Differentiation dynamics of mammary epithelial cells during normal and malignant development

The mammary gland has been central to the evolution of mammals by being capable of undergoing multiple rounds of substantial remodelling during pregnancy and providing nourishment to the offspring. This plasticity does not come without a cost. 1 in 7 females will be diagnosed with breast cancer in their lifetime in the UK, making it the most common form of cancer. To be able to understand why this tissue is highly susceptible to tumour formation, it is necessary to study its development, only then can we understand how and when the tissue deviates from normal homeostasis. To this end, we used single-cell RNA-sequencing (scRNAseq) to determine the gene expression profile of individual mammary epithelial cells across adult development. Our data identify 15 distinct mammary epithelial cell populations and allows their hierarchical structure across development to be charted. Interestingly, the effect of gestation and lactation appeared to be more pronounced for some cell types, in particular luminal progenitors. The data also showed that few clusters could be fully characterized by a single marker gene. We argue instead that the epithelial cells – especially in the luminal compartment – should rather be conceptualized as being part of a continuous spectrum of differentiation. This view highlights the plasticity of the tissue and might help to explain some of the conflicting results from lineage tracing studies. This transcriptional catalogue of cell types in the murine mammary gland allowed us to study how the tissue deviates from homeostasis during tumourigenesis. It is currently unclear how genetic aberrations impact the cell state of nascent tumour cells and their mi- croenvironment. BRCA1 driven triple negative breast cancer (TNBC), for example, has been shown to arise from luminal progenitor cells yet little is known about how BRCA1 loss-of-function (LOF) and concomitant mutations affect the luminal progenitor cell state. Here we demonstrate how time-resolved single-cell profiling of genetically engineered mouse models before tumour formation can address this challenge. We found that the perturbation of Brca1/p53 in luminal progenitors induces an aberrant alveolar differentiation pre-malignancy. Unlike alveolar differentiation occurring during gestation, this process is cell autonomous and characterised by the dysregulation of transcription factors driving alveologenesis. Our experimental approach has allowed us to further identify responses in the stromal and immune cell compartments during the early steps of tumourigenesis. Based on our data, we propose a model where transcriptional and epigenetic changes driven by Brca1/p53 inadvertently promote a differentiation program hardwired in luminal progenitors, highlighting the deter- ministic role of the cell of origin and offering a potential explanation for the tissue specificity of BRCA1 tumours. Moving forward, the phenotype identified herein should be evaluated as a target for early detection and treatment of TNBC. In summary, this work demonstrates how we can improve our understanding of mammary gland biology and breast cancer development using single-cell profiling of murine models across stages of normal and malignant development.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2020

Author dc:creator

• Bach, Karsten

Advisors dc:contributor.advisor

• Khaled, Walid
• Marioni, John C

• Genomics
• Biology
• Breast cancer
• scRNA-seq
• Computational Biology
• Transcriptomics

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/41180882-50fa-41cd-89c7-0cf3ed8e2df8/download
• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

eng

Author Identifier

0000-0003-3622-1115
0000-0001-9068-5776

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/323270