Multiple advantages of alpha-fetoprotein as a marker for in vivo gene transfer

Ferret and pig models of cystic fibrosis: prospects and promise for gene therapy

Large animal models of genetic diseases are rapidly becoming integral to biomedical research as technologies to manipulate the mammalian genome improve. The creation of cystic fibrosis (CF) ferrets and pigs is an example of such progress in animal modeling, with the disease phenotypes in the ferret and pig models more reflective of human CF disease than mouse models. The ferret and pig CF models also provide unique opportunities to develop and assess the effectiveness of gene and cell therapies to treat affected organs. In this review, we examine the organ disease phenotypes in these new CF models and the opportunities to test gene therapies at various stages of disease progression in affected organs. We then discuss the progress in developing recombinant replication-defective adenoviral, adeno-associated viral, and lentiviral vectors to target genes to the lung and pancreas in ferrets and pigs, the two most affected organs in CF. Through this review, we hope to convey the potential of these new animal models for developing CF gene and cell therapies.

Gene therapy: light is finally in the tunnel

After two decades of ups and downs, gene therapy has recently achieved a milestone in treating patients with Leber's congenital amaurosis (LCA). LCA is a group of inherited blinding diseases with retinal degeneration and severe vision loss in early infancy. Mutations in several genes, including RPE65, cause the disease. Using adeno-associated virus as a vector, three independent teams of investigators have recently shown that RPE65 can be delivered to retinal pigment epithelial cells of LCA patients by subretinal injections resulting in clinical benefits without side effects. However, considering the whole field of gene therapy, there are still major obstacles to clinical applications for other diseases. These obstacles include innate and immune barriers to vector delivery, toxicity of vectors and the lack of sustained therapeutic gene expression. Therefore, new strategies are needed to overcome these hurdles for achieving safe and effective gene therapy. In this article, we shall review the major advancements over the past two decades and, using lung gene therapy as an example, discuss the current obstacles and possible solutions to provide a roadmap for future gene therapy research.

Rational design of murine secreted alkaline phosphatase for enhanced performance as a reporter gene in mouse gene therapy preclinical studies

Many preclinical gene therapy studies use a reporter gene to evaluate vector design and performance in mouse models of human disease. Unfortunately, most commonly used reporter genes are immunogenic in mice, which confounds accurate evaluation of vector function. In previous studies, we showed that the murine secreted alkaline phosphatase (mSEAP) gene functions well as a simple and sensitive reporter gene in mice. In this study, we have used rational design to enhance mSEAP performance. The majority of native mSEAP remains attached to the outer surface of the cell through glycan phosphatidylinositol linkage; removal of the carboxy-terminal tail of mSEAP resulted in a dramatic enhancement of release of the protein into cell culture medium and into mouse plasma in whole animal experiments. We increased the heat stability of mSEAP through mutation of a key residue in the crown domain of the protein (H451E), thus allowing us to reduce endogenous, background AP activity through heat inactivation for enhanced sensitivity. We show that these alterations in mSEAP result in enhanced performance in tissue culture and mouse studies. Taken together, these data illustrate that mSEAP is a sensitive, nonimmunogenic reporter for preclinical mouse studies.

α-Fetoprotein gene delivery to the nasal epithelium of nonhuman primates by human parainfluenza viral vectors

Over the last two decades, enormous effort has been focused on developing virus-based gene delivery vectors to target the respiratory airway epithelium as a potential treatment for cystic fibrosis (CF) lung disease. However, amongst other problems, the efficiency of gene delivery to the differentiated airway epithelial cells of the lung has been too low for clinical benefit. Although not a target for CF therapy, the nasal epithelium exhibits cellular morphology and composition similar to that of the lower airways, thus representing an accessible and relevant tissue target for evaluating novel and improved gene delivery vectors. We previously reported that replication-competent human parainfluenza virus (PIV)-based vectors efficiently deliver the cystic fibrosis transmembrane conductance regulator gene to sufficient numbers of cultured CF airway epithelial cells to completely correct the bioelectric function of CF cells to normal levels, resulting in restoration of mucus transport. Here, using an in vitro model of rhesus airway epithelium, we demonstrate that PIV mediates efficient gene transfer in rhesus epithelium as in the human counterpart. Naive rhesus macaques were inoculated intranasally with a PIV vector expressing rhesus macaque α-fetoprotein (rhAFP), and expression was monitored longitudinally. rhAFP was detected in nasal lavage fluid and in serum samples, indicating that PIV-mediated gene transfer was effective and that rhAFP was secreted into both mucosal and serosal compartments. Although expression was transient, lasting up to 10 days, it paralleled virus replication, suggesting that as PIV was cleared, rhAFP expression was lost. No adverse reactions or signs of discomfort were noted, and only mild, transient elevations of a small number of inflammatory cytokines were measured at the peak of virus replication. In summary, rhAFP proved suitable for monitoring in vivo gene delivery over time, and PIV vectors appear to be promising airway-specific gene transfer vehicles that warrant further development.

Activation of transgene-specific T cells following lentivirus-mediated gene delivery to mouse lung

Integrating lentiviral vectors based on the human immunodeficiency virus type-1 (HIV-1) can transduce quiescent cells, which in lung account for almost 95% of the epithelial cell population. Pseudotyping lentiviral vectors with the envelope glycoprotein from the Ebola Zaire virus, the lymphocytic choriomeningitis virus (LCMV), the Mokola virus, and the vesicular stomatitis virus (VSV-G) resulted in transduction of mouse alveolar epithelium, but gene expression in the lung of C57BL/6 and BALB/c mice waned within 90 days of vector injection. Intratracheal delivery of the four pseudotyped lentiviral vectors resulted in transgene-specific T-cell activation in both mouse strains, albeit lower than that achieved by intramuscular injection of the vectors. We performed an adoptive transfer of luciferase-specific T cells, isolated from spleen or lung of donor mice injected with VSV-G-pseudotyped lentivirus vector expressing luciferase into the muscle or lung, respectively, into recipient recombination-activating gene (RAG)–deficient mice transduced in lung with adenovirus expressing firefly luciferase (ffluc2). Gene expression declined within 7 days of adoptive transfer approaching background levels by day 36. Taken together, our results suggest that the loss of transduced cells in lung is due to VSV-G.HIV vector–mediated activation of transgene-specific T cells rather than as result of normal turnover of airway cells.

Biomarker system for studying muscle, stem cells, and cancer in vivo

Bioluminescent reporter genes are sensitive in situ tools for following disease progression in preclinical models, albeit they are subject to scattering and absorption in deep tissues. We have generated a bicistronic Cre/LoxP reporter mouse line that pairs the expression of firefly luciferase with quantifiable expression of a human placental alkaline phosphatase that is secreted into the serum (SeAP). With the use of this dual-modality bioreporter with a novel, inducible Pax7-CreER line for tracking muscle satellite cells, we demonstrate the longitudinal kinetics of muscle stem cell turnover, accounting for a doubling of the signal from satellite cell and progeny every 3.93 wk in the transition from adolescence to early adulthood. We also show that this dual-modality bioreporter can be incorporated in preclinical cancer models, whereby SeAP activity is reflective of tumor burden. Thus, this dual bioreporter permits both spatial localization and accurate quantification of biological processes in vivo even when the tissue of interest is deep within the animal.

Improved hepatic transduction, reduced systemic vector dissemination, and long-term transgene expression by delivering helper-dependent adenoviral vectors into the surgically isolated liver of nonhuman primates

Helper-dependent adenoviral vectors (HDAds) are attractive vectors for liver-directed gene therapy because they can mediate sustained, high-level transgene expression without chronic toxicity. However, high vector doses are required to achieve efficient hepatic transduction by systemic delivery because of a nonlinear dose response. Unfortunately, such high doses result in systemic vector dissemination and dose-dependent acute toxicity with potentially severe and lethal consequences. We hypothesize that the threshold to efficient hepatic transduction may be circumvented by delivering the vector into the surgically isolated liver via the portal vein. Total hepatic isolation was achieved by occluding hepatic inflow from the portal vein and hepatic artery and by occluding hepatic venous outflow at the inferior vena cava. We demonstrate in nonhuman primates that this approach resulted in significantly higher efficiency hepatic transduction with reduced systemic vector dissemination compared with systemic intravascular delivery. This method of delivery was associated with transient acute toxicity, the severity of which was variable. Importantly, stable, high levels of transgene expression were obtained for at least 665 days for one baboon and for at least 560 days for two baboons with no evidence of long-term toxicity.

Readministration of helper-dependent adenovirus to mouse lung

Adenovirus vectors (Ad) are widely used in gene therapy studies, including those aimed at treating cystic fibrosis lung disease. Various approaches have been investigated to blunt the host immune response to Ad, including development of helper-dependent (HD) Ad. The host cytotoxic T-cell response to HD-Ad is generally lower than to earlier-generation Ad. However, antibodies are formed which could inhibit the efficacy of HD-Ad readministration. In this first study of HD-Ad readministration to the lung, we found that a second administration of HD-Ad to mice was possible with minimal loss of transgene expression. In contrast, when first-generation (FG) Ad was administered initially, followed by HD-Ad or FG-Ad, transgene expression was reduced. Significantly lower concentrations of antibodies against Ad were found in lung lavage fluid and serum from mice that received two doses of HD-Ad (when the initial HD-Ad lacked a transgene), compared to mice that received FG-Ad followed by HD-Ad. These data suggest that readministration of HD-Ad for lung gene therapy may be feasible.

Use of a murine secreted alkaline phosphatase as a non-immunogenic reporter gene in mice

BACKGROUND

The development of any vector system as a gene delivery system requires its optimization in vitro and in vivo. Preliminary studies frequently involve the use of a reporter gene, which allows for the rapid and simple assay of vector function through monitoring expression levels of the reporter gene. However, evaluation of vector efficacy can be compromised by immune responses directed against immunogenic reporter proteins.

METHODS

We have cloned a murine secreted alkaline phosphatase (mSEAP), and explored its use as a reporter gene in the context of an early region 1 (E1)-deleted adenovirus (Ad) vector. Studies involved characterization of gene expression in vitro and in vivo, and immunological responses after gene delivery to mice.

RESULTS

In tissue culture, we show that mSEAP is easily measured quantitatively using a sensitive, commercially available chemiluminescent assay, or visualized directly using histological staining. The level of transgene expression from AdmSEAP was similar to that observed for an Ad vector encoding the human placental secreted alkaline phosphatase (hSEAP). After intravenous administration in mice, AdmSEAP continued to express at high levels for the duration of the experiment (1 month), whereas expression from AdhSEAP declined to background levels over the course of the experiment. Although cytotoxic T-lymphocytes were not detected against either the murine or human SEAP proteins in mice, antibodies were readily detected against the human protein. No antibodies were detected to mSEAP.

CONCLUSIONS

Taken together, these data illustrate that mSEAP is a sensitive, non-immunogenic reporter gene for preclinical mouse studies.

Reduced inflammation and improved airway expression using helper-dependent adenoviral vectors with a K18 promoter

Efforts have been made to deliver transgenes to the airway epithelia of laboratory animals and humans to develop gene therapy for cystic fibrosis. These investigations have been disappointing due to combinations of transient and low-level gene expression, acute toxicity, and inflammation. We have developed new helper-dependent adenoviral vectors to deliver an epithelial cell-specific keratin 18 expression cassette driving the beta-galactosidase (beta-gal) or human alpha-fetoprotein (AFP) reporter genes. Following intranasal administration to mice, we found that the reporter genes were widely expressed in airway epithelial and submucosal cells, and secreted human AFP was also detectable in serum. In contrast to a first-generation adenoviral vector, inflammation was negligible at doses providing efficient transduction, and expression lasted longer than typically reported-up to 28 days with beta-gal and up to 15 weeks with human AFP. These results suggest that delivery to the airway of helper-dependent adenoviral vectors utilizing a tissue-specific promoter could be a significant advance in the development of gene therapy for cystic fibrosis.

MUSEAP, a novel reporter gene for the study of long-term gene expression in immunocompetent mice

The improvement of gene therapy vectors would benefit from the availability of a reporter gene that can be used for long-term studies in immunocompetent laboratory animals. We describe the construction and characterization of a novel reporter gene, murine secreted embryonic alkaline phosphatase (MUSEAP). We demonstrate by gene transfer in skeletal muscle of immunocompetent mice that MUSEAP is efficiently secreted and detected in the bloodstream and that injection of an increasing dose of DNA leads to a dose-dependent increase of plasma MUSEAP activity. We also show that the expression of MUSEAP under the control of a constitutive promoter is stable for 1 year and that the activity of MUSEAP in the bloodstream reflects the changes in the transcription rate of its gene. These properties make MUSEAP the only reporter gene that can be used for somatic gene transfer into immunocompetent mice in order to study the impact of gene transfer vectors of metabolic, developmental or environmental factors on long-term gene expression.