Expanding the differential of superficial tumors with round-cell morphology: Report of three cases of CIC-rearranged sarcoma, a potentially under-recognized entity

Molecular and therapeutic advancements in Capicua (CIC)-rearranged sarcoma

Capicua (CIC)-rearranged sarcomas are an aggressive subset of undifferentiated round cell sarcomas. CIC::DUX4, the proto-typical CIC fusion oncoprotein is associated with rapid clinical progression and chemotherapy resistance leading to poor clinical outcomes. Recent studies have identified additional CIC fusions (CIC::NUTM1, CIC::FOXO4, and CIC::LEUTX) that largely retain CIC-binding specificity but leverage C-terminal binding partners (NUTM1, FOXO4, and LEUTX) to potentially activate transcriptional programs that drive oncogenesis. Moreover, the recent development of preclinical models to study CIC::DUX4 sarcoma have advanced our understanding of the underlying biological mechanisms and uncovered key dependencies that can be translated into rational therapies. In this review, we will highlight these recent advancements in CIC-rearranged sarcoma biology with a vision for clinical translation to improve patient outcomes.

Application of immunohistochemical studies in diagnosing emerging superficial mesenchymal neoplasms

Molecular diagnostics, with the subsequent development of novel immunohistochemical markers, continues to advance and expand the field of soft tissue pathology. As such, the ever-evolving molecular diagnostic landscape will continue to shape and refine our understanding and classification of neoplasms. This article reviews the current literature on various tumors of mesenchymal origin, including fibroblastic/fibrohistiocytic, adipocytic, vascular, and tumors of uncertain origin. We aim to give the reader a detailed understanding and pragmatic approach to various new and established immunohistochemical stains in diagnosing these neoplasms and also discuss various pitfalls with significant repercussions.

Small round cell sarcomas

Undifferentiated small round cell sarcomas (SRCSs) of bone and soft tissue comprise a heterogeneous group of highly aggressive tumours associated with a poor prognosis, especially in metastatic disease. SRCS entities mainly occur in the third decade of life and can exhibit striking disparities regarding preferentially affected sex and tumour localization. SRCSs comprise new entities defined by specific genetic abnormalities, namely EWSR1-non-ETS fusions, CIC-rearrangements or BCOR genetic alterations, as well as EWSR1-ETS fusions in the prototypic SRCS Ewing sarcoma. These gene fusions mainly encode aberrant oncogenic transcription factors that massively rewire the transcriptome and epigenome of the as yet unknown cell or cells of origin. Additional mutations or copy number variants are rare at diagnosis and, depending on the tumour entity, may involve TP53, CDKN2A and others. Histologically, these lesions consist of small round cells expressing variable levels of CD99 and specific marker proteins, including cyclin B3, ETV4, WT1, NKX3-1 and aggrecan, depending on the entity. Besides locoregional treatment that should follow standard protocols for sarcoma management, (neo)adjuvant treatment is as yet ill-defined but generally follows that of Ewing sarcoma and is associated with adverse effects that might compromise quality of life. Emerging studies on the molecular mechanisms of SRCSs and the development of genetically engineered animal models hold promise for improvements in early detection, disease monitoring, treatment-related toxicity, overall survival and quality of life.

CIC-DUX4 sarcomas

PURPOSE OF REVIEW

CIC-DUX4 sarcoma (CDS) is a high-grade undifferentiated round cells sarcoma that belongs to the undifferentiated round cell sarcomas family. It represents less than one percent of sarcomas, defining a rarest among rare malignancies. It affects young adults, displaying soft tissue mass. Considered very aggressive, a high proportion of cases display an advanced disease with lung metastasis at diagnosis. Here we discuss recent progress in molecular characterization of CDS, the main tracks of CDS biology and the current and future prospects of therapeutic approaches.

RECENT FINDINGS

CDS is characterized by a specific oncogenic translocation CIC::DUX4 that induce ETV4 overexpression. Patients with CDS show an aggressive clinical course and have a significantly unfavorable outcome compared to Ewing sarcoma. As of today, there is a lack of consensus on whether they should be treated with an Ewing-like approach, as currently done by most sites, or regarded as high-grade soft tissue sarcoma (STS). Anyway, when feasible, combination regimens including anthracycline and alkylating agents should be favored and patients should not benefit from a therapeutic de-escalation. Overall, registration within clinical trials and prospective registries is recommended.

SUMMARY

Overall, CDS showed a poor prognosis regardless of the patterns of treatment that warrant biological studies to better understand the disease.

Advances in the classification of round cell sarcomas

Round cell sarcomas represent a diagnostic challenge for pathologists, owing to the poorly differentiated features of these high-grade tumours. The diagnosis of round cell sarcoma requires large immunohistochemical panels and molecular testing in many cases. This spectrum of malignancies is largely dominated by Ewing sarcomas (ESs), which represent the most common family of these tumours. Nonetheless, new families have been delineated in the past few years, with the addition of two additional families in the 2020 World Health Organization classification of bone and soft tissue tumours, namely sarcomas with CIC rearrangements and sarcomas with BCOR alterations. EWSR1, one of the genes involved in the driver fusion of ESs, is also implicated in the translocation of many other tumours with heterogeneous lineages and variable levels of aggressiveness. Round cell sarcomas associated with fusions inwhichEWSR1is partnered with genes encoding transcription factors distinct from those of the 'Ewing family' represent a heterogeneous group of rare tumours that require further study to determine whether their fusions may or not define a specific subgroup. They include mainly sarcomas with NFATc2 rearrangements and sarcomas with PATZ1 rearrangements. At this point, PATZ1 fusions seem to be associated with tumours of high clinical and morphological heterogeneity. Molecular studies have also helped in the identification of more consistent biomarkers that give tremendous help to pathologists in triaging, if not diagnosing, these tumours in practice. This review compiles the latest accumulated evidence regarding round cell sarcomas, and discusses the areas that are still under investigation.

Adding Perplexity to Rarity: Diffuse S100-Protein and SOX10 Expression in a Molecularly Confirmed PAX7-Positive Primary Cutaneous Ewing Sarcoma

ABSTRACT

Primary cutaneous Ewing sarcoma (EWS) is a very rare neoplasm that shares similar morphologic, immunohistochemical, and molecular features with its osseous counterpart. Herein, we present an extraordinarily rare case of PAX7-positive cutaneous EWS in a 9-year-old girl that was also diffusely positive for SOX10 and S100-protein. Next generation sequencing detected the EWSR1-FLI1 fusion supporting the diagnosis, which was further validated by break-apart EWSR1 fluorescence in situ hybridization. Diffuse S100-protein and SOX10 expression has been reported only in a handful of cases of EWS and may pose significant diagnostic challenges for dermatopathologists. PAX7 is a recently introduced marker, which is highly sensitive for EWS and can potentially have discriminatory power in the differential diagnosis of cutaneous undifferentiated round blue cell tumors.

Non-rhabdomyosarcoma soft tissue sarcomas diagnosed in patients at a young age. An overview of clinical, pathological, and molecular findings

OBJECTIVE

Disease spectrum in pediatric sarcoma differs substantially from adults. We report a cohort of very young children with non-rhabdomyosarcoma soft tissue sarcoma (NRSTS) detailing their molecular features, treatment, and outcome.

METHODS

We report features of consecutive children (age <2 years) with NRSTS (2000-2017). Archival pathological material was re-reviewed, with additional molecular techniques applied where indicated.

RESULTS

Twenty-nine patients (16 females, 55%) were identified (median age 6 months; range 0-23). Most common diagnoses included infantile fibrosarcoma (IFS, n = 14, 48%), malignant rhabdoid tumor (MRT, n = 4, 14%), and undifferentiated sarcoma (n = 4, 14%). Twenty-seven of 29 (93%) had tumor molecular characterization to confirm diagnosis. Clinical presentation included a swelling/mass (n = 23, 79%). Disease extent was localized (n = 20, 69%), locoregional (n = 6, 21%), or metastatic (n = 3, 10%). Seventeen of 29 (59%) who underwent surgery achieved complete resection (R0). Other treatments included conventional chemotherapy (n = 26, 90%), molecularly targeted therapies (n = 3, 10%), and radiation (n = 5, 17%). At last follow-up (median 3 years; range 0.3-16.4), 23 (79%) were alive, disease-free and six (21%) had died of disease. All patients with IFS were alive and all those with MRT died. A cancer predisposition syndrome (CPS) was confirmed in three of 10 (30%) genetically tested patients.

CONCLUSION

We recommend tumor molecular characterization in all young patients including evaluation for CPS to optimize treatment options and prognostication.

Subcutaneous desmoplastic small round-cell tumor: An unusual primary location expanding the differential of superficial round-cell tumors

Desmoplastic small round-cell tumor (DSRCT) is a rare, aggressive malignant tumor, which in the great majority of cases arises at abdominal-pelvic sites. Nevertheless, rare cases of primary extra-abdominal tumors have been reported. In challenging cases, its molecular hallmark, the EWSR1-WT1 reciprocal translocation, can be exploited diagnostically by various molecular techniques. Herein, we report an extremely rare case of primary subcutaneous DSRCT in an effort to raise awareness among our dermatopathology colleagues by expanding the differential of superficial round-cell tumors.