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1 From the Hôpital Jules Gonin, Department of Ophthalmology, 2 Division Autonome de Génétique Médicale, CHUV, Lausanne, Switzerland; 3 Augenklinik, Inselspital, Bern, Switzerland; the 4 Department of Ophthalmology and Visual Sciences, Vanderbilt University School of Medicine, Nashville, Tennessee; 5 The Hospital for Sick Children, University of Toronto, Toronto, Canada; 6 Academic Unit, Manchester Royal Eye Hospital and Department of Molecular Genetics, St. Marys Hospital, Manchester, United Kingdom; 7 Clinica Oculistica, Università de LAquila, LAquila, Italy; 8 Klinik und Poliklinik für Augenheilkunde, Universität Regensburg, Regensburg, Germany; 9 Centro di Oftalmologia Barraquer, Barcelona, Spain; 10 Institute of Human Genetics, University of Greifswald, Greifswald, Germany; 11 Universitäts-Augenklinik, Zurich, Switzerland; 12 Clinica Oculistica, Università degli Studi di Cagliari, Cagliari, Italy; and the 13 Wills Eye Hospital, Philadelphia, Pennsylvania.
| Abstract |
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METHODS. Sixty-one index patients with CDs were subjected to phenotypic and genotypic characterization. The corneal phenotypes of all patients were assessed by biomicroscopy and documented by slit lamp photography. The BIGH3 gene was amplified exon by exon from constitutional DNA to perform single-strand conformation polymorphism (SSCP) analysis, followed by direct bidirectional sequencing of abnormal conformers.
RESULTS. The phenotypes of CDs were classified as lattice CD in 30 patients, Groenouw type I in 12 (CDGGI), Avellino in 7 (CDA), Reis-Bückler in 8 (CDRB), and Thiel-Behnke in 4 (CDTB). Fifty occurrences of 16 distinct mutations were identified, including 8 novel mutations responsible for lattice type IIIA in three patients (CDLIIA), intermediate type I/IIIA (CDLI/IIIA) in four patients, and atypical CDL with deep deposits in one patient (CDL-deep).
CONCLUSIONS. Disease-causing mutations were identified in 80% of the patients (50/61). All mutations localize in two regions of kerato-epithelin: the amino acid R124 and BIGH3 fasc domain 4. This study also confirms the mutation hot spot at positions R124 and R555 with nearly 50% of the mutations targeting these two amino acids (24/50). In addition the corneal phenotypes induced by changes at R124 and R555 are amino acid specific: R124C in CDLI, R555W and R124S in CDGGI, R124H in CDA, R124L in CRRB, and R555Q in CDTB. In CDLIIIA, CDLI/IIIA, and CDL-deep the genotype-phenotype correlation is domain specific, with all changes occurring at the boundary or within the fasc4 domain.
| Introduction |
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Histologically, these eight clinical entities can be classified into four categories, based on the type of deposits: hyalin in CDGGI and in the three superficial variants of granular dystrophy; amyloid in CDLI and CDLIIIA; hyalin and amyloid in CDA; and fibrocellular in TBCD. Genetically, all these mutations, except P501T in exon 11, target the two arginine residues at positions 124 and 555 in exons 4 and 12, respectively. This relatively simple picture gained in complexity when four additional mutations were reported in atypical and/or asymmetrical late-onset forms of CDL with deep stromal deposits (CDL-deep) and intermediate type I/IIIA (CDLI/IIIA).7 8 9
The purpose of this study was to further characterize the pathologic molecular characteristics underlying 5q31-linked CDs by reporting novel disease-causing mutations and to clarify the nature of genotype-phenotype correlations.
| Patients and Methods |
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| Results |
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540), 12 with CDGG1 (R555W, R124S), 6 with CDA (R124H), 2 with CDRB (R124L), and 4 with CDTB (R555Q). In addition, eight novel mutations were identified in 11 families: 3 CDLIIIA, 3 CDL-deep, and 5 CDLI/IIIA (Fig. 2)
. None of these mutations were present in the 200 control chromosomes.
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Wild-type: VITNVLQPPANRPQERGDELADSALEIFKQASAFSRASQRSVRLAPVYQKLLERMKH*
Mutation: (V627S) SSPMFCSLQPTDLRKEGMNLQTLRLRSSNKHQRFPGLPRGLCD*
These three patients had a similar history of late-onset progressive loss of vision and recurrent corneal erosions in the fourth and fifth decades of life. Slit lamp examination documented the presence of large, ropy lattice lines in the anterior stroma (Fig. 1I) . Successful surgery was performed in both patients with the N622K mutations (T1913G and T1913A) consisting of lamellar and perforating keratoplasty, respectively. Histologic examination using Congo red staining showed large amyloid deposits in the anterior stroma and notably beneath the Bowman layer. Excimer-mediated therapeutic photoablation was performed in the patient with delG1926.
Intermediate Type CDLI/IIIA.
Another four novel mutations occurred in four presumed unrelated patients from France, (n = 1), Switzerland (n = 2), and Italy (n = 1). All patients had a positive family history of CD. The phenotype was atypical and the age of onset delayed to between the third and fifth decades of life. In two patients from one family from the United States, we observed a C-to-G transversion at position 1660, which generated a T538R mutation. The patients had experienced corneal erosions as teenagers. Histopathology obtained after perforating keratoplasty in a 17-year-old patient confirmed the presence of amyloid deposits predominating subepithelially.
Two index patients were found to have a G-to-A transition at base 1915, resulting in a G623D mutation. The first symptoms were red, painful eyes and photophobia at approximately the third and fourth decades of life, sometimes complicated by corneal erosions. Biomicroscopic examination showed discrete subepithelial and very tiny linear deposits in the anterior stroma, leaving the middle and posterior third of the stroma free of opacifications (Fig. 1H) .
One index patient had an A-to-C transversion at cDNA position 1924, resulting in an H626P mutation. The corneal phenotype was characterized by a dense haze associated with lattice lines. Histology from the first perforating keratoplasty was not available, but analysis of a subsequent corneal button showed multiple fusiform amyloid deposits in the corneal stroma.
The last novel mutation identified consisted of a T-to-G transversion in position 1600, causing an L518R substitution at the protein level in a patient from Italy. This mutation is associated with a severe phenotype characterized by subepithelial and stromal amyloid deposits, as was documented by light microscopy after perforating keratoplasty performed in the patient at age 47.
Finally, the
F540 mutation was observed in a patient originating from Arbus, Sardinia, who initially had a diagnosis of CDRB.11
Closer examination of the cornea revealed lattice I/IIIA CD composed of tiny branching subepithelial deposits, leaving the stroma entirely free of lesion. Taking into consideration the delayed age of onset with corneal erosions during the third decade of life, this phenotype should not be classified as CDRB, as previously reported, but rather as a lattice-intermediate form of CD, although there is no available histopathologic proof.
CDL-Deep.
We also identified a T-to-A transversion at nucleotide 1939 resulting in a V631D mutation. This mutation is in total linkage disequilibrium with a C-to-T single nucleotide polymorphism at position 452 (P135P) in all 15 affected members of three Italian families from Andria, Italy. The geographical location of these three families, together with the linkage disequilibrium observed for P135P, suggests that these families share a common ancestor. In this CD, the first clinical symptoms appear between 45 and 50 years of age. Typically, the patients report photophobia, sometimes followed by corneal erosions and, ultimately, visual loss. Clinically, the disease is initially characterized by pre-Descemet stellate deposition associated with round Descemet indentations, producing an irregular appearance of the posterior corneal surface reminiscent of the polymorphic degeneration phenotype.12
13
These features could explain the photophobia. Radial lattice lines appear later within the midstroma. This posterior-to-anterior progression of the disease is in contrast to the anterior-to-posterior evolution of typical CDLI. In addition the corneal involvement may be asymmetrical, because of asynchronous onset with unilateral disease, at least, at initial examination. The anterior third of the stroma is mostly intact, as is the corneal epithelium. Penetrating keratoplasty is usually necessary at approximately 50 years of age.
| Discussion |
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CDLI, CDTB, CDGGI, CDA, and CDRB belong in group A. Among the 42 families with these phenotypes, the disease-causing mutations were identified in 35 (83%) of them. All 11 families with the classic forms of CDLI exhibited the R124C classic mutation. Similar results were observed for CDTB in which the classic R555Q mutation was identified in all four families examined. So far, we have associated CDGGI with only two very specific mutations, R555W and R124S, and only the C124H and the R124L mutations were observed in CDA and CDRB, respectively, although in several families with these two phenotypes, no mutation could be identified. A review of the literature showed that only one other mutation representing most probably a "private" event, was associated with CD from group A: a complex mutation (R124L and
T125-E126 on the same chromosome) in a form of CDGGI.14
No other mutation has so far been reported in association with CDs in group A.
The CDs in group BCDLIIIA, CDLI/IIIA, and CDL-deepare more heterogeneous, and molecular analysis detected mutations in 79% (15/19) of the families. Ten different mutations, of which eight are novel, were identified in 19 families with CDLIIIA, CDLI/IIIA, or CDL-deep. In addition, the Sardinian
F540 mutation, previously associated with CDRB,3
was clinically reassessed and ultimately reclassified as CDLI/IIIA (late onset during the third decade of life and presence of tiny linear subepithelial deposits). A review of the literature indicates that group B is also associated with the largest variety of mutations: 17 of the 24 mutations reported so far in BIGH3 are associated with a large spectrum of atypical and/or asymmetrical lattice CDs covering a continuum of phenotypes between CDLI and CDLIIIA (CDLI excluded)the common denominator being a disease-causing mutation in the fasc4 domain of KE.
No mutation was observed in 11 patients, including 4 with lattice I/IIIA, 1 with CDA, and 6 with CDRB. Family history was present in only five of them. The amyloid nature of the deposits was documented histologically in three of four of those with lattice-type CD, of which two had unusual posterior amyloid deposits. Gelsolin mutations responsible for lattice CD type II (D187N and D187Y) were excluded by sequencing in all four patients. The patient with CDA had a phenotype indistinguishable from its BIGH3-related counterpart. The six patients with CDRB were poorly characterized clinically, and none had the diagnosis confirmed histologically. It is possible that mutations in introns or in the promoter could be responsible for several of them. It is also possible, that cases have been misdiagnosed and actually represent phenocopies or even do not represent cases of ADCD5. We are analyzing the available corneal samples from these cases to investigate by immunohistology whether the deposits are made of KE.
From Figure 3 showing the position of the 24 mutations of BIGH3 reported so far, two important regions are discernible: the amino acid R124 and BIGH3 fasc domain 4. The importance of R124 has already been stressed,3 whereas there has not yet been a systematic worldwide study undertaken to analyze the implication of mutations at that position, but from discussion with colleagues conducting investigations in this field, we can estimate that mutations at R124 are present in more than half of all the patients with ADCD5. R124 represents therefore a key position for the generation of intracorneal amyloid deposits. Computer analysis of the structure at that position shows a high hydrophilic region and suggests that substitution of R124 by a cysteine would induce a ß turn in the protein. Additional studies on the structure of KE will help in understanding the role of this region in the making of amyloid.
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Submitted for publication May 25, 2001; revised October 8, 2001; accepted October 24, 2001.
Commercial relationships policy: N.
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: Francis L Munier, Hôpital Jules Gonin, Unité dOculogénétique, Avenue de France 15, CH-1004 Lausanne, Switzerland; francis.munier{at}chuv.hospvd.ch
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