The Clinicopathological Characteristics And Molecular Genetic Study Of Two Rare Corneal Stromal Dystrophies

Purpose:To report clinicopathological characteristics and morphological evaluation of gelatinous drop-like corneal dystrophy(GDLD)and Schnyder corneal dystrophy (SCD) by laser scanning confocal microscopy and Fourier-domain optical coherence tomography (OCT) and identify the genetic defect of these two rare corneal stromal dystrophies in TACSTD2 and UBIAD1 gene.Methods:Three GDLD pedigrees and one SCD pedigree were collected. The patients were subjected to a complete ophthalmic examination that included slit-lamp examination and slit-lamp photography to assess and document the corneal lesions. In vivo laser scanning confocal microscopy and Fourier-domain OCT were performed on both eyes of these patients. Deep lamellar keratoplasty using fresh donor cornea was carried out on four patients and the corneal buttons were examined by light and electron microscopy. Blood samples were taken for subsequent genetic analysis.Genetic analysis included polymerase chain reaction (PCR) amplification and direct nucleotide sequencing of the coding region of the TACSTD2 and UBIAD1 gene in DNA from the patients and their relatives.Results:The characteristic clinical presentation of GDLD is a bilateral, axial, elevated, mulberry-like gelatinous lesion. Corneal buttons of GDLD cases were manifested with subepithelial accumulations of a Congo red positive substance, which showed typical fibrillar structure of amyloid fiber under electron microscopy. Bowman's membrane was disrupted and replaced by these amorphous deposits. Scanning electron microscopy revealed prominent gaps between adjacent corneal epithelial cells.Transmission electron microscopy revealed that the basal epithelial cells contained numerous spike-like cell processes which were invaginated in the underlying amyloid deposits. The morphological evaluation on GDLD by confocal microscopy and OCT highlighted pathological observations at the level of epithelium and anterior stroma. The epithelial cells of GDLD cornea were irregular inshape and often elongated.Large accumulations of brightly reflective amyloid material was noted within or beneath the epithelium and within the anterior stroma. The characteristic clinical manifestations of one SCD patient is the bilateral ring-shaped or disciform opacities consist of fine, polychromatic, needle-shaped crystals, but in the other SCD patient a disciform opacity is present without evidence of crystals.Transmission electron microscopy revealed that numerous electron lucent spaces in the corneal Bowman's membrane and anterior stroma caused by dissolved cholesterol crystal.In vivo laser scanning confocal microscopy revealed pathological alterations of the normal corneal anatomy in the patient with crystals, such as highly reflective needle-shaped or rectangular crystalline structures in the anterior-and mid-stroma. The images acquired from Fourier-domain OCT indicated that the main presence of crystalline deposits was localized within the anterior stroma. Sequencing of the TACSTD2 gene of the GDLD patients revealed three homozygous mutations, c.526₅76del51,C.509C>A and c.356G>A.Sequencing of the UBIAD1 gene of the SCD patient revealed a novel heterozygous mutation, c.292G>A.The identified molecular defect cosegregates with the disease among the affected members of the pedigrees and is not found in 50 healthy controls.Conclusions:The characteristic manifestation of GDLD is subepithelial accumulations of Congo red positive amyloid substance. Schnyder corneal dystrophy with or without crystals can be revealed by in vivo confocal microscopy. We describe in vivo confocal microscopy and Fourier-domain OCT study of GDLD and SCD for the first time in China. The advantages of confocal microscopy and OCT for microstructural analysis of a living human cornea might provide useful information about the process of abnormal deposition and the recurrence after keratoplasty. The newly identified mutations expand the spectrum of mutations in TACSTD2 and UBIAD1 gene. Molecular genetics study improves the understanding of the genetic basis of these two rare corneal stromal dystrophies.