Molecular and prognostic studies of HER2 in breast cancer

The biomarker status in breast cancer is crucial for treatment decisions in breast cancer diagnostics. In clinical routine, estrogen receptor (ER), progesterone receptor (PR), Ki67 proliferation index, and human epidermal growth factor receptor 2 (HER2) are routinely analyzed. This thesis focuses on HER2, with in-depth investigations of diagnostic procedures, how to refine future HER2 routine diagnostics, investigations on new methods for HER2 scoring, and research about biological patterns in HER2-positive breast cancer.

In paper I, the importance of repeated testing of HER2 and other biomarkers were investigated retrospectively by reviewing core needle results compared to the corresponding surgical specimens. As a result of this study, the Swedish breast cancer guidelines issued a new recommendation to perform an assessment of HER2 biomarkers on both core needle biopsies and surgical specimens.

In paper II, breast tumor diagnostics and treatment data were compared within and between pathology departments in Sweden. Results generally showed good agreement between laboratories for HER2, ER, and PR, but minor discrepancies between HER2 and ER are of clinical importance since those biomarkers are predictive of therapy. There was a variability for Ki67 and tumor grade. The results showed that refined Ki67 assessment might support Ki67 scoring, including image analysis. This paper showed that following pathology reporting results over time might reveal variability that could be fast-detected and thereby reduce the risk of variances in treatment between different departments in Sweden. Even small differences between laboratories and pathologists are of high relevance for HER2-positive patients.

In Paper III, we manually investigated HER2 biomarker levels in re-stained and re-scored HER2 in situ hybridization (ISH) slides with manual scoring and a digital image analysis algorithm. In addition, we investigated the ERBB2 mRNA levels using RT-PCR. The biomarker levels were examined continuously using Cutoff Finder and STEPP analysis, and the material was divided into three subgroups: low, intermediate, and high biomarker levels of HER2. We found significant differences in prognosis between patients with tumors with different levels of HER2. Those with tumors with low and high levels had a worse prognosis than those with intermediate levels, which showed that these patients might need additional or more effective targeted treatment.

In Paper IV, we investigated biological patterns in HER2-positive breast cancer and related them to prognosis. By performing unsupervised agglomerative hierarchical clustering, we found four differentially enriched tumor pattern subgroups in the cohort. The subgroup with low stromal markers and low immune markers had a worse prognosis. These results showed that the tumor microenvironment plays a crucial role in treatment response, which should be considered in future studies of HER2-positive breast cancer.

In conclusion, all papers have addressed HER2 diagnostics in breast cancer but from different angles. We provided important information for improving HER2 diagnostics and revealed patterns in HER2 tumor biology to consider in future studies.

List of scientific papers

I. Robertson S, Rönnlund C, de Boniface J, Hartman J. Re-testing of predictive biomarkers on surgical breast cancer specimens is clinically relevant. Breast Cancer Research and Treatment. 2019 Apr;174(3):795-805. https://doi.org/10.1007/s10549-018-05119-2

II. Acs B*, Fredriksson I*, Rönnlund C, Hagerling C, Ehinger A, Kovács A, Røge R, Bergh J, Hartman J. Variability in Breast Cancer Biomarker Assessment and the Effect on Oncological Treatment Decisions: A Nationwide 5-Year Population-Based Study. Cancers. 2021 Mar;13(5):1166. *Shared first authorship, equal contributions. https://doi.org/10.3390/cancers13051166

III. Rönnlund C, Sifakis EG, Schagerholm C, Yang Q, Karlsson E, Chen X, Foukakis T, Weidler J, Bates M, Fredriksson I, Robertson S*, Hartman J *. Prognostic impact of HER2 biomarker levels in trastuzumab-treated early HER2-positive breast cancer. Breast Cancer Research. 2024 Feb;26(1):24. *Shared last authorship, equal contributions. https://doi.org/10.1186/s13058-024-01779-9

IV. Rönnlund C*, Yang Q*, Schagerholm Stanev C, Chen X, Foukakis T, Fredriksson I, Robertson S, Sifakis EG ** , Hartman J **. Prognostic potential of molecular-derived subgroups in early HER2-positive breast cancer. *Shared first authorship, equal contributions. ** Shared last authorship, equal contributions. [Manuscript]