Small eye (Sey): cloning and characterization of the murine homolog of the human aniridia gene

Ghrelin Regulates Expression of the Transcription Factor Pax6 in Hypoxic Brain Progenitor Cells and Neurons

The nature of brain impairment after hypoxia is complex and recovery harnesses different mechanisms, including neuroprotection and neurogenesis. Experimental evidence suggests that hypoxia may trigger neurogenesis postnatally by influencing the expression of a variety of transcription factors. However, the existing data are controversial. As a proof-of-principle, we subjected cultured cerebral cortex neurons, cerebellar granule neurons and organotypic cerebral cortex slices from rat brains to hypoxia and treated these cultures with the hormone ghrelin, which is well-known for its neuroprotective functions. We found that hypoxia elevated the expression levels and stimulated nuclear translocation of ghrelin’s receptor GHSR1 in the cultured neurons and the acute organotypic slices, whereas ghrelin treatment reduced the receptor expression to normoxic levels. GHSR1 expression was also increased in cerebral cortex neurons of mice with induced experimental stroke. Additional quantitative analyses of immunostainings for neuronal proliferation and differentiation markers revealed that hypoxia stimulated the proliferation of neuronal progenitors, whereas ghrelin application during the phase of recovery from hypoxia counteracted these effects. At the mechanistic level, we provide a link between the described post-ischemic phenomena and the expression of the transcription factor Pax6, an important regulator of neural progenitor cell fate. In contrast to the neurogenic niches in the brain where hypoxia is known to increase Pax6 expression, the levels of the transcription factor in cultured hypoxic cerebral cortex cells were downregulated. Moreover, the application of ghrelin to hypoxic neurons normalised the expression levels of these factors. Our findings suggest that ghrelin stimulates neurogenic factors for the protection of neurons in a GHSR1-dependent manner in non-neurogenic brain areas such as the cerebral cortex after exposure to hypoxia.

Morphometric analysis of the lens in human aniridia and mouse Small eye

Congenital aniridia is caused by heterozygous mutations in the PAX6 gene. In this disease, congenital iris and foveal hypoplasia is associated with juvenile onset cataract, glaucoma, and corneal keratopathy. In rodents, Pax6 mutations result in a congenital reduction in ocular size that is not typically described in human aniridia. Here, the ocular morphometry of aniridia patients is compared with the lens phenotype of Pax6^+/tm1/Pgr mice to reveal whether there are species differences in Pax6 regulation of lens development and homeostasis. Ultrasound biometry (UBM) revealed that eleven percent of aniridia patients exhibited mild microphthalmia while the anterior chamber depth of aniridic eyes was significantly reduced from 6 months of age onward. Although aniridic lens thickness was normal from birth, it was significantly decreased in aniridic lenses older than 30. Notably, 86% of aniridic lenses exhibited cataractous changes in this cohort. In addition, a significant proportion of aniridia patients develop lens subluxation as they age associated with reduced lens diameter as measured by anterior segment optical coherence tomography (AS-OCT). Analysis of young adult Pax6^+/tm1/Pgr mouse lenses by micro-computed tomography (microCT), bright field and dark field imaging revealed that they are reduced in size but did not exhibit overt cataracts at this age. Overall, this study reveals that congenital microphthalmia as assessed by axial length, or microphakia, as assessed by lens thickness, are not typical in human aniridia, although these are primary manifestations of Pax6 mutations in mice, suggesting that PAX6 regulates some aspects of lens development differently between these species.

PAX6 mutation alters circadian rhythm and β cell function in mice without affecting glucose tolerance

The transcription factor PAX6 is involved in the development of the eye and pancreatic islets, besides being associated with sleep–wake cycles. Here, we investigated a point mutation in the RED subdomain of PAX6, previously described in a human patient, to present a comprehensive study of a homozygous Pax6 mutation in the context of adult mammalian metabolism and circadian rhythm. Pax6^Leca2 mice lack appropriate retinal structures for light perception and do not display normal daily rhythmic changes in energy metabolism. Despite β cell dysfunction and decreased insulin secretion, mutant mice have normal glucose tolerance. This is associated with reduced hepatic glucose production possibly due to altered circadian variation in expression of clock and metabolic genes, thereby evading hyperglycemia. Hence, our findings show that while the RED subdomain is important for β cell functional maturity, the Leca2 mutation impacts peripheral metabolism via loss of circadian rhythm, thus revealing pleiotropic effects of PAX6.

Nirav Chhabra et al. characterize adult mice carrying a homozygous mutation in Pax6 that was identified in a patient with foveal hypoplasia. They find that the Pax6 point mutation has pleiotropic effects, including defects in the mouse retinal structures, loss of the optic nerve, changes in energy metabolism and circadian rhythms, and dysregulation of genes expressed in the pancreas.

Global and age-related neuroanatomical abnormalities in a Pax6-deficient mouse model of aniridia suggests a role for Pax6 in adult structural neuroplasticity

PAX6 encodes a highly conserved transcription factor necessary for normal development of the eyes and central nervous system. Heterozygous loss-of-function mutations in PAX6 cause the disorder aniridia in humans and the Small eye trait in mice. Aniridia is a congenital and progressive disorder known for ocular phenotypes; however, recently, consequences of PAX6 haploinsufficiency in the brains of aniridia patients have been identified. These findings span structural and functional abnormalities, including deficits in cognitive and sensory processing. Furthermore, some of these abnormalities are accelerated as aniridia patients age. Although some functional abnormalities may be explained by structural changes, variability of results remain, and the effects of PAX6 heterozygous loss-of-function mutations on neuroanatomy, particularly with regard to aging, have yet to be resolved. Our study used high-resolution magnetic resonance imaging (MRI) and histology to investigate structural consequences of such mutations in the adult brain of our aniridia mouse model, Small eye Neuherberg allele (Pax6SeyNeu/+), at two adult age groups. Using both MRI and histology enables a direct comparison with human studies, while providing higher resolution for detection of more subtle changes. We show volumetric changes in major brain regions of the the Pax6SeyNeu/+ mouse compared to wild-type including genotype- and age-related olfactory bulb differences, age-related cerebellum differences, and genotype-related eye differences. We also show alterations in thickness of major interhemispheric commissures, particularly those anteriorly located within the brain including the optic chiasm, corpus callosum, and anterior commissure. Together, these genotype and age related changes to brain volumes and structures suggest a global decrease in adult brain structural plasticity in our Pax6SeyNeu/+ mice.

Age-dependent alterations in expression and co-localization of Pax6 and Ras-GAP in brain of aging mice

As the brain ages, the survival and plasticity of neurons and glia are compromised. The data-mining and in silico studies suggest interactions of Pax6 with Ras and binding sites in Ras-GAP promoter. The Pax6 also shows age-dependent alterations. Therefore, it is presumed that Pax6 may be associated with the Ras-GAP, a synaptic protein, either directly or indirectly in brain. The expression, co-localization and interaction of Pax6 and Ras-GAP in different regions of brain of mice during aging were investigated through immunofluorescence assay, co-immunoprecipitation and western blotting, respectively. The co-localization of Pax6 and Ras-GAP were observed in dentate gyrus (DG) and sub-granular zone (SGZ) of hippocampus, in glomerular (GlLa) and mitral cells (MiCe) of olfactory lobe, granular cells (GrCe), Purkinje cell (PuCe) and molecular cell layer (MoLa) of cerebellum, internal plexiform layer (InPl), molecular layer (MoLa) of cerebral cortex and in intercalated cells of amygdala (ITC), caudate nucleus regions in brain of aging mice. The expression of Pax6 and Ras-GAP was altered in hippocampus, amygdala, caudate nucleus, olfactory lobe, cerebral cortex and cerebellum from young to old mice. The Pax6 interacts with Ras-GAP in brain of mice. Results indicate impact of Pax6 on Ras-GAP-mediated activities of synapses, learning and memory, emotions and fear as well as motor functions. Alterations in expression and co-localization of Pax6 and Ras-GAP during aging may be responsible for age-associated compromised survival and plasticity of neurons and glia.

Congenital Aniridia and the Ocular Surface

Aniridia is a congenital pan-ocular disorder caused by haplo-insufficiency of Pax6, a crucial gene for proper development of the eye. Aniridia affects a range of eye structures, including the cornea, iris, anterior chamber angle, lens, and fovea. The ocular surface, in particular, can be severely affected by a progressive pathology termed aniridia-associated keratopathy (AAK), markedly contributing to impaired vision. The purpose of this review is to provide an update of the current knowledge of the genetic, clinical, micro-morphological, and molecular aspects of AAK. We draw upon material presented in the literature and from our own observations in large aniridia cohorts. We summarize signs and symptoms of AAK, describe current options for management, and discuss the latest research findings that may lead to better diagnosis and new treatment or prevention strategies for this debilitating ocular surface condition.

Regulation of cerebral cortical neurogenesis by the Pax6 transcription factor

Understanding brain development remains a major challenge at the heart of understanding what makes us human. The neocortex, in evolutionary terms the newest part of the cerebral cortex, is the seat of higher cognitive functions. Its normal development requires the production, positioning, and appropriate interconnection of very large numbers of both excitatory and inhibitory neurons. Pax6 is one of a relatively small group of transcription factors that exert high-level control of cortical development, and whose mutation or deletion from developing embryos causes major brain defects and a wide range of neurodevelopmental disorders. Pax6 is very highly conserved between primate and non-primate species, is expressed in a gradient throughout the developing cortex and is essential for normal corticogenesis. Our understanding of Pax6’s functions and the cellular processes that it regulates during mammalian cortical development has significantly advanced in the last decade, owing to the combined application of genetic and biochemical analyses. Here, we review the functional importance of Pax6 in regulating cortical progenitor proliferation, neurogenesis, and formation of cortical layers and highlight important differences between rodents and primates. We also review the pathological effects of PAX6 mutations in human neurodevelopmental disorders. We discuss some aspects of Pax6’s molecular actions including its own complex transcriptional regulation, the distinct molecular functions of its splice variants and some of Pax6’s known direct targets which mediate its actions during cortical development.

Molecular description of eye defects in the zebrafish Pax6b mutant, sunrise, reveals a Pax6b-dependent genetic network in the developing anterior chamber

The cornea is a central component of the camera eye of vertebrates and even slight corneal disturbances severely affect vision. The transcription factor PAX6 is required for normal eye development, namely the proper separation of the lens from the developing cornea and the formation of the iris and anterior chamber. Human PAX6 mutations are associated with severe ocular disorders such as aniridia, Peters anomaly and chronic limbal stem cell insufficiency. To develop the zebrafish as a model for corneal disease, we first performed transcriptome and in situ expression analysis to identify marker genes to characterise the cornea in normal and pathological conditions. We show that, at 7 days post fertilisation (dpf), the zebrafish cornea expresses the majority of marker genes (67/84 tested genes) found also expressed in the cornea of juvenile and adult stages. We also characterised homozygous pax6b mutants. Mutant embryos have a thick cornea, iris hypoplasia, a shallow anterior chamber and a small lens. Ultrastructure analysis revealed a disrupted corneal endothelium. pax6b mutants show loss of corneal epithelial gene expression including regulatory genes (sox3, tfap2a, foxc1a and pitx2). In contrast, several genes (pitx2, ctnnb2, dcn and fabp7a) were ectopically expressed in the malformed corneal endothelium. Lack of pax6b function leads to severe disturbance of the corneal gene regulatory programme.

A review of the clinical and genetic aspects of aniridia

Aniridia classically presents with a bilateral congenital absence or malformation of the irides, foveal hypoplasia, and nystagmus, and patients tend to develop visually significant pre-senile cataracts and keratopathy. Additionally, they are at high risk for developing glaucoma. Classic aniridia can be genetically defined as the presence of a PAX6 gene deletion or loss-of-function mutation that results in haploinsufficiency. Variants of aniridia, which include a condition previously referred to as autosomal dominant keratitis, are likely due to PAX6 mutations that lead to partial loss of PAX6 function. Aniridia-associated keratopathy (AAK) is a progressive and potentially debilitating problem affecting aniridic patients. The current treatments for AAK are to replace the limbal stem cells through keratolimbal allograft (KLAL) with or without subsequent keratoplasty for visual rehabilitation, or to implant a Boston type 1 keratoprosthesis. Future therapies for AAK may be aimed at the genetic modification of corneal limbal stem cells.

The role of Pax6 in forebrain development

Pax6 encodes a highly conserved transcriptional regulator with two DNA-binding motifs, a paired domain and a paired-like homeodomain. Humans carrying PAX6 loss-of-function mutations suffer from abnormal development of the eyes (congenital aniridia) and brain. Small eye mice carrying Pax6 loss-of-function mutations provide a good model for these human conditions. Their analysis has demonstrated the critical importance of this transcription factor in multiple cell types and at several key stages of forebrain development. In the forebrain, Pax6 is critical for the establishment of the pallial-subpallial boundary, which separates dorsal (future cerebral cortex) and ventral (future striatum) telencephalic regions. Levels of Pax6 expression are critically important for cortical progenitor proliferation and its presence in a rostro-lateral(high) to caudo-medial(low) gradient in the cortex is necessary to establish rostro-lateral identities. Furthermore, axon guidance is disrupted in Pax6⁻/⁻ mutants: the majority of thalamocortical axons fail to enter the ventral telencephalon and those that do are unable to innervate their cortical targets. The extent to which the effects of Pax6 later in development are secondary to its effects in early patterning and proliferation remains largely unknown. This is likely to be clarified by future studies on the molecular mechanisms of action of Pax6 and, in particular, the identification of its downstream target genes. Such studies should also help generate an increasingly coherent understanding of how this pleiotropic transcription factor becomes involved in so many facets of neural development.

Reduced expression of Pax6 in lens and cornea of mutant mice leads to failure of chamber angle development and juvenile glaucoma

Heterozygous mutations in PAX6 are causative for aniridia, a condition that is frequently associated with juvenile glaucoma. Defects in morphogenesis of the iridocorneal angle, such as lack of trabecular meshwork differentiation, absence of Schlemm's canal and blockage of the angle by iris tissue, have been described as likely causes for glaucoma, and comparable defects have been observed in heterozygous Pax6-deficient mice. Here, we employed Cre/loxP-mediated inactivation of a single Pax6 allele in either the lens/cornea or the distal optic cup to dissect in which tissues both alleles of Pax6 need to be expressed to control the development of the tissues in the iridocorneal angle. Somatic inactivation of one allele of Pax6 exclusively from epithelial cells of lens and cornea resulted in the disruption of trabecular meshwork and Schlemm's canal development as well as in an adhesion between iris periphery and cornea in juvenile eyes, which resulted in the complete closure of the iridocorneal angle in the adult eye. Structural changes in the iridocorneal angle presumably caused a continuous increase in intraocular pressure leading to degenerative changes in optic nerve axons and to glaucoma. In contrast, the inactivation of a single Pax6 allele in the distal optic cup did not cause obvious changes in iridocorneal angle formation. We conclude that the defects in iridocorneal angle formation are caused by non-autonomous mechanisms due to Pax6 haploinsufficiency in lens or corneal epithelial cells. Pax6 probably controls the expression of signaling molecules in lens cells that regulate the morphogenetic processes during iridocorneal angle formation.

Tumor suppressor PAX6 functions as androgen receptor co-repressor to inhibit prostate cancer growth

BACKGROUND

PAX6, a transcription factor, has currently been suggested to function as a tumor suppressor in glioblastoma and to act as an early differentiation marker for neuroendocrine cells. The androgen receptor (AR) plays a pivotal role in prostate cancer development and progression due to its transcriptional activity in regulating genes involved in cell growth, differentiation, and apoptosis. To determine the role of PAX6 in prostate cancer, we investigated whether PAX6 interacts with AR to affect prostate cancer development.

METHODS

We used immunostaining, RT-PCR, and Western blotting assays to show the expression status of PAX6 in prostate tissue and human prostate cancer cell lines. The role of PAX6 in cell growth and colony regeneration potential of LNCaP cells were evaluated by MTT assay and soft agar assay with PAX6-overexpressed LNCaP cells. Mammalian two-hybrid and co-immunoprecipitation (Co-IP) assays were used to demonstrate the interaction between PAX6 and AR. Reporter gene and Q-RT-PCR assays were performed to determine the effects of PAX6 on the function of AR.

RESULTS

In prostate cancer tissues, PAX6 expression was stronger in normal epithelial cells than cancer cells, and decreased in LNCaP cells compared to that of DU145 and PC3 cells. Enforced expression of PAX6 suppressed the cell growth of LNCaP cells and also inhibited the colony formation of LNCaP cells. PAX 6 interacted with AR and repressed its transcriptional activity. PAX6 overexpression decreased the expression of androgen target gene PSA in LNCaP cells.

CONCLUSIONS

In this study, we found that PAX6 may act as a prostate cancer repressor by interacting with AR and repressing the transcriptional activity and target gene expression of AR to regulate cell growth and regeneration.

Progressive loss of PAX6, TBR2, NEUROD and TBR1 mRNA gradients correlates with translocation of EMX2 to the cortical plate during human cortical development

The transcription factors Emx2 and Pax6 are expressed in the proliferating zones of the developing rodent neocortex, and gradients of expression interact in specifying caudal and rostral identities. Pax6 is also involved in corticoneurogenesis, being expressed by radial glial progenitors that give rise to cells that also sequentially express Tbr2, NeuroD and Tbr1, genes temporally downstream of Pax6. In this study, using in situ hybridization, we analysed the expression of EMX2, PAX6, TBR2, NEUROD and TBR1 mRNA in the developing human cortex between 8 and 12 postconceptional weeks (PCW). EMX2 mRNA was expressed in the ventricular (VZ) and subventricular zones (SVZ), but also in the cortical plate, unlike in the rodent. However, gradients of expression were similar to that of the rodent at all ages studied. PAX6 mRNA expression was limited to the VZ and SVZ. At 8 PCW, PAX6 was highly expressed rostrally but less so caudally, as has been seen in the rodent, however this gradient disappeared early in corticogenesis, by 9 PCW. There was less restricted compartment-specific expression of TBR2, NEUROD and TBR1 mRNA than in the rodent, where the gradients of expression were similar to that of PAX6 prior to 9 PCW. The gradient disappeared for TBR2 by 10 PCW, and for NEUROD and TBR1 by 12 PCW. These data support recent reports that EMX2 but not PAX6 is more directly involved in arealization, highlighting that analysis of human development allows better spatio-temporal resolution than studies in rodents.

Corneal epithelial stem cells: deficiency and regulation

The corneal epithelium is continuously renewed by a population of stem cells that reside in the corneoscleral junction, otherwise known as the limbus. These limbal epithelial stem cells (LESC) are imperative for corneal maintenance with deficiencies leading to in-growth of conjunctival cells, neovascularisation of the corneal stroma and eventual corneal opacity and visual loss. One such disease that has traditionally been thought to be due to LESC deficiency is aniridia, a pan-ocular congenital eye disease due to mutations in the PAX6 gene. Corneal changes or aniridia related keratopathy (ARK) seen in aniridia are typical of LESC deficiency. However, the pathophysiology behind ARK is still ill defined, with current theories suggesting it may be caused by a deficiency in the stem cell niche and adjacent corneal stroma, with altered wound healing responses also playing a role (Ramaesh et al, International Journal of Biochemistry & Cell Biology 37:547-557, 2005) or abnormal epidermal differentiation of LESC (Li et al., The Journal of Pathology 214:9, 2008). PAX6 is considered the master control gene for the eye and is required for normal eye development with expression continuing in the adult cornea, thus inferring a role for corneal repair and regeneration (Sivak et al., Developments in Biologicals 222:41-54, 2000). Studies of models of Pax6 deficiency, such as the small eyed (sey) mouse, should help to reveal the intrinsic and extrinsic mechanisms involved in normal LESC function.

Mechanisms controlling Pax6 isoform expression in the retina have been conserved between teleosts and mammals

The Pax6 gene plays several roles in retinal development, including control of cell proliferation, maintenance of the retinogenic potential of progenitor cells, and cell fate specification. Emerging evidence suggests that these different aspects of Pax6 gene function are mediated by different isoforms of the Pax6 protein; however, relatively little is known about the spatiotemporal expression of Pax6 isoforms in the vertebrate retina. Using bacterial artificial chromosome (BAC) technology, we modified a zebrafish Pax6a BAC such that we could distinguish paired-containing Pax6a transcripts from paired-less Pax6a transcripts. In the zebrafish, the spatial and temporal onset of expression of these transcripts suggests that the paired-less isoform is involved in the cell fate decision leading to the generation of amacrine cells; however, because of limitations associated with transient transgenic analysis, it was not feasible to establish whether this promoter was active in all amacrine cells or in a specific population of amacrine cells. By making mice transgenic for the zebrafish Pax6a BAC reporter transgene, we were able to show that paired-containing and paired-less Pax6a transcripts were differentially expressed in amacrine subpopulations. Our study also directly demonstrates the functional conservation of the regulatory mechanisms governing Pax6 transcription in teleosts and mammals.

Analysis of Pax6 expression using a BAC transgene reveals the presence of a paired-less isoform of Pax6 in the eye and olfactory bulb

Pax6, a member of the paired-family of transcription factors, exhibits restricted expression and essential functions in the developing eye, olfactory system, central nervous system, and pancreas. To understand Pax6 function, which critically depends on induction of proper expression levels during development, it is necessary to elucidate the molecular mechanisms governing Pax6 transcription. Although previous studies using classic transgenic approaches have provided a wealth of information about the distribution and types of regulatory elements involved in Pax6 regulation, genetic studies in both humans and mice indicate that these enhancers alone are not sufficient for fully regulated Pax6 expression. We report here our analysis of mice transgenic for a 160 kb mouse Pax6 BAC transgene, which was generated as a necessary first step towards testing the long-range control of Pax6 expression in vivo. We show that this BAC transgene replicates Pax6 expression in the eye. This is the first time that a reporter transgene has been expressed in a normal Pax6-like pattern in all of the tissues of the eye and defines an eye regulatory region within the Pax6 downstream regulatory region (DRR). Second, we show that this BAC transgene contains all of the cis regulatory elements required for normal Pax6 expression within the developing embryo, except for within the diencephalon and olfactory bulb. Third, we show that this transgene is subject to Pax6 autoregulation. Lastly, we identify, for the first time in mammals, an isoform of the Pax6 protein lacking the paired domain. This isoform is expressed in the developing olfactory bulb and eye. Over-expression of Pax6DeltaPD causes a microphthalmic phenotype in both Pax6(+/+) mice and Pax6(+/-) mice. These results demonstrate a role for Pax6DeltaPD in eye development, which appears to be different than that ascribed to either canonical Pax6 or Pax6(5a).

Drosophila retinal homeobox (drx) is not required for establishment of the visual system, but is required for brain and clypeus development

The possibility that mechanisms of retinal determination may be similar between vertebrates and Drosophila has been supported by the observations that Pax6/eyeless genes are necessary and sufficient for retinal development. These studies suggest that the function of other gene families, operating during early eye development, might also be conserved. One candidate is the retinal homeobox (Rx) family of transcription factors. Vertebrate Rx is expressed in the prospective eye and forebrain and is required for eye morphogenesis, retinal precursor appearance, and normal forebrain development, indicating that it is an essential regulator of early eye and brain formation. Here, we test the hypothesis that Drosophila Rx (drx) is required for adult and larval eye development. We have isolated a drx null allele and demonstrate that the mutant compound eye and larval visual system is not detectably abnormal. However, we find that drx is required for development of a central brain structure, the ellipsoid body, suggesting that Rx function in the brain may be conserved. Finally, we characterize a novel anterior head phenotype and demonstrate that drx is required for clypeus development. Thus, our data suggest that drx may be required for the regulation of genes involved in brain morphogenesis and clypeus precursor development. We propose that differences in insect and vertebrate eye development may be explained by changes in gene regulation and/or the tissue of origin for eye precursor cells.

Pax6 heterozygous eyes show defects in chamber angle differentiation that are associated with a wide spectrum of other anterior eye segment abnormalities

The development of the chamber angle was studied in the eyes of heterozygous Pax6(lacZ/+) mutant mice (Nature 387 (1997) 406). Mutations in PAX6 cause aniridia, a condition that is frequently associated with glaucoma, a blinding disease that may be associated with chamber angle defects. Mesenchymal cells were seen in the chamber angle at P1-P5. In wild-type mice, these cells differentiated into typical trabecular meshwork (TM) cells next to Schlemm's canal. In Pax6(lacZ/+) mice, TM cells remained undifferentiated and Schlemm's canal was absent. From P1 to P4, staining for beta-galactosidase and immunoreactivity for Pax6 were observed in chamber angle mesenchyme, but were absent later. Cultured murine TM cells expressed Pax6. The defects in chamber angle and TM differentiation were associated with a wide spectrum of other anterior eye defects, which included various degrees of iris hypoplasia and corneal haze, isolated iridocorneal adhesions and atypical coloboma, and a vascularized cornea in all adult animals. A third of the animals showed Peters' anomaly including corneal opacity and iridocorneal adhesions. The separation of the lens from the cornea was incomplete, and epithelial layers of lens and cornea were continuous. Pax6 activity is directly required for differentiation of the chamber angle. Variations in phenotype of Pax6(lacZ/+) mice appear not to involve direct dominant-negative or dose-dependent effects.

The homeobox gene mbx is involved in eye and tectum development

The homeobox gene mbx is first activated at the end of gastrulation in zebrafish in the presumptive forebrain and midbrain region. During somitogenesis stages, the anterior expression of mbx, which partly overlaps the future eye field, gradually decreases, while midbrain expression intensifies and becomes restricted to the presumptive tectum. Knockdown of mbx expression by morpholino antisense oligonucleotides (mbx-MO) leads to a reduction in the size of the eyes and tectum. Expression domains of rx1 and pax6 in the eye field and of mab21l2 in the eye field and tectum anlage were reduced in size in mbx-MO-injected embryos by somitogenesis stages. Further, induction of islet1 and lim3 expression in the eye at 2 days postfertilization (dpf) was suppressed in mbx-MO-injected embryos. In mbx-MO-injected embryos at 2-5 dpf, the lamination of the eye was disorganized and the number of retinal axons was substantially reduced, but the few remaining axons navigated appropriately to the contralateral tectum. A chimeric protein composed of the Mbx DNA-binding domain and the VP16 activation domain affected eye and tectum development similarly to mbx-MO knockdown, suggesting that Mbx acts as a transcriptional repressor in the zebrafish embryo. Based on these data, we propose that the mbx homeobox gene is required for the development of the eyes and tectum.

Dach1, a vertebrate homologue of Drosophila dachshund, is expressed in the developing eye and ear of both chick and mouse and is regulated independently of Pax and Eya genes

We have cloned a chick homologue of Drosophila dachshund (dac), termed Dach1. Dach1 is the orthologue of mouse and human Dac/Dach (hereafter referred to as Dach1). We show that chick Dach1 is expressed in a variety of sites during embryonic development, including the eye and ear. Previous work has demonstrated the existence of a functional network and genetic regulatory hierarchy in Drosophila in which eyeless (ey, the Pax6 orthologue), eyes absent (eya), and dac operate together to regulate Drosophila eye development, and that ey regulates the expression of eya and dac. We find that in the developing eye of both chick and mouse, expression domains of Dach1 overlap with those of Pax6, a gene required for normal eye development. Similarly, in the developing ear of both mouse and chick, Dach1 expression overlaps with the expression of another Pax gene, Pax2. In the mouse, Dach1 expression in the developing ear also overlaps with the expression of Eya1 (an eya homologue). Both Pax2 and Eya1 are required for normal ear development. Our expression studies suggest that the Drosophila Pax-eya-dac regulatory network may be evolutionarily conserved such that Pax genes, Eya1, and Dach1 may function together in vertebrates to regulate neural development. To address the further possibility that a regulatory hierarchy exists between Pax, Eya, and Dach genes, we have examined the expression of mouse Dach1 in Pax6, Pax2 and Eya1 mutant backgrounds. Our results indicate that Pax6, Pax2, and Eya1 do not regulate Dach1 expression through a simple linear hierarchy.

Characterization of mouse Dach2, a homologue of Drosophila dachshund

The Drosophila genes eyeless, eyes absent, sine oculis and dachshund cooperate as components of a network to control retinal determination. Vertebrate homologues of these genes have been identified and implicated in the control of cell fate. We present the cloning and characterization of mouse Dach2, a homologue of dachshund. In situ hybridization studies demonstrate Dach2 expression in embryonic nervous tissues, sensory organs and limbs. This pattern is similar to mouse Dach1, suggesting a partially redundant role for these genes during development. In addition, we determine that Dach2 expression in the forebrain of Pax6 mutants and dermamyotome of Pax3 mutants is not detectably altered. Finally, genetic mapping experiments place mouse Dach2 on the X chromosome between Xist and Esx1. The identification of human DACH2 sequences at Xq21 suggests a possible role for this gene in Allan-Herndon syndrome, Miles-Carpenter syndrome, X-linked cleft palate and/or Megalocornea.

Rieger syndrome is associated with PAX6 deletion

PURPOSE

Rieger syndrome is an autosomal dominant condition defined by anterior segment dysgenesis in combination with facial, dental, skeletal and umbilical abnormalities. To date Rieger syndrome has been associated with mutations in the PITX2 gene at chromosome 4q25 and a second locus has been found at chromosome 13q14.

METHODS

We describe a Rieger syndrome case with all the typical dysmorphic features and the molecular genetic finding by use of FISH analysis of the PAX6 gene.

RESULTS

An eight-year-old girl had iris stroma hypoplasia, corectopia and iridogoniodysgenesis. She had an underdeveloped premaxilla and a congenital absence of nine teeth in the maxilla. The front teeth in the mandible were peg-shaped and all teeth were small. There was failure of involution of the periumbilical skin. FISH analysis using probes for the PAX6 gene showed a small deletion for the PAX6 gene on one homologue of chromosome 11.

CONCLUSION

Rieger syndrome can -- in addition to PITX2 gene mutations and abnormalities at chromosome 13q14 -- be associated with PAX6 gene abnormalities.

Dach1 mutant mice bear no gross abnormalities in eye, limb, and brain development and exhibit postnatal lethality

Drosophila dachshund is necessary and sufficient for compound eye development and is required for normal leg and brain development. A mouse homologue of dachshund, Dach1, is expressed in the developing retina and limbs, suggesting functional conservation of this gene. We have generated a loss-of-function mutation in Dach1 that results in the abrogation of the wild-type RNA and protein expression pattern in embryos. Homozygous mutants survive to birth but exhibit postnatal lethality associated with a failure to suckle, cyanosis, and respiratory distress. The heart, lungs, kidneys, liver, and skeleton were examined to identify factors involved in postnatal lethality, but these organs appeared to be normal. In addition, blood chemistry tests failed to reveal differences that might explain the lethal phenotype. Gross examination and histological analyses of newborn eyes, limbs, and brains revealed no detectable abnormalities. Since Dach1 mutants die shortly after birth, it remains possible that Dach1 is required for postnatal development of these structures. Alternatively, an additional Dach homologue may functionally compensate for Dach1 loss of function.

PAX6 mutations reviewed

Mutations in PAX6 are responsible for human aniridia and have also been found in patients with Peter's anomaly, with congenital cataracts, with autosomal dominant keratitis, and with isolated foveal hypoplasia. No locus other than chromosome 11p13 has been implicated in aniridia, and PAX6 is clearly the major, if not only, gene responsible. Twenty-eight percent of identified PAX6 mutations are C-T changes at CpG dinucleotides, 20% are splicing errors, and more than 30% are deletion or insertion events. There is a noticeably elevated level of mutation in the paired domain compared with the rest of the gene. Increased mutation in the homeodomain is accounted for by the hypermutable CpG dinucleotide in codon 240. Very nearly all mutations appear to cause loss of function of the mutant allele, and more than 80% of exonic substitutions result in nonsense codons. In a gene with such extraordinarily high sequence conservation throughout evolution, there are presumed undiscovered missense mutations, these are hypothesized to exist in as-yet unidentified phenotypes.

PAX6 intronic sequence targets expression to the spinal cord

PAX6, a member of the family of highly conserved paired-box and homeobox genes, is highly conserved at both the protein and DNA levels. A conserved 216-bp Pax6 intron 4 sequence was found in human, mouse, and quail. Our transgenic mice experiments indicated that when under control of the human PAX6 promoter, the human 216-bp conserved sequence (ele4H) functioned as a spinal cord-specific enhancer. This enhancer can drive lacZ expression at the thoracic and lumbar levels of the spinal cord only when linked to a functional PAX6 promoter. These studies also suggested that PAX6 was not only conserved at the functional level, but at the transcriptional level as well.

FGF10 is an inducer and Pax6 a competence factor for lacrimal gland development

We investigated the mechanism of tissue induction and specification using the lacrimal gland as a model system. This structure begins its morphogenesis as a bud-like outgrowth of the conjunctival epithelium and ultimately forms a branched structure with secretory function. Using a reporter transgene as a specific marker for gland epithelium, we show that the transcription factor Pax6 is required for normal development of the gland and is probably an important competence factor. In investigating the cell-cell signaling required, we show that fibroblast growth factor (FGF) 10 is sufficient to stimulate ectopic lacrimal bud formation in ocular explants. Expression of FGF10 in the mesenchyme adjacent to the presumptive lacrimal bud and absence of lacrimal gland development in FGF10-null mice strongly suggest that it is an endogenous inducer. This was supported by the observation that inhibition of signaling by a receptor for FGF10 (receptor 2 IIIb) suppressed development of the endogenous lacrimal bud. In explants of mesenchyme-free gland epithelium, FGF10 stimulated growth but not branching morphogenesis. This suggested that its role in induction is to stimulate proliferation and, in turn, that FGF10 combines with other factors to provide the instructive signals required for lacrimal gland development.

Mutation in the PAX6 gene in twenty patients with aniridia

This is a report on the nature of the mutations in the PAX6 gene in twenty patients with aniridia. Five of the twenty patients had sporadic aniridia with deletions in chromosome 11p13. Three of the five had WAGR syndrome (Wilms tumor, aniridia, genitourinary anomalies, mental retardation), and the other two had deletions whose breakpoints occurred between the PAX6 and the WT1 genes. Allelic losses at PAX6 were of paternal origin. The remaining fifteen patients with aniridia had intragenic mutations in the PAX6 gene, with mutations found from exon 5 to exon 12. Twelve cases of dysfunctional PAX6 were due to premature termination of the protein by nonsense mutations (five cases), splicing defect (one case), deletion (two cases), deletion-insertions (two cases), and tandem repeat insertions (two cases). One patient (P2) had a PAX6 protein with de novo in-frame deletion of alanine, arginine, and proline at codon positions 37, 38, and 39. These codons are in the paired box region, and codon 38 is in contact with the phosphate group of the sugar-phosphate backbone of the target DNA. Another patient (P8) had a single nucleotide transition at c.1182 (nucleotide number, Genbank accession #M93650, used as in Glaser et al. [1992]), which generated both a missense mutation (Q255H) and a splicing defect. A missense mutation was found at G387E in a third patient (P10). All observed mutations support the notion that haploinsufficiency in PAX6 results in aniridia and associated eye anomalies.

Regulation of PAX-6 gene transcription: alternate promoter usage in human brain

We have isolated and characterized the 5'-flanking regulatory region of the human PAX-6 gene. Mapping of transcription initiation sites revealed the existence of an additional non-coding 5' exon, exon 1A. Functional analyses indicated that PAX-6 transcription is regulated by two distinct promoters, A and B, resulting in alternative transcription of exon 1A or 1B and joint transcription of exons 2 to 13. While a single initiation site was identified for exon 1A, transcription of exon 1B appears to be initiated from more than one site downstream of the promoter B-associated TATA motif. Multiple potential binding sites for transcription factors were found in the regions of promoter A and B. Although a 1.1-kb fragment of promoter A and a 1.5 kb fragment of promoter B, which had been fused to a reporter gene and transiently expressed in cell lines, displayed constitutive promoter activity, transcription of PAX-6 driven by promoter B was considerably higher than by promoter A in various regions of human postmortem brain. Transcript PAX-6B was primarily expressed in cerebellar cortex, whereas relatively low concentrations were detected in other brain areas. Functional dissection by serial deletions revealed several clusters of both activating elements and cell-selective silencers within the regulatory regions upstream of exon 1A and 1B. Coexpression of the promoter B constructs with a vector expressing PAX-6 modulated promoter B activity, thus indicating autoregulation by PAX-6 transcription. In conclusion, our findings suggest that PAX-6 transcription is regulated by alternate usage of promoter A and B, and that in adult human brain expression of PAX-6 is primarily controlled by promoter B. Alternate promoter usage and differential PAX-6 transcription are likely to play a critical role in brain development and neuroplasticity.

Truncation mutations in the transactivation region of PAX6 result in dominant-negative mutants

PAX6 is a transcription factor with two DNA-binding domains (paired box and homeobox) and a proline-serine-threonine (PST)-rich transactivation domain. PAX6 regulates eye development in animals ranging from jellyfish to Drosophila to humans. Heterozygous mutations in the human PAX6 gene result in various phenotypes, including aniridia, Peter's anomaly, autosomal dominant keratitis, and familial foveal dysplasia. It is believed that the mutated allele of PAX6 produces an inactive protein and aniridia is caused due to genetic haploinsufficiency. However, several truncation mutations have been found to occur in the C-terminal half of PAX6 in patients with Aniridia resulting in mutant proteins that retain the DNA-binding domains but have lost most of the transactivation domain. It is not clear whether such mutants really behave as loss-of-function mutants as predicted by haploinsufficiency. Contrary to this theory, our data showed that these mutants are dominant-negative in transient transfection assays when they are coexpressed with wild-type PAX6. We found that the dominant-negative effects result from the enhanced DNA binding ability of these mutants. Kinetic studies of binding and dissociation revealed that various truncation mutants have 3-5-fold higher affinity to various DNA-binding sites when compared with the wild-type PAX6. These results provide a new insight into the role of mutant PAX6 in causing aniridia.

The gonadotropin-releasing hormone system does not develop in Small-Eye (Sey) mouse phenotype

This study examined the development of the gonadotropin releasing-hormone (GnRH) system in a spontaneous mouse mutation, Small-Eye (Sey). This phenotype is due to a point mutation in the developmental control gene Pax-6 and results in failed development of the eye and olfactory placodes in homozygous (Sey/Sey) embryos and a variety of eye abnormalities in heterozygotes (Sey/+). Therefore, Sey/Sey embryos provided a naturally occurring olfactory placode ablation to ask whether all of the GnRH neurons found in the adult mouse forebrain arise from the olfactory epithelium. In Sey/Sey embryos, GnRH-immunoreactive neurons were not present in either the presumptive nasal regions or in any area of the brain at any embryonic age. In contrast, in Sey/+ embryos, there was no apparent effect on either GnRH cell proliferation or migration. These data support and extend the hypothesis that GnRH neurons in mice originate in the olfactory placodes and also demonstrate that two normal alleles of Pax-6 are not required for GnRH system development.

Dissection of the transactivation function of the transcription factor encoded by the eye developmental gene PAX6

PAX6 is a transcription activator that regulates eye development in animals ranging from Drosophila to human. The C-terminal region of PAX6 is proline/serine/threonine-rich (PST) and functions as a potent transactivation domain when attached to a heterologous DNA-binding domain of the yeast transcription factor, GAL4. The PST region comprises 152 amino acids encoded by four exons. The transactivation function of the PST region has not been defined and characterized in detail by in vitro mutagenesis. We dissected the PST domain in two independent systems, a heterologous system using a GAL4 DNA-binding site and the native system of PAX6. Our data consistently showed that in both systems all four constituent exons of the PST domain are responsible for the transactivation function. The four exon fragments act synergistically to stimulate transcription, although none of them can function individually as an independent transactivation domain. Combinations of two or more exon fragments can reconstitute substantial transactivation activity when fused to the DNA-binding domain of GAL4, but they surprisingly do not produce much activity in the context of native PAX6, although the mutant PAX6 proteins are stable and their DNA-binding function remains unaffected. Our data suggest that these mutants may antagonize the wild-type PAX6 activity by competing for target DNA-binding sites. We conclude that the PAX6 protein contains an unusually large transactivation domain that is evolutionarily conserved to a high degree and that its full transactivation activity relies on the synergistic action of the four exon fragments.

Pax-6 functions in boundary formation and axon guidance in the embryonic mouse forebrain

The Pax-6 gene encodes a transcription factor that is expressed in regionally restricted patterns in the developing brain and eye. Here we describe Pax-6 expression in the early forebrain (prosencephalon) on embryonic day 9.5 (E9.5) to E10.5 using both whole-mount in situ hybridization and antibody labeling. We find close correlations between Pax-6+ domains and initial neural patterning, and identify corresponding defects in embryos homozygous for the Pax-6 allele, Small eye (Sey). Pax-6 expression defines the prosencephalon-mesencephalon boundary, and mutant embryos lack this morphological boundary. Markers of the caudal prosencephalon are lost (Pax-6, Lim-1, Gsh-1) and a marker for mesencephalon is expanded rostrally into the prosencephalon (Dbx). We conclude that the caudal prosencephalon (prosomere 1) is at least partially transformed to a mesencephalic fate. This transformation results in a specific deficit of posterior commissure axons. Sey/Sey embryos also exhibit an axon pathfinding defect specific to the first longitudinal tract in the prosencephalon (tpoc, tract of the postoptic commissure). In wild type, tpoc axons fan out upon coming in contact with a superficial patch of Pax-6+ neuron cell bodies. In the mutant, the tpoc axons have normal initial projections, but make dramatic errors where they contact the neuron cell bodies, and fail to pioneer this first tract. Thus Pax-6 is required for local navigational information used by axons passing through its domain of expression. We conclude that Pax-6 plays multiple roles in forebrain patterning, including boundary formation, regional patterning, neuron specification and axon guidance.

Functional analysis of paired box missense mutations in the PAX6 gene

Mutations in the human PAX6 gene produce various phenotypes, including aniridia, Peters' anomaly, autosomal dominant keratitis and familial foveal dysplasia. The various phenotypes may arise from different mutations in the same gene. To test this theory, we performed a functional analysis of two missense mutations in the paired domain: the R26G mutation, previously reported in a case of Peters' anomaly, and an unreported I87R mutation, which we identified in a patient with aniridia. While both the R26 and the I87 positions are conserved in the paired boxes of all known PAX genes, X-ray crystallography has shown that only R26 makes contact with DNA. We showed that the R26G mutant failed to bind a subset of paired domain binding sites but, surprisingly, bound other sites and successfully transactivated promoters containing those sites. In contrast, the I87R mutant had lost the ability to bind DNA at all tested sites and failed to transactivate promoters. Our data support the haploid-insufficiency hypothesis of aniridia, and the hypothesis that R26G is a hypomorphic allele.

Transcriptional regulation of the human PAX6 gene promoter

PAX6, a member of the highly conserved paired-type homeobox gene family, is expressed in a spatially and temporally restricted pattern during early embryogenesis, and its mutation is responsible for human aniridia. Here we examined the transcriptional regulation of the PAX6 gene by transient transfection assays and identified multiple cis-regulatory elements that function differently in different cell lines. The transcriptional initiation site was identified by RNase protection and primer extension assay. Examination of the genomic DNA sequence indicated that the PAX6 promoter has a TATA like-box (ATATTTT) at -26 base pairs (bp), and two CCAAT boxes are positioned at -70 and -100 bp. A 38-bp poly(CA) sequence was located 992 bp upstream from the initiation site. Transient transfection assays in glioblastoma cells and leukemia cells indicate that a 92-bp region was required for basal level PAX6 promoter activity. A negative transcriptional element, silencer (bases -1518 to -1268), functioned differently in different cell lines. The activation of the promoter is positively correlated with the expression of PAX6 transcripts in all cells tested. These results indicate that a cis-regulatory element or elements is responsible for selective activation of the PAX6 promoter in cells that can express PAX6 mRNA.

PAX-6 in development and evolution

Pax-6 is a member of the Pax gene class and encodes a protein containing a paired domain and a homeodomain. The molecular characterization of Pax-6 genes from species of different animal phyla and the analysis of Pax-6 function in the developing eyes and central nervous system of vertebrates, Drosophila melanogaster, and Caenorhabditis elegans suggest that Pax-6 homologues share conserved functions. In this review, we present recent data on the structural and functional characterization of Pax-6 homologues from species of different animal phyla. We discuss the implications of these findings for our understanding of the development and evolution of eyes and nervous systems.

Influence of PAX6 gene dosage on development: overexpression causes severe eye abnormalities

Aniridia in man and Small eye in mice are semidominant developmental disorders caused by mutations within the paired box gene PAX6. Whereas heterozygotes suffer from iris hypoplasia, homozygous mice lack eyes and nasal cavities and exhibit brain abnormalities. To investigate the role of gene dosage in more detail, we have generated yeast artificial chromosome transgenic mice carrying the human PAX6 locus. When crossed onto the Small eye background, the transgene rescues the mutant phenotype. Strikingly, mice carrying multiple copies on a wild-type background show specific developmental abnormalities of the eye, but not of other tissues expressing the gene. Thus, at least five different eye phenotypes are associated with changes in PAX6 expression. We provide evidence that not only reduced, but also increased levels of transcriptional regulators can cause developmental defects.

Ocular surface abnormalities in aniridia

PURPOSE

The purpose of this study was to examine ocular surface abnormalities in aniridia.

METHODS

We prospectively studied the ocular surface in nine consecutive patients (18 eyes) with virtually total aniridia. Of the 18 eyes, two eyes (two patients) were excluded from the study because severe surgical damage was likely to be the main cause of their corneal abnormalities. Six volunteers whose eye examinations yielded normal results served as control subjects. Detailed slit-lamp biomicroscopic examinations were performed to assess corneal findings and confirm the presence or absence of the palisades of Vogt. Goblet cell density on the peripheral corneal epithelium and inferior conjunctival epithelium was calculated via impression cytology and was statistically compared between aniridic eyes and control eyes.

RESULTS

All 16 aniridic eyes had superficial corneal opacification and vascularization of either the peripheral or entire cornea. Palisades of Vogt were completely absent all around the limbal area. Goblet cell density on the peripheral cornea and inferior conjunctiva showed a statistically significant increase in the aniridic eyes (226 +/- 169 cells/mm2 and 406 +/- 203 cells/mm2), as compared with the control eyes (O cells/mm2 and 181 +/- 89 cells/mm2) (P = .0005 and P = .0102).

CONCLUSIONS

These findings suggest that, in the aniridic eye, conjunctival epithelium invades the cornea because of corneal epithelial stem cell deficiency.

The PAX3-FKHR fusion protein created by the t(2;13) translocation in alveolar rhabdomyosarcomas is a more potent transcriptional activator than PAX3

Alveolar rhabdomyosarcomas are pediatric solid tumors with a hallmark cytogenetic abnormality: translocation of chromosomes 2 and 13 [t(2;13) (q35;q14)]. The genes on each chromosome involved in this translocation have been identified as the transcription factor-encoding genes PAX3 and FKHR. The NH2-terminal paired box and homeodomain DNA-binding domains of PAX3 are fused in frame to COOH-terminal regions of the chromosome 13-derived FKHR gene, a novel member of the forkhead DNA-binding domain family. To determine the role of the fusion protein in transcriptional regulation and oncogenesis, we identified the PAX3-FKHR fusion protein and characterized its function(s) as a transcription factor relative to wild-type PAX3. Antisera specific to PAX3 and FKHR were developed and used to examine PAX3 and PAX3-FKHR expression in tumor cell lines. Sequential immunoprecipitations with anti-PAX3 and anti-FKHR sera demonstrated expression of a 97-kDa PAX3-FKHR fusion protein in the t(2;13)-positive rhabdomyosarcoma Rh30 cell line and verified that a single polypeptide contains epitopes derived from each protein. The PAX3-FKHR protein was localized to the nucleus in Rh30 cells, as was wild-type PAX3, in t(2;13)-negative A673 cells. In gel shift assays using a canonical PAX binding site (e5 sequence), we found that DNA binding of PAX3-FKHR was significantly impaired relative to that of PAX3 despite the two proteins having identical PAX DNA-binding domains. However, the PAX3-FKHR fusion protein was a much more potent transcriptional activator than PAX3 as determined by transient cotransfection assays using e5-CAT reporter plasmids. The PAX3-FKHR protein may function as an oncogenic transcription factor by enhanced activation of normal PAX3 target genes.

PAX6 gene dosage effect in a family with congenital cataracts, aniridia, anophthalmia and central nervous system defects

The human eye malformation aniridia results from haploinsufficiency of PAX6, a paired box DNA-binding protein. To study this dosage effect, we characterized two PAX6 mutations in a family segregating aniridia and a milder syndrome consisting of congenital cataracts and late onset corneal dystrophy. The nonsense mutations, at codons 103 and 353, truncate PAX6 within the N-terminal paired and C-terminal PST domains, respectively. The wild-type PST domain activates transcription autonomously and the mutant form has partial activity. A compound heterozygote had severe craniofacial and central nervous system defects and no eyes. The pattern of malformations is similar to that in homozygous Sey mice and suggests a critical role for PAX6 in controlling the migration and differentiation of specific neuronal progenitor cells in the brain.

Defects of neuronal migration and the pathogenesis of cortical malformations are associated with Small eye (Sey) in the mouse, a point mutation at the Pax-6-locus

The mouse Small eye (Sey) locus is situated on chromosome 2. Molecular analyses have shown that SeyNeu represents a point mutation leading to a splice site error and loss of the functional gene product. The Sey locus has been shown to be identical with the paired box (Pax)-6 gene, which contains paired-like and homoeobox domains and is a developmental control gene. Pax-6 expression occurs in many parts of the central nervous system during embryogenesis. Therefore, we may expect the Sey mutation to result in abnormal development of the central nervous system. The present study shows that Pax-6 mutation has a bimodal effect upon neurogenesis in mouse: it causes a delay of premigratory neurons in a stage-, region-, and gene-dose-dependent manner. Additionally, Sey mutation impairs axonal growth and differentiation. Neurons of the cortical plate cease differentiation on approximately day 16 of gestation and appear to have increased cohesion: their cytoplasm is swollen and vacuolated. These changes coincide both with reduced formation of axons and with the onset of vacuolar degeneration in existing axons, glial cells and radial glial fibers. Consequently, there is an impairment of the peripheral migration of putative neurons so that the neonatal lesion pattern of the neocortical roof becomes dominated by a broad spectrum of neuronal migration disorders.