TP53 drives invasion through expression of its Δ133p53ß variant.

TP53 is conventionally thought to prevent cancer formation and progression to metastasis, while mutant TP53 has transforming activities. However, in the clinic, TP53 mutation status does not accurately predict cancer progression. Here we report, based on clinical analysis corroborated with experimental data, that the p53 isoform Δ133p53ß promotes cancer cell invasion, regardless of TP53 mutation status. Δ133p53ß increases risk of cancer recurrence and death in breast cancer patients. Furthermore Δ133p53ß is critical to define invasiveness in a panel of breast and colon cell lines, expressing WT or mutant TP53. Endogenous mutant Δ133p53ß depletion prevents invasiveness without affecting mutant full-length p53 protein expression. Mechanistically WT and mutant Δ133p53ß induces EMT. Our findings provide explanations to 2 long-lasting and important clinical conundrums: how WT TP53 can promote cancer cell invasion and reciprocally why mutant TP53 gene does not systematically induce cancer progression.

Detecting and quantifying p53 isoforms at mRNA level in cell lines and tissues.

The TP53 gene expresses at least nine different mRNA variants (p53 isoform mRNAs), including the one encoding the canonical p53 tumor suppressor protein. We have developed scientific tools to specifically detect and quantify p53 isoform expression at mRNA level by nested RT-PCR (reverse transcription-polymerase chain reaction) and quantitative real-time RT-PCR (RT-qPCR using the TaqMan(®) chemistry). Here, we describe these two methods, while highlighting essential points with regard to the analysis of p53 isoform mRNA expression.

Detecting p53 isoforms at protein level.

The human p53 protein isoforms are expressed in several cell lines and modulate p53 tumor suppressor -activity, mainly through modulation of gene expression (1-4). Thus, identifying the pattern of p53 isoforms expression in cell lines is a key step for future studies of the p53 network (5). At the moment, the detection of p53 protein isoforms is based on the use of a panel of antibodies allowing their identification by comparing their molecular weights and their detection pattern by different antibodies (6). Here, classical protocols supplemented with technical know-how are described to detect p53 protein isoforms at protein level by Western blotting and immunoprecipitation. Furthermore, a simple method to study the impact of p53 protein isoforms on p53 transcriptional activity through luciferase reporter gene assays is provided.

Uncovering the role of p53 splice variants in human malignancy: a clinical perspective.

Thirty-five years of research on p53 gave rise to more than 68,000 articles and reviews, but did not allow the uncovering of all the mysteries that this major tumor suppressor holds. How p53 handles the different signals to decide the appropriate cell fate in response to a stress and its implication in tumorigenesis and cancer progression remains unclear. Nevertheless, the uncovering of p53 isoforms has opened new perspectives in the cancer research field. Indeed, the human TP53 gene encodes not only one but at least twelve p53 protein isoforms, which are produced in normal tissues through alternative initiation of translation, usage of alternative promoters, and alternative splicing. In recent years, it became obvious that the different p53 isoforms play an important role in regulating cell fate in response to different stresses in normal cells by differentially regulating gene expression. In cancer cells, abnormal expression of p53 isoforms contributes actively to cancer formation and progression, regardless of TP53 mutation status. They can also be associated with response to treatment, depending on the cell context. The determination of p53 isoform expression and p53 mutation status helps to define different subtypes within a particular cancer type, which would have different responses to treatment. Thus, the understanding of the regulation of p53 isoform expression and their biological activities in relation to the cellular context would constitute an important step toward the improvement of the diagnostic, prognostic, and predictive values of p53 in cancer treatment. This review aims to summarize the involvement of p53 isoforms in cancer and to highlight novel potential therapeutic targets.

p53 Isoforms: An Intracellular Microprocessor?

Normal function of the p53 pathway is ubiquitously lost in cancers either through mutation or inactivating interaction with viral or cellular proteins. However, it is difficult in clinical studies to link p53 mutation status to cancer treatment and clinical outcome, suggesting that the p53 pathway is not fully understood. We have recently reported that the human p53 gene expresses not only 1 but 12 different p53 proteins (isoforms) due to alternative splicing, alternative initiation of translation, and alternative promoter usage. p53 isoform proteins thus contain distinct protein domains. They are expressed in normal human tissues but are abnormally expressed in a wide range of cancer types. We have recently reported that p53 isoform expression is associated with breast cancer prognosis, suggesting that they play a role in carcinogenesis. Indeed, the cellular response to damages can be switched from cell cycle arrest to apoptosis by only manipulating p53 isoform expression. This may provide an explanation to the hitherto inconsistent relationship between p53 mutation, treatment response, and outcome in breast cancer. However, the molecular mechanism is still unknown. Recent reports suggest that it involves modulation of gene expression in a p53-dependent and -independent manner. In this review, we summarize our current knowledge about the biological activities of p53 isoforms and propose a molecular mechanism conciliating our current knowledge on p53 and integrating p63 and p73 isoforms in the p53 pathway.

p53 mutant breast cancer patients expressing p53γ have as good a prognosis as wild-type p53 breast cancer patients.

INTRODUCTION: Normal function of the p53 network is lost in most cancers, often through p53 mutation. The clinical impact of p53 mutations in breast cancer remains uncertain, especially where p53 isoforms may modify the effects of these p53 mutations. METHODS: Expression of p53ß and p53γ isoforms, the isoforms identified in normal breast tissue, was detected by reverse transcription polymerase chain reaction from a cohort of 127 primary breast tumours. Expression of p53ß and p53γ isoforms was analysed in relation to clinical markers and clinical outcomes (5 years) by binary logistic regression, Cox proportional hazards regression and Kaplan-Meier survival analyses. RESULTS: p53ß and p53γ were not randomly expressed in breast cancer. p53ß was associated with tumour oestrogen receptor (ER) expression, and p53γ was associated with mutation of the p53 gene. The patient group with the mutant p53 breast tumour-expressing p53γ isoform had low cancer recurrence and an overall survival as good as that of patients with wild-type p53 breast cancer. Conversely, patients expressing only mutant p53, without p53γ isoform expression, had a particularly poor prognosis. CONCLUSIONS: The determination of p53γ expression may allow the identification, independently of the ER status, of two subpopulations of mutant p53 breast cancer patients, one expressing p53γ with a prognosis as good as the wild-type p53 breast cancer patients and a second one not expressing p53γ with a particularly poor prognosis. The p53γ isoform may provide an explanation of the hitherto inconsistent relationship between p53 mutation, treatment response and outcome in breast cancer.

The isoforms of the p53 protein.

p53 is a transcription factor with a key role in the maintenance of genetic stability and therefore preventing cancer formation. It belongs to a family of genes composed of p53, p63, and p73. The p63 and p73 genes have a dual gene structure with an internal promoter in intron-3 and together with alternative splicing, can express 6 and 29 mRNA variants, respectively. Such a complex expression pattern had not been previously described for the p53 gene, which was not consistent with our understanding of the evolution of the p53 gene family. Consequently, we revisited the human p53 gene structure and established that it encodes nine different p53 protein isoforms because of alternative splicing, alternative promoter usage, and alternative initiation sites of translation. Therefore, the human p53 gene family (p53, p63, and p73) has a dual gene structure. We determined that the dual gene structure is conserved in Drosophila and in zebrafish p53 genes. The conservation through evolution of the dual gene structure suggests that the p53 isoforms play an important role in p53 tumor-suppressor activity. We and others have established that the p53 isoforms can regulate cell-fate outcome in response to stress, by modulating p53 transcriptional activity in a promoter and stress-dependent manner. We have also shown that the p53 isoforms are abnormally expressed in several types of human cancers, suggesting that they play an important role in cancer formation. The determination of p53 isoforms' expression may help to link clinical outcome to p53 status and to improve cancer patient treatment.

Fibroblast growth factor 1 induced during myogenesis by a transcription-translation coupling mechanism.

Fibroblast growth factor 1 (FGF1) is involved in muscle development and regeneration. The FGF1 gene contains four tissue-specific promoters allowing synthesis of four transcripts with distinct leader regions. Two of these transcripts contain internal ribosome entry sites (IRESs), which are RNA elements allowing mRNA translation to occur in conditions of blockade of the classical cap-dependent mechanism. Here, we investigated the function and the regulation of FGF1 during muscle differentiation and regeneration. Our data show that FGF1 protein expression is induced in differentiating myoblasts and regenerating mouse muscle, whereas siRNA knock-down demonstrated FGF1 requirement for myoblast differentiation. FGF1 induction occurred at both transcriptional and translational levels, involving specific activation of both promoter A and IRES A, whereas global cap-dependent translation was inhibited. Furthermore, we identified, in the FGF1 promoter A distal region, a cis-acting element able to activate the IRES A-driven translation. These data revealed a mechanism of molecular coupling of mRNA transcription and translation, involving a unique process of IRES activation by a promoter element. The crucial role of FGF1 in myoblast differentiation provides physiological relevance to this novel mechanism. This finding also provides a new insight into the molecular mechanisms linking different levels of gene expression regulation.