FISH in Triple-negative Breast Cancer: A Possible Strategy for the Future?

Elisa Rigon; Chiara Saggia; Valentina Rossi; Silvia Genestroni; Erica Gaudino; Paola Campisi; Claudia Veggiani; Renzo Luciano Boldorini; Oscar Alabiso


Future Oncol. 2015;11(7):1023-1026. 

In This Article


Triple-negative breast cancer (TNBC) is a specific subset of breast cancer characterized by tumors that do not express estrogen receptor or progesterone receptor, and do not overexpress HER2.[1] This type of breast cancer, which accounts for nearly 15–20% of all breast cancer cases, often progresses to a metastatic form and is usually associated with a poor prognosis within the first 3–5 years.[2]

Significant heterogeneity exists within the TNBC class from the morphological and clinical point of view, and in terms of chemotherapeutic sensitivity. A recent analysis of 97 TNBCs identified six different TNBC subtypes characterized by specific gene expression profiles. In particular, two basal-like (BL1 and BL2), an immunomodulatory, a mesenchymal, a mesenchymal stem-like and a luminal androgen receptor subtype were identified.[2] In addition, a considerable overlap has been reported between TNBC and BRCA1-mutated tumors; in fact nearly 75–85% of women with BRCA1 germline mutation are expected to develop a TNBC breast cancer during their life.[3]

Since no specific target is available to treat this class of patients, chemotherapy is the only currently validated and potentially effective treatment. The identification of new molecular targets and the development of new target-based therapies is therefore crucial for TNBC patients, and it first requires a better understanding of the heterogenic features presented by TNBCs. One crucial point is the identification of HER2 'false negatives' among TNBCs: about 8–10% of breast cancers with HER2 amplification are misclassified as negative (immunohistochemistry [IHC] 0 or 1+), regardless of the actual presence of hormonal receptors.[4] As a consequence, these patients are denied potential benefits given by the anti-HER2 adjuvant and neoadjuvant therapies, such as trastuzumab and lapatinib.[5]

HER2 is an important prognostic and predictive factor for response to trastuzumab in the metastatic and adjuvant settings; the development of reliable methods for its identification is therefore of paramount importance.[6] IHC and FISH are currently the most commonly used methods to assess HER2 status: the former measures the HER2 expression, while the latter directly assesses HER2 gene amplification through a single (HER2) or double probe (HER2 and CEP17).

The IHC test uses a scoring system (0, 1+, 2+, 3+) to express the amount of HER2 receptor on cell surface, taking into account the proportion of positive cells and the completeness and intensity of membrane staining. The FISH test is used when IHC results are equivocal. According to the 2013 ASCO/CAP guidelines an unequivocally positive HER2 result is obtain when IHC assay shows a complete/intense circumferential membrane staining within >10% of invasive tumor cells observed in a homogeneous and contiguous population (IHC 3+), or when the FISH assay reveals HER2 gene amplification. In single-probe assays gene amplification is defined as a mean number of HER2 copy number ≥6.0 signals per cell, while in the dual-probe assay the FISH is positive when HER2/CEP17 ratio is ≥2.0, with an average HER2 copy number ≥4.0 signals per cell; HER2/CEP17 ratio is ≥2.0, with an average HER2 copy number <4.0 signals per cell; or when HER2/CEP17 ratio is <2.0, with an average HER2 copy number ≥6.0 signals per cell.[7]

Tumor-negative breast cancers misclassified as HER2 negative by IHC (0 or 1+) could therefore be reclassified by FISH, with a remarkable impact on medical treatment. Adjuvant trastuzumab has significantly improved the prognosis of breast cancer patients both in terms of disease-free survival and overall survival (OS); moreover it has increased progression-free survival and OS in HER2-positive breast patients in the metastatic setting, as confirmed by a number of clinical trials.[8–10]