TP53 drives invasion through expression of its Δ133p53β variant

Combining TP53 mutation and isoform has the potential to improve clinical practice

The clinical importance of assessing and combining data on TP53 mutations and isoforms is discussed in this article. It gives a succinct overview of the structural makeup and key biological roles of the isoforms. It then provides a comprehensive summary of the roles that p53 isoforms play in cancer development, therapy response and resistance. The review provides a summary of studies demonstrating the role of p53 isoforms as potential prognostic indicators. It further provides evidence on how the presence of TP53 mutations may affect one or more of these activities and the association of p53 isoforms with clinicopathological data in various tumour types. The review gives insight into the present diagnostic hurdles for identifying TP53 isoforms and makes recommendations to improve their evaluation. In conclusion, this review offers suggestions for enhancing the identification and integration of TP53 isoforms in conjunction with mutation data within the clinical context.

Widespread variation in molecular interactions and regulatory properties among transcription factor isoforms

Most human Transcription factors (TFs) genes encode multiple protein isoforms differing in DNA binding domains, effector domains, or other protein regions. The global extent to which this results in functional differences between isoforms remains unknown. Here, we systematically compared 693 isoforms of 246 TF genes, assessing DNA binding, protein binding, transcriptional activation, subcellular localization, and condensate formation. Relative to reference isoforms, two-thirds of alternative TF isoforms exhibit differences in one or more molecular activities, which often could not be predicted from sequence. We observed two primary categories of alternative TF isoforms: "rewirers" and "negative regulators", both of which were associated with differentiation and cancer. Our results support a model wherein the relative expression levels of, and interactions involving, TF isoforms add an understudied layer of complexity to gene regulatory networks, demonstrating the importance of isoform-aware characterization of TF functions and providing a rich resource for further studies.

It is not all about the alpha: elevated expression of p53β variants is associated with lower probability of survival in a retrospective melanoma cohort

BACKGROUND

Melanoma is the deadliest type of skin cancer and despite improvements in treatment outcomes, melanoma claimed 57,043 lives in 2020. In most malignancies, p53 mutation rates are above 50% and provide prognostic indications. However, in melanoma where less than a quarter of cases harbour a p53 mutation, the significance of the tumour suppressor may be questioned. Instead, p53 isoforms, which modulate p53's canonical function, may be of greater clinical importance.

METHODS

The expression of p53 isoforms was evaluated in 123 melanoma specimens by immunohistochemistry using p53 isoform-specific antibodies (DO-1, KJC8, KJC40, and KJC133). To determine whether TP53 mutations may be driving p53 isoform expression, TP53 was sequenced in 30 FFPE melanoma samples.

RESULTS

The C-terminally truncated p53β isoforms (KJC8) were found to be the most highly expressed p53 isoforms compared to all other isoforms. Further, elevated KJC8 staining was found to correlate with reduced probability of melanoma-specific survival, while KJC40 staining (Δ40p53) positively correlated with reduced melanoma thickness. TAp53 isoforms (p53 retaining both transactivation domains, DO-1), were the second highest p53 isoforms expressed across all samples. Elevated DO-1 staining was also associated with worse survival outcomes and more advanced stages of cancer. Given that the isoforms are likely to work in concert, composite isoform profiles were generated. Composite biomarker profiles revealed that elevated TAp53 (DO-1) and p53β (KJC8) expression, accompanied by low Δ40p53 (KJC40) and Δ133p53 (KJC133) expression was associated with the worst survival outcomes. Supporting the lack of predictive biomarker potential of TP53 in melanoma, no clinicopathological or p53 isoform expression associations could be linked to TP53 status.

CONCLUSIONS

Given the lack of prognostic biomarker potential derived from TP53 status, this study highlights how p53 isoform expression might progress this field and, pending further validation, may provide additional information to treating oncologists that might be factored into treatment decisions.

p53 Dysregulation in Breast Cancer: Insights on Mutations in the TP53 Network and p53 Isoform Expression

In breast cancer, p53 expression levels are better predictors of outcome and chemotherapy response than TP53 mutation. Several molecular mechanisms that modulate p53 levels and functions, including p53 isoform expression, have been described, and may contribute to deregulated p53 activities and worse cancer outcomes. In this study, TP53 and regulators of the p53 pathway were sequenced by targeted next-generation sequencing in a cohort of 137 invasive ductal carcinomas and associations between the identified sequence variants, and p53 and p53 isoform expression were explored. The results demonstrate significant variability in levels of p53 isoform expression and TP53 variant types among tumours. We have shown that TP53 truncating and missense mutations modulate p53 levels. Further, intronic mutations, particularly polymorphisms in intron 4, which can affect the translation from the internal TP53 promoter, were associated with increased Δ133p53 levels. Differential expression of p53 and p53 isoforms was associated with the enrichment of sequence variants in p53 interactors BRCA1, PALB2, and CHEK2. Taken together, these results underpin the complexity of p53 and p53 isoform regulation. Furthermore, given the growing evidence associating dysregulated levels of p53 isoforms with cancer progression, certain TP53 sequence variants that show strong links to p53 isoform expression may advance the field of prognostic biomarker study in breast cancer.

How Driver Oncogenes Shape and Are Shaped by Alternative Splicing Mechanisms in Tumors

The development of RNA sequencing methods has allowed us to study and better understand the landscape of aberrant pre-mRNA splicing in tumors. Altered splicing patterns are observed in many different tumors and affect all hallmarks of cancer: growth signal independence, avoidance of apoptosis, unlimited proliferation, invasiveness, angiogenesis, and metabolism. In this review, we focus on the interplay between driver oncogenes and alternative splicing in cancer. On one hand, oncogenic proteins-mutant p53, CMYC, KRAS, or PI3K-modify the alternative splicing landscape by regulating expression, phosphorylation, and interaction of splicing factors with spliceosome components. Some splicing factors-SRSF1 and hnRNPA1-are also driver oncogenes. At the same time, aberrant splicing activates key oncogenes and oncogenic pathways: p53 oncogenic isoforms, the RAS-RAF-MAPK pathway, the PI3K-mTOR pathway, the EGF and FGF receptor families, and SRSF1 splicing factor. The ultimate goal of cancer research is a better diagnosis and treatment of cancer patients. In the final part of this review, we discuss present therapeutic opportunities and possible directions of further studies aiming to design therapies targeting alternative splicing mechanisms in the context of driver oncogenes.

Activated CD90/Thy-1 fibroblasts co-express the Δ133p53β isoform and are associated with highly inflamed rheumatoid arthritis

BACKGROUND

The p53 isoform Δ133p53β is known to be associated with cancers driven by inflammation. Many of the features associated with the development of inflammation in rheumatoid arthritis (RA) parallel those evident in cancer progression. However, the role of this isoform in RA has not yet been explored. The aim of this study was to determine whether Δ133p53β is driving aggressive disease in RA.

METHODS

Using RA patient synovia, we carried out RT-qPCR and RNAScope-ISH to determine both protein and mRNA levels of Δ133p53 and p53. We also used IHC to determine the location and type of cells with elevated levels of Δ133p53β. Plasma cytokines were also measured using a BioPlex cytokine panel and data analysed by the Milliplex Analyst software.

RESULTS

Elevated levels of pro-inflammatory plasma cytokines were associated with synovia from RA patients displaying extensive tissue inflammation, increased immune cell infiltration and the highest levels of Δ133TP53 and TP53β mRNA. Located in perivascular regions of synovial sub-lining and surrounding ectopic lymphoid structures (ELS) were a subset of cells with high levels of CD90, a marker of 'activated fibroblasts' together with elevated levels of Δ133p53β.

CONCLUSIONS

Induction of Δ133p53β in CD90⁺ synovial fibroblasts leads to an increase in cytokine and chemokine expression and the recruitment of proinflammatory cells into the synovial joint, creating a persistently inflamed environment. Our results show that dysregulated expression of Δ133p53β could represent one of the early triggers in the immunopathogenesis of RA and actively perpetuates chronic synovial inflammation. Therefore, Δ133p53β could be used as a biomarker to identify RA patients more likely to develop aggressive disease who might benefit from targeted therapy to cytokines such as IL-6.

Increased Expression of the Δ133p53β Isoform Enhances Brain Metastasis

The Δ133p53β isoform is increased in many primary tumors and has many tumor-promoting properties that contribute to increased proliferation, migration and inflammation. Here we investigated whether Δ133p53β contributed to some of the most aggressive tumors that had metastasized to the brain. Δ133p53β mRNA expression was measured in lung, breast, melanoma, colorectal metastases and, where available, the matched primary tumor. The presence of Δ133p53β expression was associated with the time for the primary tumor to metastasize and overall survival once the tumor was detected in the brain. Δ133p53β was present in over 50% of lung, breast, melanoma and colorectal metastases to the brain. It was also increased in the brain metastases compared with the matched primary tumor. Brain metastases with Δ133p53β expressed were associated with a reduced time for the primary tumor to metastasize to the brain compared with tumors with no Δ133p53β expression. In-vitro-based analyses in Δ133p53β-expressing cells showed increased cancer-promoting proteins on the cell surface and increased downstream p-AKT and p-MAPK signaling. Δ133p53β-expressing cells also invaded more readily across a mock blood-brain barrier. Together these data suggested that Δ133p53β contributes to brain metastases by making cells more likely to invade the brain.

p53 Family in Resistance to Targeted Therapy of Melanoma

Metastatic melanoma is one of the most aggressive tumors, with frequent mutations affecting components of the MAPK pathway, mainly protein kinase BRAF. Despite promising initial response to BRAF inhibitors, melanoma progresses due to development of resistance. In addition to frequent reactivation of MAPK or activation of PI3K/AKT signaling pathways, recently, the p53 pathway has been shown to contribute to acquired resistance to targeted MAPK inhibitor therapy. Canonical tumor suppressor p53 is inactivated in melanoma by diverse mechanisms. The TP53 gene and two other family members, TP63 and TP73, encode numerous protein isoforms that exhibit diverse functions during tumorigenesis. The p53 family isoforms can be produced by usage of alternative promoters and/or splicing on the C- and N-terminus. Various p53 family isoforms are expressed in melanoma cell lines and tumor samples, and several of them have already shown to have specific functions in melanoma, affecting proliferation, survival, metastatic potential, invasion, migration, and response to therapy. Of special interest are p53 family isoforms with increased expression and direct involvement in acquired resistance to MAPK inhibitors in melanoma cells, implying that modulating their expression or targeting their functional pathways could be a potential therapeutic strategy to overcome resistance to MAPK inhibitors in melanoma.

Δ133p53 coordinates ECM-driven morphogenesis and gene expression in three-dimensional mammary epithelial acini

Growing evidence indicates that p53 (encoded by TP53) has a crucial role in normal tissue development. The role of the canonical p53 (p53α) and its 12 isoforms in development and homeostasis of healthy tissue remains poorly understood. Here, we demonstrate that the Δ133p53 isoforms, the three short isoforms of p53, respond specifically to laminin-111 and play an important regulatory role in formation of mammary organoids in concert with p53α. We demonstrate that down-modulation of Δ133p53 isoforms leads to changes in gene expression of the extracellular matrix molecules fibronectin (FN), EDA+-FN, laminin α5 and laminin α3 in human breast epithelial cells. These changes resulted in increased actin stress fibers and enhanced migratory behavior of cells in two-dimensional culture. We found that α5β1-integrin coupled with the extracellularly deposited EDA+-FN activates the Akt signaling pathway in three-dimensional (3D) culture when Δ133p53 is dysregulated. Cells that do not express detectable Δ133p53 isoforms or express low levels of these isoforms failed to form polarized structures in 3D. These results uncover that Δ133p53 isoforms coordinate expression and deposition of organ-specific ECM molecules that are critical for maintenance of tissue architecture and function.

Systematic identification of intron retention associated variants from massive publicly available transcriptome sequencing data

Many disease-associated genomic variants disrupt gene function through abnormal splicing. With the advancement of genomic medicine, identifying disease-associated splicing associated variants has become more important than ever. Most bioinformatics approaches to detect splicing associated variants require both genome and transcriptomic data. However, there are not many datasets where both of them are available. In this study, we develop a methodology to detect genomic variants that cause splicing changes (more specifically, intron retention), using transcriptome sequencing data alone. After evaluating its sensitivity and precision, we apply it to 230,988 transcriptome sequencing data from the publicly available repository and identified 27,049 intron retention associated variants (IRAVs). In addition, by exploring positional relationships with variants registered in existing disease databases, we extract 3,000 putative disease-associated IRAVs, which range from cancer drivers to variants linked with autosomal recessive disorders. The in-silico screening framework demonstrates the possibility of near-automatically acquiring medical knowledge, making the most of massively accumulated publicly available sequencing data. Collections of IRAVs identified in this study are available through IRAVDB (https://iravdb.io/).

This paper proposed a novel in-silico framework for automatically screening disease-related variants and applied it to over 200,000 transcriptomes, providing an example to acquire medically relevant knowledge from publicly available sequence data.

Characterization of Vemurafenib-Resistant Melanoma Cell Lines Reveals Novel Hallmarks of Targeted Therapy Resistance

Regardless of the significant improvements in treatment of melanoma, the majority of patients develop resistance whose mechanisms are still not completely understood. Hence, we generated and characterized two melanoma-derived cell lines, primary WM793B and metastatic A375M, with acquired resistance to the RAF inhibitor vemurafenib. The morphology of the resistant primary WM793B melanoma cells showed EMT-like features and exhibited a hybrid phenotype with both epithelial and mesenchymal characteristics. Surprisingly, the vemurafenib-resistant melanoma cells showed a decreased migration ability but also displayed a tendency to collective migration. Signaling pathway analysis revealed the reactivation of MAPK and the activation of the PI3K/AKT pathway depending on the vemurafenib-resistant cell line. The acquired resistance to vemurafenib caused resistance to chemotherapy in primary WM793B melanoma cells. Furthermore, the cell-cycle analysis and altered levels of cell-cycle regulators revealed that resistant cells likely transiently enter into cell cycle arrest at the G0/G1 phase and gain slow-cycling cell features. A decreased level of NME1 and NME2 metastasis suppressor proteins were found in WM793B-resistant primary melanoma, which is possibly the result of vemurafenib-acquired resistance and is one of the causes of increased PI3K/AKT signaling. Further studies are needed to reveal the vemurafenib-dependent negative regulators of NME proteins, their role in PI3K/AKT signaling, and their influence on vemurafenib-resistant melanoma cell characteristics.

The Therapeutic Potential of the Restoration of the p53 Protein Family Members in the EGFR-Mutated Lung Cancer

Despite the recent development of precision medicine and targeted therapies, lung cancer remains the top cause of cancer-related mortality worldwide. The patients diagnosed with metastatic disease have a five-year survival rate lower than 6%. In metastatic disease, EGFR is the most common driver of mutation, with the most common co-driver hitting TP53. EGFR-positive patients are offered the frontline treatment with tyrosine kinase inhibitors, yet the development of resistance and the lack of alternative therapies make this group of patients only fit for clinical trial participation. Since mutant p53 is the most common co-driver in the metastatic setting, therapies reactivating the p53 pathway might serve as a promising alternative therapeutic approach in patients who have developed a resistance to tyrosine kinase inhibitors. This review focuses on the molecular background of EGFR-mutated lung cancer and discusses novel therapeutic options converging on the reactivation of p53 tumor suppressor pathways.

p53 Isoforms as Cancer Biomarkers and Therapeutic Targets

Simple Summary

The well-known tumor suppressor protein p53 plays important roles in tumor prevention through transcriptional regulation of its target genes. Reactivation of p53 activity has been a potent strategy for cancer treatment. Accumulating evidences indicate that p53 isoforms truncated/modified in the N- or C-terminus can modulate the p53 pathway in a p53-dependent or p53-independent manner. It is thus imperative to characterize the roles of the p53 isoforms in cancer development. This review illustrates how p53 isoforms participate in tumor development and/or suppression. It also summarizes the knowledge about the p53 isoforms as promising cancer biomarkers and therapeutic targets.

Abstract

This review aims to summarize the implications of the major isoforms of the tumor suppressor protein p53 in aggressive cancer development. The current knowledge of p53 isoforms, their involvement in cell-signaling pathways, and their interactions with other cellular proteins or factors suggests the existence of an intricate molecular network that regulates their oncogenic function. Moreover, existing literature about the involvement of the p53 isoforms in various cancers leads to the proposition of therapeutic solutions by altering the cellular levels of the p53 isoforms. This review thus summarizes how the major p53 isoforms Δ40p53α/β/γ, Δ133p53α/β/γ, and Δ160p53α/β/γ might have clinical relevance in the diagnosis and effective treatments of cancer.

Cytoplasmic p53β Isoforms Are Associated with Worse Disease-Free Survival in Breast Cancer

TP53 mutations are associated with tumour progression, resistance to therapy and poor prognosis. However, in breast cancer, TP53′s overall mutation frequency is lower than expected (~25%), suggesting that other mechanisms may be responsible for the disruption of this critical tumour suppressor. p53 isoforms are known to enhance or disrupt p53 pathway activity in cell- and context-specific manners. Our previous study revealed that p53 isoform mRNA expression correlates with clinicopathological features and survival in breast cancer and may account for the dysregulation of the p53 pathway in the absence of TP53 mutations. Hence, in this study, the protein expression of p53 isoforms, transactivation domain p53 (TAp53), p53β, Δ40p53, Δ133p53 and Δ160p53 was analysed using immunohistochemistry in a cohort of invasive ductal carcinomas (n = 108). p53 isoforms presented distinct cellular localisation, with some isoforms being expressed in tumour cells and others in infiltrating immune cells. Moreover, high levels of p53β, most likely to be N-terminally truncated β variants, were significantly associated with worse disease-free survival, especially in tumours with wild-type TP53. To the best of our knowledge, this is the first study that analysed the endogenous protein levels of p53 isoforms in a breast cancer cohort. Our findings suggest that p53β may be a useful prognostic marker.

Expression of p53 protein isoforms predicts survival in patients with multiple myeloma

Loss and/or mutation of the TP53 gene are associated with short survival in multiple myeloma, but the p53 landscape goes far beyond. At least 12 p53 protein isoforms have been identified as a result of a combination of alternative splicing, alternative promoters and/or alternative transcription site starts, which are grouped as α, β, γ, from transactivation domain (TA), long, and short isoforms. Nowadays, there are no studies evaluating the expression of p53 isoforms and its clinical relevance in multiple myeloma (MM). We used capillary nanoimmunoassay to quantify the expression of p53 protein isoforms in CD138-purified samples from 156 patients with newly diagnosed MM who were treated as part of the PETHEMA/GEM2012 clinical trial and investigated their prognostic impact. Quantitative real-time polymerase chain reaction was used to corroborate the results at RNA levels. Low and high levels of expression of short and TAp53β/γ isoforms, respectively, were associated with adverse prognosis in MM patients. Multivariate Cox models identified high levels of TAp53β/γ (hazard ratio [HR], 4.49; p < .001) and high-risk cytogenetics (HR, 2.69; p < .001) as independent prognostic factors associated with shorter time to progression. The current cytogenetic-risk classification was notably improved when expression levels of p53 protein isoforms were incorporated, whereby high-risk MM expressing high levels of short isoforms had significantly longer survival than high-risk patients with low levels of these isoforms. This is the first study that demonstrates the prognostic value of p53 isoforms in MM patients, providing new insights on the role of p53 protein dysregulation in MM biology.

p53 Signaling on Microenvironment and Its Contribution to Tissue Chemoresistance

Chemoresistance persists as a significant, unresolved clinical challenge in many cancer types. The tumor microenvironment, in which cancer cells reside and interact with non-cancer cells and tissue structures, has a known role in promoting every aspect of tumor progression, including chemoresistance. However, the molecular determinants of microenvironment-driven chemoresistance are mainly unknown. In this review, we propose that the TP53 tumor suppressor, found mutant in over half of human cancers, is a crucial regulator of cancer cell-microenvironment crosstalk and a prime candidate for the investigation of microenvironment-specific modulators of chemoresistance. Wild-type p53 controls the secretion of factors that inhibit the tumor microenvironment, whereas altered secretion or mutant p53 interfere with p53 function to promote chemoresistance. We highlight resistance mechanisms promoted by mutant p53 and enforced by the microenvironment, such as extracellular matrix remodeling and adaptation to hypoxia. Alterations of wild-type p53 extracellular function may create a cascade of spatial amplification loops in the tumor tissue that can influence cellular behavior far from the initial oncogenic mutation. We discuss the concept of chemoresistance as a multicellular/tissue-level process rather than intrinsically cellular. Targeting p53-dependent crosstalk mechanisms between cancer cells and components of the tumor environment might disrupt the waves of chemoresistance that spread across the tumor tissue, increasing the efficacy of chemotherapeutic agents.

Altered Expression of Shorter p53 Family Isoforms Can Impact Melanoma Aggressiveness

Cutaneous melanoma is the most aggressive form of skin cancer. Despite the significant advances in the management of melanoma in recent decades, it still represents a challenge for clinicians. The TP53 gene, the guardian of the genome, which is altered in more than 50% of human cancers, is rarely mutated in melanoma. More recently, researchers started to appreciate the importance of shorter p53 isoforms as potential modifiers of the p53-dependent responses. We analyzed the expression of p53 and p73 isoforms both at the RNA and protein level in a panel of melanoma-derived cell lines with different TP53 and BRAF status, in normal conditions or upon treatment with common anti-cancer DNA damaging agents or targeted therapy. Using lentiviral vectors, we also generated stable clones of H1299 p53 null cells over-expressing the less characterized isoforms Δ160p53α, Δ160p53β, and Δ160p53γ. Further, we obtained two melanoma-derived cell lines resistant to BRAF inhibitor vemurafenib. We observed that melanoma cell lines expressed a wide array of p53 and p73 isoforms, with Δ160p53α as the most variable one. We demonstrated for the first time that Δ160p53α, and to a lesser extent Δ160p53β, can be recruited on chromatin, and that Δ160p53γ can localize in perinuclear foci; moreover, all Δ160p53 isoforms can stimulate proliferation and in vitro migration. Lastly, vemurafenib-resistant melanoma cells showed an altered expression of p53 and p73 isoforms, namely an increased expression of potentially pro-oncogenic Δ40p53β and a decrease in tumor-suppressive TAp73β. We therefore propose that p53 family isoforms can play a role in melanoma cells' aggressiveness.

Δ133p53β isoform pro-invasive activity is regulated through an aggregation-dependent mechanism in cancer cells

The p53 isoform, Δ133p53β, is critical in promoting cancer. Here we report that Δ133p53β activity is regulated through an aggregation-dependent mechanism. Δ133p53β aggregates were observed in cancer cells and tumour biopsies. The Δ133p53β aggregation depends on association with interacting partners including p63 family members or the CCT chaperone complex. Depletion of the CCT complex promotes accumulation of Δ133p53β aggregates and loss of Δ133p53β dependent cancer cell invasion. In contrast, association with p63 family members recruits Δ133p53β from aggregates increasing its intracellular mobility. Our study reveals novel mechanisms of cancer progression for p53 isoforms which are regulated through sequestration in aggregates and recruitment upon association with specific partners like p63 isoforms or CCT chaperone complex, that critically influence cancer cell features like EMT, migration and invasion.

Translation of human Δ133p53 mRNA and its targeting by antisense oligonucleotides complementary to the 5'-terminal region of this mRNA

The p53 protein is expressed as at least twelve protein isoforms. Within intron 4 of the human TP53 gene, a P2 transcription initiation site is located and this transcript encodes two p53 isoforms: Δ133p53 and Δ160p53. Here, the secondary structure of the 5'-terminal region of P2-initiated mRNA was characterized by means of the SHAPE and Pb2+-induced cleavage methods and for the first time, a secondary structure model of this region was proposed. Surprisingly, only Δ133p53 isoform was synthetized in vitro from the P2-initiated p53 mRNA while translation from both initiation codons occurred after the transfection of vector-encoded model mRNA to HCT116 cells. Interestingly, translation performed in the presence of the cap analogue suggested that the cap-independent process contributes to the translation of P2-initiated p53 mRNA. Subsequently, several antisense oligonucleotides targeting the 5'-terminal region of P2-initiated p53 mRNA were designed. The selected oligomers were applied in in vitro translation assays as well as in cell lines and their impact on the Δ133p53 synthesis and on cell viability was investigated. The results show that these oligomers are attractive tools in the modulation of the translation of P2-initiated p53 mRNA through attacking the 5' terminus of the transcript. Since cell proliferation is also reduced by antisense oligomers that lower the level of Δ133p53, this demonstrates an involvement of this isoform in tumorigenesis.

p53/p73 Protein Network in Colorectal Cancer and Other Human Malignancies

Simple Summary

The p53 family of proteins comprises p53, p63, and p73, which share high structural and functional similarity. The two distinct promoters of each locus, the alternative splicing, and the alternative translation initiation sites enable the generation of numerous isoforms with different protein-interacting domains and distinct activities. The co-expressed p53/p73 isoforms have significant but distinct roles in carcinogenesis. Their activity is frequently impaired in human tumors including colorectal carcinoma due to dysregulated expression and a dominant-negative effect accomplished by some isoforms and p53 mutants. The interactions between isoforms are particularly important to understand the onset of tumor formation, progression, and therapeutic response. The understanding of the p53/p73 network can contribute to the development of new targeted therapies.

Abstract

The p53 tumor suppressor protein is crucial for cell growth control and the maintenance of genomic stability. Later discovered, p63 and p73 share structural and functional similarity with p53. To understand the p53 pathways more profoundly, all family members should be considered. Each family member possesses two promoters and alternative translation initiation sites, and they undergo alternative splicing, generating multiple isoforms. The resulting isoforms have important roles in carcinogenesis, while their expression is dysregulated in several human tumors including colorectal carcinoma, which makes them potential targets in cancer treatment. Their activities arise, at least in part, from the ability to form tetramers that bind to specific DNA sequences and activate the transcription of target genes. In this review, we summarize the current understanding of the biological activities and regulation of the p53/p73 isoforms, highlighting their role in colorectal tumorigenesis. The analysis of the expression patterns of the p53/p73 isoforms in human cancers provides an important step in the improvement of cancer therapy. Furthermore, the interactions among the p53 family members which could modulate normal functions of the canonical p53 in tumor tissue are described. Lastly, we emphasize the importance of clinical studies to assess the significance of combining the deregulation of different members of the p53 family to define the outcome of the disease.

ΔNp73, TAp73 and Δ133p53 Extracellular Vesicle Cargo as Early Diagnosis Markers in Colorectal Cancer

Simple Summary

The survival of colorectal cancer patients largely relies on the stage at diagnosis. The identification of early and non-invasive biomarkers to be used in screening programs for the diagnosis of the disease at the premalignant stage is mandatory. The aim of this study is to validate in plasma-derived extracellular vesicles secreted by malignant cells the diagnostic potential of well-known tumor-associated genes, ΔNp73, TAp73, and Δ133p53, in healthy subjects (n = 29), individuals with premalignant lesions (n = 49), and colorectal cancer patients (n = 42). Our data support ΔNp73 levels contained in extracellular vesicles as such a non-invasive and premature biomarker for the early diagnosis of colorectal cancer.

Abstract

The early diagnosis of colorectal cancer is a key factor in the overall survival of the patients. The actual screening programs include different approaches with significant limitations such as unspecificity, high invasiveness, and detection at late stages of the disease. The specific content of extracellular vesicles derived from malignant cells may represent a non-invasive technique for the early detection of colorectal cancer. Here, we studied the mRNA levels of ΔNp73, TAp73, and Δ133p53 in plasma-derived extracellular vesicles from healthy subjects (n = 29), individuals with premalignant lesions (n = 49), and colorectal cancer patients (n = 42). Extracellular vesicles’ ΔNp73 levels were already significantly high in subjects with premalignant lesions. Δ133p53 levels were statistically increased in colorectal cancer patients compared to the other two groups and were associated with patients’ survival. Remarkably, TAp73 mRNA was not detected in any of the individuals. The evaluation of ΔNp73, Δ133p53 and CEA sensitivity, specificity and AUC values supports ΔNp73 as a better early diagnosis biomarker and CEA as the best to identify advanced stages. Thus, low levels of CEA and a high content of ΔNp73 may identify in screening programs those individuals at higher risk of presenting a premalignant lesion. In addition, Δ133p53 emerges as a potential prognosis biomarker in colorectal cancer.

Breast Cancer Patient Prognosis Is Determined by the Interplay between TP53 Mutation and Alternative Transcript Expression: Insights from TP53 Long Amplicon Digital PCR Assays

Simple Summary

The TP53 gene, the most commonly mutated gene in human cancers, is capable of producing multiple RNAs (transcripts). The aim of our study was to measure the abundance of each TP53 transcript, combined with TP53 gene mutation information, to determine the interplay between these in a cohort of breast tumors from New Zealand patients. To do this, we devised a new assay which then enabled the measurement of all known TP53 transcripts. We showed how TP53 gene mutations influenced the levels of specific TP53 transcripts in breast tumors. We evaluated whether a combination of TP53 tumor information, including TP53 mutation status and the levels of certain TP53 transcripts, with standard clinical and pathological information, was associated with breast cancer patient outcome. We recommend that a truly comprehensive analysis of TP53 needs to incorporate data about both TP53 DNA mutations and the expression of the alternative TP53 transcripts.

Abstract

The TP53 gene locus is capable of producing multiple RNA transcripts encoding the different p53 protein isoforms. We recently described multiplex long amplicon droplet digital PCR (ddPCR) assays to quantify seven of eight TP53 reference transcripts in human tumors. Here, we describe a new long amplicon ddPCR assay to quantify expression of the eighth TP53 reference transcript encoding ∆40p53α. We then applied these assays, alongside DNA sequencing of the TP53 gene locus, to tumors from a cohort of New Zealand (NZ) breast cancer patients. We found a high prevalence of mutations at TP53 splice sites in the NZ breast cancer cohort. Mutations at TP53 intron 4 splice sites were associated with overexpression of ∆133TP53 transcripts. Cox proportional hazards survival analysis showed that interplay between TP53 mutation status and expression of TP53 transcript variants was significantly associated with patient outcome, over and above standard clinical and pathological information. In particular, patients with no TP53 mutation and a low ratio of TP53 transcripts t2 to t1, which derive from alternative intron 1 acceptor splice sites, had a remarkably good outcome. We suggest that this type of analysis, integrating mutation and transcript expression, provides a step-change in our understanding of TP53 in cancer.

The Δ133p53 Isoforms, Tuners of the p53 Pathway

Simple Summary

TP53, the most frequently mutated gene in human cancers, has a key role in the maintenance of the genetic stability and, thus, in preventing tumor development. The p53-dependent responses were long thought to be solely driven by canonical p53α. However, it is now known that TP53 physiologically expresses at least 12 p53 isoforms including Δ133p53α, Δ133p53β and Δ133p53γ. The Δ133p53 isoforms are potent modulators of the p53 pathway that regulate critical functions in cancer, physiological and premature aging, neurodegenerative diseases, immunity and inflammation, and tissue repair. This review aims to summarize the current knowledge on the Δ133p53 isoforms and how they contribute to multiple physiological and pathological mechanisms. Critically, further characterization of p53 isoforms may identify novel regulatory modes of p53 pathway functions that contribute to disease progression and facilitate the development of new therapeutic strategies.

Abstract

The TP53 gene is a critical tumor suppressor and key determinant of cell fate which regulates numerous cellular functions including DNA repair, cell cycle arrest, cellular senescence, apoptosis, autophagy and metabolism. In the last 15 years, the p53 pathway has grown in complexity through the discovery that TP53 differentially expresses twelve p53 protein isoforms in human cells with both overlapping and unique biologic activities. Here, we summarize the current knowledge on the Δ133p53 isoforms (Δ133p53α, Δ133p53β and Δ133p53γ), which are evolutionary derived and found only in human and higher order primates. All three isoforms lack both of the transactivation domains and the beginning of the DNA-binding domain. Despite the absence of these canonical domains, the Δ133p53 isoforms maintain critical functions in cancer, physiological and premature aging, neurodegenerative diseases, immunity and inflammation, and tissue repair. The ability of the Δ133p53 isoforms to modulate the p53 pathway functions underscores the need to include these p53 isoforms in our understanding of how the p53 pathway contributes to multiple physiological and pathological mechanisms. Critically, further characterization of p53 isoforms may identify novel regulatory modes of p53 pathway functions that contribute to disease progression and facilitate the development of new therapeutic strategies.

Intronic TP53 Polymorphisms Are Associated with Increased Δ133TP53 Transcript, Immune Infiltration and Cancer Risk

We investigated the influence of selected TP53 SNPs in exon 4 and intron 4 on cancer risk, clinicopathological features and expression of TP53 isoforms. The intron 4 SNPs were significantly over-represented in cohorts of mixed cancers compared to three ethnically matched controls, suggesting they confer increased cancer risk. Further analysis showed that heterozygosity at rs1042522(GC) and either of the two intronic SNPs rs9895829(TC) and rs2909430(AG) confer a 2.34-5.35-fold greater risk of developing cancer. These SNP combinations were found to be associated with shorter patient survival for glioblastoma and prostate cancer. Additionally, these SNPs were associated with tumor-promoting inflammation as evidenced by high levels of infiltrating immune cells and expression of the Δ133TP53 and TP53β transcripts. We propose that these SNP combinations allow increased expression of the Δ133p53 isoforms to promote the recruitment of immune cells that create an immunosuppressive environment leading to cancer progression.

Δ133p53/FLp53 Predicts Poor Clinical Outcome in Esophageal Squamous Cell Carcinoma

Background

p53 isoform Δ133p53 is directly transactivated by p53 and antagonizes p53 activities in cancer progression. However, its correlation with prognosis and cancer recurrence in esophageal squamous cell carcinoma (ESCC) is still unclear.

Patients and Methods

Expression of Δ133p53 and Δ133p53/full-length p53 (FLp53) in tissues and serums of 180 ESCC patients was evaluated using qRT-PCR. Patients were divided into high- and low-expression groups according to the cutoff value determined by X-tile 3.6.1 software. Survival analysis was performed by the Kaplan-Meier method. Univariate and multivariate Cox survival analyses were applied to assess the hazard ratios (HRs).

Results

Tissue Δ133p53 expression and Δ133p53/FLp53 ratio were significantly increased in ESCC tissue compared with adjacent normal tissue. Pre-operative Δ133p53 expression and Δ133p53/FLp53 ratio in tissue or serum samples were positively associated with TNM stage and post-operative recurrence. Kaplan-Meier curve and multivariate cox regression analyses revealed that the tissue and serum Δ133p53/FLp53 ratios (cutoff value: 2.9160) were independent prognostic factors for overall survival (OS) and progression-free survival (PFS) in ESCC patients and showed no statistical difference in receiver-operating characteristic curve (ROC) analysis, while serum Δ133p53 showed no significant prognostic value. More importantly, the serum Δ133p53/FLp53 ratio in ESCC patients was significantly decreased within 72 h post tumor resection and patients with a consistently high serum Δ133p53/FLp53 ratio (≥2.9160) had higher recurrence rates than those with consistently low ratio values. In addition, dynamic detection in each follow-up timepoint showed that serum Δ133p53/FLp53 ratios were higher than 2.9160 upon recurrence, and they even increased prior to radiologic progression.

Conclusion

The serum Δ133p53/FLp53 ratio can be a novel predictor for survival outcome and may serve as a real-time parameter for monitoring recurrence in ESCC patients after surgery.

Accessing a New Dimension in TP53 Biology: Multiplex Long Amplicon Digital PCR to Specifically Detect and Quantitate Individual TP53 Transcripts

TP53, the most commonly-mutated gene in cancer, undergoes complex alternative splicing. Different TP53 transcripts play different biological roles, both in normal function and in the progression of diseases such as cancer. The study of TP53’s alternative RNA splice forms and their use as clinical biomarkers has been hampered by limited specificity and quantitative accuracy of current methods. TP53 RNA splice variants differ at both 5’ and 3’ ends, but because they have a common central region of 618 bp, the individual TP53 transcripts are impossible to specifically detect and precisely quantitate using standard PCR-based methods or short-read RNA sequencing. Therefore, we devised multiplex probe-based long amplicon droplet digital PCR (ddPCR) assays, which for the first time allow precise end-to-end quantitation of the seven major TP53 transcripts, with amplicons ranging from 0.85 to 1.85 kb. Multiple modifications to standard ddPCR assay procedures were required to enable specific co-amplification of these long transcripts and to overcome issues with secondary structure. Using these assays, we show that several TP53 transcripts are co-expressed in breast cancers, and illustrate the potential for this method to identify novel TP53 transcripts in tumour cells. This capability will facilitate a new level of biological and clinical understanding of the alternatively-spliced TP53 isoforms.

The Emerging Landscape of p53 Isoforms in Physiology, Cancer and Degenerative Diseases

p53, first described four decades ago, is now established as a master regulator of cellular stress response, the “guardian of the genome”. p53 contributes to biological robustness by behaving in a cellular-context dependent manner, influenced by several factors (e.g., cell type, active signalling pathways, the type, extent and intensity of cellular damage, cell cycle stage, nutrient availability, immune function). The p53 isoforms regulate gene transcription and protein expression in response to the stimuli so that the cell response is precisely tuned to the cell signals and cell context. Twelve isoforms of p53 have been described in humans. In this review, we explore the interactions between p53 isoforms and other proteins contributing to their established cellular functions, which can be both tumour-suppressive and oncogenic in nature. Evidence of p53 isoform in human cancers is largely based on RT-qPCR expression studies, usually investigating a particular type of isoform. Beyond p53 isoform functions in cancer, it is implicated in neurodegeneration, embryological development, progeroid phenotype, inflammatory pathology, infections and tissue regeneration, which are described in this review.

Expression profiles of p53/p73, NME and GLI families in metastatic melanoma tissue and cell lines

Unlike other tumours, TP53 is rarely mutated in melanoma; however, it fails to function as a tumour suppressor. We assume that its functions might be altered through interactions with several families of proteins, including p53/p73, NME and GLI. To elucidate the potential interplay among these families we analysed the expression profiles of aforementioned genes and proteins in a panel of melanoma cell lines, metastatic melanoma specimens and healthy corresponding tissue. Using qPCR a higher level of NME1 gene expression and lower levels of Δ40p53β, ΔNp73, GLI1, GLI2 and PTCH1 were observed in tumour samples compared to healthy tissue. Protein expression of Δ133p53α, Δ160p53α and ΔNp73α isoforms, NME1 and NME2, and N'ΔGLI1, GLI1FL, GLI2ΔN isoforms was elevated in tumour tissue, whereas ∆Np73β was downregulated. The results in melanoma cell lines, in general, support these findings. In addition, we correlated expression profiles with clinical features and outcome. Higher Δ133p53β and p53α mRNA and both GLI1 mRNA and GLI3R protein expression had a negative impact on the overall survival. Shorter overall survival was also connected with lower p53β and NME1 gene expression levels. In conclusion, all examined genes may have implications in melanoma development and functional inactivity of TP53.

The Δ133p53β isoform promotes an immunosuppressive environment leading to aggressive prostate cancer

Prostate cancer is the second most common cancer in men, for which there are no reliable biomarkers or targeted therapies. Here we demonstrate that elevated levels of Δ133TP53β isoform characterize prostate cancers with immune cell infiltration, particularly T cells and CD163+ macrophages. These cancers are associated with shorter progression-free survival, Gleason scores ≥ 7, and an immunosuppressive environment defined by a higher proportion of PD-1, PD-L1 and colony-stimulating factor 1 receptor (CSF1R) positive cells. Consistent with this, RNA-seq of tumours showed enrichment for pathways associated with immune signalling and cell migration. We further show a role for hypoxia and wild-type p53 in upregulating Δ133TP53 levels. Finally, AUC analysis showed that Δ133TP53β expression level alone predicted aggressive disease with 88% accuracy. Our data identify Δ133TP53β as a highly accurate prognostic factor for aggressive prostate cancer.

Nucleolar and Ribosomal DNA Structure under Stress: Yeast Lessons for Aging and Cancer

Once thought a mere ribosome factory, the nucleolus has been viewed in recent years as an extremely sensitive gauge of diverse cellular stresses. Emerging concepts in nucleolar biology include the nucleolar stress response (NSR), whereby a series of cell insults have a special impact on the nucleolus. These insults include, among others, ultra-violet radiation (UV), nutrient deprivation, hypoxia and thermal stress. While these stresses might influence nucleolar biology directly or indirectly, other perturbances whose origin resides in the nucleolar biology also trigger nucleolar and systemic stress responses. Among the latter, we find mutations in nucleolar and ribosomal proteins, ribosomal RNA (rRNA) processing inhibitors and ribosomal DNA (rDNA) transcription inhibition. The p53 protein also mediates NSR, leading ultimately to cell cycle arrest, apoptosis, senescence or differentiation. Hence, NSR is gaining importance in cancer biology. The nucleolar size and ribosome biogenesis, and how they connect with the Target of Rapamycin (TOR) signalling pathway, are also becoming important in the biology of aging and cancer. Simple model organisms like the budding yeast Saccharomyces cerevisiae, easy to manipulate genetically, are useful in order to study nucleolar and rDNA structure and their relationship with stress. In this review, we summarize the most important findings related to this topic.

Influence of p53 Isoform Expression on Survival in High-Grade Serous Ovarian Cancers

High-grade serous ovarian carcinoma (HGSOC) is characterised by alterations in the p53 pathway. The expression levels of p53 isoforms have been shown to be associated with patient survival in several cancers. This study examined the predictive and prognostic effects of the expression levels of TP53 pre-mRNA splicing isoforms and TP53 mutations in tumour tissues in 40 chemotherapy responders and 29 non-responders with HGSOC. The mRNA expression levels from total p53, and total Δ133p53, p53β, p53γ isoforms were determined by RT-qPCR, and TP53 mutation status by targeted massive parallel sequencing. The results from these analyses were correlated with the clinical outcome parameters. No differential expression of p53 isoforms could be detected between the chemosensitive and chemoresistant subgroups. In a multivariate Cox regression model, high levels of total Δ133p53 were found to be an independent prognosticator for improved overall survival (HR = 0.422, p = 0.018, 95% CI: 0.207–0.861) and reached borderline significance for progression-free survival (HR = 0.569, p = 0.061, 95% CI: 0.315–1.027). TP53 mutations resulting in loss of function or located at known hotspots were predictive of tumour characteristics and disease progression. These findings suggest that total Δ133p53 mRNA can be a biomarker for survival in HGSOC.

Influence of p53 Isoform Expression on Survival in High-Grade Serous Ovarian Cancers

High-grade serous ovarian carcinoma (HGSOC) is characterised by alterations in the p53 pathway. The expression levels of p53 isoforms have been shown to be associated with patient survival in several cancers. This study examined the predictive and prognostic effects of the expression levels of TP53 pre-mRNA splicing isoforms and TP53 mutations in tumour tissues in 40 chemotherapy responders and 29 non-responders with HGSOC. The mRNA expression levels from total p53, and total Δ133p53, p53β, p53γ isoforms were determined by RT-qPCR, and TP53 mutation status by targeted massive parallel sequencing. The results from these analyses were correlated with the clinical outcome parameters. No differential expression of p53 isoforms could be detected between the chemosensitive and chemoresistant subgroups. In a multivariate Cox regression model, high levels of total Δ133p53 were found to be an independent prognosticator for improved overall survival (HR = 0.422, p = 0.018, 95% CI: 0.207-0.861) and reached borderline significance for progression-free survival (HR = 0.569, p = 0.061, 95% CI: 0.315-1.027). TP53 mutations resulting in loss of function or located at known hotspots were predictive of tumour characteristics and disease progression. These findings suggest that total Δ133p53 mRNA can be a biomarker for survival in HGSOC.

Conformational stability and dynamics of the cancer-associated isoform Δ133p53β are modulated by p53 peptides and p53-specific DNA

p53 is a tumor suppressor protein that maintains genome stability, but its Δ133p53β and Δ160p53β isoforms promote breast cancer cell invasion. The sequence truncations in the p53 core domain raise key questions related to their physicochemical properties, including structural stabilities, interaction mechanisms, and DNA-binding abilities. Herein, we investigated the conformational dynamics of Δ133p53β and Δ160p53β with and without binding to p53-specific DNA by using molecular dynamics simulations. We observed that the core domains of the 2 truncated isoforms are much less stable than wild-type (wt) p53β, and the increased solvent exposure of their aggregation-triggering segment indicates their higher aggregation propensities than wt p53. We also found that Δ133p53β stability is modulable by peptide or DNA interactions. Adding a p53 peptide (derived from truncated p53 sequence 107-129) may help stabilize Δ133p53. Most importantly, our simulations of p53 isomer-DNA complexes indicate that Δ133p53β dimer, but not Δ160p53β dimer, could form a stable complex with p53-specific DNA, which is consistent with recent experiments. This study provides physicochemical insight into Δ133p53β, Δ133p53β-DNA complexes, Δ133p53β's pathologic mechanism, and peptide-based inhibitor design against p53-related cancers.-Lei, J., Qi, R., Tang, Y., Wang, W., Wei, G., Nussinov, R., Ma, B. Conformational stability and dynamics of the cancer-associated isoform Δ133p53β are modulated by p53 peptides and p53-specific DNA.

p53 Isoforms and Their Implications in Cancer

In this review we focus on the major isoforms of the tumor-suppressor protein p53, dysfunction of which often leads to cancer. Mutations of the TP53 gene, particularly in the DNA binding domain, have been regarded as the main cause for p53 inactivation. However, recent reports demonstrating abundance of p53 isoforms, especially the N-terminally truncated ones, in the cancerous tissues suggest their involvement in carcinogenesis. These isoforms are ∆40p53, ∆133p53, and ∆160p53 (the names indicate their respective N-terminal truncation). Due to the lack of structural and functional characterizations the modes of action of the p53 isoforms are still unclear. Owing to the deletions in the functional domains, these isoforms can either be defective in DNA binding or more susceptive to altered ‘responsive elements’ than p53. Furthermore, they may exert a ‘dominant negative effect’ or induce more aggressive cancer by the ‘gain of function’. One possible mechanism of p53 inactivation can be through tetramerization with the ∆133p53 and ∆160p53 isoforms—both lacking part of the DNA binding domain. A recent report and unpublished data from our laboratory also suggest that these isoforms may inactivate p53 by fast aggregation—possibly due to ectopic overexpression. We further discuss the evolutionary significance of the p53 isoforms.

Elevation of the TP53 isoform Δ133p53β in glioblastomas: an alternative to mutant p53 in promoting tumor development

As tumor protein 53 (p53) isoforms have tumor‐promoting, migration, and inflammatory properties, this study investigated whether p53 isoforms contributed to glioblastoma progression. The expression levels of full‐length TP53α (TAp53α) and six TP53 isoforms were quantitated by RT‐qPCR in 89 glioblastomas and correlated with TP53 mutation status, tumor‐associated macrophage content, and various immune cell markers. Elevated levels of Δ133p53β mRNA characterised glioblastomas with increased CD163‐positive macrophages and wild‐type TP53. In situ‐based analyses found Δ133p53β expression localised to malignant cells in areas with increased hypoxia, and in cells with the monocyte chemoattractant protein C‐C motif chemokine ligand 2 (CCL2) expressed. Tumors with increased Δ133p53β had increased numbers of cells positive for macrophage colony‐stimulating factor 1 receptor (CSF1R) and programmed death ligand 1 (PDL1). In addition, cells expressing a murine ‘mimic’ of Δ133p53 (Δ122p53) were resistant to temozolomide treatment and oxidative stress. Our findings suggest that elevated Δ133p53β is an alternative pathway to TP53 mutation in glioblastoma that aids tumor progression by promoting an immunosuppressive and chemoresistant environment. Adding Δ133p53β to a TP53 signature along with TP53 mutation status will better predict treatment resistance in glioblastoma. © 2018 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Influence of transcriptional variants on metastasis

Cancer metastasis is defined as the dissemination of malignant cells from the primary tumor site, leading to colonization of distant organs and the establishment of a secondary tumor. Metastasis is frequently associated with chemoresistance and is the major cause of cancer-related mortality. Metastatic cells need to acquire the ability to resist to stresses provided by different environments, such as reactive oxygen species, shear stress, hemodynamic forces, stromal composition, and immune responses, to colonize other tissues. Hence, only a small population of cells has a metastasis-initiating potential. Several studies have revealed the misregulation of transcriptional variants during cancer progression, and many splice events can be used to distinguish between normal and tumoral tissue. These variants, which are abnormally expressed in malignant cells, contribute to an adaptive response of tumor cells and the success of the metastatic cascade, promoting an anomalous cell cycle, cellular adhesion, resistance to death, cell survival, migration and invasion. Understanding the different aspects of splicing regulation and the influence of transcriptional variants that control metastatic cells is critical for the development of therapeutic strategies. In this review, we describe how transcriptional variants contribute to metastatic competence and discuss how targeting specific isoforms may be a promising therapeutic strategy.

A mouse model of the Δ133p53 isoform: roles in cancer progression and inflammation

This review paper outlines studies on the Δ122p53 mouse, a model of the human Δ133p53 isoform, together with studies in other model organisms, cell culture, and where available, clinical investigations. In general, these studies imply that, in contrast to the canonical p53 tumor suppressor, Δ133p53 family members have oncogenic capability. Δ122p53 is multi-functional, conferring survival and proliferative advantages on cells, promoting invasion, metastasis and vascularization, as does Δ133p53. Cancers with high levels of Δ133p53 often have poor prognosis. Δ122p53 mediates its effects through the JAK-STAT and RhoA-ROCK signaling pathways. We propose that Δ133p53 isoforms have evolved as inflammatory signaling molecules to deal with the consequent tissue damage of p53 activation. However, if sustained expression of the isoforms occur, pathologies may result.

Regulation of the interferon-gamma (IFN-γ) pathway by p63 and Δ133p53 isoform in different breast cancer subtypes

The TP53 family consists of three sets of transcription factor genes, TP53, TP63 and TP73, each of which expresses multiple RNA variants and protein isoforms. Of these, TP53 is mutated in 25-30% of breast cancers. How TP53 mutations affect the interaction of TP53 family members and their isoforms in breast cancer is unknown. To investigate this, 3 independent breast cancer cohorts were stratified into 4 groups based on oestrogen receptor (ER) and TP53 mutation status. Using bioinformatic methodologies, principal signalling pathways associated with the expression of TP53 family members were identified. Results show an enrichment of IFN-γ signalling associated with TP63 RNA in wild type TP53 (wtTP53), ER negative (ER-) tumours and with Δ133TP53 RNA in mutant TP53 (mTP53) ER positive (ER+) tumours. Moreover, tumours with low IFN-γ signalling were associated with significantly poorer patient outcome. The predicted changes in expression of a subset of RNAs involved in IFN-γ signalling were confirmed in vitro. Our data show that different members of the TP53 family can drive transcription of genes involved in IFN-γ signalling in different breast cancer subgroups.

High expression of the p53 isoform γ is associated with reduced progression-free survival in uterine serous carcinoma

Background

Uterine serous carcinoma (USC) is a rare but aggressive subtype of endometrial carcinoma. Large-scale comprehensive efforts have resulted in an improved molecular understanding of its pathogenesis, and the p53 pathway has been proposed as a key player and is potentially targetable. Here we attempt to further portray the p53 pathway in USC by assessing p53 isoform expression.

Methods

We applied quantitative Real-Time PCRs (RT-qPCR) for expression analyses of total p53 mRNA as well as quantitative distinction of p53β, p53γ, and the total mRNA of amino-terminal truncated Δ40p53 and Δ133p53 in a retrospective cohort of 37 patients with USC. TP53 mutation status was assessed by targeted massive parallel sequencing. Findings were correlated with clinical data.

Results

The p53 isoform expression landscape in USCs was heterogeneous and dominated by total Δ133p53, while the distinct p53β and p53γ variants were found at much lower levels. The isoform expression profiles varied between samples, while their expression was independent of TP53 mutation status. We found high relative p53γ expression to be associated with reduced progression-free survival (PFS).

Conclusions

This is the first indication that elevated p53γ expression is associated with reduced PFS in USC. This single-center study may offer some insight in the landscape of p53 isoform expression in USC, but further validation studies are crucial to understand the context-dependent and tissue-specific role of the p53 isoform network in gynecological cancer.

Electronic supplementary material

The online version of this article (10.1186/s12885-018-4591-3) contains supplementary material, which is available to authorized users.

∆133p53 isoform promotes tumour invasion and metastasis via interleukin-6 activation of JAK-STAT and RhoA-ROCK signalling

∆122p53 mice (a model of ∆133p53 isoform) are tumour-prone, have extensive inflammation and elevated serum IL-6. To investigate the role of IL-6 we crossed ∆122p53 mice with IL-6 null mice. Here we show that loss of IL-6 reduced JAK-STAT signalling, tumour incidence and metastasis. We also show that ∆122p53 activates RhoA-ROCK signalling leading to tumour cell invasion, which is IL-6-dependent and can be reduced by inhibition of JAK-STAT and RhoA-ROCK pathways. Similarly, we show that Δ133p53 activates these pathways, resulting in invasive and migratory phenotypes in colorectal cancer cells. Gene expression analysis of colorectal tumours showed enrichment of GPCR signalling associated with ∆133TP53 mRNA. Patients with elevated ∆133TP53 mRNA levels had a shorter disease-free survival. Our results suggest that ∆133p53 promotes tumour invasion by activation of the JAK-STAT and RhoA-ROCK pathways, and that patients whose tumours have high ∆133TP53 may benefit from therapies targeting these pathways.

Aberrant expression of the Δ133p53 isoform is linked to many cancers. Here, the authors utilise a model of the Δ133p53 isoform that is prone to tumours and inflammation, showing that Δ133p53 promotes tumour cell invasion by activation of the JAK-STAT and RhoA-ROCK pathways in an IL-6 dependent manner.

Identification of nonsense-mediated mRNA decay pathway as a critical regulator of p53 isoform β

Human TP53 gene encodes the tumor suppressor p53 and, via alternative splicing, the p53β and γ isoforms. Numerous studies have shown that p53β/γ can modulate p53 functions and are critically involved in regulation of cellular response to stress conditions. However, it is not fully understood how the β and γ isoforms are regulated following splicing. Using gene targeting and RNAi, we showed that depletion of the nonsense-mediated mRNA decay (NMD) factor SMG7 or UPF1 significantly induced p53β but had minimal effect on p53γ. Sequence analysis reveals the presence of unique features – key hallmarks of NMD targets in the p53β transcript, which was further confirmed in NMD reporter gene assays. By manipulating splicing components, we found that NMD activities are crucial to control p53β levels under conditions that favor its splicing. Our data demonstrate that the NMD and alternative splicing pathways regulate p53β in a synergistic manner, and NMD plays a critical role in the determination of the p53β following its splicing. As aberrant p53β expression and dysfunctional NMD are both implicated in cancers, our studies may provide a novel insight into the regulation of p53β in tumorigenic settings.

The p53 isoform delta133p53ß regulates cancer cell apoptosis in a RhoB-dependent manner

The TP53 gene plays essential roles in cancer. Conventionally, wild type (WT) p53 is thought to prevent cancer development and metastasis formation, while mutant p53 has transforming abilities. However, clinical studies failed to establish p53 mutation status as an unequivocal predictive or prognostic factor of cancer progression. The recent discovery of p53 isoforms that can differentially regulate cell cycle arrest and apoptosis suggests that their expression, rather than p53 mutations, could be a more clinically relevant biomarker in patients with cancer. In this study, we show that the p53 isoform delta133p53ß is involved in regulating the apoptotic response in colorectal cancer cell lines. We first demonstrate delta133p53ß association with the small GTPase RhoB, a well-described anti-apoptotic protein. We then show that, by inhibiting RhoB activity, delta133p53ß protects cells from camptothecin-induced apoptosis. Moreover, we found that high delta133p53 mRNA expression levels are correlated with higher risk of recurrence in a series of patients with locally advanced rectal cancer (n = 36). Our findings describe how a WT TP53 isoform can act as an oncogene and add a new layer to the already complex p53 signaling network.