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

Background and Clinical Significance: Superficially serrated adenoma (SuSA) is a recently recognized colorectal serrated lesion characterized by superficial serration and adenoma-like deep glands. Its clinicopathological significance in ulcerative colitis (UC) remains unclear. Case Presentation: We report a rectal SuSA in a 57-year-old Japanese woman with a 30-month history of UC and a history of follicular lymphoma. UC was in a mild, resolving phase at the time of detection. A 7 mm pale, sessile rectal lesion was completely removed by cold snare polypectomy. Histopathologically, the lesion showed superficial serration confined to the upper one-third to one-half of the glands, adenoma-like deep glands, and a basal-to-middle proliferative zone. No ectopic crypt formation, basal crypt dilation, L-/T-shaped crypt architecture, or adjacent dysplasia was identified. Molecular analysis revealed a KRAS p.Q61H mutation and PTPRK::RSPO3 fusion retaining the RSPO3 coding region. The morphological and molecular features were consistent with sporadic SuSA rather than inflammatory bowel disease-associated non-conventional serrated dysplasia. The short duration of UC and mild, resolving inflammation did not support a causal relationship between UC and SuSA development. To our knowledge, this is the first genetically characterized case of SuSA in a patient with UC in Japan. Conclusions: This case expands the limited molecular data on SuSA arising in UC. Whether SuSA occurs more frequently in patients with UC than in the general population remains unknown and will require population-based and multicenter clinicopathological and molecular studies.