Genes That Contribute to Cancer

The signals that regulate cell division fall into two basic types: molecules that stimulate cell division and those that inhibit it. These control mechanisms are similar to the accelerator and brake of an automobile. In normal cells (but hopefully not your car), both accelerators and brakes are applied at the same time, causing cell division to proceed at the proper speed.

Because cell division is affected by both accelerators and brakes, cancer can arise from mutations in either type of signal, and there are several fundamentally different routes to cancer (IFigure 21.24). A stimulatory gene can be made hyperactive or active at inappropriate times, analogously to having the accelerator of an automobile stuck in the floored position. Mutations in stimulatory genes are usually dominant, because a mutation in a single copy of the gene is usually sufficient to produce a stimulatory effect. Dominant-acting stimulatory genes that cause cancer are termed oncogenes. Cell division may also be stimulated when inhibitory genes are made inactive, analogously to having a defective brake in an automobile. Mutated inhibitory genes generally have recessive effects, because both copies must be mutated to remove all inhibition. Inhibitory genes in cancer are termed tumor-suppressor genes.

Although oncogenes or mutated tumor-suppressor genes or both are required to produce cancer, mutations in DNA repair genes can increase the likelihood of acquiring mutations in these genes. Having mutated DNA repair genes is analogous to having a lousy car mechanic who does not make the necessary repairs to a broken accelerator or brake.

Oncogenes and tumor-suppressor genes Oncogenes were the first cancer-causing genes to be identified. In 1910, Peyton Rous described a virus that caused connective-tissue tumors (sarcomas) in chickens; this virus became known as the Rous sarcoma virus. A number of other cancer-causing

Table 21.7 Examples of geographic variation in the incidence of cancer

Type of Cancer

Location

Incidence Rate*

Lip

Canada (Newfoundland)

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