Abstract P4-04-04: Loss of epigenetic lineage fidelity with age primes breast epithelia for malignant transformation

Background: Aging is the largest risk factor for breast cancer, with 80% of new cases diagnosed in women over 50y. However, the molecular mechanism underlying age-associated cancer susceptibility is still not well understood. While aging-associated phenotypes in a number of tissues have been correlated with transcriptional and epigenetic changes, few studies have looked at epigenetic regulation at the resolution of individual cell populations. The bilayered epithelia of the mammary gland is composed of an apical layer of secretory luminal epithelial cells (LEP) surrounded by a basal layer of contractile and tumor suppressive myoepithelial cells (MEP). Breast cancer cells of origin are thought to reside mainly in the luminal or supra-basal regions directly adjacent to the apical surfaces of the MEP. Thus, an understanding of how LEP and MEP interact and maintain lineage-specific expression, and how this is dysregulated with age may elucidate a potential mechanism regulating increased breast cancer susceptibility with age. Previously, we showed that loss of lineage fidelity of LEP and MEP is a key feature of aging. Moreover, this loss of lineage fidelity can be imposed by old MEP onto young LEP in a cell non-autonomous manner (Miyano & Sayaman, et al, Aging 2017). Methods: We present a detailed analysis of genome-wide DNA methylation (Infinium Human Methylation 450K) changes at lineage-specific resolution from FACS-sorted MEP and LEP populations isolated from finite lifespan human mammary epithelial cells (HMEC) derived from reduction mammoplasties of younger ( 55y). Results: In younger epithelia, maintenance of lineage fidelity is associated with lineage-specific differential methylation (DM) of CpG sites at key regulatory regions. We find that loss of lineage fidelity is recapitulated in genome-wide DNA methylation, with loss of DM at canonical proximal regions associated with transcriptional regulation, specifically annotated CpG island groups, enhancer regions, and CTCF binding sites thought to be involved in chromatin remodeling. Strikingly, DM changes with age occur almost exclusively in LEP population, with older LEP acquiring more MEP-like DNA methylation patterns at the dysregulated sites. Aged LEP and MEP also exhibit significant variability in DNA methylation at both hypo- and hypermethylated regions that show little variance in young samples. Furthermore, predictions of physiologic age using Horvath’s clock show MEP to have older physiologic ages than their isogenic LEP counterparts. Titus et al, found that DM regions (DMRs) between early stage breast tumors and normal-adjacent breast samples in The Cancer Genome Atlas (TCGA) are significantly more enriched in the LUMA, LUMB and HER2 than the Basal molecular subtypes (Sci Reports, 2017). We show that CpG sites associated with the luminal-subtype DMRs are lineage specific and that a significant fraction of the epigenetic changes that occur in these early stage breast cancers are already present during aging. Conclusions: The lineage bias of age-specific of DNA methylation changes suggests that epithelial lineages age via different mechanisms. That LEP are significantly more differentially regulated via DNA methylation with age than MEP raises possibilities that DNA methylation affects aging-associated breast cancer risk in a lineage-specific manner. This lineage bias, along with age-specific changes in breast composition, may underlie the differences in incidence rates of breast cancer subtypes with age and the prevalence of ER+ luminal-subtype breast cancers in women post-menopause. We suggest that these epigenetic changes in the luminal lineage may be priming events that make breast cells more vulnerable to further dysregulation that leads to cancer. Citation Format: Rosalyn W. Sayaman, Masaru Miyano, Mark A. LaBarge. Loss of epigenetic lineage fidelity with age primes breast epithelia for malignant transformation [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P4-04-04.