Expanding the Molecular Diversity of CIC-Rearranged Sarcomas With Novel and Very Rare Partners

Upregulation of POLE and proficient DNA repair are features of CIC::DUX4 sarcomas

CIC::DUX4 translocation-positive sarcomas (CDS) are rare, highly proliferative tumors associated with chemotherapy resistance. Without a clear understanding of the molecular pathways driving CDS, no effective treatment regimens have been developed. Here, we identify protection against DNA damage through upregulation of POLE and DNA repair pathways specific to CDS. These data may explain the high proliferation rate and chemotherapy resistance of these tumors and suggest targeted treatment strategies for CDS.

CIC-rearranged sarcoma in the breast: A series of 3 rare cases with literature review

Capticua transcriptional repressor (CIC)-rearranged sarcoma represents a distinct highly aggressive, undifferentiated round cell sarcomas, which most commonly affects young adults. The tumor typically arises in the deep soft tissues of the limbs and trunk, followed by the head and neck region, and is rarely found in visceral organs. CIC-rearranged sarcoma originating in the breast is extremely rare with only one case having been reported in the literature. We report three cases of CIC-rearranged sarcoma in the breast, all occurring in young females and exhibiting aggressive clinical behavior. Microscopically, all three cases showed diffuse or lobular growth of small round blue tumor cells with vesicular nuclei, prominent nucleoli, and clear cytoplasm. Focal reticular growth pattern and myxoid stroma were also observed. The diagnosis of CIC-rearranged sarcoma was confirmed through either fluorescence in situ hybridization (FISH) or RNA sequencing; two of the three cases were confirmed to harbor CIC::DUX4 fusion. This case series, to the best of our knowledge, represents the largest report of CIC-rearranged sarcoma in the breast. It highlights the importance of recognizing this rare entity in the breast due to its aggressive clinical course, poor response to chemotherapy, and high tendency for metastasis. It also emphasizes the utility of molecular studies in distinguishing CIC-rearranged sarcoma from poorly differentiated carcinoma, as CIC-rearranged sarcoma has significantly worse prognosis than poorly differentiated carcinoma of breast.

Ewing Sarcoma, Desmoplastic Small Round Cell Tumor, and Other Round Cell Sarcomas

The classification of small round cell sarcomas encompasses a spectrum of diseases ranging from those that are more widely understood, such as Ewing sarcoma and desmoplastic small round cell tumor, to more newly characterized diseases such as capicua (CIC)-rearranged sarcomas, BCOR-altered sarcomas, and sarcomas with EWSR1-non-ETS fusions. This article provides background, unique molecular features, and current and emerging therapies for each round cell sarcoma subtype.

Ancillary Tools for the Diagnosis of CIC -Rearranged Sarcoma: A Comprehensive Review

CIC-rearranged sarcoma is a rare and aggressive undifferentiated round cell sarcoma that presents significant diagnostic challenges due to its histologic overlap with other round cell sarcomas. This review, conducted on behalf of the American Society of Dermatopathology Appropriate Use Criteria Committee (soft tissue subgroup), provides an overview of current immunohistochemical, cytogenetic, and molecular tests used to support the diagnosis of CIC-rearranged sarcoma. This comprehensive analysis included 36 studies, encompassing 436 CIC-rearranged sarcomas. The immunohistochemical markers, CD99 (typically non-diffuse), nuclear WT1, ETV4, and DUX4, were found to be relatively highly sensitive for CIC-rearranged sarcoma (CD99: 87%, WT1: 83%, ETV4: 85%, DUX4: 97%). However, the specificity of these markers is variable, with CD99 being highly non-specific, while WT1 (81%-90%), ETV4 (95%), and DUX4 (100%) offering greater specificity. CIC break-apart FISH can be a helpful and cost-effective assay for detection of CIC-rearrangements, but has a false-negative rate that ranges from 26% to 43%. Next-generation sequence RNA fusion analysis also carries a risk of false negatives, which may be partly mitigated through manual data review. Ultimately, an accurate diagnosis of CIC-rearranged sarcoma requires careful morphologic assessment in combination with immunohistochemical studies and cytogenetics/molecular assays.

Nuclear DUX4 immunohistochemistry is a highly sensitive and specific marker for the presence of CIC::DUX4 fusion in CIC-rearranged sarcomas: a study of 48 molecularly confirmed cases

AIMS

CIC-rearranged sarcomas (CRS) are clinically aggressive undifferentiated round cell sarcomas (URCS), commonly driven by CIC::DUX4. Due to the repetitive nature of DUX4 and the variability of the fusion breakpoints, CIC::DUX4 fusion may be missed by molecular testing. Immunohistochemical (IHC) stains have been studied as surrogates for the CIC::DUX4 fusion. We aim to assess the performance of DUX4 IHC in the work-up of CRS and its expression in non-CRS round cell or epithelioid neoplasms.

METHODS AND RESULTS

Cases of molecularly confirmed CRS (n = 48) and non-CRS (n = 105) were included. CRS cases consisted of 35 females and 13 males, with ages ranging from less than 1 year to 67 years (median = 41 years). Among the molecularly confirmed non-CRS cases, C-terminal DUX4 expression was investigated in Ewing sarcomas (38 cases), alveolar rhabdomyosarcomas (18 cases), desmoplastic small round cell tumours (12 cases) and synovial sarcomas (n = five), as well as in non-mesenchymal neoplasms such as SMARCA4/SMARCB1-deficient tumours (n = five), carcinomas of unknown primary (n = three) and haematolymphoid neoplasms (four cases). DUX4 IHC was considered positive when strong nuclear expression was detected in more than 50% of neoplastic cells. When used as a surrogate for the diagnosis of CRS, the sensitivity and specificity of DUX4 IHC was 98 and 100%, respectively. Only one CRS case was negative for DUX4 IHC and harboured a CIC::FOXO4 fusion.

CONCLUSIONS

DUX4 IHC is a highly sensitive and specific surrogate marker for the presence of CIC::DUX4 fusion, demonstrating its utility in establishing a diagnosis of CRS.

[Undifferentiated small round cell sarcomas of bone and soft tissue]

In the 2020 5th edition of the World Health Organization classification of soft tissue and bone tumours a major reorganization of Undifferentiated Small Round Cell Sarcomas (USRCS) took place based on the underlying molecular features. The classification now recognizes Ewing sarcoma, round cell sarcoma with EWSR1-non-ETS fusions, CIC-rearranged sarcoma and sarcoma with BCOR alterations. The focus on these genetic alterations highlights the importance of molecular techniques in the diagnosis of these entities. Knowledge of these features can drastically reduce the time to diagnosis and avoid potential misdiagnosis. Molecular diagnostic capabilities should not be limited to an overall small number of centres worldwide as is reflected by the WHO's recognition of 'essential' and 'desirable' diagnostic criteria. A good knowledge of the usual histomorphology, uncommon variants and diagnostic pitfalls remains essential even in centres with access to a full molecular testing arsenal. This review aims to give an overview of the current classification of USRCS not by going over each entity, but instead going over the molecular, morphological, immunophenotypic and clinical features step by step to allow easy comparison of these features between the separate entities.

Whole genome profiling of rare pediatric thoracic tumors elucidates a YAP1::LEUTX fusion in an unclassified biphasic embryonal neoplasm

Malignant biphasic tumors of the lungs are rare, more so in the pediatric population. Here, we present the whole-genome characterization of a pleuropulmonary blastoma Type III and an unclassified biphasic thoracic embryonal neoplasm. The pleuropulmonary blastoma harbored pathogenic DICER1 germline and somatic mutations, and additional somatic variants in TP53 and BCOR. The other malignant tumor demonstrated a t(11;19) balanced translocation with a YAP1::LEUTX fusion that was confirmed by fluorescence in situ hybridization. No DICER1 germline or somatic mutation was present. YAP1 and LEUTX have been implicated in tumorigenesis of various neoplasms, and YAP1 fusion genes are an emerging oncogenic entity in a variety of malignancies. In this study we highlight the importance of whole-genome characterization of rare and unclassified tumors to identify biologic mechanisms and potential therapeutic targets.

A potential conundrum in dermatopathology: molecularly confirmed superficial ossifying fibromyxoid tumors with unusual histomorphologic findings and a novel fusion

Ossifying fibromyxoid tumor (OFMT) is a rare soft tissue neoplasm of uncertain histogenesis, primarily arising in subcutaneous tissues of the extremities, head and neck, or trunk. Most cases present as well-circumscribed masses with a characteristic morphologic appearance, comprising cytologically bland ovoid cells with fibromyxoid stroma, a peripheral rim of metaplastic bone, and lobulated architecture. Nevertheless, tumors displaying unusual morphologic characteristics pose significant diagnostic challenges, requiring the detection of a pathogenic fusion for a definitive diagnosis. The majority of OFMTs exhibits PHF1 gene rearrangements. Herein, we present six cases of molecularly confirmed OFMTs with uncommon histomorphologic features, including the absence of myxoid stroma, extensive chondroid differentiation, prominent clear cell morphology, collagen entrapment, interdigitating fibrocollagenous and fibromyxoid stromal elements, and conspicuous red blood cell extravasation. One case harbored a novel fusion (EPC1::SUZ12). This study emphasizes the broad range of morphologic manifestations that can be encountered in OFMT and the crucial role of molecular testing in establishing a conclusive diagnosis in such cases.

GRM1-Rearranged Chondromyxoid Fibroma With FGF23 Expression: A Potential Pitfall in Small Biopsies

The clinical, radiological, and histopathological features of chondromyxoid fibroma can sometimes resemble those of other benign or malignant tumors. Recently, recurrent GRM1 rearrangements have been identified in chondromyxoid fibroma, and GRM1 positivity by immunohistochemistry has emerged as a dependable surrogate marker for this molecular alteration. Phosphaturic mesenchymal tumor is a rare tumor that often exhibits overexpression of fibroblastic growth factor 23 (FGF23) through various mechanisms. In this report, we present a case of GRM1-rearranged chondromyxoid fibroma that also exhibited FGF23 expression via in situ hybridization, posing significant diagnostic challenges during workup of the initial core biopsy. We hope that this case can serve as an educational resource, shedding light on a rare diagnostic pitfall.

Capicua transcriptional repressor (CIC)-rearranged sarcoma harboring CIC-LEUTX fusion with renal involvement: a rare case report

Background

Capicua transcriptional repressor (CIC)-rearranged sarcoma (CRS) is a rare and highly aggressive undifferentiated small round cell sarcoma (USRCS), which genetically displays a characteristic gene fusion between CIC gene with other genes such as DUX4.

Case Description

We report a rare case with CIC-LEUTX fusion. The 45-year-old male patient presented to our department with frequent dry cough and lumbar pain. Computed tomography (CT) scan indicated multiple pulmonary nodules on both sides, hilar lymph node enlargement, left lower lobe infection and presence of pleural effusion in left quadrant. Enlargement and uneven density were seen in the left kidney, together with perinephric exudate, renal calculi, thickening and filling deficiency in renal veins, and small retroperitoneal lymph nodes. Immunohistochemistry (IHC) showed negativity in CK7, CK20, CD10, vimentin, PAX-8, P63, MelanA and synapatophysin, but GATA3 and CK were positive. RNA-based next generation sequencing (NGS) detection revealed CRS of gene fusion in CIC (exon 20) and LEUTX (exon 3). After treatment with a variety of targeted and chemotherapy drugs, the patient showed a poor response with a survival time of merely 7 months.

Conclusions

This case of USRCS harboring CIC-LEUTX fusion with renal involvement could help to expand our understanding on the diagnosis and treatment of CRS harboring CIC-LEUTX fusion.

Myoepithelial tumors of soft tissue and bone in children and young adults: A clinicopathologic study of 40 cases occurring in patients ≤ 21 Years of age

Myoepithelial tumors of the soft tissue and bone occurring in patients 21 years of age and younger are rare, and their clinicopathologic features remain incompletely understood. We studied a well-characterized series of 40 such tumors. Cases were retrieved from our archives for the period 2009-2022 and re-reviewed. Available immunohistochemical and molecular genetic data was collected. Clinical information including available follow-up was obtained. The tumors occurred in 18 males and 22 females, ranging from 3 months to 21 years of age (median 11.5 years), and involved a wide variety of soft tissue (n = 36) and bone (n = 4) locations. Histologically benign myoepithelial tumors tended to occur in adolescents (median age 14.5 years; range 5-21 years), whereas myoepithelial carcinomas occurred in younger patients (median age 8.5 years; range 3 months-20 years). Microscopically, the tumors showed a complex admixture of epithelioid, plasmacytoid and spindled cells in a variably hyalinized, myxoid, chondroid or chondromyxoid background. Small subsets of histologically malignant tumors had rhabdoid or "round cell" features. Immunohistochemistry showed 35/40 (88%) cases to be positive with at least one keratin antibody. The 5 keratin-negative tumors were uniformly positive for S100 protein and/or SOX10 and expressed EMA (4 cases) and/or p63 (3 cases). EMA, SMA and GFAP were positive in 21/25 (84%), 13/21 (62%), and 8/21 (38%) tumors, respectively. SMARCB1 and SMARCA4 expression was retained in 29/31 (94%) and 22/22 (100%) of cases, respectively. FISH for EWSR1 gene rearrangement was positive in 6/18 (33%) tested cases. Two EWSR1-negative tumors were also FUS-negative. NGS identified EWSR1::POU5F1, FUS::KLF17, and BRD4::CITED1 gene fusions in 3 tested cases. Clinical follow-up (22 patients; median 23 months; range 1-119 months) showed 3 patients with local recurrences and 5 with distant metastases (lymph nodes, lung, and brain). Three patients died of disease, 3 were alive with recurrent or unresectable disease, and 16 were disease-free. Adverse clinical outcomes were seen only in patients with malignant tumors. We conclude that myoepithelial neoplasms of soft tissue and bone are over-repesented in patients ≤21 years of age, more often histologically malignant, and potentially lethal. Histologic evaluation appears to reliably predict the behavior of these rare tumors.

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.

Updates on WHO classification for small round cell tumors: Ewing sarcoma vs. everything else

The WHO Classification of Soft Tissue and Bone Tumours currently recognizes four categories of undifferentiated small round cell sarcoma: Ewing sarcoma, round cell sarcoma with EWSR1-non-ETS fusions including NFATc2 and PATZ1, CIC-rearranged sarcoma, and sarcoma with BCOR genetic alterations. These neoplasms frequently pose significant diagnostic challenges due to rarity and overlapping morphologic and immunohistochemical findings. Further, molecular testing, with accompanying pitfalls, may be needed to establish a definitive diagnosis. This review summarizes the clinical, histologic, immunohistochemical, and molecular features of these neoplasms. In addition, differential diagnosis and areas of uncertainty and ongoing investigation are discussed.

CIC-Rearranged Sarcoma

CIC-rearranged sarcoma is a rare type of small round cell sarcoma. The tumors often affect the deep soft tissues of patients in a wide age range. They are highly aggressive, respond poorly to chemotherapy, and have a worse outcome than Ewing sarcoma. CIC-rearranged sarcoma has characteristic and recognizable histology, including lobulated growth, focal myxoid changes, round to epithelioid cells, and minimal variation of nuclear size and shape. Nuclear ETV4 and WT1 expression are useful immunohistochemical findings. CIC fusion can be demonstrated using various methods; however, even next-generation sequencing suffers from imperfect sensitivity, especially for CIC::DUX4.

Case Report: Colon malignant tumor caused by retroperitoneal small round cell undifferentiated sarcoma

Small round cell undifferentiated sarcoma is a rare and highly invasive group of malignant bone and soft tissue tumors, often associated with a high misdiagnosis rate. The patient in this case was a 34-year-old male who presented with a two-month history of abdominal pain that worsened over the past two weeks. Elevated levels of tumor markers CA19-9 and CA72-4 were observed. Imaging revealed a substantial, well-vascularized mass in the lower left abdomen, located in the posterior abdominal cavity, invading the descending colon and the root of the small mesentery, and infiltrating the serous layer. The lesion was extensively resected without any postoperative complications. Microscopic examination indicated a combination of mucinous adenocarcinoma (approximately 30%) and small round cell undifferentiated sarcoma (approximately 70%). The patient was followed up for six months, and one month after surgery, a recurrence of the tumor was observed in the left paracolonic sulcus area, with metastases to the abdominal wall, peritoneum, and medial iliac muscles. Chemotherapy and targeted therapy were administered, and the patient currently survives with the presence of tumors. Small round cell undifferentiated sarcoma is an uncommon and highly invasive tumor, and clinical surgeons need to raise their awareness and realize to the maximum extent possible that this disease can be described through a multi-modal combination of immunohistochemistry and genetic test to improve diagnostic accuracy and reduce missed diagnoses. Further research in the field of biology is necessary to explore targeted drugs specifically suitable for this disease.

Novel HNRNPM::LEUTX fusion resulting from chromothripsis of chromosome 19 in a pediatric undifferentiated small round cell neoplasm

Small round cell neoplasms comprise a diverse group of tumors characterized by a primitive/undifferentiated appearance. Although several entities are associated with recurrent gene fusions, many of these neoplasms have not been fully characterized, and novel molecular alterations are being discovered. Here, we report an undifferentiated small round cell neoplasm arising in the anterior mediastinum of a 17-month-old female. The tumor harbored a novel HNRNPM::LEUTX fusion resulting from chromothripsis of chromosome 19, which was identified by whole transcriptome sequencing, but not by targeted sequencing. The structural variations caused by the chromothripsis event also challenged the interpretation of the targeted sequencing findings. This report expands the spectrum of gene partners involved in LEUTX fusions and underscores the value of whole transcriptome sequencing in the diagnostic workup of undifferentiated small round cell tumors. It also highlights the interpretive challenges associated with complex genomic alterations. A careful evidence-based analysis of sequencing data along with histopathologic correlation is essential to ensure correct categorization of fusions.

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.

Gene fusions in superficial mesenchymal neoplasms: Emerging entities and useful diagnostic adjuncts

Cutaneous mesenchymal neoplasms are diagnostically challenging because of their overlapping morphology, and, often, the limited tissue in skin biopsy specimens. Molecular and cytogenetic techniques have identified characteristic gene fusions in many of these tumor types, findings that have expanded our understanding of disease pathogenesis and motivated development of useful ancillary diagnostic tools. Here, we provide an update of new findings in tumor types that can occur in the skin and superficial subcutis, including dermatofibrosarcoma protuberans, benign fibrous histiocytoma, epithelioid fibrous histiocytoma, angiomatoid fibrous histiocytoma, glomus tumor, myopericytoma/myofibroma, non-neural granular cell tumor, CIC-rearranged sarcoma, hybrid schwannoma/perineurioma, and clear cell sarcoma. We also discuss recently described and emerging tumor types that can occur in superficial locations and that harbor gene fusions, including nested glomoid neoplasm with GLI1 alterations, clear cell tumor with melanocytic differentiation and ACTIN::MITF translocation, melanocytic tumor with CRTC1::TRIM11 fusion, EWSR1::SMAD3-rearranged fibroblastic tumor, PLAG1-rearranged fibroblastic tumor, and superficial ALK-rearranged myxoid spindle cell neoplasm. When possible, we discuss how fusion events mediate the pathogenesis of these tumor types, and we also discuss the related diagnostic and therapeutic implications of these events.

First Female Patient with a Rare CIC-FOXO4-Translocated Sarcoma: A Case Report

Small round cell sarcoma is a group of undifferentiated malignancies arising in the bone and soft tissue, notable for Ewing sarcoma. Recently, a new World Health Organization classification has been introduced, including an additional subset of these sarcomas, named CIC-rearranged sarcoma. Within this group, CIC-FOXO4 translocation is an exceedingly rare fusion that has been reported only 4 times in the literature. Herein, we report in-depth the pathological, clinical, and molecular features of a CIC-FOXO4 translocation-driven tumor in a 46-year-old woman.