Cancer incidence considerably increases in the aged population. Although chronological ageing reflects the general changes experienced by an organism with increasing age, it does not accurately reflect an individual’s biological or cellular age, which can vary across individuals of the same chronological age. Currently, epigenetic alterations are one of the primary determinants of cellular age, and DNA methylation sequencing has proven to be a valuable tool to assess this. However, given that the technique typically requires the disruption of the cell structure, it impedes longitudinal studies aimed at understanding cellular ageing within individuals.
We developed imaging-based chromatin and epigenetic age (ImAge), an approach that captures age-related trajectories of chromatin and epigenetic marks in single nuclei for longitudinal analyses. We used immunolabelling with antibodies specific to histone modifications associated with ageing to capture cell-specific epigenetic landscapes in cell nuclei isolated from mouse peripheral blood mononuclear cells and flash-frozen samples of mouse brain, heart, kidney, liver and skeletal muscle. The imaging signatures were extracted using threshold adjacency statistics, an image texture analysis to capture the heterogeneity of the spatial distribution of each histone modification. We generated the ImAge axis by creating a line between the average positions of the samples from youngest to oldest. Then, by projecting a sample onto this axis, we could measure how far along the ageing trajectory it is, and defined this positioning as the ImAge.