A recurrent novel fusion identifies a new subtype of high-grade spindle cell sarcoma.

-rearranged tumors are defined by the presence of a gene fusion between and various gene partners and typically follow a clinically aggressive disease course with poor outcomes despite conventional multimodality therapy. -rearranged tumors display histologic features of a poorly differentiated carcinoma with areas of focal squamous differentiation and typically express the fusion gene defining a distinct clinicopathologic entity-NUT carcinoma (NC). NCs with mesenchymal differentiation have rarely been described in the literature. In this report, we describe the characterization of two cases of high-grade spindle cell sarcoma harboring a novel fusion. Whole-genome sequencing identified the presence of complex rearrangements resulting in a fusion gene in the absence of other significant somatic mutations. Genetic rearrangement was confirmed by fluorescence in situ hybridization, and expression of the fusion gene product was confirmed by transcriptomic analysis. The fusion protein was predicted to retain nearly the entire protein sequence of both MGA (exons 1-22) and NUTM1 (exons 3-8). Histopathologically, both cases were high-grade spindle cell sarcomas without specific differentiation markers. In contrast to typical cases of NC, these cases were successfully treated with aggressive local control measures (surgery and radiation) and both patients remain alive without disease. These cases describe a new subtype of -rearranged tumors warranting expansion of diagnostic testing to evaluate for the presence of or alternative gene fusions in the diagnostic workup of high-grade spindle cell sarcomas or small round blue cell tumors of ambiguous lineage.