The Genetics of Cancer

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 more about the link between cancer and genetics. Viruses, ultraviolet (UV) radiation, and chemicals can all damage genes in the human body. If particular genes are affected, a person can develop cancer.

Genes and chromosomes

Genes are the basic functional and physical unit of heredity that is passed on from parent to child. They are made of deoxyribonucleic acid (DNA) and are located on structures called chromosomes in every cell of a person's body. Genes direct much of what happens in the body, such as the determination of eye color, blood type, and in some cases, the development of cancer.

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

How genes cause cancer

For various reasons, genes often undergo mutations (changes). Some mutations have no effect on a cell, while other mutations are harmful or helpful to the cell. There are two basic kinds of genetic mutations. If the mutation is passed from one of the parents to the child, it is called a germline mutation. When a germline mutation is passed on from a parent to the child, it 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. This is also called familial (occurring in families) cancer.

Most cancers are caused by a series of mutations that develop during a person's lifetime called acquired mutations. Acquired mutations are caused by tobacco, over-exposure to UV radiation, and other toxins and chemicals. These mutations are not in every cell of the body and are not passed from parent to child. Cancer caused by this type of mutation is called sporadic 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. These are protective genes. Normally, they suppress (limit) cell growth by monitoring the rate at which cells divide, repairing mismatched DNA, 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. Close to 30 tumor suppressor genes have already been identified, including BRCA1, BRCA2, and p53. In fact, nearly 50% of all cancers involve a missing or damaged p53 gene.

Oncogenes. These genes turn a healthy cell into a cancerous one. HER2/neu and ras are two common oncogenes.

DNA repair genes. These are genes that 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.