CSF1R inhibition depletes tumor-associated macrophages and attenuates tumor progression in a mouse sonic Hedgehog-Medulloblastoma model.

The immune microenvironment of tumors can play a critical role in promoting or inhibiting tumor progression depending on the context. We present evidence that tumor-associated macrophages/microglia (TAMs) can promote tumor progression in the sonic hedgehog subgroup of medulloblastoma (SHH-MB). By combining longitudinal manganese-enhanced magnetic resonance imaging (MEMRI) and immune profiling of a sporadic mouse model of SHH-MB, we found the density of TAMs is higher in the ~50% of tumors that progress to lethal disease. Furthermore, reducing regulatory T cells or eliminating B and T cells in Rag1 mutants does not alter SHH-MB tumor progression. As TAMs are a dominant immune component in tumors and are normally dependent on colony-stimulating factor 1 receptor (CSF1R), we treated mice with a CSF1R inhibitor, PLX5622. Significantly, PLX5622 reduces a subset of TAMs, prolongs mouse survival, and reduces the volume of most tumors within 4 weeks of treatment. Moreover, concomitant with a reduction in TAMs the percentage of infiltrating cytotoxic T cells is increased, indicating a change in the tumor environment. Our studies in an immunocompetent preclinical mouse model demonstrate TAMs can have a functional role in promoting SHH-MB progression. Thus, CSF1R inhibition could have therapeutic potential for a subset of SHH-MB patients.

Monitoring of intracerebellarly-administered natural killer cells with fluorine-19 MRI.

PURPOSE: Medulloblastoma (MB) is the most common malignant brain tumor in children. Recent studies have shown the ability of natural killer (NK) cells to lyse MB cell lines in vitro, but in vivo successes remain elusive and the efficacy and fate of NK cells in vivo remain unknown. METHODS: To address these questions, we injected MB cells into the cerebellum of immunodeficient mice and examined tumor growth at various days after tumor establishment via bioluminescence imaging. NK cells were labeled with a fluorine-19 (19F) MRI probe and subsequently injected either intratumorally or contralaterally to the tumor in the cerebellum and effect on tumor growth was monitored. RESULTS: The 19F probe efficiently labeled the NK cells and exhibited little cytotoxicity. Fluorine-19 MRI confirmed the successful and accurate delivery of the labeled NK cells to the cerebellum of the mice. Administration of 19F-labeled NK cells suppressed MB growth, with the same efficacy as unlabeled cells. Immunohistochemistry confirmed the presence of NK cells within the tumor, which was associated with induction of apoptosis in tumor cells. NK cell migration to the tumor from a distal location as well as activation of apoptosis was also demonstrated by immunohstochemistry. CONCLUSIONS: Our results show that NK cells present a novel opportunity for new strategies in MB treatment. Further, 19F-labeled NK cells can suppress MB growth while enabling 19F MRI to provide imaging feedback that can facilitate study and optimization of therapeutic paradigms.

Reprogramming Medulloblastoma-Propagating Cells by a Combined Antagonism of Sonic Hedgehog and CXCR4.

The CXCR4 chemokine and Sonic Hedgehog (SHH) morphogen pathways are well-validated therapeutic targets in cancer, including medulloblastoma. However, single-agent treatments with SHH or CXCR4 antagonists have not proven efficacious in clinical trials to date. Here, we discovered that dual inhibition of the SHH and CXCR4 pathways in a murine model of SHH-subtype medulloblastoma exerts potent antitumor effects. This therapeutic synergy resulted in the suppression of tumor-propagating cell function and correlated with increased histone H3 lysine 27 trimethylation within the promoters of stem cell genes, resulting in their decreased expression. These results demonstrate that CXCR4 contributes to the epigenetic regulation of a tumor-propagating cell phenotype. Moreover, they provide a mechanistic rationale to evaluate the combination of SHH and CXCR4 inhibitors in clinical trials for the treatment of medulloblastoma, as well as other cancers driven by SHH that coexpress high levels of CXCR4. Cancer Res; 77(6); 1416-26. ©2016 AACR.

The BET bromodomain inhibitor I-BET151 acts downstream of smoothened protein to abrogate the growth of hedgehog protein-driven cancers.

Epigenetic enzymes modulate signal transduction pathways in different biological contexts. We reasoned that epigenetic regulators might modulate the Hedgehog (HH) signaling pathway, a main driver of cell proliferation in various cancers including medulloblastoma. To test this hypothesis, we performed an unbiased small-molecule screen utilizing an HH-dependent reporter cell line (Light2 cells). We incubated Light2 cells with small molecules targeting different epigenetic modulators and identified four histone deacetylase inhibitors and a bromodomain and extra terminal domain (BET) protein inhibitor (I-BET151) that attenuate HH activity. I-BET151 was also able to inhibit the expression of HH target genes in Sufu(-/-) mouse embryonic fibroblasts, in which constitutive Gli activity is activated in a Smoothened (Smo)-independent fashion, consistent with it acting downstream of Smo. Knockdown of Brd4 (which encodes one of the BET proteins) phenocopies I-BET151 treatment, suggesting that Brd4 is a regulator of the HH signaling pathway. Consistent with this suggestion, Brd4 associates with the proximal promoter region of the Gli1 locus, and does so in a manner that can be reversed by I-BET151. Importantly, I-BET151 also suppressed the HH activity-dependent growth of medulloblastoma cells, in vitro and in vivo. These studies suggest that BET protein modulation may be an attractive therapeutic strategy for attenuating the growth of HH-dependent cancers, such as medulloblastoma.

Inhibition of BRD4 attenuates tumor cell self-renewal and suppresses stem cell signaling in MYC driven medulloblastoma.

Medulloblastoma is a pediatric brain tumor with a variable prognosis due to clinical and genomic heterogeneity. Among the 4 major genomic sub-groups, patients with MYC amplified tumors have a particularly poor prognosis despite therapy with surgery, radiation and chemotherapy. Targeting the MYC oncogene has traditionally been problematic. Here we report that MYC driven medulloblastoma can be targeted by inhibition of the bromodomain protein BRD4. We show that bromodomain inhibition with JQ1 restricts c-MYC driven transcriptional programs in medulloblastoma, suppresses medulloblastoma cell growth and induces a cell cycle arrest. Importantly JQ1 suppresses stem cell associated signaling in medulloblastoma cells and inhibits medulloblastoma tumor cell self-renewal. Additionally JQ1 also promotes senescence in medulloblastoma cells by activating cell cycle kinase inhibitors and inhibiting activity of E2F1. Furthermore BRD4 inhibition displayed an anti-proliferative, pro-senescence effect in a medulloblastoma model in vivo. In clinical samples we found that transcriptional programs suppressed by JQ1 are associated with adverse risk in medulloblastoma patients. Our work indicates that BRD4 inhibition attenuates stem cell signaling in MYC driven medulloblastoma and demonstrates the feasibility BET domain inhibition as a therapeutic approach in vivo.

Optimizing the radiation therapy dose prescription for pediatric medulloblastoma: minimizing the life years lost attributable to failure to control the disease and late complication risk.

BACKGROUND: A mathematical framework is presented for simultaneously quantifying and evaluating the trade-off between tumor control and late complications for risk-based radiation therapy (RT) decision-support. To demonstrate this, we estimate life years lost (LYL) attributable to tumor recurrence, late cardiac toxicity and secondary cancers for standard-risk pediatric medulloblastoma (MB) patients and compare the effect of dose re-distribution on a common scale. METHODS: Total LYL were derived, based on the LYL attributable to radiation-induced late complications and the LYL from not controlling the primary disease. We compared the estimated LYL for three different treatments in 10 patients: 1) standard 3D conformal RT; 2) proton therapy; 3) risk-adaptive photon treatment lowering the dose to part of the craniospinal (CS) target volume situated close to critical risk organs. RESULTS: Late toxicity is important, with 0.75 LYL (95% CI 0.60-7.2 years) for standard uniform 24 Gy CS irradiation. However, recurrence risk dominates the total LYL with 14.2 years (95% CI 13.4-16.6 years). Compared to standard treatment, a risk-adapted strategy prescribing 12 Gy to the spinal volume encompassing the 1st-10th thoracic vertebrae (Th1-Th10), and 36 Gy to the remaining CS volume, estimated a LYL reduction of 0.90 years (95% CI -0.18-2.41 years). Proton therapy with 36 Gy to the whole CS volume was associated with significantly fewer LYL compared to the risk-adapted photon strategies, with a mean LYL difference of 0.50 years (95% CI 0.25-2.60 years). CONCLUSIONS: Optimization of RT prescription strategies considering both late complications and the risk of recurrence, an all-cause mortality dose painting approach, was demonstrated. The risk-adapted techniques compared favorably to the standard, and although in this context, the gain is small compared to estimated uncertainty, this study demonstrates a framework for all-cause mortality risk estimation, rather than evaluates direct clinical applicability of risk-adapted strategies.

Long-term outcomes and role of chemotherapy in adults with newly diagnosed medulloblastoma.

OBJECTIVE: To assess the survival and role of adjuvant chemotherapy in adult medulloblastoma. METHODS: We reviewed outcomes of 66 patients (aged 18 y or more; median age, 33 y) with medulloblastoma. Forty-four (67%) patients had M0 disease, 9 had M1-M4, and 13 had MX. Thirty-one patients each for whom risk stratification was available were classified as high risk or standard risk. Fifty-six patients had histologic results: classic histology was the most common (n=46 [84%]), followed by desmoplastic (n=9), and large cell/anaplastic (n=1). Overall survival (OS) and progression-free survival (PFS) were estimated with Kaplan-Meier curves and log-rank tests. Cox regression analysis was used to compare recurrences. RESULTS: Median follow-up was 6.7 years. The estimated 5-year OS and PFS were 74% and 59%, respectively. High-risk versus standard-risk classification was associated with worse OS (61% vs. 86%; P=0.03) and recurrence (hazard ratio, 2.56; P=0.05) and a trend for worse PFS (49% vs. 69%; P=0.13). Gross total resection was associated with improved OS (P=0.03) and a trend toward improved PFS (P=0.09). No chemotherapy benefit could be demonstrated for the group as a whole. For high-risk patients with classic histology (n=25), chemotherapy was associated with a trend for improvement in 5-year PFS from 36% to 71% (P=0.10) and in 5-year OS from 49% to 100% (P=0.08). CONCLUSIONS: In adult patients with medulloblastoma, the extent of resection and risk classification predicts the outcome. These results suggest a chemotherapy benefit for high-risk patients with classic histology.

Expression of miR-124 inhibits growth of medulloblastoma cells.

Medulloblastoma is the most common malignant brain tumor in children, and a substantial number of patients die as a result of tumor progression. Overexpression of CDK6 is present in approximately one-third of medulloblastomas and is an independent poor prognostic marker for this disease. MicroRNA (miR)-124 inhibits expression of CDK6 and prevents proliferation of glioblastoma and medulloblastoma cells in vitro. We examined the effects of miR-124 overexpression on medulloblastoma cells both in vitro and in vivo and compared cell lines that have low and high CDK6 expression. MiR-124 overexpression inhibits the proliferation of medulloblastoma cells, and this effect is mediated mostly through the action of miR-124 upon CDK6. We further show that induced expression of miR-124 potently inhibits growth of medulloblastoma xenograft tumors in rodents. Further testing of miR-124 will help define the ultimate therapeutic potential of preclinical models of medulloblastoma in conjunction with various delivery strategies for treatment.

Pharmacological activation of the p53 pathway by nutlin-3 exerts anti-tumoral effects in medulloblastomas.

Medulloblastomas account for 20% of pediatric brain tumors. With an overall survival of 40%-70%, their treatment is still a challenge. The majority of medulloblastomas lack p53 mutations, but even in cancers retaining wild-type p53, the tumor surveillance function of p53 is inhibited by the oncoprotein MDM2. Deregulation of the MDM2/p53 balance leads to malignant transformation. Here, we analyzed MDM2 mRNA and protein expression in primary medulloblastomas and normal cerebellum and assessed the mutational status of p53 and MDM2 expression in 6 medulloblastoma cell lines. MDM2 expression was elevated in medulloblastomas, compared with cerebellum. Four of 6 medulloblastoma cell lines expressed wild-type p53 and high levels of MDM2. The tumor-promoting p53-MDM2 interaction can be inhibited by the small molecule, nutlin-3, restoring p53 function. Targeting the p53-MDM2 axis using nutlin-3 significantly reduced cell viability and induced either cell cycle arrest or apoptosis and expression of the p53 target gene p21 in these 4 cell lines. In contrast, DAOY and UW-228 cells harboring TP53 mutations were almost unaffected by nutlin-3 treatment. MDM2 knockdown in medulloblastoma cells by siRNA mimicked nutlin-3 treatment, whereas expression of dominant negative p53 abrogated nutlin-3 effects. Oral nutlin-3 treatment of mice with established medulloblastoma xenografts inhibited tumor growth and significantly increased survival. Thus, nutlin-3 reduced medulloblastoma cell viability in vitro and in vivo by re-activating p53 function. We suggest that inhibition of the MDM2-p53 interaction with nutlin-3 is a promising therapeutic option for medulloblastomas with functional p53 that should be further evaluated in clinical trials.

Full p53 transcriptional activation potential is dispensable for tumor suppression in diverse lineages.

Over half of all human cancers, of a wide variety of types, sustain mutations in the p53 tumor suppressor gene. Although p53 limits tumorigenesis through the induction of apoptosis or cell cycle arrest, its molecular mechanism of action in tumor suppression has been elusive. The best-characterized p53 activity in vitro is as a transcriptional activator, but the identification of numerous additional p53 biochemical activities in vitro has made it unclear which mechanism accounts for tumor suppression. Here, we assess the importance of transcriptional activation for p53 tumor suppression function in vivo in several tissues, using a knock-in mouse strain expressing a p53 mutant compromised for transcriptional activation, p53(25,26). p53(25,26) is severely impaired for the transactivation of numerous classical p53 target genes, including p21, Noxa, and Puma, but it retains the ability to activate a small subset of p53 target genes, including Bax. Surprisingly, p53(25,26) can nonetheless suppress tumor growth in cancers derived from the epithelial, mesenchymal, central nervous system, and lymphoid lineages. Therefore, full transactivation of most p53 target genes is dispensable for p53 tumor suppressor function in a range of tissue types. In contrast, a transcriptional activation mutant that is completely defective for transactivation, p53(25,26,53,54), fails to suppress tumor development. These findings demonstrate that transcriptional activation is indeed broadly critical for p53 tumor suppressor function, although this requirement reflects the limited transcriptional activity observed with p53(25,26) rather than robust transactivation of a full complement of p53 target genes.

Inhibition of neurotrophin receptor p75 intramembran proteolysis by gamma-secretase inhibitor reduces medulloblastoma spinal metastasis.

Medulloblastoma (MB) is the most devastating and common pediatric brain tumor. Tumor cells invading into surrounding tissue and disseminating through cerebrospinal fluid make treatment extremely difficult. Identifying the mechanisms of MB cells is therefore imperative for the development of novel treatments. A research group demonstrated recently that the multifunctional signaling protein neurotrophin receptor p75(NTR) is a central regulator for glioma invasion. γ-secretase mediated processing of the p75(NTR) is a major contributor to the highly invasive nature of malignant gliomas. In this study we examine the p75(NTR) expression and processing in medulloblastoma cells. Results show that p75(NTR) is a critical regulator of medulloblastoma spinal metastasis. γ-secretase inhibitor, which blocks p75(NTR) proteolytic processing, significantly abrogates p75(NTR) induced medulloblastoma migration and invasion in vitro and in vivo. This data suggests that p75(NTR) is also an important therapeutic target for MB. γ-secretase inhibitor may be a potentially effective clinical application for the treatment of medulloblastoma spinal metastasis.

Arsenic antagonizes the Hedgehog pathway by preventing ciliary accumulation and reducing stability of the Gli2 transcriptional effector.

Aberrant Hedgehog (Hh) pathway activation has been implicated in cancers of diverse tissues and organs, and the tumor growth-inhibiting effects of pathway antagonists in animal models have stimulated efforts to develop pathway antagonists for human therapeutic purposes. These efforts have focused largely on cyclopamine derivatives or other compounds that mimic cyclopamine action in binding to and antagonizing Smoothened, a membrane transductory component. We report here that arsenicals, in contrast, antagonize the Hh pathway by targeting Gli transcriptional effectors; in the short term, arsenic blocks Hh-induced ciliary accumulation of Gli2, the primary activator of Hh-dependent transcription, and with prolonged incubation arsenic reduces steady-state levels of Gli2. Arsenicals active in Hh pathway antagonism include arsenic trioxide (ATO), a curative agent in clinical use for acute promyelocytic leukemia (APL); in our studies, ATO inhibited growth of Hh pathway-driven medulloblastoma allografts derived from Ptch+/-p53-/- mice within a range of serum levels comparable to those achieved in treatment of human APL. Arsenic thus could be tested rapidly as a therapeutic agent in malignant diseases associated with Hh pathway activation and could be particularly useful in such diseases that are inherently resistant or have acquired resistance to cyclopamine mimics.

Protective role of 17 ß-estradiol on medulloblastoma development in Patched 1 heterozygous mice.

Medulloblastoma (MB) is the most common pediatric tumor of the CNS, representing ∼20% of all childhood CNS tumors. Although in recent years many molecular mechanisms that control MB development have been clarified, the effects of biological factors such as sex on this tumor remain to be explained. Epidemiological data, in fact, indicate a significant difference in the incidence of MB between the 2 sexes, with considerably higher susceptibility of males than females. Besides this different susceptibility, female sex is also a significant favorable prognostic factor in MB, with girls having a much better outcome. Despite these literature data, there has been little investigation into estrogen influence on MB development. In our study, we evaluated how hormone deficiency resulting from ovariectomy and hormone replacement influences the development of early and advanced MB stages in Patched1 heterozygous mice, a well-characterized mouse model of radiation-induced MB. Susceptibility to MB development was significantly increased in ovariectomized Ptch1(+/-) females and restored to levels observed in control mice after estrogen replacement. We next investigated the molecular mechanisms by which estrogen might influence tumor progression and show that ERß, but not ERα, is involved in modulation of MB development by estrogens. Finally, our study shows that a functional interaction between estrogen- and IGF-I-mediated pathways may be responsible for the effects observed.

Deletion of Atoh1 disrupts Sonic Hedgehog signaling in the developing cerebellum and prevents medulloblastoma.

Granule neuron precursors (GNPs) are the most actively proliferating cells in the postnatal nervous system, and mutations in pathways that control the GNP cell cycle can result in medulloblastoma. The transcription factor Atoh1 has been suspected to contribute to GNP proliferation, but its role in normal and neoplastic postnatal cerebellar development remains unexplored. We show that Atoh1 regulates the signal transduction pathway of Sonic Hedgehog, an extracellular factor that is essential for GNP proliferation, and demonstrate that deletion of Atoh1 prevents cerebellar neoplasia in a mouse model of medulloblastoma. Our data shed light on the function of Atoh1 in postnatal cerebellar development and identify a new mechanism that can be targeted to regulate medulloblastoma formation.

Inhibition of Hsp90 via 17-DMAG induces apoptosis in a p53-dependent manner to prevent medulloblastoma.

Elevated expression of HSP90 is observed in many tumor types and is associated with a limited clinical response. Targeting HSP90 using inhibitors such as 17-DMAG (17-desmethoxy-17-N,N-dimethylaminoethylaminogeldanamycin) has shown limited therapeutic success. HSP90 regulates the function of several proteins implicated in tumorigenesis although the precise mechanism through which 17-DMAG regulates tumor cell survival remains unclear. We observed a requirement for p53 in mediating 17-DMAG-induced cell death. The sensitivity of primary mouse embryonic fibroblasts and tumor cells to 17-DMAG-induced apoptosis depended on the p53 status. Wild-type MEFs underwent 17-DMAG-induced caspase-dependent cell death, whilst those lacking p53 failed to do so. Interestingly p53-dependent cell death occurred independently of Atm or Arf. Primary tumor cells derived from two models of murine medulloblastoma (Ptch1(+/-);Ink4c(-/-) and p53(FL/FL);Nestin-Cre(+); Ink4c(-/-)) that retain and lack p53 function, respectively, displayed a dependence on functional p53 to engage 17-DMAG-induced apoptosis. Strikingly, 17-DMAG treatment in an allograft model of Ptch1(+/-);Ink4c(-/-) but not p53(FL/FL);Nestin-Cre(+); Ink4c(-/-) tumor cells prevented tumor growth in vivo. Our data suggest that p53 status is a likely predictor of the sensitivity of tumors to 17-DMAG.

Two tumor suppressors, p27Kip1 and patched-1, collaborate to prevent medulloblastoma.

Two cyclin-dependent kinase inhibitors, p18(Ink4c) and p27(Kip1), are required for proper cerebellar development. Loss of either of these proteins conferred a proliferative advantage to granule neuron progenitors, although inactivation of Kip1 exerted a greater effect. Mice heterozygous for Patched-1 (Ptc1+/-) that are either heterozygous or nullizygous for Kip1 developed medulloblastoma rapidly and with high penetrance. All tumors from Ptc1+/-;Kip1+/- or Ptc1+/-;Kip1-/- mice failed to express the wild-type Ptc1 allele, consistent with its role as a canonical "two-hit" tumor suppressor. In contrast, expression of the wild-type p27(Kip1) protein was invariably maintained in medulloblastomas arising in Ptc1+/-;Kip1+/- mice, indicating that Kip1 is haploinsufficient for tumor suppression. Although medulloblastomas occurring in Ptc1+/- mice were histopathologically heterogeneous and contained intermixed regions of both rapidly proliferating and nondividing more differentiated cells, tumors that also lacked Kip1 were uniformly less differentiated, more highly proliferative, and invasive. Molecular analysis showed that the latter medulloblastomas exhibited constitutive activation of the Sonic hedgehog signaling pathway without loss of functional p53. Apart from gains or losses of single chromosomes, with gain of chromosome 6 being the most frequent, no other chromosomal anomalies were identified by spectral karyotyping, and half of the medulloblastomas so examined retained a normal karyotype. In this respect, this mouse medulloblastoma model recapitulates the vast majority of human medulloblastomas that do not sustain TP53 mutations and are not aneuploid.

Sonic Hedgehog signaling impairs ionizing radiation-induced checkpoint activation and induces genomic instability.

The Sonic Hedgehog (Shh) pathway plays important roles in embryogenesis, stem cell maintenance, tissue repair, and tumorigenesis. Haploinsufficiency of Patched-1, a gene that encodes a repressor of the Shh pathway, dysregulates the Shh pathway and increases genomic instability and the development of spontaneous and ionizing radiation (IR)-induced tumors by an unknown mechanism. Here we show that Ptc1(+/-) mice have a defect in the IR-induced activation of the ATR-Chk1 checkpoint signaling pathway. Likewise, transient expression of Gli1, a downstream target of Shh signaling, disrupts Chk1 activation in human cells by preventing the interaction of Chk1 with Claspin, a Chk1 adaptor protein that is required for Chk1 activation. These results suggest that inappropriate Shh pathway activation promotes tumorigenesis by disabling a key signaling pathway that helps maintain genomic stability and inhibits tumorigenesis.

PARP-1 cooperates with Ptc1 to suppress medulloblastoma and basal cell carcinoma.

The patched (Ptc1) protein is a negative regulator of sonic hedgehog signaling, a genetic pathway whose perturbation causes developmental defects and predisposition to specific malignant tumors. Humans and mice with mutated Ptc1 are prone to medulloblastoma and basal cell carcinoma (BCC), both tumors showing dependence on radiation damage for rapid onset and high penetrance. Poly(ADP-ribose) polymerase (PARP-1) is a nuclear enzyme that plays a multifunctional role in DNA damage signaling and repair. In healthy and fertile PARP-1-null mice, radiation exposure reveals an extreme sensitivity and a high genomic instability. To test for interactions between PARP-1 and sonic hedgehog signaling, PARP-1-null mice were crossed to Ptc1 heterozygous mice. PARP-1 deletion further accelerated medulloblastoma development in irradiated Ptc1(+/-) mice, showing that PARP-1 inactivation sensitizes cerebellar cells to radiation tumorigenic effects. In addition to increased formation and slowed down kinetics of disappearance of gamma-H2AX foci, we observed increased apoptosis in PARP-1-deficient granule cell progenitors after irradiation. Double-mutant mice were also strikingly more susceptible to BCC, with >50% of animals developing multiple, large, infiltrative tumors within 30 weeks of age. The results provide genetic evidence that PARP-1 function suppresses sonic hedgehog pathway-associated tumors arising in response to environmental stress.

Post-transcriptional down-regulation of Atoh1/Math1 by bone morphogenic proteins suppresses medulloblastoma development.

Bone morphogenic proteins 2 and 4 (BMP2 and BMP4) inhibit proliferation and induce differentiation of cerebellar granule neuron progenitors (GNPs) and primary GNP-like medulloblastoma (MB) cells. This occurs through rapid proteasome-mediated degradation of Math1 (Atoh1), a transcription factor expressed in proliferating GNPs. Ectopic expression of Atoh1, but not of Sonic hedgehog (Shh)-regulated Gli1 or Mycn, cancels these BMP-mediated effects and restores Shh-dependent proliferation of GNPs and MB cells in vitro and in vivo. Genes regulating the BMP signaling pathway are down-regulated in mouse MBs. Thus, BMPs are potent inhibitors of MB and should be considered as novel therapeutic agents.

Inhibition of medulloblastoma tumorigenesis by the antiproliferative and pro-differentiative gene PC3.

Medulloblastoma, the most common brain tumor in childhood, appears to originate from cerebellar granule cell precursors (GCPs), located in the external granular layer (EGL) of the cerebellum. The antiproliferative gene PC3 (Tis21/BTG2) promotes cerebellar neurogenesis by inducing GCPs to shift from proliferation to differentiation. To assess whether PC3 can prevent the neoplastic transformation of GCPs and medulloblastoma development, we crossed transgenic mice conditionally expressing PC3 (TgPC3) in GCPs with Patched1 heterozygous mice (Ptc(+/-)), a model of medulloblastoma pathogenesis characterized by hyperactivation of the Sonic Hedgehog pathway. Perinatal up-regulation of PC3 in Ptc(+/-)/TgPC3 mice results in a decrease of medulloblastoma incidence of approximately 40% and in a marked reduction of preneoplastic abnormalities, such as hyperplastic EGL areas and lesions. Moreover, overexpression of cyclin D1, hyperproliferation, and defective differentiation--observed in Ptc(+/-) GCPs--are restored to normality in Ptc(+/-)/TgPC3 mice. The PC3-mediated inhibition of cyclin D1 expression correlates with recruitment of PC3 to the cyclin D1 promoter, which is accompanied by histone deacetylation. Remarkably, down-regulation of PC3 is observed in preneoplastic lesions, as well as in human and murine medulloblastomas. As a whole, this indicates that PC3 may prevent medulloblastoma development by controlling cell cycle and promoting differentiation of GCPs.