Microsatellite instability (MSI), a common alteration in endometrial cancers (EC) is known as a biomarker for immune checkpoint therapy response alongside screening for Lynch Syndrome (LS). However, former studies described challenging MSI profiles in EC hindering analysis by using MSI testing methods intensively validated for colorectal cancer (CRC) only. In order to reduce false negatives, this study examined four different PCR-based approaches for MSI testing using 25 EC samples already tested for mismatch repair deficiency (dMMR). In a follow up validation set of 75 EC samples previously tested both for MMR and MSI, the efficiency of a seven-marker system corresponding to the Idylla system was further analyzed. Both Bethesda and Promega marker panels require trained operators to overcome interpretation complexities caused by either hardly visible additional peaks of one and two nucleotides, or small shifts in microsatellite repeat length. Using parallel sequencing adjustment of bioinformatics is needed. Applying the Idylla MSI assay, an evaluation of input material is more crucial for reliable results and is indispensable. Following MMR deficiency testing as a first-line screening procedure, additional testing with a PCR-based method is necessary if inconclusive staining of immunohistochemistry (IHC) must be clarified.

Over 420,242 women were diagnosed with uterine cancer worldwide in 2022

Objective Nearly 30% of unselected endometrial cancer (EC) are mismatch repair deficient (MMRd).The majority resulting from epigenetic changes due to MLH1 promoter hypermethylation (MLH1-PM) and only a fraction from mutations in the Lynch genes (MLH1/MSH2/MSH6/PMS2). Since less is known about the clinical characteristics of MMRd tumors especially based on MLH1-PM we aimed at clarifying the clinical features of EC with MLH1-PM. Methods EC patients treated between 2015-2022 who underwent MMR(IHC) +/- MLH1-PM (PCR)-testing were included. Three groups where defined. A)MMR proficient(p) B) MMRd/MLH1-PM 3) “probable” Lynch (defined as MMRd not due to MLH1-PM). Results MMR-testing was performed in 337/365 cases (279 MMRp and 58 MMRd [17,2%]). 36 of 45 tumors with MLH1 +/- PMS2 deficiency had MLH1-PM analysis, identifying MLH1-PM in 28 (77.8%) and classifying 8 (22.2%) without PM as “probable” Lynch. MMRd tumors were detected at higher stages, more often showed angioinvasion and endometrioid subtype and less abnormal p53 expression compared to MMRp. Patients with MMRd/MLH1-PM more often had endometrioid histology (89% vs 76%), showed no abnormal p53 expression (0% vs 19%) compared to “probable” Lynch. 5-year PFS was 73, 81 and 100 months (p=0.343) for MMRp, MMRd/MLH1-PM, and “probable” Lynch patients, respectively. Conclusion Significant differences in clinical characteristics were detected between MMRd and MMRp tumors, as well as between MMRd tumors depending on MLH1-PM status. Thus, this analysis supports evidence of heterogeneity in MMRd tumors concerning prognosis. Article published online: 11 October 2022 © 2022. Thieme. All rights reserved. Georg Thieme Verlag Rüdigerstraße 14, 70469 Stuttgart, Germany