KIT Receptor Research Articles

Role of LIM Kinase in Oncogenic Signaling from FLT3 and KIT Receptors and Its Targeting in Myeloid Leukemia

Acute Myeloid leukemia (AML) has a poor prognosis in part due to lack of elimination of leukemic cells and acquisition of drug resistance. In particular mutations in the activation loop of KIT and internal tandem duplication (ITD) in Flt3 receptor are associated with poor prognosis in AML. Interestingly, more than 90% patients with aggressive systemic mastocytosis (ASM) carry the KITD816V mutation which is highly resistant to conventional therapeutics and invariably associated with poor survival. Since the efforts to target the kinase activity of FLT3 and KIT have met with little success; the focus has shifted to targeting critical downstream signaling regulators. We have elucidated the role of a novel downstream regulator of FLT3-ITD and KITD816V mediated leukemic transformation - LIM kinase and evaluated the efficacy of a novel inhibitor of LIM kinase (LIMK), Liminib. LIMK, a serine/threonine kinase has 2 isoforms; LIMK1 and LIMK2. They are substrates of the Rho associated kinase (ROCK) which we have shown to play a critical role in leukemic transformation (Cancer Cell, 2011). They in turn act on cofilin and phosphorylation by LIMK inactivates cofilin and thereby modulates F-actin reorganization. Western blot analysis showed marked increase in phospho cofilin levels in cells expressing FLT3-ITD or KITD814V oncogenes as compared to their WT counterparts. Dependence of cofilin phosphorylation on activation of ROCK and LIMK was confirmed by treating the cells with an inhibitor of ROCK, H1152 and Liminib, inhibitor of LIMK. Both the inhibitors effectively reduced oncogene-induced phospho-cofilin levels. Liminib, the LIMK inhibitor also inhibited cytokine independent proliferation driven by FLT3-ITD or KITD814V (>90% at 100 nM). It also showed concentration dependent inhibition of proliferation in mutant oncogene bearing patient derived human cell lines such as MV411, Kasumi and HMC. FLT3-ITD expressing leukemic cells become resistant to Flt3 kinase inhibitor AC220 by acquiring additional point mutations. Interestingly, the cells expressing AC220 resistant FLT3-ITDD835F or FLT3-ITDF691L mutation showed similar sensitivity to Liminib as cells with FLT3-ITD. The growth inhibitory effect of Liminib was also validated in primary blasts from AML and SM patients. To further elucidate the mechanism of growth inhibition by Liminib in leukemic cells, we assayed for apoptosis induction in the treated cells. Cells expressing FLT3-ITD were significantly more susceptible (~ 73%) to induction of apoptosis as compared the WT-FLT3 cells (~33%) on treatment with Liminib. The extent of apoptosis induction by Liminib was similar in FLT3-ITD and AC220 resistant FLT3-ITDD835F or FLT3-ITDF691L mutation bearing cells. In addition to the effects of Liminib on F-actin polymerization; experimental evidence indicates involvement of mitochondrial pathway in the induction of apoptosis by Liminib. There was increase in accumulation of cofilin in the mitochondrial fraction in Liminib treated oncogene expressing cells as compared to the untreated cells. Phosphorylation of cofilin on Ser3 suppresses its mitochondrial translocation and treatment with LIMK inhibitor would inhibit cofilin phosphorylation leading to increased mitochondrial transport. Mitochondrial transport of cofilin has been observed with other inducers of apoptosis such as staurosporine and it possibly contributes to mitochondrial dysfunction during induction of apoptosis. To validate our pharmacologic findings using Liminib genetically and to assess which of the 2 isoforms of LIMK contributes to leukemogenesis, we generated LIMK1 and LIMK2 single and double knockout mice. 3 H-thymidine incorporation assay showed that deletion of either of the LIMK significantly reduced KIT D814V induced growth factor independent proliferation by approximately 50%. The extent of inhibition was more pronounced in Limk1-/- Limk2+/- or Limk1+/- Limk2-/- bone marrow cells (~80%) as compared to cells with deletion of a single isoforms of LIMK. Thus, there is some functional redundancy between the 2 isoforms and inhibition of both is likely to have better therapeutic value. Taken together we have identified a novel signaling pathway involving LIMK and cofilin that appear to be selectively activated in regulating oncogenic signaling in AML and MPN but not in normal hematopoiesis. DisclosuresNo relevant conflicts of interest to declare.

Abstract A40: MicroRNA-494 inhibits growth of the gastrointesintal stromal tumor by targeting multiple cell cycle regulatory genes

Abstract Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal neoplasms occurring throughout the entire region of the gastrointestinal tract. Two oncogenes of the receptor tyrosine kinase family, KIT and PDGFRA, have gain-of-function mutations in approximately 70% and 15% of GISTs. Mutations of these 2 genes cause sustained activation, resulting in constant stimulation of the downstream signaling pathways of KIT and platelet-derived growth factor receptor, alpha polypeptide. Although KIT mutations are present in approximately 70% of GISTs,KIT overexpression is observed in more than 90% of GISTs. Dysregulation of microRNAs(miRNA)-494 was demonstrated to be a major cause for the overexppression of mutatnt KIT proteins in GISTs. It was also demonstrated that miR-494 treatment inhibits proliferation of GISTs and promotes apoptosis however, no details mechanisms are not known. The purpose of this study was to provide evidence regarding the roles of miR-494 in GIST suppression. To elucidate the specific pathways affected by miRNA-494 treatment on GIST cell lines, transcriptome analysis of GIST430 and GIST882 cells transfected with nontargeting microRNA or with miR-494 was performed and subsequently validated by qRT-PCR. Based on the transcriptome analysis, we found that multiple cell cycle relevant pathways were down-regulated by miR-494 treatment and importantly, core genes such as KIT, CASP2, CDCA2, AURKB, CDCA8, E2F7, BIRC5 and CKS1B involved in the regulation of cell cycle pathways were identified as novel targets by miR-494. Using reporter assays, we further validated that miR-494 directly binds to the 3'UTR regions of BRAC5, CKS1B and E2F7 and induces biological alterations. To demonstrate the perturbation of cell cycle pathways by miR-494, FACS analysis using five GIST cell lines was performed and we found that all of the five GIST cell lines transfected with miR-494 subjected to cell cycle arrest and apoptosis. Colony forming and proliferation assays also demonstrated that miR-494 treatment induced the growth inhibition of GIST cells which was derived from the alterations of cell cycle pathways. Taken together, our findings suggest that the efficient suppression of GISTs by miR-494 is driven not only by targeting KIT, but also multiple genes involved in cell cycle pathways Citation Format: SeongJu Yun, Won Kyu Kim, Hoguen Kim. MicroRNA-494 inhibits growth of the gastrointesintal stromal tumor by targeting multiple cell cycle regulatory genes. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr A40.

Somatic loss of function mutations in neurofibromin 1 and MYC associated factor X genes identified by exome-wide sequencing in a wild-type GIST case.

BackgroundApproximately 10–15 % of gastrointestinal stromal tumors (GISTs) lack gain of function mutations in the KIT and platelet-derived growth factor receptor alpha (PDGFRA) genes. An alternate mechanism of oncogenesis through loss of function of the succinate-dehydrogenase (SDH) enzyme complex has been identified for a subset of these “wild type” GISTs.MethodsPaired tumor and normal DNA from an SDH-intact wild-type GIST case was subjected to whole exome sequencing to identify the pathogenic mechanism(s) in this tumor. Selected findings were further investigated in panels of GIST tumors through Sanger DNA sequencing, quantitative real-time PCR, and immunohistochemical approaches.ResultsA hemizygous frameshift mutation (p.His2261Leufs*4), in the neurofibromin 1 (NF1) gene was identified in the patient’s GIST; however, no germline NF1 mutation was found. A somatic frameshift mutation (p.Lys54Argfs*31) in the MYC associated factor X (MAX) gene was also identified. Immunohistochemical analysis for MAX on a large panel of GISTs identified loss of MAX expression in the MAX-mutated GIST and in a subset of mainly KIT-mutated tumors.ConclusionThis study suggests that inactivating NF1 mutations outside the context of neurofibromatosis may be the oncogenic mechanism for a subset of sporadic GIST. In addition, loss of function mutation of the MAX gene was identified for the first time in GIST, and a broader role for MAX in GIST progression was suggested.Electronic supplementary materialThe online version of this article (doi:10.1186/s12885-015-1872-y) contains supplementary material, which is available to authorized users.

Allelic Mutations of KITLG, Encoding KIT Ligand, Cause Asymmetric and Unilateral Hearing Loss and Waardenburg Syndrome Type 2

Linkage analysis combined with whole-exome sequencing in a large family with congenital and stable non-syndromic unilateral and asymmetric hearing loss (NS-UHL/AHL) revealed a heterozygous truncating mutation, c.286_303delinsT (p.Ser96Ter), in KITLG. This mutation co-segregated with NS-UHL/AHL as a dominant trait with reduced penetrance. By screening a panel of probands with NS-UHL/AHL, we found an additional mutation, c.200_202del (p.His67_Cys68delinsArg). Invitro studies revealed that the p.His67_Cys68delinsArg transmembrane isoform of KITLG is not detectable at the cell membrane, supporting pathogenicity. KITLG encodes a ligand for the KIT receptor. Also, KITLG-KIT signaling and MITF are suggested to mutually interact in melanocyte development. Because mutations in MITF are causative of Waardenburg syndrome type 2 (WS2), we screened KITLG in suspected WS2-affected probands. A heterozygous missense mutation, c.310C>G (p.Leu104Val), that segregated with WS2 was identified in a small family. Invitro studies revealed that the p.Leu104Val transmembrane isoform of KITLG is located at the cell membrane, as is wild-type KITLG. However, in culture media of transfected cells, the p.Leu104Val soluble isoform of KITLG was reduced, and no soluble p.His67_Cys68delinsArg and p.Ser96Ter KITLG could be detected. These data suggest that mutations in KITLG associated with NS-UHL/AHL have a loss-of-function effect. We speculate that the mechanism of the mutation underlying WS2 and leading to membrane incorporation and reduced secretion of KITLG occurs via a dominant-negative or gain-of-function effect. Our study unveils different phenotypes associated with KITLG, previously associated with pigmentation abnormalities, and will thereby improve the genetic counseling given to individuals with KITLG variants.

Regulation of FOXO3 subcellular localization by Kit ligand in the neonatal mouse ovary.

Foxo3 protein is required in the oocyte nucleus for the maintenance of primordial follicles in a dormant state. PI3K/AKT-dependent phosphorylation of Foxo3 leads to its relocalization to the cytoplasm and subsequent follicular activation. However, the nature of the upstream signals controlling Foxo3 activity and subcellular localization remains unknown. We aimed to study the in vitro effects of Kit ligand (stem cell factor) on the subcellular localization of Foxo3 in primordial follicles within the postnatal mouse ovary. This was an in vitro study using explants of intact neonatal mouse ovaries. The study was performed in laboratory animal facility and basic science research laboratory at a University Hospital. The animals used for this study were FVB mice. Neonatal FVB mice ovaries at postnatal day 7 (PD7) were harvested and incubated in culture medium (DMEM) at 37 °C and 5 % CO(2) for 60-90 min with (n = 3) or without (n = 3) Kit ligand at 150 ng/mL (8 nM). Similar experimental conditions were used to establish a dose-response curve for the effects of Kit ligand and assess the effects of imatinib (small molecule inhibitor of the Kit receptor). Immunofluorescence was used to identify the subcellular location of Foxo3 in oocytes. Proportions of cytoplasmic versus nuclear Foxo3 in primordial follicles were determined. Kit ligand treatment increased the cytoplasmic localization of Foxo3 from 40 % in the untreated ovaries to 74 % in the treated group (p = 0.007 in paired samples and p = 0.03 in unpaired samples). Furthermore, this effect was reversible with imatinib (p = 0.005). A dose-response curve for Kit ligand treatment showed that maximum effect was seen at 150 ng/mL. Kit ligand treatment in vitro increases the proportion of cytoplasmic Foxo3 in primordial follicles at PD7, lending support to the idea that Kit receptor/ligand controls Foxo3 activity in the context of primordial follicle activation.

KIT exon 10 variant (c.1621 A > C) single nucleotide polymorphism as predictor of GIST patient outcome.

BackgroundTumor genotype plays a crucial role in clinical management of GIST. Whether genetic polymorphism of KIT may influence GIST patient outcome is unclear.MethodsWe investigated the biological and clinical significance of the presence of KIT exon 10 variant (c.1621 A > C), KITL541, in a transfected cell line (3 T3 L541) and in two retrospectively collected series of 109 GIST patients in total. The control group consisted of 60 healthy donors collected at the French department of blood transfusion.ResultsIn the 3 T3 L541 cell line, KITL541 protein exhibited a spontaneous phosphorylation status comparable to that of wild-type KIT but displayed a phosphorylation pattern of AKT and ERK1/2 that was found similar to that of the classical mutated forms of the KIT receptor. Of 109 patients enrolled in this retrospective translational research study, 24 (22 %) harboured KITL541, similarly to the control group of healthy donors (n = 10 of 60, 17 %). A higher prevalence of the variant KITL541 was observed in patients with metastatic status at diagnosis (KITL541 correlated nine of 22 versus 15 of 87, p = 0.02). In addition, patients with KITL541 and localized GIST had a higher rate of relapse at 5 years and lower relapse free survival at 5 years in univariate, as well as in multivariate analysis. Response rate and duration of response to imatinib was similar in KITL541 and KITM541 patients.ConclusionKITL541 genotype is associated with a higher risk of metastasis at diagnosis and a higher risk of relapse in GIST patients.Electronic supplementary materialThe online version of this article (doi:10.1186/s12885-015-1817-5) contains supplementary material, which is available to authorized users.

Structure of Full-Length Human PDGFRβ Bound to Its Activating Ligand PDGF-B as Determined by Negative-Stain Electron Microscopy

Members of the receptor tyrosine kinases (RTKs) regulate important cellular functions such as cell growth and migration, which are key steps in angiogenesis, in organ morphogenesis and in the unregulated states, cancer formation. One long-standing puzzle regarding RTKs centers on how the extracellular domain (ECD), which detects and binds to growth factors, is coupled with the intracellular domain kinase activation. While extensive structural works on the soluble portions of RTKs have provided critical insights into RTK structures and functions, lack of a full-length receptor structure has hindered a comprehensive overview of RTK activation. In this study, we successfully purified and determined a 27-Å-resolution structure of PDGFRβ [a full-length human platelet-derived growth factor receptor], in complex with its ligand PDGF-B. In the ligand-stimulated complex, two PDGFRβs assemble into a dimer via an extensive interface essentially running along the full-length of the receptor, suggesting that the membrane-proximal region, the transmembrane helix and the kinase domain of PDGFRβ are involved in dimerization. Major structural differences are seen between the full-length and soluble ECD structures, rationalizing previous experimental data on how membrane-proximal domains modulate receptor ligand-binding affinity and dimerization efficiency. Also, in contrast to the 2-fold symmetry of the ECD, the intracellular kinase domains adopt an asymmetric dimer arrangement, in agreement with prior observations for the closely related KIT receptor. In essence, the structure provides a first glimpse into how platelet-derived growth factor receptor ECD, upon ligand stimulation, is coupled to its intracellular domain kinase activation.

Histopathological Features of Gastrointestinal Stromal Tumors and the Contribution of DOG1 Expression to the Diagnosis.

Gastrointestinal stromal tumors (GIST) have KIT or platelet-derived growth factor receptor α (PDGFRα) mutations affecting receptor tyrosine kinase activity and do not benefit from classic treatment regimens. The aim of this study was to review the algorithm that may be followed for the diagnosis and differential diagnosis in GISTs by investigating the histomorphological parameters and expression characteristics of classical immunohistochemical antibodies used in routine tests in addition to DOG1 expression. Diagnostic accuracy study. We reevaluated the histological and immunohistochemical parameters of 37 GISTs. The standard immunohistochemical diagnosis and differential diagnosis panel antibodies (CD117, PDGFRα, CD34, vimentin, desmin, SMA, S-100, and Ki67) were studied on the tumor sections. We also used the popular marker DOG1 antibody with accepted sensitivity for GISTs in recent years and the PDGFRα immune marker for which the benefit in routine practice is discussed. Classification according to progressive disease risk groups of the 37 cases revealed that 54% were in the high risk, 19% in the moderate risk, 16% in the low risk, 8% in the very low risk and 8% in the no risk group. Cytological atypia, necrosis, mucosal invasion and the Ki67 index were found to be related to the progressive disease risk groups of the tumors (p<0.05). Positive immunoreaction was observed with CD117 and PDGFRα in all GISTs in the study (100%). Positivity with the DOG1 antibody was found in 33 (89%) cases. CD34 was positive in 62% (23) of the cases. The CD117 antibody still plays a key role in GIST diagnosis. However, the use of DOG1 and PDGFRα antibodies combined with CD117 as sensitive markers can be beneficial.

An Unusual Case of Sigmoid Colon Gastrointestinal Stromal Tumor

Gastrointestinal stromal tumors (GIST), uncommon mesenchymal tumors, are driven by KIT or plateletderived growth factor receptor alpha (PDGFRA) mutations. They are most commonly diagnosed in older adults (median 62-63 years) presenting with GI bleeding or peptic ulcer-like symptoms, and most are found in the stomach (approximately 60%). GISTs are much more rarely diagnosed in the colon ( < 1%), with the sigmoid colon most commonly affected. We review the case of a 90-year-old white man who presented with one day of painless bright red blood per rectum. He denied abdominal pain, nausea, and vomiting. Past medical history included atrial fibrillation on warfarin, stroke, prostate cancer with retropubic radical prostatectomy, and hypertension. The patient was hemodynamically stable with a benign abdominal examination. Rectal examination revealed maroon blood but no hemorrhoids, fissures, or masses. Laboratory evaluation revealed hemoglobin 7.3 g/dL, WBC 7.6x10(9), INR 2.73, and creatinine 0.6 mg/dL. Electrolytes were unremarkable. A CT abdomen/pelvis revealed extensive bowel wall thickening focally, at the junction of the sigmoid and descending colon, with adjacent stranding (Figure 1). The patient underwent his index colonoscopy revealing a nearly-obstructing, necrotic, ulcerated mass with overlying clot 45 cm from the anal verge. (Figure 2). Several cold forceps biopsies were taken diagnosing a gastrointestinal stromal tumor, spindle cell subtype. Within the biopsy specimen there was no mucosal ulceration or mitoses and the ki-67 proliferative index was < 1%. By immunohistochemistry, the tumor cells expressed CD117 (c-kit), CD34, and SMA, and were negative for S100. Neither the c-KIT nor PDGFRA gene mutation was detected. Given the extent of disease and the patient's comorbidities, surgical resection was not pursued and he was initiated on imatinib 400 mg daily. At six months the patient was tolerating therapy well aside from lower extremity edema, a common side effect of imatinib.Figure 1Figure 2An estimated 12-15% of adult GIST lack KIT and PDGFRA mutations, which are mutually exclusive, and are classified as wild type. This case highlights the importance of using several histopathology markers to aid in the diagnosis of GIST. Over 95% of GIST express CD117 and 70-90% also express CD34, as was seen in this case. Those with the mutations are more likely to respond to imatinib therapy, a tyrosine kinase inhibitor., which allows therapy to be individualized.

Targeting c-kit receptor in neuroblastomas and colorectal cancers using stem cell factor (SCF)-based recombinant bacterial toxins.

Autocrine activation of c-kit (KIT receptor tyrosine kinase) has been postulated to be a potent oncogenic driver in small cell lung cancer, neuroblastoma (NB), and poorly differentiated colorectal carcinoma (CRC). Although targeted therapy involving tyrosine kinase inhibitors (TKIs) such as imatinib mesylate is highly effective for gastrointestinal stromal tumor carrying V560G c-kit mutation, it does not show much potential for targeting wild-type KIT (WT-KIT). Our study demonstrates the role of stem cell factor (SCF)-based toxin conjugates for targeting WT-KIT-overexpressing malignancies such as NBs and CRCs. We constructed SCF-based recombinant bacterial toxins by genetically fusing mutated form of natural ligand SCF to receptor binding deficient forms of Diphtheria toxin (DT) or Pseudomonas exotoxin A (ETA') and evaluated their efficacy in vitro. Efficient targeting was achieved in all receptor-positive neuroblastoma (IMR-32 and SHSY5Y) and colon cancer cell lines (COLO 320DM, HCT 116, and DLD-1) but not in receptor-negative breast carcinoma cell line (MCF-7) thereby proving specificity. While dose- and time-dependent cytotoxicity was observed in both neuroblastoma cell lines, COLO 320DM and HCT 116 cells, only an anti-proliferative effect was observed in DLD-1 cells. We prove that these novel targeting agents have promising potential as KIT receptor tyrosine kinase targeting system.

Fisetin and hesperetin induced apoptosis and cell cycle arrest in chronic myeloid leukemia cells accompanied by modulation of cellular signaling.

Fisetin and hesperetin, naturally occurring flavonoids, have been reported as novel antioxidants with chemopreventive/chemotherapeutic potential against various types of cancer. However, their mechanism of action in CML is still unknown. This particular study aims to evaluate the therapeutic potentials of fisetin and hesperetin and their effects on cell proliferation, apoptosis, and cell cycle progression in human K562 CML cells. The results indicated that fisetin and hesperetin inhibited cell proliferation and triggered programmed cell death in these cells. The latter was confırmed by mitochondrial membrane depolarization and an increase in caspase-3 activation. In addition to that, we have detected S and G2/M cell cycle arrests and G0/G1 arrest upon fisetin and hesperetin treatment, respectively. To identify the altered genes and genetic networks in response to fisetin and hesperetin, whole-genome microarray analysis was performed. The microarray gene profiling analysis revealed some important signaling pathways including JAK/STAT pathway, KIT receptor signaling, and growth hormone receptor signaling that were altered upon fisetin and hesperetin treatment. Moreover, microarray data suggested potential candidate genes for targeted CML therapy. Fisetin and hesperetin significantly modulated the expression of genes involved in cell proliferation and division, apoptosis, cell cycle regulation, and other significant cellular processes such as replication, transcription, and translation. In conclusion, our results suggest that fisetin and hesperetin as potential natural agents for CML therapy.

Management of Gastrointestinal Stromal Tumors

Gastrointestinal stromal tumors (GISTs) are the most common tumors of mesenchymal origin in the gastrointestinal tract. These tumors are believed to arise from the interstitial cells of Cajal. The oncogenic activation of KIT or platelet-derived growth factor receptor–α is common to these tumors, and GISTS are considered to be a successful model for rational development of personalized treatments against oncogenic driver mutations in cancer. This review covers the epidemiology, etiology and genetics, pathophysiology and pathogenesis, diagnosis, differential diagnosis, treatment, complications, and prognosis of GISTs. Figures show KIT primary and secondary mutations, the clinical and molecular progression of GISTs, a contrast-enhanced computed tomographic scan showing a gastric GIST presenting as a huge abdominal mass, a magnetic resonance image showing a rectal GIST at baseline and responding to neoadjuvant imatinib after 8 months of treatment, and hematoxylin-eosin stain of a fusocellular and an epithelioid GIST. Tables list demographics and clinical characteristics, associated genetic syndromes, relevant differential diagnosis, risk stratification systems used in GIST, and the comparative activity of approved regimens for GIST. This review contains 5 highly rendered figures, 5 tables, and 74 references.

Abstract 3957: Integrated molecular characterization of mast cell leukemia reveals recurrent inactivation of the SETD2 tumor suppressor gene

Abstract Systemic mastocytosis (SM) includes a heterogeneous group of disorders ranging from indolent SM to the rare and aggressive mast cell leukemia (MCL). Somatic mutations in the KIT receptor kinase (most frequently, D816V) can be detected in &amp;gt;90% of patients and are thought to play an important pathogenetic role. Nevertheless, morphological and clinical diversity, as well as the fact that some patients are negative for KIT mutations, suggest that the underlying molecular picture is far from being fully elucidated. To shed further light on this issue, we undertook an integrated molecular genetic study of a KIT gene mutation-negative MCL case who came to our attention in 2012. After having obtained written informed consent, we extracted genomic DNA and total RNA from purified mast cells (MCs) isolated from bone marrow at diagnosis and at progression, as well as DNA from saliva, and performed whole exome sequencing (WES) and RNA-seq on an HiSeq1000 (Illumina, San Diego CA). High resolution karyotyping was also performed with Cytoscan HD arrays (Affymetrix, Santa Clara CA). Among the mutated genes detected in MCs but not in saliva by WES, SETD2 stood out among others because two loss-of-function mutations (a nonsense and a frameshift mutation) inactivating both alleles of the gene were identified. Western Blotting (WB) confirmed the expression of the truncated SETD2 isoform resulting from the nonsense mutation. The SETD2 gene encodes a histone methyltransferase responsible for trimethylation of Lysine 36 of histone H3 (H3K36Me3), a key hystone mark associated not only with active chromatin but also with transcriptional elongation, alternative splicing, DNA replication and repair. Loss of the highly conserved WW and SRI domains was predicted to impair SETD2 binding with RNA polymeraseII and hNRNP L, as confirmed by co-immunoprecipitation. Accordingly, RNA-seq showed evidence of spurious transcripts initiated from cryptic promoter-like sequences within genes as well as non-canonical splice isoforms. More importantly, WB confirmed that H3K36Me3 was completely abrogated. In line with the role of SETD2-dependent H3K36Me3 in DNA repair and genome stability, Cytoscan HD arrays and WES showed that several losses at many chromosomal loci, together with more than 70 additional point mutations, undetectable at diagnosis, were acquired at the time of progression. Absence of SETD2 protein expression and/or reduced H3K36Me3 were detected in 3/3 additional MCLs and in 6/8 aggressive SMs so far screened. Our data point to epigenetic regulation and/or DNA repair as candidate pathways deserving further investigation in SM - in an attempt to elucidate the mechanisms underlying enhanced clinical aggressiveness and to identify more effective treatment modalities. Molecular characterization of 10 additional MCLs and aggressive SMs is ongoing. Supported by FP7 NGS-PTL project and Progetto Regione-Università 2010-12(L. Bolondi) Citation Format: Simona Soverini, Caterina De Benedittis, Michela Rondoni, Manuela Mancini, Cristina Papayannidis, Luca Zazzeroni, Viviana Guadagnuolo, Elisa Zago, Francesca Griggio, Alberto Ferrarini, Marianna Garonzi, Massimo Delledonne, Giorgina Specchia, Roberta Zanotti, Omar Perbellini, Livio Pagano, Michele Cavo, Peter Valent, Giovanni Martinelli. Integrated molecular characterization of mast cell leukemia reveals recurrent inactivation of the SETD2 tumor suppressor gene. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3957. doi:10.1158/1538-7445.AM2015-3957

Cell cycle and dystrophin dysregulation in GIST

Gastrointestinal stromal tumors (GISTs) are the most common sarcomas in humans, and generally result from constitutively activating mutations in the KIT or PDGFRA receptor tyrosine kinases as the initiating oncogenetic events.1,2 For this reason, most metastatic GISTs respond dramatically to therapies with KIT/PDGFRA inhibitors, such as imatinib, sunitinib and regorafenib.1,2 Asymptomatic and mitotically-inactive KIT/PDGFRA-mutant “microGISTs” are found in one third of adults, but most of these small tumors never progress to malignancy, underscoring that a progression of oncogenic mutations is required, before early innocuous GISTs transform into lethal metastasizing cancers. Mutations causing cell cycle dysregulation are one of the key events driving GIST oncogenic progression, and this is reflected in the clinically useful classification of GISTs into “low-risk” vs. “high-risk” categories, which are GISTs, respectively, with low vs. high risk of metastatic recurrence after surgical removal of the primary tumor. Mitotic activity is one crucial indicator of “risk,” underscoring the crucial role of cell cycle dysregulation mutations in progression of early GIST to a disease stage which can acquire metastatic ability. Biallelic inactivation of the CDKN2A/p16 tumor suppressor often enables GIST progression to the high-risk stage, as supported by associations between p16 loss and aggressive clinical behavior in GIST.3 Likewise, MDM2 or TP53 mutations, resulting in TP53 inactivation, are found in a subset of advanced GISTs.4 These cell cycle dysregulation mutations contribute substantially to GIST genetic progression, and are likely necessary for transition from low-risk to high-risk GIST. In a recent publication, we reported the discovery of highly recurrent DMD mutations, inactivating the dystrophin protein and thereby conferring metastatic potential, as late events in GIST progression.5 Dystrophin inactivation is responsible for the most common types of muscular dystrophy, and dystrophin-related biology and therapeutics have been studied intensively in the dystrophies. Genomic mechanisms of dystrophin inactivation were identified in 96 percent of metastatic GISTs, whereas dystrophin inactivation was not found in low-risk GISTs. The dystrophin 427kDa myogenic isoform was expressed robustly in interstitial cells of Cajal, which are the nonneoplastic cell counterpart to GIST, as well as in low-risk GISTs. Previous studies also suggested that dystrophin inactivation predisposes to development of myogenic cancers: as one example, rhabdomyosarcomas occur at increased frequency in mice with dystrophin deficiency. Our study advances these concepts by providing evidence that dystrophin has ubiquitous tumor suppressor roles in certain human cancers with myogenic features, particularly in GIST, rhabdomyosarcoma and leiomyosarcoma. Notably, dystrophin inactivation in GISTs was the last identifiable genomic event during tumor progression, arising within subclones of high-risk GISTs and present in all metastases arising from those GISTs. These findings jibe well with functional studies in dystrophin deficient GIST lines, where dystrophin restoration inhibited cell migration, invasion, anchorage independence, and invadopodia formation.5 In turn, the findings fit with the known biology of dystrophin, as a structural link between the actin-based cytoskeleton and extracellular matrix. Together, these observations establish a model of oncogenic progression in GIST, in which KIT/PDGFRA oncogenic mutations are the initiating events, creating a clonal albeit minimally proliferative benign pre-cancer, after which mutations of cell cycle regulators cause transition from low-risk to high-risk GIST, and – finally – dystrophin inactivation enables metastatic spread. Based on this new model for myogenic cancer progression, one can envision effective combination therapeutic strategies, targeting cell cycle and dystrophin defects. Demonstration of dystrophin inactivating mutations in GIST and other myogenic cancers has therapeutic relevance, because targeted therapies are already under development for muscular dystrophies, with the aim of restoring dystrophin function. These therapeutic approaches in muscular dystrophies provide a ready departure point for engaging similar strategies in dystrophin-deficient cancers. Dystrophin functional restoration can be accomplished by various approaches, including viral delivery of a functional DMD gene, exon skipping to restore the DMD reading frame, read-through of translation stop codons, and increased expression of the compensatory utrophin gene. Each of these strategies is undergoing preclinical and/or clinical evaluation as a potential therapy for Duchenne muscular dystrophy.6,7 These approaches warrant evaluation as potential therapeutic agents in oncology, particularly in myogenic cancers in which tumorigenic progression is fostered by dystrophin inactivation.

Personalized Medicine in Gastrointestinal Stromal Tumor (GIST): Clinical Implications of the Somatic and Germline DNA Analysis.

Gastrointestinal stromal tumors (GIST) are the most common mesenchymal tumors of the gastrointestinal tract. They are characterized by gain of function mutations in KIT or PDGFRA tyrosine kinase receptors, with their consequent constitutive activation. The gold standard therapy is imatinib that offers a good and stable response for approximately 18–36 months. However, resistance is very common and it is vital to identify new biomarkers. Up until now, there have been two main approaches with focus to characterize novel targets. On the one hand, the focus is on the tumor genome, as the final clinical outcome depends mainly from the cancer specific mutations/alterations patterns. However, the germline DNA is important as well, and it is inconceivable to think the patients response to the drug is not related to it. Therefore the aim of this review is to outline the state of the art of the personalized medicine in GIST taking into account both the tumor DNA (somatic) and the patient DNA (germline).

Mastocytosis: a mutated KIT receptor induced myeloproliferative disorder.

Although more than 90% systemic mastocytosis (SM) patients express gain of function mutations in the KIT receptor, recent next generation sequencing has revealed the presence of several additional genetic and epigenetic mutations in a subset of these patients, which confer poor prognosis and inferior overall survival. A clear understanding of how genetic and epigenetic mutations cooperate in regulating the tremendous heterogeneity observed in these patients will be essential for designing effective treatment strategies for this complex disease. In this review, we describe the clinical heterogeneity observed in patients with mastocytosis, the nature of relatively novel mutations identified in these patients, therapeutic strategies to target molecules downstream from activating KIT receptor and finally we speculate on potential novel strategies to interfere with the function of not only the oncogenic KIT receptor but also epigenetic mutations seen in these patients.

Molecular features and genetic markers of gastrointestinal stromal tumors

Gastrointestinal stromal tumors (GISTs) are the most spread mesenchymal tumors located within the gastrointestinal tract that have particular clinico-morphological, immunohistochemical and molecular characteristics. The distinguishing mark of GISTs is the presence of the cell-surface antigen CD117 (KIT receptor tyrosine kinase), identified by immunohistochemistry. GISTs consist of tumors with various activating mutations in KIT (75–80 %) or PDGFRA (5–15 %) receptor tyrosine kinases. Numerous KIT and PDGFRA mutations are associated with specific GIST morphology, histologic phenotype, metastasizing and prognosis. 10–15 % of GISTs contain KIT and PDGFRA wild type genes, some of them have driver BRAF, IGF1R or PIK3CA mutations. The other GISTs patients have familial syndromes (neurofibromatosis type 1, Carney–Stratakis syndrome, Carney triad) and contain germline mutations of NF1 or the genes coding for the succinate dehydrogenase subunits SDHA, SDHB, SDHC, and SDHD. GISTs are first and the most studied model for development of principles and methods of personalized targeted therapy of solid tumors with tyrosine kinase inhibitors.

Morphological Features and KIT Receptor Expression in Canine Cutaneous Mast Cell Tumor and Systemic Mastocytosis / Morfološke Karakteristike I Ekspresija KIT Receptora Kod Kutanih Mastocitoma I Sistemske Mastocitoze Pasa

Abstract Mast cell neoplasia in dogs can occur in two different forms: common as cutaneous tumor, or less common as a systemic form of neoplastic mast cell proliferation - systemic mastocytosis. The aim of this study was to compare the histological and cytological features, KIT receptor expression and presence of c-KIT proto-oncogene mutations in neoplastic cells of dogs with canine cutaneous mast cell tumor (CMCT) and systemic mastocytosis. Microscopical examination of the cytological smears obtained from all selected dogs revealed that cellular specimens were constituted mostly of round cells with a central nuclei and fine to coarse purple cytoplasmic granules. Histopathological examination of skin samples of dogs with CMCT and a dog with systemic mastocytosis showed proliferation of the neoplastic mast cells in the superficial and/or deep dermis. Similar findings were observed in tissue samples derived from lymph nodes, spleen, liver, myocardium and kidneys of a dog with systemic mastocytosis. Three dogs with high grade CMCT as well as one dog with systemic mastocytosis showed cytoplasmic CD117 expression, while 3 dogs with low grade CMCT, had membranous expression of CD117. Based on our study, histological features and cytoplasmic CD117 expression in neoplastic cells of dogs with systemic mastocytosis are similar to those in dogs with high grade CMCTs. Nevertheless, mutations of c-KIT proto-oncogene were not found in tumor samples either from dogs with CMCT or dog with systemic mastocytosis.

A point mutation in the extracellular domain of KIT promotes tumorigenesis of mast cells via ligand-independent auto-dimerization.

Mutations in the juxtamembrane and tyrosine kinase domains of the KIT receptor have been implicated in several cancers and are known to promote tumorigenesis. However, the pathophysiological manifestations of mutations in the extracellular domain remain unknown. In this study, we examined the impact of a mutation in the extracellular domain of KIT on mast cell tumorigenesis. A KIT mutant with an Asn508Ile variation (N508I) in the extracellular domain derived from a canine mast cell tumor was introduced into IC-2 cells. The IC-2N508I cells proliferated in a cytokine-independent manner and showed KIT auto-phosphorylation. Subcutaneous injection of IC-2N508I cells into the dorsal area of immunodeficient BALB/c-nu/nu mice resulted in the formation of solid tumors, but tumor progression was abrogated by treatment with a tyrosine kinase inhibitor (STI571). In addition, the N508I mutant KIT protein dimerized in the absence of the natural ligand, stem cell factor. Structure modeling indicates that the increased hydrophobicity of the mutant led to the stabilization of KIT dimers. These results suggest that this extracellular domain mutation confers a ligand-independent tumorigenic phenotype to mast cells by KIT auto-dimerization that is STI571-sensitive. This is the first report demonstrating the tumorigenic potential of a mutation in the extracellular domain of KIT.

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.