Evolving concepts of normal breast heterogeneity and its impact on tumor/metastasis characterization
Harikrishna Nakshatri, B.V.Sc., Ph.D.
November 17 in the Fralin Auditorium, Fralin Hall 102
Hosted by Dr. J. Jostan
Several studies have described cell surface markers that phenotypically define stem-progenitor-mature cell hierarchy of the normal breast. Same markers have been used to identify subpopulation of cancer cells with enhanced tumor initiating capacity. These subpopulation of cells, also called cancer stem cells, have been the focus of intense research for the last few years. Distinguishing global cancer-specific differences in cancer stem cells from their normal counter part has been difficult due to two major technical limitations. One is the recent discovery of widespread genetic variation in humans leading to functional transcriptome diversity, which makes the task of defining “normal” very difficult. The second is the non-availability of replenishable source of primary normal and cancer stem cells to perform functional assays. We have overcome these limitations by adapting human mammary epithelial cell reprogramming growth conditions. We observed remarkable variability in the number of CD49f+/EpCAM- stem-like, CD49f+/EpCAM+ luminal progenitor, and CD49f-/EpCAM+ mature cells among cells propagated from healthy donors, adjoining normal breast tissue of breast cancer patients, and patients with BRCA1 or BRCA2 mutations. Profile of CD271±EpCAM, Jam-A±EpCAM and ALDEFLUOR+ stem cells also varied among the “normal cells” between individuals. Furthermore, there is ethnicity-dependent variation in subpopulation of cells, particularly CD201+/EpCAM- cells, which are believed to possess multi-potent stem cells activity. Thus, “normal” breast epithelial hierarchy and corresponding gene expression profiles have to be defined at individual patient but not global level for comparison with cancer. Consistent with this possibility, comparison of adjoining normal and cancer cells from the same patient revealed predominant accumulation of cancer cells in luminal progenitor state (CD49f+/EpCAM+), whereas normal cells were either distributed into all three subtypes or predominantly mature cells. This observation has important implications as cancer-specific defect in differentiation alone could account for majority of gene expression differences observed between cancer and normal cells. We have used single cell qRT-PCR to further refine tumor characterization at individual patient level and to precisely identify genes deregulated in cancer. Primary cell culturing technique has been extended to tissues samples collected from different sites of metastasis and genomic analyses of these cells showed organ-site specific genomic aberrations in metastasis. Collectively, we propose that the magnitude of tumor heterogeneity and cancer stem cell phenotype is the product of individual’s epithelial cell plasticity and cancer-specific mutation.