Role of protein-protein interactions in the antiapoptotic function of EWS-Fli-1

Non-chemotherapy adjuvant agents in TP53 mutant Ewing sarcoma

Ewing sarcoma (EWS) is a malignant tumor arising in bone or soft tissue that occurs in adolescent and young adult patients as well as adults later in life. Although non-metastatic EWS is typically responsive to treatment when newly diagnosed, relapsed cases have an unmet need for which no standard treatment approach exists. Recent phase III clinical trials for EWS comparing 7 vs 5 chemotherapy drugs have failed to improve survival. To extend the durability of remission for EWS, we investigated 3 non-chemotherapy adjuvant therapy drug candidates to be combined with chemotherapy. The efficacy of these adjuvant drugs was investigated via anchorage-dependent growth assays, anchorage-independent soft-agar colony formation assays and EWS xenograft mouse models. Enoxacin and entinostat were the most effective adjuvant drug in both long-term in vitro and in vivo adjuvant studies. In the context that enoxacin is an FDA-approved antibiotic, and that entinostat is an investigational agent not yet FDA-approved, we propose enoxacin as an adjuvant drug for further preclinical and clinical investigation in EWS patients.

Targeted Therapy for EWS-FLI1 in Ewing Sarcoma

Ewing sarcoma (EwS) is a rare and predominantly pediatric malignancy of bone and soft tissue in children and adolescents. Although international collaborations have greatly improved the prognosis of most EwS, the occurrence of macrometastases or relapse remains challenging. The prototypic oncogene EWS-FLI1 acts as an aberrant transcription factor that drives the cellular transformation of EwS. In addition to its involvement in RNA splicing and the DNA damage response, this chimeric protein directly binds to GGAA repeats, thereby modifying the transcriptional profile of EwS. Direct pharmacological targeting of EWS-FLI1 is difficult because of its intrinsically disordered structure. However, targeting the EWS-FLI1 protein complex or downstream pathways provides additional therapeutic options. This review describes the EWS-FLI1 protein partners and downstream pathways, as well as the related target therapies for the treatment of EwS.

ETV6 dependency in Ewing sarcoma by antagonism of EWS-FLI1-mediated enhancer activation

The EWS-FLI1 fusion oncoprotein deregulates transcription to initiate the paediatric cancer Ewing sarcoma. Here we used a domain-focused CRISPR screen to implicate the transcriptional repressor ETV6 as a unique dependency in this tumour. Using biochemical assays and epigenomics, we show that ETV6 competes with EWS-FLI1 for binding to select DNA elements enriched for short GGAA repeat sequences. Upon inactivating ETV6, EWS-FLI1 overtakes and hyper-activates these cis-elements to promote mesenchymal differentiation, with SOX11 being a key downstream target. We show that squelching of ETV6 with a dominant-interfering peptide phenocopies these effects and suppresses Ewing sarcoma growth in vivo. These findings reveal targeting of ETV6 as a strategy for neutralizing the EWS-FLI1 oncoprotein by reprogramming of genomic occupancy.

SPOP and OTUD7A Control EWS-FLI1 Protein Stability to Govern Ewing Sarcoma Growth

Chromosomal translocation results in development of an Ewing sarcoma breakpoint region 1-Friend leukemia integration 1 (EWS-FLI1) fusion oncogene in the majority of Ewing sarcoma. The persistent dependence of the tumor for this oncoprotein points to EWS-FLI1 as an ideal drug target. Although EWS-FLI1 transcriptional targets and binding partners are evaluated, the mechanisms regulating EWS-FLI1 protein stability remain elusive. Speckle-type POZ protein (SPOP) and OTU domain-containing protein 7A (OTUD7A) are identified as the bona fide E3 ligase and deubiquitinase, respectively, that control EWS-FLI1 protein turnover in Ewing sarcoma. Casein kinase 1-mediated phosphorylation of the VTSSS degron in the FLI1 domain enhances SPOP activity to degrade EWS-FLI1. Opposing this process, OTUD7A deubiquitinates and stabilizes EWS-FLI1. Depletion of OTUD7A in Ewing sarcoma cell lines reduces EWS-FLI1 protein abundance and impedes Ewing sarcoma growth in vitro and in mice. Performing an artificial-intelligence-based virtual drug screen of a 4-million small molecule library, 7Ai is identified as a potential OTUD7A catalytic inhibitor. 7Ai reduces EWS-FLI1 protein levels and decreases Ewing sarcoma growth in vitro and in a xenograft mouse model. This study supports the therapeutic targeting of OTUD7A as a novel strategy for Ewing sarcoma bearing EWS-FLI1 and related fusions, and may also be applicable to other cancers dependent on aberrant FLI1 expression.

Development of an Ewing sarcoma cell line with resistance to EWS‑FLI1 inhibitor YK‑4‑279

Despite Ewing sarcoma (ES) being the second most common pediatric malignancy of bone and soft tissue, few novel therapeutic approaches have been introduced over the past few decades. ES contains a pathognomonic chromosomal translocation that leads to a fusion protein between EWSR1 and an ets family member, most often FLI1. EWS‑FLI1 is the most common type of fusion protein and is a well‑vetted therapeutic target. A small molecule inhibitor of EWS‑FLI1, YK‑4‑279 (YK) was developed with the intention to serve as a targeted therapy option for patients with ES. The present study investigated resistance mechanisms by developing an ES cell line specifically resistant to YK. The ES cell line A4573 was treated with YK to create resistant cells by long term continuous exposure. The results revealed that resistance in A4573 was robust and sustainable, with a >27‑fold increase in IC50 lasting up to 16 weeks in the absence of the compound. Resistant ES cells were still sensitive to standard of care drugs, including doxorubicin, vincristine and etoposide, which may be valuable in future combination treatments in the clinic. Resistant ES cells revealed an increased expression of CD99. RNA sequencing and qPCR validation of resistant ES cells confirmed an increased expression of ANO1, BRSK2 and IGSF21, and a reduced expression of COL24A1, PRSS23 and RAB38 genes. A functional association between these genes and mechanism of resistance remains to be investigated. The present study created a cell line to investigate YK resistance.

Identifying the druggable interactome of EWS-FLI1 reveals MCL-1 dependent differential sensitivities of Ewing sarcoma cells to apoptosis inducers

Ewing sarcoma (EwS) is an aggressive pediatric bone cancer in need of more effective therapies than currently available. Most research into novel targeted therapeutic approaches is focused on the fusion oncogene EWSR1-FLI1, which is the genetic hallmark of this disease. In this study, a broad range of 3,325 experimental compounds, among them FDA approved drugs and natural products, were screened for their effect on EwS cell viability depending on EWS-FLI1 expression. In a network-based approach we integrated the results from drug perturbation screens and RNA sequencing, comparing EWS-FLI1-high (normal expression) with EWS-FLI1-low (knockdown) conditions, revealing novel interactions between compounds and EWS-FLI1 associated biological processes. The top candidate list of druggable EWS-FLI1 targets included genes involved in translation, histone modification, microtubule structure, topoisomerase activity as well as apoptosis regulation. We confirmed our in silico results using viability and apoptosis assays, underlining the applicability of our integrative and systemic approach. We identified differential sensitivities of Ewing sarcoma cells to BCL-2 family inhibitors dependent on the EWS-FLI1 regulome including altered MCL-1 expression and subcellular localization. This study facilitates the selection of effective targeted approaches for future combinatorial therapies of patients suffering from Ewing sarcoma.

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.

Investigational therapies for Ewing sarcoma: a search without a clear finding

INTRODUCTION

Ewing sarcoma family tumors (ESFT) are a group of aggressive diseases, characterized histologically by small, round, blue cells and genetically by translocation involving EWS and ETS partner genes. The current treatment of localized Ewing sarcoma (ES) requires a multi-disciplinary approach, including multidrug chemotherapy, administrated before and after local treatment, surgery and radiation therapy. Unfortunately, the cure rate of metastatic or refractory/recurrent disease is still very poor.

AREAS COVERED

In this review, the authors summarize the new types of therapy and strategies aimed to improve the prognosis or cure ES. Herein, the authors discuss several preclinical and phase I-II studies with new-targeted therapies. The most studied therapies are insulin-like growth factor receptor (IGF1R) inhibitors but have limited efficacy. Other strategies include Mammalian Target of Rapamycin (mTOR) Inhibition, poly ADP ribose polymerase (PARP) inhibition, vascular endothelial growth factor (VEGF) inhibition, tyrosine kinase inhibitors and telomerase inhibitors, all with limited effectiveness.

EXPERT OPINION

Future treatment strategies should combine one or more targeted therapies with conventional chemotherapy. Some combined modality treatments are under clinical study. However, treatment breakthroughs are still needed to improve the relatively poor prognosis of recurrent/metastatic ESFT.

Molecular Mechanism of β-Catenin Signaling Pathway Inactivation in ETV1-Positive Prostate Cancers

In the United States of America, prostate cancer is the second most common age-related cancer among men. African-American men have the highest incidence of, and mortality rate from this disease in the United States. According to the American Cancer Society, 29% of all cancer cases and 9% of all cancer deaths are a result of prostate cancer. Individuals who are at highest risk include African-American men, men over 60 years of age, and those with a family history of the disease. African-Americans also have twice the risk of developing prostate cancer as compared to Caucasians. Erythroblastosis virus E26 transformation-specific (ETS) factors play an important role in human cancers. ETS Variant 1 (ETV1), an ETS factor, is notable for its association in prostate cancers, where truncated ETV1 (dETV1) or its full length counterpart is overexpressed in approximately 10% of the prostate cancer patients. Prostate cancer tumorigenesis may be initiated by deregulation of the Wnt/β-catenin pathway. Mutations that stabilize β-catenin were shown to contribute to the loss of cell-growth control in tumorigenesis. We hypothesized that ETV1's interaction with components of the Wnt/β-catenin pathway may alter β-catenin's interaction with downstream tumor-suppressor genes, which are critical in regulating apoptosis and cell-growth properties of prostate cells. Our results demonstrate for the first time that ETV1 alters β-catenin activity by activating kinases that regulate Wnt/β-catenin activity through post-translational modification in prostate cancer cells. We further demonstrate that therapeutic agents such as PD98059, that reverse effect of ETV1 on Wnt/β-catenin signaling pathway, can be used to target ETV1-positive prostate cancer cells. These therapeutic agents could have a profound impact on prevention and treatment of prostate cancer which may help to reduce health disparity seen in minority patients. Understanding the role of ETV1 in Wnt/β-catenin pathway will also allow us to develop better diagnostic tools, which can be used as a biomarker for ETV1-positive prostate cancers.

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.

EWS-FLI1 utilizes divergent chromatin remodeling mechanisms to directly activate or repress enhancer elements in Ewing sarcoma

The aberrant transcription factor EWS-FLI1 drives Ewing sarcoma, but its molecular function is not completely understood. We find that EWS-FLI1 reprograms gene regulatory circuits in Ewing sarcoma by directly inducing or repressing enhancers. At GGAA repeat elements, which lack evolutionary conservation and regulatory potential in other cell types, EWS-FLI1 multimers induce chromatin opening and create de novo enhancers that physically interact with target promoters. Conversely, EWS-FLI1 inactivates conserved enhancers containing canonical ETS motifs by displacing wild-type ETS transcription factors. These divergent chromatin-remodeling patterns repress tumor suppressors and mesenchymal lineage regulators while activating oncogenes and potential therapeutic targets, such as the kinase VRK1. Our findings demonstrate how EWS-FLI1 establishes an oncogenic regulatory program governing both tumor survival and differentiation.

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.

Anti-Epileptic Drug Targets Ewing Sarcoma

Ewing Sarcoma (ES) is a rare form of bone cancer that most commonly affects children and adolescents. Chromosomal translocations are fundamental to the development of Ewing Sarcoma, linked to the changes in gene expression affecting transcription factors. Histone acetyl transferases (HATs) and histone deacetylases (HDACs) regulate transcription by modifying acetylation of both histones and transcription factors. Despite the use of multimodal therapeutic approaches current therapies are associated with significant short and long-term side effects. Hence, new therapeutic approaches are needed. In this study, we show that ERG/EWS-ERG, inhibits transcriptional activation properties of RXRα. These results suggest that ERG/EWS-ERG/EWS-Fli-1 may target transcriptional co-activators and transcriptional repressors and thereby regulate RXRα transcriptional activity. To understand the molecular mechanism of action, how the fusion protein targets nuclear receptor function, and to provide a clue for the cancer health disparity seen in Ewing Sarcoma, we hypothesized that the aberrant fusion protein, EWS-ERG/EWS-Fli-1 regulates HDACs-mediated repressor complex and inhibits the binding of transcriptional activator complex causing transcriptional repression of RXRα activity. Since it is known that HDACs regulate nuclear receptors, we proposed that HDAC inhibitor, valproic acid (VPA), an anti-epileptic drug, may reverse the inhibitory properties of EWS-ERG/EWS-Fli-1 oncoprotein on RXRα transcriptional activity and might therefore be used as therapeutic agent in ES. We demonstrate that VPA reverses the inhibitory effect of EWSERG/EWS-Fli-1 on RXRα transcriptional activity and also inhibits the cell growth. Furthermore, VPA induces apoptosis and restored the expression of RXRα target genes RARβ, CRABPII and p21 activity and repressed the expression of aberrant fusion proteins, EWS-ERG and EWS-Fli-1 in Ewing Sarcoma cells. Thus, therapeutic regulation of transcriptional repressor properties of EWS-ERG/EWS-Fli-1 with an anti-epileptic drug with a promising new potential might have a profound impact on prevention, management and treatment of Ewing Sarcoma. Therapeutic use of VPA in minority patients may help reduce the health disparity.

Mechanism and relevance of EWS/FLI-mediated transcriptional repression in Ewing sarcoma

Ewing sarcoma provides an important model for transcription-factor mediated oncogenic transformation because of its reliance on the ETS-type fusion oncoprotein EWS/FLI. EWS/FLI functions as a transcriptional activator and transcriptional activation is required for its oncogenic activity. Here we demonstrate that a previously less-well characterized transcriptional repressive function of the EWS/FLI fusion is also required for the transformed phenotype of Ewing sarcoma. Through comparison of EWS/FLI transcriptional profiling and genome-wide localization data, we define the complement of EWS/FLI direct downregulated target genes. We demonstrate that LOX is a previously undescribed EWS/FLI-repressed target that inhibits the transformed phenotype of Ewing sarcoma cells. Mechanistic studies demonstrate that the NuRD co-repressor complex interacts with EWS/FLI, and that its associated histone deacetylase and LSD1 activities contribute to the repressive function. Taken together, these data reveal a previously unknown molecular function for EWS/FLI, demonstrate a more highly coordinated oncogenic transcriptional hierarchy mediated by EWS/FLI than previously suspected, and implicate a new paradigm for therapeutic intervention aimed at controlling NuRD activity in Ewing sarcoma tumors.

Factors Affecting EWS-FLI1 Activity in Ewing's Sarcoma

Ewing's sarcoma family tumors (ESFT) are characterized by specific chromosomal translocations, which give rise to EWS-ETS chimeric proteins. These aberrant transcription factors are the main pathogenic drivers of ESFT. Elucidation of the factors influencing EWS-ETS expression and/or activity will guide the development of novel therapeutic agents against this fatal disease.

Molecular pathogenesis of Ewing sarcoma: new therapeutic and transcriptional targets

Approximately one-third of sarcomas contain specific translocations. Ewing sarcoma is the prototypical member of this group of sarcomas; it was the first to be recognized pathologically as a singular entity and to have its signature translocation defined cytogenetically, which led to the identification of its key driver alteration, the EWS-FLI1 gene fusion that encodes this aberrant, chimeric transcription factor. We review recent progress in selected areas of Ewing sarcoma research, including the application of genome-wide chromatin immunoprecipitation analyses, to provide a comprehensive view of the EWS-FLI1 target gene repertoire, the identification of EWS-FLI1 target genes that may also point to therapeutically targetable pathways, and data from model systems as they relate to the elusive cell of origin of Ewing sarcoma and its possible similarities to mesenchymal stem cells.

Regulation of endothelial cell development by ETS transcription factors

The ETS family of transcription factors plays an essential role in controlling endothelial gene expression. Multiple members of the ETS family are expressed in the developing endothelium and evidence suggests that the proteins function, to some extent, redundantly. However, recent studies have demonstrated a crucial non-redundant role for ETV2, as a primary player in specification and differentiation of the endothelial lineage. Here, we review the contribution of ETS factors, and their partner proteins, to the regulation of embryonic vascular development.

Ecteinascidin 743 interferes with the activity of EWS-FLI1 in Ewing sarcoma cells

ET-743 (trabectedin; Yondelis) is approved in Europe for the treatment of soft tissue sarcomas. Emerging phase 1 and 2 clinical data have shown high response rates in myxoid liposarcoma in part owing to the inhibition of the FUS-CHOP transcription factor. In this report, we show that modulation of specific oncogenic transcription factors by ET-743 may extend to other tumor types. We demonstrate that, among a panel of pediatric sarcomas, Ewing sarcoma family of tumors (ESFTs) cell lines bearing the EWS-FLI1 transcription factor are the most sensitive to treatment with ET-743 compared with osteosarcoma, rhabdomyosarcoma, and synovial sarcoma. We show that ET-743 reverses a gene signature of induced downstream targets of EWS-FLI1 in two different ESFT cell lines (P = .001). In addition, ET-743 directly suppresses the promoter activity of a known EWS-FLI1 downstream target NR0B1 luciferase reporter construct without changing the activity of a constitutively active control in ESFT cells. Furthermore, the effect is specific to EWS-FLI1, as forced expression of EWS-FLI1 in a cell type that normally lacks this fusion protein, HT1080 cells, induces the same NR0B1 promoter, but this activation is completely blocked by ET-743 treatment. Finally, we used gene set enrichment analysis to confirm that other mechanisms of ET-743 are active in ESFT cells. These results suggest a particular role for ET-743 in the treatment of translocation-positive tumors. In addition, the modulation of EWS-FLI1 makes it a novel targeting agent for ESFT and suggests that further development of this compound for the treatment of ESFT is warranted.

Histone deacetylase inhibitors, valproic acid and trichostatin-A induce apoptosis and affect acetylation status of p53 in ERG-positive prostate cancer cells

An ETS family member, ETS Related Gene (ERG) is involved in the Ewing family of tumors as well as leukemias. Rearrangement of the ERG gene with the TMPRSS2 gene has been identified in the majority of prostate cancer patients. Additionally, overexpression of ERG is associated with unfavorable prognosis in prostate cancer patients similar to leukemia patients. Histone acetyltransferases (HATs) and histone deacetylases (HDACs) regulate transcription as well as epigenetic status of genes through acetylation of both histones and transcription factors. Deregulation of HATs and HDACs is frequently seen in various cancers, including prostate cancer. Many cellular oncogenes as well as tumor viral proteins are known to target either or both HATs and HDACs. Several studies have demonstrated that there are alterations of HDAC activity in prostate cancer cells. Recently, we found that ERG binds and inhibits HATs, which suggests that ERG is involved in deregulation of protein acetylation. Additionally, it has been shown that ERG is associated with a higher expression of HDACs. In this study, we tested the effect of the HDAC inhibitors valproic acid (VPA) and trichostatin-A (TSA) on ERG-positive prostate cancer cells (VCaP). We found that VPA and TSA induce apoptosis, upregulate p21/Waf1/CIP1, repress TMPRSS2-ERG expression and affect acetylation status of p53 in VCaP cells. These results suggest that HDAC inhibitors might restore HAT activity through two different ways: by inhibiting HDAC activity and by repressing HAT targeting oncoproteins such as ERG.

Dr. Jekyll and Mr. Hyde: The Two Faces of the FUS/EWS/TAF15 Protein Family

FUS, EWS, and TAF15 form the FET family of RNA-binding proteins whose genes are found rearranged with various transcription factor genes predominantly in sarcomas and in rare hematopoietic and epithelial cancers. The resulting fusion gene products have attracted considerable interest as diagnostic and promising therapeutic targets. So far, oncogenic FET fusion proteins have been regarded as strong transcription factors that aberrantly activate or repress target genes of their DNA-binding fusion partners. However, the role of the transactivating domain in the context of the normal FET proteins is poorly defined, and, therefore, our knowledge on how FET aberrations impact on tumor biology is incomplete. Since we believe that a full understanding of aberrant FET protein function can only arise from looking at both sides of the coin, the good and the evil, this paper summarizes evidence for the central function of FET proteins in bridging RNA transcription, processing, transport, and DNA repair.

Oncogenic partnerships: EWS-FLI1 protein interactions initiate key pathways of Ewing's sarcoma

Targeted therapy for cancer, which is specifically directed toward the cancer without any potential for effects outside of controlling the tumor, is a gold standard for treatment. Ewing's sarcoma contains the potential target EWS-FLI1, as a result of a pathognomonic chromosomal translocation. The EWS-FLI1 fusion protein includes the EWS domain, a potent transcriptional activator alongside the highly conserved FLI1 ets DNA-binding domain. Because of the combination of these domains, the EWS-FLI1 fusion protein acts as an aberrant transcription factor whose expression results in cellular transformation. EWS-FLI1 functions by binding to normal cellular protein partners in transcription and splicing, similar to how a virus would corrupt normal cellular machinery for virion production. Therefore, understanding the protein-protein interactions of EWS-FLI1 and the pathways that are regulated by these partnerships will inform both oncogenesis and therapeutics. This review describes the known protein partners and transcriptional targets of EWS-FLI1, while proposing strategies for exploiting these partnerships with targeted therapy.

Context matters: the hen or egg problem in Ewing's sarcoma

Ewing's sarcoma and related tumors (ESFT) are characterized by rearrangements of EWS with ets family genes. While detection of these gene fusions greatly facilitated diagnosis, it has not provided any clues about the tissue of origin. Immunological and gene expression profiling studies favour a neuroectodermal histogenesis. These investigations did not appreciate the impact of EWS-ets proteins on the tumor phenotype. Introduction of EWS-ets into different cellular models resulted in diverse outcomes ranging from the induction of cell cycle arrest or apoptosis to transformation and tumorigenicity, and from blocked differentiation to trans-differentiation. Thus, the molecular signature of EWS-ets proteins depends on the cell type. The hen or egg problem in ESFT, therefore, is whether ESFT reflect the phenotype of the tumor stem cell that is blocked in differentiation by the activity of the EWS-ets gene fusion or if the oncogene imposes an incomplete differentiation program on a pluripotent precursor cell. This article addresses the problem by considering the tissue distribution of FLI1 and ERG expression and by reviewing evidence for combinatorial control of EWS-ets activity.

EWS–ETS oncoproteins: The linchpins of Ewing tumors

Ewing tumors, which comprise Ewing's sarcoma and peripheral primitive neuroectodermal tumors, are highly aggressive and mostly affect children and adolescents. Their molecular signature is a chromosomal translocation leading to the generation of EWS-ETS (or very rarely FUS-ETS) fusion proteins that are capable of transforming cells. These oncoproteins act as aberrant transcription factors due to the fusion of an ETS DNA binding domain to a highly potent EWS (or FUS) transactivation domain. Accordingly, many EWS-ETS target genes have been identified whose dysregulation could contribute to the development of tumor formation. Furthermore, EWS-ETS oncoproteins may impact on RNA splicing or affect other proteins through disturbing their ability to form functional complexes. The molecular knowledge gained so far from studying EWS-ETS oncoproteins has not only broadened our understanding of Ewing tumors but also improved the diagnosis of these highly undifferentiated tumors. In addition, several potential prognostic markers have been uncovered and novel therapies are suggested that may improve the still dismal survival rate of Ewing tumor patients.

The oncogenic TLS-ERG fusion protein exerts different effects in hematopoietic cells and fibroblasts

The oncogenic TLS-ERG fusion protein is found in human myeloid leukemia and Ewing's sarcoma as a result of specific chromosomal translocation. To unveil the potential mechanism(s) underlying cellular transformation, we have investigated the effects of TLS-ERG on both gene transcription and RNA splicing. Here we show that the TLS protein forms complexes with RNA polymerase II (Pol II) and the serine-arginine family of splicing factors in vivo. Deletion analysis of TLS-ERG in both mouse L-G myeloid progenitor cells and NIH 3T3 fibroblasts revealed that the RNA Pol II-interacting domain of TLS-ERG resides within the first 173 amino acids. While TLS-ERG repressed expression of the luciferase reporter gene driven by glycoprotein IX promoter in L-G cells but not in NIH 3T3 cells, the fusion protein was able to affect splicing of the E1A reporter in NIH 3T3 cells but not in L-G cells. To identify potential target genes of TLS-ERG, the fusion protein and its mutants were stably expressed in both L-G and NIH 3T3 cells through retroviral transduction. Microarray analysis of RNA samples from these cells showed that TLS-ERG activates two different sets of genes sharing little similarity in the two cell lines. Taken together, these results suggest that the oncogenic TLS-ERG fusion protein transforms hematopoietic cells and fibroblasts via different pathways.

Mechanisms of Sarcomagenesis

Sarcomas comprise a heterogeneous group of malignancies that are derived from mesenchymal cells, which under normal circumstances lead to the development of connective tissues such as bone, muscle, fat, and cartilage. During the past decade, insight has been gained regarding the aberrancies that occur during normal development that result in mesenchymal cells transforming into sarcomas. More recently, these insights have led to the development of successful therapies that target the specific mechanisms inherent to individual sarcomas. This overview discusses some of the aberrant molecular mechanisms shared in sarcomas and reviews several sarcoma subtypes in which the most advances have been made. Finally, the ways in which these advances in basic science are translating into and redefining clinical practice are highlighted.