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1From the Center for Corneal Genetics, Cornea and External Disease Service, and the 2Laboratory of Developmental Genetics, The Wilmer Eye Institute, The Johns Hopkins University School of Medicine, Baltimore, Maryland; and the 3Department of Biostatistics, The Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland.
| Abstract |
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METHODS. Genotypes of small tandem repeat polymorphisms were obtained from 17 affected and 3 unaffected family members, followed by genetic linkage analysis.
RESULTS. In this family, classic late-onset FCD appeared to be inherited as a single, dominant Mendelian trait. In two exceptional sibships, however, children aged 10 and 13 years had FCD. In each sibship, both parents were found to be affected, opening the possibility that this unusually early age of onset was the result of homozygosity for an FCD mutation. Genotype results, however, were not consistent with consanguinity of the parents, who appear to have independent cases of FCD. A whole-genome linkage scan mapped FCD to a single locus at 13pTel-13q12.13, with significant two-point LOD scores of 3.91 at D13S1236 and 3.80 at D13S1304. The 26.4-Mb disease interval contains the chromosome 13 nucleolus organizer (RNR1), the centromere, and 44 annotated protein-encoding genes. So far, exons of 10 of these genes have been screened, but no mutations have been found.
CONCLUSIONS. FCD1 is the first genetic locus to be identified for late-onset FCD, a common disease of the aging cornea. The exceptional early onset of the disease observed in two children is unusual and might be the result of digenic interaction between FCD1 and an independent late-onset FCD mutation.
Although Fuchs first described this disease in the early 20th century, its pathogenesis remains poorly understood. With regard to its genetics, large families exhibiting autosomal dominant inheritance of FCD have long been reported in the literature.12 13 14 The first progress toward identifying loci and gene mutations associated with familial FCD has been provided by a rare subtype of FCD characterized by atypical histopathology and disease onset in early childhood.14 This rare condition is caused by point mutations in the COL8A2 gene,8 15 which encodes the
2 subtype of collagen VIII, a major component of Descemets membrane. Although COL8A2 disease has provided important evidence that implicates Descemets membrane as the site of primary defects that cause FCD, we still have no convincing evidence that COL8A2 mutations are involved in the more common late-onset form of this disease.8 16
For late-onset familial FCD, the existence of additional gender-related genetic or physiological factors are suggested by the consistent observation of a
2.5:1 ratio of affected females to males.5 8 This ratio is not explained by the greater frequency of women in older populations, or by simple autosomal dominant inheritance of the disease. Even though a combination of genetic, gender-related and environmental factors are thought to underlie most common diseases, large families with apparent dominant inheritance of FCD are surprisingly prevalent. On closer examination, as many as 50% of clinical patients with FCD have siblings, parents, or offspring who are also affected.5 8 In this study, we mapped the first genetic locus for late-onset FCD. In one large family it followed Mendelian inheritance as a single autosomal dominant trait.
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Determination of Phenotype, Disease Severity, and Corneal Photography
Individuals were examined and photographed to document guttae by slit lamp biomicroscopy. A confocal specular microscope17 (ConfoScan3; Nidek Technologies, Vigonza, Italy) was used to view the guttae. Grading followed the scale proposed by Krachmer et al.,5 with Grade 0 indicating no disease, as defined by fewer than 11 central guttae. In this study, none of the unaffected cases had more than two central guttae. Grade 1, which represented definitive onset of the disease, was indicated by 12 or more central, nonconfluent guttae in at least one eye. Grade 2 patients exhibited a zone of confluent central guttae 1 to 2 mm in horizontal width, whereas in grade 3, this area of confluence had expanded to 2 to 5 mm, and in grade 4 its width was greater than 5 mm. Grade 5 was the same as grade 4, but also exhibited edema of the corneal stroma and/or epithelium.
Genotyping and Linkage Analysis
Blood (
10 mL) was collected and frozen at 20°C. DNA was extracted (Qiagen, Santa Clara, CA), and genotyped for 399 polymorphic linkage markers, using the MD10 set of fluorescent primers (Applied Biosystems, Inc. [ABI], Foster City, CA) using an automated DNA sequencer (Prism 377; ABI). Data processing and genotype assignment were performed on computer (GeneScan 3.1 and Genotyper 2.1, respectively; ABI). RelCheck confirmed pedigree relationships, and Pedcheck18 identified Mendelian inconsistencies. Two-point linkage was performed using MLINK.19 SimWalk2 version 2.8920 was used for multipoint linkage and haplotype analysis. Unaffected individuals under 50 years of age were excluded from the analysis because of the late-onset nature of the disease. Marker allele frequencies were estimated by counting alleles of all genotyped members of the pedigree. Genetic markers and linkage distances were from the Marshfield maps,21 and map positions were from the physical genome map.
Screening Gene Exons for Mutations
Coding exons and their flanking splice-sites, the most common sites for disease-causing mutations,22 were PCR amplified from genomic DNA with custom primer pairs, purified and then sequenced with an automated sequencer (model 3730; ABI). Trace chromatogram data were viewed on computer (Sequencher 3.0 software) and manually screened for heterozygous sequence variants. Experimentally determined nucleotide sequences were compared with published sequences by alignment and coassembly, using the software. Genes already screened and their cDNA accession numbers are CRYL1 NM_015974; GJA3 NM_021954; GJB2 NM_004004; GJB6 NM_006783; FGF9 NM_002010; LOC387911 NM_001007537; MGC48915 NM_178540; MRP63 NM_024026; SGCG NM_000231; and TUBA2 NM_006001.
| Results |
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Exceptional Cases of Early-Onset of Disease
Two affected women of generation III had husbands who were also affected with FCD (Fig. 1 , III-10, III-16). Of special interest was the unusual diagnosis of FCD in two of their children. In one family, a 10-year-old boy (IV-2) was affected with grade 2 disease, whereas in the other family, a 13-year-old girl (IV-5) had grade 1 FCD. Although at first we suspected distant consanguinity of the parents, family members maintained that the two husbands were completely unrelated, a statement that was later supported by haplotype analysis of the genotype data.
With regard to clinical features of these families, the mother of the 10-year-old boy exhibited extensive corneal guttae when viewed by slit lamp microscopy, as indicated by white arrows in Figure 2A . Specular microscopy of the same eye (Fig. 2B) demonstrated moderate to severe cornea guttae. Specular microscopy of the father, who had grade-1 FCD, revealed scattered corneal guttae of conventional appearance (Fig. 2C) . Slit lamp examination of the 10-year-old boy (Fig. 2D) , revealed many scattered, individual central guttae, and specular microscopy (Fig. 2E) established that these were of conventional FCD morphology. Examination of the 13-year-old girl from the other sibship indicated a slightly earlier stage of the disease (Fig. 2F) . Although the ages of these two patients classified them as having early-onset FCD, the clinical appearance of the guttae was indistinguishable from those of classic late-onset FCD, and did not have the unusual, fine-grained appearance that we and others have characterized in patients with FCD with COL8A2 mutations.8 15
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Revised Genetic Linkage Model
Analysis of the genotype data for familial relationships and the construction of haplotypes for chromosome 1318 20 confirmed that the fathers of each sibship (III-10 and III-16) were not consanguineous with their wives. This favored the alternative explanation that III-10 and III-16 had independently developed FCD, perhaps because of an unlinked mutation. In this second model, the affected children were assumed to have had early-onset FCD because they had inherited one dominant FCD1 mutant allele from their mothers and a second, unmapped FCD gene from their fathers.
For purposes of linkage calculation, the two fathers (Fig. 1 , sectored symbols) were classified as unaffected because they did not carry the same FCD1 mutation inherited by descendants of the founders, I-1 and I-2. Maximum LOD scores for each of the 399 markers (Fig. 3) revealed only one region with scores higher than 3.0. Scores for individual markers were calculated with MLINK using the same parameters described in Table 1 , where linkage data is presented in more detail. As shown in Table 1 , the first five markers of the chromosome 13 linkage group all gave maximum LOD scores at
= 0, indicating that crossovers between these markers and the disease locus were not detected. The
score at D13S1304 indicated obligatory crossovers between D13S1243 and D13S1304.
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= 0.
Genes in the Disease Interval
The linkage data (Table 1) and haplotype map (Fig. 4) formally place the disease interval between the P telomere and D13S1304. Although they constitute most of the interval, there is no assembled DNA sequence available for the centromere and the entire short (p) arm of chromosome 13. As one proceeds down the q arm of 13, the first genome assembly DNA sequence begins at 17.9 Mb from the p telomere, and the first known gene, TUBA2, is located at 18.65 Mb. Further down, there are a total of 44 known or predicted genes within the disease interval (Fig. 5) . We have sequenced the exons of 10 of these (Fig. 5 , stars; see also Methods), but have not yet found FCD-associated mutations.
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| Discussion |
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The portion of the FCD1 interval that is represented in the sequenced genome assembly spans 13q12.11-q12.13 and covers 7.6 Mb. It includes 44 known and predicted protein-encoding genes (Fig. 5) . We have screened the exons of 10 of these (see Methods for details), but so far have found no variants beyond common polymorphisms. Of early interest were two closely related genes, LOC387911 and MGC48915. Each of these has internal GXX collagen helix repeats and a C-terminal C1Q domain, features shared by other basal lamina collagens including COL8A2.7 8 15 Unfortunately, they contain no coding region mutations associated with FCD. These two candidate genes also do not appear to be major components of the cornea or Descemets membrane. They are not represented in expressed sequence tag (EST) clones from cornea, or in SAGE libraries from the corneal endothelium.25
Possible difficulties in identifying the FCD1 mutation are its dominant nature and its mild, late-onset phenotype.22 The defect could be a noncoding region promoter mutation that causes a modest change in mRNA levels. Another possibility is that the FCD1 mutation is a gene deletion that causes a phenotype through haploinsufficiency, which in turn causes a dominant mode of inheritance, with a potentially lethal homozygous phenotype. Unfortunately, conventional exon screening usually does not detect the deletion of a whole exon or gene. Testing this possibility will require an examination of the relative kinetics of PCR for each gene amplimer using quantitative PCR, or the observation of apparent non-Mendelian inheritance of polymorphisms within the gene. It may also be necessary to examine the disease chromosome for other large-scale rearrangements, such as inversions.22 26 Such chromosome breakage and nonhomologous reunion are known to cause dominant gain-of-function alleles by fusing coding sequences of a gene with novel promoter elements.
An intriguing feature of this particular pedigree is the finding that two parents with late-onset FCD can have a child with the early stages of classic FCD. This has yet to be reported elsewhere and was noted only during the course of a detailed examination of this particular family. Families with mutations in COL8A2 show consistent early-onset of FCD, but it is a form of the disease with a very distinct clinical appearance and histopathologic abnormalities.8 14 15 In the two children wereport in this study, the guttae were indistinguishable from those described for conventional late-onset FCD.8 17 These findings open the possibility that the incidence of FCD in children is more frequent than previously thought. Another alternative is that that FCD1 has a tendency toward earlier onset and that this tendency is sensitive to unlinked genetic modifiers or environmental factors. In two instances, it is possible that digenic interaction with dominant FCD-causing mutations in other genes has accelerated onset of the disease by two to three decades.
| Acknowledgements |
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| Footnotes |
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Submitted for publication May 11, 2005; revised July 10 and August 17, 2005; accepted November 29, 2005.
Disclosure: O.H. Sundin, None; A.S. Jun, None; K.W. Broman, None; S.H. Liu, None; S.E. Sheehan, None; E.C.L. Vito, None; W.J. Stark, None; J.D. Gottsch, None
The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked "advertisement" in accordance with 18 U.S.C.
1734 solely to indicate this fact.
Corresponding author: John D. Gottsch, 321 Maumenee Building, The Wilmer Eye Institute, Johns Hopkins Hospital, 600 N. Wolfe Street, Baltimore, MD 21287; jgottsch{at}jhmi.edu.
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N. A. Afshari, Y.-J. Li, M. A. Pericak-Vance, S. Gregory, and G. K. Klintworth Genome-wide Linkage Scan in Fuchs Endothelial Corneal Dystrophy Invest. Ophthalmol. Vis. Sci., March 1, 2009; 50(3): 1093 - 1097. [Abstract] [Full Text] [PDF] |
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E. N. Vithana, P. E. Morgan, V. Ramprasad, D. T.H. Tan, V. H.K Yong, D. Venkataraman, A. Venkatraman, G. H.F. Yam, S. Nagasamy, R. W.K. Law, et al. SLC4A11 mutations in Fuchs endothelial corneal dystrophy Hum. Mol. Genet., March 1, 2008; 17(5): 656 - 666. [Abstract] [Full Text] [PDF] |
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J. S. Mehta, E. N. Vithana, D. T. H. Tan, V. H. K. Yong, G. H. F. Yam, R. W. K. Law, W. G. W. Chong, C. P. Pang, and T. Aung Analysis of the Posterior Polymorphous Corneal Dystrophy 3 Gene, TCF8, in Late-Onset Fuchs Endothelial Corneal Dystrophy Invest. Ophthalmol. Vis. Sci., January 1, 2008; 49(1): 184 - 188. [Abstract] [Full Text] [PDF] |
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P Liskova, Q Prescott, S S Bhattacharya, and S J Tuft British family with early-onset Fuchs' endothelial corneal dystrophy associated with p.L450W mutation in the COL8A2 gene Br. J. Ophthalmol., December 1, 2007; 91(12): 1717 - 1718. [Full Text] [PDF] |
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A. J. Aldave and B. Sonmez Elucidating the Molecular Genetic Basis of the Corneal Dystrophies: Are We There Yet? Arch Ophthalmol, February 1, 2007; 125(2): 177 - 186. [Abstract] [Full Text] [PDF] |
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O. H. Sundin, K. W. Broman, H. H. Chang, E. C. L. Vito, W. J. Stark, and J. D. Gottsch A Common Locus for Late-Onset Fuchs Corneal Dystrophy Maps to 18q21.2-q21.32. Invest. Ophthalmol. Vis. Sci., September 1, 2006; 47(9): 3919 - 3926. [Abstract] [Full Text] [PDF] |
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