The requirement for evasion of programmed cell death in neuroblastomas with MYCN amplification

Identification of hub genes and their correlation with infiltration of immune cells in MYCN positive neuroblastoma based on WGCNA and LASSO algorithm

Background

The prognosis of MYCN positive NB is poor, and there is no targeted drug for N-myc at present. This study aims to screen out hub genes closely related to MYCN, analyze the relationship between hub genes and NB microenvironment, and provide basis for molecular targeted therapy of MYCN positive NB.

Methods

We combined the microarray data of GSE45547 (n=649) and GSE49710 (n=498), screened the DEGs between MYCN positive (n=185) and MYCN negative NB (n=951), performed WGCNA, Lasso regression and Roc analyses on the merged matrix, and obtained the hub genes related to MYCN in the training group. We performed ssGSEA on the experimental group to calculate the infiltration level of 28 kinds of immune cells in each sample, compared the differences of immune cell infiltration between MYCN positive and MYCN negative group. The influences of hub genes on the distribution of each immune cell were also analyzed by ssGSEA. The expression differences of the three hub genes were verified in the E-MTAB-8248 cohort (n=223), and the correlation between hub genes and prognosis of NB was calculated by Kaplan-Meier method in GSE62564 (n=498) and the validation group. We also verified the expression differences of hub genes by qRT-PCR in SK-N-BE(2), SKNDZ, Kelly and SH-SY5Y cell lines.

Results

Here were 880 DEGs including 420 upregulated and 460 downregulated genes in MYCN positive NB in the training group. Overlap of the DEGs and WGCNA networks identified four shared genes, namely, ZNF695, CHEK1, C15ORF42 and EXO1, as candidate hub genes in MYCN positive NB. Three core genes, ZNF695, CHEK1 and C15ORF42, were finally identified by Lasso regression and Roc analyses. ZNF695, CHEK1 and C15ORF42 were highly expressed in MYCN positive NB tissues and cell lines. These three genes were closely related to the prognosis of children with NB. Except that Activated CD4 T cell and Type2 T helper cell increased, the infiltration levels of the other 26 cells decreased significantly in MYCN positive NB tissues. The infiltration levels of Type2 T helper cell and Activated CD4 T cell were also significantly positively correlated with the expression levels of the three hub genes.

Conclusion

ZNF695, CHEK1 and C15ORF42 are highly expressed in MYCN positive NB, and their expression levels are negatively correlated with the prognosis of children with NB. The infiltration levels of Activated CD4 T cell and Type2 T helper cell increased in the microenvironment of MYCN positive NB and were significantly positively correlated with the expression levels of the three hub genes. The results of this study provide that ZNF695, CHEK1 and C15ORF42 may be potential prognostic markers and immunotherapy targets for MYCN positive NB.

Iron Chelator VLX600 Inhibits Mitochondrial Respiration and Promotes Sensitization of Neuroblastoma Cells in Nutrition-Restricted Conditions

Neuroblastoma, the most common solid tumor in children, is characterized by amplification of the MYCN proto-oncogene, a high-risk aggressive clinical marker associated with treatment failure. MYCN plays an important role in cell growth, proliferation, metabolism, and chemoresistance. Here, we show for the first time that in neuroblastoma, iron chelator VLX600 inhibits mitochondrial respiration, decreases expression levels of MYCN/LMO1, and induces an efficient cell death regardless of MYCN status in both 2D and 3D culture conditions. Moreover, insufficient induction of autophagy was observed in cells treated with VLX600, which is essential as a protective response in the event of ATP synthesis disruption. Further inhibition of glucose uptake using DRB18, a pan-GLUT (glucose transporter) inhibitor, synergized the effect of VLX600 and no significant cell death was found in immortalized epithelial cells under this combination treatment. Our results demonstrate that inhibition of mitochondrial respiration by iron chelator VLX600 accompanied by autophagy deficiency promotes sensitivity of neuroblastoma cells in a nutrition-restricted microenvironment regardless of MYCN status, indicating that MYCN expression level is an essential clinical marker but might not be a necessary target for the treatment of neuroblastoma which warrants further investigation. VLX600 has been studied in Phase I clinical trials; combining VLX600 with conventional chemotherapy could be an innovative therapeutic strategy for neuroblastoma.

Identification of RNA-Binding Proteins as Targetable Putative Oncogenes in Neuroblastoma

Neuroblastoma is a common childhood cancer with almost a third of those affected still dying, thus new therapeutic strategies need to be explored. Current experimental therapies focus mostly on inhibiting oncogenic transcription factor signalling. Although LIN28B, DICER and other RNA-binding proteins (RBPs) have reported roles in neuroblastoma development and patient outcome, the role of RBPs in neuroblastoma is relatively unstudied. In order to elucidate novel RBPs involved in MYCN-amplified and other high-risk neuroblastoma subtypes, we performed differential mRNA expression analysis of RBPs in a large primary tumour cohort (n = 498). Additionally, we found via Kaplan–Meier scanning analysis that 685 of the 1483 tested RBPs have prognostic value in neuroblastoma. For the top putative oncogenic candidates, we analysed their expression in neuroblastoma cell lines, as well as summarised their characteristics and existence of chemical inhibitors. Moreover, to help explain their association with neuroblastoma subtypes, we reviewed candidate RBPs’ potential as biomarkers, and their mechanistic roles in neuronal and cancer contexts. We found several highly significant RBPs including RPL22L1, RNASEH2A, PTRH2, MRPL11 and AFF2, which remain uncharacterised in neuroblastoma. Although not all RBPs appear suitable for drug design, or carry prognostic significance, we show that several RBPs have strong rationale for inhibition and mechanistic studies, representing an alternative, but nonetheless promising therapeutic strategy in neuroblastoma treatment.

Targeting the DNA Damage Response for the Treatment of High Risk Neuroblastoma

Despite intensive multimodal therapy, the survival rate for high risk neuroblastoma (HR-NB) remains <50%. Most cases initially respond to treatment but almost half will subsequently relapse with aggressive treatment resistant disease. Novel treatments exploiting the molecular pathology of NB and/or overcoming resistance to current genotoxic therapies are needed before survival rates can significantly improve. DNA damage response (DDR) defects are frequently observed in HR-NB including allelic deletion and loss of function mutations in key DDR genes, oncogene induced replication stress and cell cycle checkpoint dysfunction. Exploiting defects in the DDR has been a successful treatment strategy in some adult cancers. Here we review the genetic features of HR-NB which lead to DDR defects and the emerging molecular targeting agents to exploit them.

Alternative NHEJ pathway proteins as components of MYCN oncogenic activity in human neural crest stem cell differentiation: implications for neuroblastoma initiation

Neuroblastoma is a cancer of neural crest stem cell (NCSC) lineage. Signaling pathways that regulate NCSC differentiation have been implicated in neuroblastoma tumorigenesis. This is exemplified by MYCN oncogene targets that balance proliferation, differentiation, and cell death similarly in normal NCSC and in high-risk neuroblastoma. Our previous work discovered a survival mechanism by which MYCN-amplified neuroblastoma circumvents cell death by upregulating components of the error-prone non-canonical alternative nonhomologous end-joining (alt-NHEJ) DNA repair pathway. Similar to proliferating stem cells, high-risk neuroblastoma cells have enhanced DNA repair capacity, overcoming DNA damage with higher repair efficiency than somatic cells. Adequate DNA maintenance is required for lineage protection as stem cells proliferate and during tumor progression to overcome oncogene-induced replication stress. On this basis, we hypothesized that alt-NHEJ overexpression in neuroblastoma is a cancer cell survival mechanism that originates from DNA repair systems of NCSC, the presumed progenitor cell of origin. A human NCSC model was generated in which inducible MYCN triggered an immortalized phenotype capable of forming metastatic neuroectodermal tumors in mice, resembling human neuroblastoma. Critical alt-NHEJ components (DNA Ligase III, DNA Ligase I, and Poly [ADP-ribose polymerase 1]) were highly expressed in normal early NCSC, and decreased as cells became terminally differentiated. Constitutive MYCN expression maintained high alt-NHEJ protein expression, preserving the expression pattern of the immature neural phenotype. siRNA knockdown of alt-NHEJ components reversed MYCN effects on NCSC proliferation, invasion, and migration. DNA Ligase III, Ligase I, and PARP1 silencing significantly decreased neuroblastoma markers expression (TH, Phox2b, and TRKB). These results utilized the first human NCSC model of neuroblastoma to uncover an important link between MYCN and alt-NHEJ expression in developmental tumor initiation, setting precedence to investigate alt-NHEJ repair mechanics in neuroblastoma DNA maintenance.

KLHL21, a novel gene that contributes to the progression of hepatocellular carcinoma

BACKGROUND

Hepatocellular carcinoma (HCC) has very high prevalence and associated-mortality. However, targeted therapies that are currently used in clinical practice for HCC have certain limitations, in part because of the lack of reliable and clinically applicable biomarkers that can be used for diagnosis and prognosis assessments and for the surveillance of treatment effectiveness.

METHODS

Meta-analysis was used to analyze the integrated microarray data for global identification of a set of robust biomarkers for HCC. Quantitative RT-PCR (qRT-PCR) was performed to validate the expression levels of selected genes. Gene expression was inhibited by siRNA. CellTiter 96^® AQueous One Solution Cell Proliferation assays were used to determine cell proliferation, and Transwell assays were used to determine cell migration and invasion potential.

RESULTS

Meta-analysis of the expression data provided a gene expression signature from a total of 1525 patients with HCC, showing 1529 up-regulated genes and 478 down-regulated genes in cancer samples. The expression levels of genes having strong clinical significance were validated by qRT-PCR using primary HCC tissues and the paired adjacent noncancerous liver tissues. Up-regulation of VPS45, WIPI1, TTC1, IGBP1 and KLHL21 genes and down-regulation of FCGRT gene were confirmed in clinical HCC samples. KLHL21 was the most promising gene for potential use as a bioclinical marker in this analysis. Abrogating expression of it significantly inhibited cell proliferation, migration and invasion.

CONCLUSIONS

Our study suggests that KLHL21 is a potential target for therapeutic intervention. Our findings also provide novel candidate genes on a genome-wide scale, which may have significant impact on the design and execution of effective therapy of HCC patients.

Epigenetic dysregulation in neuroblastoma: A tale of miRNAs and DNA methylation

In neuroblastoma, the epigenetic landscape is more profoundly altered in aggressive compared to lower grade tumors and the concomitant hypermethylation of many genes, defined as "methylator phenotype", has been associated with poor outcome. DNA methylation can interfere with gene expression acting at distance through the methylation or demethylation of the regulatory regions of miRNAs. The multiplicity of miRNA targets may result in the simultaneous alteration of many biological pathways like cell proliferation, apoptosis, migration and differentiation. We have analyzed the methylation status of a set of miRNAs in a panel of neuroblastoma cell lines and identified a subset of hypermethylated and down-regulated miRNAs (miRNA 34b-3p, miRNA 34b-5p, miRNA34c-5p, and miRNA 124-2-3p) involved in the regulation of cell cycle, apoptosis and in the control of MYCN expression. These miRNAs share, in part, some of the targets whose expression is inversely correlated to the methylation and expression of the corresponding miRNA. To simulate the effect of the demethylation of miRNAs, we transfected the corresponding miRNA-mimics in the same cell lines and observed the down-regulation of a set of their target genes as well as the partial block of the cell cycle and the activation of the apoptotic pathway. The epigenetic alterations of miRNAs described in the present study were found also in a subset of patients at high risk of progression. Our data disclosed a complex network of interactions between epigenetically altered miRNAs and target genes, that could interfere at multiple levels in the control of cell homeostasis.

Exploitation of the Apoptosis-Primed State of MYCN-Amplified Neuroblastoma to Develop a Potent and Specific Targeted Therapy Combination

Fewer than half of children with high-risk neuroblastoma survive. Many of these tumors harbor high-level amplification of MYCN, which correlates with poor disease outcome. Using data from our large drug screen we predicted, and subsequently demonstrated, that MYCN-amplified neuroblastomas are sensitive to the BCL-2 inhibitor ABT-199. This sensitivity occurs in part through low anti-apoptotic BCL-xL expression, high pro-apoptotic NOXA expression, and paradoxical, MYCN-driven upregulation of NOXA. Screening for enhancers of ABT-199 sensitivity in MYCN-amplified neuroblastomas, we demonstrate that the Aurora Kinase A inhibitor MLN8237 combines with ABT-199 to induce widespread apoptosis. In diverse models of MYCN-amplified neuroblastoma, including a patient-derived xenograft model, this combination uniformly induced tumor shrinkage, and in multiple instances led to complete tumor regression.

Targeting MYCN IRES in MYCN-amplified neuroblastoma with miR-375 inhibits tumor growth and sensitizes tumor cells to radiation

The MYCN oncogene is amplified in 20% of neuroblastomas, leading to its overexpression at both the mRNA and protein levels. MYCN overexpression is strongly associated with advanced disease stage, rapid tumor progression and a worse prognosis. In the present study, we identified microRNA-375 (miR-375) as a negative regulator of MYCN: enforced expression of miR-375 inhibited MYCN-amplified neuroblastoma in vitro and in vivo. Upon searching the website miRbase for possible miR-375 binding sites within the whole MYCN mRNA, we found that the MYCN 5'-UTR had significant sequence complementarity to miR-375, yet no complementary sequences existed within the MYCN 3'-UTR and coding regions. Enforced overexpression of miR-375 efficiently inhibited MYCN mRNA translation and protein synthesis, via an IRES-dependent mechanism. In athymic nude mouse model with human MYCN-amplified neuroblastoma, MYCN downregulation by miR-375 led to inhibition of tumor cell growth and tumorigenicity. In particular, miR-375-regulated inhibition of MYCN translation was enhanced when MYCN-amplified neuroblastoma cells were exposed to stress stimulation, such as ionizing irradiation (IR), resulting in a remarkable increase in the neuroblastoma's sensitivity to IR-induced cell death. Our results identified a novel mechanism by which IRES-dependent translation of MYCN is repressed by miR-375, particularly during cellular stress, highlighting a potential anticancer strategy: the development of miR-375 as a novel therapeutic agent to treat MYCN-amplified neuroblastoma.

Alternative NHEJ Pathway Components Are Therapeutic Targets in High-Risk Neuroblastoma

In neuroblastoma, MYCN genomic amplification and segmental chromosomal alterations including 1p or 11q loss of heterozygocity and/or 17q gain are associated with progression and poor clinical outcome. Segmental alterations are the strongest predictor of relapse and result from unbalanced translocations attributable to erroneous repair of chromosomal breaks. Although sequence analysis of affected genomic regions suggests that these errors arise by nonhomologous end-joining (NHEJ) of DNA double-strand breaks (DSB), abnormalities in NHEJ have not been implicated in neuroblastoma pathogenesis. On this basis, the hypothesis that an error-prone mechanism of NHEJ is critical for neuroblastoma cell survival was tested. Plasmid-based DSB repair assays demonstrated efficient NHEJ activity in human neuroblastoma cells with repair products that were error-prone relative to nontransformed cells. Neuroblastoma cells derived from tumorigenic neuroblastic phenotypes had differential DNA repair protein expression patterns compared with nontumorigenic cells. Tumorigenic neuroblastoma cells were deficient in DNA ligase IV (Lig4) and Artemis (DCLRE1C), mediators of canonical NHEJ. Conversely, enzymes required for an error-prone alternative NHEJ pathway (alt-NHEJ), DNA Ligase IIIα (Lig3), DNA Ligase I (Lig1), and PARP1 protein were upregulated. Inhibition of Lig3 and Lig1 led to DSB accumulation and cell death, linking alt-NHEJ to cell survival in neuroblastoma. Neuroblastoma cells demonstrated sensitivity to PARP1 inhibition (PARPi) that paralleled PARP1 expression. In a dataset of human neuroblastoma patient tumors, overexpression of genes encoding alt-NHEJ proteins associated with poor survival.

IMPLICATIONS

These findings provide an insight into DNA repair fidelity in neuroblastoma and identify components of the alt-NHEJ pathway as promising therapeutic targets.

TWIST1 is a direct transcriptional target of MYCN and MYC in neuroblastoma

In neuroblastoma, MYCN amplification is associated with a worse prognosis and is a criterion used in the clinic to provide intensive treatments to children even with localized disease. In correlation with MYCN amplification, upregulation of TWIST1, a transcription factor playing a crucial role in inhibition of apoptosis and differentiation, was previously reported. Clinical data set analysis of MYCN, MYC and TWIST1 expression permits us to confirm that TWIST1 expression is upregulated in MYCN amplified neuroblastoma but also in a subset of neuroblastoma harboring high expression of MYCN or MYC without gene amplification. In silico analyses reveal the presence of several MYC regulatory motifs (E-Boxes and INR) within the TWIST1 promoter. Using gel shift assay and reporter activity assays, we demonstrate that both N-Myc and c-Myc proteins can bind and activate the TWIST1 promoter. Therefore, we propose TWIST1 as a direct MYC transcriptional target.

Absence of MGST1 mRNA and protein expression in human neuroblastoma cell lines and primary tissue

A recent study identified a haplotype on a small region of chromosome 12, between markers D12S1725 and D12S1596, shared by all patients with familial neuroblastoma (NB). We previously localized the human MGST1 gene, whose gene product protects against oxidative stress, to this very same chromosomal region (12p112.1-p13.33). Owing to the chromosomal location of MGST1; its roles in tumorigenesis, drug resistance, and oxidative stress; and the known sensitivity of NB cell lines to oxidative stress, we considered a role for MGST1 in NB development. Surprisingly there was no detectable MGST1 mRNA or protein in either NB cell lines or NB primary tumor tissue, although all other human tissues, cell lines, and primary tumor tissue examined to date express MGST1 at high levels. The mechanism behind the failure of NB cells and tissue to express MGST1 mRNA is unknown and involves the failure of MGST1 pre-mRNA expression, but does not involve chromosomal rearrangement or nucleotide variation in the promoter, exons, or 3' untranslated region of MGST1. MGST1 provides significant protection against oxidative stress and constitutes 4 to 6% of all protein in the outer membrane of the mitochondria. As NB cells are extremely sensitive to oxidative stress, and often used as a model system to investigate mitochondrial response to endogenous and exogenous stress, these findings may be due to the lack of expression MGST1 protein in NB. The significance of this finding to the development of neuroblastoma (familial or otherwise), however, is unknown and may even be incidental. Although our studies provide a molecular basis for previous work on the sensitivity of NB cells to oxidative stress, and possibly marked variations in NB mitochondrial homeostasis, they also imply that the results of these earlier studies using NB cells are not transferable to other tumor and cell types that express MGST1 at high concentrations.

USP7 inhibitor P22077 inhibits neuroblastoma growth via inducing p53-mediated apoptosis

Neuroblastoma (NB) is a common pediatric cancer and contributes to more than 15% of all pediatric cancer-related deaths. Unlike adult tumors, recurrent somatic mutations in NB, such as tumor protein 53 (p53) mutations, occur with relative paucity. In addition, p53 downstream function is intact in NB cells with wild-type p53, suggesting that reactivation of p53 may be a viable therapeutic strategy for NB treatment. Herein, we report that the ubiquitin-specific protease 7 (USP7) inhibitor, P22077, potently induces apoptosis in NB cells with an intact USP7-HDM2-p53 axis but not in NB cells with mutant p53 or without human homolog of MDM2 (HDM2) expression. In this study, we found that P22077 stabilized p53 by inducing HDM2 protein degradation in NB cells. P22077 also significantly augmented the cytotoxic effects of doxorubicin (Dox) and etoposide (VP-16) in NB cells with an intact USP7-HDM2-p53 axis. Moreover, P22077 was found to be able to sensitize chemoresistant LA-N-6 NB cells to chemotherapy. In an in vivo orthotopic NB mouse model, P22077 significantly inhibited the xenograft growth of three NB cell lines. Database analysis of NB patients shows that high expression of USP7 significantly predicts poor outcomes. Together, our data strongly suggest that targeting USP7 is a novel concept in the treatment of NB. USP7-specific inhibitors like P22077 may serve not only as a stand-alone therapy but also as an effective adjunct to current chemotherapeutic regimens for treating NB with an intact USP7-HDM2-p53 axis.

MYCN and survivin cooperatively contribute to malignant transformation of fibroblasts

The oncogenes MYCN and survivin (BIRC5) maintain aggressiveness of diverse cancers including sarcomas. To investigate whether these oncogenes cooperate in initial malignant transformation, we transduced them into Rat-1 fibroblasts. Indeed, survivin enhanced MYCN-driven contact-uninhibited and anchorage-independent growth in vitro. Importantly, upon subcutaneous transplantation into mice, cells overexpressing both instead of either one of the oncogenes generated tumors with shortened latency, marked anaplasia and an increased proliferation-to-apoptosis ratio resulting in accelerated growth. Mechanistically, the increased tumorigenicity was associated with an enhanced Warburg effect and a hypoxia inducible factor 1α linked vascular remodeling. This cooperation between MYCN and survivin may be important in the genesis of several cancers.

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.

Direct effects of Bmi1 on p53 protein stability inactivates oncoprotein stress responses in embryonal cancer precursor cells at tumor initiation

Embryonal cancer can arise from postnatally persistent embryonal remnant or rest cells, which are uniquely characterized by the absence of p53 mutations. Perinatal overexpression of the MycN oncoprotein in embryonal cancer precursor cells causes postnatal rests, and later tumor formation through unknown mechanisms. However, overexpression of Myc in adult tissues normally activates apoptosis and/or senescence signals as an organismal defense mechanism against cancer. Here, we show that perinatal neuroblastoma precursor cells exhibited a transiently diminished p53 response to MycN oncoprotein stress and resistance to trophic factor withdrawal, compared with their adult counterpart cells from the TH-MYCN(+/+) transgenic mouse model of neuroblastoma. The adult stem cell maintenance factor and Polycomb group protein, Bmi1 (B-cell-specific Moloney murine leukemia virus integration site), had a critical role at neuroblastoma initiation in the model, by repressing p53 responses in precursor cells. We further show in neuroblastoma tumor cells that Bmi1 could directly bind p53 in a complex with other Polycomb complex proteins, Ring1A or Ring1B, leading to increased p53 ubiquitination and degradation. Repressed p53 signal responses were also seen in precursor cells for other embryonal cancer types, medulloblastoma and acute lymphoblastic leukemia. Collectively, these date indicate a general mechanism for p53 inactivation in some embryonal cell types and consequent susceptibility to MycN oncogenesis at the point of embryonal tumor initiation.

MDM2 regulates MYCN mRNA stabilization and translation in human neuroblastoma cells

The MYCN gene has a critical role in determining the clinical behavior of neuroblastoma. Although it is known that genomic amplification occurs in high-risk subsets, it remains unclear how MYCN expression is regulated in the pathogenesis of neuroblastomas. Here, we report that MYCN expression was regulated by the oncoprotein MDM2 at the post-transcriptional level and was associated with neuroblastoma cell growth. Increasing MDM2 by ectopic overexpression in the cytoplasm enhanced both mRNA and protein expression of MYCN. Mechanistic studies found that the C-terminal RING domain of the MDM2 protein bound to the MYCN mRNA's AREs within the 3'UTR and increased MYCN 3'UTR-mediated mRNA stability and translation. Conversely, MDM2 silencing by specific siRNA rendered the MYCN mRNA unstable and reduced the abundance of the MYCN protein in MYCN-amplified neuroblastoma cell lines. Importantly, this MDM2 silencing resulted in a remarkable inhibition of neuroblastoma cell growth and induction of cell death through a p53-independent pathway. Our results indicate that MDM2 has a p53-independent role in the regulation of both MYCN mRNA stabilization and its translation, suggesting that MDM2-mediated MYCN expression is one mechanism associated with growth of MYCN-associated neuroblastoma and disease progression.

Retinoic acid-induced differentiation increases the rate of oxygen consumption and enhances the spare respiratory capacity of mitochondria in SH-SY5Y cells

Retinoic acid (RA) is used in differentiation therapy to treat a variety of cancers including neuroblastoma. The contributing factors for its therapeutic efficacy are poorly understood. However, mitochondria (MT) have been implicated as key effectors in RA-mediated differentiation process. Here we utilize the SH-SY5Y human neuroblastoma cell line as a model to examine how RA influences MT during the differentiation process. We find that RA confers an approximately sixfold increase in the oxygen consumption rate while the rate of glycolysis modestly increases. RA treatment does not increase the number of MT or cause measurable changes in the composition of the electron transport chain. Rather, RA treatment significantly increases the mitochondrial spare respiratory capacity. We propose a competition model for the therapeutic effects of RA. Specifically, the high metabolic rate in differentiated cells limits the availability of metabolic nutrients for use by the undifferentiated cells and suppresses their growth. Thus, RA treatment provides a selective advantage for the differentiated state.

p53 in the CNS: Perspectives on Development, Stem Cells, and Cancer

The p53 tumor suppressor potently limits the growth of immature and mature neurons under conditions of cellular stress. Although loss of p53 function contributes to the pathogenesis of central nervous system (CNS) tumors, excessive p53 function is implicated in neural tube defects, embryonic lethality, and neuronal degeneration. Thus, p53 function must be tightly controlled. The anti-proliferative properties of p53 are mediated, in part, through the induction of apoptosis, cell cycle arrest, and senescence. Although there is still much to be learned about the role of p53 in these processes, recent evidence supports exciting new roles for p53 in a wide range of processes, including neural precursor cell self-renewal, differentiation, and cell fate decisions. Understanding the full repertoire of p53 function in CNS development and tumorigenesis may provide us with novel points of therapeutic intervention for human diseases of the CNS.

MYCN sensitizes neuroblastoma to the MDM2-p53 antagonists Nutlin-3 and MI-63

MYCN amplification is a major biomarker of poor prognosis, occurring in 25-30% of neuroblastomas. MYCN plays contradictory roles in promoting cell growth and sensitizing cells to apoptosis. We have recently shown that p53 is a direct transcriptional target of MYCN in neuroblastoma and that p53-mediated apoptosis may be an important mechanism of MYCN-induced apoptosis. Although p53 mutations are rare in neuroblastoma at diagnosis, the p53/MDM2/p14^ARF pathway is often inactivated through MDM2 amplification or p14^ARF inactivation. We hypothesised that reactivation of p53 by inhibition of its negative regulator MDM2, using the MDM2-p53 antagonists Nutlin-3 and MI-63, will result in p53-mediated growth arrest and apoptosis especially in MYCN amplified cells. Using the SHEP Tet21N MYCN regulatable system, MYCN(−) cells were more resistant to both Nutlin-3 and MI-63 mediated growth inhibition and apoptosis compared to MYCN(+) cells and siRNA mediated knockdown of MYCN in 4 MYCN amplified cell lines resulted in decreased p53 expression and activation, as well as decreased levels of apoptosis following treatment with MDM2-p53 antagonists. In a panel of 18 neuroblastoma cell lines treated with Nutlin-3 and MI-63, the sub-set amplified for MYCN had a significantly lower mean GI₅₀ value and increased caspase 3/7 activity compared to the non MYCN amplified group of cell lines, but p53 mutant cell lines were resistant to the antagonists regardless of MYCN status. We conclude that amplification or overexpression of MYCN sensitizes neuroblastoma cell lines with wildtype p53 to MDM2-p53 antagonists and that these compounds may therefore be particularly effective in treating high risk MYCN amplified disease.

Percentage tumor necrosis following chemotherapy in neuroblastoma correlates with MYCN status but not survival

The percentage of chemotherapy-induced necrosis in primary tumors corresponds with outcome in several childhood malignancies, including high-risk metastatic diseases. In this retrospective pilot study, the authors assessed the importance of postchemotherapy necrosis in high-risk neuroblastoma with a histological and case notes review of surgically resected specimens. The authors reviewed all available histology of 31 high-risk neuroblastoma cases treated with COJEC (dose intensive etoposide and vincristine with either cyclophosphamide, cisplatin or carboplatin) or OPEC/OJEC (etoposide, vincristine and cyclophosphamide with alternating cisplatin [OPEC] or carboplatin [OJEC]) induction chemotherapy in 2 Children's Cancer & Leukaemia Group (CCLG) pediatric oncology centers. The percentage of postchemotherapy necrosis was assessed and compared with MYCN amplification status and overall survival. The median percentage of postchemotherapy tumor necrosis was 60%. MYCN status was available for 28 cases, of which 12 were amplified (43%). Survival in cases with ≥ 60% necrosis or ≥ 90% necrosis was not better than those with less necrosis, nor was percentage necrosis associated with survival using Cox regression. However, MYCN-amplified tumors showed a higher percentage of necrosis than non-MYCN-amplified tumors, 71.3% versus 37.2% (P = .006). This effect was not related to prechemotherapy necrosis and did not confer improved overall survival. Postchemotherapy tumor necrosis is higher in patients with MYCN amplification. In this study, postchemotherapy necrosis did not correlate with overall survival and should not lead to modification of postoperative treatment. However, these findings need to be confirmed in a larger prospective study of children with high-risk neuroblastoma.

TP53 Mutation Is Frequently Associated With CTNNB1 Mutation or MYCN Amplification and Is Compatible With Long-Term Survival in Medulloblastoma

Purpose

The role of TP53 mutations in the tumorigenesis of sporadic medulloblastoma (MB) and the value of TP53 mutation status as a prognostic marker are not yet definitely elucidated. A recent report identified TP53 mutations in MB as an adverse prognostic marker. Hence, the current study was conducted to validate the prognostic role of TP53 mutation in MB and to understand its contribution to tumorigenesis.

Methods

A comprehensive genetic analysis of 310 MB samples was performed by screening for TP53 mutations and further relating the TP53 mutation status to p53 immunostaining, cytogenetic aberrations, and clinical variables.

Results

Mutation analysis of TP53 revealed mutations in 21 (6.8%) of 310 samples. Germline TP53 mutations were found in two patients with a history suggestive of a hereditary cancer syndrome. TP53 mutation status was not associated with unfavorable prognosis (P = .63) and was not linked to 17p allelic loss but was over-represented in the prognostically favorable WNT subgroup of MB as defined by CTNNB1 mutation (seven of 35 TP53-mutated tumors v 14 of 271 TP53 wild-type tumors; P = .005) and in tumors carrying high-level MYCN amplification (seven of 21 TP53-mutated tumors v 14 of 282 TP53 wild-type tumors; P = .001).

Conclusion

The contradictory results in the recent literature concerning the prognostic value of TP53 mutation might be explained by different frequencies of WNT MBs, different frequencies of patients with Li-Fraumeni syndrome, and different cumulative doses of alkylating drugs applied in these studies.

MYC in oncogenesis and as a target for cancer therapies

MYC proteins (c-MYC, MYCN, and MYCL) regulate processes involved in many if not all aspects of cell fate. Therefore, it is not surprising that the MYC genes are deregulated in several human neoplasias as a result from genetic and epigenetic alterations. The near "omnipotency" together with the many levels of regulation makes MYC an attractive target for tumor intervention therapy. Here, we summarize some of the current understanding of MYC function and provide an overview of different cancer forms with MYC deregulation. We also describe available treatments and highlight novel approaches in the pursuit for MYC-targeting therapies. These efforts, at different stages of development, constitute a promising platform for novel, more specific treatments with fewer side effects. If successful a MYC-targeting therapy has the potential for tailored treatment of a large number of different tumors.

MYCN oncoprotein targets and their therapeutic potential

The MYCN oncogene encodes a transcription factor which is amplified in up to 40% of high risk neuroblastomas. MYCN amplification is a well-established poor prognostic marker in neuroblastoma, however the role of MYCN expression and the mechanisms by which it acts to promote an aggressive phenotype remain largely unknown. This review discusses the current evidence identifying the direct and indirect downstream transcriptional targets of MYCN from recent studies, with particular reference to how MYCN affects the cell cycle, DNA damage response, differentiation and apoptosis in neuroblastoma.

p53 is a direct transcriptional target of MYCN in neuroblastoma

MYCN amplification occurs in approximately 25% of neuroblastomas, where it is associated with rapid tumor progression and poor prognosis. MYCN plays a paradoxical role in driving cellular proliferation and inducing apoptosis. Based on observations of nuclear p53 accumulation in neuroblastoma, we hypothesized that MYCN may regulate p53 in this setting. Immunohistochemical analysis of 82 neuroblastoma tumors showed an association of high p53 expression with MYCN expression and amplification. In a panel of 5 MYCN-amplified and 5 nonamplified neuroblastoma cell lines, and also in the Tet21N-regulatable MYCN expression system, we further documented a correlation between the expression of MYCN and p53. In MYCN-amplified neuroblastoma cell lines, MYCN knockdown decreased p53 expression. In Tet21N MYCN+ cells, higher levels of p53 transcription, mRNA, and protein were observed relative to Tet21N MYCN- cells. In chromatin immunoprecipitation and reporter gene assays, MYCN bound directly to a Myc E-Box DNA binding motif located close to the transcriptional start site within the p53 promoter, where it could initiate transcription. E-Box mutation decreased MYCN-driven transcriptional activation. Microarray analysis of Tet21N MYCN+/- cells identified several p53-regulated genes that were upregulated in the presence of MYCN, including MDM2 and PUMA, the levels of which were reduced by MYCN knockdown. We concluded that MYCN transcriptionally upregulates p53 in neuroblastoma and uses p53 to mediate a key mechanism of apoptosis.

BH3 response profiles from neuroblastoma mitochondria predict activity of small molecule Bcl-2 family antagonists

Bcl-2 family proteins regulate mitochondrial apoptosis downstream of diverse stressors. Cancer cells frequently deregulate Bcl-2 proteins leading to chemoresistance. We have optimized a platform for solid tumors in which Bcl-2 family resistance patterns are inferred. Functional mitochondria were isolated from neuroblastoma cell lines, exposed to distinct BH3-domain peptides, and assayed for cytochrome c release. Such BH3 profiles revealed three patterns of cytochrome c response. A subset had a dominant NoxaBH3 response implying Mcl1-dependence. These cells were more sensitive to small molecules that antagonize Mcl1 (AT-101) than those that antagonize Bcl-2, Bcl-x_L and Bcl-w (ABT-737). A second subset had a dominant BikBH3 response, implying a Bcl-x_L/-w dependence, and was exquisitely sensitive to ABT-737 (IC50 <200 nM). Finally, most neuroblastoma cell lines derived at relapse were relatively resistant to pro-death BH3 peptides and Bcl-2 antagonists. Our findings define heterogeneity for apoptosis resistance in neuroblastoma, help triage emerging Bcl-2 antagonists for clinical use, and provide a platform for studies to characterize post-therapy resistance mechanisms for neuroblastoma and other solid tumors.

Mcl1 downregulation sensitizes neuroblastoma to cytotoxic chemotherapy and small molecule Bcl2-family antagonists

Neuroblastoma (NB) is a common, highly lethal pediatric cancer, with treatment failures largely attributable to the emergence of chemoresistance. The pro-survival Bcl2 homology (BH) proteins critically regulate apoptosis, and may represent important therapeutic targets for restoring drug sensitivity in NB. We used a human NB tumor tissue microarray to survey the expression of pro-survival BH proteins Mcl1 and Bcl2, and correlated expression to clinical prognostic factors and survival. Primary NB tumors heterogeneously expressed Mcl1 or Bcl2, with high expression correlating to high-risk phenotype. Co-expression is infrequent (11%), but correlates to reduced survival. Using RNA interference, we investigated the functional relevance of Mcl1 and Bcl2 in high-risk NB cell lines (SK-N-AS, IMR-5, NLF). Mcl1 knockdown induced apoptosis in all NB cell lines, while Bcl2 knockdown inhibited only NLF, suggesting functional heterogeneity. Finally, we determined the relevance of Mcl1 in resistance to conventional chemotherapy (etoposide, doxorubicin) and small molecule Bcl2-family antagonists (ABT-737 and AT-101). Mcl1 silencing augmented sensitivity to chemotherapeutics 2- to 300-fold, while Bcl2 silencing did not, even in Bcl2-sensitive NLF cells. Resistance to ABT-737, which targets Bcl2/-w/-x, was overcome by Mcl1 knockdown. AT-101, which also neutralizes Mcl1, had single-agent cytotoxicity, further augmented by Mcl1 knockdown. In conclusion, Mcl1 appears a predominant pro-survival protein contributing to chemoresistance in NB, and Mcl1 inactivation may represent a novel therapeutic strategy. Optimization of compounds with higher Mcl1 affinity, or combination with additional Mcl1 antagonists, may enhance the clinical utility of this approach.

N-myc augments death and attenuates protective effects of Bcl-2 in trophically stressed neuroblastoma cells

N-myc has proapoptotic functions, yet it acts as an oncogene in neuroblastoma. Thus, antiapoptotic mechanisms have to be operative in neuroblastoma cells that antagonize the proapoptotic effects of N-myc. We conditionally activated N-myc in SH-EP neuroblastoma cells subjected to the trophic stress of serum or nutrient deprivation while changing the expression of Bcl-2, survivin and FLIP(L), antiapoptotic molecules often overexpressed in poor prognosis neuroblastomas. Bcl-2 protected SH-EP cells from death during nutritional deprivation by activating energetically advantageous oxidative phosphorylation. N-myc overrode the metabolic protection provided by Bcl-2-induced oxidative phosphorylation by reestablishing the glycolytic phenotype and attenuated the antiapoptotic effect of Bcl-2 during metabolic stress. Survivin partially antagonized the growth suppressive function of N-myc in SH-EP neuroblastoma cells during serum deprivation whereas FLIP(L) did not. These findings advance our understanding of the functions of N-myc in neuroblastoma cells.

Effects of arsenic trioxide on the cellular proliferation, apoptosis and differentiation of human neuroblastoma cells

A human neuroblastoma cell line, IMR-32, was used as an in vitro model system to study the effects of arsenic trioxide (As(2)O(3)) on aggressive human neuroblastoma. From 0.5 micro M, As(2)O(3) exhibited a dose-dependent inhibition of IMR-32 proliferation. At concentrations of 1.5 micro M or higher, As(2)O(3) up-regulated caspase 3, leading to cellular apoptosis. However, neurofilament-200 kDa and tyrosine hydroxylase were not up-regulated, implying minimal neuronal differentiation. Concomitantly, TrkA was down-regulated and TrkB up-regulated. Pre-treatment with the protein kinase C (PKC) inhibitor Ro-31-8220 partially blocked As(2)O(3)-mediated apoptosis, meaning that As(2)O(3) might signal through PKC activation. The results suggest that As(2)O(3) might be potentially useful in neuroblastoma.

Camptothecin-induced apoptosis is enhanced by Myc and involves PKCdelta signaling

The MYC oncogene is frequently deregulated in human tumors, indicative of a poor prognosis because of enhanced resistance to treatment. In such cases, the cellular sensitivity to chemotherapy could be restored by reactivation of Myc-driven apoptosis. We have analyzed apoptosis induced by the cytotoxic agents camptothecin (CPT) and paclitaxel (PTX) using Rat1 fibroblasts with different c-myc status and human Tet21N neuroblastoma cells with conditional MYCN expression. In these cell lines, the drug sensitivity was enhanced by Myc in line with previous reports showing that Myc sensitizes to apoptosis induction by many different apoptosis inducers. CPT-induced apoptosis involved cleavage and activation of proapoptotic Bid and Bax, induction of mitochondrial membrane depolarization, activation of caspase-9 and caspase-3, protein kinase c delta (PKCdelta) signaling and upregulation of p53. We also observed reduced transcriptional activity by Myc and other transcription factors in response to CPT. In contrast, the manner by which Myc potentiates the apoptosis induced by PTX differs from that of CPT and remains to be explored. In summary, our findings revealed that activation of PKCdelta in response to CPT treatment requires Myc and is important in CPT-mediated apoptosis signaling.

MDM2 inhibition sensitizes neuroblastoma to chemotherapy-induced apoptotic cell death

Novel therapeutic approaches are urgently needed for high-stage neuroblastoma, a major therapeutic challenge in pediatric oncology. The majority of neuroblastoma tumors are p53 wild type with intact downstream p53 signaling pathways. We hypothesize that stabilization of p53 would sensitize this aggressive tumor to genotoxic chemotherapy via inhibition of MDM2, the primary negative upstream regulator of p53. We used pharmacologic inhibition of the MDM2-p53 interaction with the small-molecule inhibitor Nutlin and studied the subsequent response to chemotherapy in neuroblastoma cell lines. We did 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide and terminal deoxynucleotidyl transferase assays to measure proliferation and apoptosis in several cell lines (IMR32, MYCN3, and JF) treated with combinations of cisplatin, etoposide, and Nutlin. We found consistent and robust decreases in proliferation and increases in apoptosis with the addition of Nutlin 3a to etoposide or cisplatin in all cell lines tested and no response to the inactive Nutlin 3b enantiomer. We also show a rapid and robust accumulation of p53 protein by Western blot in these cells within 1 to 2 hours of treatment. We conclude that MDM2 inhibition dramatically enhances the activity of genotoxic drugs in neuroblastoma and should be considered as an adjuvant to chemotherapy for this aggressive pediatric cancer and for possibly other p53 wild-type solid tumors.

Mutations in PIK3CA are infrequent in neuroblastoma

BACKGROUND

Neuroblastoma is a frequently lethal pediatric cancer in which MYCN genomic amplification is highly correlated with aggressive disease. Deregulated MYC genes require co-operative lesions to foster tumourigenesis and both direct and indirect evidence support activated Ras signaling for this purpose in many cancers. Yet Ras genes and Braf, while often activated in cancer cells, are infrequent targets for activation in neuroblastoma. Recently, the Ras effector PIK3CA was shown to be activated in diverse human cancers. We therefore assessed PIK3CA for mutation in human neuroblastomas, as well as in neuroblastomas arising in transgenic mice with MYCN overexpressed in neural-crest tissues. In this murine model we additionally surveyed for Ras family and Braf mutations as these have not been previously reported.

METHODS

Sixty-nine human neuroblastomas (42 primary tumors and 27 cell lines) were sequenced for PIK3CA activating mutations within the C2, helical and kinase domain "hot spots" where 80% of mutations cluster. Constitutional DNA was sequenced in cases with confirmed alterations to assess for germline or somatic acquisition. Additionally, Ras family members (Hras1, Kras2 and Nras) and the downstream effectors Pik3ca and Braf, were sequenced from twenty-five neuroblastomas arising in neuroblastoma-prone transgenic mice.

RESULTS

We identified mutations in the PIK3CA gene in 2 of 69 human neuroblastomas (2.9%). Neither mutation (R524M and E982D) has been studied to date for effects on lipid kinase activity. Though both occurred in tumors with MYCN amplification the overall rate of PIK3CA mutations in MYCN amplified and single-copy tumors did not differ appreciably (2 of 31 versus 0 of 38, respectively). Further, no activating mutations were identified in a survey of Ras signal transduction genes (including Hras1, Kras2, Nras, Pik3ca, or Braf genes) in twenty-five neuroblastic tumors arising in the MYCN-initiated transgenic mouse model.

CONCLUSION

These data suggest that activating mutations in the Ras/Raf-MAPK/PI3K signaling cascades occur infrequently in neuroblastoma. Further, despite compelling evidence for MYC and RAS cooperation in vitro and in vivo to promote tumourigenesis, activation of RAS signal transduction does not constitute a preferred secondary pathway in neuroblastomas with MYCN deregulation in either human tumors or murine models.

Increased expression of LGI1 gene triggers growth inhibition and apoptosis of neuroblastoma cells

The LGI1 gene has been implicated in the malignant progression of glioblastoma and it has also been genetically linked to a form of partial epilepsy (ADLTE). In this study, we investigated the relevance of LGI1 expression for neuroblastoma cells. The analysis of two cell lines (SH-SY5Y and SK-N-BE) revealed unpredictably low levels of LGI1 and stable cell transfection with LGI1 cDNA yielded moderate increases of LGI1 expression. Neuroblastoma cell clones exhibited impaired cell growth and survival ability in relation to LGI1 levels. The process of growth inhibition could be discerned under experimental conditions of low cell density, since conditions of elevated cell density, which enhance the requirement for survival stimuli, resulted in massive cellular death. At high cell density, spontaneous apoptosis of LGI1 cells was clearly shown by the release of cytochrome c and apoptosis inducing factor (AIF) from mitochondria and by phosphatydil serine exposure and nuclear fragmentation. Activation of apoptotic effectors caspase-3/7 also occurred, however, the broad caspase inhibitor Z-VAD-FMK substantially failed to block cell death. Thus the possibility that LGI1-triggered apoptosis may involve initiator caspases linked to activation of death receptors, appears unlikely. The decreased ratio of Bcl-2 to Bax suggests that apoptosis is initiated by the intrinsic mitochondrial pathway through the release of caspase-dependent and -independent apoptogenic molecules. This study provides the first evidence that LGI1 controls neuronal cell survival, suggesting its role in the development of the nervous system in relation to the pathogenesis of neuroblastoma and ADLTE.

Increased occurrence of caspase-dependent apoptosis in unfavorable neuroblastomas

Neuroblastoma frequently shows spontaneous regression in which two distinct types of programmed cell death, ie, caspase-dependent apoptosis and H-Ras-mediated autophagic degeneration, have been suggested to play a key role. The current study was conducted to determine which of these cell suicide pathways predominated in this tumor regression. Periodic acid-Schiff (PAS) staining and immunostaining for H-Ras and for the full-length and cleaved forms of caspase-3, poly (ADP-ribose) polymerase (PARP), and lamin A were carried out on 55 archival tumor specimens. The incidence of caspase-dependent apoptosis in each tumor was quantified by cleaved lamin A staining and compared with clinicopathologic prognostic factors. Although a recent report has shown that neuroblastic cells undergoing autophagic degeneration were readily detectable by PAS and H-Ras staining, we could not confirm this result in any of our samples with the exception of one tumor. Instead, many of our neuroblastoma samples showed nonspecific PAS and Ras staining in areas of necrosis, suggesting that autophagic "degeneration" indeed corresponds to coagulation necrosis or oncosis. Unexpectedly, the incidence of caspase-dependent apoptosis was significantly correlated with indicators of a poor prognosis in these tumors, including Shimada's unfavorable histology, MYCN amplification, and a higher mitosis-karyorrhexis index, but not with factors related to tumor regression such as clinical stage and mass screening. These results indicate that neither caspase-dependent apoptosis nor autophagic "degeneration" may be involved in spontaneous neuroblastoma regression. This suggests that other mechanisms, perhaps such as tumor maturation, may be responsible for this phenomenon.

MYCN deregulation as a potential target for novel therapies in rhabdomyosarcoma

Rhabdomyosarcoma is the most common soft-tissue sarcoma of childhood. Treatment requires a multimodality approach combining chemotherapy with surgery and radiotherapy. Although overall outcomes have improved considerably, the outlook for patients with high-risk disease, particularly the alveolar subtype, remains bleak and there is a clear need for new chemotherapeutic strategies. This review focuses on the possibilities for interventions targeting myc myelocytomatosis viral related oncogene, neuroblastoma derived (MYCN). The importance of aberrant expression of this oncogene is well established in neuroblastoma and recent data indicate that MYCN deregulation also occurs in up to a quarter of alveolar subtype cases. A range of possible approaches to target MYCN is discussed, including nucleic acid-based and immunotherapy strategies.

Successful treatment of MYCN amplified, progressive stage 4S neuroblastoma in a neonate with hepatic artery embolization in addition to multimodality treatment

Stage 4S metastatic neuroblastoma (NB) has a favorable prognosis due to a high rate of spontaneous regression. Young infants risk lethal complications arising from hepatomegaly, which can develop rapidly despite treatment. MYCN oncogene amplification confers a significantly worse prognosis. We describe a 4-week-old neonate with MYCN-amplified stage 4S NB complicated by gross hepatomegaly causing rapidly progressive respiratory, hepatic, and renal failure. The child remains in remission 3 years after hepatic artery embolization, radiotherapy, standard, and high-dose chemotherapy. Embolization of the hepatic artery, with classical treatment, is feasible and safe at this age and may contribute substantially to the management of high-risk patients.

Oncogenes as novel targets for cancer therapy (part III): transcription factors

This is the third paper in a four-part serial review on potential therapeutic targeting of oncogenes. The previous parts described the involvement of oncogenes in different aspects of cancer growth and development, and considered the new technologies responsible for the advancement of oncogene identification, target validation, and drug design. Because of such advances, new specific and more efficient therapeutic agents can be developed for cancer. This part of the review continues the exploration of various oncogenes that we have grouped within seven categories: growth factors, tyrosine kinases, intermediate signaling molecules, transcription factors, cell cycle regulators, DNA damage repair genes, and genes involved in apoptosis. Part one discussed growth factors and tyrosine kinases and part two discussed intermediate signaling molecules. This portion of the review covers transcription factors and the various strategies being used to inhibit their expression or decrease their activities.

Targeting programmed cell death pathways with experimental therapeutics: opportunities in high-risk neuroblastoma

Neuroblastoma is a pediatric solid tumor with high morbidity and mortality in association with particular high-risk biological and clinical features (such as MYCN proto-oncogene amplification or advanced tumor stage). Such high-risk neuroblastomas may be initially responsive to cytoreductive therapies, yet the majority will ultimately demonstrate de novo or acquired chemoresistance leading to tumor progression and death. Insight into the genetic alterations responsible for these phenotypes are beginning to be gained, and subversion of inherent programmed cell death pathways is a common theme. Intact apoptosis pathways protect cells against neoplastic transformation and provide the mechanisms by which cytotoxic agents exert their effects. When these pathways are abolished through alterations in the cell death machinery, they complement deregulated oncogenes to promote tumor initiation and therapy resistance. Currently, therapeutic intensity for high-risk neuroblastoma has been advanced to near-tolerance with only modest gains in survival, and it is likely that further improvements in outcome will require innovative approaches that target key regulatory pathways that potentiate currently available therapies. Efforts to abrogate the cancer cell 'survival bias' engendered by alterations in death pathways are now a major focus in experimental cancer therapeutics, and their application to the problem of high-risk neuroblastoma form the basis of this review. These include agents that activate death receptors (TRAIL-agonists) or restore DISC competency (CDDO, DNA methyltransferase and HDAC inhibitors); reduce pro-survival Bcl2 homologues (Oblimersen sodium [AS-Bcl2], AS-Mcl1) or deliver a pro-apoptotic BH3 protein burden (BH3 peptides, gossypol, ABT737); or repress IAPs (Smac/Diablo peptides, AS-XIAP, AS-Survivin). As our knowledge of apoptosis dysregulation in neuroblastoma evolves, the possibilities for pro-apoptotic therapeutics seems not only promising, but a realistic adjunct to conventional treatments.

Reactive oxygen species: Biological stimuli of neuroblastoma cell response

Reactive oxygen species play a critical role in differentiation, proliferation and apoptosis acting as 'second messengers' able to regulate sulphydryl groups in signaling molecules as protein kinase C, a family of isoenzymes involved in many cellular responses and implicated in cell transformation. Neuroblastoma is characterised by the production of oxygen intermediates and L-buthionine-S,R-sulfoximine, a glutathione-depleting agent that has been tested in the clinics, exploits this biological peculiarity to induce cell death. The latter process is mediated by the oxidative activation of PKC delta which might be involved also in the production of reactive oxygen species, thus amplifying the apoptotic cascade.

MDM2 as MYCN transcriptional target: Implications for neuroblastoma pathogenesis

MYCN amplification is associated with an exceptionally poor prognosis in neuroblastoma. Furthermore, the crucial effectors of MYCN responsible for this aggressive subset of neuroblastoma await characterization. A critical negative regulator of the p53 tumor suppressor, MDM2, has been recently characterized in neuroblastoma cell lines as a transcriptional target of MYCN. Targeted inhibition of MYCN results in reduced MDM2 expression levels, with concomitant stabilization of p53 and stimulation of apoptosis in MYCN amplified neuroblastoma cell lines. These data suggest the possibility that MYCN-driven expression of MDM2 might play a role in counterbalancing the p53-dependent apoptotic pathways concurrently stimulated by over expression of MYC proteins. Mouse models of lymphoma have demonstrated that MDM2 expression, with decreased p53 activity, is critical for complete MYCC driven tumorigenesis. Our data suggest that a similar situation may apply for MYCN in neuroblastoma. Strategies for pharmacologic and genetic inhibition of MDM2 may prove to be an important new therapeutic approach in neuroblastoma.

Increased progenitor proliferation and apoptotic cell death in the sensory lineage of mice overexpressing N-myc

N-myc, a member of the myc family of bHLH transcription factors, is expressed mainly in the nervous system, including derivatives of neural crest cells in the periphery during development, such as the sensory dorsal root ganglion (DRG). Previous studies suggest that N-myc is involved in the proliferation of progenitor cells in the sensory lineage. To address the role of N-myc in the development of peripheral sensory neurons, we have overexpressed N-myc in sensory progenitor cells. The overexpression of N-myc did not significantly affect the number of multipotent neural crest cells or glial differentiation but caused a brief and marked increase of both proliferation and apoptosis in the DRG at embryonic day 11 (E11), thus coinciding with the stage of cell-cycle exit. At E17, the total number of cells in the lumbar DRG of mice with forced expression of N-myc was significantly reduced compared with that in wild-type mice. Among the different DRG subpopulations examined, the number of parvalbumin-positive neurons representing large-diameter proprioceptive neurons increased significantly. Our results indicate that forced expression of N-myc in the sensory lineage leads to unscheduled cell-cycle re-entry and excessive apoptosis and show that N-myc can affect the composition of different functional subtypes of sensory neurons in the DRG.

Histone deacetylase inhibitor BL1521 induces a G1-phase arrest in neuroblastoma cells through altered expression of cell cycle proteins

Histone deacetylase inhibitors (HDACi) have been discovered as potential drugs for cancer treatment. The effect of BL1521, a novel HDACi, on the cell cycle distribution and the induction of apoptosis was investigated in a panel of MYCN single copy and MYCN amplified neuroblastoma cell lines. BL1521 arrested neuroblastoma cells in the G1 phase and induced up to 30% apoptosis. Downregulation of CDK4, upregulation of p21(WAF1/CIP1) and an increase of hypophosphorylated retinoblastoma protein were observed, indicating a possible mechanism for the cell-cycle arrest. BL1521 also induced downregulation of p27, which may underlie the observed induction of apoptosis.

The p53 regulatory gene MDM2 is a direct transcriptional target of MYCN in neuroblastoma

The MYCN oncogene is the major negative prognostic marker in neuroblastoma with important roles in both the pathogenesis and clinical behavior of this aggressive malignancy. MYC oncogenes activate both proliferative and apoptotic cellular pathways and, accordingly, inhibition of p53-mediated apoptosis is a prerequisite for MYC-driven tumorigenesis. To identify novel transcriptional targets mediating the MYCN-dependent phenotype, we screened a MYCN-amplified neuroblastoma cell line by using chromatin immunoprecipitation (ChIP) cloning. We identified the essential p53 inhibitor and protooncogene MDM2 as a putative target. MDM2 has multiple p53-independent functions modulating cell cycle and transcriptional events. Standard ChIP with MYCN antibodies established the binding of MYCN to a consensus E-box within the human MDM2 promoter. Oligonucleotide pull-down assays further established the capacity of MYCN to bind to this promoter region, confirming the ChIP results. Luciferase reporter assays confirmed the E-box-specific, MYCN-dependent regulation of the MDM2 promoter in MYCN-inducible neuroblastoma cell lines. Real-time quantitative PCR and Western blot analysis demonstrated a rapid increase in endogenous MDM2 mRNA and MDM2 protein upon induction of MYCN. Targeted inhibition of MYCN in a MYCN-amplified neuroblastoma cell line resulted in decreased MDM2 expression levels with concomitant stabilization of p53 and induction of apoptosis. Our finding that MYCN directly modulates baseline MDM2 levels suggests a mechanism contributing to the pathogenesis of neuroblastoma and other MYC-driven malignancies through inhibition of MYC-stimulated apoptosis.

Clinical and Molecular Evidence for c-kit Receptor as a Therapeutic Target in Neuroblastic Tumors

Purpose: Clinicobiological characteristics of neuroblastic tumor (NT) expressing c-kit tyrosine kinase receptor and/or its ligand, stem cell factor (SCF), are debated. This study aimed at investigating the clinicobiological features of primary NTs expressing c-kit and/or SCF in order to define the clinical relevance of selective therapeutic targeting.

Experimental Design: c-Kit and SCF expression was studied in 168 NTs using immunohistochemistry and in 106 of 168 using Northern blot. Quantitative determination of c-kit expression in 54 additional NTs was also done using real-time reverse transcription-PCR. Correlations between c-kit and SCF expression and clinicobiological features were analyzed using χ2 test, univariate, and multivariate regression analyses.

Results: c-Kit protein was detected in 21 of 168 NTs (13%) and its mRNA in 23 of 106 NTs (22%). SCF protein was shown in 30 of 106 NTs (28%) and its mRNA in 33 of 106 NTs (31%). No mutations in exon 11 of c-kit gene were identified. By univariate analysis, c-kit and SCF expression correlated with advanced stage, MYCN amplification, and 1p36 allelic loss. Cox simple regression analysis showed that overall survival probability was 17% in the c-kit–positive subset versus 68% in the negative (P &lt; 0.001), 43% in the SCF-positive subset versus 78% in the negative (P &lt; 0.001). When using real-time reverse transcription-PCR, significant levels of c-kit mRNA were found in 35 of 54 NTs (65%), but the correlations with clinicobiological features were no longer documented.

Conclusions: c-Kit expression can be detected in the majority of primary NTs. High levels of expression are preferentially found in tumors with unfavorable clinicobiological variables. c-Kit may represent a useful therapeutic target in a subset of otherwise untreatable NTs.

Oncogenic cooperation between H-Twist and N-Myc overrides failsafe programs in cancer cells

N-Myc oncogene amplification is a frequent event in neuroblastoma and is strongly correlated with advanced disease stage and treatment failure. Similarly to c-Myc oncogenic activation, N-Myc deregulation promotes both cell proliferation and p53-dependent apoptosis by sensitizing cells to a variety of insults. Intriguingly, p53 mutations are uncommon in neuroblastomas, strongly suggesting that an alternative cooperating event circumvents this safeguard against oncogene-driven neoplasia. By performing a pangenomic cDNA microarray analysis, we demonstrate that human Twist is constantly overexpressed in N-Myc-amplified neuroblastomas. H-Twist overexpression is responsible for the inhibition of the ARF/p53 pathway involved in the Myc-dependent apoptotic response. This oncogenic cooperation of two key regulators of embryogenesis causes cell transformation and malignant outgrowth.

Microarray transcript profiling distinguishes the transient from the acute type of megakaryoblastic leukaemia (M7) in Down's syndrome, revealing PRAME as a specific discriminating marker

Transient myeloproliferative disorder (TMD) is a unique, spontaneously regressing neoplasia specific to Down's syndrome (DS), affecting up to 10% of DS neonates. In 20-30% of cases, it reoccurs as progressive acute megakaryoblastic leukaemia (AMKL) at 2-4 years of age. The TMD and AMKL blasts are morphologically and immuno-phenotypically identical, and have the same acquired mutations in GATA1. We performed transcript profiling of nine TMD patients comparing them with seven AMKL patients using Affymetrix HG-U133A microarrays. Similar overall transcript profiles were observed between the two conditions, which were only separable by supervised clustering. Taqman analysis on 10 TMD and 10 AMKL RNA samples verified the expression of selected differing genes, with statistical significance (P < 0.05) by Student's t-test. The Taqman differences were also reproduced on TMD and AMKL blasts sorted by a fluorescence-activated cell sorter. Among the significant differences, CDKN2C, the effector of GATA1-mediated cell cycle arrest, was increased in AMKL but not TMD, despite the similar level of GATA1. In contrast, MYCN (neuroblastoma-derived oncogene) was expressed in TMD at a significantly greater level than in AMKL. MYCN has not previously been described in leukaemogenesis. Finally, the tumour antigen PRAME was identified as a specific marker for AMKL blasts, with no expression in TMD. This study provides markers discriminating TMD from AMKL-M7 in DS. These markers have the potential as predictive, diagnostic and therapeutic targets. In addition, the study provides further clues into the pathomechanisms discerning self-regressive from the progressive phenotype.