Norrin mediates tumor-promoting and -suppressive effects in glioblastoma via Notch and WNT.

Glioblastoma (GBM) contains a subpopulation of cells, GBM stem cells (GSCs), that maintain the bulk tumor and represent a key therapeutic target. Norrin is a Wnt ligand that binds the Frizzled4 (FZD4) receptor to activate canonical Wnt signaling. While Norrin, encoded by NDP, has a well- described role in vascular development, its function in human tumorigenesis is largely unexplored. Here, we show that NDP expression is enriched in neurological cancers, including GBM, and its levels positively correlated with survival in a GBM subtype defined by low expression of ASCL1, a proneural factor. We investigated the function of Norrin and FZD4 in GSCs and found that it mediated opposing tumor-promoting and -suppressive effects on ASCL1lo and ASCL1hi GSCs. Consistent with a potential tumor suppressive effect of Norrin suggested by the tumour outcome data, we found that Norrin signaling through FZD4 inhibited growth in ASCL1lo GSCs. In contrast, in ASCL1hi GSCs Norrin promoted Notch signaling, independently of WNT, to promote tumor progression. Forced ASCL1 expression reversed the tumor suppressive effects of Norrin in ASCL1lo GSCs. Our results identify Norrin as a modulator of human brain cancer progression and reveal an unanticipated Notch mediated function of Norrin in regulating cancer stem cell biology.