Deciphering mismatch repair protein expression variability in gastric cancer: clinical and prognostic implications

Gastric cancer (GC) exhibits diverse molecular characteristics, among which microsatellite instability (MSI) plays a pivotal role in determining treatment responses, particularly to immune checkpoint inhibitors (ICIs). Mismatch repair (MMR) protein immunohistochemical (IHC) staining is a primary screening method for MSI; however, heterogeneity in staining patterns can complicate interpretation. This study aimed to explore the clinicopathological features of GCs with heterogeneous mismatch repair(hMMR) protein expression, specifically (sub)clonal pattern (the coexistence of retained/lost staining in the tumor area), and to assess its association with MSI status. We analyzed 1049 cases of gastric adenocarcinoma from the First Affiliated Hospital of Zhejiang University School of Medicine between 2018 and 2024. Heterogeneous staining for MMR proteins was observed in seven cases, characterized by areas of sudden staining loss alongside retained stain. These cases were subjected to tumor microdissection to ascertain MSI status accurately. Also, the association between MSI status and clinicopathological features was investigated in a cohort of 107 selected patients. Our findings indicated that GCs with hMMR, classified as proficient MMR (pMMR) by IHC, paradoxically exhibited MSI-H status upon microdissection. This discovery underscores the potential for misdiagnosis if intratumoral heterogeneity is overlooked. Furthermore, patients with MSI status displayed distinct clinicopathological features, including older age, distal stomach location, intestinal adenocarcinoma, lower lymphatic metastasis, lower perineural invasion and lower grade of clinical stage. No statistically significant difference in 45-month disease-free survival (DFS) was observed between MSS and MSI cohorts (p = 0.122). The age (p = 0.023, HR:6.017, 95%CI:1.281–28.265) and pTNM stage (p = 0.015, HR:6.632, 95%CI:1.451–30.316) are identified as significant prognostic factors for PFS in patients with GC. Accurate diagnosis of MSI status in GC is critical for optimizing therapeutic strategies. We advocate for detailed reporting of MMR protein heterogeneity patterns and quantification of IHC staining in clinical practice to enhance diagnosis precision and enable personalized treatment approaches for GCs.