Id1 as a prospective marker for cancer stem cells in triple-negative breast cancer

Abstract

For breast cancer patients, cancer recurrence and secondary metastasis still remain serious clinical problems that are inadequately prevented or managed. Part of the underlying cause may be due to the incomplete eradication of the rare sub-population of self-renewing, highly tumourigenic cancer stem cells (CSC) found within breast cancer tumours. Hence, breast CSCs need to be identified, isolated at high purity, and studied to find therapeutic targets. The transcriptional regulator protein Id1 is known to mediate self-renewal and differentiation processes in normal stem cells, and has also been implicated in breast cancer cell proliferation and invasion. Thus, the purpose of this thesis was to investigate whether Id1 marks the sub-population of rare cancer stem cells in triple-negative breast cancer (TNBC), an aggressive type of breast cancer with poor prognosis and high risk of relapse. A p53-null mouse model of mammary carcinoma that was previously developed in our lab was used for these studies. To understand the characteristics of the mouse model, p53-null tumours were analysed through array CGH and gene expression profiling which revealed the tumours to represent the claudin-low molecular subtype of TNBC. Strikingly, copy number amplification of Met was found in almost all tumours, and shown to be functionally important for cell proliferation in certain tumours. To functionally characterise the tumour-forming capacity of Id1+ cells, p53-null tumour cells were firstly transduced with an Id1GFP reporter lentivirus to label Id1+ cells as GFP+. Two rounds of limiting dilution tumour formation assays were then performed using FACS-purified populations of Id1GFP+, Id1GFP- and unsorted cells. Id1GFP+ enriched for a tumourigenic subset of cells in one p53-null tumour clone, but showed mixed results or no effect in two other tumour clones. Interestingly, Id1 expression did not correlate with the CD29+/CD24+ phenotype, which identified tumourigenic cells in other reported studies of p53-null mouse mammary carcinoma. The findings presented in this thesis demonstrate that Id1 marks a sub-set of prospective CSCs in certain cases of claudin-low breast cancer. Further investigation is required to determine the specific biological contexts of breast cancer under which Id1 is, or is not, a useful CSC marker.