Circulating Tumor Cell Clusters More Likely to Cause Metastasis Than Single Cells
Circulating tumor cell clusters—clumps of from 2 to 50 tumor cells that break off a primary tumor and are carried through the bloodstream—appear to be much more likely to cause metastasis than are single circulating tumor cells, according to a study from investigators at the Massachusetts General Hospital (MGH) Cancer Center. The report by Aceto et al, published in Cell, also suggests that a cell adhesion protein binding circulating tumor cell clusters together is a potential therapeutic target.
“While circulating tumor cells are considered to be precursors of metastasis, the significance of circulating tumor cell clusters, which are readily detected using devices developed here at MGH, has remained elusive,” said co–senior author Shyamala Maheswaran, PhD, of the MGH Cancer Center.“Our findings that the presence of circulating tumor cell clusters in the blood of cancer patients is associated with poor prognosis may identify a novel and potentially targetable step in the blood-borne spread of cancer.”
Study Details
In their experiments the team used two versions of a microfluidic device called the CTC-Chip that captures circulating tumor cells from blood samples in ways that make the cells accessible for scientific testing. One version—the HBCTC-Chip—can efficiently capture extremely rare circulating tumor cells in a blood sample. Another version, theCTC-iChip, rapidly isolates circulating tumor cells in a way that does not rely on preidentified tumor antigens, allowing capture of cells with gene expression patterns that may be missed by the antibodies used in the HBCTC-Chip.
A series of experiments in animal models of breast cancer revealed that circulating tumor cell clusters are made up of cells that probably were adjacent to each other in the primary tumor, not cells that proliferated after entering the bloodstream.
Although circulating tumor cell clusters make up only 2% to 5% of all circulating tumor cells, they contributed to around half of lung metastases resulting from implanted breast tumors, indicating a metastatic potential 23 to 50 times greater than single circulating tumor cells.
Circulating tumor cell clusters injected into mice survived in greater numbers than did single circulating tumor cells, and metastases developing from clusters led to significantly reduced survival. The clusters disappear from the animals’ bloodstreams more rapidly than do single circulating tumor cells, probably because they become lodged in capillaries where they give rise to metastases.
Reduced Survival in Patients With Circulating Tumor Cell Clusters
Analysis of blood samples taken at several points in time from a group of patients with different forms of advanced metastatic breast cancer found circulating tumor cell clusters in the blood of 35% of patients and that the survival of those with more clusters in their blood was significantly reduced. Similar analysis of samples from a group of prostate cancer patients also found an association between the presence of circulating tumor cell clusters and dramatically reduced survival.
RNA sequencing of both single and clustered circulating tumor cells from breast cancer patients identified several genes expressed at elevated levels in circulating tumor cell clusters, one of which—a protein called plakoglobin—also was overexpressed in the primary tumors of patients with reduced survival. Analysis of blood and tissue samples from one patient revealed that plakoglobin was expressed in circulating tumor cell clusters but not single circulating tumor cells and also was expressed in some portions of both the primary tumor and metastases. Plakoglobin is a component of two important structures involved in cell-to-cell adhesion, and the investigators found that suppressing its expression caused circulating tumor cell clusters to fall apart, reducing their metastatic potential, and also disrupted cell-to-cell contact between breast cancer cells but not normal breast tissue.
“It is possible that therapeutically targeting plakoglobin or pathways involved in cell-to-cell adhesion within cancer cells could be clinically useful, especially in patients in whom circulating tumor cell clusters are found,” said Nicola Aceto, PhD, of the MGH Cancer Center and lead author of the Cell paper. “We need to investigate that possibility along with determining whether further characterization of both single circulating tumor cells and circulating tumor cell clusters will provide further insight into differences in their biology, drug responsiveness and their contribution to the process of metastasis.”
Dr. Maheswaran and Daniel Haber, MD, PhD, of the MGH Cancer Center and the Harvard Medical School, are the corresponding authors for the Cell article.
The study was supported by Janssen Diagnostics, Stand Up to Cancer, the Breast Cancer Research Foundation, the National Foundation for Cancer Research, the National Institutes of Health, Susan G. Komen for the Cure, ESSCO Breast Cancer Foundation, and the Howard Hughes Medical Institute.
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