Nucci P (ed): Pediatric Cataract. Dev Ophthalmol. Basel, Karger, 2016, vol 57, pp 1–14 (DOI: 10.1159/000442495)
Genetics of Congenital Cataract Francesco Pichi a, b Andrea Lembo b Massimiliano Serafino b Paolo Nucci b a Cole Eye Institute, Cleveland Clinic, Cleveland, Ohio, USA; b University Eye Clinic, San Giuseppe Hospital, Milan, Italy
Abstract
Cataract is the term used to describe opacification of the lens of the eye [1]. Congenital cataract is the leading cause of reversible blindness during childhood. Its occurrence, depending on the regional socioeconomic development, is 1–6 cases per 10,000 live births in industrialized countries and 5–15 per 10,000 in the poorest areas of the world [2]. Congenital cataract appears at birth or during the first decade of life. About 20,000–40,000 new cases of bilateral congenital cataract are diagnosed each year [3, 4]. There are many different causes, including intrauterine infections, metabolic disorders (galactosemia), and chromosomal abnormalities. Cataract may be an isolated anomaly, seen in association with another ocular developmental abnormality, or it may be part of a multisystem syndrome, such as Down syndrome,
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Congenital cataract is a type of cataract that presents at birth or during early childhood, and it is one of the most easily treatable causes of visual impairment and blindness during infancy, with an estimated prevalence of 1–6 cases per 10,000 live births. Approximately 50% of all congenital cataract cases may have a genetic cause, and such cases are quite heterogeneous. Although congenital nuclear cataract can be caused by multiple factors, genetic mutation remains the most common cause. All three types of Mendelian inheritance have been reported for cataract; however, autosomal dominant transmission seems to be the most frequent. The transparency and high refractive index of the lens are achieved by the precise architecture of fiber cells and homeostasis of the lens proteins in terms of their concentrations, stabilities, and supramolecular organization. Research on hereditary congenital cataract has led to the identification of several classes of candidate genes that encode proteins such crystallins, lens-specific connexins, aquaporin, cytoskeletal structural proteins, and developmental regulators. In this review, we highlight the identified genetic mutations that account © 2016 S. Karger AG, Basel for congenital nuclear cataract.
Wilson’s disease, or myotonic dystrophy. The syndromic forms of cataract are not covered in this paper. Inherited cataracts correspond to 8–25% of congenital cataracts, particularly bilateral cataracts. Rahi and Dezateux found that 27% of children with bilateral isolated congenital cataracts had a genetic basis compared with 2% of those with unilateral cataract. In nonconsanguineous populations, the majority of cases of inherited nonsyndromic cataract show autosomal dominant (AD) inheritance, but X-linked and autosomal recessive forms are also seen [5]. The identification of the mutations causing childhood cataract should lead to a greater understanding of the mechanisms implicated in cataractogenesis and provide further insights into normal lens development and physiology [6–8]. Moreover, gene mapping in congenital cataract is an important step in understanding the molecular defects of age-related cataract, which also has a strong genetic component to its etiology, and over the longer term may lead to the development of a medical therapy to slow lens opacification.
The vertebrate eye lens has remained a challenge for structural and evolutionary biologists because it represents a biological system that functions as an optical device, with the added property of being elastic. Lens formation is the result of a series of inductive processes. Studies of the embryology and morphogenesis of the ocular lens in animal models and humans have provided insights into the temporal and spatial disturbances that may result in the different ocular phenotypes found in inherited congenital cataract. The lens forms from surface ectodermal cells overlying the optic vesicle. The lens placode appears on the optic vesicle, which protrudes from the forebrain, at around the 25th day of gestation. It is a thickening of the surface ectoderm, composed of a single layer of cuboidal cells that invaginate into the neural ectoderm of the optic vesicle as the lens pit, becoming free from the surface by the 33rd day. The anterior cells remain as a single layer of cuboidal epithelial cells, whereas the posterior cells elongate to form primary lens fiber cells and obliterate the lumen of the vesicle. These cells are called primary lens fibers. The process of formation of secondary fiber cells begins thereafter and continues throughout life. Anterior epithelial cells divide to form secondary fiber cells at the lens equator, where they elongate both anteriorly and posteriorly to surround the earlier fiber cells in onion-like layers. Lens sutures appear during the second month at the anterior and posterior poles of the spherical embryonal nucleus as a result of the terminal ends of the secondary lens fibers abutting each other. The anterior lens suture has an upright Yconfiguration, while the posterior suture has an inverted Y-configuration. The secondary lens fibers are laid down in a strictly ordered manner to minimize the scattering of light. These secondary lens fibers are continually produced after birth and form the lens cortex.
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Nucci P (ed): Pediatric Cataract. Dev Ophthalmol. Basel, Karger, 2016, vol 57, pp 1–14 (DOI: 10.1159/000442495)
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Embryology
The adult lens is aneural, avascular and alymphatic, and it contains a large concentration of proteins. Due to the loss of nuclei and other organelles, these proteins do not turn over and are present throughout life. The lens maintains a gradient of refractive index from the center to the periphery that correlates with the difference in the water (and protein) content of the cortex (80% water) versus that of the nucleus (68% water). Proteins constitute about 30–35% of the entire mass of the lens, being present at high concentrations of above 450 mg/ml, which leads to a higher refractive index, thereby giving the lens functional transparency. Dilution of proteins to a concentration of below 450 mg/ml results in increased light scattering. Loss of transparency occurs as a result of aging, partly due to the cumulative effect of alterations in lens proteins in response to light or oxidizing agents and partly due to age-related changes in the cytoskeleton and cell membranes. The crystallin family of proteins deserves particular mention, not only because crystallins are the major structural proteins of the lens, constituting about 80–90% of the lens soluble proteins, but also because a number of recent studies have disclosed the involvement of crystallins in pediatric cataract. Morner first described crystallins in 1893. He fractionated bovine lens crystallins into three soluble and one insoluble fraction. The soluble fractions contained α, β and γ crystallins, which are found in all vertebrate lenses. α-Crystallins make up 40% of human lens crystallins and exist as a large complex with a molecular mass of 800–1,000 kDa composed of αA- and αB-crystallins, which are arranged to confer maximum stability and solubility to the complex. The molecular weight of each individual subunit is around 20 kDa, and the two proteins are encoded by separate genes: the CRYAA and CRYAB genes, respectively. Both genes consist of three exons (coding regions) separated by two introns (noncoding regions). The αA protein is 173 amino acids long, and αB is 175 amino acids long. These two proteins exist at a ratio of three (αA) to one (αB) in the lens. Although they are both expressed at very high levels in the lens, αA-crystallin is essentially a lens-specific protein, being expressed at a very low level in the spleen. On the other hand, αB-crystallin is expressed ubiquitously and is found at high levels in the brain, muscles, lungs, thymus, and kidneys, in addition to a number of cell lines. Apart from being a structural component of the lens, α-crystallins have distinct protective roles in maintaining solubility of intracellular lens proteins and promoting resistance of cells to stress. αA-Crystallin shows sequence homology to a heat shock protein from humans (sHSP27) and mediates the cellular responses to various types of stress, such as thermal and oxidative stresses. It may even function as a regulator in apoptotic pathways. Essentially, α-crystallins mimic the action of a molecular chaperone. In the lens, light-induced damage and oxidative or thermal stress effectively bring about loss of transparency through the modification of proteins so that they undergo denaturation. α-Crystallins execute their chaperone-like functions by bind-
Genetics of Congenital Cataract
Nucci P (ed): Pediatric Cataract. Dev Ophthalmol. Basel, Karger, 2016, vol 57, pp 1–14 (DOI: 10.1159/000442495)
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Major Structural Proteins of the Lens
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ing to denatured proteins and keeping them in solution. Because the lens does not possess any mechanism for the degradation or extrusion of damaged proteins out of cells, this chaperone-like activity is crucial for preserving lens transparency. The β- and γ-crystallins share common structural elements and are therefore regarded as members of a superfamily. The native forms of β-crystallins consist of oligomers ranging from 40 to 200 kDa that form heterodimers of acidic (βA1/A3, located on chromosome 17q11, βA2, located on chromosome 2q33, and βA4, located on chromosome 22q11) and basic (βB1, βB2 and βB3, located on chromosome 22q11) subunits. Each subunit is encoded by a separate gene, except for βA1/A3, which arise from a single gene with two different initiation codons. β-crystallins also have nonconserved N- and C-terminal extensions, which are absent from γ-crystallins. γ-Crystallins exist as monomers of about 20 kDa. They differ from β-crystallins in that the linker peptide between the two domains is folded so that the domains interact intramolecularly, giving rise to the very compact structure attributed to γ-crystallins. As a result of intramolecular interactions between the globular domains, these proteins are monomeric. They are found specifically in lens fibers and are present at a very high concentration in the lens nucleus, which is the hardest, most dehydrated part of the lens. Thus, the γ-crystallin structure is optimal for high-density molecular packing. The γ-crystallin gene cluster includes γA to γF, although polypeptides are encoded only by γA-D in humans. γE and γF are pseudogenes and are not normally expressed in the human lens. Other proteins that are important for the maintenance of lens architecture are as follows: (a) membrane proteins, which make up 2% of the lens proteins; (b) gap junction proteins, termed connexins, which form gated channels that are required for cellcell communication; and (c) cytoskeletal proteins, such as actin, myosin, and vimentin, which are common to all tissues, not only the lens. Because of its unique function and anatomy, the mammalian lens is critically dependent on the proper functioning of gap-junction proteins. Each gap-junction channel is composed of two hemi-channels, or connexons, which dock in the extracellular spaces between adjacent cells. Each connexon is comprised of six integral transmembrane protein subunits, known as connexins. Connexins belong to a multigene family consisting of >20 members, three of which are expressed in the lens (Cx43, Cx46, and Cx50). Lens epithelial cells show predominant expression of connexin 43 (Cx43). During differentiation into fibers, Cx43 expression is downregulated and is replaced by connexin 46 (Cx46) and connexin 50 (Cx50). Connexins are especially important for nutrition and intercellular communication in the avascular lens. They associate into heterogeneous oligomeric transmembrane structures with a central voltage gated ion channel, the connexon. Connexons bridge the extracellular space, allowing for the intercellular transportation of small biomolecules, including ions, nutrients, and metabolites, between adjacent cells. These functions of connexins are important for the maintenance of both lens metabolic homeostasis and the transparency of fibers within the ocular lens.
Major intrinsic protein (MIP) is an integral membrane protein that is a member of the aquaporin family of transporters of water and small select molecules that is localized to the plasma membrane. It is the most highly expressed membrane protein in the lens, representing almost 80% of the transport proteins. The architecture of lens cells is a result of interactions among the cytoskeleton, crystallin proteins, and cytoplasm. The cytoskeleton is a network of various cytoplasmic proteins that are involved in providing structural support, cell motility, and the determination and maintenance of cell volume and shape. Lens cells contain three different filaments, which are differentiated by diameter, the types of subunits, and molecular organization into microfilaments, microtubules, and intermediate filaments. Microfilaments and microtubules facilitate changes in ions, while intermediate filaments aid lens cells in overcoming physical stresses by promoting lens accommodation and adaptation to changes in temperature. Beaded filament structural proteins (BFSPs) are unique eye lens-specific intermediate filaments that form cytoskeletal structures containing two core components of BFSP1 (also called filensin) and BFSP2 (also called CP49), which are highly divergent intermediate filament proteins that combine in the presence of Îą-crystallins to form the appropriate beaded structure.
Animal models suggest that the genes implicated thus far in cataractogenesis are expressed in a time ordered, sequential fashion. Therefore, categorization weighted more toward the location of opacification will best reflect the underlying genotype [9]. Cataract affecting the nucleus is common and suggestive of an abnormality in gene expression during early development. Affected subjects show bilateral symmetrical involvement with variable expressivity. The concentric deposition of secondary lens fibers that occurs during growth of the normal lens results in the formation of lamellae. Opacities confined to a specific lamella therefore reflect a short period of developmental disturbance (usually during the fetal period), resulting in symmetrical bilateral lens opacification [10]. Lamellar cataracts have also been called zonular, perinuclear, and polymorphic. The degree of opacification is variable, and visual acuity may be well preserved or reduced enough to require surgical intervention. Cataract limited to the cortex is rare and differs from lamellar cataract because opacification is restricted to a sector of the outer cortical, often superior, lens fibers adjacent to the lens capsule. The pathogenesis is unknown, but its distribution and subsequent progression are suggestive of an abnormality arising during the later stages of lens development [11]. The presence of families with cataract limited to either the anterior or posterior pole of the lens is less amenable to explanation in terms of lens development. Ante-
Genetics of Congenital Cataract
Nucci P (ed): Pediatric Cataract. Dev Ophthalmol. Basel, Karger, 2016, vol 57, pp 1–14 (DOI: 10.1159/000442495)
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Inherited Nonsyndromic Cataract Phenotypes
rior polar cataracts are bilateral, usually symmetrical, well-circumscribed lens opacities that are rarely progressive and can be inherited as dominant, recessive, or Xlinked traits. Associations with microphthalmia and astigmatism implicate a gene involved in anterior segment development. Families with posterior polar cataracts are reasonably common [12]. Affected subjects have bilateral, symmetrical lens opacities that are usually inherited as a dominant trait. Because opacification is close to the optically crucial, nodal point of the eye, vision is commonly reduced. Cerulean cataract is not truly congenital but develops during childhood and progresses throughout early life. The discrete, pinhead-shaped, blue-white opacities are distributed throughout the lens, becoming more numerous in the cortex, where they may form wedge-like shapes in the mid-periphery. Within a pedigree, this phenotype is consistent in its distribution but variable in its severity. Visual acuity is usually well preserved, and cataract extraction is rarely necessary before adulthood and is usually associated with a good outcome.
Genetic mapping is performed by means of linkage analysis, which involves the use of specific markers whose positions on a chromosome are known relative to each other to identify the approximate location of the disease gene on a chromosome. To be able to effectively link the disease gene to a marker locus, one has to be able to unambiguously trace the parental origin of the marker allele. For this purpose, the markers have to be highly polymorphic, i.e., have many different versions within a population so that individuals have a higher chance of being heterozygous for the marker, and they have to be inherited in a Mendelian fashion [13]. Earlier mapping studies used protein or antigen markers, such as blood groups (ABO, Rh, Duffy, haptoglobin, etc.). However, these markers have limited use because protein loci are not sufficiently polymorphic. With the invention of restriction enzymes, they were replaced by restriction fragment length polymorphism markers, which have subsequently been largely replaced by microsatellite markers, consisting of units of repeated sequences of 2–12 base pairs. Variability in the number of repeat units in microsatellites, ranging from one to several hundred, gives rise to an enormous scope for polymorphism. With the advent of the Human Genome Project, the human genetic map has been demonstrated to contain about 6,000 microsatellite markers located at closely spaced intervals, thus enhancing the potential to map disease genes accurately [14]. The underlying basis for the assumption of physical proximity of genetically linked markers stems from the phenomenon of crossing over or recombination (the exchange of chromosomal material between homologous chromosomes), which occurs during the first stage of meiosis. The closer together that two genes are on a chromosome, the less likely they are to recombine. Thus, if a disease locus and marker locus are close together on a chromosome, then they are not likely to be separated by recombination
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Molecular Genetic Approaches
and will show linkage. On the other hand, if two loci are far apart, then recombination is likely to occur between them, and they are considered to be unlinked. In linkage analysis, one essentially looks at the inheritance of markers at different locations throughout the genome in all members of a pedigree and compares it with that of the disease in question to determine whether the disease is coinherited (or linked) with any set of markers. Coinheritance implies that the disease gene is present at the same chromosomal locus as the linked marker(s). There are, however, several practical considerations when mapping human cataract genes [15]. A significant proportion of cataract mutations appear de novo, often making the family size small. While the penetrance of all phenotypes is high, expressivity, age of onset, and rate of progression are variable, making careful ophthalmic evaluation critical. In addition, surgical modification of the disease can make it difficult to describe the phenotype accurately.
Genes Implicated in Cataractogenesis
Crystallin Proteins α-, β-, and γ-Crystallins constitute the main cytoplasmic proteins of the human lens. It is not surprising that mutations in crystallin genes, resulting in proteins with abnormal structures, can result in lens opacity. Maintenance of normal structures, as well as normal amounts of various proteins, both with respect to monomers and larger multimeric complexes, is essential for lens transparency [19]. The overall effect of a mutant structural protein such as a crystallin could conceivably be to perturb the equilibrium with respect to the monomers and/or multimers, apart from the insolubility of the mutant protein itself. Mutations in α-crystallin genes (CRYA) are now known to be cataractogenic [20]. Eight mutations in the CRYAA gene have been described. A missense mutation in the
Genetics of Congenital Cataract
Nucci P (ed): Pediatric Cataract. Dev Ophthalmol. Basel, Karger, 2016, vol 57, pp 1–14 (DOI: 10.1159/000442495)
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In 1963, Renwick and Lawler described the cosegregation of inherited cataract with the Duffy blood group locus [Online Mendelian Inheritance in Man (OMIM) reference number 110700]. The subsequent development of advanced molecular biological techniques has facilitated the identification of 25 further independent cataract loci, including 10 mutations [16]. Cases of hereditary cataract that have been characterized so far show Mendelian inheritance and either result from a single-gene mutation or from a chromosomal translocation. They can be inherited in an AD, recessive or Xlinked mode [17]. Mutations in distinct genes encoding the main cytoplasmic proteins of the human lens have been associated with cataracts of various morphologies and include those encoding crystallins (CRYA, CRYB, and CRYG), lens-specific connexins (Cx43, Cx46, and Cx50), MIP or aquaporin, cytoskeletal structural proteins, paired-like homeodomain transcription factor 3 (PITX3), avian musculoaponeurotic fibrosarcoma (MAF), and heat shock transcription factor 4 (HSF4) [18].
Connexin Proteins Mutations in specific connexin genes have been associated with several diseases, including genetic deafness, skin diseases, peripheral neuropathies, heart defects and cataracts. Recently, mutations in both the Cx46 (a single exon encoding a 435-amino acid protein localized to 13q11–q13) and Cx50 genes (on chromosome 1q22, a mis-
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αA-crystallin gene has been associated with zonular central cataract. This mutation, which results in an amino acid substitution in a conserved region of the protein, is predicted to alter its overall charge and therefore its tertiary structure. The effect of these changes might be impaired function as a chaperone or precipitation of the protein due to the formation of large aggregates. The first mutation (R116C), a replacement of arginine with cysteine at position 116, has been associated with congenital nuclear cataract, microcornea, and microphthalmia. This substitution results in abnormal oligomerization of α- and β-crystallins, causing opacification of the lens. The second mutation is characterized by the substitution of a threonine with a premature stop codon (W9X), resulting in a truncated protein, and it has a recessive inheritance pattern [21]. Another three mutations, which are inherited in an AD manner, that have also been described in association with total cataract are R21L, R49C, and G98R. Recently, three new mutations, R12C, R21W, and R116H, have been associated with posterior polar, lamellar, and nuclear cataracts, respectively. Further, AD isolated posterior polar cataract has been mapped to the CRYAB gene locus on 11q22 and has been associated with a deletion mutation (450delA) [22]. Two mutations in the CRYBA1 gene that encode the βA1-crystallin protein have been identified (IVS3 + 1G→A and c.271–273delGGA). The IVS3 + 1G→A mutation causes different cataract phenotypes, including lamellar and sutural cataracts and nuclear cataract. Three mutations in the CRYBB2 gene have been identified, including Q155X, D128V, and W151C, which remove the final 51 amino acids, resulting in an unstable molecule [23]. βB2-crystallin is the only member of the β-crystallin gene cluster in chromosome 22q to be highly transcribed in the lens. Missense mutations in this gene are now known to result in the development of cerulean and pulverulent cataracts. The γ-crystallin-encoding genes (CRYG genes, 2q33–q35) consist of the γA, B, C, D, E, and F genes and a gene fragment, γG. Only γC and γD encode abundant proteins, while γE and γF are pseudogenes by virtue of in-frame stop codons (γF lacks a promoter as well). An increasing number of mutations in the CRYG genes have been described in association with human congenital cataract [24]. To date, five mutations in the CRYGC gene (T5P, 225–226insGCGGC, C109X, W157X, and R168W) have been reported. Of the missense mutations, T5P has been associated with nuclear cataract, and R168W has been reported to cause lamellar and nuclear cataracts. In addition, ten mutations in the CRYGD gene have been described. Although the resulting phenotypes can vary significantly, mutations in γ-crystallins tend to result in nuclear or lamellar cataract, consistent with their high level of expression in the lens nucleus.
sense mutation in codon 88 leading to the substitution of proline with serine) have been reported to cause phenotypically similar AD lamellar pulverulent cataracts [25]. To date, another thirteen mutations in Cx50 have been found in different hereditary cataract pedigrees, including V44E, V64G, V79L, P88Q, Q48K, P189S, R198Q, R23T, W45S, D47N, D47Y, S276F, and I274M. In addition, fourteen mutations in the Cx46 gene involving the different domains been reported to date in association with AD congenital cataract in humans. Major Intrinsic Protein or Aquaporin-0 Lamellar, cortical and polymorphic cataracts have been associated with missense mutations in the aquaporin-0 (AQP0) gene. Recently, two such mutations have been identified; the first mutation, T138R, has been associated with progressive congenital lamellar and polar cataracts, and the second, E134G, has been associated with lamellar cataract [26]. Both of these mutations appear to act by interfering with the normal trafficking of AQP0 to the plasma membrane and thus with water channel activity [27].
Developmental Regulators Embryonic lens development is predicted by the spatial and temporal interactions of several genes and their products. The genes involved in this complex process encode growth factors and transcription factors. These proteins regulate the transcription of a number of tissue-specific genes during differentiation and play a crucial role in lens plan specification [29]. Depending on the nature of the genetic defect involved, mutations in genes that drive development can result in multiple abnormalities of the eye, particularly congenital cataract. Mutations in transcription factor-encoding genes have been implicated in anterior segment dysgenesis, but only three (PITX3, MAF, and HSF4) have been associated with isolated cataract [30]. Mutations in the PITX3 gene result predominantly in AD congenital cataract associated with dysgenesis of the anterior segment, including corneal opacity, iris adhesions, microcornea, and microphthalmia. Two mutations have been described, a 17bp insertion in the coding sequence (656–657ins17bp), resulting in a frame shift, and a G→A transition in exon 2 of the PITX3 gene, resulting in a serine to asparagine substitution at codon 13 (S13N). A R288P substitution within the MAF gene on chromosome 16q22–q23 has been described in association with AD juvenile pulverulent cataract, iris coloboma, and
Genetics of Congenital Cataract
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Cytoskeletal Proteins The gene encoding BFSP2 is located on human chromosome 3q21. Actually, two mutations in this gene have been identified to be related to the occurrence of congenital cataract [28]. The first mutation (R278W) has been associated with juvenile-onset cataract, while the second mutation (delE233) has been associated with congenital nuclear, sutural, and cortical cataracts, with varying severities among different individuals.
Homeobox Another class of mutations that result in cataract are those that interfere with lens development, resulting in a loss of cellular organization. Depending on the nature of the genetic defect involved, mutations in genes that drive development can result in multiple abnormalities of the eye and nervous system. An important class of developmental genes consists of the homeobox genes. These genes encode proteins with a highly conserved 60-amino acid motif known as the ‘homeodomain’ [32]. These proteins regulate transcription of a number of tissue-specific genes during differentiation and play a crucial role in body plan specification. Two homeobox genes that are associated with congenital cataract in humans in conjunction with other developmental defects are the Pax6 and PITX3 genes. The Pax6 gene encodes a protein that is required for development of the eye. It is expressed in the neural tube, areas of the developing forebrain and hindbrain, eye, olfactory epithelium, pituitary and cerebellum. The Pax6 protein is also required for expression of several crystallin genes. Mutations in Pax6 result in aniridia, a syndrome that includes cataract and is characterized by iris hypoplasia, absence of fovea, and associated abnormalities, such as lens dislocation and optic nerve hypoplasia [33]. Aniridia is inherited as an AD disorder. The PITX3 gene is a homeobox gene that belongs to the PITX/RIEG family of homeobox genes. Members of this family are known to play roles in eye development, although the function of PITX3 itself remains to be elucidated. Mutations in PITX3 result in AD congenital cataracts and anterior segment mesenchymal dysgenesis. Congenital cataract associated with PITX3 mutations is described as total cataract with no other anterior segment anomalies. Anterior segment mesenchymal dysgenesis is a very uncommon disorder that includes all malformations of the first, second and third mesenchymal waves of the neural crest (i.e., the corneal endothelium and stroma, trabecular meshwork and iris stroma). It is of interest that many cataract families have been mapped to loci for which there is no known candidate gene. Significantly, exclusion data on other families with AD cataract have been reported, strongly supporting the supposition that further genetic loci remain to be identified. [34]. The existence of X-linked nonsyndromic congenital cataract remains contentious. A number of pedigrees have been reported, although many other modes of inheritance appear more likely. It has been suggested, however, that X-linked cataract is either synonymous or closely related to Nance-Horan syndrome, which has been mapped to Xp. Furthermore, the recognition of chromosomal deletions of varying sizes within this region and the resulting phenotypes suggest that a cataract locus may reside within Xp22.3–21.1.
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microcornea. The HSF4 gene regulates the expression of heat shock proteins, which may be important components of lens development. Currently, six mutations in this gene had been associated with congenital cataract, resulting in lamellar progressive and nuclear phenotypes [31].
Cataracts Arising from Chromosomal Aberrations
Cataracts resulting from chromosomal translocations have been described [35]. They are generally associated with other abnormalities because more than one gene may be disrupted by the translocation. The breakpoint of the translocation should generally contain the candidate gene. AD congenital cataract has been described in a threegeneration family with a reciprocal translocation between chromosomes 2p22 and 16pl3 [36]. Congenital anterior polar cataract has been found to result from an unbalanced translocation between chromosomes 3 and 18, and yet another family has been reported with dominantly inherited anterior polar cataract in association with a balanced translocation between chromosomes 2 and 14. Identification of the genes located at the breakpoints in these cases should lead to the increased knowledge of pathways related to cataractogenesis. Genotype-Phenotype Correlation The presence of several clearly distinguishable human cataract phenotypes and a number of probable subtypes within each category correlate well with the complex underlying genotypes shown by human linkage studies [37]. Furthermore, evidence that each phenotype maps to more than one locus suggests that mutations in different genes may give rise to similar phenotypes. In contrast, only the γC- and βB2-crystallin genes have been implicated in more than one phenotype to date. It is possible, however, that allelic heterogeneity will be shown to be more prevalent, as different mutations within the same gene may affect the regulatory ability of the protein product or its ability to bind to other lens proteins [38]. An example of this might be α-crystallin, which is known to have both structural and chaperone-like functions. It remains to be determined whether the Volkmann and posterior polar cataract loci identified in 1p36 are indeed allelic. Lens development and growth throughout life result from the temporal and sequential expression of a number of genes [39]. There is some correlation between what is known about the distribution of proteins in the lens and the positioning of opacities observed in cataract. An example is blue dot (cerulean) cataract, resulting from a mutation in β-crystallin and known to be localized to the cortical region of the lens. Much information remains to be elucidated in lens biology, but the identification of further underlying gene mutations in patients with cataract will be beneficial.
Genetic counseling in congenital cataract is usually straightforward when the abnormality is confined to the lens and there is a positive family history. Most families show AD inheritance, and the statuses of at-risk subjects can readily be determined by careful slit lamp examination after pupillary dilatation [40]. Variability in disease expres-
Genetics of Congenital Cataract
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Genetic Counseling
sion is common, and asymptomatic subjects should not be assumed to be unaffected. X-linked and recessive forms of inherited cataract are rare and may be recognized when there is an appropriate family history. Genetic counseling in isolated cases is more problematic. Most cases of unilateral cataract are nongenetic, but patients with bilateral cataracts for whom there is no family history should undergo further investigation to elucidate the cause [41]. First, both of the parents and any siblings should undergo dilated slit lamp examination to exclude mild congenital opacities; the presence of such opacities will confirm the familial nature of the cataract and allow for accurate counseling regarding recurrence risks. If other family members are normal, then the child should be reviewed by a pediatrician to rule out any other clinical features that may be suggestive of a multisystem disorder associated with cataract. Routine investigations include measurements of the concentrations of plasma urea and electrolytes, urinary amino acids (to exclude Lowe syndrome in male infants), and urinary reducing sugars (to exclude galactosemia) and a screen for congenital infection, particularly rubella [42]. Other investigations may be required depending on other clinical findings. In the absence of a family history and when the investigations yield normal results, the risk of recurrence in subsequent pregnancies is extremely small. When counseling adults with congenital cataract about the risk to their offspring, it is again important to review other relatives and when possible to examine clinical records to exclude any syndromic forms of cataract or cataract of nongenetic etiology [43]. In adults without a family history, the risk of having an affected child is very small if the cataract is unilateral. The risk is higher in bilateral cases because some may be caused by a novel AD mutation [44], but the precise risk is difficult to quantify. Many adults seeking advice will have had multiple operations during childhood and still have severe visual impairment, and they may have reservations about putting their own child through a similar experience. However, improvements in cataract surgery and optical management have resulted in greatly improved visual outcomes, and multiple operations are rarely necessary. This improved prognosis should be discussed, and it is important that the newborn child is examined by an ophthalmologist during the first few weeks of life to exclude cataract, as the long-term prognosis in infants that require early surgery is improved if surgery is performed promptly.
Molecular genetic approaches to studying cataract are yielding clues regarding the structural and metabolic requirements for the maintenance of lens transparency. Essentially, cataracts can result from changes in lens architecture, from disruption of the intracellular ordered arrangement of proteins or from changes in the organization of lens fibers due to aberrations in growth or differentiation. Alternatively, alterations in the intracellular environment due to changes in the water content or
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Conclusions
small molecules can result in opacification of the lens due to breakdown of lens homeostasis. The latter type of alteration can result from abnormalities in cell-cell communication or from damage to the cell membrane. The study of genes related to congenital cataract and knowledge about the underlying molecular mechanisms could be extended to age-related cataract in the near future, which remains the leading cause of blindness worldwide, allowing for the development of new treatments and techniques to prevent this type of cataract. Although the pathways leading to genetic cataract that show simple Mendelian inheritance are relatively easier to dissect, senile cataract, which represents the overwhelming majority of cataract cases worldwide, is a complex disease. A number of environmental influences, such as UV light and oxidative and osmotic insults to the lens, act in a cumulative fashion to precipitate cataractous changes.
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References
Francesco Pichi, MD Cole Eye Institute, Cleveland Clinic 9500 Euclid Avenue Cleveland, OH 44106 (USA) E-Mail ilmiticopicchio @ gmail.com
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