When mothers breastfeed their babies, they depend upon a unique interaction of genes and hormones to produce milk and deliver it to their hungry little tyke. Unfortunately, this same cocktail of genes and hormones can also lead to breast cancer, especially if the mother has her first pregnancy after age 30.
A medical research group at the Medical College of Georgia at Georgia Regents University has established that a gene called DNMT1 plays a central role in the maintenance of the breast (mammary gland) stem cells that enable normal rapid growth of the breasts during pregnancy. This same gene, however, can also maintain those cancer stem cells that enable breast cancer. According to their work, the DNMT1 gene is highly expressed in the most common types of breast cancer.
Also, another gene called ISL1, which encodes a protein that puts the brakes on the growth of breast stem cells, is nearly silent in the breasts during pregnancy and in breast cancer stem cells.
Dr. Muthusamy Thangaraju, a biochemist at the Medical College of Georgia, who is the corresponding author of this study, which was published in the journal Nature Communications, said, “DNMT1 directly regulates ISL1. If the DNMT1 expression is high, this ISL1 gene is low.” Thangaraju and his team first observed the connection between DNMT1 and ISL1 when they knocked out the DNMT1 gene mice and noted an increase in the expression of ISL1. These results inspired them to examine the relationship of these two genes in human breast cancer cells.
Thangaraju and his co-workers discovered that ISL1 is silent in most human breast cancers. Furthermore, they demonstrated that restoring higher levels of ISL1 to human breast cancer cells dramatically reduced cancer stem cell populations, the growth of these cells, and their ability to spread throughout the tissue; all of which are hallmarks of cancer.
Conversely, Thangaraju and his team knocked out the DNMT1 gene in a breast-cancer mouse model, the breast will not develop as well. However, according to Thangaraju, this same deletion will also prevent the formation of about 80 percent of breast tumors. In fact, DNMT1 also down-graded super-aggressive, triple-negative breast cancers, which are negative for the estrogen receptor (ER-), progesterone receptor (PR-), and HER2 (HER2-).
The findings from this work also point toward new therapeutic targets for breast cancer and new strategies to diagnose early breast cancer. For example, a blood test for ISL1 might provide a marker for the presence of early breast cancer. Additionally, the anti-seizure medication valproic acid is presently being used in combination with chemotherapy to treat breast cancer, and this drug increases the expression of ISL1. This might explain why valproic acid works for these patients, according to Thangaraju. Workers in Thangaraju’s laboratory are already screening other small molecules that might work as well or better than valproic acid.
Mammary stem cells maintain the breasts during puberty as well as pregnancy, which are both periods of dynamic breast cell growth. During pregnancy, breasts may generate 300 times more cells as they prepare for milk production. Unfortunately, these increased levels of cell growth might also include the production of tumor cells, and the mutations that lead to breast cancer increase in frequency with age. If the developing fetus dies before she comes to term, immature breast cells that were destined to become mature mammary gland cells can more easily become cancer, according to Rajneesh Pathania, a GRU graduate student who is the first author of this study.
DNMT1 is essential for maintaining a variety of stem cell types, such as hematopoietic stem cells, which produce all types of blood cells. However, the role of DNMT1 in the regulation of breast-specific stem cells that make mammary gland tissue and may enable breast cancer has not been studied to this point.
For reasons that unclear, there is an increased risk of breast cancer if the first pregnancy occurs after age 30 or if mothers lose their baby during pregnancy or have an abortion. Women who never have children also are at increased risk, but multiple term pregnancies further decrease the risk, according to data compiled and analyzed by the American Cancer Society.
Theories to explain these phenomena include the coupling of the hormone-induced maturation of breast cells that occurs during pregnancy with an increase in the potential to produce breast cancer stem cells. Most breast cancers thrive on estrogen and progesterone, which are both highly expressed during pregnancy and help fuel stem cell growth. During pregnancy, stem cells also divide extensively and as their population increases, DNMT1 levels also increase.
In five different types of human breast cancer, researchers found high levels of DNMT1 and ISL1 turned off. Even in a laboratory dish, when they reestablished normal expression levels of ISL1, human breast cancer cells and stem cell activity were much reduced, Thangaraju said.