The development of mutant BRAF inhibitors has improved the outcome for
melanoma patients with BRAFV600E mutations. Although the initial response to these inhibitors can be dramatic, sometimes resulting in complete
tumor regression, the majority of
melanomas become resistant. To study resistance to BRAF inhibition, we developed a novel mouse model of
melanoma using a
tetracycline/
doxycycline-regulated system that permits control of mutant BRAF expression. Treatment with
doxycycline leads to loss of mutant BRAF expression and
tumor regression, but
tumors recur after a prolonged period of response to treatment.
Vemurafenib,
encorafenib and
dabrafenib induce cell cycle arrest and apoptosis in BRAF
melanoma cell lines; however, a residual population of
tumor cells survive. Comparing gene expression in human cell lines and mouse
tumors can assist with the identification of novel mechanisms of resistance. Accordingly, we conducted
RNA sequencing analysis and immunoblotting on untreated and
doxycycline-treated dormant mouse
melanomas and human mutant BRAF
melanoma cell lines treated with 2 μM
vemurafenib for 20 days. We found conserved expression changes in
histone methyltransferase genes ASH2, EZH2, PRMT5, SUV39H1, SUV39H2, and SYMD2 in P-ERK low, p-38 high
melanoma cells following prolonged BRAF inhibition. Quantitative mass spectrometry, determined a corresponding reduction in
histone Lys9 and Lys27 methylation and increase in Lys36 methylation in
melanoma cell lines treated with 2 μM
vemurafenib for 20 days. Thus, these changes as are part of the initiate response to BRAF inhibition and likely contribute to the survival of
melanoma cells.