KIT and BRAF heterogeneous mutations in gastrointestinal stromal tumors after secondary imatinib resistance.

BACKGROUND AND AIMS: Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumor of the digestive tract and characterized by expression of KIT protein. Imatinib is the frontline therapy for metastatic and unresectable GIST patients showing clinical responses in 80 % of cases. Despite the often long-lasting clinical benefit seen in most patients treated with imatinib, many will eventually suffer disease progression. The most frequent mechanism of imatinib resistance in GIST is the acquisition of secondary mutations in either KIT or PDGFRA. There are also some imatinib-resistant GIST patients lacking an identifiable mechanism of treatment failure. Recently, activating BRAF mutation was detected in a small percentage of GISTs. In this study, we report a case of GIST with acquired resistance to imatinib during therapy. METHODS: Histological, immunohistochemical, Western blot and mutational analyses were performed on GIST tissues before and after imatinib resistance. RESULTS: The imatinib-resistant tumor showed not only heterogeneous mutations of KIT and BRAF besides the primary mutation, but also transdifferentiation into a rhabdomyosarcoma phenotype. According to Western blot analysis, in imatinib-resistant GIST with both KIT V559D and BRAF V600E mutations, the inhibition of KIT V559D by imatinib caused a strong decrease of AKT phosphorylation, while ERK1/2 phosphorylation was not affected. CONCLUSIONS: This finding, in combination with the loss of KIT expression, suggests the possibility of activation of RAS-RAF-MEK-ERK pathways driven by a KIT-independent oncogenic mechanism. Understanding the genetic aberrations beyond KIT and PDGFRA may lead to the identification of additional therapeutic targets for GISTs.

Rhabdomyosarcomatous differentiation in gastrointestinal stromal tumors after imatinib resistance: a potential diagnostic pitfall.

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumor of the digestive tract and characterized by expression of protein-tyrosine kinase (KIT) protein. Treatment of advanced GISTs has been improved dramatically following the development of imatinib. Despite the often long-lasting clinical benefit seen in most patients treated with imatinib, many will eventually suffer disease progression. In general, progressing GISTs retain their typical morphology. In this study, we present a patient with metastatic GISTs, who received more than 16 months of treatment with imatinib and whose tumors changed their morphological and immunohistochemical characteristics after imatinib-resistance. Histological, immunohistochemical and mutational analysis was performed on the prior and post-imatinib treatment GIST samples. The imatinib-resistant tumor cells in the progressing metastases showed marked pleomorphism which proved to be rhabdomyoblastic differentiation with Desmin and Myogenin immunopositivity. However, there was no secondary mutation of KIT, PDGFRA, KRAS and BRAF genes found in the imatinib-resistant lesion, except primary KIT V559D mutation. To our knowledge, this case represents the few reports on this unusual type of transdifferentiation in GISTs under imatinib therapy. Awareness of this phenomenon would help to avoid diagnostic confusion when evaluating post-imatinib samples from GISTs.

[Establishment and pathologic analysis of imatinib-resistant gastrointestinal stromal tumor xenografts].

OBJECTIVE: To establish and characterize imatinib-resistant gastrointestinal stromal tumor (GIST) xenografts. Further provided an ideal experimental platform through the imatinib-resistant GIST xenografts to investigate the mechanism of resistance to imatinib. METHODS: Imatinib-resistant GIST cells were injected under the skin of athymic nude mice to establish animal models of human imatinib-resistant GIST. The molecular and histopathologic features of GIST xenografts were also analysed and compared with their counterpart of cell lines. RESULTS: The xenograft tumor models had been established by subcutaneously injection of GIST cells into nude mice. Immunohistochemistry results showed CD117 expression was positive in GIST-PR2 xenograft tumor, but negative in GIST-R. In GIST-PR1, tumor areas showing rhabdomyoblastic differentiation were presented next to areas with classic GIST morphology. The rhabdomyoblastic component demonstrated consistently positivity for desmin and myogenin, whereas CD117 was completely negative. The mutation profiles of these xenograft tumors were the same as their counterpart of cell lines. CONCLUSIONS: Human GIST xenografts with mutation in c-kit have been established from imatinib-resistant GIST lines. Those models will enable further studies on mechanisms of resistance, combination therapies and allow testing of novel targeted therapies.

Gene mutations and prognostic factors analysis in extragastrointestinal stromal tumor of a Chinese three-center study.

BACKGROUND: Most gastrointestinal stromal tumors (GISTs) have gain-of-function mutation of the c-kit gene, and some have mutation of the platelet-derived growth factor receptor-α (PDGFR-α) gene. Extragastrointestinal stromal tumors (EGISTs) are mesenchymal tumors that occur outside the digestive tract. But the clinicopathologic characteristics of EGISTs are still poorly understood. METHODS: Paraffin-embedded tissues from 25 cases of EGIST were analyzed for CD117, CD34, Ki-67, S-100, smooth muscle actin, and desmin expression by immunohistochemical method. These cases of EGISTs were also evaluated for the presence of c-kit exons 9, 11, 13, and 17 mutations and PDGFR-α exons 12 and 18 mutations. Survival analysis was used to evaluate the prognostic factors. RESULTS: c-kit mutations were detected in 44% of EGIST patients and all were exon 11 mutations. PDGFR-α mutations were found in 12% of the 25 cases and all were exon 18 mutations. Survival analysis indicated that mitotic count and Ki-67 labeling index (Ki-67 LI) were significant predictors of survival. CONCLUSION: The pattern of c-kit and PDGFR-α mutation in EGISTs was essentially similar to that in GISTs. From the molecular genetics aspect, EGISTs may be a special subtype of GISTs. The results also show that the combination of mitotic counts and Ki-67 LI may be useful for predicting the prognosis of EGISTs.

Survival and prognostic factors analysis in surgically resected gastrointestinal stromal tumor patients.

BACKGROUND/AIMS: Gastrointestinal stromal tumor (GIST) is the most common mesenchymal tumor of the digestive tract and predicting the clinical behavior and prognosis of GISTs has still been problem for both pathologists and clinicians. The aim of this study was to investigate the survival and prognostic factors of gastrointestinal stromal tumors after surgery. METHODOLOGY: Hematoxylin and eosin (H&E) stained histopathological slides of tumors from patients with GISTs were reviewed. Immunohistochemical staining was performed to demonstrate CD117, CD34, platelet -derived growth factor receptor (PDGFR-alpha) and Ki-67 protein expression. Clinicopathologic features (age, sex, tumor location and size, cell type, mitotic count, risk category, necrosis, surgical method, expression of CD117, CD34, PDGFR-alpha and Ki-67 protein) were evaluated by univariate and multivariate analyses in 135 patients with resected primary GISTs to identify independent prognostic factors. RESULTS: The overall disease-specific survival of 135 patients was 94.1% at 1 year, 76.3% at 3 years and 65.9% at 5 years. Multivariate analyses indicated that the tumor size, primary location, mitotic count, risk category, necrosis and Ki-67 index were independent significant predictors of survival (p < 0.05). Ki-67 index was strong poor predictors of survival as tumor size and mitotic count. CONCLUSIONS: Fletcher's biological behavior ranking method was a good approach to predict prognosis of GIST patients and had significant clinical value. It's better to combine other factors such as Ki-67 index and tumor primary location et al to predict prognosis accurately. Accurate prognostic prediction could provide evidence for postoperative adjuvant targeted therapy.

Secondary C-kit mutation is a cause of acquired resistance to imatinib in gastrointestinal stromal tumor.

C-kit gene gain of function mutations are important in the pathogenesis of gastrointestinal stromal tumors (GISTs). Imatinib is a selective tyrosine kinase inhibitor of KIT and achieves a partial response or stable disease in most patients with metastatic GIST, but there is increasing evidence of acquired resistance. We report a case of GIST with acquired resistance to imatinib during therapy and secondary c-kit mutation besides the primary mutation.