Control of Erythropoietin Delivery by Doxycycline in Mice After Intramuscular Injection of Adeno-Associated Vector

Mechanism of an animal toxin-antidote system

Toxin-antidote systems are selfish genetic elements composed of a linked toxin and antidote. The peel-1 zeel-1 toxin-antidote system in C. elegans consists of a transmembrane toxin protein PEEL-1 which acts cell autonomously to kill cells. Here we investigate the molecular mechanism of PEEL-1 toxicity. We find that PEEL-1 requires a small membrane protein, PMPL-1, for toxicity. Together, PEEL-1 and PMPL-1 are sufficient for toxicity in a heterologous system, HEK293T cells, and cause cell swelling and increased cell permeability to monovalent cations. Using purified proteins, we show that PEEL-1 and PMPL-1 allow ion flux through lipid bilayers and generate currents which resemble ion channel gating. Our work suggests that PEEL-1 kills cells by co-opting PMPL-1 and creating a cation channel.

Engineered Regulatory Systems Modulate Gene Expression of Human Commensals in the Gut

The gut microbiota is implicated in numerous aspects of health and disease, but dissecting these connections is challenging because genetic tools for gut anaerobes are limited. Inducible promoters are particularly valuable tools because these platforms allow real-time analysis of the contribution of microbiome gene products to community assembly, host physiology, and disease. We developed a panel of tunable expression platforms for the prominent genus Bacteroides in which gene expression is controlled by a synthetic inducer. In the absence of inducer, promoter activity is fully repressed; addition of inducer rapidly increases gene expression by four to five orders of magnitude. Because the inducer is absent in mice and their diets, Bacteroides gene expression inside the gut can be modulated by providing the inducer in drinking water. We use this system to measure the dynamic relationship between commensal sialidase activity and liberation of mucosal sialic acid, a receptor and nutrient for pathogens. VIDEO ABSTRACT.

Generation and in vivo evaluation of IL10-treated dendritic cells in a nonhuman primate model of AAV-based gene transfer

Preventing untoward immune responses against a specific antigen is a major challenge in different clinical settings such as gene therapy, transplantation, or autoimmunity. Following intramuscular delivery of recombinant adeno-associated virus (rAAV)-derived vectors, transgene rejection can be a roadblock to successful clinical translation. Specific immunomodulation strategies potentially leading to sustained transgene expression while minimizing pharmacological immunosuppression are desirable. Tolerogenic dendritic cells (TolDC) are potential candidates but have not yet been evaluated in the context of gene therapy, to our knowledge. Following intramuscular delivery of rAAV-derived vectors expressing an immunogenic protein in the nonhuman primate model, we assessed the immunomodulating potential of autologous bone marrow-derived TolDC generated in the presence of IL10 and pulsed with the transgene product. TolDC administered either intradermally or intravenously were safe and well tolerated. While the intravenous route showed a modest ability to modulate host immunity against the transgene product, intradermally delivery resulted in a robust vaccination of the macaques when associated to intramuscular rAAV-derived vectors-based gene transfer. These findings demonstrate the critical role of TolDC mode of injection in modulating host immunity. This study also provides the first evidence of the potential of TolDC-based immunomodulation in gene therapy.

Use of engineered bone marrow stem cells to deliver brain derived neurotrophic factor under the control of a tetracycline sensitive response element in experimental allergic encephalomyelitis

Brain derived neurotrophic factor (BDNF) has neuroprotective properties but its use has been limited by poor penetration of the blood brain barrier. Treatment using bone marrow stem cells (BMSC) or retroviruses as vectors reduces the clinical and pathological severity of experimental allergic encephalomyelitis (EAE). We have refined the BMSC based delivery system by introducing a tetracycline sensitive response element to control BDNF expression. We have now tested that construct in EAE and have shown a reduction in both the clinical and pathological severity of the disease. Further, we looked for changes in sirtuin1 and nicotinamide phosphoribosyltransferase expression that would be consistent with a neuroprotective effect.

Gene therapy for heart failure

Congestive heart failure accounts for half a million deaths per year in the United States. Despite its place among the leading causes of morbidity, pharmacological and mechanic remedies have only been able to slow the progression of the disease. Today's science has yet to provide a cure, and there are few therapeutic modalities available for patients with advanced heart failure. There is a critical need to explore new therapeutic approaches in heart failure, and gene therapy has emerged as a viable alternative. Recent advances in understanding of the molecular basis of myocardial dysfunction, together with the evolution of increasingly efficient gene transfer technology, have placed heart failure within reach of gene-based therapy. The recent successful and safe completion of a phase 2 trial targeting the sarcoplasmic reticulum calcium ATPase pump (SERCA2a), along with the start of more recent phase 1 trials, opens a new era for gene therapy for the treatment of heart failure.

Lack of humoral immune response to the tetracycline (Tet) activator in rats injected intracranially with Tet-off rAAV vectors

The ability to safely control transgene expression from viral vectors is a long-term goal in the gene therapy field. We have previously reported tight regulation of GFP expression in rat brain using a self-regulating tet-off rAAV vector. The immune responses against tet regulatory elements observed by other groups in nonhuman primates after intramuscular injection of tet-on encoding vectors raise concerns about the clinical value of tet-regulated vectors. However, previous studies have not examined immune responses following injection of AAV vectors into brain. Therefore, rat striatum was injected with tet-off rAAV harboring a therapeutic gene for Parkinson's disease, either hAADC or hGDNF. The expression of each gene was tightly controlled by the tet-off regulatory system. Using an ELISA developed with purified GST-tTA protein, no detectable immunogenicity against tTA was observed in sera of rats that received an intrastriatal injection of either vector. In contrast, sera from rats intradermally injected with an adenovirus containing either tTA or rtTA, as positive controls, had readily detectable antibodies. These observations suggest that tet-off rAAV vectors do not elicit an immune response when injected into rat brain and that these may offer safer vectors for Parkinson's disease than vectors with constitutive expression.

AAV9-mediated erythropoietin gene delivery into the brain protects nigral dopaminergic neurons in a rat model of Parkinson's disease

We have recently shown that intrastriatal injection of recombinant human erythropoietin (EPO) protects dopaminergic (DA) neurons in the substantia nigra (SN) from 6-hydroxydopamine (6-OHDA) toxicity in a rat model of Parkinson's disease. However, systemic administration of EPO did not protect nigral DA neurons, suggesting that the blood-brain barrier limits the passage of EPO protein into the brain. In the present study, we used an adeno-associated viral (AAV) serotype 9 (AAV9) vector to deliver the human EPO gene into the brain of 6-OHDA-lesioned rats. We observed that expression of the human EPO gene was robust and stable in the striatum and the SN for up to 10 weeks. EPO-immunoreactive (IR) cells were widespread throughout the injected striatum, and EPO-IR neurons and fibers were also found in the ipsilateral SN. Enzyme-linked immunosorbent assay and western blot analyses exhibited dramatic levels of EPO protein in the injected striatum. As a result, nigral DA neurons were protected against 6-OHDA-induced toxicity. Amphetamine-induced rotational asymmetry and spontaneous forelimb use asymmetry were both attenuated. Interestingly, we also observed that intrastriatal injection of AAV9-EPO vectors led to increased numbers of red blood cells in peripheral blood. This highlights the importance of using an inducible gene delivery system for EPO gene delivery.

Adeno-associated viral vectors engineered for macrolide-adjustable transgene expression in mammalian cells and mice

Background

Adjustable gene expression is crucial in a number of applications such as de- or transdifferentiation of cell phenotypes, tissue engineering, various production processes as well as gene-therapy initiatives. Viral vectors, based on the Adeno-Associated Virus (AAV) type 2, have emerged as one of the most promising types of vectors for therapeutic applications due to excellent transduction efficiencies of a broad variety of dividing and mitotically inert cell types and due to their unique safety features.

Results

We designed recombinant adeno-associated virus (rAAV) vectors for the regulated expression of transgenes in different configurations. We integrated the macrolide-responsive E.REX systems (E_ON and E_OFF) into rAAV backbones and investigated the delivery and expression of intracellular as well as secreted transgenes for binary set-ups and for self- and auto-regulated one-vector configurations. Extensive quantitative analysis of an array of vectors revealed a high level of adjustability as well as tight transgene regulation with low levels of leaky expression, both crucial for therapeutical applications. We tested the performance of the different vectors in selected biotechnologically and therapeutically relevant cell types (CHO-K1, HT-1080, NHDF, MCF-7). Moreover, we investigated key characteristics of the systems, such as reversibility and adjustability to the regulating agent, to determine promising candidates for in vivo studies. To validate the functionality of delivery and regulation we performed in vivo studies by injecting particles, coding for compact self-regulated expression units, into mice and adjusting transgene expression.

Conclusion

Capitalizing on established safety features and a track record of high transduction efficiencies of mammalian cells, adeno- associated virus type 2 were successfully engineered to provide new powerful tools for macrolide-adjustable transgene expression in mammalian cells as well as in mice.

Adeno-associated viral vector-mediated hypoxia-regulated VEGF gene transfer promotes angiogenesis following focal cerebral ischemia in mice

Uncontrolled expression of vascular endothelial growth factor (VEGF) in vivo may cause unexpected side effects, such as brain hemangioma or tumor growth. Because hypoxia-inducible factor-1 (HIF-1) is upregulated during cerebral ischemia and regulates gene expression by binding to a cis-acting hypoxia-responsive element (HRE), we therefore used a novel HRE, originating in the 3'-end of the erythropoietin (Epo) gene, to control gene expression in the ischemic brain. A concatemer of nine copies (H9) of the consensus sequence of HRE was used to mediate hypoxia induction. Three groups of adult CD-1 mice received AAVH9-VEGF, AAVH9-lacZ or saline injection, and then underwent 45 min of transient middle cerebral artery occlusion (tMCAO). Results show that HIF-1 was persistently expressed in the ischemic brain. VEGF was overexpressed in the ischemic perifocal region in AAVH9-VEGF-transduced mice. Double-labeled immunostaining showed that VEGF expressed in neurons and astrocytes but not endothelial cells, suggesting that adeno-associated virus (AAV) vectors transduced neurons and astrocytes predominantly. The total number of microvessels/enlarged microvessels was greatly increased in the AAVH9-VEGF-transduced mice (180+/-29/27+/-4) compared to the AAVH9-lacZ (118+/-19/14+/-3) or saline-treated (119+/-20/14+/-2) mice after tMCAO (P<0.05). Cell proliferation examination demonstrated that these microvessels were newly formed. Regional cerebral blood flow recovery in the AAVH9-VEGF-transduced mice was also better than in AAVH9-lacZ or saline-treated mice (P<0.05). Our data indicated that HRE is a novel trigger for the control of VEGF expression in the ischemic brain. VEGF overexpression through AAVH9-VEGF gene transfer showed stable focal angiogenic effects in post-ischemic repair process, providing an opportunity to rebuild injured brain tissue.

Recombinant AAV-mediated gene transfer to the retina: gene therapy perspectives

Retinal degenerative diseases such as retinal macular degeneration and retinitis pigmentosa constitute a broad group of diseases that all share one critical feature, the progressive apoptotic loss of cells in the retina. There is currently no effective treatment available by which the course of these disorders can be modified, and visual dysfunction often progresses to total blindness. Gene therapy represents an attractive approach to treating retinal degeneration because the eye is easily accessible and allows local application of therapeutic vectors with reduced risk of systemic effects. Furthermore, transgene expression within the retina and effects of treatments may be monitored by a variety of noninvasive examinations. An increasing number of strategies for molecular treatment of retinal disease rely on recombinant adeno-associated virus (rAAV) as a therapeutic gene delivery vector. Before rAAV-mediated gene therapy for retinal degeneration becomes a reality, there are a number of important requirements that include: (1) evaluation of different rAAV serotypes, (2) screening of vectors in large animals in order to ensure that they mediate safe and long-term gene expression, (3) appropriate regulation of therapeutic gene expression, (4) evaluation of vectors carrying a therapeutic gene in relevant animal models, (5) identification of suitable patients, and finally (6) manufacture of clinical grade vector. All these steps towards gene therapy are still being explored. Outcomes of these studies will be discussed in the order in which they occur, from vector studies to preclinical assessment of the therapeutic potential of rAAV in animal models of retinal degeneration.

Regulated, electroporation-mediated delivery of pro-opiomelanocortin gene suppresses chronic constriction injury-induced neuropathic pain in rats

We previously reported that intrathecal pro-opiomelanocortin gene electroporation could reduce pain sensitivity induced by chronic constriction injury (CCI) of the sciatic nerve. For optimal use of antinociceptive gene therapy, it might be important to control the expression of the transfected gene extrinsically. For this purpose, a doxycycline-controlled transrepressor system composed of two plasmids coding, respectively, for pro-opiomelanocortin gene (pTRE2-POMC) and the silencer (pTel-off) was employed. The regulation of beta-endorphin expression was first assessed in spinal neuronal culture, then we electrotranfected this plasmid into the spinal cord of mononeuropathic rats and evaluated the analgesic potential of this therapy in vivo by thermal and mechanical withdrawal latency. Intraperitoneal injections of various doses of doxycycline were made to elucidate the possible exogenous downregulation of transfected beta-endorphin gene expression in vivo. The levels of beta-endorphin were analyzed by intrathecal microdialysis and radioimmunoassay. Intrathecal pTRE2-POMC/pTel-off electroporation elevated spinal beta-endorphin levels, as manifested in a significantly elevated pain threshold for chronic constriction injury limbs. Intraperitoneal doxycycline decreased the antinociceptive effect and spinal beta-endorphin levels in a dose-dependent manner. We concluded that intrathecal pTRE2-POMC/pTel-off electroporation alleviates CCI-induced limb pain, and can be controlled by intraperitoneal doxycycline administration.

Primer on medical genomics. Part X: Gene therapy

Gene therapy is defined as any therapeutic procedure in which genes are intentionally introduced into human somatic cells. Both preclinical and clinical gene therapy research have been progressing rapidly during the past 15 years; gene therapy is now a highly promising new modality for the treatment of numerous human disorders. Since the first clinical test of gene therapy in 1989, more than 600 gene therapy protocols have been approved, and more than 3000 patients have received gene therapy. However, at the time of writing this article, no gene therapy products have been approved for clinical use. This article explains the potential clinical scope of gene therapy and the underlying pharmacological principles, describes some of the major gene transfer systems (or vectors) that are used to deliver genes to their target sites, and discusses the various strategies for controlling expression of therapeutic transgenes. Safety issues regarding clinical use of gene therapy are explored, and the most important technical challenges facing this field of research are highlighted. This review should serve as an introduction to the subject of gene therapy for clinician investigators, physicians and medical scientists in training, practicing clinicians, and other students of medicine.

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.

Adeno-associated viral vector-mediated hypoxia response element-regulated gene expression in mouse ischemic heart model

Intramyocardial injection of genes encoding angiogenic factors could provide a useful approach for the treatment of ischemic heart disease. However, uncontrolled expression of angiogenic factors in vivo may cause some unwanted side effects, such as hemangioma formation, retinopathy, and arthritis. It may also induce occult tumor growth and artherosclerotic plaque progression. Because hypoxia-inducible factor 1 is up-regulated in a variety of hypoxic conditions and it regulates gene expression by binding to a cis-acting hypoxia-responsive element (HRE), we propose to use HRE, found in the 3' end of the erythropoietin gene to control gene expression in ischemic myocardium. A concatemer of nine copies of the consensus sequence of HRE isolated from the erythropoietin enhancer was used to mediate hypoxia induction. We constructed two adeno-associated viral vectors in which LacZ and vascular endothelial growth factor (VEGF) expressions were controlled by this HRE concatemer and a minimal simian virus 40 promoter. Both LacZ and VEGF expression were induced by hypoxia and/or anoxia in several cell lines transduced with these vectors. The functions of these vectors in ischemic myocardium were tested by injecting them into normal and ischemic mouse myocardium created by occlusion of the left anterior descending coronary artery. The expression of LacZ gene was induced eight times and of VEGF 20 times in ischemic myocardium compared with normal myocardium after the viral vector transduction. Hence, HRE is a good candidate for the control of angiogenic factor gene expression in ischemic myocardium.

Insulators coupled to a minimal bidirectional tet cassette for tight regulation of rAAV-mediated gene transfer in the mammalian brain

Recombinant AAV is increasingly becoming the vector of choice for many gene therapy applications in the CNS, due to its lack of toxicity and high level of sustained expression. With recent improvements in the generation of pure, high titer vector stocks, the regulation of gene expression is now a key issue for successful translation of gene therapy-based treatments to the clinic. The level of the transgene protein may need to be maintained within a narrow therapeutic window for the successful treatment of human disease. The doxycycline responsive system directs a dose-responsive, tightly regulated level of gene expression and has been used successfully in transgenic mouse models. Here, we have optimized an autoregulatory, bidirectional doxycyline responsive cassette specifically for use in rAAV. We minimized the size of the cassette and decreased the basal leakiness of the system, leading to tight regulation in the rat

A light and electron microscopic study of ectopic tendon and ligament formation induced by bone morphogenetic protein-13 adenoviral gene therapy

OBJECT

Bone morphogenetic proteins (BMPs) are involved in the growth and development of many tissues, but it is their role in skeletal development and their unique ability to induce ectopic and orthotopic osteogenesis that have attracted the greatest interest. Expression of the BMP-13 gene is predominantly localized to hypertrophic chondrocytes in regions of endochondral bone formation during development, as well as in mature articular cartilage in the adult. In addition, the application of BMP-13 on a collagen carrier induces neotendon/neoligament formation when delivered subcutaneously or intramuscularly in rodents. The aim of the present study was to determine the histological and ultrastructural changes that occur after the intramuscular injection of a first-generation BMP-13 adenoviral vector.

METHODS

Athymic nude rats were injected with 3.75 x 10(10) plaque-forming units of adenovirus (Ad)-BMP-13 or Ad-beta-galactosidase in the thigh musculature, and the region was examined using light and electron microscopy at various time points between 2 days and 100 days postinjection. As early as 2 days after injection of Ad-BMP-13, progenitor cells were observed infiltrating between the transduced muscle fibers. These cells subsequently proliferated, differentiated, and secreted large amounts of collagenous extracellular matrix. By 100 days postinjection, the treated tissue displayed the histological and ultrastructural appearance of neotendon/neoligament, which was clearly demarcated from the surrounding muscle. Small foci of bone and fibrocartilage were also seen within the treated tissue. A short-term bromodeoxyuridine study also demonstrated rapid mesenchymal cell proliferation at the Ad-BMP-13 injection site as early as 48 hours postinjection. At all time points, the control AD-beta-gal injection sites were found to contain only normal muscle, without evidence of inflammation or mesenchymal cell proliferation.

CONCLUSIONS

The results of this study indicate that in the future the use of the BMP-13 gene may have therapeutic utility for the healing of tendon and ligament tears and avulsion injuries.

Critical Aspects of Viral Vectors for Gene Transfer into the Kidney

Abstract.Viral vectors have been usedin vitroandin vivofor more than a decade, with some significant results in specific situations,e.g., when recombinant adeno-associated virus is used for the long-term transduction of skeletal muscle in coagulation factor IX-deficient patients. However, the kidney has been quite difficult to transduce with any viral vector currently available. When viral transduction occurs, it is often heterogeneous, transient, and eventually associated with immune and toxic side effects. However, recombinant adeno-associated virus and lentiviral vectors remain to be fully evaluated in the kidney; the former is small enough to be filtered through the glomerular basement membrane. This may be critical, because glomerular filtration is required for DNA complex-mediated transduction of tubular cells. An alternative toin siturenal gene transfer is secretion of a therapeutic protein from a distant site, such as skeletal muscle. Several examples provide evidence that this could be a clinically relevant approach. It also may allow accurate determination of the pathophysiologic mechanisms involved in the establishment and maintenance of experimental glomerulonephritis.

Delivering Erythropoietin through Genetically Engineered Cells

Abstract. Erythropoietin (Epo) is a glycoprotein hormone produced by genetic engineering. Many pathologic conditions could benefit from its administration, such as chronic renal failure or hemoglobinopathies. Epo secretion from genetically modified tissued could be proposed to patients only if the protocol is low cost and low risk. For that purpose, retroviral vectors and adeno-associated vectors expressing the Epo cDNA were developed. Gene transfer was performed into skeletal muscles. To avoid polycythemia, a tetracycline-regulated system was used to control the levels of protein secretion in vivo. β-thalassemias are among diseases that could benefit from an Epo gene transfer. β-thalassemias are attributable to deficient synthesis of β-globin and accumulation of unpaired α-chains. Stimulation of fetal globin synthesis is one strategy to correct the globin chain imbalance. There is evidence that Epo could play this role. In a mouse model of β-thalassemia, an adeno-associated vector expressing the Epo cDNA was injected intramuscularly. Epo was secreted continuously during at least 1 yr. Erythropoiesis was improved in those mice by increasing the synthesis of fetal hemoglobin.

Novel transcriptional regulatory signals in the adeno-associated virus terminal repeat A/D junction element

Adeno-associated virus (AAV) type 2 vectors transfer stable, long-term gene expression to diverse cell types in vivo. Many gene therapy applications require the control of long-term transgene expression, and AAV vectors, similar to other gene transfer systems, are being evaluated for delivery of regulated gene expression cassettes. Previously, we (R. P. Haberman, T. J. McCown, and R. J. Samulski, Gene Ther. 5:1604-1611, 1998) demonstrated the use of the tetracycline-responsive system for long-term regulated expression in rat brains. In that study, we also observed residual expression in the "off" state both in vitro and in vivo, suggesting that the human cytomegalovirus (CMV) major immediate-early minimal promoter or other cis-acting elements (AAV terminal repeats [TR]) were contributing to this activity. In the present study, we identify that the AAV TR, minus the tetracycline-responsive minimal CMV promoter, will initiate mRNA expression from vector templates. Using deletion analysis and specific PCR-derived TR reporter gene templates, we mapped this activity to a 37-nucleotide stretch in the A/D elements of the TR. Although the mRNA derived from the TR is generated from a non-TATA box element, the use of mutant templates failed to identify function of canonical initiator sequences as previously described. Finally, we demonstrated the presence of green fluorescent protein expression both in vitro and in vivo in brain by using recombinant virus carrying only the TR element. Since the AAV terminal repeat is a necessary component of all recombinant AAV vectors, this TR transcriptional activity may interfere with all regulated expression cassettes and may be a problem in the development of novel TR split gene vectors currently being considered for genes too large to be packaged.

Transfer of the feline erythropoietin gene to cats using a recombinant adeno-associated virus vector

Chronic renal failure and the associated erythropoietin-responsive anemia afflicts over 2 million domestic cats in the United States, resulting in morbidity that can affect the owner-pet relationship. Although treatment of cats with recombinant human erythropoietin (Epo) protein can be effective, response to the drug often dissipates over time, probably due to the development of antibodies reactive with the human protein. As an alternate approach to the treatment of this disease, we have developed a recombinant adeno-associated virus vector containing the feline erythropoietin gene (rAAV/feEpo). This vector, when administered intramuscularly to normal healthy cats, caused a dose-related increase in hematocrit over a 7-week period after injection. Thus, the rAAV/feEpo vector holds promise as a simple, safe and effective therapy for the anemia of chronic renal failure in domestic cats.

Regulation of protein secretion through controlled aggregation in the endoplasmic reticulum

A system for direct pharmacologic control of protein secretion was developed to allow rapid and pulsatile delivery of therapeutic proteins. A protein was engineered so that it accumulated as aggregates in the endoplasmic reticulum. Secretion was then stimulated by a synthetic small-molecule drug that induces protein disaggregation. Rapid and transient secretion of growth hormone and insulin was achieved in vitro and in vivo. A regulated pulse of insulin secretion resulted in a transient correction of serum glucose concentrations in a mouse model of hyperglycemia. This approach may make gene therapy a viable method for delivery of polypeptides that require rapid and regulated delivery.

AAV vectors: is clinical success on the horizon?

Potential applications and impact of the adeno-associated virus (AAV) as a gene transfer vector have expanded rapidly in the last decade. Recent advances in the production of high-titer purified rAAV vector stocks have made the transition to human clinical trials a reality in the last moments of the millenium. Production improvements will be complemented in the coming years with understanding of and innovations in the targeting and packaging of rAAV, the design of transgene cassettes, and the host immune response to the vectors. These expected areas of progress are discussed, with special attention to clinical applications for which rAAV vectors may help close the gap towards successful gene therapy. Gene Therapy (2000) 7, 24-30.

Regulated gene expression systems

Most gene therapy research to date has focused on solving the delivery problem--getting genes efficiently and stably into target cells and tissues. Those working on systems for regulating the expression of genes once delivered have often been accused of trying to run before they can walk. Yet regulation is likely to be essential if gene therapy is to realize its full potential as a mainstream clinical option for delivering proteins. Dramatic progress has been made in designing and testing systems in which expression is controlled by orally active drugs. The next few years should see the first clinical trials of drug-regulated gene therapies.

Long-term regulated expression of growth hormone in mice after intramuscular gene transfer

Effective delivery of secreted proteins by gene therapy will require a vector that directs stable delivery of a transgene and a regulatory system that permits pharmacologic control over the level and kinetics of therapeutic protein expression. We previously described a regulatory system that enables transcription of a target gene to be controlled by rapamycin, an orally bioavailable drug. Here we demonstrate in vivo regulation of gene expression after intramuscular injection of two separate adenovirus or adeno-associated virus (AAV) vectors, one encoding an inducible human growth hormone (hGH) target gene, and the other a bipartite rapamycin-regulated transcription factor. Upon delivery of either vector system into immunodeficient mice, basal plasma hGH expression was undetectable and was induced to high levels after administration of rapamycin. The precise level and duration of hGH expression could be controlled by the rapamycin dosing regimen. Equivalent profiles of induction were observed after repeated administration of single doses of rapamycin over many months. AAV conferred stable expression of regulated hGH in both immunocompetent and immunodeficient mice, whereas adenovirus-directed hGH expression quickly extinguished in immunocompetent animals. These studies demonstrate that the rapamycin-based regulatory system, delivered intramuscularly by AAV, fulfills many of the conditions necessary for the safe and effective delivery of therapeutic proteins by gene therapy.

Efficient and regulated erythropoietin production by naked DNA injection and muscle electroporation

We show that an electric treatment in the form of high-frequency, low-voltage electric pulses can increase more than 100-fold the production and secretion of a recombinant protein from mouse skeletal muscle. Therapeutical erythopoietin (EPO) levels were achieved in mice with a single injection of as little as 1 microgram of plasmid DNA, and the increase in hematocrit after EPO production was stable and long-lasting. Pharmacological regulation through a tetracycline-inducible promoter allowed regulation of serum EPO and hematocrit levels. Tissue damage after stimulation was transient. The method described thus provides a potentially safe and low-cost treatment for serum protein deficiencies.