Defining the Mode of Medulloblastoma Growth using the Ptch1 Heterozygous Mouse Model

Single cancers can be comprised of highly heterogeneous cell populations. In brain tumours, including the malignant pediatric brain tumour medulloblastoma, how the distinct cell types that comprise a tumour contribute to growth and relapse are unclear. Transplantation of human and mouse medulloblastomas have prospectively identified cells with the cardinal stem cell properties of self-renewal and differentiation capacity, but the identity, biology and relevance of these cells in primary tumours are unknown. Here, using Ptch1 heterozygous mice irradiated at birth, I define the cellular mechanism of mouse medulloblastoma growth. Kinetic studies using thymidine analogues showed that rare, Sox2+ cells are relatively quiescent compared to the common, proliferating progenitors expressing Doublecortin (DCX) that differentiate into post-mitotic NeuN+ cells. Transplantation and lineage tracing experiments show that Sox2+ cells act as medulloblastoma stem cells: self-renewing and differentiating to drive growth in transplants and primary tumours. Lineage tracing revealed that tumours grow as a caricature of a neurogenic system. Investigating cell-type specific drug responses revealed that Sox2+ cells are selected for by anti-mitotic and Shh pathway-targeted therapies, creating a reservoir for relapse. Accordingly, high expression of a Sox2+ cell gene signature and high frequencies of Sox2+ cells in human tumours predict poor prognosis. Sox2-expressing primary medulloblastoma cultures were screened in serum free conditions in vitro to identify compounds that inhibit Sox2+ medulloblastoma cell growth. The aureolic acid mithramycin triggered Sox2+ cell apoptosis in vitro, blocked self-renewal and extended Ptch1+/- mouse survival in vivo, and completely prevented tumour regrowth in transplantation experiments. Therefore, targeting self-renewal in medulloblastoma by disrupting the stem cell hierarchy may be of therapeutic benefit. These findings confirm the hierarchical growth paradigm described for medulloblastoma based on transplantation experiments, define the biology of tumours’ constituent cell types and identify a novel approach to prolong medulloblastoma remission by targeting self-renewing cells.Ph.D