Abstract B49: Analysis of mammary stem cells from subjects of African versus European white ancestry

African American (AA) women have a higher incidence of breast cancer (BC) than white women before age 40, while the opposite is true after age 40. AA women are more likely to die from BC at every age. Although socio-economic factors have in the past been attributed to explain these differences, data exists to support the less widely held hypothesis that there may be intrinsic biological differences in AA breast tissue compared with white breast tissue. Evidence for this hypothesis includes the fact that triple negative (ER-/PR-/Her2-) breast tumors are more prevalent in AA women than in non-Hispanic white women, although neither population is monolithic in its BC phenotypes. Our laboratory has developed a novel tissue engineering system for Human Mammary Epithelial Cells (HMEC), both normal and malignant. This system allows for long-term establishment of non-diseased primary cultures that begin as 3-dimensional attached epispheres, that are structures made up of 40–100 epithelial cells. These nondiseased epi-spheres subsequently differentiate into functional, organotypic branching ducts and lobules that demonstrate ESA, CK-18, -19 staining, lumen, polarized nuclei, desmosomes, microvilli on apical surfaces and casein secretion. We hypothesize that intrinsic biological differences exist between women of African ancestry and those of European-white ancestry that will affect the ability and/or kinetics of this in vitro differentiation. We further hypothesize that this difference will be due to a difference in the proportion and/or potency of breast stem cells present. We have established primary HMEC cultures from 48/48 breast reduction mammoplasty tissues, a success rate that is markedly higher than the rate shown in the literature (less than 2%). 12 of the subjects from whom this tissue came, were AA and matched in socioeconomic status with the white women. We have found: 1. The more pregnancies a woman had, the less likely her breast tissue was to form ductal structures in vitro. This finding is consistent with the idea that lactational differentiation decreases the number of pluripotent stem cells in the breast, and that stem cells provide the capacity to form ductal structures in our system. 2. Highly proliferative breast tissue was more likely to form ductal structures. 3. Several factors, including ancestry, acted as modifying factors in the ability to form ductal architecture in culture, significantly affecting the rate of ductal differentiation. While the samples from white women took 22 days on average to form ducts, the average time for the samples from socioeconomically-matched AA women was only 11 days, a significant difference (P = 0.02). Putative mammary stem cells were analyzed by flow cytometry [CD44+, CD24−, CD227− (MUC1−), CD49f+(alpha6 integrin+)] and cloning. Stem cell proportions varied widely (range 0.3–21%), and had no association with ancestry. We are presently evaluating the potency of epithelial stem cells in these cultures by testing their ability to differentiate into diverse cell types under specific, inductive, culture conditions.