ATM immunohistochemistry as an effective screening method for POLE variants among endometrial carcinomas lacking mismatch repair deficiency and p53 abnormalities

Abstract The molecular classification of endometrial carcinomas (ECs) is now integrated into clinical practice. However, identification of polymerase‐ε (POLE) variants remains reliant on DNA sequencing, which limits broader implementation. Given the strong prognostic value of pathogenic POLE mutations and the established efficacy of immunohistochemistry (IHC) for detecting mismatch repair (MMR) deficiency and p53 abnormalities, there is a clear need for IHC‐based screening strategies to identify patients likely to carry POLE variants and prioritize them for confirmatory sequencing. In this study, we analyzed 24 cases with POLE pathogenic mutations (POLEmut ECs), 3 with benign POLE variants, and 32 matched cases with no specific molecular profile (NSMP) from a cohort of 378 ECs. IHC evaluation of the ataxia telangiectasia mutated (ATM) protein revealed that POLE‐mutated ECs (with pathogenic or benign POLE variants) exhibited significantly higher frequencies of non‐diffuse positive staining patterns, including null, heterogeneous positive, and subclonal loss, compared with NSMP cases. Targeted next‐generation sequencing of all exons across 474 cancer‐related genes in the 27 POLE‐mutated ECs and 20 NSMP cases with ATM non‐diffuse positive staining patterns confirmed that POLE‐mutated ECs typically had high tumor mutational burden and were enriched for ATM truncating variants. ATM molecular alterations, including various variant subtypes and multisite mutations, also closely correlated with these staining patterns. Based on these findings, we refined the ATM IHC interpretation framework to integrate staining patterns with sequencing data for improved molecular correlation. Specifically, the null and subclonal loss patterns showed high specificity (96.9%), positive predictive value (94.1%), and accuracy (79.7%) for identifying POLE variants. Notably, the null pattern appeared exclusively in ECs with pathogenic POLE mutations. These results suggest that ATM IHC staining is an effective screening tool for identifying patients who may benefit from confirmatory POLE sequencing among those lacking MMR deficiency or p53 abnormalities.