Chromosomal locations of the genes for the beaded filament proteins CP 115 and CP 47

A novel p.G112E mutation in BFSP2 associated with autosomal dominant pulverulent cataract with sutural opacities

PURPOSE

To identify the genetic defect in a Chinese family with bilateral pulverulent sutural cataract.

MATERIALS AND METHODS

A three-generation family with congenital cataract was recruited in the study. The study protocol followed the principles of the Declaration of Helsinki. Detailed family history and clinical data were recorded. Genomic DNA was extracted from peripheral blood leukocytes. Candidate gene sequencing was performed to identify the disease-causing mutation. The effects of amino acid changes on the structure and function of proteins were predicted by bioinformatics analysis.

RESULTS

All affected individuals presented pulverulent opacities in the embryonal nucleus and sutures. Direct candidate gene sequencing revealed a heterozygous c. 335 G>A variation in the beaded filament structural protein 2(BFSP2) gene, which resulted in the replacement of a highly conserved glycine by glutamic at codon 112 (p. G112E). Haplotype analysis indicated that the affected members shared a common haplotype with markers near BFSP2. This mutation co-segregated with all affected individuals and was not observed in unaffected members or in 120 ethnically matched controls. Bioinformatic analyses confirmed that the mutation altered the hydrophobic and secondary structure of the protein around the substitution site.

CONCLUSIONS

We report a novel mutation (p.G112E) in the BFSP2 gene, underscoring the physiological importance of the beaded filament protein and supporting its role in human cataract formation.

Molecular characteristics of inherited congenital cataracts

Congenital cataracts are a major cause of induced blindness in children, and inherited cataracts are the major cause of congenital cataracts. Inherited congenital cataracts have been associated with mutations in specific genes, including those of crystallins, gap junction proteins, membrane transport and channel proteins, the cytoskeleton, and growth and transcription factors. Locating and identifying the genes and mutations involved in cataractogenesis are essential to gaining an understanding of the molecular defects and pathophysiologic characteristics of inherited congenital cataracts. In this review, we summarize the current research in this field.

Functions of the intermediate filament cytoskeleton in the eye lens

Intermediate filaments (IFs) are a key component of the cytoskeleton in virtually all vertebrate cells, including those of the lens of the eye. IFs help integrate individual cells into their respective tissues. This Review focuses on the lens-specific IF proteins beaded filament structural proteins 1 and 2 (BFSP1 and BFSP2) and their role in lens physiology and disease. Evidence generated in studies in both mice and humans suggests a critical role for these proteins and their filamentous polymers in establishing the optical properties of the eye lens and in maintaining its transparency. For instance, mutations in both BFSP1 and BFSP2 cause cataract in humans. We also explore the potential role of BFSP1 and BFSP2 in aging processes in the lens.

Mapping of the human CP49 gene and identification of an intragenic polymorphic marker to allow genetic linkage analysis in autosomal dominant congenital cataract

The CP49 protein is an intermediate filament protein expressed specifically in the lens fibre cells of the lens, where it is an important cytoplasmic structural component. Dominant-negative mutations in other intermediate filament proteins, such as keratins, cause disorders characterised by dense cytoplasmic aggregates in specific cell types. The CP49 gene is therefore a good candidate for dominantly inherited forms of cataract. To allow genetic linkage analysis of families with autosomal dominant cataract with respect to CP49, a highly polymorphic intragenic microsatellite marker for this gene has been developed. In addition, both low and high resolution radiation hybrid mapping of the CP49 gene has been completed, placing it very close to microsatellite marker D3S1290 on human chromosome 3q. Furthermore, using the intragenic CP49 microsatellite, linkage was excluded in four families with genetically uncharacterized forms of autosomal dominant congenital cataract.

A juvenile-onset, progressive cataract locus on chromosome 3q21-q22 is associated with a missense mutation in the beaded filament structural protein-2

Juvenile-onset cataracts are distinguished from congenital cataracts by the initial clarity of the lens at birth and the gradual development of lens opacity in the second and third decades of life. Genomewide linkage analysis in a multigenerational pedigree, segregating for autosomal dominant juvenile-onset cataracts, identified a locus in chromosome region 3q21.2-q22.3. Because of the proximity of the gene coding for lens beaded filament structural protein-2 (BFSP2) to this locus, we screened for mutations in the coding sequence of BFSP2. We observed a unique C-->T transition, one that was not observed in 200 normal chromosomes. We predicted that this led to a nonconservative R287W substitution in exon 4 that cosegregated with cataracts. This mutation alters an evolutionarily conserved arginine residue in the central rod domain of the intermediate filament. On consideration of the proposed function of BFSP2 in the lens cytoskeleton, it is likely that this alteration is the cause of cataracts in the members of the family we studied. This is the first example of a mutation in a noncrystallin structural gene that leads to a juvenile-onset, progressive cataract.

Autosomal-dominant congenital cataract associated with a deletion mutation in the human beaded filament protein gene BFSP2

Congenital cataracts are a common major abnormality of the eye that frequently cause blindness in infants. At least one-third of all cases are familial; autosomal-dominant congenital cataract appears to be the most-common familial form in the Western world. Elsewhere, in family ADCC-3, we mapped an autosomal-dominant cataract gene to chromosome 3q21-q22, near the gene that encodes a lens-specific beaded filament protein gene, BFSP2. By sequencing the coding regions of BFSP2, we found that a deletion mutation, DeltaE233, is associated with cataracts in this family. This is the first report of an inherited cataract that is caused by a mutation in a cytoskeletal protein.

The eye lens cytoskeleton

During lens cell differentiation there are a number of very characteristic morphological changes that occur. These include a 50- to 100-fold increase in cell length as the equatorial lens epithelial cells differentiate into fibre cells and the loss of the cellular organelles such as mitochondria, nuclei, Golgi apparatus and endoplasmic reticulum. Coincident with these changes are dramatic alterations in the organisation of the lens fibre cell cytoskeleton and in particular the lens-specific intermediate filament network comprising CP49 and filensin. Cell shape and cell polarisation as well as tissue integrity are all processes that depend upon the cytoskeleton and are therefore important to the lens. The unique aspects of the lenticular cytoskeleton are the subject of this review.

Primary sequence, secondary structure, gene structure, and assembly properties suggests that the lens-specific cytoskeletal protein filensin represents a novel class of intermediate filament protein

The ocular lens fiber cell assembles a novel cytoskeletal element, the Beaded Filament, from CP49 and filensin, two proteins expressed only in the differentiated lens fiber cell. We report the primary sequence, secondary structural analysis, gene structure and Yeast Two Hybrid interaction data for human filensin, and develop a consensus model of filensin from the human and previously reported bovine and chicken filensin sequences. This consensus model, combined with gene structure and Yeast Two Hybrid studies establish that filensin is a member of the Intermediate Filament family of proteins. Specifically, filensin exhibits (1) divergence at amino acid sequence motifs otherwise highly conserved among intermediate filament proteins, (2) a loss of 29 amino acids from the central rod domain which is unique among cytoplasmic intermediate filament proteins, (3) an absence of sequence identity with any existing class of intermediate filament protein, (4) a gene structure unique among intermediate filament family, (5) an inability to dimerize with representatives of Type I, II, and III intermediate filament proteins. Thus, at each level of analysis, we find that filensin is similar to the consensus model of intermediate filament proteins, supporting our conclusion that filensin's relatedness to the IF family is not the consequence of convergent evolution. However, filensin also shows unique or extreme distinctions from the consensus intermediate filament protein at each level of analysis, indicating that filensin constitutes a novel class of IF protein. Some of filensin's unique features are incompatible with current models of IF assembly. Analysis of filensin gene structure suggests that the 29 amino acid reduction in the central rod domain was not the result of a single splice site mutation, the mechanism suggested for the transition between nuclear lamins and cytoplasmic intermediate filament proteins.

The chicken CP49 gene contains an extra exon compared to the human CP49 gene which identifies an important step in the evolution of the eye lens intermediate filament proteins

The gene structure for chicken CP49 gene is presented. It differs from the human CP49 gene with the presence of an extra exon in helix IB and the apparent loss of an intron, intron H. The CP49 gene localises to chromosome 2 in the chicken genome where it is flanked by homologues that map to human chromosome 10p13 (VIM) 6p24-p23 (BMP6). Two transcripts, CP49 and CP49ins, are produced from the single chicken CP49 gene. The difference is a 49-amino-acid insertion in helix IB of CP49 that is encoded by a novel exon found in the chicken CP49 gene. An extended helix IB is believed to be a characteristic of the ancestral intermediate filament protein as it is found in many invertebrate intermediate filament proteins but has been lost from all vertebrate intermediate filament proteins except the nuclear lamins. Although the intron position and length of the helix IB insert sequences in CP49ins differ to those found both in the invertebrate intermediate filament proteins and the vertebrate lamins, the CP49 gene is the first vertebrate cytoplasmic intermediate filament protein to be described with an extended helix IB. The chicken CP49 gene is also the first where differential splicing can remove such a feature. Human and bovine CP49 appear to have lost the helix IB insert sequences, and so the avian CP49 gene provides an interesting evolutionary link between the eye lens proteins and the ancestral intermediate filament protein.

Gene structure and cDNA sequence identify the beaded filament protein CP49 as a highly divergent type I intermediate filament protein

The fiber cell of the vertebrate ocular lens assembles a cytoskeletal structure, the beaded filament, which contains two proteins unique to the fiber cell: CP49 (phakinin) and CP115/CP95 (filensin). We report here the complete primary sequence and gene structure for human CP49. These data show that CP49 is a member of the intermediate filament family, but highly unusual in several regards. 1) CP49 primary sequence does not permit unambiguous assignment to any existing class of intermediate filament protein, but exhibits a gene structure that is identical to the Type I cytokeratins. 2) CP49 essentially lacks one of the three major domains that characterize all intermediate filament proteins, the carboxyl-terminal tail domain. 3) CP49 shows substitutions at 3 of 4 residues in the otherwise highly conserved intermediate filament protein motif LNDR. Notably, this divergence includes an Arg to Cys substitution that has only been observed in the mutant human cytokeratin K14, a mutation shown to cause the skin blistering seen in the genetic disorder Dowling-Meara epidermolysis bullosa simplex.