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Abstract

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.

Keywords

Meesmann epithelial corneal dystrophy autosomal dominant

Introduction

Meesmann epithelial corneal dystrophy (MECD) (OMIM 122 100) is a dominantly inherited disorder of the corneal epithelium, characterized by intraepithelial microcysts of uniform size and shape. It was first reported in three families in northern Germany by Meesmann and Wilke¹ in 1939 and was later described in Irish,² Polish,³ Swiss,⁴ British,⁵ American,^6–9 Japanese,¹⁰ Taiwanese¹¹ and Chinese¹² families. MECD is a rare disease and registries of affected cases have not been established, therefore the prevalence and expected incidence of new cases are unknown. The clinical presentation of MECD is variable, ranging from mild to recurrent epithelial erosions, leading to lacrimation, photophobia and deterioration in visual acuity. Slit-lamp examination is considered to be the standard method for diagnosis, showing the typical corneal changes of surface irregularities and epithelial cysts, concentrated most commonly in the interpalpebral zone. Currently there is no specific therapy for MECD; treatment involves palliative relief and prevention of deterioration in visual acuity.

Genetic studies have associated MECD with specific coding variations in the keratin-3 gene (KRT3) or the keratin-12 gene (KRT12). A protein heterodimer, composed of keratin-3 (a type II keratin) and keratin-12 (a type I keratin), is expressed specifically in the corneal epithelium and is important for keratocyte stability.^13,14 Patterns of kinship-specific mutations in KRT3 or KRT12 have been demonstrated in patients with MECD (Table 1). Substitution mutations have only been reported in exon 7 of KRT3,^2,3,11 affecting the helix-termination domain of the keratin-3 protein, but mutational ‘hotspots’ appear in both exon 1^{1,2,5,7–10,12,14–19} and exon 6^{4,6,7,10,11,20} of KRT12, affecting the helix-initiation and helix-termination domains, respectively, of the keratin-12 protein; in addition, an insertion mutation has been described in the helix-termination domain of the keratin-12 protein.⁷ The only heritable mutations in MECD are mis-sense mutations that give rise to amino acid substitutions, in what are relatively specific domains of the keratin-3 and keratin-12 proteins.

Table 1.

Meesmann epithelial corneal dystrophy (MECD) kindreds reported in the literatur.

Reference	Sex of patients with MECD		Proband ancestry	Reported KRT3/KRT12 mutation
Reference	Male, n	Female, n	Gene	Exon	Nucleotide	Codon
Meesmann and Wilke¹	9	17	German	KRT12	1	428G >C	R135T
Ehlers et al.¹⁴	0	8	KRT12	1	428G >C	R135T
Corden et al.¹⁵	7	5	German	KRT12	1	428G >C	R135T
Corden et al.¹⁶	4	3	German	KRT12	1	413A >C	Q130P
Irvine et al.¹⁷	Unknown		German	KRT12	1	423T >G	N133K
Clausen et al.¹⁸	2	1	German	KRT12	1	409G >A	M129V
Irvine et al.²	5	3	Irish	KRT3	7	1525G >A	E509K
KRT12	1	451G >C	V143L
Szaflik et al.³	2	2	Polish	KRT3	7	1493A >T	E498V
Nichini et al.⁴	4	4	Swiss	KRT12	6	1301T >G	I426S
Liao et al.⁵	1	4	British	KRT12	1	395T >C	L132P
Corden et al.¹⁵	4	4	American	KRT12	1	410T >C	M129T
Yoon et al.⁷	0	1	American	KRT12	1	429A >C	R135S
0	3	American	KRT12	6	1222ins27	400ins- 9ISNLEAQLL
Coleman et al.⁶	1	0	American	KRT12	6	1300A >G	I426V
Sullivan et al.²⁰	9	8	American	KRT12	6	1287G >C	A430P
Stocker and Holt,⁸ Klintworth⁹	11	9	American	KRT12	1	Not listed	R19I
Nishida et al.¹⁰	5	1	Japanese	KRT12	1	427A >G	R135G
1	2	Japanese	KRT12	1	428G >T	R135I
1	1	Japanese	KRT12	6	4064T >G	Y429D
0	1	Japanese	KRT12	1	443T >G	L140R
Takahashi et al.¹⁹	1	2	Japanese	KRT12	1	433G >C	A137P
Chen et al.¹¹	1	1	Taiwanese	KRT3	7	1508G >C	R503P
2	2	Taiwanese	KRT12	6	1286A >G	Y429C
Wang et al.¹²	7	5	Chinese	KRT12	1	419T >A	L132H
Current case report	8	0	Chinese	No mutation found
Total	85	87

Direct sequencing of the coding exons of KRT3 and KRT12 appears to be an effective strategy to confirm the genetic cause of dominant MECD. Sequencing both KRT3 and KRT12 exons is critical, since sequence variants do not always co-segregate with MECD within a kinship. For example, a KRT12 exon mutation was detected in one family but did not co-segregate with the MECD phenotype; instead an exon 7 mutation in KRT3 co-segregated with the MECD trait and was considered to be the causative mutation.³ There is therefore the potential for genomic heterogeneity to be uncovered in MECD, but to date only a few, rare, dominant-negative mutations appear to co-segregate with the MECD phenotype.

The present case report describes a four-generation Chinese kindred with MECD but no detectable exon mutation of either KRT3 or KRT12, suggesting that this rare dominant-negative genetic disorder may have undefined genetic variants in a noncoding region, or in regulatory elements, that will require further genomic analysis.

Case report

A four-generation Chinese family including six living individuals with MECD was studied (Figure 1). The proband was a 20-year-old male (IV-4) who presented to the People's Hospital of Zhengzhou, Zhengzhou, Henan, China, in May 2011 with blurred vision, irritation, tearing and frequent photophobia. Visual acuity was normal (1.2 in both eyes). Multiple small cysts were seen on slit-lamp examination. His father (III-4) had a similar eye problem, which drew the family to clinical attention. Subsequently, the extended four-generation family was assessed. A complete ophthalmological assessment, including slit-lamp examination, was performed in 14 family members (II2, II3, III1, III4, III5, III6, III7, III11, III12, IV2, IV3, IV4, IV5 and IV6).

Figure 1.

Pedigree of a Chinese family with Meesmann epithelial corneal dystrophy, showing autosomal dominant inheritance. Affected individuals are represented by a solid symbol; the proband is IV-4.

Eight individuals were found to be affected by MECD (Figure 1). All were males and ranged between 20 (IV-4) and 80 (II-2) years old; affected individuals I-1 and III-3 were no longer alive but were identified from family description. Vision was very poor in both eyes for individuals II-2, II-3, III-4, III-6 and III-12. In addition, two individuals who did not have MECD were diagnosed with pathological myopia (III-1 and IV-2). Slit-lamp examination showed multiple fine vesicles (microcysts) within the corneal epithelia in both eyes (Figure 2) in all affected living individuals, leading to a primary diagnosis of MECD.

Figure 2.

Slit-lamp photography of the left eye of patient III-12 with Meesmann epithelial corneal dystrophy. The corneal epithelium has an irregular surface with multiple microcysts, with erupted cysts forming opaque spots. The black arrow indicates a representative cyst within the corneal epithelium.

After obtaining written informed consent according to clinical research guidelines approved by the Institutional Review Board of the People's Hospital of Zhengzhou, Zhengzhou, Henan, China, 3-ml peripheral blood samples were obtained in tubes containing ethylenediamine tetra-acetic acid from all six affected living patients (II-2, II-3, III-4, III-6, III-12 and IV-4), six unaffected individuals (III-1, III-11, IV-2, IV-3, IV-5 and IV-6) and two nongenetic relatives (III-5 and III-7), for the isolation of genomic DNA and assessment of exon mutations in KRT3 and KRT12.The tube was inverted to mix the sample. Blood samples were stored at 4°C and analysed within 8 h of collection. To separate the peripheral blood leucocytes, 1 ml blood of blood was transferred to a 1.5-ml Eppendorf tube. The sample was then centrifuged for 10 min at 1200 r.p.m., at room temperature, (Model 5430, Eppendorf AG, Hamburg, Germany) to pellet red blood cells from plasma. The thin layer of buffy coat was carefully aspirated from between the pelleted red blood cells and plasma, and was transferred to a fresh tube for DNA isolation.

Genomic DNA was isolated from peripheral blood leucocytes by standard procedures using a blood DNA kit (Qiagen, Valencia, CA, USA). Coding regions of KRT3 (NM_057088) and KRT12 (NM_000223) were screened for mutations by direct sequencing, using DNA amplified with the primers shown in Table 2. Polymerase chain reaction (PCR) was performed at 94°C for 5 min followed by 35 cycles at 94°C for 30 s, 57°C for 30 s, 72°C for 30 s and 72°C for 5 min. PCR-amplified products were examined on 1% agarose gels, then sequenced using an ABI Prism® 377 DNA sequencer (Applied Biosystems, Foster City, CA, USA) and BigDye® termination cycle-sequencing kit version 3.1 (Applied Biosystems).

Table 2.

Primers used in polymerase chain reaction amplification for direct sequencing of KRT3 and KRT12 exon.

Target		Forward primer sequence	Reverse primer sequence	Product size, bp	T_m, °C
Gene	Exon
KRT3	1-1	5′-TGGAACAAACTTATTGCCTTG-3′	5′-CACCAGGACTGCCCAAGC-3′	680	57.8
1-2	5′-CAGTCGCAGCCTCTACAACC-3′	5′-ACAAACTCCTCAACCCTGGA-3′	647	59.6
2	5′-CGCTGCAAGTGTAGTGTCCT-3′	5′-GTGAGGGGGAACTGAATGAA-3′	580	59.1
3	5′-CCCCCAACTAACAGGGAGAT-3′	5′-ATAGGCACTGTCCACATCCTG-3′	595	60.0
4	5′-CCGTTCAAGTGGAGACTGGT-3′	5′-GTATCAGCCACACCCTGTCC-3′	422	60.1
5	5′-GAATGAGCCTGGAGATGAGG-3′	5′-TTTCTGCAACTGAACCACCA-3′	531	59.8
6	5′-CCCTGCTGAAACACAATGC-3′	5′-CTCCCCTAGTGCCTCCATCT-3′	426	60.3
7	5′-CACAGGGCAGTATCTCCACA-3′	5′-GCAAATGCTTCACCACAAGG-3′	499	59.7
8	5′-CTCCTTGCTACAGCCCAAAG-3′	5′-AGCCAATCACTTCCCTCTCC-3′	327	60.0
9-1	5′-GCTCGTGGTGAACATCCTG-3′	5′-AATTCTCGTGACTGGGCTTG-3′	627	60.2
9-2	5′-GCTTTGGCTCCATCTCTGG-3′	5′-AGACGAACTGCCTTTGTTTCA-3′	639	59.9
KRT12	1-1	5′-CCACCCAGCTTTCTTTTTCA-3′	5′-CGCACCTTATCCAGGTAGGA-3′	683	60.1
1-2	5′-GTTATGGGGGAAGTGCCTTT-3′	5′-TGAAACAACCTGGGATGAGA-3′	588	59.1
2	5′-CCCAGGTTGTTTCACTCCAC-3′	5′-TTGAGGTTTAGGACACTGGTAGG-3′	632	59.6
3	5′-CAACAGCAAAAACTGCATCA-3′	5′-CCTTAGCAGGAACCACCATC-3′	520	58.5
4 and 5	5′-CCAACTTACCCATTAGGCACA-3′	5′-TCCAGGGATTTCTTCTGCAC-3′	848	59.9
6	5′-GCGCAAACAGACGTAGCAT-3′	5′-TCCCAGGCATATCTTTACTAGACTT-3′	485	58.8
7	5′-GCCACCTGAACCACCTACTC-3′	5′-AGATGGGGTCTCGCTATTTG-3′	544	59.2
8-1	5′-GGCAAAGCAGAAGGGAAAA-3′	5′-TGTTGGAATCAAGGCATACA-3′	776	57.5
8-2	5′-TGGTGAATGGTGAGGTGGT-3′	5′-CCTGACATCTTCATCCCAGTT-3′	676	59.0

T_m, melting temperature.

No sequence variants were identified: all individuals were homozygous for the exons of KRT3 and KRT12.

Discussion

This large Chinese kindred is the first published report of an autosomal-dominant MECD trait that is not associated with an exon mutation in either KRT3 or KRT12. MECD is an autosomal-dominant genetic disorder affecting the corneal epithelium that was first identified in Germany in 1939¹ and subsequently has been reported worldwide (Table 1). The kindred presented here focuses attention on several genetic properties of MECD.

First, in autosomal-dominant MECD, equal numbers of affected males and females would be expected, as shown in Table 1. In the kindred presented here, there were eight affected males and no affected females. However, the absence of affected females in this family is not biologically significant, as the sex ratio and the number of affected children produced by the six affected MECD males who did have children did not show a statistical difference from expected values (P = 0.05, using a two-tailed Fisher exact probability test).

Secondly, all well-documented and reported MECD cases have an identified dominant-negative exon mutation in one of the two keratin genes, KRT3 or KRT12. Mutations in KRT3 have been identified in three MECD ancestries; mutations in KRT12 have been identified in 22 ancestries (Table 1). Although MECD is rare, the same mutation occurs in all the affected individuals in each reported kindred. Therefore, sequencing the coding exons of KRT3 and KRT12 appeared to be an appropriate genetic test to confirm the clinical diagnosis. However, it is now conceivable that there may be an undefined genomic variation in MECD that is not a KRT3 or KRT12 exon mutation, but is instead a mutation in the noncoding intron or regulatory elements of these genes.

Thirdly, despite the presence of a kinship-specific dominant-negative mutation in various reported MECD families, clinical phenotype can be variable within the kindred.¹⁴ This further suggests that mechanisms unrelated to KRT3 or KRT12 mutations may contribute to the phenotypic expression of the MECD trait.^3,14

Further investigation of the kindred presented here should include sequencing the noncoding regions of KRT3 and KRT12 in affected individuals and genome-wide association analysis.

Footnotes

Acknowledgements

We thank the members of the Chinese family presented for their enthusiastic participation in this research and their consent for us to publish the results.

Declaration of conflicting interest

The authors declare that there are no conflicts of interest.

Funding

This work was supported by the National Natural Science Foundation of China (no. 81171992) and the Zhengzhou Science and Technology Programme (121PPTGG494-8).

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

Abstract

Keywords

Introduction

Case report

Discussion

Footnotes

Acknowledgements

Declaration of conflicting interest

Funding

References