Myxoid epithelioid smooth muscle tumor with a novel MEF2A::NCOA2 fusion arising in the rectum: case report and literature review.

Mesenchymal tumors of the gastrointestinal (GI) tract include a range of entities such as gastrointestinal stromal tumor (GIST), schwannoma, and leiomyoma; however, the genetic background of GI leiomyomas has not been elucidated. Herein, we report a case of rectal myxoid epithelioid smooth muscle tumor harboring a novel MEF2A::NCOA2 gene fusion. A 67-year-old male presented with a 16 mm tumor in the lower rectum. Pathological examination revealed a well-circumscribed lesion within the muscularis mucosae, characterized by epithelioid to spindle cells arranged in a myxoid matrix. The tumor cells displayed minimal cytological atypia and no mitotic figures. Immunohistochemistry showed strong positivity for desmin, α-SMA, h-caldesmon, and CD34, and weak DOG1 positivity. RNA sequencing revealed an in-frame MEF2A::NCOA2 fusion, which was confirmed by direct sequencing. Fluorescence in situ hybridization further verified NCOA2 rearrangement. This novel fusion gene may have been functional in this case, serving as a driver. Recently, 5 myxoid epithelioid smooth muscle tumors with MEF2D::NCOA2 gene fusion were reported in the vulvovaginal region. Given the characteristic histological and molecular findings, this case may represent a new entity in the gastrointestinal tract related to the vulvovaginal tumor, serving as a foundation for future studies.