Abstract P3-07-10: The role of CBP/FOXM1 in triple negative breast cancer
Authors: Ring A , Nguyen C , Lenz H-J , Tripathy D , Lang JE , Kahn M 
Published: March 2017

Abstract
Background: Accurate assessment of prognostic and predictive biomarkers (estrogen receptor, progesterone receptor, HER2) plays a critical role in the clinical management of breast cancer. Triple negative breast cancers (TNBCs) lack the expression of all three targets, and no targetable molecular pathways have been identified to date. Hence, TNBCs are treated with non-targeted, cytotoxic chemotherapeutic agents (e.g. paclitaxel), and are characterized by high rates of drug resistance and metastatic relapse. CREB binding protein (CBP) has been implicated in cell growth and malignant transformation in various cancers. CBP is an important co-activator in the β-catenin driven transcription, including the Wnt signaling pathway, which has been implicated in TNBC biology. The Kahn lab has developed a specific CBP-binding small molecule inhibitor, ICG-001. We hypothesized that CBP-signaling plays an important role in TNBC biology and may provide a novel therapeutic target.
Methods: We used TOP-flash assay to quantify Wnt signaling activity in TNBC. ICG-001 treatment combined with RNA Seq was used to characterize the role of CBP in TNBC cell line models. Co-immunoprecipitation (CoIP), protein and gene expression studies, as well as gene knock-down were used for validation. In vitro drug resistant cell line models as well as in vivo cell line (n=40 mice) and patient derived xenografts (PDX) in NOD scid gamma mouse models (2 patients, n=40 mice per patient) (treatmenent groups: control, paclitaxel, ICG-001, paclitaxel+ICG-001, n=5 mice per condition, primary and secondary implantation) were used to establish the effect of CBP inhibition via ICG-001 in TNBC on drug resistance and metastasis. We used the TCGA breast cancer data set to substantiate the experimental results.
Results: We demonstrated that gene expression in TNBC is CBP, but not Wnt signaling dependent, and can be disrupted via ICG-001. RNA Seq analysis of TNBC cells treated with ICG-001 revealed Forkhead box M1 (FOXM1) as a potential downstream regulator. CoIP demonstrated that CBP binding to the FOXM1/β-catenin transcriptional complex. Treatment with ICG-001 revealed that CBP/FOXM1 binding, but not FOXM1/ β-catenin binding, is critical for FOXM1 expression. The PDX mouse models demonstrated that FOXM1 expression correlates with response to chemotherapy and disease recurrence in vivo. Treatment with ICG-001 sensitized FOXM1 high tumors to chemotherapeutic treatment and statistically significantly reduced tumor growth in serial transplantation experiments. Comparison of clinical data with FOXM1 expression in tumor samples from patients indicated that high levels of FOXM1 were associated with disease relapse and poor survival outcomes.
Conclusion: CBP/FOXM1 binding is critical for FOXM1 expression. Targeting CBP/FOXM1 binding via ICG-001 could provide a novel therapeutic strategy in TNBC. The use of clinically annotated tissue microarrays containing a total of 430 breast tissue cores (51 TNBC cases) is currently pending to correlate nuclear protein expression of CBP and FOXM1 with survival outcomes in TNBC. FOXM1 and CBP could potentially be of value as predictive biomarkers in TNBC. These results could provide a clinical-translational rational for patient stratification based on CBP and FOXM1 expression for clinical trials exploring the therapeutic potential of FOXM1 inhibition via ICG-001 in combination with chemotherapy.
Citation Format: Ring A, Nguyen C, Lenz H-J, Tripathy D, Lang JE, Kahn M. The role of CBP/FOXM1 in triple negative breast cancer [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P3-07-10.