Deposits of transforming growth factor-beta-induced protein in granular corneal dystrophy type II after LASIK

IC3D Classification of Corneal Dystrophies-Edition 3

PURPOSE

The International Committee for the Classification of Corneal Dystrophies (IC3D) was created in 2005 to develop a new classification system integrating current information on phenotype, histopathology, and genetic analysis. This update is the third edition of the IC3D nomenclature.

METHODS

Peer-reviewed publications from 2014 to 2023 were evaluated. The new information was used to update the anatomic classification and each of the 22 standardized templates including the level of evidence for being a corneal dystrophy [from category 1 (most evidence) to category 4 (least evidence)].

RESULTS

Epithelial recurrent erosion dystrophies now include epithelial recurrent erosion dystrophy, category 1 ( COL17A1 mutations, chromosome 10). Signs and symptoms are similar to Franceschetti corneal dystrophy, dystrophia Smolandiensis, and dystrophia Helsinglandica, category 4. Lisch epithelial corneal dystrophy, previously reported as X-linked, has been discovered to be autosomal dominant ( MCOLN1 mutations, chromosome 19). Classic lattice corneal dystrophy (LCD) results from TGFBI R124C mutation. The LCD variant group has over 80 dystrophies with non-R124C TGFBI mutations, amyloid deposition, and often similar phenotypes to classic LCD. We propose a new nomenclature for specific LCD pathogenic variants by appending the mutation using 1-letter amino acid abbreviations to LCD. Pre-Descemet corneal dystrophies include category 1, autosomal dominant, punctiform and polychromatic pre-Descemet corneal dystrophy (PPPCD) ( PRDX3 mutations, chromosome 10). Typically asymptomatic, it can be distinguished phenotypically from pre-Descemet corneal dystrophy, category 4. We include a corneal dystrophy management table.

CONCLUSIONS

The IC3D third edition provides a current summary of corneal dystrophy information. The article is available online at https://corneasociety.org/publications/ic3d .

Deterioration of Avellino corneal dystrophy in a Chinese family after LASIK

AIM

To reveal the importance of TGFBI gene screening for candidates with a family history of corneal disease or granular opacities in corneal stroma before refractive surgery.

METHODS

A 37-year-old male (proband) underwent bilateral laser-assisted in situ keratomileusis (LASIK) in 2002, with right vision decreased significantly in 2006. The proband and other 32 members of the family underwent a detailed ophthalmic examination, including vision acuity, intraocular pressure, slit-lamp photograph, fundus examination, optical coherence tomography (OCT) of cornea, and in vivo confocal microscope (IVCM) and peripheral blood was used for genomic DNA extraction. Seventeen TGFBI gene exons were analyzed via polymerase chain reaction amplification and direct sequencing.

RESULTS

Slit-lamp, IVCM, and OCT images showed that a large amount of dense and confluent granular opaque were seen at the interfaces of the flap and remnant stromal bed in right and light degree in left eye. Sanger sequencing showed that there was a 371G>A mutation (CGC>CAC) in exon 4, which indicated that he harbored a heterozygote R124H mutation, identifying the diagnosis of Avellino corneal dystrophy (ACD). Among the other 32 family members, 6 of them harbored the identical mutation to that in the proband.

CONCLUSION

ACD will worsen and recur after LASIK. Preoperative gene-screening for TGFBI mutations is important in diagnosing ACD.

Laser refractive surgery in corneal dystrophies

Twenty-eight case reports and case series published between 2000 and 2019 concerning laser refractive surgery in patients with corneal dystrophies, resulting in 173 eyes from 94 patients, were included in this systematic review. Best results were achieved in posterior corneal polymorphous and Cogan dystrophy. Unfavorable results were found in Avellino dystrophy and Fuchs endothelial corneal dystrophy (FECD). Photorefractive keratectomy was not indicated in Meesmann and Avellino dystrophy. Laser in situ keratomileusis was indicated in posterior polymorphous corneal dystrophy but not in FECD, Avellino, or Cogan dystrophy. Small-incision lenticule extraction and other dystrophies such as lattice, fleck, Lisch, or François did not achieve enough scientific evidence to report any recommendation.

Effect of osmolytes on in-vitro aggregation properties of peptides derived from TGFBIp

Protein aggregation has been one of the leading triggers of various disease conditions, such as Alzheimer’s, Parkinson’s and other amyloidosis. TGFBI-associated corneal dystrophies are protein aggregation disorders in which the mutant TGFBIp aggregates and accumulates in the cornea, leading to a reduction in visual acuity and blindness in severe cases. Currently, the only therapy available is invasive and there is a known recurrence after surgery. In this study, we tested the inhibitory and amyloid dissociation properties of four osmolytes in an in-vitroTGFBI peptide aggregation model. The 23-amino acid long peptide (TGFBIp 611–633 with the mutation c.623 G>R) from the 4th FAS-1 domain of TGFBIp that rapidly forms amyloid fibrils was used in the study. Several biophysical methods like Thioflavin T (ThT) fluorescence, Circular Dichroism (CD), fluorescence microscopy and Transmission electron microscopy (TEM) were used to study the inhibitory and amyloid disaggregation properties of the four osmolytes (Betaine, Raffinose, Sarcosine, and Taurine). The osmolytes were effective in both inhibiting and disaggregating the amyloid fibrils derived from TGFBIp 611–633 c.623 G>R peptide. The osmolytes did not have an adverse toxic effect on cultured human corneal fibroblast cells and could potentially be a useful therapeutic strategy for patients with TGFBIp corneal dystrophies.

Long-term Clinical Outcomes of Phototherapeutic Keratectomy in Corneas With Granular Corneal Dystrophy Type 2 Exacerbated After LASIK

PURPOSE

To investigate the long-term clinical outcomes and recurrence patterns of phototherapeutic keratectomy (PTK) in patients with granular corneal dystrophy type 2 (GCD2) exacerbated after LASIK.

METHODS

Fifty-one patients (76 eyes) with GCD2 exacerbated after LASIK who underwent PTK between January 2007 and February 2017 were included. Participants underwent ophthalmic examination, including slit-lamp microscopy, corrected distance visual acuity (CDVA), slit-lamp photography, and Fourier domain optical coherence tomography at preoperative and postoperative visits. PTK was performed using VISX S4 IR (VISX, Inc., Santa Clara, CA). Visual acuity, complications, interval, and contributing factors of recurrence were evaluated.

RESULTS

The follow-up period ranged from 1 to 108 months (mean: 35.22 months). The mean logMAR CDVA was 0.55 ± 0.43 (Snellen equivalent 20/80) preoperatively and 0.09 ± 0.43 (Snellen equivalent 20/25) at 3 months postoperatively. Forty-five (61.6%) eyes developed biomicroscopic recurrence at a mean of 18.6 months after PTK; 20 (27.4%) eyes developed significant recurrence at a mean of 31.3 months after PTK. The flap removal group demonstrated better CDVA at 3 years after surgery and lower recurrence and complication rates than the flap conservation group. Multivariate analysis revealed that flap removal remarkably reduced the risk of both any sign of and significant recurrence.

CONCLUSIONS

PTK improved corneal transparency and visual acuity in patients with GCD2 exacerbated after LASIK, although GCD2 eventually recurred. PTK with flap removal was superior to PTK with flap conservation in terms of visual acuity, recurrence, and complications. [J Refract Surg. 2018;34(2):132-139.].

Lithium chloride (LiCl) induced autophagy and downregulated expression of transforming growth factor β-induced protein (TGFBI) in granular corneal dystrophy

This study evaluated whether lithium chloride (LiCl) prevents cytoplasmic accumulation of mutant-transforming growth factor β-induced protein (Mut-TGFBI) in granular corneal dystrophy (GCD) via activation of the autophagy pathway. Levels of TGFBI and microtubule-associated protein 1A/1B-light chain 3 (LC3) in 3 GCD patients and healthy controls were analyzed by immunohistochemistry (IHC) staining and Western blot. Primary corneal fibroblasts were isolated and transfected with wild type or mutant type TGFBI over-expressed vectors, and then treated with LiCl and/or autophagy inhibitor 3-methyladenine (3-MA). Then, levels of TGFBI, glycogen synthase kinase-3 (GSK-3) and LC3-I/-II were detected. Cell viability and transmission electron microscopy assay were also performed. Levels of TGFBI and LC3 were significantly increased in GCD patients. Over-expression of mutant type TGFBI inhibited cell viability and induced autophagy in corneal fibroblasts. LiCl downregulated the expression of TGFBI in mutant type TGFBI over-expressed cells in a dose- and time-dependent manner. LiCl enhanced autophagy in mutant type TGFBI over-expressed cells and recovered cell viability in those cells. However, the effects of LiCl were partly attenuated when autophagy was suppressed by 3-MA. To summarize, treatment with LiCl inhibited the expression of TGFBI and recovery the inhibitory of mutant type TGFBI in cell viability, at least part through enhancing of autophagy. These data strongly suggest that LiCl may be useful in the treatment of GCD.

Association Between Visual Acuity and the Corneal Area Occupied by Granular Lesions, Linear Lesions, or Diffuse Haze in Patients With Granular Corneal Dystrophy Type 2

PURPOSE

In granular corneal dystrophy type 2 (GCD2), corneal deposits containing fragments of transforming growth factor-β-induced protein appear in sequence as granular lesions (GLs), linear lesions (LLs), and diffuse haze (DH). We aimed to investigate the association between visual acuity and age-related changes in lesion types in patients with GCD2.

METHODS

We retrospectively evaluated the medical records and slit-lamp photographs of 533 patients (1066 eyes) with genetically confirmed GCD2 (heterozygous). Deposits were classified into GLs, LLs, and DH, after which the area occupied by each lesion type was measured. The associations between visual acuity and each lesion area were also evaluated using univariable and multivariable models.

RESULTS

The area occupied by GLs increased beginning in the teen years, whereas the area occupied by DH began to increase after age 40. However, the area occupied by LLs increased until patients had reached their 40s, after which no further increases occurred. Multivariable analyses revealed no association between GLs and low visual acuity after adjusting for age and sex. Increases in the LL area were associated with lower odds for low visual acuity (adjusted odds ratio = 0.4 for the third tertile, 95% confidence interval, 0.2-0.97), whereas increases in the DH area were positively associated with low visual acuity (adjusted odds ratio = 3.3 for the third tertile of the DH area, 95% confidence interval, 1.3-8.3).

CONCLUSIONS

Our findings indicated that LLs associated with GCD2 do not produce significant decreases in visual acuity, whereas DH exerts the most significant negative impact on visual acuity. Late-onset DH may be associated with the termination of LL formation.

Vortex Pattern of Corneal Deposits in Granular Corneal Dystrophy Associated With the p.(Arg555Trp) Mutation in TGFBI

PURPOSE

To describe 2 unrelated families with multiple members demonstrating a less commonly recognized vortex pattern of corneal deposits confirmed to be granular corneal dystrophy type 1 (GCD1) after identification of the p.(Arg555Trp) mutation in the transforming growth factor β-induced gene (TGFBI).

METHODS

A slit-lamp examination was performed on individuals from 2 families, one of Mexican descent and a second of Italian descent. After DNA extraction from affected individuals and their unaffected relatives, TGFBI screening was performed.

RESULTS

Eight of 20 individuals in the Mexican family and 20 of 55 in the Italian family demonstrated corneal stromal opacities. Seven of the 8 affected individuals in the Mexican family and 4 of the 20 affected individuals in the Italian family demonstrated a phenotype characterized by a "sea fan" or vortex pattern of superficial stromal corneal deposits originating from the inferior aspect of the cornea. Screening of TGFBI in both families revealed a heterozygous missense mutation [p.(Arg555Trp)] in exon 12, confirming the diagnosis of GCD1.

CONCLUSIONS

Our findings demonstrate that GCD1 may present with a vortex pattern of anterior stromal deposits. Although this pattern of dystrophic deposits is not recognized by clinicians as a typical phenotype of GCD1, it is consistent with the production of the majority of the TGFBI protein by the corneal epithelium.

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.

Fibril Core of Transforming Growth Factor Beta-Induced Protein (TGFBIp) Facilitates Aggregation of Corneal TGFBIp

Mutations in the transforming growth factor beta-induced (TGFBI) gene result in a group of hereditary diseases of the cornea that are collectively known as TGFBI corneal dystrophies. These mutations translate into amino acid substitutions mainly within the fourth fasciclin 1 domain (FAS1-4) of the transforming growth factor beta-induced protein (TGFBIp) and cause either amyloid or nonamyloid protein aggregates in the anterior and central parts of the cornea, depending on the mutation. The A546T substitution in TGFBIp causes lattice corneal dystrophy (LCD), which manifests as amyloid-type aggregates in the corneal stroma. We previously showed that the A546T substitution renders TGFBIp and the FAS1-4 domain thermodynamically less stable compared with the wild-type (WT) protein, and the mutant FAS1-4 is prone to amyloid formation in vitro. In the present study, we identified the core of A546T FAS1-4 amyloid fibrils. Significantly, we identified the Y571-R588 region of TGFBIp, which we previously found to be enriched in amyloid deposits in LCD patients. We further found that the Y571-R588 peptide seeded fibrillation of A546T FAS1-4, and, more importantly, we demonstrated that native TGFBIp aggregates in the presence of fibrils formed by the core peptide. Collectively, these data suggest an involvement of the Y571-R588 peptide in LCD pathophysiology.

Biochemical properties and aggregation propensity of transforming growth factor-induced protein (TGFBIp) and the amyloid forming mutants

TGFBI-associated corneal dystrophies are characterized by accumulation of insoluble deposits of the mutant protein transforming growth factor β-induced protein (TGFBIp) in the cornea. Depending on the nature of mutation, the lesions appear as granular (non-amyloid) or lattice lines (amyloid) in the Bowman's layer or in the stroma. This review article emphasizes the structural biology aspects of TGFBIp. We discuss the tinctorial properties and ultrastructure of deposits observed in granular and lattice corneal dystrophic mutants with amyloid and non-amyloid forms of other human protein deposition diseases and review the biochemical and putative functional role of the protein. Using bioinformatics tools, we identify intrinsic aggregation propensity and discuss the possible protective role of gatekeepers close to the "aggregation-prone" regions of native TGFBIp. We describe the relative aggregation rates of lattice corneal dystrophy (LCD) and granular corneal dystrophy (GCD2) mutants using the three-parameter model, which is based on intrinsic properties of polypeptide chains. The predictive power of this model is compared with two other algorithms. We conclude that the model is able to predict the aggregation rate of mutants which do not alter overall net charge of the protein. The need to understand the mechanism of corneal dystrophies from the structural biology viewpoint is emphasized.

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.

Thermodynamic and structural characterization of an antibody gel

Although extensively studied, protein-protein interactions remain highly elusive and are of increasing interest in drug development. We show the assembly of a monoclonal antibody, using multivalent carboxylate ions, into highly-ordered structures. While the presence and function of similar structures in vivo are not known, the results may present a possible unexplored area of antibody structure-function relationships. Using a variety of tools (e.g., mechanical rheology, electron microscopy, isothermal calorimetry, Fourier transform infrared spectroscopy), we characterized the physical, biochemical, and thermodynamic properties of these structures and found that citrate may interact directly with the amino acid residue histidine, after which the individual protein units assemble into a filamentous network gel exhibiting high elasticity and interfilament interactions. Citrate interacts exothermically with the monoclonal antibody with an association constant that is highly dependent on solution pH and temperature. Secondary structure analysis also reveals involvement of hydrophobic and aromatic residues.

Altered mitochondrial function in type 2 granular corneal dystrophy

Type 2 granular corneal dystrophy (GCD2) is caused by point mutation R124H in the transforming growth factor-β-induced gene (TGFBI) and is characterized by age-dependent progression of corneal deposits. Mitochondrial features in heterozygous GCD2 and normal corneal tissues was evaluated using electron microscopy. Primary corneal fibroblasts of homozygous and normal corneas were cultured to passage 4 or 8. Keratocytes of normal corneal tissue are narrow, and details of their intracellular organelles are difficult to distinguish. Keratocytes of heterozygous GCD2 tissues exhibited many degenerative mitochondria. MitoTracker and cytochrome c staining demonstrated increased mitochondrial activity in mutated cells at early passages. Decreases in depolarized mitochondria, cellular proliferation, and expression of complexes I to V and increases in apoptotic change were observed in late-passage mutant fibroblasts. PGC-1α, ANT-1, p-Akt, and p-mTOR but not NF-κB expression demonstrated a passage-dependent decrease in all cells. Increased passage- or mutation-related intracellular reactive oxygen species and delayed proliferation of methanethiosulfonate (MTS) were recovered using application of antioxidant butylated hydroxyanisole. Mitochondrial features and function were altered in mutated GCD2 keratocytes, in particular in older cells. Alteration of mitochondrial function is critical for understanding the pathogenesis of GCD2.

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.

Clinical findings and treatments of granular corneal dystrophy type 2 (avellino corneal dystrophy): a review of the literature

OBJECTIVES

To review the literature about clinical findings and treatments of granular corneal dystrophy type 2 (GCD2).

METHODS

Various literatures on clinical findings, exacerbations after refractive corneal surgery, and treatment modalities of GCD2 were reviewed.

RESULTS

GCD2 is an autosomal dominant disease. Mutation of transforming growth factor beta-induced gene, TGFBI, or keratoepithelin gene in human chromosome 5 (5q31) is the key pathogenic process in patient with GCD2. Corneal trauma activates TGFBI and then it overproduces transforming growth factor beta-induced gene protein (TGFBIp), which is main component of the corneal opacity. Refractive corneal surgery is a popular procedure to correct refractive error worldwide. However, several cases about exacerbation of GCD2 after corneal refractive surgery such as photorefractive keratectomy, laser in situ keratomileusis, and laser epithelial keratomileusis have been reported. The opacities deteriorate patient's best-corrected visual acuity. Recurrence-free interval varies many factors such as the type of procedure the patient had received and the genotype of the patient. To treat the opacities in GCD2, phototherapeutic keratectomy, lamellar keratoplasty, deep lamellar keratoplasty, and penetrating keratoplasty (PKP) were used. However, the recurrence is still an unsolved problem.

CONCLUSIONS

Perfect treatment of exacerbation after corneal surface ablation does not exist until now. To prevent exacerbation, refractive surgeons must do a careful preoperative examination of candidates in refractive surgeries.