Microsatellites as EWS/FLI response elements in Ewing's sarcoma.

The ETS gene family is frequently involved in chromosome translocations that cause human cancer, including prostate cancer, leukemia, and sarcoma. However, the mechanisms by which oncogenic ETS proteins, which are DNA-binding transcription factors, target genes necessary for tumorigenesis is not well understood. Ewing's sarcoma serves as a paradigm for the entire class of ETS-associated tumors because nearly all cases harbor recurrent chromosomal translocations involving ETS genes. The most common translocation in Ewing's sarcoma encodes the EWS/FLI oncogenic transcription factor. We used whole genome localization (ChIP-chip) to identify target genes that are directly bound by EWS/FLI. Analysis of the promoters of these genes demonstrated a significant over-representation of highly repetitive GGAA-containing elements (microsatellites). In a parallel approach, we found that EWS/FLI uses GGAA microsatellites to regulate the expression of some of its target genes including NR0B1, a gene required for Ewing's sarcoma oncogenesis. The microsatellite in the NR0B1 promoter bound EWS/FLI in vitro and in vivo and was both necessary and sufficient to confer EWS/FLI regulation to a reporter gene. Genome wide computational studies demonstrated that GGAA microsatellites were enriched close to EWS/FLI-up-regulated genes but not down-regulated genes. Mechanistic studies demonstrated that the ability of EWS/FLI to bind DNA and modulate gene expression through these repetitive elements depended on the number of consecutive GGAA motifs. These findings illustrate an unprecedented route to specificity for ETS proteins and use of microsatellites in tumorigenesis.

Emergent Properties of EWS/FLI Regulation via GGAA Microsatellites in Ewing's Sarcoma.

ETS proteins are a family of transcription factors that play important roles in the development of cancer. The Ewing's sarcoma EWS/ETS fusion oncoproteins control a number of cancer-relevant phenotypes in that disease. We recently demonstrated that EWS/FLI, the most common EWS/ETS fusion in Ewing's sarcoma, regulates a portion of its target genes, including the critical target NR0B1, via GGAA-containing microsatellites in their promoters. Given the unusual nature of microsatellites as EWS/FLI response elements, we sought to elucidate the mechanism of EWS/FLI activity at these sites. We found that the ability to bind GGAA microsatellites is shared by multiple ETS family members from distinct phylogenetic subfamilies. Importantly, however, only EWS/ETS-containing fusions are capable of mediating transcriptional activation via these elements, highlighting a neomorphic function of the Ewing's sarcoma fusion proteins. Additional analysis revealed that the GGAA microsatellite binds EWS/FLI with an affinity that is 2 to 3 orders of magnitude lower than previously identified high-affinity consensus/redundant binding sites. The stoichiometry of this interaction is 2 protein molecules for each DNA molecule, suggesting that EWS/FLI binds these elements as a homodimer. The isolated FLI ETS domain bound microsatellite sequences in a nearly identical fashion to full-length EWS/FLI, thus indicating that residues required for homodimeric binding are localized to the ETS domain. These data suggest a new paradigm for an ETS family member binding to DNA at cancer-relevant genetic loci and highlight emergent properties of EWS/FLI that are required for the development of Ewing's sarcoma.

Microsatellites are EWS/FLI response elements: genomic "junk" is EWS/FLI's treasure.

Ewing's sarcoma is a solid tumor of the bone that primarily occurs in children and young adults. Most cases harbor the (11;22) (q24;q12) chromosomal translocation that encodes the EWS/FLI oncoprotein. EWS/FLI is an aberrant ETS-type transcription factor that dysregulates a number of genes important in the development of Ewing's sarcoma. Because EWS/FLI is the key oncoprotein in this tumor and ETS proteins are often dysregulated in various human cancers, Ewing's sarcoma serves as a useful paradigm for ETS-mediated oncogenesis. We recently showed that EWS/FLI interacts with GGAA-microsatellites to regulate some of its target genes, including NR0B1, an EWS/FLI-regulated gene that is required for the oncogenic phenotype of Ewing's sarcoma. While microsatellites typically have no ascribed function, and are sometimes considered "junk" DNA, our findings provide a unique role for microsatellites in cancer development. Furthermore, these findings may indicate a novel mechanism for normal ETS protein function as well. Finally, it is tempting to speculate that microsatellite polymorphisms may confer differences in susceptibility to Ewing's sarcoma, both between individuals and between populations, and other diseases mediated by ETS transcription factors. The observation of microsatellites as transcriptional response elements for EWS/FLI suggest that these elements may not be "junk" after all.