Case Report: High-grade supratentorial CNS neoplasm with EP300::BCOR fusion in an adult: expanding the morphologic and molecular spectrum of BCOR-fused tumors.

Tumors of the central nervous system harboring alterations of BCOR comprise a heterogeneous and evolving group of neoplasms. CNS tumors with BCOR internal tandem duplication are recognized as a distinct WHO entity, whereas tumors harboring EP300/CREBBP::BCOR/BCORL1 fusions represent an emerging and less clearly defined group with overlapping but non-identical morphologic and immunophenotypic features. We report a 42-year-old man with a supratentorial high-grade CNS neoplasm initially interpreted as a glial tumor with features suggestive of supratentorial anaplastic ependymoma. Histologically, the tumor showed marked intratumoral heterogeneity, with expansile/circumscribed ependymoma-like areas containing prominent perivascular pseudorosettes, and infiltrative high-grade areas with glioblastoma-like features, including extensive necrosis, marked microvascular proliferation, brisk mitotic activity and a Ki-67 index reaching approximately 70%. Immunohistochemistry showed GFAP negativity in the ependymoma-like areas and GFAP positivity in the high-grade glioblastoma-like component. OLIG2 was negative, EMA showed dot-like/paranuclear staining, and BCOR demonstrated moderate diffuse nuclear expression. RNA-based targeted sequencing identified reciprocal EP300::BCOR and BCOR::EP300 fusion junctions. FISH demonstrated homozygous CDKN2A/B deletion and 1q32 gain, findings subsequently supported by DNA-based copy-number analysis within an aneuploid profile. No reportable SNV or indel alterations of established diagnostic significance were identified. DNA methylation profiling was not performed. Accordingly, the integrated diagnostic interpretation was based on morphology, immunophenotype and RNA-based fusion detection, without methylation-class confirmation. This case expands the recognized morphologic and immunophenotypic spectrum of CNS tumors with EP300::BCOR fusion and highlights the diagnostic value of fusion testing in ependymoma-like supratentorial tumors with additional high-grade glioma-like features.