Dosage requirement and allelic expression of <i>PAX6</i> during lens placode formation

Pax6 is a member of the mammalian Pax transcription factor family. Many of the Pax genes display semi-dominant loss-of-function heterozygous phenotypes, yet the underlying cause for this dosage requirement is not known. Mice heterozygous for Pax6 mutations exhibit small eyes (Sey) and in embryos the most obvious defect is a small lens. We have studied lens development in Pax6(Sey)(-1Neu)/+ embryos to understand the basis of the haploinsufficiency. The formation of the lens pre-placode appears to be unaffected in heterozygotes, as deduced from the number of cells, the mitotic index, the amount of apoptosis and the expression of SOX2 and Pax6 in the pre-placode. However, the formation of the lens placode is delayed. The cells at the edge of the lens cup fail to express N-cadherin and undergo apoptosis and the lens fails to detach completely from the surface ectoderm. After formation, the lens, which has 50% of the cells found in wild-type embryos, grows at a rate that is indistinguishable from wild type. We rule out the possibility that monoallelic expression of Pax6 at the time of lens placode formation accounts for the 50% reduction in cell number by showing that expression of Pax6 is biallelic in the lens placode and optic vesicle. We propose instead that a critical threshold of PAX6 protein is required for lens placode formation and that the time in development at which this level is reached is delayed in heterozygotes.