S. Das1,2, M. Wu2, M. Srikanth6,7, H. Kim4, A. Celebre2, D. Brat5, J. Kessler6, J. Karamchandani2,3, M. Bredel5 1St. Michael’s Hospital, University Of Toronto,Neurosurgery/Surgery,Toronto, ON, Canada 2Ann And Arthur Labatt Brain Tumour Centre, Hospital For SickKids,Cell Biology,Toronto, ON, Canada 3Montreal Neurological Institute,Laboratory Medicine,Montreal, QC, Canada 4University Of Alabama,Radiation Oncology,Birmingham, Alabama, USA 5Emory University School Of Medicine,Pathology,Atlanta, GA, USA 6Northwestern University,Chicago, IL, USA 7Harvard School Of Medicine,Brookline, MA, USA
Introduction: The role of intratumoral heterogeneity as a determinant of treatment failure and disease recurrence has become increasingly appreciated. In glioblastoma, intratumoral heterogeneity—on the functional, genetic, and transcriptional level—likely accounts for the inability of conventional and targeted therapies to achieve long-term remission. In the neural stem cell (NSC) niche, NSCs have been shown to shuttle between a quiescent and activated state. This dynamic has also been shown to be present in the hair follicle stem cell niche, where oscillating levels of bone morphogenetic protein (BMP) and TGF-β2 expression regulate activation of hair follicle stem cell proliferation following physiologic quiescence. We postulated that glioma stem cells (GSCs) could also shuttle between a quiescent and activated state, and that these fate choices could be directed by TGF-β and BMP signaling within the tumor microenvironment. Further, we postulated that TGF-β and BMP might construe cell identities in glioblastoma that could be relevant to glioblastoma treatment resistance, disease evolution and disease progression.
Methods: We performed immunohistochemical analysis for in situ evidence of TGF-β and BMP signaling in glioblastoma using surgical specimens, then confirmed our findings using in vitro stem cell assay systems. Using knockdown and overexpression stuides, we characterized the quiescent and activated GSC states. We then confirmed the basis for our phenomonological hypothesis in mouse glioblastoma long-term label retaining and chemoradiation treatment models.
Results: We demonstrate that TGF-β and BMP signaling are active in the glioblastoma microenvironment, and regulate the shuttling of GSCs from an activated to a quiescent state through their effects on p21, Stat3 and EGFR. In vivo, BMPhi GSCs are long-term retaining cells, consistent with the quiescent phenotype. BMP-mediated quiescence protects GSCs from genotoxic stress and treatment-associated DNA damage. Further, BMPhi GSCs serve as a cellular reservoir for tumour recurrence following chemotherapy.
Conclusion: Our findings demonstrate a role for BMP-mediated quiescence in glioblastoma disease resistance and recurrence, and suggest that targeted inhibition of BMP during chemoradiation and TGF-β following its conclusion could favorably alter the natural history of this disease.