DNA methylation aging and transcriptomic studies in horses
Human DNA methylation profiles have been used successfully to develop highly accurate biomarkers of aging ("epigenetic clocks"). Here, we describe epigenetic clocks for horses, based on methylation profiles of CpGs with flanking DNA sequences that are highly conserved between multiple mammalian species. Methylation levels of these CpGs were measured using a custom-designed Infinium array (HorvathMammalMethylChip40). We generated 336 DNA methylation profiles from 42 different horse tissues and body parts, which we used to develop five epigenetic clocks for horses: a multi-tissue clock, a blood clock, a liver clock and two dual-species clocks that apply to both horses and humans. Epigenetic age measured by these clocks show that while castration affects the basal methylation levels of individual cytosines, it does not exert a significant impact on the epigenetic aging rate of the horse. We observed that most age-related CpGs are adjacent to developmental genes. Consistently, these CpGs reside in bivalent chromatin domains and polycomb repressive targets, which are elements that control expression of developmental genes. The availability of an RNA expression atlas of these tissues allowed us to correlate CpG methylation, their corresponding contextual chromatin features and gene expression. This analysis revealed that while increased methylation of CpGs in enhancers is likely to repress gene expression, methylation of CpGs in bivalent chromatin domains on the other hand is likely to stimulate expression of the corresponding downstream loci, which are often developmental genes. This supports the notion that aging may be accompanied by increased expression of developmental genes. It is expected that the epigenetic clocks will be useful for identifying and validating anti-aging interventions for horses.