These cells show properties of cancer stem cells and are characterized by three surface proteins. Patients with large numbers of these cells in the blood show a rather unfavorable disease progression. The pattern of the three molecules may therefore be used as a biomarker for disease progression. The scientists plan to investigate whether the characteristic surface molecules may be used as targets for specific therapies for patients with advanced breast cancer.
Individual cancer cells that break away from the original tumor and circulate through the blood stream are considered responsible for the development of metastases. These dreaded secondary tumors are the main cause of cancer-related deaths. Circulating tumor cells (CTCs) detectable in a patient’s blood are associated with a poorer prognosis. However, up to now, experimental evidence was lacking whether the “stem cell” of metastasis can be found among CTCs.
“We were convinced that only very few of the various circulating tumor cells are capable of forming a secondary tumor in a different organ, because many patients do not develop metastases even though they have cancer cells circulating in their blood,” says Prof. Andreas Trumpp, a stem cell expert. Trumpp is head of DKFZ’s Division of Stem Cells and Cancer and director of the Heidelberg Institute for Stem Cell Technology and Experimental Medicine (HI-STEM) at DKFZ. “Metastasis is a complex process and cancer cells need to have very specific properties to initiate the process. We hypothesized that the characteristics of cancer stem cells, which are resistant to therapy and very mobile, are best suited,” says Trumpp.
Irène Baccelli from Trumpp’s team developed a transplantation test for experimental detection of metastasis-initiating cells. In collaboration with Prof. Andreas Schneeweiss from the National Center for Tumor Diseases (NCT) Heidelberg along with colleagues from the Institute of Tumor Biology in Hamburg and the Institute of Pathology of Heidelberg University Hospital, the researchers analyzed the blood of more than 350 breast cancer patients. Using specific surface molecules, Baccelli isolated circulating tumor cells from the blood and directly transplanted them into the bone marrow of mice with defective immune systems. “Bone marrow is a perfect niche for tumor cells to colonize,” Trumpp explains. After more than one hundred transplantations, metastases actually started forming in the bones, lungs and livers of some of the animals.
This proved that CTCs do contain metastasis initiating cells – even though apparently with a low frequency. What are the hallmarks of these cells? To characterize their molecular properties, the researchers analyzed the surface molecules of those CTCs which had been capable to initiate metastasis after cell transplantation.
Three molecules characterize the metastasis stem cell
By systematic screening, Baccelli first isolated cells from the CTCs carrying a typical protein of breast cancer stem cells (CD44) on their surface. This protein facilitates settling of cells in the bone marrow. Next, the researchers screened this cell population for specific surface markers supporting survival of the cells in foreign tissue. These markers include a signaling molecule that protects against immune system attacks (CD47) and a surface receptor that enhances the cells’ migratory and invasive capabilities (MET).
Using a cell sorter, the researchers were able to isolate CTCs carrying all three markers (CD44, CD47, MET). Another round of transplantation tests showed that these were the cells from which the metastases originated.
Differing from patient to patient, cells exhibiting all three surface molecules (“triple-positive” cells) made up between 0.6 and 33 percent of all CTCs. “Interestingly, only cells with stem cell marker CD44 carry the combination of the other two surface molecules,” says Irène Baccelli. “The triple-positive cells seem to be a specialized subtype of breast cancer stem cells circulating in the blood.”
Triple-positive cells as prognostic biomarkers
Do the triple-positive cells mark more precisely breast cancer progression than the number of CTCs? In a small patient group, the researchers observed the number of triple-positive cells rising while the disease advanced, whereas the total number of CTCs did not. In addition, patients with large numbers of triple-positive cells had more metastases and a much poorer prognosis than women with only few of these metastasis-inducing cells. “As a whole, triple-positive cells seem to have a substantially higher biological relevance for disease progression than previously studied CTCs,” Andreas Schneeweiss explains. The researchers plan to confirm these new results in a large clinical trial.
Andreas Trumpp considers the two proteins CD47 and MET characterizing metastasis-initiating cells as good news. Therapeutic antibodies targeting CD47 are currently under development. A substance inhibiting the MET receptor has already been approved and shows good effectiveness against a certain type of lung cancer. The substance may also help breast cancer patients with metastasis-inducing cells. “The triple-positive cells turn out to be not only a promising biomarker of disease progression but should also serve as potential new therapeutic targets for patients with advanced breast cancer,” says Andreas Trumpp.
Irène Baccelli, Andreas Schneeweiss, Sabine Riethdorf, Albrecht Stenzinger, Anja Schillert, Vanessa Vogel, Corinna Klein, Massimo Saini, Tobias Bäuerle, Markus Wallwiener, Tim Holland-Letz, Thomas Höfner, Martin Sprick, Martina Scharpff, Frederik Marmé, Hans Peter Sinn, Klaus Pantel, Wilko Weichert and Andreas Trumpp: Identification of a population of blood circulating tumor cells from breast cancer patients that initiates metastasis in a xenograft assay. Nature Biotechnology 2013, DOI: 10.1038/nbt.2576
A picture for this press release is available at:
Circulating tumor cells isolated from the blood of breast cancer patients form a metastatic tumor in the bone marrow of mice. The stem cell marker CD44 is dyed red.
Source: Irène Baccelli, DKFZ
The German Cancer Research Center (Deutsches Krebsforschungszentrum, DKFZ) with its more than 2,500 employees is the largest biomedical research institute in Germany. At DKFZ, more than 1,000 scientists investigate how cancer develops, identify cancer risk factors and endeavor to find new strategies to prevent people from getting cancer. They develop novel approaches to make tumor diagnosis more precise and treatment of cancer patients more successful. The staff of the Cancer Information Service (KID) offers information about the widespread disease of cancer for patients, their families, and the general public. Jointly with Heidelberg University Hospital, DKFZ has established the National Center for Tumor Diseases (NCT) Heidelberg, where promising approaches from cancer research are translated into the clinic. In the German Consortium for Translational Cancer Research (DKTK), one of six German Centers for Health Research, DKFZ maintains translational centers at seven university partnering sites. Combining excellent university hospitals with high-profile research at a Helmholtz Center is an important contribution to improving the chances of cancer patients. DKFZ is a member of the Helmholtz Association of National Research Centers, with ninety percent of its funding coming from the German Federal Ministry of Education and Research and the remaining ten percent from the State of Baden-Württemberg.