Project Summary We have identified a novel mode of autonomous programmed cell death that is controlled by p53 in small cell lung cancer (SCLC) — the most deadly form of lung cancer. We capitalized on a genetically engineered mouse model that we created to regulate endogenous p53 inactivation and its temporally controlled reactivation in established mouse models of SCLC. We found that p53 controls a canonical senescence program in approximately half of the tumors and a non-apoptotic cell death program in the rest. The mechanism of cell death is distinct from known forms of programmed cell death but is dependent on multiple members of the cyclophilin protein family of peptydyl-prolyl isomerases. We demonstrate that p53 regulates a distinct transcriptional program in dying SCLC tumors not active in senescing SCLC. Moreover, the activation of this transcriptional program is abolished by small molecule inhibitors of cyclophilins. In aim 1 of our project, we will interrogate the molecular underpinnings of this p53-cyclophilin interaction. At the cell physiological level, p53-mediated SCLC cell death is associated with features of paraptosis; a poorly defined mode of cell death not previously associated with p53 but characterized by excessive accumulation of cytoplasmic vacuoles, cell swelling, and plasma membrane dysfunction. Additionally, we observed massive induction of an endoplasmic reticulum-directed autophagy program (ER-phagy) during p53-mediated SCLC death. In aim 2 of our project we will interrogate the molecular features and dissect the molecular determinants of paraptosis and ER-phagy in this context. Finally, we present comparative genomics that implicates that the `classical' molecular subtype of SCLC is represented by SCLC tumors that die after p53 restoration and that the `variant' molecular subtype of SCLC is represented by SCLC tumors that senesce after p53 restoration. In aim 3 of our project, we will determine if the effects of p53 restoration are distinct based on either the cell of origin, or the collection of driver mutations as each of these impacts the molecular subtype that develops in SCLC. Together, our project will elucidate an important molecular mechanism for controlling SCLC, shed light on targetable vulnerabilities, and identify specific subsets of SCLC patients that may be susceptible to such approaches.