Whole exome sequencing in dominant cataract identifies a new causative factor, CRYBA2, and a variety of novel alleles in known genes

A novel base substitution mutation of the CRYBA2 gene is associated with autosomal dominant congenital cataract

Congenital cataract is one of the leading causes of vision loss in children, and a large proportion of cases are related to genetics. In a Chinese family, we reported a new missense mutation in CRYBA2 (c.223T>C: p.Tyr75His), which can cause autosomal dominant congenital bilateral cataract. We collected blood samples from family members (mother and two sons) and extracted DNA. Through whole-exome sequencing, we discovered a novel unreported mutation. According to relevant ACMG guidelines, this mutation was determined to be a variant of unknown clinical significance. This article further expands the site information on the CRYBA2 mutations.

Biallelic variants in Plexin B2 (PLXNB2) cause amelogenesis imperfecta, hearing loss and intellectual disability

BACKGROUND

Plexins are large transmembrane receptors for the semaphorin family of signalling proteins. Semaphorin-plexin signalling controls cellular interactions that are critical during development as well as in adult life stages. Nine plexin genes have been identified in humans, but despite the apparent importance of plexins in development, only biallelic PLXND1 and PLXNA1 variants have so far been associated with Mendelian genetic disease.

METHODS

Eight individuals from six families presented with a recessively inherited variable clinical condition, with core features of amelogenesis imperfecta (AI) and sensorineural hearing loss (SNHL), with variable intellectual disability. Probands were investigated by exome or genome sequencing. Common variants and those unlikely to affect function were excluded. Variants consistent with autosomal recessive inheritance were prioritised. Variant segregation analysis was performed by Sanger sequencing. RNA expression analysis was conducted in C57Bl6 mice.

RESULTS

Rare biallelic pathogenic variants in plexin B2 (PLXNB2), a large transmembrane semaphorin receptor protein, were found to segregate with disease in all six families. The variants identified include missense, nonsense, splicing changes and a multiexon deletion. Plxnb2 expression was detected in differentiating ameloblasts.

CONCLUSION

We identify rare biallelic pathogenic variants in PLXNB2 as a cause of a new autosomal recessive, phenotypically diverse syndrome with AI and SNHL as core features. Intellectual disability, ocular disease, ear developmental abnormalities and lymphoedema were also present in multiple cases. The variable syndromic human phenotype overlaps with that seen in Plxnb2 knockout mice, and, together with the rarity of human PLXNB2 variants, may explain why pathogenic variants in PLXNB2 have not been reported previously.

Genetics of bilateral pediatric cataract in the Israeli and Palestinian populations

PURPOSE

Bilateral pediatric cataract (BPC) is one of the most common causes of childhood visual impairment and blindness worldwide. A significant percentage of pediatric cataracts are caused by genetic alterations. We aim to characterize the set of genes and variants that cause BPC in the Israeli and Palestinian populations and to assess genotype-phenotype correlation.

METHODS

Retrospective study in a multidisciplinary center for visual impairment, located in a tertiary medical center. Medical charts of families who underwent genetic counseling because of BPC in a family member were reviewed. Clinical characteristics and genetic tests results were obtained from medical records of affected subjects.

RESULTS

Twenty-two families (47 patients) underwent genetic counseling and completed genetic testing. Causative variants were identified in 18/22 (81.8%) of the families, including 3 novel variants. Genetic testing used included mainly panel for congenital cataracts and whole exome sequencing. Eleven families performed genetic testing with the intention of future pregnancy planning. Main causative genes identified were crystalline genes followed by transcription factor genes. BCOR gene variants were associated with persistent fetal vasculature (PFV) cataract in two of three families.

CONCLUSIONS

Combined molecular techniques are useful in identifying variants causing pediatric cataracts and showed a high detection rate in our population. BCOR gene variants might be associated with PFV type of cataracts. The study of pathogenic variants may aid in family planning and prevention of pediatric cataracts in future pregnancies. Additionally, in certain cases, it assists in diagnosing non-suspected syndromic types of pediatric cataracts.

Novel molecular, structural and clinical findings in an Italian cohort of congenital cataract

The current genetic diagnostic workup of congenital cataract (CC) is mainly based on NGS panels, whereas exome sequencing (ES) has occasionally been employed. In this multicentre study, we investigated by ES the detection yield, mutational spectrum and genotype-phenotype correlations in a CC cohort recruited between 2020 and mid-2022. The cohort consisted of 67 affected individuals from 51 unrelated families and included both non-syndromic (75%) and syndromic (25%) phenotypes, with extra-CC ocular/visual features present in both groups (48% and 76%, respectively). The functional effect of variants was predicted by 3D modelling and hydropathy properties changes. Variant clustering was used for the in-depth assessment of genotype-phenotype correlations. A diagnostic (pathogenic or likely pathogenic) variant was identified in 19 out of 51 probands/families (~37%). In a further 14 probands/families a candidate variant was identified: in 12 families a VUS was detected, of which 9 were considered plausibly pathogenic (i.e., 4 or 5 points according to ACMG criteria), while in 2 probands ES identified a single variant in an autosomal recessive gene associated with CC. Eighteen probands/families, manifesting primarily non-syndromic CC (15/18, 83%), remained unsolved. The identified variants (8 P, 12 LP, 10 VUS-PP, and 5 VUS), half of which were unreported in the literature, affected five functional categories of genes involved in transcription/splicing, lens formation/homeostasis (i.e., crystallin genes), membrane signalling, cell-cell interaction, and immune response. A phenotype-specific variant clustering was observed in four genes (KIF1A, MAF, PAX6, SPTAN1), whereas variable expressivity and potential phenotypic expansion in two (BCOR, NHS) and five genes (CWC27, KIF1A, IFIH1, PAX6, SPTAN1), respectively. Finally, ES allowed to detect variants in six genes not commonly included in commercial CC panels. These findings broaden the genotype-phenotype correlations in one of the largest CC cohorts tested by ES, providing novel insights into the underlying pathogenetic mechanisms and emphasising the power of ES as first-tier test.

Association of variants in GJA8 with familial acorea-microphthalmia-cataract syndrome

Congenital acorea is a rare disease with the absence of a pupil in the eye. To date, only one family and two isolated cases with congenital acorea have been reported. The gene associated with acorea has not been identified. In this study, we recruited a Chinese family acorea-microphthalmia-cataract syndrome. By analyzing the whole-exome sequencing (WES) data of this Chinese family, we revealed the association of a novel heterozygous variant, NM_005267.5:c.137G>A (p.G46E) in the gap junction protein alpha 8 (GJA8) gene encoding connexin 50 or CX50, with familial acorea-microphthalmia-cataract syndrome. Additionally, another variant, NM_005267.5:c.151G>A (p.D51N) in GJA8, was identified to co-segregate with this syndrome in an unrelated Japanese family. Ectopic expression of p.G46E and p.D51N mutant GJA8 genes in cultured cells caused protein mislocalization, suggesting that the p.G46E and p.D51N mutations in GJA8 impaired the function of the gap junction channels. These results established GJA8 as the first gene associated with familial acorea-microphthalmia-cataract syndrome.

Novel CRYGC Mutation in Conserved Ultraviolet-Protective Tryptophan (p.Trp131Arg) Is Linked to Autosomal Dominant Congenital Cataract

Congenital cataract (CC), the most prevalent cause of childhood blindness and amblyopia, necessitates prompt and precise genetic diagnosis. The objective of this study is to identify the underlying genetic cause in a Swiss patient with isolated CC. Whole exome sequencing (WES) and copy number variation (CNV) analysis were conducted for variant identification in a patient born with a total binocular CC without a family history of CC. Sanger Sequencing was used to confirm the variant and segregation analysis was used to screen the non-affected parents. The first de novo missense mutation at c.391T>C was identified in exon 3 of CRYGC on chromosome 2 causing the substitution of a highly conserved Tryptophan to an Arginine located at p.Trp131Arg. Previous studies exhibit significant changes in the tertiary structure of the crystallin family in the following variant locus, making CRYGC prone to aggregation aggravated by photodamage resulting in cataract. The variant can be classified as pathogenic according to the American College of Medical Genetics and Genomics (ACMG) criteria (PP3 + PM1 + PM2 + PS2; scoring 10 points). The identification of this novel variant expands the existing knowledge on the range of variants found in the CRYGC gene and contributes to a better comprehension of cataract heterogeneity.

A novel missense mutation in the CRYBA2 caused autosomal dominant presenile cataract in a Chinese family

Presenile cataract is a relatively rare type of cataract, but its genetic mechanisms are currently not well understood. The precise identification of these causative genes is crucial for effective genetic counseling for patients and their families. The aim of our study was to identify the causative gene associated with presenile cataract in a Chinese family. In February 2020, a four-generation pedigree of presenile cataract patients was recruited at the 2nd Affiliated Hospital of Kunming Medical University. One patient and her healthy husband from the family underwent whole exome sequencing. The variant was validated through sanger sequencing, and co-segregation analysis was conducted in all family members to assess its pathogenicity. Molecular dynamics simulation (MDS) was used to analyze the conformation of both the wild type and pathogenic mutant loci p.Y153H of CRYBA2. We identified presenile cataract in the pedigree, which follows an autosomal-dominant pattern of inheritance. The family includes five clinically affected patients who all developed presenile cataract between the ages from 24 to 30. We confirmed the pathogenicity of a heterozygous missense variant (NM_057093:c.457T >C) in CRYBA2 within this family. The affected amino acid demonstrates high conservation across species. Subsequent sanger sequencing confirmed co-segregation of the disease in all family members. MDS analysis revealed that the p.Y153H mutant disrupted hydrogen bond formation between Y153 and R193 within the two β-strands of the fourth Greek key domain, leading to destabilization of the βA2-crystallin. In conclusion, a novel causative mutation (NM_057093:c.457T>C) in CRYBA2 might contribute to autosomal dominant presenile cataract.

Pitfalls of whole exome sequencing in undefined clinical conditions with a suspected genetic etiology

BACKGROUND

Whole-Exome Sequencing (WES) is a valuable tool for the molecular diagnosis of patients with a suspected genetic condition. In complex and heterogeneous diseases, the interpretation of WES variants is more challenging given the absence of diagnostic handles and other reported cases with overlapping clinical presentations.

OBJECTIVE

To describe candidate variants emerging from trio-WES and possibly associated with the clinical phenotype in clinically heterogeneous conditions.

METHODS

We performed WES in ten patients from eight families, selected because of the lack of a clear clinical diagnosis or suspicion, the presence of multiple clinical signs, and the negative results of traditional genetic tests.

RESULTS

Although we identified ten candidate variants, reaching the diagnosis of these cases is challenging, given the complexity and the rarity of these syndromes and because affected genes are already associated with known genetic diseases only partially recapitulating patients' phenotypes. However, the identification of these variants could shed light into the definition of new genotype-phenotype correlations. Here, we describe the clinical and molecular data of these cases with the aim of favoring the match with other similar cases and, hopefully, confirm our diagnostic hypotheses.

CONCLUSION

This study emphasizes the major limitations associated with WES data interpretation, but also highlights its clinical utility in unraveling novel genotype-phenotype correlations in complex and heterogeneous undefined clinical conditions with a suspected genetic etiology.

Pathogenic genetic variants identified in Australian families with paediatric cataract

Objective

Paediatric (childhood or congenital) cataract is an opacification of the normally clear lens of the eye and has a genetic basis in at least 18% of cases in Australia. This study aimed to replicate clinical gene screening to identify variants likely to be causative of disease in an Australian patient cohort.

Methods and analysis

Sixty-three reported isolated cataract genes were screened for rare coding variants in 37 Australian families using genome sequencing.

Results

Disease-causing variants were confirmed in eight families with variant classification as ‘likely pathogenic’. This included novel variants PITX3 p.(Ter303LeuextTer100), BFSP1 p.(Glu375GlyfsTer2), and GJA8 p.(Pro189Ser), as well as, previously described variants identified in genes GJA3, GJA8, CRYAA, BFSP1, PITX3, COL4A1 and HSF4. Additionally, eight variants of uncertain significance with evidence towards pathogenicity were identified in genes: GJA3, GJA8, LEMD2, PRX, CRYBB1, BFSP2, and MIP.

Conclusion

These findings expand the genotype–phenotype correlations of both pathogenic and benign variation in cataract-associated genes. They further emphasise the need to develop additional evidence such as functional assays and variant classification criteria specific to paediatric cataract genes to improve interpretation of variants and molecular diagnosis in patients.

Case Report: A de novo Variant of CRYGC Gene Associated With Congenital Cataract and Microphthalmia

Background: Congenital cataract is one of the most common causes of blindness in children. A rapid and accurate genetic diagnosis benefit the patients in the pediatric department. The current study aims to identify the genetic defects in a congenital cataract patient without a family history.

Case presentation: A congenital cataract patient with microphthalmia and nystagmus was recruited for this study. Trio-based whole-exome sequencing revealed a de novo variant (c.394delG, p.V132Sfs*15) in CRYGC gene. According to the American College of Medical Genetics and Genomics (ACMG) criteria, the variant could be annontated as pathogenic.

Conclusion: Our findings provide new knowledge of the variant spectrum of CRYGC gene and are essential for understanding the heterogeneity of cataracts in the Chinese population.

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.

Cataract-Causing S93R Mutant Destabilized Structural Conformation of βB1 Crystallin Linking With Aggregates Formation and Cellular Viability

Cataract, opacity of the eye lens, is the leading cause of visual impairment worldwide. The crucial pathogenic factors that cause cataract are misfolding and aggregation of crystallin protein. βB1‐crystallin, which is the most abundant water‐soluble protein in mammalian lens, is essential for lens transparency. A previous study identified the missense mutation βB1‐S93R being responsible for congenital cataract. However, the exact pathogenic mechanism causing cataract remains unclear. The S93 residue, which is located at the first Greek‐key motif of βB1‐crystallin, is highly conserved, and its substitution to Arginine severely impaired hydrogen bonds and structural conformation, which were evaluated via Molecular Dynamic Simulation. The βB1‐S93R was also found to be prone to aggregation in both human cell lines and Escherichia coli. Then, we isolated the βB1‐S93R variant from inclusion bodies by protein renaturation. The βB1-S93R mutation exposed more hydrophobic residues, and the looser structural mutation was prone to aggregation. Furthermore, the S93R mutation reduced the structural stability of βB1-crystallin when incubated at physiological temperature and made it more sensitive to environmental stress, such as UV irradiation or oxidative stress. We also constructed a βB1-S93R cellular model and discovered that βB1-S93R was more sensitive to environmental stress, causing not only aggregate formation but also cellular apoptosis and impaired cellular viability. All of the results indicated that lower solubility and structural stability, sensitivity to environmental stress, vulnerability to aggregation, and impaired cellular viability of βB1-S93R might be involved in cataract development.

A Novel Mutation in CRYGC Mutation Associated with Autosomal Dominant Congenital Cataracts and Microcornea

Purpose

Crystallin protein mutations are associated with congenital cataract (CC), and several disease-causing mutations in the CRYGC gene have been identified. We present the location of a new mutation in CRYGC in members of a Chinese family who presented with CCs with or without microcornea.

Design

Observational study.

Participants

A Chinese family diagnosed with autosomal dominant (AD) CCs with or without microphthalmia.

Methods

Because this was an observational study, it was not registered as a clinical trial. The proband and her 2 children were diagnosed with AD CCs and microcornea and were recruited for the study. Participants underwent complete ophthalmological examinations, and blood samples were used for genomic extraction.

Main Outcome Measures

We detected 1 disease-associated variant using Exomiser analysis by matching the proband’s phenotype and the inheritance pattern. The variant was determined to be pathogenic according to American College of Medical Genetics and Genomics (ACMG) guidelines.

Results

We detected 1 disease-associated variant using Exomiser analysis by matching the proband’s phenotype and the inheritance pattern. The variant was determined to be pathogenic according to the American College of Medical Genetics and Genomics guidelines. Next-generation sequencing was verified using Sanger sequencing, and we confirmed that the proband and her children carried the same mutation. We identified the heterozygous variant c.389_390insGCTG (p.C130fs), which includes a frameshift mutation. The residues in p.C130fs are all highly conserved across species. This disease-causing frameshift mutation in the CRYGC gene is not currently present in the ClinVar database.

Conclusions

Our findings expand the repertoire of known mutations in the CRYGC gene that cause CCs and provide new insights into the etiology and molecular diagnosis of CCs; however, the molecular mechanism of this mutation warrants further investigation.

Inherited cataracts: Genetic mechanisms and pathways new and old

Cataract(s) is the clinical equivalent of lens opacity and is caused by light scattering either by high molecular weight protein aggregates in lens cells or disruption of the lens microarchitecture itself. Genetic mutations underlying inherited cataract can provide insight into the biological processes and pathways critical for lens homeostasis and transparency, classically including the lens crystallins, connexins, membrane proteins or components, and intermediate filament proteins. More recently, cataract genes have been expanded to include newly identified biological processes such as chaperone or protein degradation components, transcription or growth factors, channels active in the lens circulation, and collagen and extracellular matrix components. Cataracts can be classified by age, and in general congenital cataracts are caused by severe mutations resulting in major damage to lens proteins, while age related cataracts are associated with variants that merely destabilize proteins thereby increasing susceptibility to environmental insults over time. Thus there might be separate pathways to opacity for congenital and age-related cataracts whereby congenital cataracts induce the unfolded protein response (UPR) and apoptosis to destroy the lens microarchitecture, while in age related cataract high molecular weight (HMW) aggregates formed by denatured crystallins bound by α-crystallin result in light scattering without severe damage to the lens microarchitecture.

The utility of genomic testing in the ophthalmology clinic: A review

Genomic testing assesses many genes in one test. It is often used in the diagnosis of heterogeneous single gene disorders where pathogenic variation in one of many genes are known to cause similar phenotypes, or where a clinical diagnosis is difficult to reach. In the ophthalmic setting, genomic testing can be used to diagnose several groups of diseases, including inherited retinal dystrophies, paediatric cataract, glaucoma and anterior segment dysgenesis and other syndromic developmental disorders with eye involvement. The testing can encompass several modalities ranging from whole genome sequencing to exome sequencing or targeted gene panels. The advantages to the patient of receiving a molecular diagnosis include an end to the diagnostic odyssey, determination of prognosis and clarification of treatment, access to accurate genetic counselling, and confirming eligibility for clinical trials or genetic specific therapies. Genomic testing is a powerful addition to the diagnosis and management of inherited eye disease.

Genetic Analysis in a Swiss Cohort of Bilateral Congenital Cataract

Importance

Identification of geographic population-based differences in genotype and phenotype heterogeneity are important for targeted and patient-specific diagnosis and treatment, counseling, and screening strategies.

Objective

To report disease-causing variants and their detailed phenotype in patients with bilateral congenital cataract from a single center in Switzerland and thereby draw a genetic map and perform a genotype-phenotype comparison of this cohort.

Design, Setting, and Participants

This clinical and molecular-genetic cohort study took place through the collaboration of the Department of Ophthalmology at the University Hospital Zurich and the Institute of Medical Molecular Genetics, University of Zurich, Schlieren, Switzerland. Thirty-seven patients from 25 families with different types of bilateral congenital cataract were included. All participating family members received a comprehensive eye examination. Whole exome sequencing was performed in the index patients, followed by a filtering process to detect possible disease-associated variants in genes previously described in association with congenital cataract. Probable disease-causing variants were confirmed by Sanger sequencing in available family members. All data were collected from January 2018 to June 2020, and the molecular-genetic analyses were performed from January 2019 to July 2020.

Main Outcomes and Measures

Identification of the underlying genetic causes of bilateral congenital cataract, including novel disease-causing variants and phenotype correlation.

Results

Among the 37 patients (18 [49%] male and 19 [51%] female; mean [SD] age, 17.3 [15.9] years) from 25 families, pathogenic variants were detected in 20 families (80% detection rate), which included 13 novel variants in the following genes: BCOR, COL4A1, CRYBA2, CRYBB2, CRYGC, CRYGS, GJA3, MAF, NHS, and WFS1. Putative disease-causing variants were identified in 14 of 20 families (70%) as isolated cases and in 6 of 20 families (30%) with syndromic cases. A recessive variant in the CRYBB2 gene in a consanguineous family with 2 affected siblings showing a nuclear and sutural cataract was reported in contrast to previously published reports. In addition, the effect on splicing in a minigene assay of a novel splice site variant in the NHS gene (c.[719-2A>G]) supported the pathogenicity of this variant.

Conclusions and Relevance

This study emphasizes the importance of genetic testing of congenital cataracts. Known dominant genes need to be considered for recessive inheritance patterns. Syndromic types of cataract may be underdiagnosed in patients with mild systemic features.

Congenital Cataract and Its Genetics: The Era of Next-Generation Sequencing

Congenital cataract is a challenging ophthalmological disorder which can cause severe visual loss. It can be diagnosed at birth or during the first year of life. Early diagnosis and treatment are crucial for the visual prognosis. It can be associated with various ocular and systemic abnormalities. Determining whether congenital cataract is isolated or associated with other pathology is an indispensable step for the prediction of potential vision as well as early diagnosis and treatment of conditions that can cause morbidity or mortality. Many genes have been identified in the molecular etiology of congenital cataract. Most mutations have been reported in the crystallin genes. Determination of the genetic cause may not only enable individualized genetic counseling but also help to identify concomitant ocular and/or systemic disorders depending on the characteristics of the genetic test used. Recently, next-generation sequencing in particular has become an evolving technology for determining the molecular etiology of congenital cataract and furthering our knowledge of the disease.

The genetic landscape of crystallins in congenital cataract

Background

The crystalline lens is mainly composed of a large family of soluble proteins called the crystallins, which are responsible for its development, growth, transparency and refractive index. Disease-causing sequence variants in the crystallins are responsible for nearly 50% of all non-syndromic inherited congenital cataracts, as well as causing cataract associated with other diseases, including myopathies. To date, more than 300 crystallin sequence variants causing cataract have been identified.

Methods

Here we aimed to identify the genetic basis of disease in five multi-generation British families and five sporadic cases with autosomal dominant congenital cataract using whole exome sequencing, with identified variants validated using Sanger sequencing. Following bioinformatics analysis, rare or novel variants with a moderate to damaging pathogenicity score, were filtered out and tested for segregation within the families.

Results

We have identified 10 different heterozygous crystallin variants. Five recurrent variants were found: family-A, with a missense variant (c.145C>T; p.R49C) in CRYAA associated with nuclear cataract; family-B, with a deletion in CRYBA1 (c.272delGAG; p.G91del) associated with nuclear cataract; and family-C, with a truncating variant in CRYGD (c.470G>A; W157*) causing a lamellar phenotype; individuals I and J had variants in CRYGC (c.13A>C; T5P) and in CRYGD (c.418C>T; R140*) causing unspecified congenital cataract and nuclear cataract, respectively. Five novel disease-causing variants were also identified: family D harboured a variant in CRYGC (c.179delG; R60Qfs*) responsible for a nuclear phenotype; family E, harboured a variant in CRYBB1 (c.656G>A; W219*) associated with lamellar cataract; individual F had a variant in CRYGD (c.392G>A; W131*) associated with nuclear cataract; and individuals G and H had variants in CRYAA (c.454delGCC; A152del) and in CRYBB1 (c.618C>A; Y206*) respectively, associated with unspecified congenital cataract. All novel variants were predicted to be pathogenic and to be moderately or highly damaging.

Conclusions

We report five novel variants and five known variants. Some are rare variants that have been reported previously in small ethnic groups but here we extend this to the wider population and record a broader phenotypic spectrum for these variants.

The relationship between major intrinsic protein genes and cataract

BACKGROUND

Genetic factors play an essential role in the development of cataracts, and the major intrinsic protein (MIP) gene is a type of causative genes. Our study aims to discuss the current research progress of MIP genes responsible for cataractogenesis in DNA and protein levels, which is essential in achieving a response to the molecular deficiencies and pathophysiologic features of cataract.

METHODS

We developed a search strategy using a combination of the words "Cataract", "Mutation", "MIP gene", and "AQP0" to identify all articles from PubMed, Web of Science, Scopus, and Google Scholar up to December 2019. To find more articles and to ensure that databases were thoroughly searched, the reference lists of selected items were also reviewed.

RESULTS

A total of 29 MIP gene mutations causing congenital cataract were obtained by searching these databases and analyzing the results of genetic mutation pathogenicity prediction software tools; most of them caused amino acid codon changes in the H4, H5, H6, C-TIDs, and loop C in the structure of the MIP protein. However, there was no clear causality between lens morphology, phenotypes, and genotypes. The genotype TC in polymorphism c.-4T > C and haplotype CCG of rs2269348, c.-4T > C, and rs74641138 in MIP may attach an additional genetic risk factor for age-related cataract.

CONCLUSION

These single-base mutations and single nucleotide polymorphisms might be importantly involved in the pathogenesis of congenital cataract and age-related cataract, respectively. This review provides a significant reference for clinical trials and theoretical studies.

Connexin-46/50 in a dynamic lipid environment resolved by CryoEM at 1.9 Å

Gap junctions establish direct pathways for cells to transfer metabolic and electrical messages. The local lipid environment is known to affect the structure, stability and intercellular channel activity of gap junctions; however, the molecular basis for these effects remains unknown. Here, we incorporate native connexin-46/50 (Cx46/50) intercellular channels into a dual lipid nanodisc system, mimicking a native cell-to-cell junction. Structural characterization by CryoEM reveals a lipid-induced stabilization to the channel, resulting in a 3D reconstruction at 1.9 Å resolution. Together with all-atom molecular dynamics simulations, it is shown that Cx46/50 in turn imparts long-range stabilization to the dynamic local lipid environment that is specific to the extracellular lipid leaflet. In addition, ~400 water molecules are resolved in the CryoEM map, localized throughout the intercellular permeation pathway and contributing to the channel architecture. These results illustrate how the aqueous-lipid environment is integrated with the architectural stability, structure and function of gap junction communication channels.

A novel CRYGC E128* mutation underlying an autosomal dominant nuclear cataract in a south Indian kindred

PURPOSE

To identify the mutation causing an autosomal dominant congenital nuclear cataract in a south Indian family by whole exome sequencing and to characterize further phenotypically the same in a zebra fish model.

METHODS

A six-generation family (DKEC1) with several affected members registered at the Regional Institute of Ophthalmology (RIO), Chennai was documented to have congenital nuclear cataract. Detailed clinical history and blood samples were collected from all available family members. Genomic DNA of the proband was subjected to whole exome sequencing. Sequence variations suggestive of putative mutations were further confirmed by bidirectional sequencing and restriction site analysis. Functional analysis of the mutant CRYGC E128* in zebrafish embryos was done to dissect out the pathogenicity.

RESULTS

A unique variation viz., c.382 G > T in the coding region of the CRYGC gene, resulting in a premature stop codon at position 128 (E128*) was documented in the affected family members. The same was absent in unaffected family members and in 120 unrelated population controls checked. Bioinformatic tools predicted that the mutation might cause a deleterious effect on protein structure and function. Molecular function analysis of this novel mutation (p. E128*, CRYGC) in the zebrafish indicated this mutation to impair lens transparency.

CONCLUSION

This study identified a novel CRYGC mutation, E128* to cause autosomal dominant congenital nuclear cataract in a large south Indian family. Our study provides a new insight onto how the mutation might affect the γC-crystallin structure and function besides emphasizing the need for genetic diagnosis toward vision restoration.

A novel missense mutation of CRYBB1 causes congenital cataract in a Chinese family

OBJECTIVE OF THE STUDY

To identify the pathogenic gene and mutation site of a Chinese family with congenital cataract.

METHODS

Eight family members and 100 controls were employed, and targeted exome sequencing was used to identify the genetically pathogenic factor of the proband.

RESULTS

Targeted next-generation sequencing identified a novel missense mutation c.209A>C (p.Q70P) of CRYBB1 gene in the family. Sanger sequencing results showed that this heterozygous mutation was a causative mutation, which was not found in unaffected family members and healthy controls. Bioinformatics predicts that the effect of this mutation on protein function is probably harmful.

CONCLUSION

We demonstrate that c.209A>C of CRYBB1 gene is a pathogenic mutation in the family of congenital nuclear cataract in this study. This is the first report that this mutation leads to congenital nuclear cataract, which broadens the mutation spectrum of CRYBB1 gene in congenital nuclear cataract.

Inherited cataracts: molecular genetics, clinical features, disease mechanisms and novel therapeutic approaches

Cataract is the most common cause of blindness in the world; during infancy and early childhood, it frequently results in visual impairment. Congenital cataracts are phenotypically and genotypically heterogeneous and can occur in isolation or in association with other systemic disorders. Significant progress has been made in identifying the molecular genetic basis of cataract; 115 genes to date have been found to be associated with syndromic and non-syndromic cataract and 38 disease-causing genes have been identified to date to be associated with isolated cataract. In this review, we briefly discuss lens development and cataractogenesis, detail the variable cataract phenotypes and molecular mechanisms, including genotype-phenotype correlations, and explore future novel therapeutic avenues including cellular therapies and pharmacological treatments.

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.

Understanding the Role of ztor in Aging-related Diseases Using the Zebrafish Model

The mammalian target of rapamycin (mTOR), a 289 kDa serine/threonine protein kinase of the phosphoinositide 3-kinase (PI3K)-related family is known for its role in regulating lifespan and the aging process in humans and rodents. Aging in zebrafish very much resembles aging in humans. Aged zebrafish often manifest with spinal curvature, cataracts and cognitive frailty, akin to human age-related phenotypical effects such as osteoarthritis, dwindling vision and cognitive dysfunction. However, the role of the zebrafish orthologue of mTOR, ztor, is less defined in these areas. This review paper discusses the tale of growing old in the zebrafish, the physiological roles of ztor in normal developmental processes and its involvement in the pathogenesis of aging-related diseases such as metabolic disorders and cancers.

A Novel Mutation p.S93R in CRYBB1 Associated with Dominant Congenital Cataract and Microphthalmia

Purpose: To identify the pathogenetic mutations in a four-generation Chinese family with dominant congenital cataracts and microphthalmia.Methods: A four-generation Chinese family with dominant congenital cataracts were recruited. Genomic DNAs were collected from their peripheral blood leukocytes and subjected to whole exome sequencing. The genetic mutations were identified by bioinformatic analyses and verified by Sanger sequencing.Results: Whole exome sequencing revealed a c.279C>G point mutation in the CRYBB1 gene which was further verified by Sanger sequencing. The nucleotide replacement results in a novel mutation p.S93R in a conserved residue of βB1 crystallin which is predicted to disrupt normal βB1 structure and function.Conclusions: We identified a novel missense mutation p.S93R in CRYBB1 in a Chinese family with autosomal dominant congenital cataracts and microphthalmia. This serine residue is extremely conserved evolutionarily in more than 50 βγ-crystallins of many species. These data will be very helpful to further understand the structural and functional features of crystallins.

Silver nanoparticles affect lens rather than retina development in zebrafish embryos

Silver nanoparticles (AgNPs) have been reported to inhibit specification and differentiation of erythroid cells, chromatophores, and myofibrils during zebrafish embryogenesis. However, the knowledge of biological effects of AgNPs on eye development, especially on lens development is scarce. In this study, embryos were exposed to or injected with 0.4 mg/L AgNPs, and the results indicate that no obvious morphological changes in eye formation were observed in the stressed embryos compared to the controls. However, clefts and vacuoles were observed in lens of embryos from AgNPs stressed group. Additionally, the down-regulated expressions of different lens crystallin isoform genes and the normal expression of retinal genes were observed in AgNPs stressed embryos, suggesting AgNPs might inhibit the development of lens rather than the development of retina in zebrafish embryos. Moreover, no obvious cell apoptosis was observed, but normal nuclear DNA and RNA export was observed in lens cells. Together, the data in this study reveal that AgNPs damage the development of lens rather than retina resulting in eye abnormalities via some unknown mechanisms rather than via triggering cells apoptosis or blocking nuclear DNA or RNA export.

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.

Novel mutations in CRYBB1/CRYBB2 identified by targeted exome sequencing in Chinese families with congenital cataract

AIM

To summarize the phenotypes and identify the underlying genetic cause of the CRYBB1 and CRYBB2 gene responsible for congenital cataract in two Chinese families.

METHODS

Detailed family histories and clinical data were collected from patients during an ophthalmologic examination. Of 523 inheritable genetic vision system-related genes were captured and sequenced by targeted next-generation sequencing, and the results were confirmed by Sanger sequencing. The possible functional impacts of an amino acid substitution were performed with PolyPhen-2 and SIFT predictions.

RESULTS

The patients in the two families were affected with congenital cataract. Sixty-five (FAMILY-1) and sixty-two (FAMILY-2) single-nucleotide polymorphisms and indels were selected by recommended filtering criteria. Segregation was then analyzed by applying Sanger sequencing with the family members. A heterozygous CRYBB1 mutation in exon 4 (c.347T>C, p.L116P) was identified in sixteen patients in FAMILY-1. A heterozygous CRYBB2 mutation in exon 5 (c.355G>A, p.G119R) was identified in three patients in FAMILY-2. Each mutation co-segregated with the affected individuals and did not exist in unaffected family members and 200 unrelated normal controls. The mutation was predicted to be highly conservative and to be deleterious by both PolyPhen-2 and SIFT.

CONCLUSION

The CRYBB1 mutation (c.347T>C) and CRYBB2 mutation (c.355G>A) are novel in patients with congenital cataract. We summarize the variable phenotypes among the patients, which expanded the phenotypic spectrum of congenital cataract in a different ethnic background.

Genetic landscape of isolated pediatric cataracts: extreme heterogeneity and variable inheritance patterns within genes

Pediatric cataract represents an important cause of pediatric visual impairment. While both genetic and environmental causes for pediatric cataract are known, a large proportion remains idiopathic. The purpose of this review is to discuss genes involved in isolated pediatric cataract, with a focus on variable inheritance patterns within genes. Mutations in over 52 genes are known to cause isolated pediatric cataract, with a major contribution from genes encoding for crystallins, transcription factors, membrane proteins, and cytoskeletal proteins. Interestingly, both dominant and recessive inheritance patterns have been reported for mutations in 13 different cataract genes. For some genes, dominant and recessive alleles represent distinct types of mutations, but for many, especially missense variants, there are no clear patterns to distinguish between dominant and recessive alleles. Further research into the functional effects of these mutations, as well as additional data on the frequency of the identified variants, is needed to clarify variant pathogenicity. Exome sequencing continues to be successful in identifying novel genes associated with congenital cataract but is hindered by the extreme genetic heterogeneity of this condition. The large number of idiopathic cases suggests that more genes and potentially novel mechanisms of gene disruption remain to be identified.

Whole-Exome Sequencing in Searching for New Variants Associated With the Development of Parkinson's Disease

Background: Parkinson’s disease (PD) is a complex disease with its monogenic forms accounting for less than 10% of all cases. Whole-exome sequencing (WES) technology has been used successfully to find mutations in large families. However, because of the late onset of the disease, only small families and unrelated patients are usually available. WES conducted in such cases yields in a large number of candidate variants. There are currently a number of imperfect software tools that allow the pathogenicity of variants to be evaluated.

Objectives: We analyzed 48 unrelated patients with an alleged autosomal dominant familial form of PD using WES and developed a strategy for selecting potential pathogenetically significant variants using almost all available bioinformatics resources for the analysis of exonic areas.

Methods: DNA sequencing of 48 patients with excluded frequent mutations was performed using an Illumina HiSeq 2500 platform. The possible pathogenetic significance of identified variants and their involvement in the pathogenesis of PD was assessed using SNP and Variation Suite (SVS), Combined Annotation Dependent Depletion (CADD) and Rare Exome Variant Ensemble Learner (REVEL) software. Functional evaluation was performed using the Pathway Studio database.

Results: A significant reduction in the search range from 7082 to 25 variants in 23 genes associated with PD or neuronal function was achieved. Eight (FXN, MFN2, MYOC, NPC1, PSEN1, RET, SCN3A and SPG7) were the most significant.

Conclusions: The multistep approach developed made it possible to conduct an effective search for potential pathogenetically significant variants, presumably involved in the pathogenesis of PD. The data obtained need to be further verified experimentally.

A zebrafish model of foxe3 deficiency demonstrates lens and eye defects with dysregulation of key genes involved in cataract formation in humans

The Forkhead box E3 (FOXE3) gene encodes a transcription factor with a forkhead/winged helix domain that is critical for development of the lens and anterior segment of the eye. Monoallelic and biallelic deleterious sequence variants in FOXE3 cause aphakia, cataracts, sclerocornea and microphthalmia in humans. We used clustered regularly interspaced short palindromic repeats/Cas9 injections to target the foxe3 transcript in zebrafish in order to create an experimental model of loss of function for this gene. Larvae that were homozygous for an indel variant, c.296_300delTGCAG, predicting p.(Val99Alafs*2), demonstrated severe eye defects, including small or absent lenses and microphthalmia. The lenses of the homozygous foxe3 indel mutants showed more intense staining with zl-1 antibody compared to control lenses, consistent with increased lens fiber cell differentiation. Whole genome transcriptome analysis (RNA-Seq) on RNA isolated from wildtype larvae and larvae with eye defects that were putative homozygotes for the foxe3 indel variant found significant dysregulation of genes expressed in the lens and eye whose orthologues are associated with cataracts in human patients, including cryba2a, cryba1l1, mipa and hsf4. Comparative analysis of this RNA-seq data with iSyTE data identified several lens-enriched genes to be down-regulated in foxe3 indel mutants. We also noted upregulation of lgsn and crygmxl2 and downregulation of fmodb and cx43.4, genes that are expressed in the zebrafish lens, but that are not yet associated with an eye phenotype in humans. These findings demonstrate that this new zebrafish foxe3 mutant model is highly relevant to the study of the gene regulatory networks conserved in vertebrate lens and eye development.

Identification of a novel MIP frameshift mutation associated with congenital cataract in a Chinese family by whole-exome sequencing and functional analysis

Purpose

To detect the underlying pathogenesis of congenital cataract in a four-generation Chinese family.

Methods

Whole-exome sequencing (WES) of family members (III:4, IV:4, and IV:6) was performed. Sanger sequencing and bioinformatics analysis were subsequently conducted. Full-length WT-MIP or K228fs-MIP fused to HA markers at the N-terminal was transfected into HeLa cells. Next, quantitative real-time PCR, western blotting and immunofluorescence confocal laser scanning were performed.

Results

The age of onset for nonsyndromic cataracts in male patients was by 1-year old, earlier than for female patients, who exhibited onset at adulthood. A novel c.682_683delAA (p.K228fs230X) mutation in main intrinsic protein (MIP) cosegregated with the cataract phenotype. The instability index and unfolded states for truncated MIP were predicted to increase by bioinformatics analysis. The mRNA transcription level of K228fs-MIP was reduced compared with that of WT-MIP, and K228fs-MIP protein expression was also lower than that of WT-MIP. Immunofluorescence images showed that WT-MIP principally localized to the plasma membrane, whereas the mutant protein was trapped in the cytoplasm.

Conclusions

Our study generated genetic and primary functional evidence for a novel c.682_683delAA mutation in MIP that expands the variant spectrum of MIP and help us better understand the molecular basis of cataract.

Mutations in crystallin genes result in congenital cataract associated with other ocular abnormalities

Purpose

This study aims to describe the phenotypes and identify pathogenic mutations in Chinese patients who have congenital cataracts associated with other ocular abnormalities.

Methods

Eleven patients from four unrelated Chinese families plus two simplex cases were enrolled in this study. Detailed ophthalmic examinations were performed. DNA samples were isolated from peripheral blood collected from the patients. Next-generation sequencing of known ocular genes was applied to the proband of each family and two simplex cases to find pathogenic variances. PCR and Sanger sequencing were conducted for validation and segregation tests.

Results

All 13 patients had congenital cataracts, and other ocular abnormalities were found in some cases. Microcornea was found in 12 subjects, and ocular coloboma was observed in five. Various types of coloboma, including iris, choroid, macular, and optic disc, were described. Five mutations in crystallin genes were identified. Four of the mutations are novel: CRYBB1: p.(Arg230Cys), CRYBB2: p.(Gly149Val), CRYGC: p.(Met44CysfsTer59), and CRYGC: p.(Tyr144Ter). One mutation was reported previously: CRYAA: p.(Arg21Trp).

Conclusions

We examined a cohort of Chinese patients with congenital cataracts and studied the phenotypes and genotypes. Extralenticular abnormalities, such as microcornea and ocular coloboma, can also be found in patients with congenital cataracts. The phenotype of congenital cataracts associated with macular and optic disc coloboma was reported for the first time in this study. Four novel mutations and one previously reported mutation were identified. These data expand the mutation spectrum in crystallin genes and enhance our understanding of the phenotypes of congenital cataracts.

Systems genetics identifies a role for Cacna2d1 regulation in elevated intraocular pressure and glaucoma susceptibility

Glaucoma is a multi-factorial blinding disease in which genetic factors play an important role. Elevated intraocular pressure is a highly heritable risk factor for primary open angle glaucoma and currently the only target for glaucoma therapy. Our study helps to better understand underlying genetic and molecular mechanisms that regulate intraocular pressure, and identifies a new candidate gene, Cacna2d1, that modulates intraocular pressure and a promising therapeutic, pregabalin, which binds to CACNA2D1 protein and lowers intraocular pressure significantly. Because our study utilizes a genetically diverse population of mice with known sequence variants, we are able to determine that the intraocular pressure-lowering effect of pregabalin is dependent on the Cacna2d1 haplotype. Using human genome-wide association study (GWAS) data, evidence for association of a CACNA2D1 single-nucleotide polymorphism and primary open angle glaucoma is found. Importantly, these results demonstrate that our systems genetics approach represents an efficient method to identify genetic variation that can guide the selection of therapeutic targets.

Elevated intraocular pressure (IOP) is a heritable risk factor for primary open angle glaucoma. Using forward mouse genetics, cell biology, pharmacology and human genetic data, the authors identify CACNA2D1 as an IOP risk gene that can be therapeutically targeted by the drug pregabalin in animal models.

Identification of de novo germline mutations and causal genes for sporadic diseases using trio-based whole-exome/genome sequencing

Whole-genome or whole-exome sequencing (WGS/WES) of the affected proband together with normal parents (trio) is commonly adopted to identify de novo germline mutations (DNMs) underlying sporadic cases of various genetic disorders. However, our current knowledge of the occurrence and functional effects of DNMs remains limited and accurately identifying the disease-causing DNM from a group of irrelevant DNMs is complicated. Herein, we provide a general-purpose discussion of important issues related to pathogenic gene identification based on trio-based WGS/WES data. Specifically, the relevance of DNMs to human sporadic diseases, current knowledge of DNM biogenesis mechanisms, and common strategies or software tools used for DNM detection are reviewed, followed by a discussion of pathogenic gene prioritization. In addition, several key factors that may affect DNM identification accuracy and causal gene prioritization are reviewed. Based on recent major advances, this review both sheds light on how trio-based WGS/WES technologies can play a significant role in the identification of DNMs and causal genes for sporadic diseases, and also discusses existing challenges.

Analysis and Annotation of Whole-Genome or Whole-Exome Sequencing Derived Variants for Clinical Diagnosis

Over the last 10 years, next-generation sequencing (NGS) has transformed genomic research through substantial advances in technology and reduction in the cost of sequencing, and also in the systems required for analysis of these large volumes of data. This technology is now being used as a standard molecular diagnostic test in some clinical settings. The advances in sequencing have come so rapidly that the major bottleneck in identification of causal variants is no longer the sequencing or analysis (given access to appropriate tools), but rather clinical interpretation. Interpretation of genetic findings in a complex and ever changing clinical setting is scarcely a new challenge, but the task is increasingly complex in clinical genome-wide sequencing given the dramatic increase in dataset size and complexity. This increase requires application of appropriate interpretation tools, as well as development and application of appropriate methodologies and standard procedures. This unit provides an overview of these items. Specific challenges related to implementation of genome-wide sequencing in a clinical setting are discussed. © 2017 by John Wiley & Sons, Inc.

A novel truncation mutation in CRYBB1 associated with autosomal dominant congenital cataract with nystagmus

PURPOSE

To identify the potential candidate genes for a large Chinese family with autosomal dominant congenital cataract (ADCC) and nystagmus, and investigate the possible molecular mechanism underlying the role of the candidate genes in cataractogenesis.

METHODS

We combined the linkage analysis and direct sequencing for the candidate genes in the linkage regions to identify the causative mutation. The molecular and bio-functional properties of the proteins encoded by the candidate genes was further explored with biophysical and biochemical studies of the recombinant wild-type and mutant proteins.

RESULTS

We identified a c. C749T (p.Q227X) transversion in exon 6 of CRYBB1, a cataract-causative gene. This nonsense mutation changes a phylogenetically conserved glutamine to a stop codon and is predicted to truncate the C-terminus of the wild-type protein by 26 amino acids. Comparison of the biophysical and biochemical properties of the recombinant full-length and truncated βB1-crystallins revealed that the mutation led to the insolubility and the phase separation phenomenon of the truncated protein with a changed conformation. Meanwhile, the thermal stability of the truncated βB1-crystallin was significantly decreased, and the mutation diminished the chaperoning ability of αA-crystallin with the mutant under heating stress.

CONCLUSIONS

Our findings highlight the importance of the C-terminus in βB1-crystallin in maintaining the crystalline function and stability, and provide a novel insight into the molecular mechanism underlying the pathogenesis of human autosomal dominant congenital cataract.

A novel splice donor site mutation in EPHA2 caused congenital cataract in a Chinese family

Background:

Congenital cataract is a rare disorder characterized by crystallin denaturation, which becomes a major cause of childhood blindness. Although more than fifty pathogenic genes for congenital cataract have been reported, the genetic causes of many cataract patients remain unknown. In this study, the aim is to identify the genetic cause of a five-generation Chinese autosomal dominant congenital cataract family.

Methods:

Whole exome sequencing (WES) was performed on three affected and one unaffected member of the family, known causative genes were scanned first. Sanger sequencing was used to validate co-segregation of the candidate variant in the family. The impact on the transcript and amino acid sequences of the variant was further analyzed.

Results:

We identified a novel splice donor site mutation c. 2825+1G >A in EPHA2 that was absent in public and in-house databases and showed co-segregation in the family. This variant resulted in an altered splice that led to protein truncation.

Conclusions:

The mutation we identified was responsible for congenital cataract in our studied family. Our findings broaden the spectrum of causative mutations in EPHA2 gene for congenital cataract and suggest that WES is an efficient strategy to scan variants in known causative genes for genetically heterogeneous diseases.

New cataract markers: Mechanisms of disease

Cataract is caused by nutritional, metabolic, environmental, and genetic factors, and is a significant cause of blindness and visual impairment. In recent years, extensive research into the human genome has revealed that numerous genetic mutations are associated with cataract. These mutations affect a variety of genes, including those encoding crystallin, membrane proteins, cytoskeletal proteins, transcription factors, and metabolism-related proteins. Elucidation of these mutations and the genetic and molecular mechanisms has helped clarify the etiology of cataract and may facilitate its early diagnosis and treatment. This review summarizes recent advances in our knowledge and potential clinical of genetic markers of cataract.

Impact of obesity with impaired glucose tolerance on retinal degeneration in a rat model of metabolic syndrome

PURPOSE

Metabolic syndrome (MetS) is associated with several degenerative diseases, including retinal degeneration. Previously, we reported on progressive retinal degeneration in a spontaneous obese rat (WNIN/Ob) model. In this study, we investigated the additional effect of impaired glucose tolerance (IGT), an essential component of MetS, on retinal degeneration using the WNIN/GR-Ob rat model.

METHODS

The retinal morphology and ultrastructure of WNIN/GR-Ob and age-matched littermate lean rats were studied by microscopy and immunohistochemistry. The retinal transcriptome of WNIN/GR-Ob was compared with the respective lean controls and with the WNIN/Ob model using microarray analysis. Expression of selected retinal marker genes was studied via real-time PCR.

RESULTS

Progressive loss of photoreceptor cells was observed in WNIN/GR-Ob rats with an onset as early as 3 months. Similarly, thinning of the inner nuclear layer was observed from 6 months in these rats. Immunohistochemical analysis showed decreased levels of rhodopsin and postsynaptic density protein-95 (PSD-95) proteins and increased levels of glial fibrillary acidic protein (GFAP), vascular endothelial growth factor (VEGF), and calretinin in WNIN/GR-Ob rats compared with the age-matched lean controls, further supporting cellular stress/damage and retinal degeneration. The retinal transcriptome analysis indicated altered expression profiles in both the WNIN/GR-Ob and WNIN/Ob rat models compared to their respective lean controls; these pathways are associated with activation of pathways like cellular oxidative stress response, inflammation, apoptosis, and phototransduction, although the changes were more prominent in WNIN/GR-Ob than in WNIN/Ob animals.

CONCLUSIONS

WNIN/GR-Ob rats with added glucose intolerance developed retinal degeneration similar to the parent line WNIN/Ob. The severity of retinal degeneration was greater in WNIN/GR-Ob rats compared to WNIN/Ob, suggesting a possible role for IGT in this model. Hence, the WNIN/GR-Ob model could be a valuable tool for investigating the impact of MetS on retinal degeneration pathology.

Novel mutations in CRYGC are associated with congenital cataracts in Chinese families

Congenital cataract (CC), responsible for about one-third of blindness in infants, is a major cause of vision loss in children worldwide. 10–25% of CC cases are attributed to genetic causes and CC is a clinically and genetically highly heterogeneous lens disorder in children. Autosomal dominant (AD) inheritance is the most commonly pattern. 195 unrelated non-syndromic ADCC families in this study are recruited from 15 provinces of China. Sanger sequencing approach followed by intra-familial co-segregation, in Silico analyses and interpretation of the variations according to the published guidelines of American College of Medical Genetics (ACMG), were employed to determine the genetic defects. Two mutations (p.Tyr139X and p.Ser166Phe) identified in two unrelated families were associated with their congenital nuclear cataracts and microcornea respectively, which are also reported previously. Six novel CRYGC mutations (p.Asp65ThrfsX38, p.Arg142GlyfsX5, p.Arg142AlafsX22, p.Tyr144X, p.Arg169X, and p.Tyr46Asp) were identified in other six families with congenital nuclear cataracts, respectively. Mutations in the CRYGC were responsible for 4.1% Chinese ADCC families in our cohort. Our results expand the spectrum of CRYGC mutations as well as their associated phenotypes.

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.

Targeted Genes Sequencing Identified a Novel 15 bp Deletion on GJA8 in a Chinese Family with Autosomal Dominant Congenital Cataracts

Background:

Congenital cataract (CC) is the leading cause of visual impairment or blindness in children worldwide. Because of highly genetic and clinical heterogeneity, a molecular diagnosis of the lens disease remains a challenge.

Methods:

In this study, we tested a three-generation Chinese family with autosomal dominant CCs by targeted sequencing of 45 CC genes on next generation sequencing and evaluated the pathogenicity of the detected mutation by protein structure, pedigree validation, and molecular dynamics (MD) simulation.

Results:

A novel 15 bp deletion on GJA8 (c.426_440delGCTGGAGGGGACCCT or p. 143_147delLEGTL) was detected in the family. The deletion, concerned with an in-frame deletion of 5 amino acid residues in a highly evolutionarily conserved region within the cytoplasmic loop domain of the gap junction channel protein connexin 50 (Cx50), was in full cosegregation with the cataract phenotypes in the family but not found in 1100 control exomes. MD simulation revealed that the introduction of the deletion destabilized the Cx50 gap junction channel, indicating the deletion as a dominant-negative mutation.

Conclusions:

The above results support the pathogenic role of the 15 bp deletion on GJA8 in the Chinese family and demonstrate targeted genes sequencing as a resolution to molecular diagnosis of CCs.

Identification of a PRX variant in a Chinese family with congenital cataract by exome sequencing

BACKGROUND

Congenital cataract is a common cause of childhood vision impairment or blindness with genetic and clinical heterogeneity. The aim of this study was to identify the disease-associated gene in a Chinese family with congenital cataract.

METHODS

A four-generation Chinese family with three enrolled patients suffering from congenital cataract was studied. Detailed family history and clinical data of all the members were collected and recorded. Exome sequencing was applied in the proband to screen potential genetic variants, and then Sanger sequencing was used to verify the variant within the family.

RESULTS

A heterozygous variant, c.3673G > A (p.V1225M), in the periaxin gene (PRX) was identified in three patients and two asymptomatic individuals of the family. The variant was absent in the other three unaffected family members and in 3290 ethnically matched in-house controls from BGI-Shenzhen.

CONCLUSIONS

By utilizing both exome sequencing and Sanger sequencing, we identified a missense variant in the PRX gene that is possibly associated with disease in this family. Our finding may broaden the spectrum of genes associated with congenital cataract, and may provide insights into lens development, pathogenic mechanism, future clinical genetic diagnosis and therapy of congenital cataract.

Case report of homozygous deletion involving the first coding exons of GCNT2 isoforms A and B and part of the upstream region of TFAP2A in congenital cataract

BACKGROUND

Congenital cataracts affect 3-6 per 10,000 live births and represent one of the leading causes of blindness in children. Congenital cataracts have a strong genetic component with high heterogeneity and variability.

CASE PRESENTATION

Analysis of whole exome sequencing data in a patient affected with congenital cataracts identified a pathogenic deletion which was further defined by other techniques. A ~98-kb homozygous deletion of 6p24.3 involving the first three exons (two non-coding and one coding) of GCNT2 isoform A, the first exon (coding) of GCNT2 isoform B, and part of the intergenic region between GCNT2 and TFAP2A was identified in the patient and her brother while both parents were found to be heterozygous carriers of the deletion. The exact breakpoints were identified and revealed the presence of Alu elements at both sides of the deletion, thus indicating Alu-mediated non-homologous end-joining as the most plausible mechanism for this rearrangement. Recessive mutations in GCNT2 are known to cause an adult i blood group phenotype with congenital cataracts in some cases. The GCNT2 gene has three differentially expressed transcripts, with GCNT2B being the only isoform associated with lens function and GCNT2C being the only isoform expressed in red blood cells based on earlier studies; previously reported mutations/deletions have either affected all three isoforms (causing blood group and cataract phenotype) or the C isoform only (causing blood group phenotype only). Dominant mutations in TFAP2A are associated with syndromic anophthalmia/microphthalmia and other ocular phenotypes as part of Branchio-Ocular-Facial-Syndrome (BOFS). While the patients do not fit a diagnosis of BOFS, one sibling demonstrates mild overlap with the phenotypic spectrum, and therefore an effect of this deletion on the function of TFAP2A cannot be ruled out.

CONCLUSIONS

To the best of our knowledge, this is the first case reported in which disruption of the GCNT2 gene does not involve the C isoform. The congenital cataracts phenotype in the affected patients is consistent with the previously defined isoform-specific roles of this gene. The GCNT2-TFAP2A region may be prone to rearrangements through Alu-mediated non-homologous end-joining.

A Common Ancestral Mutation in CRYBB3 Identified in Multiple Consanguineous Families with Congenital Cataracts

PURPOSE

This study was performed to investigate the genetic determinants of autosomal recessive congenital cataracts in large consanguineous families.

METHODS

Affected individuals underwent a detailed ophthalmological examination and slit-lamp photographs of the cataractous lenses were obtained. An aliquot of blood was collected from all participating family members and genomic DNA was extracted from white blood cells. Initially, a genome-wide scan was performed with genomic DNAs of family PKCC025 followed by exclusion analysis of our familial cohort of congenital cataracts. Protein-coding exons of CRYBB1, CRYBB2, CRYBB3, and CRYBA4 were sequenced bidirectionally. A haplotype was constructed with SNPs flanking the causal mutation for affected individuals in all four families, while the probability that the four familial cases have a common founder was estimated using EM and CHM-based algorithms. The expression of Crybb3 in the developing murine lens was investigated using TaqMan assays.

RESULTS

The clinical and ophthalmological examinations suggested that all affected individuals had nuclear cataracts. Genome-wide linkage analysis localized the causal phenotype in family PKCC025 to chromosome 22q with statistically significant two-point logarithm of odds (LOD) scores. Subsequently, we localized three additional families, PKCC063, PKCC131, and PKCC168 to chromosome 22q. Bidirectional Sanger sequencing identified a missense variation: c.493G>C (p.Gly165Arg) in CRYBB3 that segregated with the disease phenotype in all four familial cases. This variation was not found in ethnically matched control chromosomes, the NHLBI exome variant server, or the 1000 Genomes or dbSNP databases. Interestingly, all four families harbor a unique disease haplotype that strongly suggests a common founder of the causal mutation (p<1.64E-10). We observed expression of Crybb3 in the mouse lens as early as embryonic day 15 (E15), and expression remained relatively steady throughout development.

CONCLUSION

Here, we report a common ancestral mutation in CRYBB3 associated with autosomal recessive congenital cataracts identified in four familial cases of Pakistani origin.

Mutations of RagA GTPase in mTORC1 Pathway Are Associated with Autosomal Dominant Cataracts

Cataracts are a significant public health problem with no proven methods for prevention. Discovery of novel disease mechanisms to delineate new therapeutic targets is of importance in cataract prevention and therapy. Herein, we report that mutations in the RagA GTPase (RRAGA), a key regulator of the mechanistic rapamycin complex 1 (mTORC1), are associated with autosomal dominant cataracts. We performed whole exome sequencing in a family with autosomal dominant juvenile-onset cataracts, and identified a novel p.Leu60Arg mutation in RRAGA that co-segregated with the disease, after filtering against the dbSNP database, and at least 123,000 control chromosomes from public and in-house exome databases. In a follow-up direct screening of RRAGA in another 22 families and 142 unrelated patients with congenital or juvenile-onset cataracts, RRAGA was found to be mutated in two unrelated patients (p.Leu60Arg and c.-16G>A respectively). Functional studies in human lens epithelial cells revealed that the RRAGA mutations exerted deleterious effects on mTORC1 signaling, including increased relocation of RRAGA to the lysosomes, up-regulated mTORC1 phosphorylation, down-regulated autophagy, altered cell growth or compromised promoter activity. These data indicate that the RRAGA mutations, associated with autosomal dominant cataracts, play a role in the disease by acting through disruption of mTORC1 signaling.

A group of guanine nucleotide-binding molecules called Rag GTPases are known to play a crucial role in regulation of mTORC1 signaling cascade. In the current study, whole exome sequencing has led to the identification of the RagA GTPase (RRAGA) gene for cataracts and we proceeded to study properties of the RRAGA protein. We captured and sequenced the whole exome for four affected patients from a family with autosomal dominant juvenile-onset posterior cataracts, and found a novel rare mutation in RagA GTPase (RRAGA). To validate this finding, we then sequenced more families and patients, and observed RRAGA mutations in unrelated patients with related phenotypes, suggesting that RRAGA could be mutated in congenital and juvenile-onset cataracts. We further demonstrated supporting evidence that in human lens epithelial cells the RRAGA mutations exerted deleterious effects on relocation of RRAGA to the lysosomes, mTORC1 phosphorylation, autophagy and cell growth. This study gives important new insight into the roles of RRAGA and mTROC1 signaling in the etiology of cataracts.

Distribution of gene mutations in sporadic congenital cataract in a Han Chinese population

PURPOSE

This study aimed to investigate the genetic effects underlying non-familial sporadic congenital cataract (SCC).

METHODS

We collected DNA samples from 74 patients with SCC and 20 patients with traumatic cataract (TC) in an age-matched group and performed genomic sequencing of 61 lens-related genes with target region capture and next-generation sequencing (NGS). The suspected SCC variants were validated with MassARRAY and Sanger sequencing. DNA samples from 103 healthy subjects were used as additional controls in the confirmation examination.

RESULTS

By filtering against common variants in public databases and those associated with TC cases, we identified 23 SCC-specific variants in 17 genes from 19 patients, which were predicted to be functional. These mutations were further confirmed by examination of the 103 healthy controls. Among the mutated genes, CRYBB3 had the highest mutation frequency with mutations detected four times in four patients, followed by EPHA2, NHS, and WDR36, the mutation of which were detected two times in two patients. We observed that the four patients with CRYBB3 mutations had three different cataract phenotypes.

CONCLUSIONS

From this study, we concluded the clinical and genetic heterogeneity of SCC. This is the first study to report broad spectrum genotyping for patients with SCC.