SMARCA4-Deficient Undifferentiated Carcinoma of the Gastroesophageal Junction: A Case Report

Abstract Introduction/Objective To report a rare case of the newly characterized variant of gastric carcinoma; undifferentiated carcinoma with intact SMARCB1 (INI1) but lost SMARCA4/SMARCA2, and approach to diagnosis. Methods/Case Report A case of gastric mass extending to the gastroesophageal junction, microscopically, showing sheets of atypical, medium to large, discohesive, polygonal cells with large eccentric and pleomorphic nuclei, variably prominent nucleoli, and eosinophilic cytoplasm with brisk mitosis, in the background of marked necrosis and granulomatous inflammation. And having focally positive staining for cytokeratin and negative for melanoma, diffuse large B-cell lymphoma, high grade neuroendocrine tumor, sarcoma, rhabdoid/myoid-specific immunohistochemistry (IHC) markers. The case was further characterized with immunophenotypic, cytogenetic, and molecular findings. Results (if a Case Study enter NA) By immunohistochemistry (IHC), the large tumor cells were positive for vimentin, FLi-1 and patchy CD71. A subset of tumor cells showed CD34 positivity. The tumor cells were mainly negative (with rare focal positivity) for all cytokeratin including (CK AE1/AE3, EMA, and CK OSCAR). Tumor cells were negative for specific markers for melanoma, GIST, sarcomatous tumors, squamous cell carcinoma, hematolymphoid malignancies, and neuroendocrine tumors. EBER-ISH and HHV-8 were negative. The Ki67 showed a proliferative index of ~60%. The tumor cells demonstrated retention of INI1 (SMARCB1) with loss of both SMARCA2 and SMARCA4 by IHC. Molecular studies detected an inactivating mutation of the retinoblastoma-1 gene (RB1 I124Rfs*6) and TP53 with altered function compared to wild-type (TP53-G245S) with no mutations or deletion of SMARCB1 gene. The status of SMARCA4 gene came as indeterminate. No microsatellite Instability detected. Conclusion Our study demonstrates the challenge of diagnosing the SMARCA4/SMARCA1-deficient undifferentiating gastric carcinoma especially with negative cytokeratin stains, showing the immunohistochemistry profile of the tumor and its molecular findings in our case.