Runx2 (Cbfa1/AML3) is a master transcription factor that activates/represses target genes dependent on nuclear architecture. Runx2 functional activity requires the fidelity of a unique nuclear matrix targeting signal (NMTS) that organizes in subnuclear domains Runx2 complexes with co-regulatory proteins that include chromatin remodeling factors and mediators of ECM signal transduction pathways (e.g., IGF-1, integrin, TGF/BMP and c-Src) for expression of Runx2 target genes. Runx2 was discovered as essential for skeletal formation and regulates genes that control cell growth (p21), recruitment of blood vessels(VEGF), differentiation of mesenchymal lineage cells and expression of proteins for ECM formation and degradation (osteopontin, bone sialoprotein, matrix metalloproteinases). Recently, high Runx2 expression has been reported in metastatic breast cancer cell lines (MDA-MB-231 and LCC15) that result in destructive osteolytic lesions in bone. Based on our findings that mutations in Runx2 which block Runx2 targeting functions and inhibit tumor progression and osteolysis in bone, we postulate that Runx2 controls expression of genes inducing metastatic properties associated with tumor progression and osteolysis in the bone micfoenvironment. Our specific aims are directed to understanding the mechanisms by which 1) endogenous Runx2 functions in metastatic breast cancer cells and 2) the subnuclear targeting deficient Runx2 mutant protein (STDmRunx2) inhibits expression of Runx2 target genes and blocks tumor growth and osteolysis in vivo. Using in vitro molecular and in vivo assays of human MDA-MB-231 metastatic breast cancer cells expressing WT and STDmutant Runx2 proteins, we will address: 1) the Runx2 response to signaling pathways hyperactive in metastatic cells (with Project 1);2) the role of Runx2 mediated histone modifying (HDACs and HATs) and SWI/SNF chromatin remodeling factors in the control of target genes in breast cancer cells (with Project 2);3) the phenotype change induced by WT and STDmRunx2 proteins and the in vivo fate of the cells lines (with Projects 1,2 and 3);and 4) a potential role of Runx2 in development of primary mammary tumors (with Project 2). Lay Summary: Metastatic breast carcinomas most frequently occur in bone. An estimated 70% morbidity of patients with metastatic breast cancer is associated with the destruction of bone that occurs when the tumor cells grow in bone. We have discovered that blocking a unique property of a master regulatory protein Runx2 that activates genes highly expressed in metastatic cells, can prevent tumor growth and osteolytic lesions in bone. By addressing the mechanisms accounting for this remarkable effect, a potential therapeutic strategy can be developed to prevent this end-stage of breast cancer.