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

Genotype/phenotype correlations in aniridia

PURPOSE

To detect and characterize mutations in cases of familial and sporadic aniridia in Maritime Canada, and to look for indications of genotype/phenotype correlation within the cohort.

METHODS

Twelve consecutive and unrelated patients (probands) who had total or nearly complete absence of irides, and four affected relatives, were recruited from Maritime Canada. Clinical data were obtained by chart review and electroretinogram testing. Mutations in the PAX6 gene were detected by single-strand conformation polymorphism and characterized by sequence analysis.

RESULTS

Eleven different PAX6 mutations, 10 of which are novel, were found. The four patients with congenital cataracts all had mutations in the C-terminal proline-serine-threonine (PST)-rich domain of the PAX6 protein. Electroretinograms of nine of 11 patients displayed depressed scotopic maximum response b-wave amplitudes. The greatest decrease in b-wave amplitudes was seen in patients in whom the paired domain was disrupted by mutation.

CONCLUSION

Some aspects of the phenotype of aniridia appear to correlate with the predicted effect of point mutations on the paired and PST domains of the PAX6 protein.

Molecular regulators involved in vertebrate eye development

Development of the eye can be subdivided into three phases. The first phase is the formation of the major structures of the eye by the processes of induction and regional specification. The second is the maturation of these structures to form the functional eye, and the third phase is the formation of neuronal connections between retina and the optic tectum. These processes are tightly regulated by signalling cascades that direct axonal outgrowth, cellular proliferation and differentiation. Some members of these signalling cascades have been identified in recent studies. These include secreted factors which transmit signals extracellularly, and receptors and transcription factors which are members of intracellular signalling pathways that respond to extracellular signals. This review summarizes the recent research that has implicated these factors in playing a role in eye development on the basis of functional or expression criteria.

PAX 6 is normal in most cases of Peters' anomaly

Mutations in the PAX 6 gene are known to cause many cases of inherited and sporadic aniridia. Although embryologically similar to aniridia, the cause of Peters' anomaly has received far less attention. Two reports have been published demonstrating mutations in the PAX 6 gene in Peters' anomaly. We have analysed the PAX 6 gene in 15 individuals with Peters' anomaly (7 familial, 8 sporadic). This is the largest cohort of Peters' anomaly described. The PAX 6 gene was screened using a combination of single-strand conformational polymorphism gel electrophoresis and direct sequencing. No mutations were found in the coding region of the PAX 6 gene. We feel that Peters' anomaly is a heterogeneous condition and that for the majority of cases PAX 6 is not the 'Peters' anomaly gene'.

A promoter-associated polymorphic repeat modulates PAX-6 expression in human brain

The PAX-6 gene plays a critical role in neurodevelopment and brain plasticity. While transcription of human PAX-6 is regulated by alternate usage of two distinct promoters termed A and B, expression in adult human brain is primarily controlled by promoter B. We now report that a novel polymorphic dinucleotide repeat sequence with the structure (AC)m(AG)n is located approximately 1 kb upstream of the transcription initiation site associated with promoter B. PCR-based systematic screening for length variations in a caucasian population showed a skewed distribution of multiple alleles containing between 24 and 36 repeat units. In 217 unrelated individuals, the frequency of alleles in the range between 25 and 29 repeats was 90%, with the 26 repeat allele alone accounting for 50%; the heterozygosity rate was 65%. Variants of this PAX-6 gene-linked polymorphic region (PAX-6LPR) had different transcriptional efficiencies when fused to a luciferase reporter gene and transfected into Cos-7 cells. Promoter activity of variants with >/=29 repeats was 4- to 9-fold higher than that of the 26 repeat allele. The influence of the PAX-6LPR on PAX-6 expression was confirmed in postmortem cerebellum from individuals with different genotypes. mRNA levels were 2-fold higher in genotypes with long alleles compared to those with short alleles. Allelic variation in PAX-6 expression may be a determinant of interindividual differences in brain plasticity and function.

Signalling interactions during facial development

The development of the vertebrate face is a dynamic multi-step process which starts with the formation of neural crest cells in the developing brain and their subsequent migration to form, together with mesodermal cells, the facial primordia. Signalling interactions co-ordinate the outgrowth of the facial primordia from buds of undifferentiated mesenchyme into the intricate series of bones and cartilage structures that, together with muscle and other tissues, form the adult face. Some of the molecules that are thought to be involved have been identified through the use of mouse mutants, data from human craniofacial syndromes and by expression studies of signalling molecules during facial development. However, the way that these molecules control the epithelial-mesenchymal interactions which mediate facial outgrowth and morphogenesis is unclear. The role of neural crest cells in these processes has also not yet been well defined. In this review we discuss the complex interaction of all these processes during face development and describe the candidate signalling molecules and their possible target genes.

Anterior segment dysgenesis and congenital glaucoma associated with partial trisomy of chromosome 1 (1q32-qter)

A child born with partial trisomy of chromosome 1 (1q32-qter) survived and was seen for anterior segment dysgenesis and congenital glaucoma. Pure trisomy 1q is rarely seen in live-born infants and has not previously been described in association with congenital glaucoma. The genetic basis for glaucoma is complicated and multifactorial and probably determined by a number of genes on a variety of chromosomes. The current case provides some evidence that part of chromosome 1 may be involved with the etiology of a glaucomatous process.

A novel homeobox gene PITX3 is mutated in families with autosomal-dominant cataracts and ASMD

We report here the identification of a new human homeobox gene, PITX3, and its involvement in anterior segment mesenchymal dysgenesis (ASMD) and congenital cataracts in humans. The PITX3 gene is the human homologue of the mouse Pitx3 gene and is a member of the RIEG/PITX homeobox gene family. The protein encoded by PITX3 shows 99% amino-acid identity to the mouse protein, with 100% identity in the homeodomain and approximately 70% overall identity to other members of this family. We mapped the human PITX3 gene to 10q25 using a radiation-hybrid panel. A collection of 80 DNA samples from individuals with various eye anomalies was screened for mutations in the PITX3 gene. We identified two mutations in independent patients. A 17-bp insertion in the 3'-end of the coding sequence, resulting in a frame shift, occurred in a patient with ASMD and cataracts, and a G-->A substitution, changing a codon for serine into a codon for asparagine, in the 5'-end of the gene occurred in a patient with congenital cataracts. Both mutations cosegregate with the disease phenotype in families, and neither were found in up to 300 control individuals studied. Further expression analysis of Pitx3 in the mouse supports a unique role in early ocular development, with later expression extending to the midbrain, tongue, incisors, sternum, vertebrae and limbs. These data strongly suggest a role for PITX3 in ASMD and cataracts and provide new evidence of the contribution of the RIEG/PITX gene family to the developmental program underpinning normal eye formation.

A highly conserved lens transcriptional control element from the Pax-6 gene

We have identified a short segment of the mouse Pax-6 gene 5' flanking region that is necessary and sufficient for reporter construct expression in components of the eye derived from non-neural ectoderm. This transcriptional control element has a highly conserved nucleotide sequence over 341 bp and is located approximately 3.5 kb upstream of the start-point for transcription from the most proximal promoter (PO) of the Pax-6 gene. The level of identity between the human and mouse Pax-6 genes in this region is 93%. When combined either with its natural promoter or a heterologous minimal promoter, this element directs reporter construct expression to a region of surface ectoderm overlying the optic cup beginning at E8.5-9.0 (12-14 somites). Subsequently, expression is restricted to the lens (primarily the lens epithelium) and the corneal epithelium. This element will provide an important tool in future transgenic analyses of lens formation and will allow identification of transcription factors with a central function in lens development.

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

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

Summary of ocular genetic disorders and inherited systemic conditions with eye findings

Of the close to 10,000 known inherited disorders that affect humankind, a disproportionately high number affect the eye. The total number of genes responsible for the normal structure, function, and differentiation of the eye is unknown, but the list of these genes is rapidly and constantly growing. The objective of this paper is to provide a current list of mapped and/or cloned human eye genes that are responsible for inherited diseases of the eye. The ophthalmologist should be aware of recent advances in molecular technology which have resulted in significant progress towards the identification of these genes. The implications of this new knowledge will be discussed herein.

Alternatively spliced insertions in the paired domain restrict the DNA sequence specificity of Pax6 and Pax8

Transcription factors of the Pax family bind to their target genes via the paired domain which is known to be composed of two subdomains each recognizing distinct half-sites in adjacent major grooves of the DNA helix. We now demonstrate that the mammalian Pax8 gene gives rise, by alternative mRNA splicing, to a protein isoform containing an extra serine residue in the recognition alpha-helix 3 of the paired domain. This Pax8(S) protein does not interact with bipartite paired domain-binding sites, indicating that inactivation of the N-terminal DNA-binding motif severely restricts the sequence specificity of the paired domain. However, the Pax8(S) protein binds in vitro and in vivo to the 5aCON sequence which was previously identified as a high-affinity binding site for the Pax6(5a) splice variant carrying a 14-amino-acid insertion in the paired domain. The 5aCON sequence is shown to consist of four interdigitated 5' half-sites of the bipartite consensus sequence and is thus bound by four Pax8(S) molecules via the intact C-terminal DNA-binding motif of the paired domain. Together these data suggest that inactivation of the N-terminal region of the paired domain by alternative splicing is used in vivo to selectively target Pax transcription factors to gene regulatory regions containing highly specialized 5aCON-like sequences.

Ultrastructural studies of primary congenital glaucoma in rabbits

BACKGROUND

The cause of congenital glaucoma is unknown.

METHODS

To determine whether the site of impaired aqueous outflow is the entrance to the trabecular meshwork (TM), within the TM, the aqueous drainage plexus, or a combination thereof, the process of TM development was examined by scanning and transmission electron microscopy on postnatal day 3 and weeks 1, 2, 3, 4, and 6 in New Zealand rabbits homozygous for the buphthalmic (bu/bu) gene compared with age-matched controls.

RESULTS

Openings to the entrance of the TM in congenital glaucoma were observed, and there was no evidence of an endothelial membrane occluding aqueous flow to the TM. The morphology of the congenital glaucoma TM was abnormal in all bu/bu rabbits by 2 weeks and was characterized by a smaller entrance to the TM at the iris base, smaller intertrabecular openings within and between the trabecular lamellae, and at 6 weeks, iris pillars with extensive lateral extensions in the angle recess. Most intertrabecular spaces were open, however, the inner intertrabecular spaces adjacent to the aqueous plexus were compressed.

CONCLUSION

These results suggest the development of congenital glaucoma, which involves a mutation in an autosomal recessive gene and leads to loss of function of a gene(s) required for the differentiation of the TM.

Isolation of a new homeobox gene belonging to the Pitx/Rieg family: expression during lens development and mapping to the aphakia region on mouse chromosome 19

We recently reported the positional cloning of a homeobox gene involved in the pathogenesis of Rieger syndrome, RIEG1 , and its mouse homolog, Rieg1 . Rieg1 (also independently described as Pitx2) is highly homologous to the Ptx1/Potx gene product, suggesting that there may be additional members of this novel Pitx family. The Pitx genes play an important role in eye, tooth, pituitary and umbilical region development as evidenced by Rieger syndrome and iris hypoplasia phenotypes, resulting from mutations in the RIEG1 gene and by expression studies. In order to characterize further the Pitx gene family we searched mouse cDNA libraries to identify additional members. A new gene was isolated which encodes a homeoprotein with strong homology to the other Pitx proteins and 97-100% identity in the homeodomain itself, suggesting that this is a third member of the family, Pitx3 . In whole mount in situ hybridization on mouse embryos ranging from 8.5 to 11.5 days post-coitum (d.p.c.), Pitx3 mRNA was seen only in the developing lens starting at day 11. Hybridization on cross-sections revealed strong signals in the lens vesicle in 11 d.p.c. embryos and throughout the lens, particularly in the anterior epithelium and equator region in 15 d.p.c. embryos. Pitx3 was mapped close to aphakia on mouse chromosome 19. The aphakia homozygous mouse is characterized by small eyes lacking a lens, which fail to develop beyond 11 d.p.c. These data make Pitx3 a strong candidate gene for the aphakia phenotype in the mouse and suggest a role for the human homolog in congenital lens malformations.

Dominant coloboma-microphthalmos syndrome associated with sensorineural hearing loss, hematuria, and cleft lip/palate

Ocular colobomas and microphthalmos, isolated or as part of a syndrome, are usually sporadic and only rarely found in large families. A 4-generation family with autosomal dominant uveal coloboma and microphthalmos associated with cleft lip and palate was re-evaluated. Wide variability in expression is evident and more recently recognized manifestations include a complete spectrum of eye involvement, impairment of extraocular movement, mid-frequency sensorineural hearing loss, and hematuria. Learning difficulties requiring remedial teaching were present in one third of those affected and a neural tube defect has occurred in one presumed affected member. This family appears to present a unique phenotype, which provides an opportunity to identify a genetic locus involved in eye, ear, renal, primary palate, and brain development.

Congenital aphakia in Peters' anomaly syndrome. A case report

The authors report a case of congenital bilateral corneal opacities in which one of the eyes was enucleated because of malignant glaucoma and corneal perforation. Corneal defects and iridocorneal adhesion were found, but aphakia was the major pathologic ocular finding. The clinical picture and pathology study indicated this case as a Peters' anomaly presenting congenital aphakia.

Reciprocal effect of Waardenburg syndrome mutations on DNA binding by the Pax-3 paired domain and homeodomain

The Pax-3 protein contains two DNA-binding domains, a paired domain and a homeodomain. Mutations in Pax-3 cause Waardenburg syndrome (WS) in humans and the mouse Splotch (Sp) phenotype. In the Sp-delayed mouse, a mutation in the Pax-3 paired domain (G9R) abrogates the DNA-binding activity of both the paired domain and the homeodomain, suggesting that they may functionally interact. To investigate this possibility further, we have analyzed the DNA-binding properties of additional point mutants in the Pax-3 paired domain and homeodomain that occur in WS patients (F12L, N14H, G15S, P17L, R23L, G48A, S51F and G66D in the paired domain, V47F and R53G in the homeodomain), the Pax-1 un mutation (G15A) and a substitution associated with Peters' anomaly in the PAX-6 gene (R23G). Within the paired domain, seven of 10 mutations were found to abrogate DNA-binding by the paired domain. Remarkably, these seven mutations also affected DNA binding by the homeodomain, causing either a complete loss (P17L and G66D), a reduction (R23G, R23L, G15S and G15A) or an increase in DNA-binding activity (N14H). In addition, the effect of paired domain mutations occurred at the level of monomer formation by the homeodomain, while the dimerization potential of this domain seemed unaffected in mutants where it could be analyzed. Furthermore, while both homeodomain mutations were found to abolish DNA binding by this domain, the R53G mutation also abrogated DNA binding by the paired domain. The important observation that independent mutations in either domain can affect DNA binding by the other in the intact Pax-3 protein strongly suggests that the two domains are not functionally independent but bind DNA through cooperative interactions. Modeling the deleterlous mutations on the three-dimensional structure of the paired domain of Drosophila Prd shows that these mutations cluster at the DNA interface, thus suggesting that a series of DNA contacts are essential for DNA binding by both the paired domain and the homeodomain of Pax-3.

Molecular genetics is revolutionizing our understanding of ophthalmic disease

PURPOSE

To inform ophthalmologists of the extraordinary progress in molecular genetics that is revolutionizing our understanding of ophthalmic disease and of the crucial role of the clinician in facilitating genetic discovery.

METHOD

Review of relevant articles.

RESULTS

Genes for many mendelian-inherited eye diseases have been localized and some identified using three general approaches: positional cloning, which requires no knowledge of underlying pathophysiology; a candidate gene approach, which examines genes based on their likely function; and a positional candidate approach, which uses map location as well as candidate genes in the linked region to isolate a gene. In positional cloning, once linkage is obtained, the gene can eventually be isolated, cloned, and sequenced and mutations identified. Techniques in molecular biology and other disciplines can then be used to unravel the pathophysiology of a disease.

CONCLUSIONS

Molecular genetics is advancing our understanding of the classification and pathophysiology of ophthalmic diseases. The present classification system, based largely on clinical description of disease, is being replaced with a more rational classification based on genetic causes. Future research will determine the function of known genes and identify susceptibility loci for complex diseases such as chronic open-angle glaucoma and age-related macular degeneration. More specific diagnostic, therapeutic, and preventive strategies for ophthalmic disease will be developed. Clinicians play a crucial role by inquiring about the family history of all patients, identifying individuals and families with a genetic trait, and, when appropriate, referring them for further investigation.

Familial glaucoma iridogoniodysplasia maps to a 6p25 region implicated in primary congenital glaucoma and iridogoniodysgenesis anomaly

Familial glaucoma iridogoniodysplasia (FGI) is a form of open-angle glaucoma in which developmental anomalies of the iris and irido-corneal angle are associated with a juvenile-onset glaucoma transmitted as an autosomal dominant trait. A single large family with this disorder was examined for genetic linkage to microsatellite markers. A peak LOD score of 11.63 at a recombination fraction of 0 was obtained with marker D6S967 mapping to chromosome 6p25. Haplotype analysis places the disease gene in a 6.4-cM interval between the markers D6S1713 and D6S1600. Two novel clinical appearances extend the phenotypic range and provide evidence of variable expressivity. The chromosome 6p25 region is now implicated in FGI, primary congenital glaucoma, and iridogoniodysgenesis anomaly. This may indicate the presence of a common causative gene or, alternatively, a cluster of genes involved in eye development/function.

Modular organization of Pax/homeodomain proteins in transcriptional regulation

Specificity in transcriptional regulation lies in a large part in the specificity of DNA binding by transcription factors. One group of transcription factors which are of great interest for studying transcriptional specificity is the Pax/Homeodomain (Pax/HD) proteins which contain two conserved DNA binding domains, a paired domain (PD) and a Paired-class homeodomain (HD). The Pax/HD proteins can bind to at least three types of specific DNA sequences: the PD binding sites, the dimeric HD binding sites and a composite HD and PD binding site. We propose that Pax/HD proteins regulate different subsets of their target genes through modular binding to one of these three specific sequences. We show that, in a tissue culture system, a member of the Pax/HD family, Paired, is able to activate transcription after binding through either its PD or its HD. The transactivation mediated by one domain does not require DNA binding of the other domain. Furthermore, binding sites specific for the PD of Paired are sufficient to mediate embryonic expression of a reporter gene in a paired-like pattern. The expression of the reporter gene is dependent on wild type paired function and, in a prd mutant background, it can be rescued by an exogenous paired gene encoding a protein whose HD is not able to bind to DNA. Finally, we show that the Paired protein uses differently its C-terminal activation domain when transactivation is mediated through its PD or its HD. These results and recent evidence from other Pax/HD proteins strongly suggest that this class of proteins is able to achieve specific and modular transcriptional regulation through its multiple DNA binding domains.

Delimiting the Wolf-Hirschhorn syndrome critical region to 750 kilobase pairs

Wolf-Hirschhorn syndrome (WHS) is a multiple anomaly condition characterized by mental and developmental defects, resulting from the absence of the distal segment of one chromosome 4 short arm (4p16.3). Owing to the complex and variable expression of this disorder, it is thought that the WHS is a contiguous gene syndrome with an undefined number of genes contributing to the phenotype. The 2.2 Mbp genomic segment previously defined as the critical region by the analyses of patients with terminal or interstitial deletions is extremely gene dense and an intensive investigation of the developmental role of all the genes contained within it would be daunting and expensive. Further refinement in the definition of the critical region would be valuable but depends on available patient material and accurate clinical evaluation. In this study, we have utilized fluorescence in situ hybridization to further characterize a WHS patient previously demonstrated to have an interstitial deletion and demonstrate that the distal breakpoint occurs between the loci FGFR3 and D4S168. This reduces the critical region for this syndrome to less than 750 kbp. This has the effect of eliminating several genes previously proposed as contributing to this syndrome and allows further research to focus on a more restricted region of the genome and a limited set of genes for their role in the WHS syndrome.

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

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

Homeodomain protein IDX-1: a master regulator of pancreas development and insulin gene expression

The homeodomain protein IDX-1 appears to be a "master regulator" of pancreas development and beta-cell differentiation and function. In murine gene inactivation models and in a human subject with a homozygous mutation of the IDX-1 gene, the pancreas fails to develop. In the adult endocrine pancreas, IDX-1 is primarily expressed in beta cells, where it is a key factor in the upregulation of insulin gene transcription and appears to have a role in the regulation of the somatostatin, glucokinase, glucose transporter-2, and islet amyloid polypeptide genes. Recent studies also suggest a role for IDX-1 in the neogenesis and proliferation of beta cells. The observed functions of IDX-1 and its downregulation in parallel with insulin in glucose-toxicity models implicate IDX-1 as a potential factor contributing to the pathogenesis of diabetes mellitus. Future directions include the use of conditional gene inactivation to determine more precisely the role of IDX-1 throughout endocrine pancreas differentiation and the exploration of IDX-1 as a potential target for gene therapy of diabetes mellitus.

rax, a novel paired-type homeobox gene, shows expression in the anterior neural fold and developing retina

Development of the vertebrate eye has been found to require the activity of several genes encoding homeodomain proteins (Freund, C., Horsford, D. J. & McInnes, R. R. (1996) Hum. Mol. Genet. 5, 1471-1488). Some of these genes, or portions thereof, are highly conserved across phyla. In this paper, we report the identification of a novel homeobox gene, rax (retina and anterior neural fold homeobox), whose expression pattern suggests an important role in eye development. The predicted amino acid sequence of Rax comprises a protein with a paired-type homeobox, as well as the octapeptide that is found in many paired-type homeobox genes. In addition, in the C terminus of Rax, we found a 15-aa domain that we have named the OAR domain. This domain is also found in several other homeobox genes. In the early mouse embryo, rax is expressed in the anterior neural fold, including areas that will give rise to the ventral forebrain and optic vesicles. Once the optic vesicles form, rax expression is restricted to the ventral diencephalon and the optic vesicles. At later stages, rax expression is found only in the developing retina. After birth, the expression of rax is restricted to the zone of proliferating cells within the retina, and expression gradually decreases as proliferation declines. These findings suggest that rax is one of the molecules that define the eye field during early development and that it has a role in the proliferation and/or differentiation of retinal cells.

The incidence of PAX6 mutation in patients with simple aniridia: an evaluation of mutation detection in 12 cases

Twelve aniridia patients, five with a family history and seven presumed to be sporadic, were exhaustively screened in order to test what proportion of people with aniridia, uncomplicated by associated anomalies, carry mutations in the human PAX6 gene. Mutations were detected in 90% of the cases. Three mutation detection techniques were used to determine if one method was superior for this gene. The protein truncation test (PTT) was used on RT-PCR products, SSCP on genomic PCR amplifications, and chemical cleavage of mismatch on both RT-PCR and genomic amplifications. For RT-PCR products, only the translated portion of the gene was screened. On genomic products exons 1 to 13 (including 740 bp of the 3' untranslated sequence and all intron/exon boundaries) were screened, as was a neuroretina specific enhancer in intron 4. Ten of the possible 12 mutations in the five familial cases and five of the sporadic patients were found, all of which conformed to a functional outcome of haploinsufficiency. Five were splice site mutations (one in the donor site of intron 4, two in the donor site of intron 6, one in each of the acceptor sites of introns 8 and 9) and five were nonsense mutations in exons 8, 9, 10, 11, and 12. SSCP analysis of individually amplified exons, with which nine of the 10 mutations were seen, was the most useful detection method for PAX6.

Functional analysis of paired box missense mutations in the PAX6 gene

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

A human homologue of the Drosophila eyes absent gene underlies branchio-oto-renal (BOR) syndrome and identifies a novel gene family

A candidate gene for Branchio-Oto-Renal (BOR) syndrome was identified at chromosome 8q13.3 by positional cloning and shown to underlie the disease. This gene is a human homologue of the Drosophila eyes absent gene (eya), and was therefore called EYA1. A highly conserved 271-amino acid C-terminal region was also found in the products of two other human genes (EYA2 and EYA3), demonstrating the existence of a novel gene family. The expression pattern of the murine EYA1 orthologue, Eya1, suggests a role in the development of all components of the inner ear, from the emergence of the otic placode. In the developing kidney, the expression pattern is indicative of a role for Eya1 in the metanephric cells surrounding the 'just-divided' ureteric branches.

Congenital aphakia: a clinicopathologic report of three cases

BACKGROUND

Congenital aphakia is a rare condition that has been classified as primary when no lens induction of the surface ectoderm occurs and secondary when lens development takes place but later is resorbed or expelled in utero.

METHODS

The authors report the clinical and pathologic findings in three infants with congenital aphakia whose eyes were enucleated either at surgery at 11 months or at autopsy after 1 and 3 days of life.

RESULTS

Two cases classified as primary congenital aphakia had severe microphthalmos, anterior segment aplasia, or anomalous development and posterior choroidal and optic disc colobomas. One was in a case believed to be Waardenburg's recessive anophthalmia syndrome and the other had 18 trisomy. A case of secondary congenital aphakia had findings of Peter's syndrome and features suggesting rubella, which had been observed in some previous reports.

CONCLUSIONS

Primary congenital aphakia can result from a variety of teratogenic events in the first 4 weeks of embryogenesis and results in microphthalmos and severe anterior segment aplasia/dysplasia. Secondary congenital aphakia is associated with less severe ocular anomalies. The possible role of deletion or mutation involving the PAX6 gene in anterior segment anomalies and induction of lens development is discussed. In addition to chromosomal influences, in utero viral infection, particularly rubella, may play a role in some cases.

PAX-6 in development and evolution

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

Forebrain patterning defects in Small eye mutant mice

Pax6 is a member of the Pax gene family of transcriptional regulators that exhibits a restricted spatiotemporal expression in the developing central nervous system, eye and nose. Mutations in Pax6 are responsible for inherited malformations in man, rat and mouse. To evaluate the role of Pax6 in forebrain development, we studied in detail mouse Small eye/Pax6 mutant brains. This analysis revealed severe defects in forebrain regions where Pax6 is specifically expressed. The establishment of some expression boundaries along the dorsoventral axis of the secondary prosencephalon is distorted and the specification of several ventral structures and nuclei is abolished. Specifically, the development of the hypothalamo-telencephalic transition zone and the ventral thalamus is distorted. Our detailed analysis included a comparison of the expression of Pax6, Dlx1 and several other genes during embryonic mouse brain development in wild-type and in the mutant Small eye (Sey) brain. The results from the analysis of normal brain development show that the restricted expression of Pax6 and Dlx1 at E12.5 dpc respect domains within the forebrain, consistent with the implications of the prosomeric model for the organisation of the forebrain (L. Puelles and J. L. R. Rubenstein (1993) Trends Neurosci. 16, 472–479). Furthermore, we found an early restriction of Pax6 and Dlx1 expression into presumptive histogenetic fields that correlate with the formation of distinct forebrain structures and nuclei. Our results are discussed in light of changes in adhesive properties in the Sey brain that might control segregation, assembly and cell migration of progenitors of specific forebrain regions.

Autosomal-dominant iridogoniodysgenesis and Axenfeld-Rieger syndrome are genetically distinct

PURPOSE

To determine whether there is a locus for iridogoniodysgenesis (IGD)/ familial iris hypoplasia in the region of the known Axenfeld-Rieger syndrome (ARS) locus at 4q25 and to determine the ocular phenotype within the autosomal-dominant iris hypoplasia group of disorders.

METHODS

Clinical examinations were performed on 27 members, with 11 affected from one family in which the IGD occurred in association with the nonocular features of ARS, and on 70 members with 30 affected from a second IGD family with ocular features only. Family members were genotyped for markers within the 4q25 region known to contain a locus for ARS. LOD scores were calculated with the MLINK option of the LINKAGE program.

RESULTS

The iris hypoplasia in each IGD family was similar. In the IGD family with only ocular features (IGD anomaly), however, a majority of those affected had a goniodysgenesis with excess tissue in the angle and anomalous angle vascularity. These findings were absent in the IGD family with syndromic features (IGD syndrome). Linkage to the 4q25 region was excluded in the IGD anomaly family, whereas the family with IGD syndrome was found to be completely linked to the 4q25 region (peak LOD score with D4S407 of 7.827 at theta = 0.00).

CONCLUSIONS

The authors' results suggest that mutations at the 4q25 locus can result in variable ocular features that also occur in combination with nonocular (dental and jaw) anomalies. Mutation of a separate locus must underlie IGD with ocular features only. A re-evaluation of the relation between the various forms of autosomal-dominant iris hypoplasia, therefore, may be warranted.

Pax-6 and alphaB-crystallin/small heat shock protein gene regulation in the murine lens. Interaction with the lens-specific regions, LSR1 and LSR2

We have demonstrated previously that a transgene comprising the -164/+44 fragment of the murine alphaB-crystallin gene fused to the bacterial chloramphenicol acetyltransferase (cat) gene is lens-specific in transgenic mice. The -147 to -118 sequence was identified as a lens-specific regulatory region and is called here LSR1 for lens-specific region 1. In the present experiments, a -115/+44-cat transgene was also lens-specific in transgenic mice, although the average activity was 30 times lower than that derived from the -164/+44-cat transgene. The -115/+44 alphaB-crystallin fragment contains a highly conserved region (-78 to -46) termed here LSR2. A -68/+44-cat transgene, in which LSR2 is truncated, was inactive in transgenic mice. DNase I footprinting indicated that LSR1 and LSR2 bind partially purified nuclear proteins from either alphaTN4-1 lens cells or the mouse lens as well as the purified paired domain of Pax-6. Site-specific mutation of LSR1 eliminated both Pax-6 binding and promoter activity of the -164/+44-cat transgene in transgenic mice. Finally antibody/electrophoretic mobility shift assays and cotransfection experiments indicated that Pax-6 can activate the alphaB-crystallin promoter via LSR1 and LSR2. Our data strengthen the idea that Pax-6 has had a major role in recruiting genes for high expression in the lens.

Cooperative interactions between paired domain and homeodomain

The Pax proteins are a family of transcriptional regulators involved in many developmental processes in all higher eukaryotes. They are characterized by the presence of a paired domain (PD), a bipartite DNA binding domain composed of two helix-turn-helix (HTH) motifs, the PAI and RED domains. The PD is also often associated with a homeodomain (HD) which is itself able to form homo- and hetero-dimers on DNA. Many of these proteins therefore contain three HTH motifs each able to recognize DNA. However, all PDs recognize highly related DNA sequences, and most HDs also recognize almost identical sites. We show here that different Pax proteins use multiple combinations of their HTHs to recognize several types of target sites. For instance, the Drosophila Paired protein can bind, in vitro, exclusively through its PAI domain, or through a dimer of its HD, or through cooperative interaction between PAI domain and HD. However, prd function in vivo requires the synergistic action of both the PAI domain and the HD. Pax proteins with only a PD appear to require both PAI and RED domains, while a Pax-6 isoform and a new Pax protein, Lune, may rely on the RED domain and HD. We propose a model by which Pax proteins recognize different target genes in vivo through various combinations of their DNA binding domains, thus expanding their recognition repertoire.

Transcription factor genes and the developing eye: a genetic perspective

We review the current knowledge of transcription factors in mammallan eye development. The 14 transcription factors presently known to be required for eye formation are examined in some detail, incorporating data from both humans and rodents. Aspects of the biochemistry, expression patterns, genetics, mutant phenotypes, and biological insights acquired from the examination of loss-of-function mutations are summarized. The other 32 tissue-restricted transcription factors that are currently known to be expressed in the developing or mature mammallan eye are tabulated, together with the timing and site of their ocular expression; the requirement for most of these genes in the eye is unknown. Contributions to mammallan eye development from the study of the genetics of the Drosophila eye are discussed briefly. Identification of the entire cohort of transcription factors required for eye development is an essential first step towards understanding the mechanisms underlying eye morphogenesis and differentiation, and the molecular basis of inherited eye abnormalities in humans.

Lens development and crystallin gene expression: many roles for Pax-6

The vertebrate eye lens has been used extensively as a model for developmental processes such as determination, embryonic induction, cellular differentiation, transdifferentiation and regeneration, with the crystallin genes being a prime example of developmentally controlled, tissue-preferred gene expression. Recent studies have shown that Pax-6, a transcription factor containing both a paired domain and homeodomain, is a key protein regulating lens determination and crystallin gene expression in the lens. The use of Pax-6 for expression of different crystallin genes provides a new link at the developmental and transcriptional level among the diverse crystallins and may lead to new insights into their evolutionary recruitment as refractive proteins.

Colobomatous microphthalmia with midfacial clefting: part of the spectrum of branchio-oculo-facial syndrome?

A young male infant was noted at birth to have bilateral cleft lip and palate, bilateral microphthalmos and ocular colobomata, and a dysplastic left kidney. His mother had similar ophthalmological findings and milder facial anomalies which included abnormality of the philtrum and bilateral congenital nasolacrimal duct obstruction. His maternal grandmother had mild facial anomalies including a short philtrum and bilateral congenital nasolacrimal duct obstruction but had no evidence of any ocular abnormalities. The spectrum of abnormalities seen in this family are similar to those described in the branchio-oculo-facial syndrome, a rare dominantly inherited syndrome in which there are a number of developmental abnormalities of the eye, face, and kidney. Although the precise cause of this syndrome is unknown, it is likely to be caused by mutations in a gene responsible for the ordered closure of the foetal fissure and fusion of facial structures.

Peters' anomaly

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The revolution in molecular genetics and its impact on ophthalmology

Rapid advances in molecular technology have led to the identification of many genes responsible for inherited disease in ophthalmology. These discoveries also portend an understanding of the pathogenesis of more common ophthalmic disorders which have a genetic component, such as open-angle glaucoma and age-related macular degeneration. This review comprises a summary of these advances in molecular genetics, particularly the contribution of the Human Genome Project; a tabulation of the genes recently proven to be mutated in hereditary ocular conditions; and a discussion of the implications for the practising ophthalmologist.

Multiple functions for Pax6 in mouse eye and nasal development

Mouse embryos, homozygous for the small eye (Sey) mutation die soon after birth with severe facial abnormalities that result from the failure of the eyes and nasal cavities to develop. Mutations in the Pax6 gene are responsible for the Sey phenotype. As a general disruption of eye and nasal development occurs in the homozygous Sey embryos, it is unclear, from the mutant phenotype alone, which tissues require functional Psx6. To examine the roles for Pax6 in eye and nasal development we produced chimeric mouse embryos composed of wild-type and Sey mutant cells. In these embryos we found that mutant cells were excluded from both the lens and nasal epithelium. Both of these tissues were smaller, and in some cases absent, in chimeras with high proportions of mutant cells. The morphology of the optic cup was also severely affected in these chimeras; mutant cells were excluded from the retinal pigmented epithelium and did not intermix with wild-type cells in other regions. The evidence shows that Pax6 has distinct roles in the nasal epithelium and the principal tissue components of the embryonic eye, acting directly and cell autonomously in the optic cup and lens. We suggest that Pax6 may promote cell surface changes in the optic cup and control the fate of the ectoderm from which the lens and nasal epithelia are derived.

Pax proteins and eye development

Homologous members of the Pax gene family are required for eye development in Drosophila and vertebrates. Despite superficial similarities in the phenotypes of vertebrates with mutations in pax-6 and Drosophila eyeless mutants, it remains uncertain whether the two proteins encoded by these genes have comparable functions. The genetic cascade triggered by eyeless leads to eye formation, whereas pax-6 is not necessary for optic vesicle formation, but is required at other stages of eye development. A second vertebrate Pax gene, pax-2, is also required during eye development and appears to play a role during closure of the choroid fissure.

Genetic aspects of embryonic eye development in vertebrates

The vertebrate eye comprises tissues from different embryonic origins, e.g., iris and ciliary body are derived from the wall of the diencephalon via optic vesicle and optic cup. Lens and cornea, on the other hand, come from the overlying surface ectoderm. The timely action of transcription factors and inductive signals ensure the correct development of the different eye components. Establishing the genetic basis of eye defects has been an important tool for the detailed analysis of this complex process. One of the main control genes for eye development was discovered by the analysis of the allelic series of the Small eye mouse mutants and characterized as Pax6. It is involved in the interaction between the optic cup and the overlaying ectoderm. The central role for Pax6 in eye development is conserved throughout the animal kingdom as the murine Pax6 gene induces ectopic eyes in transgenic Drosophila despite the obvious diverse organization of the eye in the fruit fly compared to vertebrates. In human, mutations in the PAX6 gene are responsible for aniridia and Peter's anomaly. In addition to Pax6, other mutations affecting the interaction of the optic cup and the lens placode have been documented in the mouse. For the differentiation of the retina from the optic cup several genes are responsible: Mi leads to microphthalmia, if mutated, and encodes for a transcription factor, which is expressed in the melanocytes of the pigmented layer of the retina. In addition, further genes are implicated in the correct development of the retina, e.g., Chx10, Dlx1, GH6, Msx1 and -2, Otx1 and -2, or Wnt7b. Mutations within the retinoblastoma gene (RB1) are responsible for retinal tumors. Knock-out mutants of RB1 exhibit a block of lens differentiation prior to the retinal defect. Besides the influence of Rb1, the lens differentiates under the influence of growth factors (e.g., FGF, IGF, PDGF, TGF), and specific genes become activated encoding cytoskeletal proteins (e.g., filensin, phakinin, vimentin), structural proteins (e.g., crystallins) or membrane proteins (e.g., Mip). The optic nerve originates from the neural retina; ganglion cells grow to the optic stalk, forming the optic nerve. Its retrograde walk to the brain through the rudiment of the optic stalk depends on the correct Pax2 expression.

Clinical features of five pedigrees genetically linked to the juvenile glaucoma locus on chromosome 1q21-q31

BACKGROUND

Primary juvenile glaucoma is a rare form of glaucoma that typically affects individuals between 3 and 20 years of ages and is inherited as an autosomal dominant trait. One gene responsible for this condition has been localized to the 1q21-q31 region of chromosome 1. To investigate the clinical features of this form of glaucoma, the authors have examined the affected members of five pedigrees demonstrating genetic linkage to the 1q21-q31 locus.

METHODS

Clinical characterization of 23 affected patients was performed. Genetic linkage to the 1q21-q31 locus was confirmed by segregation of the disease trait in each pedigree with genetic markers located in the 1q21-q31 region.

RESULTS

The clinical features of affected members of the five pedigrees presented are generally homogeneous. The average age of diagnosis was 18.5 years (range, 5-30 years), and the average initial intraocular pressure was 38.5 mmHg (range, 30-53 mmHg). Eighty-seven percent of affected individuals were myopic and 83% of affected individuals required surgical treatment for glaucoma. There were no uniformly associated systemic or ocular conditions. One possible nonpenetrant carrier was identified and a difference in phenotypic expression of the presumed disease gene was observed in a pair of affected monozygotic twins. We also identified two pedigrees with juvenile glaucoma and three pedigrees affected by the pigment dispersion syndrome that are not genetically linked to the 1q21-q31 region.

CONCLUSION

The form of juvenile glaucoma caused by a gene located in the q21-q31 region of chromosome 1 is generally phenotypically homogeneous. The severe elevation of intraocular pressure typically seen in affected patients suggests the product of the predisposing gene may participate in the outflow function of the eye.