HER2 (Human Epidermal growth factor Receptor 2) is a protein found on the surface of all human cells. It functions as a receiver that helps regulate cell growth, division, and repair. In certain cancers, particularly breast and gastric cancer, the gene coding for this protein becomes overactive, leading to an abnormal increase in HER2 proteins on the cell surface. This overabundance, known as HER2 overexpression, drives uncontrolled cell proliferation and is a powerful biomarker guiding specific treatment strategies.
Understanding the HER2 Scoring System
The initial test to measure the amount of HER2 protein on the surface of cancer cells is Immunohistochemistry (IHC). Pathologists apply special antibodies to a tissue sample, which bind to the HER2 protein and create a visible color stain. The intensity and completeness of this staining form the basis of the HER2 scoring system, which ranges from 0 to 3+.
A score of 0 or 1+ is classified as HER2-negative, indicating the cancer cells do not overexpress the protein and will not respond to HER2-targeted therapy. A 0 score shows no staining, while a 1+ score shows faint, incomplete membrane staining in more than 10% of the tumor cells. Conversely, a score of 3+ is considered HER2-positive, characterized by intense, complete staining around the entire membrane of more than 10% of the invasive tumor cells.
The Indeterminate Nature of HER2 2+
The HER2 2+ score occupies a unique position in this diagnostic spectrum, classified as “equivocal” or “indeterminate.” This result signifies that the initial IHC test provided a moderate level of protein staining that does not clearly meet the criteria for a positive or a negative result. Pathologists define a 2+ score as weak to moderate complete membrane staining in more than 10% of the tumor cells.
The ambiguity of the 2+ result arises because the moderate staining pattern suggests a HER2 protein level higher than the negative range but short of the definitive overexpression seen in 3+ tumors. Since a truly positive HER2 status is required to predict a strong response to targeted therapies, an equivocal 2+ score is insufficient for making a definitive treatment decision. This intermediate finding mandates a follow-up analysis to determine the cancer’s true biological nature.
Resolving the Ambiguity with Molecular Testing
When a tumor receives an equivocal IHC 2+ score, the next step is molecular testing, shifting the focus from the protein on the cell surface to the underlying genetics. The standard techniques used are Fluorescence In Situ Hybridization (FISH) or Chromogenic In Situ Hybridization (CISH/ISH). These methods do not measure the protein itself but rather count the number of HER2 gene copies inside the cell nucleus.
The FISH or CISH test determines if the HER2 gene has undergone amplification, where multiple extra copies of the gene are present. This gene amplification is the driver of HER2 overexpression and defines a tumor as HER2-positive. These gene-based tests provide a definitive result, resolving the IHC 2+ score into a final status of either positive (gene amplification confirmed) or negative (no gene amplification detected). CISH offers an advantage over FISH by allowing pathologists to view the gene copies and the tissue morphology simultaneously using a standard bright-field microscope.
Treatment Implications of Final HER2 Status
The final, confirmed HER2 status dictates the selection of targeted therapies, whether determined by a 3+ IHC score or a positive ISH test following a 2+ IHC result. Patients confirmed to be HER2-positive benefit from anti-HER2 treatments, such as monoclonal antibodies like trastuzumab. These drugs directly bind to the HER2 receptors on the cell surface to block growth signals and have improved the prognosis for patients with this aggressive subtype of cancer.
Conversely, patients whose tumors are confirmed as HER2-negative (including those with a 2+ IHC and a negative ISH result) are treated with standard chemotherapy, hormone therapy, or immunotherapy, as targeted HER2 drugs would be ineffective. This classification is evolving due to the emerging category of “HER2-low” tumors, which includes those scored as IHC 1+ or IHC 2+ with a negative ISH result. This new status is responsive to a newer class of drugs called Antibody-Drug Conjugates (ADCs). ADCs use an antibody to deliver a potent chemotherapy agent directly to the cancer cell, expanding treatment options for many patients previously considered HER2-negative.