Breast cancer metastasis to the liver; a new mouse model system

While 30% of breast cancer metastasis occurrs in the liver, the majority of genetically engineered mouse models only metastasize to the lungs. Using a bioinformatic approach and human TCGA data, we predicted genetic pathways that were involved in metastasis to other locations and examined these genes in mouse model systems. We repeatedly observed a role for the E2F5 transcription factor loss in metastasis. We therefore generated a conditional knockout of E2F5 in the mammary epithelium and this alone was sufficient for development of mammary tumors after a long latency that were highly metastatic, including metastases to the lymph, liver and lung among other sites. Using a serial transplantation approach, we enriched for either lymph or liver metastasis and examined these tumors through RNAseq to determine which genetic pathways were involved with liver specific metastasis. Our gene expression analysis revealed a concerted alteration to the primary tumor cells through multiple mechanisms, both metabolomic and genetic, to allow the tumor cells to use the coagulation cascade to drive liver metastasis. We observed that the E2F5 transcription factor normally repressed several genes in the coagulation cascade and with the conditional knockout, these coagulation genes were derepressed. This resulted in a striking ability of these tumor cells to induce a fibrin clot in an in vitro clotting assay with human pooled plasma. However, expression of the clotting cascade genes alone is insufficient to induce a fibrin clot and we noted that the tumor cells could clot due to a negative charge on the membrane, were also positive for external phospholipids and had altered lipid uptake and reactive oxygen species. To definitively test whether clotting was essential for breast cancer liver metastasis, we treated mice with low molecular weight heparin as we implanted a metastatic tumor into the fat pad. In several lines, this resulted in a near complete blockage of liver metastasis. To test whether this treatment and pathway might also be functional in human cancer, we turned to the Truveta electronic health record data. This revealed that human pancreatic cancer patients that had been administered heparin for other reasons had a 40% reduction in liver metastasis rates. Thus, our work suggests that inhibition of the clotting cascade on the fibrin side may also be beneficial for prevention of breast cancer liver metastasis.