The Human Epidermal growth factor Receptor 2 (HER2) is a protein used as a biomarker in cancer diagnosis, particularly in breast and gastric malignancies. Determining the HER2 status of a tumor is essential for personalized oncology, as it directly influences treatment selection and patient prognosis. The primary method used to assess this status is Immunohistochemistry (IHC), a laboratory technique that visually measures the amount of HER2 protein present on the surface of cancer cells. This test provides a standardized score that guides oncologists in deciding whether a patient will benefit from HER2-targeted therapies.
The Biological Function of HER2
HER2 is a receptor tyrosine kinase protein located on the outer membrane of human cells. This receptor is a member of the epidermal growth factor receptor (EGFR) family, which plays a role in regulating fundamental cellular activities. Under normal conditions, HER2 receives growth signals from outside the cell and transmits them inward, stimulating cell growth, division, and repair.
In some cancers, a genetic event called gene amplification leads to the production of an excessive number of HER2 genes. This results in protein overexpression, where there are too many HER2 receptors coating the surface of the cancer cell. This abundance causes the cell to receive constant, strong growth signals. This uncontrolled signaling drives aggressive, rapid cell proliferation, which is characteristic of HER2-positive malignancies.
HER2 is unique among its family members because it is the preferred partner for forming pairs, or heterodimers, with other EGFR receptors. These HER2-containing pairs generate particularly potent and long-lived signals, enhancing the tumor cell’s ability to grow, survive, and spread.
How Immunohistochemistry Testing Works
Immunohistochemistry is a tissue-based assay that pathologists use to visualize the HER2 protein directly on a tumor sample. The process begins after a small tissue sample, usually obtained via biopsy or surgical removal, is prepared and thinly sliced onto a glass slide. This prepared tissue is then incubated with specialized antibodies designed to recognize and bind to the HER2 protein.
If the HER2 protein is present on the cell surfaces, the antibodies attach to these receptors. A secondary step involves adding a chemical that reacts with the bound antibody complex to produce a visible color change, often brown. The intensity and pattern of this staining are then analyzed by a pathologist under a microscope.
The final IHC score is determined by observing the intensity of the stain and the percentage of tumor cells that exhibit membrane staining. This technique provides a quick, cost-effective initial screen to estimate the level of HER2 protein overexpression in the tumor.
Decoding the HER2 IHC Scoring System
The HER2 IHC test uses a standardized four-point scoring system, ranging from 0 to 3+, to classify the level of protein expression on the cancer cells. This numerical score translates directly into the tumor’s initial HER2 status, which guides the subsequent diagnostic and therapeutic approach. The scores of 0 and 1+ are grouped together as HER2 Negative, indicating that targeted HER2 therapies are unlikely to be effective.
A score of 0 means there is no membrane staining visible in the tumor cells, or only faint, incomplete staining in ten percent or less of the cells. A score of 1+ is defined by faint or barely perceptible incomplete membrane staining that is observed in more than ten percent of the tumor cells. These tumors are generally not driven by HER2 signaling.
The highest score, 3+, classifies the tumor as HER2 Positive, which confirms eligibility for targeted treatment. This score is assigned when there is strong, intense, and complete circumferential membrane staining visible in more than ten percent of the tumor cells. This intense staining pattern confirms the significant overexpression of the HER2 protein that is driving the cancer’s growth.
The score of 2+ is designated as Equivocal or Borderline, meaning the result is uncertain and requires further investigation. An equivocal score is characterized by weak to moderate complete membrane staining observed in more than ten percent of the tumor cells. Because the staining intensity is not definitively strong enough to be called positive, this result requires a more precise molecular test to confirm the true HER2 status.
Confirmatory Testing and Treatment Pathways
When an IHC test returns an equivocal 2+ score, the next mandatory step is to perform a molecular confirmatory test to resolve the HER2 status. The most common tests used for this purpose are Fluorescence In Situ Hybridization (FISH) or Silver In Situ Hybridization (SISH). Unlike IHC, which measures the amount of protein, these techniques directly count the number of HER2 genes within the cell nucleus.
FISH and SISH use specialized probes that attach to the HER2 gene and a control gene, creating a visual signal, often fluorescent or colored, that can be counted. The ratio of HER2 genes to the control gene determines if the cell has gene amplification, which is the underlying cause of protein overexpression. This second test is the definitive method for classifying a 2+ tumor as either truly HER2-positive or HER2-negative.
The final HER2 status dictates the patient’s eligibility for targeted therapies, which selectively attack the HER2 protein. Patients whose tumors are classified as HER2-positive, either via an IHC 3+ score or a confirmed positive FISH/SISH result, are candidates for drugs like Trastuzumab (Herceptin) or Pertuzumab. These monoclonal antibodies work by blocking the HER2 receptor, thereby interrupting the growth signals that fuel the cancer.
For patients whose tumors are ultimately determined to be HER2-negative (IHC 0, 1+, or 2+ with a negative confirmatory test), the treatment plan shifts away from HER2-targeted agents. These individuals are typically treated with conventional chemotherapy, hormone therapy, or immunotherapy, depending on other specific tumor characteristics. The precise determination of HER2 status, facilitated by the IHC scoring system and follow-up molecular testing, is necessary for ensuring personalized therapeutic regimens.