Activation of mTORC1/mTORC2 signaling in pediatric low-grade glioma and pilocytic astrocytoma reveals mTOR as a therapeutic target

Previous studies support a role for mitogen-activated protein kinase pathway signaling, and more recently Akt/mammalian target of rapamycin (mTOR), in pediatric low-grade glioma (PLGG), including pilocytic astrocytoma (PA). Here we further evaluate the role of the mTORC1/mTORC2 pathway in order to better direct pharmacologic blockade in these common childhood tumors. We studied 177 PLGGs and PAs using immunohistochemistry and tested the effect of mTOR blockade on 2 PLGG cell lines (Res186 and Res259) in vitro. Moderate (2) to strong (3) immunostaining was observed for pS6 in 107/177 (59) PAs and other PLGGs, while p4EBP1 was observed in 35/115 (30), pElF4G in 66/112 (59), mTOR (total) in 53/113 (47), RAPTOR (mTORC1 component) in 64/102 (63), RICTOR (mTORC2 component) in 48/101 (48), and pAkt (S473) in 63/103 (61). Complete phosphatase and tensin homolog protein loss was identified in only 7/101 (7) of cases. In PA of the optic pathways, compared with other anatomic sites, there was increased immunoreactivity for pS6, pElF4G, mTOR (total), RICTOR, and pAkt (P .05). We also observed increased pS6 (P .01), p4EBP1 (P .029), and RICTOR (P .05) in neurofibromatosis type 1 compared with sporadic tumors. Treatment of the PLGG cell lines Res186 (PA derived) and Res259 (diffuse astrocytoma derived) with the rapalog MK8669 (ridaforolimus) led to decreased mTOR pathway activation and growth. These findings suggest that the mTOR pathway is active in PLGG but varies by clinicopathologic subtype. Additionally, our data suggest that mTORC2 is differentially active in optic pathway and neurofibromatosis type 1associated gliomas. MTOR represents a potential therapeutic target in PLGG that merits further investigation.