Current Gene Therapy - Volume 2, Issue 4, 2002

Radioiodide uptake (RAIU) in thyroid follicular epithelial cells, mediated by a plasma membrane transporter, sodium iodide symporter (NIS), provides a first step mechanism for thyroid cancer detection by radioiodide injection and effective radioiodide treatment for patients with invasive, recurrent, and / or metastatic thyroid cancers after total thyroidectomy. NIS gene transfer to tumor cells may significantly and specifically enhance internal radioactive accumulation of tumors following radioiodide administration, and result in better tumor control. NIS gene transfers have been successfully performed in a variety of tumor animal models by either plasmid-mediated transfection or virus (adenovirus or retrovirus)-mediated gene delivery. These animal models include nude mice xenografted with human melanoma, glioma, breast cancer or prostate cancer, rats with subcutaneous thyroid tumor implantation, as well as the rat intracranial glioma model. In these animal models, non-invasive imaging of in vivo tumors by gamma camera scintigraphy after radioiodide or technetium injection has been performed successfully, suggesting that the NIS can serve as an imaging reporter gene for gene therapy trials. In addition, the tumor killing effects of 131I after NIS gene transfer have been demonstrated in in vitro clonogenic assays and in vivo radioiodide therapy studies, suggesting that NIS gene can also serve as a therapeutic agent when combined with radioiodide injection. Better NIS-mediated tumor treatment by radioiodide requires a more efficient and specific system of gene delivery with better retention of radioiodide in tumor. Results thus far are, however, promising, and suggest that NIS gene transfer followed by radioiodide treatment will allow non-invasive in vivo imaging to assess the outcome of gene therapy and provide a therapeutic strategy for a variety of human cancers.

Type 1 or insulin-dependent diabetes mellitus is caused by autoimmune attack and selective destruction of the pancreatic ß cells. Despite the development of various insulin replacement therapies, insulin injection still remains the mainstay treatment for type 1 diabetes. However, exogenous insulin administration cannot achieve the same degree of glycemic control as provided by endogenous insulin produced from the pancreatic ß cells. Insulin gene transfer is being developed to improve the quality of glycemic control by restoring endogenous insulin production in type 1 diabetes. Nevertheless, attempts to achieve adequately regulated insulin production are stymied by the lack of appropriate surrogate cells that are able to detect blood glucose variations and release insulin in a glucose-dependent manner. Although limited success has been made to control insulin gene expression in ectopic cells using hormone / glucose regulated expression systems, these transcriptionally regulated systems are relatively slow in the “on-” and “off”-kinetics of insulin production, raising a serious safety concern for clinical application. In this article, we will review recent advances made to address this concern and highlight the importance of insulin gene transfer to cell types that possess an intrinsic ability to kinetically mimic the pancreatic ß cells in terms of glucose-responsive insulin secretion.

Herpes Simplex Virus (HSV) has a number of advantages as a gene delivery vector, particularly for the nervous system. Thus, it naturally establishes asymptomatic latent infections of neuronal cells. Moreover, it is readily grown in culture to high titre and has a large genome so allowing it to be used to deliver multiple or very large genes. Considerable progress has been made in effectively disabling the virus so that it does not damage the cells it infects but can still deliver an inserted gene effectively. In addition, it is now possible to obtain long-term expression of the transgene in the nervous system, using regulatory elements derived from the latency-associated transcript of the virus.As well as its use in the nervous system, the virus has also been used to successfully deliver genes to a variety of other cell types, including peripheral blood mononuclear cells and cardiac myocytes within the intact heart. In particular, its ability to deliver genes effectively to replicating cancer cells and to dendritic cells offers considerable potential for the use of this virus in cancer therapy.

Established modalities of treatment for obstructive coronary artery disease include medical therapy, bypass surgery and percutaneous coronary intervention. Similarly, conventional treatment of congestive heart failure is also limited to medical therapy, temporary assist devices and transplantation. A significant subset of patients with severe symptomatic coronary artery disease and end stage heart failure is not eligible for these traditional methods of treatment. In spite of maximal medical and revascularization therapy, these patients may not get adequate symptomatic benefit. After a decade of investigations, gene therapy has emerged as a promising therapeutic option for this group of patients. This review discusses newer modalities of therapy for this subset, including therapeutic angiogenesis with growth factors and cell transplantation.

For more than two decades, there has been a concerted effort to define the biology of, and develop the clinical applications for, cytokines that influence the immune system. However, intrinsic potency and toxicity have complicated application of cytokines as therapeutic agents when applied systemically. Indeed, one of the major characteristics of most cytokines is that they regulate immunity at a local or regional level, and systemic levels provided by most conventional schema fail to mimic the induction of an effective immune response. IL-4 has pleiotropic effects on immune cells of multiple lineages, endothelial cells and tumor cells. Accumulating data in pre-clinical studies demonstrate that sustained expression of IL-4 at the targeted organs or tissues may provide an effective means for therapy of variety of diseases including cancers and immunologic disorders. This review discusses biological properties and therapeutic applications of IL-4, particularly when it is delivered as a transgene in the settings of gene therapy.

Parkinson's disease (PD) is a neurodegenerative disease characterised by a progressive loss of the dopaminergic neurones in the substantia nigra pars compacta.Accumulating evidence indicates that apoptosis contributes to neuronal cell death in PD patients' brain. Excitotoxicity, oxidative stress, and mitochondrial respiratory failure are thought to be the key inducers of the apoptotic cascade. Even though the initial cause and the mechanism of degeneration are poorly understood, neuroprotection can be achieved by interfering with neuronal cell death either directly or by preventing neuronal dysfunction. Potential agents for neuroprotection are neurotrophic factors, inhibitors of apoptosis or anti-oxidative agents. However, the existence of the blood-brain barrier precludes systemic delivery of these factors. In situ gene delivery provides strategies for local and sustained administration of protective factors at physiologically relevant doses.Viral vectors mediating stable gene expression in the central nervous system exist and are still under development. Efficacy of these vectors has repeatedly been demonstrated in the animal models both ex vivo and in vivo. Ex vivo gene delivery could furthermore be combined with cell replacement therapies by transplanting genetically modified cells compensating for the lost neuronal cell population in order to provide neuroprotection to both the grafted cells and degenerating host neurones.However, several aspects of gene transfer, such as uncontrolled diffusion, axonal transport, unpredictable site of integration and immunological responses, still raise safety concerns and justify further development of viral and non-viral vectors as well as genetic elements with tightly controlled gene expression. Various relevant animal models for Parkinson's disease are available for the evaluation of gene therapy strategies. These include induction of cell death in specific neurone population through administration of toxins either directly in the brain or systemically, as well as transgenic mice expressing human disease-associated mutations.

Recombinant adeno-associated virus (rAAV) vectors have emerged as highly promising for use in gene transfer for a variety of reasons, including lack of pathogenicity and wide host range. In addition, all virus-encoded genes have been removed from standard rAAV vectors, resulting in their comparatively low intrinsic immunogenicity. For gene replacement strategies, transgenes encoded by rAAV vectors may induce less robust host immune responses than other vectors in vivo. However, under appropriate conditions, host immune responses can be generated against rAAV encoded transgenes, raising the potential for their use in vaccine development. In this review, we have summarized current understanding of the generation of both undesirable and beneficial host immune responses directed against rAAV and encoded transgenes, and how they might be exploited for optimal use of this promising vector system.