Abstract 2142: Novel oncogenic BRaf deletions functioning as BRaf homodimer and sensitive to inhibition by LY3009120, a pan Raf and Raf dimer inhibitor
Authors: Chen Shih-Hsun , Buchanan Sean , Zhang Youyan , Van Horn Robert , Yin Tinggui , Yadav Vipin , Wong Swee Seong , Huber Lysiane , Henry James , Conti Ilaria , Starling James J. , Plowman Gregory D. , Peng Sheng-Bin 
Published: August 2015

Abstract
BRAF mutations, particularly the somatic hot spot BRAF V600E mutation, were discovered as major oncogenic mutations in many cancer types. It was shown that BRAF V600E is a potent oncogene that activates the MAPK pathway and functions as a BRaf monomer. BRaf selective inhibitors, vemurafenib and dabrafenib, which are effective in inhibiting the kinase activity of BRaf monomer, have demonstrated robust anti-tumor activities in BRAF mutant xenograft models and significant clinical benefit among BRAF mutant melanoma patients. In this study, we have identified and characterized novel BRaf aberrant variants, which have in-frame deletions within or adjacent to the L485-P490 region in patient samples and/or cell lines of lung, pancreatic, and ovarian cancers. Tumor cells with these endogenous BRaf deletions are resistant to BRaf monomer inhibitor vemurafenib based on inhibition of phospho-MEK and phospho-ERK, cell proliferation, and cell cycle progression. However, these cells are sensitive to LY3009120, a pan Raf and Raf dimer inhibitor. Further analysis using siRNA showed that the MEK-ERK activity in these cells is mainly dependent on BRaf, not CRaf or ARaf. Ectopical expression of the L485-P490 deleted BRaf in mouse NIH3T3 cells is able to transform the cells and form colonies comparable to BRaf V600E mutation in three-dimensional soft agar growth. More importantly, the Raf dimer disrupting mutation BRafR509H abolished the transforming activity of the L485-P490 deleted BRaf, suggesting that this BRaf deletion functions as a dimer. Further, ectopical expression of the L485-P490 deleted BRaf promotes primarily BRaf homodimerization as revealed by proximity ligation assays (PLA). It was also confirmed by PLA that BRaf homodimer is the dominant form of Raf dimers in tumor cells harboring these BRaf deletions. In lung and pancreatic tumor xenograft models developed with tumor cells with these BRaf deletions, LY3009120 treatment demonstrated significant tumor growth inhibition and regression, whereas vemurafenib treatment showed no in vivo activity. Overall, we have identified novel oncogenic BRaf deletions that function as BRaf homodimer and are sensitive to pan Raf and Raf dimer inhibitor LY3009120.
Citation Format: Shih-Hsun Chen, Sean Buchanan, Youyan Zhang, Robert Van Horn, Tinggui Yin, Vipin Yadav, Swee Seong Wong, Lysiane Huber, James Henry, Ilaria Conti, James J. Starling, Gregory D. Plowman, Sheng-Bin Peng. Novel oncogenic BRaf deletions functioning as BRaf homodimer and sensitive to inhibition by LY3009120, a pan Raf and Raf dimer inhibitor. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2142. doi:10.1158/1538-7445.AM2015-2142