EWS-FLI1 induces developmental abnormalities and accelerates sarcoma formation in a transgenic mouse model

YAP1 is a key regulator of EWS::FLI1-dependent malignant transformation upon IGF-1-mediated reprogramming of bone mesenchymal stem cells

Ewing sarcoma (EwS) is an aggressive cancer of adolescents in need of effective treatment. Insulin-like growth factor (IGF)-1 is an autocrine growth factor for EwS, but only 10% of patients respond to IGF-1 receptor (IGF-1R) blockade. Although EwS is presumed to originate from mesenchymal progenitors during bone development, targeting of the EwS driver oncogene EWS::FLI1 to the mesenchymal lineage in a mouse model does not result in tumor formation but in skeletal malformations and perinatal death. We report that transient exposure to IGF-1 concentrations mimicking serum levels during puberty reprograms limb-derived mesenchymal cells of EWS::FLI1-mutant mice to stable transformation and tumorigenicity. We identify a modular mechanism of IGF-1-driven tumor promotion in the early steps of EwS pathogenesis, in which Yap1 plays a central role. Pharmacologic Yap1/Tead inhibition reverses the transformed phenotype of EWS::FLI1-expressing cells. Our data provide a rationale for combined IGF-1R and YAP/TEAD inhibition in the treatment of EwS patients.

Primary Bone Tumors and Breast Cancer-Induced Bone Metastases: In Vivo Animal Models and New Alternative Approaches

Bone is the preferential site of metastasis for the most common tumors, including breast cancer. On the other hand, osteosarcoma is the primary bone cancer that most commonly occurs and causes bone cancer-related deaths in children. Several treatment strategies have been developed so far, with little or no efficacy for patient survival and with the development of side effects. Therefore, there is an urgent need to develop more effective therapies for bone primary tumors and bone metastatic disease. This almost necessarily requires the use of in vivo animal models that better mimic human pathology and at the same time follow the ethical principles for the humane use of animal testing. In this review we aim to illustrate the main and more suitable in vivo strategies employed to model bone metastases and osteosarcoma. We will also take a look at the recent technologies implemented for a partial replacement of animal testing.

Preclinical models for the study of pediatric solid tumors: focus on bone sarcomas

Sarcomas comprise between 10-15% of all pediatric malignancies. Osteosarcoma and Ewing sarcoma are the two most common pediatric bone tumors diagnosed in children and young adults. These tumors are commonly treated with surgery and/or radiation therapy and combination chemotherapy. However, there is a strong need for the development and utilization of targeted therapeutic methods to improve patient outcomes. Towards accomplishing this goal, pre-clinical models for these unique malignancies are of particular importance to design and test experimental therapeutic strategies prior to being introduced to patients due to their origination site and propensity to metastasize. Pre-clinical models offer several advantages for the study of pediatric sarcomas with unique benefits and shortcomings dependent on the type of model. This review addresses the types of pre-clinical models available for the study of pediatric solid tumors, with special attention to the bone sarcomas osteosarcoma and Ewing sarcoma.

Mesenchymal loss of p53 alters stem cell capacity and models human soft tissue sarcoma traits

Soft tissue sarcomas (STSs) are a heterogeneous group of tumors that originate from mesenchymal cells. p53 is frequently mutated in human STS. In this study, we found that the loss of p53 in mesenchymal stem cells (MSCs) mainly causes adult undifferentiated soft tissue sarcoma (USTS). MSCs lacking p53 show changes in stem cell properties, including differentiation, cell cycle progression, and metabolism. The transcriptomic changes and genetic mutations in murine p53-deficient USTS mimic those seen in human STS. Furthermore, single-cell RNA sequencing revealed that MSCs undergo transcriptomic alterations with aging-a risk factor for certain types of USTS-and that p53 signaling decreases simultaneously. Moreover, we found that human STS can be transcriptomically classified into six clusters with different prognoses, different from the current histopathological classification. This study paves the way for understanding MSC-mediated tumorigenesis and provides an efficient mouse model for sarcoma studies.

Human EWS-FLI protein recapitulates in Drosophila the neomorphic functions that induce Ewing sarcoma tumorigenesis

Ewing sarcoma (EwS) is a human malignant tumor typically driven by the Ewing sarcoma-Friend leukemia integration (EWS-FLI) fusion protein. A paucity of genetically modified animal models, partially owed to the high toxicity of EWS-FLI, hinders research on EwS. Here, we report a spontaneous mutant variant, EWS-FLI_1FS, that circumvents the toxicity issue in Drosophila. Through proteomic and genomic analyses, we show that human EWS-FLI_1FS interacts with the Drosophila homologues of EWS-FLI human protein partners, including core subunits of chromatin remodeling complexes, the transcription machinery, and the spliceosome; brings about a massive dysregulation of transcription that affects a significant fraction of known targets of EWS-FLI in human cells; and modulates splicing. We also show that EWS-FLI_1FS performs in Drosophila the two major neomorphic activities that it is known to have in human cells: activation of transcription from GGAA microsatellites and out competition of ETS transcription factors. We conclude that EWS-FLI_1FS reproduces in Drosophila the known oncogenic activities of EWS-FLI that drive EwS tumorigenesis in humans. These results open up an unprecedented opportunity to investigate EWS-FLI's oncogenic pathways in vivo in a genetically tractable organism.

Challenges in modeling EWS-FLI1-driven transgenic mouse model for Ewing sarcoma

EWS-FLI1 is a master regulator of Ewing sarcoma (ES) oncogenesis. Although EWS-FLI1 represents a clear therapeutic target, targeted therapeutic inhibitors are lacking. Scientific literature has indicated accumulating information pertaining to EWS-FLI1 translocation, pathogenesis, function, oncogenic partnerships, and potential clinical relevance. However, attempts to develop EWS-FLI1-driven human-like ES mouse models or in vivo systems ended up with limited success. Establishing such models as preclinical screening tools may accelerate the development of EWS-FLI1 targeted therapeutic inhibitors. This review summarizes the current scenario, which focuses on the limitations, challenges, and possible reasons for past failures in model development and also plausible interim alternatives.

Fli1 Promotes Vascular Morphogenesis by Regulating Endothelial Potential of Multipotent Myogenic Progenitors

[Figure: see text].

Extracellular Vesicles in Reprogramming of the Ewing Sarcoma Tumor Microenvironment

Ewing sarcoma (EwS) is a highly aggressive cancer and the second most common malignant bone tumor of children and young adults. Although patients with localized disease have a survival rate of approximately 75%, the prognosis for patients with metastatic disease remains dismal (<30%) and has not improved in decades. Standard-of-care treatments include local therapies such as surgery and radiotherapy, in addition to poly-agent adjuvant chemotherapy, and are often associated with long-term disability and reduced quality of life. Novel targeted therapeutic strategies that are more efficacious and less toxic are therefore desperately needed, particularly for metastatic disease, given that the presence of metastasis remains the most powerful predictor of poor outcome in EwS. Intercellular communication within the tumor microenvironment is emerging as a crucial mechanism for cancer cells to establish immunosuppressive and cancer-permissive environments, potentially leading to metastasis. Altering this communication within the tumor microenvironment, thereby preventing the transfer of oncogenic signals and molecules, represents a highly promising therapeutic strategy. To achieve this, extracellular vesicles (EVs) offer a candidate mechanism as they are actively released by tumor cells and enriched with proteins and RNAs. EVs are membrane-bound particles released by normal and tumor cells, that play pivotal roles in intercellular communication, including cross-talk between tumor, stromal fibroblast, and immune cells in the local tumor microenvironment and systemic circulation. EwS EVs, including the smaller exosomes and larger microvesicles, have the potential to reprogram a diversity of cells in the tumor microenvironment, by transferring various biomolecules in a cell-specific manner. Insights into the various biomolecules packed in EwS EVs as cargos and the molecular changes they trigger in recipient cells of the tumor microenvironment will shed light on various potential targets for therapeutic intervention in EwS. This review details EwS EVs composition, their potential role in metastasis and in the reprogramming of various cells of the tumor microenvironment, and the potential for clinical intervention.

Expression of the EWSR1-FLI1 fusion oncogene in pancreas cells drives pancreatic atrophy and lipomatosis

BACKGROUND

Pancreatic ductal adenocarcinoma (PDAC) harbors mutant KRAS as the most common driver mutation. Studies on mouse models have uncovered the tumorigenic characteristics of the Kras oncogene driving pancreatic carcinogenesis. Similarly, Ewing sarcoma predominantly depends on the occurrence of the EWSR1-FLI1 fusion oncogene. The expression of EWSR1-FLI1 affects pro-tumorigenic pathways and induces cell transformation. In this study, we investigated whether mutant Kras could be exchanged by another potent oncogene, such as EWSR1-FLI1, to initiate pancreatic cancer development.

METHODS

We generated two conditional mouse models expressing mutant KrasG12D (KC) or the EWSR1-FLI1 oncogene (E/F) in pancreas cells. Pancreatic tissue was collected from the mice at 4-6 weeks and 11-13 weeks of age as well as from survival cohorts to determine the development of spontaneous acinar-to-ductal metaplasia (ADM) and neoplastic lesions. Immunohistochemistry and immunofluorescence staining were performed to characterize and quantify changes in tissue morphology.

RESULTS

The expression of the EWSR1-FLI1 fusion protein in pancreas cells was confirmed by positive FLI1 immunohistochemistry staining. Notably, the EWSR1-FLI1 expression in pancreas cells resulted in a strong depletion of the acinar cell mass and an extensive lipomatosis. Although the E/F mice exhibited spontaneous ADM formation and a shorter overall survival rate compared to KC mice, no development of neoplastic lesion was observed in aging E/F mice.

CONCLUSIONS

The expression of the EWSR1-FLI1 oncogene leads to a strong pancreatic atrophy and lipomatosis. ADM formation indicates that pancreatic acinar cells are susceptible for EWSR1-FLI1-mediated oncogenic transformation to a limited extent. However, the EWSR1-FLI1 oncogene is insufficient to induce pancreatic cancer development.

CDKs in Sarcoma: Mediators of Disease and Emerging Therapeutic Targets

Sarcomas represent one of the most challenging tumor types to treat due to their diverse nature and our incomplete understanding of their underlying biology. Recent work suggests cyclin-dependent kinase (CDK) pathway activation is a powerful driver of sarcomagenesis. CDK proteins participate in numerous cellular processes required for normal cell function, but their dysregulation is a hallmark of many pathologies including cancer. The contributions and significance of aberrant CDK activity to sarcoma development, however, is only partly understood. Here, we describe what is known about CDK-related alterations in the most common subtypes of sarcoma and highlight areas that warrant further investigation. As disruptions in CDK pathways appear in most, if not all, subtypes of sarcoma, we discuss the history and value of pharmacologically targeting CDKs to combat these tumors. The goals of this review are to (1) assess the prevalence and importance of CDK pathway alterations in sarcomas, (2) highlight the gap in knowledge for certain CDKs in these tumors, and (3) provide insight into studies focused on CDK inhibition for sarcoma treatment. Overall, growing evidence demonstrates a crucial role for activated CDKs in sarcoma development and as important targets for sarcoma therapy.

Loss of Stag2 cooperates with EWS-FLI1 to transform murine Mesenchymal stem cells

BACKGROUND

Ewing sarcoma is a malignancy of primitive cells, possibly of mesenchymal origin. It is probable that genetic perturbations other than EWS-FLI1 cooperate with it to produce the tumor. Sequencing studies identified STAG2 mutations in approximately 15% of cases in humans. In the present study, we hypothesize that loss of Stag2 cooperates with EWS-FLI1 in generating sarcomas derived from murine mesenchymal stem cells (MSCs).

METHODS

Mice bearing an inducible EWS-FLI1 transgene were crossed to p53^-/- mice in pure C57/Bl6 background. MSCs were derived from the bone marrow of the mice. EWS-FLI1 induction and Stag2 knockdown were achieved in vitro by adenovirus-Cre and shRNA-bearing pGIPZ lentiviral infection, respectively. The cells were then treated with ionizing radiation to 10 Gy. Anchorage independent growth in vitro was assessed by soft agar assays. Cellular migration and invasion were evaluated by transwell assays. Cells were injected with Matrigel intramuscularly into C57/Bl6 mice to test for tumor formation.

RESULTS

Primary murine MSCs with the genotype EWS-FLI1 p53^-/- were resistant to transformation and did not form tumors in syngeneic mice without irradiation. Stag2 inhibition increased the efficiency and speed of sarcoma formation significantly in irradiated EWS-FLI1 p53^-/- MSCs. The efficiency of tumor formation was 91% for cells in mice injected with Stag2-repressed cells and 22% for mice receiving cells without Stag2 inhibition (p < .001). Stag2 knockdown reduced survival of mice in Kaplan-Meier analysis (p < .001). It also increased MSC migration and invasion in vitro but did not affect proliferation rate or aneuploidy.

CONCLUSION

Loss of Stag2 has a synergistic effect with EWS-FLI1 in the production of sarcomas from murine MSCs, but the mechanism may not relate to increased proliferation or chromosomal instability. Primary murine MSCs are resistant to transformation, and the combination of p53 null mutation, EWS-FLI1, and Stag2 inhibition does not confer immediate conversion of MSCs to sarcomas. Irradiation is necessary in this model, suggesting that perturbations of other genes beside Stag2 and p53 are likely to be essential in the development of EWS-FLI1-driven sarcomas from MSCs.

Newly identified LMO3-BORCS5 fusion oncogene in Ewing sarcoma at relapse is a driver of tumor progression

Recently, we detected a new fusion transcript LMO3-BORCS5 in a patient with Ewing sarcoma within a cohort of relapsed pediatric cancers. LMO3-BORCS5 was as highly expressed as the characteristic fusion oncogene EWS/FLI1. However, the expression level of LMO3-BORCS5 at diagnosis was very low. Sanger sequencing depicted two LMO3-BORCS5 variants leading to loss of the functional domain LIM2 in LMO3 gene, and disruption of BORCS5. In vitro studies showed that LMO3-BORCS5 (i) increases proliferation, (ii) decreases expression of apoptosis-related genes and treatment sensitivity, and (iii) downregulates genes involved in differentiation and upregulates proliferative and extracellular matrix-related pathways. Remarkably, in vivo LMO3-BORCS5 demonstrated its high oncogenic potential by inducing tumors in mouse fibroblastic NIH-3T3 cell line. Moreover, BORCS5 probably acts, in vivo, as a tumor-suppressor gene. In conclusion, functional studies of fusion oncogenes at relapse are of great importance to define mechanisms involved in tumor progression and resistance to conventional treatments.

Discovery of New Fusion Transcripts in a Cohort of Pediatric Solid Cancers at Relapse and Relevance for Personalized Medicine

We hypothetized that pediatric cancers would more likely harbor fusion transcripts. To dissect the complexity of the fusions landscape in recurrent solid pediatric cancers, we conducted a study on 48 patients with different relapsing or resistant malignancies. By analyzing RNA sequencing data with a new in-house pipeline for fusions detection named ChimComp, followed by verification by real-time PCR, we identified and classified the most confident fusion transcripts (FTs) according to their potential biological function and druggability. The majority of FTs were predicted to affect key cancer pathways and described to be involved in oncogenesis. Contrary to previous descriptions, we found no significant correlation between the number of fusions and mutations, emphasizing the particularity to study pre-treated pediatric patients. A considerable proportion of FTs containing tumor suppressor genes was detected, reflecting their importance in pediatric cancers. FTs containing non-receptor tyrosine kinases occurred at low incidence and predominantly in brain tumors. Remarkably, more than 30% of patients presented a potentially druggable high-confidence fusion. In conclusion, we detected new oncogenic FTs in relapsing pediatric cancer patients by establishing a robust pipeline that can be applied to other malignancies, to detect and prioritize experimental validation studies leading to the development of new therapeutic options.

Translocations involving ETS family proteins in human cancer

The ETS transcription factors regulate expression of genes involved in normal cell development, proliferation, differentiation, angiogenesis, and apoptosis, consisting of 28 family members in humans. Dysregulation of these transcription factors facilitates cell proliferation in cancers, and several members participate in invasion and metastasis by activating certain gene transcriptions. ETS1 and ETS2 are the founding members of the ETS family and regulate transcription by binding to ETS sequences. Three chimeric genes involving ETS genes have been identified in human cancers, which are EWS-FLI1 in Ewing's sarcoma, TMPRSS2-ERG in prostate cancer, and ETV6-RUNX1 in acute lymphocytic leukemia. Although these fusion transcripts definitely contribute to the pathogenesis of the disease, the impact of these fusion transcripts on patients' prognosis is highly controversial. In the present review, the roles of ETS protein translocations in human carcinogenesis are discussed.

Ewing sarcoma

Ewing sarcoma is the second most frequent bone tumour of childhood and adolescence that can also arise in soft tissue. Ewing sarcoma is a highly aggressive cancer, with a survival of 70-80% for patients with standard-risk and localized disease and ~30% for those with metastatic disease. Treatment comprises local surgery, radiotherapy and polychemotherapy, which are associated with acute and chronic adverse effects that may compromise quality of life in survivors. Histologically, Ewing sarcomas are composed of small round cells expressing high levels of CD99. Genetically, they are characterized by balanced chromosomal translocations in which a member of the FET gene family is fused with an ETS transcription factor, with the most common fusion being EWSR1-FLI1 (85% of cases). Ewing sarcoma breakpoint region 1 protein (EWSR1)-Friend leukaemia integration 1 transcription factor (FLI1) is a tumour-specific chimeric transcription factor (EWSR1-FLI1) with neomorphic effects that massively rewires the transcriptome. Additionally, EWSR1-FLI1 reprogrammes the epigenome by inducing de novo enhancers at GGAA microsatellites and by altering the state of gene regulatory elements, creating a unique epigenetic signature. Additional mutations at diagnosis are rare and mainly involve STAG2, TP53 and CDKN2A deletions. Emerging studies on the molecular mechanisms of Ewing sarcoma hold promise for improvements in early detection, disease monitoring, lower treatment-related toxicity, overall survival and quality of life.

Combined experience of six independent laboratories attempting to create an Ewing sarcoma mouse model

Ewing sarcoma (ES) involves a tumor-specific chromosomal translocation that produces the EWS-FLI1 protein, which is required for the growth of ES cells both in vitro and in vivo. However, an EWS-FLI1-driven transgenic mouse model is not currently available. Here, we present data from six independent laboratories seeking an alternative approach to express EWS-FLI1 in different murine tissues. We used the Runx2, Col1a2.3, Col1a3.6, Prx1, CAG, Nse, NEFL, Dermo1, P0, Sox9 and Osterix promoters to target EWS-FLI1 or Cre expression. Additional approaches included the induction of an endogenous chromosomal translocation, in utero knock-in, and the injection of Cre-expressing adenovirus to induce EWS-FLI1 expression locally in multiple lineages. Most models resulted in embryonic lethality or developmental defects. EWS-FLI1-induced apoptosis, promoter leakiness, the lack of potential cofactors, and the difficulty of expressing EWS-FLI1 in specific sites were considered the primary reasons for the failed attempts to create a transgenic mouse model of ES.

Small animal models for the study of bone sarcoma pathogenesis:characteristics, therapeutic interests and limitations

Osteosarcoma, Ewing sarcoma and chondrosarcoma are the three main entities of bone sarcoma which collectively encompass more than 50 heterogeneous entities of rare malignancies. In contrast to osteosarcoma and Ewing sarcoma which mainly affect adolescents and young adults and exhibit a high propensity to metastasise to the lungs, chondrosarcoma is more frequently observed after 40 years of age and is characterised by a high frequency of local recurrence. The combination of chemotherapy, surgical resection and radiotherapy has contributed to an improved outcome for these patients. However, a large number of patients still suffer significant therapy related toxicities or die of refractory and metastatic disease. To better delineate the pathogenesis of bone sarcomas and to identify and test new therapeutic options, major efforts have been invested over the past decades in the development of relevant pre-clinical animal models. Nowadays, in vivo models aspire to mimic all the steps and the clinical features of the human disease as accurately as possible and should ideally be manipulable. Considering these features and given their small size, their conduciveness to experiments, their affordability as well as their human-like bone-microenvironment and immunity, murine pre-clinical models are interesting in the context of these pathologies. This chapter will provide an overview of the murine models of bone sarcomas, paying specific attention for the models induced by inoculation of tumour cells. The genetically-engineered mouse models of bone sarcoma will also be summarized.

RING1B contributes to Ewing sarcoma development by repressing the NaV1.6 sodium channel and the NF-κB pathway, independently of the fusion oncoprotein

Ewing sarcoma (ES) is an aggressive tumor defined by EWSR1 gene fusions that behave as an oncogene. Here we demonstrate that RING1B is highly expressed in primary ES tumors, and its expression is independent of the fusion oncogene. RING1B-depleted ES cells display an expression profile enriched in genes functionally involved in hematological development but RING1B depletion does not induce cellular differentiation. In ES cells, RING1B directly binds the SCN8A sodium channel promoter and its depletion results in enhanced Nav1.6 expression and function. The signaling pathway most significantly modulated by RING1B is NF-κB. RING1B depletion results in enhanced p105/p50 expression, which sensitizes ES cells to apoptosis by FGFR/SHP2/STAT3 blockade. Reduced NaV1.6 function protects ES cells from apoptotic cell death by maintaining low NF-κB levels. Our findings identify RING1B as a trait of the cell-of-origin and provide a potential targetable vulnerability.

Efficacy of ATR inhibitors as single agents in Ewing sarcoma

Ewing sarcomas (ES) are pediatric bone tumors that arise from a driver translocation, most frequently EWS/FLI1. Current ES treatment involves DNA damaging agents, yet the basis for the sensitivity to these therapies remains unknown. Oncogene-induced replication stress (RS) is a known source of endogenous DNA damage in cancer, which is suppressed by ATR and CHK1 kinases. We here show that ES suffer from high endogenous levels of RS, rendering them particularly dependent on the ATR pathway. Accordingly, two independent ATR inhibitors show in vitro toxicity in ES cell lines as well as in vivo efficacy in ES xenografts as single agents. Expression of EWS/FLI1 or EWS/ERG oncogenic translocations sensitizes non-ES cells to ATR inhibitors. Our data shed light onto the sensitivity of ES to genotoxic agents, and identify ATR inhibitors as a potential therapy for Ewing Sarcomas.

Therapeutic opportunities in Ewing sarcoma: EWS-FLI inhibition via LSD1 targeting

Ewing sarcoma is an aggressive primary pediatric bone tumor, often diagnosed in adolescents and young adults. A pathognomonic reciprocal chromosomal translocation results in a fusion gene coding for a protein which derives its N-terminus from a FUS/EWS/TAF15 (FET) protein family member, commonly EWS, and C-terminus containing the DNA-binding domain of an ETS transcription factor, commonly FLI1. Nearly 85% of cases express the EWS-FLI protein which functions as a transcription factor and drives oncogenesis. As the primary genomic lesion and a protein which is not expressed in normal cells, disrupting EWS-FLI function is an attractive therapeutic strategy for Ewing sarcoma. However, transcription factors are notoriously difficult targets for the development of small molecules. Improved understanding of the oncogenic mechanisms employed by EWS-FLI to hijack normal cellular programming has uncovered potential novel approaches to pharmacologically block EWS-FLI function. In this review we examine targeting the chromatin regulatory enzymes recruited to conspire in oncogenesis with a focus on the histone lysine specific demethylase 1 (LSD1). LSD1 inhibitors are being aggressively investigated in acute myeloid leukemia and the results of early clinical trials will help inform the future use of LSD1 inhibitors in sarcoma. High LSD1 expression is observed in Ewing sarcoma patient samples and mechanistic and preclinical data suggest LSD1 inhibition globally disrupts the function of EWS-ETS proteins.

Relevance of Fusion Genes in Pediatric Cancers: Toward Precision Medicine

Pediatric cancers differ from adult tumors, especially by their very low mutational rate. Therefore, their etiology could be explained in part by other oncogenic mechanisms such as chromosomal rearrangements, supporting the possible implication of fusion genes in the development of pediatric cancers. Fusion genes result from chromosomal rearrangements leading to the juxtaposition of two genes. Consequently, an abnormal activation of one or both genes is observed. The detection of fusion genes has generated great interest in basic cancer research and in the clinical setting, since these genes can lead to better comprehension of the biological mechanisms of tumorigenesis and they can also be used as therapeutic targets and diagnostic or prognostic biomarkers. In this review, we discuss the molecular mechanisms of fusion genes and their particularities in pediatric cancers, as well as their relevance in murine models and in the clinical setting. We also point out the difficulties encountered in the discovery of fusion genes. Finally, we discuss future perspectives and priorities for finding new innovative therapies in childhood cancer.

Discovering novel oncogenic pathways and new therapies using zebrafish models of sarcoma

Sarcoma is a type of cancer affecting connective, supportive, or soft tissue of mesenchymal origin. Despite rare incidence in adults (<1%), over 15% of pediatric cancers are sarcoma. Sadly, both adults and children with relapsed or metastatic disease have devastatingly high rates of mortality. Current treatment options for sarcoma include surgery, radiation, and/or chemotherapy; however, significant limitations exist with respect to the efficacy of these strategies. Strong impetus has been placed on the development of novel therapies and preclinical models for uncovering mechanisms involved in the development, progression, and therapy resistance of sarcoma. Over the past 15 years, the zebrafish has emerged as a powerful genetic model of human cancer. High genetic conservation when combined with a unique susceptibility to develop sarcoma has made the zebrafish an effective tool for studying these diseases. Transgenic and gene-activation strategies have been employed to develop zebrafish models of rhabdomyosarcoma, malignant peripheral nerve sheath tumors, Ewing's sarcoma, chordoma, hemangiosarcoma, and liposarcoma. These models all display remarkable molecular and histopathological conservation with their human cancer counterparts and have offered excellent platforms for understanding disease progression in vivo. Short tumor latency and the amenability of zebrafish for ex vivo manipulation, live imaging studies, and tumor cell transplantation have allowed for efficient study of sarcoma initiation, growth, self-renewal, and maintenance. When coupled with facile chemical genetic approaches, zebrafish models of sarcoma have provided a strong translational tool to uncover novel drug pathways and new therapeutic strategies.

Widespread Chromatin Accessibility at Repetitive Elements Links Stem Cells with Human Cancer

Chromatin regulation is critical for differentiation and disease. However, features linking the chromatin environment of stem cells with disease remain largely unknown. We explored chromatin accessibility in embryonic and multipotent stem cells and unexpectedly identified widespread chromatin accessibility at repetitive elements. Integrating genomic and biochemical approaches, we demonstrate that these sites of increased accessibility are associated with well-positioned nucleosomes marked by distinct histone modifications. Differentiation is accompanied by chromatin remodeling at repetitive elements associated with altered expression of genes in relevant developmental pathways. Remarkably, we found that the chromatin environment of Ewing sarcoma, a mesenchymally derived tumor, is shared with primary mesenchymal stem cells (MSCs). Accessibility at repetitive elements in MSCs offers a permissive environment that is exploited by the critical oncogene responsible for this cancer. Our data demonstrate that stem cells harbor a unique chromatin landscape characterized by accessibility at repetitive elements, a feature associated with differentiation and oncogenesis.

Increased survival and cell cycle progression pathways are required for EWS/FLI1-induced malignant transformation

Ewing sarcoma (ES) is the second most frequent childhood bone cancer driven by the EWS/FLI1 (EF) fusion protein. Genetically defined ES models are needed to understand how EF expression changes bone precursor cell differentiation, how ES arises and through which mechanisms of inhibition it can be targeted. We used mesenchymal Prx1-directed conditional EF expression in mice to study bone development and to establish a reliable sarcoma model. EF expression arrested early chondrocyte and osteoblast differentiation due to changed signaling pathways such as hedgehog, WNT or growth factor signaling. Mesenchymal stem cells (MSCs) expressing EF showed high self-renewal capacity and maintained an undifferentiated state despite high apoptosis. Blocking apoptosis through enforced BCL2 family member expression in MSCs promoted efficient and rapid sarcoma formation when transplanted to immunocompromised mice. Mechanistically, high BCL2 family member and CDK4, but low P53 and INK4A protein expression synergized in Ewing-like sarcoma development. Functionally, knockdown of Mcl1 or Cdk4 or their combined pharmacologic inhibition resulted in growth arrest and apoptosis in both established human ES cell lines and EF-transformed mouse MSCs. Combinatorial targeting of survival and cell cycle progression pathways could counteract this aggressive childhood cancer.

Loss of Ewing sarcoma EWS allele promotes tumorigenesis by inducing chromosomal instability in zebrafish

The Ewing sarcoma family of tumors expresses aberrant EWSR1- (EWS) fusion genes that are derived from chromosomal translocation. Although these fusion genes are well characterized as transcription factors, their formation leaves a single EWS allele in the sarcoma cells, and the contribution that the loss of EWS makes towards disease pathogenesis is unknown. To address this question, we utilized zebrafish mutants for ewsa and tp53. The zebrafish tp53(M214K)^w/m line and the ewsa^w/m, zygotic ewsa^m/m, and Maternal-Zygotic (MZ) ewsa^m/m lines all displayed zero to low incidence of tumorigenesis. However, when the ewsa and tp53 mutant lines were crossed with each other, the incidence of tumorigenesis drastically increased. Furthermore, 27 hour post fertilization (hpf) MZ ewsa^m/m mutant embryos displayed a higher incidence of aberrant chromosome numbers and mitotic dysfunction compared to wildtype zebrafish embryos. Consistent with this finding, tumor samples obtained from ewsa^m/m;tp53^w/m zebrafish displayed loss of heterozygosity (LOH) for the wildtype tp53 locus. These results suggest that wildtype Ewsa inhibits LOH induction, possibly by maintaining chromosomal stability. We propose that the loss of ewsa promotes tumorigenesis, and EWS deficiency may contribute to the pathogenesis of EWS-fusion-expressing sarcomas.

An EWS-FLI1-Induced Osteosarcoma Model Unveiled a Crucial Role of Impaired Osteogenic Differentiation on Osteosarcoma Development

EWS-FLI1, a multi-functional fusion oncogene, is exclusively detected in Ewing sarcomas. However, previous studies reported that rare varieties of osteosarcomas also harbor EWS-ETS family fusion. Here, using the doxycycline-inducible EWS-FLI1 system, we established an EWS-FLI1-dependent osteosarcoma model from murine bone marrow stromal cells. We revealed that the withdrawal of EWS-FLI1 expression enhances the osteogenic differentiation of sarcoma cells, leading to mature bone formation. Taking advantage of induced pluripotent stem cell (iPSC) technology, we also show that sarcoma-derived iPSCs with cancer-related genetic abnormalities exhibited an impaired differentiation program of osteogenic lineage irrespective of the EWS-FLI1 expression. Finally, we demonstrate that EWS-FLI1 contributed to secondary sarcoma development from the sarcoma iPSCs after osteogenic differentiation. These findings demonstrate that modulating cellular differentiation is a fundamental principle of EWS-FLI1-induced osteosarcoma development. This in vitro cancer model using sarcoma iPSCs should provide a unique platform for dissecting relationships between the cancer genome and cellular differentiation.

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EWS-FLI1 induces small-cell osteosarcoma from murine bone marrow stromal cells

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EWS-FLI1 inhibits osteogenic differentiation of sarcoma cells in vivo

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Modulating differentiation is a fundamental principle of osteosarcoma development

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Sarcoma iPSCs should provide a unique platform for dissecting cancer biology

Overexpression of HOX genes is prevalent in Ewing sarcoma and is associated with altered epigenetic regulation of developmental transcription programs

The polycomb proteins BMI-1 and EZH2 are highly overexpressed by Ewing sarcoma (ES), a tumor of stem cell origin that is driven by EWS-ETS fusion oncogenes, most commonly EWS-FLI1. In the current study we analyzed expression of transcription programs that are controlled by polycomb proteins during embryonic development to determine if they are abnormal in ES. Our results show that polycomb target gene expression in ES deviates from normal tissues and stem cells and that, as expected, most targets are relatively repressed. However, we also discovered a paradoxical up regulation of numerous polycomb targets and these were highly enriched for homeobox (HOX) genes. Comparison of HOX profiles between malignant and non-malignant tissues revealed a distinctive HOX profile in ES, which was characterized by overexpression of posterior HOXD genes. In addition, ectopic expression of EWS-FLI1 during stem cell differentiation led to aberrant up regulation of posterior HOXD genes. Mechanistically, this up regulation was associated with altered epigenetic regulation. Specifically, ES and EWS-FLI1+ stem cells displayed a relative loss of polycomb-dependent H3K27me3 and gain of trithorax-dependent H3K4me3 at the promoters of posterior HOXD genes and also at the HOXD11.12 polycomb response element. In addition, a striking correlation was evident between HOXD13 and other genes whose regulation is coordinately regulated during embryonic development by distal enhancer elements. Together, these studies demonstrate that epigenetic regulation of polycomb target genes, in particular HOXD genes, is altered in ES and that these changes are mediated downstream of EWS-FLI1.

EWS/FLI utilizes NKX2-2 to repress mesenchymal features of Ewing sarcoma

In Ewing sarcoma, NKX2-2 is a critical activated target of the oncogenic transcription factor EWS/FLI that is required for transformation. However, its biological function in this malignancy is unknown. Here we provide evidence that NKX2-2 mediates the EWS/FLI-controlled block of mesenchymal features. Transcriptome-wide RNA sequencing revealed that NKX2-2 represses cell adhesion and extracellular matrix organization genes. NKX2-2-depleted cells form more focal adhesions and organized actin stress fibers, and spread over a wider area—hallmarks of mesenchymally derived cells. Furthermore, NKX2-2 represses the actin-stabilizing protein zyxin, suggesting that these morphological changes are attributable to zyxin de-repression. In addition, NKX2-2-knockdown cells display marked increases in migration and substrate adhesion. However, only part of the EWS/FLI phenotype is NKX2-2-dependent; consequently, NKX2-2 is insufficient to rescue EWS/FLI repression of mesenchymalization. Strikingly, we found that EWS/FLI-and NKX22-repressed genes are activated by ZEB2, which was previously shown to block Ewing sarcoma epithelialization. Together, these data support an emerging theme wherein Ewing sarcoma cells highly express transcription factors that maintain an undifferentiated state. Importantly, co-opting epithelial and mesenchymal traits by Ewing sarcoma cells may explain how the primary tumor grows rapidly while also “passively” metastasizing, without the need for transitions toward differentiated states, as in carcinomas.

The cell cycle regulator CCDC6 is a key target of RNA-binding protein EWS

Genetic translocation of EWSR1 to ETS transcription factor coding region is considered as primary cause for Ewing sarcoma. Previous studies focused on the biology of chimeric transcription factors formed due to this translocation. However, the physiological consequences of heterozygous EWSR1 loss in these tumors have largely remained elusive. Previously, we have identified various mRNAs bound to EWS using PAR-CLIP. In this study, we demonstrate CCDC6, a known cell cycle regulator protein, as a novel target regulated by EWS. siRNA mediated down regulation of EWS caused an elevated apoptosis in cells in a CCDC6-dependant manner. This effect was rescued upon re-expression of CCDC6. This study provides evidence for a novel functional link through which wild-type EWS operates in a target-dependant manner in Ewing sarcoma.

The impact of osteoblastic differentiation on osteosarcomagenesis in the mouse

Osteosarcomas remain an enigmatic group of malignancies that share in common the presence of transformed cells producing osteoid matrix, even if these cells comprise a minority of the tumor volume. The differentiation state of osteosarcomas has therefore become a topic of interest and challenge to those who study this disease. In order to test how the cell of origin contributes to the final state of differentiation in the transformed cells, we compared the relative tumorigenicity of Cre-LoxP conditional disruption of the cell cycle checkpoint tumor-suppressor genes Trp53 and Rb1 using Prx1-Cre, Collagen-1α1-Cre and Osteocalcin-Cre to transform undifferentiated mesenchyme, preosteoblasts and mature osteoblasts, respectively. The Prx1 and Col1α1 lineages developed tumors with nearly complete penetrance, as anticipated. Osteosarcomas also developed in 44% of Oc-Cre;Rb1(fl/fl);Trp53(fl/fl) mice. We confirmed using 5-ethynyl-2'-deoxyuridine click chemistry that the Oc-Cre lineage includes very few actively cycling cells. By assessing radiographic mineralization and histological osteoid production, the differentiation state of tumors did not correlate with the differentiation state of the lineage of origin. Some of the osteocalcin-lineage-derived osteosarcomas were among the least osteoblastic. Osteocalcin immunohistochemistry in tumors correlated well with the expression of DNA methyl transferases, suggesting that silencing of these epigenetic regulators may influence the final differentiation state of an osteosarcoma. Transformation of differentiated, minimally proliferative osteoblasts is possible but may require such an epigenetic reprogramming that the tumors no longer resemble their differentiated origins.

The role of mesenchymal stem/progenitor cells in sarcoma: update and dispute

Sarcoma is the collective name for a relatively rare, yet heterogeneous group of cancers, most probably derived from mesenchymal tissues. There are currently over 50 sarcoma subtypes described underscoring the clinical and biologic diversity of this group of malignant cancers. This wide lineage range might suggest that sarcomas originate from either many committed different cell types or from a multipotent cell. Mesenchymal stem/progenitor cells (MSCs) are able to differentiate into many cell types and these multipotent cells have been isolated from several adult human tumors, making them available for research as well as potential beneficial therapeutical agents. Recent accomplishments in the field have broadened our knowledge of MSCs in relation to sarcoma origin and sarcoma treatment in therapeutic settings. However, numerous concerns and disputes have been raised about whether they are the putative originating cells of sarcoma and their questionable role in sarcomagenesis and progression. We summarize the update and dispute about MSC investigations in sarcomas including the definition, cell origin hypothesis, functional and descriptive assays, roles in sarcomagenesis and targeted therapy, with the purpose to give a comprehensive view of the role of MSCs in sarcomas.

Molecular pathways: targeting ETS gene fusions in cancer

Rearrangements, or gene fusions, involving the ETS family of transcription factors are common driving events in both prostate cancer and Ewing sarcoma. These rearrangements result in pathogenic expression of the ETS genes and trigger activation of transcriptional programs enriched for invasion and other oncogenic features. Although ETS gene fusions represent intriguing therapeutic targets, transcription factors, such as those comprising the ETS family, have been notoriously difficult to target. Recently, preclinical studies have demonstrated an association between ETS gene fusions and components of the DNA damage response pathway, such as PARP1, the catalytic subunit of DNA protein kinase (DNAPK), and histone deactylase 1 (HDAC1), and have suggested that ETS fusions may confer sensitivity to inhibitors of these DNA repair proteins. In this review, we discuss the role of ETS fusions in cancer, the preclinical rationale for targeting ETS fusions with inhibitors of PARP1, DNAPK, and HDAC1, as well as ongoing clinical trials targeting ETS gene fusions.

Blocking the road, stopping the engine or killing the driver? Advances in targeting EWS/FLI-1 fusion in Ewing sarcoma as novel therapy

INTRODUCTION

Ewing sarcoma (ES) represents the paradigm of an aberrant E-twenty-six (ETS) oncogene-driven cancer. It is characterized by specific rearrangements of one of five alternative ETS family member genes with EWSR1. There is experimental evidence that the resulting fusion proteins act as aberrant transcription factors driving ES pathogenesis. The transcriptional gene regulatory network driven by EWS-ETS proteins provides the oncogenic engine to the tumor. Therefore, EWS-ETS and their downstream machinery are considered ideal tumor-specific therapeutic targets.

AREAS COVERED

This review critically discusses the literature on the development of EWS-ETS-directed ES targeting strategies considering current knowledge of EWS-ETS biology and cellular context. It focuses on determinants of EWS-FLI1 function with an emphasis on interactions with chromatin structure. We speculate about the relevance of poorly investigated aspects in ES research such as chromatin remodeling and DNA damage repair for the development of targeted therapies.

EXPERT OPINION

This review questions the specificity of signature-based screening approaches to the identification of EWS-FLI1-targeted compounds. It challenges the view that targeting the downstream gene regulatory network carries potential for therapeutic breakthroughs because of resistance-inducing network rewiring. Instead, we propose to combine targeting of the fusion protein with epigenetic therapy as a future treatment strategy in ES.

EWS-WT1 oncoprotein activates neuronal reprogramming factor ASCL1 and promotes neural differentiation

The oncogenic fusion gene EWS-WT1 is the defining chromosomal translocation in desmoplastic small round-cell tumors (DSRCT), a rare but aggressive soft tissue sarcoma with a high rate of mortality. EWS-WT1 functions as an aberrant transcription factor that drives tumorigenesis, but the mechanistic basis for its pathogenic activity is not well understood. To address this question, we created a transgenic mouse strain that permits physiologic expression of EWS-WT1 under the native murine Ews promoter. EWS-WT1 expression led to a dramatic induction of many neuronal genes in embryonic fibroblasts and primary DSRCT, most notably the neural reprogramming factor ASCL1. Mechanistic analyses demonstrated that EWS-WT1 directly bound the proximal promoter of ASCL1, activating its transcription through multiple WT1-responsive elements. Conversely, EWS-WT1 silencing in DSRCT cells reduced ASCL1 expression and cell viability. Notably, exposure of DSRCT cells to neuronal induction media increased neural gene expression and induced neurite-like projections, both of which were abrogated by silencing EWS-WT1. Taken together, our findings reveal that EWS-WT1 can activate neural gene expression and direct partial neural differentiation via ASCL1, suggesting agents that promote neural differentiation might offer a novel therapeutic approach to treat DSRCT.

Expression of FUS-CHOP fusion protein in immortalized/transformed human mesenchymal stem cells drives mixoid liposarcoma formation

Increasing evidence supports that mesenchymal stromal/stem cells (MSCs) may represent the target cell for sarcoma development. Although different sarcomas have been modeled in mice upon expression of fusion oncogenes in MSCs, sarcomagenesis has not been successfully modeled in human MSCs (hMSCs). We report that FUS-CHOP, a hallmark fusion gene in mixoid liposarcoma (MLS), has an instructive role in lineage commitment, and its expression in hMSC sequentially immortalized/transformed with up to five oncogenic hits (p53 and Rb deficiency, hTERT over-expression, c-myc stabilization, and H-RAS(v12) mutation) drives the formation of serially transplantable MLS. This is the first model of sarcoma based on the expression of a sarcoma-associated fusion protein in hMSC, and allowed us to unravel the differentiation processes and signaling pathways altered in the MLS-initiating cells. This study will contribute to test novel therapeutic approaches and constitutes a proof-of-concept to use hMSCs as target cell for modeling other fusion gene-associated human sarcomas.

ZEB2 Represses the Epithelial Phenotype and Facilitates Metastasis in Ewing Sarcoma

The vast majority of cancer-related deaths are attributable to metastasis. Effective treatment of metastatic disease will be improved by a better understanding of the molecular mechanisms contributing to this phenomenon. Much of the work in this field has focused on metastasis of carcinomas, tumors of epithelial origin, while metastasis of sarcomas, tumors of mesenchymal origin, remains poorly understood. Experimental evidence from studies in carcinomas, coupled with clinical observations, highlights the importance of both epithelial and mesenchymal characteristics in these cancer cells that make them competent for metastasis. We set out to test if similar cellular plasticity contributes to sarcoma metastasis. We found that the transcription factor, ZEB2, repressed epithelial gene expression in Ewing sarcoma cells, and this, in turn, repressed the epithelial phenotype. When ZEB2 was experimentally reduced in these cells, epithelial characteristics including decreased migratory ability and cytoskeleton rearrangements were observed. Furthermore, ZEB2 reduction in Ewing sarcoma cells resulted in a decreased metastatic potential using a mouse metastasis model. Our data show that Ewing sarcoma cells may have more epithelial plasticity than previously appreciated. This coupled with previous data demonstrating Ewing sarcoma cells also have mesenchymal features primes these cells to successfully metastasize. This is clinically relevant for 2 important reasons. First, this may offer a therapeutic opportunity to induce characteristics of one cell type or the other depending on the stage of the disease. Second, and more broadly, this raises questions about the cell of origin in Ewing sarcoma and may inform future animal models of the disease.

The adolescent and young adult with cancer: state of the art -- bone tumors

Primary malignant bone tumors in the pediatric to young adult populations are relatively uncommon and account for about 6 % of all cancers in those less than 20 years old [1] and 3 % of all cancers in adolescents and young adults (AYA) within the age range of 15 to 29 years [2]. Osteosarcoma (OS) and Ewing's sarcoma (ES) comprise the majority of malignant bone tumors. The approach to treatment for both tumors consists of local control measures (surgery or radiation) as well as systemic therapy with high-dose chemotherapy. Despite earlier advances, there have been no substantial improvements in outcomes over the past several decades, particularly for patients with metastatic disease. This review summarizes the major advances in the treatment of OS and ES and the standard therapies available today, current active clinical trials, and areas of investigation into molecularly targeted therapies.

The adolescent and young adult with cancer: state of the art -- bone tumors

Primary malignant bone tumors in the pediatric to young adult populations are relatively uncommon and account for about 6 % of all cancers in those less than 20 years old [1] and 3 % of all cancers in adolescents and young adults (AYA) within the age range of 15 to 29 years [2]. Osteosarcoma (OS) and Ewing's sarcoma (ES) comprise the majority of malignant bone tumors. The approach to treatment for both tumors consists of local control measures (surgery or radiation) as well as systemic therapy with high-dose chemotherapy. Despite earlier advances, there have been no substantial improvements in outcomes over the past several decades, particularly for patients with metastatic disease. This review summarizes the major advances in the treatment of OS and ES and the standard therapies available today, current active clinical trials, and areas of investigation into molecularly targeted therapies.

Mesenchymal stem cell transformation and sarcoma genesis

MSCs are hypothesized to potentially give rise to sarcomas after transformation and therefore serve as a good model to study sarcomagenesis. Both spontaneous and induced transformation of MSCs have been reported, however, spontaneous transformation has only been convincingly shown in mouse MSCs while induced transformation has been demonstrated in both mouse and human MSCs. Transformed MSCs of both species can give rise to pleomorphic sarcomas after transplantation into mice, indicating the potential MSC origin of so-called non-translocation induced sarcomas. Comparison of expression profiles and differentiation capacities between MSCs and sarcoma cells further supports this. Deregulation of P53- Retinoblastoma-, PI3K-AKT-and MAPK pathways has been implicated in transformation of MSCs. MSCs have also been indicated as cell of origin in several types of chromosomal translocation associated sarcomas. In mouse models the generated sarcoma type depends on amongst others the tissue origin of the MSCs, the targeted pathways and genes and the differentiation commitment status of MSCs. While some insights are glowing, it is clear that more studies are needed to thoroughly understand the molecular mechanism of sarcomagenesis from MSCs and mechanisms determining the sarcoma type, which will potentially give directions for targeted therapies.

Xenograft and genetically engineered mouse model systems of osteosarcoma and Ewing's sarcoma: tumor models for cancer drug discovery

INTRODUCTION

There are > 75 histological types of solid tumors that are classified into two major groups: bone and soft-tissue sarcomas. These diseases are more prevalent in children, and pediatric sarcomas tend to be highly aggressive and rapidly progressive. Sarcomas in adults may follow a more indolent course, but aggressive tumors are also common. Sarcomas that are metastatic at diagnosis, or recurrent following therapy, remain refractory to current treatment options with dismal overall survival rates. A major focus of clinical trials, for patients with sarcoma, is to identify novel and more effective therapeutic strategies targeted to genomic or proteomic aberrations specific to the malignant cells. Critical to the understanding of the potential for targeted therapies are models of disease that are representative of clinical disease and predictive of relevant clinical responses.

AREAS COVERED

In this article, the authors discuss the use of mouse xenograft models and genetically engineered mice in cancer drug discovery. The authors provide a special focus on models for the two most common bone sarcomas: osteosarcoma (OS) and Ewing's sarcoma (ES).

EXPERT OPINION

Predicting whether a new anticancer agent will have a positive therapeutic index in patients with OS and ES remains a challenge. The use of mouse sarcoma models for understanding the mechanisms involved in the response of tumors to new treatments is an important step in the process of drug discovery and the development of clinically relevant therapeutic strategies for these diseases.

Salient features of mesenchymal stem cells-implications for Ewing sarcoma modeling

Despite a heightened appreciation of the many defining molecular aberrations in Ewing sarcoma, the cooperative genetic environment and permissive cell of origin essential for EWS/ETS-mediated oncogenesis remain elusive. Consequently, inducible animal and in vitro models of Ewing sarcoma from a native cellular context are unable to fully recapitulate malignant transformation. Despite these shortcomings, human, and murine mesenchymal stem cells (MSCs) are the closest working in vitro systems available. MSCs are tolerant of ectopic EWS/FLI expression, which is accompanied by a molecular signature most similar to Ewing sarcoma. Whether MSCs are the elusive cell of origin or simply a tolerant platform of the EWS/FLI transcriptome, these cells have become an excellent molecular tool to investigate and manipulate oncogenesis in Ewing sarcoma. Our understanding of the biological complexity and heterogeneity of human MSCs (hMSCs) has increased substantially over time and as such, appreciation and utilization of these salient complexities may greatly enhance the efficient use of these cells as surrogate models for Ewing sarcoma tumorigenesis.

Mouse models of sarcomas: critical tools in our understanding of the pathobiology

Sarcomas are neoplastic malignancies that typically arise in tissues of mesenchymal origin. The identification of novel molecular mechanisms leading to sarcoma formation and the establishment of new therapies has been hampered by several critical factors. First, this type of cancer is rarely observed in the clinic with fewer than 15,000 newly cases diagnosed each year in the United States. Another complicating factor is that sarcomas are extremely heterogeneous as they arise in a multitude of tissues from many different cell lineages (e.g. bone (osteosarcoma), fat (liposarcoma), and muscle (myosarcoma)). The scarcity of clinical samples coupled with its inherent heterogeneity creates a challenging experimental environment for clinicians and scientists. Faced with these challenges, there has been extremely limited advancement in treatment options available to patients as compared to other cancers. In order to glean insight into the pathobiology of sarcomas, scientists are now using in vivo mouse models whose genomes have been specifically tailored to carry gene deletions, gene amplifications, and point mutations commonly observed in human sarcomas. The use of these model organisms has been successful in increasing our knowledge and understanding of how alterations in relevant oncogenic, tumor suppressive, and signaling pathways directly impact sarcomagenesis. It is the goal of many in the biological community that the use of these mouse models will serve as powerful in vivo tools to further our understanding of sarcomagenesis and potentially identify new therapeutic strategies.

EWS/FLI1 regulates EYA3 in Ewing sarcoma via modulation of miRNA-708, resulting in increased cell survival and chemoresistance

Ewing sarcoma is an aggressive pediatric cancer of the bone and soft tissue, in which patients whose tumors have a poor histologic response to initial chemotherapy have a poor overall prognosis. Therefore, it is important to identify molecules involved in resistance to chemotherapy. Herein, we show that the DNA repair protein and transcriptional cofactor, EYA3, is highly expressed in Ewing sarcoma tumor samples and cell lines compared with mesenchymal stem cells, the presumed cell-of-origin of Ewing sarcoma, and that it is regulated by the EWS/FLI1 fusion protein transcription factor. We further show that EWS/FLI1 mediates upregulation of EYA3 via repression of miR-708, a miRNA that targets the EYA3 3'-untranslated region, rather than by binding the EYA3 promoter directly. Importantly, we show that high levels of EYA3 significantly correlate with low levels of miR-708 in Ewing sarcoma samples, suggesting that this miR-mediated mechanism of EYA3 regulation holds true in human cancers. Because EYA proteins are important for cell survival during development, we examine, and show, that loss of EYA3 decreases survival of Ewing sarcoma cells. Most importantly, knockdown of EYA3 in Ewing sarcoma cells leads to sensitization to DNA-damaging chemotherapeutics used in the treatment of Ewing sarcoma, and as expected, after chemotherapeutic treatment, EYA3 knockdown cells repair DNA damage less effectively than their control counterparts. These studies identify EYA3 as a novel mediator of chemoresistance in Ewing sarcoma and define the molecular mechanisms of both EYA3 overexpression and of EYA3-mediated chemoresistance.

Ewing's Sarcoma: Development of RNA Interference-Based Therapy for Advanced Disease

Ewing's sarcoma tumors are associated with chromosomal translocation between the EWS gene and the ETS transcription factor gene. These unique target sequences provide opportunity for RNA interference(i)-based therapy. A summary of RNAi mechanism and therapeutically designed products including siRNA, shRNA and bi-shRNA are described. Comparison is made between each of these approaches. Systemic RNAi-based therapy, however, requires protected delivery to the Ewing's sarcoma tumor site for activity. Delivery systems which have been most effective in preclinical and clinical testing are reviewed, followed by preclinical assessment of various silencing strategies with demonstration of effectiveness to EWS/FLI-1 target sequences. It is concluded that RNAi-based therapeutics may have testable and achievable activity in management of Ewing's sarcoma.

PRAS40 is a functionally critical target for EWS repression in Ewing sarcoma

Ewing sarcoma family tumors (ESFT) are highly aggressive and highly metastatic tumors caused by a chromosomal fusion between the Ewing sarcoma protein (EWS) with the transcription factor FLI-1. However, expression of the EWS/FLI-1 chimeric oncogene by itself is insufficient for carcinogenesis, suggesting that additional events are required. Here, we report the identification of the Akt substrate PRAS40 as an EWS target gene. EWS negatively regulates PRAS40 expression by binding the 3' untranslated region in PRAS40 mRNA. ESFT cell proliferation was suppressed by treatment with an Akt inhibitor, and ESFT cell proliferation and metastatic growth were suppressed by siRNA-mediated PRAS40 knockdown. Furthermore, PRAS40 knockdown was sufficient to reverse an increased cell proliferation elicited by EWS knockdown. In support of a pathologic role for PRAS40 elevation in EFST, we documented inverse protein levels of EWS and PRAS40 in ESFT cells. Together, our findings suggest that PRAS40 promotes the development of ESFT and might therefore represent a novel therapeutic target in this aggressive disease.