Deciphering ELOVL4 mutant activity in retinal degeneration

Abstract

Purpose : ELOngation of Very Long-chain fatty acids-4 (ELOVL4) is an enzyme that catalyzes the biosynthesis of very long chain saturated (VLC-SFA) and polyunsaturated (VLC-PUFA) fatty acids, collectively known as very long chain fatty acids (VLC-FA). ELOVL4 is only expressed in the retina, brain, skin, testis, and Meibomian glands. Different mutations in ELOVL4 affect these tissues in different ways. Some known mutations in humans only affect the retina causing a juvenile form of macular degeneration known as Autosomal Dominant Stargardt-like Macular Dystrophy (STGD3), while other mutations affect the brain and/or skin leading to Spinocerebellar Ataxia-34 (SCA34) and skin lesions. How the different ELOVL4 mutations cause such different tissue-specific pathologies are not well understood. We hypothesize that the various mutant ELOVL4 enzymes synthesize less and varying amounts of VLC-FA in a tissue-specific manner to cause the different tissue-specific pathologies.

Methods : We transduced ELOVL4 variants (L168S, L168F, W246G, and STGD3) in human embryonic kidney cells (HEK 293T) or human retinal epithelial cells (ARPE-19) and treated the cells with VLC-FA precursors lignoceric acid (24:0) and eicosapentaenoic acid (EPA; 20:5n3). The cells were harvested for fatty acid analyses to determine the level of VLC-FA biosynthesis. The effect of the different ELOVL4 variants on cell health was also examined using cell-based assays.

Results : The truncated ELOVL4 mutations that cause STGD3 did not synthesize VLC-FA. However, the SCA34 proteins, although enzymatically active, synthesized lesser quantities of VLC-FA than that of wild-type ELOVL4. Further, the accumulation of mutant ELOVL4 protein triggers endoplasmic reticulum (ER) stress through the eIF2α–ATF4 pathway and leads to cell death. We were able to alleviate the ER stress and cell death by supplementation with unsaturated fatty acids (EPA) while saturated fatty acids (24:0) exacerbated cell death.

Conclusions : The SCA34 ELOVL4 proteins make less VLC-FA product and promote cell death via ER stress, which can be alleviated with EPA supplementation. Future experiments will delve deeper into understanding the mechanisms of retinal degeneration in ELOVL4-related diseases.

This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.