Molecular Vision 2005; 11:245-248 <http://www.molvis.org/molvis/v11/a28/> Received 12 January 2005 | Accepted 28 March 2005 | Published 1 April 2005

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KIF21A gene c.2860C>T mutation in congenital fibrosis of extraocular muscles type 1 and 3

Long-Kuang Lin,^1,2 Yin-Hsiu Chien,^3,4 Jer-Yuarn Wu,⁵ Ai-Hou Wang,^1,2 Shu-Chuan Chiang,^3,4 Wuh-Liang Hwu^3,4
 
 

Departments of ¹Ophthalmology and ³Medical Genetics and Pediatrics, National Taiwan University Hospital, Taipei, Taiwan; Departments of ²Ophthalmology and ⁴Medical Genetics and Pediatrics, National Taiwan University College of Medicine, Taipei, Taiwan; ⁵Institute of Biomedical Sciences, Academia Sinica, Taipei, Taiwan

Correspondence to: Wuh-Liang Hwu, Department of Pediatrics, National Taiwan University Hospital, 7 Chung-Shan South Road, Taipei 100, Taiwan; Phone: 886-2-23123456 ext 7541; FAX: 886-2-23314518; email: hwu@ha.mc.ntu.edu.tw

Abstract

Purpose: To determine, in Taiwanese patients, the genetic basis of congenital fibrosis of the extraocular muscles (CFEOM) type 1 and 3, a group of diseases characterized by congenital restrictive ophthalmoplegia affecting muscles innervated by the oculomotor nerve.

Methods: Linkage analysis with microsatellite markers at chromosome 12q and direct sequence analysis of the KIF21A gene were performed on three families and one sporadic CFEOM case.

Results: Two of the families were classified by clinical criteria as CFEOM1 and another family as CFEOM3. All three families were shown to be linked to the 12q CFEOM1 locus. Sequence analysis disclosed a heterozygous c.2860C>T mutation in the KIF21A gene in all families and in the sporadic case. Affected family members were further confirmed by a BsrDI polymorphism.

Conclusions: CFEOM is present in Chinese populations. Both CFEOM1 and CFEOM3 can be caused by the same mutation at the KIF21A gene. The occurrence of this mutation in different ethnic groups suggests a mutation hot spot.

Introduction

The congenital cranial dysinnervation disorders (CCDDs) encompass congenital, nonprogressive, sporadic, or familial abnormalities of cranial musculature that result from developmental abnormalities of, or the complete absence of, one or more cranial nerves with primary or secondary muscle dysinnervation [1]. Among the CCDDs, congenital fibrosis of the extraocular muscles (CFEOM, OMIM 135700), also called general fibrosis syndrome or congenital external ophthalmoplegia, includes a group of eye movement disorders that are characterized by restrictive ophthalmoplegia affecting muscles innervated by the oculomotor and trochlear nerves [2]. Since its first description in 1879 [3], this syndrome has been identified in many different ethnic groups [4]. CFEOM can result in amblyopia, loss of binocular vision, and strict monofixator. Treatment for CFEOM is primarily surgical and aims at the elimination or improvement of an unacceptable head position, the reduction of ptosis, and/or the elimination or reduction of significant misalignment of the eyes [5]. However, in some patients, satisfactory results are not achieved, probably reflecting variable clinical features of the CFEOM even within the same family [5].

Three types of CFEOM have been identified as CFEOM1-CFEOM3 [2]. Although the primary position of both eyes is infraducted, there are variations in the secondary position of each eye (exotropic, esotropic, or orthotropic) and in the degree of residual horizontal movement within the lower quadrants (full to complete restriction). If all affected members of a family have classic CFEOM in a dominantly inherited way, the phenotype is classified as CFEOM1 (OMIM 135700). CFEOM2 (OMIM 602078) is a recessive disease associated with bilateral ptosis with eyes fixed in an exotropic position. In families with CFEOM3 (OMIM 600638), one or more affected individuals do not have classic findings of the disorder. Their eyes may not be infraducted or may elevate above the midline, the individual may be unilaterally affected, or ptosis may be absent. CFEOM1 typically maps to 12p11.2-q12 [6,7], and CFEOM2 maps distal to 11q13 [8]. CFEOM3 initially mapped to 16qter [9], but some of the CFEOM3 families were later found to map to the 12q CFEOM locus [10]. CFEOM2 was first found to be caused by homozygous mutations of the ARIX gene [11]. In CFEOM1, the ARIX gene mutation was not found [12], and later heterozygous mutations in the KIF21A gene were identified in probands with CFEOM1 [13]. Recently, KIF21A mutations were also identified as a rare cause of CFEOM3 [14].

However, there have not been any reports about CFEOM in Chinese populations. In this study we performed both linkage analysis and molecular studies on three Taiwanese families with CFEOM and a sporadic case of CFEOM.

Methods

Pedigree collection

Pedigrees and affected status data were obtained from medical records. All affected individuals were examined by ophthalmologists. The study was approved by the Ethical Committee of our hospital, and informed consent was obtained from each individual who entered the study. Genomic DNA was extracted from peripheral blood leukocytes by a QIAamp DNA Blood Mini Kit (Qiagen, Valencia, CA).

Linkage analysis

Polymorphic microsatellite markers were from the ABI PRISM® Linkage Mapping Set version 2.5 HD5 (Applied Biosystems, Foster city, CA). Two point analyses for these markers were calculated using the MLINK program of the LINKAGE package. Markers used at chromosome 12 included D12S345, D12S1663, D12S85, D12S368, D12S83, D12S326, and D12S1708.

Mutation analysis

Each exon of the KIF21A gene (GenBank NM_017641) coding region was amplified as described elsewhere [13]. The translation start site of the cDNA is numbered as +1. The products of amplified exons were subjected to direct sequencing. The c.2860C>T change creates a new BsrDI site. Therefore the mutant sequence will be cut from a 1,099 bp sequence to 810 bp and 289 bp fragments.

Results

Three families (Families 1 to 3) and one sporadic case (Patient 4) were enrolled in the study. All affected members in Families 1 and 2 were born with bilateral ptosis and varying degrees of ophthalmoplegia with both eyes fixed in a hypotropic position. The inheritance patterns were dominant (Figure 1). None were able to elevate either eyes above the horizontal midline, and they all presented with a compensatory chin up and head turn prior to surgical intervention. No intraocular lesions were identified in these patients. Seven among the total 21 patients underwent surgical procedures for their eye disease. Families 1 and 2 were classified as CFEOM1. Family 3 was different because of intra-family variation in disease severity (Figure 2). Members III-3 and IV-9 had classical disease with severe ptosis and fixed eyes. Members II-1, III-4, and IV-6 were all affected, but these individuals could still move their eyes, and severity between the two eyes also differed (Table 1). Therefore, this family was compatible with the diagnosis of CFEOM3. Patient 4 had classical symptoms, but neither his three siblings nor his parents were affected.

Linkage analyses were done for the three families with markers at chromosome 12 around the CFEOM1 locus. Linkage analysis suggested linkage to that locus for all three families but the LOD score did not reach significance. The highest LOD score occurred in Family 1 and was 1.951 at D12S368 and D12S83.

After cloning the CFEOM1 gene KIF21A, we started to sequence the gene from these patients. Each exon was sequenced, and a heterozygous c.2860C>T (R954W) mutation was found in all families and the sporadic case (data not shown). Since this nucleotide change creates a new BsrDI site, we used this restriction fragment length polymorphism to examine all available family members. We found that all affected individuals, including the mildly affected patients in Family 3, carried this mutation (Figure 2, lower panel). None of the unaffected individuals had the mutation (Figure 2, lower panel).

Discussion

In this study, we found that all families, including the sporadic case and one family with CFEOM3, harbored the same mutation in the KIF21A gene. This is the first report of CFEOM and KIF21A mutation in Chinese, and the results further confirm the phenotype variation in the KIF21A mutation.

Kinesins and dyneins are molecular motors responsible for microtubule dependent anterograde and retrograde axonal transport in neurons. KIF21A, KIF21B, and KIF4 comprise one of 14 classes of human kinesins [15]. Mouse Kif21a was found to be enriched in adult neural tissues [16]. It is likely, although not proved yet, that KIF21A mutations may affect the development, migration, and survival of the motor neurons controlling eye movement. KIF21A has an N-terminal motor domain, a central coil-coil stalk, and a C-terminal tail that interacts with the transported cargo [15], and most of the known KIF21A mutations alter amino acid residues in the coil-coil region of the KIF21A stalk [13]. The stalk is the site of kinesin dimerization and protein association, and mutation at this region may have a dominant negative effect by interfering with the interaction between KIF21A and itself or its partners, which can explain the dominant inheritance of the disease.

There have not been any previous publications about CFEOM in Chinese populations. The populations of Taiwan are derived from different areas in mainland China. In this paper, we report both familial and sporadic cases, encompassing both CFEOM1 and CFEOM3. In the work by Yamada et al, the c.2860C>T mutation represented 72% of the familial cases and 69% of the sporadic cases [13]. Observing a c.2860C>T and c.2861G>A mutations in several different ethnic groups, and now in the Chinese population, suggests that this CpG dinucleotide is a hot spot for new mutations. We expect that cases of CFEOM in the Chinese population may have been under diagnosed.

CFEOM3 is a more heterogeneous disease, and from previous linkage and mutation analysis a portion of CFEOM3 cases are related to KIF21A mutations. The current study gives a second example that CFEOM3 can be caused by the common KIF21A mutation c.2860C>T. Since c.2860C>T represents the majority of the CFEOM1 cases, this report further confirms that in rare instances the expression of this mutation can be variable. Actually, Family 3 may be better described as CFEOM1 with variability in expression. This variation in phenotype-genotype relationship could be related to the molecular mechanism of KIF21A mutations, since the mutated KIF21A protein is likely to exert a dominant negative effect on the normal KIF21A protein through protein-protein interactions. We have shown in other cases that unstable mutated proteins may also demonstrate dominant negative effects by decreasing the stability of the protein complex, thus destroying the interacting proteins as innocent bystanders [17]. Because those molecular mechanisms involving protein-protein interaction and protein instability are more likely to be affected by cellular machineries of protein folding and protein degradation systems, large symptom variations can often be seen [18]. Further studies to understand the characteristics of the c.2860C>T mutation should give insights into the molecular mechanism of this mutation, and probably shed light on new treatments for this disease.

Acknowledgements

The authors thank the families for their contribution. This work was supported by a National Taiwan University Hospital Grant (NTUH 90S10).

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