Novel MAP4::RAF1 Fusion in a Primary Bone Sarcoma: Expanding the spectrum of RAF1 Fusion Sarcoma

Soft tissue tumors with RAF1 fusion had been emerging as a group of tumors with peculiar histology and immunoprofile. While a case series and rare case reports of RAF1 translocated sarcoma had been reported, to our knowledge a primary bone tumor with RAF1 translocation and fusion partner with MAP4 had not been described in the literature. The patient was a 60-year-old lady, with strong family history of breast cancer, who presented with pathological fracture of right humerus. X-ray revealed a 9.7 cm juxta-articular lesion of the proximal humerus, which was expansile and lytic with a non-sclerotic well defined border distally, radiologically suggestive of a giant cell tumor of bone. Excision was performed after initial biopsy. Histology showed a monomorphic low grade spindle cell lesion with prominent hyalinized stroma. Immunohistochemistry demonstrated diffuse CD34 staining, with focal staining for S100. Gene sequencing for histone 3 H3 genes was negative for hotspot mutation. Targeted RNA-seq sequencing revealed the presence of MAP4::RAF1 fusion, which was confirmed by reverse transcriptase-polymerase chain reaction (RT-PCR) and fluorescence in-situ hybridization (FISH) break-apart probes involving both genes. The overall features were consistent with a primary bone sarcoma with RAF1 fusion. This report expanded the spectrum of RAF1 fusion sarcoma and was the first report documenting its primary occurrence in bone.