Introduction
Schnyder corneal dystrophy (SCD; OMIM #121800) is a rare autosomal dominant disease classified within the group of stromal dystrophies (IC3D 2015, [
1]) and caused by
UBIAD1 pathogenic variants [
2‐
4]. SCD is characterized by progressive opacification of both corneas resulting from excessive cellular and intracellular accumulation of cholesterol and phospholipids in the corneal stroma. Lipid deposits may take form of crystals, non-crystalline stromal opacity, or
arcus lipoides [
2]. Chemical analysis of SCD corneas shows a tenfold higher content of cholesterol and fivefold higher content of lipids compared to healthy corneas [
5]. SCD prevalence in the general population remains unknown. Early stages of the disease may be asymptomatic and the diagnosis may be delayed until the occurrence of a distinct haze or crystals, commonly in the second decade of life [
6]. While SCD scotopic vision usually remains preserved until the late stages of the disease, photopic vision deteriorates more rapidly with the progression of corneal opacification [
5,
7]. Patients complain of decreasing visual acuity (VA) and glare which is caused by light scattering from the surfaces of corneal crystals. In order to recover vision quality in advanced stages of SCD, penetrating keratoplasty (PKP) is performed. In a group of 115 individuals from 34 SCD families, 54% of patients of at least 50 years and 77% of patients aged 70 or over were subjected to PKP [
5].
To date, 27 non-synonymous point alterations of the
UBIAD1 gene causative of SCD have been described [
8‐
11]. The most frequent pathogenic variants include p.Asn102Ser, p.Gly177Glu/Arg, and p.Leu121Phe. In 2016, p.Thr103Ile, the first de novo
UBIAD1 gene pathogenic variant associated with SCD was identified [
9]. UBIAD1 protein is predicted to contain ten transmembrane (TM) helices, nine of which lie within a functional prenyltransferase domain [
12], which is a key part of the UBIAD1 protein enzymatic activity [
10,
12‐
14]. TM helices emerge from the lipid bilayer into three soluble polypeptide loops. All of the so far identified pathogenic variants leading to SCD encompass this domain [
10]. The first loop is most frequently affected by SCD pathogenic variants which appear to disturb its hydrophilic property [
14].
The purpose of the study was to report two known and a novel UBIAD1 gene variant causative of SCD and present a clinical and molecular characterization of the disease in the context of systemic findings in four previously unreported Polish SCD families.
Discussion
In this study we have identified p.Thr120Arg, a novel heterozygous point alteration in the
UBIAD1 gene causative of SCD. The second pathogenic variant p.Asp112Asn reported here was previously published by Nickerson et al. [
14] in the context of in vitro functional studies. To the best of our knowledge, no detailed clinical characterization of SCD patients with this pathogenic variant has been provided so far. The current study delivers independent evidence for the pathogenic potential of UBIAD1 p.Asp112Asn and reports the genetic variant for the first time in Polish SCD patients. The third
UBIAD1 pathogenic variant detected in the study, p.Asn102Ser, is identified in the majority of unrelated SCD families in different ethnic groups and it is believed that this variant represents a hot spot change for SCD [
4].
We have identified the pathogenic variant p.Asn102Ser in two unrelated Polish SCD families in as many as 12 out of 18 genetically confirmed SCD patients. One of the families (Ped. no. 272, Fig.
1d) represents one of the most numerous SCD kindreds, so far reported, for which we have conducted thorough ophthalmological and genetic examinations. Our findings contribute to a previous study describing two other Polish families with SCD as a result of
UBIAD1 p.Asn102Ser [
11] and confirm that the pathogenic variant is also the most common genetic alteration found in SCD patients from Central Europe.
The last decade brought the discovery of several fundamental functions of the
UBIAD1-encoded protein. These encompass (i) synthesis of human endogenous form of vitamin K
2 (MK-4) from derivates of a plant form vitamin K
1 [
10,
20‐
25], (ii) prevention of oxidative damage in tissues by synthesis of non-mitochondrial coenzyme Q10 [
26,
27], and (iii) direct and indirect interaction with proteins that regulate cholesterol synthesis and transport (HMGCR, SOAT1, apoE) [
10,
28,
29]. UBIAD1 protein plays a crucial role in maintaining lipid-cholesterol homeostasis in different cell types [
10,
12,
21,
30,
31] but the molecular mechanism by which UBIAD1 pathogenic variants affect the cornea leading to lipid deposition in SCD patients has yet to be determined.
Defective function of UBIAD1 protein results in reduction of the local synthesis of endogenous form of vitamin K
2 in cells and impairment of cholesterol and lipid metabolism leading to a continuous steroidogenesis stimulation and tissue-specific cholesterol and lipid deposition [
10,
29]. Codon p.Thr120 of the UBIAD1 protein is placed directly between two other codons which were previously identified to be altered in SCD — p.Arg119Gly and p.Leu121Phe/Val [
10]. All of these amino acids are placed within the first aspartate–rich motif (FARM) which localizes to the first polypeptide loop of the UBIAD1 protein. It is a highly conserved region that may play a crucial role in synthesis of sterols and isoprenoid lipids, as well as cellular cholesterol binding, storage, and transport [
10,
12]. Accordingly, p.Thr120Arg along with other SCD causing pathogenic variants is predicted to strongly affect UBIAD1 protein folding and stability, protein enzymatic function, and protein–protein interactions. These alterations may have deleterious impact on cholesterol metabolism in the cornea, contributing to lipid deposition and cholesterol esterification, which may lead to corneal haze and crystalline formation, characteristic features of the SCD phenotype.
Some of our SCD patients reported cardiovascular system disorders (6/18; 33%) and/or cholelithiasis (Table
1). The most frequent systemic finding in SCD is an elevated cholesterol level in blood plasma. Generally, hypercholesterolemia is shown to be present in 66% of patients with SCD [
5]. In the 2013–2014 survey on Polish population, the prevalence of hypercholesterolemia averaged 67.3%, (70.3% for men, 64.3% for women) [
32]. Occurrence of cholesterol deposits in the cornea is described to show no relation with severity of systemic dyslipidemia [
6]. Moreover, SCD corneas present a greater tendency to accumulate high-density lipoproteins (HDL) than low-density lipoproteins (LDL) [
33]. Progression of the corneal opacification is also not related to the level of lipids in the blood plasma [
5]. It is shown that statin treatment and control of systemic cholesterol do not inhibit the progression of SCD [
34].
In line with other reports, we have observed a gradual loss of VA in SCD patients which was progressing along with the severity of dystrophic changes and corresponded with the age of affected individuals [
5,
6,
14,
35]. Unlike the moderate severity stage of SCD, which is usually recognized on the basis of slit-lamp biomicroscopy, initial and advanced stages of the disease tend to cause a more significant diagnostic problem. In the early SCD stage, the signs can be easily overlooked and in the advanced stage, fused corneal opacities and stromal deposits may resemble other corneal dystrophies or corneal degeneration. The majority of our patients presented a moderate severity stage of SCD and slit-lamp examination demonstrated a characteristic clinical picture of the dystrophy. Interestingly, at the end of the fifth decade of life in the proband with
UBIAD1 p.Thr120Arg initially mild dystrophic changes progressed rapidly from an early to advanced SCD stage only within a 3-year observation period. It is a quite unusual finding as SCD generally progresses gradually [
5,
6,
14,
35].
In general, the appearance of deposits, their reflectance, location, and the images of corneal epithelium and endothelium in IVCM imaging in our patients is in line with the descriptions by other authors [
33,
36‐
38]. However, in patients with p.Asp112Asn, we also observed small cysts with hyperreflective content in the corneal epithelium (Fig.
2j); such changes are only rarely observed in SCD patients [
11,
39]. To the best of our knowledge, there are only two other reports on SCD visualized by AS-OCT [
11,
40]. Both studies described stromal hyperreflective opacities limited to the anterior parts of the cornea corresponding with the localization of crystalline formation visible in IVCM images, which is consistent with our observations. In AS-OCT, the appearance of corneal changes was similar in all subjects, but the quantity of deposits was noticeably different and appropriate to SCD stage.
Corneal imaging with IVCM and AS-OCT is proven to be helpful in differential diagnosis of inapparent SCD cases. However, IVCM may not be a conclusive approach as corneal crystalline formations in SCD are similar to those observed, e.g., in cystinosis or infectious crystalline keratopathy. Along with the increasing availability of genetic testing, identification of an UBIAD1 pathogenic variant has become a necessary complement to ophthalmological examinations as it provides a definitive confirmation of clinical SCD diagnosis.