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Cancer-relevant mutations in the oligomerization domain (OD) of the p53 tumor suppressor protein, unlike those in the DNA binding domain, have not been well elucidated. Here, we characterized the germline OD mutant p53(A347D), which occurs in cancer-prone Li–Fraumeni syndrome (LFS) patients. Unlike wild-type p53, mutant p53(A347D) cannot form tetramers and exists as a hyperstable dimeric protein. Further, p53(A347D) cannot bind or transactivate the majority of canonical p53 target genes. Isogenic cell lines harboring either p53(A347D) or no p53 yield comparable tumorigenic properties, yet p53(A347D) displays remarkable neomorphic activities. Cells bearing p53(A347D) possess a distinct transcriptional profile and undergo metabolic reprogramming. Further, p53(A347D) induces striking mitochondrial network aberration and associates with mitochondria to drive apoptotic cell death upon topoisomerase II inhibition in the absence of transcription. Thus, dimer-forming p53 demonstrates both loss-of-function (LOF) and gain-of-function (GOF) properties compared with the wild-type form of the protein. Significance: A mutant p53 (A347D), which can only form dimers, is associated with increased cancer susceptibility in LFS individuals. We found that this mutant wields a double-edged sword, driving tumorigenesis through LOF while gaining enhanced apoptogenic activity as a new GOF, thereby yielding a potential vulnerability to select therapeutic approaches. See related commentary by Stieg et al., p. 1046. See related article by Gencel-Augusto et al., p. 1230. This article is highlighted in the In This Issue feature, p. 1027

Supplementary Figures 1-9 demonstrating the GOF and LOF activities of p53(A347D)

Differentially expressed genes in U2OS cells varying in p53 allelic status combined from two independent RNA library preparations

p53 ChIPseq peaks from U2OS cells varying in p53 allelic status

p53 ChIPseq peaks from osteoblasts derived from normal and LFS individuals

Characteristics of LFS patients' fibroblasts and derivatives

Cancer-relevant mutations in the oligomerization domain (OD) of the p53 tumor suppressor protein, unlike those in the DNA binding domain, have not been well elucidated. Here, we characterized the germline OD mutant p53(A347D), which occurs in cancer-prone Li–Fraumeni syndrome (LFS) patients. Unlike wild-type p53, mutant p53(A347D) cannot form tetramers and exists as a hyperstable dimeric protein. Further, p53(A347D) cannot bind or transactivate the majority of canonical p53 target genes. Isogenic cell lines harboring either p53(A347D) or no p53 yield comparable tumorigenic properties, yet p53(A347D) displays remarkable neomorphic activities. Cells bearing p53(A347D) possess a distinct transcriptional profile and undergo metabolic reprogramming. Further, p53(A347D) induces striking mitochondrial network aberration and associates with mitochondria to drive apoptotic cell death upon topoisomerase II inhibition in the absence of transcription. Thus, dimer-forming p53 demonstrates both loss-of-function (LOF) and gain-of-function (GOF) properties compared with the wild-type form of the protein.Significance:A mutant p53 (A347D), which can only form dimers, is associated with increased cancer susceptibility in LFS individuals. We found that this mutant wields a double-edged sword, driving tumorigenesis through LOF while gaining enhanced apoptogenic activity as a new GOF, thereby yielding a potential vulnerability to select therapeutic approaches.See related commentary by Stieg et al., p. 1046.See related article by Gencel-Augusto et al., p. 1230.This article is highlighted in the In This Issue feature, p. 1027

Differentially expressed genes in U2OS cells varying in p53 allelic status

p53 ChIPseq peaks from U2OS cells varying in p53 allelic status

Summary of GOF vs. LOF activities of p53(A347D) in U2OS cells

This file contain Supplementary Information that is relevant to the main body of the text. Supplementary Figure S1. Supplementary Figure S2. Half-lives of WT and mutant I332fs*14 p53 in U2OS and HCT116 cells. Supplementary Figure S3. Subcellular localization of p53 FS mutants expressed in U2OS and HCT116 cells. Supplementary Figure S4. Cross-linked species detected in cells grown in the absence or presence of Photo-Met. Supplementary Figure S5. Transcriptional activity of p53 FS mutants in U2OS and HCT116 cells. Supplementary Figure S6. Transactivation potential of N345K* and N345fs*26 mutants in U2OS cells. Supplementary Figure S7. Cell cycle profiles of I332fs*14-expressing HCT116 and U2OS cells. Supplementary Figure S8. Pro-apoptotic activity of I332fs*14 p53.

p53 mutations that result in loss of transcriptional activity are commonly found in numerous types of cancer. While the majority of these are missense mutations that map within the central DNA-binding domain, truncations and/or frameshift mutations can also occur due to various nucleotide substitutions, insertions, or deletions. These changes result in mRNAs containing premature stop codons that are translated into a diverse group of C-terminally truncated proteins. Here we characterized three p53 frameshift mutant proteins expressed from the endogenous TP53 locus in U2OS osteosarcoma and HCT116 colorectal cancer cell lines. These mutants retain intact DNA-binding domains but display altered oligomerization properties. Despite their abnormally high expression levels, they are mostly transcriptionally inactive and unable to initiate a stimuli-induced transcriptional program characteristic of wild-type p53. However, one of these variant p53 proteins, I332fs*14, which resembles naturally expressed TAp53 isoforms β and γ, retains some residual antiproliferative activity and can induce cellular senescence in HCT116 cells. Cells expressing this mutant also display decreased motility in migration assays. Hence, this p53 variant exhibits a combination of loss-of-gain and gain-of-function characteristics, distinguishing it from both wild type p53 and p53 loss.Implications:p53 frameshift mutants display a mixture of residual antiproliferative and neomorphic functions that may be differentially exploited for targeted therapy.

Supplementary Table 3

Supplementary Table 2

Acute leukemia is a rapidly progressing blood cancer with low survival rates. Unfavorable prognosis is attributed to insufficiently characterized subpopulations of leukemia stem cells (LSC) that drive chemoresistance and leukemia relapse. Here we utilized a genetic reporter that assesses stemness to enrich and functionally characterize LSCs. We observed heterogeneous activity of the ERG+85 enhancer–based fluorescent reporter in human leukemias. Cells with high reporter activity (tagBFPHigh) exhibited elevated expression of stemness and chemoresistance genes and demonstrated increased clonogenicity and resistance to chemo- and radiotherapy as compared with their tagBFPNeg counterparts. The tagBFPHigh fraction was capable of regenerating the original cellular heterogeneity and demonstrated increased invasive ability. Moreover, the tagBFPHigh fraction was enriched for leukemia-initiating cells in a xenograft assay. We identified the ubiquitin hydrolase USP9X as a novel ERG transcriptional target that sustains ERG+85–positive cells by controlling ERG ubiquitination. Therapeutic targeting of USP9X led to preferential inhibition of the ERG-dependent leukemias. Collectively, these results characterize human leukemia cell functional heterogeneity and suggest that targeting ERG via USP9X inhibition may be a potential treatment strategy in patients with leukemia.Significance:This study couples a novel experimental tool with state-of-the-art approaches to delineate molecular mechanisms underlying stem cell-related characteristics in leukemia cells.

Supplementary figure legends and full list of materials and methods references

Supplementary Figures

The p63 transcription factor, a member of the p53 family, plays an oncogenic role in squamous cancers, while in breast cancers its expression is often repressed. In the canonical conserved Hippo pathway, known to play a complex role in regulating growth of cancer cells, the protein kinases MST1/2 and LATS1/2 act sequentially to phosphorylate and inhibit the YAP/TAZ transcription factors. We found that in the MCF10A mammary epithelial cell line as well as in squamous and breast cancer cell lines, expression of ΔNp63 RNA and protein is strongly repressed by inhibition of the Hippo pathway protein kinases in a manner that is independent of p53. While MST1/2 and LATS1 are required for p63 expression, the next step of the pathway, namely phosphorylation and degradation of the YAP/TAZ transcriptional activators is not required for repression of p63. This suggests that regulation of p63 expression occurs by a non-canonical version of the Hippo pathway. We additionally identified additional genes that were similarly regulated suggesting the broader importance of this pathway. Interestingly, we observed that experimentally lowering p63 expression leads to increased YAP protein levels, thereby constituting a feedback loop. These results, which reveal the intersection of the Hippo and p63 pathways, may prove useful for the control of their activities in cancer cells.One Sentence SummaryRegulation of p63 expression occurs by a non-canonical version of the Hippo pathway in mammary epithelial, breast carcinoma and head and neck squamous carcinoma cells

Mutations that occur within the oligomerization domain (OD) of the tumor suppressor p53 generally abolish p53 tetramerization and are associated with increased cancer susceptibility in Li-Fraumeni syndrome (LFS). Despite their prevalence, the impact of OD-mutant p53 proteins in cancer, especially beyond a loss of canonical p53 activity, has not been well elucidated. We sought to delineate the gain-of-function (GOF) vs. loss-of-function (LOF) activities of OD-mutant p53, specifically focusing on the LFS tumor-derived p53(A347D) variant. We obtained LFS patient-derived dermal fibroblasts heterozygous for the mutation and generated an allelic series of U2OS cancer cell lines expressing wild-type p53, heterozygous (p53+/AD) or homozygous (p53AD/AD) mutant p53 or no p53 (p53 KO). In contrast to wild-type p53, mutant p53(A347D) exclusively forms dimers in both fibroblasts and cancer cells and has lost the ability to bind and transactivate the majority of canonical p53 target genes, which yields comparable tumorigenic properties between mutant p53 and p53 KO cells. Mutant p53(A347D), however, displays neomorphic activities. Glycolysis and oxidative phosphorylation pathways are enriched in cells bearing p53(A347D) relative to both wild-type p53 and p53 KO cells. Furthermore, dimeric mutant p53 induces striking mitochondrial network aberration and preferentially associates with mitochondria to drive apoptotic cell death upon topoisomerase II inhibition in the absence of transcription. Ultimately, we describe dimeric mutant p53 as wielding a double-edged sword: driving tumorigenesis through LOF while gaining enhanced apoptogenic activity, thereby providing a potential basis for select therapeutic approaches. Citation Format: Joshua H. Choe, Tatsuya Kawase, An Xu, Asja Guzman, David R. Tong, Aleksandar Z. Obradovic, Ana M. Low-Calle, Bita Alaghebandan, Ananya Raghavan, Kaitlin Long, Paul M. Hwang, Joshua D. Schiffman, Yan Zhu, Ruiying Zhao, Dung-Fang Lee, Chen Katz, Carol L. Prives. Li-Fraumeni syndrome-associated dimer-forming mutant p53 promotes transactivation-independent mitochondrial cell death [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr LB361.

V-domain Immunoglobulin Suppressor of T cell Activation (VISTA/PD-1H) is a B7 family member highly expressed on circulating and intra-tumoral myeloid cells. It is a negative checkpoint inhibitor that inhibits anti-tumor T cell response. In patients, VISTA is associated with poor overall survival in multiple tumor indications and is also a potential mediator of resistance to anti-CTLA-4 and anti-PD1 therapies. Therefore, VISTA is a unique target for cancer immunotherapy. Kineta has developed a fully human monoclonal antibody targeting VISTA, KVA12123, that is currently being evaluated in a Phase 1/2 clinical trial in cancer patients with advanced solid tumors. This trial also includes a combination arm with pembrolizumab. In order to inform which patients may be susceptible to respond to our anti-VISTA antibody, we hypothesized that the best responders should be associated with a high expression of the target in the tumor microenvironment (TME). Therefore, after assay validation of VISTA labelling by immuno-histochemistry, we analyzed a large set of tumor samples and showed that VISTA was highly expressed on tumor infiltrating immune cells. This was particularly true for patients with non-small cell lung cancers, colorectal cancers, head and neck squamous cell carcinomas, hepatocellular carcinomas, melanomas and squamous cell carcinoma of the skin, and cervical cancers as well as ovarian cancers. VISTA expression was detected mostly on CD163 positive macrophages infiltrating the tumor. These macrophages potentially promote immunosuppression present in the TME and contribute to treatment failure with current immune checkpoint inhibitors like anti-PD1/PD-L1 and CTLA-4. While previous studies reported VISTA expression on cancer cells, we were not able to confirm these results. In all tumor tissues tested, only infiltrating immune cells were labelled for VISTA. We have investigated in parallel the expression level of soluble VISTA in the serum collected from cancer patients independent of this clinical trial. Sera were screened for patients with multiple tumor types. Patients of diverse sex and age were compared to healthy donors. After validation of the ELISA assay, we showed that sera derived from cancer patients exhibit high levels of soluble VISTA, and these levels tend to correlate with age. More data are needed to confirm that high levels of soluble VISTA are associated with advanced disease. In the ongoing Phase 1/2 clinical trial, tumor tissues and serum samples will be collected from cancer patients prior to treatment with KVA12123 to inform the possible significance of these biomarkers. This work could help to better understand the response to KVA12123 in relation to the expression level of VISTA in cancer tissues as well as in the blood. Citation Format: Neda Kabi, Chen Katz, Remington Lance, Jessica Cross, Nathan Eyde, Emily Frazier, Kurt Lustig, Yulia Ovechkina, David Peckham, Shaarwari Sridhar, Carla Talbaux, Isabelle Tihista, Mei Xu, Shawn Iadonato, Thierry Guillaudeux. VISTA expression in patients with advanced solid tumors: A potential biomarker in VISTA-101 clinical trial [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 972.

The p53 tumor suppressor protein, known to be critically important in several processes including cell-cycle arrest and apoptosis, is highly regulated by multiple mechanisms, most certifiably the Murine Double Minute 2–Murine Double Minute X (MDM2–MDMX) heterodimer. The role of MDM2–MDMX in cell-cycle regulation through inhibition of p53 has been well established. Here we report that in cells either lacking p53 or expressing certain tumor-derived mutant forms of p53, loss of endogenous MDM2 or MDMX, or inhibition of E3 ligase activity of the heterocomplex, causes cell-cycle arrest. This arrest is correlated with a reduction in E2F1, E2F3, and p73 levels. Remarkably, direct ablation of endogenous p73 produces a similar effect on the cell cycle and the expression of certain E2F family members at both protein and messenger RNA levels. These data suggest that MDM2 and MDMX, working at least in part as a heterocomplex, may play a p53-independent role in maintaining cell-cycle progression by promoting the activity of E2F family members as well as p73, making them a potential target of interest in cancers lacking wild-type p53.

The devastating impact of infectious bronchitis (IB) triggered by the IB virus (IBV), on poultry farms is generally curbed by livestock vaccination with live attenuated or inactivated vaccines. Yet, this approach is challenged by continuously emerging variants and by time limitations of vaccine preparation techniques. This work describes the design and evaluation of an anti-IBV vaccine comprised of E. coli expressing and secreting viral spike 1 subunit (S1) and nucleocapsid N-terminus and C-terminus polypeptides fused to heat-labile enterotoxin B (LTB) (LS1, LNN, LNC, respectively). Following chicken oral vaccination, anti-IBV IgY levels and cellular-mediated immunity as well as protection against virulent IBV challenge, were evaluated 14 days following the booster dose. Oral vaccination induced IgY levels that exceeded those measured following vaccination with each component separately. Following exposure to inactivated IBV, splenocytes isolated from chicks orally vaccinated with LNN or LNC -expressing bacteria, showed a higher percentage of CD8+ cells as compared to splenocytes isolated from chicks vaccinated with wild type or LTB-secreting E. coli and to chicks subcutaneously vaccinated. Significant reduction in viral load and percent of shedders in the vaccinated chicks was evident starting 3 days following challenge with 107.5 EID50/ml virulent IBV. Taken together, orally delivered LTB-fused IBV polypeptide-expressing bacteria induced virus-specific IgY antibody production and was associated with significantly shorter viral shedding on challenge with a live IBV. The proposed vaccine design and delivery route promise an effective and rapidly adaptable means of protecting poultry farms from devastating IB outbreaks.HighlightsMucosal vaccination was shown particularly beneficial against respiratory viruses.An anti-IBV vaccine composed of three IBV polypeptides fused to LTB was designed.Vaccine composed of bacteria secreting polypeptides was orally delivered.Vaccine induced specific immune responses and shortened viral shedding duration.