The influence of cellular transitions on breast cancer development

Abstract

Actively self-renewing tissues need to maintain a delicate balance between providing sufficient cells to maintain the integrity of a tissue and preventing excessive proliferation resulting in tumorigenesis. Consequently, tumors inhibit signals that limit proliferation and stimulate signals that promote survival. A common mechanism that enables tumor growth are changes in tumor suppressor genes and oncogenes. In this dissertation, I aim to further uncover the relationship between mammary gland self-renewal, proliferation, and oncogenesis. Furthermore, we develop a mouse model to increase our understanding of drug resistance. In chapter 1 I introduce the concepts discussed in this thesis. Namely, I introduce the mammary gland, the concepts of self-renewal and proliferation, and the role of tumor suppressors and oncogenes in breast cancer precision oncology in breast cancer treatment. Furthermore, I show how a large single cell RNA sequencing project can provide us information on the different cell types present in the mammary gland. Lastly, I introduce the RAS/RAF/MEK/ERK pathway with an emphasis on KRAS, the most commonly mutated oncogene in cancer. In chapter 2 we show that depleting the mammary gland of tumor suppressors Trp53, p16Ink4a, and p19Arf amplifies TNFa-induced proliferation of mammary epithelial cells. In chapter 3 we describe a novel breast cancer relapse mouse model that mimics the appearance of chemoresistance to mutated Kras inhibition. A subset of Trp53-/- mice develops breast tumors upon the activation of oncogenic KrasG12D. Once KrasG12D is inactivated, the tumors declines followed by remission and eventually relapses. We show that the relapsed tumor has transitioned from an epithelial to a mesenchymal phenotype and has active MAPK/ERK signaling, the pathway activated by Kras. In chapter 4 we report that the innate immune system receptor TLR2 also has a cell-intrinsic role in the mammary gland and the intestine. We show that components of the TLR2 pathway - including TLR2, the TLR2 adaptor protein MYD88, and the TLR2 co-receptor CD14 – stimulate the regeneration of both tissues. Furthermore, blocking the TLR2 pathway slows down tumor formation in breast cancer and in intestinal adenoma. In chapter 5 we share the work of creating Tabula Muris, a single-cell transcriptomic compendium of the tissues and organs of the Mus musculus, the mouse. In chapter 6, we continue this work by adding the single-cell transcriptomic data collected at different time points, aiming to provide a public resource of the aging mouse.