Quantifying site-specific chromatin mechanics and DNA damage response.

DNA double-strand breaks pose a direct threat to genomic stability. Studies of DNA damage and chromatin dynamics have yielded opposing results that support either increased or decreased chromatin motion after damage. In this study, we independently measure the dynamics of transcriptionally active or repressed chromatin regions using particle tracking microrheology. We find that the baseline motion of transcriptionally repressed regions of chromatin are significantly less mobile than transcriptionally active chromatin, which is statistically similar to the bulk motion of chromatin within the nucleus. Site specific DNA damage using KillerRed tags induced in loci within repressed chromatin causes an increased motion, while loci within transcriptionally active regions remains unchanged at similar time scales. We also observe a time-dependent response associated with a further increase in chromatin decondensation. Global induction of damage with bleocin displays similar trends of chromatin decondensation and increased mobility only at 53BP1-labeled damage sites but not at non-damaged sites, indicating that chromatin dynamics are tightly regulated locally after damage. These results shed light on the evolution of the local and global DNA damage response associated with chromatin remodeling and dynamics, with direct implications for their role in repair.