Molecular genetics of age-related cataract

Genetic variations and polymorphisms in the ezrin gene are associated with age-related cataract

PURPOSE

Age-related cataract (ARC) is a complex multifactorial disorder, including genetic and environmental factors. Ezrin (EZR), a member of the ezrin/radixin/moesin (ERM) protein family, plays a crucial role in the development of the lens as a plasma membrane-cytoskeleton linker. We conducted this study to investigate the role of genetic variations of ezrin and the relationship between single nucleotide polymorphisms (SNPs) in EZR and susceptibility to ARC in a Chinese population.

METHODS

A total of 205 sporadic age-related cataract patients and 218 unrelated random healthy controls participated in our study. Genomic DNA was extracted from peripheral blood leukocytes. All exons of EZR were sequenced after being amplified with polymerase chain reaction. The functional consequences of the mutations were analyzed using PolyPhen2. SNP statistical analysis was performed using SNPstats.

RESULTS

We found three novel variations in 205 patients. None presented in the 218 controls, including c.441C>G, c.924G>C, and c.1503G>A. PolyPhen2 predicted that the c.924G>C mutation probably had pathogenicity. Compared with the healthy controls, the rs5881286 -/GT genotype and - allele frequencies (p=0.0012; odds ratio [OR]=3.37; 95% confidence interval [CI]=1.70-6.70; p=3.96e-5; χ(2)=18.98, respectively), rs2242318 T/C genotype and C allele frequencies (p=0.0045; OR=3.40; 95% CI=1.70-6.79; p=8.82e-6; χ(2)=21.86, respectively), and rs144581330 A/G genotype and G allele frequencies (p=0.0472; OR=14.46; 95% CI=1.29-162.43; p=0.0244, χ(2)=6.99, respectively) were higher in the patients with age-related cataract. SNP rs144581330 in exon 2 was also predicted to be probably damaging by PolyPhen2. Haplotype association including the - allele of rs5881286, C allele of rs2242318, and A allele of rs144581330 exhibited significantly higher distribution in the patients with ARC (p=8.0e-4; OR=3.38; 95% CI=1.66-6.87).

CONCLUSIONS

This study suggests that the genetic variations and SNPs in the gene EZR possibly contribute to the development of age-related cataracts in the Chinese population.

Protective effects of ebselen on sodium-selenite-induced experimental cataract in rats

PURPOSE

To determine whether ebselen has a protective effect or antioxidative potential in a sodium-selenite-induced experimental cataract model.

SETTING

Fırat University, Elazığ, Turkey.

DESIGN

Experimental study.

METHODS

Twenty-one Sprague-Dawley rat pups were randomly divided into a control group, a sodium-selenite-induced-cataract group, and an ebselen-treated group; each group contained 7 rat pups. Rats in the control group received dimethyl sulfoxide (DMSO) intraperitoneally only and rats in the sodium-selenite-induced-cataract group received 30 nmol/g body weight sodium selenite subcutaneously and DMSO intraperitoneally 10 days postpartum. Rats in the ebselen group received 30 nmol/g body weight sodium selenite subcutaneously 10 days postpartum and were treated with 5 mg/kg body weight ebselen once a day for 4 consecutive days. Cataract development was assessed weekly for 3 weeks by slitlamp examination and graded using a scale. Reduced glutathione (GSH), total nitrite, and malondialdehyde (MDA) levels in lens supernatants were measured at the end of 3 weeks.

RESULTS

In the control group, all lenses were clear. In the ebselen-treated group, the mean cataract stage was significantly lower than in the sodium-selenite-induced-cataract group (P = .022). The GSH levels were significantly lower in the sodium-selenite-induced-cataract group than in the control and ebselen groups (P < .001). The MDA levels were lower in the ebselen group than in the sodium-selenite-induced-cataract group (P < .001). The mean total nitrite level was significantly lower in the sodium-selenite-induced-cataract group than in the ebselen group (P = .001).

CONCLUSIONS

Ebselen had a protective effect on cataract development in a sodium-selenite-induced experimental model. The protective effect of ebselen appears to be due to inhibition of oxidative stress.

FINANCIAL DISCLOSURE

No author has a financial or proprietary interest in any material or method mentioned.

Patterns of gene expression in microarrays and expressed sequence tags from normal and cataractous lenses

In this contribution, we have examined the patterns of gene expression in normal and cataractous lenses as presented in five different papers using microarrays and expressed sequence tags. The purpose was to evaluate unique and common patterns of gene expression during development, aging and cataracts.

Association between cataract and genetic polymorphisms of GSTM1, GSTT1, and GSTO2 with respect of work place

PURPOSE

To investigate whether genetic polymorphisms of glutathione S-transferases (GSTM1, GSTT1, and GSTO2) in relation to the work place contribute to the development of cataract.

METHODS

The present case-control study consisted of 186 patients (108 females, 78 males) with cataract and 195 gender-matched healthy controls (111 females, 84 males) were randomly selected from unrelated volunteers in the same clinic. The GSTM1, GSTT1, and GSTO2 genotypes were determined using polymerase chain reaction (PCR) based methods.

RESULTS

The null genotype of GSTM1 increased the risk of cataract (OR=1.51, 95%CI: 1.01-2.26, p=0.045). The prevalence of GSTT1 and GSTO2 genotypes was similar between cases and controls. There was significant difference between cases and controls for work place (χ(2)=4.16, df=1, p=0.041). Genetic polymorphisms (GSTM1, GSTO2) and work place that were significant by p<0.3 in the univariate analysis were included in the analysis for investigating the additive effects of the genotypes and work place on risk of cataract. Statistical analysis showed that the risk of cataract increased as a function of number of putative high risk factors (χ(2)=8.001, p=0.005).

CONCLUSIONS

This finding suggests that the polymorphisms of GSTM1 and GSTO2 and also work place may act additively for developing cataract.

The Impact of Senile Cataract Maturity on Blood Oxidative Stress Markers and Glutathione-Dependent Antioxidants: Relations with Lens Variables

Oxidative stress is implicated in senile cataract (SC) genesis, although the impact of SC maturity on blood oxidative stress markers is unclear. Total hydroperoxides, malondialdehyde (MDA), glutathione (GSH), glutathione peroxidase (GPx), glutathione reductase (GR) and glutathione S-transferase (GST) were measured in the blood and lens samples of patients having either immature (n=31) or mature SC (n=50), and in 22 blood samples from noncataract controls. Compared to controls, SC patients had higher plasma MDA and serum GST, and decreased plasma GR and GSH levels. Plasma GPx as well as hydroperoxides differed from control values only in patients with mature SC. The multivariate logistic regression analysis showed that the fall of plasma GR activity (OR 5.14; CI 1.82-14.51;p=0.0020), as well as serum GST activity (OR 3.84; CI 1.36-10.83;p=0.0108) were independently associated with the maturity of SC. Lens hydroperoxides, MDA and GST, showed no correlation with correspondent blood values, in contrast to GPx (r=0.715; p<0.001) and GR (r=0.703; p<0.001). This study showed that the severity of SC is associated with increased systemic oxidative stress, which could be due to the fall of GSH-dependent antioxidant enzymes activities.

Spatial expression patterns of autophagy genes in the eye lens and induction of autophagy in lens cells

PURPOSE

Mutation of the autophagy gene FYVE (named after the four cysteine-rich proteins: Fab 1 [yeast orthologue of PIKfyve], YOTB, Vac 1 [vesicle transport protein], and EEA1) and coiled coil containing 1 (fyco1) causes human cataract suggesting a role for autophagy in lens function. Here, we analyzed the range and spatial expression patterns of lens autophagy genes and we evaluated whether autophagy could be induced in lens cells exposed to stress.

METHODS

Autophagy gene expression levels and their spatial distribution patterns were evaluated between microdissected human lens epithelium and fibers at the mRNA and protein levels by microarray data analysis, real-time PCR and western blot analysis. Selected autophagy protein spatial expression patterns were also examined in newborn mouse lenses by immunohistochemistry. The autophagosomal content of cultured human lens epithelial cells was determined by counting the number of microtubule-associated protein 1 light chain 3B (LC3B)-positive puncta in cells cultured in the presence or absence of serum.

RESULTS

A total of 42 autophagy genes were detected as being expressed by human lens epithelium and fibers. The autophagosomal markers LC3B and FYCO1 were detected throughout the newborn mouse lens. Consistently, the autophagy active form of LC3B (LC3B II) was detected in microdissected human lens fibers. An increased number of LC3B-positive puncta was detected in cultured lens cells upon serum starvation suggesting induction of autophagy in lens cells under stress conditions.

CONCLUSIONS

The data provide evidence that autophagy is an important component for the function of lens epithelial and fiber cells. The data are consistent with the notion that disruption of lens autophagy through mutation or inactivation of specific autophagy proteins could lead to loss of lens resistance to stress and/or loss of lens differentiation resulting in cataract formation.

The locus for an inherited cataract in sheep maps to ovine chromosome 6

PURPOSE

Cataracts are an important cause of blindness in humans but there are few large animal models available. One of these animal models is Ovine Heritable Cataract, a bilateral cortical cataract which develops after birth. This cataract has been used as a model for human cataracts in drug trials, but the gene responsible for the cataract trait is unknown. A genetic test for cataract would improve the efficiency of the model by predicting which animals would develop cataracts. Identifying the genetic basis of the cataract would indicate its relevance to human cataract.

METHODS

A genome scan was performed on 20 sheep chromosomes, representing 86% of the genome, to determine the position of the cataract locus. Additional microsatellite markers were tested on chromosome 6 using a larger pedigree. Fine mapping was performed using a breakpoint panel of 36 animals and novel microsatellite markers taken from the bovine genome assembly. All exons of the candidate gene nudix (nucleoside diphosphate linked moiety X)-type motif 9 (NUDT9) were sequenced in normal and affected sheep.

RESULTS

Significant linkage was found between cataract status and markers on chromosome 6. Linkage analysis on the larger pedigree showed the most likely position of the cataract locus was between 112.3 and 132.9 cM from the centromere. During fine mapping, NUDT9 was considered as a positional candidate for the cataract gene because it was located within the linked interval and is expressed in the lens. The gene was ruled out as the cataract gene after extensive genotype analysis, but a single nucleotide polymorphism (SNP) inside it provided a useful restriction fragment length polymorphism (RFLP) marker for further fine mapping. Twelve new markers were found and used to map the cataract locus to between 131.1 and 131.8 cM from the centromere.

CONCLUSIONS

A region of ovine chromosome 6 strongly linked to cataract has been identified, and a genetic test for cataract based on a SNP within this region has been developed. The best candidate gene within this region is AF4/FMR2 family, member 1 (AFF1), the mouse equivalent of which is associated with an inherited cataract.

Total activity of glutathione-S-transferase (GST) and polymorphisms of GSTM1 and GSTT1 genes conferring risk for the development of age related cataracts

The pathogenesis of cataract is influenced by a number of factors including oxidative stress. Glutathione-S-transferase (GST) catalyses the nucleophilic addition of the thiol of GSH to electrophilic acceptors. It is important for detoxification of xenobiotics in order to protect tissues from oxidative damage. In humans, GSTT1 and GSTM1 deletion genotypes are associated with a variety of pathological conditions including certain ophthalmic diseases. In the present study, it is aimed to determine the risk of genetic polymorphisms of GSTM1 and GSTT1 isoforms of GST for developing of age related cataracts (ARCs). We compared the prevalence of GSTT1 and GSTM1 deletion genotypes, which were determined by multiplex polymerase chain reaction, in 455 patients with ARCs (108 with nuclear (NC), 105 with cortical (CC), 96 with posterior subcapsular, (PSC) and 146 with mixed type (MT)) and 205 age and sex matched controls. The GST activity in erythrocytes (RBC) and cataractous lenses was measured spectrophotometrically using 1-chloro-2,4-dinitrobenzene (CDNB) as substrate. The frequency of GSTM1 positive individuals was significantly higher in MT cataracts followed by NC, CC and PSC types with corresponding decrease in the GSTM1 null genotypes as compared to controls. Considering the GSTT1 locus, GSTT1 null genotypes showed high frequency in patients in general as compared to controls with corresponding reduction in the GSTT1 positive genotype. The activity of GST in RBC was higher in all the types of cataracts as compared to that in controls and in cataractous lenses the mean values were slightly higher in cases of NC cataracts as compared to CC, PSC and MT. The data suggests that GSTM1 positive, GSTT1 null and double null (GSTM1 null and GSTT1 null) genotypes may confer risk for the development of ARC. The increased activity of GST found in the present study could be due to a compensatory mechanism operating in response to increased oxidative stress.

EPHA2 polymorphisms and age-related cataract in India

OBJECTIVE

We investigated whether previously reported single nucleotide polymorphisms (SNPs) of EPHA2 in European studies are associated with cataract in India.

METHODS

We carried out a population-based genetic association study. We enumerated randomly sampled villages in two areas of north and south India to identify people aged 40 and over. Participants attended a clinical examination including lens photography and provided a blood sample for genotyping. Lens images were graded by the Lens Opacification Classification System (LOCS III). Cataract was defined as a LOCS III grade of nuclear ≥4, cortical ≥3, posterior sub-capsular (PSC) ≥2, or dense opacities or aphakia/pseudophakia in either eye. We genotyped SNPs rs3754334, rs7543472 and rs11260867 on genomic DNA extracted from peripheral blood leukocytes using TaqMan assays in an ABI 7900 real-time PCR. We used logistic regression with robust standard errors to examine the association between cataract and the EPHA2 SNPs, adjusting for age, sex and location.

RESULTS

7418 participants had data on at least one of the SNPs investigated. Genotype frequencies of controls were in Hardy-Weinberg Equilibrium (p>0.05). There was no association of rs3754334 with cataract or type of cataract. Minor allele homozygous genotypes of rs7543472 and rs11260867 compared to the major homozygote genotype were associated with cortical cataract, Odds ratio (OR) = 1.8, 95% Confidence Interval (CI) (1.1, 3.1) p = 0.03 and 2.9 (1.2, 7.1) p = 0.01 respectively, and with PSC cataract, OR = 1.5 (1.1, 2.2) p = 0.02 and 1.8 (0.9, 3.6) p = 0.07 respectively. There was no consistent association of SNPs with nuclear cataract or a combined variable of any type of cataract including operated cataract.

CONCLUSIONS

Our results in the Indian population agree with previous studies of the association of EPHA2 variants with cortical cataracts. We report new findings for the association with PSC which is particularly prevalent in Indians.

A spontaneous mutation in Srebf2 leads to cataracts and persistent skin wounds in the lens opacity 13 (lop13) mouse

Lens opacity 13 (lop13) is a spontaneous, autosomal recessive mouse mutant that exhibits nuclear cataracts. Histological analysis revealed swollen lens fiber cells and the presence of bladder cells within the lens cortex, as well as morgagnian globules and liquefied material at the lens posterior. At 3 months of age, in addition to cataracts, lop13 mice also develop persistent skin wounds. Linkage analysis assigned the lop13 locus to a 1.1-Mb region on mouse Chr 15, encompassing 19 candidate genes. Sequence analysis identified a C3112T mutation in exon 18 of Sterol Regulatory Element Binding-Transcription Factor 2 (Srebf2) resulting in the R1038C substitution of a highly conserved arginine within the Srebf2 regulatory domain. Srebf2 belongs to a family of membrane-bound basic helix-loop-helix leucine zipper transcription factors that control the expression of genes involved in the biosynthesis and uptake of cholesterol and fatty acids. The lack of complementation observed in Srebf2 ( lop13/GT ) compound heterozygotes carrying the Srebf2 gene trapped allele (Srebf2 ( GT )) provides genetic evidence that the identified C3112T substitution in Srebf2 is responsible for the lop13 phenotype. Gas chromatography analysis identified lower levels of cholesterol in the lop13 brain, liver, and lens when compared to wild-type mice. These findings suggest that lop13 is a hypomorphic mutation in Srebf2. As such, the lop13 mouse presents an invaluable in vivo model for studying the contribution of Srebf2 and cholesterol to maintaining the homeostasis of the lens and skin.

Temperature-dependent structural and functional properties of a mutant (F71L) αA-crystallin: molecular basis for early onset of age-related cataract

Previously we identified a novel mutation (F71L) in the αA-crystallin gene associated with early onset of age-related cataract. However, it is not known how the missense substitution translates into reduced chaperone-like activity (CLA), and how the structural and functional changes lead to early onset of the disease. Herein, we show that under native conditions the F71L-mutant is not significantly different from wild-type with regard to secondary and tertiary structural organization, hydrophobicity and the apparent molecular mass of oligomer but has substantial differences in structural and functional properties following a heat treatment. Wild-type αA-crystallin demonstrated increased CLA, whereas the F71L-mutant substantially lost its CLA upon heat treatment. Further, unlike the wild-type αA-subunit, F71L-subunit did not protect the αB-subunit in hetero-oligomeric complex from heat-induced aggregation. Moreover, hetero-oligomer containing F71L and αB in 3:1 ratio had significantly lower CLA upon thermal treatment compared to its unheated control. These results indicate that α-crystallin complexes containing F71L-αA subunits are less stable and have reduced CLA. Therefore, F71L may lead to earlier onset of cataract due to interaction with several environmental factors (e.g., temperature in this case) along with the aging process.

Major intrinsic protein (MIP) polymorphism is associated with age-related cataract in Chinese

PURPOSE

Age-related cataract (ARC) is a complex multi-factorial disorder involving several genetic and environmental factors. The major intrinsic protein of lens fiber gene (MIP) encodes the most abundant junctional membrane protein in the mature lens and plays a critical role in maintainace of lens normal structure and internal circulation. To determine the relationship between single nucleotide polymorphisms (SNPs) in MIP and the susceptibility to ARC in a Chinese population, we conducted this case-control study.

METHODS

A total of 164 unrelated ARC patients and 132 normal controls were involved in the study. All participants completed full physical and ophthalmic examinations and provided a blood sample for DNA extraction. Seven SNPs (rs2269348, rs61759527, c.-4T>C, rs77163805, rs74641138, rs35033450, and rs36032520) in MIP were amplified by polymerase chain reaction (PCR) and then sequenced. Statistical analysis was performed using SNPstats.

RESULTS

Polymorphisms rs61759527, rs77163805, rs35033450, and rs36032520 were not detected in all 296 subjects. There were no statistical differences in genotype or allele frequency of rs2269348 and rs74641138 between ARC cases and controls. But in c.-4C>T, cataract patients had a higher TC genotype and C allele frequencies (p=0.0018 and p=0.017, respectively) compared to healthy controls. The haplotype CCG of rs2269348, c.-4T>C and rs74641138 also exhibited a significantly higher distribution in cases than controls (OR=8.83, p=0.0024).

CONCLUSIONS

Our findings indicate that the genotype TC in polymorphism c.-4T>C and haplotype CCG of rs2269348, c.-4T>C, and rs74641138 in MIP may attach an additional genetic risk factor for ARC in Chinese. This is the first association study about SNPs in MIP and susceptibility to ARC in Chinese population.

Excessive milk intake as a risk factor, probably associated with oxidative stress, in experimental naphthalene-initiated cataract in rats

AIM

To determine whether a diet containing excessive amounts of milk aggravates naphthalene-initiated cataracts in a common animal model of age-related human cataract.

METHODS

Ninety Sprague-Dawley rats were fed a natural diet supplemented with either water (group A), normal amounts of milk (group B), excessive amounts of milk (group C), naphthalene plus water (group D), naphthalene plus normal amounts of milk (group E), naphthalene plus excessive amounts of milk (group F). Cataract development was monitored weekly using a slit lamp and lens gray value analysis. Concentrations of reactive oxygen species (ROS), reduced glutathione (GSH) and malondialdehyde (MDA) in rat lenses were measured to determine the role of oxidative stress in cataract induction.

RESULTS

By week 4, the cortical gray value was significantly higher in group F than that in group D, and the cortical gray value was significantly higher in group D than in group A. However, by week 8, no significant differences were observed among groups C, F, B, E and A. ROS concentrations in lenses of rats of groups C and F were slightly higher than in those of groups B, E and A, but ROS concentrations in group F were significantly higher than in the other groups receiving naphthalene (i.e. groups D and E). GSH concentrations in group F were significantly lower than in the other groups. MDA concentrations in group F were significantly higher than in the other groups receiving naphthalene, indicating increased lipid peroxidation induced by naphthalene plus excessive intake of milk.

CONCLUSIONS

Our results provide quantitative evidence that excessive intake of milk aggravates naphthalene-initiated cataracts, which is probably due to oxidative damage caused by increased ROS.

Association of G>A transition in exon-1 of alpha crystallin gene in age-related cataracts

Aim

To identify the presence of a known or novel mutation/SNP in Exon-1 (ex-1) of alpha crystallin (CRYAA) gene in different types of age-related cataract (ARC) patients.

Materials and Methods

Single strand Conformation Polymorphism (SSCP) analysis was carried for the detection of single nucleotide polymorphism (SNP) in ex-1 of alpha crystallin (CRYAA) gene which was confirmed by sequencing.

Results

The SSCP analysis of ex-1 of CRYAA gene revealed mobility shift in patients and controls, which was due to G>A transition at 6^th position in exon-1 of CRYAA gene. All the three genotypes, GG, AA and GA, were detected in patients and controls indicating that G>A substitution is polymorphic. The analysis showed significant risk for heterozygotes (GA) as compared to pooled frequencies of homozygotes (GG + AA), which was 1.81 times for all the types of cataracts in general and 2.5 times for Nuclear Cataract and twice for Cortical Cataract.

Conclusion

The GA heterozygotes were at higher risk for developing NC and CC types of cataracts, where as the GG homozygotes for MT and AA homozygotes for PSC types were at risk. To our knowledge, an association of G>A transition found in ex-1 of CRYAA gene with ARC, with differential risk of genotypes for individual type of cataracts has not been reported previously.

Genetic variations in GJA3, GJA8, LIM2, and age-related cataract in the Chinese population: a mutation screening study

PURPOSE

To investigate the role of genetic variations in three known cataract-associated genes, gap junction protein α3 (GJA3), gap junction protein α8 (GJA8), lens intrinsic membrane protein 2 (LIM2), encoding lens fiber cell membrane proteins in the development of age-related cataracts.

METHODS

One hundred and forty-five sporadic age-related cataract patients and one hundred and fifty-six unrelated random healthy controls participated in this study. Genomic DNA was extracted from peripheral blood leukocytes. All exons of GJA3, GJA8, and LIM2 were sequenced after being amplified by polymerase chain reaction (PCR). The functional consequences of the mutations were analyzed using PolyPhen.

RESULTS

We found five novel variations in 145 patients and none of them presented in the 156 controls. There are two variations in GJA3 (c.-39C>G, c. 415G>A); one in GJA8 (c. 823G>A), and two in LIM2 (c.57G>A, c.67A>C). PolyPhen predicted that the LIM2 c.67A>C mutation may have potential pathogenicity.

CONCLUSIONS

The genetic mutation in GJA3, GJA8, and LIM2 may slightly contribute to the development of age-related cataracts. This study showed a potential relationship between lens fiber cell membrane protein genes and the development of age-related cataracts in the Chinese population.

TXNL6 is a novel oxidative stress-induced reducing system for methionine sulfoxide reductase a repair of α-crystallin and cytochrome C in the eye lens

A key feature of many age-related diseases is the oxidative stress-induced accumulation of protein methionine sulfoxide (PMSO) which causes lost protein function and cell death. Proteins whose functions are lost upon PMSO formation can be repaired by the enzyme methionine sulfoxide reductase A (MsrA) which is a key regulator of longevity. One disease intimately associated with PMSO formation and loss of MsrA activity is age-related human cataract. PMSO levels increase in the eye lens upon aging and in age-related human cataract as much as 70% of total lens protein is converted to PMSO. MsrA is required for lens cell maintenance, defense against oxidative stress damage, mitochondrial function and prevention of lens cataract formation. Essential for MsrA action in the lens and other tissues is the availability of a reducing system sufficient to catalytically regenerate active MsrA. To date, the lens reducing system(s) required for MsrA activity has not been defined. Here, we provide evidence that a novel thioredoxin-like protein called thioredoxin-like 6 (TXNL6) can serve as a reducing system for MsrA repair of the essential lens chaperone α-crystallin/sHSP and mitochondrial cytochrome c. We also show that TXNL6 is induced at high levels in human lens epithelial cells exposed to H(2)O(2)-induced oxidative stress. Collectively, these data suggest a critical role for TXNL6 in MsrA repair of essential lens proteins under oxidative stress conditions and that TXNL6 is important for MsrA defense protection against cataract. They also suggest that MsrA uses multiple reducing systems for its repair activity that may augment its function under different cellular conditions.

DNA-binding and transcriptional activities of human HSF4 containing mutations that associate with congenital and age-related cataracts

Heat shock transcription factor HSF4 is necessary for ocular lens development and fiber cell differentiation. Mutations of the human HSF4 gene have been implicated in congenital and age-related cataracts. Here, we show that HSF4 activates transcription of genes encoding crystallins and beaded filament structural proteins in lens epithelial cells. Five missense mutations that have been associated with congenital cataract inhibited DNA-binding of HSF4, which demonstrates the relationship between HSF4 mutations, loss of lens protein gene expression, and cataractogenesis. However, two missense mutations that have been associated with age-related cataract did not or only slightly alter HSF4 activity, implying that other genetic and environmental factors affect the functions of these mutant proteins.

Autosomal recessive congenital cataract in consanguineous Pakistani families is associated with mutations in GALK1

PURPOSE

To identify the pathogenic mutations responsible for autosomal recessive congenital cataracts in consanguineous Pakistani families.

METHODS

All affected individuals underwent detailed ophthalmologic and medical examination. Blood samples were collected and genomic DNA was extracted. A genome-wide scan was performed with polymorphic microsatellite markers on genomic DNA from affected and unaffected family members and logarithm of odds (LOD) scores were calculated. All coding exons of galactokinase (GALK1) were sequenced to identify pathogenic lesions.

RESULTS

Clinical records and ophthalmological examinations suggested that affected individuals have nuclear cataracts. Linkage analysis localized the critical interval to chromosome 17q with a maximum LOD score of 5.54 at theta=0, with D17S785 in family PKCC030. Sequencing of GALK1, a gene present in the critical interval, identified a single base pair deletion: c.410delG, which results in a frame shift leading to a premature termination of GALK1: p.G137fsX27. Additionally, we identified a missense mutation: c.416T>C, in family PKCC055 that results in substitution of a leucine residue at position 139 with a proline residue: p.L139P, and is predicted to be deleterious to the native GALK1 structure.

CONCLUSIONS

Here, we report pathogenic mutations in GALK1 that are responsible for autosomal recessive congenital cataracts in consanguineous Pakistani families.

EPHA2 is associated with age-related cortical cataract in mice and humans

Age-related cataract is a major cause of blindness worldwide, and cortical cataract is the second most prevalent type of age-related cataract. Although a significant fraction of age-related cataract is heritable, the genetic basis remains to be elucidated. We report that homozygous deletion of Epha2 in two independent strains of mice developed progressive cortical cataract. Retroillumination revealed development of cortical vacuoles at one month of age; visible cataract appeared around three months, which progressed to mature cataract by six months. EPHA2 protein expression in the lens is spatially and temporally regulated. It is low in anterior epithelial cells, upregulated as the cells enter differentiation at the equator, strongly expressed in the cortical fiber cells, but absent in the nuclei. Deletion of Epha2 caused a significant increase in the expression of HSP25 (murine homologue of human HSP27) before the onset of cataract. The overexpressed HSP25 was in an underphosphorylated form, indicating excessive cellular stress and protein misfolding. The orthologous human EPHA2 gene on chromosome 1p36 was tested in three independent worldwide Caucasian populations for allelic association with cortical cataract. Common variants in EPHA2 were found that showed significant association with cortical cataract, and rs6678616 was the most significant in meta-analyses. In addition, we sequenced exons of EPHA2 in linked families and identified a new missense mutation, Arg721Gln, in the protein kinase domain that significantly alters EPHA2 functions in cellular and biochemical assays. Thus, converging evidence from humans and mice suggests that EPHA2 is important in maintaining lens clarity with age.

Cataract is the leading cause of blindness. Cataract may form at any age, but the peak incidence is bimodal—in the perinatal period or later than 50 years of age. The early onset forms follow Mendelian inheritance patterns and are rare. Age-related cataract accounts for 18 million cases of blindness and 59 million cases of reduced vision worldwide. Among three types of age-related cataract, cortical cataract is known to be highly heritable, although few genes have been linked to its etiology. We report here that EPHA2 is associated with cortical cataract. EPHA2 is expressed in mouse and human cortical lens fiber cells, and homozygous deletion of Epha2 in two independent strains of mice led to development of cataract that progressed with age. Common and rare variants including a missense mutation in the EPHA2 gene were associated for cortical cataract in three different Caucasian populations. Our study identified EPHA2 as a gene for human age-related cataract and established Epha2 knockout mice as a model for progressive cortical cataract.

A novel mutation (F71L) in alphaA-crystallin with defective chaperone-like function associated with age-related cataract

Age-related cataract (ARC) is a multifactorial disease and the leading cause of blindness worldwide. Genetic predisposition in association with other etiological factors may contribute to ARC. However, gene mutation studies on ARC are scanty. In the present work, we identified a genetic variation (F71L) in the exon-2 of CRYAA (alphaA-crystallin) gene in three unrelated female sporadic cases among 711 ARC patients but not in 265 normal non-cataractous controls by SSCP and RFLP analysis. By comparing human recombinant wild-type and F71L-alphaA-crystallin, we characterized the functional significance of this missense mutation. Chromatography, fluorescence and far- and near-UV CD studies indicated that F71L missense mutation did not significantly affect the apparent molecular mass, secondary and tertiary structures and hydrophobicity of alphaA-crystallin. While the mutant alphaA-crystallin displayed significant (35-90%) loss of chaperone-like activity (CLA) in thermal aggregation of carbonic anhydrase, betaL- and gamma-crystallins, it showed moderate (10-50%) loss in CLA in DTT-induced aggregation of insulin and lysozyme. This is the first report of an alphaA-F71L mutation being associated with ARC and suggests that ARC in individuals carrying this mutation (F71L) might be due to the overall loss of in vivo chaperone activity due to interaction with other environmental factors.

Functions of the intermediate filament cytoskeleton in the eye lens

Intermediate filaments (IFs) are a key component of the cytoskeleton in virtually all vertebrate cells, including those of the lens of the eye. IFs help integrate individual cells into their respective tissues. This Review focuses on the lens-specific IF proteins beaded filament structural proteins 1 and 2 (BFSP1 and BFSP2) and their role in lens physiology and disease. Evidence generated in studies in both mice and humans suggests a critical role for these proteins and their filamentous polymers in establishing the optical properties of the eye lens and in maintaining its transparency. For instance, mutations in both BFSP1 and BFSP2 cause cataract in humans. We also explore the potential role of BFSP1 and BFSP2 in aging processes in the lens.

Phytochelatins inhibit the metal-induced aggregation of alpha-crystallin

Phytochelatins (PCs) are heavy-metal-binding peptides found in some eukaryotes. This study investigates the use of plant-derived PCs for the inhibition of metal-induced protein aggregation. The results of this study show that PCs inhibit zinc-induced alpha-crystallin aggregation, and suggest that PCs might be useful as anti-cataract agents.

Structural function of MIP/aquaporin 0 in the eye lens; genetic defects lead to congenital inherited cataracts

Aquaporin 0 (AQP0) was originally characterized as a membrane intrinsic protein, specifically expressed in the lens fibers of the ocular lens and designated MIP, for major intrinsic protein of the lens. Once the gene was cloned, an internal repeat was identified, encoding for the amino acids Asp-Pro-Ala, the NPA repeat. Shortly, the MIP gene family was emerging, with members being characterized in mammals, insects, and plants. Once Peter Agre's laboratory developed a functional assay for water channels, the MIP family became the aquaporin family and MIP became known as aquaporin 0. Besides functioning as a water channel, aquaporin 0 also plays a structural role, being required for maintaining the transparency and optical accommodation of the ocular lens. Mutations in the AQP0 gene in human and mice result in genetic cataracts; deletion of the MIP/AQP0 gene in mice results in lack of suture formation required for maintenance of the lens fiber architecture, resulting in perturbed accommodation and focus properties of the ocular lens. Crystallography studies support the notion of the double function of aquaporin 0 as a water channel (open configuration) or adhesion molecule (closed configuration) in the ocular lens fibers. The functions of MIP/AQP0, both as a water channel and an adhesive molecule in the lens fibers, contribute to the narrow intercellular space of the lens fibers that is required for lens transparency and accommodation.

The EPHA2 gene is associated with cataracts linked to chromosome 1p

PURPOSE

Cataracts are a clinically and genetically heterogeneous disorder affecting the ocular lens, and the leading cause of treatable vision loss and blindness worldwide. Here we identify a novel gene linked with a rare autosomal dominant form of childhood cataracts segregating in a four generation pedigree, and further show that this gene is likely associated with much more common forms of age-related cataracts in a case-control cohort.

METHODS

Genomic DNA was prepared from blood leukocytes, and genotyping was performed by means of single nucleotide polymorphism (SNP) markers, and short tandem repeat (STR) markers. Linkage analyses were performed with the GeneHunter and MLINK programs, and association analyses were performed with the Haploview and Exemplar programs. Mutation detection was achieved by PCR amplification of exons and di-deoxy cycle-sequencing.

RESULTS

Genome-wide linkage analysis with SNP markers, identified a likely disease-haplotype interval on chromosome 1p (rs707455-[approximately 10 Mb]-rs477558). Linkage to chromosome 1p was confirmed using STR markers D1S2672 (LOD score [Z]=3.56, recombination distance [theta]=0), and D1S2697 (Z=2.92, theta=0). Mutation profiling of positional-candidate genes detected a heterozygous transversion (c.2842G>T) in exon 17 of the gene coding for Eph-receptor type-A2 (EPHA2) that cosegregated with the disease. This missense change was predicted to result in the non-conservative substitution of a tryptophan residue for a phylogenetically conserved glycine residue at codon 948 (p.G948W), within a conserved cytoplasmic domain of the receptor. Candidate gene association analysis further identified SNPs in the EPHA2 region of chromosome 1p that were suggestively associated with age-related cataracts (p=0.007 for cortical cataracts, and p=0.01 for cortical and/or nuclear cataracts).

CONCLUSIONS

These data provide the first evidence that EPHA2, which functions in the Eph-ephrin bidirectional signaling pathway of mammalian cells, plays a vital role in maintaining lens transparency.

Prediction of Gene Expression Levels and the Role of Cis-Acting Elements in Age-Related Cataract by Applying a Promoter-Based Modeling Approach

Cataract is a dynamical process of lens opacity formation involving many inter- and intracellular regulations, as well as metabolic genes and transcription factors. Using a series of microarray-derived mRNA profiles for human cataractogenesis (Hawse et al. Mol. Vision 2003, 9, 515-537), we develop a promoter-based system-theoretic modeling to demonstrate model-driven prediction of gene expression levels and to identify the role of critical cis-acting elements. In this study, 14 key mRNA expression data from the structural and pathological molecules of age-related cataract samples are used. The first seven genes consist of structural molecules, and the second half of genes are composed of heat shock proteins, filensin, and glutathione peroxidase 3. The presented result demonstrates that mRNA expression levels of structural proteins such as crystallins can be successfully predicted from 5' flanking regulatory DNA sequences. In addition, predicted gene expression levels of heat shock protein, beta-tubulin, and alphaA-crystallin accurately estimate the stimulatory or inhibitory role of distributed cis-acting elements, i.e., c-Myc, GATA-1, GR, NE-E, and Pit-1. Although it is difficult to predict the overall gene expression levels in cataract samples, the present study shows the potential use of promoter-based modeling and prediction of the gene expression levels for age-related cataract.

Density of common complex ocular traits in the aging eye: analysis of secondary traits in genome-wide association studies

Genetic association studies are identifying genetic risks for common complex ocular traits such as age-related macular degeneration (AMD). The subjects used for discovery of these loci have been largely from clinic-based, case-control studies. Typically, only the primary phenotype (e.g., AMD) being studied is systematically documented and other complex traits (e.g., affecting the eye) are largely ignored. The purpose of this study was to characterize these other or secondary complex ocular traits present in the cases and controls of clinic-based studies being used for genetic study of AMD. The records of 100 consecutive new patients (of any diagnosis) age 60 or older for which all traits affecting the eye had been recorded systematically were reviewed. The average patient had 3.5 distinct diagnoses. A subset of 10 complex traits was selected for further study because they were common and could be reliably diagnosed. The density of these 10 complex ocular traits increased by 0.017 log-traits/year (P = 0.03), ranging from a predicted 2.74 at age 60 to 4.45 at age 90. Trait-trait association was observed only between AMD and primary vitreomacular traction (P = 0.0009). Only 1% of subjects age 60 or older had no common complex traits affecting the eye. Extrapolations suggested that a study of 2000 similar subjects would have sufficient power to detect genetic association with an odds ratio of 2.0 or less for 4 of these 10 traits. In conclusion, the high prevalence of complex traits affecting the aging eye and the inherent biases in referral patterns leads to the potential for confounding by undocumented secondary traits within case-control studies. In addition to the primary trait, other common ocular phenotypes should be systematically documented in genetic association studies so that adjustments for potential trait-trait associations and other bias can be made and genetic risk variants identified in secondary analyses.

Teleost lens development and degeneration

The transparent properties of the lens and its ability to focus light onto the retina are critical for normal vision. Optical clarity of the lens is achieved and maintained by a unique, highly regulated integration of lens cell proliferation and differentiation that persists throughout life. Zebrafish is a powerful genetic model for studying vertebrate lens differentiation and growth because the structural organization of the lens and gene functions are largely conserved with mammals, including humans. However, some features of zebrafish lens developmental morphology and gene expression are different from those of mammals and other terrestrial vertebrates. For example, the presumptive zebrafish lens delaminates from the surface ectoderm to form a solid mass of cells, in which the primary fibers differentiate by elongating in circular fashion. Both mutational and candidate gene analyses have identified and characterized developmental gene functions of the lens in zebrafish. This chapter presents the recent morphological analysis of zebrafish lens formation. In addition, the roles of Pitx3, Foxe3, and the lens-specific protein Lengsin (LENS Glutamine SYNthetase-like) in lens development are analyzed. Selected zebrafish lens mutants defective in early developmental processes and the maintenance of lens transparency are also discussed.

Fgf19 is required for zebrafish lens and retina development

Fgf signaling plays crucial roles in morphogenesis. Fgf19 is required for zebrafish forebrain development. Here, we examined the roles of Fgf19 in the formation of the lens and retina in zebrafish. Knockdown of Fgf19 caused a size reduction of the lens and the retina, failure of closure of the choroids fissure, and a progressive expansion of the retinal tissue to the midline of the forebrain. Fgf19 expressed in the nasal retina and lens was involved in cell survival but not cell proliferation during embryonic lens and retina development. Fgf19 was essential for the differentiation of lens fiber cells in the lens but not for the neuronal differentiation and lamination in the retina. Loss of nasal fate in the retina caused by the knockdown of Fgf19, expansion of nasal fate in the retina caused by the overexpression of Fgf19 and eye transplantation indicated that Fgf19 in the retina was crucial for the nasal-temporal patterning of the retina that is critical for the guidance of retinal ganglion cell axons. Knockdown of Fgf19 also caused incorrect axon pathfinding. The present findings indicate that Fgf19 positively regulates the patterning and growth of the retina, and the differentiation and growth of the lens in zebrafish.

Rybp, a polycomb complex-associated protein, is required for mouse eye development

Background

Rybp (Ring1 and YY1 binding protein) is a zinc finger protein which interacts with the members of the mammalian polycomb complexes. Previously we have shown that Rybp is critical for early embryogenesis and that haploinsufficiency of Rybp in a subset of embryos causes failure of neural tube closure. Here we investigated the requirement for Rybp in ocular development using four in vivo mouse models which resulted in either the ablation or overexpression of Rybp.

Results

Our results demonstrate that loss of a single Rybp allele in conventional knockout mice often resulted in retinal coloboma, an incomplete closure of the optic fissure, characterized by perturbed localization of Pax6 but not of Pax2. In addition, about one half of Rybp-/- <-> Rybp+/+ chimeric embryos also developed retinal colobomas and malformed lenses. Tissue-specific transgenic overexpression of Rybp in the lens resulted in abnormal fiber cell differentiation and severe lens opacification with increased levels of AP-2α and Sox2, and reduced levels of βA4-crystallin gene expression. Ubiquitous transgenic overexpression of Rybp in the entire eye caused abnormal retinal folds, corneal neovascularization, and lens opacification. Additional changes included defects in anterior eye development.

Conclusion

These studies establish Rybp as a novel gene that has been associated with coloboma. Other genes linked to coloboma encode various classes of transcription factors such as BCOR, CBP, Chx10, Pax2, Pax6, Six3, Ski, Vax1 and Vax2. We propose that the multiple functions for Rybp in regulating mouse retinal and lens development are mediated by genetic, epigenetic and physical interactions between these genes and proteins.

Light scattering in the C57BL/6 mouse lens

PURPOSE

To characterize inherent light scattering in the C57BL/6 mouse lens.

METHODS

Lenses from 20 6-week-old female C57BL/6 mice were extracted from freshly enucleated globes and microsurgically cleaned of remnants of the ciliary body. Lens light scattering was measured quantitatively with a light dissemination meter (LDM). Morphological properties of the mouse lenses were documented using grid- and dark-field illumination photography. Analysis of variance was performed to establish variance for animals, variance between left and right eyes and variance for measurements.

RESULTS

Average inherent light scattering in the C57BL/6 mouse lens is 0.16 +/- 0.02 tEDC (transformed equivalent diazepam concentration). The mean size of a mouse lens at 6 weeks is 1.9 mm in diameter. Two lenses featured pre-existing cortical lens opacities. Variance for animals was assessed to be 7.9 10(- 4) tEDC(2), variance for measurements was 1.6 10(- 4) tEDC(2), and variance between left and right eyes was 8.8 10(- 4) tEDC(2). The tolerance limit for non-pathological light scattering was determined to 0.26 tEDC. No significant difference in light scattering between left and right mouse lenses was found. The minimum number of C57BL/6 mice required for detection of a 10% experimentally induced change in light scattering intensity was estimated to be 50 for independent group experiments and 25 for paired design experiments.

CONCLUSIONS

The C57BL/6 mouse is a suitable animal in which to conduct experiments on light scattering or cataractogenesis with high precision at reasonable sample sizes. Before including C57BL/6 mice into a study on cataractogenesis, pre-existing lens opacities such as congenital cataract must be excluded.

Prevalence of feline cataract: results of a cross-sectional study of 2000 normal animals, 50 cats with diabetes and one hundred cats following dehydrational crises

OBJECTIVE

In this study 2000 normal cats, 50 cats with diabetes and 100 cats with a history of dehydrational crises were examined ophthalmoscopically to determine presence of cataract.

MATERIALS AND METHODS

The cats examined were predominantly from veterinary hospital populations but also from re-homing facilities and breeding catteries. Prevalence of cataract was determined for different age groups (year cohorts). The age at which prevalence of cataract was 50% (C(50)) was determined indirectly from a fitted prevalence curve as previously described. C(50) was determined for animals of different genders and different breeds as well as for those with diabetes and histories of dehydrational episodes related to chronic renal failure, chronic vomiting or chronic diarrhea.

RESULTS

The mean +/- standard deviation of C(50) for all normal cats in the study was 12.7 +/- 3.4 years. All cats over 17.5 years were affected by some degree of lens opacity. C(50) for cats with diabetes was 5.6 +/- 1.9 years (significantly different from normal cats at P < 0.0001). For cats with a history of dehydrational crises C(50) was 9.9 +/- 2.5 (difference from normal cats nearing statistical significance at P = 0.06).

CONCLUSIONS

The study yields novel findings regarding the prevalence of age-related cataract in normal cats together with cats with diabetes and history of previous dehydrational episodes in which prevalence of cataract is increased.

A novel function of DNA repair molecule Nbs1 in terminal differentiation of the lens fibre cells and cataractogenesis

The Nbs1 protein, hypomorphic mutant in Nijmegen breakage syndrome (NBS), is a component of the Mre11/Rad50/Nbs1 (M/R/N) complex that acts as a DNA double-strand break sensor and functions in cell cycle checkpoint in response to DNA damage and DNA repair. Here we report that targeted disruption of murine NBS1 gene (Nbn) in the lens alters the M/R/N complex nuclear localization and results in microphthalmia in mice due to reduced proliferation of the lens epithelial cells. Unexpectedly, all Nbn-deficient lenses develop cataracts at an early age due to altered lens fibre cell differentiation, including disruption of normal lens epithelial and fibre cell architecture and incomplete denucleation of fibre cells, and these changes are independent of the p53 pathway. In addition, Nbn-deficient lenses show dysregulated transcription of various crystallins. Thus, this study implicates a novel function of Nbs1 in terminal differentiation of the lens fibre cells and in cataractogenesis.

Application of genetic approaches to ocular disease

The human eye is a complex organ whose development requires extraordinary coordination of developmental processes. Multiple genes responsible for the proper development and maintenance of the vertebrate eye have been identified and shown to be involved in a variety of debilitating ocular conditions. Genetic diseases involving the eye represent a leading cause of blindness in children and adults. This article summarizes current genetic approaches and their application to studies of ocular disease.

Ordered subset analysis supports a glaucoma locus at GLC1I on chromosome 15 in families with earlier adult age at diagnosis

Open angle glaucoma (OAG) is a complex disorder with varying etiologies due to multiple genes and environmental effects. This genetic heterogeneity can confound efforts to map loci. Increased homogeneity in a sample can be achieved using either ordered subset analysis (OSA) which groups families, or individual OSA (IOSA), which groups individuals based on disease related covariates. Recently, GLC1I was mapped to 15q11-13 in families with early adult onset of OAG. We tested for linkage to GLC1I in an independent sample of 167 individuals in 25 multiplex OAG families of European descent. We carried out nonparametric linkage analysis on the complete set of 25 families and obtained a maximum LOD score of 1.00 at 9.0 cM. Using mean age at diagnosis (AAD) across the affected individuals within each family to order the families as a proxy for age at onset, we found a maximum OSA LOD score of 2.09 (p=0.021) at 26.1 cM. The mean (+/-s.d.) AAD across the 14 earlier AAD families that contributed to the OSA LOD score was 50.6 years (+/-5.38); the mean AAD for the other 1210 later AAD families that did not contribute to the OSA LOD score (the high-AAD) was 61.7 years (+/-3.50). We also ran IOSA on our families using AAD as our covariate on which to subset affected individuals. The maximum LOD score was 1.01 at 14.3 cM when ordering subjects from early to late AAD. Ordered subset analysis of this sample has provided evidence of linkage close to the previously identified GLC1I glaucoma locus on 15q11-13 in families with middle-aged mean age at diagnosis.

Radiation hybrid mapping of cataract genes in the dog

PURPOSE

To facilitate the molecular characterization of naturally occurring cataracts in dogs by providing the radiation hybrid location of 21 cataract-associated genes along with their closely associated polymorphic markers. These can be used for segregation testing of the candidate genes in canine cataract pedigrees.

METHODS

Twenty-one genes with known mutations causing hereditary cataracts in man and/or mouse were selected and mapped to canine chromosomes using a canine:hamster radiation hybrid RH5000 panel. Each cataract gene ortholog was mapped in relation to over 3,000 markers including microsatellites, ESTs, genes, and BAC clones. The resulting independently determined RH-map locations were compared with the corresponding gene locations from the draft sequence of the canine genome.

RESULTS

Twenty-one cataract orthologs were mapped to canine chromosomes. The genetic locations and nearest polymorphic markers were determined for 20 of these orthologs. In addition, the resulting cataract gene locations, as determined experimentally by this study, were compared with those determined by the canine genome project. All genes mapped within or near chromosomal locations with previously established homology to the corresponding human gene locations based on canine:human chromosomal synteny.

CONCLUSIONS

The location of selected cataract gene orthologs in the dog, along with their nearest polymorphic markers, serves as a resource for association and linkage testing in canine pedigrees segregating inherited cataracts. The recent development of canine genomic resources make canine models a practical and valuable resource for the study of human hereditary cataracts. Canine models can serve as large animal models intermediate between mouse and man for both gene discovery and the development of novel cataract therapies.

Presbyopia: the first stage of nuclear cataract?

Presbyopia, the inability to accommodate, affects almost everyone at middle age. Recently, it has been shown that there is a massive increase in the stiffness(1) of the lens with age and, since the shape of the lens must change during accommodation, this could provide an explanation for presbyopia. In this review, we propose that presbyopia may be the earliest observable symptom of age-related nuclear (ARN) cataract. ARN cataract is a major cause of world blindness. The genesis of ARN cataract can be traced to the onset of a barrier within the lens at middle age. This barrier restricts the ability of small molecules, such as antioxidants, to penetrate into the centre of the lens leaving the proteins in this region susceptible to oxidation and post-translational modification. Major protein oxidation and colouration are the hallmarks of ARN cataract. We postulate that the onset of the barrier, and the hardening of the nucleus, are intimately linked. Specifically, we propose that progressive age-dependent hardening of the lens nucleus may be responsible for both presbyopia and ARN cataract.

Detection of gender differences in rat lens proteins using 2-D-DIGE

The glass-like transparency of the human eye lens is achieved by the tight packing of abundant crystallin proteins. However, the precise role of the accessory non-crystallin proteins is not well understood. We have carried out 2-DE mapping of these proteins in rat lens. This showed the presence of the high molecular weight filamentous structural proteins spectrin, filensin, tubulin, vimentin, actin and phakinin as well as several forms of potential crystallin oligomers comprised of alphaA, betaB1, betaA1 and betaA4 chains. Other proteins that were present include, heat shock protein 71, WD repeat protein 1, and several enzymes including alpha-enolase, pyruvate kinase, transketolase and aldose reductase. 2-D-DIGE analysis revealed several expression differences between the lens proteomes of male and female rats. Female rat lenses contained lower levels of aldose reductase, increased proteolyic fragments of the structural proteins filensin, vimentin and phakinin and higher levels of potential alphaA, betaB1 and betaA1 crystallin oligomers. Taken together these findings suggest that there are potential differences in oxidative stress regulation between male and female rat lenses, which may have implications on susceptibility to cataract formation. Future studies aimed at elucidating pre-cataractic changes in the non-crystallin proteins described here may facilitate identification of novel markers involved in cataractogenesis.

Reduced expression of 1-cys peroxiredoxin in oxidative stress-induced cataracts

1-cys peroxiredoxin (1-cysPrx), a member of the peroxiredoxin family with a single conserved cysteine residue, reduces a broad spectrum of hydroperoxides. This study was undertaken to examine changes in 1-cysPrx expression in human cataract samples, human lens epithelial (HLE B3) cell line, and rat organ-cultured lenses in response to oxidative insult induced by H2O2 or transforming growth factor-beta1 (TGF-beta1). Expression of 1-cysPrx mRNA and protein in HLE B3 cells increased in response to 2-8 ng ml(-1) TGF-beta1 and 50-75 microm H2O2 and then decreased below the control level at high doses (10 ng ml(-1) TGF-beta1 and 100-150 microm H2O2), as determined by Northern blot and immunoblot analysis. This reduction coincided with the decrease of cell viability. Immunoreactive 1-cysPrx protein was measured in capsulorrhexis specimens obtained from patients with anterior subcapsular cataract (ASC), nuclear sclerosis (NS), cortical spokes (CS), posterior subcapsular cataract (PSC), or white mature cataract (WC) at the time of cataract surgery. Significant reduction of 1-cysPrx protein was observed in ASC, PSC, and WC samples, but there was no statistical difference in CS and NS samples relative to normal control. Also, rat lens explants were cultured with 10 ng ml(-1) TGF-beta1 for approximately 5 days or 500 microm H2O2 for approximately 2 days. Subsequently, expression of 1-cysPrx mRNA and protein in the lens capsules was evaluated. Rat lens explants treated with TGF-beta1 or H2O2 developed a cataract similar to human ASC or WC, respectively, which resulted in a markedly decreased expression of 1-cysPrx mRNA and protein. Collectively, these findings show that expression patterns of 1-cysPrx gene in the lens are changed in response to oxidative stress, a major factor in the etiology of cataract.

Gene structure, localization and role in oxidative stress of methionine sulfoxide reductase A (MSRA) in the monkey retina

MSRA (EC 1.8.4.6) is a member of the methionine sulfoxide reductase family that can reduce methionine sulfoxide (MetO) in proteins. This repair function has been shown to protect cells against oxidative damage. In this study we have assembled the complete gene structure of msrA and identified the presence of two distinct putative promoters that generate three different transcripts. These transcripts were cloned by 5'RACE and code for three MSRA isoforms with different N-termini. The different forms of MSRA target to distinct intracellular regions. The main MSRA transcript (msrA1) had been previously shown to target the mitochondria. MsrA2 and 3 originate from a second promoter and target the cytosol and nuclei. In the monkey retina msrA message was detected mainly in the macular RPE-choroid region while its activity was measured mainly in the soluble fractions of fractionated neural retina and RPE-choroid. The MSRA protein is found throughout the retina but is especially abundant at the photoreceptor synapses, ganglion and Müller cells. Interestingly, MSRA was not detected in the mitochondria of the photoreceptor inner segments. The RPE in the peripheral retina shows very low levels of expression but the RPE in the macular region is strongly labeled. Targeted silencing of msrA message rendered cultured RPE cells more sensitive to oxidative damage suggesting a role for MSRA in RPE protection against oxidative stress. Collectively these data suggest MSRA may play an important role in protecting macular RPE from oxidative damage.

Lens opacity and photoreceptor degeneration in the zebrafish lens opaque mutant

The zebrafish lens opaque (lop) mutant was identified in a chemical mutagenesis screen. The lop mutant, which develops normally through 4 days postfertilization (dpf), exhibits several signs of lens and retinal degeneration at 7 dpf. Histology revealed disrupted lens fibers and increased numbers of nucleated cells within the mutant lens and anterior chamber. The mutant lens also exhibited aberrant epithelial cell morphologies and lacked a definitive transition zone, which suggests that secondary fiber differentiation was interrupted. In addition, the mutant exhibits severely reduced photoreceptors and a reduction in the number of horizontal cells at 7 dpf. Other retinal cell classes appeared unaffected in the mutant. Transmission electron microscopy and opsin immunohistochemistry showed that the different photoreceptor types were generated at the retinal margin, but the rods and cones failed to mature and disappeared. The mutant lens and retina also displayed increased cell proliferation based on proliferating cell nuclear antigen immunolabeling, suggesting that the lens opacity was due to unregulated cell proliferation and undifferentiated cell accumulation within the mutant lens. The lop mutant phenotype supports recent studies showing the lens has a role in regulating teleost retinal development.

Age-related nuclear cataract—oxidation is the key

Age is by far the biggest risk factor for cataract, and it is sometimes assumed that cataract is simply an amplification of this aging process. This appears not to be the case, since the lens changes associated with aging and cataract are distinct. Oxidation is the hallmark of age-related nuclear (ARN) cataract. Loss of protein sulfhydryl groups, and the oxidation of methionine residues, are progressive and increase as the cataract worsens until >90% of cysteine and half the methionine residues are oxidised in the most advanced form. By contrast, there may be no significant oxidation of proteins in the centre of the lens with advancing age, even past age 80. The key factor in preventing oxidation seems to be the concentration of nuclear glutathione (GSH). Provided that nuclear GSH levels can be maintained above 2 mm, it appears that significant protein oxidation and posttranslational modification by reactive small molecules, such as ascorbate or UV filter degradation products, is not observed. Adequate coupling of the metabolically-active cortex, the source of antioxidants such as GSH, to the quiescent nucleus, is crucial especially since it would appear that the cortex remains viable in old lenses, and even possibly in ARN cataract lenses. Therefore it is vital to understand the reason for the onset of the lens barrier. This barrier, which becomes apparent in middle age, acts to impede the flow of small molecules between the cortex and the nucleus. The barrier, rather than nuclear compaction (which is not observed in human lenses), may contribute to the lowered concentration of GSH in the lens nucleus after middle age. By extending the residence time within the lens centre, the barrier also facilitates the decomposition of intrinsically unstable metabolites and may exacerbate the formation of H(2)O(2) in the nucleus. This hypothesis, which is based on the generation of reactive oxygen species and reactive molecules within the nucleus itself, shifts the focus away from theories for cataract that postulated a primary role for oxidants generated outside of the lens. Unfortunately, due to marked variability in the lenses of different species, there appears at present to be no ideal animal model system for studying human ARN cataract.