Genes that promote tumors: What turns them on?

Studies on tumor-producing viruses in chicken and mice showed the presence of crucial genes that changed premalignant cells into malignant cells; these were termed oncogenic genes, or oncogenes. Later studies clarified that they were mutated forms of host-cell genes, or proto-oncogenes. There are a wide variety of such proto-oncogenes, all of which are well regulated by the cell and are necessary for normal cellular function. However, as it has been shown experimentally, ionizing radiation (IR) can alter the function of these genes. In one noted mechanism, IR breaks the proto-oncogene, another unrelated gene in another part of the genome breaks simultaneously, and the broken proto-oncogene is somehow fused to the unrelated gene. In certain cases, if the fusion occurs at the right place, this can cause activation of the proto-oncogene into an oncogene; that is, it is “turned on” and becomes unresponsive to the “off” signal. Proto-oncogenes are normally associated with the proliferation of cells. So, for example, an “on” signal will tell the cell to divide, and without an “off” signal, the cell will continue to divide.

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