Diagnostic yield of candidate genes in an Australian corneal dystrophy cohort

Abstract

Corneal dystrophies describe a clinically and genetically heterogeneous group of inherited disorders. The International Classification of Corneal Dystrophies (IC3D) lists 22 types of corneal dystrophy, 17 of which have been demonstrated to result from pathogenic variants in 19 identified genes. In this study, we investigated the diagnostic yield of genetic testing in a well-characterised cohort of 58 individuals from 44 families with different types of corneal dystrophy. Individuals diagnosed solely with Fuchs endothelial corneal dystrophy were excluded. Clinical details were obtained from the treating ophthalmologist. Participants and their family members were tested using a gene candidate and exome sequencing approach. We identified a likely molecular diagnosis in 70.5% families (31/44). The detection rate was significantly higher among probands with a family history of corneal dystrophy (15/16, 93.8%) than those without (16/28, 57.1%, p = .015), and among those who had undergone corneal graft surgery (9/9, 100.0%) compared to those who had not (22/35, 62.9%, p = .041). We identified eight novel variants in five genes and identified five families with syndromes associated with corneal dystrophies. Our findings highlight the genetic heterogeneity of corneal dystrophies and the clinical utility of genetic testing in reaching an accurate clinical diagnosis.

Keywords: TGFBI; corneal dystrophy; genetic testing; molecular diagnosis.

FIGURE 1

Clinical photographs. (a) Star, icicle‐shaped, spiny and ring‐like opacities (granular corneal dystrophy type 2, CDSA001.2, TGFBI:p.Arg124His). (b) Diffuse irregular subepithelial and anterior stroma honeycomb opacities (Thiel‐Behnke corneal dystrophy, CDSA175, TGFBI:p.Arg555Gln). (c) Thin branching refractile lines and whitish branching stromal opacities (lattice corneal dystrophy, CDSA316, TGFBI:p.Arg124Cys). (d) Thick central ropy‐appearing lattice lines (lattice corneal dystrophy variant CDSA135, TGFBI:p.Asn622His). (e) Diffuse confluent stromal opacities with multiple translucent thin lattice lines (lattice corneal dystrophy variant, CDSA324, TGFBI:p.Thr538Pro). (f) Diffuse solitary microcysts of the epithelium (Meesmann corneal dystrophy, CDSA360, KRT12:p.Leu384Pro). (g) Central irregular whitish opacities with diffuse stromal haze of the entire cornea (macular corneal dystrophy, CDSA305.1, CHST6:p.Glu283Ter/p.Cys102Gly). (h) Disc‐shaped central opacity with no crystals and peripheral arcus lipoides (Schnyder corneal dystrophy, CDSA336, UBIAD1:p.Asp240Asn). (i) Infrequent small discrete opacities at various levels in the cornea (fleck corneal dystrophy, CDSA314, PIKFYVE:p.Arg782Ter). (j) Polymorphic grey opacities in deep stroma just anterior to Descemet membrane (posterior polymorphous corneal dystrophy, CDSA368, ZEB1:c.688‐1G>A). (k) Diffuse ground‐glass milky haze opacities with thickening of the cornea (congenital hereditary corneal dystrophy, CDSA319, SLC4A11:p.Glu170Lys/p.Glu170Lys). (l) Linear and punctate stromal deposits that appear opaque under direct illumination but translucent under indirect illumination (lattice corneal dystrophy with familial amyloidosis CDSA262.1, GSN:p.Trp493Arg).

FIGURE 2

Corneal histopathology and electron microscopy. (a–d) Granular corneal dystrophy type 2 (CDSA001.3). (a,b) Trichrome staining demonstrated intense trichrome positive subepithelial deposits, morphologically indistinguishable from the deposits seen in granular corneal dystrophy (a, 40X magnification) and stromal (b, 40X magnification) deposits. (c,d) Electron microscopy revealed these deposits to be abundant with large, electron dense granules. (e–h) Thiel‐Behnke corneal dystrophy (CDSA175). (e,f) A thick and irregular subepithelial layer observed in a haematoxylin and eosin (H&E) stained corneal section (e, 20X magnification) demonstrated negative periodic acid‐Schiff (PAS) staining (f, 20X magnification). (g) Undifferentiated Mason trichrome staining revealed subepithelial deposit in a pannus‐like pattern in the region of Bowman's layer (20X magnification). (h) Electron microscopy demonstrated an abundance of thick (14 nm diameter) curly collagen fibres within these depots, distinguishing the phenotype of Thiel‐Behnke from Reis‐Bücklers corneal dystrophy. (i–l) Lattice corneal dystrophy variant (CDSA135). (i) Subepithelial thickening and anterior to mid‐stromal eosinophilic extracellular deposits were observed in an H&E stained specimen (20X magnification). (j) Loss of PAS staining in the region of Bruch's membrane suggested subepithelial deposition (20X magnification). (k) Congo red staining further highlighted the characteristic features of amyloidosis, including with well‐delineated red colouration of amyloid deposits (20X magnification). (l) Apple‐green birefringence under polarised light (20X magnification). (m–p) Macular corneal dystrophy (CDSA160). (m) H&E slides demonstrated patchy loss of subepithelial haematoxylin uptake. (n) Alcian blue (pH 2.5) showed these areas corresponded to positive fine granular deposits, typical of acid mucopolysaccharide accumulation within keratocytes throughout the entire thickness of the corneal stroma (20X magnification). (o,p) Electron microscopy revealed corneal collagen with deposition of electron‐dense lysosomal granules, affirming the diagnosis of macular corneal dystrophy.