The contribution of partial and full epithelial-to-mesenchymal transition to breast cancer progression

Breast cancer is a leading cause of mortality in women worldwide. The lethality associated with the disease is primarily a consequence of the systemic spread of cancer cells and the formation of metastasis, particularly in the lung, liver, brain and bones. Multiple cellular and molecular mechanisms may contribute to the complex process of metastasis formation. In order to devise clinically effective strategies to combat metastatic disease, these need to be better understood.
Epithelial-to-mesenchymal transition (EMT) is a cellular program of trans- differentiation which is critical for embryonic development and wound healing but which may also contribute to the dissemination of cancer cells. During an EMT, epithelial cancer cells lose their cell-cell adhesion, dedifferentiate, and acquire a migratory and invasive phenotype. Experimental induction of an EMT is sufficient for metastasis formation. However, whether a spontaneous EMT is required for metastasis in an unperturbed system in vivo is a major focus of current research. Within recent years, it has become clear that EMT is not a binary switch but covers a spectrum of intermediate EMT hybrid states which differ in their functional characteristics and metastatic potential. However, due to the transient and reversible nature of the process, the extent to which cancer cells spontaneously undergo a partial or full EMT in vivo and the functional consequences regarding metastasis remain unknown.
With my PhD work, I have aimed at assessing the contribution of partial and full EMT to breast cancer progression and metastasis. To this end, I have established two novel color-switching lineage tracing models in transgenic mice. Based on an irreversible switch from mCherry to GFP expression, this model allows to visualize and track cancer cells that have undergone a partial or full EMT, even if they re-differentiate by undergoing a mesenchymal-to-epithelial transition (MET). I show that cancer cells mostly transition between epithelial/mesenchymal hybrid states but rarely undergo a full EMT. Furthermore, cells which have undergone a partial EMT are highly enriched in lung metastases compared to primary tumors. In particular, metastasis with a mosaic composition of mCherry and GFP positive cells are observed, pointing towards a collective dissemination of cells that have undergone a partial EMT together with epithelial cancer cells. In contrast, cells that have undergone a full EMT retain a more quiescent mesenchymal phenotype and do not colonize the lung. In conclusion, these data suggest that although a full EMT may not be required, a partial EMT contributes to experimental breast cancer metastasis.
In addition, I have further characterized the mammary gland-specific flippase driver-line which we have generated for the lineage tracing experiments. These mice may serve as a versatile tool for studying mammary gland biology and breast carcinogenesis.
In summary, my PhD work provides novel insights into the dynamics of EMT and MET in vivo, as well as the contribution of partial and full EMT to breast cancer metastasis. Furthermore, our newly established mouse models offer novel opportunities to study the contribution of partial and full EMT towards distinct aspects of breast cancer progression in vivo.