Mutant ELOVL4 that causes autosomal dominant stargardt-3 macular dystrophy is misrouted to rod outer segment disks.
OBJECTIVE
Autosomal dominant Stargardt macular dystrophy caused by mutations in the Elongation of Very Long Chain fatty acids (ELOVL4) gene results in macular degeneration, leading to early childhood blindness. Transgenic mice and pigs expressing mutant ELOVL4 develop progressive photoreceptor degeneration. The mechanism by which these mutations cause macular degeneration remains unclear, but have been hypothesized to involve the loss of an ER-retention dilysine motif located in the extreme C-terminus. Dominant negative mechanisms and reduction in retinal polyunsaturated fatty acids also have been suggested. To understand the molecular mechanisms involved in disease progression in vivo, we addressed the hypothesis that the disease-linked C-terminal truncation mutant of ELOVL4 exerts a dominant negative effect on wild-type (WT) ELOVL4, altering its subcellular localization and function, which subsequently induces retinal degeneration and loss of vision.


METHODS
We generated transgenic Xenopus laevis that overexpress HA-tagged murine ELOVL4 variants in rod photoreceptors.


RESULTS
Tagged or untagged WT ELOVL4 localized primarily to inner segments. However, the mutant protein lacking the dilysine motif was mislocalized to post-Golgi compartments and outer segment disks. Coexpression of mutant and WT ELOVL4 in rods did not result in mislocalization of the WT protein to outer segments or in the formation of aggregates. Full-length HA-tagged ELOVL4 lacking the dilysine motif (K308R/K310R) necessary for targeting the WT ELOVL4 protein to the endoplasmic reticulum was similarly mislocalized to outer segments.


CONCLUSIONS
We propose that expression and outer segment mislocalization of the disease-linked 5-base-pair deletion mutant ELOVL4 protein alters photoreceptor structure and function, which subsequently results in retinal degeneration, and suggest three possible mechanisms by which mutant ELOVL4 may induce retinal degeneration in STGD3.
Department of Ophthalmology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, United States Dean McGee Eye Institute, Oklahoma City, Oklahoma, United States.
Martin-Paul
MP
Agbaga
Department of Ophthalmology and Vancouver Eye Care Center, University of British Columbia, Vancouver, Canada.
Beatrice M
BM
Tam
Department of Ophthalmology and Vancouver Eye Care Center, University of British Columbia, Vancouver, Canada.
Jenny S
JS
Wong
Department of Ophthalmology and Vancouver Eye Care Center, University of British Columbia, Vancouver, Canada.
Lee Ling
LL
Yang
Department of Ophthalmology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma, United States.
Robert E
RE
Anderson
Department of Ophthalmology and Vancouver Eye Care Center, University of British Columbia, Vancouver, Canada.
Orson L
OL
Moritz
D000818:Animals; D004247:DNA / Q000235:genetics*; D004252:DNA Mutational Analysis; D004195:Disease Models, Animal; D005136:Eye Proteins / Q000235:genetics* / Q000378:metabolism; D007150:Immunohistochemistry; D008268:Macular Degeneration / Q000151:congenital* / Q000235:genetics / Q000378:metabolism / Q000473:pathology; D008565:Membrane Proteins / Q000235:genetics* / Q000378:metabolism; D051379:Mice; D008822:Mice, Transgenic; D018613:Microscopy, Confocal; D009154:Mutation; D012374:Rod Cell Outer Segment / Q000378:metabolism* / Q000473:pathology; D014982:Xenopus laevis / Q000235:genetics
C498834:Elovl4 protein, mouse; D005136:Eye Proteins; D008565:Membrane Proteins; D004247:DNA
autosomal dominant Stargardt-like macular dystrophy (STGD3); elongation of very long chain fatty acids-4 (ELOVL4); photoreceptor outer segment; retinal degeneration