Autosomal-dominant congenital cataract associated with a deletion mutation in the human beaded filament protein gene BFSP2

Congenital cataracts are a common major abnormality of the eye that frequently cause blindness in infants. At least one-third of all cases are familial; autosomal-dominant congenital cataract appears to be the most-common familial form in the Western world. Elsewhere, in family ADCC-3, we mapped an autosomal-dominant cataract gene to chromosome 3q21-q22, near the gene that encodes a lens-specific beaded filament protein gene, BFSP2. By sequencing the coding regions of BFSP2, we found that a deletion mutation, DeltaE233, is associated with cataracts in this family. This is the first report of an inherited cataract that is caused by a mutation in a cytoskeletal protein.     

Autosomal recessive juvenile onset cataract associated with mutation in BFSP1

A genome wide scan in a consanguineous family of Indian origin with autosomal recessive developmental cataracts was performed by two-point linkage analysis with 382 microsatellite markers. It showed linkage to markers on chromosome 20q, between D20S852 and D20S912, with a maximum lod score of 5.4 obtained with D20S860. This region encompasses the beaded filament structural protein 1 (BFSP1) gene. Direct sequencing revealed a 3343 bp deletion including exon 6 (c.736-1384_c.957-66 del) predicted to result in a shift of the open reading frame. This mutation was absent in 50 control individuals from south India. This is the first report of a mutation in the BFSP1 gene associated with human inherited cataracts. This further increases the genetic heterogeneity of inherited cataracts and provides clues as to the importance of BFSP1 in the cell biology of intermediate filaments and their role in the eye lens.     

Insights into the beaded filament of the eye lens

Filensin (BFSP1) and CP49 (BFSP2) represent two members of the IF protein superfamily that are thus far exclusively expressed in the eye lens. Mutations in both proteins cause lens cataract and careful consideration of the detail of these cataract phenotypes alerts us to several interesting features concerning the function of filensin (BFSP1) and CP49 (BFSP2) in the lens. With the first filensin (BFSP1) mutation now having been reported to cause a recessive cataract phenotype, there is the suggestion that the mutation could predispose heterozygote carriers to the early onset of age-related nuclear cataract. In the case of CP49 (BFSP2), there are now three unrelated families who have been identified with a common E233 Delta mutation. Very interestingly this is linked to myopia in one family. Despite the apparent phenotypic differences of the filensin (BFSP1) and CP49 (BFSP2) mutations, the data are still consistent with the beaded filament proteins being essential for lens function and specifically contributing to the optical properties of the lens. The fact that none of the mutations thus far reported affect either the conserved LNDR or TYRKLLEGE motifs that flank the central rod domain supports the view that this pair of IF proteins have unusual structural features and a distinctive assembly mechanism. The multiple sequence divergences suggest these proteins have been adapted to the specific functional requirements of lens fibre cells, a function that can be traced from squid to man.     

A deletion in a cis element of Foxe3 causes cataracts and microphthalmia in rct mice

The Rinshoken cataract (rct) mutation, which causes congenital cataracts, is a recessive mutation found in SJL/J mice. All mutants present with opacity in the lens by 2?months of age. The rct locus was mapped to a 1.6-Mb region in Chr 4 that contains the Foxe3 gene. This gene is responsible for cataracts in humans and mice, and it plays a crucial role in the development of the lens. Furthermore, mutation of Foxe3 causes various ocular defects. We sequenced the genomic region of Foxe3, including the coding exons and UTRs; however, no mutations were discovered in these regions. Because there were no differences in Foxe3 sequences between the rct/rct and wild-type mice, we inferred that a mutation was located in the regulatory regions of the Foxe3 gene. To test this possibility, we sequenced a 5' noncoding region that is highly conserved among vertebrates and is predicted to be the major enhancer of Foxe3. This analysis revealed a deletion of 22-bp located approximately 3.2-kb upstream of the start codon of Foxe3 in rct mice. Moreover, we demonstrated by RT-PCR and in situ hybridization that the rct mutant has reduced expression of Foxe3 in the lens during development. We therefore suggest that cataracts in rct mice are caused by reduced Foxe3 expression in the lens and that this decreased expression is a result of a deletion in a cis-acting regulatory element.     

A role for lengsin, a recruited enzyme, in terminal differentiation in the vertebrate lens

Lengsin is an eye lens-specific member of the glutamine synthetase (GS) superfamily. Lengsin has no GS activity, suggesting that it has a structural rather than catalytic role in lens. In situ hybridization and immunofluorescence showed that lengsin is expressed in terminally differentiating secondary lens fiber cells. Yeast two-hybrid (Y2H) and recombinant protein experiments showed that full-length lengsin can bind the 2B filament region of vimentin. In affinity chromatography, lengsin also bound the equivalent region of CP49 (BFSP2; phakinin), a related intermediate filament protein specific to the lens. Both the vimentin and CP49 2B fragments bound lengsin in surface plasmon resonance spectroscopy with fast association and slow dissociation kinetics. Lengsin expression correlates with a transition zone in maturing lens fiber cells in which cytoskeleton is reorganized. Lengsin and lens intermediate filament proteins co-localize at the plasma membrane in maturing fiber cells. This suggests that lengsin may act as a component of the cytoskeleton itself or as a chaperone for the reorganization of intermediate filament proteins during terminal differentiation in the lens.     

The oxidized thiol proteome in aging and cataractous mouse and human lens revealed by ICAT labeling

Age‐related cataractogenesis is associated with disulfide‐linked high molecular weight (HMW) crystallin aggregates. We recently found that the lens crystallin disulfidome was evolutionarily conserved in human and glutathione‐depleted mouse (LEGSKO) cataracts and that it could be mimicked by oxidation in vitro (Mol. Cell Proteomics, 14, 3211‐23 (2015)). To obtain a comprehensive blueprint of the oxidized key regulatory and cytoskeletal proteins underlying cataractogenesis, we have now used the same approach to determine, in the same specimens, all the disulfide‐forming noncrystallin proteins identified by ICAT proteomics. Seventy‐four, 50, and 54 disulfide‐forming proteins were identified in the human and mouse cataracts and the in vitro oxidation model, respectively, of which 17 were common to all three groups. Enzymes with oxidized cysteine at critical sites include GAPDH (hGAPDH, Cys247), glutathione synthase (hGSS, Cys294), aldehyde dehydrogenase (hALDH1A1, Cys126 and Cys186), sorbitol dehydrogenase (hSORD, Cys140, Cys165, and Cys179), and PARK7 (hPARK7, Cys46 and Cys53). Extensive oxidation was also present in lens‐specific intermediate filament proteins, such as BFSP1 and BFSP12 (hBFSP1 and hBFSP12, Cys167, Cys65, and Cys326), vimentin (mVim, Cys328), and cytokeratins, as well as microfilament and microtubule filament proteins, such as tubulin and actins. While the biological impact of these modifications for lens physiology remains to be determined, many of these oxidation sites have already been associated with either impaired metabolism or cytoskeletal architecture, strongly suggesting that they have a pathogenic role in cataractogenesis. By extrapolation, these findings may be of broader significance for age‐ and disease‐related dysfunctions associated with oxidant stress.     

Relationship between proteins encoded by three human gamma-crystallin genes and distinct polypeptides in the eye lens.

Although individual gamma-crystallins from the human eye lens have not been successfully purified and sequenced, most of the genes coding for these lens-specific structural proteins have been cloned and characterized. To investigate the relationship between these genes and the gamma-crystallins of the human lens, we made use of mouse cell lines which contain stably integrated copies of the coding sequences for three of the human gamma-crystallin genes coupled to the human metallothionein IIA promoter. The proteins produced by these hybrid genes in cell culture were detected immunologically and compared by physical characteristics with the gamma-crystallins from the human lens. The protein encoded by the G3 gene showed properties identical to those of the 21,000-molecular-weight gamma-crystallin from 11-month-old lens. The protein isolated from the cells expressing the G4 gene was similar to a 19,000-molecular-weight lens gamma-crystallin, while gene G5 encodes a highly basic gamma-crystallin which may be synthesized in only limited amounts in the human lens. These correlations provide a basis for future investigations on the relationship between putative mutations in human gamma-crystallin genes and altered proteins in hereditary lens cataracts.     

An Alu repeat-mediated genomic GCNT2 deletion underlies congenital cataracts and adult i blood group

We performed homozygosity mapping in a consanguineous Pakistani family segregating autosomal-recessive congenital cataracts and identified linkage to a 3.03 Mb locus on chromosome 6p24 containing the GCNT2 gene. GCNT2 encodes glucosaminyl (N-acetyl) transferase 2, an enzyme responsible for the formation of the blood group I antigen. Rare biallelic GCNT2 mutations have been shown to cause the association of congenital cataracts and the adult i blood group, making GCNT2 the prime candidate gene for the observed phenotype. Indeed, we identified a homozygous deletion segregating with cataracts that encompasses exons 1B, 1C, 2 and 3 of GCNT2. Long-range polymerase chain reaction and breakpoint sequencing revealed that affected individuals in this and in a second, apparently unrelated Pakistani family segregating congenital cataracts are homozygous for the same 93 kb deletion. The deletion is flanked by Alu repeats of the AluS family on both sides and microsatellite genotyping suggested that its occurrence in the two families was the product of recurrent Alu-Alu repeat-mediated nonhomologous recombinations or an old founder effect. Subsequently, we showed that cataract-affected individuals in both families have the adult i blood group, whereas unaffected individuals have blood group I as the vast majority of the population. Because the GCNT2 locus is rich in Short INterspersed Elements (SINE repeats) and thus likely prone to genomic rearrangements, microdeletions or microduplications at this locus might cause a larger than currently anticipated fraction of apparently isolated autosomal-recessive cataracts.     

Formins filter modified actin subunits during processive elongation

Fission yeast cells reject actin subunits tagged with a fluorescent protein from the cytokinetic contractile ring, so cytokinesis fails and the cells die when the native actin gene is replaced by GFP-actin. The lack of a fluorescent actin probe has prevented a detailed study of actin filament dynamics in contractile rings, and left open questions regarding the mechanism of cytokinesis. To incorporate fluorescent actin into the contractile ring to study its dynamics, we introduced the coding sequence for a tetracysteine motif (FLNCCPGCCMEP) at 10 locations in the fission yeast actin gene and expressed the mutant proteins from the native actin locus in diploid cells with wild-type actin on the other chromosome. We labeled these tagged actins inside live cells with the FlAsH reagent. Cells incorporated some of these labeled actins into actin patches at sites of endocytosis, where Arp2/3 complex nucleates all of the actin filaments. However, the cells did not incorporate any of the FlAsH-actins into the contractile ring. Therefore, formin Cdc12p rejects actin subunits with a tag of ~2 kDa, illustrating the stringent structural requirements for this formin to promote the elongation of actin filament barbed ends as it moves processively along the end of a growing filament.     

Roles of the 15-kDa Selenoprotein (Sep15) in Redox Homeostasis and Cataract Development Revealed by the Analysis of Sep 15 Knockout Mice

The 15-kDa selenoprotein (Sep15) is a thioredoxin-like, endoplasmic reticulum-resident protein involved in the quality control of glycoprotein folding through its interaction with UDP-glucose:glycoprotein glucosyltransferase. Expression of Sep15 is regulated by dietary selenium and the unfolded protein response, but its specific function is not known. In this study, we developed and characterized Sep15 KO mice by targeted removal of exon 2 of the Sep15 gene coding for the cysteine-rich UDP-glucose:glycoprotein glucosyltransferase-binding domain. These KO mice synthesized a mutant mRNA, but the shortened protein product could be detected neither in tissues nor in Sep15 KO embryonic fibroblasts. Sep15 KO mice were viable and fertile, showed normal brain morphology, and did not activate endoplasmic reticulum stress pathways. However, parameters of oxidative stress were elevated in the livers of these mice. We found that Sep15 mRNA was enriched during lens development. Further phenotypic characterization of Sep15 KO mice revealed a prominent nuclear cataract that developed at an early age. These cataracts did not appear to be associated with severe oxidative stress or glucose dysregulation. We suggest that the cataracts resulted from an improper folding status of lens proteins caused by Sep15 deficiency.     

The γ-Crystallins and Human Cataracts: A Puzzle Made Clearer

Despite the fact that cataracts constitute the leading cause of blindness worldwide, the mechanisms of lens opacification remain unclear. We recently mapped the aculeiform cataract to the gamma-crystallin locus (CRYG) on chromosome 2q33-35, and mutational analysis of the CRYG-genes cluster identified the aculeiform-cataract mutation in exon 2 of gamma-crystallin D (CRYGD). This mutation occurred in a highly conserved amino acid and could be associated with an impaired folding of CRYGD. During our study, we observed that the previously reported Coppock-like-cataract mutation, the first human cataract mutation, in the pseudogene CRYGE represented a polymorphism seen in 23% of our control population. Further analysis of the original Coppock-like-cataract family identified a missense mutation in a highly conserved segment of exon 2 of CRYGC. These mutations were not seen in a large control population. There is no direct evidence, to date, that up-regulation of a pseudogene causes cataracts. To our knowledge, these findings are the first evidence of an involvement of CRYGC and support the role of CRYGD in human cataract formation.     

Overexpression of the vimentin gene in transgenic mice inhibits normal lens cell differentiation

To investigate the role of the intermediate filament protein vimentin in the normal differentiation and morphogenesis of the eye lens fiber cells, we generated transgenic mice bearing multiple copies of the chicken vimentin gene. In most cases, the vimentin transgene was overexpressed in the lenses of these animals, reaching up to 10 times the endogenous levels. This high expression of vimentin interfered very strongly with the normal differentiation of the lens fibers. The normal fiber cell denucleation and elongation processes were impaired and the animals developed pronounced cataracts, followed by extensive lens degeneration. The age of appearance and extent of these abnormalities in the different transgenic lines were directly related to the vimentin level. Electron microscopic analysis revealed that the accumulated transgenic protein forms normal intermediate filaments.     

Increased O-GlcNAc causes disrupted lens fiber cell differentiation and cataracts

Diminished proteolytic functionality in the lens may cause cataracts. We have reported that O-GlcNAc is an endogenous inhibitor of the proteasome. We hypothesize that in the lens there is a cause-and-effect relationship between proteasome inhibition by O-GlcNAc, and cataract formation. To demonstrate this, we established novel transgenic mouse models to over-express a dominant-negative form of O-GlcNAcase, GK-NCOAT, in the lens. Expression of GK-NCOAT suppresses removal of O-GlcNAc from proteins, resulting in increased levels of O-GlcNAc in the lenses of our transgenic mice, along with decreased proteasome function. We observed that transgenic mice developed markedly larger cataracts than controls and lens fiber cell denucleation was inhibited. Our study suggests that increased O-GlcNAc in the lens could lead to cataract formation and attenuation of lens fiber cell denucleation by inhibition of proteasome function. These findings may explain why cataract formation is a common complication of diabetes since O-GlcNAc is derived from glucose.     

Vertebrate crystallins- from proteins to genes

A review of literature on tissue-specific proteins of the vertebrate eye lens and genes coding for these proteins is presented. Particular attention is paid to the most heterogeneous family of crystallins: beta- and gamma-crystallins, their nomenclature, and the structure of their genes. It is pointed out that mutations in gene coding for ubiquitous crystallins may be related to some forms of cataracts.     

Mechanism of insolubilization by a single-point mutation in alphaA-crystallin linked with hereditary human cataracts

AlphaA-crystallin is a small heat shock protein that functions as a molecular chaperone and a lens structural protein. The R49C single-point mutation in alphaA-crystallin causes hereditary human cataracts. We have previously investigated the in vivo properties of this mutant in a gene knock-in mouse model. Remarkably, homozygous mice carrying the alphaA-R49C mutant exhibit nearly complete lens opacity concurrent with small lenses and small eyes. Here we have investigated the 90 degrees light scattering, viscosity, refractive index, and bis-ANS fluorescence of lens proteins isolated from the alphaA-R49C mouse lenses and found that the concentration of total water-soluble proteins showed a pronounced decrease in alphaA-R49C homozygous lenses. Light scattering measurements on proteins separated by gel permeation chromatography showed a small amount of high-molecular mass aggregated material in the void volume which still remains soluble in alphaA-R49C homozygous lens homogenates. An increased level of binding of beta- and gamma-crystallin to the alpha-crystallin fraction was observed in alphaA-R49C heterozygous and homozygous lenses but not in wild-type lenses. Quantitative analysis with the hydrophobic fluorescence probe bis-ANS showed a pronounced increase in fluorescence yield upon binding to alpha-crystallin from mutant as compared with the wild-type lenses. These results suggest that the decrease in the solubility of the alphaA-R49C mutant protein was due to an increase in its hydrophobicity and supra-aggregation of alphaA-crystallin that leads to cataract formation. Our study further shows that analysis of mutant proteins from the mouse model is an effective way to understand the mechanism of protein insolubilization in hereditary cataracts.     

Assignment of the mouse desmin gene to chromosome 1 band C3

The chromosomal localization of the mouse gene coding for desmin, one of the muscle-specific intermediate filament subunits, was determined by in situ hybridization using a specific 3H-labelled DNA probe. There is only one copy of the desmin gene and it is located on chromosome 1 in the band C3. This result adds an eleventh locus to a conserved gene cluster and confirms the partial homology that exists between the long arm of human chromosome 2 and chromosome 1 of the mouse.