Sulfation patterns of keratan sulfate in different macular corneal dystrophy immunophenotypes using three different probes

Imaging Keratan Sulfate in Ocular Tissue Sections by Immunofluorescence Microscopy and LA-ICP-MS

Carbohydrate-specific antibodies can serve as valuable tools to monitor alterations in the extracellular matrix resulting from pathologies. Here, the keratan sulfate-specific monoclonal antibody MZ15 was characterized in more detail by immunofluorescence microscopy as well as laser ablation ICP-MS using tissue cryosections and paraffin-embedded samples. Pretreatment with keratanase II prevented staining of samples and therefore demonstrated efficient enzymatic keratan sulfate degradation. Random fluorescent labeling and site-directed introduction of a metal cage into MZ15 were successful and allowed for a highly sensitive detection of the keratan sulfate landscape in the corneal stroma from rats and human tissue.

CHST6 mutations identified in Iranian MCD patients and CHST6 mutations reported worldwide identify targets for gene editing approaches including the CRISPR/Cas system

PURPOSE

To identify CHST6 mutations in Iranians macular corneal dystrophy (MCD) patients and also to assess distribution of amino acids in the encoded protein that are affected by CHST6 mutations reported hitherto in various populations in order to predict gene regions that may be appropriate targets for gene editing approaches including the CRISPR/Cas system. The analysis will also reveal biologically and functionally important regions of the protein.

METHODS

Mutation screening of CHST6 by sequencing was performed on 21 Iranian MCD-affected probands. Previously reported MCD causing CHST6 mutations were identified by searches in NCBI.

RESULTS

Nineteen CHST6 mutations were found among the 21 Iranian patients, most of which were missense mutations and six of which were novel. Totally, 189 mutations among 375 MCD patients have been found worldwide, and 134 of these are missense mutations. The distribution of 88 amino acids affected by missense mutations along the length of the encoded protein was not random, and four regions of possible mutation clustering were noted. 25% of patients harbored mutations in a DNA region consisting of only 36 nucleotides.

CONCLUSION

Similar to most populations, CHST6 mutations among Iranians are very heterogeneous as indicated by finding 19 different mutations among 21 MCD patients. Nevertheless, identification of four potential mutation clusters identifies regions that are most suitable for gene therapy targeting by the CRISPR/Cas approach. Additionally, the mutation clusters identify regions with potential structural and/or functional importance. Consistent with this, the amino acids in these regions are well conserved among various membrane-bound sulfotransferases.

CHST6 mutation screening and endoplasmatic reticulum stress in macular corneal dystrophy

Macular corneal dystrophy (MCD) is an autosomal recessive disorder mainly caused by gene mutations of carbohydrate sulfotransferase (CHST6) leading to bilateral visual impairment. Because the mechanism underlying this degeneration remains poorly understood, we investigated molecular alterations and pathways that may be involved in MCD in this issue. Different mutation sites were screened by direct sequencing of the coding region of CHST6. In addition, we described morphological changes in MCD keratocytes by light microscopy and electron microscopy and determined the relationship between the development of this disease and the occurrence of apoptosis through flow cytometry, cell counting kit-8, colony formation assay and other experiments. Western blotting and quantitative real-time polymerase chain reaction were used to determine if endoplasmic reticulum (ER) stress was activated. We found 10 kinds of mutations among these families with 3 novel mutations included. The percentage of apoptotic keratocytes increased in MCD patients; furthermore, the expression of apoptosis related protein B-cell lymphoma-2 (Bcl-2) was down-regulated while Bcl-2 associated X protein was upregulated. Finally, ER stress was activated with the upregulation of glucose-regulated protein 78 and CCAAT-enhancer-binding protein homologous protein. Our clinical and in vitro results suggest that the CHST6 mutation associated with MCD is associated with apoptosis, and ER stress is probably involved in this apoptosis pathway.

Sulfation of Glycosaminoglycans and Its Implications in Human Health and Disorders

Sulfation is a dynamic and complex posttranslational modification process. It can occur at various positions within the glycosaminoglycan (GAG) backbone and modulates extracellular signals such as cell-cell and cell-matrix interactions; different sulfation patterns have been identified for the same organs and cells during their development. Because of their high specificity in relation to function, GAG sulfation patterns are referred to as the sulfation code. This review explores the role of GAG sulfation in different biological processes at the cell, tissue, and organism levels. We address the connection between the sulfation patterns of GAGs and several physiological processes and discuss the misregulation of GAG sulfation and its involvement in several genetic and metabolic disorders. Finally, we present the therapeutic potential of GAGs and their synthetic mimics in the biomedical field.

Genetic analysis of CHST6 and TGFBI in Turkish patients with corneal dystrophies: Five novel variations in CHST6

PURPOSE

To identify pathogenic variations in carbohydrate sulfotransferase 6 (CHST6) and transforming growth factor, beta-induced (TGFBI) genes in Turkish patients with corneal dystrophy (CD).

METHODS

In this study, patients with macular corneal dystrophy (MCD; n = 18), granular corneal dystrophy type 1 (GCD1; n = 12), and lattice corneal dystrophy type 1 (LCD1; n = 4), as well as 50 healthy controls, were subjected to clinical and genetic examinations. The level of antigenic keratan sulfate (AgKS) in the serum samples of patients with MCD was determined with enzyme-linked immunosorbent assay (ELISA) to immunophenotypically subtype the patients as MCD type I and MCD type II. DNA was isolated from venous blood samples from the patients and controls. Variations were analyzed with DNA sequencing in the coding region of CHST6 in patients with MCD and exons 4 and 12 in TGFBI in patients with LCD1 and GCD1. Clinical characteristics and the detected variations were evaluated to determine any existing genotype-phenotype correlations.

RESULTS

The previously reported R555W mutation in TGFBI was detected in 12 patients with GCD1, and the R124C mutation in TGFBI was detected in four patients with LCD1. Serum AgKS levels indicated that 12 patients with MCD were in subgroup I, and five patients with MCD were in subgroup II. No genetic variation was detected in the coding region of CHST6 for three patients with MCD type II. In other patients with MCD, three previously reported missense variations (c. 1A>T, c.738C>G, and c.631 C>T), three novel missense variations (c.164 T>C, c.526 G>A, c. 610 C>T), and two novel frameshift variations (c.894_895 insG and c. 462_463 delGC) were detected. These variations did not exist in the control chromosomes, 1000 Genomes, and dbSNP.

CONCLUSIONS

This is the first molecular analysis of TGFBI and CHST6 in Turkish patients with different types of CD. We detected previously reported, well-known hot spot mutations in TGFBI in the patients with GCD1 and LCD1. Eight likely pathogenic variations in CHST6, five of them novel, were reported in patients with MCD, which enlarges the mutational spectrum of MCD.

Novel mutation in the CHST6 gene causes macular corneal dystrophy in a black South African family

BACKGROUND

Macular corneal dystrophy (MCD) is a rare autosomal recessive disorder that is characterized by progressive corneal opacity that starts in early childhood and ultimately progresses to blindness in early adulthood. The aim of this study was to identify the cause of MCD in a black South African family with two affected sisters.

METHODS

A multigenerational South African Sotho-speaking family with type I MCD was studied using whole exome sequencing. Variant filtering to identify the MCD-causal mutation included the disease inheritance pattern, variant minor allele frequency and potential functional impact.

RESULTS

Ophthalmologic evaluation of the cases revealed a typical MCD phenotype and none of the other family members were affected. An average of 127 713 variants per individual was identified following exome sequencing and approximately 1.2 % were not present in any of the investigated public databases. Variant filtering identified a homozygous E71Q mutation in CHST6, a known MCD-causing gene encoding corneal N-acetyl glucosamine-6-O-sulfotransferase. This E71Q mutation results in a non-conservative amino acid change in a highly conserved functional domain of the human CHST6 that is essential for enzyme activity.

CONCLUSION

We identified a novel E71Q mutation in CHST6 as the MCD-causal mutation in a black South African family with type I MCD. This is the first description of MCD in a black Sub-Saharan African family and therefore contributes valuable insights into the genetic aetiology of this disease, while improving genetic counselling for this and potentially other MCD families.

Novel CHST6 gene mutations in 2 unrelated cases of macular corneal dystrophy

PURPOSE

To investigate the possible mutations in the carbohydrate sulfotransferase 6 (CHST6) gene of 2 unrelated cases of macular corneal dystrophy (MCD) and to report atypical stromal deposits in one of them.

METHODS

Corneal tissues were stained with antisulfated keratan sulfate (KS), antitransforming growth factor beta 1-induced protein (TGFBIp), thioflavin-T, alcian blue, and Masson trichrome. Sequencing was performed to identify potential mutations in the CHST6 gene and the fourth and twelfth exons of the TGFBI gene.

RESULTS

Alcian blue staining revealed the presence of multiple subepithelial and intrastromal mucopolysaccharide deposits, confirming the diagnosis of MCD in both cases. Immunofluorescence staining in case 1 revealed the presence of sulfated KS only in the keratocytes and select endothelial cells, consistent with MCD type IA. Preferential expression of sulfated KS was observed in keratocytes and extracellular stromal matrix in case 2, consistent with MCD type II. Atypical subepithelial and superficial stromal deposits were observed in case 1, which stained positively with alcian blue, eosin, Masson trichrome, and thioflavin-T indicating the presence of hyaline and amyloid materials. CHST6 gene sequencing revealed 2 heterozygous mutations in case 1 (a p.Arg211Gln and a novel mutation of p.Arg177Gly) and a novel homozygous mutation of p.Pro186Arg in case 2. No mutations were found in exons 4 or 12 of the TGFBI gene in case 1.

CONCLUSIONS

Secondary hyalinosis and amyloidosis occur in a case of MCD type IA with a novel p.Arg177Gly mutation in CHST6. A novel p.Pro186Arg mutation in CHST6 is associated with MCD type II in an African American.

Structural collagen alterations in macular corneal dystrophy occur mainly in the posterior stroma

PURPOSE

Collagen fibrils in the corneal stroma in macular corneal dystrophy, on average, are more closely spaced than in the normal cornea. This study was conducted to investigate if this occurs uniformly across the stroma or is more prevalent at certain stromal depths.

METHODS

Microbeam synchrotron X-ray fiber diffraction patterns were obtained in 25 microm steps across the whole thickness of a thin strip of a macular corneal dystrophy cornea obtained at keratoplasty. Data were analyzed for mean collagen interfibrillar spacing at all positions. Serum was analyzed immunochemically to determine immunophenotype, and transmission electron microscopy was carried out to visualize stromal ultrastructure.

RESULTS

Keratan sulphate was not detectable in blood serum, classifying the disease as macular corneal dystrophy type I. Collagen interfibrillar spacing dropped linearly with stromal depth from the anterior to posterior cornea, measuring 5-10% less in the posterior 100 microm of the MCD stroma compared to the anterior 100 microm (p < 0.001). Isolated pockets of collagen fibrils with unusually large diameters were identified in the deep stroma.

CONCLUSIONS

Collagen fibril spacing is reduced and large-diameter collagen fibrils are seen in macular corneal dystrophy type I, with the deep stroma affected more. We speculate that the ultrastructural abnormalities are more prevalent in the posterior stroma because the structural influence of sulphated keratan sulphate glycosaminoglycans/proteoglycans is high in this region of the cornea.

Structural and biochemical aspects of keratan sulphate in the cornea

Keratan sulphate (KS) is the predominant glycosaminoglycan (GAG) in the cornea of the eye, where it exists in proteoglycan (PG) form. KS-PGs have long been thought to play a pivotal role in the establishment and maintenance of the array of regularly-spaced and uniformly- thin collagen fibrils which make up the corneal stroma. This characteristic arrangement of fibrils allows light to pass through the cornea. Indeed, perturbations to the synthesis of KS-PG core proteins in genetically altered mice lead to structural matrix alterations and corneal opacification. Similarly, mutations in enzymes responsible for the sulphation of KS-GAG chains are causative for the inherited human disease, macular corneal dystrophy, which is manifested clinically by progressive corneal cloudiness starting in young adulthood.

Immunophenotypes of macular corneal dystrophy in India and correlation with mutations in CHST6

PURPOSE

To determine the immunophenotypes of macular corneal dystrophy (MCD) in Indian patients and to correlate them with mutations in the carbohydrate 6-sulfotransferase (CHST6) gene.

METHODS

Sixty-four patients from 53 families with MCD that were previously screened for mutations in CHST6 were included in an immunophenotype analysis. Antigenic keratan sulfate (AgKS) in serum as well as corneal tissue was evaluated in 31 families. Only cornea was evaluated in 11 families, and only serum was evaluated in 11 families. AgKS was detected in formalin-fixed, paraffin-embedded corneal sections by immunohistochemistry and in serum by ELISA using a monoclonal antibody against sulfated forms of KS in patients with MCD as well as normal controls.

RESULTS

Analysis of corneal and/or serum AgKS disclosed MCD type I (27 families), MCD type IA (5 families), and MCD type II (3 families) in the cases studied. An additional 10 families were either MCD type I or MCD type IA since only serum AgKS data were available. Seven families manifested atypical immunophenotypes since the corneal AgKS expression was either of MCD type I or MCD type IA, but serum AgKS levels ranged from 19 ng/ml to 388 ng/ml. More than one immunophenotype was detected amongst siblings in two families. Each immunophenotype was associated with mutational heterogeneity in CHST6.

CONCLUSIONS

MCD type I was the predominant immunophenotype in the Indian population studied followed by MCD type IA and then MCD type II. We detected further immunophenotypic heterogeneity by finding atypical patterns of AgKS reactivity in a subset of families. There were no simple correlations between immunophenotypes and specific mutations in CHST6, suggesting that factors other than CHST6 mutations may be contributing to the immunophenotypes in MCD.