Accumulation of DNA Damage-Induced Chromatin Alterations in Tissue-Specific Stem Cells: The Driving Force of Aging?

Abstract

Accumulation of DNA damage leading to stem cell exhaustion has been proposed to be a principal mechanism of aging. Using 53BP1-foci as a marker for DNA double-strand breaks (DSBs), hair follicle stem cells (HFSCs) in mouse epidermis were analyzed for age-related DNA damage response (DDR). We observed increasing amounts of 53BP1-foci during the natural aging process independent of telomere shortening and after protracted low-dose radiation, suggesting substantial accumulation of DSBs in HFSCs. Electron microscopy combined with immunogold-labeling showed multiple small 53BP1 clusters diffusely distributed throughout the highly compacted heterochromatin of aged HFSCs, but single large 53BP1 clusters in irradiated HFSCs. These remaining 53BP1 clusters did not colocalize with core components of non-homologous end-joining, but with heterochromatic histone modifications. Based on these results we hypothesize that these lesions were not persistently unrepaired DSBs, but may reflect chromatin rearrangements caused by the repair or misrepair of DSBs. Flow cytometry showed increased activation of repair proteins and damage-induced chromatin modifications, triggering apoptosis and cellular senescence in irradiated, but not in aged HFSCs. These results suggest that accumulation of DNA damage-induced chromatin alterations, whose structural dimensions reflect the complexity of the initial genotoxic insult, may lead to different DDR events, ultimately determining the biological outcome of HFSCs. Collectively, our findings support the hypothesis that aging might be largely the remit of structural changes to chromatin potentially leading to epigenetically induced transcriptional deregulation.

Radiation Sensitivity of Human CD34+Cells Versus Peripheral Blood T Lymphocytes of Newborns and Adults: DNA Repair and Mutagenic Effects

“…This study showed that the number of residual DSBs 24 h after exposure is significantly lower in CD34 þ cells than in newborn T lymphocytes, pointing to enhanced repair of DNA DSBs. This is possibly linked to differences in chromatin structure, which has a major influence on the cellular response to DNA damage (33). Indeed, there is a growing body of intriguing evidence to suggest that stem cells use specific DDR mechanisms for ensuring genomic integrity over a lifetime, and a possible defense mechanism could be their chromatin structure.…”

Stem Cell-Specific Mechanisms Ensure Genomic Fidelity within HSCs and upon Aging of HSCs

“…In murine hair follicle stem cells, 53BP1 foci, a mechanistic marker for DNA DSBs, did not overlap with γH2AX foci, and instead γH2AX foci were identified as a sign for chromatin alterations upon aging (Schuler and Rube, 2013). Very recently it could be shown that elevated levels of γH2AX foci in aged HSCs can be also associated with replication and ribosomal biogenesis stress and might therefore not be an optimal marker for persistent DNA damage (Flach et al, 2014).…”

Skin mirrors human aging

“…Experiments with mouse skin have shown that epidermal stem cells are resistant to cellular aging [95,96], highlighting the role of the microenvironment (stem cell niche) in agerelated stem cell exhaustion [80,88,97]. Accumulation of 53BP1 foci throughout the highly compacted heterochromatin of aged hair-follicle stem cells confirmed that DNA damage is between the primary mechanisms of stem cell exhaustion [98].…”