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

Absent iris stroma, narrow body build and small facial bones: a new association or variant of SHORT syndrome?

We report four patients from two unrelated families with strikingly similar facial appearance, short stature, narrow body build and, in two of the patients, abnormalities of the iris stroma. The birth of an affected offspring suggests that this syndrome is likely to have autosomal dominant inheritance. The facial appearance and some of the features resemble the SHORT syndrome, the name being an acronym for Short stature, Hyperextensible joints, Ocular depression, Rieger anomaly and abnormalities of the Teeth. The relationship of the syndrome to the SHORT syndrome is discussed.

New perspectives on eye evolution

The highly complex eyes of vertebrates, insects and molluscs have long been considered to be of independent evolutionary origin. Recently, however, Pax-6, a highly conserved transcription factor, has been identified as a key regulator of eye development in both mammals and flies. Homologues of Pax-6 have also been identified in species from other phyla, including molluscs. The wide variety of eyes in the animal kingdom may, therefore, have evolved from a single ancestral photosensitive origin.

Aniridia: recent achievements in paediatric practice

Aniridia is a rare panocular disorder which primarily involves not only the iris, but also the retina, optic nerve, lens and cornea. Visual acuity deteriorates as a result of nystagmus, glaucoma, cataract, corneal opacities and retinal hypoplasia. Aniridia may appear as an isolated disorder, most often familial with autosomal dominance or sporadically in association with at least 12 syndromes. Both familial isolated and Wilms tumour, bilateral sporadic aniridia, genitourinary abnormalities and mental retardation syndrome-associated aniridia have been traced to a mutation of the PAX6 gene on band 11p13. Since genetic diagnosis of this disorder is already possible, counselling affected families should be preceded by karyotype studies and linkage analysis in familial cases of isolated aniridia. In sporadic cases of isolated aniridia or WAGR syndrome, we suggest that PAX6 mutation analysis be employed.

An unusual pedigree with microcornea-cataract syndrome

A pedigree with 14 patients affected with microcornea-cataract (MC) syndrome is described. MC syndrome is very rare and this pedigree is probably the sixth to be reported. Transmission is most likely autosomal dominant, but the unusual feature of the present family is the fact that the proportion of affected members is so high that one is tempted to suspect preferential transmission of the chromosome carrying the MC gene.

Patterning of the Caenorhabditis elegans head region by the Pax-6 family member vab-3

The Pax-6 genes are important for eye development in both vertebrates and Drosophila. Mutations in the human PAX6 gene are found in patients with a variety of eye disorders, including aniridia and Peters' anomaly, and mutations in the Drosophila Pax-6 homologue cause the eyeless phenotype. In the nematode Caenorhabditis elegans, vab-3 mutants display many defects in head-region development, including aberrant morphogenesis, transformation of hypodermal (epidermal-like) cell fates to those of posterior homologues, and abnormal specification of neurons. Here we show that vab-3 is a member of the paired-domain-containing Pax-6 gene family and is expressed in head-region cells. This C. elegans Pax-6 locus can also encode proteins lacking the paired domain. Our results suggest that a primordial role of the Pax-6 gene family could have been to pattern part of the head region, and that Pax-6 genes subsequently evolved to be more specifically involved in eye development.

Transgenic models for eye malformations

Several lines of transgenic mice developing eye malformations have been described in the literature and appear to be of increasing interest for the study of eye teratology in humans, since gene expression and regulation can be studied in the developing animal. Transgenic applications are briefly described here and an overview of existing transgenic mouse models carrying different eye abnormalities is given according to the major diagnosis (e.g., cataract, microphthalmia, anterior segment dysgenesis, retinal dysplasia). Interestingly, many transgenic models exhibit pathological findings similar to those observed in human pediatric ophthalmology. Unfortunately, detailed embryological studies in transgenic mice bearing congenital eye malformations are not available for all lines. Thus, the importance of creating further transgenic models to study the function of morphogenes and growth factors in eye development is also discussed.

Pax-6 and lens-specific transcription of the chicken delta 1-crystallin gene

The abundance of delta-crystallin in the chicken eye lens provides an advantageous marker for tissue-specific gene expression during cellular differentiation. The lens-specific expression of the delta 1-crystallin gene is governed by an enhancer in the third intron, which binds a positive (delta EF2) and negative (delta EF1) factor in its core region. Here we show by DNase I footprinting, electrophoretic mobility-shift assays, and cotransfection experiments with the delta 1-promoter/enhancer fused to the chloramphenicol acetyltransferase reporter gene that the delta 1-crystallin enhancer has two adjacent functional Pax-6 binding sites. We also demonstrate by DNase I footprinting that the delta EF1 site can bind the transcription factor USF, raising the possibility that USF may cooperate with Pax-6 in activation of the chicken delta 1- and alpha A-crystallin genes. These data, coupled with our recent demonstration that Pax-6 activates the alpha A-crystallin gene, suggest that Pax-6 may have been used extensively throughout evolution to recruit and express crystallin genes in the lens.

DNA-binding and transactivation properties of Pax-6: three amino acids in the paired domain are responsible for the different sequence recognition of Pax-6 and BSAP (Pax-5)

Pax-6 is known to be a key regulator of vertebrate eye development. We have now isolated cDNA for an invertebrate Pax-6 protein from sea urchin embryos. Transcripts of this gene first appear during development at the gastrula stage and are later expressed at high levels in the tube foot of the adult sea urchin. The sea urchin Pax-6 protein is highly homologous throughout the whole protein to its vertebrate counterpart with the paired domain and homeodomain being virtually identical. Consequently, we found that the DNA-binding and transactivation properties of the sea urchin and mouse Pax-6 proteins are very similar, if not identical. A potent activation domain capable of stimulating transcription from proximal promoter and distal enhancer positions was localized within the C-terminal sequences of both the sea urchin and mouse Pax-6 proteins. The homeodomain of Pax-6 was shown to cooperatively dimerize on DNA sequences consisting of an inverted repeat of the TAAT motif with a preferred spacing of 3 nucleotides. The consensus recognition sequence of the Pax-6 paired domain deviates primarily only at one position from that of BSAP (Pax-5), and yet the two proteins exhibit largely different binding specificities for individual, naturally occurring sites. By creating Pax-6-BSAP fusion proteins, we were able to identify a short amino acid stretch in the N-terminal part of the paired domain which is responsible for these differences in DNA-binding specificity. Mutation of three Pax-6-specific residues in this region (at positions 42, 44, and 47 of the paired domain) to the corresponding amino acids of BSAP resulted in a complete switch of the DNA-binding specificity from Pax-6 to BSAP. These three amino acids were furthermore shown to discriminate between the Pax-6- and BSAP-specific nucleotide at the divergent position of the two consensus recognition sequences.

The role of Pax-6 in eye and nasal development

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

Crystal structure of a paired domain-DNA complex at 2.5 A resolution reveals structural basis for Pax developmental mutations

The 2.5 A resolution structure of a cocrystal containing the paired domain from the Drosophila paired (prd) protein and a 15 bp site shows structurally independent N-terminal and C-terminal subdomains. Each of these domains contains a helical region resembling the homeodomain and the Hin recombinase. The N-terminal domain makes extensive DNA contacts, using a novel beta turn motif that binds in the minor groove and a helix-turn-helix unit with a docking arrangement surprisingly similar to that of the lambda repressor. The C-terminal domain is not essential for prd binding and does not contact the optimized site. All known developmental missense mutations in the paired box of mammalian Pax genes map to the N-terminal subdomain, and most of them are found at the protein-DNA interface.

Congenital hereditary endothelial dystrophy associated with glaucoma

BACKGROUND

Three children, ranging in age from 2 to 6 months, had diffuse and homogeneously opaque corneas, clinically consistent with congenital hereditary endothelial dystrophy. Bilateral elevated intraocular pressure (IOP) was a feature in all three children.

METHODS

Initially, all patients underwent glaucoma surgery to reduce IOP. Subsequently, a penetrating keratoplasty was performed in one eye of each patient to clear the visual axis. The excised corneal button was examined by light microscopy and by transmission and scanning electron microscopy.

RESULTS

Postoperatively, all patients maintained clear corneal grafts. Results of histopathologic examination showed an absence of the endothelial cell layer in all patients. The presence of a variably thick collagenous layer posterior to the anterior banded zone of Descemet's membrane and the absence of endothelial cells were noted on transmission electron microscopy. Scanning electron microscopy confirmed absent, or scanty, and abnormal endothelial cells.

CONCLUSION

The authors describe three patients with a clear association between congenital glaucoma and congenital hereditary endothelial dystrophy. This combination should be suspected where persistent and total corneal opacification fails to resolve after normalization of IOP.

A high-resolution integrated physical, cytogenetic, and genetic map of human chromosome 11: distal p13 to proximal p15.1

We describe a detailed physical map of human chromosome 11, extending from the distal part of p13 through the entirety of p14 to proximal p15.1. The primary level of mapping is based on chromosome breakpoints that divide the region into 20 intervals. At higher resolution YACs cover approximately 12 Mb of the region, and in many places overlapping cosmids are ordered in contiguous arrays. The map incorporates 18 known genes, including precise localization of the GTF2H1 gene encoding the 62-kDa subunit of TFIIH. We have also localized four expressed sequences of unknown function. The physical map incorporates genetic markers that allow relationships between physical and genetic distance to be examined, and similarly includes markers from a radiation hybrid map of 11. The cytogenetic location of cosmids has been examined on high-resolution banded chromosomes by fluorescence in situ hybridization, and FLpter values have been determined. The map therefore fully integrates physical, genic, genetic, and cytogenetic information and should provide a robust framework for the rapid and accurate assignment of new markers at a high level of resolution in this region of 11p.

Three novel aniridia mutations in the human PAX6 gene

Aniridia (iris hypoplasia) is an autosomal dominant congenital disorder of the eye. Mutations in the human aniridia (PAX6) gene have now been identified in many patients from various ethnic groups. In the study reported here we describe PAX6 mutations in one sporadic and five familial cases with aniridia. Of the four different mutations identified, one was identical to a previously reported mutation (C-->T transition at codon 240), and three were novel: two in the glycine-rich region and one in the proline/serine/threonine-rich (PST) region. One PAX6 mutation found in the PST region was associated with cataracts in an aniridia family. Another splice mutation in the PST domain occurred in an aniridia patient with anosmia (inability to smell). The six new aniridia cases reported here have mutations predicted to generate incomplete PAX6 proteins. These results support the theory that human aniridia is caused by haploinsufficiency of PAX6.

Abnormal electroretinogram associated with developmental brain anomalies

PURPOSE

We have encountered abnormal ERGs associated with optic nerve hypoplasia, macular, optic nerve and chorioretinal colobomata and developmental brain anomalies. Brain anomalies include cortical dysgenesis, lissencephaly, porencephaly, cerebellar and corpus callosum hypoplasia. We describe six exemplar cases.

METHODS

Scotopic and photopic ERGs adherent to international standards were performed as well as photopic ERGs to long-duration stimuli. CT or MRI studies were also done. The ERGs were compared to age-matched normal control subjects.

RESULTS

ERG changes include reduced amplitude b-waves to blue and red stimuli under scotopic testing conditions. Implicit times were often delayed. The photopic responses also showed reduced amplitude a- and b-waves with implicit time delays. The long-duration photopic ERG done in one case shows attenuation of both ON- and OFF-responses.

CONCLUSIONS

Common underlying developmental genetic or environmental unifying casualties are speculated to be at fault in causing these cases of associated retinal and brain abnormalities. No single etiology is expected. Multiple potential causes acting early in embryogenesis effecting neuronal induction, migration and differentiation are theorized. These occur at a time when brain and retinal cells are sufficiently undifferentiated to be similarly effected. We call these cases examples of Brain Retina Neuroembryodysgenesis (BRNED). Homeobox and PAX genes with global neuronal developmental influences are gene candidates to unify the observed disruption of brain and retinal cell development. The ERG can provide a valuable clinical addition in understanding and ultimately classifying these disorders.

Comparative development. Seeing eye to eye

Comparative studies of homologous developmental genes in mouse and Drosophila are suggesting that organs in these species may have closer evolutionary relationships than was hitherto suspected.

PAX genes

PAX genes are developmental control genes that encode transcription factors containing a DNA-binding paired domain. Mutations in three of the nine mouse genes (Pax1, Pax3 and Pax6) and two of the nine human genes (PAX3 and PAX6) are known to cause developmental defects. These defects are caused by loss-of-function alleles; pathogenesis occurs as a result of a half dosage of the PAX gene product in particular cells. Gain-of-function mutations have been implicated in cancer.