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

Pediatric low-grade gliomas (PLGGs) represent the most common group of tumors involving the central nervous system in children.1 Although they are characterized by slow growth and a low frequency of malignant progression, morbidity may be considerable, particularly when located in anatomic regions not amenable to surgical resection. Traditional therapies, such as irradiation and conventional chemotherapy, are not curative and often fail in preventing clinical progression. The most common low-grade glioma in children is pilocytic astrocytoma (PA), a World Health Organization grade I tumor with low proliferative potential. Until recently, little was known about the biology of PLGG. In PA arising in patients with neurofibromatosis type 1 (NF1), there is inactivation of both NF1 gene copies,2 leading to hyperactive Ras and downstream mitogen-activated protein kinase (MAPK) pathway activation. More recently, independent high resolution genomic studies have identified a tandem duplication of BRAF at 7q34 in the majority (53%–72%) of PAs leading to an oncogenic fusion gene.3–6 In a smaller proportion of cases, BRAF activating point mutations and rearrangements of other BRAF family members (eg, RAF1) may also be encountered.7 These alterations seem to vary by anatomic site, with the highest frequency of BRAF duplications occurring in PA of the cerebellum.8 However, a recent study using archival material from tumors of the optic nerve proper showed that the majority of tumors (73%) also had duplications of the BRAF kinase domain, with the remainder being associated with NF1.9 These genetic aberrations share an increased MAPK/extracellular signal-regulated kinase pathway activation, which is active in the majority of PAs. More recent whole genome sequencing efforts of PLGG and PA have found that these neoplasms are characterized in many cases by single somatic mutations, with pediatric diffuse low-grade gliomas containing alterations in FGFR1, MYB, or MYBL1.10,11 The phosphatidylinositol-3 kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) pathway is active in a variety of cancers, and effective pharmacologic inhibitors such as rapamycin and analogs (rapalogs) are widely available. The importance of the PI3K/Akt/mTOR signaling axis has been highlighted in diffuse and high-grade gliomas.12 One of the key downstream mediators of PI3K/Akt activation is mTOR, which exists in 2 complexes: mTORC1 and mTORC2, which possess distinct regulatory inputs and outputs within the signaling network (Fig. 1). In the mTORC1 complex, mTOR interacts with proline rich Akt substrate, 40 kDa; regulatory associated protein of mTOR, complex 1 (RAPTOR); and mammalian lethal with SEC13 protein 8 (mLST8)/G protein beta subunit-like (GBL), and upon activation increases protein translation, cell growth, and survival. The function of the mTORC2 complex (mTOR, rapamycin-insensitive companion of mTOR [RICTOR], mSin1, Protor [protein observed with Rictor], and mLST8) is less well known, but it appears to regulate metabolism and survival through activation of Akt, as well as mTORC1, through protein kinase C, and also plays a role in cytoskeletal organization.13 Fig. 1. mTOR pathway signaling cascade. The mTOR pathway exists as part of either of 2 multiprotein complexes (mTORC1 and mTORC2), which are integral components of a signaling cascade that leads to increased cell growth and survival. Of interest, recent studies have highlighted a role for mTOR activation in NF1-associated PA and murine models of NF1–optic nerve glioma.14 Recent data also suggest that it may mediate phenotypic variations in NF1-associated low-grade gliomas,15 with its activation more frequent in the rare PA with anaplastic features.16 Furthermore, recent evidence suggests that mTOR also mediates the proliferative activity of cerebellar murine stem cells expressing BRAF fusions.17 Of therapeutic relevance, mTOR inhibition has proven to be of great clinical benefit and is well tolerated in pediatric patients with subependymal giant cell astrocytoma, another subtype of PLGG.18 A possible role for mTOR signaling in the biology of the most common PLGG, that is, sporadic PA, is of interest, given the abundance of pharmacologic data with mTOR inhibitors in a variety of tumor types. Therefore, we decided to investigate the effects of MK8669 (ridaforolimus) in PLGG-derived cell lines.