A team of Toronto researchers has pinpointed the molecular mechanism that may explain why women who undergo menopause late in life face a higher risk of breast cancer.

Scientists from the Ontario Cancer Institute have found the ovarian hormone progesterone drives the production of stem cells in breast tissue, which creates an environment ripe for cancer to take root.

The finding provides an important clue to why a woman’s reproductive history is a risk factor for developing breast cancer in later life and opens new doors to understanding how breast cancer begins.

Previous research has established exposure to estrogen and progesterone correlates with a risk of developing breast cancer.

Epidemiological studies have shown women who have more than the average number of menstrual cycles — women who menstruate early and those who undergo menopause later in life — are at an increased risk of developing the disease.

“Our work establishes a direct connection between these hormones and the stem cells in the breast,” said the study’s principal investigator, Rama Khokha, a molecular biologist at the Ontario Cancer Institute.

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