Triple-Negative Breast Cancer’s Metastatic Drivers Revealed

In triple-negative breast cancer (TNBC), tumors express molecular markers of the epithelial-mesenchymal transition (EMT)—during which cells acquire the ability to move and migrate. Now, a new study from researchers at the the Johns Hopkins Kimmel Cancer Center has revealed an important role played by EMT programs in TNBC metastasis.

This findings are published in Science Translational Medicine.

“We have long needed new treatment targets and options for triple-negative breast cancers,” said Andrew Ewald, PhD, professor in basic science research and director, department of cell biology at the Johns Hopkins University School of Medicine and co-leader of the Cancer Invasion and Metastasis Program at the Johns Hopkins Kimmel Cancer Center. “These cancers often return within three years of diagnosis, and treatments used for other breast cancers don’t typically work for triple-negative.”

An estimated 10% to 20% of the 280,000 breast cancers diagnosed in the United States each year are triple negative, and the rate is higher among African American women, who are twice as likely as others to experience this form of the disease.

The researchers used a combination of machine learning, cellular imaging, and biochemical analysis to identify differences in the genetic expression patters of initial and metastatic tumors, which revealed that spontaneous TNBC tumors had large populations of hybrid epithelial-mesenchymal cells that lead invasion while expressing both epithelial and mesenchymal characteristics. They used a genetically engineered mouse model (GEMM), multiple patient-derived xenografts, and archival patient samples in the research.

In addition, the researchers identified that the mesenchymal protein vimentin promoted invasion and repressed metastatic outgrowth. And, the tumors gain survival advantages by producing E-cadherin.

“The bad news from our study is that cells from metastatic sites are super optimized for migration and resisting treatment,” said Ewald. “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, and we may be able to design new therapies that target these transcription factors.”

More specifically, they observed distinct patterns of utilization of five EMT transcription factors (Grhl2, Foxc2, Zeb1, Zeb2, and Ovol1) during invasion and colony formation. This suggested a sequential activation of multiple EMT molecular programs during the metastatic cascade.

The lab of Elana Fertig, PhD, division director and associate director of quantitative sciences and co-director of the Convergence Institute at the Johns Hopkins Kimmel Cancer Center, found patterns of gene expression.

The scRNA-seq data revealed three different EMT-related molecular patterns and that TNBC cancer cells “progressed from epithelial to hybrid epithelial/mesenchymal states during both invasion and colony formation, with different transcription factors being required for invasion versus colony formation.”

Ewald’s team then validated these states in samples from eight patients with triple-negative tumors, examining both primary tumors and tissues from metastatic sites of the same patients.

“The molecular differences between metastatic and primary tumors are likely the reason why metastatic tumor cells are so resistant to current treatments,” said Ewald. They observed heterogeneity between and within metastases in the same individual and “a complex spectrum of epithelial, hybrid E/M, and mesenchymal cell states within metastases, suggesting that there are multiple successful molecular strategies for distant organ colonization.”

His team is now studying ways to block the transcription factors’ genes or their resulting proteins to halt metastatic cancer growth. They are also examining whether the same molecular and cellular changes happen in other cancers, such as those in the colon, adrenal glands, stomach, and small intestine.