Pediatric Mesenchymal Tumor With MN1::TAF3 Fusion.

Area of Science:

• Oncology
• Genetics
• Pathology

Background:

• MN1 fusion is increasingly recognized as a driver of oncogenesis in soft-tissue tumors.
• Detailed clinicopathological characterization of tumors with specific gene fusions is crucial for understanding their behavior.

Purpose of the Study:

• To provide a comprehensive clinicopathological description of a pediatric soft-tissue tumor harboring an MN1::TAF3 fusion.
• To investigate the clinical behavior and molecular underpinnings of this rare tumor entity.

Main Methods:

• Histopathological examination and immunohistochemistry of the tumor.
• Targeted RNA sequencing to identify gene fusions.
• Validation of the MN1::TAF3 fusion using reverse transcription-polymerase chain reaction, Sanger sequencing, and fluorescence in situ hybridization.

Main Results:

• A unique soft-tissue tumor in an 8-year-old boy exhibited an in-frame MN1 (exon 1)::TAF3 (exon 3) fusion transcript.
• The tumor displayed epithelioid morphology with high mitotic activity and focal necrosis, and showed no recurrence or metastasis for 5 years post-surgery without adjuvant therapy.
• Immunohistochemistry revealed positivity for cytokeratin AE1/AE3 in the epithelioid component, with otherwise nonspecific findings.

Conclusions:

• The MN1::TAF3 fusion may represent a distinct oncogenic event in pediatric soft-tissue tumors.
• The favorable clinical course suggests that some MN1::TAF3-driven tumors may have an indolent behavior.
• Further research is warranted to elucidate the spectrum and clinical significance of MN1::TAF3 fusions in pediatric sarcomas.