Triple-negative breast cancer (TNBC) is an aggressive subtype associated with early metastatic recurrence and worse patient outcomes. The tumor expresses molecular markers of the epithelial-mesenchymal transition, but its requirement during spontaneous TNBC metastasis in vivo remains incompletely understood.
Scientists from Johns Hopkins Medicine have identified significant molecular variations between cancer cells that cling to an initial tumor and those that spread out to form distant tumors.
Andrew Ewald, Ph.D., the Virginia DeAcetis Professor in Basic Science Research and Director, Department of Cell Biology at the Johns Hopkins University School of Medicine, said, “We have long needed new treatment targets and options for triple-negative breast cancers. These cancers often return within three years of diagnosis, and treatments used for other breast cancers don’t typically work for triple-negative.”
Scientists conducted the study on mouse models and human tissues. This form of cancer is particularly deadly because it lacks molecular signals on its surface that link with the hormones progesterone and estrogen, as well as the cancer-promoting protein Her2-neu. Most breast cancer treatments today target those markers, making them ineffective for those with triple-negative tumors.
Scientists, in this study, carefully observed molecular differences between initial, or primary, triple-negative breast cancer sites and areas where it spread, or metastatic sites, among three different types of cells: mouse models, human cancers implanted into mice, and samples of both primary and metastatic tissues taken from eight patients.
Using techniques such as machine learning, cellular imaging, and biochemical analysis, scientists identified differences between the primary and metastatic cancers’ genomic expression patterns.
Ewald said, “The bad news from our study is that cells from metastatic sites are super optimized for migration and resisting treatment. The good news is that we identified several proteins called transcription factors that these cells require to handle the challenges of migrating and thriving at metastatic sites. We may be able to design new therapies that target these transcription factors.”
Scientists noticed several distinctive characteristics in the cells of mice implanted with human triple-negative breast cancer tumors or mice modified to have the mouse-version of the disease. Most important, they found that invasion of triple-negative breast cancer cells to other tissues on of another part of the body, the cells get two cellular properties: better movement and survival.
To achieve this, breast cancer cells acquire the vimentin cellular skeleton protein, which improves the capacity of so-called mesenchymal cells to migrate and generate new cells. Mesenchymal cells are a type of cell that is generally found in bones and bone marrow.
The production of a protein called cadherin offers survival benefits to triple-negative breast cancer cells. The protein is typically found in epithelial cells that line the ducts and coverings of organs and frequently renew themselves.
Scientists classify their cellular state as so-called hybrid epithelial-mesenchymal (EMT) cells when triple-negative breast cancer cells gain such survival and migratory qualities.
With the help of Elana Fertig, Ph.D., division director and associate director of quantitative sciences, and co-director of the Convergence Institute at the Johns Hopkins Kimmel Cancer Center, scientists carefully observed molecules involved in hybrid EMT states. They also tracked the molecular patterns of individual cells in cell assays that model invasion out of the primary tumor and formation of a colony in a metastatic site.
Researchers employed machine learning techniques to identify patterns in each cell’s expression of RNA, a relative of DNA involved in protein synthesis. The majority of metastatic cells, the researchers discovered, transform into the hybrid EMT state, which is more migratory and more resilient. Then, scientists examined primary tumors and tissues from the same patients’ metastatic sites to validate similar states in samples from eight patients with triple-negative malignancies.
At the molecular level, most metastatic cells produce five proteins called transcription factors (Grhl2, Foxc2, Zeb1, Zeb2, and Ovol1) that foster the making of proteins involved in either cancer cell invasion or colony formation.
Ewald said, “The molecular differences between metastatic and primary tumors are likely the reason why metastatic tumor cells are so resistant to current treatments.”
Scientists are now studying ways to block the transcription factors’ genes or their resulting proteins to halt metastatic cancer growth and whether the same molecular and cellular changes happen in other cancers, such as those in the colon, adrenal glands, stomach, and small intestine.