Transcriptomic Analysis of Genes Associated with Nucleic Acid and Histone Methylation and One-Carbon Metabolism in a Mouse Cone Photoreceptor-Derived Cell Line Treated with 7-Dehydrocholesterol-Derived Oxysterols

Methylations of nucleic acids and histones are epigenetic modifications that are integral to normal development and function, as well as disease and dysfunction, of the nervous system, including the retina. One such disease is Smith-Lemli-Opitz syndrome (SLOS). 661W cells (a mouse cone photoreceptor-derived cell line) were incubated with cytotoxic concentrations of either 5,9-endoperoxycholest-7-ene-3β,6α-diol (EPCD) or 7-ketocholesterol (7kCHOL), two oxidation products of 7-dehydrocholesterol (7DHC); the latter is the cholesterol (CHOL) precursor that accumulates in SLOS as a result of mutations affecting the last step of CHOL synthesis. Preceding loss of cell integrity, RNA was harvested and analyzed by gene array, using CHOL as a nontoxic control. Differentially expressed genes (DEGs) were defined by comparison with vehicle control (VC)-treated cells. Both oxysterols induced DEGs involved in DNA demethylation and increased expression of Mettl3, coding for a signature mRNA methyltransferase. Treatments with both oxysterols upregulated histone MT genes, while 7kCHOL incubation yielded a profile of numerous histone demethylase DEGs, including some previously linked with photoreceptor cell death. EPCD and 7kCHOL both upregulated signature genes of one-carbon metabolism (OCM), which generates S-adenosylmethionine, the primary methyl donor for methyltransferases. These findings suggest new directions for basic and translational studies of hereditary retinal diseases.