The Frizzled-ligand Norrin acts as a tumour suppressor linking oncogenic RAS signalling to p53

Tumour suppressor genes are frequently affected by somatic alterations in cancer, and the impairment of their normal function provides a strong contribution to tumourigenesis. Short-hairpin (sh) RNA library screens have been employed as powerful genetic tools to uncover important new players in human cancer 1-5. To identify potential novel tumour suppressor genes acting in the p53 pathway, we performed an shRNA screen using a cell-based model in which only a single additional genetic event disrupting the p53 pathway is required to obtain in vitro transformation. By using this approach, we report here on the identification of the Frizzled-ligand Norrin (Norrie disease protein) as a candidate tumour suppressor. Inhibition of Norrin expression promotes anchorage-independent growth, confers a strong growth advantage to cells and causes a reduction in p53 protein levels. Conversely, recombinant human Norrin increases p53 levels in a {beta}-catenin dependent fashion. Interestingly, Norrin expression is stimulated by oncogenic H-RAS and BRAF, suggesting that Norrin is part of an early fail-safe mechanism to suppress transformation, and that mutation or down regulation of Norrin could contribute to tumour progression. Indeed, we found that Norrin expression is significantly decreased in melanoma, breast, prostate and ovarian cancer. These findings support the existence of a novel autocrine/paracrine feedback loop that constrains tumourigenesis, in which the crosstalk between the RAS and {beta}-catenin pathways play an unanticipated role.