2025 false infectious agents are thought to

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20–25 False. Infectious agents are thought to participate in the formation of roughly 15% of human cancers. In most cases, the specific mechanism is unknown, but there are strong associations, for example, between hepatitis viruses type B and type C and liver cancer, between H. pylori infection and stomach cancer, and between blood fluke infection and bladder cancer. 20–26 False. Although it is popular to think so, there is scant evidence to support those ideas. However, we certainly know of specific instances—such as 2- naphthylamine and asbestos—where industrial products cause human cancers. THOUGHT PROBLEMS 20–27 These data are consistent with the idea that cancer is a multi-step process in which cancer-causing changes accumulate over time. The 25-year delay between exposure and cancer reflects the time it takes for lung cells to accu- mulate a sufficient number of changes to become cancerous. From other studies it is known that cigarette smoke contains tumor initiators and tumor promoters, both of which contribute to the progression from normal to can- cerous cells. Your uncle’s suggestion that there is a genetically predisposed fraction of the population that is prone to lung cancer does not match the data. If a fixed fraction of the population were genetically predisposed, the incidence of lung cancer would be relatively constant over time. It would not be expected to track with per capita smoking. DATA HANDLING 20–28 The highly rearranged karyotypes and their similarity from tumor to tumor suggest that the cancer cells themselves are being transmitted from devil to devil. It is extremely unlikely that an infectious agent such as a virus or a microorganism could induce the same set of complicated rearrangements in different animals. Most importantly, the existence of a chromosome-5 inversion in one Tasmanian devil, which is not present in chromosome 5 of its tumor cells, argues strongly that the tumors are not generated from the host devil’s own cells. It appears that this cancer has arisen from a rogue line of cancer cells, from a tumor of unknown origin, that has acquired the capa- bility for parasitic existence. This is one of just two examples of natural transmission of cancer by tumor cells, the other being a venereal disease in dogs. A special case of such transmission occurs occasionally during organ transplantation in humans. But the requirements for organ transplanta- tion—matching tissue and immune suppression—highlight just how
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FINDING THE CANCER-CRITICAL GENES A461 unusual natural transmission is. The cancer cells responsible for facial tumors in Tasmanian devils must somehow evade the new host’s immune defenses. Reference: Pearse A-M & Swift K (2006) Transmission of devil facial-tumour disease. Nature 439, 549.
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