Chromatin relaxation in response to DNA double-strand breaks is modulated by a novel ATM and KAP-1 dependent pathway

The cellular DNA-damage response is a signaling network that is vigorously activated by cytotoxic DNA lesions, such as double-strand breaks ( DSBs) 1. The DSB response is mobilized by the nuclear protein kinase ATM, which modulates this process by phosphorylating key players in these pathways(2). A long-standing question in this field is whether DSB formation affects chromatin condensation. Here, we show that DSB formation is followed by ATM-dependent chromatin relaxation. ATM's effector in this pathway is the protein KRAB-associated protein ( KAP-1, also known as TIF1 beta, KRIP-1 or TRIM28), previously known as a corepressor of gene transcription(3,4). In response to DSB induction, KAP-1 is phosphorylated in an ATM-dependent manner on Ser 824. KAP-1 is phosphorylated exclusively at the damage sites, from which phosphorylated KAP-1 spreads rapidly throughout the chromatin. Ablation of the phosphorylation site of KAP- 1 leads to loss of DSB-induced chromatin decondensation and renders the cells hypersensitive to DSB-inducing agents. Knocking down KAP-1, or mimicking a constitutive phosphorylation of this protein, leads to constitutive chromatin relaxation. These results suggest that chromatin relaxation is a fundamental pathway in the DNA-damage response and identify its primary mediators.