendon are two major problems after tendon surgery. Novel biological therapies which enhance healing and reduce adhesions are goals of many investigations. Gene therapy offers a new and promising approach to tackle these difficult problems. In the past decade, we sought to develop methods to augment tendon healing and reduce tendon adhesion through gene therapy.

AREAS COVERED

This review discusses the methods and results of adeno-associated viral (AAV) type 2 vector gene therapy to increase tendon healing strength and reduce adhesions in a chicken model. Micro-RNA related gene therapy is also discussed. We also developed a controlled release system, which incorporates nanoparticles to deliver micro-RNAs to regulate tendon healing.

EXPERT OPINION

We obtained promising results of enhancement of tendon healing strength in a chicken model using AAV2-mediated gene transfer. AAV2-mediated micro-RNA transfer also limited adhesions around the tendon. Controlled release systems incorporating nanoparticles have ideally delivered genes to the healing tendons and resulted in a moderate (but incomplete) reduction of adhesions. It remains to be determined what the best doses are and what other factors are in play in adhesion formation. These are two targets in our future investigations.

Regulation of gene expression in human prostate cancer cells with artificially constructed promoters that are activated in response to ultrasound stimulation

We chose promoters responsive to sonication in LNCap cells, a prostate cancer cell line, out of a library composed of DNA fragments constructed by linking the TATA box sequence to randomly combined cis-acting elements of transcription factors activated in response to radiation in prostate cancer cells. When a plasmid containing the luciferase gene under control of a promoter was transfected into LNCap cells and sonicated with 1 MHz ultrasound at 0.5 W/cm(2), 10% DF for 60s, 13 promoters showed more than 10-fold enhancement compared with their counterparts without sonication 12h after sonication. As to their responsiveness to sonication, the best two promoters were then compared to clone 880-8, a derivative from clone 880 that was created by random introduction of point mutations and was shown to have an improved response to X-ray irradiation. We then took clone 880-8 for further analyses since it showed the highest enhancement to sonication, though not statistically significant from the others. Next, we employed a retrovirus vector and stably introduced the luciferase gene under control of clone 880-8 into LNCap cells to establish a cell line. When the cell line was sonicated with 1 MHz ultrasound at 0.5 W/cm(2), 10% DF for 60s, luciferase expression was enhanced up to 14.8-fold 12h after sonication. We then established another cell line by replacing the luciferase gene with the fcy::fur gene, a suicide gene, and when the cell line was sonicated with 1 MHz ultrasound at 0.5 W/cm(2), 10% DF for 60s, expression of the gene was enhanced, showing the maximum expression 12-24h after sonication. When the cells were incubated in medium containing 5-fluorocytosine, cell survival ratio decreased dose dependently with 5-fluorocytosine only after sonication treatment, suggesting this promoter could be utilized for gene expression control with ultrasound.

Effects of anticancer agents and scavengers on CMV-promoter-driven exogenous gene expression in genetically modified cells

Objectives

This study aimed to investigate whether the levels of rsGFP mRNA and the fluorescence levels of cytomegalovirus (CMV)-promoter-driven rsGFP (red-shifted green fluorescent protein) could be changed by using anticancer agents and also to examine the effects of co-treatment with anticancer agents and scavengers.

Methods

The pQBI25 vector, which encodes the CMV promoter and the cDNA for rsGFP, was transfected into FR cells (rat skin fibroblast cell line). FR-pQBI25 cells were then exposed to doxorubicin, 5-fluorouracil, methotrexate or paraquat with or without scavengers such as N-acetyl cysteine (NAC) and edaravone for 48 h.

Key findings

The levels of rsGFP mRNA were found to be significantly higher following doxorubicin, 5-fluorouracil and paraquat treatment but were not changed by methotrexate. These levels of rsGFP mRNA were found to be significantly lower after paraquat/edaravone co-treatment compared with paraquat alone. The fluorescence levels of rsGFP were found to be significantly higher following doxorubicin and paraquat treatment but were not changed by 5-fluorouracil and methotrexate. The levels were also found to be significantly lower after paraquat/edaravone co-treatment compared with paraquat alone and also after doxorubicin/NAC co-treatment compared with doxorubicin alone.

Conclusions

These findings suggest that CMV-promoter-driven exogenous gene expression may be partly regulated by reactive oxygen species.

Semi-synthetic ecdysteroids as gene-switch actuators: synthesis, structure-activity relationships, and prospective ADME properties

The ligand-inducible, ecdysteroid receptor (EcR) gene-expression system can add critical control features to protein expression in cell and gene therapy. However, potent natural ecdysteroids possess absorption, distribution, metabolism and excretion (ADME) properties that have not been optimised for use as gene-switch actuators in vivo. Herein we report the first systematic synthetic exploration of ecdysteroids toward modulation of gene-switch potency. Twenty-three semi-synthetic O-alkyl ecdysteroids were assayed in both a natural insect system (Drosophila B(II) cells) and engineered gene-switch systems in mammalian cells using Drosophila melanogaster, Choristoneura fumiferana, and Aedes aegypti EcRs. Gene-switch potency is maintained, or even enhanced, for ecdysteroids methylated at the 22-position in favourable cases. Furthermore, trends toward lower solubility, higher permeability, and higher blood-brain barrier penetration are supported by predicted ADME properties, calculated using the membrane-interaction (MI)-QSAR methodology. The structure-activity relationship (SAR) of alkylated ecdysteroids indicates that 22-OH is an H-bond acceptor, 25-OH is most likely an H-bond donor, and 2-OH and 3-OH are donors and/or acceptors in network with each other, and with the EcR. The strategy of alkylation points the way to improved ecdysteroidal actuators for switch-activated gene therapy.

Regulation of CMV promoter-driven exogenous gene expression with doxorubicin in genetically modified cells

The regulation of gene expression after the introduction of an exogenous gene is a problematic aspect of gene therapy. The purpose of this study was to use doxorubicin to regulate exogenous gene expression in a vector containing the cytomegalovirus (CMV) promoter. The pQBI25 vector, which encodes the CMV promoter and the cDNA for red-shifted green fluorescent protein (rsGFP), was transfected into a rat skin fibroblast cell line (FR cells). The pEGFP vector, encoding the CMV promoter and enhanced green fluorescent protein (EGFP) cDNA, was transfected into human hepatoma HepG2 cells. FR-pQBI25 cells were then continuously exposed to doxorubicin and methotrexate for 96 and 48 h, respectively; HepG2-pEGFP cells were continuously exposed to doxorubicin for 48 h. The levels of c-fos, c-jun and rsGFP mRNA, as well as the levels of rsGFP protein, in the FR-pQBI25 cells were found to be significantly higher following exposure to doxorubicin. However, the level of rsGFP protein was not changed by exposure to methotrexate. The level of EGFP protein in the HepG2-pEGFP cells was also significantly higher following exposure to doxorubicin. To examine the effect of cessation of doxorubicin exposure, FR-pQBI25 cells that had been exposed to doxorubicin for 48 h were re-plated in fresh medium without doxorubicin for a further 48 h. The increased levels of c-fos, c-jun and rsGFP mRNA and rsGFP protein seen after treatment with doxorubicin had reduced by 48 h after the cessation of exposure to doxorubicin. These findings suggest that CMV-driven exogenous gene expression may be regulated by doxorubicin.

Inhibition of cardiomyocyte automaticity by electrotonic application of inward rectifier current from Kir2.1 expressing cells

A biological pacemaker might be created by generation of a cellular construct consisting of cardiac cells that display spontaneous membrane depolarization, and that are electrotonically coupled to surrounding myocardial cells by means of gap junctions. Depending on the frequency of the spontaneously beating cells, frequency regulation might be required. We hypothesized that application of Kir2.1 expressing non-cardiac cells, which provide I (K1) to spontaneously active neonatal cardiomyocytes (NCMs) by electrotonic coupling in such a cellular construct, would generate an opportunity for pacemaker frequency control. Non-cardiac Kir2.1 expressing cells were co-cultured with spontaneously active rat NCMs. Electrotonic coupling between the two cell types resulted in hyperpolarization of the cardiomyocyte membrane potential and silencing of spontaneous activity. Either blocking of gap-junctional communication by halothane or inhibition of I (K1) by BaCl(2) restored the original membrane potential and spontaneous activity of the NCMs. Our results demonstrate the power of electrotonic coupling for the application of specific ion currents into an engineered cellular construct such as a biological pacemaker.

IPRO: an iterative computational protein library redesign and optimization procedure

A number of computational approaches have been developed to reengineer promising chimeric proteins one at a time through targeted point mutations. In this article, we introduce the computational procedure IPRO (iterative protein redesign and optimization procedure) for the redesign of an entire combinatorial protein library in one step using energy-based scoring functions. IPRO relies on identifying mutations in the parental sequences, which when propagated downstream in the combinatorial library, improve the average quality of the library (e.g., stability, binding affinity, specific activity, etc.). Residue and rotamer design choices are driven by a globally convergent mixed-integer linear programming formulation. Unlike many of the available computational approaches, the procedure allows for backbone movement as well as redocking of the associated ligands after a prespecified number of design iterations. IPRO can also be used, as a limiting case, for the redesign of a single or handful of individual sequences. The application of IPRO is highlighted through the redesign of a 16-member library of Escherichia coli/Bacillus subtilis dihydrofolate reductase hybrids, both individually and through upstream parental sequence redesign, for improving the average binding energy. Computational results demonstrate that it is indeed feasible to improve the overall library quality as exemplified by binding energy scores through targeted mutations in the parental sequences.

Creating new specific ligand-receptor pairs for transgene regulation

The creation of specifically matched ligand-receptor pairs that are orthogonal to naturally present interacting pairs is essential for the development of small molecule-regulated gene expression systems for biotechnological applications. However, for many years this task has represented a significant challenge for synthetic chemists and protein engineers. Recently, Doyle and colleagues demonstrated that highly specific ligand-receptor pairs can be engineered in a rapid fashion by creating large libraries of protein variants and applying a selection scheme to identify variants with improved activation by the target synthetic ligand.

Directed evolution of specific receptor-ligand pairs for use in the creation of gene switches

Despite their versatility and power in controlling gene regulation in nature, nuclear hormone receptors (NHRs) have largely eluded utility in heterologous gene regulation applications such as gene therapy and metabolic engineering. The main reason for this void is the pleiotropic interference of the receptor-ligand combination with regulatory networks in the host organism. In recent years, numerous strategies have been developed to engineer ligand-receptor pairs that do not cross-interact with host regulatory pathways. However, these strategies have either met with limited success or cannot be readily extended to other ligand-receptor pairs. Here, we present a simple, effective, and readily generalizable strategy for reengineering NHRs to respond specifically to a selected synthetic ligand. The method involves generation of genetic diversity by stepwise individual site saturation mutagenesis of a fixed set of ligand-contacting residues and random point mutagenesis, followed by phenotypic screening based on a yeast two-hybrid system. As a test case, this method was used to alter the specificity of the NHR human estrogen receptor alpha in favor of the synthetic ligand 4,4'-dihydroxybenzil, relative to the natural ligand 17beta-estradiol, by >10(7)-fold. The resulting ligand-receptor pair is highly sensitive to the synthetic ligand in human endometrial cancer cells and is essentially fully orthogonal to the wild-type receptor-natural ligand pair. This method should provide a powerful, broadly applicable tool for engineering receptors/enzymes with improved or novel ligand/substrate specificity.

Stringent rosiglitazone-dependent gene switch in muscle cells without effect on myogenic differentiation

We have developed a gene switch based on the human transcription factor peroxisome proliferator-activated receptor gamma (PPARgamma) and its activation by rosiglitazone. However, ectopic expression of PPARgamma has been demonstrated to convert myogenic cells into adipocyte-like cells and, more generally, may interfere with the physiology of the target tissue. Consequently we modified the DNA-binding specificity of PPARgamma, resulting in a transcription factor that we named PPAR*. We demonstrated by histological and molecular assessment of cell phenotype that the overexpression of PPAR* did not alter the myogenic differentiation program of G8 myoblasts. We showed that PPAR* does not transactivate promoters containing PPARgamma-responsive elements but transactivates promoters containing PPAR*-responsive elements that are at least 80% identical to a 20-bp consensus. We improved the rosiglitazone-dependent gene switch by tuning PPAR* expression with a scaffold/matrix attachment region and by expressing both PPAR* and the reporter gene under the control of PPAR*-responsive elements. Treatment of cultured murine muscle cells (myotubes) with rosiglitazone induced reporter gene expression from assay background up to the level attained by a CMV I/E promoter-enhancer. These results indicate the potential of the PPAR* gene switch for use in gene therapy applications.

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.

Potential of gene therapy for pediatric neurotransmitter diseases: lessons from Parkinson's disease

Gene therapy methods have continued to develop rapidly, and many initial limitations that hampered clinical application have been overcome. Thus serious consideration of clinical application of gene therapy is warranted for selected disorders in which the pathogenesis is well defined. Parkinson's disease has been the most extensively studied target of gene therapy for central nervous system disorders and shares many features with pediatric neurotransmitter diseases. Neurotransmitter replacement therapy using catecholamine-synthesizing genes and delivery of neurotrophic factors such as glial cell line-derived neurotrophic factors has been successful in animal models of Parkinson's disease. Application of gene therapy for pediatric neurotransmitter diseases will require delineating the optimal set of genes to correct the consequences of the deficiencies. The optimal anatomical targets and proper timing of the gene replacement must be understood. Safety of gene therapy vehicles and the ability to regulate gene expression will be essential for eventual clinical application.

Manipulation of gene expression by an ecdysone-inducible gene switch in tumor xenografts

BACKGROUND

Rapid, robust and reversible induction of transgene expression would significantly facilitate cancer gene therapy as well as allow the in vivo functional study of newly discovered genes in tumor formation and progression. The popularity of the ecdysone inducible gene switch system has led us to investigate whether such a system can successfully regulate gene expression in a syngeneic tumor system in vivo.

RESULTS

MBT-2 and Panc02 carcinoma cells were transfected with components of a modification of the ecdysone switch system driving firefly luciferase (F-Luc). In vitro luciferase expression +/- ecdysone analog GS-E indicated a robust induction with minimal baseline activity and complete decay after 24 hours without drug. In vitro selection of MBT-2 transfected cell clones which had complete absence of F-Luc expression in the absence of stimulation but which expressed this gene at high levels in response to GS-E were chosen for in vivo evaluation. Tumors from engineered MBT-2 cells were grown to 5 mm in diameter prior to GS-E administration, animals euthanized and tumors removed at 6, 12 and 24 hours after GS-E administration and assayed for F-Luc activity. GS-E resulted in a maximal induction of F-Luc activity at 6 hours in tumor tissue with almost complete reversion to control levels by 12 hours.

CONCLUSIONS

This study is the first demonstration that robust and reversible transgene expression in tumors is feasible using the ecdysone system, allowing future rapid in vivo functional characterization of gene function or gene therapy applications.

Gene therapy of monogenic and cardiovascular disorders

The concept of gene therapy involves the introduction of genetic material into patient cells to cure or alleviate the symptoms of a disease by complementing a damaged gene or by giving the cell a new function. The belief that gene therapy would soon reach the clinic has been widely spread, frequently resulting in controversies when these expectations were not met. Nevertheless, over the last 10-year period, the experience from a number of clinical trials has taught us that gene transfer is technically feasible, but that the gene delivery vehicles, or vectors, for the transfer of genetic material are still suboptimal and that treatment may have severe side effects. This review will provide examples of different genetic disorders for which gene therapy is an option and has been attempted. It will also briefly discuss the existing vector systems and mention their advantages and drawbacks.

p73 can suppress the proliferation of cells that express mutant p53

Mutation of the p53 tumor suppressor gene is the most common genetic alteration in human cancer. p73, a member of the p53 family, has been found to exhibit activity similar to that of p53, including the ability to induce growth arrest and apoptosis. p53 and p73 have a high percentage of similarity at several domains, including the DNA binding domain. This domain in p53 is the location of missense mutations in many human cancers. Mutant p53, which cannot suppress cell proliferation, has been found to have a dominant-negative activity that inactivates wild-type p53. To determine the effects of mutant p53 on wild-type p73, we have established cell lines expressing both mutant p53 and wild-type p73 in a dual-inducible system. This system expresses mutant p53 in a tetracycline-repressible system and p73beta in an ecdysone-inducible system in a p53-null lung carcinoma parental cell line. We have found that wild-type p73beta, in the presence of mutant p53, retains the ability to transactivate p21 and suppresses cell growth through induction of both cell cycle arrest and apoptosis. In addition, in cell lines expressing wild-type p53 and wild-type p73beta, we have found that these proteins cooperate to additively transactivate p21 and suppress cell proliferation.

Modulation of translation-initiation in CHO-K1 cells by rapamycin-induced heterodimerization of engineered eIF4G fusion proteins

Translation-initiation is a predominant checkpoint in mammalian cells which controls protein synthesis and fine-tunes the flow of information from gene to protein. In eukaryotes, translation-initiation is typically initiated at a 7-methyl-guanylic acid cap posttranscriptionally linked to the 5' end of mRNAs. Alternative cap-independent translation-initiation involves 5' untranslated regions (UTR) known as internal ribosome entry sites, which adopt a particular secondary structure. Translation-initiating ribosome assembly at cap or IRES elements is mediated by a multiprotein complex of which the initiation factor 4F (eIF4F) consisting of eIF4A (helicase), eIF4E (cap-binding protein), and eIF4G is a major constituent. eIF4G is a key target of picornaviral protease 2A, which cleaves this initiation factor into eIF4G(Delta) and (Delta)eIF4G to redirect the cellular translation machinery exclusively to its own IRES-containing transcripts. We have designed a novel translation control system (TCS) for conditional as well as adjustable translation of cap- and IRES-dependent transgene mRNAs in mammalian cells. eIF4G(Delta) and (Delta)eIF4G were fused C- and N-terminally to the FK506-binding protein (FKBP) and the FKBP-rapamycin-binding domain (FRB) of the human FKBP-rapamycin-associated protein (FRAP), respectively. Rapamycin-induced heterodimerization of eIF4G(Delta)-FKBP and FRB-(Delta)eIF4G fusion proteins reconstituted a functional chimeric elongation factor 4G in a dose-dependent manner. Rigorous quantitative expression analysis of cap- and IRES-dependent SEAP- (human placental secreted alkaline phosphatase) and luc- (Photinus pyralis luciferase) encoding reporter constructs confirmed adjustable translation control and revealed increased production of desired proteins in response to dimerization-induced heterologous eIF4G in Chinese hamster ovary (CHO-K1) cells.

A novel, inducible, eukaryotic gene expression system based on the quorum-sensing transcription factor TraR

Bacteria adapt their pattern of gene expression in response to a variety of external cues, including fluctuations in population density. This type of bacterial cell-to-cell communication is referred to as quorum-sensing. Quorum-sensing systems are present in many bacterial species and constitute a large collection of ligands and cognate receptors. The availability of such diversity offers interesting opportunities for biotechnological exploitation. We describe here the transformation of the quorum-sensing system of Agrobacterium tumefaciens into a transcription regulatory system that works in mammalian cells. The A. tumefaciens TraR protein was fused to the eukaryotic activation domain of NF-kappaB p65, generating a novel chimaeric transcriptional activator that stimulates gene transcription in different human cell lines from a minimal promoter containing the TraR DNA recognition sequence in the presence of the Agrobacterium quorum-sensing signal molecule N-(3-oxo-octanoyl)homoserine lactone (3-oxo-C(8)-HSL). The basal level of transcription was low in the absence of 3-oxo-C(8)-HSL, and gene expression was stimulated up to 1,000-fold at a saturating concentration of 3-oxo-C(8)-HSL.

Novel CNBP- and La-based translation control systems for mammalian cells

Throughout the development of Xenopus, production of ribosomal proteins (rp) is regulated at the translational level. Translation control is mediated by a terminal oligopyrimidine element (TOP) present in the 5' untranslated region (UTR) of rp-encoding mRNAs. TOP elements adopt a specific secondary structure that prevents ribosome-binding and translation-initiation of rp-encoding mRNAs. However, binding of CNBP (cellular nucleic acid binding protein) or La proteins to the TOP hairpin structure abolishes the TOP-mediated transcription block and induces rp production. Based on the specific CNBP-TOP/La-TOP interactions we have designed a translation control system (TCS) for conditional as well as adjustable translation of desired transgene mRNAs in mammalian cells. The generic TCS configuration consists of a plasmid encoding CNBP or La under control of the tetracycline-responsive expression system (TET(OFF)) and a target expression vector containing a TOP module between a constitutive P(SV40) promoter and the human model product gene SEAP (human secreted alkaline phosphatase) (P(SV40)-TOP-SEAP-pA). The TCS technology showed excellent SEAP regulation profiles in transgenic Chinese hamster ovary (CHO) cells. Alternatively to CNBP and La, TOP-mediated translation control can also be adjusted by artificial phosphorothioate anti-TOP oligodeoxynucleotides. Confocal laser-scanning microscopy demonstrated cellular uptake of FITC-labeled oligodeoxynucleotides and their localization in perinuclear organelles within 24 hours. Besides their TOP-based translation-controlling capacity, CNBP and La were also shown to increase cap-independent translation from polioviral internal ribosomal entry sites (IRES) and La alone to boost cap-dependent translation initiation. CNBP and La exemplify for the first time the potential of RNA-binding proteins to exert translation control of desired transgenes and to increase heterologous protein production in mammalian cells. We expect both of these assets to advance current gene therapy and biopharmaceutical manufacturing strategies.

Gene therapy in cardiovascular disease. Current status

Cardiovascular disease is the leading cause of mortality and morbidity in developed countries. Most conventional therapy is often inefficacious and tends to treat the symptoms rather than the underlying causes of the disorder. Gene therapy offers a novel approach for prevention and treatment of cardiovascular diseases. Technical advances in viral vector systems and the development of fusigenic liposome vectors have been crucial to the development of effective gene therapy strategies directed at the vasculature and myocardium in animal models. Gene transfer techniques are being evaluated as potential treatment alternatives for both genetic (familial hypercholesterolemia) and acquired occlusive vascular diseases (atherosclerosis, restenosis, arterial thrombosis) as well as for cardiac disorders including heart failure, myocardial ischemia, graft coronary arteriosclerosis and hypertension. Continued technologic advances in vector systems and promising results in human and animal gene transfer studies make the use of gene therapy a promising strategy for the treatment of cardiovascular disorders.

Shear-induced degradation of plasmid DNA

The majority of gene therapy clinical trials use plasmid DNA that is susceptible to shear-induced degradation. Many processing steps in the extraction, purification, and preparation of plasmid-based therapeutics can impart significant shear stress that can fracture the phosphodiester backbone of polynucleotides, and reduce biological activity. Much of the mechanistic work on shear degradation of DNA was conducted over 30 years ago, and we rely heavily on this early work in an attempt to explain the empirical observations of more recent investigations concerning the aerosolization of plasmids. Unfortunately, the sporadic reports of shear degradation in the literature use different experimental systems, making it difficult to quantitatively compare results and reach definitive mechanistic conclusions. In this review, we describe the forces imparted to DNA during shear stress, and use published data to quantitatively evaluate their relative effects. In addition, we discuss the effects of molecular weight, strain rate, particle size, flexibility, ionic strength, gas-liquid interfaces, and turbulence on the fluid flow degradation of supercoiled plasmid DNA. Finally, we speculate on computational methods that might allow degradation rates in different experimental systems to be predicted.

Ecdysone-controlled expression of transgenes

Inducible expression systems show great potential for use in human gene therapy and systems based on insect ecdysone receptors are particularly promising candidates. This article describes such systems and reviews actual and potential uses of ecdysone-controlled transgenes in vitro and in vivo. The ligand specificity of ecdysone receptor-based systems is considered, along with the safety and efficacy of the ecdysteroid and non-steroidal compounds used to activate them.

Glucocorticoid-regulated VEGF expression in ischemic skeletal muscle

Vascular endothelial growth factor (VEGF) is a potent neovascular inducer. Gene therapeutic delivery of a plasmid DNA encoding VEGF has been shown to impart collateral vessel development in animal models of hindlimb ischemia. Constitutive, long-lived expression of VEGF through gene transfer, however, may result in hypervascularization and/or leaky blood vessels. To that end, the introduction of regulated VEGF gene transfer technology may provide a safer and more controlled therapy for ischemic tissues. We developed a glucocorticoid-regulated plasmid vector (pNGVL-hAP/GRE(5)-vegf-pA) for modulating VEGF gene expression. This plasmid possessed five tandem repeats of the glucocorticoid-responsive element and adenovirus major-late promoter driving the expression of the VEGF(165) cDNA. Intramuscular delivery of this plasmid to mice, and subsequent treatment with the synthetic glucocorticoid dexamethasone (DEX), led to greatly enhanced VEGF expression. Similar delivery to the gracillis muscle of New Zealand white rabbits that had undergone ligation of their femoral artery to induce ischemia exhibited increased VEGF expression and collateral vessel development only in the presence of DEX. Additionally, reintroduction of DEX at a time point during which initial VEGF transgene levels had subsided resulted in a vigorous reinduction of VEGF transgene expression. This new iteration of VEGF gene delivery provides for fine-tuned angiogenic factor-based therapy for tissues requiring neovascularization.

Conditional gene targeting for cancer gene therapy

Current treatment of solid tumors is limited by severe adverse effects, resulting in a narrow therapeutic index. Therefore, cancer gene therapy has emerged as a targeted approach that would significantly reduce undesired side effects in normal tissues. This approach requires a clear understanding of the molecular biology of both the malignant clone and the biological vectors that serve as vehicles to target cancer cells. In this review we discuss novel approaches for conditional gene expression in cancer cells. Targeting transgene expression to malignant tissues requires the use of specific regulatory elements including promoters based on tumor biology, tissue-specific promoters and inducible regulatory elements. We also discuss the regulation of both replication and transgene expression by conditionally-replicative viruses. These approaches have the potential to restrict the expression of transgenes exclusively to tissues of interest and thereby to increase the therapeutic index of future vectors for cancer gene therapy.

Genetically engineered mice and their use in aging research

Genetically engineered animal models have been and will continue to be invaluable for exploring the basic mechanisms involved in the aging process as well as in extending our understanding of diseases found to be more prevalent in the older human population. Continued development of such in vivo systems will allow scientists to further dissect the role genetic and environmental factors play in aging and in age-related disease states and to enhance our understanding of these processes. In this article we discuss techniques involved in the development of such models and review some examples of laboratory mouse strains that have been used to study either normal aging or select diseases associated with aging.

Genetic reshuffling reconstitutes functional expression cassettes in retroviral vectors

BACKGROUND

A major prerequisite for the design of retroviral vectors encoding cell toxic or harmful genes is the possibility to tightly control gene expression, thus limiting activity to the relevant target cells and protecting the packaging cell used for production of recombinant viral particles.

METHODS

In the present study a system was developed in which genetic reshuffling during the retroviral life cycle is exploited, allowing reconstitution of functional expression cassettes from separate elements exclusively in transduced target cells. For construction of these murine leukaemia virus (MLV)-based reconstituting viral vectors (ReCon), a promoterless inverted enhanced green fluorescent protein (EGFP) reporter gene cassette was inserted in place of the U3 region of the 3' LTR. Subsequently, the human ubiquitin promoter was inserted in the inverse orientation into the R/U5 border of the 5' LTR of the vector.

RESULTS

PA317 packaging cells stably transfected with ReCon vectors were established and EGFP expression was analysed by fluorescence-activated cell sorting (FACS). After detection of low-level background expression, an additional polyadenylation signal was introduced in antisense orientation into the 3' LTR at the R/U5 border to prevent accidental read-through transcription from neighbouring cellular promoters. Virus-containing cell culture supernatants were then used to infect NIH3T3 target cells. EGFP expression, recloning and sequencing of integrated proviruses demonstrated the correct reassembly of the transduced ubiquitin/EGFP transcription unit in these infected cells.

CONCLUSIONS

This facile and convenient system should allow production of retroviral vectors encoding potentially toxic proteins, cell cycle inhibitors or inducers of apoptosis, all of which would interfere with vector production if expressed in the retroviral packaging cell.

Regulated gene expression in glioma cells: a comparison of three inducible systems

Transcription control systems enable experimental regulation of transgene expression in eukaryotic cells by application of specific repressors or inducers. Currently used inducible systems include tetracycline (tet-off), dimerizer and ecdysone systems. While numerous studies have utilized a single system, their comparative performance under identical conditions remains unclear. We therefore compared the efficiency of these three systems in C6 glioma cells by using transient transfection and the lacZ reporter gene. Highest induced activity was found with the ecdysone system, followed by tet-off and dimerizer systems. Both lowest repressed activity and highest regulation were revealed with the dimerizer system, followed by ecdysone and tet-off systems. Our data suggest that the most appropriate system depends on the experimental procedures, the application and the gene to be regulated.

Identification of ligands and coligands for the ecdysone-regulated gene switch

The ecdysone-inducible gene switch is a useful tool for modulating gene expression in mammalian cells and transgenic animals. We have identified inducers derived from plants as well as certain classes of insecticides that increase the versatility of this gene regulation system. Phytoecdysteroids share the favorable kinetics of steroids, but are inert in mammals. The gene regulation properties of one of these ecdysteroids have been examined in cell culture and in newly developed strains of ecdysone-system transgenic mice. Ponasterone A is a potent regulator of gene expression in cells and transgenic animals, enabling reporter genes to be turned on and off rapidly. A number of nonsteroidal insecticides have been identified that also activate the ecdysone system. Because the gene-controlling properties of the ecdysone switch are based on a heterodimer composed of a modified ecdysone receptor (VgEcR) and the retinoid X receptor (RXR), we have tested the effect of RXR ligands on the VgEcR/RXR complex. Used alone, RXR ligands display no activity on the ecdysone switch. However, when used in combination with a VgEcR ligand, RXR ligands dramatically enhance the absolute levels of induction. This property of the heterodimer has allowed the development of superinducer combinations that increase the dynamic range of the system.

Glucocorticoid-inducible retrovector for regulated transgene expression in genetically engineered bone marrow stromal cells

Transplantable bone marrow stromal cells can be utilized for cell therapy of mesenchymal disorders. They can also be genetically engineered to express synthetic transgenes and subsequently serve as a platform for systemic delivery of therapeutic proteins in vivo. Inducible production of therapeutic proteins would markedly enhance the usefulness of stromal cells for cell therapy applications. We determined whether synthetic corticosteroid hormones can be used to tightly control transgene expression via the glucocorticoid response pathway in primary bone marrow stromal cells. This regulatory mechanism does not require the presence of potentially immunogenic prokaryotic or chimeric "Trans-activators." Further, synthetic corticosteroids are pharmaceutical agents that can be readily used in vivo. We designed a self-inactivating retroviral vector in which expression of the green fluorescent protein (GFP) reporter is controlled by a minimal synthetic promoter composed of five tandem glucocorticoid response elements upstream of a TATAA box. Vesicular stomatitis virus G-pseudotyped retroparticles were synthesized and utilized to transduce cultured cell lines and primary rat bone marrow stromal cells. We have shown that primary rat bone marrow stromal cells could be efficiently engineered with our GRE-containing retrovector, basal reporter expression was low in the absence of exogenous synthetic corticosteroids, and GFP expression was dexamethasone inducible and reversible. To summarize, this strategy allows dexamethasone-induced, "on-demand" transgene expression from transplantable genetically engineered bone marrow stromal cells.

A novel means of drug delivery: myoblast-mediated gene therapy and regulatable retroviral vectors

A potentially powerful approach to drug delivery in the treatment of disease involves the use of cells to introduce genes encoding therapeutic proteins into the body. Candidate genes for delivery include those encoding secreted factors that could have broad applications ranging from treatment of inherited single-gene deficiencies to acquired disorders of the vasculature or cancer. Myoblasts, the proliferative cell type of skeletal muscle tissues, are potent tools for stable delivery of a gene of interest into the body, as they become an integral part of the muscle into which they are injected, in close proximity to the circulation. The recent development of improved tetracycline-inducible retroviral vectors allows for fine control of recombinant gene expression levels. The combination of ex vivo gene transfer using myoblasts and regulatable retroviral vectors provides a powerful toolbox with which to develop gene therapies for a number of human diseases.

Applications of developmental biology to medicine and animal agriculture

With the complete sequence of the human genome expected by winter 2001, genomic-based drug discovery efforts of the pharmaceutical industry are focusing on finding the relatively few therapeutically useful genes from among the total gene set. Methods to rapidly elucidate gene function will have increasing value in these investigations. The use of model organisms in functional genomics has begun to be recognized and exploited and is one example of the emerging use of the tools of developmental biology in recent drug discovery efforts. The use of protein products expressed during embryo-genesis and the use of certain pluripotent cell populations (stem cells) as candidate therapeutics are other applications of developmental biology to the treatment of human diseases. These agents may be used to repair damaged or diseased tissues by inducing or directing developmental programs that recapitulate embryonic processes to replace specialized cells. The activation or silencing of embryonic genes in the disease state, particularly those encoding transcription factors, is another avenue of exploitation. Finally, the direct drug-induced manipulation of embryonic development is a unique application of developmental biology in animal agriculture.

Gene therapy and metabolic engineering

Gene therapy involves the introduction of normal, healthy genes into cells to correct the underlying cause of a wide variety of inherited and acquired diseases. Future progress in developing effective clinical protocols involving gene therapy for the treatment of cellular dysfunction associated with disease may incorporate metabolic engineering. Metabolic engineering can be applied to gene therapy for the successful identification of disease genes; elucidation of disease pathways; development of safe and efficient gene-delivery systems; and regulation and control of gene expression. Cystic fibrosis, cancer, and diabetes are reviewed as examples of diseases where gene therapy approaches are being studied.

Gene therapy: designer promoters for tumour targeting

One of the biggest challenges facing cancer therapy is to generate tumour-specific treatment strategies. Gene therapy hopes to achieve this by directing the activity of therapeutic genes specifically to the sites of disease. Of paramount importance for the success of this approach is the availability of tumour-specific delivery systems: both the transductional targeting of the vector vehicle and the restriction of transgene expression to the tumour are promising strategies towards this goal. This review will focus on the recent achievements in the field of transcriptional targeting and the different strategies to improve or design promoters with the desired specificities.

Vectors for gene therapy of cardiovascular disease

Several phase I/II clinical trials are currently ongoing in gene therapy of cardiovascular disease. Whereas the indications vary, including peripheral artery disease, ischemic heart disease, post-angioplasty restenosis, and vein graft failure, these trials are mostly based on the use of adenoviral vectors and nonviral vectors. Novel vectors aimed at improving the efficacy and safety of gene delivery in target organs, such as heart, skeletal muscle, vasculature, and liver, have been recently generated. Some of them have already been successfully validated in preclinical models of cardiovascular disease. This review focuses on the most recent advances in vector development that could substantially increase the spectrum of cardiovascular pathologies amenable to gene transfer-based treatments.

Components of vectors for gene transfer and expression in mammalian cells

Progress in diverse scientific fields has been realized partly by the continued refinement of mammalian gene expression vectors. A growing understanding of biological processes now allows the design of vector components to meet specific objectives. Thus, gene expression in a tissue-selective or ubiquitous manner may be accomplished by selecting appropriate promoter/enhancer elements; stabilization of labile mRNAs may be effected through removal of 3' untranslated regions or fusion to heterologous stabilizing sequences; protein targeting to selected tissues or different organelles is carried out using specific signal sequences; fusion moieties effect the detection, enhanced yield, surface expression, prolongation of half-life, and facile purification of recombinant proteins; and careful tailoring of the codon content of heterologous genes enhances protein production from poorly translated transcripts. The use of viral as well as nonviral genetic elements in vectors allows the stable replication of episomal elements without the need for chromosomal integration. The development of baculovirus vectors for both transient and stable gene expression in mammalian cells has expanded the utility of such vectors for a broad range of cell types. Internal ribosome entry sites are now widely used in many applications that require coexpression of different genes. Progress in gene targeting techniques is likely to transform gene expression and amplification in mammalian cells into a considerably less labor-intensive operation. Future progress in the elucidation of eukaryotic protein degradation pathways holds promise for developing methods to minimize proteolysis of specific recombinant proteins in mammalian cells and tissues.

Long-term doxycycline-controlled expression of human tyrosine hydroxylase after direct adenovirus-mediated gene transfer to a rat model of Parkinson's disease

Developments of technologies for delivery of foreign genes to the central nervous system are opening the field to promising treatments for human neurodegenerative diseases. Gene delivery vectors need to fulfill several criteria of efficacy and safety before being applied to humans. The ability to drive expression of a therapeutic gene in an adequate number of cells, to maintain long-term expression, and to allow exogenous control over the transgene product are essential requirements for clinical application. We describe the use of an adenovirus vector encoding human tyrosine hydroxylase (TH) 1 under the negative control of the tetracycline-sensitive gene regulatory system for direct injection into the dopamine-depleted striatum of a rat model of Parkinson's disease. This vector mediated synthesis of TH in numerous striatal cells and transgene expression was observed in a large proportion of them for at least 17 weeks. Furthermore, doxycyline, a tetracycline analog, allowed efficient and reversible control of transgene expression. Thus, the insertion of a tetracycline-sensitive regulatory cassette into a single adenovirus vector provides a promising system for the development of successful and safe therapies for human neurological diseases. Our results also confirm that future effective gene replacement approaches to Parkinson's disease will have to consider the concomitant transfer of TH and GTP-cyclohydrolase transgenes because the synthesis of the TH cofactor tetrahydrobiopterin may be crucial for restoration of the dopaminergic deficit.

Muscle regeneration: molecular aspects and therapeutic implications

With respect to diverse clinical applications for muscle regeneration, this paper discusses the latest markers for identifying skeletal muscle precursor cells in regenerating muscle, the implications of alternative sources of myogenic precursor cells and putative stem cells, and the current status of administration of exogenous factors to enhance muscle repair.