The development of bone metastases is one of the most debilitating complications of advanced breast cancer. Patients suffer from severe pain, spontaneous fractures and possible spinal cord compression caused by development of osteolytic lesions. We don't understand how breast cancer cells develop the ability to metastasize and proliferate in bone. As a consequence, therapeutic options at this advanced stage of breast cancer are extremely limited. Breast cancer bone metastatic cells undergo striking changes, expressing genes involved in bone development such as OPN, MMPs, VEGF, all known markers of tumor growth, invasion and metastasis. Importantly, these genes are controlled by the master regulator of bone formation RUNX2, and RUNX2 becomes overexpressed in breast cancer metastatic cells. We identified that the WWOX tumor suppressor protein physically interacts with and represses RUNX2 transcriptional activity. We hypothesize that loss of WWOX expression (a common event in breast cancer) is key for the progression of breast cancer cells towards development of a bone metastatic phenotype. Our current research suggests that loss of WWOX in breast cancer cells leads to unrestrained RUNX2 activity, Osteopontin overexpression and these maybe key determinants for metastasis development. We expect aborting development of bone metastases by restoring function of the WWOX-RUNX2 axis. We have also created a conditional Wwox KO mouse that will allow in vivo study of effects in mammary gland growth, tumor development and metastasis. Therefore we will pursue the following Specific Aims: Aim 1: We will A. determine whether WWOX depletion alters the phenotype of normal mammary epithelial and non-metastatic breast cancer cells and B. determine the effects of restoring WWOX expression in metastatic breast cancer cells. Aim 2: A. To determine whether WWOX restoration suppresses spontaneous bone metastasis and osteolytic bone disease in breast cancer metastatic cells. B. To determine if WWOX depletion induces a metastatic and osteolytic phenotype in non-metastatic breast cancer cells. C. To determine whether WWOX influences the ability of breast cancer cells to crosstalk with the bone microenvironment by affecting osteogenic differentiation. Aim 3: To determine whether the integrity of the WWOX-RUNX2 physical interaction is relevant for impeding bone metastases and associated osteolytic disease. Aim 4: To determine whether targeted Wwox ablation in the mammary gland leads to enhanced mammary tumorigenesis and increases metastasis. The proposed studies will provide novel information on the mechanistic basis of bone metastasis development and identify potential novel targets for therapeutic intervention.