Genetic linkage analysis of autosomal dominant congenital cataracts

Mutation screening in autosomal dominant congenital cataract families from North India

Congenital cataract an opacity of the eye lens is present at birth and results in visual impairment during early childhood. If left untreated, it can lead to permanent blindness. Its prevalence is ten times higher in developing countries like India. Thus, we aimed to investigate the underlying genetic defects in three autosomal dominant congenital cataract (ADCC) families from North India. Detailed family histories were collected, pedigrees drawn followed by slit-lamp examination and lens photography. Mutation screening was performed in the candidate genes for crystallins, connexins, and membrane proteins by Sanger sequencing. Pathogenicity of novel variant was assessed bioinformatically. In an ADCC (CC-3006) family with bilateral membranous cataract and microcornea, a novel change (c.1114C>T;p.P372S) in GJA3 has been detected. In other two ADCC families affected with subcapsular (CC-286) and shrunken membranous hypermature cataract (CC-3014), a nonsense mutation (c.463C>T;p.Q155X) in CRYβB2 and a frameshift deletion (c.590_591delAG;p.E197VfsX22) in CRYβA1/A3 respectively, are observed. These variants segregated completely with the phenotypes in respective families and were absent in their unaffected family members and unrelated controls (tested for novel variant in GJA3). Earlier p.Q155X (CRYβB2) and p.E197VfsX22 (CRYβA1/A3) are reported with entirely different phenotypes. Thus, findings in present study expand the mutation spectrum and phenotypic heterogeneity linked with GJA3, CRYβB2, and CRYβA1/A3 for congenital cataracts. Identifying underlying genetic defects is essential for disease management and appropriate genetic counseling.

A Novel CRYBB2 Stopgain Mutation Causing Congenital Autosomal Dominant Cataract in a Chinese Family

Congenital cataract is the most common cause of the visual disability and blindness in childhood. This study aimed to identify gene mutations responsible for autosomal dominant congenital cataract (ADCC) in a Chinese family using next-generation sequencing technology. This family included eight unaffected and five affected individuals. After complete ophthalmic examinations, the blood samples of the proband and two available family members were collected. Then the whole exome sequencing was performed on the proband and Sanger sequencing was applied to validate the causal mutation in the two family members and control samples. After the whole exome sequencing data were filtered through a series of existing variation databases, a heterozygous mutation c.499T<G (p.E167X) in CRYBB2 gene was found. And the results showed that the mutation cosegregated with the disease phenotype in the family and was absolutely absent in 1000 ethnicity-matched control samples. Thus, the heterozygous mutation c.499T<G (p.E167X) in CRYBB2 was the causal mutation responsible for this ADCC family. In conclusion, our findings revealed a novel stopgain mutation c.499T<G (p.E167X) in the exon 6 of CRYBB2 which expanded the mutation spectrum of CRYBB2 in Chinese congenital cataract population and illustrated the important role of CRYBB2 in the genetics research of congenital cataract.

Role of ultrasound and biomicroscopy in evaluation of anterior segment anatomy in congenital and developmental cataract cases

PURPOSE

To investigate the role of ultrasound biomicroscopy (UBM) in the evaluation of anterior segment anatomy in cases of congenital and developmental cataract.

SETTING

Alexandria Main University Hospital, Ophthalmology Department, Faculty of Medicine, Alexandria University, Alexandria, Egypt.

METHODS

In this cross-sectional nonrandomized unmasked study, ultrasound biomicroscopy (UBM) was used to evaluate the anterior segment anatomy of 32 eyes of 21 children with congenital and developmental cataract. The parents were questioned for details of the children's history. An initial office examination was done to detect visual function, pupil color, lens morphology, corneal clarity, and presence of gross anterior segment anomaly. If the fundus was visible, it was examined in cooperative children after mydriasis. Under general anesthesia, each child was examined by UBM.

RESULTS

The parameters detected by UBM included anterior chamber depth, lens thickness, zonular fiber length, ciliary process length, anterior chamber angle, and lens morphology. A-scan biometry was also performed to detect anterior chamber biometric characteristics.

CONCLUSION

Ultrasound biomicroscopy is a valuable tool for evaluating childhood cataracts and associated ocular anomalies as well as anterior segment biometric characteristics.

Autosomal dominant cataract: intrafamilial phenotypic variability, interocular asymmetry, and variable progression in four Chilean families

PURPOSE

To document intrafamilial and interocular phenotypic variability of autosomal dominant cataract (ADC).

DESIGN

Prospective observational case series.

METHODS

We performed ophthalmologic examination in four Chilean ADC families.

RESULTS

The families exhibited variability with respect to morphology, location with the lens, color and density of cataracts among affected members. We documented asymmetry between eyes in the morphology, location within the lens, color and density of cataracts, and a variable rate of progression.

CONCLUSIONS

The cataracts in these families exhibit wide intrafamilial and interocular phenotypic variability, supporting the premise that the mutated genes are expressed differentially in individuals and between eyes; other genes or environmental factors may be the bases for this variability. Marked progression among some family members underscores the variable clinical course of a common mutation within a family. Like retinitis pigmentosa, classification of ADC will be most useful if based on the gene and specific mutation.

Novel single-base deletional mutation in major intrinsic protein (MIP) in autosomal dominant cataract

PURPOSE

To further elucidate the cataract phenotype, and identify the gene and mutation for autosomal dominant cataract (ADC) in an American family of European descent (ADC2) by sequencing the major intrinsic protein gene (MIP), a candidate based on linkage to chromosome 12q13.

DESIGN

Observational case series and laboratory experimental study.

METHODS

We examined two at-risk individuals in ADC2. We PCR-amplified and sequenced all four exons and all intron-exon boundaries of the MIP gene from genomic and cloned DNA in affected members to confirm one variant as the putative mutation.

RESULTS

We found a novel single deletion of nucleotide (nt) 3223 (within codon 235) in exon four, causing a frameshift that alters 41 of 45 subsequent amino acids and creates a premature stop codon.

CONCLUSIONS

We identified a novel single base pair deletion in the MIP gene and conclude that it is a pathogenic sequence alteration.

Linkage study between congenital cataracts and five crystallin loci

Congenital cataracts are a common cause of preventable blindness in children. We studied autosomal dominant congenital cataracts in 38 families and examined linkage between cataract loci and the crystallin genes on chromosomes 2, 11, 17, 21, and 22. We used clinical information to group families with phenotypically similar cataracts and analyzed the genetic data in these groups. Although LOD scores > 3.0 were not obtained, we found some support for linkage to four of the chromosomal regions examined, namely 2q33-35, 17q11.2-12, 21q22.3, and 22q11.2.

The morphology and natural history of childhood cataracts

The morphology of congenital cataract reflects a combination of the timing and nature of the cause, the anatomy of the lens including its capsule, its development, and changes that take place with time. Morphology may variably affect prognosis, give a clue to the etiology and the age of onset and, in an isolated case, sometimes suggest heritability. The spectrum of morphological variations is enormous and can be complex. A comprehensive approach is to classify the variations according to the area of the lens involved, and sub-dividing them by a detailed description of the shape and appearance. Each specific morphological type is then analyzed determining the etiology, visual prognosis, and management. The use of gene markers has allowed many of these variations to be identified and categorized. Cataracts in childhood can involve the whole lens, in which case they are called total, Morgagnian, or disk-like. They can affect only the center of the lens: lamellar, nuclear, oil droplet, cortical, or coronary. They can be anterior: anterior polar, anterior subcapsular, or anterior lenticonus. The posterior aspect of the lens can also be affected in different fashions: Mittendorf's dot, posterior lenticonus, posterior cortical cataracts, or posterior subcapsular. There are five more forms that must be described separately: punctuate lens opacities, sutural cataracts, coralliform or crystalline, wedge-shaped, and persistent hyperplastic primary vitreous.

Genetic analysis of adult-onset cataract in a city-based ophthalmic hospital

Adult-onset cataract (AOC) is a major ocular health problem and is the number one cause of blindness in the world. It is interesting to note that if the development of cataract is delayed by 10 years, the number of cataract surgeries needed would decrease by 45%. To prevent or delay cataract, the molecular pathological mechanisms underlying the lens change have to be understood, and this requires that the genes involved in such mechanisms should be identified. Hence, in this study we aim to identify AOC families which show a clear mendelian inheritance pattern, as only these families would be ideal for mapping the genes responsible. Over a period of 8 months, from September 1995-April 1996, 17 families with two or more affected members were identified. Segregation analysis showed autosomal dominant inheritance in multiple affected families. We propose to map the genes responsible for cataract in these families by linkage analysis and mutational screening of candidate genes.

Pediatric cataracts

The presentation, etiology, diagnostic examination, and management of pediatric cataracts are discussed in this article. If untreated, cataracts result in significant visual deprivation. Evaluation of the pupillary light reflex should become an integral part of every pediatrician's physical examination. Patients should be referred for a complete eye examination whenever a cataract is suspected because early intervention may be the most important factor in determining a good visual outcome.

Assignment of the lens intrinsic membrane protein MP19 structural gene to human chromosome 19

We have isolated and characterized a bovine cDNA clone encoding the bovine lens intrinsic membrane protein, MP19. This cDNA was used as a probe to analyze a panel of Southern blots of human-Chinese hamster somatic cell hybrid DNAs to assign the gene coding for MP19 to its human chromosome. Control human and Chinese hamster DNAs displayed a distinct EcoR1 restriction fragment pattern when hybridized with the bovine MP19 cDNA. When somatic cell hybrid DNAs were restricted with Eco R1 and Southern blots hybridized with the bovine MP19 cDNA, the characteristic human restriction pattern was observed only when human chromosome 19 was present in the hybrid panel. This assignment was confirmed using a human chromosome 19-specific genomic library. A clone from this human chromosome 19-specific library was identified and further characterized. This clone contained a 7.9 kilobase fragment that contained identical DNA sequences with that of the authentic bovine MP19 cDNA, and with a separate human genomic clone containing the MP19 gene.

Autosomal dominant congenital cataract; linkage relations; clinical and genetic heterogeneity

Congenital cataract is a heterogeneous disorder. Approximately one third of the cases are hereditary. A large family with autosomal dominant congenital cataract is described here. Clinical examinations showed variable expressivity, but all affected persons were eventually operated, most of them in the first or second decade of life. Linkage relations with a number of polymorphic marker systems were studied, all of them being negative. Among the 21 systems studied were Fy, HP, D16S4 and CRYG. The present autosomal dominant congenital cataract is termed the Volkman cataract, after the ancestor in the pedigree, and is genotypically different from the Marner cataract found in another large Danish pedigree.

Restriction fragment length polymorphisms associated with the gene for the major intrinsic protein of eye-lens fibre cell membranes in mice with hereditary cataracts

Cloned cDNAs coding for eye-lens fibre cell-membrane proteins, MIP and MP70, were used to detect restriction fragment length polymorphisms (RFLPs) in genomic DNA from inbred mice with autosomally inherited cataracts. Whereas distinct RFLPs associated with the MIP gene were identified in the Cba Cat and Nct mutants, no such genetic variation was associated with the MP70 gene. RFLPs associated with the mouse MIP gene may provide informative DNA markers in gene linkage studies of murine hereditary cataracts.

Microphthalmos in the presumed homozygous offspring of a first cousin marriage and linkage analysis of a locus in a family with autosomal dominant cerulean congenital cataracts

A family with autosomal dominant congenital cataracts was studied to determine clinical variability. A total of 159 relatives was ascertained; 17 affected and 19 normal individuals were evaluated and their blood sampled for inclusion in the linkage analysis. The disease was compatible with normal to mildly decreased visual acuity until adult life in all affected except the product of a consanguineous marriage of affected first cousins who was born with bilateral microphthalmos and dense congenital cataracts, attributed to homozygosity of the cataract gene. There were no extraocular abnormalities; the patient was of normal intelligence. Twenty-three markers were typed, 18 of which were informative. Linkage could be excluded for all 18 markers at short distances.

Autosomal dominant congenital cataract. Morphology and genetic mapping

We studied a large Danish pedigree with autosomal dominant congenital cataracts (ADCC) in 9 generations. Morphological characteristics of the cataracts are described and documented by photos. In contrast with several other types of ADCC the cataracts studied were progressive during infancy and childhood. Linkage analysis with 14 marker systems revealed close linkage between this Marner cataract locus (CAM) and the locus of Haptoglobin (HP) on chromosome 16. Genetic heterogeneity in ADCC has now established with the genetic mapping of cataract loci to chromosome 1, chromosome 2, and chromosome 16.

Marner's cataract (CAM) assigned to chromosome 16: linkage to haptoglobin

In the present study the kindred with Marner's cataract (CAM), which now comprises nine generations, was examined for linkage with 14 marker loci. We found strong evidence for linkage to haptoglobin, zmax = 8.33 at thetamax = 0.05 for males and thetamax = 0.04 for females.

Genetic linkage analysis of autosomal dominant congenital cataracts with lens-specific DNA probes and polymorphic phenotypic markers

The authors studied a four-generation family with autosomal dominant congenital cataracts (ADCCs) using linkage analysis with 23 polymorphic phenotypic markers and DNA restriction fragment length polymorphisms (RFLPs) detected by lens-specific DNA probes. A total of 19 family members were studied and the ten affected members had embryonal lens opacities. Close linkage was rejected with DNA probes encoding beta-crystallin, gamma-crystallin, and the major intrinsic protein of the lens fiber membrane (MIP) excluding defects of these genes as the cause of the cataract in this family. No statistically significant lod scores were produced with the polymorphic phenotypic markers. These results support the genetic heterogeneity of ADCCs.

Selected eye defects of special importance in pediatrics

This article reviews six defects of special importance in the care of children: aniridia, ectopia lentis, cataracts, glaucoma, colobomata, and optic nerve hypoplasia. In addition to causing potentially serious impairment of vision, these ocular disorders may be associated with significant systemic disease and genetic abnormalities.

Nucleotide sequence for the cDNA of the bovine beta B2 crystallin and assignment of the orthologous human locus to chromosome 22

We have identified and characterized two over-lapping bovine cDNA clones corresponding to the bovine crystallin beta Bp. The longer of the two clones, which contains the entire coding and 3' untranslated region as well as 54 nucleotides of the 5' untranslated sequence was used to identify and map an orthologous human gene, Hu beta B2, to chromosome 22, q11.2-q12.2. As one other human beta-crystallin, Hu beta A3/A1, has been mapped to chromosome 17, our results indicate that, unlike the tightly linked gamma-crystallins, the human beta-crystallins are not syntenic within the genome.

Molecular genetic approaches to the analysis of human ophthalmic disease

In this review of the recent literature, the contribution that the new techniques of molecular genetics has made in the analysis and diagnosis of human ophthalmic conditions is presented and discussed. Among the disorders reviewed are X-linked retinitis pigmentosa, Norrie's disease, gyrate atrophy and retinoblastoma, and there are also sections on crystallins and visual pigments.

A locus for a human hereditary cataract is closely linked to the gamma-crystallin gene family

Within the human gamma-crystallin gene cluster polymorphic Taq I sites are present. These give rise to three sets of allelic fragments from the gamma-crystallin genes. Together these restriction fragment length polymorphisms define eight possible haplotypes, three of which (Q, R, and S) were found in the Dutch and English population. A fourth haplotype (P) was detected within a family in which a hereditary Coppock-like cataract of the embryonic lens nucleus occurs in heterozygotes. Haplotype P was found only in family members who suffered from cataract, and all family members who suffered from cataract had haplotype P. The absolute correlation between the presence of haplotype P and cataract within this family shows that the gamma-crystallin gene cluster and the locus for the Coppock-like cataract are closely linked [logarithm of odds (lod) score of 7.58 at its maximum at phi = 0]. This linkage provides genetic evidence that the primary cause of a cataract in humans could possibly be a lesion in a crystallin gene.