The discovery that a subset of human tumours is dependent on mutationally deregulated BRAF kinase intensified the development of RAF inhibitors to be used as potential therapeutics. The US Food and Drug Administration (FDA)-approved second-generation RAF inhibitors vemurafenib and dabrafenib have elicited remarkable responses and improved survival of patients with BRAF-V600E/K melanoma, but their effectiveness is limited by resistance. Beyond melanoma, current clinical RAF inhibitors show modest efficacy when used for colorectal and thyroid BRAF-V600E tumours or for tumours harbouring BRAF alterations other than the V600 mutation. Accumulated experimental and clinical evidence indicates that the complex biochemical mechanisms of RAF kinase signalling account both for the effectiveness of RAF inhibitors and for the various mechanisms of tumour resistance to them. Recently, a number of next-generation RAF inhibitors, with diverse structural and biochemical properties, have entered preclinical and clinical development. In this Review, we discuss the current understanding of RAF kinase regulation, mechanisms of inhibitor action and related clinical resistance to these drugs. The recent elucidation of critical structural and biochemical aspects of RAF inhibitor action, combined with the availability of a number of structurally diverse RAF inhibitors currently in preclinical and clinical development, will enable the design of more effective RAF inhibitors and RAF-inhibitor-based therapeutic strategies, tailored to different clinical contexts.

SUMMARY The complex biochemical effects of RAF inhibitors account for both the effectiveness and mechanisms of resistance to these drugs, but a unified mechanistic model has been lacking. Here we show that RAF inhibitors exert their effects via two distinct allosteric mechanisms. Drug resistance due to dimerization is determined by the position of the αC-helix stabilized by inhibitor, whereas inhibitor-induced RAF priming and dimerization are the result of inhibitor-induced formation of the RAF/RAS-GTP complex. The biochemical effect of RAF inhibitor in cells is the combined outcome of the two mechanisms. Therapeutic strategies including αC-helix-IN inhibitors are more effective in multiple mutant BRAF-driven tumor models, including colorectal and thyroid BRAFV600E cancers, in which first generation RAF inhibitors have been ineffective.

SUMMARYThe complex biochemical effects of RAF inhibitors account for both the effectiveness and mechanisms of resistance to these drugs, but a unified mechanistic model has been lacking. HereCorrespondence and requests for materials should be addressed to: Poulikos.poulikakos@mssm.edu or evripidis.gavathiotis@einstein.yu.edu. * These authors contributed equally to this work Accession numbers. Structural coordinates and parameters have been submitted to the Protein Database Bank under the following accession codes: 4RZV for BRAF R509H /VEM, 4RZW for BRAF R509H /AZ and 5ITA for BRAF WT /AZ-VEM. Other structural coordinates used in this study are the following: PDB ID: 4KSP for TAK bound to BRAF dimer, PDB ID: 3OG7 for VEM bound to BRAF V600E dimer, PDB ID: 4MNF for GDC bound to BRAF V600E dimer, PDB ID: 2FB8 for SB bound to BRAF dimer, PDB ID: 4XV2 for DAB bound to BRAF V600E dimer and PDB ID: 4XV1 for PB bound to BRAF V600E dimer and PDB 4MNE for the BRAF/MEK complex. AUTHOR CONTRIBUTIONSP.I.P., E.G., C.K., M.H., A.L., Z.K., Y.W. and T.A.A designed experiments. Z.K., T.A.A conducted biochemical and cellular studies. E.G., Y.W. performed structural determination and structural analysis. X.W. generated the CRAF-V5 CRISPR cell line. Q.X. synthesized the AZ-VEM compound. C.K., M.H., J.A.F., A.L., E.G., P.I.P. designed animal studies. Z.K., T.A.A and J.B. conducted animal experiments. C.Z. and G.B. provided reagents and analyzed data. P.I.P. and E.G. designed research, analyzed data and wrote the manuscript, which was edited by all authors.C.Z. and G.B. are employees of Plexxikon Inc. All other authors declare no competing financial interests.Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. we show that RAF inhibitors exert their effects via two distinct allosteric mechanisms. Drug resistance due to dimerization is determined by the position of the αC-helix stabilized by inhibitor, whereas inhibitor-induced RAF priming and dimerization are the result of inhibitor-induced formation of the RAF/RAS-GTP complex. The biochemical effect of RAF inhibitor in cells is the combined outcome of the two mechanisms. Therapeutic strategies including αC-helix-IN inhibitors are more effective in multiple mutant BRAF-driven tumor models, including colorectal and thyroid BRAF V600E cancers, in which first generation RAF inhibitors have been ineffective. HHS Public Access

SUMMARY Pharmacologic targeting of components of ERK signaling in ERK-dependent tumors is often limited by adaptive resistance, frequently mediated by feedback-activation of RTK signaling and rebound of ERK activity. Here, we show that combinatorial pharmacologic targeting of ERK signaling and the SHP2 phosphatase prevents adaptive resistance in defined subsets of ERK-dependent tumors. In each tumor that was sensitive to combined treatment, p(Y542) SHP2 induction was observed in response to ERK signaling inhibition. The strategy was broadly effective in TNBC models and tumors with RAS mutations at G12, whereas tumors with RAS(G13D) or RAS(Q61X) mutations were resistant. In addition, we identified a subset of BRAF(V600E) tumors that were resistant to the combined treatment, in which FGFR was found to drive feedback-induced RAS activation, independently of SHP2. Thus, we identify molecular determinants of response to combined ERK signaling and SHP2 inhibition in ERK-dependent tumors.

Hogstad et al. show that the somatic BRAFV600E mutation in myeloid dendritic cell precursors in Langerhans cell histiocytosis promotes lesion formation through impaired dendritic cell migration and resistance to apoptosis, which can be rescued with targeted MAPK pathway inhibition.

Langerhans cell histiocytosis (LCH) is a potentially fatal condition characterized by granulomatous lesions with characteristic clonal mononuclear phagocytes (MNP) harboring activating somatic mutations in MAPK pathway genes, most notably BRAFV600E. We recently discovered that the BRAFV600E mutation can also affect multipotent hematopoietic progenitor cells (HPC) in multisystem LCH disease. How BRAFV600E mutation in HPC leads to LCH is not known. Here we show that enforced expression of the BRAFV600E mutation in early mouse and human multipotent HPC induced a senescence program that led to HPC growth arrest, apoptosis resistance and senescence-associated secretory phenotype (SASP). SASP, in turn, promoted HPC skewing towards the MNP lineage leading to the accumulation of senescent MNP in tissue and the formation of LCH lesions. Accordingly, elimination of senescent cells using INK-ATTAC transgenic mice as well as pharmacologic blockade of SASP improved LCH disease in mice. These results identify senescent cells as a novel target for the treatment of LCH.

The phosphatase and transactivator EYA family proteins are overexpressed in many cancer cell lines and are abundantly distributed in undifferentiated cells during development. Loss-of-function studies have shown that EYA1 is required for cell proliferation and survival during mammalian organogenesis. However, how EYA1 is regulated during development is unknown. Here, we report that EYA1 is regulated throughout the cell cycle via ubiquitin-mediated proteolysis. The level of EYA1 protein fluctuates in the cell cycle, peaking during mitosis and dropping drastically as cells exit into G 1 . We found that EYA1 is efficiently degraded during mitotic exit in a Cdh1-dependent manner and that these two proteins physically interact. Overexpression of Cdh1 reduces the protein levels of ectopically expressed or endogenous EYA1, whereas depletion of Cdh1 by RNA interference stabilizes the EYA1 protein. Together, our results indicate that anaphase-promoting complex/cyclosome (APC/C)-Cdh1 specifically targets EYA1 for degradation during M-to-G 1 transition, failure of which may compromise cell proliferation and survival. The eyes absent (EYA) family proteins is composed of four members (EYA1 to EYA4) defined by a conserved C-terminal Eya domain, which interacts with other proteins and has an intrinsic phosphatase activity (1-3). The EYA proteins possess a transactivation domain in their N-terminal regions (4) and act as transcriptional coactivators by interacting with DNA-binding proteins, such as the homeodomain SIX family proteins, to transactivate genes that are essential for normal development during mammalian organogenesis (4-7). Mutations in the human EYA1 cause branchio-oto-renal (BOR) and branchio-oto (BO) syndromes, which are characterized by branchial arch abnormalities and hearing loss with or without kidney defects (8-11). Deletion of either gene in mice results in the absence of the inner ear, kidney, and thymus as well as reduction of other tissues (10,12,13).During mouse embryonic development, Eya1 is expressed in early progenitor cells in several organ primordia and regulates cell proliferation and survival, as its inactivation in mice leads to reduced proliferation and increased apoptosis in several organ primordia (10,(12)(13)(14)(15). In Drosophila, overexpression of EYA results in overproliferation, while their loss leads to tissue reduction (7,16). Recent studies have found that the levels of EYA proteins are elevated in several cancer cells (17)(18)(19)(20). While a recent study reported that EYA may promote DNA repair by dephosphorylating histone ␥H2AX (21), how EYA acts to regulate cell proliferation and its precise mode of action in cell cycle regulation remain largely unknown. Furthermore, although the biochemical functions of EYA proteins and the spatiotemporal expression pattern of their mRNAs during mouse development have been well studied, it is currently unknown how the levels of EYA proteins are regulated during development.Most eukaryotic cell cycle regulators require targeted degradation to ma...

CDK4/6 inhibitors (CDK4/6i) are effective in metastatic breast cancer, but they have been only modestly effective in most other tumor types. Here we show that tumors expressing low CDK6 rely on CDK4 function, and are exquisitely sensitive to CDK4/6i. In contrast, tumor cells expressing both CDK4 and CDK6 have increased reliance on CDK6 to ensure cell cycle progression. We discovered that CDK4/6i and CDK4/6 degraders potently bind and inhibit CDK6 selectively in tumors in which CDK6 is highly thermo-unstable and strongly associated with the HSP90/CDC37 complex. In contrast, CDK4/6i and CDK4/6 degraders are ineffective in antagonizing tumor cells expressing thermostable CDK6, due to their weaker binding to CDK6 in these cells. Thus, we uncover a general mechanism of intrinsic resistance to CDK4/6i and CDK4/6i-derived degraders and the need for novel inhibitors targeting the CDK4/6i-resistant, thermostable form of CDK6 for application as cancer therapeutics.