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We report an exceptional case of a spindle cell mesenchymal tumor with S100 and CD34 co-reactivity, which harbored a SLMAP::RAF1 fusion. To the best of our knowledge, this is the second case of a spindle cell mesenchymal tumor with S100 and CD34 co-reactivity with this specific fusion. Remarkable is the presence of calcification and heterotopic ossification in the center of our lesion, a feature that, to our knowledge, has not been described yet in RAF1 -rearranged spindle cell mesenchymal tumors., Competing Interests: Declaration of Conflicting InterestsThe author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

We assess the performance of mRNA capture sequencing to identify fusion transcripts in FFPE tissue of different sarcoma types, followed by RT-qPCR confirmation. To validate our workflow, six positive control tumors with a specific chromosomal rearrangement were analyzed using the TruSight RNA Pan-Cancer Panel. Fusion transcript calling by FusionCatcher confirmed these aberrations and enabled the identification of both fusion gene partners and breakpoints. Next, whole-transcriptome TruSeq RNA Exome sequencing was applied to 17 fusion gene-negative alveolar rhabdomyosarcoma (ARMS) or undifferentiated round cell sarcoma (URCS) tumors, for whom fluorescence in situ hybridization (FISH) did not identify the classical pathognomonic rearrangements. For six patients, a pathognomonic fusion transcript was readily detected, i.e., PAX3 - FOXO1 in two ARMS patients, and EWSR1 - FLI1 , EWSR1 - ERG , or EWSR1 - NFATC2 in four URCS patients. For the 11 remaining patients, 11 newly identified fusion transcripts were confirmed by RT-qPCR, including COPS3 - TOM1L2 , NCOA1 - DTNB , WWTR1 - LINC01986 , PLAA - MOB3B , AP1B1 - CHEK2, and BRD4 - LEUTX fusion transcripts in ARMS patients. Additionally, recurrently detected secondary fusion transcripts in patients diagnosed with EWSR1 - NFATC2 -positive sarcoma were confirmed ( COPS4 - TBC1D9 , PICALM - SYTL2 , SMG6 - VPS53 , and UBE2F - ALS2 ). In conclusion, this study shows that mRNA capture sequencing enhances the detection rate of pathognomonic fusions and enables the identification of novel and secondary fusion transcripts in sarcomas.

Due to the increased application of RNA-based next-generation sequencing techniques on bone and soft tissue round cell sarcomas new fusions are frequently found, thereby expanding the molecular landscape of these tumors. In this report, we describe and discuss the finding of an undifferentiated sarcoma of the bone with a round to epithelioid cell phenotype harboring a novel EWSR1-SSX2 fusion. Treatment of this new bone tumor entity according to the Euro Ewing 2012 protocol led to complete pathologic response., (© 2021 Wiley Periodicals LLC.)

Poorly differentiated synovial sarcoma (PDSS) is a less common subtype of synovial sarcoma (SS) associated with a poor prognosis. We present a case of a SS with a poorly differentiated component that resembles Ewing sarcoma (ES). Initial immunohistochemical staining revealed a characteristic and strong expression of transducin-like enhancer of split 1 (TLE1) and weak to absent expression of integrase integrator 1 (INI1) staining. Stainings for keratin and epithelial membrane antigen (EMA) were negative in the tumoral lesion. Fluorescence In Situ Hybridization (FISH) analysis showed a rearrangement of the synaptotagmin ( SYT ) gene, confirming the diagnosis of SS. FISH analysis for the EWS RNA-binding protein 1 ( EWSR1 ) gene revealed monoallelic loss of EWSR1 . This finding was confirmed by an array comparative genomic hybridization (aCGH), showing complete loss of chromosome 22. Based on literature review, showing only a handful of cases of cytogenetically studied SS with loss of chromosome 22, this is probably a rare event in SS. Therefore, we assume that monoallelic loss of chromosome 22 cannot fully elaborate the underlying mechanism of the INI1 staining pattern in all SS, but it could account for the weak to absent INI1 staining in at least some cases. [ABSTRACT FROM AUTHOR]

We report the case of a sclerosing perineurioma with conspicious collagen rosette formation in a 20-year-old male presenting with a firm, painless nodule on the palmar side of his right ring finger. The main differential diagnosis is a low-grade fibromyxoid sarcoma. The distinction between these entities is important because low-grade fibromyxoid sarcoma has a metastatic potential, while, as a rule, perineuriomas are benign. The presence of collagen rosettes in this current case makes this distinction even more difficult given that approximately 30% of low-grade fibromyxoid sarcoma cases show the focal presence of collagen rosettes. The demonstration of the characteristic t(7;16), t(11;16) or t(11;22) translocations (resulting in the FUS-CREB3L2, FUS-CREB3L1 or EWSR1-CREB3L1 fusion genes, respectively) or immunoreactivity for MUC4, a recently described sensitive and specific marker for low-grade fibromyxoid sarcoma, remain the gold standard in the diagnosis of low-grade fibromyxoid sarcoma, differentiating it from perineurioma. This case is, to our knowledge, the first report on collagen rosettes in sclerosing perineurioma, extremely well mimicking low-grade fibromyxoid sarcoma, and further expanding the morphological spectrum of this rare subtype of perineurioma. [ABSTRACT FROM AUTHOR]