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

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. Gene espression profiles of Ptch1+/- MBs with or without p53 mutation were compared to identify gene signatures reminiscent of specific molecular subtypes of MBs