The D816V mutation of c-Kit circumvents a requirement for Src family kinases in c-Kit signal transduction

Four and a half LIM domains 2 (FHL2) attenuates tumorigenesis of gastrointestinal stromal tumors (GISTs) by negatively regulating KIT signaling

Gastrointestinal stromal tumors (GISTs) are predominately induced by KIT mutants. In this study, we found that four and a half LIM domains 2 (FHL2) was highly expressed in GISTs and KIT signaling dramatically increased FHL2 transcription while FHL2 inhibited KIT transcription. In addition, our results showed that FHL2 associated with KIT and increased the ubiquitination of both wild-type KIT and primary KIT mutants in GISTs, leading to decreased expression and activation of KIT although primary KIT mutants were less inhibited by FHL2 than wild-type KIT. In the animal experiments, loss of FHL2 expression in mice carrying germline KIT/V558A mutation which can develop GISTs resulted in increased tumor growth, but increased sensitivity of GISTs to imatinib treatment which is used as the first-line targeted therapy of GISTs, suggesting that FHL2 plays a role in the response of GISTs to KIT inhibitor. Unlike wild-type KIT and primary KIT mutants, we further found that FHL2 didn't alter the expression and activation of drug-resistant secondary KIT mutants. Taken together, our results indicated that FHL2 acts as the negative feedback of KIT signaling in GISTs while primary KIT mutants are less sensitive and secondary KIT mutants are resistant to the inhibition of FHL2.

RAF1 facilitates KIT signaling and serves as a potential treatment target for gastrointestinal stromal tumor

The aberrant activation of RAS/RAF/MEK/ERK signaling is important for KIT mutation-mediated tumorigenesis of gastrointestinal stromal tumor (GIST). In this study, we found that inhibition of RAF1 suppresses the activation of both wild-type KIT and primary KIT mutations in GIST, with primary KIT mutations showing greater sensitivity. This suggests a positive feedback loop between KIT and RAF1, wherein RAF1 facilitates KIT signaling. We further demonstrated that RAF1 associates with KIT and the kinase activity of RAF1 is necessary for its contribution to KIT activation. Accordingly, inhibition of RAF1 suppressed cell survival, proliferation, and cell cycle progression in vitro mediated by both wild-type KIT and primary KIT mutations. Inhibition of RAF1 in vivo suppressed GIST growth in a transgenic mouse model carrying germline KIT/V558A mutation, showing a similar treatment efficiency as imatinib, the first-line targeted therapeutic drug of GIST, while the combination use of imatinib and RAF1 inhibitor further suppressed tumor growth. Acquisition of drug-resistant secondary mutation of KIT is a major cause of treatment failure of GIST following targeted therapy. Like wild-type KIT and primary KIT mutations, inhibition of RAF1 suppressed the activation of secondary KIT mutation, and the cell survival, proliferation, cell cycle progression in vitro, and tumor growth in vivo mediated by secondary KIT mutation. However, the activation of secondary KIT mutation is less dependent on RAF1 compared with that of primary KIT mutations. Taken together, our results revealed that RAF1 facilitates KIT signaling and KIT mutation-mediated tumorigenesis of GIST, providing a rationale for further investigation into the use of RAF1 inhibitors alone or in combination with KIT inhibitor in the treatment of GIST, particularly in cases resistant to KIT inhibitors.

SPRY4 inhibits and sensitizes the primary KIT mutants in gastrointestinal stromal tumors (GISTs) to imatinib

BACKGROUND

KIT is frequently mutated in gastrointestinal stromal tumors (GISTs), and the treatment of GISTs largely relies on targeting KIT currently. In this study, we aimed to investigate the role of sprouty RTK signaling antagonist 4 (SPRY4) in GISTs and related mechanisms.

METHODS

Ba/F3 cells and GIST-T1 cell were used as cell models, and mice carrying germline KIT/V558A mutation were used as animal model. Gene expression was examined by qRT-PCR and western blot. Protein association was examined by immunoprecipitation.

RESULTS

Our study revealed that KIT increased the expression of SPRY4 in GISTs. SPRY4 was found to bind to both wild-type KIT and primary KIT mutants in GISTs, and inhibited KIT expression and activation, leading to decreased cell survival and proliferation mediated by KIT. We also observed that inhibition of SPRY4 expression in KITV558A/WT mice led to increased tumorigenesis of GISTs in vivo. Moreover, our results demonstrated that SPRY4 enhanced the inhibitory effect of imatinib on the activation of primary KIT mutants, as well as on cell proliferation and survival mediated by the primary KIT mutants. However, in contrast to this, SPRY4 did not affect the expression and activation of drug-resistant secondary KIT mutants, nor did it affect the sensitivity of secondary KIT mutants to imatinib. These findings suggested that secondary KIT mutants regulate a different downstream signaling cascade than primary KIT mutants.

CONCLUSIONS

Our results suggested that SPRY4 acts as negative feedback of primary KIT mutants in GISTs by inhibiting KIT expression and activation. It can increase the sensitivity of primary KIT mutants to imatinib. In contrast, secondary KIT mutants are resistant to the inhibition of SPRY4.

Penttinen syndrome-associated PDGFRB Val665Ala variant causes aberrant constitutive STAT1 signalling

Penttinen syndrome is a rare progeroid disorder caused by mutations in platelet‐derived growth factor (PDGF) receptor beta (encoded by the PDGFRB proto‐oncogene) and characterized by a prematurely aged appearance with lipoatrophy, skin lesions, thin hair and acro‐osteolysis. Activating mutations in PDGFRB have been associated with other human diseases, including Kosaki overgrowth syndrome, infantile myofibromatosis, fusiform aneurysms, acute lymphoblastic leukaemia and myeloproliferative neoplasms associated with eosinophilia. The goal of the present study was to characterize the PDGFRB p.Val665Ala variant associated with Penttinen syndrome at the molecular level. This substitution is located in a conserved loop of the receptor tyrosine kinase domain. We observed that the mutant receptor was expressed at a lower level but showed constitutive activity. In the absence of ligand, the mutant activated STAT1 and elicited an interferon‐like transcriptional response. Phosphorylation of STAT3, STAT5, AKT and phospholipase Cγ was weak or undetectable. It was devoid of oncogenic activity in two cell proliferation assays, contrasting with classical PDGF receptor oncogenic mutants. STAT1 activation was not sensitive to ruxolitinib and did not rely on interferon‐JAK2 signalling. Another tyrosine kinase inhibitor, imatinib, blocked signalling by the p.Val665Ala variant at a higher concentration compared with the wild‐type receptor. Importantly, this concentration remained in the therapeutic range. Dasatinib, nilotinib and ponatinib also inhibited the mutant receptor. In conclusion, the p.Val665Ala variant confers unique features to PDGF receptor β compared with other characterized gain‐of‐function mutants, which may in part explain the particular set of symptoms associated with Penttinen syndrome.

Novel potential oncogenic and druggable mutations of FGFRs recur in the kinase domain across cancer types

Fibroblast growth factor receptors (FGFRs) are recurrently altered by single nucleotide variants (SNVs) in many human cancers. The prevalence of SNVs in FGFRs depends on the cancer type. In some tumors, such as the urothelial carcinoma, mutations of FGFRs occur at very high frequency (up to 60%). Many characterized mutations occur in the extracellular or transmembrane domains, while fewer known mutations are found in the kinase domain. In this study, we performed a bioinformatics analysis to identify novel putative cancer driver or therapeutically actionable mutations of the kinase domain of FGFRs. To pinpoint those mutations that may be clinically relevant, we exploited the recurrence of alterations on analogous amino acid residues within the kinase domain (PK_Tyr_Ser-Thr) of different kinases as a predictor of functional impact. By exploiting MutationAligner and LowMACA bioinformatics resources, we highlighted novel uncharacterized mutations of FGFRs which recur in other protein kinases. By revealing unanticipated correspondence with known variants, we were able to infer their functional effects, as alterations clustering on similar residues in analogous proteins have a high probability to elicit similar effects. As FGFRs represent an important class of oncogenes and drug targets, our study opens the way for further studies to validate their driver and/or actionable nature and, in the long term, for a more efficacious application of precision oncology.

A novel anti-c-Kit antibody-drug conjugate to treat wild-type and activating-mutant c-Kit-positive tumors

c‐Kit overexpression and activating mutations, which are reported in various cancers, including gastrointestinal stromal tumor (GIST), small‐cell lung cancer (SCLC), acute myeloid leukemia, acral melanoma, and systemic mastocytosis (SM), confer resistance to tyrosine kinase inhibitors (TKIs). To overcome TKI resistance, an anti‐c‐Kit antibody–drug conjugate was developed in this study to treat wild‐type and mutant c‐Kit‐positive cancers. NN2101, a fully human IgG1, was conjugated to DM1, a microtubule inhibitor, through N‐succinimidyl‐4‐(N‐maleimidomethyl) cyclohexane‐1‐carboxylate (SMCC) (to give NN2101‐DM1). The antitumor activity of NN2101‐DM1 was evaluated in vitro and in vivo using various cancer cell lines. NN2101‐DM1 exhibited potent growth‐inhibitory activities against c‐Kit‐positive cancer cell lines. In a mouse xenograft model, NN2101‐DM1 exhibited potent growth‐inhibitory activities against imatinib‐resistant GIST and SM cells. In addition, NN2101‐DM1 exhibited a significantly higher anti‐cancer effect than carboplatin/etoposide against SCLC cells where c‐Kit does not mediate cancer pathogenesis. Furthermore, the combination of NN2101‐DM1 with imatinib in imatinib‐sensitive GIST cells induced complete remission compared with treatment with NN2101‐DM1 or imatinib alone in mouse xenograft models. These results suggest that NN2101‐DM1 is a potential therapeutic agent for wild‐type and mutant c‐Kit‐positive cancers.

Computational studies of anaplastic lymphoma kinase mutations reveal common mechanisms of oncogenic activation

High-risk tumors are genomically heterogeneous, harboring gene amplifications and mutations. The activation status of mutated proteins in cancer can profoundly impact disease progression, patient response, and drug sensitivity. Yet, outside of a few hotspot mutations, functional studies of clinically observed mutations are not commonly pursued. We report a combined experimental profiling and computational analysis of the effects of clinically observed and “test” mutations in the kinase domain of anaplastic lymphoma kinase (ALK), a known oncogenic driver in pediatric neuroblastoma. We find that the activation status of the mutated protein is a good indicator of the transforming ability in NIH 3T3 cells. We also report biophysical as well as data-driven models with predictive power to profile these mutant kinases in silico.

Kinases play important roles in diverse cellular processes, including signaling, differentiation, proliferation, and metabolism. They are frequently mutated in cancer and are the targets of a large number of specific inhibitors. Surveys of cancer genome atlases reveal that kinase domains, which consist of 300 amino acids, can harbor numerous (150 to 200) single-point mutations across different patients in the same disease. This preponderance of mutations—some activating, some silent—in a known target protein make clinical decisions for enrolling patients in drug trials challenging since the relevance of the target and its drug sensitivity often depend on the mutational status in a given patient. We show through computational studies using molecular dynamics (MD) as well as enhanced sampling simulations that the experimentally determined activation status of a mutated kinase can be predicted effectively by identifying a hydrogen bonding fingerprint in the activation loop and the αC-helix regions, despite the fact that mutations in cancer patients occur throughout the kinase domain. In our study, we find that the predictive power of MD is superior to a purely data-driven machine learning model involving biochemical features that we implemented, even though MD utilized far fewer features (in fact, just one) in an unsupervised setting. Moreover, the MD results provide key insights into convergent mechanisms of activation, primarily involving differential stabilization of a hydrogen bond network that engages residues of the activation loop and αC-helix in the active-like conformation (in >70% of the mutations studied, regardless of the location of the mutation).

Evolution of KIPPIS as a versatile platform for evaluating intracellularly functional peptide aptamers

Chimeric proteins have been widely used to evaluate intracellular protein–protein interactions (PPIs) in living cells with various readouts. By combining an interleukin-3-dependent murine cells and chimeric proteins containing a receptor tyrosine kinase c-kit, we previously established a c-kit-based PPI screening (KIPPIS) system to evaluate and select protein binders. In the KIPPIS components, proteins of interest are connected with a chemically inducible helper module and the intracellular domain of the growth-signaling receptor c-kit, which detects PPIs based on cell proliferation as a readout. In this system, proteins of interest can be incorporated into chimeric proteins without any scaffold proteins, which would be advantageous for evaluating interaction between small peptides/domains. To prove this superiority, we apply KIPPIS to 6 peptide aptamer–polypeptide pairs, which are derived from endogenous, synthetic, and viral proteins. Consequently, all of the 6 peptide aptamer–polypeptide interactions are successfully detected by cell proliferation. The detection sensitivity can be modulated in a helper ligand-dependent manner. The assay results of KIPPIS correlate with the activation levels of Src, which is located downstream of c-kit-mediated signal transduction. Control experiments reveal that KIPPIS clearly discriminates interacting aptamers from non-interacting ones. Thus, KIPPIS proves to be a versatile platform for evaluating the binding properties of peptide aptamers.

Protein tyrosine phosphatase receptor type E (PTPRE) regulates the activation of wild-type KIT and KIT mutants differently

Activation of receptor tyrosine kinases needs tight control by tyrosine phosphatases to keep their normal function. In this study, we investigated the regulation of activation of the type III receptor tyrosine kinase KIT by protein tyrosine phosphatase receptor type E (PTPRE). We found that PTPRE can associate with wild-type KIT and inhibit KIT activation in a dose-dependent manner, although the activation of wild-type KIT is dramatically inhibited even when PTPRE is expressed at low level. The D816V mutation of KIT is the most frequently found oncogenic mutation in mastocytosis, and we found that PTPRE can associate and inhibit the activation of KIT/D816V in a dose dependent manner, but the inhibition is much weaker compared with wild-type KIT. Similar to mastocytosis, KIT mutations are the main oncogenic mutations in gastrointestinal stromal tumors (GISTs) although GISTs carry different types of KIT mutations. We further studied the regulation of the activation of GISTs-type KIT mutants and other mastocytosis-type KIT mutants by PTPRE. Indeed, PTPRE can almost block the activation of GISTs-type KIT mutants, while the activation of mastocytosis-type KIT mutants is more resistant to the inhibition of PTPRE. Taken together, our results suggest that PTPRE can associate with KIT, and inhibit the activation of both wild-type KIT and GISTs-type KIT mutants, while the activation of mastocytosis-type KIT mutants is more resistant to PTPRE.

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PTPRE associates with wild-type KIT and KIT mutants.

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PTPRE inhibits the activation of both wild-type KIT and GISTs-type KIT mutants dramatically.

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The activation of mastocytosis-type KIT mutants are more resistant to the inhibition of PTPRE.

Persistent Human KIT Receptor Signaling Disposes Murine Placenta to Premature Differentiation Resulting in Severely Disrupted Placental Structure and Functionality

Activating mutations in the human KIT receptor is known to drive severe hematopoietic disorders and tumor formation spanning various entities. The most common mutation is the substitution of aspartic acid at position 816 to valine (D816V), rendering the receptor constitutively active independent of ligand binding. As the role of the KIT receptor in placental signaling cascades is poorly understood, we analyzed the impact of KIT^D816V expression on placental development using a humanized mouse model. Placentas from KIT^D816V animals present with a grossly changed morphology, displaying a reduction in labyrinth and spongiotrophoblast layer and an increase in the Parietal Trophoblast Giant Cell (P-TGC) layer. Elevated differentiation to P-TGCs was accompanied with reduced differentiation to other Trophoblast Giant Cell (TGC) subtypes and by severe decrease in proliferation. The embryos display growth retardation and die in utero. KIT^D816V-trophoblast stem cells (TSC) differentiate much faster compared to wild type (WT) controls. In undifferentiated KIT^D816V-TSCs, levels of Phosphorylated Extracellular-signal Regulated Kinase (P-ERK) and Phosphorylated Protein Kinase B (P-AKT) are comparable to wildtype cultures differentiating for 3–6 days. Accordingly, P-TGC markers Placental Lactogen 1 (PL1) and Proliferin (PLF) are upregulated as well. The results reveal that KIT signaling orchestrates the fine-tuned differentiation of the placenta, with special emphasis on P-TGC differentiation. Appropriate control of KIT receptor action is therefore essential for placental development and nourishment of the embryo.

SRC-Family Kinases in Acute Myeloid Leukaemia and Mastocytosis

Protein tyrosine kinases have been recognized as important actors of cell transformation and cancer progression, since their discovery as products of viral oncogenes. SRC-family kinases (SFKs) play crucial roles in normal hematopoiesis. Not surprisingly, they are hyperactivated and are essential for membrane receptor downstream signaling in hematological malignancies such as acute myeloid leukemia (AML) and mastocytosis. The precise roles of SFKs are difficult to delineate due to the number of substrates, the functional redundancy among members, and the use of tools that are not selective. Yet, a large num ber of studies have accumulated evidence to support that SFKs are rational therapeutic targets in AML and mastocytosis. These two pathologies are regulated by two related receptor tyrosine kinases, which are well known in the field of hematology: FLT3 and KIT. FLT3 is one of the most frequently mutated genes in AML, while KIT oncogenic mutations occur in 80-90% of mastocytosis. Studies on oncogenic FLT3 and KIT signaling have shed light on specific roles for members of the SFK family. This review highlights the central roles of SFKs in AML and mastocytosis, and their interconnection with FLT3 and KIT oncoproteins.

FMS-like Tyrosine Kinase 3/FLT3: From Basic Science to Clinical Implications

FMS-like tyrosine kinase 3 (FLT3) is a receptor tyrosine kinase that is expressed almost exclusively in the hematopoietic compartment. Its ligand, FLT3 ligand (FL), induces dimerization and activation of its intrinsic tyrosine kinase activity. Activation of FLT3 leads to its autophosphorylation and initiation of several signal transduction cascades. Signaling is initiated by the recruitment of signal transduction molecules to activated FLT3 through binding to specific phosphorylated tyrosine residues in the intracellular region of FLT3. Activation of FLT3 mediates cell survival, cell proliferation, and differentiation of hematopoietic progenitor cells. It acts in synergy with several other cytokines to promote its biological effects. Deregulated FLT3 activity has been implicated in several diseases, most prominently in acute myeloid leukemia where around one-third of patients carry an activating mutant of FLT3 which drives the disease and is correlated with poor prognosis. Overactivity of FLT3 has also been implicated in autoimmune diseases, such as rheumatoid arthritis. The observation that gain-of-function mutations of FLT3 can promote leukemogenesis has stimulated the development of inhibitors that target this receptor. Many of these are in clinical trials, and some have been approved for clinical use. However, problems with acquired resistance to these inhibitors are common and, furthermore, only a fraction of patients respond to these selective treatments. This review provides a summary of our current knowledge regarding structural and functional aspects of FLT3 signaling, both under normal and pathological conditions, and discusses challenges for the future regarding the use of targeted inhibition of these pathways for the treatment of patients.

Computational algorithms for in silico profiling of activating mutations in cancer

Methods to catalog and computationally assess the mutational landscape of proteins in human cancers are desirable. One approach is to adapt evolutionary or data-driven methods developed for predicting whether a single-nucleotide polymorphism (SNP) is deleterious to protein structure and function. In cases where understanding the mechanism of protein activation and regulation is desired, an alternative approach is to employ structure-based computational approaches to predict the effects of point mutations. Through a case study of mutations in kinase domains of three proteins, namely, the anaplastic lymphoma kinase (ALK) in pediatric neuroblastoma patients, serine/threonine-protein kinase B-Raf (BRAF) in melanoma patients, and erythroblastic oncogene B 2 (ErbB2 or HER2) in breast cancer patients, we compare the two approaches above. We find that the structure-based method is most appropriate for developing a binary classification of several different mutations, especially infrequently occurring ones, concerning the activation status of the given target protein. This approach is especially useful if the effects of mutations on the interactions of inhibitors with the target proteins are being sought. However, many patients will present with mutations spread across different target proteins, making structure-based models computationally demanding to implement and execute. In this situation, data-driven methods-including those based on machine learning techniques and evolutionary methods-are most appropriate for recognizing and illuminate mutational patterns. We show, however, that, in the present status of the field, the two methods have very different accuracies and confidence values, and hence, the optimal choice of their deployment is context-dependent.

The upregulation of Pim kinases is essential in coordinating the survival, proliferation, and migration of KIT D816V-mutated neoplastic mast cells

About ˜80% of mast cell neoplasm patients harbor the c-Kit activating mutation D816 V, which is associated with c-Kit inhibitor resistance and poor prognosis. However, the molecular basis for these effects is not fully known. To address this issue, in this study we screened molecules whose expression is altered by KIT D816 V mutation and found that Pim kinases were overexpressed in D816V-mutant neoplastic mast cells. This was accompanied by upregulation of signal transducer and activator of transcription (STAT) and mammalian target of rapamycin (mTOR) and downregulation of Akt and extracellular signal-regulated kinase (ERK1/2). Activated Pim kinases promoted the survival of D816 V cells by maintaining mTOR and p70S6K activation even under nutrient starvation. Conversely, cell proliferation was suppressed by inhibiting Pim kinases. The mRNA level of C-X-C chemokine receptor type 4 (CXCR4) was about 2-fold higher in D816 V cells; this was associated with a 2-fold increase in migratory capacity, which was modulated by Pim kinases. We also confirmed that upregulation of Pim kinases is a feature specific to cells with the D816 V mutation and is not observed in cells with the c-Kit activating N822 K mutation. These data suggest Pim kinases as a promising therapeutic target for the treatment of mast cell neoplasms with KIT D816 V mutation.

Myo1f, an Unconventional Long-Tailed Myosin, Is a New Partner for the Adaptor 3BP2 Involved in Mast Cell Migration

Mast cell chemotaxis is essential for cell recruitment to target tissues, where these cells play an important role in adaptive and innate immunity. Stem cell factor (SCF) is a major chemoattractant for mast cells. SCF binds to the KIT receptor, thereby triggering tyrosine phosphorylation in the cytoplasmic domain and resulting in docking sites for SH2 domain-containing molecules, such as Lyn and Fyn, and the subsequent activation of the small GTPases Rac that are responsible for cytoskeletal reorganization and mast cell migration. In previous works we have reported the role of 3BP2, an adaptor molecule, in mast cells. 3BP2 silencing reduces FcεRI-dependent degranulation, by targeting Lyn and Syk phosphorylation, as well as SCF-dependent cell survival. This study examines its role in SCF-dependent migration and reveals that 3BP2 silencing in human mast cell line (LAD2) impairs cell migration due to SCF and IgE. In that context we found that 3BP2 silencing decreases Rac-2 and Cdc42 GTPase activity. Furthermore, we identified Myo1f, an unconventional type-I myosin, as a new partner for 3BP2. This protein, whose functions have been described as critical for neutrophil migration, remained elusive in mast cells. Myo1f is expressed in mast cells and colocalizes with cortical actin ring. Interestingly, Myo1f-3BP2 interaction is modulated by KIT signaling. Moreover, SCF dependent adhesion and migration through fibronectin is decreased after Myo1f silencing. Furthermore, Myo1f silencing leads to downregulation of β1 and β7 integrins on the mast cell membrane. Overall, Myo1f is a new 3BP2 ligand that connects the adaptor to actin cytoskeleton and both molecules are involved in SCF dependent mast cell migration.

Tyrosine Kinase Inhibition in Mastocytosis: KIT and Beyond KIT

Mastocytosis is a group of rare disorders characterized by abnormal accumulation of mast cells in one or several organs. Mastocytosis can be seen at any age; but, in adults, the disease is usually systemic and chronic. Patients with indolent systemic mastocytosis (SM) are usually treated symptomatically, but cytoreductive treatments are needed in more advanced SM. In most patients with SM, an activating KIT D816V mutation is found. Thus, patients with advanced SM benefit from treatment with KIT-targeting tyrosine kinase inhibitors. However, none of these drugs are curative; new targeted drugs or combinations are still needed to improve patients' outcome.

Gene networks in basal cell carcinoma of the eyelid, analyzed using gene expression profiling

Basal cell carcinoma (BCC) is the most frequent malignant tumor of the eyelid; it progresses slowly and rarely metastasizes. However, BCC of the eyelid is partially invasive and can extend to the surrounding ocular adnexa even if appropriate treatment is performed. To understand the molecular mechanism underlying its pathogenesis, global gene expression analysis of surgical tissue samples of BCC of the eyelid (n=2) and normal human epidermal keratinocytes was performed using a GeneChip^® system. The histopathological examination of surgically removed eyelid tissues showed the tumor nest composed with small basaloid. In the samples from patients 1 and 2, 687 and 713 genes were identified, respectively, demonstrating ≥5.0-fold higher expression than that noted in normal human epidermal keratinocytes. For the 640 genes with upregulated expression in both patient samples, Ingenuity^® pathway analysis showed that the gene network in BCC of the eyelid included many BCC-associated genes, such as the following: BCL2 apoptosis regulator; Patched-1; and SRY-box 9. In addition, unique gene networks related to cancer cell growth, tumorigenesis, and cell survival were identified. These results of integrating microarray analyses provide further insights into the molecular mechanisms involved in BCC of the eyelid and may provide a therapeutic approach for this disease.

Preclinical human models and emerging therapeutics for advanced systemic mastocytosis

Mastocytosis is a term used to denote a group of rare diseases characterized by an abnormal accumulation of neoplastic mast cells in various tissues and organs. In most patients with systemic mastocytosis, the neoplastic cells carry activating mutations in KIT Progress in mastocytosis research has long been hindered by the lack of suitable in vitro models, such as permanent human mast cell lines. In fact, only a few human mast cell lines are available to date: HMC-1, LAD1/2, LUVA, ROSA and MCPV-1. The HMC-1 and LAD1/2 cell lines were derived from patients with mast cell leukemia. By contrast, the more recently established LUVA, ROSA and MCPV-1 cell lines were derived from CD34⁺ cells of non-mastocytosis donors. While some of these cell lines (LAD1/2, LUVA, ROSA^{KIT WT} and MCPV-1) do not harbor KIT mutations, HMC-1 and ROSA^{KIT D816V} cells exhibit activating KIT mutations found in mastocytosis and have thus been used to study disease pathogenesis. In addition, these cell lines are increasingly employed to validate new therapeutic targets and to screen for effects of new targeted drugs. Recently, the ROSA^{KIT D816V} subclone has been successfully used to generate a unique in vivo model of advanced mastocytosis by injection into immunocompromised mice. Such a model may allow in vivo validation of data obtained in vitro with targeted drugs directed against mastocytosis. In this review, we discuss the major characteristics of all available human mast cell lines, with particular emphasis on the use of HMC-1 and ROSA^{KIT D816V} cells in preclinical therapeutic research in mastocytosis.

XK-related protein 5 (XKR5) is a novel negative regulator of KIT/D816V-mediated transformation

In order to investigate the molecular mechanisms by which the oncogenic mutant KIT/D816V causes transformation of cells, we investigated proteins that selectively bind KIT/D816V, but not wild-type KIT, as potential mediators of transformation. By mass spectrometry several proteins were identified, among them a previously uncharacterized protein denoted XKR5 (XK-related protein 5), which is related to the X Kell blood group proteins. We could demonstrate that interaction between XKR5 and KIT/D816V leads to phosphorylation of XKR5 at Tyr 369, Tyr487, and Tyr 543. Tyrosine phosphorylated XKR5 acts as a negative regulator of KIT signaling, which leads to downregulation of phosphorylation of ERK, AKT, and p38. This led to reduced proliferation and colony forming capacity in semi-solid medium. Taken together, our data demonstrate that XKR5 is a novel type of negative regulator of KIT-mediated transformation.

Evolution of oncogenic signatures of mutation hotspots in tyrosine kinases supports the atavistic hypothesis of cancer

Cancer has been shown as an evolutionary process emerging hallmarks that are reminiscent of unicellular organisms. Since cancer is mostly driven by somatic mutations, especially by oncogenic hotspot mutations, we proposed a molecular atavism of cancer caused by gain-of-function mutations in oncogenes. As tyrosine kinase (TK) family contains the largest subgroup of oncogenes with hotspot mutations, we traced the most predominant mutation hotspots of TK oncogenes across phylogeny with the domain information and adjacent sequences integrated as onco-signatures. We detected 9 out of 17 TK oncogenes with onco-homologs possessing an onco-signature, which could be divided into two classes by whether their onco-homologs existed in mammals or not. In Class I we identified mammalian onco-homologs assuming oncogenic functions with onco-signatures always intact in cancer, such as HCK and LYN. In Class II with no bona fide mammalian onco-homologs, Pyk2, a protist onco-homolog with an onco-signature of BRAF was found assuming oncogenic-like functions. Onco-signatures in both classes root deep in the primitive system. Together, these evidences supported our proposal that cancer can be driven by reverse evolution of oncogenes through gain-of-function mutations. And also for the first time, we provided the specific targets for experimental verification of the atavistic hypothesis of cancer.

A change in structural integrity of c-Kit mutant D816V causes constitutive signaling

Several signaling pathways, ligands, and genes that regulate proliferative and self-renewal properties of the Hematopoietic Stem Cells (HSCs) have been studied meticulously. One of the signaling pathways that play a crucial role in the process of hematopoiesis is the Stem Cell Factor (SCF) mediated c-Kit pathway. The c-Kit is a Receptor Tyrosine Kinase (RTK), which is expressed in the cells including HSCs. It undergoes dimerization upon binding with its cognate ligand SCF. As a result, phosphorylation of the Juxtamembrane (JM) domain of c-Kit takes place at Tyr568 and Tyr570 residues. These phosphorylated residues become the docking sites for protein tyrosine phosphatases (PTPs) namely SHP-1 and SHP-2, which in turn cause dephosphorylation and negative regulation of the downstream signaling responsible for the cell proliferation. Interestingly, it has been reported that the mutation of c-Kit at D816V makes it independent of SCF stimulation and SHP-1/SHP-2 inhibition, thereby, causing its constitutive activation. The present study was commenced to elucidate the structural behavior of this mutation in the JM and A-loop region of c-Kit using Molecular Dynamics (MD) simulations of the wild-type and mutant c-Kit in unphosphorylated and phosphorylated states. The energy difference computed between the wild type and mutant (D816V) c-Kit, and protein-protein docking and complex analysis revealed the impact of this single residue mutation on the integrity dynamics of c-Kit that makes it independent of SHP-1/SHP-2 negative regulation.

Internal tandem duplication mutations in the tyrosine kinase domain of FLT3 display a higher oncogenic potential than the activation loop D835Y mutation

Acute myeloid leukemia (AML) remains the most common form of acute leukemia among adults and accounts for a large number of leukemia-related deaths. Mutations in FMS-like tyrosine kinase 3 (FLT3) is one of the most prevalent findings in this heterogeneous disease. The major types of mutations in FLT3 can be categorized as internal tandem duplications (ITD) and point mutations. Recent studies suggest that ITDs not only occur in the juxtamembrane region as originally described, but also in the kinase domain. Although the juxtamembrane ITDs have been well characterized, the tyrosine kinase domain ITDs have not yet been thoroughly studied due to their recent discovery. For this reason, we compared ITD mutations in the juxtamembrane domain with those in the tyrosine kinase domain, as well as with the most common activating point mutation in the tyrosine kinase domain, D835Y. The purpose of this study was to understand whether it is the nature of the mutation or the location of the mutation that plays the main role in leukemogenesis. The various FLT3 mutants were expressed in the murine pro-B cell line Ba/F3 and examined for their capacity to form colonies in semisolid medium. The size and number of colonies formed by Ba/F3 cells expressing either the internal tandem duplication within juxtamembrane domain of the receptor (JMD-ITD) or the tyrosine kinase domain (TKD)-ITD were indistinguishable, while Ba/F3 cells expressing D835Y/FLT3 failed to form colonies. Cell proliferation and cell survival was also significantly higher in TKD-ITD expressing cells, compared to cells expressing D835Y/FLT3. Furthermore, TKD-ITD is capable of inducing phosphorylation of STAT5, while D835Y/FLT3 fails to induce tyrosine phosphorylation of STAT5. Other signal transduction pathways such as the RAS/ERK and the PI3K/AKT pathways were activated to the same level in TKD-ITD cells as compared to D835Y/FLT3 expressing cells. Taken together, our data suggest that TKD-ITD displays similar oncogenic potential to the JMD-ITD but a higher oncogenic potential than the D835Y point mutation.

Oncogenic Kit signalling on the Golgi is suppressed by blocking secretory trafficking with M-COPA in gastrointestinal stromal tumours

Most gastrointestinal stromal tumours (GISTs) are caused by constitutively active mutations in Kit tyrosine kinase. The drug imatinib, a specific Kit inhibitor, improves the prognosis of metastatic GIST patients, but these patients become resistant to the drug by acquiring secondary mutations in the Kit kinase domain. We recently reported that a Kit mutant causes oncogenic signals only on the Golgi apparatus in GISTs. In this study, we show that in GIST, 2-methylcoprophilinamide (M-COPA, also known as "AMF-26"), an inhibitor of biosynthetic protein trafficking from the endoplasmic reticulum (ER) to the Golgi, suppresses Kit autophosphorylation at Y703/Y721/Y730/Y936, resulting in blockade of oncogenic signalling. Results of our M-COPA treatment assay show that Kit Y703/Y730/Y936 in the ER are dephosphorylated by protein tyrosine phosphatases (PTPs), thus the ER-retained Kit is unable to activate downstream molecules. ER-localized Kit Y721 is not phosphorylated, but not due to PTPs. Importantly, M-COPA can inhibit the activation of the Kit kinase domain mutant, resulting in suppression of imatinib-resistant GIST proliferation. Our study demonstrates that Kit autophosphorylation is spatio-temporally regulated and may offer a new strategy for treating imatinib-resistant GISTs.

KITD816V Induces SRC-Mediated Tyrosine Phosphorylation of MITF and Altered Transcription Program in Melanoma

The oncogenic D816V mutation of the KIT receptor is well characterized in systemic mastocytosis and acute myeloid leukemia. Although KIT^D816V has been found in melanoma, its function and involvement in this malignancy is not understood. Here we show that KIT^D816V induces tyrosine phosphorylation of MITF through a triple protein complex formation between KIT, MITF, and SRC family kinases. In turn, phosphorylated MITF activates target genes that are involved in melanoma proliferation, cell-cycle progression, suppression of senescence, survival, and invasion. By blocking the triple protein complex formation, thus preventing MITF phosphorylation, the cells became hypersensitive to SRC inhibitors. We have therefore delineated a mechanism behind the oncogenic effects of KIT^D816V in melanoma and provided a rationale for the heightened SRC inhibitor sensitivity in KIT^D816V transformed cells.Implications: This study demonstrates that an oncogenic tyrosine kinase mutant, KIT^D816V, can alter the transcriptional program of the transcription factor MITF in melanoma Mol Cancer Res; 15(9); 1265-74. ©2017 AACR.

Germline mutations of KIT in gastrointestinal stromal tumor (GIST) and mastocytosis

Somatic mutations of KIT are frequently found in mastocytosis and gastrointestinal stromal tumor (GIST), while germline mutations of KIT are rare, and only found in few cases of familial GIST and mastocytosis. Although ligand-independent activation is the common feature of KIT mutations, the phenotypes mediated by various germline KIT mutations are different. Germline KIT mutations affect different tissues such as interstitial cells of Cajal (ICC), mast cells or melanocytes, and thereby lead to GIST, mastocytosis, or abnormal pigmentation. In this review, we summarize germline KIT mutations in familial mastocytosis and GIST and discuss the possible cellular context dependent transforming activity of KIT mutations.

Oncogenic KIT mutations in different exons lead to specific changes in melanocyte phospho-proteome

Mutations in the proto-oncogene c-KIT (KIT) are found in several cancers, and the site of these mutations differs markedly between cancer types. We used site directed mutagenesis to induce KIT(559), KIT(642) and KIT(816) mutations in primary human melanocytes (PHM) and we investigated the impact of each mutation on KIT function. We studied canonical KIT-signaling pathways by immunoblotting, and we used stable isotope labeling by amino acids in cell culture (SILAC) and kinase prediction models to identify kinases differently activated in respective mutants. We validated our results with the analysis of phosphorylation levels of selected substrates for each kinase. We concluded that CK1 ε and δ are more active in cell clones harboring KIT(559) and KIT(642) mutations, whereas PAK4 is more active in clones with KIT(816) mutation. Our findings might help to develop further therapeutic options for tumors with specific KIT mutations in different domains.

BIOLOGICAL SIGNIFICANCE

Different types of cancers harbor mutations in the oncogene KIT. The use of small molecules inhibitors directly targeting KIT had a limited success in the treatment of patients with KIT mutant cancers. Our study describes specific phospho-proteome changes due to different KIT mutations, and provides targets of further therapeutic options.

PI3 kinase is indispensable for oncogenic transformation by the V560D mutant of c-Kit in a kinase-independent manner

Oncogenic mutants of c-Kit are often found in mastocytosis, gastrointestinal stromal tumors and acute myeloid leukemia. The activation mechanism of the most commonly occurring mutation, D816V in exon 17 of c-Kit, has been well-studied while other mutations remain fairly uncharacterized in this respect. In this study, we show that the constitutive activity of the exon 11 mutant V560D is weaker than the D816V mutant. Phosphorylation of downstream signaling proteins induced by the ligand for c-Kit, stem cell factor, was stronger in c-Kit/V560D expressing cells than in cells expressing c-kit/D816V. Although cells expressing c-Kit/V560D showed increased ligand-independent proliferation and survival compared to wild-type c-Kit-expressing cells, these biological effects were weaker than in c-Kit/D816V-expressing cells. In contrast to cells expressing wild-type c-Kit, cells expressing c-Kit/V560D were independent of Src family kinases for downstream signaling. However, the independence of Src family kinases was not due to a Src-like kinase activity that c-Kit/D816V displayed. Point mutations that selectively block the association of PI3 kinase with c-Kit/V560D inhibited ligand-independent activation of the receptor, while inhibition of the kinase activity of PI3 kinase with pharmacological inhibitors did not affect the kinase activity of the receptor. This suggests a lipid kinase-independent key role of PI3 kinase in c-Kit/V560D-mediated oncogenic signal transduction. Thus, PI3 kinase is an attractive therapeutic target in malignancies induced by c-Kit mutations independent of its lipid kinase activity.

The adaptor 3BP2 is required for KIT receptor expression and human mast cell survival

SH3-binding protein 2 (3BP2) is a cytoplasmic adaptor protein that acts as a positive regulator in mast cell FcεRI-dependent signaling. The KIT receptor whose ligand is the stem cell factor is necessary for mast cell development, proliferation, and survival as well as for optimal IgE-dependent signal. Activating mutations in KIT have been associated with several diseases including mastocytosis. In the present work, we found that 3BP2 silencing impairs KIT signaling pathways, thus affecting phosphoinositide 3-kinase and MAPK pathways in human mast cells (huMCs) from HMC-1, LAD2 (huMC lines), and CD34(+)-derived mast cells. Unexpectedly, silencing of 3BP2 reduces KIT expression in normal huMCs as well as in HMC-1 cells where KIT is mutated, thus increasing cellular apoptosis and caspase-3/7 activity. 3BP2 silencing reduces KIT transcription expression levels. Interestingly, 3BP2 silencing decreased microphthalmia-associated transcription factor (MITF) expression, a transcription factor involved in KIT expression. Reconstitution of 3BP2 in knockdown cells leads to reversal of KIT expression as well as survival phenotype. Accordingly MITF reconstitution enhances KIT expression levels in 3BP2-silenced cells. Moreover, downregulation of KIT expression by miRNA-221 overexpression or the proteasome inhibitor bortezomib also reduced 3BP2 and MITF expression. Furthermore, KIT tyrosine activity inhibition reduced 3BP2 and MITF expression, demonstrating again a tight and reciprocal relationship between these molecules. Taken together, our results show that 3BP2 regulates huMC survival and participates in KIT-mediated signal transduction by directly controlling KIT receptor expression, suggesting its potential as a therapeutic target in mast cell-mediated inflammatory diseases and deregulated KIT disorders.

The activation loop tyrosine 823 is essential for the transforming capacity of the c-Kit oncogenic mutant D816V

Oncogenic c-Kit mutations have been shown to display ligand-independent receptor activation and cell proliferation. A substitution of aspartate to valine at amino acid 816 (D816V) is one of the most commonly found oncogenic c-Kit mutations and is found in >90% of cases of mastocytosis and less commonly in germ-cell tumors, core-binding factor acute myeloid leukemia and mucosal melanomas. The mechanisms by which this mutation leads to constitutive activation and transformation are not fully understood. Previous studies have shown that the D816V mutation causes a structural change in the activation loop (A-loop), resulting in weaker binding of the A-loop to the juxtamembrane domain. In this paper, we have investigated the role of Y823, the only tyrosine residue in the A-loop, and its role in oncogenic transformation by c-Kit/D816V by introducing the Y823F mutation. Although dispensable for the kinase activity of c-Kit/D816V, the presence of Y823 was crucial for cell proliferation and survival. Furthermore, mutation of Y823 selectively downregulates the Ras/Erk and Akt pathways as well as the phosphorylation of STAT5 and reduces the transforming capacity of the D816V/c-Kit in vitro. We further show that mice injected with cells expressing c-Kit/D816V/Y823F display significantly reduced tumor size as well as tumor weight compared with controls. Finally, microarray analysis, comparing Y823F/D816V cells with cells expressing c-Kit/D816V, demonstrate that mutation of Y823 causes upregulation of proapoptotic genes, whereas genes of survival pathways are downregulated. Thus, phosphorylation of Y823 is not necessary for kinase activation, but essential for the transforming ability of the c-Kit/D816V mutant.

The tyrosine phosphatase SHP2 is required for cell transformation by the receptor tyrosine kinase mutants FIP1L1-PDGFRα and PDGFRα D842V

Activated forms of the platelet derived growth factor receptor alpha (PDGFRα) have been described in various tumors, including FIP1L1-PDGFRα in patients with myeloproliferative diseases associated with hypereosinophilia and the PDGFRα(D842V) mutant in gastrointestinal stromal tumors and inflammatory fibroid polyps. To gain a better insight into the signal transduction mechanisms of PDGFRα oncogenes, we mutated twelve potentially phosphorylated tyrosine residues of FIP1L1-PDGFRα and identified three mutations that affected cell proliferation. In particular, mutation of tyrosine 720 in FIP1L1-PDGFRα or PDGFRα(D842V) inhibited cell growth and blocked ERK signaling in Ba/F3 cells. This mutation also decreased myeloproliferation in transplanted mice and the proliferation of human CD34(+) hematopoietic progenitors transduced with FIP1L1-PDGFRα. We showed that the non-receptor protein tyrosine phosphatase SHP2 bound directly to tyrosine 720 of FIP1L1-PDGFRα. SHP2 knock-down decreased proliferation of Ba/F3 cells transformed with FIP1L1-PDGFRα and PDGFRα(D842V) and affected ERK signaling, but not STAT5 phosphorylation. Remarkably, SHP2 was not essential for cell proliferation and ERK phosphorylation induced by the wild-type PDGF receptor in response to ligand stimulation, suggesting a shift in the function of SHP2 downstream of oncogenic receptors. In conclusion, our results indicate that SHP2 is required for cell transformation and ERK activation by mutant PDGF receptors.

The PI3-kinase isoform p110δ is essential for cell transformation induced by the D816V mutant of c-Kit in a lipid-kinase-independent manner

PI3-kinase has a crucial role in transformation mediated by the oncogenic c-Kit mutant D816V. In this study, we demonstrate that the c-Kit/D816V-mediated cell survival is dependent on an intact direct binding of PI3-kinase to c-Kit. However, mutation of this binding site had little effect on the PI3-kinase activity in the cells, suggesting that c-Kit/D816V-mediated cell survival is dependent on PI3-kinase but not its kinase activity. Furthermore, inhibition of the lipid kinase activity of PI3-kinase led only to a slight inhibition of cell survival. Knockdown of the predominant PI3-kinase isoform p110δ in c-Kit/D816V-expressing Ba/F3 cells led to reduced cell transformation both in vitro and in vivo without affecting the overall PI3-kinase activity. This suggests that p110δ has a lipid-kinase-independent role in c-Kit/D816V-mediated cell transformation. We furthermore demonstrate that p110δ is phosphorylated at residues Y524 and S1039 and that phosphorylation requires an intact binding site for PI3-kinase in c-Kit/D816V. Overexpression of p110δ carrying the Y523F and S1038A mutations significantly reduced c-Kit/D816V-mediated cell survival and proliferation. Taken together, our results demonstrate an important lipid-kinase-independent role of p110δ in c-Kit/D816V-mediated cell transformation. This furthermore suggests that p110δ could be a potential diagnostic factor and selective therapeutic target for c-Kit/D816V-expressing malignancies.

Phosphorylation of the activation loop tyrosine 823 in c-Kit is crucial for cell survival and proliferation

The receptor tyrosine kinase c-Kit, also known as the stem cell factor receptor, plays a key role in several developmental processes. Activating mutations in c-Kit lead to alteration of these cellular processes and have been implicated in many human cancers such as gastrointestinal stromal tumors, acute myeloid leukemia, testicular seminomas and mastocytosis. Regulation of the catalytic activity of several kinases is known to be governed by phosphorylation of tyrosine residues in the activation loop of the kinase domain. However, in the case of c-Kit phosphorylation of Tyr-823 has been demonstrated to be a late event that is not required for kinase activation. However, because phosphorylation of Tyr-823 is a ligand-activated event, we sought to investigate the functional consequences of Tyr-823 phosphorylation. By using a tyrosine-to-phenylalanine mutant of tyrosine 823, we investigated the impact of Tyr-823 on c-Kit signaling. We demonstrate here that Tyr-823 is crucial for cell survival and proliferation and that mutation of Tyr-823 to phenylalanine leads to decreased sustained phosphorylation and ubiquitination of c-Kit as compared with the wild-type receptor. Furthermore, the mutated receptor was, upon ligand-stimulation, quickly internalized and degraded. Phosphorylation of the E3 ubiquitin ligase Cbl was transient, followed by a substantial reduction in phosphorylation of downstream signaling molecules such as Akt, Erk, p38, Shc, and Gab2. Thus, we propose that activation loop tyrosine 823 is crucial for activation of both the MAPK and PI3K pathways and that its disruption leads to a destabilization of the c-Kit receptor and decreased survival of cells.

The tyrosine kinase CSK associates with FLT3 and c-Kit receptors and regulates downstream signaling

Type III receptor tyrosine kinases (RTKs), FLT3 and c-Kit play important roles in a variety of cellular processes. A number of SH2-domain containing proteins interact with FLT3 and c-Kit and regulate downstream signaling. The SH2-domain containing non-receptor protein tyrosine kinase CSK is mainly studied in the context of regulating Src family kinases. Here we present an additional role of this kinase in RTK signaling. We show that CSK interacts with FLT3 and c-Kit in a phosphorylation dependent manner. This interaction is facilitated through the SH2-domain of CSK. Under basal conditions CSK is mainly localized throughout the cytosolic compartment but upon ligand stimulation it is recruited to the inner side of cell membrane. CSK association did not alter receptor ubiquitination or phosphorylation but disrupted downstream signaling. Selective depletion of CSK using siRNA, or inhibition with CSK inhibitor, led to increased phosphorylation of Akt and Erk, but not p38, upon FLT3 ligand (FL) stimulation. Stem cell factor (SCF)-mediated Akt and Erk activation was also elevated by CSK inhibition. However, siRNA mediated CSK knockdown increased SCF stimulated Akt phosphorylation but decreased Erk phosphorylation. CSK depletion also significantly increased both FL- and SCF-induced SHC, Gab2 and SHP2 phosphorylation. Furthermore, CSK depletion contributed to oncogenic FLT3- and c-Kit-mediated cell proliferation, but not to cell survival. Thus, the results indicate that CSK association with type III RTKs, FLT3 and c-Kit can have differential impact on receptor downstream signaling.

KIT-D816V oncogenic activity is controlled by the juxtamembrane docking site Y568-Y570

Mutation of KIT receptor tyrosine kinase at residue D816 results in ligand-independent constitutive kinase activity. This mutation occurs in most patients with mastocytosis, a myeloproliferative neoplasm, and is detected at lower frequencies in acute myeloid leukemia and in germ cell tumors. Other KIT mutations occur in gastrointestinal stromal tumors (GIST) and mucosal melanoma. KIT is considered as a bona fide therapeutic target as c-kit mutations are driving oncogenes in these pathologies. However, several evidences suggest that KIT-D816V mutant is not as aggressive as other KIT mutants. Here, we show that an intracellular docking site in the juxtamembrane region of KIT maintains a negative regulation on KIT-D816V transforming potential. Sixteen signaling proteins were shown to interact with this motif. We further demonstrate that mutation of this site results in signaling modifications, altered gene expression profile and increased transforming activity of KIT-D816V mutant. This result was unexpected as mutations of the homologous sites on wild-type (WT) KIT, or on the related oncogenic FLT3-ITD receptor, impair their function. Our results support the hypothesis that, KIT-D816V mutation is a mild oncogenic event that is sufficient to confer partial transforming properties, but requires additional mutations to acquire its full transforming potential.

A novel KIT-deficient mouse mast cell model for the examination of human KIT-mediated activation responses

Activation of KIT, by its ligand, stem cell factor (SCF), results in the initiation of signal transduction pathways that influence mast cell survival and proliferation. Activating mutations in KIT have thus been linked to clonal MC proliferation associated with systemic mastocytosis. SCF also modulates MC function by inducing MC chemotaxis and by potentiating antigen (Ag)/IgE-mediated MC degranulation. Thus, mutations in KIT also have the potential to affect these processes in allergic and other mast cell-related diseases. Studies to determine how native and mutated KIT may modulate MC chemotaxis and activation have, however, been limited due to the lack of availability of a suitable functional MC line lacking native KIT which would allow transduction of KIT constructs. Here we describe a novel mouse MC line which allows the study of normal and mutated KIT constructs. These cells originated from a bone marrow-derived mouse MC culture out of which a rapidly dividing mast cell sub-population spontaneously arose. Over time, these cells lost KIT expression while continuing to express functional high affinity receptors for IgE (FcεRI). As a consequence, these cells degranulated in response to Ag/IgE but did not migrate nor show any evidence of potentiation of Ag/IgE degranulation in response to SCF. Retroviral transduction of the cells with a human (hu)KIT construct resulted in surface expression of huKIT which responded to huSCF by potentiation of Ag/IgE-induced degranulation and chemotaxis. This cell line thus presents a novel system to delineate how MC function is modulated by native and mutated KIT and for the identification of novel inhibitors of these processes.

Src-Like Adaptor Protein (SLAP) differentially regulates normal and oncogenic c-Kit signaling

The Src-Like Adaptor Protein (SLAP) is an adaptor protein sharing considerable structural homology with Src. SLAP is expressed in variety of cells regulating receptor tyrosine kinase signaling by direct association. In this report, we show that SLAP associates with both wild-type and oncogenic c-Kit (c-Kit-D816V). The association involves SLAP SH2 domain and receptor phosphotyrosine residues different from those mediating Src interaction. Association of SLAP triggers c-Kit ubiquitination which, in turn, is followed by receptor degradation. Although SLAP depletion potentiates c-Kit downstream signaling by stabilizing the receptor, it remains non-functional in c-Kit-D816V signaling. Ligand-stimulated c-Kit or c-Kit-D816V did not alter membrane localization of SLAP. Interestingly oncogenic c-Kit-D816V, but not wild-type c-Kit, phosphorylates SLAP on Y120, Y258 and Y273 residues. Physical interaction between c-Kit-D816V and SLAP is mandatory for the phosphorylation to take place. Although tyrosine phosphorylated SLAP does not affect c-Kit-D816V signaling, mutation of these tyrosine sites to phenylalanine can restore SLAP activity. Taken together the data demonstrate that SLAP negatively regulates wild-type c-Kit signaling, but not its oncogenic counterpart, indicating a possible mechanism by which the oncogenic c-Kit bypasses the normal cellular negative feedback control.

Therapeutic targeting of c-KIT in cancer

INTRODUCTION

Mutated forms of the receptor tyrosine kinase c-KIT are "drivers" in several cancers and are attractive targets for therapy. While benefits have been obtained from use of inhibitors of KIT kinase activity such as imatinib, especially in gastrointestinal stromal tumours (GIST), primary resistance occurs with certain oncogenic mutations. Furthermore, resistance frequently develops due to secondary mutations. Approaches to addressing both of these issues as well as combination therapies to optimise use of KIT kinase inhibitors are discussed.

AREAS COVERED

This review covers the occurrence of oncogenic KIT mutations in different cancers and the molecular basis of their action. The action of KIT kinase inhibitors, especially imatinib, sunitinib, dasatinib and PKC412, on different primary and secondary mutants is discussed. Outcomes of clinical trials in GIST, acute myeloid leukaemia (AML), systemic mastocytosis and melanoma and their implications for future directions are considered.

EXPERT OPINION

Analysis of KIT mutations in individual patients is an essential prerequisite to the use of kinase inhibitors for therapy, and monitoring for development of secondary mutations that confer drug resistance is necessary. However, it is unlikely that KIT inhibitors alone can lead to cure. KIT mutations alone do not seem to be sufficient for transformation; thus identification and co-targeting of synergistic oncogenic pathways should lead to improved outcomes.

Effects of endoplasmic reticulum stressors on maturation and signaling of hemizygous and heterozygous wild-type and mutant forms of KIT

Gain of function mutations of KIT are frequent in some human tumors, and are sensible to tyrosine kinase inhibitors. In most tumors, oncogenic mutations are heterozygous, however most in vitro data of KIT activation have been obtained with hemizygous mutation. This study aimed to investigate the maturation and activation of wild-type (WT) and mutant (M) forms of KIT in hemizygous and heterozygous conditions. WT and two types of exon 11 deletions M forms of human KIT were expressed in NIH3T3 cell lines. Membrane expression of KIT was quantified by flow cytometry. Quantification of glycosylated forms of KIT and phosphorylated forms of AKT and ERK were performed by western blot. Simultaneous activation of WT KIT and treatment with endoplasmic reticulum (ER) inhibitors, tunicamycin or brefeldin A induced a complete inhibition of membrane expression of the 145 kDa form of KIT. By contrast activation or ER inhibitors alone, only partly inhibited this form. ER inhibitors also inhibited KIT activation-dependent phosphorylation of AKT and ERK1/2. Brefeldin A induced a complete down regulation of the 145 kDa form in hemizygous M, and induced an intra-cellular accumulation of the 125 kDa form in WT but not in hemizygous M. Heterozygous cells had glycosylation and response to ER inhibitors patterns more similar to WT than to hemizygous M. Phosphorylated AKT was reduced in hemizygous cells in comparison to WT KIT cells and heterozygous cells, and in the presence of brefeldin A in all cell lines. Effects of ER inhibitors are significantly different in hemizygous and heterozygous mutants. Differences in intra-cellular trafficking of KIT forms result in differences in downstream signaling pathways, and activation of PI3K/AKT pathway appears to be tied to the presence of the mature 145 kDa form of KIT at the membrane surface.

Reactivating PP2A by FTY720 as a novel therapy for AML with C-KIT tyrosine kinase domain mutation

The tyrosine kinase domain (TKD) mutations of receptor tyrosine kinase C-KIT are associated with a poor prognosis in acute myeloid leukemia (AML). However, the underlying mechanisms are not fully understood. We found the activity of protein phosphatase 2A (PP2A), a human tumor suppressor whose dysfunction contributes to malignant cell behavior, was significantly decreased in AML subgroups harboring C-KIT/D816V and AML cell line Kasumi-1 bearing C-KIT/N822K mutation. Primary AML cells and various AML cell lines were treated with PP2A activator FTY720. FTY720 showed a toxic effect in all leukemic cells, especially for cells harboring C-KIT/TKD mutation. Furthermore, FTY720-induced toxicity in AML leukemic cells was mediated by restoration of PP2A activity, via down-regulation of PP2A inhibitor SET, dephosporylation of PP2A-C(TYR307), and up-regulation of relevant PP2A subunit A and B55α. Our research indicates that the decreased PP2A activity in AML harboring C-KIT/TKD mutation may make the restoration of PP2A activity a novel therapy for AML patients with C-KIT/TKD mutation.

Stem Cell Factor Receptor/c-Kit: From Basic Science to Clinical Implications

Stem cell factor (SCF) is a dimeric molecule that exerts its biological functions by binding to and activating the receptor tyrosine kinase c-Kit. Activation of c-Kit leads to its autophosphorylation and initiation of signal transduction. Signaling proteins are recruited to activated c-Kit by certain interaction domains (e.g., SH2 and PTB) that specifically bind to phosphorylated tyrosine residues in the intracellular region of c-Kit. Activation of c-Kit signaling has been found to mediate cell survival, migration, and proliferation depending on the cell type. Signaling from c-Kit is crucial for normal hematopoiesis, pigmentation, fertility, gut movement, and some aspects of the nervous system. Deregulated c-Kit kinase activity has been found in a number of pathological conditions, including cancer and allergy. The observation that gain-of-function mutations in c-Kit can promote tumor formation and progression has stimulated the development of therapeutics agents targeting this receptor, e.g., the clinically used inhibitor imatinib mesylate. Also other clinically used multiselective kinase inhibitors, for instance, sorafenib and sunitinib, have c-Kit included in their range of targets. Furthermore, loss-of-function mutations in c-Kit have been observed and shown to give rise to a condition called piebaldism. This review provides a summary of our current knowledge regarding structural and functional aspects of c-Kit signaling both under normal and pathological conditions, as well as advances in the development of low-molecular-weight molecules inhibiting c-Kit function.

Structural and functional analysis of KIT gene encoding receptor tyrosine kinase and its interaction with sunitinib and HDAC inhibitors: an in silico approach

KIT is a growth factor receptor, important for normal germ cell migration and development. The malfunction of KIT gene results in constitutive activation of the tyrosine kinase activity of c-KIT which is believed to be the major oncogenic event in stomach, small intestine mastocytosis, acute leukemias, melanomas and colon tumors. The genetics of these diseases could be better understood by knowing the functional relevance of their SNP variation. In this study, a computational analysis to detect the most deleterious nonsynonymous SNPs of KIT gene was performed and investigated its binding affinity to native and predicted mutant protein structure (D816V) with sunitinib and HDAC (Trichostatin A and Panobinostat) inhibitors was investigated. Out of 1,288 SNPs retrieved from dbSNP database against KIT gene, 11 non-synonymous SNPs were detected to be damaging and deleterious by SIFT, PolyPhen and I-Mutant2.0 servers. Further, we modeled the mutant protein based on the deleterious nsSNP (rs121913507) and showed that the mutation from Aspartic acid to Valine at 816 position exhibit greatest impact on stability. The RMSD values of mutant and native structures are found to be 0.40 and 1.9 A, respectively. Furthermore, the binding affinity of sunitinib and HDAC inhibitors were compared with native and mutant protein. In this regard, it was found that trichostatin A has a high binding efficacy towards the mutant protein with a binding energy of -35.274 kcal mol(-1), as compared to the native structure which has a binding energy of -25.996 kcal mol(-1). Also, the FastSNP tool suggested that 3 SNPs found to affect protein splicing site and splicing regulation. From present results, it was clear that the non-synonymous SNP rs121913507 (D816V) could be the most deleterious SNP for KIT gene and HDAC inhibitors can serve as a best drug for the mutant protein.

Activated leukemic oncogenes AML1-ETO and c-kit: role in development of acute myeloid leukemia and current approaches for their inhibition

Acute myeloid leukemia (AML) is a malignant blood disease caused by different mutations that enhance the proliferative activity and survival of blood cells and affect their differentiation and apoptosis. The most frequent disorders in AML are translocations between chromosomes 21 and 8 leading to production of a chimeric oncogene, AML1-ETO, and hyperexpression of the receptor tyrosine kinase KIT. Mutations in these genes often occur jointly. The presence in cells of two activated oncogenes is likely to trigger their malignization. The current approaches for treatment of oncologic diseases (bone marrow transplantation, radiotherapy, and chemotherapy) have significant shortcomings, and thus many laboratories are intensively developing new approaches against leukemias. Inhibiting expression of activated leukemic oncogenes based on the principle of RNA interference seems to be a promising approach in this field.

New insights into the mechanisms of hematopoietic cell transformation by activated receptor tyrosine kinases

A large number of alterations in genes encoding receptor tyrosine kinase (RTK), namely FLT3, c-KIT, platelet-derived growth factor (PDGF) receptors, fibroblast growth factor (FGF) receptors, and the anaplastic large cell lymphoma kinase (ALK), have been found in hematopoietic malignancies. They have drawn much attention after the development of tyrosine kinase inhibitors. RTK gene alterations include point mutations and gene fusions that result from chromosomal rearrangements. In both cases, they activate the kinase domain in the absence of ligand, producing a permanent signal for cell proliferation. Recently, this simple model has been refined. First, by contrast to wild-type RTK, many mutated RTK do not seem to signal from the plasma membrane, but from various locations inside the cell. Second, their signal transduction properties are altered: the pathways that are crucial for cell transformation, such as signal transducer and activator of transcription (STAT) factors, do not necessarily contribute to the physiologic functions of these receptors. Finally, different mechanisms prevent the termination of the signal, which normally occurs through receptor ubiquitination and degradation. Several mutations inactivating CBL, a key RTK E3 ubiquitin ligase, have been recently described. In this review, we discuss the possible links among RTK trafficking, signaling, and degradation in leukemic cells.

Mast cell homeostasis and the JAK-STAT pathway

The Janus kinase/signal transducer and activator of transcription (JAK-STAT) pathway mediates important responses in immune cells. Activation of any of the four JAK family members leads to phosphorylation of one or more of seven STAT family members. Phosphorylation of STAT family members leads to their dimerization and translocation into the nucleus, in which they bind specific DNA sequences to activate gene transcription. Regulation of JAKs and STATs therefore has a significant effect on signal transduction and subsequent cellular responses. Mast cells are important mediators of allergic disease and asthma. These cells have the ability to cause profound inflammation and vasodilation upon the release of preformed mediators, as well as subsequent synthesis of new inflammatory mediators. The regulation of mast cells is therefore of intense interest for the treatment of allergic disease. An important regulator of mast cells, STAT5, is activated downstream of the receptors for immunoglobulin E, interleukin-3 and stem cell factor. STAT5 contributes to mast cell homeostasis, by mediating proliferation, survival, and mediator release. Regulators of the JAK-STAT pathway, such as the suppressors of cytokine signaling (SOCS) and protein inhibitor of activated STAT (PIAS) proteins, are required to fine tune the immune response and maintain homeostasis. A better understanding of the role and regulation of JAKs and STATs in mast cells is vital for the development of new therapeutics.