The genetics of cataract: our vision becomes clearer

Adenosine Triphosphate (ATP) and Protein Aggregation in Age-Related Vision-Threatening Ocular Diseases

Protein aggregation is the etiopathogenesis of the three most profound vision-threatening eye diseases: age-related cataract, presbyopia, and age-related macular degeneration. This perspective organizes known information on ATP and protein aggregation with a fundamental unrecognized function of ATP. With recognition that maintenance of protein solubility is related to the high intracellular concentration of ATP in cells, tissues, and organs, we hypothesize that (1) ATP serves a critical molecular function for organismal homeostasis of proteins and (2) the hydrotropic feature of ATP prevents pathological protein aggregation while assisting in the maintenance of protein solubility and cellular, tissue, and organismal function. As such, the metabolite ATP plays an extraordinarily important role in the prevention of protein aggregation in the leading causes of vision loss or blindness worldwide.

Evaluation of the frequency of ABO and Rh-Hr blood-group systems in different acquired cataracts type

OBJECTIVES

This study evaluated the relationship between acquired cataract's different types and the ABO and Rh blood classes.

METHODS

Overall, 520 patients, by randomized sampling method, participated in this retrospective cross-sectional study. After reviewing the patient's medical records and laboratory results, the patient's demographics, ABO group, Rh, and cataract type were documented.

RESULTS

A total of 520 patients were included in the research, with a mean age of 67.57 ± 11.85. Most of them were female (n = 286, 55%). Mix (n = 230, 44%) and nuclear sclerotic (NS) (n = 167, 32%) cataracts were the most common types. The posterior subcapsular cataract (PSC) prevalence in females was significantly higher than in males (16.1% vs.7.3% p = 0.002). Also, men had more NS cataracts than females (89, 38% vs. 78, 27.3%) (p = 0.009). Patients with PSC were significantly younger than others (all p-values < 0.001). Our results showed that cataract types are independent of blood group types and Rh (P > 0.05).

CONCLUSION

Although our findings showed that cataract types are independent of blood group types and Rh, they can be compared with future studies on the association of other Blood-Group Systems in developing acquired cataracts.

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.

A novel missense mutation in the HSF4 gene of giant pandas with senile congenital cataracts

Cataracts are a common cause of visual impairment and blindness in mammals. They are usually associated with aging, but approximately one third of cases have a significant genetic component. Cataracts are increasingly prevalent among aging populations of captive giant pandas (Ailuropoda melanoleuca) and it is therefore important to identify genetic determinants that influence the likelihood of cataract development in order to distinguish between congenital and age-related disease. Here we screened for cataract-related genetic effects using a functional candidate gene approach combined with bioinformatics to identify the underlying genetic defect in a giant panda with congenital cataracts. We identified a missense mutation in exon 10 of the HSF4 gene encoding heat shock transcription factor 4. The mutation causes the amino acid substitution R377W in a highly conserved segment of the protein between the isoform-specific and downstream hydrophobic regions. Predictive modeling revealed that the substitution is likely to increase the hydrophobicity of the protein and disrupt interactions with spatially adjacent amino acid side chains. The mutation was not found in 13 unaffected unrelated animals but was found in an unrelated animal also diagnosed with senile congenital cataract. The novel missense mutation in the HSF4 gene therefore provides a potential new genetic determinant that could help to predict the risk of cataracts in giant pandas.

A novel missense mutation in the gene encoding major intrinsic protein (MIP) in a Giant panda with unilateral cataract formation

Background

Cataracts are defects of the lens that cause progressive visual impairment and ultimately blindness in many vertebrate species. Most cataracts are age-related, but up to one third have an underlying genetic cause. Cataracts are common in captive zoo animals, but it is often unclear whether these are congenital or acquired (age-related) lesions.

Results

Here we used a functional candidate gene screening approach to identify mutations associated with cataracts in a captive giant panda (Ailuropoda melanoleuca). We screened 11 genes often associated with human cataracts and identified a novel missense mutation (c.686G > A) in the MIP gene encoding major intrinsic protein. This is expressed in the lens and normally accumulates in the plasma membrane of lens fiber cells, where it plays an important role in fluid transport and cell adhesion. The mutation causes the replacement of serine with asparagine (p.S229N) in the C-terminal tail of the protein, and modeling predicts that the mutation induces conformational changes that may interfere with lens permeability and cell–cell interactions.

Conclusion

The c.686G > A mutation was found in a captive giant panda with a unilateral cataract but not in 18 controls from diverse regions in China, suggesting it is most likely a genuine disease-associated mutation rather than a single-nucleotide polymorphism. The mutation could therefore serve as a new genetic marker to predict the risk of congenital cataracts in captive giant pandas.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07386-8.

Application of Circular Dichroism and Fluorescence Spectroscopies To Assess Photostability of Water-Soluble Porcine Lens Proteins

The eye lens is mainly composed of the highly ordered water-soluble (WS) proteins named crystallins. The aggregation and insolubilization of these proteins lead to progressive lens opacification until cataract onset. Although this is a well-known disease, the mechanism of eye lens protein aggregation is not well understood; however, one of the recognized causes of proteins modification is related to the exposure to UV light. For this reason, the spectroscopic properties of WS lens proteins and their stability to UV irradiation have been evaluated by different biophysical methods including synchrotron radiation circular dichroism, fluorescence, and circular dichroism spectroscopies. Moreover, dynamic light scattering, gel electrophoresis, transmission electron microscopy, and protein digestion followed by tandem LC-MS/MS analysis were used to study the morphological and structural changes in protein aggregates induced by exposure to UV light. Our results clearly indicated that the exposure to UV radiation modified the protein conformation, inducing a loss of ordered structure and aggregation. Furthermore, we confirmed that these changes were attributable to the generation of reactive oxygen species due to the irradiation of the protein sample. This approach, involving the photodenaturation of proteins, provides a benchmark in high-throughput screening of small molecules suitable to prevent protein denaturation and aggregation.

Temperature-Dependent Interactions Explain Normal and Inverted Solubility in a γD-Crystallin Mutant

Protein crystal production is a major bottleneck in the structural characterization of proteins. To advance beyond large-scale screening, rational strategies for protein crystallization are crucial. Understanding how chemical anisotropy (or patchiness) of the protein surface, due to the variety of amino-acid side chains in contact with solvent, contributes to protein-protein contact formation in the crystal lattice is a major obstacle to predicting and optimizing crystallization. The relative scarcity of sophisticated theoretical models that include sufficient detail to link collective behavior, captured in protein phase diagrams, and molecular-level details, determined from high-resolution structural information, is a further barrier. Here, we present two crystal structures for the P23T + R36S mutant of γD-crystallin, each with opposite solubility behavior: one melts when heated, the other when cooled. When combined with the protein phase diagram and a tailored patchy particle model, we show that a single temperature-dependent interaction is sufficient to stabilize the inverted solubility crystal. This contact, at the P23T substitution site, relates to a genetic cataract and reveals at a molecular level the origin of the lowered and retrograde solubility of the protein. Our results show that the approach employed here may present a productive strategy for the rationalization of protein crystallization.

Biochemical characterization of G64W mutant of acidic beta-crystallin 4

Crystallins are structural proteins in the lens that last a lifetime with little turnover. Deviant in crystallins can cause rare but severe visual impairment, namely, congenital cataracts. It is reported that several mutations in the acidic β-crystallin 4 (CRYBA4) are related to congenital cataracts. However, the pathogenesis of these mutants is not well understood at molecular level. Here we evaluate the biochemical properties of wild type CRYBA4 (CRYBA4^WT) and a pathogenic G64W mutant (CRYBA4^G64W) including protein folding, polymerization state and protein stability. Furthermore, we explore the differences in their interactions with α-crystallin A (CRYAA) and basic β-crystallin 1 (CRYBB1) via yeast two-hybrid and pull-down assay in vitro, through which we find that G64W mutation leads to protein misfolding, decreases protein stability, blocks its interaction with CRYBB1 but maintains its interaction with CRYAA. Our results deepen our understanding of the pathogenesis of congenital cataracts.

A novel mutation in the CRYAA gene associated with congenital cataract and microphthalmia in a Chinese family

Background

Congenital cataract is the leading cause of blindness in children worldwide. Approximately half of all congenital cataracts have a genetic basis. Protein aggregation is the single most important factor in cataract formation.

Methods

A four-generation Chinese family diagnosed with autosomal dominant congenital cataracts and microphthalmia was recruited at the Shengjing Hospital of China Medical University. Genomic DNA was extracted from the peripheral blood of the participants. All coding exons and flanking regions of seven candidate genes (CRYAA, CRYBA4, CRYBB2, CRYGC, GJA8, MAF, and PITX3) were amplified and sequenced. Restriction fragment length polymorphism (RFLP) assays were performed to confirm the candidate causative variant, c.35G > T in the CRYAA gene. We constructed pcDNA3.1(+)-CRYAA expression plasmids containing either the wild-type or the R12L mutant alleles and respectively transfected them into HEK293T cells and into HeLa cells. Western blotting was performed to determine protein expression levels and protein solubility. Immunofluorescence was performed to determine protein sub-cellular localization.

Results

A heterozygous variant c.35G > T was identified in exon 1 of CRYAA, which resulted in a substitution of arginine to leucine at codon 12 (p.R12L). The nucleotide substitution c.35G > T was co-segregated with the disease phenotype in the family. The mutant R12L-CRYAA in HEK293T cells showed a significant increase in the expression level of the CRYAA protein compared with the wild-type cells. Moreover, a large amount of the mutant protein aggregated in the precipitate where the wild-type protein was not detected. Immunofluorescence studies showed that the overexpressed mutant CRYAA in HeLa cells formed large cytoplasmic aggregates and aggresomes.

Conclusions

In summary, we described a case of human congenital cataract and microphthalmia caused by a novel mutation in the CRYAA gene, which substituted an arginine at position 12 in the N-terminal region of αA-crystallin. The molecular mechanisms that underlie the pathogenesis of human congenital cataract may be characterized by the prominent effects of the p.R12L mutation on αA-crystallin aggregation and solubility. Our study also expands the spectrum of known CRYAA mutations.

A novel splice site mutation of CRYBA3/A1 gene associated with congenital cataract in a Chinese family

AIM

To identify the disease-causing mutation responsible for the presence of congenital cataract in a Chinese family.

METHODS

The study recruited a four-generation Chinese pedigree affected by autosomal dominant congenital cataract (ADCC). Family history and the history of cataract extraction were recorded. Blood samples were collected from individuals for DNA extraction. Direct sequencing of congenital cataract-associated genes was performed. Single-strand conformational polymorphism and bioinformatic analysis were conducted to further study the mutation.

RESULTS

Direct sequencing revealed a novel splice site mutation of c.30-2 A>G in the CRYBA3/A1 gene. The mutation co-segregated within all affected individuals in the family and was not found in unaffected members or 100 unrelated normal controls. These results were further confirmed by single-strand conformational polymorphism and bioinformatic analysis using the Human Splicing Finder and MaxEnt online software and Annovar computer software.

CONCLUSION

c.30-2 A>G mutation of CRYBA3/A1 gene is a novel mutation and broadens the genetic spectrum of ADCC.

A novel mutation of p.F32I in GJA8 in human dominant congenital cataracts

AIM

To identify a causative mutation in a three-generation family with autosomal dominant congenital total cataract and dissect the molecular consequence of the identified mutation.

METHODS

Clinical and ophthalmological examinations were performed on the affected and unaffected family members. Mutation were screened in recruited family members by polymerase chain reaction (PCR) of the two reported genes (CRYAA and GJA8) which were linked to human total cataracts and direct sequencing of the PCR product. The molecular consequences of the identified mutation was dissected. The plasmids carrying wild-type and mutant mouse ORF of Gja8, coding for connexin 50 (Cx50), were generated and ectopic expressed in 293 cells. Recombinant protein expression and cellular localization of recombinated Cx50 were assessed by confocal microscopy.

RESULTS

Clinical and ophthalmological examinations were performed on the affected and unaffected family members. Mutation were screened in recruited family members by PCR of the two reported genes (CRYAA and GJA8) which were linked to human total cataracts and direct sequencing of the PCR product. The molecular consequences of the identified mutation was dissected. The plasmids carrying wild-type and mutant mouse ORF of Gja8, coding for Cx50, were generated and ectopic expressed in 293 cells. Recombinant protein expression and cellular localization of recombinated Cx50 were assessed by confocal microscopy.

CONCLUSION

This study has identified a novel cataract mutation in GJA8, which adds a novel mutation to the existing spectrum of Cx50 mutations with cataract. The molecular consequences of p.F32I mutation in GJA8 exclude instability and the mislocalization of mutant Cx50 protein.

[Triggering factors and pathogenesis of radiation cataract]

Cataract is one of the most common eye diseases in the world. Many factors, including genetic, metabolic, nutritional, and environmental, are involved in its formation. It is necessary to know main causal agents and cellular mechanism of cataractogenesis. The tissue of the lens is considered radiosensitive, thus, lens opacities are possible late effects of exposure to ionizing radiation. The use of medical radiation for both diagnostic and therapeutic purposes grows worldwide. At the same time, there continues to be much confusion regarding the actual threshold dose of radiation exposure for cataract formation. Eye safety and the risk of cataract development are not entirely clear. However, with an ever-widening range of interventional procedures, one should take attempts to reduce the risk for radiation cataract.

Molecular Genetics of Cataract

Lens opacities or cataract(s) represent a universally important cause of visual impairment and blindness. Typically, cataract is acquired with aging as a complex disorder involving environmental and genetic risk factors. Cataract may also be inherited with an early onset either in association with other ocular and/or systemic abnormalities or as an isolated lens phenotype. Here we briefly review recent advances in gene discovery for inherited and age-related forms of cataract that are providing new insights into lens development and aging.

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.

Bilateral cataract induced by unilateral UVR-B exposure -- evidence for an inflammatory response

PURPOSE

To investigate whether unilateral in vivo UVR-B exposure of one eye affects the fellow eye in a co-cataractogenic, sympathetic reaction and to determine whether an inflammatory response could be involved in the pathogenesis.

METHODS

C57BL/6 mice were unilaterally exposed in vivo to UVR-B for 15 min. In the group of 24 animals each received 0×/2×/3×/or 4× cataract threshold equivalent dose. Following 48-hr UVR-B exposure, cataract morphology was documented in dark-field illumination photography, and light scattering was quantified, in both lenses in vitro. Serum levels of pro-inflammatory cytokines IL-1ß, IL-6 and TNF-α were analysed with ELISA. Immunohistochemistry was performed for inflammatory infiltration in exposed and contralateral eyes.

RESULTS

UVR-B exposure induced cataract in all exposed lenses. There was additionally a significant UVR dose-dependent increase in light scattering in the lenses of the non-exposed fellow eye. Inflammatory infiltration was detected immunohistochemically in the anterior segment of both eyes. IL-1β serum concentration increased with increasing UVR-B exposure dose. There was a similar trend for serum IL-6 but not for TNF-α.

CONCLUSION

Unilateral UVR-B exposure to one eye is associated with intraocular inflammation and an increase in lens light scattering also in the unexposed, fellow eye. A resulting systemic inflammatory response might be mediated by IL-1β and possibly IL-6. The finding that an inflammatory response may play a role in UVR-B-induced cataract development might initiate new strategies in the prevention of the disease.

Differential proteomics and functional research following gene therapy in a mouse model of Leber congenital amaurosis

Leber congenital amaurosis (LCA) is one of the most severe forms of inherited retinal degeneration and can be caused by mutations in at least 15 different genes. To clarify the proteomic differences in LCA eyes, a cohort of retinal degeneration 12 (rd12) mice, an LCA2 model caused by a mutation in the RPE65 gene, were injected subretinally with an AAV vector (scAAV5-smCBA-hRPE65) in one eye, while the contralateral eye served as a control. Proteomics were compared between untreated rd12 and normal control retinas on P14 and P21, and among treated and untreated rd12 retinas and control retinas on P42. Gene therapy in rd12 mice restored retinal function in treated eyes, which was demonstrated by electroretinography (ERG). Proteomic analysis successfully identified 39 proteins expressed differently among the 3 groups. The expression of 3 proteins involved in regulation of apoptosis and neuroptotection (alpha A crystallin, heat shock protein 70 and peroxiredoxin 6) were investigated further. Immunofluorescence, Western blot and real-time PCR confirmed the quantitative changes in their expression. Furthermore, cell culture studies suggested that peroxiredoxin 6 could act in an antioxidant role in rd12 mice. Our findings support the feasibility of gene therapy in LCA2 patients and support a role for alpha A crystallin, heat shock protein 70 and peroxiredoxin 6 in the pathogenetic mechanisms involved in LCA2 disease process.

A novel T→G splice site mutation of CRYBA1/A3 associated with autosomal dominant nuclear cataracts in a Chinese family

PURPOSE

The purpose of this study was to identify the disease-causing mutation and the molecular phenotype that are responsible for the presence of an autosomal dominant congenital nuclear cataract disease in a Chinese family.

METHODS

The family history and clinical data were recorded. The patients were given a physical examination and their blood samples were collected for DNA extraction. Direct sequencing was used to detect the mutation. Transcription analysis of the mutant crystallin, beta A1 (CRYBA1/A3) gene was performed to verify whether the defective mutation had influenced the splice of the mature mRNA.

RESULTS

The phenotype of the congenital cataract in the family was identified as a nuclear cataract type, by using slit-lamp photography. Direct sequencing revealed a novel mutation IVS3+2 T→G in CRYBA1/A3. This mutation co-segregated with all affected individuals in the family, but was not found in unaffected family members nor in the 100 unrelated controls. Transcription analysis of the mutant CRYBA1/A3 gene indicated that this mutation had influenced the splice of the mature mRNA.

CONCLUSIONS

Our study identified a novel splice site mutation in CRYBA1/A3. This mutation was responsible for aberrant splicing of the mature mRNA and had caused the congenital nuclear cataracts in the family. This is the first report relating an IVS3+2 T→G mutation of CRYBA1/A3 to congenital cataracts.

Identification of the p. R116H mutation in a Chinese family with novel variable cataract phenotype: evidence for a mutational hot spot in αA-crystallin gene

PURPOSE

To report the recurrent p.R116H mutation in the αA-crystallin gene (CRYAA) which causes a novel variable cataract phenotype, and to determine whether this mutation represents a mutational hot spot.

METHODS

Family history and clinical data were recorded. The genomic DNA was extracted from peripheral blood leukocytes. Microsatellite markers at loci considered to be associated with autosomal dominant cataracts were selected and genotyped for two-point linkage analysis. Direct sequencing was performed to identify the disease-causing mutation. Haplotype analysis was constructed to compare the affected haplotype in this family and in another Chinese family previously reported by us.

RESULTS

Clinical features of cataract in this family were asymmetric in two eyes of some affected subjects. Evidence of linkage was obtained with marker D21S1411 (logarithm of odds [LOD] score [Z] = 2.42, recombination fraction [θ] = 0.0). Sequencing of the candidate CRYAA gene revealed a single base alteration c.347 G > A in exon 3, which resulted in the substitution of highly conserved arginine by histidine at codon 116 (p.R116H). This mutation co-segregated with all affected individuals and was not observed in unaffected family members or 100 normal unrelated individuals. The comparative haplotype analysis showed that the affected haplotypes in the two families were different.

CONCLUSIONS

This study identified a novel cataract-microcornea phenotype caused by the recurrent mutation p.R116H in CRYAA, and suggested that this mutation site is not likely the consequence of a founder effect, but probably a result of a mutational hot spot.

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.

Molecular genetic analysis of autosomal dominant late-onset cataract in a Chinese Family

Congenital cataract is a highly heterogeneous disorder at both the genetic and the clinical-phenotypic levels. A unique cataract was observed in a 4-generation Chinese family, which was characterized by autosomal dominant inheritance and late-onset. Mutations in the 13 known genes (CRYAA, CRYAB, CRYBB1, CRYBB2, CRYGC, CRYBA1/A3, CRYGD, Connexin50, Connexin46, intrinsic membrane protein LIM2, cytoskeletal protein BFSP2, the major intrinsic protein-MIP and the heat shock factor HSF4) have previously been demonstrated to be the frequent reason for isolated congenital cataracts, but the exact molecular basis and underlying mechanisms of congenital cataract still remain unclear. This study was designed to find whether these 13 genes developed any mutation in the family members and to identify the disease-causing gene. Polymerase chain reaction (PCR) and direct DNA sequence analysis were carried out to detect the 13 genes. The results showed that no mutation causing amino acid alternations was found in these potential candidate genes among all patients in the family, and only several single-nucleotide polymorphisms (SNPs) were identified. A transitional mutation in the fourth intron of CRYBB2 and some silent mutations in the first exon of BFSP2 and CRYGD were found in the cataract family, but further study showed that these mutations could also be found in normal controls. It was concluded that some unidentified genes may underlie the occurrence of late-onset cataract in this family. A genome-wide screening will be carried out in the next study.

Autosomal dominant congenital nuclear cataracts caused by a CRYAA gene mutation

PURPOSE

We sought to identify the genetic defect in a four-generation Chinese family with autosomal dominant congenital nuclear cataracts, examine the clinical features in detail and demonstrate the functional analysis of a candidate gene in the family.

METHODS

Family history data were recorded. Clinical and ophthalmological examinations were performed on affected and unaffected family members. All the members were genotyped with microsatellite markers at loci considered to be associated with cataracts. Two-point LOD scores were calculated using the LINKAGE program package after genotyping. A mutation was detected by dilff521229rect sequencing and verified by denaturing high-performance liquid chromatography (DHPLC). Wild-type and mutant proteins were analyzed with online softwares.

RESULTS

All affected members of this family had nuclear cataracts. Genetic analysis revealed a heterozygous previously described Arg116Cys mutation in the CRYAA gene in all of the affected members of the family but not in unaffected or 100 normal, unrelated individuals. Data generated with online software revealed that the different amino acid side chain, impact the aa116 interaction with other amino acids, thereby affecting the proteins secondary structure.

CONCLUSIONS

This study identified a mutation in the CRYAA gene causing autosomal dominant nuclear cataracts and some patients show nystagmus or small blepharophimosis clinical features. These results provide evidence that CRYAA is a pathogenic gene for congenital cataracts, congenital cataracts are a clinically and genetically heterogeneous lens condition; at the same time, demonstrates a possible mechanism of action for the mutant gene.

A new mutation in BFSP2 (G1091A) causes autosomal dominant congenital lamellar cataracts

PURPOSE

We sought to identify the genetic defect in a four-generation Chinese family with autosomal dominant congenital lamellar cataracts and demonstrate the functional analysis with biosoftware of a candidate gene in the family.

METHODS

Family history data were recorded. Clinical and ophthalmologic examinations were performed on family members. All the members were genotyped with microsatellite markers at loci considered to be associated with cataracts. Two-point LOD scores were calculated by using the Linkage Software after genotyping. A mutation was detected by using gene-specific primers in direct sequencing. Wild type and mutant proteins were analyzed with Online Bio-Software.

RESULTS

Affected members of this family had lamellar cataracts. Linkage analysis was obtained at markers D3S2322 (LOD score [Z]=7.22, recombination fraction [theta]=0.0) and D3S1541 (Z=5.42, theta=0.0). Haplotype analysis indicated that the cataract gene was closely linked to these two markers. Sequencing the beaded filament structural protein 2 (BFSP2) gene revealed a G>A transversion in exon 5, which caused a conservative substitution of Arg to His at codon 339 (P.R339H). This mutation cosegregated with the disease phenotype in all affected individuals and was not observed in the unaffected family members or in 100 normal, unrelated individuals. Bioinformatic analyses showed that a highly conserved region was located around Arg339. Data generated with Online Bio-Software revealed that the mutation altered the protein's hydrophobicity, hydrophobic moment, and chaperone and regulation activities.

CONCLUSIONS

This is the first reported case of a congenital lamellar cataract phenotype associated with the mutation of Arg339His (P.R339H) in BFSP2. It highlights the physiologic importance of the beaded filament protein and demonstrates a possible mechanism of action for the mutant gene.

A nonsense mutation in CRYGC associated with autosomal dominant congenital nuclear cataract in a Chinese family

PURPOSE

To identify the genetic defect associated with autosomal dominant congenital nuclear cataract in a Chinese family.

METHODS

Family history and phenotypic data were recorded, and the phenotypes were documented by slit lamp photography. The genomic DNA was extracted from peripheral blood leukocytes. All the exons and flanking intronic sequences of CRYGC and CRYGD were amplified by polymerase chain reaction (PCR) and screened for mutation by direct DNA sequencing. Structural models of the wild type and mutant gammaC-crystallin were generated and analyzed by SWISS-MODEL.

RESULTS

Sequencing of the coding regions of CRYGC and CRYGD showed the presence of a heterozygous C>A transversion at c.327 of the coding sequence in exon 3 of CRYGC (c.327C>A), which results in the substitution of a wild type cysteine to a nonsense codon (C109X). One and a half Greek key motifs at the COOH-terminus were found to be absent in the structural model of the mutant truncated gammaC-crystallin.

CONCLUSIONS

A novel nonsense mutation in CRYGC was detected in a Chinese family with consistent autosomal dominant congenital nuclear cataract, providing clear evidence of a relationship between the genotype and the corresponding cataract phenotype.

Nonsense mutation in the CRYBB2 gene causing autosomal dominant progressive polymorphic congenital coronary cataracts

PURPOSE

We sought to identify the genetic defect in a large, five-generation Chinese family with autosomal dominant progressive polymorphic congenital coronary cataracts and to examine the clinical features in detail.

METHODS

Clinical and ophthalmologic examinations were conducted on family members. All members were genotyped with microsatellite markers at loci previously associated with cataracts. Two-point LOD scores were calculated using a linkage package after genotyping. A mutation was detected by direct sequencing and verified by denaturing high-performance liquid chromatography (DHPLC).

RESULTS

Clinical observations showed that all affected family members had progressive polymorphic coronary cataracts. Linkage analysis was obtained at markers, D22S303 (LOD score [Z]=2.11, recombination fraction [theta]=0.0) and D22S1167 (Z=1.20, theta=0.0). Haplotype analysis indicated that the cataract gene was closely linked with these two markers. Sequencing the betaB-crystallin gene (CRYBB2) revealed a C --> T transition in exon 6, which changed a codon from Gln to a stop codon (P.Q155X). This mutation cosegregated with all affected individuals and was not observed in any unaffected family member or 100 normal, unrelated individuals.

CONCLUSIONS

This study identified a mutation in CRYBB2 in a large Chinese family with autosomal dominant progressive polymorphic congenital coronary cataracts. These results provide evidence that CRYBB2 is a pathogenic gene for congenital cataracts; at the same time, congenital cataracts are a clinically and genetically heterogeneous lens condition.

Mutation G61C in the CRYGD gene causing autosomal dominant congenital coralliform cataracts

PURPOSE

We sought to identify the genetic defect in a four-generation Chinese family with autosomal dominant congenital coralliform cataracts and demonstrate the functional analysis of a candidate gene in the family.

METHODS

Family history data were recorded. Clinical and ophthalmologic examinations were performed on affected and unaffected family members. All the members were genotyped with microsatellite markers at loci considered to be associated with cataracts. Two-point LOD scores were calculated using the Linkage software after genotyping. A mutation was detected by direct sequencing, using gene-specific primers. Wild-type and mutant proteins were analyzed with online software.

RESULTS

Affected members of this family had coralliform cataracts. Linkage analysis was obtained at markers, D2S72 (LOD score [Z]=3.31, recombination fraction [theta]=0.0) and D2S1782 (Z=3.01, theta=0.0). Haplotype analysis indicated that the cataract gene was closely linked to these two markers. Sequencing the gammaD-crystallin gene (CRYGD) revealed a G>T transversion in exon 2, which caused a conservative substitution of Gly to Cys at codon 61 (P.G61C). This mutation co-segregated with the disease phenotype in all affected individuals and was not observed in any of the unaffected or 100 normal, unrelated individuals. Bioinformatic analyses showed that a highly conserved region was located around Gly61. Data generated with online software revealed that the mutation altered the protein's stability, solvent-accessibility, and interactions with other proteins.

CONCLUSIONS

This is the first reported case of a congenital coralliform cataract phenotype associated with the mutation of Gly61Cys (P.G61C) in the CRYGD gene; it demonstrates a possible mechanism of action for the mutant gene.

Formation of amyloid fibrils in vitro by human gammaD-crystallin and its isolated domains

PURPOSE

Amyloid fibrils are associated with a variety of human protein misfolding and protein deposition diseases. Previous studies have shown that bovine crystallins form amyloid fibers under denaturing conditions and amyloid fibers accumulate in the lens of mice carrying mutations in crystallin genes. Within differentiating lens fiber cells, crystallins may be exposed to low pH lysosome compartments. We have investigated whether human gammaD-crystallin forms amyloid fibrils in vitro, when exposed to low pH partially denaturing conditions.

METHODS

Human gammaD-crystallin expressed and purified from E. coli, is stable and soluble at 37 degrees C, pH7, and refolds from the fully denatured state back to the native state under these conditions. Purified Human gammaD-crystallin as well as its isolated NH2- and COOH-terminal domains were incubated at acid pH and subsequently examined by transmission electron microscopy, absorption spectroscopy in the presence of Congo red, FTIR, and low-angle X-ray scattering.

RESULTS

Incubation of the intact protein at 37 degrees C in 50 mM acetate buffer pH 3 at 50 mg/ml for 2 days, led to formation of a viscous, gel-like solution. Examination of negatively stained samples by transmission electron microscopy revealed linear, non-branching fibrils of variable lengths, with widths ranging from 15 to 35 nm. Incubation with the dye Congo red generated the spectral red shift associated with dye binding to amyloid. Low-angle X-ray scattering from samples showed clear meridional reflection at 4.7 A and a more diffuse reflection on the equator between 10 and 11 A which is the typical "cross-beta" X-ray fiber diffraction pattern for amyloid fibers. FTIR was used to follow the evolution of the secondary structure of gammaD-crystallin with time during incubation of the protein at pH 3. The native protein displayed a major band at 1640 cm-1 that converted during incubation at 37 degrees C to a band at 1616 cm-1. An additional band at 1689 cm-1 also appeared with time. The presence of bands in the regions about 1620 cm-1 and about 1680 cm-1 has been attributed to the formation of intermolecular beta-sheet structure that characterizes the fibrillar amyloid motif. The isolated NH2-terminal 1-82 and COOH-terminal 86-174 domains of HgammaD-crystallin also formed amyloid fibrils after incubation under the same conditions, but to a lesser extent than the full length.

CONCLUSIONS

HgammaD-crystallin, as well as its isolated NH2-terminal 1-82 and COOH-terminal 86-174 domains of HgammaD-crystallin formed amyloid fibrils upon incubation at acid pH. Investigations of early stages in cataract formation within the lens will be required to assess whether amyloid fibrils play a role in the initiation of cataract in vivo.

Altered phase diagram due to a single point mutation in human gammaD-crystallin

The P23T mutant of human gammaD-crystallin (HGD) is associated with cataract. We have previously investigated the solution properties of this mutant, as well as those of the closely related P23V and P23S mutants, and shown that although mutations at site 23 of HGD do not produce a significant structural change in the protein, they nevertheless profoundly alter the solubility of the protein. Remarkably, the solubility of the mutants decreases with increasing temperature, in sharp contrast to the behavior of the native protein. This inverted solubility corresponds to a strong increase in the binding energy with temperature. Here we have investigated the liquid-liquid coexistence curve and the diffusivity of the P23V mutant and find that these solution properties are unaffected by the mutation. This means that the chemical potentials in the solution phase are essentially unaltered. The apparent discrepancy between the interaction energies in the solution phase, as compared with the solid phase, is explicable in terms of highly anisotropic interprotein interactions, which are averaged out in the solution phase but are fully engaged in the solid phase.

Evaluation of three canine gamma-crystallins (CRYGB, CRYGC, and CRYGS) as candidates for hereditary cataracts in the dachshund

PURPOSE

We analyzed the gamma-crystallin genes CRYGB, CRYGC, and CRYGS in the dog and tested single nucleotide polymorphisms (SNPs) for linkage and association with primary noncongenital cataract (CAT) in the dachshund, a popular dog breed. The crystallin genes may be involved in the pathogenesis of canine CAT as shown in humans and mice.

METHODS

We sequenced all exons and their flanking intronic regions of the CRYGB, CRYGC, and CRYGS genes and in addition, the complete cDNA of these three genes using lens tissue from CAT-affected and unaffected dogs of several breeds. After examining BLASTN analyses, we compared the gene structure with the predicted genes in the current dog genome assembly and the orthologs of humans and mice.

RESULTS

The search for SNPs within these crystallin genes revealed a total of five polymorphisms. As both CAT-affected and unaffected dogs shared identical haplotypes, there was no cosegregation of the SNP alleles with the affected animals. Expression did not differ among CAT-affected and unaffected dogs.

CONCLUSIONS

The polymorphisms reported for CRYGB, CRYGC, and CRYGS can be excluded as causative mutations for the CAT phenotype in the wire- and smooth-haired dachshund. The canine cataract gene orthologs described here may serve as a valuable resource for further studies in other dog breeds to develop a canine model. Many different dog breeds are affected by CAT. The use of the SNPs presented in this paper can facilitate the screening of more dog breeds.

Identification of mutations in HSF4 in dogs of three different breeds with hereditary cataracts

Cataracts are a leading cause of blindness in both dogs and humans. Mutations in several genes have been associated with inherited forms of human cataract, but no mutations have been identified as the cause of any form of canine inherited cataract. We have used a candidate gene approach to investigate 20 genes, known to be associated with cataract in humans, for their potential association with the development of hereditary cataract (HC) in dogs. We have identified mutations in the HSF4 gene in Staffordshire Bull Terriers, Boston Terriers and Australian Shepherds affected by HC. Interestingly, different mutations in this single gene may be causing a recessive form of cataract in Staffordshire Bull Terriers and Boston Terriers and a dominant cataract in Australian Shepherds. Identification of the mutations that cause HC in these three breeds provides a method of controlling the disease within populations at risk using a simple diagnostic test, and also establishes cataract in these breeds as models for their human counterparts.

CRYBA4, a novel human cataract gene, is also involved in microphthalmia

Genetic analysis of a large Indian family with an autosomal dominant cataract phenotype allowed us to identify a novel cataract gene, CRYBA4. After a genomewide screen, linkage analysis identified a maximum LOD score of 3.20 (recombination fraction [theta] 0.001) with marker D22S1167 of the beta -crystallin gene cluster on chromosome 22. To date, CRYBA4 was the only gene in this cluster not associated with either human or murine cataracts. A pathogenic mutation was identified in exon 4 that segregated with the disease status. The c.317T-->C sequence change is predicted to replace the highly conserved hydrophobic amino acid phenylalanine94 with the hydrophilic amino acid serine. Modeling suggests that this substitution would significantly reduce the intrinsic stability of the crystalline monomer, which would impair its ability to form the association modes critical for lens transparency. Considering that CRYBA4 associates with CRYBB2 and that the latter protein has been implicated in microphthalmia, mutational analysis of CRYBA4 was performed in 32 patients affected with microphthalmia (small eye). We identified a c.242T-->C (Leu69Pro) sequence change in exon 4 in one patient, which is predicted here to disrupt the beta -sheet structure in CRYBA4. Protein folding would consequently be impaired, most probably leading to a structure with reduced stability in the mutant. This is the first report linking mutations in CRYBA4 to cataractogenesis and microphthalmia.

The NEIBank project for ocular genomics: data-mining gene expression in human and rodent eye tissues

NEIBank is a project to gather and organize genomic resources for eye research. The first phase of this project covers the construction and sequence analysis of cDNA libraries from human and animal model eye tissues to develop an overview of the repertoire of genes expressed in the eye and a resource of cDNA clones for further studies. The sequence data are grouped and identified using the tools of bioinformatics and the results are displayed through a web site where they can be interrogated by keyword search, chromosome location, by Blast (sequence comparison) or by alignment on completed genomes. Many novel proteins and novel splice forms of known genes have already emerged from analysis of the accumulating data. This review provides an overview of the current state of the database for human eye tissues, with specific comparisons to some parallel data from mouse and rat, and with illustrative examples of the kinds of insights and discoveries these data can produce. One of the major themes that emerges is that at the molecular level human eye tissues have significant differences from those of rodents, encompassing species specific genes, alternative splice forms and great variation in levels of gene expression. These point to specific adaptations and mechanisms in the human eye and emphasize that care needs to be taken in the application of appropriate animal model systems.

Eye lens proteomics: from global approach to detailed information about phakinin and gamma E and F crystallin genes

Exploration of the lenticular proteome poses a challenging and worthwhile undertaking as cataracts, the products of a disease phenotype elicited by this proteome, remains the leading cause of vision impairment worldwide. The complete ten day old lens proteome of Mus musculus C57BL/6J was resolved into 900 distinct spots by large gel carrier ampholyte based 2-DE. The predicted amino acid sequences of all 16 crystallins ubiquitous in mammals were corroborated by mass spectrometry (MS). In detailed individual spot analyses, the primary structure of the full murine C57BL/6J beaded filament component phakinin CP49 was sequenced by liquid chromatography/electrospray ionization-tandem MS and amended at two positions. This definitive polypeptide sequence was aligned to the mouse genome, thus identifying the entire C57BL/6J genomic coding region. Also, two murine C57/6J polypeptides, both previously classified as gamma F crystallin, were clearly distinguished by MS and electrophoretic mobility. Both were assigned to their respective genes, one of the polypeptides was reclassified as C57BL/6J gamma E crystallin. Building on these data and previous investigations an updated crystallin reference map was put forth and several non crystallin lenticular components were examined. These results represent the first part of a comprehensive investigation of the mouse lens proteome (http://www.mpiib-berlin.mpg.de/2D-PAGE) with emphasis on understanding genetic effects on proteins and disease development.

The P23T Cataract Mutation Causes Loss of Solubility of Folded γD-Crystallin

Mutations in the human gammaD-crystallin gene have been linked to several types of congenital cataracts. In particular, the Pro23 to Thr (P23T) mutation of human gammaD crystallin has been linked to cerulean, lamellar, coralliform, and fasciculiform congenital cataracts. We have expressed and purified wild-type human gammaD, P23T, and the Pro23 to Ser23 (P23S) mutant. Our measurements show that P23T is significantly less soluble than wild-type human gammaD, with P23S having an intermediate solubility. Using synchrotron radiation circular dichroism spectroscopy, we have determined that the P23T mutant has a slightly increased content of beta-sheet, which may be attributed to the extension of an edge beta-strand due to the substitution of Pro23 with a residue able to form hydrogen bonds. Neither of the point mutations appears to have reduced the thermal stability of the protein significantly, nor its resistance to guanidine hydrochloride-induced unfolding. These results suggest that insolubility, rather than loss of stability, is the primary basis for P23T congenital cataracts.

Ocular genetics: current understanding

Over the past decade, there has been an exponential increase in our knowledge of heritable eye conditions. Coincidentally, our ability to provide accurate genetic diagnoses has allowed appropriate counseling to patients and families. A summary of our current understanding of ocular genetics will prove useful to clinicians, researchers, and students as an introduction to the subject.

Molecular genetic basis of inherited cataract and associated phenotypes

Congenital cataract is a leading cause of visual disability in children. Inherited isolated (non-syndromic) cataract represents a significant proportion of cases and recently many causative genetic mutations have been identified. Inherited cataract is known to be clinically and genetically heterogeneous. Eleven clear-cut cataract phenotypes have been described. Cataract may be inherited as autosomal dominant, autosomal recessive, or X-linked recessive traits, and 12 loci and 15 specific genes associated with inherited isolated cataract have been identified to date; it is likely that more genes remain to be discovered. The identification of remaining genes will not only improve our understanding of the mechanism of cataract formation but will shed new light on the developmental biology and biochemistry of the lens. Furthermore, it is possible that some of these genes will be implicated in the more common age related cataract, which also has a genetic component to its etiology.

Fine Localization of Nefl and Nef3 and its Exclusion as Candidate Gene for Lens Rupture 2(lr2)

Cataract causing lr2 gene is found in the CXSD mouse, which is a recombinant inbred strain of BALB/c and STS mice. For the process of positional cloning of lr2, several candidate genes were selected in the middle region of chromosome 14, but most of them were excluded by combination of recombination and homozygosity mapping. Components of neurofilament proteins, neurofilament light polypeptide (Nefl) and neurofilament3 medium (Nef3), were linked to D14Mit87 which was not separated from the lr2 locus in the homozygosity mapping. When the expression levels of Nefl and Nef3 in eyes were compared in CXSD and BALB/c mice, there were no differences in expression levels. The cDNA sequences of the two genes from CXSD, BALB/c and STS mice were subsequently compared. Several nucleotide differences in cDNA sequences were detected between the mice strains but the majority of the changes were silent mutations that did not alter the amino acids. The sole amino acid difference, E567K in the glutamate rich region of Nfm, between BALB/c and CXSD was found to be a simple genetic polymorphism because the same substitution existed in STS, a non-cataract mouse strain. Therefore we excluded Nefl and Nef3 from the candidate genes for lr2 based on expression and mutation analyses.

Molecular analyses of patients with hyperferritinemia and normal serum iron values reveal both L ferritin IRE and 3 new ferroportin (slc11A3) mutations

Unexplained hyperferritinemia is a common clinical finding, even in asymptomatic persons. When early onset bilateral cataracts are also present, the hereditary hyperferritinemia-cataract syndrome (HHCS), because of heterozygous point mutation in the L ferritin iron-responsive element (IRE) sequence, can be suspected. We sequenced the L ferritin exon 1 in 52 DNA samples from patients referred to us for molecular diagnosis of HHCS. We identified 24 samples with a point mutation/deletion in the IRE. For the 28 samples in which no IRE mutation was present, we also genotyped HFE mutations and sequenced both H ferritin and ferroportin genes. We found an increased frequency of His63Asp heterozygotes (12 of 28) but no H ferritin mutations. We identified 3 new ferroportin mutations, producing, respectively, Asp157Gly, Gln182His, and Gly323Val amino acid replacements, suggesting that these patients have dominant type 4 hemochromatosis. This study demonstrates that both L ferritin IRE and ferroportin mutations can account for isolated hyperferritinemia. The presence of cataract does not permit the unambiguous identification of patients with HHCS, although the existence of a family history of cataract was only encountered in these patients. This raises the intriguing possibility that lens ferritin accumulation might be a factor contributing to age-related cataract in the general population. Additional causes of isolated hyperferritinemia remain to be identified.

Methylation and carbamylation of human gamma-crystallins

Accessible sulfhydryls of cysteine residues are likely sites of reaction in long-lived proteins such as human lens crystallins. Disulfide bonding between cysteines is a major contributor to intermolecular cross-linking and aggregation of crystallins. A recently reported modification of gammaS-crystallins, S-methylation of cysteine residues, can prevent disulfide formation. The aim of this study was to determine whether cysteines in gammaC-, gammaD-, and gammaB-crystallins are also S-methylated. Our data show that all the gamma-crystallins are S-methylated, but only at specific cysteines. In gammaD-crystallin, methylation is exclusively at Cys 110, whereas in gammaC- and gammaB-crystallins, the principal methylation site is Cys 22 with minor methylation at Cys 79. gammaD-crystallin is the most heavily methylated gamma-crystallin. gammaD-Crystallins from adult lenses are 37%-70% methylated, whereas gammaC and gammaB are approximately 12% methylated. The specificity of gamma-crystallin methylation and its occurrence in young clear lenses supports the idea that inhibition of disulfide bonding by S-methylation may play a protective role against cataract. Another modification, not reported previously, is carbamylation of the N termini of gammaB-, gammaC-, gammaD-crystallins. N-terminal carbamylation is likely a developmentally related modification that does not negatively impact crystallin function.

Crystallins, genes and cataract

Far from being a physical entity, assembled of inanimate structural proteins, the ocular lens epitomizes the biological ingenuity that sustains an essential and near-perfect physical system of immaculate optics. Crystallins (alpha, beta, and gamma) provide transparency by dint of their high concentration, but it is debatable whether proteins that provide transparency are any different, biologically or structurally, from those that are present in non-transparent structures or tissues. It is becoming increasingly clear that crystallins may have a plethora of metabolic and regulatory functions, both within the lens as well as outside of it. Alpha-crystallins are members of a small heat shock family of proteins and beta/gamma-crystallins belong to the family of epidermis-specific differentiation proteins. Crystallin gene expression has been studied from the perspective of the lens specificity of their promoters. Mutations in alpha-, beta-, and gamma-crystallins are linked with the phenotype of the loss of transparency. Understanding catalytic, non-structural properties of crystallins may be critical for understanding the malfunction in molecular cascades that lead to cataractogenesis and its eventual therapeutic amelioration.

High-resolution X-ray crystal structures of human gammaD crystallin (1.25 A) and the R58H mutant (1.15 A) associated with aculeiform cataract

Several human cataracts have been linked to mutations in the gamma crystallin gene. One of these is the aculeiform cataract, which is caused by an R58H mutation in gammaD crystallin. We have shown previously that this cataract is caused by crystallization of the mutant protein, which is an order of magnitude less soluble than the wild-type. Here, we report the very high-resolution crystal structures of the mutant and wild-type proteins. Both proteins crystallize in the same space group and lattice. Thus, a strict comparison of the protein-protein and protein-water intermolecular interactions in the two crystal lattices is possible. Overall, the differences between the mutant and wild-type structures are small. At position 58, the mutant protein loses the direct ion-pair intermolecular interaction present in the wild-type, due to the differences between histidine and arginine at the atomic level; the interaction in the mutant is mediated by water molecules. Away from the mutation site, the mutant and wild-type lattice structures differ in the identity of side-chains that occupy alternate conformations. Since the interactions in the crystal phase are very similar for the two proteins, we conclude that the reduction in the solubility of the mutant is mainly due to the effect of the R58H mutation in the solution phase. The results presented here are also important as they are the first high-resolution X-ray structures of human gamma crystallins.

Gamma-D crystallin gene (CRYGD) mutation causes autosomal dominant congenital cerulean cataracts

Congenital cataracts are a major cause of bilateral visual impairment in childhood. We mapped the gene responsible for autosomal congenital cerulean cataracts to chromosome 2q33-35 in a four generation family of Moroccan descent. The maximum lod score (7.19 at recombination fraction theta=0) was obtained for marker D2S2208 near the gamma-crystallin gene (CRYG) cluster. Sequencing of the coding regions of the CRYGA, B, C, and D genes showed the presence of a heterozygous C>A transversion in exon 2 of CRYGD that is associated with cataracts in this family. This mutation resulted in a proline to threonine substitution at amino acid 23 of the protein in the first of the four Greek key motifs that characterise this protein. We show that although the x ray crystallography modelling does not indicate any change of the backbone conformation, the mutation affects a region of the Greek key motif that is important for determining the topology of this protein fold. Our data suggest strongly that the proline to threonine substitution may alter the protein folding or decrease the thermodynamic stability or solubility of the protein. Furthermore, this is the first report of a mutation in this gene resulting in autosomal dominant congenital cerulean cataracts.

The gamma S-crystallin gene is mutated in autosomal recessive cataract in mouse

We established a recessive cataract model from a spontaneous mutation in the KUNMING outbred mice. Lens opacity appears 11 days after birth. Slit lamp examination reveals that the opacity mainly localizes to the nuclear region of the lens. Histological analysis shows a severe degeneration of the epithelial cells underneath the anterior lens capsule, whereas those cells in the equatorial region display an excessive proliferation and migration. Within the cortical area underneath the posterior lens capsule, both vacuoles and morgagnian-like bodies are seen. Blue-stained spherical bodies are observed in the embryonic nucleus, forming a Y-like pattern. We mapped the disease locus and found a homozygous G to A nucleotide conversion at position 489 of Crygs in mutant mice, leading to a truncated gene product (Trp163Stop). This finding suggests that CRYGS is not only a lens structural protein, but is also likely to be involved in epithelial cell proliferation, apoptosis, and migration.

A temperature-sensitive mutation of Crygs in the murine Opj cataract

In Opj, an inherited cataract in mice, opacity is associated with a mutation in Crygs, the gene for gammaS-crystallin, the first mutation to be associated with this gene. A single base change causes replacement of Phe-9, a key hydrophobic residue in the core of the N-terminal domain, by serine. Despite this highly non-conservative change, mutant protein folds normally at low temperature. However, it exhibits a marked, concentration-dependent decrease in solubility, associated with loss of secondary structure, at close to physiological temperatures. This is reminiscent of processes thought to occur in human senile cataracts in which normal proteins become altered and aggregate. The Opj cataract is progressive and more severe in Opj/Opj than in Opj/+. Lens histology shows that whereas fiber cell morphology in Opj/+ mice is essentially normal, in Opj/Opj, cortical fiber cell morphology and the loss of maturing fiber cell nuclei are both severely disrupted from early stages. This may indicate a loss of function of gammaS-crystallin which would be consistent with ideas that members of the betagamma-crystallin superfamily may have roles associated with maintenance of cytoarchitecture.

A progressive autosomal recessive cataract locus maps to chromosome 9q13-q22

Cataracts are the leading cause of blindness in most countries. Although most hereditary cases appear to follow an autosomal dominant pattern of inheritance, autosomal recessive inheritance has been clearly documented and is probably underrecognized. We studied a large family-from a relatively isolated geographic region-whose members were affected by autosomal recessive adult-onset pulverulent cataracts. We mapped the disease locus to a 14-cM interval at a novel disease locus, 9q13-q22 (between markers D9S1123 and D9S257), with a LOD score of 4.7. The study of this progressive and age-related cataract phenotype may provide insight into the cause of the more common sporadic form of age-related cataracts.

Mapping of the human CP49 gene and identification of an intragenic polymorphic marker to allow genetic linkage analysis in autosomal dominant congenital cataract

The CP49 protein is an intermediate filament protein expressed specifically in the lens fibre cells of the lens, where it is an important cytoplasmic structural component. Dominant-negative mutations in other intermediate filament proteins, such as keratins, cause disorders characterised by dense cytoplasmic aggregates in specific cell types. The CP49 gene is therefore a good candidate for dominantly inherited forms of cataract. To allow genetic linkage analysis of families with autosomal dominant cataract with respect to CP49, a highly polymorphic intragenic microsatellite marker for this gene has been developed. In addition, both low and high resolution radiation hybrid mapping of the CP49 gene has been completed, placing it very close to microsatellite marker D3S1290 on human chromosome 3q. Furthermore, using the intragenic CP49 microsatellite, linkage was excluded in four families with genetically uncharacterized forms of autosomal dominant congenital cataract.

Linkage analysis in an autosomal dominant 'zonular nuclear pulverulent' congenital cataract, mapped to chromosome 13q11-13

PURPOSE

To determine by linkage analysis the chromosomal locus responsible for autosomal dominant congenital cataract (ADCC) in a four-generation Welsh pedigree.

METHODS

The family pedigree was traced through two members of the family attending the hospital for treatment of their cataracts. Twenty-five members of the family were examined ophthalmologically and blood was collected with consent for genetic linkage analysis.

RESULTS

Fifteen members of this family were known to have bilateral congenital cataracts, of whom 11 had a cataract extraction prior to examination. The youngest member of the pedigree was 5 years old and the oldest was 78 years old. Four children, unoperated at the time of this study, had a phenotypically identical morphology of their zonular pulverulent congenital cataracts. The known loci for congenital cataract were excluded. Significant lod scores for markers in the 13q11-13 region were detected with a Zmax of 3.59 D13S1236 (theta = 0.00).

CONCLUSIONS

We report linkage of an ADCC of the zonular pulverulent type to chromosome 13q11-13 in a four-generation family.

Molecular basis of a progressive juvenile-onset hereditary cataract

In a recent paper, patients with a progressive juvenile-onset hereditary cataract have been reported to have a point mutation in the human gammaD crystallin gene (Stephan, D. A., Gillanders, E., Vanderveen, D., Freas-Lutz, D., Wistow, G., Baxevanis, A. D., Robbins, C. M., VanAuken, A., Quesenberry, M. I., Bailey-Wilson, J., et al. (1999) Proc. Natl. Acad. Sci. USA 96, 1008-1012). This mutation results in the substitution of Arg-14 in the native protein by a Cys residue. It is not understood how this mutation leads to cataract. We have expressed recombinant wild-type human gammaD crystallin (HGD) and its Arg-14 to Cys mutant (R14C) in Escherichia coli and show that R14C forms disulfide-linked oligomers, which markedly raise the phase separation temperature of the protein solution. Eventually, R14C precipitates. In contrast, HGD slowly forms only disulfide-linked dimers and no oligomers. These data strongly suggest that the observed cataract is triggered by the thiol-mediated aggregation of R14C. The aggregation profiles of HGD and R14C are consistent with our homology modeling studies that reveal that R14C contains two exposed cysteine residues, whereas HGD has only one. Our CD, fluorescence, and differential scanning calorimetric studies show that HGD and R14C have nearly identical secondary and tertiary structures and stabilities. Thus, contrary to current views, unfolding or destabilization of the protein is not necessary for cataractogenesis.

Vision impairment predicts five-year mortality

PURPOSE

To describe predictors of mortality in the 5-year follow-up of the Melbourne Visual Impairment Project (VIP) cohort.

METHODS

The Melbourne VIP was a population-based study of the distribution and determinants of age-related eye disease in a cluster random sample of Melbourne residents aged 40 years and older. Baseline examinations were conducted between 1992 and 1994. In 1997, 5-year follow-up examinations of the original cohort commenced. Causes of death were obtained from the National Death Index for all reported deaths.

RESULTS

Of the original 3,271 participants, 231 (7.1%) were reported to have died in the intervening 5 years. Of the remaining 3,040 participants eligible to return for follow-up examinations, 2,594 (85% of eligible) did participate, 51 (2%) had moved interstate or overseas, 83 (3%) could not be traced, and 312 (10%) refused to participate. Best corrected visual acuity < 6/12 and cortical cataract were associated with a significantly increased risk of mortality, as were increasing age, male sex, increased duration of cigarette smoking, increased duration of hypertension, and arthritis.

CONCLUSIONS

Even mild visual impairment increases the risk of death more than twofold.