Gliomas are the most common primary central nervous system tumor, and the leading cause of cancer-related death in children, but the molecular mechanisms responsible for the development and progression of these tumors are far from being completely understood. The identification and validation of genes that cause disease is an essential first step in the drug discovery and development process. Activating mutations of BRAF have been identified in a significant number of human cancers and several recent studies have detected these lesions in both pediatric and adult gliomas of varying grades;however, the role of BRAF activation in glioma initiation, progression, and maintenance has not been validated. With the effective use of in vivo models, expensive drug-discovery experiments can be focused on the drug targets that are most likely to lead to effective therapeutics. Once a gene target or pathway is identified, the next step is to demonstrate that it plays a critical role in disease initiation, maintenance, or both. We performed a preliminary assessment of the role of activated BRAF in glioma development in vivo. We found that while BRAFV600E was not tumorigenic on its own, cooperation with cyclin-dependent kinase inhibitor 2a (Cdkn2a;Ink4a/Arf) loss or AKT activation leads to the formation of malignant gliomas that is context dependent. Based on these data, we hypothesize that BRAF or downstream signaling mediators may be relevant targets for therapy in both pediatric and adult gliomas with this alteration. Mouse reverse genetics has become a powerful approach for deciphering gene function and target validation in the natural environment of mammalian physiology and we will use this approach in combination with the tetracycline-regulated expression system to determine if BRAFV600E is required for the maintenance of gliomas in vivo in the context of Ink4a/Arf-deficiency. We will also perform mechanistic studies to determine if BRAF activation cooperates with other genetic alterations to promote gliomagenesis. Using expression profiling, a molecular signature will be generated to further define whether mutant BRAF defines a novel molecular subtype of GBM or clusters with a known glioma subtype. GBM subtypes based on differential activation of signaling pathways may not only assist with predicting response to therapy but may also allow for rational stratification of patients in clinical trials of targeted agents. Whether BRAF or relevant downstream signaling mediators can be productively targeted for therapeutic intervention in this disease has yet to be determined. Since this model can also be used to test therapeutic agents, we will evaluate the effectiveness of pharmacological inhibition of BRAFV600E in gliomas both in vitro and in vivo using pharmacological inhibitors of clinical importance. A better understanding of the biology of these gliomas will guide the development of new therapies to improve survival and reduce morbidity in these patients.
Alterations in BRAF have recently been identified in a significant percentage of both pediatric and adult brain tumors. Achieving reduced morbidity and increased survival rates for individuals with this disease requires better pre-clinical models that rapidly identify whether this genetic alteration can be productively targeted for therapeutic intervention. The experiments outlined in this proposal fulfill this need.