CCN6/WISP3 suppresses metaplastic breast carcinoma by antagonizing a WNT/b-catenin/EZH2 cascade [ChIP-seq]

Metaplastic breast carcinomas (mBrCAs) are a highly aggressive subtype of triple negative breast cancer (TNBC) with histological evidence of epithelial to mesenchymal transition (EMT) and aberrant differentiation, including varied non-glandular features. CCN6/WISP3 tumor suppressor gene inactivation and b-catenin-dependent Wnt pathway activation are often seen in mBrCAs. We have reported MMTV-Cre;Ccn6fl/fl (Ccn6-KO) mice develop spindle mBrCAs with EMT and elevated expression of the Wnt/b-catenin pathway genes HMGA2 and IGF2BP2. The mechanisms by which the secreted CCN6 protein exerts tumor suppressor functions in breast tissues are uncertain. Here, we show recombinant CCN6 protein interacts with Wnt receptor FZD8 and co-receptor LRP6 on mBrCA cells to antagonize Wnt ligand-mediated activation of b-catenin/T-cell-factor (TCF) transcription. We identify the gene for histone methyltransferase EZH2, the catalytic component of polycomb repressive complex 2, as a β-catenin/TCF transcriptional target in Ccn6-KO mBrCA cells. Inhibition of Wnt/β-catenin/TCF function in Ccn6-KO mBrCA cells led to reduced EZH2 levels and decreased histone H3 lysine 27 trimethylation, resulting in deregulation of specific gene targets. Pharmacological inhibition of EZH2 reduced growth and metastasis of Ccn6-KO mBrCA mammary tumors and induced an epithelial phenotype in vivo. In human breast cancer specimens, low CCN6 is significantly associated with activated β-catenin and high EZH2 in spindle mBrCAs compared to other subtypes. Collectively, our findings establish a novel tumor suppressor mechanism for CCN6 as a key negative regulator of a b-catenin/TCF-EZH2 axis and highlight how CCN6 restoration or b-catenin or EZH2 inhibition could have therapeutic relevance for patients with spindle mBrCAs.