Abstract 2957: Uncovering tumor-specific components of the p53 pathway using mouse models and RNAi
Authors: Sánchez-Rivera Francisco J. , Feldser David M. , Doench John , Bhutkar Arjun , Meacham Corbin E. , Root David E. , Hemann Michael T. , Jacks Tyler 
Published: August 2012

Abstract
The gene that encodes for the p53 transcription factor is the most frequently mutated gene in human cancer. It is estimated that about 50% of all human cancers harbor a mutation in the TP53 gene, and that the remaining 50% harbor mutations that inactivate other components of the pathway. p53 is known to activate a variety of tumor suppressive programs, such as cell cycle arrest, senescence and apoptosis, among others. Using genetically engineered mouse models of cancer, we and others have recently shown that a variety of established autochthonous tumors require sustained inactivation of the p53 pathway. Interestingly, the phenotypic outcome of p53 restoration in these tumors appears to be tumor specific, with some tumor types undergoing a permanent cell cycle arrest with features of cellular senescence and other tumors undergoing apoptosis. Independent of the phenotypic outcome, these studies have convincingly shown that established tumors are exquisitely sensitive to p53 restoration. Since p53 is a transcription factor, one hypothesis that may explain these tumor-specific outcomes suggests the possibility that p53 transcriptionally activates a particular cohort of genes that is specific to each tumor type, in addition to activating genes involved in a core p53 pathway. Another possibility is that p53 transcriptionally activates the same set of genes in all tumor types, but that their functions are context-dependent. Moreover, it is also possible that there are yet undiscovered genes in the p53 pathway that are not direct transcriptional targets of p53. In order to uncover both tumor-specific p53 effectors, as well as core p53 effectors, we have derived murine p53-restorable cell lines from three different tumor types: lung adenocarcinoma, sarcoma and lymphoma. These cell lines proliferate indefinitely in vitro. However, upon restoration of the endogenous Trp53 gene, these cell lines efficiently undergo cell cycle arrest (lung adenocarcinoma and sarcoma cell lines) or apoptosis (lymphoma cell lines). Moreover, this response is p53-dependent, as knockdown of either p53 or its main upstream activator p19Arf by RNAi allows the cells to bypass the effects of p53 restoration. To gain insight into the genes involved in these p53-dependent responses, we have carried out an unbiased large-scale pooled RNAi p53 bypass screen in both lung adenocarcinoma and sarcoma p53 restorable cell lines. We will describe initial results obtained from screening a library composed of approximately 40,000 shRNAs. Moreover, we are currently carrying out additional p53 bypass screens, both in the in vitro and in vivo settings, using different shRNA libraries, in order to obtain a more complete picture of the p53 tumor suppressive pathway. We envision that the results obtained from these screens will potentially uncover tumor-specific components of the p53 pathway, as well as novel components of the core p53 pathway.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2957. doi:1538-7445.AM2012-2957