Insights into glioma initiation and maintenance through the analysis of mutant BRAF and HBEGF

Gliomas are the most common primary brain tumor and are associated with a poor prognosis. Understanding the essential factors in the formation of these tumors is critical to improve the treatment of this disease. We report a thorough investigation of two potential drivers in the formation and maintenance of low-grade and high-grade gliomas â€" the kinase domain of BRAF and activated HBEGF. BRAF fusions have been observed in a majority of sporadic pilocytic astrocytomas, a low-grade glioma. In each fusion, the autoinhibitory domain of BRAF is lost, which results in activation via the retained kinase domain (BRAF-KD). Somatic cell gene transfer of the BRAF-KD alone did not cause tumors to develop; however, low-grade gliomas were detected in mice when combined with Ink4a/Arf loss. Pharmacologic inhibition of MEK and PI3K inhibited cell growth and induced apoptosis in astrocytes expressing BRAF-KD. Our findings demonstrate that the BRAF-KD can cooperate with Ink4a/Arf loss to drive the development of gliomas. Heparin-binding epidermal growth factor (EGF)-like growth factor (HBEGF) is a ligand for the epidermal growth factor receptor (EGFR), one of the most commonly amplified receptor tyrosine kinases (RTK) in glioblastoma (GBM), a high-grade glioma. While HBEGF has been found to be expressed in a subset of malignant gliomas, its sufficiency for glioma initiation has not been evaluated. In this study, we demonstrate thatHBEGF can initiate GBM in mice in the context of Ink4a/Arf loss and Pten loss, and that these tumors are similar to the classical GBM subtype observed in patients. Isogenic astrocytes from these mice showed activation not only of Egfr but also the RTK Axl in response to HBEGF stimulation. Deletion of either RTK decreased the tumorigenic properties of HBEGF transformed cells. Silencing of HBEGF in vivo resulted in significantly increased survival suggesting that HBEGF may be a clinically relevant target. The work outlined in this dissertation investigates the role of mutant BRAF and activated HBEGF in the initiation and maintenance of gliomas using a somatic cell gene transfer mouse model. These findings suggest that these proteins may be therapeutic targets in gliomas and provide valuable insight for the improvement of treatment.