Genomic instability is a conserved hallmark of eukaryotic aging and increased DNA damage load has been shown to promote both cancer and age-related diseases. Moreover, epigenetic changes such as altered histone modification patterns appear sufficient to modulate life span in model organisms. We have previously uncovered a potential link between DNA damage and age-related epigenomic changes, suggesting that DNA damage can alter chromatin structure at DNA breaks and beyond. Specifically, the histone modifier SIRT1 is redistributed across chromatin in response to DNA damage, moving away from SIRT1-regulated promoters to sites of DNA damage. This reorganization may be critical for efficient DNA repair, but comes at the cost of deregulation of SIRT1 target genes, which mirrors aspects of age-associated transcriptional deregulation. This work led to the more general hypothesis that a DNA damage-induced redistribution of chromatin modifiers (or 'RCM response') may underlie the alterations in gene expression and genomic stability that characterize eukaryotic aging. In light of these findings, it will be of great interest to determine if DNA damage-induced epigenetic changes can indeed induce age-related organ pathologies.OBJECTIVE AND RESULTS: To better understand how DNA damage and its repair affect the (epi-)genome over a lifetime, and how these changes impinge on tissue homeostasis, disease progression and mammalian aging, we generated a mouse model that allows for the conditional induction of DNA double-strand breaks (DSB-mice). In this model, the DSB-inducing homing endonuclease I-PpoI is under the control of both Cre-recombinase and a Tamoxifen-activated nuclear translocation domain, and optimal DSB induction requires both Cre and Tamoxifen. DSB-mice will be analyzed for (i) overall chromatin (re)organization following DSB induction using ChIP-seq technology and (ii) DNA damage-associated changes in tissue function using histology and biochemical approaches.We are presently investigating DSB induction and accumulation in the presence and absence of Tamoxifen following tissue-specific induction of the I-PpoI endonuclease. We further generated DSB-mice under the control of a ubiquitous Tamoxifen-inducible Cre, which allows us to investigate the impact of DSBs on a variety of cell lineages, particularly the irradiation sensitive hematopoetic system. Using these mice, we will further be able to isolate mature lymphocytes and induce DSBs in vitro to study (epi)genomic changes in primary cells. Given the dual role of macroH2A and related heterochromatic features during DSB repair and cellular senescence (see Project 1), it will be of particular interest to follow the genomic distribution of these marks in response to DSB induction in vivo. This analysis will determine the relevance of repressive chromatin in DNA repair beyond a single transgenic break site and will simultaneously address global chromatin reorganization in response to DNA damage. The latter further allows for a direct comparison with known, age-related epigenomic changes.Regarding the physiological consequences of DSB induction, we will initially focus on brain-specific DSB induction and perform a detailed histopathological analysis assessing hallmarks of brain aging such as brain atrophy, apoptosis and gliosis. We will focus on cortex and hippocampus, which show efficient transgene activation and serve as central mediators of many aspects of learning and memory. Brain sections will be analyzed at different time points after (chronic) 4OHT administration. Should we detect signs of DSB-induced brain pathology, we will determine possible changes in learning and memory formation using a variety of behavioral tests, which are established in our program.IMPLICATIONS: The potential of DNA damage to affect cell function both through direct DNA alterations and through indirect, epigenetic changes in chromatin structure puts it at a critical position to influence eukaryotic aging. A better understanding of how DNA breaks and their repair impinge on tissue homeostasis, cancer and aging is, therefore, critical for the design of targeted intervention, which may be able to prevent or improve many of the negative aspects of DNA break induced chromatin reorganization.
