The role of CtIP (RBBP8) in tamoxifen resistance and human breast cancer

Acquired resistance to the antiestrogen tamoxifen constitutes a major clinical challenge in breast cancer therapy. However, the mechanisms involved are still poorly understood. Therefore, the overall goal and focus of this dissertation are to better understand the phenomenon of tamoxifen resistance. In preliminary studies, we generated two independently derived isogenic MCF-7 breast cancer cell line variants (TAMR1 and TAMR2) that were resistant to the inhibitory growth effects of tamoxifen. Using serial analysis of gene expression (SAGE), we identified CtIP (CtBP-interacting protein), a BRCA1 (Breast cancer 1)- and CtBP (C-terminal binding protein)-interacting protein, as one of the most significantly down-regulated transcripts in the aforementioned tamoxifen resistant cells. We hypothesized that CtIP silencing constitutes a critical event for the development of tamoxifen resistance in breast cancer. We found that silencing endogenous CtIP in tamoxifen sensitive cells confers tamoxifen resistance and estrogen independence. On the other hand, re-expression of CtIP in tamoxifen resistant cells restores sensitivity to the inhibitory growth effects of tamoxifen. Importantly, poor clinical response to neo-adjuvant endocrine therapy is associated with CtIP deficiency in primary breast carcinomas. Meta-analysis of seven publicly available gene expression microarray data sets shows that CtIP expression is significantly associated with estrogen receptor (ER), disease free survival and breast cancer metastasis status. Furthermore, we found CtIP protein expression in a majority of ER positive breast cancer cell lines, but none or very little CtIP expression in ER negative lines. These findings indicate that CtIP silencing may be a novel mechanism for the development of tamoxifen resistance in breast cancer, and suggest that CtIP is likely associated with ER function and that CtIP gene and protein expression may be useful biomarkers for breast cancer prognosis and clinical management. Subsequent studies found a BRCA1-CtIP-CtBP complex in tamoxifen sensitive but not resistant cells, whereas BRCA1 is associated with ER in both cell lines. We also observed different patterns of occupancy by BRCA1, CtIP and CtBP on ERE (Estrogen Response Element) region of the pS2 promoter after E2 or tamoxifen treatment. These results support the potential involvement of a BRCA1-CtIP-CtBP complex in the development of tamoxifen resistance.