Mutations in receptor tyrosine kinase (RTK) growth factor receptors (EGFR, PDGFR, MET and ERBB2), which result in downstream activation of the RAS/RAF/MEK/ERK mitogen-activated protein kinase (MAPK) pathway and PI(3)K/AKT pathway, are found in almost all high grade gliomas and MAPK signaling is necessary for continued glioma maintenance. In addition, BRAF is mutated in the majority of low grade gliomas and its expression and activity is significantly increased in the majority of high grade gliomas. While the importance of RTKs and RAS signaling in glioma development has been demonstrated, the role of BRAF has yet to be characterized. We evaluated the effect of activated BRAF in glioma formation using the retroviral RCAS/TVA system to transfer genes encoding activated forms of BRAF, KRas, Akt and Cre to Nestin expressing neural progenitor cells in Ink4a/Arf lox/lox mice in vivo. While expression of activated BRAF alone is not sufficient for tumorigenesis, the combination of activated BRAF and Akt or BRAF with Ink4a/Arf loss is transforming. Interestingly, activated BRAF generates gliomas with characteristics similar to activated KRas in the context of Akt but not Ink4a/Arf loss. Our studies demonstrate a role for BRAF activation and signaling in glioma development and as potential target for glioma therapy.

Summary While many genetic alterations have been identified in melanoma, the relevant molecular events that contribute to disease progression are poorly understood. Most primary human melanomas exhibit loss of expression of the CDKN2A locus in addition to activation of canonical MAPK signaling including RAS, RAF, MEK, and ERK. In this study, we used a Cdkn2a-deficient mouse melanocyte cell line to screen for secondary genetic events in melanoma tumor progression. Upon investigation, intrachromosomal gene amplification of Met, a receptor tyrosine kinase implicated in melanoma progression, was identified in Cdkn2a-deficient tumors. RNA interference (RNAi) targeting Met in these tumor cells resulted in a significant delay in tumor growth in vivo compared with the control cells. MET expression is rarely detected in primary human melanoma but is frequently observed in metastatic disease. This study validates a role for Met activation in melanoma tumor progression in the context of Cdkn2a-deficiency.