Abstract A42: Inhibition of the invasive component of mucinous breast cancers by targeting c-kit

Introduction: Among the molecular subtypes of breast cancer are normal-like, luminal (A or B) estrogen receptor positive (ER+), HER2+, and triple negative (basal). In addition to the molecular subtypes, there are 18 histologic breast cancer subtypes that are classified on appearance, including mucinous breast cancers (MBC), which are ER+ tumors. MBC can be classified according to their mucin composition as pure mucinous (> 90% mucin) or mixed mucinous (50-90% mucin). We have established a new model of mucinous breast cancer using breast cancer cells derived from a pleural effusion of a patient that were xenografted into an immunocompromised mouse. The resulting cell line, BCK4, was subsequently implanted into mice supplemented with either cellulose or estrogen. Pure mucinous tumors (MUCp) formed in the presence of cellulose, however, estrogen induced a histologic switch to invasive lobular carcinoma (ILC) with mucinous features (mixed mucinous). These mixed mucinous tumors contained both high mucin (MUCm) and low mucin tumor regions (ILC). The purpose of this study was to determine the molecular underpinnings of this histologic switch in BCK4 tumors. Experimental procedures: We isolated an equal number of tumor cells from pure mucinous and both the MUCm and ILC regions of the mixed mucinous BCK4 tumors using LASER capture microdissection; samples were analyzed on Affymetrix arrays to examine differential gene regulation among the tumor regions. Summary: Approximately 3000 genes were differentially regulated between the mixed and pure mucinous tumors. PAM50 subtype calls for the different tumor regions were as follows: MUCp tumors are the Normal-like subtype, MUCm tumor regions are Normal-like/Luminal A, while the ILC regions of the mixed mucinous tumors were Luminal B/Basal. Cluster analysis of the BCK4 derived tumors compared with ER+ and ER- breast cancer cell lines shows the mixed mucinous BCK4 tumors (both MUCm and ILC regions) group with ER- breast cancers, while the pure mucinous tumors were more related to ER+ breast tumor cells. To elucidate early genes regulated by estrogen in BCK4 cells, we performed expression profiling using Illumina arrays of the BCK4 cell line, treated with or without estrogen in vitro for 24 hours. A set of ~200 genes were significantly regulated by estrogen in the BCK4 cell line and the BCK4 derived tumors. Metacore Pathway Analysis suggested the c-kit pathway might be a target to reduce estrogen-induced proliferation. Subsequent studies found that inhibition of c-kit activity using Imatinib Mesylate (Gleevec®) blocked estrogen mediated stimulation of BCK4 cells as effectively as the anti-estrogen Fulvestrant. Conclusions: Targeting c-kit may represent a new type of therapy for some mucinous breast cancers. This is the first study to examine estrogen regulated genes in a model of mucinous breast cancer, allowing identification of genetic pathways that cause more indolent pure mucinous tumors to switch to aggressive mixed mucinous tumors. Citation Format: J. Chuck Harrell, Megan J. Blatner, Britta M. Jacobsen. Inhibition of the invasive component of mucinous breast cancers by targeting c-kit. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research; Oct 17-20, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(2_Suppl):Abstract nr A42.