Gene therapy of genetic diseases and cancer

The scope of gene transfer applications in human therapy has expanded enormously over the last 15 years to include not only several types of genetic diseases but also a variety of genetic approaches to the treatment of cancer. Hematopoietic stem cells have been considered excellent targets for therapeutic gene transfer because of their capacity for self-renewal and for differentiation into multiple cellular lineages. Retrovirus-mediated gene transfer has been tested for treatment of diseases that specifically affect the hematopoietic system, such as adenosine deaminase deficiency and chronic granulomatous disease. Storage disorders such as Gaucher disease, Hurler syndrome and Hunter syndrome, genetic deficiencies that affect a broad range of tissue types, may also be amenable to treatment by gene transfer into hematopoietic cells, owing to the release of enzyme expressed in transduced cells with subsequent uptake by untransduced cells ('metabolic cross- correction'). Hematopoietic stem cells may also be targeted for introduction of drug-resistance genes for the purpose of protecting normal tissues from the toxic side-effects of cancer chemotherapeutic agents, thus allowing more effective antitumor chemotherapy. The danger of introducing drug-resistance function into tumor cells may be dealt with by including sequences specifically designed to reduce expression of oncogenes or to restore expression of tumor suppressor genes. Current limitations on the efficiency of gene transfer into hematopoietic stem cells may be alleviated by the development of new vector systems such as adeno-associated virus or lentivirus vectors, or by advances in cell processing that render hematopoietic cells more susceptible to transduction. Drug-resistance genes may also be applied for in vivo selection to expand the representation of a small proportion of transduced hematopoietic cells. These approaches toward increasing the frequency of hematopoietic cell transduction contribute to the anticipated feasibility of gene therapy for genetic diseases and cancer.