The HER2 FISH test is a specialized laboratory analysis used in oncology to examine the genetic information within cancer cells. Its primary application is in diagnosing certain types of breast cancer, though it is also relevant for some stomach and esophageal cancers. The main function of this test is to guide treatment decisions by identifying an excess number of copies of the HER2 gene inside the cancerous cells. This genetic information helps physicians determine the most effective course of therapy.
The Purpose of HER2 Testing
Every cell in the body contains genes that provide instructions for making proteins. The HER2 gene, which stands for human epidermal growth factor receptor 2, creates the HER2 protein. This protein is a receptor found on the surface of cells and is part of a system that manages cell growth, division, and repair. In a healthy cell, the number of HER2 genes and proteins is regulated to maintain normal function.
In some cancers, a process called gene amplification occurs. This results in the cell making far too many copies of the HER2 gene, which leads to an overproduction of the HER2 protein on the cell’s surface. With so many of these growth-promoting receptors, the cancer cells receive constant signals to divide and multiply uncontrollably. This can cause the tumor to grow more quickly and aggressively.
How the FISH Test Works
The HER2 FISH test is performed on a small sample of tumor tissue collected during a biopsy or surgery. The name, Fluorescence In Situ Hybridization (FISH), describes the technique used. It involves using DNA fragments called probes that have fluorescent molecules attached, which are engineered to bind to specific DNA sequences within the chromosomes of the cancer cells.
For HER2 testing, two different probes are used simultaneously. One probe attaches specifically to the HER2 gene and glows a distinct color, such as orange or red. A second probe acts as a control; it attaches to a stable part of chromosome 17, where the HER2 gene is located, and glows a different color, like green. This allows for a direct comparison within each cell.
A pathologist then uses a fluorescence microscope to view the prepared tissue sample. By observing the glowing signals, the pathologist can count the number of HER2 genes and the number of chromosome 17 copies in a population of cancer cells. This count is used to calculate a ratio that reveals whether an excessive number of HER2 genes are present.
Interpreting the Test Results
The laboratory report for a HER2 FISH test provides a quantitative result that falls into one of three categories. These results are based on calculating the ratio of HER2 gene signals to the chromosome 17 (CEP17) control signals, as well as the average number of HER2 gene copies per cell. This detailed analysis gives doctors a clear picture of the tumor’s genetic makeup.
A result is classified as HER2 positive when there is clear evidence of HER2 gene amplification. This is typically defined by a HER2/CEP17 ratio of 2.0 or greater, which indicates that there are at least twice as many copies of the HER2 gene as there are copies of chromosome 17. A positive result means the cancer’s growth is being fueled by an overabundance of the HER2 protein.
Conversely, a result is deemed HER2 negative when the test shows no evidence of HER2 gene amplification. In these cases, the ratio of HER2 genes to chromosome 17 is less than 2.0, and the average number of HER2 gene copies per cell is low. This finding signifies that the cancer is not driven by the HER2 pathway.
Occasionally, the test results are not definitive and are reported as equivocal or borderline. This can happen when the HER2/CEP17 ratio or the average gene copy number falls into a gray area that is neither clearly positive nor negative. In such situations, the pathologist may recommend further action, such as performing a different type of test on the tissue to reach a conclusive diagnosis.
The Role of IHC and When FISH is Used
While the FISH test is highly accurate, it is often not the first test performed to assess HER2 status. Another common laboratory method called Immunohistochemistry (IHC) is frequently used as an initial screening tool. The IHC test works differently; instead of counting genes, it measures the amount of HER2 protein present on the surface of the cancer cells. It uses a chemical dye that stains the proteins, and a pathologist scores the result based on the intensity of the stain.
The IHC test results are scored on a scale from 0 to 3+. A score of 0 or 1+ is considered HER2 negative, while a score of 3+ is considered HER2 positive. The FISH test becomes particularly important when the IHC result is ambiguous, which corresponds to a score of 2+.
The diagnostic pathway involves performing an IHC test first due to its speed and lower cost. If the result is a clear 0, 1+, or 3+, the HER2 status is determined. However, if the IHC result comes back as 2+, a follow-up FISH test is automatically ordered on the same tissue sample to resolve the ambiguity and confirm whether the cancer is truly HER2 positive or negative.
Treatment Implications of HER2 Status
The results of HER2 testing directly impact a patient’s treatment plan. Determining the HER2 status of a cancer allows oncologists to select therapies that are most likely to be effective. This is a form of personalized medicine, where the biological characteristics of a tumor guide the choice of medication.
If a tumor is found to be HER2 positive, the patient becomes a candidate for treatments known as HER2-targeted therapies. These drugs are engineered to find and attach to the HER2 protein on the surface of cancer cells. Medications like trastuzumab and pertuzumab work by blocking the HER2 receptors from signaling the cell to grow and can also flag the cancer cell for destruction by the immune system. These treatments have improved outcomes for patients with HER2-positive cancers.
If a cancer is determined to be HER2 negative, it means the tumor cells do not have an excess of the HER2 protein. As a result, HER2-targeted therapies would not be effective. For these patients, doctors will recommend other treatment strategies, such as chemotherapy, hormone therapies, or other targeted drugs aimed at different molecular pathways.