Abstract PO1-16-01: Clinicopathological and global transcriptional complexity of estrogen receptor low positive breast cancers in a contemporary Swedish prospective population-based cohort

Abstract Background: Estrogen receptor (ER) expression is currently the most consequential biomarker for prognostic and therapeutic decision making in clinical management of early breast cancer. Endocrine therapy (ET) is an important part of standard of care in most cases with ER-positive disease, and an effective therapy for patients with endocrine-responsive tumors, decreasing the risk of recurrence and improving the rate of survival. Unfortunately, ET comes with frequently bothersome side-effects and tend to decrease quality of life. The threshold to define ER positivity as a marker of endocrine responsiveness is non-uniform between international and local guidelines. Among ER low positive tumors (ERlow ,1-10% expression) endocrine responsiveness is uncertain. Swedish national guidelines recommend a cut-off of ≥10% for ER positivity and ET prescriptions, thus patients with ERlow tumors are often exempted from adjuvant ET. This study explores the clinicopathological characteristics, global transcriptional complexity and clinical outcome of ERlow tumors to better understand their biology and response to therapy. Methods: 9138 patients diagnosed with early breast cancer between 2010 – 2021 in Sweden with available clinicopathological data and RNA sequencing data were included. Patients were classified according to ER expression: ERneg ( 1%; n=897), ERlow (1-10%, n=158) and ERhigh ( 10%, n=8083). Adjuvant ET was provided to 3.6%, 16.8% and 91.5% of patients with ERneg, ERlow, and ERhigh tumors, respectively. Clinicopathological characteristics, overall survival (OS) and global transcriptional profiles of ERlow tumors were compared with ERneg and ERhigh tumors, respectively. Results: Generally, ERlow and ERneg tumor pathological characteristics were more similar to each other but were significantly distinct compared to ERhigh tumors. However, among patients with HER2-negative disease only, significant differences related to tumor biology persisted between ERlow compared with ERneg tumors; ERlow was enriched with tumors displaying a lobular histology, NHG grades 1 0.05 for all comparisons). Moreover, multivariable survival analyses revealed that the risk of dying was significantly lower for patients with ERlow tumors compared to patients with ERneg (p=0.005) or ERhigh tumors (p=0.011) in this cohort. The distribution of clinicopathological characteristics and overall survival between ERlow compared with ERneg or ERhigh tumors were similar among patients with HER2-positive tumors only. Global transcriptional comparisons identified only 42 genes to be differentially expressed between ERlow vs ERneg tumors (FDR 0.05); but gene set enrichment analyses failed to identify any significantly enriched cancer-related processes/pathways within the queried databases. Conclusions: Identification of optimal therapies for all subsets of breast cancer is necessary in the pursuit of personalized medicine. These results confirm that ERneg and ERlow tumors are pathologically and transcriptionally distinct from ERhigh tumors. Although subtle differences exists in the underlying biology of ERlow compared to ERneg tumors, similar therapeutic management excluding ET for patients with ERlow and ERneg disease is recommended in the Swedish context. Omission of adjuvant ET did not seem to compromise overall survival for ERlow relative to ERneg or ERhigh tumors, urging the need for prospective studies investigating the true benefit of ET for ERlow tumors. Our results also raise reasonable clinical thoughts on the benefits of new treatment strategies such as cyclin-dependent kinase 4/6 inhibitors, immunotherapy and antibody-drug conjugates for patients with ERlow expressing tumors. Citation Format: Siker Kimbung, Srinivas Veerla, Anna Ehinger, Johan Vallon-Christersson, Martin Malmberg, Niklas Loman. Clinicopathological and global transcriptional complexity of estrogen receptor low positive breast cancers in a contemporary Swedish prospective population-based cohort abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-16-01.