• Guang Yang.
• Department of Pathology, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Okayama university, Okayama, Japan.
• Medicine (Baltimore). 2022 Aug 26; 101 (34): e30311.

RationaleGastric adenocarcinoma of the fundic gland is a rare, well-differentiated variant of gastric adenocarcinoma, which has been proposed as a novel disease entity. As a result of mismatch repair deficiency, microsatellite instability has been frequently observed in various human cancers and widely performed in the area of cancer pathogenesis. Herein, we report a case of gastric adenocarcinoma of fundic gland presented with microsatellite instability phenotype.Patient ConcernsA 46-year-old man was referred to our hospital for abdominal distension and pain.DiagnosisThe patient contained 3 tumor lesions with different degrees of histologic differentiation and microsatellite instability. The lesions were located in the upper third of the stomach. The tumor size was 55 mm. Macroscopically, tumor showed an ulcerative type. In terms of depth of invasion, tumor lesion invaded into subserosa with lymphatic invasion. In addition, this patient did not present GNAS mutation but harbored AXIN2 mutation. By immunohistochemistry, the expression level of β-catenin protein in the nucleus of the carcinoma cells was obviously higher than that in normal nucleus. Compared with microsatellite instability-low lesion, PD-1, PD-L1, and CD8 were positive in the microsatellite instability-high lesions.InterventionsThe patient underwent surgical resection and postoperative chemotherapy.OutcomesThe patient experienced distant metastasis and died from severe complications after 6 months of treatment.LessonsThese results suggested that the mutation of Wnt component genes associated with Wnt/β-catenin signaling pathway activation may play a role in promoting the occurrence of gastric adenocarcinoma of fundic gland. This is the first report of a gastric adenocarcinoma of fundic gland with microsatellite instability. These findings modify our understanding of the pathophysiology of gastric adenocarcinoma of fundic gland.Copyright © 2022 the Author(s). Published by Wolters Kluwer Health, Inc.

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