Ocular coloboma: a reassessment in the age of molecular neuroscience

Increased Netrin downstream of overactive Hedgehog signaling disrupts optic fissure formation

Background

Uveal coloboma, a developmental eye defect, is caused by failed development of the optic fissure, a ventral structure in the optic stalk and cup where axons exit the eye and vasculature enters. The Hedgehog (Hh) signaling pathway regulates optic fissure development: loss-of-function mutations in the Hh receptor ptch2 produce overactive Hh signaling and can result in coloboma. We previously proposed a model where overactive Hh signaling disrupts optic fissure formation by upregulating transcriptional targets acting both cell- and non-cell-autonomously. Here, we examine the Netrin family of secreted ligands as candidate Hh target genes.

Results

We find multiple Netrin ligands upregulated in the zebrafish ptch2 mutant during optic fissure development. Using a gain-of-function approach to overexpress Netrin in a spatiotemporally specific manner, we find that netrin1a or netrin1b overexpression is sufficient to cause coloboma and disrupt wild-type optic fissure formation. We used loss-of-function alleles, CRISPR/Cas9 mutagenesis, and morpholino knockdown to test if loss of Netrin can rescue coloboma in the ptch2 mutant: loss of netrin genes does not rescue the ptch2 mutant phenotype.

Conclusion

These results suggest that Netrin is sufficient but not required to disrupt optic fissure formation downstream of overactive Hh signaling in the ptch2 mutant.

Macular corneal dystrophy with iridofundal coloboma in the same patient: a unique combination

A young a presented with painless, progressive diminution of vision in both eyes (BE). Slit lamp examination revealed the presence of a single central corneal opacity in the right eye and multiple corneal opacities of varying sizes in the left eye (LE), limited to the anterior-mid corneal stroma. Microcornea with reduced central corneal thickness and complete inferonasal iris coloboma along with inferior fundal coloboma, sparing both the disc and macula, were noted in BE. A diagnosis of BE macular corneal dystrophy (MCD) and iridofundal coloboma (IFC) was made. The patient underwent LE sutureless anterior lamellar therapeutic keratoplasty. On histopathological examination, the excised corneal tissue revealed stromal lamellar disarray with positive colloidal iron staining, strongly suggestive of MCD. Whole-exome sequencing revealed the presence of a likely pathogenic carbohydrate sulfotransferase 6 (CHST6) mutation, confirming the diagnosis of MCD. This concurrent presence of IFC with a corneal stromal dystrophy is previously unreported in the literature, to the best of our knowledge.

Ocular manifestations of congenital anomalies of the kidney and urinary tract (CAKUT)

Congenital anomalies of the kidney and urinary tract (CAKUT) are among the most common birth defects worldwide and a major cause of kidney failure in children. Extra-renal manifestations are also common. This study reviewed diseases associated with the Genomics England CAKUT-associated gene panel for ocular anomalies. In addition, each gene was examined for expression in the human retina and an ocular phenotype in mouse models using the Human Protein Atlas and Mouse Genome Informatics databases, respectively. Thirty-four (54%) of the 63 CAKUT-associated genes (55 'green' and 8 'amber') had a reported ocular phenotype. Five of the 6 most common CAKUT-associated genes (PAX2, EYA1, SALL1, GATA3, PBX1) that represent 30% of all diagnoses had ocular features. The ocular abnormalities found with most CAKUT-associated genes and with five of the six commonest were coloboma, microphthalmia, optic disc anomalies, refraction errors (astigmatism, myopia, and hypermetropia), and cataract. Seven of the CAKUT-associated genes studied (11%) had no reported ocular features but were expressed in the human retina or had an ocular phenotype in a mouse model, which suggested further possibly-unrecognised abnormalities. About one third of CAKUT-associated genes (18, 29%) had no ocular associations and were not expressed in the retina, and the corresponding mouse models had no ocular phenotype. Ocular abnormalities in individuals with CAKUT suggest a genetic basis for the disease and sometimes indicate the affected gene. Individuals with CAKUT often have ocular abnormalities and may require an ophthalmic review, monitoring, and treatment to preserve vision.

Congenital Malformations of the Eye: A Pictorial Review and Clinico-Radiological Correlations

Congenital malformations of the eye represent a wide and heterogeneous spectrum of abnormalities that may be part of a complex syndrome or be isolated. Ocular malformation severity depends on the timing of the causative event during eye formation, ranging from the complete absence of the eye if injury occurs during the first weeks of gestation, to subtle abnormalities if the cause occurs later on. Knowledge of ocular malformations is crucial to performing a tailored imaging protocol and correctly reporting imaging findings. Together with the ophthalmologic evaluation, imaging may help frame ocular malformations and identify underlying genetic conditions. The purpose of this pictorial review is to describe the imaging features of the main ocular malformations and the related ophthalmologic findings in order to provide a clinico-radiological overview of these abnormalities to the clinical radiologist. Sight is a crucial sense for children to explore the world and relate with their parents from birth. Vision impairment or even blindness secondary to ocular malformations deeply affects children's growth and quality of life.

Cell adhesion marker expression dynamics during fusion of the optic fissure

Tissue fusion is a critical process that is repeated in multiple contexts during embryonic development and shares common attributes to processes such as wound healing and metastasis. Ocular coloboma is a developmental eye disorder that presents as a physical gap in the ventral eye, and is a major cause of childhood blindness. Coloboma results from fusion failure between opposing ventral retinal epithelia, but there are major knowledge gaps in our understanding of this process at the molecular and cell behavioural levels. Here we catalogue the expression of cell adhesion proteins: N-cadherin, E-cadherin, R-cadherin, ZO-1, and the EMT transcriptional activator and cadherin regulator SNAI2, in the developing chicken embryonic eye. We find that fusion pioneer cells at the edges of the fusing optic fissure have unique and dynamic expression profiles for N-cad, E-cad and ZO-1, and that these are temporally preceded by expression of SNAI2. This highlights the unique properties of these cells and indicates that regulation of cell adhesion factors may be a critical process in optic fissure closure.

Congenital anomalies of lens shape

The crystalline lens is an important structure in the eye that starts to develop as early as the 22nd day of gestation, with further differentiation that continues after the induction. Congenital anomalies of the lens may involve the size, shape, and position of the lens. They may sometimes be associated with anterior segment dysgenesis or persistence of the tunica vasculosa lentis and hyperplastic vitreous and hyaloid system. Manifestations of anomalies of the lens shape are usually seen in early or late childhood however may sometimes be delayed into adulthood based on the level of visual impairment or the presence or absence of any syndromic associations. While lens coloboma has more often been reported in isolation, the more commonly implicated genes include the PAX6 gene, lenticonus in particular anterior is often part of Alport syndrome with extra-ocular manifestations in the kidneys and hearing abnormalities due to mutations in the alpha 5 chain of the Type IV collagen gene. Recognition of these manifestations and obtaining a genetic diagnosis is an important step in the management. The level of visual impairment and amblyopia dictates the outcomes in patients managed either conservatively with optical correction as well as surgically where deemed necessary. This review discusses the various anomalies of the lens shape with its related genetics and the management involved in these conditions.

Eye Morphogenesis in Vertebrates

Proper eye structure is essential for visual function: Multiple essential eye tissues must take shape and assemble into a precise three-dimensional configuration. Accordingly, alterations to eye structure can lead to pathological conditions of visual impairment. Changes in eye shape can also be adaptive over evolutionary time. Eye structure is first established during development with the formation of the optic cup, which contains the neural retina, retinal pigment epithelium, and lens. This crucial yet deceptively simple hemispherical structure lays the foundation for all later elaborations of the eye. Building on descriptions of the embryonic eye that started with hand drawings and micrographs, the field is beginning to identify mechanisms driving dynamic changes in three-dimensional cell and tissue shape. A combination of molecular genetics, imaging, and pharmacological approaches is defining connections among transcription factors, signaling pathways, and the intracellular machinery governing the emergence of this crucial structure.

Identification of Novel Coloboma Candidate Genes through Conserved Gene Expression Analyses across Four Vertebrate Species

Ocular coloboma (OC) is a failure of complete optic fissure closure during embryonic development and presents as a tissue defect along the proximal-distal axis of the ventral eye. It is classed as part of the clinical spectrum of structural eye malformations with microphthalmia and anophthalmia, collectively abbreviated to MAC. Despite deliberate attempts to identify causative variants in MAC, many patients remain without a genetic diagnosis. To reveal potential candidate genes, we utilised transcriptomes experimentally generated from embryonic eye tissues derived from humans, mice, zebrafish, and chicken at stages coincident with optic fissure closure. Our in-silico analyses found 10 genes with optic fissure-specific enriched expression: ALDH1A3, BMPR1B, EMX2, EPHB3, NID1, NTN1, PAX2, SMOC1, TENM3, and VAX1. In situ hybridization revealed that all 10 genes were broadly expressed ventrally in the developing eye but that only PAX2 and NTN1 were expressed in cells at the edges of the optic fissure margin. Of these conserved optic fissure genes, EMX2, NID1, and EPHB3 have not previously been associated with human MAC cases. Targeted genetic manipulation in zebrafish embryos using CRISPR/Cas9 caused the developmental MAC phenotype for emx2 and ephb3. We analysed available whole genome sequencing datasets from MAC patients and identified a range of variants with plausible causality. In combination, our data suggest that expression of genes involved in ventral eye development is conserved across a range of vertebrate species and that EMX2, NID1, and EPHB3 are candidate loci that warrant further functional analysis in the context of MAC and should be considered for sequencing in cohorts of patients with structural eye malformations.

A Brief Focus on Joubert Syndrome and Related Acute Complications

Joubert syndrome (JS) and related disorders are a group of congenital anomalies syndromes in which the obligatory hallmark is the molar tooth sign, a complex midbrain–hindbrain malformation. Moreover, JS may be associated with multiorgan involvement, mainly nephronophthisis, hepatic fibrosis, retinal dystrophy, and other abnormalities with both inter- and intra-familial variability. Therefore, these patients should be followed by both diagnostic protocol and multidisciplinary approach to assess multiorgan involvement. Here, we briefly summarize the possible complications in patients with JS.

Joubert Syndrome: Diagnostic Evaluation and Follow-up

The follow-up of a child with genetic syndrome is necessarily multidisciplinary because of the multiplicity of problems and calls for close collaboration between different specialists. The primary objective is the total care of the child and his family, regardless of the rarity and complexity of the disease, to obtain the highest possible degree of mental and physical health and autonomy.

Cell fate decisions, transcription factors and signaling during early retinal development

The development of the vertebrate eyes is a complex process starting from anterior-posterior and dorso-ventral patterning of the anterior neural tube, resulting in the formation of the eye field. Symmetrical separation of the eye field at the anterior neural plate is followed by two symmetrical evaginations to generate a pair of optic vesicles. Next, reciprocal invagination of the optic vesicles with surface ectoderm-derived lens placodes generates double-layered optic cups. The inner and outer layers of the optic cups develop into the neural retina and retinal pigment epithelium (RPE), respectively. In vitro produced retinal tissues, called retinal organoids, are formed from human pluripotent stem cells, mimicking major steps of retinal differentiation in vivo. This review article summarizes recent progress in our understanding of early eye development, focusing on the formation the eye field, optic vesicles, and early optic cups. Recent single-cell transcriptomic studies are integrated with classical in vivo genetic and functional studies to uncover a range of cellular mechanisms underlying early eye development. The functions of signal transduction pathways and lineage-specific DNA-binding transcription factors are dissected to explain cell-specific regulatory mechanisms underlying cell fate determination during early eye development. The functions of homeodomain (HD) transcription factors Otx2, Pax6, Lhx2, Six3 and Six6, which are required for early eye development, are discussed in detail. Comprehensive understanding of the mechanisms of early eye development provides insight into the molecular and cellular basis of developmental ocular anomalies, such as optic cup coloboma. Lastly, modeling human development and inherited retinal diseases using stem cell-derived retinal organoids generates opportunities to discover novel therapies for retinal diseases.

Variants in HNRNPH1 are associated with high myopia in humans and ocular coloboma in zebrafish

High myopia is one of the most common causes for blindness due to its associated complications. Genetic factor has been considered as the major cause for early-onset high myopia (eoHM), but exact genetic defects for most eoHM are yet to be identified. Through multi-step bioinformatics analysis of our in-house whole exome sequencing dataset from 5310 individuals, variants from 653 probands with eoHM were further compared with those from in-house controls as well as gnomAD database. The results showed that loss-of-function (LoF) variants in a novel gene HNRNPH1 were identified in two of 653 probands with eoHM but in none of 4657 probands with other eye conditions (P = 0.015). LoF variants in HNRNPH1 were extremely rare and intolerant, while two LoF variants in 653 eoHM were statistically higher than their frequency in gnomAD (P = 1.09×10^-3 ). These two LoF variants, c.2dupT/p.? and c.121dup/p.(Q41Pfs*20), were absent from existing database. Variants in HNRNPH1 have not been associated with any inherited eye disease before. Expression of HNRNPH1 was enriched in ganglion cell layer and inner nuclear layer in humans. Knockdown of hnrnph1 in zebrafish resulted in ocular coloboma. All these suggests that HNRNPH1 supports its potential contribution to eoHM when mutated.

Won’t You be My Neighbor: How Epithelial Cells Connect Together to Build Global Tissue Polarity

Epithelial tissues form continuous barriers to protect against external environments. Within these tissues, epithelial cells build environment-facing apical membranes, junction complexes that anchor neighbors together, and basolateral surfaces that face other cells. Critically, to form a continuous apical barrier, neighboring epithelial cells must align their apico-basolateral axes to create global polarity along the entire tissue. Here, we will review mechanisms of global tissue-level polarity establishment, with a focus on how neighboring epithelial cells of different origins align their apical surfaces. Epithelial cells with different developmental origins and/or that polarize at different times and places must align their respective apico-basolateral axes. Connecting different epithelial tissues into continuous sheets or tubes, termed epithelial fusion, has been most extensively studied in cases where neighboring cells initially dock at an apical-to-apical interface. However, epithelial cells can also meet basal-to-basal, posing several challenges for apical continuity. Pre-existing basement membrane between the tissues must be remodeled and/or removed, the cells involved in docking are specialized, and new cell-cell adhesions are formed. Each of these challenges can involve changes to apico-basolateral polarity of epithelial cells. This minireview highlights several in vivo examples of basal docking and how apico-basolateral polarity changes during epithelial fusion. Understanding the specific molecular mechanisms of basal docking is an area ripe for further exploration that will shed light on complex morphogenetic events that sculpt developing organisms and on the cellular mechanisms that can go awry during diseases involving the formation of cysts, fistulas, atresias, and metastases.

Clinical and Demographic Profile of Uveal Coloboma: A hospital-based Study of 14,371 eyes of 9557 Indian Patients

OBJECTIVE

To describe the demographics and clinical profile of uveal coloboma in patients presenting to a multi-tier ophthalmology hospital network in India.

DESIGN

Cross- sectional hospital -based study.

METHODS

This cross-sectional hospital-based study included 2,817,766 new patients presenting between August 2010 and May 2021. Patients with a clinical diagnosis of uveal coloboma in at least one eye were included as cases. The data were collected using an electronic medical record system.

RESULTS

Overall, 9,557 (0.34%) patients were diagnosed with uveal coloboma. Just over half of the patients were male (51.82%) and had bilateral (50.37%) affliction. The most common age group at presentation was during the second decade of life with 2,198 (23%) patients. The overall prevalence was higher in patients from a lower socio-economic status (0.53%) and presenting from the rural geography (0.39%). The most common type was retino-choroidal coloboma in 8,049 (84.22%) patients followed by iris coloboma in 2,129 (22.28%) patients. The most common Ida Mann classification in the eyes with retino-choroidal coloboma was type 4 in 7,049 (57.23%) eyes followed by type 2 in 3,685 (29.92%) eyes. In the 14,371 eyes, 5,696 (39.64%) eyes had a visual impairment of blindness (>20/400) followed by mild or no visual impairment (<20/70) in 2,875 (20.01%) eyes. In the 2,228 (15.5%) eyes that required a surgical/ laser intervention, cataract surgery was performed in 951 (6.62%) eyes, vitreo-retinal surgery in 661 (4.6%) eyes followed by laser photocoagulation in 357 (2.48%) eyes.

CONCLUSION

Uveal Coloboma is more common in males, is predominantly bilateral in the population studied. It is more commonly found in patients from lower socio-economic strata and from a rural background. The most common type is retino-choroidal coloboma and over a third of the eyes are affected with blindness.

Loss of zebrafish dzip1 results in inappropriate recruitment of periocular mesenchyme to the optic fissure and ocular coloboma

Cilia are essential for the development and function of many different tissues. Although cilia machinery is crucial in the eye for photoreceptor development and function, a role for cilia in early eye development and morphogenesis is still somewhat unclear: many zebrafish cilia mutants retain cilia at early stages due to maternal deposition of cilia components. An eye phenotype has been described in the mouse Arl13 mutant, however, zebrafish arl13b is maternally deposited, and an early role for cilia proteins has not been tested in zebrafish eye development. Here we use the zebrafish dzip1 mutant, which exhibits a loss of cilia throughout stages of early eye development, to examine eye development and morphogenesis. We find that in dzip1 mutants, initial formation of the optic cup proceeds normally, however, the optic fissure subsequently fails to close and embryos develop the structural eye malformation ocular coloboma. Further, neural crest cells, which are implicated in optic fissure closure, do not populate the optic fissure correctly, suggesting that their inappropriate localization may be the underlying cause of coloboma. Overall, our results indicate a role for dzip1 in proper neural crest localization in the optic fissure and optic fissure closure.

Cataract-Causing S93R Mutant Destabilized Structural Conformation of βB1 Crystallin Linking With Aggregates Formation and Cellular Viability

Cataract, opacity of the eye lens, is the leading cause of visual impairment worldwide. The crucial pathogenic factors that cause cataract are misfolding and aggregation of crystallin protein. βB1‐crystallin, which is the most abundant water‐soluble protein in mammalian lens, is essential for lens transparency. A previous study identified the missense mutation βB1‐S93R being responsible for congenital cataract. However, the exact pathogenic mechanism causing cataract remains unclear. The S93 residue, which is located at the first Greek‐key motif of βB1‐crystallin, is highly conserved, and its substitution to Arginine severely impaired hydrogen bonds and structural conformation, which were evaluated via Molecular Dynamic Simulation. The βB1‐S93R was also found to be prone to aggregation in both human cell lines and Escherichia coli. Then, we isolated the βB1‐S93R variant from inclusion bodies by protein renaturation. The βB1-S93R mutation exposed more hydrophobic residues, and the looser structural mutation was prone to aggregation. Furthermore, the S93R mutation reduced the structural stability of βB1-crystallin when incubated at physiological temperature and made it more sensitive to environmental stress, such as UV irradiation or oxidative stress. We also constructed a βB1-S93R cellular model and discovered that βB1-S93R was more sensitive to environmental stress, causing not only aggregate formation but also cellular apoptosis and impaired cellular viability. All of the results indicated that lower solubility and structural stability, sensitivity to environmental stress, vulnerability to aggregation, and impaired cellular viability of βB1-S93R might be involved in cataract development.

Pax2a, but not pax2b, influences cell survival and periocular mesenchyme localization to facilitate zebrafish optic fissure closure

Background

Pax2 is required for optic fissure development in many organisms, including humans and zebrafish. Zebrafish loss‐of‐function mutations in pax2a display coloboma, yet the etiology of the morphogenetic defects is unclear. Further, pax2 is duplicated in zebrafish, and a role for pax2b in optic fissure development has not been examined.

Results

Using a combination of imaging and molecular genetics, we interrogated a potential role for pax2b and examined how loss of pax2 affects optic fissure development. Although optic fissure formation appears normal in pax2 mutants, an endothelial‐specific subset of periocular mesenchyme (POM) fails to initially localize within the optic fissure, yet both neural crest and endothelial‐derived POM ectopically accumulate at later stages in pax2a and pax2a; pax2b mutants. Apoptosis is not up‐regulated within the optic fissure in pax2 mutants, yet cell death is increased in tissues outside of the optic fissure, and when apoptosis is inhibited, coloboma is partially rescued. In contrast to pax2a, loss of pax2b does not appear to affect optic fissure morphogenesis.

Conclusions

Our results suggest that pax2a, but not pax2b, supports cell survival outside of the optic fissure and POM abundance within it to facilitate optic fissure closure.

Zebrafish pax2a null mutants display a defect in optic fissure closure and coloboma

Loss of pax2b does not affect optic fissure development

An endothelial‐specific subset of periocular mesenchyme cells fails to initially localize to the optic fissure in pax2a mutants

At a later stage of optic fissure development both neural crest and endothelial‐derived periocular mesenchyme ectopically accumulate within the optic fissure

Pax2a mutants have increased apoptosis in surrounding tissues, but not within the optic fissure margin cells, and apoptosis in part underlies the coloboma phenotype

ZEB2, the Mowat-Wilson Syndrome Transcription Factor: Confirmations, Novel Functions, and Continuing Surprises

After its publication in 1999 as a DNA-binding and SMAD-binding transcription factor (TF) that co-determines cell fate in amphibian embryos, ZEB2 was from 2003 studied by embryologists mainly by documenting the consequences of conditional, cell-type specific Zeb2 knockout (cKO) in mice. In between, it was further identified as causal gene causing Mowat-Wilson Syndrome (MOWS) and novel regulator of epithelial-mesenchymal transition (EMT). ZEB2's functions and action mechanisms in mouse embryos were first addressed in its main sites of expression, with focus on those that helped to explain neurodevelopmental and neural crest defects seen in MOWS patients. By doing so, ZEB2 was identified in the forebrain as the first TF that determined timing of neuro-/gliogenesis, and thereby also the extent of different layers of the cortex, in a cell non-autonomous fashion, i.e., by its cell-intrinsic control within neurons of neuron-to-progenitor paracrine signaling. Transcriptomics-based phenotyping of Zeb2 mutant mouse cells have identified large sets of intact-ZEB2 dependent genes, and the cKO approaches also moved to post-natal brain development and diverse other systems in adult mice, including hematopoiesis and various cell types of the immune system. These new studies start to highlight the important adult roles of ZEB2 in cell-cell communication, including after challenge, e.g., in the infarcted heart and fibrotic liver. Such studies may further evolve towards those documenting the roles of ZEB2 in cell-based repair of injured tissue and organs, downstream of actions of diverse growth factors, which recapitulate developmental signaling principles in the injured sites. Evident questions are about ZEB2's direct target genes, its various partners, and ZEB2 as a candidate modifier gene, e.g., in other (neuro)developmental disorders, but also the accurate transcriptional and epigenetic regulation of its mRNA expression sites and levels. Other questions start to address ZEB2's function as a niche-controlling regulatory TF of also other cell types, in part by its modulation of growth factor responses (e.g., TGFβ/BMP, Wnt, Notch). Furthermore, growing numbers of mapped missense as well as protein non-coding mutations in MOWS patients are becoming available and inspire the design of new animal model and pluripotent stem cell-based systems. This review attempts to summarize in detail, albeit without discussing ZEB2's role in cancer, hematopoiesis, and its emerging roles in the immune system, how intense ZEB2 research has arrived at this exciting intersection.

Identification of missense MAB21L1 variants in microphthalmia and aniridia

Microphthalmia, coloboma, and aniridia are congenital ocular phenotypes with a strong genetic component but often unknown cause. We present a likely causative novel variant in MAB21L1, c.152G>T p.(Arg51Leu), in two family members with microphthalmia and aniridia, as well as novel or rare compound heterozygous variants of uncertain significance, c.184C>T p.(Arg62Cys)/c.-68T>C, and c.658G>C p.(Gly220Arg)/c.*529A>G, in two additional probands with microphthalmia, coloboma and/or cataracts. All variants were predicted as damaging by in silico programs. In vitro studies of coding variants revealed normal subcellular localization but variable stability for the corresponding mutant proteins. In vivo complementation assays using the zebrafish mab21l2 Q48Sfs*5 loss-of-function line demonstrated that though overexpression of wild-type MAB21L1 messenger RNA (mRNA) compensated for the loss of mab21l2, none of the coding variant mRNAs produced a statistically significant rescue, with p.(Arg51Leu) showing the highest degree of functional deficiency. Dominant variants in a close homolog of MAB21L1, MAB21L2, have been associated with microphthalmia and/or coloboma and repeatedly involved the same Arg51 residue, further supporting its pathogenicity. The possible role of p.(Arg62Cys) and p.(Gly220Arg) in microphthalmia is similarly supported by the observed functional defects, with or without an additional impact from noncoding MAB21L1 variants identified in each patient. This study suggests a broader spectrum of MAB21L1-associated disease.

Ocular coloboma-a comprehensive review for the clinician

Typical ocular coloboma is caused by defective closure of the embryonal fissure. The occurrence of coloboma can be sporadic, hereditary (known or unknown gene defects) or associated with chromosomal abnormalities. Ocular colobomata are more often associated with systemic abnormalities when caused by chromosomal abnormalities. The ocular manifestations vary widely. At one extreme, the eye is hardly recognisable and non-functional—having been compressed by an orbital cyst, while at the other, one finds minimalistic involvement that hardly affects the structure and function of the eye. In the fundus, the variability involves the size of the coloboma (anteroposterior and transverse extent) and the involvement of the optic disc and fovea. The visual acuity is affected when coloboma involves disc and fovea, or is complicated by occurrence of retinal detachment, choroidal neovascular membrane, cataract, amblyopia due to uncorrected refractive errors, etc. While the basic birth anomaly cannot be corrected, most of the complications listed above are correctable to a great extent. Current day surgical management of coloboma-related retinal detachments has evolved to yield consistently good results. Cataract surgery in these eyes can pose a challenge due to a combination of microphthalmos and relatively hard lenses, resulting in increased risk of intra-operative complications. Prophylactic laser retinopexy to the border of choroidal coloboma appears to be an attractive option for reducing risk of coloboma-related retinal detachment. However, a majority of the eyes have the optic disc within the choroidal coloboma, thus making it difficult to safely administer a complete treatment.

Outcomes of the Perplexed Surgical Management of Retinal Detachment in Eyes with Coloboma

Purpose

To determine the anatomical and visual outcomes of retinal detachment in eyes with chorioretinal coloboma managed by pars plana vitrectomy, endolaser photocoagulation and silicone oil (SO) tamponade.

Methods

Retrospective review of 29 eyes of 29 patients with retinal detachment associated with chorioretinal coloboma. All the cases were managed by vitrectomy procedures concluding with SO tamponade. Encircling band was placed based on pre-operative evaluation and/or surgeon’s discretion. Endolaser photocoagulation was applied around the peripheral retina, all around the peripheral breaks and around the colobomatous area. The outcome measures were evaluated with regard to functional and anatomical success.

Results

The average age at the time of surgery was 21.76 ± 9.58 years (range, 10–50 years). The mean follow-up duration was 12.28 ± 4.8 months (range, 6–24 months). Primary attached retina was obtained in 21 / 29 (72.4%) eyes after single surgery. Re-detachment in 8 / 29 (27.6%) eyes which required revision surgery was the most frequent postoperative complication followed by raised intraocular pressure in 4 / 29 (13.8%) with SO in situ. Out of 29 eyes, 23 were followed up after the removal of SO. The mean duration of SO removal was 7.91 ± 3.9 months (range, 4–18 months). Implantation of encircling band, lens removal and cryotherapy provided no added advantage. At the final examination, improvement in vision was observed in 21 (72.4%) eyes and the anatomical attachment of the retina was attained in 27 (93.1%) eyes.

Conclusions

Complete pars plana vitrectomy, endolaser photocoagulation along with SO tamponade is effective for retinal detachment associated with chorioretinal coloboma. This technique improves the anatomical outcome and helps in regaining significant visual acuity.

Confirming and expanding the phenotypes of FZD5 variants: Coloboma, inferior chorioretinal hypoplasia, and high myopia

PURPOSE

Two frameshift and two indel variants in FZD5 have been reported to cause coloboma in two families with incomplete penetrance and in two isolated cases in previous studies, respectively. This study aims to confirm this association and expand related specific phenotypes based on the genotype-phenotype analysis of FZD5 variants.

METHODS

Variants in FZD5 were collected from our in-house exome sequencing data of 5,845 probands with different eye conditions. Multistep bioinformatics analysis was used to classify the variants. Potential pathogenic variants and phenotypic variations were further evaluated based on family segregation and genotype-phenotype analysis.

RESULTS

In total, 63 rare variants were detected in FZD5. Multistep bioinformatics and genotype-phenotype analyses suggested that eight rare heterozygous variants in nine families should be considered potential pathogenic variants: three novel frameshift variants (c.350_356delCGCCGCT/p.Ser117*, c.1403_1406dupACCT/p.Tyr470Profs*130, and c.1428delG/p.Ser477Alafs*130) and five novel missense variants (c.388C>A/p.Arg130Ser, c.794G>T/p.Arg265Leu, c.1162G>A/p.Gly388Ser, c.1232A>G/p.Tyr411Cys, and c.1510A>T/p.Met504Leu). Among the nine families, carriers of these variants showed overlapping phenotypes, including typical uveal coloboma (12 eyes of seven patients from four families), inferior chorioretinal hypoplasia (ICH) or optic disc hypoplasia (ODH; 12 eyes of eight patients from six families), and high myopia (10 eyes of five patients from five families) within individual families or among different families.

CONCLUSIONS

The data presented in this study confirmed that variants in FZD5, not only frameshift variants but also missense variants, are a common cause of uveal coloboma. In addition, ICH, ODH, and high myopia may be variant phenotypes that are frequently associated with FZD5 variants.

Intravitreal ranibizumab for the management of serous maculopathy secondary to optic disc coloboma-associated choroidal neovascularisation

We report the case of a 19-year-old patient with symptomatic unilateral serous maculopathy associated with an optic nerve coloboma. Fluorescein angiography detected a focal late leak at the temporal edge of the coloboma which was later found to correspond with an area of choroidal neovascularisation on optical coherence tomography angiography. A course of intravitreal ranibizumab achieved good clinical and structural response. This report contributes to the evidence that maculopathies associated with cavitary optic nerve anomalies may in some instances result from choroidal neovascularisation. It also highlights the importance of angiography to identify potential choroidal neovascular membranes, particularly in the absence of haemorrhages and neovascular membranes on fundus examination and conventional optical coherence tomography.

Closing the Gap: Mechanisms of Epithelial Fusion During Optic Fissure Closure

A key embryonic process that occurs early in ocular development is optic fissure closure (OFC). This fusion process closes the ventral optic fissure and completes the circumferential continuity of the 3-dimensional eye. It is defined by the coming together and fusion of opposing neuroepithelia along the entire proximal-distal axis of the ventral optic cup, involving future neural retina, retinal pigment epithelium (RPE), optic nerve, ciliary body, and iris. Once these have occurred, cells within the fused seam differentiate into components of the functioning visual system. Correct development and progression of OFC, and the continued integrity of the fused margin along this axis, are important for the overall structure of the eye. Failure of OFC results in ocular coloboma-a significant cause of childhood visual impairment that can be associated with several complex ocular phenotypes including microphthalmia and anterior segment dysgenesis. Despite a large number of genes identified, the exact pathways that definitively mediate fusion have not yet been found, reflecting both the biological complexity and genetic heterogeneity of the process. This review will highlight how recent developmental studies have become focused specifically on the epithelial fusion aspects of OFC, applying a range of model organisms (spanning fish, avian, and mammalian species) and utilizing emerging high-resolution live-imaging technologies, transgenic fluorescent models, and unbiased transcriptomic analyses of segmentally-dissected fissure tissue. Key aspects of the fusion process are discussed, including basement membrane dynamics, unique cell behaviors, and the identities and fates of the cells that mediate fusion. These will be set in the context of what is now known, and how these point the way to new avenues of research.

Ocular coloboma combined with cleft lip and palate: a case report

Background

Ocular coloboma is an excavation of ocular structures that occurs due to abnormal fusion of the embryonic optic fissure. Further, cleft lip/palate (CL/P), a congenital midline abnormality, is caused by a defect in the fusion of the frontonasal, maxillary, and mandibular prominences. No study has reported the association between these two phenotypes in the absence of other systemic abnormalities. We present a case of ocular coloboma along with CL/P and without other neurological abnormalities.

Case presentation

A 5-year-old Asian boy presented with decreased visual acuity in his right eye. Physical examination revealed no abnormal findings except CL/P, which was surgically corrected at the age of 9 months. Best-corrected visual acuity was 20/60 in the right eye and 20/25 in the left eye. Anterior segment examination revealed iris coloboma in the inferior quadrant of his right eye as well as a large inferonasal optic disc and chorioretinal coloboma in the same eye. He was prescribed glasses based on his cycloplegic refractive errors and part-time occlusion of the left eye was recommended. After 3 months, best-corrected visual acuity improved to 20/30 in the right eye.

Conclusion

The association of ocular coloboma should be kept in mind when encountering a patient with CL/P without other neurological or systemic abnormalities.

Clinical spectrum of non-syndromic microphthalmos, anophthalmos and coloboma in the paediatric population: a multicentric study from North India

AIMS

To describe the clinical features, visual acuity and causes of ocular morbidity in children (0-18 years) with microphthalmos, anophthalmos, and coloboma (MAC) from North India.

METHODS

A retrospective study conducted between October 2017 and September 2018 in three tertiary eye institutes, part of the Bodhya Eye Consortium with consensus led common pro formas. Children with complete clinical data and without syndromic/systemic involvement were included. The clinical phenotype was divided into isolated ocular coloboma (CB), coloboma with microcornea (CBMC), colobomatous microphthalmos (CBMO), non-colobomatous microphthalmos (MO) and anophthalmos (AO).

RESULTS

A total of 532 children with MAC were examined. Seventeen records were excluded due to incomplete data (0.2%). 515 children (845 eyes) were included: 54.4% males and 45.6% females. MAC was unilateral in 36% and bilateral in 64%. CB, CBMC, CBMO, MO and AO were seen in 26.4%, 31%, 22%, 8% and 12.5% of eyes, respectively. Nystagmus was found in 40%, strabismus in 23%, cataract in 18.7% and retinal detachment in 15%. Best-corrected visual acuity (BCVA) of <3/60 was seen in 62.4% eyes. Blindness (BCVA <3/60 in better eye) was seen in 42.8% of bilateral patients. Those with microcornea or microphthalmos with coloboma had worse BCVA (p<0.001). There were regional differences in the type of MAC phenotype presenting to the three institutes.

CONCLUSION

The MAC group of disorders cause significant ocular morbidity. The presence of microcornea or microphthalmos with coloboma predicts worse BCVA. The variation of the MAC phenotype with the district of origin of the patient raises questions of aetiology and is subject to further studies.

Mitf-family transcription factor function is required within cranial neural crest cells to promote choroid fissure closure

A crucial step in eye development is the closure of the choroid fissure (CF), a transient structure in the ventral optic cup through which vasculature enters the eye and ganglion cell axons exit. Although many factors have been identified that function during CF closure, the molecular and cellular mechanisms mediating this process remain poorly understood. Failure of CF closure results in colobomas. Recently, MITF was shown to be mutated in a subset of individuals with colobomas, but how MITF functions during CF closure is unknown. To address this issue, zebrafish with mutations in mitfa and tfec, two members of the Mitf family of transcription factors, were analyzed and their functions during CF closure determined. mitfa;tfec mutants possess severe colobomas and our data demonstrate that Mitf activity is required within cranial neural crest cells (cNCCs) during CF closure. In the absence of Mitf function, cNCC migration and localization in the optic cup are perturbed. These data shed light on the cellular mechanisms underlying colobomas in individuals with MITF mutations and identify a novel role for Mitf function in cNCCs during CF closure.

Hyaloid vasculature and mmp2 activity play a role during optic fissure fusion in zebrafish

Vertebrate retinal development requires timely and precise fusion of the optic fissure (OF). Failure of this event leads to congenital vision impairment in the form of coloboma. Recent studies have suggested hyaloid vasculature to be involved in OF fusion. In order to examine this link, we analyzed OF fusion and hyaloid vasculogenesis in the zebrafish pax2a noi mutant line. We first determined that pax2a-/- embryos fail to accumulate F-actin in the OF prior to basement membrane (BM) degradation. Furthermore, using 3D and live imaging we observed reduced OF hyaloid vascularization in pax2a-/- embryos. When examining the connection between pax2a loss of function and hyaloid vasculature, we observed significant reduction of talin1 expression, a regulator of hyaloid vasculature. In addition, cranial VEGF expression was found to be reduced in pax2a-/- embryos. Pharmacological inhibition of VEGF signaling phenocopied the pax2a-/- vasculature, F-actin and BM degradation phenotypes. Lastly, we determined that OF associated hyaloid vasculature is a source of mmp2, mmp14a and mmp14b expression and showed that mmp2 is functionally necessary for degradation of OF BM. Taken together we propose a pax2a driven mechanism that ensures proper and timely hyaloid vasculature invasion of the OF in order to facilitate availability of the BM remodeler mmp2.

OUTCOMES OF VITRECTOMY WITH SILICONE OIL TAMPONADE FOR MANAGEMENT OF RETINAL DETACHMENT IN EYES WITH CHORIORETINAL COLOBOMA

PURPOSE

To estimate the outcomes of retinal detachment in eyes with chorioretinal coloboma managed by pars plana vitrectomy and silicone oil tamponade.

METHODS

A retrospective chart review of 10 eyes (10 patients) who underwent pars plana vitrectomy for retinal detachment with chorioretinal coloboma.

RESULTS

The average age at the time of the surgery was 29.8 ± 19.7 years. The mean follow-up period was 28.8 ± 28.4 months. The mean silicone oil tamponade duration was 9.8 ± 3.5 weeks. Of 10 eyes, 4 (40%) had retinal breaks outside the coloboma, 4 (40%) had breaks inside the coloboma, 1 (10%) had breaks inside and outside the coloboma, and in 1 eye (10%); the causative retinal break was not localized. Preoperatively, the mean visual acuity was 20/2,500 (n = 9), and 1 (10%) was recorded as "Not CSM." At the final examination, the mean visual acuity for the patients with measurable visual acuity was 20/200 (P = 0.06), and in the remaining eye was recorded as light perception. The retina was finally reattached in nine eyes (90%). Postoperative complications included cataract in three (30%), persistent elevated intraocular pressure in one (10%), band keratopathy in one (10%), and proliferative vitreoretinopathy in one (10%).

CONCLUSION

Complete pars plana vitrectomy with or without lensectomy, laser photocoagulation around the peripheral retina, around all the peripheral breaks and around the colobomatous area, and silicone oil tamponade is effective for retinal detachment in eyes with chorioretinal coloboma. Silicone oil removal as early as possible did not increase the risk of redetachment and seems to reduce the incidence of oil-related complications in such cases.

In Vivo Analysis of Optic Fissure Fusion in Zebrafish: Pioneer Cells, Basal Lamina, Hyaloid Vessels, and How Fissure Fusion is Affected by BMP

Colobomata, persistent optic fissures, frequently cause congenital blindness. Here, we focused on optic fissure fusion using in vivo time-lapse imaging in zebrafish. We identified the fusion initiating cells, which we termed “pioneer cells.” Based on morphology, localization, and downregulation of the neuroretinal (NR) precursor marker rx2, these cells could be considered as retinal pigment epithelial (RPE) progenitors. Notably, pioneer cells regain rx2 expression and integrate into the NR after fusion, indicating that they do not belong to the pool of RPE progenitors, supported by the lack of RPE marker expression in pioneer cells. They establish the first cellular contact between the margins in the proximal fissure region and separate the hyaloid artery and vein. After initiation, the fusion site is progressing distally, increasing the distance between the hyaloid artery and vein. A timed BMP (Bone Morphogenetic Protein) induction, resulting in coloboma, did not alter the morphology of the fissure margins, but it did affect the expression of NR and RPE markers within the margins. In addition, it resulted in a persisting basal lamina and persisting remnants of periocular mesenchyme and hyaloid vasculature within the fissure, supporting the necessity of BMP antagonism within the fissure margins. The hampered fissure fusion had severe effects on the vasculature of the eye.

Morphogenesis and axis specification occur in parallel during optic cup and optic fissure formation, differentially modulated by BMP and Wnt

Optic cup morphogenesis is an intricate process. Especially, the formation of the optic fissure is not well understood. Persisting optic fissures, termed coloboma, are frequent causes for congenital blindness. Even though the defective fusion of the fissure margins is the most acknowledged reason for coloboma, highly variable morphologies of coloboma phenotypes argue for a diverse set of underlying pathomechanisms. Here, we investigate optic fissure morphogenesis in zebrafish to identify potential morphogenetic defects resulting in coloboma. We show that the formation of the optic fissure depends on tissue flow movements, integrated into the bilateral distal epithelial flow forming the optic cup. On the temporal side, the distal flow translates into a ventral perpendicular flow, shaping the temporal fissure margin. On the nasal side, however, the distal flow is complemented by tissue derived from the optic stalk, shaping the nasal fissure margin. Notably, a distinct population of TGFβ-signalling positive cells is translocated from the optic stalk into both fissure margins. Furthermore, we show that induced BMP signalling as well as Wnt-signalling inhibition result in morphogenetic defects of the optic fissure. Our data also indicate that morphogenesis is crucial for a proper positioning of pre-specified dorsal–ventral optic cup domains.

Mutation in Bmpr1b Leads to Optic Disc Coloboma and Ventral Retinal Gliosis in Mice

Purpose

The clinical phenotype of retinal gliosis occurs in different forms; here, we characterize one novel genetic feature, (i.e., signaling via BMP-receptor 1b).

Methods

Mouse mutants were generated within a recessive ENU mutagenesis screen; the underlying mutation was identified by linkage analysis and Sanger sequencing. The eye phenotype was characterized by fundoscopy, optical coherence tomography, optokinetic drum, electroretinography, and visual evoked potentials, by histology, immunohistology, and electron-microscopy.

Results

The mutation affects intron 10 of the Bmpr1b gene, which is causative for skipping of exon 10. The expression levels of pSMAD1/5/8 were reduced in the mutant retina. The loss of BMPR1B-mediated signaling leads to optic nerve coloboma, gliosis in the optic nerve head and ventral retina, defective optic nerve axons, and irregular retinal vessels. The ventral retinal gliosis is proliferative and hypertrophic, which is concomitant with neuronal delamination and the reduction of retinal ganglion cells (RGCs); it is dominated by activated astrocytes overexpressing PAX2 and SOX2 but not PAX6, indicating that they may retain properties of gliogenic precursor cells. The expression pattern of PAX2 in the optic nerve head and ventral retina is altered during embryonic development. These events finally result in reduced electrical transmission of the retina and optic nerve and significantly reduced visual acuity.

Conclusions

Our study demonstrates that BMPR1B is necessary for the development of the optic nerve and ventral retina. This study could also indicate a new mechanism in the formation of retinal gliosis; it opens new routes for its treatment eventually preventing scar formation in the retina.

The Spectrum of PAX6 Mutations and Genotype-Phenotype Correlations in the Eye

The transcription factor PAX6 is essential in ocular development in vertebrates, being considered the master regulator of the eye. During eye development, it is essential for the correct patterning and formation of the multi-layered optic cup and it is involved in the developing lens and corneal epithelium. In adulthood, it is mostly expressed in cornea, iris, and lens. PAX6 is a dosage-sensitive gene and it is highly regulated by several elements located upstream, downstream, and within the gene. There are more than 500 different mutations described to affect PAX6 and its regulatory regions, the majority of which lead to PAX6 haploinsufficiency, causing several ocular and systemic abnormalities. Aniridia is an autosomal dominant disorder that is marked by the complete or partial absence of the iris, foveal hypoplasia, and nystagmus, and is caused by heterozygous PAX6 mutations. Other ocular abnormalities have also been associated with PAX6 changes, and genotype-phenotype correlations are emerging. This review will cover recent advancements in PAX6 regulation, particularly the role of several enhancers that are known to regulate PAX6 during eye development and disease. We will also present an updated overview of the mutation spectrum, where an increasing number of mutations in the non-coding regions have been reported. Novel genotype-phenotype correlations will also be discussed.

High-throughput custom capture sequencing identifies novel mutations in coloboma-associated genes: Mutation in DNA-binding domain of retinoic acid receptor beta affects nuclear localization causing ocular coloboma

Uveal coloboma is a potentially blinding congenital ocular malformation caused by the failure of optic fissure closure during the fifth week of human gestation. We performed custom capture high‐throughput screening of 38 known coloboma‐associated genes in 66 families. Suspected causative novel variants were identified in TFAP2A and CHD7, as well as two previously reported variants of uncertain significance in RARB and BMP7. The variant in RARB, unlike previously reported disease mutations in the ligand‐binding domain, was a missense change in the highly conserved DNA‐binding domain predicted to affect the protein's DNA‐binding ability. In vitro studies revealed lower steady‐state protein levels, reduced transcriptional activity, and incomplete nuclear localization of the mutant RARB protein compared with wild‐type. Zebrafish studies showed that human RARB messenger RNA partially reduced the ocular phenotype caused by morpholino knockdown of rarga gene, a zebrafish homolog of human RARB. Our study indicates that sequence alterations in known coloboma genes account for a small percentage of coloboma cases and that mutations in the RARB DNA‐binding domain could result in human disease.

Microphthalmos-anophthalmos-coloboma (MAC) spectrum in two brothers with Renpenning syndrome due to a truncating mutation in the polyglutamine tract binding protein 1 (PQBP1) gene

Background: Patients with intellectual disability syndromes frequently have coexisting abnormalities of ocular structures and the visual pathway system. The microphthalmos, anophthalmos, and coloboma (MAC) spectrum represent structural developmental eye defects that occur as part of a syndrome in one-third of cases. Ophthalmic examination may provide important diagnostic clues in identifying these syndromes.Purpose: To provide a detailed and comprehensive description of the microphthalmos, anophthalmos, and coloboma (MAC) spectrum in two brothers with intellectual disability and dysmorphism.Methods: The two brothers underwent a detailed ophthalmic and systemic evaluation. A family pedigree was obtained and exome sequencing was performed in the proband.Results: The two brothers aged 4 and 7 years had intellectual disability, microcephaly, short stature, and characteristic dysmorphic features. Ophthalmic evaluation revealed the presence of the MAC spectrum in both boys. Genetic testing led to the detection of an X-linked hemizygous truncating mutation in the nuclear polyglutamine-binding protein 1 (PQBP1) gene confirming the diagnosis of X-linked recessive Renpenning syndrome.Conclusion: The presence of X-linked intellectual disability and characteristic dysmorphism, in a patient with the MAC spectrum should raise the suspicion of Renpenning syndrome. PQBP1 mutation testing is confirmatory. A comprehensive systemic evaluation is mandatory in all patients with the MAC spectrum and intellectual disability.

Swept-Source OCT Analysis of the Margin of Choroidal Coloboma: New Insights

PURPOSE

To study the retinal architecture and vitreoretinal interface at the edge of choroidal coloboma using swept-source OCT.

DESIGN

Prospective observational case series at a tertiary eye care center.

PARTICIPANTS

Patients with choroidal coloboma treated at an ophthalmology department and fulfilling the inclusion criteria of the study.

METHODS

Swept-source OCT was carried out in 30 eyes of 20 patients with choroidal coloboma.

MAIN OUTCOME MEASURES

The primary objective was to describe the OCT features at the margin of the coloboma.

RESULTS

Swept-source OCT of the coloboma margin revealed new features in addition to the previously described findings. Two types of transition from normal retina into intercalary membrane (ICM) were noted: abrupt (73.33%) and gradual (26.67%). Outer retinal layers (interdigitation zone and ellipsoid zone) terminated at a variable distance before the retinal pigment epithelium in 56.67% of eyes. Cystic spaces in the ICM (46.67%), schisis-like spitting of the ICM (30%), and breaks in the ICM (6.67%) were seen as well. Subclinical retinal detachment (RD) was also noted in 1 eye. The peculiar features noted at the vitreoretinal interface included vitreous attachment at the coloboma margin (23.33%), vitreous condensation (6.67%), and hill like projections of ICM into the vitreous cavity (26.67%). In the region of the coloboma, sclera and Tenon's capsule could also be analyzed as a hyperreflective lamellar structure and an irregularly arranged less hyperreflective structure.

CONCLUSIONS

Swept-source OCT of the coloboma margin revealed various new features in addition to those described previously. The detection of subclinical RD or early termination of outer retinal layers in selected cases may be helpful in guiding new management protocols.

The Molecular Basis of Human Anophthalmia and Microphthalmia

Human eye development is coordinated through an extensive network of genetic signalling pathways. Disruption of key regulatory genes in the early stages of eye development can result in aborted eye formation, resulting in an absent eye (anophthalmia) or a small underdeveloped eye (microphthalmia) phenotype. Anophthalmia and microphthalmia (AM) are part of the same clinical spectrum and have high genetic heterogeneity, with >90 identified associated genes. By understanding the roles of these genes in development, including their temporal expression, the phenotypic variation associated with AM can be better understood, improving diagnosis and management. This review describes the genetic and structural basis of eye development, focusing on the function of key genes known to be associated with AM. In addition, we highlight some promising avenues of research involving multiomic approaches and disease modelling with induced pluripotent stem cell (iPSC) technology, which will aid in developing novel therapies.

Ubiquitin-mediated proteasome degradation regulates optic fissure fusion

Optic fissure fusion is a critical event during retinal development. Failure of fusion leads to coloboma, a potentially blinding congenital disorder. Pax2a is an essential regulator of optic fissure fusion and the target of numerous morphogenetic pathways. In our current study, we examined the negative regulator of pax2a expression, Nz2, and the mechanism modulating Nlz2 activity during optic fissure fusion. Upregulation of Nlz2 in zebrafish embryos resulted in downregulation of pax2a expression and fissure fusion failure. Conversely, upregulation of pax2a expression also led to fissure fusion failure suggesting Pax2 levels require modulation to ensure proper fusion. Interestingly, we discovered Nlz2 is a target of the E3 ubiquitin ligase Siah. We show that zebrafish siah1 expression is regulated by Hedgehog signaling and that Siah1 can directly target Nlz2 for proteasomal degradation, in turn regulating the levels of pax2a mRNA. Finally, we show that both activation and inhibition of Siah activity leads to failure of optic fissure fusion dependent on ubiquitin-mediated proteasomal degradation of Nlz2. In conclusion, we outline a novel, proteasome-mediated degradation regulatory pathway involved in optic fissure fusion.

Summary: Optic fissure fusion, a key retinal morphogenic event highly sensitive to developmental signaling, is directly regulated by ubiquitin-mediated proteasomal degradation uncovering a novel regulatory pathway potentially correlated to incidence of coloboma.

Detailed analysis of chick optic fissure closure reveals Netrin-1 as an essential mediator of epithelial fusion

Epithelial fusion underlies many vital organogenic processes during embryogenesis. Disruptions to these cause a significant number of human birth defects, including ocular coloboma. We provide robust spatial-temporal staging and unique anatomical detail of optic fissure closure (OFC) in the embryonic chick, including evidence for roles of apoptosis and epithelial remodelling. We performed complementary transcriptomic profiling and show that Netrin-1 (NTN1) is precisely expressed in the chick fissure margin during fusion but is immediately downregulated after fusion. We further provide a combination of protein localisation and phenotypic evidence in chick, humans, mice and zebrafish that Netrin-1 has an evolutionarily conserved and essential requirement for OFC, and is likely to have an important role in palate fusion. Our data suggest that NTN1 is a strong candidate locus for human coloboma and other multi-system developmental fusion defects, and show that chick OFC is a powerful model for epithelial fusion research.

Our bodies are made of many different groups of cells, which are arranged into tissues that perform specific roles. As tissues form in the embryo they must adopt precise three-dimensional structures, depending on their position in the body. In many cases this involves two edges of tissue fusing together to prevent gaps being present in the final structure.

In individuals with a condition called ocular coloboma some of the tissues in the eyes fail to merge together correctly, leading to wide gaps that can severely affect vision. There are currently no treatments available for ocular coloboma and in over 70% of patients the cause of the defect is not known. Identifying new genes that control how tissues fuse may help researchers to find what causes this condition and multiple other tissue fusion defects, and establish whether these may be preventable in the future.

Much of what is currently known about how tissues fuse has come from studying mice and zebrafish embryos. Although the extensive genetic tools available in these ‘models’ have proved very useful, both offer only a limited time window for observing tissues as they fuse, and the regions involved are very small. Chick embryos, on the other hand, are much larger than mouse or zebrafish embryos and are easier to access from within their eggs. This led Hardy et al. to investigate whether the developing chick eye could be a more useful model for studying the precise details of how tissues merge.

Examining chick embryos revealed that tissues in the base of their eyes fuse between five and eight days after the egg had been fertilised, a comparatively long time compared to existing models. Also, many of the genes that Hardy et al. found switched on in chick eyes as the tissues merged had previously been identified as being essential for tissue fusion in humans. However, several new genes were also shown to be involved in the fusing process. For example, Netrin-1 was important for tissues to fuse in the eyes as well as in other regions of the developing embryo.

These findings demonstrate that the chick eye is an excellent new model system to study how tissues fuse in animals. Furthermore, the genes identified by Hardy et al. may help researchers to identify the genetic causes of ocular coloboma and other tissue fusion defects in humans.

Regional Gene Expression Profile Comparison Reveals the Unique Transcriptome of the Optic Fissure

Purpose

The optic fissure (OF) is a transient opening in the ventral optic cup (OC) that acts as a passage for blood vessels and retinal ganglion cell axons during early eye development. Failure to close the OF is the developmental basis for uveal coloboma, a congenital blinding eye disease that significantly contributes to childhood blindness. Genes specifically expressed in the OF region may play important roles in OF development and function. The aim of this study was to characterize the transcriptome of OC cells in the OF region and investigate the function of OF-specific genes during OF closure.

Methods

Laser-assisted microdissection was used to collect different regions of OC tissues. Microarray analysis was used to obtain and compare gene expression profiles of different OC regions. RNA in situ hybridization (ISH) was used to further characterize OF-specific gene expression patterns. Morpholino knockdown in zebrafish was used to study the function of a newly discovered OF-specific gene during OF closure.

Results

Microarray comparison revealed that the OC at the OF region exhibited a unique gene expression profile. OC expression patterns of a number of newly discovered OF-specific genes were confirmed by ISH. Morpholino knockdown and downstream target expression and function analysis demonstrated that afap1l2, a newly discovered OF-specific gene, controls OF closure by regulating pax2a expression.

Conclusions

Our study characterized the unique transcriptome of the OF region of the OC and demonstrated the essential role of a newly discovered OF-specific gene in OF closure. This study provides a valuable foundation for future mechanism dissection in OF development and physiology, and for human coloboma etiology exploration.

Zebrafish mab21l2 mutants possess severe defects in optic cup morphogenesis, lens and cornea development

BACKGROUND

Mutations in MAB21L2 result in severe ocular defects including microphthalmia, anophthalmia, coloboma, microcornea, and cataracts. The molecular and cellular underpinnings of these defects are unknown, as is the normal cellular function of MAB21L2. Zebrafish mab21l2 ^au10 mutants possess ocular defects resembling those in humans with MAB21L2 mutations, providing an excellent model to characterize mab21l2 functions during eye development.

RESULTS

mab21l2 ^-/- mutants possessed a host of ocular defects including microphthalmia and colobomas as well as small, disorganized lenses and cornea dysgenesis. Decreased proliferation, increased cell death, and defects in marker gene expression were detected in the lens. Cell death in the optic stalk was elevated in mab21l2 ^-/- mutants and the basement membrane between the edges of the choroid fissure failed to break down. Neuronal differentiation in the retina was normal, however. mab21l2 ^-/- mutant corneas were disorganized, possessed an increased number of cells, some of which proliferated ectopically, and failed to differentiate the corneal stroma.

CONCLUSIONS

mab21l2 function is required for morphogenesis and cell survival in the lens and optic cup, and basement membrane breakdown in the choroid fissure. mab21l2 function also regulates proliferation in the lens and cornea; in its absence, the lens is small and mispatterned, and corneal morphogenesis and patterning are also disrupted.

An update on the genetics of ocular coloboma

Ocular coloboma is an uncommon, but often severe, sight-threatening condition that can be identified from birth. This congenital anomaly is thought to be caused by maldevelopment of optic fissure closure during early eye morphogenesis. It has been causally linked to both inherited (genetic) and environmental influences. In particular, as a consequence of work to identify genetic causes of coloboma, new molecular pathways that control optic fissure closure have now been identified. Many more regulatory mechanisms still await better understanding to inform on the development of potential therapies for patients with this malformation. This review provides an update of known coloboma genes, the pathways they influence and how best to manage the condition. In the age of precision medicine, determining the underlying genetic cause in any given patient is of high importance.

Congenital eye anomalies: More mosaic than thought?

The eye is a sensory organ that primarily captures light and provides the sense of sight, as well as delivering non-visual light information involving biological rhythms and neurophysiological activities to the brain. Since the early 1990s, rapid advances in molecular biology have enabled the identification of developmental genes, genes responsible for human congenital diseases, and relevant genes of mutant animals with various anomalies. In this review, we first look at the development of the eye, and we highlight seminal reports regarding archetypal gene defects underlying three developmental ocular disorders in humans: (1) holoprosencephaly (HPE), with cyclopia being exhibited in the most severe cases; (2) microphthalmia, anophthalmia, and coloboma (MAC) phenotypes; and (3) anterior segment dysgenesis (ASDG), known as Peters anomaly and its related disorders. The recently developed methods, such as next-generation sequencing and genome editing techniques, have aided the discovery of gene mutations in congenital eye diseases and gene functions in normal eye development. Finally, we discuss Pax6-genome edited mosaic eyes and propose that somatic mosaicism in developmental gene mutations should be considered a causal factor for variable phenotypes, sporadic cases, and de novo mutations in human developmental disorders.

Temporal characterization of optic fissure basement membrane composition suggests nidogen may be an initial target of remodeling

Fusion of the optic fissure is necessary to complete retinal morphogenesis and ensure proper function of the optic stalk. Failure of this event leads to congenital coloboma, one of the leading causes of pediatric blindness. Mechanistically it is widely accepted that the basement membrane (BM) surrounding the maturing retina needs to be remodeled within the fissure in order to facilitate subsequent epithelial sheet fusion. However, the mechanism driving BM remodeling has yet to be elucidated. As a first step to understanding this critical molecular event we comprehensively characterized the core composition of optic fissure BMs in the zebrafish embryos. Zebrafish optic fissure BMs were found to express laminin a1, a4, b1a, c1 and c3, nidogen 1a, 1b and 2a, collagen IV a1 and a2 as well as perlecan. Furthermore, we observed that laminin, perlecan and collagen IV expression persists in the fissure during fusion, up to 56 hpf, while nidogen expression is downregulated upon initiation of fusion, at 36 hpf. Using immunohistochemistry we also show that nidogen is removed from the BM prior to that of laminin, indicating that remodeling of the BM is an ordered event. Lastly, we characterized retinal morphogenesis in the absence of nidogen function and documented retinal malformation similar to what is observed in laminin mutants. Taken together, we propose a model of BM remodeling where nidogen acts as a linchpin during initiation of optic fissure fusion.

TGFβ-facilitated optic fissure fusion and the role of bone morphogenetic protein antagonism

The optic fissure is a transient gap in the developing vertebrate eye, which must be closed as development proceeds. A persisting optic fissure, coloboma, is a major cause for blindness in children. Although many genes have been linked to coloboma, the process of optic fissure fusion is still little appreciated, especially on a molecular level. We identified a coloboma in mice with a targeted inactivation of transforming growth factor β2 (TGFβ2). Notably, here the optic fissure margins must have touched, however failed to fuse. Transcriptomic analyses indicated an effect on remodelling of the extracellular matrix (ECM) as an underlying mechanism. TGFβ signalling is well known for its effect on ECM remodelling, but it is at the same time often inhibited by bone morphogenetic protein (BMP) signalling. Notably, we also identified two BMP antagonists among the downregulated genes. For further functional analyses we made use of zebrafish, in which we found TGFβ ligands expressed in the developing eye, and the ligand binding receptor in the optic fissure margins where we also found active TGFβ signalling and, notably, also gremlin 2b (grem2b) and follistatin a (fsta), homologues of the regulated BMP antagonists. We hypothesized that TGFβ is locally inducing expression of BMP antagonists within the margins to relieve the inhibition from its regulatory capacity regarding ECM remodelling. We tested our hypothesis and found that induced BMP expression is sufficient to inhibit optic fissure fusion, resulting in coloboma. Our findings can likely be applied also to other fusion processes, especially when TGFβ signalling or BMP antagonism is involved, as in fusion processes during orofacial development.

Review of Ocular Manifestations of Joubert Syndrome

Joubert syndrome is a group of rare disorders that stem from defects in a sensory organelle, the primary cilia. Affected patients often present with disorders involving multiple organ systems, including the brain, eyes, and kidneys. Common symptoms include breathing abnormalities, mental developmental delays, loss of voluntary muscle coordination, and abnormal eye movements, with a diagnostic “molar tooth” sign observed by magnetic resonance imaging (MRI) of the midbrain. We reviewed the ocular phenotypes that can be found in patients with Joubert syndrome. Ocular motor apraxia is the most frequent (80% of patients), followed by strabismus (74%) and nystagmus (72%). A minority of patients also present with ptosis (43%), chorioretinal coloboma (30%), and optic nerve atrophy (22%). Although mutations in 34 genes have been found to be associated with Joubert syndrome, retinal degeneration has been reported in only 38% of patients. Mutations in AHI1 and CEP290, genes critical to primary cilia function, have been linked to retinal degeneration. In conclusion, Joubert syndrome is a rare pleiotropic group of disorders with variable ocular presentations.

Hedgehog signaling regulates cell motility and optic fissure and stalk formation during vertebrate eye morphogenesis

Establishment of precise three-dimensional tissue structure is vital for organ function. In the visual system, optic fissure and stalk morphogenesis is a crucial yet poorly understood process, disruptions of which can lead to coloboma, a birth defect causing visual impairment. Here, we use four-dimensional imaging, cell tracking, and molecular genetics in zebrafish to define the cell movements underlying normal optic fissure and stalk formation. We determine how these events are disrupted in a coloboma model in which the Hedgehog (Hh) receptor ptch2 is lost, resulting in overactive Hh signaling. In the ptch2 mutant, cells exhibit defective motile behaviors and morphology. Cells that should contribute to the fissure do not arrive at their correct position, and instead contribute to an ectopically large optic stalk. Our results suggest that overactive Hh signaling, through overexpression of downstream transcriptional targets, impairs cell motility underlying optic fissure and stalk formation, via non-cell-autonomous and cell-autonomous mechanisms. More broadly, our cell motility and morphology analyses provide a new framework for studying other coloboma-causing mutations that disrupt optic fissure or stalk formation.

Summary: Multidimensional imaging of ptch2 mutant zebrafish uncovers a role for the Hh signaling pathway in regulating the cell and tissue dynamics underlying early eye morphogenesis.

Genetics of congenital eye malformations: insights from chick experimental embryology

Embryological manipulations in chick embryos have been pivotal in our understanding of many aspects of vertebrate eye formation. This research was particularly important in uncovering the role of tissue interactions as drivers of eye morphogenesis and to dissect the function of critical genes. Here, we have highlighted a few of these past experiments to endorse their value in searching for hitherto unknown causes of rare congenital eye anomalies, such as microphthalmia, anophthalmia and coloboma. We have also highlighted a number of similarities between the chicken and human eye, which might be exploited to address other eye pathologies, including degenerative ocular diseases.