Characterization of BRAF Thr599dup Mutation as a Targetable Driver Mutation Identified in Lung Adenocarcinoma by Comprehensive Genomic Profiling.

PURPOSE: Understanding the function of BRAF mutants is crucial for determining the best treatment strategy. This study aimed to characterize a rare BRAF variant, BRAF Thr599dup , which was identified in a patient with lung adenocarcinoma (LUAD) by comprehensive genomic profiling.

MATERIALS AND METHODS: We report a case of LUAD with BRAF Thr599dup treated with dabrafenib and trametinib. We conditionally expressed wild-type BRAF, BRAFV600E , or BRAFThr599dup in Ba/F3 cells and BEAS-2B cells. Ba/F3 cells carrying double-mutant BRAF (BRAFThr599dup/R509H , BRAFV600E/R509H , or BRAFK601E/R509H ) that lacked the dimerizing ability were also established. Knockout of endogenous BRAF or CRAF in Ba/F3-BRAFThr599dup cells and Ba/F3-BRAFV600E cells was performed using the CRISPR/Cas9 system. Cell viability, mitogen-activated protein kinase (MAPK) signaling activity, and sensitivity to dabrafenib and trametinib were evaluated.

RESULTS: The patient was revealed to have BRAF Thr599dup -positive tumor cells as a predominant clone, and dabrafenib and trametinib treatment showed modest efficacy. In Ba/F3 cells, both BRAFThr599dup and BRAFV600E similarly caused interleukin-3-independent proliferation and activated the MAPK pathway. Moreover, BRAFThr599dup and BRAFV600E similarly caused a significant increase in the anchorage-independent growth ability of BEAS-2B cells. Along with Ba/F3-BRAFV600E cells, Ba/F3-BRAFThr599dup cells were highly sensitive to a monomer-specific BRAF inhibitor, dabrafenib, with a half-maximal inhibitory concentration value of 29.7 nM. In the absence of wild-type BRAF, wild-type CRAF, or an intact dimer interface, the ability to induce oncogenic addiction and MAPK pathway activation in Ba/F3-BRAFThr599dup cells was not affected, which was in contrast to the findings in the BRAFK601E/R509H double-mutant model.

CONCLUSION: BRAF Thr599dup is a potent driver oncogene that activates the MAPK pathway without the requirement for dimerization in vitro. Because BRAF Thr599dup has been recurrently reported across various cancer types, our findings should be further investigated both mechanistically and clinically.