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.     

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.     

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.     

Clinical significance of oncogenic KIT and PDGFRA mutations in gastrointestinal stromal tumours

Gastrointestinal stromal tumours (GISTs) are the most common mesenchymal neoplasms of the gastrointestinal tract. Despite clinicopathological differences, GISTs share oncogenic KIT or platelet-derived growth factor-alpha ( PDGFRA ) mutations. Imatinib, KIT and PDGFRA inhibitor, has been successfully used in the treatment of metastatic GISTs. There are primary KIT or PDGFRA mutations diagnosed before imatinib treatment, linked to GIST pathogenesis, and secondary mutations detected during treatment, causing drug resistance. KIT exon 11 mutations are the most common. Gastric GISTs with exon 11 deletions are more aggressive than those with substitutions. KIT exon 11 mutants respond well to imatinib. Less common KIT exon 9 Ala502_Tyr503dup mutants occur predominantly in intestinal GISTs and are less sensitive to imatinib. An Asp842Val substitution in exon 18 is the most common PDGFRA mutation. GISTs with such mutation are resistant to imatinib. PDGFRA mutations are associated with gastric GISTs, epithelioid morphology and a less malignant course of disease. GISTs in neurofibromatosis 1, Carney triad and paediatric tumours generally lack KIT and PDGFRA mutations. Secondary KIT mutations affect exons 13–17. GISTs with secondary mutations in exon 13 and 14 are sensitive to sunitinib, another tyrosine kinase inhibitor. KIT and PDGFRA genotyping is important for GIST diagnosis and assessment of sensitivity to tyrosine kinase inhibitors.     

The Complexity of KIT Gene Mutations and Chromosome Rearrangements and Their Clinical Correlation in Gastrointestinal Stromal (Pacemaker Cell) Tumors

Gastrointestinal stromal (pacemaker cell) tumors (GIST/GIPACTs) are frequently associated with activating KIT mutations, primarily of exon 11 and rarely of exons 9 and 13, as well as certain chromosome rearrangements. Reports regarding the frequency and prognostic significance of KIT mutations are conflicting and few cases have been completely sequenced. Furthermore, there are few detailed analyses of chromosome alterations in GIST/GIPACTs. In a detailed analysis of 14 GIST/GIPACTs from 12 patients, we found a wider spectrum of KIT mutations than previously reported, including 11 different in-frame mutations involving exons 11, 14, and 15. No mutations were detected in four malignant tumors. The shorter (GNNK-) KIT isoform was preferentially expressed. Cytogenetic and spectral karyotype analyses of 10 tumors revealed clonal abnormalities in eight tumors; the most common were terminal 1p deletions and losses of chromosomes 14 and/or 22. Neither KIT mutation status nor chromosome aberrations correlated with tumor phenotype or clinical behavior in our series. Collectively, these findings indicate that the role of KIT mutations and chromosomal rearrangements in the pathogenesis of GIST/GIPACTs are more complex than previously recognized.     

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.     

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.     

Mutations in exons 9 and 13 of KIT gene are rare events in gastrointestinal stromal tumors. A study of 200 cases

Gastrointestinal stromal tumors (GISTs), the most common mesenchymal tumors of the gastrointestinal tract, typically express the KIT protein. Activating mutations in the juxtamembrane domain (exon 11) of the c-kit gene have been shown in a subset of GISTs. These mutations lead into ligand-independent activation of the tyrosine kinase of c-kit, and have a transforming effect in vitro. Several groups have studied the clinical implication of the c-kit mutation status of exon 11 in GISTs and a possible relationship between c-kit mutations and malignant behavior has been established. Recently, a 1530ins6 mutation in exon 9 and missense mutations, 1945A>G in exon 13 of the c-kit gene were reported. The frequency and clinical importance of these findings are unknown. In this study we evaluated 200 GISTs for the presence of mutations in exons 9 and 13 of c-kit. Six cases revealed 1530ins6 mutation in exon 9 and two cases 1945A>G mutation in exon 13. All tumors with mutations in exon 9 and 13 lacked mutations in exon 11 of c-kit. None of the analyzed tumors had more than one type of c-kit mutation. All but one of the eight tumors with mutations in exon 9 or 13 of the c-kit gene were histologically and clinically malignant. All four of six cases with exon 9 mutation of which location of primary tumor was known, were small intestinal, suggesting that this type of mutation could preferentially occur in small intestinal tumors. Exon 9 and 13 mutations seem to be rare, and they cover only a small portion (8%) of the balance of GISTs that do not have mutations in exon 11 of c-kit. This finding indicates that other genetic alterations may activate c-kit in GISTs, or that KIT is not activated by mutations in all cases.     

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.     

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/     

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.     

Prognostic significance of angiogenesis in gastrointestinal stromal tumor.

Angiogenesis is important in the growth and metastasis of various kinds of solid tumors. To investigate the potential role of angiogenesis in gastrointestinal stromal tumor (GIST), an immunohistochemical analysis was performed in 95 cases of GISTs for microvessel density (MVD) and vascular endothelial growth factor (VEGF) expression. MVD was evaluated with immunohistochemical staining for CD31. A high level of MVD was significantly correlated with overexpression of VEGF, tumor location (intestine>stomach), tumor size (> or =5 cm), tumor grade (high>intermediate>low grade) (P=<0.0001, 0.0422, 0.0006, 0.0359, respectively). Of the 70 GISTs analyzed, KIT exon 11 mutations were detected in 45 cases (64.3%) and KIT exon 9 mutations in two cases (2.9%). No mutations were found in KIT exons 13 and 17, and platelet-derived growth factor receptor-alpha exons 12 and 18. Interestingly, VEGF expression level was significantly higher in the non-KIT exon 11 mutant group than in the KIT exon 11 mutant group (P=0.0266). In univariate analysis, tumor grade (high grade), tumor size (> or =5 cm), mitotic count (> or =5/50 high-power fields), Ki-67 labeling index (> or =4.6%), MVD (> or =7.0/0.95 mm(2)) and VEGF expression (high) were significantly associated with a shorter period of disease-free survival (P=<0.0001, 0.0199, 0.0055 0.0027, 0.0028 and 0.0302, respectively). In multivariate analysis, tumor grade and MVD were identified as independent worse prognostic factors (P=0.0007, 0.0152, respectively). In conclusion, our results suggest that the evaluation of MVD and VEGF expression is useful for predicting the aggressive biologic behavior of GIST, and that angiogenesis associated with VEGF may play an important role, at least in part, in the progression of GIST.     

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.     

Differential expression of KIT/PDGFRA mutant isoforms in epithelioid and mixed variants of gastrointestinal stromal tumors depends predominantly on the tumor site.

Gastrointestinal stromal tumors (GISTs) form a distinctive group of mesenchymal neoplasms, showing differentiation towards the interstitial cells of Cajal. Morphologically, GISTs vary from cellular spindle cell tumors to epithelioid or mixed, epithelioid and spindle cell variants. The genotypic features underlying the morphologic differences of GISTs with vs without epithelioid components are not well defined. Acquisition of activating mutations in KIT and PDGFRA has been reported as alternative oncogenic events in the pathogenesis of GISTs. In this study, a comprehensive KIT and PDGFRA mutational analysis was performed in a group of 28 epithelioid/mixed type tumors, in order to explore whether a specific KIT/PDGFRA mutational status segregates these neoplasms from spindle cell variant GISTs. All GISTs were primary neoplasms, 16 (57.1%) originated from the stomach and 12 (42.8%) from other locations. Histomorphologically, 14 GISTs showed an epithelioid and 14 a mixed cell type pattern. Mutational analysis included KIT exons 9, 11, 13, and 17, and PDGFRA exons 12 and 18 prescreening by denaturing high-performance liquid chromatography, followed by direct sequencing. Activating mutations of KIT were found in 14 (50%) GISTs, the majority being within exon 11 (n=11; 39.2%), and the other comprised exon 9 AY 502-503 duplications (n=2; 7.2%) and exon 17 Lys --> Aln822 missense mutations (n=1; 3.6%). Most of the KIT mutant tumors (n=11; 78.6%) originated from nongastric sites. Seven (25.0%) GISTs with no detectable KIT mutations demonstrated PDGFRA mutant isoforms, carrying either D842 V mutations (n=5) or exon 18 deletions (n=2). All GISTs harboring PDGFRA mutant isoforms originated from the stomach. In seven tumors, no detectable mutations were found; all but one of nonmutant tumors initiated from the stomach and exhibited an epithelioid morphology. These findings indicate that the mutational status of epithelioid/mixed GISTs associates with the anatomical site of the tumor.     

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.     

Genetic and pathologic characteristics of gastrointestinal stromal tumors in extragastric lesions

The goal of this study was to investigate differences in the clinicopathologic and genetic characteristics of gastric and extragastric gastrointestinal stromal tumors (GISTs). We evaluated 13 extragastric GISTs and compared them with 56 gastric GISTs, which were described previously. DNA was extracted from paraffin-embedded tumor specimens, and exons 9, 11, 13, and 17 of the KIT gene and exons 12 and 18 of the platelet-derived growth factor receptor alpha (PDGFRA) gene were amplified by polymerase chain reaction and sequenced. Immunohistochemistry was performed for KIT, CD34, Ki-67 (as a marker of cell proliferation), and CD31 (as a marker of microvessel density), and apoptosis was assessed by in situ DNA nick end-labeling. Of the 13 extragastric GISTs 7 (54%) had a mutation in exon 11 of KIT, and 2 (15%) had a mutation in exon 13 of KIT. Deletions in exon 11 of KIT were the most common mutation encountered in the extragastric GISTs. The extragastric GISTs, especially small intestinal GISTs, showed larger deletions, leading to deletions of amino acid residues in the KIT protein, and higher vascularity than did the gastric GISTs. These data suggest that extragastric GISTs differ from gastric GISTs with respect to associated mutations and angiogenic activity.     

A great majority of GISTs with PDGFRA mutations represent gastric tumors of low or no malignant potential

Gastrointestinal stromal tumors (GISTs) are KIT expressing spindle cell, epithelioid and rarely pleomorphic mesenchymal tumors. The majority of GISTs show gain-of-function KIT mutations. However, GISTs without KIT mutations and GISTs with weak or lack of immunohistochemical KIT expression have also been reported. Recently, gain-of-function mutations in exon 18 (activation loop) and exon 12 (juxtamembrane domain) of the PDGFRA were identified in such tumors. The purpose of this study was to test the hypothesis that PDGFRA mutation may define a specific clinicopathologic subgroup of GISTs. A total of 447 KIT exon 11 (juxtamembrane domain) mutation-negative GISTs were studied. DNA samples were obtained from formaldehyde-fixed paraffin-embedded tissues. Genomic sequences of PDGFRA exons 18 and 12 were evaluated for the mutations by PCR amplification and direct sequencing. PDGFRA exon 18 mutations were identified in 122 of 346 (35.3%) gastric GISTs and two of 75 (2.7%) intestinal GISTs. A great majority of these mutations represented simple T to A missense mutation at the codon 842 leading to substitution of the valine for aspartic acid (D842 V). However, in-frame deletions and deletions with point mutations clustering between codons 841-847 were found in approximately 23% of all exon 18 mutations. Mutations in PDGFRA exon 12 were found only in 10 of 170 (5.8%) gastric and one of 54 (1.9%) intestinal GISTs negative for KIT exon 11 and PDGFRA exon 18 mutations. There were seven substitutions of aspartic acid for valine at codon 561 (V561D) and four in-frame deletions with point mutations clustering between codons 566 and 571. The majority of GISTs with PDGFRA mutations had pure or predominant epithelioid morphology. Low mitotic activity, < or =5 mitoses/50HPF was detected in 81% of analyzed GISTs including larger, >5 cm tumors. Based on long-term follow-up (average 135 months), a majority (83.5%) of GISTs with PDGFRA mutations followed a benign course.     

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.