Lipofuscin accumulation, abnormal electrophysiology, and photoreceptor degeneration in mutant ELOVL4 transgenic mice: A model for macular degeneration

Abstract

Macular degeneration is a heterogeneous group of disorders characterized by photoreceptor degeneration and atrophy of the retinal pigment epithelium (RPE) in the central retina. An autosomal dominant form of Stargardt macular degeneration (STGD) is caused by mutations in ELOVL4, which is predicted to encode an enzyme involved in the elongation of long-chain fatty acids. We generated transgenic mice expressing a mutant form of human ELOVL4 that causes STGD. In these mice, we show that accumulation by the RPE of undigested phagosomes and lipofuscin, including the fluorophore, 2-[2,6-dimethyl-8-(2,6,6-trimethyl-1-cyclohexen-1-yl)-1E,3E,5E,7E-octatetraenyl]-1-(2-hyydroxyethyl)-4-[4-methyl-6-(2,6,6,-trimethyl-1-cyclohexen-1-yl)-1E,3E,5E-hexatrienyl]-pyridinium (A2E) is followed by RPE atrophy. Subsequently, photoreceptor degeneration occurs in the central retina in a pattern closely resembling that of human STGD and age-related macular degeneration. The ELOVL4 transgenic mice thus provide a good model for both STGD and dry age-related macular degeneration, and represent a valuable tool for studies on therapeutic intervention in these forms of blindness.phagosome ͉ Stargardt disease ͉ photoreceptor ͉ retinal pigment epithelium M acular degeneration involves the death of photoreceptor cells in the central retina, which is responsible for fine-detail vision. Age-related macular degeneration (AMD) affects Ϸ30% of people over the age of 75 (1, 2), and is becoming a greater health problem with the rapidly growing elderly population of developed countries. There is no treatment to halt or reverse the disease for the dry form, which comprises Ϸ90% of AMD cases. Moreover, there is a lack of suitable animal models for experimentation on therapies for dry AMD. Stargardt macular dystrophy (STGD) shares pathological features with AMD, except that it occurs at a young age. Both AMD and STGD are characterized by the accumulation of high levels of lipofuscin in the retinal pigment epithelium (RPE), which precedes degeneration of the photoreceptors in the macula and RPE atrophy.The gene responsible for an autosomal dominant form of STGD, STDG3, was identified recently as ELOVL4 (3, 4). It is predicted to encode an enzyme involved in the elongation of very long-chain fatty acids (hence the name, ELOVL), and is highly expressed in rod and cone photoreceptor cells (5, 6). Sequence analysis of human ELOVL4 cDNA predicts a protein of 314 aa that shares homology with members of the yeast Elo (elongation of long chain fatty acid) family and the human ELO1 homolog (HELO1) (3). HELO1 and the ELO family members possess biochemical features that suggest their participation in reduction reactions occurring during fatty acid elongation (7,8). Mutational analysis of the ELOVL4 gene in five large STGD-like macular dystrophy pedigrees revealed a 5-bp deletion, resulting in a frame-shift and the introduction of a stop codon, 51 codons from the end of the coding region (3). Subsequently, two 1-bp deletions, 789delT and 794delT, in ELOVL4 were identifi...

Deciphering mutant ELOVL4 activity in autosomal-dominant Stargardt macular dystrophy

“…The current study as well as others (7,31,35) have shown that tagged-MUT is mislocalized in cultured cells. Mislocalization of the tagged-MUT to photoreceptor outer segments and inner retinal layers in transgenic pigs results in aberrations in photoreceptor structure and function (46).…”

“…Several groups have shown interaction between wildtype and mutant protein (8,31,35), suggestive of a possible dominant-negative effect in disease manifestation. To determine the influence of this interaction on the enzymatic activity of ELOVL4, we coexpressed untagged ELOVL4 with each of the HA-tagged constructs (WT, MUT, and mERET) in ARPE19 cells.…”

The Complement Regulatory Protein CD46 Deficient Mouse Spontaneously Develops Dry-Type Age-Related Macular Degeneration–Like Phenotype

“…53 The connection of the complement system to AMD has been reported in some murine models of dry-type AMD. 25,44,51,52,54,57,62 Retinal abnormalities have been described in aged FH deficient (Cfh À/À ) mice. 51 However, the disease phenotype was modest because these animals displayed a partial phenotype of dry AMD.…”

Role of Stargardt-3 macular dystrophy protein (ELOVL4) in the biosynthesis of very long chain fatty acids

“…T hree independent mutations in the last exon (exon-VI) of the ELOVL4 gene are associated with dominant Stargardt-like macular dystrophy (STGD3) in humans (1)(2)(3)(4). These mutations cause a frame-shift that introduces a stop codon, resulting in premature termination of the protein and removal of the signal sequence for targeting the protein to its putative cellular location, the endoplasmic reticulum (1,4).…”

“…These mutations cause a frame-shift that introduces a stop codon, resulting in premature termination of the protein and removal of the signal sequence for targeting the protein to its putative cellular location, the endoplasmic reticulum (1,4). As a result, the mutant protein mis-localizes and aggregates (3,5,6), and, when coexpressed with the wild type protein, the mutant and wild-type proteins associate and mis-localize (3,7).…”