In modern oncology, treatment planning relies heavily on precise diagnostic testing to understand the unique characteristics of a tumor. The Human Epidermal growth factor Receptor 2 (HER2) status is one of the most important characteristics determined in several cancers, including breast and gastric malignancies. To accurately assess this status, pathologists frequently use Fluorescence In Situ Hybridization (FISH), a laboratory technique that examines the tumor’s genetic material. This highly specific test provides the genetic blueprint required to guide targeted interventions.
Understanding the HER2 Gene and Protein
The HER2 protein (ErbB2) is a receptor found on the surface of many cells, where it regulates cellular processes like growth, division, and repair. The gene responsible for producing this protein, the HER2/neu gene, is located on chromosome 17.
In certain cancers, the HER2/neu gene undergoes amplification, creating numerous extra copies. This leads to protein overexpression, resulting in an abnormally high number of HER2 receptors on the cancer cell surface. These overexpressed receptors become hyperactive, constantly signaling the cell to grow and divide uncontrollably. This uncontrolled proliferation drives an aggressive form of cancer, seen in approximately 15 to 20% of breast cancer cases. Accurate identification of HER2-driven tumors is necessary for effective clinical management.
How the FISH Test Works
The Fluorescence In Situ Hybridization (FISH) test is a molecular technique designed to directly visualize and count the number of gene copies within a tumor cell’s nucleus. Technicians prepare tumor tissue and introduce specialized fluorescent DNA probes to the sample.
The technique uses two distinct probes, each labeled with a different colored dye. One probe binds specifically to the HER2/neu gene locus on chromosome 17. The second probe acts as a control, binding to the centromere of chromosome 17 (CEP17). After the probes bind, the slide is examined under a fluorescence microscope, allowing the pathologist to count the distinct colored signals within the nucleus of multiple tumor cells.
The control probe establishes the baseline number of chromosome 17 copies present in the cell. By comparing the count of the HER2 gene signals to the count of the CEP17 control signals, the test accounts for variations in chromosome number that can occur in cancer cells. This comparison provides the numerical data needed to calculate a ratio and determine if gene amplification has occurred.
Interpreting HER2 FISH Results
The raw counts of the two fluorescent signals generate the HER2/CEP17 ratio, the basis for the final clinical classification. Pathologists count the signals in a minimum of 20 tumor cell nuclei to ensure statistical reliability. The total number of HER2 signals is divided by the total number of CEP17 signals to produce the final ratio.
A ratio of less than 2.0 classifies the tumor as HER2-negative, suggesting the tumor is unlikely to respond to HER2-targeted treatments. Conversely, a ratio of 2.0 or greater is classified as HER2-positive, confirming gene amplification and predicting a strong likelihood of response to targeted therapy.
In some instances, the FISH test may yield an equivocal result, falling into a borderline range that requires careful re-evaluation. Guidelines define equivocal results based on specific ratio and average HER2 copy number cut-offs, often including ratios near the 2.0 threshold. In these ambiguous cases, the pathologist may recount an additional 20 nuclei or perform confirmatory testing, such as immunohistochemistry (IHC), to achieve a definitive classification.
Guiding Targeted Treatment
A definitive HER2-positive classification by the FISH test directs the patient toward personalized oncology. The presence of HER2 gene amplification is a predictive biomarker, signifying that the tumor’s growth is driven by this specific molecular mechanism. This finding makes the patient a candidate for anti-HER2 targeted therapies that specifically block the hyperactive signaling pathway.
These targeted drugs include monoclonal antibodies, such as Trastuzumab and Pertuzumab, which attach to the HER2 receptors and shut down growth signals. Other options include antibody-drug conjugates like Trastuzumab emtansine (T-DM1), which deliver a chemotherapy drug directly to the cancer cell. For patients with HER2-negative results, these targeted drugs are ineffective. Instead, those patients are guided toward other treatments, such as standard chemotherapy, hormone therapy, or immunotherapy.