Ataxia-telangiectasia mutated ( Atm ) disruption sensitizes spatially-directed H3.3K27M/TP53 diffuse midline gliomas to radiation therapy

Diffuse midline gliomas (DMGs) are lethal brain tumors characterized by p53-inactivating mutations and oncohistone H3.3K27M mutations that rewire the cellular response to genotoxic stress, which presents therapeutic opportunities. We used RCAS/tv-a retroviruses and Cre recombinase to inactivate p53 and induce K27M in the native H3f3a allele in a lineage- and spatially-directed manner, yielding primary mouse DMGs. Genetic or pharmacologic disruption of the DNA damage response kinase Ataxia-telangiectasia mutated (ATM) enhanced the efficacy of focal brain irradiation, extending mouse survival. This finding suggests that targeting ATM will enhance the efficacy of radiation therapy for p53-mutant DMG but not p53-wildtype DMG. We used spatial in situ transcriptomics and an allelic series of primary murine DMG models with different p53 mutations to identify transactivation-independent p53 activity as a key mediator of such radiosensitivity. These studies deeply profile a genetically faithful and versatile model of a lethal brain tumor to identify resistance mechanisms for a therapeutic strategy currently in clinical trials.

Abstract 3403: Leveraging a p53 “super-tumor suppressor” to understand mechanisms of lung adenocarcinoma regression

Lung cancer is the leading cause of cancer deaths worldwide, and lung adenocarcinoma (LUAD) comprises the majority of lung cancer cases. Inactivation of the TP53 tumor suppressor gene, which encodes the p53 transcription factor, is observed in ~50% of human LUADs and is associated with poor prognosis and therapeutic responses. Hence, pharmacological restoration of p53 is an appealing strategy for LUAD treatment. Previous studies have demonstrated that p53 reactivation in mouse LUAD models leads to tumor regression. Here, we seek to understand the molecular mechanisms through which p53 restoration promotes tumor regression. To this end, we sought to better understand the transcriptional programs underlying tumor regression by leveraging an allelic series of transcriptional activation domain (TAD) mutants. We generated mouse models of LUAD initiated by oncogenic Kras and loss of p53, followed by reactivation of wild-type p53 or p53 TAD mutants, including the TAD1 (p5325,26), TAD2 (p5353,54), and TAD1/2 (p5325,26,53,54) mutants. We find that restoration of wild-type p53 induces significant regression of established tumors. Interestingly, we find that restoration of the p5353,54 mutant induces tumor regression even more potently than wild-type p53, reminiscent of our previous findings showing this mutant is a “super” tumor suppressor in pancreatic cancer. In contrast, the transcriptionally-dead TAD1/2 mutant, p5325,26,53,54, is unable to drive tumor regression. To understand the basis of tumor suppression, we are currently analyzing antiproliferative cellular responses in tumors, including cell cycle arrest, apoptosis, autophagy and differentiation. We have also developed a complementary in vitro system to further explore the molecular mechanisms of p53 action in LUAD regression. RNA-sequencing analysis in this system has revealed transcriptional programs underlying the enhanced tumor regression with the p5353,54 mutant. Collectively, these findings have illuminated the individual contributions of each p53 TAD to LUAD regression. In particular, elucidating the mechanisms by which the “super” tumor suppressor p5353,54 mutant induces tumor regression will provide great insight for developing therapeutic strategies to restore and amplify p53 function.

Citation Format: Mengxiong Wang, Alyssa Kaiser, Kathryn Bieging-Rolett, Colleen Brady, Laura Attardi. Leveraging a p53 “super-tumor suppressor” to understand mechanisms of lung adenocarcinoma regression [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3403.

Absence of p53 in a mouse mammary tumor model promotes tumor cell proliferation without affecting apoptosis

Loss or mutation of p53 may have multiple biological and genetic effects that result in accelerated tumor progression. Loss of p53 in some tumors has been correlated with a marked decrease in tumor cell apoptosis. p53 loss may also accelerate tumor growth through an increase in cell proliferation rates. To examine the effects of p53 loss on tumor progression in a controlled experimental context, we previously crossed p53-deficient mice to mammary tumor-susceptible Wnt-1 transgenic (TG) mice. The resulting female Wnt-1 TG offspring of this cross all developed mammary tumors, regardless of p53 status (p53+/+, p53+/-, or p53-/-). However, female p53-/- Wnt-1 TG mice developed tumors much sooner than their p53+/+ counterparts. In this report, we demonstrate that the average growth rates of tumors missing (p53-/-) or losing p53 (p53+/- with loss of heterozygosity) are accelerated compared to tumors with both wild-type p53 alleles (p53+/+). This accelerated growth rate appears to be due primarily to increases in rates of tumor cell proliferation. Tumor cell apoptotic levels were modest and were not measurably different in the presence or absence of wild-type p53. These results differ substantially from other mouse tumor models in which p53 loss was closely correlated with accelerated growth rates through attenuated apoptosis. Thus, the mechanisms by which p53 loss influences tumor progression may differ, depending on the tissue type and/or the oncogenic pathways involved.