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1 From the Department of Ophthalmology, Tohoku University School of Medicine, Sendai, Japan; and the 2 National Rehabilitation Center For The Disabled, Saitama, Japan.
| Abstract |
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METHODS. Mutation screening by single-strand conformation polymorphism (SSCP) was performed in 120 unrelated patients with ADRP, 200 unrelated patients with autosomal recessive retinitis pigmentosa (ARRP), and 100 patients with simplex RP (SRP). The DNA fragment that showed abnormal mobility on SSCP was sequenced. The clinical features of these patients were determined by visual acuity, slit lamp biomicroscopy, electroretinography, fluorescein angiography, and kinetic visual field testing.
RESULTS. A novel 208delG mutation in the FSCN2 gene was identified in 14 patients from four unrelated families with ADRP. The ophthalmic findings were typical of RP.
CONCLUSIONS. The findings show that a 208delG mutation in the FSCN2 gene produces ADRP. This mutation was found in 3.3% of the patients with ADRP in Japan, which suggests that it may be relatively common in Japanese patients with ADRP.
| Introduction |
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The retinal fascin gene (FSCN2) is a newly identified photoreceptor-specific gene located on chromosome 17q25, which encodes 516 amino acids.8 9 The fascin gene is associated with the assemblage of the actin-based structures of the connecting cilium plasma membrane and plays an important role in photoreceptor disc formation.8 9 10 11 To date, only five polymorphic mutations in the FSCN2 gene have been reported,9 and a disease-causing mutation in the FSCN2 gene has not been published.
We report the presence of a 208delG mutation of the fascin gene in four unrelated Japanese families with ADRP and describe the clinical features of these patients.
| Materials and Methods |
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Genomic DNA was isolated from leukocytes prepared from a sample of each patients blood (1015 ml), by using a protocol previously described in detail.2 For the screenings, nine sets of oligonucleotide primer pairs were used from the genomic sequence of FSCN2. The primer sequences are given in Table 1 . PCR was performed in 50 µl of reaction mixture containing 250 ng genomic DNA, 20 µM of each primer, 200 µM of each dNTP, and 1.25 U Taq polymerase. The buffer contained 50 mM KCl, 10 mM TrisCl (pH 8.3), and 1.5 mM MgCl2.
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The amplified DNA fragment was then electrophoresed in 8% nondenaturing polyacrylamide gel containing 10% glycerol at 20 W for 8 hours at room temperature. After electrophoresis, DNA bands were visualized by silver staining. The mutation or polymorphism was observed by the presence of abnormal bands derived from a mutant allele. The DNA fragment that showed abnormal mobility on SSCP was then directly sequenced to identify the mutation in the FSCN2 gene on a sequencer (model 310; Perkin Elmer-Applied Biosystems, Foster City, CA). The product of the PCR amplification in exons 1d, 2, 3, and 5 were directly sequenced without SSCP. The PCR products were sequenced in the forward and reverse directions.
The tenets of the Declaration of Helsinki were followed, and informed consent was obtained from all subjects who participated in the study.
Clinical Examination
We examined 14 affected patients from four families (Fig. 1)
. The ophthalmic examination included best corrected visual acuity,
slit lamp biomicroscopy, kinetic visual field examination, fundus
examination, fluorescein angiography (FA), and electroretinography
(ERG). Ophthalmoscopic findings were recorded by color fundus
photography. Kinetic visual field examination was performed on a
Goldmann perimeter with V-4-e, I-4-e, I-3-e, and I-2-e isopters. The
area of each isopter was expressed in steradians.12
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| Results |
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ACA in codon 193 (Thr193Thr) and CCT
CCC in codon 228
(Pro228Pro), one of which (Pro288Pro) was the same polymorphism
previously reported.9
Clinical Characteristics
The clinical characteristics of the 14 patients from four families
associated with the 208delG mutation are summarized in Tables 2
and 3
. All the affected patients had had night blindness from childhood. The
visual acuity of patients with 208delG mutation ranged from hand motion
to 1.0 in all four families. Patients more than 40 years of age showed
a marked decrease of the visual fields and visual acuity.
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FA disclosed hyperfluorescence from the posterior pole to the peripheral retina that corresponded with the mottled retina, suggesting atrophic changes in the RPE layer. Three patients (patients III-1 and III-3 of family 1 and patient II-2 of family 2) showed a combination of diffuse hyperfluorescence and patchy hypofluorescence. In addition, cystoid macula edema was observed in patient III-2 of family 3, and sharply demarcated macular degeneration was observed in patient II-2 of family 4.
The results of the ERG recordings are presented in Table 3 , and those for the three members of family 2 are shown in Figure 4 . The scotopic, single-flash, standard-flash ERGs and 30-Hz flicker ERG were mildly reduced in patients III-1 of family 2 and III-1 of family 4, and the single-flash a- and b-waves of the ERGs were mildly reduced in patient IV-1 of family 4. The ERGs of the other patients were nonrecordable (Table 3) .
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| Discussion |
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RP17 is a novel locus for ADRP detected in two South African families.8 19 20 Because the FSCN2 gene is located on 17q25, RP17 was considered to be a candidate gene for RP type 17 (RP17). However, Tubb et al.9 reported that only five polymorphic mutations in the FSCN2 gene were found in two large RP17-carrying families, and no previous report of a disease-causing mutation in the FSCN2 gene had been published.
We evaluated 120 Japanese patients with ADRP, 200 patients with ARRP, and 100 patients with SRP. Of note, molecular genetic analysis disclosed that 14 patients from four unrelated families had an identical 208delG mutation in the FSCN2 gene and that no mutation was detected in the patients with ARRP and SRP.
This mutation resulted in a frame shift and a premature termination at codon 144, 359 bp downstream from the deletion. If translated, the mutated FSCN2 gene would not encode a functional protein.
Fundus examination of three generations of family 2 disclosed the progression of retinal degeneration with increasing age (Fig. 3) . In the early stage, a 10-year-old patient showed a mottled appearance of the RPE and attenuation of the retinal vessels. In all families, affected individuals more than 40 years old showed marked retinal degeneration.
For human fascin, Ser39 is very important in regulating actin binding, and this residue is also conserved in human retinal fascin.11 Thus, Ser39 is thought to play an important role, not only in human fascin but also in human retinal fascin. Because the 208delG mutation causes a frame shift and premature termination, patients with this mutation do not have Ser39 in the FSCN2 gene. Thus, these patients would be expected to lose the activity of actin binding and have a disorder of photoreceptor formation.
We hypothesize that the 208delG mutation in the FSCN2 gene may be relatively common in Japanese patients with ADRP, because we have found this mutation in 3.3% of unrelated patients with ADRP in Japan, and there have been no reports about pathogenic mutations in the FSCN2 gene in RP type 17 of two families in other countries.8 19 20 Additional families with ADRP, ARRP, and other retinal degenerations are being screened for this mutation to ascertain the phenotypegenotype correlation in the FSCN2 gene in the Japanese population.
| Footnotes |
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Submitted for publication October 10, 2000; revised March 13, 2001; accepted April 6, 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: Yuko Wada, Department of Ophthalmology, Tohoku University School of Medicine, 1-1, Seiryo-machi, Aoba-ku, Sendai 980-77, Japan. yukow{at}oph.med.tohoku.ac.jp
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Lys) of the rhodopsin gene in a Japanese family Ophthalmic Genet 15,61-67[Medline][Order article via Infotrieve]
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