Genetics and Cancer
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 the cancer susceptibility genes are at an increased risk for developing certain types of cancer. These individuals can also pass on the genetic variation to their children who can then be an increased risk for cancer as well.
How common is hereditary breast cancer or ovarian cancer?
Only 5 to 10 percent of all breast and ovarian cancer cases are hereditary, that is, the result of a genetic 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 thatnot all individuals who have a strong family history of breast or ovarian cancer are found to have BRCA1 or BRCA2 gene mutations. There are additional cancer genes that account for a small percentage of families with hereditary breast or ovarian cancer. There may be more genes that have yet to be identified.
What are the BRCA1 and BRCA2 genes and why are they so important?
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 genes or BRCA2 genes contains a mistake or genetic 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 gene 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 male breast cancer.
What are the chances that a person carries an altered BRCA1 or BRCA2 gene?
A person’s chance of carrying a BRCA1 or BRCA2 mutation depends upon their personal and family history of cancer. Individuals/families whohave 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 cancers
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 genes 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 gene 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 genetic mutations. 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.
Are there other genes that increase the risk of cancer?
Multiple other genes have been identified that contribute to an increased risk of many types of cancer. Mutations in some of these genes confer a relatively high risk of cancer (referred to as high risk cancer genes), while other genes increase the cancer risk by a modest amount (moderate risk cancer genes). Genetic testing can be done for more than 50 different cancer genes. Small panels that evaluate only the high risk genes can be performed or it is possible to test many high and moderate risk genes at one time with a larger panel.
Should I be tested?
The decision to have genetic 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 genemutation in your family. It can also help you explore how you and your family might feel about a positive or negative test result, and how you would use that information in your medical care.