Mutant P53 Protein Research Articles

Abstract 1295: Small molecule reactivators of Y220C mutant p53 modulate tumor infiltrating leukocytes and synergize with immune checkpoint inhibitors

Abstract Half of human cancers possess mutations in the TP53 gene, with most clustering as hotspots in the DNA binding domain. A tyrosine to cysteine substitution at amino acid 220 of the p53 protein (Y220C) is one such hotspot mutation (~1% of all solid tumors). Small molecules that reactivate Y220C mutant p53 activity to be like wild type have been developed at PMV Pharmaceuticals and the lead compound PC14586 is in Phase I clinical development. The molecules were designed to bind tightly to a crevice within the Y220C p53 mutant protein and stabilize it to a wild type conformation allowing reactivation of p53 transcriptional activity and expression of target proteins (e.g. p21, MDM2), resulting in tumor regression in Y220C p53 mutant xenograft models grown in immunocompromised mice. p53 has been documented to contribute to immune responses by activating regulators of immune signaling pathways and to result in T-cell exclusion in tumor models. To investigate the role of p53 and the immune system in tumorigenesis, a Y220C human p53 knock-in (HUPKI) mouse was generated. Mice that are homozygous mutant for the Y220C humanized alleles succumbed to lymphomas and sarcomas within 6 months and cell lines generated from these tumors were shown to be sensitive in an in vitro proliferation assay to Y220C p53 reactivators (IC50 ~ 192-722 nM) and used to generate Y220C mutant syngeneic mouse models. Administration of Y220C p53 reactivators in Y220C p53 syngeneic mouse models resulted in a dose responsive anti-tumor effect, with maximally efficacious doses resulting in durable cures in nearly all animals, more than observed in immunocompromised mouse models. Addition of checkpoint agents with sub-efficacious administration of Y220C reactivating compounds resulted in synergistic increases in mean survival time of mice, longer than with either single agent. Immunophenotyping analysis of HUPKI Y220C mutant tumors exposed to p53 reactivator compounds showed modulation of the tumor immune environment. Changes include dose responsive increases in T-cells (CD4+, CD8+), T-regulatory cells, natural killer T cells and dose responsive decreases in macrophages (M2) and g-MDSC cells. The synergy observed with an anti-PD-1 combination appears driven by an increase in CD8+ T cells, given a decrease in efficacy in mice depleted of CD8+ T cells, following exposure to anti-CD8. Further investigation into the role of Y220C p53 reactivation in immune modulation showed decreased cytokines and chemokines involved in inflammation (i.e. CXCL10, Ccl2) and an increase in a Tumor Inflammation Signature (nanoString) in a dose and time dependent manner. Taken together, these data suggest a role for p53 signaling in encouraging an immunologically hot tumor microenvironment and provide support for testing small molecule reactivators of mutant p53 in combination with immune checkpoint inhibitors. Citation Format: Anna M. Puzio-Kuter, Chris Mulligan, Brandon Russo, Amy Wiebesiek, Lizhong Xu, Hong Yang, Binh Vu, Melissa Dumble. Small molecule reactivators of Y220C mutant p53 modulate tumor infiltrating leukocytes and synergize with immune checkpoint inhibitors [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 1295.

Clinical significance and immunogenomic landscape analysis of glycolysis-associated prognostic model to guide clinical therapy in hepatocellular carcinoma.

Hepatocellular carcinoma (HCC) is a common malignant tumor with a poor prognosis and high mortality rate worldwide. Glucose metabolism disorder is one of the most important characteristics of HCC. However, as the primary risk factors for the prognosis of HCC patients are unclear, the survival prognosis and therapy response of patients cannot be accurately predicted. First, gene sets of 29 cancer hallmarks were collected from public databases. The z-score of various cancer hallmarks were quantitively analyzed by a single-sample gene set enrichment analysis (ssGSEA) of HCC patients. Next, a glycolysis-related gene signature (GRS) was constructed using a series of bioinformatics methods, which were used to predict the survival prognosis of HCC patients and the immunotherapy benefits. The prediction accuracy of the GRS was validated in different HCC cohorts and clinical subgroups. Additionally, a decision tree and nomogram were also established based on the GRS and other clinical variables. Finally, the genomic alterations and tumor immune microenvironment of the HCC patients were examined. Among the 29 cancer hallmarks, glycolysis was the most predominant risk factor for a poor prognosis in HCC. We subsequently constructed a novel GRS comprising 12 glycolysis-related genes. The high-GRS patients had a poorer survival prognosis than the low-GRS patients. The GRS exhibited a powerful ability to predict survival prognosis in different HCC cohorts and clinical feature subgroups. Additionally, the decision tree and nomogram aided in the risk stratification and prognosis evaluations of HCC patients. Further, we found that a high GRS was characterized by a severe tumor stage, pathological grade, and other clinical features. There were significant differences in the genomic alterations, immune cells, and immune checkpoints between the low- and high-GRS patients, especially in relation to the tumor protein p53 mutation and immunosuppressive cells. Notably, we also found that the GRS could be used to identify HCC patients who are more sensitive to chemotherapy and immunotherapy. In summary, the GRS may be a useful tool for predicting the prognosis and guiding the clinical therapy of HCC patients.

Mutations in DNA binding domain of p53 impede RSL1D1-p53 interaction to escape from degradation in human colorectal cancer cells

Different from the nucleolus-specific localization in some types of cancer cells, ribosomal L1 domain-containing protein 1 (RSL1D1) distributes throughout the nucleus in human colorectal cancer (CRC) cells. RSL1D1 directly interacts with DNA binding domain (aa 93–292) of wild-type p53 (p53-WT) and thereby recruits p53 to HDM2. The ensuing formation of RSL1D1/HDM2/p53 complex enhances p53 ubiquitination and decreases the protein level of p53 in CRC cells. In this study, we investigated the interaction between RSL1D1 and mutant p53 proteins. We first corroborated that aa 93–224 of p53 is a more precise domain for RSL1D1 binding and mutation in either aa 93–224 or aa 225–292 domain of p53 affects RSL1D1-p53 interaction. R175H mutated p53 does not interact with RSL1D1, whereas R273H mutated p53 still can bind to RSL1D1 but showing a remarkably decreased affinity than p53-WT. Although p53-R273H retains a weakened binding affinity with RSL1D1, it can hardly be recruited to HDM2 by RSL1D1 in HCT116 CRC cells. Accordingly, RSL1D1 loses its capacity to negatively regulate either R175H or R273H p53 mutant via directly interaction in HCT116 cells, thereby facilitating p53 mutants to accumulate and gain oncogenic function. Our findings help explain why mutant p53 proteins are more stable than p53-WT in CRC cells.

Haematococcus pluvialis Microalgae Extract Inhibits Proliferation, Invasion, and Induces Apoptosis in Breast Cancer Cells.

Breast cancer (BC) is the most common malignant cancer in females worldwide. Drug resistance, toxicity, and the failure of current therapies to completely cure BC has challenged conventional medicine. Consequently, complementary alternative medicine has become popular due to its safety and efficacy. Haematococcus pluvialis (H. pulvialis) is a green microalga living in fresh water, and its crude extract is rich of bioactives, including carotenoids, known to inhibit cancer cell growth. In the present study, we investigated the effects of a methanol crude extract called “T1” of H. pulvialis on cell growth and migration/invasion of the BC cell line MDA-MB-231 in comparison to the fibroblast control cells. TI significantly suppressed BC cell growth, inhibited migration and invasion and induced apoptosis. Interestingly, apoptosis was mediated by a significant loss of mutant p53 protein, and increased Bax/Bcl2 ratio. Our findings support our hypothesis that T1 exerts its anti-cancer effects by inhibiting BC invasion and inducing apoptosis mediated, at least, via the p53/Bax/Bcl2 pathway. Ongoing experiments aim to identify the molecular mechanisms underpinning T1-inhibited BC cell invasion using pre-designed metastasis gene-based array method.

Suppression of the Proliferation of Huh7 Hepatoma Cells Involving the Downregulation of Mutant p53 Protein and Inactivation of the STAT 3 Pathway with Ailanthoidol.

Ailanthoidol (ATD) has been isolated from the barks of Zanthoxylum ailanthoides and displays anti-inflammatory, antioxidant, antiadipogenic, and antitumor promotion activities. Recently, we found that ATD suppressed TGF-β1-induced migration and invasion of HepG2 cells. In this report, we found that ATD exhibited more potent cytotoxicity in Huh7 hepatoma cells (mutant p53: Y220C) than in HepG2 cells (wild-type p53). A trypan blue dye exclusion assay and colony assay showed ATD inhibited the growth of Huh7 cells. ATD also induced G1 arrest and reduced the expression of cyclin D1 and CDK2. Flow cytometry analysis with Annexin-V/PI staining demonstrated that ATD induced significant apoptosis in Huh7 cells. Moreover, ATD increased the expression of cleaved PARP and Bax and decreased the expression of procaspase 3/8 and Bcl-xL/Bcl-2. In addition, ATD decreased the expression of mutant p53 protein (mutp53), which is associated with cell proliferation with the exploration of p53 siRNA transfection. Furthermore, ATD suppressed the phosphorylation of the signal transducer and activator of transcription 3 (STAT3) and the expression of mevalonate kinase (MVK). Consistent with ATD, the administration of S3I201 (STAT 3 inhibitor) reduced the expression of Bcl-2/Bcl-xL, cyclin D1, mutp53, and MVK. These results demonstrated ATD’s selectivity against mutp53 hepatoma cells involving the downregulation of mutp53 and inactivation of STAT3.

Highlights from Recent Cancer Literature

Highlights from Recent Cancer Literature

Triptolide promotes degradation of the unfolded gain-of-function Tp53R175H/Y220C mutant protein by initiating heat shock protein 70 transcription in non-small cell lung cancer.

BackgroundThe mutation rate of the tumor protein P53 (TP53) has been reported to be greater than 50% in non-small cell lung cancer (NSCLC), and gain-of-function (GOF) mutations in unfolded P53 (TP53R175H and TP53Y220C) have been associated with poor prognosis. However, the best treatment for patients with NSCLC harboring unfolded mutant P53 (mutp53) remains unclear. Triptolide is a natural compound derived from Tripterygium wilfordii that has shown a strong antitumor effect in a variety of cancers. Our study aimed to explore the GOF mutations in unfolded mutp53 (TP53R175H and TP53Y220C) and to clarify the molecular mechanisms by which triptolide regulates the degradation of unfolded mutp53 proteins in NSCLC.MethodsTwo unfolded proteins harboring TP53R175H and TP53Y220C mutations were selected to explore their functions in NSCLC progression. NCI-H1299 cells (TP53-null) were transfected with wild-type TP53 (TP53WT), TP53R175H, or TP53Y220C genes and treated with triptolide or a vehicle. Wound healing and transwell assays were performed to measure cell migration and invasion in vitro. Lung metastasis models were constructed through tail vein injection of mutant cells into BALB/c nude mice to evaluate the effect of triptolide on metastasis in vivo. Western blotting, quantitative real-time polymerase chain reaction (qRT-PCR), immunoprecipitation, and dual-luciferase reporter assays were performed to explore the relevant molecular mechanisms.ResultsOur study revealed that triptolide treatment reduced TP53R175H levels and that the TP53Y220C mutation enhanced the invasion and migration of NCI-H1299 cells. Mechanistically, triptolide promoted TP53R175H and TP53Y220C protein proteasomal degradation mediated through the E3 ligase murine double minute 2 (MDM2) by directly interacting with heat shock protein 70 (HSP70). Moreover, by upregulating HSP70 transcription, triptolide contributed to the protein degradation of the GOF mutp53.ConclusionsOur study reports, for the first time, the mechanism underlying triptolide-regulated protein degradation of TP53R175H or TP53Y220C, which offers new insight into developing a better therapeutic strategy for patients with NSCLC who express the unfolded mutp53 GOF protein.

Cross-talk between mutant p53 and p62/SQSTM1 augments cancer cell migration by promoting the degradation of cell adhesion proteins

Missense mutations in the p53 tumor suppressor abound in human cancer. Common (“hotspot”) mutations endow mutant p53 (mutp53) proteins with oncogenic gain of function (GOF), including enhanced cell migration and invasiveness, favoring cancer progression. GOF is usually attributed to transcriptional effects of mutp53. To elucidate transcription-independent effects of mutp53, we characterized the protein interactome of the p53R273H mutant in cells derived from pancreatic ductal adenocarcinoma (PDAC), where p53R273H is the most frequent p53 mutant. We now report that p53R273H, but not the p53R175H hotspot mutant, interacts with SQSTM1/p62 and promotes cancer cell migration and invasion in a p62-dependent manner. Mechanistically, the p53R273H-p62 axis drives the proteasomal degradation of several cell junction–associated proteins, including the gap junction protein Connexin 43, facilitating scattered cell migration. Concordantly, down-regulation of Connexin 43 augments PDAC cell migration, while its forced overexpression blunts the promigratory effect of the p53R273H-p62 axis. These findings define a mechanism of mutp53 GOF.

Immunohistochemical expression of K-ras protein and p53 in chronic pancreatitis and precursor lesions of pancreatic carcinoma- 5 year study in a tertiary care centre

K-ras mutation is an early event and p53 mutation is a late event in the tumorigenesis pathway of carcinoma associated with Chronic Pancreatitis. To identify the immunohistochemical expression of K-ras protein and p53 in surgical specimens of pancreas with chronic pancreatitis.: A 5 year study of chronic pancreatitis with associated pancreatic lesions received in the Histopathology Department, Government Medical College, Trivandrum. All cases of chronic pancreatitis, (n=60) were histopathologically examined along with the adjacent pancreatic tissue which showed precursor lesions. Immunohistochemical expression of K-ras protein and p53 in all cases were studied. In a total 60 cases of chronic pancreatitis, adjacent pancreatic tissue showed ductal adenocarcinoma in 20 cases (30%). Pancreatic Intraepithelial neoplasm (PanIN) was present in 6 cases accounting for 10%. 1 case was IPMN and one case was associated with serous cystadenoma. 7% of chronic pancreatitis showed K-ras mutations. Immunostaining for mutated p53 protein always was negative. p53 was positive in Pan IN 1A , Pan IN 1B, IPMN and carcinoma cells in 0 of 4(0%), 0 of 2(0%), 0 of 1(0%), and 18 of 20(90%), respectively, and K-ras was positive in 3 of 4 (75%), 2 of 2(100%), 1of 1(100%) and 12 of 20(60%), cases respectively. Chronic pancreatitis and precursor lesions may progress to the development of pancreatic cancer. Further molecular studies for evaluation of oncogenes may be done in future for targeted therapy in pancreatic cancer.

TP53 copy number and protein expression inform mutation status across risk categories in acute myeloid leukemia

AbstractMutant TP53 is an adverse risk factor in acute myeloid leukemia (AML), but large-scale integrated genomic-proteomic analyses of TP53 alterations in patients with AML remain limited. We analyzed TP53 mutational status, copy number (CN), and protein expression data in AML (N = 528) and provide a compilation of mutation sites and types across disease subgroups among treated and untreated patients. Our analysis shows differential hotspots in subsets of AML and uncovers novel pathogenic variants involving TP53 splice sites. In addition, we identified TP53 CN loss in 70.2% of TP53-mutated AML cases, which have more deleterious TP53 mutations, as well as copy neutral loss of heterozygosity in 5/32 (15.6%) AML patients who had intact TP53 CN. Importantly, we demonstrate that mutant p53 protein expression patterns by immunohistochemistry evaluated using digital image-assisted analysis provide a robust readout that integrates TP53 mutation and allelic states in patients with AML. Expression of p53 by immunohistochemistry informed mutation status irrespective of TP53 CN status. Genomic analysis of comutations in TP53-mutant AML shows a muted landscape encompassing primarily mutations in genes involved in epigenetic regulation (DNMT3A and TET2), RAS/MAPK signaling (NF1, KRAS/NRAS, PTPN11), and RNA splicing (SRSF2). In summary, our data provide a rationale to refine risk stratification of patients with AML on the basis of integrated molecular and protein-level TP53 analyses.

Advanced Strategies for Therapeutic Targeting of Wild-Type and Mutant p53 in Cancer.

TP53 is a tumor suppressor gene that encodes a sequence-specific DNA-binding transcription factor activated by stressful stimuli; it upregulates target genes involved in growth suppression, cell death, DNA repair, metabolism, among others. TP53 is the most frequently mutated gene in tumors, with mutations not only leading to loss-of-function (LOF), but also gain-of-function (GOF) that promotes tumor progression, and metastasis. The tumor-specific status of mutant p53 protein has suggested it is a promising target for cancer therapy. We summarize the current progress of targeting wild-type and mutant p53 for cancer therapy through biotherapeutic and biopharmaceutical methods for (1) boosting p53 activity in cancer, (2) p53-dependent and p53-independent strategies for targeting p53 pathway functional restoration in p53-mutated cancer, (3) targeting p53 in immunotherapy, and (4) combination therapies targeting p53, p53 checkpoints, or mutant p53 for cancer therapy.

Ultrasound-triggered reactive oxygen species effector nanoamplifier for enhanced combination therapy of mutant p53 tumors

Reactive oxygen species (ROS) damage is a crucial method with which to inhibit tumor cell proliferation; however, tumor cells can reduce ROS damage by modulating multiple repair mechanisms, thus, reducing the efficacy of ROS damage in tumor therapy. In this study, we built an ultrasound-triggered ROS damage nanoamplifier using a synergistic strategy consisting of ROS damage and decreased tumor self-protection capability to enhance the treatment efficacy of mutant p53 tumors. A ROS damage nanoamplifier (PT@PTGA) was fabricated using amphiphilic polyglutamic acid (PTGA) to load with a sonosensitizer (protoporphyrin IX, PpIX) and an MTH1 inhibitor (TH287). Under ultrasonic excitation, PpIX catalyzes oxygen to produce singlet oxygen and release TH287 to inhibit MTH1 activity, thereby causing the accumulation of 8-oxo-dGTP, which enhances DNA damage and further induces cell apoptosis. In addition, TH287 allies with ROS to eliminate the mutated p53 protein in tumor cells, thus reducing the self-protective capacity of tumor cells. As a result, the "internal and external" aspects were combined to enhance sensitization for mutant p53 tumor therapy. The construction of a ROS nanoamplifier not only provides an effective strategy for the treatment of mutant p53 tumors but also supplies an integrated platform for tumor diagnosis and therapy.

Prometastatic CXCR4 and Histone Methyltransferase EZH2 Are Upregulated in SMARCB1/INI1-Deficient and TP53-Mutated Poorly Differentiated Chordoma

Background: Chordoma is a rare tumor most commonly arising in the sacrococcygeal region from notochord remnants. Usually, these tumors are locally invasive and recurrent, and they have a 5–43% ability to metastasize. A newly-described aggressive variant called poorly differentiated chordoma is different from conventional chordoma in that it does not have the well-differentiated histologic appearance of conventional chordoma and also exhibits the loss of SMARCB1/INI1. Herein, we describe a case of poorly differentiated chordoma with SMARCB1/INI1 loss, a concurrent TP53 mutation, and Rb1 loss. Methods: The patient is a middle-aged man with a history of previously resected sacrococcygeal chordoma, who was found to have new hepatic, lung, and adrenal lesions. Results: Biopsy of the liver lesion showed sheets of malignant epithelioid cells with vacuolated cytoplasm, areas of necrosis, and up to five mitoses in one high-power field. No physaliferous cytologic features or matrix material was seen. After reviewing an extensive panel of immunohistochemical markers, the origin of the metastatic tumor could not be determined; the tumor was only positive for Cam5.2, EMA, and CD56. Brachyury was performed due to the patient’s previous history and was positive. Genomic testing showed a SMARCB1 mutation, TP53 mutation, and RB1 loss. Additional markers were performed, and the tumor showed a Ki-67 proliferation index of approximately 80%, mutant p53 protein, loss of INI1, and strong expression of both the histone methyl transferase EZH2 and the chemokine receptor CXCR4. Conclusions: Poorly differentiated chordoma is a highly aggressive variant of chordoma with few cases reported. This case of SMARCB1/INI-deficient, poorly differentiated chordoma also showed a concurrent TP53 mutation and loss of RB1, which resulted in malignant transformation with upregulation of both prometastatic CXCR4 and the histone methyltransferase EZH2, causing aggressive behavior and metastasis.

An Immunohistochemical Assessment of Ki-67 and P53 Over-expression in Helicobacter pylori-positive Gastritis

AIM: Helicobacter pylori infection of the stomach is a common disease and the resulting changes from it are many and deserve to be in the focus of researchers’ attention, to assess the expression of mutant P53 protein and Ki-67 markers in patients with gastritis secondary to H. pylori. METHODS: Thirty samples with positive H. pylori gastritis were included in a retrospective study in Mosul/Iraq. The histological parameters were assessed using the Sydney system, then, the expression of Ki-67 and P53 expression was studied by immunohistochemical methods. The significance level was appointed at (0.05). RESULTS: Ki-67 and P53 expression were found in 83.3% of the total cases. The study results show that 92% of positive Ki-67 and P53 cases had chronic inflammatory cell infiltration, polymorph nuclear cells infiltration, and atrophy. Whereas 96% of positive Ki-67 cases had no metaplasia, 92% of the positive P53 cases had no metaplasia. The results also showed that only 16% of the positive Ki-67 cases had dysplastic changes, and 24% of the positive cases of P53 cases were showed dysplasia. whenever P53 was negative; there is neither metaplasia nor dysplasia in the tissue, this does not apply to Ki-67 negative cases. CONCLUSIONS: Ki-67 and P53 expressions increase with chronicity of Helicobacter pylori-positive gastritis. P53 expression is amplified when atrophy is present in these samples.

Role of p53 in regulation of hematopoiesis in health and disease.

Human aging is associated with an exponential increase in the occurrence of clonal hematopoiesis of indeterminate potential (CHIP). CHIP is associated with increased risks of de novo and therapy-related hematologic neoplasms and serves as a reservoir for leukemic relapse. Somatic mutations in the TP53 gene, which encodes the tumor suppressor protein p53, rank in the top five among genes that were mutated in CHIP. TP53 mutations in CHIP are associated with an increased incidence of myeloid neoplasms such as myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML). This review focuses on mechanisms by which mutant p53 promotes CHIP progression and drives the pathogenesis of MDS and AML. We will also discuss potential therapeutic approaches that can target mutant p53 and improve treatment outcomes of MDS and AML. TP53 was frequently mutated in individuals with CHIP as well as in patients with MDS and AML. While clinical studies suggest that p53 mutant hematopoietic stem and progenitor cell expansion may predispose the elderly to hematologic neoplasms, the underlying mechanisms are not fully understood. Recent findings suggest that mutant p53 may utilize both cell autonomous and noncell autonomous mechanisms to promote CHIP development. Furthermore, we and others have demonstrated that several gain-of-function mutant p53 proteins have enhanced oncogenic potential beyond dominant-negative and loss-of-function effects. Notably, TP53 allelic state has important implications for genome stability, clinical presentation, and outcomes in MDS. Some small molecules reactivating wild-type p53 tumor suppressor activity show promising effects on some human MDS and AML cells with TP53 mutations in preclinical and early phases of clinical studies. TP53 mutations in MDS and AML are correlated with advanced disease, poor prognosis, reduced overall survival, and dismal outcomes. Deep understanding of the functions of mutant p53 proteins is essential to devise effective therapies for patients with myeloid neoplasms and other human cancers with TP53 mutations. Targeting mutant p53 directly or pathways regulated by mutant p53 holds great potential in preventing CHIP progression and treating MDS and AML patients with TP53 mutations.

Partial p53 reactivation is sufficient to induce cancer regression

BackgroundImpaired p53 function is one of the central molecular features of a tumor cell and even a partial reduction in p53 activity can increase the cancer risk in mice and men. From a therapeutic perspective it is noteworthy that tumor cells often become addicted to the absence of p53 providing a rationale for developing p53 reactivating compounds to treat cancer patients. Unfortunately, many of the compounds that are currently undergoing preclinical and clinical testing fail to fully reactivate mutant p53 proteins, raising the crucial question: how much p53 activity is needed to elicit a therapeutic effect?MethodsWe have genetically modelled partial p53 reactivation using knock-in mice with inducible expression of the p53 variant E177R. This variant has a reduced ability to bind and transactivate target genes and consequently causes moderate cancer susceptibility. We have generated different syngeneically transplanted and autochthonous mouse models of p53-deficient acute myeloid leukemia and B or T cell lymphoma. After cancer manifestation we have activated E177R expression and analyzed the in vivo therapy response by bioluminescence or magnetic resonance imaging. The molecular response was further characterized in vitro by assays for gene expression, proliferation, senescence, differentiation, apoptosis and clonogenic growth.ResultsWe report the conceptually intriguing observation that the p53 variant E177R, which promotes de novo leukemia and lymphoma formation, inhibits proliferation and viability, induces immune cell infiltration and triggers cancer regression in vivo when introduced into p53-deficient leukemia and lymphomas. p53-deficient cancer cells proved to be so addicted to the absence of p53 that even the low-level activity of E177R is detrimental to cancer growth.ConclusionsThe observation that a partial loss-of-function p53 variant promotes tumorigenesis in one setting and induces regression in another, underlines the highly context-specific effects of individual p53 mutants. It further highlights the exquisite sensitivity of cancer cells to even small changes in p53 activity and reveals that changes in activity level are more important than the absolute level. As such, the study encourages ongoing research efforts into mutant p53 reactivating drugs by providing genetic proof-of-principle evidence that incomplete p53 reactivation may suffice to elicit a therapeutic response.

Abstract P5-09-01: Using isogenic model systems to determine mechanisms regulating mutant p53 protein stability in breast cancer cells

Abstract Two of the most common events in triple-negative breast cancers (TNBC) are mutation of the tumor suppressor gene TP53 and development of aneuploidy. This is often the case in aggressive and molecularly heterogeneous cancer subtypes such as TNBC, where ~80% of cases harbor a mutation in TP53. In addition to losing wild-type (WT) tumor-suppressive function, mutant p53 proteins are proposed to acquire gain-of-function (GOF) activity, leading to novel oncogenic phenotypes. Mutant p53 proteins can accumulate to high levels in tumors, and this stabilization is thought to be a pre-requisite for mutant p53 to exert its GOF effects. Mechanisms underlying the accumulation of mutant p53 are not well understood. To study mutant p53 GOF mechanisms and phenotypes, we used CRISPR/Cas9-mediated genome editing and developed two isogenic TNBC cell line models (one non-transformed and one tumor-derived) that express the most frequently occurring p53 missense mutations (R175H and R273H), are deficient for functional p53 protein (null), or retain the wild-type (WT) protein. In these engineered models, endogenous p53 expression is regulated by the native p53 promoter, thus providing a controlled system for rigorous functional experimentation across different p53 states. Through functional genomics analyses comparing isogenic TNBC cell lines, which initially differed only by the TP53 genotype, we have evaluated the relationship between mutant p53 and development of aneuploidy and determined that development of aneuploidy and not TP53 genotype contributed to several previously reported mutant p53 GOF phenotypes in vitro and in vivo1. Immunoblotting, immunofluorescence, and immunohistochemistry experiments revealed that mutant p53-expressing cell lines display a range of p53 protein levels, with several maintaining low levels of p53, similar to the cell lines which contain WT p53. In addition, mutant p53 levels correlate with aneuploidy, but not mean nuclear area or whole-genome doubling status, and protein levels increase over time in cell lines that become aneuploid. Using our various clonal cell lines that exhibit differing p53 protein stability, we are currently determining if p53 stability is dysregulated in aneuploid cell lines after copy number gain or loss of genes encoding critical p53 regulators, leading to the accumulation of the mutant protein. Analysis of cancer cell line genetic dependencies and human tumor data from The Cancer Genome Atlas (TCGA) has identified genetic and clinical correlates with increased mutant p53 protein levels. Utilizing biochemical and bioinformatic approaches, we aim to uncover mechanisms for increased mutant p53 stability and identify novel regulators of both WT and mutant p53 proteins. The dissection of mechanisms that contribute to stabilization of p53 protein will not only give insight into the regulation and tumor suppressive function of WT p53, but it has the potential for clinical translation in human cancers that have high-frequency p53 mutation and high levels of mutant p53 protein, as targeting mutant p53 through pharmacologic inhibition or antibody-based therapies is an active area of investigation. 1Article in press in Nature Communications Citation Format: Lindsay Redman-Rivera, Hailing Jin, Brian D Lehmann, Jennifer A Pietenpol. Using isogenic model systems to determine mechanisms regulating mutant p53 protein stability in breast cancer cells [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 P5-09-01.

Abstract P1-02-05: A genome-wide functional genomics screen reveals unique co-driver mutations of mutant TP53 promoting cellular heterogeneity during breast cancer progression

Abstract Triple-negative breast cancer (TNBC) is an aggressive subtype of breast cancer that lacks three major drug-targetable receptors, ER, PR, and HER2. TNBC patients have much worse 5-year survival rates (60%) in contrast to 90% for other breast cancer subtypes and display highly heterogeneous molecular profiles, cellular phenotypes, and drug responses, which poses major challenges in patient treatments. The tumor suppressor gene TP53 is mutated in 30% of breast tumors overall and but highly prevalent (~80%) in TNBC. Unlike mutations in other tumor suppressor genes that are predominantly loss-of-function deletions or truncations, TP53 mutations occur mostly as >100 different missense mutations within the DNA binding domain, implying that the mutant proteins may exert both loss-of-function activities and gain of distinct neomorphic functions, thus contributing to phenotypic heterogeneity of TNBC. When we characterized systematically a panel of MCF10A cell lines expressing 10 most prevalent missense mutant p53 proteins, the cell lines indeed displayed highly diverse neomorphic cellular phenotypes distinct from those of p53-knockdown cells. To investigate molecular mechanisms underlying the heterogeneity, we then performed RNA-Seq and pathway analysis and identified the key pathways, such as the Hippo/YAP pathway, that were dysregulated correlatively with phenotypic aggressiveness of the mutant p53 cell lines. In addition, ChIP-Seq analysis revealed that promoter binding capacity and preference of mutant p53 proteins associated with more aggressive phenotypes were more severely affected, especially for the genes in the dysregulated pathways identified from RNA-Seq analysis. These demonstrated collectively that different missense p53 mutations lead to heterogeneous phenotypes by exerting distinct neomorphic molecular functions. Further, given that TP53 mutations by themselves cannot drive full cancer progression, these imply that cells with different p53 missense mutations need distinct sets of additional “co-driver” mutations and alteration of cellular programs specific to each mutation for full cancer progression, representing potential molecular targets for personalized therapies. Supporting this hypothesis, when we performed genome-wide in vitro CRISPR screens in search of co-driver mutations specific to different p53 mutations, a unique set of hits was identified for each mutant p53-expressing cell lines. However, in in vivo mouse xenograft models, even the cells expressing aggressive p53 mutants such R273C failed to develop tumors upon transducing gene-deleting CRISPR gRNA libraries at high MOI. Based on reasoning that development of tumor requires mutations in both tumor suppressors and oncogenes, we then performed CRISPR screens on the p53-R273C cells overexpressing MYC, a known oncogene for TNBC, and observed tumor formation within 9 weeks, only after the CRISPR library transduction. By next-generation sequencing of the gRNA cassettes amplified from the tumors, >20 novel co-driver candidates in addition to known tumor suppressors such as NF2 and PTEN were identified. Interestingly, ARAF, a proto-oncogene, was one of the top candidates found in multiple tumors, and the targeted sequencing confirmed out-of-frame deletions resulting in truncated proteins with only the N-term Ras-binding domain. We are currently validating the functional relevance of these findings in conjunction with the dysregulated pathways identified from RNA-Seq and ChIP-Seq analysis. Taken together, our integrated approach of utilizing phenotyping, multi-omics bioinformatics analysis, and screening has revealed the molecular mechanisms underlying phenotypic and molecular heterogeneity and potential molecular targets of TNBC. Citation Format: Dustin Grief, Anasuya Pal, Laura Gonzalez-Malerva, Seron Eaton, Chenxi Xu, Grant Christensen, Joy Blain, Nicholas Mellor, Jason Steel, Chitrak Gupta, Ellen Streitwieser, Abhishek Singharoy, Jin Park, Joshua LaBaer. A genome-wide functional genomics screen reveals unique co-driver mutations of mutant TP53 promoting cellular heterogeneity during breast cancer progression [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 P1-02-05.

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.

Abstract P5-10-03: Mutant p53 and ERK1/2 MAPK cooperate with the production of TNBC inflammatory secretome

Abstract TP53 is the most frequently mutated gene in most types of human cancer, including breast cancer. The triple-negative breast cancer (TNBC) subtype in particular displays TP53 mutation in approximately 80% of patients. Unlike other breast cancer subtypes (e.g., ER/PR-positive or HER2-positive), TNBC patients currently lack an approved highly effective targeted therapy. Notably, most TNBC acquires TP53 mutations, which, in addition to the loss of canonical p53 functions, can result in gain-of-function, activating different cellular mechanisms involved in tumor phenotypes such as proliferation, metastasis, invasiveness, and angiogenesis. Here we evaluate the role of mutant p53 in cancer phenotypes of TNBC cell lines, in particular their inflammatory profile. As expected, loss of p53 protein by small interfering RNA depletion did not show a major impact on the cell viability of MDA-MB231 and Hs578t cells in MTT assay. However, depleting mutant p53 knockdown made MDA-MB231 cells more susceptible to treatment with methyl-methane sulfonate, a genotoxic alkylating agent. Interestingly, cell invasion as measured by the transwell assay demonstrated that depletion of mutant p53 depletion decreased the invasiveness potential of MDA-MB231 and Hs578t cells which was substantiated with decreased migration of MDA-MB231 cells in a scratch assay over 24 h. RNA sequencing of MDA-MB231 and Hs578t cells revealed that mutant p53 knockdown decreased the expression of several constitutively expressed pro-inflammatory cytokines such as IL8, IL6, CXCL2, and CXCL3, but not genes associated with survival to alkylating agents (NRF2 and Endoplasmic reticulum stress markers) or genes typically regulated by wild-type p53 when compared to control-silenced and MMS-treated cells as indicated by Pathway Enrichment Analysis using the Enrich R and DAVID tools. These results were confirmed by ELISA quantification of IL8, IL6, and CXCL2 in MDA-MB231 and Hs578t transfected with two sequences of siRNA targeting mutant p53. On the other hand, RNA sequencing revealed some constitutively expressed genes known to be involved in breast cancer cells malignancy, such as PTGS2 (COX-2 enzyme gene) and MMP1, which were not affected by p53 knockdown. We found that MMP1, and PGE2 (the product of PTGS2/COX-2 enzyme), are upregulated by the ERK1/2 MAPK signaling pathway as determined in MDA-MB231 cells treated with the MEK1/2 inhibitor UO126 and sorafenib. UO126 and sorafenib also decreased IL8 and IL6 production. Furthermore, combined mutant p53 knockdown with MEK/ERK1/2 pathway inhibition caused a more pronounced IL8 and IL6 inhibition when compared to either p53 knockdown or UO126/sorafenib alone, whereas MMP1 and PGE2 levels were only reduced by MEK/ERK1/2 inhibitor treatments. Interestingly, neither mutant p53 knockdown impacted ERK1/2 phosphorylation status nor did UO126/sorafenib alter mutant p53 immunocontent. Reporter assays showed that mutant p53 promotes NFkappaB reporter activation, and MEK/ERK1/2 controls both NFkappaB and AP-1 transcription factors, both associated with the expression of the secretome components evaluated herein. Functional cell assays showed that concomitant inhibition of mutant p53 and MEK/ERK1/2 pathways reduce cell proliferation, invasion, and migration, indicating that mutant p53 protein gain of function cooperates with ERK1/2 MAPK signaling pathway to promote secretome production and malignant phenotypes in TNBC cell models. Citation Format: Raquel Nascimento das Neves, Aparna Gorthi, Alexander James Roy Bishop, Alfeu Zanotto Filho. Mutant p53 and ERK1/2 MAPK cooperate with the production of TNBC inflammatory secretome [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 P5-10-03.