First mutation in the βA2-crystallin encoding gene is associated with small lenses and age-related cataracts.

PURPOSE A new mouse mutant with small lenses was identified within a mutagenesis screen. The aim of the study was to determine its molecular and morphologic characterization. METHODS The offspring of paternally N-ethyl-N-nitrosourea (ENU)-treated C57BL/6J mice were analyzed for eye-size parameters by noninvasive in vivo laser interference biometry. RESULTS A new mutant characterized by a clear, but significantly smaller lens without any changes for cornea thickness, anterior chamber depth, or aqueous humor size, was identified. The smaller size of the lens was more pronounced in the homozygous mutants, which were fully fertile and viable. The mutation was mapped to chromosome 1 between the markers D1Mit251 and D1Mit253. Using a positional candidate approach, the βA2-crystallin encoding gene Cryba2 was sequenced; a T→C exchange at cDNA position 139 led to a p.S47P amino-acid alteration. The eyes of newborn homozygous mutants showed no gross changes. At the age of three weeks, some clefts appeared at the cornea, but the lens and retina appeared without major changes. At the age of 25 weeks, the lenses of the heterozygous mutants develop a subcapsular cortical cataract, but the lenses of homozygous mutants were completely opaque. CONCLUSIONS These findings demonstrate the first mutation in the Cryba2 gene. In contrast to the closely linked Cryg gene cluster, no congenital cataract mutation could be attributed to the Cryba2 gene. Therefore, the human CRYBA2 gene should be considered as a strong candidate gene for age-related cataracts, and the slightly smaller size of the lens might be recognized as an early biomarker for age-related cataracts.