A new mutation in BFSP2 (G1091A) causes autosomal dominant congenital lamellar cataracts

Abstract

Purpose: We sought to identify the genetic defect in a four-generation Chinese family with autosomal dominant congenital lamellar cataracts and demonstrate the functional analysis with biosoftware of a candidate gene in the family.

Methods: Family history data were recorded. Clinical and ophthalmologic examinations were performed on family members. All the members were genotyped with microsatellite markers at loci considered to be associated with cataracts. Two-point LOD scores were calculated by using the Linkage Software after genotyping. A mutation was detected by using gene-specific primers in direct sequencing. Wild type and mutant proteins were analyzed with Online Bio-Software.

Results: Affected members of this family had lamellar cataracts. Linkage analysis was obtained at markers D3S2322 (LOD score [Z]=7.22, recombination fraction [theta]=0.0) and D3S1541 (Z=5.42, theta=0.0). Haplotype analysis indicated that the cataract gene was closely linked to these two markers. Sequencing the beaded filament structural protein 2 (BFSP2) gene revealed a G>A transversion in exon 5, which caused a conservative substitution of Arg to His at codon 339 (P.R339H). This mutation cosegregated with the disease phenotype in all affected individuals and was not observed in the unaffected family members or in 100 normal, unrelated individuals. Bioinformatic analyses showed that a highly conserved region was located around Arg339. Data generated with Online Bio-Software revealed that the mutation altered the protein's hydrophobicity, hydrophobic moment, and chaperone and regulation activities.

Conclusions: This is the first reported case of a congenital lamellar cataract phenotype associated with the mutation of Arg339His (P.R339H) in BFSP2. It highlights the physiologic importance of the beaded filament protein and demonstrates a possible mechanism of action for the mutant gene.

Figure 1

Slit lamp photographs of proband. The photographs of the proband (IV:25) showed that the opacities were lamellar cataracts.

Figure 2

Pedigree and haplotype of cataract family. Four-generation pedigree, with autosomal dominant lamellar cataracts. Haplotyping shows segregation of two microsatellite markers on 3q21-q25. Squares and circles indicate males and females, respectively. Blackened symbols and bars denote affected status.

Figure 3

Multiple-sequence alignment and DNA sequence chromatograms of BFSP2. A: The multiple-sequence alignment of BFSP2 from primates, rodents, cattle, and zebrafish to humans (Homo sapiens) is shown. The Arg339 residue is located within a highly conserved region. B: DNA sequence chromatograms of the P.R339H mutation in BFSP2 are shown. The G→A transversion at position 1091 resulted in the P.R339H mutation.

Figure 4

Denaturing high-performance liquid chromatography results of wild type and mutated BFSP2. DHPLC results show a variant trace for BFSP2 compared to the wild type (WT) trace. The profile in purple is the mutant protein. The profile in blue is the wild type protein.

Figure 5

Comparison of wild type and mutant BFSP2 3D structures. A: Wild type BFSP2 3D structure using Phyre software is shown, and Arg339 is indicated in yellow and a green arrow. B: Mutant BFSP2 3D structure is shown also using Phyre software, and His339 is indicated in yellow and a green arrow. The 3D structure was not overly changed due to the mutation.

Figure 6

Comparison of hydrophobicity between wild type and mutant BFSP2. BioEdit software predicted the effect of the substitution on BFSP2 protein hydrophobicity. Hydrophobicity of the mutant protein increased between 330 amino acids and 340 amino acids.

Figure 7

Comparison of hydrophobic moment between wild type and mutant BFSP2. BioEdit software predicted the effect of the substitution on the BFSP2 protein hydrophobic moment. The hydrophobic moment of the mutant protein decreased between 330 amino acids and 350 amino acids.

Figure 8

The predicted effect of substitution in BFSP2. The online Radar software was used to predict the effect of the substation in BFSP2. The substitution would have in the wild-type protein with an increase from two to three repeats, the part marked by red arrow turned into two parts.