A novel arginine substitution mutation in 1A domain and a novel 27 bp insertion mutation in 2B domain of keratin 12 gene associated with Meesmann's corneal dystrophy

A Novel Pathogenic Variant in the KRT3 Gene in a Family with Meesmann Corneal Dystrophy

Background/Objectives: to report a novel KRT3 Meesmann corneal dystrophy (MECD) mutation and its clinical findings in a Spanish family, thus completing the international database. Case series study. Methods: Two generations of three family members were studied. The clinical ophthalmologic evaluation was made including best-corrected visual acuity (BCVA), biomicroscopy with and without fluorescein, fundoscopy, Schirmer test I, non-invasive break-up time (NiBUT), and esthesiometry. In vivo confocal microscopy (IVCM), anterior segment optical coherence tomography (AS-OCT) with an epithelial map, and genetic analysis were also performed. Results: A novel heterozygous mutation in the KRT3 gene c.1527G>T (p. Glu509Asp) was identified. Biomicroscopy revealed bilateral multiple corneal intraepithelial cysts. IVCM showed numerous and relatively small microcysts (12-32 µm), hyperreflective materials, subepithelial nerve and Bowman's layer alterations. AS-OCT scan revealed diffuse hyperreflectivity and the epithelial map displayed thickening of the corneal epithelium in the interpalpebral zone (proband: 52-68 µm and father's proband: 55-71 µm) with a slightly thinned cornea. Conclusions: We identified a new mutation in the KRT3 gene-c.1527G>T (p. Glu509Asp) in a Spanish family with MECD. A comprehensive characterization of the clinical signs, using different techniques, especially an epithelial map, could be useful to diagnose and monitor epithelial changes by quantitative measures. Epithelial map changes provide better understanding of MECD differential epithelial behavior and its progression changes. Larger studies will be necessary to better understand these specific patterns and clinically evaluate new therapies.

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.

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.

Over-expression of ΔNp63α facilitates rat corneal wound healing in vivo

To investigate the roles of ΔNp63α during corneal wound healing and the genes regulated by ΔNp63α in limbal epithelial cells. Adenovirus or shRNA targeting ΔNp63α were pre-injected into the anterior chamber of rat eyeballs and the central corneal epithelium was then wounded with NaOH. The effects of ΔNp63α expression during wound healing were observed by propidium iodide staining. In addition, limbal epithelial cells were cultured and ectopically expressed ΔNp63α by transfecting Ad-ΔNp63α. Total RNA was extracted from transfected epithelial cells and subjected to a gene expression microarray assay. The results showed that over-expression of ΔNp63α accelerated the process of corneal wound healing while knockdown of ΔNp63α impaired the process. ΔNp63α positively up-regulated several cell growth promoter genes and could be referred as a positive regulator of limbal epithelial cell proliferation. It might also inhibit cell differentiation and cell death by differential target gene regulation.

Identification of presumed pathogenic KRT3 and KRT12 gene mutations associated with Meesmann corneal dystrophy

PURPOSE

To report potentially pathogenic mutations in the keratin 3 (KRT3) and keratin 12 (KRT12) genes in two individuals with clinically diagnosed Meesmann corneal dystrophy (MECD).

METHODS

Slit-lamp examination was performed on the probands and available family members to identify characteristic features of MECD. After informed consent was obtained, saliva samples were obtained as a source of genomic DNA, and screening of KRT3 and KRT12 was performed. Potentially pathogenic variants were screened for in 200 control chromosomes. PolyPhen-2, SIFT, and PANTHER were used to predict the functional impact of identified variants. Short tandem repeat genotyping was performed to confirm paternity.

RESULTS

Slit-lamp examination of the first proband demonstrated bilateral, diffusely distributed, clear epithelial microcysts, consistent with MECD. Screening of KRT3 revealed a heterozygous missense variant in exon 1, c.250C>T (p.(Arg84Trp)), which has a minor allele frequency of 0.0076 and was not identified in 200 control chromosomes. In silico analysis with PolyPhen-2 and PANTHER predicted the variant to be damaging to protein function; however, SIFT analysis predicted tolerance of the variant. The second proband demonstrated bilateral, diffusely distributed epithelial opacities that appeared gray-white on direct illumination and translucent on retroillumination. Neither parent demonstrated corneal opacities. Screening of KRT12 revealed a novel heterozygous insertion/deletion variant in exon 6, c.1288_1293delinsAGCCCT (p.(Arg430_Arg431delinsSerPro)). This variant was not present in either of the proband's parents or in 200 control chromosomes and was predicted to be damaging by PolyPhen-2, PANTHER, and SIFT. Haplotype analysis confirmed paternity of the second proband, indicating that the variant arose de novo.

CONCLUSIONS

We present a novel KRT12 mutation, representing the first de novo mutation and the first indel in KRT12 associated with MECD. In addition, we report a variant of uncertain significance in KRT3 in an individual with MECD. Although the potential pathogenicity of this variant is unknown, it is the first variant affecting the head domain of K3 to be reported in an individual with MECD and suggests that disease-causing variants associated with MECD may not be restricted to primary sequence alterations of either the helix-initiation or helix-termination motifs of K3 and K12.

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.

Identification of a novel mutation in the cornea specific keratin 12 gene causing Meesmann's corneal dystrophy in a German family

PURPOSE

To report a novel missense mutation of the cornea specific keratin 12 (KRT12) gene in two generations of a German family diagnosed with Meesmann;s corneal dystrophy.

METHODS

Ophthalmologic examination of the proband and sequencing of keratin 3 (KRT3) and KRT12 of the proband and three other family members were performed. Restriction enzyme analysis was used to confirm the detected mutation in affected individuals of the family.

RESULTS

Slit-lamp biomicroscopy of the proband revealed multiple intraepithelial microcysts comparable to a Meesmann dystrophy phenotype. A novel heterozygous A-->G transversion at the first nucleotide position of codon 129 (ATG>GTG, M129V) in exon 1 of KRT12 was detected in the proband, her two affected sons but not in her unaffected husband or 50 control individuals.

CONCLUSIONS

We have identified a novel missense mutation within the highly conserved helix-initiation motif of KRT12 causing Meesmann;s corneal dystrophy in a German family.

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.

Genetics of Meesmann corneal dystrophy: a novel mutation in the keratin 3 gene in an asymptomatic family suggests genotype-phenotype correlation

PURPOSE

Juvenile epithelial corneal dystrophy of Meesmann (MCD, OMIM 122100) is a dominantly inherited disorder characterized by fragility of the anterior corneal epithelium and intraepithelial microcyst formation. Although the disease is generally mild and affected individuals are often asymptomatic, some suffer from recurrent erosions leading to lacrimation, photophobia, and deterioration in visual acuity. MCD is caused by mutations in keratin 3 (KRT3) or keratin 12 (KRT12) genes, which encode cornea-specific cytoskeletal proteins. Seventeen mutations in KRT12 and two in KRT3 have been described so far. The purpose of this study was to investigate the genetic background of MCD in a Polish family.

METHODS

We report on a three-generation family with MCD. Epithelial lesions characteristic for MCD were visualized with slit-lamp examination and confirmed by in vivo confocal microscopy. Using genomic DNA as a template, all coding regions of KRT3 and KRT12 were amplified and sequenced. Presence of the mutation was verified with restriction endonuclease digestion.

RESULTS

In the proband, direct sequencing of the polymerase chain reaction (PCR) product from amplified coding regions of KRT3 and KRT12 revealed a novel 1493A>T heterozygous missense mutation in exon 7 of KRT3, which predicts the substitution of glutamic acid for valine at codon 498 (E498V). Using PCR-Restriction Fragment Length Polymorphism (RFLP) analysis, the mutation was demonstrated to segregate with the disease (four affected members, three non-affected) and to be absent in 100 controls from the Polish population, indicating that it is not a common polymorphism.

CONCLUSIONS

Location of the E498V mutation emphasizes the functional relevance of the highly conserved boundary motifs at the COOH-terminus of the alpha-helical rod domain in keratin 3 (K3).

A novel mutation in the cornea-specific keratin 12 gene in Meesmann corneal dystrophy

PURPOSE

To report a novel mutation in the keratin 12 gene (KRT12) found in a Japanese family in association with Meesmann corneal dystrophy (MECD).

METHODS

After informed consent was obtained, genomic DNA was extracted from the leukocytes of the peripheral blood of the proband, her affected father, normal mother, and 50 normal unrelated volunteers. Exons 1-8 of the KRT12 gene were amplified by polymerase chain reaction and directly sequenced.

RESULTS

A novel heterozygous T to G transversion at the second nucleotide position of codon 433 (CTG>CGG), resulting in the replacement of leucine by arginine at codon 433 of the KRT12 gene (L433R), was detected in the proband and her affected father but not in her normal mother or the 50 controls.

CONCLUSIONS

The novel L433R mutation of the KRT12 gene found in two members of this Japanese family caused MECD.

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.

Identifying the role of specific motifs in the lens fiber cell specific intermediate filament phakosin

PURPOSE

Phakosin and filensin are lens fiber cell-specific intermediate filament (IF) proteins. Unlike every other cytoplasmic IF protein, they assemble into a beaded filament (BF) rather than an IF. Why the lens fiber cell requires two unique IF proteins and why and how they assemble into a structure other than an IF are unknown. In this report we test specific motifs/domains in phakosin to identify changes that that have adapted phakosin to lens-specific structure and function.

METHODS

Phakosin shows the highest level of sequence identity to K18, whose natural assembly partner is K8. We therefore exchanged conserved keratin motifs between phakosin and K18 to determine whether phakosin's divergent motifs could redirect the assembly of chimeric K18 and K8. Modified proteins were bacterially expressed and purified. Assembly competence was assessed by electron microscopy.

RESULTS

Substitution of the phakosin helix initiation motif (HIM) into K18 does not alter assembly with K8, establishing that the radical divergence in phakosin HIM is not by itself the mechanism by which IF assembly is redirected to BF assembly. Unexpectedly, K18 bearing phakosin HIM resulted in normal IF assembly, despite the presence of an otherwise disease-causing R-C substitution, and two helix-disrupting glycines. This disproves the widely held belief that mutation of the R is catastrophic to IF assembly. Additional data are presented that suggest normal IF assembly is dependent on sequence-specific interactions between the IF head domain and the HIM.

CONCLUSIONS

In the lens fiber cell, two members of the IF family have evolved to produce BFs instead of IFs, a change that presumably adapts the IF to a fiber cell-specific function. The authors establish here that the most striking divergence seen in phakosin is not, as hypothesized, the cause of this altered assembly outcome. The authors further establish that the HIM of IFs is far more tolerant of mutations, such as those that cause some corneal dystrophies and Alexander disease, than previously hypothesized and that normal assembly involves sequence-specific interactions between the head domain and the HIM.

A novel mutation of the Keratin 12 gene responsible for a severe phenotype of Meesmann's corneal dystrophy

PURPOSE

To determine if a mutation within the coding region of the keratin 12 gene (KRT12) is responsible for a severe form of Meesmann's corneal dystrophy.

METHODS

A family with clinically identified Meesmann's corneal dystrophy was recruited and studied. Electron microscopy was performed on scrapings of corneal epithelial cells from the proband. Mutations in the KRT12 gene were sought using direct genomic sequencing of leukocyte DNA from two affected and two unaffected family members. Subsequently, the observed mutation was screened in all available family members using polymerase chain reaction and direct sequencing.

RESULTS

A heterozygous missense mutation (Arg430Pro) was found in exon 6 of KRT12 in all 14 affected individuals studied. Unaffected family members and 100 normal controls were negative for this mutation.

CONCLUSIONS

We have identified a novel mutation in the KRT12 gene that is associated with a symptomatic phenotype of Meesmann's corneal dystrophy. This mutation results in a substitution of proline for arginine in the helix termination motif that may disrupt the normal helix, leading to a dramatic structural change of the keratin 12 protein.

Meesmann corneal dystrophy (MECD): report of 2 families and a novel mutation in the cornea specific keratin 12 (KRT12) gene

PURPOSE

Meesmann corneal dystrophy (MECD) is an autosomal dominant disorder affecting the corneal epithelium. It is caused by heterozygous mutations in KRT3 or KRT12 gene. Actually, 14 mutations have been reported, 1 in KRT3 and 13 in KRT12. These genes were screened in several patients suffering from MECD.

METHODS

Patients from 2 families were screened for mutation in KRT3 and KRT12. Exons were PCR-amplified and directly sequenced. The new mutation was checked by DHPLC in 51 control individuals of Swiss origin.

RESULTS/CONCLUSIONS

In one family, the M129T heterozygous mutation was observed in KRT12. In the second family, we identified a novel I426S heterozygous mutation in exon 6 of KRT12.

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

PURPOSE

To analyze mutations of the keratin 3 gene (KRT3) and keratin 12 gene (KRT12) in 2 Taiwanese families with Meesmann corneal dystrophy (MCD).

METHODS

Diagnosis of MCD was confirmed by slit-lamp examination of the cornea in 4 members of family 1 and 6 members of family 2. All exons and flanking intron boundaries of KRT3 and KRT12 were amplified by polymerase chain reaction (PCR), and products were subjected to direct sequencing. Restriction fragment length polymorphism analysis (RFLP) with created mismatch primers, Bst XI and Nsp I, was used to confirm the presence of the mutations in affected individuals in family 1 and family 2, respectively.

RESULTS

A novel heterozygous missense mutation (1508G-->C), predicting the substitution of a proline for an arginine (R503P) was detected in the helix termination motif of the keratin 3 polypeptide in family 1. Another novel heterozygous missense mutation (1286A-->G), predicting the substitution of a cysteine for a tyrosine at codon 429 (Y429C) was detected in the helix termination motif of the keratin 12 polypeptide in family 2. These 2 mutations were excluded from 50 normal controls by RFLP analysis, indicating that they were not common polymorphisms.

CONCLUSIONS

A novel missense mutation (R503P) in KRT3 and another novel missense mutation (Y429C) in KRT12 lead to MCD in 2 unrelated Taiwanese families. The mutant codons in our study are all located in the highly conserved alpha-helix-termination motif, which is essential for keratin filament assembly. Mutation at this area may account for the disruption of keratin filament assembly, leading to MCD.