Role of PTEN in gastrointestinal stromal tumor progression

Refining Prognosis in Localized Gastrointestinal Stromal Tumor: Clinical Significance of Phosphatase and Tensin Homolog Low Expression and Gene Loss

To investigate the use of PTEN biomarker to improve prognostic stratification in patients with localized gastrointestinal stromal tumor (GIST).

Inhibition of AKT-Signaling Sensitizes Soft Tissue Sarcomas (STS) and Gastrointestinal Stromal Tumors (GIST) to Doxorubicin via Targeting of Homology-Mediated DNA Repair

Activation of the phosphoinositide 3-kinase (PI3K)/Akt/mTOR pathway is well documented for a broad spectrum of human malignancies supporting their growth and progression. Accumulating evidence has also implicated AKT as a potent modulator of anti-cancer therapies via regulation of DNA damage response and repair (DDR) induced by certain chemotherapeutic agents and ionizing radiation (IR). In the present study, we examined the role of AKT signaling in regulating of Rad51 turnover and cytotoxic effects of topoisomerase II inhibitor, doxorubicin (Dox) in soft tissue sarcomas (STS) and gastrointestinal stromal tumors (GIST) in vitro. Blocking of AKT signaling (MK-2206) enhanced cytotoxic and pro-apoptotic effects of Dox in vast majority of STS and GIST cell lines. The phosphorylated form of Akt co-immunoprecipitates with Rad51 after Dox-induced DNA damage, whereas Akt inhibition interrupts this interaction and decreases Rad51 protein level by enhancing protein instability via proteasome-dependent degradation. Inhibition of Akt signaling in Dox-treated cells was associated with the increased number of γ-H2AX-positive cells, decrease of Rad51 foci formation and its colocalization with γ-H2AX foci, thereby revealing unsuccessful DDR events. This was also in consistency with an increase of tail moment (TM) and olive tail moment (OTM) in Dox-treated GIST and STS cells cultured in presence of Akt inhibitor after Dox washout. Altogether, our data illustrates that inhibition of AKT signaling is STS and GIST might potentiate the cytotoxic effect of topoisomerase II inhibitors via attenuating the homology-mediated DNA repair.

The PTEN Tumor Suppressor Gene in Soft Tissue Sarcoma

Soft tissue sarcoma (STS) is a rare malignancy of mesenchymal origin classified into more than 50 different subtypes with distinct clinical and pathologic features. Despite the poor prognosis in the majority of patients, only modest improvements in treatment strategies have been achieved, largely due to the rarity and heterogeneity of these tumors. Therefore, the discovery of new prognostic and predictive biomarkers, together with new therapeutic targets, is of enormous interest. Phosphatase and tensin homolog (PTEN) is a well-known tumor suppressor that commonly loses its function via mutation, deletion, transcriptional silencing, or protein instability, and is frequently downregulated in distinct sarcoma subtypes. The loss of PTEN function has consequent alterations in important pathways implicated in cell proliferation, survival, migration, and genomic stability. PTEN can also interact with other tumor suppressors and oncogenic signaling pathways that have important implications for the pathogenesis in certain STSs. The aim of the present review is to summarize the biological significance of PTEN in STS and its potential role in the development of new therapeutic strategies.

miR-148b-3p functions as a tumor suppressor in GISTs by directly targeting KIT

BACKGROUND

Gain-of-function mutations and overexpression of KIT are characteristic features of gastrointestinal stromal tumor (GIST). Dysregulation in miRNA expression may lead to KIT overexpression and tumorigenesis.

METHODS

miRNA microarray analysis and real-time PCR were used to determine the miRNA expression profiles in a cohort of 69 clinical samples including 50 CD117IHC+/KITmutation GISTs and 19 CD117IHC-/wild-type GISTs. GO enrichment and KEGG pathway analyses were performed to reveal the predicted targets of the dysregulated miRNAs. Of the dysregulated miRNAs whose expression was inversely correlated with that of KIT miRNAs were predicted by bioinformatics analysis and confirmed by luciferase reporter assay. Cell counting kit-8 (CCK-8) and flow cytometry were used to measure the cell proliferation, cycle arrest and apoptosis. Wound healing and transwell assays were used to evaluate migration and invasion. A xenograft BALB/c nude mouse model was applied to investigate the tumorigenesis in vivo. Western blot and qRT-PCR were used to investigate the protein and mRNA levels of KIT and its downstream effectors including ERK, AKT and STAT3.

RESULTS

Of the six miRNAs whose expression was inversely correlated with that of KIT, we found that miR-148b-3p was significantly downregulated in the CD117IHC+/KITmutation GIST cohort. This miRNA was subsequently found to inhibit proliferation, migration and invasion of GIST882 cells. Mechanistically, miR-148b-3p was shown to regulate KIT expression through directly binding to the 3'-UTR of the KIT mRNA. Restoration of miR-148b-3p expression in GIST882 cells led to reduced expression of KIT and the downstream effectors proteins ERK, AKT and STAT3. However, overexpression of KIT reversed the inhibitory effect of miR-148b-3p on cell proliferation, migration and invasion. Furthermore, we found that reduced miR-148b-3p expression correlated with poor overall survival (OS) and disease-free survival (DFS) in GIST patients.

CONCLUSION

miR-148b-3p functions as an important regulator of KIT expression and a potential prognostic biomarker for GISTs.

The clinical correlation of phosphatase and tensin homolog on chromosome 10, phosphorylation of AKT to an activated state, and odontogenic ameloblast-associated protein in gastrointestinal stromal tumors

BACKGROUND

Approximately 15% of gastrointestinal stromal tumors (GISTs) will not respond to tyrosine kinase inhibitors and drug resistance can develop over time. For refractory tumors, additional therapies are needed. Odontogenic ameloblast-associated protein (ODAM) is expressed in some epithelial malignancies and can correlate with clinical outcomes. This study evaluated ODAM and its relationship to phosphatase and tensin homolog on chromosome 10 (PTEN) and phosphorylation of AKT to an activated state (pAKT) in GISTs.

MATERIALS AND METHODS

Ninety-five distinct tumor specimens from 79 patients were identified. Morphologic features and clinical data were recorded for all tumors. Risk of recurrence was calculated using the Memorial Sloan-Kettering nomogram. Immunohistochemistry was performed using antibodies to ODAM, PTEN, and pAKT. Immunoreactivity was assessed for both cytoplasmic and nuclear expression. Staining patterns were correlated with clinical outcomes.

RESULTS

Increasing cytoplasmic ODAM staining correlated with a lower recurrence score (P = 0.002), a lower mitotic rate (P = 0.0001), and smaller tumor size (P = 0.038). Increasing pAKT cytoplasmic staining correlated with a higher recurrence score (P = 0.037) and a higher mitotic rate (P = 0.036). ODAM and pAKT expression in the nucleus was associated with tumor origin. PTEN nuclear expression increased with increasing mitotic rate. pAKT expression increased in the cytoplasm and nucleus in high-risk tumors.

CONCLUSIONS

Risk of recurrence correlated with cytoplasmic expression of ODAM and pAKT, whereas nuclear expression did not predict recurrence. The staining pattern for ODAM and pAKT in the cytoplasm may further clarify the risk of recurrence beyond the available nomograms. The increased expression of pAKT in the cytoplasm and nucleus of high-risk tumors suggests a potential target for systemic therapy.

Frequent mono-allelic loss associated with deficient PTEN expression in imatinib-resistant gastrointestinal stromal tumors

Insufficiency of phosphatase and tensin homolog (PTEN) occurs in numerous tumor types and has been implicated as a resistance mechanism to receptor tyrosine kinase-targeted therapies in human cancer. In this study, we have performed a comprehensive molecular and immunohistochemical characterization of PTEN in 58 imatinib-naïve and 54 imatinib-treated gastrointestinal stromal tumors (GISTs). The findings were correlated with clinicopathological data. At the genomic level, PTEN was affected mainly by mono-allelic loss, which was significantly less frequent in imatinib-naïve vs imatinib-resistant tumors (9% vs 39%, P<0.001). Neither PTEN mutations nor PTEN promoter hyper-methylation were found. By immunohistochemistry, PTEN depletion was clearly related to GIST progression. Low PTEN protein expression was common (50%) and often paralleled with total immunonegativity in imatinib-resistant tumors. The abnormal PTEN protein expression correlated with PTEN loss at the genomic level (P=0.001). In addition, the effect of small interfering RNA (siRNA) PTEN knockdown on KIT signaling was examined in GIST-T1 and GIST430 cell lines, in the absence or presence of a dual PI3K/mTOR inhibitor NVP-BEZ235, alone or in combination with imatinib. In both cell lines, siRNA silencing of PTEN resulted in the substantial upregulation of PI3K-AKT and MAPK pathways. The MAPK hyperactivation was further potentiated by NVP-BEZ235 in the imatinib-sensitive GIST-T1 cells; yet, this effect was counteracted efficiently by combined treatment. In the imatinib-resistant GIST430 cells, neither NVP-BEZ235 alone or in combination with imatinib yielded sufficient inhibition of hyper-phosphorylated MAPK and downstream intermediate S6 protein. In conclusion, depleted PTEN expression associated with mono-allelic PTEN loss occurs frequently in imatinib-resistant GIST and might serve as a biomarker for stratifying patients for optimal treatment. In vitro, the PTEN insufficiency leads to hyperactivation of AKT and MAPK pathways in tumor cells. Novel therapies targeting multiple components of the integrated KIT receptor signaling pathways in imatinib-resistant GIST warrant further studies.

Cowden syndrome complicated by a gastrointestinal stromal tumor

To our knowledge, this is the first report of Cowden syndrome complicated by a gastrointestinal stromal tumor (GIST) of the small bowel. A 42-year-old female patient was found to have an abdominal mass that was diagnosed as the cause of anemia and was surgically extracted. The surgical specimen was found to be a GIST. During the same period, the patient underwent an endoscopic examination of the entire gastrointestinal tract. She was also diagnosed as having Cowden syndrome based on gastrointestinal polyps and skin, thyroid and breast lesions. Cowden syndrome is associated with germline mutations in the tumorsuppressor gene PTEN. PTEN expression may be essential to tumor growth and is a predictive biomarker of the prognosis of both diseases. The present report of such a case is expected to further the analysis of Cowden syndrome.

Prognostic impact of gastrointestinal bleeding and expression of PTEN and Ki-67 on primary gastrointestinal stromal tumors

BACKGROUND

Prognostic indicators for gastrointestinal stromal tumors (GISTs) are under investigation. The latest risk classification criteria may still have room for improvement. This study aims to investigate prognostic factors for primary GISTs from three aspects, including clinicopathological parameters, immunohistochemical (IHC) expression of PTEN, and Ki-67 labeling index (LI), and attempts to find valuable predictors for the malignancy potential of primary GISTs.

METHODS

Tumor samples and clinicopathological data from 84 patients with primary GISTs after R0 resection were obtained. Immunohistochemical analysis was performed based on tissue microarray (TMA) to estimate expression of PTEN and Ki-67 in tumor cells.

RESULTS

The cut-off point of Ki-67 LI was determined as 1%, using a receiver operator characteristic test with a sensitivity of 71.7% and a specificity of 64.5%. Univariate analysis demonstrated the following factors as poor prognostic indicators for relapse-free survival (RFS) against a median follow-up of 40.25 months: gastrointestinal (GI) bleeding (P = 0.009), non-gastric tumor location (P = 0.001), large tumor size (P = 0.022), high mitotic index (P < 0.001), high cellularity (P = 0.012), tumor rupture (P = 0.013), absent or low expression of PTEN (P = 0.036), and Ki-67 LI >1% (P = 0.043). Gastrointestinal bleeding (hazard ratio, 3.85; 95% confidence interval, 1.63 to 9.10; P = 0.002) was a negative independent risk predictor in multivariate analysis, in addition to tumor size (P = 0.023), and mitotic index (P = 0.002). In addition, GI bleeding showed a good ability to predict recurrence potential, when included in our re-modified risk stratification criteria.

CONCLUSIONS

This study suggests that GI bleeding is an independent predictor of poor prognosis for RFS in primary GISTs. Expression of PTEN and Ki-67 are correlated with high risk potential and may predict early recurrence in univariate analysis.

Targeted therapy for sarcomas

Sarcomas are tumors of mesenchymal origin that make up approximately 1% of human cancers. They may arise as primary tumors in either bone or soft tissue, with approximately 11,280 soft tissue tumors and 2,650 bone tumors diagnosed each year in the United States. There are at least 50 different subtypes of soft tissue sarcoma, with new ones described with ever-increasing frequency. One way to look at sarcomas is to divide them into categories on the basis of their genetic make-up. One group of sarcomas has an identifiable, relatively simple genetic signature, such as the X:18 translocation seen in synovial sarcoma or the 11:22 translocation seen in Ewing's sarcoma. These specific abnormalities often lead to the presence of fusion proteins, such as EWS-FLI1 in Ewing's sarcoma, which are helpful as diagnostic tools and may become therapeutic targets in the future. Another group of sarcomas is characterized by complex genetic abnormalities as seen in leiomyosarcoma, osteosarcoma, and undifferentiated sarcoma. It is important to keep these distinctions in mind when contemplating the development of targeted agents for sarcomas. Different abnormalities in sarcoma could be divided by tumor subtype or by the molecular or pathway abnormality. However, some existing drugs or drugs in development may interfere with or alter more than one of the presented pathways.

Exploring novel therapeutic targets in GIST: focus on the PI3K/Akt/mTOR pathway

Gastrointestinal stromal tumors (GISTs) are the most common soft tissue sarcoma, and most feature abnormalities in two genes encoding the receptor tyrosine kinases (RTKs), KIT, and PDGFRA. The RTK inhibitor imatinib revolutionized treatment in GIST; however, drug resistance remains a challenge. Constitutive autophosphorylation of RTKs is linked to phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) pathway hyperactivation, which is central to oncogenic signaling, and known to be dysregulated in GIST. Preclinical experiments have confirmed that inhibiting the PI3K/Akt/mTOR pathway is a rational target for therapy. Early studies using mTOR inhibitors have shown limited success, which may be due to the activation of Akt that occurs following mTORC1 inhibition. Therefore, targeting PI3K or Akt, which lie upstream of mTORC1, may translate into more complete pathway inhibition. Several treatment strategies are currently being developed in phase 1 and 2 clinical trials. Compounds currently in development include pan-Class I PI3K inhibitors, dual PI3K/mTOR inhibitors, and Akt inhibitors. The aim of this review is to highlight the evidence for targeting PI3K/Akt/mTOR-dependent mechanisms in GIST and to evaluate the existing preclinical and clinical data supporting this strategy.

A potent combination of the novel PI3K Inhibitor, GDC-0941, with imatinib in gastrointestinal stromal tumor xenografts: long-lasting responses after treatment withdrawal

PURPOSE

Oncogenic signaling in gastrointestinal stromal tumors (GIST) is sustained via PI3K/AKT pathway. We used a panel of six GIST xenograft models to assess efficacy of GDC-0941 as single agent or in combination with imatinib (IMA).

EXPERIMENTAL DESIGN

Nude mice (n = 136) were grafted bilaterally with human GIST carrying diverse KIT mutations. Mice were orally dosed over four weeks, grouped as follows: (A) control; (B) GDC-0941; (C) imatinib, and (D) GDC+IMA treatments. Xenografts regrowth after treatment discontinuation was assessed in groups C and D for an additional four weeks. Tumor response was assessed by volume measurements, micro-PET imaging, histopathology, and immunoblotting. Moreover, genomic alterations in PTEN/PI3K/AKT pathway were evaluated.

RESULTS

In all models, GDC-0941 caused tumor growth stabilization, inhibiting tumor cell proliferation, but did not induce apoptosis. Under GDC+IMA, profound tumor regression, superior to either treatment alone, was observed. This effect was associated with the best histologic response, a nearly complete proliferation arrest and increased apoptosis. Tumor regrowth assays confirmed superior activity of GDC+IMA over imatinib; in three of six models, tumor volume remained reduced and stable even after treatment discontinuation. A positive correlation between response to GDC+IMA and PTEN loss, both on gene and protein levels, was found.

CONCLUSION

GDC+IMA has significant antitumor efficacy in GIST xenografts, inducing more substantial tumor regression, apoptosis, and durable effects than imatinib. Notably, after treatment withdrawal, tumor regression was sustained in tumors exposed to GDC+IMA, which was not observed under imatinib. Assessment of PTEN status may represent a useful predictive biomarker for patient selection.

Prognostic significance of PTEN, Ki-67 and CD44s expression patterns in gastrointestinal stromal tumors

AIM: To develop a prognostic approach for gastrointestinal stromal tumors (GISTs) using a cluster of indicators and follow-up information.

METHODS: One hundred and four GISTs that had not been subjected to targeted therapies were collected and classified by NIH risk assessment and anatomic location. By immunohistochemistry, the expressions of PTEN, Ki-67, CD44s matrix metalloproteinase (MMP)-9 and TIMP-1 were detected on tissue microarray. Univariate and multimarker survival analyses were performed and then a COX hazard proportion model was constructed to evaluate a cluster of predictors of GIST.

RESULTS: Our data showed small intestinal GIST are more aggressive than gastric GIST. The NIH risk assessment correlated with disease-free survival for either gastric GIST or small intestinal GIST. Immunohistochemical analysis revealed that Ki-67 labeling indexes (LIs) < 5% predicted higher disease-specific survival (DSS) in gastric and small intestinal GIST. CD44s positivity and PTEN LIs ≥ 50% correlated with higher DSS in gastric GIST. MMP-9 and TIMP-1 had no correlation with survival. Multimarker analysis revealed that the expression pattern of PTEN LIs ≥ 50% combined with Ki-67 LIs < 5% and CD44s positivity reliably predicted favorable outcomes for gastric GIST (P = 0.009), as did the combination of PTEN LIs ≥ 50% and Ki-67 LIs < 5% for small intestinal GIST (P = 0.011). Authors also found that high NIH risk grade was correlated with DSS in patients with gastric GIST and disease-free survival in patients with small intestinal GIST.

CONCLUSION: PTEN LIs ≥ 50%, Ki-67 LIs < 5% and CD44s positivity provides an accurate, favorable prognosis for gastric GIST. PTEN LIs ≥ 50% and Ki-67 LIs < 5% does the same for small intestinal GIST. Ki-67 LIs enhances the NIH assessment.

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.

Long-term exposure of gastrointestinal stromal tumor cells to sunitinib induces epigenetic silencing of the PTEN gene

Although sunitinib possesses significant clinical effects on imatinib-resistant gastrointestinal stromal tumors (GISTs), the individuals with GIST eventually become resistant to treatment with this tyrosine kinase inhibitor. The mechanism of resistance to sunitinib is still under investigation. To address this issue, we have established sunitinib-resistant GIST-T1 sublines (designated as GIST-T1R) by culturing cells with increasing concentrations of sunitinib. GIST-T1R cells were also resistant to imatinib-mediated growth inhibition. Examination of intracellular signaling found that Akt/ mammalian target of rapamycin (mTOR) signaling remained activated in GIST-T1R but not in parental GIST-T1 cells, after exposure of these cells to sunitinib, as measured by immunoblotting. Further study found that the phosphatase and tensin homolog deleted on chromosome ten (PTEN) gene was silenced by methylation of the promoter region of the gene. Notably, forced-expression of PTEN in GIST-T1R cells negatively regulated the Akt/mTOR pathways and sensitized these cells to sunitinib-mediated growth arrest and apoptosis. Taken together, epigenetic silence of PTEN might be one of the mechanisms which cause drug-resistance in individuals with GIST after exposure to tyrosine kinase inhibitors. Blockade of the PI3K/Akt signaling with the specific inhibitors could be useful in such a case.

The emerging role of mammalian target of rapamycin inhibitors in the treatment of sarcomas

The mammalian target of rapamycin (mTOR) is a protein kinase that functions as a key regulator of cell growth, proliferation and differentiation, cell-cycle progression, angiogenesis, protein degradation, and apoptosis. Following activation by a number of oncogenic signals such as growth factors, energy and nutrients, mTOR stimulates several downstream effectors including the 40S ribosomal protein S6 kinase (p70s6k) and the eukaryotic initiation factor 4 E binding protein-1 (4 EBP-1), as well as a complex network of regulatory loops. Activation of the mTOR pathway plays a critical role in the development of many tumor types, including renal cell and breast carcinomas, neuroendocrine tumors, and sarcomas. Bone and soft tissue sarcomas are rare, heterogeneous tumors that are curable by local treatments if diagnosed at early stages; however advanced or metastatic sarcomas are rarely curable and very few drugs are efficacious in this setting. Several disruptions in phosphatidylinositol-3 kinase (PI3K)-Akt-mTOR signaling are associated with malignant transformation or progression in various sarcoma sub-types. The PI3K-Akt-mTOR pathway is therefore an exciting target for therapy of sarcomas, and its blockade represents an opportunity to improve outcomes in this poor-prognosis disease. Early studies with mTOR inhibitors have demonstrated promising antitumor activity in patients with metastatic sarcoma who have failed standard treatments. This article discusses the mTOR signaling pathway and summarizes the clinical experience with mTOR inhibitors in patients with advanced or metastatic sarcoma.

Skp2 expression is associated with high risk and elevated Ki67 expression in gastrointestinal stromal tumours

Background

Gastrointestinal stromal tumors (GIST) exhibit an unpredictable clinical course and can rapidly progress to lethality. Predictions about the biological behavior of GIST are based on a number of canonical clinical and pathologic parameters whose validity in distinguishing between a benign and a malignant tumour is still imperfect. The aim of our study was to investigate the role of morphologic parameters and expression of cells cycle regulators as prognosticators in GIST.

Methods

We performed an immunohistochemical analysis for Ki67, p27^Kip1, Jab1, and Skp2, on a Tissue Microarray (TMA) containing 94 GIST. Expression of the above proteins was correlated to classically used prognosticators, as well as to risk groups. Clinical significance of histologic and immunohistochemical features were evaluated in 59 patients for whom follow-up information was available.

Results

Overexpression of Ki67 and Skp2, and p27^Kip1 loss directly correlated with the high risk group (p = 0.03 for Ki67 and Skp2, p = 0.05 for p27^Kip1). Jab1 expression did not exhibit correlation with risk. In 59 cases provided with clinical follow-up, high cellularity, presence of necrosis, and Ki67 overexpression were predictive of a reduced overall survival in a univariate model. The same parameters, as well as mitotic rate, tumour size, and p27^Kip1 loss were indicative of a shortened relapse free survival interval. High cellularity, and high mitotic rate retained their prognostic significance by multivariate analysis.

Conclusion

Our data suggest that a number of histologic parameters in combination with immunohistochemical expression of cell cycle regulators can facilitate risk categorization and predict biologic behavior in GIST. Importantly this study demonstrates, for the first time, that Skp2 expression correlates with Ki67 expression and high risk in GIST.

The role of mTOR inhibitors for treatment of sarcomas

Bone and soft tissue sarcomas represent rare tumors that can be cured by local treatment at early stages of disease. However, advanced or metastatic disease is rarely cured, and very few drugs have shown efficacy in this setting. Early studies with mTOR inhibitors have demonstrated antitumor activity in patients with metastatic sarcoma who failed previous chemotherapies. The response rate and durable stable disease in early studies, as well as the tolerability profile, recommend these drugs as promising candidates for further clinical studies. This article discusses preliminary results from clinical trials in patients with advanced or metastatic sarcoma as well as future perspectives.

The molecular pathogenesis of gastrointestinal stromal tumors

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

Mammalian target of rapamycin inhibitors in sarcomas

PURPOSE OF REVIEW

Sarcomas are a rare malignancy accounting for less than 1% of all cancers diagnosed annually. Standard chemotherapy has a response rate of around 25% and newer agents are needed to improve the outcome in patients with advanced sarcomas. The mammalian target of rapamycin plays a central role in cell growth, proliferation, and apoptosis and its inhibition has demonstrated antitumor activity in many tumors and shows promise against sarcomas.

RECENT FINDINGS

Recent studies of mammalian target of rapamycin inhibitors in sarcomas have demonstrated clinical benefit response in sarcomas.

SUMMARY

Clinical benefit response uses standard Response Evaluation Criteria in Solid Tumors of complete response and partial response as well as stable disease lasting at least 4 months as an endpoint. This endpoint has been shown to select promising new agents against sarcomas. Using this endpoint, the use of the mammalian target of rapamycin inhibitor AP23573 has demonstrated activity against sarcomas. The use of the inhibitor RAD001 (everolimus) along with imatinib in patients with imatinib resistant gastrointestinal stromal tumor has shown promise. Future studies will need to be performed to determine the clinical differences among the mammalian target of rapamycin inhibitors in different subsets of sarcomas.

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

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