Clinicopathological and molecular characterisation of “multiple classifier” endometrial carcinomas

Endometrial carcinoma (EC) molecular classification based on four molecular subclasses identified in The Cancer Genome Atlas (TCGA) has gained relevance in recent years due to its prognostic utility and potential to predict benefit from adjuvant treatment. While most ECs can be classified based on a single-classifier (POLE exonuclease domain mutations - POLEmut, MMR deficiency - MMRd, p53 aberrant - p53abn), a small but clinically relevant group of tumours harbour more than one molecular classifying feature and are referred to as “Multiple classifier” ECs. We aimed to describe the clinicopathological and molecular features of multiple classifier ECs with aberrant p53. Within a cohort of 3518 molecularly profiled EC, 107 (3%) tumours displayed p53abn in addition to another classifier(s), including 64 with MMRd (MMRd-p53abn), 31 with POLEmut (POLEmut-p53abn) and 12 with all three aberrations (MMRd-POLEmut-p53abn). MMRd-p53abn EC and POLEmut-p53abn EC were mostly grade 3 endometrioid EC, early stage, and frequently showed morphologic features characteristic of MMRd or POLEmut EC.18/28 (60%) MMRd-p53abn EC and 7/15 (46.7%) POLEmut-p53abn EC showed subclonal p53 overexpression, suggesting TP53 mutation was a secondary event acquired during tumour progression. Hierarchical clustering of TCGA ECs by single nucleotide variant (SNV) type and somatic copy number alterations (SCNAs) revealed that MMRd-p53abn tumours mostly clustered with single-classifier MMRd tumours (20/23) rather than single-classifier p53abn tumours (3/23), while POLEmut-p53abn tumours mostly clustered with single-classifier POLEmut tumours (12/13) and seldom with single-classifier p53abn tumours (1/13) (both p≤0.001 Chi-square test). Finally, the clinical outcome of patients with MMRd-p53abn and POLEmut-p53abn EC (stage I 5-year recurrence free survival (RFS) of 92.2% and 94.1% respectively) was significantly different from single-classifier p53abn EC (stage I RFS 70.8%, p=0.024 and p= 0.050 respectively). Our results support the classification of MMRd-p53abn EC as MMRd and POLEmut-p53abn EC as POLEmut