All ETDs from UAB

Advisory Committee Chair

Alexa L Mattheyses

Advisory Committee Members

John M Parent

Louise T Chow

Bruce R Korf

Bradley K Yoder

Document Type

Dissertation

Date of Award

2023

Degree Name by School

Doctor of Philosophy (PhD) Heersink School of Medicine

Abstract

Tumor suppressor gene TP53 is the most frequently mutated gene across human cancers (~50%). Patients with Li-Fraumeni syndrome (LFS) who carry germline p53 mutations exhibit a diverse spectrum of childhood- and adult-onset malignancies. Despite over 40 years of dedicated studies to understand the role of p53 in tumor prevention, there are still many unanswered questions regarding the underlying mechanisms of p53. Previous studies have supported the notion that p53 exerts its tumor-suppressive function through its transcriptional activities. Therefore, strategies to enhance p53’s functions in tumor suppression via manipulating of downstream target gene activities in cancers show promising. To better investigate the p53 tumor-suppressive network, we first generated p53-/-, p63-/-, p73-/-, puma-/-, noxa-/- and analyzed their response to genotoxic, ER and ROS stresses. Our studies revealed that PUMA is a common module mediating apoptosis in these stress pathways. Therefore, the induction of PUMA expression may have promising therapeutic applications in p53-mutant human cancers. p53-mediated apoptosis (via p53 targets PUMA/BBC3 and PMAIP1/NOXA) and cell-cycle arrest (via p53 target CDKN1A/p21) are considered the primary mechanisms downstream of p53 tumor suppression. However, intriguingly, multiple animal models with deficiencies in p53-mediated apoptotic cell death and cell-cycle arrest, while still displaying intact p53 other downstream transcription, still maintain the suppression of tumor onset. These suggest that other non-canonical p53 targets and their effector functions iv may play a critical, and possibly even more significant, role in tumor suppression. To study this, we generated zebrafish triple knockouts of puma, noxa-/- and p21 and observed that these triple knockouts did not develop spontaneous tumors similar to the loss of p53. We next showed that p21 is not the only p53 downstream target in regulation of cell cycle. To identify the other p53 targets, we conducted a cross-species transcriptional analysis and defined 132 conserved p53-uprgualted transcripts. We proposed that they may contribute significantly to p53 tumor suppression. Among these conserved transcripts, our following experiments confirmed that ccng1, fbxw7 and foxo3b are important in p53-dependent cellcycle arrest. These findings highlight additional players in p53-mediated tumorsuppressive networks. Loss of heterozygosity (LOH) is a genetic event where an individual loses the remaining functional allele of a tumor suppressor gene. In the context of tumor suppression, LOH plays a critical role in cancer predisposition, particularly after the loss of one copy of tumor suppressor gene TP53. Cancers associated with LFS patients often lose the remaining wild-type p53 allele. Interestingly, recent comprehensive analysis of p53 mutant sporadic tumors have shown that inactivation of both alleles occurs in more than 90% of TCGA cancers, strongly suggesting that p53 LOH contributes to tumor initiation. The timing of tumor onset varies significantly among p53 heterozygous-null models, and the incidence and frequency of p53 LOH can impact the timing. Notably, ESCO2, which plays a crucial role in the establishment of sister chromatid cohesion during the cell cycle, has emerged as a promising modifier candidate in context of p53 LOH and timing of tumor initiation. Through studies in zebrafish and mouse p53 heterozygous nulls, we showed that Esco2 haploinsufficiency accelerates the timing of tumor onset. We also demonstrated v esco2 haploinsufficiency resulted in reduced sister chromatid cohesion (SCC) and elevated mitotic recombination(MR)-derived p53 LOH. We proposed that reduced SCC as a promising modifier, contributing to the accelerated tumor penetrance by elevating p53 LOH derived from mitotic recombination. These findings indicate the complex interplay between sister chromatid cohesion and p53 LOH events, providing valuable insights into the molecular mechanisms that govern tumor initiation and progression.

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