Ethylnitrosourea-Induced Base Pair Substitution Affects Splicing of the Mouse γE-Crystallin Encoding Gene Leading to the Expression of a Hybrid Protein and to a Cataract

A novel ENU-induced mutation in the mouse leading to a nuclear and cortical opacity of the eye lens (ENU418) was mapped to proximal chromosome 1 by a genome-wide mapping approach. It suggests that the cluster of γ-crystallin encoding genes (Cryg) and the βA2-crystallin encoding gene Cryba2 are excellent candidate genes. An A → G exchange in the middle of intron 1 of the Cryge gene was found as the only alteration cosegregating with the cataractous phenotype. The mutation was confirmed by the presence of a novel restriction site for ApaI in the corresponding genomic DNA fragment. The mutation represses splicing of intron 1; the additional 92 bp in the corresponding cDNA leads to a frameshift and the expression of a novel hybrid protein containing 3 amino acids of the γE-crystallin at the N terminus, but 153 novel amino acids. The CrygeENU418 protein has a calculated molecular mass of ∼15.6 kD and an alkaline isoelectric point (pH 10.1) and is predicted to have two hydrophobic domains. Western blot analysis using a polyclonal antibody against the hydrophilic C-terminal part of the CrygeENU418-specific protein demonstrated its stable expression in the cataractous lenses; it was not found in the wild types. Histological analysis of the cataractous lenses indicated that the expression of the new protein disrupts the cellular structure of the eye lens.