p53 Mediated Suppression of Normal and Malignant Mammary Stem Cell Self- Renewal

Accumulating evidence from a number of malignancies including breast cancer suggests that tumorigenesis is driven by a subset of tumor cells which retain stem cell properties. The cancer stem cell concept proposes that cancer originates from malignant transformation of stem/progenitor cells. A number of pathways have been implicated in maintenance of tissue specific stem/progenitor cells, including the p53 pathway. Inactivating mutations or deletions of TP53 have been reported in 50% of human malignancies. Germline mutations of p53 greatly increase the risk of breast tumors in animal models. Moreover, alterations of the p53 pathway in premalignant breast lesions have been reported and increase the risk of developing invasive ductal carcinoma. Meletis et al., demonstrated a role for p53 in suppressing self-renewal of adult neuronal stem cells. We utilized in vitro and mouse systems to examine the role of p53 in mammary stem/progenitor cells. p53 was down regulated by using a lentiviral vector carrying TP53specific short hairpin sequences. Down regulation of p53 in both normal and malignant human mammary epithelial cells increased mammosphere formation and stem/progenitor cell population that expressed the stem cell marker aldehyde dehydrogenase as detected by the Aldefluor assay. Furthermore, down regulation of p53 compared to the control cells in normal mammary epithelial cells resulted in a 5 fold increase in the amount of ductal structures (outgrowths) generated in humanized NOD/SCID mammary fat pads. Outgrowths generated from control cells were characterized by luminal and myoepithelial layers of epithelial cells. However, outgrowths generated by p53 down regulated cells displayed areas of hyperplasia and/or ductal carcinoma in situ resembling human lesions,