Mouse Model Of Medulloblastoma Research Articles

Genomic and transcriptomic analyses match medulloblastoma mouse models to their human counterparts.

Medulloblastoma is a malignant embryonal brain tumor with highly variable outcome. In order to study the biology of this tumor and to perform preclinical treatment studies, a lot of effort has been put into the generation of appropriate mouse models. The usage of these models, however, has become debatable with the advances in human medulloblastoma subgrouping. This study brings together multiple relevant mouse models and matches genetic alterations and gene expression data of 140 murine tumors with 423 human medulloblastomas in a global way. Using AGDEX analysis and k-means clustering, we show that the Blbp-cre::Ctnnb1(ex3)(Fl/+)Trp53 (Fl/Fl) mouse model fits well to human WNT medulloblastoma, and that, among various Myc- or Mycn-based mouse medulloblastomas, tumors in Glt1-tTA::TRE-MYCN/Luc mice proved to be most specific for human group 3 medulloblastoma. None of the analyzed models displayed a significant match to group 4 tumors. Intriguingly, mice with Ptch1 or Smo mutations selectively modeled SHH medulloblastomas of adulthood, although such mutations occur in all human age groups. We therefore suggest that the infantile or adult gene expression pattern of SHH MBs are not solely determined by specific mutations. This is supported by the observation that human medulloblastomas with PTCH1 mutations displayed more similarities to PTCH1 wild-type tumors of the same age group than to PTCH1-mutated tumors of the other age group. Together, we provide novel insights into previously unrecognized specificity of distinct models and suggest these findings as a solid basis to choose the appropriate model for preclinical studies on medulloblastoma.

Role of the EphA4 and EphA7 genes in mediating the growth and aggressiveness of medulloblastoma tumors in the Smo/Smo medulloblastoma mouse model.

e22137 Background: Eph/ephrin signaling in numerous cancer systems promotes aspects of tumorigenesis, including proliferation, migration, and angiogenesis. Few studies, however, have investigated t...

ASC deficiency suppresses proliferation and prevents medulloblastoma incidence.

Apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) is silenced by promoter methylation in many types of tumors, yet ASC's role in most cancers remains unknown. Here, we show that ASC is highly expressed in a model of medulloblastoma, the most common malignant pediatric brain cancer; ASC is also expressed in human medulloblastomas. Importantly, while ASC deficiency did not affect normal cerebellar development, ASC knockout mice on the Smoothened (ND2:SmoA1) transgenic model of medulloblastoma exhibited a profound reduction in medulloblastoma incidence and a delayed tumor onset. A similar decrease in tumorigenesis with ASC deficiency was also seen in the hGFAP-Cre:SmoM2 mouse model of medulloblastoma. Interestingly, hyperproliferation of the external granule layer (EGL) was comparable at P20 in both wild-type and ASC-deficient SmoA1 mice. However, while the apoptosis and differentiation markers remained unchanged at this age, proliferation makers were decreased, and the EGL was reduced in thickness and area by P60. This reduction in proliferation with ASC deficiency was also seen in isolated SmoA1 cerebellar granule precursor cells in vitro, indicating that the effect of ASC deletion on proliferation was cell autonomous. Interestingly, ASC-deficient SmoA1 cerebella exhibited disrupted expression of genes in the transforming growth factor-β pathway and increased level of nuclear Smad3. Taken together, these results demonstrate an unexpected role for ASC in Sonic hedgehog-driven medulloblastoma tumorigenesis, thus identifying ASC as a promising novel target for antitumor therapy.

MyoD Is a Tumor Suppressor Gene in Medulloblastoma

While medulloblastoma, a pediatric tumor of the cerebellum, is characterized by aberrations in developmental pathways, the majority of genetic determinants remain unknown. An unbiased Sleeping Beauty transposon screen revealed MyoD as a putative medulloblastoma tumor suppressor. This was unexpected, as MyoD is a muscle differentiation factor and not previously known to be expressed in cerebellum or medulloblastoma. In response to deletion of one allele of MyoD, two other Sonic hedgehog-driven mouse medulloblastoma models showed accelerated tumor formation and death, confirming MyoD as a tumor suppressor in these models. In normal cerebellum, MyoD was expressed in the proliferating granule neuron progenitors that are thought to be precursors to medulloblastoma. Similar to some other tumor suppressors that are induced in cancer, MyoD was expressed in proliferating medulloblastoma cells in three mouse models and in human medulloblastoma cases. This suggests that although expression of MyoD in a proliferating tumor is insufficient to prevent tumor progression, its expression in the cerebellum hinders medulloblastoma genesis.

Sleeping Beauty mutagenesis in a mouse medulloblastoma model defines networks that discriminate between human molecular subgroups

The Sleeping Beauty (SB) transposon mutagenesis screen is a powerful tool to facilitate the discovery of cancer genes that drive tumorigenesis in mouse models. In this study, we sought to identify genes that functionally cooperate with sonic hedgehog signaling to initiate medulloblastoma (MB), a tumor of the cerebellum. By combining SB mutagenesis with Patched1 heterozygous mice (Ptch1(lacZ/+)), we observed an increased frequency of MB and decreased tumor-free survival compared with Ptch1(lacZ/+) controls. From an analysis of 85 tumors, we identified 77 common insertion sites that map to 56 genes potentially driving increased tumorigenesis. The common insertion site genes identified in the mutagenesis screen were mapped to human orthologs, which were used to select probes and corresponding expression data from an independent set of previously described human MB samples, and surprisingly were capable of accurately clustering known molecular subgroups of MB, thereby defining common regulatory networks underlying all forms of MB irrespective of subgroup. We performed a network analysis to discover the likely mechanisms of action of subnetworks and used an in vivo model to confirm a role for a highly ranked candidate gene, Nfia, in promoting MB formation. Our analysis implicates candidate cancer genes in the deregulation of apoptosis and translational elongation, and reveals a strong signature of transcriptional regulation that will have broad impact on expression programs in MB. These networks provide functional insights into the complex biology of human MB and identify potential avenues for intervention common to all clinical subgroups.

MADM-ML, a Mouse Genetic Mosaic System with Increased Clonal Efficiency

Mosaic Analysis with Double Markers (MADM) is a mouse genetic system that allows simultaneous gene knockout and fluorescent labeling of sparse, clonally-related cells within an otherwise normal mouse, thereby circumventing embryonic lethality problems and providing single-cell resolution for phenotypic analysis in vivo. The clonal efficiency of MADM is intrinsically low because it relies on Cre/loxP-mediated mitotic recombination between two homologous chromosomes rather than within the same chromosome, as in the case of conditional knockout (CKO). Although sparse labeling enhances in vivo resolution, the original MADM labels too few or even no cells when a low-expressing Cre transgene is used or a small population of cells is studied. Recently, we described the usage of a new system, MADM-ML, which contains three mutually exclusive, self-recognizing loxP variant sites as opposed to a single loxP site present in the original MADM system (referred to as MADM-SL in this paper). Here we carefully compared the recombination efficiency between MADM-SL and MADM-ML using the same Cre transgene, and found that the new system labels significantly more cells than the original system does. When we established mouse medulloblastoma models with both the original and the new MADM systems, we found that, while the MADM-SL model suffered from varied tumor progression and incomplete penetrance, the MADM-ML model had consistent tumor progression and full penetrance of tumor formation. Therefore MADM-ML, with its higher recombination efficiency, will broaden the applicability of MADM for studying many biological questions including normal development and disease modeling at cellular resolution in vivo.

Notch1-Induced Brain Tumor Models the Sonic Hedgehog Subgroup of Human Medulloblastoma

While activation of the Notch pathway is observed in many human cancers, it is unknown whether elevated Notch1 expression is sufficient to initiate tumorigenesis in most tissues. To test the oncogenic potential of Notch1 in solid tumors, we expressed an activated form of Notch1 (N1ICD) in the developing mouse brain. N1ICD;hGFAP-cre mice were viable but developed severe ataxia and seizures, and died by weaning age. Analysis of transgenic embryo brains revealed that N1ICD expression induced p53-dependent apoptosis. When apoptosis was blocked by genetic deletion of p53, 30% to 40% of N1ICD;GFAP-cre;p53(+/-) and N1ICD;GFAP-cre;p53(-/-) mice developed spontaneous medulloblastomas. Interestingly, N1ICD-induced medulloblastomas most closely resembled the sonic hedgehog subgroup of human medulloblastoma at the molecular level. Surprisingly, N1ICD-induced tumors do not maintain high levels of the Notch pathway gene expression, except for Notch2, showing that initiating oncogenic events may not be decipherable by analyzing growing tumors in some cases. In summary, this study shows that Notch1 has an oncogenic potential in the brain when combined with other oncogenic hits, such as p53 loss, and provides a novel mouse model of medulloblastoma. Cancer Res; 73(17); 5381-90. ©2013 AACR.

Sonic Hedgehog-Induced Histone Deacetylase Activation Is Required for Cerebellar Granule Precursor Hyperplasia in Medulloblastoma

Medulloblastoma, the most common pediatric brain tumor, is thought to arise from deregulated proliferation of cerebellar granule precursor (CGP) cells. Sonic hedgehog (Shh) is the primary mitogen that regulates proliferation of CGP cells during the early stages of postnatal cerebellum development. Aberrant activation of Shh signaling during this time has been associated with hyperplasia of CGP cells and eventually may lead to the development of medulloblastoma. The molecular targets of Shh signaling involved in medulloblastoma formation are still not well-understood. Here, we show that Shh regulates sustained activation of histone deacetylases (HDACs) and that this activity is required for continued proliferation of CGP cells. Suppression of HDAC activity not only blocked the Shh-induced CGP proliferation in primary cell cultures, but also ameliorated aberrant CGP proliferation at the external germinal layer (EGL) in a medulloblastoma mouse model. Increased levels of mRNA and protein of several HDAC family members were found in medulloblastoma compared to wild type cerebellum suggesting that HDAC activity is required for the survival/progression of tumor cells. The identification of a role of HDACs in the early steps of medulloblastoma formation suggests there may be a therapeutic potential for HDAC inhibitors in this disease.

Characterization of IRDye 800CW chlorotoxin as a targeting agent for brain tumors

Primary brain tumors present significant challenges for surgical resection because of their location and the frequent occurrence of malignant projections extending beyond the primary tumor. Visualization of the tumor margins during surgery is critical for a favorable outcome. We report the use of IRDye 800CW chlorotoxin (CLTX) as a targeted imaging agent for brain tumors in a spontaneous mouse model of medulloblastoma, ND2:SmoA1. Specificity and functionality of the targeted agent were confirmed in cell-based assays. Tumors were detected by magnetic resonance imaging and IRDye 800CW CLTX administered to individual animals for optical imaging at 1-month increments. The integrity of the blood–brain barrier (BBB) was measured by Evan’s Blue perfusion prior to sacrifice. Results show that IRDye 800CW CLTX specifically targeted tumor tissue. The extravasation of Evan’s Blue was observed in all tumors, suggesting that the presence of the tumors can introduce alterations in the permeability of the BBB. Because increased vascular permeability was observed early in the disease model, larger dye-labeled imaging agents that exceed current BBB size restrictions may warrant renewed consideration as candidates for tumor detection and surgical resection. Our study provides data characterizing in vitro and in vivo use of IRDye 800CW CLTX as a broadly applicable tumor imaging agent.

Abstract 1560: Early tumor detection and characterization of a novel mouse model of Shh-driven medulloblastoma using contrast-enhanced micro-MRI.

Abstract Introduction: An important translational application of preclinical models is the ability to draw insights into the etiology and molecular pathways altered in cancers. This is especially true for brain tumors since the early stages of disease are rarely detectable in patients and advanced-stage tumors may not accurately reflect the mutations responsible for tumor initiation. Therefore, there is a critical need for sensitive imaging protocols that allow the study of the early stages of tumor formation. In this study we optimized an in vivo high-resolution contrast-enhanced MRI protocol for the early detection and characterization of a novel mouse model of medulloblastoma (MB), the most common pediatric brain tumor which originates in the cerebellum (Cb). Method: The Patched (Ptc1) mutant mouse is one of the most studied models of MB. In our study we generated a new variant of this model (referred as Ptc1-CKO) by breeding Ptf1acre/+ mice with mice homozygous for a floxed allele of the Ptc1 gene (Ptc1fl/fl). Using this approach, Ptc1 was deleted in less than 1,000 embryonic Ptf1a-expressing granule cell progenitors (GCPs) compared to the extensive mutation of many GCPs seen in many models. Based on our results, tumors in Ptc1-CKO mice likely initiate from a single mutated GCP, and thus better models the clonal origin of sporadic human MBs. Thus, the Ptc1-CKO mouse is an ideal platform to study the early stages of MB tumorigenesis. For in vivo imaging, we used manganese-enhanced MRI (MEMRI) to obtain images 24h after intraperitoneal (IP) injection of a manganese chloride (MnCl2) solution using a 7T micro-MRI system. We acquired three-dimensional (3D) images with a 15-minute and 2-hour protocol for screening and high-resolution images, respectively. Image analysis was performed using AMIRA software. After imaging, tumors were extracted for histological and molecular analysis. Results: The contrast obtained with our MEMRI protocol showed detailed cerebellar morphology and allowed detection of MBs in Ptc1-CKO mice at different stages. Our in vivo MRI protocol was sensitive enough to detect pre-neoplastic lesions as early as 2 weeks postnatal age (N=22) and was validated with histology (N=12). Furthermore, in vivo longitudinal micro-MRI allowed the noninvasive monitoring of individual pre-neoplastic lesions and showed that individual lesions have different tumorigenic potential. Volumetric studies were performed to analyze tumor morphology and growth rates (N=25), and the advanced phenotypes were correlated with immunohistochemistry. In addition, 3D MRI images were used to guide the dissection of tumor tissue for microarray expression analysis (N=6). Our results show that Shh-driven MBs in Ptc1-CKO mice display at least two distinct imaging and molecular phenotypes, contrary to previous reports that all Shh-driven tumors converge to a common molecular endpoint. Citation Format: Giselle A. Suero-Abreu, Praveen B. Raju, Diane Pham, Edward J. Houston, Alexandra L. Joyner, Daniel H. Turnbull. Early tumor detection and characterization of a novel mouse model of Shh-driven medulloblastoma using contrast-enhanced micro-MRI. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1560. doi:10.1158/1538-7445.AM2013-1560

Abstract 3128: Mxd3 is required for proliferation of human neuroblastoma cells.

Abstract Neuroblastoma (NB) is the most common extracranial solid tumor in children. Up to 45% of the cases are high risk at diagnosis, representing very aggressive, usually metastasized tumors that are refractory or recurrent after all available therapies. Prognosis for these patients is very poor with only 30% survival. MXD3 is a transcription factor previously shown in our lab to be a novel member of the Hedgehog (Hh) pathway (Yun, Rust, Ishimaru, Diaz, Mol Cell Biol, 2007) in granule neuron precursors. In these cells, overexpression of MXD3 results in higher MYCN expression, suggesting a functional interaction between the two transcription factors. MXD3 phenocopies MYCN expression in mouse models of medulloblastoma; we have also shown that it is expressed in human medulloblastoma and that its knockdown reduces proliferation of human medulloblastoma cell lines (Barisone et al., PLoS One, 2012). More than 20% of NBs have MYCN amplification, which is considered a marker of poor prognosis. In the current study, we investigated a possible role for MXD3 in NB cell proliferation. Using qRT-PCR we observed high levels of MXD3 mRNA expression in 5 NB cell lines: SK-N-DZ, IMR-32, SH-SY5Y, SK-N-BE and SK-N-SH. Immunoblot analysis with anti-MXD3 monoclonal antibodies confirmed that the protein was present in these cells. Furthermore, immunohistochemistry analysis showed strong immunoreactivity in a cohort of 16 primary NB tissue specimens. We investigated the role of MXD3 in NB cell proliferation by silencing MXD3 in the SK-N-DZ cell line. We used lentiviral delivery to knockdown MXD3 using an RNA interference approach. Upon transduction by the viral vector, MXD3 knockdown was confirmed at both the RNA and protein level. Within 48 hours, MXD3 protein levels were reduced >90% in cells infected with the shMXD3 virus but not with the control virus. MXD3 knockdown resulted in decreased proliferation in NB cells, supporting our hypothesis that it may be involved in the maintenance of NB. We analyzed cell cycle progression after knockdown using flow cytometry. We observed no significant differences in the G0/G1, S or G2/M populations between experimental and control samples. Our results suggest that MXD3 is important for NB cell proliferation, possibly by acting as an anti-apoptotic factor. Therefore, MXD3 is a potential candidate for targeted therapies against neuroblastoma. Citation Format: Gustavo Barisone, Noriko Satake, Carly Lewis, Connie Duong, Cathy Chan, Kit Lam, Jan Nolta, Elva Diaz. Mxd3 is required for proliferation of human neuroblastoma cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3128. doi:10.1158/1538-7445.AM2013-3128

Abstract 5055: Efficacy of cabazitaxel in mouse models of pediatric brain tumors.

Abstract Successful treatment of pediatric brain tumors is limited by significant long-term toxicities associated with surgery and radiation therapy on the developing central nervous system There is an unmet need for chemotherapy with improved efficacy in difficult to treat tumors, such as supratentorial primitive neuroectodermal tumors (sPNET) and atypical teratoid/rhabdoid tumors (ATRT), as well as diagnoses that currently have good long-term survival, such as medulloblastoma. Overcoming the blood brain barrier is a large hurdle in effectively treating brain tumors, particularly when there is a need, as with very young patients, to avoid radiation therapy. Cabazitaxel is a promising taxane derivative shown to effectively kill tumor cells in pre-clinical in vitro and in vivo experiments. Similar to docetaxel, cabazitaxel has favorable drug properties and a manageable safety profile in patients with advanced solid tumors, with the advantage of being able to cross the blood brain barrier. Additionally, cabazitaxel has shown efficacy in docetaxel resistant tumors. The experiments presented here, tested the relative efficacy of cabazitaxel and docetaxel in mouse models of pediatric brain tumors. A spontaneous mouse model of medulloblastoma driven by Smo activation was evaluated by magnetic resonance imaging for tumor growth and blood brain barrier integrity before and after treatment with cabazitaxel or docetaxel. Efficacy of cabazitaxel and docetaxel against patient-derived pediatric brain tumors was tested in flank and orthotopic xenograft models of medulloblastoma, sPNET, ATRT, and ependymoma. We observed regression in some flank xenograft tumors after treatment with cabazitaxel or docetaxel, with evidence of superior action of cabazitaxel. These data support further testing of cabazitaxel as a therapy to treat human pediatric brain tumors. This work has been funded by Sanofi Inc. Citation Format: James M. Olson, Emily Girard, Sally Ditzler, Andrew Richards, Doonhoon Lee, Patricia Vrignaud. Efficacy of cabazitaxel in mouse models of pediatric brain tumors. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5055. doi:10.1158/1538-7445.AM2013-5055

Abstract 5644: Itraconazole and arsenic trioxide inhibit hedgehog pathway activation and tumor growth associated with acquired resistance to vismodegib.

Abstract Recognition of the multiple roles of Hedgehog signaling in cancer has prompted intensive efforts to develop targeted pathway inhibitors, most recently culminating in the FDA-approval of vismodegib (Genentech/Roche) for the treatment of locally advanced or metastatic basal cell carcinoma. Vismodegib and other hedgehog pathway antagonists currently being evaluated in the clinical setting are small molecules that act by binding to a common site within Smoothened (SMO), a critical pathway component. Acquired mutations in SMO that map to these overlapping binding pockets compromise the potency and clinical efficacy of vismodegib and other clinically relevant agents in this space through an apparent class effect. As a consequence there are currently no visible therapeutic options for maintaining profound on-target efficacies in patients that present with recurrent disease harboring these point mutations. We report here that itraconazole and arsenic trioxide, two FDA-approved agents recently reported to inhibit Hedgehog signaling through mechanisms distinct from that of current SMO antagonists, maintain potent inhibition of hedgehog signaling in the face of acquired SMO mutations, both as single agents and when used in combination. Using a preclinical mouse model of medulloblastoma that mimics clinical mechanisms of vismodegib-associated acquired resistance, we demonstrate that itraconazole and arsenic trioxide inhibit pathway activation, proliferation, and tumor growth of medulloblastoma harboring SMO-D477G mutation, both in vitro and in vivo. These drugs act as effectively in syngeneic models of medulloblastoma demonstrating constitutive signaling through wild-type SMO. Combination of itraconazole and arsenic trioxide further suppress tumor growth and improves survival in an orthotopic model of vismodegib-resistant medulloblastoma compared to either single agent alone. Furthermore, itraconazole and arsenic trioxide retain activity in all reported drug-resistant SMO mutants, including those reported in relation to NVP-LDE225 (Novartis). Taken together, these results support clinical evaluation of itraconazole and arsenic trioxide for the treatment of Hh-dependent tumors, most notably those with acquired resistance to cyclopamine-mimics. These readily available FDA-approved agents have well defined pharmacokinetic and toxicity profiles. Our data support the rapid entry of itraconazole and arsenic trioxide into clinical testing to address patient populations for which vismodegib may not be available or is no longer efficacious. Citation Format: Blake T. Aftab, James Kim, Jean Y. Tang, Emily M. King, Daniel Kim, Jynho Kim, Ervin H. Epstein, Philip A. Beachy, Charles M. Rudin. Itraconazole and arsenic trioxide inhibit hedgehog pathway activation and tumor growth associated with acquired resistance to vismodegib. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5644. doi:10.1158/1538-7445.AM2013-5644

Abstract 5050: KCNJ2 constitutes a marker and therapeutic target of high-risk medulloblastomas.

Abstract Medulloblastoma comprises the most common malignant brain tumor in children. Non-WNT/SHH tumors define the most refractory medulloblastoma subgroups. Interestingly, 17q gain, the most common genetic aberration in medulloblastoma, comprises a cytogenetic hallmark of these molecular high-risk tumors detected in group 3 (62%), and group 4 (73%). The majority of recurrent tumors harbor 17q gain in the corresponding primary. Virtually all of these tumors develop resistance to current treatment protocols at relapse. The lack of a common molecular target hampers the development of urgently needed novel treatment strategies. Through mRNA expression profiling of 64 primary tumor samples, we identified potassium inwardly-rectifying channel J2 (KCNJ2) as one of the most upregulated genes on chromosome 17q in tumors with 17q gain. High KCNJ2 transcript levels were significantly associated with non-WNT/non-SHH grouping, anaplastic histology, metastatic dissemination, and poor clinical outcome. KCNJ2 protein expression was analyzed by immunohistochemistry in a large cohort of patients (n=199), and high protein expression levels were found to be strongly correlated with 17q gain, metastatic dissemination, and inferior prognosis (p<0.0001). To functionally validate the potential role of KCNJ2 in medulloblastoma biology, we performed knockdown experiments by small interfering RNA-mediated silencing in two well-characterized medulloblastoma cell lines. Transient knockdown of KCNJ2 resulted in a reduced proliferation rate and induction of apoptosis. Furthermore, treatment of the medulloblastoma cell lines and medulloblastoma stem cells with amiodarone and gambogic acid, two inhibitors of this class of Kir channels, phenocopied these effects in a time- and dose-dependent manner. Whole cell patch clamp results revealed a nearly complete current blockade upon inhibitor treatment. Subsequently, we showed that pharmacological inhibition of KCNJ2 and knockdown KCNJ2 significantly reduced tumor growth and resulted in prolonged survival in an orthotopic medulloblastoma mouse model. In summary, our data suggest that pharmacological inhibition of KCNJ2 may constitute a new therapeutic option for patients with high-risk medulloblastomas. Citation Format: Francesca Valdora, Florian Freier, Livia Garzia, Vijay Ramaswamy, Claudia Seyler, Thomas Hielscher, Nathan Brady, Paul A. Northcott, Marcel Kool, David TW Jones, Hendrik Witt, Gian Paolo Tonini, Wolfram Scheurlen, Hugo A. Katus, Andreas E. Kulozik, Edgar Zitron, Andrey Korshunov, Peter Lichter, Michael D. Taylor, Stefan M. Pfister, Marc Remke. KCNJ2 constitutes a marker and therapeutic target of high-risk medulloblastomas. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5050. doi:10.1158/1538-7445.AM2013-5050

Prognostic value and functional consequences of cell cycle inhibitor p27Kip1 loss in medulloblastoma

BackgroundThe cyclin-dependent kinase inhibitor p27Kip1 functions during normal cerebellar development and has demonstrated tumor suppressor functions in mouse models of medulloblastoma. Because P27 loss is associated with increased proliferation, we assessed whether P27 absence in surgical medulloblastoma specimens correlated with response to therapy in pediatric patients enrolled in two large studies. Additionally, we examined the functional consequence of p27Kip1 loss in the SmoA1 medulloblastoma model to distinguish whether p27Kip1 reduces tumor initiation or slows tumor progression.FindingsAnalysis of 87 well-characterized patient samples identified a threshold of P27 staining at which significant P27 loss correlated with poor patient outcome. The same criteria, applied to a second test set of tissues from 141 patients showed no difference in survival between patients with minimal P27 staining and others, suggesting that P27 levels alone are not a sufficient prognostic indicator for identifying standard-risk patients that may fail standard therapy. These findings were in contrast to prior experiments completed using a mouse medulloblastoma model. Analysis of cerebellar tumor incidence in compound mutant mice carrying the activated Smoothened (SmoA1) allele that were heterozygous or nullizygous for p27Kip1 revealed that p27Kip1 loss did not alter the frequency of tumor initiation. Tumors haploinsufficient or nullizygous for p27Kip1 were, however, more invasive and displayed a higher proliferative index, suggesting p27Kip1 loss may contribute to SmoA1 medulloblastoma progression.ConclusionsThese studies revealed P27 loss affects medulloblastoma progression rather than initiation and that this putative biomarker should not be used for stratifying children with medulloblastoma to risk-based therapeutic regimens.

The miR-183~96~182 cluster promotes tumorigenesis in a mouse model of medulloblastoma

Medulloblastoma is the most common malignant pediatric brain tumor. Some are thought to originate from cerebellar granule neuron progenitors (CGNPs) that fail to undergo normal cell cycle exit and differentiation. The contribution of microRNAs to the initiation and progression of medulloblastoma remains poorly understood. Increased expression of the miR-183∼96∼182 cluster of microRNAs has been noted in several aggressive subgroups. We identified that expression of miR-183∼96∼182 was higher in medulloblastomas with Pten gene loss in the background of the activated sonic hedgehog (Shh) signaling pathway. Ectopic miR-183∼96∼182 expression in CGNPs synergized with exogenous Shh to increase proliferation and its role depended on hedgehog signaling activation. Our findings suggest a new microRNA cluster, the miR-183∼96∼182, functionally collaborates with the Shh signaling pathway in the development of medulloblastomas in mice.

Microscopic Delineation of Medulloblastoma Margins in a Transgenic Mouse Model Using a Topically Applied VEGFR-1 Probe

The unambiguous demarcation of tumor margins is critical at the final stages in the surgical treatment of brain tumors because patient outcomes have been shown to correlate with the extent of resection. Real-time high-resolution imaging with the aid of a tumor-targeting fluorescent contrast agent has the potential to enable intraoperative differentiation of tumor versus normal tissues with accuracy approaching the current gold standard of histopathology. In this study, a monoclonal antibody targeting the vascular endothelial growth factor receptor 1 (VEGFR-1) was conjugated to fluorophores and evaluated as a tumor contrast agent in a transgenic mouse model of medulloblastoma. The probe was administered topically, and its efficacy as an imaging agent was evaluated in vitro using flow cytometry, as well as ex vivo on fixed and fresh tissues through immunohistochemistry and dual-axis confocal microscopy, respectively. Results show a preferential binding to tumor versus normal tissue, suggesting that a topically applied VEGFR-1 probe can potentially be used with real-time intraoperative optical sectioning microscopy to guide brain tumor resections.

OTX2 Represses Myogenic and Neuronal Differentiation in Medulloblastoma Cells

The brain development transcription factor OTX2 is overexpressed and/or genomically amplified in most medulloblastomas, but the mechanistic basis for its contributions in this setting are not understood. In this study, we identified OTX2 as a transcriptional repressor and a gatekeeper of myogenic and neuronal differentiation in medulloblastoma cells. OTX2 binds to the MyoD1 core enhancer through its homeobox domain, and the remarkable repressor activity exhibited by the homeobox domain renders OTX2 transcriptionally repressive. RNA interference-mediated attenuation of OTX2 expression triggered myogenic and neuronal differentiation in vitro and prolonged the survival in an orthotopic medulloblastoma mouse model. Conversely, inducing myogenic conversion of medulloblastoma cells led to the loss of OTX2 expression. In medullomyoblastoma, a medulloblastoma subtype containing muscle elements, myogenic cells share cytogenetic signatures with the primitive tumor cells and OTX2 expression was lost in the differentiated myogenic cells. Thus, OTX2 functions via its homeobox domain as a suppressor of differentiation, and the loss of OTX2 expression is linked to the myogenesis in medullomyoblastoma. Together, our findings illustrate the origin of muscle cells in medullomyoblastomas and the oncogenic mechanism of OTX2 as a repressor of diverse differentiating potential.

Abstract CN07-02: Targeting tumor-initiating cells in medulloblastoma

Abstract The growth of many tumors has been suggested to depend on a subset of tumor cells with an extensive capacity for self-renewal, termed cancer stem cells or tumor-initiating cells (TICs). These cells are not necessarily abundant or proliferative, but because they are long-lived and often resistant to conventional therapies, they are thought to contribute to tumor resistance and recurrence. Therefore, identifying these cells and finding approaches to targeting them has become an important goal in cancer research. We recently identified a population of TICs in a mouse model of medulloblastoma, the most common malignant brain tumor in children. These cells, marked by expression of the cell surface antigen CD15, are capable of propagating tumors following transplantation, whereas CD15– cells from the same donors cannot. To gain insight into the molecular mechanisms underlying the increased tumorigenic potential of CD15+ cells, we compared their gene expression profile to that of CD15– cells. Among the genes most consistently elevated in CD15+ cells were regulators of the G2/M phases of the cell cycle. This observation led us to hypothesize that CD15+ cells display altered cell cycle regulation, and that these cells might be particularly vulnerable to inhibitors of cell cycle progression. Here we show that CD15+ cells move more quickly through the G1 and S phases of the cell cycle, and as a result contain an increased proportion of cells in G2/M. Consistent with this cell cycle skewing, CD15+ cells express higher levels of the G2/M regulators Aurora Kinase (Aurk) and Polo-like Kinase (Plk). Treatment of tumor cells with small molecule antagonists of Aurk or Plk inhibits cell cycle progression, promotes apoptosis and enhances sensitivity to other modes of chemotherapy. Treatment of tumor-bearing mice with these agents also significantly inhibits tumor progression. Importantly, cells from medulloblastoma patient-derived xenografts are also sensitive to Aurk and Plk inhibitors. These findings suggest that inhibitors of Aurk and Plk may be useful for targeting TICs and may represent a novel avenue for therapy of human medulloblastoma. Citation Format: Shirley Markant, Lourdes Adriana Esparza, Kelly Barton, Jesse Sun, Robert Wechsler-Reya,. Targeting tumor-initiating cells in medulloblastoma. [abstract]. In: Proceedings of the Eleventh Annual AACR International Conference on Frontiers in Cancer Prevention Research; 2012 Oct 16-19; Anaheim, CA. Philadelphia (PA): AACR; Cancer Prev Res 2012;5(11 Suppl):Abstract nr CN07-02.

Canonical TGF‐β Pathway Activity Is a Predictor of SHH‐Driven Medulloblastoma Survival and Delineates Putative Precursors in Cerebellar Development

Medulloblastoma (MB) is the most common malignant brain tumor of childhood. Very little is known about aggressive forms of this disease, such as metastatic or recurrent MBs. In order to identify pathways involved in aggressive MB pathophysiology, we performed unbiased, whole genome microarrays on MB tumors at both the human and murine levels. Primary human MBs were compared, transcriptomically, to their patient-matched recurrent or metastatic tumors. Expression profiling was also performed on murine tumors from two spontaneously developing MB mouse models (Ptch+/- and Smo/Smo) that present with differing clinical severities. At both the human and murine levels we identified transforming growth factor-beta (TGF-β) as a potential contributor to MB progression/metastasis. Smad3, a major downstream component of the TGF-β pathway, was also evaluated using immunohistochemistry in malignant human tissues and was shown to correlate with MB metastasis and survival. Similarly, Smad3 expression during development identified a subset of cerebellar neuronal precursors as putative cells of origin for the Smad3-positive MBs. To our knowledge, this is the first study that links TGF-β to MB pathogenesis. Our research suggests that canonical activation of this pathway leads to better prognosis for patients.