Ataxia telangiectasia–mutated immunohistochemistry identified endometrial carcinomas with loss staining patterns achieving nearly 97% specificity and 80% accuracy for detecting polymerase-epsilon variants in tumors lacking mismatch repair deficiency and p53 abnormalities.
In a cohort of 378 patients with endometrial carcinoma, researchers analyzed 59 cases—24 of which had pathogenic polymerase-epsilon (POLE) variants, three of which had benign POLE variants, and 32 of which had no specific molecular profile—using immunohistochemistry (IHC) for ataxia telangiectasia–mutated (ATM) protein and targeted next-generation sequencing. Molecular classification incorporated POLE sequencing, mismatch repair protein status, and p53 expression.
Patients with POLE variants showed predominantly nondiffuse-positive ATM staining patterns. Just 4% (n = 1) demonstrated diffuse staining, whereas 96% (n = 26) showed nondiffuse patterns, including 37% (n = 10) for null, 37% (n = 10) for heterogeneous positive, and 22% (n = 6) for subclonal loss. In contrast, among patients with no specific molecular profile, 38% (n = 12) showed diffuse staining, 59% (n = 19) heterogeneous staining, and 3% (n = 1) subclonal loss; no null pattern was observed in the no specific molecular profile group.
Genomic analysis further showed that tumors with POLE variants had higher tumor mutational burden (TMB), with 63% (n = 17) classified as ultra-high and a median of 158.6 mutations per Mb, compared with a median of 7.2 mutations per Mb in tumors without a specific molecular profile. None of the comparison tumors had ultra-high TMB.
ATM alterations were identified in 82% (n = 22) of POLE-mutated endometrial cancer, with truncating variants present in 48% (n = 13) . ATM truncating variants correlated with a loss of ATM protein expression: 64% (n = 9) of such cases showed a null staining pattern, and 0% showed diffuse staining. Null staining was observed exclusively in the tumors with pathogenic POLE mutations.
When evaluated as a screening approach, nondiffuse-positive ATM staining achieved 96% sensitivity but 38% specificity for identifying POLE variants. Restricting the definition to loss patterns (null or subclonal loss) increased specificity to 97% and positive predictive value to 94%, with overall accuracy of 80%, but reduced sensitivity to 59%.
Heterogeneous staining patterns were observed across both molecular groups and in tumors with and without ATM variants, limiting their diagnostic specificity. Subclonal loss patterns were associated with both truncating and missense variants and may reflect intratumoral heterogeneity.
The study was limited by its single-center design, modest sample size, and inclusion of only endometrioid histology, which may have limited generalizability.
“ATM IHC offers a cost-effective approach to identify patients who may benefit from confirmatory molecular testing among those without [mismatch repair] deficiency or p53 abnormalities,” wrote lead study author Xinyi Huang, of the Department of Pathology at Xijing Hospital at The Fourth Military Medical University in China, and colleagues.
The study authors reported no conflicts of interest.
