BRAF Mutations in Melanoma: Biological Aspects, Therapeutic Implications, and Circulating Biomarkers

Abstract

Melanoma is an aggressive form of skin cancer resulting from the malignant transformation of melanocytes. Recent therapeutic approaches, including targeted therapy and immunotherapy, have improved the prognosis and outcome of melanoma patients. BRAF is one of the most frequently mutated oncogenes recognised in melanoma. The most frequent oncogenic BRAF mutations consist of a single point mutation at codon 600 (mostly V600E) that leads to constitutive activation of the BRAF/MEK/ERK (MAPK) signalling pathway. Therefore, mutated BRAF has become a useful target for molecular therapy and the use of BRAF kinase inhibitors has shown promising results. However, several resistance mechanisms invariably develop leading to therapeutic failure. The aim of this manuscript is to review the role of BRAF mutational status in the pathogenesis of melanoma and its impact on differentiation and inflammation. Moreover, this review focuses on the mechanisms responsible for resistance to targeted therapies in BRAF-mutated melanoma and provides an overview of circulating biomarkers including circulating tumour cells, circulating tumour DNA, and non-coding RNAs.

Keywords: BRAF V600; BRAF mutations; biomarkers; melanoma; targeted therapy resistance.

Figure 1

Types of BRAF variants detected in melanoma as reported in the COSMIC database (https://cancer.sanger.ac.uk/cosmic/ (accessed on 22 May 2023)).

Figure 2

BRAF-mediated regulation of MITF, in which expression levels are linked with melanoma phenotype (see [47,50,52]), in a complex way. BRAF modulates MITF activation through several mechanisms (left panel): (i) BRAF modulates MITF activity through ERK-dependent phosphorylation; (ii) direct interaction between BRAF (and other RAF kinases) and MITF leads to decreased nuclear translocation, increasing cytoplasmic levels of MITF; (iii) ERK phosphorylation also regulates MITF degradation through the ubiquitin pathway. MITF is a driver of phenotype switching (right panel). Parts of the figure are drawn using pictures from Servier Medical Art (https://smart.servier.com (accessed on 17 June 2023)).

Figure 3

BRAF mutation and inflammation. Dynamic interaction between BRAF V600E melanoma cells and tumour microenvironment (TME) in which activated stromal cells support cancer progression. The chronic activation of the immune system leads to an accumulation of T and B lymphocytes (T and B cells). MMPs and several cytokines alter cancer-associated fibroblast (CAF) behavior; inflammation mediators like IL-1β, IL-6, IL-8, and TGF-β are responsible for neutrophil recruitment. MMP-1, produced by melanoma cells, affects CAFs and promotes interstitial collagen degradation activating tumour invasion and angiogenesis. Melanoma cells secrete IL-8-inducing MMP-2 and MMP-9 that contribute to angiogenesis and cancer vascularization. Tumour-associated macrophages (TAMs) produce TNF-α modulating CXCL1 overexpression, which enhanced recruitment of regulatory T cells (Tregs). Tregs are also recruited by TGF-β secretion. IL-6, produced by stromal cells, can promote IL-10 secretion in melanoma cells blocking the function of antigen-presenting cells (APCs). TME cells also produce IP-10 which polarized the action of immune cells. Parts of the figure are drawn using pictures from Servier Medical Art (https://smart.servier.com (accessed on 15 June 2023)).

Figure 4

Timeline of FDA-approved targeted therapy in metastatic melanoma.

Figure 5

Targeted therapy (single and combined) for the treatment of metastatic melanoma. Binding of ligand with RTKs activates MAPK signalling cascade, leading to cell proliferation. In this context, vemurafenib or dabrafenib single therapy selectively inhibits BRAF V600E activity; trametinib regulates the activation of MEK kinases; BRAFi/MEKi combined therapy blocks BRAF V600E and MEK kinases.

Figure 6

Main BRAFi resistance mechanisms addressed in this paper.