Molecular Vision 2004; 10:37-42 <http://www.molvis.org/molvis/v10/a5/> Received 3 July 2003 | Accepted 31 December 2003 | Published 15 January 2004

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Identification of a fourth locus (EVR4) for familial exudative vitreoretinopathy (FEVR)

Carmel Toomes,¹ Louise M. Downey,¹ Helen M. Bottomley,¹ Sheila Scott,¹ Geoffrey Woodruff,² Richard C. Trembath,³ Chris F. Inglehearn¹
 
 

¹Molecular Medicine Unit, University of Leeds, St James's University Hospital, Beckett Street, Leeds LS9 7TF, UK; ²Department of Ophthalmology, Leicester Royal Infirmary, University of Leicester, UK; ³Department of Clinical Genetics, Leicester Royal Infirmary, UK

Correspondence to: Carmel Toomes, Molecular Medicine Unit, St. James's University Hospital, Beckett Street, Leeds LS9 7TF, UK; Phone: +44 (0)113 2066612; FAX: +44 (0)113 2444475; email: c.toomes@leeds.ac.uk

Abstract

Purpose: Familial exudative vitreoretinopathy (FEVR) is a genetically heterogeneous inherited blinding disorder of the retinal vascular system. To date three loci have been mapped: EVR1 on chromosome 11q, EVR2 on chromosome Xp, and EVR3 on chromosome 11p. The gene underlying EVR3 remains unidentified whilst the EVR2 gene, which encodes the Norrie disease protein (NDP), was identified over a decade ago. More recently, FZD4, the gene that encodes the Wnt receptor Frizzled-4, was identified as the mutated gene at the EVR1 locus. The purpose of this study was to screen FZD4 in a large family previously proven to be linked to the EVR1 locus.

Methods: PCR products were generated using genomic DNA from affected family members with primers designed to amplify the coding sequence of FZD4. The PCR products were screened for mutations by direct sequencing. Genotyping was performed in all available family members using fluorescently labeled microsatellite markers from chromosome 11q.

Results: Sequencing of the EVR1 gene, FZD4, in this family identified no mutation. To investigate this family further we performed high-resolution genotyping with markers spanning chromosome 11q. Haplotype analysis excluded FZD4 as the mutated gene in this family and identified a candidate region approximately 10 cM centromeric to EVR1. This new FEVR locus is flanked by markers D11S1368 (centromeric) and D11S937 (telomeric) and spans approximately 15 cM.

Conclusions: High-resolution genotyping and haplotype analysis excluded FZD4 as the defective gene in a family previously linked to the EVR1 locus. The results indicate that the gene mutated in this family lies centromeric to the EVR1 gene, FZD4, and is also genetically distinct from the EVR3 locus. This new locus has been designated EVR4 and is the fourth FEVR locus to be described.

Introduction

Familial exudative vitreoretinopathy (FEVR) is a well-defined inherited disorder of retinal vessel development (MIM 133780) [1] first reported by Criswick and Schepens in 1969 [2]. Clinical features can be highly variable, even within the same family. Severely affected patients may be registered blind during the first decade of life, while mildly affected individuals may not even be aware of symptoms and are only diagnosed by fluorescein angiography [3]. The primary pathological process is believed to be a premature arrest of retinal angiogenesis/vasculogenesis or retinal vascular differentiation, leading to incomplete vascularization of the peripheral retina [4]. This failure to vascularize the peripheral retina is seen in all affected individuals, but by itself usually causes no clinical symptoms. The visual problems in FEVR result from secondary complications due to the development of hyperpermeable blood vessels, neovascularization, and vitreo-retinal traction. These features cause a reduction in visual acuity and in 20% of cases can lead to partial or total retinal detachment [4].

FEVR is genetically heterogeneous, with X-linked [5,6], autosomal dominant [7,8], and autosomal recessive [9,10] modes of inheritance described, autosomal dominant being the most common mode [11-13]. To date, one X-linked and two autosomal dominant loci have been mapped: EVR1 on chromosome 11q [14], EVR2 on chromosome Xp [15], and EVR3 on chromosome 11p [16].

The gene underlying X-linked FEVR, EVR2, has been identified and shown to be the same gene as that mutated in Norries disease (ND) [17]. This is a rare X-linked recessive neurodevelopmental disorder characterized by congenital blindness, with a proportion of patients (30%) also suffering from sensorineural deafness and mental disturbances [18]. The ND gene encodes a secreted protein (Norries disease protein; NDP) containing a cystine knot motif [19]. The function of NDP remains elusive despite the creation of a mouse with targeted disruption of the gene [20].

The autosomal dominant EVR3 gene has been mapped within a 14 cM interval on chromosome 11p12-p13, defined by markers GATA34E08 (telomeric) and D11S4102 (centromeric) [16]. To date, no other EVR3 family has been reported and the causative gene remains unknown.

The first FEVR locus to be described was EVR1, which was localised to chromosome 11q13-q23 in two large European families, segregating FEVR in an autosomal dominant fashion [14,21]. Additional linkage studies suggested that further families mapped to this region [11-13,22], but these studies assumed autosomal dominant FEVR was homogeneous and summed lod scores to obtain statistically significant results. With the identification of the EVR3 locus, such data must now be reassessed. Of these families, only one was sufficiently large to generate a lod score of greater than 3; unfortunately this family did not refine the disease interval [22].

Last year, Robitaille and colleagues mapped a large Canadian family to the EVR1 locus, enabling them to place the disease gene within an interval of 1.55 Mb, which contained only two genes [23]. Sequencing these genes in this family led to the identification of FZD4 as the defective gene at the EVR1 locus [23]. We undertook a study to analyze a family described by Price and co-workers, in which linkage to the EVR1 locus with a lod score of 5.5 was observed [22]. However, when FZD4 was screened by direct sequencing, no mutation was identified. We therefore went on to confirm, through detailed haplotype analysis, that FZD4 was excluded as the defective gene in this family. Our results identify a new autosomal dominant FEVR locus, EVR4, that maps 10 cM centromeric to EVR1 on chromosome 11q13.

Methods

Sequence analysis of FZD4

The exons and flanking splice junctions of FZD4 were amplified by PCR from genomic DNA with the primers detailed in Table 1. Due to its large size, exon 2 was amplified in five overlapping segments as shown in Figure 1. Reactions were carried out in a 50 μl volume with 50 ng of DNA, 20 pmol of each primer, 200 μM dATP, dCTP, dGTP, and dTTP, 10 mM Tris-HCl pH 8.3, 50 mM KCl, 1.5 mM MgCl₂, 0.01% gelatin, and 1 unit Taq DNA polymerase (Invitrogen, Paisley, UK). After the initial denaturation step at 96 °C for 3 min, the samples were processed through 35 cycles of 95 °C for 30 s, 60-65 °C for 30 s, and 72 °C for 30 s. A final extension step was performed at 72 °C for 10 min. Both the forward and reverse strands of the PCR products were directly sequenced on a Li-Cor Long Read IR 4200 sequencer using the Thermo Sequenase fluorescent labeled primer cycle sequencing kit (Amersham Pharmacia Biotech, Buckinghamshire, UK). Sequencing reactions were set up according to the manufacturer's instructions using either the forward or reverse primers detailed in Table 1.

Haplotype Analysis

Genotyping was performed using fluorescently tagged microsatellite markers. The Marshfield [24] and DeCODE maps [25] were used for the selection of markers and to determine the genetic distances. PCR reactions were carried out in a 25 μl volume with 50 ng of DNA, 20 pmol of each primer (a fluorescent and an unlabeled primer in each pair), 200 μM each dATP, dCTP, dGTP, dTTP, 10 mM Tris-HCl pH 8.3, 50 mM KCl, 1.5 mM MgCl₂, 0.01% gelatin, and 1 unit Taq DNA polymerase (Invitrogen). After the initial denaturation step at 96 °C for 3 min, the samples were processed through 35 cycles of 92 °C for 30 s, 50-60 °C for 30 s and 72 °C for 30 s. A final extension step was performed at 72 °C for 10 min. Following amplification, PCR products were resolved using an ABI 377 sequencer and were analyzed using GENSCAN 2.0.2 and GENOTPER 1.1.1 software (Applied Biosystems, Warrington, UK). Haplotypes were assembled using Cyrillic 2.1 software (Cherwell Scientific, Reading, UK).

Results & Discussion

After FZD4 was identified as the FEVR gene at the EVR1 locus, we screened genomic DNA from two affected individuals from a linked family for mutations in this gene by direct sequencing. Both strands of FZD4 were sequenced using four-lane chemistry (to enable easy detection of heterozygotes) and the resulting sequences were checked by two individuals. Unexpectedly, we were unable to detect a mutation in this gene. The clinical data for this family have been reported previously and was clearly consistent with a diagnosis of FEVR [22]. Earlier genetic analysis on this family had also shown that it undoubtedly linked to the EVR1 locus with a maximum lod score of 5.55 at a recombination fraction of 0.00, obtained for marker D11S533 (D11S533 is physically located between D11S1291 and D11S937, and maps genetically alongside D11S937) [22].

We investigated this finding further by performing high-resolution genotyping in nine individuals from this family (Figure 2). We analyzed 25 polymorphic markers located on chromosome 11q12-11q14. The haplotype data for the twelve most informative markers is shown in Figure 2. The order and genetic location of these markers is as follows: 11cen - D11S1298 (59.77 cM) - D11S1983 (64.21 cM) - D11S1368 (64.93 cM) - D11S4136 (71.60 cM) - D11S4139 (72.82 cM) - D11S4207 (76.13 cM) - D11S1291 (77.78 cM) - D11S937 (79.98 cM) - D11S4172 (82.57 cM) - D11S2002 (85.48 cM) - D11S1354 (87.89 cM) - D11S896 (89.69 cM) - 11qtel (genetic distances are taken from the chromosome 11p telomere). According to the University of California, Santa Cruz (UCSC) April 2003 Genome Browser, FZD4 is contained within the same genomic clone as the marker D11S896 (RP11-736K20-AP001528).

The affected individual III:7 clearly shows a crossover between the disease haplotype and markers D11S937, D11S4172, D11S2002, D11S1354, and D11S896. This crossover excludes FZD4 genetically and defines the telomeric limit of the disease interval in this family, D11S937, which is located 10 cM centromeric to FZD4.

Similarly, the haplotype in individual V:1 reveals a recombination event between the disease phenotype and markers D11S1298, D11S1983 and D11S1368. This crossover defines the centromeric flanking limit of the disease interval at the marker D11S1368. This family therefore contains a mutation in a new autosomal dominant FEVR gene located between D11S1368 and D11S937, a genetic distance of 15 cM. We have designated this new locus EVR4. This is the third autosomal dominant FEVR locus to be identified and brings the total number of mapped FEVR loci to four.

It has long been assumed that the EVR1 locus was the major, if not the only, locus for autosomal dominant FEVR. This assumption was based on the results of a series of linkage studies that report EVR1 linked pedigrees [11-14,22]. However, the results of mutation screening in FZD4 do not corroborate this hypothesis. The original study by Robitaille and colleagues identified FZD4 mutations in only two of the five autosomal dominant FEVR families they screened [23]. Likewise, preliminary studies by our group [26] and others [27] show that FZD4 mutations account for less than 30% of autosomal dominant FEVR. Initially, we postulated that more of these "EVR1 linked pedigrees" may be attributable to the neighbouring EVR4 locus, like the family featured in this study. Taken together, both the EVR1 and EVR4 loci may account for a large proportion of autosomal dominant FEVR, thereby giving the appearance of one major single locus. However, many of the papers describing EVR1 linked families based their studies on the assumption that autosomal dominant FEVR is homogeneous. A reappraisal of the available data suggests that only three of the pedigrees described are linked to EVR1 with statistically significant lod scores: the large German family referred to as family 1 by Li and colleagues [12,14,21]; the large Asian family analyzed in this and a previous study [22]; and the Canadian family used to identify the FZD4 gene [23]. None of the other families analyzed independently generated a lod score of above 2. Given the identification of two further autosomal dominant FEVR loci on chromosome 11 and evidence for still further locus heterogeneity [28], summing the lod scores of independent families is inappropriate, since it is likely that a proportion of these families will harbour mutations in other genes. It will therefore be interesting to see if any other researchers fail to find FZD4 mutations in families unambiguously linked to EVR1. This will provide a clue as to the frequency of EVR4 mutations and may help to refine the locus interval.

The occurrence of patients harboring mutations some distance outside the coding sequence of a causative gene is a well established phenomenon. These so called "position effect" mutations are thought to exert their effect by one of two different mechanisms. One method proposed is that the mutation disrupts a distant regulatory element or, in the case of chromosomal translocation, moves the gene away from the element. The second mechanism involves altering the expression of the gene through long-range effects on chromatin structure [29]. The distance between FZD4 and the new EVR4 locus is at least 10 cM and corresponds to a minimum of 9 Mb of DNA (UCSC April 2003). This distance is much larger than any previously reported for a position effect mutation. In humans, a microdeletion of 8 kb was found to exert an effect on the POU3F4 gene which was 900 kb away [30]. In Drosophila, chromatin effects have been reported up to 2400 kb away from a heterochromatin breakpoint [31]. Although we cannot rule out the possibility of a position effect mutation in this family, the large distances involved make it highly unlikely.

Surprisingly, all three mapped autosomal dominant FEVR loci are on chromosome 11. This is most likely to be because chromosome 11 is one of the most gene dense chromosomes [32]. The April 2003 UCSC human genome assembly records that the 15 cM EVR4 locus corresponds to over 18 Mb of DNA and potentially contains over 250 genes, making the identification of the EVR4 gene a daunting task. However, the discovery that FEVR can be caused by mutations in the Wnt receptor FZD4 highlights proteins involved in the Wnt signalling pathway as potential candidates for other FEVR genes [23]. It is therefore interesting to note that WNT11, a potential ligand for FZD4, and LRP5, a Frizzled co-receptor, are both located within the EVR4 critical region. Similarly, the vascular phenotypes observed in FEVR patients suggest that genes encoding proteins with a role in vascular development are also good candidate FEVR genes. One such gene mapping within the EVR4 region is vascular endothelial growth factor-B (VEFG-B). VEGF-B shows many structural similarities to VEGF-A, a protein crucial to normal vascular growth in the retina [33-35], and has been shown to be expressed in the developing retina [36]. Although no retinal vascular defects have been observed in Vegfb^-/- knockout mice [37], this does not exclude the gene from involvement in human FEVR as many mouse models of human disease fail to replicate the human condition [38].

Chromosome 11 is known to contain a number of loci/genes implicated in inherited blindness. Within the EVR4 locus are genes for Bardet-Biedl syndrome (BBS1) [39], digenic and possibly dominant Retinitis Pigmentosa (ROM1) [40,41], Best macular dystrophy (VMD2) [42], Usher syndrome (MYO7A) [43], osteoporosis-pseudoglioma syndrome (LRP5) [44], and the locus for autosomal dominant neovascular inflammatory vitreoretinopathy (VRNI) [45]. In the past, researchers have discussed the possibility that EVR1 could be allelic with VRNI [12,45]. Although FEVR and neovascular inflammatory vitreoretinopathy have distinct phenotypes [1,46], there is some phenotypic overlap and both have been mapped to the same region of chromosome 11q [14,45]. Recent opinion, based on reassessment of the original data in light of improved chromosome 11 genetic maps, suggested that these disorders are caused by mutations in different genes [23,47]. However, the identification of a new FEVR locus in an interval overlapping the VRNI locus raises the possibility that EVR4 and VRNI are allelic.

In summary, we have identified a third autosomal dominant FEVR locus (EVR4). Further studies in this and other FEVR pedigrees should lead to the identification of the gene responsible at this locus. FEVR is a defect of the retinal vasculature and the identification of FEVR genes is therefore likely to throw new light on the pathways and processes underlying retinal vasculogenesis, which may in turn provide insights into a range of different vascular disorders.

Acknowledgements

We should like to thank the FEVR family, without whose help this study would not have been possible. The financial support of the Wellcome Trust (grants 069718/Z/02, 055145/Z/98 and 035535/Z/96) is gratefully acknowledged.

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