Normal breast stem cells, malignant breast stem cells, and the perinatal origin of breast cancer

Both experimental and epidemiological evidence support the concept that the in utero environment can influence an individual's risk of breast cancer in adult life. Recently identified breast stem cells may be the key to understanding the mechanism underlying this phenomenon. It has been theorized that breast cancers arise from breast stem cells. Our emerging view of the characteristics of normal breast stem cells and their link to malignant breast stem cells is reviewed here. It has also been postulated that factors that expand the normal breast stem cell pool in utero would increase the probability that one such cell might undergo an oncogenic mutation or epigenetic change. We discuss how a number of proposed perinatal determinants of adult breast cancer risk, including (1) in utero estrogen and IGF-1 levels, (2) birthweight, (3) breast density, and (4) early-life mutagen exposure, can be tied together by this "breast stem cell burden" hypothesis.