The role of KIT in the management of patients with gastrointestinal stromal tumors

In recent years, immunohistochemical staining for KIT (CD117) has become integral to the diagnosis of gastrointestinal stromal tumors (GISTs), nearly 90% of which harbor activating mutations in the KIT receptor tyrosine kinase gene. Approximately 80% of patients with metastatic GIST show at least some clinical response to the targeted small molecule KIT inhibitor imatinib. The response to imatinib is closely correlated with the presence and type of KIT mutation. GISTs with the most common KIT exon 11 mutations have the highest response rate by far, whereas GISTs lacking mutations in KIT or the alternative receptor tyrosine kinase PDGFRA show much lower rates of response to imatinib. Less than 5% of GISTs are KIT-immunonegative; and many of these tumors have activating mutations of PDGFRA, some of which are also inhibited by imatinib. Most patients who initially respond to imatinib become resistant and eventually progress, which coincides with the selection of imatinib-resistant secondary KIT mutations in the kinase domain. Sunitinib has recently been approved for patients with GIST, principally those who fail imatinib therapy; and additional small molecule inhibitors are in the pipeline. It is becoming evident that alternative approaches to direct KIT inhibition will be required for long-term survival of patients with advanced GISTs. This review examines the role of KIT in the diagnosis and management of patients with GIST.     

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.     

Heterogeneity of kinase inhibitor resistance mechanisms in GIST

Most GIST patients develop clinical resistance to KIT/PDGFRA tyrosine kinase inhibitors (TKI). However, it is unclear whether clinical resistance results from single or multiple molecular mechanisms in each patient. KIT and PDGFRA mutations were evaluated in 53 GIST metastases obtained from 14 patients who underwent surgical debulking after progression on imatinib or sunitinib. To interrogate possible resistance mechanisms across a broad biological spectrum of GISTs, inter- and intra-lesional heterogeneity of molecular drug-resistance mechanisms were evaluated in the following: conventional KIT (CD117)-positive GISTs with KIT mutations in exon 9, 11 or 13; KIT-negative GISTs; GISTs with unusual morphology; and KIT/PDGFRA wild-type GISTs. Genomic KIT and PDGFRA mutations were characterized systematically, using complementary techniques including D-HPLC for KIT exons 9, 11-18 and PDGFRA exons 12, 14, 18, and mutation-specific PCR (V654A, D820G, N822K, Y823D). Primary KIT oncogenic mutations were found in 11/14 patients (79%). Of these, 9/11 (83%), had secondary drug-resistant KIT mutations, including six (67%) with two to five different secondary mutations in separate metastases, and three (34%) with two secondary KIT mutations in the same metastasis. The secondary mutations clustered in the KIT ATP binding pocket and kinase catalytic regions. FISH analyses revealed KIT amplicons in 2/10 metastases lacking secondary KIT mutations. This study demonstrates extensive intra- and inter-lesional heterogeneity of resistance mutations and gene amplification in patients with clinically progressing GIST. KIT kinase resistance mutations were not found in KIT/PDGFRA wild-type GISTs or in KIT-mutant GISTs showing unusual morphology and/or loss of KIT expression by IHC, indicating that resistance mechanisms are fundamentally different in these tumours. Our observations underscore the heterogeneity of clinical TKI resistance, and highlight the therapeutic challenges involved in salvaging patients after clinical progression on TKI monotherapies.     

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.     

Imatinib in the management of multiple gastrointestinal stromal tumors associated with a germline KIT K642E mutation

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors of the gut and are distinguished by expression of CD117 (c-Kit). Oncogenic mutations in the KIT or PDGFRA gene are detected in approximately 85% of sporadic GISTs. In recent years, examples of familial GIST have been reported in which germline mutations of KIT or PDGFRA result in multiple GISTs, skin disorders, and other abnormalities. The most common germline mutations are in KIT exon 11, mutations in exons 8 and 17 have also been described, and there are 2 families with germline PDGFRA mutations. We present a case in which a germline KIT exon 13 mutation (K642E) was discovered in a patient with multiple GISTs of rectum, small intestine, and esophagus, as well as diffuse hyperplasia of the interstitial cells of Cajal. To our knowledge, this is only the second germline example of this particular mutation. The patient's esophageal tumors were stabilized with imatinib.     

Novel V600E BRAF mutations in imatinib-naive and imatinib-resistant gastrointestinal stromal tumors

BRAF and NRAS are commonly mutated in cancer and represent the most frequent genetic events in malignant melanoma. More recently, a subset of melanomas was shown to overexpress KIT and harbor KIT mutations. Although most gastrointestinal stromal tumors (GISTs) exhibit activating mutations in either KIT or PDGFRA, about 10% of the cases lack mutations in these genes. It is our hypothesis following the melanoma model that mutations in BRAF or NRAS may play a role in wild-type GIST pathogenesis. Alterations in RAS/MEK/ERK pathway may also be involved in development of imatinib resistance in GIST, particularly in tumors lacking secondary KIT or PDGFRA mutations. Imatinib-naive wild-type GISTs from 61 patients, including 15 children and 28 imatinib-resistant tumors without secondary KIT mutations were analyzed. Screening for hot spots mutations in BRAF (exons 11 and 15) and NRAS (exons 2 and 3) was performed. A BRAF exon 15 V600E was identified in 3 of 61 GIST patients, who shared similar clinical features, being 49- to 55-years-old females and having their tumors located in the small bowel. The tumors were strongly KIT immunoreactive and had a high risk of malignancy. An identical V600E BRAF mutation was also identified in one of 28 imatinib resistant GIST lacking a defined mechanism of drug resistance. In conclusion, we identified a primary BRAF V600E mutations in 7% of adult GIST patients, lacking KIT/PDGFRA mutations. The BRAF-mutated GISTs show predilection for small bowel location and high risk of malignancy. A secondary V600E BRAF mutation could represent an alternative mechanism of imatinib resistance. Kinase inhibitors targeting BRAF may be effective therapeutic options in this molecular GIST subset.     

KIT and PDGFRalpha mutational analyses of mixed cell-type gastrointestinal stromal tumours

AIMS: To determine whether the epithelioid and spindle components of a mixed cell-type gastrointestinal stromal tumour (GIST) show the same receptor tyrosine kinase mutation and, by inference, the same sensitivity to imatinib. METHODS AND RESULTS: Six mixed cell-type GISTs were identified from 108 gastric GISTs. Clinicopathological and immunohistochemical data of the six neoplasms were collated. For each neoplasm, DNA was extracted separately from the laser-microdissected epithelioid and spindle components and non-neoplastic tissue and sequenced for KIT and platelet-derived growth factor receptor (PDGFR)alpha mutations. The epithelioid component often showed less CD117 and/or CD34 immunoreactivity than the spindle component of the same mixed cell-type GIST. Four mixed cell-type GISTs showed somatic KIT mutations (deletions in exon 11 in three tumours and an insertion in exon 9 in one tumour) and one showed a somatic PDGFRalpha mutation (point mutation in exon 18); in each of the five cases, both epithelioid and spindle components showed identical mutations. CONCLUSIONS: The presence of the same receptor tyrosine kinase mutation in both components of a mixed cell-type GIST suggests that both components should be equally responsive to imatinib treatment, and that such mutation is an early key event in the pathogenesis of these neoplasms.     

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.     

The novel marker, DOG1, is expressed ubiquitously in gastrointestinal stromal tumors irrespective of KIT or PDGFRA mutation status

We recently characterized gene expression patterns in gastrointestinal stromal tumors (GISTs) using cDNA microarrays, and found that the gene FLJ10261 (DOG1, discovered on GIST-1), encoding a hypothetical protein, was specifically expressed in GISTs. The immunoreactivity of a rabbit antiserum to synthetic DOG1 peptides was assessed on two soft tissue tumor microarrays. The tissue microarrays included 587 soft tissue tumors, with 149 GISTs, including 127 GIST cases for which the KIT and PDGFRA mutation status was known. Immunoreactivity for DOG1 was found in 136 of 139 (97.8%) of scorable GISTs. All seven GIST cases with a PDGFRA mutation were DOG1-positive, while most of these failed to react for KIT. The immunohistochemical findings were confirmed with in situ hybridization probes for DOG1, KIT, and PDGFRA. Other neoplasms in the differential diagnosis of GIST, including desmoid fibromatosis (0 of 17) and Schwannoma (0 of 3), were immunonegative for DOG1. Only 4 of 438 non-GIST cases were immunoreactive for DOG1. DOG1, a protein of unknown function, is expressed strongly on the cell surface of GISTs and is rarely expressed in other soft tissue tumors. Reactivity for DOG1 may aid in the diagnosis of GISTs, including PDGFRA mutants that fail to express KIT antigen, and lead to appropriate treatment with imatinib mesylate, an inhibitor of the KIT tyrosine kinase.     

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.     

The GIST paradigm: lessons for other kinase-driven cancers

Gastrointestinal stromal tumour (GIST) is the most common sarcoma of the intestinal tract, known to be notoriously refractory to conventional chemotherapy or radiation. It is an ideal solid tumour model to apply our understanding from aberrant signal transduction to drug development, since nearly all tumours have a mutation in the KIT or, less often, the PDGFRA or BRAF genes. The constitutively activated KIT and PDGFRA oncoproteins serve as crucial diagnostic and therapeutic targets. The discovery of oncogenic KIT activation as a central mechanism of GIST pathogenesis suggested that inhibiting or blocking KIT signalling might be the milestone in the targeted therapy of GISTs. Indeed, imatinib mesylate inhibits KIT kinase activity and represents the front-line drug for the treatment of unresectable and advanced GISTs, achieving a partial response or stable disease in about 80% of patients with metastatic GIST. KIT mutation status has a significant impact on treatment response, emerging in recent years as a leading paradigm for genotype-driven targeted therapy. In this review, parallels with other models in oncology that share their addiction to a particular mutationally activated kinase are contrasted. A better understanding of oncogene addiction as a common theme across tumours of diverse histologies underlies the clinical success of targeting such kinases with several selective kinase inhibitors. Also remarkable is the similarity displayed in the mechanisms of drug failure after a successful but temporary clinical response to kinase inhibition. Reactivation of the same oncogenic kinase, often by acquisition of second site mutations, is another emerging paradigm of secondary resistance in these tumour models. The complexity of polyclonal resistance in imatinib-resistant patients argues that single next-generation kinase inhibitors will not be beneficial in all mutant clones. Other broad therapeutic strategies could include combination of kinase inhibitors with targeting KIT downstream targets, such as PI3-K or MAPK/MEK inhibitors.     

Spectrum of mutations in gastrointestinal stromal tumor patients – a population‐based study from Slovakia

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal neoplasms of gastrointestinal tract and are characterized by presence of mutations in tyrosine kinases cKIT (KIT) and PDGFRα (PDGFRA). Mutations identified are highly heterogeneous, but some mutations are associated with specific clinical features of the tumor. Samples from 278 GIST patients collected during the period 2004–2011 were screened for mutations in exons 9, 11, 13, and 17 of KIT and 12, 14 and 18 of PDGFRA. Results of mutation screening were summarized and tested for possible association with clinical parameters of tumors. Mutations were identified in 83.81% of patients. Most frequent mutations were found in KIT exon 11 reaching frequency of 62.95%. Other exons contributed to the mutation pool with frequencies 8.27%, 7.55%, 2.52%, 1.44%, 1.08%, and 0.00%, in decreasing order KIT exon 9, PDGRFA exons 18 and 12, KIT exon 13, PDGFRA exon 14, and KIT exon 17. General linear model analysis showed no effect of any individual analyzed mutation on the phenotypic variables, but we confirmed association between mutations KIT exon 9 p. 503‐504_dup2, and PDGFRA exon 18 p. D842V and intestinal and gastric localization of tumors.     

The location of KIT and PDGFRA gene mutations in gastrointestinal stromal tumours is site and phenotype associated

AIMS: To assess the relation between KIT and PDGFRA mutations and the site of origin, histological phenotype, and pathomorphologically determined risk assessment in gastrointestinal stromal tumours (GISTs). METHODS: A series of 83 clinicopathologically characterised GISTs from 79 patients was analysed for KIT and PDGFRA mutations by polymerase chain reaction amplification, single strand conformation polymorphism analysis, and direct DNA sequencing. RESULTS: KIT or PDGFRA mutations were found in 57 and 11 GISTs, respectively. Most KIT mutations involved exon 11 (46 cases), followed by exon 9 (10 cases). The PDGFRA mutations mostly affected exon 18 (eight cases), followed by exon 12 (three cases). There was a significant association between KIT exon 9 mutations and an intestinal origin of GISTs, and between PDGFRA mutations and gastric origin of the tumours. In addition, the presence of PDGFRA mutations was significantly associated with epithelioid/mixed histology, as was the absence of identified receptor tyrosine kinase mutations. Vice versa, KIT exon 11 mutations were almost exclusively found in spindle cell GISTs. Furthermore, the presence of any KIT and PDGFRA mutations and the presence of KIT mutations alone were significantly associated with high risk/malignant GISTs. CONCLUSIONS: The location of KIT and PDGFRA mutations in GISTs is associated with the site of origin and histological phenotype. Genotyping of GISTs may be a helpful additional parameter in determining the biological profile of these tumours.     

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.     

GISTs with PDGFRA exon 14 mutations represent subset of clinically favorable gastric tumors with epithelioid morphology

Gastrointestinal stromal tumors (GISTs) are common mesenchymal tumors of the gastrointestinal tract. Activating KIT or PDGFRA (platelet-derived growth factor receptor alpha) mutations have been shown to be a major force in GIST pathogenesis. Recently, a previously undescribed N659K PDGFRA exon 14 mutation has been reported in GISTs. The purpose of this study was to evaluate the frequency of GISTs with PDGFRA exon 14 mutations and define the clinicopathologic profile of such tumors. In all, 200 GISTs negative for mutations in KIT exons 9, 11, 13 and 17 and PDGFRA exons 12 and 18 were evaluated for PDGFRA exon 14 mutations by PCR amplification and direct sequencing. Mutations were found in 11 of 119 (9%) gastric GISTs. None of the 81 GISTs from other than gastric location had such a PDGFRA mutation. A majority of these mutations (eight cases) represented simple 2125C>A or C>G missense mutations, leading to substitution of the lysine for asparagine (N659K). However, in two cases, 2123A>T missense mutations leading to substitution of the tyrosine for asparagine (N659Y) was found instead. Of 11 PDGFRA N659-mutant GISTs, 10 had pure epithelioid morphology. One tumor had mixed, predominantly spindle and focally epithelioid cell morphology. Frequency of PDGFRA N659-mutant GISTs among pure epithelioid GISTs was almost 19%. Immunohistochemically, the majority (64%) of these tumors lacked KIT expression or showed only focal scattered KIT positivity. Tumor size ranged from 2.5 to 16 cm (average 7.1 cm). Low mitotic activity, 5 cm tumors. Based on mitotic activity and tumor size, six tumors were classified as probably benign with very low malignant potential. Low to moderate malignant potential and high malignant potential was suggested in three and two tumors, respectively. In four cases with moderate or high malignant potential GISTs, a long-term follow-up (average 235.5 months) showed favorable course of disease.     

Gain-of-function mutation at the extracellular domain of KIT in gastrointestinal stromal tumours

Gastrointestinal stromal tumours (GISTs) are the most common mesenchymal tumours of the human gastrointestinal tract. Previous studies of GISTs found gain-of-function mutations of the c-kit gene, which encodes a receptor tyrosine kinase (KIT). All the mutations were confined to exon 11, which encodes the juxtamembrane domain. By further examination of the whole coding region of c-kit complementary DNA in 35 GISTs, two were found to show the identical mutation at exon 9, which encodes the extracellular domain. The aims of the present study were to examine the frequency of the extracellular domain mutation and to determine whether the mutation is a gain-of-function type or not. Genomic DNA was extracted from paraffin-embedded tissues of 133 GISTs and exon 9 of the c-kit gene was amplified by polymerase chain reaction. Screening of the mutation was carried out by single-strand conformation polymorphism analysis and direct sequencing was done. Mutant c-kit cDNA was transfected into 293T human embryonic kidney cells and the magnitude of autophosphorylation of the mutant KIT was examined with or without the ligand of KIT, stem cell factor (SCF). In total, seven GIST cases (approximately 5%) were found with the identical mutation at exon 9. The mutant KIT exhibited constitutive autophosphorylation without SCF stimulation. The prognosis of the patients with the extracellular domain mutation was comparable to that of the patients with the juxtamembrane domain mutation.     

KIT mutations are common in incidental gastrointestinal stromal tumors one centimeter or less in size

Gastrointestinal stromal tumors (GISTs) are mesenchymal neoplasms of the gut wall that express the receptor tyrosine kinase KIT. Somatic mutations that result in constitutive activation of KIT kinase have been identified in a number of studies of GISTs, although the reported frequency of these mutations has varied over a wide range (20 to 92%). Several reports have suggested that KIT gene mutations are more common in malignant GISTs than in benign lesions, and it has been proposed that mutations in exon 11 of KIT are a negative prognostic factor. To maximize sensitivity for KIT mutations we have adapted denaturing high-pressure liquid chromatography as a method for screening polymerase chain reaction amplimers of exons 9, 11, 13, and 17 from GIST genomic DNA. This approach was used to assess the frequency of KIT mutations in 13 morphologically benign, incidentally discovered, GISTs identified at autopsy, endoscopy, or laparotomy for unrelated disease. Representing the smallest pathologically recognizable GISTs, these lesions ranged in size from 4 to 10 mm in diameter and were all immunohistochemically positive for KIT. Eleven of the 13 tumors had sequence-confirmed mutations in KIT, including 10 mutations in exon 11 (77%) and one mutation in exon 9 (7.7%). The remaining two tumors were wild type for exons 9, 11, and 17; one of these was also analyzed for exon 13 and was wild type in this exon as well. The mutations found in the incidental GISTs were identical to those that have been documented in larger GISTs. In addition, the overall frequency of mutations in the incidental tumors (85%) did not differ significantly from that we previously reported in a series of 72 advanced/metastatic GISTs (86%), strongly supporting the view that activating mutations in KIT are acquired very early in the development of most GISTs. The findings suggest that KIT mutations per se are of little prognostic importance in GISTs.     

Presence of homozygous KIT exon 11 mutations is strongly associated with malignant clinical behavior in gastrointestinal stromal tumors

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors of gastrointestinal tract. GISTs range from benign indolent neoplasms to highly malignant sarcomas. Gain-of-function mutations of tyrosine kinase receptors, KIT or PDGFRA, have been identified in most GISTs. In this study, we report 36 GIST patients whose tumors had homozygous KIT exon 11 mutations detected by direct sequencing of PCR products. Loss of heterozygosity in KIT locus and other chromosome 4 loci were documented in majority of these tumors. However, fluorescence in situ hybridization with KIT locus-specific probe and chromosome 4 centromeric enumeration probe showed no evidence of KIT hemizygosity in a majority of analyzed cases. These findings are consistent with duplication of chromosome 4 with KIT mutant allele. Homozygous KIT exon 11 mutations were found in 33 primary tumors and 7 metastatic lesions. In two cases, shift from heterozygosity to homozygosity was documented during tumor progression being present in metastases, but not in primary tumors. Among primary GISTs, there were 16 gastric, 18 intestinal and 2 from unknown locations. An average primary tumor size was 12 cm and average mitotic activity 32/50 HPFs. Out of 32 tumors 29 (90.6%) with complete clinicopathologic data were diagnosed as sarcomas with more than 50% risk of metastatic disease, and 26 of 29 patients with follow-up had metastases or died of disease. An average survival time among pre-imatinib patients, who died of the disease was 33.4 months. Based on these findings, we conclude that presence of homozygous KIT exon 11 mutations is associated with malignant course of disease and should be considered an adverse prognostic marker in GISTs.     

Molecular analysis of c-Kit and PDGFRA in GISTs diagnosed by EUS

Gastrointestinal stromal tumors (GISTs) are characterized by overexpression and mutations of c-Kit. Approximately 80% of c-Kit mutations occur in exon 11, being a response factor to imatinib (Gleevec) therapy. Mutations of platelet-derived growth factor receptor-alpha (PDGFRA) are observed in a subset of GISTs lacking c-Kit mutations.We aimed to assess whether c-Kit and PDGFRA mutation analysis of GISTs obtained by endoscopic ultrasound-guided fine-needle aspiration (EUS-FNA) could be routinely performed. Mutation analysis of c-Kit hotspot exons (9, 11, 13 and 17) and PDGFRA hotspot exons (12 and 18) was performed in aspirates of 33 GISTs and 18 non-GIST mesenchymal tumors.Of the GIST cases, 19 (58%) of 33 contained a mutation in exon 11, 1 (3%) in exon 9, and none in exons 13 and 17. No activating c-Kit mutations were identified in non-GIST cases. No PDGFRA mutation was detected.Mutation analysis is possible in these FNA cell blocks and can assist in the diagnosis and therapeutic decisions in GIST cases/     

Molecular pathogenesis of multiple gastrointestinal stromal tumors in NF1 patients

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors of the gastrointestinal tract. KIT and PDGFRA activating mutations are the oncogenic mechanisms in most sporadic GISTs. In addition to sporadic occurrences, GISTs are increasingly being recognized in association with neurofibromatosis type 1 (NF1), yet the underlying pathogenic mechanism remains elusive. To gain an insight into the mechanisms underlying GIST formation in NF1 patients, we studied seven GISTs from three NF1 patients with a combination of different techniques: mutation analysis (KIT, PDGFRA and NF1), western blotting, array CGH and ex vivo imatinib response experiments. We demonstrate that (i) the NF1-related GISTs do not have KIT or PDGFRA mutations, (ii) the molecular event underlying GIST development in this patient group is a somatic inactivation of the wild-type NF1 allele in the tumor and (iii) inactivation of neurofibromin is an alternate mechanism to (hyper) activate the MAP-kinase pathway, while the JAK-STAT3 and PI3K-AKT pathways are less activated in NF1-related GIST compared with sporadic GISTs. In conclusion, we report for the first time the molecular pathogenesis of GISTs in NF1 individuals and demonstrate that this type of tumor clearly belongs to the spectrum of clinical symptoms in NF1.