Bmi1 is a MYCN target gene that regulates tumorigenesis through repression of KIF1Bbeta and TSLC1 in neuroblastoma

L3MBTL2 maintains MYCN-amplified neuroblastoma cell proliferation through silencing NRIP3 and BRME1 genes

Epigenetic alterations critically affect tumor development. Polycomb-group complexes constitute an evolutionarily conserved epigenetic machinery that regulates stem cell fate and development. They are implicated in tumorigenesis, primarily via histone modification. Polycomb repressive complex 1 (PRC1) complexes 1-6 (PRC1.1-6) mediate the ubiquitination of histone H2A on lysine 119 (H2AK119ub). Here, we studied the functional roles of a PRC1.6 molecule, L3MBTL2, in neuroblastoma (NB) cells. L3MBTL2-knockout and knockdown revealed that L3MBTL2 depletion suppressed NB cell proliferation via cell-cycle arrest and gamma-H2A.X upregulation. L3MBTL2-knockout profoundly suppressed xenograft tumor formation. Transcriptome analysis revealed suppressed cell-cycle-related and activated differentiation-related pathways. Break repair meiotic recombinase recruitment factor 1 (BRME1) and nuclear receptor interacting protein 3 (NRIP3) were notably de-repressed by L3MBTL2-knockout. The deletion of L3MBTL2 reduced enrichment of H2AK119ub and PCGF6 at transcriptional start site proximal regions of the targets. Add-back studies unveiled the importance of L3MBTL2-BRME1 and -NRIP3 axes for NB cell proliferation. We further manifested the association of MYCN with de-repression of NRIP3 in an L3MBTL2-deficient context. Therefore, this study first revealed the significance of L3MBTL2-mediated gene silencing in MYCN-amplified NB cells.

CX‑5461 potentiates imatinib‑induced apoptosis in K562 cells by stimulating KIF1B expression

The selective RNA polymerase I inhibitor CX-5461 has been shown to be effective in treating some types of leukemic disorders. Emerging evidence suggests that combined treatments with CX-5461 and other chemotherapeutic agents may achieve enhanced effectiveness as compared with monotherapies. Currently, pharmacodynamic properties of the combination of CX-5461 with tyrosine kinase inhibitors remain to be explored. The present study tested whether CX-5461 could potentiate the effect of imatinib in the human chronic myeloid leukemia cell line K562, which is p53-deficient. It was demonstrated that CX-5461 at 100 nM, which was non-cytotoxic in K562 cells, potentiated the pro-apoptotic effect of imatinib. Mechanistically, the present study identified that the upregulated expression of kinesin family member 1B (KIF1B) gene might be involved in mediating the pro-apoptotic effect of imatinib/CX-5461 combination. Under the present experimental settings, however, neither CX-5461 nor imatinib alone exhibited a significant effect on KIF1B expression. Moreover, using other leukemic cell lines, it was demonstrated that regulation of KIF1B expression by imatinib/CX-5461 was not a ubiquitous phenomenon in leukemic cells and should be studied in a cell type-specific manner. In conclusion, the results suggested that the synergistic interaction between CX-5461 and imatinib may be of potential clinical value for the treatment of tyrosine kinase inhibitor-resistant chronic myeloid leukemia.

Polycomb repressor complex: Its function in human cancer and therapeutic target strategy

The Polycomb Repressor Complex (PRC) plays a pivotal role in gene regulation during development and disease, with dysregulation contributing significantly to various human cancers. The intricate interplay between PRC and cellular signaling pathways sheds light on cancer complexity. PRC presents promising therapeutic opportunities, with inhibitors undergoing rigorous evaluation in preclinical and clinical studies. In this review, we emphasize the critical role of PRC complex in gene regulation, particularly PcG proteins mediated chromatin compaction through phase separation. We also highlight the pathological implications of PRC complex dysregulation in various tumors, elucidating underlying mechanisms driving cancer progression. The burgeoning field of therapeutic strategies targeting PRC complexes, notably EZH2 inhibitors, has advanced significantly. However, we explore the need for combination therapies to enhance PRC targeted treatments efficacy, providing a glimpse into the future of cancer therapeutics.

The role of BMI1 in endometrial cancer and other cancers

Endometrial cancer (EC) is the third leading gynecological malignancy, and its treatment remains challenging. B cell-specific Moloney murine leukemia virus integration site-1 (BMI1) is one of the core members of the polycomb group (PcG) family, which plays a promoting role in the occurrence and development of various tumors. Notably, BMI1 has been found to be frequently upregulated in endometrial cancer (EC) and promote the occurrence of EC through promoting epithelial-mesenchymal transition (EMT) and AKT/PI3K pathways. This review summarizes the structure and upstream regulatory mechanisms of BMI1 and its role in EC. In addition, we focused on the role of BMI1 in chemoradiotherapy resistance and summarized the current drugs that target BMI1.

Polycomb group protein BMI1 protects neuroblastoma cells against DNA damage-induced apoptotic cell death

The overexpression of BMI1, a polycomb protein, correlates with cancer development and aggressiveness. We previously reported that MYCN-induced BMI1 positively regulated neuroblastoma (NB) cell proliferation via the transcriptional inhibition of tumor suppressors in NB cells. To assess the potential of BMI1 as a new target for NB therapy, we examined the effects of reductions in BMI1 on NB cells. BMI1 knockdown (KD) in NB cells significantly induced their differentiation for up to 7 days. BMI1 depletion significantly induced apoptotic NB cell death for up to 14 days along with the activation of p53, increases in p73, and induction of p53 family downstream molecules and pathways, even in p53 mutant cells. BMI1 depletion in vivo markedly suppressed NB xenograft tumor growth. BMI1 reductions activated ATM and increased γ-H2AX in NB cells. These DNA damage signals and apoptotic cell death were not canceled by the transduction of the polycomb group molecules EZH2 and RING1B. Furthermore, EZH2 and RING1B KD did not induce apoptotic NB cell death to the same extent as BMI1 KD. Collectively, these results suggest the potential of BMI1 as a target of molecular therapy for NB and confirmed, for the first time, the shared role of PcG proteins in the DNA damage response of NB cells.

An immature, dedifferentiated, and lineage-deconstrained cone precursor origin of N-Myc-initiated retinoblastoma

Most retinoblastomas develop from maturing cone precursors in response to biallelic RB1 loss and are dependent on cone maturation-related signaling. Additionally, ∼2% lack RB1 mutations but have MYCN amplification (MYCN^A), N-Myc protein overexpression, and more rapid and invasive growth, yet the MYCN^A retinoblastoma cell of origin and basis for its responses to deregulated N-Myc are unknown. Here, using explanted cultured retinae, we show that ectopic N-Myc induces cell cycle entry in cells expressing markers of several retinal types yet induces continuous proliferation and tumorigenesis only in cone precursors. Unlike the response to RB1 loss, both immature cone arrestin-negative (ARR3^-) and maturing ARR3⁺ cone precursors proliferate, and maturing cone precursors rapidly dedifferentiate, losing ARR3 as well as L/M-opsin expression. N-Myc-overexpressing retinal cells also lose cell lineage constraints, occasionally coexpressing the cone-specific RXRγ with the rod-specific NRL or amacrine-specific AP2α and widely coexpressing RXRγ with the progenitor and Müller cell-specific SOX9 and retinal ganglion cell-specific BRN3 and GAP43. Mechanistically, N-Myc induced Cyclin D2 and CDK4 overexpression, pRB phosphorylation, and SOX9-dependent proliferation without a retinoma-like stage that characterizes pRB-deficient retinoblastoma, despite continuous p16^INK4A expression. Orthotopic xenografts of N-Myc-overexpressing retinal cells formed tumors with retinal cell marker expression similar to those in MYCN-transduced retinae and MYCN^A retinoblastomas in patients. These findings demonstrate the MYCN^A retinoblastoma origin from immature and lineage-deconstrained cone precursors, reveal their opportunistic use of an undifferentiated retinal progenitor cell feature, and illustrate that different cancer-initiating mutations cooperate with distinct developmental stage-specific cell signaling circuitries to drive retinoblastoma tumorigenesis.

The PRC2 molecule EED is a target of epigenetic therapy for neuroblastoma

Epigenetic modifications by polycomb repressive complex (PRC) molecules appear to play a role in the tumorigenesis and aggressiveness of neuroblastoma (NB). Embryonic ectoderm development (EED) is a member of the PRC2 complex that binds to the H3K27me3 mark deposited by EZH2 via propagation on adjacent nucleosomes. We herein investigated the molecular roles of EED in MYCN-amplified NB cells using EED-knockdown (KD) shRNAs, EED-knockout sgRNAs, and the EED small molecule inhibitor EED226. The suppression of EED markedly inhibited NB cell proliferation and flat and soft agar colony formation. A transcriptome analysis using microarrays of EED-KD NB cells indicated the de-repression of cell cycle-regulated and differentiation-related genes. The results of a GSEA analysis suggested that inhibitory cell cycle-regulated gene sets were markedly up-regulated. Furthermore, an epigenetic treatment with the EED inhibitor EED226 and the HDAC inhibitors valproic acid/SAHA effectively suppressed NB cell proliferation and colony formation. This combined epigenetic treatment up-regulated cell cycle-regulated and differentiation-related genes. The ChIP sequencing analysis of histone codes and PRC molecules suggested an epigenetic background for the de-repression of down-regulated genes in MYCN-amplified/PRC2 up-regulated NB.

Molecular Basis and Clinical Features of Neuroblastoma

Neuroblastoma, a neoplasm of the sympathetic nervous system, originates from neuroblastoma stem cells during embryogenesis. It exhibits unique clinical features including a tendency for spontaneous regression of tumors in infants and a high frequency of metastatic disease at diagnosis in patients aged over 18 months. Genetic risk factors and epigenetic dysregulation also play a significant role in the development of neuroblastoma. Over the past decade, our understanding of this disease has advanced considerably. This has included the identification of chromosomal copy number aberrations specific to neuroblastoma development, risk groups, and disease stage. However, high-risk neuroblastoma remains a therapeutic challenge for pediatric oncologists. New therapeutic approaches have been developed, either as alternatives to conventional chemotherapy or in combination, to overcome the dismal prognosis. Particularly promising strategies are targeted therapies that directly affect cancer cells or cancer stem cells while exhibiting minimal effect on healthy cells. This review summarizes our understanding of neuroblastoma biology and prognostic features and focuses on novel therapeutic strategies for this intractable disease.

Neuronal Differentiation-Related Epigenetic Regulator ZRF1 Has Independent Prognostic Value in Neuroblastoma but Is Functionally Dispensable In Vitro

Simple Summary

Neuroblastoma is the most common pediatric solid tumor occurring outside the brain, and it is thought to arise from cells that acquire errors during the normal process of embryonal development. Today, we know that embryonal development is regulated by epigenetics, a mechanism that determines which genes need to be expressed in each cell type and developmental step. Epigenetic errors, therefore, are considered contributory to the appearance and progression of tumors such as neuroblastoma. Here, we aimed at finding whether ZRF1, a known epigenetic regulator, could play a significant role in the aggressiveness of neuroblastoma. Our results suggest that ZRF1 does not seem to have any relevant function in neuroblastoma cells; however, the levels of this epigenetic regulator are related to the prognostic of neuroblastoma patients and could be used to predict their progression and improve the diagnosis.

Abstract

Neuroblastoma is a pediatric tumor of the peripheral nervous system that accounts for up to ~15% of all cancer-related deaths in children. Recently, it has become evident that epigenetic deregulation is a relevant event in pediatric tumors such as high-risk neuroblastomas, and a determinant for processes, such as cell differentiation blockade and sustained proliferation, which promote tumor progression and resistance to current therapies. Thus, a better understanding of epigenetic factors implicated in the aggressive behavior of neuroblastoma cells is crucial for the development of better treatments. In this study, we characterized the role of ZRF1, an epigenetic activator recruited to genes involved in the maintenance of the identity of neural progenitors. We combined analysis of patient sample expression datasets with loss- and gain-of-function studies on neuroblastoma cell lines. Functional analyses revealed that ZRF1 is functionally dispensable for those cellular functions related to cell differentiation, proliferation, migration, and invasion, and does not affect the cellular response to chemotherapeutic agents. However, we found that high levels of ZRF1 mRNA expression are associated to shorter overall survival of neuroblastoma patients, even when those patients with the most common molecular alterations used as prognostic factors are removed from the analyses, thereby suggesting that ZRF1 expression could be used as an independent prognostic factor in neuroblastoma.

Mechanisms involved in selecting and maintaining neuroblastoma cancer stem cell populations, and perspectives for therapeutic targeting

Pediatric neuroblastomas (NBs) are heterogeneous, aggressive, therapy-resistant embryonal tumours that originate from cells of neural crest (NC) origin and in particular neuroblasts committed to the sympathoadrenal progenitor cell lineage. Therapeutic resistance, post-therapeutic relapse and subsequent metastatic NB progression are driven primarily by cancer stem cell (CSC)-like subpopulations, which through their self-renewing capacity, intermittent and slow cell cycles, drug-resistant and reversibly adaptive plastic phenotypes, represent the most important obstacle to improving therapeutic outcomes in unfavourable NBs. In this review, dedicated to NB CSCs and the prospects for their therapeutic eradication, we initiate with brief descriptions of the unique transient vertebrate embryonic NC structure and salient molecular protagonists involved NC induction, specification, epithelial to mesenchymal transition and migratory behaviour, in order to familiarise the reader with the embryonic cellular and molecular origins and background to NB. We follow this by introducing NB and the potential NC-derived stem/progenitor cell origins of NBs, before providing a comprehensive review of the salient molecules, signalling pathways, mechanisms, tumour microenvironmental and therapeutic conditions involved in promoting, selecting and maintaining NB CSC subpopulations, and that underpin their therapy-resistant, self-renewing metastatic behaviour. Finally, we review potential therapeutic strategies and future prospects for targeting and eradication of these bastions of NB therapeutic resistance, post-therapeutic relapse and metastatic progression.

Regulation of gene transcription of B lymphoma Mo-MLV insertion region 1 homolog (Review)

B lymphoma Mo-MLV insertion region 1 homolog (Bmi-1) is a core protein component of the polycomb repressive complex 1 that inhibits cell senescence and maintains the self-renewal ability of stem cells via downregulation of p16Ink4a and p19Arf expression. Bmi-1 serves an important role in hematopoietic stem cell maintenance and neurodevelopment during embryonic development, and it has been shown to enhance tumorigenesis by promoting cancer stem cell self-renewal and epithelial to mesenchymal transition. Emerging evidence suggests that Bmi-1 overexpression is closely related to the development and progression of various types of cancer, and that downregulation of Bmi-1 expression can inhibit the proliferation, invasion and metastasis of cancer cells. It is therefore important to elucidate the mechanisms underlying the regulation of Bmi-1 expression both under normal growth conditions and in malignant tissues. In the present review, the current body of knowledge pertaining to the transcriptional and post-transcriptional regulation of the BMI-1 gene is discussed, and the potential mechanisms by which Bmi-1 is dysregulated in various types of cancer are highlighted. Bmi-1 expression is primarily controlled via transcriptional regulation, and is regulated by the transcription https://www.ushuaia.pl/hyphen/?ln=en factors of the Myc family, including Myb, Twist1, SALL4 and E2F-1. Post-transcriptionally, regulation of Bmi-1 expression is inhibited by several microRNAs and certain small-molecule drugs. Thus, regulatory transcriptional factors are potential therapeutic targets to reduce Bmi-1 expression in cancer cells. Thus, the present review provides an up-to-date review on the regulation of BMI-1 gene expression at the transcriptional and post-transcriptional level.

Targeting the p53-MDM2 pathway for neuroblastoma therapy: Rays of hope

Despite being the subject of extensive research and clinical trials, neuroblastoma remains a major therapeutic challenge in pediatric oncology. The p53 protein is a central safeguard that protects cells against genome instability and malignant transformation. Mutated TP53 (the gene encoding p53) is implicated in many human cancers, but the majority of neuroblastomas have wild type p53 with intact transcriptional function. In fact, the TP53 mutation rate does not exceed 1-2% in neuroblastomas. However, overexpression of the murine double minute 2 (MDM2) gene in neuroblastoma is relatively common, and leads to inhibition of p53. It is also associated with other non-canonical p53-independent functions, including drug resistance and increased translation of MYCN and VEGF mRNA. The p53-MDM2 pathway in neuroblastoma is also modulated at several different molecular levels, including via interactions with other proteins (MYCN, p14^ARF). In addition, the overexpression of MDM2 in tumors is linked to a poorer prognosis for cancer patients. Thus, restoring p53 function by inhibiting its interaction with MDM2 is a potential therapeutic strategy for neuroblastoma. A number of p53-MDM2 antagonists have been designed and studied for this purpose. This review summarizes the current understanding of p53 biology and the p53-dependent and -independent oncogenic functions of MDM2 in neuroblastoma, and also the regulation of the p53-MDM2 axis in neuroblastoma. This review also highlights the use of MDM2 as a molecular target for the disease, and describes the MDM2 inhibitors currently being investigated in preclinical and clinical studies. We also briefly explain the various strategies that have been used and future directions to take in the development of effective MDM2 inhibitors for neuroblastoma.

Internal Ribosome Entry Sites Mediate Cap-Independent Translation of Bmi1 in Nasopharyngeal Carcinoma

Bmi1 is overexpressed in multiple human cancers. We previously reported the oncogenic function and the transcription regulation mechanisms of Bmi1 in nasopharyngeal carcinoma (NPC). In this study, we observed that the mRNA and the protein levels of Bmi1 were strictly inconsistent in NPC cell lines and cancer tissues. The inhibitors of proteasome and lysosome could not enhance the protein level of Bmi1, indicating that Bmi1 may be post-transcriptionally regulated. The IRESite analysis showed that there were two potential internal ribosome entry sites (IRESs) in the 5'-untranslated region (5'-UTR) of Bmi1. The luciferase assay demonstrated that the 5'-UTR of Bmi1 has IRES activity, which may mediate cap-independent translation. The IRES activity of the Bmi1 5'-UTR was significantly reduced after the mutation of the two IRES elements. Taken together, these results suggested that the IRES elements mediating translation is a novel post-transcriptional regulation mechanism of Bmi1.

Mediator complex subunit 16 is down-regulated in papillary thyroid cancer, leading to increased transforming growth factor-β signaling and radioiodine resistance

Mediator complex subunit 16 (MED16) is a component of the mediator complex and functions as a coactivator in transcriptional events at almost all RNA polymerase II-dependent genes. In this study, we report that the expression of MED16 is markedly decreased in papillary thyroid cancer (PTC) tumors compared with normal thyroid tissues. In vitro, MED16 overexpression in PTC cells significantly inhibited cell migration, enhanced sodium/iodide symporter expression and iodine uptake, and decreased resistance to radioactive ¹³¹I (RAI). Conversely, PTC cells in which MED16 had been further knocked down (MED16^KD) exhibited enhanced cell migration, epithelial-mesenchymal transition, and RAI resistance, accompanied by decreased sodium/iodide symporter levels. Moreover, cell signaling through transforming growth factor β (TGF-β) was highly activated after the MED16 knockdown. Similar results were obtained in MED12^KD PTC cells, and a co-immunoprecipitation experiment verified interactions between MED16 and MED12 and between MED16 and TGF-βR2. Of note, the application of LY2157299, a potent inhibitor of TGF-β signaling, significantly attenuated MED16^KD-induced RAI resistance both in vitro and in vivo In conclusion, our findings indicate that MED16 reduction in PTC contributes to tumor progression and RAI resistance via the activation of the TGF-β pathway.

MicroRNA-429 inhibits bone metastasis in breast cancer by regulating CrkL and MMP-9

Bone metastasis is common in late-stage breast cancer patients and leads to skeletal-related events that affect the quality of life and decrease survival. Numerous miRNAs have been confirmed to be involved in metastatic breast cancer, such as the miR200 family. Our previous study identified microRNA-429 (miR-429) as a regulatory molecule in breast cancer bone metastasis. However, the effects of miR-429 and its regulatory axis in the metastatic breast cancer bone microenvironment have not been thoroughly investigated. We observed a positive correlation between miR-429 expression in clinical tissues and the bone metastasis-free interval and a negative correlation between miR-429 expression and the degree of bone metastasis. We cultured bone metastatic MDA-MB-231 cells and used conditioned medium (CM) to detect the effect of miR-429 on osteoblast and osteoclast cells in vitro. We constructed an orthotopic bone destruction model and a left ventricle implantation model to examine the effect of miR-429 on the metastatic bone environment in vivo. The transfection experiments showed that the expression levels of V-crk sarcoma virus CT10 oncogene homolog-like (CrkL) and MMP-9 were negatively regulated by miR-429. The in vitro coculture experiments showed that miR-429 promoted osteoblast differentiation and that CrkL promoted osteoclast differentiation. The two animal models showed that miR-429 diminished local bone destruction and distant bone metastasis but CrkL enhanced these effects. Furthermore, CrkL and MMP-9 expression decreased simultaneously in response to increased miR-429 expression. These findings further reveal the possible mechanism and effect of the miR-429/CrkL/MMP-9 regulatory axis in the bone microenvironment in breast cancer bone metastasis.

MYCN and PRC1 cooperatively repress docosahexaenoic acid synthesis in neuroblastoma via ELOVL2

Background

The MYCN amplification is a defining hallmark of high-risk neuroblastoma. Due to irregular oncogenes orchestration, tumor cells exhibit distinct fatty acid metabolic features from non-tumor cells. However, the function of MYCN in neuroblastoma fatty acid metabolism reprogramming remains unknown.

Methods

Gas Chromatography-Mass Spectrometer (GC-MS) was used to find the potential target fatty acid metabolites of MYCN. Real-time PCR (RT-PCR) and clinical bioinformatics analysis was used to find the related target genes. The function of the identified target gene ELOVL2 on cell growth was detected through CCK-8 assay, Soft agar colony formation assay, flow Cytometry assay and mouse xenograft. Chromatin immunoprecipitation (ChIP) and Immunoprecipitation-Mass Spectrometer (IP-MS) further identified the target gene and the co-repressor of MYCN.

Results

The fatty acid profile of MYCN-depleted neuroblastoma cells identified docosahexaenoic acid (DHA), an omega-3 polyunsaturated fatty acid with anti-tumor activity, significantly increased after MYCN depletion. Compared with MYCN single-copy neuroblastoma cells, DHA level was significantly lower in MYCN-amplified neuroblastoma cells. RT-PCR and clinical bioinformatics analysis discovered that MYCN interfered DHA accumulation via ELOVL fatty acid elongase 2 (ELOVL2) which is a rate-limiting enzyme of cellular DHA synthesis. Enforced ELOVL2 expression in MYCN-amplified neuroblastoma cells led to decreased cell growth and counteracted the growth-promoting effect of MYCN overexpression both in vitro and vivo. ELOVL2 Knockdown showed the opposite effect in MYCN single-copy neuroblastoma cells. In primary neuroblastoma, high ELOVL2 transcription correlated with favorable clinical tumor biology and patient survival. The mechanism of MYCN-mediated ELOVL2 inhibition contributed to epigenetic regulation. MYCN recruited PRC1 (Polycomb repressive complex 1), catalysed H2AK119ub (histone 2A lysine 119 monoubiquitination) and inhibited subsequent ELOVL2 transcription.

Conclusions

The tumor suppressive properties of DHA and ELOVL2 are repressed by the MYCN and PRC1 jointly, which suggests a new epigenetic mechanism of MYCN-mediated fatty acid regulation and indicates PRC1 inhibition as a potential novel strategy to activate ELOVL2 suppressive functions.

Discovery, characterization and potential roles of a novel NF-YAx splice variant in human neuroblastoma

BACKGROUND

Identification of novel cancer-associated splice variants is of potential diagnostic, prognostic and therapeutic importance. NF-Y transcription factor is comprised of NF-YA, NF-YB and NF-YC subunits, binds inverted CCAAT-boxes in ≈70% of gene promoters, regulates > 1000 cancer-associated genes and proteins involved in proliferation, staminality, differentiation, apoptosis, metabolism and is subject to component alternative splicing. RT-PCR evaluation of alternative NF-YA splicing in primary human neuroblastomas (NBs), led to discovery of a novel NF-YAx splice variant, also expressed during mouse embryo development and induced by doxorubicin in NB cells. Here, we report the discovery and characterisation of NF-YAx and discus its potential roles in NB.

METHODS

NF-YAx cDNA was RT-PCR-cloned from a stage 3 NB (provided by the Italian Association of Haematology and Paediatric Oncology, Genova, IT), sequenced and expressed as a protein using standard methods and compared to known fully-spliced NF-YAl and exon B-skipped NF-YAs isoforms in: EMSAs for capacity to form NF-Y complexes; by co-transfection, co-immunoprecipitation and Western blotting for capacity to bind Sp1; by IF for localisation; in AO/EtBr cell-death and colony formation assays for relative cytotoxicity, and by siRNA knockdown, use of inhibitors and Western blotting for potential mechanisms of action. Stable SH-SY5Y transfectants of all three NF-YA isoforms were also propagated and compared by RT-PCR and Western blotting for differences in cell-death and stem cell (SC)-associated gene expression, in cell-death assays for sensitivity to doxorubicin and in in vitro proliferation, substrate-independent growth and in vivo tumour xenograft assays for differences in growth and tumourigenic capacity.

RESULTS

NF-YAx was characterized as a novel variant with NF-YA exons B, D and partial F skipping, detected in 20% of NF-YA positive NBs, was the exclusive isoform in a stage 3 NB, expressed in mouse stage E11.5-14 embryos and induced by doxorubicin in SH-SY5Y NB cells. The NF-YAx protein exhibited nuclear localisation, competed with other isoforms in CCAAT box-binding NF-Y complexes but, in contrast to other isoforms, did not bind Sp1. NF-YAx expression in neural-related progenitor and NB cells repressed Bmi1 expression, induced KIF1Bβ expression and promoted KIF1Bβ-dependent necroptosis but in NB cells also selected tumourigenic, doxorubicin-resistant, CSC-like sub-populations, resistant to NF-YAx cytotoxicity.

CONCLUSIONS

The discovery of NF-YAx in NBs, its expression in mouse embryos and induction by doxorubicin in NB cells, unveils a novel NF-YA splice mechanism and variant, regulated by and involved in development, genotoxic-stress and NB. NF-YAx substitution of other isoforms in NF-Y complexes and loss of capacity to bind Sp1, characterises this novel isoform as a functional modifier of NF-Y and its promotion of KIF1Bβ-dependent neural-lineage progenitor and NB cell necroptosis, association with doxorubicin-induced necroptosis and expression in mouse embryos coinciding with KIF1Bβ-dependent sympathetic neuroblast-culling, confirm a cytotoxic function and potential role in suppressing NB initiation. On the other hand, the in vitro selection of CSC-like NB subpopulations resistant to NF-YAx cytotoxicity not only helps to explain high-level exclusive NF-YAx expression in a stage 3 NB but also supports a role for NF-YAx in disease progression and identifies a potential doxorubicin-inducible mechanism for post-therapeutic relapse.

Cancer stem cells in neuroblastoma therapy resistance

Neuroblastoma (NB) is the most common cancer of infancy and accounts for nearly one tenth of pediatric cancer deaths. This mortality rate has been attributed to the > 50% frequency of relapse despite intensive, multimodal clinical therapy in patients with progressive NB. Given the disease’s heterogeneity and developed resistance, attaining a cure after relapse of progressive NB is highly challenging. A rapid decrease in the timeline between successive recurrences is likely due to the ongoing acquisition of genetic rearrangements in undifferentiated NB-cancer stem cells (CSCs). In this review, we present the current understanding of NB-CSCs, their intrinsic role in tumorigenesis, their function in disease progression, and their influence on acquired therapy resistance and tumor evolution. In particular, this review focus on the intrinsic involvement of stem cells and signaling in the genesis of NB, the function of pre-existing CSCs in NB progression and therapy response, the formation and influence of induced CSCs (iCSCs) in drug resistance and tumor evolution, and the development of a CSC-targeted therapeutic approach.

Increase in DNA Damage by MYCN Knockdown Through Regulating Nucleosome Organization and Chromatin State in Neuroblastoma

As a transcription factor, MYCN regulates myriad target genes including the histone chaperone FACT. Moreover, FACT and MYCN expression form a forward feedback loop in neuroblastoma. It is unclear whether MYCN is involved in chromatin remodeling in neuroblastoma through regulation of its target genes. We showed here that MYCN knockdown resulted in loss of the nucleosome-free regions through nucleosome assembly in the promoters of genes functionally enriched for DNA repair. The active mark H3K9ac was removed or replaced by the repressive mark H3K27me3 in the promoters of double-strand break repair-related genes upon MYCN knockdown. Such chromatin state alterations occurred only in MYCN-bound promoters. Consistently, MYCN knockdown resulted in a marked increase in DNA damage in the treatment with hydroxyurea. In contrast, nucleosome reorganization and histone modification changes in the enhancers largely included target genes with tumorigenesis-related functions such as cell proliferation, cell migration, and cell–cell adhesion. The chromatin state significantly changed in both MYCN-bound and MYCN-unbound enhancers upon MYCN knockdown. Furthermore, MYCN knockdown independently regulated chromatin remodeling in the promoters and the enhancers. These findings reveal the novel epigenetic regulatory role of MYCN in chromatin remodeling and provide an alternative potential epigenetic strategy for MYCN-driven neuroblastoma treatment.

Lost expression of cell adhesion molecule 1 is associated with bladder cancer progression and recurrence and its overexpression inhibited tumor cell malignant behaviors

Cell adhesion molecule 1 (CADM1) regulates cell-cell adhesion and an altered expression level is associated with tumorigenesis and progression. The present study assessed CADM1 expression level in 84 bladder tissues to investigate the association with clinicopathological parameters from bladder cancer patients and then investigated the effects of CADM1 overexpression on T24 bladder cancer cells in vitro. The results demonstrated that expression level of CADM1 protein was significantly reduced in bladder cancer tissues (0.26±0.14) compared with in normal bladder mucosa (0.69±0.092; P<0.01), and methylation of CADM1 promoter was responsible for silencing of CADM1 protein expression and significantly associated with tumor size, recurrence, pathology classification and clinical stage (P<0.05). Overexpression of CADM1 protein inhibited tumor cell proliferation, reduced tumor cell invasion capacity and induced tumor cell apoptosis in vitro. At the gene level, CADM1 expression level upregulated caspase-3 activity and expression of Bax and p27 protein and downregulated levels of B cell lymphoma-2, cyclinD1, cyclinE1 and cyclin dependent kinase 2 proteins. Furthermore, overexpression of CADM1 regulated the expression level of epithelial to mesenchymal transition markers, including increased expression level of E-cadherin and β-catenin, whereas it decreased the levels of Vimentin. The present study demonstrated that lost expression of CADM1 protein may exert an essential role in the development and progression of bladder cancer and suggested that CADM1 may be a novel molecular target for the control of this disease in clinical practice.

CFC1 is a cancer stemness-regulating factor in neuroblastoma

Background

Despite the use of aggressive therapy, survival rates among high-risk neuroblastoma (NB) patients remain poor. Cancer stem cells (CSCs) are considered to be critically involved in the recurrence and metastasis of NB and are isolated as NB spheres.

Methods

The gene expression profiling of adherent (control) and sphere-forming primary NB cells was conducted using a gene expression microarray. CFC1, which functions in the development of embryos and decides the left-right axis, was strongly expressed in sphere-forming cells only and was related to the unfavorable prognosis of NB patients. The knockdown and overexpression of CFC1 were performed using a lentiviral system in NB cell lines. Sphere formation, cell proliferation, colony formation in soft agar, and xenograft tumor formation were analyzed.

Results

The overexpression of CFC1 increased sphere formation, cell growth, and colony formation. These phenotypes, particularly sphere formation, and xenograft tumor formation were significantly suppressed by the knockdown of CFC1. CFC1 inhibited Activin A-induced NB cell differentiation and Smad2 phosphorylation in NB cell lines, indicating its involvement in tumorigenesis related to EGF-CFC co-receptor family molecule pathways. Collectively, these results indicate that CFC1 is a candidate molecule for the development of CSC-targeted therapy for NB.

Neuroblastoma

Neuroblastoma is one of the most common solid tumors in children and has a diverse clinical behavior that largely depends on the tumor biology. Neuroblastoma exhibits unique features, such as early age of onset, high frequency of metastatic disease at diagnosis in patients over 1 year of age and the tendency for spontaneous regression of tumors in infants. The high-risk tumors frequently have amplification of the MYCN oncogene as well as segmental chromosome alterations with poor survival. Recent advanced genomic sequencing technology has revealed that mutation of ALK, which is present in ~10% of primary tumors, often causes familial neuroblastoma with germline mutation. However, the frequency of gene mutations is relatively small and other aberrations, such as epigenetic abnormalities, have also been proposed. The risk-stratified therapy was introduced by the Japan Neuroblastoma Study Group (JNBSG), which is now moving to the Neuroblastoma Committee of Japan Children's Cancer Group (JCCG). Several clinical studies have facilitated the reduction of therapy for children with low-risk neuroblastoma disease and the significant improvement of cure rates for patients with intermediate-risk as well as high-risk disease. Therapy for patients with high-risk disease includes intensive induction chemotherapy and myeloablative chemotherapy, followed by the treatment of minimal residual disease using differentiation therapy and immunotherapy. The JCCG aims for better cures and long-term quality of life for children with cancer by facilitating new approaches targeting novel driver proteins, genetic pathways and the tumor microenvironment.

EZH2 regulates neuroblastoma cell differentiation via NTRK1 promoter epigenetic modifications

The polycomb repressor complex 2 molecule EZH2 is now known to play a role in essential cellular processes, namely, cell fate decisions, cell cycle regulation, senescence, cell differentiation, and cancer development/progression. EZH2 inhibitors have recently been developed; however, their effectiveness and underlying molecular mechanisms in many malignancies have not yet been elucidated in detail. Although the functional role of EZH2 in tumorigenesis in neuroblastoma (NB) has been investigated, mutations of EZH2 have not been reported. A Kaplan–Meier analysis on the event free survival and overall survival of NB patients indicated that the high expression of EZH2 correlated with an unfavorable prognosis. In order to elucidate the functional roles of EZH2 in NB tumorigenesis and its aggressiveness, we knocked down EZH2 in NB cell lines using lentivirus systems. The knockdown of EZH2 significantly induced NB cell differentiation, e.g., neurite extension, and the neuronal differentiation markers, NF68 and GAP43. EZH2 inhibitors also induced NB cell differentiation. We performed a comprehensive transcriptome analysis using Human Gene Expression Microarrays and found that NTRK1 (TrkA) is one of the EZH2-related suppression targets. The depletion of NTRK1 canceled EZH2 knockdown-induced NB cell differentiation. Our integrative methylome, transcriptome, and chromatin immunoprecipitation assays using NB cell lines and clinical samples clarified that the NTRK1 P1 and P2 promoter regions were regulated differently by DNA methylation and EZH2-related histone modifications. The NTRK1 transcript variants 1/2, which were regulated by EZH2-related H3K27me3 modifications at the P1 promoter region, were strongly expressed in favorable, but not unfavorable NB. The depletion and inhibition of EZH2 successfully induced NTRK1 transcripts and functional proteins. Collectively, these results indicate that EZH2 plays important roles in preventing the differentiation of NB cells and also that EZH2-related NTRK1 transcriptional regulation may be the key pathway for NB cell differentiation.

Regulating BMI1 expression via miRNAs promote Mesenchymal to Epithelial Transition (MET) and sensitizes breast cancer cell to chemotherapeutic drug

Polycomb group (PcG) proteinB lymphoma Mo-MLV insertion region 1 homolog (BMI1) is a transcriptional repressor that plays an important role in human carcinogenesis. MicroRNAs (miRNAs) are endogenous small non-coding RNAsthat implicate a negative regulation on gene expression. Deregulation of the expression of miRNAs has been implicated in tumorigenesis. Here, we have shown that knock-down ofBMI1increases theexpression of tumor-suppressivemiRNAs. Elevated levels of expression of miR-200a, miR-200b, miR-15a, miR-429, miR-203were observed upon knock-down of BMI1. Up-regulation of these miRNAsleads to down-regulation ofPRC1 group of proteins i.e. BMI1, RING1A, RING1B and Ub-H2A. Interestingly, overexpression of miR-200a, miR-200b and miR-15aalso produced decreased BMI1 and Ub-H2A protein expression in the CD44+ Cancer Stem Cellpopulation of MDAMB-231cells. Also,elevating the levels of BMI1 regulated miRNAspromoted Mesenchymal to Epithelial transition by regulating the expression of N-Cadherin, Vimentin, β-Catenin, Zeb, Snail thereby resulting in decreased invasion, migration and proliferation. Here, we also report that miR-200a, miR-200b, miR-203 accretes the sensitivity of MDAMB-231 cells to the histone deacetylase inhibitor (HDACi) SAHA and miR-15a sensitized breast cancer cells to the chemotherapeutic drug cisplatin leading to apoptosis. These findings suggest that modulatingspecific miRNAs may serve as a therapeutic approach for the treatment of breast cancer

Epigenetic regulation of neuroblastoma development

In recent years, technological advances have enabled a detailed landscaping of the epigenome and the mechanisms of epigenetic regulation that drive normal cell function, development and cancer. Rather than merely a structural entity to support genome compaction, we now look at chromatin as a very dynamic and essential constellation that is actively participating in the tight orchestration of transcriptional regulation as well as DNA replication and repair. The unique feature of chromatin flexibility enabling fast switches towards more or less restricted epigenetic cellular states is, not surprisingly, intimately connected to cancer development and treatment resistance, and the central role of epigenetic alterations in cancer is illustrated by the finding that up to 50% of all mutations across cancer entities affect proteins controlling the chromatin status. We summarize recent insights into epigenetic rewiring underlying neuroblastoma (NB) tumor formation ranging from changes in DNA methylation patterns and mutations in epigenetic regulators to global effects on transcriptional regulatory circuits that involve key players in NB oncogenesis. Insights into the disruption of the homeostatic epigenetic balance contributing to developmental arrest of sympathetic progenitor cells and subsequent NB oncogenesis are rapidly growing and will be exploited towards the development of novel therapeutic strategies to increase current survival rates of patients with high-risk NB.

Coexpression network analysis identifies transcriptional modules associated with genomic alterations in neuroblastoma

Neuroblastoma is a highly complex and heterogeneous cancer in children. Acquired genomic alterations including MYCN amplification, 1p deletion and 11q deletion are important risk factors and biomarkers in neuroblastoma. Here, we performed a co-expression-based gene network analysis to study the intrinsic association between specific genomic changes and transcriptome organization. We identified multiple gene coexpression modules which are recurrent in two independent datasets and associated with functional pathways including nervous system development, cell cycle, immune system process and extracellular matrix/space. Our results also indicated that modules involved in nervous system development and cell cycle are highly associated with MYCN amplification and 1p deletion, while modules responding to immune system process are associated with MYCN amplification only. In summary, this integrated analysis provides novel insights into molecular heterogeneity and pathogenesis of neuroblastoma. This article is part of a Special Issue entitled: Accelerating Precision Medicine through Genetic and Genomic Big Data Analysis edited by Yudong Cai & Tao Huang.

PRC2-Mediated Transcriptomic Alterations at the Embryonic Stage Govern Tumorigenesis and Clinical Outcome in MYCN-Driven Neuroblastoma

Pediatric cancers such as neuroblastoma are thought to involve a dysregulation of embryonic development. However, it has been difficult to identify the critical events that trigger tumorigenesis and differentiate them from normal development. In this study, we report the establishment of a spheroid culture method that enriches early-stage tumor cells from TH-MYCN mice, a preclinical model of neuroblastoma. Using this method, we found that tumorigenic cells were evident as early as day E13.5 during embryo development, when the MYC and PRC2 transcriptomes were significantly altered. Ezh2, an essential component of PRC2, was expressed in embryonic and postnatal tumor lesions and physically associated with N-MYC and we observed that H3K27me3 was increased at PRC2 target genes. PRC2 inhibition suppressed in vitro sphere formation, derepressed its target genes, and suppressed in situ tumor growth. In clinical specimens, expression of MYC and PRC2 target genes correlated strongly and predicted survival outcomes. Together, our findings highlighted PRC2-mediated transcriptional control during embryogenesis as a critical step in the development and clinical outcome of neuroblastoma. Cancer Res; 77(19); 5259-71. ©2017 AACR.

Combined Treatments with a Retinoid Receptor Agonist and Epigenetic Modulators in Human Neuroblastoma Cells

Neuroblastoma (NB) is the most common extracranial solid childhood tumor accounting for around 15% of pediatric cancer deaths and most probably originates from a failure in the development of embryonic neural crest cells. Retinoids can inhibit the proliferation and stimulate differentiation of NB cells. In addition, epigenetic events involving changes in chromatin structure and DNA methylation can mediate the effects of retinoids; hence, the scope of this study is to investigate the use of retinoids and epigenetic drugs in NB cell lines. Here, we demonstrate that the combination of retinoid all trans-retinoic acid (ATRA) with inhibitors of either histone deacetylases (HDACs) or DNA methyltransferase is more effective in impairing the proliferation of human SH-SY5Y and SK-N-BE(2) NB cells than any drug given alone. Treatments also induced differential changes on the messenger RNA (mRNA) expression of retinoid receptor subtypes and reduced the protein content of c-Myc, the neuronal markers NeuN and β-3 tubulin, and the oncoprotein Bmi1. These results suggest that the combination of retinoids with epigenetic modulators is more effective in reducing NB growth than treatment with single drugs.

p19-INK4d inhibits neuroblastoma cell growth, induces differentiation and is hypermethylated and downregulated in MYCN-amplified neuroblastomas

Uncontrolled cell cycle entry, resulting from deregulated CDK-RB1-E2F pathway activity, is a crucial determinant of neuroblastoma cell malignancy. Here we identify neuroblastoma-suppressive functions of the p19-INK4d CDK inhibitor and uncover mechanisms of its repression in high-risk neuroblastomas. Reduced p19-INK4d expression was associated with poor event-free and overall survival and neuroblastoma risk factors including amplified MYCN in a set of 478 primary neuroblastomas. High MYCN expression repressed p19-INK4d mRNA and protein levels in different neuroblastoma cell models with conditional MYCN expression. MassARRAY and 450K methylation analyses of 105 primary neuroblastomas uncovered a differentially methylated region within p19-INK4d. Hypermethylation of this region was associated with reduced p19-INK4d expression. In accordance, p19-INK4d expression was activated upon treatment with the demethylating agent, 2'-deoxy-5-azacytidine, in neuroblastoma cell lines. Ectopic p19-INK4d expression decreased viability, clonogenicity and the capacity for anchorage-independent growth of neuroblastoma cells, and shifted the cell cycle towards the G1/0 phase. p19-INK4d also induced neurite-like processes and markers of neuronal differentiation. Moreover, neuroblastoma cell differentiation, induced by all-trans retinoic acid or NGF-NTRK1-signaling, activated p19-INK4d expression. Our findings pinpoint p19-INK4d as a neuroblastoma suppressor and provide evidence for MYCN-mediated repression and for epigenetic silencing of p19-INK4d by DNA hypermethylation in high-risk neuroblastomas.

Receptor-type protein tyrosine phosphatase κ directly dephosphorylates CD133 and regulates downstream AKT activation

Although CD133 has been considered to be a molecular marker for cancer stem cells, its functional roles in tumorigenesis remain unclear. We here examined the molecular basis behind CD133-mediated signaling. Knockdown of CD133 resulted in the retardation of xenograft tumor growth of colon cancer-derived HT-29 and LoVo cells accompanied by hypophosphorylation of AKT, which diminished β-catenin/T-cell factor-mediated CD44 expression. As tyrosine residues of CD133 at positions 828 and 852 were phosphorylated in HT-29 and SW480 cells, we further addressed the significance of this phosphorylation in the tumorigenesis of SW480 cells expressing mutant CD133, with substitution of these tyrosine residues by glutamate (CD133-EE) or phenylalanine (CD133-FF). Forced expression of CD133-EE promoted much more aggressive xenograft tumor growth relative to wild-type CD133-expressing cells accompanied by hyperphosphorylation of AKT; however, CD133-FF expression had negligible effects on AKT phosphorylation and xenograft tumor formation. Intriguingly, the tyrosine phosphorylation status of CD133 was closely linked to the growth of SW480-derived spheroids. Using yeast two-hybrid screening, we finally identified receptor-type protein tyrosine phosphatase κ (PTPRK) as a binding partner of CD133. In vitro studies demonstrated that PTPRK associates with the carboxyl-terminal region of CD133 through its intracellular phosphatase domains and also catalyzes dephosphorylation of CD133 at tyrosine-828/tyrosine-852. Silencing of PTPRK elevated the tyrosine phosphorylation of CD133, whereas forced expression of PTPRK reduced its phosphorylation level markedly and abrogated CD133-mediated AKT phosphorylation. Endogenous CD133 expression was also closely associated with higher AKT phosphorylation in primary colon cancer cells, and ectopic expression of CD133 enhanced AKT phosphorylation. Furthermore, lower PTPRK expression significantly correlated with the poor prognosis of colon cancer patients with high expression of CD133. Thus, our present findings strongly indicate that the tyrosine phosphorylation of CD133, which is dephosphorylated by PTPRK, regulates AKT signaling and has a critical role in colon cancer progression.

The N-myc Oncogene: Maximizing its Targets, Regulation, and Therapeutic Potential

N-myc (MYCN), a member of the Myc family of basic-helix-loop-helix-zipper (bHLHZ) transcription factors, is a central regulator of many vital cellular processes. As such, N-myc is well recognized for its classic oncogenic activity and association with human neuroblastoma. Amplification and overexpression of N-myc has been described in other tumor types, particularly those of neural origin and neuroendocrine tumors. This review outlines N-myc's contribution to normal development and oncogenic progression. In addition, it highlights relevant transcriptional targets and mechanisms of regulation. Finally, the clinical implications of N-Myc as a biomarker and potential as a target using novel therapeutic approaches are discussed.

Estrogen receptor α-coupled Bmi1 regulation pathway in breast cancer and its clinical implications

Background

Bmi1 has been identified as an important regulator in breast cancer, but its relationship with other signaling molecules such as ERα and HER2 is undetermined.

Methods

The expression of Bmi1 and its correlation with ERα, PR, Ki-67, HER2, p16^INK4a, cyclin D1 and pRB was evaluated by immunohistochemistry in a collection of 92 cases of breast cancer and statistically analyzed. Stimulation of Bmi1 expression by ERα or 17β-estradiol (E2) was analyzed in cell lines including MCF-7, MDA-MB-231, ERα-restored MDA-MB-231 and ERα-knockdown MCF-7 cells. Luciferase reporter and chromatin immunoprecipitation assays were also performed.

Results

Immunostaining revealed strong correlation of Bmi1 and ERα expression status in breast cancer. Expression of Bmi1 was stimulated by 17β-estradiol in ERα-positive MCF-7 cells but not in ERα-negative MDA-MB-231 cells, while the expression of Bmi1 did not alter expression of ERα. As expected, stimulation of Bmi1 expression could also be achieved in ERα-restored MDA-MB-231 cells, and at the same time depletion of ERα decreased expression of Bmi1. The proximal promoter region of Bmi1 was transcriptionally activated with co-transfection of ERα in luciferase assays, and the interaction of the Bmi1 promoter with ERα was confirmed by chromatin immunoprecipitation. Moreover, in breast cancer tissues activation of the ERα-coupled Bmi1 pathway generally correlated with high levels of cyclin D1, while loss of its activity resulted in aberrant expression of p16^INK4a and a high Ki-67 index, which implied a more aggressive phenotype of breast cancer.

Conclusions

Expression of Bmi1 is influenced by ERα, and the activity of the ERα-coupled Bmi1 signature impacts p16^INK4a and cyclin D1 status and thus correlates with the tumor molecular subtype and biologic behavior. This demonstrates the important role which is played by ERα-coupled Bmi1 in human breast cancer.

Type III TGF-β receptor promotes FGF2-mediated neuronal differentiation in neuroblastoma

Growth factors and their receptors coordinate neuronal differentiation during development, yet their roles in the pediatric tumor neuroblastoma remain unclear. Comparison of mRNA from benign neuroblastic tumors and neuroblastomas revealed that expression of the type III TGF-β receptor (TGFBR3) decreases with advancing stage of neuroblastoma and this loss correlates with a poorer prognosis. Patients with MYCN oncogene amplification and low TGFBR3 expression were more likely to have an adverse outcome. In vitro, TβRIII expression was epigenetically suppressed by MYCN-mediated recruitment of histone deacetylases to regions of the TGFBR3 promoter. TβRIII bound FGF2 and exogenous FGFR1, which promoted neuronal differentiation of neuroblastoma cells. TβRIII and FGF2 cooperated to induce expression of the transcription factor inhibitor of DNA binding 1 via Erk MAPK. TβRIII-mediated neuronal differentiation suppressed cell proliferation in vitro as well as tumor growth and metastasis in vivo. These studies characterize a coreceptor function for TβRIII in FGF2-mediated neuronal differentiation, while identifying potential therapeutic targets and clinical biomarkers for neuroblastoma.

Polycomb group proteins and MYC: the cancer connection

Polycomb group proteins (PcGs) are transcriptional repressors involved in physiological processes whereas PcG deregulation might result in oncogenesis. MYC oncogene is able to regulate gene transcription, proliferation, apoptosis, and malignant transformation. MYC deregulation might result in tumorigenesis with tumor maintenance properties in both solid and blood cancers. Although the interaction of PcG and MYC in cancer was described years ago, new findings are reported every day to explain the exact mechanisms and results of such interactions. In this review, we summarize recent data on the PcG and MYC interactions in cancer, and the putative involvement of microRNAs in the equation.

Neuroblastoma and MYCN

Neuroblastoma, the most common extracranial solid tumor of childhood, is thought to originate from undifferentiated neural crest cells. Amplification of the MYC family member, MYCN, is found in ∼25% of cases and correlates with high-risk disease and poor prognosis. Currently, amplification of MYCN remains the best-characterized genetic marker of risk in neuroblastoma. This article reviews roles for MYCN in neuroblastoma and highlights recent identification of other driver mutations. Strategies to target MYCN at the level of protein stability and transcription are also reviewed.

BMI1, the polycomb-group gene, is recurrently targeted by genomic rearrangements in progressive B-cell leukemia/lymphoma

BMI1, a Polycomb-group gene located at 10p12.2, is implicated in the pathogenesis of a variety of tumors. However, the genetic molecular mechanisms underlying its aberrant expression in cancer cells remain largely unknown. In this study, we show that BMI1 is recurrently targeted by chromosomal aberrations in B-cell leukemia/lymphoma. We identified a novel t(10;14)(p12;q32)/IGH-BMI1 rearrangement and its IGL variant in six cases of chronic lymphocytic leukemia (CLL) and found that these aberrations were consistently acquired at time of disease progression and high grade transformation of leukemia (Richter syndrome). The IG-BMI1 translocations were not associated with any particular molecular subtype of CLL and the leukemias were negative for common mutations of NOTCH1 and TP53, known to increase a risk of progression and transformation in CLL. In addition, using FISH and SNP array analysis, we identified a wide range of BMI1-involving 10p12 lesions in 17 cases of mantle cell lymphoma (MCL). These aberrations included various balanced and unbalanced structural abnormalities and very frequently but not exclusively, were associated with gain of the BMI1 locus and loss of the 10p terminal sequences. These findings point to genomic instability at the 10p region in MCL which likely promotes rearrangements and deregulation of BMI1. Our findings are in line with previously published observations correlating overexpression of BMI1 with tumor progression and chemoresistance. In summary, our study provides new insights into genetic molecular mechanisms underlying aberrant expression of BMI1 in lymphoma and documents its contribution in the pathogenesis of Richter syndrome and MCL.

Apoptotic cell death in neuroblastoma

Neuroblastoma (NB) is one of the most common malignant solid tumors in childhood, which derives from the sympathoadrenal lineage of the neural crest and exhibits extremely heterogeneous biological and clinical behaviors. The infant patients frequently undergo spontaneous regression even with metastatic disease, whereas the patients of more than one year of age who suffer from disseminated disease have a poor outcome despite intensive multimodal treatment. Spontaneous regression in favorable NBs has been proposed to be triggered by nerve growth factor (NGF) deficiency in the tumor with NGF dependency for survival, while aggressive NBs have defective apoptotic machinery which enables the tumor cells to evade apoptosis and confers the resistance to treatment. This paper reviews the molecules and pathways that have been recently identified to be involved in apoptotic cell death in NB and discusses their potential prospects for developing more effective therapeutic strategies against aggressive NB.

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.

MYCN and the epigenome

It is well known that Neuroblastoma (NB) patients whose tumors have an undifferentiated histology and a transcriptome enriched in cell cycle genes have a worse prognosis. This contrasts with the good prognoses of patients whose tumors have histologic evidence of differentiation and a transcriptome enriched in differentiation genes. Tumor cell lines from poor prognosis, high-risk patients contain a number of genetic alterations, including amplification of MYCN, 1pLOH, and unbalanced 11q or gains of Chr 17 and 7, and exhibit uncontrolled growth and an undifferentiated phenotype in in vitro culture. Yet treatment of such NB cell lines with retinoic acid results in growth control and induction of differentiation. This indicates that the signaling pathways that regulate cell growth and differentiation are not functionally lost but dysregulated. Agents such as retinoic acid normalize the signaling pathways and impose growth control and induction of differentiation. Recent studies in embryonic stem cells indicate that polycomb repressor complex proteins (PRC1 and PRC2) play a major role in regulating stem cell lineage specification and coordinating the shift from a transcriptome that supports self-renewal or growth to one that specifies lineage and controls growth. We have shown that in NB, the PRC2 complex is elevated in undifferentiated NB tumors and functions to suppress a number of tumor suppressor genes. This study will review the role of MYC genes in regulating the epigenome in normal development and explore how this role may be altered during tumorigenesis.

Low p14ARF expression in neuroblastoma cells is associated with repressed histone mark status, and enforced expression induces growth arrest and apoptosis

The TP53 tumor suppressor pathway is abrogated by TP53 mutations in the majority of human cancers. Increased levels of wild-type TP53 in aggressive neuroblastomas appear paradox but are tolerated by tumor cells due to co-activation of the TP53 ubiquitin ligase, MDM2. The role of the MDM2 antagonist, p14(ARF), in controlling the TP53-MDM2 balance in neuroblastoma is unresolved. In the present study, we show that conditional p14(ARF) expression substantially suppresses viability, clonogenicity and anchorage-independent growth in p14(ARF)-deficient or MYCN-amplified neuroblastoma cell lines. Furthermore, ectopic 14(ARF) expression induced accumulation of cells in the G1 phase and apoptosis, which was paralleled by accumulation of TP53 and its targets. Comparative genomic hybridization analysis of 193 primary neuroblastomas detected one homozygous deletion of CDKN2A (encoding both p14(ARF) and p16(INK4A)) and heterozygous loss of CDKN2A in 22% of tumors. Co-expression analysis of p14(ARF) and its transactivator, E2F1, in a set of 68 primary tumors revealed only a weak correlation, suggesting that further regulatory mechanisms govern p14(ARF) expression in neuroblastomas. Intriguingly, analyses utilizing chromatin immunoprecipitation revealed different histone mark-defined epigenetic activity states of p14(ARF) in neuroblastoma cell lines that correlated with endogenous p14(ARF) expression but not with episomal p14(ARF) promoter reporter activity, indicating that the native chromatin context serves to epigenetically repress p14(ARF) in neuroblastoma cells. Collectively, the data pinpoint p14(ARF) as a critical factor for efficient TP53 response in neuroblastoma cells and assign p14(ARF) as a neuroblastoma suppressor candidate that is impaired by genomic loss and epigenetic repression.

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.

Epigenetic changes in pediatric solid tumors: promising new targets

Cancer is being reinterpreted in the light of recent discoveries related to the histone code and the dynamic nature of epigenetic regulation and control of gene programs during development, as well as insights gained from whole cancer genome sequencing. Somatic mutations in or deregulated expression of genes that encode chromatin-modifying enzymes are being identified with high frequency. Nowhere is this more relevant than in pediatric embryonal solid tumors. A picture is emerging that shows that classic genetic alterations associated with these tumors ultimately converge on the epigenome to dysregulate developmental programs. In this review, we relate how alterations in components of the transcriptional machinery and chromatin modifier genes contribute to the initiation and progression of pediatric solid tumors. We also discuss how dramatic progress in our understanding of the fundamental mechanisms that contribute to epigenetic deregulation in cancer is providing novel avenues for targeted cancer therapy.

Molecular mechanisms of MYCN-dependent apoptosis and the MDM2-p53 pathway: an Achille's heel to be exploited for the therapy of MYCN-amplified neuroblastoma

The p53 oncosuppressor is very seldom mutated in neuroblastoma, but several mechanisms cooperate to its functional inactivation in this tumor. Increased MDM2 levels, due to genetic amplification or constitutive inhibition of p14( ARF), significantly contribute to this event highlighting p53 reactivation as an attractive perspective for neuroblastoma treatment. In addition to its role in tumorigenesis, MYCN sensitizes untransformed and cancer cells to apoptosis. This is associated to a fine modulation of the MDM2-p53 pathway. Indeed MYCN induces p53 and MDM2 transcription, and, by evoking a DNA damage response (DDR), it stabilizes p53 and its proapoptotic kinase Homeodomain Interacting Protein Kinase 2 (HIPK2). Through the regulation of the HIPK2-p53 inhibitor High Mobility Group protein A1 (HMGA1) and the homeobox proteins BMI-1 and TWIST-1, MYCN establishes a delicate balance between pro- and antiapoptotic molecules that might be easily perturbed by a variety of insults, leading to cell death. MDM2-p53 antagonists, such as Nutlin-3, are strikingly prone to inducing death in MYCN-amplified neuroblastoma, by further pushing on HIPK2 accumulation. Here we discuss implications and caveats of exploiting this pathway and its connections to MYCN-induced DDR for a tailored therapy of MYCN-amplified neuroblastoma.

Evidence of asymmetric cell division and centrosome inheritance in human neuroblastoma cells

Asymmetric cell division (ACD) is believed to be a physiological event that occurs during development and tissue homeostasis in a large variety of organisms. ACD produces two unequal daughter cells, one of which resembles a multipotent stem and/or progenitor cell, whereas the other has potential for differentiation. Although recent studies have shown that the balance between self-renewal and differentiation potentials is precisely controlled and that alterations in the balance may lead to tumorigenesis in Drosophila neuroblasts, it is largely unknown whether human cancer cells directly show ACD in an evolutionarily conserved manner. Here, we show that the conserved polarity/spindle protein NuMA is preferentially localized to one side of the cell cortex during cell division, generating unequal inheritance of fate-altering molecules in human neuroblastoma cell lines. We also show that the cells with a single copy of MYCN showed significantly higher percentages of ACD than those with MYCN amplification. Moreover, suppression of MYCN in MYCN-amplified cells caused ACD, whereas expression of MYCN in MYCN-nonamplified cells enhanced symmetric cell division. Furthermore, we demonstrate that centrosome inheritance follows a definite rule in ACD: The daughter centrosome with younger mother centriole is inherited to the daughter cell with NuMA preferentially localized to the cell cortex, whereas the mother centrosome with the older mother centriole migrates to the other daughter cell. Thus, the mechanisms of cell division of ACD or symmetric cell division and centrosome inheritance are recapitulated in human cancer cells, and these findings may facilitate studies on cancer stem cells.