Ewing sarcoma (ES) and atypical teratoid rhabdoid tumor (ATRT) are high-grade malignancies of childhood, each of which is associated with genetic abnormalities on chromosome 22. ES is typically characterized by rearrangement of the EWSR1 locus and ATRT by deletion of SMARCB1. We report a case with an unusual fluorescence in situ hybridization signal pattern consistent with EWSR1 rearrangement that was shown to have loss of INI1 expression by immunohistochemistry due to deletion in the long arm of one chromosome 22. In light of the unusual findings in this case as well as the proximity of the EWSR1 locus and SMARCB1 locus on chromosome 22 and frequent CD99 staining in both tumors, we examined 16 ES cases and 17 ATRT, renal rhabdoid tumor (RRT), and extrarenal rhabdoid tumor (ERRT) cases for CD99 and INI1 staining and for EWSR1 rearrangement. Staining with INI1 was negative in ATRT, RRT, and ERRT and positive in ES cases; CD99 was positive in ES cases and variable in ATRT cases. All but 2 cases of ES, and no cases of ATRT, showed rearrangement of EWSR1. The present case appears to be best classified as a unique variant of ATRT based on immunohistochemistry, EWSR1 fluorescence in situ hybridization and RT-PCR, and SMARCB1 gene sequencing.
Keywords: CD99; Ewing sarcoma; INI1; atypical teratoid rhabdoid tumor; fluorescence in situ hybridization.