Confirmation of the OVOL2 Promoter Mutation c.-307T>C in Posterior Polymorphous Corneal Dystrophy 1

Ovol2 promoter mutations in mice and human illuminate species-specific phenotypic divergence

Pathogenic variants in the highly conserved OVOL2 promoter region cause posterior polymorphous corneal dystrophy (PPCD) 1 by inducing an ectopic expression of the endothelial OVOL2 mRNA. Here we produced an allelic series of Ovol2 promoter mutations in the mouse model including the heterozygous c.-307T>C variant (RefSeq NM_021220.4) causing PPCD1 in humans. Despite the high evolutionary conservation of the Ovol2 promoter, only some alterations of its sequence had phenotypic consequences in mice. Four independent sequence variants in the distal part of the Ovol2 promoter had no significant effect on endothelial Ovol2 mRNA level or caused any ocular phenotype. In contrast, the mutation c.-307T>C resulted in increased Ovol2 expression in the corneal endothelium. However, only a small fraction of adult mice c.-307T>C heterozygotes developed ocular phenotypes such as irido-corneal adhesions, and corneal opacity. Interestingly, phenotypic penetrance was increased at embryonic stages. Notably, c.-307T>C mutation is located next to the Ovol1/Ovol2 transcription factor binding site. Mice carrying an allele with a deletion encompassing the Ovol2 binding site c.-307_-320del showed significant Ovol2 gene upregulation in the cornea endothelium and exhibited phenotypes similar to the c.-307T>C mutation. In conclusion, although the mutations c.-307T>C and -307_-320del lead to a comparably strong increase in endothelial Ovol2 expression as seen in PPCD1 patients, endothelial dystrophy was not observed in the mouse model, implicating species-specific differences in endothelial cell biology. Nonetheless, the emergence of dominant ocular phenotypes associated with Ovol2 promoter variants in mice implies a potential role of this gene in eye development and disease.

OVOL2: an epithelial lineage determiner with emerging roles in energy homeostasis

Ovo like zinc finger 2 (OVOL2) is an evolutionarily conserved regulator of epithelial lineage determination and differentiation during embryogenesis. OVOL2 binds to DNA using zinc-finger domains to suppress epithelial-mesenchymal transition (EMT), which is critical for tumor metastasis. However, recent studies have suggested some noncanonical roles of OVOL2 that do not rely on the DNA binding of zinc-finger domains or regulation of EMT. OVOL2 and EMT regulators have emerging roles in adipogenesis, thermogenesis, and lipid metabolism. Here, we review different roles of OVOL2 from embryo development to adult tissue homeostasis, and discuss how OVOL2 and other EMT regulators orchestrate a regulatory network to control energy homeostasis. Last, we propose potential applications of targeting OVOL2 to reduce human obesity.

Systematic review of SLC4A11, ZEB1, LOXHD1, and AGBL1 variants in the development of Fuchs' endothelial corneal dystrophy

Introduction

The pathogenic role of variants in TCF4 and COL8A2 in causing Fuchs' endothelial corneal dystrophy (FECD) is not controversial and has been confirmed by numerous studies. The causal role of other genes, SLC4A11, ZEB1, LOXHD1, and AGBL1, which have been reported to be associated with FECD, is more complicated and less obvious. We performed a systematic review of the variants in the above-mentioned genes in FECD cases, taking into account the currently available population frequency information, transcriptomic data, and the results of functional studies to assess their pathogenicity.

Methods

Search for articles published in 2005-2022 was performed manually between July 2022 and February 2023. We searched for original research articles in peer-reviewed journals, written in English. Variants in the genes of interest identified in patients with FECD were extracted for the analysis. We classified each presented variant by pathogenicity status according to the ACMG criteria implemented in the Varsome tool. Diagnosis, segregation data, presence of affected relatives, functional analysis results, and gene expression in the corneal endothelium were taken into account. Data on the expression of genes of interest in the corneal endothelium were extracted from articles in which transcriptome analysis was performed. The identification of at least one variant in a gene classified as pathogenic or significantly associated with FECD was required to confirm the causal role of the gene in FECD.

Results

The analysis included 34 articles with 102 unique ZEB1 variants, 20 articles with 64 SLC4A11 variants, six articles with 26 LOXHD1 variants, and five articles with four AGBL1 variants. Pathogenic status was confirmed for seven SLC4A11 variants found in FECD. No variants in ZEB1, LOXHD1, and AGBL1 genes were classified as pathogenic for FECD. According to the transcriptome data, AGBL1 and LOXHD1 were not expressed in the corneal endothelium. Functional evidence for the association of LOXHD1, and AGBL1 with FECD was conflicting.

Conclusion

Our analysis confirmed the causal role of SLC4A11 variants in the development of FECD. The causal role of ZEB1, LOXHD1, and AGBL1 variants in FECD has not been confirmed. Further evidence from familial cases and functional analysis is needed to confirm their causal roles in FECD.

New Endothelial Corneal Dystrophy in a Chinese Family

PURPOSE

The aim of this study was to characterize the clinical presentation of atypical endothelial corneal dystrophy (ECD) and to identify possible associated genetic variants in a Chinese family.

METHODS

Six affected members, 4 unaffected first-degree relatives, and 3 spouses who were enrolled in this study underwent ophthalmic examinations. Genetic linkage analysis was performed for 4 affected and 2 unaffected members, and whole-exome sequencing (WES) was performed for 2 patients to identify disease-causing variants. Candidate causal variants were verified using Sanger sequencing in family members and 200 healthy controls.

RESULTS

The mean age at disease onset was 16.5 years. The early phenotype of this atypical ECD was characterized by multiple small white translucent spots located in Descemet membrane of the peripheral cornea. These spots coalesced to form opacities with variable shapes, and eventually merged along the limbus. Subsequently, translucent spots appeared in central Descemet membrane and accumulated, causing diffuse polymorphous opacities over time. Finally, significant endothelial decompensation led to diffuse corneal edema. A heterozygous missense variant in the KIAA1522 gene (c.1331G>A; p.R444Q) was identified by WES, which was present in all 6 patients but was absent in the unaffected members and healthy controls.

CONCLUSIONS

The clinical features of atypical ECD are unique compared with those of known corneal dystrophies. Moreover, genetic analysis identified the c.1331G>A variant in KIAA1522 , which may be responsible for the pathogenesis of this atypical ECD. Thus, we propose this is a new form of ECD based on our clinical findings.

Update on the genetics of corneal endothelial dystrophies

Corneal endothelial dystrophies are a heterogeneous group of diseases with different modes of inheritance and genetic basis for each dystrophy. The genes associated with these diseases encode transcription factors, structural components of the stroma and Descemet membrane, cell transport proteins, and others. Congenital hereditary endothelial dystrophy (CHED) is associated with mutations in two genes, OVOL2 and SLC4A11, for dominant and recessive forms of CHED, respectively. Mutations in three genes are known to cause posterior polymorphous corneal dystrophy (PPCD). They are OVOL2 (PPCD1), ZEB1 (PPCD3), and GRHL1 (PPCD4). The PPCD2 locus involving the collagen gene COL8A2 on chromosome 1 is disputed due to insufficient evidence. Mutations in the COL8A2 gene are associated with early-onset Fuchs’ endothelial corneal dystrophy (FECD). Several genes have been associated with the more common, late-onset FECD. Alterations in each of these genes occur in a fraction of patients, and the most prevalent genetic alteration in FECD patients across the world is a triplet repeat expansion in the TCF4 gene. Knowledge of the genetics of corneal endothelial dystrophies has considerably advanced within the last decade and has contributed to better diagnosis of these dystrophies as well as opened up the possibility of novel therapeutic approaches based on the molecular mechanisms involved. The functions of genes identified to date provide insights into the pathogenic mechanisms involved in each disorder.

c.-61G>A in OVOL2 is a Pathogenic 5' Untranslated Region Variant Causing Posterior Polymorphous Corneal Dystrophy 1

PURPOSE

The purpose of this study was to investigate the clinical and genetic features of a man and his daughter with posterior polymorphous corneal dystrophy (PPCD), referred to our clinic for Descemet membrane endothelial keratoplasty. No other known relatives were affected.

METHODS

Ophthalmic examination and histology, including electron microscopy, were performed. Genetic testing was conducted by means of whole exome sequencing, and variant analysis was achieved by using an internal in silico pipeline. Molecular tests included a dual-luciferase assay.

RESULTS

Slowly progressive blurred vision was reported from childhood by the daughter. The father's symptoms started at age 55. Best-corrected visual acuity was reduced in both patients (0.2-0.4). Slit-lamp examination in both patients revealed bilateral corneal clouding with gray endothelial lesions; other family members had no ophthalmological signs. Descemet membrane endothelial keratoplasty was performed uneventfully in both patients. Histology showed thickened Descemet membrane and abnormal endothelium resembling epithelial-like cells. Both patients carried the OVOL2 5' untranslated region NM_021220.4.c.-61G>A variant in the heterozygous state. This change was associated with increased promoter activity and was not present in the unaffected members of the family.

CONCLUSIONS

The 5' untranslated region mutation c.-61G>A in OVOL2 has been previously found in 1 individual with PPCD1 and reported as a variant of unknown significance because of insufficient evidence supporting its pathogenicity. Identification of the second family with 2 individuals affected by PPCD1 carrying this change, together with functional data, provides further proofs that it is disease-causing.

Corneal ectasia associated with posterior lamellar opacification

BACKGROUND

Concomitant corneal ectasia and posterior lamellar corneal opacification is rare, and the genetic relationship between these two conditions is unclear. We report the genetic and clinical characterization of this phenotype in three unrelated individuals.

MATERIALS AND METHODS

One previously reported affected individual and two unreported, unrelated, affected individuals were recruited for the study. Subjects and unaffected relatives underwent slit lamp examination, refraction, and multi-modal imaging. Saliva samples were obtained from two of the three affected individuals, from which DNA was extracted. Sanger sequencing was performed to identify mutations in genes associated with posterior amorphous corneal dystrophy (PACD), brittle cornea syndrome (BCS), and posterior polymorphous corneal dystrophy (PPCD), while copy number variation (CNV) analysis was used to identify CNV in the PACD locus.

RESULTS

Affected individuals demonstrated bilateral corneal steepening, stromal thinning and lamellar posterior corneal opacification. Corneal topography and tomography revealed conical or globular corneal steepening and decreased thickness. Anterior segment optical coherence tomography demonstrated hyperreflectivity of the posterior stroma in each of the affected individuals. Genetic testing did not detect a heterozygous deletion involving the PACD locus on chromosome 12 or a pathogenic mutation in the genes associated with BCS or PPCD.

CONCLUSIONS

Corneal ectasia may be associated with posterior lamellar stromal opacification that appears consistent with PACD. However, genetic testing for PACD as well as BCS and PPCD in affected individuals fails to reveal pathogenic deletions or mutations, indicating that other genetic factors are involved.

Diseases of the corneal endothelium

The corneal endothelial monolayer and associated Descemet's membrane (DM) complex is a unique structure that plays an essential role in corneal function. Endothelial cells are neural crest derived cells that rest on a special extracellular matrix and play a major role in maintaining stromal hydration within a narrow physiologic range necessary for clear vision. A number of diseases affect the endothelial cells and DM complex and can impair corneal function and vision. This review addresses different human corneal endothelial diseases characterized by loss of endothelial function including: Fuchs endothelial corneal dystrophy (FECD), posterior polymorphous corneal dystrophy (PPCD), congenital hereditary endothelial dystrophy (CHED), bullous keratopathy, iridocorneal endothelial (ICE) syndrome, post-traumatic fibrous downgrowth, glaucoma and diabetes mellitus.

Direct Reprogramming Into Corneal Epithelial Cells Using a Transcriptional Network Comprising PAX6, OVOL2, and KLF4

In its early stages, an embryo polarizes to form cell subpopulations that subsequently produce specific organ cell types. These cell subpopulations are defined by transcription factors (TFs) that activate or repress specific genes. Although an embryo comprises thousands of TFs, surprisingly few are needed to determine the fate of a given cell. The ectoderm divides into the neuroectoderm and surface ectoderm, the latter of which gives rise to epidermal keratinocytes and corneal epithelial cells (CECs). Meanwhile, neuroectoderm cells give rise to other parts of the eye such as the corneal endothelium and retina. To investigate the regulatory role of TFs in CECs, we overexpressed the "core TFs" (PAX6, OVOL2, and KLF4) in human fibroblasts and found that the cells adopted a CEC-like quality. OVOL2 overexpression was even able to directly induce cells with a neuroectoderm fate toward a surface ectoderm fate, designated "direct reprogramming." Conversely, suppression of OVOL2 or PAX6 expression induced CECs to show qualities consistent with neural lineage cells or epidermal keratinocytes, respectively. This suggests that these core TFs can maintain the CEC phenotype through reciprocal gene regulation. Direct reprogramming has important implications for cell therapies. The potential benefits of cells derived by direct reprogramming compared with induced pluripotent stem cells include the fact that it requires less time than reprogramming a cell back to the pluripotent state and then to another cell type. Further understanding of the reciprocally repressive mechanism of action for core TFs could lead to alternative treatments for regenerative medicine not requiring cell transplantation.

CUGC for posterior polymorphous corneal dystrophy (PPCD)

Name of the disease (synonyms) CUGC for posterior polymorphous corneal dystrophy (PPCD).OMIM# of the disease 122000; 609141; 618031.Name of the analysed genes or DNA/chromosome segments OVOL2 (PPCD1); ZEB1 (PPCD3); GRHL2 (PPCD4).OMIM# of the gene(s) 616441; 189909; 608576. Review of the analytical and clinical validity as well as of the clinical utility of DNA-based testing for variants in theOVOL2, ZEB1andGRHL2gene(s) in a diagnostic setting, predictive and parental settings and for risk assesment in relatives.

ZEB1 insufficiency causes corneal endothelial cell state transition and altered cellular processing

The zinc finger e-box binding homeobox 1 (ZEB1) transcription factor is a master regulator of the epithelial to mesenchymal transition (EMT), and of the reverse mesenchymal to epithelial transition (MET) processes. ZEB1 plays an integral role in mediating cell state transitions during cell lineage specification, wound healing and disease. EMT/MET are characterized by distinct changes in molecular and cellular phenotype that are generally context-independent. Posterior polymorphous corneal dystrophy (PPCD), associated with ZEB1 insufficiency, provides a new biological context in which to understand and evaluate the classic EMT/MET paradigm. PPCD is characterized by a cadherin-switch and transition to an epithelial-like transcriptomic and cellular phenotype, which we study in a cell-based model of PPCD generated using CRISPR-Cas9-mediated ZEB1 knockout in corneal endothelial cells (CEnCs). Transcriptomic and functional studies support the hypothesis that CEnC undergo a MET-like transition in PPCD, termed endothelial to epithelial transition (EnET), and lead to the conclusion that EnET may be considered a corollary to the classic EMT/MET paradigm.

Investigating the Pathogenicity of VSX1 Missense Mutations and Their Association With Corneal Disease

Purpose

Despite numerous studies associating Visual System Homeobox 1 (VSX1), with posterior polymorphous corneal dystrophy and keratoconus, its role in these diseases is unclear. Here we examine the pathogenicity of VSX1 missense mutations in vitro and in a mouse genetic model.

Methods

Vsx1 transcriptional repressor activity, protein stability, and subcellular localization activity, was examined using luciferase reporter-based assays, western blotting and immunolabeling, respectively, in transfected human embryonic kidney 293T cells. A genetic model for VSX1 p.P247R was generated to investigate pathogenicity of the mutation, in vivo. A wholemount confocal imaging approach on unfixed intact eyes was developed to examine corneal morphology, curvature, and thickness. Immunolabeling and electroretinography was used to examine retinal phenotype.

Results

A mutation corresponding to human VSX1 p.P247R led to enhanced transcriptional repressor activity, in vitro. A mouse model for VSX1 p.P247R did not have any observable corneal defect, but did exhibit an abnormal electroretinogram response characterized by a more prominent ON as opposed to OFF panretinal responsiveness. In vitro analysis of additional VSX1 missense mutations showed that they either enhanced repressor activity or did not alter activity.

Conclusions

Our results indicate that although VSX1 sequence variants can alter transcriptional activity, in the context of a mouse genetic model, at least one of these changes does not lead to corneal abnormalities. While we cannot exclude a role for VSX1 as a risk factor for corneal disease, on its own, it does not appear to play a major causative role.

Ectopic GRHL2 Expression Due to Non-coding Mutations Promotes Cell State Transition and Causes Posterior Polymorphous Corneal Dystrophy 4

In a large family of Czech origin, we mapped a locus for an autosomal-dominant corneal endothelial dystrophy, posterior polymorphous corneal dystrophy 4 (PPCD4), to 8q22.3-q24.12. Whole-genome sequencing identified a unique variant (c.20+544G>T) in this locus, within an intronic regulatory region of GRHL2. Targeted sequencing identified the same variant in three additional previously unsolved PPCD-affected families, including a de novo occurrence that suggests this is a recurrent mutation. Two further unique variants were identified in intron 1 of GRHL2 (c.20+257delT and c.20+133delA) in unrelated PPCD-affected families. GRHL2 is a transcription factor that suppresses epithelial-to-mesenchymal transition (EMT) and is a direct transcriptional repressor of ZEB1. ZEB1 mutations leading to haploinsufficiency cause PPCD3. We previously identified promoter mutations in OVOL2, a gene not normally expressed in the corneal endothelium, as the cause of PPCD1. OVOL2 drives mesenchymal-to-epithelial transition (MET) by directly inhibiting EMT-inducing transcription factors, such as ZEB1. Here, we demonstrate that the GRHL2 regulatory variants identified in PPCD4-affected individuals induce increased transcriptional activity in vitro. Furthermore, although GRHL2 is not expressed in corneal endothelial cells in control tissue, we detected GRHL2 in the corneal "endothelium" in PPCD4 tissue. These cells were also positive for epithelial markers E-Cadherin and Cytokeratin 7, indicating they have transitioned to an epithelial-like cell type. We suggest that mutations inducing MET within the corneal endothelium are a convergent pathogenic mechanism leading to dysfunction of the endothelial barrier and disease.

Elucidating the molecular basis of PPCD: Effects of decreased ZEB1 expression on corneal endothelial cell function

PURPOSE

To investigate the functional role that the zinc e-box binding homeobox 1 (ZEB1) gene, which underlies the genetic basis of posterior polymorphous corneal dystrophy 3 (PPCD3), plays in corneal endothelial cell proliferation, apoptosis, migration, and barrier function.

METHODS

A human corneal endothelial cell line (HCEnC-21T) was transfected with siRNA targeting ZEB1 mRNA. Cell proliferation, apoptosis, migration, and barrier assays were performed: Cell proliferation was assessed with cell counting using a hemocytometer; cell apoptosis, induced by either ultraviolet C (UVC) radiation or doxorubicin treatment, was quantified by measuring cleaved caspase 3 (cCASP3) protein levels; and cell migration and barrier function were monitored with electric cell-substrate impedance sensing (ECIS).

RESULTS

ZEB1 knockdown in HCEnC-21T cells transfected with siRNA targeting ZEB1 did not result in a significant difference in cell proliferation when compared with control. Although knockdown of ZEB1 in HCEnC-21T cells sensitized the cells to UV-induced apoptosis, ZEB1 knockdown did not alter the cells' susceptibility to doxorubicin-induced apoptosis, as measured with cCASP3 protein levels, compared with controls. Similarly, no difference was observed in cell migration following ZEB1 knockdown. However, cell barrier function increased significantly following ZEB1 knockdown.

CONCLUSIONS

The corneal endothelium in PPCD3 is characterized by morphologic, anatomic, and molecular features that are more consistent with an epithelial-like rather than an endothelial-like phenotype. Although these characteristics have been well documented, we demonstrate for the first time that susceptibility to UV-induced apoptosis and cell barrier function are significantly altered in the setting of reduced ZEB1. The significance of an altered cellular response to apoptotic stimuli and increased cell barrier function in the pathobiology of PPCD remains to be fully elucidated.

Retinal pathology in the PPCD1 mouse

Retinal phenotypes of the PPCD1 mouse, a mouse model of posterior polymorphous corneal dystrophy, have been characterized. PPCD1 mice on the DBA/2J background (D2.Ppcd1) have previously been reported to develop an enlarged anterior chamber due to epithelialization and proliferation of the corneal endothelium and subsequent blockage of the iridocorneal angle. Results presented here show that D2.Ppcd1 mice develop increased intraocular pressure (IOP), with measurements at three months of age revealing significant increases in IOP. Significant retinal ganglion cell layer cell loss is observed at five months of age. D2.Ppcd1 animals also exhibit marked degeneration of the outer nuclear layer in association with hyperplasia of the retinal pigment epithelium. Evidence of retinal detachment is present as early as three weeks of age. By 3.5 months of age, focal areas of outer nuclear layer loss are observed. Although the Gpnmb^R150X mutation leads to increased IOP and glaucoma in DBA/2J mice, development of anterior segment and retinal defects in D2.Ppcd1 animals does not depend upon presence of the Gpnmb^R150X mutation.

Transcriptomic Profiling of Posterior Polymorphous Corneal Dystrophy

Purpose

To investigate the molecular basis of posterior polymorphous corneal dystrophy (PPCD) by examining the PPCD transcriptome and the effect of decreased ZEB1 expression on corneal endothelial cell (CEnC) gene expression.

Methods

Next-generation RNA sequencing (RNA-seq) analyses of corneal endothelium from two PPCD-affected individuals (one with PPCD3 and one of unknown genetic cause) compared with two age-matched controls, and primary human CEnC (pHCEnC) transfected with siRNA-mediated ZEB1 knockdown. The expression of selected differentially expressed genes was validated by quantitative polymerase chain reaction (qPCR) and/or assessed by in situ hybridization in the corneal endothelium of four independent cases of PPCD (one with PPCD3 and three of unknown genetic cause).

Results

Expression of 16% and 46% of the 104 protein-coding genes specific to ex vivo corneal endothelium was lost in the endothelium of two individuals with PPCD. Thirty-two genes associated with ZEB1 and 3 genes (BMP4, CCND1, ZEB1) associated with OVOL2 were differentially expressed in the same direction in both individuals with PPCD. Immunohistochemistry staining and RNA-seq analyses demonstrated variable expression of type IV collagens in PPCD corneas. Decreasing ZEB1 expression in pHCEnC altered expression of 711 protein-coding genes, many of which are associated with canonical pathways regulating various cellular processes.

Conclusions

Identification of the altered transcriptome in PPCD and in a cell-based model of PPCD provided insight into the molecular alterations characterizing PPCD. Further study of the differentially expressed genes associated with ZEB1 and OVOL2 is expected to identify candidate genes for individuals with PPCD and without a ZEB1 or OVOL2 mutation.