A morphologic and molecular reappraisal of myoepithelial tumors of soft tissue, bone, and viscera with EWSR1 and FUS gene rearrangements

Chondromyxoid Fibroma of the Temporal Bone: Case Report and Literature Review

Chondromyxoid fibroma (CMF) is a rare, benign, cartilaginous bone tumor that commonly affects the metaphysis of the lower-extremity long bones in young adults. Involvement of the craniofacial bones, especially the temporal bone, is exceedingly rare. We report a case of a 64-year-old man who presented with a left ear mass, but on subsequent imaging was found to have a more extensive mass centered in the temporal bone. Initial biopsy raised the concern for a low-grade chondrosarcoma. However, following surgical resection, a diagnosis of CMF was rendered. The clinical findings, radiographic features, histology, and differential diagnoses are discussed.

Primary Pulmonary Myxoid Sarcoma and Myxoid Angiomatoid Fibrous Histiocytoma: A Unifying Continuum With Shared and Distinct Features

Primary pulmonary myxoid sarcoma (PPMS) is a recently reported, exceedingly rare low-grade lung neoplasm characterized by reticular/lace-like growth of spindle to epithelioid cells embedded in an abundant myxoid matrix. Morphologically, it overlaps with a myxoid variant of angiomatoid fibrous histiocytoma (AFH) of the soft tissue. Genetically, they were both reported to harbor EWSR1-CREB1 fusion, while EWSR1-ATF1 has only been reported in AFH thus far. We report a case of primary pulmonary low-grade myxoid spindle cell tumor with morphologic and immunohistochemical features of PPMS but with an EWSR1-ATF1 fusion gene. In addition, we also encountered a case of endobronchial AFH with EWSR1-CREB1 translocation but also focal morphologic features of PPMS. These findings provide new evidence supporting the concept that PPMS and a myxoid variant of AFH represent a continuum with overlapping histologic, immunohistochemical, and genetic features.

EWSR1/FUS-CREB fusions define a distinctive malignant epithelioid neoplasm with predilection for mesothelial-lined cavities

Gene fusions constitute pivotal driver mutations often encoding aberrant chimeric transcription factors. However, an increasing number of gene fusion events have been shown not to be histotype specific and shared among different tumor types, otherwise completely unrelated clinically or phenotypically. One such remarkable example of chromosomal translocation promiscuity is represented by fusions between EWSR1 or FUS with genes encoding for CREB-transcription factors family (ATF1, CREB1, and CREM), driving the pathogenesis of various tumor types spanning mesenchymal, neuroectodermal, and epithelial lineages. In this study, we investigate a group of 13 previously unclassified malignant epithelioid neoplasms, frequently showing an epithelial immunophenotype and marked predilection for the peritoneal cavity, defined by EWSR1/FUS-CREB fusions. There were seven females and six males, with a mean age of 36 (range 9-63). All except three cases occurred intra-abdominally, including one each involving the pleural cavity, upper, and lower limb soft tissue. All tumors showed a predominantly epithelioid morphology associated with cystic or microcystic changes and variable lymphoid cuffing either intermixed or at the periphery. All except one case expressed EMA and/or CK, five were positive for WT1, while being negative for melanocytic and other mesothelioma markers. Nine cases were confirmed by various RNA-sequencing platforms, while in the remaining four cases the gene rearrangements were detected by FISH. Eleven cases showed the presence of CREM-related fusions (EWSR1-CREM, 7; FUS-CREM, 4), while the remaining two harbored EWSR1-ATF1 fusion. Clinically, seven patients presented with and/or developed metastases, confirming a malignant biologic potential. Our findings expand the spectrum of tumors associated with CREB-related fusions, defining a novel malignant epithelioid neoplasm with an immunophenotype suggesting epithelial differentiation. This entity appears to display hybrid features between angiomatoid fibrous histiocytoma (cystic growth and lymphoid cuffing) and mesothelioma (peritoneal/pleural involvement, epithelioid phenotype, and cytokeratin and WT1 co-expression).

Aggressive myoepithelial carcinoma with EWSR1-POU5F1 fusion highly responsive to Ewing sarcoma combination chemotherapy

This article describes a case of metastatic, bulky, high‐grade, rapidly progressive myoepithelial carcinoma with EWSR1‐POU5F1 fusion originating from the left kidney that demonstrated a dramatic, deep response within 2 cycles of treatment with a regimen used for patients with Ewing sarcoma and an ongoing sustained response of >10 months. To the author's knowledge, this is the first report of a rapid and deep response using a systemic therapy regimen in a patient with myoepithelial carcinoma with EWSR1‐POU5F1 fusion.

Pigmented (melanotic) myoepithelial tumor of soft tissue with EWSR1-KLF17 fusion

Myoepithelial tumors of soft tissue are rare, morphologically and biologically heterogeneous tumors. EWSR1 fusion is found in about half of the cases, followed by PLAG1 and FUS fusions. EWSR1-KLF17 fusion has so far been reported in one benign myoepithelial tumor. Using next generation sequencing we identified another myoepithelial tumor of soft tissue with EWSR1-KLF17 fusion, located on the foot in a 55-year-old male. It was composed predominantly of spindle cells with multiple small areas of epithelioid and multinucleated cells in myxohyaline stroma and areas of melanin pigment in the cytoplasm of tumor cells. The pigmented tumor cells were positive for HMB45 and, ultrastructurally, melanosomes were identified in their cytoplasm. Melanin production has not been previously documented in myoepithelial tumors of soft tissue. Our case extends the spectrum of myoepithelial tumors of soft tissue and emphasizes the importance of molecular characterization of fusions, including determination of fusion partners in myoepithelial tumors and their mimics.

Undifferentiated round cell sarcomas with novel SS18-POU5F1 fusions

Despite significant recent advances in characterizing the molecular pathogenesis of undifferentiated round cell neoplasms, rare cases remain unclassified. Here, we report two distinctive undifferentiated round cell tumors occurring in young adults. One tumor presented intrabdominally and the other arose within the abdominal wall. One patient died of disease following local and distance recurrence, despite aggressive chemotherapy and radiotherapy. Morphologically, both tumors were similarly composed of primitive round to epithelioid cells arranged in nests, sheets, and trabecular patterns. The cytoplasm was scant and amphophilic, while the nuclei were round and uniform with brisk mitotic activity. Focal necrosis was present. Immunohistochemically, both tumors were variably positive for S100 and EMA, and one case focally expressed cytokeratin and TLE1. Targeted RNA sequencing revealed in both an identical SS18-POU5F1 fusion gene. Fluorescence in situ hybridization was performed which confirmed SS18 and POU5F1 gene rearrangements. Expression data, relative to over 200 other mesenchymal neoplasms that had undergone targeted RNA sequencing on the same platform, suggested the SS18-POU5F1 tumors cluster with EWSR1/FUS-POU5F1-positive myoepithelial tumors. In view of our limited sample size, additional studies are needed to characterize the breadth of clinical and pathologic findings in these neoplasms. In addition, further investigation is necessary to determine whether this entity represents a clinically aggressive and phenotypically undifferentiated variant of myoepithelial tumors, or perhaps an altogether novel category of undifferentiated round cell sarcoma.