Background: Altered dosage of the transcription factor PAX6 causes multiple human eye pathophysiologies. PAX6 +/- heterozygotes suffer from aniridia and aniridia-related keratopathy (ARK), a corneal deterioration that probably involves a limbal epithelial stem cell (LESC) deficiency. Heterozygous Pax6 +/Sey-Neu (Pax6 +/-) mice recapitulate the human disease and are a good model of ARK. Corneal pathologies also occur in other mouse Pax6 mutants and in PAX77 Tg/- transgenics, which over-express Pax6 and model human PAX6 duplication.
Methodology/Principal Findings: We used electron microscopy to investigate ocular defects in Pax6 +/- heterozygotes (low Pax6 levels) and PAX77 Tg/- transgenics (high Pax6 levels). As well as the well-documented epithelial defects, aberrant Pax6 dosage had profound effects on the corneal stroma and endothelium in both genotypes, including cellular vacuolation, similar to that reported for human macular corneal dystrophy. We used mosaic expression of an X-linked LacZ transgene in X-inactivation mosaic female (XLacZ Tg/-) mice to investigate corneal epithelial maintenance by LESC clones in Pax6 +/- and PAX77 Tg/- mosaic mice. PAX77 Tg/- mosaics, over-expressing Pax6, produced normal corneal epithelial radial striped patterns (despite other corneal defects), suggesting that centripetal cell movement was unaffected. Moderately disrupted patterns in Pax6 +/- mosaics were corrected by introducing the PAX77 transgene (in Pax6 +/-, PAX77 Tg/- mosaics). Pax6 Leca4/+, XLacZ Tg/- mosaic mice (heterozygous for the Pax6 Leca4 missense mutation) showed more severely disrupted mosaic patterns. Corrected corneal epithelial stripe numbers (an indirect estimate of active LESC clone numbers) declined with age (between 15 and 30 weeks) in wild-type XLacZ Tg/- mosaics. In contrast, corrected stripe numbers were already low at 15 weeks in Pax6 +/- and PAX77 Tg/- mosaic corneas, suggesting Pax6 under- and over-expression both affect LESC clones.
Conclusions/Significance: Pax6 +/- and PAX77 Tg/- genotypes have only relatively minor effects on LESC clone numbers but cause more severe corneal endothelial and stromal defects. This should prompt further investigations of the pathophysiology underlying human aniridia and ARK.