Mutational Inactivation Of P53 Research Articles

Altered p53/p16 expression is linked to urothelial carcinoma progression but largely unrelated to prognosis in muscle-invasive tumors

Background Most inactivating p53 mutations result in a nuclear p53 accumulation – detectable by immunohistochemistry (IHC). p53 alterations leading to a complete lack of p53 protein and absence of immunostaining do also occur – not easily detectable by IHC. p16 is upregulated in p53 inactivated cells. We hypothesized that a positive p16 IHC may help to distinguish p53 inactivation in IHC negative cases. Material and methods We investigated p53 and p16 immunostaining on 2710 urothelial bladder carcinomas in a tissue microarray format to understand their impact in relation to clinicopathological parameters of disease progression and patient outcome. Results p16 immunostaining was absent in normal urothelium but occurred in 63.5% (30.4% strong) of cancers. p16 strongly positive cases increased from pTaG2 low-grade (9.6%) to pTaG3 high-grade tumors (46.5%, p < .0001) but decreased from pTaG3 to pT4 (33.3%; p = .0030). Among pT2-4 carcinomas, p16 positivity was linked to high-grade (p = .0005) but unrelated to overall survival. p53 staining was negative in 8.4%, very weak in 15.4%, weak in 55.3%, strong in 4.7%, and very strong in 16.2% cancers. p53 negative (potentially p53 null phenotype), strong, and very strong p53 positivity increased from pTaG2 low-grade to pTaG3 high-grade tumors (p < .0001) and from pTaG3 to pT2-4 cancers (p = .0007). p53 staining was largely unrelated to histopathological parameters or patient prognosis among pT2-4 carcinomas, except of p53 strong/very strong immunostaining. p16 expression predominated in tumors with very strong, strong, and negative p53 staining and the combination of p53 negative/p16 strongly positive cancers was linked to features of tumor aggressiveness. Conclusion Aberrant p53 and p16 immunostaining increases during grade and stage progression although p53 negative and p16 positive immunostaining lack prognostic significance in pT2–4 carcinomas. Potential diagnostic features are that high level p16 expression is limited to neoplastic urothelium and p53 null phenotype to aggressive cancers (grade 3 and invasive).

Butein inhibits cancer cell growth by rescuing the wild-type thermal stability of mutant p53

p53 is a transcription factor that activates the expression of various genes involved in the maintenance of genomic stability, and more than 50% of cancers harbor inactivating p53 mutations, which are indicative of highly aggressive cancer and poor prognosis. Pharmacological targeting of mutant p53 to restore the wild-type p53 tumor-suppressing function is a promising strategy for cancer therapy. In this study, we identified a small molecule, Butein, that reactivates mutant p53 activity in tumor cells harboring the R175H or R273H mutation. Butein restored wild-type-like conformation and DNA-binding ability in HT29 and SK-BR-3 cells harboring mutant p53-R175H and mutant p53-R273H, respectively. Moreover, Butein enabled the transactivation of p53 target genes and decreased the interactions of Hsp90 with mutant p53-R175H and mutant p53-R273H proteins, while Hsp90 overexpression reversed targeted p53 gene activation. In addition, Butein induced thermal stabilization of wild-type p53, mutant p53-R273H and mutant p53-R175H, as determined via CETSA. From docking study, we further proved that Butein binding to p53 stabilized the DNA-binding loop-sheet-helix motif of mutant p53-R175H and regulated its DNA-binding activity via an allosteric mechanism, conferring wild-type-like the DNA-binding activity of mutant p53. Collectively, the data suggest that Butein is a potential antitumor agent that restores p53 function in cancers harboring mutant p53-R273H or mutant p53-R175H. SignificanceButein restores the ability of mutant p53 to bind DNA by reversing its transition to the Loop3 (L3) state, endows p53 mutants with thermal stability and re-establishes their transcriptional activity to induce cancer cell death.

Abstract 5555: Altered p53/p16 expression is linked to urothelial carcinoma progression but is unrelated to prognosis in muscle-invasive tumors

Abstract Background: Most inactivating p53 mutations result in a nuclear accumulation of the defective p53 protein. However, p53 alterations that result in a complete lack of cellular p53 and complete absence of p53 immunostaining do also occur. As p16 is upregulated in p53 inactivated cells, p16 immunohistochemistry may be a surrogate marker for p53 inactivation. Design: In this study, we investigated p53 and p16 immunostaining on more than 2,500 urothelial bladder carcinomas in a tissue microarray format to better understand their impact in relation to clinicopathological parameters of disease progression and patient outcome. Results: p16 immunostaining was not observed in normal urothelium but occurred in 1,576 (63.5%) of cancers including 755 (30.4%) with a strong staining. The fraction of p16 positive cases increased markedly from pTaG2 low grade (9.6%) to pTaG3 high grade tumors (46.5% strongly positive, p&amp;lt;0.0001 for pTaG2 low vs. pTaG3) but continuously decreased from pTaG3 to pT2 (41.3% strongly positive), pT3 (36.5%) and pT4 (33.3%; p=0.0030). Within pT2-4 carcinomas, p16 positivity was also linked to high grade (p=0.0005) but unrelated to overall survival. p53 staining has been recorded as negative in 203 (8.4%), very weak in 373 (15.4%), weak in 1,341 (55.3%), strong in 115 (4.7%), and very strong in 393 (16.2%) cancers. The fraction of tumors with negative (potentially p53 null phenotype), strong, and very strong p53 positivity increased markedly from pTaG2 low grade to pTaG3 high grade tumors (p&amp;lt;0.0001) and from pTaG3 to muscle-invasive pT2-4 cancers (p=0.0007). p53 staining pattern was unrelated to histopathological parameters of malignancy or patient prognosis within pT2-4 carcinomas, however. There was a significant overall association between p53 and p16 expression but strong p16 expression predominated in tumors with very strong, strong, and negative p53 staining. Subset analyses showed that the combination of p53 negative/p16 strongly positive cancers was particularly linked to features of tumor aggressiveness. Conclusion: Our data show that altered function of p53 and p16 immunostaining increases during grade and stage progression although these alterations lack prognostic significance in pT2-4 carcinomas. That high level p16 expression is limited to neoplastic urothelium and that the p53 null phenotype is largely limited to grade 3 and invasive urothelial carcinomas are features with potential diagnostic utility. Citation Format: Simon Schallenberg, Henning Plage, Sebastian Hofbauer, Kira Kornienko, Sarah Weinberger, Paul G. Bruch, Florian Roßner, Sefer Elezkurtaj, Martina Kluth, Maximilian Lennartz, Tim Mandelkow, Elena Bady, Niclas C. Blessin, Andreas H. Marx, Henrik Samtleben, Margit Fisch, Michael Rink, Marcin Slojewski, Krystian Kaczmarek, Thorsten Ecke, Steffen Hallmann, Stefan Koch, Nico Adamini, Sarah Minner, Ronald Simon, Guido Sauter, David Horst, Tobias Klatte, Thorsten Schlomm, Henrik Zecha. Altered p53/p16 expression is linked to urothelial carcinoma progression but is unrelated to prognosis in muscle-invasive tumors. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5555.

EXTH-19. OPTIMIZING MDM2 INHIBITION FOR THE TREATMENT OF GLIOBLASTOMA

Abstract Over 60% of glioblastomas retain wild-type p53, resulting in potential susceptibility to MDM2 inhibitors. These molecules disrupt the interaction between p53 and its negative regulator MDM2, allowing downstream pathway signaling and cell fate decisions including growth inhibition and death resulting from cellular stress. We have analyzed clinical samples from 10 patients newly diagnosed with glioblastoma multiforme (GBM) and treated with the MDM2 inhibitor KRT-232. We detected upregulation of CDKN1A transcript (a target of p53) in all p53 wild-type tumors (8 of 10), but not in p53-mutant tumors (2 of 10). In patient-derived cell lines, treatment with KRT-232 at the clinically detected concentration was only sufficient to stall tumor growth. Cell death via apoptosis can be achieved when MDM2 inhibition is combined with the chemotherapeutic agent temozolomide. As the effects and resistance mechanisms of MDM2 inhibition remain poorly understood in GBM, we have additionally performed genomic and transcriptomic analyses in patient-derived cell lines to better characterize sensitive tumors and identify putative biomarkers of drug response. Dose response curves and growth assays showed that tumors with inactivating p53 mutations are highly resistant to treatment, but those that retain wild-type p53 exhibit various degrees of drug sensitivity. This suggests that other factors, in addition to p53 mutational status, mediate response to MDM2 inhibition in gliomas. Transcriptional analyses of patient samples following drug treatment suggest that cell division, chromatin reorganization, cell differentiation state and immune response programs become deregulated under MDM2 inhibition and in the absence of p53-inactivating mutations.

656MO Phase I study of M6620 (VX-970, berzosertib) in combination with cisplatin and XRT in patients with locally advanced head and neck squamous cell carcinoma

M6620 is a highly potent and selective ATP-competitive ATR inhibitor. It showed strong synergy with cisplatin and radiation in preclinical cancer models. The significant vulnerability to ATR inhibition by cancer cells harboring inactivating p53 mutations provided a rationale to test the safety of M6620 along with definitive chemoradiation in Head and Neck Squamous Cell Carcinoma (HNSCC) where such alterations are common.

CRISPR/Cas9-Induced DNA Damage Enriches for Mutations in a p53-Linked Interactome: Implications for CRISPR-Based Therapies.

Inactivating p53 mutations are the most abundant genetic alterations found in cancer. Here we show that CRISPR/Cas9-induced double-stranded DNA breaks enrich for cells deficient in p53 and in genes of a core CRISPR-p53 tumor suppressor interactome. Such enrichment could predispose to cancer development and thus pose a challenge for clinical CRISPR use. Transient p53 inhibition could suppress the enrichment of cells with these mutations. The level of DNA damage response induced by an sgRNA influenced the enrichment of p53-deficient cells and could be a relevant parameter in sgRNA design to limit cellular enrichment. Furthermore, a dataset of >800 human cancer cell lines identified additional factors influencing the enrichment of p53-mutated cells, including strong baseline CDKN1A expression as a predictor for an active CRISPR-p53 axis. Taken together, these data provide details about p53 biology in the context of CRISPR-induced DNA damage and identify strategies to enable safer CRISPR use. SIGNIFICANCE: CRISPR-mediated DNA damage enriches for cells with escape mutations in a core CRISPR-p53 interactome, which can be suppressed by transient inhibition of p53.

RasGRP1 induces autophagy and transformation-associated changes in primary human keratinocytes

Ras mutations are present in only a subset of sporadic human cutaneous squamous cell carcinomas (cSCC) even though Ras is activated in most. This suggests that other mechanisms of Ras activation play a role in the disease. The aberrant expression of RasGRP1, a guanyl nucleotide exchange factor for Ras, is critical for mouse cSCC development through its ability to increase Ras activity. However, the role of RasGRP1 in human keratinocyte carcinogenesis remains unknown. Here we report that RasGRP1 is significantly elevated in human cSCC and that high RasGRP1 expression in human primary keratinocytes triggered activation of endogenous Ras and significant morphological changes including cytoplasmic vacuole formation and growth arrest. Moreover, RasGRP1-expressing cells were autophagic as indicated by LC3-II increase and the formation of LC3 punctae. In an in vitro organotypic skin model, wild type keratinocytes generated a well-stratified epithelium, while RasGRP1-expressing cells failed to do so. Finally, RasGRP1 induced transformation-like changes in skin cells from Li-Fraumeni patients with inactivating p53 mutations, demonstrating the oncogenic potential of this protein. These results support a role for RasGRP1 in human epidermal keratinocyte carcinogenesis and might serve as an important new therapeutic target.

High RIG‐I expression in ovarian cancer associates with an immune‐escape signature and poor clinical outcome

Ovarian cancer (OC) is the most lethal gynecological malignancy, with platinum‐based chemotherapy remaining the mainstay for adjuvant treatment after surgery. The lack of indication for immunotherapy may at least in part result from the lack of suitable biomarkers allowing stratification of potentially responding patients. In this monocentric study of 141 cases with OC, we used real‐time quantitative PCR to assess the expression of retinoic acid‐inducible gene‐I (RIG‐I) in primary tumor and healthy ovarian control tissues. RIG‐I expression was correlated to various clinicopathological characteristics as well as to a set of molecular and immunological markers. The prognostic significance of RIG‐I expression was queried in univariate and multivariate analyses and validated in an independent cohort. RIG‐I was overexpressed in the cancerous ovary and correlated with a higher tumor grade. The more aggressive Type‐II cancers and cancers with inactivating p53 mutations exhibited higher RIG‐I expression. RIG‐I levels were also elevated in cancers that recurred after remission or were platinum‐refractory. Survival analyses disclosed RIG‐I as an independent marker of poor outcome in OC. Continuative analyses revealed the molecular and immunological correlates of RIG‐I expression in the tumor microenvironment, including interferon production and a distinct immune‐regulatory signature involving checkpoint molecules (PD‐L1/PD‐1), the RNA‐editing enzyme ADAR1 and the regulatory T cell‐specific transcription factor FoxP3. We conclude that high RIG‐I expression associates with poor outcome in OC, which is explainable by local immunosuppression in the tumor bed. RIG‐I expression may inform checkpoint blockade and/or RIG‐I agonistic targeting in a subset of high‐risk OC patients.

Abstract 4266: Endoplasmic reticulum (ER), a potential therapeutic target for mutant p53 colorectal cancer

Abstract Introduction: Inactivating p53 mutations contribute to tumor progression and treatment-resistance, resulting in poor patient survival in colorectal cancer patients (CRC). The goal of this study is to identify novel small molecules that therapeutically target mutant p53 in CRC. Methods: We analyzed the effects of small molecule (ASR458) on p53-wild type (p53-wt; HCT116) and mutant p53 (p53-mut; SW620) colon cancer cells by phenotypic, molecular and in vivo assays. Results: ASR458 treatment significantly inhibited the proliferation of both HCT116 and SW620 cells at nM concentrations. In p53-wt HCT116 cells, ASR458 caused induction of p53 that resulted in caspase-mediated cell death in both in vitro and in vivo models. On the contrary, ASR458 treatment induced ER-stress signaling (i.e., phosphorylation of ERK and eIF2-α) in p53-mut SW620 cells, which triggered ATF4 activation and subsequent induction of cascade of autophagy events (Atg family proteins, LC3B and Lamp1), causing autophagy-mediated cell death. Silencing ER stress marker ATF-4, a key regulator of autophagy, caused resistance to ASR458 and abrogated autophagy signaling in SW620 cells. This suggested that induction of ER-stress is critical for the cytotoxic effects of ASR458 in p53-mut CRC. Preliminary knockdown studies indicate that silencing of ER markers causes resistance to ASR458 in vitro, further confirming ER-stress as the mechanism of ASR458 action in p53-mut CRC. ASR458 significantly inhibited the growth of SW620 tumors in xenograft. Tissue analysis confirmed the ER-stress signaling observed in vitro. Conclusion: Our results demonstrate ASR458 as a potential therapeutic agent with distinct targets in p53-wt and p53-mut CRC. This study also suggests that ATF4 mediated autophagy in unmanaged ER stress can reduce CRC pathogenesis. Further investigation into the pharmacokinetics and pharmacodynamics of ASR458 should help clinical translation of this agent. Citation Format: Ashish Tyagi, Balaji chandrasekaran, Venkatesh Kolluru, Becca V. Baby, Alatassi Houda, Srinivasa Ramisetti, Arun Sharma, Murali Ankem, Chendil Damodaran. Endoplasmic reticulum (ER), a potential therapeutic target for mutant p53 colorectal cancer [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 4266.

Anti-cancer Potential of Captopril and Botulinum Toxin Type-A and Associated p53 Gene Apototic Stimulating Activity.

Mutational inactivation of p53 is a key player in the development of human cancer. Thus, retrieving the tumor suppressor activity of p53 gene is considered a novel strategy in cancer therapy. Current study aimed to investigate the anti-cancer potentials of botulinum toxin type-A (BTX-A) and captopril as a trial to shed light on effective anti-cancer therapy with lower side effects. Cytotoxic effect of captopril and BTX-A was determined using MTT assay against colon (HCT116) and prostate cancer (DU145) cells compared to their effect on normal vero cells. Anti-proliferation assay and anti-metastatic effect were carried out using trypan blue exclusion method and wound scratch migration test, respectively. The ability of test drugs to induce apoptosis in cancer cells was examined using real time PCR. Recorded data revealed that captopril exhibited a statistically significant cytotoxicity (P < 0.05) to cancer cells (IC50 values of 1.5 and 1.2 mg/mL) with much lower toxicity to normal cells. At the same time, IC50 values post BTX-A treatment were 7.2 and 6.4 U/mL for HCT116 and DU145 cells, respectively without any toxicity to vero cells. Both drugs showed inhibitory potentials on cellular proliferation and the ability of cancer cells to migrate in scratched monolayers was obviously inhibited along with increasing their concentrations. P53 expression levels in captopril and BTX-A treated DU145 cells were elevated by 4 and 2.5 folds, respectively, while lower level of apoptosis induction in HCT116 cells was observed. Accordingly, BTX-A and captopril could present potential anti-cancer candidates through triggering cancer cells towards self-destruction.

MA04.05 P53 Non-Disruptive Mutation is a Negative Predictive Factor for OS and PFS in EGFR M+ NSCLC Treated with TKI

P53 mutations are common in lung cancer, and have also been described in EGFR mutated patients The impact of p53 mutations in EGFR M+ patients is controversial, especially if classified as “disruptive” and “non-disruptive” according to their functional effect on the p53 protein as proposed by Poeta and colleagues. The aim of the study was therefore to systematically analyze EGFR and p53 mutations within a cohort of patients with lung cancer stage IV (UICC 7), to correlate alterations with clinical characteristics and to investigate a potential impact of p53 mutations on treatment outcome. 484 patients diagnosed with lung cancer stage IV were studied for the presence of EGFR as well as inactivating p53 mutations. Methods for the detection of EGFR mutations included Sanger Sequencing and hybridization based COBAS testing, hybrid cage next generation sequencing. P53 mutations were detected by Sanger Sequencing and either Miseq or hybrid cage NGS. Clinical characteristics including smoking status were available for more than 97%. 484 consecutive patients were studied. The overall EGFR M+ rate was 17.8% (86/484) in all patients, 84.9% (73/86) showing common mutations of exon 19 or 21. In 21/86 (24.4%) patients’ p53 analysis was not successful. P53 disruptive mutations were demonstrated in 24.6% (16/65) of successfully tested patients, and p53 non-disruptive mutation occurred in 27.7% (18/65) whereas p53 WT configuration was found in 47.7% (31/65). Median OS was 28 months in p53 disruptive mutation and 44 month in p53 WT compared to 23 months in p53 non-disruptive mutation (p<0.023). PFS on 1st line TKI therapy was 14 months in p53 disruptive mutation, 27 months in p53 WT and 10 months in p53 non-disruptive mutation (p<0.040). Similar results were shown in the EGFR common mutation subgroup. 11/16 (68.8%) patients with a disruptive p53 M+ and 25/29 (86.2%) patients with a p53 WT constellation achieved an objective response on the 1st line TKI therapy compared to 7/13 (53.8%) patients with a non-disruptive p53 status. The patients with an unknown p53 status achieved an objective response on the 1st line TKI therapy of 82.4.8% (14/17). Significant differences in PFS and OS in EGFR M+ patients were observed depending on p53 M+ status. P53 mutational status is predictive when disruptive and non-disruptive p53 M+ are differentiated. A p53 WT constellation has a positive effect on OS and PFS. P53 should be tested prospectively in EGFR M+ patients as management of patients on 1st line TKI may be different.

A mechanistic evaluation of the Syrian hamster embryo cell transformation assay (pH 6.7) and molecular events leading to senescence bypass in SHE cells

The implementation of the Syrian hamster embryo cell transformation assay (SHE CTA) into test batteries and its relevance in predicting carcinogenicity has been long debated. Despite prevalidation studies to ensure reproducibility and minimise the subjective nature of the assay’s endpoint, an underlying mechanistic and molecular basis supporting morphological transformation (MT) as an indicator of carcinogenesis is still missing. We found that only 20% of benzo(a)pyrene-induced MT clones immortalised suggesting that, alone, the MT phenotype is insufficient for senescence bypass. From a total of 12 B(a)P- immortalised MT lines, inactivating p53 mutations were identified in 30% of clones, and the majority of these were consistent with the potent carcinogen’s mode of action. Expression of p16 was commonly silenced or markedly reduced with extensive promoter methylation observed in 45% of MT clones, while Bmi1 was strongly upregulated in 25% of clones. In instances where secondary events to MT appeared necessary for senescence bypass, as evidenced by a transient cellular crisis, clonal growth correlated with monoallelic deletion of the CDKN2A/B locus. The findings further implicate the importance of p16 and p53 pathways in regulating senescence while providing a molecular evaluation of SHE CTA −derived variant MT clones induced by benzo(a)pyrene.

TPL2-NPM-p53 pathway monitors nucleolar stress.

TPL2-NPM-p53 pathway monitors nucleolar stress.

Reactivating p53 functions by suppressing its novel inhibitor iASPP: a potential therapeutic opportunity in p53 wild-type tumors.

Although mutational inactivation of p53 is found in 50% of all human tumors, a subset of tumors display defective p53 function, but retain wild-type (WT) p53. Here, direct and indirect mechanisms leading to the loss of WT p53 activities are discussed. We summarize the oncogenic roles of iASPP, an inhibitor of WT p53, in promoting proliferation, invasion, drug or radiation-resistance and metastasis. From the therapeutic view, we highlight promising perspectives of microRNA-124, peptide and small molecules that reduce or block iASPP for the treatment of cancer. High iASPP expression enhances proliferation, aggressive behavior, the resistance to radiation/chemotherapy and correlates with poor prognosis in a range of human tumors. Overexpression of iASPP accelerates tumorigenesis and invasion through p53-dependent and p53-independent mechanisms. MicroRNA-124 directly targets iASPP and represses the growth and invasiveness of cancer cells. The disruption of iASPP-p53 interaction by a p53-derived peptide A34 restores p53 function in cancer cells. The inhibition of iASPP phosphorylation with small molecules induces p53-dependent apoptosis and growth suppression. The mechanisms underlying aberrant expression of iASPP in human tumors should be further investigated. Reactivating WT p53 functions by targeting its novel inhibitor iASPP holds promise for potential therapeutic interventions in the treatment of WT p53-containing tumors.

PRMT5 is required for lymphomagenesis triggered by multiple oncogenic drivers.

Protein arginine methyltransferase 5 (PRMT5) has been implicated as a key modulator of lymphomagenesis. Whether PRMT5 has overt oncogenic function in the context of leukemia/lymphoma and whether it represents a therapeutic target remains to be established. We demonstrate that inactivation of PRMT5 inhibits colony-forming activity by multiple oncogenic drivers, including cyclin D1, c-MYC, NOTCH1, and MLL-AF9. Furthermore, we demonstrate that PRMT5 overexpression specifically cooperates with cyclin D1 to drive lymphomagenesis in a mouse model, revealing inherent neoplastic activity. Molecular analysis of lymphomas revealed that arginine methylation of p53 selectively suppresses expression of crucial proapoptotic and antiproliferative target genes, thereby sustaining tumor cell self-renewal and proliferation and bypassing the need for the acquisition of inactivating p53 mutations. Critically, analysis of human tumor specimens reveals a strong correlation between cyclin D1 overexpression and p53 methylation, supporting the biomedical relevance of this pathway. We have identified and functionally validated a crucial role for PRMT5 for the inhibition of p53-dependent tumor suppression in response to oncogenic insults. The requisite role for PRMT5 in the context of multiple lymphoma/leukemia oncogenic drivers suggests a molecular rationale for therapeutic development.

88 Genome-wide drug sensitivity screens in haploid mouse embryonic stem cells

osteosarcoma cell line in vitro and in vivo. The SJSA-1 cell line possesses an amplified MDM2 gene and wild-type p53 and is sensitive to SAR405838 and other MDM2 inhibitors in vitro and in vivo. In vitro treatment of the SJSA-1 cells with SAR405838 using two different treatment protocols resulted in acquired resistance to the drug. Analysis of these resistant cells showed that p53 is mutated in the DNA binding domain and cannot be activated by SAR405838. Treatment of mice bearing the parental SJSA-1 xenograft tumors with SAR405838 led to rapid tumor regression, but tumors eventually returned after treatment cessation. A number of cell lines were established by culturing the regrown tumors and, surprisingly, these sublines showed minimal loss of sensitivity to SAR405838 as compared to the parental SJSA-1 cell line. Consistently, analysis of thesesublines showed that p53 retains its wild-type status, with the exception of one subline, that harbored a single heterozygous mutation C176F. Computational modeling suggested that the p53 C176F mutant may still be capable of binding to target DNA to elicit gene transcription, which was confirmed by in vitro treatment with SAR405838. Xenografts of representative sublines lacking p53 C176F mutation obtained from regrown tumors treated with one-round SAR405838 were still responsive to SAR405838 treatment. Interestingly, after a second round of in vivo treatment with SAR405838, all sublines established from harvested tumors uniformly harbored a heterozygous C176F p53 mutation, which were still responsive to SAR405838, albeit with reduced sensitivity. Our study showed that the SJSA-1 tumor cells acquired profound resistance to MDM2 inhibitor SAR405838 in vitro and developed inactivating p53 mutations in its DNA binding domain. But in vivo, SJSA-1 tumor cells only acquired minimal or modest resistance when treated with either oneor two-rounds of SAR405838. Our study suggests that tumor cells respond differently to drug treatment under in vitro and in vivo environments and consequently develop different types of resistance.

Attenuated Expression of Apoptosis Stimulating Protein of p53-2 (ASPP2) in Human Acute Leukemia Is Associated with Therapy Failure

Inactivation of the p53 pathway is a universal event in human cancers and promotes tumorigenesis and resistance to chemotherapy. Inactivating p53 mutations are uncommon in non-complex karyotype leukemias, thus the p53-pathway must be inactivated by other mechanisms. The Apoptosis Stimulating Protein of p53-2 (ASPP2) is a damage-inducible p53-binding protein that enhances apoptosis at least in part through a p53-mediated pathway. We have previously shown, that ASPP2 is an independent haploinsufficient tumor suppressor in vivo. Now, we reveal that ASPP2 expression is significantly attenuated in acute myeloid and lymphoid leukemia – especially in patients with an unfavorable prognostic risk profile and patients who fail induction chemotherapy. In line, knock down of ASPP2 in expressing leukemia cell lines and native leukemic blasts attenuates damage-induced apoptosis. Furthermore, cultured blasts derived from high-risk leukemias fail to induce ASPP2 expression upon anthracycline treatment. The mechanisms of ASPP2 dysregulation are unknown. We provide evidence that attenuation of ASPP2 is caused by hypermethylation of the promoter and 5′UTR regions in native leukemia blasts. Together, our results suggest that ASPP2 contributes to the biology of leukemia and expression should be further explored as a potential prognostic and/or predictive biomarker to monitor therapy responses in acute leukemia.

Methylation Of The 5'UTR Of Apoptosis Stimulating Protein Of p53-2 (ASPP2) Impairs Gene Expression and Associates With Aggressive Disease and Therapy Failure In Acute Leukemia

IntroductionInactivation of the p53 pathway is a frequent event in human cancers promoting tumorigenesis and resistance to chemotherapy. Inactivating p53 mutations are uncommon in non-complex karyotype leukemias suggesting that the p53-pathway must be inactivated by other mechanisms. ASPP2 is a damage-inducible p53-binding protein that enhances apoptosis at least in part through a p53-mediated pathway. We have previously shown that ASPP2 is an independent haploinsufficient tumor suppressor in vivo and that ASPP2 expression is attenuated in a proportion of acute leukemia patients. We now provide evidence that attenuation of ASPP2 is exclusively linked to patients failing to obtain complete remission after induction chemotherapy. Furthermore, we link attenuated ASPP2 expression to hypermethylation of the promoter and 5'UTR regions of ASPP2. MethodsASPP2 levels were determined using standard qRT-PCR assays as well as FACS and immunoblot analyses. Expression levels in bone marrow samples of 51 leukemia patients were normalized to a control cohort comprising bone marrow samples of 17 different healthy donors. Statistical analyses were performed using the non-parametric Wilcoxon rank-sum test and Kruskal-Wallis test. To silence ASPP2 expression, siRNA experiments were performed using leukemia cell lines and primary leukemic patient blasts. gDNA methylation arrays were performed using patient derived leukemia cells. ResultsASPP2 mRNA expression was found to be significantly lower (p=0.02) in acute myeloid (AML) and lymphoid leukemia (ALL) samples (n=51) compared to mononuclear cells from healthy bone marrow donors (n=17). Notably, lower ASPP2 mRNA and protein expression levels were statistically significantly (p=0.02) associated with clinical unfavorable disease according to ELN. We further tested whether ASPP2 levels differ between patients responding or not to induction chemotherapy and found significantly lower overall ASPP2 expression levels in the non-responder cohort (p=0.01). Importantly, 27% of the patients in the non-responding cohort displayed ASPP2 levels below 0.8 (on a normalized scale with the levels in healthy donors set to 1). Such low levels were not observed in good risk patients or in higher-risk patients achieving complete remission after the first cycle of induction chemotherapy. Receiver operating characteristic (ROC) analysis determined<0.8 as an optimal threshold level for safely detecting patients who are likely to fail induction chemotherapy – with no patient of the responder cohort classified as false-positive at this cut-off value. Knockdown of ASPP2 in leukemia cell lines and primary leukemic blasts treated ex vivo with anthracyclines confirmed a functional role of ASPP2 in therapy response. Tantalizingly, analyses of gDNA methylation patterns in high-risk leukemia patients revealed hypermethylation of the promoter and 5'UTR region of ASPP2 arguing for epigenetic dysregulation of ASPP2 thereby identifying it as target for demethylating agents. ConclusionOur findings provide evidence that attenuation of ASPP2 is associated with high-risk disease as well as therapy failure and is caused by hypermethylation of ASPP2 gene regions crucial for initiation of transcription. Prospective analysis of ASPP2 expression profiles is warranted to define its role as a biomarker for risk stratification in leukemia patients and for monitoring therapy responses. Disclosures:No relevant conflicts of interest to declare.

CD44 Expression in Intestinal Epithelium and Colorectal Cancer Is Independent of p53 Status

CD44 marks stem cell-like cells in a number of tumour types, including colorectal cancer (CRC), while aberrant CD44 expression conveys increased tumourigenic, invasive, and metastatic potential. Previous data indicate that CD44 is a direct target of p53-mediated transcriptional repression in breast cancer. Since inactivating p53 mutations are frequent genetic events in CRC these could unleash expression of CD44. In the present study, we therefore explored the relation between p53 mutational status and CD44 expression in a cohort of 90 localized primary CRCs and studied the effect of radiation-induced p53 activation on CD44 expression. Interestingly, we observed that, in contrast to breast cancer, loss of function p53 mutations were not associated with elevated CD44 expression in colon cancer. Moreover, DNA-damage induced p53 activation did not result in repression of CD44 expression, neither in colon cancer cells nor in normal intestinal epithelial cells. Our data demonstrate that CD44 expression in normal and malignant intestinal epithelial cells is not regulated by p53, implying that regulation of this potentially important therapeutic target is tissue and cancer-type specific.

Abstract 2713: The Pten and p53 tumor suppressors cooperate to constrain the PI3K/mTOR pathway activation.

Abstract Loss of Pten expression and mutational inactivation of p53 are frequent molecular events in liver cancer associated with advanced disease stage and poor prognosis. The tumor suppressor PTEN is a phosphoinositide phosphatase that antagonizes the class IA PI3K-engaged oncogenic signaling by dephosphorylating PtdIns(3,4,5)P(3); however, the specific roles of class IA PI3K isoforms, p110a and p110b, have not been assessed in genetically engineered mouse models of liver cancer with Pten inactivation. Furthermore, effective treatment strategies for Pten and p53 deficient liver malignancy remain to be defined. In this study, we showed that combined deletion of Pten and p53 in liver led to rapid development of hepatocellular carcinoma and cholangiocellular carcinoma, the two most common types of liver cancer, with complete penetrance. While a single ablation of p110a or p110b markedly reduced the tumor burden, inactivation of both p110a and p110b completely blocked the liver tumorigenesis. Although liver tumors driven by inactivation of Pten and p53 showed a good response to GDC-0941, a pan-PI3K inhibitor, the combined treatment with both GDC-0941 and Rad001, an mTOR inhibitor or single-agent BEZ235, a dual pan-PI3K and mTOR inhibitor, led to marked tumor regression. These studies suggest that Pten and p53 cooperate to constrain the PI3K/mTOR pathway activation and inhibitors targeting both PI3K and mTOR may effectively treat Pten and p53 deficient liver cancers. Citation Format: Pixu Liu, Hailing Cheng, Lynn Symonds, Erbo Xu, Sauveur-Michel Maira, Emmanuelle di Tomaso, Thomas Roberts, Jean Zhao. The Pten and p53 tumor suppressors cooperate to constrain the PI3K/mTOR pathway activation. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2713. doi:10.1158/1538-7445.AM2013-2713