KIT extracellular and kinase domain mutations in gastrointestinal stromal tumors

Targeting c-Kit mutations: basic science to novel therapies

The Kit receptor tyrosine kinase is a transmembrane receptor that is expressed in a variety of different tissues and mediates pleiotropic biological effects through its ligand stem cell factor (SCF). Sporadic mutations of Kit as well as autocrine/paracrine activation mechanisms of the SCF/Kit pathway have been implicated in a variety of malignancies, where its primary contribution to metastases is in enhancing tumor growth and reducing apoptosis. For example, Kit is frequently mutated and activated in gastrointestinal stromal tumors (GISTs) and there is ligand-mediated activation of Kit in some lung cancers. Kit is a convenient target in Kit-induced tumors and inhibition of this receptor with the small molecule drug Gleevec (imatinib mesylate, STI571) in GIST has shown dramatic efficacy. Unfortunately, past experience has demonstrated that chemotherapy of cancers with a single drug often leads to resistance of the cancer. Further understanding of the molecular mechanisms underlying Kit-mediated transformation is therefore important and may lead to the identification of further novel drug targets. These Kit-specific signaling pathways may then be targeted to overcome potential drug resistance. This review will focus on our understanding of the molecular mechanisms involved in transformation by Kit. The potential mechanisms by which Kit induces cellular transformation are described. We will also discuss the role and expression of Kit in various malignancies. Ultimately, the understanding of c-Kit biology, biochemistry, and mutational analysis will lead to better therapeutics.

Biology of gastrointestinal stromal tumors: KIT mutations and beyond

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors in the digestive tract. Aspects of the morphology and immunophenotype in GISTs resemble those in the interstitial cells of Cajal (ICC), which are a specialized cell type responsible for coordinating peristaltic activity throughout the gastrointestinal tract. Therefore, it is possible that GISTs result from transformation of nonneoplastic progenitor cells that would normally differentiate towards an ICC endpoint. Activation of the KIT receptor tyrosine kinase is required for differentiation and proliferation of nonneoplastic ICC, and oncogenic KIT mutations are a crucial event in the development of most GISTs. These mutations can involve either the extracellular or intracellular domains of the KIT receptor, giving rise to conformational changes that enable constitutive, ligand-independent, activation of the KIT protein. Oncogenic KIT activation leads to phosphorylation of various substrate proteins and, in turn, to activation of signal transduction cascades regulating cell proliferation, apoptosis, chemotaxis, and adhesion. Recently, a small molecule tyrosine kinase inhibitor (STI571, imatinib mesylate, Gleevec) directed against the enzymatic (kinase) domain of the KIT protein was found to produce dramatic clinical responses as monotherapy for metastatic GISTs. This review focuses on the biological and molecular genetic principles of GISTs, and particularly the role of mutant KIT as a therapeutic target.

Expression of Kit and platelet-derived growth factor receptors alpha and beta in cholangiocarcinoma, and case report of therapy with imatinib mesylate (STI571)

We have evaluated the expression of Kit, a receptor encoded by the c-kit protooncogene, and platelet-derived growth factor-receptors (PDGF-R) alpha and beta in cholangiocarcinoma specimens from 13 separate patients, and provide a case report of a therapeutic trial of imatinib mesylate in one patient. Archived pathologic samples from 13 patients with cholangiocarcinoma were obtained. Tissue sections were hybridized with anti-Kit, anti-PDGF-Ralpha and anti-PDGF-Rbeta monoclonal antibodies. Kit, PDGF-Ralpha and PDGF-Rbeta expression was seen in 31, 69 and 46% of samples, respectively. All patients with PDGF-Rbeta expression also expressed PDGF-Ralpha. Three out of four patients with Kit expression did not express either PDGF receptor and only one patient exhibiting expression of PDGF expressed Kit. Cohen's kappa statistic demonstrated that PDGF and Kit expression were inversely correlated with borderline significance (p=0.052; kappa=-0.4742, 95% confidence interval -0.9821 to 0.03364). In one case, strong Kit expression was noted in a tumor from a metastatic lymph node, but was absent in the primary tumor, suggesting that Kit may be related to invasive or metastatic potential. Given the high level of expression defined in this study, a prospective clinical trial incorporating imatinib mesylate, alone or in combination with cytotoxic chemotherapy, and especially in chemotherapy-naive patients, should be considered for patients with cholangiocarcinoma.

Pleomorphic Characteristics of a Germ-Line KIT Mutation in a Large Kindred with Gastrointestinal Stromal Tumors, Hyperpigmentation, and Dysphagia

PURPOSE

Somatic mutations that result in the activation of the growth factor receptor KIT are commonly found in gastrointestinal stromal tumors (GISTs). Six families have been reported in which a germ-line mutation in KIT is associated with an autosomal dominant predisposition to the development of GISTs. Hyperpigmentation, urticaria pigmentosa, and dysphagia have been described in some, but not all, families. Preliminary correlations between the site of mutation and the clinical phenotype have been proposed, but the strength of these associations is not defined.

DESIGN

A large kindred with multiple GISTs, hyperpigmentation, and dysphagia was identified after the index case presented with multiple GISTs. A germ-line mutation in KIT (W557R) was identified in an affected cousin, after which a large family meeting was held and testing offered. Clinical data were obtained by interview and, whenever possible, medical record documentation.

RESULTS

To date, 19 individuals have been tested, and the mutation has been shown to cosegregate with the syndrome. The phenotypic expression, however, is variable. GISTs, often presenting as upper gastrointestinal bleeding, and hyperpigmentation are common, but not diagnosed in all documented or obligate carriers. Dysphagia is a less prevalent complaint. The diagnosis of GISTs appears to be made at a younger age in more recent generations. Metastatic disease is uncommon.

CONCLUSIONS

A germ-line mutation in KIT resulting in an amino acid substitution in the juxtamembrane region is associated with a syndrome of GIST, hyperpigmentation, and dysphagia, although the prominence of each component varies.

KIT 1530ins6 mutation defines a subset of predominantly malignant gastrointestinal stromal tumors of intestinal origin

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors of the gastrointestinal tract. GISTs express KIT and show gain-of-function KIT mutations. Most of these mutations affect the KIT juxtamembrane domain, but other KIT domains are mutated at a lower frequency. In this study, frequency of GCC TAT insertion mutation (1530ins6) in KIT exon 9 (extracellular domain) and its possible clinicopathologic significance was investigated. Screening of 520 GISTs identified 26 cases with 1530ins6 KIT mutation and confirmed the previously reported low frequency of this type of KIT mutation among GISTs of different locations. Of the 26 tumors with 1530ins6 KIT mutation studied, 21 originated from the small intestine, 1 from the colon, and 3 from the rectum. In 1 case, primary small intestinal versus colonic localization could not be clearly established because of intra-abdominal dissemination. No distinctive morphological features were identified for the cohort of tumors defined by 1530ins6 KIT mutations. Most of the tumors showed predominant spindle cell morphology, and a few cases had epithelioid or pleomorphic histological features. Following previously published criteria based on tumor size and mitotic rate, 22 of 26 (85%) tumors were classified as malignant or potentially malignant, and 4 (15%) were classified as probably benign. A malignant clinical course was documented in 18 of 19 tumors from the malignant category. The survival times of 11 patients who died of disseminated GISTs ranged from 1 month to 105 months (median survival time, 26 months). In contrast, 2 of 4 GISTs assigned as probably benign tumors with follow-up information had long disease-free survival. GISTs carrying 1530ins6 occur exclusively in the intestinal location, and a great majority of these tumors follow a malignant course.

Malignant gastrointestinal stromal tumor originating in the lesser omentum, complicated by rapidly progressive glomerulonephritis and gastric carcinoma

A 69-year-old man was admitted with a large elastic mass in the upper abdomen. Computed tomography revealed a massive tumor in contact with the liver and gastrointestinal endoscopy revealed a gastric adenocarcinoma. He developed acute renal failure with massive proteinuria and died with a marked enlargement of the tumor. Autopsy revealed a tumor located in the lesser omentum. The tumor was considered to be a Gastrointestinal Stromal Tumor (GIST) because it was positive for c-kit. In addition, crescent formations and immune complexes in glomeruli were observed. We report the first case of GIST complicated by rapidly progressive glomerulonephritis and gastric carcinoma.

Serum KIT and KIT ligand levels in patients with gastrointestinal stromal tumors treated with imatinib

Imatinib mesylate is a selective inhibitor of a few tyrosine kinases including KIT, and it is the first effective treatment for gastrointestinal stromal tumors (GISTs). We monitored the serum levels of KIT, KIT ligand (stem cell factor, SCF), and the vascular endothelial growth factor (VEGF) in patients with advanced GISTs treated with imatinib in a prospective randomized trial. Patients with GISTs (n = 66) had elevated pretreatment serum KIT and VEGF levels as compared with controls (median, 292 AU/mL [409 ng/mL] vs 238 AU/mL [333 ng/mL], P =.037; and median, 303 pg/mL vs 190 pg/mL, P =.013, respectively), but lower levels of SCF (median, 645 pg/mL vs 950 pg/mL; P < or =.0001). After 1 and 6 months of imatinib treatment the average serum KIT levels decreased 31% and 52% from pretreatment levels, whereas SCF levels increased 11% and 33%, respectively. Serum VEGF levels decreased during treatment in responding patients. The median serum SCF/KIT ratio increased with treatment duration, and was 7.7-fold higher after 12 months of treatment than at baseline (range, 3.1-259-fold). A high serum SCF/KIT ratio may increase SCF-induced cell signaling with prolonged imatinib treatment, at the time when imatinib treatment is withdrawn, and in patients whose GIST has wild-type receptors.

Mechanisms and implications of imatinib resistance mutations in BCR-ABL

PURPOSE OF REVIEW

Aside from bone marrow transplantation, a definitive cure for Philadelphia (Ph) chromosome-positive chronic myeloid leukemia (CML) has yet to be developed. Although Imatinib, the first molecularly targeted drug developed for CML has achieved a remarkable success, the emergence of resistance to this agent mitigates the prospect of a cure for this leukemia. Though a variety of resistance mechanisms can arise, in the majority of patients resistance coincides with reactivation of the tyrosine kinase activity of the BCR-ABL fusion oncoprotein. This can result from gene amplification and, more importantly, point mutations that disrupt the bind of imatinib to BCR-ABL itself. In this review, we aim to define and illuminate mechanisms of resistance and describe how drug resistance is shedding new light on kinase domain regulation.

RECENT FINDINGS

In light of recent studies and publications, it is now clear that Imatinib exerts its inhibitory action by stabilizing the inactive non ATP-binding conformation of BCR-ABL and that mutations even outside the kinase domain can lead to enhanced autophosphorylation of the kinase, thereby stabilizing the active conformation that resists imatinib binding. So far, 25 different substitutions of 21 amino acid residues of BCR-ABL have been detected in CML patients. In addition, it has been recently illustrated that mutations preexist the onset of treatment and that some confer a more aggressive disease phenotype. Finally it has been shown that molecular remission is almost never reached through Imatinib therapy.

SUMMARY

The most common mechanism of relapse for CML patients treated with Imatinib is the appearance of point mutations in the BCR-ABL oncogene that confer resistance to this drug. Insights into the emerging problem of resistance should promote the rational development of alternative, synergistic, and potentially curative treatment strategies.

Different inhibitory effect of imatinib on phosphorylation of mitogen-activated protein kinase and Akt and on proliferation in cells expressing different types of mutant platelet-derived growth factor receptor-?

Most gastrointestinal stromal tumors (GISTs) have gain-of-function mutations of the c-kit gene. Previously, we found 2 types of gain-of-function mutation of the PDGFRA gene, Val561 to Asp and Asp842 to Val, in about half of GISTs without c-kit gene mutations. Although the inhibitory effect of imatinib on various types of activating mutant KIT has been well examined, that on the activating mutant PDGFRA has not been fully investigated. In the present study, we examined the effect of imatinib on autophosphorylation of mutant PDGFRA, phosphorylation of MAPK and of Akt and in vitro cell proliferation using murine Ba/F3 cells stably transfected with one of the 2 murine-type mutated PDGFRA cDNAs. Imatinib almost completely inhibited autophosphorylation of mutant PDGFRA, phosphorylation of MAPK and Akt as well as in vitro cell proliferation at the concentration of 0.01 microM in cells expressing mutant PDGFRA with Val561 to Asp. However, in cells expressing mutant PDGFRA with Asp842 to Val, imatinib almost completely inhibited autophosphorylation of mutant PDGFRA and phosphorylation of MAPK and Akt at 1.0 microM. The concentration contributing to complete inhibition of in vitro cell proliferation was 10 microM. Ba/F3 cells expressing mutant PDGFRA are a good model to investigate the mechanism of cell proliferation or growth inhibition by imatinib in mutant PDGFRA-driven cells.

Kinase mutations and imatinib response in patients with metastatic gastrointestinal stromal tumor

PURPOSE

Most gastrointestinal stromal tumors (GISTs) express constitutively activated mutant isoforms of KIT or kinase platelet-derived growth factor receptor alpha (PDGFRA) that are potential therapeutic targets for imatinib mesylate. The relationship between mutations in these kinases and clinical response to imatinib was examined in a group of patients with advanced GIST.

PATIENTS AND METHODS

GISTs from 127 patients enrolled onto a phase II clinical study of imatinib were examined for mutations of KIT or PDGFRA. Mutation types were correlated with clinical outcome.

RESULTS

Activating mutations of KIT or PDGFRA were found in 112 (88.2%) and six (4.7%) GISTs, respectively. Most KIT mutations involved exon 9 (n = 23) or exon 11 (n = 85). All KIT mutant isoforms, but only a subset of PDGFRA mutant isoforms, were sensitive to imatinib, in vitro. In patients with GISTs harboring exon 11 KIT mutations, the partial response rate (PR) was 83.5%, whereas patients with tumors containing an exon 9 KIT mutation or no detectable mutation of KIT or PDGFRA had PR rates of 47.8% (P =.0006) and 0.0% (P <.0001), respectively. Patients whose tumors contained exon 11 KIT mutations had a longer event-free and overall survival than those whose tumors expressed either exon 9 KIT mutations or had no detectable kinase mutation.

CONCLUSION

Activating mutations of KIT or PDGFRA are found in the vast majority of GISTs, and the mutational status of these oncoproteins is predictive of clinical response to imatinib. PDGFRA mutations can explain response and sensitivity to imatinib in some GISTs lacking KIT mutations.

Advanced gastrointestinal stromal tumors successfully treated with imatinib mesylate: a report of two cases

Activation of kit-receptor tyrosine kinase occurs in all cases of gastrointestinal stromal tumors, regardless of the mutation status of kit. Imatinib mesylate (STI 571,Gleevec) is a selective inhibitor of certain protein tyrosine kinases. It has been shown in preclinical models and clinical studies to have activity against such tumors. The aim of the present study was to report the efficacy of imatinib mesylate in the treatment of advanced gastrointestinal stromal tumors. Two adults with histologically confirmed, unresectable, and metastatic gastrointestinal stromal tumors that expressed CD117 (a marker of kit-receptor tyrosine kinase) were identified at our institution during 2000-2002. As the diseases were advanced and not amenable to surgery, chemotherapy, or radiation therapy, imatinib mesylate was used, because this targeted inhibitor has been shown to be active against advanced gastrointestinal stromal tumors and has a mild toxicity profile. Imatinib mesylate induced a sustained response in both patients with advanced unresectable or metastatic gastrointestinal stromal tumors. Inhibition of the KIT signal-transduction pathway is a promising treatment for advanced gastrointestinal stromal tumors, which resist conventional chemotherapy.

CD117, but Not Lysozyme, Is Positive in Cutaneous Plasmacytoma

Context.—CD117 (c-Kit) and lysozyme are frequently expressed by myeloblasts and are sensitive markers for the diagnosis of extramedullary myeloid tumor. The diagnosis of cutaneous plasmacytoma presents a degree of difficulty, particularly with the plasmablastic variant, which can mimic hematologic as well as epithelioid malignancies. Approximately 25% of multiple myelomas express CD117 in the bone marrow by flow cytometry. Lysozyme immunoreactivity has been previously shown in 30% of poorly differentiated myelomas, while it is nonreactive in nonmalignant plasma cells.

Objective.—To ascertain whether CD117 and lysozyme can aid in the diagnosis of cutaneous plasmacytomas, particularly the plasmablastic type.

Design.—Pathology reports of 2357 patients with a diagnosis of multiple myeloma were reviewed to find 13 cutaneous plasmacytomas (8 Bartl grade II, 5 Bartl grade III). Formalin-fixed, paraffin-embedded tissue sections were stained with CD117 and lysozyme on the Dako Autostainer system.

Setting.—Patients with the diagnosis of multiple myeloma who developed cutaneous plasmacytoma(s).

Results.—The cutaneous plasmacytomas uniformly expressed CD117 in a cytoplasmic or membranous and cytoplasmic distribution with varying degrees of staining intensity unrelated to the Bartl grade of the lesion, while they were uniformly negative for lysozyme.

Conclusions.—CD117 is a sensitive marker for malignant plasma cells in paraffin-embedded tissue, while lysozyme does not help identify poorly differentiated malignant plasma cells. While CD117 alone does not distinguish extramedullary myeloid tumor from poorly differentiated myeloma, the combination of CD117 and lysozyme may allow their differentiation. The possibility of c-kit inhibitors being used in the treatment of other hematopoietic malignancies allows speculation regarding implications for the treatment of multiple myeloma.

Expression and mutation of c-kit gene in gastrointestinal stromal tumors

AIM: To investigate the expression and mutation of c-kit gene and its correlation with the clinical pathology and prognosis of gastrointestinal stromal tumors (GISTs).

METHODS: A total of 94 cases of GISTs, 10 leiomyomas and 2 schwannomas were studied for the expression of KIT by immunohistochemistry. The c-kit gene mutations in exon 11 of these specimens were detected by PCR-SSCP technique.

RESULTS: Of the 94 cases of GISTs, 91 (96.8%) expressed the KIT protein. Leiomyomas and schwannomas were negative for KIT. The c-kit gene mutations of exon 11 were found in 38 out of the 94 cases of GISTs (40.4%). The mutations involved point mutations (Val560-Asp, Ile563-Met), del 557-559 and 579ins12. No mutations were detectable in benign GISTs, leiomyomas or schwannomas. The patients with mutation-positive GISTs showed more frequent recurrences, invasion and metastasis in adjacent tissues than those with mutation-negative ones.

CONCLUSION: KIT is a useful marker for diagnosis of GISTs. Mutation of the c-kit gene may play a significant role in the pathogenesis of GISTs and may be associated with poor prognosis in patients with GISTs.

Endoscopic ultrasonography-guided fine needle aspiration biopsy in follow-up patients with gastrointestinal stromal tumours

OBJECTIVES

Since all gastrointestinal stromal tumours (GISTs) are practically considered to be potentially malignant, they must be clinically differentiated from other submucosal tumours (SMTs). In this study, we carried out endoscopic ultrasonography-guided fine needle aspiration biopsy (EUS-FNAB) against follow-up SMTs to examine whether the method is sufficient for pathological diagnosis and analysis of c-kit mutation. The relationship between tumour growth and c-kit mutation was also examined.

METHODS

The tumour tissues were obtained by EUS-FNAB from 10 Japanese follow-up cases with SMT. Paraffin sections of tissues were used for haematoxylin and eosin staining, and for immunohistochemistry. Genomic DNA was extracted from the sections, and c-kit gene fragments of exons 9, 11, 13 and 17 were amplified by polymerase chain reaction and directly sequenced.

RESULTS

Nine SMTs were diagnosed as GIST and one was diagnosed as schwannoma by immunohistochemistry. Mutation at exon 11 was detected in six of nine GISTs, but no mutations were detected at exons 9, 13 and 17 in all GISTs. The case of schwannoma did not have any mutations at exons 9, 11, 13 and 17. Statistical significance was not observed between the average growth rate of six GISTs with the mutation and that of three GISTs without the mutation (P = 0.694).

CONCLUSIONS

EUS-FNAB against SMTs was useful for the differential diagnosis of SMT and the analysis of c-kit mutation. The c-kit mutation might not be one of the predictable markers for rapid tumour growth.

Gain-of-function mutations of platelet-derived growth factor receptor alpha gene in gastrointestinal stromal tumors

BACKGROUND & AIMS

Most gastrointestinal stromal tumors (GISTs) have gain-of-function mutations of c-kit receptor tyrosine kinase (KIT) gene, but some GISTs do not. We investigated the cause of GISTs without KIT mutations. Because GISTs apparently expressed platelet-derived growth factor receptor (PDGFR) alpha, we examined whether GISTs without KIT mutations had a mutation of PDGFR alpha.

METHODS

Whole coding region of PDGFR alpha complementary DNA (cDNA) was sequenced in GISTs with or without KIT mutations. Mutant PDGFR alpha cDNA was transfected into 293T human embryonic kidney cells, and autophosphorylation of PDGFR alpha was examined. Proliferation of Ba/F3 murine lymphoid cells stably transfected with mutant PDGFR alpha cDNA was estimated by tritium thymidine incorporation. Wild-type KIT cDNA was cotransfected with mutant PDGFR alpha cDNA, and immunoprecipitation by anti-KIT antibody was performed. Inhibitory effect of Imatinib mesylate on activated PDGFR alpha was examined.

RESULTS

We found 2 types of constitutively activated mutations of PDGFR alpha, Val-561 to Asp or Asp-842 to Val, in 5 of 8 GISTs without KIT mutations but not in 10 GISTs with KIT mutations. Stable transfection of each mutation induced autonomous proliferation of Ba/F3 cells. Constitutively activated mutant PDGFR alpha bound and activated the cotransfected wild-type KIT. The constitutive activation of PDGFR alpha with Val-561 to Asp was inhibited effectively by Imatinib mesylate but that of PDGFR alpha with Asp-842 to Val was inhibited only weakly, even at the concentration of 10 micromol/L.

CONCLUSIONS

The gain-of-function mutations of PDGFR alpha appear to play an important role in development of GISTs without KIT mutations.

Imatinib in small cell lung cancer

PURPOSE

To evaluate the clinical efficacy of Imatinib Mesylate in untreated or sensitive relapsed small cell lung carcinoma patients. Secondary endpoints included assessment of the safety and tolerance of Imatinib, and an analysis of KIT expression in tumor samples.

PATIENTS AND METHODS

Patients with previously untreated small cell lung cancer (SCLC) or with a sensitive relapse (one prior regimen with a sustained response for over 60 days) were eligible. Imatinib was delivered at 600 mg on a daily basis. Response was evaluated at 6 weeks using SWOG criteria.

RESULTS

As planned in the study design, a total of 19 patients were included in the trial (9 chemonaive patients with extensive disease and 10 sensitive relapse SCLC patients). No objective responses were observed with most previously untreated patients staying on study for less than 30 days. The median time to progression was 1 and 1.2 month in the untreated and sensitive relapse groups, respectively. Only 4 out of 19 tumor samples (21%) stained for KIT by means of immunohistochemistry.

CONCLUSIONS

No evidence of antitumor activity was observed in this small phase II trial including 19 SCLC patients treated with Imatinib Mesylate. KIT positivity in tumor samples appeared to be far less common than anticipated (21 vs. 70%). Future trials should include a screening procedure to evaluate KIT expression in SCLC samples. A better evaluation of KIT contribution to SCLC tumor progression needs to be achieved.

c-Kit is not expressed in malignant mesothelioma

Overexpression of KIT protein (CD117), the product of the c-kit gene, has been shown to have important prognostic and therapeutic implications for a number of malignant neoplasms. Previous studies have shown conflicting results regarding the expression of c-kit in malignant mesothelioma. To determine whether malignant mesothelioma expresses KIT, immunohistochemistry and RT-PCR were used to analyze archived tissue from 37 cases of mesothelioma. Although a subset of mesotheliomas demonstrated specific staining with the DAKO anti-KIT antibody, in each case staining was nuclear. We could not detect c-kit mRNA by a sensitive RT-PCR assay, even in cases with strong nuclear staining. Furthermore, a second anti-KIT antibody (Cell-Marque) only demonstrated staining in a single mesothelioma case and in none of the cases that demonstrated nuclear staining. We conclude that immunoreactivity for KIT in mesothelioma does not represent expression of the c-kit gene and may represent antibody cross-reaction with nuclear proteins. Our results raise doubt about previously reported expression of KIT in mesothelioma and consequently, the applicability of therapeutic agents that target the kinase activity of KIT.

Sensitivity toward tyrosine kinase inhibitors varies between different activating mutations of the FLT3 receptor

Activating mutations of FLT3 have been detected in patients with acute myeloid leukemia (AML). Two distinct types of FLT3 mutations are most common: internal tandem duplication (ITD) of sequences coding for the juxtamembrane domain and point mutations at codon 835 (Asp835) within the kinase domain. Both types of mutations constitutively activate the tyrosine kinase activity of FLT3 in experimental systems and result in factor-independent proliferation of Ba/F3 and 32D cells. Recently, novel mutations within the activation loop were identified in patients with AML: deletion of isoleucine 836 (Ile836del) and an exchange of isoleucine 836 to methionine plus an arginine insertion (Ile836Met+Arg). To examine whether the Ile836 mutations result in constitutive activation of the FLT3 receptor, we introduced both mutant FLT3 cDNAs transiently into HEK 293 cells. Both mutant FLT3 receptors were constitutively autophosphorylated in the absence of ligand and kinase activity led to constitutive activation of downstream signaling cascades as determined by activation of the STAT5 (signal transducer and activator of transcription 5) pathway. When stably expressed in the growth factor-dependent cell lines Ba/F3 and 32D, both deletion and insertion mutants led to factor-independent proliferation, indicating that both mutants have transforming capabilities. We then examined the sensitivity of the FLT3 ITD, FLT3 Asp835Tyr, and the novel FLT3 receptor mutants toward the kinase inhibitors AG1296, PKC412, and SU5614. We show that these FLT3 kinase inhibitors have distinct inhibitory potencies against different activating FLT3 receptor mutants. These results suggest that it may be useful to determine the exact kind of FLT3 mutation when applying receptor kinase inhibitors in clinical trials.

Alterations of the c-kit gene in testicular germ cell tumors

Expression and gain-of-function mutation of the c-kit gene, that encodes a receptor tyrosine kinase (KIT), have been reported in mast cell tumors and gastrointestinal stromal tumors (GISTs). Among human testicular germ cell tumors (GCTs), seminomas and seminoma components of mixed GCTs have also been shown to express KIT, but only one study has found the c-kit gene mutation at exon 17 in seminoma. To elucidate the frequency and location of the c-kit gene mutation of testicular GCTs, we analyzed the whole coding region of the c-kit complementary DNA along with 4 mutational hot spots (exons 9, 11, 13 and 17) of the c-kit genomic DNA by polymerase chain reaction and direct sequencing. Somatic mutations were found in 4 pure seminomas of 34 testicular GCTs (11.8%). One mutation was found in exon 11 (W557R) and the others were observed in exon 17 (D816H and D816V). These types of mutations were reported in GISTs (W557R), seminoma (D816H) and mastocytosis (D816V) and were considered to be gain-of-function mutations, although there were no differences of any clinicopathological factors or outcome between patients with and without mutations. Additionally, we also demonstrated coexpression of Gly-Asn-Asn-Lys510-513 (GNNK) + and GNNK - isoforms of the c-kit gene with dominance of the GNNK - transcript in all testicular GCTs. The mutations and/or preferential expression of GNNK - isoform of the c-kit gene might play an important role in the development of testicular GCTs, and these tumors may also be targets for STI571, which is a promising drug for advanced and metastatic GISTs.

STI571 as a targeted therapy for CML

Chronic myelogenous leukemia (CML) is a clonal hematopoietic stem cell disorder that progresses through distinct phases as the malignant clone progressively loses the capacity for terminal differentiation. It is characterized by the (9;22) translocation and resultant production of the Bcr-Abl tyrosine kinase. Bcr-Abl functions as a constitutively activated tyrosine kinase, and this kinase activity is absolutely required for the transforming function of the Bcr-Abl protein. In preclinical studies, STI571 (Gleevec, imatinib mesylate), a Bcr-Abl tyrosine kinase inhibitor, specifically inhibited the proliferation of Bcr-Abl-expressing cells in vitro and in vivo. STI571 has shown remarkable results in all phases of CML. Although responses are seen in all phases of the disease, durable responses are most common in earlier stage patients. Thus, STI571 has emerged as a paradigm for gene product targeted therapy, offering expanded treatment options for patients with CML.

Gastrointestinal stromal tumors, intramural leiomyomas, and leiomyosarcomas in the duodenum: a clinicopathologic, immunohistochemical, and molecular genetic study of 167 cases

In this study we analyzed the clinicopathologic features of duodenal smooth muscle or stromal tumors, including 156 GISTs, 6 leiomyomas (LMs), and 5 leiomyosarcomas (LMSs) from the files of the Armed Forces Institute of Pathology and the Haartman Institute of the University of Helsinki. GISTs were documented as KIT positive (n = 109); 47 tumors were also included because of their histologic identity to KIT-positive cases. GIST-specific c-kit gene mutations were documented in exon 11 in 9 of 30 cases (30%) and exon 9 in 4 of 30 cases (13%). The GISTs occurred in patients with an age range of 10-88 years (median 56 years); 54% were male. Ten patients had neurofibromatosis type I; six of them had multiple GISTs. The GISTs ranged from small asymptomatic intramural or external nodules to large masses that extended into the retroperitoneum (median size 4.5 cm). They were mostly spindle cell tumors; three malignant GISTs had an epithelioid morphology, and 81 cases had skeinoid fibers. The tumors often coexpressed CD34 and KIT (54%) and were variably positive for smooth muscle actin (39%) and S-100 protein (20%) but never for desmin. A total of 86% of patients with tumors >5 cm with >5 mitoses/50 high power fields (HPF) (n = 21) died of disease, whereas no tumor <2 cm with <5 mitoses/50 HPF (n = 12) recurred or caused death. Long latency was common between primary operation and recurrences or metastases; either one occurred in 49 of 140 patients with follow-up (35%). No formula could accurately predict metastases, which occasionally developed even if mitotic activity was <5/50 HPF and size <5 cm. Metastases were in the abdominal cavity, liver, and rarely in bones and lungs but never in lymph nodes. Four actin- and desmin-positive and KIT-negative benign intramural LMs were similar to those more often seen in the esophagus. There were five LMSs, one of which formed a polypoid intraluminal mass; all were actin positive and KIT negative. The great majority of duodenal mesenchymal tumors are GISTs, which have a spectrum from small indolent tumors to overt sarcomas. LMs and LMSs are rare.

Mast cell tumors in the dog

The most common skin tumor in dogs is the mast cell tumor (MCT), with an incidence of close to 20% in the canine population. MCTs range from relatively benign to extremely aggressive, leading to metastasis and eventual death from systemic disease. Although surgical removal with or without radiation therapy may cure most patients with low-grade MCTs, there are no effective treatments for dogs with aggressive high-grade MCTs. This article reviews the current understanding of MCT biology with regard to diagnosis, staging, identification of prognostic indicators, and appropriate treatment planning.

The reappraisal of gastrointestinal stromal tumors: from Stout to the KIT revolution

For five decades gastrointestinal stromal tumors (GISTs) truly have represented one of the most confusing as well as neglected areas of both surgical pathology and clinical oncology. The recognition of the central role played by KIT expression in the development of the interstitial cell of Cajal and of the activating KIT mutations in the pathogenesis of GIST have been the keys for a more precise categorization of this long elusive clinicopathological entity. A Consensus Conference held at the National Institutes of Health in 2001 provided both an evidence-based definition and a practical scheme for the assessment of the risk of aggressive clinical behavior. This scheme is based on evaluation of the size and mitotic rate of the tumors, and its use is strongly advocated. On the basis of current data GISTs can be defined as a distinctive group of KIT-expressing mesenchymal neoplasms of the gastrointestinal tract, showing differentiation towards the interstitial cell of Cajal, also known as the gastrointestinal pacemaker cells. Metastatic GISTs have been a virtually incurable disease until the elucidation of the role of KIT mutations. STI-571 (imatinib mesylate) is a molecule that inhibits the function of various receptors with tyrosine kinase activity, such as abl, the bcr-abl chimeric product, platelet-derived growth factor receptor, and KIT. Following its successful use in the treatment of chronic myeloid leukemia, STI-571 has also proved extremely effective in targeting metastatic GIST. Data regarding the duration of the response to this therapy are not yet available, and therefore any overenthusiasm should be avoided. Nonetheless, the GIST story remains paradigmatic of a totally innovative approach to cancer therapy which until now is the most elegant translation of cancer biology experimental knowledge into clinical practice.

High prognostic value of p16INK4 alterations in gastrointestinal stromal tumors

PURPOSE

Gastrointestinal stromal tumors (GISTs) represent a distinctive (but histologically heterogeneous) group of neoplasms, the malignant potential of which is often uncertain. To determine the prognostic relevance of p16INK4 alterations in GISTs, we investigated a larger group of GISTs and correlated the genetic findings with clinicopathological factors and patient survival.

MATERIAL AND METHODS

We evaluated the methylation status of the promotor by methylation-specific polymerase chain reaction (PCR), the presence of mutations by PCR-SSCP-sequencing, the loss of heterozygosity at the p16INK4 locus (using the c5.1 marker), and the immunohistochemical expression of p16INK4 protein in 43 GISTs in 39 patients.

RESULTS

p16INK4 alterations were found in 25 of 43 GISTs (58.1%), with benign, borderline, or malignant GISTs showing no differences in the type and frequency of alteration. p16INK4 alterations were correlated with a loss of p16INK4 protein expression (P <.01). Patients who had tumors with p16INK4 alterations had a poorer prognosis than patients with tumors without such alterations (P =.02). There was a high predictive value for p16INK4 alterations only in the group of benign and borderline GISTs (P <.01) with regard to clinical outcome. Univariate Cox's proportional hazard regression analysis revealed a strong correlation between p16INK4 alterations, tumor size, mitotic index, and overall survival (P <.02), whereas multivariate Cox's analysis confirmed only p16INK4 alterations as an independent prognostic factor.

CONCLUSION

We believe that the evaluation of p16INK4 alteration status is a helpful prognosticator, particularly in the benign and borderline groups of GISTs.

Immunohistochemical expression of receptor-tyrosine kinase c-kit protein in invasive ductal carcinoma of the pancreas

The expression of receptor tyrosine kinase c-kit and its biologic significance in pancreatic cancer are unclear. We studied the expression of c-kit protein (c-KIT) in resectable invasive ductal carcinomas (IDCs) of the pancreas, in order to assess whether a selective c-kit inhibitor, STI571 (Glivec), may be applied for the treatment of pancreatic IDCs. This study included 72 pancreatic IDC patients who received a pancreatectomy between 1982 and 2002. The expression of c-KIT was analyzed retrospectively by immunohistochemistry. c-KIT was expressed in 78% (56/72) of the pancreatic IDCs. c-KIT expression did not correlate with any clinicopathological factor of pancreatic IDC and c-KIT expression had no significant influence on the survival of the patients. The survival rate of the adjuvant chemotherapy (ACT) (+) group was significantly higher than that of the ACT (-) group, but c-KIT expression had no significant effects on the efficacy of the ACT. Multivariate analysis indicated that the pTNM stage, grade and ACT were all significant variables for survival in IDCs overall. As c-KIT was expressed in 78% of the pancreatic IDCs, it suggests that STI571 may be a beneficial agent for chemotherapy against human pancreatic IDCs.

C-kit immunoreactivity in endometrial adenocarcinomas and its clinicopathologic significance

The proto-oncogene c-kit is a transmembrane-tyrosine-kinase receptor that is structurally related to the platelet-derived growth-factor receptor (PDGFR) and is involved in cell differentiation. C-kit has been found to be expressed in certain solid tumors and may play a role in their tumorigenesis. Recently, a tyrosine-kinase inhibitor specific for the PDGFR family, bcr-abl, and c-kit (STI571) has been reported to have therapeutic effects in tumors expressing the aberrant forms or high quantities of target proteins. Expression of c-kit has not been well evaluated in endometrial adenocarcinomas. In this study, c-kit immunoreactivity was evaluated on paraffin sections of 72 endometrial adenocarcinomas (57 endometrioid, 10 serous, and 5 clear cell) with a polyclonal antibody. Immunoreactivity of c-kit was analyzed semiquantitatively and correlated with various clinicopathologic factors. Cytoplasmic c-kit immunoreactivity was detected in 42 (58%) tumors. Thirty-four (60%) endometrioid, 8 (80%) serous, and 0 of the 5 clear-cell adenocarcinomas were c-kit positive. There was a significant correlation between c-kit positivity and the depth of myometrial invasion. Patients with c-kit-positive endometrial adenocarcinomas more frequently had metastases and shorter disease-free survival. Expression of c-kit may be a potentially adverse prognostic feature in endometrial adenocarcinoma. Patients with c-kit-positive advanced endometrial adenocarcinoma might benefit from tyrosine-kinase-inhibitor therapy.

Gastrointestinal stromal tumors (GIST): a model for molecule-based diagnosis and treatment of solid tumors

Gastrointestinal stromal tumor (GIST) is the most common mesenchymal tumor of the human gastrointestinal (GI) tract. The c-kit receptor tyrosine kinase (KIT) is expressed by practically all GISTs, and gain-of-function mutations of KIT are present in most GISTs. Interstitial cells of Cajal (ICC) are the pacemaker of the peristaltic movement of the GI tract. Since signals through KIT are essential for development of ICC and since multiple GISTs develop from the hyperplastic lesion of ICCs in familial GIST patients with germline mutations of KIT, GISTs are considered to originate from ICC. Imatinib mesylate, which was developed for treatment of chronic myeloid leukemia (CML), was found to be useful for treatment of GISTs. Imatinib mesylate inhibits BCR-ABL fused tyrosine kinase that causes CML. Imatinib mesylate also inhibits the mutated KIT observed in most GISTs, and this explains the effectiveness of Imatinib mesylate on GISTs. GISTs appear to serve as a model for molecule-based diagnosis and treatment of solid tumors.

Imatinib: a targeted clinical drug development

Imatinib (Gleevec) (formerly STI571) is an orally bioavailable rationally developed inhibitor of the tyrosine kinases Bcr-Abl, Kit, and platelet-derived growth factor receptor (PDGFR). In 4 years of clinical development, more than 12,000 patients have been treated in the clinical development program. Imatinib was first shown to be highly effective in the treatment of all stages of chronic myelogenous leukemia (CML). Moreover, preliminary results of a randomized study have demonstrated superior efficacy and safety of first-line imatinib therapy compared with a combination of interferon and cytarabine. Imatinib has also been shown to be the only effective drug therapy in the treatment of patients with metastatic gastrointestinal stromal tumors expressing the stem cell factor (SCF) receptor Kit. This review outlines the successive steps in the clinical development of this new, targeted anticancer agent.

U.S. Food and Drug Administration drug approval summaries: imatinib mesylate, mesna tablets, and zoledronic acid

The purpose of this report is to summarize information on drugs recently approved by the U.S. Food and Drug Administration. Three drugs have recently been approved: Gleevec (imatinib mesylate) at a starting dose of 400 or 600 mg daily for the treatment of malignant unresectable and/or metastatic gastrointestinal stromal tumors; Mesnex (mesna) tablets as a prophylactic agent to reduce the incidence of ifosfamide-induced hemorrhagic cystitis, and Zometa (zoledronic acid) for the treatment of patients with multiple myeloma and for patients with documented bone metastases from solid tumors, in conjunction with standard antineoplastic therapy. Prostate cancer should have progressed after treatment with at least one hormonal therapy. The recommended dose and schedule is 4 mg infused over 15 minutes every 3-4 weeks. These three drugs represent three different types of drug approval: Gleevec is an accelerated approval and supplemental new drug application (NDA); Mesnex tablets represent an oral formulation of a drug approved 14 years ago as an intravenous formulation, and Zometa represents a standard NDA for a noncytotoxic, supportive-care drug. Information provided includes rationale for drug development, study design, efficacy and safety results, and pertinent literature references.

Gastrointestinal stromal tumors: radiologic features with pathologic correlation

Gastrointestinal stromal tumors (GISTs) are unique neoplasms that occur throughout the gastrointestinal tract, mesentery, omentum, and retroperitoneum. They are the most common mesenchymal neoplasm of the gastrointestinal tract and are defined by their expression of KIT (CD117), a tyrosine kinase growth factor receptor. The expression of KIT is important to distinguish GISTs from other mesenchymal neoplasms such as leiomyomas, leiomyosarcomas, schwannomas, and neurofibromas and to determine the appropriateness of KIT-inhibitor therapy. The series described herein was accumulated over 2 years and includes 64 pathologically proved GISTs (28 gastric, 27 small intestinal, six anorectal, one colonic, one esophageal, and one from the small bowel mesentery). Radiologic features of GISTs vary depending on tumor size and organ of origin. Since most GISTs arise within the muscularis propria of the stomach or intestinal wall, they most commonly have an exophytic growth pattern and manifest as dominant masses outside the organ of origin. Dominant intramural and intraluminal masses are less common radiologic manifestations. GISTs occurring in the gastrointestinal tract and mesentery characteristically have hemorrhage, necrosis, or cyst formation that appears as focal areas of low attenuation on computed tomographic images. Although the radiologic features of GISTs are often distinct from those of epithelial tumors, criteria to separate GISTs radiologically from other nonepithelial tumors have not yet been fully developed.

Immunophenotype of desmoplastic small round cell tumors as detected in cases with EWS-WT1 gene fusion product

Desmoplastic small round cell tumor is a rare tumor typically involving peritoneum. Although the histogenesis of desmoplastic small round cell tumor has yet to be elucidated, immunophenotypical and morphological analysis shows a characteristic divergent phenotype overlapping with other round cell tumors such as Ewing's sarcoma/primitive neuroectodermal tumor, rhabdomyosarcoma, small cell mesothelioma, and carcinoma. Detection of the EWS-WT1 gene fusion is characteristic of desmoplastic small round cell tumor and has been used reliably in tumor diagnosis. In this study, we evaluated the immunophenotype of 23 desmoplastic small round cell tumor cases with the EWS-WT1 gene fusion product identified by reverse transcription-polymerase chain reaction. Paraffin sections were stained with antibodies against calretinin, WT1 (C19), desmin, myoglobin, MyoD, Myf5, myogenin, placental alkaline phosphatase, cytokeratins, MIC2, HER2/neu and c-kit using standard immunohistochemical methods. Immunoreactivity was evaluated semiquantitively by light microscopy. Desmoplastic small round cell tumors showed reactivity with calretinin in 4/21, desmin in 21/23, myoglobin in 5/17, placental alkaline phosphatase in 17/21, HER2/neu in 7/18 (3+ in 1 and 1+ in 6), c-kit in 2/14, MIC2 in 13/23, WT1 in 16/23, CAM5.2 in 21/23, and AE1/3 in 16/23 cases. The most sensitive myogenic and epithelial markers are desmin and CAM 5.2. Although nuclear reactivity of the early myogenic regulatory factors (MyoD, myogenin, Myf5) was not detected, myoglobin immunoreactivity was present in 29% of desmoplastic small round cell tumors. HER2/neu overexpression (3+) and c-kit expression are uncommon in desmoplastic small round cell tumors. A panel of myogenic and epithelial markers should be used to detect the divergent phenotype in desmoplastic small round cell tumors, a key feature in the differential diagnosis. Detection of EWS-WT1 fusion becomes critical for the diagnosis when the characteristic divergent phenotype cannot be detected immunohistochemically.

A phase II and pharmacokinetic study of ecteinascidin 743 in patients with gastrointestinal stromal tumors

PURPOSE

To assess the efficacy, tolerability, and pharmacokinetics of ecteinascidin 743 (ET-743) in patients with advanced gastrointestinal stromal tumors (GISTs).

PATIENTS AND METHODS

The study was confined to adult patients with radiographically measurable GISTs. ET-743 was administered as a 24-hour continuous i.v. infusion at a dose of 1.5 mg/m(2) repeated every 3 weeks. Pharmacokinetic blood sampling was performed during the first cycle of therapy. Tumors were restaged after every second cycle of therapy.

RESULTS

A total of 20 patients was enrolled in the study, 19 of whom were treated with 47 cycles of ET-743 (median 2, range 1-10). Severe toxicities were limited to reversible grade 3 transaminitis in 10 patients and grade 3 fatigue in one patient. There were no objective responses, and disease stabilization occurred in two patients lasting for periods of 4 and 10 months. The 1-year survival rate was 71.1%. Mean +/- standard deviation values of the maximum plasma concentration and total plasma clearance were 1.1 +/- 0.4 ng/ml and 44 +/- 16 l/h/m(2), respectively, for 19 of the 20 patients.

CONCLUSION

This study is the first report of a prospective phase II trial to evaluate a cytotoxic agent in patients with GISTs. This study underscores the primary resistance of GISTS to chemotherapy and stands in stark contrast to the encouraging results recently achieved with STI571. The lack of response may be associated with a therapeutically ineffective exposure to the drug based upon the lower incidence of severe toxicities and greater clearance than described in phase I and II trials of ET-743.

C-kit expression in uterine leiomyosarcomas: an immunocytochemical study of 29 cases of malignant smooth muscle tumors of the uterus

Uterine malignant stromal tumors are rare neoplasms characterized by fatal prognosis. At the moment no effective systemic treatment is available for metastases or recurrent disease. The drugs employed in advanced neoplasms are iposfamide, doxorubicin or epidoxorubicin, but the clinical response to chemotherapy is poor. Recent studies have shown that cells in gastrointestinal stromal tumors express a growth factor receptor with tyrosine kinase activity termed c-kit. Lately reports of efficacy of a specific anticancer drug with imatinib (ST1571) based on specific molecular abnormalities of proto-oncogene c-kit present in gastrointestinal stromal tumors induced us to identify the c-kit phenotype also in uterine leiomyosarcomas. These data may be useful for treating metastatic uterine leiomyosarcomas with increased c-kit kinase activity.

c-kit gene mutation at exon 17 or 13 is very rare in sporadic gastrointestinal stromal tumors

BACKGROUND AND AIM

Gastrointestinal stromal tumors (GIST) are the most common mesenchymal tumors of the human gut. They frequently have gain-of-function mutations of the c-kit gene, which encodes a receptor, tyrosine kinase. The mutations were found at exon 11 in most cases, and either at exon 9 or at exon 13 in rare cases. Recently, we found a family with multiple GIST and a gain-of-function mutation at exon 17. The family was the first reported GIST case with c-kit gene mutation at exon 17 including sporadic GIST. Although we previously reported that the c-kit gene mutation at exon 17 was not detected in 124 sporadic GIST by single-strand conformation polymorphism (SSCP) analysis, the mutation at exon 17 observed in the familial GIST was detectable by the use of direct sequencing but not by our SSCP method. In the present study, we examined the mutations at exon 17 and exon 13 by using direct sequencing.

METHODS

Genomic DNA was extracted from formalin-fixed, paraffin-embedded GIST tissues. We could obtain 143 sporadic GIST cases appropriate for DNA analysis at exon 17 and 141 at exon 13. Exons 17 and 13 were amplified by using polymerase chain reaction and direct sequencing was conducted.

RESULTS

No mutation was found at exon 17, and only one case with the mutation at exon 13 was observed. The GIST with the mutation at exon 13 was large and showed frequent mitosis, and the patient died of the recurrent GIST 3 years after the first operation.

CONCLUSION

The mutation at exons 17 or 13 was considered to be very rare in sporadic GIST.

Gastrointestinal stromal tumors (GIST): C-kit mutations, CD117 expression, differential diagnosis and targeted cancer therapy with Imatinib

Gastrointestinal stromal tumors (GISTs) have been recognised as a biologically distinctive tumor type, different from smooth muscle and neural tumors of the gastrointestinal tract. They constitute the majority of gastrointestinal mesenchymal tumors. They are defined and diagnosed by the expression of a protooncogene protein called CD117 detected by immunohistochemistry. It is now believed that GISTs originate from gastrointestinal pacemaker cells known as interstitial cells of Cajal, that control gut motility or from a precursor of these cells. The identification of mutations mostly in exon 11 and to a lesser extent in exons 9 and 13 of the c-kit protooncogene coding for c-kit (CD117) in many GISTs, has resulted in a better understanding of their oncogenic mechanisms. The finding of remarkable antitumor effects of the molecular inhibitor, imatinib (Glivec trade mark ) in metastatic and inoperable GISTs, has necessitated accurate diagnosis of GISTs and their distinction from other gastrointestinal mesenchymal tumors. To achieve this, pathologists need to be familiar with the spectrum of histological appearances shown by GISTs and have a high index of suspicion for these tumors. This review summarises recent advances in knowledge regarding the histogenesis, pathology, molecular biology, genetics and differential diagnosis of GISTs and the basis for the novel targeted cancer therapy with imatinib.

Gastrointestinal stromal tumors and leiomyomas in the dog: a histopathologic, immunohistochemical, and molecular genetic study of 50 cases

Fifty canine gastrointestinal (GI) mesenchymal tumors were examined to determine the occurrence of leiomyomas (LM) and GI stromal tumors and to compare their clinicopathologic features. Twenty-one tumors (42%) were histologically reclassified as gastrointestinal stromal tumors (GISTs) and 29 tumors (58%) as LMs on the basis of their histologic similarity with homologous human tumors. The GISTs occurred equally in males and females, with a mean age of 11 years (range 5-14 years). Five GISTs (24%) were associated with clinical signs and six (29%) had metastasis in liver or abdominal cavity. The GISTs occurred in large intestine (10, 48%), small bowel (six, 29%), stomach (four, 19%), and mesentery of small intestine (one, 5%). Histologically, they were highly cellular spindle, or less commonly epithelioid tumors with mitotic rates ranging from 0 to 19 per 10 HPF. Eleven tumors (52%) were positive for CD117 (KIT); seven (33%) were positive for smooth muscle actin but none for desmin and S-100 protein. Sequences of KIT exon 11, often mutated in human GISTs, were evaluated from four GISTs. Deletion of Try556-Lys557 coexisting with duplication of Gln555 in one case of GIST and T to C transition resulting in substitution of Pro for Leu575 in another were identified. The LMs occurred predominantly in males (82%) with a mean age of 11 years (range 8-17 years). Nine tumors (31%) had associated clinical signs. They occurred in the stomach (22, 76%), esophagus (four, 14%), and intestines (three, 10%); all were paucicellular, had no mitoses, and were composed of mature smooth muscle cells. Twenty-eight (97%) were positive for smooth muscle actin and 18 (62%) for desmin but none for CD117 and S-100. Both GISTs and true LMs occur in the GI tract of dogs. Both tumors have distinctive pathologic features.

Prevalence and importance of internal tandem duplications in exons 11 and 12 of c-kit in mast cell tumors of dogs

OBJECTIVE

To determine the prevalence of activating internal tandem duplications (ITDs) in exons 11 and 12 of c-kit in mast cell tumors (MCTs) of dogs and to correlate these mutations with prognosis.

SAMPLE POPULATION

157 formalin-fixed, paraffin-embedded MCTs from dogs in the pathology database of the Veterinary Medical Teaching Hospital at the University of California, Davis.

PROCEDURE

Genomic DNA was isolated from tumor specimens and a polymerase chain reaction procedure was performed to determine whether there were ITDs in exons 11 and 12.

RESULTS

We identified ITDs in 1 of 12 (8%) grade-I, 42 of 119 (35%) grade-lI, and 9 of 26 (35%) grade-ll tumors (overall prevalence, 52 of 157 [33%]). Logistic regression analysis revealed that the odds of grade-II and -III tumors possessing an ITD were approximately 5 times greater than that for grade-I tumors, although these odds did not differ significantly. Although MCTs possessing an ITD were twice as likely to recur after excision and twice as likely to result in metastasis as those without an ITD, these values also did not differ significantly.

CONCLUSIONS AND CLINICAL RELEVANCE

These results provide evidence that ITDs in c-kit occur frequently in MCTs of dogs. The high prevalence of c-kit activating mutations in MCTs of dogs combined with the relative abundance of mast cell disease in dogs provide an ideal naturally developing tumor in which to test the safety and efficacy of novel small-molecule kinase inhibitors such as imatinib mesylate.

The GIST of targeted cancer therapy: a tumor (gastrointestinal stromal tumor), a mutated gene (c-kit), and a molecular inhibitor (STI571)

Although gastrointestinal stromal tumor (GIST) is the most frequent mesenchymal neoplasm of the gastrointestinal tract, until recently it has been an obscure disease. Now, there is widespread scientific and clinical interest in GIST because its principal pathogenetic defect has been identified and a specific molecular inhibitor of GIST has been developed. Most GISTs contain a gain-of-function mutation in the c-kit proto-oncogene. Mutation results in constitutive activation of the Kit receptor tyrosine kinase, which induces cellular proliferation. STI571 is an oral agent that selectively inhibits Kit. It is a landmark development in cancer treatment and marks a new era of targeted molecular therapy. Its efficacy proves that a specific inhibitor can counteract the effects of a genetic defect responsible for neoplasia. Although STI571 was first applied to GIST only 2 years ago, it has already revolutionized the treatment of patients with metastatic disease and is also currently being tested as an adjuvant therapy after the resection of primary GIST.

Gastrointestinal stromal tumors: rationale for surgical adjuvant trials with imatinib

Gastrointestinal stromal tumors (GISTs) constitute the majority of mesenchymal tumors involving the gastrointestinal tract. Over the past decade, it has been recognized that these tumors have distinctive immunohistochemical and genetic features. The expression of c-Kit (CD117), a transmembrane growth factor receptor, has emerged as an important defining feature of GISTs, and the pathogenesis of these tumors may be related to c-Kit mutations. Promising preclinical results have provided the driving force for the rapid clinical development of imatinib mesylate, a selective tyrosine kinase inhibitor of c-Kit. This novel molecularly targeted therapy has produced impressive clinical responses in a large proportion of patients with advanced GISTs and is under study as an adjuvant therapy in patients with localized resectable GISTs.

Management of malignant gastrointestinal stromal tumours

Gastrointestinal stromal tumours (GISTs) are the most common form of mesenchymal tumour of the gastrointestinal tract. Clinically, they range from small indolent tumours curable with surgery alone to aggressive cancers. Making a distinction between an indolent and a malignant GIST is unreliable with conventional histopathological techniques. The presence of metastases at the time of diagnosis confirms malignancy, but all GISTs should be regarded as having malignant potential. GISTs characteristically express the KIT protein, a transmembrane tyrosine kinase receptor for stem-cell factor. Most GISTs have a mutation in the KIT proto-oncogene that translates into a gain-of-function constitutive activation of the KIT kinase. KIT activation seems to be an early tumour-promoting event in pathogenesis. Commonly, malignant GISTs show high-level primary resistance to conventional chemotherapy. Imatinib mesylate is an orally administered selective inhibitor of certain tyrosine kinases including KIT. Most patients with advanced malignant GISTs achieve clinical benefit and significant antitumour responses with imatinib mesylate. Responses have been durable, and most patients tolerate the drug well at clinically effective doses. Imatinib mesylate is the first effective systemic therapy for advanced GIST.

Gastrointestinal stromal tumors: chemotherapy and imatinib

Gastrointestinal stromal tumors (GISTs), previously thought to arise from the smooth muscles of the gastrointestinal tract, have recently been identified as a separate clinicopathologic entity. This new entity was revealed when investigations using more refined techniques suggested that GISTs had neural rather than smooth muscle differentiation and expressed the cell surface receptor Kit (CD117). Ensuing research led to development of a new molecularly targeted therapy, imatinib mesylate, which showed significant response among GIST patients in initial clinical trials. This review describes treatment of GIST before and after imatinib and the significance of this accomplishment.