Myxoinflammatory Fibroblastic Sarcoma, Nodular-Necrotizing Variant With Two YAP1::MAML2 Fusions and TRIM24::BRAF Fusion.

Abstract

Myxoinflammatory fibroblastic sarcoma (MIFS) is a rare malignant soft tissue neoplasm typically found in the distal extremities of middle-aged adults. Histologically, MIFS presents with a myxoid stroma, a mixed inflammatory infiltrate, epithelioid/spindle cells resembling virocytes, pseudolipoblasts, and emperipolesis. Genetic alterations commonly include TGFBR3-MGEA5 rearrangements and VGLL3 amplifications. Recently, YAP1::MAML2 fusions have been identified in nodular necrotizing variants of MIFS. We report a case of a 40-year-old man presenting with right thumb pain initially suspected to be tendovaginitis. Histopathologic examination revealed a variably cellular proliferation in a fibrosclerotic-myxoid stroma with epithelioid cells displaying vesicular nuclei, spindle cells, and inflammatory infiltrates. Focal extensive necrosis was present. Immunohistochemical analysis was negative for SOX10, S100, SMA, desmin, MDM2, ALK, and CD20. Molecular genetic testing identified 2 variants of YAP1::MAML2 gene fusions and an additional TRIM24::BRAF fusion. Although BRAF rearrangements have previously been reported in MIFS, identification of TRIM24 as a fusion partner represents a novel finding. The presence of YAP1::MAML2 fusions, especially in combination with a TRIM24::BRAF fusion, has not been previously described in MIFS. TRIM24 is a transcriptional coregulator involved in p53 ubiquitination and tumorigenesis. The TRIM24::BRAF fusion has been detected in various malignancies, suggesting its potential oncogenic role. This case underscores the genetic heterogeneity of MIFS and suggests the need for further studies to elucidate the clinical implications of these molecular alterations. This report expands the molecular spectrum of MIFS and highlights the diagnostic utility of advanced sequencing technologies in identifying rare gene fusions. The TRIM24::BRAF fusion may represent a potential therapeutic target, warranting further investigation.