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1From the Kallam Anji Reddy Molecular Genetics Laboratory, the 2Cornea and Anterior Segment Service, and the 3Ophthalmic Pathology Service, L. V. Prasad Eye Institute, Hyderabad, India.
| Abstract |
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METHODS. Thirty-seven unrelated patients were studied, 18 with LCD and 19 with GCD. The diagnosis of LCD or GCD was made on the basis of clinical and/or histopathological evaluation. Exons and flanking intron sequences of the TGFBI gene were amplified by PCR with specific primers. PCR products were screened by the method of single-strand conformation polymorphism followed by sequencing. Mutations were confirmed by screening at least 100 unrelated normal control subjects.
RESULTS. Mutations were identified in 14 of 18 patients with LCD and in all 19 patients with GCD. In LCD, three novel heterozygous mutations found were glycine-594-valine (Gly594Val) in 2 of 18 patients, valine-539-aspartic acid (Val539Asp) in 1 patient, and deletion of valine 624, valine 625 (Val624-Val625del) in 1 patient. In addition, mutation of arginine 124-to-cysteine (Arg124Cys) was found in 8 of 18 patients and histidine 626-to-arginine (His626Arg) in 2 of 18 patients. Atypical clinical features for LCD were noted in patients with the Gly594Val and Val624-Val625del mutations. In GCD, 18 patients with GCD type I had a mutation of arginine 555-to-tryptophan (Arg555Trp) and 1 patient with GCD type III (Reis-Bücklers dystrophy), had the Arg124Leu mutation. Seven novel single-nucleotide polymorphisms (SNPs) were also found, of which a change of leucine 269 to phenylalanine (Leu269Phe) was found in 12 of 18 patients with the Arg555Trp mutation.
CONCLUSIONS. Arg124Cys and Arg555Trp appear to be the predominant mutations causing LCD and GCD, respectively, in the population studied. The novel mutations identified in this study are associated with distinct phenotypes.
LCD (OMIM 122200; On-line Mendelian Inheritance in Man; http://www.ncbi.nlm.nih.gov/Omim/ provided in the public domain by the National Center for Biotechnology Information [NCBI], Bethesda, MD) is a primary, usually bilateral, corneal amyloidosis characterized by refractile lines that are in the form of a fine, branching network. Histologically, the deposits in LCD stain positively with Congo red and are birefringent under polarized light. LCD has at least four different subtypes (reviewed in Ref. 6 ). LCD type I7 is an autosomal dominant, bilaterally symmetrical corneal disorder that is characterized by numerous translucent fine lattice lines that are associated with white dots and faint haze in the superficial and middle layers of the central stroma. LCD type III8 is a late-onset disease of autosomal recessive inheritance that appears with decreased vision in the fifth to seventh decades of life. Asymmetrical findings are common. Lattice lines extend up to the limbus, are thicker, and are more easily seen with direct illumination than those in LCD type I. LCD type IIIA differs from type III in the presence of erosions and an autosomal dominant inheritance pattern.9 Recently, LCD type IV has been described, with deep stromal opacities and late onset of disease.6
GCD type I (OMIM 121900) is characterized by small, discrete, sharply demarcated grayish white opacities in the anterior central stroma resembling bread crumbs or snowflakes.10 Histologically, the corneal deposits stain positively with Masson trichrome and are nonamyloid.11 As the condition advances, individual lesions increase in size and number and may coalesce, extending into the deeper and more peripheral stroma. GCD type II (Avellino corneal dystrophy, OMIM 607541), shares features of lattice and granular dystrophies and has both granular and amyloid types of deposits.12 It is clinically similar to GCD type I. GCD type III is a superficial variant of GCD13 (Reis-Bücklers dystrophy, OMIM 608470) and the deposits are morphologically similar to those in GCD type I, but are present mainly in the Bowmans layer and beneath the epithelium.
Phenotype-specific mutations have been characterized in the TGFBI gene. Most patients have mutations at mutational hot spots corresponding to arginine 124 and arginine 555 of the keratoepithelin protein. These include the arginine 124-to-cysteine (Arg124Cys) mutation in LCD type I, arginine 555-to-tryptophan (Arg555Trp) in GCD type I, arginine 124-to-histidine (Arg124His) in Avellino corneal dystrophy,2 14 and arginine124-to-leucine (Arg124Leu) in Reis-Bücklers corneal dystrophy.15 In addition to these common mutations, mutational heterogeneity exists, particularly in different forms of lattice dystrophy.16
We screened the TGFBI gene for mutations in Indian patients with LCD and GCD to determine the range of mutations underlying these diseases and to characterize the associated phenotypes. LCD and GCD account for approximately 15% to 25% of all patients with corneal dystrophy requiring corneal grafts in our institution, a tertiary-care referral center in southern India.17 No genetic studies have been reported so far on lattice and granular dystrophies in Indians. We report the results of our study on 37 unrelated patients (18 with LCD, 19 with GCD).
| Materials and Methods |
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Genomic DNA was isolated from blood leukocytes by standard procedures.18 Individual exons of the TGFBI gene were amplified with primers designed by us, specific for flanking intron sequences (primer sequences available on request). PCR-amplified products of all 17 exons were screened for sequence changes by the method of single-strand conformation polymorphism (SSCP), as previously described.19 Fragments showing altered mobility relative to control subjects were sequenced bidirectionally. Sequencing of purified PCR products was performed (BigDye Terminator Kit on a model 310 Prism sequencer; Applied Biosystems, Inc. [ABI], Foster City, CA). Sequences were compared with the published sequence of TGFBI (GenBank accession no. for genomic sequence, AY149344; mRNA sequence- NM_000358, version NM_000358.1; http://www.ncbi.nlm.nih.gov/Genbank; provided in the public domain by the National Center for Biotechnology Information, Bethesda, MD). All samples that were negative for mutations on screening by SSCP, were sequenced directly in all exons to identify mutations. At least 100 normal unrelated control individuals as well as family members were screened for identified mutations to confirm pathogenicity. PCR-restriction fragment length polymorphism (RFLP) was used to test for the presence of various mutations in unrelated control subjects and family members. Restriction site changes are detailed in Table 1 . PCR products of normal and mutant DNAs were digested with the relevant restriction enzyme and resolved on polyacrylamide or agarose gels. DNA was visualized by staining with ethidium bromide.
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| Results |
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Mutations
Five mutations, of which three are novel, were identified in 14 patients with a diagnosis of LCD. Eight patients had a mutation of arginine 124 to cysteine (Arg124Cys), two patients had a mutation of histidine 626 to arginine (His626Arg), one patient had a mutation of valine 539 to aspartic acid (Val539Asp), two patients had a mutation of glycine 594 to valine (Gly594Val), and one patient had an in-frame deletion of two amino acids, valine 624 and valine 625 (Val624-Val625del). All the mutations detected were heterozygous in probands and were absent in 100 unrelated unaffected individuals. No mutations were identifiable after sequencing of all exons of TGFBI in the remaining four patients. Eighteen patients with a diagnosis of GCD type I had a mutation of arginine 555 to tryptophan (Arg555Trp), and one patient with a diagnosis of GCD type III (or Reis-Bücklers dystrophy) had a mutation of arginine 124 to leucine (Arg124Leu). Pedigrees with segregation of novel mutations in available family members are shown in Figure 1 .
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Val539Asp.
The slit lamp photograph of the cornea of the proband is shown in Figure 2A . The opacities were in the form of lattice lines in the anterior stroma. Cosegregation analysis of the mutation in this pedigree, shown in Figure 1A , revealed that his older son, aged 34 years, affected but asymptomatic, was heterozygous for the mutation.
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Val624-Val625del.
One patient had this mutation and showed manifestations that are atypical of LCD (Figs. 3A 3B) . He had diffuse corneal opacities with no clear lattice lines. He complained of progressive loss of vision and photophobia during his 20s and at the age of 30 years, underwent corneal grafting. The diagnosis of LCD was based on the histopathology of the corneal button, which showed amyloid deposits in the anterior stroma (Fig. 3C) . The deposits were negative for Masson trichrome. His father and two siblings were reported to be similarly affected in their 20s. The brother of the proband, who was also examined in our institution, had scarring and deposits in the superficial stroma. He was diagnosed to have corneal dystrophy of an unspecified type. After a corneal graft at the age of 30 years, histopathology revealed amyloid deposits in the Bowmans layer and anterior stroma. The pedigree was analyzed for cosegregation of the mutation with disease, and details are shown in Figure 1D .
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One patient with the Arg124Leu mutation, received a clinical diagnosis of Reis-Bücklers dystrophy (GCD type III) with stromal involvement, based on the presence of multiple opacities in a honeycomb pattern in the subepithelial and superficial stromal layers (Figs. 4A 4B) . Disease had its onset during the second decade. The patient underwent corneal transplantation in both eyes with recurrence of the opacities within a few years of surgery. Histopathological evaluation revealed granular Masson-positive deposits in the stroma.
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T variation at cDNA position 852, and results in a change of CTT (coding for leucine 269) to TTT (phenylalanine).This change was heterozygous in 12 of 18 patients with GCD type I and in 3 of 100 unrelated normal control subjects. It cosegregated with the Arg555Trp mutation in some affected families and in one family (not included in this series), affected members were homozygous for both the Arg555Trp mutation and the Leu269Phe variant (data not shown), suggesting that the two sequence changes may be in cis. This polymorphism was absent in the patients that we studied who had LCD and Reis-Bücklers dystrophy. | Discussion |
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We observed a broad correspondence similar to that reported in earlier studies, between mutations at arginine 124 and arginine 555 residues in TGFBI and their associated phenotypes.2 14 In addition, our study demonstrates further mutational heterogeneity in TGFBI and brings to light unusual phenotypes of TGFBI-linked corneal dystrophies (Table 1) .
The three novel mutations identified in this study were each associated with different phenotypes of LCD. All three mutations involve the fourth fasciclin-like domain in which most pathogenic alterations in TGFBI have been found. The two missense changes at valine 539 and glycine 594, as well as the in-frame deletion at valine residues 624 and 625, involve highly conserved residues, conserved among fasciclin-like domains of several proteins (NCBI Conserved Domain Database; available in the public domain at http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=cdd).
For the Gly594Val mutation, the clinical features in both probands (Table 1) were late onset of disease and the presence of deep stromal opacities that extended up to the limbus. These manifestations are similar to those reported for two other mutationsnamely, Leu527Arg24 and Val631Asp21 and have been classified as LCD type IV.6 The Gly594Val mutation, to our knowledge, represents the third mutation causing this form of LCD. Cosegregation analysis in the family members revealed that two offspring of one of the patients (Fig. 1B) carried the mutation but did not manifest disease. It is possible that the mutation carriers in this family may manifest disease at a more advanced age or that this mutation has incomplete penetrance. The high degree of conservation of the residue mutated, as well as the absence of the change in 100 unrelated control subjects support the conclusion that it is pathogenic.
The novel deletion of Val624-Val625 occurred in a patient having diffuse corneal opacities with no clinically evident lattice-type pattern and amyloid deposits in the stroma (Fig. 3) . A nonlattice pattern of corneal opacification has also been observed in association with the Arg124Cys mutation,25 thus raising the idea that factors such as advanced stage disease, ageing, environmental factors, or modifier genes contribute to the phenotype. In addition, a nonlattice phenotype resulting from a mutation in TGFBI was reported in a family with polymorphic corneal amyloidosis.26 These data together enlarge the range of phenotypic variability associated with TGFBI gene mutations and suggest that the lattice and nonlattice types of stromal amyloidoses showing autosomal dominant inheritance may be part of a spectrum of phenotypes of the same disease.
An interesting novel polymorphism identified in this study is a change of leucine269 to phenylalanine (Leu269Phe). Two thirds of the patients with GCD (12/18) with the Arg555Trp mutation were heterozygous for Leu269Phe as were 3% of normal control subjects. Analysis of a larger cohort of families with GCD is necessary to determine whether the Leu269Phe polymorphism is in linkage disequilibrium with the Arg555Trp mutation in this population. To examine the possibility of a common origin of the Arg555Trp allele in patients with both Arg555Trp and Leu269Phe changes, we looked at the haplotypes of the other SNPs that we identified in the TGFBI gene, as well as of flanking microsatellite markers. We found that there was more than one haplotype in this group of 12 patients (data not shown). The small number of patients with GCD studied did not permit a conclusion as to whether there is a significant difference in the frequency of any haplotype between patients and control subjects.
No mutations were identified in four patients diagnosed with LCD clinically and histopathologically (Table 1) . It is possible that mutations in these cases lie within the introns or promoter of the TGFBI gene.
The keratoepithelin protein has four internal repeat domains, the FAS1 domains with homology to fasciclin-1, an insect cell-adhesion molecule.27 Most of the mutations so far reported lie in the fourth fasciclin-like domain. Structural modeling of the fourth FAS1 domain in TGFBI has predicted that the mutations in this domain possibly disrupt the structure by leading to misfolding of the protein.28 Mutant keratoepithelin especially for the Arg124 mutations, appears to undergo abnormal processing and/or turnover, resulting in the accumulation of mutant protein or fragments thereof.29 30 It may be speculated that the mutants within FAS1 domain 4 also follow a similar route. Leucine 269 is present in the second FAS1 domain of keratoepithelin (NCBI Conserved Domain Database). Because no pathogenic alterations have been found in this region, it is possible that sequence variations, especially replacement of one hydrophobic residue with another, as in the case of Leu269Phe, are tolerated. Knowledge of the functions of the different domains of keratoepithelin may eventually provide insight into the pathogenesis of the different forms of TGFBI-linked corneal dystrophies.
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Submitted for publication April 19, 2004; revised September 2, 2004; accepted September 10, 2004.
Disclosure: S.V.V.K. Chakravarthi, None; C. Kannabiran, None; M.S. Sridhar, None; G.K. Vemuganti, 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: Chitra Kannabiran, Kallam Anji Reddy Molecular Genetics Laboratory, L. V. Prasad Eye Institute, L. V. Prasad Marg, Banjara Hills, Hyderabad 500 034, India; chitra{at}lvpei.org.
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