Combinations of BRAF inhibitor and MEK inhibitor have been widely used to treat melanoma, but some patients quickly develop resistance to the treatment. Now, scientists at Thomas Jefferson University’s Sidney Kimmel Cancer Center have uncovered how one BRAF site contributes to that resistance, and hope the finding will lead to new combination therapies.
Targeting the common BRAF V600E mutation and its downstream MEK enzymes has been adopted by many FDA-approved therapies against melanoma. These include Novartis’ Tafinlar and Mekinist, Roche’s Zelboraf and Cotellic, and most recently Array BioPharma’s Braftovi and Mektovi. However, about 13% to 30% of patients carrying aberrantly spliced BRAF V600E variants would progress on those BRAF-MEK inhibitor combos.
Previous studies have suggested increased contact between BRAF and its substrate MEK is the reason behind such resistance, but the mechanism wasn’t further understood until new findings from the Jefferson team were made in Cell Reports.
RAF kinase phosphorylates and activates MEK that’s part of a signaling cascade involved in the regulation of cell proliferation. A defect in the pathway leads to uncontrolled cell growth, i.e., cancer. Those resistant types of melanoma produce BRAF proteins that form complex, or dimerization, with MEK.
When the researchers tried to identify sites in the aberrantly spliced BRAF V600E that contribute to the increase in BRAF-MEK association, they set eyes on a phosphor-binding site called serine 729 (S729).
According to the researchers, RAF inhibitors increased phosphorylation at S729. In their study, the researchers used a mutation of S729 to a nonphosphorylatable state. That blocked MEK binding, reduced dimerization, and most importantly, restored the potency of a BRAF inhibitor, the team reported.
“The work helps explain dual hypotheses for RAF-inhibitor resistance, one which focused on MEK and the other on dimerization,” said Andrew Aplin, the study’s lead author, in a statement. “This work weaves the two together mechanistically. The results may also help guide the design of better combination therapies for melanoma.”
Other scientists have also been trying to solve the mystery of BRAF inhibitor resistance in melanoma. One team at Mount Sinai School of Medicine recently used CRISPR-Cas9 to show that a protein called SIRT6 that’s key in DNA repair boosts expression of the IGFBP2 gene and hence triggers resistance.
Aplin and his teammates now call for additional studies to understand how S729 contributes to MEK interaction, whether directly or through other proteins.
“Furthermore, although the disruption of BRAF dimerization has been associated with favorable results in preclinical models, directly testing how these treatments effect substrate binding is necessary to broadly apply such strategies,” they wrote in the study.