Sarcomas With CIC-rearrangements Are a Distinct Pathologic Entity With Aggressive Outcome: A Clinicopathologic and Molecular Study of 115 Cases

Abstract

CIC-DUX4 gene fusion, resulting from either a t(4;19) or t(10;19) translocation, is the most common genetic abnormality detected in EWSR1-negative small blue round cell tumors. Following their discovery it was debated if these tumors should be classified as variants of Ewing sarcoma (ie, atypical Ewing sarcoma) or as a stand-alone pathologic entity. As such the WHO classification temporarily grouped the CIC-rearranged tumors under undifferentiated sarcomas with round cell phenotype, until further clinical evidence was available. However, most studies reported so far include small series with limited follow-up information, which preclude a more definitive assessment. The present work investigates the clinicopathologic features of a large cohort of sarcomas with CIC gene rearrangement, to define their clinical presentation, morphologic spectrum, and outcome. Our study further examines the overall survival of the CIC-positive cohort compared with a control group of EWSR1-rearranged Ewing sarcoma matched for age and stage. The study cohort included 115 patients, with a mean age of 32 years and a slight male predominance. Most tumors occurred in the soft tissue (86%), predominantly deep-seated and equally divided among trunk and extremity, followed by visceral locations (12%) and rarely in the bone (3%). Microscopically, most tumors showed round to ovoid cytomorphology but half of the cases showed also focal areas of spindling and epithelioid/rhabdoid phenotype, with frequent myxoid stromal changes. Variable CD99 reactivity was seen in 84% cases, with a diffuse pattern only in 23% of cases, whereas nuclear WT1 was seen in 92%. A CIC-DUX4 fusion was detected in 57% of cases, with either DUX4 on 4q35 (35%) or on 10q26 in 25 (22%) cases. No FOXO4 gene rearrangements were present in 39 cases tested. Clinical follow-up was available in 57 patients, with a 5-year survival of 43%, which was significantly lower than the 77% 5-year survival in the control Ewing sarcoma group (P=0.002). Our findings show that CIC-DUX4 sarcomas occur most commonly in young adults within the somatic soft tissues, having a wide spectrum of morphology including round, epithelioid and spindle cells, and associated with an aggressive clinical course, with an inferior overall survival compared with Ewing sarcoma. The results support the classification of CIC-rearranged tumors as an independent molecular and clinical subset of small blue round cell tumors distinct from Ewing sarcoma.

Figure 1. Morphologic spectrum of CIC-rearranged sarcomas

Tumors showed a solid and often nodular growth pattern (A). Less common architectural patterns included intersecting short fascicles in tumors with a spindle cell component (B), a rare reticular growth (C). Myxoid stromal component was seen in a third of cases (D). The predominant phenotype was that of solid sheets of round to ovoid cells (E), however, focal areas of more epithelioid or plasmacytoid appearance was also noted with light eosinophilic cytoplasm and eccentric nuclei (F). The nuclear features showed variable chromatin patterns, with either fine (G), dark, hyperchromatic (H) or vesicular (I). Most cases showed minor variability in nuclear size and shape, except for a small subset revealing moderate degree of nuclear pleomorphism (H). Small to medium sized nucleoli were a common finding (G,I).

Figure 2. Immunohistochemical and FISH ancillary methods in diagnosis of CIC fusion positive sarcomas

CD99 reactivity a hallmark for Ewing sarcoma was present with a diffuse membranous pattern in a minority of cases (A), while most showed patchy, focal staining (B). WT1 nuclear staining was a consistent finding in overwhelming majority of cases (C). 3-color fusion FISH assay showing CIC (green, telomeric) fused to DUX4 (red, centromeric) gene located on 4q35 (D) or 10q26 (E). FISH break-apart showing split red centromeric and green telomeric CIC signals, in an SBRCT lacking DUX4 or FOXO4 gene abnormalities.

Figure 3. Survival analysis of CIC-rearranged sarcoma patients

Overall survival of the 57 patients with follow-up information showed a 2-year OS: 53% and a 5-year OS: 43% (A). Patients who presented with metastatic disease at diagnosis followed an unfavorable clinical course compared to patients with localized disease (p<0.0001) (B). Distant recurrence was associated with an inferior survival (p<0.0001) (C). Patients with CIC-rearranged sarcoma have a significantly worse outcome compared to patients with Ewing sarcoma (matched for age/stage) (p=0.002) (D).