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Distinct transcriptional signature and immunoprofile of CIC‐DUX4 fusion–positive round cell tumors compared to EWSR1‐rearranged ewing sarcomas: Further evidence toward distinct pathologic entities

Round cell sarcomas harboring CIC‐DUX4 fusions have recently been described as highly aggressive soft tissue tumors of children and young adults. Due to partial morphologic and immunohistochemical overlap with Ewing sarcoma (ES), CIC‐DUX4‐positive tumors have generally been classified as ES‐like and managed similarly; however, a systematic comparison at the molecular and immunohistochemical levels between these two groups has not yet been conducted. Based on an initial observation that CIC‐DUX4‐positive tumors show nuclear immunoreactivity for WT1 and ETS transcription factors, FLI1 and ERG, we performed a detailed immunohistochemical and molecular analysis including these markers, to further investigate the relationship between CIC‐DUX4 tumors and ES. The study group included 21 CIC‐DUX4‐positive sarcomas and 20 EWSR1‐rearranged ES. Immunohistochemically, CIC‐DUX4 sarcomas showed membranous CD99 positivity in 18 (86%) cases, but only 5 (24%) with a diffuse pattern, while WT1 and FLI1 were strongly positive in all cases. ERG was positive in 18% of cases. All ES expressed CD99 and FLI1, while ERG positivity was only seen in EWSR1‐ERG fusion positive ES. WT1 was negative in all ES. Expression profiling validated by q‐PCR revealed a distinct gene signature associated with CIC‐DUX4 fusion, with upregulation of ETS transcription factors (ETV4, ETV1, and ETV5) and WT1, among top overexpressed genes compared to ES, other sarcomas and normal tissue. In conclusion, the distinct gene signature and immunoprofile of CIC‐DUX4 sarcomas suggest a distinct pathogenesis from ES. The consistent WT1 expression may provide a useful clue in the diagnosis in the context of round cell sarcomas negative for EWSR1 rearrangement. © 2014 Wiley Periodicals, Inc.