Abstract PD5-10: Mutational burden of the normal breast during age and pregnancy
Authors: Cereser Biancastella , Tabassum Neha , Del Bel Belluz Lisa , Zagorac Sladjana , Yiu Angela , Carter Philip , Miere Cristian , Jeffries-Jones Alicia R , Stebbing Justin 
Published: February 2021

Abstract
Background: The potential for accumulation of somatic mutations in the healthy breast throughout life and pregnancy is poorly understood. In particular, the unique mutational landscape of both epithelial and stromal components of the mammary gland has not been investigated in depth. As cancer risk correlates with both age, age of first-time pregnancy and other factors including pregnancy itself, we wished to study mutational rate over time, using these landmarks. Methods: Here, using whole genome sequencing, we determined how the rate of mutations in both cancer drivers and passenger mutations are affected by both age and pregnancy. We aimed to describe for the first time how the mammary epithelium and stroma differ in their mutational burden. Results: Our analysis of epithelial and stromal laser-capture micro-dissected DNA from 25 normal breast samples of nulliparous and age-matched early- and late-parous women collected from Komen Tissue Bank, University of Indiana, shows that the mammary gland is characterised by known COSMIC signatures SBS1 and SBS5, both of which correlate with age (p&lt;0.05). Using a non-negative matrix factorization approach, we identified a novel signature HB1 for a subset of samples which could not be fitted into the known signature database. Differences between the two cellular compartments were observed mostly in the enrichment pathway of mutated genes, notably a significant enrichment in mutations in the PI3K-Akt signalling pathway in the epithelium. We found that parous stroma is associated with a significant enrichment in mutations in the 3’UTR of genes, suggesting that regulation of the environment in the post-partum breast could preferably occur via these pathways. Alike other normal tissue, the mutational burden of the mammary gland significantly increases with age (P&lt;0.05) and we observed parity-associated patterns of mutational burden, particularly evident in the epithelial compartment. The nulliparous epithelium is characterised by a significant increase of mutations (p&lt;0.05), but mutated clones are maintained at a consistently small size. Conversely, the number of somatic mutations in the parous epithelium is not significantly affected by age, but age is positively correlated with bigger clone sizes (P&lt;0.05) in this group. This trend suggests that possible cancer-associated mutations may have a lower probability of occurring but higher chance of expanding within the parous breast with age, compared to the nulliparous breast. To confirm this, we detected mutations in known driver genes in all the healthy samples, with some occurrence of known individual pathogenic variants.Conclusions: We show the mutational landscape of the healthy breast and highlight differences in the epithelial and stromal cellular compartments. We show how mutated cells, including mutations in driver genes for breast cancer, and genetic alterations change in the contest of pregnancy and age, provide a possible explanation for pregnancy-associated breast cancer risk.
Citation Format: Biancastella Cereser, Neha Tabassum, Lisa Del Bel Belluz, Sladjana Zagorac, Angela Yiu, Philip Carter, Cristian Miere, Alicia R Jeffries-Jones, Justin Stebbing. Mutational burden of the normal breast during age and pregnancy [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PD5-10.