Identification and functional analysis of two novel connexin 50 mutations associated with autosome dominant congenital cataracts

Four mutations identified in Chinese families with autosomal dominant congenital cataracts by next-generation sequencing

BACKGROUND

Congenital cataracts, which can arise due to a combination of factors like environmental influences and genetic predisposition, significantly impact children's visual health globally. The occurrence rate of congenital cataracts varies from 0. 63 to 9.74 per 10,000 births. There are 7.4 instances per 10,000 children, with the highest occurrence seen in Asia. Symptoms of the disease include clouding of the lens and visual impairment. Timely identification of the condition plays a crucial role in the management and outlook of pediatric patients.

OBJECTIVE

This investigation aimed to discover causative mutations in four separate Chinese family lineages.

METHODS

The detailed clinical data and family history of four Chinese families with autosomal dominant congenital cataracts were carefully documented. Examination of the Whole Exome Sequencing was utilized to identify the genetic anomalies present in the familial cases. Subsequent validation of the identified mutations was carried out using PCR and Sanger sequencing. Following this, various computational predictive programs were utilized to evaluate how the mutations impact the structure and function of the protein.

RESULTS

The sequencing results reveal four potential disease-causing mutations: c.436G > A (p.V146M) of CRYBB2 Family 1, c.26G > T (p.R9I) of GJA3 in family 2, c.227G > A (p.R76H) of GJA8 in family 3, c.-168G > T of FTL in family 4. Among them, the causative mutation in Family GJA3 is novel, and Family FTL is a rare cataract syndrome. These familial mutations showed complete co-segregation with the affected individuals, with no presence in unaffected family members or the 100 controls. Several bioinformatic prediction tools also support the likely pathogenicity of these mutations.

CONCLUSION

Our findings expand the mutational and phenotypic spectrum of genes associated with congenital cataracts and provide clues to the pathogenesis of congenital cataracts. These data also demonstrate the importance of NGS technology for the molecular diagnosis of congenital cataract patients.

Novel and known variants in GJA3 and LIM2 in congenital cataract families from North India

BACKGROUND

To identify the underlying genetic defects in autosomal dominant (ADCC) and autosomal recessive (ARCC) congenital cataract families from North India.

METHODS

Detailed family histories were collected, pedigrees drawn followed by slit-lamp examination and lens photography. Mutation screening was performed using Sanger sequencing in the known candidate genes for crystallins, connexins, and membrane proteins. The pathogenicity of identified variants was assessed bioinformatically.

RESULTS

In two ADCC families (CC-281 and CC-3015) with posterior lenticonus cataract, a novel change c.263C > T (p.P88L) in GJA3 in CC-281 family and a previously reported substitution c.388C > T (p.R130C) in LIM2 in CC-3015 family was observed. In an ARCC family (CC-3005) having central pulverulent cataract, a novel frameshift deletion (c.764delT;p.L255R46fs) in GJA3 was detected. The observed variants segregated completely with phenotypes in the affected members and were neither present in unaffected family members nor in the ethnically matched 150 controls (tested for two novel variants), hence excluding these as polymorphisms.

CONCLUSIONS

Present study identified two novel mutations i.e., c.263C > T;p.P88L and c.764delT;p.L255R46fs in GJA3 in an ADCC and an ARCC family having posterior lenticonus and central pulverulent cataract, respectively. In another ADCC family with posterior lenticonus cataract, a previously reported mutation c.388C > T;p.R130C in LIM2 was observed. R130 may be a mutation hotspot as previously ADCC families from different ethnicities (UK/Czechia, China, Spain, Japan) also harbored the same substitution, however, with different phenotypes i.e., nuclear pulverulent, membranous, nuclear, lamellar, and sutural/lamellar. Findings in present study thus expand the mutation spectrum and phenotypic heterogeneity linked with GJA3 and LIM2.

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.

Identification and functional analysis of two GJA8 variants in Chinese families with eye anomalies

In this study, we report on two different GJA8 variants related to congenital eye anomalies in two unrelated families, respectively. GJA8 (or Cx50) encoding a transmembrane protein to form lens connexons has been known as a common causative gene in congenital cataracts and its variants have recently been reported related to a wide phenotypic spectrum of eye defects. We identified two GJA8 variants, c.134G>T (p.Try45Leu, W45L) detected in a cataract family by Sanger sequencing and c.281G>A (p.Gly94Glu, G94E) found in a family with severe eye malformations including microphthalmia by whole-exome sequencing. These two variants were absent in healthy population and predicted deleterious by bioinformatic analysis. Furthermore, we compared the expression in cell lines between these mutants and the wildtype to explore their potential mechanism. Cell counting kit-8 assay showed that overexpression of either W45L or G94E decreased cell viability compared with wild-type Cx50 and the control. A lower protein level in W45L found by western blotting and fewer punctate fluorescent signals showed by fluorescence microscopy suggested that W45L may have less protein expression. A higher G94E protein level and abundant dotted distribution indicated that G94E may cause aberrant protein degradation and accumulation. Such results from in vitro assays confirmed the impact of these two variants and gave us a hint about their different pathogenic roles in different phenotypes. In conclusion, our study is the first to have the functional analysis of two GJA8 variants c.134G>T and c.281G>A in Chinese pedigrees and explore the impact of these variants, which can help in prenatal diagnosis and genetic counseling as well in basic studies on GJA8.

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.

Congenital cataract - clinical and morphological aspects

Congenital cataract is one of the main causes of blindness in newborns and children. According to the World Health Organization (WHO), there are about 14 million children suffering from congenital cataract. Our study is based on 82 children, males – 46 (56.1%) and females – 36 (43.9%), with congenital cataract operated in the same ophthalmological centre in Bucharest, Romania. Of the 82 patients, 49 (59.76%) had bilateral cataract and 33 (40.24%) unilateral cataract. Clinically, the most frequent was the total cataract, followed by lamellar, nuclear and cerulean. We employed nine surgical approaches in our patients, depending on the type of intraocular lens (IOL). Morphologically, obvious changes were rendered evident at the level of anterior and posterior capsules, as well as subcapsular.

A new heterozygous mutation in the stop codon of CRYAB (p.X176Y) is liable for congenital posterior pole cataract in a Chinese family

Background: The present study aims to identify the underlying genetic defects in a Chinese family with autosomal dominant congenital cataracts (ADCC).Methods: Detailed family histories and clinical data were recorded. Targeted exome sequencing of 54 known cataract-associated genes combined with high-throughput next-generation sequencing was conducted followed by Sanger sequencing and bioinformatic analysis to identify the causative gene lesion for the family.Results: A four-generation Chinese family with posterior pole type cataract were enrolled. Enrichment of targeted genes revealed a new heterozygous p.X176Y mutation in the stop codon of αB-crystallin (CRYAB) gene, which resulted in the loss of the stop codon and prolongation of the mutant protein by 19 amino acid residues (p.X176Yfs19*). Sanger sequencing showed complete co-segregation with the disease. The elongated mutant protein was predicted to be pathogenic by forming new α-helix and random-coil in the secondary structure as well as producing an extended strand in the tertiary structure, potentially leading to increased hydrophobicity and reduced protein stability.Conclusions: Our report added a new mutation in the spectrum of congenital cataracts. The data suggested that X176 residue in the COOH-terminal is of crucial importance for the αB-crystallin protein function which was valuable for further study of the pathogenesis of congenital cataracts.Abbreviations: CRYAB: αB-crystallin; DNA: deoxyribonucleic acid; PCR: polymerase chain reaction; TES: targeted exome sequencing; ACD: αB-crystallin domain.

Mutational analysis of BFSP1, CRYBB1, GALK1, and GJA8 in captive-bred vervet monkeys (Chlorocebus aethiops)

BACKGROUND

Congenital cataract has been reported in a colony of captive-bred vervet monkeys (Chlorocebus aethiops).

METHODS

Molecular tools such as genotyping and gene expression were used to identify mutations associated with congenital cataract in this vervet colony. Beaded filament structural protein 1 (BFSP1), beta-crystallin B1 (CRYBB1), galactokinase1 (GALK1), and gap junction alpha-8 protein (GJA8) were screened, sequenced, and analyzed for mutations in 24 vervet monkeys (control and cataract).

RESULTS

Five missense sequence variants were identified (V147E, A167P, L212F, N55K, and T247A), three of which were found to be potentially disease-causing. Furthermore, downregulation was observed in BFSP1, CRYBB1, and GALK1 genes.

CONCLUSION

This study reports two cases of incomplete penetrance and/or uniparental disomy (L212F and T247A) in BSFP1. Mutations in BSFP1 together with three mutations in GALK1 and GJA8 were predicted to be disease-causing.

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.

Characterization of a p.R76H mutation in Cx50 identified in a Chinese family with congenital nuclear cataract

BACKGROUND/PURPOSE

A three-generation Chinese family with autosomal dominant congenital nuclear cataract was recruited. This study aimed to identify the disease-causing gene for nuclear cataract with functional dissections of the identified mutant.

METHODS

Detailed clinical data and family history were recorded. Candidate gene sequencing was performed to identify the disease-causing mutation. Recombinant connexin50 (Cx50) wild type and mutant constructs were synthesized. Triton X-100 solubility and subcellular localization of the recombinant Cx50 proteins were analyzed in HeLa cells. Apoptosis was assayed as the percentage of fragmented nuclei in transfected cells.

RESULTS

All affected individuals in the family displayed clear phenotypes of dense nuclear cataracts. A c.227 G > A variation was found in the coding region of Cx50, which arginine residue at position 76 was substituted by histidine (p.R76H). This mutation was co-segregated with the disease in the family, and was not observed in 110 unrelated Chinese controls. No statistically significant differences were found in the Triton X-100 solubility and apoptosis rate between wild type and mutant Cx50 in HeLa cells. However, Cx50 mutant was unable to form gap junctional plaques between adjacent cells as the wild type proteins did.

CONCLUSION

This study identified a novel cataract phenotype caused by the p.R76H mutation in Cx50, providing evidence of further phenotypic heterogeneity associated with this mutation. Functional analysis showed that the mutation affected the formation of gap junction channels and led to opacity in the lens.

A novel missense mutation of GJA8 causes congenital cataract in a large Mauritanian family

OBJECTIVE OF THE STUDY

Inborn lens opacity is the most frequent cause of childhood blindness. In this study, we aimed to define the presumed genetic cause of a congenital cataract present in a Mauritanian family over the last nine generations.

METHODS

A family history of the disease and eye examination were carried out for the family members. Next-generation sequencing using a panel of 116 cataract underlying genes was selectively conducted on the proband's DNA. Nucleotide and amino acid changes and their impact on the phenotype were evaluated using various data analyzing software.

RESULTS

Congenital nuclear cataract, with autosomal dominant mode, was observed in the family. All patients had consequences on their vision in the first 2 years of life. Genetic screening revealed a new mutation c.166A>C (p.Thr56Pro) in GJA8, encoding the Cx50 α-connexin protein. This mutation co-segregated in all patients and was not observed in the unaffected family members and controls. The predicted secondary structure impacted by p.Thr56Pro revealed a localized disruption, in the first extra membrane loop of the wild-type sheet, which is replaced in the mutant protein by a turn then a coil. This conformational change was functionally predicted as probably damaging.

CONCLUSION

A new mutation (c.166A>C) in GJA8 underlying a nuclear congenital cataract was identified in this study. Its segregation with the phenotype might be useful as a predicting marker of the disease.

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.

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

Congenital cataract is a clinically and genetically heterogeneous disease. The present study was undertaken to find the genetic cause of congenital cataract families. DNA samples of a large consanguineous Pakistani family were genotyped with a high resolution single nucleotide polymorphism Illumina microarray. Homozygosity mapping identified a homozygous region of 4.4 Mb encompassing the gene GJA3. Sanger sequence analysis of the GJA3 gene revealed a novel homozygous variant c.950dup p.(His318ProfsX8) segregating in an autosomal recessive (AR) manner. The previously known mode of inheritance for GJA3 gene mutations in cataract was autosomal dominant (AD) only. The screening of additional probands (n = 41) of cataract families revealed a previously known mutation c.56C>T p.(Thr19Met) in GJA3 gene. In addition, sequencing of the exon-intron boundaries of the GJA8 gene in 41 cataract probands revealed two additional mutations: a novel c.53C>T p.(Ser18Phe) and a known c.175C>G p.(Pro59Ala) mutation, both co-segregating with the disease phenotype in an AD manner. All these mutations are predicted to be pathogenic by in silico analysis and were absent in the control databases. In conclusion, results of the current study enhance our understanding of the genetic basis of cataract, and identified the involvement of the GJA3 in the disease etiology in both AR and AD manners.

Identification of a novel GJA3 mutation in a large Chinese family with congenital cataract using targeted exome sequencing

Autosomal dominant congenital cataract (ADCC) is a clinically and genetically heterogeneous ocular disease in children that results in serious visual impairments or even blindness. Targeted exome sequencing (TES) is an efficient method used for genetic diagnoses of inherited diseases. In the present study, we used a custom-made TES panel to identify the genetic defect of a four-generation Chinese family with bilateral pulverulent nuclear cataracts. A novel heterozygous missense mutation c.443C>T (p. T148I) in GJA3 was identified. The results of the bioinformatic analysis showed that the mutation was deleterious to the structure and hemichannel function of Cx46 encoded by GJA3. Plasmids expressing wild-type and mutant human Cx46 were constructed and ectopically expressed in human lens epithelial cells (HLECs) or human embryonic kidney (HEK-293) cells. Fluorescent images indicated aggregated signals of mutant protein in the cytoplasm, and a higher protein level was also detected in T148I stable cell lines. In summary, we identified a novel mutation in GJA3 for ADCC, which provided molecular insights into the pathogenic mechanism of ADCC.

Screening, genetics, risk factors, and treatment of neonatal cataracts

Neonatal cataracts remain the most common cause of visual loss in children worldwide and have diverse, often unknown, etiologies. This review summarizes current knowledge about the detection, treatment, genetics, risk factors, and molecular mechanisms of congenital cataracts. We emphasize significant progress and topics requiring further study in both clinical cataract therapy and basic lens research. Advances in genetic screening and surgical technologies have improved the diagnosis, management, and visual outcomes of affected children. For example, mutations in lens crystallins and membrane/cytoskeletal components that commonly underlie genetically inherited cataracts are now known. However, many questions still remain regarding the causes, progression, and pathology of neonatal cataracts. Further investigations are also required to improve diagnostic criteria for determining the timing of appropriate interventions, such as the implantation of intraocular lenses and postoperative management strategies, to ensure safety and predictable visual outcomes for children. Birth Defects Research 109:734-743, 2017. © 2017 Wiley Periodicals, Inc.

Inherited Congenital Cataract: A Guide to Suspect the Genetic Etiology in the Cataract Genesis

Cataracts are the principal cause of treatable blindness worldwide. Inherited congenital cataract (CC) shows all types of inheritance patterns in a syndromic and nonsyndromic form. There are more than 100 genes associated with cataract with a predominance of autosomal dominant inheritance. A cataract is defined as an opacity of the lens producing a variation of the refractive index of the lens. This variation derives from modifications in the lens structure resulting in light scattering, frequently a consequence of a significant concentration of high-molecular-weight protein aggregates. The aim of this review is to introduce a guide to identify the gene involved in inherited CC. Due to the manifold clinical and genetic heterogeneity, we discarded the cataract phenotype as a cardinal sign; a 4-group classification with the genes implicated in inherited CC is proposed. We consider that this classification will assist in identifying the probable gene involved in inherited CC.