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

Understanding wild-type and mutant p53 activities in human cancer: new landmarks on the way to targeted therapies

Three decades of p53 research have led to many advances in understanding the basic biology of normal and cancer cells. Nonetheless, the detailed functions of p53 in normal cells, and even more so in cancer cells, remain obscure. A major breakthrough is the realization that mutant p53 has a life of its own: it contributes to cancer not only through loss of activity, but also through gain of specific 'mutant functions'. This new focus on mutant p53 is the rationale behind the meeting series dedicated to advances on mutant p53 biology. This review provides an overview of results presented at the Fourth International Workshop on Mutant p53, held in Akko, Israel in March 2009. New roles and functions of p53 relevant for tumor suppressions were presented, including the regulation of microRNAs networks, the modulation of cell-stroma interactions and the induction of senescence. A main focus of the meeting was the rapidly growing body of knowledge on autonomous properties of mutant p53 and on their oncogenic 'gain of function' impact. Importantly, the meeting highlighted that, 30 years after p53 discovery, research on mutant p53 is entering the clinical and translational era. Two major steps forward in this respect are a better understanding of the active mechanism of small drugs targeting mutant p53 in tumor cells and an improved definition of the prognostic and predictive value of mutant p53 in human cancer.

NFκB/p53 crosstalk-a promising new therapeutic target

The transcription factors p53 and NFκB determine cellular fate and are involved in the pathogenesis of most-if not all-cancers. The crosstalk between these transcription factors becomes increasingly appreciated as an important mechanism operative during all stages of tumorigenesis, metastasis, and immunological surveillance. In this review, we summarize molecular mechanisms regulating cross-signaling between p53 and NFκB proteins and how dysregulated interactions between p53 and NFκB family members contribute to oncogenesis. We furthermore analyze how such signaling modules represent targets for the design of novel intervention strategies using established compounds and powerful combination therapies.

Transcription factor NF-Y induces apoptosis in cells expressing wild-type p53 through E2F1 upregulation and p53 activation

The CCAAT-binding transcription factor NF-Y plays a central role in regulating cellular proliferation by controlling the expression of genes required for cell-cycle progression such as cyclin A, cyclin B1, cyclin B2, cdc25A, cdc25C, and cdk1. Here we show that unrestricted NF-Y activity leads to apoptosis in an E2F1- and wild-type p53 (wtp53)-dependent manner. Unrestricted NF-Y activity induced an increase in E2F1 mRNA and protein levels. Furthermore, NF-Y directly bound the E2F1 promoter and this correlated with the appearance of open chromatin marks. The ability of NF-Y to induce apoptosis was impaired in cells lacking E2F1 and wtp53. Moreover, NF-Y overexpression elicited phosphorylation of wt p53Ser18 in an E2F1-dependent manner. Our findings establish that NF-Y acts upstream of E2F1 in p53-mediated apoptosis.

p53 status in stromal fibroblasts modulates tumor growth in an SDF1-dependent manner

The p53 tumor suppressor exerts a variety of cell-autonomous effects that are aimed to thwart tumor development. In addition, however, there is growing evidence for cell nonautonomous tumor suppressor effects of p53. In the present study, we investigated the impact of stromal p53 on tumor growth. Specifically, we found that ablation of p53 in fibroblasts enabled them to promote more efficiently the growth of tumors initiated by PC3 prostate cancer-derived cells. This stimulatory effect was dependent on the increased expression of the chemokine SDF-1 in the p53-deficient fibroblasts. Notably, fibroblasts harboring mutant p53 protein were more effective than p53-null fibroblasts in promoting tumor growth. The presence of either p53-null or p53-mutant fibroblasts led also to a markedly elevated rate of metastatic spread of the PC3 tumors. These findings implicate p53 in a cell nonautonomous tumor suppressor role within stromal fibroblasts, through suppressing the production of tumor stimulatory factors by these cells. Moreover, expression of mutant p53 by tumor stroma fibroblasts might exert a gain of function effect, further accelerating tumor development.

Restoration of DNA-binding and growth-suppressive activity of mutant forms of p53 via a PCAF-mediated acetylation pathway

Tumor-derived mutant forms of p53 compromise its DNA binding, transcriptional, and growth regulatory activity in a manner that is dependent upon the cell-type and the type of mutation. Given the high frequency of p53 mutations in human tumors, reactivation of the p53 pathway has been widely proposed as beneficial for cancer therapy. In support of this possibility p53 mutants possess a certain degree of conformational flexibility that allows for re-induction of function by a number of structurally different artificial compounds or by short peptides. This raises the question of whether physiological pathways for p53 mutant reactivation also exist and can be exploited therapeutically. The activity of wild-type p53 is modulated by various acetyl-transferases and deacetylases, but whether acetylation influences signaling by p53 mutant is still unknown. Here, we show that the PCAF acetyl-transferase is down-regulated in tumors harboring p53 mutants, where its re-expression leads to p53 acetylation and to cell death. Furthermore, acetylation restores the DNA-binding ability of p53 mutants in vitro and expression of PCAF, or treatment with deacetylase inhibitors, promotes their binding to p53-regulated promoters and transcriptional activity in vivo. These data suggest that PCAF-mediated acetylation rescues activity of at least a set of p53 mutations. Therefore, we propose that dis-regulation of PCAF activity is a pre-requisite for p53 mutant loss of function and for the oncogenic potential acquired by neoplastic cells expressing these proteins. Our findings offer a new rationale for therapeutic targeting of PCAF activity in tumors harboring oncogenic versions of p53.

You can win by losing: p53 mutations in rhabdomyosarcomas

Rhabdomyosarcomas are soft tissue sarcomas of mesenchymal origin. Unlike rhabdomyosarcomas observed in paediatric patients which typically respond well to chemotherapeutic treatment, adults generally present with pleomorphic rhabdomyosarcomas that are typically associated with poor prognosis. Therefore, understanding the molecular biology that gives rise to pleomorphic rhabdomyosarcomas is critical. In this issue of The Journal of Pathology, Doyle and colleagues have generated elegant tissue-specific Cre/loxP-dependent mouse models that mimic pleomorphic rhabdomyosarcoma development in humans. In this report, the authors employed KRas(G12V)-expressing mouse models that concomitantly either express mutant p53 (p53R172H) or have deleted the p53 gene. Mice that express mutant p53 have decreased survival with development of aggressive metastases as compared to mice that have simply lost wild-type p53. The data presented herein provide the first in vivo evidence that in rhabdomyosarcomas, expression of mutant p53 results in a more aggressive p53R172H-dependent gain-of-function phenotype.

Clinical outcomes and correlates of TP53 mutations and cancer

The initial observation that p53 accumulation might serve as a surrogate biomarker for TP53 mutation has been the cornerstone for vast translational efforts aimed at validating its clinical use for the diagnosis, prognosis, and treatment of cancer. Early on, it was realized that accurate evaluation of p53 status and function could not be achieved through protein-expression analysis only. As our understanding of the p53 pathway has evolved and more sophisticated methods for assessment of p53 functional integrity have become available, the clinical and molecular epidemiological implications of p53 abnormalities in cancers are being revealed. They include diagnostic testing for germline p53 mutations, and the assessment of selected p53 mutations as biomarkers of carcinogen exposure and cancer risk and prognosis. Here, we describe the strengths and limitations of the most frequently used techniques for determination of p53 status in tumors, as well as the most remarkable latest findings relating to its clinical and epidemiological value.

Mutant p53 gain-of-function in cancer

In its wild-type form, p53 is a major tumor suppressor whose function is critical for protection against cancer. Many human tumors carry missense mutations in the TP53 gene, encoding p53. Typically, the affected tumor cells accumulate excessive amounts of the mutant p53 protein. Various lines of evidence indicate that, in addition to abrogating the tumor suppressor functions of wild-type p53, the common types of cancer-associated p53 mutations also endow the mutant protein with new activities that can contribute actively to various stages of tumor progression and to increased resistance to anticancer treatments. Collectively, these activities are referred to as mutant p53 gain-of-function. This article addresses the biological manifestations of mutant p53 gain-of-function, the underlying molecular mechanisms, and their possible clinical implications.

RNA interference targeting mutant p53 inhibits growth and induces apoptosis in DU145 human prostate cancer cells

p53 is the most frequently mutated tumor suppressor gene in human cancer. Recent studies have indicated that p53 mutants not only lose tumor suppression activity but also gain novel oncogenic functions that contribute to tumor malignancy. In this study, we explored mutant p53 as a target for novel anti-cancer treatment in prostate cancer. Using the DU145 human androgen-independent prostate cancer cell line, we show that silencing of mutant p53 gene by RNA interference led to significant inhibition of cell viability and growth, which was associated with cell cycle arrest at G1 and G2/M phase, and ultimately induced massive apoptosis. Mechanistically, p53-siRNA inhibited phosphatidylinositol 3'-kinase/Akt signaling pathway, which might be responsible for the reduced proliferation and apoptosis induction. These findings suggest that RNA interference targeting mutant p53 may serve as a novel therapeutic strategy for the treatment of androgen-independent prostate cancer.

Single-nucleotide polymorphisms in p53 pathway and aggressiveness of prostate cancer in a Caucasian population

PURPOSE

The tumor suppressor p53 plays a crucial role in maintaining genomic stability and tumor prevention. Mdm2, Mdm4, and Hausp are all critical regulators of the p53 protein. Despite the importance of the p53 pathway in prostate cancer development and progression, little is known about the association of functional single-nucleotide polymorphisms (SNP) in the p53 pathway genes and prostate cancer aggressiveness.

EXPERIMENTAL DESIGN

In this study, we analyze the association of SNPs in p53, Mdm2, Mdm4, and Hausp genes with prostate cancer clinicopathologic variables in a large hospital-based Caucasian prostate cancer cohort (N = 4,073).

RESULTS

We found that the Mdm2 SNP309 T allele was associated with earlier onset prostate cancer (P = 0.004), higher Gleason scores (P = 0.004), and higher stages in men undergoing a radical prostatectomy (P = 0.011). Both the Mdm4 and Hausp SNPs (rs1380576 and rs1529916) were found to be associated with higher D'Amico risk prostate cancer category at the time of diagnosis (P = 0.023 and P = 0.046, respectively). Mdm4 SNP was also found to be associated with higher Gleason score at radical prostatectomy (P = 0.047). We did not observe any statistically significant association between the p53 Arg72Pro polymorphism and prostate cancer aggressiveness or pathologic variables.

CONCLUSIONS

These results suggested the importance of these p53 regulators in prostate cancer development and progression.

Degradation of p53 by human Alphapapillomavirus E6 proteins shows a stronger correlation with phylogeny than oncogenicity

Background

Human Papillomavirus (HPV) E6 induced p53 degradation is thought to be an essential activity by which high-risk human Alphapapillomaviruses (alpha-HPVs) contribute to cervical cancer development. However, most of our understanding is derived from the comparison of HPV16 and HPV11. These two viruses are relatively distinct viruses, making the extrapolation of these results difficult. In the present study, we expand the tested strains (types) to include members of all known HPV species groups within the Alphapapillomavirus genus.

Principal Findings

We report the biochemical activity of E6 proteins from 27 HPV types representing all alpha-HPV species groups to degrade p53 in human cells. Expression of E6 from all HPV types epidemiologically classified as group 1 carcinogens significantly reduced p53 levels. However, several types not associated with cancer (e.g., HPV53, HPV70 and HPV71) were equally active in degrading p53. HPV types within species groups alpha 5, 6, 7, 9 and 11 share a most recent common ancestor (MRCA) and all contain E6 ORFs that degrade p53. A unique exception, HPV71 E6 ORF that degraded p53 was outside this clade and is one of the most prevalent HPV types infecting the cervix in a population-based study of 10,000 women. Alignment of E6 ORFs identified an amino acid site that was highly correlated with the biochemical ability to degrade p53. Alteration of this amino acid in HPV71 E6 abrogated its ability to degrade p53, while alteration of this site in HPV71-related HPV90 and HPV106 E6s enhanced their capacity to degrade p53.

Conclusions

These data suggest that the alpha-HPV E6 proteins' ability to degrade p53 is an evolved phenotype inherited from a most recent common ancestor of the high-risk species that does not always segregate with carcinogenicity. In addition, we identified an amino-acid residue strongly correlated with viral p53 degrading potential.

Oncogenes and tumor suppressor genes

Breast cancer progression involves multiple genetic events, which can activate dominant-acting oncogenes and disrupt the function of specific tumor suppressor genes. This article describes several key oncogene and tumor suppressor signaling networks that have been implicated in breast cancer progression. Among the tumor suppressors, the article emphasizes BRCA1/2 and p53 tumor suppressors. In addition to these well characterized tumor suppressors, the article highlights the importance of PTEN tumor suppressor in counteracting PI3K signaling from activated oncogenes such as ErbB2. This article discusses the use of mouse models of human breast that recapitulate the key genetic events involved in the initiation and progression of breast cancer. Finally, the therapeutic potential of targeting these key tumor suppressor and oncogene signaling networks is discussed.

Mutant p53 facilitates somatic cell reprogramming and augments the malignant potential of reprogrammed cells

p53 deficiency enhances the efficiency of somatic cell reprogramming to a pluripotent state. As p53 is usually mutated in human tumors and many mutated forms of p53 gain novel activities, we studied the influence of mutant p53 (mut-p53) on somatic cell reprogramming. Our data indicate a novel gain of function (GOF) property for mut-p53, which markedly enhanced the efficiency of the reprogramming process compared with p53 deficiency. Importantly, this novel activity of mut-p53 induced alterations in the characteristics of the reprogrammed cells. Although p53 knockout (KO) cells reprogrammed with only Oct4 and Sox2 maintained their pluripotent capacity in vivo, reprogrammed cells expressing mutant p53 lost this capability and gave rise to malignant tumors. This novel GOF of mut-p53 is not attributed to its effect on proliferation, as both p53 KO and mut-p53 cells displayed similar proliferation rates. In addition, we demonstrate an oncogenic activity of Klf4, as its overexpression in either p53 KO or mut-p53 cells induced aggressive tumors. Overall, our data show that reprogrammed cells with the capacity to differentiate into the three germ layers in vitro can form malignant tumors, suggesting that in genetically unstable cells, such as those in which p53 is mutated, reprogramming may result in the generation of cells with malignant tumor-forming potential.

Binding of pro-prion to filamin A: by design or an unfortunate blunder

Over the last decades, cancer research has focused on tumor suppressor genes and oncogenes. Genes in other cellular pathways has received less attention. Between 0.5% to 1% of the mammalian genome encodes for proteins that are tethered on the cell membrane via a glycosylphosphatidylinositol (GPI)-anchor. The GPI modification pathway is complex and not completely understood. Prion (PrP), a GPI-anchored protein, is infamous for being the only normal protein that when misfolded can cause and transmit a deadly disease. Though widely expressed and highly conserved, little is known about the functions of PrP. Pancreatic cancer and melanoma cell lines express PrP. However, in these cell lines the PrP exists as a pro-PrP as defined by retaining its GPI anchor peptide signal sequence (GPI-PSS). Unexpectedly, the GPI-PSS of PrP has a filamin A (FLNA) binding motif and binds FLNA. FLNA is a cytolinker protein, and an integrator of cell mechanics and signaling. Binding of pro-PrP to FLNA disrupts the normal FLNA functions. Although normal pancreatic ductal cells lack PrP, about 40% of patients with pancreatic ductal cell adenocarcinoma express PrP in their cancers. These patients have significantly shorter survival time compared with patients whose cancers lack PrP. Pro-PrP is also detected in melanoma in situ but is undetectable in normal melanocyte, and invasive melanoma expresses more pro-PrP. In this review, we will discuss the underlying mechanisms by which binding of pro-PrP to FLNA disrupts normal cellular physiology and contributes to tumorigenesis, and the potential mechanisms that cause the accumulation of pro-PrP in cancer cells.

Drug discovery and mutant p53

Missense mutations in the p53 gene are commonly selected for in developing human cancer cells. These diverse mutations in p53 can inactivate its normal sequence-specific DNA-binding and transactivation function, but these mutations can also stabilize a mutant form of p53 with pro-oncogenic potential. Recent multi-disciplinary advances have demonstrated exciting and unexpected potential in therapeutically targeting the mutant p53 pathway, including: the development of biophysical models to explain how mutations inactivate p53 and strategies for refolding and reactivation of mutant p53, the ability of mutant p53 protein to escape MDM2-mediated degradation in human cancers, and the growing 'interactome' of mutant p53 that begins to explain how the mutant p53 protein can contribute to diverse oncogenic and pro-metastatic signaling. Our rapidly accumulating knowledge on mutant p53-signaling pathways will facilitate drug discovery programmes in the challenging area of protein-protein interactions and mutant protein conformational control.

Gene expression profiling of mouse p53-deficient epidermal carcinoma defines molecular determinants of human cancer malignancy

Background

The epidermal specific ablation of Trp53 gene leads to the spontaneous development of aggressive tumors in mice through a process that is accelerated by the simultaneous ablation of Rb gene. Since alterations of p53-dependent pathway are common hallmarks of aggressive, poor prognostic human cancers, these mouse models can recapitulate the molecular features of some of these human malignancies.

Results

To evaluate this possibility, gene expression microarray analysis was performed in mouse samples. The mouse tumors display increased expression of cell cycle and chromosomal instability associated genes. Remarkably, they are also enriched in human embryonic stem cell gene signatures, a characteristic feature of human aggressive tumors. Using cross-species comparison and meta-analytical approaches, we also observed that spontaneous mouse tumors display robust similarities with gene expression profiles of human tumors bearing mutated TP53, or displaying poor prognostic outcome, from multiple body tissues. We have obtained a 20-gene signature whose genes are overexpressed in mouse tumors and can identify human tumors with poor outcome from breast cancer, astrocytoma and multiple myeloma. This signature was consistently overexpressed in additional mouse tumors using microarray analysis. Two of the genes of this signature, AURKA and UBE2C, were validated in human breast and cervical cancer as potential biomarkers of malignancy.

Conclusions

Our analyses demonstrate that these mouse models are promising preclinical tools aimed to search for malignancy biomarkers and to test targeted therapies of prospective use in human aggressive tumors and/or with p53 mutation or inactivation.

Update on human polyomaviruses and cancer

Over 50 years of polyomavirus research has produced a wealth of insights into not only general biologic processes in mammalian cells, but also, how conditions can be altered and signaling systems tweaked to produce transformation phenotypes. In the past few years three new members (KIV, WUV, and MCV) have joined two previously known (JCV and BKV) human polyomaviruses. In this review, we present updated information on general virologic features of these polyomaviruses in their natural host, concentrating on the association of MCV with human Merkel cell carcinoma. We further present a discussion on advances made in SV40 as the prototypic model, which has and will continue to inform our understanding about viruses and cancer.

The p53 tumor suppressor: a master regulator of diverse cellular processes and therapeutic target in cancer

The tumor suppressor p53 has been implicated in a growing number of biological processes, including cell cycle arrest, senescence, apoptosis, autophagy, metabolism, and aging. Activation of p53 in response to oncogenic stress eliminates nascent tumor cells by apoptosis or senescence. p53 is regulated at the protein level by posttranslational modifications such as phosphorylation and acetylation. A p53 antisense gene, Wrap53, enhances p53 mRNA levels via the 5'UTR. Lack of Wrap53 transcripts that overlap with p53 abrogates the p53 DNA damage response. Around half of all human tumors carry p53 mutation that disrupt p53 specific DNA binding, and transcriptional transactivation of target genes. Reactivation of mutant p53 is a promising strategy for novel cancer therapy. The small molecule PRIMA-1 restores wild type conformation and DNA binding to mutant p53, induces mutant p53-dependent apoptosis, and inhibits tumor growth in vivo. The PRIMA-1 analog APR-246 is currently tested in a phase I clinical trial. Improved understanding of the p53 pathway should lead to better diagnosis and treatment of cancer in the future.

p53 family: Therapeutic targets in neuroblastoma

Survival rates for metastatic neuroblastoma remain poor, despite significant increase in the intensity of therapy. Although it represents approximately 7% of pediatric cancer, neuroblastoma accounts for approximately 15% of childhood cancer deaths. Thus, novel approaches to enhance neuroblastoma chemotherapy sensitivity and prevent or bypass chemoresistance are required. Disruption of the p53 pathway is a common mechanism leading to defects in apoptosis in cancer cells. Increasing evidence suggests that the p53 pathway may be inactivated in neuroblastoma. Inactivation of the p53 pathway occurs most commonly at the time of relapse, and probably contributes to chemoresistance. The p53 family proteins, p73 and p63, can also induce apoptosis, and early studies suggest that p73 may be important in neuroblastoma pathogenesis and response to treatment. This article focuses on current therapies and novel drugs targeting p53 and p73 signaling pathways in neuroblastoma. Understanding the balance between the p53 family proteins in neuroblastoma and how their expression and activity are regulated will hopefully lead to the discovery of agents that target these pathways to induce neuroblastoma cell death, alone or in combination with chemotherapies.

Effects of the TP53 p.R249S mutant on proliferation and clonogenic properties in human hepatocellular carcinoma cell lines: interaction with hepatitis B virus X protein

Aflatoxin B(1) (AFB(1)) is a risk factor for hepatocellular carcinoma (HCC) in many low-resource countries. Although its metabolites bind at several positions in TP53, a mutation at codon 249 (AGG to AGT, arginine to serine, p.R249S) accounts for 90% of TP53 mutations in AFB(1)-related HCC. This specificity suggests that p.R249S confers a selective advantage during hepatocarcinogenesis. Using HCC cell lines, we show that p.R249S has lost the capacity to bind to p53 response elements and to transactivate p53 target genes. In p53-null Hep3B cells, stable transfection of p.R249S or of another mutant, p.R248Q, did not induce significant changes in cell proliferation and survival after cytotoxic stress. In contrast, in a cell line that constitutively expresses both p.R249S and the hepatitis B virus antigen HBx (PLC/PRF/5), silencing of either p.R249S or HBx by RNA interference slowed down proliferation, with no additive effects when both factors were silenced. Furthermore, the two proteins appear to form a complex. In human HCC samples, mutation at codon 249 did not correlate with p.R249S protein accumulation or HBx truncation status. We suggest that p.R249S may contribute to hepatocarcinogenesis through interaction with HBx, conferring a subtle growth advantage at early steps of the transformation process, but that this interaction is not required for progression to advanced HCC.

SNF5, a core component of the SWI/SNF complex, is necessary for p53 expression and cell survival, in part through eIF4E

SNF5, a core component of the SWI/SNF chromatin remodeling complex, is expressed as two isoforms, SNF5a and SNF5b. SNF5 is a tumor suppressor as mutation of SNF5 leads to tumor formation and cooperates with p53 deficiency to enhance cancer susceptibility. Interestingly, lack of SNF5 inhibits cell survival and embryonic development potentially via abnormal activation of p53. To further examine this, we generated cell lines in that SNF5a, SNF5b, or both can be inducibly knocked down. We found that SNF5 knockdown leads to cell cycle arrest in G1, and SNF5a and SNF5b are functionally redundant. We also showed that SNF5 knockdown impairs p53-dependent transcription of p21 and MDM2. However, contrary to earlier reports that p53 is activated by SNF5 knockout in murine cells, SNF5 knockdown leads to decreased, but not increased, expression of both basal and stress-induced p53 in multiple human cell lines. In addition, we showed that SNF5 knockdown induces AMPK activation and inhibits eIF4E expression. Finally, we demonstrated that SNF5 knockdown inhibits p53 translation via eIF4E and replacement of eIF4E in SNF5-knockdown cells restores p53 expression and cell survival. Together, our results suggest that the p53 pathway is regulated by, and mediates the activity of, SNF5 in tumor suppression and pro-survival.

Driver mutations in TP53 are ubiquitous in high grade serous carcinoma of the ovary

Numerous studies have tested the association between TP53 mutations in ovarian cancer and prognosis but these have been consistently confounded by limitations in study design, methodology, and/or heterogeneity in the sample cohort. High-grade serous (HGS) carcinoma is the most clinically important histological subtype of ovarian cancer. As these tumours may arise from the ovary, Fallopian tube or peritoneum, they are collectively referred to as high-grade pelvic serous carcinoma (HGPSC). To identify the true prevalence of TP53 mutations in HGPSC, we sequenced exons 2-11 and intron-exon boundaries in tumour DNA from 145 patients. HGPSC cases were defined as having histological grade 2 or 3 and FIGO stage III or IV. Surprisingly, pathogenic TP53 mutations were identified in 96.7% (n = 119/123) of HGPSC cases. Molecular and pathological review of mutation-negative cases showed evidence of p53 dysfunction associated with copy number gain of MDM2 or MDM4, or indicated the exclusion of samples as being low-grade serous tumours or carcinoma of uncertain primary site. Overall, p53 dysfunction rate approached 100% of confirmed HGPSCs. No association between TP53 mutation and progression-free or overall survival was found. From this first comprehensive mapping of TP53 mutation rate in a homogeneous group of HGPSC patients, we conclude that mutant TP53 is a driver mutation in the pathogenesis of HGPSC cancers. Because TP53 mutation is almost invariably present in HGPSC, it is not of substantial prognostic or predictive significance.

ID4: a new player in the cancer arena

Id proteins (Id-1 to 4) are dominant negative regulators of basic helix-loop-helix transcription factors. They play a key role during development, preventing cell differentiation while inducing cell proliferation. They are poorly expressed in adult life but can be reactivated in tumorigenesis. Several evidences indicate that Id proteins are associated with loss of differentiation, unrestricted proliferation and neoangiogenesis in diverse human cancers. Recently, we identified Id4 as a transcriptional target of the protein complex mutant p53/E2F1/p300 in breast cancer. Id4 protein binds, stabilizes and enhances the translation of mRNAs encoding proangiogenic cytokines, such as IL8 and GRO-alpha, increasing the angiogenic potential of cancer cells. We present here an overview of the current experimental data that links Id4 to cancer. We provide evidence also of the induction of Id4 following anticancer treatments in mutant p53- carrying cells. Indeed, mutant p53 is recruited to a specific region of the Id4 promoter upon DNA damage. Our findings indicate that Id4, besides its proangiogenic role, might also participate in the chemoresistance associated to mutant p53 proteins exerting gain of function activities.

Classical and Novel Prognostic Markers for Breast Cancer and their Clinical Significance

The use of biomarkers ensures breast cancer patients receive optimal treatment. Established biomarkers such as estrogen receptor (ER) and progesterone receptor (PR) have been playing significant roles in the selection and management of patients for endocrine therapy. HER2 is a strong predictor of response to trastuzumab. Recently, the roles of ER as a negative and HER2 as a positive indicator for chemotherapy have been established. Ki67 has traditionally been recognized as a poor prognostic factor, but recent studies suggest that measurement of Ki67-positive cells during treatment will more effectively predict treatment efficacy for both anti-hormonal and chemotherapy. p53 mutations are found in 20–35% of human breast cancers and are associated with aggressive disease with poor clinical outcome when the DNA-binding domain is mutated. The utility of cyclin D1 as a predictor of breast cancer prognosis is controversial, but cyclin D1b overexpression is associated with poor prognosis. Likewise, overexpression of the low molecular weight form of cyclin E1 protein predicts poor prognosis. Breast cancers from BRCA1/2 carriers often show high nuclear grades, negativity to ER/PR/HER2, and p53 mutations, and thus, are associated with poor prognosis. The prognostic values of other molecular markers, such as p14^ARF, TBX2/3, VEGF in breast cancer are also discussed. Careful evaluation of these biomarkers with current treatment modality is required to determine whether their measurement or monitoring offer significant clinical benefits.

Measurement of tumor-associated mutations in the nasal mucosa of rats exposed to varying doses of formaldehyde

This study examined the potential induction of tumor-associated mutations in formaldehyde-exposed rat nasal mucosa using a sensitive method, allele-specific competitive blocker-PCR (ACB-PCR). Levels of p53 codon 271 CGT to CAT and K-Ras codon 12 GGT to GAT mutations were quantified in nasal mucosa of rats exposed to formaldehyde. In addition, nasal mucosa cell proliferation was monitored because regenerative cell proliferation is considered a key event in formaldehyde-induced carcinogenesis. Male F344 rats (6-7 weeks old, 5 rats/group) were exposed to 0, 0.7, 2, 6, 10, and 15 ppm formaldehyde for 13 weeks (6 h/day, 5 days/week). ACB-PCR was used to determine levels of p53 and K-Ras mutations. Although two of five untreated rats had measureable spontaneous p53 mutant fractions (MFs), most nasal mucosa samples had p53 MFs below 10(-5). All K-Ras MF measurements were below 10(-5). No dose-related increases in p53 or K-Ras MF were observed, even though significant increases in bromodeoxyuridine incorporation demonstrated induced cell proliferation in the 10 and 15 ppm formaldehyde-treatment groups. Therefore, induction of tumor-associated p53 mutation likely occurs after several other key events in formaldehyde-induced carcinogenesis.

Regulation of tumor suppressor p53 at the RNA level

p53 is a key tumor suppressor that triggers cell cycle arrest, senescence, or apoptosis in response to cellular stress. Frequent p53 mutation in human tumors allows survival, sustained growth, and tumor progression. p53 is expressed at low levels under normal conditions, due to rapid protein turnover. Stress signaling induces p53 protein stabilization through phosphorylation and other post-translational modifications. However, recent studies have demonstrated critical regulation of p53 at the mRNA level, mediated via both the 5'UTR and the 3'UTR and affecting both the stability and the translation efficiency of the p53 mRNA. Both proteins and microRNAs have been implicated in such regulation. The p53 target gene Wig-1 encodes a zinc finger protein that binds to double-stranded RNA and enhances p53 mRNA stability by binding to the 3'UTR in a positive feedback loop. Here, we shall summarize current knowledge about regulation of the p53 mRNA and discuss possible implications for cancer therapy.

Driver mutations in TP53 are ubiquitous in high grade serous carcinoma of the ovary

Numerous studies have tested the association between TP53 mutations in ovarian cancer and prognosis but these have been consistently confounded by limitations in study design, methodology, and/or heterogeneity in the sample cohort. High-grade serous (HGS) carcinoma is the most clinically important histological subtype of ovarian cancer. As these tumours may arise from the ovary, Fallopian tube or peritoneum, they are collectively referred to as high-grade pelvic serous carcinoma (HGPSC). To identify the true prevalence of TP53 mutations in HGPSC, we sequenced exons 2-11 and intron-exon boundaries in tumour DNA from 145 patients. HGPSC cases were defined as having histological grade 2 or 3 and FIGO stage III or IV. Surprisingly, pathogenic TP53 mutations were identified in 96.7% (n = 119/123) of HGPSC cases. Molecular and pathological review of mutation-negative cases showed evidence of p53 dysfunction associated with copy number gain of MDM2 or MDM4, or indicated the exclusion of samples as being low-grade serous tumours or carcinoma of uncertain primary site. Overall, p53 dysfunction rate approached 100% of confirmed HGPSCs. No association between TP53 mutation and progression-free or overall survival was found. From this first comprehensive mapping of TP53 mutation rate in a homogeneous group of HGPSC patients, we conclude that mutant TP53 is a driver mutation in the pathogenesis of HGPSC cancers. Because TP53 mutation is almost invariably present in HGPSC, it is not of substantial prognostic or predictive significance.

Driver mutations in TP53 are ubiquitous in high grade serous carcinoma of the ovary

Numerous studies have tested the association between TP53 mutations in ovarian cancer and prognosis but these have been consistently confounded by limitations in study design, methodology, and/or heterogeneity in the sample cohort. High-grade serous (HGS) carcinoma is the most clinically important histological subtype of ovarian cancer. As these tumours may arise from the ovary, Fallopian tube or peritoneum, they are collectively referred to as high-grade pelvic serous carcinoma (HGPSC). To identify the true prevalence of TP53 mutations in HGPSC, we sequenced exons 2-11 and intron-exon boundaries in tumour DNA from 145 patients. HGPSC cases were defined as having histological grade 2 or 3 and FIGO stage III or IV. Surprisingly, pathogenic TP53 mutations were identified in 96.7% (n = 119/123) of HGPSC cases. Molecular and pathological review of mutation-negative cases showed evidence of p53 dysfunction associated with copy number gain of MDM2 or MDM4, or indicated the exclusion of samples as being low-grade serous tumours or carcinoma of uncertain primary site. Overall, p53 dysfunction rate approached 100% of confirmed HGPSCs. No association between TP53 mutation and progression-free or overall survival was found. From this first comprehensive mapping of TP53 mutation rate in a homogeneous group of HGPSC patients, we conclude that mutant TP53 is a driver mutation in the pathogenesis of HGPSC cancers. Because TP53 mutation is almost invariably present in HGPSC, it is not of substantial prognostic or predictive significance.

Interference with the p53 family network contributes to the gain of oncogenic function of mutant p53 in hepatocellular carcinoma

Whereas the hallmark of wild-type p53 is its tumor suppressor activity, tumor-associated mutant p53 proteins can exert novel anti-apoptotic gain-of-function activities, which confer a selective advantage upon tumor cells harboring such mutations. We investigated the molecular mechanisms of mutant p53 gain-of-function in hepatocellular carcinoma with special emphasis on the interaction of mutant p53 gain-of-function proteins with the p53 family members p63 and p73. Mutant forms of p53, namely the hot-spot mutants p53R143A, p53R175D, p53R175H, p53R248W, and p53R273H, acquire anti-apoptotic gain-of-function in hepatocellular carcinoma by repressing the activity of genes regulating both, the extrinsic apoptosis pathway initiated by ligation of death receptors and the intrinsic/mitochondrial apoptosis pathway. In the presence of mutated p53, the CD95L-CD95 apoptotic pathway is markedly attenuated. This is due to repression of CD95 gene transcription by mutant p53. In addition, these mutants repress the expression of the Bax gene and attenuate mitochondria-mediated apoptosis signaling. Furthermore, and of clinical relevance, these gain-of-function mutants are anti-apoptotic due to their inhibitory interaction with the pro-apoptotic p53 family members TAp63 and TAp73. p53 gain-of-function mutants significantly decrease activation of pro-apoptotic target genes by wild-type p53, TAp63, and TAp73. This contributes to the ability of cancer cells to withstand DNA damage-induced apoptosis. Interference with the interaction of p53 gain-of-function mutants with TAp63 or TAp73 may thus sensitize hepatocellular carcinoma to elimination by therapy.

Estrogen levels act as a rheostat on p53 levels and modulate p53-dependent responses in breast cancer cell lines

A large proportion of breast cancers expresses the estrogen receptor alpha (ERα) and are dependent on estrogens for their proliferation and survival. The tumor suppressor TP53 encodes the p53 protein, an important mediator of the anti-proliferative and apoptotic effects of several treatments used for breast cancer. A significant proportions of breast tumors (20-30%) carry mutations in TP53 gene and these mutations are associated with poor survival and poor response to several types of chemotherapeutic treatments. While there is mounting evidence for functional interactions between p53 and ERα pathways in breast and other tissues, the impact of these interactions on response to chemotherapy and anti-hormone treatments remain largely unknown. Here, using estrogen-dependent breast cancer cell lines with different p53 status, we show that estrogens, through ERα, influence p53 protein levels and activities. Estrogen deprivation reduced, while estradiol increased p53 levels, in a time and dose-dependent manner. Both wild-type and endogenously expressed mutant p53 proteins were affected. This reduction in p53 protein levels resulted in reduced p53-dependent responses induced by DNA damage in p53 wild-type cells, lowering the capacity of doxorubicine to induce apoptosis. The p53 response appeared to be quantitatively but not qualitatively affected. These results suggest that ERα activity is required for a strong p53 response in estrogen-dependent breast cancer cells. These results are in line with previous observations that we made in a clinical series, where a larger effect of TP53 mutation status was found for patient survival in cases with progesterone receptor positive status, a marker of a functional ERα pathway. It would thus be important to further characterize the influence of ERα pathway on the predictive value of TP53 mutation status in specifically designed clinical trials, as it may open perspectives for improving breast cancer treatment.

Characterization of functional domains necessary for mutant p53 gain of function

Tumor cells, including SW480 carcinoma cells that carry a mutant p53, are addicted to the mutant for their survival and resistance to growth suppression by chemotherapeutic agents. Here, we investigated whether various classes of p53 mutants share a common property and functional domains necessary for mutant p53 gain of function. To test this, we generated SW480 cell lines in which endogenous mutant R273H/P309S can be inducibly or stably knocked down, whereas a small interfering RNA-resistant mutant p53 along with a mutated functional domain can be inducibly or stably expressed. We found that both contact-site (R248W and R273H) and conformation (G245S and R249S) mutants are able to maintain the transformed phenotypes of SW480 cells conferred by endogenous mutant p53. We also found that activation domains 1-2 and the proline-rich domain are required for mutant p53 gain of function. Interestingly, we showed that the C-terminal basic domain, which is required for wild-type p53 activity, is an inhibitory domain for mutant p53. Furthermore, we showed that deletion of the basic domain enhances, whereas a mutation in activation domains 1-2 and deletion of the proline-rich domain abolish mutant p53 to regulate Gro1 and Id2, both of which are regulated by and mediate endogenous mutant p53 gain of function. These results indicate that both conformation and contact-site mutants share a property for cell transformation, and the domains critical for wild-type p53 tumor suppression are also required for mutant p53 tumor promotion. Thus, the inhibitory basic domain and the common property for p53 mutants can be explored for targeting tumors with mutant p53.

The molecular genetic basis of ovarian cancer and its roadmap towards a better treatment

OBJECTIVES

Ovarian cancer remains a major health problem for women. Although there is considerable clinico-pathological heterogeneity, the molecular genetic basis of ovarian cancer remains poorly understood. Recently, high-resolution genomic maps generated by genome-wide SNP analyses and novel sequencing technologies, have started to dissect the genetic basis of ovarian cancer.

METHODS

Here, we will describe our first insights on how somatic mutations may contribute to the diagnostic re-classification of ovarian cancer. We will discuss how copy number alterations and epigenetic changes represent promising biomarkers to predict resistance to treatment in ovarian cancer, and will also highlight how some of the recently-discovered microRNAs might represent interesting therapeutic targets for ovarian cancer.

RESULTS AND CONCLUSIONS

Future studies, such as the Cancer Genome Atlas Project, involving a large number of ovarian tumors and combining various high-throughput genetic technologies with sophisticated integrative bioinformatic analyses, will be required and are expected to fine-map the full genetic spectrum of ovarian cancer. It is hoped, however, that once the molecular genetic basis of ovarian cancer is understood, this will lead to better and personalized treatments for ovarian cancer.

Massively regulated genes: the example of TP53

Intensive study of the TP53 gene over the last three decades has revealed a highly complex network of factors that regulate its performance. The gene has several promoters, alternative splicing occurs and there are alternative translation initiation sites. Up to 10 p53 isoforms have been identified. At the post-translational level, p53 activity depends on its quantity in the cell and on qualitative changes in its structure, intracellular localization, DNA-binding activity and interactions with other proteins. Both accumulation and activation are regulated by an intricate pattern of post-translational modifications, including phosphorylation, acetylation, ubiquitination, sumoylation, neddylation, methylation and glycosylation. The Mdm2 protein, a negative regulator of p53, is the most important determinant of p53 abundance and subcellular localization. Enzymes that post-translationally modify p53 by phosphorylation, methylation and acetylation fine-tune p53 binding to recognition sequences in DNA and p53 interactions with transcription cofactors at promoters of target genes, thereby exerting a discriminatory role in p53 function. This multitude of parameters determining expression, modification, accumulation and localization of p53 proteins may explain how a single gene can display an extensive repertoire of activities. Presumably this is needed, because the p53 protein can have such profound consequences for a cell.

The contribution of gene expression profiling to breast cancer classification, prognostication and prediction: a retrospective of the last decade

In the last decade, the development of microarrays and the ability to perform massively parallel gene expression analysis of human tumours were received with great excitement by the scientific community. The promise of microarrays was of apocalyptic dimensions, with some experts envisaging that it would be a matter of a few years for this technology to replace traditional clinicopathological markers in clinical practice and treatment decision-making. The replacement of histopathology by high-tech and more objective approaches to cancer diagnosis, prognostication and prediction was, at that time, a foregone conclusion. Ten years after the initial publications of translational research studies using microarrays, one cannot deny that this technology has changed the way breast cancer is perceived. It has brought the concept of breast cancer heterogeneity to the forefront of cancer research, and the fact that distinct subtypes of breast cancer are completely different diseases that affect the same anatomical site. Furthermore, it has led to the development of prognostic and predictive 'gene signatures', which are yet to be fully incorporated into clinical practice. Importantly, though, the prognostic and predictive power of microarrays has been shown to be complementary to, rather than a replacement for, traditional clinicopathological parameters. Here we endeavour to provide a fair and balanced assessment of what microarray-based gene expression analysis has taught us in the last decade and its contribution to breast cancer classification, prognostication and prediction.

Barrett's oesophagus and oesophageal adenocarcinoma: time for a new synthesis

The public health importance of Barrett's oesophagus lies in its association with oesophageal adenocarcinoma. The incidence of oesophageal adenocarcinoma has risen at an alarming rate over the past four decades in many regions of the Western world, and there are indications that the incidence of this disease is on the rise in Asian populations in which it has been rare. Much has been learned of host and environmental risk factors that affect the incidence of oesophageal adenocarcinoma, and data indicate that patients with Barrett's oesophagus rarely develop oesophageal adenocarcinoma. Given that 95% of oesophageal adenocarcinomas arise in individuals without a prior diagnosis of Barrett's oesophagus, what strategies can be used to reduce late diagnosis of oesophageal adenocarcinoma?

Integrins and mutant p53 on the road to metastasis

Understanding how tumor cells invade tissues is key to developing drugs to block metastasis. In this issue, Muller et al. (2009) report that a mutant form of the tumor suppressor p53 in cancer cells boosts the endocytic recycling of the adhesion molecule integrin alpha5beta1 and of epidermal growth factor receptor, promoting invasion and metastasis.

Mutant p53 drives invasion by promoting integrin recycling

p53 is a tumor suppressor protein whose function is frequently lost in cancers through missense mutations within the Tp53 gene. This results in the expression of point-mutated p53 proteins that have both lost wild-type tumor suppressor activity and show gain of functions that contribute to transformation and metastasis. Here, we show that mutant p53 expression can promote invasion, loss of directionality of migration, and metastatic behavior. These activities of p53 reflect enhanced integrin and epidermal growth factor receptor (EGFR) trafficking, which depends on Rab-coupling protein (RCP) and results in constitutive activation of EGFR/integrin signaling. We provide evidence that mutant p53 promotes cell invasion via the inhibition of TAp63, and simultaneous loss of p53 and TAp63 recapitulates the phenotype of mutant p53 in cells. These findings open the possibility that blocking alpha5/beta1-integrin and/or the EGF receptor will have therapeutic benefit in mutant p53-expressing cancers.

Environmental factors and genetic susceptibility promote urinary bladder cancer

Cancer of the urinary bladder is the second most common malignancy of the genitourinary tract, currently accounting for up to 5% of all newly diagnosed tumours in the western world. Urinary bladder carcinogenesis seems to develop from the interaction of environmental exposure and genetic susceptibility. Smoking, specific industrial chemicals, dietary nitrates and arsenic represent the most important exogenous risk factors. Chromosomal abnormalities, silencing of certain genes by abnormal methylation of their promoter region, alterations in tumour suppressor genes and proto-oncogenes that induce uncontrolled cell proliferation and reduced apoptosis, are molecular mechanisms that have been reported in bladder carcinogenesis. In this article, we discuss the environmental risk factors of bladder cancer and we review the genetic and epigenetic alterations, including aberrant DNA methylation and deregulation of microRNAs expression. We also discuss the role of p53 and retinoblastoma suppressor genes in disease progression. Finally, we present recent reports on the use of molecular profiling to predict disease stage and grade and direct targeted therapy.

Recent advances in the biology of heavy-ion cancer therapy

Superb biological effectiveness and dose conformity represent a rationale for heavy-ion therapy, which has thus far achieved good cancer controllability while sparing critical normal organs. Immediately after irradiation, heavy ions produce dense ionization along their trajectories, cause irreparable clustered DNA damage, and alter cellular ultrastructure. These ions, as a consequence, inactivate cells more effectively with less cell-cycle and oxygen dependence than conventional photons. The modes of heavy ion-induced cell death/inactivation include apoptosis, necrosis, autophagy, premature senescence, accelerated differentiation, delayed reproductive death of progeny cells, and bystander cell death. This paper briefly reviews the current knowledge of the biological aspects of heavy-ion therapy, with emphasis on the authors' recent findings. The topics include (i) repair mechanisms of heavy ion-induced DNA damage, (ii) superior effects of heavy ions on radioresistant tumor cells (intratumor quiescent cell population, TP53-mutated and BCL2-overexpressing tumors), (iii) novel capacity of heavy ions in suppressing cancer metastasis and neoangiogenesis, and (iv) potential of heavy ions to induce secondary (especially breast) cancer.

Forkhead box M1 transcription factor: a novel target for cancer therapy

FoxM1 signaling has been reported to be associated with carcinogenesis. Therefore, the FoxM1 may represent a novel therapeutic target, and thus the development of agents that will target FoxM1 is likely to have significant therapeutic impact on human cancer. This review describes the mechanisms of signal transduction associated with FoxM1 and provides emerging evidence in support of its role in the carcinogenesis. Further, we summarize data on several FoxM1 inhibitors especially "chemopreventive agents" and these agents could be useful for targeted inactivation of FoxM1, which indeed could become a novel approach for the prevention and/or treatment of human cancer.

Overexpression of RAD51 suppresses recombination defects: a possible mechanism to reverse genomic instability

RAD51, a key protein in the homologous recombinational DNA repair (HRR) pathway, is the major strand-transferase required for mitotic recombination. An important early step in HRR is the formation of single-stranded DNA (ss-DNA) coated by RPA (a ss-DNA-binding protein). Displacement of RPA by RAD51 is highly regulated and facilitated by a number of different proteins known as the 'recombination mediators'. To assist these recombination mediators, a second group of proteins also is required and we are defining these proteins here as 'recombination co-mediators'. Defects in either recombination mediators or co-mediators, including BRCA1 and BRCA2, lead to impaired HRR that can genetically be complemented for (i.e. suppressed) by overexpression of RAD51. Defects in HRR have long been known to contribute to genomic instability leading to tumor development. Since genomic instability also slows cell growth, precancerous cells presumably require genomic re-stabilization to gain a growth advantage. RAD51 is overexpressed in many tumors, and therefore, we hypothesize that the complementing ability of elevated levels of RAD51 in tumors with initial HRR defects limits genomic instability during carcinogenic progression. Of particular interest, this model may also help explain the high frequency of TP53 mutations in human cancers, since wild-type p53 represses RAD51 expression.

20 years studying p53 functions in genetically engineered mice

Cell and molecular biological studies of p53 functions over the past 30 years have been complemented in the past 20 years by studies that use genetically engineered mice. As expected, mice that have mutant Trp53 alleles usually develop cancers of various types more rapidly than their counterparts that have wild-type Trp53 genes. These mouse studies have been instrumental in providing important new insights into p53 tumour suppressor function. Such studies have been facilitated by the development of increasingly sophisticated genetic engineering approaches, which allow the more precise manipulation of p53 structure and function in a mammalian model.

The first 30 years of p53: growing ever more complex

Thirty years ago p53 was discovered as a cellular partner of simian virus 40 large T-antigen, the oncoprotein of this tumour virus. The first decade of p53 research saw the cloning of p53 DNA and the realization that p53 is not an oncogene but a tumour suppressor that is very frequently mutated in human cancer. In the second decade of research, the function of p53 was uncovered: it is a transcription factor induced by stress, which can promote cell cycle arrest, apoptosis and senescence. In the third decade after its discovery new functions of this protein were revealed, including the regulation of metabolic pathways and cytokines that are required for embryo implantation. The fourth decade of research may see new p53-based drugs to treat cancer. What is next is anybody's guess.

p53 ancestry: gazing through an evolutionary lens

Evolutionary patterns indicate that primordial p53 genes predated the appearance of cancer. Therefore, wild-type tumour suppressive functions and mutant oncogenic functions that give celebrity status to this gene family were probably co-opted from unrelated primordial activities. Is it possible to deduce what these early functions might have been? And might this knowledge provide a platform for therapeutic opportunities?

The expanding universe of p53 targets

The p53 tumour suppressor is modified through mutation or changes in expression in most cancers, leading to the altered regulation of hundreds of genes that are directly influenced by this sequence-specific transcription factor. Central to the p53 master regulatory network are the target response element (RE) sequences. The extent of p53 transactivation and transcriptional repression is influenced by many factors, including p53 levels, cofactors and the specific RE sequences, all of which contribute to the role that p53 has in the aetiology of cancer. This Review describes the identification and functionality of REs and highlights the inclusion of non-canonical REs that expand the universe of genes and regulation mechanisms in the p53 tumour suppressor network.