The development of epigenetic clocks for the assessment of biological age is a popular area of study, but connecting characteristic age-related epigenetic changes at specific CpG sites on the genome to specific underlying mechanisms of aging is slow going at best. There are many such sites and only so many scientists and only so much funding. An example of this sort of work is presented here, illustrative of the complexity involved in this area of research. The gene ELOVL2 is associated with a few sites that are strongly linked to age. There is a lot to say about potentially relevant mechanisms, and a great many gaps left to be filled in, even when just focused down on a single small part of the body, the retina in this case.
Epigenetic aging of tissues and organs has been tightly correlated with global genome DNA methylation changes in specific regions, called CpG islands. A number of recent studies have shown that CpG methylation (CpGme) patterns progressively change during aging in a variety of tissues and cells such as blood, muscle, brain, lung, and colon. One major question is whether these methylation changes merely correlate with aging, or if there any functional role of these epigenetic changes in regulating aging. Interestingly, within the top ten markers predictive of human epigenetic age, four are localized in the CpG islands in the regulatory element of the ELOVL2 gene, accounting for over 70% of the one "methylation clock" model. Consequently, methylation of the ELOVL2 regulatory region has been shown in many studies to correlate strongly with the biological age of individuals, as well as in rodents.
ELOVL2 is an enzyme that elongates long-chain omega-3 and omega-6 polyunsaturated fatty acids (LC-PUFAs), precursors of docosahexaenoic acid (DHA) and very-long-chain PUFAs (VLC-PUFAs), playing important role in retina biology. The fatty acids composition in the retina is unique - the retina is particularly enriched in PUFAs, with DHA constitutes 40-50% of the total fatty acids in the photoreceptor outer disc membranes. PUFAs are well known to play important roles in the retina and deficiency of LC-PUFAs has been shown to be associated with increased risk of the dry form of age-related macular degeneration (AMD), a highly prevalent retinal disease. Recent studies suggest that individuals who self-reported intake of foods rich in omega 3 PUFAs were 30% less likely to develop central geographic atrophy (GA) and 50% less likely to develop AMD than subjects with the lowest self-reported intake.
While methylation of the ELOVL2 promoter is highly correlated with chronological age, whether ELOVL2 protein has a functional role in aging has not been investigated. We observed an age-dependent increase in Elovl2 regulatory region methylation associated with concomitant downregulation of Elovl2 expression on mRNA and protein levels. Next we observed Elovl2 expression in cone and rod photoreceptors, as well as the retinal pigment epithelium. We also observed a significant age-related decline of the expression of the Elovl2 in the eye. The same age-dependent changes of Elovl2 methylation and gene expression were observed in the mouse liver, indicating that age-associated methylation of Elovl2 occurs in multiple tissues in the mouse, similarly to what was observed previously in humans.
Next, we investigated the function of Elovl2 in aging in vivo. As Elovl2 heterozygous mice are infertile, we created a knock-in point mutation using Crispr-Cas9 technology, Elovl2-C234W. Inhibiting Elovl2 accelerates aging in the mouse retina. Using lipidomics, we confirmed that Elovl2-C234W mutation results in loss of ELOVL2-specific function. We further investigated the effect of Elovl2-C234W mice on both anatomic and functional surrogates of aging in the mouse eye. These included autofluorescent (AF) deposits in the fundus, which increases with age, as well as the electroretinogram (ERG), which shows a decrease in the maximum scotopic response with age. In Elovl2-C234W mice, we noticed an increase in AF deposits as well as a decrease in ERG compared to age-matched controls, suggesting that inhibiting Elovl2 accelerates aging in the mouse retina.