Abstract B10: Evaluating YAP and TAZ as therapeutic targets for treating malignant mesothelioma with Hippo pathway disruptions
Authors: Kulkarni Aishwarya S. , Vissers Joseph H.A. , Harvey Kieran F. 
Published: September 2020

Abstract
Malignant mesotheliomas (MMs) are tumors originating from the mesothelial lining of internal organs. Hippo pathway disruptions are common in MMs; up to 50% of cases exhibit NF2 deficiency and other pathway genes such as LATS2 are also mutated (1). This suggests hyperactivity of YAP and TAZ as a likely oncogenic driver in MMs and a potential axis to exploit therapeutically (2). Before committing to such an approach, a dependence of MMs on YAP/TAZ for survival needs to be tested. We investigated this using different MM cell line models cultured in vitro. The impact of Hippo pathway mutations on YAP and TAZ activity in mesothelial cells was assessed using an NF2 mutant isogenic system in an immortalized mesothelial cell line (Met5A.) YAP activation levels were probed through immunoblotting for YAP S127 phosphorylation, immunofluorescence, and RNA sequencing (RNAseq). YAP and TAZ were inhibited through RNAi-mediated depletion or treatment with simvastatin or verteporfin in a panel of 7 MM cell lines, including 3 with mutations in known Hippo pathway genes and 4 lacking such mutations, and cell viability was measured using an Alamar Blue assay. We found that Hippo pathway-mediated phosphorylation of YAP at S127, and YAP cytoplasmic abundance, was reduced in NF2 mutant Met5A cells compared to isogenic nonmutant cells. Consistent with this, RNAseq analysis showed elevated YAP activity in NF2 mutant Met5A cells. However, RNAi-mediated depletion of YAP and TAZ in a panel of mesothelioma cell lines did not reveal a clear correlation between dependence on YAP/TAZ for survival and aberrant Hippo signaling. There was also a comparable level of sensitivity of NF2 wild-type and NF2 mutant Met5A cells to YAP/TAZ knockdown. Additionally, treatment with verteporfin and simvastatin did not show increased toxicity in cell lines with Hippo pathway mutations relative to nonmutant cells. Our findings confirm that Hippo pathway mutations drive YAP/TAZ hyperactivity in mesothelial cells. However, they show that pathway mutation status does not always correlate with a dependence on YAP and TAZ for mesothelioma cell survival, at least when cultured in vitro. Clearly, a more in-depth analysis of the Hippo pathway in MMs is required to determine the potential of this pathway as a therapeutic target for this disease.
References
1. Bueno R, Stawiski EW, Goldstein LD, et al. Comprehensive genomic analysis of malignant pleural mesothelioma identifies recurrent mutations, gene fusions and splicing alterations. Nat Genet 2016;48(4):407-16.
2. Huang J, Wu S, Barrera J, Matthews K, Pan D. The Hippo signaling pathway coordinately regulates cell proliferation and apoptosis by inactivating Yorkie, the Drosophila homolog of YAP. Cell 2005;122(3):421-34. doi:10.1016/j.cell.2005.06.007.
Citation Format: Aishwarya S. Kulkarni, Joseph H.A. Vissers, Kieran F. Harvey. Evaluating YAP and TAZ as therapeutic targets for treating malignant mesothelioma with Hippo pathway disruptions [abstract]. In: Proceedings of the AACR Special Conference on the Hippo Pathway: Signaling, Cancer, and Beyond; 2019 May 8-11; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2020;18(8_Suppl):Abstract nr B10.