Novel mutations in the helix termination motif of keratin 3 and keratin 12 in 2 Taiwanese families with Meesmann corneal dystrophy

[Intracorneal selective stromal transplantation]

Deep anterior lamellar keratoplasty or penetrating keratoplasty are currently considered the optimal methods of surgical treatment of stromal dystrophies and corneal degeneration. Despite certain advantages and benefits of these methods, they also have significant limitations: involvement of superficial corneal layers in the surgery, need for suturing, development of post-keratoplasty astigmatism etc.

PURPOSE

This study aimed to test and describe the new method of closed sutureless keratoplasty (intracorneal selective stromal transplantation), which was indicated in isolated dystrophic and degenerative pathology of the stroma.

MATERIAL AND METHODS

Intracorneal selective stromal transplantation was performed in a 62-year-old patient with stromal degeneration and intact corneal layers between the altered stroma and the Descemet's membrane posteriorly, and the Bowman's layer anteriorly. The patient also had immature senile cataract. Corneal stroma was removed and replaced with a graft in the optical center of the lens, while the endothelium, the Descemet's membrane and the Bowman's layer remained intact.

RESULTS

The proposed technique of intracorneal selective stromal transplantation makes it possible to replace only the pathologically altered stroma through closed surgical approach, without affecting the anterior and posterior surfaces of the cornea. Best-corrected visual acuity has increased in the patient from 0.01 to 0.6, while mean endothelial cell density has not changed in the course of 24-months follow-up.

CONCLUSION

The proposed keratoplasty method can be used in patients with dystrophy or degeneration of the corneal stroma and preserved endothelial cells, intact Descemet's membrane and Bowman layer. Since the superficial corneal layers are not involved during the surgery, intracorneal selective stromal transplantation combined the advantages of both deep anterior lamellar keratoplasty and endothelial keratoplasty. The biologically favorable result in this first clinical case allows a preliminary conclusion on the technical possibility and functional effectiveness of the proposed method, but further long-term observation and more clinical cases are required.

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.

Identification of a Heterozygous Mutation in the TGFBI Gene in a Hui-Chinese Family with Corneal Dystrophy

Background/Aims

Corneal dystrophies (CDs) belong to a group of hereditary heterogeneous corneal diseases which result in visual impairment due to the progressive accumulation of deposits in different corneal layers. So far, mutations in several genes have been responsible for various CDs. The purpose of this study is to identify gene mutations in a three-generation Hui-Chinese family associated with granular corneal dystrophy type I (GCD1).

Methods

A three-generation Hui-Chinese pedigree with GCD1 was recruited for this study. Slit-lamp biomicroscopy, optical coherence tomography, and confocal microscopy were performed to determine the clinical features of available members. Whole exome sequencing was performed on two patients to screen for potential disease-causing variants in the family. Sanger sequencing was used to test the variant in the family members.

Results

Clinical examinations demonstrated bilaterally abundant multiple grayish-white opacities in the basal epithelial and superficial stroma layers of corneas of the two patients. Whole exome sequencing revealed that a heterozygous missense mutation (c.1663C > T, p.Arg555Trp) in the transforming growth factor beta-induced gene (TGFBI) was shared by the two patients, and it cosegregated with this disease in the family confirmed by Sanger sequencing.

Conclusions

The results suggested that the heterozygous TGFBI c.1663C > T (p.Arg555Trp) mutation was responsible for GCD1 in the Hui-Chinese family, which should be of great help in genetic counseling for this family.

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.

KRT12 mutations and in vivo confocal microscopy in two Japanese families with Meesmann corneal dystrophy

PURPOSE

To identify genetic mutations and study the corneal epithelium in Japanese patients with Meesmann corneal dystrophy.

DESIGN

Laboratory investigation and prospective observational case series.

METHODS

Slit-lamp biomicroscopy with fluorescein vital staining and in vivo confocal microscopy were performed. Mutation screening of the KRT3 and KRT12 genes was performed via polymerase chain reaction and direct sequencing for 5 patients in 2 families.

RESULTS

Slit-lamp biomicroscopy revealed multiple corneal intraepithelial microcysts in all patients. A clear zone was seen in the younger generation, whereas mild subepithelial opacity was seen in the older generation. In the in vivo confocal microscopy, numerous corneal intraepithelial microcysts and hyperreflective materials, which were believed to be degenerative cells, were detected closer to the basal layer of the corneal epithelium in older patients. The superficial layer contained more enlarged microcysts, and the hyperreflective materials showed atrophic changes, as compared to the basal layer. The demarcation line between the microcysts and normal epithelial cells was clearly visualized by in vivo confocal microscopy and corresponded to the demarcation line of the clear zone observed by the slit-lamp examination. Two heterozygous mutations (Q130P, L140Q) in the KRT12 gene, one of which (L140Q) was novel, were identified only in the affected patients of the families.

CONCLUSIONS

We identified a novel missense mutation of the KRT12 gene in Meesmann corneal dystrophy. The in vivo confocal microscopy examinations revealed previously unreported depth-dependent ultrastructural changes in the living cornea of Meesmann corneal dystrophy patients.

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.

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.

A novel mutation as the basis for asymptomatic meesmann dystrophy in a Danish family

PURPOSE

Meesmann dystrophy is a rare inherited corneal disease. This is the description of a unique family in Denmark.

METHODS

The family members were examined by biomicroscopy. Blood samples were collected. DNA from the leukocyte population was isolated, and the cytokeratin 12 (KRT12) gene was partially sequenced.

RESULTS

This Danish family harbors a 451G-->T mutation. All patients in this family that harbor mutations also show microcysts, but none have any symptoms.

CONCLUSIONS

This is the second family recently diagnosed with Meesmann dystrophy in Denmark. The family represents its own distinct genotype, independent of previously reported ones. All patients with microcysts were asymptomatic.

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

PURPOSE

To describe the phenotypic variability in Meesmann's microcystic dystrophy of the corneal epithelium based on a review of the literature and the presentation of a Danish family.

METHODS

We carried out a clinical examination of the family and genetic sequencing of DNA.

RESULTS

Subjective symptoms often include blurred vision and ocular irritation. Typical cases may be entirely free of complaints. Intermittent pain episodes, such as occur in recurrent erosion syndrome, are not the rule. Genetic sequencing indicated a familial relationship with the originally described Meesmann family. Clinical variability was similar. Approximately 85% of cases showed microcysts in the entire epithelium. The remaining 15% demonstrated variants with microcysts in the upper or lower part of the cornea, or in the central or peripheral cornea, as well as subepithelial opacities.

CONCLUSIONS

Meesmann's dystrophy occurs worldwide. The largest family described is the original German one, now supplemented with a Danish branch. Despite the presence of an identical genetic defect, the clinical phenotype varies. This suggests that non-KRT12-related mechanisms are responsible for the variation.

Genetics of corneal dystrophies: the evolving landscape

PURPOSE OF REVIEW

Major advances and developments in corneal molecular genetics have revolutionized our fundamental understanding of corneal dystrophies. At the same time, this knowledge is allowing for improved ways to classify these dystrophies. New genes and mutations responsible for corneal dystrophies are being discovered at an accelerating rate. Ophthalmologists must keep abreast of all the new information, as our basic understanding as well as our classification systems are changing. We present a current review of the genetics of corneal dystrophies.

RECENT FINDINGS

After the discovery of the BIGH3 (TGFbeta1) gene responsible for several corneal dystrophies, there has been an explosion of new information. New mutations are discovered every day for many of the corneal dystrophies located on the BIGH3 gene. In addition, new genetic sites are also being realized. Additionally, corneal dystrophies which have never been linked to any genetic site are now beginning to be uncovered.

SUMMARY

As new mutations and genetic sites are discovered for the various corneal dystrophies, new information will arise, allowing researches to develop innovative methods to study these gene products and their function. This will open the door for novel diagnostic and therapeutic approaches. Ultimately, gene therapy may be possible, leading to cures for these sight-threatening diseases.