Moderate reduction of Norrin signaling activity associated with the causative missense mutations identified in patients with familial exudative vitreoretinopathy
Mutations in Norrin signaling genes (NDP, FZD4 and LRP5) have been found in patients with familial exudative vitreoretinopathy (FEVR) and the altered signaling is suspected to play a critical role in its pathogenesis. To better understand this relationship, we systematically performed functional analyses on previously identified single nucleotide variants of LRP5, FZD4 and NDP, utilizing the Norrin dependent Topflash reporter assay. Cell surface binding assays and protein electrophoresis analysis of Norrin were also performed. Seven causative mutations and five possibly causative but indecisive variants were examined. We found: (1) a nonsense mutation in FZD4 completely abolished its signaling activity, while single missense mutations in LRP5 and FZD4 caused a moderate level of reduction (ranging from 26 to 48, 36% on average) and a double missense mutation in both genes caused a severe reduction in activity (71%). These observations correlated roughly with clinical phenotypes. (2) A mutational effect is suggested in four of five indecisive variants by signaling reductions comparable to those of missense mutations. (3) Norrin mutants demonstrated variable effects on signal transduction, and no apparent correlation with clinical phenotypes was observed. (4) The Norrin mutants examined demonstrated impaired cell surface binding, and some may have partially lost their ability to form a complex with unknown high molecular weight material(s). Our results illustrate the nature of FEVR in relation to Norrin signaling and further suggest the complexity of its disease causing mechanism.
We thank Dr. Randal Moon for kindly providing the reporter plasmid. Special thanks go to Dr. Jeremy Nathans for his generosity of sharing plasmids and cell line. This project was supported by a Grant-in-aid 15591883 for Scientific Research, Japan to H.K. and a Grant-in-Aid for Research Revolution 2002 from The Ministry of Education, Culture, Sports, Science and Technology, Japan to K.H. in the Division of Genome Analysis, Research Center for Genetic Information, Medical Institute of Bioregulation, Kyushu University, Japan.
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