Abstract A17: p53 missense mutants R172H and R270H exhibit differential effects on tumorigenesis

p53 is a well-characterized transcription factor that is mutated in about 50% of human cancers. The majority of p53 genomic alterations are missense mutations which result in expression of p53 protein isoforms with deficient functionality. Such missense mutations cluster in several hotspots, with the most frequent alterations located at amino acids 175, 248 and 273 of human p53 (mouse amino acids 172, 245 and 270). p53 missense mutants disrupt the function of p53 tetramers, thereby behaving similarly to loss of p53. However, there is evidence that p53 missense mutants have additional tumor-promoting functions that differ from p53 deletions: p53 missense mutants can transform p53 null cells in vitro and in mouse models, and the p53 missense mutants induce a different tumor spectrum from that induced by p53 loss or other p53 missense mutants. The information obtained from the engineered murine models featuring p53 missense mutations has greatly elucidated the role of p53 mutagenesis in cancer. However, the currently-available p53 models bear several drawbacks that make them sub-optimal for preclinical studies. These models contain a global p53 haploinsufficiency, potentially impacting the biology of tumor-modifying structures like stroma, the immune system, and the vasculature. Furthermore, activation of the p53 missense mutant alleles results in a transition from a single functional wild-type allele to a combination of a wild-type and a mutant allele. These features substantially deviate from those of observed in human tumorigenesis, where somatic p53 mutagenesis in cells undergoing transformation results in a transition from two wild-type alleles to a combination of a wild-type and a mutant alleles, while tumor-modifying structures retain an unaltered p53 biallelic configuration. Here we describe the molecular and phenotypic characterization of a new allelic series of conditional p53 missense mutant mouse lines in which Cre-mediated recombination converts p53 from a wild-type to a missense mutant (R172H, R270H or R270C) configuration. To characterize the gradual molecular changes induced by expression of R172H and R270H mutants, we derived MEF lines harboring the alleles in a heterozygous conditional (p53-R172H fl/+ or p53-R270H fl/+) configuration. We observed that the wild-type p53 mRNA was lost but p53 protein abundance was increased after Cre-mediated activation of the mutant allele. Interestingly, we found that the R270H mutant allele provided a greater proliferative advantage, a distinct growth pattern and a greater ability to grow in vitro under starvation conditions than a similarly engineered R172H mutant allele. Consistent with published evidence, these observations suggest that the acquired functional changes are dissimilar between different p53 missense mutants. We further characterized the transcriptome profiles in MEF lines harboring R172H and R270H alleles and identified a number of unique and common transcriptional changes that could be causal for gain-of-function phenotypes and informative for uncovering additional mechanisms for carcinogenesis driven by p53 missense mutants. We used the newly-derived p53 mutant alleles to establish improved autochthonous and orthotopic pancreatic cancer models suitable for preclinical efficacy studies and we are currently developing lung cancer, ovarian cancer, and medullary thyroid carcinoma models. Citation Format: Tomas Vilimas, Keith Collins, Theresa Guerin, Roackie Awasthi, Lionel Feigenbaum, Guillermina Lozano, Terry Van Dyke, Serguei Kozlov. p53 missense mutants R172H and R270H exhibit differential effects on tumorigenesis. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr A17.