A comparison between immunohistochemistry and mRNA expression to detect HER2-low breast cancer

Abstract

Background: Tumors with low levels of HER2 expression (HER2-low) have become a targetable subset of breast cancer (BC) thanks to the efficacy of novel anti-HER2 antibody-drug conjugates like trastuzumab deruxtecan (T-DXd). HER2 immunohistochemistry (IHC) is the current recommended assay for HER2 classification in daily clinical practice. However, this essay is associated with limited interobserver agreement concerning the diagnosis of HER2-low BC. We investigated whether mRNA expression quantified via qRT-PCR could serve as a valuable complementary and more objective method to IHC for detecting HER2-low BC. Method(s): Twenty-four biopsy cases were randomly selected from our previously published interobserver study of non-overexpressing HER2 BC. In this study, each case underwent HER2 IHC evaluation by 16 pathologists to establish a consensus on the IHC scores. In addition, fluorescence in situ hybridization (FISH) was performed on all cases. For this current study, mRNA was extracted from microdissected invasive tumor cells of formalin-fixed paraffin-embedded material. qRT-PCR was employed for quantitative evaluation of HER2 using the cut-off values of the essay, resulting in the following MammaTyper classes: HER2-0, (including HER2-0 and ultralow), HER2-low and HER2-positive. We compared the mRNA expression levels with the consensus IHC scores (IHC 0, 1+, 2+), the HER2 subcategories: HER2-0 (IHC 0) and HER2-low (IHC 1+ and 2+/FISH negative). Result(s): Based on the IHC consensus, 5 cases were HER2-0 and 19 were HER2-low, while MammaTyper identified 4 cases as HER2-0 or ultralow, 17 as HER2-low, and 3 as HER2-positive, resulting in a concordance of 75%. Two discordant cases had a IHC consensus score of 0 and were classified as HER2-low by MammaTyper. One case had a consensus IHC score of 2+ and was identified as HER2-0 or ultralow by MammaTyper. Three MammaTyper HER2-positive cases were scored as IHC 2+ without amplification as determined by FISH. The mRNA levels strongly correlated with the consensus IHC scores (r = 0.585, p = 0.003) and the HER2 subcategories (r = 0.547, p = 0.006). When comparing mRNA levels within HER2 subgroups, a significant difference in mean mRNA expression was found between HER2-0 and HER2-low (p = 0.03). Conclusion(s): Our findings indicate a strong correlation between mRNA expression quantified by MammaTyper RT-qPCR and HER2 IHC consensus scores, but there was a substantial proportion of discordant HER2-results between both methods. Additional research with larger numbers including clinical outcome is needed to determine its added value. Conflict of interest: Advisory Board: C. van Deurzen is part of the Advisory Board of AstraZeneca. Other Substantive Relationships: Sysmex Nederland B.V. is the subsidizing party of this project by providing the PCR kits. However, they have no involvement in the processing, analysis or interpretation of the results or the writing of the abstract.Copyright © 2024 Elsevier LTD. All rights reserved.