H3F3A K36M-mutant Epithelioid Neoplasm: A Report of Two Novel Cases of a Non-chondrogenic H3K36-altered Mesenchymal Tumor.

Background: Identifying the genetic signatures in bone and soft tissue tumors enhances our understanding of tumor biology and aids in the subclassification of tumors for personalized treatment. Histone H3.3 alterations play a pivotal role in H3F3A/B K36M-mutant chondroblastomas and H3F3A G34W/L-mutant giant cell tumors of the bone.

Methods and results: In this report, we describe 2 cases of a distinct epithelioid neoplasm with H3F3A K36M mutation but lacking features of chondroblastoma, which extends the spectrum of H3.3-mutant mesenchymal tumors. The 2 cases occurred in pediatric patients, had an aggressive clinical presentation, distinct epithelioid histomorphology with diffuse cytokeratin and TFE3 expression, and identical H3F3A K36M mutations. No gene fusions were identified. Methylation analysis using the DKFZ sarcoma classifier v12.3 pipeline did not classify these tumors with known entities, suggesting the existence of non-chondrogenic mesenchymal tumors within the H3.3-mutant tumor spectrum.

Conclusions: The distinctive histological and molecular features of these 2 cases expand the spectrum of H3.3-mutant tumors and call for further investigation of the biological underpinnings of this group of tumors.