Molecular Vision 2007; 13:1280-1284 <http://www.molvis.org/molvis/v13/a139/> Received 24 May 2007 | Accepted 20 July 2007 | Published 24 July 2007

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Novel FBN1 mutations associated with predominant ectopia lentis and marfanoid habitus in Chinese patients

Chongfei Jin, Ke Yao, Jin Jiang, Xiajing Tang, Xingchao Shentu, Renyi Wu
 
 

Eye Center, Second Affiliated Hospital of Medical College, Zhejiang University, Hangzhou, China

Correspondence to: Ke Yao, Ph.D., M.D., Eye Center, Second Affiliated Hospital of Medical College, Zhejiang University, Hangzhou, 310009, China; Phone: +86 571 87783897; FAX: +86 571 87783908; email: xlren@zju.edu.cn

Abstract

Purpose: To identify mutations in the fibrillin-1 gene (FBN1) and provide further information about genotype-phenotype correlations in Chinese patients with predominant ectopia lentis (EL) and marfanoid habitus.

Methods: Patients from seven Chinese families underwent complete physical, ophthalmic, and cardiovascular examination. Genomic DNA was extracted from leukocytes of peripheral blood from the patients. The 65 exons and flanking intronic sequences of FBN1 were amplified by polymerase chain reaction (PCR) and screened for mutation by direct DNA sequencing.

Results: Three novel mutations, c.203G>T in exon 2, c.502T>C in exon 5, and c.2096G>C in exon 16 as well as four known mutations, c.364C>T in exon 4, c.1633C>T in exon 13, c.1879C>T in exon 15, and c.4588C>T in exon37, were identified in FBN1.

Conclusions: We identified three novel mutations and four known mutations in FBN1 and found cysteine substitution highly related to EL. These results expand the mutation spectrum in FBN1 and enrich our knowledge of genotype-phenotype correlations due to FBN1 mutations. To our knowledge, this is the first report of cysteine residue loss in the unique NH₂-terminal domain of fibrillin-1.

Introduction

Ectopia lentis (EL; OMIM 129600) is an autosomal dominant connective disorder characterized by lenses dislocation with stretched or disrupted zonular filaments [1]. Mutations in the fibrillin-1 gene (FBN1) are assumed to cause Marfan syndrome (MFS; OMIM 154700) [2]. However, in 1994, Edwards et al. [3] reported the linkage of FBN1 to EL patient in the absence of clear indications of Marfan syndrome in other systems. Therefore, EL and MFS are linked to the same FBN1 gene. A series of fibrillinopathies with FBN1 mutation has been reported including MFS, isolated EL, MASS syndrome (OMIM 604308), autosomal dominant Weill-Marchesani syndrome (OMIM 608328), Shprintzen-Goldberg syndrome (OMIM 182212), isolated skeletal features of MFS, and thoracic aortic aneurysms [4,5]. MFS is clinically diagnosed according to the Ghent criteria, which describe pleiotropic manifestations cardinally including lens dislocation, proximal aortic aneurysm, and long-bone overgrowth [6]. Isolated EL or predominant EL with some skeletal features do not satisfy the Ghent criteria for absence of aortic dilatation or dissection.

FBN1 contains 65 exons spanning 235 kb of genomic DNA encoding a secreted 350 kDa glycoprotein, which is highly conserved among different species [7]. Fibrillin-1 is a mosaic protein comprising interspersed repeated modules; this protein contains 47 epidermal growth factor (EGF)-like modules (43 calcium binding (or cb) EGF-like modules and 4 non-cb EGF-like modules) and seven transforming growth factor-binding (or TB) protein-like modules (8-Cys/TB) [8]. Each of the EGF-like domains contains six highly conserved cysteine residues that form three disulfide bridges between C1 and C3, between C2 and C4, and between C5 and C6. These bridges create an antiparallel beta-pleated sheet conformation that enhances calcium binding [9]. Each 8-Cys/TB module is characterized by eight highly conserved cysteine residues, which serve to hold TGF-β in an inactive complex in various tissues including the extracellular matrix [10]. Identified mutations to date are located throughout the length of FBN1 with limited clear evidence of genotype-phenotype correlations [11].

Genotype-phenotype correlations of FBN1 mutations even in selected groups of individuals are complex [12]. Moreover, some patients meet Ghent criteria when genotype for a mutation is included in the analysis. In mild cases, the symptoms and signs are not always pronounced thus, the diagnosis is difficult. Furthermore, there is considerable variation in the clinical phenotype between families and even within the same family. More information is still needed to clarify the genotype-phenotype correlations of FBN1 mutations.

In our study, genomic DNA was collected from seven unrelated families with predominant EL and marfanoid habitus. In two of these cases, there was evidence of family history while the other five were sporadic cases. All of the seven probands and available family members were screened for genetic mutations.

Methods

Patients and clinical data

All the patients participating in this study were diagnosed and treated in the Eye Center of the Second Affiliated Hospital of Medical College, Zhejiang University. Informed consent in accordance with the Zhejiang Institutional Review Board approval was obtained from all participants prior to the study. We investigated seven predominant EL patients with or without skeletal signs of MFS but without aortic dilatation or dissection. The patients underwent complete physical, ophthalmic, and cardiovascular examinations. Fifty control subjects without diagnostic features of MFS were also recruited.

Genomic DNA preparation

Blood specimens (5 ml) were collected in EDTA and genomic DNA was extracted from leukocytes of peripheral blood from the patients and available family members by the QIAmp Blood kit (Qiagen, Hilden, Germany).

Mutation screening

The 65 exons and flanking intronic sequences of FBN1 were amplified by polymerase chain reaction (PCR), using previously reported intron-specific primers [13] with the exception of exon 12, 19, and 42 (primer sequences seen in Table 1). The mutation analysis was performed by direct sequencing, using the BigDye Terminator Cycle sequencing kit V 3.1(ABI Applied Biosystems; Sangon Co.,Shanghai,China) on an ABI PRISM 3730 Sequence Analyzer according to the manufacturer's directions. Each mutation was confirmed by bidirectional sequencing. One hundred chromosomes from 50 unrelated controls were tested for the identified mutations by using direct sequencing to determine recurrent mutations or polymorphisms.

Results

Clinical findings

In all affected patients, bilateral lenses dislocation was present as a predominant manifestation. Lens deformation was observed in addition to lens dislocation in patient Lsy (Figure 1). Flat cornea was detected in four patients by keratometry. Only one patient displayed increased axial length. No patients in this study were diagnosed with retinal detachment. In addition, aortic dilatation or dissection was absent in all the patients by echocardiogram. The echocardiogram of 15-year-old patient Yl demonstrated atrial septal defect (ASD) and mild mitral valve regurgitation (MVR). The echocardiogram of Khx was not available while an echocardiogram of her mother demonstrated no positive signs in the cardiovascular system. Patient Lq received orthopedic surgery for severe scoliosis two years ago when she was 17 years old (Table 2).

Mutation analysis

Direct sequencing of FBN1 revealed three novel heterozygous mutations, including c.203G>T, c.502T>C, and c.2096G>C as well as four known mutations, c.364C>T, c.1633C>T, c.1879C>T and c.4588C>T (Figure 2). The mutation c.502T>C discovered in patient Khx was also found in her affected mother while c.4588C>T was detected in Lq's affected mother. None of the seven mutations were detected in the 50 unrelated control subjects with no diagnostic features of MFS. Genetic information analysis was summarized in Table 2.

Discussion

We report here three novel and four known FBN1 mutations related to disruptions in the unique NH₂-terminal domain, non-cb EGF-like modules, cb EGF modules, and 8-Cys/TB modules. At the phenotypic level, recent studies have shown that patients with cysteine substitutions have a significantly greater incidence of EL [5,14]. Similar to these previous reports, cysteine substitutions are detected in all the patients with predominant EL in our study, suggesting that cysteine residues may play an important role in suspensory ligaments formation or development. Cysteine substitutions disrupt one of the three disulfide bridges that covalently connect three pairs of cysteine residues that are highly conserved in EGF-like domains [15]. This has a predictable detrimental effect on the domain and results in a change to the secondary structure of the protein.

The NH₂-terminal domains of fibrillin-1 are thought to direct the formation of dimer intermediates, which aggregate to form the functional microfibril [16]. Four cysteine residues in the unique NH₂-terminal domain (Fib-4-cys) have been determined to form intramolecular disulfide bonds. No mutations of the cysteine residues in this unique NH₂-terminal domain have previously been reported, suggesting the possibility that loss of these cysteine residues is not compatible with life [17]. However, in our study, patient Yl has survived the presence of the novel mutation c.203G>T, which alters the third wild-type cysteine residue to a phenylalanine residue in the unique NH₂-terminal domain. Besides EL, the patient suffers myopia in the ocular system, pectus carinatum, arachnodactyly, and dental crowding in the skeletal system whereas neither aortic dilatation nor dissection was shown in the echocardiogram. Since this patient is only 15 years old, we can not exclude the possibility of late onset aortic dilatation and/or dissection. In addition, it is intriguing that the echocardiogram indicates atrial septal defect (ASD), which is rarely reported in MFS.

It is clear that EGF-like domains play a major role in the pathogenesis of fibrillinopathies with most of the mutations in these domains associated with classic MFS [12]. As three disulfide bonds are required to maintain cb EGF-like module-fold, the loss or addition of cysteine residues would result in module misfolding. In our study, we reported two mutations in non-cb EGF-like modules and two other mutations in cb EGF modules. Substitutions between cysteine and arginine alter the intramolecular disulphide bond arrangement and therefore change the secondary structure of the protein. The presence of two mutations in non-cb EGF-like modules demonstrate that disturbance in the calcium-binding capacity of the EGF motif is not required for a marfanoid phenotype [18].

Although more than 600 FBN1 mutations have been registered in the UMD-FBN1 database for MFS and its associated disorders, little information is available for predominant EL. Our study is the largest predominant EL series yet reported in Chinese patients. Our results support prior data [19] that FBN1 variants in patients with predominant EL are clustered in the first 15 exons of the gene (5/7 in our study and 5/5 in the previous study). This is in contrast to the FBN1 mutations associated with MFS, which are distributed over the entire FBN1 gene; only 20% (120/601) of these mutations are within the first 15 exons (p=0.005, Fisher exact test). This suggests that disruption of the NH₂-terminal domains of fibrillin-1 is inclined to result in EL.

Interestingly, the majority of mutations (4/7) in our present work resulted in a substitution of a cysteine for arginine. By analyzing the eight mutations resulting in arginine to cysteine substitutions in UMD-FBN1 database, we find that all eight mutations (R62C, R122C, R240C, R545C, R627C, R1530C, R1832C, and R2680C) are related to predominant EL with exception of R1832C in which clinical manifestation is not available. This suggests that arginine to cysteine substitutions are highly related to predominant EL regardless of which modules they are located. Furthermore, the recurrence of mutations resulting in arginine to cysteine substitutions is significantly higher (p<0.001, Mann-Whitney U test) than other types of mutations in UMD-FBN1 database (Table 3).

In summary, our data add three novel mutations to the existing spectrum of FBN1 gene mutations and enrich our knowledge of genotype-phenotype correlations due to FBN1 mutations. Our present work demonstrates a significantly greater incidence of cysteine substitutions in marfanoid patients with predominant EL. To our knowledge, this is the first report of cysteine residue loss in the unique NH₂-terminal domain of fibrillin-1.

Acknowledgements

We thank the patients and families for their participation and Professor Ming Qi at Zhejiang University-Adinovo Center for Genetic & Genomic Medicine and University of Rochester Medical Center for his critical reading of this manuscript. This work was supported by National Natural Science Foundation of China (30640075) and Science Fund of Health Bureau of Zhejiang Province (2006A064), China.

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