Marli E. Ebus

Sequencing of neuroblastoma identifies chromothripsis and defects in neuritogenesis genes

Neuroblastoma is a childhood tumour of the peripheral sympathetic nervous system. The pathogenesis has for a long time been quite enigmatic, as only very few gene defects were identified in this often lethal tumour. Frequently detected gene alterations are limited to MYCN amplification (20%) and ALK activations (7%). Here we present a whole-genome sequence analysis of 87 neuroblastoma of all stages. Few recurrent amino-acid-changing mutations were found. In contrast, analysis of structural defects identified a local shredding of chromosomes, known as chromothripsis, in 18% of high-stage neuroblastoma. These tumours are associated with a poor outcome. Structural alterations recurrently affected ODZ3, PTPRD and CSMD1, which are involved in neuronal growth cone stabilization. In addition, ATRX, TIAM1 and a series of regulators of the Rac/Rho pathway were mutated, further implicating defects in neuritogenesis in neuroblastoma. Most tumours with defects in these genes were aggressive high-stage neuroblastomas, but did not carry MYCN amplifications. The genomic landscape of neuroblastoma therefore reveals two novel molecular defects, chromothripsis and neuritogenesis gene alterations, which frequently occur in high-risk tumours.

LIN28B induces neuroblastoma and enhances MYCN levels via let-7 suppression

LIN28B regulates developmental processes by modulating microRNAs (miRNAs) of the let-7 family. A role for LIN28B in cancer has been proposed but has not been established in vivo. Here, we report that LIN28B showed genomic aberrations and extensive overexpression in high-risk neuroblastoma compared to several other tumor entities and normal tissues. High LIN28B expression was an independent risk factor for adverse outcome in neuroblastoma. LIN28B signaled through repression of the let-7 miRNAs and consequently resulted in elevated MYCN protein expression in neuroblastoma cells. LIN28B–let-7–MYCN signaling blocked differentiation of normal neuroblasts and neuroblastoma cells. These findings were fully recapitulated in a mouse model in which LIN28B expression in the sympathetic adrenergic lineage induced development of neuroblastomas marked by low let-7 miRNA levels and high MYCN protein expression. Interference with this pathway might offer therapeutic perspectives.

Relapsed neuroblastomas show frequent RAS-MAPK pathway mutations

The majority of patients with neuroblastoma have tumors that initially respond to chemotherapy, but a large proportion will experience therapy-resistant relapses. The molecular basis of this aggressive phenotype is unknown. Whole-genome sequencing of 23 paired diagnostic and relapse neuroblastomas showed clonal evolution from the diagnostic tumor, with a median of 29 somatic mutations unique to the relapse sample. Eighteen of the 23 relapse tumors (78%) showed mutations predicted to activate the RAS-MAPK pathway. Seven of these events were detected only in the relapse tumor, whereas the others showed clonal enrichment. In neuroblastoma cell lines, we also detected a high frequency of activating mutations in the RAS-MAPK pathway (11/18; 61%), and these lesions predicted sensitivity to MEK inhibition in vitro and in vivo. Our findings provide a rationale for genetic characterization of relapse neuroblastomas and show that RAS-MAPK pathway mutations may function as a biomarker for new therapeutic approaches to refractory disease.

Inactivation of CDK2 is synthetically lethal to MYCN over-expressing cancer cells

Two genes have a synthetically lethal relationship when the silencing or inhibiting of 1 gene is only lethal in the context of a mutation or activation of the second gene. This situation offers an attractive therapeutic strategy, as inhibition of such a gene will only trigger cell death in tumor cells with an activated second oncogene but spare normal cells without activation of the second oncogene. Here we present evidence that CDK2 is synthetically lethal to neuroblastoma cells with MYCN amplification and over-expression. Neuroblastomas are childhood tumors with an often lethal outcome. Twenty percent of the tumors have MYCN amplification, and these tumors are ultimately refractory to any therapy. Targeted silencing of CDK2 by 3 RNA interference techniques induced apoptosis in MYCN-amplified neuroblastoma cell lines, but not in MYCN single copy cells. Silencing of MYCN abrogated this apoptotic response in MYCN-amplified cells. Inversely, silencing of CDK2 in MYCN single copy cells did not trigger apoptosis, unless a MYCN transgene was activated. The MYCN induced apoptosis after CDK2 silencing was accompanied by nuclear stabilization of P53, and mRNA profiling showed up-regulation of P53 target genes. Silencing of P53 rescued the cells from MYCN-driven apoptosis. The synthetic lethality of CDK2 silencing in MYCN activated neuroblastoma cells can also be triggered by inhibition of CDK2 with a small molecule drug. Treatment of neuroblastoma cells with roscovitine, a CDK inhibitor, at clinically achievable concentrations induced MYCN-dependent apoptosis. The synthetically lethal relationship between CDK2 and MYCN indicates CDK2 inhibitors as potential MYCN-selective cancer therapeutics.

Cyclin D1 and CDK4 Activity Contribute to the Undifferentiated Phenotype in Neuroblastoma

Genomic aberrations of Cyclin D1 (CCND1), CDK4, and CDK6 in neuroblastoma indicate that dysregulation of the G(1) entry checkpoint is an important cell cycle aberration in this pediatric tumor. Here, we report that analysis of Affymetrix expression data of primary neuroblastic tumors shows an extensive overexpression of Cyclin D1, which correlates with histologic subgroups. Immunohistochemical analysis showed overexpression of Cyclin D1 in neuroblasts and low Cyclin D1 expression in all cell types in ganglioneuroma. This suggests an involvement of G(1)-regulating genes in neuronal differentiation processes which we further evaluated using RNA interference against Cyclin D1 and its kinase partners CDK4 and CDK6 in several neuroblastoma cell lines. The Cyclin D1 and CDK4 knockdown resulted in pRb pathway inhibition as shown by an almost complete disappearance of CDK4/CDK6-specific pRb phosphorylation, reduction of E2F transcriptional activity, and a decrease of Cyclin A protein levels. Phenotype analysis showed a significant reduction in cell proliferation, a G(1)-specific cell cycle arrest, and, moreover, an extensive neuronal differentiation. Affymetrix microarray profiling of small interfering RNA-treated cells revealed a shift in expression profile toward a neuronal phenotype. Several new potential downstream players are identified. We conclude that neuroblastoma functionally depend on overexpression of G(1)-regulating genes to maintain their undifferentiated phenotype.

Knockdown of survivin (BIRC5) causes apoptosis in neuroblastoma via mitotic catastrophe

BIRC5 (survivin) is one of the genes located on chromosome arm 17q in the region that is often gained in neuroblastoma. BIRC5 is a protein in the intrinsic apoptotic pathway that interacts with XIAP and DIABLO leading to caspase-3 and caspase-9 inactivation. BIRC5 is also involved in stabilizing the microtubule-kinetochore dynamics. Based on the Affymetrix mRNA expression data, we here show that BIRC5 expression is strongly upregulated in neuroblastoma compared with normal tissues, adult malignancies, and non-malignant fetal adrenal neuroblasts. The over-expression of BIRC5 correlates with an unfavorable prognosis independent of the presence of 17q gain. Silencing of BIRC5 in neuroblastoma cell lines by various antisense molecules resulted in massive apoptosis as measured by PARP cleavage and FACS analysis. As both the intrinsic apoptotic pathway and the chromosomal passenger complex can be therapeutically targeted, we investigated in which of them BIRC5 exerted its essential anti-apoptotic role. Immunofluorescence analysis of neuroblastoma cells after BIRC5 silencing showed formation of multinucleated cells indicating mitotic catastrophe, which leads to apoptosis via P53 and CASP2. We show that BIRC5 silencing indeed resulted in activation of P53 and we could rescue apoptosis by CASP2 inhibition. We conclude that BIRC5 stabilizes the microtubules in the chromosomal passenger complex in neuroblastoma and that the apoptotic response results from mitotic catastrophe, which makes BIRC5 an interesting target for therapy.

Copy number defects of G1-cell cycle genes in neuroblastoma are frequent and correlate with high expression of E2F target genes and a poor prognosis.

The tightly controlled network of cell cycle genes consists of a core of cyclin dependent kinases (CDKs) that are activated by periodically expressed cyclins. The activity of the cyclin‐CDK complexes is regulated by cyclin dependent kinase inhibitors (CDKIs) and multiple signal transduction routes that converge on the cell cycle. Neuroblastoma are pediatric tumors that belong to the group of small round blue cell tumors, characterized by a fast proliferation. Here, we present high throughput analyses of cell cycle regulating genes in neuroblastoma. We analyzed a series of 82 neuroblastomas by comparative genomic hybridization arrays, single nucleotide polymorphism arrays, and Affymetrix expression arrays and analyzed the datasets in parallel with the R2 bioinformatic tool (http://r2.amc.nl). About 30% of the tumors had genomic amplifications, gains, or losses with shortest regions of overlap that suggested implication of a series of G1 cell cycle regulating genes. CCND1 (cyclin D1) and CDK4 were amplified or gained and the chromosomal regions containing the CDKN2 (INK4) group of CDKIs were frequently deleted. Cluster analysis showed that tumors with genomic aberrations in G1 regulating genes over‐expressed E2F target genes, which regulate S and G2/M phase progression. These tumors have a poor prognosis. Our findings suggest that pharmacological inhibition of cell cycle genes might bear therapeutic promises for patients with high risk neuroblastoma.

Oncogenic activation of FOXR1 by 11q23 intrachromosomal deletion-fusions in neuroblastoma

Neuroblastoma tumors frequently show loss of heterozygosity of chromosome 11q with a shortest region of overlap in the 11q23 region. These deletions are thought to cause inactivation of tumor suppressor genes leading to haploinsufficiency. Alternatively, micro-deletions could lead to gene fusion products that are tumor driving. To identify such events we analyzed a series of neuroblastomas by comparative genomic hybridization and single-nucleotide polymorphism arrays and integrated these data with Affymetrix mRNA profiling data with the bioinformatic tool R2 (http://r2.amc.nl). We identified three neuroblastoma samples with small interstitial deletions at 11q23, upstream of the forkhead-box R1 transcription factor (FOXR1). Genes at the proximal side of the deletion were fused to FOXR1, resulting in fusion transcripts of MLL–FOXR1 and PAFAH1B2–FOXR1. FOXR1 expression has only been detected in early embryogenesis. Affymetrix microarray analysis showed high FOXR1 mRNA expression exclusively in the neuroblastomas with micro-deletions and rare cases of other tumor types, including osteosarcoma cell line HOS. RNAi silencing of FOXR1 strongly inhibited proliferation of HOS cells and triggered apoptosis. Expression profiling of these cells and reporter assays suggested that FOXR1 is a negative regulator of fork-head box factor-mediated transcription. The neural crest stem cell line JoMa1 proliferates in culture conditional to activity of a MYC-ER transgene. Over-expression of the wild-type FOXR1 could functionally replace MYC and drive proliferation of JoMa1. We conclude that FOXR1 is recurrently activated in neuroblastoma by intrachromosomal deletion/fusion events, resulting in overexpression of fusion transcripts. Forkhead-box transcription factors have not been previously implicated in neuroblastoma pathogenesis. Furthermore, this is the first identification of intrachromosomal fusion genes in neuroblastoma.

Cyclin D1 is a direct transcriptional target of GATA3 in neuroblastoma tumor cells

Almost all neuroblastoma tumors express excess levels of Cyclin D1 (CCND1) compared to normal tissues and other tumor types. Only a small percentage of these neuroblastoma tumors have high-level amplification of the Cyclin D1 gene. The other neuroblastoma tumors have equally high Cyclin D1 expression without amplification. Silencing of Cyclin D1 expression was previously found to trigger differentiation of neuroblastoma cells. Overexpression of Cyclin D1 is therefore one of the most frequent mechanisms with a postulated function in neuroblastoma pathogenesis. The cause for the Cyclin D1 overexpression is unknown. Here we show that Cyclin D1 overexpression results from transcriptional upregulation. To identify upstream regulators, we searched in mRNA profiles of neuroblastoma tumor series for transcription factors with expression patterns correlating to Cyclin D1. GATA3 most consistently correlated to Cyclin D1 in four independent data sets. We identified a highly conserved GATA3 binding site 27 bp upstream of the Cyclin D1 transcriptional start. Chromatin immune precipitation confirmed binding of GATA3 to the Cyclin D1 promoter. Overexpression of GATA3 induced Cyclin D1 promoter activity, which decreased after site-directed mutagenesis of the GATA3 binding site in the Cyclin D1 promoter. Silencing of GATA3 resulted in reduced Cyclin D1 promoter activity and reduced Cyclin D1 mRNA and protein levels. Moreover, GATA3 silencing caused differentiation that was similar to that caused by Cyclin D1 inhibition. These finding implicate GATA3 in Cyclin D1 overexpression in neuroblastoma.

Neuroblastoma is composed of two super-enhancer-associated differentiation states

Neuroblastoma and other pediatric tumors show a paucity of gene mutations, which has sparked an interest in their epigenetic regulation. Several tumor types include phenotypically divergent cells, resembling cells from different lineage development stages. It has been proposed that super-enhancer-associated transcription factor (TF) networks underlie lineage identity, but the role of these enhancers in intratumoral heterogeneity is unknown. Here we show that most neuroblastomas include two types of tumor cells with divergent gene expression profiles. Undifferentiated mesenchymal cells and committed adrenergic cells can interconvert and resemble cells from different lineage differentiation stages. ChIP–seq analysis of isogenic pairs of mesenchymal and adrenergic cells identified a distinct super-enhancer landscape and super-enhancer-associated TF network for each cell type. Expression of the mesenchymal TF PRRX1 could reprogram the super-enhancer and mRNA landscapes of adrenergic cells toward a mesenchymal state. Mesenchymal cells were more chemoresistant in vitro and were enriched in post-therapy and relapse tumors. Two super-enhancer-associated TF networks, which probably mediate lineage control in normal development, thus dominate epigenetic control of neuroblastoma and shape intratumoral heterogeneity.