LAUSR

Recently, Charville et al. [1] reported that EWSR1-rearranged fusion proteins mediate the expression of the pairedbox transcription factor PAX7 in Ewing sarcoma. Based on an analysis of a published gene expression microarray dataset (accession code GSE60740), they state having identified PAX7 to be significantly overexpressed in Ewing sarcoma in comparison to CIC-DUX4-positive round cell sarcomas [1]. In that microarray analysis they compared CIC-DUX4-positive sarcomas with EWSR1-NFATc2-positive sarcomas, assuming that EWSR1-NFATc2-positive sarcomas belong to the family of Ewing sarcomas [1], which are typically characterized by EWSR1-ETS fusion oncogenes [2]. Accordingly, Charville et al. [1] summarized in a schematic EWSR1-FLI1-, EWSR1-ERG-, and EWSR1NFATc2-positive sarcomas as “Ewing sarcoma,” referring to Szuhai et al. (2009) [3], and did not take into account more recent reports in the literature that EWSR1-NFATc2-positive sarcomas may constitute an own entity [2]. Comparison of the dataset (GSE60740) used by Charville et al. to a published transcriptome reference dataset of genetically defined EWSR1-ETS-positive Ewing sarcomas (GSE34620) [4] and 13 other malignancies that may constitute morphological mimics [5] shows that EWSR1-NFATc2-positive sarcomas do not cluster with any other analyzed tumor entity including EWSR1-ETS-positive Ewing sarcoma (Fig. 1a). Furthermore, Charville et al. noted that (in addition to PAX7) the genes FOXG1, NR5A2, SOX5, VDR, and TFAP2 were highly overexpressed in EWSR1-NFATc2-positive sarcomas as compared to CIC-DUX4-positive sarcomas [1]. Interestingly, all these genes are similarly lowly expressed in EWSR1-ETS-positive Ewing sarcomas and CIC-DUX4positive sarcomas (Fig. 1b). Collectively, these analyses strongly suggest that EWSR1-NFATc2-positive sarcomas constitute a tumor entity distinct from Ewing sarcoma [5]. Hence, the correctness of extrapolation from EWSR1NFATc2-positive sarcomas to Ewing sarcoma remains to be substantiated. Based on their disputable extrapolation from EWSR1NFATc2-positive sarcomas to Ewing sarcomas, Charville et al. [1] performed immunohistochemical analyses of tissue samples, which were “diagnosed as Ewing sarcoma using a combination of morphologic, immunohistochemical, and molecular features.” For the majority of cases (102/103), which were all “PAX7-positive” by immunohistochemistry, no details or references were provided on how the diagnosis of Ewing sarcoma was molecularly confirmed. Only for one sample, which was classified as “PAX7-negative,” details were provided, but the diagnosis of Ewing sarcoma could not be molecularly confirmed [1]. The authors conclude from this sample set that “immunohistochemical detection of anti-PAX7 immunohistochemistry is a sensitive marker for Ewing sarcoma” [1]. Yet, it remains unclear whether the sensitivity of PAX7 is superior to that of the established Ewing sarcoma marker CD99 [6]. In their transcriptome analysis of EWSR1-NFATc2-positive sarcomas, Charville et al. noted that CD99 was not significantly overexpressed Michaela C. Baldauf and Julia S. Gerke contributed equally to this work.