Characterization of a variant of gap junction protein α8 identified in a family with hereditary cataract

Longitudinal study of microphthalmia in connexin 50 knockout mice using spectral-domain optical coherence tomography

Connexin 50 (Cx50) mediated signaling is essential for controlling the lens growth and size. Cx50 mutations cause microphthalmia, smaller lenses, and cataracts in humans and animals. These ocular defects have never been investigated in live Cx50 mutant mice by using non-invasive imaging techniques. Here, we report a longitudinal study of the ocular defects in Cx50 knockout (Cx50KO) mice from the ages of 3 weeks to 12 months by using spectral-domain optical coherence tomography (SD-OCT). The anterior chamber depth (ACD), lens thickness (LT), vitreous chamber depth (VCD), and axial length (AL) were measured along the visual axis and adjusted with corresponding refractive indices. The SD-OCT image data confirm age-related reductions of LT and AL in live Cx50KO mice compared to age-matched wild-type (WT) controls, and the reduction values are comparable to the in vitro measurements of Cx50KO eyeballs and lenses reported previously. Moreover, reductions of ACD were observed in Cx50KO mice at all ages studied while VCD changes are statistically insignificant in comparison to the WT controls. Therefore, Cx50KO's microphthalmia with small lens is selectively associated with delayed ACD development but not the vitreous formation. This work supports the notion that lens size and/or growth is important for anterior chamber development.

Genetic changes and testing associated with childhood glaucoma: A systematic review

Many forms of childhood glaucoma have been associated with underlying genetic changes, and variants in many genes have been described. Currently, testing is variable as there are no widely accepted guidelines for testing. This systematic review aimed to summarize the literature describing genetic changes and testing practices in childhood glaucoma. This systematic review was conducted in accordance with the Preferred Reporting Items for Systematic review and Meta-Analyses (PRISMA) 2020 guidelines and registered with Prospero (ID CRD42023400467). A comprehensive review of Pubmed, Embase, and Cochrane databases was performed from inception through March 2, 2023 using the search terms: (glaucoma) AND (pediatric OR childhood OR congenital OR child OR infant OR infantile) AND (gene OR genetic OR genotype OR locus OR genomic OR mutation OR variant OR test OR screen OR panel). Information was extracted regarding genetic variants including genotype-phenotype correlation. Risk of bias was assessed using the Newcastle-Ottawa Scale. Of 1,916 records screened, 196 studies met inclusion criteria and 53 genes were discussed. Among study populations, mean age±SD at glaucoma diagnosis was 8.94±9.54 years and 50.4% were male. The most common gene discussed was CYP1B1, evaluated in 109 (55.6%) studies. CYP1B1 variants were associated with region and population-specific prevalence ranging from 5% to 86% among those with primary congenital glaucoma. MYOC variants were discussed in 31 (15.8%) studies with prevalence up to 36% among patients with juvenile open angle glaucoma. FOXC1 variants were discussed in 25 (12.8%) studies, which demonstrated phenotypic severity dependent on degree of gene expression and type of mutation. Overall risk of bias was low; the most common domains of bias were selection and comparability. Numerous genes and genetic changes have been associated with childhood glaucoma. Understanding the most common genes as well as potential genotype-phenotype correlation has the potential to improve diagnostic and prognostic outcomes for children with glaucoma.

Characterization of a Novel Gja8 (Cx50) Mutation in a New Cataract Rat Model

Purpose

To describe a novel spontaneous cataract inbred strain isolated from large-scale breeding SD rats, identify the responsible gene mutation, and understand how this mutation affects lens function.

Methods

Exome sequencing of 12 cataract-associated genes was performed in the affected and healthy relatives. Sequences of rat wild-type or mutant gap junction protein alpha 8 gene (Gja8) were transfected into cells. The expression level of protein was assayed by Western blot analysis. Subcellular localization of connexin 50 (Cx50) was analyzed in confocal fluorescent images. Wound-healing, 5-ethynyl-2'-deoxyuridine incorporation, and attachment assay were performed to characterize the cell migration, proliferation and adhesion.

Results

The abnormality was found to be inheritable in an autosomal semi-dominant pattern through different mating patterns. We found a G to T transversion at codon 655 in Gja8, leading to a substitution of valine by phenylalanine (p.V219F). Gja8V219F/+ heterozygotes expressed nuclear cataract while Gja8V219F/V219F homozygotes manifested microphthalmia in addition to cataract. Histology revealed fiber disorders and loss of organelle-free zone in the mutant lens. Cx50V219F altered its location in HeLa cells and inhibited the proliferation, migration and adhesion abilities of HLEB3 cells. The mutation also reduced the expression of focal adhesion kinase and its phosphorylation.

Conclusions

The c.655G>T mutation (p.V219F) is a novel mutation in Gja8, inducing semi-dominant nuclear cataracts in a new spontaneous cataract rat model. The p.V219F mutation altered Cx50 distribution, inhibited lens epithelial cell proliferation, migration, and adhesion, and disrupted fiber cell differentiation. As a consequence, the nuclear cataract and small lens formed.

Identification of a New Mutation p.P88L in Connexin 50 Associated with Dominant Congenital Cataract

Congenital hereditary cataract is genetically heterogeneous and the leading cause of visual impairment in children. Identification of hereditary causes is critical to genetic counselling and family planning. Here, we examined a four-generation Chinese pedigree with congenital dominant cataract and identified a new mutation in GJA8 via targeted exome sequencing. A heterozygous missense mutation c.263C > T, leading to a proline-to-Leucine conversion at the conserved residue 88 in the second transmembrane domain of human connexin 50 (Cx50), was identified in all patients but not in unaffected family members. Functional analyses of the mutation revealed that it disrupted the stability of Cx50 and had a deleterious effect on protein function. Indeed, the mutation compromised normal membrane permeability and gating of ions, and impeded cell migration when overexpressed. Together, our results expand the pathogenic mutation spectrum of Cx50 underlying congenital cataract and lend more support to clinical diagnosis and genetic counseling.

Expressional and functional involvement of gap junctions in aqueous humor outflow into the ocular trabecular meshwork of the anterior chamber

PURPOSE

The ocular trabecular meshwork (TM) responsible for aqueous humor (AH) drainage is crucial for regulating intraocular pressure (IOP) of the eye. An IOP elevation that causes distended TM is involved in the pathogenesis of glaucoma, suggesting intercellular connections important for the TM pathophysiology. The goal of this study was to examine whether gap junction proteins between endothelial cells in the TM are expressional and functional.

METHODS

The expression levels of the gap junction channels in normal human TM cells were determined with real-time PCR and western blot assays. Immunohistochemistry (IHC) staining was performed to examine the localization of gap junction proteins in normal human TM cells and tissues. IOP and the outflow of AH were measured after intercameral injection of gap junction blockers in C57/BL6 mice.

RESULTS

Gap junction proteins GJA1, GJA8, GJB6, and GJC1 were robustly expressed in human TM cells from three individuals. Among the four gap junction channels, GJA1 and GJA8 exhibited the most abundance in the TM. The IHC analysis further confirmed that these proteins were expressed on the membrane between adjacent cells. In the human TM tissues, GJA1, GJA8, GJB6, and GJC1 were also observed along the trabecular beams. Inhibition of gap junctions with intracameral injection of blockers resulted in a statistically significant increase in aqueous humor outflow resistance and IOP elevation in mice.

CONCLUSIONS

The GJA1 and GJA8 gap junction proteins, in particular, are robustly expressed in human TM cells and tissues. Pharmacological inhibition of gap junction channels causes an increased resistance of AH outflow and an elevation of IOP in mice. The present findings suggest the functional role of gap junction channels for regulation of AH outflow in the TM, and activation of gap junctions might represent a therapeutic strategy for treatment of glaucoma.

New GJA8 variants and phenotypes highlight its critical role in a broad spectrum of eye anomalies

GJA8 encodes connexin 50 (Cx50), a transmembrane protein involved in the formation of lens gap junctions. GJA8 mutations have been linked to early onset cataracts in humans and animal models. In mice, missense mutations and homozygous Gja8 deletions lead to smaller lenses and microphthalmia in addition to cataract, suggesting that Gja8 may play a role in both lens development and ocular growth. Following screening of GJA8 in a cohort of 426 individuals with severe congenital eye anomalies, primarily anophthalmia, microphthalmia and coloboma, we identified four known [p.(Thr39Arg), p.(Trp45Leu), p.(Asp51Asn), and p.(Gly94Arg)] and two novel [p.(Phe70Leu) and p.(Val97Gly)] likely pathogenic variants in seven families. Five of these co-segregated with cataracts and microphthalmia, whereas the variant p.(Gly94Arg) was identified in an individual with congenital aphakia, sclerocornea, microphthalmia and coloboma. Four missense variants of unknown or unlikely clinical significance were also identified. Furthermore, the screening of GJA8 structural variants in a subgroup of 188 individuals identified heterozygous 1q21 microdeletions in five families with coloboma and other ocular and/or extraocular findings. However, the exact genotype-phenotype correlation of these structural variants remains to be established. Our data expand the spectrum of GJA8 variants and associated phenotypes, confirming the importance of this gene in early eye development.