mTORC1 promotes malignant large cell/anaplastic histology and is a targetable vulnerability in SHH-TP53 mutant medulloblastoma
Valentina Conti, Manuela Cominelli, Valentina Pieri, Alberto L. Gallotti, Ilaria Pagano, Matteo Zanella, Stefania Mazzoleni, Flavia Pivetta, Monica Patanè, Giulia M. Scotti, Ignazio S. Piras, Bianca Pollo, Andrea Falini, Alessio Zippo, Antonella Castellano, Roberta Maestro, Pietro L. Poliani, Rossella Galli
Valentina Conti, Manuela Cominelli, Valentina Pieri, Alberto L. Gallotti, Ilaria Pagano, Matteo Zanella, Stefania Mazzoleni, Flavia Pivetta, Monica Patanè, Giulia M. Scotti, Ignazio S. Piras, Bianca Pollo, Andrea Falini, Alessio Zippo, Antonella Castellano, Roberta Maestro, Pietro L. Poliani, Rossella Galli
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Research Article Neuroscience Oncology

mTORC1 promotes malignant large cell/anaplastic histology and is a targetable vulnerability in SHH-TP53 mutant medulloblastoma

Abstract

Medulloblastoma (MB), one of the most malignant brain tumors of childhood, comprises distinct molecular subgroups, with p53 mutant sonic hedgehog–activated (SHH-activated) MB patients having a very severe outcome that is associated with unfavorable histological large cell/anaplastic (LC/A) features. To identify the molecular underpinnings of this phenotype, we analyzed a large cohort of MB developing in p53-deficient Ptch+/– SHH mice that, unexpectedly, showed LC/A traits that correlated with mTORC1 hyperactivation. Mechanistically, mTORC1 hyperactivation was mediated by a decrease in the p53-dependent expression of mTORC1 negative regulator Tsc2. Ectopic mTORC1 activation in mouse MB cancer stem cells (CSCs) promoted the in vivo acquisition of LC/A features and increased malignancy; accordingly, mTORC1 inhibition in p53-mutant Ptch+/– SHH MB and CSC-derived MB resulted in reduced tumor burden and aggressiveness. Most remarkably, mTORC1 hyperactivation was detected only in p53-mutant SHH MB patient samples, and treatment with rapamycin of a human preclinical model phenocopying this subgroup decreased tumor growth and malignancy. Thus, mTORC1 may act as a specific druggable target for this subset of SHH MB, resulting in the implementation of a stringent risk stratification and in the potentially rapid translation of this precision medicine approach into the clinical setting.

Authors

Valentina Conti, Manuela Cominelli, Valentina Pieri, Alberto L. Gallotti, Ilaria Pagano, Matteo Zanella, Stefania Mazzoleni, Flavia Pivetta, Monica Patanè, Giulia M. Scotti, Ignazio S. Piras, Bianca Pollo, Andrea Falini, Alessio Zippo, Antonella Castellano, Roberta Maestro, Pietro L. Poliani, Rossella Galli

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Figure 3

The expression of the negative mTORC1 regulator Tsc2 is significantly decreased in HN MB and is regulated by p53.

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The expression of the negative mTORC1 regulator Tsc2 is significantly de...
(A) Protein level of Tsc1, Tsc2, and Tbc1d7 in autochthonous MB (Western blot). Densitometric quantification of Tsc2 in HW (n = 14) and HN MB (n = 17). (B) TSC2 expression is low in the human SHHα subtype (SHHα versus SHHβ: 1.44 × 10–3; SHHα versus SHHδ: 7.32 × 10–5; SHHα versus SHHγ: 3.3 × 10–3; Welch t test). (C) DNp53 HW CSCs show decreased Tsc2 and enhanced pS6 levels. (D) Tumors from DNp53 HW CSCs show high expression of the functionally inactive WT p53 (nuclear staining, brown), LC/A histology, and pS6 hyperactivation (cytoplasmic staining, blue). Scale bars: 50 μm. (E) LC/A features are found in HW MB with spontaneous p53 mutations (nuclear staining, brown) and correlate with the presence of pS6-IR cells that often colabeled with p53. Scale bars: 50 μm; insets: 10 μm. (F) Quantification of pS6 staining indicates that mTORC1 is hyperactivated in p53 mutant HW MB. (G) Tsc2 levels in p53 mutant HW MB are significantly lower than in HW MB, while pS6 levels are increased, as in HN MB. (H) Anaplastic features and p53-IR cells are detected in tumors derived from a p53 mutant HW CSC line (L68). Many pS6-IR tumor cells are p53-IR. Scale bars: 50 μm; insets: 10 μm. (I) Tsc2 levels in L68 MB are significantly lower than in HW MB, with pS6 levels being increased. Quantitative data are represented as a box-and-whisker plot, with bounds from 25th to 75th percentile, median line, and whiskers ranging from minimum to maximum values. Student’s t test, unpaired (A), and 1-way ANOVA followed by Tukey’s multiple comparison test (F). ***P < 0.005; ****P < 0.001.