Lipid Keratopathy: Histopathology, Major Differential Diagnoses and The Importance of Clinical Correlation

Lipid keratopathy (LK) is a rare ophthalmological condition characterized by a progressive reduction in visual acuity caused by corneal opacification due to central lipid accumulation. LK is characterized by lipid deposits, cholesterol clefts, and neovascularization (NV) leading to disruption in corneal optical quality. LK classification includes a primary and secondary form which depend on pre-existing corneal or systemic disorders and the evidence of NV. Secondary LK is typically associated with a prior occurrence of herpetic infection, such as herpes zoster keratitis. Patients with LK usually present with progressive vision loss and dense cream-colored corneal opacification. Treatment modalities include conservative and surgical approaches focused on corneal NV elimination. When evaluating corneal lipidosis, it is crucial to consider a range of differential diagnoses, including corneal arcus, Schnyder corneal dystrophy, and other corneal deposit conditions. We report a case of a 62-year-old male with herpes zoster keratitis complicated with LK. He presented with painless progressive vision loss and corneal scarring, which raised suspicion about LK diagnosis. This paper emphasizes the importance of correlating clinical and histological findings for accurate LK diagnosis.

UbiA prenyltransferase domain-containing protein 1 (UBIAD1) variant c.695 A > G identified in a multigenerational Japanese family with Schnyder corneal dystrophy

PURPOSE

We aimed to identify pathogenic variations in the UbiA prenyltransferase domain-containing protein 1 (UBIAD1) gene in a Japanese family with Schnyder corneal dystrophy (SCD).

STUDY DESIGN

Clinical study METHODS: Three clinically diagnosed SCD patients from a single pedigree participated. Patients 1 and 2 were 69- and 65-year-old sisters, and patient 3 was the 42-year-old daughter of patient 1. Blood samples from the patients were obtained for genetic analysis. Mutation screening of the two UBIAD1 exons was performed using polymerase chain reaction (PCR)-based DNA sequencing.

RESULTS

All participants were found to be heterozygous for the pathogenic missense variation c.695 A > G (p.Asn232Ser) in exon 2 of UBIAD1.

CONCLUSION

This is the first report on the pathogenic UBIAD1 variation c.695 A > G (p.Asn232Ser) in a Japanese population. SCD is a rare corneal dystrophy, and further research on additional cases will aid in the elucidation of disease mechanisms and development of therapeutic strategies.

Naturally occurring UBIAD1 mutations differentially affect menaquinone biosynthesis and vitamin K-dependent carboxylation

UbiA prenyltransferase domain-containing protein-1 (UBIAD1) is responsible for the biosynthesis of menaquinone-4 (MK-4), a cofactor for extrahepatic carboxylation of vitamin K-dependent (VKD) proteins. Genetic variations of UBIAD1 are mainly associated with Schnyder corneal dystrophy (SCD), a disease characterized by abnormal accumulation of cholesterol in the cornea. Results from in vitro studies demonstrate that SCD-associated UBIAD1 mutations are defective in MK-4 biosynthesis. However, SCD patients do not exhibit typical phenotypes associated with defects of MK-4 or VKD carboxylation. Here, we coupled UBIAD1's biosynthetic activity of MK-4 with VKD carboxylation in HEK293 cells that stably express a chimeric VKD reporter protein. The endogenous Ubiad1 gene in these cells was knocked out by CRISPR-Cas9-mediated genome editing. The effect of UBIAD1 mutations on MK-4 biosynthesis and VKD carboxylation was evaluated in Ubiad1-deficient reporter cells by determining the production of MK-4 or by measuring the efficiency of reporter-protein carboxylation. Our results show that the hot-spot mutation N102S has a moderate impact on MK-4 biosynthesis (retained ˜ 82% activity) but does not affect VKD carboxylation. However, the G186R mutation significantly affected both MK-4 biosynthesis and VKD carboxylation. Other mutations exhibit varying degrees of effects on MK-4 biosynthesis and VKD carboxylation. These results are consistent with in vivo results obtained from gene knock-in mice and SCD patients. Our findings suggest that UBIAD1's MK-4 biosynthetic activity does not directly correlate with the phenotypes of SCD patients. The established cell-based assays in this study provide a powerful tool for the functional studies of UBIAD1 in a cellular milieu.

High expression of Matrix Gla Protein in Schnyder corneal dystrophy patients points to an active role of vitamin K in corneal health

PURPOSE

Schnyder corneal dystrophy (SCD) is a rare autosomal dominant disorder characterized by corneal lipid accumulation and caused by UBIAD1 pathogenic variants. UBIAD1 encodes a vitamin K (VK) biosynthetic enzyme. To assess the corneal and vascular VK status in SCD patients, we focused on matrix Gla protein (MGP), a VK-dependent protein.

METHODS

Conformation-specific immunostainings of different MGP maturation forms were performed on corneal sections and primary keratocytes from corneal buttons of two SCD patients with UBIAD1 p.Asp112Asn and p.Asn102Ser pathogenic variants and unrelated donors. Native or UBIAD1-transfected keratocytes were used for gene expression analysis. Plasma samples from SCD patients (n = 12) and control individuals (n = 117) were subjected for inactive desphospho-uncarboxylated MGP level measurements with an ELISA assay.

RESULTS

Substantial amounts of MGP were identified in human cornea and most of it in its fully matured and active form. The level of mature MGP did not differ between SCD and control corneas. In primary keratocytes from SCD patients, a highly increased MGP expression and presence of immature MGP forms were detected. Significantly elevated plasma concentration of inactive MGP was found in SCD patients.

CONCLUSION

High amount of MGP and the predominance of mature MGP forms in human cornea indicate that VK metabolism is active in the visual system. Availability of MGP seems of vital importance for a healthy cornea and may be related to protection against corneal calcification. Systemic MGP findings reveal a poor vascular VK status in SCD patients and indicate that SCD may lead to cardiovascular consequences.

Schnyder corneal dystrophy-associated UBIAD1 inhibits ER-associated degradation of HMG CoA reductase in mice

Autosomal-dominant Schnyder corneal dystrophy (SCD) is characterized by corneal opacification owing to overaccumulation of cholesterol. SCD is caused by mutations in UBIAD1, which utilizes geranylgeranyl pyrophosphate (GGpp) to synthesize vitamin K₂. Using cultured cells, we previously showed that sterols trigger binding of UBIAD1 to the cholesterol biosynthetic enzyme HMG CoA reductase (HMGCR), thereby inhibiting its endoplasmic reticulum (ER)-associated degradation (ERAD) (Schumacher et al. 2015). GGpp triggers release of UBIAD1 from HMGCR, allowing maximal ERAD and ER-to-Golgi transport of UBIAD1. SCD-associated UBIAD1 resists GGpp-induced release and is sequestered in ER to inhibit ERAD. We now report knockin mice expressing SCD-associated UBIAD1 accumulate HMGCR in several tissues resulting from ER sequestration of mutant UBIAD1 and inhibition of HMGCR ERAD. Corneas from aged knockin mice exhibit signs of opacification and sterol overaccumulation. These results establish the physiological significance of UBIAD1 in cholesterol homeostasis and indicate inhibition of HMGCR ERAD contributes to SCD pathogenesis.

Clinical diversity in patients with Schnyder corneal dystrophy-a novel and known UBIAD1 pathogenic variants

PURPOSE

Schnyder corneal dystrophy (SCD) is a rare inherited disease that leads to gradual vision loss by the deposition of lipids in the corneal stroma. The aim of this study is to report a novel pathogenic variant in the UBIAD1 gene and present clinical and molecular findings in Polish patients with SCD.

METHODS

Individuals (n = 37) originating from four Polish SCD families were subjected for a complete ophthalmological check-up and genetic testing. Corneal changes were visualized by slit-lamp examination, anterior segment optical coherent tomography (AS-OCT), and in vivo confocal microscopy (IVCM).

RESULTS

In a proband with primarily mild SCD that progressed rapidly at the end of the fifth decade of life, a novel missense pathogenic variant in UBIAD1 (p.Thr120Arg) was identified. The other studied SCD family represents the second family reported worldwide with the UBIAD1 p.Asp112Asn variant. SCD in the remaining two families resulted from a frequently identified p.Asn102Ser pathogenic variant. All affected subjects presented a crystalline form of SCD. The severity of corneal changes was age-dependent, and their morphology and localization are described in detail.

CONCLUSION

The novel p.Thr120Arg is the fourth SCD-causing variant lying within the FARM motif of the UBIAD1 protein, which underlines a high importance of this motif for SCD pathogenesis. The current study provides independent evidence for the pathogenic potential of UBIAD1 p.Asp112Asn and new information useful for clinicians.

Schnyder corneal dystrophy and associated phenotypes caused by novel and recurrent mutations in the UBIAD1 gene

Background

The purpose of this study was to identify the genetic cause and describe the clinical phenotype of Schnyder corneal dystrophy (SCD) in six unrelated probands.

Methods

We identified two white Czech, two white British and two South Asian families with a clinical diagnosis of SCD. Ophthalmic assessment included spectral domain optical coherence tomography (SD-OCT) of one individual with advanced disease, and SD-OCT and confocal microscopy of a child with early stages of disease. UBIAD1 coding exons were amplified and Sanger sequenced in each proband. A fasting serum lipid profile was measured in three probands. Paternity testing was performed in one family.

Results

A novel heterozygous c.527G>A; p.(Gly176Glu) mutation in UBIAD1 was identified in one Czech proband. In the second Czech proband, aged 6 years when first examined, a previously described de novo heterozygous c.289G>A; p.(Ala97Thr) mutation was found. Two probands of South Asian descent carried a known c.305G>A; p.(Asn102Ser) mutation in the heterozygous state. Previously reported heterozygous c.361C>T; p.(Leu121Phe) and c.308C>T; p.(Thr103Ile) mutations were found in two white British families. Although crystalline deposits were present in all probands the affected area was small in some individuals. Corneal arcus and stromal haze were the most prominent phenotypical feature in two probands. In the Czech probands, SD-OCT confirmed accumulation of reflective material in the anterior stroma. Crystalline deposits were visualized by confocal microscopy. Mild dyslipidemia was found in all three individuals tested.

Conclusion

Although de novo occurrence of mutations in UBIAD1 is extremely rare, SCD should be considered in the differential diagnosis of bilateral corneal haze and/or crystal deposition, especially in children.

Mutational spectrum of Korean patients with corneal dystrophy

Corneal dystrophy typically refers to a group of rare hereditary disorders with a heterogeneous genetic background. A comprehensive molecular genetic analysis was performed to characterize the genetic spectrum of corneal dystrophies in Korean patients. Patients with various corneal dystrophies underwent thorough ophthalmic examination, histopathologic examination, and Sanger sequencing. A total of 120 probands were included, with a mean age of 50 years (SD = 18 years) and 70% were female. A total of 26 mutations in five genes (14 clearly pathogenic and 12 likely pathogenic) were identified in 49 probands (41%). Epithelial-stromal TGFBI dystrophies, macular corneal dystrophy and Schnyder corneal dystrophy (SCD) showed 100% mutation detection rates, while endothelial corneal dystrophies showed lower detection rates of 3%. Twenty six non-duplicate mutations including eight novel mutations were identified and mutations associated with SCD were identified genetically for the first time in this population. This study provides a comprehensive characterization of the genetic aberrations in Korean patients and also highlights the diagnostic value of molecular genetic analysis in corneal dystrophies.

Molecular Pathogenesis of Corneal Dystrophies: Schnyder Dystrophy and Granular Corneal Dystrophy type 2

The International Committee for Classification of Corneal Dystrophies (IC3D) provides updated data to ophthalmologists by incorporating traditional definitions of corneal dystrophies with new genetic, clinical, and pathologic information. Recent advances in the genetics of corneal dystrophies facilitate more precise classifications and elucidate each classification's molecular mechanisms. Unfortunately, the molecular mechanisms and underlying pathogenic mechanisms have remained obscure, with the exception of Schnyder corneal dystrophy (CD), granular CD type 2 (GCD2), and Fuch's endothelial CD. Here, we review the pathogenesis of Schnyder CD and GCD2.

Mitochondrial damage and cholesterol storage in human hepatocellular carcinoma cells with silencing of UBIAD1 gene expression

Heterozygous mutations in the UBIAD1 gene cause Schnyder corneal dystrophy characterized by abnormal cholesterol and phospholipid deposits in the cornea. Ubiad1 protein was recently identified as Golgi prenyltransferase responsible for biosynthesis of vitamin K2 and CoQ10, a key protein in the mitochondrial electron transport chain. Our study shows that silencing UBIAD1 in cultured human hepatocellular carcinoma cells causes dramatic morphological changes and cholesterol storage in the mitochondria, emphasizing an important role of UBIAD1 in mitochondrial function.