e17642 Background: Molecular characterization of endometrial cancer (EC) is essential for therapeutic stratification and prognostic assessment. 1/3 EC cases are mismatch repair deficient (dMMR) in the MLH1, MSH2, MSH6, or PMS2 genes. 10 % EC harbor pathogenic variants within exonuclease domain of DNA polymerase epsilon ( POLE ). Generally, dMMR and POLE mut ECs exhibit favorable survival outcomes due to responsiveness to immune checkpoint inhibitors (ICIs). However, chemotherapy resistance to ICIs has been reported in dMMR and POLEmut subsets,and attributed to potential molecular heterogeneity. The aim of the present study; delineate potential molecular differences among EC subsets. Methods: 214 ECs submitted for routine clinical testing for MMR status and p53 by immunohistochemistry (IHC). POLE hotspot mutations tested via Sanger Sequencing. Comprehensive targeted Next Generation Sequencing (NGS) for microsatellite instability (MSI), tumor mutation burden (TMB), and EC oncogenic mutations. Results: 214 ECs: 30.8% dMMR and 69.2% pMMR. 5.6 % POLE mut cases. 33 samples were selected (11 POLE mut and 22 POLE neg) for comprehensive NGS testing. Herein preliminary findings: 1) POLE mut: all were pMMR. 9 microsatellite stable (MS-S) and 2 microsatellite instability-high (MSI-H). p53 IHC: 3 of 9 MSS p53 aberrant and 6 p53 wt. 2 MSI-H cases p53 aberrant. All p53 IHC results consistent with TP53 mutation. TMB very high in POLE mut (500 Mut/Mb MSI-H, 315 MSS/p53 aberrant, and 201 MSS/p53 wt). All harbor PTEN mutations 2) POLE neg: 8 pMMR and 14 dMMR. Among dMMR, 11 MSI-H, 3 MSS. p53 IHC consistent with TP53 NGS in 3 MSS and 7 out of 8 cases of MSI-H (p53 wt). But p53 IHC and TP53 NGS diverged in 1 of the 8 MSI-H/p53 wt and in MSI-H/p53 aberrant. For pMMR, all 8 cases MSS. However, overall p53 IHC not consistent with TP53 NGS: whereas all 8 cases p53 aberrant, 7 out of the 8 noTP53 mutation. In contrast to POLE mut, TMB in POLE neg significantly lower (avg = 28Mut/Mb), lowest in POLE neg/pMMR (8Mut/Mb). POLE neg/pMMR mostly harbor ARID1A mutations. Conclusions: Cohort demonstrates remarkable molecular heterogeneity among EC cases. In present study, we established molecular classification based on current NCCN guidelines for EC into two main groups based on POLEmut . We further subdivided these two groups based on MMR and microsatellite instability. Then, correlated p53 IHC results with NGS TP53 mutation. Previous studies have associated POLE mut, microsatellite instability (MSI-H), and dMMR with favorable prognosis. These preliminary findings clearly show the presence of additional EC subsets POLE mut/TP53-positive and POLE neg/pMMR/TP53-negative which may point to a poor prognosis (TP53 mutation). We demonstrated the inconsistencies in IHC assay. Taken together, there is necessity for comprehensive and robust molecular testing for accurate prognosis and clinical management.
Safina et al. (Thu,) studied this question.