Kunieda Junko, Yamashita Kyoko, Dobashi Akito, Togashi Yuki, Baba Satoko, Inoue Norihito, Nakano Kaoru, Chino Akiko, Saito Shoichi, Yamamoto Noriko, Kawachi Hiroshi, Takeuchi Kengo
Division of Pathology, Institute of Science Tokyo Hospital, Tokyo, Japan.
Division of Pathology, Cancer Institute, Japanese Foundation for Cancer Research, Tokyo, Japan.
Virchows Arch. 2025 Sep 24. doi: 10.1007/s00428-025-04275-3.
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.
