BACKGROUND: Liposarcomas are the most common class of soft tissue sarcomas, and 
myxoid liposarcoma is the second most common liposarcoma. EWSR1-DDIT3 is a 
chimeric fusion protein generated by the myxoid liposarcoma-specific chromosomal 
translocation t(12;22)(q13;q12). Current studies indicate that multipotent 
mesenchymal cells are the origin of sarcomas. The mechanism whereby EWSR1-DDIT3 
contributes to the phenotypic selection of target cells during oncogenic 
transformation remains to be elucidated.
METHODOLOGY/PRINCIPAL FINDINGS: Reporter assays showed that the EWSR1-DDIT3 
myxoid liposarcoma fusion protein, but not its wild-type counterparts EWSR1 and 
DDIT3, selectively repressed the transcriptional activity of cell 
lineage-specific marker genes in multipotent mesenchymal C3H10T1/2 cells. 
Specifically, the osteoblastic marker Opn promoter and chondrocytic marker 
Col11a2 promoter were repressed, while the adipocytic marker Ppar-γ2 promoter 
was not affected. Mutation analyses, transient ChIP assays, and treatment of 
cells with trichostatin A (a potent inhibitor of histone deacetylases) or 
5-Aza-2'-deoxycytidine (a methylation-resistant cytosine homolog) revealed the 
possible molecular mechanisms underlying the above-mentioned selective 
transcriptional repression. The first is a genetic action of the EWSR1-DDIT3 
fusion protein, which results in binding to the functional C/EBP site within Opn 
and Col11a2 promoters through interaction of its DNA-binding domain and 
subsequent interference with endogenous C/EBPβ function. Another possible 
mechanism is an epigenetic action of EWSR1-DDIT3, which enhances histone 
deacetylation, DNA methylation, and histone H3K9 trimethylation at the 
transcriptional repression site. We hypothesize that EWSR1-DDIT3-mediated 
transcriptional regulation may modulate the target cell lineage through target 
gene-specific genetic and epigenetic conversions.
CONCLUSIONS/SIGNIFICANCE: This study elucidates the molecular mechanisms 
underlying EWSR1-DDIT3 fusion protein-mediated phenotypic selection of putative 
target multipotent mesenchymal cells during myxoid liposarcoma development. A 
better understanding of this process is fundamental to the elucidation of 
possible direct lineage reprogramming in oncogenic sarcoma transformation 
mediated by fusion proteins.