Dedifferentiation-like tubular and solid carcinoma of the stomach shows phenotypic divergence and association with deficient SWI/SNF complex

Gastric carcinoma showing an abrupt transition from a tubular to solid pattern is an unusual phenomenon reminiscent of dedifferentiation. The phenotypic and molecular characteristics of this transition are still unclear. We retrospectively collected 41 gastric carcinomas exhibiting dedifferentiation-like tubular to solid transition and applied an array of immunohistochemical stains, including neuroendocrine and hepatocytic markers, to delineate their lineage. The status of Epstein-Barr virus (EBV) infections, mismatch repair proteins, SWI/SNF complex proteins and p53 expression levels were examined. The clinicopathologic differences were assessed by statistical analysis. Except for 10 cases with neuroendocrine differentiation and 2 EBV-associated carcinomas, we identified 8 hepatoid carcinomas and 21 solid adenocarcinomas with loss of CDX2 and/or hep-par1 expression in solid part (12/29). A subset of solid adenocarcinoma was associated with MSI (8) and mutant p53 expression was frequent in non-MSI cases (10/13). We found hepatoid carcinomas usually harbored SMARCA2 loss (5/8), MSI-associated cases commonly had ARID1A loss (6/8), and non-MSI solid adenocarcinomas frequently showed SMARCA2/A4 loss (7/13) with a high rate of concurrent ARID1A loss (4/7). Spatial correlation between solid transition and loss of SWI/SNF complex subunits were seen in 63% of tumors (12/19). Dedifferentiation-like tubular and solid carcinoma was associated with a propensity to inferior survival outcomes (p = 0.034), especially hepatoid carcinoma and in the non-MSI/EBV intestinal subgroup. In conclusion, gastric cancer exhibiting dedifferentiation-like tubular to solid transition is a phenotypically divergent group that shares common alterations in the SWI/SNF complex.