Norrin-dependent gene expression in Patched mouse medulloblastoma. Mus musculus

Medulloblastoma (MB), a tumor of the cerebellum, is the most common malignant brain tumor in children. One third of all human MB exhibits a gene expression signature of Sonic hedgehog (Shh) signaling. Hedgehog (Hh) pathway inhibitors have shown efficacy in clinical trials for MB, however, tumors develop resistance to these compounds, highlighting the need to identify additional therapeutic targets for treatment. We have identified a role for Norrin signaling in tumor initiation in the Patched (Ptch) mouse model of MB. Norrin is a secreted factor that functions as an atypical Wnt by binding to the Frizzled4 (Fzd4) receptor on endothelial cells to activate canonical beta-catenin-mediated Wnt signaling pathway. In the cerebellum, activation of Norrin/Fzd4 signaling is required for the establishment and maintenance of the blood brain barrier (BBB). We have identified a role for Norrin signaling in the stroma as a potent tumor inhibitory signal. Inactivation of Norrin in Ptch+/- mice significantly shortens latency and increases MB incidence. This phenotype is associated with an increased frequency of pre-tumor lesions and their conversion to malignancy. In this context, loss of Norrin signalling in endothelial cells is associated with an accelerated transition to a pro-tumor stroma characterized by vascular permeability, inflammation and angiogenic remodelling. Accordingly, loss of Ndp significantly alters the stromal gene expression signature of established Ptch MB. Overall design: Ptch+/- mice were maintained on a C57Bl/6 background. NdpKO mice, generated by disruption of the Ndp locus by a lacZ-containing cassette were obtained from Lexicon Pharmaceuticals (Junge et al., 2009) and maintained by interbreeding on a mixed background. Ndp is an X-linked gene and Ndp+/- females were crossed with Ptch+/- males, to generate male (Ndp-/y;Ptch+/-) NdpKO;Ptch+/- compound mutants. All compound mutants were compared to single Ptch+/- or wild-type controls from the same breeding cohort to ensure matched backgrounds. Granule neuron progenitors (GNPs) were isolated from postnatal day 6 (P6) cerebella of WT, Ptch+/- and Ndp-/y;Ptch+/- mice. MB tumors were dissected from the cerebella of symptomatic Ptch and Ndp-/y;Ptch+/- mice. To identify an NDP-dependent gene signature in tumors we compare gene expression in GNPs and MB from mice of the indicated genotypes.