Round Cell Sarcoma With EWSR1-Non-ETS Fusions

Round cell sarcoma with EWSR1–non-ETS fusions is staged under the Union for International Cancer Control (UICC) and American Joint Committee on Cancer (AJCC) TNM systems.

At the cytogenetic level, the translocation event resulting in fusion of the EWSR1 (22q12.2) and NFATC2 (20q13.2) genes typically arises within a derivative 22 ring chromosome as unbalanced and amplified. The EWSR1-NFATC2 fusion induces activation of the NFATC2 transcription factor consequent to the loss of the N-terminal regulatory domain, leading to the relocation in the nucleus of the chimeric transcription factor. Expression and methylome profiling has shown that EWSR1-NFATC2 and FUS-NFATC2 sarcomas differ from Ewing sarcomas, EWSR1-PATZ1 sarcomas, and CIC-fused and BCOR-rearranged sarcomas. In contrast to EWSR1-NFATC2, the FUS-NFATC2 fusion gene has not demonstrated amplification. Clustering analyses also suggest that FUS-NFATC2 sarcomas are transcriptionally distinct from EWSR1-NFATC2 sarcomas.

PATZ1 encodes a zinc finger protein with a Cys2-His2 motif with tumor suppressive functions involved in transcriptional regulation. PATZ1 resides in close proximity (~2 Mb distance) to EWSR1 on chromosome 22. The EWSR1-PATZ1 fusion event is most likely the result of a submicroscopic intrachromosomal paracentric inversion (as the genes are normally transcribed in opposite directions), although genesis of this fusion via more-complex structural alterations at least for some cases cannot be fully excluded. An in-frame fusion between exon 8 or 9 of EWSR1 and exon 1 of PATZ1 results in removal of the putative transcriptional repressor domain and the AT-hook domain at the N-terminus of PATZ1 and converts a transcriptional repressor into a transcriptional activator. EWSR1-PATZ1 sarcomas possess a unique expression signature, but they have not been compared with the CNS cases. In one study, comprehensive DNA and RNA sequencing revealed loss/deletion of CDKN2A/CDKN2B in 5 of 7 EWSR1-PATZ1 sarcomas, with concurrent MDM2 amplification in one (representing alterations that may have prognostic significance, because these events were evidenced in the clinically most aggressive tumors for which follow-up data was available and contrasted with absence of these alterations in a surgically resected, well-encapsulated EWSR1-PATZ1 sarcoma that demonstrated no evidence of disease at 19 months after resection).

Gross examination of primary soft tissue or bone EWSR1-NFATC2 sarcoma reveals a solid mass with a wide size range (4–18 cm in greatest dimension) and a yellowish-tan, firm or fleshy cut surface. Most of these sarcomas are ill defined and are locally destructive or infiltrate into adjacent tissues; however, a few primary soft tissue and bone lesions have been described as being well circumscribed or confined to the intramedullary cavity, respectively. EWSR1-PATZ1 sarcoma is a solid-cystic mass with reported tumor sizes of 3.5 to > 10 cm in greatest dimension.

EWSR1-NFATC2 and FUS-NFATC2 sarcomas are composed of small to medium-sized round and/or spindled cells with limited eosinophilic or clear cytoplasm, predominantly arranged in cords, small nests, trabeculae, and pseudoacinar structures in a fibrohyaline or myxohyaline stromal background. Less commonly, matrix-poor sheets of cells are encountered focally or diffusely. Small round monotonous to markedly pleomorphic nuclei featuring smooth or irregular nuclear contours, dense hyperchromatic or vesicular chromatin, and small or prominent nucleoli represent the morphological spectrum reported. Tumor necrosis and mitotic activity are variable. Tumor cells diffusely express CD99 in half of cases; PAX7 and NKX2-2 may be expressed. Focal dot-like staining for keratin AE1/AE3 is possible, as is focal staining for CD138. Cases have been mainly misdiagnosed as myoepithelial tumor, plasmacytoma, and lymphoma.

The histopathological and immunophenotypic features described for EWSR1-PATZ1 sarcomas are fairly diverse. Tumor cells are small, round, and/or spindled and are often accompanied by a fibrous stroma. Necrosis and mitotic activity may or may not be evident. Coexpression of myogenic markers (desmin, myogenin, MYOD1) and neurogenic markers (S100P, SOX10, MITF, GFAP) is seen at variable levels. CD34 can be positive. CD99 is not consistently expressed.

Not clinically relevant

The clinical course may include local recurrences and/or metastatic disease. Disease control was achieved in 11 of the 14 EWSR1-NFATC2 patients managed with surgical resection with a median follow-up of 45 months. Lung, cutaneous, and bone metastases have been reported as long as 10.5 years after initial diagnosis and clinical indolence; two patients died of disease, at 4 and 94 months. Little to no histological response has been observed in patients treated with neoadjuvant chemotherapy. Outcome data of FUS-NFATC2 sarcomas are limited, with one unfavorable outcome at 15 months reported in a congenital case that displayed high-grade features, whereas other patients were free of disease after surgical management. Evidence of metastatic disease at the time of diagnosis, of development of metastatic disease 5–24 months after diagnosis, and of patient death due to disease within 5–32 months of initial diagnosis underscores the aggressive behavior characterizing a subset of EWSR1-PATZ1 sarcomas for which follow-up data are available. Responses to conventional systemic chemotherapies have been negligible to modest.

Identification of the EWSR1-NFATC2, FUS-NFATC2, and EWSR1-PATZ1 fusions can be attained via diverse molecular approaches. However, for the EWSR1-PATZ1 fusion, the sensitivity of an EWSR1 break-apart probe may be compromised by the short genomic distance between PATZ1 and EWSR1 on 22q12.2 and corresponding interpretive challenges in visualizing the subtle dissociation or breaking apart of signals in balanced alterations; additional studies would be required for identification of the PATZ1 partnership.

Essential: spindled to rounded cytomorphology; mostly low-grade features, but high-grade cases are reported; fibrohyaline stromal changes are common; EWSR1 break-apart FISH shows amplification of the 5′ probe in EWSR1-NFATC2–rearranged sarcomas; identification of the fusion transcript remains the gold standard.

Desirable: most NFATC2-rearranged tumors are located in long bone; PATZ1-rearranged sarcomas: round to spindled cells with divergent phenotype, both myogenic and neurogenic.