Purpose: Hereditary cataract is a clinically and genetically heterogeneous lens disorder that usually presents as a sight-threatening trait in childhood. The purpose of this study was to map and identify the mutation underlying an autosomal dominant form of coral-shaped cataract segregating in a three generation Caucasian pedigree.
Methods: Genomic DNA was prepared from blood leucocytes, genotyping was performed using microsatellite markers, and LOD scores were calculated using the LINKAGE programs. Mutation detection was performed using direct sequencing and primer extension analysis. Following site-directed mutagenesis, mutant and wild type expression constructs were transfected into a human lens epithelial cell line (HLE B-3) and recombinant protein was detected by immunoblotting, imunofluorescence, and immunogold microscopy. Cell death was monitored by fluorescence activated cell sorting.
Results: Significant evidence of linkage was detected at markers D2S371 (LOD score [Z]=3.81, recombination fraction [theta]=0) and D2S369 (Z=3.64, theta=0). Haplotyping indicated that the disease gene lay in the approximate 10 Mb physical interval between D2S1384 and D2S128, containing the gamma-crystallin gene (CRYGA-CRYGD) cluster on chromosome 2q33.3-q34. Sequencing of the CRYGA-CRYGD cluster identified a C->A transversion in exon 2 of CRYGD that was predicted to result in the non-conservative substitution of threonine for proline at amino-acid residue 23 (P23T) in the processed CRYGD protein. Transfection studies suggested that the P23T mutant was less soluble than its wild type counterpart when expressed in HLE B-3 cells.
Conclusions: This study has identified an eighth type of cataract morphology associated with CRYGD and suggests that a CRYGD mutation may underlie the historically important "coralliform" cataract first reported in 1895.