[The different opacity patterns of Lisch corneal dystrophy]

Lisch Epithelial Corneal Dystrophy Is Caused by Heterozygous Loss-of-Function Variants in MCOLN1

PURPOSE

To report the genetic etiology of Lisch epithelial corneal dystrophy (LECD).

DESIGN

Multicenter cohort study.

METHODS

A discovery cohort of 27 individuals with LECD from 17 families, including 7 affected members from the original LECD family, 6 patients from 2 new families and 14 simplex cases, was recruited. A cohort of 6 individuals carrying a pathogenic MCOLN1 (mucolipin 1) variant was reviewed for signs of LECD. Next-generation sequencing or targeted Sanger sequencing were used in all patients to identify pathogenic or likely pathogenic variants and penetrance of variants.

RESULTS

Nine rare heterozygous MCOLN1 variants were identified in 23 of 27 affected individuals from 13 families. The truncating nature of 7 variants and functional testing of 1 missense variant indicated that they result in MCOLN1 haploinsufficiency. Importantly, in the homozygous and compound-heterozygous state, 4 of 9 LECD-associated variants cause the rare lysosomal storage disorder mucolipidosis IV (MLIV). Autosomal recessive MLIV is a systemic disease and comprises neurodegeneration as well as corneal opacity of infantile-onset with epithelial autofluorescent lysosomal inclusions. However, the 6 parents of 3 patients with MLIV confirmed to carry pathogenic MCOLN1 variants did not have the LECD phenotype, suggesting MCOLN1 haploinsufficiency may be associated with reduced penetrance and variable expressivity.

CONCLUSIONS

MCOLN1 haploinsufficiency is the major cause of LECD. Based on the overlapping clinical features of corneal epithelial cells with autofluorescent inclusions reported in both LECD and MLIV, it is concluded that some carriers of MCOLN1 haploinsufficiency-causing variants present with LECD.

In vivo confocal microscopic findings in Lisch corneal dystrophy

PURPOSE

To describe the in vivo confocal microscopic and clinicopathologic correlations in Lisch corneal dystrophy.

METHODS

This is a retrospective case series of 2 patients with Lisch corneal dystrophy. The diagnosis was made based on clinical findings in both cases and was confirmed histopathologically following epithelial debridement in case 1. In vivo laser scanning confocal microscopy using the Heidelberg Retina Tomograph III with the Rostock Cornea Module was carried out in both cases.

RESULTS

Clinical examination of the corneas revealed areas of epithelial opacification that were sharply demarcated in juxtaposition with normal corneal epithelium. The gray feathery appearance of the epithelial lesions in both cases was characteristic of Lisch corneal dystrophy. The central visual axis was involved in case 1, and corneal topography showed irregular astigmatism. Histological analysis of the epithelial cells in this case showed intracytoplasmic vacuoles, confirming the diagnosis of Lisch corneal dystrophy. In vivo confocal microscopy in both cases demonstrated highly hyperreflective epithelial cytoplasm with hypo-reflective nuclei. There was involvement of all epithelial layers extending to the limbus and findings on imaging were confined to the clinically observed areas of corneal opacity. The lesion in case 1 recurred after epitheliectomy of the central cornea without removal of affected limbal cells.

CONCLUSIONS

The unique features on in vivo confocal microscopy correlated with the clinical and histopathologic features of Lisch corneal dystrophy may be used to distinguish this disorder from other corneal epithelial conditions. The affected epithelial cells appear to originate from abnormal limbal stem cells.

The IC3D classification of the corneal dystrophies

BACKGROUND

The recent availability of genetic analyses has demonstrated the shortcomings of the current phenotypic method of corneal dystrophy classification. Abnormalities in different genes can cause a single phenotype, whereas different defects in a single gene can cause different phenotypes. Some disorders termed corneal dystrophies do not appear to have a genetic basis.

PURPOSE

The purpose of this study was to develop a new classification system for corneal dystrophies, integrating up-to-date information on phenotypic description, pathologic examination, and genetic analysis.

METHODS

The International Committee for Classification of Corneal Dystrophies (IC3D) was created to devise a current and accurate nomenclature.

RESULTS

This anatomic classification continues to organize dystrophies according to the level chiefly affected. Each dystrophy has a template summarizing genetic, clinical, and pathologic information. A category number from 1 through 4 is assigned, reflecting the level of evidence supporting the existence of a given dystrophy. The most defined dystrophies belong to category 1 (a well-defined corneal dystrophy in which a gene has been mapped and identified and specific mutations are known) and the least defined belong to category 4 (a suspected dystrophy where the clinical and genetic evidence is not yet convincing). The nomenclature may be updated over time as new information regarding the dystrophies becomes available.

CONCLUSIONS

The IC3D Classification of Corneal Dystrophies is a new classification system that incorporates many aspects of the traditional definitions of corneal dystrophies with new genetic, clinical, and pathologic information. Standardized templates provide key information that includes a level of evidence for there being a corneal dystrophy. The system is user-friendly and upgradeable and can be retrieved on the website www.corneasociety.org/ic3d.