As a genetic counselor, I talk about genetic testing with individuals who have a personal or family history of cancer. We discuss the chance that their cancer or their relative’s cancer might be inherited, and review the option of testing to determine if they carry an inherited genetic mutation that may increase their risk of cancer. Occasionally, someone with a cancer diagnosis will say, “My doctor already did genetic testing”. In most cases, the tests already done were genomic, rather than genetic tests.

What does genetic testing look at?

Genetic testing has traditionally referred to the analysis of genes that a person was born with. This is “germline” genetic testing, so called because what is being analyzed are genes as they were present in the germ cells (eggs and sperm) and therefore in the fertilized egg from which the embryo and fetus developed. A mutation present in the fertilized egg (a single cell) will be present in every cell that develops from that egg, and therefore, in (almost) every cell in a person’s body. 

Hereditary cancer testing is genetic testing that is focused on finding an inherited mutation in a single gene, like BRCA1 or BRCA2. Genetic testing is available to individuals with or without cancer, and can be performed on multiple different tissues from the body, blood or saliva being the most common. Testing for a gene mutation associated with hereditary breast cancer is typically offered with a cancer diagnosis at age 50 or younger, triple negative cancer, personal history or close relative with ovarian, pancreatic or metastatic prostate cancer, male breast cancer, Ashkenazi Jewish ancestry, or known mutation in the family. The number of genes analyzed varies, but may be anywhere from 1 to around 80 or more, depending on what is known about personal and family history, and how much information is desired.

How genetic testing helps with breast health planning

If genetic testing identifies a germline mutation, this provides the individual and their healthcare team with information about the risk to have cancer in the future, type of cancer, and the chance for close relatives to have the same mutation. People who discover they are carriers of harmful genetic mutations prior to a cancer diagnosis can work with a medical team to create a risk reduction plan. If cancer develops, knowing there is a germline mutation may help oncologists determine the best course of treatment. For example, an individual with a germline BRCA1 or BRCA2 mutation may be eligible for treatment with a PARP inhibitor after other cancer treatments are completed.

Germline vs Somatic (Sporadic) Mutation

Adapted from the National Cancer Institute and the American Society of Clinical Oncologists.

What does genomic testing focus on?

Genomic testing is focused on finding acquired mutations in multiple genes in cancer tissue. (Acquired mutations are also referred to as somatic or sporadic mutations. Somatic mutations cannot be passed on to children.) In most of the cells in a body, genes are unchanged from birth. However, genetic changes, or damage to DNA, can happen after birth, due to aging and environmental exposures. Common exposures that cause genetic changes, or somatic mutations, are sunlight, use of tobacco, use of alcohol, occupational exposures (such as asbestos or benzene) and viruses (such as HPV or HIV). Somatic changes do not always lead to cancer. Cells with DNA damage often die naturally. But a cell with DNA damage that acquires more damage in multiple genes, and creates more cells with the same abnormal DNA, may develop into a tumor.

What does genomic testing have to do with breast cancer?

Each tumor has a unique pattern of DNA. Analysis of this pattern creates a genomic tumor profile, with information that enables oncologists to determine a course of treatment that will target the features of the cancer cells that are allowing them to multiply. Oncotype and Mammaprint are two genomic tumor profiling tests that may be ordered as part of analysis of breast tumors. These tests can help predict whether chemotherapy will be beneficial. 

In addition to detecting somatic mutations, genomic testing may also analyze tumor tissue to determine the activity or expression of certain genes, or biomarkers, that can be targeted with a specific drug or drugs. The identification of and attention to specific DNA patterns within a tumor, and creation of targeted therapies, is referred to as personalized medicine, or precision medicine. With the development of personalized and targeted treatment, cancer treatment is no longer “one size fits all.” Fewer patients need chemotherapy, or may be able to receive a type of chemotherapy that is less toxic.

While genomic tumor testing is focused on identification of somatic mutations, it occasionally identifies a mutation that is possibly germline. Individuals who have a possible germline mutation on tumor testing are referred for genetic testing to rule out or confirm that it is germline. Using a combination of genetic and genomic information, oncologists are able to better predict which treatments will be effective and well tolerated.