Metastatic cells, mechanisms of drug resistance identified for triple-negative breast cancer

Researchers at The University of Texas MD Anderson Cancer Center have discovered important mechanisms of drug resistance and metastasis in triple-negative breast cancer. Gloria Echeverria, Ph.D., instructor in Experimental Radiation Oncology, presents these findings today at the 2019 San Antonio Breast Cancer Symposium, which provide possible leads for improving breast cancer treatment.

Triple-negative breast cancer is extremely aggressive and highly heterogeneous. It lacks hormonal and protein targets that make other forms of the disease treatable.

“We do not have universal targeted therapies for this type of cancer, so women are first treated with chemotherapy,” Echeverria says. “Of these women, 50% will have substantial residual disease, and of these, 50% will have metastatic disease within 3 to 5 years.”

Identifying metastatic cell populations

The researchers used patient-derived xenograft models, in which a patient’s biopsied tissue is xenografted into a mouse’s mammary gland, to study the clonal diaspora of metastatic triple-negative breast cancer. Helen Piwnica-Worms, Ph.D., professor of Experimental Radiation Oncology and senior investigator of the research group, has collected a unique bank of PDX models of triple-negative breast cancer from 83 patients.

The researchers first engineered bioluminescent tumors and then labeled each tumor cell with a unique DNA barcode, which allowed the researchers to track the glowing tumor cells in real time as they moved throughout the mouse’s body. The researchers then dissected the metastatic tissue and analyzed the barcodes.

“In the original tumor tissue, we had thousands of different barcodes, but we found the same 10 uniquely barcoded lineages in the metastatic tissue, regardless of whether the tissue was removed from the lung, liver, or brain,” Echeverria says. “These barcoded tumor cell populations were able to survive in distinct microenvironments. A major question that we’re trying to address is what is going on in these cells that allows them to survive and thrive in different environments.”

To answer this question, the researchers are performing single cell RNA sequencing on the metastatic cells and comparing the gene expression profiles to identify similarities or differences. These gene expression profiles might help the researchers find a targetable molecular pathway in one or all of these lineages.

Mechanisms of drug resistance

The researchers also used cellular barcoding and PDX models to investigate chemotherapy resistance, which affects 50% of these patients. Residual triple-negative breast cancer tissue can become  temporarily tolerant to chemotherapy; when treatment stops, the tumor cells start growing again, without overt evidence that they had been exposed to the drug.

“For the drug resistance experiments, our findings were opposite of what we found for our metastasis experiments,” Echeverria says. “Instead of clonal selection, we found that any of the cell lineages could survive in the residual tumor tissue.”

The researchers found that glycolysis was downregulated in all three PDX models of residual disease. Piwnica-Worms says, “Typically, cells that downregulate glycolysis depend on another pathway, such as oxidative phosphorylation, to survive.”

The research team exposed the residual tumor cells to IACS-10759, a drug developed by MD Anderson’s Institute of Applied Cancer Sciences that targets the oxidative phosphorylation (OXPHOS) pathway. Although the drug did not completely eradicate the residual tumor cells, it significantly delayed regrowth. These findings were published earlier this year in Science Translational Medicine.

Next steps in this project include testing combination therapies to improve the effectiveness of IACS-10759. The researchers also hope to discover biomarkers that will identify triple-negative breast cancer that is resistant to chemotherapy and patients who might benefit from combination therapy.

This research is part of the ARTEMIS (A Robust TNBC Evaluation Framework to Improve Survival) clinical trial, headed by Stacy Moulder, M.D., professor in the Department of Breast Medical Oncology at MD Anderson, and funded by the Breast Cancer Moon Shot®, part of MD Anderson’s Moon Shots Program®.

“This project involves so many groups at MD Anderson, including pathologists, breast medical oncologists, surgeons, radiologists, bioinformaticists, and IACS. We have an incredible environment at MD Anderson that facilitates this translational work and provides the necessary funding and support,” Piwnica-Worms says. “Most importantly, we are grateful to our patients who donate their biopsies for research like this. These findings that Gloria presented at the San Antonio Breast Cancer Symposium will help us move toward developing combination therapies for triple-negative breast cancer.”