Eye phenotypes were investigated in Le-CreTg/-; Pax6fl/+ 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-CreTg/- and heterozygous Pax6fl/+ 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-CreTg/-; Pax6+/+ controls (without a floxed Pax6fl allele) as well as experimental Le-CreTg/-; Pax6fl/+ mice. However, no abnormalities were seen in Le-Cre-/-; Pax6fl/+ or Le-Cre-/-; Pax6+/+ controls (without the Le-Cre transgene). The severity and frequency of the eye abnormalities in Le-CreTg/-; Pax6+/+ control mice diminished after backcrossing Le-CreTg/- 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-CreTg/-; 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-CreTg/-; Pax6+/+ mice, in the absence of a floxed allele, demonstrates the importance of including all the relevant genetic controls in Cre-loxP experiments.
Bibliographical noteFunding: This work was supported the Wellcome Trust (http://www.wellcome.ac.uk/); grant reference 088876/Z/09/Z. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
This paper is dedicated to the memory of our friend and colleague, Dr. Thaya Ramaesh, who taught us so much and died far too young. She is sorely missed by us all. We thank Paul Devenney for technical help including genotyping mice, staff at BRR, University of Edinburgh, for specialised technical services, Joseph Russell for help with analysis of the E12.5P10 developmental series and Ronnie Grant for help with preparation of figures. We thank Dr Ruth Ashery-Padan and Prof Peter Gruss (Max Plank Institute for Biophysical Chemistry, Eye abnormalities in Le-Cre Mice PLOS ONE | www.plosone.org 17 October 2014 | Volume 9 | Issue 10 | e109193 Goettingen, Germany), Prof David Price University of Edinburgh, UK) and Prof Veronica van Heyningen and Dr Dirk A. Kleinjan (MRC Human Genetics Unit, Edinburgh) for kindly providing founder stocks of Le-CreTg/2, Pax6fl/+ , Z/AP and PAX77Tg/2 mice.