Abstract B07: Regaining epigenetic control of the Hippo pathway to inhibit sarcomagenesis

Nearly 11,000 individuals in the United States and 200,000 individuals worldwide are diagnosed with a form of soft-tissue sarcoma (STS) every year. STS is a rare heterogeneous and complex family of disease associated with no uniform underlying genetic abnormality. However, sarcoma treatment has not changed significantly in 25 years. Patients are limited to surgery, radiotherapy, and chemotherapy. Therefore, the discovery of novel targets and mechanisms is critical. The Hippo pathway functions as a signaling hub facilitating cellular responses to multiple stimuli during tumorigenesis. The most important downstream effector of the Hippo pathway is Yes-activated protein (YAP), a transcriptional coactivator that promotes pro-proliferation and suppresses apoptosis. Inactivation of the Hippo pathway promotes nuclear localization of YAP. Whereas many studies have defined YAP19s transcriptional targets in epithelial tumors and normal tissues, its role in mesenchymal tumors is unclear. Here, we confirm that increased YAP1 mRNA expression correlates with worse overall survival in undifferentiated pleomorphic sarcoma (UPS) patients and that YAP1 protein levels are frequently elevated in human sarcomas. Furthermore, immunohistochemical (IHC) analysis of human sarcoma tissue microarray (TMA) showed that YAP levels are dramatically increased in the nuclei of high-grade UPS tumor cells, compared with normal skeletal, adipose, and arterial tissue. UPS is one of the more commonly diagnosed and aggressive subtypes of sarcomas found in adults. To define mechanisms of YAP1-mediated sarcomagenesis, we developed a novel genetic mouse model in which YAP1 is conditionally deleted from KrasG12D+; Trp53fl/fl (KP) UPS tumors (KPY). Microarray analysis of both tumors revealed that YAP1 loss inhibits expression of NF-κB signaling components and transcriptional targets. Consistent with these findings, ChIP-seq and super-enhancer analysis of human UPS tumors (n=3) revealed that many of the 900 identified UPS super enhancers regulate expression of NF-κB targets dependent on YAP1 in our system. This finding is particularly relevant as NF-κB is a key regulator of muscle cell proliferation and suppresses differentiation by inhibiting expression of the myogenic transcription factor, MYOD1. Together, these data suggest that upregulation of YAP1 activity promotes NF-κB-mediated proliferation and inhibits differentiation. Importantly, we have discovered that a combination of JQ1, a BET family inhibitor, and vorinostat (SAHA), a histone deacetylase (HDAC) inhibitor, decreases YAP1 expression, YAP1 protein stability, and sarcoma cell proliferation. Mechanistically, SAHA/JQ1 treatment significantly increases expression of the tumor suppressor angiomotin (AMOT), which binds YAP1, sequesters it in the cytoplasm, and facilitates its degradation. We have also found that AMOT expression is lost in sarcomas compared with normal muscle tissue, likely due to epigenetic suppression. SAHA/JQ1 treatment also decreased gene expression of YAP1 targets and caused reexpression of the muscle differentiation markers p57, MEF2C, and MYOD1. Based on these data we will investigate the efficacy of SAHA/JQ1 against UPS and other sarcomas in vivo using subcutaneous and autochthonous mouse models. We will probe the ability of SAHA/JQ1 treatment to inhibit YAP1 in vivo, to suppress NF-κB dependent mechanisms of cell proliferation and tumorigenesis, and to promote terminal differentiation of sarcoma cells. Ultimately, we will determine whether this approach represents a novel course of targeted treatment for sarcoma patients. Citation Format: Shuai Ye, Koreana Pak, Jennifer Shah, Susan Chor, Shaun Egolf, Gloria Marino, T.S. Karin Eisinger-Mathason. Regaining epigenetic control of the Hippo pathway to inhibit sarcomagenesis [abstract]. In: Proceedings of the AACR Conference on Advances in Sarcomas: From Basic Science to Clinical Translation; May 16-19, 2017; Philadelphia, PA. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(2_Suppl):Abstract nr B07.