Delineation of Novel Autosomal Recessive Mutation in GJA3 and Autosomal Dominant Mutations in GJA8 in Pakistani Congenital Cataract Families

GJA3 Genetic Variation and Autosomal Dominant Congenital Cataracts and Glaucoma Following Cataract Surgery

Importance

The p.Asp67Tyr genetic variant in the GJA3 gene is responsible for congenital cataracts in a family with a high incidence of glaucoma following cataract surgery.

Objective

To describe the clinical features of a family with a strong association between congenital cataracts and glaucoma following cataract surgery secondary to a genetic variant in the GJA3 gene (NM_021954.4:c.199G>T, p.Asp67Tyr).

Design, Setting, and Participants

This was a retrospective, observational, case series, genetic association study from the University of Iowa spanning 61 years. Examined were the ophthalmic records from 1961 through 2022 of the family members of a 4-generation pedigree with autosomal dominant congenital cataracts.

Main Outcomes and Measures

Frequency of glaucoma following cataract surgery and postoperative complications among family members with congenital cataract due to the p.Asp67Tyr GJA3 genetic variant.

Results

Medical records were available from 11 of 12 family members (7 male [63.6%]) with congenital cataract with a mean (SD) follow-up of 30 (21.7) years (range, 0.2-61 years). Eight of 9 patients with congenital cataracts developed glaucoma, and 8 of 8 patients who had cataract surgery at age 2 years or younger developed glaucoma following cataract surgery. The only family member with congenital cataracts who did not develop glaucoma had delayed cataract surgery until 12 and 21 years of age. Five of 11 family members (45.5%) had retinal detachments after cataract extraction and vitrectomy. No patients developed retinal detachments after prophylactic 360-degree endolaser.

Conclusions and Relevance

The GJA3 genetic variant, p.Asp67Tyr, was identified in a 4-generation congenital cataract pedigree from Iowa. This report suggests that patients with congenital cataract due to some GJA3 genetic variants may be at especially high risk for glaucoma following cataract surgery. Retinal detachments after cataract extraction in the first 2 years of life were also common in this family, and prophylactic retinal endolaser may be indicated at the time of surgery.

Identification of a Missense Mutation in GJA8 Gene in an Iranian Family with Autosomal Dominant Congenital Cataract

Purpose

To identify the causative mutations of autosomal dominant (AD) congenital cataracts in a large Iranian family.

Methods

The complete and accurate family history and clinical information of participants were collected. A total of 51 family members, including 22 affected and 29 unaffected individuals, were recruited in this study. We performed whole exome sequencing to reveal pathogenic mutation. We used amplification refractory mutation system polymerase chain reaction and Sanger sequencing techniques to confirm segregation in patients and also to rule it out in the healthy participants.

Results

A known missense mutation, c.827C>T (S276F), in GJA8 was identified. This mutation was confirmed in all patients. Neither all healthy family members nor 100 healthy individuals who served as controls from general population had this mutation.

Conclusion

The missense mutation c. 827C>T in the GJA8 gene is associated with AD congenital lamellar cataract with complete penetrance in a six-generation Iranian family.

Targeted gene sequencing of FYCO1 identified a novel mutation in a Pakistani family for autosomal recessive congenital cataract

Background

Congenital cataract is causing one‐third of blindness worldwide. Congenital cataract is heterogeneous in its inheritance patterns. The current study is aimed to explore the unknown genetic causes underlying congenital cataracts.

Methods

Blood samples from affected and normal individuals of n = 25 Pakistani families identified with congenital cataracts were collected. Genomic DNA was extracted and Sanger sequencing was performed to identify novel pathogenic variants in the FYCO1 (MIM#607182) gene. Later structural bioinformatics tools and molecular dynamics simulations were performed to analyze the impact of these variants on protein structure and function.

Results

Sanger sequencing resulted in the identification of a novel splice site mutation (NM_024513.3: c.3151‐29_3151‐7del) segregating in an autosomal recessive manner. This novel variant was confirmed to be absent in the n = 300 population controls. Further, bioinformatics tools revealed the formation of a mutant protein with a loss of the Znf domain. In addition, we also found a previously known (c.4127 T > C; p.Leu1376Pro) mutation in four families. We also report a novel heterozygous variant (c.3419G > A; p.Arg1140Gln) in another family.

Conclusions

In conclusion, we report a novel deletion (NM_024513.3: c.3151‐29_3151‐7del) in one family and a frequent homozygous missense mutation (c.4127 T > C; p.Leu1376Pro) in four Pakistani families. The current research highlights the importance of autophagy in lens development and maintaining its transparency.

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.

Novel Homozygous Missense Variant in GJA3 Connexin Domain Causing Congenital Nuclear and Cortical Cataracts

Congenital cataracts (CC) are responsible for approximately one-tenth of childhood blindness cases globally. Here, we report an African American family with a recessively inherited form of CC. The proband demonstrated decreased visual acuity and bilateral cataracts, with nuclear and cortical cataracts in the right and left eye, respectively. Exome sequencing revealed a novel homozygous variant (c.563A > G; p.(Asn188Ser)) in GJA3, which was predicted to be pathogenic by structural analysis. Dominantly inherited variants in GJA3 are known to cause numerous types of cataracts in various populations. Our study represents the second case of recessive GJA3 allele, and the first report in African Americans. These results validate GJA3 as a bona fide gene for recessively inherited CC in humans.

Connexin 46 and connexin 50 gap junction channel properties are shaped by structural and dynamic features of their N-terminal domains

KEY POINTS

Gap junctions formed by different connexins are expressed throughout the body and harbour unique channel properties that have not been fully defined mechanistically. Recent structural studies by cryo-electron microscopy have produced high-resolution models of the related but functionally distinct lens connexins (Cx50 and Cx46) captured in a stable open state, opening the door for structure-function comparison. Here, we conducted comparative molecular dynamics simulation and electrophysiology studies to dissect the isoform-specific differences in Cx46 and Cx50 intercellular channel function. We show that key determinants Cx46 and Cx50 gap junction channel open stability and unitary conductance are shaped by structural and dynamic features of their N-terminal domains, in particular the residue at the 9th position and differences in hydrophobic anchoring sites. The results of this study establish the open state Cx46/50 structural models as archetypes for structure-function studies targeted at elucidating the mechanism of gap junction channels and the molecular basis of disease-causing variants.

ABSTRACT

Connexins form intercellular communication channels, known as gap junctions (GJs), that facilitate diverse physiological roles, from long-range electrical and chemical coupling to coordinating development and nutrient exchange. GJs formed by different connexin isoforms harbour unique channel properties that have not been fully defined mechanistically. Recent structural studies on Cx46 and Cx50 defined a novel and stable open state and implicated the amino-terminal (NT) domain as a major contributor for isoform-specific functional differences between these closely related lens connexins. To better understand these differences, we constructed models corresponding to wildtype Cx50 and Cx46 GJs, NT domain swapped chimeras, and point variants at the 9th residue for comparative molecular dynamics (MD) simulation and electrophysiology studies. All constructs formed functional GJ channels, except the chimeric Cx46-50NT variant, which correlated with an introduced steric clash and increased dynamical behaviour (instability) of the NT domain observed by MD simulation. Single channel conductance correlated well with free-energy landscapes predicted by MD, but resulted in a surprisingly greater degree of effect. Additionally, we observed significant effects on transjunctional voltage-dependent gating (Vj gating) and/or open state dwell times induced by the designed NT domain variants. Together, these studies indicate intra- and inter-subunit interactions involving both hydrophobic and charged residues within the NT domains of Cx46 and Cx50 play important roles in defining GJ open state stability and single channel conductance, and establish the open state Cx46/50 structural models as archetypes for structure-function studies targeted at elucidating GJ channel mechanisms and the molecular basis of cataract-linked connexin variants.

Autosomal recessive cataract (CTRCT18) in the Yakut population isolate of Eastern Siberia: a novel founder variant in the FYCO1 gene

Congenital autosomal recessive cataract with unknown genetic etiology is one of the most common Mendelian diseases among the Turkic-speaking Yakut population (Eastern Siberia, Russia). To identify the genetic cause of congenital cataract spread in this population, we performed whole-exome sequencing (Illumina NextSeq 500) in one Yakut family with three affected siblings whose parents had preserved vision. We have revealed the novel homozygous c.1621C>T transition leading to premature stop codon p.(Gln541*) in exon 8 of the FYCO1 gene (NM_024513.4). Subsequent screening of c.1621C>T p.(Gln541*) revealed this variant in a homozygous state in 25 out of 29 Yakut families with congenital cataract (86%). Among 424 healthy individuals from seven populations of Eastern Siberia (Russians, Yakuts, Evenks, Evens, Dolgans, Chukchi, and Yukaghirs), the highest carrier frequency of c.1621C>T p.(Gln541*) was found in the Yakut population (7.9%). DNA samples of 25 homozygous for c.1621C>T p.(Gln541*) patients with congenital cataract and 114 unaffected unrelated individuals without this variant were used for a haplotype analysis based on the genotyping of six STR markers (D3S3512, D3S3685, D3S3582, D3S3561, D3S1289, and D3S3698). The structure of the identified haplotypes indicates a common origin for all of the studied mutant chromosomes bearing c.1621C>T p.(Gln541*). The age of the с.1621C>T p.(Gln541*) founder haplotype was estimated to be approximately 260 ± 65 years (10 generations). These findings characterize Eastern Siberia as the region of the world with the most extensive accumulation of the unique variant c.1621C>T p.(Gln541*) in the FYCO1 gene as a result of the founder effect.

Molecular and Genetic Mechanism of Non-Syndromic Congenital Cataracts. Mutation Screening in Spanish Families

Our purpose was to identify mutations responsible for non-syndromic congenital cataracts through the implementation of next-generation sequencing (NGS) in our center. A sample of peripheral blood was obtained from probands and willing family members and genomic DNA was extracted from leukocytes. DNA was analyzed implementing a panel (OFTv2.1) including 39 known congenital cataracts disease genes. 62 probands from 51 families were recruited. Pathogenic or likely pathogenic variants were identified in 32 patients and 25 families; in 16 families (64%) these were de novo mutations. The mutation detection rate was 49%. Almost all reported mutations were autosomal dominant. Mutations in crystallin genes were found in 30% of the probands. Mutations in membrane proteins were detected in seven families (two in GJA3 and five in GJA8). Mutations in LIM2 and MIP were each found in three families. Other mutations detected affected EPHA2, PAX6, HSF4 and PITX3. Variants classified as of unknown significance were found in 5 families (9.8%), affecting CRYBB3, LIM2, EPHA2, ABCB6 and TDRD7. Mutations lead to different cataract phenotypes within the same family.

Elongated axial length and myopia-related fundus changes associated with the Arg130Cys mutation in the LIM2 gene in four Chinese families with congenital cataracts

Background

Congenital cataract (CC) is a congenital abnormality characterized by lens opacity present at birth and is associated with highly heterogeneous clinical manifestations. Lens-specific integral membrane protein (LIM2) gene expression is localized to tight junctional domains of different lens fiber membranes. To date, only four mutations in LIM2 have been reported to be associated with congenital or presenile cataracts. Due to the rarity of variants detected in the gene, there is limited progress in understanding the correlation between the genotype and phenotype of patients with mutations in LIM2.

Methods

A total of four Chinese families with CCs were recruited for this study, including three families inheriting in an autosomal dominant (AD) pattern and one sporadic case. Genomic DNA was extracted from the leukocytes of peripheral blood collected from all available patients. Whole-exome sequencing (WES) was performed on all probands and at least one of their parents. Bioinformatics analysis was performed to evaluate the pathogenicity of the candidate variants. Exon 4 of LIM2 was amplified by polymerase chain reaction and directly sequenced. All patients underwent full ocular examinations. This was an observational study to explore the genotype-phenotype relationships in the four families with a common candidate variant.

Results

Various ocular phenotypes were detected in these families, mainly including CCs, elongated axial length, and myopia-related fundus changes. The LIM2 gene mutation, p.Arg130Cys, was detected in all patients. This was further confirmed by Sanger sequencing. The proportion of probands with this mutation in our CCs database was 3.1% (4/130), which indicated that this mutation appears to be a frequent cause of cataracts in the Han Chinese population. This variation has been reported by other investigators before and was correlated with isolated cataracts.

Conclusions

This is the first study that reports various ocular phenotypes associated with the p.Arg130Cys mutation in the LIM2 gene, which indicated the phenotypic heterogeneity of this gene. LIM2 might not only function as an integral membrane protein in lens fiber cells but also be associated with the axial development of the eyeball. Functional studies of the LIM2 gene are important and should receive more attention.

Mutations in FYCO1 identified in families with congenital cataracts

Purpose

This study was designed to identify the pathogenic variants in three consanguineous families with congenital cataracts segregating as a recessive trait.

Methods

Consanguineous families with multiple individuals manifesting congenital cataracts were ascertained. All participating members underwent an ophthalmic examination. A small aliquot of the blood sample was collected from all participating individuals, and genomic DNAs were extracted. Homozygosity-based linkage analysis was performed using short tandem repeat (STR) markers. The haplotypes were constructed with alleles of the STR markers, and the two-point logarithm of odds (LOD) scores were calculated. The candidate gene was sequenced bidirectionally to identify the disease-causing mutations.

Results

Linkage analysis localized the disease interval to chromosome 3p in three families. Subsequently, bidirectional Sanger sequencing identified two novel mutations-a single base deletion resulting in a frameshift (c.3196delC; p.His1066IlefsTer10) mutation and a single base substitution resulting in a nonsense (c.4270C>T; p.Arg1424Ter) mutation-and a known missense (c.4127T>C, p.Leu1376Pro) mutation in FYCO1. All three mutations showed complete segregation with the disease phenotype and were absent in 96 ethnically matched control individuals.

Conclusions

We report two novel mutations and a previously reported mutation in FYCO1 in three large consanguineous families. Taken together, mutations in FYCO1 contribute nearly 15% to the total genetic load of autosomal recessive congenital cataracts in this cohort.

GJA8 missense mutation disrupts hemichannels and induces cell apoptosis in human lens epithelial cells

Autosomal dominant congenital cataract (ADCC), the most common hereditary disease, is a major cause of eye disease in children. Due to its high genetic and clinical heterogeneity, the identification of ADCC-associated gene mutations is essential for the development of molecular therapies. In this study, we examined a four-generation Chinese pedigree with ADCC and identified putative mutations in ADCC candidate genes via next-generation sequencing (NGS) followed by Sanger sequencing. A novel missense mutation in GJA8 (c.T217C) in ADCC patients causes a serine-to-proline substitution at residue 73 of connexin 50 (Cx50); no mutation was found in unaffected family members and unrelated healthy individuals. Functional analysis revealed that this missense mutation disrupts protein function in human lens epithelial cells (HLEpiCs), which fails to form calcium-sensitive hemichannels. Furthermore, mutant Cx50 leads to decreased ROS scavenging by inhibiting G6PD expression and thus induces cell apoptosis via aberrant activation of the unfolded protein response (UPR). In conclusion, we report a novel GJA8 heterozygous mutation in a Chinese family with a vital role in ADCC, broadening the genetic spectrum of this disease.

Autosomal recessive congenital cataracts linked to HSF4 in a consanguineous Pakistani family

PURPOSE

To investigate the genetic basis of autosomal recessive congenital cataracts (arCC) in a large consanguineous Pakistani family.

METHODS

All participating members of family, PKCC074 underwent an ophthalmic examination. Slit-lamp photographs were ascertained for affected individuals that have not been operated for the removal of the cataractous lens. A small aliquot of the blood sample was collected from all participating individuals and genomic DNAs were extracted. A genome-wide scan was performed with polymorphic short tandem repeat (STR) markers and the logarithm of odds (LOD) scores were calculated. All coding exons and exon-intron boundaries of HSF4 were sequenced and expression of Hsf4 in mouse ocular lens was investigated. The C-terminal FLAG-tagged wild-type and mutant HSF4b constructs were prepared to examine the nuclear localization pattern of the mutant protein.

RESULTS

The ophthalmological examinations suggested that nuclear cataracts are present in affected individuals. Genome-wide linkage analyses localized the critical interval to a 10.95 cM (14.17 Mb) interval on chromosome 16q with a maximum two-point LOD score of 4.51 at θ = 0. Sanger sequencing identified a novel missense mutation: c.433G>C (p.Ala145Pro) that segregated with the disease phenotype in the family and was not present in ethnically matched controls. Real-time PCR analysis identified the expression of HSF4 in mouse lens as early as embryonic day 15 with a steady level of expression thereafter. The immunofluorescence tracking confirmed that both wild-type and mutant HSF4 (p.Ala145Pro) proteins localized to the nucleus.

CONCLUSION

Here, we report a novel missense mutation in HSF4 associated with arCC in a familial case of Pakistani descent.

Molecular genetics of congenital cataracts

Congenital cataracts, the most common cause of visual impairment and blindness in children worldwide, have diverse etiologies. According to statistics analysis, about one quarter of congenital cataracts caused by genetic defects. Various mutations of more than one hundred genes have been identified in hereditary cataracts so far. In this review, we briefly summarize recent developments about the genetics, molecular mechanisms, and treatments of congenital cataracts. The studies of these pathogenic mutations and molecular genetics is making it possible for us to comprehend the underlying mechanisms of cataractogenesis and providing new insights into the preventive, diagnostic and therapeutic approaches of cataracts.

Identification and preliminary functional analysis of two novel congenital cataract associated mutations of Cx46 and Cx50

Background: Congenital cataract is a significant cause of visual impairment and blindness. The present study examined the disease-causing mutations in three Chinese families with autosomal dominant congenital cataract (ADCC) to provide the preliminary evidence of the mechanisms underlying congenital cataract formation.Methods: Three pedigrees affected with ADCC were recruited. All participants underwent detailed ophthalmic examinations. Leucocyte DNA was extracted from venous blood for direct sequencing of candidate genes. In silico bioinformatics analysis was conducted to verify the functional impacts of the mutant proteins. Distribution patterns of connexin proteins were assessed through fluorescence microscopy using an enhanced green fluorescent protein (EGFP)-labeled expression vector in stably transfected Hek293 cells.Results: We identified three Chinese pedigrees with ADCC. Family 1 and family 2 presented with pulverized cataract and family 3 with an unknown phenotype. Direct sequencing of family 1 and family 2 revealed a missense mutation of c.64G>A encoding for G22S of connexin46 (Cx46), while a similar c.64G>A encoding for G22S of connexin50 (Cx50) was found in family 3; both mutations co-segregated well within all affected individuals in their families and were absent from 100 unrelated controls. Bioinformatics analysis revealed with high confidence that both mutations were deleterious. Confocal microscopy revealed the accumulation of both mutant connexins in the cytoplasm with punctate staining and a failure of gap junction formation between adjacent cells.Conclusions: Two novel G22S mutations of Cx46 and Cx50 were identified, and preliminary functional analysis revealed a potential deleterious effect of these mutations due to the malfunction of connexins.Abbreviations: ADCC: autosomal dominant congenital cataract; Cx26: connexin26; Cx32: connexin32; Cx46: connexin46; Cx46WT: wild-type connexin46; Cx50: Connexin50; Cx50WT: wild-type connexin50; DAPI: 4',6-diamidino-2-phenylindole; EGFP: enhanced green fluorescent protein; FBS: fetal bovine serum; GJA-:gap junction alpha-; PCR: polymerase chain reaction; PolyPhen: polymorphism phenotyping; PSIC: position-specific independent count; RPMI: Roswell Park Memorial Institute; TM1: first transmembrane.

The impact of GJA3 SNPs on susceptibility to age-related cataract

AIM

To determine the association of gap junction protein alpha 3 (GJA3) gene tag single-nucleotide polymorphisms (SNPs) with susceptibility to age-related cataract (ARC).

METHODS

In total, 486 ARC patients were matched with 500 healthy controls. All the participants underwent complete ophthalmic examinations. Haplotype-tagging SNPs of GJA3 gene were selected from the HapMap Beijing Han Chinese population. Genomic DNA was extracted from the peripheral blood leukocytes of all the subjects. Under three different genetic models: dominant, recessive, and additive, the association between SNPs and ARC was examined. After adjusting for age and sex, the genetic effects of the GJA3 SNPs were evaluated with logistic regression analysis.

RESULTS

Four tag GJA3 SNPs (rs6490519, rs9506430, rs9509053, and rs9552089) were included in the present study. None of the SNPs showed a significant relationship with an altered risk of total ARC under the dominant, recessive, or additive models. In the subgroup analysis, rs9506430 had a significant effect on the formation of a posterior subcapsular cataract (P=0.002, OR: 0.227, 95%CI: 0.088-0.590) under the recessive model.

CONCLUSION

Our study indicates that GJA3 variants may influence the development of posterior subcapsular cataracts. Further studies need to be designed to confirm this possibility.