Abstract 911: BRM loss promotes tumor progression through extracellular matrix remodeling and elevated mammary epithelial stem/progenitor activity

BRM expression is progressively lost in breast cancer and low levels are predictive of poor patient prognosis. We have previously shown that oncogenic downregulation of BRM promotes malignancy of mammary epithelial cells (MECs) through the CEBP/â-mediated induction of a5-integrin. BRM and BRG1 are necessary but mutually exclusive subunits of the SWI/SNF ATP-dependent chromatin remodeling complex that disrupts DNA-histone interactions to alter accessibility of specific genomic regions. To explore the physiological role of BRM in the normal mammary gland and in breast cancer, we studied BRM germline knockout mice (BRM KO). We first backcrossed BRM KO mice onto a clean genetic background and observed a 40% increase in animal size compared to wild type littermates. Histological examination of developing BRM KO mammary glands revealed enhanced ductal branching and a 60% increase in numbers of terminal end buds. Concordantly, immunohistochemistry for phospho-histone H3 exhibited significantly elevated proliferation of BRM KO MECs. Picrosirius red staining of mammary glands further demonstrated an accumulation of collagen around epithelial ducts in BRM KO mice that correlated with an upregulation of genes encoding the extracellular matrix (ECM) proteins Collagen-I and Fibronectin as well as the collagen remodeling enzymes Loxl1 and Loxl2 as assessed by quantitative PCR (qPCR). Subsequent flow cytometry analysis of mammary glands to sort luminal versus basal MEC populations demonstrated a preference for expansion of the basal lineage in BRM KO mice compared to controls. Basal MECs are in direct contact with the ECM and mammary stem cells are proposed to exist within this population. To investigate the possibility that BRM KO augments mammary stemness, we performed colony formation and limiting dilution transplantation assays, which served to confirm that the expanded basal population was indeed associated with elevated MEC stem/progenitor activity and also to verify the MEC dependence of these effects. To gain more mechanistic insight, we sorted luminal/basal MEC populations for qPCR analysis and found that MECs from BRM KO mice produce more abundant transcripts of genes associated with stemness. Our preliminary data now suggests a compensatory upregulation of BRG1 in BRM KO MECs and elevated expression of several known YAP/TAZ target genes involved in the control of cell proliferation. Finally, to describe a role for BRM in tumor progression, we examined the outcome of BRM KO in MMTV-NEU mice and observed more rapid tumor growth with a trend to higher tumor incidence and metastasis in mice lacking BRM. Taken together, our data indicate that SWI/SNF chromatin remodeling regulates YAP/TAZ control over MEC growth and a loss of BRM expression leads to BRG1-directed ECM remodeling, MEC proliferation and elevated mammary stemness, resulting in heightened mammary tumor aggression. Citation Format: Jason J. Northey, Laura Damiano, Valerie M. Weaver. BRM loss promotes tumor progression through extracellular matrix remodeling and elevated mammary epithelial stem/progenitor activity. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 911.