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

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.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.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).Our findings highlighted the importance of accurately interpreting heterogeneous MMR protein staining patterns for developing a more efficient personalized genetic investigation strategy.