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MTORC1 promotes malignant large cell/anaplastic histology and is a therapeutically targetable vulnerability in SHH-TP53 mutant medulloblastoma

ABSTRACT: Distinct molecular subgroups and subtypes have been identified for medulloblastoma (MB), one of the most malignant brain tumors of childhood, with the sonic hedgehog (SHH)-activated subgroup of patients with mutations in p53 having a very severe outcome, associated with unfavorable histological large cell/anaplastic (LC/A) features. Aim of this study was to determine whether hyperactivation of mechanistic Target Of Rapamycin Complex 1 (mTORC1) pathway, whose role in molecular subtypes of MB is currently undefined, underlies the acquisition of LC/A histology in this subtype. MBs developing in p53-deficient Ptch+/- SHH mice show LC/A features that correlate with mTORC1 hyperactivation, which, mechanistically, is mediated by a decrease in the p53-dependent expression of the negative mTORC1 regulator Tsc2. Ectopic mTORC1 activation in mouse MB cancer stem cells (CSCs) promotes the acquisition of the LC/A phenotype. Furthermore, mTORC1 inhibition in both autochthonous p53-mutant SHH MBs and CSC-derived LC/A MBs results in reduced tumor burden and enhanced survival. Most remarkably, mTORC1 hyperactivation in tumor cells is detected only in human p53-mutant SHH MB samples and treatment with rapamycin of a human preclinical model phenocopying this subgroup decreases tumor growth and malignancy. As such, mTORC1 may act as a new and specific druggable target for this subset of SHH MB, thus resulting in the implementation of a more stringent MB risk stratification and in the potentially rapid translation of this precision medicine approach into the clinical setting.

ORGANISM(S): Mus musculus

PROVIDER: GSE183901 | GEO | 2021/10/21

REPOSITORIES: GEO

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Project description:Medulloblastomas (MBs) are cerebellar tumors that can be classified into molecularly distinct subgroups that differ in pathology and prognosis. The mechanisms that underlie subgroup specification are largely unknown. While human SHH MBs express MYCN, Group3 (G3) MBs are associated with c-MYC (MYC) overexpression and often show metastasis that confers a poor prognosis. Although MYC proteins are thought to be functionally exchangeable, ectopic expression of Myc or N-myc in Trp53-/-;Cdkn2c-/- cerebellar granule neuron progenitors (GNPs) induces G3 and SHH MBs, respectively, demonstrating that each Myc protein has distinct biological properties. We now show that Myc and N-myc differ in their affinity to Miz1 and that Myc, but not N-myc, effectively recruits Miz1 to its target sites on chromatin. The interaction of Myc with Miz1 is required for the genesis of G3 MB. Myc suppresses ciliogenesis and âreprogramsâ the transcriptome of SHH-dependent GNPs to stem-like cells by repressing genes highly expressed in SHH MB via Miz1. Consistently, target genes of Myc/Miz1 are repressed in human G3 MBs but not in other MB subgroups. Collectively, the data show that the interaction of Myc with Miz1 is a defining hallmark of G3 MB development. In this study, we show that Myc and N-myc differ in their affinity for Miz1, and Myc/Miz1 interaction is required for Group3 medulloblastoma (MB). The gene expression profiles of these tumors were compared to our previously published Group3 MB model as well as SHH model of MB (Kawachi et al., 2012, Cancer Cell). Cerebellar granule neuron progenitors (GNPs) [dka220-222] from postnatal (P) 6 Math1-GFP;Trp53-/-;Cdkn2c-/-. For SHH medulloblastomas, [dka204-206] and [blm015-017 or dka050-dka051], spontaneous medulloblastomas from Cdkn2c-/-;Trp53Fl/Fl;Nestin-Cre (Kawachi et al., 2012, Cancer Cell) and Ptch1+/-;Cdkn2c-/- (Uziel et al., 2005, Genes Dev) were used, respectively. Myc- [dka201-203] and MycV394D (termed MycVD thereafter)- [bvo017-bvo023] tumors were from Trp53-/-;Cdkn2c-/- cerebella of P6-P7 pups. Myc/ÎPOZ tumors [bvo002-bvo006] were obtained from Trp5Fl/Fl;Miz1ÎPOZ/POZ;Nestin-Cre cerebella of P6-P7 pups.

Project description:Medulloblastomas (MBs) are the most common brain tumors in children. Some are thought to originate from cerebellar granule neuron progenitors (GNPs) that fail to undergo normal cell cycle exit and differentiation. Since microRNAs regulate numerous aspects of cellular physiology and development, we reasoned that alterations in miRNA expression might contribute to MB. We tested this hypothesis using two spontaneous mouse MB models with specific initiating mutations, Ink4c-/-; Ptch1+/- and Ink4c-/-; p53-/-. We found that 26 miRNAs showed increased expression and 24 miRNAs showed decreased expression in proliferating mouse GNPs and MBs relative to mature mouse cerebellum, regardless of genotype. Among the 26 overexpressed miRNAs, nine were encoded by the miR-17~92 cluster family, a group of microRNAs implicated as oncogenes in several tumor types. Analysis of human MBs demonstrated that three miR-17~92 cluster miRNAs (miR-92, miR-19a and miR-20) were also overexpressed in human MBs with a constitutively activated SHH signaling pathway, but not in other forms of the disease. To test whether the miR-17~92 cluster could promote MB formation, we enforced expression of these miRNAs in GNPs isolated from cerebella of postnatal (P) day P6 Ink4c-/-; Ptch1+/- mice. These, but not similarly engineered cells from Ink4c-/-; p53-/- mice, formed MBs in orthotopic transplants with complete penetrance. Interestingly, orthotopic mouse tumors ectopically expressing miR-17~92 lost expression of the wild-type Ptch1 allele. Our findings suggest a functional collaboration between the miR-17~92 cluster and the SHH signaling pathway in the development of MBs in mouse and man. Samples: purified CGNP-like mouse medulloblastoma cells; purified CGNPs from age day 6 (P6) mice; whole cerebellum from P6 mice; whole cerebellum from 1 month old mice. Each from three backgrounds (except tumors) - wt; Ptc+/-,Ink4c-/-; p53-/-,Ink4c-/-. Two to five biological replicates each.

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MTORC1 promotes malignant large cell/anaplastic histology and is a therapeutically targetable vulnerability in SHH-TP53 mutant medulloblastoma

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