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Microcystic, elongated fragmented (MELF) pattern of myometrial invasion is a distinct histologic feature occasionally seen in low-grade endometrial carcinomas (EC). The prognostic relevance of MELF invasion was uncertain due to conflicting data, and it had not yet appropriately been studied in the context of the molecular EC classification. We aimed to determine the relation of MELF invasion with clinicopathological and molecular characteristics, and define its prognostic relevance in early-stage low/intermediate risk EC. Single whole tumor slides of 979 (85.8%) out of 1141 (high)intermediate-risk EC of women who participated in the PORTEC-1/−2 trials were available for review. Clinicopathological and molecular features were compared between MELF invasion positive and negative cases. Time-to-event analyses were done by Kaplan-Meier method, log-rank tests and Cox' proportional hazards models. MELF invasion was found in 128 (13.1%) cases, and associated with grade 1–2 histology, deep myometrial invasion and substantial lymph-vascular space invasion (LVSI). 85.6% of MELF invasion positive tumors were no-specific-molecular-profile (NSMP) EC. NSMP EC with MELF invasion were CTNNB1 wild type in 92.2% and KRAS mutated in 24.4% of cases. Risk of recurrence was lower for MELF invasion positive as compared to MELF invasion negative cases (4.9% vs. 12.7%, p = 0.026). However, MELF invasion had no independent impact on risk of recurrence (HR 0.65, p = 0.30) after correction for clinicopathological and molecular factors. MELF invasion has no independent impact on risk of recurrence in early-stage EC, and is frequently observed in low-grade NSMP tumors. Routine assessment of MELF invasion has no clinical implications and is not recommended. • MELF pattern of invasion is identified in 13.1% of early-stage endometrial cancer. • MELF invasion is associated with low-grade endometrial cancer. • MELF invasion is predominantly seen in CTNNB1 wild type no-specific-molecular-profile endometrial cancer. • MELF invasion has no independent prognostic impact. [ABSTRACT FROM AUTHOR]

Background The morphological classification of high-risk endometrial cancer is of limited prognostic value. Recent attempts to stratify tumours according to molecular signatures have shown considerable promise. Here we attempted to further refine molecular classifications using markers of the p53 pathway. Methods We analysed the expression of p53 as well as three downstream markers of the p53 pathway, p21, mdm2 and phospho-p63 ( p p63), by immunohistochemistry in a series of 114 endometrial cancers (86 endometrioid, 28 non-endometrioid subtype) with high-risk features (such as high tumour grade and deep myometrial invasion) and correlated results with clinical outcome. The Cancer Genome Atlas (TCGA) data were used to analyse TP63 mutations and copy-number alterations using cBioPortal. TP53 was silenced in two endometrial cancer cell lines to study its effect on p21 and p63. Results About half of the tumours showed a p53 mutant phenotype and there was a strong negative correlation with p21 expression. Being marker positive for p p63 or mdm2 was associated with a significantly increased likelihood of dying, [hazard ratios 5.93 (95% CI 2.37–7.27) and 7.48 (95% CI 3.04–9.39), respectively]. These findings were seen in both p53 wildtype and p53 mutant tumours. Only 11% of TCGA endometrial cancers had a functional TP63 alteration. Upon silencing of TP53, p21 expression was decreased in one cell line, but no effects on p63 were observed. Conclusion Markers of the p53 pathway improve stratification of endometrial cancers and provide novel insights into the role of this pathway in the disease. [ABSTRACT FROM AUTHOR]