Previous studies have established a strong linkage between female hormone levels and breast cancer, with mutations in mammary stem cells being the likely source of breast cancer cells. In the current study, published in the April 11, 2010, online edition of the journal Nature, investigators at the Walter and Eliza Hall Institute (Parkville, Victoria, Australia) showed in a mouse model that removal of the ovaries or treatment with hormone inhibitors reduced breast stem cell numbers and induced a state of dormancy in these cells.

In contrast, pregnancy led to a transient 11-fold increase in breast stem cell numbers, probably mediated through signaling via the RANK ligand pathway. The augmented breast stem cell pool indicated a cellular basis for the short-term increase in breast cancer incidence that accompanies pregnancy.

"There is clear evidence that the more menstrual cycles a woman has the greater her breast cancer risk,” said senior author Dr. Jane Visvader, associate professor of molecular genetics at the Walter and Eliza Hall Institute. "There is even an increase in breast cancer risk in the short-term following pregnancy. However the cellular basis for these observations has been poorly understood.”

Overall, the findings generated in this study suggest that successful chemical treatment to prevent development of breast cancer may be achieved, in part, through suppression of stem breast-cell function.

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