Mutations at the PAX6 locus are found in heterogeneous anterior segment malformations including Peters' anomaly

The upstream ectoderm enhancer in Pax6 has an important role in lens induction

The Pax6 gene has a central role in development of the eye. We show, through targeted deletion in the mouse, that an ectoderm enhancer in the Pax6 gene is required for normal lens formation. Ectoderm enhancer-deficient embryos exhibit distinctive defects at every stage of lens development. These include a thinner lens placode, reduced placodal cell proliferation, and a small lens pit and lens vesicle. In addition, the lens vesicle fails to separate from the surface ectoderm and the maturing lens is smaller and shows a delay in fiber cell differentiation. Interestingly, deletion of the ectoderm enhancer does not eliminate Pax6 production in the lens placode but results in a diminished level that, in central sections, is apparent primarily on the nasal side. This argues that Pax6 expression in the lens placode is controlled by the ectoderm enhancer and at least one other transcriptional control element. It also suggests that Pax6 enhancers active in the lens placode drive expression in distinct subdomains, an assertion that is supported by the expression pattern of a lacZ reporter transgene driven by the ectoderm enhancer. Interestingly, deletion of the ectoderm enhancer causes loss of expression of Foxe3, a transcription factor gene mutated in the dysgenetic lens mouse. When combined, these data and previously published work allow us to assemble a more complete genetic pathway describing lens induction. This pathway features (1) a pre-placodal phase of Pax6 expression that is required for the activity of multiple, downstream Pax6 enhancers; (2) a later, placodal phase of Pax6 expression regulated by multiple enhancers; and (3) the Foxe3 gene in a downstream position. This pathway forms a basis for future analysis of lens induction mechanism.

Haploinsufficient Bmp4 ocular phenotypes include anterior segment dysgenesis with elevated intraocular pressure

BACKGROUND

Glaucoma is a blinding disease usually associated with high intraocular pressure (IOP). In some families, abnormal anterior segment development contributes to glaucoma. The genes causing anterior segment dysgenesis and glaucoma in most of these families are not identified and the affected developmental processes are poorly understood. Bone morphogenetic proteins (BMPs) participate in various developmental processes. We tested the importance of Bmp4 gene dosage for ocular development and developmental glaucoma.

RESULTS

Bmp4+/- mice have anterior segment abnormalities including malformed, absent or blocked trabecular meshwork and Schlemm's canal drainage structures. Mice with severe drainage structure abnormalities, over 80% or more of their angle's extent, have elevated IOP. The penetrance and severity of abnormalities is strongly influenced by genetic background, being most severe on the C57BL/6J background and absent on some other backgrounds. On the C57BL/6J background there is also persistence of the hyaloid vasculature, diminished numbers of inner retinal cells, and absence of the optic nerve.

CONCLUSIONS

We demonstrate that heterozygous deficiency of BMP4 results in anterior segment dysgenesis and elevated IOP. The abnormalities are similar to those in human patients with developmental glaucoma. Thus, BMP4 is a strong candidate to contribute to Axenfeld-Rieger anomaly and other developmental conditions associated with human glaucoma. BMP4 also participates in posterior segment development and wild-type levels are usually critical for optic nerve development on the C57BL/6J background. Bmp4+/- mice are useful for studying various components of ocular development, and may allow identification of strain specific modifiers affecting a variety of ocular phenotypes.

A case of Peters' anomaly in a springer spaniel

An 8-week-old springer spaniel presented with a large central corneal opacity of the left globe, which was accompanied by cords of tissue spanning from the iris collarette to the posterior cornea. A posterior cortical cataract was noted in the right eye. At the owner's request the puppy was humanely destroyed, and a necropsy was performed. Upon sectioning the left globe in the vertical plane, a circle of pigmented strands of tissue was observed spanning the anterior chamber from the iris to the posterior aspect of the cornea. The right globe appeared normal when inspected grossly. Histologically, a membrane of pigmented tissue covered the posterior aspect of the broad central corneal leukoma of the left globe. This membrane and the cords traversing the anterior chamber were composed of vascular uveal tissue. Descemet's membrane and the corneal endothelium were reduced or absent in the zone of corneal opacity. Other than the changes associated with cataract, the right globe was histologically normal. The clinical and histological findings in the left globe were identical with those described for Peters>> anomaly in human beings.

Targeted Disruption of the Myocilin Gene (Myoc) Suggests that Human Glaucoma-Causing Mutations Are Gain of Function

Glaucoma is a heterogeneous eye disease and a major cause of blindness worldwide. Recently, primary open angle glaucoma (POAG)-associated mutations have been found in the trabecular meshwork inducible glucocorticoid response gene (TIGR), also known as the myocilin gene (MYOC), at the GLC1A locus on chromosome 1q21-q31. These mutations occurred in a subset of patients with juvenile- and adult-onset POAG and exhibited autosomal dominant inheritance. Ocular expression and its involvement in POAG suggest that TIGR/MYOC may have a role(s) in regulating intraocular pressure (IOP). Here, we report the generation and analysis of mice heterozygous and homozygous for a targeted null mutation in Myoc. Our study shows that Myoc mutant mice are both viable and fertile. Our in vivo findings further demonstrate that Myoc is not required for normal IOP or normal ocular morphology. The lack of a discernable phenotype in both Myoc-heterozygous and Myoc-null mice suggests that haploinsufficiency is not a critical mechanism for POAG in individuals with mutations in MYOC. Instead, disease-causing mutations in humans likely act by gain of function.

Activation of the human PAX6 gene through the exon 1 enhancer by transcription factors SEF and Sp1

PAX6 is a transcription factor that plays a major role in ocular morphogenesis. PAX6 is expressed in the eye, central nervous system and pancreas. Two alternative promoters, P0 and P1, which are differentially regulated during development, drive PAX6 transcription. We identified a 57 bp cis-regulatory element in exon 1 of the human PAX6 gene exon 1 enhancer (EIE). EIE enhances P1-driven PAX6 expression. Three regions in E1E (E1E-1, E1E-2 and E1E-3) have sequence similarities with binding sites of transcription factors ARP-1, Isl-1 and SEF, respectively. As shown by electrophoretic mobility shift assays, E1E-3, but not E1E-1 or E1E-2, bound to proteins in nuclear extracts of human glioma cells and transcription factor SEF bound to E1E-3. As shown by transient transfection experiments, deletion or site-specific mutations in E1E-3 dramatically decreased P1 promoter activity. Mutations in E1E-2, however, did not affect function of the P1 promoter. Co-transfection of SEF and PAX6 promoter-reporter constructs showed that SEF up-regulates PAX6 gene expression through the P1 promoter. Two Sp1 sites in the E1E region were also shown to be important by transient co-transfection assays. Data from immunoprecipitation and transient transfection assays demonstrated that SEF and Sp1 interacted in vitro and may act together in vivo to regulate PAX6 expression.

Renal coloboma syndrome

OBJECTIVE

To characterize the ocular features of renal coloboma syndrome.

DESIGN

Prospective, observational case series.

PARTICIPANTS

Twelve patients referred by the pediatric nephrology clinic and the ophthalmic records of five additional patients.

METHODS

For each patient, age at the time of examination, gender, renal function, and presence of a mutation in the PAX2 gene were noted. All patients underwent measurement of visual acuity and anterior and posterior segment examination with fundus photography. Goldmann visual fields were tested in four cases.

MAIN OUTCOME MEASURES

Visual acuity, optic disc abnormalities, and mutation in the PAX2 gene.

RESULTS

Mean age was 21.5 years. Renal failure was mild in 6 patients and severe in 11 patients. A mutation in the PAX2 gene was identified in nine patients, without correlation to the ocular phenotype. Ocular features could be divided into five groups: optic disc dysplasia limited to an unusual pattern of retinal vessels without functional consequence; optic disc pit with normal visual acuity and blind spot enlargement; large optic disc coloboma; large coloboma of the optic disc and adjacent retina; morning glory anomaly (these last three conditions were accompanied by poor visual acuity). Fundus abnormalities were symmetrical in most cases and unrelated to renal status.

CONCLUSIONS

Ophthalmic and renal characteristics of the renal coloboma syndrome are highly variable. The need for dialysis or renal transplantation can occur early in life or several years later. A wide range of ocular abnormalities located in the posterior segment can be observed. Mild optic disc dysplasia or pit have no functional consequence and can be underdiagnosed. More severe colobomas or related abnormalities, such as morning glory anomaly, often lead to poor visual acuity. Molecular biology allows detection of the mutations in the PAX2 gene, but can be negative in approximately 50% of cases. The observation of an optic disc coloboma or related abnormality stimulates the ophthalmologist to propose simple nephrologic investigations to check for renal hypoplasia, a potentially life-threatening disease. Conversely, renal hypoplasia stimulates the nephrologist to ask for a fundus examination to confirm the diagnosis and check for complications such as retinal detachment.

Primary defects in the lens underlie complex anterior segment abnormalities of the Pax6 heterozygous eye

We describe lens defects in heterozygous small eye mice, and autonomous deficiencies of Pax6(+/-) cells in the developing lens of Pax6(+/+) <--> Pax6(+/-) chimeras. Two separate defects of the lens were identified by analyzing the distribution of heterozygous cells in chimeras: Pax6(+/-) cells are less readily incorporated into the lens placode than wild type, and those that are incorporated into the lens are not maintained efficiently in the proliferating lens epithelium. The lens of chimeric eyes is, therefore, predominantly wild type from embryonic day 16.5 onwards, whereas heterozygous cells contribute normally to all other eye tissues. Eye size and defects of the iris and cornea are corrected in fetal and adult chimeras with up to 80% mutant cells. Therefore, these aspects of the phenotype may be secondary consequences of primary defects in the lens, which has clinical relevance for the human aniridia (PAX6(+/-)) phenotype.

Molecular evolution of the homeodomain family of transcription factors

The homeodomain family of transcription factors plays a fundamental role in a diverse set of functions that include body plan specification, pattern formation and cell fate determination during metazoan development. Members of this family are characterized by a helix-turn-helix DNA-binding motif known as the homeodomain. Homeodomain proteins regulate various cellular processes by specifically binding to the transcriptional control region of a target gene. These proteins have been conserved across a diverse range of species, from yeast to human. A number of inherited human disorders are caused by mutations in homeodomain-containing proteins. In this study, we present an evolutionary classification of 129 human homeodomain proteins. Phylogenetic analysis of these proteins, whose sequences were aligned based on the three-dimensional structure of the homeodomain, was performed using a distance matrix approach. The homeodomain proteins segregate into six distinct classes, and this classification is consistent with the known functional and structural characteristics of these proteins. An ancestral sequence signature that accurately describes the unique sequence characteristics of each of these classes has been derived. The phylogenetic analysis, coupled with the chromosomal localization of these genes, provides powerful clues as to how each of these classes arose from the ancestral homeodomain.

The eyeless homeodomain is dispensable for eye development in Drosophila

Pax-6 genes, known to be essential for eye development, encode an evolutionarily conserved transcription factor with two DNA-binding domains. To corroborate the contribution of each DNA-binding domain to eye formation, we generated truncated forms of the Drosophila Pax-6 gene eyeless and tested their capacity to rescue the ey(2) mutant. Surprisingly, EY deleted of the homeodomain rescued the ey(2) mutant and triggered ectopic eyes morphogenesis. In contrast, EY lacking the paired domain failed to rescue the ey(2) mutant, led to truncation of appendages, and repressed Distal-less when misexpressed. This result suggests distinct functions mediated differentially by the two DNA-binding domains of eyeless.

A 76-bp deletion in the Mip gene causes autosomal dominant cataract in Hfi mice

Hfi is a dominant cataract mutation where heterozygotes show hydropic lens fibers and homozygotes show total lens opacity. The Hfi locus was mapped to the distal part of mouse chromosome 10 close to the major intrinsic protein (Mip), which is expressed only in cell membranes of lens fibers. Molecular analysis of Mip revealed a 76-bp deletion that resulted in exon 2 skipping in Mip mRNA. In Hfi/Hfi this deletion resulted in a complete absence of the wildtype Mip. In contrast, Hfi/+ animals had the same amount of wildtype Mip as +/+. Results from pulse-chase expression studies excluded hetero-oligomerization of wildtype and mutant Mip as a possible mechanism for cataract formation in the Hfi/+. We propose that the cataract phenotype in the Hfi heterozygote mutant is due to a detrimental gain of function by the mutant Mip resulting in either cytotoxicity or disruption in processing of other proteins important for the lens. Cataract formation in the Hfi/Hfi mouse is probably a combined result of both the complete loss of wildtype Mip and a gain of function of the mutant Mip.

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.

The mouse anterior chamber angle and trabecular meshwork develop without cell death

BACKGROUND

The iridocorneal angle forms in the mammalian eye from undifferentiated mesenchyme between the root of the iris and cornea. A major component is the trabecular meshwork, consisting of extracellular matrix organized into a network of beams, covered in trabecular endothelial cells. Between the beams, channels lead to Schlemm's canal for the drainage of aqueous humor from the eye into the blood stream. Abnormal development of the iridocorneal angle that interferes with ocular fluid drainage can lead to glaucoma in humans. Little is known about the precise mechanisms underlying angle development. There are two main hypotheses. The first proposes that morphogenesis involves mainly cell differentiation, matrix deposition and assembly of the originally continuous mesenchymal mass into beams, channels and Schlemm's canal. The second, based primarily on rat studies, proposes that cell death and macrophages play an important role in forming channels and beams. Mice provide a potentially useful model to understand the origin and development of angle structures and how defective development leads to glaucoma. Few studies have assessed the normal structure and development of the mouse angle. We used light and electron microscopy and a cell death assay to define the sequence of events underlying formation of the angle structures in mice.

RESULTS

The mouse angle structures and developmental sequence are similar to those in humans. Cell death was not detectable during the period of trabecular channel and beam formation.

CONCLUSIONS

These results support morphogenic mechanisms involving organization of cellular and extracellular matrix components without cell death or atrophy.

Superactivation of Pax6-mediated transactivation from paired domain-binding sites by dna-independent recruitment of different homeodomain proteins

The Pax6 genes encode evolutionary conserved transcription factors that act high up in the regulatory hierarchy controlling development of central organs such as the eyes and the central nervous system. These proteins contain two DNA-binding domains. The N-terminal paired domain is separated from a paired-type homeodomain by a linker region, and a transactivation domain is located C-terminal to the homeodomain. Vertebrate Pax6 genes express a paired-less isoform of Pax6 (Pax6DeltaPD) from an internal start codon in the coding region between the paired domain and homeodomain. We now provide evidence for an interaction between the full-length isoform and Pax6DeltaPD, which enhances the transactivation activity of Pax6 from paired domain-binding sites. The paired-like homeodomain protein Rax behaved similarly to Pax6DeltaPD. Both Pax6DeltaPD and Rax bound to the homeodomain of Pax6 in vitro in the absence of specific DNA binding. Coimmunoprecipitation experiments following cotransfection confirmed the existence of complexes between Pax6 and Pax6DeltaPD, Pax6 and Rax, and Pax6DeltaPD and Rax in vivo. Interestingly, the C-terminal subdomain of the paired domain and the homeodomain can interact with each other. The paired domain can also interact with itself. Surprisingly, GST pull-down assays revealed that the homeodomains of such diverse proteins as Chx10, Six3, Lhx2, En-1, Prep1, Prox1, and HoxB1 could all bind to Pax6, and several of these enhanced Pax6-mediated transactivation upon coexpression. Since many homeodomain proteins are coexpressed with Pax6 in several tissues during development, our results indicate the existence of novel regulatory interactions that may be important for fine tuning of gene regulation.

Dosage requirement and allelic expression of PAX6 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.

3' deletions cause aniridia by preventing PAX6 gene expression

Aniridia is a panocular human eye malformation caused by heterozygous null mutations within PAX6, a paired-box transcription factor, or cytogenetic deletions of chromosome 11p13 that encompass PAX6. Chromosomal rearrangements also have been described that disrupt 11p13 but spare the PAX6 transcription unit in two families with aniridia. These presumably cause a loss of gene expression, by removing positive cis regulatory elements or juxtaposing negative DNA sequences. We report two submicroscopic de novo deletions of 11p13 that cause aniridia but are located >11 kb from the 3' end of PAX6. The clinical manifestations are indistinguishable from cases with chain-terminating mutations in the coding region. Using human x mouse retinoblastoma somatic cell hybrids, we show that PAX6 is transcribed only from the normal allele but not from the deleted chromosome 11 homolog. Our findings suggest that remote 3' regulatory elements are required for initiation of PAX6 expression.

Identification of RALDH-3, a novel retinaldehyde dehydrogenase, expressed in the ventral region of the retina

In the developing retina, a retinoic acid (RA) gradient along the dorso-ventral axis is believed to be a prerequisite for the establishment of dorso-ventral asymmetry. This RA gradient is thought to result from the asymmetrical distribution of RA-generating aldehyde dehydrogenases along the dorso-ventral axis. Here, we identified a novel aldehyde dehydrogenase specifically expressed in the chick ventral retina, using restriction landmark cDNA scanning (RLCS). Since this molecule showed enzymatic activity to produce RA from retinaldehyde, we designated it retinaldehyde dehydrogenase 3 (RALDH-3). Structural similarity suggested that RALDH-3 is the orthologue of human aldehyde dehydrogenase 6. We also isolated RALDH-1 which is expressed in the chick dorsal retina and implicated in RA formation. Raldh-3 was preferentially expressed first in the surface ectoderm overlying the ventral portion of the prospective eye region and then in the ventral retina, earlier than Raldh-1 in chick and mouse embryos. High level expression of Raldh-3 was also observed in the nasal region. In addition, we found that Pax6 mutants are devoid of Raldh-3 expression. These results suggested that Raldh-3 is the key enzyme in the formation of an RA gradient along the dorso-ventral axis during the early eye development, and also in the development of the olfactory system.

Genetics of the corneal dystrophies: what we have learned in the past twenty-five years

PURPOSE

To indicate important changes in our understanding of the corneal dystrophies.

METHODS

A review of the literature of the last quarter of a century.

RESULTS

The earliest clinical classifications of the corneal dystrophies were based on the application of clinical, biological, histochemical, and ultrastructural methods. Since then, the first great impetus to our understanding has come from the application of techniques to map disorders to specific chromosome loci, using polymorphic markers. More recently, using candidate gene and related approaches, it has been possible to identify genes causing several of the corneal dystrophies and the mutations responsible for their phenotypic variation. A notable success has been to show that several important "stromal" dystrophies result from mutations in the gene beta ig-h3, which encodes for the protein keratoepithelin (beta ig-h3).

CONCLUSIONS

For the corneal dystrophies, as with other inherited disorders, there is room for two sorts of classification system, one based mainly on clinical presentation and the other on an up-to-date understanding of the genetic mechanisms. They are not mutually exclusive. Some developmental corneal disorders are also discussed.

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.

Ten novel mutations found in Aniridia

Aniridia (AN) is a sight-threatening congenital ocular disorder characterized by iris hypoplasia, corneal pannus, foveal and optic nerve hypoplasia, cataract formation, and glaucoma. In two-thirds of the patients, AN is inherited in an autosomal dominant fashion with almost complete penetrance but variable expression. The remaining cases are sporadic. Aniridia has been shown to be associated with mutations in the PAX6 gene, located on chromosome 11p13, telomeric to the Wilms' tumor predisposition gene (WT1). This paper describes 14 mutations in the PAX6 gene in patients with AN. Among these 14 mutations, 10 have been unpublished until now. They result most probably in haploinsufficiency and consequently in a reduced protein level of functional PAX6 protein. The mutations reported here are scattered all over the gene, including the paired-box, the glycine-rich region, the homeobox, and the proline-serine-threonine (PST)-rich region.

A new set of primers for mutation analysis of the human PAX6 gene

Mutations in the human PAX6 gene are an important cause of dominantly inherited congenital malformations of the eye, including aniridia, Peters' anomaly, keratitis, and isolated foveal hypoplasia. To satisfy the need for efficient detection of PAX6 mutations, we have developed a new set of oligonucleotides for genomic SSCP based on the recently completed genomic sequence of the entire human PAX6 gene. We also describe PAX6 mutations in eight aniridia patients, five of which are novel.

Perspectives on eye development

The lens of the vertebrate eye was the classic model used to demonstrate the concepts of inductive interactions controlling development. However, it is in the Drosophila model that the greatest progress in understanding molecular mechanisms of eye development have most recently been mode. This progress can be attributed to the power of molecular genetics, an approach that was once confined to simpler systems like worms and flies, but is now becoming possible in vertebrates. Thus, the use of transgenic and knock-out gene technology, coupled with the availability of new positional cloning methods, has recently initiated a surge of progress in the mouse genetic model and has also led to the identification of genes involved in human inherited disorders. In addition, gene transfer techniques have opened up opportunities for progress using chick, Xenopus, and other classic developmental systems. Finally, a new vertebrate genetic model, zebrafish, appears very promising for molecular studies. As a result of the opportunities presented by these new approaches, eye development has come into the limelight, hence the timeliness of this focus issue of Developmental Genetics. In this introductory review, we discuss three areas of current work arising through the use of these newer genetic approaches, and pertinent to research articles presented herein. We also touch on related studies reported at the first Keystone Meeting on Ocular Cell and Molecular Biology, recently held in Tamarron Springs, Colorado, January 7-12, 1997.

Forkhead Foxe3 maps to the dysgenetic lens locus and is critical in lens development and differentiation

Here we report the isolation of a novel forkhead gene, Foxe3, that plays an important role in lens formation. During development Foxe3 is expressed in all undifferentiated lens tissues, and is turned off upon fiber cell differentiation. Foxe3 maps to a chromosomal region containing the dysgenetic lens (dyl) mutation. Mice homozygous for dyl display several defects in lens development. dyl mice also show altered patterns of crystallin expression suggesting a dysregulation of lens differentiation. We have identified mutations in Foxe3 that cosegregate with the dyl phenotype and are a likely cause of the mutant phenotype. Head ectoderm expression of Foxe3 is absent in Rx-/- and Small eye embryos indicating that Rx and Pax6 activity are necessary for Foxe3 expression.

Pax-6 expression and activity are induced in the reepithelializing cornea and control activity of the transcriptional promoter for matrix metalloproteinase gelatinase B

Recent evidence supports the idea that matrix metalloproteinases (MMPs) act as morphogenetic regulators in embryonic and adult events of tissue remodeling. MMP activity is controlled primarily at the level of gene expression. In a recent study we characterized the transcriptional promoter of the MMP gene, gelatinase B (gelB), in transgenic mice, demonstrating the requirement for DNA sequences between -522 and +19 for appropriate activity. In this study we investigated factors required for gelB promoter activity in the developing eye and reepithelializing adult cornea. Pax-6 is a homeobox and paired domain transcription factor that acts at the top of the hierarchy of genes controlling eye development. Pax-6 is also expressed in the adult eye. We show here that the tissue expression pattern of Pax-6 overlaps extensively with gelB promoter activity in the developing and adult eye. In addition Pax-6 is observed to be upregulated in repairing corneal epithelium, as is gelB promoter activity. In cell culture transfection experiments, we identified two promoter regions which mediate positive response to Pax-6. By electrophoretic mobility shift assay, we further pinpoint two Pax-6 binding sites within these response regions and demonstrate direct interaction of the Pax-6 paired domain with one of these sites. These data suggest a mechanism by which Pax-6 may direct gelB expression in an eye-specific manner.

Prenatal diagnosis of aniridia

OBJECTIVE

To use molecular genetic techniques to prenatally screen for aniridia.

DESIGN

Case report.

METHODS

DNA was extracted from cultured fibroblasts obtained through amniocentesis. Two mutation detection methods, Ava1 restriction digestion and single-strand conformational polymorphism electrophoresis, were used to screen the PAX6 gene.

MAIN OUTCOME MEASURES

The results from the amniocentesis sample were compared with DNA obtained from the affected father, firstborn infant, and unaffected mother to determine whether the fetus carried the PAX6 mutation.

RESULTS

DNA from the fetus demonstrated the same banding pattern as the affected father and firstborn infant.

CONCLUSIONS

The fetus carried the mutated PAX6 allele and was predicted to develop aniridia. This was later confirmed when the child was born. This case report illustrates an important use of genetic mutation screening in the clinical setting.

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.

Two nonsense mutations of PAX6 in two Japanese aniridia families: case report and review of the literature

PURPOSE

To identify PAX6 mutations in patients from four Japanese families with aniridia.

METHODS

Polymerase chain reaction (PCR)-single stand conformational polymorphism (SSCP) analysis (SSCA) was performed in probands of the families, and restriction analysis using MaeIII or AvaI was carried out in other affected family members.

RESULTS

PCR-SSCA demonstrated in the proband from one family an extra-band in the PCR product for PAX6 exon 8. Base sequence analysis revealed that the patient is a heterozygote for a C to T transition mutation at codon 203. DNAs from the patient and another affected member in the same family were cut with MaeIII into two fragments, while non-affected members in the family showed only one MaeIII fragment, the result confirmed the mutation. In another family, PCR-SSCA revealed an extra-band in the PCR product for exon 9. Sequencing detected a C-->T substitution at codon 240 in the patient, the mutation resulted in loss of an AvaI site. AvaI cleavage analysis confirmed the mutation in the patient. The two transition mutations observed in the two families also predict the conversion of arginine to a stop codon (R203X and R240X, respectively) around the homeodomain (HD), leading to the truncation of the PAX6 protein within its glycine-rich region. No abnormal SSCP bands or abnormal restriction fragments were detected in patients from the other two families.

CONCLUSIONS

The two mutations sites identified in the two families, one at codon 203 and the other at codon 240, are those most frequently observed among 118 previously reported PAX6 mutations. This indicates that the two mutations are two hot-spots in the gene.

Syndromes associated with Homo sapiens pol II regulatory genes

The molecular basis of human characteristics is an intriguing but an unresolved problem. Human characteristics cover a broad spectrum, from the obvious to the abstract. Obvious characteristics may include morphological features such as height, shape, and facial form. Abstract characteristics may be hidden in processes that are controlled by hormones and the human brain. In this review we examine exaggerated characteristics presented as syndromes. Specifically, we focus on human genes that encode transcription factors to examine morphological, immunological, and hormonal anomalies that result from deletion, insertion, or mutation of genes that regulate transcription by RNA polymerase II (the Pol II genes). A close analysis of abnormal phenotypes can give clues into how sequence variations in regulatory genes and changes in transcriptional control may give rise to characteristics defined as complex traits.

A novel Pax-6 binding site in rodent B1 repetitive elements: coevolution between developmental regulation and repeated elements?

Pax-6 encodes a transcription factor that is important in the development of eye and CNS. Identification of Pax-6 target genes is crucial for understanding the gene regulatory network in these developmental processes. Using an in-vitro approach of cyclic amplification of the protein binding sequences (CAPBS), we isolated a PAX6 binding sequence from a human single-copy (sc) DNA library. Characterization of this PAX6 binding sequence revealed a 15bp region (hGCalpha1BLs5) that is sufficient for PAX6 specific binding. From a homology search in the GenBank, we found that an hGCalpha1BLs5-like Pax-6 binding site exists in 21 genes (16 from rodent), 15 of which were shown to be able to bind Pax-6 in vitro. Interestingly, some of these sites occur in B1 repetitive elements. Although hGCalpha1BLs5 is highly similar to a region in B1 repetitive elements, PAX6 does not bind to the consensus sequence in B1. However, a single-step mutation in some B1 elements can lead to a gain of function for PAX6 binding. This experimental evidence and phylogenetic analysis raise an interesting speculation for the coevolution between PAX6 regulation and repeat elements. Since a (Pax-6-binding) null B1 element can be re-activated by even a single-step mutation, it has the potential to recruit gene targets for Pax-6 if it is inserted into the regulatory region, and therefore may play a role for evolutionary modification of Pax-6 regulation.

Formation of corneal endothelium is essential for anterior segment development - a transgenic mouse model of anterior segment dysgenesis

The anterior segment of the vertebrate eye is constructed by proper spatial development of cells derived from the surface ectoderm, which become corneal epithelium and lens, neuroectoderm (posterior iris and ciliary body) and cranial neural crest (corneal stroma, corneal endothelium and anterior iris). Although coordinated interactions between these different cell types are presumed to be essential for proper spatial positioning and differentiation, the requisite intercellular signals remain undefined. We have generated transgenic mice that express either transforming growth factor (alpha) (TGF(alpha)) or epidermal growth factor (EGF) in the ocular lens using the mouse (alpha)A-crystallin promoter. Expression of either growth factor alters the normal developmental fate of the innermost corneal mesenchymal cells so that these cells often fail to differentiate into corneal endothelial cells. Both sets of transgenic mice subsequently manifest multiple anterior segment defects, including attachment of the iris and lens to the cornea, a reduction in the thickness of the corneal epithelium, corneal opacity, and modest disorganization in the corneal stroma. Our data suggest that formation of a corneal endothelium during early ocular morphogenesis is required to prevent attachment of the lens and iris to the corneal stroma, therefore permitting the normal formation of the anterior segment.

A forkhead gene, FoxE3, is essential for lens epithelial proliferation and closure of the lens vesicle

In the mouse mutant dysgenetic lens (dyl) the lens vesicle fails to separate from the ectoderm, causing a fusion between the lens and the cornea. Lack of a proliferating anterior lens epithelium leads to absence of secondary lens fibers and a dysplastic, cataractic lens. We report the cloning of a gene, FoxE3, encoding a forkhead/winged helix transcription factor, which is expressed in the developing lens from the start of lens placode induction and becomes restricted to the anterior proliferating cells when lens fiber differentiation begins. We show thatFoxE3 is colocalized with dyl in the mouse genome, thatdyl mice have mutations in the part of FoxE3 encoding the DNA-binding domain, and that these mutations cosegregate with thedyl phenotype. During embryonic development, the primordial lens epithelium is formed in an apparently normal way in dylmutants. However, instead of the proliferation characteristic of a normal lens epithelium, the posterior of these cells fail to divide and show signs of premature differentiation, whereas the most anterior cells are eliminated by apoptosis. This implies that FoxE3 is essential for closure of the lens vesicle and is a factor that promotes survival and proliferation, while preventing differentiation, in the lens epithelium.

Missense mutation in the alternative splice region of the PAX6 gene in eye anomalies

The PAX6 gene is involved in ocular morphogenesis, and PAX6 mutations have been detected in various types of ocular anomalies, including aniridia, Peters anomaly, corneal dystrophy, congenital cataract, and foveal hypoplasia. The gene encodes a transcriptional regulator that recognizes target genes through its paired-type DNA-binding domain. The paired domain is composed of two distinct DNA-binding subdomains, the N-terminal subdomain (NTS) and the C-terminal subdomain (CTS), which bind respective consensus DNA sequences. The human PAX6 gene produces two alternative splice isoforms that have the distinct structure of the paired domain. The insertion, into the NTS, of 14 additional amino acids encoded by exon 5a abolishes the DNA-binding activity of the NTS and unmasks the DNA-binding ability of the CTS. Thus, exon 5a appears to function as a molecular switch that specifies target genes. We ascertained a novel missense mutation in four pedigrees with Peters anomaly, congenital cataract, Axenfeldt anomaly, and/or foveal hypoplasia, which, to our knowledge, is the first mutation identified in the splice-variant region. A T-->A transition at the 20th nucleotide position of exon 5a results in a Val-->Asp (GTC-->GAC) substitution at the 7th codon of the alternative splice region. Functional analyses demonstrated that the V54D mutation slightly increased NTS binding and decreased CTS transactivation activity to almost half.

Ocular anterior chamber dysgenesis in craniosynostosis syndromes with a fibroblast growth factor receptor 2 mutation

Fibroblast growth factor receptor (FGFR) mutations have been found in craniosynostosis syndromes with and without limb and/or dermatologic anomalies. Ocular manifestations of FGFR2 syndromes are reported to include shallow orbits, proptosis, strabismus, and hypertelorism, but no ocular anterior chamber, structural abnormalities have been reported until now. We evaluated three unrelated patients with severe Crouzon or Pfeiffer syndrome. Two of them had ocular findings consistent with Peters anomaly, and the third patient had opaque corneae, thickened irides and ciliary bodies, and shallow anterior chambers with occluded angles. Craniosynostosis with and without cloverleaf skull deformity, large anterior fontanelle, hydrocephalus, proptosis, depressed nasal bridge, choanal stenosis/ atresia, midface hypoplasia, and elbow contractures were also present. These patients had airway compromise, seizures, and two died by age 15 months. All three cases were found to have the same FGFR2 Ser351Cys (1231C to G) mutation predicted to form an aberrant disulfide bond(s) and affect ligand binding. Seven patients with isolated Peters anomaly, two patients with Peters plus syndrome, and three cases with typical Antley-Bixler syndrome were screened for this mutation, but none was found. These phenotype/genotype data demonstrate that FGFR2 is involved in the development of the anterior chamber of the eye and that the Ser351Cys mutation is associated with a severe phenotype and clinical course.

Association analysis of a PAX-6 gene promoter-associated polymorphic repeat with alcohol dependence

The human paired box-containing gene PAX-6 participates in the development and plasticity of the brain including the limbic system, the neural system that plays a crucial role in reward processes. We have reported recently a polymorphic dinucleotide repeat sequence with the structure (AC)m(AG)n, which is located approximately 1 kb upstream of the transcription initiation site associated with promoter B and confers allelic variation of PAX-6 expression in the human brain. In the present association study we tested whether length variation of PAX-6 gene-linked polymorphic region (PAX-6 LPR) influences susceptibility to alcohol dependence.The repeat length of the PAX-6 LPR was assessed in 354 control subjects and 328 alcohol-dependent patients, including four subgroups with a presumed substantial genetic predisposition: (a) with a history of withdrawal complications (n=100); (b) with a history of parental alcoholism (n=115); (c) with early onset (n=67) and (d) with dissocial personality disorders (n=54). Allelic distribution of the PAX-6 LPR did not differ significantly between the controls and the entire group of alcohol-dependent patients χ²=0.015, df 1, p=0.904), or any of the subgroups of patients with severe alcoholism. Our results do not provide evidence that length variation of the PAX-6 LPR contributes to the pathogenesis of alcohol dependence.

PAX6 expression in the developing human eye

AIMS

To investigate the changes in PAX6 expression in the developing human eye.

METHODS

Six developing human eyes from 6 to 22 weeks' gestation were evaluated. Frozen sections were immunohistochemically stained with monoclonal antibody to chick Pax6 (amino acids 1-223). To verify antibody specificity, western blot analysis was carried out using cell lysates from P19 cells transfected with the human PAX6 gene.

RESULTS

Western blot analysis demonstrated that the antibody reacted to human PAX6 protein. Positive immunostainings for PAX6 were seen in the surface ectoderm, lens vesicle, inner and outer layers of the optic cup, and optic stalk at 6 weeks, and in the corneal epithelia and conjunctiva, lens, and non-pigmented ciliary epithelia from 8 to 22 weeks. In the retina, positive cells were seen in the entire retina from 8 to 10 weeks, and were restricted to the ganglion cell layer and the inner and outer portions of the inner nuclear layer after 21 weeks.

CONCLUSIONS

PAX6 is expressed on the surface and neuroectoderms at an early stage, then in the differentiating cells in the cornea, lens, ciliary body, and retina through development. PAX6 may play a role in determining cell fate in the morphogenesis of various human ocular tissue.

Monoallelic expression of Pax5: a paradigm for the haploinsufficiency of mammalian Pax genes?

It is generally assumed that most mammalian genes are transcribed from both alleles. Hence, the diploid state of the genome offers the advantage that a loss-of-function mutation in one allele can be compensated for by the remaining wild-type allele of the same gene. Indeed, the vast majority of human disease syndromes and engineered mutations in the mouse genome are recessive, indicating that recessiveness is the 'default' state. However, a minority of genes are semi-dominant, as heterozygous loss-of-function mutation in these genes leads to phenotypic abnormalities. This condition, known as haploinsufficiency, has been described for five of the nine mammalian Pax genes, which are associated with mouse developmental mutants and human disease syndromes. Recently we have reported that the Pax5 gene is subject to allele-specific regulation during B cell development. Pax5 is predominantly transcribed from only one of its two alleles in early B-lymphoid progenitors and mature B cells, while it transiently switches to a biallelic mode of transcription in pre-B and immature B cells. As a consequence, B-lymphoid tissues are mosaic with regard to the transcribed allele, and heterozygous mutation of Pax5 therefore results in deletion of B lymphocytes expressing only the mutant allele. The allele-specific regulation of Pax5 raises the intriguing possibility that monoallelic expression may also be the mechanism causing the haploinsufficiency of other Pax genes. In this review, we discuss different models accounting for the haploinsufficiency of mammalian Pax genes, provide further evidence in support of the allele-specific regulation of Pax5 and discuss the implication of these findings in the context of the recent literature describing the stochastic and monoallelic activation of other hematopoietic genes.

Crystal structure of the human Pax6 paired domain-DNA complex reveals specific roles for the linker region and carboxy-terminal subdomain in DNA binding

Pax6, a transcription factor containing the bipartite paired DNA-binding domain, has critical roles in development of the eye, nose, pancreas, and central nervous system. The 2.5 A structure of the human Pax6 paired domain with its optimal 26-bp site reveals extensive DNA contacts from the amino-terminal subdomain, the linker region, and the carboxy-terminal subdomain. The Pax6 structure not only confirms the docking arrangement of the amino-terminal subdomain as seen in cocrystals of the Drosophila Prd Pax protein, but also reveals some interesting differences in this region and helps explain the sequence specificity of paired domain-DNA recognition. In addition, this structure gives the first detailed information about how the paired linker region and carboxy-terminal subdomain contact DNA. The extended linker makes minor groove contacts over an 8-bp region, and the carboxy-terminal helix-turn-helix unit makes base contacts in the major groove. The structure and docking arrangement of the carboxy-terminal subdomain of Pax6 is remarkably similar to that of the amino-terminal subdomain, and there is an approximate twofold symmetry axis relating the polypeptide backbones of these two helix-turn-helix units. Our structure of the Pax6 paired domain-DNA complex provides a framework for understanding paired domain-DNA interactions, for analyzing mutations that map in the linker and carboxy-terminal regions of the paired domain, and for modeling protein-protein interactions of the Pax family proteins.

Autosomal dominant cataracts and Peters anomaly in a large Australian family

Peters anomaly is a congenital corneal opacity with underlying defects in the posterior stroma, Descemets membrane and corneal endothelium. It is a disorder resulting from abnormal migration or function of neural crest cells and may include abnormalities of other anterior segment structures, such as the lens and iris. We report a family in which anterior segment abnormalities, including Peters anomaly and cataracts, were inherited in an autosomal dominant fashion. Although the PAX6 gene on chromosome 11 has been shown to be involved in some cases of anterior segment developmental defects, we found no evidence that the condition in this family is linked to the PAX6 gene. Identification of this gene will indicate another gene with major involvement in the development of the anterior segment of the eye.

A mutation in the RIEG1 gene associated with Peters’ anomaly

Mutations within the RIEG1 homeobox gene on chromosome 4q25 have previously been reported in association with Rieger syndrome. We report a 3′ splice site mutation within the 3rd intron of the RIEG1 gene which is associated with unilateral Peters’ anomaly. The mutation is a single base substition of A to T at the invariant -2 site of the 3′ splice site. Peters’ anomaly, which is characterised by ocular anterior segment dysgenesis and central corneal opacification, is distinct from Rieger anomaly. This is the first description of a RIEG1 mutation associated with Peters’ anomaly.

AP-2alpha transcription factor is required for early morphogenesis of the lens vesicle

AP-2 transcription factors are a family of retinoic acid-responsive genes, which are involved in complex morphogenetic processes. In the current study, we determine the requirement for AP-2alpha in early morphogenesis of the eye by examining the nature of the ocular defects in AP-2alpha null and chimeric mice. AP-2alpha null embryos exhibited ocular phenotypes ranging from a complete lack of eyes (anophthalmia) to defects in the developing lens involving a persistent adhesion of the lens to the overlying surface ectoderm. Two genes involved in lens development and differentiation, Pax6 and MIP26 were also misexpressed. AP-2alpha mutants also exhibited defects in the optic cup consisting of transdifferentiation of the dorsal retinal pigmented epithelium into neural retina and the absence of a defined ganglion cell layer. Newly generated chimeric embryos consisting of a population of AP-2alpha-/- and AP-2alpha+/+ cells exhibit ocular defects similar to those seen in the knockout embryos. Immunolocalization of AP-2 proteins (alpha, beta, and gamma) to the normal developing eye revealed both unique and overlapping expression patterns, with AP-2alpha expressed in a number of the ocular tissues that exhibited defects in the mutants, including the developing lens where AP-2alpha is uniquely expressed. Together these findings demonstrate a requirement for AP-2alpha in early morphogenesis of the eye.

BMP4 is essential for lens induction in the mouse embryo

Vertebrate lens development is a classical model system for studying embryonic tissue interactions. Little is known, however, about the molecules mediating such inductive events. Here, we show that Bmp4, which is expressed strongly in the optic vesicle and weakly in the surrounding mesenchyme and surface ectoderm, has crucial roles during lens induction. In Bmp4(tm1) homozygous null mutant embryos, lens induction is absent, but the process can be rescued by exogenous BMP4 protein applied into the optic vesicle in explant cultures. This is associated with rescue of ectodermal expression of Sox2, an early lens placode marker. Substituting the optic vesicle in explant cultures with BMP4-carrying beads, however, does not lead to lens induction, indicating that other factors produced by the optic vesicle are involved. BMP4 appears to regulate expression of a putative downstream gene, Msx2, in the optic vesicle. No change in Pax6 expression is seen in Bmp4(tm1) mutant eyes, and Bmp4 expression appears unaffected in the eyes of homozygous Pax6(Sey-1Neu), suggesting that PAX6 and BMP4 function independently. Based on these results we propose that BMP4 is required for the optic vesicle to manifest its lens-inducing activity, by regulating downstream genes and/or serving as one component of multiple inductive signals.

Autosomal dominant nanophthalmos (NNO1) with high hyperopia and angle-closure glaucoma maps to chromosome 11

Nanophthalmos is an uncommon developmental ocular disorder characterized by a small eye, as indicated by short axial length, high hyperopia (severe farsightedness), high lens/eye volume ratio, and a high incidence of angle-closure glaucoma. We performed clinical and genetic evaluations of members of a large family in which nanophthalmos is transmitted in an autosomal dominant manner. Ocular examinations of 22 affected family members revealed high hyperopia (range +7.25-+13.00 diopters; mean +9.88 diopters) and short axial length (range 17.55-19.28 mm; mean 18.13 mm). Twelve affected family members had angle-closure glaucoma or occludable anterior-chamber angles. Linkage analysis of a genome scan demonstrated highly significant evidence that nanophthalmos in this family is the result of a defect in a previously unidentified locus (NNO1) on chromosome 11. The gene was localized to a 14.7-cM interval between D11S905 and D11S987, with a maximum LOD score of 5. 92 at a recombination fraction of .00 for marker D11S903 and a multipoint maximum LOD score of 6.31 for marker D11S1313. NNO1 is the first human locus associated with nanophthalmos or with an angle-closure glaucoma phenotype, and the identification of the NNO1 locus is the first step toward the cloning of the gene. A cloned copy of the gene will enable examination of the relationship, if any, between nanophthalmos and less severe forms of hyperopia and between nanophthalmos and other conditions in which angle-closure glaucoma is a feature.

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.

A series of no isthmus (noi) alleles of the zebrafish pax2.1 gene reveals multiple signaling events in development of the midbrain-hindbrain boundary

Generation of cell diversity in the vertebrate central nervous system starts during gastrulation stages in the ectodermal germ layer and involves specialized cell groups, such as the organizer located at the midbrain-hindbrain boundary (MHB). Mutations in the zebrafish no isthmus (noi) gene alter development of the MHB, and affect the pax2.1 gene (formerly pax(zf-b)). Analysis of the structure of pax2.1 reveals at least 12 normal splice variants. The noi alleles can be arranged, by molecular and phenotypic criteria, into a series of five alleles of differing strength, ranging from a null allele to weak alleles. In keeping with a role in development of the MHB organizer, gene expression is already affected in the MHB primordium of the gastrula neural ectoderm in noi mutants. eng3 activation is completely and eng2 activation is strongly dependent on noi function. In contrast, onset of wnt1, fgf8 and her5 expression occurs normally in the null mutants, but is eliminated later on. Our observations suggest that three signaling pathways, involving pax2.1, wnt1 and fgf8, are activated independently in early anterior-posterior patterning of this area. In addition, analysis of the allelic series unexpectedly suggests that noi activity is also required during dorsal-ventral patterning of the MHB in somitogenesis stages, and possibly in a later eng expression phase. We propose that noi/pax2.1 participates in sequential signaling processes as a key integrator of midbrain-hindbrain boundary development.

Vsx-2, a gene encoding a paired-type homeodomain, is expressed in the retina, hindbrain, and spinal cord during goldfish embryogenesis

Vsx-2 encodes a paired-type homeodomain and is the goldfish ortholog of the murine Chx10 gene. During development, Vsx-2 is expressed at high levels in goldfish and zebrafish retina. In addition to the retina, in situ hybridization studies using whole mount and cryosection embryos demonstrate that Vsx-2 is also expressed in subsets of neurons in the hindbrain and in the spinal cord. Expression begins approximately at the metencephalon-myelencephalon border and continues in a restricted lateral zone along the rostral-caudal axis of the spinal cord. These observations suggest a potential requirement for Vsx-2 in the specification and/or the maintenance of neurons in specific CNS regions during embryogenesis. Also discussed are other transcription factors which may act combinatorially with Vsx-2 to regulate neuronal differentiation.