Genetic ablation of the TET family in retinal progenitor cells impairs photoreceptor development and leads to blindness

Rod and cone photoreceptors are critical for vision, and their loss leads to blindness. Photoreceptors are epigenetically unique, because the promoters of the genes required for the development and function of these neurons are hypermethylated in retinal progenitor cells (RPCs) and hypomethylated in photoreceptors. However, the mechanism responsible for DNA demethylation during the differentiation of RPCs into photoreceptors and its role in photoreceptor development and function were unknown. We hypothesized that the Ten-Eleven Translocation (TET) family of dioxygenases plays a key role in this mechanism. To this end, we knocked out all TET genes in RPCs and characterized the TET-deficient and control retinas using various approaches including electron microscopy, electroretinogram (ERG) tests, RNA-seq, whole genome bisulfite sequencing (WGBS), and 5hmC-Seal. We found that genetic ablation of the TET family prevents demethylation of the promoters of genes essential for rod specification and for rod and cone maturation during the differentiation of RPCs into photoreceptors. Preservation of methylated cytosines in the promoters of these genes significantly reduced their expression, which was confirmed by western blot analysis. This impaired expression leads to the underdevelopment or complete absence of outer segments and synaptic termini in the photoreceptors of TET-deficient retinas, which results in loss of rod and cone function, as assayed by ERG. These function-deprived, underdeveloped photoreceptors die over time, which leads to blindness.