Diverse Mechanisms of BRAF Inhibitor Resistance in Melanoma Identified in Clinical and Preclinical Studies

Abstract

BRAF inhibitor therapy may provide profound initial tumor regression in metastatic melanoma with BRAF V600 mutations, but treatment resistance often leads to disease progression. A multi-center analysis of BRAF inhibitor resistant patient tissue samples detected genomic changes after disease progression including multiple secondary mutations in the MAPK/Erk signaling pathway, mutant BRAF copy number gains, and BRAF alternative splicing as the predominant putative mechanisms of resistance, but 41.7% of samples had no known resistance drivers. In vitro models of BRAF inhibitor resistance have been developed under a wide variety of experimental conditions to investigate unknown drivers of resistance. Several in vitro models developed genetic alterations observed in patient tissue, but others modulate the response to BRAF inhibitors through increased expression of receptor tyrosine kinases. Both secondary genetic alterations and expression changes in receptor tyrosine kinases may increase activation of MAPK/Erk signaling in the presence of BRAF inhibitors as well as activate PI3K/Akt signaling to support continued growth. Melanoma cells that develop resistance in vitro may have increased dependence on serine or glutamine metabolism and have increased cell motility and metastatic capacity. Future studies of BRAF inhibitor resistance in vitro would benefit from adhering to experimental parameters that reflect development of BRAF inhibitor resistance in patients through using multiple cell lines, fully characterizing the dosing strategy, and reporting the fold change in drug sensitivity.

Keywords: BRAF inhibitor; cell line; dabrafenib; drug resistance; invasion; melanoma; metabolism; vemurafenib.

Figure 1

Mechanisms supporting BRAF inhibitor resistance in melanoma. Receptor tyrosine kinases (RTK) include AXL receptor tyrosine kinase (AXL), epidermal growth factor receptor (EGFR), fibroblast growth factor receptor 1 (FGFR1), fibroblast growth factor receptor 3 (FGFR3), platelet-derived growth factor receptor beta (PDGFRB), MET proto-oncogene receptor tyrosine kinase (MET), and KIT proto-oncogene receptor tyrosine kinase (KIT). Growth factors (GF) correspond to the specific receptor tyrosine kinase. The MAPK/Erk pathway includes the Ras GTPases (N/K/HRAS), Serine/threonine-protein kinase B-raf (BRAF), RAF proto-oncogene serine/threonine-protein kinase (CRAF), mitogen-activated and extracellular signal-regulated kinase kinase 1 or 2 (MEK1/2), extracellular signal-regulated kinase 1 or 2 (ERK1/2), cancer Osaka thyroid (COT), and dual specificity protein phosphatase 4 (DUSP4). The PI3K/Akt pathway includes phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA), phosphatidylinositol 3-kinase regulatory subunit 1 or 2 (PIK3R1/2), phosphatidylinositol 4,5-bisphosphate (PIP2), phosphatidylinositol 3,4,5-trisphosphate (PIP3), phosphatase and tensin homolog (PTEN), AKT serine/threonine kinase 1 or 2 (AKT1/2), mammalian target of rapamycin complex 1 (mTORC1). Src signaling factors include SRC proto-oncogene non-receptor tyrosine kinase (SRC) and focal adhesion kinase 1 (FAK1). Transcription factors include signal transducer and activator of transcription 3 (STAT3), TEA domain transcription factor protein family (TEAD), activator protein 1 complex (AP-1), Jun proto-oncogene AP-1 transcription factor subunit (JUN), SRY-box 10 (SOX10), melanocyte inducing transcription factor (MITF), cyclic AMP responsive element binding protein family (CREB), FOS like 1 AP-1 transcription factor subunit (FOSL1), GLI family zinc finger 1 or 2 (GLI1/2), transforming growth factor beta (TGFβ), SMAD family member 3 (SMAD3). Cell cycle regulators included cyclin D1 (CCND1), cyclin dependent kinase 4 or 6 (CDK4/6). Non-canonical Wnt signaling mediators include receptor like tyrosine kinase (RYK), frizzled class receptor 7 (FZD7), and Wnt family member 5A (WNT5A).