A novel deletion variant of γD-crystallin responsible for congenital nuclear cataract

Purpose: To investigate a novel deletion variant of γD-crystallin (CRYGD) identified in a Chinese family with nuclear congenital cataract. Methods: A Chinese family with five affected members diagnosed with nuclear cataract and four unaffected members were recruited for the mutational screening of 15 known candidate genes for autosomal dominant congenital cataract. Two-point linkage analysis with single nucleotide polymorphism markers and microsatellite markers flanking these genes together with direct sequencing was applied to identify the disease-causing mutation. Recombinant NH₂-terminal FLAG-tagged wildtype or mutant γD-crystallin was expressed in COS-7 cells. The expression pattern, protein solubility and intracellular distribution were analyzed by western blotting and confocal double immunofluorescence. Results: Linkage analysis located the candidate region in the γC-crystallin and γD-crystallin gene cluster. Direct sequencing identified a c.494delG in CRYGD, which cosegregated with the disease in all affected members. Neither the unaffected family members nor the 103 unrelated controls carried this deletion mutation, which causes a frameshift and an early termination of polypeptide to become G165fs. A significantly reduced solubility was observed for this mutant. Unlike wildtype γD-crystallin, which existed in both the nucleus and cytoplasm, G165fs was colocalized with lamin A/C on the nuclear envelope. Conclusions: We have identified a novel mutation, c.494delG, in CRYGD, which was associated with nuclear cataract. This is the first deletion mutation of CRYGD found to cause autosomal dominant congenital cataract. The mutant protein with loss of solubility and localization to the nuclear envelope is hypothesized to impair nuclear transfiguration and degradation in lens fiber cell differentiation, leading to opacity formation during lens development.