Abstract IA007: The mutational landscape of normal human endometrial epithelium

Abstract

Abstract Human endometrium is a highly dynamic tissue that undergoes numerous cycles of breakdown, rapid repair and remodelling in response to the oscillating levels of oestrogen and progesterone. The marked regenerative capacity of the tissue’s epithelial compartment is maintained by the intra-glandular adult stem cells (ASCs) that are retained during menstruation. Although the endometrial epithelial ASCs were first described over a decade ago, they remain poorly characterised in comparison to their counterparts in other tissues, such as the small and arge intestine. In particular, the size of the stem cell pool within individual glands, clonal dynamics and genomic landscape are largely unknown. In this study, we laser-capture microdissected 257 morphologically normal endometrial glands from pre- and post-menopausal women (aged 19 to 81). In selected donors, we also obtained samples of normal endocervical and Fallopian tubular epithelium. Isolated cellular material was subjected to our modified library preparation protocol and subsequent whole genome sequencing (WGS). Analysis of the WGS data identified that the majority of the glands were clonal cell populations sharing a recent common ancestor. Somatic mutations accumulated at a rate of 29 substitutions per year. Total mutation burden in the normal endometrial epithelium is higher than in other normal cells and manyfold lower than in endometrial cancers. Overall, despite the heterogeneity in age, reproductive history and BMI, we found relatively homogenous mutational processes to be operative in this tissue with only occasional outliers. Remarkably, we not only identify recurrent acquisitions of certain cancer-associated mutations, particularly those that are advantageous to cell growth, proliferation and migration, but also show that such events occur early in life, potentially even before adolescence. Using a generalised linear mixed effect model, we found that age has a positive association with the accumulation of driver mutations, while parity has a negative association. Over time, the mutant ASCs serve as a reservoir for the acquisition of further driver mutations to the extent that in some cases, the entire sampled endometrium becomes ‘neoplastic’ on the genomic level while still retaining the apparently normal phenotype. Our data identify a distinct mutational landscape in normal endometrium that is in keeping with the presence of early positive selection in this highly regenerative tissue. Citation Format: Luiza Moore. The mutational landscape of normal human endometrial epithelium [abstract]. In: Proceedings of the AACR Virtual Special Conference: Endometrial Cancer: New Biology Driving Research and Treatment; 2020 Nov 9-10. Philadelphia (PA): AACR; Clin Cancer Res 2021;27(3_Suppl):Abstract nr IA007.