Gene therapy is a newly hatched field of biomedical research aimed at introducing therapeutically important genes into somatic cells of patients for the treatment of human disease. Whereas for inborn errors of metabolism transfer of a single gene can correct the disorder, cancer is a complex disease involving mutations in a number of protooncogenes and tumor suppressor genes as well as an imbalance and disarray in phosphorylation events and regulatory circuits of the cell cycle; transfer of the wild-type p53 or p21 tumor suppressor genes is a successful gene therapy approach leading to apoptotic death of cancer cells or in restrain of their chaotic growth. A different promising approach is transfer of the herpes simplex virus thymidine kinase (HSV-tk) gene (suicide gene) and systemic treatment with the prodrug ganciclovir which is converted by HSV-tk into a toxic drug killing dividing cells. Expression of suicide genes, p53, and other therapeutic genes preferentially in cancer cells can be achieved by regulatory elements from tumor-specific genes such as carcinoembryonic antigen, BRCA1, and PSA.

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