P05 Yes-associated protein engages RIF1 to dampen replication stress-induced DNA damage in squamous cell carcinoma

Introduction and aims Squamous cell carcinomas (SCCs) are among the most common solid tumours, with cutaneous SCC being the most prevalent type, imposing a major healthcare burden owing to its high incidence. Head and neck SCC (HNSCC) is less frequent; however, it has a higher mortality rate. SCC cells undergo high levels of replication stress owing to oncogene-induced cell cycle deregulation. How such cells sustain rapid proliferation despite replication stress is still not fully understood. The transcriptional coregulators Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ, also known as WWTR1) are critical oncoproteins in SCC. In fact, ∼50% of patients with HNSCC harbour somatic alterations in components of the regulatory Hippo signalling pathway or YAP1/WWTR1 copy number gains, which invariably lead to hyperactivation of YAP/TAZ. How YAP/TAZ control oncogenic signalling in human SCC at the molecular level, and how human SCC cells deal with YAP/TAZ-induced replication stress remains poorly understood. Methods Using a multiomics approach involving RNA sequencing and chromatin-immunoprecipitation coupled with mass spectrometry, and combined with various molecular cell biology techniques, we have characterized transcriptional and nontranscriptional mechanisms of the oncogenic functions of YAP in human cutaneous SCC and HNSCC cell lines. Results We identified RIF1, a key regulator of DNA replication and DNA damage repair under replication stress, as an important binding partner of YAP in human SCC cells. RIF1 is highly expressed in human SCC cell lines and tissues and upregulated during tumour progression. Depletion of RIF1 in SCC cells exacerbates their endogenous replication stress. Mechanistically, we show that YAP engages with RIF1 specifically at broken replication forks, and that YAP depletion impairs DNA damage repair under replication stress. Conclusions Our results demonstrate that the oncogenic functions of YAP in squamous cancers involve both transcriptional and nontranscriptional mechanisms, the latter through interaction with RIF1 to dampen replication stress.