Expanding the molecular landscape of undifferentiated sarcomas of bone with a novel EWSR1-SSX3 gene fusion

EWSR1::SSX1 Fusion-Driven Synovial Sarcoma: A Case Presentation and Review of the Literature

According to the 5th edition of the WHO Classification of Soft Tissue and Bone Tumors, the diagnosis of synovial sarcoma relies on morphology, immunohistochemistry, and the detection of a specific fusion involving the SS18 gene with a member of the SSX gene family. However, few cases of synovial sarcoma that do not harbor such molecular alterations have been recently reported. We present the case of a patient with a diffuse pleural mass and pleural effusion that showed in a core needle biopsy a spindle cell neoplasia morphologically suggestive of synovial sarcoma. An SS18 break-apart FISH was performed with a negative result. Afterwards, an EWSR1::SSX1 fusion was detected by next-generation sequencing. There is scarce literature on non-SS18 fusion-driven synovial sarcomas, and no study has evaluated whether these novel molecular alterations have a relevant clinical impact on patients beyond the diagnostic value.

SS18-SSX and SSX c-terminus antibodies for identification of specific fusion oncoprotein in mesenchymal neoplasms

Chromosomal rearrangement can be identified by direct methods or by using immunohistochemistry for a component of the fusion oncoprotein as a surrogate marker. Our aim was to gain insights into the staining profile using novel SS18-SSX and SSX c-terminus antibodies in SS18 fusion tumours and to investigate their potential use in soft tissue tumours harbouring SSX fusion partner outside the spectrum of synovial sarcoma. A retrospective analysis of 310 soft tissue sarcomas [via tissue microarray (TMA)] diagnosed at our Institution between 1999 and 2019 was performed. As controls, whole tissue sections from 14 genetically confirmed synovial sarcomas and one EWSR1::SSX2 rearranged sarcoma diagnosed between 2020 and 2023 were included. Two different antibodies for SSX locus were used: SSX c-terminus and SS18-SSX. Twenty-one of 310 (6.8%) and 25 of 310 (8.1%) sarcomas on the TMA showed nuclear staining with SS18-SSX and SSX, respectively. From the 24 synovial sarcomas, 17 (70.8%) stained positive for both antibodies, and in five of these cases, nuclear staining for SSX was weak. In four (16.7%) cases, only SS18-SSX was positive, and in three (12.5%) cases, only SSX staining was found. Furthermore, SSX nuclear expression was only found in four of 62 (6.5%) myxofibrosarcomas. In the control cohort, 11 of 14 synovial sarcomas (78.6%) showed positive staining for both antibodies. The remaining three cases were negative for SS18-SSX, but demonstrated at least focally strong positivity for SSX. The EWSR1::SSX2 rearranged sarcoma showed strong nuclear positivity for SSX. SS18-SSX and SSX c-terminus antibodies are reliable diagnostic markers that can be used as a surrogate marker for identification of a specific fusion. The SS18-SSX antibody is more specific and shows strong nuclear staining in synovial sarcomas, whereas SSX can present with weak staining and is less specific. However, the latter can be used in soft tissue tumours harbouring SSX fusion partner outside the spectrum of synovial sarcoma.

Identification of Novel/Rare EWSR1 Fusion Partners in Undifferentiated Mesenchymal Neoplasms

Recurrent gene fusions (GFs) in translocated sarcomas are recognized as major oncogenic drivers of the disease, as well as diagnostic markers whose identification is necessary for differential diagnosis. EWSR1 is a 'promiscuous' gene that can fuse with many different partner genes, defining different entities among a broad range of mesenchymal neoplasms. Molecular testing of EWSR1 translocation traditionally relies on FISH assays with break-apart probes, which are unable to identify the fusion partner. Therefore, other ancillary molecular diagnostic modalities are being increasingly adopted for accurate classification of these neoplasms. Herein, we report three cases with rare GFs involving EWSR1 in undifferentiated mesenchymal neoplasms with uncertain differential diagnoses, using targeted RNA-seq and confirming with RT-PCR and Sanger sequencing. Two GFs involved hormone nuclear receptors as 3' partners, NR4A2 and RORB, which have not been previously reported. NR4A2 may functionally replace NR4A3, the usual 3' partner in extraskeletal myxoid chondrosarcoma. The third GF, EWSR1::BEND2, has previously been reported in a subtype of astroblastoma and other rare entities, including a single case of a soft-tissue tumor that we discuss in this work. In conclusion, our findings indicate that the catalogue of mesenchymal neoplasm-bearing EWSR1 fusions continues to grow, underscoring the value of using molecular ancillary techniques with higher diagnostic abilities in the routine clinical setting.