Mechanisms of Acquired Resistance to BRAF V600E Inhibition in Colon Cancers Converge on RAF Dimerization and Are Sensitive to Its Inhibition

Abstract

BRAF V600E colorectal cancers (CRC) are insensitive to RAF inhibitor monotherapy due to feedback reactivation of receptor tyrosine kinase signaling. Combined RAF and EGFR inhibition exerts a therapeutic effect, but resistance invariably develops through undefined mechanisms. In this study, we determined that CRC progression specimens invariably harbored lesions in elements of the RAS-RAF-MEK-ERK pathway. Genetic amplification of wild-type RAS was a recurrent mechanism of resistance in CRC patients that was not seen in similarly resistant melanomas. We show that wild-type RAS amplification increases receptor tyrosine kinase-dependent activation of RAS more potently in CRC than in melanoma and causes resistance only in the former. Currently approved RAF inhibitors inhibit RAF monomers but not dimers. All the drug-resistant lesions we identified activate BRAF V600E dimerization directly or by elevating RAS-GTP. Overall, our results show that mechanisms of resistance converge on formation of RAF dimers and that inhibiting EGFR and RAF dimers can effectively suppress ERK-driven growth of resistant CRC.

Encorafenib, Binimetinib, and Cetuximab in BRAF V600E–Mutated Colorectal Cancer

“…[17][18][19][20][21][22][23]25 Analyses of mechanisms of resistance in several studies reiterated the dependency of BRAF V600E-mutated metastatic colorectal cancer on the MAPK pathway, which is composed most prominently of KRAS, NRAS, BRAF, and MEK. 17,21,22,35,36 Suppression of MAPK signaling with encorafenib, binimetinib, and cetuximab represented the logical therapeutic strategy to address this persistent dependency. Despite the documented activity of the triplet regimen, resistance ultimately develops in many patients, and further characterization of these mechanisms is needed to further improve outcomes.…”

Targeting Alterations in the RAF–MEK Pathway

“…These drugs should have a therapeutic index because they inhibit mutant RAF dimers and monomers with greater potency than they inhibit wild-type dimers, and thus would not inhibit MAPK signaling to the same degree in normal cells (17). Mechanistically, these RAF inhibitors would be expected to inhibit tumors with mutant RAF dimers, including non-V600 BRAFactivating mutants and BRAF V600 tumors that have become resistant to RAF inhibitors through genomic alterations, such as RAS mutation/amplification, BRAF V600E amplification, or intragenic deletion or splice variants of BRAF, that lead to BRAF V600 dimerization (17,39,41). Next-generation RAF inhibitors that do not induce paradoxical ERK activation in RAF wild-type cells have also been developed (42), .…”

Colorectal Cancer: Genetic Abnormalities, Tumor Progression, Tumor Heterogeneity, Clonal Evolution and Tumor-Initiating Cells

“…Recent studies have explored the mechanism of resistance of BRAF V600E -mutated colorectal cancers to BRAF-inhibitors, showing that the mechanism of resistance implies alterations in elements of the RAS-RAF-MEK-ERK pathway. In this context, the genetic amplification of wild-type RAS is a recurrent mechanism of resistance, determining increased receptor kinase-dependent activation of RAS [ 122 ]. RAS gene amplification causes resistance to RAF/EGFR inhibition by increasing cellular RAS/GTP levels to levels sufficient to drive BRAF V600E dimerization; in fact, currently approved RAF inhibitors inhibit RAF monomers, but not dimers [ 122 ].…”