Metastasis to distant organs is the major cause of breast cancer-related death. Bone is the most frequent destination of metastasis. Over 45% first-site metastases occur to bone, as compared to 19% to lung, 5% to liver and 2% to brain. Patients with skeleton as the first site of metastasis usually have better prognosis than those with visceral organs as first site. However, in more than two third of cases, bone metastases will not be confined to the skeleton, but rather subsequently occur to other organs and eventually kill patients. This raises the possibility of secondary metastatic dissemination from the initial bone lesions to other sites. In fact, some metastases first found in non-bone organs may be seeded from subclinical bone micrometastases as well, as suggested by the finding that cancer cells arrived in the bone can acquire more aggressive phenotypes even before establishing overt metastases. Recent genomic analyses indeed concluded that the majority of metastases result from seeding from other metastases, rather than primary tumors. Thus, it is of imperative importance to investigate further metastatic seeding from bone lesions, as it might lead to prevention of the terminal stage, multi-organ metastases that ultimately cause the vast majority of deaths. Despite the potential relevance, we know very little about metastasis-to-metastasis seeding. Current pre-clinical models focus on seeding from primary tumors, but cannot distinguish further dissemination. Taking advantage of a recently developed approach that selectively deliver cancer cells to hind limb bones, we have uncovered frequent metastatic seeding from established bone lesions to multiple other organs. This seeding is hypothetically enabled by the bone microenvironment-induced effects that confer more stem-like properties through a combination of clonal selection and epigenomic adaptation. In this application, we will elucidate the underlying mechanisms and temporal course of this process in order to provide the first ever insights into time window and strategies of potential therapeutic interventions. We hypothesize that clonal selection and epigenetic adaptation driven by the bone microenvironment engender the ability of disseminated breast cancer cells to further metastasize and blockade of the microenvironment-induced alterations may confine bone metastases and prevent further dissemination to other fetal organs. Our specific aims are 1) to characterize the kinetics of metastatic seeding from bone lesions and determine potential therapeutic windows accordingly, and 2) to identify key druggable targets and design therapeutic strategies against secondary metastatic seeding from bone lesions. This project is innovative and impactful because it is the first in the field that focuses on secondary metastasis and the profound reprogramming effects of bone microenvironment on metastatic seeding. The outcome will likely generate significant impact on our understanding of metastatic evolution and provide insights into novel therapies confining metastases for ultimate cure.

Many metastases of breast cancer may further disseminate to seed more metastases, but the pre-clinical models, the underlying mechanisms, and the potential therapeutic targets are all lacking. Herein, based on several lines of preliminary evidence, we propose to test the hypothesis that the bone microenvironment invigorates metastatic seeds for further dissemination. Specifically, we will focus on delineation of temporal course of metastatic seeding from bone to other organs, and identification of druggable pathways driving this process.