Mouse models of forebrain defects caused by Pax6 haploinsufficiency

点击翻译按钮获取中文摘要

In the human population, ~1/50,000 live births suffer from a condition called congenital aniridia, which results from mutations that disrupt the function of a gene called PAX6. This gene, which codes for a protein that is a high-level regulator of the activity of other genes, is expressed in developing eye and brain and is critical for their normal formation. Here, we are interested in understanding the consequences of the loss of function of one copy of PAX6 for development of the most advanced part of the brain, the forebrain. If both copies of the PAX6 gene are mutated so that there is no functional PAX6 at all, the result is fatal before or shortly after birth and there are severe abnormalities of the brain. Patients with mutations causing one copy of PAX6 to lose function have normal life expectancy but show abnormalities of brain structure and function. These abnormalities include a reduced volume of the major structure that links the two hemispheres of the brain, the corpus callosum, and a range of neurological and psychiatric defects. It is well-recognized that mutant mice that have lost Pax6 gene function provide excellent models of at least some of the defects found in congenital aniridia. Heterozygous mutant mice (i.e. mutants in which just one of the two copies of Pax6 is abnormal, designated Pax6+/-) are the best mouse models for congenital aniridia, since humans surviving with the condition are heterozygous for PAX6 loss-of-function (PAX6+/-). Whereas defects of the eye, olfactory bulbs and specific hindbrain structures have been found before in Pax6+/- mice, until now forebrain defects that model those in human patients have not been identified. We now have new pilot data showing that heterozygous Pax6 mutations in mice cause specific forebrain defects. These findings are potentially very relevant for understanding human congenital aniridia. We have found: (i) A significant narrowing of the corpus callosum in adult Pax6+/- mice, a result that fits well with observations of corpus callosum abnormalities in heterozygous PAX6+/- humans with congenital aniridia. (ii) An increased density of a set of Pax6+/- cells of a specific type (cells that activate, or in their past activated, a gene called Zic4: i.e. so-called Zic4-lineage cells) in a region of the forebrain, known as the geniculate nuclei, that connects with the cerebral cortex; studies of corresponding defects in PAX6+/- humans have not yet been reported. Our objective in this proposal is to gain a better understanding of these forebrain defects and their causes in mice, with a view to improving our knowledge of the forebrain abnormalities associated with human congenital aniridia.In the first part, we shall study callosal defects in Pax6+/- mice, their age of onset, their cellular causes and whether Pax6+/- mice show behavioural abnormalities that have been linked by previous authors with callosal abnormalities. In the second part, we shall study the effects of mutating one copy of Pax6 on the development of Zic4-lineage cells in the geniculate nuclei. We shall first study the effects of mutating Pax6 specifically in Zic4-lineage cells on the postnatal development of these cells and their connections to the cortex. We shall then study what happens to Zic4-lineage cells in mice carrying one mutant copy of Pax6 in all cells, a condition that most closely models human congenital aniridia. In summary, our objective is to capitalize on our recent identification of defects that are likely to be highly relevant for understanding forebrain abnormalities in congenital aniridia. Once we know the answers to the questions posed here (e.g. are defects early or late onset, are they cell autonomous or cell non-autonomous?) we shall be able to design future experiments to provide molecular explanations for our discoveries and to test ways of rescuing abnormalities that might be applicable to humans.