Novel mutations in the gamma-crystallin genes cause autosomal dominant congenital cataracts

Improving the Yield of Genetic Diagnosis through Additional Genetic Panel Testing in Hereditary Ophthalmic Diseases

Numerous hereditary ophthalmic diseases display significant genetic diversity. Consequently, the utilization of gene panel sequencing allows a greater number of patients to receive a genetic diagnosis for their clinical manifestations. We investigated how to improve the yield of genetic diagnosis through additional gene panel sequencing in hereditary ophthalmic diseases. A gene panel sequencing consisting of a customized hereditary retinopathy panel or hereditary retinitis pigmentosa (RP) panel was prescribed and referred to a CAP-accredited clinical laboratory. If no significant mutations associated with hereditary retinopathy and RP were detected in either panel, additional gene panel sequencing was requested for research use, utilizing the remaining panel. After additional gene panel sequencing, a total of 16 heterozygous or homozygous variants were identified in 15 different genes associated with hereditary ophthalmic diseases. Of 15 patients carrying any candidate variants, the clinical symptoms could be tentatively accounted for by genetic mutations in seven patients. However, in the remaining eight patients, given the in silico mutation predictive analysis, variant allele frequency in gnomAD, inheritance pattern, and genotype-phenotype correlation, fully elucidating the clinical manifestations with the identified rare variant was challenging. Our study highlights the utility of gene panel sequencing in achieving accurate diagnoses for hereditary ophthalmic diseases and enhancing the diagnostic yield through additional gene panel sequencing. Thus, gene panel sequencing can serve as a primary tool for the genetic diagnosis of hereditary ophthalmic diseases, even in cases where a single genetic cause is suspected. With a deeper comprehension of the genetic mechanisms underlying these diseases, it becomes feasible.

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.

Congenital coralliform cataract is the predominant consequence of a recurrent mutation in the CRYGD gene

BACKGROUND

Congenital cataract is a leading cause of treatable childhood blindness and both clinically and genetically heterogeneous. Among the already characterized phenotypes, coralliform cataract is a rare special form of congenital cataracts. Although previous studies had shown that mutations in the γD-crystallin (CRYGD) can result in congenital coralliform cataracts, no conclusive genotype-phenotype correlation might be drawn. Here we aimed to identify the spectrum and frequency of CRYGD gene mutations in congenital coralliform cataracts of Chinese origin.

METHODS

The medical records of 392 Chinese families with congenital cataracts were reviewed between January 2011 and December 2021. The families, clinically documented to have congenital coralliform cataracts, were screened for mutations in candidate CRYGD gene. The genomic DNA of all subjects was extracted from peripheral blood leukocytes. PCR amplified and direct sequencing were performed to identify the disease-causing mutation.

RESULTS

A total of 12 families with coralliform cataracts were recruited in this study in the past 10 years, accounting for 3.1% of the families with congenital cataracts. Of the 12 families, all affected individuals presented with bilateral non-progressive coralliform cataracts since birth, with the best-corrected Snellen visual acuities ranging from 20/200 to 20/25. A recurrent c.70 C > A (p. P24T) mutation in CRYGD was identified in 10 families (83.3%) with congenital cataract, which co-segregated with all affected individuals and was not observed in unaffected family members or ethnically matched normal controls.

CONCLUSIONS

The coralliform cataract is characterized by being bilateral, non-progressive and present at birth. A recurrent p.P24T CRYGD mutation occurs independently in 83.3% of the Chinese families with congenital coralliform cataracts and most likely represents a mutational hot spot, which underscore the relations between coralliform cataract and p.P24T CRYGD.

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.

Association of single nucleotide polymorphism variations in CRYAA and CRYAB genes with congenital cataract in Pakistani population

Background

The purpose of present study was to analyze the association of single nucleotide polymorphism (SNPs) variant in CRYAA and CRYAB genes with Congenital Cataract.

Method

Total 196 blood samples of children were collected, out of which 102 samples were congenital cataract (case group) and 94 samples were normal individuals (control group). Genomic DNA was extracted by using optimized inorganic method. Tetra primers for SNPs were designed and TETRA-ARMs assay was performed on both groups. Genotypic, allelic frequency and haplotype analyses were obtained by using SNPstats software.

Results

The coordination of genotypic and allelic frequencies of CRYAA and CRYAB genes variants and the association between case and control groups showed increased risk of congenital cataract in children who contained rs13053109 G > C variant of CRYAA in all models (all P > 0.05). This depicts the evident difference between the frequencies of case and control groups. The haplotype analysis of SNPs rs3761382, rs7278468 and rs13051039 of CRYAA gene showed weak linkage disequilibrium between the 3 SNPs (r2 < 0.8). The haplotype CTC indicated the high risk of congenital cataract in infants based of its p value (OR = 1.60 95% CI = 0.11-22.64, P > 0.05).

Conclusion

The variation in CRYAA gene can be the risk factor for congenital cataract in infants.

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.

Deciphering the association of intronic single nucleotide polymorphisms of crystallin gene family with congenital cataract

Purpose

Introns play an important role in gene regulation and expression. Single nucleotide polymorphisms (SNPs) in introns have the potential to cause disease and alter the genotype-phenotype association. Hence, this study aimed to decipher the association of SNPs in the introns of the crystallin gene in congenital cataracts.

Methods

SNPs in the introns of crystallin gene family - CRYAA (rs3788059), CRYAB (rs2070894), CRYBA4 (rs2071861), and CRYBB2 (rs5752083, rs5996863) - were genotyped in 248 participants consisting of 141 congenital cataracts and 107 healthy controls by allele-specific oligonucleotide polymerase chain reaction method. Around 10% of samples for each SNPs were sequenced to confirm the genotypes. The allele, genotype, and haplotype frequency were evaluated by the SHEsis online tool.

Results

Using dominant model, the "A" allele of rs3788059 was found to have an increased risk toward congenital cataract development whereas the "G" allele was found to be protective (AA + AG vs. GG; odds ratio [OR] 95% confidence interval [CI] = 3.73 [1.71, 8.15], P = 0.0009). The "A" allele of both rs2070894 (AA + AG vs. GG; OR [95% CI] = 0.49 [0.29, 0.84], P = 0.012) and rs5752083 (AA + AC vs. CC; OR [95% CI] = 0.25 [0.08, 0.76], P = 0.016) were suggested to have a protective role by the dominant model. The A-C-T haplotype (rs2071861, rs5752083, and rs5996863) was found to be a significant risk factor for the development of congenital cataract.

Conclusion

Intronic SNPs in crystallin genes may play a role in the predisposition toward congenital cataract. However, the present findings need to be replicated in a large cohort with more number of samples.

A mutated CRYGD associated with congenital coralliform cataracts in two Chinese pedigrees

AIM

To investigate the causal gene mutation and clinical characteristics for two Chinese families with autosomal dominant congenital coralliform cataract.

METHODS

Two Chinese pedigrees with congenital cataract were investigated. Routine ophthalmic examinations were performed on all patients and non-affected family members. Peripheral blood samples were collected, and the genomic DNAs were extracted. The coding regions of proband's DNAs were analyzed with cataract gene panel. The identified mutation was amplified by polymerase chain reaction, and automated sequencing was performed in other members of two families to verify whether the mutated gene was co-segregated with the disease.

RESULTS

Congenital coralliform cataract was inherited in an autosomal dominant mode in both pedigrees. For each family, more than half of the family members were affected. All patients presented with severe visual impairment after birth as a result of bilateral symmetric coralliform lens opacification. An exact the same defect in the same gene, a heterozygous mutation of c.70C>A (p. P24T) in exon 2 of γD-crystallin gene, was detected in both probands from each family. Sanger sequencing analysis demonstrated that the mutated CRYGD was co-segregated in these two families.

CONCLUSION

A c.70C>A (p. P24T) variant in CRYGD gene was reconfirmed to be the causal gene in two Chinese pedigrees. It is known that mutated CRYGD caused most of the congenital coralliform cataracts, suggesting that the CRYGD gene is associated with coralliform congenital cataract.

Double Domain Swapping in Human γC and γD Crystallin Drives Early Stages of Aggregation

Human γD (HγD) and γC (HγC) are two-domain crystallin (Crys) proteins expressed in the nucleus of the eye lens. Structural perturbations in the protein often trigger aggregation, which eventually leads to cataract. To decipher the underlying molecular mechanism, it is important to characterize the partially unfolded conformations, which are aggregation-prone. Using a coarse grained protein model and molecular dynamics simulations, we studied the role of on-pathway folding intermediates in the early stages of aggregation. The multidimensional free energy surface revealed at least three different folding pathways with the population of partially structured intermediates. The two dominant pathways confirm sequential folding of the N-terminal [Ntd] and the C-terminal domains [Ctd], while the third, least favored, pathway involves intermediates where both the domains are partially folded. A native-like intermediate (I*), featuring the folded domains and disrupted interdomain contacts, gets populated in all three pathways. I* forms domain swapped dimers by swapping the entire Ntds and Ctds with other monomers. Population of such oligomers can explain the increased resistance to unfolding resulting in hysteresis observed in the folding experiments of HγD Crys. An ensemble of double domain swapped dimers are also formed during refolding, where intermediates consisting of partially folded Ntds and Ctds swap secondary structures with other monomers. The double domain swapping model presented in our study provides structural insights into the early events of aggregation in Crys proteins and identifies the key secondary structural swapping elements, where introducing mutations will aid in regulating the overall aggregation propensity.

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.

Molecular Etiology of Isolated Congenital Cataract Using Next-Generation Sequencing: Single Center Exome Sequencing Data from Turkey

Congenital cataract, which refers to lenticular opacity diagnosed at birth or more commonly during the first year of life, is one of the leading causes of childhood blindness. Molecular understanding of the disease pathogenesis has evolved thanks to many studies based on modern technologies. In this study, we aimed to identify and discuss the molecular etiology of nonsyndromic or nonmetabolic bilateral congenital cataract by whole-exome sequencing (WES). Patients with bilateral congenital cataract presumed to be isolated after metabolic and genetic evaluation were enrolled in the study. All patients underwent detailed ophthalmological examination and bilateral cataract surgery. DNA samples of the probands, parents, and available affected family members were analyzed by WES. Variants were validated and confirmed by Sanger sequencing in all probands and in available affected family members. A total of 4 patients (3 girls and 1 boy) were recruited. Two patients had nuclear, 1 patient had total, and 1 patient had combined lamellar and sutural cataract. One family had consanguinity. A heterozygous c.215+1G>A mutation in CRYBA1, heterozygous c.432C>G (p.Tyr144Ter) mutation in CRYGC, heterozygous c.70A>C (p.Pro24Thr) mutation in CRYGD, and a heterozygous c.466G>A (p.Gly156Arg) mutation in CRYBB3 were detected. All these mutations were confirmed by Sanger sequencing in selected affected individuals. The current study identified all causative mutations of congenital cataract in the crystalline genes. The results confirmed that WES is a very useful tool in the investigation of the diseases with heterogeneous genetic background.

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.

MS/MS in silico subtraction-based proteomic profiling as an approach to facilitate disease gene discovery: application to lens development and cataract

While the bioinformatics resource-tool iSyTE (integrated Systems Tool for Eye gene discovery) effectively identifies human cataract-associated genes, it is currently based on just transcriptome data, and thus, it is necessary to include protein-level information to gain greater confidence in gene prioritization. Here, we expand iSyTE through development of a novel proteome-based resource on the lens and demonstrate its utility in cataract gene discovery. We applied high-throughput tandem mass spectrometry (MS/MS) to generate a global protein expression profile of mouse lens at embryonic day (E)14.5, which identified 2371 lens-expressed proteins. A major challenge of high-throughput expression profiling is identification of high-priority candidates among the thousands of expressed proteins. To address this problem, we generated new MS/MS proteome data on mouse whole embryonic body (WB). WB proteome was then used as a reference dataset for performing "in silico WB-subtraction" comparative analysis with the lens proteome, which effectively identified 422 proteins with lens-enriched expression at ≥ 2.5 average spectral counts, ≥ 2.0 fold enrichment (FDR < 0.01) cut-off. These top 20% candidates represent a rich pool of high-priority proteins in the lens including known human cataract-linked genes and many new potential regulators of lens development and homeostasis. This rich information is made publicly accessible through iSyTE (https://research.bioinformatics.udel.edu/iSyTE/), which enables user-friendly visualization of promising candidates, thus making iSyTE a comprehensive tool for cataract gene discovery.

Chemical Modification Alters Protein-Protein Interactions and Can Lead to Lower Protein Solubility

The chemical modification of proteins is at the frontier of developments in biological imaging and biopharmaceutics. With the advent of more sensitive and higher resolution imaging techniques,  researchers increasingly rely on the functionalization of proteins to enable these techniques to capture cellular processes. For biopharmaceutical therapies, chemically modified proteins, for example, antibody-drug conjugates (ADCs) offer the possibility of more tailored treatments for the disease with lower toxicities than traditional small molecule therapies. However, relatively little consideration is paid to how chemical modifications impact protein-protein interactions and solution stability. Using human γD-crystallin as a model, we demonstrate that chemical modification of the protein surface alters protein-protein interactions, which can result in lower solubility depending on the chemical nature of the modifier and the position on the protein where the modification is made. Understanding these effects is essential to ensure that modifying proteins effectively occurs with minimum self-association and that studies carried out using labeled proteins accurately reflect those of unmodified proteins.

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.

Clinical and genetic characteristics of Chinese patients with familial or sporadic pediatric cataract

Background

Pediatric cataract is a clinically and genetically heterogeneous disease which is a significant cause of lifelong visual impairment and treatable blindness. Our study aims to investigate the genotype spectrum in a group of Chinese patients with pediatric cataract.

Methods

We enrolled 39 families with pediatric cataract from October 2015 to April 2016. DNA samples of the probands were analyzed by target next-generation sequencing. Variants were validated using Sanger sequencing in the probands and available family members.

Results

In our cohort of 39 cases with different types of pediatric cataract, 23 cases were found to harbor putative pathogenic variants in 15 genes: CRYAA, CRYBA1, CRYBA4, CRYBB1, CRYGC, CRYGD, MIP, GCNT2, IARS2, NHS, BCOR, BFSP2, FYCO1, MAF, and PAX6. The mutation detection rates in the familial and sporadic cases were 75 and 47.8%, respectively. Of the 23 causative variants, over half were novel.

Conclusions

This is a rare report of systematic mutation screening analysis of pediatric cataract in a comparably large cohort of Chinese patients. Our observations enrich the mutation spectrum of pediatric cataract. Next-generation sequencing provides significant diagnostic information for pediatric cataract cases, especially when considering sporadic and subtle syndromal cases.

Electronic supplementary material

The online version of this article (10.1186/s13023-018-0828-0) contains supplementary material, which is available to authorized users.

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 Combined NMR and SAXS Analysis of the Partially Folded Cataract-Associated V75D γD-Crystallin

A cataract is a pathological condition characterized by the clouding of the normally clear eye lens brought about by deposition of crystallin proteins in the lens fiber cells. These protein aggregates reduce visual acuity by scattering or blocking incoming light. Chemical damage to proteins of the crystallin family, accumulated over a lifetime, leads to age-related cataract, whereas inherited mutations are associated with congenital or early-onset cataract. The V75D mutant of γD-crystallin is associated with congenital cataract in mice and was previously shown to un/fold via a partially folded intermediate. Here, we structurally characterized the stable equilibrium urea unfolding intermediate of V75D at the ensemble level using solution NMR and small-angle x-ray scattering. Our data show that, in the intermediate, the C-terminal domain retains a folded conformation that is similar to the native wild-type protein, whereas the N-terminal domain is unfolded and comprises an ensemble of random conformers, without any detectable residual structural propensities.

Targeted Exome Sequencing of Congenital Cataracts Related Genes: Broadening the Mutation Spectrum and Genotype-Phenotype Correlations in 27 Chinese Han Families

Congenital cataract is the most frequent inherited ocular disorder and the most leading cause of lifelong visual loss. The screening of pathogenic mutations can be very challenging in some cases, for congenital cataracts are clinically and genetically heterogeneous diseases. The aim of this study is to investigate the mutation spectrum and frequency of 54 cartaract-associated genes in 27 Chinese families with congenital cataracts. Variants in 54 cataract-associated genes were screened by targeted next-generation sequencing (NGS) and then validated by Sanger sequencing. We identified pathogenic variants in 62.96% (17/27) of families, and over 52.94% (9/17) of these variants were novel. Among them, three are splicing site mutations, four are nonsense mutations, seven are missense mutations, two are frame shift mutations and one is intronic mutation. This included identification of: complex ocular phenotypes due to two novel PAX6 mutations; progressive cortical cataract and lamellar cataract with lens subluxation due to two novel CRYGS mutations. Mutations were also found in rarely reported genes including CRYBA4, CRYBA2, BFSP1, VIM, HSF4, and EZR. Our study expands the mutation spectrum and frequency of genes responsible for congenital cataracts. Targeted next-generation sequencing in inherited congenital cataract patients provided significant diagnostic information.

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.

Aggregation of Trp > Glu point mutants of human gamma-D crystallin provides a model for hereditary or UV-induced cataract

Numerous mutations and covalent modifications of the highly abundant, long-lived crystallins of the eye lens cause their aggregation leading to progressive opacification of the lens, cataract. The nature and biochemical mechanisms of the aggregation process are poorly understood, as neither amyloid nor native-state polymers are commonly found in opaque lenses. The βγ-crystallin fold contains four highly conserved buried tryptophans, which can be oxidized to more hydrophilic products, such as kynurenine, upon UV-B irradiation. We mimicked this class of oxidative damage using Trp→Glu point mutants of human γD-crystallin. Such substitutions may represent a model of UV-induced photodamage-introduction of a charged group into the hydrophobic core generating "denaturation from within." The effects of Trp→Glu substitutions were highly position dependent. While each was destabilizing, only the two located in the bottom of the double Greek key fold-W42E and W130E-yielded robust aggregation of partially unfolded intermediates at 37°C and pH 7. The αB-crystallin chaperone suppressed aggregation of W130E, but not W42E, indicating distinct aggregation pathways from damage in the N-terminal vs C-terminal domain. The W130E aggregates had loosely fibrillar morphology, yet were nonamyloid, noncovalent, showed little surface hydrophobicity, and formed at least 20°C below the melting temperature of the native β-sheets. These features are most consistent with domain-swapped polymerization. Aggregation of partially destabilized crystallins under physiological conditions, as occurs in this class of point mutants, could provide a simple in vitro model system for drug discovery and optimization.

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

Congenital cataract disease is a clinically and genetically heterogeneous lens disorder. The purpose of this study was to identify the genetic defects and to investigate the relationships between disease-causing genes and lens morphology in congenital cataracts. Patients were given a physical examination, and their blood samples were collected for DNA extraction. Mutation analysis was performed by direct sequencing of the following candidate genes: CRYGC, CRYGD, CRYGS, GJA8, GJA3 and CRYAA. Mutational analysis of CRYGD identified a recurrent (p.P24T) mutation in two unrelated families with congenital coralliform cataracts and three novel (p.Q101X, p.E104fsX4 and p.E135X) mutations in three families with congenital nuclear cataracts. The p.E135X mutation is a de novo mutation. Haplotype analysis showed patients inherited the same CRYGD allele originated from father. The p.E135X mutation seen in two siblings suggests a mechanism of gonadal mosaicism in the father.

Single-molecule Force Spectroscopy Predicts a Misfolded, Domain-swapped Conformation in human γD-Crystallin Protein

Cataract is a protein misfolding disease where the size of the aggregate is directly related to the severity of the disorder. However, the molecular mechanisms that trigger the onset of aggregation remain unknown. Here we use a combination of protein engineering techniques and single-molecule force spectroscopy using atomic force microscopy to study the individual unfolding pathways of the human γD-crystallin, a multidomain protein that must remain correctly folded during the entire lifetime to guarantee lens transparency. When stretching individual polyproteins containing two neighboring HγD-crystallin monomers, we captured an anomalous misfolded conformation in which the β1 and β2 strands of the N terminus domain of two adjacent monomers swap. This experimentally elusive domain-swapped conformation is likely to be responsible for the increase in molecular aggregation that we measure in vitro. Our results demonstrate the power of force spectroscopy at capturing rare misfolded conformations with potential implications for the understanding of the molecular onset of protein aggregation.

Gamma crystallins of the human eye lens

BACKGROUND

Protein crystallins co me in three types (α, β and γ) and are found predominantly in the eye, and particularly in the lens, where they are packed into a compact, plastic, elastic, and transparent globule of proper refractive power range that aids in focusing incoming light on to the retina. Of these, the γ-crystallins are found largely in the nuclear region of the lens at very high concentrations (>400 mg/ml). The connection between their structure and inter-molecular interactions and lens transparency is an issue of particular interest.

SCOPE OF REVIEW

We review the origin and phylogeny of the gamma crystallins, their special structure involving the use of Greek key supersecondary structural motif, and how they aid in offering the appropriate refractive index gradient, intermolecular short range attractive interactions (aiding in packing them into a transparent ball), the role that several of the constituent amino acid residues play in this process, the thermodynamic and kinetic stability and how even single point mutations can upset this delicate balance and lead to intermolecular aggregation, forming light-scattering particles which compromise transparency. We cite several examples of this, and illustrate this by cloning, expressing, isolating and comparing the properties of the mutant protein S39C of human γS-crystallin (associated with congenital cataract-microcornea), with those of the wild type molecule. In addition, we note that human γ-crystallins are also present in other parts of the eye (e.g., retina), where their functions are yet to be understood.

MAJOR CONCLUSIONS

There are several 'crucial' residues in and around the Greek key motifs which are essential to maintain the compact architecture of the crystallin molecules. We find that a mutation that replaces even one of these residues can lead to reduction in solubility, formation of light-scattering particles and loss of transparency in the molecular assembly.

GENERAL SIGNIFICANCE

Such a molecular understanding of the process helps us construct the continuum of genotype-molecular structural phenotype-clinical (pathological) phenotype. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

Personalized diagnosis and management of congenital cataract by next-generation sequencing

PURPOSE

To assess the utility of integrating genomic data from next-generation sequencing and phenotypic data to enhance the diagnosis of bilateral congenital cataract (CC).

DESIGN

Evaluation of diagnostic technology.

PARTICIPANTS

Thirty-six individuals diagnosed with nonsyndromic or syndromic bilateral congenital cataract were selected for investigation through a single ophthalmic genetics clinic.

METHODS

Participants underwent a detailed ophthalmic examination, accompanied by dysmorphology assessment where appropriate. Lenticular, ocular, and systemic phenotypes were recorded. Mutations were detected using a custom-designed target enrichment that permitted parallel analysis of 115 genes associated with CC by high-throughput, next-generation DNA sequencing (NGS). Thirty-six patients and a known positive control were tested. Suspected pathogenic variants were confirmed by bidirectional Sanger sequencing in relevant probands and other affected family members.

MAIN OUTCOME MEASURES

Molecular genetic results and details of clinical phenotypes were identified.

RESULTS

Next-generation DNA sequencing technologies are able to determine the precise genetic cause of CC in 75% of individuals, and 85% patients with nonsyndromic CC were found to have likely pathogenic mutations, all of which occurred in highly conserved domains known to be vital for normal protein function. The pick-up rate in patients with syndromic CC also was high, with 63% having potential disease-causing mutations.

CONCLUSIONS

This analysis demonstrates the clinical utility of this test, providing examples where it altered clinical management, directed care pathways, and enabled more accurate genetic counseling. This comprehensive screen will extend access to genetic testing and lead to improved diagnostic and management outcomes through a stratified medicine approach. Establishing more robust genotype-phenotype correlations will advance knowledge of cataract-forming mechanisms.

A nonsense mutation of γD-crystallin associated with congenital nuclear and posterior polar cataract in a Chinese family

OBJECTIVE

The goal of this study was to characterize the disease-causing mutations in a Chinese family with congenital nuclear and posterior polar cataracts.

METHODS

Clinical data of patients in the family were recorded using slit-lamp photography and high definition video. Genomic DNA samples were extracted from the peripheral blood of the pedigree members and 100 healthy controls. Mutation screening was performed in the candidate genes by bi-directional sequencing of the amplified products.

RESULTS

The congenital cataract phenotype of the pedigree was identified by slit-lamp examinations and observation during surgery as nuclear and posterior polar cataracts. Through the sequencing of the candidate genes, a heterozygous c. 418C>T change was detected in the coding region of the γD-crystallin gene (CRYGD). As a result of this change, a highly conserved arginine residue was replaced by a stop codon (p. R140X). This change was discovered among all of the affected individuals with cataracts, but not among the unaffected family members or the 100 ethnically matched controls.

CONCLUSIONS

This study identified a novel congenital nuclear and posterior polar cataract phenotype caused by the recurrent mutation p. R140X in CRYGD.

Study of the γD-crystallin protein using two-dimensional infrared (2DIR) spectroscopy: experiment and simulation

Cataracts is a misfolding protein disease in which one of the major components is the γD-crystallin protein. The conformational structure of the aggregated γD-crystallin and the interactions that cause aggregation are largely unknown. A recent experimental two-dimensional infrared (2DIR) spectroscopy study determined that the C-terminal domain has a high propensity to form β-sheets whereas the N-terminal domain forms a disordered structure in the fiber state. We present a combined computational molecular dynamics and infrared spectroscopy study of the local dynamics of these domains. The computed 2DIR signals agree remarkably well with experiment. We show that the two domains, both of which have a Greek key structural fold, experience different electrostatic environments, which may be related to the fact that the C-terminal domain is more structurally stable than the N-terminal domain. We correlate the vibrational couplings to known energy dissipation mechanisms and reveal their origin.

Amyloid fiber formation in human γD-Crystallin induced by UV-B photodamage

γD-Crystallin is an abundant structural protein of the lens that is found in native and modified forms in cataractous aggregates. We establish that UV-B irradiation of γD-Crystallin leads to structurally specific modifications and precipitation via two mechanisms: amorphous aggregates and amyloid fibers. UV-B radiation causes cleavage of the backbone, in large measure near the interdomain interface, where side chain oxidations are also concentrated. 2D IR spectroscopy and expressed protein ligation localize fiber formation exclusively to the C-terminal domain of γD-Crystallin. The native β-sandwich domains are not retained upon precipitation by either mechanism. The similarities between the amyloid forming pathways when induced by either UV-B radiation or low pH suggest that the propensity for the C-terminal β-sandwich domain to form amyloid β-sheets determines the misfolding pathway independent of the mechanism of denaturation.

A CRYGC gene mutation associated with autosomal dominant pulverulent cataract

PURPOSE

To describe at molecular level a family with pulverulent congenital cataract associated with a CRYGC gene mutation.

METHODS

One family with several affected members with pulverulent congenital cataract and 230 healthy controls were examined. Genomic DNA from leukocytes was isolated to analyze the CRYGA-D cluster, CX46, CX50 and MIP genes through high-resolution melting curve and DNA sequencing.

RESULTS

DNA sequencing in the affected members revealed the c.143G>A mutation (p.R48H) in exon 2 of the CRYGC gene; 230 healthy controls and ten healthy relatives were also analyzed and none of them showed the c.143G>A mutation. No other polymorphisms or mutations were found to be present.

CONCLUSION

In the present study, we described a family with pulverulent congenital cataract that segregated the c.143G>A mutation (p.R48H) in the CRYGC gene. A few mutations have been described in the CRYGC gene in autosomal dominant cataract, none of them with pulverulent cataract making clear the clinical heterogeneity of congenital cataract. This mutation has been associated with the phenotype of congenital cataract but also is considered an SNP in the NCBI data base. Our data and previous report suggest that p.R48H could be a disease-causing mutation and not an SNP.

Crystal structure of the cataract-causing P23T γD-crystallin mutant

Up to now, efforts to crystallize the cataract-associated P23T mutant of human γD-crystallin have not been successful. Therefore, insights into the light scattering mechanism of this mutant have been exclusively obtained from solution work. Here we present the first crystal structure of the P23T mutant at 2.5 Å resolution. The protein exhibits essentially the same overall structure as seen for the wild-type protein. Based on our structural data, we confirm that no major conformational changes are caused by the mutation, and that solution phase properties of the mutant appear exclusively associated with cataract formation.

The human crystallin gene families

Crystallins are the abundant, long-lived proteins of the eye lens. The major human crystallins belong to two different superfamilies: the small heat-shock proteins (α-crystallins) and the βγ-crystallins. During evolution, other proteins have sometimes been recruited as crystallins to modify the properties of the lens. In the developing human lens, the enzyme betaine-homocysteine methyltransferase serves such a role. Evolutionary modification has also resulted in loss of expression of some human crystallin genes or of specific splice forms. Crystallin organization is essential for lens transparency and mutations; even minor changes to surface residues can cause cataract and loss of vision.

A nonsense mutation of CRYGC associated with autosomal dominant congenital nuclear cataracts and microcornea in a Chinese pedigree

PURPOSE

To report the identification of a nonsense mutation in γC-crystallin (CRYGC) associated with autosomal dominant congenital nuclear cataracts and microcornea in a Chinese family.

METHODS

We investigated four generations of a Chinese family six of whose members were affected by nuclear cataracts and microcornea. The genomic DNA was extracted from peripheral blood leukocytes. All reported nuclear cataract-related candidate genes were screened for causative mutations by direct DNA sequencing. The effects of amino acid changes on the structure and function of proteins were predicted by bioinformatics analysis.

RESULTS

All affected individuals in this family exhibited nuclear cataracts and microcornea. Direct sequencing of the candidate gene cluster showed a c.471G>A transition in exon 3 of CRYGC, which co-segregated according to family members with cataracts, and was not observed in 100 normal controls. This single nucleotide change was predicted to introduce a translation stop codon at tryptophan 157 (W157X). Bioinformatics analysis showed that the mutation was predicted to affect the function and secondary structure of the CRYGC protein.

CONCLUSIONS

This study identified a disease-causing mutation c.471G>A in CRYGC in a Chinese family with cataracts, expanding the mutation spectrum of CRYGC causing congenital cataracts.

Racemization of two proteins over our lifespan: deamidation of asparagine 76 in γS crystallin is greater in cataract than in normal lenses across the age range

PURPOSE

Long-lived proteins are widespread in man, yet little is known about the processes that affect their function over time, or their role in age-related diseases.

METHODS

Racemization of two proteins from normal and cataract human lenses were compared with age using tryptic digestion and LC/mass spectrometry. Asp 151 in αA crystallin and Asn 76 in γS crystallin were studied.

RESULTS

Age-dependent profiles for the two proteins from normal lenses were different. In neither protein did the modifications increase linearly with age. For αA crystallin, racemization occurred most rapidly during the first 15 years of life, with approximately half of L-Asp 151 converted to D-isoAsp, L-isoAsp, and D-Asp in a ratio of 3:1:0.5. Values then changed little. By contrast, racemization of Asn 76 in γS crystallin was slow until age 15, with isoAsp accounting for only 5%. Values remained relatively constant until age 40 when a linear increase (1%/year) took place. When cataract lenses were compared with age-matched normal lenses, there were marked differences in the time courses of the two crystallins. For αA crystallin, there was no significant difference in Asp 151 racemization between cataract and normal lenses. By contrast, in γS crystallin the degree of conversion of Asn 76 to isoAsp in cataract lenses was approximately double that of normals at every age.

CONCLUSIONS

Modification of Asn and Asp over time may contribute to denaturation of proteins in the human lens. An accelerated rate of deamidation/racemization at selected sites in proteins, such as γS crystallin, may contribute to cataract formation.

A missense mutation in CRYGD linked with autosomal dominant congenital cataract of aculeiform type

To detect the underlying genetic defects in two autosomal dominant congenital cataract (ADCC) families, having respectively twenty and four members affected with bilateral congenital cataract. Detailed family history and clinical data were recorded. Mutation screening in twenty three candidate genes including crystallins (CRYAA, CRYAB, CRYBA1/A3, CRYBA2, CRYBA4, CRYBB1, CRYBB2, CRYBB3, CRYGA, CRYGB, CRYGC, CRYGD, and CRYGS), gap junctional channels; connexins (GJA8, GJA3), beaded filament chain proteins (BFSP1, BFSP2), major intrinsic protein (MIP), lens intrinsic membrane protein-2 (LIM2), transcriptional factor (MAF), and in genes encoding for membrane-associated proteins (TMEM114, CHMP4B, EPHA2) was performed by bi-directional sequence analysis of the amplified products. In family A twenty members in six generations were affected by bilateral aculeiform type cataract and in family B four affected members in three generations had granular nuclear cataract. Mutation screening in already known candidate genes by sequence analyses revealed proline to threonine substitution at codon 23 (p.Pro23Thr) in CRYGD for aculeiform type cataract in family A. The family B with four members affected by granular nuclear cataract, however, could not be linked with any of these analyzed 23 candidate genes. The present study describes identification of p.Pro23Thr mutation in CRYGD for aculeiform type cataract in an ADCC family of Indian origin. The identical mutation has previously been reported to be linked with different phenotypes; lamellar cataract, cerulean cataract, coralliform cataract, flaky silica-like nuclear cataract and fasciculiform type cataract in different ADCC families. Interestingly, a mutation of different codon, i.e., p.Arg58His in CRYGD has been reported to be linked with aculeiform cataract in four different families; two from Switzerland, one from Macedonia and in a Mexican family. The findings in present study thus expand the genetic heterogeneity for aculeiform type cataract. Further, exclusion of these twenty three known candidate genes in family B having ADCC of granular nuclear type indicates the role of some other gene apart from for crystallins, gap junction channels, beaded filaments and membrane-associated proteins, and MAF for this phenotype.

A novel CRYGD mutation (p.Trp43Arg) causing autosomal dominant congenital cataract in a Chinese family

To identify the genetic defect associated with autosomal dominant congenital nuclear cataract in a Chinese family, molecular genetic investigation via haplotype analysis and direct sequencing were performed Sequencing of the CRYGD gene revealed a c.127T>C transition, which resulted in a substitution of a highly conserved tryptophan with arginine at codon 43 (p.Trp43Arg). This mutation co-segregated with all affected individuals and was not observed in either unaffected family members or in 200 normal unrelated individuals. Biophysical studies indicated that the p.Trp43Arg mutation resulted in significant tertiary structural changes. The mutant protein was much less stable than the wild-type protein, and was more prone to aggregate when subjected to environmental stresses such as heat and UV irradiation. © 2010 Wiley-Liss, Inc.

A recurrent mutation in CRYGD is associated with autosomal dominant congenital coralliform cataract in two unrelated Chinese families

PURPOSE

Congenital cataract is a clinically and genetically heterogeneous lens disorder. The purpose of this study was to identify the mutation responsible for autosomal dominant congenital coralliform cataracts in two Chinese families and to investigate the relationship between virulence genes and lens morphology.

METHODS

Patients received a physical examination, and blood samples were collected for DNA extraction. Mutation analysis was performed by direct sequencing of the candidate genes: gammaC-crystallin (CRYGC), gammaD-crystallin (CRYGD), gammaS-crystallin (CRYGS), gap-junction protein, alpha 8 (GJA8), gap-junction protein, alpha 3 (GJA3), and alphaA-crystallin (CRYAA).

RESULTS

The affected individuals in two families had congenital coralliform cataracts. Mutational analysis of the CRYGD identified a C→A transversion at nucleotide position c.70 in exon 2, which resulted in a threonine substitution for proline at amino-acid residue 24 (P24T). This mutation was identified in all affected individuals but was not found in healthy relatives or 100 control chromosomes from the same ethnic background.

CONCLUSIONS

Our results indicated that the P24T mutation of CRYGD was responsible for two Chinese pedigrees with congenital coralliform cataracts. CRYGD and coralliform cataracts are highly related, and P24T may be a hot-point mutation for this disorder.

A novel mutation in γD-crystallin associated with autosomal dominant congenital cataract in a Chinese family

PURPOSE

To identify the pathogenic gene mutation in a Chinese family with autosomal dominant congenital nuclear cataract.

METHODS

After obtaining informed consent, detailed ophthalmic examinations were performed and genomic DNAs were obtained from eleven family members in a three-generation Chinese family with five affected. All exons of candidate genes associated with congenital nuclear cataract were amplified by polymerase chain reaction (PCR) and the PCR products were sequenced in both directions. The hydrophobic property of the mutant protein was analyzed with bioinformatics program ProtScale. The structure homology modeling of the mutant protein was based on Swiss-Model Serve, and its structure was displayed and compared with native γD-crystallin (CRYGD) using the RasMol software.

RESULTS

By sequencing the encoding regions of the candidate genes, a novel mutation (c.110G>C) was detected in exon 2 of CRYGD, which resulted in the substitution of a highly conserved arginine by proline at codon 36 (p.R36P). The mutation co-segregated with all patients and was absent in 100 normal Chinese controls. Bioinformatics analysis showed an obvious increase of the local hydrophilicity of the R36P mutant γD-crystallin. The homology modeling showed that the structure of the mutant protein was similar with that of native human γD-crystallin.

CONCLUSIONS

The study identified a novel mutation (c. 110G>C) in CRYGD associated with autosomal dominant congenital cataract in a Chinese family. It expands the mutation spectrum of CRYGD in association with congenital cataract.

Mutation screening and genotype phenotype correlation of α-crystallin, γ-crystallin and GJA8 gene in congenital cataract

PURPOSE

To screen α-crystallin (CRYAB), γ-crystallin (CRYGC and CRYGD), and Connexin 50 (Cx-50 or GJA8) genes in congenital cataract patients and controls.

METHODS

Thirty clinically diagnosed congenital cataract cases below 3 years of age from northern India, presenting at Dr. R. P. Centre for Ophthalmic Sciences (AIIMS, New Delhi, India) were enrolled in this study. Genomic DNA was extracted from peripheral blood, all coding and exon/intron regions were amplified using PCR and direct sequencing was performed to detect any nucleotide variation. ProtScale and Discovery Studio programs were used for insilico and structural analysis of non-synonymous mutations.

RESULTS

DNA sequencing analysis of CRYAB, CRYGC, CRYGD, and GJA8 showed a total of six variations of which two were novel (CRYGC:p.R48H and GJA8:p.L281C) and four have been previously reported (CRYAB: rs11603779T>G, GJA8: p.L268L, CRYGD: p.R95R, and c.T564C). Both the novel changes, in CRYGC and GJA8 were found in 16.6% of the patients. Previously reported nucleotide alterations (CRYGD:p.R95R and c.T564C) were found in 90% of the patients. Insilico and structural analysis data suggested that two novel non-synonymous mutations altered the stability and solvent accessibility of γC-crystallin and Cx-50 proteins which may lead to lens opacification.

CONCLUSIONS

We observed two novel nonsynonymous variations and four reported variations in CRYAB, CRYGC, CRYGD, and GJA8. The p.R48H variation in γC-crystallin may disrupt the normal structure of lens and can cause cataract. Cx50 is responsible for joining the lens cells into a functional syncytium and a mutation (p.L281C) in GJA8 may lead to lens opacification resulting in cataract formation. This study further expands the mutation spectrum of congenital cataract and help understanding how mutant proteins lead to opacification of lens.

Examining the influence of ultraviolet C irradiation on recombinant human γD-crystallin

PURPOSE

Human γD crystallin is a principal protein component of the human eye lens and associated with the development of juvenile and mature-onset cataracts. Exposure to ultraviolet (UV) light is thought to perturb protein structure and eventually lead to aggregation. This work is aimed at exploring the effects of UV-C irradiation on recombinant human γD-crystallin (HGDC).

METHODS

Recombinant HGDC proteins were expressed in E. coli strain BL21(DE3) harboring plasmid pEHisHGDC and purified using chromatographic methods. The proteins were then exposed to UV-C light (λ(max)=254 nm, 15 W) at the intensity of 420, 800, or 1850 μW/cm(2). The UV-C-unexposed, supernatant fraction of UV-C-exposed, and re-dissolved precipitated fraction of UV-C exposed preparations were characterized by SDS-PAGE, turbidity measurement, CD spectroscopy, tryptophan fluorescence spectroscopy, acrylamide fluorescence quenching analysis, and sulfhydryl group measurements.

RESULTS

The turbidity of the HGDC sample solution was found to be positively correlated with HGDC concentration, UV-C irradiation intensity, and UV-C irradiation duration. When exposed to UV-C, HGDC sample solutions became visibly turbid and a noticeable amount of larger protein particle, perceptible to the naked eye, was observed upon prolonged irradiation. The precipitated fraction of irradiated HGDC sample was found to be re-dissolved by guanidine hydrochloride. Electrophoresis, acrylamide fluorescence quenching, and spectroscopic analyses revealed differences in structures among the non-irradiated HGDC, the supernatant fraction of irradiated HGDC, and the re-dissolved precipitated fraction of irradiated HGDC. Through the use of L-cysteine, the measurements of sulfhydryl contents, and the reducing as well as non-reducing SDS-PAGE, our data further suggested that disulfide bond formation and/or cleavage probably play an important role in aggregation and/or precipitation of HGDC elicited by UV-C irradiation.

CONCLUSIONS

Our findings highlight the close connections among disulfide bond cleavage and/or formation, intermolecular interactions, and the resultant formation of aggregates of HGDC induced by UV-C irradiation. The results from this research may not only contribute to the understanding of the environmental factors causing protein aggregation but also have implications for deciphering the molecular mechanism of cataractogenesis.

Molecular characteristics of inherited congenital cataracts

Congenital cataracts are a major cause of induced blindness in children, and inherited cataracts are the major cause of congenital cataracts. Inherited congenital cataracts have been associated with mutations in specific genes, including those of crystallins, gap junction proteins, membrane transport and channel proteins, the cytoskeleton, and growth and transcription factors. Locating and identifying the genes and mutations involved in cataractogenesis are essential to gaining an understanding of the molecular defects and pathophysiologic characteristics of inherited congenital cataracts. In this review, we summarize the current research in this field.

Molecular analysis of cataract families in India: new mutations in the CRYBB2 and GJA3 genes and rare polymorphisms

PURPOSE

The aim of the study was to resolve the genetic etiology in families having inherited cataracts.

METHODS

Families afflicted with congenital/childhood cataracts were registered in Chennai and Orissa (India). Blood samples were collected from the probands and available family members. Selected functional candidate genes were amplified by polymerase chain reaction (PCR) and characterized by direct sequencing. Putative mutations were confirmed in healthy controls.

RESULTS

We observed interesting new polymorphisms of ethnic specificity, some of frequent nature, such as a 3-bp deletion in intron 3 of CRYBB2 (encoding βB2-crystallin) and IVS1+9 c>t variation in HSF4 (encoding heat-shock factor 4). Some rare single nucleotide polymorphisms (SNPs) co-segregate with the respective phenotype such as IVS3+120c>a of CRYBB2, while M44V of CRYGD (encoding γD-crystallin), although found in association with blue dot opacity was seen in a few healthy controls too. We identified two new mutations co-segregating along with the respective cataract phenotype within the families that were not seen in healthy controls from India or Germany. These include two missense mutations; one in GJA3 (encoding gap junction protein α3, which is also referred to as connexin 46); the mutation affects codon 19 (T19M), and the corresponding phenotype is a posterior-polar cataract. The other missense mutation affects CRYBB2 (W59C; total cataract). Additionally, a cDNA variation (G54A) identified in a zonular cataract affects a highly conserved splice site of CRYBB2. This mutation, however, showed reduced penetrance in the family, which might be explained by different molecular consequences in the affected family members: nonsense-mediated decay of the mutated mRNA might have no clinical phenotype in heterozygotes, whereas the translation of the mutated mRNA is predicted to lead to a small hybrid protein (consisting of 16 amino acids of the βB2-crystallin and 18 new amino-acids), which might have a dominant-negative function in the lens.

CONCLUSIONS

This report identifies in families with childhood cataract some new alleles, which may be considered as causative for cataracts. Furthermore, we report some geographically restricted rare polymorphic sites, whose significance might be considered in some context as modifiers or alleles in sensitizing ocular lens toward cataractogenesis.

Increase in surface hydrophobicity of the cataract-associated P23T mutant of human gammaD-crystallin is responsible for its dramatically lower, retrograde solubility

The cataract-associated Pro23 to Thr (P23T) mutation in human gammaD-crystallin (HGD) has a variety of phenotypes and is geographically widespread. Therefore, there is considerable interest in understanding the molecular basis of cataract formation due to this mutation. We showed earlier [Pande, A., et al. (2005) Biochemistry 44, 2491-2500] that the probable basis of opacity in this case is the severely compromised, retrograde solubility and aggregation of P23T relative to HGD. The dramatic solubility change occurs even as the structure of the mutant protein remains essentially unchanged in vitro. We proposed that the retrograde solubility and aggregation of P23T were mediated by net hydrophobic, protein-protein interactions. On the basis of these initial findings for P23T and related mutants, and the subsequent finding that they show atypical phase behavior [McManus, J. J., et al. (2007) Proc. Natl. Acad. Sci. U.S.A. 104, 16856-16861], we concluded that the protein clusters formed in solutions of the mutant proteins were held together by net hydrophobic, anisotropic interactions. Here we show, using chemical probes, that the surface hydrophobicities of these mutants are inversely related to their solubility. Furthermore, by probing the isolated N-terminal domains of HGD and P23T directly, we find that the increase in the surface hydrophobicity of P23T is localized in the N-terminal domain. Modeling studies suggest the presence of sticky patches on the surface of the N-terminal domain that could be engaged in the formation of protein clusters via hydrophobic protein-protein interactions. This work thus provides direct evidence of the dominant role played by net hydrophobic and anisotropic protein-protein interactions in the aggregation of P23T.

A novel human CRYGD mutation in a juvenile autosomal dominant cataract

PURPOSE

Identification of causal mutation in the crystallin, connexin, and paired box gene 6 (PAX6) genes associated with childhood cataract in patients from India.

METHODS

In this study, forty eight members from seventeen families and 148 sporadic cases of childhood cataract were evaluated. Clinical and ophthalmologic examinations were performed on available affected and unaffected family members. Samples of genomic DNA were PCR amplified to screen for mutations in the candidate genes viz., alpha-A crystallin (CRYAA), beta- B2 crystallin (CRYBB2), gamma-A crystallin (CRYGA), gamma-B crystallin (CRYGB), gamma-C crystallin (CRYGC), gamma-D crystallin (CRYGD), gap junction alpha-3 (GJA3), gap junction alpha-8 (GJA8), and PAX6 based on polymerase chain reaction and single strand conformation polymorphism (PCR-SSCP) analysis. Samples showing any band mobility shift were subjected to bidirectional sequencing to confirm the variation. Co-segregation of the observed change with the disease phenotype was further tested by restriction fragment length polymorphism (RFLP) for the appropriate restriction site.

RESULTS

DNA sequencing analysis of CRYAA, CRYBB2, CRYGA-D, GJA3, GJA8, and PAX6 of the affected members of a family (C-35) showed a novel heterozygous missense mutation C>A at position 229 in CRYGD in three affected members of family C-35 with anterior polar coronary cataract. This variation C229A substitution created a novel restriction site for AluI and resulted in a substitution of highly conserved arginine at position 77 by serine (R77S). AluI restriction site analysis confirmed the transversion mutation. Analysis of the available unaffected members of the family (C-35) and 100 unrelated control subjects (200 chromosomes) of the same ethnic background did not show R77S variation. Data generated using ProtScale and PyMOL programs revealed that the mutation altered the stability and solvent-accessibility of the CRYGD protein.

CONCLUSIONS

We describe here a family having anterior polar coronary cataract that co-segregates with the novel allele R77S of CRYGD in all the affected members. The same was found to be absent in the ethnically matched controls (n=100) studied. Interestingly the residue Arg has been frequently implicated in four missense (R15C, R15S, R37S, and R59H) and in one truncation mutation (R140X) of CRYGD. In two of the reported mutations Arg residues have been replaced with Serine. This finding further expands the mutation spectrum of CRYGD in association with childhood cataract and demonstrates a possible mechanism of cataractogenesis. Screening of other familial (n=48) and sporadic (n=148) cases of childhood cataract, did not reveal any previously reported or novel mutation in the candidate genes screened.

A single destabilizing mutation (F9S) promotes concerted unfolding of an entire globular domain in gammaS-crystallin

Conformational change and aggregation of native proteins are associated with many serious age-related and neurological diseases. gammaS-Crystallin is a highly stable, abundant structural component of vertebrate eye lens. A single F9S mutation in the N-terminal domain of mouse gammaS-crystallin causes the severe Opj cataract, with disruption of cellular organization and appearance of fibrillar structures in the lens. Although the mutant protein has a near-native fold at room temperature, significant increases in hydrogen/deuterium exchange rates were observed by NMR for all the well-protected beta-sheet core residues throughout the entire N-terminal domain of the mutant protein, resulting in up to a 3.5-kcal/mol reduction in the free energy of the folding/unfolding equilibrium. No difference was detected for the C-terminal domain. At a higher temperature, this effect further increases to allow for a much more uniform exchange rate among the N-terminal core residues and those of the least well-structured surface loops. This suggests a concerted unfolding intermediate of the N-terminal domain, while the C-terminal domain stays intact. Increasing concentrations of guanidinium chloride produced two transitions for the Opj mutant, with an unfolding intermediate at approximately 1 M guanidinium chloride. The consequence of this partial unfolding, whether by elevated temperature or by denaturant, is the formation of thioflavin T staining aggregates, which demonstrated fibril-like morphology by atomic force microscopy. Seeding with the already unfolded protein enhanced the formation of fibrils. The Opj mutant protein provides a model for stress-related unfolding of an essentially normally folded protein and production of aggregates with some of the characteristics of amyloid fibrils.

The structure of the cataract-causing P23T mutant of human gammaD-crystallin exhibits distinctive local conformational and dynamic changes

Crystallins are major proteins of the eye lens and essential for lens transparency. Mutations and aging of crystallins cause cataracts, the predominant cause of blindness in the world. In human γD-crystallin, the P23T mutant is associated with congenital cataracts. Until now, no atomic structural information has been available for this variant. Biophysical analyses of this mutant protein have revealed dramatically reduced solubility compared to that of the wild-type protein due to self-association into higher-molecular weight clusters and aggregates that retain a nativelike conformation within the monomers [Pande, A., et al. (2005) Biochemistry 44, 2491−2500]. To elucidate the structure and local conformation around the mutation site, we have determined the solution structure and characterized the protein’s dynamic behavior by NMR. Although the global structure is very similar to the X-ray structure of wild-type γD-crystallin, pivotal local conformational and dynamic differences are caused by the threonine substitution. In particular, in the P23T mutant, the imidazole ring of His22 switches from the predominant Nε2 tautomer in the wild-type protein to the Nδ1 tautomer, and an altered motional behavior of the associated region in the protein is observed. The data support structural changes that may initiate aggregation or polymerization by the mutant protein.