Abstract B08: Therapeutic inhibition of YAP1 expression by next-generation antisense oligonucleotides leads to antitumor activity in head and neck squamous cell carcinoma with YAP1 activation

As the final effector of the Hippo pathway, YAP1 plays a driver role in promoting growth of a variety of tumor types. Unlike other oncogenic pathways, however, genetic alterations on the core components of the Hippo pathway leading to YAP1 activation are rare and have not been well characterized. Therefore, identifying alternate pathway alterations leading to YAP1-dependent tumors will be critical for clinical success of potential YAP1-targeted therapies. Recent reports have suggested that frequent mutations of tumor suppressor FAT1 might contribute to the progression of YAP1-dependent head and neck squamous cell carcinoma (HNSCC). In this study, we have investigated the underlying mechanisms for YAP1 activation in HNSCC, developed a therapeutic agent for the specific suppression of YAP expression, and identified a HNSCC subtype that is highly sensitive to YAP1 inhibition. We first screened and identified active human YAP1 antisense oligonucleotides (ASOs) with next-generation (constrained ethyl=cEt) chemistry and then evaluated them in a broad panel of human HNSCC lines in vitro. The majority of HNSCC lines were highly amenable to ASO free uptake (without any transfection agents), resulting in strong reductions of YAP1 and its downstream target genes such as CTGF, Cyr61, and Survivin in a dose-dependent manner. However, phenotypic consequences of YAP1 depletion varied significantly among the cell lines and were dependent upon the levels of functional FAT1 protein. Cell proliferation was strongly reduced by YAP1 ASOs in the cell lines harboring homozygous copy loss or nonsense mutations of FAT1, while minimal antiproliferative activity was observed in cells with wild-type FAT1 or YAP1 amplification. Importantly, the nuclear localization of YAP1 appeared to be a key determining factor for the sensitivity of HNSCC cells to YAP1 ASOs. Unexpectedly, in contrast to oral tongue SCC (OTSCC) lines that were generally sensitive to YAP1 ASOs, proliferation of FAT1 mutant HNSCC lines originated from larynx were minimally affected by YAP1 inhibition even with near-complete depletion of YAP1, suggesting variable dependency of tumor cells on YAP1 for their growth among different subtypes of HNSCC. Moreover, when systemically administered into mice bearing OTSCC with FAT1 mutations, YAP1 ASOs greatly reduced YAP1 expression in tumors and suppressed tumor growth in subcutaneous and more notably oral orthotopic xenograft models. These results suggest that OTSCC with YAP1 activation due to FAT1 loss is highly dependent on YAP1 for growth and that YAP1 ASOs have promise for the treatment of this cancer. Finally, through an extensive selection process the human YAP1 ASO clinical development compound, YAP1Rx, has been identified. Citation Format: Xiaolin Luo, Youngsoo Kim, Joanna Schmidt, Jian Li, Rachel Fleming, Shivani Malik, Stephanie Klein, A. Robert MacLeod. Therapeutic inhibition of YAP1 expression by next-generation antisense oligonucleotides leads to antitumor activity in head and neck squamous cell carcinoma with YAP1 activation [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 B08.