Loss of Zebrafish Mfrp Causes Nanophthalmia, Hyperopia, and Accumulation of Subretinal Macrophages

Heterozygous variants c.781G>A and c.1066dup of serine protease 56 cause familial nanophthalmos by impairing serine-type endopeptidase activity

BACKGROUND/AIMS

Nanophthalmos is a rare developmental, bilateral, sporadic or hereditary form of microphthalmos. In this study, the heterozygous variants c.781G>A and c.1066dup of the PRSS56 gene were identified in two patients with nanophthalmos. This study reports the clinical manifestation and the underlying pathogenic mechanism.

METHODS

Whole-exome sequencing was performed to identify the pathogenic genes in a Chinese family with nanophthalmos. The molecular simulation was used to predict the structures of wild-type or mutant PRSS56. The PRSS56 wild-type or mutation overexpression cellular models have been constructed accordingly. The subcellular localisation was then observed using immunofluorescence and Western-blot techniques. The Folin-Ciocalteu assay was carried out to evaluate serine-type endopeptidase activity, and a wound-healing assay was used to examine the cellular migratory ability.

RESULTS

The whole-exome sequencing revealed that heterozygous variants c.781G>A and c.1066dup of the PRSS56 gene might contribute to nanophthalmos. Both variants were not identified in the dbSNP, 1000 Genome project or ESP6500 databases. Furthermore, the variants were highly conserved and were involved in biological functions. The mutations result in destructive protein structure and impede serine-type endopeptidase activity, thereby impairing subcellular localisation and cellular migration.

CONCLUSION

The c.781G>A and c.1066dup variants of the PRSS56 gene might negatively affect protein structures, subcellular localisation, serine-type endopeptidase activity and cellular migratory ability. Together, these changes could lead to the development of nanophthalmos. This study identifies the PRSS56 gene as a potential target for nanophthalmos diagnosis and treatment.

The Genetic Determinants of Axial Length: From Microphthalmia to High Myopia in Childhood

The axial length of the eye is critical for normal visual function by enabling light to precisely focus on the retina. The mean axial length of the adult human eye is 23.5 mm, but the molecular mechanisms regulating ocular axial length remain poorly understood. Underdevelopment can lead to microphthalmia (defined as a small eye with an axial length of less than 19 mm at 1 year of age or less than 21 mm in adulthood) within the first trimester of pregnancy. However, continued overgrowth can lead to axial high myopia (an enlarged eye with an axial length of 26.5 mm or more) at any age. Both conditions show high genetic and phenotypic heterogeneity associated with significant visual morbidity worldwide. More than 90 genes can contribute to microphthalmia, and several hundred genes are associated with myopia, yet diagnostic yields are low. Crucially, the genetic pathways underpinning the specification of eye size are only now being discovered, with evidence suggesting that shared molecular pathways regulate under- or overgrowth of the eye. Improving our mechanistic understanding of axial length determination will help better inform us of genotype-phenotype correlations in both microphthalmia and myopia, dissect gene-environment interactions in myopia, and develop postnatal therapies that may influence overall eye growth.

Optical Coherence Tomography Is a Promising Tool for Zebrafish-Based Research-A Review

The zebrafish is an established vertebrae model in the field of biomedical research. With its small size, rapid maturation time and semi-transparency at early development stages, it has proven to be an important animal model, especially for high-throughput studies. Three-dimensional, high-resolution, non-destructive and label-free imaging techniques are perfectly suited to investigate these animals over various development stages. Optical coherence tomography (OCT) is an interferometric-based optical imaging technique that has revolutionized the diagnostic possibilities in the field of ophthalmology and has proven to be a powerful tool for many microscopic applications. Recently, OCT found its way into state-of-the-art zebrafish-based research. This review article gives an overview and a discussion of the relevant literature and an outlook for this emerging field.

A Novel Mutation in the Membrane Frizzled-Related Protein Gene for Posterior Microphthalmia, Non-pigmented Retinitis Pigmentosa, Optic Nerve Drusen, and Retinoschisis in a Consanguineous Family

Background

Microphthalmos (MCO) is a rare developmental defect characterized by small malformed eyes. Our study aimed to describe the clinical characteristics of posterior microphthalmos syndrome caused by a novel variant in MFRP gene in a Chinese patient.

Methods

Complete ophthalmologic examinations were performed for the proband and proband's family members. Whole exon sequencing (WES) and Sanger sequencing were used to identify the mutated genes, and bioinformatic analysis was undertaken to predict the effect of this variant.

Results

Clinical analysis showed that the proband had reduced axial length (17.95 and 17.98 mm) with normal-size corneas and shallow anterior chamber depth. Fundus photography showed scattered yellowish-white spots in the whole retina with cup-to-disc ratios of 0.95 in both eyes. Retinoschisis in the inner nuclear layer and reduced outer retina thickness were apparent on OCT examination, and optic nerve drusen demonstrated increased autofluorescence in fundus autofluorescence (FAF). Perimeter examination revealed a tubular visual field for the right eye, and electroretinography (ERG) revealed a moderately reduced rod response combined with compromised cone response. Ocular examinations of the patient's family members were unremarkable. WES revealed that the proband had homozygous mutations in c.55-1 (IVS1) G>A in intron 1 for the MFRP gene. Both the proband's parents and offspring were confirmed to be heterozygous by Sanger sequencing. Bioinformatic analysis showed this mutation was deleterious.

Conclusion

We reported autosomal recessive posterior microphthalmia, atypical retinitis pigmentosa, and retinoschisis caused by a novel mutation in the MFRP gene in this consanguineous marriage family. Our study further broadens the mutation and phenotype spectrum of the MFRP gene in microphthalmia.

Genetic and Neurological Deficiencies in the Visual System of mct8 Mutant Zebrafish

Thyroid hormones (THs; T3 and T4) enter cells using specific transporters and regulate development and metabolism. Mutation in the TH transporter monocarboxylate transporter 8 (MCT8, SLC16A2) is associated with brain hypothyroidism and neurological impairment. We established mct8 mutant (mct8-/-) zebrafish as a model for MCT8 deficiency, which causes endocrinological, neurological, and behavioral alterations. Here, we profiled the transcriptome of mct8-/- larvae. Among hundreds of differentially expressed genes, the expression of a cluster of vision-related genes was distinct. Specifically, the expression of the opsin 1 medium wave sensitive 2 (opn1mw2) decreased in two mct8 mutants: mct8-/- and mct8^-25bp-/- larvae, and under pharmacological inhibition of TH production. Optokinetic reflex (OKR) assays showed a reduction in the number of conjugated eye movements, and live imaging of genetically encoded Ca²⁺ indicator revealed altered neuronal activity in the pretectum area of mct8^-25bp-/- larvae. These results imply that MCT8 and THs regulate the development of the visual system and suggest a mechanism to the deficiencies observed in the visual system of MCT8-deficiency patients.

Comparative Study in Zebrafish and Medaka Unravels the Mechanisms of Tissue Regeneration

Tissue regeneration has been in the spotlight of research for its fascinating nature and potential applications in human diseases. The trait of regenerative capacity occurs diversely across species and tissue contexts, while it seems to decline over evolution. Organisms with variable regenerative capacity are usually distinct in phylogeny, anatomy, and physiology. This phenomenon hinders the feasibility of studying tissue regeneration by directly comparing regenerative with non-regenerative animals, such as zebrafish (Danio rerio) and mice (Mus musculus). Medaka (Oryzias latipes) is a fish model with a complete reference genome and shares a common ancestor with zebrafish approximately 110–200 million years ago (compared to 650 million years with mice). Medaka shares similar features with zebrafish, including size, diet, organ system, gross anatomy, and living environment. However, while zebrafish regenerate almost every organ upon experimental injury, medaka shows uneven regenerative capacity. Their common and distinct biological features make them a unique platform for reciprocal analyses to understand the mechanisms of tissue regeneration. Here we summarize current knowledge about tissue regeneration in these fish models in terms of injured tissues, repairing mechanisms, available materials, and established technologies. We further highlight the concept of inter-species and inter-organ comparisons, which may reveal mechanistic insights and hint at therapeutic strategies for human diseases.

Genetic Interaction between Mfrp and Adipor1 Mutations Affect Retinal Disease Phenotypes

Adipor1tm1Dgen and Mfrprd6 mutant mice share similar eye disease characteristics. Previously, studies established a functional relationship of ADIPOR1 and MFRP proteins in maintaining retinal lipidome homeostasis and visual function. However, the independent and/or interactive contribution of both genes to similar disease phenotypes, including fundus spots, decreased axial length, and photoreceptor degeneration has yet to be examined. We performed a gene-interaction study where homozygous Adipor1tm1Dgen and Mfrprd6 mice were bred together and the resulting doubly heterozygous F1 offspring were intercrossed to produce 210 F2 progeny. Four-month-old mice from all nine genotypic combinations obtained in the F2 generation were assessed for white spots by fundus photo documentation, for axial length by caliper measurements, and for photoreceptor degeneration by histology. Two-way factorial ANOVA was performed to study individual as well as gene interaction effects on each phenotype. Here, we report the first observation of reduced axial length in Adipor1tmlDgen homozygotes. We show that while Adipor1 and Mfrp interact to affect spotting and degeneration, they act independently to control axial length, highlighting the complex functional association between these two genes. Further examination of the molecular basis of this interaction may help in uncovering mechanisms by which these genes perturb ocular homeostasis.

Ablation of mpeg+ Macrophages Exacerbates mfrp-Related Hyperopia

Purpose

Proper refractive development of the eye, termed emmetropization, is critical for focused vision and is impacted by both genetic determinants and several visual environment factors. Improper emmetropization caused by genetic variants can lead to congenital hyperopia, which is characterized by small eyes and relatively short ocular axial length. To date, variants in only four genes have been firmly associated with human hyperopia, one of which is MFRP. Zebrafish mfrp mutants also have hyperopia and, similar to reports in mice, exhibit increased macrophage recruitment to the retina. The goal of this research was to examine the effects of macrophage ablation on emmetropization and mfrp-related hyperopia.

Methods

We utilized a chemically inducible, cell-specific ablation system to deplete macrophages in both wild-type and mfrp mutant zebrafish. Spectral-domain optical coherence tomography was then used to measure components of the eye and determine relative refractive state. Histology, immunohistochemistry, and transmission electron microscopy were used to further study the eyes.

Results

Although macrophage ablation does not cause significant changes to the relative refractive state of wild-type zebrafish, macrophage ablation in mfrp mutants significantly exacerbates their hyperopic phenotype, resulting in a relative refractive error 1.3 times higher than that of non-ablated mfrp siblings.

Conclusions

Genetic inactivation of mfrp leads to hyperopia, as well as abnormal accumulation of macrophages in the retina. Ablation of the mpeg1-positive macrophage population exacerbates the hyperopia, suggesting that macrophages may be recruited in an effort help preserve emmetropization and ameliorate hyperopia.

Retinal Stem Cell 'Retirement Plans': Growth, Regulation and Species Adaptations in the Retinal Ciliary Marginal Zone

The vertebrate retina develops from a specified group of precursor cells that adopt distinct identities and generate lineages of either the neural retina, retinal pigmented epithelium, or ciliary body. In some species, including teleost fish and amphibians, proliferative cells with stem-cell-like properties capable of continuously supplying new retinal cells post-embryonically have been characterized and extensively studied. This region, termed the ciliary or circumferential marginal zone (CMZ), possibly represents a conserved retinal stem cell niche. In this review, we highlight the research characterizing similar CMZ-like regions, or stem-like cells located at the peripheral margin, across multiple different species. We discuss the proliferative parameters, multipotency and growth mechanisms of these cells to understand how they behave in vivo and how different molecular factors and signalling networks converge at the CMZ niche to regulate their activity. The evidence suggests that the mature retina may have a conserved propensity for homeostatic growth and plasticity and that dysfunction in the regulation of CMZ activity may partially account for dystrophic eye growth diseases such as myopia and hyperopia. A better understanding of the properties of CMZ cells will enable important insight into how an endogenous generative tissue compartment can adapt to altered retinal physiology and potentially even restore vision loss caused by retinal degenerative conditions.

Identification of MFRP and the secreted serine proteases PRSS56 and ADAMTS19 as part of a molecular network involved in ocular growth regulation

Precise regulation of ocular size is a critical determinant of normal visual acuity. Although it is generally accepted that ocular growth relies on a cascade of signaling events transmitted from the retina to the sclera, the factors and mechanism(s) involved are poorly understood. Recent studies have highlighted the importance of the retinal secreted serine protease PRSS56 and transmembrane glycoprotein MFRP, a factor predominantly expressed in the retinal pigment epithelium (RPE), in ocular size determination. Mutations in PRSS56 and MFRP constitute a major cause of nanophthalmos, a condition characterized by severe reduction in ocular axial length/extreme hyperopia. Interestingly, common variants of these genes have been implicated in myopia, a condition associated with ocular elongation. Consistent with these findings, mice with loss of function mutation in PRSS56 or MFRP exhibit a reduction in ocular axial length. However, the molecular network and cellular processes involved in PRSS56- and MFRP-mediated ocular axial growth remain elusive. Here, we show that Adamts19 expression is significantly upregulated in the retina of mice lacking either Prss56 or Mfrp. Importantly, using genetic mouse models, we demonstrate that while ADAMTS19 is not required for ocular growth during normal development, its inactivation exacerbates ocular axial length reduction in Prss56 and Mfrp mutant mice. These results suggest that the upregulation of retinal Adamts19 is part of an adaptive molecular response to counteract impaired ocular growth. Using a complementary genetic approach, we show that loss of PRSS56 or MFRP function prevents excessive ocular axial growth in a mouse model of early-onset myopia caused by a null mutation in Irbp, thus, demonstrating that PRSS56 and MFRP are also required for pathological ocular elongation. Collectively, our findings provide new insights into the molecular network involved in ocular axial growth and support a role for molecular crosstalk between the retina and RPE involved in refractive development.

During ocular refractive development, the eye’s growth is modulated, such that the ocular axial length matches the optical power enabling the eyes to achieve optimal focus. Alterations in ocular growth mainly contribute to refractive errors. Mutations in human PRSS56 and MFRP are responsible for nanophthalmos that exhibit a severe reduction in ocular axial length, and high hyperopia. Importantly, mutant mouse models lacking either Müller glia expressed PRSS56, or retinal pigment epithelium (RPE) localized MFRP exhibit ocular axial length reduction. Here, we have identified Adamts19 as a factor whose levels were significantly upregulated in the retina of mice lacking either Prss56 or Mfrp. Importantly, utilizing Adamts19 knockout mice we demonstrate that upregulation of retinal Adamts19 expression constitutes a compensatory mechanism that provides partial protection against ocular axial reduction due to mutation in Prss56 and Mfrp. Next, utilizing a mouse model of early-onset myopia, we demonstrate that the mutant Irbp induced ocular axial elongation is completely dependent on Prss56 as well as Mfrp, suggesting an interplay between Müller glia and RPE in the regulation of ocular axial growth. Collectively, these findings suggest that ocular refractive development relies on complex interactions occurring between genetic factors in the retina and RPE.

Overview of how N32 and N34 elovanoids sustain sight by protecting retinal pigment epithelial cells and photoreceptors

The essential fatty acid DHA (22:6, omega-3 or n-3) is enriched in and required for the membrane biogenesis and function of photoreceptor cells (PRCs), synapses, mitochondria, etc. of the CNS. PRC DHA becomes an acyl chain at the sn-2 of phosphatidylcholine, amounting to more than 50% of the PRC outer segment phospholipids, where phototransduction takes place. Very long chain PUFAs (n-3, ≥ 28 carbons) are at the sn-1 of this phosphatidylcholine molecular species and interact with rhodopsin. PRC shed their tips (DHA-rich membrane disks) daily, which in turn are phagocytized by the retinal pigment epithelium (RPE), where DHA is recycled back to PRC inner segments to be used for the biogenesis of new photoreceptor membranes. Here, we review the structures and stereochemistry of novel elovanoid (ELV)-N32 and ELV-N34 to be ELV-N32: (14Z,17Z,20R,21E,23E,25Z,27S,29Z)-20,27-dihydroxydo-triaconta-14,17,21,23,25,29-hexaenoic acid; ELV-N34: (16Z,19Z,22R,23E,25E,27Z,29S,31Z)-22,29-dihydroxytetra-triaconta-16,19,23,25,27,31-hexaenoic acid. ELVs are low-abundance, high-potency, protective mediators. Their bioactivity includes enhancing of antiapoptotic and prosurvival protein expression with concomitant downregulation of proapoptotic proteins when RPE is confronted with uncompensated oxidative stress. ELVs also target PRC/RPE senescence gene programming, the senescence secretory phenotype in the interphotoreceptor matrix, as well as inflammaging (chronic, sterile, low-grade inflammation). An important lesson on neuroprotection is highlighted by the ELV mediators that target the terminally differentiated PRC and RPE, sustaining a beautifully synchronized renewal process. The role of ELVs in PRC and RPE viability and function uncovers insights on disease mechanisms and the development of therapeutics for age-related macular degeneration, Alzheimer's disease, and other pathologies.

Animal and cellular models of microphthalmia

Microphthalmia is a rare developmental eye disorder affecting 1 in 7000 births. It is defined as a small (axial length ⩾2 standard deviations below the age-adjusted mean) underdeveloped eye, caused by disruption of ocular development through genetic or environmental factors in the first trimester of pregnancy. Clinical phenotypic heterogeneity exists amongst patients with varying levels of severity, and associated ocular and systemic features. Up to 11% of blind children are reported to have microphthalmia, yet currently no treatments are available. By identifying the aetiology of microphthalmia and understanding how the mechanisms of eye development are disrupted, we can gain a better understanding of the pathogenesis. Animal models, mainly mouse, zebrafish and Xenopus, have provided extensive information on the genetic regulation of oculogenesis, and how perturbation of these pathways leads to microphthalmia. However, differences exist between species, hence cellular models, such as patient-derived induced pluripotent stem cell (iPSC) optic vesicles, are now being used to provide greater insights into the human disease process. Progress in 3D cellular modelling techniques has enhanced the ability of researchers to study interactions of different cell types during eye development. Through improved molecular knowledge of microphthalmia, preventative or postnatal therapies may be developed, together with establishing genotype-phenotype correlations in order to provide patients with the appropriate prognosis, multidisciplinary care and informed genetic counselling. This review summarises some key discoveries from animal and cellular models of microphthalmia and discusses how innovative new models can be used to further our understanding in the future.

Plain language summary

Animal and Cellular Models of the Eye Disorder, Microphthalmia (Small Eye) Microphthalmia, meaning a small, underdeveloped eye, is a rare disorder that children are born with. Genetic changes or variations in the environment during the first 3 months of pregnancy can disrupt early development of the eye, resulting in microphthalmia. Up to 11% of blind children have microphthalmia, yet currently no treatments are available. By understanding the genes necessary for eye development, we can determine how disruption by genetic changes or environmental factors can cause this condition. This helps us understand why microphthalmia occurs, and ensure patients are provided with the appropriate clinical care and genetic counselling advice. Additionally, by understanding the causes of microphthalmia, researchers can develop treatments to prevent or reduce the severity of this condition. Animal models, particularly mice, zebrafish and frogs, which can also develop small eyes due to the same genetic/environmental changes, have helped us understand the genes which are important for eye development and can cause birth eye defects when disrupted. Studying a patient's own cells grown in the laboratory can further help researchers understand how changes in genes affect their function. Both animal and cellular models can be used to develop and test new drugs, which could provide treatment options for patients living with microphthalmia. This review summarises the key discoveries from animal and cellular models of microphthalmia and discusses how innovative new models can be used to further our understanding in the future.

The majority of autosomal recessive nanophthalmos and posterior microphthalmia can be attributed to biallelic sequence and structural variants in MFRP and PRSS56

This study aimed to genetically and clinically characterize a unique cohort of 25 individuals from 21 unrelated families with autosomal recessive nanophthalmos (NNO) and posterior microphthalmia (MCOP) from different ethnicities. An ophthalmological assessment in all families was followed by targeted MFRP and PRSS56 testing in 20 families and whole-genome sequencing in one family. Three families underwent homozygosity mapping using SNP arrays. Eight distinct MFRP mutations were found in 10/21 families (47.6%), five of which are novel including a deletion spanning the 5' untranslated region and the first coding part of exon 1. Most cases harbored homozygous mutations (8/10), while a compound heterozygous and a monoallelic genotype were identified in the remaining ones (2/10). Six distinct PRSS56 mutations were found in 9/21 (42.9%) families, three of which are novel. Similarly, homozygous mutations were found in all but one, leaving 2/21 families (9.5%) without a molecular diagnosis. Clinically, all patients had reduced visual acuity, hyperopia, short axial length and crowded optic discs. Retinitis pigmentosa was observed in 5/10 (50%) of the MFRP group, papillomacular folds in 12/19 (63.2%) of MCOP and in 3/6 (50%) of NNO cases. A considerable phenotypic variability was observed, with no clear genotype-phenotype correlations. Overall, our study represents the largest NNO and MCOP cohort reported to date and provides a genetic diagnosis in 19/21 families (90.5%), including the first MFRP genomic rearrangement, offering opportunities for gene-based therapies in MFRP-associated disease. Finally, our study underscores the importance of sequence and copy number analysis of the MFRP and PRSS56 genes in MCOP and NNO.

Pxr- and Nrf2- mediated induction of ABC transporters by heavy metal ions in zebrafish embryos

Transcription factors including pregnane X receptor (Pxr) and nuclear factor-erythroid 2-related factor-2 (Nrf2) are important modulators of Adenosine triphosphate-binding cassette (ABC) transporters in mammalian cells. However, whether such modulation is conserved in zebrafish embryos remains largely unknown. In this manuscript, pxr- and nrf2-deficient models were constructed with CRISPR/Cas9 system, to evaluate the individual function of Pxr and Nrf2 in the regulation of ABC transporters and detoxification of heavy metal ions like Cd²⁺ and Ag⁺. As a result, both Cd²⁺ and Ag⁺ conferred extensive interactions with ABC transporters in wild type (WT) embryos: their accumulation and toxicity were affected by the activity of ABC transporters, and they significantly induced the mRNA expressions of ABC transporters. These induction effects were reduced by the mutation of pxr and nrf2, but elevations in the basal expression of ABC transporters compensated for the loss of their inducibility. This could be an explanation for remaining transporter function in both mutant models as well as the unaltered toxicity of metal ions in pxr-deficient embryos. However, mutation of nrf2 disrupted the production of glutathione (GSH), resulting in the enhanced toxicity of Cd²⁺/Ag⁺ in zebrafish embryos. In addition, elevated expressions of other transcription factors like aryl hydrocarbon receptor (ahr) 1b, peroxisome proliferator-activated receptor (ppar)-β, and nrf2 were found in pxr-deficient models without any treatment, while enhanced induction of ahr1b, ppar-β and pxr could only be seen in nrf2-deficient embryos after the treatment of metal ions, indicating different compensation phenomena for the absence of transcription factors. After all, pxr-deficient and nrf2-deficient zebrafish embryos are useful tools in the functional investigation of Pxr and Nrf2 in the early life stages of aquatic organisms. However, the compensatory mechanisms should be taken into consideration when interpreting the results and need in-depth investigations.

Membrane-type frizzled-related protein regulates lipidome and transcription for photoreceptor function

Molecular decision‐makers of photoreceptor (PRC) membrane organization and gene regulation are critical to understanding sight and retinal degenerations that lead to blindness. Using Mfrp^rd6mice, which develop PRC degeneration, we uncovered that membrane‐type frizzled‐related protein (MFRP) participates in docosahexaenoic acid (DHA, 22:6) enrichment in a manner similar to adiponectin receptor 1 (AdipoR1). Untargeted imaging mass spectrometry demonstrates cell‐specific reduction of phospholipids containing 22:6 and very long‐chain polyunsaturated fatty acids (VLC‐PUFAs) in Adipor1^−/−and Mfrp^rd6 retinas. Gene expression of pro‐inflammatory signaling pathways is increased and gene‐encoding proteins for PRC function decrease in both mutants. Thus, we propose that both proteins are necessary for retinal lipidome membrane organization, visual function, and to the understanding of the early pathology of retinal degenerative diseases.

Genetics of anophthalmia and microphthalmia. Part 1: Non-syndromic anophthalmia/microphthalmia

Eye formation is the result of coordinated induction and differentiation processes during embryogenesis. Disruption of any one of these events has the potential to cause ocular growth and structural defects, such as anophthalmia and microphthalmia (A/M). A/M can be isolated or occur with systemic anomalies, when they may form part of a recognizable syndrome. Their etiology includes genetic and environmental factors; several hundred genes involved in ocular development have been identified in humans or animal models. In humans, around 30 genes have been repeatedly implicated in A/M families, although many other genes have been described in single cases or families, and some genetic syndromes include eye anomalies occasionally as part of a wider phenotype. As a result of this broad genetic heterogeneity, with one or two notable exceptions, each gene explains only a small percentage of cases. Given the overlapping phenotypes, these genes can be most efficiently tested on panels or by whole exome/genome sequencing for the purposes of molecular diagnosis. However, despite whole exome/genome testing more than half of patients currently remain without a molecular diagnosis. The proportion of undiagnosed cases is even higher in those individuals with unilateral or milder phenotypes. Furthermore, even when a strong gene candidate is available for a patient, issues of incomplete penetrance and germinal mosaicism make diagnosis and genetic counseling challenging. In this review, we present the main genes implicated in non-syndromic human A/M phenotypes and, for practical purposes, classify them according to the most frequent or predominant phenotype each is associated with. Our intention is that this will allow clinicians to rank and prioritize their molecular analyses and interpretations according to the phenotypes of their patients.

Regeneration of the zebrafish retinal pigment epithelium after widespread genetic ablation

The retinal pigment epithelium (RPE) is a specialized monolayer of pigmented cells within the eye that is critical for maintaining visual system function. Diseases affecting the RPE have dire consequences for vision, and the most prevalent of these is atrophic (dry) age-related macular degeneration (AMD), which is thought to result from RPE dysfunction and degeneration. An intriguing possibility for treating RPE degenerative diseases like atrophic AMD is the stimulation of endogenous RPE regeneration; however, very little is known about the mechanisms driving successful RPE regeneration in vivo. Here, we developed a zebrafish transgenic model (rpe65a:nfsB-eGFP) that enabled ablation of large swathes of mature RPE. RPE ablation resulted in rapid RPE degeneration, as well as degeneration of Bruch’s membrane and underlying photoreceptors. Using this model, we demonstrate for the first time that zebrafish are capable of regenerating a functional RPE monolayer after RPE ablation. Regenerated RPE cells first appear at the periphery of the RPE, and regeneration proceeds in a peripheral-to-central fashion. RPE ablation elicits a robust proliferative response in the remaining RPE. Subsequently, proliferative cells move into the injury site and differentiate into RPE. BrdU incorporation assays demonstrate that the regenerated RPE is likely derived from remaining peripheral RPE cells. Pharmacological disruption using IWR-1, a Wnt signaling antagonist, significantly reduces cell proliferation in the RPE and impairs overall RPE recovery. These data demonstrate that the zebrafish RPE possesses a robust capacity for regeneration and highlight a potential mechanism through which endogenous RPE regenerate in vivo.

Diseases resulting in retinal pigment epithelium (RPE) degeneration are among the leading causes of blindness worldwide, and no therapy exists that can replace RPE or restore lost vision. One intriguing possibility is the development of therapies focused on stimulating endogenous RPE regeneration. For this to be possible, we must first gain a deeper understanding of the mechanisms underlying RPE regeneration. Here, we develop a transgenic zebrafish system through which we ablate large swathes of mature RPE and demonstrate that zebrafish regenerate RPE after widespread injury. Injury-adjacent RPE proliferate and regenerate RPE, suggesting that they are the source of regenerated tissue. Finally, we demonstrate that Wnt signaling may be involved in RPE regeneration. These findings establish a versatile in vivo model through which the molecular and cellular underpinnings of RPE regeneration can be further characterized.

Long-Term Effects of Gene Therapy in a Novel Mouse Model of Human MFRP-Associated Retinopathy

Patients harboring homozygous c.498_499insC mutations in MFRP demonstrate hyperopia, microphthalmia, retinitis pigmentosa, retinal pigment epithelial atrophy, variable degrees of foveal edema, and optic disc drusen. The disease phenotype is variable, however, with some patients maintaining good central vision and cone function till late in the disease. A knock-in mouse model with the c.498_499insC mutation in Mfrp (Mfrp KI/KI) was developed to understand the effects of these mutations in the retina. The model shares many of the features of human clinical disease, including reduced axial length, hyperopia, retinal degeneration, retinal pigment epithelial atrophy, and decreased electrophysiological responses. In addition, the eyes of these mice had a significantly greater refractive error (p < 0.01) when compared to age-matched wild-type control animals. Administration of recombinant adeno-associated virus-mediated Mfrp gene therapy significantly prevented thinning from retinal neurodegeneration (p < 0.005) and preserved retinal electrophysiology (p < 0.001) when treated eyes were compared to contralateral sham-treated control eyes. The Mfrp KI/KI mice will serve as a useful tool to model human disease and point to a potential gene therapeutic approach for patients with preserved vision and electrophysiological responses in MFRP-related retinopathy.

CRISPR GENOME SURGERY IN THE RETINA IN LIGHT OF OFF-TARGETING

PURPOSE

Recent concerns regarding the clinical utilization of clustered regularly interspaced short palindromic repeats (CRISPR) involve uncertainties about the potential detrimental effects that many arise due to unintended genetic changes, as in off-target mutagenesis, during CRISPR genome surgery. This review gives an overview of off-targeting detection methods and CRISPR's place in the clinical setting, specifically in the field of ophthalmology.

RESULTS

As CRISPR utilization in the laboratory setting has increased, knowledge regarding CRISPR mechanisms including its off-target effects has also increased. Although a perfect method for achieving 100% specificity is yet to be determined, the past few years have seen many developments in off-targeting detection and in increasing efficacy of CRISPR tools.

CONCLUSION

The CRISPR system has high potential to be an invaluable therapeutic tool as it has the ability to modify and repair pathogenic retinal lesions. Although it is not yet a perfect system, with further efforts to improve its specificity and efficacy along with careful screening of off-target mutations, CRISPR-mediated genome surgery potential can become maximized and applied to patients.

CRISPR/Cas9 disease models in zebrafish and Xenopus: The genetic renaissance of fish and frogs

The speed by which clinical genomics is currently identifying novel potentially pathogenic variants is outperforming the speed by which these can be functionally (genotype-phenotype) annotated in animal disease models. However, over the past few years the emergence of CRISPR/Cas9 as a straight-forward genome editing technology has revolutionized disease modeling in vertebrate non-mammalian model organisms such as zebrafish, medaka and Xenopus. It is now finally possible, by CRISPR/Cas9, to rapidly establish clinically relevant disease models in these organisms. Interestingly, these can provide both cost-effective genotype-phenotype correlations for gene-(variants) and genomic rearrangements obtained from clinical practice, as well as be exploited to perform translational research to improve prospects of disease afflicted patients. In this review, we show an extensive overview of these new CRISPR/Cas9-mediated disease models and provide future prospects that will allow increasingly accurate modeling of human disease in zebrafish, medaka and Xenopus.

Versatile Genome Engineering Techniques Advance Human Ocular Disease Researches in Zebrafish

Over recent decades, zebrafish has been established as a sophisticated vertebrate model for studying human ocular diseases due to its high fecundity, short generation time and genetic tractability. With the invention of morpholino (MO) technology, it became possible to study the genetic basis and relevant genes of ocular diseases in vivo. Many genes have been shown to be related to ocular diseases. However, the issue of specificity is the major concern in defining gene functions with MO technology. The emergence of the first- and second-generation genetic modification tools zinc-finger nucleases (ZFNs) and TAL effector nucleases (TALENs), respectively, eliminated the potential phenotypic risk induced by MOs. Nevertheless, the efficiency of these nucleases remained relatively low until the third technique, the clustered regularly interspersed short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) system, was discovered. This review highlights the application of multiple genome engineering techniques, especially the CRISPR/Cas9 system, in the study of human ocular diseases in zebrafish.

Translation of CRISPR Genome Surgery to the Bedside for Retinal Diseases

In recent years, there has been accelerated growth of clustered regularly interspaced short palindromic repeats (CRISPR) genome surgery techniques. Genome surgery holds promise for diseases for which a cure currently does not exist. In the field of ophthalmology, CRISPR offers possibilities for treating inherited retinal dystrophies. The retina has little regenerative potential, which makes treatment particularly difficult. For such conditions, CRISPR genome surgery methods have shown great potential for therapeutic applications in animal models of retinal dystrophies. Much anticipation surrounds the potential for CRISPR as a therapeutic, as clinical trials of ophthalmic genome surgery are expected to begin as early as 2018. This mini-review summarizes preclinical CRISPR applications in the retina and current CRISPR clinical trials.

Nanophthalmos: A Review of the Clinical Spectrum and Genetics

Nanophthalmos is a clinical spectrum of disorders with a phenotypically small but structurally normal eye. These disorders present significant clinical challenges to ophthalmologists due to a high rate of secondary angle-closure glaucoma, spontaneous choroidal effusions, and perioperative complications with cataract and retinal surgeries. Nanophthalmos may present as a sporadic or familial disorder, with autosomal-dominant or recessive inheritance. To date, five genes (i.e., MFRP, TMEM98, PRSS56, BEST1, and CRB1) and two loci have been implicated in familial forms of nanophthalmos. Here, we review the definition of nanophthalmos, the clinical and pathogenic features of the condition, and the genetics of this disorder.

Molecular genetics and targeted therapy of WNT-related human diseases (Review)

Canonical WNT signaling through Frizzled and LRP5/6 receptors is transduced to the WNT/β-catenin and WNT/stabilization of proteins (STOP) signaling cascades to regulate cell fate and proliferation, whereas non-canonical WNT signaling through Frizzled or ROR receptors is transduced to the WNT/planar cell polarity (PCP), WNT/G protein-coupled receptor (GPCR) and WNT/receptor tyrosine kinase (RTK) signaling cascades to regulate cytoskeletal dynamics and directional cell movement. WNT/β-catenin signaling cascade crosstalks with RTK/SRK and GPCR-cAMP-PKA signaling cascades to regulate β-catenin phosphorylation and β-catenin-dependent transcription. Germline mutations in WNT signaling molecules cause hereditary colorectal cancer, bone diseases, exudative vitreoretinopathy, intellectual disability syndrome and PCP-related diseases. APC or CTNNB1 mutations in colorectal, endometrial and prostate cancers activate the WNT/β-catenin signaling cascade. RNF43, ZNRF3, RSPO2 or RSPO3 alterations in breast, colorectal, gastric, pancreatic and other cancers activate the WNT/β-catenin, WNT/STOP and other WNT signaling cascades. ROR1 upregulation in B-cell leukemia and solid tumors and ROR2 upregulation in melanoma induce invasion, metastasis and therapeutic resistance through Rho-ROCK, Rac-JNK, PI3K-AKT and YAP signaling activation. WNT signaling in cancer, stromal and immune cells dynamically orchestrate immune evasion and antitumor immunity in a cell context-dependent manner. Porcupine (PORCN), RSPO3, WNT2B, FZD5, FZD10, ROR1, tankyrase and β-catenin are targets of anti-WNT signaling therapy, and ETC-159, LGK974, OMP-18R5 (vantictumab), OMP-54F28 (ipafricept), OMP-131R10 (rosmantuzumab), PRI-724 and UC-961 (cirmtuzumab) are in clinical trials for cancer patients. Different classes of anti-WNT signaling therapeutics are necessary for the treatment of APC/CTNNB1-, RNF43/ZNRF3/RSPO2/RSPO3- and ROR1-types of human cancers. By contrast, Dickkopf-related protein 1 (DKK1), SOST and glycogen synthase kinase 3β (GSK3β) are targets of pro-WNT signaling therapy, and anti-DKK1 (BHQ880 and DKN-01) and anti-SOST (blosozumab, BPS804 and romosozumab) monoclonal antibodies are being tested in clinical trials for cancer patients and osteoporotic post-menopausal women. WNT-targeting therapeutics have also been applied as reagents for in vitro stem-cell processing in the field of regenerative medicine.

CRISPR-Cas Genome Surgery in Ophthalmology

Genetic disease affecting vision can significantly impact patient quality of life. Gene therapy seeks to slow the progression of these diseases by treating the underlying etiology at the level of the genome. Clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated systems (Cas) represent powerful tools for studying diseases through the creation of model organisms generated by targeted modification and by the correction of disease mutations for therapeutic purposes. CRISPR-Cas systems have been applied successfully to the visual sciences and study of ophthalmic disease - from the modification of zebrafish and mammalian models of eye development and disease, to the correction of pathogenic mutations in patient-derived stem cells. Recent advances in CRISPR-Cas delivery and optimization boast improved functionality that continues to enhance genome-engineering applications in the eye. This review provides a synopsis of the recent implementations of CRISPR-Cas tools in the field of ophthalmology.

The use of mrp1-deficient (Danio rerio) zebrafish embryos to investigate the role of Mrp1 in the toxicity of cadmium chloride and benzo[a]pyrene

Previous studies in our lab have revealed that both P-glycoprotein (Pgp) and multi-resistance associated protein (Mrp) 1 played important roles in the detoxification of heavy metals and polycyclic aromatic hydrocarbon (PAH) in zebrafish embryos. This paper aims to extend this research by using mrp1-deficient model to illustrate the individual function of Mrp1. In this respect, CRISPR/Cas9 system was employed to generate a frameshift mutation in zebrafish mrp1 causing premature translational stops in Mrp1. Significant reduction on the efflux function of Mrps was found in mutant zebrafish embryos, which correlated well with the significantly enhanced accumulation and toxicity of cadmium chloride (CdCl₂) and benzo[a]pyrene (BαP), indicating the protective role of the corresponding protein. The different alteration on the accumulation and toxicity of Cd²⁺ and BαP could be attributed to the fact that Cd²⁺ and its metabolites were mainly excreted by Mrp1, while BαP was primarily pumped out by Pgp. More importantly, the compensation mechanism for the absence of Mrp1, including elevated glutathione (GSH) level and up-regulated expression of pgp and mrp2 were also found. Thus, mrp1-deficient zebrafish embryo could be a useful tool in the investigation of Mrp1 functions in the early life stages of aquatic organisms. However, compensation mechanism should be taken into consideration in the interpretation of results obtained with mrp1-deficient fish.

Genome Editing to Study Ca2+ Homeostasis in Zebrafish Cone Photoreceptors

Photoreceptors are specialized sensory neurons with unique biological features. Phototransduction is well understood due in part to the exclusive expression and function of the molecular components of this cascade. Many other processes are less well understood, but also extremely important for understanding photoreceptor function and for treating disease. One example is the role of Ca²⁺ in the cell body and overall compartmentalization and regulation of Ca²⁺ within the cell. The recent development of CRISPR/Cas9 genome editing techniques has made it possible to rapidly and cheaply alter specific genes. This will help to define the biological function of elusive processes that have been more challenging to study. CRISPR/Cas9 has been optimized in many systems including zebrafish, which already has some distinct advantages for studying photoreceptor biology and function. These new genome editing technologies and the continued use of the zebrafish model system will help advance our understanding of important understudied aspects of photoreceptor biology.