CHAPTER 8 Cancer—Genetics Gone Awry
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How Oncogenes Cause Cancer Like every other gene, a protooncogene provides the instructions for constructing a particular form of protein. Around 1980, scientists discovered that the protooncogene form of src made the protein kinase, which adds phosphate groups to certain amino acids, a process called phosphorylation. Phosphorylation is a normal part of cell activity. In fact, it’s essential to some activities, especially cell growth. Normally, however, kinases only kick in when signaled to do so by other cell chemicals. Kinase-producing oncogenes produce kinases constantly. Other protooncogenes carry out similar important tasks, all of which can be subverted in a way that promotes cancer. Some make growth factors, which make cells grow and reproduce; others make receptors, proteins to which growthpromoting substances attach, and which in turn send signals that activate the production of additional growth-related chemicals. The oncogene versions of all of these keep promoting growth when they should be shutting down. As the much simplified diagram in Fig. 8-2 shows, cell growth is triggered by a complex process involving many different chemical signals. Protooncogenes are involved at every step of the path.
Fig. 8-2. Cell growth begins with a signal from growth factors—chemicals sent to the cell from other cells. The growth factors attach to receptors in the cell membrane, which promote chemicals in the cytoplasm, which activate kinases, which activate chemicals that can pass through the wall of the cell nucleus to turn on transcription factors, which then turn on the genes that make the cell divide. Changes in any of the proteins involved in this complex cascade may trigger uncontrolled growth.