Sarcoma with MGA::NUTM1 fusion: a report of three cases and literature review.

AIMS: NUTM1-rearranged sarcoma is an emerging entity that differs from NUT carcinoma at the molecular level, with most of the former tumours harbouring fusions involving genes in the MYC-associated factor X dimerization (MAD) transcription family (MXD1, MXD4, MXI1 [or MXD2], and MGA). MGA::NUTM1 is one of the most recently described novel gene fusions associated with NUTM1-rearranged sarcoma. Herein we describe the clinicopathologic features of three sarcomas with an MGA::NUTM1 fusion. METHODS AND RESULTS: The three study patients were male, with an age range of 10-28 years. The tumour sites were deep soft tissue of the thigh, the chest wall, and the pelvis. All three tumours were aggressive, with multiple recurrences and metastases. Histologically, the tumours were composed of monotonous spindle, round, or epithelioid cells in variably hyalinized stroma and prominent aggregates of amianthoid fibre-like collagen or collagen rosettes. Mitotic activity was relatively low (5-12 mitotic figures per 10 hhpf). All tumours tested expressed NUT, with one tumour having S100 protein expression and two tumours having CD99 and CD56 expression. The genetic breakpoints were MGA exon 21, MGA exon 22, and NUTM1 exon 3. CONCLUSION: MGA::NUTM1 sarcoma often exhibits hyalinized stroma with amianthoid fibre-like collagen or collagen rosettes in the presence of monotonous round, epithelioid, or spindle cell morphology. NUT immunohistochemistry and molecular testing can help confirm the diagnosis.