Molecular Testing’s Vital Role in Determining Treatment for Advanced Breast Cancer

The DNA of a cell is the code or blueprint that tells a cell how to act, grow and respond to the surrounding environment. When there are specific changes to the DNA (mutations), cells can act abnormally and grow unregulated, gathering more and more changes to the DNA leading to the formation of cancer.

When is molecular testing used for breast cancer?

In part due to education and breast cancer screening with mammograms, a majority of breast cancers are diagnosed at an early stage.

When patients have an abnormal mammogram, they will undergo additional testing, which can include:

• Breast ultrasound or MRI to determine the extent of disease locally
• If there is suspicion for cancer a biopsy will be performed on the breast mass and possibly lymph nodes in the axilla (armpit)
• When a biopsy confirms the presence of cancer, the tissue is analyzed for specific markers that can determine the behavior of the cancer and response to specific treatments
• Depending on the other information collected, you can then be assessed for distant spread of disease with additional imaging

All of this information together helps to determine the stage of the disease, with early stage disease being localized to the breast tissue, while more advanced disease typically involves nearby lymph nodes. Stage IV, or metastatic disease, refers to breast cancer that has spread distantly.

Depending on the stage of the disease, the results of the testing performed on the cancer tissue itself, and individual factors (such as overall health) different options for cancer treatment that are specific and effective for your disease are recommended.

What testing is performed on the cancer biopsy?

The testing performed in the initial workup of breast cancer can vary depending on where it is performed, but in breast cancer your doctor most commonly looks for levels of the following:

• Estrogen receptor (ER)
• Progesterone receptor (PR)
• Human epidermal growth factor receptor 2 (HER2)
• Ki-67

Your doctor may refer to the analysis of these hormone receptors and proteins as “tumor marker testing,” “biomarker testing,” “pathology testing,” or just “marker testing.”  The presence or absence of these markers is measured by a technique called immunohistochemistry (IHC). Results are usually reported as a percentage from 0-100% with a 100% meaning all cells express these molecules, while a low percentage or 0% would mean very few or none of the cells have the tested molecule.

In the case of  estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) the presence or absence of these markers tells us not only how the cancer will act, but what targeted therapies specific to these markers are effective. Ki-67 tells us how fast the cells are dividing, with a higher percentage meaning the cells are dividing more quickly and the cancer is typically more aggressive.

The most common type of breast cancer is actually ER positive, HER2 negative which represents approximately 70% of diagnosed breast cancers. When cells do not have an ER, PR, or HER2 present, it is called triple negative breast cancer which represents around 15% of cases. HER2 positive breast cancer (which can be ER or PR positive), represents another 15 to 20% of cases.

In particular, the presence of estrogen receptors and HER2 help to provide additional treatment options for patients. Therapies for the estrogen receptor can target the receptor itself or the production of estrogen, which interacts with the receptor to stimulate the cancer cell. There are also many targeted treatments for HER2 that can be used when HER2 is present on the cells.

Again, this is all determined by tests that look at the presence of these markers on the cancer cells.

“These treatments are extremely effective, and as I often tell patients, they’re as important as treatment with chemotherapy, which may or may not be indicated,” says Dr. Kate Tkaczuk, a professor of Medicine at the University of Maryland and a breast medical oncologist at the University of Maryland Greenebaum Comprehensive Cancer Center.

The type of targeted therapy used and their role in your cancer treatment depends on many factors including the stage of disease and individual factors such as age and overall health.

What role does tumor marker testing have in treatment of advanced breast cancer?

Tumor marker testing for ER, PR, HER2, and Ki-67 happens on initial diagnosis of any breast cancer and helps determine potential targeted therapies for early or advanced stage disease.

“It is important to mention that the majority of patients who present with advanced stage four breast cancer have prior diagnosis of early stage breast cancer,” says Dr. Tkaczuk. And in patients that have this prior history of breast cancer, “We know actually a lot about the prior diagnosis, and the tumor markers, which are typically required for all patients who are diagnosed with early stage breast cancer.”

In the uncommon situation where a patient presents with a diagnosis of metastatic disease and no prior history of breast cancer, much of the workup and tumor marker testing which helps guide future therapies is the same as described above.

If there is a prior history of breast cancer, a new biopsy is often obtained and compared to the old biopsy from the prior breast cancer. Even if a cancer recurs or spreads distantly, often the expression of these tumor markers (ER, PR, HER2) remains the same. If there is a difference in these receptors or protein levels it can mean there is new primary disease or separate breast cancer that developed, which is unrelated to the prior breast cancer. This can change treatment significantly if it is thought to be a new breast cancer versus a recurrence of prior disease.

Therefore, most breast cancers — even if they have recurrence or spread to distant sites  — will have the same tumor markers. Despite this, the tumor markers (ER, PR, HER2, Ki67) can still be targeted by systemic therapy and guide treatment options.

Next-generation sequencing

In addition to standard molecular testing (ER, PR, HER2 and Ki67) performed in metastatic breast cancer, the biopsy should also be tested for other mutations using tests such as next generation sequencing (NGS).

When diagnosed with metastatic disease, Dr. Tkaczuk says “next-generation sequencing is actually one of those tests that I think if it was not done, the patient should ask for it.”

Next-generation sequencing, or NGS, analyzes multiple portions of the tumor DNA to identify additional potential markers or DNA mutations that can be targeted for treatment. Typically, NGS looks for targets for alternative therapies that are considered if the initial treatment stops working or is not tolerated well and a new therapy is needed.

NGS can be done via tissue biopsy which tests a piece of the tumor, but if there is a need for repeat biopsy and an additional biopsy procedure is not feasible or unsafe, NGS can also be performed with a liquid biopsy. A liquid biopsy collects a sample of your blood and looks for pieces of tumor DNA or ctDNA circulating in your bloodstream.

Hereditary cancer molecular testing

When your doctor talks about molecular testing they may be referring to germline testing or tumor based somatic testing. Germline testing (hereditary cancer testing) looks at inheritable DNA that we pass on from generation to generation. Somatic tumor testing attempts to understand what mutations are in the DNA in the tumor itself that are driving the cancer.

When initially diagnosed with breast cancer, patients with specific personal history, family history, or features specific to their cancer can be recommended to undergo germline testing which seeks to identify these potential inherited DNA mutations.

If you did not already receive germline testing during your initial diagnosis of breast cancer, any patient with recurrent or metastatic disease should undergo this germline testing.

By testing for inherited DNA mutations, you can uncover valuable information. If the testing is negative for inherited DNA mutations, it gives family peace of mind that there is no clear inherited increased risk of cancer. If results are positive other related family can then be tested to evaluate their risk for subsequently developing cancer. In addition, certain inherited mutations such as BRCA1, BRCA2, and PALB2 can help guide future treatment with additional targeted therapies.

If you feel like you would be a candidate for germline genetic testing and have not been offered testing, please consult with your healthcare team.

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