The intermediate-activity L597V BRAF mutant acts as an epistatic modifier of oncogenic RAS by enhancing signaling through the RAF/MEK/ERK pathway

Abstract

L597VBRAF mutations are acquired somatically in human cancer samples and are frequently coincident with RAS mutations. Germline L597VBRAF mutations are also found in several autosomal dominant developmental conditions known as RASopathies, raising the important question of how the same mutation can contribute to both pathologies. Using a conditional knock-in mouse model, we show that endogenous expression of L597VBraf leads to approximately twofold elevated Braf kinase activity and weak activation of the Mek/Erk pathway. This is associated with induction of RASopathy hallmarks including cardiac abnormalities and facial dysmorphia but is not sufficient for tumor formation. We combined L597VBraf with G12DKras and found that L597VBraf modified G12DKras oncogenesis such that fibroblast transformation and lung tumor development were more reminiscent of that driven by the high-activity V600EBraf mutant. Mek/Erk activation levels were comparable with those driven by V600EBraf in the double-mutant cells, and the gene expression signature was more similar to that induced by V600EBraf than G12DKras. However, unlike V600EBraf, Mek/Erk pathway activation was mediated by both Craf and Braf, and ATP-competitive RAF inhibitors induced paradoxical Mek/ Erk pathway activation. Our data show that weak activation of the Mek/Erk pathway underpins RASopathies, but in cancerL597VBraf epistatically modifies the transforming effects of driver oncogenes. © 2012 by Cold Spring Harbor Laboratory Press.