The ancient Greeks believed that cancer was caused by too much body fluid they called “black bile.” Doctors in the seventeenth and eighteenth centuries suggested that parasites caused cancer. Today, doctors understand that cancer is genetic, meaning that changes to various genes in the body can lead to cancer.

Genes and chromosomes

Genes are the basic functional and physical units of heredity that are passed from parent to child. They are made of deoxyribonucleic acid (DNA) and are located on structures called chromosomes located in every cell of a person’s body. Genes provide the instructions to keep your body functioning. Most people are familiar with genes for their role in determining a person’s physical characteristics, such as eye color.

The human body contains nearly 30,000 genes. They are located on 46 chromosomes and arranged in 23 pairs. Chromosome pairs are numbered 1 through 22 and are called autosomes. The remaining two chromosomes (the 23rd pair) are the sex chromosomes, which determine whether someone is born female or male. Because genes come in pairs, a person inherits one gene in each pair from the mother and the other from the father.

The role of genes in cancer

In a person’s body, cells are continually dying and being replaced. For example, cells in the skin, liver, and intestines are replaced every few weeks. The body makes new cells by copying the old cells, and in this process (called cell division), there may be mistakes in how the genetic material is copied. These are called mutations. Some mutations have no effect on a cell, while other mutations are harmful or helpful to the cell. If a mutation is not corrected, and it occurs in a critical part of the gene, it may lead to cancer. However, the chance of one mutation leading to cancer is rare. Usually it takes multiple mutations over a lifetime to produce a cancer cell. This is why cancer occurs more often in older people—there have been more opportunities for mutations to accumulate.

There are two basic kinds of genetic mutations. If the mutation is directly passed from a parent to a child, it is called a germline mutation. This means that the mutation is present in every cell of the child’s body, including the reproductive sperm and egg cells. Because the mutation affects reproductive cells, it is passed from generation to generation. Germline mutations are responsible for 5% to 10% of cancer cases, which is called familial (occurring in families) cancer.

Acquired mutations occur during a person’s life and are not passed from parent to child. These mutations are caused by tobacco, UV radiation, viruses, age, and other factors. Cancer caused by this type of mutation is called sporadic cancer, which is much more common than familial cancer.

Most scientists believe that cancer happens when several genes of a particular group of cells become mutated. Some people may have more inherited mutations than others, and even with the same amount of environmental exposure, some people are simply more likely to develop cancer.

Genes that play a role in cancer

The following types of genes contribute to cancer:

• Tumor suppressor genes are protective genes. Normally, they suppress (limit) cell growth by monitoring the rate at which cells divide into new cells, repairing mismatched DNA (a cause of mutations), and controlling cell death. When a tumor suppressor gene is mutated (due to heredity or environmental factors), cells continue to grow and can eventually form a tumor. BRCA1, BRCA2, and p53 are examples of tumor suppressor genes. In fact, nearly 50% of all cancers involve a missing or damaged p53 gene.
  
  
• Oncogenes turn a healthy cell into a cancerous one. HER2/neu and ras are two common oncogenes.
  
  
• DNA repair genes fix any mistakes made when DNA is replicated (copied). Mistakes that aren’t fixed become mutations, which may eventually lead to cancer, especially if the mutation occurs in a tumor suppressor gene or oncogene.

Cancer develops when several genes in a cell become mutated in a way that overrides the checks and balances of the cell. However, many cancers cannot be tied to a specific gene, and some genes may interact in unpredictable ways with other genes or factors in the environment to cause cancer. In the future, doctors hope to learn more about the role of genetic changes in the development of cancer, which may lead to improved cancer treatment and prevention strategies.

More Information

Explaining Cancer Genome Research

Genetics

Additional resources

National Cancer Institute: Cancer Genetics

Talking Glossary of Genetic Terms: Find more than 200 genetic terms from the National Human Genome Research Institute

Last Updated: August 13, 2009