If you have breast cancer, chances are it’s not caused by a faulty gene you were born with. Most of the time, genes that lead to the disease misfire sometime during your life and aren’t an inherited problem.
But in about 5% to 10% of cases, the cause is hereditary. This means that the cancer is due to a gene change, called a “mutation,” that’s passed down from a parent.
Researchers have identified hundreds of genes associated with breast cancer, but some seem to play more of a role than others.
Just because you have an inherited gene mutation doesn’t mean you’ll definitely develop breast cancer. It only means you have a greater chance of it happening.
How Are Gene Mutations Inherited?
The way that you inherit a mutation for hereditary breast cancer depends on the specific gene that affects the disease in you.
Many mutations, such as those found in the well-known BRCA1 and BRCA2 genes, are passed down in what’s called an autosomal dominant pattern. This means that inheriting one bad gene from one parent may lead to an increased chance of disease.
If your parent has an autosomal dominant gene mutation, there’s a 50% chance you’ll inherit it.
Common Breast Cancer Mutations
Researchers have identified several key gene changes linked to breast cancer. Some of these pose a high risk, while others seem to be less significant.
BRCA1 and BRCA2
The BRCA1 (breast cancer gene one) and BRCA2 (breast cancer gene two) inherited gene mutations are the most common cause of hereditary breast cancer. Mistakes in these genes account for up to 10% of all breast cancers. BRCA mutations also raise your chances for ovarian cancer, pancreatic cancer, and, in men, prostate cancer. A BRCA mutation may raise your chances of colon cancer. Women with a BRCA1 or BRCA2 mutation have up to a 72% chance of breast cancer during their lifetime.
Normally, the PALB2 gene makes a protein that works with the BRCA2 gene protein to repair damaged DNA and stop tumor growth. But defects in the gene can lead to a higher likelihood of breast cancer. Some studies suggest that women with a PALB2 mutation have a 14% chance of breast cancer by age 50 and a 35% chance by age 70.
The PIK3CA gene gives instructions to make a protein that’s important for many cell functions. A PIK3CA mutation isn’t inherited -- you can’t pass it down to your children. Instead, it’s a mutation that you develop during your life, called a sporadic mutation. PIK3CA gene mutations are found in about 30% to 40% of breast cancers. It’s important to know your tumor’s PIK3CA status because it may affect how your doctor treats your cancer. Surgeons can remove tissue in your tumor (called a biopsy) to test your breast tumor for a PIK3CA mutation, or in some cases, a blood test can find the defect.
The HER2 gene makes a protein called HER2 (human epidermal growth factor receptor 2). This protein is found on the surface of all breast cells and that helps them grow. If the HER2 gene malfunctions and makes too many copies of itself, it tells cells to make too much HER2 protein. This causes the cells to grow out of control.
Like PIK3CA, HER2 is not an inherited gene mutation. Most breast cancer is HER2-negative. Research suggests about only 10% to 20% of cases are HER2-positive. This means there’s a change in the HER2 gene that makes breast cells grow and divide uncontrollably. If you have invasive breast cancer, your doctor will probably test your tumor tissue for HER2 after you have a biopsy.
PTEN is a gene that helps control cell growth. An inherited change in PTEN can cause Cowden syndrome, a disorder that puts you at risk for cancerous and noncancerous breast tumors and other growths. Women with a PTEN mutation have a lifetime breast cancer chance of between 25% and 50%, though some studies suggest the odds are even higher.
The TP53 gene helps stop the growth of cells that have damaged DNA. An inherited TP53 mutation causes Li-Fraumeni syndrome, a disorder that ups your chances of breast cancer, leukemia, brain tumors, and cancers called sarcomas. One study found women with Li-Fraumeni syndrome have a 54% chance of breast cancer by age 70.
The ATM gene normally helps repair damaged DNA, but some people who inherit one bad copy of the gene are at high risk for breast cancer and pancreatic cancer. Research suggest the lifetime chance of breast cancer for those who carry an ATM mutation is between 33% and 38%. Those who have a type of mutation that affects a specific location on the ATM gene have a 69% lifetime chance.
CDH1 makes a protein that helps cells bind together to create tissue. People with a faulty CDH1 gene are more likely to develop a rare type of stomach cancer. Women with this mutation also have a 39% to 52% lifetime chance of invasive lobular breast cancer (breast cancer that starts in the lobules of the breast, the glands that make milk).
Researchers have discovered many other genes related to breast cancer.
Changes in the following genes may also raise your chances:
Genetic Testing for Breast Cancer Genes
Genetic tests can tell you if you have an inherited gene mutation for breast cancer. You can have a test that looks for an individual gene abnormality or a larger panel test that examines several genes at one time. Experts usually recommend gene testing if you have breast cancer or strong risk factors, such as a family history of hereditary cancers. Talk to your doctor if you’d like to learn more about genetic testing.