David M. Langenau, PhD
Massachusetts General Hospital
Associate Professor of Pathology
Harvard Medical School
Self-renewal is responsible for the indefinite replicative potential found in malignant cells and is likely found in residual cell populations that evade conventional therapy and lead to relapse. Uncovering downstream molecular pathways that regulate self-renewal will be integral to identifying novel drugs for the treatment of cancer.
I. Identifying the genetic programs underlying self-renewal of rhabdomyosarcoma cancer stem cells
Rhabdomyosarcoma (RMS) is the most common soft-tissue sarcoma of childhood, and analysis of human disease suggests that the RAS pathway is activated in a majority of embryonal RMS –the most prevalent pediatric subtype of disease. Using transgenic zebrafish, we have shown that expression of activated k-RAS in early muscle cells is sufficient to induce embryonal rhabdomyosarcoma (ERMS). Zebrafish tumors are morphologically similar to human ERMS and express the clinical diagnostic markers of this malignancy. Using fluorescence activated cell sorting and cell transplantation, we have identified the self-renewing cancer stem cell in ERMS, a cell that is most similar to a non-transformed, activated muscle satellite cell.
Using fluorescent transgenic zebrafish and cutting-edge live cell imaging, zebrafish ERMS development can be visualized in real-time. Moreover, our fluorescent transgenic approaches allow for the identification of malignant cell sub-types and should afford unprecedented opportunities to visualize self-renewal within a tumor directly. Together with these imaging studies, we are interrogating which pathways modulate self-renewal, cancer stem cell number, and tumor growth in ERMS.
II. Uncovering self-renewal programs in T-cell acute lymphoblastic leukemia
T-cell acute lymphoblastic leukemia (T-ALL) is a devastating disease of childhood, accounting for approximately 20% of pediatric ALLs and is associated with transformation of thymic precursor cells. Using a transgenic zebrafish model of T-ALL, large-scale transplantation experiments have been used to functionally assess self-renewal and tumor growth. Our results in zebrafish T-ALL reveal that leukemia-initiating cell number is quite high in primary leukemias (0.4%-1.4%), signifying that self-renewal is a much more common attribute in malignant T-ALL cells than previously suggested. Additional experiments show that T-ALLs can evolve increased percentages of tumor-initiating cells as leukemias continue to grow. Two models could explain this data: 1) either malignant clones present in the primary leukemia mass are selected that have increased leukemia-initiating potential or 2) clones continue to amass new genetic or epigenetic modifications that led to increased self-renewal. These two models are currently being tested experimentally and will likely lead to new insights into the general mechanisms governing tumor growth and relapse in patients.