Recurrent gene fusions involving EWSR1 with members of the cAMP response element binding protein (CREB) family (ATF1 and CREB1) have been reported in a diverse group of tumors including angiomatoid fibrous histiocytoma (AFH), soft tissue and gastrointestinal clear cell sarcoma (CCS), primary pulmonary myxoid sarcoma (PPMS) and hyalinizing clear cell carcinoma of salivary gland. We have recently encountered a group of 5 myxoid mesenchymal tumors positive for EWSR1 fusions with one of the CREB family member (ATF1, CREB1 and CREM), with histologic features distinct from any of the previously described pathologic entities. Tumors occurred in children or young adults (12–23 years; mean 18), with equal gender distribution. All except one were intracranial (intra-axial, 2; meningeal, 2) while one was perirectal. Histologically, the tumors were well-circumscribed, often lobulated, composed of uniform ovoid to round cells, and arranged in cord-like or reticular structures in a myxoid background. All except one displayed unique sunburst amianthoid fibers. Immunohistochemically, tumors were positive for EMA (5/5; 4 focal, 1 diffuse) and desmin (3/5). A novel EWSR1-CREM fusion was identified by RNA sequencing in the peri-rectal tumor, which was further confirmed by FISH and RT-PCR. A 2nd case with similar EWSR1-CREM fusion was identified by RT-PCR and FISH in a meningeal tumor. The remaining cases studied by FISH showed the presence of EWSR1-CREB1 fusion in 2 cases and EWSR1-ATF1 in one. In conclusion, we report a distinct group of myxoid mesenchymal neoplasms occurring in children or young adults with a predilection for intracranial locations. Although the immunoprofile (EMA, desmin) and the fusion type raise the possibility of a myxoid AFH, none of the typical histologic findings of AFH were present, suggesting a novel entity.

Small blue round cell tumors (SBRCTs) are a heterogenous group of tumors that are difficult to diagnose due to overlapping morphologic, immunohistochemical and clinical features. About two-thirds of EWSR1-negative SBRCTs are associated with CIC-DUX4 related fusions, while another small subset shows BCOR-CCNB3 X-chromosomal paracentric inversion. Applying paired-end RNA sequencing to an SBRCT index case of a 44 year-old male, we identified a novel BCOR-MAML3 chimeric fusion, which was validated by RT-PCR and FISH techniques. We then screened a total of 75 SBRCTs lacking EWSR1, FUS, SYT, CIC and BCOR-CCNB3 abnormalities, for BCOR break-apart probes by FISH to detect potential recurrent BCOR gene rearrangements, outside the typical X-chromosomal inversion. Indeed, 8/75 (11%) SBRCTs showed distinct BCOR gene rearrangements, with 2 cases each showing either a BCOR-MAML3 or the alternative ZC3H7B-BCOR fusion, while no fusion partner was detected in the remaining 4 cases. Gene expression of the BCOR-MAML3 positive index case showed a distinct transcriptional profile with upregulation of HOX-gene signature, compared to classic Ewing sarcoma or CIC-DUX4-positive SBRCTs. The clinicopathologic features of the SRBCTs with alternative BCOR rearrangements were also compared with a group of BCOR-CCNB3 inversion positive cases, combining 11 from our files with a meta-analysis of 42 published cases. The BCOR-CCNB3-positive tumors occurred preferentially in children and in bone, in contrast to alternative BCOR-rearranged SBRCTs which presented in young adults, with a variable anatomic distribution. Furthermore BCOR-rearranged tumors often displayed spindle cell areas, either well-defined in intersecting fascicles or blending with the round cell component, which appears distinct from most other fusion-positive SBRCTs and shares histologic overlap with poorly differentiated synovial sarcoma.