RESEARCH HIGHLIGHTS

expression, and, although HDAC in resistance mechanisms. The availa‑ inhibitors have other effects, they bility of many post-treatment samples reversed forskolin-mediated resistance from several patients allowed these to ERK pathway inhibitors. authors to determine that melanomas IMAGE STOCK IMAGE Two other studies from this underwent extensive branched evolu‑ group also looked at resistance tion during progression or relapse to RAF or combined RAF–MEK (one patient had at least five distinct inhibitors in BRAF‑V600E melanoma. resistance mechanisms at relapse). Van Allen et al. conducted whole- In addition, altered mutational exome sequencing (WES) of tumours spectra (non-ultraviolet light-related with early or acquired resistance mutations) were observed during to RAF inhibition from 45 patients disease progression. Further analysis pretreatment and post-treatment. of AKT activation in melanomas Nearly 50% of the resistant tumours treated with RAF inhibitors indicated had mutations in ERK pathway that early in treatment these tumours or downstream effectors, including have increased levels of phosphory­ previously unobserved mutations in lated AKT relative to pretreatment. THERAPEUTICS MEK2, which were verified to cause Experiments in cell lines showed that resistance in cell lines. MITF amplifi‑ this can occur via adaptive upregu­ cation was observed in one patient at lation of tyrosine kinases, Delving deeper into relapse, and consistent with the results leading to increased recruitment of from Johannessen et al. MITF over‑ AKT to the cell surface. PI3K–AKT resistance expression induced resistance to ERK signalling can also be increased via pathway inhibitors in BRAF‑V600E selection for activating mutations Five recent papers have looked more melanoma cell lines. Some patients following longer term inhibitor closely at mechanisms that underlie had PI3K pathway mutations, treatment, which increases the highlighting resistance to inhibitors of the ERK although how these contributed to dependency of cells on this pathway the MAPK pathway in melanomas that resistance was unclear from this study. for growth. Inhibition of both RAF have the BRAF‑V600E mutation. Importantly, three patients had several and AKT repressed cell growth more heterogeneous Johannessen et al. conducted a independent resistance mechanisms than either agent alone. The observed nature of screen for genes that induced resist‑ within the same tumour, highlighting early or late activation of AKT, as well RAF inhibitor ance to inhibitors targeting RAF, MEK the heterogeneous nature of RAF as the pattern of branched evolution, resistance or ERK, or RAF and MEK combined inhibitor resistance. suggest that initial therapy with ERK in a BRAF‑V600E melanoma cell Although RAF–MEK combina‑ pathway and AKT inhibitors might be line. Many of the hits were involved in tion therapy improves progression- more efficacious than ERK pathway G -coupled receptor (GPCR) free survival, resistance still occurs inhibition alone. signalling. GPCRs induce production in most patients. Wagle et al. looked These papers indicate that several of cyclic AMP (cAMP) from ATP by specifically at the mechanisms of this mechanisms probably contribute to adenyl cyclase, and cAMP activates resistance using WES and transcrip‑ resistance following ERK pathway protein kinase A (PKA). Both cAMP tome sequencing (RNA-seq) in five inhibition in melanoma, even within and an adenyl cyclase activator patients. They also found a MEK2 the same patient. They also argue that (forskolin) induced PKA-dependent mutation (which was different from a deeper understanding of resistance resistance to ERK pathway inhibitors. those found by Van Allen et al.), as mechanisms and evolutionary adap‑ Downstream of PKA, the transcrip‑ well as expression of a BRAF splice tation to ERK pathway suppression is tion factor cAMP response element- isoform and BRAF amplification. needed to improve therapy. binding protein (CREB) was also Other possible, but unconfirmed, Sarah Seton-Rogers required for ERK pathway inhibitor mechanisms of resistance were muta‑ ORIGINAL RESEARCH PAPERS Johannessen, resistance. CREB activation was high tion of the ETS2 C. M. et al. A melanocyte lineage program confers in pretreatment biopsy samples from and amplification of the putative resistance to MAP kinase pathway inhibition. Nature 504, 138–142 (2013) | Shi, H. et al. Acquired patients with BRAF‑V600E melanoma, transcriptional repressor SAMD4B. resistance and clonal evolution in melanoma during was suppressed during RAF inhibitor Two papers by Shi et al. also looked BRAF inhibitor therapy. Cancer Discov. http://dx. therapy and then increased to pretreat‑ at resistance to RAF inhibitors. WES of doi.org/10.1158/2159-8290.CD-13-0642 (2013) | Shi, H. et al. A novel AKT1 mutant amplifies an ment levels following relapse. One of 100 pretreatment and post-treatment adaptive melanoma response to BRAF inhibition. the genes activated by CREB in cell tumour samples from 44 patients Cancer Discov. http://dx.doi.org/10.1158/2159- lines was microphthalmia-associated with melanoma identified activating 8290.CD-13-0279 (2013) | Van Allen, E. M. et al. The genetic landscape of clinical resistance to RAF transcription factor (MITF), a lineage- alterations in the ERK pathway in 70% inhibition in metastatic melanoma. Cancer Discov. survival oncogene in melanoma, and of post-treatment tumours, and gain- http://dx.doi.org/10.1158/2159-8290.CD-13-0617 (2013) | Wagle, N. et al. MAP kinase pathway MITF knockdown blocked ERK path‑ of-function mutations in PI3K–AKT alterations in BRAF-mutant melanoma patients with way inhibitor resistance mediated by pathway genes in 22%. These often acquired resistance to combined RAF/MEK forskolin. Finally, histone deacetylase co-occurred with ERK pathway altera‑ inhibition. Cancer Discov. http://dx.doi. org/10.1158/2159-8290.CD-13-0631 (2013) (HDAC) inhibitors can reduce MITF tions, again suggesting heterogeneity

NATURE REVIEWS | CANCER VOLUME 14 | JANUARY 2014

© 2014 Macmillan Publishers Limited. All rights reserved