Inappropriate Hedgehog (Hh) signaling underlies development of a subset of 
medulloblastomas, and tumors with elevated HH signaling activity express the 
stem cell self-renewal gene BMI1. To test whether Bmi1 is required for Hh-driven 
medulloblastoma development, we varied Bmi1 gene dosage in transgenic mice 
expressing an oncogenic Hh effector, SmoA1, driven by a glial fibrillary acidic 
protein (GFAP) promoter. Whereas 100% of SmoA1; Bmi1(+/+) or SmoA1;Bmi1(+/-) 
mice examined between postnatal (P) days 14 and 26 had typical medulloblastomas 
(N = 29), tumors were not detected in any of the SmoA1;Bmi1(-/-) animals 
examined (N = 6). Instead, small ectopic collections of cells were present in 
the region of greatest tumor load in SmoA1 animals, suggesting that 
medulloblastomas were initiated but failed to undergo expansion into frank 
tumors. Cells within these Bmi1(-/-) lesions expressed SmoA1 but were largely 
nonproliferative, in contrast to cells in Bmi1(+/+) tumors (6.2% vs 81.9% 
PCNA-positive, respectively). Ectopic cells were negative for the progenitor 
marker nestin, strongly GFAP-positive, and highly apoptotic, relative to 
Bmi1(+/+) tumor cells (29.6% vs 6.3% TUNEL-positive). The alterations in 
proliferation and apoptosis in SmoA1;Bmi1(-/-) ectopic cells are associated with 
reduced levels of Cyclin D1 and elevated expression of cyclin-dependent kinase 
inhibitor p19(Arf), two inversely regulated downstream targets of Bmi1. These 
data provide the first demonstration that Bmi1 is required for spontaneous de 
novo development of a solid tumor arising in the brain, suggest a crucial role 
for Bmi1-dependent, nestin-expressing progenitor cells in medulloblastoma 
expansion, and implicate Bmi1 as a key factor required for Hh pathway-driven 
tumorigenesis.