Heterogeneous expression of mismatch repair proteins and interpretation of immunohistochemical results in colorectal cancer and endometrial cancer.

To investigate the heterogeneous expression patterns of four mismatch repair (MMR) proteins in colorectal cancer (CRC) and endometrial cancer (EC), and their effects on the interpretation of immunohistochemical (IHC) results. A total of 1636 CRC and EC specimens were collected from two hospitals. Seventy-eight cases had heterogeneous expression of MMR proteins, including 49 CRC and 29 EC cases. Polymerase chain reaction-capillary electrophoresis (PCR-CE) was then performed to detect the microsatellite instability (MSI) status, and 44 cases were further verified by targeted next-generation sequencing (NGS). Heterogeneous expression of MMR proteins was observed in 66 cases (66/78, 84.6%) of proficient MMR (pMMR) and 12 cases (12/78, 15.4%) of deficient MMR (dMMR). The proportion of heterogeneous MMR protein expression in EC (12.0%) was higher than that in CRC (3.5%). The heterogeneous expression patterns were divided into focal clonal heterogeneity (6/78, 7.7%) and glandular mosaic heterogeneity (72/78, 92.3%). Surprisingly, three pMMR CRC cases showed isolated small focal clonal heterogeneity of mutS homologue 6 (MSH6), with < 15% positive tumour cells, which was validated as high MSI (MSI-H) with PCR-CE and NGS. However, the other three EC pMMR cases with > 50% focal clonal heterogeneity of MMR proteins were verified as microsatellite stable (MSS) or low MSI (MSI-L). Fifteen EC cases with glandular mosaic heterogeneous expression of MMR proteins included two MSI-H cases, which were validated using PCR-CE and NGS. Among the dMMR cases, only two EC cases with mutL homologue 1 (MLH1)/PMS1 homologue 2, mismatch repair system component (PMS2) loss and MSH2/MSH6 mosaic heterogeneous expression were confirmed as MSS using PCR-CE and NGS, which may be related to the mechanism of MLH1 promoter methylation. Thus, in CRC, only cases with small focal clonal heterogeneous expression of MSH6 have a high likelihood of MSI-H, and further PCR-CE or NGS testing is recommended. The possibility of MSI-H cannot be ruled out in EC cases with glandular mosaic heterogeneous expression of MMR proteins; PCR-CE or NGS detection is therefore necessary.