Early Pregnancy May Offer Long-Term Protection Against Breast Cancer
A study by cell biologists at the University of California, Santa Cruz (UCSC), suggests that an early first pregnancy may provide protection against breast cancer decades later. Researchers observed that pregnancy alters the aging process of mammary tissue, reducing the accumulation of abnormal cells linked to tumor formation. The findings, published in Nature Communications, address the long-standing observation that while aging increases breast cancer risk, early-life pregnancy offers long-term protection.
Study Details and Methodology
The research utilized a mouse model designed to simulate human aging and reproductive history. Researchers compared the breast tissue of aged mice that had experienced an early pregnancy with those that had not. This approach aimed to model long-term risk, examining mammary glands at a stage equivalent to postmenopausal age in humans.
This stage corresponds to women having their first child between ages 20-30 and being studied after age 50. Approximately 75% of breast cancer diagnoses occur after age 50, and most first pregnancies in the U.S. occur between ages 20 and 33.
Single-cell RNA sequencing technology was employed to analyze mammary epithelial cells, tracking how aging and pregnancy influence cell populations and gene activity.
Identification of "Confused" Hybrid Cells
The study found that in the absence of pregnancy, aging breast tissue accumulates a population of "confused" hybrid cells. These cells express markers of both luminal and basal mammary lineages simultaneously and are located in the basal layer of the mammary gland. Many studies indicate that breast tumors often originate from cells that lose their normal identity over time, particularly with age.
These hybrid cells also displayed an inflammatory signal known as IL-33. Researchers determined that IL-33 can stimulate uncontrolled cell growth, a preliminary step toward tumor development.
To investigate this, mammary epithelial cells from young mice were treated with IL-33, causing them to behave similarly to cells from aged, never-pregnant animals, exhibiting increased cell proliferation and enhanced formation of organoids, especially when the tumor-suppressor gene Trp53 was suppressed.
Pregnancy as a "Cellular Reset"
The study indicated that pregnancy functions as a "cellular reset button," effectively preventing the buildup of these hybrid cells.
Shaheen Sikandar, an assistant professor of molecular, cell, and developmental biology and a corresponding author, stated that pregnancy compels cells to commit to specific functions, thereby maintaining the 'lineage integrity' of the tissue.
Beyond reducing the presence of hybrid cells, pregnancy also corrected broader age-related imbalances in mammary tissue. In aged mice that had been pregnant, the age-associated expansion of basal cells was normalized, and both basal and luminal cells displayed a reduced capacity to form organoids.
Additionally, luminal cells in these mice retained molecular signatures linked to "post-pregnancy involution," which may enhance immune surveillance and further reduce cancer risk.
Long-Term Impact and Future Directions
Andrew Olander, a graduate student and lead author, noted that these findings could help explain why the protective effect of pregnancy takes years to manifest and persists into later life, by demonstrating how early reproductive events leave a lasting impact on the aging breast.
While the study was conducted in mice, the researchers suggest the biological principles are likely relevant to humans due to similarities in mammary gland structure and cancer epidemiology. The study identifies hybrid cells as a plausible factor in age-related breast cancer risk and a potential target for future prevention strategies. Future research will focus on further understanding the role of these 'confused' hybrid cells in breast cancer development.
Research Team and Funding
Other contributing authors from UC Santa Cruz included Paloma Medina, Veronica Haro Acosta, Sara Kaushik, and Matijs Dijkgraaf. Funding for the research was provided by the Hellman Foundation, a National Institutes of Health/National Cancer Institute fellowship, and a grant to Professor Sikandar.