A Novel Splice Variant to NCOR2 as Biomarker for Tamoxifen Resistant Breast Cancer

Abstract

More than two-thirds of all breast cancers are estrogen receptor (ER) positive for which Tamoxifen revolutionized their management as adjuvant treatment to prevent cancer recurrence. Almost half of these patients eventually develop resistance. The mechanisms underlying tamoxifen resistance are not yet well understood. There is no robust biomarker to reliably predict those who will be resistant. By the time drug resistance is established, the cancer has already progressed and sometimes metastasized. We have identified a novel splice variant, BQ323636.1 which retains only the N-terminus repression domain 1 of the NCOR2 wildtype protein. Overexpression of BQ conferred resistance to tamoxifen in both in vitro and in orthotopic mouse model. Mechanistically, co-immunoprecipitation showed BQ could bind to NCOR2 and inhibit the formation of co-repressor complex for suppression of the ER signaling. Consistently, BQ overexpression compromised the suppressive role of NCOR2 in regulating estrogen-response element activity and rescued transcriptional suppression of tamoxifen on ER-target genes. We generated a monoclonal antibody specific for BQ used to predict patients’ responses to tamoxifen treatment. Immunohistochemistry was performed on tissue microarray of 355 patients with clinical follow-up data of more than 10 years, who had ER positive primary breast carcinoma and had received adjuvant tamoxifen treatment. Nuclear BQ overexpression was significantly associated with tamoxifen resistance by Chi-square test, p= 3.90 x 10-6 with a sensitivity of 51.4% and specificity of 72.9%. In tamoxifen treated patients, nuclear BQ overexpression was significantly correlated with cancer metastasis, p= 1.72 x 10-6, and also significantly associated with disease relapse, p=3.47 x 10-4. Consistent with its role in predicting tamoxifen resistance, nuclear BQ was significantly associated with poorer survival by Kaplan-Meier estimate (Log-rank test) p=6.28 x 10-5 and p=1.31 x 10-4 for overall survival and disease-specific survival respectively. The development of such reliable biomarker would enable appropriate alternative therapy to be given at an early stage, thus saving the patient from the side effects as well as risk of inappropriate treatment by tamoxifen.