TITLE: Identification of pro-differentiation p53 target genes and evaluation of expression in normal and malignant mammary gland PRINCIPAL INVESTIGATOR:

Tumor stem cell theory implies that causative lesions underlying human malignancies occur and are harbored in multi-potent progenitors with retained proliferative capacity and a prolonged lifespan. Transcriptional profiling of human breast cancers has identified five distinct subtypes of which the basal-epithelial subtype is most aggressive, correlates with poor prognosis, and lacks established molecular targets such as ERα, PR and Her2overexpression. These tumors display a high degree of cellular heterogeneity suggesting that they may arise as the result of unregulated self-renewal in a multipotent cell. Clinically basal epithelial tumors are recognized by their negativity for ERα, PR and Her2, suggesting that the identification and validation of markers that definitively define this subtype will improve the diagnosis of these tumors. We present evidence that nestin, a previouslydescribed neural progenitor marker is expressed in basal epithelia of the normal human mammary gland. Co-localization studies indicate two distinct populations of nestin-positive cells; one that expresses cytokeratin 14 and p63 and another that expresses desmin. Oncogenic transformation of a mammary progenitor cell culture model leads to increased expression of nestin. Immunohistochemical analysis of basal-epithelial breast tumors indicates robust expression of nestin, and CK14, punctate expression of p63, and low-toundetectable levels of desmin expression. Nestin was not detected in other breast cancer sub-types, indicating selectivity for basal-epithelial breast tumors including those known to carry BRCA1 mutations. These studies coupled to the established role for p63 in the preservation of self-renewal suggest that basal breast tumors display biochemical features of mammary progenitors.