Current Gene Therapy - Volume 1, Issue 2, 2001

Despite the fact that adeno-associated virus type 2 (AAV2) is an extremely attractive gene therapy vector, its application has been limited to certain tissues such as muscle and the brain. In an attempt to broaden the array of target organs for this vector, molecular studies on the mechanism(s) of AAV transduction have expanded over the past several years. These studies have led to the development of innovative strategies capable of overcoming intracellular barriers to AAV2 transduction. The basis of these technologic breakthroughs has stemmed from a better understanding of the molecular processes that control AAV entry and intracellular trafficking to the nucleus. This review will focus on the identification of molecular components important for recombinant AAV (rAAV) transduction while highlighting the techniques used to discover them and potential clinical application of research findings.

Human gene therapy promises to change the practice of medicine by treating the causes of disease rather than the symptoms. Since the first clinical trial made its debut ten years ago, there are over 400 approved protocols in the United States alone, most of which have failed to show convincing data of clinical efficacy. This setback is largely due to the lack of efficient and adequate gene transfer vehicles. With the recent progress in elucidating the molecular mechanisms of human diseases and the imminent arrival of the post genomic era, there are increasing numbers of therapeutic genes or targets that are available for gene therapy. Therefore, the urgency and need for efficacious gene therapies are greater than ever. Clearly, the current fundamental obstacle is to develop delivery vectors that exhibit high efficacy and specificity of gene transfer. Recombinant adenoviruses have provided a versatile system for gene expression studies and therapeutic applications. Of late, there has been a remarkable increase in adenoviral vector-based clinical trials. Recent endeavors in the development of recombinant adenoviral vectors have focused on modification of virus tropism, accommodation of larger genes, increase in stability and control of transgene expression, and down-modulation of host immune responses. These modifications and continued improvements in adenoviral vectors will provide a great opportunity for human gene therapy to live up to its enormous potential in the second decade.

Clinical gene therapy needs non invasive tools to evaluate the efficiency of gene transfer. This includes the evaluation of infection efficiency as well as the verification of successfull gene transfer in terms of gene transcription. These informations can be used for therapy planning, follow up studies in treated tumors and as an indicator of prognosis. Therapy planning is performed by the assessment of gene expression for example using radiolabeled specific substrates to determine the activity of suicide enzymes as the Herpes Simplex Virus thymidine kinase or cytosine deaminase. Furthermore, other in vivo reporter genes as receptors, antigens or transport proteins may be used in bicistronic vector systems for the evaluation of gene transduction and expression. This is done using radiolabeled ligands, antigens or substrates. Follow up studies with magnetic resonance imaging, single photon emission tomography or positron emission tomography may be done to evaluate early or late effects of gene therapy on tumor volume, metabolism or proliferation. Finally, enhancement of radioactive isotope accumulation in tumors by transfer of the appropriate genes may be used for the treatment of malignant tumors.

In the last few years, significant advances in gene therapy have been made as a result of advances in many areas of molecular and cell biology, including the improvement of both viral and nonviral gene delivery systems, discovery of new therapeutic genes, better understanding of mechanism of disease progression, exploration of tissue specific promoter, receptor- and antibody-mediated targeting delivery, and development of better prodrug enzyme / prodrug systems. In this article, viral based gene therapy for prostate cancer will be reviewed and discussed. The areas of emphasis in this review are choice of viral vectors, comparison of delivery routes, development of prostate-targeted viruses, choice of therapeutic genes and strategies including corrective gene therapy (tumor suppressor gene and anti-oncogene gene approaches), suicide gene therapy, programmed cell death therapy, immunomodulation therapy, and conditional oncolytic virus approach. Among them, several examples will be discussed in detail for the scientific basis and therapeutic applications. In addition, prostate cancer gene therapy clinical trials, unresolved problems and future directions in this field will also be described.

The last 10 years have seen substantial progress in the development and application of nonviral vectors in gene therapy. Several novel nonviral methods have been developed that approach viruses with respect to transfection efficiency. A variety of nonviral delivery systems that can be used for gene therapy in different clinical settings are also available. In this review article, we will detail all of the major nonviral vectors that are currently used in gene therapy while highlighting some recent developments, particularly the progress towards the understanding of the cellular and in vivo barriers in gene transfer. Recent advancement in achieving sustained and regulated gene expression will also be addressed. Finally, this review will briefly cover targeted gene repair using nonviral delivery systems. Their impact in gene therapy will also be discussed.

Over the past decade, bone marrow transplantation has come to be considered an ideal therapeutic strategy for the treatment of certain diseases affecting the hematopoietic system such as hemophilia, and several clinical trials have Been performed. Although traditionally used for the treatment of lethal diseases, it is speculated that this approach could also be used in the treatment of non-lethal but much more common diseases, which are resistant to conventional therapies, and affect a large number of patients physically and even financially.Inflammation may be one target for transplantation-based gene therapy, since macrophages and neutrophils, which are basically derived from hematopoietic stem cells, have been identified as key determinants in the development of diseases.This article focuses on the glomerulonephritis as a model of local inflammation and reviews recent investigations on transplantation-based gene therapy for inflammatory diseases.