Breast-cancer stromal cells with TP53 mutations

Cancer Cells Shuttle Extracellular Vesicles Containing Oncogenic Mutant p53 Proteins to the Tumor Microenvironment

Simple Summary

In addition to the classical cell-to-cell communication patterns, extracellular vesicles (EVs) are instrumental in conveying molecular messages across cell types and have the potential to mediate changes at a tissue level. Since it is now appreciated that carcinomas are fundamentally reliant on two-way communication with activated cells in the tumor microenvironment, elucidating the roles of EVs exchange and of the cargo that is transferred is essential to obtain a thorough understanding of tumor progression. This study reveals that mutant p53 proteins—the result of the most frequent mutated gene in human cancer—are packed into EVs and delivered to neighboring cells with the potential to reprogram immune cells and subsequently establish a positive feedback loop that will enhance tumor progression. This non-cell autonomous role of mutant p53 is evidence of an extra layer of communication that is orchestrated by smaller vesicles that transfer oncogenic elements between cellular entities. Building on the foundation of our work on mutant p53, future studies may aim to characterize the potential activation of additional oncogenes, thus opening new paths of research at the interface of extracellular vesicles, cancer, and evolution.

Abstract

Extracellular vesicles (EVs) shed by cancer cells play a major role in mediating the transfer of molecular information by reprogramming the tumor microenvironment (TME). TP53 (encoding the p53 protein) is the most mutated gene across many cancer types. Mutations in TP53 not only result in the loss of its tumor-suppressive properties but also results in the acquisition of novel gain-of-functions (GOF) that promote the growth of cancer cells. Here, we demonstrate that GOF mutant p53 proteins can be transferred via EVs to neighboring cancer cells and to macrophages, thus modulating them to release tumor supportive cytokines. Our data from pancreatic, lung, and colon carcinoma cell lines demonstrate that the mutant p53 protein can be selectively sorted into EVs. More specifically, mutant p53 proteins in EVs can be taken up by neighboring cells and mutant p53 expression is found in non-tumor cells in both human cancers and in non-human tissues in human xenografts. Our findings shed light on the intricate methods in which specific GOF p53 mutants can promote oncogenic mechanisms by reprogramming and then recruiting non-cancerous elements for tumor progression.

Missing links - epigenetic regulators of the pancreatic cancer-associated inflammation

Pancreatic ductal adenocarcinoma (PDAC) features a hostile tumor microenvironment (TME) that renders it remarkably resistant to most therapeutic interventions. Consequently, survival remains among the poorest compared with other gastrointestinal cancers. Concerted efforts are underway to decipher the complex PDAC TME, break down barriers to efficacious therapies and identify novel treatment strategies. In the recent Clinical Science, Li and colleagues identify the long noncoding RNA KLHDC7B-DT as a crucial epigenetic regulator of IL-6 transcription in PDAC and illustrate its potent influences on the pancreatic TME. In this commentary, we introduce epigenetics in pancreatic cancer and put the findings by Li et al. in context with current knowledge.

Gain-of-function p53 protein transferred via small extracellular vesicles promotes conversion of fibroblasts to a cancer-associated phenotype

Tumor and stromal interactions consist of reciprocal signaling through cytokines, growth factors, direct cell-cell interactions, and extracellular vesicles (EVs). Small EVs (≤200 nm) have been considered critical messengers of cellular communication during tumor development. Here, we demonstrate that gain-of-function (GOF) p53 protein can be packaged into small EVs and transferred to fibroblasts. GOF p53 protein is selectively bound by heat shock protein 90 (HSP90), a chaperone protein, and packaged into small EVs. Inhibition of HSP90 activity blocks packaging of GOF, but not wild-type, p53 in small EVs. GOF p53-containing small EVs result in their conversion to cancer-associated fibroblasts. In vivo studies reveal that GOF p53-containing small EVs can enhance tumor growth and promote fibroblast transformation into a cancer-associated phenotype. These findings provide a better understanding of the complex interactions between cancer and stromal cells and may have therapeutic implications.

Ma et al. report that gain-of-function (GOF) p53 protein can be packaged into small EVs and transferred to stromal fibroblasts. The packaging of GOF p53 into small EVs is regulated by HSP90. Small EVs with GOF p53 activate Nrf2-mediated pathways in fibroblasts and induce their conversion to a cancer-associated phenotype.

Mechanisms that drive inflammatory tumor microenvironment, tumor heterogeneity, and metastatic progression

Treatment of cancer metastasis has been largely ineffective. It is paramount to understand the mechanisms underlying the metastatic process, of which the tumor microenvironment is an indispensable participant. What are the critical cellular and molecular players at the primary tumor site where metastatic cascade initiates? How is tumor-associated inflammation regulated? How do altered vasculatures contribute to metastasis? What is the dynamic nature or heterogeneity of primary tumors and what are the challenges to catch a moving target? This review summarizes recent progress, mechanistic understanding, and options for metastasis-targeted therapy.

Oncogenic Mutant p53 Gain of Function Nourishes the Vicious Cycle of Tumor Development and Cancer Stem-Cell Formation

More than half of human tumors harbor an inactivated p53 tumor-suppressor gene. It is well accepted that mutant p53 shows an oncogenic gain-of-function (GOF) activity that facilitates the transformed phenotype of cancer cells. In addition, a growing body of evidence supports the notion that cancer stem cells comprise a seminal constituent in the initiation and progression of cancer development. Here, we elaborate on the mutant p53 oncogenic GOF leading toward the acquisition of a transformed phenotype, as well as placing mutant p53 as a major component in the establishment of cancer stem cell entity. Therefore, therapy targeted toward cancer stem cells harboring mutant p53 is expected to pave the way to eradicate tumor growth and recurrence.

Cells Comprising the Prostate Cancer Microenvironment Lack Recurrent Clonal Somatic Genomic Aberrations

Prostate cancer-associated stroma (CAS) plays an active role in malignant transformation, tumor progression, and metastasis. Molecular analyses of CAS have demonstrated significant changes in gene expression; however, conflicting evidence exists on whether genomic alterations in benign cells comprising the tumor microenvironment (TME) underlie gene expression changes and oncogenic phenotypes. This study evaluates the nuclear and mitochondrial DNA integrity of prostate carcinoma cells, CAS, matched benign epithelium and benign epithelium-associated stroma by whole-genome copy-number analyses, targeted sequencing of TP53, and FISH. Array comparative genomic hybridization (aCGH) of CAS revealed a copy-neutral diploid genome with only rare and small somatic copy-number aberrations (SCNA). In contrast, several expected recurrent SCNAs were evident in the adjacent prostate carcinoma cells, including gains at 3q, 7p, and 8q, and losses at 8p and 10q. No somatic TP53 mutations were observed in CAS. Mitochondrial DNA (mtDNA) extracted from carcinoma cells and stroma identified 23 somatic mtDNA mutations in neoplastic epithelial cells, but only one mutation in stroma. Finally, genomic analyses identified no SCNAs, LOH, or copy-neutral LOH in cultured cancer-associated fibroblasts, which are known to promote prostate cancer progression in vivo

IMPLICATIONS

The gene expression changes observed in prostate cancer-adjacent stroma and the attendant contribution of the stroma to the development and progression of prostate cancer are not due to frequent or recurrent genomic alterations in the TME.

Roles of tumor suppressors in regulating tumor-associated inflammation

Loss or silencing of tumor suppressors (TSs) promotes neoplastic transformation and malignant progression. To date, most work on TS has focused on their cell autonomous effects. Recent evidence, however, demonstrates an important noncell autonomous role for TS in the control of tumor-associated inflammation. We review evidence from clinical data sets and mouse model studies demonstrating enhanced inflammation and altered tumor microenvironment (TME) upon TS inactivation. We discuss clinical correlations between tumor-associated inflammation and inactivation of TS, and their therapeutic implications. This review sets forth the concept that TS can also suppress tumor-associated inflammation, a concept that provides new insights into tumor-host interactions. We also propose that in some cases the loss of TS function in cancer can be overcome through inhibition of the resulting inflammatory response, regardless whether it is a direct or an indirect consequence of TS loss.

Roles of tumor suppressors in regulating tumor-associated inflammation

Loss or silencing of tumor suppressors (TSs) promotes neoplastic transformation and malignant progression. To date, most work on TS has focused on their cell autonomous effects. Recent evidence, however, demonstrates an important noncell autonomous role for TS in the control of tumor-associated inflammation. We review evidence from clinical data sets and mouse model studies demonstrating enhanced inflammation and altered tumor microenvironment (TME) upon TS inactivation. We discuss clinical correlations between tumor-associated inflammation and inactivation of TS, and their therapeutic implications. This review sets forth the concept that TS can also suppress tumor-associated inflammation, a concept that provides new insights into tumor-host interactions. We also propose that in some cases the loss of TS function in cancer can be overcome through inhibition of the resulting inflammatory response, regardless whether it is a direct or an indirect consequence of TS loss.

Interleukin-1β promotes ovarian tumorigenesis through a p53/NF-κB-mediated inflammatory response in stromal fibroblasts

Cancer has long been considered a disease that mimics an "unhealed wound," with oncogene-induced secretory activation signals from epithelial cancer cells facilitating stromal fibroblast, endothelial, and inflammatory cell participation in tumor progression. However, the underlying mechanisms that orchestrate cooperative interaction between malignant epithelium and the stroma remain largely unknown. Here, we identified interleukin-1β (IL-1β) as a stromal-acting chemokine secreted by ovarian cancer cells, which suppresses p53 protein expression in cancer-associated fibroblasts (CAFs). Elevated expression of IL-1β and cognate receptor IL-1R1 in ovarian cancer epithelial cells and CAFs independently predicted reduced overall patient survival, as did repressed nuclear p53 in ovarian CAFs. Knockdown of p53 expression in ovarian fibroblasts significantly enhanced the expression and secretion of chemokines IL-8, growth regulated oncogene-alpha (GRO-α), IL-6, IL-1β, and vascular endothelial growth factor (VEGF), significantly increased in vivo mouse xenograft ovarian cancer tumor growth, and was entirely dependent on interaction with, and transcriptional up-regulation of, nuclear factor-kappaB (NF-κB) p65. Our results have uncovered a previously unrecognized circuit whereby epithelial cancer cells use IL-1β as a communication factor instructing stromal fibroblasts through p53 to generate a protumorigenic inflammatory microenvironment. Attenuation of p53 protein expression in stromal fibroblasts generates critical protumorigenic functionality, reminiscent of the role that oncogenic p53 mutations play in cancer cells. These findings implicate CAFs as an important target for blocking inflammation in the tumor microenvironment and reducing tumor growth.

Data-driven unbiased curation of the TP53 tumor suppressor gene mutation database and validation by ultradeep sequencing of human tumors

Cancer mutation databases are expected to play central roles in personalized medicine by providing targets for drug development and biomarkers to tailor treatments to each patient. The accuracy of reported mutations is a critical issue that is commonly overlooked, which leads to mutation databases that include a sizable number of spurious mutations, either sequencing errors or passenger mutations. Here we report an analysis of the latest version of the TP53 mutation database, including 34,453 mutations. By using several data-driven methods on multiple independent quality criteria, we obtained a quality score for each report contributing to the database. This score can now be used to filter for high-confidence mutations and reports within the database. Sequencing the entire TP53 gene from various types of cancer using next-generation sequencing with ultradeep coverage validated our approach for curation. In summary, 9.7% of all collected studies, mostly comprising numerous tumors with multiple infrequent TP53 mutations, should be excluded when analyzing TP53 mutations. Thus, by combining statistical and experimental analyses, we provide a curated mutation database for TP53 mutations and a framework for mutation database analysis.

Understanding the role of stromal fibroblasts in cancer progression

The major cellular components of tumor microenvironment, referred to as the cancer stroma, are composed of cancer-associated fibroblasts that support tumor epithelial growth, invasion and therapeutic resistance. Thus when we speak of developing therapies that address tumor heterogeneity it is not only a matter of different mutations within the tumor epithelia. While individual mutations in the stromal compartment are controversial, the heterogeneity in fibroblastic population in a single tumor is not up for debate. Cooperative interaction among heterotypic fibroblasts and tumor cells contribute to cancer progression. Therefore to tackle solid tumors, we need to understand its complex microenvironment. Here we review some seminal developments in the field of tumor microenvironment, mainly focusing on cancer-associated fibroblast.

The tumor microenvironment: part 1

For years the mutated, highly proliferating neoplastic cells were presented as the only important agent in tumors; however, during the last 3–4 decades it has become clear that the microenvironment of the cancer cells plays a determinative role in the malignant evolution of neoplasia. Cancers are in fact heterogeneous entities containing, in addition to the neoplastic cell component, cells derived of multiple lineages (fibroblasts, endothelial cells lining blood and lymphatic vessels, pericytes, adipocytes and immune system cells belonging to both innate and adaptive responses), as well as the extracellular matrix, with a large variety of soluble molecules of biological importance, constituting a complex organ-like structure. The tumor microenvironment consists in a tissue that may have a predictive significance for tumor behavior and response to therapy.

TP53 mutations in human cancer: database reassessment and prospects for the next decade

TP53 mutations are the most frequent genetic alterations found in human cancer. For more than 20 years, TP53 mutation databases have collected over 30,000 somatic mutations from various types of cancer. Analyses of these mutations have led to many types of studies and have improved our knowledge about the TP53 protein and its function. The recent advances in sequencing methodologies and the various cancer genome sequencing projects will lead to a profound shift in database curation and data management. In this paper, we will review the current status of the TP53 mutation database, its application to various fields of research, and how data quality and curation can be improved. We will also discuss how the genetic data will be stored and handled in the future and the consequences for database management.

Breast cancer stem cells, cytokine networks, and the tumor microenvironment

Many tumors, including breast cancer, are maintained by a subpopulation of cells that display stem cell properties, mediate metastasis, and contribute to treatment resistance. These cancer stem cells (CSCs) are regulated by complex interactions with the components of the tumor microenvironment - including mesenchymal stem cells, adipocytes, tumor associated fibroblasts, endothelial cells, and immune cells - through networks of cytokines and growth factors. Since these components have a direct influence on CSC properties, they represent attractive targets for therapeutic development.

Genetic changes in tumour microenvironments

Numerous in vitro and in vivo studies have established that carcinoma-associated fibroblasts differ phenotypically from fibroblasts associated with normal tissue but the mechanisms underlying these differences are unclear. Since carcinoma-associated fibroblasts can be propagated in vitro for extended periods and still maintain their cancer-promoting phenotype, some investigators have proposed that they might have acquired somatic genetic alterations analogous to those observed in malignant epithelium. Early molecular genetic studies appeared to validate this hypothesis by demonstrating remarkably high frequencies of clonal somatic genetic alterations in carcinoma-associated fibroblasts, including loss of heterozygosity, gene amplification, and point mutations in tumour suppressor genes such as TP53 and PTEN. The initial excitement of these paradigm-changing studies overshadowed concerns that there may have been a more mundane explanation for these observations. In addition to the fact that the data would necessarily invoke an unlikely scenario of the simultaneous generation of two symbiotic malignancies, subsequent molecular genetic studies found no evidence of frequent genomic aberrations. One striking common trait of those studies reporting frequent clonal somatic alterations in carcinoma-associated fibroblasts is the use of tissues and techniques which are well known to be highly prone to generating artefacts such as limiting and poor quality DNA followed by highly multiplexed PCR-based analyses. It is now clear that clonal somatic mutations are not the biological basis of the cancer-promoting attributes of carcinoma-associated fibroblasts.

Mutations in bone marrow-derived stromal stem cells unmask latent malignancy

Neoplastic epithelia may remain dormant and clinically unapparent in human patients for decades. Multiple risk factors including mutations in tumor cells or the stromal cells may affect the switch from dormancy to malignancy. Gene mutations, including p53 mutations, within the stroma of tumors are associated with a worse clinical prognosis; however, it is not known if these stromal mutations can promote tumors in genetically at-risk tissue. To address this question, Apc(Min/+) and Apc(Min/+) Rag2(-/-) mice, which have a predilection to mammary carcinoma (as well as wild-type (wt) mice), received mesenchymal stem cells (MSC) with mutant p53 (p53MSC) transferred via tail vein injection. In the wt mouse, p53MSC circulated in the periphery and homed to the marrow cavity where they could be recovered up to a year later without apparent effect on the health of the mouse. No mammary tumors were found. However, in mice carrying the Apc(Min/+) mutation, p53MSC homed to mammary tissue and significantly increased the incidence of mammary carcinoma. Tumor necrosis factor (TNF)-alpha-dependent factors elaborated from mesenchymal cells converted quiescent epithelia into clinically apparent disease. The increased cancer phenotype was completely preventable with neutralization of TNF-alpha or by transfer of CD4(+) regulatory T cells from immune competent donors, demonstrating that immune competency to regulate inflammation was sufficient to maintain neoplastic dormancy even in the presence of oncogenic epithelial and stromal mutations. The significant synergy between host immunity and mesenchymal cells identified here may restructure treatments to restore an anticancer microenvironment.

MUT-TP53 2.0: a novel versatile matrix for statistical analysis of TP53 mutations in human cancer

Analysis of the literature reporting p53 mutations shows that 8% of report display typographical mistakes with a notable increase in recent years. These errors are sometimes isolated, but in some cases, they concern several or even all mutations described in a single article. Furthermore, some works report unusual profile of p53 mutations whose accuracy is difficult to assess. To handle these problems we have developed MUT-TP53 2.0, an accurate and powerful tool that will automatically handle p53 mutations and generate tables ready for publication that will lower the risk of typographical errors. Furthermore, using functional and statistical information issued from the UMD p53 database, it allows to assess the biological activity and the likelihood of every p53 mutant.

Jekyll and Hyde: the role of the microenvironment on the progression of cancer

It is now recognized that the host microenvironment undergoes extensive change during the evolution and progression of cancer. This involves the generation of cancer-associated fibroblasts (CAFs), which, through release of growth factors and cytokines, lead to enhanced angiogenesis, increased tumour growth and invasion. It has also been demonstrated that CAFs may modulate the cancer stem cell (CSC) phenotype, which has therapeutic implications. The altered fibroblast phenotype also contributes to the development of an altered extracellular matrix (ECM), with synthesis of ECM isoforms rarely found in normal tissues, including tenascin-C isoforms and the fibronectin EDA isoform. There is also emerging evidence of how the tensile strength of the tumour-associated ECM may be modified and lead to altered signalling in tumour cells. The hypoxic environment of the tumour stimulates angiogenesis and also impacts on other aspects of cell signalling, including the c-met pathway and lysyl oxidase-mediated signalling, which can directly promote tumour cell invasion. The inflammatory infiltrate associated with many solid tumours also modulates tumour function, having both anti- and pro-tumour effects. All of these components of the microenvironment provide potential targets for therapeutic attack, with a number of molecules already in clinical trials. It is also becoming evident that characterizing the tumour microenvironment can provide important prognostic and predictive information about tumours, independent of the tumour cell phenotype.

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.

Breast carcinoma-associated fibroblasts rarely contain p53 mutations or chromosomal aberrations

It has become increasingly clear that the cells within the tumor microenvironment play a critical role in cancer growth and metastasis. Studies in experimental models suggest that carcinoma-associated fibroblasts (CAF) differ from normal fibroblasts and are capable of promoting cancer progression through a variety of mechanisms. At present, a definitive view is lacking on whether genomic abnormalities are present and whether they might underlie the observed phenotypic differences. This study reports the molecular analysis of the largest series of breast CAFs reported to date, with an array comparative genomic hybridization-based DNA copy number analysis of cultured CAFs derived from 25 freshly resected human breast cancers. We found DNA copy number changes consisting of the whole arm of chromosomes 6p and 9p plus interstitial 4q loss in only one sample. No abnormalities were observed in non-tumor-associated fibroblast counterparts. Karyotyping of the same CAF revealed further chromosomal abnormalities, which included clonal loss of chromosomes, chromosomal duplications, and less frequent chromosomal rearrangements. These abnormalities were not associated with alterations in the global gene expression profile of this particular CAF, relative to its non-tumor-associated fibroblast counterpart. Moreover, this particular patient's CAF also displayed the only p53 mutation in the cohort, the first time such a mutation has been reported in freshly cultured human CAFs. These findings argue that the procancerous effects of CAFs are unlikely to be due to DNA copy number-type genomic abnormalities in the CAFs themselves. As such, breast CAFs should be mainly regarded as genomically stable cellular constituents that exist within complex cancer microenvironments.

2-methoxyestradiol inhibits Barrett's esophageal adenocarcinoma growth and differentiation through differential regulation of the beta-catenin-E-cadherin axis

The purpose of this study was to evaluate whether 2-methoxyestradiol (2-ME(2)), a promising anticancer agent, modulates Barrett's esophageal adenocarcinoma (BEAC) cell growth and behavior through a cellular pathway involving beta-catenin in partnership with E-cadherin, which seems to play a critical role in the induction of antitumor responses in cancer cells. We found that 2-ME(2) markedly reduced the BEAC cell proliferation through regulating apoptotic machinery such as Bcl-2 and Bax. It may nullify the aggressive behavior of the cells by reducing the migratory behavior. Expressions of beta-catenin and E-cadherin and binding of these two proteins is activated in a 2-ME(2)-dependent fashion in Bic-1 cells. Moreover, overexpressions of these two proteins may be due to the stabilization of these proteins by 2-ME(2). We found that 2-ME(2)-induced antimigratory effects are mediated through the beta-catenin-E-cadherin signaling pathways. In view of these results, we determined whether 2-ME(2) reduces BEAC tumor growth. Administration of 2-ME2 significantly decreased the growth of BEAC cells xenografted on the flank of nude mice. The evidence presented points out that the effect of 2-ME(2) on beta-catenin-orchestrated signal transduction plausibly plays a multifaceted functional role to inhibit the proliferation and cell migration of 2-ME(2)-treated malignant cells and it could be a potential candidate in novel treatment strategies for Barrett's esophageal adenocarcinoma.

Heterogeneity of gene expression in stromal fibroblasts of human breast carcinomas and normal breast

Breast carcinoma invasion is associated with prominent alterations in stromal fibroblasts. Carcinoma-associated fibroblasts (CAF) support and promote tumorigenesis, whereas normal mammary fibroblasts (NF) are thought to suppress tumor progression. Little is known about the difference in gene expression between CAF and NF or the patient-to-patient variability in gene expression. Paired CAF and NF were isolated from six primary human breast carcinoma specimens. RNA was extracted from low-passage cultures of CAF and NF and analyzed with Affymetrix Human Genome U133 Plus 2.0 arrays. The array data were examined with an empirical Bayes model and filtered according to the posterior probability of equivalent expression and fold difference in expression. Twenty-one genes (27 probe sets) were up-regulated in CAF, as compared to NF. Known functions of these genes relate to paracrine or intracellular signaling, transcriptional regulation, extracellular matrix and cell adhesion/migration. Ten genes (14 probe sets) were down-regulated in CAF, including the pluripotency transcription factor KLF4. Quantitative RT-PCR analysis of 10 genes validated the array results. Immunohistochemical staining for three gene products confirmed stromal expression in terms of location and relative quantity. Surprisingly, the variability of gene expression was slightly higher in NF than in CAF, suggesting inter-individual heterogeneity of normal stroma.

Involvement of stromal p53 in tumor-stroma interactions

p53 is a major tumor-suppressor gene, inactivated by mutations in about half of all human cancer cases, and probably incapacitated by other means in most other cases. Most research regarding the role of p53 in cancer has focused on its ability to elicit apoptosis or growth arrest of cells that are prone to become malignant owing to DNA damage or oncogene activation, i.e. cell-autonomous activities of p53. However, p53 activation within a cell can also exert a variety of effects upon neighboring cells, through secreted factors and paracrine and endocrine mechanisms. Of note, p53 within cancer stromal cells can inhibit tumor growth and malignant progression. Cancer cells that evolve under this inhibitory influence acquire mechanisms to silence stromal p53, either by direct inhibition of p53 within stromal cells, or through pressure for selection of stromal cells with compromised p53 function. Hence, activation of stromal p53 by chemotherapy or radiotherapy might be part of the mechanisms by which these treatments cause cancer regression. However, in certain circumstances, activation of stromal p53 by cytotoxic anti-cancer agents might actually promote treatment resistance, probably through stromal p53-mediated growth arrest of the cancer cells or through protection of the tumor vasculature. Better understanding of the underlying molecular mechanisms is thus required. Hopefully, this will allow their manipulation towards better inhibition of cancer initiation, progression and metastasis.

Molecular biology of cancer-associated fibroblasts: can these cells be targeted in anti-cancer therapy?

It is increasingly recognized that the non-neoplastic stromal compartment in most solid cancers plays an active role in tumor proliferation, invasion and metastasis. Cancer-associated fibroblasts (CAFs) are one of the most abundant cell types in the tumor stroma, and these cells are pro-tumorigenic. Evidence that CAFs are epigenetically and possibly also genetically distinct from normal fibroblasts is beginning to define these cells as potential targets of anti-cancer therapy. Here, we review the cell-of-origin and molecular biology of CAFs, arguing that such knowledge provides a rational basis for designing therapeutic strategies to coordinately and synergistically target both the stromal and malignant epithelial component of human cancers.

Limited copy number-high resolution melting (LCN-HRM) enables the detection and identification by sequencing of low level mutations in cancer biopsies

Background

Mutation detection in clinical tumour samples is challenging when the proportion of tumour cells, and thus mutant alleles, is low. The limited sensitivity of conventional sequencing necessitates the adoption of more sensitive approaches. High resolution melting (HRM) is more sensitive than sequencing but identification of the mutation is desirable, particularly when it is important to discriminate false positives due to PCR errors or template degradation from true mutations.

We thus developed limited copy number - high resolution melting (LCN-HRM) which applies limiting dilution to HRM. Multiple replicate reactions with a limited number of target sequences per reaction allow low level mutations to be detected. The dilutions used (based on Ct values) are chosen such that mutations, if present, can be detected by the direct sequencing of amplicons with aberrant melting patterns.

Results

Using cell lines heterozygous for mutations, we found that the mutations were not readily detected when they comprised 10% of total alleles (20% tumour cells) by sequencing, whereas they were readily detectable at 5% total alleles by standard HRM. LCN-HRM allowed these mutations to be identified by direct sequencing of those positive reactions.

LCN-HRM was then used to review formalin-fixed paraffin-embedded (FFPE) clinical samples showing discordant findings between sequencing and HRM for KRAS exon 2 and EGFR exons 19 and 21. Both true mutations present at low levels and sequence changes due to artefacts were detected by LCN-HRM. The use of high fidelity polymerases showed that the majority of the artefacts were derived from the damaged template rather than replication errors during amplification.

Conclusion

LCN-HRM bridges the sensitivity gap between HRM and sequencing and is effective in distinguishing between artefacts and true mutations.

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

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.

Clonal mutations in the cancer-associated fibroblasts: the case against genetic coevolution

It is well established that carcinoma-associated fibroblasts (CAFs) differ phenotypically from fibroblasts associated with normal tissue, but the mechanisms underlying these differences remain controversial. Because CAFs can be propagated in vitro for extended periods and still maintain their cancer promoting phenotype, it has been proposed that they might have acquired somatic genetic alterations analogous to those observed in malignant epithelium. Whereas some investigators have reported frequent and profound genomic alterations in CAFs, other groups have found no such evidence. One striking common trait of those studies reporting frequent clonal somatic alterations in CAFs is the use of tissues and techniques which are well known to be highly prone to generating artefacts, such as limiting and poor quality DNA followed by highly multiplexed PCR-based analysis. We conclude that reported frequent clonal somatic mutations in CAFs are likely to be artefacts and are not the biological basis of the cancer promoting attributes of CAFs. [corrected]

Global hypomethylation of genomic DNA in cancer-associated myofibroblasts

Global hypomethylation has long been recognized as a feature of the malignant epithelial component in human carcinomas. Here we show evidence for this same type of epigenetic alteration in cancer-associated stromal myofibroblasts. We used methylation-sensitive SNP array analysis (MSNP) to profile DNA methylation in early-passage cultures of stromal myofibroblasts isolated from human gastric cancers. The MSNP data indicated widespread hypomethylation in these cells, with rare focal gains of methylation, conclusions that were independently validated by bisulfite sequencing and by a methylation-sensitive cytosine incorporation assay. Immunohistochemistry with anti-5-methylcytosine (anti-5-methyl-C) in a series of gastrectomy specimens showed frequent loss of methylation in nuclei of both the malignant epithelial cells and alpha-smooth muscle actin (ASMA)-positive stromal myofibroblasts of both intestinal-type and diffuse carcinomas. We confirmed this phenomenon and established its onset at the stage of noninvasive dysplastic lesions by immunohistochemistry for anti-5-methyl-C in a transgenic mouse model of multistage gastric carcinogenesis. These findings indicate similar general classes of epigenetic alterations in carcinoma cells and their accompanying reactive stromal cells and add to accumulating evidence for biological differences between normal and cancer-associated myofibroblasts.

Cancer cells suppress p53 in adjacent fibroblasts

The p53 tumor suppressor serves as a crucial barrier against cancer development. In tumor cells and their progenitors, p53 suppresses cancer in a cell-autonomous manner. However, p53 also possesses non-cell-autonomous activities. For example, p53 of stromal fibroblasts can modulate the spectrum of proteins secreted by these cells, rendering their microenvironment less supportive of the survival and spread of adjacent tumor cells. We now report that epithelial tumor cells can suppress p53 induction in neighboring fibroblasts, an effect reproducible by tumor cell-conditioned medium. The ability to suppress fibroblast p53 activation is acquired by epithelial cells in the course of neoplastic transformation. Specifically, stable transduction of immortalized epithelial cells by mutant H-Ras and p53-specific short inhibitory RNA endows them with the ability to quench fibroblast p53 induction. Importantly, human cancer-associated fibroblasts are more susceptible to this suppression than normal fibroblasts. These findings underscore a mechanism whereby epithelial cancer cells may overcome the non-cell-autonomous tumor suppressor function of p53 in stromal fibroblasts.

Co-evolution of tumor cells and their microenvironment

Increasing evidence indicates that tumor-stromal cell interactions have a crucial role in tumor initiation and progression. These interactions modify cellular compartments, leading to the co-evolution of tumor cells and their microenvironment. Although the importance of microenvironmental alterations in tumor development is recognized, the molecular mechanisms underlying these changes are only now beginning to be understood. Epigenetic and gene expression changes have consistently been reported in cancer-associated stromal cells and the influence of the host genotype on tumorigenesis is also well documented. However, the presence of clonally selected somatic genetic alterations within the tumor microenvironment has been controversial. A thorough understanding of the co-evolution of these two cellular compartments will require carefully executed molecular studies combined with mathematical modeling.