Oncogene (2015) 34, 2951–2957 © 2015 Macmillan Publishers Limited All rights reserved 0950-9232/15 www.nature.com/onc REVIEW Tumor heterogeneity and plasticity as elusive drivers for resistance to MAPK pathway inhibition in melanoma A Roesch Despite the recent success of MAPK signaling-targeted drugs in melanoma, the majority of patients with metastatic melanoma still undergo disease progression after initial tumor shrinkage indicating gradually developing therapy resistance. This review will give an overview on currently suggested concepts of resistance to MAPK pathway inhibitors in melanoma with particular focus on inter- and intraindividual as well as intratumor heterogeneity. The high plasticity of melanoma cells promotes both the clonal evolution of genetic resistance, for example, because of mutations in the MAPK or PI3K/AKT/PTEN pathways, and the emergence of cell phenotypes that functionally and metabolically overcome MAPK inhibition. Like a ‘moving target’, melanoma cells are shifting between different metabolic, cell cycle and differentiation states reflecting a highly dynamic potential to adapt to exogenous stressors including drugs. The introduction of MAPK inhibitors into the clinics has tremendously pushed the field of melanoma research not just because of the historic therapeutic success but also by providing a new tool to study human melanoma in its natural microenvironment. Oncogene (2015) 34, 2951–2957; doi:10.1038/onc.2014.249; published online 11 August 2014 INTRODUCTION when applied to patients. Thus, the examination of drug Melanoma is an aggressive tumor causing the majority of skin resistance mechanisms increases our knowledge on melanoma cancer-related deaths in Caucasians with increasing incidence, at biology and provides a plethora of future therapeutic the moment 15–25 per 100 000 in Western countries. Unlike early innovations. In this respect, the first combinatorial therapies of stage melanomas that can be cured by surgical excision, advanced BRAF and MEK inhibitors are already being tested in clinical trials melanomas (AJCC 2009 stage IV) are associated with a median aiming to increase the number of cells being killed and to survival of 6–12 months.1,2 Until 2010, all major chemotherapeutic minimize the risk for development of resistance originating from and immunological approaches as well as radiotherapeutic surviving cells. A recent phase I/II trial suggests that the median interventions have failed to increase survival rates of stage IV progression-free survival in the combination group (9.4 months) could be improved by approximately 3.6 months as compared patients in randomized clinical trials. Recent phase III trials with 7 MAPK pathway-targeted drugs showed significant improvements with the monotherapy group (5.8 months). But also here, in progression-free survival and also overall survival when development of resistance has been observed with a delay of a compared with the former therapeutic standard dacarbazine.2 few months (see below). This review will give a brief overview on For example, mutant-specific BRAF kinase inhibitors like vemur- currently suggested concepts of resistance to MAPK pathway afenib or dabrafenib achieve ~ 50% objective response rate and inhibitors in melanoma with the particular focus on tumor an overall survival benefit of ~ 4 months in BRAFV600E-positive heterogeneity and plasticity. individuals (representing ~ 50% of all melanomas).3–5 The MEK1/2 inhibitor trametinib improved the median progression-free survival in BRAFV600E/K-positive individuals by ∼ 3 months when CURRENTLY DISCUSSED CATEGORIES OF RESISTANCE compared with dacarbazine. At 6 months of therapy, the rate of MECHANISMS IN MELANOMA overall survival was 81% in the trametinib group and 67% in the The number of suggested resistance mechanisms to MAPK dacarbazine group.6 pathway inhibition in melanoma is rapidly increasing. These Nevertheless, in the broad majority of patients treated with mechanisms can be subdivided into genomic vs phenotypic MAPK pathway inhibitors, metastases recur after a median mechanisms and according to their temporal occurrence into duration of ∼ 5–7 months indicating gradually developing intrinsic, adaptive and acquired resistance. Intrinsic resistance, for therapy resistance after initial (complete or partial) tumor example, to vemurafenib, according to RECIST criteria occurs in – shrinkage.3 6 Despite—or even because of—such developing ∼ 50% of all patients with 15% of patients showing no tumor resistance, those drugs represent valuable tools enabling shrinkage at all and 35% of patients achieving only limited tumor researchers to study melanoma and its evolutionary capability in reduction that does not fulfill the RECIST criteria for partial its natural environment. For example, melanoma cells that are response. The other 50% of patients initially show tumor shrinkage addicted to activated MAPK signaling are enforced to disclose 430% but subsequently undergo progressive disease with tumor their full genomic and phenotypic plasticity to survive the outgrowth because of acquired resistance.3,8–10 Adaptive resistance therapeutic selection pressure of BRAF or MEK inhibitors occurs already within hours after drug exposure and reflects the University Hospital Essen, Department of Dermatology, Essen, Germany. Correspondence: Professor A Roesch, University Hospital Essen, Department of Dermatology, Hufelandstr. 55, D-45122 Essen, Germany. E-mail: [email protected] Received 29 April 2014; revised 22 May 2014; accepted 23 May 2014; published online 11 August 2014 Resistance to MAPK pathway inhibition in melanoma A Roesch 2952 dynamic re-adjustment of signaling pathways on the cellular level, other positive and negative regulators of this pathway (PI3K3CA, for example, ERK-controlled feedback loops regulating upstream PIK3CG, PIK3R2, PTEN and PHLPP1) were detected at an overall activators like RAS.11 However, all categorizations, which are based frequency of 22% of patients with BRAF inhibitor resistance. Of on clinical responses, have to be used with caution because they them, 18% also had overlapping alterations in the MAPK core do not fully reflect the resistance state of single melanoma cells. pathway.21 However, as is the case for MEK1 mutations, also PI3K For example, also, most responders still have remaining tumor pathway alterations do not necessarily preclude clinical response — — masses, which on the cellular level certainly display relative if detected in pre-treatment tumors. Although a recent tumor resistance compared to cells, which are being killed. Thus, as long genetic analysis suggested a correlative trend (P = 0.059) between as the technical resolution of our diagnostic and molecular PTEN loss/mutation status and progression-free survival in screening procedures in patients is too low to routinely assess the dabrafenib-treated patients,22 single patients with PTEN altera- single cell level, a comprehensive categorization of the temporal fi and spatial development of resistance is difficult. More clinical tions may still bene t from BRAFV600E inhibition. Very lately, a studies are highly needed to address this question. Besides, future patient with a pre-existing PTENK128T mutation was reported to therapeutic concepts in melanoma will increasingly involve have passed 18 weeks of stable disease before developing 15 immunotherapeutic approaches such as anti-PD-1 or anti-CTLA4 progressive disease. checkpoint inhibition, which most likely will lead to additional With regard to the first clinical trials on combinatorial BRAF plus changes in the definition of resistance. MEK inhibition, two groups have identified a mutation in MAP2K2/ MEK2 (Q60P) in patients who progressed under dabrafenib/ trametinib.23,24 Because melanoma cells that had been chronically THE HETEROGENEITY AND EVOLUTION OF GENOMIC exposed to dabrafenib/trametinib remained sensitive to either RESISTANCE ERK inhibition24 or PI3K/mTOR inhibition,23 hope is raised that Interindividual genomic heterogeneity future triple therapies could also overcome double resistance in In 2010, Nazarian et al.12 initially demonstrated that melanoma the clinics. cells can maintain high levels of ERK1/2 phosphorylation despite Next to the MAPK and PI3K-AKT core pathways, a number of the presence of the BRAF inhibitor vemurafenib because of the other genomic modifications have been identified till date expression of mutant NRASQ61K. In the presence of BRAF including NF1, RAC1, HOXD8 and prominent melanoma drivers inhibitors, mutated RAS promotes dimerization of BRAFV600E such as the master lineage transcription factor MITF, which was with other (wild-type) RAF isoforms, which restores strong found focally amplified in 2 of 45 patients.21 Subsequent in vitro 11 activation of the MAPK pathway. Somatic mutations in NRAS experiments demonstrated that enforced overexpression of MITF (Q61K/R/L, G12D/R and G13R) were detected till date by whole- in BRAFV600E mutant cells promotes resistance towards inhibitors exome sequencing in ∼ 8–18% of BRAF inhibitor-resistant patients; 15 13–15 of BRAF, MEK and ERK. This observation is supported by a recent in most cases, as a late event beyond 12 weeks of therapy. systematic gain-of-function study, which individually expressed This is interesting because in primary melanomas co-existence of 16 more than 15 500 ORFs of a lentiviral expression library in a BRAF and NRAS mutations occurs rather rarely. Also, KRAS BRAFV600E mutant melanoma cell line. This approach revealed a mutations (G12C, G12R,