A Comprehensive Study of Heterogeneous Mismatch Repair Expression in Solid Tumors Reveals Different Immunohistochemical Patterns and Distinct Genetic Mechanisms

Objective: Immunohistochemistry is routinely performed to detect mismatch repair deficiency in solid tumors. Heterogeneous MMR expression (MMR-het) has been reported occasionally but not systemically studied. Methods: In this study, we depicted MMR-het patterns of 40 tumors of different anatomical sites and analyzed MMR genetic alterations and tumor mutational burdens (TMB) through comprehensive genomic profiling. Results: The MMR-het patterns were classified into 4 subgroups: “single-loss” (3 cases), “MLH1/PMS2 double-loss” (16 cases), “MSH2/MSH6 double-loss” (8 cases), and “triple/tetra-loss” (13 cases). Seventeen MMR-het cases exhibited histological heterogeneity, in which MMR protein loss was generally confined to either poorly differentiated or well-differentiated tumor areas. All “single-loss” tumors had MMR somatic mutations and coexisting POLE exonuclease domain mutations. “MLH1/PMS2 double-loss” tumors unexceptionally harbored MLH1 hypermethylation without MMR germline mutations. In the “MSH2/MSH6 double-loss” subgroup, 4 cases had MSH2/MSH6 germline mutations, while another 4 cases had multiple MSH2/MSH6 somatic mutations. Additional POLE exonuclease domain mutations were identified in 2 cases. Tumors in the “triple/tetra-loss” subgroup generally had MLH1 abnormalities (8 MLH1 hypermethylation, 4 MLH1 germline mutation, 1 MLH1 double somatic mutations), and coexistent somatic mutations on MSH2/MSH6 . Thirty-one cases (83.8%) were TMB-H, and all POLE -mutated cases exhibited ultra-high TMB (111.4 to 524.2 mut/Mb). Conclusion: Our findings highlighted the importance of accurately interpreting heterogeneous MMR protein staining patterns for developing a more efficient personalized genetic investigation strategy.