How Cancer BeginsCancer begins when a cell undergoes a mutation, that is, one or more of its genes is damaged or lost.
Mutations can occur when a cell is reproducing, but it’s not easy for a cell to become cancerous. A number of different mutations have to happen before the cell becomes a cancer cell. And, if a cell carries a mutation, it usually either destroys itself or is recognized as being abnormal by the immune system and killed.
This is why cancer usually occurs in older people: there has been more time for exposure to carcinogens and for chance mutations to occur.
Genes at RiskThere are three different types of genes that can have an important part in making a cell turn malignant:
- Oncogenes (“cancer genes”) encourage the cell to multiply. Normally, in adults, cells divide only rarely, for instance to replace cells damaged by a wound. If these genes become abnormal, however, they can instruct the cell to multiply without stopping, protect it against programmed cell death, and give it the ability to live in diverse tissues.
- Tumor-suppressor genes tell the cell to stop multiplying. If a tumor-suppressor gene is damaged, the cell won’t receive the instruction to stop doubling and will multiply without stopping. This infinite lifespan is one of the hallmarks of a cancer cell. The best-known tumor-suppressor gene is called p53. p53 is damaged or missing in most human cancers.
- DNA repair genes repair damage to the DNA of other cells’ genes. If the repair genes themselves are damaged, other mutations aren’t repaired and the abnormal cell copies its mutations into its daughter cells.
How Genes Are DamagedGenes become damaged in several ways:
- Damage due to free radicals produced in the normal process of metabolism
- Exposure to carcinogens, such as radiation, chemicals, tobacco, and infectious agents
- Random errors in DNA replication
- Inherited mutated gene