A Pygopus 2-histone interaction is critical for cancer cell de-differentiation and progression in malignant breast cancer.

Pygopus 2 (Pygo2) is a co-activator of Wnt/β-catenin signaling that can bind bi- or trimethylated lysine 4 of histone-3 (H3K4me2/3) and participate in chromatin reading and writing. It remains unknown whether the Pygo2- H3K4me2/3 association has a functional relevance in breast cancer progression in vivo. To investigate the functional relevance of histone binding activity of Pygo2 in malignant progression of breast cancer, we generated a knock-in mouse model where binding of Pygo2 to H3K4me2/3 was rendered ineffective. Loss of Pygo2-histone interaction resulted in smaller, differentiated, and less metastatic tumors, due in part to decreased canonical Wnt/β-catenin signaling. RNA and ATAC sequencing analyses of tumor-derived cell lines revealed downregulation of TGFβ signaling and upregulation of differentiation pathways such as PDGFR signaling. Increased differentiation correlated with a luminal cell fate which could be reversed by inhibition of PDGFR activity. Mechanistically, the Pygo2-histone interaction potentiated Wnt/ β-catenin signaling in part by repressing the expression of Wnt signaling antagonists. Furthermore, Pygo2 and β-catenin regulated the expression of miR-29 family members which in turn repressed PDGFR expression to promote de- differentiation of wildtype Pygo2 mammary epithelial tumor cells. Collectively, these results demonstrate that the histone binding function of Pygo2 is important for driving de-differentiation and malignancy of breast tumors, and loss of this binding activates various differentiation pathways which attenuate primary tumor growth and metastasis formation. Interfering with the Pygo2- H3K4me2/3 interaction may therefore serve as an attractive therapeutic target for metastatic breast cancer.