Molecular subtypes of clear cell carcinoma of the endometrium: Opportunities for prognostic and predictive stratification

Highlights

• There is molecular heterogeneity within clear cell carcinoma (CCC) of the endometrium.

• P53wt CCC of endometrium show aggressive features and are distinct from other p53wt endometrial cancers.

• Molecular classification of CCC of endometrium elicits prognostic parameters that are not apparent with histology alone.

Abstract

Objective

Our aim was to characterize the pathological, molecular and clinical outcomes of clear cell carcinoma of the endometrium (CCC).

Methods

CCC underwent ProMisE (Proactive Molecular Risk Classifier for Endometrial Cancer) classification identifying four molecular subtypes: i) ‘POLEmut’ for ECs with pathogenic POLE mutations, ii) ‘MMRd’, if there is loss of mismatch repair proteins by immunohistochemistry (IHC), iii) ‘p53wt’ or iv) ‘p53abn’ based on p53 IHC staining. Clinicopathologic parameters, immune markers (CD3, CD8, CD79a, CD138, PD-1), ER, L1CAM, and outcomes were assessed.

Results

52 CCCs were classified, including 1 (2%) POLEmut, 5 (10%) MMRd, 28 (54%) p53wt and 18 (35%) p53abn. Women with p53abn and p53wt CCCs were older than women with MMRd and POLEmut subtypes. p53wt CCC were distinct from typical p53wt endometrioid carcinomas; more likely to arise in older, thinner women, with advanced stage disease, LVSI and lymph node involvement, and a higher proportion ER negative, L1CAM overexpressing tumors with markedly worse outcomes. High levels of immune infiltrates (TIL^high) were observed in 75% of the CCC cohort. L1CAM overexpression was highest within p53abn and p53wt subtypes of CCC.

Conclusion

CCC is a heterogeneous disease encompassing all four molecular subtypes and a wide range of clinical outcomes. Outcomes of patients with POLEmut, MMRd and p53abn CCC are not distinguishable from those of other patients with these respective subtypes of EC; p53wt CCC, however, differ from endometrioid p53wt EC in clinical, pathological, molecular features and outcomes. Thus, p53wt CCC of endometrium appear to be a distinct clinicopathological entity within the larger group of p53wt ECs.

Introduction

Endometrial carcinoma (EC) is the most common gynecologic malignancy in North America [1]. Clear cell carcinoma (CCC) of the endometrium accounts for <6% of all endometrial carcinomas and has historically been considered an aggressive ‘Type II’ endometrial cancer, with a propensity for extrauterine spread and poor prognosis [[2], [3], [4]]. Surgical management and adjuvant therapy for CCC usually follows the same treatment algorithms of uterine serous carcinoma although the rationale for this strategy has been questioned in recent years [4].

Challenges of histomorphological categorization of endometrial carcinoma, particularly in high grade tumors are well known [5,6] with inter-observer disagreement in up to one-third of cases [6]. This lack of reproducibility is a major barrier to risk stratification as treatment recommendations are primarily based on these unreliable pathological parameters (histotype, grade), and variation in treatment within and across centers can be dramatic [4]. A move to more reproducible methods of classification through characterization of molecular features has proven to be feasible, consistent, and provides biologically relevant information for patients and clinicians [[7], [8], [9], [10], [11]].

The Cancer Genome Atlas (TCGA) was the first study to comprehensively classify endometrial cancers into genomic subgroups. Pragmatic classification systems using low cost clinically applicable tools on standard formalin fixed paraffin-embedded (FFPE) material e.g. the ProMisE classifier, followed [8,9] identifying 4 molecular subtypes with distinct prognostic outcomes. Key distinguishing parameters include i) identifying pathogenic mutations in the exonuclease domain of DNA polymerase epsilon involved in DNA proofreading/repair (POLEmut), ii) the presence or absence of proteins associated with DNA mismatch repair where ‘loss’ identifies MMR deficiency (MMRd), and iii) p53 status where p53 wildtype has normal expression on IHC (‘p53wt’, also termed ‘no specific molecular profile’ or ‘NSMP’ by the PORTEC/Leiden investigators) and abnormal expression includes both complete loss of detectable p53 and/or overexpression (‘p53abn’). The majority of work on the molecular classification of endometrial cancers has been in endometrioid and serous histotypes and the TCGA excluded other histologies entirely. The PORTEC 1 and 2 series, Vancouver discovery and confirmation cohorts, and German validation series classified only 18, 5 and 6 CCC in total respectively and demonstrated that CCCs can show features of different molecular subtypes [8,[10], [11], [12], [13]]. Additional series that have used selected mutational profiling and/or IHC independent of TCGA-based classification systems also support the molecular diversity of CCC [[13], [14], [15], [16]]. Yet treatment algorithms direct the same aggressive surgery and adjuvant therapy for all CCC. It is time to ask whether the clinical behavior of CCC is also diverse and whether treatment can be tailored according to molecular subtype.

Herein, we sought to characterize the molecular diversity of endometrial CCC, assessing prognostic and predictive biomarkers within the framework of the ProMisE molecular classifier.