Selective ARID1A Loss Restricted to the Undifferentiated Component of a Mismatch Repair-Deficient Gastric Carcinoma: A Case Report.

Mismatch repair-deficient (dMMR) gastric carcinomas often harbor ARID1A alteration, but a sharply demarcated undifferentiated/rhabdoid component with selective ARID1A loss is uncommon and may create a diagnostic dilemma. An 80-year-old man underwent esophagogastroduodenoscopy for anemia, which revealed a circumferential Borrmann Type 3 lesion in the gastric body, and distal gastrectomy was performed. Histologically, the tumor was composed predominantly of undifferentiated carcinoma with focal rhabdoid features and a minute well-differentiated adenocarcinoma component, with an abrupt transition between the two. Immunohistochemistry showed loss of nuclear MLH1 and PMS2 in both components, whereas loss of ARID1A expression was confined to the undifferentiated component; SMARCB1 (INI1), SMARCA2 (BRM), and SMARCA4 (BRG1) were retained. EBER in situ hybridization was negative. Because gene-level testing, MSI testing, and MLH1 promoter methylation analysis were not performed, the molecular basis of the dMMR phenotype and ARID1A loss could not be determined. The restricted scope of molecular testing limits the ability to draw broad or generalizable conclusions and to fully establish clinicopathological correlations. The value of this report is, therefore, not mechanistic proof but recognition of a practical morphologic-immunophenotypic observation: When a gastric carcinoma shows a sharply demarcated shift from differentiated to undifferentiated/rhabdoid morphology, dMMR should be considered, and selective ARID1A loss in the undifferentiated component may be associated with dedifferentiation. These findings should be interpreted with caution as preliminary, hypothesis-generating observations that require validation in larger studies with more extensive molecular profiling.