The Role of the WWTR1(TAZ)-CAMTA1 Gene Fusion in Epithelioid Hemangioendothelioma

Epithelioid hemangioendothelioma (EHE) is a devastating and mysterious vascular cancer which has no known definitive treatment. Due to a lack of valid animal or cell-based models of EHE, progress toward understanding and treating this cancer has been severely limited. However, recent studies have determined that 90% of patients exhibit a lone, characteristic in-frame gene fusion, TAZ(WWTR1)-CAMTA1. While expression of the TAZ-CAMTA1 fusion protein has been validated as a biomarker of EHE, it remains unknown whether this genetic abnormality is a passenger or a driver of EHE. In this project, I present the first genetically-engineered mouse model (GEMM) of EHE, showing that the expression of the TAZ-CAMTA1 protein in endothelial cells is sufficient to drive the formation of EHE-like tumors in the lungs of mice. Furthermore, I demonstrate that the cessation of TAZ-CAMTA1 expression leads to the regression of these vascular tumors. I also demonstrate that TAZ-CAMTA1 transforms the MS1 endothelial cell line and that subcutaneous transplantation of these cells into nude mice leads to the formation of solid, progressive EHE-like vascular tumors that have the capacity to metastasize to the lung. Utilizing these two novel models of EHE, I unravel the gene program of TAZ-CAMTA1 and demonstrate that TAZ-CAMTA1 drives a gene signature similar to TAZ, the key effector of the Hippo pathway. Expression of an activated TAZ in endothelial cells is also sufficient to drive EHE-like vascular tumors in mice, and genetic blockade of the transcriptional partners of TAZ, the TEAD family of transcription factors, prevents the formation of TAZ-CAMTA1-induced vascular tumors. Next, I show that TAZ-CAMTA1 induces an angiogenic and regenerative-like gene program in endothelial cells. I validate that TAZ-CAMTA1 exhibits gain-of-function activities by having increased resistance to proteasomal degradation and increased nuclear enrichment over TAZ. Lastly, I show that TAZ-CAMTA1 still maintains its binding to the Hippo pathway proteins which are known to negatively regulate TAZ. In summary, I generate two novel models that pinpoint TAZ-CAMTA1 as the key driver of EHE and utilize these models to suggest several new lines of investigation for the treatment of patients with EHE.