ERβ Isoforms Have Differential Clinical Significance in Breast Cancer Subtypes and Subgroups

SIMPLE SUMMARY: ERβ, an ER subtype first identified in 1996, is significantly expressed in ERα-negative breast cancer (BCa) and TNBC. Many studies investigated mostly ERβ1 protein expression in the entire cohort of BCa, and the results are inconsistent. In this study, we simultaneously investigated both ERβ mRNA and three ERβ 1, 2, and 5 protein isoforms in various subtypes and subgroups of BCa. Each ERβ isoform’s mRNA and protein expression seemingly plays a significant role in BCa subtypes and subgroups, and ERβ2 mRNA expression is risk factor for poor outcome. Studies in a large cohort of BCa are needed to explore the potential usefulness of ERβ as a prognostic and predictive marker and a therapeutic target in BCa. Furthermore, the standardization of a ERβ testing protocol may be required for ERβ testing to be utilized in a clinical setting. ABSTRACT: ERβ, an ER subtype first identified in 1996, is highly expressed in different types of BCa including ERα-negative BCa and TNBC. Many studies on ERβ expression investigated mostly on ERβ1 protein expression in ERα-positive and ERα-negative BCa combined. The results are conflicting. This may be due to the complexity of ERβ isoforms, subject heterogeneity, and various study designs targeting different ERβ isoforms and either ERβ protein or mRNA expression, as well as to the lack of a standardized testing protocol. Herein, we simultaneously investigated both mRNA and protein expression of ERβ isoforms 1, 2, and 5 in different BCa subtypes and clinical characteristics. Patient samples (138) and breast cancer cell lines (BCC) reflecting different types of BCa were tested for ERα and ERβ mRNA expression using quantitative real-time PCR, as well as for protein expression of ERα, ERβ1, ERβ2, and ERβ5 isoforms, PR, HER2/neu, Ki-67, CK 5/6, and p53 using immunohistochemistry. Associations of ERβ isoform expression with clinical characteristics and overall survival (OS) were analyzed. ERβ1, 2, and 5 isoforms are differentially expressed in different BCa subtypes including ERα-negative and TNBC. Each ERβ isoform seemingly plays a distinct role and is associated with clinical tumor characteristics and patient outcomes. ERβ isoform expression is significantly associated with >15% Ki-67 positivity and poor prognostic markers, and it predicts poorer OS, mostly in the subgroups. High ERβ2 and 5 isoform expression in ERα-negative BCa and TNBC is predictive of poor OS. Further investigation of ERβ isoforms in a larger cohort of BCa subgroups is needed to evaluate the role of ERβ for the potential usefulness of ERβ as a prognostic and predictive marker and for therapeutic use. The inconsistent outcomes of ERβ isoform mRNA or protein expression in many studies suggest that the standardization of ERβ testing would facilitate the use of ERβ in a clinical setting.