Disruption of eyelid and cornea development by targeted overexpression of the glucocorticoid receptor

Glucocorticoid hormones act through the glucocorticoid receptor (GR) and they affect almost all physiological systems in the organism. We have previously reported that transgenic mice overexpressing GR under the control of the keratin k5 promoter (K5-GR mice) display severe phenotypic alterations in the epidermis and other ectoderm derivatives (Perez et al., 2001). In this work, we aimed to characterize the pathological consequences of GR targeted overexpression in the eyelid and cornea at late developmental stages. Despite glucocorticoids being widely prescribed as a topical treatment in ophthalmology, their potential role during ocular development in the embryo is not well understood. As shown by scanning electron microscopy analysis as well as by our histopathological and immunohistochemical data, long-term and newborn transgenic embryos showed unfused eyelids, along with proptosis of the globe and exposure of the anterior surface. In addition, epithelial defects were evident at the cornea. Our results indicate that GR overexpression affected the proliferation rate of targeted epithelia of the cornea and eyelid, thus demonstrating that GR was responsible for the arrest of epithelial proliferation of the developing eyelid edges, as well as for their destruction. We conclude that constitutive targeted overexpression of GR in the eyelid and corneal epithelium dramatically impairs ocular function in these transgenic mice.

Connexin 43 (GJA1) mutations cause the pleiotropic phenotype of oculodentodigital dysplasia

Gap junctions are assemblies of intercellular channels that regulate a variety of physiologic and developmental processes through the exchange of small ions and signaling molecules. These channels consist of connexin family proteins that allow for diversity of channel composition and conductance properties. The human connexin 43 gene, or GJA1, is located at human chromosome 6q22-q23 within the candidate region for the oculodentodigital dysplasia locus. This autosomal dominant syndrome presents with craniofacial (ocular, nasal, and dental) and limb dysmorphisms, spastic paraplegia, and neurodegeneration. Syndactyly type III and conductive deafness can occur in some cases, and cardiac abnormalities are observed in rare instances. We found mutations in the GJA1 gene in all 17 families with oculodentodigital dysplasia that we screened. Sixteen different missense mutations and one codon duplication were detected. These mutations may cause misassembly of channels or alter channel conduction properties. Expression patterns and phenotypic features of gja1 animal mutants, reported elsewhere, are compatible with the pleiotropic clinical presentation of oculodentodigital dysplasia.

Novel mutation in sonic hedgehog in non-syndromic colobomatous microphthalmia

Ocular (uveoretinal) colobomas occur in one in 10,000 individuals and present a substantive cause of congenital poor vision. The genetic bases of most forms of uveoretinal coloboma are elusive; mutations in PAX2 are found in only a few cases of coloboma of the retina and optic nerve that occur with renal anomalies as part of the renal-coloboma syndrome (MIM#120330; #167409). From experimental data that upstream expression of sonic hedgehog (SHH) controls Pax2 expression in mice and zebrafish, and from clinical experience that colobomas are observed frequently in patients with holoprosencephaly, we hypothesized that SHH could be a candidate for non-syndromic ocular colobomas (NSOC). We identified a three-generation family in which both a proband and his mother presented with iris and uveoretinal colobomas without optic nerve involvement. A novel 24 bp deletion in the gene SHH was identified in these affected family members, and cosegregated with the phenotype. This is the first report of the association of SHH mutations and uveoretinal coloboma.

Ccd1, a novel protein with a DIX domain, is a positive regulator in the Wnt signaling during zebrafish neural patterning

Wnt signaling plays a crucial role in directing cell differentiation, polarity, and growth. In the canonical pathway, Wnt receptors activate Dishevelled (Dvl), which then blocks the degradation of a key signal transducer, beta-catenin, leading to the nuclear accumulation of beta-catenin and induction of Wnt target genes through TCF/LEF family transcription factors. Here we identified a novel zebrafish gene encoding Ccd1, which possesses a DIX (Dishevelled-Axin) domain. DIX domains are essential for the signal transduction of two major Wnt downstream mediators, Dvl and Axin. Ccd1 formed homomeric and heteromeric complexes with Dvl and Axin and activated TCF-dependent transcription in vitro. In addition, overexpression of ccd1 in zebrafish embryos led to a reduction in the size of the eyes and forebrain (posteriorization), as seen with wnt8 overexpression, whereas a dominant-negative ccd1 (DN-ccd1) caused the opposite phenotype. Furthermore, the Wnt activation phenotype induced by ccd1 was inhibited by the expression of axin1 or DN-ccd1, and the wnt8 overexpression phenotype was rescued by DN-ccd1, suggesting that Ccd1 functions downstream of the Wnt receptor and upstream of Axin. These results indicate that Ccd1 is a novel positive regulator in this Wnt signaling pathway during zebrafish neural patterning.

In frame fibrillin-1 gene deletion in autosomal dominant Weill-Marchesani syndrome

Weill-Marchesani syndrome (WMS) is a connective tissue disorder characterised by short stature, brachydactyly, joint stiffness, and characteristic eye anomalies including microspherophakia, ectopia of the lenses, severe myopia, and glaucoma. Both autosomal recessive (AR) and autosomal dominant (AD) modes of inheritance have been described and a gene for AR WMS has recently been mapped to chromosome 19p13.3-p13.2. Here, we report on the exclusion of chromosome 19p13.3-p13.2 in a large AD WMS family and show that, despite clinical homogeneity, AD and AR WMS are genetically heterogeneous entities. Because two AD WMS families were consistent with linkage to chromosome 15q21.1, the fibrillin-1 gene was sequenced and a 24 nt in frame deletion within a latent transforming growth factor-beta1 binding protein (LTBP) motif of the fibrillin-1 gene was found in a AD WMS family (exon 41, 5074_5097del). This in frame deletion cosegregated with the disease and was not found in 186 controls. This study strongly suggests that AD WMS and Marfan syndrome are allelic conditions at the fibrillin-1 locus and adds to the remarkable clinical heterogeneity of type I fibrillinopathies.

Evidence for an epigenetic mechanism by which Hsp90 acts as a capacitor for morphological evolution

Morphological alterations have been shown to occur in Drosophila melanogaster when function of Hsp90 (heat shock 90-kDa protein 1alpha, encoded by Hsp83) is compromised during development. Genetic selection maintains the altered phenotypes in subsequent generations. Recent experiments have shown, however, that phenotypic variation still occurs in nearly isogenic recombinant inbred strains of Arabidopsis thaliana. Using a sensitized isogenic D. melanogaster strain, iso-Kr(If-1), we confirm this finding and present evidence supporting an epigenetic mechanism for Hsp90's capacitor function, whereby reduced activity of Hsp90 induces a heritably altered chromatin state. The altered chromatin state is evidenced by ectopic expression of the morphogen wingless in eye imaginal discs and a corresponding abnormal eye phenotype, both of which are epigenetically heritable in subsequent generations, even when function of Hsp90 is restored. Mutations in nine different genes of the trithorax group that encode chromatin-remodeling proteins also induce the abnormal phenotype. These findings suggest that Hsp90 acts as a capacitor for morphological evolution through epigenetic and genetic mechanisms.

[Formation of congenital defects during ocular development. II. Genetic background of oculogenesis and developmental birth defects]

The course of oculogenesis is genetically controlled by genes, that are selectively expressed in the developing eye. Many of them, first recognized in studies on model animals, such as fruit fly (Drosophila melanogaster) proved, that they play similar roles in humans. The crucial role in cascade of genetic control of oculogenesis plays a system of genes PAX6-EYA1/SIX3-DACH1. In this work there are described also other genes, that form a network of interactions and regulations, such as: BMP4, BMP7, OPTX2, SOX1, PAX2, PITX2, PAX3, CHX10 and other genes. Describing function of each gene, author presented simultaneously all known congenital defects and syndromes, that are caused by mutations of above mentioned genes, such as: aniridia, anophthalmia, Peters anomaly and others.

Rieger syndrome: case report

Rieger syndrome is a dysembryogenetic disease in which labyrinthic damage can be associated with other genetic anomalies. The case presented here is of a patient who has bilateral dysgenesis of the iris, with bulbar atrophy and dyscoria. The patient does not present any malformation of the craniofacial structures, of the periumbilical skin, or of the skeletal bones. The case is, therefore, a variant of the Rieger syndrome labeled Axenfeld-Rieger syndrome. The patient reported a progressive sensation of auricular fullness, and liminal audiometry revealed a sensorineural hearing loss. Computed tomography scanning of the temporal bone revealed a bilateral dysmorphism of the acoustic channels. The presence of a bilateral cochlcopathy in a patient suffering from the Axenfeld-Rieger syndrome could be the expression of a genetic "disorder." We cannot exclude the possibility also that this genetic anomaly is responsible for the bony dysmorphism of the inner ear channels shown by the computed tomography scan of the temporal bone.

A Turkish case of subcortical/subependymal heterotopia associated with corpus callosum dysgenesis, craniofacial dysmorphism, severe eye abnormalities, and growth-mental retardation

The patient is a 12-year-old boy with a history of learning disability, growth retardation, and strabismus. Weight, height and head circumference were below the 3rd percentile. A café-au-lait spot, 1x1 cm a diameter, on the back region and pectus excavatum deformity were diagnosed. He had facial asymmetry, a broad nose, sparse eyebrows and eyelashes, a rudimentary frontal sinus, deviation of the nasal septum, and bilateral small maxillary bones. The left orbital fossa was also mildly rudimentary. On eye examination the movements of the left globe to the upward and lateral side were limited and internal strabismus was noted at this side. Visual acuity was 1/10, bilaterally. Bilateral choroid coloboma, glaucoma, vertical and horizontal nystagmus were diagnosed. Fundoscopic examination revealed bilateral optic atrophy and macular and paramacular granulation tissues on the left side. Intelligence quotient was 46. Electroencephalography revealed bilateral frontal slow-wave activity. Visual evoked potential revealed prolonged p100 wave latencies bilaterally. Magnetic resonance imaging of the brain demonstrated corpus callosum dysgenesis, bilateral subcortical heterotopia in the frontal lobes and subependymal heterotopia in the posterior horn of the left ventricle. Chromosomal analysis revealed a normal male karyotype, 46, XY. Although several cases of heterotopia in association with mental retardation, craniofacial dysmorphism, cerebral, and eye abnormalities have been described the combination of abnormalities diagnosed in our case has not previously been reported. We hypothesize that the combination of subcortical/subependymal heterotopia, corpus callosum dysgenesis, craniofacial dysmorphism, severe eye abnormalities, and growth-mental retardation may be a new syndrome.

Novel mutation in FOXC1 wing region causing Axenfeld-Rieger anomaly

PURPOSE

To determine the possible molecular genetic defect underlying Axenfeld-Rieger anomaly (ARA) and to identify the pathogenic mutation causing this anterior segment dysgenesis in an Indian pedigree.

METHODS

The FOXC1 gene was amplified from genomic DNA of members of an ARA-affected family and control subjects using four novel sets of primers. The amplicons were directly sequenced, and the sequences were analyzed to identify the disease-causing mutation.

RESULTS

A heterozygous novel missense mutation was identified in the coding region of the FOXC1 gene in all three patients in this family. Consistent with the autosomal dominant inheritance pattern, the mutation segregated with the disease phenotype and was fully penetrant. The mutation was found in the wing region of the highly conserved forkhead domain of the FOXC1 gene and resulted in a very severe phenotype leading to blindness.

CONCLUSIONS

This is the first study to demonstrate that a mutation in the FOXC1 wing region can cause an anterior segment dysgenesis of the eye. This mutation resulted in blindness in the ARA-affected family, and the findings suggest that the FOXC1 wing region has a functional role in the normal development of the eye. Moreover, this is the first study from India to report the genetic etiology of Axenfeld-Rieger anomaly. Genotype-phenotype correlations of FOXC1 may help in establishing the disease prognosis and also in understanding the clinical and genetic heterogeneity associated with various anterior segment dysgenesis caused by this gene.

Molecular analysis of collagen XVIII reveals novel mutations, presence of a third isoform, and possible genetic heterogeneity in Knobloch syndrome

Knobloch syndrome (KS) is a rare disease characterized by severe ocular alterations, including vitreoretinal degeneration associated with retinal detachment and occipital scalp defect. The responsible gene, COL18A1, has been mapped to 21q22.3, and, on the basis of the analysis of one family, we have demonstrated that a mutation affecting only one of the three COL18A1 isoforms causes this phenotype. We report here the results of the screening of both the entire coding region and the exon-intron boundaries of the COL18A1 gene (which includes 43 exons), in eight unrelated patients with KS. Besides 20 polymorphic changes, we identified 6 different pathogenic changes in both alleles of five unrelated patients with KS (three compound heterozygotes and two homozygotes). All are truncating mutations leading to deficiency of one or all collagen XVIII isoforms and endostatin. We have verified that, in exon 41, the deletion c3514-3515delCT, found in three unrelated alleles, is embedded in different haplotypes, suggesting that this mutation has occurred more than once. In addition, our results provide evidence of nonallelic genetic heterogeneity in KS. We also show that the longest human isoform (NC11-728) is expressed in several tissues (including the human eye) and that lack of either the short variant or all of the collagen XVIII isoforms causes similar phenotypes but that those patients who lack all forms present more-severe ocular alterations. Despite the small sample size, we found low endostatin plasma levels in those patients with mutations leading to deficiency of all isoforms; in addition, it seems that absence of all collagen XVIII isoforms causes predisposition to epilepsy.

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.

The gene for the intermediate chain subunit of cytoplasmic dynein is essential in Drosophila

The microtubule motor cytoplasmic dynein powers a variety of intracellular transport events that are essential for cellular and developmental processes. A current hypothesis is that the accessory subunits of the dynein complex are important for the specialization of cytoplasmic dynein function. In a genetic approach to understanding the range of dynein functions and the contribution of the different subunits to dynein motor function and regulation, we have identified mutations in the gene for the cytoplasmic dynein intermediate chain, Dic19C. We used a functional Dic transgene in a genetic screen to recover X-linked lethal mutations that require this transgene for viability. Three Dic mutations were identified and characterized. All three Dic alleles result in larval lethality, demonstrating that the intermediate chain serves an essential function in Drosophila. Like a deficiency that removes Dic19C, the Dic mutations dominantly enhance the rough eye phenotype of Glued(1), a dominant mutation in the gene for the p150 subunit of the dynactin complex, a dynein activator. Additionally, we used complementation analysis to identify an existing mutation, shortwing (sw), as an allele of the dynein intermediate chain gene. Unlike the Dic alleles isolated de novo, shortwing is homozygous viable and exhibits recessive and temperature-sensitive defects in eye and wing development. These phenotypes are rescued by the wild-type Dic transgene, indicating that shortwing is a viable allele of the dynein intermediate chain gene and revealing a novel role for dynein function during wing development.

Okihiro syndrome is caused by SALL4 mutations

Okihiro syndrome refers to the association of forearm malformations with Duane syndrome of eye retraction. Based on the reported literature experience, clinical diagnosis of the syndrome can be elusive, owing to the variable presentation in families reported. Specifically, there is overlap of clinical features with other conditions, most notably Holt-Oram syndrome, a condition resulting from mutation of the TBX5 locus and Townes-Brocks syndrome, known to be caused by mutations in the SALL1 gene. Arising from our observation of several malformations in Okihiro syndrome patients which are also described in Townes-Brocks syndrome, we postulated that Okihiro syndrome might result from mutation of another member of the human SALL gene family. We have characterized the human SALL4 gene on chromosome 20q13.13-q13.2. Moreover, we have identified literature reports of forelimb malformations in patients with cytogenetically identifiable abnormalities of this region. We here present evidence in 5 of 8 affected families that mutation at this locus results in the Okihiro syndrome phenotype.

Mutations in the O-mannosyltransferase gene POMT1 give rise to the severe neuronal migration disorder Walker-Warburg syndrome

Walker-Warburg syndrome (WWS) is an autosomal recessive developmental disorder characterized by congenital muscular dystrophy and complex brain and eye abnormalities. A similar combination of symptoms is presented by two other human diseases, muscle-eye-brain disease (MEB) and Fukuyama congenital muscular dystrophy (FCMD). Although the genes underlying FCMD (Fukutin) and MEB (POMGnT1) have been cloned, loci for WWS have remained elusive. The protein products of POMGnT1 and Fukutin have both been implicated in protein glycosylation. To unravel the genetic basis of WWS, we first performed a genomewide linkage analysis in 10 consanguineous families with WWS. The results indicated the existence of at least three WWS loci. Subsequently, we adopted a candidate-gene approach in combination with homozygosity mapping in 15 consanguineous families with WWS. Candidate genes were selected on the basis of the role of the FCMD and MEB genes. Since POMGnT1 encodes an O-mannoside N-acetylglucosaminyltransferase, we analyzed the possible implication of O-mannosyl glycan synthesis in WWS. Analysis of the locus for O-mannosyltransferase 1 (POMT1) revealed homozygosity in 5 of 15 families. Sequencing of the POMT1 gene revealed mutations in 6 of the 30 unrelated patients with WWS. Of the five mutations identified, two are nonsense mutations, two are frameshift mutations, and one is a missense mutation. Immunohistochemical analysis of muscle from patients with POMT1 mutations corroborated the O-mannosylation defect, as judged by the absence of glycosylation of alpha-dystroglycan. The implication of O-mannosylation in MEB and WWS suggests new lines of study in understanding the molecular basis of neuronal migration.

[Ocular signs in Rubinstein-Taybi's syndrome]

Rubinstein-Taybi syndrome, known also as "Broad Thumb-Hallux syndrome", was first recognized by Rubinstein and Taybi in 1963 and is characterized by somatic and ophthalmologic signs. Most commonly somatic changes are: broad thumbs and hallux, craniofacial dysmorphism, growth and psychomotor retardation. Most frequently reported eye anomalies are: antimongoloid slant of the palpebral fissures, strabismus, congenital obstruction of the lacrimal excretory system, colobomas of the iris and of the optic nerve head, ametropia.

Members of the synaptobrevin/vesicle-associated membrane protein (VAMP) family in Drosophila are functionally interchangeable in vivo for neurotransmitter release and cell viability

Synaptobrevins or VAMPs are vesicle-associated membrane proteins, often called v-SNARES, that are important for vesicle transport and fusion at the plasma membrane. Drosophila has two characterized members of this gene family: synaptobrevin (syb) and neuronal synaptobrevin (n-syb). Mutant phenotypes and gene-expression patterns indicate that n-Syb is exclusively neuronal and required only for synaptic vesicle secretion, whereas Syb is ubiquitous and, as shown here, essential for cell viability. When the eye precursor cells were made homozygous for syb(-), the eye failed to develop. In contrast, n-syb(-) eye clones developed appropriately but failed to activate downstream neurons. To determine whether the two proteins are structurally specialized to accomplish these distinct in vivo functions, we have driven the expression of each gene in the absence of the other to look for phenotypic rescue. We find that expression of n-syb during eye development can rescue the cell lethality of the syb mutations, as can rat VAMP2 and cellubrevin. Expression of syb can restore synaptic transmission to n-syb mutants as assayed both by electroretinogram and recordings of excitatory junctional currents at the neuromuscular junction. Therefore, we find that Syb, which usually is not involved in synaptic function, can mediate Ca(2+)-triggered synaptic activity and that no particular specialization of the v-SNARE is required to differentiate synaptic exocytosis from other forms.

Bilateral ocular malformations in a newborn with normal karyotype: histologic findings

Microphthalmos with cyst is a rare condition characterized by a small globe and an inferior uveoretinal coloboma. There is also a defect in the posterior aspect of the eye through which a cyst lined by neuroectodermically derived tissue protrudes into the orbit. A case of isolated bilateral colobomatous and cystic microphthalmos is reported in an otherwise healthy child, showing no evidence of chromosomal abnormalities. Microscopic findings in the enucleated eye consisted of iris and retinal dysgenesis, ectopia lentis, persistent anterior tunica vasculosa lentis and pupillary membrane, intrachoroidal smooth muscle, and optic nerve hypoplasia. In the orbital cyst, a thick membrane reminiscent of the retinal inner limiting membrane lay between the fibroadipose and vascularised outer wall and the inner neuroectodermal lining.

Suppression of polyglutamine toxicity by a Drosophila homolog of myeloid leukemia factor 1

The toxicity of an abnormally long polyglutamine [poly(Q)] tract within specific proteins is the molecular lesion shared by Huntington's disease (HD) and several other hereditary neurodegenerative disorders. By a genetic screen in Drosophila, devised to uncover genes that suppress poly(Q) toxicity, we discovered a Drosophila homolog of human myeloid leukemia factor 1 (MLF1). Expression of the Drosophila homolog (dMLF) ameliorates the toxicity of poly(Q) expressed in the eye and central nervous system. In the retina, whether endogenously or ectopically expressed, dMLF co-localized with aggregates, suggesting that dMLF alone, or through an intermediary molecular partner, may suppress toxicity by sequestering poly(Q) and/or its aggregates.

Eye suppression, a novel function of teashirt, requires Wingless signaling

teashirt (tsh) encodes a Drosophila zinc-finger protein. Misexpression of tsh has been shown to induce ectopic eye formation in the antenna. We report that tsh can suppress eye development. This novel function of tsh is due to the induction of homothorax (hth), a known repressor of eye development, and requires Wingless (WG) signaling. Interestingly, tsh has different functions in the dorsal and ventral eye, suppressing eye development close to the ventral margin, while promoting eye development near the dorsal margin. It affects both growth of eye disc and retinal cell differentiation.

Four novel mutations in the PITX2 gene in patients with Axenfeld-Rieger syndrome

Mutational screening and sequence analysis of the PITX2 gene was performed in four families previously diagnosed with Rieger syndrome. The results of this analysis identified four novel mutations within the coding sequence of PITX2. These mutations were not identified in the sequence of 50 control individuals. Two mutations were found in the homeobox and would be expected to result in nonconservative amino acid changes within the second and third helixes. The remaining two mutations were found in the region downstream of the homeobox and are also predicted to result in missense mutations. In conclusion, mutations within the homeobox sequence and the adjacent coding sequence of PITX2 lead to various Rieger syndrome phenotypes characterized by a high incidence of glaucoma.

salvador Promotes both cell cycle exit and apoptosis in Drosophila and is mutated in human cancer cell lines

The number of cells in an organism is determined by regulating both cell proliferation and cell death. Relatively few mechanisms have been identified that can modulate both of these processes. In a screen for Drosophila mutations that result in tissue overgrowth, we identified salvador (sav), a gene that promotes both cell cycle exit and cell death. Elevated Cyclin E and DIAP1 levels are found in mutant cells, resulting in delayed cell cycle exit and impaired apoptosis. Salvador contains two WW domains and binds to the Warts (or LATS) protein kinase. The human ortholog of salvador (hWW45) is mutated in three cancer cell lines. Thus, salvador restricts cell numbers in vivo by functioning as a dual regulator of cell proliferation and apoptosis.