RREB1-MKL2 fusion in a spindle cell sinonasal sarcoma: biphenotypic sinonasal sarcoma or ectomesenchymal chondromyxoid tumor in an unusual site?

An extralingual Ectomesenchymal chondromyxoid tumor with RREB1::MRTFB fusion: a rare case report of plantar fascia involvement

BACKGROUND

Ectomesenchymal chondromyxoid tumor (ECT) is a rare benign intraoral tumor that almost exclusively presents as a small mass on the anterior dorsal tongue. Recently, the ras-responsive element-binding protein 1::myocardin-related transcription factor B (RREB1::MRTFB; previously known as MKL2) fusion gene has been identified in 90% of ECTs, all localized to the tongue, highlighting its genetic distinctiveness. Herein, we report a mesenchymal tumor involving the plantar fascia of the left foot in a young woman, harboring the RREB1::MRTFB fusion gene.

CASE PRESENTATION

The tumor presented as a well-circumscribed mass. Following complete excision, no recurrence was observed at the six-month follow-up. Histological examination revealed tumor cells exhibiting mild nuclear atypia and very low mitotic activity. Immunohistochemical analysis showed diffuse positive staining for S100, glial fibrillary acidic protein (GFAP), and CD56, variable expression of smooth muscle actin, and negative staining for SOX10 and P63. Targeted RNA sequencing identified RREB1 (exon 8)-MRTFB (exon 11) fusion transcripts. Collectively, these findings suggest the possibility of a previously unreported extralingual ECT involving the plantar fascia. However, its atypical morphology and uncommon anatomical location posed significant diagnostic challenges.

CONCLUSIONS

We report, for the first time, a mesenchymal chondromyxoid tumor with an RREB1::MRTFB fusion gene occurring in the foot. This case expands the known distribution of ECT beyond the tongue. Accurate differential diagnosis should rely on thorough histological assessment, combined with immunohistochemical and molecular analyses.

Spindle Cell Tumors of the Sinonasal Tract: A Diagnostic Update with Focus on Ancillary Workup

Spindle cell neoplasms arising in the head and neck may be challenging to recognize due to their relative rarity. While underlying molecular alterations are increasingly elucidated, testing for these features may not be readily available. In most cases, combinations of key morphologic features and diagnostic immunohistochemical markers can be used to replace molecular diagnostics. Conversely, some molecular alterations and expression of their surrogate biomarkers are not specific for any one entity, and it is important to recognize these to avoid diagnostic pitfalls. In this review, we discuss both old and new spindle cell tumors of the sinonasal tract, with an emphasis on histologic features and clinically relevant immunohistochemical markers serving as surrogate markers for underlying genomic alterations.

A Low-grade Sinonasal Sarcoma Harboring EWSR1::BEND2: Expanding the Differential Diagnosis of Sinonasal Spindle Cell Neoplasms

BACKGROUND

Molecular diagnostics has greatly refined sinonasal tumor pathology over the past decade. While much of the attention has focused on carcinomas, it is becoming clear that there are emerging mesenchymal neoplasms which have previously defied classification.

METHODS

Here, we present a 33-year-old woman with a multiply recurrent sinonasal spindle cell tumor exhibiting distinctive features, and not easily classifiable into a specific category.

RESULTS

The hypercellular tumor was composed of plump spindled cells, with uniform vesicular chromatin arranged as vague fascicles around a prominent hemangiopericytoma-like vasculature. The mitotic rate was brisk at 10 per 10 high power fields. By immunohistochemistry, it was only positive for EMA (focal) and SATB2 (diffuse, weak). Fusion analysis uncovered EWSR1::BEND2, a fusion which is best known for being seen in astroblastoma, but which has not yet been reported in sarcomas.

CONCLUSION

This case underscores the utility of fusion analysis when confronted with a sinonasal spindle cell neoplasm which does not neatly fit into any specific category. It remains to be seen if EWSR1::BEND2 sinonasal sarcoma represents a distinct entity.

Translocations and Gene Fusions in Sinonasal Malignancies

PURPOSE OF REVIEW

During the past few years there has been an expansion in our understanding of gene fusions and translocations involved in cancer of the sinonasal tract. Here we review the downstream biologic effects, clinical characteristics, and pathologic features of these tumors. The molecular consequences and neo-antigens resulting from these chromosomal aberrations are considered and targets for current and future clinical trials discussed.

RECENT FINDINGS

Several new, clinically relevant, chromosomal aberrations have been discovered and evaluated to varying degrees in sinonasal tumors including DEK::AFF2, BRD4::NUT, ADCK4::NUMBL, and ETV6::NTRK3. Sinonasal malignancies demonstrate a diverse genetic landscape and varying clinical courses. Recent studies illustrate that gene fusions and translocations may play a role in carcinogenesis in certain sinonasal tumor subtypes and may be used to develop new biomarker-driven and patient-centered treatments.

Ectomesenchymal chondromyxoid tumor of the oral cavity: a report of 5 new cases with comprehensive review of the literature and clinicohistopathologic features

OBJECTIVE

An ectomesenchymal chondromyxoid tumor (ECT) is an uncommon soft tissue tumor with an enigmatic histogenesis and striking predilection for the tongue. We present 5 new cases and review the literature.

STUDY DESIGN

We performed a retrospective search for ECTs within the University of Kentucky Oral Pathology Biopsy Service and the published literature.

RESULTS

Five new cases from the biopsy archives and 103 well-documented ECT cases from the literature were compiled and reviewed. Whereas 89.8% of ECT are found on the anterior/dorsal/lateral/unspecified tongue, 4.6% are on the posterior/base of tongue. Six extralingual cases are reported. The age ranges from 2.3 to 78 years with an average of 40. Most ECT react with GFAP (92.8%) and S-100 protein (91.3%). Whereas 21/23 cases demonstrated a RREB1-MKL2 fusion, EWSR1 gene mutations are identified in 4 cases.

CONCLUSIONS

Most ECT are readily diagnosed on routine histopathology in combination with tumor site, immunohistochemical findings, and molecular findings; however, a subset share overlapping features with myoepithelioma of soft parts. As further molecular analysis is performed on this tumor, we may find that a subset of previously diagnosed ECT relate to or represent myoepithelioma or conversely fall under the spectrum of the pluripotent ECT.

Biphenotypic sinonasal sarcoma with PAX3::MAML3 fusion transforming into high-grade rhabdomyosarcoma: report of an emerging rare phenomenon

We report a case of a 67-year-old male patient with a sinonasal tumor that showed areas of classic biphenotypic sinonasal sarcoma (BSNS) which in some sections sharply transitioned into high-grade rhabdomyosarcoma. Immunohistochemically, the conventional BSNS parts showed S100 protein, SMA, PAX7, and focal MyoD1 expression, whereas desmin and myogenin were negative. In contrast, the cells in high-grade areas expressed desmin, MyoD1, myogenin, and PAX7, while being negative for S100 protein and SMA. Using the Archer FusionPlex assay, the classical PAX3::MAML3 gene fusion was detected. FISH for PAX3 and MAML3 confirmed a break of these genes in both components. Despite aggressive therapy, the tumor progression resulted in the patient's death. The herein presented case, together with 2 previously published cases of BSNS with high-grade transformation, helps to better understand this novel phenomenon. Although the risk for such transformation appears low, it has important clinical and diagnostic implications which are discussed.

RREB1::MRTFB fusion-positive extra-glossal mesenchymal neoplasms: A series of five cases expanding their anatomic distribution and highlighting significant morphological and phenotypic diversity

The RREB1::MRTFB (former RREB1::MKL2) fusion characterizes ectomesenchymal chondromyxoid tumors (EMCMT) of the tongue. Only five molecularly confirmed extra-glossal EMCMT cases have been reported recently; all occurring at head and neck or mediastinal sites. We herein describe five new cases including the first two extracranial/extrathoracic cases. The tumors occurred in three male and two female patients with an age ranging from 18 to 61 years (median, 28). Three tumors were located in the head and neck (jaw, parapharyngeal space, and nasopharyngeal wall) and two in the soft tissue (inguinal and presacral). The tumor size ranged from 3.3 to 20 cm (median, 7). Treatment was surgical without adjuvant treatment in all cases. Two cases were disease-free at 5 and 17 months; other cases were lost to follow-up. Histologically, the soft tissue cases shared a predominant fibromyxoid appearance, but with variable cytoarchitectural pattern (cellular perineurioma-like whorls and storiform pattern in one case and large polygonal granular cells embedded within a chondromyxoid stroma in the other). Two tumors (inguinal and parapharyngeal) showed spindled to ovoid and round cells with a moderately to highly cellular nondescript pattern. One sinonasal tumor closely mimicked nasal chondromesenchymal hamartoma (NCMH). Mitotic activity was low (0-5 mitoses/10 hpfs). Immunohistochemical findings were heterogeneous with variable expression of S100 (2/5), EMA (2/3), CD34 (1/4), desmin (1/4), and GFAP (1/3). Targeted RNA sequencing revealed the same RREB1::MRTFB fusion in all cases, with exon 8 of RREB1 being fused to exon 11 of MRTFB. This study expands the topographic spectrum of RREB1::MRTFB fusion-positive mesenchymal neoplasms, highlighting a significant morphological and phenotypic diversity. Overall, RREB1::MRTFB-rearranged neoplasms seem to fall into two subcategories: tumors with lobulated, chondroid, or myxochondroid epithelioid morphology (Cases 2 and 3) and those with more undifferentiated hypercellular spindle cell phenotype (Cases 1, 4, and 5). Involvement of extracranial/extrathoracic sites and the NCMH-like pattern are novel. The biology of these likely indolent or benign tumors remains to be verified in the future.

RNA-sequencing of myxoinflammatory fibroblastic sarcomas reveals a novel SND1::BRAF fusion and 3 different molecular aberrations with the potential to upregulate the TEAD1 gene including SEC23IP::VGLL3 and TEAD1::MRTFB gene fusions

Myxoinflammatory fibroblastic sarcoma (MIFS) has been shown to harbor various recurrent molecular aberrations; most of which, however, seem to be present in only a minority of cases. In order to better characterize the molecular underpinnings of MIFS, fourteen cases were analyzed by targeted RNA-sequencing (RNA-seq), VGLL3 enumeration FISH probe, and BRAF break-apart and enumeration probes. Neither t(1;10)(p22;q24) nor BRAF gene amplifications were found. However, VGLL3 gene amplification was detected in 5 cases by FISH which corresponded with an increase in VGLL3 expression detected by RNA-seq. In 1 of these cases, RNA-seq additionally revealed a novel SND1::BRAF fusion. Two of the 9 cases lacking VGLL3 amplification harbored either a SEC23IP::VGLL3 or a TEAD1::MRTFB rearrangement by RNA-seq, both confirmed by RT-PCR and Sanger sequencing. The detected molecular aberrations have a potential to either activate the expression of genes regulated by the transcription factors of the TEAD family, which are involved in tumor initiation and progression, or switch on the MEK/ERK signaling cascade, which plays an important role in cell cycle progression. Our results broaden the molecular genetic spectrum of MIFS and point toward the importance of the VGLL3-TEAD interaction, as well as the deregulation of the MEK/ERK pathway in the pathogenesis of MIFS, and may represent a potential target for therapy of recurrent or advanced disease.