Richmond, C. S. | Search bioRxiv

bioRxiv2021

Melanomagenesis driven by a kinase-dead mutant BRAF lacking codons 600-618

While considerable success has been achieved with treating BRAF-V600E mutant tumors in several organ systems, tumors expressing non-V600E BRAF mutations remain a significant clinical challenge. Such atypical BRAF mutations are classified based on their kinase activity, RAS-dependence and their requirement for dimerization to function. We identified an unusually large in-frame deletion in a melanoma patient, encompassing codons 600-618 in the kinase domain. This patient experienced rapid disease progression when treated with dual BRAF/MEK inhibition. To determine whether and how such a large in-frame BRAF deletion might be activating, we ectopically expressed it in HEK293 cells and monitored BRAF pathway activation and response to BRAF and MEK inhibitors. We demonstrate that this mutation results in a kinase-deficient BRAF mutant which, nevertheless, activates MEK/ERK signaling in a dimerization-dependent manner, placing this mutation among the class III BRAF mutants.

cancer biology↗

bioRxiv2021

The MKRN1-BRAF exon4-exon9 fusion is a targetable oncogenic driver

BackgroundBRAF, when mutated at V600E, is a potent oncogenic driver in melanoma, lung and colorectal cancer with well understood signaling mechanisms and established treatment guidelines. Non-V600E mutations are less common, more functionally diverse and do not yet have clear treatment guidelines. One class of non-V600E mutations are BRAF fusion genes which typically involve the C-terminal kinase domain of BRAF joined to one of a wide repertoire of potential N-terminal fusion partners. Here, we functionally characterized an MKRN1-BRAF fusion gene which we detected in multi-gene panel sequencing of a metastatic colorectal tumor. MethodsLevels of MEK/ERK pathway activity were evaluated by western blotting in HEK293 cells ectopically expressing MKRN1-BRAF or a series of other BRAF constructs. Dependence on dimerization was evaluated by introducing a dimerization deficiency mutation and drug sensitivity was evaluated by treatment with sorafenib, dabrafenib and trametinib. ResultsMKRN1-BRAF potently activated MEK/ERK signaling and did not require dimerization for activity. Among the inhibitors evaluated, trametinib most effectively suppressed MKRN1-BRAF driven pathway activity. ConclusionOur data demonstrate that the MKRN1-BRAF fusion gene encodes an oncoprotein that strongly activates MEK/ERK signaling in a trametinib-sensitive manner.

cancer biology↗