Collagen Fibrils and Proteoglycans of Macular Dystrophy Cornea: Ultrastructure and 3D Transmission Electron Tomography

Loss of carbohydrate sulfotransferase 6 function leads to macular corneal dystrophy phenotypes and skeletal defects in zebrafish

The carbohydrate sulfotransferase 6 (chst6) gene is linked to macular corneal dystrophy (MCD), a rare disease that leads to bilateral blindness due to the accumulation of opaque aggregates in the corneal stroma. chst6 encodes for a keratan sulfate proteoglycan (KSPG) specific sulfotransferase. MCD patients lose sulfated KSPGs (cKS) in the cornea and the serum. The significance of serum cKS loss has not been understood. Zebrafish cornea structure is similar to that of humans and it contains high levels of sulfated cKS in the stroma. Here, zebrafish chst6 is shown to be expressed in the cornea and head structures of the embryos. An animal model of MCD is developed by generating chst6 mutant animals with CRISPR/Cas9-mediated gene editing. The dramatic decrease in cKS epitopes in the mutants was shown with ELISA and immunofluorescence. Morphological defects or alterations of jaw cartilage were detected in a minor fraction of the mutant larvae. Loss of cKS epitopes and morphological defects was fully rescued with wild-type chst6. Mutant adult zebrafish displayed all clinical manifestations of MCD, while a fraction also displayed jaw and skeleton defects. Opaque accumulations formed in the eye, which were alcian blue positive. Loss of cKS in the corneal stroma and a decrease in corneal thickness were shown. Interestingly, alteration of transforming growth factor beta-induced (BIGH3) expression which was not described in patients was also observed. This is the first report of an MCD model in a genetically tractable organism, providing a preclinical model and insight into the importance of KSPG sulfation for proper skeletal morphogenesis.

Quantitative Assessment of Biomechanical Properties of the Human Keratoconus Cornea Using Acoustic Radiation Force Optical Coherence Elastography

Purpose

Quantification of biomechanical properties of keratoconus (KC) corneas has great significance for early diagnosis and treatment of KC, but the corresponding clinical measurement remains challenging. Here, we developed an acoustic radiation force (ARF) optical coherence elastography technique and explored its potential for evaluating biomechanical properties of KC corneas.

Methods

An ARF system was used to induce the tissue deformation, which was detected by an optical coherence tomography system, and thus the localized point-by-point Young's modulus measurements were achieved. Then, two healthy rabbit eyes were imaged to test the system, after which the human keratoconus cornea was evaluated by using the same method. Three regions were selected for biomechanics analysis: the conical region, the transitional region, and the peripheral region.

Results

Young's moduli of transitional region ranged from 53.3 to 58.5 kPa. The corresponding values for the peripheral region were determined to be 58.6 kPa and 63.2 kPa, respectively. Young's moduli of the conical region were gradually increased by 18.3% from the center to the periphery, resulting in the minimum and maximum values of 44.9 kPa and 53.1 kPa, respectively. Furthermore, Young's moduli of the anterior and posterior of the center were determined to be 44.9 kPa and 50.7 kPa, respectively.

Conclusions

Differences in biomechanical properties between the three regions and slight variations within the conical region were clearly distinguished. Biomechanical weakening of the keratoconus cornea was mainly localized in the conical region, especially in the vertex position.

Translational Relevance

The system may provide a promising clinical tool for the noninvasive evaluation of local corneal biomechanics and thus may have potential applications in early keratoconus detection with further optimization.

The Communication between Ocular Surface and Nasal Epithelia in 3D Cell Culture Technology for Translational Research: A Narrative Review

There is a lack of knowledge regarding the connection between the ocular and nasal epithelia. This narrative review focuses on conjunctival, corneal, ultrastructural corneal stroma, and nasal epithelia as well as an introduction into their interconnections. We describe in detail the morphology and physiology of the ocular surface, the nasolacrimal ducts, and the nasal cavity. This knowledge provides a basis for functional studies and the development of relevant cell culture models that can be used to investigate the pathogenesis of diseases related to these complex structures. Moreover, we also provide a state-of-the-art overview regarding the development of 3D culture models, which allow for addressing research questions in models resembling the in vivo situation. In particular, we give an overview of the current developments of corneal 3D and organoid models, as well as 3D cell culture models of epithelia with goblet cells (conjunctiva and nasal cavity). The benefits and shortcomings of these cell culture models are discussed. As examples for pathogens related to ocular and nasal epithelia, we discuss infections caused by adenovirus and measles virus. In addition to pathogens, also external triggers such as allergens can cause rhinoconjunctivitis. These diseases exemplify the interconnections between the ocular surface and nasal epithelia in a molecular and clinical context. With a final translational section on optical coherence tomography (OCT), we provide an overview about the applicability of this technique in basic research and clinical ophthalmology. The techniques presented herein will be instrumental in further elucidating the functional interrelations and crosstalk between ocular and nasal epithelia.

Macular corneal dystrophy related to novel mutations of CHST6 in a Chinese family and clinical observation after penetrating keratoplasty

BACKGROUND

Macular corneal dystrophy (MCD) is a rare corneal stromal dystrophy with bilateral progressive vision loss. The pathogenic gene of MCD is carbohydrate sulfotransferase 6 (CHST6). Herein, we report a novel missense mutation and a rare exon deletion mutation in the CHST6 gene in a Chinese family with MCD.

METHODS

Genomic DNA was extracted from the peripheral blood, and next generation sequencing was used to analyse the gene sequence. The pathogenic mutations were identified in all affected family members. The proband successively received binocular penetrating keratoplasty (PKP), and the corneas were examined by histopathology and colloidal iron staining to prove the diagnosis. A long-term follow-up was made to observe the changes after PKP.

RESULTS

Genetic analysis demonstrated hemizygous mutations in the proband, including a novel c.520A>C (p.K174Q) missense mutation and a rarely reported exon 3 deletion mutation, which were co-segregated with the MCD phenotypes in the pedigree. The positive colloidal iron staining confirmed the diagnosis of MCD in the proband. However, the clinical phenotype and pathological manifestation of both eyes were different from each other because of complicated keratitis in the left eye. During the nine years of follow-up, visual acuity was improved significantly, and the cornea was transparent without rejection and postoperative recurrence in both eyes.

CONCLUSIONS

The novel hemizygous mutations were thought to contribute to the loss of CHST6 function, which induced typical clinical and pathological features of MCD. PKP was an effective treatment for MCD.

Macular Corneal Dystrophy: An Updated Review

Macular Corneal Dystrophy is an autosomal recessive form of corneal dystrophy due to a mutation in CHST6 gene, which results in abnormal proteoglycan synthesis. There is accumulation of abnormal glycosaminoglycans in the corneal stroma and endothelium. The deposition results in progressive loss of corneal transparency and visual acuity. The histopathology shows characteristic alcian blue positive deposits. Management in the cases with visual loss requires keratoplasty either full thickness or lamellar. The decision about the ideal type of keratoplasty depends on age and pre-operative clinical features. Although prognosis after keratoplasty is good, recurrences can occur. Future research should be targeted towards gene therapy in this condition.

Characterization of In Vivo Biomechanical Properties in Macular Corneal Dystrophy

PURPOSE

To measure and compare corneal biomechanics in patients with macular corneal dystrophy (MCD), versus those who underwent penetrating keratoplasty (PK) for MCD, versus normal subjects.

DESIGN

Cross-sectional study.

METHODS

This study enrolled 24 eyes with MCD, 25 eyes that underwent PK in patients with preoperative diagnosis of MCD, and 28 normal eyes. The ocular response analyzer was used to measure corneal biomechanical properties, including corneal resistance factor (CRF) and corneal hysteresis (CH). Intraocular pressure (IOP) was measured using a Goldmann applanation tonometer, and central corneal thickness (CCT) was measured using an ultrasonic pachymeter. The CCT, CRF, CH, and IOP were compared among the study group.

RESULTS

CCT was significantly lower in the MCD group (423 ± 47 mμ) than in the PK group (541 ± 31 mμ; P < .001) and the controls (540 ± 26 mμ; P < .001). Meanwhile, CCT was comparable between the PK and control groups (P = .98). CRF did not differ between the MCD (8.34 ± 2.12 mm Hg) and the PK (8.66 ± 1.66 mm Hg) groups (P = .89); however, both of these groups had lower CRF values than the control (9.76 ± 1.83 mm Hg, P = .02) group. No significant differences were observed among the 3 groups in CH (P = .13). IOP values did not differ between the MCD (11.25 ± 1.69 mm Hg) and PK (12.0 ± 2.67 mm Hg) groups (P = .95); however, the IOP values for both of these groups differed significantly from those of the controls (13.46 ± 2.17 mm Hg; P = .006).

CONCLUSIONS

CRF was significantly reduced in MCD and did not return to normal values even after PK. The reduced corneal rigidity could result in underestimating IOP in patients with MCD and in those undergoing PK for this stromal dystrophy.

A comprehensive evaluation of 181 reported CHST6 variants in patients with macular corneal dystrophy

Macular corneal dystrophy (MCD) is an autosomal recessive disease featured by bilateral progressive stromal clouding and loss of vision, consequently necessitating corneal transplantation. Variants in CHST6 gene have been recognized as the most critical genetic components in MCD. Although many CHST6 variants have been described until now, the detailed mechanisms underlying MCD are still far from understood. In this study, we integrated all the reported CHST6 variants described in 408 MCD cases, and performed a comprehensive evaluation to better illustrate the causality of these variants. The results showed that majority of these variants (165 out of 181) could be classified as pathogenic or likely pathogenic. Interestingly, we also identified several disease causal variants with ethnic specificity. In addition, the results underscored the strong correlation between mutant frequency and residue conservation in the general population (Spearman's correlation coefficient = -0.311, P = 1.20E-05), thus providing potential candidate targets for further genetic manipulation. The current study highlighted the demand of further functional investigations to evaluate the causality of CHST6 variants, so as to promote earlier accurate diagnosis of MCD and future development of potential targets for genetic therapy.

The structural response of the cornea to changes in stromal hydration

The primary aim of this study was to quantify the relationship between corneal structure and hydration in humans and pigs. X-ray scattering data were collected from human and porcine corneas equilibrated with polyethylene glycol (PEG) to varying levels of hydration, to obtain measurements of collagen fibril diameter, interfibrillar spacing (IFS) and intermolecular spacing. Both species showed a strong positive linear correlation between hydration and IFS² and a nonlinear, bi-phasic relationship between hydration and fibril diameter, whereby fibril diameter increased up to approximately physiological hydration, H = 3.0, with little change thereafter. Above H = 3.0, porcine corneas exhibited a larger fibril diameter than human corneas (p < 0.001). Intermolecular spacing also varied with hydration in a bi-phasic manner but reached a maximum value at a lower hydration (H = 1.5) than fibril diameter. Human corneas displayed a higher intermolecular spacing than porcine corneas at all hydrations (p < 0.0001). Human and porcine corneas required a similar PEG concentration to reach physiological hydration, suggesting that the total fixed charge that gives rise to the swelling pressure is the same. The difference in their structural responses to hydration can be explained by variations in molecular cross-linking and intra/interfibrillar water partitioning.

Genetic testing for corneal dystrophies and other corneal Mendelian diseases

We studied the scientific literature and disease guidelines in order to summarize the clinical utility of the genetic test for corneal dystrophies and other Mendelian corneal diseases (CDs). CDs are mostly inherited in an autosomal dominant manner (autosomal recessive inheritance is rare). The overall prevalence is currently unknown. CDs are caused by mutations in the AGBL1, CHST6, COL8A2, DCN, GSN, KRT12, KRT3, NLRP1, PAX6, PIKFYVE, PRDM5, SLC4A11, TACSTD2, TCF4, TGFBI, UBIAD1, VSX1, ZEB1, and ZNF469 genes. Clinical diagnosis is based on clinical findings, ophthalmological examination, confocal microscopy and slit-lamp biomicroscopy. The genetic test is useful for confirming diagnosis and for differential diagnosis, couple risk assessment and access to clinical trials.

Ultrastructure Features and Three-Dimensional Transmission Electron Tomography of Dhub Lizard (Uromastyx Aegyptia) Cornea and Its Adaptation to a Desert Environment

We report ultrastructural features and transmission electron tomography of the dhub lizard (Uromastyx aegyptia) cornea and its adaptation to hot and dry environments. Six corneas of dhub lizards were fixed in 2.5% glutaraldehyde and processed for electron microscopy and tomography. The ultrathin sections were observed with a JEOL 1400 transmission electron microscope. The cornea of the dhub lizard is very thin (~28-30 µm). The epithelium constitutes ~14% of the cornea, whereas the stroma constitutes 80% of the cornea. The middle stromal lamellae are significantly thicker than anterior and posterior stromal lamellae. Collagen fibril (CF) diameters in the anterior stroma are variable in size (25-75 nm). Proteoglycans (PGs) are very large in the middle and posterior stroma, whereas they are small in the anterior stroma. Three-dimensional electron tomography was carried out to understand the structure and arrangement of the PG and CFs. The presence of large PGs in the posterior and middle stroma might help the animal retain a large amount of water to protect it from dryness. The dhub corneal structure is equipped to adapt to the dry and hot desert environment.

Characteristics of corneal dystrophies: a review from clinical, histological and genetic perspectives

Corneal dystrophy is a common type of hereditary corneal diseases. It includes many types, which have varied pathology, histology and clinical manifestations. Recently, the examination techniques of ophthalmology and gene sequencing advance greatly, which do benefit to our understanding of these diseases. However, many aspects remain still unknown. And due to the poor knowledge of these diseases, the results of the treatments are not satisfoctory. The purpose of this review was to summarize the clinical, histological and genetic characteristics of different types of corneal dystrophies.