Current Gene Therapy - Volume 8, Issue 3, 2008

The increasing knowledge of the molecular and genetic background of many different human diseases has led to the vision that genetic engineering might be used one day for their phenotypic correction. The main goal of gene therapy is to treat loss-of-function genetic disorders by delivering correcting therapeutic DNA sequences into the nucleus of a cell, allowing its long-term expression at physiologically relevant levels. Manifold different vector systems for the therapeutic gene delivery have been described over the recent years. They all have their individual advantages but also their individual limitations and must be judged on a careful risk/benefit analysis. Integrating vector systems can deliver genetic material to a target cell with high efficiency enabling long-term expression of an encoded transgene. The main disadvantage of integrating vector systems, however, is their potential risk of causing insertional mutagenesis. Episomal vector systems have the potential to avoid these undesired side effects, since they behave as separate extrachromosomal elements in the nucleus of a target cell. Within this article we present a comprehensive survey of currently available episomal vector systems for the genetic modification of mammalian cells. We will discuss their advantages and disadvantages and their applications in the context of basic research, biotechnology and gene therapy.

Viral vector systems are widely being used in the development of new genetic approaches for a variety of human diseases. Oncolytic viruses have shown great potential as cancer therapeutics. The ideal viral vector for cancer gene therapy eradicates a clinically significant fraction of malignant cells and leaves normal tissues unharmed. The Edmonston vaccine strain of measles virus is a replicating RNA virus which is characterized by its tumor selectivity and oncolysis. Its strong tumor suppressive potential combined with its excellent safety record as a viral vaccine makes it an optimal platform for oncolytic virotherapy of cancer. Recent advances in genetic engineering of measles virus allow insertion of therapeutic and diagnostic transgenes as well as complete retargeting of measles virus. These strategies resulted in the generation of recombinant measles viruses allowing non-invasive monitoring of viral replication and viral spread. The immune defense is a significant barrier for efficient viral gene therapy. Immune-evasive strategies have successfully been developed for measles virus enhancing its efficacy. This review gives an overview of measles virus as an anticancer agent; in particular, its use in oncologic virotherapy as well as new developments in targeting and immune evasive strategies.

The inherited porphyrias are inborn errors of haem biosynthesis, each resulting from the deficient activity of a specific enzyme of the haem biosynthetic pathway. Porphyrias are divided into erythropoietic and hepatic according to the predominant porphyrin-accumulating tissue. Three different erythropoietic porphyrias (EP) have been described: erythropoietic protoporphyria (EPP, MIM 177000) the most frequent, congenital erythropoietic porphyria (CEP, MIM 263700), and the very rare hepatoerythropoietic porphyria (HEP, MIM 176100). Bone marrow transplantation is considered as the only curative treatment for severe cases of erythropoietic porphyria (especially CEP), if donors are available. Some EPP patients who undergo liver failure may require hepatic transplantation. Murine models of EPP and CEP have been developed and mimic most of the human disease features. These models allow a better understanding of the pathophysiological mechanisms involved in EP as well as the development of new therapeutic strategies. The restoration of deficient enzymatic activity in the bone marrow compartment following gene therapy has been extensively studied. Murine oncoretroviral, and recently, lentiviral vectors have been successfully used to transduce hematopoietic stem cells, allowing full metabolic and phenotypic correction of both EPP and CEP mice. In CEP, a selective survival advantage of corrected cells was demonstrated in mice, reinforcing the arguments for a gene therapy approach in the human disease. These successful results form the basis for gene therapy clinical trials in severe forms of erythropoietic porphyrias.

Gene therapy for gastric cancer and gastric ulcer is a rationalized strategy since various genes correlate with these diseases. Since gene expressions in non-target tissues/cells cause side effects, a selective gene delivery system targeted to the stomach and/or cancer must be developed. The route of vector transfer (direct injection, systemic, intraperitoneal, gastric serosal surface and oral administration) is an important issue which can determine efficacy and safety. Strategies for cancer gene therapy can be categorized as suicide gene therapy, growth inhibition and apoptosis induction, immunotherapy, anti-angiogenesis, and others. Combination of the target gene with other genes and/or strategies such as chemotherapy and virotherapy is promising. Candidates for treatment of gastric ulcer are vascular endothelial growth factor, angiopoietin-1, serum response factor, and cationic host defense peptide cathelicidin. In this review, we discuss stomachand cancer-targeted gene transfer methods and summarize gene therapy trials for gastric cancer and gastric ulcer.

Gene delivery in cystic fibrosis is hampered by extracellular and intracellular biological barriers and inefficient vectors. Although progress is evident, continued bioengineering of DNA, vectors, and delivery technologies will be critical to ensure biocompatibility, safety, and therapeutic effectiveness. Both viral and nonviral vectors demonstrate insufficient gene expression to adequately correct chloride ion and respiratory homeostasis, but vector modifications and novel vector types continue to advance understanding of transfection processes, immunobiological responses, and cystic fibrosis pathology. Interactions of toll-like receptors and other coreceptors may be critical components of cystic fibrosis immunobiology but additional research will be needed before causative associations are widely established; however, receptor modulation provides a theoretical framework to develop new therapeutic approaches. Clinical-phase pharmacotherapies offer short-term promise to restore electrolyte imbalance and/or symptomatology, but it may be many years before gene therapy offers a curative solution for the disease.

Oncolytic HSV-1 has been developed as a novel anticancer agent. According to the properties and functions of HSV-1 encoded proteins, several genes have been targeted for engineering of oncolytic HSV-1. As a result, a variety of strategies have been applied to the engineering of oncolytic HSV-1. Success in cancer therapy for solid tumors requires a maximal oncolytic effect; however, recombinant HSV-1 that has been adapted to meet neurotoxicity requirements for the treatment of brain tumors may be too highly attenuated for effective use in solid tumors outside the brain. Recently, there has been renewed interest in the high potency of naturally oncolytic viruses. In this review, we will overview the engineered oncolytic HSV developed thus far, as well as its mechanism of selectivity and its mode of spreading within tumors. We also discuss the preclinical and clinical studies of HF-10, a non-engineered oncolytic HSV-1 virus, and its potential for use in cancer gene therapy.