YAP1-TFE3 mediates endothelial to mesenchymal transition in epithelioid hemangioendothelioma

Epithelioid Hemangioendothelioma (EHE) is an ultra-rare vascular sarcoma with a high metastatic rate and a lack of reliable prognostic markers. Because of its rarity, EHE is an understudied disease with limited research tools and few effective treatment options. A thorough understanding of the molecular underpinnings that drive EHE tumorigenesis and facilitate metastasis is essential to discovery of druggable targets that could improve patient outcomes. To better understand this EHE biology we generated transgenic endothelial EHE cell models and observed robust transformation of YAP1-TFE3 (YT) expressing cells resulting in a pro-migratory, anti-apoptotic phenotype. We identified endothelial to mesenchymal transition (EndMT) as a contributing process to this transformation as evidenced by the loss of enothelial gene expression and strong increases in EndMT biomarkers Acta2 and Tgm2. To understand the underlying drivers of this process, we utilized mutant versions of YT and pharmacologic inhibitors to determine that the DNA-binding and dimerization bHLH-LZ domain of YT was necessary for EndMT to occur while canonical TEAD activation was dispensable. This project sheds light on molecular mechanisms of EHE which directly contribute to endothelial transformation.