| Abstract [eng] |
The aim of this work was to investigate whether epigenetic clock results are influenced by extrinsic (cell number) and/or intrinsic (DNA modification) variability in the modification levels of cytosines over the course of a day. Firstly, it is shown that the results provided by 13 out of 17 different epigenetic clocks oscillate statistically significantly over the course of 24 hours and therefore depend on the time of day. This variation is partly explained by extrinsic variability as the proportions of NK, B and CD4 T cells were shown to oscillate statistically significantly over the course of the day in the samples tested and the Horwath pan-tissue 2013 clock gives different predicted ages for these cell types compared to the whole blood sample. However, internal variability is also a factor, as some clocks still retained oscillations in their results in the purified T-cell samples. The proportion of CD4, but not CD8, T-cells oscillated statistically significantly in these samples as well. In addition, it was found that for neutrophils on average 4.3% and for T cells 7.7% of the cytosines used by epigenetic clocks overlap with those whose level of modification oscillates over the course of the day. Finally, it has been shown that internal variability is not explained by variations in the levels of transcripts of proteins regulating DNA methylation over 24 h., but further studies are needed to confirm this conclusion. In summary, in future, the proportions of cell types and the timing of sample collection must be taken into account to ensure the accuracy of studies using epigenetic clocks. |
