Abstract B084: Comparative Functional Analysis of TP53 Alleles in Pancreatic Ductal Adenocarcinoma in Vivo

Abstract TP53 is the second most mutated gene in pancreatic ductal adenocarcinoma (PDAC), bearing both truncal loss-of-function and missense point mutations. TP53 encodes a stress-induced transcription factor (p53) capable of activating tumor suppressive pathways mediating cell cycle arrest, apoptosis, senescence, and metabolic reprogramming. Despite high mutational frequency, how TP53 mutants function to suppress pancreatic tumorigenesis in vivo remains unclear. Previous studies indicate that the most frequent point mutations observed in human PDAC (TP53 R175H and R273H; mouse homologues Trp53 R172H and R270H) exhibit both dominant-negative (e.g., response to radiation in leukemia cells) and gain-of-function (e.g., increased liver metastasis in advanced PDAC models) roles compared to p53 null cells in various contexts. To rigorously explore the differential functions of p53 point and deletion mutants in PDAC progression, we performed an isogenic comparative analysis of Trp53 alleles in vivo using Mosaic Analysis with Double Markers (MADM) in mice. MADM uses stochastic mitotic recombination to induce two genotypically distinct daughter cells – created at 1-to-1 ratios – expressing different Trp53 variants dependent on the genotype of the mouse (+/+ vs. -/-, R172H/R172H vs. +/+, R172H/R172H vs. -/-, R270H/R270H vs. +/+, R270H/R270H vs. -/-) and simultaneously label them with unique genetically encoded fluorescent markers (TdTomato vs. GFP). We integrated MADM into a faithful Kras-driven model of pancreatic tumorigenesis (KC: Kras LSL-G12D/+ ; Pdx1-Cre) and used fluorescence microscopy to trace subclonal populations with various Trp53 alleles within the same mouse. By analyzing the ratio of TdTomato+ to GFP+ cells, we studied the functional differences between alleles in driving tumor initiation and expansion at various stages of progression. Strikingly, we found that Trp53 -/- cells exhibited greater expansion in preinvasive pancreatic intraepithelial neoplasias (PanINs) compared to not only Trp53 +/+ cells but also cells harboring either point mutation (Trp53 R172H/R172H or Trp53 R270H/R270H ). While Trp53 R270H/R270H cells predominated over Trp53 +/+ in PanINs, Trp53 R172H/R172H cells were surprisingly indistinguishable from Trp53 +/+ cells, suggesting that p53R172H retains tumor suppressive properties in early PDAC progression. Conversely, both Trp53 R172H/R172H and Trp53 R270H/R270H cells progressed to advanced PDAC with comparable PDAC incidence and survival as Trp53 -/- , arguing that Trp53 R172H is a separation-of-function mutant concordant with a distinct transcriptional profile relative to Trp53 -/- tumor cells. Together, these studies offer the strongest evidence to date for stage-specific functional differences between TP53 mutant variants in cancer progression in vivo and provide a faithful MADM-based PDAC model to define molecular mechanisms (through ongoing transcriptomic, epigenomic, and metabolomic studies) that govern functional differences amongst these diverse alleles. Citation Format: Sherry S. Agabiti, Andy Tang, Timothy Nottoli, Mandar D. Muzumdar. Comparative functional analysis of TP53 alleles in pancreatic ductal adenocarcinoma in vivo [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Pancreatic Cancer Research; 2024 Sep 15-18; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2024;84(17 Suppl_2):Abstract nr B084.