Molecular Vision 2007; 13:2023-2029 <http://www.molvis.org/molvis/v13/a228/> Received 18 June 2007 | Accepted 19 October 2007 | Published 24 October 2007

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The E233del mutation in BFSP2 causes a progressive autosomal dominant congenital cataract in a Chinese family

Xiaobo Cui,¹ Linghan Gao,² Yan Jin,¹ Yi Zhang,¹ Jing Bai,¹ Guoyin Feng,² Weiqi Gao,³ Ping Liu,³ Lin He,² Songbin Fu¹
 
 

The first three authors contributed equally to this publication.

¹Laboratory of Medical Genetics, Harbin Medical University, Harbin, China; ²Bio-X Life Science Research Center, Shanghai Jiao Tong University, 1954 Huashan Road, Shanghai, China; ³Affiliated First Hospital of Harbin Medical University, Harbin, China

Correspondence to: Professor Songbin Fu, Harbin Medical University, Laboratory of Medical Genetics, No. 194, Xuefu Road, Nangang District, Harbin, Heilongjiang 150086, China; Phone: +86-451-86676560; FAX: +86-451-86632768; email: fusb@ems.hrbmu.edu.cn

Abstract

Purpose: Congenital cataract is a fundamental cause of blindness throughout the world. A large multi-generational family in northern China was investigated to determine the genetic cause of a progressive autosomal dominant congenital cataract.

Methods: Slit-lamp photography was conducted to provide definite data for cataract diagnosis. A genome wide scan, linkage analysis, and haplotype analysis were performed to shield the linkage region on the chromosome. BFSP2 was investigated by direct sequencing and detection of fluorescent labeled polymerase chain reaction (PCR) products.

Results: Two-point linkage analysis mapped this autosomal dominant congenital cataract (ADCC) locus to D3S1292 in 3q22.1 with a LOD score Z_max=3.99 (θ=0.00). Haplotype analysis located the cosegregating region between marker D3S1551 and D3S3617. In this region, BFSP2 is a powerful candidate gene. Direct sequencing identified the cosegregating E233del mutation in exon 3 of BFSP2. This mutation was not detected in 100 unrelated controls.

Conclusions: The E233del mutation in BFSP2 is the cause of the cataract phenotype in this pedigree. The progressive phenotype has provided more evidence for the heterogeneity of congenital cataract caused by BFSP2 mutations and for the important role BFSP2 plays in cataract formation.

Introduction

Cataract refers to the opacity of all or part of the lens in the eye. Of all types of cataracts, congenital cataract is the most common cause of blindness for infants, with an incidence of 1-2 per 10,000 [1]. Congenital cataract generally occurs as a solitary disorder in association with some developmental defects or as a fraction of a complicated multisystem syndrome [2]. About one-quarter to one-third of congenital cataract are hereditary cataract, which are usually inherited in an autosomal dominant manner [3].

The clinical phenotype of cataract can be various. According to current criterion, congenital cataract can be divided into several categories based on the location and appearance of the opacities. These categories include total cataract, disk-like cataract, anterior polar cataract, anterior subcapsular cataract, anterior lenticonus cataract, posterior cortical cataract, posterior subcapsular cataract, lamellar (zonular) cataract, nuclear cataract, central pulverulent cataract, cortical cataract, cerulean cataract, coraliform cataract, wedge-shaped cataract, punctuate cataract, and the list continues on [4].

Congenital cataract shows considerable genotypic and phenotypic variations. This heterogeneity can be present within the same family, among different families, or even between two lenses of one single patient. According to previous reports, different mutations can lead to the same phenotype while different types of lens opacities can also arise from the same mutation [2,5]. So far, loci for genes leading to congenital cataract have been identified by linkage analysis across the genome, such as 1p36, 1q21-25, 2q33-36, 2p12, 3q21-22, 10q24-25, 11q22.3-23.1, 12q13, 13cen-q13, 15q21-22, 16q22.1, 17p13, 17q11-12, 17q24, 20p12-q12, q22.3, and 22q11.2 [6,7]. Recently, many of the genes participating in the formation of congenital cataract have been identified, and their coding proteins range from structural proteins (crystallins), lens membrane proteins (connexins and MIP), cytoskeleton proteins (CP49), transcriptional factors (gene symbol HSF4) to development-related proteins (gene symbol PITX3) [2,5]. In general, different genes may make an effect during specific phases of lens development. Early events in lens formation are usually associated with genes coding for transcription factors like PAX6, PITX3, MAF, and SOX. In the mature lens, mutations destroying the function of the lens membrane or the structural proteins of the lens fiber cells are more frequent [8].

Sutural cataract is defined as an opacity affecting the whole or part of the anterior or posterior suture of one or both eyes. Most sutural cataracts have been reported to be congenital without progression [9]. Cortical cataract is unusual in childhood and is restricted to the outer cortex [4]. Cortical cataract is seldom described as an autosomal dominant trait [7]. In this study, we mapped the locus to 3q22.1 by linkage analysis in a multi-generational Chinese family associated with an autosomal dominant congenital cataract, exhibiting progressive sutural cataract combined with cortical opacity. In this region, we performed direct sequencing of BFSP2 and found an E233del mutation in the exon 3 of this gene. This result has supplied further evidence for the heterogeneity of congenital cataract.

Methods

Clinical evaluation

The congenital cataract family, a five-generation family with 18 affected members, were collected in Heilongjiang province in northern China. Blood samples were obtained from the family members with informed consent acquired from 23 participants, which adhered to the tenets of the Declaration of Helsinki. DNA was isolated using a QIAampDNA mini kit (Qiagene, Hiaden, Germany).

Genotyping and linkage analysis

First, we performed a partial genome scan around the loci reported to be associated with congenital cataract. We chose 80 microsatellite markers altogether with three or four markers for each known locus. These microsatellite markers were analyzed by Multiplexed PCR. Microsatellites were first amplified by "Touch-down PCR" using a Thermocyclers (GeneAmp PCR system 9700, Applied Biosystems, Singapore), using fluorescently labeled primers (Perkin-Elmer, Applied Biosystems). Polymerase chain reaction (PCR) products were mixed with size standard (Genescan-400HD ROX, Perkin Elmer, Foster City, CA), denatured at 95 °C for 5 min, and electrophoresed using a 96-capillary automated DNA sequencer (MegaBACE 1000, Amersham, Freiburg, Germany). Second, data collection and analysis were fulfilled by genetic profiler v1.1. Genotyping was performed by Linkage Designer software v1.0. Finally, two-point linkage analysis was performed by the MLINK in the linkage program package. After the 80-marker partial genome-wide scan, 13 microsatellite markers were chosen for fine mapping after we obtained a positive LOD score at D3S1292 (Table 1). The haplotype was constructed using the Cyrillic program to define the borders of the cosegregating region.

Mutation analysis

Mutation analysis was carried out for all the seven exons of BFSP2 (GenBank NM_003571) in 3q22.1. Gene specific primers were designed to amplify the coding regions and splice sites of the transcript of the entire gene (Table 2). Genomic DNA from two affected and two unaffected individuals was amplified. Purified PCR products were sequenced bidirectionally with ABI BigDye Terminator Cycle Sequencing Kit v3.1 (Applied Biosystems, Foster City, CA). After being purified by standard isopropanol precipitation, the sequencing reaction products were resuspended in 10 μl of formamide (Hi-Di-Formamide, Applied Biosystems), denatured at 95 °C for 5 min, and electrophoresed on an automated DNA sequencer (3100 genetic analyzer, Perkin-Elmer, Applied Biosystems). Sequencing data were collected and analyzed with Sequence Scanner v1.0. Alignment with population genomic sequence information was performed by the Blast engine on the NCBI database.

Results

This five-generation family was recruited through an index case in a large obligatory medical care activity in Heilongjiang province, China (Figure 1). Tracing of the pedigree revealed a clear autosomal dominant inheritance with high penetrance of the disease. All family members dwell in rural Mingshui county in Heilongjiang province. The morphology was acquired through slit-lamp photography before cataract extraction. Imaging data combined with previously confirmed clinical diagnosis indicated bilateral progressive cataract with sutures and cortex affected. Clinical records demonstrate that the opacity happened after birth or during infancy, the severity among individuals tends to worsen with age (Figure 2, Figure 3). Lens opacity of elder patients exhibited more profoundly and seriously than younger individuals. In addition, no history of other ocular abnormality and systematic disorders were found in this family.

Of all the 80 microsatellite markers analyzed in this study, the two-point linkage analysis mapped the locus to D3S1292 in 3q22.1 with a positive LOD score Z_max=3.99 (θ=0.00; Table 1). The cosegregating region was located between D3S1551 to D3S3617 by haplotype analysis (Figure 4). Although we used different identities to symbolize the family members in the haplotype analysis compared with the pedigree, the result is definite that recombination events were found. Individual 12 shows recombination distal to marker D3S1551 while individual 13 shows recombination proximal to marker D3S3617. These critical recombination events have defined the disease gene to a region of 9.78 Mb between marker D3S1551 and D3S3617. As was reported in previous studies, BFSP2, which lies in this region, became a powerful candidate gene for this family. By direct sequencing, we identified an in-frame deletion mutation that cosegregated with the cataract phenotype in this family (Figure 5). Moreover, this anomaly was observed neither in the unaffected family members nor in the 100 unrelated control individuals, strongly suggesting that BFSP2 is the pathogenic agent in this family associated with progressing autosomal dominant congenital cataract.

Discussion

In this study, we found an E233del mutation in BFSP2 in a Chinese family living in northern China affected with an autosomal dominant congenital cataract. In our family, the phenotype takes on shape in a progressive way from early and mild sutural cataract to serious sutural cataract, and the cortical opacity can be seen in both phases. No progressive sign was seen in the cortical opacity. Morever, no other ocular or systemic disorders have been found. As was reported, cortical cataract is unusual in childhood and is seldom described as an autosomal dominant trait [4,7]. Most sutural cataracts have also been reported to be congenital without progression [9]. Our study observed a unique phenotype of cortical cataract accompanying progressive sutural cataract appearing in childhood and inherited in an autosomal dominant manner. This has not been previously reported in the study of BFSP2.

The causative gene associated with autosomal dominant congenital cataract has previously been located to BFSP2 on 3q22.1 in four families. One family had a nonconservative R287W mutation associated with juvenile-onset, progressive cataract, and the earliest sign of the phenotype was a general haze with a prominent suture. Of all the cases reported in this family, three had lamellar cataract, three had cortical cataract, two had nuclear cataract, and one had sutural cataract [10]. However, no sign of lamellar and nuclear phenotype was observed in our family. Morever, the mutation in our family happened in the conservative region of BFSP2 [11], which is quite different between these two families. The one other family shows nuclear, sutural, stellate, or spoke-like anterior and posterior subcapsular cortical cataracts with an E233del mutation in BFSP2, and the mildest sign is spoke-like cortical opacities with radially-oriented fine vacuoles [12]. The phenotype in this family is not progressive but more polymorphic compared with that of our family. Another family in southern China demonstrated solitary Y-sutural cataract accompanied by myopia with the same E233del mutation [13]. In this familiy, the sutural cataract forms in a nonprogressive way. Moreover, no myopia has been found in our family members. Since family members in our study have been living in Mingshui county, an isolated rural area in Heilongjiang province for several decades, no gene exchange could have happened between our group and these populations. At this point of view, it is strongly believed that the E233del mutation in our pedigree is an independent event and not a result of founder effect.

More recently, another family in northern China has identified the same E233del mutation associated with progressive sutural cataract [14]. However, we still hold the view that the E233del mutation in our pedigree is not a recurrent event. Our population is collected from an isolated rural part of northern China where communication with the outside is rare. Inquiries of the family history found no evidence of gene exchange between these two families. Therefore we think that the genetic background are different between these two families. The patients in Zhang's family show simple progressive sutural cataract without the cortex affected. Additionally, seriously affected sutures in our family members appear to be scalpel-like, which is different from the appearance of Zhang's family [14]. Analysis of all the affected families reveals a universal pattern that sutural cataract may be the early step of congenital cataract caused by a BFSP2 mutation, and the suture opacity can be evolved in a progressive way. Moreover, the suture and cortex seems to be most likely to be affected in congenital cataract caused by BFSP2 mutation. Until this study, no BFSP2 mutation has been reported to be associated with progressive sutural, together with cortical cataract. This result adds to the genotypic-phenotypic spectrum in congenital cataract caused by BFSP2 mutation.

Cataract is generally correlated to the destruction of the lens' microarchitectures [15]. Intermediate filaments (IF) as an important constituent of cytoskeleton has been proven to be a vital part to maintain lens lucidity. The affected gene product, phakinin, belongs to the IF protein family [16]. Phakinin has only one kind of transcript for all vertebrates except the chicken [17,18]. Two domains constitute the phakinin molecular: a central rod domain and an amino head domain of which the former is composed of runs of α-helix. During the first stage of IF assembly, the central rod domain formed a coiled-coil dimer through the alignment of two α-helix strands [12]. Homology analysis has made it clear that the central rod domain is highly conserved in mammals like humans, rats, mice, and horses, is essential for the phakinin biological function, and plays an important role in IF assembly. The deleted glutamic acid (E) residual is located at the B-coil of the central rod domain, which affects the coiled-coil formation of the phakinin tertiary structure [11,15,19]. The mouse model has provided further evidence for the association of phakinin with the function of lens cells. The targeted deletion of BFSP2 in mice leads to the disassembly of beaded filaments and progressive light scattering, which indicates the fundamental alteration of lens fiber cells and its function [20]. Natural mutation in mouse 129X1/SvJ strains also mimicked similar cytoskeleton changes to phakinin knockout mice. Although no cataract phenotype in the mouse model is yet observed, the close relationship between phakinin and lens function is undeniable. We can reasonably presume the possible existence of other genetic or even environmental factors that may be involved in cataract manifestation caused by the dysfunction of phakinin [21].

More and more studies have indicated the interplay between phakinin and other components in lens fiber cells. In vitro studies have indicated that phakinin and filensin can assemble and constitute the basic microstructure unit of beaded filament (a cytoskeletal element) [15,22,23]. Vimentin, another member of cytoskeleton proteins, also has connection with this assembly by providing a scaffold for beaded filament components. Furthermore, the distribution of phakinin/filensin complex is in line with that of the vimentin in lens cells [21]. Apart from this, other elements in lens cells like α-crystallin also take part in the beaded filament network by cooperating with phakinin [8]. All these studies indicate that phakinin is in close relationship with many other factors in lens cells and may have functional crosstalk with them. Although BFSP2 knock out mice have presented no evident visible changes to lens cells under microscope, filensin was reported to be sharply reduced in the same mouse model [20]. Due to the insolubility feature of filensin, the absence of phakinin will undoubtedly lead to the disassembly of phakinin/filensin complex, which will result in the instability of filensin and ultimately cause the accumulation of insoluble filensin in lens cells. Similarly, point mutations in phakinin, especially those at the central domain, may also cause abortive assembly and successive accumulation of insoluble materials [11,21]. All these experimental facts have suggested that the transparency and integrity of lens fiber cells actually is not modulated solely by phakinin but by a more complicated network, which calls for further mechanistic studies.

In summary, we report a progressive autosomal dominant congenital cataract in a multi-generational Chinese family. The causative gene was mapped to BFSP2 on 3q22.1. This outcome further accentuates the understanding of the heterogeneity of phenotype to genotype association in congenital cataractogenesis and facilitates more evidence to the previous finding of BFSP2 function. Since no animal model with a BFSP2 mutation has mimicked the phenotype as reported, more investigation is needed to learn the role of BFSP2 in cataract formation.

Acknowledgements

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