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

Novel CRYGC Mutation in Conserved Ultraviolet-Protective Tryptophan (p.Trp131Arg) Is Linked to Autosomal Dominant Congenital Cataract

Congenital cataract (CC), the most prevalent cause of childhood blindness and amblyopia, necessitates prompt and precise genetic diagnosis. The objective of this study is to identify the underlying genetic cause in a Swiss patient with isolated CC. Whole exome sequencing (WES) and copy number variation (CNV) analysis were conducted for variant identification in a patient born with a total binocular CC without a family history of CC. Sanger Sequencing was used to confirm the variant and segregation analysis was used to screen the non-affected parents. The first de novo missense mutation at c.391T>C was identified in exon 3 of CRYGC on chromosome 2 causing the substitution of a highly conserved Tryptophan to an Arginine located at p.Trp131Arg. Previous studies exhibit significant changes in the tertiary structure of the crystallin family in the following variant locus, making CRYGC prone to aggregation aggravated by photodamage resulting in cataract. The variant can be classified as pathogenic according to the American College of Medical Genetics and Genomics (ACMG) criteria (PP3 + PM1 + PM2 + PS2; scoring 10 points). The identification of this novel variant expands the existing knowledge on the range of variants found in the CRYGC gene and contributes to a better comprehension of cataract heterogeneity.

Network Hamiltonian Models for Unstructured Protein Aggregates, with Application to γD-Crystallin

Network Hamiltonian models (NHMs) are a framework for topological coarse-graining of protein-protein interactions, in which each node corresponds to a protein, and edges are drawn between nodes representing proteins that are noncovalently bound. Here, this framework is applied to aggregates of γD-crystallin, a structural protein of the eye lens implicated in cataract disease. The NHMs in this study are generated from atomistic simulations of equilibrium distributions of wild-type and the cataract-causing variant W42R in solution, performed by Wong, E. K.; Prytkova, V.; Freites, J. A.; Butts, C. T.; Tobias, D. J. Molecular Mechanism of Aggregation of the Cataract-Related γD-Crystallin W42R Variant from Multiscale Atomistic Simulations. Biochemistry2019, 58 (35), 3691-3699. Network models are shown to successfully reproduce the aggregate size and structure observed in the atomistic simulation, and provide information about the transient protein-protein interactions therein. The system size is scaled from the original 375 monomers to a system of 10000 monomers, revealing a lowering of the upper tail of the aggregate size distribution of the W42R variant. Extrapolation to higher and lower concentrations is also performed. These results provide an example of the utility of NHMs for coarse-grained simulation of protein systems, as well as their ability to scale to large system sizes and high concentrations, reducing computational costs while retaining topological information about the system.

Generation of Lens Progenitor Cells and Lentoid Bodies from Pluripotent Stem Cells: Novel Tools for Human Lens Development and Ocular Disease Etiology

In vitro differentiation of human pluripotent stem cells (hPSCs) into specialized tissues and organs represents a powerful approach to gain insight into those cellular and molecular mechanisms regulating human development. Although normal embryonic eye development is a complex process, generation of ocular organoids and specific ocular tissues from pluripotent stem cells has provided invaluable insights into the formation of lineage-committed progenitor cell populations, signal transduction pathways, and self-organization principles. This review provides a comprehensive summary of recent advances in generation of adenohypophyseal, olfactory, and lens placodes, lens progenitor cells and three-dimensional (3D) primitive lenses, "lentoid bodies", and "micro-lenses". These cells are produced alone or "community-grown" with other ocular tissues. Lentoid bodies/micro-lenses generated from human patients carrying mutations in crystallin genes demonstrate proof-of-principle that these cells are suitable for mechanistic studies of cataractogenesis. Taken together, current and emerging advanced in vitro differentiation methods pave the road to understand molecular mechanisms of cataract formation caused by the entire spectrum of mutations in DNA-binding regulatory genes, such as PAX6, SOX2, FOXE3, MAF, PITX3, and HSF4, individual crystallins, and other genes such as BFSP1, BFSP2, EPHA2, GJA3, GJA8, LIM2, MIP, and TDRD7 represented in human cataract patients.

A native chemical chaperone in the human eye lens

Cataract is one of the most prevalent protein aggregation disorders and still the most common cause of vision loss worldwide. The metabolically quiescent core region of the human lens lacks cellular or protein turnover; it has therefore evolved remarkable mechanisms to resist light-scattering protein aggregation for a lifetime. We now report that one such mechanism involves an unusually abundant lens metabolite, myo-inositol, suppressing aggregation of lens crystallins. We quantified aggregation suppression using our previously well-characterized in vitro aggregation assays of oxidation-mimicking human γD-crystallin variants and investigated myo-inositol’s molecular mechanism of action using solution NMR, negative-stain TEM, differential scanning fluorometry, thermal scanning Raman spectroscopy, turbidimetry in redox buffers, and free thiol quantitation. Unlike many known chemical chaperones, myo-inositol’s primary target was not the native, unfolded, or final aggregated states of the protein; rather, we propose that it was the rate-limiting bimolecular step on the aggregation pathway. Given recent metabolomic evidence that it is severely depleted in human cataractous lenses compared to age-matched controls, we suggest that maintaining or restoring healthy levels of myo-inositol in the lens may be a simple, safe, and globally accessible strategy to prevent or delay lens opacification due to age-onset cataract.

The mechanism for thermal-enhanced chaperone-like activity of α-crystallin against UV irradiation-induced aggregation of γD-crystallin

Exposure to solar UV irradiation damages γ-crystallin, leading to cataract formation via aggregation. α-Crystallin, as a small heat-shock protein (sHsps), efficiently suppresses this irreversible aggregation by selectively binding the denatured γ-crystallin monomer. In this study, liquid chromatography tandem mass spectrometry (LC-MS) was used to evaluate UV-325 nm irradiation-induced photodamage of human γD-crystallin in the presence of bovine α-crystallin, atomic force microscope (AFM) and dynamic light scattering (DLS) techniques were used to detect the quaternary structure changes of α-crystallin oligomer, and Fourier transform infrared (FTIR) spectroscopy and temperature-jump (T-jump) nanosecond time-resolved IR absorbance difference spectroscopy were used to probe the secondary structure changes of bovine α-crystallin. We find that the thermal-induced subunit dissociation of α-crystallin oligomer involves the breaking of hydrogen bonds at the dimeric interface, leading to three different spectral components at varied temperature regions as resolved from temperature-dependent IR spectra. Under UV-325 nm irradiation, unfolded γD-crystallin binds to the dissociated α-crystallin subunit to form αγ-complex, then follows the reassociation of αγ-complex to the partially dissociated α-crystallin oligomer. This prevents the aggregation of denatured γD-crystallin. The formation of the γD-bound α-crystallin oligomer is further confirmed by AFM and DLS analysis, which reveals an obvious size expansion in the reassociated αγ-oligomers. In addition, UV-325 nm irradiation causes a peptide bond cleavage of γD-crystallin at Ala158 in presence of α-crystallin. Our results suggest a very effective protection mechanism for subunits dissociated from α-crystallin oligomers against UV irradiation-induced aggregation of γD-crystallin, at an expense of a loss of a short C-terminal peptide in γD-crystallin.

Replica exchange molecular dynamics simulations reveal self-association sites in M-crystallin caused by mutations provide insights of cataract

Crystallins are ubiquitous, however, prevalence is seen in eye lens. Eye lens crystallins are long-lived and structural intactness is required for maintaining lens transparency and protein solubility. Mutations in crystallins often lead to cataract. In this study, we performed mutations at specific sites of M-crystallin, a close homologue of eye lens crystallin and studied by using replica exchange molecular dynamics simulation with generalized Born implicit solvent model. Mutations were made on the Ca²⁺ binding residues (K34D and S77D) and in the hydrophobic core (W45R) which is known to cause congenital cataract in homologous γD-crystallin. The chosen mutations caused large motion of the N-terminal Greek key, concomitantly broke the interlocking Greek keys interactions and perturbed the compact core resulting in several folded and partially unfolded states. Partially unfolded states exposed large hydrophobic patches that could act as precursors for self-aggregation. Accumulation of such aggregates is the potential cause of cataract in homologous eye lens crystallins.

A novel F30S mutation in γS-crystallin causes autosomal dominant congenital nuclear cataract by increasing susceptibility to stresses

Despite of increasingly accumulated genetic variations of autosomal dominant congenital cataracts (ADCC), the causative genes of many ADCC patients remains unknown. In this research, we identified a novel F30S mutation in γS-crystallin from a three-generation Chinese family with ADCC. The patients possessing the F30S mutation exhibited nuclear cataract phenotype. The potential molecular mechanism underlying ADCC by the F30S mutation was investigated by comparing the structural features, stability and aggregatory potency of the mutated protein with the wild type protein. Spectroscopic experiments indicated that the F30S mutation did not affect γS-crystallin secondary structure compositions, but modified the microenvironments around aromatic side-chains. Thermal and chemical denaturation studies indicated that the mutation destabilized the protein and increased its aggregatory potency. The mutation altered the two-state unfolding of γS-crystallin to a three-state unfolding with the accumulation of an unfolding intermediate. The almost identical values in the changes of Gibbs free energies for transitions from the native state to intermediate and from the intermediate to unfolded state suggested that the mutation probably disrupted the cooperativity between the two domains during unfolding. Our results expand the genetic variation map of ADCC and provide novel insights into the molecular mechanism underlying ADCC caused by mutations in β/γ-crystallins.

Cataract-Associated New Mutants S175G/H181Q of βΒ2-Crystallin and P24S/S31G of γD-Crystallin Are Involved in Protein Aggregation by Structural Changes

β/γ-Crystallins, the main structural protein in human lenses, have highly stable structure for keeping the lens transparent. Their mutations have been linked to cataracts. In this study, we identified 10 new mutations of β/γ-crystallins in lens proteomic dataset of cataract patients using bioinformatics tools. Of these, two double mutants, S175G/H181Q of βΒ2-crystallin and P24S/S31G of γD-crystallin, were found mutations occurred in the largest loop linking the distant β-sheets in the Greek key motif. We selected these double mutants for identifying the properties of these mutations, employing biochemical assay, the identification of protein modifications with nanoUPLC-ESI-TOF tandem MS and examining their structural dynamics with hydrogen/deuterium exchange-mass spectrometry (HDX-MS). We found that both double mutations decrease protein stability and induce the aggregation of β/γ-crystallin, possibly causing cataracts. This finding suggests that both the double mutants can serve as biomarkers of cataracts.

Effects of Mutating Trp42 Residue on γD-Crystallin Stability

Oligomerization and aggregation of γD-crystallins (HγDC) in the eye lens is one of the main causes of cataract development. To date, several congenital mutations related to this protein are known to promote the formation of aggregates. Previous studies have demonstrated that mutations in W42 residue of HγDC lead to the generation of partially unfolded intermediates that are more prone to aggregate. To understand the role of W42 in the stability of HγDC, we performed alchemical free-energy calculations and all-atom molecular dynamics simulations of different W42 mutant models. Our results suggest that substitution of W42 by small size and/or polar residues promotes HγDC denaturation due to the entry of water molecules into the hydrophobic core of the N-terminal domain. Similar behavior was observed in the C-terminal domain of HγDC when mutating the W130 residue located in a homologous position. Moreover, the exposure of the hydrophobic core residues could lead to the formation of aggregation-prone partially unfolded species. Overall, this study takes a step toward understanding the role of HγDC in cataract development.

Molecular genetics of congenital cataracts

Congenital cataracts, the most common cause of visual impairment and blindness in children worldwide, have diverse etiologies. According to statistics analysis, about one quarter of congenital cataracts caused by genetic defects. Various mutations of more than one hundred genes have been identified in hereditary cataracts so far. In this review, we briefly summarize recent developments about the genetics, molecular mechanisms, and treatments of congenital cataracts. The studies of these pathogenic mutations and molecular genetics is making it possible for us to comprehend the underlying mechanisms of cataractogenesis and providing new insights into the preventive, diagnostic and therapeutic approaches of cataracts.

Molecular Mechanism of Aggregation of the Cataract-Related γD-Crystallin W42R Variant from Multiscale Atomistic Simulations

The mechanisms leading to aggregation of the crystallin proteins of the eye lens remain largely unknown. We use atomistic multiscale molecular simulations to model the solution-state conformational dynamics of γD-crystallin and its cataract-related W42R variant at both infinite dilution and physiologically relevant concentrations. We find that the W42R variant assumes a distinct conformation in solution that leaves the Greek key domains of the native fold largely unaltered but lacks the hydrophobic interdomain interface that is key to the stability of wild-type γD-crystallin. At physiologically relevant concentrations, exposed hydrophobic regions in this alternative conformation become primary sites for enhanced interprotein interactions leading to large-scale aggregation.

Thermodynamic Stability of Human γD-Crystallin Mutants Using Alchemical Free-Energy Calculations

γD-Crystallin (HγDC) is a key structural protein in the human lens, whose aggregation has been associated with the development of cataracts. Single-point mutations and post-translational modifications destabilize HγDC interactions, forming partially folded intermediates, where hydrophobic residues are exposed and thus triggering its aggregation. In this work, we used alchemical free-energy calculations to predict changes in thermodynamic stability (ΔΔG) of 10 alanine-scanning variants and 12 HγDC mutations associated with the development of congenital cataract. Our results show that W42R is the most destabilizing mutation in HγDC. This has been corroborated through experimental determination of ΔΔG employing differential scanning calorimetry. Calculations of hydration free energies from the HγDC WT and the W42R mutant suggested that the mutant has a higher aggregation propensity. Our combined theoretical and experimental results contribute to understand HγDC destabilization and aggregation mechanisms in age-onset cataracts.

Dissimilarity in the Contributions of the N-Terminal Domain Hydrophobic Core to the Structural Stability of Lens β/γ-Crystallins

Vertebrate lens β/γ-crystallins share a conserved tertiary structure consisting of four Greek-key motifs divided into two globular domains. Numerous inherited mutations in β/γ-crystallins have been linked to cataractogenesis. In this research, the folding mechanism underlying cataracts caused by the I21N mutation in βB2 was investigated by comparing the effect of mutagenesis on the structural features and stability of four β/γ-crystallins, βB1, βB2, γC, and γD. Our results showed that the four β/γ-crystallins differ greatly in solubility and stability against various stresses. The I21N mutation greatly impaired βB2 solubility and native structure as well as its stability against denaturation induced by guanidine hydrochloride, heat treatment, and ultraviolet irradiation. However, the deleterious effects were much weaker for mutations at the corresponding sites in βB1, γC, and γD. Molecular dynamics simulations indicated that the introduction of a nonnative hydrogen bond contributed to twisting Greek-key motif I outward, which might direct the misfolding of the I21N mutant of βB2. Meanwhile, partial hydration of the hydrophobic interior of the domain induced by the mutation destabilized βB1, γC, and γD. Our findings highlight the importance of nonnative hydrogen bond formation and hydrophobic core hydration in crystallin misfolding caused by inherited mutations.

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.

Dynamic disulfide exchange in a crystallin protein in the human eye lens promotes cataract-associated aggregation

Increased light scattering in the eye lens due to aggregation of the long-lived lens proteins, crystallins, is the cause of cataract disease. Several mutations in the gene encoding human γD-crystallin (HγD) cause misfolding and aggregation. Cataract-associated substitutions at Trp⁴² cause the protein to aggregate in vitro from a partially unfolded intermediate locked by an internal disulfide bridge, and proteomic evidence suggests a similar aggregation precursor is involved in age-onset cataract. Surprisingly, WT HγD can promote aggregation of the W42Q variant while itself remaining soluble. Here, a search for a biochemical mechanism for this interaction has revealed a previously unknown oxidoreductase activity in HγD. Using in vitro oxidation, mutational analysis, cysteine labeling, and MS, we have assigned this activity to a redox-active internal disulfide bond that is dynamically exchanged among HγD molecules. The W42Q variant acts as a disulfide sink, reducing oxidized WT and forming a distinct internal disulfide that kinetically traps the aggregation-prone intermediate. Our findings suggest a redox "hot potato" competition among WT and mutant or modified polypeptides wherein variants with the lowest kinetic stability are trapped in aggregation-prone intermediate states upon accepting disulfides from more stable variants. Such reactions may occur in other long-lived proteins that function in oxidizing environments. In these cases, aggregation may be forestalled by inhibiting disulfide flow toward mutant or damaged polypeptides.

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

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

Congenital microcornea-cataract syndrome-causing mutation X253R increases βB1-crystallin hydrophobicity to promote aggregate formation

The high solubility and lifelong stability of crystallins are crucial to the maintenance of lens transparency and optical properties. Numerous crystallin mutations have been linked to congenital cataract, which is one of the leading causes of newborn blindness. Besides cataract, several crystallin mutations have also been linked to syndromes such as congenital microcornea-cataract syndrome (CMCC). However, the molecular mechanism of CMCC caused by crystallin mutations remains elusive. In the present study, we investigated the mechanism of CMCC caused by the X253R mutation in βB1-crystallin. The exogenously expressed X253R proteins were prone to form p62-negative aggregates in HeLa cells, strongly inhibited cell proliferation and induced cell apoptosis. The intracellular X253R aggregates could be successfully redissolved by lanosterol but not cholesterol. The extra 26 residues at the C-terminus of βB1-crystallin introduced by the X253R mutation had little impact on βB1-crystallin structure and stability, but increased βB1-crystallin hydrophobicity and decreased its solubility. Interestingly, the X253R mutant fully abolished the aggregatory propensity of βB1- and βA3/βB1-crystallins at high temperatures, suggesting that X253R was an aggregation-inhibition mutation of β-crystallin homomers and heteromers in dilute solutions. Our results suggest that an increase in hydrophobicity and a decrease in solubility might be responsible for cataractogenesis induced by the X253R mutation, while the cytotoxic effect of X253R aggregates might contribute to the defects in ocular development. Our results also highlight that, at least in some cases, the aggregatory propensity in dilute solutions could not fully mimic the behaviours of mutated proteins in the crowded cytoplasm of the cells.

Copper and Zinc Ions Specifically Promote Nonamyloid Aggregation of the Highly Stable Human γ-D Crystallin

Cataract is the leading cause of blindness in the world. It results from aggregation of eye lens proteins into high-molecular-weight complexes, causing light scattering and lens opacity. Copper and zinc concentrations in cataractous lens are increased significantly relative to a healthy lens, and a variety of experimental and epidemiological studies implicate metals as potential etiological agents for cataract. The natively monomeric, β-sheet rich human γD (HγD) crystallin is one of the more abundant proteins in the core of the lens. It is also one of the most thermodynamically stable proteins in the human body. Surprisingly, we found that both Cu(II) and Zn(II) ions induced rapid, nonamyloid aggregation of HγD, forming high-molecular-weight light-scattering aggregates. Unlike Zn(II), Cu(II) also substantially decreased the thermal stability of HγD and promoted the formation of disulfide-bridged dimers, suggesting distinct aggregation mechanisms. In both cases, however, metal-induced aggregation depended strongly on temperature and was suppressed by the human lens chaperone αB-crystallin (HαB), implicating partially folded intermediates in the aggregation process. Consistently, distinct site-specific interactions of Cu(II) and Zn(II) ions with the protein and conformational changes in specific hinge regions were identified by nuclear magnetic resonance. This study provides insights into the mechanisms of metal-induced aggregation of one of the more stable proteins in the human body, and it reveals a novel and unexplored bioinorganic facet of cataract disease.

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

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

Lanosterol reverses protein aggregation in cataracts

The human lens is comprised largely of crystallin proteins assembled into a highly ordered, interactive macro-structure essential for lens transparency and refractive index. Any disruption of intra- or inter-protein interactions will alter this delicate structure, exposing hydrophobic surfaces, with consequent protein aggregation and cataract formation. Cataracts are the most common cause of blindness worldwide, affecting tens of millions of people, and currently the only treatment is surgical removal of cataractous lenses. The precise mechanisms by which lens proteins both prevent aggregation and maintain lens transparency are largely unknown. Lanosterol is an amphipathic molecule enriched in the lens. It is synthesized by lanosterol synthase (LSS) in a key cyclization reaction of a cholesterol synthesis pathway. Here we identify two distinct homozygous LSS missense mutations (W581R and G588S) in two families with extensive congenital cataracts. Both of these mutations affect highly conserved amino acid residues and impair key catalytic functions of LSS. Engineered expression of wild-type, but not mutant, LSS prevents intracellular protein aggregation of various cataract-causing mutant crystallins. Treatment by lanosterol, but not cholesterol, significantly decreased preformed protein aggregates both in vitro and in cell-transfection experiments. We further show that lanosterol treatment could reduce cataract severity and increase transparency in dissected rabbit cataractous lenses in vitro and cataract severity in vivo in dogs. Our study identifies lanosterol as a key molecule in the prevention of lens protein aggregation and points to a novel strategy for cataract prevention and treatment.

A Novel Insertion Variant of CRYGD Is Associated with Congenital Nuclear Cataract in a Chinese Family

Objective

To investigate a novel insertion variant of CRYGD identified in a Chinese family with nuclear congenital cataract.

Methods

A Chinese family with congenital nuclear cataract was recruited for the mutational screening of candidate genes by direct sequencing. Recombinant N-terminal Myc tagged wildtype or mutant CRYGD was expressed in HEK293T cells. The expression pattern, protein solubility and subcellular distribution were analyzed by western blotting and immunofluorescence.

Principal Findings

A novel insertion variant, c.451_452insGACT, in CRYGD was identified in the patients. It causes a frameshift and a premature termination of the polypeptide to become Y151*. A significantly reduced solubility was observed for this mutant. Unlike wildtype CRYGD, which existed mainly in the cytoplasm, Y151* was mis-located in the nucleus.

Conclusions

We have identified a novel mutation, c.451_452insGACT, in CRYGD, which is associated with nuclear cataract. This is the first insertion mutation of CRYGD found to cause autosomal dominant congenital cataract. The mutant protein, with loss of solubility and localization to the nucleus, is hypothesized to be the major cause of cataract in these patients.

Gamma crystallins of the human eye lens

BACKGROUND

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

SCOPE OF REVIEW

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

MAJOR CONCLUSIONS

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

GENERAL SIGNIFICANCE

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

Wild-type human γD-crystallin promotes aggregation of its oxidation-mimicking, misfolding-prone W42Q mutant

Non-native protein conformers generated by mutation or chemical damage template aggregation of wild-type, undamaged polypeptides in diseases ranging from amyotrophic lateral sclerosis to cancer. We tested for such interactions in the natively monomeric human eye lens protein γd-crystallin, whose aggregation leads to cataract disease. The oxidation-mimicking W42Q mutant of γd-crystallin formed non-native polymers starting from a native-like state under physiological conditions. Aggregation occurred in the temperature range 35-45 °C, in which the mutant protein began to lose the native conformation of its N-terminal domain. Surprisingly, wild-type γd-crystallin promoted W42Q polymerization in a catalytic manner, even at mutant concentrations too low for homogeneous nucleation to occur. The presence of wild-type protein also downshifted the temperature range of W42Q aggregation. W42Q aggregation required formation of a non-native intramolecular disulfide bond but not intermolecular cross-linking. Transient WT/W42Q binding may catalyze this oxidative misfolding event in the mutant. That a more stable variant in a mixture can specifically promote aggregation of a less stable one rationalizes how extensive aggregation of rare damaged polypeptides can occur during the course of aging.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Molecular genetics of congenital nuclear cataract

A cataract is defined as opacification of the normally transparent crystalline lens. Congenital cataract (CC) is a type of cataract that presents at birth or during early childhood. CC is one of the most common causes of visual impairment or blindness in children worldwide. Approximately 50% of all CC cases may have a genetic cause which is quite heterogeneous. CC occurs in a variety of morphologic configurations, including polar/subcapsular, nuclear, lamellar, sutural, cortical, membranous/capsular and complete. Nuclear cataract refers to the opacification limited to the embryonic and/or fetal nuclei of the lens. Although congenital nuclear cataract can be caused by multiple factors, genetic mutation remains to be the most common cause. It can be inherited in one of the three patterns: autosomal dominant, autosomal recessive, or X-linked transmission. Autosomal dominant inheritance is the most frequent mode with high penetrance. There may be no obvious correlation between the genotype and phenotype of congenital nuclear cataract. Animal models have been established to study the pathogenesis of congenital nuclear cataract and to identify candidate genes. In this review, we highlight identified genetic mutations that account for congenital nuclear cataract. Our review may be helpful for genetic counseling and prenatal diagnosis.

The congenital cataract-linked A2V mutation impairs tetramer formation and promotes aggregation of βB2-crystallin

β/γ-Crystallins, the major structural proteins in human lens, are highly conserved in their tertiary structures but distinct in the quaternary structures. The N- and C-terminal extensions have been proposed to play a crucial role in mediating the size of β-crystallin assembly. In this research, we investigated the molecular mechanism underlying the congenital hereditary cataract caused by the recently characterized A2V mutation in βB2-crystallin. Spectroscopic experiments indicated that the mutation did not affect the secondary and tertiary structures of βB2-crystallin. The mutation did not affect the formation of βB2/βA3-crystallin heteromer as well as the stability and folding of the heteromer, suggesting that the mutation might not interfere with the protein interacting network in the lens. However, the tetramerization of βB2-crystallin at high protein concentrations was retarded by the A2V mutation. The mutation slightly decreased the thermal stability and promoted the thermal aggregation of βB2-crystallin. Although it did not influence the stability of βB2-crystallin against denaturation induced by chemical denaturants and UV irradiation, the A2V mutant was more prone to be trapped in the off-pathway aggregation process during kinetic refolding. Our results suggested that the A2V mutation might lead to injury of lens optical properties by decreasing βB2-crystallin stability against heat treatment and by impairing βB2-crystallin assembly into high-order homo-oligomers.

The human W42R γD-crystallin mutant structure provides a link between congenital and age-related cataracts

Some mutants of human γD-crystallin are closely linked to congenital cataracts, although the detailed molecular mechanisms of mutant-associated cataract formation are generally not known. Here we report on a recently discovered γD-crystallin mutant (W42R) that has been linked to autosomal dominant, congenital cataracts in a Chinese family. The mutant protein is much less soluble and stable than wild-type γD-crystallin. We solved the crystal structure of W42R at 1.7 Å resolution, which revealed only minor differences from the wild-type structure. Interestingly, the W42R variant is highly susceptible to protease digestion, suggesting the presence of a small population of partially unfolded protein. This partially unfolded species was confirmed and quantified by NMR spectroscopy. Hydrogen/deuterium exchange experiments revealed chemical exchange between the folded and unfolded species. Exposure of wild-type γD-crystallin to UV caused damage to the N-terminal domain of the protein, resulting in very similar proteolytic susceptibility as observed for the W42R mutant. Altogether, our combined data allowed us to propose a model for W42R pathogenesis, with the W42R mutant serving as a mimic for photodamaged γD-crystallin involved in age-related cataract.

The congenital cataract-linked G61C mutation destabilizes γD-crystallin and promotes non-native aggregation

γD-crystallin is one of the major structural proteins in human eye lens. The solubility and stability of γD-crystallin play a crucial role in maintaining the optical properties of the lens during the life span of an individual. Previous study has shown that the inherited mutation G61C results in autosomal dominant congenital cataract. In this research, we studied the effects of the G61C mutation on γD-crystallin structure, stability and aggregation via biophysical methods. CD, intrinsic and extrinsic fluorescence spectroscopy indicated that the G61C mutation did not affect the native structure of γD-crystallin. The stability of γD-crystallin against heat- or GdnHCl-induced denaturation was significantly decreased by the mutation, while no influence was observed on the acid-induced unfolding. The mutation mainly affected the transition from the native state to the intermediate but not that from the intermediate to the unfolded or aggregated states. At high temperatures, both proteins were able to form aggregates, and the aggregation of the mutant was much more serious than the wild type protein at the same temperature. At body temperature and acidic conditions, the mutant was more prone to form amyloid-like fibrils. The aggregation-prone property of the mutant was not altered by the addition of reductive reagent. These results suggested that the decrease in protein stability followed by aggregation-prone property might be the major cause in the hereditary cataract induced by the G61C mutation.