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

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.

Vps35-deficiency impairs SLC4A11 trafficking and promotes corneal dystrophy

Vps35 (vacuolar protein sorting 35) is a major component of retromer that selectively promotes endosome-to-Golgi retrieval of transmembrane proteins. Dysfunction of retromer is a risk factor for the pathogenesis of Parkinson’s disease (PD) and Alzheimer’s disease (AD). However, Vps35/retromer’s function in the eye or the contribution of Vps35-deficiency to eye degenerative disorders remains to be explored. Here we provide evidence for a critical role of Vps35 in mouse corneal dystrophy. Vps35 is expressed in mouse and human cornea. Mouse cornea from Vps35 heterozygotes (Vps35^+/-) show features of dystrophy, such as loss of both endothelial and epithelial cell densities, disorganizations of endothelial, stroma, and epithelial cells, excrescences in the Descemet membrane, and corneal edema. Additionally, corneal epithelial cell proliferation was reduced in Vps35-deficient mice. Intriguingly, cell surface targeting of SLC4A11, a membrane transport protein (OH^- /H⁺ /NH₃ /H₂O) of corneal endothelium, whose mutations have been identified in patients with corneal dystrophy, was impaired in Vps35-deficient cells and cornea. Taken together, these results suggest that SLC4A11 appears to be a Vps35/retromer cargo, and Vps35-regulation of SLC4A11 trafficking may underlie Vps35/retromer regulation of corneal dystrophy.