p53 immunohistochemistry in endometrial cancer: clinical and molecular correlates in the PORTEC-3 trial

Abstract

Standard molecular classification of endometrial cancers (EC) is now endorsed by the WHO and identifies p53-abnormal (p53abn) EC as the subgroup with the poorest prognosis and the most likely to benefit from adjuvant chemo(radio)therapy. P53abn EC are POLE wildtype, mismatch repair proficient and show abnormal immunohistochemical (IHC) staining for p53. Correct interpretation of routinely performed p53 IHC has therefore become of paramount importance. We aimed to comprehensively investigate abnormal p53 IHC patterns and their relation to clinicopathological and molecular features. Tumor material of 411 molecularly classified high-risk EC from consenting patients from the PORTEC-3 clinical trial were collected. p53 IHC was successful in 408 EC and was considered abnormal when the tumor showed a mutant expression pattern (including subclonal): overexpression, null or cytoplasmic. The presence of pathogenic mutations was determined by next generation sequencing (NGS). Abnormal p53 expression was observed in 131/408 (32%) tumors. The most common abnormal p53 IHC pattern was overexpression (n = 89, 68%), followed by null (n = 12, 9%) and cytoplasmic (n = 3, 2%). Subclonal abnormal p53 staining was observed in 27 cases (21%), which was frequently but not exclusively, associated with POLE mutations and/or MMRd (n = 22/27; p < 0.001). Agreement between p53 IHC and TP53 NGS was observed in 90.7%, resulting in a sensitivity and specificity of 83.6% and 94.3%, respectively. Excluding POLEmut and MMRd EC, as per the WHO-endorsed algorithm, increased the accuracy to 94.5% with sensitivity and specificity of 95.0% and 94.1%, respectively. Our data shows that awareness of the abnormal p53 IHC patterns are prerequisites for correct EC molecular classification. Subclonal abnormal p53 expression is a strong indicator for POLEmut and/or MMRd EC. No significant differences in clinical outcomes were observed among the abnormal p53 IHC patterns. Our data support use of the WHO-endorsed algorithm and combining the different abnormal p53 IHC patterns into one diagnostic entity (p53abn EC).

Fig. 1. Representative examples of abnormal p53 immunohistochemical staining patterns.

A Mutant overexpression, B null mutant, C cytoplasmic, and D subclonal abnormal p53 expression.

Fig. 2. Flowchart of cohort selection.

FFPE, formalin-fixed, paraffin-embedded; EC, endometrial cancer; IHC, immunohistochemistry; NGS, next generation sequencing.

Fig. 3. p53 immunohistochemical staining (IHC) of cases with discordant results between p53 IHC and TP53 next generation sequencing (NGS), as a result of erroneously interpreted p53 IHC.

Cases shown in (A–C) were scored p53 wildtype but did have a frameshift TP53 mutation (A, B) and nonsense TP53 mutation (C). In retrospect, p53 IHC should have been scored as null (A, B) and cytoplasmic (C). The case shown in (D) was assigned ‘p53 mutant overexpression’, however did not have a TP53 mutation. Given the high expression in the stromal cells, this case was probably overstained.

Fig. 4. Histopathological and molecular characteristics of all high-risk endometrial cancers (n = 128) with abnormal p53 immunohistochemistry and/or a pathogenic TP53 mutation.

IHC immunohistochemistry, G grade, MMRd mismatch repair deficient, NSMP no specific molecular profile; p53abn, p53-abnormal.

Fig. 5. Kaplan–Meier curves for time to recurrence for patients with p53-abnormal high-risk endometrial cancers with mutant overexpression and null-mutant p53 immunohistochemical staining patterns (n = 93).

EC endometrial cancer.