A novel variant of combined granular-lattice corneal dystrophy associated with the Met619Lys mutation in the TGFBI gene

A pathogenic variant in the transforming growth factor beta I (TGFBI) in four Iranian extended families segregating granular corneal dystrophy type II: A literature review

OBJECTIVES

Granular and lattice corneal dystrophies (GCDs & LCDs) are autosomal dominant inherited disorders of the cornea. Due to genetic heterogeneity and large genes, unraveling the mutation is challenging.

MATERIALS AND METHODS

Patients underwent comprehensive clinical examination, and targeted next-generation sequencing (NGS) was used for mutation detection. Co-segregation and in silico analysis was accomplished.

RESULTS

Patients suffered from GCD. NGS disclosed a known pathogenic variant, c.371G>A (p.R124H), in exon 4 of TGFBI. The variant co-segregated with the phenotype in the family. Homozygous patients manifested with more severe phenotypes. Variable expressivity was observed among heterozygous patients.

CONCLUSION

The results, in accordance with previous studies, indicate that the c.371G>A in TGFBI is associated with GCD. Some phenotypic variations are related to factors such as modifier genes, reduced penetrance and environmental effects.

TGF-β1 enhanced myocardial differentiation through inhibition of the Wnt/β-catenin pathway with rat BMSCs

OBJECTIVES

To investigate and test the hypotheses that TGF-β1 enhanced myocardial differentiation through Wnt/β-catenin pathway with rat bone marrow mesenchymal stem cells (BMSCs).

MATERIALS AND METHODS

Lentiviral vectors carrying the TGF-β1 gene were transduced into rat BMSCs firstly. Then several kinds of experimental methods were used to elucidate the related mechanisms by which TGF-β1 adjusts myocardial differentiation in rat BMSCs.

RESULTS

Immunocytochemistry revealed that cTnI and Cx43 expressed positively in the cells that were transduced with TGF-β1. The results of Western blot (WB) test showed that the levels of intranuclear β-catenin and total β-catenin were all significantly decreased. However, the cytoplasmic β-catenin level was largely unchanged. Moreover, the levels of GSK-3β were largely unchanged in BMSCs, whereas phosphorylated GSK-3β was significantly decreased in BMSCs. When given the activator of Wnt/β-catenin pathway (lithium chloride, LiCl) to BMSCs transducted with TGF-β1, β-catenin was increased, while phosphorylated β-catenin was decreased. In addition, cyclinD1, MMP-7, and c-Myc protein in BMSCs transducted with Lenti-TGF-β1-GFP were significantly lower.

CONCLUSION

These results indicate that TGF-β1 promotes BMSCs cardiomyogenic differentiation by promoting the phosphorylation of β-catenin and inhibiting cyclinD1, MMP-7, and c-Myc expression in Wnt/β-catenin signaling pathway.

Mutation update: TGFBI pathogenic and likely pathogenic variants in corneal dystrophies

Human transforming growth factor β-induced (TGFBI), is a gene responsible for various corneal dystrophies. TGFBI produces a protein called TGFBI, which is involved in cell adhesion and serves as a recognition sequence for integrins. An alteration in cell surface interactions could be the underlying cause for the progressive accumulation of extracellular deposits in different layers of the cornea with the resulting changes of refractive index and transparency. To this date, 69 different pathogenic or likely pathogenic variants in TGFBI have been identified in a heterozygous or homozygous state in various corneal dystrophies, including a novel variant reported here. All disease-associated variants were inherited as autosomal-dominant traits but one; this latter was inherited as an autosomal recessive trait. Most corneal dystrophy-associated variants are located at amino acids Arg124 and Arg555. To keep the list of corneal dystrophy-associated variant current, we generated a locus-specific database for TGFBI (http://databases.lovd.nl/shared/variants/TGFBI) containing all pathogenic and likely pathogenic variants reported so far. Non-disease-associated variants are described in specific databases, like gnomAD and ExAC but are not listed here. This article presents the most recent up-to-date list of disease-associated variants.

Cornea guttata associated with special phenotypic variants of granular corneal dystrophy type 2 in a Chinese family

PURPOSE

The aim of this study was to analyze the relevant gene mutations in a Chinese family with special phenotypic variants of granular corneal dystrophy type 2 with cornea guttata.

METHODS

A total of 11 individuals from the affected family underwent complete ophthalmic examination. Genomic DNA was extracted from peripheral leukocytes of affected and unaffected family members. High-throughput sequencing was performed to screen for mutations in 290 genes associated with inherited ophthalmic diseases. Results were validated by bidirectional Sanger sequencing.

RESULTS

An Arg124His (R124H) mutation of the transforming growth factor beta-induced gene was identified in three members of the affected family: the proband (II-1), his mother (I-2), and his son (III-1). The eyes of the proband and his mother had bilateral superficial whitish ring patches with clear centers occupying their central corneas and appeared to be discoid or ring shaped. In addition, specular microscopic examination showed the presence of dark, round bodies. In vivo confocal microscopy showed some hyporeflective round images (cornea guttata), containing occasionally central highlight, in the proband, his mother, and one of his elder sisters.

CONCLUSION

We report, for the first time, atypical granular corneal dystrophy type 2 with cornea guttata associated with a single R124H mutation in a Chinese family. Our findings emphasize that genotyping is essential for the accurate diagnosis and classification of granular corneal dystrophy type 2.

Corneal Dystrophy Mutations Drive Pathogenesis by Targeting TGFBIp Stability and Solubility in a Latent Amyloid-forming Domain

Numerous mutations in the corneal protein TGFBIp lead to opaque extracellular deposits and corneal dystrophies (CDs). Here we elucidate the molecular origins underlying TGFBIp's mutation-induced increase in aggregation propensity through comprehensive biophysical and bioinformatic analyses of mutations associated with every major subtype of TGFBIp-linked CDs including lattice corneal dystrophy (LCD) and three subtypes of granular corneal dystrophy (GCD 1-3). LCD mutations at buried positions in the C-terminal Fas1-4 domain lead to decreased stability. GCD variants show biophysical profiles distinct from those of LCD mutations. GCD 1 and 3 mutations reduce solubility rather than stability. Half of the 50 positions within Fas1-4 most sensitive to mutation are associated with at least one known disease-causing mutation, including 10 of the top 11 positions. Thus, TGFBIp aggregation is driven by mutations that despite their physico-chemical diversity target either the stability or solubility of Fas1-4 in predictable ways, suggesting straightforward general therapeutic strategies.

Structural and Functional Implications of Human Transforming Growth Factor β-Induced Protein, TGFBIp, in Corneal Dystrophies

A major cause of visual impairment, corneal dystrophies result from accumulation of protein deposits in the cornea. One of the proteins involved is transforming growth factor β-induced protein (TGFBIp), an extracellular matrix component that interacts with integrins but also produces corneal deposits when mutated. Human TGFBIp is a multi-domain 683-residue protein, which contains one CROPT domain and four FAS1 domains. Its structure spans ∼120 Å and reveals that vicinal domains FAS1-1/FAS1-2 and FAS1-3/FAS1-4 tightly interact in an equivalent manner. The FAS1 domains are sandwiches of two orthogonal four-stranded β sheets decorated with two three-helix insertions. The N-terminal FAS1 dimer forms a compact moiety with the structurally novel CROPT domain, which is a five-stranded all-β cysteine-knot solely found in TGFBIp and periostin. The overall TGFBIp architecture discloses regions for integrin binding and that most dystrophic mutations cluster at both molecule ends, within domains FAS1-1 and FAS1-4.

Effect of position-specific single-point mutations and biophysical characterization of amyloidogenic peptide fragments identified from lattice corneal dystrophy patients

Corneal stromal dystrophies are a group of genetic disorders that may be caused by mutations in the transforming growth factor β-induced (TGFBI) gene which results in the aggregation and deposition of mutant proteins in various layers of the cornea. The type of amino acid substitution dictates the age of onset, anatomical location of the deposits, morphological features of deposits (amyloid, amorphous powder or a mixture of both forms) and the severity of disease presentation. It has been suggested that abnormal turnover and aberrant proteolytic processing of the mutant proteins result in the accumulation of insoluble protein deposits. Using mass spectrometry, we identified increased abundance of a 32 amino acid-long peptide in the 4th fasciclin-like domain-1 (FAS-1) domain of transforming growth factor β-induced protein (amino acid 611-642) in the amyloid deposits of the patients with lattice corneal dystrophies (LCD). In vitro studies demonstrated that the peptide readily formed amyloid fibrils under physiological conditions. Clinically relevant substitution (M619K, N622K, N622H, G623R and H626R) of the truncated peptide resulted in profound changes in the kinetics of amyloid formation, thermal stability of the amyloid fibrils and cytotoxicity of fibrillar aggregates, depending on the position and the type of the amino acid substitution. The results suggest that reduction in the overall net charge, nature and position of cationic residue substitution determines the amyloid aggregation propensity and thermal stability of amyloid fibrils.

In Vitro Topographical Model of Fuchs Dystrophy for Evaluation of Corneal Endothelial Cell Monolayer Formation

A common indication for corneal transplantation, which is the most transplanted tissue, is a dysfunctional corneal endothelium due to Fuchs' endothelial dystrophy (FED). FED is diagnosed by the presence of in vivo pathological microtopography on the Descemet membrane, which is called corneal guttata. Minimally invasive corneal endothelial cell regenerative procedures such as endothelial cell injection therapy and Rho kinase inhibitor pharmacotherapy have been proposed as alternatives to conventional corneal transplantation for FED patients. However, the effect of guttata on monolayer reformation following such therapies is unknown and there is no equivalent in vitro or animal model to study monolayer reformation. Using a synthetic guttata FED disease model, the formation of the monolayer is investigated to evaluate the efficacy of both therapies. Results obtained suggest that guttata dimensions, density, and spacing greatly affect the fate of corneal endothelial cells in terms of migratory behavior and monolayer reformation. Densely packed synthetic guttata mimicking late-stage FED hinders monolayer reformation, while synthetic guttata of lower height and density show improved monolayer formation. These results suggest that severity of the FED, as determined by height and density of existing guttata, can potentially attenuate corneal endothelial monolayer formation of corneal cell injection therapy and pharmacotherapy.

Pathogenesis and treatments of TGFBI corneal dystrophies

Transforming growth factor beta-induced (TGFBI) corneal dystrophies are a group of inherited progressive corneal diseases. Accumulation of transforming growth factor beta-induced protein (TGFBIp) is involved in the pathogenesis of TGFBI corneal dystrophies; however, the exact molecular mechanisms are not fully elucidated. In this review article, we summarize the current knowledge of TGFBI corneal dystrophies including clinical manifestations, epidemiology, most common and recently reported associated mutations for each disease, and treatment modalities. We review our current understanding of the molecular mechanisms of granular corneal dystrophy type 2 (GCD2) and studies of other TGFBI corneal dystrophies. In GCD2 corneal fibroblasts, alterations of morphological characteristics of corneal fibroblasts, increased susceptibility to intracellular oxidative stress, dysfunctional and fragmented mitochondria, defective autophagy, and alterations of cell cycle were observed. Other studies of mutated TGFBIp show changes in conformational structure, stability and proteolytic properties in lattice and granular corneal dystrophies. Future research should be directed toward elucidation of the biochemical mechanism of deposit formation, the relationship between the mutated TGFBIp and the other materials in the extracellular matrix, and the development of gene therapy and pharmaceutical agents.

Lattice corneal dystrophy type IIIA with hyaline component from a novel A620P mutation and distinct surgical treatments

PURPOSE

The aim of this study was to report a lattice corneal dystrophy (LCD) family with a novel mutation of A620P in the TGFBI gene, its long-term treatment, follow-up data, and related pathologic findings.

METHODS

A total of 28 family members were clinically examined, and blood samples or buccal epithelial cells were taken for DNA analysis. All exons from the entire TGFBI gene coding region were analyzed for mutations in 3 affected members. Exon 14 was amplified in other family members and in 100 normal Korean persons as control. Corneal tissues from 1 affected family member were examined using light and electron microscopy.

RESULTS

Clinical examination revealed relatively late-onset LCD with asymmetric progression and recurrent corneal erosion. The affected family members have been treated with penetrating keratoplasty, deep lamellar keratoplasty, and phototherapeutic keratectomy for up to 19 years. Screening of the TGFBI gene revealed a novel A620P mutation, which was found in all affected members. The amyloid origin of deposits was confirmed by Congo red and was also partially stained with Masson trichrome. Although there were no electron-dense bodies as in granular dystrophy, transmission electron microscopy demonstrated that the stromal deposits were not homogenous and contained a variety of constituents with different electron densities.

CONCLUSIONS

We present the characteristics and surgical treatment of corneas with a novel A620P mutation in TGFBI showing LCD type IIIA with hyaline component.

TGFBI, CHST6, and GSN gene analysis in Mexican patients with stromal corneal dystrophies

OBJECTIVES

The purpose of our study was to describe the results of molecular screening of TGFBI, CHST6, and GSN genes in a group of Mexican patients with different stromal corneal dystrophies (CD).

MATERIAL AND METHODS

A total of 16 CD Mexican patients pertaining to nine different pedigrees were subjected to a complete ophthalmological investigation. A clinical diagnosis of lattice CD was performed in 10 patients from five pedigrees. Three patients from two pedigrees were diagnosed with granular CD type 2, two patients with unrelated probands had Finnish-type corneal amyloidosis, and one patient had macular CD. Genetic analysis included DNA isolation from blood leukocytes and polymerase chain reaction (PCR) amplification and direct nucleotide sequencing of TGFBI, CHST6, and GSN genes.

RESULTS

Seven lattice CD patients from four unrelated families had an identical p.H626R mutation in TGFBI, three patients from a single lattice CD family carried a p.R124C substitution in TGFBI, and a granular type 2 CD pedigree was demonstrated to carry a heterozygous TGFBI p.M619K substitution. A patient having Finnish-type corneal amyloidosis had a p.D187N mutation in GSN. Finally, molecular analysis of CHST6 in a patient with macular CD disclosed the presence of a homozygous p.Y110C change.

CONCLUSIONS

This study improves the knowledge of the genetic features of Mexican patients with corneal stromal dystrophies by identifying mutations in the TGFBI, CHST6, and GSN genes. Genetic screening of larger samples of patients from distinct ethnic groups would be of great importance for a better understanding of the mutational spectrum of stromal CD.

Clinical and genetic aspects of the TGFBI-associated corneal dystrophies

Corneal dystrophies are a group of inherited disorders localized to various layers of the cornea that affect corneal transparency and visual acuity. The deposition of insoluble protein materials in the form of extracellular deposits or intracellular cysts is pathognomic. Mutations in TGFBI are responsible for superficial and stromal corneal dystrophies. The gene product, transforming growth factor β induced protein (TGFBIp) accumulates as insoluble deposits in various forms. The severity, clinicopathogenic variations, age of the onset, and location of the deposits depend on the type of amino acid alterations in the protein. Until 2006, 38 different pathogenic mutants were reported for the TGFBI-associated corneal dystrophies. This number has increased to 63 mutants, reported in more than 30 countries. There is no effective treatment to prevent, halt, or reverse the deposition of TGFBIp. This review presents a complete mutation update, classification of phenotypes, comprehensive reported incidents of various mutations, and current treatment options and their shortcomings. Future research directions and possible approaches to inhibiting disease progression are discussed.

TGFB1-induced extracellular expression of TGFBIp and inhibition of TGFBIp expression by RNA interference in a human corneal epithelial cell line

PURPOSE

To report the increased production of extracellular transforming growth factor β-induced protein (TGFBIp) by human corneal epithelial cells (HCECs) after induction by TGFB1 and the inhibition of TGFBIp production in induced and noninduced HCECs by RNA interference (RNAi).

METHODS

HCECs were cultured in serum-free medium and treated with 0 or 10 ng/mL TGFB1 over a period of 72 hours. Commercially available siRNAs targeting TGFBI mRNA were mixed with a transfection reagent and used to reverse transfect TGFB1-induced and noninduced HCECs. Extracellular and intracellular concentrations of TGFBIp were measured by ELISA and Western blot analysis, respectively, and TGFBI RNA was assayed using semiquantitative RT-PCR.

RESULTS

HCECs constitutively express TGFBIp, and treatment with TGFB1 results in up to a fourfold increase in the amount of extracellular TGFBIp. Four commercially available siRNAs targeting TGFBI mRNA produced a >70% decrease in extracellular TGFBIp within 48 hours after transfection of noninduced HCECs but a <25% decrease in extracellular TGFBIp by 48 hours after transfection of TGFB1-induced HCECs. The suppression of extracellular TGFBIp production correlated with a decrease in intracellular TGFBIp production and TGFBI mRNA expression after transfection.

CONCLUSIONS

Extracellular TGFBIp expression by HCECs is increased several fold after exposure to TGFB1. Both HCEC-constitutive and HCEC-induced TGFBIp production can be inhibited with RNA interference, though the effect was greater and lasted longer for constitutive than induced TGFBIp production. Given that the corneal deposits in the TGFBI dystrophies consist of TGFBIp derived from HCECs, RNAi represents a potential means to inhibit primary dystrophic deposit formation and recurrence after surgical intervention.

Keratoconus associated with corneal stromal amyloid deposition containing TGFBIp

PURPOSE

To report the identification and characterization of stromal amyloid deposits in patients with keratoconus.

METHODS

The excised corneal buttons from 2 patients diagnosed clinically with keratoconus underwent histochemical analysis with Masson trichrome, Congo red, Alcian blue, and periodic acid-Schiff stains, and immunohistochemical analysis for the transforming growth factor beta-induced gene (TGFBI) protein product (TGFBIp), prealbumin, lysozyme, and kappa and lambda light chain expression. After the collection of DNA from both patients, exons 4, 11, 12, 13 and 14 of TGFBI were amplified and sequenced to search for mutations previously associated with dystrophic corneal stromal amyloid deposition.

RESULTS

Light microscopic examination of the corneal buttons revealed stromal thinning, epithelial basement membrane abnormalities, and focal disruption of Bowman layer. Multiple stromal deposits were identified that stained red with Masson trichrome, pink with periodic acid-Schiff, and red with Congo red; the Congo red-stained deposits demonstrated birefringence and dichroism with crossed polarizing lenses. Immunohistochemical staining demonstrated reactivity of the stromal deposits with antibodies to TGFBIp but no reactivity with antibodies against prealbumin, lysozyme, or kappa and lambda light chains. Screening of TGFBI exons 4, 11, 12, 13, and 14 revealed 2 previously identified single nucleotide polymorphisms present in the heterozygous state in both individuals but no other coding region variants.

CONCLUSIONS

Two cases of keratoconus with clinically unsuspected, presumed secondary stromal amyloid deposition are described. Although TGFBIp is identified in the stromal deposits, no previously reported amyloidogenic mutations are identified in TGFBI in either affected individual, indicating a previously undescribed mechanism of stromal amyloid deposition.

The IC3D classification of the corneal dystrophies

BACKGROUND

The recent availability of genetic analyses has demonstrated the shortcomings of the current phenotypic method of corneal dystrophy classification. Abnormalities in different genes can cause a single phenotype, whereas different defects in a single gene can cause different phenotypes. Some disorders termed corneal dystrophies do not appear to have a genetic basis.

PURPOSE

The purpose of this study was to develop a new classification system for corneal dystrophies, integrating up-to-date information on phenotypic description, pathologic examination, and genetic analysis.

METHODS

The International Committee for Classification of Corneal Dystrophies (IC3D) was created to devise a current and accurate nomenclature.

RESULTS

This anatomic classification continues to organize dystrophies according to the level chiefly affected. Each dystrophy has a template summarizing genetic, clinical, and pathologic information. A category number from 1 through 4 is assigned, reflecting the level of evidence supporting the existence of a given dystrophy. The most defined dystrophies belong to category 1 (a well-defined corneal dystrophy in which a gene has been mapped and identified and specific mutations are known) and the least defined belong to category 4 (a suspected dystrophy where the clinical and genetic evidence is not yet convincing). The nomenclature may be updated over time as new information regarding the dystrophies becomes available.

CONCLUSIONS

The IC3D Classification of Corneal Dystrophies is a new classification system that incorporates many aspects of the traditional definitions of corneal dystrophies with new genetic, clinical, and pathologic information. Standardized templates provide key information that includes a level of evidence for there being a corneal dystrophy. The system is user-friendly and upgradeable and can be retrieved on the website www.corneasociety.org/ic3d.