Heterogeneity Of Resistance Mechanisms Research Articles

Acquired Resistance and Clonal Evolution in Melanoma during BRAF Inhibitor Therapy

BRAF inhibitors elicit rapid antitumor responses in the majority of patients with BRAF(V600)-mutant melanoma, but acquired drug resistance is almost universal. We sought to identify the core resistance pathways and the extent of tumor heterogeneity during disease progression. We show that mitogen-activated protein kinase reactivation mechanisms were detected among 70% of disease-progressive tissues, with RAS mutations, mutant BRAF amplification, and alternative splicing being most common. We also detected PI3K-PTEN-AKT-upregulating genetic alterations among 22% of progressive melanomas. Distinct molecular lesions in both core drug escape pathways were commonly detected concurrently in the same tumor or among multiple tumors from the same patient. Beyond harboring extensively heterogeneous resistance mechanisms, melanoma regrowth emerging from BRAF inhibitor selection displayed branched evolution marked by altered mutational spectra/signatures and increased fitness. Thus, melanoma genomic heterogeneity contributes significantly to BRAF inhibitor treatment failure, implying upfront, cotargeting of two core pathways as an essential strategy for durable responses.

In Vivo MAPK Reporting Reveals the Heterogeneity in Tumoral Selection of Resistance to RAF Inhibitors

Activation of the ERK1/2 mitogen-activated protein kinases (MAPK) confers resistance to the RAF inhibitors vemurafenib and dabrafenib in mutant BRAF-driven melanomas. Methods to understand how resistance develops are important to optimize the clinical use of RAF inhibitors in patients. Here, we report the development of a novel ERK1/2 reporter system that provides a noninvasive, quantitative, and temporal analysis of RAF inhibitor efficacy in vivo. Use of this system revealed heterogeneity in the level of ERK1/2 reactivation associated with acquired resistance to RAF inhibition. We identified several distinct novel and known molecular changes in resistant tumors emerging from treatment-naïve cell populations including BRAF V600E variants and HRAS mutation, both of which were required and sufficient for ERK1/2 reactivation and drug resistance. Our work offers an advance in understanding RAF inhibitor resistance and the heterogeneity in resistance mechanisms, which emerge from a malignant cell population.

Acquired Resistance to BRAF Inhibition Can Confer Cross-Resistance to Combined BRAF/MEK Inhibition

Aberrant activation of the BRAF kinase occurs in ∼60% of melanomas, and although BRAF inhibitors have shown significant early clinical success, acquired resistance occurs in most patients. Resistance to chronic BRAF inhibition often involves reactivation of mitogen-activated protein kinase (MAPK) signaling, and the combined targeting of BRAF and its downstream target MAPK/ERK kinase (MEK) may delay or overcome resistance. To investigate the efficacy of combination BRAF and MEK inhibition, we generated melanoma cell clones resistant to the BRAF inhibitor GSK2118436. These BRAF inhibitor-resistant sublines acquired resistance through several distinct mechanisms, including the acquisition of activating N-RAS mutations and increased accumulation of COT1. These alterations uniformly promoted MAPK reactivation and most conferred resistance to MEK inhibition and to the concurrent inhibition of BRAF and MEK. These data indicate that melanoma tumors are likely to develop heterogeneous mechanisms of resistance, many of which will confer resistance to multiple MAPK inhibitory therapies.