LAUSR

Approximately two hundred mutant BRAF alleles have been identified in human tumors. Physiologic activation of RAF isoforms requires RAS-dependent induction of their dimerization. Activating BRAF mutants cause ERK dependent feedback inhibition of RAS.GTP and are RAS independent. As we have shown previously, they signal either as active monomers or RAS-independent constitutively active dimers. Here, we characterized a third class of BRAF mutants—those that have impaired kinase activity or are kinase dead. These class 3 BRAF mutants are sensitive to ERK-mediated feedback and they function in a RAS-dependent manner. In tumors, they bind more tightly to active RAS, thus enhancing their heterodimerization with CRAF. This is associated with the amplification of RAS-RAF-MEK-ERK signaling. Since these mutants are sensitive to ERK-dependent feedback inhibition of RAS, their enhancement of ERK signaling in tumors requires concurrent dysregulation of RAS activation. Thus, melanomas with Class 3 mutations usually harbor coexistent RAS mutation or NF1 mutants/deletion, whereas receptor tyrosine kinase signaling is activated in lung and colorectal cancers with these mutants. Our model suggests that these tumors will be sensitive to the inhibition of RAS activation. Currently, no direct inhibitors of RAS activation are available. However, in support of this idea, inhibitors of activated RTK signaling in carcinomas with Class 3 BRAF mutants and wild type RAS is sufficient to markedly inhibit ERK signaling and their growth in in vivo murine models and in patients. We have thus defined a third subset of BRAF mutants, which is RAS-dependent. Tumors harboring such mutants are sensitive to tyrosine kinase inhibitors in tumors expressing wild type RAS and NF1. Citation Format: Zhan Yao, Rona Yaeger, Vanessa S. Rodrik-Outmezguine, Anthony Tao, Neilawattie M. Torres, Matthew T. Chang, Matthias Drosten, Huiyong Zhao, Fabiola Cecchi, Todd Hembrough, Judith Michels, Herve Baumert, Linde Miles, Naomi M. Campbell, Elisa de Stanchina, David B. Solit, Mariano Barbacid, Barry S. Taylor, Neal Rosen. Tumors with class 3 BRAF mutants are sensitive to the inhibition of activated RAS [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4078. doi:10.1158/1538-7445.AM2017-4078