Investigating β-catenin and the Norrin signalling pathway in FEVR

Familial exudative vitreoretinopathy (FEVR) is a rare inherited blindness disorder characterised by peripheral retinal avascularity. FEVR has a highly variable clinical presentation with a phenotypic spectrum ranging from asymptomatic to blindness at birth. The Norrin/β-catenin pathway plays a key role in angiogenesis and retinal vascular development and is implicated in vascular retinopathies including FEVR. This study investigated the functional impact of two FEVR mutations affecting the C-terminal domain (CTD) of β-catenin, p.(Arg710Cys) and p.(His720*). Results from a range of in-vitro cell-based assays questioned the pathogenicity of the p.(Arg710Cys) missense variant. However, TOPflash assay results indicated that the p.(His720*) nonsense variant causes transcription defects and localization studies showed these were not due to defects in nuclear translocation, suggesting β-catenin’s CTD may have key functional roles in transcription. This work led to the development of a CRISPR-Cas9 mouse model (CTNNB1-H720X-EM1-B6) to further explore the pathogenic effects of truncated β-catenin in-vivo. Also in this study, a high-throughput RNAi screen was developed and trialled to characterise components of the Norrin/β-catenin pathway. A kinase screen identified both expected and potentially novel pathway components including an uncharacterised protein, Wnt/β-catenin related hits (PRKAR2B, TRIB2, RAF1 and RIOK3), hits with roles in angiogenesis and Wnt signalling (FLT1, VEGF Receptor 1) and a hit linked to the FEVR phenotype of retinal peripheral avascularity (IKBKG). Altogether, these findings are expected to enhance our understanding of the complex Norrin/β-catenin pathway. The H720X mouse model will enable investigation of the transcriptional and binding profile of variant β-catenin. Newly discovered components will shed light to the molecular mechanisms of Norrin signalling and FEVR, providing future opportunities for new therapeutic approaches.