FGFR2 fusion proteins drive oncogenic transformation of mouse liver organoids towards cholangiocarcinoma.

BACKGROUND & AIMS: About 15% of intrahepatic cholangiocarcinomas (iCCAs) express fibroblast growth factor receptor 2 (FGFR2) fusion proteins (FFs), usually alongside mutational inactivation of TP53, CDKN2A or BAP1. In FFs, FGFR2 residues 1-768 fuse to sequences encoded by a diverse array of partner genes (>60) causing oncogenic FF activation. While FGFR-specific tyrosine kinase inhibitors (F-TKI) provide clinical benefit in FF+ iCCA, responses are partial and/or limited by resistance mechanisms, such as the V565F substitution in the FGFR2 gatekeeper residue. Improving on FF targeting in iCCA therefore remains a critical unmet need. Herein, we aimed to generate a murine model of FF-driven iCCA and use this to uncover actionable FF-associated dependencies. METHODS: Four iCCA FFs carrying different fusion sequences were expressed in Tp53-/- mouse liver organoids. Tumorigenic properties of genetically modified liver organoids were assessed by transplantation into immuno-deficient mice. Cellular models derived from neoplastic lesions were exploited for pre-clinical studies. RESULTS: Transplantation of FF-expressing liver organoids yielded tumors diagnosed as CCA based on histological, phenotypic and transcriptomic analyses. The penetrance of this tumorigenic phenotype was influenced by FF identity. Tumor organoids and 2D cell lines derived from CCA lesions were addicted to FF signaling via Ras-Erk, regardless of FF identity or V565F mutation. Dual blockade of FF and the Ras-Erk pathway by concomitant pharmacological inhibition of FFs and Mek1/2 provided greater therapeutic efficacy than single agent F-TKI in vitro and in vivo. CONCLUSIONS: FF-driven iCCA pathogenesis was successfully modeled on a Tp53-/- murine background, revealing biological heterogeneity among structurally different FFs. Double blockade of FF-ERK signaling deserves consideration for precision-based approaches against human FF+ iCCA. LAY SUMMARY: Intrahepatic cholangiocarcinoma (iCCA) is a rare cancer that is difficult to treat. A subtype of iCCA is caused by genomic alterations that generate oncogenic drivers known as FGFR2 fusions. Patients with FGFR2 fusions respond to FGFR inhibitors, but clinical responses are often of modest duration. We used animal and cellular models to show that FGFR2 fusions require the activity of a downstream effector named Mek1/2. We found that dual blockade of FGFR2 fusions and Mek1/2 was more effective than isolated inhibition of FGFR2 fusions, pointing to the potential clinical utility of dual FGFR2-MEK1/2 blockade in patients with iCCA.

Making sense of Cbp/p300 loss of function mutations in skin tumorigenesis.

CBP and p300 are highly homologous lysine acetyltransferases involved in cell cycle regulation, DNA synthesis and DNA repair. Loss of function mutations of CBP and p300 are found in about one-third of cutaneous squamous cell carcinoma (cSCC) and often co-occur, yet their role in cSCC pathogenesis is unclear. Writing in The Journal of Pathology, Ichise and colleagues modeled combined heterozygous loss of Cbp/p300 in mouse keratinocytes expressing a transgenic HrasS35 allele that allows selective coupling of Hras to the Erk pathway. Epidermal thickening caused by expression of HrasS35 was exacerbated by reduced dosage of Cbp/p300 and eventually resulted in development of skin papillomas. This phenotype was associated with reduced expression of Mig6, an Egfr feedback inhibitor, and attendant enhancement of Egfr signaling to the Ras-Erk pathway. This model provides a mechanistic framework for understanding how Cbp/p300 loss of function mutations impact on skin tumorigenesis and suggests potential therapeutic options in CBP/p300 mutated human cSCC. © 2019 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

HSP90 Inhibition Drives Degradation of FGFR2 Fusion Proteins: Implications for Treatment of Cholangiocarcinoma.

About 15% of intrahepatic cholangiocarcinomas (ICCs) express constitutively active fibroblast growth factor receptor 2 (FGFR2) fusion proteins (FFs) generated by chromosomal translocations. FFs have been nominated as oncogenic drivers because administration of FGFR tyrosine kinase inhibitors (F-TKIs) can elicit meaningful objective clinical responses in patients carrying FF-positive ICC. Thus, optimization of FF targeting is a pressing clinical need. Herein, we report that three different FFs, previously isolated from ICC samples, are heat shock protein 90 (HSP90) clients and undergo rapid degradation upon HSP90 pharmacological blockade by the clinically advanced HSP90 inhibitor ganetespib. Combining catalytic suppression by the F-TKI BGJ398 with HSP90 blockade by ganetespib suppressed FGFR2-TACC3 (transforming acidic coiled-coil containing protein 3) signaling in cultured cells more effectively than either BGJ398 or ganetespib in isolation. The BGJ398 + ganetespib combo was also superior to single agents when tested in mice carrying subcutaneous tumors generated by transplantation of FGFR2-TACC3 NIH3T3 transformants. Of note, FF mutants known to enforce clinical resistance to BGJ398 in ICC patients retained full sensitivity to ganetespib in cultured cells. Conclusion: Our data provide a proof of principle that upfront treatment with the BGJ398 + ganetespib combo improves therapeutic targeting of FGFR2 fusions in an experimental setting, which may be relevant to precision medicine approaches to FF-driven ICC.

miR-205 mediates adaptive resistance to MET inhibition via ERRFI1 targeting and raised EGFR signaling.

The onset of secondary resistance represents a major limitation to long-term efficacy of target therapies in cancer patients. Thus, the identification of mechanisms mediating secondary resistance is the key to the rational design of therapeutic strategies for resistant patients. MiRNA profiling combined with RNA-Seq in MET-addicted cancer cell lines led us to identify the miR-205/ERRFI1 (ERBB receptor feedback inhibitor-1) axis as a novel mediator of resistance to MET tyrosine kinase inhibitors (TKIs). In cells resistant to MET-TKIs, epigenetically induced miR-205 expression determined the downregulation of ERRFI1 which, in turn, caused EGFR activation, sustaining resistance to MET-TKIs. Anti-miR-205 transduction reverted crizotinib resistance in vivo, while miR-205 over-expression rendered wt cells refractory to TKI treatment. Importantly, in the absence of EGFR genetic alterations, miR-205/ERRFI1-driven EGFR activation rendered MET-TKI-resistant cells sensitive to combined MET/EGFR inhibition. As a proof of concept of the clinical relevance of this new mechanism of adaptive resistance, we report that a patient with a MET-amplified lung adenocarcinoma displayed deregulation of the miR-205/ERRFI1 axis in concomitance with onset of clinical resistance to anti-MET therapy.

Regulation of the ErbB network by the MIG6 feedback loop in physiology, tumor suppression and responses to oncogene-targeted therapeutics.

The ErbB signaling network instructs the execution of key cellular programs, such as cell survival, proliferation and motility, through the generation of robust signals of defined strength and duration. In contrast, unabated ErbB signaling disrupts tissue homeostasis and leads to cell transformation. Cells oppose the threat inherent in excessive ErbB activity through several mechanisms of negative feedback regulation. Inducible feedback inhibitors (IFIs) are expressed in the context of transcriptional responses triggered by ErbB signaling, thus being uniquely suited to regulate ErbB activity during the execution of complex cellular programs. This review focuses on MIG6, an IFI that restrains ErbB signaling by mediating ErbB kinase suppression and receptor down-regulation. We will review key issues in MIG6 function, regulation and tumor suppressor activity. Subsequently, the role for MIG6 loss in the pathogenesis of tumors driven by ErbB oncogenes as well as in the generation of cellular addiction to ErbB signaling will be discussed. We will conclude by analyzing feedback inhibition by MIG6 in the context of therapies directed against ErbB and non-ErbB oncogenes.

Regulation of epidermal growth factor receptor signalling by inducible feedback inhibitors.

Signalling by the epidermal growth factor receptor (EGFR) controls morphogenesis and/or homeostasis of several tissues from worms to mammals. The correct execution of these programmes requires the generation of EGFR signals of appropriate strength and duration. This is obtained through a complex circuitry of positive and negative feedback regulation. Feedback inhibitory mechanisms restrain EGFR activity in time and space, which is key to ensuring that receptor outputs are commensurate to the cell and tissue needs. Here, we focus on the emerging field of inducible negative feedback regulation of the EGFR in mammals. In mammalian cells, four EGFR inducible feedback inhibitors (IFIs), namely LRIG1, RALT (also known as MIG6 and ERRFI1), SOCS4 and SOCS5, have been discovered recently. EGFR IFIs are expressed de novo in the context of early or delayed transcriptional responses triggered by EGFR activation. They all bind to the EGFR and suppress receptor signalling through several mechanisms, including catalytic inhibition and receptor downregulation. Here, we review the mechanistic basis of IFI signalling and rationalise the function of IFIs in light of gene-knockout studies that assign LRIG1 and RALT an essential role in restricting cell proliferation. Finally, we discuss how IFIs might participate in system control of EGFR signalling and highlight the emerging roles for IFIs in the suppression of EGFR-driven tumorigenesis.

A two-tiered mechanism of EGFR inhibition by RALT/MIG6 via kinase suppression and receptor degradation.

Signaling by epidermal growth factor receptor (EGFR) must be controlled tightly because aberrant EGFR activity may cause cell transformation. Receptor-associated late transducer (RALT) is a feedback inhibitor of EGFR whose genetic ablation in the mouse causes phenotypes due to EGFR-driven excess cell proliferation. RALT inhibits EGFR catalytic activation by docking onto EGFR kinase domain. We report here an additional mechanism of EGFR suppression mediated by RALT, demonstrating that RALT-bound EGF receptors undergo endocytosis and eventual degradation into lysosomes. Moreover, RALT rescues the endocytic deficit of EGFR mutants unable to undergo either endocytosis (Dc214) or degradation (Y1045F) and mediates endocytosis via a domain distinct from that responsible for EGFR catalytic suppression. Consistent with providing a scaffolding function for endocytic proteins, RALT drives EGFR endocytosis by binding to AP-2 and Intersectins. These data suggest a model in which binding of RALT to EGFR integrates suppression of EGFR kinase with receptor endocytosis and degradation, leading to durable repression of EGFR signaling.

Loss of RALT/MIG-6 expression in ERBB2-amplified breast carcinomas enhances ErbB-2 oncogenic potency and favors resistance to Herceptin.

An emerging paradigm holds that loss of negative signalling to receptor tyrosine kinases (RTKs) is permissive for their oncogenic activity. Herein, we have addressed tumor suppression by RALT/MIG-6, a transcriptionally controlled feedback inhibitor of ErbB RTKs, in breast cancer cells. Knockdown of RALT expression by RNAi enhanced the EGF-dependent proliferation of normal breast epithelial cells, indicating that loss of RALT signalling in breast epithelium may represent an advantageous condition during ErbB-driven tumorigenesis. Although mutational inactivation of the RALT gene was not detected in human breast carcinomas, RALT mRNA and protein expression was strongly and selectively reduced in ERBB2-amplified breast cancer cell lines. Reconstitution of RALT expression in ERBB2-amplified SKBr-3 and BT474 cells inhibited ErbB-2-dependent mitogenic signalling and counteracted the ability of ErbB ligands to promote resistance to the ErbB-2-targeting drug Herceptin. Thus, loss of RALT expression cooperates with ERBB2 gene amplification to drive full oncogenic signalling by the ErbB-2 receptor. Moreover, loss of RALT signalling may adversely affect tumor responses to ErbB-2-targeting agents.

Feedback inhibition by RALT controls signal output by the ErbB network.

The ErbB-2 interacting protein receptor-associated late transducer (RALT) was previously identified as a feedback inhibitor of ErbB-2 mitogenic signals. We now report that RALT binds to ligand-activated epidermal growth factor receptor (EGFR), ErbB-4 and ErbB-2.ErbB-3 dimers. When ectopically expressed in 32D cells reconstituted with the above ErbB receptor tyrosine kinases (RTKs) RALT behaved as a pan-ErbB inhibitor. Importantly, when tested in either cell proliferation assays or biochemical experiments measuring activation of ERK and AKT, RALT affected the signalling activity of distinct ErbB dimers with different relative potencies. RALT deltaEBR, a mutant unable to bind to ErbB RTKs, did not inhibit ErbB-dependent activation of ERK and AKT, consistent with RALT exerting its suppressive activity towards these pathways at a receptor-proximal level. Remarkably, RALT deltaEBR retained the ability to suppress largely the proliferative activity of ErbB-2.ErbB-3 dimers over a wide range of ligand concentrations, indicating that RALT can intercept ErbB-2.ErbB-3 mitogenic signals also at a receptor-distal level. A suppressive function of RALT deltaEBR towards the mitogenic activity of EGFR and ErbB-4 was detected at low levels of receptor occupancy, but was completely overcome by saturating concentrations of ligand. We propose that quantitative and qualitative aspects of RALT signalling concur in defining identity, strength and duration of signals generated by the ErbB network.