Genetics & Cancer

What are the BRCA1 and BRCA2 genes and why are they so important?

Alterations in specific genes have been linked to an increased risk for some cancers. These genes are often referred to as cancer susceptibility genes, or “cancer genes”. Individuals who carry an alteration in a cancer susceptibility gene are at an increased risk for developing certain types of cancer. These individuals can also pass on the alteration to their children who can then be an increased risk for cancer as well.

Cancer occurs when cells grow in an uncontrolled way. The BRCA1 and BRCA2 genes are two important genes that control cell growth and division. All individuals have two copies of BRCA1 and two copies of BRCA2. If one of the BRCA1 or BRCA1 genes contains a mistake or mutation, certain cells in the body lose their ability to control cell growth and division. Therefore, an altered or mutated BRCA1 or BRCA2 gene increases the likelihood that cancer will develop. However, it is important to realize that a person who carries a BRCA1 or BRCA2 mutation is not at risk for all types of cancer. Also, having a BRCA1 or BRCA2 mutation does not guarantee that cancer will develop. In women who carry BRCA1 or BRCA2 mutations, the most common cancers are breast and ovarian cancer. In men, the most common cancers are prostate and possibly colon cancer.

How common is hereditary breast or ovarian cancer?

Only 5 to 10 percent of all breast and ovarian cancer cases are hereditary, that is, the result of a mutation in a single gene that is being passed down through a family. Most cases of breast and ovarian cancer (90 to 95 percent) are due to a combination of a person’s genetic makeup and environmental factors.

Of the 5 to 10 percent of breast and ovarian cancer cases that are thought to be inherited, the majority (up to 80 percent) are caused by mutations in the BRCA1 or BRCA2 genes. This means that not all individuals who have a strong family history of breast or ovarian cancer are found to have mutations in BRCA1 or BRCA2. As a result, researchers believe that there may be additional cancer genes that account for a small percentage of families with hereditary breast or ovarian cancer. These genes have yet to be identified.

What are the chances that a person carries an altered BRCA1 or BRCA2 gene?

A person’s chance of carrying a BRCA1 or BRCA2 alteration depends upon their personal and family history of cancer. Individuals/families that have an increased chance of carrying a BRCA1 or BRCA2 alteration usually have the following characteristics:

  • More than two individuals with either breast or ovarian cancer
  • More than one generation affected with breast and/or ovarian cancer
  • An earlier age of onset (<50 years)
  • Individuals with more than one cancer, such as bilateral breast cancer or both breast and ovarian cancer

There are also certain ethnic groups in which BRCA1/2 mutations are more common. Among Ashkenazi Jews, there are three specific mutations in the BRCA1 and BRCA2 genes that are more common. It has been estimated that 1 in 40 (2.5 percent) Ashkenazi Jewish individuals carry a BRCA1 or BRCA2 mutation. This chance is higher in Ashkenazi Jewish families with multiple cases of breast and/or ovarian cancer.

What are the chances of inheriting an altered BRCA1 or BRCA2 gene?

If an individual (male or female) is found to carry an altered BRCA1 or BRCA2 gene, their children and siblings have a 50 percent chance of also being carriers. Likewise, each child and sibling would have a 50 percent chance of not being a carrier. In the latter case, they would have cancer risks similar to the general population, and they could not pass the altered gene on to their children.

How are BRCA1 and BRCA2 mutations detected?

It is now possible to test for BRCA1 and BRCA2 mutations. By taking a blood sample from someone who has had breast or ovarian cancer, the laboratory will look for alterations in these genes. Once an alteration has been identified in a family, other family members who have not had cancer can be tested. This is called predictive genetic testing.

Should I be tested?

The decision to have BRCA1 or BRCA2 testing is a complicated one. Individuals and families must not only weigh the risks and benefits of testing, they must also consider their unique situations. It may be helpful to discuss the risks and benefits of testing with a genetic professional, such as a genetic counselor or a geneticist. Genetic counseling can help you determine if testing is appropriate for your family, and can clarify the actual likelihood of a BRCA1 or BRCA2 mutation in your family. It can also help you explore how you and your family might feet about a positive or negative test result, and how you would use that information in your medical care.

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