PROJECT SUMMARY The long-term objective of this project is to identify new, more effective, and less toxic therapies for Head and Neck Squamous Cell Carcinoma (HNSCC) and related cancers, by understanding the function and regulation of the p53 family members p63 and p73. The p53 family of proteins plays a key role in the pathogenesis of HNSCC. The p53 gene itself is a target of mutational inactivation in the majority of these tumors, and p53-mutant tumors are highly lethal regardless of treatment modality. In contrast, the related family member p63 is not mutated but is dramatically overexpressed and/or subject to genomic amplification in the majority of cases. P63 is a lineage-specific master regulator of normal epithelial development that we and others have demonstrated to function at the nexus of pathways controlling proliferation, differentiation and survival of malignant epithelial cells in HNSCC. By developing a physiologic mouse model of HNSCC we demonstrated that these tumors are addicted to high levels of p63, which critically activates a paracrine FGFR signaling axis that is essential for tumor cell survival. The physiologic significance of these observations is further supported by the demonstration that p63/p73 are direct mediators of chemosensitivity in HNSCC which is abrogated by Bcl-2 up-regulation, a mediator of therapeutic resistance. HNSCC exhibits extensive cellular heterogeneity within the neoplastic compartment, and successful therapy must target the regenerative population that maintains the self-renewing capacity of the tumor. Here we describe a novel and powerful transcriptional program mediated by p63 that drives regenerative proliferation in HNSCC. In Aim 1 we will perform in vivo studies to credential a key p63-associated co-factor that is required for the regenerative program, test effects on chromatin, and identify the downstream transcriptional and functional contributions. In Aim 2 we will validate an additional p63-associated co-factor and pharmacologic target which we hypothesize to mediate a central transcriptional program relevant to tumor progression. In Aim 3 we describe molecular analysis of patient samples and in vitro drug screens, leading to our discovery of a new approach to treat platinum-resistant HNSCC that leverages cross-talk between oncogenic and MAP kinase pathways. We will pursue this approach by employing genetic models, clinical pharmacologic inhibitors, and correlative tissue studies with patients enrolled in early-phase clinical trials. In addition to improving our knowledge of the basic biology of HNSCC, these studies will advance the goal of uncovering novel and viable therapeutic targets to improve treatment outcomes in this disease.