Inhibition of a new differentiation pathway in neuroblastoma by copy number defects of N-myc, Cdc42, and nm23 genes

Dickkopf-1 is down-regulated by MYCN and inhibits neuroblastoma cell proliferation

Neuroblastomas are tumors of the developing peripheral sympathetic nervous system, which originates from the neural crest. Twenty percent of neuroblastomas show amplification of the MYCN oncogene, which correlates with poor prognosis. The MYCN transcription factor can activate and repress gene expression. To broaden our insight in the spectrum of genes down-regulated by MYCN, we generated gene expression profiles of the neuroblastoma cell lines SHEP-21N and SKNAS-NmycER, in which MYCN activity can be regulated. In this study, we show that MYCN suppresses the expression of Dickkopf-1 (DKK1) in both cell lines. DKK1 is a potent inhibitor of the wnt/beta-catenin signalling cascade, which is known to function in neural crest cell migration. We generated a DKK1 inducible cell line, IMR32-DKK1, which showed impaired proliferation upon DKK1 expression. Surprisingly, DKK1 expression did not inhibit the canonical wnt/beta-catenin signalling, suggesting a role of DKK1 in an alternative route of the wnt pathway. Gene expression profiling of two IMR32-DKK1 clones showed that only a few genes, amongst which SYNPO2, were up-regulated by DKK1. SYNPO2 encodes an actin-binding protein and was previously found to inhibit proliferation and invasiveness of prostate cancer cells. These results suggest that MYCN might stimulate cell proliferation by inhibiting the expression of DKK1. DKK1 might exert part of its growth suppressive effect by induction of SYNPO2 expression.

Direct regulation of the minichromosome maintenance complex by MYCN in neuroblastoma

The c-Myc and MYCN oncogenes strongly induce cell proliferation. Although a limited series of cell cycle genes were found to be induced by the myc transcription factors, it is still unclear how they mediate the proliferative phenotype. We therefore analysed a neuroblastoma cell line with inducible MYCN expression. We found that all members of the minichromosome maintenance complex (MCM2-7) and MCM8 and MCM10 were up-regulated by MYCN. Expression profiling of 110 neuroblastoma tumours revealed that these genes strongly correlated with MYCN expression in vivo. Extensive chromatin immunoprecipitation experiments were performed to investigate whether the MCM genes were primary MYCN targets. MYCN was bound to the proximal promoters of the MCM2 to -8 genes. These data suggest that MYCN stimulates the expression of not only MCM7, which is a well defined MYCN target gene, but also of the complete minichromosome maintenance complex.

Heterogeneity in retinoic acid signaling in neuroblastomas: Role of matrix metalloproteinases in retinoic acid-induced differentiation

Causes of retinoid resistance often observed in neuroblastomas are unknown. We studied all trans-retinoic acid (RA) signaling in neuroblastoma cells differing in N-myc levels in terms of neurite formation, expression of tissue transglutaminase, neuronal marker proteins, matrix metalloproteinases (MMPs), and activation of Rac1 and Cdc42. Poor invasiveness observed in SH-SY5Y, LA-N-5, and SMS-KCNR cells was associated with RA-induced neurite formation, Cdc42 activation and N-myc down regulation; expression of constitutively active Cdc42 down regulated N-myc expression and reduced invasion in RA-resistant SK-N-BE(2) and IMR32 cells. RA treatment for 24 h transiently increased invasion and expression of MMP9 in SH-SY5Y, LA-N-5 and MMP2 in SMS-KCNR cells. MMP inhibition prevented RA-induced neurite formation indicating a role in differentiation. Variation in RA signaling thus follows a defined pattern and relates to invasive potential. A defective RA signaling might result in retinoid resistance and unpredictable clinical outcome observed in some neuroblastomas.

Are chromosomal imbalances important in cancer?

Tumor-specific patterns of large-scale chromosomal imbalances characterize most forms of cancer. Based on evidence primarily from neuroblastomas, it can be argued that large-scale chromosomal imbalances are crucial for tumor pathogenesis and have an impact on the global transcriptional profile of cancer cells, and that some imbalances even initiate cancer. The genes and genetic pathways that have been dysregulated by such imbalances remain surprisingly elusive. Many genes are affected by the regions of gain and loss, and there are complex interactions and relationships that occur between these genes, hindering their identification. The study of untranslated RNA sequences, such as microRNAs, is in its infancy, and it is likely that such sequences are also dysregulated by chromosomal imbalance, contributing to pathogenesis.

Mechanisms of invasion and metastasis in human neuroblastoma

Neuroblastoma is the second most common solid tumor in children that is metastatic in 70% of patients at the time of diagnosis. The ability of neuroblastoma cells to colonize distant organs like the bone marrow and the bone is the result of close interactions between tumor cells and the microenvironment. Significant progress has been recently made in our understanding of the mechanisms that promote the colonization and invasion of the bone by neuroblastoma cells and these mechanisms are reviewed in this article. How this understanding is now allowing us to test new therapeutic agents specifically targeted at interfering with neuroblastoma metastasis is then discussed.

Differential patterns of microRNA expression in neuroblastoma are correlated with prognosis, differentiation, and apoptosis

Neuroblastoma accounts for 15% of pediatric cancer deaths, and although a few protein-coding genes, such as MYCN, are involved with aggressive pathogenicity, the identification of novel biological targets for therapeutic intervention is still a necessary prerequisite for improving patient survival. Expression profiling of 157 microRNA (miRNA) loci in 35 primary neuroblastoma tumors indicates that 32 loci are differentially expressed in favorable and unfavorable tumor subtypes, indicating a potential role of miRNAs in neuroblastoma pathogenesis. Many of these loci are significantly underexpressed in tumors with MYCN amplification, which have particularly poor prognoses. Interestingly, we found that miRNA expression levels substantially change in a MYCN-amplified cell line following exposure to retinoic acid, a compound which is well known for causing reductions in MYCN expression and for inducing neuroblastoma cell lines to undergo neuronal differentiation. We also show that small interfering RNA inhibition of MYCN by itself causes similar alterations in the expression of miRNA loci. In vitro functional studies of one locus, miR-184, indicate that it plays a significant role in apoptosis. The association of experimentally induced alterations of miRNA expression in neuroblastoma cell lines with differentiation or apoptosis leads us to conclude that these loci play important roles in neuroblastoma pathogenesis. We further suggest that MYCN may mediate a tumorigenic effect, in part, through directly or indirectly regulating the expression of miRNAs that are involved with neural cell differentiation and/or apoptosis, warranting substantial further studies of miRNAs as potential therapeutic targets.

MicroRNA-34a functions as a potential tumor suppressor by inducing apoptosis in neuroblastoma cells

Neuroblastoma (NB) is one of the most common forms of cancer in children, accounting for 15% of pediatric cancer deaths. The clinical course of these tumors is highly variable and is dependent on such factors as age at presentation, stage, ploidy and genomic abnormalities. Hemizygous deletion of chromosome 1p occurs in approximately 30% of advanced stage tumors, is associated with a poor prognosis, and likely leads to the loss of one or more tumor suppressor genes. We show here that microRNA (miRNA)-34a (1p36.23) is generally expressed at lower levels in unfavorable primary NB tumors and cell lines relative to normal adrenal tissue and that reintroduction of this miRNA into three different NB cell lines causes a dramatic reduction in cell proliferation through the induction of a caspase-dependent apoptotic pathway. As a potential mechanistic explanation for this observation, we demonstrate that miR-34a directly targets the messenger ribonucleic acid (mRNA) encoding E2F3 and significantly reduces the levels of E2F3 protein, a potent transcriptional inducer of cell-cycle progression. Furthermore, miR-34a expression increases during retinoic acid-induced differentiation of the SK-N-BE cell line, whereas E2F3 protein levels decrease. Thus, adding to the increasing role of miRNAs in cancer, miR-34a may act as a suppressor of NB tumorgenesis.

Nm23-H1: a metastasis-associated gene

The protein product of nm23-H1 gene has activity of nucleoside diphosphate (NDP) kinase, which catalyzes the phosphorylation of nucleoside diphosphates to the corresponding nucleoside triphosphates. Reductions in nm23 expression have been significantly associated with aggressive behavior in melanoma, breast, colon, and gastric carcinomas. On the contrary, high levels of nm23 gene expression are noted in the advanced stage of thyroid carcinomas and associated with significant reductions in survival for neuroblastoma and osteosarcoma patients. Although expression of nm23/NDP kinase is divergent in various malignant tumors, its reduced expression seems to be related to increased metastatic potential in most carcinoma types. However, it is hypothesized that nm23 may play a tissue-specific role, and that different regulatory mechanisms may act in different tumors. In ovarian carcinoma, nm23-H1/NDP kinase may be correlated with some clinicopathologic characteristics. In cervical cancer, nm23-H1 is probably involved in cervical carcinogenesis and correlated with some aggressive parameters. Overexpression of nm23-H1 protein may indicate poor survival for cervical cancer patients. Other than histidine 118 residue (amino acid sequence 118: histidine) concerned with NDP kinase activity of nm23-H1, serine 120 (amino acid sequence 120: serine) related activity of histidine-dependent protein phosphotransfer was recently reported to be responsible for its biological suppressive effects. To inhibit metastatic potential, nm23-H1 is also demonstrated to co-immunoprecipitate the kinase suppressor of Ras and phosphorylate it, and therefore reduce activation of the extracellular signal-regulated kinase mitogen-activated protein kinase pathway in response to signaling.

Human fetal neuroblast and neuroblastoma transcriptome analysis confirms neuroblast origin and highlights neuroblastoma candidate genes

Transcriptome profiling of neuroblasts and neuroblastoma tumor cells provides strong support for a neuroblast origin of neuroblastoma and highlights new candidate neuroblastoma genes

Background

Neuroblastoma tumor cells are assumed to originate from primitive neuroblasts giving rise to the sympathetic nervous system. Because these precursor cells are not detectable in postnatal life, their transcription profile has remained inaccessible for comparative data mining strategies in neuroblastoma. This study provides the first genome-wide mRNA expression profile of these human fetal sympathetic neuroblasts. To this purpose, small islets of normal neuroblasts were isolated by laser microdissection from human fetal adrenal glands.

Results

Expression of catecholamine metabolism genes, and neuronal and neuroendocrine markers in the neuroblasts indicated that the proper cells were microdissected. The similarities in expression profile between normal neuroblasts and malignant neuroblastomas provided strong evidence for the neuroblast origin hypothesis of neuroblastoma. Next, supervised feature selection was used to identify the genes that are differentially expressed in normal neuroblasts versus neuroblastoma tumors. This approach efficiently sifted out genes previously reported in neuroblastoma expression profiling studies; most importantly, it also highlighted a series of genes and pathways previously not mentioned in neuroblastoma biology but that were assumed to be involved in neuroblastoma pathogenesis.

Conclusion

This unique dataset adds power to ongoing and future gene expression studies in neuroblastoma and will facilitate the identification of molecular targets for novel therapies. In addition, this neuroblast transcriptome resource could prove useful for the further study of human sympathoadrenal biogenesis.

Integrative genomics identifies distinct molecular classes of neuroblastoma and shows that multiple genes are targeted by regional alterations in DNA copy number

Neuroblastoma is remarkable for its clinical heterogeneity and is characterized by genomic alterations that are strongly correlated with tumor behavior. The specific genes that influence neuroblastoma biology and are targeted by genomic alterations remain largely unknown. We quantified mRNA expression in a highly annotated series of 101 prospectively collected diagnostic neuroblastoma primary tumors using an oligonucleotide-based microarray. Genomic copy number status at the prognostically relevant loci 1p36, 2p24 (MYCN), 11q23, and 17q23 was determined by PCR and was aberrant in 26, 20, 40, and 38 cases, respectively. In addition, 72 diagnostic neuroblastoma primary tumors assayed in a different laboratory were used as an independent validation set. Unsupervised hierarchical clustering showed that gene expression was highly correlated with genomic alterations and clinical markers of tumor behavior. The vast majority of samples with MYCN amplification and 1p36 loss of heterozygosity (LOH) clustered together on a terminal node of the sample dendrogram, whereas the majority of samples with 11q deletion clustered separately and both of these were largely distinct from the copy number neutral group of tumors. Genes involved in neurodevelopment were broadly overrepresented in the more benign tumors, whereas genes involved in RNA processing and cellular proliferation were highly represented in the most malignant cases. By combining transcriptomic and genomic data, we showed that LOH at 1p and 11q was associated with significantly decreased expression of 122 (61%) and 88 (27%) of the genes mapping to 1p35-36 and all of 11q, respectively, suggesting that multiple genes may be targeted by LOH events. A total of 71 of the 1p35-36 genes were also differentially expressed in the independent validation data set, providing a prioritized list of candidate neuroblastoma suppressor genes. Taken together, these data are consistent with the hypotheses that the neuroblastoma transcriptome is a sensitive marker of underlying tumor biology and that chromosomal deletion events in this cancer likely target multiple genes through alteration in mRNA dosage. Lead positional candidates for neuroblastoma suppressor genes can be inferred from these data, but the potential multiplicity of transcripts involved has significant implications for ongoing gene discovery strategies.

Read-through transcript from NM23-H1 into the neighboring NM23-H2 gene encodes a novel protein, NM23-LV

NM23-H1 and NM23-H2 are neighboring genes on chromosome 17q. They encode nucleoside diphosphate kinases that have additional roles in signal transduction, transcription, and apoptosis. NM23-H1 expression is a strong marker for prognosis and metastatic behavior in many tumor types. A new bioinformatic tool, TranscriptView, identified read-through transcripts that start in the NM23-H1 gene and continue in the neighboring NM23-H2 gene. Splicing results in a transcript containing exons 1 to 4 of NM23-H1 and exons 2 to 5 of NM23-H2. The resulting mRNA encodes a novel and long variant of the NM23 protein family, NM23-LV, which contains part of NM23-H1 and the complete NM23-H2 protein. The transcript was amplified and sequenced from two neuroblastoma cell lines, confirming the presence of the predicted NM23-LV mRNA in vivo. Tissue analysis showed that NM23-LV is ubiquitously expressed, with the exception of the kidney. Neuroblastoma tumors show high-level expression of NM23-H1 and-H2 as well as NM23-LV mRNA. In neuroblastoma cells, the NM23-LV protein has mainly a cytoplasmic localization, but some nuclear staining was observed as well.