Inhibiteurs de BRAF dans le traitement du cancer : Contribution à l’étude des mécanismes de résistance et des effets secondaires paradoxaux

摘要

BRAFV600- inhibitors, including vemurafenib, are efficient against tumors harboring this MAPK pathway activating mutation, which is the case of ~50% of melanomas. But most of the patients under treatment progress within a year, and develop paradoxical secondary tumors. Most resistance mechanisms to drugs that target the BRAF and/or MEK kinases in cancer rely on reactivation of the RAS-RAF-MEK-ERK signal transduction pathway (ERK-dependent), on activation of the alternative PI3K-AKT-mTOR pathway (ERK-independent) or on modulation of the caspase-dependent apoptotic cascade. All three pathways converge to regulate the formation of the eIF4F translation initiation complex that binds to the 7-methyl-guanine cap at the 5’ end of mRNAs, thereby modulating mRNA translation of specific mRNAs. We show here that persistent formation of the eIF4F complex, comprising the eIF4E cap binding protein, the eIF4G scaffolding protein and the eIF4A RNA helicase, is associated with resistance to anti-BRAF, anti-MEK and to anti-BRAF + anti-MEK combinations in BRAFV600- mutant melanoma, colon and thyroid cell lines. Unresponsiveness to treatment and maintenance of eIF4F complex formation is associated with either reactivation of MAPK signaling or absence of ERK-independent decreased phosphorylation of the inhibitory eiF4E binding protein 4EBP1 or increased pro-apoptotic Bcl-2 modifying factor (BMF)-dependent degradation of eIF4G. Development of an in situ method shows by proximity ligation assay (PLA) that the formation of the eIF4F complex is decreased in tumors responding to anti-BRAF therapy and increased in resistant metastases. Strikingly, inhibiting the eIF4F complex, either by blocking the eIF4E-eIF4G interaction or by targeting eIF4A with small compounds is synergistic with BRAFV600- inhibition. The other main problem arising during anti-BRAF treatment is the frequent induction of secondary cutaneous and extra-cutaneous tumors, through the formation of BRAF-CRAF dimers that we visualized in vivo for the first time. In conclusion, we have identified by PLA a novel biomarker of resistance against BRAF inhibitors, which is also a promising therapeutic target. Combinations of drugs targeting BRAF (and/or MEK) and eIF4F may overcome most of the resistance mechanisms arising in BRAFV600- cancers. In parallel, we established a BRAF-CRAF PLA method in paradoxical secondary tumors induced by BRAF inhibitors, leading to the identification of several subpopulations more at risk of developing this type of tumors.