Decrease in breast stem cells after birth by young animals may protect against breast cancer

By Ruth SoRelle, M.P.H.

For many years, researchers and physicians have known that women who give birth before the age of 24 face a dramatically decreased risk of developing breast cancer. However, they did not know why.

Now research at Baylor College of Medicine may provide a clue.

In a recent issue of the journal Stem Cells, Yi Li, Ph.D., a professor in the Lester and Sue Smith Breast Center at BCM and his colleagues have found that after a young animal has given birth, the number of breast or mammary stem cells decreases.

Promising work

"If we can figure out the mechanism behind this, we could develop a pill that we could offer young women in high school and college that could significantly reduce their risk of breast cancer," he said.

However, there are many steps to be taken before he and his colleagues can determine how best to do that, Li said. Understanding why stem cells decrease in women who have their children young could prove an important advance.

In studies in mice, Li and his colleagues compared the numbers of mammary or breast stem cells (early cells that can differentiate into breast tissue) found in mice that had had babies at an age equivalent to the teens to mice that had never had babies.

Stem cell reduction

Using proven scientific techniques, they found that the mice that had had early pregnancies had half the numbers of mammary cells found in the mice that had never had babies, Li said.

They found that 1 in approximately 2,500 mammary cells were stem cells in the "virgin" mice while 1 in 5,000 were stem cells in the mice that had given birth.

Why having fewer stems cells protects against breast cancer remains unproven, said Li.

"Stem cells are long-living cells. One theory is that they can thus accumulate more mutations and are probably the most susceptible to giving rise to breast cancer," said Li. Thus, the more stem cells an animal has, the more likely the animal is to develop breast cancer.

Questions remain

However, Li stressed that this is just a theory.

He also noted that the protective effect of pregnancy is seen across the lifetime. The effect is not immediate. He studied the effect in mice that were 10 months old – the equivalent to 50 to 60 years old in humans.

"We saw that the stem cells are reduced," he said. "We know that breast cancer risk is reduced. This is an association. We have not proven that reducing the number of stem cells actually reduces the risk of breast cancer."

That is the next step he plans to take in his research.

Others who took part in this research include Stefan K. Siwko, Jie Dong, Michael T. Lewis, Hao Liu and Susan G. Hilsenbeck, all of BCM and the Dan L. Duncan Cancer Center at BCM.

Funding for this work came from the National Institutes of Health and the National Cancer Institute. View the full article.