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Novel Allele of Crybb2 in the Mouse and Its Expression in the Brain

¹From the Institutes of Developmental Genetics, ³Human Genetics, and ⁴Experimental Genetics, Helmholtz Center Munich, German Research Center for Environmental Health, Neuherberg, Germany; and the Institutes of ⁵Experimental Genetics, ⁶Developmental Genetics, and ⁷Genetics, Technical University Munich, Center of Life and Food Sciences Weihenstephan, Weihenstephan, Germany. ²Present affiliation: Institute of Inhalation Biology, Helmholtz Center Munich, German Research Center for Environmental Health, Neuherberg, Germany.

PURPOSE. O377 was identified as a new dominant cataract mutation in mice after radiation experiments. The purpose of this study was to genetically characterize the mutation and to analyze its biological consequences.

METHODS. Linkage analysis of the O377 mouse mutant was performed; candidate genes including Crybb2 were sequenced. The authors analyzed eyes and brains of the mutants by histology and the expression domains of Crybb2 by in situ hybridization and immunohistochemistry. RNA was isolated from whole brains of heterozygous and homozygous O377 mutants, and differential expression arrays were performed. All studies were compared with age- and strain-matched wild-type mice.

RESULTS. The mutation was mapped to chromosome 5 and characterized as an AT substitution at the end of intron 5 of the Crybb2 gene. It led to alternative splicing with a 57-bp insertion in the mRNA and to 19 additional amino acids in the protein. In the brain, βB2-crystallin was expressed in the cerebellum, olfactory bulb, cerebral cortex, and hippocampus. The only morphologic difference in the brain is the increased number of Purkinje cells in the cerebellum of homozygous strain-matched mutants. Differential expression analysis revealed the upregulation of calpain-3 in the brain of homozygous mutants, which was confirmed by quantitative real-time PCR.

CONCLUSIONS. These results confirm the third allele of Crybb2 in the mouse that also affected exon 6 and the fourth Greek key motif. Moreover, expression analysis of Crybb2 identified for the first time distinct regions of expression in the brain, and the differential expression analysis points to the participation of Ca²⁺ in the corresponding pathologic processes.