p53 is best known as a tumour suppressor, although recent studies have challenged the view that this is its only role. Instead, p53 has important functions in organismal development, and might contribute to a number of diseases other than cancer.

p53 in health and disease.

In the past fifteen years, it has become apparent that tumour-associated p53 mutations can provoke activities that are different to those resulting from simply loss of wild-type tumour-suppressing p53 function. Many of these mutant p53 proteins acquire oncogenic properties that enable them to promote invasion, metastasis, proliferation and cell survival. Here we highlight some of the emerging molecular mechanisms through which mutant p53 proteins can exert these oncogenic functions.

p53 mutations in cancer

https://www.cell.com/cancer-cell/pdf/S1535-6108(14)00037-3.pdf

Mutant p53 in Cancer: New Functions and Therapeutic Opportunities

Ample data indicate that mutant p53 proteins not only lose their tumour suppressive functions, but also gain new abilities that promote tumorigenesis. Moreover, recent studies have modified our view of mutant p53 proteins, portraying them not as inert mutants, but rather as regulated proteins that influence the cancer cell transcriptome and phenotype. This influence is clinically manifested as association of TP53 mutations with poor prognosis and drug resistance in a growing array of malignancies. Here, we review recent studies on mutant p53 regulation, gain-of-function mechanisms, transcriptional effects and prognostic association, with a focus on the clinical implications of these findings.

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When mutants gain new powers: news from the mutant p53 field

There is now strong evidence that mutation not only abrogates p53 tumor-suppressive functions, but in some instances can also endow mutant proteins with novel activities. Such neomorphic p53 proteins are capable of dramatically altering tumor cell behavior, primarily through their interactions with other cellular proteins and regulation of cancer cell transcriptional programs. Different missense mutations in p53 may confer unique activities and thereby offer insight into the mutagenic events that drive tumor progression. Here we review mechanisms by which mutant p53 exerts its cellular effects, with a particular focus on the burgeoning mutant p53 transcriptome, and discuss the biological and clinical consequences of mutant p53 gain of function.

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Mutant p53: one name, many proteins

An accurate, nonsurgical diagnostic test for brain tumors is currently unavailable, and the methods of monitoring disease progression are not fully reliable. MicroRNA profiling of biological fluids has recently emerged as a diagnostic tool for several pathologic conditions. Here we tested whether microRNA profiling of cerebrospinal fluid (CSF) enables detection of glioblastoma, discrimination between glioblastoma and metastatic brain tumors, and reflects disease activity. We determined CSF levels of several cancer-associated microRNAs for 118 patients diagnosed with different types of brain cancers and nonneoplastic neuropathologies by quantitative reverse transcription PCR analysis. The levels of miR-10b and miR-21 are found significantly increased in the CSF of patients with glioblastoma and brain metastasis of breast and lung cancer, compared with tumors in remission and a variety of nonneoplastic conditions. Members of the miR-200 family are highly elevated in the CSF of patients with brain metastases but not with any other pathologic conditions, allowing discrimination between glioblastoma and metastatic brain tumors. Quantification of as few as 7 microRNAs in CSF enables differential recognition of glioblastoma and metastatic brain cancer using computational machine learning tools (Support Vector Machine) with high accuracy (91%‐99%) on a test set of samples. Furthermore, we show that disease activity and treatment response can be monitored by longitudinal microRNA profiles in the CSF of glioblastoma and non‐small cell lung carcinoma patients. This study demonstrates that microRNA-based detection of brain malignancies can be reliably performed and that microRNAs in CSF can serve as biomarkers of treatment response in brain cancers.

/pdf/micrornas-in-cerebrospinal-fluid-identify-glioblastoma-and-4wbd1fwvx7.pdf

MicroRNAs in cerebrospinal fluid identify glioblastoma and metastatic brain cancers and reflect disease activity.

The dominant oncogenic properties of mutant p53 have been recognized as a phenomenon associated with tumor progression a long time ago, even before it was realized that the major function of wild type p53 is that of a tumor suppressor. Recent advances in this fascinating area in tumor cell biology reveal that the community of mutant p53 proteins is comprised of proteins that are extremely diverse both structurally and functionally, and elicit a multitude of cellular responses that not only are entirely distinct from those mediated by wild type p53, but also vary among different mutant p53 proteins. Aberrant regulation of transcription is one of the mechanisms underlying the ability of some mutant p53 proteins to act as oncogenic factors. Systematic analyses of the transcriptional activities of mutant p53 suggest that not the loss of transcriptional activity as such, but alterations of target DNA selectivity may be the driving force of mutant p53 specific transcription underlying the growth-promoting effects of mutant p53. This article focuses on mechanistic aspects of mutp53 "gain-of-function" with the emphasis on possible mechanisms underlying transcriptional activation by mutp53.

Transcriptional activities of mutant p53: When mutations are more than a loss

Despite the loss of sequence-specific DNA binding, mutant p53 (mutp53) proteins can induce or repress transcription of mutp53-specific target genes. To date, the molecular basis for transcriptional modulation by mutp53 is not understood, but increasing evidence points to the possibility that specific interactions of mutp53 with DNA play an important role. So far, the lack of a common denominator for mutp53 DNA binding, i.e. the existence of common sequence elements, has hampered further characterization of mutp53 DNA binding. Emanating from our previous discovery that DNA structure is an important determinant of wild-type p53 (wtp53) DNA binding, we analyzed the binding of various mutp53 proteins to oligonucleotides mimicking non-B DNA structures. Using various DNA-binding assays we show that mutp53 proteins bind selectively and with high affinity to non-B DNA. In contrast to sequence-specific and DNA structure-dependent binding of wtp53, mutp53 DNA binding to non-B DNA is solely dependent on the stereo-specific configuration of the DNA, and not on DNA sequence. We propose that DNA structure-selective binding of mutp53 proteins is the basis for the well-documented interaction of mutp53 with MAR elements and for transcriptional activities mediates by mutp53.

Mutant p53 proteins bind DNA in a DNA structure-selective mode.

Cancer formation and progression is a complex process determined by several mechanisms that promote cell growth, invasiveness, neo-angiogenesis, and render neoplastic cells resistant to apoptosis. The tumor suppressor p53 and the proto-oncogenic factor ets-1 are important regulators of such mechanisms. While it is well established that p53 and ets-1 influence various aspects of cell behavior by regulating the transcription of specific genes, little is known about the functional relationship between these transcription factors. We found that the gene encoding thromboxane synthase (TXSA), which we recently identified as a factor promoting invasion and resistance to apoptosis in gliomas, is a novel target gene for both p53 and ets-1. We demonstrate that p53 and ets-1 coregulate TXSA in an antagonistic and inter-related manner, with ets-1 being a potent transcriptional activator and p53 inhibiting ets-1-dependent transcription. Negative interference with ets-1 transcription requires functional p53 and is lost in mutant p53 proteins. We show that ets-1 and p53 associate physically in vitro and in vivo and that their interaction, rather than a direct binding of p53 to the TXSA promoter, is required for transcriptional repression of TXSA by wild-type p53. An important implication of our findings is that the loss of p53-mediated negative control over ets-1-dependent transcription may lead to the acquisition of an invasive phenotype in tumor cells.

/pdf/tumor-suppressor-p53-inhibits-transcriptional-activation-of-5f3r3lsdvr.pdf

Ella Kim

Papers

MicroRNAs in cerebrospinal fluid identify glioblastoma and metastatic brain cancers and reflect disease activity.

Transcriptional activities of mutant p53: When mutations are more than a loss

Mutant p53 proteins bind DNA in a DNA structure-selective mode.

Tumor suppressor p53 inhibits transcriptional activation of invasion gene thromboxane synthase mediated by the proto-oncogenic factor ets-1