Identification of mutations in the coding sequence of the proto-oncogene c-kit in a human mast cell leukemia cell line causing ligand-independent activation of c-kit product

CD72 negatively regulates KIT-mediated responses in human mast cells

KIT activation, through binding of its ligand, stem cell factor, is crucial for normal mast cell growth, differentiation, and survival. Furthermore, KIT may also contribute to mast cell homing and cytokine generation. Activating mutations in KIT lead to the dysregulated mast cell growth associated with the myeloproliferative disorder, mastocytosis. We investigated the potential of downregulating such responses through mast cell inhibitory receptor activation. In this study, we report that the B cell-associated ITIM-containing inhibitory receptor, CD72, is expressed in human mast cells. Ligation of CD72 with the agonistic Ab, BU40, or with recombinant human CD100 (rCD100), its natural ligand, induced the phosphorylation of CD72 with a resulting increase in its association with the tyrosine phosphatase SH2 domain-containing phosphatase-1. This, in turn, resulted in an inhibition of KIT-induced phosphorylation of Src family kinases and extracellular-regulated kinases (ERK1/2). As a consequence of these effects, KIT-mediated mast cell proliferation, chemotaxis, and chemokine production were significantly reduced by BU40 and rCD100. Furthermore, BU40 and rCD100 also downregulated the growth of the HMC1.2 human mast cell line. Thus, targeting CD72 may provide a novel approach to the suppression of mast cell disease such as mastocytosis.

The antitumor activity of homoharringtonine against human mast cells harboring the KIT D816V mutation

Gain-of-function mutations of the receptor tyrosine kinase KIT play a critical role in the pathogenesis of systemic mastocytosis (SM) and gastrointestinal stromal tumors. The various juxtamembrane type of KIT mutations, including V560G, are found in 60% to 70% of patients with gastrointestinal stromal tumors; loop mutant D816V, which exists in approximately 80% of SM patients, is completely resistant to imatinib. In the present study, we hypothesized that homoharringtonine (HHT), a protein synthesis inhibitor, would decrease the level of KIT protein by inhibiting translation, resulting in a decreased level of phospho-KIT and abrogating its constitutive downstream signaling. Imatinib-sensitive HMC-1.1 cells harboring the mutation V560G in the juxtamembrane domain of KIT, imatinib-resistant HMC-1.2 cells harboring both V560G and D816V mutations, and murine P815 cells were treated with HHT and analyzed in terms of growth, apoptosis, and signal transduction. The in vivo antitumor activity was evaluated by using the murine mast cell leukemia model. Our results indicated that HHT effectively inhibited the growth and induced apoptosis in cells bearing both V560G and D816V or D814Y KIT. Additionally, HHT inhibited the KIT-dependent phosphorylation of downstream signaling molecules Akt, signal transducer and activator of transcription 3 and 5, and extracellular signal-regulated kinase 1/2. Furthermore, HHT significantly prolonged the survival duration of mice with aggressive SM or mast cell leukemia by inhibiting the expansion and infiltration of imatinib-resistant mast tumor cells harboring imatinib-resistant D814Y KIT. Collectively, we show that HHT circumvents D816V KIT-elicited imatinib resistance. Our findings warrant a clinical trial of HHT in patients with SM harboring D816V or D814Y KIT.

Receptor tyrosine kinase alterations in AML - biology and therapy

Acute myeloid leukemia (AML) is the most common form of leukemia in adults, and despite some recent progress in understanding the biology of the disease, AML remains the leading cause of leukemia-related deaths in adults and children. AML is a complex and heterogeneous disease, often involving multiple genetic defects that promote leukemic transformation and drug resistance. The cooperativity model suggests that an initial genetic event leads to maturational arrest in a myeloid progenitor cell, and subsequent genetic events induce proliferation and block apoptosis. Together, these genetic abnormalities lead to clonal expansion and frank leukemia. The purpose of this chapter is to review the biology of receptor tyrosine kinases (RTKs) in AML, exploring how RTKs are being used as novel prognostic factors and potential therapeutic targets.

Expression of activated STAT5 in neoplastic mast cells in systemic mastocytosis: subcellular distribution and role of the transforming oncoprotein KIT D816V

Recent data suggest that the signal transducer and activator of transcription (STAT)5 contributes to differentiation and growth of mast cells. It has also been described that constitutively phosphorylated STAT5 (pSTAT5) plays a pro-oncogenic role in various myeloid neoplasms. We examined the expression of pSTAT5 in neoplastic mast cells in systemic mastocytosis and asked whether the disease-related oncoprotein KIT D816V is involved in STAT5 activation. As assessed by immunohistochemistry using the anti-pSTAT5 antibody AX1, neoplastic mast cells were found to display pSTAT5 in all SM patients examined (n = 40). Expression of pSTAT5 was also demonstrable in the KIT D816V-positive mast cell leukemia cell line HMC-1. Using various staining-protocols, pSTAT5 was found to be located in both the cytoplasmic and nuclear compartment of mast cells. To define the functional role of KIT D816V in STAT5-activation, Ba/F3 cells with doxycycline-inducible expression of KIT D816V were used. In these cells, induction of KIT D816V resulted in an increased expression of pSTAT5 without substantial increase in total STAT5. Moreover, the KIT D816V-targeting kinase-inhibitor PKC412 was found to counteract expression of pSTAT5 in HMC-1 cells as well as doxycycline-induced expression of pSTAT5 in Ba/F3 cells. Finally, a dominant negative STAT5-construct was found to inhibit growth of HMC-1 cells. Together, our data show that neoplastic mast cells express cytoplasmic and nuclear pSTAT5, that KIT D816V promotes STAT5-activation, and that STAT5-activation contributes to growth of neoplastic mast cells.

Identification of proapoptotic Bim as a tumor suppressor in neoplastic mast cells: role of KIT D816V and effects of various targeted drugs

Systemic mastocytosis (SM) is a myeloid neoplasm involving mast cells (MCs) and their progenitors. In most cases, neoplastic cells display the D816V-mutated variant of KIT. KIT D816V exhibits constitutive tyrosine kinase (TK) activity and has been implicated in increased survival and growth of neoplastic MCs. Recent data suggest that the proapoptotic BH3-only death regulator Bim plays a role as a tumor suppressor in various myeloid neoplasms. We found that KIT D816V suppresses expression of Bim in Ba/F3 cells. The KIT D816-induced down-regulation of Bim was rescued by the KIT-targeting drug PKC412/midostaurin. Both PKC412 and the proteasome-inhibitor bortezomib were found to decrease growth and promote expression of Bim in MC leukemia cell lines HMC-1.1 (D816V negative) and HMC-1.2 (D816V positive). Both drugs were also found to counteract growth of primary neoplastic MCs. Furthermore, midostaurin was found to cooperate with bortezomib and with the BH3-mimetic obatoclax in producing growth inhibition in both HMC-1 subclones. Finally, a Bim-specific siRNA was found to rescue HMC-1 cells from PKC412-induced cell death. Our data show that KIT D816V suppresses expression of proapoptotic Bim in neoplastic MCs. Targeting of Bcl-2 family members by drugs promoting Bim (re)-expression, or by BH3-mimetics such as obatoclax, may be an attractive therapy concept in SM.

Sequence and structure signatures of cancer mutation hotspots in protein kinases

Protein kinases are the most common protein domains implicated in cancer, where somatically acquired mutations are known to be functionally linked to a variety of cancers. Resequencing studies of protein kinase coding regions have emphasized the importance of sequence and structure determinants of cancer-causing kinase mutations in understanding of the mutation-dependent activation process. We have developed an integrated bioinformatics resource, which consolidated and mapped all currently available information on genetic modifications in protein kinase genes with sequence, structure and functional data. The integration of diverse data types provided a convenient framework for kinome-wide study of sequence-based and structure-based signatures of cancer mutations. The database-driven analysis has revealed a differential enrichment of SNPs categories in functional regions of the kinase domain, demonstrating that a significant number of cancer mutations could fall at structurally equivalent positions (mutational hotspots) within the catalytic core. We have also found that structurally conserved mutational hotspots can be shared by multiple kinase genes and are often enriched by cancer driver mutations with high oncogenic activity. Structural modeling and energetic analysis of the mutational hotspots have suggested a common molecular mechanism of kinase activation by cancer mutations, and have allowed to reconcile the experimental data. According to a proposed mechanism, structural effect of kinase mutations with a high oncogenic potential may manifest in a significant destabilization of the autoinhibited kinase form, which is likely to drive tumorigenesis at some level. Structure-based functional annotation and prediction of cancer mutation effects in protein kinases can facilitate an understanding of the mutation-dependent activation process and inform experimental studies exploring molecular pathology of tumorigenesis.

In vivo effect of imatinib on progression of cecal GIST-like tumors in exon 17-type c-kit knock-in mice

Two families with a germline Asp820Tyr mutation at exon 17 of the c-kit gene and multiple gastrointestinal stromal tumors (GISTs) have been reported. Recently, we generated a knock-in mouse model of the family, and mice with KIT-Asp818Tyr corresponding to human KIT-Asp820Tyr showed a cecal GIST-like tumor. In this report, we examined the in vivo effect of imatinib on tumor progression in knock-in mice. Imatinib of 100 microg/g body weight was administered to heterozygous (KIT-Asp818Tyr/+) mice orally for 7, 14 and 28 days, and cecal tumors were dissected. Both macroscopic size and the measured volume of cecal tumors were not significantly reduced after a 7-, 14- and 28-day administration of imatinib when compared with those before imatinib administration. Cell proliferation was assessed by Ki-67 immunohistochemistry and the labeling index significantly decreased after imatinib administration, but the value of the index after imatinib was only about half compared with that before imatinib. Western blotting and real-time PCR revealed that KIT expression was almost equivalent, but KIT phosphorylation was significantly but not completely inhibited in tumor tissues after 7, 14 and 28 days of imatinib administration when compared with that before imatinib administration. Phosphorylation of Akt and Stat1 was accordingly inhibited after imatinib administration. Thus, imatinib seemed to inhibit in vivo tumor proliferation but not decrease tumor volume on this mouse model, probably because of an insufficient inhibition of phosphorylation of KIT and its downstream signaling molecules. These results suggested that progression of multiple GISTs in patients with germline Asp820Tyr might be partially controlled by imatinib and that model mice provide an opportunity to examine the effect of various other targeted drugs on in vivo tumor progression.

IL-33 synergizes with IgE-dependent and IgE-independent agents to promote mast cell and basophil activation

OBJECTIVE

Mast cell and basophil activation contributes to inflammation, bronchoconstriction, and airway hyperresponsiveness in asthma. Because IL-33 expression is inflammation inducible, we investigated IL-33-mediated effects in concert with both IgE-mediated and IgE-independent stimulation.

METHODS

Because the HMC-1 mast cell line can be activated by GPCR and RTK signaling, we studied the effects of IL-33 on these pathways. The IL-33- and SCF-stimulated HMC-1 cells were co-cultured with human lung fibroblasts and airway smooth muscle cells in a collagen gel contraction assay. IL-33 effects on IgE-mediated activation were studied in primary mast cells and basophils.

RESULT

IL-33 synergized with adenosine, C5a, SCF, and NGF receptor activation. IL-33-stimulated and SCF-stimulated HMC-1 cells demonstrated enhanced collagen gel contraction when cultured with fibroblasts or smooth muscle cells. IL-33 also synergized with IgE receptor activation of primary human mast cells and basophils.

CONCLUSION

IL-33 amplifies inflammation in both IgE-independent and IgE-dependent responses.

Splenic Mastocytosis: Report of Two Cases and Detection of the Transforming SomaticC-KITMutation D816V

In the vast majority of patients with systemic mastocytosis (SM), the bone marrow is the primary extracutaneous site of disease. In addition to bone marrow involvement, other visceral organs such as the spleen, liver or the gastrointestinal tract, may also be affected. However, isolated involvement of a single extramedullary organ is rarely seen in SM. We report on two patients with SM with splenic involvement, lack of 'diagnostic' mast cell (MC) infiltrates in the bone marrow, and absence of skin lesions. In one patient, a myelodysplastic syndrome was diagnosed prior to the detection of SM. Both patients presented with massive splenomegaly and multifocal MC infiltrates in splenic tissues. These MCs also expressed CD25 as well as the C-KIT mutation D816V. In consecutive examinations, the mutation was also detected in the bone marrow in both patients suggesting diffuse infiltration with neoplastic cells. In summary, our data show that the spleen can be a primary site of disease in rare cases of SM. Mastocytosis should therefore be considered as a (rare) differential diagnosis in patients with splenomegaly of unknown etiology.

Gastric leiomyosarcoma manifesting peculiar findings: radiological-pathological correlation

Reported herein is a gastric leiomyosarcoma, which, nowadays, is extremely rare. Attention was focused not only on pathological findings but also on the histological basis of magnetic resonance imaging (MRI) findings. The patient was a 29-year-old Japanese woman. Preoperative T2-weighted MRI showed a large high-intensity gastric tumor with isointense streaks. The tumor was a broad-based large polypoid lesion and histologically consisted of fascicles of spindled cells having eosinophilic cytoplasm and cigar-shaped nuclei. Immunoreactivity for several smooth muscle markers including desmin on tumor cells, low amplification of both c-kit and PDGFRA cDNA on polymerase chain reaction, and absence of c-kit gene mutation in exons 9 and 11 strongly suggested that the tumor was not a gastrointestinal stromal tumor but a true leiomyosarcoma. In vitro MRI of the fresh tumor was obtained to explain the radiological findings on a morphological basis. In vitro MRI clearly depicted the very high-intensity areas separated by radially extended isointense lines. This radiological finding corresponded best to the most characteristic histological feature, that is, linearly extended fascicles of the tumor cells often with myxedematous change separated by radially elongated thin fibrovascular stroma: in other words, spouting appearance.

Activity of triptolide against human mast cells harboring the kinase domain mutant KIT

Gain-of-function mutations of the receptor tyrosine kinase KIT can cause systemic mastocytosis (SM) and gastrointestinal stromal tumors. Most of the constitutively active KIT can be inhibited by imatinib; D816V KIT cannot. In this study, we investigated the activity of triptolide, a diterpenoid isolated from the Chinese herb Tripterygium wilfordii Hook. f., in cells expressing mutant KIT, including D816V KIT. Imatinib-sensitive HMC-1.1 cells harboring the mutation V560G in the juxtamembrane domain of KIT, imatinib-resistant HMC-1.2 cells harboring both V560G and D816V mutations, and murine P815 cells, were treated with triptolide, and analyzed in terms of growth, apoptosis, and signal transduction. The in vivo antitumor activity was evaluated by using the nude mouse xenograft model. Our results demonstrated that triptolide potently inhibits the growth of both human and murine mast cells harboring not only imatinib-sensitive KIT mutation but also imatinib-resistant D816V KIT. Triptolide markedly inhibited KIT mRNA levels and strikingly reduced the levels of phosphorylated and total Stat3, Akt, and Erk1/2, downstream targets of KIT. Triptolide triggered apoptosis by inducing depolarization of mitochondrial potential and release of cytochrome c, downregulation of Mcl-1 and XIAP. Furthermore, triptolide significantly abrogated the growth of imatinib-resistant HMC-1.2 cell xenografts in nude mice and decreased KIT expression in xenografts. Our data demonstrate that triptolide inhibits imatinib-resistant mast cells harboring D816V KIT. Further investigation of triptolide for treatment of human neoplasms driven by gain-of-function KIT mutations is warranted.

Aleukemic Mast Cell Leukemia with Abnormal Immunophenotype and C-kit Mutation D816V

Mastocytosis comprises a heterogeneous group of disorders characterized by proliferation and accumulation of mast cells in 1 or more organ systems. Mast cell leukemia (MCL) is an extremely rare subtype of mastocytosis in which a leukemic spread of mast cells and a rapid progression of disease is seen. In typical cases, mast cells are found in the peripheral blood. However, an aleukemic variant of MCL (formerly termed malignant mastocytosis) has also been described. We here report a case of aleukemic MCL with abnormal immunophenotype of mast cells and the classical c-kit point mutation Asp-816-Val (=D816V). The 75-year-old male patient had a short history of weight loss and lymphadenopathy. There were no urticaria pigmentosa-like skin lesions. The bone marrow was diffusely infiltrated with atypical mast cells that comprised more than 80% of all nucleated cells on a bone marrow smears. As assessed by immunohistochemistry, neoplastic mast cells expressed tryptase, chymase, CD2, CD25, CD68, and the KIT protein (CD117). Mutation analysis revealed the c-kit mutation D816V. Since circulating mast cells could not be detected in the peripheral blood, the diagnosis of aleukemic MCL was established in accordance to the updated WHO consensus classification. This case further supports the notion that the pathogenesis (c-kit mutation D816V) in MCL is closely related to that found in indolent mast cell disorders. However, additional (but yet unknown) molecular (genetic) defects have to be considered to explain the extremely heterogenous clinical course in these patients.

JAK2 V617F in myeloid disorders: What do we know now, and where are we headed?

Activating tyrosine kinase (TK) mutations disrupt cellular proliferation and survival pathways and are increasingly recognized as a fundamental cause of human cancers. Until very recently, the only TK mutations widely observed in myeloid neoplasia were the BCR/ABL1 fusions characteristic of chronic myeloid leukemia and some acute leukemias, and FLT3 activating mutations in a minority of acute myeloid leukemias. Several rare TK mutations are found in various atypical myeloproliferative disorders, but big pieces of the pathobiological puzzle were glaringly missing. In the first half of 2005, one gap was filled in: 7 studies identified the same acquired amino acid substitution (V617F) in the Janus kinase 2 (JAK2) TK in large numbers of patients with diverse clonal myeloid disorders. Most affected patients suffer from the classic BCR/ABL1-negative myeloproliferative disorders (MPD), especially polycythemia vera (74% of n = 506), but a subset of people with essential thrombocythemia (36% of n = 339) or myelofibrosis with myeloid metaplasia (44% of n = 127) bear the identical mutation, as do a few individuals with myelodysplastic syndromes or an atypical myeloid disorder (7% of n = 556). This long-sought common mutation in BCR/ABL1-negative MPD raises many provocative biological and clinical questions, and demands re-evaluation of prevailing diagnostic algorithms for erythrocytosis and thrombocytosis. JAK2 V617F may provide novel molecular targets for drug therapy, and suggests other places to seek cooperating mutations or mutations associated with similar phenotypes. The story of this exciting finding will unfold rapidly in the years ahead, and ongoing developments will be important for all hematologists to understand.

Updating old ideas and recent advances regarding the Interstitial Cells of Cajal

Since their discovery by Cajal in 1889, the Interstitial Cells of Cajal (ICC) have generated much controversy in the scientific community. Indeed, the nervous, muscle or fibroblastic nature of the ICC has remained under debate for more than a century, as has their possible physiological function. Cajal and his colleagues considered them to be neurons, while contemporary histologists like Kölliker and Dogiel categorized these cells as fibroblasts. More recently, the role of ICC in the origin of slow-wave peristaltism has been elucidated, and several studies have shown that they participate in neurotransmission (intercalation theory). The fact that ICC assemble in the circular muscular layer and that they originate from cells which emerge from the ventral neural tube (VENT cells), a source of neurons, glia and ICC precursors other than the neural crest, suggests a neural origin for this particular subset of ICC. The discovery that ICC express the Kit protein, a type III tyrosine kinase receptor encoded by the proto-oncogene c-kit, has helped better understand their physiological role and implication in pathological conditions. Gleevec, a novel molecule designed to inhibit the mutant activated version of c-Kit receptors, is the drug of choice to treat the so-called gastrointestinal stromal tumours (GIST), the most common non-epithelial neoplasm of the gastrointestinal tract. Here we review Cajal's original contributions with the aid of unique images taken from Cajal's histological slides (preserved at the Cajal Museum, Cajal Institute, CSIC). In addition, we present a historical review of the concepts associated with this particular cell type, emphasizing current data that has advanced our understanding of the role these intriguing cells fulfil.

Multi-center, placebo-controlled, double-blind, randomized study of oral toceranib phosphate (SU11654), a receptor tyrosine kinase inhibitor, for the treatment of dogs with recurrent (either local or distant) mast cell tumor following surgical excision

PURPOSE

The purpose of this study was to determine the objective response rate (ORR) following treatment of canine mast cell tumors (MCT) with toceranib phosphate (Palladia, SU11654), a kinase inhibitor with both antitumor and antiangiogenic activity through inhibition of KIT, vascular endothelial growth factor receptor 2, and PDGFRbeta. Secondary objectives were to determine biological response rate, time to tumor progression, duration of objective response, health-related quality of life, and safety of Palladia.

EXPERIMENTAL DESIGN

Dogs were randomized to receive oral Palladia 3.25 mg/kg or placebo every other day for 6 weeks in the blinded phase. Thereafter, eligible dogs received open-label Palladia.

RESULTS

The blinded phase ORR in Palladia-treated dogs (n = 86) was 37.2% (7 complete response, 25 partial response) versus 7.9% (5 partial response) in placebo-treated dogs (n = 63; P = 0.0004). Of 58 dogs that received Palladia following placebo-escape, 41.4% (8 complete response, 16 partial response) experienced objective response. The ORR for all 145 dogs receiving Palladia was 42.8% (21 complete response, 41 partial response); among the 62 responders, the median duration of objective response and time to tumor progression was 12.0 weeks and 18.1 weeks, respectively. Palladia-treated responders scored higher on health-related quality of life versus Palladia-treated nonresponders (P = 0.030). There was no significant difference in the number of dogs with grade 3/4 (of 4) adverse events; adverse events were generally manageable with dose modification and/or supportive care.

CONCLUSIONS

Palladia has biological activity against canine MCTs and can be administered on a continuous schedule without need for routine planned treatment breaks. This clinical trial further shows that spontaneous tumors in dogs are good models to evaluate therapeutic index of targeted therapeutics in a clinical setting.

Growth-inhibitory effects of four tyrosine kinase inhibitors on neoplastic feline mast cells exhibiting a Kit exon 8 ITD mutation

Systemic mastocytosis (SM) in felines is a rare neoplasm defined by increased growth and accumulation of immature mast cells (MC) in various organs including the spleen. Although in many cases splenectomy is an effective approach, relapses may occur. In these patients, treatment options are limited. Recent data suggest that various Kit tyrosine kinase inhibitors (TKI) interfere with growth of neoplastic MC in humans. In the current study, we examined the effects of four TKI, imatinib, midostaurin, nilotinib, and dasatinib, on growth of spleen-derived feline neoplastic MC in three SM patients. Expression of Kit in neoplastic MC was confirmed by flow cytometry and/or Western blotting. In all three cases, a 12-bp internal tandem duplication in exon 8, resulting in a four amino acid-insertion between residues Thr418 and His419 in Kit, was detectable. As assessed by (3)H-thymidine incorporation experiments, all four TKI were found to inhibit the growth of feline neoplastic MC in a dose-dependent manner. The growth-inhibitory TKI effects were found to be associated with morphologic signs of apoptosis in MC. In conclusion, various Kit-targeting TKI can inhibit the in vitro growth and survival of feline neoplastic MC in SM.

CD203c is overexpressed on neoplastic mast cells in systemic mastocytosis and is upregulated upon IgE receptor cross-linking

The ectoenzyme E-NPP3 (CD203c) has recently been identified as a novel activation-linked cell surface antigen on basophils. In the present study, we examined expression of CD203c on normal mast cells (MC)and bone marrow (bm) MC derived from 85 patients with systemic mastocytosis (SM), including cases with indolent SM (ISM, n=72), SM with associated clonal hematologic non-MC-lineage disease (SM-AHNMD, n=6), aggressive SM (ASM, n=3), and mast cell leukemia (MCL, n=4). Surface expression of CD203c was analyzed by multicolor flow cytometry. In patients with SM, bm MC expressed significantly higher amounts of CD203c compared to normal bm MC (median MFI in controls: 260 versus median MFI in SM: 516, p<0.05). Slightly lower amounts of CD203c were detected on MC in SM-AHNMD and ASM compared to ISM. To demonstrate CD203c expression in MC at the mRNA level, neoplastic MC were highly enriched by cell sorting, and were found to express CD203c mRNA in RT-PCR analysis. Cross-linking of the IgE receptor on MC resulted in a substantial upregulation of CD203c, whereas the KIT-ligand stem cell factor (SCF) showed no significant effects. In conclusion, CD203c is a novel activation-linked surface antigen on MC that is upregulated in response to IgE receptor cross-linking and is overexpressed on neoplastic MC in patients with SM.

Gastrointestinal stromal tumors (GISTs): a review

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal neoplasms in the gastrointestinal tract, which, over the last 10 years, have emerged from a poorly understood neoplasm to a well-defined tumor entity exhibiting particular molecular abnormalities and for which promising novel treatment modalities have been developed. GISTs probably arise from the precursor cell of the interstitial cell of Cajal, express KIT tyrosine kinase in most of the cases and harbor mutations of importance for individualized treatment. The molecular targets for therapeutic interventions are not only of importance for the treatment of GIST patients but also useful for in the development of novel drug modalities and new strategies in basic cancer therapy.

Amelanotic Anorectal Malignant Melanoma: Case Report with Immunohistochemical Study and Literature Review

Epithelioid cell tumors presenting in the gastrointestinal tract are uncommon, but when they arise, arriving at a correct diagnosis is important. We report a case of anal malignant melanoma in an 82-year-old man who microscopically showed an epithelioid malignant tumor simulating a gastrointestinal stromal tumor. C-kit stain and Melan-A were diffusely and strongly positive, while HMB-45 was focally positive. This case illustrates the potential pitfall of relying on a single antibody or inadequate panel of immunohistochemical stains to confirm the diagnosis. We recommend to apply an adequate immunohistochemical panel which includes S-100 protein, HMB-45 and Melan-A in order to make an accurate diagnosis, and discuss the differential diagnosis and surgical treatment modalities.

Phase II study of imatinib mesylate as therapy for patients with systemic mastocytosis

Gain-of-function D816V point mutation within the kinase domain of the transmembrane receptor KIT is found in the great majority of patients with systemic mastocytosis (SM) and is attractive therapeutic target. Twenty patients with SM were enrolled during 2003-2005 in phase II clinical trial with imatinib mesylate (400mg daily), a KIT inhibitor. Median time on therapy was 9 months (range, 0.5-44+). Only one patient, with D816V KIT mutation-negative FIP1L1-PDGFRalpha-negative SM-HES, achieved complete remission (now lasting for 44 months). Six other patients reported symptomatic improvement, including two with D816V KIT mutation-positive SM (one reported improvement in diarrhea and the other in fatigue). Other patients had no benefit. Imatinib was relatively well tolerated. Our study confirms that imatinib therapy does not result in appreciable clinical activity in patients with D816V mutation-positive SM, but may result in a significant benefit in occasional patient with D816V mutation-negative SM.

Interleukin-13 promoter gene polymorphism -1112C/T is associated with the systemic form of mastocytosis

BACKGROUND

Mastocytosis is a heterogenous disease involving mast cells (MC) and their progenitors. Cutaneous and systemic variants of the disease have been reported. In contrast to cutaneous mastocytosis (CM), patients with systemic mastocytosis (SM) are at risk to develop disease progression or a nonMC-lineage haematopoietic neoplasm. Little is known, however, about factors predisposing for the development of SM. One factor may be cytokine regulation of MC progenitors.

METHODS

We examined the role of the interleukin-13 (IL-13) promoter gene polymorphism -1112C/T, known to be associated with increased transcription, in mastocytosis using allele-specific polymerase chain reaction method. Serum tryptase and IL-13 levels were determined by immunoassay, and expression of the IL-13 receptor in neoplastic MC by reverse transcription-polymerase chain reaction and flow cytometry.

RESULTS

The frequency of the -1112T allele of the IL-13 promoter was significantly higher in patients with SM compared with CM (P < 0.008) and in mastocytosis patients compared with healthy controls (P < 0.0001). Correspondingly, the polymorphism was found to correlate with an elevated serum tryptase level (P = 0.004) and with adult-onset of the disease (P < 0.0015), both of which are almost invariably associated with SM. Serum IL-13 levels were also higher in SM patients compared with CM (P = 0.011), and higher in CT- than in CC carriers (P < 0.05). Finally, we were able to show that neoplastic human MC display IL-13 receptors and grow better in IL-13-containing medium.

CONCLUSIONS

The -1112C/T IL-13 gene polymorphism and the resulting 'hypertranscription' may predispose for the development of SM.

Canine mast cell tumours: a review of the pathogenesis, clinical features, pathology and treatment

Mast cells (MCs) are well known for their neoplastic transformation in solitary and multiple cutaneous mast cell tumours (MCTs), as well as visceral and systemic mastocytosis. Dogs have a unique risk of developing cutaneous MCTs, and they account for 7% to 21% of all canine skin tumours. The aetiology of canine MCTs is unknown but is probably multifactorial. This article reviews up-to-date knowledge on the pathogenesis, the clinical presentation, the clinical prognostic factors, the diagnostic workup including clinical staging, cytological findings, histological findings and the various grading systems which have been evaluated based on morphology, the assessment of proliferation markers and other factors such as vessel density. Furthermore, detailed information about current treatment protocols for canine cutaneous MCTs is provided.

FIP1L1-PDGFRalpha imposes eosinophil lineage commitment on hematopoietic stem/progenitor cells

Although leukemogenic tyrosine kinases (LTKs) activate a common set of downstream molecules, the phenotypes of leukemia caused by LTKs are rather distinct. Here we report the molecular mechanism underlying the development of hypereosinophilic syndrome/chronic eosinophilic leukemia by FIP1L1-PDGFRalpha. When introduced into c-Kit(high)Sca-1(+)Lineage(-) cells, FIP1L1-PDGFRalpha conferred cytokine-independent growth on these cells and enhanced their self-renewal, whereas it did not immortalize common myeloid progenitors in in vitro replating assays and transplantation assays. Importantly, FIP1L1-PDGFRalpha but not TEL-PDGFRbeta enhanced the development of Gr-1(+)IL-5Ralpha(+) eosinophil progenitors from c-Kit(high)Sca-1(+)Lineage(-) cells. FIP1L1-PDGFRalpha also promoted eosinophil development from common myeloid progenitors. Furthermore, when expressed in megakaryocyte/erythrocyte progenitors and common lymphoid progenitors, FIP1L1-PDGFRalpha not only inhibited differentiation toward erythroid cells, megakaryocytes, and B-lymphocytes but aberrantly developed eosinophil progenitors from megakaryocyte/erythrocyte progenitors and common lymphoid progenitors. As for the mechanism of FIP1L1-PDGFRalpha-induced eosinophil development, FIP1L1-PDGFRalpha was found to more intensely activate MEK1/2 and p38(MAPK) than TEL-PDGFRbeta. In addition, a MEK1/2 inhibitor and a p38(MAPK) inhibitor suppressed FIP1L1-PDGFRalpha-promoted eosinophil development. Also, reverse transcription-PCR analysis revealed that FIP1L1-PDGFRalpha augmented the expression of C/EBPalpha, GATA-1, and GATA-2, whereas it hardly affected PU.1 expression. In addition, short hairpin RNAs against C/EBPalpha and GATA-2 and GATA-3KRR, which can act as a dominant-negative form over all GATA members, inhibited FIP1L1-PDGFRalpha-induced eosinophil development. Furthermore, FIP1L1-PDGFRalpha and its downstream Ras inhibited PU.1 activity in luciferase assays. Together, these results indicate that FIP1L1-PDGFRalpha enhances eosinophil development by modifying the expression and activity of lineage-specific transcription factors through Ras/MEK and p38(MAPK) cascades.

Constitutive JAK3 activation induces lymphoproliferative syndromes in murine bone marrow transplantation models

The tyrosine kinase JAK3 plays a well-established role during normal lymphocyte development and is constitutively phosphorylated in several lymphoid malignancies. However, its contribution to lymphomagenesis remains elusive. In this study, we used the newly identified activating JAK3A572V mutation to elucidate the effect of constitutive JAK3 signaling on murine lymphopoiesis. In a bone marrow transplantation model, JAK3A572V induces an aggressive, fatal, and transplantable lymphoproliferative disorder characterized by the expansion of CD8(+)TCRalphabeta(+)CD44(+)CD122(+)Ly-6C(+) T cells that closely resemble an effector/memory T-cell subtype. Compared with wild-type counterparts, these cells show increased proliferative capacities in response to polyclonal stimulation, enhanced survival rates with elevated expression of Bcl-2, and increased production of interferon-gamma (IFNgamma) and tumor necrosis factor-alpha (TNFalpha), correlating with enhanced cytotoxic abilities against allogeneic target cells. Of interest, the JAK3A572V disease is epidermotropic and produces intraepidermal microabscesses. Taken together, these clinical features are reminiscent of those observed in an uncommon but aggressive subset of CD8(+) human cutaneous T-cell lymphomas (CTCLs). However, we also observed a CD4(+) CTCL-like phenotype when cells are transplanted in an MHC-I-deficient background. These data demonstrate that constitutive JAK3 activation disrupts T-cell homeostasis and induces lymphoproliferative diseases in mice.

Guidelines for Safe Surgery in Patients with Systemic Mastocytosis

Systemic mastocytosis (SM) is a rare disorder with important perioperative implications. The physiological stress of operative procedures and a variety of anesthetic and analgesic medications can be triggers of acute exacerbation of this condition. We present two patients with systemic mastocytosis. One underwent open left inguinal hernia and umbilical hernia repair and the other laparoscopic ventral hernia repair. The literature for perioperative management of patients with SM is extensively reviewed. Both patients were treated preoperatively with intravenous antihistamines and steroids to minimize SM reactions. The first patient underwent uneventful open left inguinal hernia repair and umbilical hernia repair under spinal anesthesia. The second patient underwent general anesthesia. A composite mesh was used to repair a 9 x 12-cm Swiss cheese incisional hernia at the site of previous surgery. After the administration of neostigmine and glycopyrrolate, she developed a generalized rash without any hemodynamic instability. The patient was treated with intravenous Solu-Medrol and Benadryl and was extubated successfully and had an unremarkable postoperative course. Patients with SM require careful perioperative management for surgery under spinal and general anesthesia. These patients can undergo surgical procedures safely and effectively without compromising the standard of care.

Non-solid oncogenes in solid tumors: EML4-ALK fusion genes in lung cancer

It is generally accepted that recurrent chromosome translocations play a major role in the molecular pathogenesis of hematological malignancies but not of solid tumors. However, chromosome translocations involving the e26 transformation-specific sequence transcription factor loci have been demonstrated recently in many prostate cancer cases. Furthermore, through a functional screening with retroviral cDNA expression libraries, we have discovered the fusion-type protein tyrosine kinase echinoderm microtubule-associated protein like-4 (EML4)-anaplastic lymphoma kinase (ALK) in non-small cell lung cancer (NSCLC) specimens. A recurrent chromosome translocation, inv(2)(p21p23), in NSCLC generates fused mRNA encoding the amino-terminal half of EML4 ligated to the intracellular region of the receptor-type protein tyrosine kinase ALK. EML4-ALK oligomerizes constitutively in cells through the coiled coil domain within the EML4 region, and becomes activated to exert a marked oncogenicity both in vitro and in vivo. Break and fusion points within the EML4 locus may diverge in NSCLC cells to generate various isoforms of EML4-ALK, which may constitute approximately 5% of NSCLC cases, at least in the Asian ethnic group. In the present review I summarize how detection of EML4-ALK cDNA may become a sensitive diagnostic means for NSCLC cases that are positive for the fusion gene, and discuss whether suppression of ALK enzymatic activity could be an effective treatment strategy against this intractable disorder.

Gastrointestinal stromal tumors: past, present, and future

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors of the gastrointestinal tract. The name "GIST" was proposed in 1983, but the cell origin of GIST remained unclear until 1998, when my colleagues and I reported immunohistochemical evidence that GIST originated from interstitial cells of Cajal or their precursors. At the same time, we reported gain-of-function mutations of the Kit gene in GISTs. The Kit gene encodes KIT receptor tyrosine kinase, whose structure is similar to that of platelet-derived growth factor receptor (PDGFR). Imatinib mesylate was initially developed as an inhibitor of PDGFR. Then, it was found to be a potent inhibitor of BCR-ABL. Imatinib was successfully used for the treatment of chronic myeloid leukemia. When we reported gain-of-function mutations of the Kit gene in GISTs, the inhibitory effect of imatinib on KIT was already known. Imatinib was then successfully applied to the treatment of GISTs. The interrelationship between the type of Kit gain-of-function mutation and the therapeutic effect of imatinib has been well characterized in GISTs. Although various mutations of Kit and Pdgfr-alpha genes have been found in GISTs, most GISTs are luckily imatinibsensitive. After long-term administration of imatinib, however, new imatinib-resistant clones develop a secondary mutation of the Kit or Pdgfr-alpha gene. New drugs and adjuvant regimens against such secondary progression are now being intensively explored.

The c-Kit/D816V mutation eliminates the differences in signal transduction and biological responses between two isoforms of c-Kit

Activating mutations of codon 816 of the Kit gene have been implicated in malignant cell growth of acute myeloid leukemia (AML), systemic mastocytosis and germ cell tumors. Substitution of aspartic acid with valine (D816V) renders the receptor independent of ligand for activation and signaling. Wild-type c-Kit is a tyrosine kinase receptor that requires its ligand, stem cell factor (SCF), for activation. Several isoforms of c-Kit exist as a result of alternative mRNA splicing, of which two are characterized by the presence or absence of four amino acids (GNNK- and GNNK+, respectively) in the extracellular domain. The two isoforms show differences in signal transduction and biological activities and the shorter isoform seems to be highly expressed than the longer isoform in human malignancies. In this study we analysed the signal transduction downstream of the oncogenic c-Kit mutant D816V in an isoform specific context, using the hematopoietic cell line Ba/F3 stably transfected with the different versions of isoform and mutant receptor. Our data show that in contrast to the differences shown in the activation of wild-type c-Kit isoforms, both isoforms of c-Kit/D816V are constitutively phosphorylated to the same extent. By the use of Western blot analysis we investigated the activation of different signaling proteins and found that both D816V/GNNK- and D816V/GNNK+ constitutively phosphorylated Gab2, Shc, SHP-2 and Cbl to almost the same extent as c-Kit/GNNK-. In addition, both isoforms of c-Kit/D816V induced SCF-independent cell survival and proliferation equally well. This is in contrast to wild-type c-Kit, where c-Kit/GNNK- induced better cell survival and stronger proliferation than c-Kit/GNNK+, and both required stimulation with SCF. Taken together, these findings reveal that the differences in downstream signal transduction and biological responses between the two GNNK isoforms are eliminated by the D816V mutant.

The identification and characterisation of novel KIT transcripts in aggressive mast cell malignancies and normal CD34+ cells

KIT mutations have been identified in several malignancies, including acute myeloid leukemia (AML) and systemic mastocytosis (SM). Mast cell leukemia (MCL) is the most aggressive mast cell neoplasm, but has not been well studied due to its rarity. We identified novel KIT transcripts in two patients with MCL and two patients with SM with an associated hematological disorder, but not from two patients with SM. Similar novel KIT transcripts were also observed in normal CD34+ cells from bone marrow and umbilical cord blood, suggesting that altered KIT isoforms may be specific to the blast stage of hematopoietic precursors. The novel KIT proteins lack several domains including the ATP binding site, and one was inactive in a functional test for autophosphorylation. Our discovery of novel KIT transcripts underscores the importance of analysing entire protein encoding regions when studying genes of interest.

Treatment responses to cladribine and dasatinib in rapidly progressing aggressive mastocytosis

BACKGROUND

Systemic mastocytosis (SM) is a mast cell neoplasm in which neoplastic cells usually display the D816V-mutated variant of KIT. Cladribine (2CdA) and dasatinib are two drugs that counteract the in vitro growth of neoplastic mast cells in SM. However, only little is known about the in vivo effects of these drugs in SM.

PATIENT AND METHODS

We report on a patient with highly aggressive interferon-alpha-resistant SM who was treated with 2CdA and dasatinib. In vitro pretesting revealed a response of neoplastic mast cells to both compounds with reasonable IC(50) values.

RESULTS

The patient was treated with six cycles of 2CdA (0.13 mg kg(-1) intravenously daily on 5 consecutive days). Despite a short-lived major clinical response and a decrease in serum tryptase, the patient progressed to mast cell leukaemia after the sixth cycle of 2CdA. The patient then received two further courses of 2CdA followed by treatment with dasatinib (100 mg per os daily). However, no major response was obtained and the patient died from disease progression after 2 months.

CONCLUSIONS

In a patient with rapidly progressing aggressive SM, neither 2CdA nor dasatinib produced a long-lasting response in vivo, despite encouraging in vitro results. For such patients, alternative treatment strategies have to be developed.

Molecular drug targets in myeloproliferative neoplasms: mutant ABL1, JAK2, MPL, KIT, PDGFRA, PDGFRB and FGFR1

Therapeutically validated oncoproteins in myeloproliferative neoplasms (MPN) include BCR-ABL1 and rearranged PDGFR proteins. The latter are products of intra- (e.g. FIP1L1-PDGFRA) or inter-chromosomal (e.g.ETV6-PDGFRB) gene fusions. BCR-ABL1 is associated with chronic myelogenous leukaemia (CML) and mutant PDGFR with an MPN phenotype characterized by eosinophilia and in addition, in case of FIP1L1-PDGFRA, bone marrow mastocytosis. These genotype-phenotype associations have been effectively exploited in the development of highly accurate diagnostic assays and molecular targeted therapy. It is hoped that the same will happen in other MPN with specific genetic alterations: polycythemia vera (JAK2V617F and other JAK2 mutations), essential thrombocythemia (JAK2V617F and MPL515 mutations), primary myelofibrosis (JAK2V617F and MPL515 mutations), systemic mastocytosis (KITD816V and other KIT mutations) and stem cell leukaemia/lymphoma (ZNF198-FGFR1 and other FGFR1 fusion genes). The current review discusses the above-listed mutant molecules in the context of their value as drug targets.

Generation and characterization of novel canine malignant mast cell line CL1

Studies using the currently available malignant canine mast cell lines and bone marrow-derived cultured mast cells (BMCMCs) have provided an in-depth understanding of normal and neoplastic canine mast cell biology. However, many of the currently available malignant canine mast cell lines possess limitations, including loss of cell surface markers and inability to bind canine IgE. We have recently generated a novel mast cell line, CL1, from an 11-year-old spayed female Labrador retriever diagnosed with systemic mastocytosis and neoplastic effusion. The CL1 cells express KIT, FcepsilonRI, CD44, CD45, CD14, CD11a, CD11b and CD18 as well as chymase. Interestingly, these cells express wild-type KIT, with no evidence of autophosphorylation, but are able to proliferate independently without the addition of exogenous stem cell factor (SCF), KIT ligand. However, stimulation of CL1 cells with SCF induces KIT phosphorylation promoting cell proliferation. The CL1 cells retain functional properties of mast cells, degranulating in a dose-dependent manner in response to both IgE cross-linking and chemical stimulation. Lastly, cytogenetic evaluation revealed several recurrent tumor-associated chromosome copy number imbalances in the CL1 line. In summary, the CL1 cell line possesses phenotypic and functional properties similar to those found in canine BMCMCs, and will likely be a useful tool to study mast cell biology, factors regulating transformation of mast cells, cytogenetic abnormalities in mast cell tumors, and novel preclinical therapies.

Phase II study of dasatinib in Philadelphia chromosome-negative acute and chronic myeloid diseases, including systemic mastocytosis

PURPOSE

Molecular characterization of Philadelphia chromosome-negative (Ph-) chronic myeloproliferative disorders, such as systemic mastocytosis (SM), has provided a clear rationale for investigating novel targeted therapies. The tyrosine kinase (TK) inhibitor dasatinib is 325-fold more potent against Bcr-Abl TK than imatinib in vitro, significantly inhibiting wild-type KIT and platelet-derived growth factor receptor beta TKs, and is active against cells carrying the mutant KIT-D816V gene.

EXPERIMENTAL DESIGN

In this phase 2, open-label study, the efficacy of dasatinib (140 mg/d) was investigated in 67 patients with various Ph- myeloid disorders, including SM (n = 33; 28 KIT-D816V positive).

RESULTS

The overall response rate to dasatinib in patients with SM was 33%. Only two patients, one with SM-myelofibrosis and one with SM-chronic eosinophilic leukemia, achieved complete response (elimination of mastocytosis) lasting for 5 and 16 months, respectively. Both patients were negative for KIT-D816V mutation, had low tryptase levels, abnormal WBC counts, and anemia, and had failed prior therapy with erythropoietin. Additional nine SM patients had symptomatic response, lasting 3 to 18+ months. Complete responses were achieved in two other patients (acute myeloid leukemia and hypereosinophilic syndrome). No responses were observed among patients with myelodysplastic syndromes and primary myelofibrosis. The majority of adverse events were grade 1/2.

CONCLUSION

These data show that dasatinib therapy may benefit a selected group of SM patients, primarily by improving their symptoms, but it does not eliminate the disease in the patients with KIT-D816V mutation.

Expression of proto-oncogene C-kit and correlation with morphological evaluations in canine cutaneous mast cell tumors

Canine cutaneous mast cell tumor (MCT) is very common disease in dogs, this is more aggressive than in other species. The biologic behavior of MCT is highly variable and a more accurate prognosis for these tumors needs to performed. The proto-oncogene c-kit is known to play a critical role in development and function of mast cells (MC). The aim of this study was to evaluate the expression of immunohistochemical pattern of c-kit in MCTs and to correlate these results with MC density (MCD) and intratumoral microvessel density (MVD). Our results confirm that a more aggressive biologic behavior of canine MCT is associated with the increased c-kit expression, further suggesting a new role for c-kit, as a useful marker, in diagnostic pathology and in tumor progression.

Association of paediatric mastocytosis with a polymorphism resulting in an amino acid substitution (M541L) in the transmembrane domain of c-KIT

BACKGROUND

The receptor tyrosine kinase c-KIT plays a key role in normal mast cell development. Point mutations in c-KIT have been associated with sporadic or familial mastocytosis.

OBJECTIVES

Two unrelated pairs of apparently identical twins affected by cutaneous mastocytosis attending the Mastocytosis Clinic at the Royal Children's Hospital, Melbourne, provided an opportunity to assess the possible contribution of c-KIT germline mutations or polymorphisms in this disease.

METHODS

Tissue biopsy, blood and/or buccal swab specimens were collected from 10 children with mastocytosis. To detect germline mutations/polymorphisms in c-KIT, we studied all coding exons by denaturing high pressure liquid chromatography. Exons showing mismatches were examined by direct sequencing. The influence of the substitution identified was further examined by expressing the variant form of c-KIT in factor-dependent FDC-P1 cells.

RESULTS

In both pairs of twins, a heterozygous ATG to CTG transition in codon 541 was observed, resulting in the substitution of a methionine residue in the transmembrane domain by leucine (M541L). In each case, one parent was also heterozygous for this allele. Expression of M541L KIT in FDC-P1 cells enabled them to grow in human KIT ligand (stem cell factor, SCF) but did not confer factor independence. Compared with cells expressing wild-type KIT at a similar level, M541L KIT-expressing cells displayed enhanced growth at low levels of SCF, and heightened sensitivity to the KIT inhibitor, imatinib mesylate.

CONCLUSIONS

The data suggest that the single nucleotide polymorphism resulting in the substitution M541L may predispose to paediatric mastocytosis.

Mast cells and mastocytosis

Mast cells have been recognized for well over 100 years. With time, human mast cells have been documented to originate from CD34+ cells, and have been implicated in host responses in both innate and acquired immunity. In clinical immunology, they are recognized for their central role in IgE-mediated degranulation and allergic inflammation by virtue of their expression of the high-affinity receptor for IgE and release of potent proinflammatory mediators. In hematology, the clinical disease of mastocytosis is characterized by a pathologic increase of mast cells in tissues, often associated with mutations in KIT, the receptor for stem cell factor. More recently, and with increased understanding of how human mast cells are activated through receptors including the high-affinity receptor for IgE and KIT, specific tyrosine kinase inhibitors have been identified with the potential to interrupt signaling pathways and thus limit the proliferation of mast cells as well as their activation through immunoglobulin receptors.

Expression of stem cell factor (SCF), a KIT ligand, in gastrointestinal stromal tumors (GISTs): a potential marker for tumor proliferation

Gastrointestinal stromal tumors (GISTs) show a high incidence of gain-of-function mutations of the c-kit gene, which encodes a receptor tyrosine kinase KIT. This mutation is seen independently of metastasis and/or recurrence of tumors; thus, the factors involved in tumor proliferation rate and malignancy are still not known. Some mesenchymal and epithelial tumors have been reported to co-express KIT and its ligand, stem cell factor (SCF), for autonomous proliferation by the autocrine mechanism. The purpose of this study is to clarify whether GIST cells produce SCF, despite mutated KIT with constitutive activation. Immunohistochemically, we examined the co-expression of KIT and SCF in 36 GIST cases. All cases were found to be KIT-positive, and of these, 21 cases, including four recurrent or metastatic GISTs, showed co-expression of SCF. MIB-1 labeling index was significantly higher, and the average tumor size was larger in SCF-positive cases. By confocal microscopy, KIT was expressed on the cellular membrane, around which SCF was distributed less densely. Western blot analysis revealed that the membrane-bound SCF of 31 kDa was found to be approximately 10 times more abundant than the soluble SCF of 18.5 kDa, suggesting continuous KIT activation. These results indicate that proliferation of GIST cells can be caused not only by the gain-of-function mutation of c-kit, but also by the autocrine mechanism of the SCF/KIT system. Thus, SCF expression would be a useful marker for tumor proliferation.

Unique effects of KIT D816V in BaF3 cells: induction of cluster formation, histamine synthesis, and early mast cell differentiation antigens

Oncogenic tyrosine kinases (TK) usually convert growth factor-dependent cells to factor independence with autonomous proliferation. However, TK-driven neoplasms often are indolent and characterized by cell differentiation rather than proliferation. A prototype of an indolent TK-driven neoplasm is indolent systemic mastocytosis. We found that the D816V-mutated variant of KIT, a TK detectable in most patients with systemic mastocytosis, induces cluster formation and expression of several mast cell differentiation and adhesion Ags, including microphthalmia transcription factor, IL-4 receptor, histamine, CD63, and ICAM-1 in IL-3-dependent BaF3 cells. By contrast, wild-type KIT did not induce cluster formation or mast cell differentiation Ags. Additionally, KIT D816V, but not wild-type KIT, induced STAT5 activation in BaF3 cells. However, despite these intriguing effects, KIT D816V did not convert BaF3 cells to factor-independent proliferation. Correspondingly, BaF3 cells with conditional expression of KIT D816V did not form tumors in nude mice. Together, the biologic effects of KIT D816V in BaF3 cells match strikingly with the clinical course of indolent systemic mastocytosis and with our recently established transgenic mouse model, in which KIT D816V induces indolent mast cell accumulations but usually does not induce a malignant mast cell disease. Based on all these results, it is hypothesized that KIT D816V as a single hit may be sufficient to cause indolent systemic mastocytosis, whereas additional defects may be required to induce aggressive mast cell disorders.

KIT and mastocytosis

KIT is a receptor tyrosine kinase that is functionally relevant for hematopoiesis, mast cell development and function, gametogenesis and melanogenesis. Normal KIT signaling requires binding to stem cell factor, and PI3K-Akt is one of the putative effector pathways. In humans, germline loss-of-function KIT mutations have been associated with piebaldism - an autosomal dominant condition characterized by depigmented patches of skin and hair. Gain-of-function KIT mutations are usually acquired and have been associated with myeloid malignancies including core binding factor acute myeloid leukemia and systemic mastocytosis (SM), germ cell tumors, gastrointestinal stromal tumors and sinonasal T cell lymphomas. KITD816V is the most prevalent KIT mutation in mast cell disease and occurs in more than 90% of the cases that fulfill the World Health Organization diagnostic criteria for SM. However, its precise pathogenetic contribution is not well understood. In clinical practice, SM is considered either indolent or aggressive depending on the respective absence or presence of symptomatic target organ dysfunction aside from skin disease. In general, conventional therapy for SM is suboptimal, and efforts are under way to develop and employ small molecule drugs that target mutant KIT.

Pharmacological targeting of the KIT growth factor receptor: a therapeutic consideration for mast cell disorders

KIT is a member of the tyrosine kinase family of growth factor receptors which is expressed on a variety of haematopoietic cells including mast cells. Stem cell factor (SCF)-dependent activation of KIT is critical for mast cell homeostasis and function. However, when KIT is inappropriately activated, accumulation of mast cells in tissues results in mastocytosis. Such dysregulated KIT activation is a manifestation of specific activating point mutations within KIT, with the human D816V mutation considered as a hallmark of human systemic mastocytosis. A number of other activating mutations in KIT have recently been identified and these mutations may also contribute to aberrant mast cell growth. In addition to its role in mast cell growth, differentiation and survival, localized concentration gradients of SCF may control the targeting of mast cells to specific tissues and, once resident within these tissues, mast cell activation by antigen may also be amplified by SCF. Thus, KIT inhibitors may have potential application in multiple conditions linked to mast cells including systemic mastocytosis, anaphylaxis, and asthma. In this review, we discuss the role of KIT in the context of mast cells in these disease states and how recent advances in the development of inhibitors of KIT activity and function may offer novel therapies for the treatment of these disorders.

KIT mutations induce intracellular retention and activation of an immature form of the KIT protein in gastrointestinal stromal tumors

PURPOSE

Gastrointestinal stromal tumors (GIST) are frequently associated with gain-of-function mutations of KIT, which can be inhibited by imatinib both in vitro and in vivo. The survival of patients with GIST, following imatinib therapy, has been correlated with the nature of mutations but not with KIT expression.

EXPERIMENTAL DESIGN

Subcellular localization, activation, and trafficking of the mature and the immature forms of KIT were investigated in GIST samples and in NIH3T3 cells infected with two different GIST-type exon 11-mutated human KIT cDNA.

RESULTS

Paranuclear dot expression of KIT was more frequent in GISTs with homozygous KIT mutations than in those with heterozygous (P = 0.01) or no mutations (P < 0.01). Activation of the immature 125 kDa form of KIT was detected in most GISTs with KIT mutations but not in GISTs without KIT mutations. In NIH3T3 cells, mutant KIT was mainly retained within endoplasmic reticulum and Golgi compartments in an immature constitutively phosphorylated form, whereas the wild-type KIT was expressed at the plasma membrane, in a mature nonphosphorylated form. Imatinib-induced inhibition of the phosphorylation of immature and mature mutant KIT proteins resulted in the restoration of KIT expression at the cell surface.

CONCLUSIONS

These results show that GIST-type KIT mutations induce an activation-dependent alteration of normal maturation and trafficking, resulting in the intracellular retention of the activated kinase within the cell. These observations likely account for the absence of correlation between response to imatinib and KIT expression using immunohistochemistry and may deserve to be investigated in other tyrosine kinase-activated tumors.

Allelic heterogeneity at the equine KIT locus in dominant white (W) horses

White coat color has been a highly valued trait in horses for at least 2,000 years. Dominant white (W) is one of several known depigmentation phenotypes in horses. It shows considerable phenotypic variation, ranging from approximately 50% depigmented areas up to a completely white coat. In the horse, the four depigmentation phenotypes roan, sabino, tobiano, and dominant white were independently mapped to a chromosomal region on ECA 3 harboring the KIT gene. KIT plays an important role in melanoblast survival during embryonic development. We determined the sequence and genomic organization of the approximately 82 kb equine KIT gene. A mutation analysis of all 21 KIT exons in white Franches-Montagnes Horses revealed a nonsense mutation in exon 15 (c.2151C>G, p.Y717X). We analyzed the KIT exons in horses characterized as dominant white from other populations and found three additional candidate causative mutations. Three almost completely white Arabians carried a different nonsense mutation in exon 4 (c.706A>T, p.K236X). Six Camarillo White Horses had a missense mutation in exon 12 (c.1805C>T, p.A602V), and five white Thoroughbreds had yet another missense mutation in exon 13 (c.1960G>A, p.G654R). Our results indicate that the dominant white color in Franches-Montagnes Horses is caused by a nonsense mutation in the KIT gene and that multiple independent mutations within this gene appear to be responsible for dominant white in several other modern horse populations.