Cancer stem cells and survival pathways - BrainLife.org

[Cell Cycle 7:10, 1371-1378; 15 May 2008]; ©2008 Landes Bioscience

Perspective

Cancer stem cells and survival pathways Dolores Hambardzumyan,1,2,† Oren J. Becher1-3,† and Eric C. Holland1,2,4,* 1Department of Cancer Biology and Genetics; 2Brain Tumor Center; 3Department of Pediatrics; 4Departments of Neurosurgery, Neurology, Surgery; Memorial Sloan-Kettering Cancer Center; New York, New York USA †These

authors have contributed equally to this work.

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are performed in cell lines or xenografts that have been cultured as a monolayer on plastic for decades, but studies have shown that tumor cells cultured in such conditions exhibit substantially different gene expression profiles, genotype and biology compared to their parental primary tumors within a short time.3 Within the last several years, evidence from different laboratories have shown that cancers are most effectively serially passaged by rare fractions of cancer stem cells (CSCs), that exhibit self-renewing, multipotent, and tumorinitiating capacity.4-7 Despite intensive studies in the field regarding the origin of brain tumors, the response to therapy of cancer stem cells and tumor bulk in their native microenvironment, has not been studied. In this review we will concentrate on Sonic Hedgehog (SHH) pathway driven medulloblastomas, and discuss our recent study on their therapeutic response to radiation in the context of genetic mouse models of this disease.1

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Gliomas and medulloblastomas are the most frequent malignant brain tumors in adult and children respectively. Although both tumors arise in the CNS, there is a significant difference in their therapeutic response. Medulloblastomas are relatively curable, while glioblastomas are basically incurable. During the last decade several reports have demonstrated the existence of cancer stem cells in brain tumors, their location and their response to treatment. We have recently described the therapeutic response of medulloblastomas to radiation in their native microenvironment, illustrating how the p53 and Pi3K signaling pathways lead to the evasion of cell death by the nestin-expressing cells in the perivascular stem cell niche, even while the bulk of the tumor succumbs to apoptosis.1 It remains to be determined whether this mechanism of tumor resistance applies to the more complex stem-cell niche and tumor bulk of gliomas.

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Key words: medulloblastoma, mouse models, stem cells, PI3K pathway

Introduction

Neural Stem Cells

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Brain tumors are one of the most lethal forms of human cancer. The most common malignant brain tumors of adulthood and childhood respectively are gliomas and medulloblastomas. A particularly aggressive form of brain tumor is glioblastoma multiformed (GBM), which is characterized by chemo- and radiation resistance with median survival approximately 12 months.2 Medulloblastoma is a malignant tumor typically occurring in the cerebellum of children and young adults. Clinical management of medulloblastoma is more successful than gliomas with 5 year event-free survival of around 70%. One of the reasons why brain tumors remain so difficult to treat has been the limited understanding of the underlying biology, and a lack of accurate models that histologically and genetically mimic human gliomas and medulloblastomas. Another reason for poor responsiveness is tumor heterogeneity, as there is increasing evidence from different laboratories that brain tumors arise from diverse set of molecular abnormalities. However, all patients with a diagnosis of a particular brain tumor and stage are treated identically, without regard to the underlying genetic differences. Most preclinical studies *Correspondence to: Eric C. Holland; 1275 York Avenue; New York, New York 10021 USA; Fax: 646.422.0231; Email: [email protected] Submitted: 03/18/08; Accepted: 03/19/08 Previously published online as a Cell Cycle E-publication: http://www.landesbioscience.com/journals/cc/article/5954 1371

Researchers in the field of radiation biology were the first to formulate the concept of stem cells. The term stem cell was coined in the context of clonogenic cells that have retained the capacity for producing a large number of progeny after treatments that can cause cell reproductive death as a result of damage to chromosomes, apoptosis, etc.8 In 1961 stem cells (hematopoetic stem cells) from the bone marrow of mice were shown to produce colonies reproducibly in spleens of heavily irradiated recipient animals.9 Later it was shown that skin stem cells were able to produce nodules in the irradiated skin of mice after doses that impair proliferation of the majority of cells in a selected small area. In addition, stem cells in the crypts of mouse jejunum were shown to produce colonies in heavily irradiated regions of the intestine.10 In the normal adult brain there are several areas that are thought to harbor stem cells. Nestin and GFAP double positive astrocytes in the subventricular zone (SVZ) are thought to be the stem cells, which divide to generate transit-amplifying cell, which in turn generate the neuroblasts that migrate to the olfactory bulb.11 In addition, several other groups have shown neurogenesis in the subgranular zone (SGZ) of dentate gyrus in the hippocampus.12-15 The murine postnatal cerebellum was also reported to contain multipotent stem cells located in the white matter that can be isolated based on expression of CD133.16 Other neurogenic regions are thought to exist in the adult brain and further studies to identify them are in process. Thus it appears that the brain harbors multipotent stem cells which

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Radiation resistance and cancer stem like cells

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Figure 1. Developmental pathways and stem cells. (A) An illustration of the Notch signaling pathway. Notch is activated by binding to its ligands Delta and Jagged family members. Binding to a ligand results in cleavage of the Notch extracellular domain by TACE, followed by cleavage of the intracellular domain from the membrane by γ-secretase. The Notch intracellular domain translocates into the nucleus, where it interacts with CSL members and activates transcription of downstream target genes. (B) An illustration of the SHH signaling pathway. Ptc is a receptor for SHH. When there is no SHH, Ptc inhibits Smo and keeps the pathway in the “off” state. When SHH binds to Ptc, the inhibition is released and Smo activates Gli, which translocates into the nucleus and activates the target genes. (C) An illustration of the Wnt signaling pathway. A key component of the canonical Wnt signaling pathway is β-catenin. Wnt signaling is initiated when a Wnt protein binds to its receptor frizzled and co-receptor LPR, which leads to inhibition of APC/Axin/CK1/GSK3β complex, resulting in stabilization of β-catenin and its translocation to the nucleus. In the nucleus β-catenin interacts with TCF/LEF family transcription factors, leading to activation of target genes.

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reside in specialized locations that support self-renewal, and differentiation.17 Stem cells in association with other cells create a special microenvironment, which in turn determines their behavior. Those structures have been termed the stem cell “niche”18 Adult stem cell niches differ in their cellular composition, structure and location in different tissues. The stem cell niche is a complex structure that contains different cell types and is critical for maintaining stem cell properties. For example, it was shown that in adult bone marrow, hematopoetic stem cells are located in the trabecular endosteum (osteoblastic niche) or sinusoidal perivascular (vascular niche) areas.19,20 Studies on the mouse brain suggest that neurogenesis occurs within a vascular niche where endothelial cells regulate stem cell renewal.21-23 There are several reports showing that developmental pathways such as Notch, SHH and Wnt, are active in stem cell niches. For example, activated Notch signaling appears particularly capable of affecting both tumorigenesis and stem cell development, and it is necessary for maintaining a pool of undifferentiated stem cells.24,25 Deletion of Notch1 results in a decrease in neural stem cells and reduction in their proliferation.26 Later studies have expanded the known functions of Notch to include important cell fate decisions www.landesbioscience.com

throughout glial and neuronal development. In the oligodendrocyte lineage, Notch activation has been shown to suppress terminal oligodendrocyte differentiation but also support the specification of oligodendrocyte precursors from the initial stem cell pool.27,28 Notch signaling can promote the specification of Muller glia in the rat retina,29,30 radial glia in the telencephalon from cortical stem cells,31 and astrocytes in the adult brain from hippocampal multipotent progenitors.32 A simplified schematic of the Notch signaling pathway is illustrated in Figure 1A. SHH signaling regulates multiple aspects of CNS development, controlling both cell proliferation and cell differentiation. SHH is required for the differentiation of floor plate cells and ventral neurons in the early neural tube.33,34 SHH also regulates proliferation of granule cell precursors in the developing cerebellum35 and contributes to oligodendrocyte specification in the mammalian forebrain.36,37 Furthermore, SHH is necessary and sufficient for oligodendrocyte precursor production in cortical neuroepithelial cultures,38 and drives proliferation in CNS precursor cells.39 Lastly the SHH pathway is active in both the SVZ and SGZ of adult mice, where astrocytes are thought to release SHH that stimulate neuronal progenitors to re-enter the cell cycle and generate new neurons in

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Several studies were initiated to study the effect of radiation therapy and different small molecule inhibitors on CSCs from different tumor types. After CD133 was identified as a marker for brain tumor stem cells, several reports studying response of CSCs to different anti-cancer strategies followed. Bao et al., demonstrated that CD133+ cells contribute to glioma radio-resistance via preferential activation of DNA damage checkpoints, and that their resistance could be partially reversed with specific inhibitors of Chk1 and Chk2.68,69 Two other groups have shown that the bone morphogenetic proteins (BMPs) and cannabinoids inhibit the tumorogenic potential of CD133+ cancer stem cells and promote their differentiation. These results are intriguing because they present an opportunity to develop a new approach for GBM treatment based on the induction of differentiation of CD133+ cells, rather than killing them.70,71 DAOY medulloblastoma cells expressing CD133 showed more radioresistance as compared to the CD133- negative population, and in hypoxic conditions the number of CD133 expressing cells were increased.72 Besides their relative radioresistance, CSCs express high levels of multiple drug-resistance genes than the differentiated tumor bulk73 and show significant resistance to the chemotherapeutic agents.74 SHH pathway inhibitors can inhibit the growth of medulloblastoma but not other brain tumors in mice,75,76 and inhibition of the Notch pathway by using γ-secretase inhibitors blocks CSC self-renewal and decreases mouse medulloblastoma growth.77 Most of these studies were performed in isolated cancer cells in in vitro conditions in the absence of the perivascular niche. The niche is known to provide an umbrella that protects stem cells from differentiation stimuli, apoptosis and other stresses that affect stem cells.78 Therefore, it is important to study the response of stem cells to therapy in their intact niche. As gliomas are more complex as well as more resistant to radiation treatment (unpublished observations), we investigated the radiation resistance of CSCs in a brain tumor model that is known to be radiation sensitive: medulloblastoma.

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The concept of cancer stem cells (CSCs) has been used over 50 years, but evidence supporting their existence has strengthened significantly in the last few years. In 1989, Hill and Milas suggested that murine tumors contain stem cells and that the number of these cells within the tumor can be important determinants of prognosis.45 In the last several years, evidence has grown in support of the theory that cancers have their own stem cells, supporting the notion that the neoplastic process maintains the hierarchical structure of the normal tissue from which it is derived. CSCs have been identified in leukemias, breast, pancreatic, prostate, head and neck and colon cancer.6,46-50 Since then active discussion regarding cancer stem cell theory of tumor formation was initiated.51-54 The presence of CSCs was also shown in brain tumors.5 More recently several studies have identified CD133 (prominin-1) as a stem cell surface marker for gliomas and demonstrated that the number of these cells varies greatly, ranging from 1% in low grade brain tumors to 25% in high grade gliomas.7,55,56 Cancer stem cells in the brain (CSCs) express not only CD133 but also nestin, which is activated by Notch signaling.57,58 Nestin is known to be a marker for poor prognosis in gliomas, and a neuronal stem/progenitor marker in both the adult and developing brain.59 CD133+ subpopulations isolated from glioma, medulloblastoma and ependymoma were able to initiate and sustain tumor growth in xenograft experiments.7,56,60 Besides CD133, the side population (SP) phenotype, defined by its ability to efflux the nucleic acid-staining dye Hoechst 33342, has been used to prospectively isolate a small percentage of cells in brain tumors uniquely able to generate tumor neurospheres and xenografts.61 Finally, Calabrese et al. showed that brain tumor stem cells exist in a perivascular niche, providing evidence that the microvasculature forms scaffolding that may be necessary for maintaining or promoting the cancer stem cell phenotype.57,62 Later it was demonstrated that the SHH signaling pathway is active in gliomas and it correlates with tumor grade. SHH expressing reactive astrocytes were shown to reside in the perivascular stem cell niche in a mouse model of PDGF-driven gliomas next to nestin expressing stem cells suggesting similarities between normal and cancer stem cells.40,63 Finally CD133+ cells have been termed tumor-initiating cells (CD133- cells did not form tumors when implanted in immunocompromised mice) based on data obtained in the above-mentioned publications. The uniformity of CD133 as a marker for brain tumor stem cells has been questioned however. A recent study demonstrated that a series of secondary glioblastomas do not have any CD133- expressing cells, while 70% of primary glioblastomas have cells that are CD133 positive with CSC-like properties.64 More recently, a separate group demonstrated that

Cancer Stem Cells and Resistance to Therapy

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Cancer Stem Cells and the Niche

CD133- tumor cells are capable of self-renewal and can give rise to tumors with CD133+ cells.65 These data together suggest that CD133 is expressed on cells with stem cell properties in a subset of gliomas. Finally, two recent studies identified molecular differences between histologically identical glial cell tumors arising in distinct CNS locations, with molecular similarity to the normal cells in the location of the tumor suggesting heterogeneity in cells that can fit the definition of cancer stem cells.56,66,67

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vivo and in vitro.40 A schematic of the SHH signaling pathway is illustrated in the (Fig. 1B). The role of Wnt canonical signaling in the CNS stem cell niches is also well defined. It has been shown that Wnt signaling transiently stimulates proliferation and enhances neurogenesis in neonatal neural progenitor cultures.41 It was suggested that beta-catenin signaling plays a role in the proliferation of progenitor cells in the SVZ of the adult mouse,42 and regulate stem cell fate and differentiation in vivo.43 Wnt3a was shown promotes neuronal differentiation of neuronal stem cells derived from adult mouse spinal cord.44 The Wnt signaling pathway is illustrated in (Fig. 1C).

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Medulloblastoma Mouse Models Medulloblastoma is an embryonal round blue cell tumor that arises in the cerebellum, mostly in children. There are about 350 new diagnoses of medulloblastoma in the United States per year. Over the last twenty years there has been an increasing awareness of the genetic alterations that drive medulloblastoma pathogenesis. This has been achieved mainly through genetically engineered mouse models. We now know that about of a third of medulloblastomas are due to abnormal activation of the hedgehog pathway. About 25% are due to alterations of the wingless pathway (Wnt), which is interestingly associated with a better prognosis.79 One additional genetic alteration that is common but of unknown significance is isochromosome

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Another medulloblastoma model involves overexpression of interferon gamma in astrocytes99 and CXCR6-/- mice also develop medulloblastomas.100 There is only one medulloblastoma model that demonstrates Wnt pathway activation—a transgenic mouse that overexpresses the human neurotropic virus (JCV virus).101,102 In addition there are now mouse models of medulloblastomas of the LCA subtype. The first one was generated by crossing double conditional p53 and Rb mice with GFAP-Cre mice.103 The second one uses the RCAS/tv-a system whereby overexpression of c-Myc and β-catenin in GFAP expressing cerebellar progenitors in GFAP-tva mice in a p53 null background results in a large cell anaplastic (LCA) histology.104 Both of these models target the same cell of origin (GFAP-expressing cells) with loss of p53. This is consistant with human LCA meduilloblastomas having alterations in p53.82 Table 1 presents a summary of existing genetically engineered mouse models for medulloblastoma. To ensure that our observations were not specific to a single model of medulloblastoma, we used two mouse models to study the response of medulloblastoma to radiation: Ptc+/- mice and the RCAS/ tv-a system. When we irradiated medulloblastoma-bearing mice with a single dose of 2 Gy, we noted a dramatic apoptotic response by 6 hours. This response can be visualized using a Gli-luc reporter mouse, which provides a bioluminescent imaging readout of the hedgehog pathway in vivo to measure the therapeutic response. In Fig. 2A and B, we imaged a medulloblastoma-bearing mouse before and six hours post-2 Gy single-dose radiation. While the Gli-luc output was dramatically reduced, histologic analysis demonstrated that there are surviving perivascular cells. Further analysis of the surviving cells demonstrated that they express nestin (a subset also expresses GFAP as well as Olig-2) and undergo cell cycle arrest in response to radiation. Furthermore, these cells re-enter the cell cycle at 72 hours and give rise to tumor recurrence. Although the complete mechanism by which nestin-expressing stem cells resist radiation is not known, these cells activate the Akt pathway in response to radiation. The upstream and downstream consequences of this event are unclear but not surprisingly the nestin-expressing stem cells activate p53 in response to radiation at 6 hours, and tumor bulk at 4 hours.

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17.80 Additional less common genetic alterations include c-myc amplification, and mutations in p53. There are several common histologies of human medulloblastomas: desmoplastic, classic, extensive nodularity histology and large cell anaplastic histologies. The desmoplastic histology correlates with activation of the hedgehog pathway,81 while the large cell anaplastic (LCA) histology correlates with c-myc amplification and/or p53 mutations.82,83 One additional genetic alteration that has been associated with prognosis is ERB-B2 overexpression but it has not been shown to correlate with a particular histology.84 From a clinical standpoint, the management of medulloblastoma is mainly based on the stage at which it is diagnosed or the extent of disease. Newly diagnosed patients with medulloblastoma undergo a brain and spine MRI, a lumbar puncture and sometimes additional tests as clinically indicated. Patients are classified as standard risk if all three of the following conditions are met: complete surgical resection leaving less than 0.5 cm2 of residual tumor, patients are between the ages of 3–10 years of age, and patients do not have other foci of disease. High-risk patients are those that do not meet all of the criteria for standard risk disease. Standard risk patients receive 2340 cGy of craniospinal radiation fractionated into daily dose of 180 cGy. In addition, they receive a boost to the primary tumor bed up to 55.8 cGy. Patients also receive chemotherapy after completion of radiation with most commonly eight cycles of vincristine, lomustine and cisplatin. Of these three chemotherapeutic agents, cisplatin is thought to be the most active agent. In general the prognosis for patients with standard risk medulloblastoma is good, currently with 5-year eventfree survival (EFS) of around 85%. In contrast, the prognosis for high-risk patients is around 60–70% 5-year EFS and they require more intensive therapy. Patient with recurrent medulloblastoma following treatment have a significantly worse prognosis.85 There are numerous mouse models of medulloblastoma most of which involve activation of the sonic hedgehog pathway.80 The first reported model for this tumor was the Ptc+/- model in mice where lacZ is knocked-into the Ptc locus.86 These mice develop medulloblastomas at a rate of 10%–15% by 12 months of age. When crossed with p53 null mice, these mice generate medulloblastomas at a high rate of 95% by 3 months.87 Another transgenic mouse where an activated smoothened allele is expressed under the control of the NeuroD2 promoter develop medulloblastomas at a rate of 48% by approximately 6 months.88 Another SHH driven medulloblastoma model uses the RCAS/tv-a system where overexpression of SHH in nestin-expressing progenitor cells of the cerebellum (there are nestin expressing cells in the external granule layer at P1–P2) results in 15% incidence of medulloblastomas by 3 months.89 The incidence of medulloblastomas using this model can be increased by co-infecting nestin progenitor cells with RCAS vectors expressing Akt, N-myc, IGF2 or BCL-2.90-92 Lastly, the minor subtype of medulloblastoma, namely medulloblastoma with extensive nodularity can be generated using the RCAS/tv-a system by overexpressing SHH with loss of PTEN in nestin-expressing progenitor cells from the external granular layer (EGL).1 There is also an in utero medulloblastoma model where SHH is injected via a retrovirus into 13.5-day embryos.93 In addition, there have been numerous models of medulloblastoma which are generated by crossing p53 null mice with mice lacking DNA repair pathway proteins or key cell cycle regulators such as Lig4, Ink4c, PARP, BRCA2 and XRCC4.94-98 www.landesbioscience.com

PI3K/Akt Pathway in Medulloblastoma Akt is activated by many mechanisms including loss of PTEN. In the extensive nodularity medulloblastoma model, PTEN loss in the stem cell niche activates Akt, mTOR and S6K in the-nestin expressing stem cells and thus mediates radiation resistance. In the tumor bulk, loss of PTEN results in the differentiation of the tumor cells to express NeuN82 (Fig. 3A and B). Akt/PKB is 57 KDa Ser/Thr kinase downstream of phosphatidylinositol 3'-kinases (PI3K). It represents a key mediator of the signal transduction of different growth-controlling pathways. Upon stimulation by different growth factor and ionizing radiation PI3K catalyzes the generation of phosphatidylinositol-3,4,5-triphosphate (PIP3) from phosphatidylinositol-4,5-triphosphate (PIP2). PIP3 levels are regulated by lipid and protein phosphatase PTEN/MMAC. PIP3 recruits Akt into the plasma membrane where it is activated via phosphorylation. Several downstream targets of Akt have been identified (e.g., the BCL-2 family member BAD, caspase-9, MDM2, the transcription factor Forkhead, and glycogen synthase kinase 3-β (GSK-3β), thus conferring to Akt an important role in the regulation of growth

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Table 1 Summary of genetically engineered mouse models of medulloblastoma presently available Gene

Genetic approach

Genotype

Penetration

Latency period

References

Ptc

Knockout

Ptc+/-

10–15%

25 weeks

86

Ptc & p53

Knockout

Ptc+/-,p53-/-

95%

12 weeks

87

Rb & p53 Conditional compound mutant

Rb-/-, p53-/- In GFAP+ cells

100%

20 weeks

103

Lig4 & p53

Knockout

Lig4-/-, p53-/-

100%

9 weeks

94

PARP1 & p53

Knockout

PARP1-/-, p53-/-

49%

16 weeks

96

p53-/-

98

XRCC4 & p53

Knockout

XRCC4-/-,

87%

13 weeks

BRCA2 & p53

Knockout

BRCA2-/-,p53-/-

83%

13 weeks

97

Ink4c & p53

Knockout

Ink4c-/-, p53-/-

8%

12–17 weeks

95

JCV T antigen

Transgenic

50%

36–52 weeks

101, 102

ND2:

CXCR6+/- or

Knockout

3–4%

in utero

RCAS/tv-a

GFAP + progenitors, p53-/-

SHH

RCAS/tv-a

SHH & c-Myc

RCAS/tv-a

SHH & IGF2

26 weeks

88

16–20 weeks

100

76%

2–3 weeks

93

34%

14 weeks

104

Nestin + progenitors

15%

12 weeks

89


23%

12 weeks

89

RCAS/tv-a


39%

12 weeks

92

SHH & Akt

RCAS/tv-a


48%

12 weeks

92

SHH & BCL-2

RCAS/tv-a


78%

6–8 weeks

91

SHH & N-myc

RCAS/tv-a


78%

3 weeks

90

SHH & PTEN loss

RCAS/tv-a


5%

8–12 weeks

1

Double transgenic

GFAP/tTA&TRE/IFNγ

80%

3–4 weeks

99

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Gene transfer

c-Myc+ β-catenin

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48%

-/-

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Transgenic

CXCR6

-/-

SmoA2+/-

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JCV T Ag+/- or

Figure 2. The Gli-Luciferase mouse is a useful tool for monitoring the response of SHH-driven medulloblastoma response to therapy in vivo. (A) The image on the left is a Gli-Luc mouse with no tumor; there is background Gli-luc activity in the skin which is apparent in denuded areas. The right panel is a bioluminescent image of a medulloblastoma-bearing mouse with a corresponding H&E stained tissue section. (B) A medulloblastoma-bearing Gli-luc mouse before and after 2 Gy radiation.

and cell survival signaling pathways.105-109 In studies of RCAS/t-va mouse model by Fults et al. it was demonstrated that PI3K/Akt signaling pathway contributes to SHH induced medulloblastoma formation.92 Activation of PI3K/Akt pathway is an important step in the molecular pathogenesis of human medulloblastoma and it is associated with reduced expression of PTEN110 through deletion or methylation.111 1375

The first study demonstrating the role of the Akt pathway specifically in cancer stem cell resistance to chemotherapy was a study showing, that CD133 positive CSCs from hepatocellular carcinoma (HCC) contribute to chemoresistance through preferential activation of the Akt/PKB and BCL-2 cell survival pathways.112 Using SHH driven murine medulloblastoma models, we demonstrate that the Akt pathway is responsible for radiation resistance of CSCs in their native

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Figure 3. The PI3K/Akt signal transduction pathway and its role in formation of medulloblastoma with extensive nodularity. (A) Radiation and growth factors can activate the PI3K pathway, which leads to activation of several downstream components. Akt/PKB is one of the key components of this pathway. Activated Akt/PKB regulates several downstream components that are involved in regulation of apoptosis and cell cycle, for example p53. (B) Loss of PTEN in combination with SHH overexpression in nestin expressing progenitors of the EGL leads to the formation of medulloblastoma with extensive nodularity. In the perivascular stem cell niche, activation of Akt via loss of PTEN leads to proliferation and correlates with increased Nestin expression while in the tumor bulk activated AKT leads to differentiation and correlates increased NeuN expression.

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microenviroment. By using the Akt inhibitor perifosine we were able to sensitize a subset of CSCs to radiation induced apoptosis. Our data together with others demonstrate the role of the Akt pathway not only in tumor initiation but also in determining CSC resistance to radiation.1,92 As the Akt pathway seems to be a general survival pathway in cancers, inhibition of this pathway to address the biology of specific cell types may have benefit for the treatment of patients with cancer in general. Acknowledgements

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Grant Support: NIH grants R01 CA 5R01CA099489-1 and R01 5R01CA100688-2. References

1. Hambardzumyan D, Becher OJ, Rosenblum MK, Pandolfi PP, Manova-Todorova K, Holland EC. PI3K pathway regulates survival of cancer stem cells residing in the perivascular niche following radiation in medulloblastoma in vivo. Genes Dev 2008; 22:436-48. 2. Stupp R, Mason WP, van den Bent MJ, Weller M, Fisher B, Taphoorn MJ, Belanger K, Brandes AA, Marosi C, Bogdahn U, Curschmann J, Janzer RC, Ludwin SK, Gorlia T, Allgeier A, Lacombe D, Cairncross JG, Eisenhauer E, Mirimanoff RO. Radiotherapy plus concomitant and adjuvant temozolomide for glioblastoma. N Engl J Med 2005; 352:987-96. 3. Lee J, Kotliarova S, Kotliarov Y, Li A, Su Q, Donin NM, Pastorino S, Purow BW, Christopher N, Zhang W, Park JK, Fine HA. Tumor stem cells derived from glioblastomas cultured in bFGF and EGF more closely mirror the phenotype and genotype of primary tumors than do serum-cultured cell lines. Cancer Cell 2006; 9:391-403. 4. Lapidot T, Sirard C, Vormoor J, Murdoch B, Hoang T, Caceres Cortes J, Minden M, Paterson B, Caligiuri MA, Dick JE. A cell initiating human acute myeloid leukaemia after transplantation into SCID mice. Nature 1994; 367:645-8.

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5. Ignatova TN, Kukekov VG, Laywell ED, Suslov ON, Vrionis FD, Steindler DA. Human cortical glial tumors contain neural stem-like cells expressing astroglial and neuronal markers in vitro. Glia 2002; 39:193-206. 6. Al-Hajj M, Wicha MS, Benito-Hernandez A, Morrison SJ, Clarke MF. Prospective identification of tumorigenic breast cancer cells. Proc Natl Acad Sci USA 2003; 100:3983-8. 7. Singh SK, Hawkins C, Clarke ID, Squire JA, Bayani J, Hide T, Henkelman RM, Cusimano MD, Dirks PB. Identification of human brain tumour initiating cells. Nature 2004; 432:396-401. 8. Fisher HW, Puck TT. On The Functions Of X-Irradiated “Feeder” Cells In Supporting Growth Of Single Mammalian Cells. Proc Natl Acad Sci USA 1956; 42:900-6. 9. Till JE, Mc CE. A direct measurement of the radiation sensitivity of normal mouse bone marrow cells. Radiat Res 1961; 14:213-22. 10. Franken NA, Rodermond HM, Stap J, Haveman J, van Bree C. Clonogenic assay of cells in vitro. Nat Protoc 2006; 1:2315-9. 11. Lois C, Alvarez Buylla A. Proliferating subventricular zone cells in the adult mammalian forebrain can differentiate into neurons and glia. Proc Natl Acad Sci USA 1993; 90:2074-7. 12. Altman J, Das GD. Autoradiographic and histological evidence of postnatal hippocampal neurogenesis in rats. J Comp Neurol 1965; 124:319-35. 13. Kaplan MS, Bell DH. Mitotic neuroblasts in the 9-day-old and 11-month-old rodent hippocampus. J Neurosci 1984; 4:1429-41. 14. Cameron HA, Woolley CS, McEwen BS, Gould E. Differentiation of newly born neurons and glia in the dentate gyrus of the adult rat. Neuroscience 1993; 56:337-44. 15. Kuhn HG, Dickinson Anson H, Gage FH. Neurogenesis in the dentate gyrus of the adult rat: age-related decrease of neuronal progenitor proliferation. J Neurosci 1996; 16:2027-33. 16. Lee A, Kessler JD, Read TA, Kaiser C, Corbeil D, Huttner WB, Johnson JE, Wechsler Reya RJ. Isolation of neural stem cells from the postnatal cerebellum. Nat Neurosci 2005; 8:723-9. 17. Doetsch F. A niche for adult neural stem cells. Curr Opin Genet Dev 2003; 13:543-50. 18. Schofield R. The relationship between the spleen colony-forming cell and the haemopoietic stem cell. Blood Cells 1978; 4:7-25.

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.

50. O’Brien CA, Pollett A, Gallinger S, Dick JE. A human colon cancer cell capable of initiating tumour growth in immunodeficient mice. Nature 2007; 445:106-10. 51. Campbell LL, Polyak K. Breast tumor heterogeneity: cancer stem cells or clonal evolution? Cell Cycle 2007; 6:1684-90. 52. Blagosklonny MV. Cancer stem cell and cancer stemloids: From biology to therapy. Cell Cycle 2007; 6:2997-3003. 53. Dingli D, Traulsen A, Pacheco JM. Stochastic dynamics of hematopoietic tumor stem cells. Cell Cycle 2007; 6:461-6. 54. Sakariassen PO, Immervoll H, Chekenya M. Cancer stem cells as mediators of treatment resistance in brain tumors: status and controversies. Neoplasia 2007; 9:882-92. 55. Singh SK, Clarke ID, Terasaki M, Bonn VE, Hawkins C, Squire J, Dirks PB. Identification of a cancer stem cell in human brain tumors. Cancer Res 2003; 63:5821-8. 56. Taylor MD, Poppleton H, Fuller C, Su X, Liu Y, Jensen P, Magdaleno S, Dalton J, Calabrese C, Board J, Macdonald T, Rutka J, Guha A, Gajjar A, Curran T, Gilbertson RJ. Radial glia cells are candidate stem cells of ependymoma. Cancer Cell 2005; 8:323-35. 57. Calabrese C, Poppleton H, Kocak M, Hogg TL, Fuller C, Hamner B, Oh EY, Gaber MW, Finklestein D, Allen M, Frank A, Bayazitov IT, Zakharenko SS, Gajjar A, Davidoff A, Gilbertson RJ. A perivascular niche for brain tumor stem cells. Cancer Cell 2007; 11:69-82. 58. Shih AH, Holland EC. Notch signaling enhances nestin expression in gliomas. Neoplasia 2006; 8:1072-82. 59. Strojnik T, Rosland GV, Sakariassen PO, Kavalar R, Lah T. Neural stem cell markers, nestin and musashi proteins, in the progression of human glioma: correlation of nestin with prognosis of patient survival. Surg Neurol 2007; 68:133-43. 60. Gilbertson RJ. Brain tumors provide new clues to the source of cancer stem cells: does oncology recapitulate ontogeny? Cell Cycle 2006; 5:135-7. 61. Patrawala L, Calhoun T, Schneider-Broussard R, Zhou J, Claypool K, Tang DG. Side population is enriched in tumorigenic, stem-like cancer cells, whereas ABCG2+ and ABCG2cancer cells are similarly tumorigenic. Cancer Res 2005; 65:6207-19. 62. Gilbertson RJ, Rich JN. Making a tumour’s bed: glioblastoma stem cells and the vascular niche. Nat Rev Cancer 2007; 7:733-6. 63. Becher OJ, Hambardzumyan D, Fomchenko EI, Momota H, Mainwaring L, Bleau AM, Katz AM, Edgar M, Kenney AM, Cordon Cardo C, Blasberg RG, Holland EC. Gli activity correlates with tumor grade in PDGF-induced gliomas. Cancer Res 2008; 68:2241-9. 64. Beier D, Hau P, Proescholdt M, Lohmeier A, Wischhusen J, Oefner PJ, Aigner L, Brawanski A, Bogdahn U, Beier CP. CD133(+) and CD133(-) glioblastoma-derived cancer stem cells show differential growth characteristics and molecular profiles. Cancer Res 2007; 67:4010-5. 65. Wang J, Sakariassen PO, Tsinkalovsky O, Immervoll H, Boe SO, Svendsen A, Prestegarden L, Rosland G, Thorsen F, Stuhr L, Molven A, Bjerkvig R, Enger PO. CD133 negative glioma cells form tumors in nude rats and give rise to CD133 positive cells. Int J Cancer 2008; 122:761-8. 66. Sharma MK, Mansur DB, Reifenberger G, Perry A, Leonard JR, Aldape KD, Albin MG, Emnett RJ, Loeser S, Watson MA, Nagarajan R, Gutmann DH. Distinct genetic signatures among pilocytic astrocytomas relate to their brain region origin. Cancer Res 2007; 67:890-900. 67. Gilbertson RJ, Gutmann DH. Tumorigenesis in the brain: location, location, location. Cancer Res 2007; 67:5579-82. 68. Bao S, Wu Q, McLendon RE, Hao Y, Shi Q, Hjelmeland AB, Dewhirst MW, Bigner DD, Rich JN. Glioma stem cells promote radioresistance by preferential activation of the DNA damage response. Nature 2006; 444:756-60. 69. Hambardzumyan D, Squatrito M, Holland EC. Radiation resistance and stem-like cells in brain tumors. Cancer Cell 2006; 10:454-6. 70. Piccirillo SG, Reynolds BA, Zanetti N, Lamorte G, Binda E, Broggi G, Brem H, Olivi A, Dimeco F, Vescovi AL. Bone morphogenetic proteins inhibit the tumorigenic potential of human brain tumour-initiating cells. Nature 2006; 444:761-5. 71. Aguado T, Carracedo A, Julien B, Velasco G, Milman G, Mechoulam R, Alvarez L, Guzman M, Galve-Roperh I. Cannabinoids induce glioma stem-like cell differentiation and inhibit gliomagenesis. J Biol Chem 2007; 282:6854-62. 72. Blazek ER, Foutch JL, Maki G. Daoy medulloblastoma cells that express CD133 are radioresistant relative to CD133- cells, and the CD133+ sector is enlarged by hypoxia. Int J Radiat Oncol Biol Phys 2007; 67:1-5. 73. Dean M, Fojo T, Bates S. Tumour stem cells and drug resistance. Nat Rev Cancer 2005; 5:275-84. 74. Liu G, Yuan X, Zeng Z, Tunici P, Ng H, Abdulkadir IR, Lu L, Irvin D, Black KL, Yu JS. Analysis of gene expression and chemoresistance of CD133+ cancer stem cells in glioblastoma. Mol Cancer 2006; 5:67. 75. Berman DM, Karhadkar SS, Hallahan AR, Pritchard JI, Eberhart CG, Watkins DN, Chen JK, Cooper MK, Taipale J, Olson JM, Beachy PA. Medulloblastoma growth inhibition by hedgehog pathway blockade. Science 2002; 297:1559-61. 76. Romer JT, Kimura H, Magdaleno S, Sasai K, Fuller C, Baines H, Connelly M, Stewart CF, Gould S, Rubin LL, Curran T. Suppression of the Shh pathway using a small molecule inhibitor eliminates medulloblastoma in Ptc1(+/-)p53(-/-) mice. Cancer Cell 2004; 6:229-40. 77. Fan X, Matsui W, Khaki L, Stearns D, Chun J, Li YM, Eberhart CG. Notch pathway inhibition depletes stem-like cells and blocks engraftment in embryonal brain tumors. Cancer Res 2006; 66:7445-52. 78. Moore KA, Lemischka IR. Stem cells and their niches. Science 2006; 311:1880-5.

©

20

08

LA

ND

ES

BIO

SC

IEN CE

.D

ON

19. Sugiyama T, Kohara H, Noda M, Nagasawa T. Maintenance of the hematopoietic stem cell pool by CXCL12-CXCR4 chemokine signaling in bone marrow stromal cell niches. Immunity 2006; 25:977-88. 20. Zou GM. Cancer stem cells in leukemia, recent advances. J Cell Physiol 2007; 213:440-4. 21. Palmer TD, Willhoite AR, Gage FH. Vascular niche for adult hippocampal neurogenesis. J Comp Neurol 2000; 425:479-94. 22. Louissaint A Jr, Rao S, Leventhal C, Goldman SA. Coordinated interaction of neurogenesis and angiogenesis in the adult songbird brain. Neuron 2002; 34:945-60. 23. Ramirez Castillejo C, Sanchez Sanchez F, Andreu Agullo C, Ferron SR, Aroca Aguilar JD, Sanchez P, Mira H, Escribano J, Farinas I. Pigment epithelium-derived factor is a niche signal for neural stem cell renewal. Nat Neurosci 2006; 9:331-9. 24. Nye JS, Kopan R, Axel R. An activated Notch suppresses neurogenesis and myogenesis but not gliogenesis in mammalian cells. Development 1994; 120:2421-30. 25. Morrison SJ, Perez SE, Qiao Z, Verdi JM, Hicks C, Weinmaster G, Anderson DJ. Transient Notch activation initiates an irreversible switch from neurogenesis to gliogenesis by neural crest stem cells. Cell 2000; 101:499-510. 26. Hitoshi S, Alexson T, Tropepe V, Donoviel D, Elia AJ, Nye JS, Conlon RA, Mak TW, Bernstein A, van der Kooy D. Notch pathway molecules are essential for the maintenance, but not the generation, of mammalian neural stem cells. Genes Dev 2002; 16:846-58. 27. Genoud S, Lappe-Siefke C, Goebbels S, Radtke F, Aguet M, Scherer SS, Suter U, Nave KA, Mantei N. Notch1 control of oligodendrocyte differentiation in the spinal cord. The Journal of cell biology 2002; 158:709-18. 28. Wang S, Sdrulla AD, diSibio G, Bush G, Nofziger D, Hicks C, Weinmaster G, Barres BA. Notch receptor activation inhibits oligodendrocyte differentiation. Neuron 1998; 21:63-75. 29. Bao ZZ, Cepko CL. The expression and function of Notch pathway genes in the developing rat eye. J Neurosci 1997; 17:1425-34. 30. Furukawa T, Mukherjee S, Bao ZZ, Morrow EM, Cepko CL. rax, Hes1, and notch1 promote the formation of Muller glia by postnatal retinal progenitor cells. Neuron 2000; 26:383-94. 31. Gaiano N, Nye JS, Fishell G. Radial glial identity is promoted by Notch1 signaling in the murine forebrain. Neuron 2000; 26:395-404. 32. Tanigaki K, Nogaki F, Takahashi J, Tashiro K, Kurooka H, Honjo T. Notch1 and Notch3 instructively restrict bFGF-responsive multipotent neural progenitor cells to an astroglial fate. Neuron 2001; 29:45-55. 33. Lee J, Platt KA, Censullo P, Ruiz i Altaba A. Gli1 is a target of Sonic hedgehog that induces ventral neural tube development. Development 1997; 124:2537-52. 34. Roelink H, Augsburger A, Heemskerk J, Korzh V, Norlin S, Ruiz i Altaba A, Tanabe Y, Placzek M, Edlund T, Jessell TM, et al. Floor plate and motor neuron induction by vhh-1, a vertebrate homolog of hedgehog expressed by the notochord. Cell 1994; 76:761-75. 35. Wechsler Reya RJ, Scott MP. Control of neuronal precursor proliferation in the cerebellum by Sonic Hedgehog. Neuron 1999; 22:103-14. 36. Nery S, Wichterle H, Fishell G. Sonic hedgehog contributes to oligodendrocyte specification in the mammalian forebrain. Development 2001; 128:527-40. 37. Tekki Kessaris N, Woodruff R, Hall AC, Gaffield W, Kimura S, Stiles CD, Rowitch DH, Richardson WD. Hedgehog-dependent oligodendrocyte lineage specification in the telencephalon. Development 2001; 128:2545-54. 38. Alberta JA, Park SK, Mora J, Yuk D, Pawlitzky I, Iannarelli P, Vartanian T, Stiles CD, Rowitch DH. Sonic hedgehog is required during an early phase of oligodendrocyte development in mammalian brain. Mol Cell Neurosci 2001; 18:434-41. 39. Rowitch DH, B SJ, Lee SM, Flax JD, Snyder EY, McMahon AP. Sonic hedgehog regulates proliferation and inhibits differentiation of CNS precursor cells. J Neurosci 1999; 19:8954-65. 40. Jiao J, Chen DF. Induction of Neurogenesis in Non-conventional Neurogenic Regions of the Adult CNS by Niche Astrocyte-Produced Signals. Stem Cells 2008. 41. Hirsch C, Campano LM, Wohrle S, Hecht A. Canonical Wnt signaling transiently stimulates proliferation and enhances neurogenesis in neonatal neural progenitor cultures. Exp Cell Res 2007; 313:572-87. 42. Adachi K, Mirzadeh Z, Sakaguchi M, Yamashita T, Nikolcheva T, Gotoh Y, Peltz G, Gong L, Kawase T, Alvarez Buylla A, Okano H, Sawamoto K. Beta-catenin signaling promotes proliferation of progenitor cells in the adult mouse subventricular zone. Stem Cells 2007; 25:2827-36. 43. Prakash N, Wurst W. A Wnt signal regulates stem cell fate and differentiation in vivo. Neurodegener Dis 2007; 4:333-8. 44. Yin ZS, Zhang H, Wang W, Hua XY, Hu Y, Zhang SQ, Li GW. Wnt-3a protein promote neuronal differentiation of neural stem cells derived from adult mouse spinal cord. Neurol Res 2007; 29:847-54. 45. Hill RP, Milas L. The proportion of stem cells in murine tumors. Int J Radiat Oncol Biol Phys 1989; 16:513-8. 46. Bonnet D, Dick JE. Human acute myeloid leukemia is organized as a hierarchy that originates from a primitive hematopoietic cell. Nat Med 1997; 3:730-7. 47. Li C, Heidt DG, Dalerba P, Burant CF, Zhang L, Adsay V, Wicha M, Clarke MF, Simeone DM. Identification of pancreatic cancer stem cells. Cancer Res 2007; 67:1030-7. 48. Maitland NJ, Bryce SD, Stower MJ, Collins AT. Prostate cancer stem cells: a target for new therapies. Ernst Schering Found Symp Proc 2006; 5:155-79. 49. Prince ME, Sivanandan R, Kaczorowski A, Wolf GT, Kaplan MJ, Dalerba P, Weissman IL, Clarke MF, Ailles LE. Identification of a subpopulation of cells with cancer stem cell properties in head and neck squamous cell carcinoma. Proc Natl Acad Sci USA 2007; 104:973-8. 1377

Cell Cycle

2008; Vol. 7 Issue 10


OT D

IST RIB

UT E

.

106. Cardone MH, Roy N, Stennicke HR, Salvesen GS, Franke TF, Stanbridge E, Frisch S, Reed JC. Regulation of cell death protease caspase-9 by phosphorylation. Science 1998; 282:1318-21. 107. Mayo LD, Donner DB. A phosphatidylinositol 3-kinase/Akt pathway promotes translocation of Mdm2 from the cytoplasm to the nucleus. Proc Natl Acad Sci USA 2001; 98:11598-603. 108. Brunet A, Bonni A, Zigmond MJ, Lin MZ, Juo P, Hu LS, Anderson MJ, Arden KC, Blenis J, Greenberg ME. Akt promotes cell survival by phosphorylating and inhibiting a Forkhead transcription factor. Cell 1999; 96:857-68. 109. Cross DA, Alessi DR, Cohen P, Andjelkovich M, Hemmings BA. Inhibition of glycogen synthase kinase-3 by insulin mediated by protein kinase B. Nature 1995; 378:785-9. 110. Hartmann W, Digon-Sontgerath B, Koch A, Waha A, Endl E, Dani I, Denkhaus D, Goodyer CG, Sorensen N, Wiestler OD, Pietsch T. Phosphatidylinositol 3'-kinase/AKT signaling is activated in medulloblastoma cell proliferation and is associated with reduced expression of PTEN. Clin Cancer Res 2006; 12:3019-27. 111. Inda MM, Mercapide J, Munoz J, Coullin P, Danglot G, Tunon T, Martinez-Penuela JM, Rivera JM, Burgos JJ, Bernheim A, Castresana JS. PTEN and DMBT1 homozygous deletion and expression in medulloblastomas and supratentorial primitive neuroectodermal tumors. Oncol Rep 2004; 12:1341-7. 112. Ma S, Lee TK, Zheng BJ, Chan KW, Guan XY. CD133(+) HCC cancer stem cells confer chemoresistance by preferential expression of the Akt/PKB survival pathway. Oncogene 2007.

©

20

08

LA

ND

ES

BIO

SC

IEN CE

.D

ON

79. Clifford SC, Lusher ME, Lindsey JC, Langdon JA, Gilbertson RJ, Straughton D, Ellison DW. Wnt/Wingless pathway activation and chromosome 6 loss characterize a distinct molecular sub-group of medulloblastomas associated with a favorable prognosis. Cell Cycle 2006; 5:2666-70. 80. Marino S. Medulloblastoma: developmental mechanisms out of control. Trends Mol Med 2005; 11:17-22. 81. Pietsch T, Waha A, Koch A, Kraus J, Albrecht S, Tonn J, Sorensen N, Berthold F, Henk B, Schmandt N, Wolf HK, von Deimling A, Wainwright B, Chenevix Trench G, Wiestler OD, Wicking C. Medulloblastomas of the desmoplastic variant carry mutations of the human homologue of Drosophila patched. Cancer Res 1997; 57:2085-8. 82. Frank AJ, Hernan R, Hollander A, Lindsey JC, Lusher ME, Fuller CE, Clifford SC, Gilbertson RJ. The TP53-ARF tumor suppressor pathway is frequently disrupted in large/ cell anaplastic medulloblastoma. Brain Res Mol Brain Res 2004; 121:137-40. 83. Stearns D, Chaudhry A, Abel TW, Burger PC, Dang CV, Eberhart CG. c-myc overexpression causes anaplasia in medulloblastoma. Cancer Res 2006; 66:673-81. 84. Gilbertson RJ, Perry RH, Kelly PJ, Pearson AD, Lunec J. Prognostic significance of HER2 and HER4 coexpression in childhood medulloblastoma. Cancer Res 1997; 57:3272-80. 85. Gilbertson RJ. Medulloblastoma: signalling a change in treatment. Lancet Oncol 2004; 5:209-18. 86. Goodrich LV, Milenkovic L, Higgins KM, Scott MP. Altered neural cell fates and medulloblastoma in mouse patched mutants. Science 1997; 277:1109-13. 87. Wetmore C, Eberhart DE, Curran T. Loss of p53 but not ARF accelerates medulloblastoma in mice heterozygous for patched. Cancer Res 2001; 61:513-6. 88. Hallahan AR, Pritchard JI, Hansen S, Benson M, Stoeck J, Hatton BA, Russell TL, Ellenbogen RG, Bernstein ID, Beachy PA, Olson JM. The SmoA1 mouse model reveals that notch signaling is critical for the growth and survival of sonic hedgehog-induced medulloblastomas. Cancer Res 2004; 64:7794-800. 89. Rao G, Pedone CA, Coffin CM, Holland EC, Fults DW. c-Myc enhances sonic hedgehoginduced medulloblastoma formation from nestin-expressing neural progenitors in mice. Neoplasia 2003; 5:198-204. 90. Browd SR, Kenney AM, Gottfried ON, Yoon JW, Walterhouse D, Pedone CA, Fults DW. N-myc can substitute for insulin-like growth factor signaling in a mouse model of sonic hedgehog-induced medulloblastoma. Cancer Res 2006; 66:2666-72. 91. McCall TD, Pedone CA, Fults DW. Apoptosis suppression by somatic cell transfer of Bcl2 promotes Sonic hedgehog-dependent medulloblastoma formation in mice. Cancer Res 2007; 67:5179-85. 92. Rao G, Pedone CA, Del Valle L, Reiss K, Holland EC, Fults DW. Sonic hedgehog and insulin-like growth factor signaling synergize to induce medulloblastoma formation from nestin-expressing neural progenitors in mice. Oncogene 2004; 23:6156-62. 93. Weiner HL, Bakst R, Hurlbert MS, Ruggiero J, Ahn E, Lee WS, Stephen D, Zagzag D, Joyner AL, Turnbull DH. Induction of medulloblastomas in mice by sonic hedgehog, independent of Gli1. Cancer Res 2002; 62:6385-9. 94. Lee Y, McKinnon PJ. DNA ligase IV suppresses medulloblastoma formation. Cancer Res 2002; 62:6395-9. 95. Zindy F, Nilsson LM, Nguyen L, Meunier C, Smeyne RJ, Rehg JE, Eberhart C, Sherr CJ, Roussel MF. Hemangiosarcomas, medulloblastomas, and other tumors in Ink4c/p53-null mice. Cancer Res 2003; 63:5420-7. 96. Tong WM, Ohgaki H, Huang H, Granier C, Kleihues P, Wang ZQ. Null mutation of DNA strand break-binding molecule poly(ADP-ribose) polymerase causes medulloblastomas in p53(-/-) mice. Am J Pathol 2003; 162:343-52. 97. Frappart PO, Lee Y, Lamont J, McKinnon PJ. BRCA2 is required for neurogenesis and suppression of medulloblastoma. Embo J 2007; 26:2732-42. 98. Yan CT, Kaushal D, Murphy M, Zhang Y, Datta A, Chen C, Monroe B, Mostoslavsky G, Coakley K, Gao Y, Mills KD, Fazeli AP, Tepsuporn S, Hall G, Mulligan R, Fox E, Bronson R, De Girolami U, Lee C, Alt FW. XRCC4 suppresses medulloblastomas with recurrent translocations in p53-deficient mice. Proc Natl Acad Sci USA 2006; 103:7378-83. 99. Lin W, Kemper A, McCarthy KD, Pytel P, Wang JP, Campbell IL, Utset MF, Popko B. Interferon-gamma induced medulloblastoma in the developing cerebellum. J Neurosci 2004; 24:10074-83. 100. Sasai K, Romer JT, Kimura H, Eberhart DE, Rice DS, Curran T. Medulloblastomas derived from Cxcr6 mutant mice respond to treatment with a smoothened inhibitor. Cancer Res 2007; 67:3871-7. 101. Krynska B, Otte J, Franks R, Khalili K, Croul S. Human ubiquitous JCV(CY) T-antigen gene induces brain tumors in experimental animals. Oncogene 1999; 18:39-46. 102. Gan DD, Reiss K, Carrill T, Del Valle L, Croul S, Giordano A, Fishman P, Khalili K. Involvement of Wnt signaling pathway in murine medulloblastoma induced by human neurotropic JC virus. Oncogene 2001; 20:4864-70. 103. Shakhova O, Leung C, van Montfort E, Berns A, Marino S. Lack of Rb and p53 delays cerebellar development and predisposes to large cell anaplastic medulloblastoma through amplification of N-Myc and Ptch2. Cancer Res 2006; 66:5190-200. 104. Momota H, Shih AH, Edgar MA, Holland EC. c-Myc and -catenin Cooperate with Loss of p53 to Generate Multiple Members of the Primitive Neuroectodermal Tumor (PNET) Family in Mice. Oncogene 2008; in press. 105. Datta SR, Dudek H, Tao X, Masters S, Fu H, Gotoh Y, Greenberg ME. Akt phosphorylation of BAD couples survival signals to the cell-intrinsic death machinery. Cell 1997; 91:231-41.

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