Oncogenes, Tumor Suppressor Genes, and Cancer

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What are mutations?

Genes are made up of DNA. The arrangement of the DNA building blocks (called bases) determines the gene and its function. Mutations are gene defects. They are abnormal changes in the DNA of a gene. Mutations involve changes in the arrangement of the bases that make up a gene. Even a change in just one base among the thousands of bases that make up a gene can have a major effect.

A mutation can affect the cell in many ways. Some mutations stop a protein from being made at all. Others may change the protein that is made so that it no longer works the way it should or it may not even work at all. Some mutations may cause a gene to be turned on, and make more of the protein than usual. Some mutations don't have a noticeable effect, but others may lead to a disease. For example, a certain mutation in the gene for hemoglobin causes the disease, sickle cell anemia.

Hereditary mutations

Hereditary mutations (also called germline mutations) are gene defects that are passed from a parent to child. Hereditary mutations are present in the egg or sperm that join during fertilization and develop into a fetus. Because the mutation is present at the beginning, it exists in all cells of the body, including reproductive cells (the cells that make sperm in males or the egg cells in females). This means the mutation can be passed from generation to generation.

A hereditary mutation is a major factor in about 5% to 10% of all cancers.

Some people are more likely to develop cancer than others simply because they are born with mutations in their genes. To learn more about this, see our document, Heredity and Cancer.

Acquired mutations

Most cancers are caused by DNA changes that happen during the person's life. These are called acquired, sporadic, or somatic mutations. An acquired mutation can be caused by things in the environment such as exposure to radiation or toxins. But for most acquired mutations, no specific cause can be found.

Unlike the inherited mutations, acquired mutations start in one cell of the body and are found only in the offspring of that cell. They are not in every cell of the body. Because they are not in the reproductive cells, acquired mutations cannot be passed on to the next generation.

It is important to realize that mutations in our cells happen all the time. Usually, the cell detects the change and repairs it. If it can’t be repaired, the cell will get a signal telling it to die in a process called apoptosis. But if the cell doesn't die and the mutation is not repaired, it may lead to a person developing cancer. This is more likely if the mutation affects a gene involved with cell division or a gene that normally causes a defective cell to die.

Most scientists today believe that cancer develops in a process that has more than one, and likely several, mutations. We have 2 copies of each gene (one from each chromosome in a pair). So, even when a person inherits a mutation, at least one more mutation is needed to "knock out" the other copy of that gene (so that it doesn't function). This acquired mutation is needed before a person develops a heredity-related cancer. Sometimes acquired mutations in other genes (such as oncogenes) are needed as well. For a person who is not born with a mutation, 2 acquired mutations (one in each copy of the same gene) are needed to knock out that gene.

Last Medical Review: 12/27/2011
Last Revised: 12/27/2011