Scientists at the Walter and Eliza Hall Institute have found that breast stem cells are extremely sensitive to the female hormones oestrogen and progesterone, a finding that opens the way for the development of new preventions and treatments for breast cancer.

The discovery made by scientists in the institute's Stem Cells and Cancer and Bioinformatics divisions, explains decades of evidence tying breast cancer risk to exposure to female hormones.

The led researcher Dr Jane Visvader along with Dr Geoff Lindeman, said sustained exposure to oestrogen and progesterone was a well established risk factor for breast cancer.

There is clear evidence that the more menstrual cycles a woman has the greater her breast cancer risk, said Dr Visvader.

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 misunderstood.

Dr Visvader and Dr Lindeman discovered breast stem cells in both mice and human back in the mid-2000s. They also unexpectedly found that breast stem cells lacked receptors that would allow them to be directly controlled by the female hormones oestrogen and progesterone.

Just recently, collaborative work between Dr Visvader and Linderman with doctors, Marie-Liesse Asslin-Labat, Gordon Smyth and others at the institutes has revealed that despite lacking receptors for the female hormones, breast stem cells are still remarkably sensitive to them.

Utilizing mouse models, they found that when the ovaries were removed or the animals were treated with hormone inhibitors (which are in clinical use as anti-breast cancer agents), breast stem cell numbers dropped and the cells appeared to become dormant.

This finding helps to explain why the effects of chemoprevention - a treatment aimed at breast cancer prevention continued long after anti-estrogen tablets have been stopped, according to Dr Lindeman, who is also a medical oncologist at the Royal Melbourne Hospital.

Our research also revealed that during pregnancy there is profound increase in breast stem cell numbers, Dr Lindeman said.

This might account for the short-term increase in cancer risk associated with pregnancy.

The findings of the study have been published online in the international journal Nature.