Genetic analyses of undifferentiated small round cell sarcoma identifies a novel sarcoma subtype with a recurrent CRTC1‐SS18 gene fusion

Abstract

In recent years, undifferentiated small round cell sarcomas (USRCSs) have been divided into a variety of new, rare, sarcoma subtypes, including the group of Ewing-like sarcomas, which have the morphological appearance of Ewing sarcomas, but carry CIC-DUX4, BCOR-CCNB3 and other gene fusions different from the classic EWSR1-ETS gene fusion. Using high-throughput RNA-sequencing (RNA-seq) analyses, we identified a novel recurrent gene fusion, CRTC1-SS18, in two cases of USRCS that lacked any known translocation. RNA-seq results were confirmed by reverse transcription polymerase chain reaction, long-range polymerase chain reaction, and fluorescence in situ hybridization. In vitro, we showed that the cells expressing the gene fusion were morphologically distinct and had enhanced oncogenic potential as compared with control cells. Expression profile comparisons with tumours of other sarcoma subtypes demonstrated that both cases clustered close to EWSR1-CREB1-positive tumours. Moreover, these analyses indicated enhanced NTRK1 expression in CRTC1-SS18-positive tumours. We conclude that the novel gene fusion identified in this study adds a new subtype to the USRCSs with unique gene signatures, and may be of therapeutic relevance. Copyright © 2018 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Undifferentiated round cell sarcomas with novel SS18‐POU5F1 fusions

“…These arose in the soft tissue of the lower extremity of young adults. Interestingly, the two tumors did not cluster together with other synovial sarcomas with canonical SS18‐SSX fusion, and thus, are presumed unlikely to represent molecular variants of poorly differentiated synovial sarcoma 4 …”

“…Given a primitive round cell morphology and nonspecific immunophenotype, the differential would include novel molecular variants of synovial sarcoma, malignant myoepithelial tumor, or perhaps an altogether novel round cell sarcoma. There is recent precedence for alternative SS18 gene partners in sarcoma 3,4 . First, an SS18‐NEDD4 fusion was recently described as a case report of a renal tumor occurring in a young adult, with a morphology reminiscent of myxoid synovial sarcoma 3 .…”

“…However, with the advent of next generation sequencing, several themes are emerging, including: (a) molecular pleiotropy, whereby the same genetic event may result in multiple phenotypes, driving the pathogenesis of histologically unrelated neoplasms—such as the EWSR1 ‐ CREB family of tumors, spanning clear cell sarcoma/clear cell sarcoma‐like tumor of the gastrointestinal tract, angiomatoid fibrous histiocytoma, and hyalinizing clear cell carcinoma of salivary gland 1 ; (b) significant infidelity among genes participating in fusions, resulting in numerous possible fusion product permutations, particularly involving EWSR1 and FUS genes, which can associate with tens of different partners resulting in a wide spectrum of tumor types; and, (c) variability in the fusion transcript structure may result in various domain compositions of the fusion oncoproteins. For the latter, one example is a 5′ SS18 ‐, which may fuse with 3′ ‐ SSX1 , ‐ SSX2 , ‐ SSX4 , or ‐ NEDD4 , resulting in synovial sarcoma; 2,3 alternatively, a 3′ ‐ SS18 may fuse with 5′ CRTC1‐ resulting in an undifferentiated small round cell sarcoma 4 …”

An update of molecular findings in uterine tumor resembling ovarian sex cord tumor and GREB1‐rearranged uterine sarcoma with variable sex‐cord differentiation

“…SSXT, encoded by SS18 at 18q11.2, is a transcriptional coactivator and a component of the SWI/SNF chromatin remodeling complex. Best known as the 5′ fusion partner in the majority of synovial sarcoma, the role of SS18 as a 3′ partner, hitherto found in exceptionally rare tumors, is poorly understood 38‐40 . The expression of SMARCB1/INI1 and SMARCA4/BRG1, key members of SWI/SNF complex, did not seem to be affected in the UTROSCT with GREB1‐SS18 20 .…”

Molecular genetics of Ewing sarcoma, model systems and finding novel (immuno-) therapeutic targets

“…[53] holding chromosome pairs together during cell division, and in the context of cancer also affects genome organization and transcriptional regulation [77] . STAG2 is functionally redundant with STAG1, and depletion of STAG1 resulted in lethality in EWS cells with mutated STAG2, but not in cells with wildtype STAG2, making it an interesting target for therapeutics [78] .…”