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

Molecular landscape and clinical significance of exon 11 mutations in KIT gene among patients with gastrointestinal stromal tumor: a retrospective exploratory study

Objective

This aim of this study was to investigate the prognostic significance of KIT exon 11 mutation subtypes in patients with GISTs.

Methods

A total of 233 consecutive patients diagnosed with GISTs at the First Affiliated Hospital of Zhengzhou University from January 2013 to August 2018 were included in this study. The prevalence and mutation landscape of exon 11 in KIT was presented. The clinicopathological characteristics and prognosis among the different mutation subtypes were analyzed. All the statistical analyses were performed by SPSS22.0.

Results

Somatic mutational analysis indicated that point mutations were the most frequently detected mutations followed by deletions & compound mutations and insertion and tandem duplication mutations in the stomach. Point mutations showed a low mitotic count and a high risk of recurrence, and deletions and compound mutations have a high mitotic count while insertions and tandem duplication mutations showed a low mitotic count with an intermediate recurrence risk. Point mutations and deletions frequently occurred in sequence region codons 550-560 of exon 11, while compound mutations, insertion, and tandem duplication were mainly detected in codons 557-559, 572-580, and 577-581, respectively. The multi-variation analysis demonstrated that tumor diameter and high recurrence risk groups had worse prognostic values. However, mutation types were not significant predictors of relapse-free survival (RFS) in GISTs. Survival analysis suggested no significant difference in RFS between the 557/558 deletion and the other deletions.

Conclusion

This study suggested that mutations in exon 11 of the KIT gene were common with intermediate/high recurrence risk in GISTs patients. Tumor diameter ≥5 cm, and deletions mutations might predict a worse prognosis.

Emerging Targeted Therapeutic Strategies to Overcome Imatinib Resistance of Gastrointestinal Stromal Tumors

Gastrointestinal stromal tumors (GISTs) are the most common malignant mesenchymal neoplasms of the gastrointestinal tract. The gold standard for the diagnosis of GISTs is morphologic analysis with an immunohistochemical evaluation plus genomic profiling to assess the mutational status of lesions. The majority of GISTs are driven by gain-of-function mutations in the proto-oncogene c-KIT encoding the tyrosine kinase receptor (TKR) known as KIT and in the platelet-derived growth factor-alpha receptor (PDGFRA) genes. Approved therapeutics are orally available as tyrosine kinase inhibitors (TKIs) targeting KIT and/or PDGFRA oncogenic activation. Among these, imatinib has changed the management of patients with unresectable or metastatic GISTs, improving their survival time and delaying disease progression. Nevertheless, the majority of patients with GISTs experience disease progression after 2-3 years of imatinib therapy due to the development of secondary KIT mutations. Today, based on the identification of new driving oncogenic mutations, targeted therapy and precision medicine are regarded as the new frontiers for GISTs. This article reviews the most important mutations in GISTs and highlights their importance in the current understanding and treatment options of GISTs, with an emphasis on the most recent clinical trials.

Gastrointestinal Stromal Tumors (GISTs): From Science to Targeted Therapy

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors of the gastrointestinal tract. GISTs represent a distinct category of tumors characterized by oncogenic mutations of the KIT receptor tyrosine kinase in a majority of patients. KIT is useful not only for the diagnosis but also for targeted therapy of this disease. Imatinib, a tyrosine kinase inhibitor, is widely used in advanced and metastatic GISTs. This agent revolutionized the treatment strategy of advanced disease and is being tested in the neoadjuvant and adjuvant settings with encouraging results. New therapeutic agents like sunitinib have now been approved, enriching the treatment scenario for imatinib-resistant GISTs. The present review reports on the peculiar characteristics of this disease through its biology and molecular patterns, focusing on the predictive value of KIT mutations and their correlation with clinical outcome as well as on the activity of and resistance to approved targeted drugs.

A Case of Sinonasal Melanoma With Unusual Primary Exon 17 KIT D820G Mutation

Sinonasal melanomas are rare neoplasms with poor prognosis that may harbor KIT and NRAS genes mutations. Molecular alterations represent possible targets of tailored therapeutic approaches. We describe the case of a 74-year-old patient with primary melanoma of the nasal cavity. Mutational analysis of KIT demonstrated a point missense mutation D820G in exon 17. This represents, to our knowledge, the first case of sinonasal melanoma harboring this specific KIT mutation. Although KIT mutations confer sensibility to thyrosine-kinase inhibitor, it has been proved that this is strongly dependent on the region in which this alteration occurs. Thus it seems very important to perform an accurate gene mutational analysis to provide a biological rationale to the tailored therapy.

Gastrointestinal stromal tumor - an evolving concept

Gastrointestinal stromal tumors (GISTs) are the most frequent mesenchymal tumors of the gastrointestinal tract. The discovery that these tumors, formerly thought of smooth muscle origin, are indeed better characterized by specific activating mutation in genes coding for the receptor tyrosine kinases (RTKs) CKIT and PDGFRA and that these mutations are strongly predictive for the response to targeted therapy with RTK inhibitors has made GISTs the typical example of the integration of basic molecular knowledge in the daily clinical activity. The information on the mutational status of these tumors is essential to predict (and subsequently to plan) the therapy. As resistant cases are frequently wild type, other possible oncogenic events, defining other "entities," have been discovered (e.g., succinil dehydrogenase mutation/dysregulation, insuline growth factor expression, and mutations in the RAS-RAF-MAPK pathway). The classification of disease must nowadays rely on the integration of the clinico-morphological characteristics with the molecular data.

Regorafenib in gastrointestinal stromal tumors: clinical evidence and place in therapy

Gastrointestinal stromal tumors (GISTs) are rare malignancies, and historically had a poor prognosis, with little benefit from traditional anticancer therapies. The management of GISTs has undergone a paradigm change in recent years with the detection of activating mutations in the majority of these tumors. This knowledge has led to the development of targeted treatments which have dramatically improved benefit rates and survival. The tyrosine kinase inhibitor, imatinib, has become the standard of care for both those with high-risk resected GIST, and as first-line therapy in metastatic GIST. However, some patients demonstrate innate resistance to imatinib or, for many, resistance develops despite an initial response. Other tyrosine kinase inhibitors with a broader spectrum of action, such as sunitinib and sorafenib, have been investigated and show some benefit after the use of imatinib. Regorafenib, an orally available multitargeted tyrosine kinase inhibitor with antiangiogenic activity, has also demonstrated preclinical evidence of activity against a number of solid tumors and further studies have proven it to be effective in GISTs following failure of standard therapy, with manageable toxicity profile. It has now received licensing approval in a number of countries both for the treatment of GISTs and for colorectal cancer, and further research is ongoing. With a number of potential agents now available to treat this disease, clinicians must now consider questions of timing and sequencing to maximize the benefit from these treatments, and the role that new agents such as regorafenib could play in further advancing changes.

Extracellular domain c-kit mutation with duplication of Ser501Ala502 found in gastrointestinal stromal tumors is more imatinib- and nilotinib-sensitive than that with duplication of Ala502Tyr503

The great majority of gastrointestinal stromal tumors (GISTs) have gain-of-function mutations of the c-kit gene, which encodes KIT receptor tyrosine kinase. Most of the mutations are located at exon 11, but some are at exon 9 or at other exons. Mutation types at exon 11 vary, while most mutations at exon 9 are a particular duplication of Ala502Tyr503 (KIT-Dup-Ala502Tyr503). Recently a duplication of Ser501Ala502 (KIT-Dup-Ser501Ala502) at exon 9 has been reported in two cases of pediatric mastocytosis and one case of adult mast cell leukemia. Although KIT-Dup-Ser501Ala502 had not been reported in GISTs, we found two GIST cases possessing the mutation in 45 GIST cases with exon 9 c-kit gene mutations, among a total of approximately 500 GIST cases examined. In this report, we briefly summarize clinicopathological findings of the two cases, and characterize the biology of the mutation. When autophosphorylation of KIT-Dup-Ser501Ala502 was examined by transient transfection of c-kit cDNA with Dup-Ser501Ala502 into CHO-K1 cells, KIT-Dup-Ser501Ala502 was ligand-independently activating. The inhibitory effect of selective tyrosine kinase inhibitors, imatinib and nilotinib, on KIT-Dup-Ser501Ala502 was examined and compared with that of KIT-Dup-Ala502Tyr503. Imatinib efficiently inhibited constitutive activation of KIT-Dup-Ser501Ala502 at a concentration of 0.1 μM, whereas it inhibited that of KIT-Dup-Ala502Tyr503 at a concentration of 10 μM. Constitutive activation of KIT-Dup-Ser502Ala503 was not inhibited by nilotinib even at a concentration of 10 μM but that of KIT-Dup-Ala501Tyr502 was almost completely inhibited at a concentration of 1 μM. The results suggest that imatinib and nilotinib could be more effective on GISTs with KIT-Dup-Ser501Ala502 than those with KIT-Dup-Ala502Tyr503. In fact, a patient with KIT-Dup-Ser501Ala502 showed long-term stable disease with administration of the usual dose of 400 mg imatinib. Although mutation sites of KIT-Dup-Ser501Ala502 and KIT-Dup-Ala502Tyr503 are closely located, imatinib- and nilotinib-sensitive KIT-Dup-Ser501Ala502 are distinguishable from KIT-Dup-Ala502Tyr503.

Contribution of DOG1 expression to the diagnosis of gastrointestinal stromal tumors

Gastrointestinal stromal tumors (GIST) are the most common mesenchymal tumors of the gastrointestinal tract, and the majority contain KIT or PDGFRA-activating mutations. However, up to 10% of GISTs are c-kit-negative. Antibodies with increased sensitivity and specificity for the detection of c-kit-negative GIST cases may be of value, especially because some of these cases may also benefit from tyrosine kinase inhibitor therapy. Hematoxylin and Eosin sections of 33 GISTs were re-examined in order to define histopathological criteria used in risk assessment of these tumors. Immunohistochemistry with a panel of antibodies [c-kit, DOG1 (discovered on GIST 1), CD34, smooth muscle actin (SMA), Desmin, S100 and Ki67] was performed on 5μm-thick paraffin sections of all tumors. Statistical analysis of immunohistochemical studies showed that DOG1 and CD117 were the most sensitive and specific antibodies in the diagnosis of GISTs. Other antibodies were unhelpful in confirming a diagnosis of GIST, but were particularly useful in the differential diagnosis. Reactivity for DOG1 may aid in the diagnosis of GISTs, which fail to express c-kit antigen, and lead to appropriate treatment with imatinib mesylate, an inhibitor of the KIT tyrosine kinase.

Pharmacogenetics and pharmacogenomics: role of mutational analysis in anti-cancer targeted therapy

The goal of cancer pharmacogenomics is to obtain benefit from personalized approaches of cancer treatment and prevention. Recent advances in genomic research have shed light on the crucial role of genetic variants, mainly involving genes encoding drug-metabolizing enzymes, drug transporters and targets, in driving different treatment responses among individuals, in terms of therapeutic efficacy and safety. Although a considerable amount of new targeted agents have been designed based on a finely understanding of molecular alterations in cancer, a wide gap between pharmacogenomic knowledge and clinical application still persists. This review focuses on the relevance of mutational analyses in predicting individual response to antitumor therapy, in order to improve the translational impact of genetic information on clinical practice.

Characterization of novel germline c-kit gene mutation, KIT-Tyr553Cys, observed in a family with multiple gastrointestinal stromal tumors

We found a novel type germline mutation at exon 11 of the c-kit gene, which results in a substitution of Tyr to Cys at codon 553 of the c-kit gene product (KIT-Tyr553Cys), in a 68-year-old female patient with multiple gastrointestinal stromal tumors (GISTs). In the present study, we carried out mutational analysis in her family members to determine the carriers and characterized the mutation by introducing the corresponding mutation (murine KIT-Tyr552Cys) into expression vector possessing murine c-kit cDNA. Mutational analysis of peripheral blood leukocytes of her family members revealed that a 44-year-old son had the same mutation, but at present he had neither apparent symptoms nor images of multiple GISTs. By transfection with the expression vector possessing the murine mutant c-kit cDNA, interleukin-3-dependent Ba/F3 murine lymphoid cells started growing autonomously without any growth factors, indicating that the mutation was considered to be of gain-of-function. Imatinib, a small molecule of tyrosine kinase inhibitor, effectively inhibited autophosphorylation of KIT-Tyr552Cys. Nilotinib, another small molecule of the KIT inhibitor, also effectively inhibited autophosphorylation of KIT-Tyr552Cys. In fact, proliferation of Ba/F3 cells expressing KIT-Tyr552Cys was effectively inhibited by both imatinib and nilotinib. These findings indicate that the novel type human KIT-Tyr553Cys mutation is the cause of the present familial and multiple GISTs, and that both imatinib and nilotinib might effectively inhibit the growth of GISTs developing in the patients of this family.

Evaluation of prognostic factors and their capacity to predict biological behavior in gastrointestinal stromal tumors

Gastrointestinal stromal tumors (GISTs) are c-KIT-signaling-driven mesenchymal tumors of the human digestive tract, many of which have c-KIT or PDGFRα activating mutations. The authors studied the immunohistochemical markers, c-KIT and PDGFRα mutations, in GISTs and their association with the clinicopathological and clinical follow-up in 145 GISTs. Tumors were located mainly in the stomach, the median tumor size being 7.5 cm. The mitotic index was ≤5 mitoses per 50 high-power fields in 61% of cases, 96% expressed CD117, and c-KIT or PDGFRα mutations were detected in 68% of cases. The median follow-up of the series was 52 months (range = 1 to 244.9 months). Tumor size, cell morphology, mitotic index, incomplete resection, Fletcher's risk classification, Ki-67 overexpression, and c-KIT mutations were associated with progression-free survival. Incomplete resection and mitotic activity also provide information about overall survival. In conclusion, complete clinicopathological, immunohistochemical, and genetic descriptions are necessary to characterize this disease and optimize its clinical management.

Gain-of-Function Mutations of Receptor Tyrosine Kinases in Gastrointestinal Stromal Tumors

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors in human gastrointestinal tract. We first found that most GISTs expressed KIT, a receptor tyrosine kinase encoded by protooncogene c-kit and that approximately 90% of the sporadic GISTs had somatic gain-of-function mutations of the c-kit gene. Since both GISTs and interstitial cells of Cajal (ICCs) were double-positive for KIT and CD34, GISTs were considered to originate from ICCs or their precursor cells. We also found that germline gain-of-function mutations of the c-kit gene resulted in familial and multiple GISTs with diffuse hyperplasia of ICCs as the preexisting lesion. Moreover, we found that about half of the sporadic GISTs without c-kit gene mutations had gain-of-function mutations of platelet-derived growth factor receptor alpha (PDGFRA) gene that encodes another receptor tyrosine kinase. Imatinib which is known to inhibit constitutively activated BCR-ABL tyrosine kinase in chronic myelogenous leukemia also inhibits constitutive activation of mutated KIT and PDGFRA, and is now being used for metastatic or unresectable GISTs as a molecular target drug. Mutational analyses of c-kit and PDGFRA genes are considered to be significant for prediction of effectiveness of imatinib and newly developed/developing other agents on GISTs. Some mouse models of familial and multiple GISTs have been genetically created, and may be useful for further investigation of GIST biology.

Frequency, phenotype, and genotype of minute gastrointestinal stromal tumors in the stomach: an autopsy study

Gastrointestinal stromal tumors are the most common mesenchymal tumors of the human digestive tract. Up to 85% of these tumors show somatic gain-of-function mutation of the receptor tyrosine kinase c-KIT gene. A recent study has shown a high frequency (22.5%) of minute gastrointestinal stromal tumors in stomachs examined during routine autopsies. The aims of our study were to confirm the previously reported incidence of gastric gastrointestinal stromal tumors in routine autopsies and to investigate their molecular alterations. Gastrointestinal stromal tumors were collected prospectively from 578 autopsies over an 18-month period. After recording the size and location of each lesion, representative tissue samples were processed for hematoxylin and eosin staining and immunohistochemically stained for CD117 and CD34. Microdissected DNA from all identified gastrointestinal stromal tumors was studied for c-KIT and platelet-derived growth factor receptor α mutations. We identified 17 gastrointestinal stromal tumors in 578 consecutive autopsies (2.9%) located in the gastric body (47%) and fundus (47%). One tumor location was not recorded. All tumors were immunohistochemically positive for CD117 and CD34. DNA analysis showed c-KIT mutations in 11 cases. One platelet-derived growth factor receptor α mutation was found. The incidence of gastric minute gastrointestinal stromal tumors (2.9%) is higher than the reported clinical incidence. All are benign tumors, and most, including minute tumors, contain c-KIT mutations. This finding highlights the fact that c-KIT mutations are an early event in the evolution of gastrointestinal stromal tumors but are not sufficient per se for clinically relevant disease.

Colorectal gastrointestinal stromal tumor

Gastrointestinal stromal tumor (GIST) is the most common mesenchymal neoplasm arising in the digestive tract, with an estimated prevalence of 15-20 per 1,000,000. GISTs are related to the interstitial cells of Cajal and are characterized by constitutive over-expression of the transmembrane tyrosine kinase receptor KIT. This is produced by a patognomonic mutation of the proto-oncogene c-kit that occurs in up to 90% of cases. Exon 11 is affected most frequently; exons 9 and 13 are less commonly involved. One-third of GISTs lacking KIT mutations exhibits alternative activating mutations in the PDGFRα gene. Colorectal GISTs represent about 5-10% of the cases, mainly located in the rectum that is the third common site. Benign GISTs are more common, but many tumors are of uncertain malignant potential; tumor size and rate of mitosis are still the most reliable criteria for assessing the risk of an aggressive behavior. Surgery is the first-line treatment for resectable non-metastatic colorectal GIST. Standard oncologic resection is inappropriate because skip metastases and lymphatic spread are rarely reported. Segmental colectomy with negative margins is recommended, and local excision is oncologically adequate in highly selected rectal tumors. Radical surgery alone is not always curative especially in high-risk GISTs, and half of patients develops local recurrences or distant metastases after R0 operation. Medical therapeutic strategies have rapidly evolved after the introduction of targeted molecular therapy. Efficacy and safety of imatinib mesylate was first demonstrated in patients with metastatic and unresectable disease. Adjuvant and neoadjuvant use of imatinib are promising therapeutic options to improve the outcome of surgery to downstage unresectable lesions and to allow less extensive resections.

KIT amplification and gene mutations in acral/mucosal melanoma in Korea

Mucosal and acral melanomas have demonstrated different genetic alterations and biological behavior compared with more common cutaneous melanomas. It was recently reported that gain-of-function KIT mutations and/or copy number increases are more common in mucosal and acral melanomas. Thus, we studied the frequency and pattern of KIT aberrations in mucosal and acral melanomas in Korea. We analyzed 97 patients who were pathologically confirmed with mucosal or acral melanoma between 1997 and 2010 at Samsung Medical Center. Of the 97 melanoma patients, 92 were screened for mutations in KIT exons 11, 13, 17, and 18, BRAF and NRAS genes. KIT copy number was assessed by quantitative, real-time PCR. Of the 97 patients, 55 (56.7%) were mucosal, 40 (41.2%) were acral melanoma, and two were of unknown primary origin. Among seven cases with KIT mutation, five (60.0%) occurred in exon 11, one (20.0%) in exon 17, and one (20.0%) in exon 13. Point mutations were the most common, resulting in substitutions in exon 11 (K558R, T574A, L576P, and V559A), exon 13 (N655K), and exon 17 (N822K). A novel Thr574Ala (c.1720A>G) KIT mutation, which has not been reported in melanoma or other tumor types, was identified in one genital melanoma case. Of the 97 mucosal or acral melanoma specimens, 49 were tested for KIT gene copy number changes using quantitative PCR. Increased KIT copy number was identified in 15 patients: seven (40%) of 20 acral melanomas and eight (31%) of 26 mucosal melanomas. Our study implicates that a significant proportion of acral and mucosal melanomas have KIT mutations in Asian population.

Targeted therapy for gastrointestinal stromal tumors: current status and future perspectives

Gastrointestinal stromal tumors (GISTs) present 80% of gastrointestinal tract mesenchymal tumors, with systemic chemotherapy and radiotherapy being unable to improve survival of patients with advanced disease. The identification of activating mutations in either KIT cell surface growth factor receptor or platelet-derived growth factor receptor alpha, which lead to ligand-independent signal transduction, paved the way for the development of novel agents that selectively inhibit key molecular events in disease pathogenesis. The development of imatinib mesylate in the treatment of metastatic GIST represents a therapeutic breakthrough in molecularly targeted strategies, which crucially improved patients' prognosis while its usefulness in adjuvant and neoadjuvant setting is under study. Sunitinib malate is available in the second-line setting, with ongoing studies evaluating its role in an earlier disease stage, while other targets are under intense investigation in order to enrich the therapeutical armamentarium for this disease. GIST phenotype seems to be an essential indicator of treatment response; thus, obtaining genotype information of each patient may be critical in order to tailor individualized treatment strategies and achieve maximal therapeutic results.

Pathological characteristics of gastrointestinal stromal tumours with PDGFRA mutations

AIMS

The aim of this study was to identify the characteristics of PDGFRA-mutated gastrointestinal stromal tumours (GISTs) in comparison with KIT-mutated GISTs.

METHODS

A total of 151 GISTs were examined for KIT and PDGFRA mutations, and clinical and histopathological parameters were evaluated and analysed statistically according to mutation status.

RESULTS

KIT and PDGFRA mutations were detected in 123 (81.5%) and 15 cases (9.9%), respectively. Clinically, all PDGFRA-mutated GISTs were located in the stomach with no recurrences and tumour-related deaths were noted. Pathological parameters associated with PDGFRA mutations were epithelioid and mixed type, low to moderate cellularity, moderate to severe intratumoural lymphocytic infiltration, prominent myxoid change, frequent rhabdoid cells and multinucleated giant cells, increased cellular pleomorphism, low mitotic count, and lower risk assessment (p < 0.05). Compared with KIT-mutated GISTs, PDGFRA-mutated GISTs had unique histopathological patterns, intermingling of spindle and epithelioid cells, and/or a jigsaw puzzle-like arrangement of epithelioid cells.

CONCLUSIONS

PDGFRA-mutated GISTs have distinctive histological patterns that are differentiated from KIT-mutated GISTs. The algorithmic approach by a combination of several distinguishing histological and immunohistochemical features between KIT- and PDGFRA-mutated GISTs might be helpful in predicting the mutated gene of each GIST on pathological examination.

Imatinib treatment for gastrointestinal stromal tumour (GIST)

Gastrointestinal stromal tumour (GIST) is the most common mesenchymal neoplasm of the gastrointestinal tract. GISTs are believed to originate from intersticial cells of Cajal (the pacemaker cells of the gastrointestinal tract) or related stem cells, and are characterized by KIT or platelet-derived growth factor receptor alpha (PDGFRA) activating mutations. The use of imatinib has revolutionized the management of GIST and altered its natural history, substantially improving survival time and delaying disease progression in many patients. The success of imatinib in controlling advanced GIST led to interest in the neoadjuvant and adjuvant use of the drug. The neoadjuvant (preoperative) use of imatinib is recommended to facilitate resection and avoid mutilating surgery by decreasing tumour size, and adjuvant therapy is indicated for patients at high risk of recurrence. The molecular characterization (genotyping) of GISTs has become an essential part of the routine management of the disease as KIT and PDGFRA mutation status predicts the likelihood of achieving response to imatinib. However, the vast majority of patients who initially responded to imatinib will develop tumour progression (secondary resistance). Secondary resistance is often related to secondary KIT or PDGFRA mutations that interfere with drug binding. Multiple novel tyrosine kinase inhibitors may be potentially useful for the treatment of imatinib-resistant GISTs as they interfere with KIT and PDGFRA receptors or with the downstream-signalling proteins.

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.

Does imatinib turn recurrent and/or metastasized gastrointestinal stromal tumors into a chronic disease? - single center experience

BACKGROUND

Gastrointestinal stromal tumors (GIST) are mesenchymal tumors of the gastrointestinal tract supposed to arise from the cells of Cajal because of gain-of-function mutations of the tyrosine receptor kinases c-kit or platelet-derived growth factor receptor A. Imatinib selectively inhibits the kinase activity of both receptors. Despite this breakthrough in the treatment of GIST, resistance against imatinib has been reported to be as high as 50% after the first 2 years of treatment.

AIM

Outcome of 13 consecutive patients with relapsed or metastasized GIST who were treated with imatinib was analyzed.

RESULTS

Mean duration of treatment was 53.5 months. Four patients developed progressive disease and died after a mean treatment time of 31 months in spite of increase of imatinib dosages to 800 mg daily. Two patients (23%) developed a progressive disease after 46 months or 52 months of treatment. Two patients had a stable disease and five had a partial response. The overall progression rate was 46%, the mean survival time since primary diagnosis was 85.8 months.

CONCLUSION

From our experience, frequency of resistance development to imatinib may be below that given in the literature (50% after 2 years). Individual treatment in specialized centers may improve compliance.

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.

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.

Familial gastrointestinal stromal tumors caused by the novel KIT exon 17 germline mutation N822Y

Gastrointestinal stromal tumors (GISTs) are most often associated with oncogenic mutations of the KIT gene resulting in activation of the tyrosine kinase receptor KIT. Familial GIST syndrome based on a hereditary predisposition to develop GIST owing to a germline mutation is exceedingly rare. We describe a kindred with familial GIST displaying a novel germline mutation in exon 17. Three siblings (2 females, 1 male; 42 to 49 y) underwent surgery for multiple intra-abdominal tumors within a 3-year period. Their father had been operated on for gastric and jejunal tumors 20 years previously. The GIST was confirmed by immunohistochemistry in each sibling. Tumor and blood samples of the family members were analyzed for mutations in KIT and platelet-derived growth factor receptor (PDGFRalpha) genes. All examined lesions were of spindle cell type with expression of CD117. The tumor material exhibited a novel point mutation in codon 822 in exon 17 resulting in the replacement of asparagine by tyrosine (N822Y). The same mutation was detected in the father's blood sample. One healthy brother of the 3 siblings showed a wild-type sequence of the KIT gene. The germline mutation in exon 17 of the KIT gene identified in this kindred is very different from previously reported mutations of the KIT gene in familial GIST. Although the penetrance of KIT mutations is as yet unknown, assessment of the unaffected kindred of GIST patients for the presence of this mutation could help to distinguish individuals at high risk from those at virtually no risk.

The analysis of status and clinical implication of KIT and PDGFRA mutations in gastrointestinal stromal tumor (GIST)

BACKGROUND AND OBJECTIVE

To analyze the frequency and spectrum of KIT and platelet-derived growth factor receptor-alpha (PDGFRA) gene in a large series of study and to explore the clinical implication of mutations in the gastrointestinal stromal tumors (GISTs) in China.

METHOD

A total of 141 GISTs were subject to mutation analysis of KIT (exons 9, 11, 13, and 17) and PDGFRA (exons 12 and 18) using PCR amplification and direct sequencing. Clinicopathologic characteristics were correlated to gene mutations.

RESULTS

Of the 141 tumors studied, approximately 76.6% had KIT gene mutations, 2.8% had PDGFRA gene mutations and 20.6% had a wild-type gene of KIT and PDGFRA. Among those with KIT gene mutations, 70.2% occurred in exon 11, 5.7% in exon 9, 0.7% in exon 13, and no mutation was detected in exons 17. The most frequent sites of mutation were in exon 11 and the mutations clustered in the classic "hot spot" at the 5'end of the exon mostly heterogeneous and the second "hot spot" were internal tandem duplications (ITD) at the 3'end of the exon. The overall mutation rate was significantly lower in GISTs originated from colorectum or extra-gastrointestinal tract (chi(2) = 6.728, P = 0.009; chi(2) = 4.059, P = 0.044), however, mutation rate on exon 11 was significantly increased in gastric stromal tumor (chi(2) = 5.713, P = 0.017; chi(2) = 4.341, P = 0.037). There were no significant differences in terms of age, gender, tumor size, mitotic counts, grade of malignant potential and liver metastasis in patients with or without gene mutations.

CONCLUSION

KIT and PDGFRA gene were frequently found in patients with GISTs. Gene mutation rate varies in originated organ.

Clinicopathologic profile of gastrointestinal stromal tumors (GISTs) with primary KIT exon 13 or exon 17 mutations: a multicenter study on 54 cases

Gastrointestinal stromal tumors (GISTs) are mesenchymal neoplasms driven by oncogenic, mutational activation of KIT or platelet-derived growth factor receptor alpha (PDGFRA). GIST-specific KIT or PDGFRA mutations have been linked to tumor location, tumor cell morphology and clinical behavior. The purpose of this study was to evaluate the clinicopathologic profile of GISTs that have KIT exon 13 or exon 17 mutations. Through the collaboration of several GIST research groups, we gathered 54 cases from the pre-imatinib era that had such primary mutations. From our observations and those in the literature, we estimate that the frequency of these mutations is no higher than 1-2%. Almost all (32 of 33, 97%) of the KIT exon 13 mutations were the 1945A>G substitution leading to Lys642Glu. A majority (15 of 21, 71.4%) of the KIT exon 17 mutations were the 2487T>A substitution leading to Asn822Lys. Demographic and clinicopathologic data were available for 26 and 14 KIT exon 13 and exon 17 mutant GISTs, respectively. Median age and male to female ratio were similar to ones reported in other GIST studies. Small intestinal tumors were two times more frequent than gastric ones among KIT exon 17 mutants. Also, intestinal tumors were slightly overrepresented among KIT exon 13 mutants when compared with population-based studies. The majority of KIT exon 13 or exon 17 mutants had a spindle-cell morphology and only a few had epithelioid features. Tumor size varied from 1.2 to 25 cm and average mitotic rates were 9.5 and 4.2 for KIT exon 13 and exon 17 mutants, respectively. Gastric KIT exon 13 mutant GISTs tend to be slightly larger and more aggressive than gastric GISTs in average, whereas the behavior of small intestinal GISTs with KIT exon 13 mutations does not differ from other small intestinal GISTs. The latter is also true for all KIT exon 17 mutant GISTs.

A mouse model of a human multiple GIST family with KIT-Asp820Tyr mutation generated by a knock-in strategy

Several families exhibiting multiple gastrointestinal stromal tumours (GISTs) and germline c-kit gene mutations at exons 8, 11, 13, or 17 have been reported. These patients also exhibit diffuse hyperplasia of the interstitial cells of Cajal (ICCs) as a pre-existing lesion of multiple GISTs. We generated a mouse model of a family with germline c-kit gene mutation at exon 17, and compared the phenotypes between the mice and humans. The mouse counterpart (KIT-Asp818Tyr) of the human KIT-Asp820Tyr mutation was transmitted into germline by a knock-in strategy. Mating of male and female heterozygotes (KIT-Asp818Tyr/+) resulted in the generation of homozygotes (KIT-Asp818Tyr/KIT-Asp818Tyr). Histological examination revealed that all heterozygotes had both a small KIT-positive mesenchymal tumour at the caecum, consistent with GIST, and KIT-positive diffuse spindle-shaped cell proliferation in the distal oesophagus, stomach, proximal duodenum, and colon consistent with ICC hyperplasia. All homozygotes exhibited a larger caecal tumour and more prominent spindle-shaped cell proliferation compared with the heterozygous mice, and they usually died within 10 weeks after birth, likely due to ileus. The small intestine of both genotypes showed no apparent morphological abnormality, and autonomous contraction of the ileal segments appeared normal. Western blotting demonstrated that the caecal tumours expressed phosphorylated KIT, MAPK, Stat1, and Stat5. These mutant mice are considered to be useful for further investigation of the mechanism of GIST development as a result of ICC hyperplasia and for assessment of the in vivo effects of drugs against molecular targets.

Mutation analysis of gastrointestinal stromal tumors: increasing significance for risk assessment and effective targeted therapy

Molecular characterization of gastrointestinal stromal tumors (GISTs) plays an increasing role not only for the patient's prognosis but also for treatment options and in the context of resistance to therapy. Several mutational subtypes in KIT or platelet-derived growth factor receptor-alpha (PDGFRalpha) have been identified to be correlated with a different clinical behavior of GISTs. In KIT exon 11, deletions in the proximal part are associated with a high metastatic risk, whereas duplications in the distal part lead to a less aggressive phenotype. GISTs of the small bowel with a duplication in KIT exon 9 are often high risk tumors. In contrast, PDGFRalpha exon 18 mutated GISTs tend to have a low malignant potential. The authors suggest to include these molecular data together with classical parameters such as mitotic count and tumor size into the risk assessment of GISTs. The first choice for treatment of GISTs is still the surgical resection. In advanced tumors, which cannot be R0 resected, the neoadjuvant treatment with the tyrosine kinase inhibitor imatinib is now well established. Furthermore, an adjuvant treatment of locally R0-resected intermediate and high risk tumors is evaluated in several international clinical trials. For metastatic disease, treatment with imatinib is still the first option, but with new upcoming substances, the molecular characterization of GISTs may become mandatory. Very recently, it has been shown that sunitinib may be especially effective in GISTs with KIT exon 9 mutation, whereas these tumors show only an intermediate response to imatinib. A European Organisation for Research and Treatment of Cancer clinical trial randomizing patients according to their mutational status is under preparation. Secondary resistance to imatinib treatment is increasing, at least partly due to secondary mutations in the tyrosine kinase domain of the KIT receptor. Once a lesion has been shown to carry such a mutation, the local excision may be useful, mean while still responding metastases are further controlled by continuing imatinib. Taken together, the molecular characterization of GISTs turns out to play a central role before and during the treatment with tyrosine kinase inhibitors, which have improved the treatment of GIST patients dramatically.

Mutations in gastrointestinal stromal tumors--a population-based study from Northern Norway

Gastrointestinal stromal tumor (GIST) is the most common mesenchymal tumor of the gastrointestinal tract. This tumor typically expresses KIT, and has KIT or PDGFRA activating mutation. In this study we evaluated 89 GISTs diagnosed in Northern Norway during a 30-year period. KIT exons 8, 9, 11, 13, and 17 were analyzed by PCR amplification and direct sequencing. Subsequently PDGRA exons 12, 14, and 18 were evaluated in KIT wild-type cases. KIT mutations were found in 66 cases (75%), and PDGFRA mutations in 9 cases (10%). Most common were KIT exon 11 mutations, with 58 cases. Tumors with Kit exon 11 point mutations had a significantly better prognosis than those with deletions. There were five KIT exon 9 duplications, three exon 13 point mutations, and one point mutation in exon 17. There were nine PDGFGRA mutations: seven in exon 18 and two in exon 12. All but one PDGFRA mutant GISTs were gastric tumors with epithelioid morphology, and these tumors were on average smaller than those with KIT mutations. KIT and PDGFRA wild type was found in 15% of cases. Analysis of KIT and PDGFRA mutations is of significance for treatment with tyrosine kinase inhibitors, and may also have value when assessing the biological potential of GIST.

The molecular pathogenesis of gastrointestinal stromal tumors

Gastrointestinal stromal tumors (GISTs) are clinically diagnosed by positive immunohistochemical staining of KIT, a type III receptor tyrosine kinase. Most GISTs contain gain-of-function, ie, oncogenic mutations in c-KIT or in platelet-derived growth factor receptor-alpha (PDGFR-alpha), which appears to be the major initiating event that drives the pathogenesis for GIST. Furthermore, mutations in either of these genes appear to be required for tumor growth and progression. This scenario can be thought of as "oncogenic addiction" and is one of the major reasons why some GISTs respond significantly to therapies that target these mutant receptors. In addition to mutations in c-KIT or PDGFR-alpha, genomic alterations contribute to disease progression. Moreover, GISTs that harbor different c-KIT or PDGFR-alpha mutations have different molecular signatures at the level of gene expression, which further contributes to the complexity of GIST biology and variable responses to treatment. This article will discuss the molecular basis of pathogenesis and genetic and genomic alterations that contribute to GIST tumorigenesis and disease progression as well as the heterogeneity of this disease.

Juxtamembrane-type c-kit gene mutation found in aggressive systemic mastocytosis induces imatinib-resistant constitutive KIT activation

Aggressive systemic mastocytosis (ASM) is a very rare form of mast cell neoplasm that does not benefit from conventional chemotherapy. The majority of adult mast cell neoplasms and gastrointestinal stromal tumors (GISTs) have mutations in the proto-oncogene c-kit, which encodes the KIT receptor tyrosine kinase. The c-kit gene mutations are generally confined to the tyrosine kinase II domain in mast cell neoplasms, but are often observed at the juxtamembrane domain in GISTs. We found a case of ASM with a juxtamembrane-type mutation, Val559Ile, and in this report the mutation was characterized through transfection of the mutated c-kit cDNA into human embryonic kidney cells. Phosphorylation of KIT and its possible downstream signaling molecules were examined in the presence or absence of imatinib, a selective tyrosine kinase inhibitor. Ligand-independent autophosphorylation was observed in the mutant KIT with Val559Ile as well as that with Val559Asp, as found in GISTs. Imatinib, at a concentration of 10 microM, inhibited autophosphorylation of the mutant KIT with Val559Asp, but not that with the Val559Ile. Phosphorylation of MAPK and STAT5 was also inhibited by imatinib at the same concentration, in cells expressing Val559Asp but not in those expressing Val559Ile. These results suggest that different mutations, even at the same codon, in juxtamembrane domain of the c-kit gene show different inhibitory effects of imatinib, and that patients with GISTs or mast cell neoplasms possessing this Val559Ile mutation are resistant to imatinib therapy.

KIT and PDGFRA mutations in gastrointestinal stromal tumors (GISTs)

Mutually exclusive KIT and PDGFRA mutations are central events in GIST pathogenesis, and their understanding is becoming increasingly important, because specific treatment targeting oncogenic KIT and PDGFRA activation (especially imatinib mesylate) has become available. KIT mutations in GIST are clustered in four exons. Most common are exon 11 (juxtamembrane domain) mutations that include deletions, point mutations (affecting a few codons), and duplications (mostly in the 3' region). The latter mutations most often occur in gastric GISTs. Among gastric GISTs, tumors with deletions are more aggressive than those with point mutations; this does not seem to hold true in small intestinal GISTs. Exon 9 mutations (5-10%) usually are 2-codon 502-503 duplications, and these occur predominantly in intestinal versus gastric GISTs. Lesser imatinib sensitivity of these tumors has been noted. Kinase domain mutations are very rare; GISTs with such mutations are variably sensitive to imatinib. PDGFRA mutations usually occur in gastric GISTs, especially in the epithelioid variants; their overall frequency is approximately 30% to 40% of KIT mutation negative GISTs. Most common is exon 18 mutation leading Asp842Val at the protein level. This mutation causes imatinib resistance. Exon 12 and 14 mutations are rare. Most mutations are somatic (in tumor tissue only), but patients with familial GIST syndrome have consitutitonal KIT/PDGFRA mutations; >10 families have been reported worldwide with mutations generally similar to those in sporadic GISTs. GISTs in neurofibromatosis 1 patients, children, and Carney triad seem to lack GIST-specific KIT and PDGFRA mutations and may have a different disease mechanism. Secondary mutations usually occur in KIT kinase domains in patients after imatinib treatment resulting in resistance to this drug. Mutation genotyping is a tool in GIST diagnosis and in assessment of sensitivity to kinase inhibitors. This is a US government work. There are no restrictions on its use.

Ovarian dysgerminomas are characterised by frequent KIT mutations and abundant expression of pluripotency markers

BACKGROUND

Ovarian germ cell tumours (OGCTs) typically arise in young females and their pathogenesis remains poorly understood. We investigated the origin of malignant OGCTs and underlying molecular events in the development of the various histological subtypes of this neoplasia.

RESULTS

We examined in situ expression of stem cell-related (NANOG, OCT-3/4, KIT, AP-2gamma) and germ cell-specific proteins (MAGE-A4, NY-ESO-1, TSPY) using a tissue microarray consisting of 60 OGCT tissue samples and eight ovarian small cell carcinoma samples. Developmental pattern of expression of NANOG, TSPY, NY-ESO-1 and MAGE-A4 was determined in foetal ovaries (gestational weeks 13-40). The molecular genetic part of our study included search for the presence of Y-chromosome material by fluorescence in situ hybridisation (FISH), and mutational analysis of the KIT oncogene (exon 17, codon 816), which is often mutated in testicular GCTs, in a subset of tumour DNA samples. We detected a high expression of transcription factors related to the embryonic stem cell-like pluripotency and undifferentiated state in OGCTs, but not in small cell carcinomas, supporting the view that the latter do not arise from a germ cell progenitor. Bilateral OGCTs expressed more stem cell markers than unilateral cases. However, KIT was mutated in 5/13 unilateral dysgerminomas, whereas all bilateral dysgerminomas (n = 4) and all other histological types (n = 22) showed a wild type sequence. Furthermore, tissue from five phenotypic female patients harbouring combined dysgerminoma/gonadoblastoma expressed TSPY and contained Y-chromosome material as confirmed by FISH.

CONCLUSION

This study provides new data supporting two distinct but overlapping pathways in OGCT development; one involving spontaneous KIT mutation(s) leading to increased survival and proliferation of undifferentiated oogonia, the other related to presence of Y chromosome material and ensuing gonadal dysgenesis in phenotypic females.

Clinical implications of C-kit gene mutation in patients with large gastrointestinal stromal tumors

BACKGROUND AND AIM

To evaluate the clinical implications of C-kit gene mutation in patients with gastrointestinal stromal tumors (GIST) greater than 10 cm in size.

METHODS

All cases of pathologically diagnosed GIST with positive CD117 immunostaining from one hospital were retrospectively reviewed. Tissue from the 25 patients with tumors greater than 10 cm in diameter were collected and DNA was extracted. Exons 9, 11, and 13 of the C-kit gene were analyzed and the mutations compared with the clinical and pathological characteristics of the corresponding tumors.

RESULTS

Of the 25 tumors studied, 16 had C-kit gene mutations and nine did not. Of the 16 with mutations, there were four with exon 9 mutations, 12 with exon 11 mutations, and none with exon 13 mutations. Gene mutations were more frequent in male than female patients (12/13, 92% vs 4/12, 33%). There were no significant differences in age, resectability, recurrence rate, tumor characteristics (ulceration, necrosis, hemorrhage and mitotic counts), or survival in patients with or without gene mutations.

CONCLUSIONS

C-kit gene mutations were frequently found in patients with large GIST, more commonly in men than in women. However, the presence of a mutation was not predictive of prognosis in patients with large GIST.

An update on molecular genetics of gastrointestinal stromal tumours

Gastrointestinal stromal tumours (GISTs) are the most common primary mesenchymal tumours of the gastrointestinal tract. Most of them show activating mutations of the genes coding for KIT or platelet-derived growth factor receptor alpha (PDGFRalpha), two receptor tyrosine kinases (RTKs). The RTK inhibitor Imatinib (Gleevec, Novartis, Switzerland), induces regression of the tumour. The level of response to treatment, together with other clinicopathological parameters is related to the type and site of the activating mutation, thus suggesting that these tumours should be classified according to the molecular context. This is confirmed also by the phenomenon of the resistance to treatment, which arises because of different mechanisms (second mutation, amplification, activation of other RTKs) and can be fought only by specific RTK inhibitors, that are at present under development. RTK activation involves an homogeneous transduction pathway whose components (MAPK, AKT, PI3K, mTOR and RAS) are possible targets of new molecular treatment. A new paradigm of classification integrating the classic pathological criteria with the molecular changes will permit personalised prognosis and treatment.

Gastrointestinal stromal tumours: consensus statement on diagnosis and treatment

In the multidisciplinary management of gastrointestinal stromal tumours (GISTs), there is a need to coordinate the efforts of pathology, radiology, surgery and oncology. Surgery is the mainstay for resectable nonmetastatic GISTs, but virtually all GISTs are associated with a risk of metastasis. Imatinib 400 mg/day with or without surgery is the recommended first-line treatment for recurrent or metastatic GIST; a higher dose may be considered in patients who progress, develop secondary resistance or present with specific genotypic characteristics. Adjuvant or neoadjuvant imatinib is not advised for resectable nonmetastatic GISTs. Neoadjuvant imatinib may be considered when surgery would result in significant morbidity or loss of organ function. Follow-up computed tomography imaging is recommended every three to six months for at least five years. Patients with metastatic disease should be continued on imatinib due to the high risk of recurrence on discontinuation of therapy. Treatment should be continued until there is progression or intolerable adverse effects. If dose escalation with imatinib fails, a clinical trial with novel agents alone or in combination may be considered. The present recommendations were developed at a surgical subcommittee meeting and a subsequent full Advisory Committee meeting held in Toronto, Ontario, in April 2005, under the sponsorship of Novartis Pharmaceuticals Canada Inc.

[Gastrointestinal stromal tumors: definition, histological, immunohistochemical, and molecular features, and diagnostic strategy]

Gastrointestinal stromal tumors (GISTs) are the most frequent mesenchymal tumors of the gastrointestinal tract. Major advances in their definition and classification and the understanding of their molecular mechanisms have recently been made. These advances have resulted in the delineation of a treatment that has become a model of targeted therapy in oncology. GISTs are defined as tumors of the gastrointestinal tract, but also of the mesentery and peritoneum, constituted by a proliferation of usually spindle-shaped, rarely epithelioid cells, usually, but not consistently expressing the KIT protein. Most GISTs are associated with molecular abnormalities in two target genes: KIT (which encodes the KIT protein) and PDGFRA (which encodes the A chain of the PDGF receptor). The diagnosis of GIST relies on histological arguments (proliferation of spindle-shaped cells in 70% of cases, of epithelioid cells in 20%; histological variants are rare and sometimes misleading) and on immunohistochemical arguments (expression of KIT in 95%, usually associated with CD34 expression in 60%-70% of cases). The demonstration of mutations in target genes is required only in cases that are histologically suggestive but KIT-negative; beyond this indication, this is only undertaken in research protocols. The differential diagnosis of GIST includes the other mesenchymal tumors of the gastrointestinal tract, such as leiomyomas and leiomyosarcomas, and the digestive locations of some sarcomas; it relies on both histological and immunohistochemical arguments. The evaluation of the prognosis is essential. According to the current concept, every GIST carries a risk of malignancy, which may vary from very low to very high. Prognosis is based on a simple algorithm using two histoprognostic parameters, i.e., tumor size and mitotic index. The treatment of localized GIST is surgical resection, which must be complete; that of advanced or unresectable GIST is based on the use of a targeted therapy, imatinib, which is a pharmacological antagonist of the KIT protein. Proper understanding and utilisation of the diagnostic criteria and classification of GIST by pathologists are essential for good patient management.

Novel NF1 gene mutation in a Japanese patient with neurofibromatosis type 1 and a gastrointestinal stromal tumor

Many mutations of the NF1 gene have been reported in patients with neurofibromatosis type 1 (NF1); however, there have been no documented NF1 gene mutations in Japanese NF1 patients. In the present study, we used the polymerase chain reaction (PCR) and DNA sequencing analysis to characterize the NF1 gene in a 53-year-old Japanese patient with NF1 who suffered from neurofibroma, pheochromocytoma, and gastrointestinal stromal tumor (GIST). Direct sequence analyses revealed a single base substitution in the splicing donor site of intron 6 (IVS6 888+1, G --> A) in one NF1 allele, resulting in an altered splice site (ss) in the mutated allele. Splicing at the cryptic 5' ss in the mutated allele generated mRNA with an insertion of 60 nucleotides. In addition, we screened for mutations in exons 9, 11, 13, and 17 of the c-kit gene in GIST and the succinate dehydrogenase subunit D (SDHD) gene in the pheochromocytoma, but we did not detect any somatic mutations. We report here the first case of an NF1 patient with four neoplasms: neurofibroma, pheochromocytoma, astrocytoma and GIST. Our results suggest that the molecular pathogenesis of GISTs in NF1 patients is different from that in non-NF1 patients.

Pathology of gastrointestinal stromal tumors

Gastrointestinal stromal tumors (GIST) are the most common mesenchymal tumors in the gastrointestinal tract. It was found that most GIST expressed KIT, a receptor tyrosine kinase encoded by protooncogene c-kit. In normal gastrointestinal wall, KIT is expressed by interstitial cells of Cajal (ICC), which are a pacemaker for autonomous gastrointestinal movement. Because both GIST and ICC are double-positive for KIT and CD34, and because familial and multiple GIST appear to develop from diffuse hyperplasia of ICC, GIST are considered to originate from ICC or their precursor cells. It was also found that approximately 90% of the sporadic GIST have somatic gain-of-function mutations of the c-kit gene, and that the patients with familial and multiple GIST have germline gain-of-function mutations of the c-kit gene. These facts strongly suggest that the c-kit gene mutations are a cause of GIST. Approximately half of the sporadic GIST without c-kit gene mutations were demonstrated to have gain-of-function mutations in platelet-derived growth factor receptor-alpha (PDGFRA) gene that encodes another receptor tyrosine kinase. Because KIT is immunohistochemically negative in a minority of GIST, especially in PDGFRA gene mutation-harboring GIST, mutational analyses of c-kit and PDGFRA genes may be required to diagnose such GIST definitely. Imatinib mesylate was developed as a selective tyrosine kinase inhibitor. It inhibits constitutive activation of mutated KIT and PDGFRA, and is now being used for KIT-positive metastatic or unresectable GIST as a molecular target drug. Confirmation of KIT expression by immunohistochemistry is necessary for application of the drug. The effect of imatinib mesylate is different in various types of c-kit and PDGFRA gene mutations, and the secondary resistance against imatinib mesylate is often acquired by the second mutation of the identical genes. Mutational analyses of c-kit and PDGFRA genes are also significant for prediction of effectiveness of drugs including newly developed agents.

Differential diagnosis of gastrointestinal leiomyoma versus gastrointestinal stromal tumor

INTRODUCTION

Strategies for the diagnosis of tumors arising in the intestinal muscular wall are rapidly evolving. Immunoreactivity for CD117 (KIT) usually supports the diagnosis of gastrointestinal stromal tumor (GIST), but a small subset of GISTs lacks KIT expression. In these cases the differential diagnosis of KIT-negative GIST versus one of their morphological mimics is difficult and bears critical implications for therapeutic management.

CASE REPORT

Here, we report a case of a KIT-negative smooth muscle cell tumor of the colon in a 21-year-old man with the clinical appearance of GIST. Mutations of the KIT and platelet-derived growth factor receptor alpha (PDGFRA) gene could be ruled out. No chromosomal imbalances characteristic of GIST were found. However, cytogenetic analysis revealed losses at 7q, which has previously been reported in cases of uterine leiomyoma.

DISCUSSION

We discuss current approaches to the differential diagnosis of true gastrointestinal smooth muscle cell tumor versus GIST.

Gastrointestinal stromal tumours: an update

Recently, there has been intense interest in the study of gastrointestinal stromal tumour (GIST); one might call it a virtual GIST revolution. This is due largely to the realization that most GISTs express KIT and harbour activating c-KIT (KIT) or platelet-derived growth factor receptor-alpha (PDGFRA) receptor tyrosine kinase mutations that can be targeted by small molecule pharmacological inhibitors. Pathologists have benefited greatly from this revolution, mainly in the form of an improved ability to classify GISTs and, even more recently, in understanding the molecular underpinnings that underlie many fascinating clinical and pathological correlations. It is the purpose of this review to summarize recent developments in GIST classification and the molecular pathogenesis of GIST.

Differential expression of connexin 43 in gastrointestinal stromal tumours of gastric and small intestinal origin

Gastrointestinal stromal tumours (GISTs) are considered to originate from interstitial cells of Cajal (ICCs). ICCs are classified into several subtypes according to their location or roles. Several reports indicate that GISTs of the small intestine appear to have different clinical and pathological characteristics from gastric GISTs. We previously found using a cDNA expression chip that connexin 43, a component of gap junctions, is expressed specifically in small intestinal GISTs but not in gastric GISTs. To confirm the specificity of connexin 43 expression, we analysed 10 small intestinal GISTs and 15 gastric GISTs by northern blotting, western blotting and immunohistochemistry in this study. Northern blotting was performed in five small intestinal GISTs and five gastric GISTs, and revealed connexin 43 mRNA expression in all of the five small intestinal GISTs, but in none of the gastric GISTs. By western blotting, bands corresponding to connexin 43 were easily detected in all of the five small intestinal GISTs studied but were absent in all five gastric GISTs analysed. Immunohistochemistry showed that all of the 10 small intestinal GISTs were positive for connexin 43 but only one of 15 gastric GISTs, which exhibited a mutation in exon 9 of the KIT gene, was connexin 43-positive. We also examined the localization of connexin 43 in the normal stomach and small intestine. Immunoreactivity for connexin 43 was present in both normal gastric and small intestinal circular muscle layers, but it was unclear which cell type was positive. These results suggest that GISTs are divided into at least two groups, namely the gastric subtype and the small intestinal subtype, through phenotype but not location. Furthermore, these data indicate that the gastric and the small intestinal subtypes of GIST may originate from different subtypes of ICC.

Pathophysiology, diagnosis, and treatment of gastrointestinal stromal tumors

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors of the gastrointestinal tract. Activating mutations of KIT or the platelet-derived growth factor receptor alpha gene (PDGFRA) have been identified in the vast majority of GISTs. The respective oncoproteins exhibit constitutive tyrosine kinase activity and promote cell growth. KIT and PDGFRA mutations are rarely found in GISTs in patients with neurofibromatosis type 1 (NF1) suggesting that the pathogenesis of GIST in NF1 patients is different from that in non-NF1 patients. Endoscopic diagnosis of GIST is usually difficult. Endoscopic ultrasonography (EUS)-guided fine-needle aspiration biopsy (EUS-FNAB) is a useful method for the diagnosis of GIST and for the detection of KIT or PDGFRA mutations. Imatinib mesylate, a tyrosine kinase inhibitor known to inhibit the activities of BCR-ABL, KIT, and PDGFR, is currently being used for the treatment of both chronic myeloid leukemia and metastatic GIST. The clinical response to imatinib therapy correlates with the types of mutations of KIT and PDGFRA, and the determination of KIT and PDGFRA mutations is useful for predicting the effect of imatinib. Resistance to imatinib after an initial response has been reported; secondary point mutations in KIT or PDGFRA that confer imatinib resistance are the most common mechanisms responsible for acquired resistance to imatinib. The continued development of target-specific therapies should increase the probability of cure in most patients with GISTs.

Gastrointestinal stromal tumors of neurofibromatosis type I (von Recklinghausen's disease)

Gastrointestinal stromal tumor (GIST), as well as the hyperplastic lesions of intestinal neural tissue and its supporting structures, is a gastrointestinal complication of type 1 neurofibromatosis (NF1) (von Recklinghausen's disease). In the present study, we analyzed the histologic and immunohistochemical features, and the c-kit and PDGFRA gene mutations of 36 GISTs derived from 9 NF1 patients. Distinctively, multiple GISTs arose preferentially in the small intestine. The histologic features of NF1-associated GISTs are almost similar to those of non-NF1 GISTs, but characteristically most of the NF1-associated GISTs contained skeinoid fibers. Thirty-three GISTs (92%) showed immunoreactivity for KIT, and 23 tumors (64%) showed diffuse or mosaic-like immunoreactivity for S-100 protein. Hyperplasic lesions, which may be the hyperplasia of interstitial cells of Cajal, were observed around some GISTs. Exons 9, 11, 13, and 17 of the c-kit gene and exons 12 and 18 of the PDGFRA gene were amplified and directly sequenced. Point mutations of c-kit gene or PDGFRA gene were identified only in three (8%) and two (6%) tumors, respectively. NF1-associated GISTs, showing the dual differentiation of interstitial cells of Cajal and Schwann cells, develop in close association with the myenteric nerve structure of gastrointestinal tract of NF1 patients. The point mutations of c-kit and PDGFRA gene may play a limited role in the tumorigenesis of NF1-associated GISTs.

A recurrent duodenal gastrointestinal stromal tumor with a frameshift mutation resulting in a stop codon in KIT exon 13

Gastrointestinal stromal tumors (GISTs) are a specific and rare subset of human gastrointestinal tract tumors. Most GISTs show gain-of-function mutations of KIT, mainly in exon 11, that always maintain the reading frame. We report on data from a 43-year-old Japanese man with recurrent duodenal GIST and a frameshift mutation in KIT exon 13 together with an in-frame deletion in KIT exon 11 detected by genomic DNA sequencing. Deletion of 48 base pairs of KIT exon 11, which preserved the reading frame, was identified in both primary and recurrent tumors, whereas deletion of one nucleotide of codon 642 of KIT exon 13, which changed the reading frame and induced a novel stop codon at amino acid 644, was found only in the recurrent tumor. The predicted protein resulting from the latter would lack part of the kinase domain. To the best of our knowledge, this is the first documentation of a GIST with a frameshift mutation of KIT.

A mutation-created novel intra-exonic pre-mRNA splice site causes constitutive activation of KIT in human gastrointestinal stromal tumors

We report a new mechanism of aberrant pre-mRNA splicing resulting in constitutive activation of a mis-spliced oncoprotein (KIT) leading to malignancy (gastrointestinal stromal tumor) in contrast to loss of function of mis-spliced proteins resulting in diverse human diseases in the literature. The mechanisms of three consecutive molecular events, deletion of noncoding and coding regions encompassing the 3' authentic splice site, creation of a novel intra-exonic pre-mRNA 3' splice acceptor site leading to in-frame loss of 27 nucleotides (nine amino acids; Lys550-Lys558), and the mechanism of constitutive activation of the mis-spliced KIT are elucidated. Loss of a peptide in a critical location unleashed the protein from autoinhibition (as evidenced by three-dimensional structural analysis), causing KIT to become constitutively activated and resulting in the GIST phenotype. We also demonstrated that only one of the following two exonic splicing enhancers is sufficient for inclusion of the KIT exon 11 in vivo: AACCCATGT (nucleotides 2-10 from the 5' end, which are recognized by SC35, SRp55, and SF2/ASF) or GGTTGTTGAGG (nucleotides 27-37 from the 5' end, which are recognized by SC35 and SRp55), suggestive of exonic enhancer redundancy.

Analysis of KIT Mutations in Sporadic and Familial Gastrointestinal Stromal Tumors: Therapeutic Implications through Protein Modeling

PURPOSE

Gastrointestinal stromal tumors (GIST) are characterized by expressing a gain-of-function mutation in KIT, and to a lesser extent, PDGFR. Imatinib mesylate, a tyrosine kinase inhibitor, has activity against GISTs that contain oncogenic mutations of KIT. In this study, KIT and PDGFRalpha mutation status was analyzed and protein modeling approaches were used to assess the potential effect of KIT mutations in response to imatinib therapy.

EXPERIMENTAL DESIGN

Genomic DNA was isolated from GIST tumors. Exons 9, 11, 13, and 17 of c-KIT and exons 12, 14, and 18 of PDGFRalpha were evaluated for oncogenic mutations. Protein modeling was used to assess mutations within the juxtamembrane region and the kinase domain of KIT.

RESULTS

Mutations in KIT exons 9, 11, and 13 were identified in GISTs with the majority of changes involving the juxtamembrane region of KIT. Molecular modeling indicates that mutations in this region result in disruption of the KIT autoinhibited conformation, and lead to gain-of-function activation of the kinase. Furthermore, a novel germ-line mutation in KIT was identified that is associated with an autosomal dominant predisposition to the development of GIST.

CONCLUSIONS

We have used protein modeling and structural analyses to elucidate why patients with GIST tumors containing exon 11 mutations are the most responsive to imatinib mesylate treatment. Importantly, mutations detected in this exon and others displayed constitutive activation of KIT. Furthermore, we have found tumors that are both KIT and PDGFRalpha mutation negative, suggesting that additional, yet unidentified, abnormalities may contribute to GIST tumorigenesis.

Multiple gastrointestinal stromal tumors in type I neurofibromatosis: a pathologic and molecular study

Multiple gastrointestinal stromal tumors typically occur in familial form associated with KIT receptor tyrosine kinase or platelet-derived growth factor receptor-alpha (PDGFRA) germline mutations, but may also develop in the setting of type 1 neurofibromatosis. The molecular abnormalities of gastrointestinal stromal tumors arising in neurofibromatosis have not been extensively studied. We identified three patients with type 1 neuro-fibromatosis and multiple small intestinal stromal tumors. Immunostains for CD117, CD34, desmin, actins, S-100 protein, and keratins were performed on all of the tumors. DNA was extracted from representative paraffin blocks from separate tumor nodules in each case and subjected to a nested polymerase chain reaction, using primers for KIT exons 9, 11, 13, and 17 and PDGFRA exons 12 and 18, followed by direct sequencing. The mean patient age was 56 years (range: 37-86 years, male/female ratio: 2/1). One patient had three tumors, one had five, and one had greater than 10 tumor nodules, all of which demonstrated histologic features characteristic of gastrointestinal stromal tumors and stained strongly for CD117 and CD34. One patient died of disease at 35 months, one was disease free at 12 months and one was lost to follow-up. DNA extracts from 10 gastrointestinal stromal tumors (three from each of two patients and four from one patient) were subjected to polymerase chain reactions and assessed for mutations. All of the tumors were wild type for KIT exons 9, 13, and 17 and PDGFRA exons 12 and 18. Three tumors from one patient had identical point mutations in KIT exon 11, whereas the other tumors were wild type at this locus. We conclude that, although most patients with type 1 neurofibromatosis and gastrointestinal stromal tumors do not have KIT or PDGFRA mutations, KIT germline mutations might be implicated in the pathogenesis of gastrointestinal stromal tumors in some patients.