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1From the Departments of Human Genetics and 3Ophthalmology, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands; the 2Department of Ophthalmology, Ghent University Hospital, Ghent, Belgium; the 4Centre de Genetique ULB, Hopital Erasme, Bruxelles, Belgium; and the 5Department of Ophthalmology and Visual Science and the 6Program in Human Molecular Biology and Genetics, Eccles Institute of Human Genetics, University of Utah, Salt Lake City, Utah.
| Abstract |
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METHODS. Ophthalmic examination and mutation screening by direct sequencing of the ELOVL4 gene was performed in two affected individuals. Wild-type and mutant ELOVL4 genes were expressed as enhanced green fluorescent protein (EGFP) fusion proteins in transient transfection in NIH-3T3 and HEK293 cells. To determine the subcellular localization of ELOVL4, an endoplasmic-reticulum (ER)specific marker for pDsRed2-ER was cotransfected with ELOVL4 constructs. Transfected cells were viewed by confocal microscopy. Western blot analysis was performed to assess protein expression using an anti-GFP antibody.
RESULTS. Affected patients exhibited macular atrophy with surrounding flecks characteristic of adSTGD-like MD. A novel ELOVL4 p.Tyr270X mutation was detected in affected individuals. In cell-transfection studies, wild-type ELOVL4 localized preferentially to the ER. In contrast, the mutant protein appeared to be mislocalized within transfected cells.
CONCLUSIONS. In a European family with adSTGD-like MD, a novel ELOVL4 mutation was found to underlie the disorder. Transfection studies indicated that, unlike wild-type ELOVL4, the mutant protein does not localize to the ER but rather appears to be sequestered elsewhere in an aggregated pattern in the cytoplasm. Further analysis of the function of normal and mutant ELOVL4 will provide insight into the mechanism of macular degeneration.
Herein, we describe the identification of a third ELOVL4 mutation in a European family with adSTGD-like macular dystrophy. To assess the functional consequence of this ELOVL4 mutation, we investigated the subcellular location of normal and mutant ELOVL4. We demonstrated that the wild-type enhanced green fluorescent protein (EGFP)-ELOVL4 fusion protein localizes to the ER compartment in transfected cells. In contrast, the mutant EGFP-ELOVL4 fusion protein does not localize to the ER but rather appears to be sequestered elsewhere in an aggregate pattern in the cytoplasm.
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Generation of Expression Constructs.
Wild-type and mutant ELOVL4 cDNAs were cloned separately into a pEGFPC1 vector (BD-Clontech, Palo Alto, CA). This vector utilizes a cytomegalovirus (CMV) promoter and expresses EGFP after transfection into mammalian cells. PCR was performed with one forward primer containing a KpnI site (5'-CGGGGTACCGCGATGGGGCTCCTGGACTC-3'), and two reverse primers containing a BamH1 site (5'-CGGGATCCCGTTAATCTCCTTTTGCTTTTC-3' for the wild-type ELOVL4 and 5'-CGGGATCCCGCTATGTCCGAATGTAGAAG-3' for the mutant 270X ELOVL4), with wild-type cDNAs used as a template. The resultant PCR products were digested with KpnI and BamHI, and cloned into the KpnI and BamHI sites of a pEGFPC1 vector in frame at the C-terminal end of EGFP. The recombinant plasmids containing EGFP-ELOVL4 fusion constructs were verified by direct DNA sequencing, amplified and purified with a plasmid isolation kit (Qiagen, Inc., Valencia, CA).
Cell Transfection and Imaging.
NIH3T3 and HEK293 were used for all transfection studies. The recombinant plasmids were transfected into the cell lines using Lipofectamine Reagent 2000 (Invitrogen-Gibco) according to the manufacturers protocol. Cells were monitored for fluorescence between 7 and 36 hours after transfection, by epifluorescence microscopy.
NIH3T3 cells were seeded onto poly-L-lysinecoated, four-well chamber glass slides at a confluence of 20% to 30% and transfected with recombinant plasmids containing wild-type or mutant ELOVL4. Cotransfection with an ER-organellespecific marker pDsRed2ER was performed with wild-type and mutant ELOVL4 to determine the subcellular localization of ELOVL4. Transfected cells were incubated at 37°C for 24 hours, washed with PBS, fixed in methanol-acetone (50:50, vol/vol), and mounted on glass slides with antifade medium (Vectashield; Vector Laboratories, Inc., Burlingame, CA) for microscopy. To assess subcellular localization of wild-type and mutant ELOVL4 protein, transfected cells were observed at different time intervals. However, all data were collected at approximately the same time point after transfection.
Images of green fluorescence were collected with a confocal laser scanning microscope (IX70; Olympus, Tokyo, Japan) using a 488-nm excitation source and a 505- to 550-nm band-pass barrier filter. A red fluorescence (DsRed2) marker for ER was examined using 568-nm excitation light from the Kn laser, a 575-nm dichromic mirror, and a 580- to 625-nm filter. The cells were illuminated sequentially to avoid bleed through (3.7 seconds per frame for EGFP and DsRed2), and images were collected in a single optical section of 0.35 µm, where they were compared for colocalization analyses.
Electrophoresis and Immunoblot Analysis.
To analyze the expression of wild-type and mutant ELOVL4, we grew transfected cells for 24 hours, harvested them from the plates, and briefly washed them with PBS. Cells were lysed on ice for 20 minutes with a buffer containing 1% Triton X-100, 0.01% SDS, 0.05 M Tris-HCl, and 1 mM EDTA (pH 7.5). The cell lysates were centrifuged at 4000 rpm for 5 minutes and the supernatants used for electrophoresis.
SDS-polyacrylamide gel electrophoresis (SDS-PAGE) was performed according to the method of Laemmli.12 Ten microliters of sample (
7 µg) was loaded onto a 9% polyacrylamide gel and electrophoresed at 110 V for 1 hour. The resolved proteins were transferred to a polyvinylidene difluoride (PVDF) membrane (Millipore, Billerica, MA) and blocked for 2 hours at room temperature with 5% skim milk in Tris-buffered saline containing 0.05% Tween 20 (TTBS). The membrane was incubated for 2 hours with monoclonal anti-GFP antibody diluted 1:2000 in 5% milk containing TTBS, and then probed with peroxidase-conjugated anti-mouse antibody (1:4000 dilution in TTBS; Amersham Biosciences, Piscataway, NJ) for 1 hour and developed with a chemiluminescence detection kit according to the manufacturers protocol (ECL; Amersham Bioscience).
| Results |
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Characterization of EGFP-ELOVL4 Fusion Proteins
We expressed wild-type and mutant ELOVL4 as EGFP fusion proteins to facilitate direct visualization of subcellular localization. Western blot analysis confirmed the synthesis of EGFP-ELOVL4 fusion proteins, and single bands were visualized for each construct at
61 kDa for wild-type ELOVL4 and
56 kDa for the EGFP-ELOVL4 truncated mutant (Fig. 3) .
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| Discussion |
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Herein, we describe the identification of a novel ELOVL4 mutation in a family of European origin with adSTGD-like MD. This c.810C>G mutation predicts a p.Tyr270X change.
Both previously identified ELOVL4 mutations were small deletions that generate a frameshift leading to a truncated protein lacking the last 51 amino acids.7 8 The p.Tyr270X mutation results in the absence of the last 45 amino acids. In all three cases, a KXKXX dilysine-targeting signal at the carboxyl terminus of the ELOVL4 protein is deleted. This signal is known to be responsible for the retention of transmembrane proteins in the ER,14 the site of very-long-chain fatty acid biosynthesis.15 As expected, subcellular localization studies showed that, unlike wild-type ELOVL4, the mutant does not localize to the ER. The mislocalized protein seems to be sequestered in another subcellular compartment exhibiting dense fluorescence-positive aggregates. Similar results were also observed with 5-bp ELOVL4 mutants.16 17 Future studies with additional cell markers and/or electron microscopy will elucidate the subcompartmental localization and underlying mechanism of the ELOVL4 mutant proteins.
It is interesting that all three ELOVL4 mutations were clustered in the C terminus; so far, no mutations in the N terminus have been reported. This may suggest a dominant negative nature of the mutant ELOVL4 protein, rather than a mechanism of haploinsufficiency. Consistent with this notion, the 5-bp deletion mutant causes cell death when transfected into cultured cells.17
The identification of the third mutation in the ELOVL4 gene further supports its important role in macular degeneration, and provides another entry point from which ELOVL4 function in retinal physiology and disease can be investigated.
| Acknowledgements |
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| Footnotes |
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Supported by the British Retinitis Pigmentosa Society (AM, FPMC). KZ is supported by Grants R01EY14428 and R01EY14448 from the National Eye Institute; the American Health Assistance Foundation; the Karl Kirchgessner Foundation; The Ruth and Milton Steinbach Fund; Ronald McDonald House Charities; Macular Vision Research Foundation; Val and Edith Green Foundation; and the Simmons Family Foundation. GK and ZY are supported by grants from Fight for Sight and the Knights Templar Eye Research Foundation.
Submitted for publication January 27, 2004; revised June 28, 2004; accepted August 9, 2004.
Disclosure: A. Maugeri, None; F. Meire, None; C.B. Hoyng, None; C. Vink, None; N. Van Regemorter, None; G. Karan, None; Z. Yang, None; F.P.M. Cremers, None; K. Zhang, None
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: Alessandra Maugeri, Department of Human Genetics, Radboud University Nijmegen Medical Centre, Geert Grooteplein 10, PO Box 9101, 6500 HB Nijmegen, The Netherlands; a.maugeri{at}antrg.umcn.nl.
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