A Secondary Mutation inBRAFConfers Resistance to RAF Inhibition in aBRAFV600E-Mutant Brain Tumor

Abstract

BRAF hyperactivates ERK and signals as a RAF inhibitor-sensitive monomer. Although RAF inhibitors can produce impressive clinical responses in patients with mutant tumors, the mechanisms of resistance to these drugs are incompletely characterized. Here, we report a complete response followed by clinical progression in a patient with a-mutant brain tumor treated with dabrafenib. Whole-exome sequencing revealed a secondary mutation at progression that was not present in the pretreatment tumor. Expressing BRAF induces ERK signaling, promotes RAF dimer formation, and is sufficient to confer resistance to dabrafenib. Newer RAF dimer inhibitors and an ERK inhibitor are effective against BRAF-mediated resistance. Collectively, our results validate a novel biochemical mechanism of RAF inhibitor resistance mediated by a secondary mutation, emphasizing that, like driver mutations in cancer, the spectrum of mutations that drive resistance to targeted therapy are heterogeneous and perhaps emerge with a lineage-specific prevalence. In contrast to receptor tyrosine kinases, in which secondary mutations are often responsible for acquired resistance, second-site mutations in have not been validated in clinically acquired resistance to RAF inhibitors. We demonstrate a secondary mutation in (V600E/L514V) following progression on dabrafenib and confirm functionally that this mutation is responsible for resistance. .

Biological Rationale for Targeting MEK/ERK Pathways in Anti-Cancer Therapy and to Potentiate Tumour Responses to Radiation

“…Until recently, secondary BRAF mutations such as to cause BRAF and MEK inhibitor resistance could not be detected, whereas the selective amplification of mutant BRAF was detected in colorectal cancer cell lines that are resistant to the MEK inhibitor [77] as a result of an underlying mechanism of MEK1 hyperactivation. However, whole exome sequencing recently led to the detection of a secondary mutation in mutant BRAF, which confers resistance to RAF inhibitor [78], in a brain tumour that was progressing though not before the tumour was treated. A secondary BRAF resistant mutation was also detected in metastatic BRAF mutant melanoma [79] using detailed imaging studies and genetic analyses.…”

CRISPR Screens Identify Essential Cell Growth Mediators in BRAF Inhibitor-Resistant Melanoma

“…The IC 50 value of the resistant line M238R1 was significantly higher than that of the parental line M238. Previous studies indicated that secondary mutations in BRAF could lead to BRAFi resistance [10] . To rule out the possibility that secondary mutations in BRAF lead to BRAFi resistance in M238R1, we sequenced the BRAF coding region in M238R1.…”

“…For this reason, it is critical to investigate the mechanisms underlying drug resistance and design alternative therapeutic strategies to overcome drug resistance. Resistance to kinase inhibitors is often associated with secondary mutations in the target genes, which render the kinase insensitive to the inhibitor [10] . However, we did not find secondary mutations in BRAF coding regions in the M238R1.…”

“…Numerous mechanisms of acquired BRAFi resistance have been reported. Amplification of the BRAF locus, BRAF alternative splicing, and secondary mutations in BRAF such as L514V and L505H confer resistance to BRAF inhibitors [6] , [7] , [10] . Hyper-activation of components in the RTK-Ras-ERK pathway [11] , [12] and the persistent expression of the RTK platelet-derived growth factor receptor-β (PDGFRβ) or insulin growth factor-1 receptor (IGF-1R) [11] , [13] can contribute to BRAFi resistance.…”

Targeted Therapy in Melanoma and Mechanisms of Resistance

“…The overexpression of the BRAF gene leads to the formation of large amounts of the BRAF protein, and this may result in dimerization. Surprisingly, to the best of our knowledge, no secondary mutations in BRAF have been found in melanomas, though there is an example of a BRAF secondary mutation in a V600E brain tumor [ 118 ], namely L514V, and in this case it conferred resistance to dabrafenib. Moreover, BRAF inhibitors bind one BRAF and transactivate the other one, decreasing the effectiveness of treatment with BRAF inhibitors.…”