Homozygosity for the Le–Cre Transgene is Associated With Microphthalmia and Cataracts in the Absence of Other Engineered Genetic Changes

Abstract

Abstract: : Purpose: Le–Cre mice were previously produced with a transgene construct containing the mouse Pax6 P0 promoter upstream of Cre recombinase (Ashery–Padan et al., 2000 Genes & Dev 14:2701–2711). These mice are a valuable tool to delete loxP flanked alleles in the surface ectoderm–derived ocular tissues and endocrine pancreas. We noticed a proportion of mice with microphthalmia and cataracts in our breeding colony of Le–Cre transgenic animals in the absence of any loxP flanked alleles. The purpose of this investigation was to determine if these eye defects were a characteristic of the Cre transgene or a mutation unlinked to the transgene. Methods:Microphthalmic Le–Cre transgenic mice were analyzed histologically. Le–Cre transgenic mice on two different genetic backgrounds were obtained from two different sources. Le–Cre mice on both a mixed genetic background and an FVB/N inbred genetic background were indpendently crossed to determine segregation of the microphthalmic trait. Results: When hemizygous Le–Cre mice on the mixed genetic background were intercrossed approximately 25% of the pups exhibited microphthalmia and cataracts. When microphthalmic mice on the mixed background were crossed to non–transgenic littermates, all resulting pups were transgenic and none exhibited microphthalmia (N= 62). When microphthalmic Le–Cre mice on the mixed genetic background were intercrossed, all resulting pups were transgenic and microphthalmic N= 7. In contrast, when known hemizygous Le–Cre mice an inbred FVB/N genetic background were intercrossed, only 2 of 30 pups exhibited microphthalmia and cataracts. Lenses from microphthalmic mice were smaller than wildtype and exhibited epithelial disruptions and abnormal fiber differentiation at birth and these degenerative changes progressed with age. Conclusions: These findings are consistent with microphthalmia and cataracts being the direct result of homozygosity for the Le–Cre transgene or for a linked spontaneous mutation. While histologically similar microphthalmia and cataracts were seen in Le–Cre mice from both genetic backgrounds, alleles present in the mixed background appear more permissive for the penetrance of the microphthalmic trait. While Le–Cre transgenic mice have great utility for revealing the role of specific genes in the eye, homozygosity for the locus containing the transgene can result in an independent ocular phenotype.

Keywords: genetics • transgenics/knock-outs • gene modifiers