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.

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.

siRNA silencing of the mutant keratin 12 allele in corneal limbal epithelial cells grown from patients with Meesmann's epithelial corneal dystrophy.

PURPOSE: The aim of this study is to further assess our previously reported keratin 12 (K12)-Leu132Pro specific siRNA in silencing the mutant allele in Meesmann's Epithelial Corneal Dystrophy (MECD) in experimental systems more akin to the in vivo situation through simultaneous expression of both wild-type and mutant alleles. METHODS: Using KRT12 exogenous expression constructs transfected into cells, mutant allele specific knockdown was quantified using pyrosequencing and infrared Western blot analysis, while the silencing mechanism was assessed by a modified rapid amplification of cDNA ends (5'RACE) method. Corneal limbal biopsies taken from patients suffering from MECD were used to establish cultures of MECD corneal limbal epithelial stem cells and the ability of the siRNA to silence the endogenous mutant KRT12 allele was assessed by a combination of pyrosequencing, qPCR, ELISA, and quantitative-fluorescent immunohistochemistry (Q-FIHC). RESULTS: The siRNA displayed a potent and specific knockdown of K12-Leu132Pro at both the mRNA and protein levels with exogenous expression constructs. Analysis by the 5'RACE method confirmed siRNA-mediated cleavage. In the MECD cells, an allele-specific knockdown of 63% of the endogenous mutant allele was observed without effect on wild-type allele expression. CONCLUSIONS: Combined with an effective delivery vehicle this siRNA approach represents a viable treatment option for prevention of the MECD pathology observed in K12-Leu132Pro heterozygous individuals.

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.

Allele-specific siRNA silencing for the common keratin 12 founder mutation in Meesmann epithelial corneal dystrophy.

PURPOSE: To identify an allele-specific short interfering RNA (siRNA), against the common KRT12 mutation Arg135Thr in Meesmann epithelial corneal dystrophy (MECD) as a personalized approach to treatment. METHODS: siRNAs against the K12 Arg135Thr mutation were evaluated using a dual luciferase reporter gene assay and the most potent and specific siRNAs were further screened by Western blot. Off-target effects on related keratins were assessed and immunological stimulation of TLR3 was evaluated by RT-PCR. A modified 5' rapid amplification of cDNA ends method was used to confirm siRNA-mediated mutant knockdown. Allele discrimination was confirmed by quantitative infrared immunoblotting. RESULTS: The lead siRNA, with an IC(50) of thirty picomolar, showed no keratin off-target effects or activation of TLR3 in the concentration ranges tested. We confirmed siRNA-mediated knockdown by the presence of K12 mRNA fragments cleaved at the predicted site. A dual tag infrared immunoblot showed knockdown to be allele-specific, with 70% to 80% silencing of the mutant protein. CONCLUSIONS: A potent allele-specific siRNA against the K12 Arg135Thr mutation was identified. In combination with efficient eyedrop formulation delivery, this would represent a personalized medicine approach, aimed at preventing the pathology associated with MECD and other ocular surface pathologies with dominant-negative or gain-of-function pathomechanisms.

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.

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.

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.