Abstract
In humans, parity without breastfeeding increases risk of estrogen receptor-negative (ER-) breast cancer and is associated with hypermethylation of FOXA1, a pioneer factor regulating lineage commitment of mammary gland luminal progenitor cells. We postulate that pregnancy-associated repression of FOXA1 results in the accumulation of aberrant, differentiation-arrested luminal progenitor cells which, following additional genetic and epigenetic insults, may give rise to ER- tumors. Consistent with this hypothesis, we show that deletion of Foxa1 in the mouse mammary gland results in a two-fold increase in the proportion of luminal progenitor cells and a reduction in mammary gland epithelial cells that stain positive for ER. These results provide compelling support for the notion that reduced Foxa1 expression is sufficient to alter mammary gland luminal cell fate determination in vivo, which could be a mechanism linking parity with ER- breast cancer.