Phenotypic variability in Meesmann's dystrophy: clinical review of the literature and presentation of a family genetically identical to the original family

Genetic predisposition to ocular surface disorders and opportunities for gene-based therapies

The ocular surface, comprised of the corneal and conjunctival epithelium, innervation system, immune components, and tear-film apparatus, plays a key role in ocular integrity as well as comfort and vision. Gene defects may result in congenital ocular or systemic disorders with prominent ocular surface involvement. Examples include epithelial corneal dystrophies, aniridia, ectrodactyly-ectodermal dysplasia-clefting (EEC) syndrome, xeroderma pigmentosum (XP), and hereditary sensory and autonomic neuropathy. In addition, genetic factors may interact with environmental risk factors in the development of several multifactorial ocular surface disorders (OSDs) such as autoimmune disorders, allergies, neoplasms, and dry eye disease. Advanced gene-based technologies have already been introduced in disease modelling and proof-of-concept gene therapies for monogenic OSDs. For instance, patient-derived induced pluripotent stem cells have been used for modelling aniridia-associated keratopathy (AAK), XP, and EEC syndrome. Moreover, CRISPR/Cas9 genome editing has been used for disease modelling and/or gene therapy for AAK and Meesmann's epithelial corneal dystrophy. A better understanding of the role of genetic factors in OSDs may be helpful in designing personalized disease models and treatment approaches. Gene-based approaches in monogenic OSDs and genetic predisposition to multifactorial OSDs such as immune-mediated disorders and neoplasms with known or possible genetic risk factors has been seldom reviewed. In this narrative review, we discuss the role of genetic factors in monogenic and multifactorial OSDs and potential opportunities for gene therapy.

Identification of a Novel Missense KRT12 Mutation in a Vietnamese Family with Meesmann Corneal Dystrophy

Meesmann epithelial corneal dystrophy (MECD) is a rare dominantly inherited disorder that is characterized by corneal epithelial microcysts and is associated with mutations in the keratin 3 (KRT3) and keratin 12 (KRT12) genes. In this study, we report a novel mutation in the KRT12 gene in a Vietnamese pedigree with MECD. Slit-lamp examination was performed on each of the 7 recruited members of a Vietnamese family to identify characteristic features of MECD. After informed consent was obtained from each individual, genomic DNA was isolated from saliva samples and screening of KRT3and KRT12 genes was performed by Sanger sequencing. The proband, a 31-year-old man, complained of a 1-year history of eye irritation and photophobia. Slit-lamp examination revealed intraepithelial microcysts involving only the corneal periphery in each eye with clear central corneas and no stromal or endothelial involvement. Three family members demonstrated similar intraepithelial microcysts, but with diffuse involvement, extended from limbus to limbus. Sanger sequencing of KRT3 (exon 7) and KRT12 (exons 1 and 6) in the proband revealed a novel heterozygous KRT12 variant (c.1273G>A [p.Glu425Lys]) that was present in the three affected family members but was absent in the three family members with clear corneas. This study is the first report of a Vietnamese family affected with MECD, associated with an atypical peripheral corneal epithelial phenotype in the proband and a novel mutation in KRT12.

New Frontiers of Corneal Gene Therapy

Corneal diseases are among the most prevalent causes of blindness worldwide. The transparency and clarity of the cornea are guaranteed by a delicate physiological, anatomic, and functional balance. For this reason, all the disorders, including those of genetic origin, that compromise this state of harmony can lead to opacity and eventually vision loss. Many corneal disorders have a genetic etiology, and some are associated with rather rare and complex syndromes. Conventional treatments, such as corneal transplantation, are often ineffective, and to date, many of these disorders are still incurable. Gene therapy carries the promise of being a potential cure for many of these diseases, with solutions and strategies that did not seem possible until a few years ago. With its potential to treat genetic disease by means of deletion, replacement, or editing of a defective gene, the challenge can also be extended to corneal disorders in order to achieve long-term, if not definitive, relief. The aim of this paper is to review the state of the art of the different gene therapy approaches as potential treatments for corneal diseases and the future perspectives for the development of personalized gene-based medicine.

In vivo histology and p.L132V mutation in KRT12 gene in Japanese patients with Meesmann corneal dystrophy

PURPOSE

To report genetic mutational analysis and in vivo histology of Meesmann corneal dystrophy.

STUDY DESIGN

Prospective, case control study.

METHODS

Six patients from three independent families with clinically diagnosed Meesmann corneal dystrophy were enrolled in this study. Slit-lamp biomicroscopy with fluorescein vital staining, anterior segment optical coherence tomography (AS-OCT), and in vivo laser confocal microscopy (IVCM) were performed on selected patients. Mutational screening for the keratin genes KRT3 and KRT12 was performed in all six patients and selected unaffected family members.

RESULTS

Slit-lamp biomicroscopy revealed numerous intraepithelial microcysts in all affected individuals. AS-OCT revealed hyperreflectivity and high corneal epithelial layer thickness (mean, 64.8μm) in all individuals tested (3/3). By using IVCM, multiple epithelial microcysts and hyperreflective materials (6/6), subepithelial nerve abnormalities (6/6), tiny punctate hyperreflective material (6/6), and needle-like hyperreflective materials (4/6) were observed in the corneal stromal layer. A heterozygous genetic mutation in the KRT12 gene (c.394 C>G, p.L132V) was identified in all six patients. No pathological mutation was observed in the KRT3 gene.

CONCLUSION

We identified a heterozygous genetic mutation (c.394 C>G, p.L132V) in the KRT12 gene in six Japanese patients with inherited Meesmann corneal dystrophy. This is the first study to confirm this genetic mutation in Japanese Meesmann corneal dystrophy patients. This mutation has been independently reported in an American Meesmann corneal dystrophy patient, confirming its pathogenicity. AS-OCT and IVCM proved to be useful tools for observing corneal epithelial layer pathology in this dystrophy. Furthermore, IVCM reveals corneal stromal layer pathological changes not previously reported in this dystrophy.

Autosomal-dominant Meesmann epithelial corneal dystrophy without an exon mutation in the keratin-3 or keratin-12 gene in a Chinese family

Meesmann epithelial corneal dystrophy (MECD) is a dominantly inherited disorder, characterized by fragility of the anterior corneal epithelium and formation of intraepithelial microcysts. It has been described in a number of different ancestral groups. To date, all reported cases of MECD have been associated with either a single mutation in one exon of the keratin-3 gene (KRT3) or a single mutation in one of two exons of the keratin-12 gene (KRT12). Each mutation leads to a predicted amino acid change in the respective keratin-3 or keratin-12 proteins that combine to form the corneal-specific heterodimeric intermediate filament protein. This case report describes a four-generation Chinese kindred with typical autosomal-dominant MECD. Exon sequencing of KRT3 and KRT12 in six affected and eight unaffected individuals (including two spouses) did not detect any mutations or nucleotide sequence variants. This kindred demonstrates that single mis-sense mutations may be sufficient but are not required in all individuals with the MECD phenotype. It provides a unique opportunity to investigate further genomic and functional heterogeneity in MECD.

Severe Meesmann's epithelial corneal dystrophy phenotype due to a missense mutation in the helix-initiation motif of keratin 12

PURPOSE

To describe a severe phenotype of Meesmann's epithelial corneal dystrophy (MECD) and to determine the underlying molecular cause.

METHODS

We identified a 30-member family affected by MECD and examined 11 of the 14 affected individuals. Excised corneal tissue from one affected individual was examined histologically. We used PCR and direct sequencing to identify mutation of the coding regions of the KRT3 and KRT12 genes.

RESULTS

Cases had an unusually severe phenotype with large numbers of intraepithelial cysts present from infancy and they developed subepithelial fibrosis in the second to third decade. In some individuals, the cornea became superficially vascularized, a change accompanied by the loss of clinically obvious epithelial cysts. Visual loss from amblyopia or corneal opacity was common and four individuals were visually impaired (≤6/24 bilaterally) and one was blind (<6/60 bilaterally). In all affected family members, there was a heterozygous missense mutation c. 395T>C (p. L132P) in exon 1 of the KRT12 gene, which codes for the helix-initiation motif of the K12 polypeptide. This sequence change was not found in unaffected family members or in 100 unaffected controls.

CONCLUSIONS

The Leu132Pro missense mutation is within the helix-initiation motif of the keratin and is predicted to result in a significant structural change of the K12 protein. The clinical effects are markedly more severe than the phenotype usually associated with the Arg135Thr mutation within this motif, most frequently seen in European patients with MECD.