Resistance to BRAF and MEK inhibition occurs by amplifying or combining BRAFi resistance mechanisms.
Major finding: Resistance to BRAF and MEK inhibition occurs by amplifying or combining BRAFi resistance mechanisms.
Mechanism: BRAFV600E ultra-amplification or MEK mutants with enhanced BRAFV600E interaction reactivate ERK.
Impact: CRAF underlies hyper-MAPK reactivation, and double-drug addiction may be therapeutically exploited.
Acquired resistance to single-agent BRAF inhibitor (BRAFi) therapy has been shown to arise via multiple mechanisms that converge on reactivation of MAPK signaling, supporting the use of combined BRAFi and MEK inhibitor (MEKi) treatment. However, although dual treatment shows increased clinical efficacy, acquired resistance frequently occurs, prompting Moriceau and colleagues to dissect the genetic mechanisms that underlie resistance to BRAF and MEK co-inhibition (BRAFi+MEKi). Genomic characterization of melanomas from patients whose tumors progressed while on BRAFi+MEKi revealed a higher amplitude and/or combinations of genetic aberrations in the MAPK and PI3K–PTEN–AKT pathways known to be associated with single-agent BRAFi resistance. In line with this data, creation of isogenic BRAFi+MEKi–resistant human BRAFV600E melanoma cell lines via either sequential or combined up-front drug treatment highlighted that progression from single-drug resistance (SDR) to double-drug resistance (DDR) was accelerated by tunable augmentation of preexisting MAPK reactivation mechanisms, including further BRAFV600E amplification and overexpression of mutant NRAS. In contrast, simultaneous up-front treatment with BRAFi+MEKi generated drug-addicted DDR subclones that were characterized by ultra-amplification of BRAFV600E or low-level amplification of BRAFV600E in combination with MEK1 mutation. Mechanistically, BRAFV600E amplification led to increased dimerization with and activation of CRAF and elevated ERK activity. In addition, MEK1 mutation enhanced its interaction with BRAFV600E, and disruption of this scaffold-like interaction via mutation of BRAFR662 led to reduced downstream ERK signaling and reversed DDR. In general, DDR cells with amplified MAPK signaling could be resensitized to BRAFi+MEKi by treatment with optimal levels of an ERK inhibitor or CRAF depletion. Of note, suboptimal levels of ERK inhibition rescued DDR cells from BRAFi+MEKi drug addiction. These results indicate that augmentation and combination of single-agent BRAFi resistance mechanisms underlie BRAFi+MEKi resistance in BRAF-mutant melanoma and suggest that interfering with BRAF–CRAF or BRAF–MEK interactions, inhibiting CRAF activity, or intermittent dosing may be exploited for potential therapeutic gains.