Understanding the molecular genetics of congenital cataract may have wider implications for age related cataract

Through the Cat-Map Gateway: A Brief History of Cataract Genetics

Clouding of the transparent eye lens, or cataract(s), is a leading cause of visual impairment that requires surgical replacement with a synthetic intraocular lens to effectively restore clear vision. Most frequently, cataract is acquired with aging as a multifactorial or complex trait. Cataract may also be inherited as a classic Mendelian trait-often with an early or pediatric onset-with or without other ocular and/or systemic features. Since the early 1990s, over 85 genes and loci have been genetically associated with inherited and/or age-related forms of cataract. While many of these underlying genes-including those for lens crystallins, connexins, and transcription factors-recapitulate signature features of lens development and differentiation, an increasing cohort of unpredicted genes, including those involved in cell-signaling, membrane remodeling, and autophagy, has emerged-providing new insights regarding lens homeostasis and aging. This review provides a brief history of gene discovery for inherited and age-related forms of cataract compiled in the Cat-Map database and highlights potential gene-based therapeutic approaches to delay, reverse, or even prevent cataract formation that may help to reduce the increasing demand for cataract surgery.

Potential of GJA8 gene variants in predicting age-related cataract: A comparison of supervised machine learning methods

Cataracts are the problems associated with the crystallins proteins of the eye lens. Any perturbation in the conformity of these proteins results in a cataract. Age-related cataract is the most common type among all cataracts as it accounts for almost 80% of cases of senile blindness worldwide. This research study was performed to predict the role of single nucleotide polymorphisms (SNPs) of the GJA8 gene with age-related cataracts in 718 subjects (400 age-related cataract patients and 318 healthy individuals). A comparison of supervised machine learning classification algorithm including logistic regression (LR), random forest (RF) and Artificial Neural Network (ANN) were presented to predict the age-related cataracts. The results indicated that LR is the best for predicting age-related cataracts. This successfully developed model after accounting different genetic and demographic factors to predict cataracts will help in effective disease management and decision-making medical practitioner and experts.

Oxysterol Compounds in Mouse Mutant αA- and αB-Crystallin Lenses Can Improve the Optical Properties of the Lens

Purpose

To investigate how cataract-linked mutations affect the gradient refractive index (GRIN) and lens opacification in mouse lenses and whether there is any effect on the optics of the lens from treatment with an oxysterol compound.

Methods

A total of 35 mice including wild-type and knock-in mutants (Cryaa-R49C and Cryab-R120G) were used in these experiments: 26 mice were treated with topical VP1-001, an oxysterol, in one eye and vehicle in the other, and nine mice were untreated controls. Slit lamp biomicroscopy was used to analyze the lens in live animals and to provide apparent cataract grades. Refractive index in the lenses of 64 unfixed whole mouse eyes was calculated from measurements with X-ray phase tomography based on X-ray Talbot interferometry with a synchrotron radiation source.

Results

Heterozygous Cryaa-R49C lenses had slightly irregularly shaped contours in the center of the GRIN and distinct disturbances of the gradient index at the anterior and posterior poles. Contours near the lens surface were denser in homozygous Cryab-R120G lenses. Treatment with topical VP1-001, an oxysterol, showed an improvement in refractive index profiles in 61% of lenses and this was supported by a reduction in apparent lens opacity grade by 1.0 in 46% of live mice.

Conclusions

These results indicate that α-crystallin mutations alter the refractive index gradient of mouse lenses in distinct ways and suggest that topical treatment with VP1-001 may improve lens transparency and refractive index contours in some lenses with mutations.

Alpha-crystallin mutations alter lens metabolites in mouse models of human cataracts

Cataracts are a major cause of blindness worldwide and commonly occur in individuals over 70 years old. Cataracts can also appear earlier in life due to genetic mutations. The lens proteins, αA- and αB-crystallins, are chaperone proteins that have important roles maintaining protein solubility to prevent cataract formation. Mutations in the CRYAA and CRYAB crystallin genes are associated with autosomal dominant early onset human cataracts. Although studies about the proteomic and genomic changes that occur in cataracts have been reported, metabolomics studies are very limited. Here, we directly investigated cataract metabolism using gas-chromatography-mass spectrometry (GC-MS) to analyze the metabolites in adult Cryaa-R49C and Cryab-R120G knock-in mouse lenses. The most abundant metabolites were myo-inositol, L-(+)-lactic acid, cholesterol, phosphate, glycerol phosphate, palmitic and 9-octadecenoic acids, α-D-mannopyranose, and β-D-glucopyranose. Cryaa-R49C knock-in mouse lenses had a significant decrease in the number of sugars and minor sterols, which occurred in concert with an increase in lactic acid. Cholesterol composition was unchanged. In contrast, Cryab-R120G knock-in lenses exhibited increased total amino acid content including valine, alanine, serine, leucine, isoleucine, glycine, and aspartic acid. Minor sterols, including cholest-7-en-3-ol and glycerol phosphate were decreased. These studies indicate that lenses from Cryaa-R49C and Cryab-R120G knock-in mice, which are models for human cataracts, have unique amino acid and metabolite profiles.

Cataracts in the Norwegian Buhund-current prevalence and characteristics

OBJECTIVE

To evaluate prevalence and characteristics of cataracts in the Norwegian Buhund breed 20 years after high reported prevalence of especially pulverulent nuclear cataracts (PNCs).

ANIMALS STUDIED

Two hundred and fifty Norwegian Buhund dogs in Norway, Sweden, and Denmark (117 males and 133 females) with previously unknown eye health status were included. Forty-five dogs had multiple examinations (two to six times over a 6-year period). Median age was 4.4 years [0.2-15.2] at first examination and 5.3 years [0.2-15.2] at last examination.

PROCEDURES

All dogs underwent regular screening for inherited eye diseases.

RESULTS

At the last observation of each dog, 52.4% were affected by PNC, categorized as minimal (33 of 250 dogs; 13.2%), mild (31 dogs; 12.4%), moderate (38 dogs; 15.2%), or pronounced (29 dogs; 11.6%). Moderate or pronounced changes were only seen in older dogs, and progressive changes were identified in some of the re-examined dogs. Some dogs, free of lenticular changes at early examinations, were affected by PNC at re-examinations. The odds for finding PNC increased with dog's age up to approximately 8 years. Presumably inherited cataracts other than PNC were found in 53 dogs (21.2%) with cortical (17.6%) and posterior polar (6.4%) locations as the most common ones.

CONCLUSIONS

The high prevalence of PNC in the breed reported 20 years ago persists. PNCs are not always visible in young dogs, and the rate of progression varies. The prevalence of other types of cataract is also high, but cataracts rarely cause loss of vision in this breed.

The Polymorphisms with Cataract Susceptibility Impair the EPHA2 Receptor Stability and Its Cytoprotective Function

Despite accumulating evidence revealing susceptibility genes for age-related cataract, its pathophysiology leading to visual impairment at the cellular and molecular level remains poorly understood. Recent bioinformatic studies uncovered the association of two single nucleotide polymorphisms in human EPHA2, rs2291806 and rs1058371, with age-related cataract. Here we investigated the role of EPHA2 in counteracting oxidative stress-induced apoptosis of lens epithelial cells. The cataract-associated missense mutations resulted in the destabilization of EPHA2 receptor without altering the mRNA transcription. The cytoprotective and antiapoptotic function of EPHA2 in lens epithelial cells was abolished by the functional polymorphisms. Furthermore, our results suggest that the downstream signaling of activated EPHA2 promotes the antioxidative capacity of lens epithelial cells to eradicate the overproduction of reactive oxygen species. In contrast, the overexpression of EPHA2 with nonsynonymous mutations in the lens epithelial cells offered limited antioxidative protection against oxidative stress. Thus, our study not only sheds the light on the potential cytoprotective function of EPHA2 signaling in lens but also provides the cellular mechanisms underlying the pathogenesis of age-related cataract.

The effect of GSTT1, GSTM1 and GSTP1 gene polymorphisms on the susceptibility of age-related cataract in Chinese Han population

OBJECTIVE

Age-related cataract (ARC) is one of the most common eye diseases in the elderly worldwide, especially in China. The genetic polymorphisms of many glutathione S-transferases coding genes are likely to be closely related to the development of ARC, especially the GSTT1, the GSTM1 and the GSTP1. This investigation is aimed to determine the possible associations of GSTT1, GSTM1 and GSTP1 polymorphisms with the susceptibility of ARC in Chinese Han Population.

METHODS

A case-control study including ARC cases (n = 312) and controls (n = 256) in Chinese Han Population was performed. GSTT1 and GSTM1 polymorphisms were detected by duplex polymerase chain reaction (PCR), and two SNPs (rs1695, A/G and rs1138272, C/T) in GSTP1 gene were genotyped by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method, all the results were verified by sequencing method.

RESULTS

The GSTT1 null genotype carriers had a much higher risk of ARC compared with non-null genotype (χ(2) = 14.091, P<0.001), and the allele G carriers also had a increased risk over the allele A carriers in the SNP (rs1695, A/G) in GSTP1 gene (χ(2) = 7.696, P = 0.006), while the GSTM1 polymorphism and the SNP (rs1138272, C/T) in GSTP1 gene seem had no association with the susceptibility of ARC in Chinese Han Population.

CONCLUSIONS

These preliminary results indicated carriage of null GSTT1 and GSTP1 Val/Val genotypes may contribute to genetic susceptibility to ARC in Chinese Han Population, and these genetic polymorphisms might be used as molecular markers for detecting ARC susceptibility.

Autophagy and UPR in alpha-crystallin mutant knock-in mouse models of hereditary cataracts

BACKGROUND

Knock-in mice provide useful models of congenital and age-related cataracts caused by α-crystallin mutations. R49C αA-crystallin and R120G αB-crystallin mutations are linked with hereditary cataracts. Knock-in αA-R49C+/- heterozygotes develop cataracts by 1-2months, whereas homozygote mice have cataracts at birth. The R49C mutation drastically reduces lens protein water solubility and causes cell death in knock-in mouse lenses. Mutant crystallin cannot function as a chaperone, which leads to protein aggregation and lens opacity. Protein aggregation disrupts the lens fiber cell structure and normal development and causes cell death in epithelial and fiber cells. We determined what aspects of the wild-type phenotype are age-dependently altered in the mutant lens.

METHODS

Wild-type, heterozygote (αA-R49C+/-), and homozygote (αA-R49C+/+) mouse lenses were assessed pre- and postnatally for lens morphology (electron microscopy, immunohistochemistry), and autophagy or unfolded protein response markers (immunoblotting).

RESULTS

Morphology was altered by embryonic day 17 in R49C+/+ lenses; R49C+/- lens morphology was unaffected at this stage. Active autophagy in the lens epithelium of mutant lenses was indicated by the presence of autophagosomes using electron microscopy. Protein p62 levels, which are degraded specifically by autophagy, increased in αA-R49C mutant versus wild-type lenses, suggesting autophagy inhibition in the mutant lenses. The unfolded protein response marker XBP-1 was upregulated in adult lenses of αB-R120G+/+ mice, suggesting its role in lens opacification.

CONCLUSIONS

Mutated crystallins alter lens morphology, autophagy, and stress responses.

GENERAL SIGNIFICANCE

Therapeutic modulation of autophagic pathways may improve protein degradation in cataractous lenses and reduce lens opacity. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

Electronic medical records and genomics (eMERGE) network exploration in cataract: several new potential susceptibility loci

PURPOSE

Cataract is the leading cause of blindness in the world, and in the United States accounts for approximately 60% of Medicare costs related to vision. The purpose of this study was to identify genetic markers for age-related cataract through a genome-wide association study (GWAS).

METHODS

In the electronic medical records and genomics (eMERGE) network, we ran an electronic phenotyping algorithm on individuals in each of five sites with electronic medical records linked to DNA biobanks. We performed a GWAS using 530,101 SNPs from the Illumina 660W-Quad in a total of 7,397 individuals (5,503 cases and 1,894 controls). We also performed an age-at-diagnosis case-only analysis.

RESULTS

We identified several statistically significant associations with age-related cataract (45 SNPs) as well as age at diagnosis (44 SNPs). The 45 SNPs associated with cataract at p<1×10(-5) are in several interesting genes, including ALDOB, MAP3K1, and MEF2C. All have potential biologic relationships with cataracts.

CONCLUSIONS

This is the first genome-wide association study of age-related cataract, and several regions of interest have been identified. The eMERGE network has pioneered the exploration of genomic associations in biobanks linked to electronic health records, and this study is another example of the utility of such resources. Explorations of age-related cataract including validation and replication of the association results identified herein are needed in future studies.

Identification of a novel GJA8 (Cx50) point mutation causes human dominant congenital cataracts

Hereditary cataracts are clinically and genetically heterogeneous lens diseases that cause a significant proportion of visual impairment and blindness in children. Human cataracts have been linked with mutations in two genes, GJA3 and GJA8, respectively. To identify the causative mutation in a family with hereditary cataracts, family members were screened for mutations by PCR for both genes. Sequencing the coding regions of GJA8, coding for connexin 50, revealed a C > A transversion at nucleotide 264, which caused p.P88T mutation. To dissect the molecular consequences of this mutation, plasmids carrying wild-type and mutant mouse ORFs of Gja8 were generated and ectopically expressed in HEK293 cells and human lens epithelial cells, respectively. The recombinant proteins were assessed by confocal microscopy and Western blotting. The results demonstrate that the molecular consequences of the p.P88T mutation in GJA8 include changes in connexin 50 protein localization patterns, accumulation of mutant protein, and increased cell growth.

p62 expression and autophagy in αB-crystallin R120G mutant knock-in mouse model of hereditary cataract

The formation of cataracts is associated with the accumulation of protein aggregates in the ocular lens, suggesting that defective protein degradation plays a role in cataract pathogenesis. Accumulation of the p62 protein has recently been identified as a marker for impaired autophagy in a variety of tissues; however, little information exists on its expression in the ocular lens and in cataracts. In the present study we examined the expression of p62 in the mouse lens and compared its expression in wild-type lenses with that in lenses from knock-in mice with an arginine to glycine mutation in αB-crystallin (αB-R120G) that is known to cause human hereditary cataract. Immunohistochemical, immunoblotting, and transmission electron microscopic analyses of wild type and αB-R120G mutant mice were performed. To assess the effect of increased protein aggregation on autophagy, immunohistochemical staining was performed with an anti-p62 antibody, revealing the presence of p62-positive punctate staining in a band of denucleated cortical fiber cells. The number and size of p62 puncta were significantly greater in αB-R120G homozygous mutant lenses than in wild type and heterozygous mutant lenses. p62 staining was also abundant in lens epithelial cells and was concentrated around the nuclear membrane. Double-membraned structures similar to autophagosomes containing cellular cytoplasmic content were detected in lens epithelial cells by transmission electron microscopy. The autophagosomes in lens epithelial cells from αB-R120G homozygous mutant mice were larger than those in wild type mice. Double-membraned structures that are probably autophagosomes were also detected in cortical fiber cells and were more abundant in the αB-R120G homozygous mutant lens than the wild type lens. This study demonstrates p62 distribution as speckles in the lens fiber cells, altered levels of p62 expression, and the presence of autophagosomes in the ocular lens of αB-R120G mutant mice. We propose that autophagy is inhibited in the αB-R120G mutant lenses because of a defect in protein degradation after autophagosome formation. Further work is necessary to determine the relationship between αB-crystallin function, autophagy, and cataract formation.

Novel crystallin gamma B mutations in a Kuwaiti family with autosomal dominant congenital cataracts reveal genetic and clinical heterogeneity

PURPOSE

To explore the disease locus and causative mutation for autosomal dominant congenital cataracts (ADCC) in a Kuwaiti family. There were seven affected and three unaffected subjects in the family.

METHODS

Whole-genome linkage analysis was performed using Gene Chip Human Mapping 250 K Arrays to identify regions of linkage. Potential genes within this region were cloned and sequenced to identify the disease-causing mutation.

RESULTS

The highest logarithm of odds score (1.5) region 2q34-36.1, spanning the crystallin beta A2 (CRYBA2) gene, showed no sequence changes. Thus, the second highest logarithm of odds score (1.49) region, 2q33-37, spanning the gamma crystalline gene cluster (CRYG), was considered. Sequencing of the CRYGA, B, C, and D genes revealed two novel heterozygous deletions and one trinucleotide polymorphism in the CRYGB gene. These mutations included a heterozygous g.67delG, intron 1 deletion in four of the affected family members with lamellar cataracts and a heterozygous g.167delC, exon 2 deletion inherited from the Egyptian grandmother by her granddaughter, resulting in anterior polar cataracts. Another patient with complete cataracts was a compound heterozygote with both of the above-mentioned mutations. In addition, the novel trinucleotide polymorphism g.20-22 GGT>AAA was detected in three of the family members.

CONCLUSIONS

We report the linkage of ADCC to chromosome 2q33-37, which spans the CRYGB gene. This study is the first to report complex heterogeneous mutations in the CRYGB gene resulting in ADCC with three distinct phenotypes (lamellar, anterior polar, and complete cataracts) in the same family.

A knock-in mouse model for the R120G mutation of αB-crystallin recapitulates human hereditary myopathy and cataracts

An autosomal dominant missense mutation in αB-crystallin (αB-R120G) causes cataracts and desmin-related myopathy, but the underlying mechanisms are unknown. Here, we report the development of an αB-R120G crystallin knock-in mouse model of these disorders. Knock-in αB-R120G mice were generated and analyzed with slit lamp imaging, gel permeation chromatography, immunofluorescence, immunoprecipitation, histology, and muscle strength assays. Wild-type, age-matched mice were used as controls for all studies. Both heterozygous and homozygous mutant mice developed myopathy. Moreover, homozygous mutant mice were significantly weaker than wild-type control littermates at 6 months of age. Cataract severity increased with age and mutant gene dosage. The total mass, precipitation, and interaction with the intermediate filament protein vimentin, as well as light scattering of αB-crystallin, also increased in mutant lenses. In skeletal muscle, αB-R120G co-aggregated with desmin, became detergent insoluble, and was ubiquitinated in heterozygous and homozygous mutant mice. These data suggest that the cataract and myopathy pathologies in αB-R120G knock-in mice share common mechanisms, including increased insolubility of αB-crystallin and co-aggregation of αB-crystallin with intermediate filament proteins. These knock-in αB-R120G mice are a valuable model of the developmental and molecular biological mechanisms that underlie the pathophysiology of human hereditary cataracts and myopathy.

Cat-Map: putting cataract on the map

Lens opacities, or cataract(s), may be inherited as a classic Mendelian disorder usually with early-onset or, more commonly, acquired with age as a multi-factorial or complex trait. Many genetic forms of cataract have been described in mice and other animal models. Considerable progress has been made in mapping and identifying the genes and mutations responsible for inherited forms of cataract, and genetic determinants of age-related cataract are beginning to be discovered. To provide a convenient and accurate summary of current information focused on the increasing genetic complexity of Mendelian and age-related cataract we have created an online chromosome map and reference database for cataract in humans and mice (Cat-Map).

Abnormal expression of collagen IV in lens activates unfolded protein response resulting in cataract

Human diseases caused by mutations in extracellular matrix genes are often associated with an increased risk of cataract and lens capsular rupture. However, the underlying mechanisms of cataract pathogenesis in these conditions are still unknown. Using two different mouse models, we show that the accumulation of collagen chains in the secretory pathway activates the stress signaling pathway termed unfolded protein response (UPR). Transgenic mice expressing ectopic Col4a3 and Col4a4 genes in the lens exhibited activation of IRE1, ATF6, and PERK associated with expansion of the endoplasmic reticulum and attenuation of general protein translation. The expression of the transgenes had adverse effects on lens fiber cell differentiation and eventually induced cell death in a group of transgenic fiber cells. In Col4a1(+/Deltaex40) mutant mice, the accumulation of mutant chains also caused low levels of UPR activation. However, cell death was not induced in mutant lenses, suggesting that low levels of UPR activation are not proapoptotic. Collectively, the results provide in vivo evidence for a role of UPR in cataract formation in response to accumulation of terminally unfolded proteins in the endoplasmic reticulum.

Mutation analysis of congenital cataract in a Basotho family identified a new missense allele in CRYBB2

PURPOSE

To identify the causative genetic mutation among the known cataract candidate genes underlying the observed phenotype in a Basotho family, with congenital nuclear cataracts.

METHODS

Because of the small family size, we used the functional candidate gene analysis approach. We screened a Basotho family, clinically documented to have congenital nuclear cataracts, for mutation in the candidate genes CRYG (C & D; Crystallin, gamma C and Crystallin, gamma D), GJA8 (Gap junction protein, alpha 8), CRY (AA & AB; Crystallin, alpha A and Crystallin, alpha B), CRYBA (Crystallin, beta A) and CRY (BB1 & BB2; Crystallin, beta B1 and Crystallin, beta B2) through polymerase chain reaction analyses and sequencing.

RESULTS

Mutation screening identified only one significant alteration in exon 6 (607G>A) of CRYBB2, with a substitution of Valine to Methionine at position 187. This mutation segregated in all five affected family members but it was not observed in any of the unaffected persons of the family. The putative mutation led also to the appearance of a new NIaIII restriction site in the samples of the affected family members that was not present in 100 randomly selected DNA samples from ophthalmologically normal individuals and in 40 unrelated senile cataract patients of the same ethnic background as the family members.

CONCLUSIONS

This study identified a missense mutation in CRYBB2 in a family of Basotho with autosomal dominant congenital cataract (ADCC). In summary, we believe this new missense allele is the probable causative molecular lesion for the observed phenotype in this family.

A heterozygous c-Maf transactivation domain mutation causes congenital cataract and enhances target gene activation

MAF, one of a family of large Maf bZIP transcription factors, is mutated in human developmental ocular disorders that include congenital cataract, microcornea, coloboma and anterior segment dysgenesis. Expressed early in the developing lens vesicle, it is central to regulation of lens crystallin gene expression. We report a semi-dominant mouse c-Maf mutation recovered after ENU mutatgenesis which results in the substitution, D90V, at a highly conserved residue within the N-terminal 35 amino-acid minimal transactivation domain (MTD). Unlike null and loss-of-function c-Maf mutations, which cause severe runting and renal abnormalities, the phenotype caused by the D90V mutation is isolated cataract. In reporter assays, D90V results in increased promoter activation, a situation similar to MTD mutations of NRL that also cause human disease. In contrast to wild-type protein, the c-Maf D90V mutant protein is not inhibited by protein kinase A-dependent pathways. The MTD of large Maf proteins has been shown to interact with the transcriptional co-activator p300 and we demonstrate that c-Maf D90V enhances p300 recruitment in a cell-type dependent manner. We observed the same for the pathogenic human NRL MTD mutation S50T, which suggests a common mechanism of action.

2005 Gregg Lecture: Congenital cataract--from rubella to genetics

On the 65th anniversary of Gregg's observation "Congenital cataract following German measles in the mother", rubella has retired as the leading cause of congenital cataract, from 87% of Gregg's cohort to less than 3% over the last 25 years and almost zero now in Australia and other developed countries. However, people must keep vigilance in maintaining immunization rates and encourage immunization in developing countries. At least one-fifth of congenital cataract is familial. Understanding the genetics of familial cataract will lead to better treatment of congenital as well as age-related cataract.

Transparency and non-refractive functions of crystallins--a proposal

Based on the premise that all crystallins have cellular and metabolically relevant catalytic activities, we propose that aberrant changes in non-crystallin (non-refractive) functions presage the appearance of cataractous pathologies in an otherwise highly stable edifice of transparency. This proposal is based on accumulating evidence from developmental, molecular and genetic studies that have established that crystallins are more than inanimate building blocks of the transparent lens fiber mass. The published work does not support the perceived dichotomy in the relevance of crystallin function (as essential) and non-crystallin function (as either of secondary importance or not essential at all), to the emergence and maintenance of the phenotype of transparency. A number of crystallin mutations have stage-specific phenotypes at developmental times when their concentrations have not reached 'crystallin' (high) proportions. There is heterogeneity in the cataract phenotypes associated with similar or identical mutations in different populations; the cataracts have disparate phenotypes even when the mutations are in the same gene. These data suggest that non-crystallin function is not merely a non-lens activity of a crystallin but an essential requirement within the lens itself.