ABSTRACT
Diabetes Research Group, Institute of Cellular Medicine, University of Newcastle, Newcastle upon Tyne, U.K.
UNDERLYING CONCEPTS AND POSSIBILITIES
Gene therapy encompasses transfer of any prophylactic or therapeutic gene to human or animal cells resulting in subsequent expression in patients. Transfer of DNA can be achieved in situ by direct delivery to the host cells of the individual to be treated or performed ex vivo in cells that are subsequently transplanted. Cells or organs to be genetically manipulated prior to transplantation may be harvested and returned to the patient. Alternative sources include living or deceased human donors, animal donors, and proliferative transformed cell lines. Cell lines may be of embryonic or adult human or animal origin. The DNA sequence encoding the transgene can be incorporated into plasmid or viral vectors. In addition to employing naked plasmid DNA or a viral vector alone, approaches to achieve and enhance successful gene transfer in situ and ex vivo include plasmid complexation with a liposomal transfection agent or a wide range of other adjuvants in addition to electroporation. A wide range of targeting strategies in addition to natural tropisms associated with various viral vectors have been harnessed to attain a degree of selectivity in targeting delivery to specific organs following intravenous injection. Expression restricted to specific organs or cell types can be achieved by employing tissue-specific promoters to initiate and regulate transgene expression. These promoters may enable physiological regulation at the level of transcription. Alternatively, strong viral promoters may enable efficient constitutive expression regardless of cell type. Incompletely understood mechanisms that
may have evolved as a defense against pathogenic viral expression in eukaryotic cells may, however, lead to downregulation of viral promoters [reviewed in (1)]. Promoters have been further engineered to enable pharmacological regulation of transcription following administration or withdrawal of a specific small molecule ligand (2). Regulators include tetracycline or related antibiotics, FK506 analogues, and progestogens. Each system has potential advantages and disadvantages but there has been little experience of any regulated gene therapy approach in humans to date.
Viral Vectors
