Heterozygote loss-of-function variants in the LRP5 gene cause familial exudative vitreoretinopathy

Characterization of a novel heterozygous frameshift variant in NDP gene that causes familial exudative vitreoretinopathy in female patients

Familial exudative vitreoretinopathy (FEVR) is a severe inherited disease characterized by defective retinal vascular development. With genetic and clinical heterogeneity, FEVR can be inherited in different patterns and characterized by phenotypes ranging from moderate visual defects to complete vision loss. This study was conducted to unravel the genetic and functional etiology of a 4-month-old female FEVR patient. Targeted gene panel and Sanger sequencing were utilized for genetic evaluation. Luciferase assays, western blot, quantitive real-time PCR, and immunocytochemistry were performed to verify the functional defects in the identified candidate variant. Here, we report a 4-month-old girl with bilateral retinal folds and peripheral avascularization, and identified a novel frameshift heterozygous variant c.37dup (p.Leu13ProfsTer13) in NDP. In vitro experiments revealed that the Leu13ProfsTer13 variant led to a prominent decrease in protein levels instead of mRNA levels, resulting in compromised Norrin/β-catenin signaling activity. Human androgen receptor assay further revealed that a slight skewing of X chromosome inactivation could partially cause FEVR. Thus, the pathogenic mechanism by which heterozygous frameshift or nonsense variants in female carriers cause FEVR might largely result from a loss-of-function variant in one X chromosome allele and a slightly skewed X-inactivation. Further recruitment of more FEVR-affected females carrying NDP variants and genotype-phenotype correlation analysis can ultimately offer valuable information for the prognosis prediction of FEVR.

Defective EMC1 drives abnormal retinal angiogenesis via Wnt/β-catenin signaling and may be associated with the pathogenesis of familial exudative vitreoretinopathy

Endoplasmic reticulum (ER) membrane protein complex (EMC) is required for the co-translational insertion of newly synthesized multi-transmembrane proteins. Compromised EMC function in different cell types has been implicated in multiple diseases. Using inducible genetic mouse models, we revealed defects in retinal vascularization upon endothelial cell (EC) specific deletion of Emc1, the largest subunit of EMC. Loss of Emc1 in ECs led to reduced vascular progression and vascular density, diminished tip cell sprouts, and vascular leakage. We then performed an unbiased transcriptomic analysis on human retinal microvascular endothelial cells (HRECs) and revealed a pivotal role of EMC1 in the β-catenin signaling pathway. Further in-vitro and in-vivo experiments proved that loss of EMC1 led to compromised β-catenin signaling activity through reduced expression of Wnt receptor FZD4, which could be restored by lithium chloride (LiCl) treatment. Driven by these findings, we screened genomic DNA samples from familial exudative vitreoretinopathy (FEVR) patients and identified one heterozygous variant in EMC1 that co-segregated with FEVR phenotype in the family. In-vitro expression experiments revealed that this variant allele failed to facilitate the expression of FZD4 on the plasma membrane and activate the β-catenin signaling pathway, which might be a main cause of FEVR. In conclusion, our findings reveal that variants in EMC1 gene cause compromised β-catenin signaling activity, which may be associated with the pathogenesis of FEVR.

Five novel dysfunctional variants in the TSPAN12 gene in familial exudative vitreoretinopathy

Familial exudative vitreoretinopathy (FEVR) is an inheritable vitreoretinal disease characterized by incomplete retinal vascular development, which often leads to multiple retinal complications and causes severe vision loss in children. We reported the TSPAN12 variants' frequency in a cohort of FEVR and five novel TSPAN12 variants and related clinical features in six Chinese families. Seven hundred thirty-four families' genetic in-house data were reviewed. Whole-exome sequencing (WES) was performed in all probands; Sanger sequencing was conducted in the family members. Five novel variants from six families were noted, and clinical data were collected. Luciferase assays were applied to test the activity of the Norrin/β-catenin signal caused by the mutant TSPAN12 genes. The frequency of TSPAN12 variants in FEVR is 8.79% (50/569). Five novel variants in TSPAN12 were identified in six families, including two missense variants, c.476G > A(p.Cys159Tyr) and c.81T > G(p.Ser27Arg), two frameshift variants, c.628_629insA(p.Met210Asnfs*42) and c.251delG(p.Gly84Glufs*3) and one nonsense, c.352G > T(p.Glu118*). Low vision, high myopia, nystagmus, and leukocoria are the common symptom at the first presentation. All variants were also predicted as pathogenic in silico. Moreover, the luciferase assay demonstrated that all variants caused severely compromised Norrin/β-catenin signaling activity. In conclusion, the frequency of TSPAN12 variants in FEVR was 8.79% in our cohort. Five novel variants of TSPAN12 were identified. Moreover, we demonstrated the dysfunction of mutant variants via the downregulation of Norrin/β-catenin signaling. These findings expanded the genetic and clinical spectrum of FEVR with TSPAN12 variants.

A comprehensive functional analysis on the pathogenesis of novel TSPAN12 and NDP variants in familial exudative vitreoretinopathy

Familial exudative vitreoretinopathy (FEVR) is an inherited blinding disorder; however, the known FEVR-associated variants account for approximately only 50% cases. Currently, the pathogenesis of most reported variants is not well studied, we aim to identify novel variants from FEVR-associated genes and perform a comprehensive functional analysis to uncover the pathogenesis of variants that cause FEVR. Using targeted gene panel and Sanger sequencing, we identified six novel and three known variants in TSPAN12 and NDP. These variants were demonstrated to cause significant inhibition of Norrin/β-catenin pathway by dual-luciferase reporter assay and western blot analysis. Structural analysis and co-immunoprecipitation revealed compromised interactions between missense variants and binding partners in the Norrin/β-catenin pathway. Immunofluorescence and subcellular protein extraction were performed to reveal the abnormal subcellular trafficking. Additionally, over-expression of TSPAN12 successfully enhanced the Norrin/β-catenin signaling activity by strengthening the binding affinity of mutant Norrin with FZD4 or LRP5. Together, these observations expanded the spectrum of FEVR-associated variants for the genetic counseling and prenatal diagnosis of FEVR, as well providing a potential therapeutic strategy for the treatment of FEVR.

A novel frameshift variant in the TSPAN12 gene causes autosomal dominant FEVR

BACKGROUND

Familial exudative vitreoretinopathy (FEVR) is an inherited blinding eye disease with abnormal retinal vascular development. We aim to broaden the variant spectrum of FEVR and provide a basis for molecular diagnosis and genetic consultation.

METHODS

We recruited five FEVR patients from one large Chinese family. Whole-exome sequencing (WES) and Sanger sequencing were applied to sequence, analyze, and verify variants on genomic DNA samples. Immunocytochemistry, western blot, qPCR, and luciferase assay were performed to test the influence of the variant on the protein expression and activity of the Norrin/β-catenin pathway.

RESULTS

We identified a novel heterozygous frameshift variant c.533dupC (p.D179Rfs*6) in Tetraspanin 12 (TSPAN12) gene that is related to FEVR. This variant caused degradation of the entire TSPAN12 protein, which failed to activate Norrin/β-catenin signaling, possibly causing FEVR.

CONCLUSION

Our study revealed a novel frameshift variant D179Rfs*6 in TSPAN12 that is inherited in an autosomal dominant manner. We found that D179Rfs*6 caused a failure to activate Norrin/β-catenin signaling. This finding broadens the variant spectrum of TSPAN12 and provides invaluable information for the molecular diagnosis of FEVR.