A Drosophila gene that contains both a paired box and a homeobox and has extensive sequence homology to the mouse Pax-6 (Small eye) gene was isolated and mapped to chromosome IV in a region close to the eyeless locus. Two spontaneous mutations, ey2 and eyR, contain transposable element insertions into the cloned gene and affect gene expression, particularly in the eye primordia. This indicates that the cloned gene encodes ey. The finding that ey of Drosophila, Small eye of the mouse, and human Aniridia are encoded by homologous genes suggests that eye morphogenesis is under similar genetic control in both vertebrates and insects, in spite of the large differences in eye morphology and mode of development.

https://science.sciencemag.org/content/sci/265/5173/785.full.pdf

Homology of the eyeless gene of Drosophila to the Small eye gene in mice and Aniridia in humans

The faithful execution of biological processes requires a precise and carefully orchestrated set of steps that depend on the proper spatial and temporal expression of genes. Here we review the various classes of transcriptional regulatory elements (core promoters, proximal promoters, distal enhancers, silencers, insulators/boundary elements, and locus control regions) and the molecular machinery (general transcription factors, activators, and coactivators) that interacts with the regulatory elements to mediate precisely controlled patterns of gene expression. The biological importance of transcriptional regulation is highlighted by examples of how alterations in these transcriptional components can lead to disease. Finally, we discuss the methods currently used to identify transcriptional regulatory elements, and the ability of these methods to be scaled up for the purpose of annotating the entire human genome.

/pdf/transcriptional-regulatory-elements-in-the-human-genome-4ey4yyl1i0.pdf

Transcriptional Regulatory Elements in the Human Genome

Complete block of early B cell differentiation and altered patterning of the posterior midbrain in mice lacking Pax5/BSAP

Greig cephalopolysyndactyly syndrome (GCPS) is an autosomal dominant disorder affecting limb and craniofacial development. Recently, the human GLI3 gene has been proposed to be a candidate gene for GCPS. Here we describe the molecular characterization of extra-toes (Xt), which is a mouse model of GCPS. The Xt heterozygotes show craniofacial defects and a polydactyly phenotype similar to GCPS. We show that a deficiency of Gli3 expression in the XtJ mutant is due to a deletion within the 3' end of the gene. Furthermore, structures affected in the mouse mutant and human syndrome were found to correlate with expression domains of Gli3 in mouse. These results strongly suggest that the deficiency of GLI3 function leads to GCPS.

A Mouse Model of Greig Cephalopolysyndactyly Syndrome: The extra-toesJ Mutation Contains an Intragenic Deletion of the Gli3 Gene

Small eye (Sey) mice homozygous for mutations in the Pax-6 gene have no lenses and no nasal cavities. We have examined the ontogeny of eye and nasal defects in Sey/Sey embryos and have related the defects seen to the pattern of Pax-6 mRNA expression in the mouse during normal eye and nasal development. There are two principal components of the early eye, the neural ectoderm of the optic vesicle, which forms the retina, and the overlying surface ectoderm, which forms the lens and cornea. By studying these interacting tissues in normal and Sey/Sey embryos, we have identified processes for which Pax-6 is important and can thus suggest possible roles for the Pax-6 gene. Pax-6 is essential for the formation of lens placodes from surface ectoderm. In normal development, early Pax-6 mRNA expression in a broad domain of surface ectoderm is downregulated, but expression is specifically maintained in the developing lens placode. Moreover, other Pax-6-expressing tissues are frequently those that have can transdifferentiate into lens. Thus, phenotype and expression together suggest a role for Pax-6 in lens determination. At least some functions of Pax-6 can be separated from the influence of other tissues. Early Sey/Sey optic vesicles are abnormally broad and fail to constrict proximally. These defects occur prior to the time of lens placode formation and probably reflect a requirement for Pax-6 in neural ectoderm. In surface ectoderm domains, where Pax-6 expression is known to be independent of the presence of an optic vesicle, Pax-6 function is required for the maintenance of its own transcription. The mutual dependency of lens and optic vesicle development can also be studied using the Small eye mutation. Using region-specific markers we find that, in the morphologically abnormal Sey/Sey optic vesicles, aspects of normal proximo-distal specification nevertheless persist, despite the complete absence of lens. Like the lens, the nasal cavities develop from ectodermal placodes that normally express Pax-6 mRNA, fail to form in Sey/Sey mice and show Pax-6-dependent Pax-6 mRNA regulation. Analysis of patterns of programmed cell death and absence of nasal region expression from an Msx-1 transgene in Sey/Sey embryos suggest a requirement for Pax-6 in the transition from presumptive nasal ectoderm to placode, and that Msx-1, or genes regulating it, are possible targets for Pax-6.

The role of Pax-6 in eye and nasal development

Aniridia is an inherited ocular disorder of variable expressivity characterized by iris hypoplasia. A candidate aniridia gene, AN, which is the human homologue of the mouse Pax-6 gene, has recently been isolated by positional cloning from the WAGR region of 11p13. Here we describe mutations in this gene in two cases of sporadic aniridia, one detected at the DNA level and one at the RNA level, both of which are predicted to affect protein function. Mutations in Pax-6 have been described previously in Small eye, the proposed mouse model for aniridia. We present new phenotypic evidence for the validity of this mouse model.

The human PAX6 gene is mutated in two patients with aniridia.

Aniridia is a congenital malformation of the eye, chiefly characterised by iris hypoplasia, which can cause blindness. The PAX6 gene was isolated as a candidate aniridia gene by positional cloning from the smallest region of overlap of aniridia-associated deletions. Subsequently PAX6 intragenic mutations were demonstrated in Smalleye, a mouse mutant which is an animal model for aniridia, and six human aniridia patients. In this paper we describe four additional PAX6 point mutations in aniridia patients, both sporadic and familial. These mutations highlight regions of the gene which are essential for normal PAX6 function. In addition, the frequency at which we have found PAX6 mutations suggests that lesions in PAX6 will account for most cases of aniridia.

Dmitri Zaletayev

Papers

The human PAX6 gene is mutated in two patients with aniridia.

PAX6 mutations in aniridia