Gene Therapy and Cancer
In 1997, an estimated 1.38 million Americans will be newly diagnosed with cancer (Blaese, 1997). The treatments available only cure half of them. Many new strategies, including gene therapy are in developmental stages for treating cancer. Nearly half of all gene therapy trials currently under way deal with cancer and experts believe a number of these applications will be in use within the next three to five years (Lyon, 1997).
Cancer is considered a genetic disorder. Studies have identified a small number of genes that must be mutated to bring about development of cancer or maintain the growth of malignant cells (Klug, 1996). Two main properties of cancer are uncontrollable cell division and the ability to spread or metastasize. Both are results of genetic alterations. Mutations in the cells that lead to certain forms of cancer, can be identified as inherited in some families. In most cases, however, it is difficult to identify a simple pattern of inheritance.
There are two ways to regulate cell division. One way is with tumor suppressor genes, which usually function to inactivate or repress cell division. These genes or their products or both, must be inactivated sporadically for cell division of take place. If they are permanently inactivated or lost through mutation, uncontrolled cell division occurs. Another way cell division is regulated is by proto-oncogenes, which usually promotes cell division also. These genes can be in an "on" or "off" mode and when in the "on" mode, cell division is promoted. When the genes or their products or both are inactivated, cell division is stopped. If they are permanently switched "on", cell regulation is stopped and tumor formation begins.
Oncogenes are the mutant form of proto-oncogenes. An example of a transformation of a proto-oncogene to an oncogene is the p53 gene, which encodes a nuclear protein that acts as a transcription factor. The p53 gene is usually a tumor suppressor gene that controls passage of the cell from one phase of mitosis to another. The mutations in p53 gene are estimated to be associated with over half of all cancers.
The most prevalent cause of death in cancer patients is metastasis, where cancer cells detach from the original tumor site and settle elsewhere in the body, to grow and divide producing another tumor. There are two kinds of tumors, benign and malignant. Benign tumors can be removed and usually do not return.