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

Potential roles for BMP and Pax genes in the development of iris smooth muscle

The embryonic optic cup generates four types of tissue: neural retina, pigmented epithelium, ciliary epithelium, and iris smooth muscle. Remarkably little attention has focused on the development of the iris smooth muscle since Lewis ([1903] J. Am. Anat. 2:405-416) described its origins from the anterior rim of the optic cup neuroepithelium. As an initial step toward understanding iris smooth muscle development, I first determined the spatial and temporal pattern of the development of the iris smooth muscle in the chick by using the HNK1 antibody, which labels developing iris smooth muscle. HNK1 labeling shows that iris smooth muscle development is correlated in time and space with the development of the ciliary epithelial folds. Second, because neural crest is the only other neural tissue that has been shown to generate smooth muscle (Le Lievre and Le Douarin [1975] J. Embryo. Exp. Morphol. 34:125-154), I sought to determine whether iris smooth muscle development shares similarities with neural crest development. Two members of the BMP superfamily, BMP4 and BMP7, which may regulate neural crest development, are highly expressed by cells at the site of iris smooth muscle generation. Third, because humans and mice that are heterozygous for Pax6 mutations have no irides (Hill et al. [1991] Nature 354:522-525; Hanson et al. [1994] Nat. Genet. 6:168-173), I determined the expression of Pax6. I also examined the expression of Pax3 in the developing anterior optic cup. The developing iris smooth muscle coexpresses Pax6 and Pax3. I suggest that some of the eye defects caused by mutations in Pax6, BMP4, and BMP7 may be due to abnormal iris smooth muscle.

The prevalence and associated features of posterior embryotoxon in the general ophthalmic clinic

PURPOSE

To estimate the prevalence of posterior embryotoxon (PE) in the general ophthalmic clinic and to identify any features of PE that suggest that it is pathological rather than physiological.

METHODS

Over 700 consecutive patients of all ages were examined with the slit lamp during their routine eye clinic appointment. Patients with posterior embryotoxon were invited to return for full ocular examination including keratometry, corneal topography, and gonioscopy.

RESULTS

In all, 49 out of 723 patients were found to have PE. This gives a prevalence of 6.8% with an age range of 18 months to 95 years. There was a higher prevalence in the younger age group of 22.5% (age range 18 months to 20 years) compared to 5.9% in the older age range (21-95 years). A total of 29 patients with PE returned for further examination. Six patients had glaucoma (two with Axenfeld's syndrome and one with aniridia), and one had ocular hypertension. In all, 20 patients had bilateral PE on slit-lamp examination, which increased to 24 with gonioscopy. The majority of the PE was seen temporally (97.9%) and limited to a few clock hours. Gonioscopy showed that eight patients with PE had associated inferior pigmentation of schwalbes line.

CONCLUSION

This large series found the prevalence of PE the general ophthalmic clinic to be 6.8%. Its presence should prompt careful anterior segment examination, including gonioscopy, to identify any associated abnormalities that may carry a risk of glaucoma. Children should also be assessed for any associated systemic or genetic abnormality.

Inherited corneal disease: the evolving molecular, genetic and imaging revolution

Advances in molecular genetics and in vivo ocular imaging modalities have enhanced our understanding of the corneal dystrophies. To date at least 11 genes have been identified, in which mutations manifest in corneal disease. In addition there are at least eight other loci identified to which corneal dystrophies have been linked. The information gained from the knowledge of gene function, aberrant protein production, or altered enzyme activity in the cornea, has resulted in greater knowledge of the pathophysiological mechanisms in these disorders. In vivo confocal microscopy has recently enabled microstructural study of dystrophic corneas throughout the disease course, rather than being limited to histopathological analysis of tissue removed at corneal transplantation. This perspective article summarizes the current knowledge, with emphasis on the genes, mutant proteins and resultant mechanisms that lead to manifestations of disease, along with characteristic findings with in vivo confocal microscopy.

Genetics of Corneal Disease for the Ocular Surface Clinician

Advances in the understanding of inherited corneal and external diseases may allow interventions that prevent the substantial vision impairment currently caused by these diseases. The observant clinician may first recognize inherited corneal and external diseases based on clinical examination and a careful family history. Researchers using positional cloning and candidate gene techniques have identified several disease-causing genes. Identification of the genes responsible for inherited corneal and external diseases will lead to more definitive diagnoses and represent the first step in development of effective therapies. Future endeavors are directed toward identifying additional inherited corneal and external diseases, the genes that cause them, and possible gene therapies to improve visual outcomes.

Genetic dissection of Pax6 dosage requirements in the developing mouse eye

Haploinsufficiency of the transcription factor Pax6/PAX6 has been implicated in a number of congenital eye disorders in humans and mice, such as aniridia and Small-eye, which affect the development and function of the lens, cornea, anterior eye segment and neuroretina. However, the widespread distribution of Pax6/PAX6 protein within the developing and adult eye preclude the identification and direct study of the ocular tissues affected by a reduction in Pax6/PAX6 dosage. Here, we employed Cre/loxP-mediated inactivation of a single Pax6 allele in either the lens/cornea or the distal optic cup to dissect the tissue-specific sensitivity to Pax6 haploinsufficiency. Exclusive inactivation of a single Pax6 allele in the lens recapitulates the Small-eye lens and corneal defects, while only mildly affects iris morphology in a non-cell-autonomous fashion. Conversely, selective inactivation of a single Pax6 allele in the distal optic cup revealed primarily cell-autonomous dosage requirements for proper iris differentiation, with no affects on either lens or corneal morphology. Pax6 dosage within the distal optic cup is found here to influence the number of progenitors destined for the anterior ocular structures, the timing of iris muscle-cell differentiation and iris stroma development. Taken together, we genetically dissected the complex mouse Small-eye phenotype, thereby pinpointing the underlying Pax6/PAX6 haploinsufficiency to autonomous dosage requirements within the developing iris and lens/cornea tissues.

Evolving concepts on the pathogenic mechanisms of aniridia related keratopathy

Heterozygosity for PAX6 deficiency (PAX6+/-) results in aniridia. Corneal changes in aniridia-related keratopathy (ARK) include corneal vascular pannus formation, conjunctival invasion of the corneal surface, corneal epithelial erosions and epithelial abnormalities, which eventually result in corneal opacity and contribute to visual loss. Corneal changes in aniridia have been attributed to congenital deficiency of corneal limbal stem cells. The aim of this paper is to review the potential mechanisms that may underlie the pathogenesis of aniridia related keratopathy. Current evidence, based on clinical observations and an animal model of aniridia suggest that the proliferative potential of the corneal limbal stem cells may not primarily be impaired. The corneal changes in aniridia may be related to an abnormality within the limbal stem cell niche. The mechanisms underlying progressive corneal pathology in aniridia appear multi-factorial and include: (1) abnormal corneal healing responses secondary to anomalous extracellular matrix metabolism; (2) abnormal corneal epithelial differentiation leading to fragility of epithelial cells; (3) reduction in cell adhesion molecules in the PAX6 heterozygous state, rendering the cells susceptible to natural shearing forces; and (4) conjunctival and corneal changes leading to the presence of cells derived from conjunctiva on the corneal surface.

Expression of FoxE4 and Rx Visualizes the Timing and Dynamics of Critical Processes Taking Place during Initial Stages of Vertebrate Eye Development

Several transcription factors have a critical function during initial stages of vertebrate eye formation. In this paper, we discuss the role of the Rx subfamily of homeobox-containing genes in retinal development, and the role of the Foxe3 and FoxE4 subfamily of forkhead box-containing genes in lens development. Rx genes are expressed in the initial stages of retinal development and they play a critical role in eye formation. Elimination of Rx function in mice results in lack of eye formation. Abnormal eye development observed in the mouse mutation eyeless (ey1), the medakatemperature-sensitive mutation eyeless (el), and the zebrafish mutation chokh are caused by abnormal regulation or function of Rx genes. In humans, a mutation in Rx leads to anophthalmia. In contrast, Foxe3 and FoxE4 genes are expressed in the lens and they play an essential role in its formation. Mutations in the Foxe3 gene are the cause of the mouse mutation dysgenetic lens (dyl) and in humans, mutation in FOXE3 leads to anterior segment dysgenesis and cataracts. Since Rx and FoxE4 are expressed in the earliest stages of retina and lens development, their expression visualizes the timing and dynamics of the crucial processes that comprise eye formation. In this paper we present a model of eye development based on the expression pattern of these two genes.

Transdifferentiation of the retinal pigment epithelia to the neural retina by transfer of the Pax6 transcriptional factor

The Pax6 gene plays an important role in eye morphogenesis throughout the animal kingdom. The Pax6 gene and its homologue could form ectopic eyes by targeted expression in Drosophila and Xenopus. Thus, this gene is a master gene for the eye morphogenesis at least in these animals. In the early development of the vertebrate eye, Pax6 is required for the instruction of multipotential progenitor cells of the neural retina (NR). Primitive retinal pigment epithelial (RPE) cells are able to switch their phenotype and differentiate into NR under exogenous intervention, including treatment with fibroblast growth factors (FGFs), and surgical removal of endogenous NR. However, the molecular basis of phenotypic switching is still controversial. Here, we show that Pax6 alone is sufficient to induce transdifferentiation of ectopic NR from RPE cells without addition of FGFs or surgical manipulation. Pax6-mediated transdifferentiation can be induced even at later stages of development. Both in vivo and in vitro studies show that the Pax6 lies downstream of FGF signaling, highlighting the central roles of Pax6 in NR transdifferentiation. Our results provide an evidence of retinogenic potential of nearly mature RPE and a cue for new therapeutic approaches to regenerate functional NR in patients with a visual loss.

Signaling circuitries in development: insights from the retinal determination gene network

Context-specific integration of information received from the Notch, Transforming growth factor beta, Wingless/Wnt, Hedgehog and Epidermal growth factor receptor signaling pathways sets the stage for deployment of the retinal determination gene network (RDGN), a group of transcription factors that collectively directs the formation of the eye and other tissues. Recent investigations have revealed how these transcription factors are regulated by their interactions with each other and with effectors of the above signaling pathways. Further study of the RDGN may provide insights into how common cues can generate context-specific responses, a key aspect of developmental regulation that remains poorly understood.

The Pax6 isoform bearing an alternative spliced exon promotes the development of the neural retinal structure

The vertebrate retina has an area where visual cells are closely packed for proper vision that is known as a fovea, an area centralis or a visual streak. The molecular mechanism that regulates the formation of these structures and visual cell gradients is unknown. The transcription factor Pax6 is a master regulator of eye development. A Pax6 isoform that contains an exon 5a-encoded 14 amino acid insertion in its paired domain, Pax6(+5a), has different DNA-binding properties compared with the Pax6(-5a) isoform. Little is known about the functional significance of Pax6(+5a). Here, we show that Pax6(+5a) is expressed especially in the retinal portion where visual cells accumulate during eye development and, when overexpressed, induces a remarkable well-differentiated retina-like structure. Pax6(+5a) proteins that bear point mutations that are found in patients with foveal hypoplasia are unable to induce these ectopic retina-like structures. We propose that Pax6(+5a) induces a developmental cascade in the prospective fovea, area centralis or visual streak region that leads to the formation of a retinal architecture bearing densely packed visual cells.

Epidermal growth factor-induced proliferation requires down-regulation of Pax6 in corneal epithelial cells

Growth factors play important roles in regulating corneal epithelial cell proliferation/differentiation during wound healing. It is suggested that PAX6 involves corneal epithelium lineage-specific differentiation (Liu, J. J., Kao, W. W., and Wilson, S. E. (1999) Exp. Eye Res. 68, 295-301); however, the regulatory mechanism and function of Pax6 in growth factor-induced corneal epithelial responses is still unknown. In the present study, we found that the mitogenic effect of epidermal growth factor (EGF) in corneal epithelial cells required suppression of PAX6 activity through cellular mechanisms involving Erk-signaling pathway-mediated increase in CTCF expression. EGF-induced CCCTC binding factor (CTCF) activation subsequently inhibited Pax6 expression by interacting with a CTCF-specific region upstream of the pax6 P0 promoter. Suppression of EGF-induced Erk activation by specific inhibitor or by the dominant expression of a silent Erk mutant effectively abolished the effects of EGF stimulation on regulations of CTCF and pax6. Apparently, down-regulation of Pax6 expression induced by EGF is required for corneal epithelial proliferation, because overexpression of pax6 in these cells attenuated EGF-induced proliferation. In contrast, knockdown of mRNA expression with pax6- or CTCF-specific small interfering RNA in corneal epithelial cells significantly promoted or attenuated EGF-induced proliferation, respectively. Thus, our results revealed a new regulatory mechanism that involves cellular signaling events and pax6 transcription regulation in growth factor-mediated proliferation. In corneal epithelial cells, this suggests that inhibition of pax6 expression is a prerequisite for EGF to elicit controls of cell growth and fate.

Novel phenotype of craniosynostosis and ocular anterior chamber dysgenesis with a fibroblast growth factor receptor 2 mutation

Fibroblast growth factor receptor 2 (FGFR2) mutations are associated with syndromic and non-syndromic craniosynostoses. More recently it has been recognized that FGFR2 may have a role in the development of the anterior chamber of the eye following the finding of a specific FGFR2 mutation (p.Ser351Cys, c.1231 C --> G) with anterior chamber dysgenesis. Affected patients had a severe craniofacial phenotype and clinical course. A child with a different FGFR2 mutation (p.Ala344Ala, c1032 G --> A heterozygote), premature fusion of the sagittal suture, and an Axenfeld-Rieger anomaly but otherwise normal clinical course is reported. The case provides further evidence that FGFR2 has a role in anterior chamber embryogenesis.

Autosomal dominant brachydactyly, coloboma and anterior segment dysgenesis

A three-generation family presenting with ocular developmental abnormalities, including anterior segment dysgenesis and coloboma, associated with brachydactyly and clinodactyly is presented. Several conditions incorporating ocular and bony limb abnormalities have been described. However, we believe that this family manifests a previously undescribed syndrome due to autosomal dominant or possibly x-linked inheritance with variable expression.

Variable phenotype related to a novel PAX 6 mutation (IVS4+5G>C) in a family presenting congenital nystagmus and foveal hypoplasia

PURPOSE

Several ocular defects have been identified as a consequence of the PAX6 gene mutations. With regard to the implication of this gene in unusual phenotypes, we report a family presenting with congenital nystagmus, foveal hypoplasia, and iris hypoplasia or atypical coloboma.

DESIGN

Observational case report.

METHODS

The entire transcribed region of the PAX6 gene was submitted to mutation search at the DNA and mRNA levels in five affected members of a French family in test with 82 normal subjects.

RESULTS

A novel heterozygous PAX6 gene splice mutation (IVS4 + 5G>C) was identified. The mutation is located in IVS4 within the consensus donor splice site. A mutant mRNA lacking exon 4 as the sole defect was evidenced. The resultant protein was predicted to contain a cryptic ATG initiation codon in exon 3 and a slightly altered paired-domain in an open reading frame extended by 13 amino acids.

CONCLUSIONS

The association of anterior segment anomalies and foveal hypoplasia with one of the slightest alterations of the PAX6 protein described to date confirms the association of variant phenotypes with hypomorphic alleles. Mutation screening of the PAX6 gene could be useful in elucidating the origin of complex ocular malformations.

Molecular basis of Peters anomaly in Saudi Arabia

Peters anomaly (PA) and primary congenital glaucoma (PCG) are genetically and phenotypically distinct conditions. Mutations in cytochrome P4501B1 (CYP1B1) are the most common cause of PCG in Saudi Arabia. Recent evidence suggests that there may be common genetic factors to these conditions. To determine the molecular basis of PA, 11 study subjects with PA from 10 Saudi Arabian families were recruited. Experienced ophthalmologists examined all affected subjects and most of their available unaffected relatives. The diagnosis of PA was confirmed by pathological examination of excised corneal buttons in seven subjects. The coding exons of FOXC1, PITX2, and PAX6 were screened and those of CYP1B1 and FOXE3 sequenced. Homozygous CYP1B1 mutations were identified in six individuals in five families. Five individuals were homozygous for G61E and one was homozygous for 143del10. No mutations were identified in FOXC1, PITX2, PAX6, or FOXE3. The clinical or pathologic phenotype of the subjects with CYP1B1 mutations was not different from that of the other PA patients in this study. Two families included at least one individual with homozygous CYP1B1 mutations and no ocular anomalies (nonpenetrant). Mutations in CYP1B1 may be a substantive cause for PA in this population. Thus, PA and PCG may share a common molecular pathophysiology. Indeed, PA and PCG may share the same spectrum of anterior segment dysgenesis. Finally, the occurrence of PA, PCG, and unaffected individuals with identical homozygous CYP1B1 mutations in the same sibship suggests the presence of modifiers that modulate the clinical severity of the phenotypic expression of the same CYP1B1 mutation(s).

Roles of Maf family proteins in lens development

Lens provides a good model for studying developmental cues relevant to cellular and molecular interactions. Basic region/leucine zipper (bZIP) transcription factors have been found to play key roles during eye formation in various species, including human, mouse, rat, Xenopus, zebrafish, chick, and quail. Different ocular developmental anomalies associated with MAF mutation in human implicate its active role during eye development. Several members of the maf gene family with this bZIP motif participate directly in lens morphogenesis. One vital Maf protein, L-Maf, is expressed in developing lens cells of chick embryos. Its homolog recently has been detected in lens placode of Xenopus embryos and regulates expression of lens fiber-specific genes in this species. Ectopic expression of L-Maf can induce lens-specific genes in cultured retina cells and embryonic ectoderm. The dominant-negative form of L-Maf causes the suppression of crystallin expression and subsequently inhibits lens formation, indicating that L-Maf plays a central role in chick lens development.

A missense mutation in PAX9 in a family with distinct phenotype of oligodontia

Mutations in PAX9 have been described for families in which inherited oligodontia characteristically involves permanent molars. Our study analysed one large family with dominantly inherited oligodontia clinically and genetically. In addition to permanent molars, some teeth were congenitally missing in the premolar, canine, and incisor regions. Measurements of tooth size revealed the reduced size of the proband's and his father's deciduous and permanent teeth. This phenotype is distinct from oligodontia phenotypes associated with mutations in PAX9. Sequencing of the PAX9 gene revealed a missense mutation in the beginning of the paired domain of the molecule, an arginine-to-tryptophan amino-acid change occurring in a position absolutely conserved in all sequenced paired box genes. A mutation of the homologous arginine of PAX6 has been shown to affect the target DNA specificity of PAX6. We suggest that a similar mechanism explains these distinct oligodontia phenotypes.

Paired-type homeodomain transcription factors are imported into the nucleus by karyopherin 13

We report that the paired homeodomain transcription factor Pax6 is imported into the nucleus by the Karyopherin beta family member Karyopherin 13 (Kap13). Pax6 was identified as a potential cargo for Kap13 by a yeast two-hybrid screen. Direct binding of Pax6 to Kap13 was subsequently confirmed by in vitro assays with recombinant proteins, and binding in vivo was shown by coimmunoprecipitation. Ran-dependent import of Pax6 by Kap13 was shown to occur by using a digitonin-permeabilized cells assay. Kap13 binds to Pax6 via a nuclear localization sequence (NLS), which is located within a segment of 80 amino acid residues that includes the homeodomain. Kap13 showed reduced binding to Pax6 when either region located at each end of the homeodomain (208 to 214 and 261 to 267) was deleted. The paired-type homeodomain transcription factor family includes more than 20 members. All members contain a region similar to the NLS found in Pax6 and are therefore likely to be imported by Kap13. We confirmed this hypothesis for Pax3 and Crx, which bind to and are imported by Kap13.

Ocular genetics: current understanding

Over the past decade, there has been an exponential increase in our knowledge of heritable eye conditions. Coincidentally, our ability to provide accurate genetic diagnoses has allowed appropriate counseling to patients and families. A summary of our current understanding of ocular genetics will prove useful to clinicians, researchers, and students as an introduction to the subject.

Anterior eye development and ocular mesenchyme: new insights from mouse models and human diseases

During development of the anterior eye segment, cells that originate from the surface epithelium or the neuroepithelium need to interact with mesenchymal cells, which predominantly originate from the neural crest. Failures of proper interaction result in a complex of developmental disorders such Peters' anomaly, Axenfeld-Rieger's syndrome or aniridia. Here we review the role of transcription factors that have been identified to be involved in the coordination of anterior eye development. Among these factors is PAX6, which is active in both epithelial and mesenchymal cells during ocular development, albeit at different doses and times. We propose that PAX6 is a key element that synchronizes the complex interaction of cell types of different origin, which are all needed for proper morphogenesis of the anterior eye. We discuss several molecular mechanisms that might explain the effects of haploinsufficiency of PAX6 and other transcription factors, and the broad variation of the resulting phenotypes.

Regulation of eye development by transcription control of CCCTC binding factor (CTCF)

CCCTC binding factor (CTCF), a transcriptional regulator, plays important roles in epigenetics and development. In the present study, we report that overexpression of CTCF in transgenic mice during embryonic development suppresses Pax6 gene expression. This effect causes defects in ocular development that result in microophthalmia. In eye-derived cells transfected with a tetracycline turn-on CTCF system, up-regulation of CTCF expression significantly suppressed Pax6 expression. In contrast, the knockdown of CTCF mRNA resulted in the down-regulation of CTCF protein expression, which in turn enhanced the Pax6 expression. CTCF controls Pax6 transcription by interacting with a repressor element located in the 5'-flanking region upstream of the Pax6 P0 promoter. This interaction suppressed Pax6 gene transcription by blocking the effect of an ectoderm enhancer located 3.5 kb upstream from the P0 promoter. We also found an 80-bp sequence in a region -1.2 kbp upstream from the P0 promoter that contained multiple CTCF binding sites and interacted with nuclear proteins obtained from eye-derived cells forming electrophoretic mobility shift assay complexes with CTCF. We conclude that a novel function of CTCF is to regulate Pax6 transcription by binding to the repressor element, which in turn blocks the effect of the ectoderm enhancer resulting in the inhibition of P0 promoter activity.

Functional interactions between alternatively spliced forms of Pax6 in crystallin gene regulation and in haploinsufficiency

Pax6 is essential for development of the eye, olfactory system, brain and pancreas. Haploinsufficiency of Pax6 causes abnormal eye development. Two forms of Pax6 protein, PAX6 and PAX6(5a), differ in a 14 amino acid insertion encoded by an alternatively spliced exon 5a in the N-terminal DNA-binding paired domain (PD), and they are simultaneously expressed. Here, we show that PAX6 and PAX6(5a) together synergistically activate transcription from promoters recognized by Pax6 PD and PD5a, but not by their homeodomain. This synergism promotes activation of transcription by c-Maf and MafA on the alphaB-crystallin promoter, and is required for transcriptional co-activation by RARbeta/RXRbeta and PAX6/PAX6(5a) on the gammaF-crystallin promoter. To determine the role of this synergism in haploinsufficiency, we tested four human missense (G18W, R26G, G64V and R128C) and one nonsense (R317X) mutants, with reporters driven by Pax6 PD consensus binding sites and the alphaB-crystallin promoter. The simultaneous activity of Pax6 proteins [PAX6, mutated PAX6, PAX6(5a) and mutated PAX6(5a)] modeling haploinsufficiency yielded results not predicted by properties of individual PAX6 or PAX6(5a). Taken together, these results indicate that complex ocular phenotypes due to Pax6 haploinsufficiency originate, at least partially, from functional interactions between alternatively spliced PAX6 and PAX6(5a) variants and other factors, e.g. MafA/c-Maf.

Functional properties of natural human PAX6 and PAX6(5a) mutants

PURPOSE

Pax6 is essential for development of the eye, brain, and pancreas. Two major products of PAX6 are specific DNA-binding proteins, PAX6 and PAX6(5a). PAX6(5a) contains a short insertion influencing its DNA-binding activity. Heterozygous mutations in PAX6 result in abnormal eye development implicating haploinsufficiency. Deletions of one PAX6 allele result in aniridia characterized by severe ocular phenotypes. Approximately 10% of PAX6 mutations encode missense mutations. These mutations usually cause less severe abnormalities than does aniridia. The moderate phenotypes raise the possibility that different ocular tissues are differently sensitive to specific mutations. To test this hypothesis, we probed functional properties of individual mutated Pax6 proteins in a variety of conditions.

METHODS

Mutations in PAX6 and PAX6(5a) were introduced by site-directed mutagenesis and tested by transfections in four cell lines using reporters containing three different Pax6 binding sites. Pax6 binding to DNA was studied by electrophoretic mobility shift assays.

RESULTS

Functional studies of PAX6 and PAX6(5a) and their eight natural missense (G18W, R26G, A33P, S43P, G64V, I87R, V126D and R128C) and two nonsense (R317X and S353X) disease-causing mutants revealed unexpected pleiotropic effects in gene regulation, not predicted by the PAX6-DNA crystal structure. Transactivation by PAX6 and PAX6(5a) was dependent on the location of mutation, type of DNA-binding site, and cellular environment.

CONCLUSIONS

This work provides evidence that activation by PAX6 and PAX6(5a) is modulated by specific cellular environments. It is likely that moderate phenotypes associated with PAX6 missense mutations originate from abnormal protein function in a restricted number of ocular cell types.

Transcription Factors Pax6 and AP-2alpha Interact To Coordinate Corneal Epithelial Repair by Controlling Expression of Matrix Metalloproteinase Gelatinase B

Pax6 is a paired box containing transcription factor that resides at the top of a genetic hierarchy controlling eye development. It continues to be expressed in tissues of the adult eye, but its role in this capacity is unclear. Pax6 is present in the adult corneal epithelium, and we showed that the amount of Pax6 is increased at the migrating front as the epithelium resurfaces the cornea after injury. We also showed that Pax6 controls activity of the transcriptional promoter for the matrix metalloproteinase, gelatinase B (gelB; MMP-9) in cell culture transfection studies. gelB expression is turned on at the migrating epithelial front in the cornea, and it coordinates and effects aspects of epithelial regeneration. We define here two positively acting Pax6 response elements in the gelB promoter. Pax6 binds directly to one of these sites through the paired DNA-binding domain. It binds the second site indirectly by interaction with AP-2alpha, a transcription factor that also exerts control over eye development. Pax6 control of gelB expression was examined in vivo by using a corneal reepithelialization model in mice heterozygous for a Pax6 paired-domain mutation (Sey(+/-)). A reduced Pax6 dosage in these mice resulted in a loss of gelB expression at the migrating epithelial front. This effect was correlated with an increase in inflammation and the rate of reepithelialization, a finding consistent with the phenotype of gelB knockout mice. Together, these data indicate that Pax6 controls activity of the gelB promoter through cooperative interactions with AP-2alpha and support an active role for Pax6 in maintenance and repair of the adult corneal epithelium.

Surgical management of glaucoma in infants and children with Peters' anomaly

OBJECTIVE

To determine the long-term outcome of surgery for congenital glaucoma in infants and children with Peters' anomaly.

DESIGN

Retrospective review of a consecutive interventional case series.

SETTING

An urban academic tertiary referral institution.

PARTICIPANTS

Thirty-four eyes of 19 children are subjects of this report. Included are all children 12 years of age or younger with Peters' anomaly who underwent surgery for primary congenital glaucoma between January 1971 and December 1992 and completed a minimum of 3 years of follow-up from the date of the first glaucoma surgery.

INTERVENTION

The surgical procedures performed were trabeculectomy, trabeculotomy, goniotomy, Molteno shunt implantation, cyclodialysis, and cyclocryotherapy.

MAIN OUTCOME MEASURES

Primary outcome measures were intraocular pressure (IOP) control and final postoperative visual acuity. Intraocular pressure control was defined as complete success (IOP</=21 mmHg without antiglaucoma medication), qualified success (IOP</=21 mmHg with antiglaucoma mediation), or failure (IOP>21 mmHg with or without antiglaucoma medication, inoperable retinal detachment, phthisis, or chronic hypotony, defined as an IOP of </=6 mmHg).

RESULTS

A total of 126 glaucoma procedures were performed on 34 eyes of 19 patients. The median age at time of first glaucoma surgery was 2.1 months (range, 2 days to 8.5 years). The median follow-up was 11.0 years (range, 3.2 to 22.8 years) from the time of first glaucoma surgery. Intraocular pressure control with or without antiglaucoma medication was achieved in 11 eyes (32%) after 1 or more surgical procedures. Major postoperative complications included graft failure in 26 eyes (76%), cataract in 6 eyes (18%), inoperable retinal detachment with phthisis in 12 eyes (35%), and phthisis alone in 6 eyes (18%). Final vision was 20/200 or better in 3 eyes (9%), 20/400 to hand motion in 12 eyes (35%), light perception in 7 eyes (21%), and no light perception in 12 eyes (35%).

CONCLUSIONS

Glaucoma surgery, combined with medical therapy, may result in adequate, long-term IOP control in 32% of eyes with glaucoma associated with Peters' anomaly. Visual results are poor due to uncontrolled glaucoma, amblyopia, neurologic impairment, and other anterior and posterior segment anomalies that may accompany Peters' anomaly. Postoperative complications, including graft failure, cataract, inoperable retinal detachment, and phthisis, also contribute to decreased visual acuity.

PAX6 and congenital eye malformations

The PAX6 gene is a paradigm for our understanding of the molecular genetics of mammalian eye development. Twelve years after its identification it is one of the most intensively studied genes, both in terms of its diverse and complex functions during oculogenesis and its role in an ever-increasing variety of human congenital eye malformations. The PAX6 field has benefited greatly from the continued input of clinicians, human geneticists and developmental biologists. This review summarizes the latest data on the PAX6 mutation spectrum and recent insights into Pax6 function from the mouse.

Mutations in PAX1 may be associated with Klippel-Feil syndrome

Pax genes are a highly conserved family of developmental control genes that encode transcription factors. In vertebrates, Pax genes play a role in pattern formation during embryogenesis. Mutations in Pax genes have been associated with both spontaneous mouse mutants and congenital human diseases. The mouse Pax1 mutant phenotype undulated is characterised by vertebral segmentation defects reminiscent of the human disorder Klippel-Feil syndrome (KFS). To determine whether PAX1 haploinsufficiency plays a role in KFS, we have defined the gene structure of the human PAX1 gene and screened 63 KFS patients for mutations in this gene. Differences in the PAX1 sequence were detected in eight patients. Two patients had a silent change within the paired box that was also seen in 2/303 control chromosomes. The other variants were missense, silent or intronic changes not represented in the control panel tested. The significance of these results and the possible role of PAX1 in the pathogenesis of KFS are discussed.

Requirement for Pax6 in corneal morphogenesis: a role in adhesion

The Pax6 transcription factor functions early during embryogenesis to control key steps in brain, pancreas, olfactory and ocular system development. A requirement for Pax6 in proper formation of lens, iris and retina is well documented. By examining the corneas of heterozygous Small eye (SEY) mice, this report shows that Pax6 is also necessary for normal corneal morphogenesis. In particular, the epithelial component of the postnatal and adult SEY (+/-) cornea is thinner owing to a reduction in the number of cell layers, despite a tenfold increase in the proliferative index and no change in TUNEL labeling. Ultrastructural views revealed large gaps between corneal epithelial cells and a change in the appearance of desmosomes, suggesting that adhesion abnormalities contribute to the corneal phenotype of SEY (+/-) mice. Western blot analysis and immunofluorescence showed equivalent amounts and normal localization of E-cadherin in SEY (+/-) corneas, and the actin cytoskeleton appeared normal as judged by phalloidin staining. By contrast, the levels of desmoglein, beta-catenin and gamma-catenin were reduced in the SEY (+/-) cornea. In addition, the amount of keratin-12 mRNA and protein, the major intermediate filament, was reduced in SEY (+/-) corneal epithelium as shown by in situ hybridization and immunohistochemistry. Finally, the SEY (+/-) corneal epithelium adheres less well than wild-type when challenged with gentle rubbing using a microsponge. In conclusion, our results indicate that cellular adhesion is compromised in the SEY (+/-) corneal epithelium and suggests a role for Pax6 in the proper generation and maintenance of the adult cornea.

Three novel PAX6 mutations in patients with aniridia

AIMS

To describe mutations in the PAX6 gene in five patients with aniridia from three unrelated families.

METHODS

The PAX6 gene was analysed using single stranded conformational polymorphism analysis and direct sequencing.

RESULTS

In one family, three individuals from two generations had aniridia, whereas in each of the other families only one member was affected. The first patient had the heterozygous Q221X (1023C --> T) nonsense mutation in exon 8. The same mutation was found in his mother and sister. Another patient had a heterozygous Q297X (1252C --> T) mutation in exon 10. The third patient carried a heterozygous IVS5+2T --> C mutation leading to aberrant splicing of mRNA.

CONCLUSIONS

These findings provide further examples of haploinsufficiency of PAX6 in aniridia.

Molecular characterization of a familial translocation implicates disruption of HDAC9 and possible position effect on TGFbeta2 in the pathogenesis of Peters' anomaly

Peters' anomaly (PA) is a congenital defect of the anterior chamber of the eye. We identified a family in which an apparently balanced chromosomal translocation t(1;7) (q41;p21) was associated with PA. Based on this observation, detailed molecular characterizations of the breakpoint regions and candidate genes were carried out. A candidate gene from each breakpoint was identified: on chromosome 7, histone deacetylase 9 (HDAC9), disrupted by the translocation breakpoint, and on chromosome 1, transforming growth factor-beta2 (TGFbeta2) located 500 kb proximal to the breakpoint. An additional lysophospholipase-like 1 gene (LYPLAL1), localized approximately 200 kb distal to the chromosome 1 breakpoint, was also identified and characterized. Although only the HDAC9 gene is disrupted by the breakpoint, we consider that TGFbeta2 represents the main candidate gene in this family, which is elicited in mice by the Tgfbeta2-null status and by the TGFbeta2-induced cataractus changes in animal models. As an alternative scenario, which is supported by the ability of class II HDACs to mediate extracellular TGF-beta stimuli to core histone deacetylation in promoter-adjacent regions, we propose the hypothesis of digenic inheritance. Inappropriate or inadequate TGFbeta2 expression, together with deficient mediation of these signals at the transcription level, due to an altered HDAC9 isoforms ratio, may also lead to the observed ocular phenotype.

A family with Axenfeld-Rieger syndrome and Peters Anomaly caused by a point mutation (Phe112Ser) in the FOXC1 gene

PURPOSE

Mutations of the forkhead transcription factor gene FOXC1 result in anterior segment anomalies. No description of the spectrum of defects resulting from a single point mutation of this gene exists in the ophthalmology literature. We have screened all available patients with Axenfeld-Rieger genes (PITX2 and FOXC1). In this report, we clinically characterize the spectrum of ocular and systemic manifestations in one family resulting from a previously reported point mutation (Phe112Ser) in FOXC1.

DESIGN

Observational case series.

METHODS

Ten members of a multigenerational family were examined for signs of glaucoma, anterior segment abnormalities, and systemic features of Axenfeld-Rieger syndrome. The examinations were performed in an ophthalmology examination room or in the patients' homes. Blood was obtained from 10 members and screened for mutations in FOXC1 using direct DNA sequencing.

RESULTS

A single mutation causing a T to C change in codon 112 (Phe112Ser) of FOXC1 was present in six members of the family. Five of these six patients were examined and all demonstrated anterior segment anomalies. One patient had Axenfeld anomaly, one had Rieger syndrome, and one had both Axenfeld anomaly and Peters anomaly. Additionally, some members demonstrated cardiac abnormalities, which may be secondary to their FOXC1 mutation.

CONCLUSIONS

A wide spectrum of clinical phenotypes can result from a single point mutation of FOXC1. This report confirms that Rieger syndrome (with dental and facial abnormalities) can be caused by a mutation in FOXC1. It is also the first report of Peters anomaly being caused by a FOXC1 mutation.

Missense mutations in the DNA-binding region and termination codon in PAX6

We have identified nine novel intragenic mutations of the PAX6 gene in 30 patients with aniridia. One patient with Wilms' tumor, aniridia, genitourinary anomalies, and mental retardation (WAGR syndrome) had deletion of 11p and had lost the paternal PAX6 allele. Two patients had small deletions: a frameshift that should result in early termination of the PAX6 protein, and a frameshift that leads to a termination-site change and run-on into the 3' untranslated region (UTR). The other 27 patients had single base-pair mutations. Four had splicing defects; three had IVS6+1G>A, which was at a mutation hotspot in the PAX6 gene; 10 had premature termination (four 1024C>T [R203X], also at a mutation hotspot); and six had missense mutations. Missense mutation A321T (1378G>A) was a polymorphic change; the other five missense mutations were L46R, C52R, I56T, G73D, and I87K. These five codons are in the PAX6 paired domain and are highly conserved throughout the entire paired family. Seven patients had a mutation in the normal stop codon (TAA). This change leads to run-on into the 3' UTR and is also at a mutation hotspot. All 30 mutations should result in PAX6 haploinsufficiency. No correlation was observed between mutation sites and phenotypes.

Screening for PAX6 gene mutations is consistent with haploinsufficiency as the main mechanism leading to various ocular defects

PAX6, a paired box transcriptional factor, is considered as the master control gene for morphogenesis of the eye. Human PAX6 mutations have been associated with a range of eye abnormalities, including aniridia, various anterior segment defects and foveal hypoplasia. We carried out a mutational analysis of the PAX6 gene in 54 unrelated patients with aniridia or related syndromes. A deleterious variation was evidenced in 17 sporadic cases (50%) and in 13 (72%) familial cases. Twenty-four different mutations, 17 of which are novel, were found. The spectrum of PAX6 mutations was highly homogeneous: 23 mutations (96%) leading to premature stop codons (eight nonsense and four splice site mutations, 11 insertions and deletions) and only one (4%) missense mutation. Twenty-two mutations were associated with aniridia phenotypes whereas two were associated with atypical phenotypes. These latter encompassed a missense mutation (R19P) in an individual with a microphthalmia-sclerocornea and a splice site mutation (IVS4+5G > C) in a family presenting with a congenital nystagmus. Both represented the most probably hypomorphic alleles. Aniridia cases were associated with nonsense or frameshifting mutations. A careful examination of the phenotypes did not make it possible to recognise significant differences whenever the predicted protein was deprived of one or another of its functional domains. This strongly suggested that most of the truncating mutations generated null alleles by nonsense mediated mRNA decay. Our observations support the concept of dosage effects of the PAX6 mutations as well as presenting evidence for variable expressivity.

PAX6, paired domain influences sequence recognition by the homeodomain

PAX6 functions as a transcription factor and has two DNA-binding domains, a paired domain (PD) and a homeodomain (HD), joined by a glycine-rich linker and followed by a proline-serine-threonine-rich (PST) transactivation region at the C terminus. The mechanism of PAX6 function is not clearly understood, and few target genes in vertebrates have been identified. In this report we described the functional analyses of patient missense mutations from the paired domain region of PAX6 and a paireddomain-less isoform (PD-less) of Pax6 that lacks the paired domain and part of the glycine-rich linker. The PD-less was expressed in the brain, eyes, and pancreas of mouse. The level of expression of this isoform was relatively higher in brain. The mutation sites PAX6-L46R and -C52R were located in the PD of PAX6 on either end of the 5a-polypeptide insert of the alternatively spliced form of PAX6, PAX6-5a. Another PAX6 mutant V53L described in this report was adjacent to C52R. We created corresponding mutations in PAX6 and PAX6-5a, and evaluated their transcriptional activation and DNA binding properties. The PD mutants of PAX6 (L46R, C52R, and V53L) exhibited lower transactivation activities and variable DNA binding ability than wild-type PAX6 with PD DNA-binding consensus sequences. The mutated amino acids containing PAX6-5a isoforms showed unexpected transactivation properties with a reporter containing HD DNA-binding sequences. PAX6-5a-C52R, and -V53L showed lower transactivation activities, but PAX6-5a-L46R had greater transactivation ability than PAX6-5a. The PD-less isoform of Pax6 lost its transactivational ability but could bind to the HD DNA-binding sequences. Functional analysis of the PD-less isoform of Pax6 as well as findings related to missense mutations in the PD suggest that the PD of PAX6 is required for HD function.

Developmental neurogenetics and neuro-ophthalmology

The field of developmental neurogenetics has burgeoned over the past decade. Through the combined efforts of developmental biologists, geneticists, and clinicians, genetic defects resulting in neuro-ophthalmic disorders such as holoprosencephaly, microphthalmia, dominant optic atrophy, and optic nerve colobomas have been identified and characterized at the molecular level. Experimental studies in model organisms are continuing to identify novel genes critical for ocular and central nervous system development. Mutations in some of these genes have revealed a spectrum of pathology similar to that observed in septo-optic dysplasia, Möebius syndrome, and Duane retraction syndrome. This review examines our current knowledge of the molecular genetics of neuro-ophthalmic disease and focuses on several candidate genes for afferent and efferent visual system disorders.

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.

Pax6 is required for establishing naso-temporal and dorsal characteristics of the optic vesicle

The establishment of polarity is an important step during organ development. We assign a function for the paired and homeodomain transcription factor Pax6 in axis formation in the retina. Pax6 is a key factor of the highly conserved genetic network implicated in directing the initial phases of eye development. We recently demonstrated that Pax6 is also essential for later aspects of eye development, such as lens formation and retinogenesis. In this study, we present evidence that a highly conserved intronic enhancer, α, in the Pax6 gene is essential for the establishment of a distalhigh-proximallow gradient of Pax6 activity in the retina. In the mature retina, the activity mediated by the α-enhancer defines a population of retinal ganglion cells that project to two sickle-shaped domains in the superior colliculus and lateral geniculate nucleus. Deletion of the α-enhancer in vivo revealed that retinal Pax6 expression is regulated in two complementary topographic domains. We found that Pax6 activity is required for the establishment, as well as the maintenance of dorsal and nasotemporal characteristics in the optic vesicle and, later, the optic cup.

Meis homeoproteins directly regulate Pax6 during vertebrate lens morphogenesis

Pax6 is a pivotal regulator of eye development throughout Metazoa, but the direct upstream regulators of vertebrate Pax6 expression are unknown. In vertebrates, Pax6 is required for formation of the lens placode, an ectodermal thickening that precedes lens development. Here we show that the Meis1 and Meis2 homeoproteins are direct regulators of Pax6 expression in prospective lens ectoderm. In mice, Meis1 and Meis2 are developmentally expressed in a pattern remarkably similar to Pax6 and their expression is Pax6-independent. Biochemical and transgenic experiments reveal that Meis1 and Meis2 bind a specific sequence in the Pax6 lens placode enhancer that is required for its activity. Furthermore, Pax6 and Meis2 exhibit a strong genetic interaction in lens development, and Pax6 expression is elevated in lenses of Meis2-overexpressing transgenic mice. When expressed in embryonic lens ectoderm, dominant-negative forms of Meis down-regulate endogenous Pax6. These results contrast with those in Drosophila, where the single Meis homolog, Homothorax, has been shown to negatively regulate eye formation. Therefore, despite the striking evolutionary conservation of Pax6 function, Pax6 expression in the vertebrate lens is uniquely regulated.

Development and pathology: the Pax gene

During development, many genes support different functions at different times, when they are expressed in different tissues, or in response to different transcription factors. Pathologists should be cautious about ascribing functions to such genes in pathological processes.

Aetiology of congenital and paediatric cataract in an Australian population

BACKGROUND/AIM

Paediatric cataract is a major cause of childhood blindness. Several genes associated with congenital and paediatric cataracts have been identified. The aim was to determine the incidence of cataract in a population, the proportion of hereditary cataracts, the mode of inheritance, and the clinical presentation.

METHODS

The Royal Children's Hospital and the Royal Victorian Eye and Ear Hospital have a referral base for almost all paediatric patients with cataracts in south eastern Australia. The database contains cases seen over the past 25 years. The medical histories of these patients were reviewed.

RESULTS

421 patients with paediatric cataract were identified, which gives an estimated incidence of 2.2 per 10,000 births. Of the 342 affected individuals with a negative family history, 50% were diagnosed during the first year of life, and 56/342 (16%) were associated with a recognised systemic disease or syndrome. Unilateral cataract was identified in 178/342 (52%) of sporadic cases. 79 children (from 54 nuclear families) had a positive family history. Of these 54 families, 45 were recruited for clinical examination and DNA collection. Ten nuclear families were subsequently found to be related, resulting in four larger pedigrees. Thus, 39 families have been studied. The mode of inheritance was autosomal dominant in 30 families, X linked in four, autosomal recessive in two, and uncertain in three. In total, 178 affected family members were examined; of these 8% presented with unilateral cataracts and 43% were diagnosed within the first year of life.

CONCLUSIONS

In the paediatric cataract population examined, approximately half of the patients were diagnosed in the first year of life. More than 18% had a positive family history of cataracts. Of patients with hereditary cataracts 8% presented with unilateral involvement. Identification of the genes that cause paediatric and congenital cataract should help clarify the aetiology of some sporadic and unilateral cataracts.

The Peters' plus syndrome: a review

Peters' plus syndrome is an infrequently described entity that combines anomalies in the anterior chamber of the eye with other multiple congenital anomalies, and a developmental delay. Major symptoms are extremely variable anterior chamber anomalies, cupid bow of the upper lip, cleft lip and palate, short stature, broad hands and feet, and variable mental delay. The syndrome follows an autosomal recessive pattern of inheritance. The etiology is unknown, but may involve abnormal neural crest development. A review of the pertinent literature is provided.

Clinical and ultrastructural features of a novel hereditary anterior segment dysgenesis

OBJECTIVE

To describe the clinical, histopathologic, and hereditary features of a novel familial anterior segment dysgenesis.

DESIGN

Prospective, observational case series and interventional case report.

PARTICIPANTS

Ten individuals from three generations of a single family with iris and corneal abnormalities associated with congenital cataracts.

MAIN OUTCOME MEASURES

An ophthalmic evaluation including slit-lamp examination, corneal topography, pachymetry, and specular biomicroscopy of all family members, and histopathologic and ultrastructural evaluation of one excised corneal button.

RESULTS

The proband was an 81-year-old man with bilateral aphakia and diffuse corneal haze, and thinning associated with corneal guttae. His pupils were small, mildly eccentric, and difficult to dilate. Pachymeter readings were 335 microm (right eye) and 330 microm (left eye). Topography confirmed advanced steepening of both corneas. Light microscopic and transmission electron microscopic examinations of the corneal button revealed an attenuated endothelium with prominent intracellular random aggregates of small-diameter filaments staining positively for cytokeratin. Descemet's membrane was thickened and had marked posterior nodularity. Various-sized polymorphic vacuoles containing layered electron-dense material were present within and between collagen lamellae and within keratocytes throughout the stroma and Bowman's membrane. Secondary bullous changes of the epithelium with thickening of the basement membrane were also observed. The family pedigree demonstrated an autosomal dominant inheritance pattern.

CONCLUSIONS

This constellation of autosomal dominantly inherited corneal endothelial and stromal disorder, with congenital cataracts and iris abnormalities, represents a novel anterior segment disorder. Its etiology may involve an abnormal migration of the secondary mesenchyme.

Differential occurrence of mutations causative of eye diseases in the Chinese population

Ethnic differences and geographic variations affect the frequencies and nature of human mutations. In the literature, descriptions of causative mutations of eye diseases in the Chinese population are few. In this paper we attempt to reveal molecular information on genetic eye diseases involving Chinese patients from published and unpublished works by us and other groups. Our studies on candidate genes of eye diseases in the Chinese population in Hong Kong include MYOC and TISR for primary open angle glaucoma, RHO and RP1 for retinitis pigmentosa, ABCA4 and APOE for age-related macular degeneration, RB1 for retinoblastoma, APC for familial adenomatous polyposis with congenital hypertrophy of retinal pigment epithelium, BIGH3/TGFBI for corneal dystrophies, PAX6 for aniridia and Reiger syndrome, CRYAA and CRYBB2 for cataracts, and mtDNA for Leber hereditary optic neuropathy. We have revealed novel mutations in most of these genes, and in RHO, RP1, RB1, BIGH3, and PAX6 we have reported mutations that contribute to better understanding of the functions and properties of the respective gene products. We showed absence of MYOC does not necessarily cause glaucoma. No disease causative mutations have been identified in MYOC or ABCA4. There are similarities in the patterns of sequence alterations and phenotype-genotype associations in comparison with other ethnic groups, while the MYOC, RB1, APC, and PAX6 genes have more Chinese-specific sequence alterations. Establishment of a mutation database specific for the Chinese is essential for identification of genetic markers with diagnostic, prognostic, or pharmacological values.

Bilateral congenital corneal keloids and anterior segment mesenchymal dysgenesis in a case of Rubinstein-Taybi syndrome

PURPOSE

To report the unusual association of bilateral corneal keloids and anterior segment mesenchymal dysgenesis in a child with Rubinstein-Taybi syndrome.

METHODS

Case report of a 2-year-old boy.

RESULTS

Excision of the epicorneal mass in the right eye was followed by recurrence of the lesion. Multiple penetrating keratoplasties were unsuccessful in reconstructing the anterior segment because of recurrent corneal epithelial breakdown, suggesting limbal stem cell insufficiency. Histopathology and electron microscopy of the excised mass lesion showed features typical of a corneal keloid: thickened keratinized epithelium, absent Bowman's layer, and fibrovascular hyperplasia, with haphazard orientation of the collagen lamellae. Ultrasound biomicroscopy and intraoperative findings suggested a diagnosis of Peter anomaly, but genetic analysis did not show a PAX6 mutation.

CONCLUSION

The findings in our patient add to the spectrum of ocular changes described in Rubinstein-Taybi syndrome and confirm earlier reports of poor ocular prognosis in corneal keloids and Rubinstein-Taybi syndrome.

PAX6 mutation as a genetic factor common to aniridia and glucose intolerance

A paired homeodomain transcription factor, PAX6, is a well-known regulator of eye development, and its heterozygous mutations in humans cause congenital eye anomalies such as aniridia. Because it was recently shown that PAX6 also plays an indispensable role in islet cell development, a PAX6 gene mutation in humans may lead to a defect of the endocrine pancreas. Whereas heterozygous mutations in islet-cell transcription factors such as IPF1/IDX-1/STF-1/PDX-1 and NEUROD1/BETA2 serve as a genetic cause of diabetes or glucose intolerance, we investigated the possibility of PAX6 gene mutations being a genetic factor common to aniridia and diabetes. In five aniridia and one Peters' anomaly patients, all of the coding exons and their flanking exon-intron junctions of the PAX6 gene were surveyed for mutations. The results of direct DNA sequencing revealed three different mutations in four aniridia patients: one previously reported type of mutation and two unreported types. In agreement with polypeptide truncation and a lack of the carboxyl-terminal transactivation domain in all of the mutated PAX6 proteins, no transcriptional activity was found in the reporter gene analyses. Oral glucose tolerance tests revealed that all of the patients with a PAX6 gene mutation had glucose intolerance characterized by impaired insulin secretion. Although we did not detect a mutation within the characterized portion of the PAX6 gene in one of the five aniridia patients, diabetes was cosegregated with aniridia in her family, and a single nucleotide polymorphism in intron 9 of the PAX6 gene was correlated with the disorders, suggesting that a mutation, possibly located in an uncharacterized portion of the PAX6 gene, can explain both diabetes and aniridia in this family. In contrast, the patient with Peters' anomaly, for which a PAX6 gene mutation is a relatively rare cause, showed normal glucose tolerance (NGT) and did not show a Pax6 gene mutation. Taken together, our present observations suggest that heterozygous mutations in the PAX6 gene can induce eye anomaly and glucose intolerance in individuals harboring these mutations.

Clinicopathological correlation of congenital corneal opacification using ultrasound biomicroscopy

AIM

To investigate the correlation between clinical, high frequency ultrasound biomicroscopy (UBM) and, where possible, histological findings in cases of congenital corneal opacification presenting to the departments of ophthalmology, Great Ormond Street Hospital for Children, London, and the Hospital for Sick Children, Toronto, Canada.

METHOD

22 eyes of 13 children (age range 3-225 days) with congenitally opaque corneas were examined. UBM was performed using the ultrasound biomicroscope (Allergan-Humphrey). All eyes underwent penetrating keratoplasties (PKP) except five. The host corneas were all sent for histological examination.

RESULTS

The final diagnosis in our series was Peters' anomaly in nine cases (70%), corneal dystrophy in two cases (15%), and sclerocornea in two cases (15%). The UBM findings changed the clinical diagnosis in five cases (38%). In these five cases histology was available in four and confirmed the UBM diagnosis in each case. In no case of the 13 where histology was available did it contradict the UBM findings. In two cases a hypoechoic region in the anterior stroma was seen on UBM which correlated histologically with absent Bowman's layer and oedema. In two cases UBM revealed aniridia and in one, congenital aphakia, which was not apparent clinically.

CONCLUSION

UBM examination is not only very useful in evaluating the clinical diagnosis in congenital corneal opacification, it also acts as a preoperative guide in cases undergoing PKP by detecting keratolenticular and iridocorneal adhesions and other ocular abnormalities such as aniridia and congenital aphakia. In all cases where PKP was performed the UBM diagnosis was confirmed histologically. The clinical diagnosis was incorrect in five cases. This has important implications in studies of phenotype/genotype correlation of congenital corneal opacification.

Molecular characterization of Pax6(2Neu) through Pax6(10Neu): an extension of the Pax6 allelic series and the identification of two possible hypomorph alleles in the mouse Mus musculus

Phenotype-based mutagenesis experiments will increase the mouse mutant resource, generating mutations at previously unmarked loci as well as extending the allelic series at known loci. Mapping, molecular characterization, and phenotypic analysis of nine independent Pax6 mutations of the mouse recovered in mutagenesis experiments is presented. Seven mutations result in premature termination of translation and all express phenotypes characteristic of null alleles, suggesting that Pax6 function requires all domains to be intact. Of major interest is the identification of two possible hypomorph mutations: Heterozygotes express less severe phenotypes and homozygotes develop rudimentary eyes and nasal processes and survive up to 36 hr after birth. Pax6(4Neu) results in an amino acid substitution within the third helix of the homeodomain. Three-dimensional modeling indicates that the amino acid substitution interrupts the homeodomain recognition alpha-helix, which is critical for DNA binding. Whereas cooperative dimer binding of the mutant homeodomain to a paired-class DNA target sequence was eliminated, weak monomer binding was observed. Thus, a residual function of the mutated homeodomain may explain the hypomorphic nature of the Pax6(4Neu) allele. Pax6(7Neu) is a base pair substitution in the Kozak sequence and results in a reduced level of Pax6 translation product. The Pax6(4Neu) and Pax6(7Neu) alleles may be very useful for gene-dosage studies.

Fgf receptor signaling plays a role in lens induction

We describe experiments showing that fibroblast growth factor receptor (Fgfr) signaling plays a role in lens induction. Three distinct experimental strategies were used: (1) using small-molecule inhibitors of Fgfr kinase activity, we showed that both the transcription level and protein expression of Pax6, a transcription factor critical for lens development, was diminished in the presumptive lens ectoderm; (2) transgenic mice (designated Tfr7) that expressed a dominant-negative Fgf receptor exclusively in the presumptive lens ectoderm showed defects in formation of the lens placode at E9.5 but in addition, showed reduced levels of expression for Pax6, Sox2 and Foxe3, all markers of lens induction; (3) by performing crosses between Tfr7 transgenic and Bmp7-null mice, we showed that there is a genetic interaction between Fgfr and Bmp7 signaling at the induction phases of lens development. This manifested as exacerbated lens development defects and lower levels of Pax6 and Foxe3 expression in Tfr7/Tfr7, Bmp7+/– mice when compared with Tfr7/Tfr7 mice alone. As Bmp7 is an established lens induction signal, this provides further evidence that Fgfr activity is important for lens induction. This analysis establishes a role for Fgfr signaling in lens induction and defines a genetic pathway in which Fgfr and Bmp7 signaling converge on Pax6 expression in the lens placode with the Foxe3 and Sox2 genes lying downstream.