Isolated Superficially Serrated Adenoma in Ulcerative Colitis: A Case Report with Molecular Analysis in Japan

Background/Objectives: Superficially serrated adenoma is a recently recognized subtype of colorectal serrated lesions. Case Reports: We report a case of a rectal superficially serrated adenoma in a 57-year-old Japanese woman with ulcerative colitis and a history of follicular lymphoma. Endoscopically, the lesion presented as a superficial sessile lesion without a nodule component and was completely removed by cold snare polypectomy. Histopathological examination revealed a characteristic combination of superficially serrated epithelium and vertically oriented adenoma-like glands. Immunohistochemistry confirmed serrated differentiation of the superficial epithelial component. Molecular analyses identified a KRAS Q61H mutation and a novel PTPRK::RSPO3 fusion that retained the entire RSPO3 coding region. Conclusions: To our knowledge, this represents the first reported case of RSPO3 fusion in a superficially serrated adenoma associated with ulcerative colitis in Japan. Although some aspects of the molecular pathogenesis of superficially serrated adenoma have been increasingly investigated, its clinicopathological features, particularly in the setting of ulcerative colitis, remain poorly characterized. Additional clinicopathological and molecular studies are required to determine whether superficially serrated adenomas arising in patients with ulcerative colitis represent incidental sporadic lesions or a distinct subset associated with the inflammatory bowel disease setting.