Head and neck squamous cell carcinomas (HNSCC) are a diverse group of tumors from the upper aerodigestive tract with relatively poor outcomes and limited targeted therapeutic options. Most patients are treated with a combination of DNA damaging agents (platinum, XRT). Recent genomic characterization of these tumors has not identified targetable oncogenic drivers, thus emphasizing the need to develop rational genomic-based approaches to increase sensitivity to treatments that induce DNA damage. We performed an in vivo functional genomic screen in genomically characterized HNSCC cell lines alone or in combination with DNA damaging agents. Knock-down of CREBBP or EP300 was identified as a potential mechanism to sensitize cells to the DNA damage response. CREBBP and EP300 are homologous multifunctional bromodomain-containing acetyltransferases that can regulate many proteins and pathways. Importantly, CREBBP and EP300 are mutated in 13% of HNSCC and sensitivity seems to be associated with those alterations, suggesting a synthetic cytotoxicity relationship. Additionally, these genes are druggable and small molecule inhibition of CREBBP increases DNA damage induction and persistence in response to radiation treatment and increased apoptotic cell death. We hypothesize that the CREBBP/EP300 pathway is both a biomarker and a therapeutically relevant target to sensitize HNSCC to currently used DNA damaging treatments. We propose to examine the genomic basis for this synthetic cytotoxicity and understand how mutations modulate the phenotype. We will also identify global changes to histone acetylation caused by modulation of CREBBP or EP300 and aim to understand how those changes impact the response to DNA damage. Additionally, therapeutic agents that modulate these genes will be tested for their efficacy in preclinical models with the goal of generating sufficient data to justify a clinical trial. Finally, we will perform another in vivo screen to identify other targets that can sensitize to inhibition of CREBBP with reduced toxicity. Overall, this project will examine an exciting new therapeutic target and candidate biomarker for a tumor type that is driven by loss of tumor suppressor genes and has proven difficult to target.