The ERK mitogen-activated protein kinase signaling network: the final frontier in RAS signal transduction

Defining the KRAS- and ERK-dependent transcriptome in KRAS-mutant cancers

How the KRAS oncogene drives cancer growth remains poorly understood. Therefore, we established a systemwide portrait of KRAS- and extracellular signal-regulated kinase (ERK)-dependent gene transcription in KRAS-mutant cancer to delineate the molecular mechanisms of growth and of inhibitor resistance. Unexpectedly, our KRAS-dependent gene signature diverges substantially from the frequently cited Hallmark KRAS signaling gene signature, is driven predominantly through the ERK mitogen-activated protein kinase (MAPK) cascade, and accurately reflects KRAS- and ERK-regulated gene transcription in KRAS-mutant cancer patients. Integration with our ERK-regulated phospho- and total proteome highlights ERK deregulation of the anaphase promoting complex/cyclosome (APC/C) and other components of the cell cycle machinery as key processes that drive pancreatic ductal adenocarcinoma (PDAC) growth. Our findings elucidate mechanistically the critical role of ERK in driving KRAS-mutant tumor growth and in resistance to KRAS-ERK MAPK targeted therapies.

Determining the ERK-regulated phosphoproteome driving KRAS-mutant cancer

To delineate the mechanisms by which the ERK1 and ERK2 mitogen-activated protein kinases support mutant KRAS-driven cancer growth, we determined the ERK-dependent phosphoproteome in KRAS-mutant pancreatic cancer. We determined that ERK1 and ERK2 share near-identical signaling and transforming outputs and that the KRAS-regulated phosphoproteome is driven nearly completely by ERK. We identified 4666 ERK-dependent phosphosites on 2123 proteins, of which 79 and 66%, respectively, were not previously associated with ERK, substantially expanding the depth and breadth of ERK-dependent phosphorylation events and revealing a considerably more complex function for ERK in cancer. We established that ERK controls a highly dynamic and complex phosphoproteome that converges on cyclin-dependent kinase regulation and RAS homolog guanosine triphosphatase function (RHO GTPase). Our findings establish the most comprehensive molecular portrait and mechanisms by which ERK drives KRAS-dependent pancreatic cancer growth.

RASopathies - what they reveal about RAS/MAPK signaling in skeletal muscle development

RASopathies are rare developmental genetic syndromes caused by germline pathogenic variants in genes that encode components of the RAS/mitogen-activated protein kinase (MAPK) signal transduction pathway. Although the incidence of each RASopathy syndrome is rare, collectively, they represent one of the largest groups of multiple congenital anomaly syndromes and have severe developmental consequences. Here, we review our understanding of how RAS/MAPK dysregulation in RASopathies impacts skeletal muscle development and the importance of RAS/MAPK pathway regulation for embryonic myogenesis. We also discuss the complex interactions of this pathway with other intracellular signaling pathways in the regulation of skeletal muscle development and growth, and the opportunities that RASopathy animal models provide for exploring the use of pathway inhibitors, typically used for cancer treatment, to correct the unique skeletal myopathy caused by the dysregulation of this pathway.

Hyperactivation of MEK1 in cortical glutamatergic neurons results in projection axon deficits and aberrant motor learning

Abnormal extracellular signal-regulated kinase 1/2 (ERK1/2, encoded by Mapk3 and Mapk1, respectively) signaling is linked to multiple neurodevelopmental diseases, especially the RASopathies, which typically exhibit ERK1/2 hyperactivation in neurons and non-neuronal cells. To better understand how excitatory neuron-autonomous ERK1/2 activity regulates forebrain development, we conditionally expressed a hyperactive MEK1 (MAP2K1) mutant, MEK1S217/221E, in cortical excitatory neurons of mice. MEK1S217/221E expression led to persistent hyperactivation of ERK1/2 in cortical axons, but not in soma/nuclei. We noted reduced axonal arborization in multiple target domains in mutant mice and reduced the levels of the activity-dependent protein ARC. These changes did not lead to deficits in voluntary locomotion or accelerating rotarod performance. However, skilled motor learning in a single-pellet retrieval task was significantly diminished in these MEK1S217/221E mutants. Restriction of MEK1S217/221E expression to layer V cortical neurons recapitulated axonal outgrowth deficits but did not affect motor learning. These results suggest that cortical excitatory neuron-autonomous hyperactivation of MEK1 is sufficient to drive deficits in axon outgrowth, which coincide with reduced ARC expression, and deficits in skilled motor learning. Our data indicate that neuron-autonomous decreases in long-range axonal outgrowth may be a key aspect of neuropathogenesis in RASopathies.

Safety and tolerability of topical trametinib in rosacea: Results from a phase I clinical trial

Purpose

Overactivation of the mitogen activated kinase pathway has been associated with rosacea. We hypothesised that inhibitors of this pathway can be repurposed to alleviate rosacea symptoms.

Methods

In order to test this hypothesis, we designed a double-blind, randomised, placebo-controlled phase I clinical trial to assess the safety and tolerability of a first-in-kind topical formulation of a MEK kinase inhibitor, trametinib. Subjects applied daily trametinib-containing cream (0.05 mg in 0.5 mL) to one cheek and cream without inhibitor to the other for consecutive 21 days. Skin irritation scores and blood samples were obtained during visits on days 8, 15 and 22.

Results

On analysis of high-performance liquid chromatography, no systemic trametinib absorption was detected during this treatment period. Subjects demonstrated a slight but significant improvement in both cheeks, regardless of drug contents. No adverse effects were reported during this time.

Conclusions

Topical trametinib was well tolerated at a dose of 0.05 mg per day without meaningful systemic absorption or local adverse events. A dose escalation trial is warranted to determine optimal dosing to treat rosacea while avoiding the adverse effects of systemic treatment.

CRISPRing KRAS: A Winding Road with a Bright Future in Basic and Translational Cancer Research

Once considered "undruggable" due to the strong affinity of RAS proteins for GTP and the structural lack of a hydrophobic "pocket" for drug binding, the development of proprietary therapies for KRAS-mutant tumors has long been a challenging area of research. CRISPR technology, the most successful gene-editing tool to date, is increasingly being utilized in cancer research. Here, we provide a comprehensive review of the application of the CRISPR system in basic and translational research in KRAS-mutant cancer, summarizing recent advances in the mechanistic understanding of KRAS biology and the underlying principles of drug resistance, anti-tumor immunity, epigenetic regulatory networks, and synthetic lethality co-opted by mutant KRAS.

K-RAS Is…Complicated

There is little argument that the K-RAS onco-protein is the most important single oncoprotein in human cancer [...].

Mind the GAP: RASA2 and RASA3 GTPase-activating proteins as gatekeepers of T cell activation and adhesion

Following stimulation, the T cell receptor (TCR) and its coreceptors integrate multiple intracellular signals to initiate T cell proliferation, migration, gene expression, and metabolism. Among these signaling molecules are the small GTPases RAS and RAP1, which induce MAPK pathways and cellular adhesion to activate downstream effector functions. Although many studies have helped to elucidate the signaling intermediates that mediate T cell activation, the molecules and pathways that keep naive T cells in check are less understood. Several recent studies provide evidence that RASA2 and RASA3, which are GAP1-family GTPase-activating proteins (GAPs) that inactivate RAS and RAP1, respectively, are crucial molecules that limit T cell activation and adhesion. In this review we describe recent data on the roles of RASA2 and RASA3 as gatekeepers of T cell activation and migration.

Identifying Fibroblast Growth Factor Receptor 3 as a Mediator of Periosteal Osteochondral Differentiation through the Construction of microRNA-Based Interaction Networks

Human periosteum-derived progenitor cells (hPDCs) have the ability to differentiate towards both the chondrogenic and osteogenic lineages. This coordinated and complex osteochondrogenic differentiation process permits endochondral ossification and is essential in bone development and repair. We have previously shown that humanised cultures of hPDCs enhance their osteochondrogenic potentials in vitro and in vivo; however, the underlying mechanisms are largely unknown. This study aimed to identify novel regulators of hPDC osteochondrogenic differentiation through the construction of miRNA-mRNA regulatory networks derived from hPDCs cultured in human serum or foetal bovine serum as an alternative in silico strategy to serum characterisation. Sixteen differentially expressed miRNAs (DEMis) were identified in the humanised culture. In silico analysis of the DEMis with TargetScan allowed for the identification of 1503 potential miRNA target genes. Upon comparison with a paired RNAseq dataset, a 4.5% overlap was observed (122 genes). A protein-protein interaction network created with STRING interestingly identified FGFR3 as a key network node, which was further predicted using multiple pathway analyses. Functional analysis revealed that hPDCs with the activating mutation FGFR3N540K displayed increased expressions of chondrogenic gene markers when cultured under chondrogenic conditions in vitro and displayed enhanced endochondral bone formation in vivo. A further histological analysis uncovered known downstream mediators involved in FGFR3 signalling and endochondral ossification to be upregulated in hPDC FGFR3N540K-seeded implants. This combinational approach of miRNA-mRNA-protein network analysis with in vitro and in vivo characterisation has permitted the identification of FGFR3 as a novel mediator of hPDC biology. Furthermore, this miRNA-based workflow may also allow for the identification of drug targets, which may be of relevance in instances of delayed fracture repair.

Quantitative proteomics and phosphoproteomics of PP2A-PPP2R5D variants reveal deregulation of RPS6 phosphorylation via converging signaling cascades

Genetic germline variants of PPP2R5D (encoding: phosphoprotein phosphatase 2 regulatory protein 5D) result in PPP2R5D-related disorder (Jordan's Syndrome), which is characterized by intellectual disability, hypotonia, seizures, macrocephaly, autism spectrum disorder, and delayed motor skill development. The disorder originates from de novo single nucleotide mutations, generating missense variants that act in a dominant manner. Pathogenic mutations altering 13 different amino acids have been identified, with the E198K variant accounting for ∼40% of reported cases. However, the generation of a heterozygous E198K variant cell line to study the molecular effects of the pathogenic mutation has been challenging. Here, we use CRISPR-PRIME genomic editing to introduce a transition (c.592G>A) in a single PPP2R5D allele in HEK293 cells, generating E198K-heterozygous lines to complement existing E420K variant lines. We generate global protein and phosphorylation profiles of WT, E198K, and E420K cell lines and find unique and shared changes between variants and WT cells in kinase- and phosphatase-controlled signaling cascades. We observed ribosomal protein S6 (RPS6) hyperphosphorylation as a shared signaling alteration, indicative of increased ribosomal protein S6-kinase activity. Treatment with rapamycin or an RPS6-kinase inhibitor (LY2584702) suppressed RPS6 phosphorylation in both, suggesting upstream activation of mTORC1/p70S6K. Intriguingly, our data suggests ERK-dependent activation of mTORC1 in both E198K and E420K variant cells, with additional AKT-mediated mTORC1 activation in the E420K variant. Thus, although upstream activation of mTORC1 differs between PPP2R5D-related disorder genotypes, inhibition of mTORC1 or RPS6 kinases warrants further investigation as potential therapeutic strategies for patients.

RAS nanoclusters are cell surface transducers that convert extracellular stimuli to intracellular signalling

Mutations of rat sarcoma virus (RAS) oncogenes (HRAS, KRAS and NRAS) can contribute to the development of cancers and genetic disorders (RASopathies). The spatiotemporal organization of RAS is an important property that warrants further investigation. In order to function, wild-type or oncogenic mutants of RAS must be localized to the inner leaflet of the plasma membrane (PM), which is driven by interactions between their C-terminal membrane-anchoring domains and PM lipids. The isoform-specific RAS-lipid interactions promote the formation of nanoclusters on the PM. As main sites for effector recruitment, these nanoclusters are biologically important. Since the spatial distribution of lipids is sensitive to changing environments, such as mechanical and electrical perturbations, RAS nanoclusters act as transducers to convert external stimuli to intracellular mitogenic signalling. As such, effective inhibition of RAS oncogenesis requires consideration of the complex interplay between RAS nanoclusters and various cell surface and extracellular stimuli. In this review, we discuss in detail how, by sorting specific lipids in the PM, RAS nanoclusters act as transducers to convert external stimuli into intracellular signalling.

Dynamic regulation of RAS and RAS signaling

RAS proteins regulate most aspects of cellular physiology. They are mutated in 30% of human cancers and 4% of developmental disorders termed Rasopathies. They cycle between active GTP-bound and inactive GDP-bound states. When active, they can interact with a wide range of effectors that control fundamental biochemical and biological processes. Emerging evidence suggests that RAS proteins are not simple on/off switches but sophisticated information processing devices that compute cell fate decisions by integrating external and internal cues. A critical component of this compute function is the dynamic regulation of RAS activation and downstream signaling that allows RAS to produce a rich and nuanced spectrum of biological outputs. We discuss recent findings how the dynamics of RAS and its downstream signaling is regulated. Starting from the structural and biochemical properties of wild-type and mutant RAS proteins and their activation cycle, we examine higher molecular assemblies, effector interactions and downstream signaling outputs, all under the aspect of dynamic regulation. We also consider how computational and mathematical modeling approaches contribute to analyze and understand the pleiotropic functions of RAS in health and disease.

Combination Therapies with CDK4/6 Inhibitors to Treat KRAS-Mutant Pancreatic Cancer

Mutational loss of CDKN2A (encoding p16INK4A) tumor-suppressor function is a key genetic step that complements activation of KRAS in promoting the development and malignant growth of pancreatic ductal adenocarcinoma (PDAC). However, pharmacologic restoration of p16INK4A function with inhibitors of CDK4 and CDK6 (CDK4/6) has shown limited clinical efficacy in PDAC. Here, we found that concurrent treatment with both a CDK4/6 inhibitor (CDK4/6i) and an ERK-MAPK inhibitor (ERKi) synergistically suppresses the growth of PDAC cell lines and organoids by cooperatively blocking CDK4/6i-induced compensatory upregulation of ERK, PI3K, antiapoptotic signaling, and MYC expression. On the basis of these findings, a Phase I clinical trial was initiated to evaluate the ERKi ulixertinib in combination with the CDK4/6i palbociclib in patients with advanced PDAC (NCT03454035). As inhibition of other proteins might also counter CDK4/6i-mediated signaling changes to increase cellular CDK4/6i sensitivity, a CRISPR-Cas9 loss-of-function screen was conducted that revealed a spectrum of functionally diverse genes whose loss enhanced CDK4/6i growth inhibitory activity. These genes were enriched around diverse signaling nodes, including cell-cycle regulatory proteins centered on CDK2 activation, PI3K-AKT-mTOR signaling, SRC family kinases, HDAC proteins, autophagy-activating pathways, chromosome regulation and maintenance, and DNA damage and repair pathways. Novel therapeutic combinations were validated using siRNA and small-molecule inhibitor-based approaches. In addition, genes whose loss imparts a survival advantage were identified (e.g., RB1, PTEN, FBXW7), suggesting possible resistance mechanisms to CDK4/6 inhibition. In summary, this study has identified novel combinations with CDK4/6i that may have clinical benefit to patients with PDAC.

SIGNIFICANCE

CRISPR-Cas9 screening and protein activity mapping reveal combinations that increase potency of CDK4/6 inhibitors and overcome drug-induced compensations in pancreatic cancer.

Illuminating DEPDC1B in Multi-pronged Regulation of Tumor Progression

DEPDC1B (aliases BRCC3, XTP8, XTP1) is a DEP (Dishevelled, Egl-1, Pleckstrin) and Rho-GAP-like domains containing predominately membrane-associated protein. Earlier, we and others have reported that DEPDC1B is a downstream effector of Raf-1 and long noncoding RNA lncNB1, and an upstream positive effector of pERK. Consistently, DEPDC1B knockdown is associated with downregulation of ligand-stimulated pERK expression. We demonstrate here that DEPDC1B N-terminus binds to the p85 subunit of PI3K, and DEPDC1B overexpression results in decreased ligand-stimulated tyrosine phosphorylation of p85 and downregulation of pAKT1. Collectively, we propose that DEPDC1B is a novel cross-regulator of AKT1 and ERK, two of the prominent pathways of tumor progression. Our data showing high levels of DEPDC1B mRNA and protein during the G2/M phase have significant implications in cell entry into mitosis. Indeed, DEPDC1B accumulation during the G2/M phase has been associated with disassembly of focal adhesions and cell de-adhesion, referred to as a DEPDC1B-mediated de-adhesion mitotic checkpoint. DEPDC1B is a direct target of transcription factor SOX10, and SOX10-DEPDC1B-SCUBE3 axis has been associated with angiogenesis and metastasis. The Scansite analysis of the DEPDC1B amino acid sequence shows binding motifs for three well-established cancer therapeutic targets CDK1, DNA-PK, and aurora kinase A/B. These interactions and functionalities, if validated, may further implicate DEPDC1B in regulation of DNA damage-repair and cell cycle progression processes. Finally, a survey of the publicly available datasets indicates that high DEPDC1B expression is a viable biomarker in breast, lung, pancreatic and renal cell carcinomas, and melanoma. Currently, the systems and integrative biology of DEPDC1B is far from comprehensive. Future investigations are necessary in order to understand how DEPDC1B might impact AKT, ERK, and other networks, albeit in a context-dependent manner, and influence the actionable molecular, spatial, and temporal vulnerabilities within these networks in cancer cells.

Silencing effects of mutant RAS signalling on transcriptomes

Mutated genes of the RAS family encoding small GTP-binding proteins drive numerous cancers, including pancreatic, colon and lung tumors. Besides the numerous effects of mutant RAS gene expression on aberrant proliferation, transformed phenotypes, metabolism, and therapy resistance, the most striking consequences of chronic RAS activation are changes of the genetic program. By performing systematic gene expression studies in cellular models that allow comparisons of pre-neoplastic with RAS-transformed cells, we and others have estimated that 7 percent or more of all transcripts are altered in conjunction with the expression of the oncogene. In this context, the number of up-regulated transcripts approximates that of down-regulated transcripts. While up-regulated transcription factors such as MYC, FOSL1, and HMGA2 have been identified and characterized as RAS-responsive drivers of the altered transcriptome, the suppressed factors have been less well studied as potential regulators of the genetic program and transformed phenotype in the breadth of their occurrence. We therefore have collected information on downregulated RAS-responsive factors and discuss their potential role as tumor suppressors that are likely to antagonize active cancer drivers. To better understand the active mechanisms that entail anti-RAS function and those that lead to loss of tumor suppressor activity, we focus on the tumor suppressor HREV107 (alias PLAAT3 [Phospholipase A and acyltransferase 3], PLA2G16 [Phospholipase A2, group XVI] and HRASLS3 [HRAS-like suppressor 3]). Inactivating HREV107 mutations in tumors are extremely rare, hence epigenetic causes modulated by the RAS pathway are likely to lead to down-regulation and loss of function.

The molecular genetics of RASopathies: An update on novel disease genes and new disorders

Enhanced signaling through RAS and the mitogen-associated protein kinase (MAPK) cascade underlies the RASopathies, a family of clinically related disorders affecting development and growth. In RASopathies, increased RAS-MAPK signaling can result from the upregulated activity of various RAS GTPases, enhanced function of proteins positively controlling RAS function or favoring the efficient transmission of RAS signaling to downstream transducers, functional upregulation of RAS effectors belonging to the MAPK cascade, or inefficient signaling switch-off operated by feedback mechanisms acting at different levels. The massive effort in RASopathy gene discovery performed in the last 20 years has identified more than 20 genes implicated in these disorders. It has also facilitated the characterization of several molecular activating mechanisms that had remained unappreciated due to their minor impact in oncogenesis. Here, we provide an overview on the discoveries collected during the last 5 years that have delivered unexpected insights (e.g., Noonan syndrome as a recessive disease) and allowed to profile new RASopathies, novel disease genes and new molecular circuits contributing to the control of RAS-MAPK signaling.

Functional and biological heterogeneity of KRASQ61 mutations

Missense mutations at the three hotspots in the guanosine triphosphatase (GTPase) RAS-Gly12, Gly13, and Gln61 (commonly known as G12, G13, and Q61, respectively)-occur differentially among the three RAS isoforms. Q61 mutations in KRAS are infrequent and differ markedly in occurrence. Q61H is the predominant mutant (at 57%), followed by Q61R/L/K (collectively 40%), and Q61P and Q61E are the rarest (2 and 1%, respectively). Probability analysis suggested that mutational susceptibility to different DNA base changes cannot account for this distribution. Therefore, we investigated whether these frequencies might be explained by differences in the biochemical, structural, and biological properties of KRASQ61 mutants. Expression of KRASQ61 mutants in NIH 3T3 fibroblasts and RIE-1 epithelial cells caused various alterations in morphology, growth transformation, effector signaling, and metabolism. The relatively rare KRASQ61E mutant stimulated actin stress fiber formation, a phenotype distinct from that of KRASQ61H/R/L/P, which disrupted actin cytoskeletal organization. The crystal structure of KRASQ61E was unexpectedly similar to that of wild-type KRAS, a potential basis for its weak oncogenicity. KRASQ61H/L/R-mutant pancreatic ductal adenocarcinoma (PDAC) cell lines exhibited KRAS-dependent growth and, as observed with KRASG12-mutant PDAC, were susceptible to concurrent inhibition of ERK-MAPK signaling and of autophagy. Our results uncover phenotypic heterogeneity among KRASQ61 mutants and support the potential utility of therapeutic strategies that target KRASQ61 mutant-specific signaling and cellular output.

Crosstalk between the CBM complex/NF-κB and MAPK/P27 signaling pathways of regulatory T cells contributes to the tumor microenvironment

Regulatory T cells (Tregs), which execute their immunosuppressive functions by multiple mechanisms, have been verified to contribute to the tumor microenvironment (TME). Numerous studies have shown that the activation of the CBM complex/NF-κB signaling pathway results in the expression of hypoxia-inducible factor-1 (HIF-1α) and interleukin-6 (IL-6), which initiate the TME formation. HIF-1α and IL-6 promote regulatory T cells (Tregs) proliferation and migration through the MAPK/CDK4/6/Rb and STAT3/SIAH2/P27 signaling pathways, respectively. IL-6 also promotes the production of HIF-1α and enhances the self-regulation of Tregs in the process of tumor microenvironment (TME) formation. In this review, we discuss how the crosstalk between the CARMA1–BCL10–MALT1 signalosome complex (CBM complex)/NF-κB and MAPK/P27 signaling pathways contributes to the formation of the TME, which may provide evidence for potential therapeutic targets in the treatment of solid tumors.

Antibody-Based Approaches to Target Pancreatic Tumours

Pancreatic cancer is an aggressive cancer with a dismal prognosis. This is due to the difficulty to detect the disease at an early and curable stage. In addition, only limited treatment options are available, and they are confronted by mechanisms of resistance. Monoclonal antibody (mAb) molecules are highly specific biologics that can be directly used as a blocking agent or modified to deliver a drug payload depending on the desired outcome. They are widely used to target extracellular proteins, but they can also be employed to inhibit intracellular proteins, such as oncoproteins. While mAbs are a class of therapeutics that have been successfully employed to treat many cancers, they have shown only limited efficacy in pancreatic cancer as a monotherapy so far. In this review, we will discuss the challenges, opportunities and hopes to use mAbs for pancreatic cancer treatment, diagnostics and imagery.

Neurodevelopmental disorders, immunity, and cancer are connected

Immunity could be viewed as the common factor in neurodevelopmental disorders and cancer. The immune and nervous systems coevolve as the embryo develops. Immunity can release cytokines that activate MAPK signaling in neural cells. In specific embryonic brain cell types, dysregulated signaling that results from germline or embryonic mutations can promote changes in chromatin organization and gene accessibility, and thus expression levels of essential genes in neurodevelopment. In cancer, dysregulated signaling can emerge from sporadic somatic mutations during human life. Neurodevelopmental disorders and cancer share similarities. In neurodevelopmental disorders, immunity, and cancer, there appears an almost invariable involvement of small GTPases (e.g., Ras, RhoA, and Rac) and their pathways. TLRs, IL-1, GIT1, and FGFR signaling pathways, all can be dysregulated in neurodevelopmental disorders and cancer. Although there are signaling similarities, decisive differentiating factors are timing windows, and cell type specific perturbation levels, pointing to chromatin reorganization. Finally, we discuss drug discovery.

ERK1b, a 46-kDa ERK Isoform That Is Differentially Regulated by MEK

The extracellular signal‐regulated kinases (ERK) 1 and 2 (ERK1/2) are members of the mitogen‐activated protein kinase family. Using various stimulated rodent cells and kinase activation techniques, we identified a 46‐kDa ERK. The kinetics of activation of this ERK isoform was similar to that of ERK1 and ERK2 under most but not all circumstances. We purified this isoform from rat cells followed by its cloning. The sequence of this isoform revealed that it is an alternatively spliced version of the 44‐kDa ERK1 and therefore we termed it ERK1b. Interestingly, this isoform had a 26‐amino acid insertion between residues 340 and 341 of ERK1, which results from Intron 7 insertion to the sequence. Examining the expression pattern, we found that ERK1b is detected mainly in rat and particularly in Ras‐transformed Rat1 cells. In this cell line, ERK1b was more sensitive to extracellular stimulation than ERK1 and ERK2. Moreover, unlike ERK1 and ERK2, ERK1b had a very low binding affinity to MEK1. This low interaction led to nuclear localization of this isoform when expressed together with MEK1 under conditions in which ERK1 and ERK2 are retained in the cytoplasm. In addition, ERK1b was not coimmunoprecipitated with MEK1. We identified a new, 46‐kDa ERK alternatively spliced isoform. Our results indicate that this isoform is the major one to respond to exogenous stimulation in Ras‐transformed cells, probably due to its differential regulation by MAPK/ERK kinase and by phosphatases.

Not all RAS mutations are equal: A detailed review of the functional diversity of RAS hot spot mutations

The RAS family of small GTPases are among the most frequently mutated oncogenes in human cancer. Approximately 20% of cancers harbor a RAS mutation, and >150 different missense mutations have been detected. Many of these mutations have mutant-specific biochemical defects that alter nucleotide binding and hydrolysis, effector interactions and cell signaling, prompting renewed efforts in the development of anti-RAS therapies, including the mutation-specific strategies. Previously viewed as undruggable, the recent FDA approval of a KRAS^G12C-selective inhibitor has offered real promise to the development of allele-specific RAS therapies. A broader understanding of the mutational consequences on RAS function must be developed to exploit additional allele-specific vulnerabilities. Approximately 94% of RAS mutations occur at one of three mutational "hot spots" at Gly¹², Gly¹³ and Gln⁶¹. Further, the single-nucleotide substitutions represent >99% of these mutations. Within this scope, we discuss the mutational frequencies of RAS isoforms in cancer, mutant-specific effector interactions and biochemical properties. By limiting our analysis to this mutational subset, we simplify the analysis while only excluding a small percentage of total mutations. Combined, these data suggest that the presence or absence of select RAS mutations in human cancers can be linked to their biochemical properties. Continuing to examine the biochemical differences in each RAS-mutant protein will continue to provide additional breakthroughs in allele-specific therapeutic strategies.

Targeting the ERK mitogen-activated protein kinase cascade for the treatment of KRAS-mutant pancreatic cancer

Mutational activation of the KRAS oncogene is found in ~95% of pancreatic ductal adenocarcinoma (PDAC), the major form of pancreatic cancer. With substantial experimental evidence that continued aberrant KRAS function is essential for the maintenance of PDAC tumorigenic growth, the National Cancer Institute has identified the development of effective anti-KRAS therapies as one of four major initiatives for pancreatic cancer research. The recent clinical success in the development of an anti-KRAS therapy targeting one specific KRAS mutant (G12C) supports the significant potential impact of anti-KRAS therapies. However, KRAS^G12C mutations comprise only 2% of KRAS mutations in PDAC. Thus, there remains a dire need for additional therapeutic approaches for targeting the majority of KRAS-mutant PDAC. Among the different directions currently being pursued for anti-KRAS drug development, one of the most promising involves inhibitors of the key KRAS effector pathway, the three-tiered RAF-MEK-ERK mitogen-activated protein kinase (MAPK) cascade. We address the promises and challenges of targeting ERK MAPK signaling as an anti-KRAS therapy for PDAC. In particular, we also summarize the key role of the MYC transcription factor and oncoprotein in supporting ERK-dependent growth of KRAS-mutant PDAC.

Defining RASopathy

The term RASopathy was originally created to describe a phenotypically similar group of medical genetic syndromes caused by germline pathogenic variants in components of the RAS/mitogen-activated protein kinase (RAS/MAPK) pathway. In defining a RASopathy syndrome, one needs to consider the complex nature of the RAS/MAPK pathway, the numerous genes and regulatory components involved, its crosstalk with other signaling pathways and the phenotypic spectrum among these syndromes. Three main guiding principles to the definition should be considered. First, a RASopathy is a clinical syndrome with overlapping phenotypic features caused by germline pathogenic variants associated with the RAS/MAPK pathway. Second, a RASopathy is caused by multiple pathogenetic mechanisms, all of which lead to a similar outcome of RAS/MAPK pathway activation/dysregulation. Finally, because a RASopathy has dysfunctional germline RAS/MAPK pathway activation/dysregulation, it may, therefore, be amenable to treatment with pathway modulators.

Summary: RASopathy is a term used to unify a phenotypically similar group of medical genetic syndromes, yet its definition is challenging and should consider the perspectives of clinicians, scientists and the patients.

How can same-gene mutations promote both cancer and developmental disorders?

The question of how same-gene mutations can drive both cancer and neurodevelopmental disorders has been puzzling. It has also been puzzling why those with neurodevelopmental disorders have a high risk of cancer. Ras, MEK, PI3K, PTEN, and SHP2 are among the oncogenic proteins that can harbor mutations that encode diseases other than cancer. Understanding why some of their mutations can promote cancer, whereas others promote neurodevelopmental diseases, and why even the same mutations may promote both phenotypes, has important clinical ramifications. Here, we review the literature and address these tantalizing questions. We propose that cell type–specific expression of the mutant protein, and of other proteins in the respective pathway, timing of activation (during embryonic development or sporadic emergence), and the absolute number of molecules that the mutations activate, alone or in combination, are pivotal in determining the pathological phenotypes—cancer and (or) developmental disorders.

Alternative Splicing of MAPKs in the Regulation of Signaling Specificity

The mitogen-activated protein kinase (MAPK) cascades transmit signals from extracellular stimuli to a variety of distinct cellular processes. The MAPKKs in each cascade specifically phosphorylate and activate their cognate MAPKs, indicating that this step funnels various signals into a seemingly linear pathway. Still, the effects of these cascades vary significantly, depending on the identity of the extracellular signals, which gives rise to proper outcomes. Therefore, it is clear that the specificity of the signals transmitted through the cascades is tightly regulated in order to secure the desired cell fate. Indeed, many regulatory components or processes that extend the specificity of the cascades have been identified. Here, we focus on a less discussed mechanism, that is, the role of distinct components in each tier of the cascade in extending the signaling specificity. We cover the role of distinct genes, and the alternatively spliced isoforms of MAPKKs and MAPKs, in the signaling specificity. The alternatively spliced MEK1b and ERK1c, which form an independent signaling route, are used as the main example. Unlike MEK1/2 and ERK1/2, this route’s functions are limited, including mainly the regulation of mitotic Golgi fragmentation. The unique roles of the alternatively spliced isoforms indicate that these components play an essential role in determining the proper cell fate in response to distinct stimulations.

CHK1 protects oncogenic KRAS-expressing cells from DNA damage and is a target for pancreatic cancer treatment

We apply genetic screens to delineate modulators of KRAS mutant pancreatic ductal adenocarcinoma (PDAC) sensitivity to ERK inhibitor treatment, and we identify components of the ATR-CHK1 DNA damage repair (DDR) pathway. Pharmacologic inhibition of CHK1 alone causes apoptotic growth suppression of both PDAC cell lines and organoids, which correlates with loss of MYC expression. CHK1 inhibition also activates ERK and AMPK and increases autophagy, providing a mechanistic basis for increased efficacy of concurrent CHK1 and ERK inhibition and/or autophagy inhibition with chloroquine. To assess how CHK1 inhibition-induced ERK activation promotes PDAC survival, we perform a CRISPR-Cas9 loss-of-function screen targeting direct/indirect ERK substrates and identify RIF1. A key component of non-homologous end joining repair, RIF1 suppression sensitizes PDAC cells to CHK1 inhibition-mediated apoptotic growth suppression. Furthermore, ERK inhibition alone decreases RIF1 expression and phenocopies RIF1 depletion. We conclude that concurrent DDR suppression enhances the efficacy of ERK and/or autophagy inhibitors in KRAS mutant PDAC.

Context Matters-Why We Need to Change From a One Size Fits all Approach to Made-to-Measure Therapies for Individual Patients With Pancreatic Cancer

Pancreatic cancer is one of the deadliest cancers and remains a major unsolved health problem. While pancreatic ductal adenocarcinoma (PDAC) is associated with driver mutations in only four major genes (KRAS, TP53, SMAD4, and CDKN2A), every tumor differs in its molecular landscape, histology, and prognosis. It is crucial to understand and consider these differences to be able to tailor treatment regimens specific to the vulnerabilities of the individual tumor to enhance patient outcome. This review focuses on the heterogeneity of pancreatic tumor cells and how in addition to genetic alterations, the subsequent dysregulation of multiple signaling cascades at various levels, epigenetic and metabolic factors contribute to the oncogenesis of PDAC and compensate for each other in driving cancer progression if one is tackled by a therapeutic approach. This implicates that besides the need for new combinatorial therapies for PDAC, a personalized approach for treating this highly complex cancer is required. A strategy that combines both a target-based and phenotypic approach to identify an effective treatment, like Reverse Clinical Engineering® using patient-derived organoids, is discussed as a promising way forward in the field of personalized medicine to tackle this deadly disease.

Targeting small GTPases and their downstream pathways with intracellular macromolecule binders to define alternative therapeutic strategies in cancer

The RAS superfamily of small GTPases regulates major physiological cellular processes. Mutation or deregulation of these small GTPases, their regulators and/or their effectors are associated with many diseases including cancer. Hence, targeting these classes of proteins is an important therapeutic strategy in cancer. This has been recently achieved with the approval of the first KRASG12C covalent inhibitors for the clinic. However, many other mutants and small GTPases are still considered as 'undruggable' with small molecule inhibitors because of a lack of well-defined pocket(s) at their surface. Therefore, alternative therapeutic strategies have been developed to target these proteins. In this review, we discuss the use of intracellular antibodies and derivatives - reagents that bind their antigen inside the cells - for the discovery of novel inhibitory mechanisms, targetable features and therapeutic strategies to inhibit small GTPases and their downstream pathways. These reagents are also versatile tools used to better understand the biological mechanisms regulated by small GTPases and to accelerate the drug discovery process.