Improving HER2 testing reproducibility in HER2-low breast cancer

Abstract

HER2 is a pillar biomarker in breast cancer, and it is assessed by immunohistochemistry (IHC) using a three-tier scoring system and reflex in situ hybridization (ISH) for IHC score 2+. Novel HER2-directed antibody-drug conjugates have demonstrated significant antitumor activity in breast cancers with low levels of HER2 expression, i.e. IHC score 1+ or ISH-negative IHC score 2+. Both primary and acquired resistance to anti-HER2 therapies remains a challenge in the treatment of breast cancers according to the HER2 positivity continuum. Thus, the ability to precisely discriminate among HER2-zero, HER2-low, and HER2-positive breast cancers is no longer a mere academic exercise. HER2 testing criteria, guidelines, and quality controls are re-gaining momentum for this new clinical need. Therefore, all preanalytical and analytical variables that might trouble the sensitivity and reproducibility of this test should be carefully considered to address all possible issues and open all possible therapeutic opportunities for breast cancer patients.

Pathological identification of HER2-low breast cancer: Tips, tricks, and troubleshooting for the optimal test

“…The identification of low HER2 expression levels is not a trivial task because it relies on multiple methodological and analytical variables (Table 2). These variables might trouble the testing sensitivity and reproducibility, particularly for the discrimination between HER2-low score 1+ and "HER2zero" (i.e., IHC score 0), which comprises also the subset of "HER2 ultra-low" (i.e., score 0 with incomplete and faint staining in ≤10% of tumor cells) (Sajjadi et al, 2022). The Frontiers in Molecular Biosciences frontiersin.org tissue sample handling and processing remains a crucial task, mainly subjected to the "garbage-in-garbage-out" paradigm (Perez et al, 2014).…”

“…Therefore, this method has not been developed for detecting HER2-low tumors ( Zhang and Peng, 2022 ). On the other hand, in the post-analytical phase, inter-observer variability can occur due to the lack of consistent epithelial internal positive control for HER2 in non-neoplastic breast tissue ( Sajjadi et al, 2022 ; Zhang et al, 2022 ). The subjective mode of HER2 assessment and HER2 intratumoral heterogeneity in HER2-low breast cancer are other barriers with great influence on HER2-low assessment ( Bianchi et al, 2015 ; Jensen et al, 2020 ; Zhang et al, 2020 ).…”

“…The American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) guidelines have clearly defined intra-tumor heterogeneity of HER2 amplification (i.e., HER2/CEP17 signal ratios >2.2 in 5%–50% of the neoplastic cells). However, there is a lack of consensus to define HER2 heterogeneity for IHC ( Ahn et al, 2020 ; Sajjadi et al, 2022 ). It should be noted that intratumoral HER2 IHC heterogeneity (i.e., uneven distribution of HER2 expression or different intensities of HER2 staining in tumor cells) is more frequent in the HER2-low (2 + or 1 + IHC score) samples ( Seol et al, 2012 ; Wu et al, 2023a ).…”

“…This analysis relies on a combination of immunohistochemistry (IHC) and in situ hybridization (ISH). In particular, IHC detects the expression and intensity of HER2 protein on the cell membrane by a three-tier scoring system (from score 0 to score 3+) Table 1 ; Figure 1 , while ISH detects the presence of gene amplification using HER2 and CEP17 probes ( Sajjadi et al, 2022 ). To date, HER2+ breast cancer patients are defined as IHC score 3 + or score 2+ with a positive ISH, and subsequently qualified for anti-HER2 targeted therapy ( Pondé et al, 2018 ; Wolff et al, 2018 ).…”

“…This drug contains a strong chemotherapeutic payload that is guided against tumor cells even with the presence of low traces of HER2 positivity thanks to the so-called “bystander effect” ( Zhang et al, 2020 ; Venetis et al, 2022 ). The precise identification of HER2-low, and in particular the discrimination of score 1 + vs. score 0, is impacted by methodological and analytical variables ( Sajjadi et al, 2022 ). In this respect, several studies are ongoing for the development of new technologies and implementation of dedicated guidelines that could provide more comprehensive and accurate assessments of HER2 status in view of HER2-low mBC.…”