Background
Breast cancer (BCa) incidence is increasing, with >70% of new cases being defined as the estrogen receptor positive (ER+) subtype. Defining the cell of origin for ER+ BCa will allow us to explore potential chemopreventative approaches for this subtype beyond antiestrogen therapies, which suffer from poor uptake and adherence due to minor and major side effects. The mammary epithelium consists of luminal and basal cells, and the proliferative compartment can be divided into consist of mammary stem cells, as well as lineage restricted ER- and ER+ luminal progenitors (LPs). This dissection of the mammary gland (MG) has revealed the cell of origin for some BCa subtypes, but not for ER+ tumours.
Methodology/Results
Prom1 has been shown to be expressed specifically in ER+ cells in the MG. We used Prom1-CreERT2 mice crossed with Pik3ca-loxp-H1047R Rosa-loxp-mTmG mice to trace how this oncogenic mutation affects ER+ cells of the MG using flow cytometry and immunofluorescence.
A single 3mg tamoxifen dose was sufficient for ER+ specific cre-mediated recombination, revealed by flow cytometry profiling and immunofluorescence 48h and 8 weeks post injection of Pik3ca-WT mice. 3 months post tamoxifen injection, the proportion of luminal, GFP+ ER+ progenitor cells in the MG was significantly increased in the MG of Pik3ca-H1047R mice compared to wildtype Pik3ca mice. Additionally, an intermediate, GFP+ CD49fhi EPCAMhi population with enhanced colony forming potential was present in Pik3ca-H1047R mice. GFP+ K5+ K18+ cells were present in the MGs of Pik3ca-H1047R mice whereas these markers were mutually exclusive in wildtype Pik3ca mice.
Discussion
Our lineage tracing studies reveal that Pik3ca activation in ER+ luminal cells leads to increased abundance of ER+ LPs and the emergence of a population of cells with both basal (K5) and luminal (K8) markers. This suggests that oncogenic mutations in these cells leads to increased multipotent potential, a hallmark of cancer, even at this early time point. We are now ageing conditionally activated Pik3ca mice to observe whether this increased multipotency generates ER+ tumours. This will provide evidence as to whether ER+ LPs are a cell of origin for ER+ BCa.