The HER2 test is a critical diagnostic tool used to assess the presence of the HER2 gene or protein in cancer cells, especially for breast cancer. HER2 (human epidermal growth factor receptor 2) is a gene that plays a key role in the growth of cancer cells. When this gene is overexpressed or amplified, it can result in rapid cancer cell growth, and patients with HER2-positive cancer may benefit from targeted therapies.
Types of HER2 Tests:
- Immunohistochemistry (IHC):
- Purpose: This test measures the amount of HER2 protein on the surface of the cancer cells. It is the most commonly used initial screening method.
- Procedure: A sample of cancer tissue is stained, and the presence and intensity of HER2 protein are graded from 0 to 3+:
- 0 or 1+: Negative for HER2
- 2+: Equivocal (uncertain result) – further testing is recommended
- 3+: Positive – high likelihood of HER2 amplification, indicating the cancer may respond well to HER2-targeted therapies.
- Limitations: IHC can sometimes provide unclear results (e.g., IHC score of 2+), requiring further testing with ISH.
- In Situ Hybridization (ISH):
- Fluorescence In Situ Hybridization (FISH): This method detects the number of HER2 gene copies in cancer cells. It is used when IHC results are equivocal (score of 2+). The FISH test uses fluorescent probes to bind to the HER2 gene, and if more than 6 copies of HER2 per cell are found, the result is considered positive.
- HER2:Chromosome 17 ratio >2.0 is typically considered positive.
- Chromogenic In Situ Hybridization (CISH): Similar to FISH, CISH uses chromogenic dyes rather than fluorescent probes. It’s useful in settings where FISH equipment is unavailable.
- Benefits: FISH and CISH can more clearly define HER2 gene amplification compared to IHC.
- Fluorescence In Situ Hybridization (FISH): This method detects the number of HER2 gene copies in cancer cells. It is used when IHC results are equivocal (score of 2+). The FISH test uses fluorescent probes to bind to the HER2 gene, and if more than 6 copies of HER2 per cell are found, the result is considered positive.
- Polymerase Chain Reaction (PCR):
- This is a more specialized test for detecting HER2 gene amplification at the DNA level, though it’s less commonly used in clinical practice compared to IHC and FISH.
- Next-Generation Sequencing (NGS):
- This technology can assess multiple genetic alterations simultaneously, including HER2 amplifications, and can provide more comprehensive information for treatment decisions.
Clinical Importance of HER2 Testing:
- HER2-Positive Breast Cancer: The test helps identify patients who are likely to benefit from HER2-targeted therapies such as trastuzumab (Herceptin), pertuzumab (Perjeta), or newer agents like tucatinib. These therapies block the HER2 protein, which helps slow or stop the growth of HER2-positive breast cancer cells.
- Treatment Decisions: For patients with breast cancer, HER2 status is a cornerstone of treatment decision-making. HER2-positive patients typically respond better to targeted therapies than chemotherapy alone. In contrast, HER2-negative breast cancers may not respond as effectively to these targeted therapies.
- Prognosis: HER2-positive cancers tend to grow faster and are more aggressive, but targeted therapies have significantly improved the outcomes for patients with HER2-positive cancers.
Interpretation of Results:
- HER2 Negative: No HER2 protein overexpression or gene amplification. These cancers typically do not respond to HER2-targeted therapies.
- HER2 Positive: Overexpression of HER2 protein or amplification of the HER2 gene. These cancers are likely to respond to targeted HER2 therapies, improving overall survival.
- Equivocal (IHC score of 2+): Further testing with FISH or CISH is required to confirm the HER2 status and guide therapy decisions.
Challenges:
- False Positives/Negatives: The accuracy of the HER2 test depends on the quality of the tissue sample and the testing method used. Poor-quality samples or improper testing procedures can lead to incorrect results.
- Tumor Heterogeneity: Some tumors may have mixed HER2 status (positive in some areas and negative in others), complicating treatment decisions.
Why HER2 Testing is Essential:
- Guiding Treatment: It helps clinicians select the most appropriate therapy, particularly for breast cancer, where HER2-targeted treatments are integral to management.
- Personalized Medicine: Knowing a patient’s HER2 status allows for tailored treatments that can improve outcomes and minimize unnecessary side effects.
For accurate HER2 testing, it is crucial to use high-quality samples and reliable testing methods like IHC, FISH, or CISH. Both the American Society of Clinical Oncology (ASCO) and the College of American Pathologists (CAP) provide guidelines for testing and interpreting HER2 status to ensure consistency and reliability across labs.
For further reading and clinical guidelines on HER2 testing, refer to studies and protocols published by ASCO and CAP.