Hemizygous Le-Cre transgenic mice have severe eye abnormalities on some genetic backgrounds in the absence of LoxPSites

Eye phenotypes were investigated in Le-Cre^Tg/-; Pax6^fl/+ mice, which were expected to show tissue-specific reduction of Pax6 in surface ectoderm derivatives. To provide a better comparison with our previous studies of Pax6^+/- eye phenotypes, hemizygous Le-Cre^Tg/- and heterozygous Pax6^fl/+ mice were crossed onto the CBA/Ca genetic background. After the Le-Cre transgene had been backcrossed to CBA/Ca for seven generations, significant eye abnormalities occurred in some hemizygous Le-Cre^Tg/-; Pax6^+/+ controls (without a floxed Pax6^fl allele) as well as experimental Le-Cre^Tg/-; Pax6^fl/+ mice. However, no abnormalities were seen in Le-Cre^-/-; Pax6^fl/+ or Le-Cre^-/-; Pax6^+/+ controls (without the Le-Cre transgene). The severity and frequency of the eye abnormalities in Le-Cre^Tg/-; Pax6^+/+ control mice diminished after backcrossing Le-Cre^Tg/- mice to the original FVB/N strain for two generations, showing that the effect was reversible. This genetic background effect suggests that the eye abnormalities are a consequence of an interaction between the Le-Cre transgene and alleles of unknown modifier genes present in certain genetic backgrounds. The abnormalities were also ameliorated by introducing additional Pax6 gene copies on a CBA/Ca background, suggesting involvement of Pax6 depletion in Le-Cre^Tg/-; Pax6^+/+ mice rather than direct action of Cre recombinase on cryptic pseudo-loxP sites. One possibility is that expression of Cre recombinase from the Pax6-Le regulatory sequences in the Le-Cre transgene depletes cofactors required for endogenous Pax6 gene expression. Our observation that eye abnormalities can occur in hemizygous Le-Cre^Tg/-; Pax6^+/+ mice, in the absence of a floxed allele, demonstrates the importance of including all the relevant genetic controls in Cre-loxP experiments.