ReviewEWS–ETS oncoproteins: The linchpins of Ewing tumors
Introduction
Ewing's sarcoma is an aggressive and frequently metastatic small round blue-cell tumor that derives its name from the description of an endothelioma of bone by James Ewing in 1921 (Ewing, 1921). Although the vast majority of Ewing's sarcomas arises in bone, especially in the pelvis, femur, fibula and tibia, in ∼ 15% of Ewing's sarcoma patients the primary tumor site is within soft tissue. Approximately half of all patients are between 10 and 20 years of age at the time of first diagnosis, making this highly undifferentiated tumor the second most common primary malignant bone tumor in children and adolescents (Grier, 1997, Denny, 1998, Kovar, 1998, de Alava and Gerald, 2000, Paulussen et al., 2001). The frequency of occurrence in children is in the range of 1–3 per million per year in the Western hemisphere; interestingly, Ewing's sarcoma is slightly more frequent in males than females as well as more frequent in people of European than Chinese or African origin (Fraumeni and Glass, 1970, Young and Miller, 1975, Li et al., 1980, Stiller and Parkin, 1996, Zucman-Rossi et al., 1997, Hense et al., 1999, Cotterill et al., 2000).
The diagnosis of Ewing's sarcoma has been difficult due to the absence of distinguishing morphological features. Although most Ewing's sarcomas express high levels of the transmembrane glycoprotein, MIC2/CD99 (Kovar et al., 1990, Perlman et al., 1994), this marker is also highly expressed in several other tumors, including small round blue-cell tumors such as rhabdomyosarcomas. Thus, MIC2/CD99 staining lacks specificity and therefore Ewing's sarcoma was often diagnosed in the past by excluding other tumors. This situation changed in the early 1990s with the discovery that Ewing's sarcomas are characterized by a chromosomal translocation involving the EWSR1 (Ewing's sarcoma breakpoint region 1) gene and an ETS (E26 transformation-specific) gene (Delattre et al., 1992, Arvand and Denny, 2001, Ladanyi, 2002). As a result of this translocation, fusion proteins composed of the N-terminus of the EWS (Ewing's sarcoma) protein and C-terminal portions of an ETS protein are expressed that are thought to be causative for the development of Ewing's sarcoma.
This molecular signature, the expression of an EWS–ETS fusion protein, was not only observed in classical Ewing's sarcoma, but also in other small round blue-cell tumors, including peripheral primitive neuroectodermal, Askin's and paravertebral small cell tumors. Thus, these tumors are now combined into the family of Ewing tumors (Kovar, 1998, de Alava and Gerald, 2000) and their diagnosis is helped by fluorescence in-situ hybridization and RT-PCR directed at detecting the underlying chromosomal translocations and resulting EWS–ETS fusion transcripts (Peter et al., 2001, Qian et al., 2005). Despite this recent progress, much remains to be learned about Ewing tumors. For instance, their cell of origin is still undefined, although many lines of evidence, including the expression of typical neuronal enzymes, the ability of some Ewing tumor cell lines to differentiate upon stimulation into primitive dendrites and a similar gene expression profile as neural crest-derived cells, support a neuroectodermal origin (Cavazzana et al., 1987, O'Regan et al., 1995, Staege et al., 2004). Furthermore, we do not know what other events apart from the chromosomal translocation generating an EWS–ETS fusion have to take place in order for a Ewing tumor to develop. And most significantly, the molecular knowledge gained so far has not yet translated into better therapies in the fight against Ewing tumors.
Section snippets
EWS–ETS oncoproteins
The realization that chromosomal translocations can be the cause of neoplasias may have provided a rationale for characterizing chromosomal abnormalities in Ewing tumors. Indeed, a recurrent t(11;22)(q24;q12) translocation has been noted in Ewing tumors for more than two decades (Aurias et al., 1983, Turc-Carel et al., 1983, Becroft et al., 1984, de Chadarevian et al., 1984, Whang-Peng et al., 1984, Whang-Peng et al., 1986, Whang-Peng et al., 1987, Vigfusson et al., 1986). In 1992, for the
EWS–ETS target genes
EWS–ETS fusion proteins are transcription factors in which a highly potent transactivation domain is fused to an ETS DNA binding domain. As such, it is expected that EWS–ETS fusion proteins upregulate many target genes in Ewing tumors and thereby lead to the development of this disease. However, it is also possible that EWS–ETS fusion proteins repress certain target genes, similar to other transcription factors that promoter–specifically up–or downregulate gene transcription; also, one has to
EWS–ETS proteins: beyond transcription regulation
Although most studies have focused on their role as aberrant transcription factors, EWS–ETS fusion proteins may have functions that are unrelated to transcription. In particular, EWS–ETS proteins may affect RNA splicing. Indeed, EWS–FLI1 has been shown to interact with the splicing factor U1C and is capable of altering splice site selection and modulating splicing efficiency (Knoop and Baker, 2000, Knoop and Baker, 2001, Yang et al., 2000). Of note, EWS–FLI1 can also interact with at least two
Functions of EWS, FUS and ETS proteins
Due to the nature of the chromosomal translocations in Ewing tumors, one of the EWS (or FUS) and one of the FLI1 (or ERG, FEV, ETV1, or ETV4) alleles will be inactivated, leading to hemizygosity. Thus, it is conceivable that dosage reduction of one of these genes contributes to the development of Ewing tumors. Furthermore, it has been shown that EWS–FLI1 can form complexes with EWS and FLI1 (Spahn et al., 2003), thereby potentially modulating their function. Finally, proteins that interact with
Therapeutic strategies
Ewing tumors are nowadays aggressively treated with surgery, radiation and chemotherapy (Meyers and Levy, 2000, Weber, 2002, Rodriguez-Galindo et al., 2003). Despite the introduction of enhanced chemotherapeutic regimens during the last decades that drastically improved therapy, the 5-year survival rate of Ewing tumor patients is still dismal at ∼ 60% and less than half of that for patients that present with metastases at diagnosis. Furthermore, long-term survival rates are probably
Conclusion
The discovery of EWS–ETS fusion proteins more than a decade ago has been instrumental for the vast expansion of our knowledge about Ewing tumors. In particular, diagnosis has benefited through the ability to detect EWS–ETS fusion proteins by molecular means, and prognosis will be helped in the future by profiling Ewing tumors through gene chip microarrays and possibly also proteomics. But therapy still awaits improvement, which may come through more insight into the molecular mechanisms of
Acknowledgement
This work was supported by Grant CA085257 from the National Cancer Institute.
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