Poster Presentation Clinical Oncology Society of Australia Annual Scientific Meeting 2019

Breast cancer mortality results largely from systemic, hematogenously-disseminated metastatic disease. To decrease metastatic risk, many breast cancer patients are treated with adjuvant chemotherapy, often unnecessarily. Several gene expression assays (e.g., Oncotype DX^®,      PAM50) provide more accurate prognostic information than classical clinicopathologic features. However, they each provide similar prognostic information driven largely by proliferation-/estrogen-regulated genes and not by the intrinsic propensity of a tumor to metastasize or interact with its microenvironment. We review here two new mechanism-based markers of metastatic risk: TMEM and Mena^calc.

Using multiphoton intravital imaging of mice with mammary tumors, we have observed that tumor cells migrate toward blood vessels accompanied by macrophages and intravasate exclusively at microanatomic sites called TMEM (Tumor Microenvironment of Metastasis), composed of a complex containing a Mena-expressing tumor cell, a pro-angiogenic macrophage, and a vascular endothelial cell, in direct contact. Gene expression profiling of migrating TMEM-associated cancer cells has led to identification of signaling pathways activated in migrating and intravasating tumor cells. These pathways involve differential expression of Mena isoforms. Mena undergoes alternative splicing resulting in multiple mRNAs that encode functionally distinct protein isoforms expressed in specific tissues and cell-types. The Mena expression pattern Mena^ClassicHi/ Mena^INVHi/Mena11a^Lo (scored as the fluorescent intensity of antibody staining of (pan-Mena – Mena 11a = Mena^calc)) is involved in breast tumor cells undergoing TMEM-mediated dissemination in vivo. Investigation of the association between TMEM and specific Mena isoforms by overexpressing either Mena^INV or Mena11a in tumor cells and observing their phenotypes by imaging in vivo has shown that Mena^INVHi-expressing tumor cells greatly enhance tumor cell migration and TMEM-mediated intravasation, while Mena11a-expressing cells suppress these phenotypes. Consistent with these findings in mice, in human breast tumors, TMEM and Mena^calc scores are individually prognostic for metastasis in ER⁺/HER2- patients and add complementary information to prognostic assays that are based on proliferation.