Low ER level is one factor in the resistance of ER-positive and HER-2-positive tumors to hormone therapy

Abstract

809 Introduction: Some clinical data have suggested that HER-2 overexpression might be associated with tamoxifen resistance in patients with hormone receptor-positive breast cancer. The purpose of this study was to evaluate the inter-site variation of ER and HER-2 expression and ER and HER-2 levels as identified using immunohistochemistry. Methods: A total of 227 women with invasive breast cancer undergoing surgical treatment between 1991 and 2001 were included in this study. Paraffin-embedded tumor tissues were used in immunohistochemical study. Double staining was performed using both ER and HER-2 antibody. ER expression was assessed using the Allred score. HER-2 score was determined using the HercepTest score. Both the ER- and HER-2-positive tumors were classified into two types; co-expressed type and separately expressed type. Results: HER-2 expression was inversely correlated with ER expression. When evaluated in ER-positive patients, HER-2 overexpression was associated with lower ER levels. ER and HER-2-positive tumors were observed in 22 (9.7%) of 227 tumors. The co-expressed type was found in 10 patients, and the separately expressed type was found in 12 patients. ER and HER-2 scores were significant prognostic factors, however, there was no difference in disease-free survival between patients with the co-expressed type tumors and the separately expressed type tumors. Conclusions: Low ER levels might be one factor in the relative resistance of ER-positive and HER-2-positive tumors to hormonal therapy. No significant financial relationships to disclose.