Chromosomal translocations of the Ets genes are found frequently in human cancer, including nearly all cases of Ewing sarcoma, and result in fused transcription factors that have an abnormal repertoire of target genes. The mechanisms of their specificity are poorly understood, but now new research implicates micro satellites near target gene promoters as specific response elements.

EWS–FLI (encoded by a fusion between EWSR1 and FLI1) is the most common fusion protein in Ewing sarcoma. Using genome-wide localization studies in Ewing sarcoma cells, Stephen Lessnick and colleagues identified 900 genes that are bound by the fusion protein, including NR0B1, which is known to be required for the transformed phenotype. Noting the absence of the known high-affinity Ets binding site, the authors tested a series of deletion mutants of the NR0B1 promoter and found a 500-bp region that was sufficient for EWS–FLI responsiveness. Within this they identified a 102-bp micro satellite containing 25 GGAA repeats (a motif to which Ets proteins bind), and this also conferred EWS–FLI responsiveness in vitro.

In parallel, the genes that were most highly bound by EWS–FLI were assessed computationally to identify enriched upstream sequence motifs. The sequence that was most enriched was the previously identified GGAA microsatellite motif and this was found in 9% of the 134 genes analysed. So do

GGAA microsatellites generally act as EWS–FLI response elements? A GGAA microsatellite region was also identified in the promoter of FCGRT, and was demonstrated to be necessary and sufficient for EWS–FLI upregulation of this gene. Further, six microsatellite-containing promoters were shown to be bound by EWS–FLI in vivo but not by two other Ets family members. The authors went on to analyse all GGAA microsatellite-containing genes in the human genome and found that EWS–FLI-upregulated genes were significantly enriched within 5 kb of GGAA regions. Moreover, gene expression datasets from Ewing sarcoma samples were found to be significantly enriched with GGAA microsatellites compared with randomly sampled gene sets or other known cancer-associated gene sets.

Next, the authors addressed the mode of binding and found for

EWS–FLI and FLI that no other proteins are needed for binding in vitro and that at least five repeat sequences are necessary for EWS–FLI to activate transcription, and six or seven repeats led to increased activity. Finally, although both proteins are able to bind the GGAA-containing microsatellites, only EWS–FLI was able to activate transcription through the NR0B1 microsatellite.

This study demonstrates the use of microsatellites in tumorigenesis and shows a novel mechanism for target gene specificity of an Ets fusion protein.

Isobel BarryORIGINAL RESEARCH PAPERS Gangwal, K. et al. Microsatellites as EWS/FLI response elements in Ewing’s sarcoma. Proc. Natl Acad. Sci. USA 105, 10149–10154 (2008)FuRtHER REAdING Kumar-Sinha, C., Tomlins, S. A. & Chinnaiyan, A. M. Recurrent gene fusions in prostate cancer. Nature Rev. Cancer 8, 497–511 (2008)

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Fusion protein guided by Sat Nav

Nature Reviews Cancer | AOP, published online 14 August 2008; doi:10.1038/nrc2473

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NATUrE rEvIEWS | cancer vOLUME 8 | SEPTEMbEr 2008