Dickkopf-3 is regulated by the MYCN-induced miR-17-92 cluster in neuroblastoma

Role of non-coding RNAs in neuroblastoma

Neuroblastoma is known as the most prevalent extracranial malignancy in childhood with a neural crest origin. It has been widely accepted that non-coding RNAs (ncRNAs) play important roles in many types of cancer, including glioma and gastrointestinal cancers. They may regulate the cancer gene network. According to recent sequencing and profiling studies, ncRNAs genes are deregulated in human cancers via deletion, amplification, abnormal epigenetic, or transcriptional regulation. Disturbances in the expression of ncRNAs may act either as oncogenes or as anti-tumor suppressor genes, and can lead to the induction of cancer hallmarks. ncRNAs can be secreted from tumor cells inside exosomes, where they can be transferred to other cells to affect their function. However, these topics still need more study to clarify their exact roles, so the present review addresses different roles and functions of ncRNAs in neuroblastoma.

MYC Hyperactivates Wnt Signaling in APC/CTNNB1-Mutated Colorectal Cancer Cells through miR-92a-Dependent Repression of DKK3

Activation of Wnt signaling is among the earliest events in colon cancer development. It is achieved either via activating mutations in the CTNNB1 gene encoding β-catenin, the key transcription factor in the Wnt pathway, or most commonly by inactivating mutations affecting APC, a major β-catenin binding partner and negative regulator. However, our analysis of recent Pan Cancer Atlas data revealed that CTNNB1 mutations significantly co-occur with those affecting Wnt receptor complex components (e.g., Frizzled and LRP6), underscoring the importance of additional regulatory events even in the presence of common APC/CTNNB1 mutations. In our effort to identify non-mutational hyperactivating events, we determined that KRAS-transformed murine colonocytes overexpressing direct β-catenin target MYC show significant upregulation of the Wnt signaling pathway and reduced expression of Dickkopf 3 (DKK3), a reported ligand for Wnt co-receptors. We demonstrate that MYC suppresses DKK3 transcription through one of miR-17-92 cluster miRNAs, miR-92a. We further examined the role of DKK3 by overexpression and knockdown and discovered that DKK3 suppresses Wnt signaling in Apc-null murine colonic organoids and human colon cancer cells despite the presence of downstream activating mutations in the Wnt pathway. Conversely, MYC overexpression in the same cell lines resulted in hyperactive Wnt signaling, acquisition of epithelial-to-mesenchymal transition markers, and enhanced migration/invasion in vitro and metastasis in a syngeneic orthotopic mouse colon cancer model. IMPLICATIONS: Our results suggest that the MYC→miR-92a-|DKK3 axis hyperactivates Wnt signaling, forming a feed-forward oncogenic loop.

MYCN in Neuroblastoma: "Old Wine into New Wineskins"

MYCN Proto-Oncogene, BHLH Transcription Factor (MYCN) has been one of the most studied genes in neuroblastoma. It is known for its oncogenetic mechanisms, as well as its role in the prognosis of the disease and it is considered one of the prominent targets for neuroblastoma therapy. In the present work, we attempted to review the literature, on the relation between MYCN and neuroblastoma from all possible mechanistic sites. We have searched the literature for the role of MYCN in neuroblastoma based on the following topics: the references of MYCN in the literature, the gene's anatomy, along with its transcripts, the protein's anatomy, the epigenetic mechanisms regulating MYCN expression and function, as well as MYCN amplification. MYCN plays a significant role in neuroblastoma biology. Its functions and properties range from the forming of G-quadraplexes, to the interaction with miRNAs, as well as the regulation of gene methylation and histone acetylation and deacetylation. Although MYCN is one of the most primary genes studied in neuroblastoma, there is still a lot to be learned. Our knowledge on the exact mechanisms of MYCN amplification, etiology and potential interventions is still limited. The knowledge on the molecular mechanisms of MYCN in neuroblastoma, could have potential prognostic and therapeutic advantages.

MEIS2 Is an Adrenergic Core Regulatory Transcription Factor Involved in Early Initiation of TH-MYCN-Driven Neuroblastoma Formation

Simple Summary

Neuroblastoma is a pediatric tumor originating from the sympathetic nervous system responsible for 10–15% of all childhood cancer deaths. Half of all neuroblastoma patients present with high-risk disease, of which nearly 50% relapse and die of their disease. In addition, standard therapies cause serious lifelong side effects and increased risk for secondary tumors. Further research is crucial to better understand the molecular basis of neuroblastomas and to identify novel druggable targets. Neuroblastoma tumorigenesis has to this end been modeled in both mice and zebrafish. Here, we present a detailed dissection of the gene expression patterns that underlie tumor formation in the murine TH-MYCN-driven neuroblastoma model. We identified key factors that are putatively important for neuroblastoma tumor initiation versus tumor progression, pinpointed crucial regulators of the observed expression patterns during neuroblastoma development and scrutinized which factors could be innovative and vulnerable nodes for therapeutic intervention.

Abstract

Roughly half of all high-risk neuroblastoma patients present with MYCN amplification. The molecular consequences of MYCN overexpression in this aggressive pediatric tumor have been studied for decades, but thus far, our understanding of the early initiating steps of MYCN-driven tumor formation is still enigmatic. We performed a detailed transcriptome landscaping during murine TH-MYCN-driven neuroblastoma tumor formation at different time points. The neuroblastoma dependency factor MEIS2, together with ASCL1, was identified as a candidate tumor-initiating factor and shown to be a novel core regulatory circuit member in adrenergic neuroblastomas. Of further interest, we found a KEOPS complex member (gm6890), implicated in homologous double-strand break repair and telomere maintenance, to be strongly upregulated during tumor formation, as well as the checkpoint adaptor Claspin (CLSPN) and three chromosome 17q loci CBX2, GJC1 and LIMD2. Finally, cross-species master regulator analysis identified FOXM1, together with additional hubs controlling transcriptome profiles of MYCN-driven neuroblastoma. In conclusion, time-resolved transcriptome analysis of early hyperplastic lesions and full-blown MYCN-driven neuroblastomas yielded novel components implicated in both tumor initiation and maintenance, providing putative novel drug targets for MYCN-driven neuroblastoma.

MicroRNA dysregulation interplay with childhood abdominal tumors

Abdominal tumors (AT) in children account for approximately 17% of all pediatric solid tumor cases, and frequently exhibit embryonal histological features that differentiate them from adult cancers. Current molecular approaches have greatly improved the understanding of the distinctive pathology of each tumor type and enabled the characterization of novel tumor biomarkers. As seen in abdominal adult tumors, microRNAs (miRNAs) have been increasingly implicated in either the initiation or progression of childhood cancer. Moreover, besides predicting patient prognosis, they represent valuable diagnostic tools that may also assist the surveillance of tumor behavior and treatment response, as well as the identification of the primary metastatic sites. Thus, the present study was undertaken to compile up-to-date information regarding the role of dysregulated miRNAs in the most common histological variants of AT, including neuroblastoma, nephroblastoma, hepatoblastoma, hepatocarcinoma, and adrenal tumors. Additionally, the clinical implications of dysregulated miRNAs as potential diagnostic tools or indicators of prognosis were evaluated.

MicroRNAs miR-25, let-7 and miR-124 regulate the neurogenic potential of Müller glia in mice

Müller glial cells (MG) generate retinal progenitor (RPC)-like cells after injury in non-mammalian species, although this does not occur in the mammalian retina. Studies have profiled gene expression in these cells to define genes that may be relevant to their differences in neurogenic potential. However, less is known about differences in micro-RNA (miRNA) expression. In this study, we compared miRNAs from RPCs and MG to identify miRNAs more highly expressed in RPCs, and others more highly expressed in MG. To determine whether these miRNAs are relevant to the difference in neurogenic potential between these two cell types, we tested them in dissociated cultures of MG using either mimics or antagomiRs to increase or reduce expression, respectively. Among the miRNAs tested, miR-25 and miR-124 overexpression, or let-7 antagonism, induced Ascl1 expression and conversion of ∼40% of mature MG into a neuronal/RPC phenotype. Our results suggest that the differences in miRNA expression between MG and RPCs contribute to their difference in neurogenic potential, and that manipulations in miRNAs provide a new tool with which to reprogram MG for retinal regeneration.

miR-373 promotes neuroblastoma cell proliferation, migration, and invasion by targeting SRCIN1

Introduction

Previous studies have shown that miR-373 functions as either a tumor suppressor or an oncogene depending on which type of cancer it’s operating in. However, the functional role of miR-373 in neuroblastoma (NB) remains largely unclear. 

Methods

Expression of miR-373 and SRC kinase signaling inhibitor 1 (SRCIN1) in 20 metastatic and 20 primary NB tissues was detected by quantitative real-time PCR (qRT-PCR) and Western blotting. MTT assay, flow cytometry analysis and transwell migration and invasion assays were performed to evaluate the influence of miR-373 inhibition on the growth, migration and invasion of NB cells, respectively. In vivo experiment was applied to determine the effect of miR-373 inhibition on tumor growth. Dual-luciferase reporter assay was used to confirm the interaction between miR-373 and SRCIN1.

Results

We observed a significant increase in the expression of miR-373 in metastatic NB samples compared with primary NB samples, and this was inversely correlated with SRCIN1 expression. Functional studies revealed that depletion of miR-373 inhibited in vitro NB cell growth, migration and invasion, and also suppressed tumor growth in an in vivo mouse model. Moreover, we identified that SRCIN1 was a direct and functional target gene of miR-373. Silencing of SRCIN1 partially rescued the antimiR-373-mediated inhibition of cell growth, migration and invasion.

Conclusion

The data from our study verified a potential oncogenic role of miR-373 in NB cells that occurs through direct targeting SRCIN1. The newly identified miR-373/SRCIN1 axis represents a new potential candidate for therapeutic intervention of malignant NB.

The Wnt/β-Catenin/LEF1 Pathway Promotes Cell Proliferation at Least in Part Through Direct Upregulation of miR-17-92 Cluster

The miR-17-92 cluster is involved in animal development and homeostasis, and its dysregulation leads to human diseases such as cancer. In the present study, we investigated the functional link between miR-17-92 cluster and Wnt/β-catenin signaling pathway in ICP2 and DF1 cells. We demonstrated that ectopic expression of either LEF1 or β-catenin increased the promoter activity of the miR-17-92 cluster host gene (MIR17HG) and combined ectopic expression of LEF1 and β-catenin further enhanced the promoter activity; while knockdown of either LEF1 or β-catenin reduced the MIR17HG promoter activity. Both LEF1 and β-catenin could directly bind to the MIR17HG promoter. Furthermore, we demonstrated that low doses of lithium chloride (LiCl), an activator of Wnt/β-catenin signaling pathway, increased MIR17HG promoter activity and the endogenous expression of the miR-17-92 cluster, while high doses of LiCl had the opposite effects. Treatment with XAV-939, an inactivator of the Wnt/β-catenin pathway, reduced the endogenous expression of miR-17-92 cluster. Finally, we found that low doses of LiCl promoted the proliferation of ICP2 and DF1 cells, while high doses of LiCl inhibited the proliferation of ICP2 and DF1 cells. Taken together, our results reveal that MIR17HG is a target of LEF1 and the Wnt/β-catenin pathway and suggest that the miR-17-92 cluster may, at least in part, mediate the proliferation-promoting effect of the Wnt/β-catenin pathway in cell proliferation.

Bioinformatics-based interaction analysis of miR-92a-3p and key genes in tamoxifen-resistant breast cancer cells

The abnormal expression of miR-92a-3p was detected in multiple cancers. However, the biological role and underlying mechanism of miR-92a-3p in tamoxifen-resistant cells are still unknown. The main objective of our study was to find potential miR-92a-3p regulating pathways involved in tamoxifen resistance and to construct their regulatory network using bioinformatics. Four gene expression profiles were retrieved from GEO database and the GEO2R tool was used for analysis. GSE41922 and GSE42072 were applied to investigate aberrant miR-92a-3p expression in breast cancer serum and tissue. We found that miR-92a-3p expression was higher in breast cancer serum or tissue than in healthy volunteer serum or adjacent normal tissue, and high expression of miR-92a-3p could predict poor prognosis of breast cancer patients. In our qRT-PCR validation, we found that miR-92a-3p was upregulated in tamoxifen-resistant cells. MiR-92a-3p might play a role in tamoxifen resistance. In order to find the relationship between miR-92a-3p and some key genes and their potential molecular mechanisms in tamoxifen-resistant cells. The microarray data GSE26459 and GSE28267 were analyzed to determine the differentially expressed genes (DEGs) or miRNAs (DEMs). Furthermore, the related long non-coding RNAs (lncRNAs) were screened with starBase v2.0. Finally,microRNA.org,miRDB, targetminer and targetscan were applied to predict the targets of miR-92a-3p. Through analysis, we find that miR-92a-3p may be used as a potential biomarker for early detection of cancer and monitoring the efficacy of endocrine therapy.

The Oncogenic Relevance of miR-17-92 Cluster and Its Paralogous miR-106b-25 and miR-106a-363 Clusters in Brain Tumors

The fundamental function of ribonucleic acids is to transfer genetic information from DNA to protein during translation process, however, this is not the only way connecting active RNA sequences with essential biological processes. Up until now, many RNA subclasses of different size, structure, and biological function were identified. Among them, there are non-coding single-stranded microRNAs (miRNAs). This subclass comprises RNAs of 19–25 nucleotides in length that modulate the activity of well-defined coding RNAs and play a crucial role in many physiological and pathological processes. miRNA genes are located both in exons, introns, and also within non-translated regions. Several miRNAs that are transcribed from the adjacent miRNA genes are called cluster. One of the largest ones is miR-17-92 cluster known as OncomiR-1 due to its strong link to oncogenesis. Six miRNAs from the OncomiR-1 have been shown to play important roles in various physiological cellular processes but also through inhibition of cell death in many cancer-relevant processes. Due to the origin and similarity of the sequence, miR-17-92 cluster and paralogs, miR-106b-25 and miR-106a-363 clusters were defined. Here we discuss the oncogenic function of those miRNA subgroups found in many types of cancers, including brain tumors.

WNT signaling, the development of the sympathoadrenal-paraganglionic system and neuroblastoma

Neuroblastoma (NB) is a tumor of the sympathoadrenal system arising in children under 15 years of age. In Germany, NB accounts for 7% of childhood cancer cases, but 11% of cancer deaths. It originates from highly migratory progenitor cells that leave the dorsal neural tube and contribute neurons and glial cells to sympathetic ganglia, and chromaffin and supportive cells to the adrenal medulla and paraganglia. Clinically, histologically and molecularly, NBs present as extremely heterogeneous, ranging from very good to very poor prognosis. The etiology of NB still remains unclear and needs to be elucidated, however, aberrant auto- and paracrine embryonic cell communications seem to be likely candidates to initiate or facilitate the emergence, progression and regression of NB. The wingless-type MMTV integration site (WNT) family of proteins represents an evolutionary highly conserved signaling system that orchestrates embryogenesis. At least 19 ligands in the human, numerous receptors and co-receptors are known, which control not only proliferation, but also cell polarity, migration and differentiation. Here we seek to interconnect aspects of WNT signaling with sympathoadrenal and paraganglionic development to define new WNT signaling cues in the etiology and progression of NB.

Dickkopf homolog 3 (DKK3): A candidate for detection and treatment of cancers?

Wnt signaling is an evolutionary highly conserved pathway that is modulated by several inhibitors and activators, and plays a key role in numerous physiological processes. One of the extracellular Wnt inhibitors is the DKK (Dickkopf Homolog) family which has four members (Dkk1-4) and a unique Dkk3-related gene, Dkkl1 (soggy). DKK3 is a divergent member of the DKK protein family. Evidence suggests that DKK3 may serve as a potential therapeutic target in several types of human cancers. We review here the biological role of DKK3 as a tumor suppressor gene (TSG) or oncogene, and its correlation with various miRNAs. In addition, we discuss the role of polymorphisms and promoter methylation of the DKK3 gene, and of its expression in regulating cancer cell proliferation. Finally, we propose that DKK3 may be considered as both a biomarker and a therapeutic target in different cancers.

Neuroblastoma: A Tough Nut to Crack

Neuroblastoma, an embryonal tumor arising from neural crest-derived progenitor cells, is the most common solid tumor in childhood, with more than 700 cases diagnosed per year in the United States. In the past several decades, significant advances have been made in the treatment of neuroblastoma. Treatment advances reflect improved understanding of the biology of neuroblastoma. Although amplification of MYCN was discovered in the early 1980s, our understanding of neuroblastoma oncogenesis has advanced in the last decade as a result of high-throughput genomic analysis, exome and whole-genome sequencing, genome-wide association studies, and synthetic lethal drug screens. Our refined understanding of neuroblastoma biology and genetics is reflected in improved prognostic stratification and appropriate tailoring of therapy in recent clinical trials. Moreover, for high-risk neuroblastoma, a disease that was uniformly fatal 3 decades ago, recent clinical trials incorporating autologous hematopoietic transplant and immunotherapy utilizing anti-GD2 antibody plus cytokines have shown improved event-free and overall survival. These advances have resulted in a growing population of long-term survivors of neuroblastoma. Examination of the late effects and second malignant neoplasms (SMNs) in both older generations of survivors and more recently treated survivors will inform both design of future trials and surveillance guidelines for long-term follow-up. As a consequence of advances in understanding of the biology of neuroblastoma, successful clinical trials, and refined understanding of the late effects and SMNs of survivors, the promise of precision medicine is becoming a reality for patients with neuroblastoma.

Towards diagnostic application of non-coding RNAs in neuroblastoma

INTRODUCTION

Neuroblastoma is a solid cancer of childhood, which is devastating upon recurrence. Markers for minimal residual disease and early detection of relapse are eagerly awaited to improve the outcome of affected patients. Several miRNAs have been identified as key regulators of neuroblastoma pathogenesis. Areas covered: Here, we focus on miRNAs that have been linked to MYCN, a prominent oncogenic driver, and we review the hitherto known interactions between miRNAs and other important players in neuroblastoma. Expert commentary: Existing diagnostic miRNA signatures remain to be established in clinical settings. Moreover, inhibition of individual oncogenic miRNAs or enhancement of tumor suppressive miRNA function could represent a new therapeutic approach in cancer treatment, including NB.

Onco-GPCR signaling and dysregulated expression of microRNAs in human cancer

The G-protein-coupled receptor (GPCR) family is the largest family of cell-surface receptors involved in signal transduction. Aberrant expression of GPCRs and G proteins are frequently associated with prevalent human diseases, including cancer. In fact, GPCRs represent the therapeutic targets of more than a quarter of the clinical drugs currently on the market. MiRNAs (miRNAs) are also aberrantly expressed in many human cancers, and they have significant roles in the initiation, development and metastasis of human malignancies. Recent studies have revealed that dysregulation of miRNAs and their target genes expression are associated with cancer progression. The emerging information suggests that miRNAs play an important role in the fine tuning of many signaling pathways, including GPCR signaling. We summarize our current knowledge of the individual functions of miRNAs regulated by GPCRs and GPCR signaling-associated molecules, and miRNAs that regulate the expression and activity of GPCRs, their endogenous ligands and their coupled heterotrimeric G proteins in human cancer.

E2F1-miR-20a-5p/20b-5p auto-regulatory feedback loop involved in myoblast proliferation and differentiation

miR-17 family microRNAs (miRNAs) are crucial for embryo development, however, their role in muscle development is still unclear. miR-20a-5p and miR-20b-5p belong to the miR-17 family and are transcribed from the miR-17~92 and miR-106a~363 clusters respectively. In this study, we found that miR-20a-5p and miR-20b-5p promoted myoblast differentiation and repressed myoblast proliferation by directly binding the 3' UTR of E2F transcription factor 1 (E2F1) mRNA. E2F1 is an important transcriptional factor for organism's normal development. Overexpression of E2F1 in myoblasts promoted myoblast proliferation and inhibited myoblast differentiation. Conversely, E2F1 inhibition induced myoblast differentiation and repressed myoblast proliferation. Moreover, E2F1 can bind directly to promoters of the miR-17~92 and miR-106a~363 clusters and activate their transcription, and E2F1 protein expression is correlated with the expression of pri-miR-17~92 and pri-miR-106a~363 during myoblast differentiation. These results suggested an auto-regulatory feedback loop between E2F1 and miR-20a-5p/20b-5p, and indicated that miR-20a-5p, miR-20b-5p and E2F1 are involved in myoblast proliferation and differentiation through the auto-regulation between E2F1 and miR-20a-5p/20b-5p. These findings provide new insight into the mechanism of muscle differentiation, and further shed light on the understanding of muscle development and muscle diseases.

MYCN-targeting miRNAs are predominantly downregulated during MYCN‑driven neuroblastoma tumor formation

MYCN is a transcription factor that plays key roles in both normal development and cancer. In neuroblastoma, MYCN acts as a major oncogenic driver through pleiotropic effects regulated by multiple protein encoding genes as well as microRNAs (miRNAs). MYCN activity is tightly controlled at the level of transcription and protein stability through various mechanisms. Like most genes, MYCN is further controlled by miRNAs, but the full complement of all miRNAs implicated in this process has not been determined through an unbiased approach. To elucidate the role of miRNAs in regulation of MYCN, we thus explored the MYCN-miRNA interactome to establish miRNAs controlling MYCN expression levels. We combined results from an unbiased and genome-wide high-throughput miRNA target reporter screen with miRNA and mRNA expression data from patients and a murine neuroblastoma progression model. We identified 29 miRNAs targeting MYCN, of which 12 miRNAs are inversely correlated with MYCN expression or activity in neuroblastoma tumor tissue. The majority of MYCN-targeting miRNAs in neuroblastoma showed a decrease in expression during murine MYCN-driven neuroblastoma tumor development. Therefore, we provide evidence that MYCN-targeting miRNAs are preferentially downregulated in MYCN-driven neuroblastoma, suggesting that MYCN negatively controls the expression of these miRNAs, to safeguard its expression.

MicroRNA and pediatric tumors: Future perspectives

A better understanding of pediatric tumor biology is needed to allow the development of less toxic and more efficient therapies, as well as to provide novel reliable biomarkers for diagnosis and risk stratification. The emerging role of microRNAs in controlling key pathways implicated in tumorigenesis makes their use in diagnostics a powerful novel tool for the early detection, risk assessment and prognosis, as well as for the development of innovative anticancer therapies. This perspective would be more urgent for the clinical management of pediatric cancer. In this review, we focus on the involvement of microRNAs in the biology of the main childhood tumors, describe their clinical significance and discuss their potential use as novel therapeutic tools and targets.

Effect of autonomic nerves on Dickkopf-3 expression in the uterus during early pregnancy of rats

To explore how uterine innervations affect expression of Dickkopf-3 (DKK-3) during peri-implantation, we first examined the consequence of uterine neurectomy on embryo implantation events. We observed that amputation of autonomic nerves innervating the uterus led to the failure of on-time implantation in rats. We then analyzed the effect of neurectomy on expression of DKK-3 further using immunohistochemistry and quantitative real-time reverse transcription polymerase chain reaction. We observed that disconnection of autonomic nerve innervation significantly increased DKK-3 expression in the endometrium before and during invasion of the blastocyst. We also observed high levels of DKK-3 immunoreactivity in the vasculature of the uterus during peri-implantation. Thus, we speculate that DKK-3 may relate to implantation. Besides, our findings provide a new line of evidence that DKK-3 may be regulated by the autonomic nervous system.

Pax6 regulates the expression of Dkk3 in murine and human cell lines, and altered responses to Wnt signaling are shown in FlpIn-3T3 cells stably expressing either the Pax6 or the Pax6(5a) isoform

Pax6 is a transcription factor important for early embryo development. It is expressed in several cancer cell lines and tumors. In glioblastoma, PAX6 has been shown to function as a tumor suppressor. Dickkopf 3 (Dkk3) is well established as a tumor suppressor in several tumor types, but not much is known about the regulation of its expression. We have previously found that Pax6 and Pax6(5a) increase the expression of the Dkk3 gene in two stably transfected mouse fibroblast cell lines. In this study the molecular mechanism behind this regulation is looked at. Western blot and reverse transcriptase quantitative PCR (RT-qPCR) confirmed higher level of Dkk3 expression in both Pax6 and Pax6(5a) expressing cell lines compared to the control cell line. By the use of bioinformatics and electrophoretic mobility shift assay (EMSA) we have mapped a functional Pax6 binding site in the mouse Dkk3 promoter. The minimal Dkk3 promoter fragment required for transcriptional activation by Pax6 and Pax6(5a) was a 200 bp region just upstream of the transcriptional start site. Mutation of the evolutionary conserved binding site in this region abrogated transcriptional activation and binding of Pax6/Pax6(5a) to the mouse Dkk3 promoter. Since the identified Pax6 binding site in this promoter is conserved, RT-qPCR and Western blot were used to look for regulation of Dkk3/REIC expression in human cell lines. Six of eight cell lines tested showed changes in Dkk3/REIC expression after PAX6 siRNA knockdown. Interestingly, we observed that the Pax6/Pax6(5a) expressing mouse fibroblast cell lines were less responsive to canonical Wnt pathway stimulation than the control cell line when TOP/FOP activity and the levels of active β-catenin and GSK3-β Ser9 phosphorylation were measured after LiCl stimulation.

Significance and therapeutic value of miRNAs in embryonal neural tumors

Embryonal tumors of the nervous system are the leading cause of childhood cancer-related morbidity and mortality. Medulloblastoma, supratentorial primitive neuroectodermal tumors, atypical teratoid/rhabdoid tumor and neuroblastoma account for more than 20% of childhood malignancies and typify the current neural embryonal tumor model in pediatric oncology. Mechanisms driving the formation of these tumors point towards impaired differentiation of neuronal and neuron-associated cells during the development of the nervous system as an important factor. The importance of microRNAs (miRNAs) for proper embryonic cell function has been confirmed and their aberrant expressions have been linked to tumor development. The role of miRNAs in controlling essential regulators of key pathways implicated in tumor development makes their use in diagnostics a powerful tool to be used for early detection of cancer, risk assessment and prognosis, as well as for the design of innovative therapeutic strategies. In this review we focus on the significance of miRNAs involved in the biology of embryonal neural tumors, delineate their clinical significance and discuss their potential as a novel therapeutic target.

The roles of microRNAs in neuroblastoma

BACKGROUND

Neuroblastoma (NB) is the most common extracranial solid tumor in childhood and displays remarkable heterogeneity in clinical behaviors, ranging from spontaneous regression to rapid progression or resistance to multimodal treatment. Recent evidence has shown that microRNAs (miRNAs), a class of small non-coding RNAs, are involved in tumor development and progression. This article aimed to review recent advances in investigating the roles of miRNAs in NB.

METHODS

We searched the PubMed/MEDLINE database for articles about the expression profile, functions and target genes of miRNAs in NB.

RESULTS

We reviewed the most recent evidence regarding the functional roles of oncogenic and tumor suppressive miRNAs in NB and application of novel miRNA-based methods for diagnostic, prognostic and therapeutic purposes.

CONCLUSIONS

Deregulation of miRNAs is associated with the development and progression of NB, suggesting that miRNAs may serve as novel targets for the treatment of high-risk NB patients. However, their precise functions and underlying mechanisms still warrant further studies.

Role of Intracellular and Extracellular MicroRNA-92a in Colorectal Cancer

Colorectal cancer is one of the leading causes of cancer-related death worldwide. Previous studies have shown that miR-92a has an oncogenic function in several cancers and that its up-regulation is correlated with malignant clinicopathologic behaviors of colorectal cancer. It also has been suggested that circulating miR-92a in patients' plasma can be a potential biomarker for colorectal cancer. However, the precise roles of intracellular and extracellular miR-92a are not yet understood. In this study, we examined the expression levels of miR-92a in colorectal tumors (38 cancer specimens and 56 adenoma specimens) and paired adjacent nontumorous tissues. Increased expression of miR-92a was frequently observed in the cancers compared with that in the adenomas and was correlated with advanced clinical stages, tumor depth, and size. We also demonstrated that the levels of miR-92a within microvesicles (MVs) in the plasma of mice bearing colon cancer xenografts were significantly increased compared with those in control mice. One of the roles of intracellular and extracellular miR-92a was shown to be down-regulation of Dickkopf-3 (Dkk-3), a presumed tumor suppressor gene. Within the colon cancer cells, suppression of Dkk-3 by miR-92a contributed to the cell proliferation. Extracellular miR-92a packed within MVs secreted by colon cancer cells was delivered into endothelial cells and contributed to the proliferation and motility of these cells through down-regulation of the same target gene, Dkk-3. These data suggest that intracellular and extracellular miR-92a had important roles in tumor growth and the tumor microenvironment in colorectal cancer.

Cidofovir selectivity is based on the different response of normal and cancer cells to DNA damage

Background

Cidofovir (CDV) proved efficacious in treatment of human papillomaviruses (HPVs) hyperplasias. Antiproliferative effects of CDV have been associated with apoptosis induction, S-phase accumulation, and increased levels of tumor suppressor proteins. However, the molecular mechanisms for the selectivity and antitumor activity of CDV against HPV-transformed cells remain unexplained.

Methods

We evaluated CDV drug metabolism and incorporation into cellular DNA, in addition to whole genome gene expression profiling by means of microarrays in two HPV⁺ cervical carcinoma cells, HPV^- immortalized keratinocytes, and normal keratinocytes.

Results

Determination of the metabolism and drug incorporation of CDV into genomic DNA demonstrated a higher rate of drug incorporation in HPV⁺ tumor cells and immortalized keratinocytes compared to normal keratinocytes. Gene expression profiling clearly showed distinct and specific drug effects in the cell types investigated. Although an effect on inflammatory response was seen in all cell types, different pathways were identified in normal keratinocytes compared to immortalized keratinocytes and HPV⁺ tumor cells. Notably, Rho GTPase pathways, LXR/RXR pathways, and acute phase response signaling were exclusively activated in immortalized cells. CDV exposed normal keratinocytes displayed activated cell cycle regulation upon DNA damage signaling to allow DNA repair via homologous recombination, resulting in genomic stability and survival. Although CDV induced cell cycle arrest in HPV^- immortalized cells, DNA repair was not activated in these cells. In contrast, HPV⁺ cells lacked cell cycle regulation, leading to genomic instability and eventually apoptosis.

Conclusions

Taken together, our data provide novel insights into the mechanism of action of CDV and its selectivity for HPV-transformed cells. The proposed mechanism suggests that this selectivity is based on the inability of HPV⁺ cells to respond to DNA damage, rather than on a direct anti-HPV effect. Since cell cycle control is deregulated by the viral oncoproteins E6 and E7 in HPV⁺ cells, these cells are more susceptible to DNA damage than normal keratinocytes. Our findings underline the therapeutic potential of CDV for HPV-associated malignancies as well as other neoplasias.

MYCN-mediated transcriptional repression in neuroblastoma: the other side of the coin

Neuroblastoma is the most common extra cranial solid tumor in childhood and the most frequently diagnosed neoplasm during the infancy. MYCN amplification and overexpression occur in about 25% of total neuroblastoma cases and this percentage increases at 30% in advanced stage neuroblastoma. So far, MYCN expression profile is still one of the most robust and significant prognostic markers for neuroblastoma outcome. MYCN is a transcription factor that belongs to the family of MYC oncoproteins, comprising c-MYC and MYCL genes. Deregulation of MYC oncoprotein expression is a crucial event involved in the occurrence of different types of malignant tumors. MYCN, as well as c-MYC, can heterodimerize with its partner MAX and activate the transcription of several target genes containing E-Box sites in their promoter regions. However, recent several lines of evidence have revealed that MYCN can repress at least as many genes as it activates, thus proposing a novel function of this protein in neuroblastoma biology. Whereas the mechanism by which MYCN can act as a transcriptional activator is relatively well known, very few studies has been done in the attempt to explain how MYCN can exert its transcription repression function. Here, we will review current knowledge about the mechanism of MYCN-mediated transcriptional repression and will emphasize its role as a repressor in the recruitment of a precise set of proteins to form complexes capable of down-regulating specific subsets of genes whose function is actively involved in apoptosis, cell differentiation, chemosensitivity, and cell motility. The finding that MYCN can also act as a repressor has widen our view on its role in oncogenesis and has posed the bases to search for novel therapeutic drugs that can specifically target its transcriptional repression function.

A systemic transcriptome analysis reveals the regulation of neural stem cell maintenance by an E2F1-miRNA feedback loop

Stem cell fate decisions are controlled by a molecular network in which transcription factors and miRNAs are of key importance. To systemically investigate their impact on neural stem cell (NSC) maintenance and neuronal commitment, we performed a high-throughput mRNA and miRNA profiling and isolated functional interaction networks of involved mechanisms. Thereby, we identified an E2F1–miRNA feedback loop as important regulator of NSC fate decisions. Although E2F1 supports NSC proliferation and represses transcription of miRNAs from the miR-17∼92 and miR-106a∼363 clusters, these miRNAs are transiently up-regulated at early stages of neuronal differentiation. In these early committed cells, increased miRNAs expression levels directly repress E2F1 mRNA levels and inhibit cellular proliferation. In mice, we demonstrated that these miRNAs are expressed in the neurogenic areas and that E2F1 inhibition represses NSC proliferation. The here presented data suggest a novel interaction mechanism between E2F1 and miR-17∼92 / miR-106a∼363 miRNAs in controlling NSC proliferation and neuronal differentiation.

The role of genetic and epigenetic alterations in neuroblastoma disease pathogenesis

Neuroblastoma is a highly heterogeneous tumor accounting for 15 % of all pediatric cancer deaths. Clinical behavior ranges from the spontaneous regression of localized, asymptomatic tumors, as well as metastasized tumors in infants, to rapid progression and resistance to therapy. Genomic amplification of the MYCN oncogene has been used to predict outcome in neuroblastoma for over 30 years, however, recent methodological advances including miRNA and mRNA profiling, comparative genomic hybridization (array-CGH), and whole-genome sequencing have enabled the detailed analysis of the neuroblastoma genome, leading to the identification of new prognostic markers and better patient stratification. In this review, we will describe the main genetic factors responsible for these diverse clinical phenotypes in neuroblastoma, the chronology of their discovery, and the impact on patient prognosis.

Focal DNA copy number changes in neuroblastoma target MYCN regulated genes

Neuroblastoma is an embryonic tumor arising from immature sympathetic nervous system cells. Recurrent genomic alterations include MYCN and ALK amplification as well as recurrent patterns of gains and losses of whole or large partial chromosome segments. A recent whole genome sequencing effort yielded no frequently recurring mutations in genes other than those affecting ALK. However, the study further stresses the importance of DNA copy number alterations in this disease, in particular for genes implicated in neuritogenesis. Here we provide additional evidence for the importance of focal DNA copy number gains and losses, which are predominantly observed in MYCN amplified tumors. A focal 5 kb gain encompassing the MYCN regulated miR-17∼92 cluster as sole gene was detected in a neuroblastoma cell line and further analyses of the array CGH data set demonstrated enrichment for other MYCN target genes in focal gains and amplifications. Next we applied an integrated genomics analysis to prioritize MYCN down regulated genes mediated by MYCN driven miRNAs within regions of focal heterozygous or homozygous deletion. We identified RGS5, a negative regulator of G-protein signaling implicated in vascular normalization, invasion and metastasis, targeted by a focal homozygous deletion, as a new MYCN target gene, down regulated through MYCN activated miRNAs. In addition, we expand the miR-17∼92 regulatory network controlling TGFß signaling in neuroblastoma with the ring finger protein 11 encoding gene RNF11, which was previously shown to be targeted by the miR-17∼92 member miR-19b. Taken together, our data indicate that focal DNA copy number imbalances in neuroblastoma (1) target genes that are implicated in MYCN signaling, possibly selected to reinforce MYCN oncogene addiction and (2) serve as a resource for identifying new molecular targets for treatment.

p53, SKP2, and DKK3 as MYCN Target Genes and Their Potential Therapeutic Significance

Neuroblastoma is the most common extra-cranial solid tumor of childhood. Despite significant advances, it currently still remains one of the most difficult childhood cancers to cure, with less than 40% of patients with high-risk disease being long-term survivors. MYCN is a proto-oncogene implicated to be directly involved in neuroblastoma development. Amplification of MYCN is associated with rapid tumor progression and poor prognosis. Novel therapeutic strategies which can improve the survival rates whilst reducing the toxicity in these patients are therefore required. Here we discuss genes regulated by MYCN in neuroblastoma, with particular reference to p53, SKP2, and DKK3 and strategies that may be employed to target them.

N-myc and noncoding RNAs in neuroblastoma

Neuroblastoma is a pediatric tumor of the sympathetic nervous system. Amplification and overexpression of the MYCN proto-oncogene occurs in approximately 20% of neuroblastomas and is associated with advanced stage disease, rapid tumor progression, and poor prognosis. MYCN encodes the transcriptional regulator N-myc, which has been shown to both up- and downregulate many target genes involved in cell cycle, DNA damage, differentiation, and apoptosis in neuroblastoma. During the last years, it has become clear that N-myc also modulates the expression of several classes of noncoding RNAs, in particular microRNAs. MicroRNAs are the most widely studied noncoding RNA molecules in neuroblastoma. They function as negative regulators of gene expression at the posttranscriptional level in diverse cellular processes. Aberrant regulation of miRNA expression has been implicated in the pathogenesis of neuroblastoma. While the N-myc protein is established as an important regulator of several miRNAs involved in neuroblastoma tumorigenesis, tumor suppressor miRNAs have also been documented to repress MYCN expression and inhibit cell proliferation of MYCN-amplified neuroblastoma cells. It is now becoming increasingly evident that N-myc also regulates the expression of long noncoding RNAs such as T-UCRs and ncRAN. This review summarizes the current knowledge about the interplay between N-myc and noncoding RNAs in neuroblastoma and how this contributes to neuroblastoma tumorigenesis.

Targeting the Wnt pathway in cancer: the emerging role of Dickkopf-3

Aberrant activation of the Wnt signaling pathway is a major trait of many human cancers. Due to its vast implications in tumorigenesis and progression, the Wnt pathway has attracted considerable attention at several molecular levels, also with respect to developing novel cancer therapeutics. Indeed, research in Wnt biology has recently provided numerous clues, and evidence is accumulating that the secreted Wnt antagonist Dickkopf-related protein 3 (Dkk-3) and its regulators may constitute interesting therapeutic targets in the most important human cancers. Based on the currently available literature, we here review the knowledge on the biological role of Dkk-3 as an antagonist of the Wnt signaling pathway, the involvement of Dkk-3 in several stages of tumor development, the genetic and epigenetic mechanisms disrupting DKK3 gene function in cancerous cells, and the potential clinical value of Dkk-3 expression/DKK3 promoter methylation as a biomarker and molecular target in cancer diseases. In conclusion, Dkk-3 rapidly emerges as a key player in human cancer with auspicious tumor suppressive capacities, most of all affecting apoptosis and proliferation. Its gene expression is frequently downregulated by promoter methylation in almost any solid and hematological tumor entity. Clinically, evidence is accumulating of Dkk-3 being both a potential tumor biomarker and effective anti-cancer agent. Although further research is needed, re-establishing Dkk-3 expression in cancer cells holds promise as novel targeted molecular tumor therapy.

Neuroblastoma genetics and phenotype: a tale of heterogeneity

Cancer is a complex disease driven by multiple genetic and epigenetic alterations. Understanding the (epi-)genetic changes and consequent deregulation of regulatory networks controlling the various normal critical cellular phenotypes that are perturbed in cancer cells can provide clues to new therapeutic opportunities. Moreover, such insights into the molecular pathology of a given cancer type can offer clinical relevant genetic markers or molecular signatures for assessment of prognosis and response to therapy, and prediction of risk for relapse. Therefore, as for many other tumour entities, neuroblastoma (NB) has been the subject of intensive ongoing genomic research. Here we will summarize the current state-of-the-art of these studies with focus on genome wide DNA copy number and gene expression analyses in relation to the relevance for present and future clinical management of NB patients.