Recent scientific breakthroughs in the genomics field and our understanding of the important role of genes in disease has made gene therapy one of the most rapidly advancing fields of biotechnology with great promise for treating inherited and acquired diseases.
Many human diseases are caused by the absence or inappropriate presence of a protein. The protein could then be administered to patients in order to compensate for its absence. Today, gene therapy is the ultimate method of protein delivery, in which the delivered gene enters the body's cells and turns them into small "factories" that produce a therapeutic protein for a specific disease over a prolonged period.
As gene therapy has moved from the laboratory into the clinic, several issues have emerged as central to the development of this technology: gene identification, gene expression and gene delivery. A number of disease-related genes with direct clinical have already been identified, and this number is growing as the field rapidly advances. Genes with broader clinical application are also being utilized to make cells express immune activating agents locally at the disease site or to become susceptible to further drug treatment or to immune response recognition.
Molecular genetics has spawned an impressive outpouring of insights into the biology of neoplastic transformation and the host-tumor relationship. This deeper understanding of cancer pathogenesis presents a rich opportunity to develop novel therapeutic agents with improved selectivity for cancer cells.
One promising approach involves gene therapy, which is the introduction of genetic material into a patient's tissues with the intent to achieve therapeutic benefit. A number of gene transfer systems have been designed that enable the genetic modification of relevant target cells, albeit with varying strengths and limitations. Several strategies to exploit gene transfer as a tool to target specific molecular defects intrinsic to cancer cells, enhance tumor chemosensitivity, and augment tumor immunogenicity are under intensive investigation. A number of these approaches have entered initial clinical testing and already provide intriguing new information about the biology of cancer in patients.sickle cell anemia where a variant globin causes hemoglobin to polymerize under low oxygen tension, thereby damaging the red blood cell. In this situation, gene transfer and expression of a normal globin chain is still expected to benefit the patient.
In yet other instances, such as in dominantly inherited connective tissue disorders in which the presence of an abnormal molecule interferes with normal tissue development and function, only selective silencing of the mutant gene would be expected to be of benefit to the patient.
Although "gene addition" is the simplest strategy for somatic gene therapy, several practical difficulties need to be addressed. Particularly important among these is the need in many instances to deliver the appropriate gene to a specific cell type or tissue. Other challenges includes gaining access to the relevant cell type for correction, assessing the total fraction of cells in a tissue that need to be corrected, achieving the level of expression required for correction, and regulating expression of the added gene once it is transferred into appropriate target cells.Multifactorial disorders
For a variety of more common diseases (e.g., coronary heart disease, diabetes), typically several genes are involved, making a single gene mechanism exceptional. Knowledge of pathophysiology is beginning to suggest how in particular instances the introduction of specific genes might reverse or retard disease processes at the cellular level. This general approach may prove effective regardless of genetic etiology and without the need to replace a single, missing gene product. For instance, in restenosis following angioplasty, local transfer into vascular cells of genes reducing proliferative and thrombotic processes might prevent reocclusion.
The possibilities for gene transfer as a treatment for common multifactorial diseases are vast. The precise approach needs to be assessed in each instance by considering how specific gene products influence cellular physiology. We can expect many different, sometimes speculative, strategies to be proposed. Each will need to be judged in comparison with conventional treatment approaches.In principle, a number of chronic infectious diseases, including several types of hepatitis and herpesvirus infections, may be suitable targets for gene therapy approaches. However, only HIV infection has received much attention to date. Current efforts focus on two general areas: postexposure vaccination in an attempt to boost the host immune response to the infection and attempts to express genes in target cells that render them unable to be infected or of supporting HIV replication. Although a handful of trials are ongoing at present, they are in very early stages, and no results have been published.
In vaccination trials, modified HIV genes are introduced directly into infected individuals following ex vivo treatment of target CD4 or precursor cells, typically with retroviral vectors that express genes encoding antiviral products. Several such products are being tested: mutant proteins that inhibit virus replication; antisense RNA that blocks translation of HIV gene products or causes destruction of the HIV genome; ribozymes that attack HIV RNA at specific unique sites; "decoy" RNAs that efficiently compete for binding of viral proteins; and singlechain antibodies that prevent key HIV enzymes from functioning. Although these approaches block HIV replication in cell culture systems, serious obstacles to their practical application remain. Most importantly, it is not yet known what cell types to target, much less how they will be isolated, treated, and returned to the patient. Furthermore, it is unknown whether resistant mutants-the major obstacle to successful drug therapy-will also present a serious problem. Nevertheless, the pursuit of gene therapy remains an active area of acquired immunodeficiency syndrome research, and one that also promises to provide important insights into HIV pathogenesis.
The past two decades have seen an explosion both in our understanding of mammalian gene structure and also in our ability to manipulate DNA sequences. This, coupled with an increased understanding of the nuances of gene transfer, has clearly ushered in an era of gene therapy. The treatment of clinical cardiovascular disease has lagged somewhat behind the treatment of infectious and proliferative cellular disorders because of the polygenic nature and complex biology of many of the more common cardiac disorders, however, strategies are now developing that should allow the application of gene therapy approaches to many cardiovascular disorders.
The following reviews discussed several central issues in gene therapy, including the choice and design of gene targeting vectors and the practical and theoretical limitations of gene therapy, current progress toward gene therapy for a variety of cardiovascular diseases .
Currently, this application lag behind cancer and single-gene metabolic diseases due to its inherent difficulties.
The major difficulties preventing successful gene-based therapies relate to accessibility of target tissues and the multifactorial nature of many neurological disorders. The central nervous system is generally not amenable to the types of ex vivo interventions successful with blood cells or hepatocytes. Such cells are also difficult to easily target with in vivo approaches. For example, one cannot remove adult neuronal tissue, modify the cells in vitro and reinsert them into the patient because such calls are no longer actively dividing. However, it is possible that Parkinson's disease may be amenable to gene therapy if autologous, actively dividing cells, perhaps bone marrow stem cells or fibroblasts which may be able to successfully populate certain brain regions, can be modified to express the gene for tyrosine hydroxylase. Such cells, when reinfused directly into the brain would then be producing L-dopa or dopamine locally at sufficient concentrations to alleviate the disease phenotype.
Another approach for neurological disease gene therapy is the use of in vivo viral vectors carrying the gene of interest which are targeted to the appropriate region of the CNS. Obviously, for most such diseases, direct injection into the target tissue may be required, as the blood-brain barrier is a substantial obstacle to such an approach. Another problem is that most neural tissues can not be treated with retroviruses which, although they are very efficient at gene delivery, require their target cells to be actively dividing. Therefore, other vector systems, such as herpes simplex or adenoviral vectors are being developed.
