CHAPTER 9

Altered Genes

PART 1 How Cancer Begins

Cancer 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.

PART 2 Genes at Risk

There 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.

LESS

PART 3 How Genes Are Damaged

Genes 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

PART 4 Genetic Predisposition

People who are said to have a genetic predisposition to certain types of cancer have a greater chance of developing that cancer than other people. This is because they were born with one of the mutations that can lead to a cell becoming cancerous. They may never develop that cancer, because the further mutations that cause the cancer may never take place. But they are at higher risk, because fewer mutations need to take place in their genes for the disease to arise.