Wt P53 Function Research Articles

CSIG-28. DISSECTING THE FUNCTIONAL IMPAIRMENT OF WILD-TYPE P53 IN HUMAN GLIOBLASTOMA

Abstract Glioblastoma (GBM) is the most common malignant brain tumor in adults and is characterized by heterogeneous nature, invasive potential, and poor treatment response. Work from The Cancer Genome Atlas (TCGA) identified the tumor suppressor gene TP53 as a GBM driver. In contrast to several cancers, only ~35% of GBM cases carry mutations in TP53. Work from our lab showed that TP53 mutations emerge as ‘late’ events during GBM growth. Therefore, we hypothesized that wild-type (wt) p53 function is impaired in GBM. We used the TCGA data of 591 GBM patients to compare the clinical and molecular characteristics of wt and mutant TP53 patients. Gene expression data were analyzed to compare the correlation of TP53 with the expression of key p53 regulators and identify differentially expressed regulators between our groups. Furthermore, we evaluated the expression of p53 and selected regulators in patient-derived cells (PDCs) and control lines with known TP53 status. Our results show that wt TP53 patients live less than mutant patients and both groups respond similarly to treatment. No differences in age, sex, race, TP53 levels, and MGMT methylation status were observed between the groups. Gene expression analyses revealed that MAPK8, a p53 activator, negatively correlates with TP53 in the wt group. Moreover, ~21% of p53 regulators are differentially expressed between wt and mutant patients. Among these, we selected SIRT3, a p53 repressor, for experimental validation. We verified the p53 genetic status of PDCs through western blot and confirmed Sirt3 expression in wt and mutant TP53 PDCs. By CRISPR-Cas9-based genomic editing, we are currently evaluating the impact of SIRT3 knockout in GBM-PDC and patient-derived xenografts. We expect that SIRT3 knockout will restore p53 function in the wt group. In conclusion, our results suggest that p53 function is impaired in wt patients and provide insights into mechanisms affecting its activity.

A phase (Ph) 0/Ia study of brigimadlin concentration in brain tissue and a non-randomized, open-label, dose escalation study of brigimadlin in combination with radiotherapy (RT) in patients (pts) with newly diagnosed glioblastoma (GBM).

2017 Background: MDM2 inhibits tumor suppressor p53. Brigimadlin, a potent MDM2–p53 antagonist, restores wild-type (wt) p53 function and has shown early efficacy in pts with solid tumors (LoRusso et al Cancer Disc 2023). GBM is an area of unmet need with 5-year survival <10%. In p53 wt GBM pt-derived xenograft models, brigimadlin promotes tumor cell apoptosis and extends survival in combination with RT. Methods: NCT05376800 is a Ph 0/Ia open-label, single-arm trial that aims to measure brigimadlin concentration in brain tumor tissue in pts with histologically or radiologically newly diagnosed GBM eligible for resection (Ph 0) and determine the maximum tolerated dose of brigimadlin plus RT in pts with TP53wt, IDH wt, MGMT promoter unmethylated GBM (Ph Ia). In Ph 0, pts received one brigimadlin dose (30 mg or 45 mg) ~12–24 h before resection. Ph 0 primary endpoints are the measured total concentration and the calculated unbound concentration of brigimadlin in brain tissue homogenate from non-contrast enhancing (NCE) and contrast enhancing (CE) regions. The predefined threshold for trial continuation is 0.5 nmol/L (corresponding to IC50 in GBM cell lines) unbound brigimadlin in CE samples in ≥50% of pts. Brigimadlin concentration was measured using LC/MS and corrected for amount of brigimadlin in residual blood. Unbound concentration was calculated using an in vitro estimate of unbound fraction (fu): 0.654% (rat brain slice). fu in human plasma was 0.22%. Kp,uu (ratio of unbound concentration in brain vs plasma) was calculated. Biomarker testing was performed. Results: Data are available for 11 pts (brigimadlin 30 mg: n=6; 45 mg: n=5). In the 30 mg group, median total brigimadlin concentration in NCE samples was 267 nmol/L (4 samples, range 86–316 nmol/L) and 332 nmol/L (6 samples, range 272–952 nmol/L) in CE samples. In the 45 mg group, median total brigimadlin concentration in NCE samples was 197 nmol/L (5 samples, range 140–347 nmol/L) and 603 nmol/L (5 samples, range 441–905 nmol/L) in CE samples. Unbound concentration exceeded the 0.5 nmol/L threshold in most cases (Table). Post-brigimadlin, an increase in selected p53 target gene expression was observed in CE vs NCE tissue. Conclusions: Unbound brigimadlin concentrations in CE regions in all pts receiving the low dose of 30 mg brigimadlin exceeded the 0.5 nmol/L threshold. Kp,uu in most patients was close to 1 in CE regions. Biomarker data support target engagement in brain tissue. Our findings support continued investigation of brigimadlin in GBM. Recruitment is ongoing. Updated data will be presented. Clinical trial information: NCT05376800 . [Table: see text]

Complexity of the Genetic Background of Oncogenesis in Ovarian Cancer-Genetic Instability and Clinical Implications.

Ovarian cancer is a leading cause of death among women with gynecological cancers, and is often diagnosed at advanced stages, leading to poor outcomes. This review explores genetic aspects of high-grade serous, endometrioid, and clear-cell ovarian carcinomas, emphasizing personalized treatment approaches. Specific mutations such as TP53 in high-grade serous and BRAF/KRAS in low-grade serous carcinomas highlight the need for tailored therapies. Varying mutation prevalence across subtypes, including BRCA1/2, PTEN, PIK3CA, CTNNB1, and c-myc amplification, offers potential therapeutic targets. This review underscores TP53's pivotal role and advocates p53 immunohistochemical staining for mutational analysis. BRCA1/2 mutations' significance as genetic risk factors and their relevance in PARP inhibitor therapy are discussed, emphasizing the importance of genetic testing. This review also addresses the paradoxical better prognosis linked to KRAS and BRAF mutations in ovarian cancer. ARID1A, PIK3CA, and PTEN alterations in platinum resistance contribute to the genetic landscape. Therapeutic strategies, like restoring WT p53 function and exploring PI3K/AKT/mTOR inhibitors, are considered. The evolving understanding of genetic factors in ovarian carcinomas supports tailored therapeutic approaches based on individual tumor genetic profiles. Ongoing research shows promise for advancing personalized treatments and refining genetic testing in neoplastic diseases, including ovarian cancer. Clinical genetic screening tests can identify women at increased risk, guiding predictive cancer risk-reducing surgery.

Abstract A103: Targeting mutant p53-R248W reactivates WT p53 function and alters the onco-metabolic profile

Abstract The tumor suppressor p53 is mutated (mt-p53) in over 50% of human cancers causing gain-of-function oncogenic effects, including metabolic changes that reduce tumor responsiveness to radio/chemotherapy. Common hot-spot mutations within the DNA-binding domain can be categorized as conformational (R175H) or DNA binding (R248W). NSC59984 has been characterized as a small molecule that targets mt-p53 for degradation and restores wt-p53 signaling. Using esophageal adenocarcinoma cells and CRISPR generated isogenic cell lines bearing matching hot-spot p53 mutations, we aim to understand how the molecular features of mt-p53 affect drug efficiency and enable the development of targeted therapies to limit cancer cell growth. We found that NSC59984 covalently modifies p53 by Michael addition at cysteine residues 124 and 229, which promote interactions that would stabilize the protein/DNA complex leading to increased p53 transcriptional activity. In cells, the effects of NSC59984 were substantially greater in cells harboring the R248W mutation compared with the R175H mutation. Treatment with NSC59984 reduced proliferation and increased apoptosis via the intrinsic mitochondrial pathway. It also induced changes in OXPHOS, ATP level, mitochondrial membrane potential, glycolysis, and lactate production. Furthermore, treatment of cells with NSC59984 increased reactive oxygen species production and decreased glutathione levels; effects were enhanced by the addition of buthionine sulfoximine and inhibited by N-acetyl cysteine. NSC59984 treatment increased G6PD activity, total NADPH levels, and expression of TIGAR. Knockout of TIGAR partially removed the antiproliferative effects of the drug and reduced G6PD levels in the p53-R248W cells. Incorporation of [13C6] into cellular metabolites suggests that p53-regulated transcription of TIGAR increased utilization of the pentose phosphate pathway and inhibited glycolysis at the fructose-6-P fructose-1,6-bisphosphate junction, supported by an increase in Hexokinase 2 and a decrease of phosphofructokinase-1. Thermal proteome profiling identified TIGAR as an additional reaction target of NSC59984, suggesting increased involvement in modulating these metabolic effects. Combining currently available therapeutic metabolic inhibitors with NSC59984 enhanced the antiproliferative effects in cells harboring p53-R248W creating a therapeutic window when compared to the wt-p53 expressing cells. This suggests these combinations could be used in a clinically relevant setting. Overall, this work has identified a distinctive mode of action for p53 reactivation resulting in not only transcriptional activity, but also a unique effect on cellular energetics. This study shows evidence of variation in responsiveness of different mt-p53 forms and allows the development of specific therapeutics directed to individuals for patient-centered precision medicine. Importantly, we have shown that targeting p53 signaling has significant effects on the metabolic profiles of cancer cells rendering them more vulnerable to neoadjuvant therapy. Citation Format: Kate Brown, Lisa Jenkins, Dan Crooks, Deborah Surman, Sharlyn Mazur, Yuan Xu, Bhargav Arimilli, Ye Yang, Andrew Lane, Stewart Durell, Teresa Fan, David Schrump, Marston Marston, Taylor Ripley, Ettore Appella, Gaelyn Lyons, Andrew Perciaccante, Jerry Dinan, Marco Robello, Herman Nikolayevskiy, Robert O’Connor, Daniel Appella. Targeting mutant p53-R248W reactivates WT p53 function and alters the onco-metabolic profile [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr A103.

Predicted Functional Consequences of TP53 Alterations and Prognosis in TP53-Mutated Mantle Cell Lymphoma

Background: Mantle cell lymphoma (MCL) cases with TP53 mutations ( TP53mut) are associated with poor responses to chemoimmunotherapy (Eskelund Blood 2017). Across myeloid and solid tumors, predicted functional consequences and pathogenicity of specific TP53 mutations have been linked to differential patient outcomes (Pollock Breast Cancer Res Treat 2022, Yabe Cancers 2021). We aimed to characterize the impact of these features and associated molecular alterations on treatment outcome in TP53mut MCL patients treated with standard frontline regimens and on the ongoing phase 2 BOVEN trial. The BOVEN trial aims to assess the efficacy of frontline treatment with zanubrutinib, obinutuzumab, and venetoclax in TP53mut MCL (Kumar Blood 2021). Methods: The BOVEN trial enrolled 25 patients with TP53mut MCL. Retrospective chart review was performed for 22 patients with TP53mut MCL treated at frontline with standard chemoimmunotherapy regimens (Joffe Blood 2019). Tumor histology and FISH were obtained for each case. Mutational profiling (SNV/indels, CNA) was obtained prior to therapy with various CLIA-certified targeted sequencing platforms (Cheng J Mol Diagn 2015, Dias-Santagata EMBO Mol Med 2010). Mutations in TP53 were annotated using predicted phenotypes and pathogenicity from the TP53 Database (de Andrade Cell Death Differ 2022) and from phenotypic annotation of TP53 mutations (Giacomelli Nat Genet 2018). The primary outcome measures were overall and progression-free survival (OS/PFS). Results: Each patient included in the BOVEN cohort featured variants in TP53 with 26 SNV/indels across all 25 patients. There were 25 SNV/indels across all 22 patients treated with standard regimens. Within the trial cohort 14/25 cases had evidence of biallelic inactivation of TP53 with co-occurring SNV/indels in the gene and 17p loss by FISH or TP53 copy losses by SNParray; the comparator cohort featured 11/22 cases with biallelic losses. Across cohorts, most mutations were missense (>80% in both), with the majority occurring in the DNA binding domain of the p53 protein (76% and 91%, respectively), and with >30% at known hotspot codons in both cohorts. Most mutations (>70% in both cohorts) were predicted to adversely impact WT p53 function, including nonfunctional transactivation activity and dominant negative effect on the WT allele. The majority were also predicted to be highly pathogenic by bioinformatic methods (AGVGD, SIFT, PolyPhen2, PHANTM, REVEL, BayesDel). However, none of these features correlated with outcome. In the BOVEN cohort the variant allele frequency (VAF) of the TP53 SNV/indels approached significant association with poor PFS (HR 33.2, p=0.05) but not OS (HR 28.6, p=0.09). In contrast, in the comparator cohort TP53 VAF appeared to have an opposite effect direction, though this was not significant for PFS (HR 0.98, p=0.2) or OS (HR 0.97, p=0.06). We next investigated the impact of co-occurring molecular alterations on outcome. Besides TP53, the most frequently mutated genes in the trial cohort were KMT2D (24%), ATM (16%), NOTCH2 (12%), and CCND1 (12%). Alterations in these genes did not correlate with outcome. Within the comparator cohort the recurrently mutated genes were SMARCA4 (23%), ARID1A (18%), ARID1B (14%), ATM (14%), and CCND1 (14%). As described previously in this cohort (Joffe Blood 2019), alterations in SMARCA4 were associated with worse PFS (HR 4.2, p=0.02) but not OS (HR 2.2, p=0.27). Mutational burden (SNV and CNA load) was not prognostic in either 12/25 BOVEN samples sequenced with the same panel or the comparator cohort. There was no significant difference in outcome between patients with single mutations in TP53 vs. those with biallelic losses in TP53 in either cohort. Conclusion: Across patients with TP53mut MCL treated either with standard frontline chemoimmunotherapy or on the BOVEN trial, predictions of intrinsic pathogenicity and functional consequences of the TP53 alterations were not prognostic. The impact of TP53 mutation VAF remains unclear, with seemingly opposite effect directions between the two cohorts, though neither finding was significant. As previously described in the comparator cohort, alterations in SMARCA4 are associated with worse PFS in TP53mut MCL, though this was not observed in the BOVEN cohort. There were no significant differences in outcome among patients with biallelic inactivations in TP53 or with higher burdens of somatic alterations.

Epigenetic upregulation of spleen tyrosine kinase in cancer cells through p53-dependent downregulation of DNA methyltransferase

Syk is a tumor suppressor gene in some solid tumors. Currently, it remains unknown how Syk gene hypermethylation is controlled by DNA methyltransferase (DNMT) and p53. In colorectal cancer HCT116 cells, we found that protein and mRNA levels of Syk were much higher in WT than in p53−/− cells. Both p53 inhibitor PFT-α and p53 silencing can reduce the protein and mRNA expression of Syk in WT cells, while DNMT inhibitor 5-Aza-2′-dC can increase Syk expression in p53−/− cells. Interestingly, the DNMT expression in p53−/− HCT116 cells was higher than that in WT cells. PFT-α can not only enhance Syk gene methylation but also increase DNMT1 protein and mRNA levels in WT HCT116 cells. In metastatic lung cancer cell lines A549 and PC9, which express WT p53 and gain function of p53, respectively, PFT-α can also downregulate Syk mRNA and protein expression. However, the Syk methylation level was increased by PFT-α in A549 but not in PC9 cells. Likewise, 5-Aza-2′-dC transcriptionally increased Syk gene expression in A549 cells, but not in PC9 cells. In summary methylation of Syk promoter requires DNMT1, and p53 can upregulate Syk expression via downregulation of DNMT1 at the transcriptional level.

Targeting mutant p53-R248W reactivates WT p53 function and alters the onco-metabolic profile.

TP53 is the most commonly mutated gene in cancer, and gain-of-function mutations have wide-ranging effects. Efforts to reactivate wild-type p53 function and inhibit mutant functions have been complicated by the variety of TP53 mutations. Identified from a screen, the NSC59984 compound has been shown to restore activity to mutant p53 in colorectal cancer cells. Here, we investigated its effects on esophageal adenocarcinoma cells with specific p53 hot-spot mutations. NSC59984 treatment of cells reactivated p53 transcriptional regulation, inducing mitochondrial intrinsic apoptosis. Analysis of its effects on cellular metabolism demonstrated increased utilization of the pentose phosphate pathway and inhibition of glycolysis at the fructose-1,6-bisphosphate to fructose 6-phosphate junction. Furthermore, treatment of cells with NSC59984 increased reactive oxygen species production and decreased glutathione levels; these effects were enhanced by the addition of buthionine sulfoximine and inhibited by N-acetyl cysteine. We found that the effects of NSC59984 were substantially greater in cells harboring the p53 R248W mutation. Overall, these findings demonstrate p53-dependent effects of NSC59984 on cellular metabolism, with increased activity in cells harboring the p53 R248W mutation. This research highlights the importance of defining the mutational status of a particular cancer to create a patient-centric strategy for the treatment of p53-driven cancers.

GOF Mutant p53 in Cancers: A Therapeutic Challenge.

Simple SummaryIn normal cells, p53 is a protein which regulates the cell cycle progression to ensure normal cell division, growth, and development. However, in cancer, changes in the p53 DNA sequence, called genetic mutation, results in the protein either losing its normal function or exhibiting advanced pro-tumorigenic functions that lead to cancer. Importantly, cancers with mutations in the p53 protein often represent ones which are more aggressive and more resistant to chemotherapy. As a result, many studies have and continue to investigate multiple ways to target mutant p53-bearing cancer using targeted therapy, gene therapy, immunotherapy, and combination therapies. Knowledge of these strategies is important in improving the overall therapeutic response of cancers with mutant p53. This review highlights new strategies and discusses the progression of such therapies.TP53 is mutated in the majority of human cancers. Mutations can lead to loss of p53 expression or expression of mutant versions of the p53 protein. These mutant p53 proteins have oncogenic potential. They can inhibit any remaining WTp53 in a dominant negative manner, or they can acquire new functions that promote tumour growth, invasion, metastasis and chemoresistance. In this review we explore some of the mechanisms that make mutant p53 cells resistant to chemotherapy. As mutant p53 tumours are resistant to many traditional chemotherapies, many have sought to explore new ways of targeting mutant p53 tumours and reinstate chemosensitivity. These approaches include targeting of mutant p53 stability, mutant p53 binding partners and downstream pathways, p53 vaccines, restoration of WTp53 function, and WTp53 gene delivery. The current advances and challenges of these strategies are discussed.

Abstract 1035: VCP/p97 promotes pancreatic cancer growth by enhancing mutant p53 activity

Abstract Pancreatic cancer is the most lethal malignant neoplasms across the world with the lowest overall five-year survival rate (9%). TP53 is mutated in ~70% of pancreatic ductal adenocarcinomas (PDACs). Hotspot p53 mutants promote cancer cell proliferation, metastasis and metabolism through their gain-of-function (GOF). Among the p53 “hotspot” mutations, R273 is the most frequent mutation position (~12%) with R273H as the most commonly mutated codon (~8%) in PDACs. However, the mechanisms underlying regulation of mutant p53s’ GOF in PDACs remain incompletely understood. In our attempt to address this question, we recently identified the ATPase valosin-containing protein (VCP)/p97 as a novel mutant p53-binding protein by performing immunoprecipitation (IP)-tandem mass spectrometry (LC-MS/MS) analysis. VCP bound to p53-R273H via the DNA-binding domain. VCP inhibition either by genetic depletion or pharmacological inhibition by CB-5083 resulted in the increase of MDM2-mediated ubiquitination and degradation of p53-R273H. VCP did so by binding to MDM2 and disturbing its interaction with mutant p53. As a result, VCP protected mutant p53 from degradation and enhanced its GOF in cancer development. Ablation of VCP retarded PDAC cell growth in vitro and in vivo. Our further studies also unveiled the negative regulation of wt p53 by VCP in cancer cells. Together, these results demonstrate that VCP can strengthen mutant p53’s oncogenic function by stabilizing it and negating wt p53 function, suggesting VCP as a resistant factor for chemotherapy against PDACs and thus a potential therapeutic target of the mutant p53-harboring pancreatic cancers as well as of the wt p53-containing cancer cells. Citation Format: Jieqiong Wang. VCP/p97 promotes pancreatic cancer growth by enhancing mutant p53 activity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1035.

EGFR suppresses p53 function by promoting p53 binding to DNA-PKcs: a noncanonical regulatory axis between EGFR and wild-type p53 in glioblastoma.

BackgroundEpidermal growth factor receptor (EGFR) amplification and TP53 mutation are the two most common genetic alterations in glioblastoma multiforme (GBM). A comprehensive analysis of the TCGA GBM database revealed a subgroup with near mutual exclusivity of EGFR amplification and TP53 mutations indicative of a role of EGFR in regulating wild-type-p53 (wt-p53) function. The relationship between EGFR amplification and wt-p53 function remains undefined and this study describes the biological significance of this interaction in GBM.MethodsMass spectrometry was used to identify EGFR-dependent p53-interacting proteins. The p53 and DNA-dependent protein kinase catalytic subunit (DNA-PKcs) interaction was detected by co-immunoprecipitation. We used CRISPR-Cas9 gene editing to knockout EGFR and DNA-PKcs and the Edit-R CRIPSR-Cas9 system for conditional knockout of EGFR. ROS activity was measured with a CM-H2DCFDA probe, and real-time PCR was used to quantify expression of p53 target genes.ResultsUsing glioma sphere-forming cells (GSCs), we identified, DNA-PKcs as a p53 interacting protein that functionally inhibits p53 activity. We demonstrate that EGFR knockdown increased wt-p53 transcriptional activity, which was associated with decreased binding between p53 and DNA-PKcs. We further show that inhibition of DNA-PKcs either by siRNA or an inhibitor (nedisertib) increased wt-p53 transcriptional activity, which was not enhanced further by EGFR knockdown, indicating that EGFR suppressed wt-p53 activity through DNA-PKcs binding with p53. Finally, using conditional EGFR-knockout GSCs, we show that depleting EGFR increased animal survival in mice transplanted with wt-p53 GSCs.ConclusionThis study demonstrates that EGFR signaling inhibits wt-p53 function in GBM by promoting an interaction between p53 and DNA-PKcs.

Abstract P5-09-02: Tetrameric and monomeric gain-of-function mutant p53 interacts with chromatin

Abstract In 80% of triple-negative breast cancers (TNBC) the TP53 gene is mutated, with a high occurrence of missense changed in the DNA binding domain. These mutations lead to loss of wild-type p53 (wtp53) function and some also create mutant p53 (mtp53) proteins with novel gain-of-function (GOF) tumor-promoting capabilities. We previously reported that mtp53 R273H interacts with replication-associated proteins including MCM2-7 and poly ADP-ribose polymerase (PARP1). Moreover, mtp53 is found on actively replicating nascent DNA. What remains unclear is which domains of mtp53 R273H are responsible for the protein’s chromatin-based replication activities. In this study, we tested whether the oligomerization state of mtp53 plays a role in oncogenic GOF activities. We destabilized tetramer formation by using site-directed mutagenesis to introduce oligomerization domain (OD) point mutations A347D, R337C, or L344P. The site-directed mutagenesis constructs were transiently transfected into p53-/- mammalian cell lines to evaluate the altered functions. The oligomerization state of mtp53 was assessed via glutaraldehyde crosslinking and western blot analyses. Western blot and qPCR analyses were used to test how OD mutations in plasmids expressing either wtp53 or R273H mtp53 regulated the targets p21, CDC7 and RRM2. We used chromatin fractionation and western blot analyses to compare the chromatin association of the R273H mtp53 (with variable OD mutants), PARP1, and MCM2 to assess how destabilizing tetramer formation influenced chromatin interactions. Compared to the tetrameric versions of wildtype (wt) and mutant (mt) p53, the wtp53-A347D and R273H-A347D were predominantly dimers, while wtp53-R337C, wtp53-L344P, mtp53 R273H-R337C, and mtp53R273H-L344P were predominantly monomers. When wtp53 tetramerization was blocked so was its ability to activate the cyclin-dependent kinase p21/waf. Interestingly, all oligomerization forms of mtp53 localized to the chromatin suggesting GOF does not require tetramer formation for all functions. Strikingly, all oligomerization mutants of R273H mtp53, compared to the tetrameric mtp53, had a stronger interaction with MCM2. When we examined the mtp53 GOF transcriptional target we found that only tetrameric mtp53 R273H activated the target gene RRM2. Our findings suggest that destabilizing tetramer formation reduced certain mtp53 R273H GOF oncogenic activities while activating others. These changes of functions require examination in the context of triple-negative breast cancers. Citation Format: George Kwakye Annor. Tetrameric and monomeric gain-of-function mutant p53 interacts with chromatin [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P5-09-02.

Novel CDK9 inhibitor oroxylin A promotes wild-type P53 stability and prevents hepatocellular carcinoma progression by disrupting both MDM2 and SIRT1 signaling.

Hepatocellular carcinoma (HCC) is one of the most lethal tumours worldwide. However, the effects of first-line sorafenib treatment in advanced HCC fail to prolong patients' survival due to the highly heterogeneous characteristics of HCC etiology. Cyclin-dependent kinase 9 (CDK9) is an important target in the continuous development of cancer therapy. Here, we demonstrate that CDK9 is closely associated with the progression of HCC and can serve as an HCC therapeutic target by modulating the recovery of wild-type p53 (wt-p53) function. We prove that mouse double minute 2 homologue (MDM2) and Sirtuin 1 (SIRT1) are phosphorylated by CDK9 at Ser166 and Ser47, respectively. Inhibition of CDK9 not only reduces the MDM2-mediated ubiquitination and degradation of wt-p53 but also increases wt-p53 stability by suppressing deacetylase activity of SIRT1. Thus, inhibition of CDK9 promotes the wt-p53 stabilization and prevents HCC progression. However, excessive inhibition by high concentrations of specific CDK9 inhibitors counteracts the promotion of p53 stability and reduces their anti-HCC activity because of extreme general transcription repression. The effects of a novel CDK9 inhibitor named oroxylin A (OA) from Scutellaria baicalensis are explored, with the results indicating that OA shows moderate and controlled inhibition of CDK9 activity and expression, and stabilizes wt-p53 by inhibiting CDK9-regulated MDM2 and SIRT1 signaling. These outcomes indicate the high therapeutic potential of OA against HCC and its low toxicity in normal tissue. This study demonstrates a novel mechanism for the regulation of wt-p53 by CDK9 and indicates that OA is a potential candidate for HCC therapy.

The Naturally Occurring ∆40p53 Isoform Inhibits eRNA Transcription and Enables Context-Specific Regulation During p53 Activation

The Δ40p53 isoform heterotetramerizes with WTp53 to regulate development, aging, and stress responses. How Δ40p53 alters WTp53 function remains enigmatic because their co-expression causes tetramer heterogeneity. We circumvented this issue with knock-in cell lines that generated homogeneous Δ40p53:WTp53 tetramers from the native TP53 locus, and examined with transcriptomics (PRO-seq, RNA-seq), metabolomics, and other methods. Although phenotypically similar to WTp53 under normal conditions, Δ40p53:WTp53 failed to induce growth arrest upon Nutlin-induced p53 activation. This occurred via Δ40p53:WTp53-dependent inhibition of eRNA transcription and subsequent failure to induce mRNA, despite similar genomic occupancy to WTp53. A different stimulus (5-fluorouracil) also showed Δ40p53:WTp53 defects in mRNA induction; however, other stimulus-specific TFs could then drive the response, yielding similar outcomes vs. WTp53. Our results define how Δ40p53 alters WTp53 function and identify an eRNA requirement for mRNA biogenesis. Moreover, Δ40p53:WTp53 functional distinctions uncovered herein suggest human p53 evolved as a tetramer to support eRNA transcription.

The pharmalogical reactivation of p53 function improves breast tumor cell lysis by granzyme B and NK cells through induction of autophagy

Cytotoxic T lymphocytes (CTL) and natural killer cells (NK)-mediated elimination of tumor cells is mostly dependent on Granzyme B apoptotic pathway, which is regulated by the wild type (wt) p53 protein. Because TP53 inactivating mutations, frequently found in human tumors, could interfere with Granzyme B-mediated cell death, the use of small molecules developed to reactivate wtp53 function in p53-mutated tumor cells could optimize their lysis by CTL or NK cells. Here, we show that the pharmalogical reactivation of a wt-like p53 function in p53-mutated breast cancer cells using the small molecule CP-31398 increases their sensitivity to NK-mediated lysis. This potentiation is dependent on p53-mediated induction of autophagy via the sestrin-AMPK-mTOR pathway and the ULK axis. This CP31398-induced autophagy sequestrates in autophagosomes several anti-apoptotic proteins, including Bcl-XL and XIAP, facilitating Granzyme B-mediated mitochondrial outer membrane permeabilization, caspase-3 activation and Granzyme B- or NK cell-induced apoptosis. Together, our results define a new way to increase cytotoxic lymphocyte-mediated lysis of p53-mutated breast cancer cell, through a p53-dependent autophagy induction, with potential applications in combined immunotherapeutic approaches.

Abstract LB-093: Refolding & restoring function to mutant p53: HDAC inhibitor MS275 partially restores activity to R175H mutant p53

Abstract Introduction: Most p53 mutations occur in the DNA binding domain, resulting not only in a loss of WT p53 function but also in the gain of additional onocogenic properties such as increased cellular proliferation and metastasis. Two newly developed pharmaceuticals, NSC 319726 and SCH 529074, refold mutant p53 to a WT conformation, reduce cell proliferation, decrease aggregate formation, and represent a novel therapeutic approach. Another approach to regulating p53 activity is through acetylation. This post-translational modification leads to an activating conformational change in WT p53, but it is unclear how acetylation affects mutant p53. Two histone deacetylase inhibitors (HDACi), MS275 (Entinostat) and FK228 (Romidepsin), are promising chemotherapeutics that increase acetylation of many proteins. In this study, we ask if treatment with these HDACi leads to refolding of mutant p53, reduction of proliferation, and p53 transcription factor activity. Methods: R175H, H193Y, R248Q, and R273H constructs were transfected into a p53-null cell line. Inducible p53 expression in stable subclones was quantitated by immunofluorescence, western blot, and ELISA. Proliferation was measured by incorporation of EdU after treatment with FK228, MS275, NSC 319726, and SCH 529074. Refolding of p53 was determined before and after treatment using the conformational antibodies pAb240 and pAb1620, which bind to unfolded and folded p53, respectively. Functionality of the R175H mutant p53 was assessed by measuring the ability of p53 to bind its response element. Results: We focused on the R175H conformational p53 mutation as had a higher rate of cellular proliferation measured by EdU incorporation compared to the other mutants (p< 0.05). As expected, NSC 319726 treatment reduces R175H proliferation (p< 0.0001) and promotes refolding into the WT p53 conformation (p< 0.05) while no difference was detected with the broad spectrum mutant p53 reactivator SCH 529074. The HDAC inhibitors MS275 and FK228 reduce R175H proliferation (p< 0.0001) and MS275 appears to promote refolding into the WT p53 conformation while FK228 does not. When compared to control, transcription factor activity increased 3.8-fold with NSC 319726 treatment (p < 0.05), 6.0-fold with MS275 treatment (p< 0.0001), and 4.6-fold with FK228 treatment (p<0.01). Results suggest that HDACi may activate mutant p53 and that MS275 may do so by restoring WT conformation. Conclusion: The HDAC inhibitor MS275 reactivates R175H mutant p53 as measured by increased transcription factor activity apparently through refolding, as determined by pAb1620 binding following treatment. These results imply a novel mode of action for HDACi. Clinically this suggests that MS275 may be able to restore WT p53 tumor suppressor activity. Citation Format: Margaux Baatz, Brianna Flores, Jacob Moore, Kathryn J. Leyva, Elizabeth E. Hull. Refolding & restoring function to mutant p53: HDAC inhibitor MS275 partially restores activity to R175H mutant p53 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr LB-093.

Emerging Non-Canonical Functions and Regulation by p53: p53 and Stemness.

Since its discovery nearly 40 years ago, p53 has ascended to the forefront of investigated genes and proteins across diverse research disciplines and is recognized most exclusively for its role in cancer as a tumor suppressor. Levine and Oren (2009) reviewed the evolution of p53 detailing the significant discoveries of each decade since its first report in 1979. In this review, we will highlight the emerging non-canonical functions and regulation of p53 in stem cells. We will focus on general themes shared among p53’s functions in non-malignant stem cells and cancer stem-like cells (CSCs) and the influence of p53 on the microenvironment and CSC niche. We will also examine p53 gain of function (GOF) roles in stemness. Mutant p53 (mutp53) GOFs that lead to survival, drug resistance and colonization are reviewed in the context of the acquisition of advantageous transformation processes, such as differentiation and dedifferentiation, epithelial-to-mesenchymal transition (EMT) and stem cell senescence and quiescence. Finally, we will conclude with therapeutic strategies that restore wild-type p53 (wtp53) function in cancer and CSCs, including RING finger E3 ligases and CSC maintenance. The mechanisms by which wtp53 and mutp53 influence stemness in non-malignant stem cells and CSCs or tumor-initiating cells (TICs) are poorly understood thus far. Further elucidation of p53’s effects on stemness could lead to novel therapeutic strategies in cancer research.

Sensitivity to PRIMA-1MET is associated with decreased MGMT in human glioblastoma cells and glioblastoma stem cells irrespective of p53 status

Alterations of the TP53 tumor suppressor gene occur in ~30% of primary glioblastoma (GBM) with a high frequency of missense mutations associated with the acquisition of oncogenic “gain-of-function” (GOF) mutant (mut)p53 activities. PRIMA-1MET/APR-246, emerged as a promising compound to rescue wild-type (wt)p53 function in different cancer types. Previous studies suggested the role of wtp53 in the negative regulation of the DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT), a major determinant in resistance to therapy in GBM treatment. The potential role of MGMT in expression of p53 and the efficacy of PRIMA-1MET with respect to TP53 status and expression of MGMT in GBM remain unknown. We investigated response to PRIMA-1MET of wtp53/MGMT-negative (U87MG, A172), mutp53/MGMT-positive U138, LN-18, T98/Empty vector (T98/EV) and its isogenic MGMT/shRNA gene knockdown counterpart (T98/shRNA). We show that MGMT silencing decreased expression of mutp53/GOF in T98/shRNA. PRIMA-1MET further cleared T98/shRNA cells of mutp53, decreased proliferation and clonogenic potential, abrogated the G2 checkpoint control, increased susceptibility to apoptotic cell death, expression of GADD45A and sustained expression of phosphorylated Erk1/2. PRIMA-1MET increased expression of p21 protein in U87MG and A172 and promoted senescence in U87MG cell line. Importantly, PRIMA-1MET decreased relative cell numbers, disrupted the structure of neurospheres of patient-derived GBM stem cells (GSCs) and enabled activation of wtp53 with decreased expression of MGMT in MGMT-positive GSCs or decreased expression of mutp53. Our findings highlight the cell-context dependent effects of PRIMA-1MET irrespective of p53 status and suggest the role of MGMT as a potential molecular target of PRIMA-1MET in MGMT-positive GSCs.

Abstract 2157: Enhanced in vitro activity of dianhydrogalactitol (VAL-083) in combination with platinum drugs: Impact of p53 and platinum-resistance

Abstract Cisplatin is an important frontline drug for ovarian carcinoma and non-small cell lung cancer (NSCLC). However, the initial response rates of up to 70% are usually followed by relapse due to the onset of drug resistance. Mechanistically, platinum resistance is multifactorial, with loss of p53 function (by mutation or inhibitory protein binding to wild-type (wt) p53) playing a central role. The appearance of drug resistance is a major clinical barrier and, therefore, new agents are needed to overcome this limitation. Dianhydrogalactitol (VAL-083) is a bi-functional alkylating agent that induces DNA interstrand cross-links at N7-guanine, a mechanism that is distinct from the intrastrand cross-links by platinum-based drugs. VAL-083 is clinically approved in China for lung cancer, and is undergoing clinical trial in the US for glioma. In the present study, we have examined the in vitro cytotoxicity of VAL-083 as a single agent and in combination with cisplatin or oxaliplatin using the 5-day MTT assay. In the isogenic HCT-116p53+/+ and HCT-116p53-/- colorectal models, loss of p53 increased IC50 of (or resistance to) cisplatin and oxaliplatin by 3- to 6-fold, whereas resistance to VAL-083 was increased only 1.7-fold by loss of p53. These results indicate that the cytotoxicity of VAL-083 is less impacted than the platinum drugs by loss of p53. When tested in cisplatin-sensitive (A2780) vs. cisplatin-resistant wt p53 ovarian tumor models (2780CP-16, OVCAR-10, Hey and OVCA-433) there was a 10- to 40-fold increase in IC50 for cisplatin, while the corresponding increase in IC50 for VAL-083 was only 4- to 7-fold. This indicates only partially cross-resistance between VAL-083 and cisplatin and thus suggests a distinct mode of action for VAL-083 as compared to cisplatin. To further investigate, immunoblots were developed after a 24-h exposure of isogenic A2780 or 2780CP-16 models to cisplatin or VAL-083. The two drugs were equally effective at stabilizing and activating p53 in A2780 cells. However, in cisplatin-resistant 2780CP-16 cells, VAL-083 was more effective than cisplatin at increasing p53 and p21 levels, and at inducing Ser-15 and Ser-20 phosphorylation of p53. This is consistent with the ability of VAL-083 to circumvent cisplatin resistance and demonstrated that this alkylating agent also has the capacity to partially restore wt p53 function in ovarian tumor cells. The independent mode of actions of these drugs suggested the potential for combining VAL-083 with cisplatin or oxaliplatin. These combinations in wt (H460 and A549) and mutant (H1975 and H157) p53 NSCLC models demonstrated significant super-additivity (p<0.05) and/or synergy (CI < 1). Taken together, these results demonstrate the antitumor activity of VAL-083 against both wt and mutant p53 cancers and raise the clinical potential for treatment in a combination setting with platinum drugs. Citation Format: Anne Steino, Guanghan He, Michelle Martinez-Rivera, Jeffrey A. Bacha, Dennis M. Brown, Zahid H. Siddik. Enhanced in vitro activity of dianhydrogalactitol (VAL-083) in combination with platinum drugs: Impact of p53 and platinum-resistance. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2157.

Nitric Oxide Down-Regulates Topoisomerase I and Induces Camptothecin Resistance in Human Breast MCF-7 Tumor Cells.

Camptothecin (CPT), a topoisomerase I poison, is an important drug for the treatment of solid tumors in the clinic. Nitric oxide (·NO), a physiological signaling molecule, is involved in many cellular functions, including cell proliferation, survival and death. We have previously shown that ·NO plays a significant role in the detoxification of etoposide (VP-16), a topoisomerase II poison in vitro and in human melanoma cells. ·NO/·NO-derived species are reported to modulate activity of several important cellular proteins. As topoisomerases contain a number of free sulfhydryl groups which may be targets of ·NO/·NO-derived species, we have investigated the roles of ·NO/·NO-derived species in the stability and activity of topo I. Here we show that ·NO/·NO-derived species induces a significant down-regulation of topoisomerase I protein via the ubiquitin/26S proteasome pathway in human colon (HT-29) and breast (MCF-7) cancer cell lines. Importantly, ·NO treatment induced a significant resistance to CPT only in MCF-7 cells. This resistance to CPT did not result from loss of topoisomerase I activity as there were no differences in topoisomerase I-induced DNA cleavage in vitro or in tumor cells, but resulted from the stabilization/induction of bcl2 protein. This up-regulation of bcl2 protein in MCF-7 cells was wtp53 dependent as pifithrine-α, a small molecule inhibitor of wtp53 function, completely reversed CPT resistance, suggesting that wtp53 and bcl2 proteins played important roles in CPT resistance. Because tumors in vivo are heterogeneous and contaminated by infiltrating macrophages, ·NO-induced down-regulation of topoisomerase I protein combined with bcl2 protein stabilization could render certain tumors highly resistant to CPT and drugs derived from it in the clinic.

Trans-splicing repair of mutant p53 suppresses the growth of hepatocellular carcinoma cells in vitro and in vivo.

Reactivation of wild-type p53 (wt-p53) function is an attractive therapeutic approach to p53-defective cancers. An ideal p53-based gene therapy should restore wt-p53 production and reduces mutant p53 transcripts simultaneously. In this study, we described an alternative strategy named as trans-splicing that repaired mutant p53 transcripts in hepatocellular carcinoma (HCC) cells. The plasmids which encoded a pre-trans-splicing molecule (PTM) targeting intron 6 of p53 were constructed and then transfected into HCC cells carrying p53 mutation. Phenotypic changes of HCC cells induced by p53-PTM were analyzed through cell cycle, cell apoptosis and the expression of p53 downstream target genes. Spliceosome mediated RNA trans-splicing (SMaRT) reduced mutant p53 transcripts and produced functional wt-p53 protein after the delivery of p53-PTM plasmids, which resulted in phenotype correction of HCC cells. In tumor xenografts established by p53-mutated HCC cells, adenovirus encoding p53-PTM induced cell cycle arrest and apoptosis and then blocked the growth of tumors in mice. Collectively, our results demonstrated for the first time that mutant p53 transcripts were functionally corrected in p53-defective HCC cells and xenografts using trans-splicing, which indicated the feasibility of using trans-splicing to repair p53 mutation in p53-defective cancers.