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. 2012 Feb 14;21(2):155-67.
doi: 10.1016/j.ccr.2011.12.021.

An animal model of MYC-driven medulloblastoma

Yanxin Pei  1 , Colin E Moore, Jun Wang, Alok K Tewari, Alexey Eroshkin, Yoon-Jae Cho, Hendrik Witt, Andrey Korshunov, Tracy-Ann Read, Julia L Sun, Earlene M Schmitt, C Ryan Miller, Anne F Buckley, Roger E McLendon, Thomas F Westbrook, Paul A Northcott, Michael D Taylor, Stefan M Pfister, Phillip G Febbo, Robert J Wechsler-Reya
Affiliations

An animal model of MYC-driven medulloblastoma

Yanxin Pei et al. Cancer Cell. .

Abstract

Medulloblastoma (MB) is the most common malignant brain tumor in children. Patients whose tumors exhibit overexpression or amplification of the MYC oncogene (c-MYC) usually have an extremely poor prognosis, but there are no animal models of this subtype of the disease. Here, we show that cerebellar stem cells expressing Myc and mutant Trp53 (p53) generate aggressive tumors following orthotopic transplantation. These tumors consist of large, pleiomorphic cells and resemble human MYC-driven MB at a molecular level. Notably, antagonists of PI3K/mTOR signaling, but not Hedgehog signaling, inhibit growth of tumor cells. These findings suggest that cerebellar stem cells can give rise to MYC-driven MB and identify a novel model that can be used to test therapies for this devastating disease.

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Figures

Figure 1
Figure 1. Myc promotes proliferation of cerebellar stem cells in vitro
(A), Prom1+Lin− cells sorted from cerebella of 5–7 day-old mice were infected with Myc-IRES-GFP or control (GFP only) viruses for 48h, pulsed with tritiated thymidine (3H-Td) and cultured overnight before being assayed for 3H-Td incorporation. Data represent means of triplicate samples ± SEM. (B–D) Prom1+Lin− cells infected with Myc-IRES-GFP or control viruses were cultured at low density in the presence of EGF and bFGF for 7 days. Representative fields are shown in panels C and D (scale bars = 100 μm). The number of GFP+ neurospheres is quantified in B; data represent means of triplicate samples ± SEM. The infection efficiency was 80% with Myc-IRES-GFP and 90% with control retrovirus. See also Figure S1.
Figure 2
Figure 2. Myc-infected stem cells give rise to transient hyperplastic lesions following transplantation
Prom1+Lin− cells were infected with Myc-IRES-GFP or control retroviruss for 20h and then transplanted into the cerebellum of NSG mice. Hosts were sacrificed after 2.5 weeks. Frozen sections from mice that received GFP-infected (A–B) or Myc infected cells (C–F) were stained with anti-Ki67 antibodies (A–D) or H&E (E, F). Note the large mass of proliferating (Ki67+) cells seen in animals that received Myc-infected cells (C, D). Box in E corresponds to high-power field shown in F. Panels A–D and F, scale bars = 50 μm; panel E, scale bar = 100 μm. (G–H) Prom1+ cells were infected with Myc-ires-GFP or control-GFP viruses for 20h and then transplanted into the cerebellum of NSG hosts. Mice were sacrificed after 2 weeks. Frozen sections from mice that received control (G) or Myc infected cells (H) were stained with antibodies specific for cleaved caspase-3 (CC3) to detect apoptotic cells. Scale bars = 50 μm. See also Figure S2.
Figure 3
Figure 3. Overexpression of Myc and inactivation of p53 transforms cerebellar stem cells into tumors
Prom1+Lin− cells were infected with Myc + DNp53 retroviruses, Myc alone or DNp53 alone for 20h, and transplanted into cerebella of NSG mice. Animals were sacrificed when they developed symptoms. (A), Whole mount image of tumor, with GFP expression originating from DNp53 retrovirus. (B) Survival curve of animals receiving 5 x104 cells infected with Myc viruses (blue line), DNp53 viruses (green line) or Myc + DNp53 viruses (red line) (median survival 48 days). (C–H) Sections of tumor tissue from animals transplanted with cells expressing Myc and DNp53 (C–E) or from Ptch1 mutant mice (F–H) were stained with hematoxylin and eosin. For C and F scale bars = 100 μm; for D, E, G and H, scale bars = 50 μm. Boxes in panels C and F refer to panels D and G, respectively. Asterisk in C shows an area of necrosis, and asterisk in D shows a large tumor cell with marked nuclear atypia (anaplasia). Horizontal arrows in D and E show prominent nuclear molding. Arrowheads in D, G and H show mitotic figures. Vertical arrows in E and H show normal granule neurons in the internal granule layer (igl); the majority of tumor cells in E are much larger than these cells, whereas those in H are approximately the same size. See also Figure S3.
Figure 4
Figure 4. MP tumors exhibit characteristics of human MB
Cryosections from MP tumors were stained with H&E (A) or with antibodies specific for Ki67 (B), Nestin (C), Tuj1 (D), GFAP (E) or BAF47/Ini1 (F). Panels A–E represent adjacent sections. Scale bars = 50 μm. See also Figure S4.
Figure 5
Figure 5. Myc is required for continued growth of MP tumors
(A) Strategy for generating Tet-regulatable MP tumors. (B–F) Bioluminescent imaging of animals at 1, 2 and 3 weeks after tumor cells transplantation. Panel B shows representative images of 4 animals from each group at each time point (X’s denote animals that died before they could be imaged). Graphs on right show mean percent increase in bioluminescence for all animals in the group (with the 1-week signal for each animal set at 100%). In the top graph, the 1- and 2-week time points represent the average signal intensity for all 14 animals; the 3-week time point (marked by asterisk) represents the average for the 3 animals that remained alive at the time of imaging. (C–E) H&E-stained cerebellar sections from representative animals in Groups 1 (C), 2 (D) and 3 (E) three weeks after transplantation. Arrows in D and E point to injection site. Scale bars = 250 μm. (F) Survival curve (Groups 1 and 2, n=14; Group 3, n=12).
Figure 6
Figure 6. MP tumors resemble human MYC-driven MB
A, Gene expression profiles of Myc/DNp53 tumors from Prom1+/Lin− cells (MP-pl2-6) or from Prom1+ cells (MP-P3-7) and Ptch1 mutant (Ptc1-4) tumors were compared to signatures generated from human MB subtypes: WNT (blue), SHH (red), and Group C/D (green). Each murine tumor was assigned a score denoting its similarity to each subtype of human tumor (for details see Human Tumor Analysis Supplement and Table S1). (B–G) Ptch1 and MP tumors were stained with antibodies specific for secreted frizzled-related protein 1 (SFRP1, a marker for SHH tumors), Natriuretic Peptide Receptor C (NPR3, a marker for Group C tumors), or Potassium voltage-gated channel, shaker-related subfamily, member 1 (KCNA1, a marker for Group D tumors). Scale bars = 100 μm. See also Figure S5 and Table S1.
Figure 7
Figure 7. MP tumors are molecularly distinct from stem cells and from Ptch1 tumors
(A) Principle components analysis (PCA). Three PCA coordinates describe 55.2% of total data variation (PC1 – 27.2%, PC2 – 19.8% and PC3 – 8.23%). Green, MP tumors derived from Prom1+Lin− cells; purple, MP tumors derived from Prom1+ cells; blue, Ptch1 tumors; red, normal stem cells (NSCs). (B) Unsupervised hierarchical clustering analysis. Each column represents a distinct sample and each row represents an individual gene. The normalized (log2) and standardized (each sample to mean signal = 0 and standard deviation = 1) level of gene expression is denoted by color (green = low, dark = intermediate, red = high) as indicated in the gradient at the bottom. (C–F) Genes differentially expressed between MP tumors and Ptch1 tumors were subjected to NextBio analysis, to identify Biogroups and Studies that contain similar genes. Representative Biogroups (C–D, E) and Studies (F) are shown. Venn diagrams show the number of common and unique genes in both sets. Bars on the right show the significance of overlap between gene subsets (the scale of the bar is measured in −log(p-value), so taller the bar, the higher the significance of the gene overlap). Whereas each Biogroup is represented by a single list of genes, signature genes from Studies have two lists, one for up-regulated and one for down-regulated genes. Thus, Biogroup comparisons consist of just two graphs, whereas comparisons to Studies consist of four graphs. See also Tables S2–6.
Figure 8
Figure 8. Growth of MP tumor cells is inhibited by antagonists of PI3K/mTOR signaling
(A) Effects of inhibitors on short-term proliferation. MP tumor cells were cultured in serum-free media containing no additive (ø), vehicle (DMSO), cyclopamine (0.1, 1, 2.5 μM), 10058-F4 (10, 25, 100 μM), BEZ-235 (0.2, 1, 5 μM), BKM-120 (0.2, 1, 5μM), or RAD-001 (0.2, 1, 5μM). For each inhibitor, columns are ordered from lowest to highest concentration. After 48h, cells were pulsed with 3H-Td and cultured overnight before being assayed for 3H-Td incorporation. Data represent means of triplicate samples ± SEM. (B) Effects on long-term growth. Tumor cells were cultured for 3, 7 or 14 days in the presence of different doses of inhibitors and cell number was counted at the indicated time points. (C) Effects on PI3K signaling. Tumor cells were treated with DMSO, BEZ-235 (5uM, 1uM), BKM-120 (5uM, 1uM), RAD-001(5uM, 1uM) or cyclopamine (2.5uM, 1uM) for 3 hr. Cells were lysed and protein was analyzed for phosphorylation of AKT and S6 (pAKT and pS6) or for GAPDH by Western blotting. (D) Effects on tumor growth in vivo. 500 MP tumor cells were re-transplanted into naïve NSG mice. After 7 days, mice were imaged for luciferase activity and separated into two groups randomly. Mice in Group 1 were treated with vehicle (0.5% methyl-cellulose) and those in Group 2 were treated with BKM-120 (30mg/kg per day) by oral gavage until they developed symptoms. BMK-120 treatment significantly prolonged survival compared to vehicle (p=0.001). See also Figure S6.
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