Promiscuous partnerships in Ewing's sarcoma

Abstract

Ewing's sarcoma is a highly aggressive bone and soft tissue tumor of children and young adults. At the molecular genetic level Ewing's sarcoma is characterized by a balanced reciprocal translocation, t(11;22)(q24;q12), which encodes an oncogenic fusion protein and transcription factor EWS/FLI. This tumor-specific chimeric fusion retains the amino terminus of EWS, a member of the TET (TLS/EWS/TAF15) family of RNA-binding proteins, and the carboxy terminus of FLI, a member of the ETS family of transcription factors. In addition to EWS/FLI, variant translocation fusions belonging to the TET/ETS family have been identified in Ewing's sarcoma. These studies solidified the importance of TET/ETS fusions in the pathogenesis of Ewing's sarcoma and have since been used as diagnostic markers for the disease. EWS fusions with non-ETS transcription factor family members have been described in sarcomas that are clearly distinct from Ewing's sarcoma. However, in recent years there have been reports of rare fusions in "Ewing's-like tumors" that harbor the amino-terminus of EWS fused to the carboxy-terminal DNA or chromatin-interacting domains contributed by non-ETS proteins. This review aims to summarize the growing list of fusion oncogenes that characterize Ewing's sarcoma and Ewing's-like tumors and highlights important questions that need to be answered to further support the existing concept that Ewing's sarcoma is strictly a "TET/ETS" fusion-driven malignancy. Understanding the molecular mechanisms of action of the various different fusion oncogenes will provide better insights into the biology underlying this rare but important solid tumor.

Figure 1

(A) Domain structures of wild-type EWS, wild-type FLI and the EWS/FLI fusion proteins. SYGQ: serine-tyrosine-glycine-glutamine rich transactivation region; RGG: arginine-glycine-glycine rich regions; RRM: RNA-recognition motif; Zn: putative zinc finger; PTD: pointed domain; DNA-BD: DNA binding domain; Pro: proline-rich activation domain. Arrows indicate breakpoints in wild-type EWS and FLI included in commonly observed subtypes of the EWS/FLI fusion protein. (B) Genomic structures of EWSR1 and FLI1 genes. Breakpoints in the EWSR1 and FLI1 genes occur in many introns. Following splicing, the exons join together to generate various subtypes of EWS/FLI. Some of the previously described EWS/FLI fusion subtypes are depicted here.

Figure 2

Domain structures of the TET family of RNA binding proteins: TLS (FUS), EWS, TAF15 and the ETS family of transcription factors: FLI, ERG, FEV, ETV1 and ETV4. AD: Activation domain; Ala-rich: Alanine-rich region involved in transcriptional repression. Arrows indicate breakpoints in wild-type EWS, TLS, FLI, ERG, FEV, ETV1 and ETV4 included in TET/ETS fusions in Ewing’s sarcoma. Breakpoints occur in the introns of the wild-type genes but following splicing exons are joined to generate various TET/ETS chimeric fusions.

Figure 3

Domain structures of wild-type EWS, NFATc2, POU5F1 and SMARCA5 proteins as well as EWS/NFATc2, EWS/POU5F1 and EWS/SMARCA5 fusion proteins. TAD-N: amino-terminal transactivation domain; Reg domain: Regulatory region containing multiple conserved phosphorylation sites; POU: POU-specific domain; POU HD: POU homeodomain; SNF₂: domain with helicase activity for chromatin unwinding and transcriptional regulation; SrmB: domain found in DNA and RNA helicases, harbors an ATP binding site; SANT: domain involved in DNA-binding. Arrows indicate breakpoints observed in wild-type EWS, NFATc2, POU5F1 and SMARCA5 that are included in the resulting fusion proteins observed in “Ewing’s-like” tumors.

Figure 4

Domain structures of wild-type EWS and Zinc-finger proteins, ZSG and SP3. The resultant EWS/ZSG and EWS/SP3 chimeric fusions are non-TET/ETS fusions in “Ewing’s-like” tumors. A/T: A-T hook DNA binding motif, a minor groove tether; Zn finger: Cys₂-His₂ zinc fingers; ID: Inhibitory domain. Arrows indicate breakpoints observed in wild-type EWS, ZSG and SP3 included in the EWS/ZSG and EWS/SP3 fusions in “Ewing’s-like” tumors.