Corneal dystrophies and genetics in the International Committee for Classification of Corneal Dystrophies era: a review

Methylation in cornea and corneal diseases: a systematic review

Corneal diseases are among the primary causes of blindness and vision loss worldwide. However, the pathogenesis of corneal diseases remains elusive, and diagnostic and therapeutic tools are limited. Thus, identifying new targets for the diagnosis and treatment of corneal diseases has gained great interest. Methylation, a type of epigenetic modification, modulates various cellular processes at both nucleic acid and protein levels. Growing evidence shows that methylation is a key regulator in the pathogenesis of corneal diseases, including inflammation, fibrosis, and neovascularization, making it an attractive potential therapeutic target. In this review, we discuss the major alterations of methylation and demethylation at the DNA, RNA, and protein levels in corneal diseases and how these dynamics contribute to the pathogenesis of corneal diseases. Also, we provide insights into identifying potential biomarkers of methylation that may improve the diagnosis and treatment of corneal diseases.

Corneal epithelial development and homeostasis

The corneal epithelium (CE), the most anterior cellular structure of the eye, is a self-renewing stratified squamous tissue that protects the rest of the eye from external elements. Each cell in this exquisite three-dimensional structure needs to have proper polarity and positional awareness for the CE to serve as a transparent, refractive, and protective tissue. Recent studies have begun to elucidate the molecular and cellular events involved in the embryonic development, post-natal maturation, and homeostasis of the CE, and how they are regulated by a well-coordinated network of transcription factors. This review summarizes the status of related knowledge and aims to provide insight into the pathophysiology of disorders caused by disruption of CE development, and/or homeostasis.

Maltese Allelic Variants in Corneal Dystrophy Genes in a Worldwide Setting

INTRODUCTION

This study aimed to establish which worldwide population cohorts have a genetic make-up closest to that of a large sample of the Maltese population with regard to corneal dystrophy (CD) genes.

METHODS

Single nucleotide polymorphisms (SNPs) in the Maltese cohort were compared with worldwide cohorts. Fixation index (F_ST) values were calculated to evaluate population differentiation. The genetic prevalence of CD subtypes in worldwide and Maltese cohorts were calculated, and single nucleotide missense mutations present in the Maltese cohort were evaluated for potential pathogenicity.

RESULTS

F_ST values showed that CD-related genes differ substantially among the studied cohorts. F_ST values for each SNP showed greatest differentiation between the Maltese and African cohorts and least differentiation with the Puerto Rican, Mexican, and Colombian cohorts. One TGFBI casual CD mutation, 502V, which causes a Bowman's layer CD/atypical Thiel-Behnke CD was identified in the Maltese cohort. The KRT3 NC_000012.11:g.53186088G>C mutation was potentially deleterious.

CONCLUSION

Identifying populations with least genetic differentiation can facilitate and help guide future diagnostic and treatment strategies for Maltese individuals with CDs in the absence of comparable Maltese data. Analysing the previously unknown CD genetic pool present in a large Maltese cohort adds to the global genetic bank that researchers rely on for medical progress.

Ocular Involvement in Hereditary Amyloidosis

The term amyloidosis describes a group of rare diseases caused by protein conformation abnormalities resulting in extracellular deposition and accumulation of insoluble fibrillar aggregates. So far, 36 amyloid precursor proteins have been identified, and each one is responsible for a specific disease entity. Transthyretin amyloidosis (ATTRv) is one of the most common forms of systemic and ocular amyloidosis, due to the deposition of transthyretin (TTR), which is a transport protein mainly synthesized in the liver but also in the retinal pigment epithelial cells. ATTRv amyloidosis may be misdiagnosed with several other conditions, resulting in a significant diagnostic delay. Gelsolin and keratoepithelin are other proteins that, when mutated, are responsible for a systemic amyloid disease with significant ocular manifestations that not infrequently appear before systemic involvement. The main signs of ocular amyloid deposition are in the cornea, irido-corneal angle and vitreous, causing complications related to vasculopathy and neuropathy at the local level. This review aims at describing the main biochemical, histopathological and clinical features of systemic amyloidosis associated with eye involvement, with particular emphasis on the inherited forms. We discuss currently available treatments, focusing on ocular involvement and specific ophthalmologic management and highlighting the importance of a prompt treatment for the potential sight-threatening complications derived from amyloid deposition in ocular tissues.

Topical use of alcohol in ophthalmology - Diagnostic and therapeutic indications

Alcohol (ethanol) has been used in medicine since time immemorial. In ophthalmic practice, besides as an antiseptic, it was given as retrobulbar injections to relieve severe ocular pain. Alcohol can be applied topically to the surface of neoplastic or suspicious lesions to kill cells that might desquamate and seed during surgical excision, to treat epithelial ingrowth that can occur following corneal surgeries, particularly laser in situ keratomileusis (LASIK), and to treat superficial infectious keratitis. In view of its ability to achieve a smooth cleavage plane between the epithelium and the Bowman's layer, alcohol-assisted delamination (ALD) of the corneal epithelium has been used widely and effectively for a variety of diagnostic and therapeutic indications, at times delivering both outcomes. Diagnostically, ALD yields an intact epithelial sheet which can be fixed flat to provide excellent orientation for histopathological evaluation. Therapeutically, it is most commonly used to treat recurrent corneal erosion syndrome, where its efficacy is comparable to that of phototherapeutic keratectomy but with several advantages. It has also been used to treat various forms of epithelial/anterior stromal dystrophies, which can obviate or delay the need for corneal transplantation for several years. In addition, ALD is performed in corneal collagen cross-linking and corneal refractive surgery for relatively atraumatic removal of the epithelium. In this review, we aimed to provide a comprehensive overview of the diagnostic and therapeutic use of topical alcohol in ophthalmology, to describe the surgical and fixation techniques of ALD, and to highlight our experience in ALD over the past decade.

Molecular Mechanisms of Fuchs and Congenital Hereditary Endothelial Corneal Dystrophies

The cornea, the eye's outermost layer, protects the eye from the environment. The cornea's innermost layer is an endothelium separating the stromal layer from the aqueous humor. A central role of the endothelium is to maintain stromal hydration state. Defects in maintaining this hydration can impair corneal clarity and thus visual acuity. Two endothelial corneal dystrophies, Fuchs Endothelial Corneal Dystrophy (FECD) and Congenital Hereditary Endothelial Dystrophy (CHED), are blinding corneal diseases with varied clinical presentation in patients across different age demographics. Recessive CHED with an early onset (typically age: 0-3 years) and dominantly inherited FECD with a late onset (age: 40-50 years) have similar phenotypes, although caused by defects in several different genes. A range of molecular mechanisms have been proposed to explain FECD and CHED pathology given the involvement of multiple causative genes. This critical review provides insight into the proposed molecular mechanisms underlying FECD and CHED pathology along with common pathways that may explain the link between the defective gene products and provide a new perspective to view these genetic blinding diseases.

Novel Mutations Associated With Various Types of Corneal Dystrophies in a Han Chinese Population

Aims: To study the genetic spectra of corneal dystrophies (CDs) in Han Chinese patients using next-generation sequencing (NGS).

Methods: NGS-based targeted region sequencing was performed to evaluate 71 CD patients of Han Chinese ethnicity. A custom-made capture panel was designed to capture all coding exons and untranslated regions plus 25 bp of intronic flanking sequences of 801 candidate genes for eye diseases. The Genome Analysis Tool Kit Best Practices pipeline and an intensive computational prediction pipeline were applied for the analysis of pathogenic variants.

Results: We achieved a mutation detection rate of 59.2% by NGS. Eighteen known mutations in CD-related genes were found in 42 out of 71 patients, and these cases showed a genotype–phenotype correlation consistent with previous reports. Nine novel variants that were likely pathogenic were found in various genes, including CHST6, TGFBI, SLC4A11, AGBL1, and COL17A1. These variants were all predicted to be protein-damaging by an intensive computational analysis.

Conclusions: This study expands the spectra of genetic mutations associated with various types of CDs in the Chinese population and highlights the clinical utility of targeted NGS for genetically heterogeneous CD.

A Cell Culture Approach to Optimized Human Corneal Endothelial Cell Function

Purpose

Cell-based therapies to replace corneal endothelium depend on culture methods to optimize human corneal endothelial cell (HCEC) function and minimize endothelial-mesenchymal transition (EnMT). Here we explore contribution of low-mitogenic media on stabilization of phenotypes in vitro that mimic those of HCECs in vivo.

Methods

HCECs were isolated from cadaveric donor corneas and expanded in vitro, comparing continuous presence of exogenous growth factors (“proliferative media”) to media without those factors (“stabilizing media”). Identity based on canonical morphology and expression of surface marker CD56, and function based on formation of tight junction barriers measured by trans-endothelial electrical resistance assays (TEER) were assessed.

Results

Primary HCECs cultured in proliferative media underwent EnMT after three to four passages, becoming increasingly fibroblastic. Stabilizing the cells before each passage by switching them to a media low in mitogenic growth factors and serum preserved canonical morphology and yielded a higher number of cells. HCECs cultured in stabilizing media increased both expression of the identity marker CD56 and also tight junction monolayer integrity compared to cells cultured without stabilization.

Conclusions

HCECs isolated from donor corneas and expanded in vitro with a low-mitogenic media stabilizing step before each passage demonstrate more canonical structural and functional features and defer EnMT, increasing the number of passages and total canonical cell yield. This approach may facilitate development of HCEC-based cell therapies.

Novel Identity and Functional Markers for Human Corneal Endothelial Cells

Purpose

Human corneal endothelial cell (HCEC) density decreases with age, surgical complications, or disease, leading to vision impairment. Such endothelial dysfunction is an indication for corneal transplantation, although there is a worldwide shortage of transplant-grade tissue. To overcome the current poor donor availability, here we isolate, expand, and characterize HCECs in vitro as a step toward cell therapy.

Methods

Human corneal endothelial cells were isolated from cadaveric corneas and expanded in vitro. Cell identity was evaluated based on morphology and immunocytochemistry, and gene expression analysis and flow cytometry were used to identify novel HCEC-specific markers. The functional ability of HCEC to form barriers was assessed by transendothelial electrical resistance (TEER) assays.

Results

Cultured HCECs demonstrated canonical morphology for up to four passages and later underwent endothelial-to-mesenchymal transition (EnMT). Quality of donor tissue influenced cell measures in culture including proliferation rate. Cultured HCECs expressed identity markers, and microarray analysis revealed novel endothelial-specific markers that were validated by flow cytometry. Finally, canonical HCECs expressed higher levels of CD56, which correlated with higher TEER than fibroblastic HCECs.

Conclusions

In vitro expansion of HCECs from cadaveric donor corneas yields functional cells identifiable by morphology and a panel of novel markers. Markers described correlated with function in culture, suggesting a basis for cell therapy for corneal endothelial dysfunction.

A COL17A1 Splice-Altering Mutation Is Prevalent in Inherited Recurrent Corneal Erosions

PURPOSE

Corneal dystrophies are a genetically heterogeneous group of disorders. We previously described a family with an autosomal dominant epithelial recurrent erosion dystrophy (ERED). We aimed to identify the underlying genetic cause of ERED in this family and 3 additional ERED families. We sought to characterize the potential function of the candidate genes using the human and zebrafish cornea.

DESIGN

Case series study of 4 white families with a similar ERED. An experimental study was performed on human and zebrafish tissue to examine the putative biological function of candidate genes.

PARTICIPANTS

Four ERED families, including 28 affected and 17 unaffected individuals.

METHODS

HumanLinkage-12 arrays (Illumina, San Diego, CA) were used to genotype 17 family members. Next-generation exome sequencing was performed on an uncle-niece pair. Segregation of potential causative mutations was confirmed using Sanger sequencing. Protein expression was determined using immunohistochemistry in human and zebrafish cornea. Gene expression in zebrafish was assessed using whole-mount in situ hybridization. Morpholino-induced transient gene knockdown was performed in zebrafish embryos.

MAIN OUTCOME MEASURES

Linkage microarray, exome analysis, DNA sequence analysis, immunohistochemistry, in situ hybridization, and morpholino-induced genetic knockdown results.

RESULTS

Linkage microarray analysis identified a candidate region on chromosome chr10:12,576,562-112,763,135, and exploration of exome sequencing data identified 8 putative pathogenic variants in this linkage region. Two variants segregated in 06NZ-TRB1 with ERED: COL17A1 c.3156C→T and DNAJC9 c.334G→A. The COL17A1 c.3156C→T variant segregated in all 4 ERED families. We showed biologically relevant expression of these proteins in human cornea. Both proteins are expressed in the cornea of zebrafish embryos and adults. Zebrafish lacking Col17a1a and Dnajc9 during development show no gross corneal phenotype.

CONCLUSIONS

The COL17A1 c.3156C→T variant is the likely causative mutation in our recurrent corneal erosion families, and its presence in 4 independent families suggests that it is prevalent in ERED. This same COL17A1 c.3156C→T variant recently was identified in a separate pedigree with ERED. Our study expands the phenotypic spectrum of COL17A1 disease from autosomal recessive epidermolysis bullosa to autosomal dominant ERED and identifies COL17A1 as a key protein in maintaining integrity of the corneal epithelium.

Mutations in collagen, type XVII, alpha 1 (COL17A1) cause epithelial recurrent erosion dystrophy (ERED)

Corneal dystrophies are a clinically and genetically heterogeneous group of inherited disorders that bilaterally affect corneal transparency. They are defined according to the corneal layer affected and by their genetic cause. In this study, we identified a dominantly inherited epithelial recurrent erosion dystrophy (ERED)-like disease that is common in northern Sweden. Whole-exome sequencing resulted in the identification of a novel mutation, c.2816C>T, p.T939I, in the COL17A1 gene, which encodes collagen type XVII alpha 1. The variant segregated with disease in a genealogically expanded pedigree dating back 200 years. We also investigated a unique COL17A1 synonymous variant, c.3156C>T, identified in a previously reported unrelated dominant ERED-like family linked to a locus on chromosome 10q23-q24 encompassing COL17A1. We show that this variant introduces a cryptic donor site resulting in aberrant pre-mRNA splicing and is highly likely to be pathogenic. Bi-allelic COL17A1 mutations have previously been associated with a recessive skin disorder, junctional epidermolysis bullosa, with recurrent corneal erosions being reported in some cases. Our findings implicate presumed gain-of-function COL17A1 mutations causing dominantly inherited ERED and improve understanding of the underlying pathology.

First identification of a triple corneal dystrophy association: keratoconus, epithelial basement membrane corneal dystrophy and fuchs' endothelial corneal dystrophy

Purpose

To report the observation of a triple corneal dystrophy association consisting of keratoconus (KC), epithelial basement membrane corneal dystrophy (EBMCD) and Fuchs’ endothelial corneal dystrophy (FECD).

Methods

A 55-year-old male patient was referred to our cornea service for blurred vision and recurrent foreign body sensation. He reported bilateral recurrent corneal erosions with diurnal visual fluctuations. He underwent corneal biomicroscopy, Scheimpflug tomography, in vivo HRT confocal laser scanning microscopy and genetic testing for TGFBI and ZEB1 mutations using direct DNA sequencing.

Results

Biomicroscopic examination revealed the presence of subepithelial central and paracentral corneal opacities. The endothelium showed a bilateral flecked appearance, and the posterior corneal curvature suggested a possible concomitant ectatic disorder. Corneal tomography confirmed the presence of a stage II KC in both eyes. In vivo confocal laser scanning microscopy revealed a concomitant bilateral EBMCD with hyperreflective deposits in basal epithelial cells, subbasal Bowman's layer microfolds and ridges with truncated subbasal nerves as pseudodendritic elements. Stromal analysis revealed honeycomb edematous areas, and the endothelium showed a strawberry surface configuration typical of FECD. The genetic analysis resulted negative for TGFBI mutations and positive for a heterozygous mutation in exon 7 of the gene ZEB1.

Conclusion

This is the first case reported in the literature in which KC, EBMCD and FECD are present in the same patient and associated with ZEB1 gene mutation. The triple association was previously established by means of morphological analysis of the cornea using corneal Scheimpflug tomography and in vivo HRT II confocal laser scanning microscopy.

Regenerative Cell Therapy for Corneal Endothelium

Endothelial cell dysfunction as in Fuchs dystrophy or pseudophakic bullous keratopathy, and the limited regenerative capacity of human corneal endothelial cells (HCECs), drive the need for corneal transplant. In response to limited donor corneal availability, significant effort has been directed towards cell therapy as an alternative to surgery. Stimulation of endogenous progenitors, or transplant of stem cell-derived HCECs or in vitro-expanded, donor-derived HCECs could replace traditional surgery with regenerative therapy. Ex vivo expansion of HCECs is technically challenging, and the basis for molecular identification of functional HCECs is not established. Delivery of cells to the inner layer of the human cornea is another challenge: different techniques, from simple injection to artificial corneal scaffolds, are being investigated. Despite remaining questions, corneal endothelial cell therapies, translated to the clinic, represent the future for the treatment of corneal endotheliopathies.