Reduced levels of gamma-crystallin transcripts during embryonic development of murine Cat2nop mutant lenses

Modeling congenital cataract in vitro using patient-specific induced pluripotent stem cells

Congenital cataracts are the leading cause of childhood blindness. To date, surgical removal of cataracts is the only established treatment, but surgery is associated with multiple complications, which often lead to visual impairment. Therefore, mechanistic studies and drug-candidate screening have been intrigued by the aims of developing novel therapeutic strategies. However, these studies have been hampered by a lack of an appropriate human-disease model of congenital cataracts. Herein, we report the establishment of a human congenital cataract in vitro model through differentiation of patient-specific induced pluripotent stem cells (iPSCs) into regenerated lenses. The regenerated lenses derived from patient-specific iPSCs with known causative mutations of congenital cataracts (CRYBB2 [p. P24T] and CRYGD [p. Q155X]) showed obvious opacification that closely resembled that seen in patients’ cataracts in terms of opacification severity and disease course accordingly, as compared with lentoid bodies (LBs) derived from healthy individuals. Increased protein aggregation and decreased protein solubility corresponding to the patients’ cataract severity were observed in the patient-specific LBs and were attenuated by lanosterol treatment. Taken together, the in vitro model described herein, which recapitulates patient-specific clinical manifestations of congenital cataracts and protein aggregation in patient-specific LBs, provides a robust system for research on the pathological mechanisms of cataracts and screening of drug candidates for cataract treatment.

The genetic landscape of crystallins in congenital cataract

Background

The crystalline lens is mainly composed of a large family of soluble proteins called the crystallins, which are responsible for its development, growth, transparency and refractive index. Disease-causing sequence variants in the crystallins are responsible for nearly 50% of all non-syndromic inherited congenital cataracts, as well as causing cataract associated with other diseases, including myopathies. To date, more than 300 crystallin sequence variants causing cataract have been identified.

Methods

Here we aimed to identify the genetic basis of disease in five multi-generation British families and five sporadic cases with autosomal dominant congenital cataract using whole exome sequencing, with identified variants validated using Sanger sequencing. Following bioinformatics analysis, rare or novel variants with a moderate to damaging pathogenicity score, were filtered out and tested for segregation within the families.

Results

We have identified 10 different heterozygous crystallin variants. Five recurrent variants were found: family-A, with a missense variant (c.145C>T; p.R49C) in CRYAA associated with nuclear cataract; family-B, with a deletion in CRYBA1 (c.272delGAG; p.G91del) associated with nuclear cataract; and family-C, with a truncating variant in CRYGD (c.470G>A; W157*) causing a lamellar phenotype; individuals I and J had variants in CRYGC (c.13A>C; T5P) and in CRYGD (c.418C>T; R140*) causing unspecified congenital cataract and nuclear cataract, respectively. Five novel disease-causing variants were also identified: family D harboured a variant in CRYGC (c.179delG; R60Qfs*) responsible for a nuclear phenotype; family E, harboured a variant in CRYBB1 (c.656G>A; W219*) associated with lamellar cataract; individual F had a variant in CRYGD (c.392G>A; W131*) associated with nuclear cataract; and individuals G and H had variants in CRYAA (c.454delGCC; A152del) and in CRYBB1 (c.618C>A; Y206*) respectively, associated with unspecified congenital cataract. All novel variants were predicted to be pathogenic and to be moderately or highly damaging.

Conclusions

We report five novel variants and five known variants. Some are rare variants that have been reported previously in small ethnic groups but here we extend this to the wider population and record a broader phenotypic spectrum for these variants.

Cataracts in transgenic mice caused by a human papillomavirus type 18 E7 oncogene driven by KRT1-14

Human papillomavirus type 18 (HPV18) is a common cause of cervical cancer. To create a mouse model for this common neoplastic disease, we used a human keratin 14 promoter to drive the HPV18 E7 oncogene to create transgenic mice. No mice up to a year of age developed cervical cancer. However, all transgenic mice and none of the controls developed progressive bilateral cortical cataracts. By 6 months of age, the cortex liquefied leaving the lens nucleus. Proliferation of lens epithelium formed multifocal nodules and free floating lens epithelial cells within the liquefied cortex. These cells were hyperplastic not neoplastic. Other HPV transgenic stocks develop cataracts suggesting this virus may have a broad cellular tropism.

Clinicopathological study of bilateral developmental cataracts diagnosed in utero

BACKGROUND

We report a case of unusual, bilateral developmental cataracts in a fetus with a supernumerary chromosome.

METHODS

A 42-year-old woman presented during her 6th pregnancy for assessment of fetal karyotype. This showed a supernumerary chromosome derived from chromosome 21. Subsequently fetal ultrasound suggested the presence of bilateral cataracts and the pregnancy was terminated at 19 weeks and 3 days' gestation. Both eyes were submitted for histopathological and electron microscopical examination.

RESULTS

Histopathological examination revealed unusual bilateral developmental cataracts with abnormal bladder-type cells lining the posterior aspect of the lens vesicle, a poorly formed nuclear bow and a central mass of fibrillar material associated with macrophages lying within an area of liquefaction. Transmission electron microscopy revealed the presence of peg and socket joints in both central and posterior regions and degenerate crystallins in the posterior region.

CONCLUSIONS

We described an unusual case of developmental cataract diagnosed in utero by ultrasound. The morphological appearances suggest that the defect occurred during or after formation of the secondary lens fibres. Detailed descriptions of cases such as this one may contribute to our understanding of lens development and cataract formation.

Developmental genetics in ophthalmology

Much of our knowledge about the function of genes in mammalian development has been derived from the molecular analysis of spontaneous or induced mutations in the mouse. Since mutations affecting the mouse eye can be easily identified, a remarkable number of mutant lines provide animal models for congenital anomalies in man. To understand the mechanisms of lens development in detail, the isolation of the corresponding genes and the characterization of the mutations at the molecular level are important. A prerequisite for molecular analysis is the chromosomal localization of the gene. In this review, some mutants from our institute will be discussed according to the embryological time scale of the expression of the affected genes, reflecting also their genetic hierarchy. (1) In the aphakia mouse mutant, two deletions in the promoter of the homeobox transcription factor Pitx3 lead to a loss of its function and to an arrest of eye development at the lens stalk stage. Mutations in the homologous human PITX3 gene have been demonstrated to be causative of cataracts and the dysmorphology of the anterior segment of the eye. (2) Connexin50 is present in the lens vesicle. Later on, it becomes abundant in the anterior part of the fiber cells and in the lens epithelial cells. Mutations in the connexin50-encoding gene Gja8 lead to dominant cataracts. (3) alphaA-crystallin is present in the mouse lens cup, in the posterior half of the lens vesicle, and later in a high concentration in the lens fiber cells. Mutations in the alphaA-crystallin-encoding gene Cryaa lead to recessive and dominant cataracts. (4) Mutations in the gamma-crystallin -encoding genes (Cryg) are the most frequent cause of congenital, dominant nuclear, or total cataracts in the mouse. Indications from our first studies in congenital human cataracts support these data. (5) Some postnatal, progressive cataracts have been characterized by mutations in the beta-crystallin -encoding genes (Cryb). Since at least one of them is also expressed in the retina and the brain, effects on these tissues have to be considered, too.

Antagonistic action of Six3 and Prox1 at the gamma-crystallin promoter

Gamma-crystallin genes are specifically expressed in the eye lens. Their promoters constitute excellent models to analyse tissue-specific gene expression. We investigated murine CRYGE/f promoters of different length in lens epithelial cell lines. The most active fragment extends from position -219 to +37. Computer analysis predicts homeodomain and paired-domain binding sites for all rodent CRYGD/e/f core promoters. As examples, we analysed the effects of Prox1 and Six3, which are considered important transcription factors involved in lens development. Because of endogenous Prox1 expression in N/N1003A cells, a weak stimulation of CRYGE/f promoter activity was found for PROX1. In contrast, PROX1 stimulated the CRYGF promoter 10-fold in CD5A cells without endogenous PROX1. In both cell lines Six3 repressed the CRYGF promoter to 10% of its basal activity. Our cell transfection experiments indicated that CRYG expression increases as Six3 expression decreases. Prox1 and Six3 act antagonistically on regulation of the CRYGD/e/f promoters. Functional assays using randomly mutated gammaF-crystallin promoter fragments define a Six3-responsive element between -101 and -123 and a Prox1-responsive element between -151 and -174. Since Prox1 and Six3 are present at the beginning of lens development, expression of CRYGD/e/f is predicted to remain low at this time. It increases as Six3 expression decreases during ongoing lens development.

Lop12, a mutation in mouse Crygd causing lens opacity similar to human Coppock cataract

A new cataract mutation was discovered in an ongoing program to identify new mouse models of hereditary eye disease. Lens opacity 12 (Lop12) is a semidominant mutation that results in an irregular nuclear lens opacity similar to the human Coppock cataract. Lop12 is associated with a small nonrecombining segment that maps to mouse Chromosome 1 close to the eye lens obsolescence mutation (Cryge(Cat2-Elo)), a member of the gamma-crystallin gene cluster (Cryg). Using a systemic candidate gene approach to analyze the entire Cryg cluster, a G to A transition was found in exon 3 of Crygd associated with the Lop12 mutation and has been designated Crygd(Lop12). The mutation Crygd(Lop12) leads to the formation of an in-frame stop codon that produces a truncated protein of 156 amino acids. It is predicted that the defective gene product alters protein folding of the gamma-crystallin(s) and results in lens opacity.

The gamma-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.

Mouse models of congenital cataract

Mouse mutants affecting lens development are excellent models for corresponding human disorders. The mutant aphakia has been characterised by bilaterally aphakic eyes (Varnum and Stevens, J Hered 1968;59:147-50); the corresponding gene was mapped to chromosome 19 (Varnum and Stevens, Mouse News Lett 1975;53:35). Recent investigations in our laboratory refined the linkage of 0.6 cM proximal to the marker D19Mit10. Several candidate genes have been excluded (Chuk1, Fgf8, Lbp1, Npm3, Pax2, Pitx3). The Cat3 mutations are characterised by vacuolated lenses caused by alterations in the initial secondary lens fibre cell differentiation. Secondary malformations develop at the cornea and iris, but the retina remains unaffected. The mutation has been mapped to chromosome 10 close to the markers D10Mit41 and D10Mit95. Several candidate genes have been excluded (Dcn, Elk3, Ldc, Mell8, Tr2-11). The series of Cat2 mutations have been mapped close to the gamma-crystallin genes (Cryg; Löster et al., Genomics 1994;23:240-2). The Cat2nop mutation is characterised by a mutation in the third exon of Crygb leading to a truncated gamma B-crystallin and the termination of lens fibre cell differentiation. The Cat2 mutants are interesting models for human cataracts caused by mutations in the human CRYG genes at chromosome 2q32-35.

Cataract mutations and lens development

The lens plays an essential role for proper eye development. Mouse mutants affecting lens development are excellent models for corresponding human disorders. Moreover, using mutations in particular genes the process of eye and lens development can be dissected into distinct steps. Therefore, three mouse mutants will be described in detail and discussed affecting three essential stages: formation of the lens vesicle, initiation of secondary lens fiber cell formation, and terminal differentiation of the secondary fiber cells. The mutant aphakia (ak) has been characterized by bilaterally apakic eyes [Varnum and Stevens (1968) J. Hered. 59, 147-150], and the corresponding gene was mapped to chromosome 19 [Varnum and Stevens (1975) Mouse News Letters 53, 35]. Recent investigations in our laboratory refined the linkage 0.6 +/- 0.3 N cm proximal to the microsatellite marker D19Mit10. The linked gene Pax2, responsible for proper development of the posterior part of the eye and the optic nerve, was excluded as candidate gene by sequence analysis. Histological analysis of the homozygous ak mutants revealed a persisting lens stalk and subsequently the formation of lens rudiments. The lens defects led to irregular iris development and retinal folding. Congenital aphakia is known as a rare human anomaly. Besides a corneal dystrophy (CDTB), no corresponding disease is localized at the homologous region of human chromosome 10q23. The Cat3 mutations are characterized by vacuolated lenses caused by alterations in the beginning of secondary lens fiber cell differentiation at embryonic day 12.5. Secondary malformations develop at the cornea and the iris, but the retina remains unaffected. Two mutant alleles of the Cat3 locus have been mapped to mouse chromosome 10 very close to the microsatellite markers D10Mit41 and D10Mit95 (less than 0.3 cM). Since Cat3 is mapped to a position, which is homologous to human chromosome 12q21-24, the disorder cornea plana congenita can be considered as a candidate disease. The series of Cat2 mutations have been mapped close to the locus encoding the gamma-crystallin gene cluster Cryg [Löster et al. (1994) Genomics 23, 240-242]. The Cat2nop mutation is characterized by a deletion of 11 bp and an insertion of 4 bp in the 3rd exon of Crygh leading to a truncated gamma B-crystallin. The defect in the Crygh gene is causative for the stop of lens fiber cell differentiation from embryonic day 15.5 onward. Besides the lens, no further ocular tissue is affected. The Cat2 mouse mutants are interesting models for human cataracts caused by mutations in the gamma-crystallin genes at human chromosome 2q32-35. The ak, Cat3 and Cat2 mutants are discussed in the context of other mutants affecting early eye and lens development. Additionally, human congenital cataracts are discussed, which have been characterized similar to the mouse models. The overview of the three types of mutants demonstrates that genes, which affect the early eye development, e.g. at the lens vesicle stage, have consequences for the development of the whole eye. In contrast, if the mutation influences later steps of lens differentiation, the consequences are restricted to the lens only. These data indicate a decreasing effect of the lens for the regulation of eye development during embryogenesis.

Identification of mouse crystallins in 2D protein patterns by sequencing and mass spectrometry. Application to cataract mutants

The eye lens proteins of the mouse were separated into 1940 polypeptide spots by two-dimensional electrophoresis in large gels. All 16 crystallins ubiquitous in mammals were identified by protein sequencing and mass spectrometry except for (gamma)-F, which shows an almost identical sequence with (gamma)-E. Two crystallins, (beta)-A2 and (gamma)-S, were shown for the first time to occur in the mouse lens. An investigation of the murine cataract mutant Cat2(nop)((gamma)-B gene) demonstrated that a monogenic mutation might affect a broad spectrum of proteins.

Three murine cataract mutants (Cat2) are defective in different gamma-crystallin genes

A number of murine cataract mutations have been localized to chromosome 1 close to the gamma-crystallin gene cluster (Cryg) (Everett et al., 1994, Genomics 20: 429-434; Löster et al., 1994, Genomics 23: 240-242). Based on the size of the mapping or allelism tests they have not been shown to be genetically distinct and have been assigned to locus symbol Cat2. Here we assign three mutations to the respective gamma-crystallin gene. Using a systematic candidate gene approach to analyze the entire Cryg cluster, an A-->G transition was found in exon 2 of Cryga for the ENU-436 mutation and is designated Cryga1Neu. The mutant allele Crygbnop (formerly Cat2(nop)) is caused by a replacement of 11 bp by 4 bp in the third exon of Crygb, while a C-->G transversion in exon 3 of Cryge has been found for the Cryget (formerly Cat2(t)) mutation. For the mutation Cryga1Neu, an Asp-->Gly exchange is deduced, whereas the mutations Crygbnop and Cryget lead to the formation of in-frame stop codons and give rise to truncated proteins of 144 and 143 amino acids, respectively. The effects of the mutations upon gamma-crystallin structure are likely to be quite different. The Cryga1Neu mutation is expected to affect the link between Greek-key motifs 2 and 3, whereas both Crygbnop and Cryget mutations are supposed to truncate the fourth Greek-key motif. All three mutations are predicted to alter protein folding of the gamma-crystallins and result in lens cataract, but the phenotype for each is quite distinctive.

Aphakia (ak), a mouse mutation affecting early eye development: fine mapping, consideration of candidate genes and altered Pax6 and Six3 gene expression pattern

The homozygous mouse mutant aphakia (ak) has been characterized by bilaterally aphakic eyes without a pupil [Varnum DS, Stevens, LC (1968): J Hered 59:147-150]. The mutation was mapped to chromosome 19 [Varnum DS, Stevens, LC (1975): Mouse News Lett 53:35]. Our linkage studies yielded a precise localization of the ak gene 0.6 +/- 0.3 cM proximal to the microsatellite marker D19Mit10 and 0.7 +/- 0.4 cM distal to D19Mit4 and D19Mit91. No recombination was found with the marker D19Mit9 among 418 backcross offspring tested. The developmental control gene Pax2 mapped 11.0 +/- 3.5 cM proximal to ak and is excluded as a candidate gene. Sequence analysis of Fgf8 and Chuk1, which are localized close to the marker D19Mit10, detected no mutations in the ak/ak mutants. Histological analysis of homozygous mutants suggested the arrest of lens development at the lens stalk stage, a transient morphological structure during the formation of the lens vesicle. In the lens remnants, Pax6 and Six3 are expressed, whereas in the persisting lens stalk only Pax6 was detected. The expression pattern of Pax2 appeared normal; Cryaa expression could not be detected. As a consequence of the arrested lens development, other ocular tissues that require for their development information from the intact lens, such as iris, ciliary muscle, retina, and vitreous body, are absent or formed abnormally.

Genetic aspects of embryonic eye development in vertebrates

The vertebrate eye comprises tissues from different embryonic origins, e.g., iris and ciliary body are derived from the wall of the diencephalon via optic vesicle and optic cup. Lens and cornea, on the other hand, come from the overlying surface ectoderm. The timely action of transcription factors and inductive signals ensure the correct development of the different eye components. Establishing the genetic basis of eye defects has been an important tool for the detailed analysis of this complex process. One of the main control genes for eye development was discovered by the analysis of the allelic series of the Small eye mouse mutants and characterized as Pax6. It is involved in the interaction between the optic cup and the overlaying ectoderm. The central role for Pax6 in eye development is conserved throughout the animal kingdom as the murine Pax6 gene induces ectopic eyes in transgenic Drosophila despite the obvious diverse organization of the eye in the fruit fly compared to vertebrates. In human, mutations in the PAX6 gene are responsible for aniridia and Peter's anomaly. In addition to Pax6, other mutations affecting the interaction of the optic cup and the lens placode have been documented in the mouse. For the differentiation of the retina from the optic cup several genes are responsible: Mi leads to microphthalmia, if mutated, and encodes for a transcription factor, which is expressed in the melanocytes of the pigmented layer of the retina. In addition, further genes are implicated in the correct development of the retina, e.g., Chx10, Dlx1, GH6, Msx1 and -2, Otx1 and -2, or Wnt7b. Mutations within the retinoblastoma gene (RB1) are responsible for retinal tumors. Knock-out mutants of RB1 exhibit a block of lens differentiation prior to the retinal defect. Besides the influence of Rb1, the lens differentiates under the influence of growth factors (e.g., FGF, IGF, PDGF, TGF), and specific genes become activated encoding cytoskeletal proteins (e.g., filensin, phakinin, vimentin), structural proteins (e.g., crystallins) or membrane proteins (e.g., Mip). The optic nerve originates from the neural retina; ganglion cells grow to the optic stalk, forming the optic nerve. Its retrograde walk to the brain through the rudiment of the optic stalk depends on the correct Pax2 expression.