Clinicopathological, Molecular, and DNA Methylation Analysis of Ossifying Fibromyxoid Tumors Delineates the ZC3H7B::BCOR Subset as a Distinct Entity.

Ossifying fibromyxoid tumor (OFMT) is a rare mesenchymal neoplasm of uncertain lineage of differentiation driven by a broad spectrum of gene fusions. It manifests primarily in soft tissues of the extremities. In this study, we performed a comprehensive clinicopathological, molecular-genetic, and epigenetic analysis of 70 cases of OFMT, with a specific focus on rare fusion subtypes, particularly ZC3H7B::BCOR, PHF1::TFE3, and MEAF6::PHF1. In addition, we included 7 tumors with novel fusions, namely, AFF3::PHF1, PHF1::KLF15, PHF1::PRKAG1, CREBBP::PHF1, EPC1::BMI1, MEAF6::BCOR, and EP300::BCORL1. The clinicopathological characteristics revealed a correlation between specific fusions and aggressive clinical behavior; notably, tumors with ZC3H7B::BCOR fusions were always classified as morphologically atypical or malignant and were associated with significantly higher recurrence and metastatic rates compared with other fusion groups, particularly EP400::PHF1. Immunohistochemical analysis further revealed a distinct immunophenotype in the ZC3H7B::BCOR group. Whereas immunopositivity for cytokeratins and myogenic markers was observed in approximately one-quarter and more than one-third of OFMT cases overall, respectively, ZC3H7B::BCOR-rearranged tumors were negative for both. In contrast, the majority showed immunopositivity for pan-Trk. Additionally, DNA methylation profiling distinguished ZC3H7B::BCOR-rearranged cases from other fusion-positive OFMT cases. The former group clustered closely with a subset of high-grade endometrial stromal sarcomas. This study supports the recognition of ZC3H7B::BCOR-positive tumors as a distinct clinicopathological entity, contributing to the refined molecular taxonomy of this neoplasm.