Exploiting neurofibromatosis type 1-associated cancer models to identify novel therapeutic targets

Neurofibromatosis type 1 (NF1) is an autosomal dominant tumor-predisposition syndrome caused by loss-of-function mutations in the NF1 tumor-suppressor gene. NF1 is characterized by a range of features, though the hallmark feature of the disease is the formation of benign peripheral nerve sheath tumors, which are capable of progressing to malignancy. Neurofibromin, the NF1 -encoded protein product, has been well characterized as a RasGAP--a family of proteins that negatively regulates Ras signaling. Accordingly, a substantial body of research has shown that uncontrolled Ras activation underlies neurofibromatosis type 1 and its associated tumors. Until recently, the specific signaling pathways downstream of Ras responsible for NF1 -dependent tumorigenicity were unclear. Here, we show that neurofibromin regulates the TSC2/mTOR signaling pathway in an active, Ras-dependent manner. Specifically, we describe a signaling pathway that is initiated by aberrant Ras activation and ultimately results in uncontrolled mTOR signaling via PI3K, AKT and TSC2/tuberin. Critically, we also show that this mTOR hyperactivation occurs in primary and patient-derived tumors cells and that NF1 -deficient malignant tumor cell lines are exquisitely sensitive to the mTOR inhibitor rapamycin, implying that activation of the mTOR pathway is a critical step in NF1 -dependent tumorigenesis. Further, we present data suggesting that mTOR inhibitors can be used as an effective therapy for NF1 -associated malignant peripheral nerve sheath tumors in a pre-clinical, genetically engineered animal model. Pre-clinical long term efficacy studies using this model also reveal the emergence of rapamycin-refractory lesions, an observation that may prove crucial to effective long-term treatment of this disease in the clinic. Finally, we show that the Ras/Raf/MEK/ERK/RSK signaling cascade plays a role in mTOR regulation following loss of Nf1 and/or activation of specific growth factor receptors, and suggest a novel role for RSK2 as a negative regulator of mTOR signaling. Together, these data highlight the potential of rapamycin as a therapy for neurofibromatosis type 1-associated malignant peripheral nerve sheath tumors as well as the feasibility of using genetic animal models as a pre-clinical testing ground for novel therapeutic strategies.