[Microsatellite status and minimal microsatellite shift in atypical endometrial hyperplasia and endometrial cancer:...

Objective: To analyze the occurrence and distribution of high-level microsatellite instability (MSI-H) in precancerous lesions and histological subtypes of endometrial cancer, to compare the concordance rate between mismatch repair protein deficiency (dMMR) and MSI-H, to investigate the incidence of minimal microsatellite shift in the development of endometrial cancer, and to explore potential solutions for accurate MSI-H diagnosis. Methods: A total of 848 endometrial lesion samples that underwent molecular typing at the Department of Pathology, Obstetrics and Gynecology Hospital of Fudan University from January 2023 to January 2024 were collected, including 93 cases of atypical endometrial hyperplasia (AEH) and 755 cases of endometrial carcinoma. Microsatellite status, MMR protein expression, and gene mutations (MMR, POLE, etc.) were analyzed by histology, IHC, and NGS. Microsatellite status was determined by calculating the MSI-score based on 34 microsatellite loci. An MSI-score<0.3 was defined as microsatellite stable (MSS),≥0.3 as microsatellite instability-high (MSI-H), and 0.3±0.05 as the equivocal range. Simultaneously, a visualization graph was established for minimal microsatellite shift analysis and identification. Results: The 848 lesions included 93 cases of AEH, 442 cases of low-grade endometrioid carcinoma (G1-G2), 143 cases of high-grade endometrioid carcinoma (G3), 20 cases of clear cell carcinoma, 20 cases of carcinosarcoma, 16 cases of mixed adenocarcinoma, 12 cases of dedifferentiated/undifferentiated carcinoma, and 102 cases of other types (including 94 serous carcinomas, 6 mesonephric-like adenocarcinomas, and 2 gastric-type mucinous adenocarcinomas). The results showed that the incidence of dMMR in AEH was 4.3%, and MSI-H was 3.2%, significantly lower than the 24.9% and 23.0% in endometrial cancer, respectively (P<0.001). All the 102 cases of other types, including serous carcinoma, were pMMR/MSS. The overall concordance rate between dMMR and MSI-H was 90.6%, but it varied between 75% and 100% across different stages and histological subtypes. In molecular subtyping, the concordance rate was 75% for AEH and carcinosarcoma, 87.5% for dedifferentiated/undifferentiated carcinoma, 91.5% for high-grade endometrioid carcinoma, and 100% for other high-grade carcinomas, while the POLE-mutated subtype had the lowest concordance rate of 66.7%, significantly lower than the 93.1% overall concordance rate for the MSI-H subtype (P<0.001). In MSI-H cases, up to 84.5% of endometrial cancer cases exhibited minimal microsatellite shift in at least one locus, with 67.8% showing shifts in≥3 loci. Through analysis of 34 microsatellite loci and visualization, 20 cases (11.5% of 174 MSI-H cases) were identified as borderline (MSI-score=0.3±0.05), considered diagnostically challenging. Among these, MLH1-/PMS2- co-loss accounted for 12/20 cases, with half (6/12) harboring MLH1 mutations; isolated MSH6 loss accounted for 6/20 cases, with 5 of these harboring MSH6 mutations. The 20 minimal shift MSI-H cases shared all pathological features of typical MSI-H endometrial cancer. Conclusions: The incidence and distribution of MSI-H show significant differences across histological subtypes between endometrial precancerous lesions and endometrial carcinomas. The overall concordance between dMMR and MSI-H is good, but varies across different disease stages, histological types, and molecular subtypes. Minimal microsatellite shift is commonly detected in MSI-H cases, and some cases are difficult to interpret due to their classification within the equivocal range. Increasing the number of microsatellite loci, combined with visualization graph comparison and integration of mismatch repair protein immunophenotype and histological features, can effectively improve the accuracy of MSI-H interpretation.