Overview
HER2 Neu, also known as ERBB2 or CD340, is a gene located on chromosome 17q12 that encodes the human epidermal growth factor receptor 2. It belongs to the epidermal growth factor receptor (EGFR) family and plays an important role in normal cell growth, differentiation, and survival.
HER2 is a transmembrane receptor tyrosine kinase that functions as a co-receptor within the HER family. Unlike other HER receptors, HER2 has no known ligand and acts as an “orphan receptor.” When overexpressed or amplified, HER2 becomes a major driver of tumor development and progression. HER2 overexpression is seen in several malignancies, most notably breast cancer, but also in ovarian, gastric, bladder, pancreatic, and esophageal cancers. In breast cancer, HER2 positivity defines a biologically aggressive subtype with distinct therapeutic implications.
Molecular Features and Mechanism
HER2 is composed of three main domains: an extracellular domain, a transmembrane domain, and an intracellular domain that contains intrinsic tyrosine kinase activity. The receptor promotes cell growth and survival by forming heterodimers with other HER family members, particularly HER3.
Gene amplification leads to overexpression of the HER2 protein on the cell surface. This results in continuous activation of downstream signaling pathways such as PI3K/AKT and RAS/MAPK. These pathways drive malignant transformation by promoting cell proliferation, survival, invasion, angiogenesis, and metastasis. Persistent signaling due to HER2 overexpression explains the aggressive nature and poor prognosis associated with HER2-positive tumors when left untreated.
Role in Cancer Development
It plays a direct role in the growth and spread of cancer cells. Approximately one in five breast cancers shows amplification of the HER2 gene, resulting in excessive production of the HER2 protein.
HER2 gene amplification and overexpression occur in about 15–20% of invasive breast cancers and in up to 60% of intraductal breast carcinomas. This alteration enhances signal transduction during cell growth and leads to rapid tumor progression. HER2-positive tumors tend to grow faster, recur earlier, and have a higher likelihood of metastasis compared to HER2-negative tumors.
Symptoms
HER2 overexpression itself does not cause specific symptoms. Clinical manifestations depend on the type, stage, and location of the cancer rather than HER2 status alone.
Patients may present with symptoms related to breast cancer or other malignancies, such as a breast lump, pain, skin changes, weight loss, or metastatic symptoms. HER2 testing is therefore used as a diagnostic and prognostic marker rather than a symptom-based indicator.
Causes
HER2 positivity arises from amplification of the ERBB2 gene, leading to overexpression of the HER2 protein on tumor cells. This genetic alteration is not inherited but acquired during tumor development.
The exact cause of HER2 gene amplification is not fully understood. However, once amplification occurs, it drives uncontrolled cellular signaling and tumor aggressiveness through continuous activation of growth and survival pathways.
Risk Factors
HER2 amplification is primarily associated with specific cancer subtypes rather than external risk factors. Patients with invasive breast cancer, high-grade ductal carcinoma in situ, recurrent disease, or metastatic tumors are more likely to undergo HER2 testing.
HER2 overexpression is associated with poor disease outcome, higher recurrence rates, and shorter survival if untreated. It has also been shown to be an independent predictor of prognosis on multivariate analysis in several cancers, including breast, gastric, and bladder cancers.
Clinical Significance and Indications
HER2 testing is clinically significant in newly diagnosed invasive breast cancer to determine eligibility for HER2-targeted therapy and assist in treatment planning. It is also used in high-grade ductal carcinoma in situ for risk stratification.
In recurrent or metastatic breast cancer, HER2 status guides the selection of targeted agents and helps monitor changes in receptor expression over time. Equivocal HER2 immunohistochemistry results require reflex testing with in situ hybridization techniques to confirm gene amplification.
HER2 assessment is increasingly used in non-breast cancers such as gastric, ovarian, lung, and pancreatic cancers to identify candidates for targeted therapy or clinical trial enrollment.
Methods of Detection
HER2 status is most commonly evaluated using immunohistochemistry (IHC) on tissue obtained from trucut biopsy or surgical excision. Tissue samples are processed into paraffin blocks and tested with specific antibody clones.
Fluorescence in situ hybridization (FISH) is used to detect HER2 gene amplification and is considered the confirmatory test, especially in equivocal IHC cases. These tests also function as companion diagnostics to determine eligibility for HER2-targeted therapies.
Interpretation of HER2 IHC
HER2 IHC scoring ranges from 0 to 3+. Scores of 0 and 1+ are considered negative and indicate no HER2 overexpression. A score of 2+ is considered equivocal and requires confirmation by ISH or FISH testing.
A score of 3+ shows strong, complete, circumferential membrane staining in more than 10% of tumor cells and is considered positive. Patients with HER2-positive tumors are eligible for HER2-targeted therapies.
Role in Targeted Therapy and Prognosis
HER2 status directly determines response to targeted therapy. Trastuzumab (Herceptin), a humanized monoclonal antibody, binds to the HER2 protein and inhibits tumor cell proliferation. Other therapies include pertuzumab, tyrosine kinase inhibitors such as lapatinib and neratinib, and antibody–drug conjugates like T-DM1.
HER2 amplification is associated with poorer prognosis but also predicts significant benefit from HER2-targeted therapy. With appropriate treatment, outcomes in HER2-positive cancers have improved markedly, making HER2 one of the most important predictive and prognostic biomarkers in oncology.
