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

Characterization of a Human Gastrointestinal Stromal Tumor Cell Line Established by SV40LT-Mediated Immortalization

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors in the digestive tract and originate from the interstitial cells of Cajal (ICC), which is the pacemaker for peristaltic movement in the gastrointestinal tract. Existing GIST cell lines are widely used as cell models for in vitro experimental studies because the mutation sites are known. However, the immortalization methods of these cell lines are unknown, and no Chinese patient-derived GIST cell lines have been documented. Here, we transfected simian virus 40 large T antigen (SV40LT) into primary GIST cells to establish an immortalized human GIST cell line (ImGIST) for the first time. The ImGIST cells had neuronal cell-like irregular radioactive growth and retained the fusion growth characteristics of GIST cells. They stably expressed signature proteins, maintained the biological and genomic characteristics of normal primary GIST cells, and responded well to imatinib, suggesting that ImGIST could be a potential in vitro model for research in GIST to explore the molecular pathogenesis, drug resistance mechanisms, and the development of new adjuvant therapeutic options.

Characterization of cell line with dedifferentiated GIST-like features established from cecal GIST of familial GIST model mice

Approximately 40 families with multiple gastrointestinal stromal tumors (GISTs) and germline c-kit gene mutations have been reported. Three knock-in mouse models have been generated, and all the models showed a cecal GIST. In the present study, we established a cell line derived from cecal GIST in a familial GIST model mouse with KIT-Asp818Tyr. Since the established cells showed spindle-shaped morphology with atypical nuclei, and since immunohistochemistry revealed that they were positive for α-SMA but negative for KIT, CD34 and desmin, the phenotypes of the cells were reminiscent of dedifferentiated GIST-like ones but not the usual GIST-like ones. Gene expression analysis showed that the cell line, designated as DeGISTL1 cell, did not express c-kit gene apparently, but highly expressed HSP90 families and glutaminase 1. Pathway analysis of the cells revealed that metabolic pathway might promote their survival and growth. Pimitespib, a heat shock protein 90α/β inhibitor, and Telaglenastat, a selective glutaminase 1 inhibitor, inhibited proliferation of DeGISTL1 cells and the combination of these showed an additive effect. DeGISTL1 cells might be a good model of dedifferentiated GISTs, and combination of Pimitespib and Telaglenastat could be a possible candidate for treatment strategy for them.

Advances in immunology and immunotherapy for mesenchymal gastrointestinal cancers

Mesenchymal gastrointestinal cancers are represented by the gastrointestinal stromal tumors (GISTs) which occur throughout the whole gastrointestinal tract, and affect human health and economy globally. Curative surgical resections and tyrosine kinase inhibitors (TKIs) are the main managements for localized GISTs and recurrent/metastatic GISTs, respectively. Despite multi-lines of TKIs treatments prolonged the survival time of recurrent/metastatic GISTs by delaying the relapse and metastasis of the tumor, drug resistance developed quickly and inevitably, and became the huge obstacle for stopping disease progression. Immunotherapy, which is typically represented by immune checkpoint inhibitors (ICIs), has achieved great success in several solid tumors by reactivating the host immune system, and been proposed as an alternative choice for GIST treatment. Substantial efforts have been devoted to the research of immunology and immunotherapy for GIST, and great achievements have been made. Generally, the intratumoral immune cell level and the immune-related gene expressions are influenced by metastasis status, anatomical locations, driver gene mutations of the tumor, and modulated by imatinib therapy. Systemic inflammatory biomarkers are regarded as prognostic indicators of GIST and closely associated with its clinicopathological features. The efficacy of immunotherapy strategies for GIST has been widely explored in pre-clinical cell and mouse models and clinical experiments in human, and some patients did benefit from ICIs. This review comprehensively summarizes the up-to-date advancements of immunology, immunotherapy and research models for GIST, and provides new insights and perspectives for future studies.

New C3H KitN824K/WT cancer mouse model develops late-onset malignant mammary tumors with high penetrance

Gastro-intestinal stromal tumors and acute myeloid leukemia induced by activating stem cell factor receptor tyrosine kinase (KIT) mutations are highly malignant. Less clear is the role of KIT mutations in the context of breast cancer. Treatment success of KIT-induced cancers is still unsatisfactory because of primary or secondary resistance to therapy. Mouse models offer essential platforms for studies on molecular disease mechanisms in basic cancer research. In the course of the Munich N-ethyl-N-nitrosourea (ENU) mutagenesis program a mouse line with inherited polycythemia was established. It carries a base-pair exchange in the Kit gene leading to an amino acid exchange at position 824 in the activation loop of KIT. This KIT variant corresponds to the N822K mutation found in human cancers, which is associated with imatinib-resistance. C3H Kit^N824K/WT mice develop hyperplasia of interstitial cells of Cajal and retention of ingesta in the cecum. In contrast to previous Kit-mutant models, we observe a benign course of gastrointestinal pathology associated with prolonged survival. Female mutants develop mammary carcinomas at late onset and subsequent lung metastasis. The disease model complements existing oncology research platforms. It allows for addressing the role of KIT mutations in breast cancer and identifying genetic and environmental modifiers of disease progression.

Monitoring longitudinal disease progression in a novel murine Kit tumor model using high-field MRI

Animal models are an indispensable platform used in various research disciplines, enabling, for example, studies of basic biological mechanisms, pathological processes and new therapeutic interventions. In this study, we applied magnetic resonance imaging (MRI) to characterize the clinical picture of a novel N-ethyl-N-nitrosourea-induced Kit-mutant mouse in vivo. Seven C3H KitN824K/WT mutant animals each of both sexes and their littermates were monitored every other month for a period of twelve months. MRI relaxometry data of hematopoietic bone marrow and splenic tissue as well as high-resolution images of the gastrointestinal organs were acquired. Compared with controls, the mutants showed a dynamic change in the shape and volume of the cecum and enlarged Peyer´s patches were identified throughout the entire study. Mammary tumors were observed in the majority of mutant females and were first detected at eight months of age. Using relaxation measurements, a substantial decrease in longitudinal relaxation times in hematopoietic tissue was detected in mutants at one year of age. In contrast, transverse relaxation time of splenic tissue showed no differences between genotypes, except in two mutant mice, one of which had leukemia and the other hemangioma. In this study, in vivo MRI was used for the first time to thoroughly characterize the evolution of systemic manifestations of a novel Kit-induced tumor model and to document the observable organ-specific disease cascade.

Pimitespib is effective on cecal GIST in a mouse model of familial GISTs with KIT-Asp820Tyr mutation through KIT signaling inhibition

Three families with multiple gastrointestinal stromal tumors (GISTs) caused by a germline Asp820Tyr mutation at exon 17 of the c-kit gene (KIT-Asp820Tyr) have been reported. We previously generated a knock-in mouse model of the family, and the mice with KIT-Asp818Tyr corresponding to human KIT-Asp820Tyr showed a cecal tumor equivalent to human GIST. In the model mice, we reported that tyrosine kinase inhibitor, imatinib, could stabilize but not decrease the cecal tumor volume. In this report, we examined whether a heat shock protein 90 inhibitor, pimitespib (TAS-116), has an inhibitory effect on phosphorylation of KIT-Asp818Tyr and can decrease the cecal tumor volume in the model mice. First, we showed that pimitespib inhibited KIT phosphorylation both dose- and time-dependently in KIT-Asp818Tyr transfected murine Ba/F3 cells. Then, four 1-week courses of pimitespib were orally administered to heterozygous (KIT-Asp818Tyr/+) model mice. Each course consisted of once-daily administration for consecutive 5 days followed by 2 days-off. Cecal tumors were dissected, and tumor volume was histologically analyzed, Ki-67 labeling index was immunohistochemically examined, and apoptotic figures were counted. Compared to the vehicle treated mice, pimitespib administered mice showed statistically significantly smaller cecal tumor volume, lower Ki-67 labeling index, and higher number of apoptotic figures in 10 high power fields (P = 0.0344, P = 0.0019 and P = 0.0269, respectively). Western blotting revealed that activation of KIT signaling molecules was strongly inhibited in the tumor tissues of pimitespib-administered mice compared to control mice. Thus, pimitespib seemed to inhibit in vivo tumor progression effectively in the model mice. These results suggest that the progression of multiple GISTs in patients with germline KIT-Asp820Tyr might be controllable by pimitespib.

Understanding the Biology of Human Interstitial Cells of Cajal in Gastrointestinal Motility

Millions of patients worldwide suffer from gastrointestinal (GI) motility disorders such as gastroparesis. These disorders typically include debilitating symptoms, such as chronic nausea and vomiting. As no cures are currently available, clinical care is limited to symptom management, while the underlying causes of impaired GI motility remain unaddressed. The efficient movement of contents through the GI tract is facilitated by peristalsis. These rhythmic slow waves of GI muscle contraction are mediated by several cell types, including smooth muscle cells, enteric neurons, telocytes, and specialised gut pacemaker cells called interstitial cells of Cajal (ICC). As ICC dysfunction or loss has been implicated in several GI motility disorders, ICC represent a potentially valuable therapeutic target. Due to their availability, murine ICC have been extensively studied at the molecular level using both normal and diseased GI tissue. In contrast, relatively little is known about the biology of human ICC or their involvement in GI disease pathogenesis. Here, we demonstrate human gastric tissue as a source of primary human cells with ICC phenotype. Further characterisation of these cells will provide new insights into human GI biology, with the potential for developing novel therapies to address the fundamental causes of GI dysmotility.

Constitutive STAT5 activation regulates Paneth and Paneth-like cells to control Clostridium difficile colitis

Clostridium difficile impairs Paneth cells, driving intestinal inflammation that exaggerates colitis. Besides secreting bactericidal products to restrain C. difficile, Paneth cells act as guardians that constitute a niche for intestinal epithelial stem cell (IESC) regeneration. However, how IESCs are sustained to specify Paneth-like cells as their niche remains unclear. Cytokine-JAK-STATs are required for IESC regeneration. We investigated how constitutive STAT5 activation (Ca-pYSTAT5) restricts IESC differentiation towards niche cells to restrain C. difficile infection. We generated inducible transgenic mice and organoids to determine the effects of Ca-pYSTAT5-induced IESC lineages on C. difficile colitis. We found that STAT5 absence reduced Paneth cells and predisposed mice to C. difficile ileocolitis. In contrast, Ca-pYSTAT5 enhanced Paneth cell lineage tracing and restricted Lgr5 IESC differentiation towards pYSTAT5+Lgr5-CD24+Lyso+ or cKit+ niche cells, which imprinted Lgr5hiKi67+ IESCs. Mechanistically, pYSTAT5 activated Wnt/β-catenin signaling to determine Paneth cell fate. In conclusion, Ca-pYSTAT5 gradients control niche differentiation. Lack of pYSTAT5 reduces the niche cells to sustain IESC regeneration and induces C. difficile ileocolitis. STAT5 may be a transcription factor that regulates Paneth cells to maintain niche regeneration.

LMTK3 is essential for oncogenic KIT expression in KIT-mutant GIST and melanoma

Certain cancers, including gastrointestinal stromal tumor (GIST) and subsets of melanoma, are caused by somatic KIT mutations that result in KIT receptor tyrosine kinase constitutive activity, which drives proliferation. The treatment of KIT-mutant GIST has been revolutionized with the advent of KIT-directed cancer therapies. KIT tyrosine kinase inhibitors (TKI) are superior to conventional chemotherapy in their ability to control advanced KIT-mutant disease. However, these therapies have a limited duration of activity due to drug-resistant secondary KIT mutations that arise (or that are selected for) during KIT TKI treatment. To overcome the problem of KIT TKI resistance, we sought to identify novel therapeutic targets in KIT-mutant GIST and melanoma cells using a human tyrosine kinome siRNA screen. From this screen, we identified lemur tyrosine kinase 3 (LMTK3) and herein describe its role as a novel KIT regulator in KIT-mutant GIST and melanoma cells. We find that LMTK3 regulated the translation rate of KIT, such that loss of LMTK3 reduced total KIT, and thus KIT downstream signaling in cancer cells. Silencing of LMTK3 decreased cell viability and increased cell death in KIT-dependent, but not KIT-independent GIST and melanoma cell lines. Notably, LMTK3 silencing reduced viability of all KIT-mutant cell lines tested, even those with drug-resistant KIT secondary mutations. Furthermore, targeting of LMTK3 with siRNA delayed KIT-dependent GIST growth in a xenograft model. Our data suggest the potential of LMTK3 as a target for treatment of patients with KIT-mutant cancer, particularly after failure of KIT TKIs.

Surgical Pathology of Gastrointestinal Stromal Tumors: Practical Implications of Morphologic and Molecular Heterogeneity for Precision Medicine

Gastrointestinal stromal tumor (GIST), the most common mesenchymal neoplasm of the gastrointestinal tract, exhibits diverse histologic and clinical manifestations. With its putative origin in the gastrointestinal pacemaker cell of Cajal, GIST can arise in association with any portion of the tubular gastrointestinal tract. Morphologically, GISTs are classified as spindled or epithelioid, though each of these subtypes encompasses a broad spectrum of microscopic appearances, many of which mimic other histologic entities. Despite this morphologic ambiguity, the diagnosis of GIST is aided in many cases by immunohistochemical detection of KIT (CD117) or DOG1 expression. The natural history of GIST ranges from that of a tumor cured by surgical resection to that of a locally advanced or even widely metastatic, and ultimately fatal, disease. This clinicopathologic heterogeneity is paralleled by an underlying molecular diversity: the majority of GISTs are associated with spontaneous activating mutations in KIT, PDGFRA, or BRAF, while additional subsets are driven by genetic lesions-often inherited-of NF1 or components of the succinate dehydrogenase enzymatic complex. Specific gene mutations correlate with particular anatomic or morphologic characteristics and, in turn, with distinct clinical behaviors. Therefore, prognostication and treatment are increasingly dictated not only by morphologic clues, but also by accompanying molecular genetic features. In this review, we provide a comprehensive description of the heterogenous molecular underpinnings of GIST, including implications for the practicing pathologist with regard to morphologic identification, immunohistochemical diagnosis, and clinical management.

ETV1-Positive Cells Give Rise to BRAFV600E -Mutant Gastrointestinal Stromal Tumors

Gastrointestinal stromal tumor (GIST) is the most common subtype of sarcoma. Despite clinical advances in the treatment of KIT/PDGFRA-mutant GIST, similar progress against KIT/PDGFRA wild-type GIST, including mutant BRAF-driven tumors, has been limited by a lack of model systems. ETV1 is a master regulator in the intestinal cells of Cajal (ICC), thought to be the cells of origin of GIST. Here, we present a model in which the ETV1 promoter is used to specifically and inducibly drive Cre recombinase in ICC as a strategy to study GIST pathogenesis. Using a conditional allele for Braf^V600E , a mutation observed in clinical cases of GIST, we observed that Braf^V600E activation was sufficient to drive ICC hyperplasia but not GIST tumorigenesis. In contrast, combining Braf^V600E activation with Trp53 loss was sufficient to drive both ICC hyperplasia and formation of multifocal GIST-like tumors in the mouse gastrointestinal tract with 100% penetrance. This mouse model of sporadic GIST model was amenable to therapeutic intervention, and it recapitulated clinical responses to RAF inhibition seen in human GIST. Our work offers a useful in vivo model of human sporadic forms of BRAF-mutant GIST to help unravel its pathogenesis and therapeutic response to novel experimental agents. Cancer Res; 77(14); 3758-65. ©2017 AACR.

Germline mutations of KIT in gastrointestinal stromal tumor (GIST) and mastocytosis

Somatic mutations of KIT are frequently found in mastocytosis and gastrointestinal stromal tumor (GIST), while germline mutations of KIT are rare, and only found in few cases of familial GIST and mastocytosis. Although ligand-independent activation is the common feature of KIT mutations, the phenotypes mediated by various germline KIT mutations are different. Germline KIT mutations affect different tissues such as interstitial cells of Cajal (ICC), mast cells or melanocytes, and thereby lead to GIST, mastocytosis, or abnormal pigmentation. In this review, we summarize germline KIT mutations in familial mastocytosis and GIST and discuss the possible cellular context dependent transforming activity of KIT mutations.

Nonproliferative and Proliferative Lesions of the Gastrointestinal Tract, Pancreas and Salivary Glands of the Rat and Mouse

The INHAND (International Harmonization of Nomenclature and Diagnostic Criteria for Lesions in Rats and Mice) project is a joint initiative of the Societies of Toxicologic Pathology from Europe (ESTP), Great Britain (BSTP), Japan (JSTP), and North America (STP) to develop an internationally accepted nomenclature and diagnostic criteria for nonproliferative and proliferative lesions in laboratory animals. The purpose of this publication is to provide a standardized nomenclature and diagnostic criteria for classifying lesions in the digestive system including the salivary glands and the exocrine pancreas of laboratory rats and mice. Most lesions are illustrated by color photomicrographs. The standardized nomenclature, the diagnostic criteria, and the photomicrographs are also available electronically on the Internet (http://www.goreni.org/). Sources of material included histopathology databases from government, academia, and industrial laboratories throughout the world. Content includes spontaneous and age related lesions as well as lesions induced by exposure to test items. Relevant infectious and parasitic lesions are included as well. A widely accepted and utilized international harmonization of nomenclature and diagnostic criteria for the digestive system will decrease misunderstandings among regulatory and scientific research organizations in different countries and provide a common language to increase and enrich international exchanges of information among toxicologists and pathologists.

KIT Mutation and Loss of 14q May Be Sufficient for the Development of Clinically Symptomatic Very Low-Risk GIST

The aim of this study was to determine the minimal set of genetic alterations required for the development of a very low risk clinically symptomatic gastro-intestinal stromal tumour within the stomach wall. We studied the genome of a very low-risk gastric gastro-intestinal stromal tumour by whole-genome sequencing, comparative genomic hybridisation and methylation profiling. The studied tumour harboured two typical genomic lesions: loss of the long arm of chromosome 14 and an activating mutation in exon 11 of KIT. Besides these genetic lesions, only two point mutations that may affect tumour progression were identified: A frame-shift deletion in RNF146 and a missense mutation in a zinc finger of ZNF407. Whilst the frameshift deletion in RNF146 seemed to be restricted to this particular tumour, a similar yet germline mutation in ZNF407 was found in a panel of 52 gastro-intestinal stromal tumours from different anatomical sites and different categories. Germline polymorphisms in the mitotic checkpoint proteins Aurora kinase A and BUB1 kinase B may have furthered tumour growth. The epigenetic profile of the tumour matches that of other KIT-mutant tumours. We have identified mutations in three genes and loss of the long arm of chromosome 14 as the so far minimal set of genetic abnormalities sufficient for the development of a very low risk clinically symptomatic gastric stromal tumour.

Submucosal tumors: comprehensive guide for the diagnosis and therapy of gastrointestinal submucosal tumors

Small submucosal tumors (SMT) without symptoms are frequently found by endoscopic and radiological examinations. To find proper diagnostic measures and therapeutic indications for histologically undiagnosed SMT, we reviewed published articles in PubMed between 1990 and March 2013 using the key words 'submucosal tumor' and the name of a specific disease. SMT is observed in a wide range of gastrointestinal (GI) diseases and conditions, including compression by extra-GI organs and lesions, congenital tumors, inflammation, and benign as well as malignant neoplastic lesions. In the diagnosis of diseases and decision-making for therapy, endoscopic ultrasonography (EUS) and endoscopic ultrasound-guided fine-needle aspiration (EUS-FNA) may play a key role. Symptomatic SMT and SMT histologically diagnosed as malignant or potentially malignant tumors such as gastrointestinal stromal tumor (GIST) should be treated by surgery. SMT >5 cm, SMT increasing in size and those with'high-risk features' including irregular border, heterogeneous internal echo such as anechoic area, and heterogeneous enhancement by contrast media may also be removed by surgery. Laparoscopic approach is feasible for gastric GIST <5 cm and this is considered less invasive than the open approach. Emerging techniques using flexible endoscopes appear less invasive, but require further evidence and are still under clinical study. Correct diagnosis of SMT is challenging; however, EUS and EUS-FNA are useful in the histological diagnosis and clinical decision-making. In the future, minimally invasive approaches may be a mainstream of surgical treatment for small SMT.

Effects of endoplasmic reticulum stressors on maturation and signaling of hemizygous and heterozygous wild-type and mutant forms of KIT

Gain of function mutations of KIT are frequent in some human tumors, and are sensible to tyrosine kinase inhibitors. In most tumors, oncogenic mutations are heterozygous, however most in vitro data of KIT activation have been obtained with hemizygous mutation. This study aimed to investigate the maturation and activation of wild-type (WT) and mutant (M) forms of KIT in hemizygous and heterozygous conditions. WT and two types of exon 11 deletions M forms of human KIT were expressed in NIH3T3 cell lines. Membrane expression of KIT was quantified by flow cytometry. Quantification of glycosylated forms of KIT and phosphorylated forms of AKT and ERK were performed by western blot. Simultaneous activation of WT KIT and treatment with endoplasmic reticulum (ER) inhibitors, tunicamycin or brefeldin A induced a complete inhibition of membrane expression of the 145 kDa form of KIT. By contrast activation or ER inhibitors alone, only partly inhibited this form. ER inhibitors also inhibited KIT activation-dependent phosphorylation of AKT and ERK1/2. Brefeldin A induced a complete down regulation of the 145 kDa form in hemizygous M, and induced an intra-cellular accumulation of the 125 kDa form in WT but not in hemizygous M. Heterozygous cells had glycosylation and response to ER inhibitors patterns more similar to WT than to hemizygous M. Phosphorylated AKT was reduced in hemizygous cells in comparison to WT KIT cells and heterozygous cells, and in the presence of brefeldin A in all cell lines. Effects of ER inhibitors are significantly different in hemizygous and heterozygous mutants. Differences in intra-cellular trafficking of KIT forms result in differences in downstream signaling pathways, and activation of PI3K/AKT pathway appears to be tied to the presence of the mature 145 kDa form of KIT at the membrane surface.

Inherited gastrointestinal stromal tumor syndromes: mutations, clinical features, and therapeutic implications

The discovery of underlying molecular genetic abnormalities in gastrointestinal stromal tumors (GISTs) such as activating mutations in the tyrosine kinase genes, KIT and platelet derived growth factor receptor-alpha (PDGFRA), has led to remarkable clinical advances in treatment. Small molecule inhibitors such as imatinib and sunitinib are known to inhibit the aberrantly activated KIT and PDGFRA receptor signaling and can lead to excellent clinical outcomes for patients with GIST. Though the majority of GISTs appear to arise sporadically, a number of families with high frequencies of GISTs have been reported and germline mutations have been identified. This review will highlight the various inherited mutations associated with familial GIST syndromes and describe how an improved understanding of these genetic syndromes has important clinical implications for future understanding of this heterogeneous disease.

Histopathology of gastrointestinal stromal tumor

Gastrointestinal stromal tumor (GIST), generally driven by oncogenic KIT or PDGFRA mutations, is the most common mesenchymal tumor of the gastrointestinal (GI) tract. GIST is most common in the stomach (60%) and small intestine (30%), but can occur anywhere in the GI-tract and the intra-abdominal soft tissues. GIST can show spindle cell or epithelioid morphology, and mitotic count and tumor size are most important prognostic parameters. GISTs in NF1 patients and children are distinctive clinicopathologic groups. Immunohistochemical testing for KIT and sometimes for DOG1/Ano 1 is essential in confirming the diagnosis.

Morphological and Immunohistochemical Characterization of Sarcomatous Tumors in Wild-Type and Genetically Engineered Mice

Malignant soft tissue tumors are commonly observed in wild-type and gene-targeted mice. These tumors have different degrees of differentiation, cellularity, cellular atypia, nuclear pleomorphism, normal and abnormal mitosis, and giant tumor cells with enlarged polylobulated nuclei. They are often diagnosed as pleomorphic sarcoma, undifferentiated sarcoma, fibrosarcoma, malignant fibrous histiocytoma, sarcoma, or sarcoma, not otherwise specified. Pleomorphic sarcomas have no morphological differentiation toward a differentiated mesenchymal or other tumor type in hematoxylin and eosin–stained sections. With the use of immunohistochemistry, human and mouse, tumors associated with these broad nonspecific diagnoses can often be demonstrated to be of a specific cellular lineage. With mouse models being used to delineate the molecular mechanisms, pathogenesis, and cellular origin of human sarcomas, it will be necessary to correlate the morphological and cellular lineage and the molecular profiles of the pleomorphic tumors associated with these mouse models. The results presented here show that with the use of immunohistochemistry, the cellular lineage of many mouse tumors with pleomorphic features can be determined.

Impact of gene dosage, loss of wild-type allele, and FLT3 ligand on Flt3-ITD-induced myeloproliferation

Acquisition of homozygous activating growth factor receptor mutations might accelerate cancer progression through a simple gene-dosage effect. Internal tandem duplications (ITDs) of FLT3 occur in approximately 25% cases of acute myeloid leukemia and induce ligand-independent constitutive signaling. Homozygous FLT3-ITDs confer an adverse prognosis and are frequently detected at relapse. Using a mouse knockin model of Flt3-internal tandem duplication (Flt3-ITD)-induced myeloproliferation, we herein demonstrate that the enhanced myeloid phenotype and expansion of granulocyte-monocyte and primitive Lin(-)Sca1(+)c-Kit(+) progenitors in Flt3-ITD homozygous mice can in part be mediated through the loss of the second wild-type allele. Further, whereas autocrine FLT3 ligand production has been implicated in FLT3-ITD myeloid malignancies and resistance to FLT3 inhibitors, we demonstrate here that the mouse Flt3(ITD/ITD) myeloid phenotype is FLT3 ligand-independent.

Abnormal elevated PTEN expression in the mouse antrum of a model of GIST Kit(K641E/K641E)

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors of the gastrointestinal tract. Approximately 85% of GISTs harbor activating mutations of the KIT or PDGFRA receptor tyrosine kinases. PTEN and SHIP2 are major phosphatases that dephosphorylate PI(3,4,5)P(3), one of the intracellular signal pathways downstream of KIT. PTEN is an important tumor suppressor, whereas the involvement of SHIP2 in cancer has been proposed based essentially on cell line studies. We have used a mouse model of GIST, i.e. Kit(K641E) knock-in mice, resulting in the substitution of a Lys by Glu at position 641 of Kit. In homozygous Kit(K641E) mice, PTEN-immunoreactivity (ir) in antrum was found in the hyperplastic Kit-ir layer. The same localization was found for SHIP2. Western blot analysis in antrum showed a large increase in PTEN expression in Kit(K641E) homozygous mice as compared to wild type. In contrast, SHIP2 expression was not affected between the two genotypes. Erk1, but not PKB, phosphorylation appears to be upregulated in Kit(K641E) homozygous mice. In the human GIST882 imatinib sensitive cell line, both PTEN and SHIP2 were expressed and showed, in part, a nuclear localization. The upregulation of PTEN in antrum in Kit(K641E) mice might serve as a feedback mechanism to limit PI 3-kinase activation downstream of Kit in a context of oncogenic mutation.

Preclinical evaluation of KIT/PDGFRA and mTOR inhibitors in gastrointestinal stromal tumors using small animal FDG PET

Background

Primary and secondary drug resistance to imatinib and sunitinib in patients with gastrointestinal stromal tumors (GISTs) has led to a pressing need for new therapeutic strategies such as drug combinations. Most GISTs are caused by mutations in the KIT receptor, leading to upregulated KIT tyrosine kinase activity. Imatinib and nilotinib directly inhibit the kinase activity of KIT, while RAD001 (everolimus) inhibits mTOR. We report a preclinical study on drug combinations in a xenograft model of GIST in which effects on tumor dimensions and metabolic activity were assessed by small animal PET imaging.

Methods

Rag2-/-; γcommon -/- male mice were injected s.c. into the right leg with GIST 882. The animals were randomized into 6 groups of 6 animals each for different treatment regimens: No therapy (control), imatinib (150 mg/kg b.i.d.) by oral gavage for 6 days, then once/day for another 7 days, everolimus (10 mg/kg/d.) by oral gavage, everolimus (10 mg/kg/d.) + imatinib (150 mg/kg b.i.d.) by oral gavage for 6 days, then once/day for another 7 days, nilotinib (75 mg/kg/d.) by oral gavage, nilotinib (75 mg/kg/d.) + imatinib (150 mg/kg b.i.d) by oral gavage for 6 days, then once/day for another 7 days. Tumor growth control was evaluated by measuring tumor volume (cm³). Small animal PET (GE Explore tomography) was used to evaluate tumor metabolism and performed in one animal per group at base-line then after 4 and 13 days of treatment.

Results

After a median latency time of 31 days, tumors grew in all animals (volume 0,06-0,15 cm³) and the treatments began at day 38 after cell injection. Tumor volume control (cm3) after 13 days of treatment was > 0.5 for imatinib alone and nilotinib alone, and < 0.5 for the 2 combinations of drugs and for everolimus alone. The baseline FDG uptake was positive in all animals. FDG/SUV/TBR was strongly reduced over time by everolimus both as a single agent and in combination with imatinib respectively: 3.1 vs. 2.3 vs. 1.9 and 2.5 vs 2.3 vs 0.

Conclusions

As single agents, all drugs showed an anti-tumor effect in GIST xenografts but everolimus was superior. The everolimus plus imatinib combination appeared to be the most active regimen both in terms of inhibiting tumor growth and tumor metabolism. The integration of everolimus in GIST treatment merits further investigation.

Diagnostic criteria, specific mutations, and genetic predisposition in gastrointestinal stromal tumors

In 1998, gastrointestinal stromal tumor (GIST) emerged as a distinct oncogenetic entity and subsequently became a paradigm of targeted therapies in solid tumors. Diagnosis of GIST relies on both histology and immunohistochemistry. Ninety-five percent of GISTs express either KIT or DOG-1. Approximately 80%-90% of GISTs harbor gain-of-function mutations of either KIT or platelet-derived growth factor receptor alpha polypeptide (PDGFRA) receptor tyrosine kinase (RTK). More than 100 different mutations have been described, some of which are associated with specific clinical and/or histological characteristics. Detection of KIT or PDGFRA mutations is recommended in advanced GISTs because they are highly predictive of tumor response to RTK inhibitors, as well as in KIT-negative cases to confirm diagnosis. In most cases, GISTs are sporadic, but in rare cases, they are related with genetic predisposition, such as neurofibromatosis type 1, Carney triad, Carney-Stratakis syndrome, and inherited KIT or PDGFRA germline mutations.

ETV1 is a lineage survival factor that cooperates with KIT in gastrointestinal stromal tumours

Gastrointestinal stromal tumour (GIST) is the most common human sarcoma and is primarily defined by activating mutations in the KIT or PDGFRA receptor tyrosine kinases. KIT is highly expressed in interstitial cells of Cajal (ICCs)-the presumed cell of origin for GIST-as well as in haematopoietic stem cells, melanocytes, mast cells and germ cells. Yet, families harbouring germline activating KIT mutations and mice with knock-in Kit mutations almost exclusively develop ICC hyperplasia and GIST, suggesting that the cellular context is important for KIT to mediate oncogenesis. Here we show that the ETS family member ETV1 is highly expressed in the subtypes of ICCs sensitive to oncogenic KIT mediated transformation, and is required for their development. In addition, ETV1 is universally highly expressed in GISTs and is required for growth of imatinib-sensitive and resistant GIST cell lines. Transcriptome profiling and global analyses of ETV1-binding sites suggest that ETV1 is a master regulator of an ICC-GIST-specific transcription network mainly through enhancer binding. The ETV1 transcriptional program is further regulated by activated KIT, which prolongs ETV1 protein stability and cooperates with ETV1 to promote tumorigenesis. We propose that GIST arises from ICCs with high levels of endogenous ETV1 expression that, when coupled with an activating KIT mutation, drives an oncogenic ETS transcriptional program. This differs from other ETS-dependent tumours such as prostate cancer, melanoma and Ewing sarcoma where genomic translocation or amplification drives aberrant ETS expression. It also represents a novel mechanism of oncogenic transcription factor activation.

Anti-inflammatory M2 type macrophages characterize metastasized and tyrosine kinase inhibitor-treated gastrointestinal stromal tumors

We have made a detailed inventory of the immune infiltrate of gastrointestinal stromal tumors (GISTs), which originate from mesenchymal cells in the intestinal tract. These sarcomas are heavily infiltrated with macrophages and T cells, while immune cells of other lineages were much less abundant. Dissecting the functional subtypes of T cells with multicolor fluorescent microscopy revealed substantial populations of cytotoxic T cells, helper T cells and FoxP3(+) regulatory T cells. The balance of cytotoxic T cells and FoxP3(+) T cells was toward immune suppression. Analysis of the macrophage population also showed a dominance of anti-inflammatory cells, as the M2 type scavenger receptor CD163 was abundantly present. Other subsets of macrophages (CD14(+)CD163(-)) were occasionally detected. M2 type CD163(+) macrophages were associated with the number of infiltrating FoxP3(+) regulatory T cells and twice as many macrophages were found in metastatic GIST compared to primary lesions. Most metastatic GISTs had been treated with the tyrosine kinase inhibitors imatinib and sunitinib, but the high macrophage infiltrate was not related to this treatment. However, imatinib and sunitinib did induce secretion of anti-inflammatory IL-10 in macrophage cultures, indicating that treatment with these inhibitors might contribute to an immune suppressive microenvironment in GIST. Overall, our data reveal a picture of GIST as an active site of tumor-immune interaction in which suppressive mechanisms overrule potential antitumor responses. Tyrosine kinase inhibitors might promote this negative balance.

Targeted therapies of gastrointestinal stromal tumors (GIST)--the next frontiers

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors of the gastrointestinal (GI) tract and are caused by activating KIT or PDGFRA mutations. GISTs can be successfully treated with the small molecule kinase inhibitor imatinib mesylate (Gleevec, Novartis) with response rates of up to 85%. However, complete responses are rare, and most patients will develop imatinib resistance over time. Recent results have shown that although imatinib effectively stimulates apoptotic cell death in sensitive GIST cells, a considerable proportion of cells does not undergo apoptosis, but instead enters a state of quiescence. Quiescence is characterized by a reversible withdrawal from the cell division cycle, during which the cells remain alive and metabolically active. It is conceivable that quiescence not only plays a pivotal role in the emergence of residual disease but also in creating a pool of tumor cells that survive continuous small molecule therapy and may hence represent the "seeds" for the outgrowth of resistant clones. This review will summarize the current knowledge about GIST biology and treatment response to imatinib including the induction of cellular quiescence in GIST. In addition, we will highlight future strategies to design more effective treatment options to overcome these problems with an aim towards cure of this hitherto untreatable tumor entity.

Gastrointestinal stromal tumor: a bridge between bench and bedside

Gastrointestinal stromal tumor (GIST) is considered to be driven by a gain-of-function mutation in the KIT or PDGFRA gene. Cure can be obtained only by complete surgical removal of the GIST; however, imatinib, an inhibitor of KIT and PDGFRA, is indicated for advanced, recurrent, and/or metastatic GISTs. Imatinib exhibited remarkable clinical effects on advanced GISTs, with substantial tolerability. Its efficacy greatly depends on the genotype of GIST. The drug, however, met intrinsic or acquired resistance during the treatment, of which the molecular mechanisms were mostly dependent on the genotype of GIST, including primary mutations or secondary mutations in the kinase domains of the corresponding target genes, respectively. Although sunitinib had substantial effects on imatinib-resistant GIST, this drug also encountered primary or secondary resistance depending on the genotype. Thus, advanced GIST may require multidisciplinary treatment. Because resistance mechanisms show some regularity, it is hoped that, in the near future, we may be able to develop a new drug to which resistance does not occur easily, based on scientific evidence.

The Journal of Pathology 2008 Jeremy Jass Prize for Research Excellence in Pathology

The first Jass Prize for Research Excellence has been awarded to a group from Hannover in Germany. These authors discovered the epigenetic inactivation of microRNA gene hsa-mir-9-1 in human breast cancer and characterized its biological and clinical relevance. This frequent epigenetic silencing was found to occur early in the development of breast cancer, and illustrates another mechanism by which tumour development is influenced by genes that operate without expression as 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.

Targeted mutation of p53 and Rb in mesenchymal cells of the limb bud produces sarcomas in mice

Mice bearing germ line mutations of p53 develop sarcomas at a significant rate. Since they are susceptible to a variety of other malignancies, they are not ideally suited to the study of sarcomas. To test the possibility that targeted mutation of tumor suppressor genes in early mesenchymal cells would induce formation of sarcomas, the Prx1-cre transgenic mouse was crossed to mice-bearing floxed alleles of p53 and Rb. Mice with homozygous deletion of p53 (Prx1-cre p53(lox/lox)) developed sarcomas in the extremities at a mean time of 50 weeks. Osteosarcomas (OS) were the most common type of sarcoma (61%) followed by poorly differentiated soft tissue sarcomas (PDSTS) (32%). Homozygous deletion of p53 produced sarcomas significantly more rapidly than heterozygous deletion, which resulted in sarcoma formation after a mean of 96 weeks. Mice with homozygous Rb mutation (Prx1-cre Rb(lox/lox)) developed normally and had no ostensible defects in the limbs. In contrast to p53, targeted deletion of Rb did not produce sarcomas in the limbs. However, simultaneous deletion of Rb and p53 accelerated the time to sarcoma formation, and a greater percentage of PDSTS were found. Deletion of p53 in committed osteoblasts by the Col1a1-cre transgenic mouse bearing an osteoblast-specific enhancer resulted in a high percentage of OS. These findings suggest that deletion of p53 in mesenchymal cells that give rise to osteoblasts is a powerful initiator of OS. Deletion of Rb does not initiate sarcoma formation in mice, but it accelerates formation of both soft tissue sarcomas and OS.

Interstitial cells of Cajal do not harbor c-kit or PDGFRA gene mutations in patients with sporadic gastrointestinal stromal tumors

BACKGROUND

Gastrointestinal stromal tumors (GISTs) are believed to originate from the interstitial cells of Cajal (ICCs) or from the precursors of ICCs. Most GISTs show an activating mutation in either the c-kit or platelet-derived growth factor receptor alpha (PDGFRA) gene that results in a constitutive, ligand-independent activation of receptor tyrosine kinases. These gene mutations may play an important role in transforming a GIST progenitor cell into a tumor cell during the early phase of GIST tumorigenesis. However, the precise mechanism of the tumorigenesis is not known.

METHODS

We examined ten patients with sporadic GIST for mutations in the tumor and its adjacent ICC cells, and compared the mutational status of ICC cells with that of the GIST cells in each patient. All cases were screened for mutations in the c-kit gene (exons 9, 11, 13, and 17) and in the PDGFRA gene (exons 12 and 18). Samples were limited to GIST cases from the small intestine, where ICCs were present in bundles and were considered suitable for isolation by laser capture microdissection.

RESULTS

Of the ten tumors screened, eight had mutations in the c-kit gene (all in exon 11) and two were wild-type, whereas none of the ICCs exhibited mutations in these genes.

CONCLUSIONS

Our results suggest that ICCs adjacent to overt GISTs did not have mutations in the c-kit or PDGFRA genes, and overt GISTs may develop after the local and sporadic acquisition of these gene mutations, together with additional events.

Kit K641E oncogene up-regulates Sprouty homolog 4 and trophoblast glycoprotein in interstitial cells of Cajal in a murine model of gastrointestinal stromal tumours

Gastrointestinal stromal tumours (GIST) are thought to derive from the interstitial cells of Cajal (ICC) or an ICC precursor. Oncogenic mutations of the receptor tyrosine kinase KIT are present in most GIST. KIT K642E was originally identified in sporadic GIST and later found in the germ line of a familial GIST cohort. A mouse model harbouring a germline Kit K641E mutant was created to model familial GIST. The expression profile was investigated in the gastric antrum of the Kit(K641E) murine GIST model by microarray, quantitative PCR and immunofluorescence. Gja1/Cx43, Gpc6, Gpr133, Pacrg, Pde3a, Prkar2b, Prkcq/Pkce, Rasd2, Spry4 and Tpbg/5T4 were found to be up-regulated. The proteins encoded by Gja1/Cx43, Pde3a, Prkcq/Pkce were localized in Kit-ir ICC in wild-type and Kit(K641E) animals while Spry4 and Tpbg/5T4 were detected in Kit-ir cells only in Kit(K641E), but not in Kit(WT/WT) animals. Most up-regulated genes in this mouse model belong to the gene expression profile of human GIST but also to the profile of normal Kit(+) ICC in the mouse small intestine. Spry4 and Tpbg/5T4 may represent candidates for targeted therapeutic approaches in GIST with oncogenic KIT mutations.

Inherited predisposition to gastrointestinal stromal tumor

Gastrointestinal stromal tumors (GIST) are the most common mesenchymal tumors of the gastrointestinal tract and are a relatively recently identified category of mesenchymal tumors. Germline mutations in a number of different genes predispose to GIST. This article discusses familial GIST syndrome, Carney triad, Carney-Stratakis syndrome, and neurofibromatosis type 1, and addresses the recognition of an inherited predisposition in GIST patients.

Advances in preclinical therapeutics development using small animal imaging and molecular analyses: the gastrointestinal stromal tumors model

The large use of target therapies in the treatment of gastrointestinal stromal tumors (GISTs) highlighted the urgency to integrate new molecular imaging technologies, to develop new criteria for tumor response evaluation and to reach a more comprehensive definition of the molecular target. These aspects, which come from clinical experiences, are not considered enough in preclinical research studies which aim to evaluate the efficacy of new drugs or new combination of drugs with molecular target. We developed a xenograft animal model GIST882 using nude mice. We evaluated both the molecular and functional characterization of the tumor mass. The mutational analysis of KIT receptor of the GIST882 cell lines and tumor mass showed a mutation on exon 13 that was still present after in vivo cell growth. The glucose metabolism and cell proliferation was evaluated with a small animal PET using both FDG and FLT. The experimental development of new therapies for GIST treatment requires sophisticated animal models in order to represent the tumor molecular heterogeneity already demonstrated in the clinical setting and in order to evaluate the efficacy of the treatment also considering the inhibition of tumor metabolism, and not only considering the change in size of tumors. This approach of cancer research on GISTs is crucial and essential for innovative perspectives that could cross over to other types of cancer.

Changes in the structure and function of ICC networks in ICC hyperplasia and gastrointestinal stromal tumors

BACKGROUND & AIMS

Gastrointestinal stromal tumors (GISTs) express the receptor tyrosine kinase c-kit. Approximately 90% of GISTs have gain-of-function mutations in the Kit gene, which leads to its constitutive activation and drives malignant behavior of GISTs. Interstitial cells of Cajal (ICC) express c-kit; however, it is unknown whether uncontrolled hyperplasia of ICC is responsible for GISTs. Here, we sought to determine whether gain-of-function mutations in Kit lead to hyperplasia of all classes of ICC, whether ICC hyperplasia begins before birth, and whether functional defects occur in ICC hyperplasia or the development of GISTs.

METHODS

Heterozygous mutant Kit(V558Delta)/+ mice that develop symptoms of human familial GISTs and prematurely die from pathology of the gastrointestinal tract were utilized and compared with wild-type controls. C-kit-immunohistochemistry and intracellular electrical recording of spontaneous and nerve-evoked activity were applied to examine the density and functionality of ICC in these mutants.

RESULTS

There was considerable hyperplasia in all classes of ICC throughout the GI tract of Kit(V558Delta)/+ mice, except for ICC in the deep muscular plexus of the intestine. Spontaneous electrical activity and postjunctional neural responses in hyperplastic ICC tissues appeared normal but were up-regulated in the cecum, where GISTs were commonly found.

CONCLUSIONS

Kit gain-of-function leads to hyperplasia of most classes of ICC throughout the GI tract. ICC retain normal pacemaker function and enteric neural responses well after development of hyperplasia.

Gastrointestinal stromal tumors: past, present, and future

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

Increased KIT signalling with up-regulation of cyclin D correlates to accelerated proliferation and shorter disease-free survival in gastrointestinal stromal tumours (GISTs) with KIT exon 11 deletions

Gastrointestinal stromal tumours (GISTs) with deletions in KIT exon 11 are characterized by higher proliferation rates and shorter disease-free survival times, compared to GISTs with KIT exon 11 point mutations. Up-regulation of cyclin D is a crucial event for entry into the G1 phase of the cell cycle, and links mitogenic signalling to cell proliferation. Signalling from activated KIT to cyclin D is directed through the RAS/RAF/ERK, PI3K/AKT/mTOR/EIF4E, and JAK/STATs cascades. ERK and STATs initiate mRNA transcription of cyclin D, whereas EIF4E activation leads to increased translation efficiency and reduced degradation of cyclin D protein. The aim of the current study was to analyse the mRNA and protein expression as well as protein phosphorylation of central hubs of these signalling cascades in primary GISTs, to evaluate whether tumours with KIT exon 11 deletions and point mutations differently utilize these pathways. GISTs with KIT exon 11 deletions had significantly higher mitotic counts, higher proliferation rates, and shorter disease-free survival times. In line with this, they had significantly higher expression of cyclin D on the mRNA and protein level. Furthermore, there was a significantly higher amount of phosphorylated ERK1/2, and a higher protein amount of STAT3, mTOR, and EIF4E. PI3K and phosphorylated AKT were also up-regulated, but this was not significant. Ultimately, GISTs with KIT exon 11 deletions had significantly higher phosphorylation of the central negative cell-cycle regulator RB. Phosphorylation of RB is accomplished by activated cyclin D/CDK4/6 complex, and marks a central event in the release of the cell cycle. Altogether, these observations suggest increased KIT signalling with up-regulation of cyclin D as the basis for the unfavourable clinical course in GISTs with KIT exon 11 deletions.

Knock-in murine models of familial gastrointestinal stromal tumours

In recent years, gastrointestinal stromal tumour (GIST) has emerged from a poorly understood group of mesenchymal tumours to a distinct pathological entity that has become a leading model for new therapies targeting kinases. GIST pathogenesis is driven by receptor tyrosine kinase-activating mutations most often in KIT or PDGFRA that may be sporadic or result in familial GIST syndromes. In a recent issue of The Journal of Pathology (J Pathol 2008;214:302-311), Nakai and colleagues report that mutation of the previously un-modelled tyrosine kinase II domain of KIT generates a spectrum of features very similar to that in two human GIST families. Knock-in mouse models of GIST may prove extremely useful in pre-clinical evaluation of kinase-targeted compounds and of the mechanism of drug resistance, but intriguingly it now seems clearer that the distribution of GIST in mouse models and humans is different.