A Deeper Dive into POLE -Mutated Endometrial Carcinomas : The Contributions and Consequences of Tumor Mutational...

POLE -mutated endometrial carcinomas ( POLE mut EC) have the most favorable prognosis among all TCGA molecular subgroups. Though the consequent "ultramutated" phenotype is recognized as a trademark feature, hypermutated tumors (10-100 mutations/megabase), as well as those that are histologically low-grade, have been described. Herein, we investigate 19 POLE mut ECs from a single institution to determine whether morphologic and genomic differences exist between tumors with high tumor mutational burden (TMB-H: 10-100 mutations/megabase) and ultra-high TMB (TMB-UH: >100 mutations/megabase). All tumors were FIGO stage I, of endometrioid histotype, and no patients recurred (median follow-up of 90 mo). Six previously described POLE exonuclease domain mutations were detected, with the 2 most common being P286R (n=7) and V411L (n=6). Seven ECs were TMB-H (median 76 mutations/megabase), and 12 TMB-UH (median 187 mutations/megabase). TMB-UH tumors were more frequently high-grade, with intratumoral heterogeneity, serous-like nuclei, and bizarre nuclei. Using an integrated approach, multiple classifiers were observed in both cohorts (53% of all tumors); however, those associated with MMRd were exclusive to TMB-UH ECs. All TMB-H ECs had a dominant (>50%) POLE mutational signature, in contrast to only 5 TMB-UH tumors. The remaining TMB-UH ECs demonstrated mixed signatures associated with mismatch repair deficiency (MMRd, n=4) or nonspecific signatures (n=3). All POLE mut tumors were enriched in single-nucleotide variants, while insertions-deletions were rare (maximum of 1.7%). In both groups, all harbored PTEN mutations, with other commonly recurring mutations including PIK3CA, ATRX, BRCA2, FBXW7 , and NF1 . ARID1A mutations were not identified in any TMB-H ECs. Although our cohort is small, the morphology of POLE mut ECs appears to be influenced by TMB, a phenomenon not yet described in the evolving landscape of these tumors. Taken in combination with differences in underlying genomic signatures, these findings provide an interesting springboard for better understanding POLE mut EC pathobiology.