Carolyn A. Pedone

Sonic hedgehog and insulin-like growth factor signaling synergize to induce medulloblastoma formation from nestin-expressing neural progenitors in mice

Medulloblastoma (MB) is a malignant brain tumor that arises in the cerebellum of children. Activation of the Sonic hedgehog/Patched (Shh/Ptc) signaling pathway in neural progenitor cells of the cerebellum induces MBs in mice. The incomplete penetrance of tumor formation in mice, coupled with the low frequency of mutations in Shh/Ptc pathway genes in human tumors, suggests that other signaling molecules cooperate with Shh to enhance MB formation. We modeled the ability of insulin-like growth factor (IGF) signaling to induce MB using the RCAS/tv-a system, which allows postnatal gene transfer and expression in a cell-type-specific manner. We used RCAS retroviral vectors to target expression of Shh, IGF2, and activated Akt to nestin-expressing neural progenitors in the cerebella of newborn mice. The incidence of Shh-induced tumor formation (15%) was enhanced by coexpression with IGF2 (39%) and Akt (48%). Neither IGF2 nor Akt caused tumors when expressed independently. The induced tumors showed upregulated expression of insulin receptor substrate 1 and phosphorylated forms of IGF1 receptor and Akt, mimicking activated IGF signaling found in human MBs. These results indicate that combined activation of the Shh/Ptc and IGF signaling pathways is an important mechanism in MB pathogenesis.

c-Myc Enhances Sonic Hedgehog-Induced Medulloblastoma Formation from Nestin-Expressing Neural Progenitors in Mice

Medulloblastomas are malignant brain tumors that arise in the cerebella of children. The presumed cells-of-origin are undifferentiated precursors of granule neurons that occupy the external granule layer (EGL) of the developing cerebellum. The overexpression of proteins that normally stimulate proliferation of neural progenitor cells may initiate medulloblastoma formation. Two known mitogens for neural progenitors are the c-Myc oncoprotein and Sonic hedgehog (Shh), a crucial determinant of embryonic pattern formation in the central nervous system. We modeled the ability of c-Myc and Shh to induce medulloblastoma in mice using the RCAS/tv-a system, which allows postnatal gene transfer and expression in a cell type-specific manner. We targeted the expression of Shh and c-Myc to nestin-expressing neural progenitor cells by injecting replication-competent ALV splice acceptor (RCAS) vectors into the cerebella of newborn mice. Following injection with RCAS-Shh alone, 3/32 (9%) mice developed medulloblastomas and 5/32 showed multifocal hyperproliferation of the EGL, possibly a precursor stage of medulloblastoma. Following injection with RCAS-Shh plus RCAS-Myc, 9/39 (23%) mice developed medulloblastomas. We conclude that nestin-expressing neural progenitors, present in the cerebellum at birth, can act as the cells-of-origin for medulloblastoma, and that c-Myc cooperates with Shh to enhance tumorigenicity.

N-myc Can Substitute for Insulin-Like Growth Factor Signaling in a Mouse Model of Sonic Hedgehog–Induced Medulloblastoma

Medulloblastoma is a malignant brain tumor that arises in the cerebellum in children, presumably from granule neuron precursors (GNP). Advances in patient treatment have been hindered by a paucity of animal models that accurately reflect the molecular pathogenesis of human tumors. Aberrant activation of the Sonic hedgehog (Shh) and insulin-like growth factor (IGF) pathways is associated with human medulloblastomas. Both pathways are essential regulators of GNP proliferation during cerebellar development. In cultured GNPs, IGF signaling stabilizes the oncogenic transcription factor N-myc by inhibiting glycogen synthase kinase 3beta-dependent phosphorylation and consequent degradation of N-myc. However, determinants of Shh and IGF tumorigenicity in vivo remain unknown. Here we report a high frequency of medulloblastoma formation in mice following postnatal overexpression of Shh in cooperation with N-myc. Overexpression of N-myc, alone or in combination with IGF signaling mediators or with the Shh target Gli1, did not cause tumors. Thus, Shh has transforming functions in addition to induction of N-myc and Gli1. This tumor model will be useful for testing novel medulloblastoma therapies and providing insight into mechanisms of hedgehog-mediated transformation.

Chromosome 11p15 deletions in human malignant astrocytomas and primitive neuroectodermal tumors

Chromosome 11p15 deletions occur frequently in several types of human cancer, both sporadic and familial, suggesting that a tumor suppressor gene is present within the deleted chromosome region. We carried out a restriction fragment length polymorphism analysis of chromosome 11p in two types of human brain tumors: malignant astrocytoma, the most common glial tumor in adults; and primitive neuroectodermal tumor (PNET), a malignant embryonic tumor that afflicts children. Loss of heterozygosity was found in 11/43 malignant astrocytomas (26%) and in 3/11 PNETs (27%). Deletion mapping revealed a region of loss on chromosome 11p (p15.4-pter) that was common to both tumor types. To determine whether the c-H-ras gene, located on chromosome 11p in the common region of deletion, was a candidate gene, we analyzed polymerase chain reaction products corresponding to all four c-H-ras coding exons for single-strand conformation polymorphisms. The absence of electrophoretic mobility shifts in tumor DNA compared to leukocyte DNA indicated that c-H-ras gene mutations were most likely not present. These results suggested that loss of a gene on chromosome 11p15 distinct from c-H-ras is an important step in tumorigenesis within the central nervous system in both children and adults.

Establishment and characterization of a human primitive neuroectodermal tumor cell line from the cerebral hemisphere

The primitive neuroectodermal tumors (PNET) comprise a class of malignant nervous system neoplasms that afflict children. These tumors consist of cells that are morphologically identical to the primitive neuroepithelial cells normally seen in early stages of neural embryogenesis, supporting the notion that PNET result from a disturbance in the process of normal neuronal or glial differentiation. In the central nervous system, PNET occur most commonly in the cerebellum (medulloblastomas), but only occasionally in the cerebral hemispheres. We report here the establishment and characterization of a new human cell line (PFSK) derived from a PNET from the cerebral hemisphere of a child. The growth characteristics of PFSK cells were typical of an immortalized, transformed cell line. Cytogenetic and molecular genetic studies showed that three different sublines were present. In one of these sublines, sequences from chromosome 17 had been lost during establishment in culture. Immunocytochemical studies showed that PFSK cells expressed nestin, an intermediate filament protein normally expressed by neuroepithelial stem cells during neurulation. The PFSK cells did not express antigens typically found in terminally differentiated neurons or glia, indicating that this tumor cell line might represent neuroepithelial stem cells prior to commitment to a neuronal or glial lineage.

Apoptosis Suppression by Somatic Cell Transfer of Bcl-2 Promotes Sonic Hedgehog–Dependent Medulloblastoma Formation in Mice

Medulloblastomas are malignant brain tumors that arise in the cerebellum in children. Aberrant activation of the Sonic hedgehog (Shh) signaling pathway, which normally stimulates proliferation of granule neuron precursors (GNP) during cerebellar development, induces tumors in mice that closely mimic human medulloblastomas. Shh-dependent medulloblastoma formation is enhanced by hyperactive insulin-like growth factor (IGF) signaling and ectopic expression of Myc oncogenes. This enhanced tumorigenesis stems from the sensitivity of GNPs to IGF and Myc levels in regulating proliferation. An emerging theme in cancer research is that oncogene-induced cell proliferation cannot initiate neoplastic transformation unless cellular programs that mediate apoptosis are disabled. Here, we report a high frequency of medulloblastoma formation in mice after postnatal overexpression of the antiapoptotic protein Bcl-2 in cooperation with Shh. Ectopic expression of Bcl-2 alone or in combination with N-Myc did not induce tumors, indicating that Shh has essential transforming functions in GNPs not supplied by the mitogenic stimulus of N-Myc combined with a strong antiapoptotic signal provided by Bcl-2. Expression of endogenous Bcl-2 was not up-regulated in Shh-induced tumors. Instead, elevated levels of phosphorylated Akt were found, suggesting that activated phosphatidylinositol 3-kinase signaling is one intrinsic mechanism for suppressing apoptosis in Shh-dependent medulloblastomas. Thus, blockade of apoptosis cooperates with Shh-stimulated proliferation to transform GNPs and induce aggressive medulloblastomas. These findings provide insights into the molecular signals that initiate medulloblastoma formation and they support the importance of blocking apoptosis in carcinogenesis.

Functional Genomics Identifies Drivers of Medulloblastoma Dissemination

Medulloblastomas are malignant brain tumors that arise in the cerebellum in children and disseminate via the cerebrospinal fluid to the leptomeningeal spaces of the brain and spinal cord. Challenged by the poor prognosis for patients with metastatic dissemination, pediatric oncologists have developed aggressive treatment protocols, combining surgery, craniospinal radiation, and high-dose chemotherapy, that often cause disabling neurotoxic effects in long-term survivors. Insights into the genetic control of medulloblastoma dissemination have come from transposon insertion mutagenesis studies. Mobilizing the Sleeping Beauty transposon in cerebellar neural progenitor cells caused widespread dissemination of typically nonmetastatic medulloblastomas in Patched(+/-) mice, in which Shh signaling is hyperactive. Candidate metastasis genes were identified by sequencing the insertion sites and then mapping these sequences back to the mouse genome. To determine whether genes located at transposon insertion sites directly caused medulloblastomas to disseminate, we overexpressed candidate genes in Nestin(+) neural progenitors in the cerebella of mice by retroviral transfer in combination with Shh. We show here that ectopic expression of Eras, Lhx1, Ccrk, and Akt shifted the in vivo growth characteristics of Shh-induced medulloblastomas from a localized pattern to a disseminated pattern in which tumor cells seeded the leptomeningeal spaces of the brain and spinal cord.

Hepatocyte Growth Factor and Sonic Hedgehog Expression in Cerebellar Neural Progenitor Cells Costimulate Medulloblastoma Initiation and Growth

Medulloblastomas are malignant brain tumors that arise by transformation of neural progenitor cells in the cerebellum in children. Treatment-related neurotoxicity has created a critical need to identify signaling molecules that can be targeted therapeutically to maximize tumor growth suppression and minimize collateral neurologic injury. In genetically engineered mice, activation of Sonic Hedgehog (Shh) signaling in neural stem cells in the developing cerebellum induces medulloblastomas. Hepatocyte growth factor (HGF) and its cell surface receptor c-Met are highly expressed in human medulloblastomas, and elevated levels of c-Met and HGF mRNA predict an unfavorable prognosis for patients. HGF is neuroprotective for cerebellar granule cells and promotes growth of human medulloblastoma cells in culture and in murine xenografts. We modeled the ability of HGF to induce medulloblastomas in mice using a version of the RCAS/tv-a system that allows gene transfer to cerebellar neural progenitors during their postnatal expansion phase when these cells are highly susceptible to transformation. Here, we report a high frequency of medulloblastoma formation in mice after postnatal expression of HGF in cooperation with Shh. Some tumors showed neurocytic differentiation similar to that in human nodular medulloblastomas with activated Shh signaling. Systemic administration of a monoclonal antibody against HGF prolonged survival of mice bearing Shh + HGF-induced medulloblastomas by stimulating apoptosis. These findings indicate a role for HGF in medulloblastoma initiation and growth and show efficacy of HGF-targeted therapy in a mouse model of endogenously arising tumors.

Molecular Therapy Targeting Sonic Hedgehog and Hepatocyte Growth Factor Signaling in a Mouse Model of Medulloblastoma

The use of genetically engineered mice has provided insights into the molecular pathogenesis of the pediatric brain tumor medulloblastoma and revealed promising therapeutic targets. Ectopic expression of Sonic hedgehog (Shh) in cerebellar neural progenitor cells induces medulloblastomas in mice, and coexpression of hepatocyte growth factor (HGF) enhances Shh-induced tumor formation. To determine whether Shh + HGF–driven medulloblastomas were responsive to Shh signaling blockade and whether treatment response could be enhanced by combination therapy targeting both HGF and Shh signaling pathways, we carried out a survival study in mice. We induced medulloblastomas by retrovirus-mediated expression of Shh and HGF, after which we treated the mice systemically with (a) HGF-neutralizing monoclonal antibody L2G7, (b) Shh signaling inhibitor cyclopamine, (c) Shh-neutralizing monoclonal antibody 5E1, (d) L2G7 + cyclopamine, or (e) L2G7 + 5E1. We report that monotherapy targeting either HGF signaling or Shh signaling prolonged survival and that anti-HGF therapy had a more durable response than Shh-targeted therapy. The effect of L2G7 + 5E1 combination therapy on cumulative survival was equivalent to that of L2G7 monotherapy and that of L2G7 + cyclopamine therapy was worse. The principal mechanism by which Shh- and HGF-targeted therapies inhibited tumor growth was a potent apoptotic death response in tumor cells, supplemented by a weaker suppressive effect on proliferation. Our observation that combination therapy either failed to improve or even reduced survival in mice bearing Shh + HGF–induced medulloblastomas compared with monotherapy underscores the importance of preclinical testing of molecular-targeted therapies in animal models of tumors in which the targeted pathways are known to be active. Mol Cancer Ther; 9(9); 2627–36. ©2010 AACR.

WIP1 Enhances Tumor Formation in a Sonic Hedgehog- Dependent Model of Medulloblastoma

BACKGROUND: A significant number of medulloblastomas (MBs) originate from abnormal activation of the sonic hedgehog/patched (SHH/PTC) signaling pathway. Although p53 deficiency enhances tumor formation in mice, inactivation of the p53 gene is seen in a minority of MBs. Wild-type p53-induced phosphatase 1 (WIP1) downregulates p53 expression and has been shown to be overexpressed in MBs. OBJECTIVE: We tested the hypothesis that overexpression of WIP1 enhances tumor formation in an SHH-dependent model of MB. METHODS: We used the RCAS/Ntv-a system to study the effect of WIP1 in vitro and in vivo. We transfected A375-TVA cells with RCAS-WIP1 and then exposed these cells to cisplatin to determine the effect on p53 expression. We modeled ectopic WIP1 expression independently and in combination with SHH in the cerebella of newborn mice to assess the effect on tumor formation. Mice were observed for 12 weeks or until neurological symptoms developed. The brains were examined for tumor formation. RESULTS: A375-TVA cells infected with RCAS-WIP1 demonstrated reduced p53 expression after exposure to cisplatin compared with controls. We detected tumors in 12 of 35 mice (34%) injected with RCAS-WIP1 and RCAS-SHH. Tumors were detected in 3 of 40 mice (8%) injected with RCAS-SHH alone. The difference in tumor formation rates was significant (&khgr;2 test, P = < .01). Tumors did not form in mice injected with RCAS-WIP1 alone. CONCLUSION: We show that ectopic expression of WIP1 cooperates with SHH to enhance formation of MB, although it is insufficient to induce tumors independently. Our results verify the role of WIP1 in MB formation and provide a crucial link to the inactivation of p53 in MBs.